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  1. Zhi-Hua Zhou. Machine Learning. Springer Singapore, 2021. Keyword(s): Machine Learning, Book on Machine Learning, Computer Science, Learning Algorithms, Neural Networks, Support Vector Machines, Decision Trees, Classification, Clustering, Supervised Learning, Semi-Supervised Learning, Unsupervised Learning, Metric Learning, Feature Selection, Rule Learning, Mathematical Models, Reinforcement Learning, Bayesian Networks, ML, AI, Artificial Intelligence.
    Abstract: This is an introductory-level machine learning textbook. To make the content accessible to a wider readership, the author has tried to reduce the use of mathematics. However, to gain a decent understanding of machine learning, basic knowledge of probability, statistics, algebra, optimization, and logic seems unavoidable. Therefore, this book is more appropriate for advanced undergraduate or graduate students in science and engineering, as well as practitioners and researchers with equivalent background knowledge. The book has 16 chapters that can be roughly divided into three parts. The first part includes Chapters 1-3, which introduces the basics of machine learning. The second part includes Chapters 4-10, which presents some classic and popular machine learning methods. The third part includes Chapters 11-16, which covers advanced topics. As a textbook, Chapters 1-9 and 10 can be taught in one semester at the undergraduate level, while the whole book could be used for the graduate level. This introductory textbook aims to cover the core topics of machine learning in one semester, and hence is unable to provide detailed discussions on many important frontier research works. The author believes that, for readers new to this field, it is more important to have a broad view than drill down into the very details. Hence, in-depth discussions are left to advanced courses. However, readers who wish to explore the topics of interest are encouraged to follow the further reading section at the end of each chapter. The book was originally published in Chinese and had a wide readership in the Chinese community. The author would like to thank Dr. Shaowu Liu for his great effort of translating the book into English and thank Springer for the publication.
    [bibtex-key = bookZhiHuaZhou2021MachineLearning] [bibtex-entry]


  2. Marc Peter Deisenroth, A. Aldo Faisal, and Cheng Soon Ong. Mathematics for Machine Learning. Campbridge University Press, 2020. Keyword(s): Machine Learning, Textbook, Introduction to Machine Learning, Mathematical Foundations, Linear Algebra, Analytic Geometry, Matrix Decompositions, Vector Calculus, Probability, Distributions, Optimization, Central Machine Learning Problems, Models and Data, Linear Regression, Dimensionality Reduction, Principle Components Analysis, Density Estimation, Gaussian Mixture, Support Vector Machines. [bibtex-key = bookDeisenrothFaisalOng2020MathematicsForMachineLearning] [bibtex-entry]


  3. MATLAB. v. 9.6.0.1135713 (R2019a) Update 3. The MathWorks Inc., Natick, Massachusetts, 2019. Note: Function robustfit. [bibtex-key = MATLAB2019aRobustfit] [bibtex-entry]


  4. Matthias Landgraf. Smart data for sustainable Railway Asset Management: railway track: assessment - aggregation - asset management, Monographic Series TU Graz : Railway Research. Verlag der Technischen Universität Graz, September 2018. Note: Basiert auf der Dissertation: Zustandsbeschreibung des Fahrwegs der Eisenbahn - von der Messdatenanalyse zum Anlagenmanagement, TU Graz, 2016.
    Abstract: Cost pressure forces infrastructure managers to work sustainably and efficiently. There-fore, track engineers face increasing difficulty to carry out necessary measures owing to budget restrictions. Consequently, they should be supported in prioritising. This requires an objective tool enabling proper condition monitoring as well as component-specific, preventive maintenance and renewal planning. Hence, the right measures are to be executed at the right time. This dissertation deals with a description of the railway track condition. A bottom-up approach provides an in-depth assessment of track using a variety of measurement signals and an aggregated component-specific assessment. Since the approach is based on well positioned measurement signals, it is valid for monitoring specific track sections as well as whole networks. Innovative analyses of various measurement signals form a sound basis to grasp their characteristics enabling a component specific condition evaluation of railway track. The use of historical measurement data allows for an analysis of track behaviour over time. A thorough validation process, including on-site inspections and excavations, shows that the presented approach is able to evaluate the actual condition of railway track. The assessment of the specific components condition can be used for timely maintenance as well as renewal planning. Based on correlation analyses, the component specific evaluations are aggregated into one holistic quality figure. This enables asset managers to monitor the asset condition network-wide as well as to predict future budget demands.
    [bibtex-key = landgraf2018SmartDataForSustainableRailwayAssetManagement] [bibtex-entry]


  5. Akira Ishimaru. Electromagnetic Wave Propagation Radiation and Scattering. John Wiley and Sons, Hoboken, New Jersey, 2017. Keyword(s): Scattering Theory, Wave Propagation, Radiation, Scattering, advanced analytical theory, dielectric slab, electromagnetic scattering, electromagnetic theory, electromagnetic wave propagation, excitation, Maxwell's equations, microwave waveguides, Wentzel-Kramers-Brillouim solution, boundary conditions, electromagnetic waves, energy relations, fundamental field equations, Hertz vectors, Poynting's theorem, reciprocity theorem, scalar acoustic waves, time-harmonic wave, Time-Reversal Imaging, Scattering by Turbulence, Particles, Diffuse Medium, Rough Surfaces, Coherence in Multiple Scattering, Diagram Method, Solitons, Optical Fibers, Porous Media, Permittivity, Fluid Permeability.
    Abstract: The book presents the fundamentals and the basic formulations of electromagnetic theory as well as advanced analytical theory and mathematical techniques and current new topics and applications. It starts with Maxwell's equations and covers the fundamental concepts and relationships as energy relations, potentials, Hertz vectors, and uniqueness and reciprocity theorems. Then, the book deals with microwave waveguides, dielectric waveguides, and cavities. It further presents selected topics on antennas, apertures, and arrays. The book presents discussion on are arrays with nonuniform spacings, microstrip antennas, mutual couplings, and the integral equation for current distributions on wire antennas. It also deals with excitation and scattering by sources, patches, and apertures embedded in planar structures. Excitation of a dielectric slab is discussed, followed by the Wentzel-Kramers-Brillouim (WKB) solution for the excitation of waves in inhomogeneous layers.
    [bibtex-key = bookIshimaru2017ElectromagneticWavePropagationRadiationAndScattering] [bibtex-entry]


  6. M. Heublein, F. Alshawaf, M. Mayer, S. Hinz, and B. Heck. Towards a rigorous fusion of GNSS and InSAR observations for the purpose of water vapor retrieval. 2014. Note: Cited By 0. [bibtex-key = Heublein20141] [bibtex-entry]


  7. Bruce Hapke. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, 2 edition, 2012. Keyword(s): Coherent Backscatter Opposition Effect. [bibtex-key = bookHapke2000TheoryOfReflectanceAndEmittanceSpectroscopy] [bibtex-entry]


  8. Roland Siegwart. Introduction to autonomous mobile robots, Intelligent robotics and autonomous agents series. MIT Press, Cambridge, Massachusetts, 2nd ed. / / Roland Siegwart, Illah R. Nourbakhsh, and Davide Scaramuzza edition, 2011. Keyword(s): Autonomous robots.
    Abstract: Mobile robots range from the Mars Pathfinder mission's teleoperated Sojourner to the cleaning robots in the Paris Metro. This text offers students and other interested readers an introduction to the fundamentals of mobile robotics, spanning the mechanical, motor, sensory, perceptual, and cognitive layers the field comprises. The text focuses on mobility itself, offering an overview of the mechanisms that allow a mobile robot to move through a real world environment to perform its tasks, including locomotion, sensing, localization, and motion planning. It synthesizes material from such fields as kinematics, control theory, signal analysis, computer vision, information theory, artificial intelligence, and probability theory. The book presents the techniques and technology that enable mobility in a series of interacting modules. Each chapter treats a different aspect of mobility, as the book moves from low-level to high-level details. It covers all aspects of mobile robotics, including software and hardware design considerations, related technologies, and algorithmic techniques.] This second edition has been revised and updated throughout, with 130 pages of new material on such topics as locomotion, perception, localization, and planning and navigation. Problem sets have been added at the end of each chapter. Bringing together all aspects of mobile robotics into one volume, Introduction to Autonomous Mobile Robots can serve as a textbook or a working tool for beginning practitioners.
    [bibtex-key = bookSiegwartNourbakhshScaramuzza2011AutonomousMobileRobots] [bibtex-entry]


  9. Jakob Van Zyl and Yunjin Kim. Synthetic Aperture Radar Polarimetry. John Wiley & Sons, Inc., 2011. Keyword(s): SAR Processing, SAR, Polarimetry, SAR Polarimetry, Synthetic Aperture Radar, Scattering, Microwave Remote Sensing, Remote Sensing, Polarization, Scattering Models, Microwave Scattering, Electromagnetic Scattering, calibration algorithms, polarimetric calibration, calibration, scattering from rough surfaces, surface scattering, soil moisture.
    Abstract: This book describes the application of polarimetric synthetic aperture radar to earth remote sensing based on research at the NASA Jet Propulsion Laboratory (JPL). This book synthesizes all current research to provide practical information for both the newcomer and the expert in radar polarimetry. The text offers a concise description of the mathematical fundamentals illustrated with many examples using SAR data, with a main focus on remote sensing of the earth. The book begins with basics of synthetic aperture radar to provide the basis for understanding how polarimetric SAR images are formed and gives an introduction to the fundamentals of radar polarimetry. It goes on to discuss more advanced polarimetric concepts that allow one to infer more information about the terrain being imaged. In order to analyze data quantitatively, the signals must be calibrated carefully, which the book addresses in a chapter summarizing the basic calibration algorithms. The book concludes with examples of applying polarimetric analysis to scattering from rough surfaces, to infer soil moisture from radar signals.
    [bibtex-key = VanZyl2011] [bibtex-entry]


  10. Othmar Frey. Synthetic Aperture Radar Imaging in the Time Domain for Nonlinear Sensor Trajectories and SAR Tomography, PhD Thesis. Remote Sensing Series, vol. 59, Remote Sensing Laboratories, University of Zurich, Zurich, Switzerland. 2010. Keyword(s): SAR Processing, SAR Tomography, Tomography, Focusing, Time-Domain Back-Projection, TDBP, Corridor Mapping, Curvilinear SAR, Nonlinear Flight Tracks, Synthetic Aperture Radar (SAR), Remote Sensing, Airborne SAR, Forestry, L-Band, P-Band, Interferometry, InSAR, SAR Interferometry, Polarimetry, PolInSAR, Polarimetric SAR Tomography, Array signal processing, Extended Chirp Scaling, ECS, Mosaicking, Geocoding, Integrated Focusing and Geocoding, Georeferencing, mapping, E-SAR, digital elevation model, Capon, Capon beamformer, beamforming, multibaseline, multiple signal classification, MUSIC, three-dimensional imaging, 3-D imaging, Vegetation. [bibtex-key = frey2010PhDThesis] [bibtex-entry]


  11. Adrian K. Fung and K. S. Chen. Microwave Scattering and Emission Models for Users. Artech House, Boston, London, 2010. Keyword(s): Remote Sensing, Physics, Microwaves, Scattering, Scattering Models, Emission models, Emission, actice, passive, passive microwace, microwave remote sensing, radar, radar remote sensing, radar scattering, backscatter models, microwave scattering models, microwave emission models, radar scattering models, Earth and Environmental Sciences, Computational Science and Modelling. [bibtex-key = bookFungChen2009MicrowaveScatteringAndEmissionModelsForUsers] [bibtex-entry]


  12. Shane R. Cloude. Polarisation: Applications in Remote Sensing. Oxford Press, 2009. Keyword(s): SAR Processing, Polarisation, Interferometry, Polarimetric Interferometry, PolInSAR, InSAR, SAR Interferometry, Scattering, Remote Sensing, Radar, Radar Remote Sensing. [bibtex-key = cloudeBookPolarisation2009] [bibtex-entry]


  13. Peter J. Huber and Elvezio M. Ronchetti. Robust Statistics. John Wiley & Sons, Inc., Hoboken, New Jersey, 2nd edition, 2009. Keyword(s): Statistics, Robust Statistics, robust fit, curve fitting, robust least squares estimation.
    Abstract: When Wiley asked me to undertake a revision of Robust Statistics for a second edition, I was at first very reluctant to do so. My own interests had begun to shift toward data analysis in the late 1970s, and I had ceased to work in robustness shortly after the publication of the first edition. Not only was I now out of contact with the forefront of current work, but I also disagreed with some of the directions that the latter had taken and was not overly keen to enter into polemics. Back in the 1960s, robustness theory had been created to correct the instability problems of the "optimal" procedures of classical mathematical statistics. At that time, in order to make robustness acceptable within the paradigms then prevalent in statistics, it had been indispensable to create optimally robust (i.e., minimax) alternative procedures. Ironically, by the 1980s, "optimal" robustness began to run into analogous instability problems. In particular, while a high breakdown point clearly is desirable, the (still) fashionable strife for the highest possible breakdown point in my opinion is misguided: it is not only overly pessimistic, but, even worse, it disregards the central stability aspect of robustness. But an update clearly was necessary. After the closure date of the first edition, there had been important developments not only with regard to the breakdown point, on which I have added a chapter, but also in the areas of infinitesimal robustness, robust tests, and small sample asymptotics. In many places, it would suffice to update bibliographical references, so the manuscript of the second edition could be based on a re-keyed version of the first. Other aspects deserved a more extended discussion. I was fortunate to persuade Elvezio Ronchetti, who had been one of the prime researchers working in the two last mentioned areas (robust tests and small sample asymptotics), to collaborate and add the corresponding Chapters 13 and 14. Also, I extended the discussion of regression, and I decided to add a chapter on Bayesian robustness-even though, or perhaps because, I am not a Bayesian (or only rarely so). Among other minor changes, since most readers of the first edition had appreciated the General Remarks at the beginning of the chapters, I have expanded some of them and also elsewhere devoted more space to an informal discussion of motivations. The new edition still has no pretensions of being encyclopedic. Like the first, it is centered on a robustness concept based on minimax asymptotic variance and on M-estimation, complemented by some exact finite sample results. Much of the material of the first edition is just as valid as it was in 1980. Deliberately, such parts were left intact, except that bibliographical references had to be added. Also, I hope that my own perspective has improved with an increased temporal and professional distance. Although this improved perspective has not affected the mathematical facts, it has sometimes sharpened their interpretation. Special thanks go to Amy Hendrickson for her patient help with the Wiley LATEX- macros and the various quirks of TEX.
    [bibtex-key = bookHuberRonchetti2009RobustStatistics2ndEd] [bibtex-entry]


  14. Jong-Sen Lee and Eric Pottier. Polarimetric radar imaging: from basics to applications. CRC Press, Taylor and Francis, 2009. Keyword(s): SAR Processing, Polarimetry, Polarimetric Decomposition. [bibtex-key = leePottier:PolarimetricRadarImagingBook2009] [bibtex-entry]


  15. Ingo Walterscheid. Bistatisches SAR - Signaltheoretische und experimentelle Untersuchung der bistatischen Radarbildgebung, FHR-Schriftenreihe. Shaker Verlag, 2008. Keyword(s): SAR Processing, Bistatic SAR, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Time-Domain Back-Projection, Back-Projection, NuSAR, Airborne SAR, PAMIR, Elektrotechnik Radartechnik Signalverarbeitung Radarsignalverarbeitung. [bibtex-key = walterscheid2008Diss:BistaticSAR] [bibtex-entry]


  16. Annelie Wyholt. SAR Image Focus Errors due to Incorrect Geometrical Positioning in Fast Factorized Back-Projection. Licentiatavhandling, Chalmers University of Technology, 2008. Keyword(s): SAR Processing, Autofocus, Time-Domain Back-Projection, TDBP, FFBP, SAR image processing, antenna path parameters, autofocus, fast factorized back-projection, radar imaging, synthetic aperture radar.
    Abstract: Synthetic Aperture Radar, SAR, is an aperture synthesis technique to generate an image of the ground from air or space with high resolution. Signal processing is performed in frequency domain with Fourier Transform techniques, or in time domain with Back Projection techniques. The advantage of Back-Projection processing is that any aperture shape can be handled and the processing can be performed in real time. The Fast Factorized Back Projection algorithm, FFBP, is also computationally efficient and comparable to Fast Fourier Transform methods. When the resolution is near wavelength size the FFBP algorithm is dependent on accurate positioning data and topography information to avoid defocusing due to range errors. Other SAR image formation methods can use an autofocus method to relax the demands on the positioning data or to remove residual phase errors after the image formation. However, none of the existing autofocus methods will fit the way FFBP is executed. Thus a new autofocus method which will be integrated with FFBP is needed. This thesis is focused on the analysis of different range errors that can occur in one merging step in the FFBP processing and how they can be avoided or corrected, topography errors as well as aperture errors. The analysis of aperture geometry errors is built on coordinate transformation matrixes which can be used for every type of geometry error to calculate the corresponding image shift at any location in the image. A few illustrating examples of aperture geometry errors have been analyzed and a method to track the geometry error by measuring the image shift is presented. It is concluded that defocusing due to topography errors will only arise when the subapertures are not co-linear. The defocusing increases with the angle between the subaperture directions, the subaperture length and the size of the topography error. An expression of the accuracy of topography data needed to preserve the focus for different aperture sizes and subaperture tilts is presented. Different subaperture geometry errors will differently give rise to image shifts in the resulting image. As long as there is contrast in the scene such that image shifts can be accurately determined, a focusing preserving processing geometry can be found by correlation measurements. The estimated geometry can differ from the true geometry relative to the ground. If this is the case, the image will be distorted but still be focused which means that the estimated image is just another view of the true image. Image distortions can also be detected with correlation measurements between subaperture images but only if one image is distorted differently than the other.
    [bibtex-key = Wyholt2008sif] [bibtex-entry]


  17. Alessandro Ferretti, Andrea Monti-Guarnieri, Claudio Prati, Fabio Rocca, and Didier Massonnet. InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation, Part A: Interferometric SAR image processing and interpretation. ESA, 2007. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Interferometry, ESA, ESA-TM-19.
    Abstract: Part A is for readers with a good knowledge of optical and microwave remote sensing, to acquaint them with interferometric SAR image processing and interpretation.
    [bibtex-key = InSAR_ESA_TM-19_2007PartA] [bibtex-entry]


  18. Alessandro Ferretti, Andrea Monti-Guarnieri, Claudio Prati, Fabio Rocca, and Didier Massonnet. InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation, Part B: InSAR processing: a practical approach. ESA, 2007. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Interferometry, ESA, ESA-TM-19.
    Abstract: Part B provides a practical approach and the technical background for beginners with InSAR processing.
    [bibtex-key = InSAR_ESA_TM-19_2007PartB] [bibtex-entry]


  19. Alessandro Ferretti, Andrea Monti-Guarnieri, Claudio Prati, Fabio Rocca, and Didier Massonnet. InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation, Part C: InSAR processing: a mathematical approach. ESA, 2007. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Interferometry, ESA, ESA-TM-19.
    Abstract: Part C contains a more mathematical approach, for a deeper understanding of the interferometric process. It includes themes such as super resolution and ERS/Envisat interferometry.
    [bibtex-key = InSAR_ESA_TM-19_2007PartC] [bibtex-entry]


  20. Bert M. Kampes. Radar interferometry: persistent scatterer technique. Springer, 2006. Keyword(s): SAR Processing, SAR, Interferometry, InSAR, DInsAR, Persistent Scatterer Technique, PSI, IPTA, Interferometric Point Target Analysis. [bibtex-key = kampesBookPSIInSAR] [bibtex-entry]


  21. Jian Li, Petre Stoica, Zhisong Wang, Robert G. Lorenz, Stephen P. Boyd, Alex B. Gershman, Zhi-Quan Luo, Shahram Shahbazpanahi, Xavier Mestre, Miguel A. Lagunas, Yonina C. Eldar, Arye Nehorai, Alle-Jan van der Veen, Amir Leshem, Elio D. Di Claudio, and Raffaele Parisi. Robust Adaptive Beamforming. Wiley-Interscience, 2006. Keyword(s): SAR Processing, Beamforming, Adaptive Beamforming, Robust Adaptive Beamforming, MUSIC, Capon, Subspace Methods, SAR Tomography, Tomography. [bibtex-key = bookRABLiStoicaEditors2006:Beamforming] [bibtex-entry]


  22. Christian Matzler. Thermal microwave radiation: applications for remote sensing, volume 52. Iet, 2006. [bibtex-key = Maetzler2006] [bibtex-entry]


  23. Ian G. Cumming and Frank H. Wong. Digital Processing of Synthetic Aperture Radar Data: Algorithms and Implementation. Artech House Inc., Boston, London, 2005. Keyword(s): SAR Processing, Range-Doppler Algorithm, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, SPECAN Algorithm, Comparison of Algorithms, Demodulation, Quadrature Demodulation, Hilbert Transform, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Doppler Rate Estimation, Azimuth FM Rate, Autofocus Techniques, ScanSAR, Range Compression, Pulse Compression, Pulse Compression of Linear FM Signals, Linear FM Signals, Stolt Mapping, Quality Assessment, Quality Measures, Quality Metrics, PSLR, ISLR, Spaceborne SAR.
    Abstract: This cutting-edge resource offers you complete howto guidance on digital processing of synthetic aperture radar (SAR) data. You discover how SAR is used to obtain a high-resolution image from a satellite and learn the mathematical structure and spectral properties of the signal received from a SAR system. Supported with over 600 equations and over 250 figures, the book arms you with state-of-the-art signal processing algorithms and helps you choose the best algorithm for a given SAR system and image quality requirements. This hands-on reference shows you how to process received SAR data into a well-focused image on a digital computer, using the popular range Doppler, chirp scaling, omega-K and SPECAN algorithms. In addition, the book teaches you how to process ScanSAR data using the full-aperture, SPECAN, short IFFT and extended chirp scaling algorithms. You also learn how to estimate the Doppler centroid frequency and azimuth FM rate from a geometry model or from received data. Written from a digital signal processing point of view, this authoritative volume can be fully understood by professionals with a general electrical engineering background.
    [bibtex-key = cummingWong05:SARBook] [bibtex-entry]


  24. Petre Stoica and Randolph L. Moses. Spectral Analysis of Signals. Prentice Hall, Upper Saddle River, NJ, 2005. Keyword(s): Spectral Estimation, MUSIC, Capon, Beamforming, Direction-of-arrival estimation, SAR Processing, SAR Tomography. [bibtex-key = stoicaMosesBook2005:SpectralAnalysis] [bibtex-entry]


  25. Eugene F. Knott, John F. Shaeffer, and Michael T. Tuley. Radar Cross Section (2nd Edition). 2004. Keyword(s): radar cross section, RCS, electromagnetic wave absorption, radar cross-sections, electromagnetic wave scattering, scattering mechanisms, complex body, radar cross section reduction methods, test targets, techniques for measuring absorber property, RCS prediction techniques, background information, simple body, scattering characteristics, RCS behavior, electromagnetic wave propagation, electromagnetic waves theory, Radar theory, corner reflector, trihedral corner reflector, signal-to-clutter ratio.
    Abstract: Radar cross section (RCS) is a comparison of two radar signal strengths. One is the strength of the radar beam sweeping over a target, the other is the strength of the reflected echo sensed by the receiver. This book shows how the RCS gauge can be predicted for theoretical objects and how it can be measured for real targets. Predicting RCS is not easy, even for simple objects like spheres or cylinders, but this book explains the two exact forms of theory so well that even a novice will understand enough to make close predictions. Weapons systems developers are keenly interested in reducing the RCS of their platforms. The two most practical ways to reduce RCS are shaping and absorption. This book explains both in great detail, especially in the design, evaluation, and selection of radar absorbers. There is also great detail on the design and employment of indoor and outdoor test ranges for scale models or for full-scale targets (such as aircraft). In essence, this book covers everything you need to know about RCS, from what it is, how to predict and measure, and how to test targets (indoors and out), and how to beat it.
    [bibtex-key = knottShaefferTuleyBook2004RadarCrossSection] [bibtex-entry]


  26. Ramon F. Hanssen. Radar interferometry: data interpretation and error analysis. Kluwer Academic Publishers, 2001. Keyword(s): SAR Processing, SAR, Interferometry, InSAR, Error Analysis, InSAR Error Analysis, Teaching Material, Tutorial, Teaching. [bibtex-key = RadarInterferometryBookRamonHanssen2002] [bibtex-entry]


  27. Leung Tsang and Jin Au Kong. Scattering of Electromagnetic Waves: Advanced Topics. John Wiley & Sons, Inc., April 2001. [bibtex-key = bookTsangKong2001ScatteringOfElectromagneticWavesAdvancedTopics] [bibtex-entry]


  28. Leung Tsang, Jin Au Kong, Kung-Hau Ding, and Chi On Ao. Scattering of Electromagnetic Waves: Numerical Simulations. John Wiley & Sons, Inc., May 2001. [bibtex-key = bookTsangKongDingAo2001ScatteringOfElectromagneticWavesNumericalSimulations] [bibtex-entry]


  29. Leung Tsang, Jin Au Kong, and Kung-Hau Ding. Scattering of Electromagnetic Waves: Theories and Applications, volume 1. John Wiley & Sons, Inc., July 2000.
    Abstract: Electromagnetic wave scattering is an active, interdisciplinary area of research with myriad practical applications in fields ranging from atomic physics to medical imaging to geoscience and remote sensing. In particular, the subject of wave scattering by random discrete scatterers and rough surfaces presents great theoretical challenges due to the large degrees of freedom in these systems and the need to include multiple scattering effects accurately. In the past three decades, considerable theoretical progress has been made in elucidating and understanding the scattering processes involved in such problems. Diagrammatic techniques and effective medium theories remain essential for analytical studies; however, rapid advances in computer technology have opened new doors for researchers with the full power of Monte Carlo simulations in the numerical analysis of random media scattering. Numerical simulations allow us to solve the Maxwell equations exactly without the limitations of analytical approximations, whose regimes of validity are often difficult to assess. Thus it is our aim to present in these three volumes a balanced picture of both theoretical and numerical methods that are commonly used for tackling electromagnetic wave scattering problems. While our book places an emphasis on remote sensing applications, the materials covered here should be useful for students and researchers from a variety of backgrounds as in, for example, composite materials, photonic devices, optical thin films, lasers, optical tomography, and X-ray lithography. Introductory chapters and sections are also added so that the materials can be readily understood by graduate students. We hope that our book would help stimulate new ideas and innovative approaches to electromagnetic wave scattering in the years to come. The increasingly important role of numerical simulations in solving electromagnetic wave scattering problems has motivated us to host a companion web site that contains computer codes on topics relevant to the book. These computer codes are written in the MATLAB programming language and are available for download from our web site at www.emwave.com. They are provided to serve two main purposes. The first is to supply our feaders a hands-on laboratory for performing numerical experiments, through which the concepts in the book can be more dynamically relayed. The second is to give new researchers a set of basic tools with which they could quickly build on projects of their own. The fluid nature of the web site would also allow us to regularly update the contents and keep pace with new research developments. The present volume covers the basic principles and applications of electromagnetic wave scattering and lays the groundwork for the study of more advanced topics in Volumes II and III. We start in Chapter 1 with exact and approximate solutions of wave scattering by a single particle of simple shape. Such problems can be solved exactly by expanding the fields in terms of scalar or vector waves in separable coordinates, depending on the geometry of the scatterer. When the size of the scatterer is small, Rayleigh scattering represents a simple and valid approximation. When scattering is weak, the Born approximation can be applied to the volume integral equation for the internal field. Approximate solutions are also useful when the scatterer lacks perfect symmetry as in the case of a finite cylinder. In Chapter 2, we discuss basic scattering theory. We introduce the Green's function for the wave equation and its various coordinate representations. From the vector Green's theorem, we derive the Hugyens' Principle and the extinction theorem, which are especially useful for formulating surface integral equations in scattering problems. The reciprocity principle leads to useful symmetry relations in the scattering amplitudes and the Green's function, while energy conservation leads to the optical theorem. The T-matrix formulation with the extended boundary condition technique is a popular method that can be used to calculate scattering from an arbitrarily shaped object. We give explicit results for dielectric spheres and spheroids. In Chapter 3, we begin the study of electromagnetic scattering by a random collection of scatterers. The concepts of fluctuating fields and ensemble averaging are of central importance in random media scattering. These and related ideas are explored in this chapter. The specific intensity is often used to describe energy transport through a random medium. The fully polarimetric description of the specific intensity is provided by the Stokes vector. As an application to passive remote sensing, we derive the emissivity of the four Stokes parameters using the fluctuation dissipation theorem. Basic radiative transfer (RT) theory elements including the extinction coefficient and scattering phase matrix are also introduced. In contrast to conventional RT theory, the phase functions are defined in terms of bistatic scattering cross sections. This allows for the development of the dense medium radiative transfer theory (DMRT) to be discussed in Volumes II and III. Many natural media, e.g., snow, vegetation, and ocean surfaces, can be effectively modeled in terms of simple random media. Chapter 4 is devoted to the statistical characterizations of such random discrete media and rough surfaces. Useful characterizations include the pair distribution function for volume scatterers and the power spectrum for rough surfaces. In Chapter 5, we consider scattering and emission by plane-parallel layered media, which provide simple but very useful models for geophysical remote sensing. We solve this problem in two different ways: the coherent or wave approach, which is exact, versus the incoherent or radiative transfer approach. This gives us some insights into the approximations involved in RT theory. In Chapter 6, we discuss the single scattering approximation, where each particle is assumed to scatter independently. However, we take into account demonstrate of the phase coherence in the addition of scattered fields. We the existence of an interesting correlation effect in random media scattering known as the memory effect. As will be shown in Volumes II and III, this effect persists even when multiple scattering is included. Applications of single scattering to synthetic aperture radar (SAR) and random media scattering are also discussed. In Chapters 7 and 8, we take a closer look at the radiative transfer equation and its solutions. The iterative method is useful when scattering is weak and provides physical correspondence with different orders of multiple scattering. When scattering is strong, the discrete ordinate eigenanalysis approach can be used to obtain numerically exact solutions. For scattering media with inhomogeneous profiles, the method of invariant imbedding can be applied. Diffusion approximation is useful when, upon multiple scattering, the intensities have been diffused almost uniformly in all directions. illustrate these solution techniques with extensive examples from active passive microwave remote sensing. In Chapter 9, we discuss wave scattering by random rough surfaces. Despite much theoretical and numerical efforts, the two classical analytical approximations of small perturbation method and Kirchhoff approach are still the simplest and most widely used analytical methods for solving rough surface Problems. Here they are illustrated using one-dimensional rough surfaces with Dirichlet and Neumann boundary conditions. Two-dimensional rough surface scattering problems are discussed extensively in Volumes II and III.
    [bibtex-key = bookTsangKongDing2000ScatteringOfElectromagneticWavesTheoriesAndApplications] [bibtex-entry]


  30. Mehrdad Soumekh. Synthetic Aperture Radar Signal Processing: with MATLAB Algorithms. John Wiley & Sons, 1999. Keyword(s): SAR Processing, Wavefront Reconstruction, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Back-Projection, Time-Domain Back-Projection, TDBP, Spotlight SAR, Stripmap SAR, MATLAB, Motion Compensation, Digital Spotlighting, Monopulse SAR, Stolt Mapping, Range Compression, Pulse Compression, Pulse Compression of Linear FM Signals, Linear FM Signals.
    Abstract: This book introduces the wavefront reconstruction signal theory that underlies the best SAR imaging methods and provides clear guidelines to system design, implementation, and applications in diverse areas - from airborne reconnaissance to topographic imaging of ocean floors to surveillance and air traffic control to medical imaging techniques, and numerous others. Enabling professionals in radar signal and image processing to use synthetic aperture technology to its fullest potential, this work: includes M-files to supplement this book; provides practical examples and results from real SAR, ISAR, and CSAR databases; outlines unique properties of the SAR signal that cannot be found in other information processing systems; examines spotlight SAR, stripmap SAR, circular SAR, and monopulse SAR modalities; discusses classical SAR processing issues such as motion compensation and radar calibration.
    [bibtex-key = soumekh:SARProc] [bibtex-entry]


  31. Akira Ishimaru. Wave Propagation and Scattering in Random Media. Wiley-IEEE Press, 1997. Keyword(s): Wave characteristics in aerosols and hydrometeors, Optical and acoustic scattering in sea water, Scattering from biological materials, Pulse scattering and beam wave propagation in such media, Optical diffusion in tissues and blood, Transport and radiative transfer theory, Kubelka-Munk flux theory and plane-parallel problem, Multiple scattering theory, Wave fluctuations in turbulence, Strong fluctuation theory, Rough surface scattering, Remote sensing and inversion techniques, Imaging through various media.
    Abstract: Electrical Engineering Wave Propagation and Scattering in Random Media A volume in the IEEE/OUP Series on Electromagnetic Wave Theory Donald G. Dudley, Series Editor This IEEE Classic Reissue presents a unified introduction to the fundamental theories and applications of wave propagation and scattering in random media. Now for the first time, the two volumes of Wave Propagation and Scattering in Random Media previously published by Academic Press in 1978 are combined into one comprehensive volume. This book presents a clear picture of how waves interact with the atmosphere, terrain, ocean, turbulence, aerosols, rain, snow, biological tissues, composite material, and other media. The theories presented will enable you to solve a variety of problems relating to clutter, interference, imaging, object detection, and communication theory for various media. This book is expressly designed for engineers and scientists who have an interest in optical, microwave, or acoustic wave propagation and scattering. Topics covered include: * Wave characteristics in aerosols and hydrometeors * Optical and acoustic scattering in sea water * Scattering from biological materials * Pulse scattering and beam wave propagation in such media * Optical diffusion in tissues and blood * Transport and radiative transfer theory * Kubelka--Munk flux theory and plane-parallel problem * Multiple scattering theory * Wave fluctuations in turbulence * Strong fluctuation theory * Rough surface scattering * Remote sensing and inversion techniques * Imaging through various media About the IEEE/OUP Series on Electromagnetic Wave Theory Formerly the IEEE Press Series on Electromagnetic Waves, this joint series between IEEE Press and Oxford University Press offers outstanding coverage of the field with new titles as well as reprintings and revisions of recognized classics that maintain long-term archival significance in electromagnetic waves and applications. Designed specifically for graduate students, practicing engineers, and researchers, this series provides affordable volumes that explore electromagnetic waves and applications beyond the undergraduate level. See page il of the front matter for a listing of books in this series.
    [bibtex-key = bookIshimaru1997WavePropagationAndScatteringInRandomMedia] [bibtex-entry]


  32. Charles V. Jakowatz, Daniel E. Wahl, Paul H. Eichel, Dennis C. Ghiglia, and Paul A. Thompson. Spotlight-Mode Synthetic Aperture Radar: A Signal Processing Approach. Norwell, MA: Kluwer Academic Publishers, 1996. Keyword(s): SAR Processing, Spotlight SAR, Spotlight mode, Autofocus, Phase Gradient Autofocus, PGA, Motion Compensation, MoComp, Airborne SAR, SAR Tomography, Tomography, InSAR, SAR Interferometry. [bibtex-key = jakowatzWahlEichelGhigliaThompson1996:BookSpotlightSAR] [bibtex-entry]


  33. Walter G. Carrara, Ron S. Goodman, and Ronald M. Majewski. Spotlight Synthetic Aperture Radar: Signal Processing Algorithms. Artech House Inc., 1995. Keyword(s): SAR Processing, Spotlight SAR, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Chirp Scaling Algorithm, Polar Format Algorithm, Autofocus Techniques, Demodulation, Quadrature Demodulation, Hilbert Transform.
    Abstract: The book gives an excellent theoretical and practical background of SAR in general and specifically of spotlight SAR. The rich experience of the authors in spotlight SAR processing is reflected by a very detailed summary of the associated theory as well as a lot of SAR image examples. These images illustrate the techniques described in the book and provide a valuable connection to practice. This book can be highly recommended to all scientists and engineers involved in SAR system design and SAR data evaluation, -- International Journal of Electronics and Communications, January 1996. Contents: Synthetic Aperture Radar Fundamentals. Spotlight SAR and Polar Format Algorithm. Digital Polar Format Processing. Phase Errors. Autofocus Techniques. Processor Design Examples. SAR System Performance. Spotlight Processing Applications. Range Migration Algorithm. Chirp Scaling Algorithm. Comparison of Image Formation Algorithms. Appendices.
    [bibtex-key = carrara:SARProc] [bibtex-entry]


  34. Mehrdad Soumekh. Fourier Array Imaging. Englewood Cliffs, NJ: Prentice Hall, 1994. Keyword(s): SAR Processing, Fourier Array Imaging. [bibtex-key = soumekh:FourierArrayImaging1994] [bibtex-entry]


  35. Eugene F. Knott. Radar Cross Section Measurements. Van Nostrand Reinhold, 1993. Keyword(s): Radar Cross Section, Corner Reflector, Trihedral Corner Reflector, Calibration, Signal-to-clutter ratio. [bibtex-key = bookKnott1993RadarCrossSectionMeasurements] [bibtex-entry]


  36. Albrecht Ludloff. Handbuch Radar und Radarsignalverarbeitung. Vieweg+Teubner Verlag, Wiesbaden, 1993.
    Abstract: Den Anstoss zu diesem Buch gab der in der Industriepraxis beobachtete Mangel an solcher Radar-Literatur, die den Systemplaner, den Entwicklungsingenieur und den interessierten Anwender theoretisch ausreichend tief, aber zugleich praxisorientiert mit der Radarsignalverarbeitung und dem Entwurf des Sendesignals, dem sogenannten "Waveform Design", vertraut macht. Denn diese bestimmen wesentlich die "Intelligenz" des Radars, das heisst seine Faehigkeit, einerseits gesuchte Zielobjekte zu entdecken und zu vermessen und andererseits Falschmeldungen durch andere Objekte zu vermeiden. Die Radarsignalverarbeitung ist -beguenstigt durch die rasante Entwicklung der Mikro-Elektronik und hier besonders der Speicher und der sehr schnellen Rechenwerke - immer komplexer und damit leistungsfaehiger geworden. Eine Grenze ist nicht abzusehen. Allerdings finden sich die Beschreibungen neuerer Verfahren oder Algorithmen der Radarsignalverarbeitung oft nur verstreut in Publikationen der Fachzeitschriften und in Tagungsberichten. Auch hat sich der Aufgabenbereich des auf diesem Gebiet taetigen Entwicklungsingenieurs erweitert. Er ist erstens staerker als frueher am Waveform Design beteiligt. Zweitens muss er neben seiner Hauptaufgabe, der Entwicklung von Hardware und Software fuer die Subsysteme, in steigendem Masse Monte-Carlo-Simulationen zum Entwurf und zur Optimierung von Signalverarbeitungsalgorithmen und zur Ermittlung von Performance-Parametern einsetzen. Alle diese Taetigkeiten erfordern eine theoretisch gut fundierte Kenntnis sowohl der Grundlagen als auch der praktischen Verfahren der Radarsignalverarbeitung. Hier eine gewisse Luecke zu fuellen, ist Sinn des vorliegenden Buches.
    [bibtex-key = bookLudloff1993HandbuchRadarUndRadarsignalverarbeitung] [bibtex-entry]


  37. Gunter Schreier, editor. SAR Geocoding: Data and Systems. Wichmann, 1993. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, Doppler Centroid Estimation.
    Abstract: This book gives an overview of the principles of SAR processing, geometric effects of SAR and parametric geocoding of SAR, both ellipsoid and terrain corrected. The new geocoded data sets and the systems and ancillary information to generate these data are described in detail. Basic applications and analysis procedures with the data are described in order to help to understand the content of the geocoded SAR images. Several authors from the German Processing and Archiving Facility (D-PAF) of DLR in Oberpfaffenhofen and other research groups, who actively contributed to algorithms and system aspects of SAR geocoding, contributed chapters of this reference book. An overview of current on-going activities in Europe in this domain concludes the chapters. This volume is intended to introduce new SAR data users as well as those starting with SAR data and Geographic Information Systems (GIS) to the principles of SAR data geocoding and to give experts a detailed overview of used references and algorithms.
    [bibtex-key = schreier93:SARgeocoding] [bibtex-entry]


  38. John C. Curlander and Robert N. McDonough. Synthetic Aperture Radar - Systems and Signal Processing. John Wiley & Sons, New York, 1991. Keyword(s): SAR Processing, Radar Equation, Matched Filter, Pulse Compression, Range Compression Processing, Azimuth Processing, range-Doppler, Clutterlock, Doppler Centroid Estimation, Doppler Centroid, Autofocus, Doppler Ambiguity Resolver, DAR, Doppler Rate Estimation, Azimuth FM Rate, Autofocus Techniques, Radiometric Calibration, Geometric Calibration, Rectangular Algorithm, Deramp Compression, Step Transform Processing, Polar Processing, Fourier Transform, Convolution, Interpolation, Satellite Orbits.
    Abstract: Synthetic Aperture Radar: Systems and Signal Processing specifiecally addresses these items, as applied to the design and implementation of the spaceborne SAR system with a strong emphasis on signal processing techniques. The reader will find that the book has been written with tree key goals. The first is to present a complete picture of SAR from a theoretical perspective by deriving from basic principles the SAR signal processing algorithms. The second goal is to provide some insight into the practical aspects of the sensor and ground system design and implementation. the third is to analyze the system performance given the common error sources and demonstrate their effect on the output image products. This text provieds in a single source the information required both for SAR system design as well as image data analysis. The book serves as an aid for both the radar engineer and the application scientist. The text is generously annotated with figures, plots and images in an effort to make the ideas as accessible as possible. The material is presented such that this text can be used both as an introduction to SAR for new workers in the field and as a reference to the experienced engineer involved in system design and development.
    Comments: ++ Notes
    [bibtex-key = Curlander91:Synthetic] [bibtex-entry]


  39. Merrill I. Skolnik. Radar Handbook (2nd Edition). McGraw-Hill, 1990. [bibtex-key = skolnik90:RadarHandbook] [bibtex-entry]


  40. Albert Monteith. Temporal Characteristics of Boreal Forest Radar Measurements. PhD thesis, Chalmers University of Technology, 2020. Keyword(s): SAR Tomography, BorealScat.
    Abstract: Radar observations of forests are sensitive to seasonal changes, meteorological variables and variations in soil and tree water content. These phenomena cause temporal variations in radar measurements, limiting the accuracy of tree height and biomass estimates using radar data. The temporal characteristics of radar measurements of forests, especially boreal forests, are not well understood. To fill this knowledge gap, a tower-based radar experiment was established for studying temporal variations in radar measurements of a boreal forest site in southern Sweden. The work in this thesis involves the design and implementation of the experiment and the analysis of data acquired. The instrument allowed radar signatures from the forest to be monitored over timescales ranging from less than a second to years. A purpose-built, 50 m high tower was equipped with 30 antennas for tomographic imaging at microwave frequencies of P-band (420-450 MHz), L-band (1240-1375 MHz) and C-band (5250-5570 MHz) for multiple polarisation combinations. Parallel measurements using a 20-port vector network analyser resulted in significantly shorter measurement times and better tomographic image quality than previous tower-based radars. A new method was developed for suppressing mutual antenna coupling without affecting the range resolution. Algorithms were developed for compensating for phase errors using an array radar and for correcting for pixel-variant impulse responses in tomographic images. Time series results showed large freeze/thaw backscatter variations due to freezing moisture in trees. P-band canopy backscatter variations of up to 10 dB occurred near instantaneously as the air temperature crossed 0 deg C, with ground backscatter responding over longer timescales. During nonfrozen conditions, the canopy backscatter was very stable with time. Evidence of backscatter variations due to tree water content were observed during hot summer periods only. A high vapour pressure deficit and strong winds increased the rate of transpiration fast enough to reduce the tree water content, which was visible as 0.5-2 dB backscatter drops during the day. Ground backscatter for cross-polarised observations increased during strong winds due to bending tree stems. Significant temporal decorrelation was only seen at P-band during freezing, thawing and strong winds. Suitable conditions for repeat-pass L-band interferometry were only seen during the summer. C-band temporal coherence was high over timescales of seconds and occasionally for several hours for night-time observations during the summer. Decorrelation coinciding with high transpiration rates was observed at L- and C-band, suggesting sensitivity to tree water dynamics. The observations from this experiment are important for understanding, modelling and mitigating temporal variations in radar observables in forest parameter estimation algorithms. The results also are also useful in the design of spaceborne synthetic aperture radar missions with interferometric and tomographic capabilities. The results motivate the implementation of single-pass interferometric synthetic aperture radars for forest applications at P-, L- and C-band.
    [bibtex-key = phDThesisAlbertMonteithChalmers2020TemporalCharacteristicsOfForestRadarMonitoring] [bibtex-entry]


  41. Muhammad Adnan Siddique. SAR tomography as an add-on to persistent scatterer interferometry for improved deformation coverage. PhD thesis, ETH Zurich, 2018. Keyword(s): SAR Processing, SAR Tomography, deformation analysis in urban and alpine areas, persistent scatterer interferometry, PSI, atmospheric phase corrections, radar interferometry, differential SAR tomography, SAR interferometry, InSAR, SAR, SAR tomography, Synthetic aperture radar (SAR), deformation, SAR signal processing, Radar signal processing, SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne SAR, Cosmo Skymed, interferometric stack, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Alpine Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry, Cosmo SkyMed, Matter Valley, Switzerland, Alps, mountainous terrain, layover.
    Abstract: Persistent scatterer interferometry (PSI) is a synthetic aperture radar (SAR) signal processing technique for the measurement of land surface deformation. As a conceptual derivative of differential interferometry, it strives to extract the interferometric phase variation induced by the line-of-sight component of the deformation. PSI limits the interferometric analysis to the so-called {persistent} (or synonymously {coherent}) scatterers (PS). These are single dominant scatterers exhibiting point-like, quasi-deterministic behavior. On the one hand, it implies reduced susceptibility to temporal and geometric decorrelation, but on the other hand, it incurs the limitation that range-azimuth resolution cells containing multiple scatterers are rejected in the PSI processing, even if they are individually coherent. Side-looking geometry of SAR sensors results in frequent layovers, whereby multiple scatterers situated at different elevations fall in the same resolution cell. Consequently, deformation coverage with PSI processing may remain limited in layover-affected areas. SAR tomography is a means to alleviate the aforementioned limitation. It allows 3-D reconstruction of the scene reflectivity -- a feature that offers the potential to resolve the layover problem. The coherent scatterers that are interfering in the same cell can be separated along the elevation. Additionally, differential SAR tomographic methods allow a joint spatio-temporal inversion of the coherent scatterers in layover, i.e., the position along the elevation axis as well as the deformation velocity of the interfering scatterers are simultaneously estimated. Therefore, differential SAR tomography can be used as an add-on to PSI techniques to improve deformation coverage in layover-affected areas. This dissertation provides a comprehensive assessment of the utility offered by SAR tomography as an add-on to PSI. Several aspects of a tomographic processing framework, such as different phase models for tomography, phase calibration of the interferometric stack, statistical detection of coherent scatterers, etc., need to be investigated. To this end, three core investigations have been performed. In each case, a prior PSI solution has been used as a starting point. It serves not only as a reference to compare with, but is also shown to be a natural precursor to tomographic processing. An interferometric data stack comprising 50 TerraSAR-X stripmap-mode acquisitions over an urban zone in the city of Barcelona, Spain, has been used in the first investigation. The phase models for classical SAR tomography (3-D SAR), differential tomography with the assumption of linear deformation over time, and the one further extended to simultaneously model thermal expansion, are compared against each other with respect to their suitability in resolving layovers. The results confirm that modeling thermal expansion of the scatterers, in addition to linear deformation and elevation, is indeed critical for effective layover separations, especially in the case of high-rise buildings. The quality of the scatterers obtained with tomography has been evaluated in terms of the dispersion of the residual phase and compared against the quality of the PS identified in the prior PSI processing. The results show a trade-off between the quantity and the quality of the scatterers. The second investigation focuses on the problem of phase calibration for a potential application of SAR tomography in mountainous regions. It is a case study that assesses a regression-kriging approach to functionally model height-dependent atmospheric phase variations and lateral phase trends, and consider the turbulent mixing effects in a stochastic sense. The study has been performed on a stack comprising 32 Cosmo-SkyMed acquisitions over Matter Valley in the Swiss Alps. Phase corrections with the kriging approach extend the deformation coverage to parts of a mountainside (in layover) where no PS were identified in the prior PSI processing. However, a very few double scatterers are detected on the whole. The third investigation explores how to perform scatterer detection for tomography extending the same quality considerations as used in the prior PSI processing. The outcome of this work is a detection strategy whereby quality parameters (in terms of the statistics of the phase residue or ensemble coherence) are used to determine the thresholds for hypothesis testing. The detection strategy is tested on the same data stack as for the first investigation to detect single and double scatterers in an urban area. An empirical analysis of the probability of false alarm is also provided. As a whole, this dissertation covers several aspects that collectively highlight how the synergistic use of PSI and tomography can lead to improved deformation coverage.
    [bibtex-key = phdThesisSiddiqueETH2018TomoSARasAddOnToPSI] [bibtex-entry]


  42. S. Samie Esfahany. Exploitation of distributed scatterers in synthetic aperture radar interferometry. PhD thesis, TUDelft, 2017.
    Abstract: uring the last decades, time-series interferometric synthetic aperture radar (InSAR) has emerged as a powerful technique to measure various surface deformation phenomena of the earth. Early generations of time-series InSAR methodologies, i.e. Persistent Scatterer Interferometry (PSI), focused on point targets, which are mainly man-made features with a high density in urban areas and associated infrastructure. Later, methodologies were introduced aiming to extract information from other targets known as distributed scatterers (DS), which are associated with ground resolution cells occurring mainly in rural areas. Unfortunately, the underlying properties and assumptions behind various DS-phase estimation methodologies are sometimes subjective and incomparable, which hampers the objective application of the different methods. Moreover, for some terrain types, such as agricultural terrain or pastures, the feasibility of DS-methodologies is not straightforward.In view of these challenges, the two main objectives of this study are (i) to formulate and implement the estimation methodology of DS-pixels in a standard geodetic framework and to compare it with other existing methods, and (ii) to assess the feasibility of exploiting distributed scatterers for deformation monitoring over agricultural and pasture areas.We review state-of-the-art time-series InSAR methodologies with special attention toprocessing aspects related to distributed scatterers. From an estimation theory perspective, the key processing step to extract information from DS-pixels is the equivalent single-master (ESM) phase estimation. To situate this estimation in a geodetic framework, a mathematical model is proposed in the form of a Gauss-Markov model. To evaluate the stochastic part of the model, a numerical Monte-Carlo methodology as well as an analytical approach are introduced. Regarding the functional part, the ESM-phase estimation is formulated in the form of a hybrid linear system of observation-equations with both real-value and integer unknowns. The solution of the proposed model is given by the integer least-squares (ILS) estimator. The properties of such an estimator for ESM-phase estimation are described and demonstrated using synthetic and real datasets. Furthermore, to provide a theoretical comparison between the proposed ILS estimator and other existing ESM-phase estimators, a unified mathematical model in the form of a system of observation equations is proposed. Evaluating all the existing DS-methods shows that, although they all provide specific solutions, their fundamental difference is in how they assign weights to the interferometric observations.The feasibility of exploiting PS, DS, and their combination over agricultural and rurallandscapes is assessed via a case study on a subsidence area near city of Veendam,the Netherlands, based on the coherence behavior of different types of land use. It isshown that, under the condition of using the entire time-series, agricultural and pasture areas show only limited improvement in point density compared to the results of PSonly processing. This is due to the seasonal behavior of the temporal coherence, which causes an almost complete drop in coherence during summer periods, mainly as a result of tillage, crop growth and harvesting.To model this periodicity, a new analytical model is introduced. In this model, the hypothetical movements of elementary scatterers within DS resolution cells are modeled as a stochastic process with non-stationary but periodic increments. The parameters of this model are estimated for pasture areas, and are subsequently used to assess the feasibility of exploiting DS-pixels in agricultural areas by different satellite missions. The results confirm that, assuming a three-year stack of data, the information content in DS-pixels from current C-band and X-band missions is not enough for the successful utilization of their entire time-series. However by using intermittent series, e.g., by processing individual coherent periods, the results indicate that DS-pixels can be exploited: based on the proposed decorrelation model, the short repeat times of Sentinel-1 (6 or 12 days) results in a sufficient number of coherent interferograms over each winter period, enabling DS exploitation even over agricultural and pasture areas.
    [bibtex-key = samieiEsfahanyPhDThesisDistributedScatterersInSAR] [bibtex-entry]


  43. Pooja S. Mahapatra. Geodetic network design for InSAR: Application to ground deformation monitoring. PhD thesis, TU Delft, 2015. Keyword(s): SAR Processing, InSAR Persistent Scatterer Interferometry, PSI, GNSS, Transponder, Geodetic Network.
    Abstract: For the past two decades, interferometric synthetic aperture radar (InSAR) has been used to monitor ground deformation with subcentimetric precision from space. But the applicability of this technique is limited in regions with a low density of naturally-occurring phase-coherent radar targets, e.g. vegetated nonurbanized areas. Third-party end-users of InSAR survey results cannot, in a systematic way, determine a priori whether these coherent targets have adequate spatial distribution to estimate the parameters of their interest. Additionally, InSAR deformation estimates are referred to a local datum, meaning that the technique is sensitive only to the relative deformation occurring within the SAR images. This makes it difficult to compare these estimates with those from other techniques, e.g. historical levelling data or changes in the sea level. Here we propose the design of a geodetic network for InSAR, aimed at densifying the naturally-occurring measurement network and converting from a local datum to a global one. A practical solution for improving spatial sampling is to deploy coherent target devices such as corner reflectors or transponders on ground, tailored to the specific monitoring application under consideration. The proposed method (1) provides a generic description of any deformation phenomenon; (2) determines whether the naturally-occurring InSAR measurements are adequate in terms of user-defined criteria; (3) finds the minimum number of additional devices to be deployed (if required); and (4) finds their optimal ground locations. It digests, as inputs, any prior knowledge of the deformation signal, the expected locations and quality of the existing coherent targets, and the quality of the deployed devices. The method is based on comparing different covariance matrices of the final parameters of interest with a criterion matrix (i.e., the ideal desired covariance matrix) using a predefined metric. The resulting measurement network is optimized with respect to precision, reliability and economic criteria; this is demonstrated via synthetic examples and a case of subsidence in the Netherlands. As a basis for the choice and number of deployed devices, we evaluate the measurement precision of compact active transponders and demonstrate their viability as alternatives to passive corner reflectors through three field experiments, using different satellite data and geodetic validation techniques. Transponders are shown to be usable for subcentimetre-precision geodetic applications, while improving upon the drawbacks of corner reflectors in terms of size, shape, weight and conspicuousness. For transforming the spatially-relative InSAR deformation estimates (local datum) to a standard terrestrial reference frame (global datum), we introduce a new concept involving the collocation of transponders with Global Navigation Satellite System (GNSS) measurements. The displacement of such a transponder is consequently determined in the standard reference frame used by GNSS, eliminating the need for any assumptions on reference-point stability in applications where the InSAR deformation estimates are compared with results from other techniques. The considerations, results and practical lessons learnt at several permanent GNSS stations in the Netherlands are described.
    [bibtex-key = mahapatra20015PhDThesisInSARandGNSS] [bibtex-entry]


  44. Craig L. Stringham. Developments in LFM-CW SAR for UAV Operation. PhD thesis, 2014. Keyword(s): SAR Proceessing, radar, SAR, UAV, GPU, Autofocus, SAR Autofocus, Backprojection, Time-Domain Back-Projection, Back-Projection, TDBP, fast-factorized back-projection, FFBP, LFM-CW, FMCW, MoComp, Motion Compensation, CSA, ECS, Chirp Scaling, Extended Chirp Scaling, FSA, Frequency Scaling Algorithm, Range-Doppler Algorithm, RDA, synthetic aperture radar, Brigham Young University, muSAR system, LFM-CW signal model, SAR image quality, aircraft, atmospheric turbulence, high-resolution synthetic aperture radar systems, linear frequency-modulated continuous-wave signal, motion compensation, motion correction algorithms, unmanned aerial vehicle, Airborne SAR, geophysical techniques.
    Abstract: Opportunities to use synthetic aperture radar (SAR) in scientific studies and military operations are expanding with the development of small SAR systems that can be operated on small unmanned air vehicles (UAV)s. While the nimble nature of small UAVs make them an attractive platform for many reasons, small UAVs are also more prone to deviate from a linear course due autopilot errors and external forces such as turbulence and wind. Thus, motion compensation and improved processing algorithms are required to properly focus the SAR images. The work of this dissertation overcomes some of the challenges and addresses some of the opportunities of operating SAR on small UAVs. Several contributions to SAR backprojection processing for UAV SARs are developed including: 1. The derivation of a novel SAR backprojection algorithm that accounts for motion during the pulse that is appropriate for narrow or ultra-wide-band SAR. 2. A compensation method for SAR backprojection to enable radiometrically accurate image processing. 3. The design and implementation of a real-time backprojection processor on a commercially available GPU that takes advantage of the GPU texture cache. 4. A new auto-focus method that improves the image focus by estimating motion measurement errors in three dimensions, correcting for both amplitude and phase errors caused by inaccurate motion parameters. 5. A generalization of factorized backprojection, which we call the Dually Factorized Backprojection method, that factorizes the correlation integral in both slow-time and fast-time in order to eciently account for general motion during the transmit of an LFM-CW pulse. Much of this work was conducted in support of the Characterization of Arctic Sea Ice Experiment (CASIE), and the appendices provide substantial contributions for this project as well, including: 1. My work in designing and implementing the digital receiver and controller board for the microASAR which was used for CASIE. 2. A description of how the GPU backprojection was used to improved the CASIE imagery. 3. A description of a sample SAR data set from CASIE provided to the public to promote further SAR research.
    [bibtex-key = stringhamPhDThesis2014LFMCWSARProcessingForUAVOperation] [bibtex-entry]


  45. Evan C. Zaugg. Generalized Image Formation for Pulsed and LFM-CW Synthetic Aperture Radar. PhD thesis, 2010. Keyword(s): SAR Processing, LFM-CW, LFM-CW SAR, FMCW, MoComp, motion compensation, CSA, ECS, Chirp Scaling, Extended Chirp Scaling, FSA, Frequency Scaling Algorithm, Range-Doppler Algorithm, synthetic aperture radar, Brigham Young University, muSAR system, LFM-CW signal model, SAR image quality, aircraft, atmospheric turbulence, high-resolution synthetic aperture radar systems, linear frequency-modulated continuous-wave signal, motion correction algorithms, unmanned aerial vehicle, Airborne SAR, geophysical techniques.
    Abstract: Approximations made in the traditional signal model for synthetic aperture radar (SAR) processing cause defocusing of the radar images when the system operates under conditions where the approximations lose validity. This dissertation investigates a number of these approximations and presents algorithmic improvements based on generalizations of the approxmations of the SAR signal model. These improvements result in better focused imagery from SAR systems with varied designs and parameters. Among the advancements presented is the development of a generalized chirp-scaling algorithm and a generalized frequency scaling algorithm to address the problems caused by approximations based on a Taylor series expansion of the SAR signal for both pulsed SAR and linear frequency modulated continuous wave (LFM-CW) SAR systems. These generalized algorithms extend the ability of frequency-domain algorithms to process SAR data from systems with a low frequency, a wide beamwidth, and a large bandwidth. Image formation algorithms are developed that account for the continuous platform motion and compensate for translational position errors due to the continuous non-ideal motion of real-world LFM-CW SAR systems, including a backprojection algorithm that does not rely upon the traditional stop-and-go approximation for platform motion.
    [bibtex-key = phDThesisEvanZaugg2010Generalized_Image_Formation_for_Pulsed_and_LFMCW_Synthetic_Aperture_Radar] [bibtex-entry]


  46. Adriano Meta. Signal Processing of FMCW Synthetic Aperture Radar Data. PhD thesis, TU Delft, 2006. Keyword(s): SAR Processing, FMCW, Frequency Modulated Continuous Wave, LFM-CW, SAR, Synthetic Aperture Radar, FMCW SAR, Airborne SAR, Focusing.
    Abstract: In the field of airborne earth observation, there is special attention for compact, cost effective, high resolution imaging sensors. Such sensors are foreseen to play an important role in small-scale remote sensing applications, such as the monitoring of dikes, watercourses, or highways. Furthermore, such sensors are of military interest; reconnaissance tasks could be performed with small unmanned aerial vehicles (UAVs), reducing in this way the risk for one's own troops. In order to be operated from small, even unmanned, aircrafts, such systems must consume little power and be small enough to fulfill the usually strict payload requirements. Moreover, to be of interest for the civil market, a reasonable cost is mandatory. Radar-based sensors have advantages over optical systems in their all weather capability and in the possibility to operate through smoke and at night. However, radar sensors used for imaging purposes exhibit relative low resolution in the cross-range or azimuth dimension, and furthermore it gets coarser with increasing distance due to the constant antenna beamwidth. This limitation is overcome by Synthetic Aperture Radar (SAR) techniques. Such techniques have already been successful employed in the field of radar earth observation by using coherent pulse radars. However, pulse radar systems are usually very complex instruments, and neither low cost nor compact. The fact that they are quite expensive makes them less suitable for low-cost, civil applications, while their bulkiness could prevent them from being chosen for UAV or small aircraft solutions. Frequency Modulated Continuous Wave (FMCW) radar systems are, instead, generally compact and relatively cheap to purchase and to exploit. FMCW radars consume little power and, due to the fact that they are continuously operating, they can transmit a modest power, which makes them very interesting for military applications. Consequently, FMCW radar technology is of interest for both civil and military airborne earth observation applications, specially in combination with high resolution SAR techniques. The novel combination of FMCW technology and SAR techniques leads to the development of a small, lightweight, and cost-effective high resolution imaging sensor. Research motivation SAR techniques have been successfully applied in combination with coherent pulse radars. Also the concept of synthetic aperture with FMCW sensors has already been put forward in literature, [1] [2], and some experimental systems have been described, [3] [4]. However, the practical feasibility of an airborne FMCW SAR was not evident; the experimental sensors described in literature were, in fact, radars mounted on rail supports operating in ground SAR configurations and at short distances. These FMCW radars could perform measurements in each position of the synthetic aperture and then be moved to the next one. As in conventional pulse SAR systems, the stop-and-go approximation could be used; such an approximation assumes the radar platform stationary during the transmission of the electromagnetic pulse and the reception of the corresponding echo. The approximation is valid for conventional pulse systems because the duration of the transmitted waveform is relatively short and, of course, is also valid for ground FMCW SAR where the radar can be actually fixed in a predetermined position while making the measurements. For airborne FMCW radars, however, the stop-and-go approximation can be not valid anymore because the platform is actually moving while continuously transmitting. A complete model for the deramped FMCW SAR signal derived without using the stop-and-go approximation was missing in the literature. In addition to the particular signal aspects relative to the combination of FMCW technology and SAR techniques, the use of FMCW radars for high resolution and long range applications was not evident. In practical FMCW sensors, in fact, the presence of unwanted non-linearities in the frequency modulation severely degrades the radar performances for large distances. Again, proper processing methods to overcome such limitation due to frequency non-linearities were not available to the scientific community. Therefore, the area of FMCW SAR airborne observation and related signal processing aspects was a very novel field of research. At the International Research Centre for Telecommunications and Radar (IRCTR) of the Delft University of Technology, a project was initiated to investigate the feasibility of FMCW SAR in the field of airborne earth observation and to develop proper processing algorithms to fully exploit the capability of such sensors. Research objectives Following from the motivations previously discussed, the first main objective of the project was to develop special processing SAR algorithms which could take into account the peculiar characteristics of an FMCW sensor. The features of major interest were: the presence of frequency non-linearities in the transmitted waveform and the fact that the FMCW sensor is continuously transmitting. The non-linearities represent a difference between an ideal and actual system, while the continuous motion has to be faced even when using sensors with performance close to the ideal. In the literature, some non-linearity correction algorithms were available, however they work only for very limited range intervals and, furthermore, require a reference point in each interval. For larger distance applications, as in the case for SAR, the use of these algorithms is not efficient neither robust. In FMCW SAR, the fact that the radar is continuously transmitting while moving means that the stop-and-go approximation used for the derivation of conventional SAR algorithms could not be anymore valid. These aspects needed to be analyzed and solutions had to be provided. The continuous transmission, on the other hand, can be used as an advantage in different other applications, as Moving Target Indication (MTI). In fact, in FMCW sensors, the pulse duration is considerably longer than in pulse radars, and therefore a better range frequency resolution is possible. The combination of this property and the possibility of using different kind of modulations (linear and non-linear) was investigated to see whether some FMCW SAR properties could be used to enhance the indication of moving targets. The other main objective of the project was to show the practicability of FMCW SAR under operational circumstances. Therefore, concurrently with the signal processing algorithms elaboration, the development of a fully operational airborne demonstrator system and an X-band radar front-end was started at the Delft University of Technology. A complete and detailed sensor model was required in order to estimate and analyze the performances of the system during the operational mode. In addition, the demonstrator system had to prove that an FMCW SAR sensor can indeed be operated in an efficient and cost effective manner from a very small airborne platform. The work for the initial requirements to the FMCW SAR system, the acquisition design and the development of the controlling software has been done by dr.ir. J.J.M. de Wit within the framework of the project [5]. This part will not be treated in this thesis. Novelties and main results Corresponding to the objectives set by the research project, the following novelties and main results have been reached and are presented in this thesis: - Non-linearity correction. The author has developed a very innovative processing solution, which completely solves the problem of the presence of frequency non-linearities in FMCW SAR. It corrects for the non-linearity effects for the whole range profile in one step, and it allows perfect range focusing, independently of the looking angle. The proposed method operates directly on the deramped data and it is very computationally efficient (Chapter 3, Section 3.4). - Deramping technique for non-linear Stepped Frequency Continuous Wave (SFCW) signals. An extension to non-linear continuous signals of the deramping technique, commonly used in linear FMCW sensors, has been developed. With the proposed extension, the great reduction in terms of sampling requirements can be achieved also when using non-linear waveforms, at the cost of increased computation (Chapter 3, Section 3.6.1). - A complete FMCW SAR signal model. The author has derived a detailed analytical model for the FMCW SAR signal in the two-dimensional frequency domain. Based on this model, proper algorithms are developed which guarantee the best performances when processing FMCW SAR data (Chapter 4). - MTI with slope diversity in linear FMCW SAR. The author has exploited the possibility of using triangular modulation for MTI by producing two images, respectively with the upslope and downslope part of the transmitted waveform. Based on the FMCW SAR signal model, interferometric techniques on the pair of images can be used to help distinguishing moving targets from stationary clutter (Chapter 5, Section 5.3). - MTI with randomized SFCW SAR. Based on the non-linear deramping technique previously proposed, the author has analyzed how randomized non-linear SFCW SAR can be used for MTI purposes (Chapter 5, Section 5.4). - Detailed system model. A complete model description of the X-band FMCW SAR front-end system developed at the IRCTR, Delft University of Technology, has been provided. The system has been extensively tested by the author together with P. Hakkart andW.F. van der Zwan through ground and laboratory measurements, the results showing very good consistency with the developed model (Chapter 6). - First demonstration of an X-band FMCW SAR. A flight test campaign has been organized during the last part of 2005. The results were very successful. The feasibility of an operational cheap FMCW SAR under practical circumstances has been proved. - High resolution FMCW SAR images. Thanks to the special algorithms developed, FMCW SAR images with 45 cm times 25 cm resolution (including windowing) have been obtained for the first time. Outline of the thesis The remaining of this thesis is divided in seven chapters: in the first four, the theory of FMCW SAR is introduced. Subsequently, the experimental system built at the IRCTR is described; the methods previously developed are validated by processing real FMCW SAR data collected during the flight test campaign organized in the last part of 2005. The thesis is organized as follows: Chapter 2 provides a short overview of the FMCW radar and SAR principles. It introduces aspects which are then more deeply analyzed and discussed in the subsequent chapters. Chapter 3 deals with the range processing of FMCW data and presents a novel processing solution, which completely solves the frequency non-linearity problem. It corrects for the non-linearity effects for the whole range profile and Doppler spectrum in one step, it operates directly on the deramped data and it is very computationally efficient. Nonlinear SFCW modulation is also treated in the chapter; a novel deramping technique extended to the case of non-linear signals is introduced. With the extended deramping technique proposed here, a reduced sampling frequency as for the linear case can be used also for randomized SFCW signals, at the cost of increased computation. Chapter 4 derives a complete analytical model of the FMCW SAR signal description in the two-dimensional frequency domain, starting from the deramped signal and without using the stop-and-go approximation. The model is then applied to stripmap, spotlight and single transmitter/multiple receiver Digital Beam Forming (DBF) synthetic aperture operational modes. Specially in the last two cases, the effects of the motion during the transmission and reception of the pulse can become seriously degrading for the SAR image quality, if not compensated. Chapter 5 exploits the peculiar characteristics of the complex FMCW SAR image for Moving Target Indication purposes. Two MTI methods are proposed in the chapter. The first is based on the frequency slope diversity in the transmitted modulation by using linear triangular FMCW SAR. The second makes use of the Doppler filtering properties of randomized SFCW modulations. Chapter 6 describes the X-band radar front-end developed at the Delft University of Technology. A detailed system model is provided in order to estimate and analyze the performance of the demonstrator system. Laboratory and ground based measurements show very good consistency with the calculated values, validating the model description. Chapter 7 presents the results obtained from the FMCW SAR flight test campaign organized during the last part of 2005. Thanks to the special algorithms which have been developed during the research project and described in the previous chapters, FMCW SAR images with a measured resolution up to 45 cm times 25 cm (including windowing) were obtained for the first time. Several tests performed during the flight campaign (imaging at different resolutions, varying the incident angle, MTI experiment) are reported and discussed. Chapter 8 summarizes the main results of the study which have led to this thesis; additionally, it draws conclusions and gives some recommendations for future work. Finally, as a demonstration of the increasing interest in FMCW SAR from the scientific and industry community, the chapter reports some related works started at other institutes.
    [bibtex-key = metaAdriano2006PhDThesisFMCWSARProcessing] [bibtex-entry]


  47. Hayden J. Callow. Signal Processing for Synthetic Aperture Sonar Image Enhancement. PhD thesis, University of Canterbury, 2003. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, Stripmap, Stripmap PGA, Stripmap Phase Gradient Autofocus, SAS, Synthetic Aperture Sonar.
    Abstract: This thesis contains a description of SAS processing algorithms, offering improvements in Fourier-based reconstruction, motion-compensation, and autofocus. Fourier-based image reconstruction is reviewed and improvements shown as the result of improved system modelling. A number of new algorithms based on the wavenumber algorithm for correcting second order effects are proposed. In addition, a new framework for describing multiple-receiver reconstruction in terms of the bistatic geometry is presented and is a useful aid to understanding. Motion-compensation techniques for allowing Fourier-based reconstruction in widebeam geometries suffering large-motion errors are discussed. A motion-compensation algorithm exploiting multiple receiver geometries is suggested and shown to provide substantial improvement in image quality. New motion compensation techniques for yaw correction using the wavenumber algorithm are discussed. A common framework for describing phase estimation is presented and techniques from a number of fields are reviewed within this framework. In addition a new proof is provided outlining the relationship between eigenvector-based autofocus phase estimation kernels and the phase-closure techniques used astronomical imaging. Micronavigation techniques are reviewed and extensions to the shear average single-receiver micronavigation technique result in a 3-4 fold performance improvement when operating on high-contrast images. The stripmap phase gradient autofocus (SPGA) algorithm is developed and extends spotlight SAR PGA to the wide-beam, wide-band stripmap geometries common in SAS imaging. SPGA supersedes traditional PGA-based stripmap autofocus algorithms such as mPGA and PCA -- the relationships between SPGA and these algorithms is discussed. SPGA's operation is verified on simulated and field-collected data where it provides significant image improvement. SPGA with phase-curvature based estimation is shown and found to perform poorly compared with phase-gradient techniques. The operation of SPGA on data collected from Sydney Harbour is shown with SPGA able to improve resolution to near the diffraction-limit. Additional analysis of practical stripmap autofocus operation in presence of undersampling and space-invariant blurring is presented with significant comment regarding the difficulties inherent in autofocusing field-collected data. Field-collected data from trials in Sydney Harbour is presented along with associated autofocus results from a number of algorithms.
    [bibtex-key = Callow2003PhD:PGAforSAS] [bibtex-entry]


  48. Oliver Wolfgang Hirsch. Neue Verarbeitungsverfahren von Along-Track Interferometrie Daten eines Radars mit synthetischer Apertur. PhD thesis, Universität-Gesamthochschule Siegen, 2002. Keyword(s): SAR Processing, Along Track Interferometry, Interferometry, Airborne SAR.
    Abstract: The German remote sensing project EURoPAK-B is aimed at the development and the demonstration of a special remote sensing system for the measurement of ocean surface currents and the monitoring of underwater bottom topography in coastal waters on the basis of airborne along- track interferometric (ATI) synthetic aperture radar. In contrast to across-track interferometry ATI is directly sensitive to motions of the scatterers which have a radial velocity component. The ATI phase being directly proportional to the interferometric velocity contains not only contributions resulting from the effective ocean surface currents but also from effects like e. g. orbital motions of longer waves. Thus the actual surface ocean currents differ from the measured interferometric velocities. The processed ATI phase is the starting point for the determination of interferometric velocities and has to be handled accurately. Therefore some concepts concerning a very proper processing of ATI data have to be developed. This includes the correction of phase contributions resulting from the wave guide length at different channel frequencies. Another undesirable contribution to the phase results from the existing across-track component which depends on the squint angle during data acquisition. To minimize that effect the exact positions of the phase centres of both antennas have to be determined in a theoretical way. Also some other effects on the ATI data like the azimuthal displacement of moving targets have to be taken into account for a correct geolocation of the ATI data. This work gives also reasons why the ATI phase has to be unwrapped although the phase resulting from the ocean currents should be within the order of -PI and +PI. It will also been shown how phase unwrapping errors of the ATI phase can be removed. In this work these concepts for accurately processing ATI data are discussed and results are shown. Supplementary it points out the proposed remote sensing technique for accurate measurements of both ocean surface currents and underwater bottom topography in coastal waters generated by the University of Hamburg.
    [bibtex-key = hirsch:ati] [bibtex-entry]


  49. Andreas Reigber. Airborne Polarimetric SAR Tomography. PhD thesis, University of Stuttgart, 2001. Keyword(s): SAR Processing, SAR Tomography, Tomography, Polarimettric SAR, Pol-InSAR, L-Band, E-SAR. [bibtex-key = reigberDiss2001:Tomo] [bibtex-entry]


  50. Richard T. Lord. Aspects of Stepped-Frequency Processing for Low-Frequency SAR Systems. PhD thesis, University of Cape Town, Rondebosch, South Africa, 2000. Keyword(s): SAR Processing, Stepped-Frequency Processing, RFI Suppression, Low-Frequency SAR, Wideband SAR.
    Abstract: Ultra-wideband synthetic aperture radar (SAR) systems operating in the VHF/UHF region are becoming increasingly popular because of their growing number of applications in the areas of foliage penetration radar (FOPEN) and ground-penetrating radar (GPR). The objective of this thesis is to investigate the following two aspects of low-frequency (VHF/UHF-band) SAR processing: 1. The use of stepped-frequency waveforms to increase the total radar bandwidth, thereby increasing the range resolution, and 2. Radio frequency interference (RFI) suppression. A stepped-frequency system owes its wide bandwidth to the transmission of a group of narrow-bandwidth pulses, which are then combined using a signal processing technique to achieve the wide bandwidth. Apart from providing an economically viable path for the upgrading of an existing single frequency system, stepped-frequency waveforms also offer opportunities for RFI suppression. This thesis describes three methods to process stepped-frequency waveforms, namely an IFFTmethod, a time-domainmethod and a frequency-domainmethod. Both the IFFT method and the time-domain method have been found to be unsuitable for SAR processing applications. The IFFT method produces multiple ghost targets in the high resolution range profile due to the spill-over effect of energy into consecutive coarse range bins, and the time-domain technique is computationally inefficient on account of the upsampling requirement of the narrow-bandwidth pulses prior to the frequency shift. The frequency-domain technique, however, efficiently uses all the information in the narrowband pulses to obtain high-resolution range profiles which do not contain any ghost targets, and is therefore well suited for SAR processing applications. This technique involves the reconstruction of a wider portion of the target\u2019s reflectivity spectrum by combining the individual spectra of the transmitted narrow-bandwidth pulses in the frequency domain. It is shown here how this method may be used to avoid spectral regions that are heavily contaminated with RFI, thereby alleviating the problem of receiver saturation due to RFI. Stepped-frequency waveforms also enable the A/D converter to sample the received narrow-bandwidth waveform with a larger number of bits, which increases the receiver dynamic range, thereby further alleviating the problem of receiver saturation during the presence of RFI. In addition to using stepped-frequency waveforms for RFI suppression, a number of other techniques have been investigated to suppress RFI... Of these, the notch filter and the LMS adaptive filter have been implemented and applied on real P-band data obtained from the E-SAR system of the German Aerospace Center (DLR), Oberpfaffenhofen, and on real VHF-band data obtained from the South African SAR (SASAR) system. Both methods significantly suppressed the RFI in the real images investigated. It was found that the number of range lines upon which the LMS adaptive filter could operate without adaptively changing the filter tap weights was often well above 100. This facilitated the re-writing of the LMS adaptive filter in terms of an equivalent transfer function, which was then integrated with the range-compression stage of the range-Doppler SAR processing algorithm. Since the range-compression and the interference suppression could then be performed simultaneously, large computational savings were achieved. A technique was derived for suppressing the sidelobes which arise as a result of the interference suppression of the LMS adaptive filter. This method was also integrated with the range-compression stage of the range-Doppler processor, leading to a very efficient implementation of the entire RFI suppression routine.
    Comments: +Frequency/time/IFFT processing of stepped frequencies and applications toward radio frequency interferences (RFI).
    [bibtex-key = LordDissertation00:SteppedFreqAndRFI] [bibtex-entry]


  51. Josef Mittermayer. Hochauflösende Verarbeitung von Radardaten mit synthetischer Apertur. PhD thesis, Universität-Gesamthochschule Siegen, 2000. Keyword(s): SAR Processing, Frequency Scaling Algorithm, Comparison of Algorithms, Range-Doppler Algorithm, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Polar Format Algorithm, SPECAN, Convolution Back-projection, Spotlight SAR, Stripmap SAR, ScanSAR, ESAR, Airborne SAR, Spaceborne SAR.
    Abstract: In this thesis, the novel Frequency Scaling Algorithm for efficient and accurate interpolation-free processing of Spotlight SAR data with Dechirp on Receive has been developed. An accurate analytical model of the Spotlight illumination geometry for a straight flight path of the carrier is introduced. Using this model, a valid and an illuminated target area is defined. A new analytical formulation for SAR raw data with one-dimensional Dechirp on Receive Operation is derived, which expresses the Residual Video Phase Term by a convolution with a chirp signal. Starting with this formulation, an interpolation-free range cell migration correction is developed. This range cell migration correction is called Frequency Scaling. Due to the constant target illumination start and end time in Spotlight SAR data, the SPECAN approach offers a very efficient processing in azimuth but due to the poor accuracy, it has only been used for ScanSAR and not for Spotlight data up to now. Furthermore, a SPECAN approach always requires interpolations. In the frame of this thesis, an azimuth scaling operation has been developed, which allows a highly accurate and interpolation-free processing of Spotlight data by the SPECAN approach. The combination of SPECAN approach and azimuth scaling is first applied to ScanSAR data in order to improve the Extended Chirp Scaling Algorithm. Then it is extended to Spotlight processing. Spotlight raw data with one-dimensional Dechirp on Receive have to be upsampled in azimuth before the data processing. In order to avoid this upsampling, a subaperture approach is introduced, which reduces the required azimuth sampling during the processing down to the pulse repetition frequency of the raw data. The high quality performance of the Frequency Scaling Algorithm is demonstrated by processing simulated and real raw data acquired with the E-SAR-Sensor of DLR. The phase fidelity, which is important for many applications, is proved by point target processing and by the generation of an interferogram of an E-SAR scene. As application example, a spotlight interferogram is generated from E-SAR raw data. Interesting physical properties of the Spotlight mode are demonstrated by the comparison of a Spotlight and a Stripmap interferogram, processed from the same raw data.
    [bibtex-key = mittermayer:sar] [bibtex-entry]


  52. David W. Hawkins. Synthetic aperture imaging algorithms : with application to wide bandwidth sonar. PhD thesis, University of Canterbury. Electrical and Computer Engineering, 1996. Keyword(s): SAR Processing, Azimuth Focusing, Autofocus, Synthetic Aperture Sonar, SAS, Sonar, Phase Gradient Autofocus, Stripmap, range-Doppler Algorithm, Chirp Scaling Algorithm, CSA, omega-k, Wavenumber Domain Algorithm.
    Abstract: This thesis contains the complete end-to-end simulation, development, implementation, and calibration of the wide bandwidth, low-Q, Kiwi-SAS synthetic aperture sonar (SAS). Through the use of a very stable towfish, a new novel wide bandwidth transducer design, and autofocus procedures, high-resolution diffraction limited imagery is produced. As a complete system calibration was performed, this diffraction limited imagery is not only geometrically calibrated, it is also calibrated for target cross-section or target strength estimation. Is is important to note that the diffraction limited images are formed without access to any form of inertial measurement information. Previous investigations applying the synthetic aperture technique to sonar have developed processors based on exact, but inefficient, spatial-temporal domain time-delay and sum beamforming algorithms, or they have performed equivalent operations in the frequency domain using fast-correlation techniques (via the fast Fourier transform (FFT)). In this thesis, the algorithms used in the generation of synthetic aperture radar (SAR) images are derived in their wide bandwidth forms and it is shown that these more efficient algorithms can be used to form diffraction limited SAS images. Several new algorithms are developed; accelerated chirp scaling algorithm represents an efficient method for processing synthetic aperture data, while modified phase gradient autofocus and a low-Q autofocus routine based on prominent point processing are used to focus both simulated and real target data that has been corrupted by known and unknown motion or medium propagation errors.
    [bibtex-key = hawkinsPhDThesis1996] [bibtex-entry]


  53. Jerald L. Bauck. Tomographic Processing of Synthetic Aperture Radar Signals for Enhanced Resolution. PhD thesis, 1989. Keyword(s): SAR Processing, Bistatic SAR, Back-Projection, bistatic synthetic aperture radar, Azimuth Focusing, convolution back-projection, elliptical-arc projections, final reconstructed image, ground patch, image resource, pixel, weighting, radar cross-sections, radar theory, Spotlight mode, Airborne SAR, Tomographic Processing, Tomography, Wavefront Curvature.
    Abstract: Spotlight-mode synthetic aperture radar imaging is studied from the viewpoint of tomographic signal processing which allows the relaxation of the nearly-universal assumption that plane waves pass over the ground patch. This allows high-quality image reconstruction in the face of arbitrary amounts of wavefront curvature such as would be present when the angle subtended by the ground patch, as seen by the radar,is not small. One such application is wide-area surveillance. A methodology is used which has the benefits of a wideband transmitted signal (impulse) and a sensible simulation. Image reconstruction algorithms are developed for monostatic and bistatic systems. Simulation results using these algorithms compare favorably with baseline simulations which use a more conventional algorithm operating on data which do not embody the effects of wavefront curvature. Comments on system design and computational implementation are made as necessary. A new set of problems which appear to benefit from the tomographic viewpoint is posed. This work may also find I applications in some forms of reflection tomography.
    [bibtex-key = phDThesisBauck1989TomoBistaticSAR] [bibtex-entry]


  54. J.R. Huynen. Phenomenological theory of radar targets. PhD thesis, Drukkerij Bronder-Offset NV, 1970. [bibtex-key = huynenPhDThesis1970] [bibtex-entry]


  55. Jan Freihardt and Othmar Frey. Assessing riverbank erosion in Bangladesh using time series of Sentinel-1 radar imagery in the Google Earth Engine. Nat. Hazards Earth Syst. Sci.,, 23:751-770, 2023. Keyword(s): SAR Processing, Sentinel-1, time series, Google Earth Engine, GEE, Jamuna, Jamuna River, Bangladesh, erosion, riverbank erosion, classification, natural hazard, geohazard.
    Abstract: Riverbank erosion occurs along many of the Earth's river systems, affecting riverine populations by destroying agricultural land and housing. In this study, we detected past events of riverbank erosion along the Jamuna River in Bangladesh using time series of Sentinel-1 satellite radar imagery, ground-range-detected (GRD) data with a 12 d revisit cycle, available in the Google Earth Engine (GEE). Eroded land is detected by performing a land cover classification and by detecting land cover changes from vegetated areas before the monsoon to sand or water after the monsoon. Further, settlements are detected as persistent scatterers and classified as eroded if they are located on eroded land. We found that with Sentinel-1 data, erosion locations can be determined already 1 month after the end of the monsoon and hence potentially earlier than using optical satellite images which depend on cloud-free daylight conditions. Further, we developed an interactive GEE-based online tool allowing the user to explore where riverbank erosion has destroyed land and settlements along the Jamuna in five monsoon seasons (2015-2019). The source code of our implementation is publicly available, providing the opportunity to reproduce the results, to adapt the algorithm and to transfer our results to assess riverbank erosion in other geographical settings.
    [bibtex-key = freihardtFreyNHESS2023RiverbankErosionBangladeshSentinel1GoogleEarthEngine] [bibtex-entry]


  56. Marco Manzoni, Dario Tagliaferri, Marco Rizzi, Stefano Tebaldini, Andrea Virgilio Monti Guarnieri, Claudio Maria Prati, Monica Nicoli, Ivan Russo, Sergi Duque, Christian Mazzucco, and Umberto Spagnolini. Motion Estimation and Compensation in Automotive MIMO SAR. IEEE Transactions on Intelligent Transportation Systems, 24(2):1756-1772, February 2023. Keyword(s): Synthetic aperture radar, Radar, Radar imaging, Radar polarimetry, Radar antennas, Automotive engineering, Navigation, SAR, automotive, MIMO, autofocus, motion compensation.
    Abstract: With the advent of self-driving vehicles, autonomous driving systems will have to rely on a vast number of heterogeneous sensors to perform dynamic perception of the surrounding environment. Synthetic Aperture Radar (SAR) systems increase the resolution of conventional mass-market radars by exploiting the vehicle's ego-motion, requiring very accurate knowledge of the trajectory, usually not compatible with automotive-grade navigation systems. In this setting, radar data are typically used to refine the navigation-based trajectory estimation with so-called autofocus algorithms. Although widely used in remote sensing applications, where the timeliness of the imaging is not an issue, autofocus in automotive scenarios calls for simple yet effective processing options to enable real-time environment imaging. This paper aims at providing a comprehensive theoretical and experimental analysis of the autofocus requirements in typical automotive scenarios. We analytically derive the effects of navigation-induced trajectory estimation errors on SAR imaging, in terms of defocusing and wrong targets' localization. Then, we propose a motion estimation and compensation workflow tailored to automotive applications, leveraging a set of stationary Ground Control Points (GCPs) in the low-resolution radar images (before SAR focusing). We theoretically discuss the impact of the GCPs position and focusing height on SAR imaging, highlighting common pitfalls and possible countermeasures. Finally, we show the effectiveness of the proposed technique employing experimental data gathered during open road campaign by a 77 GHz multiple-input multiple-output radar mounted in a forward-looking configuration.
    [bibtex-key = manzoniEtalTIT2023SMotionEstimationAndCompensationInAutomotiveMIMOSAR] [bibtex-entry]


  57. Marco Manzoni, Stefano Tebaldini, Andrea Virgilio Monti-Guarnieri, and Claudio Maria Prati. Multipath in Automotive MIMO SAR Imaging. IEEE Transactions on Geoscience and Remote Sensing, 61:1-12, 2023. Keyword(s): Radar imaging, Radar, Automotive engineering, MIMO communication, Automobiles, Radar antennas, Geometry, Automotive, double bounce, ghost targets, multipath, multiple-input multiple-output (MIMO), radar, synthetic aperture radar (SAR).
    Abstract: This article discusses the effect of multipath in automotive radar imaging under different sensor configurations. The study is motivated by the fact that radar technologies are becoming indispensable in the automotive scenario. Many applications such as collision avoidance systems, assisted parking, and driving assistance systems take advantage of radar technologies to accomplish their task. However, one of the main concerns about automotive radars is the possibility of detecting false targets due to multiple signal reflections. In this article, we show how different sensor layouts experience multipath differently. In particular, we demonstrate that with multiple-input multiple-output (MIMO) radars, what really matters is the physical positions of the transmitting and receiving antennas. The monostatic/bistatic equivalent configurations cannot be used to design a system and to simulate an acquisition in the presence of a multipath. We also demonstrate how vehicle-based MIMO-synthetic aperture radar (MIMO-SAR) imaging can generate a bi-dimensional aperture which significantly reduces multipath effects in the focused image, avoiding the detection of false targets. All the theoretical analyses are supported by several simulations where different sensor layouts are tested, and the capability of MIMO-SAR to reject multipath is validated.
    [bibtex-key = manzoniTebaldiniMontiGuarnieriPratiTGRS2023MultipathInAutomoticeMIMOSARImaging] [bibtex-entry]


  58. Aboveground forest biomass varies across continents, ecological zones and successional stages: Refined IPCC default values for tropical and subtropical forests. Environmental Research Letters, 17(1), 2022. [bibtex-key = Rozendaal2022] [bibtex-entry]


  59. A. Araza, S. de Bruin, M. Herold, S. Quegan, N. Labriere, P. Rodriguez-Veiga, V. Avitabile, M. Santoro, E.T.A. Mitchard, C.M. Ryan, O.L. Phillips, S. Willcock, H. Verbeeck, J. Carreiras, L. Hein, M.-J. Schelhaas, A.M. Pacheco-Pascagaza, P. da Conceicao Bispo, G.V. Laurin, G. Vieilledent, F. Slik, A. Wijaya, S.L. Lewis, A. Morel, J. Liang, H. Sukhdeo, D. Schepaschenko, J. Cavlovic, H. Gilani, and R. Lucas. A comprehensive framework for assessing the accuracy and uncertainty of global above-ground biomass maps. Remote Sensing of Environment, 272(112917), 2022. [bibtex-key = Araza2022] [bibtex-entry]


  60. A. Bertone, C. Barboux, X. Bodin, T. Bolch, F. Brardinoni, R. Caduff, H.H. Christiansen, M.M. Darrow, R. Delaloye, B. Etzelmüller, O. Humlum, C. Lambiel, K.S. Lilleoren, V. Mair, G. Pellegrinon, L. Rouyet, L. Ruiz, and T. Strozzi. Incorporating InSAR kinematics into rock glacier inventories: insights from 11 regions worldwide. Cryosphere, 16(7):2769-2792, 2022. [bibtex-key = Bertone2022] [bibtex-entry]


  61. Achille Capelli, Franziska Koch, Patrick Henkel, Markus Lamm, Florian Appel, Christoph Marty, and Jürg Schweizer. GNSS signal-based snow water equivalent determination for different snowpack conditions along a steep elevation gradient. The Cryosphere, 16(2):505-531, September 2022. Keyword(s): GNSS, Snow water equivalent, SWE, Liquid water content (LWC), snow height, Davos, Laret, Weissfluhjoch, Küblis, snow, SLF, WSL.
    Abstract: Snow water equivalent (SWE) can be measured using low-cost Global Navigation Satellite System (GNSS) sensorswith one antenna placed below the snowpack and another one serving as a reference above the snow. The underlying GNSSsignal-based algorithm for SWE determination for dry- and wet-snow conditions processes the 5 carrier phases and signalstrengths and derives additionally liquid water content (LWC) and snow depth (HS). So far, the algorithm was testedintensively for high-alpine conditions with distinct seasonal accumulation and ablation phases. In general, snow occurrence,snow amount, snow density and LWC can vary considerably with climatic conditions and elevation. Regarding alpine regions,lower elevations mean generally earlier and faster melting, more rain-on-snow events and shallower snowpack. Therefore, we10 assessed the applicability of the GNSS-based SWE measurement at four stations along a steep elevation gradient (820, 1185,1510 and 2540 m a.s.l.) in the eastern Swiss Alps during two winter seasons (2018-2020). Reference data of SWE, LWC andHS were collected manually and with additional automated sensors at all locations. The GNSS-derived SWE estimates agreedvery well with manual reference measurements along the elevation gradient and the accuracy (RMSE = 34 mm,RMSRE = 11 %) was similar under wet- and dry-snow conditions, although significant differences in snow density and15 meteorological conditions existed between the locations. The GNSS-derived SWE was more accurate than measured withother automated SWE sensors. However, with the current version of the GNSS algorithm, the determination of daily changesof SWE was found to be less suitable compared to manual measurements or pluviometer recordings and needs furtherrefinement. The values of the GNSS-derived LWC were robust and within the precision of the manual and radar measurements.The additionally derived HS correlated well with the validation data. We conclude that SWE can reliably be determined using low-cost GNSS-sensors under a broad range of climatic conditions and LWC and HS are valuable add-ons.
    [bibtex-key = capelliKochHenkelLammAppelMartySchweizerCRYOSPHERE2022GNSSsignalBasedSWEForDifferentSnowPackConditionsSLFSites] [bibtex-entry]


  62. O. Cartus, M. Santoro, U. Wegmuller, N. Labriere, and J. Chave. Sentinel-1 Coherence for Mapping Above-Ground Biomass in Semiarid Forest Areas. IEEE Geoscience and Remote Sensing Letters, 19, 2022. [bibtex-key = Cartus2022] [bibtex-entry]


  63. A. Cicoira, S. Weber, A. Biri, B. Buchli, R. Delaloye, R. Da Forno, I. Gärtner-Roer, S. Gruber, T. Gsell, A. Hasler, R. Lim, P. Limpach, R. Mayoraz, M. Meyer, J. Noetzli, M. Phillips, E. Pointner, H. Raetzo, C. Scapozza, T. Strozzi, L. Thiele, A. Vieli, D. Vonder Mühll, V. Wirz, and J. Beutel. In situ observations of the Swiss periglacial environment using GNSS instruments. Earth System Science Data, 14(11):5061-5091, 2022. [bibtex-key = Cicoira2022] [bibtex-entry]


  64. Roberto Coscione, Irena Hajnsek, Charles Werner, and Othmar Frey. Assessing the impact of positioning errors in car-borne repeat-pass SAR interferometry with a controlled rail-based experiment. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 15:8402-8415, 2022. Keyword(s): SAR Processing, SAR Interferometry, Car-borne SAR, positioning errors, rail-based SAR, experiment, terrestrial radar interferometry, TRI, Gamma L-band SAR, ground-based SAR, GB-SAR.
    Abstract: Agile synthetic aperture radar (SAR) platforms such as car-borne and UAV-borne SAR systems require combined inertial navigation systems (INS) and global navigation satellite systems (GNSS) to measure the radar sensor trajectories used for focusing and interferometric processing. Measurement inaccuracies from INS/GNSS systems lead to residual phase errors in the SAR products whose minimisation is crucial to derive accurate topographic and deformation information. In this work, we analyse the impact of residual positioning errors on car-borne repeat-pass SAR interferometry at L-band for different INS/GNSS measurement configurations and for the typical car-borne acquisition geometry. The positioning errors are evaluated both during single SAR acquisitions with long integration times and between different acquisitions as a function of the distance of the radar platform from the GNSS reference stations. We show the reduction of interferometric phase errors achievable by additionally using a GNSS receiver mounted in the vicinity of the SAR platform as compared to remote reference stations of the national network of permanent GNSS receivers. Test results obtained in a controlled setup with a rail-based SAR system equipped with a navigation-grade INS/GNSS system show maximum repeat-pass trajectory errors on the order of 1-2 cm using a local GNSS reference station and up to 10-15 cm using the remote reference stations, leading to azimuth and range phase trends in the interferometric products.
    [bibtex-key = coscioneWernerHajnsekFrey2022ImpactOfPositioningErrorsInCarborneRepeatpassINSAR] [bibtex-entry]


  65. Richard Czikhardt, Hans van der Marel, Juraj Papco, and Ramon F. Hanssen. On the Efficacy of Compact Radar Transponders for InSAR Geodesy: Results of Multiyear Field Tests. IEEE Transactions on Geoscience and Remote Sensing, 60:1-13, 2022. Keyword(s): SAR Processing, Corner Reflectors, InSAR, SAR Interferometry.
    Abstract: Compact and low-cost radar transponders are an attractive alternative to corner reflectors (CRs) for interferometric synthetic aperture radar (InSAR) deformation monitoring, datum connection, and geodetic data integration. Recently, such transponders have become commercially available for C-band sensors, which poses relevant questions on their characteristics in terms of radiometric, geometric, and phase stability. Especially for extended time series and for high-precision geodetic applications, the impact of secular or seasonal effects, such as variations in temperature and humidity, has yet to be proven. In this article, we address these challenges using a multitude of short baseline experiments with four transponders and six CRs deployed at test sites in The Netherlands and Slovakia. Combined together, we analyzed 980 transponder measurements in Sentinel-1 time series to a maximum extent of 21 months. We find an average radar cross section (RCS) of over 42 dBm2 within a range of up to 15 deg of elevation misalignment, which is comparable to a triangular trihedral CR with a leg length of 2.0 m. Its RCS shows the temporal variations of 0.3-0.7 dBm2 (standard deviation), which is partially correlated with surface temperature changes. The precision of the InSAR phase double differences over short baselines between a transponder and a stable reference CRs is found to be 0.5-1.2 mm (one sigma). We observe a correlation with surface temperature, leading to seasonal variations of up to +/-3 mm, which should be modeled and corrected for in high-precision InSAR applications. For precise SAR positioning, we observe antenna-specific constant internal electronic delays of 1.2-2.1 m in slant range, i.e., within the range resolution of the Sentinel-1 interferometric wide (IW) product, with a temporal variability of less than 20 cm. Comparing similar transponders from the same series, we observe distinct differences in performance. Our main conclusion is that these characteristics are favorable for a wide range of geodetic applications. For particular demanding applications, individual calibration of single devices is strongly recommended.
    [bibtex-key = czikhardtvanDerMarelPapcoHanssenTGRS2022CompactRadarTranspondersforInSARGeodesy] [bibtex-entry]


  66. Claudio De Luca, Francesco Casu, Michele Manunta, Giovanni Onorato, and Riccardo Lanari. Comments on ``Study of Systematic Bias in Measuring Surface Deformation With SAR Interferometry''. IEEE Transactions on Geoscience and Remote Sensing, 60:1-5, 2022.
    Abstract: In a recent publication, Ansari et al. (2021) claimed (see, in particular, the Discussion and Recommendation Section in their article) that the advanced differential SAR interferometry (InSAR) algorithms for surface deformation retrieval, based on the small baseline approach, are affected by systematic biases in the generated InSAR products. Therefore, to avoid such biases, they recommended a strategy primarily focused on excluding "the short temporal baseline interferograms and using long baselines to decrease the overall phase errors." In particular, among various techniques, Ansari et al. (2021) identified the solution presented by Manunta et al. (2019) as a small baseline advanced InSAR processing approach where the presence of the above-mentioned biases (referred to as a fading signal) compromises the accuracy of the retrieved InSAR deformation products. We show that the claim of Ansari et al. (2021) is not correct (at least) for what concerns the mentioned approach discussed by Manunta et al. (2019). In particular, by processing the Sentinel-1 dataset relevant to the same area in Sicily (southern Italy) investigated by Ansari et al. (2021), we demonstrate that the generated InSAR products do not show any significant bias.
    [bibtex-key = deLucaCasuManuntaOnoratoLanariTGRS2022CommentsOnSudyOfSystematicBiasInDInSARDeformation] [bibtex-entry]


  67. Davide Giudici, Pietro Guccione, Marco Manzoni, Andrea Monti Guarnieri, and Fabio Rocca. Compact and Free-Floating Satellite MIMO SAR Formations. IEEE Transactions on Geoscience and Remote Sensing, 60:1-12, 2022. Keyword(s): SAR Formation, MIMO, Spaceborne SAR, Multistatic SAR.
    Abstract: We discuss a coherent synthetic aperture radar (SAR) formation where N identical sensors transmit at the same time, code, and frequency. This is a particular multiple-input-multiple-output (MIMO) configuration, where the transmitted waveforms interfere together, resulting in an illumination pattern that randomly changes in space and time. Similar to the single-input-multiple-output (SIMO) formations, the diversity provided by the N receiver phase centers can be used to mitigate this interference and reduce the pulse repetition frequency (PRF) for achieving large swath coverage. The good point, in the MIMO case, is that the signal-to-noise ratio (SNR) gain of the system increases, theoretically, with the square of the number of elements. However, residual spurious sidelobes may appear as ghosts of the multiple illuminators. In practice, the power gain is to be optimized, together with ambiguity rejection, sidelobes, and azimuth resolution. The actual performances achievable by these formations in terms of impulse response function (IRF), SNR, and sensitivity to the precise positioning of the sensors are discussed theoretically and based on simulations.
    [bibtex-key = giudiciGuccioneManzoniGuarnieriRoccaTGRS2022CompactAndFreeFloatingSatelliteMIMOSARFormations] [bibtex-entry]


  68. Timo Grebner, Pirmin Schoeder, Vinzenz Janoudi, and Christian Waldschmidt. Radar-Based Mapping of the Environment: Occupancy Grid-Map Versus SAR. IEEE Microwave and Wireless Components Letters, pp 1-4, 2022. Keyword(s): SAR Processing, automotive radar, automotive SAR, synthetic aperture radar, SAR, caborne SAR, Frequency-modulated continuous-wave radar, multiple-input multiple-output (MIMO) radar, occupancy grid map, OGM.
    Abstract: For autonomous driving vehicles, highly accurate representations of the environment are essential for both trajectory planning and self-localization. Different possibilities allow to generate detailed maps of the environment based on chirp-sequence radar sensors for advanced driver assistance systems (ADASs). For the first time, this letter shows a qualitative comparison between synthetic aperture radar (SAR)- and occupancy grid map (OGM)-based environment representation using identical measurement data. The differences of existing signal processing chains as well as a visual measurement-based comparison of the resulting environmental maps is presented.
    [bibtex-key = grebnerSchoederJanoudiWaldschmidtIEEEMWCL2022CarborneMappingAt77GhzOccupancyGridVsSAR] [bibtex-entry]


  69. Fengming Hu, Freek J. van Leijen, Ling Chang, Jicang Wu, and Ramon F. Hanssen. Combined Detection of Surface Changes and Deformation Anomalies Using Amplitude-Augmented Recursive InSAR Time Series. IEEE Transactions on Geoscience and Remote Sensing, 60:1-16, 2022. Keyword(s): PSI, Deformation, Persistent Scatterer Interferometry.
    Abstract: Synthetic aperture radar (SAR) missions with short repeat times enable opportunities for near real-time deformation monitoring. Traditional multitemporal interferometric SAR (MT-InSAR) is able to monitor long-term and periodic deformation with high precision by time-series analysis. However, as time series lengthen, it is time-consuming to update the current results by reprocessing the whole dataset. Additionally, the number of coherent scatterers varies over time due to disappearing and emerging scatterers due to inevitable changes in surface scattering, and potential deformation anomalies require changes in the prevailing deformation model. Here, we propose a novel method to analyze InSAR time series recursively and detect both significant changes in scattering as well as deformation anomalies based on the new acquisitions. Sequential change detection is developed to identify temporary coherent scatterers (TCSs) using amplitude time series. Based on the predicted phase residuals, scatterers with abnormal deformation displacements are identified by a generalized ratio test, while the parameters of stable scatterers are updated using Kalman filtering. The quality of the anomaly detection is assessed based on the detectability power and the minimum detectable deformation. This facilitates (near) real-time data processing and decreases the false alarm likelihood. Experimental results show that the technique can be used for the real-time evaluation of deformation risks.
    [bibtex-key = huVanLeijenChangWuHanssenTGRS2022CombinedDetectionOfSurfaceChangesAndDeformatioUsingAmplitudeAugmentedRecursiveInSARTimeSeries] [bibtex-entry]


  70. V. Humphrey and C. Frankenberg. Continuous ground monitoring of vegetation optical depth and water content with GPS signals. Biogeosciences Discussions, 2022:1-44, 2022. Keyword(s): GNSS, forest, monitoring, microwave, radar, L-band, vegetation. [bibtex-key = humphreyFrankenberg2022VegetationOpticalDepthAndWaterContentWithGPSSignals] [bibtex-entry]


  71. Guodong Jin, Wei Wang, Yunkai Deng, He Yan, and Robert Wang. A Novel Range-Azimuth Joint Modulation Scheme for Range Ambiguity Suppression. IEEE Transactions on Geoscience and Remote Sensing, 60:1-10, 2022. Keyword(s): Azimuth, Doppler effect, Synthetic aperture radar, Modulation, Bandwidth, Radar imaging, Frequency modulation, Azimuth phase coding (APC), nonlinear frequency modulation (NLFM) waveforms, range ambiguity suppression.
    Abstract: Range ambiguity is a technical challenge for current spaceborne synthetic aperture radar (SAR) systems. To this end, a novel range-azimuth joint modulation transmission scheme is proposed, and the corresponding imaging processing and performance analysis are detailed. Compared with the azimuth phase coding (APC) technique, this scheme fully exploits the sampling margins of the range and azimuth dimensions, resulting in the range ambiguities experiencing a double suppression effect. Starting from the range-azimuth joint modulation scheme, to obtain the best ambiguity suppression performance, the design of a nonlinear frequency modulation (NLFM) waveform with a continuous piecewise linear instantaneous frequency is formulated and tackled via a MATLAB optimization toolbox. The detailed simulation results based on LuTan-1 (LT-1) parameters illustrate that the proposed methodologies outperform the APC method and provide considerable ambiguity suppression.
    [bibtex-key = Jin2022] [bibtex-entry]


  72. Josef Kellndorfer, Oliver Cartus, Marco Lavalle, Christophe Magnard, Pietro Milillo, Shadi Oveisgharan, Batu Osmanoglu, Paul A. Rosen, and Urs Wegmuller. Global seasonal Sentinel-1 interferometric coherence and backscatter data set. Scientific Data, 9(1):73, 2022.
    Abstract: This data set is the first-of-its-kind spatial representation of multi-seasonal, global C-band Synthetic Aperture Radar (SAR) interferometric repeat-pass coherence and backscatter signatures. Coverage comprises land masses and ice sheets from 82° Northern to 79° Southern latitudes. The data set is derived from multi-temporal repeat-pass interferometric processing of about 205,000 Sentinel-1 C-band SAR images acquired in Interferometric Wide-Swath Mode from 1-Dec-2019 to 30-Nov-2020. The data set encompasses three sets of seasonal (December-February, March-May, June-August, September-November) metrics produced with a pixel spacing of three arcseconds: 1) Median 6-, 12-, 18-, 24-, 36-, and 48-days repeat-pass coherence at VV or HH polarizations, 2) Mean radiometrically terrain corrected backscatter (γ0) at VV and VH, or HH and HV polarizations, and 3) Estimated parameters of an exponential coherence decay model. The data set has been produced to obtain global, spatially detailed information on how decorrelation affects interferometric measurements of surface displacement and is rich in spatial and temporal information for a variety of mapping applications.
    [bibtex-key = kellndorferEtAl2022GlobalSeasonalSentinel1InterferometricCoherenceAndBackScatter] [bibtex-entry]


  73. W. Kochtitzky, L. Copland, W. Van Wychen, R. Hugonnet, R. Hock, J.A. Dowdeswell, T. Benham, T. Strozzi, A. Glazovsky, I. Lavrentiev, D.R. Rounce, R. Millan, A. Cook, A. Dalton, H. Jiskoot, J. Cooley, J. Jania, and F. Navarro. The unquantified mass loss of Northern Hemisphere marine-terminating glaciers from 2000-2020. Nature Communications, 13(1), 2022. [bibtex-key = Kochtitzky2022] [bibtex-entry]


  74. Juha Lemmetyinen, Juval Cohen, Anna Kontu, Juho Vehvil�inen, Henna-Reetta Hannula, Ioanna Merkouriadi, Stefan Scheiblauer, Helmut Rott, Thomas Nagler, Elisabeth Ripper, Kelly Elder, Hans-Peter Marshall, Reinhard Fromm, Marc Adams, Chris Derksen, Joshua King, Adriano Meta, Alex Coccia, Nick Rutter, Melody Sandells, Giovanni Macelloni, Emanuele Santi, Marion Leduc-Leballeur, Richard Essery, Cecile Menard, and Michael Kern. Airborne SnowSAR data at X and Ku bands over boreal forest, alpine and tundra snow cover. Earth System Science Data, 14(9):3915-3945, September 2022. [bibtex-key = lemmetyinenEtAlESSD2022AirborneSARDataAtXandKuBandOverBorealForestAlpineAndTundraSnowCover] [bibtex-entry]


  75. Haoyu Lin, Yunkai Deng, Heng Zhang, Jili Wang, Da Liang, Tingzhu Fang, and Robert Wang. Estimating and Removing Ionospheric Effects for L-Band Spaceborne Bistatic SAR. IEEE Transactions on Geoscience and Remote Sensing, 60:1-16, 2022. Keyword(s): Ionosphere, Synthetic aperture radar, Imaging, Spaceborne radar, L-band, Focusing, Dispersion, Bistatic synthetic aperture radar (BiSAR), ionospheric effects correction, L-band, LuTan-1 (LT-1).
    Abstract: One of the challenges of the low-frequency spaceborne synthetic aperture radar (SAR) is that propagation through the ionosphere will introduce nonnegligible errors in the final SAR product. In the low-frequency bistatic SAR (BiSAR) system, the ionosphere will degrade the imaging performance and cause nonnegligible phase errors in the single-pass SAR interferometry application, which results in undesired errors of digital elevation model (DEM). In this article, a method that embedded into the focusing procedure is proposed, which aims to estimate and remove ionospheric effects on L-band spaceborne BiSAR system. First, the impacts of ionospheric effects on the BiSAR system are demonstrated, including the deterioration of imaging performance and the geometric distortion. Then, a method is proposed to correct ionospheric effects. Afterward, the simulations, including point targets and distributed targets, are carried out to verify the effectiveness of the proposed method. The imaging results and the DEM reconstruction results show that the proposed method can effectively estimate and remove ionospheric effects on spaceborne BiSAR systems.
    [bibtex-key = Lin2022] [bibtex-entry]


  76. Haoyu Lin, Yunkai Deng, Heng Zhang, Jili Wang, and Yongwei Zhang. An Extended Model of Ionospheric Dispersion Effects for Nonlinear Frequency Modulation Signal and Correction Method. IEEE Geoscience and Remote Sensing Letters, 19:1-5, 2022. Keyword(s): Dispersion, Synthetic aperture radar, Time-frequency analysis, Focusing, Ionosphere, Frequency modulation, Spaceborne radar, Ionospheric dispersion effects, LuTan-1 (LT-1), nonlinear frequency modulation (NLFM) signal, synthetic aperture radar (SAR).
    Abstract: Nonlinear frequency modulation (NLFM) signal can construct the signal's power spectral density (PSD) to reduce sidelobes without loss of signal-to-noise ratio. LuTan-1 (LT-1) is an L-band spaceborne synthetic aperture radar (SAR) mission which is launched in the beginning of 2022, and a high-precision NLFM signal generator is developed in LT-1. However, the existing model, i.e., the traditional frozen ionosphere model, cannot accurately describe ionospheric dispersion effects faced by the NLFM signal due to the nonlinear characteristic of the instantaneous frequency. Thus, an extended model is established in this letter to describe ionospheric dispersion effects of the NLFM signal. Then, the differences of ionospheric dispersion effects on the NLFM and linear frequency modulation (LFM) signals are compared. Afterward, a method that embedded into the focusing procedure is proposed, which aims to eliminate ionospheric dispersion effects for the NLFM signal. Finally, the hardware-in-the-loop simulations of point targets and distributed targets are performed to verify the proposed method. The method proposed in this letter is used in the ground processing system of LT-1.
    [bibtex-key = Lin2022a] [bibtex-entry]


  77. Guido Luzi, Anna Barra, Qi Gao, Pedro F.Espin-Lopez, Riccardo Palama, Oriol Monserrat, Michele Crosetto, and Xavier Colell. A low-cost active reflector and a passive corner reflector network for assisting landslide monitoring using multi-temporal InSAR. Remote Sensing Letters, 13(11):1080-1089, 2022.
    Abstract: A C-band Low-cost Active Reflector (AR)has been tested in a real experimental campaign aimed at monitoring through multi-temporal InSAR an area threatened by a landslide that occurred in 2019. To monitor and characterize the movement, a network of eight Passive Corner Reflectors and one Active Reflector were installed along a forested slope. A set of 285 interferograms obtained combining 60 Sentinel-1 SAR images were processed to evaluate the stability of the area. The AR, installed in a stable location close to the landslide, was used to provide a reference point in this low coherence area. Despite the high sensitivity of the phase response of such devices to temperature changes, the device operates with a stability of +/-2mm in deformation retrieval, a value acceptable for monitoring purposes, with a moderate range of temperature values.
    [bibtex-key = luziEtAlRSL2022TransponderAndReflectorNetworkForLandslideMonitoring] [bibtex-entry]


  78. Yasser Maghsoudi, Andrew J. Hooper, Tim J. Wright, Milan Lazecky, and Homa Ansari. Characterizing and correcting phase biases in short-term, multilooked interferograms. Remote Sensing of Environment, 275:113022, 2022. Keyword(s): InSAR, Phase bias, Fading signal, Correction, Closure Phase, SAR Interferometry, Radar interferometry, Time Series, Interferometric Stack, ground motion, deformation, subsidence, displacement, monitoring.
    Abstract: Interferometric Synthetic Aperture Radar (InSAR) is widely used to measure deformation of the Earth's surface over large areas and long time periods. A common strategy to overcome coherence loss in long-term interferograms is to use multiple multilooked shorter interferograms, which can cover the same time period but maintain coherence. However, it has recently been shown that using this strategy can introduce a bias (also referred to as a ``fading signal'') in the interferometric phase. We isolate the signature of the phase bias by constructing ``daisy chain'' sums of short-term interferograms of different length covering identical 1-year time intervals. This shows that the shorter interferograms are more affected by this phenomenon and the degree of the effect depends on ground cover types; cropland and forested pixels have significantly larger bias than urban pixels and the bias for cropland mimics subsidence throughout the year, whereas forests mimics subsidence in the spring and heave in the autumn. We, propose a method for correcting the phase bias, based on the assumption, borne out by our observations, that the bias in an interferogram is linearly related to the sum of the bias in shorter interferograms spanning the same time. We tested the algorithm over a study area in western Turkey by comparing average velocities against results from a phase linking approach, which estimates the single primary phases from all the interferometric pairs, and has been shown to be almost insensitive to the phase bias. Our corrected velocities agree well with those from a phase linking approach. Our approach can be applied to global compilations of short-term interferograms and provides accurate long-term velocity estimation without a requirement for coherence in long-term interferograms.
    [bibtex-key = maghsoudiHooperWrightLazeckyAnsariRSE2022CorrectionPhaseBiasesInShortTermMultilookedInterferogramsClosurePhase] [bibtex-entry]


  79. Michele Manunta and Yasir Muhammad. A Novel Algorithm Based on Compressive Sensing to Mitigate Phase Unwrapping Errors in Multitemporal DInSAR Approaches. IEEE Transactions on Geoscience and Remote Sensing, 60:1-20, 2022.
    Abstract: In this work, we present a new method based on the compressive sensing (CS) theory to correct phase unwrapping (PhU) errors in the multitemporal sequence of interferograms exploited by advanced differential interferometric synthetic aperture radar (DInSAR) techniques to generate deformation time series. The developed algorithm estimates the PhU errors by using a modified $L_{1}$ -norm estimator applied to the interferometric network built in the temporal/spatial baseline plane. Indeed, in order to search the minimum $L_{1}$ -norm sparse solution, we apply the iterative reweighted least-squares method with an improved weight function that takes account of the baseline characteristics of the interferometric pairs. Moreover, we also introduce a quality function to identify those solutions that have no physical meaning. Although the proposed approach can be applied to different multitemporal DInSAR approaches, our analysis is tailored to the full-resolution small baseline subset (SBAS) processing chain that we properly modify to implement the proposed CS-based algorithm. To assess the performance of the developed technique, we carry out an extended experimental analysis based on simulated and real SAR data. In particular, we process two wide SAR datasets acquired by Sentinel-1 and COSMO-SkyMed constellations over central Italy between 2011 and 2019. The achieved experimental results clearly demonstrate the effectiveness of the developed approach in retrieving PhU errors and generating displacement time series related to strongly nonlinear deformation phenomena. Indeed, the developed CS-based technique significantly increases the number of detected coherent points and improves the accuracy of the retrieved deformation time series.
    [bibtex-key = manuntaMuhammadTGRS2022CompressiveSensingBasedMitigationOfPhaseUnwrappingErrorsInDInSAR] [bibtex-entry]


  80. R. Naderpour, M. Schwank, D. Houtz, and C. Matzler. L-Band Radiometry of Alpine Seasonal Snow Cover: 4 Years at the Davos-Laret Remote Sensing Field Laboratory. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 15:8199-8220, 2022. [bibtex-key = Naderpour2022] [bibtex-entry]


  81. Reza Naderpour, Mike Schwank, Derek Houtz, Charles Werner, and Christian Mätzler. Wideband Backscattering From Alpine Snow Cover: A Full-Season Study. IEEE Transactions on Geoscience and Remote Sensing, 60(4302215):1-15, 2022. Keyword(s): Radar, remote sensing, snow, WBSCAT, wide-band scatterometer, ESA SnowLab.
    Abstract: This article experimentally investigates relationships between copol backscattering at a wide range of frequencies (L- to Ka-bands) and snow-ground state parameters (SPs) in different evolution phases during the full winter cycle of 2019/2020. Backscattering coefficients from 1 to 40 GHz, in situ snow-ground SPs, and meteorological data are measured at the Davos-Laret Remote Sensing Field Laboratory (Switzerland). Relative strengths of the snow-ground system's three primary scattering elements (air-snow interface, snow volume, and snow-ground interface) on backscattering are assessed. An anticorrelation between reasonably high snow wetness and backscattering coefficient is found, especially at higher microwave frequencies. For small amounts of snow wetness, backscatter coefficients at L- and S-bands are intensified via increasing snow volume and snow surface scattering. Snow-ground SPs influence backscattering according to their characteristic time scales of temporal evolution. Under dry snow conditions and at low and intermediate frequencies, ground permittivity is the major influencer of backscatter at a time scale of roughly two weeks. Snowfall is the major influencer of backscatter at a time scale of a few hours to a few days. The findings of this article are valuable to the development of retrieval algorithms using machine learning while maintaining a grasp on the ongoing physical processes. Another key message is that multifrequency active microwave measurements are critical to maximize the number of retrievable SPs and their estimation accuracy. For example, while Ka-band performs well in the detection of snow cover, L-band measurements are more responsive to changes of snow water equivalent (SWE) under moist or wet snow conditions.
    [bibtex-key = naderpourSchwankWernerMatzlerTGRS2022WBSCATandELBARATimeSeries] [bibtex-entry]


  82. Ahmad Naghavi, Mohammad Sadegh Fazel, Mojtaba Beheshti, and Ehsan Yazdian. A sequential MUSIC algorithm for scatterers detection in SAR tomography enhanced by a robust covariance estimator. Digital Signal Processing, pp 103621, 2022. Keyword(s): SAR Processing, SAR Tomography, TomoSAR, Tomography, MUSIC, DOA, Multiple Signal Classification, Synthetic aperture radar tomography (TomoSAR), Scatterers detection, Sequential MUSIC algorithm, Covariance matrix estimation.
    Abstract: Synthetic aperture radar (SAR) tomography (TomoSAR) is an appealing tool for the extraction of height information of urban infrastructures. Due to the widespread applications of the MUSIC algorithm in source localization, it is a suitable solution in TomoSAR when multiple snapshots (looks) are available. While the classical MUSIC algorithm aims to estimate the whole reflectivity profile of scatterers, sequential MUSIC algorithms are suited for the detection of sparse point-like scatterers. In this class of methods, successive cancellation is performed through orthogonal complement projections on the MUSIC power spectrum. In this work, a new sequential MUSIC algorithm named recursive covariance cancelled MUSIC (RCC-MUSIC), is proposed. This method brings higher accuracy in comparison with the previous sequential methods at the cost of a negligible increase in computational cost. Furthermore, to improve the performance of RCC-MUSIC, it is combined with the recent method of covariance matrix estimation called correlation subspace. Utilizing the correlation subspace method results in a denoised covariance matrix which in turn, increases the accuracy of subspace-based methods. Several numerical examples are presented to compare the performance of the proposed method with the relevant state-of-the-art methods. As a subspace method, simulation results demonstrate the efficiency of the proposed method in terms of estimation accuracy and computational load.
    [bibtex-key = naghaviFazelBeheshtiYazdianDSP2022SequentialMUSICforSARTomographyEnhancedByRobustCovarianceEstimator] [bibtex-entry]


  83. Maria I. Navarro-Hernandez, Javier Valdes-Abellan, Roberto Tomas, Juan M. Lopez-Sanchez, Pablo Ezquerro, Guadalupe Bru, Roberta Boni, Claudia Meisina, and Gerardo Herrera. ValInSAR: A Systematic Approach for the Validation of Differential SAR Interferometry in Land Subsidence Areas. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 15:3650-3671, 2022. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Validation, reflector, deformation, displacement, ground motion.
    Abstract: Land subsidence is a natural or anthropogenic process triggering the settlement of the Earth's surface. When this phenomenon is induced by groundwater withdrawal, compaction of unconsolidated sediments causes land displacement. Differential interferometric synthetic aperture radar (DInSAR) is widely used nowadays to monitor subsidence over extensive areas. However, validation of DInSAR measurements with in-situ techniques is lacking in many case studies, reducing the reliability of further analyses. The aim of this article is to propose a systematic methodology to validate DInSAR measurements with in-situ techniques to obtain reliable subsidence measurements. The article provides a literature review of the most common approaches to validate DInSAR measurements and a description of the proposed systematic methodology, which is supported by a MATLAB open-source code. The methodology allows the analysis of both DInSAR-based velocity and displacement time series. We propose a set of statistics to assess the accuracy of the DInSAR estimates. For this purpose, RMSE parameters have been normalized with the range and the average of the in-situ deformation values. Moreover, combining these normalized parameters with the Pearson correlation coefficient (R2), a classification scheme is recommended for accepting/rejecting the DInSAR data for further analyses. This methodology has been applied in three study areas characterized by very well-documented subsidence processes: The Alto Guadalent�n Valley and Murcia City in Spain, and San Luis Potosi in Mexico.
    [bibtex-key = navarroHernandezEtAlJSTARS2022ValidationOfDInSARInSubsidenceAreas] [bibtex-entry]


  84. F. Paul, L. Piermattei, D. Treichler, L. Gilbert, L. Girod, A. Kääb, L. Libert, T. Nagler, T. Strozzi, and J. Wuite. Three different glacier surges at a spot: What satellites observe and what not. Cryosphere, 16(6):2505-2526, 2022. [bibtex-key = Paul2022] [bibtex-entry]


  85. G. Picard, H. Läwe, and C. Mätzler. Brief communication: A continuous formulation of microwave scattering from fresh snow to bubbly ice from first principles. Cryosphere, 16(9):3861-3866, 2022. [bibtex-key = Picard2022] [bibtex-entry]


  86. Melody Sandells, Henning Loewe, Ghislain Picard, Marie Dumont, Richard Essery, Nicolas Floury, Anna Kontu, Juha Lemmetyinen, William Maslanka, Samuel Morin, Andreas Wiesmann, and Christian Matzler. X-Ray Tomography-Based Microstructure Representation in the Snow Microwave Radiative Transfer Model. IEEE Transactions on Geoscience and Remote Sensing, 60(4301115):1-15, 2022. Keyword(s): Snow, Microstructure, Snow microstructure, X-ray, tomography, X-ray tomography, microwave, microwave scattering, SMRT, SMRTmodel, snow, snow microwave radiative transfer (SMRT), microwave remote sensing, radar, radar remote sensing, Nordic Snow Radar Experiment, NoSREx.
    Abstract: The modular Snow Microwave Radiative Transfer (SMRT) model simulates microwave scattering behavior in snow via different selectable theories and snow microstructure representations, which is well suited to intercomparisons analyses. Here, five microstructure models were parameterized from X-ray tomography and thin-section images of snow samples and evaluated with SMRT. Three field experiments provided observations of scattering and absorption coefficients, brightness temperature, and/or backscatter with the increasing complexity of snowpack. These took place in Sodankyla, Finland, and Weissfluhjoch, Switzerland. Simulations of scattering and absorption coefficients agreed well with observations, with higher errors for snow with predominantly vertical structures. For simulation of brightness temperature, difficulty in retrieving stickiness with the Sticky Hard Sphere microstructure model resulted in relatively poor performance for two experiments, but good agreement for the third. Exponential microstructure gave generally good results, near to the best performing models for two field experiments. The Independent Sphere model gave intermediate results. New Teubner-Strey and Gaussian Random Field models demonstrated the advantages of SMRT over microwave models with restricted microstructural geometry. Relative model performance is assessed by the quality of the microstructure model fit to micro-computed tomography (CT) data and further improvements may be possible with different fitting techniques. Careful consideration of simulation stratigraphy is required in this new era of high-resolution microstructure measurement as layers thinner than the wavelength introduce artificial scattering boundaries not seen by the instrument.
    [bibtex-key = sandellsEtAlTGRS2022XRayTomographyBasedMicrosctructureInTheSnowMicrowaveRadiativeTransferModel] [bibtex-entry]


  87. M. Santoro, O. Cartus, and J.E.S. Fransson. Dynamics of the Swedish forest carbon pool between 2010 and 2015 estimated from satellite L-band SAR observations. Remote Sensing of Environment, 270(112846), 2022. [bibtex-key = Santoro2022] [bibtex-entry]


  88. M. Santoro, O. Cartus, U. Wegmüller, S. Besnard, N. Carvalhais, A. Araza, M. Herold, J. Liang, J. Cavlovic, and M.E. Engdahl. Global estimation of above-ground biomass from spaceborne C-band scatterometer observations aided by LiDAR metrics of vegetation structure. Remote Sensing of Environment, 279(113114), 2022. [bibtex-key = Santoro2022] [bibtex-entry]


  89. Gustavo H. X. Shiroma, Marco Lavalle, and Sean M. Buckley. An Area-Based Projection Algorithm for SAR Radiometric Terrain Correction and Geocoding. IEEE Transactions on Geoscience and Remote Sensing, 60:1-23, 2022.
    Abstract: This article describes a projection algorithm between radar and map coordinates based on the representation of radar samples as area elements (AEs) rather than point elements. Each AE on the map grid (geographic grid) is associated with a number of radar grid samples that intersect completely or partially the AE. The association enables the geocoding (i.e., the map projection of radar imagery) with adaptive multilooking, accurately accounting for all radar samples contributing to the geocoded elements according to topography and radar geometry. By using averaging rather than interpolation, the proposed projection does not suffer from interpolation overfitting. The area-based geocoding also enables the generation of the geocoded polarimetric covariance matrix (GCOV) and geocoded synthetic aperture radar (SAR) interferograms with adaptive multilooking. Analogously, the slant-range projection of geocoded data is improved by projecting geographic grid pixels onto the radar grid according to their corresponding location based on the radar geometry without leaving gaps. This approach is used to reduce the computation time of previously published radiometric terrain correction (RTC) algorithms, performing 3.6-6.5 times faster over multilooked data and up to 26.3 times faster over single-look data. We demonstrate the strengths of the proposed area projection (AP) algorithm for RTC and geocoding using Uninhabited Aerial Vehicle SAR (UAVSAR), Sentinel-1B, and ALOS-2/PALSAR-2 data, and evaluate the results in the context of the upcoming NASA-ISRO SAR (NISAR) mission.
    [bibtex-key = shiromaLavalleBuckleyTGRS2022AreaBasedProjectionAlgorithmRTCandGeocoding] [bibtex-entry]


  90. Marcel Stefko, Othmar Frey, Charles Werner, and Irena Hajnsek. Calibration and Operation of a Bistatic Real-Aperture Polarimetric-Interferometric Ku-Band Radar. IEEE Transactions on Geoscience and Remote Sensing, 60(5106719):1-19, 2022.
    Abstract: This article presents the bistatic operation mode and the performance analysis of KAPRI, a terrestrial frequency-modulated continuous wave (FMCW) Ku-band polarimetric radar interferometer capable of acquiring bistatic full-polarimetric datasets with high spatial and temporal resolution. In the bistatic configuration, the system is composed of two independently-operating KAPRI devices, one serving as a primary transmitter-receiver, and the other as a secondary receiver. The secondary bistatic dataset is affected by possible offsets between the two devices' reference clocks, as well as distortions arising from geometry. To correct for this, we present a two-chirp bistatic FMCW signal model which accounts for the distortions, and a reference chirp transmission procedure which allows correcting the clock offsets in the deramped signal time domain. The second challenge of operation of a bistatic polarimetric system is polarimetric calibration, since it is not possible to employ purely monostatic targets such as corner reflectors. For this purpose we developed a novel active calibration device VSPARC (Variable-Signature Polarimetric Active Radar Calibrator), designed for monostatic and bistatic calibration of all polarimetric channels. VSPARC and its associated novel polarimetric calibration method were then used to achieve full calibration of both KAPRI devices with polarimetric phase calibration accuracy of 20 deg and 30 dB polarization purity in field conditions. This article thus presents a complete measurement configuration and data processing pipeline necessary for synchronization, coregistration, and polarimetric calibration of bistatic and monostatic datasets acquired by a real-aperture FMCW radar.
    [bibtex-key = stefkoFreyWernerHajnsekTGRS2022CalibrationAndOperationOfBistaticPolGPRI] [bibtex-entry]


  91. Marcel Stefko, Silvan Leinss, Othmar Frey, and Irena Hajnsek. Coherent backscatter enhancement in bistatic Ku-/X-band radar observations of dry snow. The Cryosphere, 16(7):2859-2879, 2022. Keyword(s): SAR Processing, Snow, Coherent backscatter enhancement, bistatic, bistatic SAR, Ku-band, X-band, radar, dry snow.
    Abstract: The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of two in a very narrow cone around the direct return direction when multiple scattering occurs in a weakly absorbing, disordered medium. So far, this effect has not been investigated in terrestrial snow in the microwave spectrum. It has also received little attention in scattering models. We present the first characterization of the CBOE in dry snow using ground-based and space-borne bistatic radar systems. For a seasonal snow pack in Ku-band (17.2 GHz),we found backscatter enhancement of 50-60% (+1.8-2.0 dB) at zero bistatic angle and a peak half-width-at-half-maximum (HWHM) of 0.25 deg. In X-band (9.65 GHz), we found backscatter enhancement of at least 35% (+1.3 dB) and an estimated HWHM of 0.12 deg in the accumulation areas of glaciers in the Jungfrau-Aletsch region, Switzerland. Sampling of the peak shape at different bistatic angles allows estimating the scattering and absorption mean free paths, Lambda_T and Lambda_A. In the VV polarization, we obtained Lambda_T = 0.4 +/- 0.1 m and Lambda_A = 19 +/- 12 m at Ku-band, and Lambda_T = 2.1 +/- 0.4 m, Lambda_A = 21.8 +/- 2.7 m at X-band. The HH polarization yielded similar results. The observed backscatter enhancement is thus significant enough to require consideration in backscatter models describing monostatic and bistatic radar experiments. Enhanced backscattering beyond the Earth, on the surface of solar system bodies, has been interpreted as being caused by the presence of water ice. In agreement with this interpretation, our results confirm the presence of the CBOE at X- and Ku-band frequencies in terrestrial snow.
    [bibtex-key = stefkoLeinssFreyHanjsek2022CoherentBackscatterEnhancementInBistaticRadarObservationsOfSnow] [bibtex-entry]


  92. T. Strozzi, A. Wiesmann, A. Kääb, T. Schellenberger, and F. Paul. Ice Surface Velocity in the Eastern Arctic from Historical Satellite SAR Data. Earth System Science Data Discussions, 2022:1-42, 2022. [bibtex-key = Strozzi2022] [bibtex-entry]


  93. S. Tao, J. Chave, P.-L. Frison, T. Le Toan, P. Ciais, J. Fang, J.-P. Wigneron, M. Santoro, H. Yang, X. Li, N. Labriere, and S. Saatchi. Increasing and widespread vulnerability of intact tropical rainforests to repeated droughts. Proceedings of the National Academy of Sciences of the United States of America, 119(37)(e2116626119), 2022. [bibtex-key = Tao2022] [bibtex-entry]


  94. Leung Tsang, Michael Durand, Chris Derksen, Ana P. Barros, Do-Hyuk Kang, Hans Lievens, Hans-Peter Marshall, Jiyue Zhu, Joel Johnson, Joshua King, Juha Lemmetyinen, Melody Sandells, Nick Rutter, Paul Siqueira, Anne Nolin, Batu Osmanoglu, Carrie Vuyovich, Edward J. Kim, Drew Taylor, Ioanna Merkouriadi, Ludovic Brucker, Mahdi Navari, Marie Dumont, Richard Kelly, Rhae Sung Kim, Tien-Hao Liao, and Xiaolan Xu. Review Article: Global Monitoring of Snow Water Equivalent using High Frequency Radar Remote Sensing. The Cryosphere, 16(9):3531-3573, September 2022. Keyword(s): Snow Water Equivalent, SWE, Synthetic Aperture Radar, SAR, Review Paper. [bibtex-key = tsangEtalCryosphere2022ReviewArticleGlobalMonitoringOfSWEUsingHighFrequencyRadar] [bibtex-entry]


  95. Jianbing Xiang, Xiaolei Lv, Xikai Fu, and Ye Yun. Detection and Estimation Algorithm for Marine Target With Micromotion Based on Adaptive Sparse Modified-LV's Transform. IEEE Transactions on Geoscience and Remote Sensing, 60:1-17, 2022. Keyword(s): Clutter, Radar, Radar detection, Radar clutter, Frequency modulation, Fractals, Estimation, Adaptive sparse Fourier transform (ASFT), adaptive sparse-modified Lv's transform (ASMLVT), Mairne targets, micro-Doppler (m-D), modified-Lv's transform (MLVT), radar target detection and estimation.
    Abstract: Due to the complex marine environment and high-order frequency modulation (FM) on radar echo from the micromotion of the target, the effective and robust detection of a marine target with micromotion under heavy sea clutters' background is a challenging task. In this article, we propose a novel detection and estimation algorithm based on adaptive sparse modified-Lv's transform (ASMLVT). First, the micro-Doppler (m-D) characteristics of marine targets are employed and modeled as quadratic frequency-modulated (QFM) signals. Second, we modify the 2-D robust sparse Fourier transform (2-D-RSFT) and make it adaptive to the sea clutters' background, namely, 2-D adaptive sparse Fourier transform (2-D-ASFT). Then, we substitute the 2-D Fast Fourier transform (2-D-FFT) operation with 2-D-ASFT in the modified-Lv's transform (MLVT). The proposed algorithm can not only achieve good energy accumulation and accurate parametric estimation for marine targets with micromotion but is also robust to the heavy sea clutters and can greatly reduce false alarms. Besides, it has a good cross-term suppression ability to detect multitargets. Experiments with simulated and real radar datasets show that the proposed algorithm can effectively detect and estimate multitargets with micromotion under heavy sea clutter and low signal-to-clutter ratio (SCR) background.
    [bibtex-key = Xiang2022] [bibtex-entry]


  96. Feng Xiao, Andrea Monti Guarnieri, Zegang Ding, and Marco Manzoni. Improving the Split-Spectrum Method for Sentinel-1 Differential TOPSAR Interferometry. IEEE Geoscience and Remote Sensing Letters, 19:1-5, 2022. Keyword(s): Interferometry, Ionosphere, Synthetic aperture radar, Dispersion, Fitting, Estimation, Filtering, Differential SAR Interferometry (DInSAR), ionosphere estimation, split-spectrum method.
    Abstract: Differential SAR interferometry (DInSAR) is a useful technique used to measure small movements and surface deformation. However, ionospheric phase screens are a major error source in multipass terrain observation by progressive scans sar (TOPSAR) interferograms. In this letter, an improved split-spectrum method is proposed. First, the burst used for ionospheric phase estimation is selected through coherence, and then, the ionospheric phase of the burst is estimated based on the split-spectrum method. Finally, the TOPSAR ionospheric space-variable phase in a large scene is obtained through 2-D space-variable fitting, which avoids the complicated processing of splicing between bursts of different periods and reduces the number of unwrapping calculations for large scenes after splicing. The method can ensure that the number of calculations is reduced without loss of accuracy. Sentinel-1 TOPSAR real data processing verifies the correctness of the proposed method.
    [bibtex-key = xiaoMontiGuarnieriDingManzoniGRSL2022ImprovedSplitSpectrumMethodForSentinel1DINSARwithTOPSAR] [bibtex-entry]


  97. Chuan Xiong, Jiancheng Shi, Jinmei Pan, Haokui Xu, Tao Che, Tianjie Zhao, Yan Ren, Deyuan Geng, Tao Chen, Kaiwen Jiang, and Peng Feng. Time Series X- and Ku-Band Ground-Based Synthetic Aperture Radar Observation of Snow-Covered Soil and Its Electromagnetic Modeling. IEEE Transactions on Geoscience and Remote Sensing, 60:1-13, 2022. Keyword(s): Snow, X-band, Ku-band, times series.
    Abstract: The snow water equivalent (SWE, a measurement of the amount of water contained in snow packs) is an important variable in earth systems. Microwave remote sensing provides a possible solution for estimating the SWE globally. To support radar SWE retrieval, the snow backscattering theory needs to be studied; the forward simulation model needs to be validated against natural snow observations. In this study, a one-winter experiment to observe the time series backscattering coefficient of snow-covered bare soil is reported. This is the first long time series snow-covered soil backscattering experiment that was measured by an imaging radar. The backscattering coefficient was observed at three frequencies covering the X-band and dual-Ku bands, which are of great interest to the snow remote sensing community and are used for SWE estimation in mountains. The calibration of the synthetic aperture radar (SAR) system was conducted manually and carefully to ensure high-quality radar observation data. The observations from our experiment show that in general, the time series backscattering signature of snow-covered terrain is mainly driven by soil freezing, snow grain size growth, and snow accumulation processes. The time series observations for dry snow are modeled by backscattering models with model inputs directly calculated from field measurements. Our simulation results indicate that the time series radar backscattering at three frequencies and four polarizations can be simulated with high accuracy, including the cross-polarization channels. This study provides some key understanding of the time series signature of radar backscattering from snow and provides some key implications for SWE retrieval from radar observations.
    [bibtex-key = xiongEtAlTGRS2022TimeSeriesOfXandKuBandGBSARObservations] [bibtex-entry]


  98. Y. Xu, L. Yu, P. Ciais, W. Li, M. Santoro, H. Yang, and P. Gong. Recent expansion of oil palm plantations into carbon-rich forests. Nature Sustainability, 5(7):574-577, 2022. [bibtex-key = Xu2022] [bibtex-entry]


  99. H. Yang, P. Ciais, J.-P. Wigneron, J. Chave, O. Cartus, X. Chen, L. Fan, J.K. Green, Y. Huang, E. Joetzjer, H. Kay, D. Makowski, F. Maignan, M. Santoro, S. Tao, L. Liu, and Y. Yao. Climatic and biotic factors influencing regional declines and recovery of tropical forest biomass from the 2015/16 El Niño. Proceedings of the National Academy of Sciences of the United States of America, 119(26), 2022. [bibtex-key = Yang2022] [bibtex-entry]


  100. Ce Yang, Naiming Ou, Yunkai Deng, Dacheng Liu, Yanyan Zhang, Nan Wang, and Robert Wang. Pattern Synthesis Algorithm for Range Ambiguity Suppression in the LT-1 Mission via Sequential Convex Optimizations. IEEE Transactions on Geoscience and Remote Sensing, 60:1-13, 2022. Keyword(s): Synthetic aperture radar, Optimization, Antenna measurements, Phased arrays, Microwave antennas, Convex functions, Backscatter, Antenna pattern synthesis, array antenna, convex optimization, quadrature polarimetric (quad-pol), range ambiguity, synthetic aperture radar (SAR).
    Abstract: The innovative spaceborne Earth observation mission LuTan-1 (LT-1) deploys advanced full polarimetric L-band synthetic aperture radar (SAR) to obtain high-precision, multidimensional ground feature information. As the quad-pol SAR system, LT-1 suffers from strong ambiguities that degrade the quality of the observation products. Antenna pattern synthesis algorithm can suppress the range ambiguities without the increase of the azimuth ambiguities and the system complexity and therefore has great potential to improve the overall ambiguity performance of the quad-pol SAR system. However, the previous research on this kind of method is generally limited to specific situations and lacks of the analysis of actual measurement results. Based on the application requirements of LT-1, this article proposes a novel pattern synthesis algorithm that suppresses the range ambiguities via sequential convex optimizations. In simulation and comparison, the proposed algorithm effectively suppresses the strong co-polarized range ambiguities and shows the flexibility, efficiency, and stability that are significantly better than the previous algorithms. What is more, the analysis of practical performance loss is performed based on the measurement result of the LT-1 antenna, and the practicality and validity of the algorithm are strongly verified.
    [bibtex-key = Yang2022] [bibtex-entry]


  101. Kaiyu Zhang, Xiaolei Lv, Huiming Chai, and Jingchuan Yao. Unsupervised SAR Image Change Detection for Few Changed Area Based on Histogram Fitting Error Minimization. IEEE Transactions on Geoscience and Remote Sensing, 60:1-19, 2022. Keyword(s): Synthetic aperture radar, Remote sensing, Radar polarimetry, Change detection algorithms, Histograms, Rail transportation, Radiometry, Change detection, conditional random fields (CRFs), half-normal distribution, image thresholding, synthetic aperture radar (SAR) images, unsupervised change detection.
    Abstract: Change detection in synthetic aperture radar (SAR) images is an essential task of remote sensing image analysis. However, the thresholding procedure is the main difficulty in change detection for a few changed areas for traditional change detection methods. In this article, we propose a novel change detection method for very few changed or even none changed areas. The proposed method contains three procedures: difference image (DI) generation, thresholding, and spatial analysis. In the second procedure, a new thresholding method called histogram fitting error minimization (HFEM) is proposed for a few changed areas. HFEM is derived under the assumption that the unchanged class in the absolute-valued DI follows the half-normal distribution, and the changed class follows the Gaussian distribution. In the spatial analysis procedure, a new conditional random fields (CRF) method based on half-normal distribution is proposed to model the mutual influences among image pixels. The proposed CRF method is called half-normal CRF (HNCRF). Experiments carried out on both synthetic datasets and four real SAR datasets demonstrate the superiority of our method. Not only a few changed datasets but datasets with lots of changes are used in the experiments. The kappa coefficients of the proposed method can reach up to ten times that of the traditional method under extreme conditions. The results prove that the proposed method outperforms the traditional methods in the case of a few changed areas. Meanwhile, the proposed method can get similar results compared with traditional methods under normal conditions.
    [bibtex-key = Zhang2022] [bibtex-entry]


  102. Yujie Zheng, Heresh Fattahi, Piyush Agram, Mark Simons, and Paul Rosen. On Closure Phase and Systematic Bias in Multilooked SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 60:1-11, 2022. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, time-series analysis, closure phase, non-zero closure phase, phase consistency, systematic bias, deformation, displacement, multilooking, SBAS, PSI, persistent scatterer intereferometry, SqueeSAR.
    Abstract: In this article, we investigate the link between the closure phase and the observed systematic bias in deformation modeling with multilooked SAR interferometry. Multilooking or spatial averaging is commonly used to reduce stochastic noise over a neighborhood of distributed scatterers in interferometric synthetic aperture radar (InSAR) measurements. However, multilooking may break consistency among a triplet of interferometric phases formed from three acquisitions leading to a residual phase error called closure phase. Understanding the cause of closure phase in multilooked InSAR measurements and the impact of closure phase errors on the performance of InSAR time-series algorithms is crucial for quantifying the uncertainty of ground displacement time series derived from InSAR measurements. We develop a model that consistently explains both closure phase and systematic bias in multilooked interferometric measurements. We show that nonzero closure phase can be an indicator of temporally inconsistent physical processes that alter both phase and amplitude of interferometric measurements. We propose a method to estimate the systematic bias in the InSAR time series with generalized closure phase measurements. We validate our model with a case study in Barstow-Bristol Trough, CA, USA. We find systematic differences on the order of cm/year between InSAR time-series results using subsets of varying maximum temporal baselines. We show that these biases can be identified and accounted for.
    [bibtex-key = zhengFattahiAgramSimonsRosenTGRS2022OnClosurePhaseAndSystematicBiasInMultilookedSARInterferometry] [bibtex-entry]


  103. Homa Ansari, Francesco De Zan, and Alessandro Parizzi. Study of Systematic Bias in Measuring Surface Deformation With SAR Interferometry. IEEE Trans. Geosci. Remote Sens., 59(2):1285-1301, February 2021. Keyword(s): SAR Processing, SAR Interferometry, Time Series, Surface Displacement, Deformation, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, short temporal baseline interferograms, Earth surface deformation, SAR time series, mentioned phase component, biasing impact, quality measure, fading signal, physical signal, modern SAR missions, deformation bias, efficient deformation-signal retrieval, accurate deformation-signal retrieval, multilooked interferograms, systematic bias, measuring surface deformation, SAR interferometry, interferometric signal, multilooked synthetic aperture radar interferograms, atmospheric Earth-surface topography changes, stochastic noise, temporal decorrelation, systematic phase component, Big Data, deformation estimation, differential interferometric synthetic aperture radar, SAR, DInSAR, distributed scatterers, DS, error analysis, near real-time processing, NRT, phase inconsistencies, signal decorrelation, time-series analysis.
    Abstract: This article investigates the presence of a new interferometric signal in multilooked synthetic aperture radar (SAR) interferograms that cannot be attributed to the atmospheric or Earth-surface topography changes. The observed signal is short-lived and decays with the temporal baseline; however, it is distinct from the stochastic noise attributed to temporal decorrelation. The presence of such a fading signal introduces a systematic phase component, particularly in short temporal baseline interferograms. If unattended, it biases the esti- mation of Earth surface deformation from SAR time series. Here, the contribution of the mentioned phase component is quantita- tively assessed. The biasing impact on the deformation-signal retrieval is further evaluated. A quality measure is introduced to allow the prediction of the associated error with the fading signals. Moreover, a practical solution for the mitigation of this physical signal is discussed; special attention is paid to the efficient processing of Big Data from modern SAR missions such as Sentinel-1 and NISAR. Adopting the proposed solution, the deformation bias is shown to decrease significantly. Based on these analyses, we put forward our recommendations for efficient and accurate deformation-signal retrieval from large stacks of multilooked interferograms.
    [bibtex-key = ansariDeZanParizziTGRS2021SystematicBiasInSARInterferometry] [bibtex-entry]


  104. Andreas Baumann-Ouyang, Jemil Avers Butt, David Salido-Monzu, and Andreas Wieser. MIMO-SAR Interferometric Measurements for Structural Monitoring: Accuracy and Limitations. Remote Sensing, 13(21), 2021.
    Abstract: Terrestrial Radar Interferometry (TRI) is a measurement technique capable of measuring displacements with high temporal resolution at high accuracy. Current implementations of TRI use large and/or movable antennas for generating two-dimensional displacement maps. Multiple Input Multiple Output Synthetic Aperture Radar (MIMO-SAR) systems are an emerging alternative. As they have no moving parts, they are more easily deployable and cost-effective. These features suggest the potential usage of MIMO-SAR interferometry for structural health monitoring (SHM) supplementing classical geodetic and mechanical measurement systems. The effects impacting the performance of MIMO-SAR systems are, however, not yet sufficiently well understood for practical applications. In this paper, we present an experimental investigation of a MIMO-SAR system originally devised for automotive sensing, and assess its capabilities for deformation monitoring. The acquisitions generated for these investigations feature a 180 deg Field-of-View (FOV), distances of up to 60 m and a temporal sampling rate of up to 400 Hz. Experiments include static and dynamic setups carried out in a lab-environment and under more challenging meteorological conditions featuring sunshine, fog, and cloud-cover. The experiments highlight the capabilities and limitations of the radar, while allowing quantification of the measurement uncertainties, whose sources and impacts we discuss. We demonstrate that, under sufficiently stable meteorological conditions with humidity variations smaller than 1%, displacements as low as 25 um can be detected reliably. Detecting displacements occurring over longer time frames is limited by the uncertainty induced by changes in the refractive index.
    [bibtex-key = baumannButtSalidoMonzuWieserRemoteSensing2021MIMOSARInterferometryAccuracyAndLimitations] [bibtex-entry]


  105. E. Casalini, J. Fagir, and D. Henke. Moving Target Refocusing With the FMCW SAR System MIRANDA-35. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 14:1283-1291, 2021. Keyword(s): SAR Processing, Moving Target, Airborne SAR, Synthetic aperture radar, Radar imaging, Earth, Signal processing algorithms, Remote sensing, Radar polarimetry, Motion compensation, Frequency-modulated continuous-wave (FMCW), inverse synthetic aperture radar (ISAR), MIRANDA-35, motion compensation (MoComp), radar imaging, synthetic aperture radar (SAR).
    Abstract: Inverse synthetic aperture radar is a commonly adopted technique for producing high-resolution images of moving targets. This article investigates the imaging capabilities of high-frequency and high-bandwidth systems by means of two distinct experiments. The deployed sensor is the Fraunhofer FHR MIRANDA-35, a millimeter-wave synthetic aperture radar airborne system, which transmits frequency-modulated continuous-wave signals at the Ka-band and is capable of achieving centimeter resolution. The performances are assessed by comparing the derived estimates (e.g., radial velocity and acceleration, and dimensions) with independent ground measurements. The resulting accuracy can be summarized as follows: the mean value of the percent error is 2.05% and 2.11% for radial velocity and acceleration, respectively, and 4.27% for the target dimensions.
    [bibtex-key = casaliniFagirHenke2021MovingTargetRefocusingFMCWSARSystemMIRANDA35] [bibtex-entry]


  106. Richard Czikhardt, Hans van der Marel, Freek J. van Leijen, and Ramon F. Hanssen. Estimating Signal-to-Clutter Ratio of InSAR Corner Reflectors From SAR Time Series. IEEE Geoscience and Remote Sensing Letters, pp 1-5, 2021. Keyword(s): SAR Processing, Corner Reflectors, InSAR, SAR Interferometry.
    Abstract: The estimation of Signal-to-Clutter Ratio (SCR) of a radar point target, such as a corner reflector, is an essential step for synthetic aperture radar (SAR) interferometry and positioning, as it influences the phase measurement variance as well as the absolute positioning precision. The standard method to estimate the SCR of a point target relies on the debatable assumption of spatial ergodicity, using the clutter of the surrounding as representative of the clutter at the point target. Here, we estimate the SCR of a corner reflector using a time series of SAR measurements, i.e.,\ assuming temporal ergodicity. This assumption is often more realistic, particularly in a complex environment, in the presence of other point scatterers, and for small-sized reflectors. Empirical results on a corner reflector network, using Sentinel-1 SAR measurements, show that the temporal method yields a less biased and more precise estimate of the average SCR. A second experiment shows that the InSAR phase variance as well as positioning precision, predicted using SCR estimated by the temporal estimation method, is closer to the truth.
    [bibtex-key = czikhardtVanDerMarelVanDerLeijenHanssenGRSL2021EstimateSCRofInSARReflectorsFromSARTimeSeries] [bibtex-entry]


  107. Dyre Oliver Dammann, Mark A. Johnson, Emily R. Fedders, Andrew R. Mahoney, Charles L. Werner, Christopher M. Polashenski, Franz J. Meyer, and Jennifer K. Hutchings. Ground-Based Radar Interferometry of Sea Ice. Remote Sensing, 13(1), 2021.
    Abstract: In light of recent Arctic change, there is a need to better understand sea ice dynamic processes at the floe scale to evaluate sea ice stability, deformation, and fracturing. This work investigates the use of the Gamma portable radar interferometer (GPRI) to characterize sea ice displacement and surface topography. We find that the GPRI is best suited to derive lateral surface deformation due to mm-scale horizontal accuracy. We model interferometric phase signatures from sea ice displacement and evaluate possible errors related to noise and antenna motion. We compare the analysis with observations acquired during a drifting ice camp in the Beaufort Sea. We used repeat-scan and stare-mode interferometry to identify two-dimensional shear and to track continuous uni-directional convergence. This paper demonstrates the capacity of the GPRI to derive surface strain on the order of 10−7 and identify different dynamic regions based on sub-mm changes in displacement. The GPRI is thus a promising tool for sea ice applications due to its high accuracy that can potentially resolve pre- and post-fracture deformation relevant to sea ice stability and modeling.
    [bibtex-key = dammannJohnsonFeddersMahoneyWernerPolashenskiMeyerHutchingsREMOTESENSING2021GroundBasedRadarInterferometryOfSeaIce] [bibtex-entry]


  108. D. Feng, D. An, L. Chen, and X. Huang. Holographic SAR Tomography 3-D Reconstruction Based on Iterative Adaptive Approach and Generalized Likelihood Ratio Test. IEEE Transactions on Geoscience and Remote Sensing, 59(1):305-315, Jan. 2021. Keyword(s): Image resolution, Apertures, Image reconstruction, Signal resolution, Synthetic aperture radar, Tomography, Three-dimensional displays, 3-D imaging, generalized likelihood ratio test (GLRT), holographic synthetic aperture radar (HoloSAR) tomography, iterative adaptive approach (IAA).
    Abstract: Holographic synthetic aperture radar (HoloSAR) tomography is an attractive imaging mode that can retrieve the 3-D scattering information of the observed scene over 360 deg azimuth angle variation. To improve the resolution and reduce the sidelobes in elevation, the HoloSAR imaging mode requires many passes in elevation, thus decreasing its feasibility. In this article, an imaging method based on iterative adaptive approach (IAA) and generalized likelihood ratio test (GLRT) is proposed for the HoloSAR with limited elevation passes to achieve super-resolution reconstruction in elevation. For the elevation reconstruction in each range-azimuth cell, the proposed method first adopts the nonparametric IAA to retrieve the elevation profile with improved resolution and suppressed sidelobes. Then, to obtain sparse elevation estimates, the GLRT is used as a model order selection tool to automatically recognize the most likely number of scatterers and obtain the reflectivities of the detected scatterers inside one range-azimuth cell. The proposed method is a super-resolving method. It does not require averaging in range and azimuth, thus it can maintain the range-azimuth resolution. In addition, the proposed method is a user parameter-free method, so it does not need the fine-tuning of any hyperparameters. The super-resolution power and the estimation accuracy of the proposed method are evaluated using the simulated data, and the validity and feasibility of the proposed method are verified by the HoloSAR real data processing results.
    [bibtex-key = fengAnChenHuangTGRS2021HolographicSARTomography] [bibtex-entry]


  109. Oriane Gassot, Alain Herique, Wenzhe Fa, Jun Du, and Wlodek Kofman. Ultra-Wideband SAR Tomography on Asteroids. Radio Science, 56(8):e2020RS007186, 2021. Note: E2020RS007186 2020RS007186. Keyword(s): asteroids, Synthetic Aperture Radar, SAR tomography, simulation.
    Abstract: Abstract Our knowledge of the internal structure of asteroids is currently indirect and relies on inferences from remote sensing observations of surfaces. However, it is fundamental for understanding small bodies' history and for planetary defense missions. Radar observation of asteroids is the most mature technique available to characterize their inner structure, and Synthetic Aperture Radar Tomography (TomoSAR) allows 3D imaging of their interior. However, as the geometry of observation of small asteroids is complex, and TomoSAR studies have always been performed in the Earth observation geometry, its results in a small body geometry must be simulated to assess the methods' performances. We adopt here two different tomography algorithms and evaluate their performances in our geometry by assessing the resolution and the difference between the scatterer's position and its retrieved position. The first method, the Frequency Domain Back Projection (FDBP) is based on correcting the Fourier transform of the received signal by a phase function built from the geometry. While it can provide a good resolution, a bias remains in the imaged scatterer's position. Meanwhile, Compressive Sensing (CS) relies on the hypothesis that few scatterers lie in the same direction from the subsurface. Its application in the small body geometry is studied, which results in a slightly impoverished resolution but an improved localization of the scatterer.
    [bibtex-key = gassotEtAlRadioScience2021UltraWidebandSARTomographyOnAsteroids] [bibtex-entry]


  110. Yuta Izumi, Othmar Frey, Simone Baffelli, Irena Hajnsek, and Motoyuki Sato. Efficient Approach for Atmospheric Phase Screen Mitigation in Time Series of Terrestrial Radar Interferometry Data Applied to Measure Glacier Velocity. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 14:7734-7750, 2021. Note: Early Access.
    Abstract: The accuracy of surface displacements measured by differential radar interferometry is significantly degraded by the atmospheric phase screen (APS). This paper presents a practical and efficient approach for APS mitigation based on the coherent pixels technique (CPT) displacement velocity estimation algorithm. In the proposed approach, all motionless coherent pixels closest to the moving area are defined as seeds surrounding the moving area at the integration step of the CPT. This arrangement consequently minimizes the integration path and the APS effect in the final velocity result. It is designed for terrestrial radar interferometry (TRI) applications. As a continuous operational mode processing framework, a piecewise processing chain is further introduced to derive arbitrary temporal displacement patterns in this work. Three-day datasets measured by Ku-band TRI over a mountainous region in the canton of Valais, Switzerland, were used for validation. Through this validation, a comparative study of five algorithms was carried out. This evaluation showed the efficiency of the proposed approach. The proposed approach does not require phase unwrapping, kriging interpolation, and spatio-temporal covariance inference for APS mitigation, which is appropriate for continuous TRI operation.
    [bibtex-key = izumiFreyBaffelliHajnsekSatoJSTARS2021APSMitigationInTimeSeriesOfTRIDataGlacierVel] [bibtex-entry]


  111. D. -H. Jung, D. -H. Kim, M. T. Azim, J. Park, and S. -O. Park. A Novel Signal Processing Technique for Ku-Band Automobile FMCW Fully Polarimetric SAR System Using Triangular LFM. IEEE Transactions on Instrumentation and Measurement, 70:1-10, 2021. Keyword(s): SAR Processing, carborne SAR, Frequency modulation, Signal processing algorithms, Signal processing, Polarimetry, Automobiles, Doppler effect, Synthetic aperture radar, Automobile synthetic aperture radar (SAR), frequency modulated continuous wave (FMCW) radar, fully polarimetric SAR (PolSAR), linear frequency modulation (LFM), range Doppler algorithm (RDA), SAR, triangular waveform.
    Abstract: This article presents a novel signal technique for Ku-band automobile frequency-modulated continuous-wave fully polarimetric synthetic aperture radar (FMCW PolSAR) system using triangular linear frequency modulation (LFM). Our proposed system shows the first utilizations of triangular LFM for an FMCW PolSAR. The proposed signal processing algorithm is based on the range Doppler algorithm (RDA). We developed an FMCW PolSAR system that transmits triangular LFM signals, which are used less frequently than sawtooth LFM in an SAR sensor. Using a theoretical background, we describe its configuration and how it works. We propose the novel processing solution, which forms two kinds of single-polarization images from a raw data set and is suitable for our system. We obtained all four kinds of single-polarization images from two raw data sets while using the triangular LFM. In comparison, when using sawtooth LFM, we obtained the four images from four raw data sets by repeating the RDA four times. The proposed method simplifies the FMCW PolSAR system configuration and the processing algorithm. We collected FMCW PolSAR raw data from an experimentally equipped automobile while maintaining a constant speed on a highway. The proposed algorithm and system were validated by processing a high-resolution FMCW PolSAR image.
    [bibtex-key = jungKimAzimParkParkIEEETIM2021KuBandAutomobileFMCWFulPolSARSystem] [bibtex-entry]


  112. Da Liang, Kaiyu Liu, Heng Zhang, Yafeng Chen, Haixia Yue, Dacheng Liu, Yunkai Deng, Haoyu Lin, Tingzhu Fang, Chuang Li, and Robert Wang. The Processing Framework and Experimental Verification for the Noninterrupted Synchronization Scheme of LuTan-1. IEEE Transactions on Geoscience and Remote Sensing, 59(7):5740-5750, July 2021. Keyword(s): Synchronization, Oscillators, Satellites, Synthetic aperture radar, Receivers, Remote sensing, Bistatic synthetic aperture radar (BiSAR), internal calibration, noninterrupted, oscillator, phase synchronization.
    Abstract: The bistatic synthetic aperture radar (BiSAR) plays an important role in remote sensing. However, the deviation between the two oscillators in BiSAR systems will cause a residual modulation of the echo signal. Therefore, the phase synchronization is an important issue that must be addressed in the BiSAR system. An advanced noninterrupted phase synchronization scheme is used for LuTan-1. The synchronization pulses are exchanged immediately after the ending time of the radar echo receiving window and before the starting time of the next pulse repetition interval, which will not interrupt the normal SAR operation. In order to evaluate the accuracy of the phase synchronization scheme, the model of phase synchronization is introduced at first. The hardware design and processing flow of LuTan-1 are introduced in detail. An innovative internal calibration strategy is also described. Then, the test data acquired by the ground validation system are analyzed to verify the effectiveness of the phase synchronization scheme. The signal-to-noise ratio (SNR) and the synchronization rate are the two most important factors to influence the accuracy in phase synchronization. The conclusions have guiding significance for the synchronization module design of LuTan-1 and the future BiSAR system.
    [bibtex-key = Liang2021] [bibtex-entry]


  113. Giuseppe Parrella, Irena Hajnsek, and Konstantinos P. Papathanassiou. Retrieval of Firn Thickness by Means of Polarisation Phase Differences in L-Band SAR Data. Remote Sensing, 13(21), 2021.
    Abstract: The knowledge of glacier zones' extent and their temporal variations is fundamental for the retrieval of surface mass balance of glaciers and ice sheets. In this context, a key parameter is the firn line (FL), the lower boundary of the percolation zone, whose location is an indicator of time-integrated mass balance changes. Several approaches have been developed in the last decades to map the FL by means of Synthetic Aperture Radar (SAR) imagery, mainly exploiting backscatter intensities and their seasonal variation. In this paper, an alternative approach is proposed, based on co-polarisation phase differences (CPDs). In particular, CPDs are interpreted as the result of propagation through anisotropic firn layers and are, therefore, proposed as an indicator of the presence of firn. A model is employed to demonstrate the link between CPDs and firn depth, indicating the potential of polarimetric SAR to improve firn characterization beyond spatial extent and FL detection. The proposed approach is demonstrated on L-band airborne data, acquired on 21 May 2015 by the F-SAR sensor of DLR in West Greenland during the ARCTIC15 campaign, and validated with in-situ information available from other studies.
    [bibtex-key = parrellaHajnsekPapathanassiouRemoteSensing2021RetrievalOfFirnThicknessByCopolarPhaseDifferencesInLBandSARData] [bibtex-entry]


  114. S. T. Peters, D. M. Schroeder, M. S. Haynes, D. Castelletti, and A. Romero-Wolf. Passive Synthetic Aperture Radar Imaging Using Radio-Astronomical Sources. IEEE Trans. Geosci. Remote Sens., pp 1-16, 2021. Keyword(s): Synthetic aperture radar, Passive radar, Sun, Radar, Focusing, Signal to noise ratio, Mathematical model, Passive radar, passive radio sounding, passive synthetic aperture radar (SAR), radio echo sounding, Back-Projection, Time-Domain Back-Projection, TDBP.
    Abstract: Recent work has demonstrated a passive radio sounding approach that uses the Sun as a source for echo detection and ranging. As the Sun is a moving source with a position that is known a priori, we evaluate this technique's capabilities to measure the echo's phase history, map topography, and perform synthetic aperture radar (SAR) focusing. Here, we present our approach to implementing passive SAR using a compact, temporally incoherent radio-astronomical source as a signal of opportunity. We first evaluate the passive system's capabilities to obtain an echo from a rough surface by determining the critical signal-to-noise ratio (SNR) for reliably observing the Sun's echo reflection with our passive instrument. We then demonstrate that our technique can detect the necessary changes in range, phase, and reflectivity of an echo from the Sun. We next present the experimental results of our passive radar testing using the Sun at Dante's View, Death Valley, to highlight this technique's ability to perform 2-D imaging. Finally, with synthetic data, we demonstrate that we can use time-domain backprojection to focus a planar white noise signal, perform passive SAR imaging, and improve the measurement's SNR and azimuth resolution. The results of passive SAR focusing on white noise highlight the potential for the Sun and Jupiter's radio emissions to perform surface and subsurface imaging for planetary and terrestrial observations.
    [bibtex-key = petersSchroederHaynesCastellettiRomeroWolfTGRS2021PassiveTDBPSARImagingUsingRadioAstronomicalSources] [bibtex-entry]


  115. Fabio Rocca, Deren Li, Stefano Tebaldini, Mingsheng Liao, Lu Zhang, Fabrizio Lombardini, Timo Balz, Norbert Haala, Xiaoli Ding, and Ramon Hanssen. Three- and Four-Dimensional Topographic Measurement and Validation. Remote Sensing, 13(15), 2021. Keyword(s): SAR Processing, Synthetic Aperture Radar, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, SAR Tomography, Tomography, TomoSAR, 3-D imaging, temporal decorrelation, deformation, validation, surface displacements, topographic mapping.
    Abstract: This paper reports on the activities carried out in the context of ''Dragon project 32278: Three- and Four-Dimensional Topographic Measurement and Validation''. The research work was split into three subprojects and encompassed several activities to deliver accurate characterization of targets on land surfaces and deepen the current knowledge on the exploitation of Synthetic Aperture Radar (SAR) data. The goal of Subproject 1 was to validate topographic mapping accuracy of various ESA, TPM, and Chinese satellite system on test sites in the EU and China; define and improve validation methodologies for topographic mapping; and develop and setup test sites for the validation of different surface motion estimation techniques. Subproject 2 focused on the specific case of spatially and temporally decorrelating targets by using multi-baseline interferometric (InSAR) and tomographic (TomoSAR) SAR processing. Research on InSAR led to the development of robust retrieval techniques to estimate target displacement over time. Research on TomoSAR was focused on testing or defining new processing methods for high-resolution 3D imaging of the interior of forests and glaciers and the characterization of their temporal behavior. Subproject 3 was focused on near-real-time motion estimation, considering efficient algorithms for the digestion of new acquisitions and for changes in problem parameterization.
    [bibtex-key = roccaEtREMOTESENSING2021ThreeAnd4DTopographicMeasurementAndValidation] [bibtex-entry]


  116. Helmut Rott, Stefan Scheiblauer, Jan Wuite, Lukas Krieger, Dana Floricioiu, Paola Rizzoli, Ludivine Libert, and Thomas Nagler. Penetration of interferometric radar signals in Antarctic snow. The Cryosphere, 15(9):4399-4419, September 2021. Keyword(s): SAR Interferometry, Snow, Arctic Snow.
    Abstract: Synthetic aperture radar interferometry (InSAR) is an efficient technique for mapping the surface elevation and its temporal change over glaciers and ice sheets. However, due to the penetration of the SAR signal into snow and ice, the apparent elevation in uncorrected InSAR digital elevation models (DEMs) is displaced versus the actual surface. We studied relations between interferometric radar signals and physical snow properties and tested procedures for correcting the elevation bias. The work is based on satellite and in situ data over Union Glacier in the Ellsworth Mountains, West Antarctica, including interferometric data of the TanDEM-X mission, topographic data from optical satellite sensors and field measurements on snow structure, and stratigraphy undertaken in December 2016. The study area comprises ice- free surfaces, bare ice, dry snow and firn with a variety of structural features related to local differences in wind ex- posure and snow accumulation. Time series of laser measurements of NASA's Ice, Cloud and land Elevation Satellite (ICESat) and ICESat-2 show steady-state surface topography. For area-wide elevation reference we use the Reference Elevation Model of Antarctica (REMA). The different elevation data are vertically co-registered on a blue ice area that is not affected by radar signal penetration. Backscatter simulations with a multilayer radiative transfer model show large variations for scattering of individual snow layers, but the vertical backscatter distribution can be approximated by an exponential function representing uniform absorption and scattering properties. We obtain estimates of the elevation bias by inverting the interferometric volume correlation co- efficient (coherence), applying a uniform volume model for describing the vertical loss function. Whereas the mean values of the computed elevation bias and the elevation difference between the TanDEM-X DEMs and the REMA show good agreement, a trend towards overestimation of penetration is evident for heavily wind-exposed areas with low accumulation and towards underestimation for areas with higher accumulation rates. In both cases deviations from the uniform volume structure are the main reason. In the first case the dense sequence of horizontal structures related to internal wind crust, ice layers and density stratification causes increased scattering in near-surface layers. In the second case the small grain size of the top snow layers causes a downward shift in the scattering phase centre.
    [bibtex-key = rottEtAlCryosphere2021PenetrationOfInSARSignalsInArcticSnow] [bibtex-entry]


  117. Emanuele Santi, Marco Brogioni, Marion Leduc-Leballeur, Giovanni Macelloni, Francesco Montomoli, Paolo Pampaloni, Juha Lemmetyinen, Juval Cohen, Helmut Rott, Thomas Nagler, Chris Derksen, Joshua King, Nick Rutter, Richard Essery, Cecile Menard, Melody Sandells, and Michael Kern. Exploiting the ANN Potential in Estimating Snow Depth and Snow Water Equivalent From the Airborne SnowSAR Data at X- and Ku-Bands. IEEE Transactions on Geoscience and Remote Sensing, pp 1-16, 2021. Keyword(s): SAR Processing, Artificial neural networks (ANNs), dense medium radiative transfer (DMRT), quasi Mie scattering (QMS) model, snow depth (SD), snow water equivalent (SWE), SnowSAR, synthetic aperture radar, SAR.
    Abstract: Within the framework of European Space Agency (ESA) activities, several campaigns were carried out in the last decade with the purpose of exploiting the capabilities of multifrequency synthetic aperture radar (SAR) data to retrieve snow information. This article presents the results obtained from the ESA SnowSAR airborne campaigns, carried out between 2011 and 2013 on boreal forest, tundra and alpine environments, selected as representative of different snow regimes. The aim of this study was to assess the capability of X- and Ku-bands SAR in retrieving the snow parameters, namely snow depth (SD) and snow water equivalent (SWE). The retrieval was based on machine learning (ML) techniques and, in particular, of artificial neural networks (ANNs). ANNs have been selected among other ML approaches since they are capable to offer a good compromise between retrieval accuracy and computational cost. Two approaches were evaluated, the first based on the experimental data (data driven) and the second based on data simulated by the dense medium radiative transfer (DMRT). The data driven algorithm was trained on half of the SnowSAR dataset and validated on the remaining half. The validation resulted in a correlation coefficient R = 0.77 between estimated and target SD, a root-mean-square error (RMSE) = 13 cm, and bias = 0.03 cm. ANN algorithms specific for each test site were also implemented, obtaining more accurate results, and the robustness of the data driven approach was evaluated over time and space. The algorithm trained with DMRT simulations and tested on the experimental dataset was able to estimate the target parameter (SWE in this case) with R = 0.74, RMSE = 34.8 mm, and bias = 1.8 mm. The model driven approach had the twofold advantage of reducing the amount of in situ data required for training the algorithm and of extending the algorithm exportability to other test sites.
    [bibtex-key = santiEtAlTGRS2021ANNPotentialInEstimatingSnowDepthAndSWEfromAirborneSnowSARDataAtXandKuBand] [bibtex-entry]


  118. Ilgin Seker and Marco Lavalle. Tomographic Performance of Multi-Static Radar Formations: Theory and Simulations. Remote Sensing, 13(4), 2021. Keyword(s): SAR Processing, SAR Tomography, Multibaseline SAR, Multistatic SAR, Simulations, Spaceborne SAR, Airborne SAR.
    Abstract: 3D imaging of Earth's surface layers (such as canopy, sub-surface, or ice) requires not just the penetration of radar signal into the medium, but also the ability to discriminate multiple scatterers within a slant-range and azimuth resolution cell. The latter requires having multiple radar channels distributed in across-track direction. Here, we describe the theory of multi-static radar tomography with emphasis on resolution, SNR, sidelobes, and nearest ambiguity location vs. platform distribution, observation geometry, and different multi-static modes. Signal-based 1D and 2D simulations are developed and results for various observation geometries, target distributions, acquisition modes, and radar parameters are shown and compared with the theory. Pros and cons of multi-static modes are compared and discussed. Results for various platform formations are shown, revealing that unequal spacing is useful to suppress ambiguities at the cost of increased multiplicative noise. In particular, we demonstrate that the multiple-input multiple-output (MIMO) mode, in combination with nonlinear spacing, outperforms the other modes in terms of ambiguity, sidelobe levels, and noise suppression. These findings are key to guiding the design of tomographic SAR formations for accurate surface topography and vegetation mapping.
    [bibtex-key = sekerLavalleREMOTESENSING2021TomoSARPerformanceForMultiStaticFormationsTheoryAndSimulations] [bibtex-entry]


  119. Dario Tagliaferri, Mattia Brambilla, Monica Nicoli, and Umberto Spagnolini. Sensor-Aided Beamwidth and Power Control for Next Generation Vehicular Communications. IEEE Access, 9:56301-56317, 2021. Keyword(s): Degradation, Power control, Sensor systems, Sensors, Trajectory, Vehicle dynamics, Global Positioning System, Beam pointing, beam tracking, beamwidth and power control, on-board sensors, V2X.
    Abstract: Ultra-reliable low-latency Vehicle-to-Everything (V2X) communications are needed to meet the extreme requirements of enhanced driving applications. Millimeter-Wave (24.25-52.6 GHz) or sub-THz (>100 GHz) V2X communications are a viable solution, provided that the highly collimated beams are kept aligned during vehicles' maneuverings. In this work, we propose a sensor-assisted dynamic Beamwidth and Power Control (BPC) system to counteract the detrimental effect of vehicle dynamics, exploiting data collected by on-board inertial and positioning sensors, mutually exchanged among vehicles over a parallel low-rate link, e.g., 5G New Radio (NR) Frequency Range 1 (FR1). The proposed BPC solution works on top of a sensor-aided Beam Alignment and Tracking (BAT) system, overcoming the limitations of fixed-beamwidth systems and optimizing the performance in challenging Vehicle-to-Vehicle (V2V) scenarios, even if extensions to Vehicle-to-Infrastructure (V2I) use-cases are feasible. We evaluate the sensor-assisted dynamic BPC by simulation over real trajectories and sensors' data collected by a dedicated experimental campaign. The goal is to show the advantages of the proposed BPC strategy in a high data-rate Line-Of-Sight (LOS) V2V context, and to outline the requirements in terms of sensors' sampling time and accuracy, along with the end-to-end latency on the control channel.
    [bibtex-key = tagliaferriBrambillaNicoliSpagnoliniIEEEACCESS2021SensorAidedBeamwidthAndPowerControlForVehicularCommunications] [bibtex-entry]


  120. Dario Tagliaferri, Marco Rizzi, Monica Nicoli, Stefano Tebaldini, Ivan Russo, Andrea Virgilio Monti-Guarnieri, Claudio Maria Prati, and Umberto Spagnolini. Navigation-Aided Automotive SAR for High-Resolution Imaging of Driving Environments. IEEE Access, 9:35599-35615, 2021. Keyword(s): Spaceborne radar, Radar imaging, Sensor systems, Sensors, Synthetic aperture radar, Standards, Automotive engineering, Sensor fusion, automotive SAR, environment mapping, ADAS, in-car navigation, IMU/GNSS integration.
    Abstract: The evolution of Advanced Driver Assistance Systems (ADAS) towards the ultimate goal of autonomous driving relies on a conspicuous number of sensors, to perform a wide range of operations, from parking assistance to emergency braking and environment mapping for target recognition/classification. Low-cost Mass-Market Radars (MMRs) are today widely used for object detection at various ranges (up to 250 meters) but they might not be suited for high-precision environment mapping. In this context, vehicular Synthetic Aperture Radar (SAR) is emerging as a promising technique to augment radar imaging capability by exploiting the vehicle motion to provide two-dimensional (2D), or even three-dimensional (3D), images of the surroundings. SAR has a higher resolution compared to standard automotive radars, provided that motion is precisely known. In this regard, one of the most attractive solutions to increase the positioning accuracy is to fuse the information from multiple on-board sensors, such as Global Navigation Satellite System (GNSS), Inertial Measurement Units (IMUs), odometers and steering angle sensors. This paper proposes a multi-sensor fusion technique to support automotive SAR systems, experimentally validating the approach and demonstrating its advantages compared to standard navigation solutions. The results show that multi-sensor-aided SAR images the surrounding with centimeter-level accuracy over typical urban trajectories, confirming its potential for practical applications and leaving room for further improvements.
    [bibtex-key = tagliaferriEtAlIEEEAccess2021NavigationAidedAutomotiveSARForHighResolutionImagingDrivingEnvironments] [bibtex-entry]


  121. S. Vey, D. Al-Halbouni, M.H. Haghighi, F. Alshawaf, J. Vüllers, A. Güntner, G. Dick, M. Ramatschi, P. Teatini, J. Wickert, and M. Weber. Delayed subsidence of the Dead Sea shore due to hydro-meteorological changes. Scientific Reports, 11(1), 2021. Note: Cited By 0. [bibtex-key = Vey2021] [bibtex-entry]


  122. Marwan Younis, Felipe Queiroz de Almeida, Michelangelo Villano, Sigurd Huber, Gerhard Krieger, and Alberto Moreira. Digital Beamforming for Spaceborne Reflector-Based Synthetic Aperture Radar, Part 1: Basic imaging modes. IEEE Geoscience and Remote Sensing Magazine, 9(3):8-25, Sep. 2021. Keyword(s): SCORE, scan-on-receive, SweepSAR, Digital Beamforming, Beamforming, Wide-Swath, Synthetic aperture radar, Spaceborne SAR, Mission Concept, Antenna Concept, Imaging Concept, Antenna feeds, Transmission line measurements, Reflector antennas, Spaceborne radar, Radar imaging, Parabolic antennas, Tandem-L, Rose-L, NiSAR.
    Abstract: Deployable reflector antennas illuminated by a digital feed array enable spaceborne synthetic aperture radar (SAR) systems to image an ultrawide, continuous swath at a fine azimuth resolution. This is facilitated by the use of dedicated imaging modes and by multidimensional digital beamforming (DBF) techniques. This article is part 1 of a tutorial trilogy, where a focus is put on the required onboard functionality, operational techniques, and DBF aspects, for which a rigorous mathematical description is included. To maintain general validity, the functional implementation for the various modes is detailed, thus avoiding restricting the description to a specific realization. The reader is assumed to be familiar with the general concept of SAR and, otherwise, referred to the literature on the topic. The approach followed in the trilogy is to start with a basic imaging concept and then move to more advanced modes, thus successively increasing the complexity; the mathematical description follows the same approach.
    [bibtex-key = younisEtAlIEEEGRSM2021DigitalBeamformingForSpaceborneReflectorBasedSyntheticApertureRadarPart1] [bibtex-entry]


  123. P. Yuan, A. Hunegnaw, F. Alshawaf, J. Awange, A. Klos, F.N. Teferle, and H. Kutterer. Feasibility of ERA5 integrated water vapor trends for climate change analysis in continental Europe: An evaluation with GPS (1994–2019) by considering statistical significance. Remote Sensing of Environment, 260, 2021. Note: Cited By 7. [bibtex-key = Yuan2021] [bibtex-entry]


  124. Howard Zebker. Accuracy of a Model-Free Algorithm for Temporal InSAR Tropospheric Correction. Remote Sensing, 13(3), 2021. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, DInSAR, InSAR, atmosphere, troposphere, APS, atmospheric phase screen, correction, atmospheric correction, tropospheric correction, path delay, tropospheric path delay, time series, deformation, displacement, deformation monitoring, monitoring.
    Abstract: Atmospheric propagational phase variations are the dominant source of error for InSAR (interferometric synthetic aperture radar) time series analysis, generally exceeding uncertainties from poor signal to noise ratio or signal correlation. The spatial properties of these errors have been well studied, but, to date, their temporal dependence and correction have received much less attention. Here, we present an evaluation of the magnitude of tropospheric artifacts in derived time series after compensation using an algorithm that requires only the InSAR data. The level of artifact reduction equals or exceeds that from many weather model-based methods, while avoiding the need to globally access fine-scale atmosphere parameters at all times. Our method consists of identifying all points in an InSAR stack with consistently high correlation and computing, and then removing, a fit of the phase at each of these points with respect to elevation. A comparison with GPS truth yields a reduction of three, from a rms misfit of 5–6 to ~2 cm over time. This algorithm can be readily incorporated into InSAR processing flows without the need for outside information.
    [bibtex-key = zebkerRemoteSensing2021AccuracyOfModelFreeTemporalInSARTroposphericCorrection] [bibtex-entry]


  125. Yanyan Zhang, Hao Zhang, Shuai Hou, Yunkai Deng, Weidong Yu, and Robert Wang. An Innovative Superpolyhedron (SP) Formation for Multistatic SAR (M-SAR) Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 59(12):10136-10150, December 2021. Keyword(s): Satellites, Synthetic aperture radar, Spaceborne radar, Radar, Space vehicles, Sea measurements, Radar imaging, Genetic algorithm (GA), interferometric synthetic aperture radar (InSAR), multistatic synthetic aperture radar (M-SAR), satellite formation, superpolyhedron (SP).
    Abstract: Spaceborne multistatic synthetic aperture radar (M-SAR) has extensive applications, including multiangle imaging, digital beamforming (DBF) and cross- and along-track interferometry. However, it is difficult for classical satellite formations to meet the multimission (e.g., cross-track interferometry and along-track interferometry, i.e., XTI and ATI) requirements of M-SAR concurrently. Therefore, superpolyhedron (SP), an innovative satellite formation based on the dual-helix and pendulum formations, is proposed to maximize the coverage ratio of the effective cross- and along-track interferometric baselines at the same time. A method of constructing the SP formation is detailed. The method is a three-step procedure, in which the first two steps aim at obtaining an initial formation via exploiting the geometry of the formation dynamics; the last step solves an optimization problem. Then, the design of a supertetrahedron (ST) formation, a special case of the SP formation, is investigated as a numerical example. The merit of SP formation is represented as the coverage ratios of effective cross- and along-track interferometric baselines. The result is compared with those of the classical satellite formations. It shows that only the coverage ratios of the ST formation are greater than 70% simultaneously. Therefore, the ST formation can realize both effective XTI and ATI. It implies that the SP formation has the potential to be used in future spaceborne M-SAR interferometry.
    [bibtex-key = Zhang2021] [bibtex-entry]


  126. F. Alshawaf. A new method for reconstructing absolute water vapor maps from persistent scatterer InSAR. IEEE Transactions on Geoscience and Remote Sensing, 58(7):4951-4957, 2020. Note: Cited By 1. [bibtex-key = Alshawaf20204951] [bibtex-entry]


  127. Simone Baffelli, Othmar Frey, and Irena Hajnsek. Geostatistical Analysis and Mitigation of the Atmospheric Phase Screens in Ku-Band Terrestrial Radar Interferometric Observations of an Alpine Glacier. IEEE Transactions on Geoscience and Remote Sensing, 58(11):7533-7556, November 2020. Keyword(s): Gamma Portable Radar Interferometer, GPRI, Pol-GPRI, Atmospheric modeling, Spaceborne radar, Atmospheric measurements, Radar interferometry, Delays, Phase measurement, Atmospheric modeling, atmospheric phase screen (APS), differential radar interferometry, terrestrial radar interferometry, TRI.
    Abstract: Terrestrial radar interferometry (TRI) can measure displacements at high temporal resolution, potentially with high accuracy. An application of this method is the observation of the surface flow velocity of steep, fast-flowing aglaciers. For these observations, the main factor limiting the accuracy of TRI observations is the spatial and temporal variabilities in the distribution of atmospheric water vapor content, causing a phase delay [atmospheric phase screen (APS)] whose magnitude is similar to the displacement phase. This contribution presents a geostatistical analysis of the spatial and temporal behaviors of the APS in Ku-Band TRI. The analysis is based on the assumption of a separable spatiotemporal covariance structure, which is tested empirically using variogram analysis. From this analysis, spatial and temporal APS statistics are estimated and used in a two-step procedure combining regression-Kriging with generalized least squares (GLS) inversion to estimate a velocity time-series. The performance of this method is evaluated by cross-validation using phase observations on stable scatterers. This analysis shows a considerable reduction in residual phase variance compared with the standard approach of combining the linear models of APS stratification and interferogram stacking.
    [bibtex-key = baffelliFreyHajnsekTGRS2020GeostatisticalAtmosphereAnalysisMitigationKuBandGPRIGlacier] [bibtex-entry]


  128. Angelique Benoit, Beatrice Pinel-Puyssegur, Romain Jolivet, and Cecile Lasserre. CorPhU: an algorithm based on phase closure for the correction of unwrapping errors in SAR interferometry. Geophysical Journal International, 221(3):1959-1970, 03 2020.
    Abstract: Interferometric Synthetic Aperture Radar (InSAR) is commonly used in Earth Sciences to study surface displacements or construct high resolution topographic maps. Recent satellites such as those of the Sentinel-1 constellation allow to derive dense deformation maps with millimetric precision with high revisit frequency. However, InSAR is still limited by interferometric coherence. Interferometric phase noise resulting from a loss of coherence, due to changes in scattering properties between repeated SAR acquisitions, may lead to unwrapping errors, which then in turn lead to centimetric errors in time-series reconstruction. We present an algorithm based on interferometric phase closure to automatically correct unwrapping errors. We describe the algorithm and highlight its performances with two case studies, in Lebanon with Envisat satellite data and in Central Turkey with Sentinel-1 data. The first data set is particularly affected by unwrapping errors because of long spatial (500 m) and temporal baseline interferograms (6 yr) and decorrelation due, in particular, to vegetation. The second data set contains unwrapping errors because of temporal changes in the scattering properties of the ground. For these two examples, the algorithm allows the correction of almost all detectable unwrapping errors, without requiring visual inspection or manual deletions. Our algorithm is efficient especially on large data sets, such as with Sentinel-1 constellation, where interferometric phase is redundant and improves eventually the reconstruction of time-series.
    [bibtex-key = benoitPinelPuyssegurJolivetLasserreGJI2020CorPhuAPhaseClosureAlgorithmForCorrectionOfInSARUnwrappingErrors] [bibtex-entry]


  129. Victor Cazcarra-Bes, Matteo Pardini, Marivi Tello-Alonso, and K. P. Papathanassiou. Comparison of Tomographic SAR Reflectivity Reconstruction Algorithms for Forest Applications at L-band. IEEE Trans. Geosci. Remote Sens., 58(1):147-164, January 2020. Keyword(s): SAR Processing, SAR Tomography, Forestry, Synthetic aperture radar, Image reconstruction, Decorrelation, Estimation, Reconstruction algorithms, Capon beamforming (CB), compressive sensing (CS), forest applications, forest structure, Fourier beamforming (FB), L-band, synthetic aperture radar (SAR), tomography, airborne SAR.
    Abstract: SAR Processing, SAR Tomography, Forest structure is a key parameter for forest applications, but it is difficult to be estimated at the required spatial and temporal scales. In this context, synthetic aperture radar Tomography (TomoSAR) that allows, at lower frequencies, the 3-D imaging of natural volume scatterers with high spatial and temporal resolution may be a game changer. The aim of this article is to evaluate three TomoSAR algorithms, Fourier beamforming (FB), Capon beamforming (CB), and compressive sensing (CS) with respect to their performance in the reconstruction of the 3-D forest reflectivity. The implications of volumetric forest scattering, as well as the temporal decorrelation of scatterers, are analyzed. The algorithms are compared on a set of simulated scenarios and then evaluated on an experimental L-band data set composed by four acquisition dates, each one consisting of five tomographic tracks. The data were acquired in 2014, within a time span of two months, over the Traunstein forest (Germany) using the F-SAR system. Additionally, discrete airborne Lidar has been used for a qualitative evaluation. The results indicate that the CS reconstruction is, for many practical cases, superior when compared to FB or CB reconstructions as they achieve higher vertical resolution, especially in cases with a lower number of acquisitions and complex forest scenarios. By combining acquisitions performed at different days, the effect of temporal decorrelation on each algorithm for two different tomographic implementations (repeat-pass vs. single-pass) has been assessed. The results indicate that simultaneously acquired image pairs allow a better reconstruction of the 3-D forest reflectivity.
    [bibtex-key = cazcarraBesPardiniTelloPapathanassiouTGRS2019ComparisonSARTOMOAlgorithmsForestLBand] [bibtex-entry]


  130. Elise Colin Koeniguer and Jean-Marie Nicolas. Change Detection Based on the Coefficient of Variation in SAR Time-Series of Urban Areas. Remote Sensing, 12(13), 2020. Keyword(s): REASSESS.
    Abstract: This paper discusses change detection in SAR time-series. First, several statistical properties of the coefficient of variation highlight its pertinence for change detection. Subsequently, several criteria are proposed. The coefficient of variation is suggested to detect any kind of change. Furthermore, several criteria that are based on ratios of coefficients of variations are proposed to detect long events, such as construction test sites, or point-event, such as vehicles. These detection methods are first evaluated on theoretical statistical simulations to determine the scenarios where they can deliver the best results. The simulations demonstrate the greater sensitivity of the coefficient of variation to speckle mixtures, as in the case of agricultural plots. Conversely, they also demonstrate the greater specificity of the other criteria for the cases addressed: very short event or longer-term changes. Subsequently, detection performance is assessed on real data for different types of scenes and sensors (Sentinel-1, UAVSAR). In particular, a quantitative evaluation is performed with a comparison of our solutions with baseline methods. The proposed criteria achieve the best performance, with reduced computational complexity. On Sentinel-1 images containing mainly construction test sites, our best criterion reaches a probability of change detection of 90% for a false alarm rate that is equal to 5%. On UAVSAR images containing boats, the criteria proposed for short events achieve a probability of detection equal to 90% of all pixels belonging to the boats, for a false alarm rate that is equal to 2%.
    [bibtex-key = koeniguerNicolasREMOTESENSING2020ChangeDetectionBasedOnCoefficientOfVariationInSARTimeSeriesOfUrbanAreas] [bibtex-entry]


  131. M. Dalaison and R. Jolivet. A Kalman Filter Time Series Analysis Method for InSAR. Journal of Geophysical Research: Solid Earth, 125(7):e2019JB019150, 2020. Note: E2019JB019150 10.1029/2019JB019150.
    Abstract: Abstract Earth orbiting satellites, such as Sentinel 1A-B, build up an ever-growing set of synthetic aperture radar images of the ground. This conceptually allows for real-time monitoring of ground displacements using Interferometric Synthetic Aperture Radar (InSAR), notably in tectonically active regions such as fault zones or over volcanoes. We propose a Kalman filter for InSAR time series analysis (KFTS), an efficient method to rapidly update preexisting time series of displacement with data as they are made available, with limited computational cost. KFTS solves together for the evolution of phase change with time and for a parametrized model of ground deformation. Synthetic tests of the KFTS reveal exact agreement with the equivalent weighted least squares solution and a convergence of descriptive model parameter after the assimilation of about 1 year of data. We include the impact of sudden deformation events such as earthquakes or slow slip events on the time series of displacement. First tests of the KFTS on ENVISAT data over Mt. Etna (Sicily) and Sentinel 1 data around the Chaman fault (Afghanistan, Pakistan) show precise ( 0.05 mm) retrieval of phase change when data are sufficient. Otherwise, the optimized parametrized model is used to forecast phase change. Good agreement is found with classic time series analysis solution and GPS-derived time series. Accurate estimates are conditioned to the proper parametrization of errors so that models and observations can be combined with their respective uncertainties. This new tool is freely available to process ongoing InSAR time series.
    [bibtex-key = dalaisonJolivetJGR2020KalmanFilterTimeSeriesAnalysisMethodforInSAR] [bibtex-entry]


  132. Marion Heublein, P. E. Bradley, and Stefan Hinz. Observing geometry effects on a Global Navigation Satellite System (GNSS)-based water vapor tomography solved by least squares and by compressive sensing. Annales Geophysicae, 38(1):179-189, 2020. [bibtex-key = heubleinBradleyHinz2020GeometryEffectsInGNSSWaterVaporTomography] [bibtex-entry]


  133. M. Hoseini, F. Alshawaf, H. Nahavandchi, G. Dick, and J. Wickert. Towards a zero-difference approach for homogenizing GNSS tropospheric products. GPS Solutions, 24(1), 2020. Note: Cited By 9. [bibtex-key = Hoseini2020] [bibtex-entry]


  134. Xie Hu, Roland Burgmann, Eric J. Fielding, and Hyongki Lee. Internal kinematics of the Slumgullion landslide (USA) from high-resolution UAVSAR InSAR data. Remote Sensing of Environment, 251:112057, 2020. Keyword(s): SAR Processing, Slumgullion landslide, Kinematic boundaries, Geomorphologic features, UAVSAR, InSAR, DInSAR, Airborne SAR, L-band, Surface Displacements, Deformation.
    Abstract: Landslides represent one of the most damaging natural hazards and often lead to unexpected casualties and property damage. They also continually modify our natural environment and landscapes. Knowledge of landslide systems is largely restricted by the stochastic nature, subjective interpretation and infrequent or spatially sparse surveying of landslides. Characterized by persistent daily movements of a couple of centimeters over multi-centennial timescales and a long narrow shape as long as ~4 km, the Slumgullion landslide in Colorado, USA represents an ideal natural laboratory to study slow-moving landslides. Here we demonstrate the capability of the highly accurate, spatially continuous airborne Synthetic Aperture Radar (SAR) system of the NASA Uninhabited Aerial Vehicle SAR (UAVSAR) to characterize the kinematic details of internal deformation of the Slumgullion landslide using SAR interferometry (InSAR). We develop a phase-based approach to automatically extract the boundaries of the mobile geological structures without unwrapping. Comparison with historic field observations from 1991 reveals the 40-m advance of the frontal toe and shift of an internal fault. The UAVSAR data also resolve an internal minislide (100 by 70 m), which moves more southerly than the main body at 5 mm/day in the lower part of the landslide. A Light Detection and Ranging (LiDAR) Digital Elevation Model (DEM) shows that the minislide is associated with the opening of a 30 by 10 m pull-apart basin and bounding strike-slip faults. These extensional structures, nearby incised streams, and steepened local slopes helped establish the kinematic environment for the formation of the secondary minislide. The disparity between the UAVSAR InSAR-derived horizontal moving directions and the LiDAR DEM-derived slope aspects suggest that while the surface topography governs the first-order orientation, the local kinematics is also subject to the variable nature of heterogeneous landslide materials and the irregular basal bedrock surface. The landslide velocity and precipitation show similar multi-annual variations. Our study demonstrates that the freely available, high-resolution UAVSAR data, have great potential for characterizing landslide kinematics and other small-scale geological and geomorphological processes.
    [bibtex-key = huBurgmannFieldingLeeRSE2020UAVSARDisplacementSlumgullionLandslideDInSAR] [bibtex-entry]


  135. Y. Huang, F. Liu, Z. Chen, J. Li, and W. Hong. An Improved Map-Drift Algorithm for Unmanned Aerial Vehicle SAR Imaging. IEEE Geoscience and Remote Sensing Letters, pp 1-5, 2020. Keyword(s): Synthetic aperture radar, Unmanned aerial vehicles, Apertures, Azimuth, Trajectory, Electronics packaging, Doppler effect, Map-drift algorithm (MDA), motion compensation (MOCO), random sample consensus (RANSAC), unmanned aerial vehicle synthetic aperture radar (UAV SAR) imaging..
    Abstract: Unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) is usually sensitive to trajectory deviations that cause severe motion error in the recorded data. Because of the small size of the UAV, it is difficult to carry a high-accuracy inertial navigation system. Therefore, in order to obtain a precise SAR imagery, autofocus algorithms, such as phase gradient autofocus (PGA) method and map-drift (MD) algorithm, were proposed to compensate the motion error based on the received signal, but most of them worked on range-invariant motion error and abundant prominent scatterers. In this letter, an improved MD algorithm is proposed to compensate the range-variant motion error compared to the existed MD algorithm. In this context, in order to solve the outliers caused by homogeneous scenes or absent prominent scatterers, a random sample consensus (RANSAC) algorithm is employed to mitigate the influence resulting from the outliers, realizing robust performance for different cases. Finally, real SAR data are applied to demonstrate the effectiveness of the proposed method.
    [bibtex-key = huangLiuChenLiHongGRSL2020MapDriftAutofocusForUAVborneSARImaging] [bibtex-entry]


  136. P. Hügler, T. Grebner, C. Knill, and C. Waldschmidt. UAV-Borne 2-D and 3-D Radar-Based Grid Mapping. IEEE Geosci. Remote Sens. Lett., pp 1-5, 2020. Keyword(s): Frequency-modulated continuous-wave radar, multiple-input multiple-output (MIMO) radar, occupancy grid map (OGM), unmanned aerial vehicle (UAV)..
    Abstract: For unmanned aerial vehicles (UAVs), grid maps can be a versatile tool for navigation and self-localization. In general, payload is critical for UAVs and every additional sensor limits the flight duration. Due to its robustness and the ability to directly measure velocities, radar sensors are well suited for sense and avoid applications (SAAs) for UAVs. It would be advantageous if these sensor data could be used to generate grid maps instead of mounting additional sensors such as light detection and ranging (LiDAR). This letter demonstrates that using the data from high-resolution multiple-input-multiple-output (MIMO) imaging radars, high-resolution 2-D and 3-D radar grid maps can be created. The necessary adaption of the sensors free-space model for MIMO radar-based occupancy grid maps is presented in detail. UAV-borne measurements resulting in 2-D and 3-D grid maps with an adequate representation of the environment validate this approach.
    [bibtex-key = huglerGrebnerKnillWaldschmidtGRSL2020UAVborne2Dand3DGridmapping] [bibtex-entry]


  137. Guodong Jin, Kaiyu Liu, Dacheng Liu, Da Liang, Heng Zhang, Naiming Ou, Yanyan Zhang, Yunkai Deng, Chuang Li, and Robert Wang. An Advanced Phase Synchronization Scheme for LT-1. IEEE Transactions on Geoscience and Remote Sensing, 58(3):1735-1746, March 2020. Keyword(s): Synchronization, Satellites, Spaceborne radar, Synthetic aperture radar, Radar antennas, Oscillators, Bistatic synthetic aperture radar (SAR), phase error, phase synchronization.
    Abstract: LuTan-1 (LT-1), i.e., TwinSAR-L, mission is an innovative spaceborne bistatic synthetic aperture radar (SAR) mission that is based on two satellites operating at L-band with flexible formation flying, which is planned to launch in 2020. The primary objective of LT-1 is to generate a highly accurate global digital elevation model (DEM). Beyond that, LT-1 will serve for the demonstration of some state-of-the-art technologies in radar field and some applications, such as biomass inversion, disaster forecasting, and climate and environmental monitoring, and phase synchronization is a technical challenge in realizing the highly accurate topography and deformation measurements. The pulsed alternate synchronization scheme, which is proposed to solve the synchronization problem of TerraSAR-X add-on for Digital Elevation Measurements (TanDEM)-X, is an efficient and accurate method. However, it interrupts the normal work of the TanDEM-X, accordingly flowing a series of problems. For LT-1, a novel synchronization scheme, in which the phase synchronization signal is exchanged by virtue of a time slot between radar signals, is applied. Thus, the working efficiency and synchronization accuracy can further be improved. Furthermore, the performance prediction and phase synchronization experiment for this synchronization scheme is presented, which verifies the feasibility of the proposed scheme. Finally, in order to guarantee the transmission quality of the synchronization signal, the illumination combinations and gain variation of the synchronization antenna in an orbital period are detailed.
    [bibtex-key = Jin2020] [bibtex-entry]


  138. Jorge Jorge Ruiz, Risto Vehmas, Juha Lemmetyinen, Josu Uusitalo, Janne Lahtinen, Kari Lehtinen, Anna Kontu, Kimmo Rautiainen, Riku Tarvainen, Jouni Pulliainen, and Jaan Praks. SodSAR: A Tower-Based 1-10 GHz SAR System for Snow, Soil and Vegetation Studies. Sensors, 20(22), 2020.
    Abstract: We introduce SodSAR, a fully polarimetric tower-based wide frequency (1–10 GHz) range Synthetic Aperture Radar (SAR) aimed at snow, soil and vegetation studies. The instrument is located in the Arctic Space Centre of the Finnish Meteorological Institute in Sodankyl\"a, Finland. The system is based on a Vector Network Analyzer (VNA)-operated scatterometer mounted on a rail allowing the formation of SAR images, including interferometric pairs separated by a temporal baseline. We present the description of the radar, the applied SAR focusing technique, the radar calibration and measurement stability analysis. Measured stability of the backscattering intensity over a three-month period was observed to be better than 0.5 dB, when measuring a target with a known radar cross section. Deviations of the estimated target range were in the order of a few cm over the same period, indicating also good stability of the measured phase. Interforometric SAR (InSAR) capabilities are also discussed, and as a example, the coherence of subsequent SAR acquisitions over the observed boreal forest stand are analyzed over increasing temporal baselines. The analysis shows good conservation of coherence in particular at L-band, while higher frequencies are susceptible to loss of coherence in particular for dense vegetation. The potential of the instrument for satellite calibration and validation activities is also discussed.
    [bibtex-key = ruizVehmasLemmetyinenUusitaloLahtinenLehtinenKontuRautiainenTarvainenPulliainenPraksSENSORS2020SodSARTowerBasedSARforSnowSoilVegetation] [bibtex-entry]


  139. M. Kern, R. Cullen, B. Berruti, J. Bouffard, T. Casal, M. R. Drinkwater, A. Gabriele, A. Lecuyot, M. Ludwig, R. Midthassel, I. Navas Traver, T. Parrinello, G. Ressler, E. Andersson, C. Martin-Puig, O. Andersen, A. Bartsch, S. Farrell, S. Fleury, S. Gascoin, A. Guillot, A. Humbert, E. Rinne, A. Shepherd, M. R. van den Broeke, and J. Yackel. The Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) high-priority candidate mission. The Cryosphere, 14(7):2235-2251, 2020. Keyword(s): ice, snow, polar ice, polar snow, radar, radar altimeter, altimeter, Ku-band, Ka-band, CRISTAL, Copernicus Polar Ice and Snow Topography Altimeter, remote sensing, radar remote sensing, microwave, microwave remote sensing. [bibtex-key = kernEtAlCryosphere2020CopernicusPolarIceAndSnowTopographyAltimeterCRISTAL] [bibtex-entry]


  140. Leonidas Kosmidis, Iv�n Rodriguez, �lvaro Jover, Sergi Alcaide, J�r�me Lachaize, Jaume Abella, Olivier Notebaert, Francisco J. Cazorla, and David Steenari. GPU4S: Embedded GPUs in space - Latest project updates. Microprocessors and Microsystems, 77:103143, 2020. Keyword(s): Space, Mobile GPU, Embedded systems, GPU, Aerospace.
    Abstract: Following the trend of other safety-critical industries like automotive and avionics, the space domain is witnessing an increase in the on-board computing performance demands. This raise in performance needs comes from both control and payload parts of the spacecraft and calls for advanced electronics systems able to provide high computational power under the constraints of the harsh space environment. On the non-technical side, for strategic reasons it is mandatory to get European independence on the used computing technology. In this project, we study the applicability of embedded GPUs in space, which have shown a dramatic improvement of their performance per-watt ratio coming from their proliferation in consumer markets based on competitive European technology. To that end, we perform an analysis of the existing space application domains to identify which software domains can benefit from their use. Moreover, we survey the embedded GPU domain in order to assess whether embedded GPUs can provide the required computational power and identify the challenges which need to be addressed for their adoption in space. In this paper, we describe the steps followed in the project, as well as a summary of results obtained from our analyses so far in the project.
    [bibtex-key = KosmidisEtAl2020EmbeddedGPUsInSpace] [bibtex-entry]


  141. S. Leinss, H. Löwe, M. Proksch, and A. Kontu. Modeling the evolution of the structural anisotropy of snow. The Cryosphere, 14(1):51-75, 2020. Keyword(s): snow, anisotropy, modelling.
    Abstract: The structural anisotropy of snow characterizes the spatially anisotropic distribution of the ice and air microstructure and is a key parameter for improving parameterizations of physical properties. To enable the use of the anisotropy in snowpack models as an internal variable, we propose a simple model based on a rate equation for the temporal evolution. The model is validated with a comprehensive set of anisotropy profiles and time series from X-ray microtomography (CT) and radar measurements. The model includes two effects, namely temperature gradient metamorphism and settling, and can be forced by any snowpack model that predicts temperature and density. First, we use CT time series from lab experiments to validate the proposed effect of temperature gradient metamorphism. Next, we use SNOWPACK simulations to calibrate the model with radar time series from the NoSREx campaigns in Sodankylae, Finland. Finally we compare the simulated anisotropy profiles against field-measured full-depth CT profiles. Our results confirm that the creation of vertical structures is mainly controlled by the vertical water vapor flux through the snow volume. Our results further indicate a yet undocumented effect of snow settling on the creation of horizontal structures. Overall the model is able to reproduce the characteristic anisotropy variations in radar time series of four different winter seasons with a very limited set of calibration parameters.
    [bibtex-key = leinssLoewePorkschKontuCryosphere2020ModelingStructuralAnisotropyOfSnow] [bibtex-entry]


  142. Chuang Li, Heng Zhang, Yunkai Deng, Robert Wang, Kaiyu Liu, Dacheng Liu, Guodong Jin, and Yanyan Zhang. Focusing the L-Band Spaceborne Bistatic SAR Mission Data Using a Modified RD Algorithm. IEEE Transactions on Geoscience and Remote Sensing, 58(1):294-306, January 2020. Keyword(s): Satellites, Imaging, Synthetic aperture radar, Orbits, Azimuth, Interferometry, Spaceborne radar, 2-D principle of stationary phase (2-DPOSP) spectrum, efficient baselines, improved bistatic range-Doppler algorithm (RDA), LuTan-1 (LT-1), TwinSAR-L.
    Abstract: LuTan-1 [(LT-1), i.e., TwinSAR-L] mission is an innovative spaceborne bistatic synthetic-aperture radar (BiSAR) mission focusing mainly on differential interferometry, which will be launched in 2020. This article introduces some important aspects of the LT-1 mission for the first time, including the formation configuration, imaging mode, application scenarios, and the efficient baselines between the master satellite and the slave satellite. To realize the high accurate topography and deformation measurements, a wide swath BiSAR focusing algorithm with phase reserving ability should be developed. This article proposes a modified bistatic range-Doppler algorithm based on 2-D principle of stationary phase spectrum, which reduces the phase error introduced by the root term expansion and has an excellent focus performance and a good phase reserving ability. Finally, the spaceborne bistatic simulation experiments using orbital parameters and imaging mode of the LT-1 mission, including point targets and scene targets, are utilized to illustrate the validity and accuracy of the proposed algorithm.
    [bibtex-key = Li2020] [bibtex-entry]


  143. Da Liang, Kaiyu Liu, Heng Zhang, Yunkai Deng, Dacheng Liu, Yafeng Chen, Chuang Li, Haixia Yue, and Robert Wang. A High-Accuracy Synchronization Phase-Compensation Method Based on Kalman Filter for Bistatic Synthetic Aperture Radar. IEEE Geoscience and Remote Sensing Letters, 17(10):1722-1726, October 2020. Keyword(s): Synchronization, Satellites, Kalman filters, Synthetic aperture radar, Spaceborne radar, Oscillators, Bistatic synthetic aperture radar (BiSAR), Kalman filter, noninterrupted, phase synchronization.
    Abstract: Phase synchronization is one of the key issues that must be addressed for the bistatic synthetic aperture radar (BiSAR) system. LuTan-1 (LT-1) is an innovative spaceborne BiSAR mission based on the use of two radar satellites operating in the L-band to generate the global digital terrain models in the bistatic interferometry mode. An advanced synchronization scheme is used for the LT-1 system. The synchronization pulses are exchanged immediately after the ending time of the radar echo-receiving window and before the starting time of the next pulse-repetition interval. Therefore, it cannot interrupt the normal SAR data acquisition, further improving the synchronization accuracy and avoiding the data missing effect. In this letter, a robust phase-error estimation and compensation method is proposed to improve the accuracy of the synchronization by using the Kalman filter. The test data acquired from the ground validation system of the LT-1 synchronization module are used to demonstrate the feasibility of the proposed scheme. The results validate the effectiveness of the proposed scheme and prove the promise for its future application in LT-1.
    [bibtex-key = Liang2020] [bibtex-entry]


  144. Dieter Luebeck, Christian Wimmer, Laila F. Moreira, Marlon Alcantara, Oré Gian, Juliana A. Gós, Luciano P. Oliveira, Bárbara Teruel, Leonardo S. Bins, Lucas H. Gabrielli, and Hugo E. Hernandez-Figueroa. Drone-borne Differential SAR Interferometry. Remote Sensing, 12(5), 2020.
    Abstract: Differential synthetic aperture radar interferometry (DInSAR) has been widely applied since the pioneering space-borne experiment in 1989, and subsequently with the launch of the ERS-1 program in 1992. The DInSAR technique is well assessed in the case of space-borne SAR data, whereas in the case of data acquired from aerial platforms, such as airplanes, helicopters, and drones, the effective application of this technique is still a challenging task, mainly due to the limited accuracy of the information provided by the navigation systems mounted onboard the platforms. The first airborne DInSAR results for measuring ground displacement appeared in 2003 using L- and X-bands. DInSAR displacement results with long correlation time in P-band were published in 2011. This letter presents a SAR system and, to the best of our knowledge, the first accuracy assessment of the DInSAR technique using a drone-borne SAR in L-band. A deformation map is shown, and the accuracy and resolution of the methodology are presented and discussed. In particular, we have obtained an accuracy better than 1 cm for the measurement of the observed ground displacement. It is in the same order as that achieved with space-borne systems in C- and X-bands and the airborne systems in X-band. However, compared to these systems, we use here a much longer wavelength. Moreover, compared to the satellite experiments available in the literature and aimed at assessing the accuracy of the DInSAR technique, we use only two flight tracks with low time decorrelation effects and not a big data stack, which helps in reducing the atmospheric effects.
    [bibtex-key = luebeckEtAlMDPIRemoteSensLett2020UAVDroneDInSAR] [bibtex-entry]


  145. Marco Manzoni, Andrea Virgilio Monti-Guarnieri, Eugenio Realini, and Giovanna Venuti. Joint Exploitation of SAR and GNSS for Atmospheric Phase Screens Retrieval Aimed at Numerical Weather Prediction Model Ingestion. Remote Sensing, 12(4), 2020.
    Abstract: This paper proposes a simple and fast method to estimate Atmospheric Phase Screens (APSs) by jointly exploit a stack of Synthetic Aperture Radar (SAR) images and a dataset of GNSS-derived atmospheric product. The output of this processing is conceived to be ingested by Numerical Weather Prediction Models (NWPMs) to improve weather forecasts. In order to provide wide and dense area coverage and to respect requirements in terms of spatial resolution of ingestion products in NWPMs, both Permanent Scatterers (PSs) and Distributed Scatterers (DSs) are jointly exploited. While the formers are by definition stable targets, but unevenly distributed, the latter are ubiquitous but stable only within a certain temporal baseline that can vary depending on the operational frequency of the radar. The proposed method is thus particularly suited for C, L, and P band missions with low temporal baseline between two consecutive acquisitions of the same scene: these conditions, that are both necessary to provide the dense space-time coverage required by meteorologists, allow for a reliable and robust estimation of APSs thanks to the intrinsic limitation of temporal decorrelation. The proposed technique integrates Zenith Total Delay (ZTD) products computed on a very sparse grid from a network of GNSS stations to correct for SAR orbital errors and to provide the missing phase constant from the derived APS map. In this paper, the complete workflow is explained, and a comparison of the derived APSs is performed with phase screens derived from state-of-the-art SAR processing workflow (SqueeSAR®).
    [bibtex-key = manzoniMontiGuarnieriRealiniVenutiREMOTESENSING2020ExploitationOfSARandGNSSForAtmosphericPhaseScreensRetrievalForNumericalWeatherPrediction] [bibtex-entry]


  146. J. Matar, M. Rodriguez-Cassola, G. Krieger, P. Lopez-Dekker, and A. Moreira. MEO SAR: System Concepts and Analysis. IEEE Transactions on Geoscience and Remote Sensing, 58(2):1313-1324, February 2020. Keyword(s): Orbits, Sensitivity, Synthetic aperture radar, Antennas, Spatial resolution, Low earth orbit satellites, Coverage, medium-Earth-orbit (MEO) synthetic aperture radar (SAR), orbits, SAR, space radiation, system performance..
    Abstract: Existing microwave remote sensing instruments used for Earth observation face a clear tradeoff between spatial resolution and revisit times at global scales. The typical imaging capabilities of current systems range from daily observations at kilometer-scale resolutions provided by scatterometers to meter-scale resolutions at lower temporal rates (more than ten days) typical of synthetic aperture radars (SARs). A natural way to fill the gap between these two extremes is to use medium-Earth-orbit SAR (MEO-SAR) systems. MEO satellites are deployed at altitudes above the region of low Earth orbits (LEOs), ending at around 2000 km and below the geosynchronous orbits (GEOs) near 35,786 km. MEO SAR shows a clear potential to provide advantages in terms of spatial coverage, downlink visibility, and global temporal revisit times, e.g., providing moderate resolution images (some tens of meters) at daily rates. This article discusses the design tradeoffs of MEO SAR, including sensitivity and orbit selection. The use of these higher orbits opens the door to global coverage in one- to two-day revisit or continental/oceanic coverage with multidaily observations, making MEO SAR very attractive for future scientific missions with specific interferometric and polarimetric capabilities.
    [bibtex-key = matarRodriguezCassolaKriegerLopezDekkerMoreiraTGRS2020MEOSARConcepts] [bibtex-entry]


  147. Gian Ore, Marlon S. Alcantara, Juliana A. Goes, Luciano P. Oliveira, Jhonnatan Yepes, Barbara Teruel, Valquiria Castro, Leonardo S. Bins, Felicio Castro, Dieter Luebeck, Laila F. Moreira, Lucas H. Gabrielli, and Hugo E. Hernandez-Figueroa. Crop Growth Monitoring with Drone-Borne DInSAR. Remote Sensing, 12(4), 2020. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, UAV, Drone, Hexacopter, DJI, DJI-S900, Synthetic aperture radar (SAR), SAR interferometry, mobile mapping, airborne SAR, repeat-pass interferometry, differential interferometry, DInSAR, SAR imaging, focusing, back-projection, Time-Domain Back-Projection, TDBP.
    Abstract: Accurate, high-resolution maps of for crop growth monitoring are strongly needed by precision agriculture. The information source for such maps has been supplied by satellite-borne radars and optical sensors, and airborne and drone-borne optical sensors. This article presents a novel methodology for obtaining growth deficit maps with an accuracy down to 5 cm and a spatial resolution of 1 m, using differential synthetic aperture radar interferometry (DInSAR). Results are presented with measurements of a drone-borne DInSAR operating in three bands—P, L and C. The decorrelation time of L-band for coffee, sugar cane and corn, and the feasibility for growth deficit maps generation are discussed. A model is presented for evaluating the growth deficit of a corn crop in L-band, starting with 50 cm height. This work shows that the drone-borne DInSAR has potential as a complementary tool for precision agriculture.
    [bibtex-key = oreEtAlREMOTESENSING2020CropGrowthWithDroneBorneDInSAR] [bibtex-entry]


  148. C. Rambour, A. Budillon, A. C. Johnsy, L. Denis, F. Tupin, and G. Schirinzi. From Interferometric to Tomographic SAR: A Review of Synthetic Aperture Radar Tomography-Processing Techniques for Scatterer Unmixing in Urban Areas. IEEE Geoscience and Remote Sensing Magazine, 8(2):6-29, June 2020. Keyword(s): geophysical techniques, radar imaging, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, tomography, tomographic SAR, synthetic aperture radar tomography-processing techniques, scatterer unmixing, urban areas, cross-track synthetic aperture radar interferometry, phase shifts, interferometric processing, scatterer distribution, multiple scatterers, line of sight, resolution cell, Radar tracking, Three-dimensional displays, Synthetic aperture radar, Radar antennas, Tomography, Urban areas.
    Abstract: Cross-track synthetic aperture radar (SAR) interferometry is a powerful technique that analyzes the phase shift each pixel undergoes between acquisitions of the same scene with just a slight change of viewpoint. These phase shifts provide information about the topography and, when more than two acquisitions are available at different dates, about possible slow motions along the line of sight, related to subsidence and/or thermal dilation, of the dominant scatterer in the resolution cell. However, interferometric processing exploits phase-only data, does not provide scatterer distribution in the vertical direction, and is not able to separate multiple scatterers lying in the same range/azimuth resolution cell.
    [bibtex-key = rambourBudillonJohnsyDenisTupinSchirinziIEEEGRSMag2020FromInterferometricToTomographicSARaReview] [bibtex-entry]


  149. F. Rosu, A. Anghel, R. Cacoveanu, B. Rommen, and M. Datcu. Multiaperture Focusing for Spaceborne Transmitter/Ground-Based Receiver Bistatic SAR. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 13:5823-5832, 2020. Keyword(s): autoregressive processes, covariance matrices, eigenvalues and eigenfunctions, radar receivers, radar resolution, radar transmitters, spaceborne radar, synthetic aperture radar, Romania, Bucharest city, eigenvalues, spatial smoothing, principle component analysis, full rank covariance matrix, bistatic data, integer multiple, Akaike information criterion, multiaperture range image, back-projection focusing, autoregressive model, azimuth samples, slow time resampling, antenna pattern compensation, satellite bursts, range compressed pulses, azimuth apertures, azimuth resolution, spaceborne transmitter-stationary receiver bistatic SAR, azimuth focusing, Azimuth, Receivers, Apertures, Focusing, Synthetic aperture radar, Chirp, Satellites, Autoregressive (AR) model, bistatic synthetic aperture radar (SAR), focusing, order estimation, signal reconstruction.
    Abstract: The article proposes a methodology to perform azimuth focusing of spaceborne transmitter-stationary receiver bistatic synthetic aperture radar data across multiple along-track apertures to increase azimuth resolution. The procedure uses as input several azimuth apertures (continuous groups of range compressed pulses) from one or more satellite bursts and comprises the following stages: antenna pattern compensation, slow time resampling, reconstruction of missing azimuth samples between neighboring sets of pulses using an autoregressive (AR) model and back-projection focusing of the resulting multiaperture range image. A novel, highly efficient method is proposed to estimate the optimal order for the AR model. It differs from the traditional approach that uses the Akaike Information Criterion to directly estimate the order because the proposed method estimates the order indirectly by detecting the number of targets using principle component analysis. Spatial smoothing is used to obtain a full rank covariance matrix, whose eigenvalues are then analyzed using minimum description length. The optimal order is an integer multiple of the number of targets, which depends on signal-to-noise ratio. The approach is evaluated with real bistatic data acquired over an area of Bucharest city, Romania.
    [bibtex-key = rosuAnghelCacoveanuRommenDatcuJSTARS2020MultiapertureFocusingSpaceborneTransmitterGroundBasedReceiverBistaticSAR] [bibtex-entry]


  150. M. Schartel, R. Burr, W. Mayer, and C. Waldschmidt. Airborne Tripwire Detection Using a Synthetic Aperture Radar. IEEE Geoscience and Remote Sensing Letters, 17(2):262-266, February 2020. Keyword(s): SAR Processing, UAV, FMCW, Chirp, Synthetic aperture radar, Radar imaging, Wires, Antennas, Radar cross-sections, Antipersonnel (AP) mine, frequency-modulated continuous-wave (FMCW) radar, multicopter, synthetic aperture radar (SAR), tripwire, unmanned aerial system (UAS), Time-Domain Back-Projection, TDBP.
    Abstract: Antipersonnel fragmentation mines are relatively large metallic mines, which are only partially buried and often triggered by a metallic tripwire. In humanitarian mine clearance, the search for the wires is usually carried out manually. As a new approach, an airborne system for the detection of tripwires using a synthetic aperture radar is presented. The system consists of an industrial multicopter, a frequency-modulated continuous-wave radar, and a real time kinematic global navigation satellite system. For image formation, a backprojection algorithm is used. Measurements with tripwires attached to a dummy mine successfully demonstrate the functionality of this system approach. In addition, the influence of wire length, vegetation, and incidence angle are investigated. It is shown that several overflights with different directions of flight are required to detect randomly oriented tripwires.
    [bibtex-key = schartelBurrMayerWaldschmidtGRSL2020UAVSARMineDetection] [bibtex-entry]


  151. Endrit Shehaj, Karina Wilgan, Othmar Frey, and Alain Geiger. A Collocation Framework to Retrieve Tropospheric Delays from a Combination of GNSS and InSAR. Navigation, 67(4):823-842, 2020. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Persistent Scatterer Interferometry, PSI, GNSS, GPS Troposphere, Collocation, Retrieval of Tropospheric Delays, Combination of GNSS and InSAR.
    Abstract: High spatio-temporal variability of atmospheric water vapor affects microwave signals of Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR). A better knowledge of the distribution of water vapor in the atmosphere improves both GNSS and InSAR derived data products. Moreover, this information can potentially enhance meteorological and climatological applications. In this work, we present a collocation framework to combine and retrieve zenith and (relative) slant tropospheric delays. GNSS and InSAR meteorological products, which are complementary in terms of spatio-temporal resolution, are combined aiming at a better retrieval of the atmospheric water vapor (and therefore of the respective delays). We investigate the combination approach with synthetic and real data acquired in the Alpine region of Switzerland. This research is a contribution to improve the spatio-temporal mapping of tropospheric delays by combining GNSS-derived and InSAR-derived delays.
    [bibtex-key = shehajWilganFreyGeigerNavigation2020CollocationFrameworkForTropoRetrievalFromGNSSandInSAR] [bibtex-entry]


  152. G. H. X. Shiroma and M. Lavalle. Digital Terrain, Surface, and Canopy Height Models From InSAR Backscatter-Height Histograms. IEEE Transactions on Geoscience and Remote Sensing, 58(6):3754-3777, June 2020. Keyword(s): backscatter, forestry, optical radar, radar imaging, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, vegetation, vegetation mapping, digital terrain, canopy height models, InSAR backscatter-height histogram, interferometric synthetic aperture radar backscatter-height histograms, single-look backscatter measurements, InSAR phase height, InSAR histogram, LiDAR waveforms, ground topography, full-polarimetric L-band uninhabited aerial vehicle synthetic aperture radar data, forest height, Histograms, Laser radar, Forestry, Vegetation mapping, Backscatter, Synthetic aperture radar, Digital elevation models (DEMs), forest height, interferometry, L-band, polarimetric synthetic aperture radar (SAR) interferometry (PolInSAR), polarimetry, SAR.
    Abstract: This article demonstrates how 3-D vegetation structure can be approximated by interferometric synthetic aperture radar (InSAR) backscatter-height histograms. Single-look backscatter measurements are plotted against the InSAR phase height and are aggregated spatially over a forest patch to form a 3-D histogram, referred to as InSAR backscatter-height histogram or simply InSAR histogram. InSAR histograms resemble LiDAR waveforms, suggesting that existing algorithms used to retrieve canopy height and ground topography from radar tomograms or LiDAR waveforms can be applied to InSAR histograms. Three algorithms are evaluated to generate maps of digital terrain, surface, and canopy height models: Gaussian decomposition, quantile, and backscatter threshold. Full-polarimetric L-band uninhabited aerial vehicle synthetic aperture radar (UAVSAR) data collected over the Gabonese Lope National Park during the 2016 AfriSAR campaign are used to illustrate and compare the performance of the algorithms for the HH, HV, VV, HH+VV, and HH-VV polarimetric channels. Results show that radar-derived maps using the InSAR histograms differ by 4 m (top-canopy), 5 m (terrain), and 6 m (forest height) in terms of average root-mean-square errors (RMSEs) from standard maps derived from full-waveform laser, vegetation, and ice sensor (LVIS) LiDAR measurements.
    [bibtex-key = shiromaLavalleTGRS2020DigitalTerrainSurfaceAndCanopyHeightFromInSARBackscatterHeightHistograms] [bibtex-entry]


  153. Francescopaolo Sica, Sofie Bretzke, Andrea Pulella, Jose-Luis Bueso-Bello, Michele Martone, Pau Prats-Iraola, Maria-Jose Gonzalez-Bonilla, Michael Schmitt, and Paola Rizzoli. InSAR Decorrelation at X-Band From the Joint TanDEM-X/PAZ Constellation. IEEE Geoscience and Remote Sensing Letters, pp 1-5, 2020.
    Abstract: Decorrelation phenomena are always present in synthetic aperture radar interferometry (InSAR). While this implies a certain level of signal degradation, decorrelation is also a characteristic of the type of imaged target itself and can, therefore, be seen as a source of information. In this letter, we investigate InSAR decorrelation effects at the X-band by fitting volume and temporal decorrelation trends using the unique combination of data provided by the TanDEM-X (TDX) and PAZ spaceborne missions. The innovative use of this constellation allows for the acquisition of both single- and repeat-pass data at short revisit times. The concurrent availability of simultaneous acquisitions and the fine temporal resolution makes this constellation the ideal observation scenario for the study of decorrelation phenomena. Overall, we analyze five test sites, characterized by the presence of different land cover classes, and for each of them, we provide volume and temporal decorrelation fitting parameters. The performed analysis gives a first insight on the potential of combining bistatic and repeat-pass InSAR acquisitions also in view of future spaceborne constellations, which could benefit from the TDX/PAZ experience.
    [bibtex-key = sicaBretzkePulellaBuesoMartonePratsGonzalezSchmittRizzoliGRSL2020InSARDecorrelationAtXBandFromTanDEMXPAZConstellation] [bibtex-entry]


  154. F. Sica, G. Gobbi, Paola Rizzoli, and Lorrenzo Bruzzone. arphi-Net: Deep Residual Learning for InSAR Parameters Estimation. IEEE Transactions on Geoscience and Remote Sensing, pp 1-25, 2020. Keyword(s): Estimation, Synthetic aperture radar, Coherence, Noise reduction, Convolution, Measurement, Wavelet transforms, Coherence, convolutional neural network (CNN), deep learning (DL), denoising, interferometric phase, residual learning, synthetic aperture radar (SAR) interferometry..
    Abstract: Nowadays, deep learning (DL) finds application in a large number of scientific fields, among which the estimation and the enhancement of signals disrupted by the noise of different natures. In this article, we address the problem of the estimation of the interferometric parameters from synthetic aperture radar (SAR) data. In particular, we combine convolutional neural networks together with the concept of residual learning to define a novel architecture, named \varphi-Net, for the joint estimation of the interferometric phase and coherence. \varphi-Net is trained using synthetic data obtained by an innovative strategy based on the theoretical modeling of the physics behind the SAR acquisition principle. This strategy allows the network to generalize the estimation problem with respect to: 1) different noise levels; 2) the nature of the imaged target on the ground; and 3) the acquisition geometry. We then analyze the \varphi-Net performance on an independent data set of synthesized interferometric data, as well as on real InSAR data from the TanDEM-X and Sentinel-1 missions. The proposed architecture provides better results with respect to state-of-the-art InSAR algorithms on both synthetic and real test data. Finally, we perform an application-oriented study on the retrieval of the topographic information, which shows that \varphi-Net is a strong candidate for the generation of high-quality digital elevation models at a resolution close to the one of the original single-look complex data.
    [bibtex-key = sicaGobbiRizzoliBruzzoneTGRS2020DeepResidualLearningForInSARParameterEstimation] [bibtex-entry]


  155. Ladina Steiner, Michael Meindl, Christoph Marty, and Alain Geiger. Impact of GPS Processing on the Estimation of Snow Water Equivalent Using Refracted GPS Signals. IEEE Transactions on Geoscience and Remote Sensing, 58(1):123-135, January 2020. Keyword(s): Global Positioning System, remote sensing, snow, Swiss Alps, GPS processing parameters, SWE estimation performance, elevation-dependent weighting scheme, elevation cutoff angles, sub-snow GPS, temporal reliability, systematic overview, seasons time period, GPS refractometry, sub-snow global positioning system antennas, snowpack modeling, remote sensing data, snow hydrological monitoring, weather conditions, continuous SWE quantification, automated SWE quantification, situ snow water equivalent estimation, global navigation satellite system antennas, refracted GPS signals, daily estimates, hourly SWE estimation, Global Positioning System, Snow, Estimation, Global navigation satellite system, Antennas, Satellites, Delays, Global navigation satellite system (GNSS), global positioning system (GPS), GPS refractometry, snow, snow water equivalent (SWE), sub-snow.
    Abstract: Global navigation satellite system (GNSS) antennas buried underneath a snowpack have a high potential for in situ snow water equivalent (SWE) estimation. Automated and continuous SWE quantification independent of weather conditions could enhance snow hydrological monitoring and modeling. Accurate and reliable in situ data are needed for the calibration and validation of remote sensing data and snowpack modeling. A relative bias of less than 5% is achieved using sub-snow global positioning system (GPS) antennas (GPS refractometry) during a three full seasons time period in the Swiss Alps. A systematic overview regarding the temporal reliability of the sub-snow GPS derived results is, however, missing for this emerging technique. Moreover, GPS processing impacts the results significantly. Different GPS processing parameters are therefore selected and their influence on the SWE estimation is investigated. The impact of elevation-dependent weighting, the elevation cutoff angles, and the time intervals for SWE estimation are systematically assessed. The best results are achieved using all observations with an elevation-dependent weighting scheme. Moreover, the SWE estimation performance is equally accurate for hourly SWE estimation as for lower temporal resolutions up to daily estimates. The impact of snow on the coordinate solution is furthermore evaluated. While the east and north components are not systematically influenced by the overlying snowpack, the vertical component exhibits a significant variation and strongly depends on the SWE. The biased vertical component therefore provides an additional possibility to estimate SWE.
    [bibtex-key = steinerMeindlMartyGeigerTGRS2020GNSSprocessingOfSnowWaterEquivalentSWE] [bibtex-entry]


  156. Benjamin Thomas, Alan Hunter, and Samantha Dugelay. Phase Wrap Error Correction by Random Sample Consensus With Application to Synthetic Aperture Sonar Micronavigation. IEEE Journal of Oceanic Engineering, pp 1-15, 2020. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Delay effects, Synthetic aperture sonar, SAS, Estimation, Bandwidth, Synthetic aperture radar, Robustness, Phase unwrapping, synthetic aperture radar (SAR), synthetic aperture sonar (SAS), time delay estimation.
    Abstract: Accurate time delay estimation between signals is crucial for coherent imaging systems such as synthetic aperture sonar (SAS) and synthetic aperture radar (SAR). In such systems, time delay estimates resulting from the cross-correlation of complex signals are commonly used to generate navigation and scene height measurements. In the presence of noise, the time delay estimates can be ambiguous, containing errors corresponding to an integer number of phase wraps. These ambiguities cause navigation and bathymetry errors and reduce the quality of synthetic aperture imagery. In this article, an algorithm is introduced for the detection and correction of phase wrap errors. The random sample consensus (RANSAC) algorithm is used to fit 1-D and 2-D models to the ambiguous time delay estimates made in the time delay estimation step of redundant phase center (RPC) micronavigation. Phase wrap errors are then corrected by recalculating the phase wrap number using the best-fitting model. The approach is demonstrated using the data collected by the 270-330 kHz SAS of the NATO Centre for Maritime Research and Experimentation Minehunting unmanned underwater vehicle for Shallow water Covert Littoral Expeditions. Systems with lower fractional bandwidth were emulated by windowing the bandwidth of the signals to increase the occurrence of phase wrap errors. The time delay estimates were refined using both the RANSAC algorithms using 1-D and 2-D models and the commonly used branch-cuts method. Following qualitative assessment of the smoothness of the full-bandwidth time delay estimates after application of these three methods, the results from the 2-D RANSAC method were chosen as the reference time delay estimates. Comparison with the reference estimates shows that the 1-D and 2-D RANSAC methods outperform the branch-cuts method, with improvements of 29%-125% and 30%-150%, respectively, compared to 16%-134% for the branch-cuts method for this data set.
    [bibtex-key = thomasHunterDugelayIEEEJOE2020PhaseWrapErrorCorrectionSyntheticApertureSonar] [bibtex-entry]


  157. R. Van Malderen, E. Pottiaux, A. Klos, P. Domonkos, M. Elias, T. Ning, O. Bock, J. Guijarro, F. Alshawaf, M. Hoseini, A. Quarello, E. Lebarbier, B. Chimani, V. Tornatore, S. Zengin Kazanci, and J. Bogusz. Homogenizing GPS Integrated Water Vapor Time Series: Benchmarking Break Detection Methods on Synthetic Data Sets. Earth and Space Science, 7(5), 2020. Note: Cited By 9. [bibtex-key = VanMalderen2020] [bibtex-entry]


  158. Simona Verde, Antonio Pauciullo, Diego Reale, and Gianfranco Fornaro. Multiresolution Detection of Persistent Scatterers: A Performance Comparison Between Multilook GLRT and CAESAR. IEEE Transactions on Geoscience and Remote Sensing, pp 1-16, 2020. Keyword(s): Monitoring, Detectors, Spatial resolution, Signal resolution, Tomography, Covariance matrices, Strain, Detection, generalized likelihood ratio test (GLRT), persistent scatterers (PS), SAR tomography..
    Abstract: Persistent scatterers (PS) interferometry tools are extensively used for the monitoring of slow, long-term ground deformation. High spatial resolution is typically required in urban areas to cope with the variability of the signal, whereas in rural regions, multilook shall be implemented to improve the coverage of monitored areas. Along this line, SqueeSAR and later Component extrAction and sElection SAR (CAESAR) were introduced for the monitoring of both persistent and (decorrelating) distributed scatterers (DS). Multilook generalized likelihood ratio test (MGLRT) is a detector derived in the context of tomographic SAR processing that has been investigated for a fixed multilook degree. In this work, we address MGLRT and CAESAR in the multiresolution context characterized by a spatially variable multilook degree. We compare the two schemes for the multiresolution selection of PS and DS, highlighting the pros and cons of each scheme, particularly the peculiarities of CAESAR that have important implications at the implementation stage. A performance analysis of both detectors in case of model mismatch is also addressed. Experiments carried out with data acquired by the COSMO-SkyMed constellation support both the theoretical argumentation and the results achieved by resorting to Monte Carlo simulations.
    [bibtex-key = verdePauciulloRealeFornaroTGRS2020PSIPerformanceComparisonMultilookGLRTandCAESAR] [bibtex-entry]


  159. Yanghai Yu, Mauro Mariotti d'Alessandro, Stefano Tebaldini, and Mingsheng Liao. Signal Processing Options for High Resolution SAR Tomography of Natural Scenarios. Remote Sensing, 12(10), 2020.
    Abstract: Synthetic Aperture Radar (SAR) Tomography is a technique to provide direct three-dimensional (3D) imaging of the illuminated targets by processing SAR data acquired from different trajectories. In a large part of the literature, 3D imaging is achieved by assuming mono-dimensional (1D) approaches derived from SAR Interferometry, where a vector of pixels from multiple SAR images is transformed into a new vector of pixels representing the vertical profile of scene reflectivity at a given range, azimuth location. However, mono-dimensional approaches are only suited for data acquired from very closely-spaced trajectories, resulting in coarse vertical resolution. In the case of continuous media, such as forests, snow, ice sheets and glaciers, achieving fine vertical resolution is only possible in the presence of largely-spaced trajectories, which involves significant complications concerning the formation of 3D images. The situation gets even more complicated in the presence of irregular trajectories with variable headings, for which the one theoretically exact approach consists of going back to raw SAR data to resolve the targets by 3D back-projection, resulting in a computational burden beyond the capabilities of standard computers. The first aim of this paper is to provide an exhaustive discussion of the conditions under which high-quality tomographic processing can be carried out by assuming a 1D, 2D, or 3D approach to image formation. The case of 3D processing is then further analyzed, and a new processing method is proposed to produce high-quality imaging while largely reducing the computational burden, and without having to process the original raw data. Furthermore, the new method is shown to be easily parallelized and implemented using GPU processing. The analysis is supported by results from numerical simulations as well as from real airborne data from the ESA campaign AlpTomoSAR.
    [bibtex-key = yuDAlessandroTebaldiniREMOTESENSING2020SignalProcessingOptionsHighResolutionSARTomographyNaturalMedia] [bibtex-entry]


  160. Yanyan Zhang, Heng Zhang, Naiming Ou, Kaiyu Liu, Da Liang, Yunkai Deng, and Robert Wang. First Demonstration of Multipath Effects on Phase Synchronization Scheme for LT-1. IEEE Transactions on Geoscience and Remote Sensing, 58(4):2590-2604, April 2020. Keyword(s): Synchronization, Satellites, Frequency synchronization, Receiving antennas, Doppler effect, Rough surfaces, Surface roughness, Bistatic synthetic aperture radar (SAR), Doppler frequency shift, multipath effect, phase synchronization, scale experiment.
    Abstract: LuTan-1 (LT-1) refers to an innovative mission of spaceborne bistatic synthetic aperture radar (BiSAR) adopting the phase synchronization scheme of pulse exchange, which is scheduled to be launched in 2020. In this mission, the multipath effect caused by the reflection of satellites and Doppler frequency shift resulting from the relative motion between satellites constitute two critically latent factors deteriorating the performance of the phase synchronization scheme. This article details a loss factor to evaluate the fading of synchronization signals under the reflection of smooth and rough surfaces and estimates the influence of Doppler frequency shift on synchronization signals. Furthermore, the multipath channel impulse response is modeled, and the correlation characteristics of multipath channel are described by the coherent bandwidth and coherent time. In addition, two simulation experiments based on the system parameters of LT-1, are implemented to assess the influence of time-invariant and -varying multipath effects on the phase synchronization scheme. A scale experiment is also executed to quantify the influence of multipath effect on phase synchronization, and its result demonstrates that the phase synchronization scheme of LT-1 can not only achieve full coverage of space and backup capability of pulse exchange but also meet the application requirements under multipath effect.
    [bibtex-key = Zhang2020] [bibtex-entry]


  161. Y. Zhang, D. Zhu, X. Mao, X. Yu, J. Zhang, and Y. Li. Multirotors Video Synthetic Aperture Radar: System Development and Signal Processing. IEEE Aerospace and Electronic Systems Magazine, 35(12):32-43, December 2020. Keyword(s): SAR Processing, SAR Imaging, SAR Focusing, UAV, Video SAR, Video sequences, Unmanned aerial vehicles, Trajectory, Vehicle dynamics, Synthetic aperture radar, Field programmable gate arrays, Video signal processing.
    Abstract: 360-degree video synthetic aperture radar (VideoSAR) presents a powerful capability for the information perception of dynamic scenes. In comparison to time-consuming airborne SAR, miniature SAR (MiniSAR) mounted on an unmanned aerial vehicle platform is a cost-effective configuration that provides a higher operation flexibility for video acquisition. Nevertheless, MiniSAR system development and signal processing limited by multiple factors are still challenging issues, including restricted weight, volume, power consumption, complicated trajectory imaging, and high-efficient video processing. In this article, we develop a multirotors VideoSAR system at X-band in Nanjing University of Aeronautics and Astronautics, concerning hardware architecture, field experiment, and video signal processing. It has the capability of ultrahigh resolution imaging with three available channels up to 1.8-GHz transmission bandwidth. Field programmable gate array-based unified signal processing architecture can further accelerate the generation of massive VideoSAR sequences in terms of both circular spotlight and stripmap modes. Several experimental results have been presented to demonstrate the validity of our developed system.
    [bibtex-key = zhangZhuMaoYuZhangLiIEEEAESMagMultirotorsUAVVideoSAR] [bibtex-entry]


  162. Michele Ambrosanio, Martina T. Bevacqua, Tommaso Isernia, and Vito Pascazio. The Tomographic Approach to Ground-Penetrating Radar for Underground Exploration and Monitoring: A More User-Friendly and Unconventional Method for Subsurface Investigation. IEEE Signal Processing Magazine, 36(4):62-73, July 2019.
    Abstract: Underground exploration and characterization via nondestructive imaging is of great interest in the modern scientific community [1]. The reason for such enthusiasm is related to the methodology?s wide range of applications, which span from civil engineering to the archaeological and forensic fields and to planetary exploration. In this framework, ground-penetrating radar (GPR) is becoming one of the leading technologies because of its flexibility and ease of application. However, standard GPR provides a user-dependent map, known as a radargram, of underground and subsurface targets, the interpretation of which requires strong human expertise and does not allow for inferences about the nature of the buried targets, unless some a priori information is available.
    [bibtex-key = ambrosanioBevacquaIserniaPascazioIEEESignalProcMag2019TomographicApproachToGroundPenetratingRadarProcessing] [bibtex-entry]


  163. Simone Baffelli, Othmar Frey, and Irena Hajnsek. Polarimetric Analysis of Natural Terrain Observed With a Ku-Band Terrestrial Radar. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 12(12):5268-5288, December 2019. Keyword(s): Terrestrical Radar, Polarimetry, Radar Polarimetry, ku-band, Gamma Portable Radar Interferometer, GPRI-II, Polarimetric Gamma Portable Radar Interferometer, PolGPRI, Entropy, ground based radar, polarimetric radar.
    Abstract: Terrestrial radar interferometers (TRI) are complimentary to spaceborne synthetic aperture radar systems for deformation monitoring in natural terrain: they permit shorter revisit times and greater flexibility in acquisition mode and timing. The additional diversity offered by polarimetric data can also be beneficial for TRI observations because polarized waves are sensitive to the dielectric and geometrical properties of the scatterers. Polarimetric data helps to distinguish different scattering mechanisms in a resolution cell while at the same time estimating terrain displacements. However, the polarimetric scattering signatures of natural surfaces at Ku-Band are not as well characterized as the ones at longer wavelengths, owing to relative rarity of full polarimetric systems operating in Ku-band. This band is often employed in TRI to obtain a fine azimuth resolution with a limited aperture size. This article aims at assessing the potential of polarimetric measurements in Ku-band TRI through an experimental study of polarimetric scattering signatures of natural surfaces using two datasets acquired over a glacier and in an agricultural and urban scene. The main finding of this analysis is that the Cloude-Pottier entropy is high for all land cover types; it is only observed to be less than 0.5 for scatterers with a large radar cross section. Several plausible hypotheses for this observation are formulated and tested, the most likely assumes a combination of depolarizing scattering from natural surfaces and the effect of the large ratio of wavelength to resolution cell size.
    [bibtex-key = baffelliFreyHajnsekJSTARS2019PolarimetricAnalysisNaturalTerrainKuBandRadar] [bibtex-entry]


  164. Ilaria Catapano, Gianluca Gennarelli, Giovanni Ludeno, and Francesco Soldovieri. Applying Ground-Penetrating Radar and Microwave Tomography Data Processing in Cultural Heritage: State of the Art and Future Trends. IEEE Signal Processing Magazine, 36(4):53-61, July 2019.
    Abstract: Ground-penetrating radar (GPR) enables inspection of optically opaque media and provides information about buried or otherwise hidden targets. This makes GPR an especially useful tool for monitoring and investigating cultural-heritage (CH) and archeological sites. This article reviews theoretical and practical aspects of GPR for its effective and reliable use in CH applications. Data processing via a linearized microwave tomo- graphic (MWT) approach is our main focus. Theoretical and practical issues related to GPR imaging, such as the mitigation of clutter and artifacts as well as 3D reconstruction of large spatial domains, are addressed in this context. Finally, we explain how GPR was applied in a survey performed at the archeological site of Pompeii, Italy.
    [bibtex-key = catapanoGennarelliLudenoSoldovieriIEEESIgnalProcMag2019GroundPenetratingRadarAndMicrowaveTomographyProcessingInCulturalHeritage] [bibtex-entry]


  165. Ibrahim El Moussawi, Dinh Ho Tong Minh, Nicolas Baghdadi, Chadi Abdallah, Jalal Jomaah, Olivier Strauss, Marco Lavalle, and Yen-Nhi Ngo. Monitoring Tropical Forest Structure Using SAR Tomography at L- and P-Band. Remote Sensing, 11(16), 2019.
    Abstract: Our study aims to provide a comparison of the P- and L-band TomoSAR profiles, Land Vegetation and Ice Sensor (LVIS), and discrete return LiDAR to assess the ability for TomoSAR to monitor and estimate the tropical forest structure parameters for enhanced forest management and to support biomass missions. The comparison relies on the unique UAVSAR Jet propulsion Laboratory (JPL)/NASA L-band data, P-band data acquired by ONERA airborne system (SETHI), Small Footprint LiDAR (SFL), and NASA Land, Vegetation and Ice Sensor (LVIS) LiDAR datasets acquired in 2015 and 2016 in the frame of the AfriSAR campaign. Prior to multi-baseline data processing, a phase residual correction methodology based on phase calibration via phase center double localization has been implemented to improve the phase measurements and compensate for the phase perturbations, and disturbances originated from uncertainties in allocating flight trajectories. First, the vertical structure was estimated from L- and P-band corrected Tomography SAR data measurements, then compared with the canopy height model from SFL data. After that, the SAR and LiDAR three-dimensional (3D) datasets are compared and discussed at a qualitative basis at the region of interest. The L- and P-band’s performance for canopy penetration was assessed to determine the underlying ground locations. Additionally, the 3D records for each configuration were compared with their ability to derive forest vertical structure. Finally, the vertical structure extracted from the 3D radar reflectivity from L- and P-band are compared with SFL data, resulting in a root mean square error of 3.02 m and 3.68 m, where the coefficient of determination shows a value of 0.95 and 0.93 for P- and L-band, respectively. The results demonstrate that TomoSAR holds promise for a scientific basis in forest management activities.
    [bibtex-key = elMoussawiHoTongHoBaghdadiAbdallahJomaahStraussLavalleNgoREMOTESENSING2019SARTomoLbandPband] [bibtex-entry]


  166. D. Feng, D. An, X. Huang, and Y. Li. A Phase Calibration Method Based on Phase Gradient Autofocus for Airborne Holographic SAR Imaging. IEEE Geoscience and Remote Sensing Letters, pp 1-5, 2019. Keyword(s): Calibration, Imaging, Synthetic aperture radar, Electronics packaging, Image reconstruction, Radar polarimetry, Azimuth, Holographic synthetic aperture radar (HoloSAR) tomography, phase calibration, phase gradient autofocus (PGA), three-dimensional (3-D) imaging..
    Abstract: Holographic synthetic aperture radar tomography can realize full 3-D reconstructions of objects over 360deg with very high resolution, and thus becomes an interesting 3-D imaging technique. However, due to the uncompensated platform motion errors, phase errors among different tracks usually exist in this imaging mode, which will hinder the 3-D image focusing. In this letter, a phase calibration method based on phase gradient autofocus is proposed to mitigate the influence of these phase errors. It exploits the space-invariant characteristic of the phase errors in a limited scene and uses an efficient and robust multibaseline autofocus to calibrate the phase errors among the multiple circular tracks. Experimental results with the GOTCHA real data are carried out to demonstrate the validity and feasibility of the proposed method.
    [bibtex-key = FengAnHuangLiGRSL2019SARTomoPGAPhaseCalibration] [bibtex-entry]


  167. Weike Feng, Jean-Michel Friedt, Giovanni Nico, Suyun Wang, Gilles Martin, and Motoyuki Sato. Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results. Remote Sensing, 11(15), 2019.
    Abstract: A passive bistatic ground-based synthetic aperture radar (PB-GB-SAR) system without a dedicated transmitter has been developed by using commercial-off-the-shelf (COTS) hardware for local-area high-resolution imaging and displacement measurement purposes. Different from the frequency-modulated or frequency-stepped continuous wave signal commonly used by GB-SAR, the continuous digital TV signal broadcast by a geostationary satellite has been adopted by PB-GB-SAR. In order to increase the coherence between the reference and surveillance channels, frequency and phase synchronization of multiple low noise blocks (LNBs) has been conducted. Then, the back-projection algorithm (BPA) and the range migration algorithm (RMA) have been modified for PB-GB-SAR to get the focused SAR image. Field experiments have been carried out to validate the designed PB-GB-SAR system and the proposed methods. It has been found that different targets within 100 m (like the fence, light pole, tree, and car) can be imaged by the PB-GB-SAR system. With a metallic plate moved on a positioner, it has been observed that the displacement of the target can be estimated by PB-GB-SAR with submillimeter accuracy.
    [bibtex-key = fengFriedtNicoWangMartinSatoREMOTESENSING2019PassiveBistaticGroundbasedSAR] [bibtex-entry]


  168. Xikai Fu, Bingnan Wang, Maosheng Xiang, Shuai Jiang, and Xiaofan Sun. Residual RCM Correction for LFM-CW Mini-SAR System Based on Fast-Time Split-Band Signal Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 57(7):4375-4387, July 2019. [bibtex-key = fuWangXiangJiangSunTGRS2019ResidualMotionCorrectionForLFMCWMiniSAR] [bibtex-entry]


  169. Nan Ge and Xiao Xiang Zhu. Bistatic-Like Differential SAR Tomography. IEEE Trans. Geosci. Remote Sens., pp 1-11, 2019. Keyword(s): Synthetic aperture radar, Strain, Satellites, Satellite broadcasting, Spaceborne radar, Decorrelation, Tomography, Synthetic aperture radar (SAR), SAR tomography, Tandem-L, TerraSAR-X add-on for digital elevation measurements (TanDEM-X)..
    Abstract: Motivated by prospective synthetic aperture radar (SAR) satellite missions, this paper addresses the problem of differential SAR tomography (D-TomoSAR) in urban areas using spaceborne bistatic or pursuit monostatic acquisitions. A bistatic or pursuit monostatic interferogram is not subject to significant temporal decorrelation or atmospheric phase screen and, therefore, ideal for elevation reconstruction. We propose a framework that incorporates this reconstructed elevation as deterministic prior to deformation estimation, which uses conventional repeat-pass interferograms generated from bistatic or pursuit monostatic pairs. By means of theoretical and empirical analyses, we show that this framework is, in the pursuit monostatic case, both statistically and computationally more efficient than the standard D-TomoSAR. In the bistatic case, its theoretical bound is no worse by a factor of 2. We also show that reasonable results can be obtained by using merely six TerraSAR-X add-on for digital elevation measurements (TanDEM-X) pursuit monostatic pairs, if additional spatial prior is introduced. The proposed framework can be easily extended for multistatic configurations or external sources of scatterer's elevation.
    [bibtex-key = geZhuTGRS2019BistaticLikeDiffTOMOSAR] [bibtex-entry]


  170. Changzhan Gu, Jian Wang, and Jaime Lien. Motion Sensing Using Radar: Gesture Interaction and Beyond. IEEE Microwave Magazine, 20(8):44-57, August 2019. Keyword(s): Android (operating system), artificial intelligence, gesture recognition, iOS (operating system), mobile computing, smart phones, Android smartphones, motion sensing, iOS, mobile computing platform, gesture interaction, artificial intelligence, Radar detection, Gesture recognition, Artificial intelligence, Mobile computing, Sensors, Motion detection.
    Abstract: Since the debut of iOS and Android smartphones 10 years ago, the world has seen a mobile era wherein our phones have become a mobile computing platform deeply integrated into our lives [1]-[4]. Due to the advancement of computing, it is believed that the world is shifting to a new era where artificial intelligence (AI) is unlocking capabilities that were previously unthinkable [5]-[13]. Because computing is becoming more universally available, interaction with computing devices needs to be much more natural, intuitive, and, above all, intelligent [14].
    [bibtex-key = guWangLienIEEEMWMagazine2019MotionSensingRadarGestureInteractionGOOGLEProjectSoli] [bibtex-entry]


  171. Ping He, Yangmao Wen, Caijun Xu, and Yunguo Chen. Complete three-dimensional near-field surface displacements from imaging geodesy techniques applied to the 2016 Kumamoto earthquake. Remote Sensing of Environment, 232:111321, 2019. Keyword(s): Imaging geodesy, Three-dimensional displacement, Kumamoto earthquake, SAR interferometry, Pixel offset tracking, LiDAR data.
    Abstract: The recent development of imaging geodesy, an advanced technique with a high spatial resolution and large-scale coverage, has enabled researchers to obtain multiple high-quality surface displacement estimates at low labor-cost, thereby improving the capability to monitor and manage geological disasters. The different sources (e.g., radar, optical and LiDAR sensors) and analysis approaches (e.g., differential interferometric synthetic aperture radar, DInSAR; multiple-aperture InSAR; pixel offset tracking; and iterative closest point, ICP) in imaging geodesy used to derive displacement estimates have unique benefits and drawbacks. However, the inherent differences among these data sources and methods in the construction of three-dimensional (3D) deformation maps, particularly in the near field, remain poorly understood and require further discussion. In this study, we acquired three pairs of ALOS-2 stripmap mode images, two pairs of Sentinel-1 TOPS mode images and pre- and post-event LiDAR data for the 2016 Kumamoto earthquake to explore the 3D near-field displacements using various imaging geodesy techniques with different types of image information, i.e., SAR phase data, SAR amplitude data and LiDAR point cloud data. Our results show that each image type is independently capable of producing a high-quality 3D deformation map for the 2016 Kumamoto earthquake with an on-fault accuracy of <43 cm determined from field work measurement, and an off-fault accuracy of <14 cm determined from GNSS observations. The major contributors to the uncertainty in the 3D deformation estimates from the SAR phase, SAR amplitude, and LiDAR data methods are the effective Doppler bandwidth, pixel resolution and topographic roughness, respectively. In the near-field deformation region, more secondary fault ruptures were revealed by SAR amplitude and LiDAR information than by the SAR phase information, thereby overcoming the sensitivity of the SAR phase signal to incoherence. Finally, an integrated complete 3D map was generated to constrain the coseismic rupture behavior of the Kumamoto earthquake sequence associated with the Futagawa and Hinagu faults. Our slip model suggests the main slip rupture a length of 32 km and terminated near the rim of the Aso caldera; additionally, the slip pattern delineated by three asperities that was dominated by right-lateral strike-slip with a minor normal slip component at a depth of 7 km. Furthermore, the main rupture triggered two secondary faults with right-lateral strike-slip and normal slip. Our source model yields a geodetic moment of 5.77 x 1019 Nm, which corresponds to Mw 7.11.
    [bibtex-key = heWenXuChenRSE20193DSurfaceDisplacementsFromImagingGeodesy] [bibtex-entry]


  172. Marion Heublein, Fadwa Alshawaf, Bastian Erdnüss, Xiao Xiang Zhu, and Stefan Hinz. Compressive sensing reconstruction of 3D wet refractivity based on GNSS and InSAR observations. Journal of Geodesy, 93(2):197-217, 2019. Keyword(s): SAR Processing, Compressive Sensing, Tropospheric Path Delay, Synthetic Aperture Radar, Atmospheric Modelling, Atmospheric modeling, Meteorology, radar interferometry, synthetic aperture radar (SAR), SAR Tomography, GNSS, InSAR, Tropospheric Wet Path Delay.
    Abstract: In this work, the reconstruction quality of an approach for neutrospheric water vapor tomography based on Slant Wet Delays (SWDs) obtained from Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) is investigated. The novelties of this approach are (1) the use of both absolute GNSS and absolute InSAR SWDs for tomography and (2) the solution of the tomographic system by means of compressive sensing (CS). The tomographic reconstruction is performed based on (i) a synthetic SWD dataset generated using wet refractivity information from the Weather Research and Forecasting (WRF) model and (ii) a real dataset using GNSS and InSAR SWDs. Thus, the validation of the achieved results focuses (i) on a comparison of the refractivity estimates with the input WRF refractivities and (ii) on radiosonde profiles. In case of the synthetic dataset, the results show that the CS approach yields a more accurate and more precise solution than least squares (LSQ). In addition, the benefit of adding synthetic InSAR SWDs into the tomographic system is analyzed. When applying CS, adding synthetic InSAR SWDs into the tomographic system improves the solution both in magnitude and in scattering. When solving the tomographic system by means of LSQ, no clear behavior is observed. In case of the real dataset, the estimated refractivities of both methodologies show a consistent behavior although the LSQ and CS solution strategies differ.
    [bibtex-key = heubleinAlshawafErdnussZhuHinzJournalOfGeodesy2019WetPathDelayFromCSTomoSARandGNSS] [bibtex-entry]


  173. Zhongbo Hu and Jordi J. Mallorqui. An Accurate Method to Correct Atmospheric Phase Delay for InSAR with the ERA5 Global Atmospheric Model. Remote Sensing, 11(17), 2019.
    Abstract: Differential SAR Interferometry (DInSAR) has proven its unprecedented ability and merits of monitoring ground deformation on a large scale with centimeter to millimeter accuracy. However, atmospheric artifacts due to spatial and temporal variations of the atmospheric state often affect the reliability and accuracy of its results. The commonly-known Atmospheric Phase Screen (APS) appears in the interferograms as ghost fringes not related to either topography or deformation. Atmospheric artifact mitigation remains one of the biggest challenges to be addressed within the DInSAR community. State-of-the-art research works have revealed that atmospheric artifacts can be partially compensated with empirical models, point-wise GPS zenith path delay, and numerical weather prediction models. In this study, we implement an accurate and realistic computing strategy using atmospheric reanalysis ERA5 data to estimate atmospheric artifacts. With this approach, the Line-of-Sight (LOS) path along the satellite trajectory and the monitored points is considered, rather than estimating it from the zenith path delay. Compared with the zenith delay-based method, the key advantage is that it can avoid errors caused by any anisotropic atmospheric phenomena. The accurate method is validated with Sentinel-1 data in three different test sites: Tenerife island (Spain), Almería (Spain), and Crete island (Greece). The effectiveness and performance of the method to remove APS from interferograms is evaluated in the three test sites showing a great improvement with respect to the zenith-based approach.
    [bibtex-key = huMallorquiREMOTESENSING2019CorrectAtmosphericDelayForInSARWithERA5GlobalAtmosphericModel] [bibtex-entry]


  174. C. Hu, B. Zhang, X. Dong, and Y. Li. Geosynchronous SAR Tomography: Theory and First Experimental Verification Using Beidou IGSO Satellite. IEEE Transactions on Geoscience and Remote Sensing, pp 1-17, 2019. Keyword(s): Orbits, Synthetic aperture radar, Tomography, Satellites, Perturbation methods, Radar imaging, 3-D imaging, Beidou inclined geosynchronous orbit (IGSO), geosynchronous synthetic aperture radar (GEO SAR), tomography..
    Abstract: Synthetic aperture radar (SAR) tomography (TomoSAR) techniques exploit multipass acquisitions of the same scene with slightly different view angles, and allow generating fully 3-D images, providing an estimation of scatterers' distribution along range, azimuth, and elevation directions. This paper extends TomoSAR to geosynchronous SAR (GEO TomoSAR). First, the potential and performance of GEO TomoSAR were analyzed from the perspective of orbital perturbation and the resulting large spatial baseline. Then, the rotation-induced decorrelation problems induced by the along-track baseline component were analyzed. In addition, the optimized acquisition geometry and tomographic processing flow were given, and the computer simulation verification was also completed. Finally, the equivalent validation experiment based on Beidou inclined geosynchronous orbit (IGSO) navigation satellite was carried out to demonstrate the feasibility and effectiveness of GEO TomoSAR. The experimental system employs the Beidou IGSO satellite as illuminator of opportunity and a ground system collecting and processing reflected echoes. This is the first time to employ the data from repeat-track Beidou IGSO satellites for tomographic processing. The 3-D imaging of the urban area using this experimental system was presented and then verified using LiDAR cloud data as reference. The results show that GEO TomoSAR can form the baseline of the order of hundreds of kilometers in elevation, which has the ability to achieve a resolution of 5 m in elevation.
    [bibtex-key = huZhangDongLiTGRS2019GeosynchronousSARTomography] [bibtex-entry]


  175. Yongwei Huang, Mingkang Zhou, and Sergiy A. Vorobyov. New Designs on MVDR Robust Adaptive Beamforming Based on Optimal Steering Vector Estimation. IEEE Transactions on Signal Processing, 67(14):3624-3638, July 2019. Keyword(s): SAR Processing, Beamforming, Robust Beamforming, Capon Beamformsing, MVDR, MVDR Beamforming, Direction-of-arrival estimation, DOA, SAR Tomography, Robust Adaptive Beamforming.
    Abstract: The robust adaptive beamforming design problem based on estimation of the signal-of-interest (SOI) steering vector is considered in the paper. The common criteria to find the best estimate of the steering vector are the beamformer output signal-to-noise-plus-interference ratio (SINR) and output power, while the constraints assume as little as possible prior inaccurate knowledge about the SOI, the propagation media, and the antenna array. Herein, in order to find the optimal steering vector, a beamformer output power maximization problem is formulated and solved subject to a double-sided norm perturbation constraint, a similarity constraint, and a quadratic constraint that guarantees that the direction-of-arrival (DOA) of the SOI is away from the DOA region of all linear combinations of the interference steering vectors. The prior knowledge required is some allowable error norm bounds and approximate knowledge of the antenna array geometry and angular sector of the SOI. It turns out that the array output power maximization problem is a non-convex quadratically constrained quadratic programming problem with inhomogeneous constraints. However, we show that the problem is still solvable, and develop efficient algorithms for finding globally optimal estimate of the SOI steering vector. The results are generalized to the case when an ellipsoidal constraint is considered instead of the similarity constraint, and sufficient conditions for the global optimality are derived. In addition, a new quadratic constraint on the actual signal steering vector is proposed in order to improve the array performance. To validate our results, simulation examples are presented, and they demonstrate the improved performance of the new robust beamformers in terms of the output SINR as well as the output power.
    [bibtex-key = huangZhouVorobyovTSP2019NewDesignsOnMVDRCaponRobustAdaptiveBeamformingBasedOnOptimalSteeringVectorEstimation] [bibtex-entry]


  176. Guodong Jin, Kaiyu Liu, Yunkai Deng, Yu Sha, Robert Wang, Dacheng Liu, Wei Wang, Yajun Long, and Yongwei Zhang. Nonlinear Frequency Modulation Signal Generator in LT-1. IEEE Geoscience and Remote Sensing Letters, 16(10):1570-1574, October 2019. Keyword(s): Signal generators, Frequency modulation, Synthetic aperture radar, Field programmable gate arrays, Complexity theory, Matrix decomposition, Signal to noise ratio, Nonlinear frequency modulation (NLFM) signal, signal generator, synthetic aperture radar (SAR).
    Abstract: Generally, synthetic aperture radar (SAR) system transmits linear frequency modulation (LFM) signal to obtain the high-resolution image and weighted windowing is usually employed to suppress sidelobes. However, it will cause a 1-2-dB signal-to-noise ratio (SNR) loss. Nonlinear frequency modulation (NLFM) signal, which can construct the signal's power spectral density (PSD) to reduce sidelobes without loss of SNR, is a promising candidate. However, the real-time generation of precise NLFM signal is still a technical challenge. In this letter, a high-precision NLFM signal generator with the ability of predistortion compensation is developed, and this signal generator will be employed in LuTan-1 (LT-1, i.e., TwinSAR-L) mission which is an innovative spaceborne bistatic SAR mission and planned to launch in 2020. In addition, a two-step error compensation method is developed to compensate the system error. Finally, the ground experiment is performed to validate the designed signal generator.
    [bibtex-key = Jin2019] [bibtex-entry]


  177. Unmesh Khati, Marco Lavalle, and Gulab Singh. Spaceborne tomography of multi-species Indian tropical forests. Remote Sensing of Environment, 229:193-212, 2019. Keyword(s): SAR Processing, SAR Tomography, TomoSAR, Tomography, TanDEM-X, TerraSAR-X, Tropical, Forest, India, Spaceborne SAR, X-band.
    Abstract: Synthetic Aperture Radar (SAR) backscatter is the coherent combination of the scattering from multiple individual scatterers within the radar resolution cell, which results into a 2-D radar image. Tomographic SAR (TomoSAR) takes advantage of multiple SAR acquisitions to provide 3-D vertical structure of the imaged target. Over forests, parameters such as canopy structure, canopy density, leaf area and phenology contribute to different SAR scattering mechanisms along the vertical dimension of the trees. Tomography has been demonstrated in various earlier works for forest bio-physical parameter estimation using airborne data. In this research work, multi-polarimetric space-borne TanDEM-X tomograms are examined for the first time over a multi-species Indian tropical forest. Multiple TanDEM-X acquisitions are focused using Capon beamforming at five polarimetric channels HH, HV, VV, HH-VV, HH+VV. Four distinct forest species compartments are selected representing different canopy structure and density. The tomograms obtained in different polarizations for these species are analyzed in detail to understand the scattering patterns across different forest species. Field surveys carried out in several forest locations provide in situ observations of forest height and vertical structure. It was observed that canopy gaps and leaf density play a crucial role for X-band SAR signal penetration through ground. For dense canopy species the backscatter contributions are spread through the canopy, while in case of sparse canopy species, the ground scattering is dominant. Vertical profiles obtained at surveyed plot locations are plotted in all polarizations, and provide a good agreement with field observations. Further, the obtained TomoSAR backscatter layers have a good correlation with field measured above-ground biomass (AGB). The AGB is modeled from TomoSAR with the HH-pol TomoSAR backscatter layer at 27m canopy height leading to an AGB estimation with correlation r=0.76 and RMSE of 50.4t/ha.
    [bibtex-key = khatiLavalleSinghRSE2019SpaceborneSARTomographyIndianForest] [bibtex-entry]


  178. Franziska Koch, Patrick Henkel, Florian Appel, Lino Schmid, Heike Bach, Markus Lamm, Monika Prasch, J�rg Schweizer, and Wolfram Mauser. Retrieval of Snow Water Equivalent, Liquid Water Content, and Snow Height of Dry and Wet Snow by Combining GPS Signal Attenuation and Time Delay. Water Resources Research, 55(5):4465-4487, 2019. Keyword(s): GNSS, snow water equivalent, liquid water content, snow cover, SnowSense, GNSS signals, Global Positioning System.
    Abstract: Abstract For numerous hydrological applications, information on snow water equivalent (SWE) and snow liquid water content (LWC) are fundamental. In situ data are much needed for the validation of model and remote sensing products; however, they are often scarce, invasive, expensive, or labor-intense. We developed a novel nondestructive approach based on Global Positioning System (GPS) signals to derive SWE, snow height (HS), and LWC simultaneously using one sensor setup only. We installed two low-cost GPS sensors at the high-alpine site Weissfluhjoch (Switzerland) and processed data for three entire winter seasons between October 2015 and July 2018. One antenna was mounted on a pole, being permanently snow-free; the other one was placed on the ground and hence seasonally covered by snow. While SWE can be derived by exploiting GPS carrier phases for dry-snow conditions, the GPS signals are increasingly delayed and attenuated under wet snow. Therefore, we combined carrier phase and signal strength information, dielectric models, and simple snow densification approaches to jointly derive SWE, HS, and LWC. The agreement with the validation measurements was very good, even for large values of SWE (>1,000 mm) and HS (> 3 m). Regarding SWE, the agreement (root-mean-square error (RMSE); coefficient of determination (R2)) for dry snow (41 mm; 0.99) was very high and slightly better than for wet snow (73 mm; 0.93). Regarding HS, the agreement was even better and almost equally good for dry (0.13 m; 0.98) and wet snow (0.14 m; 0.95). The approach presented is suited to establish sensor networks that may improve the spatial and temporal resolution of snow data in remote areas.
    [bibtex-key = kochEtAlWaterResourcesResearch2019GNSSBasedSnowWaterEquivalentLiquidWaterContentAndSnowHeightDryAndWetSnow] [bibtex-entry]


  179. Martina Lagasio, Antonio Parodi, Luca Pulvirenti, Agostino N. Meroni, Giorgio Boni, Nazzareno Pierdicca, Frank S. Marzano, Lorenzo Luini, Giovanna Venuti, Eugenio Realini, Andrea Gatti, Giulio Tagliaferro, Stefano Barindelli, Andrea Monti Guarnieri, Klodiana Goga, Olivier Terzo, Alessio Rucci, Emanuele Passera, Dieter Kranzlmueller, and Bjorn Rommen. A Synergistic Use of a High-Resolution Numerical Weather Prediction Model and High-Resolution Earth Observation Products to Improve Precipitation Forecast. Remote Sensing, 11(20), 2019. Keyword(s): SAR Processing, Weather, Forecasting, Precipitation Forecasting, SAR, GNSS, Zenith Path Delay, Troposphere.
    Abstract: The Mediterranean region is frequently struck by severe rainfall events causing numerous casualties and several million euros of damages every year. Thus, improving the forecast accuracy is a fundamental goal to limit social and economic damages. Numerical Weather Prediction (NWP) models are currently able to produce forecasts at the km scale grid spacing but unreliable surface information and a poor knowledge of the initial state of the atmosphere may produce inaccurate simulations of weather phenomena. The STEAM (SaTellite Earth observation for Atmospheric Modelling) project aims to investigate whether Sentinel satellites constellation weather observation data, in combination with Global Navigation Satellite System (GNSS) observations, can be used to better understand and predict with a higher spatio-temporal resolution the atmospheric phenomena resulting in severe weather events. Two heavy rainfall events that occurred in Italy in the autumn of 2017 are studied--a localized and short-lived event and a long-lived one. By assimilating a wide range of Sentinel and GNSS observations in a state-of-the-art NWP model, it is found that the forecasts benefit the most when the model is provided with information on the wind field and/or the water vapor content.
    [bibtex-key = lagasioEtAlInclPierdiccaTagliaferroMontiGuarnieriRommenREMOTESENSING2019SynergisticUseOfHighResNumericalWeatherPredictionAndEarthObservationToImprovePrecipitationForecast] [bibtex-entry]


  180. Michele Manunta, Claudio De Luca, Ivana Zinno, Francesco Casu, Mariarosaria Manzo, Manuela Bonano, Adele Fusco, Antonio Pepe, Giovanni Onorato, Paolo Berardino, Prospero De Martino, and Riccardo Lanari. The Parallel SBAS Approach for Sentinel-1 Interferometric Wide Swath Deformation Time-Series Generation: Algorithm Description and Products Quality Assessment. IEEE Transactions on Geoscience and Remote Sensing, 57(9):6259-6281, Sep. 2019.
    Abstract: We present an advanced differential synthetic aperture radar (SAR) interferometry (DInSAR) processing chain, based on the Parallel Small BAseline Subset (P-SBAS) technique, for the efficient generation of deformation time series from Sentinel-1 (S-1) interferometric wide (IW) swath SAR data sets. We first discuss an effective solution for the generation of high-quality interferograms, which properly accounts for the peculiarities of the terrain observation with progressive scans (TOPS) acquisition mode used to collect S-1 IW SAR data. These data characteristics are also properly accounted within the developed processing chain, taking full advantage from the burst partitioning. Indeed, such data structure represents a key element in the proposed P-SBAS implementation of the S-1 IW processing chain, whose migration into a cloud computing (CC) environment is also envisaged. An extensive experimental analysis, which allows us to assess the quality of the obtained interferometric products, is presented. To do this, we apply the developed S-1 IW P-SBAS processing chain to the overall archive acquired from descending orbits during the March 2015-April 2017 time span over the whole Italian territory, consisting in 2740 S-1 slices. In particular, the quality of the final results is assessed through a large-scale comparison with the GPS measurements relevant to nearly 500 stations. The mean standard deviation value of the differences between the DInSAR and the GPS time series (projected in the radar line of sight) is less than 0.5 cm, thus confirming the effectiveness of the implemented solution. Finally, a discussion about the performance achieved by migrating the developed processing chain within the Amazon Web Services CC environment is addressed, highlighting that a two-year data set relevant to a standard S-1 IW slice can be reliably processed in about 30 h.The presented results demonstrate the capability of the implemented P-SBAS approach to efficiently and effectively process large S-1 IW data sets relevant to extended portions of the earth surface, paving the way to the systematic generation of advanced DInSAR products to monitor ground displacements at a very wide spatial scale.
    [bibtex-key = manuntaEtAlLanariTGRS2022ParallelSBASforSentinel1DeformationTimeSeriesGenerationAlgorithmDescriptionAndProductQualityAssessment] [bibtex-entry]


  181. Mauro Mariotti d'Alessandro and Stefano Tebaldini. Cross Sensor Simulation of Tomographic SAR Stacks. Remote Sensing, 11(18), 2019. Keyword(s): SAR Processing, tomography, SAR tomography, Simulation, Wavenumbers, orbits, biomass, synthetic aperture radar, P-band.
    Abstract: This paper presents an algorithm for simulating tomographic synthetic aperture radar (SAR) data based on another stack actually gathered by a real acquisition system. Through the procedure here proposed, the simulated system can be evaluated according to its capability to image complex natural media rather than reference point targets. This feature is particularly important whenever the biophysical properties of the target of interest must be preserved and cannot be easily modeled. The system to be simulated may be different from the original one concerning resolution, off-nadir angles, bandwidth and central frequency. The algorithm here proposed handles these differences by properly taking into account the wavenumbers of the target illuminated by the real survey and requested by the simulated one. The complex images constituting the synthetic stack are associated with the effective vertical interferometric wavenumber peculiar of the geometry to be simulated, regardless of the original data. Furthermore, the three-dimensional resolution cell of the simulated tomographic system is consistent with the simulated geometry concerning size and spatial orientation. These two latter features cannot be guaranteed by simply filtering the original stack. The simulator here proposed has been used to simulate the tomographic stack expected from the forthcoming European Space Agency (ESA) BIOMASS mission. The relationship between baseline distribution and 3D focusing capability was explored; special attention has been paid to the robustness of tomographic power at being a good proxy for the above ground biomass in tropical regions.
    [bibtex-key = mariottiDAlessandroTebaldiniRemoteSensing2019CrossSensorTomoStackSimulation] [bibtex-entry]


  182. Albert R. Monteith, Lars M. H. Ulander, and Stefano Tebaldini. Calibration of a Ground-Based Array Radar for Tomographic Imaging of Natural Media. Remote Sensing, 11(24), 2019. Keyword(s): SAR Tomography, BorealScat.
    Abstract: Ground-based tomographic radar measurements provide valuable knowledge about the electromagnetic scattering mechanisms and temporal variations of an observed scene and are essential in preparation for space-borne tomographic synthetic aperture radar (SAR) missions. Due to the short range between the radar antennas and a scene being observed, the tomographic radar observations are affected by several systematic errors. This article deals with the modelling and calibration of three systematic errors: mutual antenna coupling, magnitude and phase errors and the pixel-variant impulse response of the tomographic image. These errors must be compensated for so that the tomographic images represent an undistorted rendering of the scene reflectivity. New calibration methods were described, modelled and validated using experimental data. The proposed methods will be useful for future ground-based tomographic radar experiments in preparation for space-borne SAR missions.
    [bibtex-key = monteithUlanderTebaldiniRemoteSensing2019CalibrationOfGBArrayRadarForTomographicImaging] [bibtex-entry]


  183. Elias Méndez Domìnguez, Christophe Magnard, Erich Meier, David Small, Michael E. Schaepman, and Daniel Henke. A Back-Projection Tomographic Framework for VHR SAR Image Change Detection. IEEE Transactions on Geoscience and Remote Sensing, 57(7):4470-4484, July 2019. Keyword(s): Synthetic aperture radar, Tomography, Backscatter, Apertures, Laser radar, Image resolution, Detectors, Image processing, Markov processes, synthetic aperture radar (SAR), tomography, urban areas.
    Abstract: Information on 3-D structure expands the scope of change detection applications, for example, in urban studies, human activity, and forest monitoring. Current change detection methods do not fully consider the specifics of SAR data or the properties of the corresponding image focusing techniques. We propose a three-stage method complementing the properties of 2-D and 3-D very high-resolution (VHR) synthetic aperture radar imagery to improve the performance of 2-D only approaches. The method takes advantage of back-projection tomography to ease translation of the 2-D location of the targets into their corresponding 3-D location and vice versa. Detection of changes caused by objects with a small vertical extent is based on the corresponding backscatter difference, while changes caused by objects with a large vertical extent are detected with both backscatter and height difference information combined in a conditional random field. Using multitemporal images, the kappa coefficient improved by a factor of two in comparison with traditional schemes.
    [bibtex-key = dominguezMagnardMeierSmallSchaepmanHenke2019TDBPTomographyforVHRSARChangeDetection] [bibtex-entry]


  184. Stephan Palm, Rainer Sommer, Daniel Janssen, Axel Tessmann, and Uwe Stilla. Airborne Circular W-Band SAR for Multiple Aspect Urban Site Monitoring. IEEE Transactions on Geoscience and Remote Sensing, 57(9):6996-7016, September 2019.
    Abstract: This paper presents a strategy for urban site monitoring by very high-resolution circular synthetic aperture radar (CSAR) imaging of multiple aspects. We analytically derive the limits of coherent azimuth processing for nonplanar objects in CSAR if no digital surface model (DSM) is available. The result indicates the level of maximum achievable resolution of these objects in this geometry. The difficulty of constantly illuminating a specific scene in full aspect mode (360 deg) for such small wavelengths is solved by a hardware- and software-side integration of the radar in a mechanical tracking mode. This results in the first demonstration of full aspect airborne subaperture CSAR images collected with an active frequency-modulated continuous wave (FMCW) radar at W-band. We describe the geometry and the implementation of the real-time beam-steering mode and evaluate resulting effects in the CSAR processing chain. The physical properties in W-band allow the use of extremely short subapertures in length while generating high azimuthal bandwidths. We use this feature to generate full aspect image stacks for CSAR video monitoring in very high frame rates. This technique offers the capability of detecting and observing moving objects in single channel data by shadow tracking. Due to the relatively strong echo of roads, the shadows of moving cars are rich in contrast. The image stack is further evaluated to present wide angular anisotropic properties of targets and first results on multiple aspect image fusion. Both topics show huge potential for further investigations in terms of image analysis and scene classification.
    [bibtex-key = palmSommerJanssenTessmannStillaTGRS2019AirborneCircularWBandSAR] [bibtex-entry]


  185. Matteo Pardini, John Armston, Wenlu Qi, Seung Kuk Lee, Marivi Tello, Victor Cazcarra Bes, Changhyun Choi, Konstantinos P. Papathanassiou, Ralph O. Dubayah, and Lola E. Fatoyinbo. Early Lessons on Combining Lidar and Multi-baseline SAR Measurements for Forest Structure Characterization. Surveys in Geophysics, 40(4):803-837, July 2019. Keyword(s): SAR Processing, Forest, Biomass, LiDAR, SAR Tomography, Multibaseline SAR.
    Abstract: The estimation and monitoring of 3D forest structure at large scales strongly rely on the use of remote sensing techniques. Today, two of them are able to provide 3D forest structure estimates: lidar and synthetic aperture radar (SAR) configurations. The differences in wavelength, imaging geometry, and technical implementation make the measurements provided by the two configurations different and, when it comes to the sensitivity to individual 3D forest structure components, complementary. Accordingly, the potential of combining lidar and SAR measurements toward an improved 3D forest structure estimation has been recognised from the very beginning. However, until today there is no established framework for this combination. This paper attempts to review differences, commonalities, and complementarities of lidar and SAR measurements. First, vertical lidar reflectance and SAR reflectivity profiles at different wavelengths are compared in different forest types. Then, current perspectives on their combination for the generation of enhanced structure products are discussed. Two promising frameworks for combining lidar and SAR measurements are reviewed. The first one is a model-based framework where lidar-derived parameters are used to initialize SAR scattering models, and relies on both the validity of the models and on the physical equivalence of the used lidar and SAR parameters. The second one is a structure-based framework based on the ability of lidar and SAR measurements to express physical forest structure by means of appropriate indices. These indices can then be used to establish a link between the two kind of measurements. The review is supported by experimental results achieved using space- and airborne data acquired in recent relevant mission and campaigns.
    [bibtex-key = pardiniEtAlSURVEYGeophysics2019CombiningLiDARandTomoSARForestStructure] [bibtex-entry]


  186. Massimiliano Pieraccini and Lapo Miccinesi. Ground-Based Radar Interferometry: A Bibliographic Review. Remote Sensing, 11(9):1029, April 2019.
    Abstract: Ground-based/terrestrial radar interferometry (GBRI) is a scientific topic of increasing interest in recent years. This article is a bibliographic review, as much complete as possible, of the scientific papers/articles published in the last 20 years, since the pioneering works in the nineties. Some statistics are reported here about the number of publications in the years, popularity of applications, operative modalities, operative bands. The aim of this review is also to identify directions and perspectives. In the opinion of authors, this type of radar systems will move forward faster modulations, wider view angle, MIMO (Multiple Input Multiple Output) systems and radar with capability to detect the vector of displacement and not only a single component.
    [bibtex-key = pieracciniMiccinesiRemoteSensing2019GroundBasedRadarInterferometryABibliographicReview] [bibtex-entry]


  187. A. Rinke, B. Segger, S. Crewell, M. Maturilli, T. Naakka, T. Nygård, T. Vihma, F. Alshawaf, G. Dick, J. Wickert, and A.J. Keller. Trends of vertically integrated water vapor over the Arctic during 1979–2016: Consistent moistening all over?. Journal of Climate, 32(18):6097-6116, 2019. Note: Cited By 24. [bibtex-key = Rinke20196097] [bibtex-entry]


  188. Giuseppe Ruzza, Luigi Guerriero, Gerardo Grelle, Francesco Maria Guadagno, and Paola Revellino. Multi-Method Tracking of Monsoon Floods Using Sentinel-1 Imagery. Water, 11(11), 2019. Keyword(s): SAR Processing, Flood Mapping, Sentinel-1, Water.
    Abstract: Floods cause great losses in terms of human life and damages to settlements. Since the exposure is a proxy of the risk, it is essential to track flood evolution. The increasing availability of Synthetic Aperture Radar (SAR) imagery extends flood tracking capabilities because of its all-water and day/night acquisition. In this paper, in order to contribute to a better evaluation of the potential of Sentinel-1 SAR imagery to track floods, we analyzed a multi-pulse flood caused by a typhoon in the Camarines Sur Province of Philippines between the end of 2018 and the beginning of 2019. Multiple simple classification methods were used to track the spatial and temporal evolution of the flooded area. Our analysis indicates that Valley Emphasis based manual threshold identification, Otsu methodology, and K-Means Clustering have the potential to be used for tracking large and long-lasting floods, providing similar results. Because of its simplicity, the K-Means Clustering algorithm has the potential to be used in fully automated operational flood monitoring, also because of its good performance in terms of computation time.
    [bibtex-key = ruzzaGuerrieroGrelleGuadagnoRevellinoWATER2019Sentinel1MonsoonFloodsMapping] [bibtex-entry]


  189. N. Sakar, M. Rodriguez-Cassola, P. Prats-Iraola, and A. Moreira. Azimuth Reconstruction Algorithm for Multistatic SAR Formations With Large Along-Track Baselines. IEEE Transactions on Geoscience and Remote Sensing, pp 1-10, 2019. Keyword(s): Azimuth, Synthetic aperture radar, Image reconstruction, History, Receivers, Doppler effect, Reconstruction algorithms, Digital beamforming, high-resolution wide-swath (HRWS) radar, multistatic radar signal processing, synthetic aperture radar (SAR).
    Abstract: A multistatic synthetic aperture radar (SAR) system offers the potential of exceeding the capabilities of conventional SARs in various ways, one of which is to realize high-resolution imaging over wide swaths when operated under the Nyquist frequency. Due to the decrease of the operational pulse repetition frequency, the Doppler spectrum of the received data of the single channels appears strongly aliased and needs to be resolved via azimuth reconstruction. Our aim in this article is to suggest an accurate reconstruction strategy that is applicable to multistatic SAR systems also with large along-track baselines and very high resolution on curved orbits. The algorithm, which can be regarded as a generalized reconstruction in the range-Doppler domain, operates in two steps and is capable of correcting most of the polychromatic deviations over large areas, achieving accurate reconstruction in constellations with kilometric baselines, resolutions of about 15 lambda over swaths of hundreds of kilometers. The validity of our approach has been tested using point targets in two- and six-receiver multistatic configurations.
    [bibtex-key = sakarRodriguezPratsMoreiraTGRS2019AzimuthReconstructionMultistaticSARWithLargeAlongTrackBaselines] [bibtex-entry]


  190. Francescopaolo Sica, Andrea Pulella, Matteo Nannini, Muriel Pinheiro, and Paola Rizzoli. Repeat-pass SAR interferometry for land cover classification: A methodology using Sentinel-1 Short-Time-Series. Remote Sensing of Environment, 232:111277, 2019. Keyword(s): Land cover classification, SAR, Interferometric coherence, Sentinel-1, Temporal decorrelation.
    Abstract: In this paper we explore the potential of repeat-pass interferometric SAR (InSAR) for land cover classification purposes. We introduce a novel approach for the generation of large-scale thematic maps, based on the use of multi-temporal data, acquired over short observation intervals (short-time-series). A larger interferometric coherence loss is expected with the increasing time difference between two interferometric acquisitions. This phenomenon is normally indicated as temporal decorrelation whose amount differs depending on the type of imaged target on ground. The basic idea is therefore to accurately model the evolution in time of the temporal decorrelation and use the estimated parameters, together with backscatter, as input features for the Random Forest machine learning classification algorithm. The work has been carried out on the case study of land cover mapping over central Europe, considering Sentinel-1 C-band interferometric stacks, acquired over a time span of about one month. Three different land cover classes have been considered: artificial surfaces as e.g. urban areas, forests, and non-forested areas as the ensemble of low vegetation, bare soil, and agricultural areas. The results show a level of agreement above 91%, when compared to the CORINE land cover map product of 2012, which has been used as external reference for both training and testing of the classification algorithm.
    [bibtex-key = sicaPulellaNanniniPinheiroRizzoliRSE2019InSARforLandCoverClassificationSentinel1ShortTimeSeries] [bibtex-entry]


  191. Muhammad Adnan Siddique, Tazio Strozzi, Irena Hajnsek, and Othmar Frey. A Case Study on the Correction of Atmospheric Phases for SAR Tomography in Mountainous Regions. IEEE Trans. Geosci. Remote Sens., 57(1):416-431, January 2019. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, synthetic aperture radar, tomography, 3-D point cloud retrieval, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne SAR, Cosmo SkyMed, Matter Valley, Switzerland, Alps, mountainous terrain, layover, layover separation, interferometric stack, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry, deformation, displacement, subsidence, detection, radar interferometry, displacement mapping, spaceborne SAR, differential interferometry, differential tomography, coherent scatterer detection.
    Abstract: Synthetic aperture radar (SAR) tomography with repeat-pass acquisitions generally requires a priori phase calibration of the interferometric data stack by compensating for the atmosphere-induced phase delay variations. These variations act as a disturbance in tomographic focusing. In mountainous regions, the mitigation of these disturbances is particularly challenging due to strong spatial variations of the local atmospheric conditions and propagation paths through the troposphere. In this paper, we assess a data-driven approach to estimate these phase variations under a regression-kriging framework. The vertical stratification of the troposphere is modeled functionally while the impact of spatial turbulence is considered in a stochastic sense. The methodology entails an initial persistent scatterer interferometry (PSI) analysis. The atmospheric phases isolated for the persistent scatterers (PS) within the PSI processing are considered as samples of the 3-D distribution of the phase delay variations over the scene. These atmospheric phases are regressed against the spatial coordinates in map geometry at PS locations. In turn, kriging predictions are obtained at each location along the elevation profile where tomographic focusing is intended. A key point of this approach is that the requisite atmospheric corrections are incorporated within the tomographic focusing model. A case study has been performed on a data stack comprising 32 Cosmo-SkyMed stripmap images acquired over the Matter Valley in the Swiss Alps, in the summers of 2008-2013. The results show locally improved deformation sampling with tomographic methods compared to the initial PSI solution, primarily due to the improved phase calibration. In general, the work underscores the indispensability of height-dependent correction of atmospheric phases for SAR tomography.
    [bibtex-key = siddiqueStrozziHajnsekFreyTGRS2019AtmoCorrectionTomoPsiMountains] [bibtex-entry]


  192. Ladina Steiner, Michael Meindl, and Alain Geiger. Characteristics and limitations of GPS L1 observations from submerged antennas. Journal of Geodesy, 93(2):267-280, February 2019. Keyword(s): GNSS, GPS, Snow-water equivalent, SWE, Submerged antennas.
    Abstract: Observations from a submerged GNSS antenna underneath a snowpack need to be analyzed to investigate its potential for snowpack characterization. The magnitude of the main interaction processes involved in the GPS L1 signal propagation through different layers of snow, ice, or freshwater is examined theoretically in the present paper. For this purpose, the GPS signal penetration depth, attenuation, reflection, refraction as well as the excess path length are theoretically investigated. Liquid water exerts the largest influence on GPS signal propagation through a snowpack. An experiment is thus set up with a submerged geodetic GPS antenna to investigate the influence of liquid water on the GPS observations. The experimental results correspond well with theory and show that the GPS signal penetrates the liquid water up to three centimeters. The error in the height component due to the signal propagation delay in water can be corrected with a newly derived model. The water level above the submerged antenna could also be estimated.
    [bibtex-key = steinerMeindlGeigerJGeodesy2019GNSSL1observationsSubmergedAntennas] [bibtex-entry]


  193. Meng Sun, Jingjing Pan, Cedric Le Bastard, Yide Wang, and Jianzhong Li. Advanced Signal Processing Methods for Ground-Penetrating Radar: Applications to Civil Engineering. IEEE Signal Processing Magazine, 36(4):74-84, July 2019. Keyword(s): Ground-penetrating radar, GPR, GPR Signal Processing, SAR Processing, Civil Engineering Applications.
    Abstract: Ground-penetrating radar (GPR) is a common technique for evaluating the structure and quality of civil engineering materials. The ever-increasing demand for higher GPR time resolution and better interpretation of GPR data has motivated the use of advanced signal processing methods for GPR applications. In this article, we review the major advances in signal processing techniques employed in civil engineering for different tasks, such as estimation of thickness, permittivity, and roughness. Their performance is tested and compared through numerical testing and using experimental data from laboratory measurements.
    [bibtex-key = sunEtAlIEEESignalProcessinMag2019GroundPenetratingRadar] [bibtex-entry]


  194. Stefano Tebaldini, Dinh Ho Tong Minh, Mauro Mariotti d'Alessandro, Ludovic Villard, Thuy Le Toan, and Jerome Chave. The Status of Technologies to Measure Forest Biomass and Structural Properties: State of the Art in SAR Tomography of Tropical Forests. Surveys in Geophysics, May 2019. Keyword(s): SAR Processing, SAR Tomography, BIOMASS, Earth Explorer 7, EE7, Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, scattering, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation, Spaceborne SAR.
    Abstract: Synthetic aperture radar (SAR) tomography (TomoSAR) is an emerging technology to image the 3D structure of the illuminated media. TomoSAR exploits the key feature of microwaves to penetrate into vegetation, snow, and ice, hence providing the possibility to see features that are hidden to optical and hyper-spectral systems. The research on the use of P-band waves, in particular, has been largely propelled since 2007 in experimental studies supporting the future spaceborne Mission BIOMASS, to be launched in 2022 with the aim of mapping forest aboveground biomass (AGB) accurately and globally. The results obtained in the frame of these studies demonstrated that TomoSAR can be used for accurate retrieval of geophysical variables such as forest height and terrain topography and, especially in the case of dense tropical forests, to provide a more direct link to AGB. This paper aims at providing the reader with a comprehensive understanding of TomoSAR and its application for remote sensing of forested areas, with special attention to the case of tropical forests. We will introduce the basic physical principles behind TomoSAR, present the most relevant experimental results of the last decade, and discuss the potentials of BIOMASS tomography.
    [bibtex-key = tebaldiniEtAlSurveyInGeophysics2019SARTomographyTropicalForestReviewPaper] [bibtex-entry]


  195. Aaron Thompson, Richard Kelly, and Joshua King. Sensitivity of Ku- and X-Band Radar Observations to Seasonal Snow in Ontario, Canada. Canadian Journal of Remote Sensing, 45(6):829-846, 2019. Keyword(s): Microwave remote sensing, radar, radar remote sensing, Snow, Snow Water Equivalent (SWE), remote sensing of snow, seasonal snow, Ku-band, X-band.
    Abstract: Radar scatterometer observations at 17.2GHz and 9.6GHz were made of the snow cover in mid-latitude agricultural fields, using the University of Waterloo scatterometer, to determine the sensitivity of the frequency-dependent radar response to snow water equivalent. Observations were made in alfalfa fields near Maryhill, Ontario during the 2013-2014 and 2014-2015 winter seasons. Additional observations in the 2014-2015 season were made at Englehart, Ontario. The natural snowpack was scanned using polarimetric observations throughout the season as snow accumulated. Conditions typical of the mid-latitude snow environment, including non-frozen ground, snow melt, freezing rain, and agricultural vegetation were observed to confound the relationship between backscatter and SWE or snow depth. With these special cases removed from the analysis, the strongest relationship is observed at 17.2GHz where VV-polarized backscatter increased linearly by 0.35dB/cm SWE. Further, the backscatter increases linearly by 0.10dB/cm of snow depth. Relationships at 9.6GHz are weaker. These findings are unique. They highlight the suitability and challenges of radar remote sensing for estimating snow accumulation in a mid-latitude environment and demonstrate the use of polarization signatures for identifying the effects of short vegetation prevalent in thisenvironment.
    [bibtex-key = thompsonKellyKingCJRS2019SensitvityOfKuAndXbandRadarObservationsToSeasonalSnow] [bibtex-entry]


  196. Jan Torgrimsson, Patrick Dammert, Hans Hellsten, and Lars M. H. Ulander. SAR Processing Without a Motion Measurement System. IEEE Transactions on Geoscience and Remote Sensing, 57(2):1025-1039, February 2019. Keyword(s): SAR Processsing, Backprojection, Fast-factorized Back-projection, FFBP, Time-Domain Back-Projection, TDBP, Azimuth Focusing, Motion Compensation, MoComp, autofocus, geometric autofocus, radar imaging, synthetic aperture radar, synthetic aperture radar image, very high frequency band, base-2 fast factorized back-projection, track velocity error, CARABAS II system, ultrawideband data sets, innovative autofocus concept, subaperture pair, free geometry parameters, back-projection formulation, factorized geometrical autofocus, SAR processing, FGA algorithm, VHF-band, wavelength-resolution SAR image, FGA images, linear equidistant track, basic geometry model, Geometry, Synthetic aperture radar, Global Positioning System, Tracking, Apertures, Radar tracking, Autofocus, back-projection (BP), factorized geometrical autofocus (FGA), Synthetic Aperture Radar (SAR).
    Abstract: This paper leads a discussion on how to form a Synthetic Aperture Radar (SAR) image without knowing the relative track. That is, within the scope of factorized geometrical autofocus (FGA). The FGA algorithm is a base-2 fast factorized back-projection (FFBP) formulation with six free geometry parameters (per subaperture pair). These are tuned step by step until a sharp image is obtained. This innovative autofocus concept can compensate completely for an erroneous geometry. The FGA algorithm has been applied successfully on two ultrawideband (UWB) data sets, acquired by the CARABAS II system at very high frequency (VHF)-band. The relative tracks are known (measured accurately). We, however, adopt and modify a basic geometry model. A linear equidistant track at fixed altitude is initially assumed. Apart from deviations due to linearization, a ~2.5-m/s along-track velocity error is also introduced. Resulting FGA images are compared to reference images and verified to be focused. This indicates that it is feasible to form a wavelength-resolution SAR image at VHF-band without support from a motion measurement system. The execution time for the examples in this paper is about five times longer with autofocus than without. Hence, the FGA algorithm is now fit for use on a regular basis.
    [bibtex-key = torgrimssonDammertHellstenUlanderTGRS2019SARProcessingGeometricFFBPWithoutMotionMeasurementSystem] [bibtex-entry]


  197. Karina Wilgan, Muhammad Adnan Siddique, Tazio Strozzi, Alain Geiger, and Othmar Frey. Comparison of Tropospheric Path Delay Estimates from GNSS and Space-Borne SAR Interferometry in Alpine Conditions. Remote Sensing, 11(15):1-24, July 2019. Note: 1789. Keyword(s): SAR Processing, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas, alpine, rugged terrain, atmospheric phase, atmospheric phase screen, APS, mitigation of atmospheric phase, turbulent atmospheric phase in alpine areas, Cosmo-SkyMed, Zermatt, Mattertal, Matter valley, Switzerland, multi-baseline interferometry, GNSS, GPS, Comparison, tropospheric path delay, Collocation, Kriging.
    Abstract: We compare tropospheric delays from Global Navigation Satellite Systems (GNSS) and Synthetic Aperture Radar (SAR) Interferometry (InSAR) in a challenging mountainous environment in the Swiss Alps, where strong spatial variations of the local tropospheric conditions are often observed.Tropospheric delays are usually considered to be an error for both GNSS and InSAR, and are typically removed. However, recently these delays are also recognized as a signal of interest, for example for assimilation into numerical weather models or climate studies. The GNSS and InSAR are techniques of complementary nature, as one has sparse spatial but high temporal resolution, and the other very dense spatial coverage but repeat pass of only a few days. This raises expectations for a combination of these techniques. For this purpose, a comprehensive comparison between the techniques must be first performed. Due to the relative nature of InSAR estimates, we compare the difference slant tropospheric delays (dSTD) retrieved from GNSS with the dSTDs estimated using Persistent ScattererInterferometry (PSI) of 32 COSMO-SkyMed SAR images taken in a snow-free period from June to October between 2008 and 2013. The GNSS estimates calculated at permanent geodetic stations are interpolated to the locations of persistent scatterers using an in-house developed least-squares collocation software COMEDIE. The Pearson's correlation coefficient between InSAR and GNSS estimates averaged over all acquisitions is equal to 0.64 and larger than 0.8 for approximately half of the layers. Better agreement is obtained mainly for days with high variability of the troposphere(relative to the tropospheric conditions at the time of the reference acquisition), expressed as standard deviations of the GNSS-based dSTDs. On the other hand, the most common feature for the days with poor agreement is represented by very stable, almost constant GNSS estimates. In addition,there is a weak correlation between the agreement and the water vapor values in the area, as well as with the number of stations in the closest vicinity of the study area. Adding low-cost L-1 only GPS stations located within the area of the study increases the biases for most of the dates, but the standard deviations between InSAR and GNSS decrease for the limited area with low-cost stations.
    [bibtex-key = wilganSiddiqueStrozziGeigerFreyRemoteSensing2019ComparisonTropoDelayFromGNSSandPSI] [bibtex-entry]


  198. Pengfei Xie, Man Zhang, Lei Zhang, and Guanyong Wang. Residual Motion Error Correction with Backprojection Multisquint Algorithm for Airborne Synthetic Aperture Radar Interferometry. Sensors, 19(10), 2019. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, TDBP, Non-Linear Flight Tracks, Curvilinear SAR, digital elevation model, Airborne SAR, Motion Compensation, MoComp, Residual Motion Errors, Multisquint, Multi-aperture interferometry, MAI.
    Abstract: For airborne interferometric synthetic aperture radar (InSAR) data processing, it is essential to achieve precise motion compensation to obtain high-quality digital elevation models (DEMs). In this paper, a novel InSAR motion compensation method is developed, which combines the backprojection (BP) focusing and the multisquint (MSQ) technique. The algorithm is two-fold. For SAR image focusing, BP algorithm is applied to fully use the navigation information. Additionally, an explicit mathematical expression of residual motion error (RME) in the BP image is derived, which paves a way to integrating the MSQ algorithm in the azimuth spatial wavenumber domain for a refined RME correction. It is revealed that the proposed backprojection multisquint (BP-MSQ) algorithm exploits the motion error correction advantages of BP and MSQ simultaneously, which leads to significant improvements of InSAR image quality. Simulation and real data experiments are employed to illustrate the effectiveness of the proposed algorithm.
    [bibtex-key = xieZhangZhangWangRemoteSensing2019TDBPResidualMoCompMultiSquint] [bibtex-entry]


  199. M. Yang, P. Lopez-Dekker, P. Dheenathayalan, F. Biljecki, M. Liao, and R. F. Hanssen. Linking Persistent Scatterers to the Built Environment Using Ray Tracing on Urban Models. IEEE Transactions on Geoscience and Remote Sensing, 57(8):5764-5776, August 2019. Keyword(s): electromagnetic wave scattering, millimetre wave radar, radar imaging, ray tracing, specific physical objects, millimeter-scale displacements, satellite radar images, time series, coherent measurement points, urban models, fivefold-bounce scatterers, triple-bounce scatterers, identified scatterers, matched scatterers, ray tracing, point scatterers, ray-tracing synthetic aperture radar simulator, pointlike scatterers, Synthetic aperture radar, Solid modeling, Ray tracing, Scattering, Urban areas, Geometry, Object oriented modeling, Level of detail (LOD), persistent scatterers (PSs), ray tracing, simulation, synthetic aperture radar (SAR).
    Abstract: Persistent scatterers (PSs) are coherent measurement points obtained from time series of satellite radar images, which are used to detect and estimate millimeter-scale displacements of the terrain or man-made structures. However, associating these measurement points with specific physical objects is not straightforward, which hampers the exploitation of the full potential of the data. We have investigated the potential for predicting the occurrence and location of PSs using generic 3-D city models and ray-tracing methods, and proposed a methodology to match PSs to the pointlike scatterers predicted using RaySAR, a ray-tracing synthetic aperture radar simulator. We also investigate the impact of the level of detail (LOD) of the city models. For our test area in Rotterdam, we find that 10% and 37% of the PSs detected in a stack of TerraSAR-X data can be matched with point scatterers identified by ray tracing using LOD1 and LOD2 models, respectively. In the LOD1 case, most matched scatterers are at street level while LOD2 allows the identification of many scatterers on the buildings. Over half of the identified scatterers easily correspond to identify double or triple-bounce scatterers. However, a significant fraction corresponds to higher bounce levels, with approximately 25% being fivefold-bounce scatterers.
    [bibtex-key = yangLopezDekkerDheenathayalanBiljeckiLiaoHanssenTGRS2019LinkingPSItoBuiltupAreasUsingRayTracing] [bibtex-entry]


  200. Hao Zhang and Paco López-Dekker. Persistent Scatterer Densification Through the Application of Capon- and APES-Based SAR Reprocessing Algorithms. IEEE Trans. Geosci. Remote Sens., pp 1-13, 2019. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Synthetic aperture radar, Signal resolution, Spatial resolution, Estimation, Dispersion, Jitter, Amplitude and phase estimation (APES), Capon, persistent scatterer (PS) density, PS interferometry (PSI), superresolution (SR)..
    Abstract: Capon's minimum-variance method (MVM) and amplitude and phase estimation (APES) spectral estimation algorithms can be applied to synthetic aperture radar (SAR) processing to improve the resolution and suppress sidelobe levels. In this paper, we use Capon-/APES-based SAR reprocessing algorithms to increase the persistent scatterer (PS) density in PS interferometry (PSI). We propose a PS candidate (PSC) selection algorithm applicable to the superresolution reprocessed images and the corresponding processing chain. The performance of the proposed algorithm is evaluated by a number of simulations and a stack of TerraSAR-X data. The results show that the Capon algorithm outperforms others in PSC selection. We present a full PSI time-series analysis on the PSCs extracted from the Capon-reprocessed stacks. The results show that the PS density is increased between 50% and 60%, while their interferometric quality is maintained.
    [bibtex-key = zhangLopezDekkerTGARS2019PSICaponAPES] [bibtex-entry]


  201. F. Alshawaf, F. Zus, K. Balidakis, Z. Deng, M. Hoseini, G. Dick, and J. Wickert. On the Statistical Significance of Climatic Trends Estimated From GPS Tropospheric Time Series. Journal of Geophysical Research: Atmospheres, 123(19):10,967-10,990, 2018. Note: Cited By 17. [bibtex-key = Alshawaf201810967] [bibtex-entry]


  202. Homa Ansari, Francesco De Zan, and Richard Bamler. Efficient Phase Estimation for Interferogram Stacks. IEEE Transactions on Geoscience and Remote Sensing, 56(7):4109-4125, July 2018. Keyword(s): maximum likelihood estimation, phase estimation, radar interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, time series, efficient phase estimation, interferogram stacks, signal decorrelation, SAR interferometry, high-precision deformation, different techniques, short baseline subset, SqueeSAR, CAESAR, overarching schemes, different analysis approaches, deformation estimation, called Eigendecomposition, maximum-likelihood-estimator, Interferometric phase, state-of-the-art techniques, computational estimation efficiency, Sequential Estimator, efficient processing scheme, state-of-the-art approaches, Electromagnetic interference, Time series analysis, Maximum likelihood estimation, Systematics, Synthetic aperture radar, Strain, Big Data, coherence matrix, covariance estimation, differential interferometric synthetic aperture radar, distributed scatterers (DS), efficiency, error analysis, maximum-likelihood estimation, near real-time (NRT) processing.
    Abstract: SAR Processin, Signal decorrelation poses a limitation to multipass SAR interferometry. In pursuit of overcoming this limitation to achieve high-precision deformation estimates, different techniques have been developed, with short baseline subset, SqueeSAR, and CAESAR as the overarching schemes. These different analysis approaches raise the question of their efficiency and limitation in phase and consequently deformation estimation. This contribution first addresses this question and then proposes a new estimator with improved performance, called Eigendecomposition-based Maximum-likelihood-estimator of Interferometric phase (EMI). The proposed estimator combines the advantages of the state-of-the-art techniques. Identical to CAESAR, EMI is solved using eigendecomposition; it is therefore computationally efficient and straightforward in implementation. Similar to SqueeSAR, EMI is a maximum-likelihood-estimator; hence, it retains estimation efficiency. The computational and estimation efficiency of EMI renders it as an optimum choice for phase estimation. A further marriage of EMI with the proposed Sequential Estimator by Ansari et al. provides an efficient processing scheme tailored to the analysis of Big InSAR Data. EMI is formulated and verified in relation to the state-of-the-art approaches via mathematical formulation, simulation analysis, and experiments with time series of Sentinel-1 data over the volcanic island of Vulcano, Italy.
    [bibtex-key = ansariDeZanBamlerTGRS2018EfficientPhaseEstimationInSARStacks] [bibtex-entry]


  203. Simone Baffelli, Othmar Frey, Charles L. Werner, and Irena Hajnsek. Polarimetric Calibration of the Ku-Band Advanced Polarimetric Radar Interferometer. IEEE Trans. Geosci. Remote Sens., 56(4):2295-2311, 2018. Keyword(s): real-aperture radar, radar, terrestrial radar, Apertures, Calibration, interferometry, radar interferometry, Radar antennas, Radar imaging, Radar polarimetry, ground-based radar, polarimetric calibration, polarimetric, GB-RADAR, polarimetric-interferometric radar, Gamma Portable Radar Interferometer, GPRI.
    Abstract: Differential interferometry using ground-based radar systems permits to monitor displacements in natural terrain with high flexibility in location, time of acquisition, and revisit time. In combination with polarimetric imaging, discrimination of different scattering mechanisms present in a resolution cell can be obtained simultaneously with the estimation of surface displacement. In this paper, we present the preprocessing steps and the calibration procedure required to produce high-quality calibrated polarimetric single-look complex imagery with KAPRI, a new portable Ku-band polarimetric radar interferometer. The processing of KAPRI data into single look complex images is addressed, including the correction of beam squint and of azimuthal phase variations. A polarimetric calibration model adapted to the acquisition mode is presented and used to produce calibrated polarimetric covariance matrix data. The methods are validated by means of a scene containing five trihedral corner reflectors. Data preprocessing is assessed by analyzing the oversampled response of a corner reflector, and the polarimetric calibration quality is verified by computing polarimetric signatures and residual calibration parameters.
    [bibtex-key = baffelliFreyWernerHajnsekTGRS2018PolGPRICalibration] [bibtex-entry]


  204. Alessandra Budillon, Michele Crosetto, Angel Caroline Johnsy, Oriol Monserrat, Vrinda Krishnakumar, and Gilda Schirinzi. Comparison of Persistent Scatterer Interferometry and SAR Tomography Using Sentinel-1 in Urban Environment. Remote Sensing, 10(12), 2018. Keyword(s): SAR Processing, SAR Tomography, Tomography, 3D SAR data imaging, Persistent Scatterer Interferometry, PSI, Interferometry, SAR Interferometry, differential interferometry, DInSAR.
    Abstract: In this paper, persistent scatterer interferometry and Synthetic Aperture Radar (SAR) tomography have been applied to Sentinel-1 data for urban monitoring. The paper analyses the applicability of SAR tomography to Sentinel-1 data, which is not granted, due to the reduced range and azimuth resolutions and the low resolution in elevation. In a first part of the paper, two implementations of the two techniques are described. In the experimental part, the two techniques are used in parallel to process the same Sentinel-1 data over two test areas. An intercomparison of the results from persistent scatterer interferometry and SAR tomography is carried out, comparing the main parameters estimated by the two techniques. Finally, the paper addresses the complementarity of the two techniques, and in particular it assesses the increase of measurement density that can be achieved by adding the double scatterers from SAR tomography to the persistent scatterer interferometry measurements.
    [bibtex-key = budillonCrosettoJohnsyMonserratKrishnakumarSchirinziRemoteSensing2018ComparisonPSIandTomoWithSentinel1Urban] [bibtex-entry]


  205. Ning Cao, Hyongki Lee, Evan Zaugg, Ramesh Shrestha, William E. Carter, Craig Glennie, Zhong Lu, and Hanwen Yu. Estimation of Residual Motion Errors in Airborne SAR Interferometry Based on Time-Domain Backprojection and Multisquint Techniques. IEEE Trans. Geosci. Remote Sens., 56(4):2397-2407, 2018. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, DInSAR, deformation monitoring, subsidence monitoring, Displacement, Focusing, Radar antennas, Synthetic aperture radar, Time-domain analysis, Trajectory, Backprojection (BP), SAR interferometry (InSAR), motion compensation (MoCo), residual motion error (RME), synthetic aperture radar (SAR).
    Abstract: For airborne repeat-pass synthetic aperture radar interferometry (InSAR), precise trajectory information is needed to compensate for deviations of the platform movement from a linear track. Using the trajectory information, motion compensation (MoCo) can be implemented within SAR data focusing. Due to the inaccuracy of current navigation systems, residual motion errors (RMEs) exist between the real and measured trajectory, causing phase undulations in the final interferograms. Up to now, MoCo and RME estimation have usually been combined in airborne InSAR to estimate ground deformation. Conventional MoCo methods generally involve azimuthal and range resampling and phase correction. Then frequency-domain focusing techniques can be used to generate the SAR images. After focusing SAR images with MoCo, both multisquint and autofocus approaches can be used to estimate RME. In addition to the MoCo-based frequency-domain focusing, the time-domain backprojection (BP) technique can also focus the SAR data obtained from highly nonlinear platform trajectories. In this paper, we present, for the first time, the combination of BP and multisquint techniques for RME estimation. A detailed derivation of the implementation of the multisquint approach using the BP-focusing images is presented. Repeat-pass data from the SlimSAR system over Slumgullion landslide are used to demonstrate the feasibility of RME estimation for both stationary and nonstationary scenes. We conclude that the proposed method can effectively remove the RME.
    [bibtex-key = caoLeeZauggShresthaCarterGlennieLuYuTGRS2018TDBPResidualMotionInSAR] [bibtex-entry]


  206. Cosmin Danisor, Gianfranco Fornaro, Antonio Pauciullo, Diego Reale, and Mihai Datcu. Super-Resolution Multi-Look Detection in SAR Tomography. Remote Sensing, 10(12), 2018. Keyword(s): SAR Processing, SAR Tomography, Multi-look, Coherence, Detectors, Interferometry, Spatial resolution, Synthetic aperture radar, Tomography, Detection, SAR tomography, generalized likelihood ratio test, multi-look, persistent scatterers (PSs), GLRT, Super-resolution, Capon, Generalized likelihood ratio test.
    Abstract: Synthetic Aperture Radar (SAR) Tomography (TomoSAR) allows extending the 2-D focusing capabilities of SAR to the elevation direction, orthogonal to the azimuth and range. The multi-dimensional extension (along the time) also enables the monitoring of possible scatterer displacements. A key aspect of TomoSAR is the identification, in the presence of noise, of multiple persistent scatterers interfering within the same 2-D (azimuth range plane) pixel. To this aim, the use of multi-look has been shown to provide tangible improvements in the detection of single and double interfering persistent scatterers at the expense of a minor spatial resolution loss. Depending on the system acquisition characteristics, this operation may require also the detection of multiple scatterers interfering at distances lower than the Rayleigh resolution (super-resolution). In this work we further investigated the use of multi-look in TomoSAR for the detection of multiple scatterers located also below the Rayleigh resolution. A solution relying on the Capon filtering was first analyzed, due to its improved capabilities in the separation of the responses of multiple scatterers and sidelobe suppression. Moreover, in the framework of the Generalized Likelihood Ratio Test (GLRT), the single-look support based detection strategy recently proposed in the literature was extended to the multi-look case. Experimental results of tests carried out on two datasets acquired by TerraSAR-X and COSMO-SkyMED sensors are provided to show the performances of the proposed solution as well as the effects of the baseline span of the dataset for the detection capabilities of interfering scatterers.
    [bibtex-key = danisorFornaroPauciulloRealeDatcuRemoteSensing2018SuperResMLDetectionSARTomo] [bibtex-entry]


  207. Markus Even and Karsten Schulz. InSAR Deformation Analysis with Distributed Scatterers: A Review Complemented by New Advances. Remote Sensing, 10(5), 2018. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Persistent Scatterers, Persistent Scatterer Interferometry, PSI, Distributed Scatterers, Distributed Scatterer Interferometry, DS, DS Interferoemetry, preprocessing, adaptive neighborhood, covariance, coherence, deformation, displacement mapping, deformation monitoring, SqueeSAR.
    Abstract: Interferometric Synthetic Aperture Radar (InSAR) is a powerful remote sensing technique able to measure deformation of the earth’s surface over large areas. InSAR deformation analysis uses two main categories of backscatter: Persistent Scatterers (PS) and Distributed Scatterers (DS). While PS are characterized by a high signal-to-noise ratio and predominantly occur as single pixels, DS possess a medium or low signal-to-noise ratio and can only be exploited if they form homogeneous groups of pixels that are large enough to allow for statistical analysis. Although DS have been used by InSAR since its beginnings for different purposes, new methods developed during the last decade have advanced the field significantly. Preprocessing of DS with spatio-temporal filtering allows today the use of DS in PS algorithms as if they were PS, thereby enlarging spatial coverage and stabilizing algorithms. This review explores the relations between different lines of research and discusses open questions regarding DS preprocessing for deformation analysis. The review is complemented with an experiment that demonstrates that significantly improved results can be achieved for preprocessed DS during parameter estimation if their statistical properties are used.
    [bibtex-key = evenSchulzRemoteSensing2018InSARDistributedScatterers] [bibtex-entry]


  208. Patrick Henkel, Franziska Koch, Florian Appel, Heike Bach, Monika Prasch, Lino Schmid, J�rg Schweizer, and Wolfram Mauser. Snow Water Equivalent of Dry Snow Derived From GNSS Carrier Phases. IEEE Transactions on Geoscience and Remote Sensing, 56(6):3561-3572, June 2018. Keyword(s): GNSS, Snow water equivalent, SWE, Davos, Weissfluhjoch.
    Abstract: Snow water equivalent (SWE) is a key variable for various hydrological applications. It is defined as the depth of water that would result upon complete melting of a mass of snow. However, until now, continuous measurements of the SWE are either scarce, expensive, labor-intense, or lack temporal or spatial resolution especially in mountainous and remote regions. We derive the SWE for dry-snow conditions using carrier phase measurements from the Global Navigation Satellite System (GNSS) receivers. Two static GNSS receivers are used, whereby one antenna is placed below the snow and the other antenna is placed above the snow. The carrier phase measurements of both receivers are combined in double differences (DDs) to eliminate clock offsets and phase biases and to mitigate atmospheric errors. Each DD carrier phase measurement depends on the relative position between both antennas, an integer ambiguity due to the periodic nature of the carrier phase signal, and the SWE projected into the direction of incidence. The relative positions of the antennas are determined under snow-free conditions with millimeter accuracy using real-time kinematic positioning. Subsequently, the SWE and carrier phase integer ambiguities are jointly estimated with an integer least-squares estimator. We tested our method at an Alpine test site in Switzerland during the dry-snow season 2015-2016. The SWE derived solely by the GNSS shows very high correlation with conventionally measured snow pillow (root mean square error: 11 mm) and manual snow pit data. This method can be applied to dense low-cost GNSS receiver networks to improve the spatial and temporal information on snow.
    [bibtex-key = henkelKochAppelBachPraschSchmidSchweizerMauserTGRS2018SWEofDrySnowFromGNSSCarrierPhases] [bibtex-entry]


  209. P. Hügler, F. Roos, M. Schartel, M. Geiger, and C. Waldschmidt. Radar Taking Off: New Capabilities for UAVs. IEEE Microwave Magazine, 19(7):43-53, November 2018. Keyword(s): accelerometers, aircraft control, autonomous aerial vehicles, collision avoidance, gyroscopes, mobile robots, robot vision, satellite navigation, sensor fusion, SLAM (robots), stability, stereo image processing, UAVs, waypoint flights, autopilot mode, stabilization, localization, IMUs, accelerometers-barometric sensors, Global Navigation Satellite System, collision avoidance, vision-based sensors, monocular vision, stereo vision, radar sensors, multichannel radar, unmanned aerial vehicles, inertial measurement units, gyroscopes, flying sensor platforms, Radar imaging, Radar measurements, Radar antennas, Collision avoidance, Radar detection, Transmitters.
    Abstract: Modern consumer and industrial unmanned aerial vehicles (UAVs) are easy-to-use flying sensor platforms. They offer stable flight, good maneuverability, hovering, and even waypoint flights in autopilot mode. For stabilization and localization, sensors such as inertial measurement units (IMUs)-including gyroscopes and accelerometers-barometric sensors, and the Global Navigation Satellite System (GNSS) are used. To sense the UAV's direct environment, e.g., for collision avoidance or fully automated flight, additional sensors are needed. State-of-the-art combinations of infrared, ultrasonic, and vision-based sensors (monocular and/or stereo vision) capture the close vicinity. Using radar sensors is advantageous, as they are able to directly sense range and velocity and are not affected by lighting conditions and contrast. With the help of a multichannel radar, the angular information may also be extracted.
    [bibtex-key = huglerRoosSchartelGeigerWaldschmidtIEEEMicrowaveMag2018RadarOnUAVs] [bibtex-entry]


  210. R. Jolivet and M. Simons. A Multipixel Time Series Analysis Method Accounting for Ground Motion, Atmospheric Noise, and Orbital Errors. Geophysical Research Letters, 45(4):1814-1824, 2018. Keyword(s): InSAR, Time Series.
    Abstract: Abstract Interferometric synthetic aperture radar time series methods aim to reconstruct time-dependent ground displacements over large areas from sets of interferograms in order to detect transient, periodic, or small-amplitude deformation. Because of computational limitations, most existing methods consider each pixel independently, ignoring important spatial covariances between observations. We describe a framework to reconstruct time series of ground deformation while considering all pixels simultaneously, allowing us to account for spatial covariances, imprecise orbits, and residual atmospheric perturbations. We describe spatial covariances by an exponential decay function dependent of pixel-to-pixel distance. We approximate the impact of imprecise orbit information and residual long-wavelength atmosphere as a low-order polynomial function. Tests on synthetic data illustrate the importance of incorporating full covariances between pixels in order to avoid biased parameter reconstruction. An example of application to the northern Chilean subduction zone highlights the potential of this method.
    [bibtex-key = jolivetSimonsGRL2018MultipixelTimeSeriesAnalysisInSARwithGroundMotionAtmoAndOrbitErrors] [bibtex-entry]


  211. Heming Liao, Franz J. Meyer, Bernd Scheuchl, Jeremie Mouginot, Ian Joughin, and Eric Rignot. Ionospheric correction of InSAR data for accurate ice velocity measurement at polar regions. Remote Sensing of Environment, 209:166-180, 2018. Keyword(s): Synthetic aperture radar, SAR interferometry, Ice velocity, Range split spectrum, Data stacking, Ionosphere effect, Ionosphere correction.
    Abstract: Interferometric synthetic aperture radar (InSAR) has become an essential tool for measuring ice sheet velocity in the Polar Regions. At low radar frequencies, e.g. L-band (1.2 GHz) but also at higher frequency, e.g. C-band (5.6 GHz), the ionosphere has been documented to be an important source of noise in these data. In this paper, we employ a split-spectrum technique and investigate its performance for correcting ionospheric effects in InSAR-based ice velocity measurements in Greenland and Antarctica. Three case studies using ALOS PALSAR data are used to assess the performance of the split spectrum technique for ionosphere correction over a range of environmental parameters. We employ several approaches to evaluate the results, including visual inspection, profile analysis, comparison of experimental and theoretic errors, comparison with reference data from other sources, generation of double difference interferograms, and analysis of time series of multi-temporal data. Our experiments show that ionospheric distortions are observed regularly, and in our analyzed Greenland dataset and Antarctic dataset the ionospheric noise reaches 14 m/yr and 10 m/yr, respectively, which exceeds the signal associated with ice motion. Our analysis using several different approaches demonstrates that the split-spectrum technique provides an effective correction. The split spectrum technique is also found to be superior to currently used approaches such as baseline fitting and multi-temporal averaging. The noise level is reduced by a factor of 70% in Greenland test areas and 90% in Antarctic test areas.
    [bibtex-key = liaoMeyerScheuchlMouginotJoughinRignotRSE2018IonosphericCorrectionInSAR] [bibtex-entry]


  212. M. Lort, A. Aguasca, C. Lopez-Martinez, and T. M. Marin. Initial Evaluation of SAR Capabilities in UAV Multicopter Platforms. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(1):127-140, January 2018. Keyword(s): airborne radar, geophysical image processing, object detection, radar polarimetry, remote sensing by radar, remotely operated vehicles, synthetic aperture radar, topography (Earth), topographic mapping, Universitat Politecnica de Catalunya, AiR-based remote sensing, Barcelona, Spain, UAV MP, fully polarimetric SAR system, airborne systems, unmanned aerial vehicles, object detection, airborne synthetic aperture radar sensors, UAV multicopter platform, Synthetic aperture radar, Unmanned aerial vehicles, Apertures, Remote sensing, Sensor phenomena and characterization, Trajectory, Airborne synthetic aperture radar (SAR), unmanned aerial vehicle (UAV) multicopter, UAV SAR.
    Abstract: Airborne synthetic aperture radar (SAR) sensors have been commonly used during the last decades to monitor different phenomena in medium-scale areas of observation, such as object detection and characterization or topographic mapping. The use of unmanned aerial vehicles (UAVs) is a cost-effective solution that offers higher operational flexibility than airborne systems to monitor these types of scenarios. The Universitat Politecnica de Catalunya has developed the first fully polarimetric SAR system at X-band integrated into a small UAV multicopter platform (UAV MP). The sensor, called AiR-based remote sensing, has been integrated into the platform overcoming restrictions of weight, space, robustness, and power consumption. To demonstrate the validity of the developed system, some measurement campaigns have been conducted in the outskirts of Barcelona, Spain.
    [bibtex-key = lortAguascaLopezMartinezMarinJSTARS2018SARCapabilitiesUAVMulticopter] [bibtex-entry]


  213. Pooja Mahapatra, Hans van der Marel, Freek van Leijen, Sami Samie Esfahany, Roland Klees, and Ramon Hanssen. InSAR datum connection using GNSS-augmented radar transponders. Journal of Geodesy, 92(1):21, January 2018.
    Abstract: Deformation estimates from Interferometric Synthetic Aperture Radar (InSAR) are relative: they form a`` free'' network referred to an arbitrary datum, e.g. by assuming a reference point in the image to be stable. However, some applications require ``absolute'' InSAR estimates, i.e. expressed in a well-defined terrestrial reference frame, e.g. to compare InSAR results with those of other techniques. We propose a methodology based on collocated InSAR and Global Navigation Satellite System (GNSS) measurements, achieved by rigidly attaching phase-stable millimetre-precision compact active radar transponders to GNSS antennas. We demonstrate this concept through a simulated example and practical case studies in the Netherlands.
    [bibtex-key = mahapatraVanDerMarelVanLeijenSamieiEsfahanyKleesHanssenJOG2018InSARandGNSSaugmentedTransponders] [bibtex-entry]


  214. Albert R. Monteith and Lars M. H. Ulander. Temporal Survey of P- and L-Band Polarimetric Backscatter in Boreal Forests. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(10):3564-3577, October 2018. Keyword(s): SAR Tomography, backscatter, radar imaging, radar polarimetry, remote sensing by radar, spaceborne radar, synthetic aperture radar, vegetation mapping, temporal survey, L-band polarimetric backscatter, boreal forests, environmental conditions, seasonal variations, backscattered radar signal, biomass retrieval scheme, synthetic aperture radar data, electromagnetic scattering mechanisms, biomass estimation algorithms, L-band SAR missions, temporal changes, HV-polarized P, L-band radar backscatter, boreal forest site, environmental parameters, mature Norway spruce, above-ground biomass, approximately 250 tons/ha, BorealScat tower-based scatterometer, L-band backscatter, HH/VV backscatter ratio, average backscatter, double-bounce scattering, severe temporal decorrelation, P-band temporal coherence, mass 250.0 ton, time 5.0 min, noise figure 4.0 dB to 10.0 dB, Synthetic aperture radar, L-band, Backscatter, Biomass, Antenna arrays, Coherence, Backscatter, boreal forest, L-band, P-band, synthetic aperture radar (SAR), temporal coherence, time series.
    Abstract: Environmental conditions and seasonal variations affect the backscattered radar signal from a forest. This potentially causes errors in a biomass retrieval scheme using data from the synthetic aperture radar (SAR) data. A better understanding of these effects and the electromagnetic scattering mechanisms in forests is required to improve biomass estimation algorithms for current and upcoming P- and L-band SAR missions. In this paper, temporal changes in HH-, VV-, and HV-polarized P- and L-band radar backscatter and temporal coherence from a boreal forest site are analyzed in relation to environmental parameters. The radar data were collected from a stand of mature Norway spruce ( Picea abies (L.) Karst.) with an above-ground biomass of approximately 250 tons/ha at intervals of 5 min from January to August 2017 using the BorealScat tower-based scatterometer. It was observed that subzero temperatures during the winters cause large variations (4 to 10 dB) in P- and L-band backscatter, for which the HH/VV backscatter ratio offered some mitigation. High wind speeds were also seen to cause deviations in the average backscatter at P-band due to decreased double-bounce scattering. Severe temporal decorrelation was observed at L-band over timescales of days or more, whereas the P-band temporal coherence remained high (>0.9) for at least a month neglecting windy periods. Temporal coherence at P-band was highest during night times when wind speeds are low.
    [bibtex-key = monteithUlanderJSTARS2018TemporalPBandAndLBandPolarimetricBackscatter] [bibtex-entry]


  215. Andrea Monti Guarnieri, Antonio Leanza, Andrea Recchia, Stefano Tebaldini, and Giovanna Venuti. Atmospheric Phase Screen in GEO-SAR: Estimation and Compensation. IEEE Transactions on Geoscience and Remote Sensing, 56(3):1668-1679, 2018. Keyword(s): SAR Processing, geosynchronous SAR, Synthetic Aperture Radar, Autofocus, Atmospheric Modelling, Apertures, Atmospheric modeling, Azimuth, Delays, Estimation, Orbits, Synthetic aperture radar, Meteorology, radar clutter, radar imaging, radar interferometry, synthetic aperture radar (SAR).
    Abstract: We study the impact of atmospheric turbulence, specifically the wet tropospheric delay, in that synthetic aperture radar (SAR) with very long integration time, from minutes to hours, and wide swaths, such as the geosynchronous or geostationary SAR. In such systems, the atmospheric phase screen (APS) cannot be assumed frozen in time as for Low Earth Orbit or airborne SARs nor constant in space as for the ground-based SAR. The impact of space-time turbulence on SAR focusing is quantitatively assessed, and a novel focusing method that integrates APS estimation and compensation is proposed. Performances are evaluated as a function of SAR parameters, mainly the wavelength, based on a parametric model of the APS variogram, and results achieved by a simulating realistic scenario are shown.
    [bibtex-key = montiGuarnieriLeanzaRecchiaTebaldiniVenutiTGRS2018APSinGEOSAREstimationCompensation] [bibtex-entry]


  216. Andrea Virgilio Monti-Guarnieri, Maria Antonia Brovelli, Marco Manzoni, Mauro Mariotti d'Alessandro, Monia Elisa Molinari, and Daniele Oxoli. Coherent Change Detection for Multipass SAR. IEEE Transactions on Geoscience and Remote Sensing, 56(11):6811-6822, November 2018. Keyword(s): SAR Processing, Coherent Change Detection, CCD.
    Abstract: This paper focuses on the detection, from a stack of repeated-pass interferometric synthetic aperture radar (SAR) images, of such changes causing a target to completely lose the correlation between one epoch and another. This can be the consequence of human activities, such as construction, destruction, and agricultural activities, and also be the consequence of hazards, such as earthquake, landslides, or flooding, to buildings or terrains. The millimetric sensitivity of SAR makes it valuable for detecting such changes. This paper approaches two coherent change detection methods: a space coherent, time incoherent one and a full space and time coherent one, both based on the generalized likelihood ratiob (LR) test. A preliminary validation of the method is provided by processing two Sentinel-1 data stacks of 2016 Central Italy earthquake and by comparing the results with the map of damaged buildings in Amatrice and Accumoli made by Copernicus Emergency Management Service.
    [bibtex-key = montiGuarnieriEtAlTGARS2018CoherentChangeDetectionForMultipassSAR] [bibtex-entry]


  217. A. G. Mullissa, D. Perissin, V. A. Tolpekin, and A. Stein. Polarimetry-Based Distributed Scatterer Processing Method for PSI Applications. IEEE Trans. Geosci. Remote Sens., PP(99):1-12, 2018. Keyword(s): Coherence, Interferometry, Matrix decomposition, Optimization, Synthetic aperture radar, Adaptive filtering, distributed scatterers (DSs), multitemporal interferometric synthetic aperture radar (InSAR), permanent scatterer interferometry (PSI), polarimetric optimization, polarimetric synthetic aperture radar interferometry..
    Abstract: Permanent scatterer interferometry is a multitemporal interferometric synthetic aperture radar technique that produces high-accuracy ground deformation measurement. A high density of permanent scatterer (PS) is required to provide accurate results. In natural environments with low PS density, distributed scatterers (DSs) could serve as additional coherent observations. This paper introduces a polarimetric scattering property-based adaptive filtering method that preserves PS candidates and filters DS candidates. To further increase the coherence estimate of DS candidates, the technique includes a complex coherence decomposition that adaptively selects the most stable scattering mechanisms, thus improving pixel coherence estimation. The proposed method was evaluated on 11 quad-polarized ALOS PALSAR images and 21 dual-polarized Sentinel-1 images acquired over San Fernando Valley, CA, USA, and Groningen, The Netherlands, respectively. The application of this method increased the number of coherent pixels by almost a factor of eight compared with a single-polarization channel. This paper concludes that a coherence estimate can be significantly improved by applying scattering property-based adaptive filtering and coherence matrix decomposition and accurate displacement measurements can be achieved.
    [bibtex-key = mullissaPerissinTolpekinSteinTGRS2018PolarimetryBasedDistributedScatterersForPSI] [bibtex-entry]


  218. Elvira Musico, Claudio Cesaroni, Luca Spogli, John P. Merryman Boncori, De Franceschi Giorgiana, and Roberto Seu. The Total Electron Content From InSAR and GNSS: A Midlatitude Study. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(5):1725-1733, May 2018. Keyword(s): SAR Processing, Ionosphere, Global Positioning System, atmospheric techniques, radar interferometry, rain, remote sensing by radar, satellite navigation, synthetic aperture radar, ALOS-PALSAR, GNSS experimental measurements, GNSS receivers, InSAR images, L-band InSAR, RING network, Rete Integrata Nazionale GPS network, TEC variability, advanced land observing satellite, array type L-band synthetic aperture radar, correlation coefficient, dense network, global navigation satellite system receivers, interferometric phase, interferometric synthetic aperture radar, ionospheric information, midlatitude study, night-time case studies, reference true ionospheric TEC, total electron content, tropospheric contribution, Azimuth, Correlation, Earth, Global navigation satellite system, Ionosphere, Receivers, Synthetic aperture radar, Global positioning system, ionosphere, synthetic aperture radar (SAR).
    Abstract: The total electron content (TEC) measured from the interferometric synthetic aperture radar (InSAR) and from a dense network of global navigation satellite system (GNSS) receivers are used to assess the capability of InSAR to retrieve ionospheric information, when the tropospheric contribution to the interferometric phase is reasonably negligible. With this aim, we select three night-time case studies over Italy and investigate the correlation between TEC from advanced land observing satellite-phased array type L-band synthetic aperture radar (ALOS-PALSAR) and from the Rete Integrata Nazionale GPS (RING) network, the latter considered as the reference true ionospheric TEC. To retrieve the TEC variability from ALOS-PALSAR, we first investigate the correlation between the integral of the azimuth shifts and the interferometric phase in the absence of ground motions (e.g., earthquakes) and/or heavy rain events. If correlation exists (as in two out of three case studies under investigation), we can assume the tropospheric contribution to the interferometric phase as negligible and the TEC variability from L-band InSAR can be retrieved. For these two case studies, the comparison between the TEC from the InSAR images and from the RING network is quite encouraging as the correlation coefficient is R ~ 0.67 in the first case and R ~ 0.83 in the second case. This result highlights the potential to combine InSAR and GNSS experimental measurements to investigate small-scale spatial variability of TEC, in particular over regions scarcely covered by ground-based GNSS receivers.
    [bibtex-key = musicoCesaroniSpogliMerrymanDeFranceschiSeuJSTARS2018IonoTECInSARandGNSS] [bibtex-entry]


  219. NI. The Total Electron Content From InSAR and GNSS: A Midlatitude Study. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(5):1725-1733, May 2018. Keyword(s): SAR Processing, Ionosphere, Global Positioning System, atmospheric techniques, radar interferometry, rain, remote sensing by radar, satellite navigation, synthetic aperture radar, ALOS-PALSAR, GNSS experimental measurements, GNSS receivers, InSAR images, L-band InSAR, RING network, Rete Integrata Nazionale GPS network, TEC variability, advanced land observing satellite, array type L-band synthetic aperture radar, correlation coefficient, dense network, global navigation satellite system receivers, interferometric phase, interferometric synthetic aperture radar, ionospheric information, midlatitude study, night-time case studies, reference true ionospheric TEC, total electron content, tropospheric contribution, Azimuth, Correlation, Earth, Global navigation satellite system, Ionosphere, Receivers, Synthetic aperture radar, Global positioning system, ionosphere, synthetic aperture radar (SAR).
    Abstract: The total electron content (TEC) measured from the interferometric synthetic aperture radar (InSAR) and from a dense network of global navigation satellite system (GNSS) receivers are used to assess the capability of InSAR to retrieve ionospheric information, when the tropospheric contribution to the interferometric phase is reasonably negligible. With this aim, we select three night-time case studies over Italy and investigate the correlation between TEC from advanced land observing satellite-phased array type L-band synthetic aperture radar (ALOS-PALSAR) and from the Rete Integrata Nazionale GPS (RING) network, the latter considered as the reference true ionospheric TEC. To retrieve the TEC variability from ALOS-PALSAR, we first investigate the correlation between the integral of the azimuth shifts and the interferometric phase in the absence of ground motions (e.g., earthquakes) and/or heavy rain events. If correlation exists (as in two out of three case studies under investigation), we can assume the tropospheric contribution to the interferometric phase as negligible and the TEC variability from L-band InSAR can be retrieved. For these two case studies, the comparison between the TEC from the InSAR images and from the RING network is quite encouraging as the correlation coefficient is R ~ 0.67 in the first case and R ~ 0.83 in the second case. This result highlights the potential to combine InSAR and GNSS experimental measurements to investigate small-scale spatial variability of TEC, in particular over regions scarcely covered by ground-based GNSS receivers.
    [bibtex-key = musicoCesaroniSpogliMerrymanDeFranceschiSeuJSTARS2018IonoTECInSARandGNSS] [bibtex-entry]


  220. Stephan Palm, Rainer Sommer, and Uwe Stilla. Mobile Radar Mapping --- Subcentimeter SAR Imaging of Roads. IEEE Transactions on Geoscience and Remote Sensing, 56(11):6734-6746, November 2018. Keyword(s): SAR Processing, Azimuth Focusing, FMCW, Back Projection, Time-Domain Back-Projection, TDBP, FFBP, Fast-Factorized Back-Projection, CW radar, digital elevation models, FM radar, geophysical image processing, Global Positioning System, image reconstruction, image resolution, radar imaging, remote sensing by radar, synthetic aperture radar, mobile radar mapping-subcentimeter SAR imaging, ultrahigh-resolution synthetic aperture radar data, related theoretical background, imaging method, backprojection techniques, potential errors, correct geometry, imaging quality, point target simulations, suitable digital elevation model, illuminated scene, conventional roads, mobile mapping scenarios, SAR images, output data, reference targets, GPS-INS data, conventional 3-D Point Cloud Software, geometric distortions, subcentimeter SAR imaging, active frequency-modulated continuous wave radar system, frequency 300.0 GHz, Synthetic aperture radar, Sensors, Radar imaging, Roads, Laser radar, Geometry, Millimeter wave radar, radar resolution, radar signal processing, road vehicle radar.
    Abstract: In this paper, we present a strategy for focusing ultrahigh-resolution synthetic aperture radar (SAR) data for mobile radar mapping. We illustrate the related theoretical background and required extensions on the imaging method based on backprojection techniques. The influence of potential errors in estimating a correct geometry with respect to the imaging quality is investigated in detail by point target simulations. As backprojection techniques require precise knowledge of the topography in close range, the new strategy instantly uses the GPS/INS data of the trajectory to define a suitable digital elevation model of the illuminated scene. We have tested the strategy by driving on conventional roads with an active frequency-modulated continuous wave radar system operating at 300 GHz. Different reference targets were placed in the scene, and the accuracy of the method was evaluated. The results experimentally reveal that the lower terahertz band is capable of subcentimeter SAR imaging in mobile mapping scenarios at very high quality. We have shown that narrow cracks in the asphalt of roads can be detected and fine-scale objects on millimeter size can be displayed. Geometric distortions in the SAR images are significantly reduced allowing the measurements of infrastructure. The output data can at last be transferred to the conventional 3-D Point Cloud Software for further processing.
    [bibtex-key = palmSommerStillaTGRS2018MobileRadarMappingSubCentimeterImaging] [bibtex-entry]


  221. Matteo Pardini, Marivi Tello, Victor Cazcarra-Bes, K. P. Papathanassiou, and I. Hajnsek. L- and P-Band 3-D SAR Reflectivity Profiles Versus Lidar Waveforms: The AfriSAR Case. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(10):3386-3401, October 2018. Keyword(s): SAR Processing, SAR Tomography, airborne radar, backscatter, forestry, geophysical techniques, optical radar, radar imaging, radar polarimetry, remote sensing by laser beam, remote sensing by radar, synthetic aperture radar, vegetation, vegetation mapping, NASA Land, Ice Sensor lidar datasets, AfriSAR campaign, LVIS data, plot field measurements, ground-to-volume power ratio, physical forest structure descriptors, vertical structure indices, 3-D radar reflectivity, LVIS profiles, P-band 3-D, AfriSAR case, P-band vertical backscattering profiles, synthetic aperture radar tomography, light detection, DLR F-SAR, tropical forest structure types, Forestry, Synthetic aperture radar, Laser radar, Radar tracking, L-band, Vegetation, Forest structure, full waveforms, light detection and ranging (lidar), SAR tomography (TomoSAR), synthetic aperture radar (SAR), tropical forest.
    Abstract: The aim of this paper is to compare L- and P-band vertical backscattering profiles estimated by means of synthetic aperture radar (SAR) tomography and full light detection and ranging (lidar) waveforms in terms of their ability to distinguish different tropical forest structure types. The comparison relies on the unique DLR F-SAR and NASA Land, Vegetation and Ice Sensor (LVIS) lidar datasets acquired in 2016 in the frame of the AfriSAR campaign. In particular, F-SAR and LVIS data over three different test sites complemented by plot field measurements are used. First, the SAR and lidar three-dimensional (3-D) datasets are compared and discussed on a qualitative basis. The ability to penetrate into and through the canopy down to the ground is assessed at L- and P-band in terms of both the ground-to-volume power ratio and the performance to estimate the location of the underlying ground. The effect of polarimetry on the visibility of the ground is discussed as well. Finally, the 3-D measurements for each configuration are compared with respect to their ability to derive physical forest structure descriptors. For this, vertical structure indices derived from the volume-only 3-D radar reflectivity at L- and P-band and from the LVIS profiles are compared against each other as well as against plot-derived indices.
    [bibtex-key = pardiniTelloCazcarraBesPapathanassiouHajnsek2018LBandAndPBandSARTomoAgainstLiDARAfriSAR] [bibtex-entry]


  222. Antonio Pauciullo, Diego Reale, Walter Franzé, and Gianfranco Fornaro. Multi-Look in GLRT-Based Detection of Single and Double Persistent Scatterers. IEEE Trans. Geosci. Remote Sens., 56(9):5125-5137, September 2018. Keyword(s): SAR Processing, SAR Tomography, Multi-look, Coherence, Detectors, Interferometry, Spatial resolution, Synthetic aperture radar, Tomography, Detection, SAR tomography, generalized likelihood ratio test, multi-look, persistent scatterers (PSs).
    Abstract: Persistent scatterer (PS) interferometry and more recently synthetic aperture radar tomography have shown to be powerful tools in urban scenarios for providing 3-D point clouds in the reconstruction of buildings as well as in the monitoring of their possible slow temporal deformations. The detection of PSs represents a fundamental aspect, which in the literature has been mainly addressed at full resolution (single-look detection), thus considering only the scatterer coherence properties along the different acquisitions. In this paper, we investigate the benefits offered by the usage of multiple observation looks. Multi-look generalized likelihood ratio test detection schemes are derived and analyzed in terms of detection performances. The analysis shows that even a slight multi-look can provide a dramatic improvement on the detection capability both on simulated and real data, especially in the areas characterized by a low signal-to-noise ratio and in the presence of a limited number of acquisitions.
    [bibtex-key = pauciulloRealeFranzeFornaroTGRS2018MultiLookGLRTPSITomo] [bibtex-entry]


  223. Muriel Pinheiro, Andreas Reigber, Rolf Scheiber, Pau Prats-Iraola, and Alberto Moreira. Generation of Highly Accurate DEMs Over Flat Areas by Means of Dual-Frequency and Dual-Baseline Airborne SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, pp 1-30, 2018. Keyword(s): Calibration, Data models, Decorrelation, Interferometry, Standards, Surfaces, Synthetic aperture radar, Digital elevation model (DEM), SAR interferometry (InSAR)., dual frequency, repeat-pass interferometry.
    Abstract: In this paper, a dual-frequency and dual-baseline (DFDB) processing framework for the extraction of high-precision terrain information from airborne interferometric synthetic aperture radar (SAR) data is presented. Specifically, we propose the use of two single-pass data sets acquired simultaneously in two different frequency bands and two large-baseline repeat-pass data sets also acquired simultaneously in two frequency bands. The configuration profits from the stability of the single-pass derived elevation maps in relation to spatially correlated artifacts as well as from the increased sensitivity associated with large-baseline acquisitions. Moreover, the dual-frequency nature of the data set enables the tackling of the phase unwrapping issue, promoting the retrieval of unambiguous measurements. Several algorithms for the interferometric processing of the DFDB airborne data set are proposed, including the outline of multichannel phase calibration and unwrapping error correction strategies and approaches to remove spatially correlated artifacts and extract the common underlying topography. Elevation models generated from a DFDB data set acquired with the airborne F-SAR sensor over tidal flats in northern Germany are presented, and comparisons with an airborne laser scanner reference show errors with a standard deviation of around 14 cm and a mean absolute deviation of less than 10 cm.
    [bibtex-key = pinheiroReigberScheiberPratsMoreiraTGRS2018HighlyAccurateDEMsWithDualFreqDualBaselineAirborneInSAR] [bibtex-entry]


  224. Pau Prats-Iraola, Paco Lopez-Dekker, Francesco De Zan, Nestor Yague-Martinez, Mariantonietta Zonno, and Marc Rodriguez-Cassola. Performance of 3-D Surface Deformation Estimation for Simultaneous Squinted SAR Acquisitions. IEEE Transactions on Geoscience and Remote Sensing, 56(4):2147-2158, April 2018. Keyword(s): SAR Processing, deformation, radar imaging, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, 3D mean deformation map retrieval, 3D surface deformation estimation performance, C band, L band, LOSs, along-track deformation measurement, angular separations, atmospheric delay correlation, atmospheric noise, differential measurement, future spaceborne SAR missions, hybrid Cramer-Rao bound, interferograms, lines of sight, mathematical framework, multibeam low earth observation missions, multisatellite earth observation missions, north-south component sensitivity, quasisimultaneous squinted synthetic aperture radar interferometric acquisitions, repeat-pass scenario, simultaneous SAR image acquisition, simultaneous squinted SAR acquisitions, squint angles, troposphere autocorrelation function, troposphere-free estimation, Atmospheric measurements, Extraterrestrial measurements, Geometry, Satellites, Strain, Synthetic aperture radar, Terrestrial atmosphere, Atmospheric boundary layer (ABL), differential synthetic aperture radar interferometry (DInSAR), hybrid Cramer-Rao bound (HCRB), squinted synthetic aperture radar acquisitions, synthetic aperture radar (SAR), troposphere.
    Abstract: This paper addresses the performance in the retrieval of 3-D mean deformation maps by exploiting simultaneous or quasi-simultaneous squinted synthetic aperture radar (SAR) interferometric acquisitions in a repeat-pass scenario. In multisatellite or multibeam low earth observation missions, the availability of two (or more) lines of sight (LOSs) allows the simultaneous acquisition of SAR images with different squint angles, hence improving the sensitivity to the north-south component of the deformation. Due to the simultaneity of the acquisitions, the troposphere will be highly correlated and, therefore, will tend to cancel out when performing the differential measurement between the interferograms obtained with the different LOSs, hence resulting in a practically troposphere-free estimation of the along-track deformation measurement. In practice, however, the atmospheric noise in the differential measurement will increase for increasing angular separations. This paper expounds the mathematical framework to derive the performance by properly considering the correlation of the atmospheric delays between the simultaneous acquisitions. To that aim, the hybrid Cramer-Rao bound is exploited making use of the autocorrelation function of the troposphere. Some performance examples are presented in the frame of future spaceborne SAR missions at C and L band.
    [bibtex-key = pratsLopezDekkerDeZanYagueMartinezZonnoRodriguezCassolaTGRS2018Performance3DDeforamtionMultisquintedSpaceborneSAR] [bibtex-entry]


  225. Meenu Rani, S. B. Dhok, and R. B. Deshmukh. A Systematic Review of Compressive Sensing: Concepts, Implementations and Applications. IEEE Access, 6:4875-4894, 2018. Keyword(s): Compressive Sensing, CS, Sensors, Transforms, Mathematical model, Sparse matrices, Compressed sensing, Reconstruction algorithms, Image reconstruction, systematic review, compressive sensing, sensing modality, sparse representation, compressible representation, Nyquist sampling rate, varied reconstruction algorithms, compressive measurements, CS acquisition strategies, signal representation, transform domain, sparsity, random demodulator, CS reconstruction algorithms, OMP, CS applications.
    Abstract: Compressive Sensing (CS) is a new sensing modality, which compresses the signal being acquired at the time of sensing. Signals can have sparse or compressible representation either in original domain or in some transform domain. Relying on the sparsity of the signals, CS allows us to sample the signal at a rate much below the Nyquist sampling rate. Also, the varied reconstruction algorithms of CS can faithfully reconstruct the original signal back from fewer compressive measurements. This fact has stimulated research interest toward the use of CS in several fields, such as magnetic resonance imaging, high-speed video acquisition, and ultrawideband communication. This paper reviews the basic theoretical concepts underlying CS. To bridge the gap between theory and practicality of CS, different CS acquisition strategies and reconstruction approaches are elaborated systematically in this paper. The major application areas where CS is currently being used are reviewed here. This paper also highlights some of the challenges and research directions in this field.
    [bibtex-key = raniDhokDeshmukhIEEEAcess2018ASystematicReviewOfCompressiveSensingConceptsImplementationsApplications] [bibtex-entry]


  226. Nida Sakar, Marc Rodriguez-Cassola, Pau Prats-Iraola, Andreas Reigber, and Alberto Moreira. Analysis of Geometrical Approximations in Signal Reconstruction Methods for Multistatic SAR Constellations With Large Along-Track Baseline. IEEE Geoscience and Remote Sensing Letters, 15(6):892-896, June 2018. Keyword(s): Doppler effect, Geometry, History, Image reconstruction, Receivers, Spaceborne radar, Synthetic aperture radar, Digital beamforming, high-resolution wide-swath (HRWS) radar, multistatic radar signal processing, synthetic aperture radar (SAR).
    Abstract: Large along-track baselines introduce residual polychromatic quadratic phase components which decrease the performance of state-of-the-art multichannel/multiplatform SAR reconstruction algorithms. This letter investigates the impact of the geometrical approximations in signal reconstruction methods for spaceborne multistatic SAR constellations with large along-track baselines operated with a pulse repetition frequency (PRF) under the Nyquist rate required for a single platform. We characterize and quantify the impact of these approximations, especially severe in the case of kilometric baselines and resolutions around $15lambda$ . Finally, we put forward a generalized range-Doppler strategy to accommodate the geometry of distributed along-track constellations in an accurate manner.
    [bibtex-key = sakarRodriguezCassolaPratsReigberMoreiraGRSL2018GeometricalApproximationsMultistaticAcquisitions] [bibtex-entry]


  227. Francescopaolo Sica, Davide Cozzolino, Xiao Xiang Zhu, Luisa Verdoliva, and Giovanni Poggi. InSAR-BM3D: A Nonlocal Filter for SAR Interferometric Phase Restoration. IEEE Transactions on Geoscience and Remote Sensing, 56(6):3456-3467, June 2018. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Phase filtering, InSAR filter, filter, AWGN, filtering theory, image denoising, radar imaging, radar interferometry, synthetic aperture radar, InSAR-BM3D, nonlocal filter, SAR interferometric phase restoration, block-matching 3-D, nonlocal approach, additive white Gaussian noise image denoising, synthetic aperture radar, SAR interferometry signal, Synthetic aperture radar, Image restoration, Wavelet transforms, AWGN, Coherence, Estimation, Nonlocal filtering, synthetic aperture radar (SAR), SAR interferometry (InSAR).
    Abstract: The block-matching 3-D (BM3D) algorithm, based on the nonlocal approach, is one of the most effective methods to date for additive white Gaussian noise image denoising. Likewise, its extension to synthetic aperture radar (SAR) amplitude images, SAR-BM3D, is a state-of-the-art SAR despeckling algorithm. In this paper, we further extend BM3D to address the restoration of SAR interferometric phase images. While keeping the general structure of BM3D, its processing steps are modified to take into account the peculiarities of the SAR interferometry signal. Experiments on simulated and real-world Tandem-X SAR interferometric pairs prove the effectiveness of the proposed method.
    [bibtex-key = sicaCozzolinoZhuVerdolivaPoggiTGRS2018InSARBM3DNonlocalFilerForInSAR] [bibtex-entry]


  228. Muhammad Adnan Siddique, Urs Wegmuller, Irena Hajnsek, and Othmar Frey. SAR Tomography as an Add-On to PSI: Detection of Coherent Scatterers in the Presence of Phase Instabilities. Remote Sensing, 10(7):1014, 2018. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne SAR, Barcelona, interferometric stack, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry, deformation, displacement, subsidence, detection, urban deformation monitoring, radar interferometry, displacement mapping, spaceborne SAR, differential interferometry, differential tomography, coherent scatterer detection.
    Abstract: The estimation of deformation parameters using persistent scatterer interferometry (PSI) is limited to single dominant coherent scatterers. As such, it rejects layovers wherein multiple scatterers are interfering in the same range-azimuth resolution cell. Differential synthetic aperture radar (SAR) tomography can improve deformation sampling as it has the ability to resolve layovers by separating the interfering scatterers. In this way, both PSI and tomography inevitably require a means to detect coherent scatterers, i.e., to perform hypothesis testing to decide whether a given candidate scatterer is coherent. This paper reports the application of a detection strategy in the context of tomography as an add-on to PSI. As the performance of a detector is typically linked to the statistical description of the underlying mathematical model, we investigate how the statistics of the phase instabilities in the PSI analysis are carried forward to the subsequent tomographic analysis. While phase instabilities in PSI are generally modeled as an additive noise term in the interferometric phase model, their impact in SAR tomography manifests as a multiplicative disturbance. The detection strategy proposed in this paper allows extending the same quality considerations as used in the prior PSI processing (in terms of the dispersion of the residual phase) to the subsequent tomographic analysis. In particular, the hypothesis testing for the detection of coherent scatterers is implemented such that the expected probability of false alarm is consistent between PSI and tomography. The investigation is supported with empirical analyses on an interferometric data stack comprising 50 TerraSAR-X acquisitions in stripmap mode, over the city of Barcelona, Spain, from 2007-2012.
    [bibtex-key = siddiqueWegmullerHajnsekFreyRemoteSensing2018SARTomoPSIDetectionInPresenceOfPhaseInstabilities] [bibtex-entry]


  229. Ladina Steiner, Michael Meindl, Charles Fierz, and Alain Geiger. An assessment of sub-snow GPS for quantification of snow water equivalent. The Cryosphere, 12(10):3161-3175, 2018. Keyword(s): GNSS, GPS, Snow-water equivalent, SWE, Submerged antennas.
    Abstract: Global Navigation Satellite Systems (GNSS) contribute to various Earth observation applications. The present study investigates the potential and limitations of the Global Positioning System (GPS) to estimate in-situ water equivalents of the snow cover (snow water equivalent, SWE) by using buried GPS antennas. GPS-derived SWE is estimated over three seasons (2015/16-2017/18) at a high Alpine test site in Switzerland. Results are validated against state-of-the-art reference sensors: snow scale, snow pillow, and manual observations. SWE is estimated with a high correspondence to the reference sensors for all three seasons. Results agree with a median relative bias below 10% and are highly correlated to the mean of the three reference sensors. The sensitivity of the SWE quantification is assessed for different GPS ambiguity resolution techniques, as the results strongly depend on the GPS processing.
    [bibtex-key = steinerMeindlFierzGeigerCryosphere2018GNSSforSWE] [bibtex-entry]


  230. Marivi Tello-Alonso, Victor Cazcarra-Bes, Matteo Pardini, and K. Papathanassiou. Forest Structure Characterization From SAR Tomography at L-Band. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 11(10):3402-3414, October 2018. Keyword(s): SAR Processing, SAR Tomography, forestry, optical radar, radar imaging, radar interferometry, radar polarimetry, remote sensing by laser beam, remote sensing by radar, synthetic aperture radar, vegetation mapping, 3-D forest monitoring, radar reflectivity, physical forest structure, tomographic SAR data, vertical structure index, inventory data, spatial distribution, experimental tomographic L-band data, vertical structure indices, vertical profiles, forest structure indices, forest structure characterization, SAR tomography, synthetic aperture radar remote sensing configurations, high spatial resolution, temporal resolution, tomographic SAR techniques, correlation coefficients, Forestry, Vegetation, Synthetic aperture radar, Tomography, Indexes, L-band, Forest structure, synthetic aperture radar, tomography.
    Abstract: Synthetic aperture radar (SAR) remote sensing configurations are able to provide continuous measurements on global scales sensitive to the vertical structure of forests with a high spatial and temporal resolution. Furthermore, the development of tomographic SAR techniques allows the reconstruction of the three-dimensional (3-D) radar reflectivity opening the door for 3-D forest monitoring. However, the link between 3-D radar reflectivity and 3-D forest structure is not yet established. In this sense, this paper introduced a framework that allows a qualitative and quantitative interpretation of physical forest structure from tomographic SAR data at L-band. For this, forest structure is parameterized into a set of a horizontal and a vertical structure index. From inventory data, both indices can be derived from the spatial distribution and the dimensions of the trees. Similarly, two structure indices are derived from the 3-D spatial distribution of the local maxima of the reconstructed 3-D radar reflectivity profiles at L-band. The proposed methodology is tested by means of experimental tomographic L-band data acquired over the temperate forest site of Traunstein in Germany. The obtained horizontal and vertical structure indices are validated against the corresponding estimates obtained from inventory measurements and against the same indices derived from the vertical profiles of airborne Lidar data. The high correlation between the forest structure indices obtained from these three different data sources (expressed by correlation coefficients between 0.75 and 0.87) indicates the potential of the proposed framework.
    [bibtex-key = telloAlonsoCazcarraBesPardiniPapathanassiouJSTARS2018ForestStructureLBandSARTomography] [bibtex-entry]


  231. Lars M. H. Ulander, Albert R. Monteith, Macej J. Soja, and Leif E. B. Eriksson. Multiport Vector Network Analyzer Radar for Tomographic Forest Scattering Measurements. IEEE Geoscience and Remote Sensing Letters, 15(12):1897-1901, December 2018. Keyword(s): SAR Tomography, BorealScat, antenna arrays, multiport networks, network analysers, radar antennas, radar imaging, S-parameters, tomographic forest scattering measurements, C-band radar, BorealScat, radar tomography, vertical antenna array, vertical scattering distribution, temporal decorrelation, reflected signals, 20-port vector network analyzer, stepped-frequency waveform, 20-element arrays, radar measurements, hemiboreal forest, tomographic imaging capabilities, multiport VNA tomography results, 2-port VNA measurement scheme, multiport vector network analyzer radar, P-band radar, L-band radar, polarimetric time-series measurements, Antenna measurements, Antenna arrays, Forestry, Tomography, Radar, Radar antennas, Switches, BorealScat, forest, polarimetry, radar, scattering, time series, tomography, vector network analyzer (VNA).
    Abstract: We describe a P-, L- and C-band radar, BorealScat, designed for polarimetric time-series measurements of forests. Radar tomography is implemented with a vertical antenna array, which provides measurements of the vertical scattering distribution. To minimize temporal decorrelation, the radar performs simultaneous measurements of the reflected signals using all array elements. The system is implemented using a 20-port vector network analyzer (VNA) and a stepped-frequency waveform. It has two 20-element arrays: one array optimized for P- and L-bands and one for C-band. The arrays are installed on a 50-m high tower and radar measurements are collected over a hemiboreal forest stand. We discuss several design issues and demonstrate some tomographic imaging capabilities. The multiport VNA tomography results are compared with results from the system operating in the slower 2-port VNA measurement scheme.
    [bibtex-key = ulanderMonteithSojaErikssonGRSL2018MultiportVNAForBorealScatForestScattering] [bibtex-entry]


  232. Urs Wegmuller, Charles Werner, Othmar Frey, Christophe Magnard, and Tazio Strozzi. Reformulating the Split-Spectrum Method to Facilitate the Estimation and Compensation of the Ionospheric Phase in SAR Interferograms. Procedia Computer Science, pp 318-325, 2018. Keyword(s): SAR Processing, Ionosphere, Ionospheric Path Delay, split-beam interferometry, SBI, ionospheric electromagnetic wave propagation, ionospheric techniques, radar interferometry, remote sensing by radar, split beam interferograms, along track ground displacement estimation, azimuth spectrum band pass filtering, directional scattering identification, ionospheric path delay estimation, long baseline pair coherence estimation, split beam interferometry, Azimuth, Band pass filters, Coherence, Delay, Ionosphere, Time series analysis. [bibtex-key = wegmullerWernerFreyMagnardStrozziProcediaCS2018SplitSpectrumIonosphereReformulation] [bibtex-entry]


  233. Othmar Frey. Synthetic Aperture Radar. In Douglas Richardson, editor, International Encyclopedia of Geography: People, the Earth, Environment, and Technology, pages 1-24. Wiley, 2017. Keyword(s): Synthetic Aperture Radar, SAR, Imaging, microwave imaging, radar systems, biosphere, carbon sequestration, co-seismic displacement, crustal deformation, data acquisition, digital earth, earth observation, earth system science, geocomputation, geodesy, geohazards, geomatics, geophysical signal processing, geospatial information, GIScience, ground deformation/subsidence monitoring, interferometry, land cover, land use change, mapping, microwave remote sensing, polarimetry, radar remote sensing, remote sensing, topography, volcano monitoring.
    Abstract: In this entry, the basic principles of synthetic aperture radar imaging are introduced from a general physics perspective instead of tackling the topic from the more widespread signal processing point of view. Thus, the material is thought to be accessible to any scientist or the general reader with some basic knowledge of physics and calculus. The focus is on explaining the imaging concept of synthetic aperture radar. Most of the applications based on remote sensing with synthetic aperture radar systems make use of advanced technologies that have been built on top of synthetic aperture radar (SAR) imagery. Such technologies include: SAR interferometry, differential SAR interferometry, persistent scatterer interferometry, and more recently, SAR polarimetric interferometry, and SAR tomography. A plethora of applications are supported - or, partly, were made available only - by these SAR-based techniques. Examples of such applications include: topographic mapping, change detection, land cover classification, ground deformation/subsidence monitoring, measurement of co-seismic displacements, crustal deformation and volcano monitoring, remote sensing of the cryosphere, remote sensing of the biosphere, quantification of carbon sequestration, and, in general, monitoring tasks requiring day/night, all-weather availability.
    [bibtex-key = freySARChapterEncyclopediaOfGeography2016] [bibtex-entry]


  234. Andrea Monti-Guarnieri, Fabio Rocca, and Stefano Tebaldini. SAR interferometry. In Novel Radar Techniques and Applications Volume 1: Real Aperture Array Radar, Imaging Radar, and Passive and Multistatic Radar, Radar, Sonar and Navigation, pages 265-312. Institution of Engineering and Technology, October 2017. Keyword(s): SAR Processing, SAR Interferometry, InSAR, DInSAR, digital elevation models, land monitoring, passive bistatic companions, infrastructure monitoring, multibaseline acquisitions, mechanical stability, performance evaluation, interferometric data processing, electromagnetic wave penetration, radar signals, SAR interferometry, solid Earth monitoring, differential InSAR stacks processing, single-baseline acquisitions, persistent scatterers, SAR tomography, geosynchronous SAR. [bibtex-key = montiGuarnieriRoccaTebaldini2017BookChapterSARInterferometry] [bibtex-entry]


  235. F. Alshawaf, K. Balidakis, G. Dick, S. Heise, and J. Wickert. Estimating trends in atmospheric water vapor and temperature time series over Germany. Atmospheric Measurement Techniques, 10(9):3117-3132, 2017. Note: Cited By 30. [bibtex-key = Alshawaf20173117] [bibtex-entry]


  236. Homa Ansari, Francesco De Zan, and Richard Bamler. Sequential Estimator: Toward Efficient InSAR Time Series Analysis. IEEE Transactions on Geoscience and Remote Sensing, 55(10):5637-5652, October 2017. Keyword(s): SAR Processing, SAR Interferometry, InSAR, differential SAR interferometry, D-InSAR, covariance matrices, data compression, image coding, radar imaging, radar interferometry, synthetic aperture radar, time series, Big Data, InSAR time series analysis, compressed data batch artificial interferograms, data batch compression, data covariance matrix analysis, data reduction, high-precision near-real-time processing, interferometric phase estimation, recursive estimation, sequential estimator, virtual image estimator, wide-swath synthetic aperture radar mission, Coherence, Earth, Maximum likelihood estimation, Monitoring, Synthetic aperture radar, Time series analysis, Big Data, coherence estimation error, data compression, differential interferometric synthetic aperture radar (DInSAR), distributed scatterers, efficiency, error analysis, low-rank approximation, maximum-likelihood estimation (MLE).
    Abstract: Wide-swath synthetic aperture radar (SAR) missions with short revisit times, such as Sentinel-1 and the planned NISAR and Tandem-L, provide an unprecedented wealth of interferometric SAR (InSAR) time series. However, the processing of the emerging Big Data is challenging for state-of-the-art InSAR analysis techniques. This contribution introduces a novel approach, named Sequential Estimator, for efficient estimation of the interferometric phase from long InSAR time series. The algorithm uses recursive estimation and analysis of the data covariance matrix via division of the data into small batches, followed by the compression of the data batches. From each compressed data batch artificial interferograms are formed, resulting in a strong data reduction. Such interferograms are used to link the older data batches with the most recent acquisitions and thus to reconstruct the phase time series. This scheme avoids the necessity of reprocessing the entire data stack at the face of each new acquisition. The proposed estimator introduces negligible degradation compared to the Cramer-Rao lower bound under realistic coherence scenarios. The estimator may therefore be adapted for high-precision near-real-time processing of InSAR and accommodate the conversion of InSAR from an offline to a monitoring geodetic tool. The performance of the Sequential Estimator is compared to state-of-the-art techniques via simulations and application to Sentinel-1 data.
    [bibtex-key = ansariDeZanBamlerTGRS2017SequentialEstimatorNRTInSARProcessing] [bibtex-entry]


  237. A. E. A. Blomberg, T. O. Saebo, Roy E. Hansen, R. B. Pedersen, and A. Austeng. Automatic Detection of Marine Gas Seeps Using an Interferometric Sidescan Sonar. IEEE Journal of Oceanic Engineering, 42(3):590-602, July 2017. Keyword(s): Synthetic Aperture Sonar, SAS, carbon capture and storage, interferometry, oceanographic techniques, seafloor phenomena, sediments, sonar, underwater sound, volcanology, North Sea, acoustic backscatter properties, acoustic seep detection, automatic seep detection method, autonomous underwater vehicle, biogenic methane, gravel, interferometric sidescan sonar, man-made constructions, marine gas seep automatic detection, marine gas seep detection method, marine gas seep monitoring method, multibeam surveying, ocean greenhouse gases, pipelines, sandy seafloor, seafloor seeps, seafloor types, sediments, seep acoustical property, seep spatial property, sidescan surveying, signal processing techniques, silt, subseafloor CO2 storage sites, volcano CO2 release, water gas-filled bubbles, well heads, Acoustics, Backscatter, Damping, Monitoring, Resonant frequency, Sonar detection, Automatic leakage detection, carbon capture and storage (CCS) monitoring, coherence, gas seep detection, interferometric sonar, sidescan sonar.
    Abstract: There is a significant need for reliable, cost-effective, and preferably automatic methods for detecting and monitoring marine gas seeps. Seeps at the seafloor may originate from natural sources including sediments releasing biogenic methane and volcanoes releasing CO2, or from man-made constructions such as pipelines or well heads, and potentially also from subseafloor CO2 storage sites. Improved seep detection makes it possible to estimate the amount of greenhouse gases entering the oceans, and to promptly detect and address potential leaks to reduce environmental and economical consequences. Sonar is an excellent tool for seep detection due to the strong acoustic backscatter properties of gas-filled bubbles in water. Existing methods for acoustic seep detection include multibeam and sidescan surveying, as well as active and passive sensors mounted on a stationary platform. In this work, we develop a new method for automatic seep detection using an interferometric sidescan sonar. We apply signal processing techniques combined with knowledge about acoustical and spatial properties of seeps for improved detectability. The proposed method fills an important gap in existing technology-the ability to automatically detect a seep during a single pass with an autonomous underwater vehicle (AUV) equipped with an interferometric sidescan sonar. Results from simulations as well as field data from two leaking abandoned wells in the North Sea indicate that small seeps are consistently detected on a sandy seafloor even when the observation time is limited (a single pass with the AUV). We explore the detection capability for different seafloor types ranging from silt to gravel.
    [bibtex-key = blombergSaeboHansenPedersenAustengJOE2017DetectionUsingInterferometricSonar] [bibtex-entry]


  238. Alessandra Budillon, Angel Caroline Johnsy, and Gilda Schirinzi. Extension of a Fast GLRT Algorithm to 5D SAR Tomography of Urban Areas. Remote Sensing, 9(8), 2017. Keyword(s): SAR Processing, SAR Tomography, radar detection, GLRT, Fast GLRT, deformation, PSI, persistent scatterer interferometry, displacement mapping, radar receivers, radar resolution, synthetic aperture radar, tomography, GLRT detector, ROC curve, SAR tomography, Sup-GLRT, elevation direction, generalized likelihood ratio test detector, multiple scatterer detection, nominal system resolution, nominal system super-resolution, receiver operating characteristic curve, signal sparsity, signal support detection operation, signal support estimation operation, signal-to-noise ratio, subspace energy measurement, synthetic aperture radar, Earth, Estimation, Signal resolution, Spatial resolution, Synthetic aperture radar, Tomography, Generalized likelihood ratio test (GLRT), radar detection, sparse signals, synthetic aperture radar (SAR).
    Abstract: This paper analyzes a method for Synthetic Aperture Radar (SAR) Tomographic (TomoSAR) imaging, allowing the detection of multiple scatterers that can exhibit time deformation and thermal dilation by using a CFAR (Constant False Alarm Rate) approach. In the last decade, several methods for TomoSAR have been proposed. The objective of this paper is to present the results obtained on high resolution tomographic SAR data of urban areas, by using a statistical test for detecting multiple scatterers that takes into account phase variations due to possible deformations and/or thermal dilation. The test can be evaluated in terms of probability of detection (PD) and probability of false alarm (PFA), and is based on an approximation of a Generalized Likelihood Ratio Test (GLRT), denoted as Fast-Sup-GLRT. It was already applied and validated by the authors in the 3D case, while here it is extended and experimented in the 5D case. Numerical experiments on simulated and on StripMap TerraSAR-X (TSX) data have been carried out. The presented results show that the adopted method allows the detection of a large number of scatterers and the estimation of their position with a good accuracy, and that the consideration of the thermal dilation and surface deformation helps in recovering more single and double scatterers, with respect to the case in which these contributions are not taken into account. Moreover, the capability of method to provide reliable estimates of the deformations in urban structure suggests its use in structure stress monitoring.
    [bibtex-key = budillonJohnsySchirinziRemoteSensing2017FastGLRT5DTomo] [bibtex-entry]


  239. J. I. Buskenes, Roy E. Hansen, and A. Austeng. Low-Complexity Adaptive Sonar Imaging. IEEE Journal of Oceanic Engineering, 42(1):87-96, January 2017. Keyword(s): Synthetic Aperture Sonar, SAS, Sonar, array signal processing, interference suppression, sonar imaging, DAS beamformer, Hamming window function, Kaiser window, Kongsberg Maritime HISAS1030 sonar, LCA beamformer, delay-and-sum beamformer, gain -3 dB, interference minimization, low-complexity adaptive sonar imaging, minimum variance distortionless response, parallel hardware, rectangular window function, robust MVDR implementation, spatial statistics, Array signal processing, Arrays, Covariance matrices, Image resolution, Imaging, Robustness, Sonar, Active, LCA, MVDR, active filters, adaptive beamforming, adaptive filters, beamforming, complexity, computational complexity, phased arrays, sonar, spatial filters.
    Abstract: We have studied the low-complexity adaptive (LCA) beamformer in active sonar imaging. LCA can be viewed as either a simplification of the minimum variance distortionless response (MVDR) beamformer, or as an adaptive extension to the delay-and-sum (DAS) beamformer. While both LCA and MVDR attempt to minimize the power of noise and interference in the image, MVDR achieves this by computing optimal array weights from the spatial statistics of the wavefield, while LCA selects the best performing weights out of a predefined set. To build confidence in the LCA method, we show that a robust MVDR implementation typically creates weight sets with shapes spanning between a rectangular and Hamming window function. We let LCA select from a set of Kaiser windows with responses in this span, and add some steered variations of each. We limit the steering to roughly half the -3-dB width of the window's amplitude response. Using experimental data from the Kongsberg Maritime HISAS1030 sonar we find that LCA and MVDR produce nearly identical images of large scenes, both being superior to DAS. On point targets LCA is able to double the resolution compared to DAS, or provide half that of MVDR. This performance is achieved with a total of six windows: the rectangular window and the Kaiser window with beta= 5, in an unsteered version, and versions that are left and right steered to the steering limit. Slightly smoother images are produced if the window count is increased to 15, but past this we observe minimal difference. Finally, we show that LCA works just as well if Kaiser windows are substituted with trigonometric ones. All our observations and experiences point to LCA being very easy to understand and manage. It simply works, and is surprisingly insensitive to the exact type of window function, steering amount, or number of windows. It can be efficiently implemented on parallel hardware, and handles any scene without the need for parameter adjustments.
    [bibtex-key = buskenesHansenAustengJOE2017AdaptiveSonarImaging] [bibtex-entry]


  240. Jemil Butt, Andreas Wieser, and Stefan Conzett. Intrinsic random functions for mitigation of atmospheric effects in terrestrial radar interferometry. Journal of Applied Geodesy, 2017. Keyword(s): Atmospheric phase screen, radar interferometry, terrestrial radar interferometry ground-based radar, ground-based radar interferometry, intrinsic random functions, Kriging, IRF. [bibtex-key = buttWieserConzettJAG2017IntrinsicRandomFuncForAtmophere] [bibtex-entry]


  241. Ning Cao, Hyongki Lee, Evan Zaugg, R. Shrestha, W. Carter, C. Glennie, G. Wang, Z. Lu, and J. C. Fernandez-Diaz. Airborne DInSAR Results Using Time-Domain Backprojection Algorithm: A Case Study Over the Slumgullion Landslide in Colorado With Validation Using Spaceborne SAR, Airborne LiDAR, and Ground-Based Observations. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10(11):4987-5000, November 2017. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Backprojection, Repeat-Pass Interferometry, SAR Interferometry, Displacement, Deformation Measurement, Aircraft, Interferometry, L-band, Spaceborne radar, Synthetic aperture radar, Terrain factors, Trajectory, Backprojection (BP), InSAR, SAR, differential synthetic aperture radar interferometry (DInSAR), landslide, motion compensation (MoCo), residual motion error (RME), Airborne SAR.
    Abstract: The major impediment to accurate airborne repeat-pass differential synthetic aperture radar (SAR) interferometry (DInSAR) is compensating for aircraft motion caused by air turbulence. Various motion compensation (MoCo) procedures have been used in the airborne DInSAR processing to acquire reliable deformation mapping. In this paper, we present the use of time-domain backprojection (BP) algorithm for SAR focusing in an airborne DInSAR survey: No MoCo procedure is needed because the BP algorithm is inherently able to compensate for platform motion. In this study, we present the results of a pilot study aimed at demonstrating the feasibility of deformation mapping with an airborne SAR system based on the monitoring of the Slumgullion landslide in Colorado, USA between July 3 and 10 of 2015. The employed airborne SAR system is an Artemis SlimSAR that is a compact, modular, and multi-frequency radar system. Airborne light detection and ranging and global navigation satellite system (GNSS) observations, as well as spaceborne DInSAR results using COSMO-SkyMed (CSK) images, were used to verify the performance of the airborne SAR system. The surface velocities of the landslide derived from the airborne DInSAR observations showed good agreement with the GNSS and spaceborne DInSAR estimates. A three-dimensional deformation map of the Slumgullion landslide was also generated, which displayed distinct correlation between the landslide motion and topographic variation. This study shows that an inexpensive airborne L-band DInSAR system has the potential to measure centimeter level deformation with flexible temporal and spatial baselines.
    [bibtex-key = caoLeeZauggEtAlJSTARS2017AirborneSARBackprojectionDInSAR] [bibtex-entry]


  242. Victor Cazcarra-Bes, Marivi Tello-Alonso, Rico Fischer, Michael Heym, and Konstantinos Papathanassiou. Monitoring of Forest Structure Dynamics by Means of L-Band SAR Tomography. Remote Sensing, 9(12), 2017. Keyword(s): SAR Processing, SAR Tomography, Forestry, Synthetic aperture radar, Image reconstruction, Decorrelation, Estimation, forest applications, forest structure, Fourier beamforming (FB), L-band, synthetic aperture radar (SAR), tomography, L-Band, Airborne SAR.
    Abstract: Synthetic Aperture Radar Tomography (TomoSAR) allows the reconstruction of the 3D reflectivity of natural volume scatterers such as forests, thus providing an opportunity to infer structure information in 3D. In this paper, the potential of TomoSAR data at L-band to monitor temporal variations of forest structure is addressed using simulated and experimental datasets. First, 3D reflectivity profiles were extracted by means of TomoSAR reconstruction based on a Compressive Sensing (CS) approach. Next, two complementary indices for the description of horizontal and vertical forest structure were defined and estimated by means of the distribution of local maxima of the reconstructed reflectivity profiles. To assess the sensitivity and consistency of the proposed methodology, variations of these indices for different types of forest changes in simulated as well as in real scenarios were analyzed and assessed against different sources of reference data: airborne Lidar measurements, high resolution optical images, and forest inventory data. The forest structure maps obtained indicated the potential to distinguish between different forest stages and the identification of different types of forest structure changes induced by logging, natural disturbance, or forest management.
    [bibtex-key = cazcarraBesTelloAlonsoFischerHeymPapathanassiouRemoteSensing2017ForstStructureDynamicsLBandSARTomography] [bibtex-entry]


  243. Claudio De Luca, Ivana Zinno, Michele Manunta, Riccardo Lanari, and Francesco Casu. Large areas surface deformation analysis through a cloud computing P-SBAS approach for massive processing of DInSAR time series. Remote Sensing of Environment, 2017. Keyword(s): DInSAR, P-SBAS, Cloud computing, Mosaicking, ENVISAT, Sentinel-1. [bibtex-key = deLucaZinnoManuntaLanariCasuRSE2017CloudComputingPSBASDeformation] [bibtex-entry]


  244. Masato Furuya, Takato Suzuki, Jun Maeda, and Kosuke Heki. Midlatitude sporadic-E episodes viewed by L-band split-spectrum InSAR. Earth, Planets and Space, 69(1):175, December 2017. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, Split-Spectrum, Split-band, Split-Spectrum Interferometry, Split-band Interferometry, Total Electron Content Estimation, TEC Estimation, Ionospheric TEC, Faraday Rotation, Path Delay, Spaceborne SAR, L-Band, ALOS, Phased Array L-band SAR, PALSAR, Calibration, Ionosphere, Ionospheric Path Delay.
    Abstract: Sporadic-E (Es) is a layer of ionization that irregularly appears within the E region of the ionosphere and is known to generate an unusual propagation of very high frequency waves over long distances. The detailed spatial structure of Es remains unclear due to the limited spatial resolution in the conventional ionosonde observations. We detect midlatitude Es by interferometric synthetic aperture radar (InSAR), which can clarify the spatial structure of Es with unprecedented resolution. Moreover, we use the range split-spectrum method (SSM) to separate dispersive and nondispersive components in the InSAR image. While InSAR SSM largely succeeds in decomposing into dispersive and nondispersive signals, our results indicate that small-scale dispersive signals due to the total electron content anomalies are accompanied by nondispersive signals with similar spatial scale at the same locations. We also examine the effects of higher-order terms in the refractive index for dispersive media. Both of these detected Es episodes indicate that smaller-scale dispersive effects originate from higher-order effects. We interpret that the smaller-scale nondispersive signals could indicate the emergence of nitric oxide (NO) generated by the reactions of metals, Mg and Fe, with nitric oxide ion (NO+) during the Es.
    [bibtex-key = furuyaSuzukiMaedaHeki2017IonoLBandSplitSpectrumInSAR] [bibtex-entry]


  245. Matthew C. Garthwaite. On the Design of Radar Corner Reflectors for Deformation Monitoring in Multi-Frequency InSAR. Remote Sensing, 9(7), 2017.
    Abstract: Trihedral corner reflectors are being increasingly used as point targets in deformation monitoring studies using interferometric synthetic aperture radar (InSAR) techniques. The frequency and size dependence of the corner reflector Radar Cross Section (RCS) means that no single design can perform equally in all the possible imaging modes and radar frequencies available on the currently orbiting Synthetic Aperture Radar (SAR) satellites. Therefore, either a corner reflector design tailored to a specific data type or a compromise design for multiple data types is required. In this paper, I outline the practical and theoretical considerations that need to be made when designing appropriate radar targets, with a focus on supporting multi-frequency SAR data. These considerations are tested by performing field experiments on targets of different size using SAR images from TerraSAR-X, COSMO-SkyMed and RADARSAT-2. Phase noise behaviour in SAR images can be estimated by measuring the Signal-to-Clutter ratio (SCR) in individual SAR images. The measured SCR of a point target is dependent on its RCS performance and the influence of clutter near to the deployed target. The SCR is used as a metric to estimate the expected InSAR displacement error incurred by the design of each target and to validate these observations against theoretical expectations. I find that triangular trihedral corner reflectors as small as 1 m in dimension can achieve a displacement error magnitude of a tenth of a millimetre or less in medium-resolution X-band data. Much larger corner reflectors (2.5 m or greater) are required to achieve the same displacement error magnitude in medium-resolution C-band data. Compromise designs should aim to satisfy the requirements of the lowest SAR frequency to be used, providing that these targets will not saturate the sensor of the highest frequency to be used. Finally, accurate boresight alignment of the corner reflector can be critical to the overall target performance. Alignment accuracies better than 4deg in azimuth and elevation will incur a minimal impact on the displacement error in X and C-band data.
    [bibtex-key = garthwaiteRemoteSensing2017OnTheDesignOfRadarCornerReflectorsForDeformationMonitoring_v2_corrected] [bibtex-entry]


  246. Giorgio Gomba, Fernando Rodriguez Gonzalez, and Francesco De Zan. Ionospheric Phase Screen Compensation for the Sentinel-1 TOPS and ALOS-2 ScanSAR Modes. IEEE_J_GRS, 55(1):223-235, January 2017. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, ionospheric disturbances, total electron content (atmosphere), 2015 Nepal earthquake, 2016 Taiwan earthquake, ALOS-2 ScanSAR modes, ALOS-2 interferograms, C-band interferograms, Sentinel-1 TOPS, dispersive ionospheric component, interferometric measurements, ionospheric phase screen compensation, local global positioning system measurements, split-spectrum method, synthetic aperture radar acquisitions, total electron content maps, Azimuth, Correlation, Electrostatic discharges, Ionosphere, Satellites, Synthetic aperture radar, Timing, InSAR, SAR ionospheric effects, ionosphere estimation.
    Abstract: Variations of the ionosphere can significantly disrupt synthetic aperture radar (SAR) acquisitions and interferometric measurements of ground deformation. In this paper, we show how the ionosphere can also strongly modify C-band interferograms despite its smaller influence at higher frequencies. Thus, ionospheric phase screens should not be neglected: their compensation improves the estimation of ground deformation maps. The split-spectrum method is able to estimate the dispersive ionospheric component of the interferometric phase; we describe the implementation of this method for the burst modes TOPS and ScanSAR to estimate and remove ionospheric phase screens. We present Sentinel-1 interferograms of the 2016 Taiwan earthquake and ALOS-2 interferograms of the 2015 Nepal earthquake, which show strong ionospheric phase gradients, and their corrected versions. Finally, to validate the results and better understand the origin of these ionospheric variations, we compare the estimated differential ionosphere with global Total Electron Content maps and local Global Positioning System measurements.
    [bibtex-key = gombaRodriguezGonzalezDeZanTGRS2017IonoSplitSpectrumInSAR] [bibtex-entry]


  247. G. Gomba and F. De Zan. Bayesian Data Combination for the Estimation of Ionospheric Effects in SAR Interferograms. IEEE_J_GRS, 55(11):6582-6593, November 2017. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, Bayes methods, Faraday effect, fractals, inverse problems, ionospheric electromagnetic wave propagation, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, Bayesian data combination, Bayesian inverse problem, Faraday rotation method, SAR images, SAR interferograms, advanced land observing satellite phased array type L-band SAR L-band images, azimuth mutual shifts, data-based model parameter estimation, differential ionospheric phase screen, error source, estimation accuracy, information sources, interferometric pair images, ionosphere turbulence, ionospheric effects estimation, ionospheric propagation path delay, physically realistic fractal modeling, range variations, sensitive azimuth shifts, simple split-spectrum method, small-scale azimuth variations, synthetic aperture radar interferograms, Azimuth, Estimation, Extraterrestrial measurements, Ionosphere, Synthetic aperture radar, Ionosphere estimation, SAR ionospheric effects, interferometric synthetic aperture radar (SAR), methods\textquoteright combination.
    Abstract: The ionospheric propagation path delay is a major error source in synthetic aperture radar (SAR) interferograms and, therefore, has to be estimated and corrected. Various methods can be used to extract different kinds of information about the ionosphere from SAR images, with different accuracies. This paper presents a general technique, based on a Bayesian inverse problem, that combines various information sources in order to increase the estimation accuracy, and thus the correction. A physically realistic fractal modeling of the ionosphere turbulence and a data-based estimation of the model parameters allow the avoidance of arbitrary filtering windows and coefficients. To test the technique, the differential ionospheric phase screen was estimated by combining the split-spectrum method with the azimuth mutual shifts between interferometric pair images. This combination is convenient since it can benefit from the strengths of both sources: range and azimuth variations from the split-spectrum method and small-scale azimuth variations from more sensitive azimuth shifts. Therefore, the two methods can recover the long and short wavelength components of the ionospheric phase screen, respectively. A theoretical comparison between the Faraday rotation method and the split-spectrum method is also reported. For the use in the combination, precedence was then given to the split-spectrum method because of the comparable precision level, lower susceptibility to biases, and wider applicability. Finally, Advanced Land Observing Satellite Phased Array type L-band SAR L-band images are used to show how the combined result is more accurate than that obtained with the simple split-spectrum method.
    [bibtex-key = gombaDeZanTGRS2017IonoSplitSpectrumAndAzimuthShiftsBaysianComb] [bibtex-entry]


  248. Céline Lamarche, Maurizio Santoro, Sophie Bontemps, Raphaël D'Andrimont, Julien Radoux, Laura Giustarini, Carsten Brockmann, Jan Wevers, Pierre Defourny, and Olivier Arino. Compilation and Validation of SAR and Optical Data Products for a Complete and Global Map of Inland/Ocean Water Tailored to the Climate Modeling Community. Remote Sensing, 9(1), 2017. Keyword(s): Remote Sensing, Water.
    Abstract: Accurate maps of surface water extent are of paramount importance for water management, satellite data processing and climate modeling. Several maps of water bodies based on remote sensing data have been released during the last decade. Nonetheless, none has a truly (90 deg N / 90 deg S) global coverage while being thoroughly validated. This paper describes a global, spatially-complete (void-free) and accurate mask of inland/ocean water for the 2000-2012 period, built in the framework of the European Space Agency (ESA) Climate Change Initiative (CCI). This map results from the synergistic combination of multiple individual SAR and optical water body and auxiliary datasets. A key aspect of this work is the original and rigorous stratified random sampling designed for the quality assessment of binary classifications where one class is marginally distributed. Input and consolidated products were assessed qualitatively and quantitatively against a reference validation database of 2110 samples spread throughout the globe. Using all samples, overall accuracy was always very high among all products, between 98% and 100%. The CCI global map of open water bodies provided the best water class representation (F-score of 89%) compared to its constitutive inputs. When focusing on the challenging areas for water bodies' mapping, such as shorelines, lakes and river banks, all products yielded substantially lower accuracy figures with overall accuracies ranging between 74% and 89%. The inland water area of the CCI global map of open water bodies was estimated to be 3.17 million km^2 +/- 0.24 million km^2. The dataset is freely available through the ESA CCI Land Cover viewer.
    [bibtex-key = lamarcheEtALRemoteSensing2017GlobalWaterMapFromSARandOptical] [bibtex-entry]


  249. Yang Lei, Paul Siqueira, and Robert Treuhaft. A physical scattering model of repeat-pass InSAR correlation for vegetation. Waves in Random and Complex Media, 27(1):129-152, 2017. Keyword(s): SAR Processing, Scattering Model, Interferometry, Correlation, Vegetation.
    Abstract: A physical scattering model of repeat-pass InSAR correlation over forested areas is derived by accounting for the changes in the dielectric properties and positions of the scatterers in the scene between overpasses. This derivation is based on the discrete representation of a sparse random medium (such as forest canopy) along with the solution to the Foldy-Lax multiple scattering equations. In addition to taking into account the random motion of scatterers, which has been investigated in previous work, the derived repeat-pass InSAR correlation model in this paper includes the effects of moisture-induced dielectric fluctuations. This is accomplished by incorporating a separate correlation profile that takes into account these fluctuations into the framework. Once constructed, the mathematical formulation of this scattering model is cast into a modified version of the Random Volume over Ground model such that it can separately take into account dielectric fluctuations in the ground and volume components. This model is then validated using a modified version of ESA's PolSARproSim simulation to show that similar results can be obtained from both the simulation and the theoretical model.
    [bibtex-key = leiSiqueiraTreuhaft2017ScatteringModelRepeatPassInSARCorrelationVegetation] [bibtex-entry]


  250. D. Li, M. Rodriguez-Cassola, P. Prats-Iraola, M. Wu, and A. Moreira. Reverse Backprojection Algorithm for the Accurate Generation of SAR Raw Data of Natural Scenes. IEEE Geoscience and Remote Sensing Letters, 14(11):2072-2076, November 2017. Keyword(s): data acquisition, geophysical image processing, remote sensing by radar, synthetic aperture radar, tropospheric electromagnetic wave propagation, reverse backprojection algorithm, SAR raw data, natural scenes, SAR image formation sibling, multistatic SAR missions, synthetic aperture radar mission concepts, geosynchronous SAR missions, observation geometry, acquisition strategy, atmospheric propagation, Synthetic aperture radar, Low earth orbit satellites, Azimuth, Atmospheric modeling, Standards, Algorithm design and analysis, Data models, Azimuth variation, backprojection algorithm, geosynchronous (GEO) SAR, raw data simulation, synthetic aperture radar (SAR), terrain observation with progressive scan (TOPS), tropospheric propagation.
    Abstract: Future synthetic aperture radar (SAR) mission concepts often rely on locally nonlinear (e.g., high orbits and bistatic) surveys or acquisition schemes. The simulation of the raw data of natural scenes as acquired by future systems appears as one powerful tool in order to understand the particularities of these systems and assess the impact of system and propagation errors on their performance. We put forward, in this letter, a new formulation of the reverse backprojection algorithm for the accurate simulation of raw data of natural surfaces. In particular, the algorithm is perfectly suited to accommodate any kind (1-D/2-D) of temporal and spatial variation, e.g., in observation geometry, acquisition strategy, or atmospheric propagation. The algorithm is analyzed with respect to its SAR image formation sibling, and tested under different simulation scenarios. We expect the reverse backprojection algorithm to play a relevant role in the simulation of future geosynchronous and multistatic SAR missions.
    [bibtex-key = liRodriguezPratsWuMoreiraGRSL2017ReverseBackprojectionForRawDataGeneration] [bibtex-entry]


  251. Simone Mancon, Andrea Monti Guarnieri, Davide Giudici, and Stefano Tebaldini. On the Phase Calibration by Multisquint Analysis in TOPSAR and Stripmap Interferometry. IEEE Trans. Geosci. Remote Sens., 55(1):134-147, January 2017. Keyword(s): SAR Processing, SAR Interferometry, Atmospheric modeling, Interferometry, Orbits, Spaceborne radar, Synthetic aperture radar, Target tracking, Trajectory, Calibration, TOPSAR, interferometry, multisquint phase, spaceborne, synthetic aperture radar, SAR, Spaceborne SAR. [bibtex-key = ManconMontiGuarnieriGiudiciTebaldiniTGRS2017PhaseCalTOPSARMultiSquint] [bibtex-entry]


  252. Christopher R. Mannix, David P. Belcher, and Paul S. Cannon. Measurement of Ionospheric Scintillation Parameters From SAR Images Using Corner Reflectors. IEEE Trans. Geosci. Remote Sens., 55(12):6695-6702, December 2017. Keyword(s): SAR Processin, Ionosphere, Ionospheric Scintillation, Global Positioning System, ionospheric electromagnetic wave propagation, radar imaging, radiowave propagation, synthetic aperture radar, Ascension Island, L-band, PALSAR-2, SAR PSF, SAR images, analytical theory, corner reflector, corner reflectors, ionospheric scintillation parameters, ionospheric turbulence parameters p, phase scintillation, point spread function, simultaneous GPS measurements, size 5.0 m, spotlight mode, Extraterrestrial measurements, Ionosphere, Radar tracking, Satellites, Spaceborne radar, Synthetic aperture radar, Ionosphere, ionospheric electromagnetic propagation, synthetic aperture radar.
    Abstract: Space-based low-frequency (L-band and below) synthetic aperture radar (SAR) is affected by the ionosphere. In particular, the phase scintillation causes the sidelobes to rise in a manner that can be predicted by an analytical theory of the point spread function (PSF). In this paper, the results of an experiment, in which a 5 m corner reflector on Ascension Island, was repeatedly imaged by PALSAR-2 in the spotlight mode are described. Many examples of the effect of scintillation on the SAR PSF were obtained, and all fit the theoretical model. This theoretical model of the PSF has then been used to determine two ionospheric turbulence parameters p and CkL from the SAR PSF. The values obtained have been compared with those obtained from simultaneous GPS measurements. Although the comparison shows that the two measures are strongly correlated, the differing spatial and temporal scales of SAR and GPS make exact comparison difficult.
    [bibtex-key = mannixBelcherCannonTGRS2017IonosphericScintillationFromCornerReflectors] [bibtex-entry]


  253. Delwyn Moller, Konstantinos M. Andreadis, Kat J. Bormann, Scott Hensley, and Thomas H. Painter. Mapping Snow Depth From Ka-Band Interferometry: Proof of Concept and Comparison With Scanning Lidar Retrievals. IEEE Geoscience and Remote Sensing Letters, 14(6):886-890, June 2017. Keyword(s): SAR Processing, GLINT, Ka-band, airborne SAR, snow, snow depth, interferometry, SAR interferometry.
    Abstract: This letter presents the first demonstration of millimeter-wave single-pass interferometric synthetic aperture radar (InSAR) for snow-depth mapping. Maps are presented over the Tuolumne River Basin region of the Sierra Nevada, CA, USA, and compared with those collected by a scanning lidar onboard the NASA Airborne Snow Observatory for the same region on the same snow day. For this observation, the snow surface was wet and melting and as such penetration of the electromagnetic wave into the snow volume can be effectively neglected. Despite the rugged terrain, heavy tree-cover, and very low snow-volume, depth maps had a standard deviation <;1 m with the largest differences occurring on slopes exceeding 40�. While additional evaluation is needed with demonstration of the InSAR capability over a greater range of conditions and terrain, these results are promising. InSAR for snow-depth mapping holds significant advantages for a spaceborne mission if proven viable as it can operate through cloud cover, day or night, and measure snowpack when wet or melting.
    [bibtex-key = mollerEtAlTGRS2017MappingSnowDepthFromKaBandInSAR] [bibtex-entry]


  254. Andrea Monti Guarnieri and Fabio Rocca. Options for continuous radar Earth observations. Science China Information Sciences, 60(6), May 2017. Keyword(s): SAR Processing, geostationary, geostationary SAR, geosynchronous, geosynchronous SAR, Spaceborne SAR.
    Abstract: Near Real Time (minutes or hours) radar imaging of ground targets located anywhere on an hemisphere, with or without interferometric coherence with previous passes, can be obtained with different solutions that are considered here. Geosynchronous systems, from the one proposed in 1978 by Tomiyasu to telecom satellite compatible solutions, and Low, Medium or Geosynchronous Earth Orbit constellations are discussed. Their benefits, problems, and sizes are briefly summarized, and a comparative table is presented. If interfer-ometric coherence is requested, continuous imaging is obtained only if a very wide geostationary aperture is progressively scanned, eventually using a MIMO (Multiple Input Multiple Output) combination of several slow librating small satellites. Instead, fast librating, strip mapping, large geosynchronous satellites do provide high resolution imaging, but interferometry (and thus coherent change detection) is achievable only after a minimum delay of 12 h, i.e., when the target comes in sight without need to squint the antenna. Hence, both complex and simple systems reach full resolution interferometric imaging and thus coherent change detection capability only after 12 h.
    [bibtex-key = guarnieriRoccaSCICHINA2017OptionsForContinuousRadarEarthObservations] [bibtex-entry]


  255. Amy L. Parker, Will E. Featherstone, Nigel T. Penna, Mick S. Filmer, and Matthew C. Garthwaite. Practical Considerations before Installing Ground-Based Geodetic Infrastructure for Integrated InSAR and cGNSS Monitoring of Vertical Land Motion. Sensors, 17(8):1-20, 2017. Keyword(s): SAR Processing, GNSS, GPS, SAR Interferometry, Integration of GNSS Networks and SAR data, Persistent Scatterer Interferometry, PSI.
    Abstract: Continuously operating Global Navigation Satellite Systems (cGNSS) can be used to convert relative values of vertical land motion (VLM) derived from Interferometric Synthetic Aperture Radar (InSAR) to absolute values in a global or regional reference frame. Artificial trihedral corner reflectors (CRs) provide high-intensity and temporally stable reflections in SAR time series imagery, more so than naturally occurring permanent scatterers. Therefore, it is logical to co-locate CRs with cGNSS as ground-based geodetic infrastructure for the integrated monitoring of VLM. We describe the practical considerations for such co-locations using four case-study examples from Perth, Australia. After basic initial considerations such as land access, sky visibility and security, temporary test deployments of co-located CRs with cGNSS should be analysed together to determine site suitability. Signal to clutter ratios from SAR imagery are used to determine potential sites for placement of the CR. A significant concern is whether the co-location of a deliberately designed reflecting object generates unwanted multipath (reflected signals) in the cGNSS data. To mitigate against this, we located CRs >30 m from the cGNSS with no inter-visibility. Daily RMS values of the zero-difference ionosphere-free carrier-phase residuals, and ellipsoidal heights from static precise point positioning GNSS processing at each co-located site were then used to ascertain that the CR did not generate unwanted cGNSS multipath. These steps form a set of recommendations for the installation of such geodetic ground-infrastructure, which may be of use to others wishing to establish integrated InSAR-cGNSS monitoring of VLM elsewhere.
    [bibtex-key = parkerEtAlSensors2017InSARandGNSSConsiderationForReflectorInstallation] [bibtex-entry]


  256. Achille Peternier, John Peter Merryman Boncori, and Paolo Pasquali. Near-real-time focusing of ENVISAT ASAR Stripmap and Sentinel-1 TOPS imagery exploiting OpenCL GPGPU technology. Remote Sensing of Environment, 2017. Keyword(s): SAR Processing, Synthetic Aperture Radar, Azimuth Focusing, Image Focusing, GPGPU, GPU, Graphics Processing Units, General-purpose Computing, Parallelization, OpenCL, CUDA.
    Abstract: This paper describes a SAR image focuser application exploiting General-purpose Computing On Graphics Processing Units (GPGPU), developed within the European Space Agency (ESA) funded SARIPA project. Instead of relying on distributed technologies, such as clustering or High-performance Computing (HPC), the SARIPA processor is designed to run on a single computer equipped with multiple GPUs. To exploit the computational power of the latter, while retaining a high level of hardware portability, SARIPA is written using the Open Computing Language (OpenCL) framework rather than the more widespread Compute Unified Device Architecture (CUDA). This allows the application to exploit both GPUs and CPUs without requiring any code modification or duplication. A further level of optimization is achieved thanks to a software architecture, which mimics a distributed computing environment, although implemented on a single machine. SARIPA's performance is demonstrated on ENVISAT ASAR Stripmap imagery, for which a real-time performance of 8.5s is achieved, and on Sentinel-1 Interferometric Wideswath (IW) raw data products, for which a near-real time processing time of about 1min is required. Such a performance has the potential of significantly reducing the storage requirements for wide-area monitoring applications, by avoiding the need of maintaining large permanent archives of Level 1 (focused) imagery, in favor of lighter Level 0 (raw) products, which can be focused on-the-fly within the user's application processing pipelines at almost no overhead.
    [bibtex-key = peternierMerrymanPasqualiRSE2017GPUNearRealTimeAzimuthFocusingSARIPA] [bibtex-entry]


  257. M. Pieraccini and L. Miccinesi. ArcSAR: Theory, Simulations, and Experimental Verification. IEEE Transactions on Microwave Theory and Techniques, 65(1):293-301, January 2017. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar imaging, synthetic aperture radar, 3D space, ArcSAR images, antenna movement, defocusing effect, ground-based synthetic aperture radar, image synthesis, linear rail, spatial diversity, Apertures, Radar antennas, Radar imaging, Spaceborne radar, Synthetic aperture radar, Ground-based synthetic aperture radar (GBSAR), GBSAR, terrestrial radar, radar, remote sensing, synthetic aperture radar (SAR).
    Abstract: ArcSAR is a ground-based synthetic aperture radar (GBSAR) that has recently been receiving increasing interest in the scientific literature. While the conventional GBSAR exploits the movement of an antenna along a linear rail to synthesize a large aperture, an ArcSAR exploits the spatial diversity of the data acquired by an antenna fixed to a rotating arm. The great advantage of ArcSAR is its capability to synthesize images at 360 deg with a constant resolution in azimuth. In this paper, the authors propose and test a new focusing algorithm that does not require to operate in the far field and neither with narrow beam antennas; moreover, it is flexible enough to focus on any plane (not necessarily on the rotation plane) as well as in the whole 3-D space. Furthermore, the authors demonstrate theoretically and experimentally that ArcSAR images can be affected by a ?defocusing effect? of the targets far from the rotation plane, which has to be taken into consideration when designing such radars.
    [bibtex-key = pieracciniMiccinesiTMTT2017ARCSARBGSAR] [bibtex-entry]


  258. Badreddine Rekioua, Matthieu Davy, Laurent Ferro-Famil, and Stefano Tebaldini. Snowpack permittivity profile retrieval from tomographic SAR data. Comptes Rendus Physique, 18(1):57-65, 2017. Keyword(s): Snowpack, snow permittivity, SAR, SAR tomography, GB-SAR, ground-based SAR, Time-domain back-projection, TDBP.
    Abstract: This work deals with 3D structure characterisation and permittivity profile retrieval of snowpacks by tomographic SAR data processing. The acquisition system is a very high resolution ground based SAR system, developed and operated by the SAPHIR team, of IETR, University of Rennes-1 (France). It consists mainly of a vector network analyser and a multi-static antenna system, moving along two orthogonal directions, so as to obtain a two-dimensional synthetic array. Data were acquired during the AlpSAR campaign carried by the European Space Agency and led by ENVEO. In this study, tomographic imaging is performed using Time Domain Back Projection and consists in coherently combining the different recorded backscatter contributions. The assumption of free-space propagation during the focusing process is discussed and illustrated by focusing experimental data. An iterative method for estimating true refractive indices of the snow layers is presented. The antenna pattern is also compensated for. The obtained tomograms after refractive index correction are compared to the stratigraphy of the observed snowpack.
    [bibtex-key = rekiouaDavyFerroFamilTebaldini2017SnowTomography] [bibtex-entry]


  259. Zahra Sadeghi, Mohammad Javad Valadan Zoej, and Jan-Peter Muller. Combination of Persistent Scatterer Interferometry and Single-Baseline Polarimetric Coherence Optimisation to Estimate Deformation Rates with Application to Tehran Basin. PFG -- Journal of Photogrammetry, Remote Sensing and Geoinformation Science, 85(5):327-340, December 2017.
    Abstract: This study reports on the monitoring of land subsidence in a rural area located in the southwest of the Tehran basin, Iran, by combining a persistent scatterer interferometry (PSI) method with a single-baseline polarimetric coherence optimisation. Owing to vegetation coverage in this rural area, coherence level experiences a decline and the performance and coverage of conventional interferometry to estimate deformation rate reduces concomitantly. Since the launch of satellites with polarimetric information, the polarimetric InSAR (PolInSAR) technique, which is vector interferometry with different polarimetric channels, has been introduced to optimise the coherence level. One of the most common criteria to select PS pixels is coherence and maximising the coherence can lead to an increased number of selected PS pixels and enhanced PSI performance. The single-baseline polarimetric coherence optimisation method assumes equal polarisation states at the end of each baseline. In order to apply this technique in our study, two different multi-look windows for coherence calculation and also two TerraSAR-X data sets with different numbers of images are used to assess their effect on the polarimetric PSI. Combination of the single-baseline coherence optimisation method with PSI shows significant improvements (more than 50{\%}) in terms of the number of selected PS pixels in the case study even using a data set with a small number of images. A 15 x 15 multi-look window selects a greater number of PS pixels compared to a 9 x 9 multi-look window, although this entails reducing spatial resolution. The most effective PSI approach in terms of the density of the selected PS turned out to be polarimetric PSI using a data set with a large number of images and a selection of a 15 x 15 multi-look window. Validation of the PSI methods using a large number of images with 9 x 9 and 15 x 15 multi-look windows via levelling measurements confirms the accuracy and reliability of the results obtained.
    [bibtex-key = sadeghiValadanZoejMullerPFG2017PSInSARPolarimetricCoherenceOptimisation] [bibtex-entry]


  260. Zahra Sadeghi, M. J. Valadan Zoej, and J. P. Muller. Monitoring Land Subsidence in a Rural Area Using a Combination of ADInSAR and Polarimetric Coherence Optimization. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10(8):3582-3590, August 2017. Keyword(s): environmental monitoring (geophysics), geomorphology, optimisation, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, MCPO, North Iran, Tehran basin, TerraSAR-X images, agricultural fields, coherence-set based polarimetry optimization, coherent pixel density, differential synthetic aperture radar interferometry, land subsidence monitoring, modified coherence set-based polarimetry optimization, pixel phase quality, polarimetric ADInSAR, polarimetric DInSAR, polarimetric coherence optimization, rural area, satellites, search polarimetry optimization, single-baseline coherence optimization method, single-baseline coherence optimization technique, Coherence, Interferometry, Optimization methods, Polarimetry, Synthetic aperture radar, Advanced differential synthetic aperture radar interferometry (ADInSAR), coherence optimization, polarimetric differential synthetic aperture radar interferometry (DInSAR), polarimetry.
    Abstract: This paper investigates a combination of advanced differential synthetic aperture radar interferometry (ADInSAR) with different coherence optimization methods. After the launch of satellites with polarimetry capabilities, differential synthetic aperture radar interferometry (DInSAR) is feasible to generate polarimetric DInSAR to enhance pixel phase quality and increase coherent pixel (CP) density. The first method proposed in this paper, modified coherence set-based polarimetry optimization (MCPO), is a modification of a known single-baseline coherence optimization method to optimize coherence of all interferograms simultaneously. The second method, coherence-set based polarimetry optimization (CPO), was presented by Neumann et al., and is an existing revision of the single-baseline coherence optimization technique. The final method, exhaustive search polarimetry optimization, is a search-based approach to find the optimized scattering mechanism introduced by Navarro-Sanchez et al. The case study is the Tehran basin located in the North of Iran, which suffers from a high-rate of land subsidence and is covered by agricultural fields. Usually such an area would significantly decorrelate but applying polarimetric ADInSAR allows us to obtain a more CP coverage. A set of dual polarization TerraSAR-X images with 9x9 and 15 x 15 as multilook factors were used within the polarimetric ADInSAR procedure. All three coherence optimization methods with two different multilook factors are shown to have increased the density and phase quality of CPs. Moreover, the estimated deformation rates were evaluated using available levelling measurements. MCPO, which is presented in this paper, works more successful than CPO in terms of CPs density, phase quality and deformation accuracy.
    [bibtex-key = sadeghiValadanZoejMullerJSTARSDInSARandPolarimetricCoherenceOptimization] [bibtex-entry]


  261. H. Schuh, J. Wickert, G. Dick, M. Ge, M. Ramatschi, X. Li, Z. Deng, F. Alshawaf, C. Arras, and B. Männel. GNSS research at GFZ for monitoring the Earth system [GNSS-Arbeiten am GFZ zum Monitoring des Systems Erde]. AVN Allgemeine Vermessungs-Nachrichten, 124(6):175-185, 2017. Note: Cited By 0. [bibtex-key = Schuh2017175] [bibtex-entry]


  262. Tazio Strozzi, Rafael Caduff, Urs Wegmuller, Hugo Raetzo, and Marc Hauser. Widespread surface subsidence measured with satellite SAR interferometry in the Swiss alpine range associated with the construction of the Gotthard Base Tunnel. Remote Sensing of Environment, 190:1 - 12, 2017. Keyword(s): SAR interferometry, Tunnel, Surface subsidence, Landslides.
    Abstract: Abstract Drilling deep tunnels in alpine rocks might induce surface settlements of a few centimetres because of groundwater drainage and associated pore pressure reduction. Settlements of this order of magnitude are sufficient to pose a potential threat to the integrity of any large concrete structure such as arch dams located above the tunnel and an accurate survey of surface deformation before, during and after construction is of high importance. We present the spatial and temporal evolution of surface subsidence measured with satellite SAR interferometry associated with the construction of the 57km long Gotthard Base Tunnel in Switzerland. Significant deformations of 1 to 12mm/year were detected between 2003 and 2010 with ENVISAT ASAR data above the tunnel on villages and sparsely vegetated alpine slopes where no displacement was recorded between 1992 and 2000 with ERS-1/2 SAR data. Our results, available also for sectors where there is no information from any other surveying technique, are important not only to assess the hazard posed on any large concrete structure but also for the development and calibration of numerical models - to be employed to simulate the expected surface deformation before and during the construction works - and to study the effect of groundwater drainage on the dynamic of large deep-seated landslides.
    [bibtex-key = strozziCaduffWegmullerRaetzoHauserRSE2017] [bibtex-entry]


  263. S. A. V. Synnes, A. J. Hunter, Roy E. Hansen, T. O. Saebo, H. J. Callow, R. van Vossen, and A. Austeng. Wideband Synthetic Aperture Sonar Backprojection With Maximization of Wave Number Domain Support. IEEE Journal of Oceanic Engineering, 42(4):880-891, October 2017. Keyword(s): Synthetic Aperture Sonar, SAS, image filtering, image resolution, optimisation, sensor arrays, sonar imaging, synthetic aperture sonar, time-domain analysis, BP, SAS arrays, SAS image formation algorithms, TDBP access data, WD filtering, aspect-dependent scattering, data degradation, frequency-dependent scattering, generic SAS design, sensor data quality, spatial domain quality metrics, time domain backprojection access data, wave number domain counterpart, wave number domain support maximization, wideband SAS systems, wideband synthetic aperture sonar backprojection, widebeam synthetic aperture sonar backprojection, Image resolution, Imaging, Performance evaluation, Scattering, Sonar applications, Synthetic aperture sonar, Wideband, Along-track ambiguity, backprojection (BP) algorithm, grating lobes, synthetic aperture sonar (SAS), wideband sonar.
    Abstract: Wideband and widebeam synthetic aperture sonar (SAS) can provide information on the frequency- and aspect-dependent scattering in a scene. We suggest an approach to predict the quality of the sensor data over the available frequencies and aspect angles. We relate the typical spatial domain quality metrics to their wave number domain (WD) counterpart, and use these to map the data quality in WD. Because SAS arrays often are undersampled along-track, we pay particular attention to data degradation from aliasing. We use the proposed approach to examine how three SAS image formation algorithms based on time domain backprojection (TDBP) access data of different quality from wideband SAS systems. We illustrate the results with predictions for a generic SAS design and demonstrate the findings on two experimental systems. We observe that the maximum support of high-quality data is achieved through BP onto a high-resolution grid followed by WD filtering.
    [bibtex-key = synnesHunterHansenSaeboCallowVanVossenAustengJOE2017WidebandSyntheticApertureSonarBackprojection] [bibtex-entry]


  264. Karina Wilgan, Fabian Hurter, Alain Geiger, Witold Rohm, and Jaroslaw Bosy. Tropospheric refractivity and zenith path delays from least-squares collocation of meteorological and GNSS data. Journal of Geodesy, 91(2):117-134, 2017.
    Abstract: Precise positioning requires an accurate a priori troposphere model to enhance the solution quality. Several empirical models are available, but they may not properly characterize the state of troposphere, especially in severe weather conditions. Another possible solution is to use regional troposphere models based on real-time or near-real time measurements. In this study, we present the total refractivity and zenith total delay (ZTD) models based on a numerical weather prediction (NWP) model, Global Navigation Satellite System (GNSS) data and ground-based meteorological observations. We reconstruct the total refractivity profiles over the western part of Switzerland and the total refractivity profiles as well as ZTDs over Poland using the least-squares collocation software COMEDIE (Collocation of Meteorological Data for Interpretation and Estimation of Tropospheric Pathdelays) developed at ETH Zürich. In these two case studies, profiles of the total refractivity and ZTDs are calculated from different data sets. For Switzerland, the data set with the best agreement with the reference radiosonde (RS) measurements is the combination of ground-based meteorological observations and GNSS ZTDs. Introducing the horizontal gradients does not improve the vertical interpolation, and results in slightly larger biases and standard deviations. For Poland, the data set based on meteorological parameters from the NWP Weather Research and Forecasting (WRF) model and from a combination of the NWP model and GNSS ZTDs shows the best agreement with the reference RS data. In terms of ZTD, the combined NWP-GNSS observations and GNSS-only data set exhibit the best accuracy with an average bias (from all stations) of 3.7 mm and average standard deviations of 17.0 mm w.r.t. the reference GNSS stations.
    [bibtex-key = wilganHurterGeigerRohmBosyJoG2017TroposphericRefractivityAndZTDsFromLeastSquaresCollocationOfMeteoDataAndGNSS] [bibtex-entry]


  265. H. Yu, Y. Lan, J. Xu, D. An, and Hyonki Lee. Large-Scale ${L}^{0}$ -Norm and ${L}^{1}$ -Norm 2-D Phase Unwrapping. IEEE Transactions on Geoscience and Remote Sensing, 55(8):4712-4728, August 2017. Keyword(s): SAR Processing, Phase Unwrapping, radar interferometry, remote sensing by radar, signal processing, synthetic aperture radar, InSAR technology, L0-norm 2D phase unwrapping, L1-norm 2D phase unwrapping, L1-norm envelope-sparsity theorem, big-data, computer hardware, global L1-norm PU solution, local L1-norm PU solution, subinterferograms, synthetic aperture radar interferometry, tiling accuracy, tiling resolution, tiling strategy, Hardware, Laser radar, Laser theory, Memory management, Optimization, Spatial resolution, Synthetic aperture radar interferometry, 2-D phase unwrapping (PU), L1-norm, L0-norm, large scale, synthetic aperture radar interferometry (InSAR), tiling strategy.
    Abstract: Two-dimensional phase unwrapping (PU) is a crucial processing step of synthetic aperture radar interferometry (InSAR). With the rapid advance of InSAR technology, the scale of interferograms is becoming increasingly larger. When the size of the input interferogram exceeds computer hardware capabilities, PU becomes more problematic in terms of computational and memory requirements. In the case of ?big-data? PU, the input interferogram needs to be first tiled into a number of subinterferograms, unwrapped separately, and then spliced together. Hence, whether the PU result of each subinterferogram is consistent with that of the whole interferogram is critical to the large-scale PU process. To effectively solve this problem, the L1-norm envelope-sparsity theorem, which gives a sufficient condition to exactly guarantee the consistency between local and global L1-norm PU solutions, is put forward and proved. Furthermore, the L0-norm envelope-sparsity theorem, which gives a sufficient condition to exactly guarantee the consistency between local and global L0-norm PU solutions, is also proposed and proved. Afterward, based on these two theorems, two tiling strategies are put forward for the large-scale L0-norm and L1-norm PU methods. In addition, this paper presents the concepts of the tiling accuracy and the tiling resolution, which are the criteria used to evaluate the effectiveness of a tiling strategy, and we use them to quantitatively analyze the aforementioned tiling strategies. Both theoretical analysis and experimental results show that the proposed tiling strategies are effective for the largescale L0-norm and L1-norm PU problems.
    [bibtex-key = yuLanXuAnLeeTGRS2017L1NormL0Norm2DPhaseUnwrapping] [bibtex-entry]


  266. S. Zhou, L. Yang, L. Zhao, and G. Bi. Quasi-Polar-Based FFBP Algorithm for Miniature UAV SAR Imaging Without Navigational Data. IEEE Transactions on Geoscience and Remote Sensing, 55(12):7053-7065, December 2017. Keyword(s): autonomous aerial vehicles, image resolution, radar imaging, radar resolution, synthetic aperture radar, polar coordinate system, phase autofocusing, trajectory deviations, quasipolar grid image, data-driven motion compensation, back-projection algorithm, unmanned aerial vehicle synthetic aperture radar applications, time-domain algorithms, trajectory designation, flexible geometric configuration, navigational data, miniature UAV SAR imaging, FFBP algorithm, miniature UAV-SAR test bed, raw data experiments, high-resolution SAR applications, image focusing quality, analytical image spectrum, phase errors, quasipolar coordinate system, Synthetic aperture radar, Trajectory, Unmanned aerial vehicles, Signal processing algorithms, Algorithm design and analysis, Fast factorized back-projection (FFBP), motion compensation (MOCO), quasi-polar coordinate system, synthetic aperture radar (SAR), unmanned aerial vehicle (UAV).
    Abstract: Because of flexible geometric configuration and trajectory designation, time-domain algorithms become popular for unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) applications. In this paper, a new quasi-polar-coordinate-based fast factorized back-projection (FFBP) algorithm combined with data-driven motion compensation is proposed for miniature UAV-SAR imaging. By utilizing wavenumber decomposition, the analytical spectrum of a quasi-polar grid image is obtained, where the phase errors arising from the trajectory deviations can be conveniently investigated and the phase autofocusing can be compatibly incorporated. Different from the conventional FFBP based on a polar coordinate system, the proposed algorithm operates in a quasi-polar coordinate system, where the phase errors become spacial invariant and can be accurately estimated and easily compensated. Moreover, the relationship between phase errors and nonsystematic range cell migration (NsRCM) is revealed according to the analytical image spectrum, based on which the NsRCM correction is developed to further improve the image focusing quality for high-resolution SAR applications. Promising experimental results from the raw data experiments of miniature UAV-SAR test bed are presented and analyzed to validate the advantages of the proposed algorithm.
    [bibtex-key = zhouYangZhaoBiTGRS2017QuasiPolarFFBPforMiniatureUAVbasedSARImagingWithoutNavigationData] [bibtex-entry]


  267. Homa Ansari, Francesco De Zan, Alessandro Parizzi, Michael Eineder, Kanika Goel, and Nico Adam. Measuring 3-D Surface Motion With Future SAR Systems Based on Reflector Antennae. IEEE Geoscience and Remote Sensing Letters, 13(2):272-276, February 2016. Keyword(s): artificial satellites, radar interferometry, reflector antennas, remote sensing by radar, synthetic aperture radar, 3D surface motion measurement, SAR system, reflector antennae, interferometric synthetic aperture radar, 1D line-of-sight motion measurement, left-looking observation, right-looking observation, SAR acquisition mode, BiDiSAR, SuperSAR, electronic beam steering, squinted SAR geometry, satellite constellation, data processing, Synthetic aperture radar, Orbits, Geometry, Satellites, Motion measurement, Antenna measurements, Sensitivity, Azimuth shifts, error analysis, interferometric SAR (InSAR), SAR acquisition geometry, squinted SAR, 3-D surface motion, Azimuth shifts, error analysis, interferometric SAR (InSAR), SAR acquisition geometry, squinted SAR, 3-D surface motion.
    Abstract: A conventional interferometric synthetic aperture radar (SAR) system provides 1-D line-of-sight motion measurements from repeat-pass observations. Two-dimensional motions may be measured by combining two observations from ascending and descending geometries. The third motion component may be retrieved by adding a third geometry and/or by integrating along-track measurements although with much reduced precision compared to the other two components. Several options exist to improve the accuracy of retrieving the third motion component, such as combining left- and right-looking observations or exploiting recently proposed innovative SAR acquisition modes (BiDiSAR and SuperSAR). These options are, however, challenging for future SAR systems based on large reflector antennae, due to lack of capability to electronic beam steering or frequent toggle between left- and right-looking modes. Therefore, in this letter, we assess and compare the realistic acquisition scenarios for a reflector-based SAR in an attempt to optimize the achievable 3-D precision. Investigating the squinted SAR geometry as one of the feasible scenarios, we show that a squint of 13.5deg will yield comparable performance to the left-looking acquisition, while further squinting outperforms this or other feasible configurations. As an optimum configuration for 3-D retrieval, the squinted acquisition is further elaborated: the different acquisition plans considering a constellation of two satellites as well as the challenges for data processing are addressed.
    [bibtex-key = ansariDeZanParizziEinederGoelAdamGRSL3DSurfaceMotionFromFutureSARSystems] [bibtex-entry]


  268. F. Banda, J. Dall, and S. Tebaldini. Single and Multipolarimetric P-Band SAR Tomography of Subsurface Ice Structure. IEEE Transactions on Geoscience and Remote Sensing, 54(5):2832-2845, May 2016. Keyword(s): glaciology, remote sensing by radar, synthetic aperture radar, multipolarimetric P-band SAR tomography, subsurface ice structure, multipolarization synthetic aperture radar, glaciers, ice sheets, Earth Explorer mission BIOMASS, IceSAR 2012, TomoSAR techniques, southwest of Greenland, cryospheric remote sensing, Synthetic aperture radar, Ice, Tomography, Estimation, Image resolution, Scattering, BIOMASS, cryosphere, Greenland, synthetic aperture radar (SAR), tomography, BIOMASS, cryosphere, Greenland, synthetic aperture radar (SAR), tomography.
    Abstract: In this paper, first results concerning the characterization of the subsurface of ice sheets and glaciers through single and multipolarization synthetic aperture radar (SAR) tomography (TomoSAR) are illustrated. To this aim, the processing of data acquired in the framework of the European Space Agency IceSAR 2012 campaign is discussed. IceSAR 2012 was conceived so as to support the secondary objectives of the future Earth Explorer mission BIOMASS, which will be a SAR instrument with media penetration capabilities due to the use of the P-band frequency. In this regard, a tomographic study of ice was motivated by the fact that cryospheric remote sensing is of fundamental importance in order to understand more in depth the morphology and the dynamic processes regulating ice sheets. The main objective of the tomographic experiment of the campaign herein discussed was indeed to assess the capability of P-band SAR to retrieve any information about ice subsurface structure. Imaging has been achieved through TomoSAR techniques, applied to airborne multibaseline data acquired in the southwest of Greenland. Different imaging approaches are compared, and the main results achieved are presented: It is found that scattering in the upper layers of glacial subsurface can be achieved up to an extent of about 20-60 m, conditional on the different types of glaciological zone observed. Moreover, clear morphological structures have been found beneath the ice surface at one of the investigated sites.
    [bibtex-key = bandaDallTebaldiniTGRS2016SingleMultipolPBandTomoSubsurfaceIce] [bibtex-entry]


  269. David P. S. Bekaert, P. Segall, Tim J. Wright, and Andrew J. Hooper. A Network Inversion Filter combining GNSS and InSAR for tectonic slip modeling. Journal of Geophysical Research: Solid Earth, 121(3):2069-2086, March 2016. [bibtex-key = bekaertSegallWrightHooperAGUJGR2016NetworkInversionFilterCombiningGNSSandInSARforTectonicSlipModelling] [bibtex-entry]


  270. Alessandra Budillon and Gilda Schirinzi. GLRT Based on Support Estimation for Multiple Scatterers Detection in SAR Tomography. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(3):1086-1094, March 2016. Keyword(s): SAR Processing, SAR Tomography, radar detection, radar receivers, radar resolution, synthetic aperture radar, tomography, GLRT detector, ROC curve, SAR tomography, Sup-GLRT, elevation direction, generalized likelihood ratio test detector, multiple scatterer detection, nominal system resolution, nominal system super-resolution, receiver operating characteristic curve, signal sparsity, signal support detection operation, signal support estimation operation, signal-to-noise ratio, subspace energy measurement, synthetic aperture radar, Earth, Estimation, Signal resolution, Spatial resolution, Synthetic aperture radar, Tomography, Generalized likelihood ratio test (GLRT), radar detection, sparse signals, synthetic aperture radar (SAR).
    Abstract: In this paper, a generalized likelihood ratio test (GLRT) detector, based on support estimation (Sup-GLRT), for multiple scatterers detection in SAR Tomography, is proposed. It incorporates, in the statistical model, a sparsity assumption of the signal in the elevation direction, which is always verified in practical cases for scarcely vegetated areas. The test consists of sequential steps: first the presence of scatterers is detected; then, the determination of the number of scatterers and the estimation of their positons is performed sequentially, one by one, by means of a signal support detection-estimation operation. The test proposed follows an approach similar to SGLRT, where decisions are taken based on subspace energy measurements, but it is derived following a different testing order and is investigated both at the nominal system resolution and in the super-resolution cases, showing in the latter case, a detection gain with respect to SGLRT. Sup-GLRT performance is evaluated in terms of receiver operating characteristic (ROC) curves for different signal-to-noise ratio values. Experimental results obtained on real COSMO-SkyMed data are shown.
    [bibtex-key = budillonSchirinziJSTARS2016Tomo] [bibtex-entry]


  271. Mariko S. Burgin, Uday K. Khankhoje, Xueyang Duan, and Mahta Moghaddam. Generalized Terrain Topography in Radar Scattering Models. IEEE Transactions on Geoscience and Remote Sensing, 54(7):3944-3952, July 2016. Keyword(s): terrain mapping, topography (Earth), vegetation, N-layered soil structure, evergreen forest, extended boundary condition method, modular model, multilayered multispecies vegetation model, overlying vegetation, radar scattering models, radar wave interactions, terrain topography, topographic slopes, Backscatter, Radar, Radar scattering, Surface topography, Vegetation mapping, Electromagnetic scattering, modeling, radar terrain factors, remote sensing, vegetation.
    Abstract: Modeling of terrain topography is crucial for vegetated areas given that even small slopes impact and alter the radar wave interactions between the ground and the overlying vegetation. Current missions either exclude pixels with large topographic slopes or disregard the terrain topography entirely, potentially accumulating substantial modeling errors and therefore impacting the retrieval performance over such sloped pixels. The underlying terrain topography needs to be considered and modeled to obtain a truly general and accurate radar scattering model. In this paper, a flexible and modular model is developed: the vegetation is considered by a multilayered multispecies vegetation model capable of representing a wide range of vegetation cover types ranging from bare soil to dense forests. The ground is incorporated with the stabilized extended boundary condition method, allowing the representation of an N-layered soil structure with rough interfaces. Terrain topography is characterized by a 2-D slope with two tilt angles (alpha, beta). Simulation results for an evergreen forest show the impact of a 2-D slope for a range of tilt angles: a 10 deg tilt in the plane of incidence translates to a change of up to 15 dB in 1111, 10 dB in VV, and 1.5 dB in 11V for the total radar backscatter. Terrain topography is shown to be crucial for accurate forward modeling, especially over forested areas.
    [bibtex-key = burginKhankhojeDuanMoghaddamTGRS2016TerrainTopographyRadarScatteringModels] [bibtex-entry]


  272. Mariko S. Burgin and Jakob J. van Zyl. Analysis of Polarimetric Radar Data and Soil Moisture From Aquarius: Towards a Regression-Based Soil Moisture Estimation Algorithm. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(8):3497-3504, August 2016. Keyword(s): Backscatter, Data models, Estimation, L-band, Radar, Soil moisture, Vegetation mapping, Moisture, polarimetric radar, soil, synthetic aperture radar (SAR), time series.
    Abstract: Many soil moisture radar retrieval algorithms depend on substantial amounts of ancillary data, such as land cover type and soil composition. To address this issue, we examine and expand an empirical approach by Kim and van Zyl as an alternative; it describes radar backscatter of a vegetated scene as a linear function of volumetric soil moisture, thus reducing the dependence on ancillary data. We use 2.5 years of L-band Aquarius radar and radiometer derived soil moisture data to determine the two polarization dependent parameters on a global scale and on a weekly basis. We propose a look-up table based soil moisture estimation approach; it is promising due to its simplicity and independence of ancillary data. However, the estimation performance is found to be impacted by the used land cover classification scheme. Our results show that the sensitivity of the radar signal to soil moisture changes seasonally, and that the variation differs depending on vegetation class. While this seasonal variation can be relatively small, it must be properly accounted for as it impacts the soil moisture retrieval accuracy.
    [bibtex-key = burginVanZylJSTARS2016SoilMoisturePolSAR] [bibtex-entry]


  273. Ning Cao, Hyongki Lee, and H. C. Jung. A Phase-Decomposition-Based PSInSAR Processing Method. IEEE Transactions on Geoscience and Remote Sensing, 54(2):1074-1090, February 2016. Keyword(s): radar interferometry, synthetic aperture radar, coherence, distributed scatterer, eigendecomposition, measurement points, multiple scattering mechanisms, persistent scatterer network density, phase-decomposition-based persistent scatterer InSAR method, spatial density, Coherence, Covariance matrices, Decision support systems, Eigenvalues and eigenfunctions, Synthetic aperture radar, Urban areas, Differential interferometric synthetic aperture radar (DInSAR), distributed scatterer (DS) interferometry, persistent scatterer (PS) interferometry (PSI), synthetic aperture radar (SAR).
    Abstract: A phase-decomposition-based persistent scatterer (PS) InSAR (PD-PSInSAR) method is developed in this paper to improve coherence and spatial density of measurement points (MPs). In order to improve PS network density, a distributed scatterer (DS) has been utilized in some advanced PSInSAR process, such as SqueeSAR. In addition to the conventional DS that consists of independent small scatterers with a uniform scattering mechanism, processing the DSs dominated by two or more scattering mechanisms is a promising way to improve MP density. Estimating phases from DS with multiple scattering mechanisms is difficult for many DS algorithms because of the interference between different scattering mechanisms. Recently, a covariance-matrix-decomposition-based method, which is named Component extrAction and sElection SAR (CAESAR), is proposed to extract different scattering components from the analysis of the covariance matrix. Instead of using a covariance matrix, the PD-PSInSAR in this study is developed to perform eigendecomposition on a coherence matrix, in order to estimate the phases corresponding to the different scattering mechanisms, and then to implement these estimated phases in a conventional PSInSAR process. The major benefit of using a coherence matrix rather than a covariance matrix is to compensate the amplitude unbalances among SAR images. A detailed study of comparison among SqueeSAR, CAESAR, and PD-PSInSAR is also performed in this study. It has been found that these three methods share very similar mathematic forms with different weight values. The PD-PSInSAR method is implemented to estimate land deformation over the greater Houston area using Envisat ASAR images, which verifies that the proposed method can detect more MPs and provide better coherences.
    [bibtex-key = caoLeeJungTGRS2016PhaseDecompositionPSInSAR] [bibtex-entry]


  274. S. K. Chan, R. Bindlish, P. E. O'Neill, E. Njoku, T. Jackson, A. Colliander, F. Chen, Mariko S. Burgin, S. Dunbar, J. Piepmeier, S. Yueh, D. Entekhabi, M. H. Cosh, T. Caldwell, J. Walker, X. Wu, A. Berg, T. Rowlandson, A. Pacheco, H. McNairn, M. Thibeault, J. Martinez-Fernandez, Angel Gonzalez-Zamora, M. Seyfried, D. Bosch, P. Starks, D. Goodrich, J. Prueger, M. Palecki, E. E. Small, M. Zreda, J. C. Calvet, W. T. Crow, and Y. Kerr. Assessment of the SMAP Passive Soil Moisture Product. IEEE Transactions on Geoscience and Remote Sensing, 54(8):4994-5007, August 2016. Keyword(s): hydrological techniques, moisture, remote sensing by radar, soil, L-band radar, L-band radiometer, Level 2 Passive Soil Moisture Product, NASA Distributed Active Archive Center at the National Snow and Ice Data Center, NASA SMAP satellite mission, National Aeronautics and Space Administration, SMAP Passive Soil Moisture product, V-pol Single Channel Algorithm, freeze-thaw state, high-resolution soil moisture global mapping, radar irrecoverable hardware failure, radiometer-only soil moisture product, soil moisture estimates, soil moisture retrievals, Agriculture, Data models, Microwave radiometry, NASA, Soil moisture, Spatial resolution, Brightness temperature, L-band, Level 2 Passive Soil Moisture Product, Level 3 Daily Composite Version, Soil Moisture Active Passive (SMAP), land emission, passive microwave remote sensing, soil moisture, tau-omega model, validation.
    Abstract: The National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) satellite mission was launched on January 31, 2015. The observatory was developed to provide global mapping of high-resolution soil moisture and freeze-thaw state every two to three days using an L-band (active) radar and an L-band (passive) radiometer. After an irrecoverable hardware failure of the radar on July 7, 2015, the radiometer-only soil moisture product became the only operational soil moisture product for SMAP. The product provides soil moisture estimates posted on a 36 km Earth-fixed grid produced using brightness temperature observations from descending passes. Within months after the commissioning of the SMAP radiometer, the product was assessed to have attained preliminary (beta) science quality, and data were released to the public for evaluation in September 2015. The product is available from the NASA Distributed Active Archive Center at the National Snow and Ice Data Center. This paper provides a summary of the Level 2 Passive Soil Moisture Product (L2_SM_P) and its validation against in situ ground measurements collected from different data sources. Initial in situ comparisons conducted between March 31, 2015 and October 26, 2015, at a limited number of core validation sites (CVSs) and several hundred sparse network points, indicate that the V-pol Single Channel Algorithm (SCA-V) currently delivers the best performance among algorithms considered for L2_SM_P, based on several metrics. The accuracy of the soil moisture retrievals averaged over the CVSs was 0.038 m3/m3 unbiased root-mean-square difference (ubRMSD), which approaches the SMAP mission requirement of 0.040 m3/m3.
    [bibtex-key = chanEtAlTGRS2016SMAPPassiveSoilMoistureProduct] [bibtex-entry]


  275. Michele Crosetto, Oriol Monserrat, Marìa Cuevas-González, Núria Devanthéry, and Bruno Crippa. Persistent Scatterer Interferometry: A review. ISPRS Journal of Photogrammetry and Remote Sensing, 115:78-89, 2016. Note: Theme issue State-of-the-art in photogrammetry, remote sensing and spatial information science. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Remote sensing, Radar, SAR, Monitoring, Deformation.
    Abstract: Persistent Scatterer Interferometry (PSI) is a powerful remote sensing technique able to measure and monitor displacements of the Earth's surface over time. Specifically, PSI is a radar-based technique that belongs to the group of differential interferometric Synthetic Aperture Radar (SAR). This paper provides a review of such PSI technique. It firstly recalls the basic principles of SAR interferometry, differential SAR interferometry and PSI. Then, a review of the main PSI algorithms proposed in the literature is provided, describing the main approaches and the most important works devoted to single aspects of PSI. A central part of this paper is devoted to the discussion of different characteristics and technical aspects of PSI, e.g. SAR data availability, maximum deformation rates, deformation time series, thermal expansion component of PSI observations, etc. The paper then goes through the most important PSI validation activities, which have provided valuable inputs for the PSI development and its acceptability at scientific, technical and commercial level. This is followed by a description of the main PSI applications developed in the last fifteen years. The paper concludes with a discussion of the main open PSI problems and the associated future research lines.
    [bibtex-key = crosettoMonserratCuevasGonzalesDevantheryCrippaISPRS2016PSIReview] [bibtex-entry]


  276. Brent G. Delbridge, Roland Bürgmann, Eric Fielding, Scott Hensley, and William H. Schulz. Three-dimensional surface deformation derived from airborne interferometric UAVSAR: Application to the Slumgullion Landslide. Journal of Geophysical Research: Solid Earth, 121(5):3951-3977, 2016. Keyword(s): SAR Processing, UAVSAR, landslide, Slumgullion, InSAR, geodesy, inversion, DInSAR, Airborne DInSAR, Surface Displacement, Deformation, Airborne SAR, L-band.
    Abstract: Abstract In order to provide surface geodetic measurements with landslide-wide spatial coverage, we develop and validate a method for the characterization of 3-D surface deformation using the unique capabilities of the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) airborne repeat-pass radar interferometry system. We apply our method at the well-studied Slumgullion Landslide, which is 3.9 km long and moves persistently at rates up to approx. 2 cm/day. A comparison with concurrent GPS measurements validates this method and shows that it provides reliable and accurate 3-D surface deformation measurements. The UAVSAR-derived vector velocity field measurements accurately capture the sharp boundaries defining previously identified kinematic units and geomorphic domains within the landslide. We acquired data across the landslide during spring and summer and identify that the landslide moves more slowly during summer except at its head, presumably in response to spatiotemporal variations in snowmelt infiltration. In order to constrain the mechanics controlling landslide motion from surface velocity measurements, we present an inversion framework for the extraction of slide thickness and basal geometry from dense 3-D surface velocity fields. We find that the average depth of the Slumgullion Landslide is 7.5 m, several meters less than previous depth estimates. We show that by considering a viscoplastic rheology, we can derive tighter theoretical bounds on the rheological parameter relating mean horizontal flow rate to surface velocity. Using inclinometer data for slow-moving, clay-rich landslides across the globe, we find a consistent value for the rheological parameter of 0.85 +/- 0.08.
    [bibtex-key = delbridgeBurgmannFieldingHensleySchulzJGR2016AirborneDINSARwithUAVSARSlumgullionLandslide] [bibtex-entry]


  277. G. Gomba, A. Parizzi, F. De Zan, M. Eineder, and R. Bamler. Toward Operational Compensation of Ionospheric Effects in SAR Interferograms: The Split-Spectrum Method. IEEE_J_GRS, 54(3):1446-1461, March 2016. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, ionospheric electromagnetic wave propagation, synthetic aperture radar, L-band interferograms, L-band synthetic aperture radar interferometric pairs, SAR interferograms, advanced land observing satellite phased-array, differential ionospheric path delay, geophysical processes, ground deformation signals, ionospheric effects operational compensation, ionospheric phase, split-spectrum method, tropospheric path delay, Accuracy, Azimuth, Coherence, Delays, Estimation, Ionosphere, Synthetic aperture radar, Interferometric synthetic aperture radar (InSAR), ionosphere estimation, split spectrum, synthetic aperture radar (SAR) ionospheric effects.
    Abstract: The differential ionospheric path delay is a major error source in L-band interferograms. It is superimposed to topography and ground deformation signals, hindering the measurement of geophysical processes. In this paper, we proceed toward the realization of an operational processor to compensate the ionospheric effects in interferograms. The processor should be robust and accurate to meet the scientific requirements for the measurement of geophysical processes, and it should be applicable on a global scale. An implementation of the split-spectrum method, which will be one element of the processor, is presented in detail, and its performance is analyzed. The method is based on the dispersive nature of the ionosphere and separates the ionospheric component of the interferometric phase from the nondispersive component related to topography, ground motion, and tropospheric path delay. We tested the method using various Advanced Land Observing Satellite Phased-Array type L-band synthetic aperture radar interferometric pairs with different characteristics: high to low coherence, moving and nonmoving terrains, with and without topography, and different ionosphere states. Ionospheric errors of almost 1 m have been corrected to a centimeter or a millimeter level. The results show how the method is able to systematically compensate the ionospheric phase in interferograms, with the expected accuracy, and can therefore be a valid element of the operational processor.
    [bibtex-key = gombaParizziDeZanEinederBamlerTGRS2016IonoSplitSpectrumInSAR] [bibtex-entry]


  278. Scott Hensley, D. Moller, S. Oveisgharan, T. Michel, and X. Wu. Ka-Band Mapping and Measurements of Interferometric Penetration of the Greenland Ice Sheets by the GLISTIN Radar. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(6):2436-2450, June 2016. Keyword(s): Global Positioning System, geophysical image processing, hydrological techniques, ice, image segmentation, meteorological radar, optical radar, radar interferometry, snow, synthetic aperture radar, topography (Earth), Antarctica, Earth environment, GLISTIN elevation measurement, GLISTIN instrument, GLISTIN radar, Greenland ice sheet, Jakobshavn glacier area, Ka-band cross-track interferometric radar, Ka-band mapping, NASA GLISTIN Ka-band interferometric radar, NASA Wallop airborne terrain mapper lidar measurement, climate change, ice cap topography, ice surface topography, image mosaic, interferometric penetration, interferometric penetration measurement, interferometric radar mapping system, kinematic GPS survey measurement, lidar, optical system, swath topographic measurement, Ice, Instruments, Laser radar, Sea measurements, Snow, Surfaces, Glaciers, Ka-band, ice sheets, interferometry, penetration, radar.
    Abstract: Measuring ice surface topography over the major ice caps of Greenland and Antarctica is crucial to quantifying and understanding the effect of climate change on the Earth's environment. Multiple sensors including radars, lidars, and optical systems have been utilized in making these measurements. To integrate data from these multiple sensors into a coherent and self-consistent history of ice cap topography requires knowledge of where vertically within the snow volume the elevation measurement corresponds. This paper examines the penetration of a Ka-band cross-track interferometric radar into the dry firn at Greenland's summit using the NASA GLISTIN Ka-band interferometric radar. GLISTIN elevation measurements are compared to NASA Wallop's Airborne Terrain Mapper lidar and kinematic GPS survey measurements to assess the amount of relative penetration with GPS-surveyed corner reflectors deployed to establish the absolute vertical positioning of the radar data. We found an interferometric penetration depth estimate of 27 $\pm$ 0.3 cm. Moreover, we compare these penetration measurements to model derived estimates of the amount of interferometric penetration and provide sensitivity analysis of the amount of penetration to various ice properties. Interferometric radar mapping systems also have the ability to make wide swath topographic measurements over a wide range of weather conditions either day or night making them ideal instruments for wide area mapping. We illustrate this aspect of interferometric radar mapping with a mosaic of 24 passes of the GLISTIN instrument of the Jakobshavn Glacier area.
    [bibtex-key = hensleyEtAlJSTARS2016KaBandInSARMeasurementOfGreenlandIceSheetsGLISTINRadar] [bibtex-entry]


  279. S. Leinss, H. Löwe, M. Proksch, J. Lemmetyinen, A. Wiesmann, and I. Hajnsek. Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series. The Cryosphere, 10:1771-1797, 2016. Keyword(s): snow, anisotropy, SnowScat, polarimetric, copolar phase difference CPD..
    Abstract: The snow microstructure, i.e., the spatial distribution of ice and pores, generally shows an anisotropy which is driven by gravity and temperature gradients and commonly determined from stereology or computer tomography. This structural anisotropy induces anisotropic mechanical, thermal, and dielectric properties. We present a method based on radio-wave birefringence to determine the depth-averaged, dielectric anisotropy of seasonal snow with radar instruments from space, air, or ground. For known snow depth and density, the birefringence allows determination of the dielectric anisotropy by measuring the copolar phase difference (CPD) between linearly polarized microwaves propagating obliquely through the snowpack. The dielectric and structural anisotropy are linked by Maxwell-Garnett-type mixing formulas. The anisotropy evolution of a natural snowpack in Northern Finland was observed over four winters (2009-2013) with the ground-based radar instrument SnowScat. The radar measurements indicate horizontal structures for fresh snow and vertical structures in old snow which is confirmed by computer tomographic in situ measurements. The temporal evolution of the CPD agreed in ground-based data compared to space-borne measurements from the satellite TerraSAR-X. The presented dataset provides a valuable basis for the development of new snow metamorphism models which include the anisotropy of the snow microstructure.
    [bibtex-key = LeinssLoweProkschLemmetyinenWiesmannHajnsek2016] [bibtex-entry]


  280. J. Lemmetyinen, A. Kontu, J. Pulliainen, J. Vehviläinen, K. Rautiainen, A. Wiesmann, C. Mätzler, C. Werner, H. Rott, T. Nagler, M. Schneebeli, M. Proksch, D. Schüttemeyer, M. Kern, and M. W. J. Davidson. Nordic Snow Radar Experiment. Geoscientific Instrumentation, Methods and Data Systems, 5(2):403-415, 2016. [bibtex-key = lemmetyinenEtAlGeoscientificInstrumentation2016NordicSnowRadarExperimentNoSREx] [bibtex-entry]


  281. C. Lin, B. Rommen, N. Floury, D. Schüttemeyer, M. W. J. Davidson, M. Kern, A. Kontu, J. Lemmetyinen, J. Pulliainen, A. Wiesmann, C. L. Werner, C. Mätzler, M. Schneebeli, M. Proksch, and T. Nagler. Active Microwave Scattering Signature of Snowpack---Continuous Multiyear SnowScat Observation Experiments. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(8):3849-3869, August 2016. Keyword(s): remote sensing by radar, snow, snowpack active microwave scattering signature, multiyear SnowScat observation experiment, European Space Agency SnowScat instrument, aperture scatterometer, gamma remote sensing AG, AD 2009 02, Weissfluhjoch, Davos, Switzerland, alpine snowpack, snowpack physical characterization, Sodankyla, Finland, AD 2009 11, Finnish Meteorological Institute, Lapland, passive microwave observation, winter season, time-domain snow profiling experiment, snow microstructure, snow metamorphism, snow depth, snow-water-equivalent, frequency 9.15 GHz to 17.9 GHz, Snow, Instruments, Microstructure, Spaceborne radar, Backscatter, Microwave radiometry, Microwave backscatter, radar remote sensing, scatterometer, snow microstructure, snowpack, snow-water-equivalent (SWE), time-domain profiling.
    Abstract: European Space Agency's SnowScat instrument is a real aperture scatterometer which was developed by Gamma Remote Sensing AG. It operates in a continuous-wave mode, covers a frequency range of 9.15-17.9 GHz in a user-defined frequency-step and has a full polarimetric capability. The measurement campaigns were started first in February 2009 at Weissfluhjoch, in Davos, Switzerland, as an initial test of the instrument over a deep alpine snowpack. Physical characterizations of the snowpack and meteorological measurements were carried out, which formed a detailed in situ dataset. SnowScat was then moved to Sodankylae in Finland in early November 2009, a site of the Finnish Meteorological Institute in Lapland. In addition to the in situ snowpack characterizations and meteorological observations, continuous passive microwave observations were also performed. During the 2012-2013 winter period, a vertical time-domain snow profiling experiment was carried out in addition for resolving the scattering contributions from the snow layers of different physical properties. This paper summarizes the results of the SnowScat observations and initial comparisons against the in situ meteorological and snowpack data. The Sodankylae campaign data evidenced the high variability of the radar backscatter behavior of snowpack from year to year, which indicates its strong dependency on changing snow microstructure. Indeed, the snow microstructure is continuously driven by snow metamorphism, which are further affected by meteorological conditions and their interannual variability. The backscattering property of snowpack in the range X- to Ku-band for all polarizations appeared to be dominated by its microstructural morphology and underlying ground conditions, and to a lesser extent by the snow depth, or its snow-water-equivalent.
    [bibtex-key = linRommenFlourySchuettemeyerDavidsonKernKontuLemmetyinenPulliainenWiesmannWernerMaetzlerSchneebeliProkschNaglerJSTARS2016MultiyearSnowScatExperiments] [bibtex-entry]


  282. Fabrizio Lombardini and Federico Viviani. Single-look light-burden superresolution differential SAR tomography. Electronics Letters, 52(7):557-558, 2016. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, urban remote sensing, array signal processing, image sampling, radar imaging, radar interferometry, radar resolution, spectral analysis, synthetic aperture radar, 2D baseline-time spectral analysis framework, adaptive 2D spectral estimation, coherent multilooking processing, complex nonstationary scenes, complex-valued synthetic aperture radar data, elevation beamforming, full range-azimuth resolution products, full-3D imaging, height-velocity sidelobe reduction, layover scatterers, mature differential interferometry, multibaseline interferometry, satellites, single-look adaptive Diff-Tomo processor, single-look light-burden superresolution differential SAR tomography, sparse sampling, spatial spectral estimation.
    Abstract: Research and application is spreading of techniques of coherent combination of complex-valued synthetic aperture radar (SAR) data to extract rich information even on complex observed scenes, fully exploiting existing SAR data archives, and new satellites. Among such techniques, SAR tomography stems from multibaseline interferometry to achieve full-3D imaging through elevation beamforming (spatial spectral estimation). The Tomo concept has been integrated with the mature differential interferometry, producing the new differential tomography (Diff-Tomo) processing mode, that allows `opening' the SAR cells in complex non-stationary scenes, resolving multiple heights and slow deformation velocities of layover scatterers. Consequently, the operational capability limit of differential interferometry to the single scatterer case is overcome. Diff-Tomo processing is cast in a 2D baseline-time spectral analysis framework, with sparse sampling. The use of adaptive 2D spectral estimation has demonstrated to allow joint baseline-time processing with reduced sidelobes and enhanced height-velocity resolution at low computational burden. However, this method requires coherent multilooking processing, thus does not produce full range-azimuth resolution products, as it would be desirable for urban applications. A new single-look adaptive Diff-Tomo processor is presented and tested with satellite data, allowing full range-azimuth resolution together with height-velocity sidelobe reduction and superresolution capabilities and the low computational burden.
    [bibtex-key = lombardiniVivianiElectronicsLetter2016SingleLookSuperResolutionTomo] [bibtex-entry]


  283. Paco Lopez-Dekker, Marc Rodriguez-Cassola, Francesco De Zan, Gerhard Krieger, and Alberto Moreira. Correlating Synthetic Aperture Radar (CoSAR). IEEE Trans. Geosci. Remote Sens., 54(4):2268-2284, April 2016. Keyword(s): SAR Processing, geosynchronous SAR, geosynchronous orbit, Correlating SAR, CoSAR, Doppler effect, Doppler radar, Radar imaging, Sea surface, Surface topography, Synthetic aperture radar, Bistatic radar, ocean currents, sea level, sea surface, synthetic aperture radar, van Cittert-Zernike.
    Abstract: This paper presents the correlating synthetic aperture radar (CoSAR) technique, a novel radar imaging concept to observe statistical properties of fast decorrelating surfaces. A CoSAR system consists of two radars with a relative motion in the along-track (cross-range) dimension. The spatial autocorrelation function of the scattered signal can be estimated by combining quasi-simultaneously received radar echoes. By virtue of the Van Cittert-Zernike theorem, estimates of this autocorrelation function for different relative positions can be processed by generating images of several properties of the scene, including the normalized radar cross section, Doppler velocities, and surface topography. Aside from the geometric performance, a central aspect of this paper is a theoretical derivation of the radiometric performance of CoSAR. The radiometric quality is proportional to the number of independent samples available for the estimation of the spatial correlation, and to the ratio between the CoSAR azimuth resolution and the real-aperture resolution. A CoSAR mission concept is provided where two geosynchronous radar satellites fly at opposing sides of a quasi-circular trajectory. Such a mission could provide bidaily images of the ocean backscatter, mean Doppler, and surface topography at resolutions on the order of 500 m over wide areas.
    [bibtex-key = lopezDekkerRodriguezCassolaDeZanKriegerMoreiraTGRS2016CoSAR] [bibtex-entry]


  284. Jun Maeda, Takato Suzuki, Masato Furuya, and Kosuke Heki. Imaging the midlatitude sporadic E plasma patches with a coordinated observation of spaceborne InSAR and GPS total electron content. Geophysical Research Letters, 43(4):1419-1425, 2016. Keyword(s): sporadic E, GPS, total electron content, synthetic aperture radar, Kelvin-Helmholtz instability.
    Abstract: Kilometer-scale fine structures of midlatitude sporadic E (Es) plasma patches have been directly imaged for the first time by an interferogram derived from L band Advanced Land Observation Satellite/Phased Array-type L band Synthetic Aperture Radar data obtained over southwestern Japan. The synthetic aperture radar interferogram captured the eastern part of a large-scale frontal structure of daytime midlatitude Es which spans over 250 km in the east-northeast to west-southwest direction. Fine structures are characterized by frontal and disc-shaped patches which are elongated in the same direction as the large-scale frontal structure. Length and width of the disc-shaped patches are 10-20 km and 5-10 km, respectively, and they are quasi-periodically located with a typical separation of 10-15 km. The Kelvin-Helmholtz instability with the vertical shear of zonal winds is considered to be the most likely candidate for the generation mechanism of the frontal patch and disc-shaped patches aligned in the zonal direction.
    [bibtex-key = maedaSuzukiFuruyaHekiGRL2016IonophereGNSSandSARsporadicEthroughTEC] [bibtex-entry]


  285. Christophe Magnard, Max Frioud, David Small, Thorsten Brehm, and Erich Meier. Analysis of a Maximum Likelihood Phase Estimation Method for Airborne Multibaseline SAR Interferometry. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(3):1072-1085, March 2016. Keyword(s): SAR Processing, SAR Interferometry, InSAR, Multibaseline Interferometry, Ka-band, Airborne SAR, Single-pass Multibaseline Interferometry, airborne radar, image texture, maximum likelihood estimation, millimetre wave radar, motion compensation, phase estimation, radar imaging, radar interferometry, synthetic aperture radar, C2F algorithm, ML method, airborne multibaseline SAR interferometry, calibration steps, coarse-to-fine algorithm, cross-track multibaseline synthetic aperture radar interferometric data, experimental Ka-band multibaseline system, homogeneous texture, imperfect motion compensation, maximum likelihood phase estimation method, noise level, Antenna measurements, Antennas, Calibration, Maximum likelihood estimation, Motion compensation, Phase estimation, Synthetic aperture radar, Interferometry, Ka-band, maximum likelihood (ML), millimeter wave radar, millimeterwave experimental multifrequency polarimetric high-resolution interferometric system (MEMPHIS), multibaseline, phase unwrapping, synthetic aperture radar (SAR).
    Abstract: It has been shown using simulated data that phase estimation of cross-track multibaseline synthetic aperture radar (SAR) interferometric data was most efficiently achieved through a maximum likelihood (ML) method. In this paper, we apply and assess the ML approach on real data, acquired with an experimental Ka-band multibaseline system. Compared to simulated data, dealing with real data implies that several calibration steps be carried out to ensure that the data fit the model. A processing chain was, therefore, designed, including steps responsible for compensating for imperfections observed in the data, such as beam elevation angle dependent phase errors or phase errors caused by imperfect motion compensation. The performance of the ML phase estimation was evaluated by comparing it to results based on a coarse-to-fine (C2F) algorithm, where information from the shorter baselines was used only to unwrap the phase from the longest available baseline. For this purpose, flat areas with high coherence and homogeneous texture were selected in the acquired data. The results show that with only four looks, the noise level was marginally better with the C2F approach and contained fewer outliers. However, with more looks, the ML method consistently delivered better results: noise variance with the C2F approach was slightly but steadily larger than the variance obtained with ML method.
    [bibtex-key = magnardFrioudSmallBrehmMeierJSTARS2016MaxLikelihoodPhaseEstimationMBINSAR] [bibtex-entry]


  286. Timothy M. Marston and Jermaine L. Kennedy. Volumetric Acoustic Imaging via Circular Multipass Aperture Synthesis. IEEE Journal of Oceanic Engineering, 41(4):852-867, October 2016. Keyword(s): SAR Processing, SAR Tomography, SAS Tomography, Synthetic Aperture Sonar, SAS, Synthetic Aperture Radar, Circular SAR, Circular SAS, Apertures, Arrays, Navigation, Synthetic aperture radar, Synthetic aperture sonar, Tomography, Compressive sensing, multipass sonar, synthetic aperture sonar (SAS), tomography, volumetric imaging.
    Abstract: In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum.
    [bibtex-key = marstonKennedyJOE2016CircularTomoSAS] [bibtex-entry]


  287. T. G. Michael, B. Marchand, J. D. Tucker, T. M. Marston, D. D. Sternlicht, and M. R. Azimi-Sadjadi. Image-Based Automated Change Detection for Synthetic Aperture Sonar by Multistage Coregistration and Canonical Correlation Analysis. IEEE_J_OE, 41(3):592-612, July 2016. Keyword(s): backscatter, image registration, sonar imaging, synthetic aperture sonar, transforms, backscattered signals, canonical correlation analysis, image-based automated change detection, multistage coregistration, scale-invariant feature transform algorithm, scene coherence, Change detection algorithms, Coherence, Correlation, Navigation, Surges, Synthetic aperture sonar, Automated change detection, canonical correlation analysis (CCA), coherent change detection, coregistration, synthetic aperture sonar (SAS). [bibtex-key = G-Michael2016] [bibtex-entry]


  288. J. Pan, M. Durand, M. Sandells, J. Lemmetyinen, E. J. Kim, J. Pulliainen, A. Kontu, and C. Derksen. Differences Between the HUT Snow Emission Model and MEMLS and Their Effects on Brightness Temperature Simulation. IEEE Transactions on Geoscience and Remote Sensing, 54(4):2001-2019, April 2016. Keyword(s): radiative transfer, remote sensing, snow, HUT snow emission model, Helsinki University of Technology, brightness temperature simulation, snow water equivalent retrieval algorithm, passive microwave measurement, multiple-layer HUT model, Microwave Emission Model of Layered Snowpacks, scattered intensity, radiative transfer equation, one-flux equation, two-flux theory, HUT scattering coefficient, trapped-radiation, natural snow cover, Sodankyla, Finland, Churchill, Canada, Colorado, USA, snow grain size was, deep snow, Born approximation, root-mean-square error, Snow, Mathematical model, Scattering, Grain size, Microwave theory and techniques, Ice, Correlation, Model comparison, passive microwave remote sensing, snow, Model comparison, passive microwave remote sensing, snow.
    Abstract: Microwave emission models are a critical component of snow water equivalent retrieval algorithms applied to passive microwave measurements. Several such emission models exist, but their differences need to be systematically compared. This paper compares the basic theories of two models: the multiple-layer Helsinki University of Technology (HUT) model and the microwave emission model of layered snowpacks (MEMLS). By comparing the mathematical formulation side by side, three major differences were identified: 1) by assuming that the scattered intensity is mostly (96%) in the forward direction, the HUT model simplifies the radiative transfer equation in 4pi space into two one-flux equations, whereas MEMLS uses a two-flux theory; 2) the HUT scattering coefficient is much larger than the one of MEMLS; and 3) MEMLS considers the trapped radiation inside snow due to internal reflection by a six-flux model, which is not included in HUT. Simulation experiments indicate that the large scattering coefficient of the HUT model compensates for its large forward scattering ratio to some extent, but the effects of one-flux simplification and the trapped radiation still result in different TB simulations between the HUT model and MEMLS. The models were compared with observations of natural snow cover at Sodankylae, Finland; Churchill, Canada; and Colorado, USA. No optimization of the snow grain size was performed. It shows that the HUT model tends to underestimate TB for deep snow. MEMLS with the physically based improved Born approximation performed best among the models, with a bias of -1.4 K and a root-mean-square error of 11.0 K.
    [bibtex-key = panDurandSandellsLemmetyinenKimPulliainenKontuDerksenTGRS2016DifferenceOfHUTandMEMLSandEffectsOnBrightnessTempSim] [bibtex-entry]


  289. M. Pieraccini and L. Miccinesi. ArcSAR for detecting target elevation. Electronics Letters, 52(18):1559-1561, 2016. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, data acquisition, object detection, radar antennas, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, ArcSAR, digital elevation model generation, interferometric differential radar, rotating arm, spatial diversity, synthetic aperture radar, target elevation detection.
    Abstract: ArcSAR is a Synthetic Aperture Radar which recently has been receiving increasing interest in scientific literature. It operates exploiting the spatial diversity of the data acquired by an antenna fixed to a rotating arm. Its great advantage is its capability to synthesise images at 360 deg with constant resolution in azimuth. The ArcSAR, in addition to operate as interferometric differential radar, can detect the elevation of the targets, i.e. it is potentially able to generate digital elevation models of the surrounding field of view.
    [bibtex-key = pieracciniMiccinesiEL2016ArcSAR] [bibtex-entry]


  290. Octavio Ponce, Pau Prats-Iraola, Rolf Scheiber, Andreas Reigber, and Alberto Moreira. First Airborne Demonstration of Holographic SAR Tomography With Fully Polarimetric Multicircular Acquisitions at L-Band. IEEE_J_GRS, 54(10):6170-6196, October 2016. Keyword(s): geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, 2-D synthetic aperture, Fully Polarimetric Multicircular Acquisitions, German Aerospace Center, Germany, HoloSAR tomogram polarimetric analysis, Kaufbeuren, L-Band, SAR systems, airborne F-SAR sensor, coherent imaging approach, complex reflectivity, compressive sensing, fast-factorized back-projection, forest backscattering analysis, full 3-D reconstructions, generalized likelihood ratio test, geometric acquisition, geoscience community, holographic SAR tomography, holographic techniques, impulse response function, incoherent imaging, individual circular trajectories, internal structure imaging, multicircular SAR acquisitions, polarimetric L-band HoloSAR survey, radar resolution capability function, scatterer polarimetric signature, semitransparent media, sidelobe power, spatial resolution, target 3-D IRF, tomographic techniques, volumetric backscattering, Apertures, Radar imaging, Spatial resolution, Synthetic aperture radar, Tomography, Circular synthetic aperture radar (CSAR), compressive sensing (CS), fast-factorized back-projection (FFBP), holographic sar tomography (HoloSAR), phase gradient autofocus (PGA), polarimetric synthetic aperture radar (PolSAR). [bibtex-key = poncePratsScheiberReigberMoreiraTGRS2016HolographicSARTOMOLBand] [bibtex-entry]


  291. Martin Proksch, Nick Rutter, Charles Fierz, and Martin Schneebeli. Intercomparison of snow density measurements: bias, precision, and vertical resolution. The Cryosphere, 10(1):371-384, 2016. Keyword(s): Snow characterisation, snow density, density cutter, snow density retrieval, comparison of methods.
    Abstract: Density is a fundamental property of porous media such as snow. A wide range of snow properties and physical processes are linked to density, but few studies have addressed the uncertainty in snow density measurements. No study has yet quantitatively considered the recent advances in snow measurement methods such as micro-computed tomography ($\mu$CT) in alpine snow. During the MicroSnow Davos 2014 workshop, different approaches to measure snow density were applied in a controlled laboratory environment and in the field. Overall, the agreement between $\mu$CT and gravimetric methods (density cutters) was 5 to 9 %, with a bias of -5 to 2 %, expressed as percentage of the mean $\mu$CT density. In the field, density cutters overestimate (1 to 6 %) densities below and underestimate (1 to 6 %) densities above a threshold between 296 to 350 kg m^-3, dependent on cutter type. Using the mean density per layer of all measurement methods applied in the field ($\mu$CT, box, wedge, and cylinder cutters) and ignoring ice layers, the variation between the methods was 2 to 5 % with a bias of -1 to 1 %. In general, our result suggests that snow densities measured by different methods agree within 9 %. However, the density profiles resolved by the measurement methods differed considerably. In particular, the millimeter-scale density variations revealed by the high-resolution $\mu$CT contrasted the thick layers with sharp boundaries introduced by the observer. In this respect, the unresolved variation, i.e., the density variation within a layer which is lost by lower resolution sampling or layer aggregation, is critical when snow density measurements are used in numerical simulations.
    [bibtex-key = prokschRutterFierzSchneebeliCryosphere2016ComparisonSnowDensityMeasurements] [bibtex-entry]


  292. A. Recchia, Andrea Monti Guarnieri, Antonio Broquetas, and Antonio Leanza. Impact of Scene Decorrelation on Geosynchronous SAR Data Focusing. IEEE Transactions on Geoscience and Remote Sensing, 54(3):1635-1646, March 2016. Keyword(s): SAR Processing, SAR focusing, autofocus, atmospheric phase, atmospheric phase screen, APS, decorrelation, temporal decorrelation, geostationary, geosynchronous, radar clutter, radar resolution, synthetic aperture radar, Billingsley intrinsic clutter motion model, GEOSAR signal-to-clutter ratio, azimuth resolution, clutter energy, geosynchronous SAR data focusing, ground based radar experiment, power spectral density, scene decorrelation, two-way propagation losses, Azimuth, Clutter, Decorrelation, Focusing, Synthetic aperture radar, Thyristors, Focusing, Geosynchronous Synthetic Aperture Radar (GEOSAR), scene decorrelation, wind-blown clutter.
    Abstract: We discuss the effects of the clutter on geosynchronous SAR systems exploiting long integration times (from minutes to hours) to counteract for two-way propagation losses and increase azimuth resolution. Only stable targets will be correctly focused whereas unstable targets will spread their energy along azimuth direction. We derive here a generic model for the spreading of the clutter energy based on the power spectral density of the clutter itself. We then assume the Billingsley Intrinsic Clutter Motion model, representing the clutter power spectrum as an exponential decay, and derive the expected GEOSAR signal-to-clutter ratio. We also provide some results from a Ground Based RADAR experiment aimed at assessing the long-term clutter statistics for different scenarios to complement the Internal Clutter Motion model, mainly derived for windblown trees. Finally, we discuss the expected performances of two GEOSAR systems with different acquisition geometries.
    [bibtex-key = recchiaMontiGuarnieriBroquetasLeanzaTGRS2016DecorrelationInGeosynchronousSARFocusing] [bibtex-entry]


  293. Jamal Saeedi and Karim Faez. A back-projection autofocus algorithm based on flight trajectory optimization for synthetic aperture radar imaging. Multidimensional Systems and Signal Processing, 27(2):411, April 2016.
    Abstract: In this paper, a new autofocus algorithm is presented for back-projection (BP) image formation of synthetic aperture radar (SAR) imaging. The approach is based on maximizing a cost function obtained by prominent points in different sub-apertures of constructed SAR image by varying the flight trajectory parameters. While image-quality-based autofocus approach together with BP algorithm can be computationally intensive, we use approximations that allow optimal corrections to be derived. The approach is applicable for focusing different signal processing algorithms by obtaining modified flight trajectory parameters. Different examples demonstrate the effectiveness of the new autofocus approach applied to the frequency modulated continuous wave mode SAR dataset.
    [bibtex-key = Saeedi2016] [bibtex-entry]


  294. Muhammad Adnan Siddique, Urs Wegmuller, Irena Hajnsek, and Othmar Frey. Single-look SAR tomography as an add-on to PSI for improved deformation analysis in urban areas. IEEE Trans. Geosci. Remote Sens., 54(10):6119-6137, 2016. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, thermal dilation, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas, TerraSAR-X, Barcelona.
    Abstract: Persistent scatterer interferometry (PSI) is in operational use for spaceborne SAR-based deformation analysis. A limitation inherently associated with PSI is that by definition a persistent scatterer (PS) is a single dominant scatterer. Therefore, pixels containing signal contributions from multiple scatterers, as in the case of a layover, are typically rejected in the PSI processing, which in turn limits deformation retrieval. SAR tomography has the ability to resolve layovers. This paper investigates the value-addition that can be achieved by operationally combining SAR tomography with a PSI approach towards the objective of improving deformation sampling in layover-affected urban areas. Different tomographic phase models are implemented and compared as regards their suitability in resolving layovers. Single-look beamforming-based tomographic inversion and a generalized likelihood ratio (GLRT)-based detection strategy are used to detect single and double scatterers. The quantity of the detected scatterers is weighed against their quality as defined in terms of the phase deviation between the SLC measurements and the tomographic model fit. The gain in deformation sampling that can be derived with tomography relative to a PSI-based analysis is quantitatively assessed, and alongside the quality of the scatterers obtained with tomography is compared with the quality of the PSs identified with a PSI approach. The experiments are performed on an interferometric stack of 50 TerraSAR-X stripmap images. The results obtained show that, although there is a trade-off between the quantity and the quality of the detected scatterers, the tested SAR tomography approach leads to an improvement in deformation sampling in layover-affected areas.
    [bibtex-key = siddiqueWegmullerHajnsekFreyTGRS2016TOMOBarca] [bibtex-entry]


  295. N. Smitha, D. R. Ullas Bharadwaj, S. Abilash, S. N. Sridhara, and Vipula Singh. Kirchhoff and F-K migration to focus ground penetrating radar images. International Journal of Geo-Engineering, 7(1), April 2016.
    Abstract: Ground penetrating radar (GPR) based land mine detection has a main challenge of having an accurate image analysis method that is capable of reducing false alarms. However this image analysis depends on having sufficient spatial resolution in the backscattered signal. This paper aims at getting better resolution by applying two migration algorithms. One is by Kirchhoff's migration using geometrical approach and other one is F-K migration algorithms with Fourier transform. The algorithms are developed using MATLAB simulations over different scenarios for stepped frequency continuous wave (SFCW) GPR.
    [bibtex-key = smithaEtAl2016KirchhoffMirgrationAndFKMigrationToFocusGroundPenetratingRadarImages] [bibtex-entry]


  296. Xiaoshen Song and Weidong Yu. Processing video-SAR data with the fast backprojection method. IEEE Transactions on Aerospace and Electronic Systems, 52(6):2838-2848, December 2016. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, radar imaging, synthetic aperture radar, FBP algorithm, GR, Gotcha data set, O(N2 log N) complexity reduction, ROI, X-band SAR measurement, fast backprojection method, general region, image sequence, land-imaging mode, recursive procedure, region of interest, synthetic aperture radar, video framing, video-SAR data processing, video-SAR image formation, Apertures, Azimuth, Complexity theory, Image resolution, Radar imaging, Synthetic aperture radar, Time-domain analysis.
    Abstract: Video synthetic aperture radar (video-SAR) is a land-imaging mode where a sequence of images is continuously formed when the radar platform either flies by or circles the scene. In this paper, the fast backprojection (FBP) algorithm is introduced for video-SAR image formation. It avoids unnecessary duplication of processing for the overlapping parts between consecutive video frames and achieves O(N2 log N) complexity through a recursive procedure. To reduce the processing complexity in video-SAR system, the scene is partitioned into the general region (GR) and the region of interest (ROI). In different regions, different aperture lengths are used. The proposed method allows a direct trade between processing speed and focused quality for the GR, meanwhile reserving particular details in the ROI. The effectiveness is validated both for a simulated scene and for X-band SAR measurements from the Gotcha data set.
    [bibtex-key = songYuTAES2016VideoSARBackprojection] [bibtex-entry]


  297. S. Tebaldini, F. Rocca, M. Mariotti d'Alessandro, and L. Ferro-Famil. Phase Calibration of Airborne Tomographic SAR Data via Phase Center Double Localization. IEEE Trans. Geosci. Remote Sens., 54(3):1775-1792, March 2016. Keyword(s): SAR Processing, SAR tomography, Apertures, Calibration, Focusing, Sensors, Surfaces, Synthetic aperture radar, Tomography, Distributed media, phase calibration, reconstruction algorithms, synthetic aperture radar (SAR), tomography.
    Abstract: Synthetic aperture radar (SAR) data collected over a 2-D synthetic aperture can be processed to focus the illuminated scatterers in the 3-D space, using a number of signal processing techniques generally grouped under the name of SAR tomography (TomoSAR). A fundamental requirement for TomoSAR processing is to have precise knowledge of the platform position along the 2-D synthetic aperture. This requirement is not easily met in the case where the 2-D aperture is formed by collecting different flight lines (i.e., 1-D apertures) in a repeat-pass fashion, which is the typical case of airborne and spaceborne TomoSAR. Subwavelength platform position errors give rise to residual phase screens among different passes, which hinder coherent focusing in the 3-D space. In this paper, we propose a strategy for calibrating repeat-pass tomographic SAR data that allows us to accurately estimate and remove such residual phase screens in the absence of reference targets and prior information about terrain topography and even in the absence of any point- or surface-like target within the illuminated scene. The problem is tackled by observing that multiple flight lines provide enough information to jointly estimate platform and target positions, up to a roto-translation of the coordinate system used for representing the imaged scene. The employment of volumetric scatterers in the calibration process is enabled by the phase linking algorithm, which allows us to represent them as equivalent phase centers. The proposed approach is demonstrated through numerical simulations, in order to validate the results based on the exact knowledge of the simulated scatterers, and using real data from the ESA campaigns AlpTomoSAR, BioSAR 2008, and TropiSAR. A cross-check of the results from simultaneous P- and L-band acquisitions from the TropiSAR data set indicates that the dispersion of the retrieved flight trajectories is limited to a few millimeters.
    [bibtex-key = tebaldiniRoccaMariottiFerroFamilTGRS2015PhaseCalibrationTomoSAR] [bibtex-entry]


  298. Jan Torgrimsson, Patrik Dammert, Hans Hellsten, and Lars M. H. Ulander. An Efficient Solution to the Factorized Geometrical Autofocus Problem. IEEE Transactions on Geoscience and Remote Sensing, 54(8):4732-4748, August 2016. Keyword(s): SAR Processing, Autofocus, Fast-Factorized Back-Projection, FFBP, radar imaging, synthetic aperture radar, 6-D autofocus problem, FGA algorithm, adjustable geometry parameters, factorized back-projection formulation, factorized geometrical autofocus problem, geometrical variation, global autofocus solution, magnitude values, maximizing focus quality, peak-to-sidelobe ratio, point-like targets, synthetic-aperture-radar processing, ultrawideband CARABAS II data, Apertures, Geometry, Radar imaging, Radar tracking, Search problems, Synthetic aperture radar, Autofocus, back-projection, synthetic aperture radar (SAR).
    Abstract: This paper describes a new search strategy within the scope of factorized geometrical autofocus (FGA) and synthetic-aperture-radar processing. The FGA algorithm is a fast factorized back-projection formulation with six adjustable geometry parameters. By tuning the flight track step by step and maximizing focus quality by means of an object function, a sharp image is formed. We propose an efficient two-stage approach for the geometrical variation. The first stage is a low-order (few parameters) parallel search procedure involving small image areas. The second stage then combines the local hypotheses into one global autofocus solution, without the use of images. This method has been applied successfully on ultrawideband CARABAS II data. Errors due to a constant acceleration are superposed on the measured track prior to processing, giving a 6-D autofocus problem. Image results, including resolution, peak-to-sidelobe ratio and magnitude values for point-like targets, finally confirm the validity of the strategy. The results also verify the prediction that there are several satisfying autofocus solutions for the same radar data.
    [bibtex-key = torgrimssonDammertHellstenUlanderTGRS2016FFBPEfficientAutofocus] [bibtex-entry]


  299. Luigi Tosi, Cristina Da Lio, Tazio Strozzi, and Pietro Teatini. Combining L- and X-Band SAR Interferometry to Assess Ground Displacements in Heterogeneous Coastal Environments: The Po River Delta and Venice Lagoon, Italy. Remote Sensing, 8(4):308, 2016.
    Abstract: From leveling to SAR-based interferometry, the monitoring of land subsidence in coastal transitional environments significantly improved. However, the simultaneous assessment of the ground movements in these peculiar environments is still challenging. This is due to the presence of relatively small built-up zones and infrastructures, e.g., coastal infrastructures, bridges, and river embankments, within large natural or rural lands, e.g., river deltas, lagoons, and farmland. In this paper we present a multi-band SAR methodology to integrate COSMO-SkyMed and ALOS-PALSAR images. The method consists of a proper combination of the very high-resolution X-band Persistent Scatterer Interferometry (PSI), which achieves high-density and precise measurements on single structures and constructed areas, with L-band Short-Baseline SAR Interferometry (SBAS), properly implemented to raise its effectiveness in retrieving information in vegetated and wet zones. The combined methodology is applied on the Po River Delta and Venice coastland, Northern Italy, using 16 ALOS-PALSAR and 31 COSMO-SkyMed images covering the period between 2007 and 2011. After a proper calibration of the single PSI and SBAS solution using available GPS records, the datasets have been combined at both the regional and local scales. The measured displacements range from ~0 mm/yr down to 35 mm/yr. The results reveal the variable pattern of the subsidence characterizing the more natural and rural environments without losing the accuracy in quantifying the sinking of urban areas and infrastructures. Moreover, they allow improving the interpretation of the natural and anthropogenic processes responsible for the ongoing subsidence.
    [bibtex-key = tosiDaLioStrozziTeatiniRemoteSensing2016SubsidenceVeniceLbandXband] [bibtex-entry]


  300. André Twele, Wenxi Cao, Simon Plank, and Sandro Martinis. Sentinel-1-based flood mapping: a fully automated processing chain. International Journal of Remote Sensing, 37(13):2990-3004, 2016. Keyword(s): Sentinel-1, Flood Mapping.
    Abstract: This article presents an automated Sentinel-1-based processing chain designed for flood detection and monitoring in near-real-time (NRT). Since no user intervention is required at any stage of the flood mapping procedure, the processing chain allows deriving time-critical disaster information in less than 45 min after a new data set is available on the Sentinel Data Hub of the European Space Agency (ESA). Due to the systematic acquisition strategy and high repetition rate of Sentinel-1, the processing chain can be set up as a web-based service that regularly informs users about the current flood conditions in a given area of interest. The thematic accuracy of the thematic processor has been assessed for two test sites of a flood situation at the border between Greece and Turkey with encouraging overall accuracies between 94.0\% and 96.1\% and Cohen's kappa coefficients (kappa) ranging from 0.879 to 0.910. The accuracy assessment, which was performed separately for the standard polarizations (VV/VH) of the interferometric wide swath (IW) mode of Sentinel-1, further indicates that under calm wind conditions, slightly higher thematic accuracies can be achieved by using VV instead of VH polarization data.
    [bibtex-key = tweleCaoPlankMartinisIJRS2016Sentinel1FloodMapping] [bibtex-entry]


  301. Viet Thuy Vu and Mats I. Pettersson. Fast Backprojection Algorithms Based on Subapertures and Local Polar Coordinates for General Bistatic Airborne SAR Systems. IEEE Trans. Geosci. Remote Sens., 54(5):2706-2712, May 2016. Keyword(s): SAR Processing, Back-Projection, Fast Back-Projection, Fast-Factorized Back-Projection, Azimuth Focusing, airborne radar, synthetic aperture radar, bistatic CARABAS-like data, fast backprojection algorithms, general bistatic airborne SAR systems, half-power beamwidths, image quality measurements, interpolation step, local polar coordinates, peak sidelobe ratio, subapertures, Image reconstruction, Receivers, Signal processing algorithms, Synthetic aperture radar, Time-domain analysis, Transmitters, Algorithm, bistatic, fast backprojection, polar coordinates, synthetic aperture radar (SAR).
    Abstract: This paper introduces a bistatic fast backprojection synthetic aperture radar (SAR) algorithm that is available for different bistatic geometries. The proposed algorithm is tested with the bistatic CARABAS-like data, and the results indicate that the algorithm is a good candidate for bistatic SAR data processing. The image quality measurements are quite similar to the referenced values obtained with the bistatic global backprojection algorithm. That is, the peak sidelobe ratio (PSLR) is -13.7 dB in comparison to the referenced PSLR of -13.8 dB. The half-power beamwidths (HPBWs) measured on the cuts in x and y and the direction where the peak sidelobes locate are 2.8, 3.7, and 3.5 m, respectively, which are approximate to the referenced HPBWs. The small differences in the measured results mainly come from the interpolation step of the algorithm.
    [bibtex-key = vuPetterssonTGRS2016FastBackprojectionBistatic] [bibtex-entry]


  302. Daniel E. Wahl, David A. Yocky, Charles V. Jakowatz, and Katherine M. Simonson. A New Maximum-Likelihood Change Estimator for Two-Pass SAR Coherent Change Detection. IEEE Transactions on Geoscience and Remote Sensing, 54(4):2460-2469, April 2016. Keyword(s): SAR Processing, Coherent Change Detection, CCD, Synthetic aperture radar, Maximum likelihood estimation, Coherence, Charge coupled devices, Maximum likelihood detection, Clutter, Measurement, Coherent change detection, maximum likelihood estimator, radar interferometry, synthetic aperture radar, Coherent change detection, maximum likelihood estimator, radar interferometry, synthetic aperture radar.
    Abstract: In past research, two-pass repeat-geometry synthetic aperture radar (SAR) coherent change detection (CCD) predominantly utilized the sample degree of coherence as a measure of the temporal change occurring between two complex-valued image collects. Previous coherence-based CCD approaches tend to show temporal change when there is none in areas of the image that have a low clutter-to-noise power ratio. Instead of employing the sample coherence magnitude as a change metric, in this paper, we derive a new maximum-likelihood (ML) temporal change estimate-the complex reflectance change detection (CRCD) metric to be used for SAR coherent temporal change detection. The new CRCD estimator is a surprisingly simple expression, easy to implement, and optimal in the ML sense. This new estimate produces improved results in the coherent pair collects that we have tested.
    [bibtex-key = wahlEtAl2016NewMLEstimatorFor2passSARCoherentChangeDetection] [bibtex-entry]


  303. Urs Wegmuller, Charles L. Werner, Tazio Strozzi, Andreas Wiesmann, Othmar Frey, and Maurizio Santoro. Sentinel-1 Support in the GAMMA Software. Procedia Computer Science, 100:1305-1312, 2016. Keyword(s): SAR Processing, Sentinel-1, Interferometry, SAR Interferometry, InSAR, persistent scatterer interferometry, PSI, offset tracking, split-beam interferometry, GAMMA Software.
    Abstract: First results using the new Sentinel-1 SAR look very promising but the special interferometric wide-swath data acquired in the TOPS mode makes InSAR processing more challenging than for normal stripmap mode data. The steep azimuth spectra ramp in each burst results in very stringent co-registration requirements. Combining the data of the individual bursts and sub-swaths into consistent mosaics requires careful book-keeping in the handling of the data and meta data and the large file sizes and high data throughputs require also a good performance. Considering these challenges good support from software is getting increasingly important. In this contribution we describe the Sentinel-1 support in the GAMMA Software, a high-level software package used by researchers, service providers and operational users in their SAR, InSAR, PSI and offset tracking work.
    [bibtex-key = WegmullerEtAlProcediaSentinel1GammaSoftware] [bibtex-entry]


  304. C. H. Yang, B. K. Kenduiywo, and U. Soergel. Change detection based on persistent scatterer interferometry - a new method of monitoring building changes. ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, III-7:243-250, 2016. Keyword(s): Persistent Scatterer Interferometry, PSI, Change Detection. [bibtex-key = yangKenduiywoSoergelISPRSAnnals2016ChangeDetectionBasedOnPSI] [bibtex-entry]


  305. S. Zwieback and I. Hajnsek. Influence of Vegetation Growth on the Polarimetric Zero-Baseline DInSAR Phase Diversity ---Implications for Deformation Studies. IEEE_J_GRS, 54(5):3070-3082, May 2016. Keyword(s): crops, geophysical techniques, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, L-band zero-baseline data set, agricultural crop, barley, crop type, deformation study, differential SAR interferometry, displacement estimation, growing season, in situ observed biomass, maize, polarimetric DInSAR phase diversity, polarimetric coherence region, polarization diversity, sugar beet, vegetation canopy, vegetation effect, vegetation growth effect, wheat, zero-baseline DInSAR phase diversity, Coherence, Interferometry, Synthetic aperture radar, Systematics, Vegetation, Vegetation mapping, Birefringence, radar remote sensing, vegetation. [bibtex-key = Zwieback2016a] [bibtex-entry]


  306. S. Zwieback, X. Liu, S. Antonova, B. Heim, A. Bartsch, J. Boike, and I. Hajnsek. A Statistical Test of Phase Closure to Detect Influences on DInSAR Deformation Estimates Besides Displacements and Decorrelation Noise: Two Case Studies in High-Latitude Regions. IEEE_J_GRS, 54(9):5588-5601, September 2016. Keyword(s): atmospheric precipitation, displacement measurement, geophysical techniques, radar interferometry, remote sensing by radar, snow, statistical analysis, synthetic aperture radar, DInSAR deformation estimates, Finland, Ku-band, Lena Delta, Russia, Sodankylä, X-band observations, decorrelation noise, differential interferometric synthetic aperture radar, high-latitude regions, ice-rich permafrost regions, phase measurement, snow metamorphism, statistical test, summer precipitation event, Decorrelation, Optical interferometry, Scattering, Snow, Synthetic aperture radar, Displacement measurement, interferometry, remote sensing, soil, statistics. [bibtex-key = Zwieback2016] [bibtex-entry]


  307. Tazio Strozzi, Hugo Raetzo, Urs Wegmüller, Jessica Papke, Rafael Caduff, Charles L. Werner, and Andreas Wiesmann. Satellite and Terrestrial Radar Interferometry for the Measurement of Slope Deformation, chapter Satellite and Terrestrial Radar Interferometry for the Measurement of Slope Deformation, pages 161-165. Springer International Publishing, Cham, 2015.
    Abstract: Synergistic use of satellite and terrestrial radar interferometry was considered for the measurement of slope deformation in the Mattervalley (Canton of Valais, Switzerland). Highest rates of movement of more than 1 cm/day were measured only with terrestrial radar interferometry, because of the large time interval between satellite SAR observations. Summer TerraSAR-X and Cosmo-SkyMed interferograms as well as terrestrial radar interferometry campaigns repeated with a time interval of a few days were jointly considered for the study of landslides moving at rates of dm/year. Persistent scatterer interferometric analyses conducted with ERS-1/2, ENVISAT, Radarsat-2, TerraSAR-X and Cosmo-SkyMed images were finally used to detect the slowest moving landslides, with rates of movement below a few cm/yr in the line-of-sight direction.
    [bibtex-key = strozziRaetzoWegmullerPapkeCaduffWernerWiesmannEngGeology2015Deformation] [bibtex-entry]


  308. Laurent Ferro-Famil, Yue Huang, and Eric Pottier. Principles and Applications of Polarimetric SAR Tomography for the Characterization of Complex Environments. In International Association of Geodesy Symposia, pages 1-13. Springer Berlin Heidelberg, Berlin, Heidelberg, 2015. [bibtex-key = ferroFamilHuangPottierSpringer2015TomoOverview] [bibtex-entry]


  309. P. S. Agram and M. Simons. A noise model for InSAR time series. Journal of Geophysical Research: Solid Earth, 120(4):2752-2771, 2015. Keyword(s): SAR Processing, InSAR, radar, interferometry, noise budget, time series, stacking, interferometric stacks.
    Abstract: Interferometric synthetic aperture radar (InSAR) time series methods estimate the spatiotemporal evolution of surface deformation by incorporating information from multiple SAR interferograms. While various models have been developed to describe the interferometric phase and correlation statistics in individual interferograms, efforts to model the generalized covariance matrix that is directly applicable to joint analysis of networks of interferograms have been limited in scope. In this work, we build on existing decorrelation and atmospheric phase screen models and develop a covariance model for interferometric phase noise over space and time. We present arguments to show that the exploitation of the full 3-D covariance structure within conventional time series inversion techniques is computationally challenging. However, the presented covariance model can aid in designing new inversion techniques that can at least mitigate the impact of spatial correlated nature of InSAR observations.
    [bibtex-key = agramSimonsJGRSolidEarth2015NoiseModelForInSARTimeSeries] [bibtex-entry]


  310. F. Alshawaf, B. Fersch, S. Hinz, H. Kunstmann, M. Mayer, and F.J. Meyer. Water vapor mapping by fusing InSAR and GNSS remote sensing data and atmospheric simulations. Hydrology and Earth System Sciences, 19(12):4747-4764, 2015. Note: Cited By 14. [bibtex-key = Alshawaf20154747] [bibtex-entry]


  311. Fadwa Alshawaf, T. Fuhrmann, A. Knöpfler, X. Luo, Michael Mayer, Stefan Hinz, and B. Heck. Accurate Estimation of Atmospheric Water Vapor Using GNSS Observations and Surface Meteorological Data. IEEE Transactions on Geoscience and Remote Sensing, 53(7):3764-3771, July 2015. Keyword(s): atmospheric humidity, atmospheric temperature, remote sensing, satellite navigation, time series, remote sensing data, temporal variation, spatial variation, Global Navigation Satellite System, time series, precipitable water vapor content, precise point positioning, absolute precipitable water vapor, GNSS observations, GNSS site, surface temperature measurements, GNSS-based delay, MEdium Resolution Imaging Spectrometer sensor, mean RMS value, GNSS-based total precipitable water vapor, Weather Research and Forecasting Modeling System, WRF model simulations, atmospheric water vapor estimation, surface meteorological data, Global Positioning System, Delays, Temperature measurement, Atmospheric modeling, Atmospheric measurements, Satellites, Atmospheric sounding, Global Navigation Satellite System(s) (GNSS), MEdium Resolution Imaging Spectrometer (MERIS), precipitable water vapor (PWV), Weather Research and Forecasting (WRF), Atmospheric sounding, Global Navigation Satellite System(s) (GNSS), MEdium Resolution Imaging Spectrometer (MERIS), precipitable water vapor (PWV), Weather Research and Forecasting (WRF).
    Abstract: Remote sensing data have been increasingly used to measure the content of water vapor in the atmosphere and to characterize its temporal and spatial variations. In this paper, we use observations from Global Navigation Satellite System(s) (GNSS) to estimate time series of precipitable water vapor (PWV) by applying the technique of precise point positioning. For an accurate quantification of the absolute PWV, it is necessary to combine the GNSS observations with meteorological data measured directly or inferred at the GNSS site. In addition, measurements of the surface temperature are used to calculate the empirical constant required to convert the GNSS-based delay into water vapor. Our results show strong agreement between the total precipitable water estimated based on GNSS observations and that measured by the sensor MEdium Resolution Imaging Spectrometer with a mean RMS value of 0.98 mm. In a similar way, we compared the GNSS-based total PWV estimates with those produced by the Weather Research and Forecasting (WRF) Modeling System. We found that the WRF model simulations agree well with the GNSS estimates with a mean RMS value of 0.97 mm.
    [bibtex-key = alshawafFuhrmannKnoepflerLuoMayerHinzHeckTGRS2015EstimateAtmosphericWaterVapourUsingGNSSandMeteoData] [bibtex-entry]


  312. Fadwa Alshawaf, Stefan Hinz, Michael Mayer, and Franz J. Meyer. Constructing accurate maps of atmospheric water vapor by combining interferometric synthetic aperture radar and GNSS observations. Journal of Geophysical Research: Atmospheres, 120(4):1391-1403, 2015. Keyword(s): SAR Processing, atmospheric water vapor, InSAR, GNSS, Tropospheric Path Delay, Synthetic Aperture Radar, Atmospheric Modelling, Atmospheric modeling, Meteorology, radar clutter, radar imaging, radar interferometry, synthetic aperture radar (SAR).
    Abstract: AbstractOver the past 20years, repeat-pass spaceborne interferometric synthetic aperture radar (InSAR) has been widely used as a geodetic technique to generate maps of the Earth's topography and to measure the Earth's surface deformation. In this paper, we present a new approach to exploit microwave data from InSAR, particularly Persistent Scatterer InSAR (PSI), and Global Navigation Satellite Systems (GNSS) to derive maps of the absolute water vapor content in the Earth's atmosphere. Atmospheric water vapor results in a phase shift in the InSAR interferogram, which if successfully separated from other phase components provides valuable information about its distribution. PSI produces precipitable water vapor (PWV) difference maps of a high spatial density, which can be inverted using the least squares method to retrieve PWV maps at each SAR acquisition time. These maps do not contain the absolute (total) PWV along the signal path but only a part of it. The components eliminated by forming interferograms or phase filtering during PSI data processing are reconstructed using GNSS phase observations. The approach is applied to build maps of absolute PWV by combining data from InSAR and GNSS over the region of Upper Rhine Graben in Germany and France. For validation, we compared the derived PWV maps with PWV maps measured by the optical sensor MEdium-Resolution Imaging Spectrometer. The results show strong spatial correlation with values of uncertainty of less than 1.5mm. Continuous grids of PWV are then produced by applying the kriging geostatistical interpolation technique that exploits the spatial correlations between the PWV observations.
    [bibtex-key = alshawafHinzMayerMeyerJGR2015AtmosphericWaterVaporFromInSARandGNSS] [bibtex-entry]


  313. Francesco Banda and Stefano Tebaldini. Texture-Free Absolute DEM Retrieval From Opposite-Side Multibaseline InSAR Data. IEEE Geosci. Remote Sens. Lett., PP(99):1-5, 2015. Keyword(s): SAR Processing, Multibaseline InSAR, Accuracy, Azimuth, Estimation, Laser radar, Radar tracking, Synthetic aperture radar, Tomography, Digital elevation model (DEM), radargrammetry, synthetic aperture radar interferometry (InSAR).
    Abstract: In this letter, we propose a new methodology to estimate the absolute digital elevation model (DEM) of an area by radargammetric-like processing of interferometric multibaseline synthetic aperture radar (SAR) data from two opposite-side surveys. Two DEMs of the imaged area obtained from two opposite-side tomographic SAR views are coregistered, correcting residual baseline errors. This methodology combines the great accuracy of multibaseline interferometric processing with precise stereo plotting typical of opposite-side radargrammetry, requires no texture matching and no control points, and is applicable also in the case of few a priori information about the site topography.
    [bibtex-key = bandaTebaldiniGRSL2015DEMRetrievalInSAR] [bibtex-entry]


  314. D. P. S. Bekaert, A. Hooper, and T. J. Wright. A spatially variable power law tropospheric correction technique for InSAR data. Journal of Geophysical Research: Solid Earth, 120(2):1345-1356, February 2015. Keyword(s): SAR Interferometry, Spatially-variable Correction, Power law model, Troposphere, APS, Estimation of APS, Atmospheric Phase Screen, Interferometry, Tropospheric Path Delay. [bibtex-key = bekaertHooperWrightAGUJGR2015SpatiallyVariablePowerLawTroposphericCorrINSAR] [bibtex-entry]


  315. David P.S. Bekaert, R.J. Walters, Tim J. Wright, Andrew J. Hooper, and D.J. Parker. Statistical comparison of InSAR tropospheric correction techniques. Remote Sensing of Environment, 170:40-47, December 2015. Keyword(s): SAR Interferometry, Atmosphere, Tropospheric Corrections, Phase-based Spectrometers, Weather models, InSAR, Interferometry. [bibtex-key = bekaertWaltersWrightHooperParkerRSE2015StatisticalComparisonOfTroposphericCorrectionTechniques] [bibtex-entry]


  316. Silvia Bianchini, Andrea Ciampalini, Federico Raspini, Federica Bardi, Federico Di Traglia, Sandro Moretti, and Nicola Casagli. Multi-temporal evaluation of landslide movements and impacts on buildings in San Fratello (Italy) by means of C-band and X-band PSI data. Pure and Applied Geophysics, 172(11):3043-3065, 2015. [bibtex-key = bianchiniCiampaliniRaspiniBardiDiTragliaMorettiCasagli2015] [bibtex-entry]


  317. Rafael Caduff, Fritz Schlunegger, Andrew Kos, and Andreas Wiesmann. A review of terrestrial radar interferometry for measuring surface change in the geosciences. Earth Surface Processes and Landforms, 40(2):208-228, 2015. Keyword(s): SAR Processing, Gamma Portable Radar Interferometer, GPRI, Review, GBSAR, InSAR, Ground-based radar, Ground-based SAR, deformation measurement, displacement, subsidence, terrestrial radar interferometry, mass movements, surface deformation.
    Abstract: This paper presents a review of the current state of the art in the use of terrestrial radar interferometry for the detection of surface changes related to mass movement. Different hardware-types and acquisition concepts are described, which use either real or synthetic aperture for radar image formation. We present approaches for data processing procedures, paying special attention to the separation of high resolution displacement information from atmospheric phase variations. Recent case studies are used to illustrate applications in terrestrial radar interferometry for change detection. Applications range from detection and quantification of very slow moving (millimeters to centimeters per year) displacements in rock walls from repeat monitoring, to rapid processes resulting in fast displacements (~ 50 m/yr) acquired during single measurement campaigns with durations of only a few hours. Fast and episodic acting processes such as rockfall and snow avalanches can be assessed qualitatively in the spatial domain by mapping decorrelation caused by those processes. A concluding guide to best practice outlines the necessary preconditions that have to be fulfilled for successful application of the technique, as well as in areas characterized by rapid decorrelation. Empirical data from a Ku-band sensor show the range of temporal decorrelation of different surfaces after more than two years for rock-surfaces and after a few seconds to minutes in vegetated areas during windy conditions. The examples show that the displacement field can be measured for landslides in dense grassland, ice surfaces on flowing glaciers and snowpack creep. Copyright 2014 John Wiley and Sons, Ltd.
    [bibtex-key = caduffSchluneggerKosWiesmannESPL2014ReviewGBSARRADAR] [bibtex-entry]


  318. Rafael Caduff, Andreas Wiesmann, Yves Bühler, and Christine Pielmeier. Continuous monitoring of snowpack displacement at high spatial and temporal resolution with terrestrial radar interferometry. Geophysical Research Letters, 42(3):813-820, 2015. Note: 2014GL062442. Keyword(s): GPRI-II, Remote sensing, Dynamics, Instruments and techniques, Monitoring, forecasting, prediction, Methods, terrestrial radar interferometry, remote sensing of snow, snowpack displacement monitoring, full-depth snow glide avalanche. [bibtex-key = caduffWiesmannBuehlerPielmeierGRL2015SnowGPRI] [bibtex-entry]


  319. Astor T. Caicoya, Matteo Pardini, Irena Hajnsek, and Konstantinos P. Papathanassiou. Forest Above-Ground Biomass Estimation From Vertical Reflectivity Profiles at L-Band. IEEE Geosci. Remote Sens. Lett., 12(12):2379-2383, December 2015. Keyword(s): SAR Processing, SAR Tomography, L-band, Capon, forestry, vegetation mapping, L-band reflectivity profiles, SAR tomography, forest above-ground biomass estimation, forest stand densities, height measurements, root-mean-square error, vertical forest structure information, vertical radar reflectivity profiles, vertical reflectivity profiles, Biomass, Estimation, Image color analysis, L-band, Remote sensing, Synthetic aperture radar, Forest allometry, L-band, forest biomass, synthetic aperture radar (SAR) tomography, vertical forest structure, vertical reflectivity profiles.
    Abstract: Forest height is an important parameter for the allometric estimation of above-ground forest biomass (AGB). However, variable forest stand densities limit the performance of the allometric estimation of AGB from height measurements alone. Recently, the use of vertical forest structure information as an indicator for the variation of stand density has been proposed and used to improve the allometric estimation of AGB from height measurements. In this letter, the use of vertical radar reflectivity profiles at L-band obtained from SAR tomography, as a proxy for vertical forest structure for the allometric estimation of AGB, is investigated. L-band reflectivity profiles, which are reconstructed from data at different polarizations (HH and HV) and acquired under moist and dry weather conditions, are investigated. The proposed allometric AGB estimator increases the correlation factor from 0.60 to 0.81 and reduces the root-mean-square error from 50.25 to 36.30 Mg/ha when compared with the AGB estimation from forest height alone. The effect of polarization and weather conditions on the AGB estimation performance is discussed.
    [bibtex-key = caicoyaPardiniHajnsekPapathanassiouGRSL2015TomoLBandBiomass] [bibtex-entry]


  320. Ning Cao, Hyongki Lee, and H. C. Jung. Mathematical Framework for Phase-Triangulation Algorithms in Distributed-Scatterer Interferometry. IEEE Geoscience and Remote Sensing Letters, 12(9):1838-1842, Sept 2015. Keyword(s): geophysical techniques, mathematical analysis, maximum likelihood estimation, phase estimation, radar interferometry, DS interferometry procedure, coherence-weighted PT, distributed-scatterer interferometry, eigendecomposition-based phase estimator, equal-weighted PT, estimation procedure weight value, least square estimator, mathematical framework, mathematical relation analysis, maximum-likelihood phase estimator, modified PT algorithm, persistent-scatterer interferometry measurement point, phase-triangulation algorithm, published PT method, Coherence, Covariance matrices, Interferometry, Maximum likelihood estimation, Remote sensing, Synthetic aperture radar, Differential interferometric synthetic aperture radar (DInSAR), distributed scatterer (DS) interferometry, persistent scatterer (PS) interferometry (PSI), phase triangulation (PT), synthetic aperture radar (SAR).
    Abstract: To improve the spatial density of measurement points of persistent-scatterer interferometry, distributed scatterer (DS) should be considered and processed. An important procedure in DS interferometry is the phase triangulation (PT). This letter introduces two modified PT algorithms (i.e., equal-weighted PT and coherence-weighted PT) and analyzes the mathematical relations between different published PT methods (i.e., the maximum-likelihood phase estimator, least squares estimator, and eigendecomposition-based phase estimators). The analysis shows that the above five PT methods share very similar mathematical forms with different weight values in the estimation procedure.
    [bibtex-key = caoLeeJungGRSL2015FrameworkForPhaseTriangulationDSInSAR] [bibtex-entry]


  321. F. De Zan, P. Prats-Iraola, and M. Rodriguez-Cassola. On the Dependence of Delta-k Efficiency on Multilooking. IEEE Geoscience and Remote Sensing Letters, 12(8):1745-1749, August 2015. Keyword(s): estimation theory, radar imaging, radar interferometry, synthetic aperture radar, delta-k method, synthetic aperture radar imaging, differential interferogram, multilooking, interferometric fringe, incoherent cross-correlation, Synthetic aperture radar, Bandwidth, Correlation, Estimation, Coherence, Remote sensing, Interferometry, Delay estimation, Delta-k, synthetic aperture radar (SAR), synthetic aperture radar interferometry, Delay estimation, Delta-k, synthetic aperture radar (SAR), synthetic aperture radar interferometry.
    Abstract: This letter discusses some aspects of the implementation of Delta-k methods for shift estimation with synthetic aperture radar images. In particular, it shows that a common Delta-k algorithm, which postpones the multilooking to the differential interferogram and is therefore robust to the presence of interferometric fringes in the averaging window, does not reach the maximum possible performance and should be better considered as a variant of incoherent cross-correlation. A small adaptation, retaining some multilooking at interferogram level, can significantly improve the efficiency.
    [bibtex-key = 7102710] [bibtex-entry]


  322. Michael I. Duersch and David G. Long. Analysis of Multistatic Pixel Correlation in SAR. IEEE Transactions on Geoscience and Remote Sensing, 53(1):362-374, January 2015. Keyword(s): MIMO radar, correlation methods, image resolution, radar imaging, radar receivers, radar transmitters, synthetic aperture radar, MIMO technique, SAR, collocated array category, distributed array category, geometric correlation calculation, ground-plane image formation, multiple-input multiple-output technique, multistatic pixel correlation analysis, pixel image resolution, receiver-transmitter pair, synthetic aperture radar, wireless communication, Correlation, MIMO, Receiving antennas, Synthetic aperture radar, Backprojection, multiple-input multiple-output (MIMO), multistatic radar, synthetic aperture radar (SAR).
    Abstract: The field of wireless communications has benefited from multiple-input and multiple-output (MIMO) techniques. As researchers seek to apply MIMO (multistatic) techniques to radar and specifically to synthetic aperture radar (SAR), a key factor in determining MIMO application and performance is the level of correlation of signals from different receiver/transmitter pairs. The level of correlation determines whether a MIMO array falls into the category of a collocated array or a distributed array. The type of array dramatically affects which MIMO techniques may be performed and what advantages MIMO offers from conventional techniques. This paper presents models for calculating geometric correlation of multistatic SAR pixels using a ground-plane image formation. The models' results are compared to previous correlation models found in literature. A key result is that correlation depends on pixel resolution and not the number of individual scatterers. This paper concludes that most MIMO arrays operating on a single platform operate in the collocated regime.
    [bibtex-key = duerschLongTGARS2015MultistaticPixelCorrelation] [bibtex-entry]


  323. Michael I. Duersch and David G. Long. Analysis of time-domain back-projection for stripmap SAR. International Journal of Remote Sensing, 36(8):2010-2036, 2015. Keyword(s): SAR Processing, FMCW, Time-Domain Back-Projection, TDBP, LFMCW, Azimuth Focusing, Motion Compensation, Interferometry, SAR Interferometry, Airborne SAR.
    Abstract: This article explores the SAR back-projection algorithm for stripmap image formationand its characteristics. The article provides a derivation of generalized time-domain back-projection from first principles. It shows that back-projection may be considered an ideal matched filter for SAR. The article presents an analysis of the sensitivity of back-projection to its geometric parameters as well as several performance considerations: azimuth beam width, residual phase error, digital elevation map accuracy, and antenna position estimation accuracy.
    [bibtex-key = duerschLongIJRS2015TDBPforStripmapSAR] [bibtex-entry]


  324. Michael I. Duersch and David G. Long. Backprojection SAR interferometry. International Journal of Remote Sensing, 36(4):979-999, 2015. Keyword(s): SAR Processing, FMCW, Time-Domain Back-Projection, TDBP, LFMCW, Azimuth Focusing, Motion Compensation, Interferometry, SAR Interferometry, Airborne SAR.
    Abstract: Synthetic aperture radar (SAR) interferometry uses the phase difference between two SAR antennas to obtain an elevation estimate of the imaged terrain. Using an initial digital elevation model (DEM), the time-domain backprojection algorithm implicitly removes the terrain height phase from images during image formation. The use of a DEM during image formation adds additional information to the process of interferometry, resulting in different sensitivities to conventional interferometry. This article presents a novel method of SAR interferometry using backprojected imagery. It is shown that the sensitivity and performance of backprojection interferometry is significantly different to that of conventional methods. Specifically, it is shown that backprojection interferometry is much less sensitive to errors in measurement of the interferometric baseline length and angle. This comes at the expense of higher sensitivity to phase errors. We conclude that backprojection interferometry is best suited for airborne operation.
    [bibtex-key = duerschLongIJRS2015BackprojectionSARInterferometry] [bibtex-entry]


  325. Alexander G. Fore, Bruce D. Chapman, Brian P. Hawkins, Scott Hensley, Cathleen E. Jones, Thierry R. Michel, and Ronald J. Muellerschoen. UAVSAR Polarimetric Calibration. IEEE Trans. Geosci. Remote Sens., 53(6):3481-3491, June 2015. Keyword(s): SAR Processing, UAVSAR, Airborne SAR, Polarimetry, Polarimetric Calibration, calibration, radar polarimetry, remote sensing by radar, synthetic aperture radar, UAVSAR polarimetric calibration, UAVSAR radar performance, airborne repeat-track SAR data, interferometric measurements, quadpolarization mode, radiometric calibration, reconfigurable polarimetric L-band SAR, residual RMS errors, root-mean-square, stable crosstalk estimates, uninhabited aerial vehicle synthetic aperture radar, Azimuth, Calibration, Crosstalk, Image resolution, Radiometry, Synthetic aperture radar, Airborne radar, polarimetric SAR, radar cross-sections, radar imaging, radar measurements, radar polarimetry, radar remote sensing, synthetic aperture radar (SAR).
    Abstract: Uninhabited aerial vehicle synthetic aperture radar (UAVSAR) is a reconfigurable polarimetric L-band SAR that operates in quad-polarization mode and is specifically designed to acquire airborne repeat-track SAR data for interferometric measurements. In this paper, we present details of the UAVSAR radar performance, the radiometric calibration, and the polarimetric calibration. For the radiometric calibration, we employ an array of trihedral corner reflectors, as well as distributed targets. We show that UAVSAR is a well-calibrated SAR system for polarimetric applications, with absolute radiometric calibration bias better than 1 dB, residual root-mean-square (RMS) errors of ~0.7 dB, and RMS phase errors ~5.3 deg. For the polarimetric calibration, we have evaluated the methods of Quegan and Ainsworth et al. for crosstalk calibration and find that the method of Quegan gives crosstalk estimates that depend on target type, whereas the method of Ainsworth et al. gives more stable crosstalk estimates. We find that both methods estimate leakage of the copolarizations into the cross-polarizations to be on the order of -30 dB.
    [bibtex-key = foreChapmanHawkinsHensleyJonesMichelMuellerschoenTGRS2015UAVSARPolCalibration] [bibtex-entry]


  326. Gianfranco Fornaro, Nicola D'Agostino, Roberta Giuliani, Carlo Noviello, Diego Reale, and Simona Verde. Assimilation of GPS-Derived Atmospheric Propagation Delay in DInSAR Data Processing. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 8(2):784-799, 2015. Keyword(s): SAR Interferometry, Spatially-variable Correction, Power law model, Troposphere, APS, Estimation of APS, Atmospheric Phase Screen, Interferometry, Tropospheric Path Delay.
    Abstract: Microwave radiation is almost insensitive in terms of power attenuation to the presence of atmosphere; the atmosphere is however an error source in repeat pass interferometry due to propagation delay variations. This effect represents a main limitation in the detection and monitoring of weak deformation patterns in differential interferometric Synthetic Aperture Radar (DInSAR), especially in emergency conditions. Due to the wavelength reduction current, X-Band sensors are even more sensitive to such error sources: procedures adopted in classical advanced DInSAR for atmospheric filtering may fail in the presence of higher revisiting rates. In this work, we show such effect on data acquired by the COSMO-SkyMed constellation. The dataset has been acquired with very high revisiting rates during the emergency phase. This feature allows clearly showing the inability of standard filtering adopted in common processing chains in handling seasonal atmospheric delay variations over temporal intervals spanning periods shorter than 1 year. We discuss a procedure for the mitigation of atmospheric propagation delay (APD) that is based on the integration of data of GPS systems which carries out measurements with large observation angles diversity practically in continuous time. The proposed algorithm allows a robust assimilation of the GPS atmospheric delay measurements in the multipass DInSAR processing and found on a linear approximation with the height of the atmospheric delay corresponding to a stratified atmosphere. Achieved results show a significant mitigation of the seasonal atmospheric variations.
    [bibtex-key = fornaroEtAlIEEEJSTARS2015AssimilationOfGPSDerivedAtmoDelayInDInSAR] [bibtex-entry]


  327. Gianfranco Fornaro, Simona Verde, Diego Reale, and Antonio Pauciullo. CAESAR: An Approach Based on Covariance Matrix Decomposition to Improve Multibaseline - Multitemporal Interferometric SAR Processing. IEEE Trans. Geosci. Remote Sens., 53(4):2050-2065, April 2015. Keyword(s): SAR Processing, SAR Tomography, Component Extraction And selection SAR, CEASAR, Spaceborne SAR, multilook SAR tomography, X-Band, Urban, Persistent Scatterer Interferometry, PSI, covariance matrices, geophysical signal processing, matrix decomposition, principal component analysis, radar interferometry, radar signal processing, remote sensing by radar, synthetic aperture radar, tomography, CAESAR, Component Extraction and Selection SAR algorithm, SAR tomography, SqueeSAR, classical interferometric processing, coherence losses, covariance matrix analysis, covariance matrix decomposition, data covariance matrix, equivalent scattering mechanisms, ground deformation monitoring, high resolution Cosmo-SkyMed data, high resolution interferometric SAR sensors, interferometic stac filtering, multibaseline-multitemporal interferometric SAR processing, multilook operation, multiple scatterers, principal component analysis, synthetic aperture radar, Covariance matrices, Interferometry, Monitoring, Scattering, Spatial resolution, Synthetic aperture radar, Tomography, 3-D, 4-D and multidimensional (Multi-D) SAR imaging, Covariance matrix decomposition, SAR interferometry (InSAR), SAR tomography.
    Abstract: Synthetic aperture radar (SAR) tomography has been strongly developed in the last years for the analysis at fine scale of data acquired by high-resolution interferometric SAR sensors as a technique alternative to classical persistent scatterer interferometry and able to resolve also multiple scatterers. SqueeSAR is a recently proposed solution which, in the context of SAR interferometry at the coarse scale analysis stage, allows taking advantage of the multilook operation to filter interferometic stacks by extracting, pixel by pixel, equivalent scattering mechanisms from the set of all available interferometric measurement collected in the data covariance matrix. In this paper, we investigate the possibilities to extend SqueeSAR by allowing the identification of multiple scattering mechanisms from the analysis of the covariance matrix. In particular, we present a new approach, named Component extrAction and sElection SAR algorithm, that allows taking advantage of the principal component analysis to filter interferograms relevant to the decorrelating scatterer, i.e., scatterers that may exhibit coherence losses depending on the spatial and temporal baseline distributions, and to detect and separate scattering mechanisms possibly interfering in the same pixel due to layover directly at the interferogram generation stage. The proposed module allows providing options useful for classical interferometric processing to monitor ground deformations at lower resolution (coarse scale), as well as for possibly aiding the data calibration preliminary for the subsequent full-resolution interferometric/tomographic (fine scale) analysis. Results achieved by processing high-resolution Cosmo-SkyMed data, characterized by the favorable features of a large baseline span, are presented to explain the advantages and validate this new interferometric processing solution.
    [bibtex-key = fornaroVerdeRealePauciulloTGRS2014TomoCAESAR] [bibtex-entry]


  328. Roy E. Hansen, A. P. Lyons, T. O. Saebo, H. J. Callow, and D. A. Cook. The Effect of Internal Wave-Related Features on Synthetic Aperture Sonar. IEEE Journal of Oceanic Engineering, 40(3):621-631, July 2015. Keyword(s): Synthetic Aperture Sonar, SAS, autonomous underwater vehicles, bathymetry, oceanographic techniques, sonar imaging, synthetic aperture sonar, (AUV), CMRE, Centre for Maritime Research and Experimentation, Elba Island Italy, FFI, HUGIN autonomous underwater vehicle, Kjeller Norway, La Spezia Italy, NATO research vessel, Norwegian Defence Research Establishment, RV, SAS bathymetry, SAS imaging, acoustical ray model, bolus, interferometric synthetic aperture sonar, internal wave-related feature effect, moving target analysis, multiaperture processing, multilook processing, repeat pass imaging, seabed topography, sound-speed structure, water column, Geometry, Imaging, Interferometry, Shape, Surfaces, Synthetic aperture sonar, Internal waves, refraction effects, synthetic aperture sonar.
    Abstract: In October 2012, the Centre for Maritime Research and Experimentation (CMRE, La Spezia, Italy) conducted trials from the NATO research vessel (RV) Alliance, off Elba Island, Italy. During this trial, data were collected by the Norwegian Defence Research Establishment (FFI, Kjeller, Norway) using a HUGIN autonomous underwater vehicle (AUV) with interferometric synthetic aperture sonar (SAS) in repeated passes. Large linear structures (tens of meters by several meters) observed in both the SAS images and SAS bathymetry during the initial pass were absent in data taken on a repeated pass the following day. We suggest that these phenomena were not true seafloor features, but were caused by features in the water column, known as boluses, which can form after breaking internal wave events. The changes observed in acoustical intensity and phase appear to be caused by the interaction of the acoustical field with the lower average sound-speed structure of the bolus, constructing features in both SAS imagery and SAS bathymetry that looked like seabed topography. In this paper, we present examples and give an interpretation of the results based on an acoustical ray model. We discuss different techniques for recognizing these phenomena: repeat pass imaging and interferometry, multilook and multiaperture processing, and moving target analysis.
    [bibtex-key = hansenLyonsSaeboCallowCookJOE2015EffectOfInternalWavesOnSAS] [bibtex-entry]


  329. R. Iglesias, A. Aguasca, X. Fabregas, J. J. Mallorqui, D. Monells, C. Lopez-Martinez, and L. Pipia. Ground-Based Polarimetric SAR Interferometry for the Monitoring of Terrain Displacement Phenomena - Part I: Theoretical Description. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 8(3):980-993, March 2015. Keyword(s): geomorphology, radar interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, GB-InSAR techniques, GB-SAR sensors, PSI processing chains, PSI techniques, PolInSAR algorithms, PolSAR measurements, SLFMCW GB-SAR system, SLFMCW signals, Universitat Politecnica de Catalunya, acquisition time, classical single-polarimetric performances, coherent pixels technique, ground displacement episodes, ground displacement phenomena, ground-based SAR interferometry, ground-based polarimetric SAR interferometry, ground-based synthetic aperture radar, persistent scatterer interferomerty, polarimetric RiskSAR sensor, polarimetric SAR measurements, stepped linear frequency modulated continuous wave, terrain displacement phenomena monitoring, troposphere medium decorrelation, troposphere temporal homogeneity, vector network analyzers, Atmospheric measurements, Interferometry, Monitoring, Sensor phenomena and characterization, Synthetic aperture radar, Differential synthetic aperture radar (SAR) interferometry (DInSAR), GB-SAR interferometry (GB-InSAR), frequency modulated continuous wave (FMCW) radar, ground-based SAR (GB-SAR), persistent scatterer interferomerty (PSI), polarimetric SAR interferometry (PolInSAR), stepped linear FMCW (SLFMCW) radar.
    Abstract: Ground-based synthetic aperture radar (SAR) (GB-SAR) sensors represent an effective solution for the monitoring of ground displacement episodes. Initially, the most GB-SAR sensors were based on vector network analyzers (VNA). This type of solution, characterized by a slow scanning time comparable to the decorrelation of the troposphere medium, compromised in many cases the quality of final products for the application of persistent scatterer interferomerty (PSI) techniques. The development of GB-SAR sensors based on the use of stepped linear frequency modulated continuous wave (SLFMCW) signals has led to significant improvements during the last years. They have allowed fulfilling the need of temporal homogeneity of the troposphere during the acquisition time and, moreover, they have favored the acquisition of reliable polarimetric SAR (PolSAR) measurements without drastically increasing the scanning time. This fact has boosted the inclusion of polarimetric SAR interferometry (PolInSAR) algorithms in PSI processing chains, which are demonstrating to outperform classical single-polarimetric performances. The objective of this paper is twofold. On the one hand, a general overview of the polarimetric RiskSAR sensor, developed by the Universitat Politecnica de Catalunya (UPC), is put forward as an example of SLFMCW GB-SAR system implementation. On the other hand, a complete theoretical description of ground-based SAR (GB-SAR) interferometry (GB-InSAR) techniques for PSI purposes is widely discussed. The adaptation of the Coherent Pixels Technique to obtain the linear and nonlinear components of ground displacement phenomena is proposed. In the second part of this paper, the displacement maps and time series over two very different scenarios are presented in order to show the feasibility of GB-SAR sensors for terrain displacement monitoring applications.
    [bibtex-key = iglesiasEtAlJSTARS2015GBSARDisplacementPart1Theory] [bibtex-entry]


  330. R. Iglesias, A. Aguasca, X. Fabregas, J. J. Mallorqui, D. Monells, C. López-Martìnez, and L. Pipia. Ground-Based Polarimetric SAR Interferometry for the Monitoring of Terrain Displacement Phenomena --Part II: Applications. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 8(3):994-1007, March 2015. Keyword(s): geomorphology, geophysical techniques, radar interferometry, remote sensing by radar, synthetic aperture radar, Andorran Pyrenees, El Forn de Canillo, GB-InSAR techniques, GB-SAR sensors, Remote Sensing Laboratory, RiskSAR sensor, Spain, Universitat Politecnica de Catalunya, X-band, cost-effective solution, ground displacement phenomena, ground-based polarimetric SAR interferometry, ground-based synthetic aperture radar interferometry, in-field data, infrastructure safety, landslide monitoring, people safety, terrain displacement phenomena monitoring, urban landslides, urban subsidence, urban subsidence monitoring, zero-baseline configuration, Coherence, Interferometry, Monitoring, Sensitivity, Synthetic aperture radar, Terrain factors, Vectors, Differential synthetic aperture radar (SAR) interferometry (DInSAR), displacement monitoring, frequency modulated continuous wave (FMCW) radar, ground-based SAR (GB-SAR), ground-based SAR interferometry (GBInSAR), persistent scatterer interferometry (PSI), polarimetric SAR interferometry (PolInSAR), steepest linear frequency modulated continuous wave (SLFMCW) radar. [bibtex-key = iglesiasEtAlJSTARS2015GBSARDisplacementPart2Applications] [bibtex-entry]


  331. Marko Komac, Rachel Holley, Pooja Mahapatra, Hans van der Marel, and Milos Bavec. Coupling of GPS/GNSS and radar interferometric data for a 3D surface displacement monitoring of landslides. Landslides, 12(2):241-257, April 2015.
    Abstract: Persistent scatterer interferometry (PSI) is capable of millimetric measurements of ground deformation phenomena occurring at radar signal reflectors (persistent scatterers, PS) that are phase coherent over a period of time. However, there are also limitations to PSI; significant phase decorrelation can occur between subsequent interferometric radar (InSAR) acquisitions in vegetated and low-density PS areas. Here, artificial amplitude- and phase-stable radar scatterers may have to be introduced. I2GPS was a Galileo project (02/2010--09/2011) that aimed to develop a novel device consisting of a compact active transponder (CAT) with an integrated global positioning system (GPS) antenna to ensure millimetric co-registration and a coherent cross-reference. The advantages are: (1) all advantages of CATs such as small size, light weight, unobtrusiveness and usability with multiple satellites and tracks; (2) absolute calibration for PSI data; (3) high sampling rate of GPS enables detection of abrupt ground motion in 3D; and (4) vertical components of the local velocity field can be derived from single-track InSAR line-of-sight displacements. A field trial was set to test the approach at a potential landslide site in Poto{\v{s}}ka planina, Slovenia to evaluate the applicability for operational monitoring of natural hazards. Preliminary results from the trial highlight some of the key considerations for operational deployments in the field. Ground motion measurements also allowed an assessment of landslide hazard at the site and demonstrated the synergies between InSAR and GPS measurements for landslide applications. InSAR and GPS measurements were compared to assess the consistency between the methods from the slope mass movement detection aspect.
    [bibtex-key = komacHolleyMahapatraMarelBavecLandslides2015GNSSandInSARforLandslides] [bibtex-entry]


  332. K. Landmark, A. H. Schistad Solberg, F. Albregtsen, A. Austeng, and Roy E. Hansen. A Radon-Transform-Based Image Noise Filter With Applications to Multibeam Bathymetry. IEEE Transactions on Geoscience and Remote Sensing, 53(11):6252-6273, November 2015. Keyword(s): Synthetic Aperture Sonar, SAS, Radon transforms, bathymetry, geophysical image processing, image denoising, image restoration, radar imaging, remote sensing by radar, synthetic aperture radar, Chebyshev approximation, Laplacian point spread function, denoised image, fast discrete Radon transform, geomorphological type statistical classification, image transform, invariant terrain features, invertible edge detection operator, linear-image-transform-based algorithm, motion-induced errors, motion-induced noise, multibeam bathymetry, original image, processed test images, radon-transform-based image noise filter, remote sensing data, second noise signature, standard low-pass filters, synthetic aperture radar images, track line artifacts, Approximation algorithms, Image edge detection, Noise, Noise reduction, Presses, Radio frequency, Transforms, Discrete transforms, image denoising, image restoration, iterative methods, remote sensing, sonar, terrain mapping.
    Abstract: This paper describes a linear-image-transform-based algorithm for reducing stripe noise, track line artifacts, and motion-induced errors in remote sensing data. Developed for multibeam bathymetry (MB), the method has also been used for removing scalloping in synthetic aperture radar images. The proposed image transform is the composition of an invertible edge detection operator and a fast discrete Radon transform (DRT) due to Goetz, Druckmuller, and Brady. The inverse DRT is computed by using an iterative method and exploiting an approximate inverse algorithm due to Press. The edge operator is implemented by circular convolution with a Laplacian point spread function modified to render the operator invertible. In the transformed image, linear discontinuities appear as high-intensity spots, which may be reset to zero. In MB data, a second noise signature is linked to motion-induced errors. A Chebyshev approximation of the original image is subtracted before applying the transform, and added back to the denoised image; this is necessary to avoid boundary effects. It is possible to process data faster and suppress motion-induced noise further by filtering images in nonoverlapping blocks using a matrix representation for the inverse DRT. Processed test images from several MB data sets had less noise and distortion compared with those obtained with standard low-pass filters. Denoising also improved the accuracy in statistical classification of geomorphological type by 10-28% for two sets of invariant terrain features.
    [bibtex-key = landmarkSchistadSolbergAlbregtsenAustengHansenTGRS2015RadonTransformImageNoiseFilter] [bibtex-entry]


  333. Marco Lavalle and Scott Hensley. Extraction of Structural and Dynamic Properties of Forests From Polarimetric-Interferometric SAR Data Affected by Temporal Decorrelation. IEEE Trans. Geosci. Remote Sens., 53(9):4752-4767, September 2015. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, Gaussian processes, optical radar, radar imaging, radar interferometry, radar polarimetry, synthetic aperture radar, vegetation mapping, Gaussian-statistic motion model, Harvard Forest, L-band NASA Uninhabited Aerial Vehicle Synthetic Aperture Radar, Laser Vegetation and Ice Sensor, Massachussetts, NASA lidar, RMoG model, RVoG model, USA, canopy elements, canopy motion, forest biomass estimation, forest dynamic property, forest property estimation, forest structural property, forest vertical structure, least square distance minimization, lidar-derived height, multiplicative factors, polarimetric channels, polarimetric-interferometric SAR data, polarimetric-interferometric coherence, polarimetric-interferometric radar image, random-motion-over-ground model, random-volume-over-ground model, temporal coherence, temporal decorrelation effect, tree height, volumetric coherence, volumetric decorrelation effect, wave polarization, Biomass, Coherence, Data models, Decorrelation, Radar, Vegetation, Decorrelation, interferometry, polarimetry.
    Abstract: This paper addresses the important yet unresolved problem of estimating forest properties from polarimetric-interferometric radar images affected by temporal decorrelation. We approach the problem by formulating a physical model of the polarimetric-interferometric coherence that incorporates both volumetric and temporal decorrelation effects. The model is termed random-motion-over-ground (RMoG) model, as it combines the random-volume-over-ground (RVoG) model with a Gaussian-statistic motion model of the canopy elements. Key features of the RMoG model are: 1) temporal decorrelation depends on the vertical structure of forests; 2) volumetric and temporal coherences are not separable as simple multiplicative factors; and 3) temporal decorrelation is complex-valued and changes with wave polarization. This third feature is particularly important as it allows compensating for unknown levels of temporal decorrelation using multiple polarimetric channels. To estimate model parameters such as tree height and canopy motion, we propose an algorithm that minimizes the least square distance between model predictions and complex coherence observations. The algorithm was applied to L-band NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar data acquired over the Harvard Forest (Massachussetts, USA). We found that the RMS difference at stand level between estimated RMoG-model tree height and NASA's lidar Laser Vegetation and Ice Sensor tree height was within 12% of the lidar-derived height, which improved significantly the RMS difference of 37% obtained using the RVoG model and ignoring temporal decorrelation. This result contributes to our ability of estimating forest biomass using in-orbit and forthcoming polarimetric-interferometric radar missions.
    [bibtex-key = lavalleHensleyTGARS2015TempDecorrelation] [bibtex-entry]


  334. Silvan Leinss, Andreas Wiesmann, J. Lemmetyinen, and I. Hajnsek. Snow Water Equivalent of Dry Snow Measured by Differential Interferometry. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 8(8):3773-3790, August 2015. Keyword(s): radar interferometry, remote sensing by radar, snow, Finland, SnowScat instrument, Sodankyla town, Xand Ku-band, active microwave remote sensing method, differential interferogram time series, differential radar interferometry, dry snow measurement, frequency 10 GHz, frequency 16 GHz, frequency 20 GHz, passive microwave remote sensing method, phase wrapping error, reference instrument, signal delay, snow density, snow pack spatial inhomogeneity, snow volume, snow water equivalent mapping, stratigraphy, temporal decorrelation, time 30 day, Backscatter, Ice, Instruments, Interferometry, Snow, Synthetic aperture radar, Coherence loss, SnowScat, dielectric constant of snow, differential interferometry (D-InSAR), dry snow, microwave penetration of snow, real aperture radar, snow water equivalent (SWE), synthetic aperture radar (SAR).
    Abstract: Large scale mapping of snow water equivalent (SWE) is a long-lasting request in many scientific and economical fields. Active and passive microwave remote sensing methods are explored, as local methods cannot be generalized due to the spatial inhomogeneity of the snow pack. Microwaves interact with snow by absorption, scattering, and refraction. For dry snow of a few meters depth and frequencies below 20 GHz, absorption and scattering in the snow volume are negligible compared with the backscattered energy from the underlying ground. The signal delay caused by refraction can be measured with differential radar interferometry, but phase wrapping errors and temporal decorrelation must be considered. We demonstrate that large delta SWE can be accurately determined from dense time series of differential interferograms at X- and Ku-band by temporal integration. Lost phase cycles are reconstructed with a two-frequency approach. Temporal decorrelation is minimized by a temporal resolution of 4 h. A linear function between delta SWE and phase difference is derived, which deviates only a few percent from the exact solution and which depends negligibly on snow density and stratigraphy. delta SWE retrieved from observations of the SnowScat instrument (SSI) were validated against observed SWE from different reference instruments, installed at a test site near the town of Sodankyl{\"a}, Finland. An accuracy below +/- 6 mm SWE was achieved at frequencies of 10 and 16 GHz for up to 200 mm of delta SWE. An exceptionally high temporal coherence was observed for up to 30 days for dry snow, whereas for wet snow it decayed within hours.
    [bibtex-key = leinssWiesmannLemmetyinenHajnsek2015] [bibtex-entry]


  335. Jun Maeda and Kosuke Heki. Morphology and dynamics of daytime mid-latitude sporadic-E patches revealed by GPS total electron content observations in Japan. Earth, Planets and Space, 67(1):89, June 2015. Keyword(s): sporadic E, ionosphere, GPS, Global Positioning System, GNSS, Global Navigation Satellite System, GEONET, TEC, Total Electron Content.
    Abstract: Morphological characteristics of daytime mid-latitude sporadic-E (Es) patches are studied by two-dimensional total electron content (TEC) maps drawn using the Japanese dense network of Global Positioning System (GPS) receivers. By analyzing over 70 cases, we found that their horizontal shapes are characterized by frontal structure typically elongated in east-west by { extasciitilde}100 km. They are observed to migrate mainly northward in the morning and southward in the afternoon with speeds of 30-100 m/s. This may reflect the velocities of neutral winds controlled by the atmospheric tides. Such frontal structures are often found to include smaller scale structures.
    [bibtex-key = maedaHekiEPS2015IonosphereGNSSSporadicEThroughTEC] [bibtex-entry]


  336. Pooja S. Mahapatra, Sami Samie Esfahany, and Ramon F. Hanssen. Geodetic Network Design for InSAR. IEEE Trans. Geosci. Remote Sens., 53(7):3669-3680, July 2015. Keyword(s): SAR Processing, InSAR, GNSS, persistent scatterer interferometry, covariance matrices, deformation, geodesy, geomorphology, geophysical techniques, network theory (graphs), radar interferometry, remote sensing by radar, synthetic aperture radar, InSAR measurement, coherent target device deployment, covariance matrices, criterion matrix, deformation signal, densification measurement, geodesic network design methodology, ground deformation measurement, interferometric synthetic aperture radar, network optimization, optimal ground location, Covariance matrices, Deformable models, Geophysical measurements, Redundancy, Synthetic aperture radar, Coherent target, compact active transponder, corner reflector, criterion matrix, economy, geodesy, interferometric synthetic aperture radar (InSAR), network design, precision, reliability.
    Abstract: Ground deformation can be monitored with subcentimetric precision from space, using interferometric synthetic aperture radar (InSAR). This technique can sometimes be limited by a low density of naturally occurring phase-coherent radar targets. Measurement densification may be achieved through improvements in processing algorithms and new satellites with better revisit times, but there can still exist areas where very few coherent targets are detected, e.g., in vegetated nonurbanized areas. For third-party end users of InSAR survey results, there is currently no systematic method to determine a priori whether these coherent targets have adequate spatial distribution to estimate the parameters of their interest. We propose such a method, along with a practical solution for measurement densification, i.e., deployment of coherent target devices such as corner reflectors or transponders. We propose a generic network design methodology that does the following: 1) determines whether the naturally occurring InSAR measurements are adequate; 2) finds the minimum number of additional devices (if required); and 3) finds their optimal ground locations. The method digests, as inputs, the expected locations and quality of existing coherent targets, the quality of the devices being deployed, and, if available, any prior knowledge of the deformation signal. At the core of the method is a comparison of different covariance matrices of the final parameters of interest with a criterion matrix (i.e., the desired idealized covariance matrix), using a predefined metric. The resulting network is optimized with respect to precision, reliability, and cost criteria. Simulated data sets and a subsidence case study in the Netherlands are used to demonstrate this method.
    [bibtex-key = mahapatraSamieiEsfahanyHanssenTGARS2015GeodeticNetworkDesignForInSAR] [bibtex-entry]


  337. Timothy M. Marston and Daniel S. Plotnick. Semiparametric Statistical Stripmap Synthetic Aperture Autofocusing. IEEE Transactions on Geoscience and Remote Sensing, 53(4):2086-2095, April 2015. Keyword(s): SAR Processing, Autofocus, Motion Compensation, MoComp, geophysical image processing, remote sensing by radar, synthetic aperture radar, synthetic aperture sonar, SAR literature, artificially injected crabbing error, artificially injected sway error, corrupting phase function, cost function gradient, metric-maximizing solutions, semiparametric statistical stripmap synthetic aperture autofocusing, spotlight-mode SAR applications, stripmap error model, stripmap gradient expression, stripmap imagery, synthetic aperture sonar literature, to statistical quality metric, unmanned-underwater-vehicle-mounted sonar system, widebeam wideband rail-based system, Apertures, Arrays, Computational modeling, Focusing, Measurement, Synthetic aperture sonar, Synthetic aperture sonar (SAS) radar autofocus stripmap.
    Abstract: Autofocusing synthetic aperture imagery by maximizing a statistical quality metric such as contrast or sharpness is a well-documented approach in both synthetic aperture radar (SAR) literature and synthetic aperture sonar literature. It is most successfully applied in spotlight-mode SAR applications, where the assumption of spatial invariance of the corrupting phase function is strong and expressions for the gradients of various quality metrics with respect to standard error models have been calculated. Examples of application to stripmap imagery often involve sectioning images into small blocks, allowing spotlight algorithms to be patchwise applied. This paper formulates the gradient of the cost function in a manner that is consistent with the stripmap error model, inherently providing solutions that compensate for the spatial variance while simultaneously bypassing the need for subdividing an image or aperture. This paper formulates the stripmap gradient expression in conjunction with a computationally efficient imaging approach to rapidly achieve metric-maximizing solutions. Demonstrations are shown on a widebeam wideband rail-based system experiencing random jitter and on an unmanned-underwater-vehicle-mounted sonar system exhibiting artificially injected crabbing and sway errors. Results indicate that the algorithm is particularly effective at compensating for random and rapidly oscillating navigation and jitter errors, as well as sidelobes introduced by crabbing in arrayed systems.
    [bibtex-key = marstonPlotnickTGRS2015AutofocusStripmap] [bibtex-entry]


  338. Yu Morishita and Ramon F. Hanssen. Deformation Parameter Estimation in Low Coherence Areas Using a Multisatellite InSAR Approach. IEEE Trans. Geosci. Remote Sens., 53(8):4275-4283, August 2015. Keyword(s): SAR Processing, persistent scatterer interferometry, PSI, InSAR, DInSAR, Interferometry, Differential Interferometry, decorrelation, deformation, geophysical techniques, least mean squares methods, radar interferometry, remote sensing by radar, soil, synthetic aperture radar, time series, The Netherlands, drained peat soils, least squares method, local subsidence rates, low coherence areas, multisatellite InSAR, pasture periodic signal, peat periodic signal, persistent scatterer interferometry, satellite data, small baseline subset algorithms, statistically homogeneous pixels, surface deformation parameter estimation, Coherence, Decorrelation, Deformable models, Estimation, Satellites, Soil, Synthetic aperture radar, Decorrelation, radar interferometry, synthetic aperture radar (SAR).
    Abstract: Persistent scatterer (PS) interferometry and small baseline subset algorithms can be used to estimate time series of surface deformation with high precision. In areas with low coherence, and in the absence of sufficient PS, the estimation of reliable phase information can be cumbersome. Here, we report a successful approach for estimating deformation at pasture on drained peat soils using the integrated use of data from several satellite missions, a parametric deformation model with a generalized least squares method, and spatial averaging over statistically homogeneous pixels. The developed methodology is analyzed and applied on a test site in The Netherlands, where we report local subsidence rates and periodic signal over the peat and pasture areas.
    [bibtex-key = morishitaHanssenTGRS2015DInSARMultiSat] [bibtex-entry]


  339. Yu Morishita and Ramon F. Hanssen. Temporal Decorrelation in L-, C-, and X-band Satellite Radar Interferometry for Pasture on Drained Peat Soils. IEEE Trans. Geosci. Remote Sens., 53(2):1096-1104, February 2015. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, geophysical signal processing, land use, radar interferometry, remote sensing by radar, soil, synthetic aperture radar, terrain mapping, vegetation, vegetation mapping, ALOS-2 satellite, Advanced Land Observation Satellite mission, C-band SAR observations, C-band satellite radar interferometry, Envisat mission, European Remote Sensing Satellite mission, L-band SAR observations, L-band satellite radar interferometry, RADARSAT-2 mission, Sentinel-1 satellite, TerraSAR-X mission, X-band SAR observations, X-band satellite radar interferometry, a priori assessment, actual land use, climatological circumstances, coherence estimation window sizes, coherence levels, coherent information, coherent signal, drained peat soils, frequency function, generic models, interferograms, interferometric applications, nonurban areas, optimal data sets, pasture, repeat intervals, repeat orbits, satellite missions, soil types, synthetic aperture radar interferometry, temporal decorrelation model, temporal dynamics, vegetation types, Coherence, Decorrelation.
    Abstract: Temporal decorrelation is one of the main limitations of synthetic aperture radar (SAR) interferometry. For nonurban areas, its mechanism is very complex, as it is very dependent of vegetation types and their temporal dynamics, actual land use, soil types, and climatological circumstances. Yet, an a priori assessment and comprehension of the expected coherence levels of interferograms are required for designing new satellite missions (in terms of frequency, resolution, and repeat orbits), for choosing the optimal data sets for a specific application, and for feasibility studies for new interferometric applications. Although generic models for temporal decorrelation have been proposed, their parameters depend heavily on the land use in the area of interest. Here, we report the behavior of temporal decorrelation for a specific class of land use: pasture on drained peat soils. We use L-, C-, and X-band SAR observations from the Advanced Land Observation Satellite (ALOS), European Remote Sensing Satellite, Envisat, RADARSAT-2, and TerraSAR-X missions. We present a dedicated temporal decorrelation model using three parameters and demonstrate how coherent information can be retrieved as a function of frequency, repeat intervals, and coherence estimation window sizes. New satellites such as Sentinel-1 and ALOS-2, with shorter repeat intervals than their predecessors, would enhance the possibility to obtain a coherent signal over pasture.
    [bibtex-key = morishitaHanssenTGRS2015TempDecorrelation] [bibtex-entry]


  340. Antonio Pepe, Yang Yang, Mariarosaria Manzo, and Riccardo Lanari. Improved EMCF-SBAS Processing Chain Based on Advanced Techniques for the Noise-Filtering and Selection of Small Baseline Multi-Look DInSAR Interferograms. IEEE Transactions on Geoscience and Remote Sensing, 53(8):4394-4417, August 2015. Keyword(s): SAR Processing, SAR interferometry, InSAR, DInSAR, phase unwrapping, extended minimum cost flow (EMCF), EMCF phase unwrapping method, minimum cost flow, small baseline subset, SBAS, Deformation, Deformation time-series, differential synthetic aperture radar interferometry (DInSAR), small baseline subset (SBAS), EMCF-SBAS processing chain improvement, small baseline multilook DInSAR interferogram selection, deformation time-series retrieval, differential SAR interferometry, SBAS inversion technique, effective noise-filtering operation, wrapped phase vector, weighted circular variance, original interferogram, noise-filtered interferogram, exploited full-resolution SAR image, complex-valued SAR image statistics, advanced EMCF-SBAS processing chain.
    Abstract: We present in this paper a solution to drastically improve the deformation time-series retrieval capability of the small baseline differential SAR interferometry (DInSAR) processing chain based on the cascade of the extended minimum cost flow (EMCF) phase unwrapping method and of the small baseline subset (SBAS) inversion technique. This improvement relies on the inclusion of two preprocessing steps implementing an effective noise-filtering operation and an efficient interferogram selection procedure, respectively. The former step filters out the noise affecting the phase components of a redundant set of conventional multi-look small baseline interferograms. This is achieved by solving, for each pixel, a nonlinear minimization problem based on computing the wrapped phase vector that minimizes the weighted circular variance of the phase difference between the original and noise-filtered interferograms. This technique is very easy to implement because it does not require any pixel selection step to be applied to the exploited full-resolution SAR images, and it has no need of any a priori information on the statistics of the complex-valued SAR images. The latter step, implementing the interferogram selection procedure, is carried out via a computationally efficient simulated annealing algorithm and allows identifying the optimum set of previously filtered small baseline interferograms to be used as input for the original EMCF-SBAS processing chain by maximizing the (average) coherence values. The presented results, achieved by processing three data sets collected by the ENVISAT ASAR sensor over the Abruzzi region (Central Italy), Mt. Etna volcano (South Italy), and Yellowstone Caldera (WY, USA), demonstrate the effectiveness of the proposed advanced EMCF-SBAS processing chain.
    [bibtex-key = pepeYangManzoLanariTGRS2015ImprovedEMCFSBASProcessingBasedOnAdvancedNoiseFilteringAndSelectionOfSmallBaselineInterferograms] [bibtex-entry]


  341. M. Pieraccini, F. Papi, and S. Rocchio. Interferometric RotoSAR. Electron Lett, 51(18):1451-1453, 2015. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar antennas, radar imaging, radar interferometry, synthetic aperture radar, SAR images, displacement vectors, field of view, interferometric RotoSAR, plane of rotation, radar antenna. [bibtex-key = PieracciniEL2015RotoSAR] [bibtex-entry]


  342. M. Pinheiro, M. Rodriguez-Cassola, P. Prats-Iraola, A. Reigber, G. Krieger, and A. Moreira. Reconstruction of Coherent Pairs of Synthetic Aperture Radar Data Acquired in Interrupted Mode. IEEE Transactions on Geoscience and Remote Sensing, 53(4):1876-1893, April 2015. Keyword(s): calibration, image reconstruction, radar imaging, radar interferometry, synchronisation, synthetic aperture radar, coherent pairs, synthetic aperture radar, cooperative bistatic SAR systems, SAR imaging, multistatic systems, spectral estimation based interpolators, distributed scatterers, image reconstruction, TanDEM-X mission, Synthetic aperture radar, Image reconstruction, Interrupters, Azimuth, Synchronization, Apertures, Coherence, Bistatic synthetic aperture radar (SAR), cross-reconstruction, reconstruction of missing data, SAR, SAR interferometry, spectral estimators, Bistatic synthetic aperture radar (SAR), cross-reconstruction, reconstruction of missing data, SAR, SAR interferometry, spectral estimators.
    Abstract: A number of synthetic aperture radar (SAR) systems might work in interrupted operation for different purposes. Examples are cooperative bistatic SAR systems with a synchronization link between the transmitter and receiver or multistatic systems operating in receive-only mode, among others. As a direct consequence, the acquired raw data contain missing echoes presented in a periodical or random pattern. Since the missing raw data introduce artifacts in the processed images, recovery methods have to be applied. Usually, spectral-estimation-based interpolators can be used to recover data. Although such algorithms show good performance for pointlike targets, their efficiency is decreased for distributed scatterers. In this paper, we propose, for a coherent pair of SAR images, the use of the common information in one image to reconstruct the other and vice versa. The conditions required for the proper use of the approach are discussed, and the method is verified using simulated data. One special case of study is the TanDEM-X mission, where the cooperative nature of the bistatic operation requires the periodic exchange of information between the satellites in order to gather information for calibration and synchronization, creating a periodic missing data pattern in the raw data. For this case of study, the reconstruction methods based on spectral estimation are analyzed, and the proposed reconstruction using cross-information is validated.
    [bibtex-key = 6891190] [bibtex-entry]


  343. P. Prats-Iraola, M. Rodriguez-Cassola, F. De Zan, R. Scheiber, P. Lopez-Dekker, I. Barat, and D. Geudtner. Role of the Orbital Tube in Interferometric Spaceborne SAR Missions. IEEE Geoscience and Remote Sensing Letters, 12(7):1486-1490, July 2015. Keyword(s): decorrelation, Earth orbit, radar interferometry, spaceborne radar, synthetic aperture radar, interferometric spaceborne SAR mission, Earth observation satellite synthetic aperture radar mission, Earthfixed orbital tube, ground-track coverage repeatability, repeat-pass interferometric compatibility, azimuth spectral decorrelation, azimuth coregistration accuracy, ScanSAR, TOPS, terrain observation by progressive scan, Sentinel-1 mission, Orbits, Electron tubes, Azimuth, Doppler effect, Satellites, Synthetic aperture radar, Remote sensing, Coregistration, interferometric SAR (InSAR), orbital tube, ScanSAR, spectral decorrelation, synthetic aperture radar (SAR), Terrain Observation by Progressive Scans (TOPS), Coregistration, interferometric SAR (InSAR), orbital tube, ScanSAR, spectral decorrelation, synthetic aperture radar (SAR), Terrain Observation by Progressive Scans (TOPS).
    Abstract: The orbit for Earth observation satellite synthetic aperture radar (SAR) missions is maintained within an Earth-fixed orbital tube to ensure ground-track coverage repeatability and, consequently, to enable repeat-pass interferometric compatibility between data takes. In this letter, it is shown that the size of the orbital tube may affect the interferometric performance in terms of azimuth spectral decorrelation and azimuth coregistration accuracy under the presence of squint. These effects require special consideration for SAR burst modes, such as ScanSAR or TOPS (i.e., Terrain Observation by Progressive Scans). This letter presents and analyzes these aspects in the frame of the Sentinel-1 mission.
    [bibtex-key = 7065218] [bibtex-entry]


  344. Martin Proksch, Henning Löwe, and Martin Schneebeli. Density, specific surface area, and correlation length of snow measured by high-resolution penetrometry. J. Geophys. Res. Earth Surf., pp 346-362, 2015. Keyword(s): Snow characterisation, correlation length, snow density, specific surface area, SSA, Snow Micro Pen, SMP, micro-CT, Statistical model, Statistical model relating micro-CT structure to SMP force for many snow data, snow density retrieval, and SSA in the field, Efficient retrieval of spatial variability and 2-D stratigraphy of snow, 2-D stratigraphy of snow.
    Abstract: Precise measurements of snow structural parameters are crucial to understand the formation of snowpacks by deposition and metamorphism and to characterize the stratigraphy for many applications and remote sensing in particular. The area-wide acquisition of structural parameters at high spatial resolution from state-of-the-art methods is, however, still cumbersome, since the time required for a single profile is a serious practical limitation. As a remedy we have developed a statistical model to extract three major snow structural parameters: density, correlation length, and specific surface area (SSA) solely from the SnowMicroPen (SMP), a high-resolution penetrometer, which allows a meter profile to be measured with millimeter resolution in less than 1 min. The model was calibrated by combining SMP data with 3-D microstructural data from microcomputed tomography which was used to reconstruct full-depth snow profiles from different snow climates (Alpine, Arctic, and Antarctic). Density, correlation length, and SSA were derived from the SMP with a mean relative error of 10.6%, 16.4%, and 23.1%, respectively. For validation, we compared the density and SSA derived from the SMP to traditional measurements and near-infrared profiles. We demonstrate the potential of our method by the retrieval of a two-dimensional stratigraphy at Kohnen Station, Antarctica, from a 46 m long SMP transect. The result clearly reveals past depositional and metamorphic events, and our findings show that the SMP can be used as an objective, high-resolution tool to retrieve essential snow structural parameters efficiently in the field.
    [bibtex-key = prokschLoeweSchneebeliJGR2015SnowParametersFromSnowMicroPenAndMicroCT] [bibtex-entry]


  345. Shaun Quegan and M.R. Lomas. The Interaction Between Faraday Rotation and System Effects in Synthetic Aperture Radar Measurements of Backscatter and Biomass. Geoscience and Remote Sensing, IEEE Transactions on, 53(8):4299-4312, August 2015. Keyword(s): Faraday effect, backscatter, geophysical techniques, synthetic aperture radar, European space agency BIOMASS mission, P-band radar, backscatter synthetic aperture radar measurement, biomass estimation error, biomass synthetic aperture radar measurement, channel imbalance deviation magnitude, distortion term amplitude, distortion term phase, faraday rotation, first-order analysis, long-wavelength space-based radar, polarimetric backscattering coefficient, polarimetric scattering matrix measurement, power-law relation, signal-to-noise ratio, stringent condition, system distortion, Backscatter, Biomass, Crosstalk, Distortion measurement, Faraday effect, Noise, Scattering, Biomass, Faraday rotation, calibration, long-wavelength radar, polarimetric measurements, system distortion.
    Abstract: For long-wavelength space-based radars, such as the P-band radar on the recently selected European Space Agency BIOMASS mission, system distortions (crosstalk and channel imbalance), Faraday rotation, and system noise all combine to degrade the measurements. A first-order analysis of these effects on the measurements of the polarimetric scattering matrix is used to derive differentiable expressions for the errors in the polarimetric backscattering coefficients in the presence of Faraday rotation. Both the amplitudes and phases of the distortion terms are shown to be important in determining the errors and their maximum values. Exact simulations confirm the accuracy and predictions of the first-order analysis. Under an assumed power-law relation between hv and the biomass, the system distortions and noise are converted into biomass estimation errors, and it is shown that the magnitude of the deviation of the channel imbalance from unity must be 4-5 dB less than the crosstalk, or it will dominate the error in the biomass. For uncalibrated data and midrange values of biomass, the crosstalk must be less than -24 dB if the maximum possible error in the biomass is to be within 20% of its true value. A less stringent condition applies if the amplitudes and phases of the distortion terms are considered random since errors near the maximum possible are very unlikely. For lower values of the biomass, the noise becomes increasingly important because the hv signal-to-noise ratio is smaller.
    [bibtex-key = queganLomasTGRS2015FaradayRotEffectSARData] [bibtex-entry]


  346. M. Rodriguez-Cassola, P. Prats-Iraola, F. De Zan, R. Scheiber, A. Reigber, D. Geudtner, and A. Moreira. Doppler-Related Distortions in TOPS SAR Images. IEEE Trans. Geosci. Remote Sens., 53(1):25-35, January 2015. Keyword(s): SAR Processing, SAR Focusing, Azimuth Focusing, TOPS, Doppler radar, approximation theory, beam steering, compensation, distortion, geophysical image processing, radar antennas, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, SAR image formation scheme, Sentinel-1 interferometric extra wide swath mode, Sentinel-1 interferometric wide swath mode, TOPS SAR image, TerraSAR-X TOPS, azimuth distortion, burst mode acquisition, compensation strategy, focused SAR image, intrapulse motion, low Earth orbit SAR, radar antenna, range distortion, steering, stop-and-go approximation, terrain observation with progressive scan, time-varying Doppler centroid, Azimuth, Doppler effect, Geometry, Orbits, Spaceborne radar, Surfaces, Synthetic aperture radar, Burst-mode acquisitions, Sentinel-1, TerraSAR-X (TerraSAR-X), Terrain Observation with Progressive Scans (TOPS), spaceborne SAR missions, synthetic aperture radar (SAR), wide-swath SAR modes.
    Abstract: A direct consequence of the TOPS acquisition geometry and the steering in azimuth of the antenna is the time-varying Doppler centroid within bursts. If this fact is not properly accommodated during SAR image formation, undesired distortions in both azimuth and range dimensions of the focused SAR images may appear. Azimuth distortions are caused by the local mismatch of both squint and topography. Range distortions arise from the inaccurate accommodation of the intrapulse motion of the platform, usually known as the stop-and-go approximation. Conventional spaceborne SAR image formation schemes will be, in general, unable to provide accurate TOPS SAR images. These distortions are discussed and evaluated for exemplary low-Earth-orbit SAR scenarios. Compensation strategies are presented and validated with TerraSAR-X TOPS data. A discussion of the potential impact on the Sentinel-1 interferometric-wide-swath and extra-wide-swath modes (i.e, TOPS) is also given.
    [bibtex-key = rodriguezCassolaPratsDeZanScheiberReigberGeudtnerMoreiraTGRS2015TOPSDopplerRelDistorsion] [bibtex-entry]


  347. Maria J. Sanjuan-Ferrer, Irena Hajnsek, K. P. Papathanassiou, and Alberto Moreira. A New Detection Algorithm for Coherent Scatterers in SAR Data. IEEE Transactions on Geoscience and Remote Sensing, 53(11):6293-6307, November 2015. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Detector, Candidate Selection, Coherent Scatterer, remote sensing by radar, synthetic aperture radar, SAR data, TerraSAR-X acquisitions, coherent scatterers, detection algorithm, generalized likelihood ratio test approach, natural environments, natural scenarios, permanent-scatterer interferometry techniques, point-like scatterers, scattering temporal stability, single SAR image, spectral diversity techniques, sublook coherence approach, sublook entropy approach, synthetic aperture radar, urban environments, Bandwidth, Clutter, Coherence, Spatial resolution, Speckle, Synthetic aperture radar, Coherent scatterers (CSs), likelihood ratio test, signal processing, synthetic aperture radar (SAR), target detection.
    Abstract: In contrast to the random nature of synthetic aperture radar (SAR) data, it is also possible to identify bright targets whose scattering properties scarcely vary within imaging and time. These targets are commonly named point-like scatterers and can be found in both urban and natural environments. Permanent-scatterer interferometry techniques single out stable scatterers in a stack of SAR images, which preserve their backscattering stability along time. However, this methodology may not be optimum in natural scenarios, where the temporal stability of the scattering is rather reduced, or when the number of available SAR acquisitions is significantly small. Consequently, alternative methods have come out to detect stable scatters in a single SAR image, thus reducing all constraints related to their temporal behavior. Particularly, spectral diversity techniques are exploited to detect the so-called coherent scatterers. In this paper, a new detection scheme based on the generalized likelihood ratio test approach (GLRTA) is proposed, and its performance is extensively evaluated compared with three of the traditional methods, namely, the sublook coherence approach, the sublook entropy approach, and the phase variance approach. Remarkably, the GLRTA exploits both amplitude and phase information and does not need any further averaging (apart from sublooking processing with reduced signal bandwidth). The presented analysis is conducted both theoretically and with simulated data. For all scenarios, the new detector outperforms the other methods. The obtained results are validated also on real data. Finally, the proposed GLRTA is tested over different scattering scenarios, considering three TerraSAR-X acquisitions.
    [bibtex-key = sanjuanFerrerHajnsekPapathanassiouMoreiraTGRS2015CoherentScattererDetector] [bibtex-entry]


  348. Eugenio Sansosti, Michele Manunta, Francesco Casu, Manuela Bonano, Chandrakanta Ojha, Maria Marsella, and Riccardo Lanari. Radar remote sensing from space for surface deformation analysis: present and future opportunities from the new SAR sensor generation. Rendiconti Lincei, 26(1):75-84, 2015. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, DINSAR, Differential SAR Interferometry, Ground deformation, Remote Sensing, Synthetic Aperture Radar (SAR), DInSAR, Urban monitoring, Wide area monitoring, Sentinel-1, ENVISAT, ASAR, COSMO-SkyMed, X-band, C-band, Spaceborne SAR.
    Abstract: This paper discusses, through two selected case studies based on real data, how the availability of the new generation of Synthetic Aperture Radar (SAR) sensors, characterized by reduced revisiting time and improved spatial resolution or coverage, is impacting the exploitation of Differential SAR Interferometry (DInSAR) techniques for the detection and monitoring of deformation phenomena. The presented analysis is carried out using X-band data of the COSMO-SkyMed constellation satellites, as well as C-band data acquired by the Sentinel-1A sensor; furthermore, we compare the achieved results to those based on first-generation ERS-1/2 and ENVISAT satellite data. The first case study shows how the COSMO-SkyMed X-band SAR systems open new opportunities for the detection and monitoring of deformation phenomena at the scale of a single building, even when they are characterized by a rather fast dynamic. The second experiment is based on the Sentinel-1A DInSAR measurements and permits us to envisage new scenarios for deformation analysis of very wide areas. The final discussion is devoted to summarise the lessons learned through the presented case studies and to identify the main future actions needed for a full exploitation of the surface deformation measurement capability provided by the new generation of SAR sensor.
    [bibtex-key = sansostiManuntaCasuBonanoOjhaMarsellaLanari2015PSIDINSAR] [bibtex-entry]


  349. Maurizio Santoro, André Beaudoin, Christian Beer, Oliver Cartus, Johan E.S. Fransson, Ronald J. Hall, Carsten Pathe, Christiane Schmullius, Dmitry Schepaschenko, Anatoly Shvidenko, Martin Thurner, and Urs Wegmuller. Forest growing stock volume of the northern hemisphere: Spatially explicit estimates for 2010 derived from Envisat {ASAR}. Remote Sensing of Environment, 168:316 - 334, 2015. Keyword(s): Envisat ASAR, Forest, Growing stock volume, Biomass, MODIS Vegetation Continuous Fields, BIOMASAR, Northern hemisphere.
    Abstract: This paper presents and assesses spatially explicit estimates of forest growing stock volume (GSV) of the northern hemisphere (north of 10 deg N) from hyper-temporal observations of Envisat Advanced Synthetic Aperture Radar (ASAR) backscattered intensity using the BIOMASAR algorithm. Approximately 70,000 ASAR images at a pixel size of 0.01 deg were used to estimate GSV representative for the year 2010. The spatial distribution of the GSV across four ecological zones (polar, boreal, temperate, subtropical) was well captured by the ASAR-based estimates. The uncertainty of the retrieved GSV was smallest in boreal and temperate forest <30% for approximately 80% of the forest area) and largest in subtropical forest. ASAR-derived GSV averages at the level of administrative units were mostly in agreement with inventory-derived estimates. Underestimation occurred in regions of very high GSV >300 m3/ha) and fragmented forest landscapes. For the major forested countries within the study region, the relative RMSE between ASAR-derived GSV averages at provincial level and corresponding values from National Forest Inventory was between 12% and 45% (average: 29%).
    [bibtex-key = santoroEtAlRSE2015GrowingStockVolume] [bibtex-entry]


  350. Michael Schmitt, Muhammad Shahzad, and Xiao Xiang Zhu. Reconstruction of individual trees from multi-aspect TomoSAR data. Remote Sensing of Environment, 165:175-185, 2015. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), Multi-aspect, Trees, 3D reconstruction, Forested areas, Point cloud segmentation.
    Abstract: Abstract The localization and reconstruction of individual trees as well as the extraction of their geometrical parameters is an important field of research in both forestry and remote sensing. While the current state-of-the-art mostly focuses on the exploitation of optical imagery and airborne LiDAR data, modern SAR sensors have not yet met the interest of the research community in that regard. This paper presents a prototypical processing chain for the reconstruction of individual deciduous trees: First, single-pass multi-baseline InSAR data acquired from multiple aspect angles are used for the generation of a layover- and shadow-free 3D point cloud by tomographic SAR processing. The resulting point cloud is then segmented by unsupervised mean shift clustering, before ellipsoid models are fitted to the points of each cluster. From these 3D ellipsoids the relevant geometrical tree parameters are extracted. Evaluation with respect to a manually derived reference dataset prove that almost 74% of all trees are successfully segmented and reconstructed, thus providing a promising perspective for further research toward individual tree recognition from SAR data.
    [bibtex-key = schmittShahzadZhu2015RSEonTomoSARSingleTree] [bibtex-entry]


  351. Zoran Sjanic and Frederik Gustafsson. Simultaneous navigation and synthetic aperture radar focusing. IEEE Transactions on Aerospace and Electronic Systems, 51(2):1253-1266, April 2015. Keyword(s): SAR Focusing, Autofocus, SLAM, Simultaneous Localization and Mapping, autonomous aerial vehicles, image resolution, radar imaging, radar resolution, radionavigation, synthetic aperture radar, synthetic aperture radar imaging equipment, image resolution, flying platform, image focusing, real-time SAR imaging, navigation system, trajectory joint estimation, unmanned aerial vehicle navigation, azimuth position error, Synthetic aperture radar, Trajectory, Radar imaging, Navigation, Entropy, Focusing.
    Abstract: Synthetic aperture radar (SAR) equipment is a radar imaging system that can be used to create high-resolution images of a scene by utilizing the movement of a flying platform. Knowledge of the platform's trajectory is essential to get good and focused images. An emerging application field is real-time SAR imaging using small and cheap platforms where estimation errors in navigation systems imply unfocused images. This contribution investigates a joint estimation of the trajectory and SAR image. Starting with a nominal trajectory, we successively improve the image by optimizing a focus measure and updating the trajectory accordingly. The method is illustrated using simulations using typical navigation performance of an unmanned aerial vehicle. One real data set is used to show feasibility, where the result indicates that, in particular, the azimuth position error is decreased as the image focus is iteratively improved.
    [bibtex-key = sjanicGustafssonTAES2015SLAMandSARFocusing] [bibtex-entry]


  352. Maciej J. Soja, H.J. Persson, and Lars M.H. Ulander. Estimation of Forest Biomass From Two-Level Model Inversion of Single-Pass InSAR Data. IEEE Trans. Geosci. Remote Sens., 53(9):5083-5099, September 2015. Keyword(s): data acquisition, digital elevation models, forestry, radar interferometry, remote sensing by radar, synthetic aperture radar, vegetation, AD 2008, AD 2010, AD 2011, AD 2012, AD 2013, InSAR processing, Krycklan feature, Remningstorp feature, Swedish test site, VV-polarized TanDEM-X acquisition, aboveground biomass estimation, biomass predictor, canopy density, digital terrain model, forest biomass estimation, forest height, hemiboreal forest, northern Sweden, single-pass InSAR data, single-pass interferometric synthetic aperture radar data, southern Sweden, two-level model inversion, Biological system modeling, Biomass, Computational modeling, Correlation, Decorrelation, Estimation, Synthetic aperture radar, Aboveground biomass (AGB), TanDEM-X (TDM), canopy density, forest height, interferometric model, interferometric syntheticaperture radar (InSAR), two-level model (TLM).
    Abstract: A model for aboveground biomass estimation from single-pass interferometric synthetic aperture radar (InSAR) data is presented. Forest height and canopy density estimates dh and n0, respectively, obtained from two-level model (TLM) inversion, are used as biomass predictors. Eighteen bistatic VV-polarized TanDEM-X (TDM) acquisitions are used, made over two Swedish test sites in the summers of 2011, 2012, and 2013 (nominal incidence angle: 41 deg, height-of-ambiguity: 32-63 m) . Remningstorp features a hemiboreal forest in southern Sweden, with flat topography and where 32 circular plots have been sampled between 2010 and 2011 (area: 0.5 ha; biomass: 42-242 t/ha; height: 14-32 m) . Krycklan features a boreal forest in northern Sweden, 720-km north-northeast from Remningstorp, with significant topography and where 31 stands have been sampled in 2008 (area: 2.4-26.3 ha; biomass: 23-183 t/ha; height: 7-21 m) . A high-resolution digital terrain model has been used as ground reference during InSAR processing. For the aforementioned plots and stands and if the same acquisition is used for model training and validation, the new model explains 65%-89% of the observed variance, with root-mean-square error (RMSE) of 12%-19% (median: 15%) . By fixing two of the three model parameters, accurate biomass estimation can also be done when different acquisitions or different test sites are used for model training and validation, with RMSE of 12%-56% (median: 17%) . Compared with a simple scaling model computing biomass from the phase center elevation above ground, the proposed model shows significantly better performance in Remningstorp, as it accounts for the large canopy density variations caused by active management. In Krycklan, the two models show similar performance.
    [bibtex-key = sojaPerssonUlanderTGRS2015BiomassTandemX] [bibtex-entry]


  353. Maciej J. Soja, H. Persson, and Lars M. H. Ulander. Estimation of Forest Height and Canopy Density From a Single InSAR Correlation Coefficient. IEEE Geosci. Remote Sens. Lett., 12(3):646-650, March 2015. Keyword(s): digital elevation models, geophysical image processing, parameter estimation, radar interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping, AD 2011 to 2013, VV-polarized bistatic-interferometric TanDEM-X image pairs, canopy density, forest height estimation, hemiboreal test site Remningstorp, high-resolution digital terrain model, single InSAR correlation coefficient, southern Sweden, synthetic aperture radar, two-level model, vegetation, Backscatter, Coherence, Laser radar, Remote sensing, Synthetic aperture radar, Time division multiplexing, Vegetation, Canopy density, TanDEM-X, forest height, interferometric model, interferometry, synthetic aperture radar (SAR), two-level model (TLM).
    Abstract: A two-level model (TLM) is introduced and investigated for the estimation of forest height and canopy density from a single ground-corrected InSAR complex correlation coefficient. The TLM models forest as two scattering levels, namely, ground and vegetation, separated by a distance dh and with area-weighted backscatter ratio mu. The model is evaluated using eight VV-polarized bistatic-interferometric TanDEM-X image pairs acquired in the summers of 2011, 2012, and 2013 over the managed hemi-boreal test site Remningstorp, which is situated in southern Sweden. Ground phase is removed using a highresolution digital terrain model. Inverted TLM parameters for thirty-two 0.5-ha plots of four different types (regular plots, sparse plots, seed trees, and clear-cuts) are studied against reference lidar data. It is concluded that the level distance dh can be used as an estimate of the 50th percentile forest height estimated from lidar (for regular plots: r > 0.95 and root-mean-square difference (sigma) < 10%, or 1.8 m). Moreover, the uncorrected area fill factor n0 = 1/(1 + mu) can be used as an estimate of the vegetation ratio, which is a canopy density estimate defined as the fraction of lidar returns coming from the canopy to all lidar returns (for regular plots: r > 0.59 and sigma = 10%, or 0.07).
    [bibtex-key = sojaPerssonUlanderGRSL2015ForestHeightTandemX] [bibtex-entry]


  354. Alireza Tabatabaeenejad, Mariko S. Burgin, X. Duan, and Mahta Moghaddam. P-Band Radar Retrieval of Subsurface Soil Moisture Profile as a Second-Order Polynomial: First AirMOSS Results. IEEE Transactions on Geoscience and Remote Sensing, 53(2):645-658, February 2015. Keyword(s): hydrological techniques, remote sensing by radar, vegetation, AD 2012 09, AD 2012 10, AirMOSS mission flights, AirMOSS results, Airborne Microwave Observatory of Sub- canopy and Subsurface, Arizona, P-band radar data, Root Mean Squared Error, Walnut Gulch Experimental Watershed, barren terrain, discrete scattering model, radar pixel, second-order polynomial, shrubland terrain, subsurface depth function, subsurface soil moisture profile, synthetic radar data, terrain radar backscattering coefficients, vegetated terrain, Atmospheric modeling, Data models, Moisture, Polynomials, Radar, Soil moisture, Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS), discrete scattering model, quadratic function, radar, remote sensing, second-order polynomial, simulated annealing, soil moisture profile.
    Abstract: We propose a new model for estimating subsurface soil moisture using P-band radar data over barren, shrubland, and vegetated terrains. The unknown soil moisture profile is assumed to have a second-order polynomial form as a function of subsurface depth with three unknown coefficients that we estimate using the simulated annealing algorithm. These retrieved coefficients produce the value of soil moisture at any given depth up to a prescribed depth of validity. We use a discrete scattering model to calculate the radar backscattering coefficients of the terrain. The retrieval method is tested and developed with synthetic radar data and is validated with measured radar data and in situ soil moisture measurements. Both forward and inverse models are briefly explained. The radar data used in this paper have been collected during the Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) mission flights in September and October of 2012 over a 100 km by 25 km area in Arizona, including the Walnut Gulch Experimental Watershed. The study area and the ancillary data layers used to characterize each radar pixel are explained. The inversion results are presented, and it is shown that the RMSE between the retrieved and measured soil moisture profiles ranges from 0.060 to 0.099 m3/m3, with a Root Mean Squared Error (RMSE) of 0.075 m3/m3 over all sites and all acquisition dates. We show that the accuracy of retrievals decreases as depth increases. The profiles used in validation are from a fairy dry season in Walnut Gulch and so are the accuracy conclusions.
    [bibtex-key = tabatabaeenejadBurginDuanMoghaddamTGRS2015PBandSoilMoistureAIRMOSS] [bibtex-entry]


  355. Ekaterina Tymofyeyeva and Yuri Fialko. Mitigation of atmospheric phase delays in InSAR data, with application to the eastern California shear zone. Journal of Geophysical Research: Solid Earth, 120(8):5952-5963, 2015. Note: 2015JB011886. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Displacement, Surface Displacement, Atmosphere, APS, Transient deformation, Satellite geodesy: results, Satellite geodesy: technical issues, Seismic cycle related deformations, Integrations of techniques, InSAR, time series, atmospheric delays, transient deformation.
    Abstract: We present a method for estimating radar phase delays due to propagation through the troposphere and the ionosphere based on the averaging of redundant interferograms that share a common scene. Estimated atmospheric contributions can then be subtracted from the radar interferograms to improve measurements of surface deformation. Inversions using synthetic data demonstrate that this procedure can considerably reduce scatter in the time series of the line-of-sight displacements. We demonstrate the feasibility of this method by comparing the interferometric synthetic aperture radar (InSAR) time series derived from ERS-1/2 and Envisat data to continuous Global Positioning System data from eastern California. We also present results from several sites in the eastern California shear zone where anomalous deformation has been reported by previous studies, including the Blackwater fault, the Hunter Mountain fault, and the Coso geothermal plant.
    [bibtex-key = tymofyeyevaFialkoJGRB2015AtmosphereInSAR] [bibtex-entry]


  356. Alberto Villa, Lorenzo Iannini, Davide Giudici, Andrea Monti-Guarnieri, and Stefano Tebaldini. Calibration of SAR Polarimetric Images by Means of a Covariance Matching Approach. IEEE Trans. Geosci. Remote Sens., 53(2):674-686, February 2015. Keyword(s): Faraday effect, calibration, covariance analysis, numerical analysis, optimisation, parameter estimation, radar imaging, radar polarimetry, synthetic aperture radar, Faraday rotation, SAR polarimetric imaging, corner reflector, covariance matching approach, intrinsic ambiguity identification, numerical method, optimization, polarimetric calibration, repeated full polarimetric ALOS PALSAR imaging, retrieved distortion parameter stability, synthetic aperture radar, system polarimetric distortion parameter estimation, Calibration, Eigenvalues and eigenfunctions, Estimation, Faraday effect, Noise, Sensitivity, Thyristors, Covariance matching, Faraday rotation, numerical methods, polarimetric calibration.
    Abstract: In this paper, a numerical method optimizer based on covariance matching is proposed for synthetic aperture radar (SAR) polarimetric calibration. The method makes use of the information provided by a distributed target and a corner reflector in order to jointly estimate the system polarimetric distortion parameters and the Faraday rotation. A preliminary analysis is conducted to show the expected accuracy values and to identify the intrinsic ambiguities of the problem. Results from simulations are shown to assess the accuracy and convergence of the method. Finally, tests have been conducted on stack of repeated full polarimetric ALOS PALSAR images to check the stability of the retrieved distortion parameters in a realistic case.
    [bibtex-key = villaIanniniGiudiciMontiGuarnieriTebaldiniTGRS2015PolCalibration] [bibtex-entry]


  357. Viet Thuy Vu and Mats I. Pettersson. Nyquist Sampling Requirements for Polar Grids in Bistatic Time-Domain Algorithms. IEEE Transactions on Signal Processing, 63(2):457-465, January 2015. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Bistatic SAR, Bistatic Fast-Factorized Back-Projection, BiFFBP, radar signal processing, signal sampling, Nyquist sampling, airborne bistatic system, bistatic CARABAS-II like data, bistatic cases, bistatic time-domain algorithms, general bistatic geometry, polar grids, Geometry, Radar polarimetry, Receivers, Signal processing algorithms, Synthetic aperture radar, Time-domain analysis, Transmitters, Bistatic, Nyquist sampling, SAR, fast backprojection.
    Abstract: The paper presents a derivation of Nyquist sampling requirements for the polar grids in some bistatic time-domain algorithms. The derivation is based on an airborne bistatic system with general bistatic geometry. The Nyquist sampling requirements are shown to be the functions of operating radar frequency, transmitter and receiver subaperture lengths, and bistatic geometry. How to decide the Nyquist sampling requirements for different bistatic geometries and the relationship between the Nyquist sampling requirements in the monostatic and bistatic cases are also addressed in the paper. The derived Nyquist sampling requirements is examined with the bistatic CARABAS-II like data.
    [bibtex-key = vuPettersson2015SamplingReqBistaticFFBP] [bibtex-entry]


  358. Ze Yu, Zhou Li, and Shusen Wang. An Imaging Compensation Algorithm for Correcting the Impact of Tropospheric Delay on Spaceborne High-Resolution SAR. IEEE Transactions on Geoscience and Remote Sensing, 53(9):4825-4836, September 2015. Keyword(s): SAR Processing, SAR Focusing, Azimuth Focusing, Autofocus, Motion Compensation, atmospheric electromagnetic wave propagation, delays, geophysical image processing, image filtering, image resolution, radar cross-sections, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, troposphere, imaging compensation algorithm, tropospheric delay, spaceborne high-resolution SAR, atmospheric refraction, electromagnetic signalpropagation speed, propagation path delay, geometrical straight-line path, spaceborne synthetic aperture radar, imaging filter, rectilinear propagation, residual phase, focusing quality, focusing performance, spaceborne SAR echo model, range delay coefficient, European Geostationary Navigation Overlay Service model, zenith delay, Niell mapping function, looking direction, range compensation, classical imaging, azimuth compensation, Delays, Synthetic aperture radar, Atmospheric modeling, Focusing, Data models, Real-time systems, High-resolution imaging, phase compensation, synthetic aperture radar (SAR), tropospheric delay, High-resolution imaging, phase compensation.
    Abstract: Atmospheric refraction in the troposphere causes the propagation speed of electromagnetic signals to be less than the light speed. This creates a difference between the actual propagation path delay and the distance of the geometrical straight-line path, i.e, a quantity known as the tropospheric delay. As classical imaging algorithms for spaceborne synthetic aperture radar (SAR) do not take the tropospheric delay into account, imaging filters are designed based on the assumption of rectilinear propagation with the light speed. Therefore, a residual phase exists in imaging results, which affects focusing quality under the condition of high resolution. In order to compensate for the impact of tropospheric delay on focusing performance, this paper modifies the spaceborne SAR echo model and then proposes an imaging compensation algorithm. The key to this algorithm is to fit a range delay coefficient based on the European Geostationary Navigation Overlay Service model of zenith delay and Niell mapping function, which projects the zenith delay onto the looking direction. After range compensation, classical imaging, and azimuth compensation, which compose the proposed algorithm, the processed results are well focused.
    [bibtex-key = yuLiWangTGRS2015SpaceborneSARFocusingWithTropoDelayCorr] [bibtex-entry]


  359. Evan C. Zaugg and David G. Long. Generalized Frequency Scaling and Backprojection for LFM-CW SAR Processing. IEEE Trans. Geosci. Remote Sens., 53(7):3600-3614, July 2015. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Back-Projection, Fast-Factorized Back-Projection, FFBP, GPU, SAR focusing, Azimuth Focusing, GPU-based parallelized TDBP, graphics processing units, LFM-CW, FMCW, Airborne SAR, Approximation algorithms, Approximation methods, Bandwidth, Chirp, Doppler effect, Synthetic aperture radar, Radar imaging, synthetic aperture radar (SAR).
    Abstract: This paper presents a generalized treatment of image formation for a linear-frequency-modulated continuous wave (LFM-CW) synthetic aperture radar (SAR) signal, which is a key technology in making very small SAR systems viable. The signal model is derived, which includes the continuous platform motion. The effect of this motion on the SAR signal is discussed, and an efficient compensation method is developed. Processing algorithms are developed including precise and approximate backprojection methods and a generalized frequency scaling algorithm that accounts for an arbitrary number of terms of a Taylor expansion approximation of the SAR signal in the Doppler frequency domain. Together, these algorithms allow for the processing of LFM-CW SAR data for a wide variety of system parameters, even in scenarios where traditional algorithms and signal approximations break down.
    [bibtex-key = zauggLongTGRS2015Backprojection] [bibtex-entry]


  360. Rafael Caduff, Andrew Kos, Fritz Schlunegger, Brian W. McArdell, and Andreas Wiesmann. Terrestrial Radar Interferometric Measurement of Hillslope Deformation and Atmospheric Disturbances in the Illgraben Debris-Flow Catchment, Switzerland. IEEE Geoscience and Remote Sensing Letters, 11(2):434-438, February 2014. Keyword(s): Atmospheric phase delay, rockslide monitoring, terrestrial radar interferometry, Radar interferometry, Gamma Portable Radar Interferometer, GPRI, Ku-band, Deformation, Displacement, Illgraben, Debris-flow catchment. [bibtex-key = caduffKosSchluneggerMcArdellWiesmannGRSL2014TRIGPRIDisplacementAtmoIllgrabenDebriFlowCatchment] [bibtex-entry]


  361. Rafael Caduff and Dirk Rieke-Zapp. Registration And Visualisation Of Deformation Maps From Terrestrial Radar Interferometry Using Photogrammetry And Structure From Motion. The Photogrammetric Record, 29(146):167-186, 2014. Keyword(s): compact digital camera, digital photogrammetry, geo-monitoring, interferometric radar, metric camera, structure from motion, SAR Processing, Gamma Portable Radar Interferometer, GPRI, radar interferometry, ground-based radar, GBSAR, InSAR, Ground-based SAR, deformation measurement, displacement, subsidence, terrestrial radar interferometry, mass movements, surface deformation, Structure from motion, SfM, Photogrammetry.
    Abstract: This paper describes a general workflow for the registration of terrestrial radar interferometric data with 3D point clouds derived from terrestrial photogrammetry and structure from motion. After the determination of intrinsic and extrinsic orientation parameters, data obtained by terrestrial radar interferometry were projected on point clouds and then on the initial photographs. Visualisation of slope deformation measurements on photographs provides an easily understandable and distributable information product, especially of inaccessible target areas such as steep rock walls or in rockfall run-out zones. The suitability and error propagation of the referencing steps and final visualisation of four approaches are compared: (a) the classic approach using a metric camera and stereo-image photogrammetry; (b) images acquired with a metric camera, automatically processed using structure from motion; (c) images acquired with a digital compact camera, processed with structure from motion; and (d) a markerless approach, using images acquired with a digital compact camera using structure from motion without artificial ground control points. The usability of the completely markerless approach for the visualisation of high-resolution radar interferometry assists the production of visualisation products for interpretation.
    [bibtex-key = caduffRiekeZapp2014GPRIRegistrPhotogrammetry] [bibtex-entry]


  362. Estelle Chaussard, R Bürgmann, Manoochehr Shirzaei, EJ Fielding, and B Baker. Predictability of hydraulic head changes and characterization of aquifer-system and fault properties from InSAR-derived ground deformation. Journal of Geophysical Research: Solid Earth, 119(8):6572-6590, 2014. [bibtex-key = Chaussard2014] [bibtex-entry]


  363. A. Elsherbini and K. Sarabandi. Image Distortion Effects in SAR Subsurface Imaging and a New Iterative Approach for Refocusing and Coregistration. IEEE_J_GRS, 52(5):2994-3004, May 2014. Keyword(s): SAR Processing, geophysical image processing, geophysical prospecting, groundwater, image registration, image resolution, radar interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth), 3D simulations, InSAR concept, SAR subsurface imaging, archaeological surveys, ground water exploration, image aberrations, image coregistration, image distortion effects, image refocusing, interferometric synthetic aperture radar, iterative approach, oil field exploration, phase front distortion, topography estimation, Focusing, History, Radar imaging, Surfaces, Synthetic aperture radar, Interferometric synthetic aperture radar (InSAR), radar imaging, subsurface imaging, terrain mapping. [bibtex-key = Elsherbini2014] [bibtex-entry]


  364. Heresh Fattahi and Falk Amelung. InSAR uncertainty due to orbital errors. Geophysical Journal International, 199(1):549-560, August 2014. Keyword(s): SAR Processing, Interferometry, Baseline Errors, orbital errors, SAR Interferometry, Spaceborne SAR. [bibtex-key = fattahiAmelungGeophysicalJournalInt2014InSARUncertaintyDueToOrbitalErrors] [bibtex-entry]


  365. Gianfranco Fornaro, Fabrizio Lombardini, Antonio Pauciullo, Diego Reale, and Federico Viviani. Tomographic Processing of Interferometric SAR Data: Developments, applications, and future research perspectives. IEEE Signal Processing Magazine, 31(4):41-50, July 2014. Keyword(s): SAR Processing, SAR Tomography, PSI, Persistent Scatterer Interferometry, Deformation Monitoring, Deformation, radar interferometry, synthetic aperture radar, tomography, 3D reconstruction, classical multipass interferometric processing, environmental risk monitoring, interferometric SAR data tomographic processing, long-term deformation monitoring capability, natural hazard monitoring, synthetic aperture radar tomography techniques, three-dimensional reconstruction, Backscatter, Interferometry, Monitoring, Scattering, Synthetic aperture radar, Tomography.
    Abstract: Synthetic aperture radar (SAR) data processed with interferometric techniques are widely used today for environmental risk monitoring and security. SAR tomography techniques are a recent advance that provide improved three-dimensional (3-D) reconstruction and long-term deformation monitoring capabilities. This article is meant to discuss the main developments achieved in the last few years in the SAR tomography framework, with particular reference to both urban and forest scenarios. An insight on classical multipass interferometric processing is also included to summarize the importance of the technology for natural hazards monitoring and to provide the basis for the description of SAR tomography.
    [bibtex-key = fornaroLombardiniPauciulloRealeVivianiIEEESigProcMag2014TomoSAR] [bibtex-entry]


  366. Gianfranco Fornaro, Antonio Pauciullo, Diego Reale, and Simona Verde. Multilook SAR Tomography for 3-D Reconstruction and Monitoring of Single Structures Applied to COSMO-SKYMED Data. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7(7):2776-2785, July 2014. Keyword(s): SAR Processing, SAR Tomography, Component Extraction And selection SAR, CEASAR, Spaceborne SAR, multilook SAR tomography, X-Band, Urban, Persistent Scatterer Interferometry, PSI, time series, geophysical image processing, image reconstruction, image resolution, optical tomography, radar imaging, radar interferometry, radar resolution, synthetic aperture radar, 3D reconstruction, COSMO-SKYMED dataset, Italy, Naples, Rome, building monitoring, classical DInSAR analysis, fine resolution analysis, infrastructure monitoring, multilook SAR tomography, scatterer detection, single structure monitoring, Interferometry, Monitoring, Scattering, Spatial resolution, Synthetic aperture radar, Tomography, 3D, 4D, and multi-D SAR imaging, COSMO-SKYMED (CSK), SAR tomography, (CAESAR), differential SAR tomography, principal component analysis (PCA), synthetic aperture radar (SAR).
    Abstract: With reference to the application to the imaging and monitoring of infrastructures and buildings in urban areas, SAR tomography has been mainly developed and tested at full resolution. In this work, we investigate the possibility related to the use of a multilook approach for fine resolution analysis of ground structures that combines SAR tomography and a method, CAESAR, recently proposed for classical DInSAR analysis at coarse resolution over large areas. Shown results, achieved by processing two 3 m spatial resolution (stripmap mode) COSMO-SKYMED datasets relative to the urban areas of Naples and Rome (Italy), clearly indicate that the proposed multilook-based method allows achieving an impressive density of detected scatterers over buildings and infrastructures, much higher than those achievable with standard full-resolution methods.
    [bibtex-key = fornaroPauciulloRealeVerdeJSTARS2014TomoCAESAR] [bibtex-entry]


  367. Y. Gao, W. Yu, Y. Liu, R. Wang, and C. Shi. Sharpness-Based Autofocusing for Stripmap SAR Using an Adaptive-Order Polynomial Model. IEEE Geoscience and Remote Sensing Letters, 11(6):1086-1090, June 2014. Keyword(s): conjugate gradient methods, maximum likelihood estimation, optical focusing, polynomials, radar imaging, search problems, synthetic aperture radar, closed-form expression, airborne SAR data experiment, modified adaptive-order searching strategy, conjugate gradient algorithm, polynomial expansion coefficient, sharpness function gradient, maximum-posterior estimation, SAR, stripmap-mode synthetic aperture radar image data, adaptive-order polynomial model, image sharpness-based autofocusing technique, Synthetic aperture radar, Polynomials, Accuracy, Estimation, Azimuth, Adaptation models, Optimization, Autofocus, conjugate gradient method (CGM), sharpness, synthetic aperture radar (SAR), Autofocus, conjugate gradient method (CGM), sharpness, synthetic aperture radar (SAR).
    Abstract: A novel autofocusing technique is developed for image from stripmap-mode synthetic aperture radar (SAR) data. The approach is based on maximizing the image sharpness function that induces the solution to maximum-posterior estimation. In this letter, closed-form expressions are derived for the gradients of the sharpness function with respect to the coefficients of the polynomial expansion, which makes the use of conjugate gradient algorithm available. Additionally, we also design a modified adaptive-order searching strategy, and it helps to remarkably reduce the computational load while maintaining the accuracy. Real airborne SAR data experiments and comparisons demonstrate the validity and effectiveness of the proposed algorithm.
    [bibtex-key = gaoYuLiuWangShiGRSL2014SharpnessBasedAutofocusingStripmapSARUsingAdaptiveOrderPolynomialModel] [bibtex-entry]


  368. Paolo Ghelfi, Francesco Laghezza, Filippo Scotti, Giovanni Serafino, Amerigo Capria, Sergio Pinna, Daniel Onori, Claudio Porzi, Mirco Scaffardi, Antonio Malacarne, Valeria Vercesi, Emma Lazzeri, Fabrizio Berizzi, and Antonella Bogoni. A fully photonics-based coherent radar system. Nature, 507(7492):341-345, 2014. Keyword(s): Radar, Photonics, Photonics-based radar, radar design.
    Abstract: The next generation of radar systems must be photonic to obtain frequency flexibility and improved performance; here both generation and detection of radio signals are demonstrated in a successful field trial of a photonic-based radar system using aeroplanes.
    [bibtex-key = ghelfiEtAlNature2014PhotonicsBasedCoherentRadar] [bibtex-entry]


  369. Scott Hensley, S. Oveisgharan, S. Saatchi, M. Simard, R. Ahmed, and Z. Haddad. An Error Model for Biomass Estimates Derived From Polarimetric Radar Backscatter. IEEE_J_GRS, 52(7):4065-4082, July 2014. Keyword(s): air pollution, carbon capture and storage, remote sensing by radar, vegetation, above ground biomass, basic imaging physics, biomass estimation accuracy, carbon flux measurement, carbon storage, disturbance quantification, ecosystem processes, error model, forest carbon inventories, forested areas, ground carbon, instrument parameter, mission parameter, notional Earth observing mission, polarimetric radar backscatter, radar polarimetric measurements, regrowth quantification, remote sensing measurements, Backscatter, Biological system modeling, Biomass, Radar measurements, Radar polarimetry, Signal to noise ratio, Backscatter error model, forest biomass, polarimetry, synthetic aperture radar (SAR). [bibtex-key = hensleyOveisgharanSaatchiSimardAhmedHaddadTGRS2014ErrorModelforBiomassFromPolarimetricBackscatter] [bibtex-entry]


  370. R. Iglesias, X. Fabregas, A. Aguasca, J. J. Mallorqui, C. Lopez-Martinez, J. A. Gili, and J. Corominas. Atmospheric Phase Screen Compensation in Ground-Based SAR With a Multiple-Regression Model Over Mountainous Regions. IEEE_J_GRS, 52(5):2436-2449, May 2014. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, geophysical techniques, radar polarimetry, synthetic aperture radar, AD 2010 10 to 2011 10, Andorran Pyrenees, El Forn de Canillo, GB-SAR sensor, Universitat Politecnica de Catalunya, X-band, atmospheric fluctuations, atmospheric phase screen compensation process, differential-SAR-interferometry applications, ground-based SAR data, mountainous environment, mountainous regions, multiple-regression model, multitemporal GB-SAR measurements, one-year measurement campaign, polarimetric SAR data, steep topography effect, strong rain episodes, zero-baseline fully polarimetric data sets, Atmospheric phase screen (APS) compensation, differential synthetic aperture radar (SAR) interferometry (DInSAR), ground-based SAR (GB-SAR), polarimetry, steep topography. [bibtex-key = Iglesias2014] [bibtex-entry]


  371. R. Iglesias, D. Monells, X. Fabregas, J. J. Mallorqui, A. Aguasca, and C. Lopez-Martinez. Phase Quality Optimization in Polarimetric Differential SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 52(5):2875-2888, May 2014. Keyword(s): geophysical techniques, optimisation, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, ALOS, DInSAR processing, DInSAR techniques, RADARSAT-2, TerraSAR-X, advanced land observing satellite, amplitude dispersion case, amplitude dispersion maps, classical single-polarimetric approach, coherence case, differential SAR interferometry, differential synthetic aperture radar interferometry, final DInSAR result density, final DInSAR result reliability, fully polarimetric data unavailability, ground-based SAR fully polarimetric data, interferometric technique merging, orbital SAR fully polarimetric data, phase quality optimization, pixel candidate number threefold, pixel phase quality, pixel selection process, polarimetric capabilities, polarimetric differential SAR interferometry, polarimetric optimization techniques, polarimetric technique merging, polarimetrically optimized coherence, satellite launch, single-polarimetric case, Amplitude dispersion optimization, coherence optimization, differential synthetic aperture radar (SAR) interferometry (DInSAR), polarimetric DInSAR (PolDInSAR), polarimetry.
    Abstract: In this paper, a study of polarimetric optimization techniques in the frame of differential synthetic aperture radar (SAR) interferometry (DInSAR) is considered. Historically, DInSAR techniques have been limited to the single-polarimetric case, mainly due to the unavailability of fully polarimetric data. Lately, the launch of satellites with polarimetric capabilities, such as the Advanced Land Observing Satellite (ALOS), RADARSAT-2, or TerraSAR-X, allowed merging polarimetric and interferometric techniques to improve the pixels' phase quality and, thus, the density and the reliability of the final DInSAR results. The relationship between the polarimetrically optimized coherence or amplitude dispersion maps and the final DInSAR results is carefully analyzed, using both orbital and ground-based SAR fully polarimetric data. DInSAR processing using polarimetric optimization techniques in the pixel selection process is compared with the classical single-polarimetric approach, achieving up to a threefold increase of the number of pixel candidates in the coherence case and up to a factor of seven in the amplitude dispersion case.
    [bibtex-key = iglesiasMonellsFabregasMallorquiAguascaLopezMartinezTGRS2014PhaseQualityOptInPolDInSAR] [bibtex-entry]


  372. Uday K. Khankhoje, Mariko S. Burgin, and Mahta Moghaddam. On the Accuracy of Averaging Radar Backscattering Coefficients for Bare Soils Using the Finite-Element Method. IEEE Geoscience and Remote Sensing Letters, 11(8):1345-1349, August 2014. Keyword(s): finite element analysis, geophysical techniques, remote sensing by radar, soil, surface roughness, 2-D finite-element method, Gaussian correlated surfaces, Gaussian correlated surfaces possessing behavior, Gaussian type, assumption validity, average coefficients, averaging assumption, averaging radar backscattering coefficient accuracy, backscatter averaging assumption breaks, backscatter averaging assumption validity conditions, bare rough surface assumption validity, bare soils, constitutive homogeneous pixels, ensemble averaged true coefficient computation, exponential type, exponentially correlated surfaces, heterogeneity impact quantification, heterogeneity source, heterogeneous moisture pixels, heterogeneous pixels, heterogeneous roughness pixels, heterogeneous soil moisture, heterogeneous soil roughness, high-contrast pixels, higher cross-pixel coherent interactions, longest correlation lengths, soil moisture, soil surface roughness, surface correlation type, surface variety computed averages, Backscatter, Correlation, Radar, Rough surfaces, Soil moisture, Surface roughness, Electromagnetic scattering by rough surfaces, finite-element methods (FEMs).
    Abstract: Radar backscattering coefficients for heterogeneous pixels are traditionally assumed to be the average of the coefficients for the constitutive homogeneous pixels. We investigate the validity of this assumption for bare rough surfaces by using the 2-D finite-element method to compute the ensemble averaged true coefficients for heterogeneous pixels and compare these values with the computed averages for a variety of surfaces. We quantify the impact of heterogeneity in both soil moisture and surface roughness on the averaging assumption. We find that the validity of the assumption rests crucially on the surface correlation type (exponential or Gaussian) and length. In particular, when considering pixels with either heterogeneous soil moisture or roughness, we find that for high-contrast pixels, the backscatter averaging assumption breaks down by as much as 11 dB for Gaussian correlated surfaces for the longest correlation lengths considered (regardless of the source of heterogeneity), whereas for exponentially correlated surfaces, it breaks down by 6 dB for pixels with heterogeneous roughness and 2 dB for pixels with heterogeneous moisture. We attribute this behavior to Gaussian correlated surfaces possessing higher cross-pixel coherent interactions. Furthermore, conditions of validity for the backscatter averaging assumption are identified.
    [bibtex-key = khankhojeBurginMoghaddamGRSL2014RadarBackScatterFEMMethod] [bibtex-entry]


  373. S. B. Kim, Mahta Moghaddam, L. Tsang, Mariko S. Burgin, X. Xu, and E. G. Njoku. Models of L-Band Radar Backscattering Coefficients Over Global Terrain for Soil Moisture Retrieval. IEEE Transactions on Geoscience and Remote Sensing, 52(2):1381-1396, February 2014. Keyword(s): Maxwell equations, permittivity, remote sensing by radar, soil, time series, vegetation mapping, L-band radar backscattering coefficient models, Maxwell equations, RMS height, VWC, accurate soil moisture inversion, airborne data, airborne observation, bare surface, co-pol RMS errors, corn crop, datacube errors, dielectric soil constant, distorted Born approximation framework, double-bounce reflectivity, double-bounce volume-surface interaction, empirical formulae, empirical parameters, fast soil moisture inversion, field-based radar data, global land surface, global terrain, grass fields, in situ observation, independent spaceborne phased array type L-band synthetic aperture radars, input parameters, international geosphere-biosphere programme scheme, land surface class simulation, lookup tables, major crops, mean difference range, numerical solutions, physical model outputs, real-time soil moisture inversion, rice crop, shrub, single scatterer, soil moisture active passive mission data, soil moisture retrieval, soil surface root mean square, sophisticated forward model direct inversion, soybean crop, spaceborne Aquarius scatterometer data, surface scattering, theoretical models.
    Abstract: Physical models for radar backscattering coefficients are developed for the global land surface at L-band (1.26 GHz) and 40 deg incidence angle to apply to the soil moisture retrieval from the upcoming soil moisture active passive mission data. The simulation of land surface classes includes 12 vegetation types defined by the International Geosphere-Biosphere Programme scheme, and four major crops (wheat, corn, rice, and soybean). Backscattering coefficients for four polarizations (HH/VV/HV/VH) are produced. In the physical models, three terms are considered within the framework of distorted Born approximation: surface scattering, double-bounce volume-surface interaction, and volume scattering. Numerical solutions of Maxwell equations as well as theoretical models are used for surface scattering, double-bounce reflectivity, and volume scattering of a single scatterer. To facilitate fast, real-time, and accurate inversion of soil moisture, the outputs of physical model are provided as lookup tables (with three axes; therefore called datacube). The three axes are the real part of the dielectric constant of soil, soil surface root mean square (RMS) height, and vegetation water content (VWC), each of, which covers the wide range of natural conditions. Datacubes for most of the classes are simulated using input parameters from in situ and airborne observations. This simulation results are found accurate to the co-pol RMS errors of to 3.4 dB (six woody vegetation types), 1.8 dB (grass), and 2.9 dB (corn) when compared with airborne data. Validated with independent spaceborne phased array type L-band synthetic aperture radars and field-based radar data, the datacube errors for the co-pols are within 3.4 dB (woody savanna and shrub) and 1.5 dB (bare surface). Assessed with spaceborne Aquarius scatterometer data, the mean differences range from ~ 1.5 to 2 dB. The datacubes allow direct inversion of sophisticated forward models without empirical par- meters or formulae. This capability is evaluated using the time-series inversion algorithm over grass fields.
    [bibtex-key = kimMoghaddamTsangBurginXuNjokuTGRS2014ModelLBandBackscatteringOverGlobalTerrainForSoilMoisture] [bibtex-entry]


  374. Franziska Koch, Monika Prasch, Lino Schmid, J�rg Schweizer, and Wolfram Mauser. Measuring Snow Liquid Water Content with Low-Cost GPS Receivers. Sensors, 14(11):20975-20999, 2014. Keyword(s): GNSS, snow water equivalent, SWE, Liquid Water Content, LWC.
    Abstract: The amount of liquid water in snow characterizes the wetness of a snowpack. Its temporal evolution plays an important role for wet-snow avalanche prediction, as well as the onset of meltwater release and water availability estimations within a river basin. However, it is still a challenge and a not yet satisfyingly solved issue to measure the liquid water content (LWC) in snow with conventional in situ and remote sensing techniques. We propose a new approach based on the attenuation of microwave radiation in the L-band emitted by the satellites of the Global Positioning System (GPS). For this purpose, we performed a continuous low-cost GPS measurement experiment at the Weissfluhjoch test site in Switzerland, during the snow melt period in 2013. As a measure of signal strength, we analyzed the carrier-to-noise power density ratio (C/N0) and developed a procedure to normalize these data. The bulk volumetric LWC was determined based on assumptions for attenuation, reflection and refraction of radiation in wet snow. The onset of melt, as well as daily melt-freeze cycles were clearly detected. The temporal evolution of the LWC was closely related to the meteorological and snow-hydrological data. Due to its non-destructive setup, its cost-efficiency and global availability, this approach has the potential to be implemented in distributed sensor networks for avalanche prediction or basin-wide melt onset measurements.
    [bibtex-key = kochPraschSchmidSchweizerMauserSENSORS2014MeasurinSnowLWCwithLowCostGPSReceivers] [bibtex-entry]


  375. K. Landmark, A. H. Schistad Solberg, A. Austeng, and Roy E. Hansen. Bayesian Seabed Classification Using Angle-Dependent Backscatter Data From Multibeam Echo Sounders. IEEE Journal of Oceanic Engineering, 39(4):724-739, October 2014. Keyword(s): Synthetic Aperture Sonar, SAS, Bayes methods, Gaussian processes, acoustic wave scattering, approximation theory, backscatter, oceanographic techniques, pattern classification, piecewise constant techniques, sonar, statistical analysis, Bayesian seabed classification, Gaussian statistical model, North Sea data set, acoustical seabed classification, across-track spatial resolution, angle-dependent backscatter data, intrinsic scattering strength statistics, mapping seabed sediment, multibeam echo sounder, multibeam sonar data processing, piecewise constant function, piecewise function approximation, seabed scattering strength, spatial averaging, standard Bayesian theory, Bayes methods, Classification algorithms, Remote sensing, Sea floor, Sediments, Sonar, Underwater acoustics, Bayesian methods, classification algorithms, remote sensing, seafloor, sediments, sonar.
    Abstract: Acoustical seabed classification is a technology for mapping seabed sediments. Processed multibeam sonar data yield the variation of the seabed scattering strength with incidence angle, and this paper examines the effect of this on classification. A simple Gaussian statistical model is developed for the observed scattering strength, whereby an observation is represented by a piecewise constant function of incidence angle. Provided some data for which the sediment types are known (training data), the statistics for each type can be robustly estimated. Subsequently, a standard Bayesian theory is applied to classify new observations. The model was used to compute limits on classification accuracy in terms of the intrinsic scattering strength statistics of the seabed, and to predict whether a logarithmic or linear scale for the data is preferable. Systematic experiments on a North Sea data set with four sediment classes tested how the classification accuracy depends on the piecewise function approximation, incidence angle range, amount of training data, and spatial averaging (combining consecutive pings into one observation). The classifier based on Gaussian statistics performed at least as well as sophisticated algorithms with no assumptions about the data statistics. The best accuracy (95%) was attained for logarithmic data. The amount of training data needed to achieve this was about 500 pings per class; spatial averaging could be limited to 10-20 pings. Comparable across-track spatial resolution was possible by dividing the full swath into separate independent sectors, but only at reduced accuracy (87% or less). However, comparable accuracy may be possible by taking into account the spatial relationships of observations.
    [bibtex-key = landmarkSolbergAustengHansenJOE2014BayesianClassificationUsingMultibeamSounders] [bibtex-entry]


  376. H. Lee, J. H. Lee, K. E. Kim, N. H. Sung, and S. J. Cho. Development of a Truck-Mounted Arc-Scanning Synthetic Aperture Radar. IEEE Transactions on Geoscience and Remote Sensing, 52(5):2773-2779, May 2014. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, Doppler radar, antennas, geophysical equipment, radar imaging, remote sensing by radar, synthetic aperture radar, ArcSAR formulation, ArcSAR scan mode image, ArcSAR system development, SAR focusing algorithms, antenna view angle fixing, arc-scanning real aperture radar, azimuth resolution, conventional ground-based SAR system linear scanning, exemplary X-band ArcSAR spot mode image, extendable boom, ground-based arc-scanning synthetic aperture radar development, high-resolution image, horizontal circular antenna motion, image area, image processing accuracy, image processing efficiency, imaging mode, range Doppler algorithm, reduced resolution, scan mode, time domain algorithm, truck-mounted arc-scanning synthetic aperture radar development, wider image coverage, Arc-scanning synthetic aperture radar (ArcSAR), SAR, range Doppler algorithm, scan mode, spot mode, time domain algorithm.
    Abstract: This paper presents the development of a ground-based arc-scanning synthetic aperture radar (ArcSAR) system mounted on a truck. ArcSAR formulates synthetic aperture by a horizontal circular motion of antennas attached at the end of an extendable boom. The ArcSAR system is designed to operate in two different imaging modes: the spot mode and the scan mode . The spot mode obtains a high-resolution image by fixing the view angle of antennas toward a target. The scan mode obtains wider image coverage with a reduced resolution by fixing the antennas relative to the boom. Different SAR focusing algorithms were implemented for the accuracy and efficiency of image processing: the time domain algorithm for the spot mode and the range Doppler algorithm for the scan mode. An exemplary X-band ArcSAR spot mode image, obtained with a 180 deg scanning of 4-m boom, has an azimuth resolution of 0.07 deg , which is equivalent to the 12.6-m linear scanning of a conventional ground-based SAR system. An ArcSAR scan mode image was successfully obtained as well, covering a 350 deg image area at an azimuth resolution of 1.07 deg, which is 11 times better than that of arc-scanning real aperture radar that would have 11.84 deg azimuth resolution.
    [bibtex-key = leeLeeKimSungChoTGARS2014ArcSARGBSAR] [bibtex-entry]


  377. Silvan Leinss, Giuseppe Parrella, and Irena Hajnsek. Snow height determination by polarimetric phase differences in X-band SAR data. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7(9):3794-3810, Sept 2014. Keyword(s): hydrological techniques, remote sensing by radar, snow, synthetic aperture radar, AD 2012 01, AD 2012 12 to 2013 04, CPD temporal evolution, Finland, HH polarization, Sodankylae city, TanDEM-X, TanDEM-X acquisitions, TerraSAR-X acquisitions, VV polarization, X-band SAR acquisitions, X-band SAR data, aligned elliptical particles, computer tomography observations, copolar phase difference, fresh snow depth, polarimetric phase difference, snow height determination, snow microstructure, subsequent recrystallization process, temperature-gradient-driven recrystallization process, weather station data, Backscatter, Scattering, Snow, Soil, Soil measurements, Synthetic aperture radar, Temperature measurement, Birefringence, TanDEM-X, TerraSAR-X, VV-HH phase difference, copolar phase difference, dry snow, fresh snow, polarimetry, snow anisotropy, snow microstructure, synthetic aperture radar.
    Abstract: The copolar phase difference (CPD) between VV and HH polarization of X-band SAR acquisitions shows a significant dependence on the depth of fresh snow. Phase differences of 5-15 deg/10 cm fresh snow were determined at a frequency of 9.65 GHz by comparing spatial and temporal variations of snow depth (SD) with the CPD. Spatial correlations were derived from snow transect measurements during January 2012 and TanDEM-X acquisitions. Temporal correlations were derived from weather station data and TerraSAR-X acquisitions between December 2012 and April 2013. All measurements were done at a test field near the city Sodankyl{\"a}, Finland. To explain the observed CPD, a model derives birefringent properties from the microstructure of snow, which is described as aligned elliptical particles. The microscopic description is based on computer tomography observations. Different incidence angles were analyzed in consistency with the model. The temporal evolution of the CPD was linked to the temperature-gradient-driven recrystallization process. Sudden increases in the CPD indicate fresh snow. Slow decreases indicate the subsequent recrystallization process. The background signal of wet soil was considered and causes a small negative offset to the CPD. A quantitative determination of the depth of fresh snow is possible, because the specific CPD per meter of snow can be estimated. Spatial resolutions below 100x100 m are achievable with sensors such as TerraSAR-X or TanDEM-X. This paper presents a theoretical relationship between the microstructure of snow and the CPD and relates the CPD theoretically and empirically to the depth of fresh snow.
    [bibtex-key = leinssParrellaHajnsekJSTARS2014SnowHeightFromPolPhaseDifference] [bibtex-entry]


  378. Y. Luo, H. Song, R. Wang, Y. Deng, F. Zhao, and Z. Xu. Arc FMCW SAR and Applications in Ground Monitoring. IEEE Transactions on Geoscience and Remote Sensing, 52(9):5989-5998, Sept 2014. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, Apertures, Azimuth, Doppler effect, Focusing, Rails, Synthetic aperture radar, Arc frequency-modulated continuous wave (FMCW) SAR, change detection, interferometric SAR (InSAR), synthetic aperture radar (SAR).
    Abstract: As a novel mode of ground-based synthetic aperture radar (GB-SAR), Arc frequency-modulated continuous wave (FMCW) SAR is rarely discussed in the literature up to now. Compared with the conventional rail GB-SAR system, the synthetic aperture is formed by the rotation of antennas, which can scan a wide azimuth extent. Due to its convenience and potential in SAR applications, the Institute of Electronics, Chinese Academy Sciences, carried out a series of Arc FMCW SAR experiments in March 2013, and a lot of results were obtained. In this paper, we discuss the signal processing of Arc FMCW SAR and its first results in change detection and interferometric SAR. The components of the Arc FMCW SAR system are first described, and then, we focus on its signal processing, where the signal model and its properties are investigated; two focusing algorithms are developed for different purposes. The effectiveness of the algorithms is validated by both simulation and real data experiments. We also exhibit the applications of Arc FMCW SAR in landslide detection and digital-elevation-model extraction for the first time. Results show its huge potential in ground-based applications.
    [bibtex-key = luoSongWangDengZhaoXuTGARS2014ARCSARGBSAR] [bibtex-entry]


  379. Jun Maeda and Kosuke Heki. Two-dimensional observations of midlatitude sporadic E irregularities with a dense GPS array in Japan. Radio Science, 49(1):28-35, 2014. Keyword(s): sporadic E, ionosphere, GPS, Global Positioning System, GNSS, Global Navigation Satellite System, GEONET, TEC, Total Electron Content.
    Abstract: We observed two-dimensional structure and time evolution of ionospheric irregularities caused by midlatitude sporadic E (Es) over Japan as positive anomalies of total electron content (TEC) by analyzing the data from the nationwide Global Positioning System (GPS) array. In this paper we report a case study of strong Es observed in the local evening of 21 May 2010, over Tokyo, Japan. In the slant TEC time series, Es showed a characteristic pulse-like enhancement of 1.5 TEC units lasting for ~10 min. We plotted these positive TEC anomalies on the subionospheric points of station-satellite pairs to study the horizontal structure of the Es irregularity. We confirmed that the irregularity existed at the height of ~106 km by comparing the data of multiple GPS satellites, which is consistent with the local ionosonde observations. The horizontal shapes of the Es irregularity showed frontal structures elongated in E-W, spanning ~150 km in length and ~30 km in width, composed of small patches. The frontal structure appears to consist of at least two parts propagating in different directions: one moved eastward by ~60 m s-1, and the other moved southwestward by ~80 m s-1. Similar TEC signatures of Es were detected by other GPS satellites, except one satellite that had line of sight in the N-S direction which dips by 40-50deg toward north, which indicates the direction of plasma transportation responsible for the Es formation. We also present a few additional observation results of strong Es irregularities.
    [bibtex-key = maedaHekiRadioScience2014IonosphereGNSSSporadicE] [bibtex-entry]


  380. Christophe Magnard, Max Frioud, David Small, Torsten Brehm, Helmut Essen, and Erich Meier. Processing of MEMPHIS Ka-Band Multibaseline Interferometric SAR Data: From Raw Data to Digital Surface Models. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7(7):2927-2941, July 2014. Keyword(s): SAR Processing, SAR Interferometry, InSAR, Multibaseline Interferometry, Ka-band, Airborne SAR, Single-pass Multibaseline Interferometry, Focusing, Azimuth Focusing, Motion Compensation, data acquisition, geophysical image processing, image reconstruction, image resolution, millimetre wave radar, radar imaging, radar interferometry, radar polarimetry, radar resolution, synthetic aperture radar, DSM, MEMPHIS Ka-band multibaseline interferometric SAR data processing, SAR image resolution, azimuth focusing, cross-track multibaseline interferometric data acquisition, digital surface model, extended omega-K algorithm, interferogram generation, millimeter-wave synthetic aperture radar system, multibaseline antenna setup, observed depression angle-dependent interferometric phase error, phase unwrapping, phase-to-height conversion, reference ALS model, stepped-frequency SAR data reconstruction, Accuracy, Antennas, Azimuth, Chirp, Focusing, Motion compensation, Synthetic aperture radar, Digital surface model (DSM), Ka-band, MEMPHIS, interferometry, millimeter wave radar, multibaseline, stepped-frequency, synthetic aperture radar (SAR).
    Abstract: MEMPHIS is an experimental millimeter-wave synthetic aperture radar (SAR) system that acquires cross-track multibaseline interferometric data at high resolution in a single pass, using four receive horns. In this paper, we present the SAR system and navigation data, and propose a processing chain from the raw data input to a digital surface model (DSM) output. This processing chain includes full bandwidth reconstruction of the steppedfrequency SAR data, azimuth focusing with an Extended Omega-K algorithm, generation ofinterferograms for each available baseline, phase unwrapping using the multibaseline data, and phaseto-height conversion. The hardware and processing chain were validated through the analysis of experimental Ka-band data. The SAR image resolution was measured with point targets and found to be ~2% and 15% coarser than the theoretical value in range and azimuth, respectively. The geolocation accuracy was typically better than 0.1 m in range and 0.2 m in azimuth. Observed depression angledependent interferometric phase errors were successfully removed using a correction function derived from the InSAR data. Investigation of the interferometric phase noise showed the utility of a multibaseline antenna setup; the number of looks and filter size used for the DSM generation were also derived from this analysis. The results showed that in grassland areas, the height difference between the ~2 m-resolution InSAR DSMs and the reference ALS models was 0 +/- 0.25 m.
    [bibtex-key = magnardFrioudSmallBrehmEssenMeierJSTARS2014MemphisKaBandMBInSAR] [bibtex-entry]


  381. Pooja Mahapatra, Sami Samie Esfahany, Hans van der Marel, and Ramon F. Hanssen. On the Use of Transponders as Coherent Radar Targets for SAR Interferometry. IEEE Trans. Geosci. Remote Sens., 52(3):1869-1878, March 2014. Keyword(s): SAR Processing, Global Positioning System, geomorphology, geophysical equipment, radar interferometry, reliability, remote sensing by radar, synthetic aperture radar, terrain mapping, transponders, vegetation, ERS-2, Envisat, SAR interferometry, coherent radar targets, controlled environment, corner reflectors, double-difference transponder phase measurements, empirical precision range, field experiments, geometric variations, global positioning system, ground deformation monitoring, landslide monitoring, maintenance-related degradation, operational performance, passive devices, radar line of sight, radar transponders, reliability, transponder-InSAR observations, validation measurements, validation tests, vegetated nonurbanized areas, Corner reflector, geodesy, interferometry, measurement errors, persistent scatterer, phase measurement, quality control, synthetic aperture radar, transponder.
    Abstract: Monitoring ground deformation using SAR interferometry (InSAR) sometimes requires the introduction of coherent radar targets, especially in vegetated nonurbanized areas. Passive devices such as corner reflectors were used in such areas in the past. However, they suffer from drawbacks related to their large size and weight, conspicuousness, and loss of reliability because of geometric variations as well as material and maintenance-related degradation over several years of deployment. The viability of smaller, lighter, and less conspicuous radar transponders as an alternative is demonstrated via two field experiments: validation tests in a controlled environment, and operational performance for monitoring landslides in a heavily vegetated area. Comparison of 113 transponder-InSAR observations with independent validation measurements such as leveling and the global positioning system yields an empirical precision range of 1.8-4.6 mm, after outlier removal, for double-difference (spatial and temporal) transponder phase measurements in the radar line of sight, for Envisat and ERS-2.
    [bibtex-key = mahapatraSamieiEsfahanyVanDerMarelHanssenTGARS2014TranspondersAsCoherentTargetsForInSAR] [bibtex-entry]


  382. Oriol Monserrat, Michele Crosetto, and Guido Luzi. A review of ground-based SAR interferometry for deformation measurement. ISPRS Journal of Photogrammetry and Remote Sensing, 93:40-48, 2014. Keyword(s): SAR Processing, SAR, Interferometry, Terrestrial, Deformation, Monitoring, Review Paper, GBSAR, ground-based SAR, Differential Interferometry, DInSAR, Subsidence, Mointoring, Persistent Scatterer Interferometry, PSI.
    Abstract: Abstract This paper provides a review of ground-based SAR (GBSAR) interferometry for deformation measurement. In the first part of the paper the fundamentals of this technique are provided. Then the main data processing and analysis stages needed to estimate deformations starting from the \{GBSAR\} observations are described. This section introduces the two types of GBSAR acquisition modes, i.e., continuous and discontinuous GBSAR, and reviews the different GBSAR processing and analysis methods published in the literature. This is followed by a discussion of the specific technical aspects of GBSAR deformation measurement. A section then summarizes the pros and cons of GBSAR for deformation monitoring. The last part of the paper includes two reviews: one concerning the GBSAR systems described in the literature, including non-strictly SAR systems and a second one addresses the main GBSAR applications.
    [bibtex-key = monserratCrosettoLuziISPRS2014BGSARReview] [bibtex-entry]


  383. Keith Morrison and John Bennett. Tomographic Profiling - A Technique for Multi-Incidence-Angle Retrieval of the Vertical SAR Backscattering Profiles of Biogeophysical Targets. IEEE Trans. Geosci. Remote Sens., 52(2):1350-1355, February 2014. Keyword(s): SAR Processing, SAR tomography, tomography, snow, X-band, Ku-band, geophysical image processing, image retrieval, radar imaging, remote sensing by radar, synthetic aperture radar, tomography, across-track direction, airborne application, along-track direction, biogeophysical targets, forest canopies, ground-based SAR system, ice, multiincidence-angle retrieval, satellite application, snow, subaperture elements, synthetic aperture radar imaging, tomographic profiling, vegetation, vertical SAR backscattering profiles, Array signal processing, phased arrays, radar, radar imaging, synthetic aperture radar.
    Abstract: Tomographic profiling (TP) is a new imaging technique designed to provide vertical backscatter profiles through biophysical and geophysical target volumes, such as snow, ice, vegetation, and forest canopies. Data is collected as for normal synthetic aperture radar (SAR) imaging, but with the antennas aligned along the scan or along-track direction. The real antenna provides a wide beam in the along-track direction, which is sharpened by the addition of elemental measurements across a subaperture using a SAR-like processing scheme. A novelty of the scheme is the ability to produce an image transect in which the incidence angle is constant at every point. This is accomplished by incrementally sliding the subaperture across the full aperture, and utilizing the appropriate subaperture to provide the necessary viewing geometry at each pixel. This is in contrast to the SAR case, in which the angle of incidence varies across a scene. By suitable phasing between the subaperture elements, the synthesized beam can be steered in angle within the wide angular extent of the real beam, allowing post-measurement retrieval of the backscattering properties of the scene over a continuous range of incidence angles from a single scan. In the across-track direction, a narrow real beam is required to maintain good vertical resolution and limit the size of the horizontal footprint. Example TP experimental fieldwork results are provided for a 42-cm-deep snowpack, collected with a ground-based SAR system. Although the scheme was developed for ground-based applications, its application to the airborne and satellite cases is also discussed.
    [bibtex-key = morrisonBennettTGRS2014TomoProfiling] [bibtex-entry]


  384. Mahdi Nasirian and Mohammad H. Bastani. A Novel Model for Three-Dimensional Imaging Using Interferometric ISAR in Any Curved Target Flight Path. IEEE Trans. Geosci. Remote Sens., 52(6):3236-3245, June 2014. Keyword(s): radar antennas, radar imaging, radar interferometry, radar receivers, radar tracking, radio transceivers, synthetic aperture radar, target tracking, 3D imaging, InISAR, bistatic system, curved target flight path, curvy motion, flight path form, flying object, highly nonlinear motion, interferometric ISAR, inverse synthetic aperture radar, linear trajectory, main transceiver antenna, monopulse system, nonlinear flight path, second receiver antenna, target motion, target scattering point 3D positioning, Apertures, Radar imaging, Scattering, Synthetic aperture radar, Trajectory, Vectors, Data processing, image reconstruction, interferometry, modeling, synthetic aperture radar (SAR).
    Abstract: Using a second receiver antenna close to the main transceiver antenna of inverse synthetic aperture radar (ISAR), it is possible to find 3-D positions of target scattering points. Such system is called bistatic, monopulse, or interferometric ISAR (InISAR). In the conventional model of ISAR, the unknown flying object should have a linear trajectory, and only small deviations from this trajectory can be compensated. Target motions which are highly nonlinear or curvy cannot be used in the conventional model. In this paper, we propose a new model for InISAR to process all collected data from the target, regardless of the form of the flight path. More accuracy is achieved for 3-D positioning of the target scattering points by this model because all parts of the flight path contribute in it. We will show the effectiveness of the proposed model in a nonlinear flight path by simulation.
    [bibtex-key = nasirianBastani2014] [bibtex-entry]


  385. E Nikolaeva, TR Walter, Manoochehr Shirzaei, and J Zschau. Landslide observation and volume estimation in central Georgia based on L-band InSAR. Natural Hazards and Earth System Sciences, 14(3):675-688, 2014. [bibtex-key = Nikolaeva2014] [bibtex-entry]


  386. Octavio Ponce, Pau Prats-Iraola, Muriel Pinheiro, Marc Rodriguez-Cassola, Rolf Scheiber, Andreas Reigber, and Alberto Moreira. Fully Polarimetric High-Resolution 3-D Imaging With Circular SAR at L-Band. IEEE Trans. Geosci. Remote Sens., 52(6):3074-3090, June 2014. Keyword(s): SAR Processing, Circular SAR, Time-Domain Back-Projection, TDBP, MoComp, Motion Compensation, Bandwidth, Image resolution, Imaging, L-band, Synthetic aperture radar, Trajectory, Autofocus, circular synthetic aperture radar (CSAR), fast factorized back-projection, FFBP, graphics processing unit (GPU), high-resolution SAR, polarimetry, synthetic aperture radar (SAR), tomography, SAR Tomography.
    Abstract: This paper presents the first fully polarimetric highresolution circular synthetic aperture radar (CSAR) images at L-band (1.3 GHz). The circular data were acquired in 2008 by the Experimental SAR (E-SAR) airborne system of the German Aerospace Center (DLR) over the airport of Kaufbeuren, Germany. The obtained images resulting from the coherent integration of the whole circular flight are investigated and discussed in terms of two of the main CSAR properties, namely, the theoretical subwavelength resolution in the horizontal plane (x, y) and the 3-D imaging capabilities. The 3-D imaging capabilities are of special interest due to the penetration of L-band in vegetated areas. These results were compared with images processed by the incoherent addition of the full synthetic aperture. The coherent approach showed a better performance since scatterers are focused at their maximum resolution. Due to the nonlinearity of the tracks and the high-computational burden, an efficient fast factorized back-projection (FFBP) has been developed. Unlike frequencydomain processors, it accommodates azimuthal variances and topography changes. Limits and considerations of the proposed algorithm are described and discussed. To further accelerate this process, the FFBP was also implemented in a graphics processing unit (GPU). Processing performance has been assessed with the direct BP (DBP) as a reference, obtaining speedup factors up to 1800. Residual motion errors have been estimated with a new frequency-based autofocus approach for CSAR configurations based on low signal-to-clutter ratio (SCR) isotropic scatterers. High-resolution images of man-made and distributed scatterers have been analyzed and compared with a stripmap SAR, both concerning anisotropic and isotropic-like scatterers. Results include a single-channel tomogram of a Luneburg lens and a fully polarimetric tomogram of a tree.
    [bibtex-key = poncePratsIraolaPinheiroRodriguezScheiberMoreiraTGRS2014CircularTomoSAR] [bibtex-entry]


  387. O. Ponce, P. Prats-Iraola, R. Scheiber, A. Reigber, A. Moreira, and E. Aguilera. Polarimetric 3-D Reconstruction From Multicircular SAR at P-Band. IEEE_J_GRSL, 11(4):803-807, April 2014. Keyword(s): airborne radar, array signal processing, calibration, compressed sensing, image reconstruction, image resolution, optical focusing, radar imaging, radar polarimetry, radar resolution, singular value decomposition, synthetic aperture radar, 2D image stacking, 3D cone-shaped sidelobe suppression, 3D reconstruction polarimetry, BF, CS, DLR, FFBP, German Aerospace Center F-SAR system, LOS, P-band, SVD, Vordemwald Switzerland, beamforming, circular flight, complex reflectivity solution, compressive sensing, constant phase offset estimation, fast-factorized back projection, line of sight, multicircular synthetic aperture radar, phase calibration method, polarimetric MCSAR airborne experiment, Apertures, Calibration, Focusing, Image resolution, Remote sensing, Synthetic aperture radar, Circular synthetic aperture radar (SAR) (CSAR), compressive sensing (CS), phase calibration, polarimetric SAR (PolSAR), singular value decomposition (SVD), tomography. [bibtex-key = poncePratsScheiberReigberMoreiraAquileraGRSLTGRS2016CircularSARTOMOPBand] [bibtex-entry]


  388. Pau Prats-Iraola, Marc Rodriguez-Cassola, Francseco De Zan, Pau Lopez-Dekker, Rolf Scheiber, and Andreas Reigber. Efficient Evaluation of Fourier-Based SAR Focusing Kernels. IEEE Geoscience and Remote Sensing Letters, 11(9):1489-1493, Sep. 2014. Keyword(s): airborne radar, image resolution, image sensors, numerical analysis, optical focusing, optical transfer function, radar imaging, spaceborne radar, synthetic aperture radar, transient response, geometry, transfer function, airborne SAR sensor, spaceborne SAR sensor, IRF, 2D point target spectrum simulation, numerical analysis, bistatic SAR image resolution, spaceborne SAR image resolution, compact analytic expression, hyperbolic range history, residual phase error, focused impulse response function, SAR image formation, synthetic aperture radar image formation, Fourier-based SAR focusing kernel, Kernel, Synthetic aperture radar, Focusing, Azimuth, Transfer functions, Geometry, Spaceborne radar, SAR processing, SAR simulation, SAR spectrum, spotlight SAR, synthetic aperture radar (SAR), SAR processing, SAR simulation, SAR spectrum, spotlight SAR, synthetic aperture radar (SAR).
    Abstract: This letter addresses the efficient evaluation of Fourier-based kernels for synthetic aperture radar (SAR) image formation. The goal is to evaluate the quality of the focused impulse response function and the residual phase errors of the kernel without having to implement the processor itself nor perform a costly point-target simulation followed by the processing. The proposed methodology is convenient for situations where the assumption of a hyperbolic range history does not hold anymore, and hence, a compact analytic expression of the point target spectrum is not available. Examples where the hyperbolic range history does not apply include very high resolution spaceborne SAR imaging or bistatic SAR imaging. The approach first numerically computes the 2-D spectrum of a point target and then uses the transfer function of the focusing kernel to match it. The spectral support is then computed to adapt the spectrum to the output imaging geometry, so that the impulse response function (IRF) is finally obtained. The proposed approach is valid under the assumption of a large time-bandwidth product, as is usually the case for current air- and spaceborne SAR sensors. The methodology is validated by comparing the matched IRFs with the ones obtained using point-target simulations.
    [bibtex-key = pratsRodriguezDeZanLopezDekkerScheiberReigberGRSL2014EfficientEvaluationOfFourierBasedFocusingKernels] [bibtex-entry]


  389. Pau Prats-Iraola, Rolf Scheiber, Marc Rodriguez-Cassola, J. Mittermayer, S. Wollstadt, F. De Zan, B. Brautigam, M. Schwerdt, Andreas Reigber, and Alberto Moreira. On the Processing of Very High Resolution Spaceborne SAR Data. IEEE Trans. Geosci. Remote Sens., 52(10):6003-6016, October 2014. Keyword(s): SAR Processing, Spaceborne SAR, SAR Focusing, Azimuth focusing, radar antennas, remote sensing by radar, synthetic aperture radar, TerraSAR-X, bandwidth 300 MHz, chirp signal, complete focusing methodology, decimeter range, electronically steered antennas, satellite motion, synthetic aperture radar, troposphere effect, very high resolution spaceborne SAR data, Azimuth, Bandwidth, Geometry, Orbits, Satellites, Synthetic aperture radar, Terrestrial atmosphere, SAR processing, spotlight SAR, stop-and-go approximation, synthetic aperture radar (SAR), troposphere.
    Abstract: This paper addresses several important aspects that need to be considered for the processing of spaceborne synthetic aperture radar (SAR) data with resolutions in the decimeter range. In particular, it will be shown how the motion of the satellite during the transmission/reception of the chirp signal and the effect of the troposphere deteriorate the impulse response function if not properly considered. Further aspects that have been investigated include the curved orbit, the array pattern for electronically steered antennas, and several considerations within the processing itself. For each aspect, a solution is proposed, and the complete focusing methodology is expounded and validated using simulated point targets and staring spotlight data acquired by TerraSAR-X with 16-cm azimuth resolution and 300-MHz range bandwidth.
    [bibtex-key = pratsScheiberRodriguezCassolaMittermayerWollstadtDeZanBrautigamSchwerdtReigberMoreiraTGRS2014] [bibtex-entry]


  390. Craig Stringham and David G. Long. GPU Processing for UAS-Based LFM-CW Stripmap SAR. Photogrammetric Engineering & Remote Sensing, 80(12):1107-1115, 2014. Keyword(s): SAR Processing, Azimuth Focusing, Time-domain back-projection, TDBP, SAR focusing, GPU, GPU-based parallelized TDBP, graphics processing units, motion compensation, parallel processing, radar signal processing, synthetic aperture radar, 3D motion compensation, GPU based backprojection processing, NVIDIA CUDA GPU computing framework, SAR processing scheme, agile SAR platforms, parallelized backprojection processing, time domain backprojection processing, Azimuth, Focusing, Graphics processing units, Remote sensing, Synthetic aperture radar, Time-domain analysis, Azimuth focusing, CARSAR, CUDA, GPU, Parallelization, SAR imaging.
    Abstract: Unmanned air systems (UAS) provide an excellent platform for synthetic aperture radar (SAR), enabling surveillance and research over areas too difficult, dangerous, or costly to reach using manned aircraft. However, the nimble nature of the small UAS makes them more susceptible to external forces, thus requiring significant motion compensation in order for SAR images to focus properly. SAR backprojection has been found to improve the focusing of low-altitude stripmap SAR images compared to frequency domain algorithms. In this paper we describe the development and implementation of SAR backprojection appropriate for UAS based stripmap SAR that utilizes the unique architecture of a GPU in order to produce high-quality imagery in real-time.
    [bibtex-key = stringhamLongPERS2014GPUbasedTDBP] [bibtex-entry]


  391. Jan Torgrimsson, Patrik Dammert, Hans Hellsten, and Lars M. H. Ulander. Factorized Geometrical Autofocus for Synthetic Aperture Radar Processing. IEEE Trans. Geosci. Remote Sens., 52(10):6674-6687, October 2014. Keyword(s): SAR Processing, Fast-Factorized Back-Projection, FFBP, Time-Domain Back-Projection, TDBP, Airborne SAR, CARABAS Autofocus, SAR Autofocus, Geometrical Autofocus, radar imaging, radar tracking, synthetic aperture radar, ultra wideband radar, FGA algorithm, coherent all radio band system II data set, constrained problem, factorized geometrical autofocus algorithm, fast factorized back-projection chain, gain 3 dB, intensity correlation, object function, peak-to-sidelobe ratio measurement, radar imaging, residual space variant range cell migration, ultrawideband synthetic aperture radar processing, Apertures, Electronics packaging, Geometry, Radar tracking, Synthetic aperture radar, Transforms, Autofocus, back-projection, phase gradient algorithm (PGA), synthetic aperture radar (SAR).
    Abstract: This paper describes a factorized geometrical autofocus (FGA) algorithm, specifically suitable for ultrawideband synthetic aperture radar. The strategy is integrated in a fast factorized back-projection chain and relies on varying track parameters step by step to obtain a sharp image; focus measures are provided by an object function (intensity correlation). The FGA algorithm has been successfully applied on synthetic and real (Coherent All RAdio BAnd System II) data sets, i.e., with false track parameters introduced prior to processing, to set up constrained problems involving one geometrical quantity. Resolution (3 dB in azimuth and slant range) and peak-to-sidelobe ratio measurements in FGA images are comparable with reference results (within a few percent and tenths of a decibel), demonstrating the capacity to compensate for residual space variant range cell migration. The FGA algorithm is finally also benchmarked (visually) against the phase gradient algorithm to emphasize the advantage of a geometrical autofocus approach.
    [bibtex-key = torgrimssonDammertHellstenUlanderFFBPAutofocusTGRS2014] [bibtex-entry]


  392. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. Two-Dimensional Spectrum for BiSAR Derivation Based on Lagrange Inversion Theorem. IEEE Geosci. Remote Sens. Lett., 11(7):1210-1214, July 2014. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Bistatic SAR, Bistatic Fast-Factorized Back-Projection, BiFFBP, Fourier transforms, inverse problems, synthetic aperture radar, BiSAR derivation, Fourier transform, Lagrange inversion theorem, bistatic synthetic aperture radar, stationary phase, two-dimensional spectrum, Apertures, Azimuth, Focusing, Frequency-domain analysis, Remote sensing, Synthetic aperture radar, Transmitters, Bistatic synthetic aperture radar (BiSAR), LORA, Lagrange inversion theorem, two-dimensional spectrum.
    Abstract: A 2-D spectrum for bistatic synthetic aperture radar is derived in this letter. The derivation is based on the commonly used mathematic principles such as the method of stationary phase and the Fourier transform and the Lagrange inversion theorem in order to find the point of stationary phase in the method of stationary phase. Using the Lagrange inversion theorem allows minimizing the initial assumptions or the initial approximations. The derived 2-D spectrum is compared with the commonly used 2-D spectrum to verify it and illustrate its accuracy.
    [bibtex-key = vuSjogrenPetterssonGRSL2014] [bibtex-entry]


  393. TR Walter, Manoochehr Shirzaei, A Manconi, G Solaro, A Pepe, M Manzo, and E Sansosti. Possible coupling of Campi Flegrei and Vesuvius as revealed by InSAR time series, correlation analysis and time dependent modeling. Journal of Volcanology and Geothermal Research, 280:104-110, 2014. [bibtex-key = Walter2014] [bibtex-entry]


  394. Yuanyuan Wang, Xiao Xiang Zhu, and Richard Bamler. An efficient tomographic inversion approach for urban mapping using meter resolution SAR image stacks. IEEE Geoscience and Remote Sensing Letters, 11(7):1250-1254, 2014. [bibtex-key = wangZhuBamlerGRSL2014] [bibtex-entry]


  395. Zhu. Xiao Xiang and Richard Bamler. Superresolving SAR Tomography for Multidimensional Imaging of Urban Areas: Compressive sensing-based TomoSAR inversion. IEEE Signal Processing Magazine, 31(4):51-58, July 2014. Keyword(s): SAR Processing, SAR Tomography, PSI, Persistent Scatterer Interferometry, Deformation Monitoring, Deformation, compressed sensing, image motion analysis, image reconstruction, radar imaging, radar resolution, synthetic aperture radar, tomography, 4D point clouds, LiDAR, VHR spaceborne tomographic SAR inversion, building deformation, compressive sensing-based TomoSAR inversion, dynamic city model reconstruction, motion reconstruction, multicomponent nonlinear motion, multidimensional imaging, multiple scatterers, point density, shape reconstruction, synthetic aperture radar, urban areas, urban infrastructure, very high spatial resolution SAR tomography, Mathematical model, Signal processing algorithms, Signal resolution, Signal to noise ratio, Synthetic aperture radar, Tomography, Urban areas.
    Abstract: With reference to the current status of VHR spaceborne tomographic SAR inversion presented in this article, the following conclusions can be drawn: VHR tomographic SAR inversion is able to reconstruct the shape and motion of individual buildings and entire city areas. SR is crucial and possible, e.g., using CS, for VHR tomographic SAR inversion for urban infrastructure. The motion or deformation of buildings is often nonlinear (periodic, accelerating, stepwise, etc.). Multicomponent nonlinear motion of multiple scatterers can be separated. The 4-D point clouds retrieved by VHR TomoSAR has a point density comparable to LiDAR and can be potentially used for dynamic city model reconstruction.
    [bibtex-key = zhuBamlerIEEESigProcMag2014TOMOSAR] [bibtex-entry]


  396. Francesco De Zan, A. Parizzi, Pau Prats-Iraola, and Paco López-Dekker. A SAR Interferometric Model for Soil Moisture. IEEE_J_GRS, 52(1):418-425, January 2014. Keyword(s): dielectric properties, hydrological techniques, radar interferometry, remote sensing by radar, soil, synthetic aperture radar, Born approximation, L-band airborne SAR data, SAR interferometric model, coherence magnitudes, complex interferometric coherences, geometrical properties, interferogram triplets, phase consistency, plane waves, quantitatively synthetic aperture radar interferometric observables, scattering models, soil moisture, tomography, vertical complex wavenumbers, Coherence, L-band, Moisture, Soil moisture, Synthetic aperture radar, synthetic aperture radar (SAR) interferometry.
    Abstract: In this paper, a new autofocus algorithm is presented for back-projection (BP) image formation of synthetic aperture radar (SAR) imaging. The approach is based on maximizing a cost function obtained by prominent points in different sub-apertures of constructed SAR image by varying the flight trajectory parameters. While image-quality-based autofocus approach together with BP algorithm can be computationally intensive, we use approximations that allow optimal corrections to be derived. The approach is applicable for focusing different signal processing algorithms by obtaining modified flight trajectory parameters. Different examples demonstrate the effectiveness of the new autofocus approach applied to the frequency modulated continuous wave mode SAR dataset.
    [bibtex-key = Zan2014] [bibtex-entry]


  397. Lei Zhang, Hao-lin Li, Zhi-Jun Qiao, and Zhi-wei Xu. A Fast BP Algorithm With Wavenumber Spectrum Fusion for High-Resolution Spotlight SAR Imaging. IEEE Geosci. Remote Sens. Lett., 11(9):1460-1464, September 2014. Keyword(s): SAR Processing, Fast-Factorized Back-Projection, FFBP, Fast Backprojection, fast Fourier transforms, image fusion, image resolution, interpolation, radar imaging, synthetic aperture radar, transient response, AFBP algorithm, FFBP algorithm, SA fusion, UPC system, WN spectrum domain, accelerated fast backprojection algorithm, fast Fourier transform, fast factorization backprojection algorithm, high-resolution spotlight SAR imaging, image-domain interpolation, impulse response function, interpolation-based fusion, subaperture fusion, synthetic aperture radar, unified polar coordinate system, wavenumber spectrum fusion, Apertures, Azimuth, Image resolution, Imaging, Interpolation, Signal processing algorithms, Synthetic aperture radar, Accelerated fast BP (AFBP), fast backprojection (FBP), fast factorized backprojection (FFBP).
    Abstract: This letter presents the accelerated fast backprojection (AFBP) algorithm for high-resolution spotlight synthetic aperture radar (SAR) imaging. In conventional fast backprojection (FBP) algorithms, image-domain interpolation is employed in the subaperture (SA) fusion. However, in AFBP, by using a unified polar coordinate (UPC) system, the interpolation-based fusion is substituted by fusing the SA spectra in the wavenumber (WN) spectrum domain. The WN-domain SA fusion is efficiently implemented by fast Fourier transform and circular shifting. In this letter, an accurate impulse response function and the WN spectrum expression of the backprojected image in the UPC are explicitly derived, and furthermore, the implementations of AFBP are investigated in detail. Compared with conventional FBP algorithms, the AFBP can precisely focus on high-resolution SAR data with dramatically improved efficiency. Both simulation and real-measured data experiments validate the superiorities of AFBP by comparing it with the fast factorization backprojection (FFBP) algorithm.
    [bibtex-key = zhangLiQiaoXuTGRS2014FastBackProjection] [bibtex-entry]


  398. S. Zwieback and I. Hajnsek. Statistical Tests for Symmetries in Polarimetric Scattering Coherency Matrices. IEEE_J_GRSL, 11(1):308-312, January 2014. Keyword(s): airborne radar, covariance matrices, geophysical equipment, geophysical techniques, invariance, radar polarimetry, scattering, statistical testing, Freeman-Durden decomposition, SAR polarimetry, airborne L-band data, covariance matrix, likelihood-ratio testing, mature rape field, polarimetric scattering coherency matrix, reflection invariance hypothesis, reflection symmetry, second-order statistics, synthetic aperture radar polarimetry, vegetation growth, Parametric statistics, polarimetric synthetic aperture radar, radar remote sensing. [bibtex-key = Zwieback2014b] [bibtex-entry]


  399. M. \cCetin, I. Stojanovic, N.O. Önhon, K.R. Varshney, S. Samadi, W.C. Karl, and A.S. Willsky. Sparsity-Driven Synthetic Aperture Radar Imaging: Reconstruction, autofocusing, moving targets, and compressed sensing. IEEE Signal Processing Magazine, 31(4):27-40, July 2014. Keyword(s): SAR Processing, Autofocus, compressed sensing, image representation, radar imaging, synthetic aperture radar, SAR image formation, SAR sensing mission design, anisotropy characterization, compressed sensing-based analysis, joint autofocusing, joint imaging, phase errors, sparsity-based methods, sparsity-driven synthetic aperture radar imaging, synthesis-based sparse signal representation formulations, wide-angle SAR imaging, Image reconstruction, Imaging, Radar imaging, Radar polarimetry, Scattering, Synthetic aperture radar.
    Abstract: This article presents a survey of recent research on sparsity-driven synthetic aperture radar (SAR) imaging. In particular, it reviews 1) the analysis and synthesis-based sparse signal representation formulations for SAR image formation together with the associated imaging results, 2) sparsity-based methods for wide-angle SAR imaging and anisotropy characterization, 3) sparsity-based methods for joint imaging and autofocusing from data with phase errors, 4) techniques for exploiting sparsity for SAR imaging of scenes containing moving objects, and 5) recent work on compressed sensing (CS)-based analysis and design of SAR sensing missions.
    [bibtex-key = cetinStojanovicOnhonVarshneySamadiKarlWillskyIEEESigProcMag2014CompressiveSensingSAR] [bibtex-entry]


  400. Esteban Aguilera, Matteo Nannini, and Andreas Reigber. A Data-Adaptive Compressed Sensing Approach to Polarimetric SAR Tomography of Forested Areas. IEEE Geosci. Remote Sens. Lett., 10(3):543-547, May 2013. Keyword(s): SAR Processing, SAR Tomography, Compressed sensing, Compressive sensing, Covariance matrix, Synthetic aperture radar, Tomography, data compression, forestry, radar polarimetry, tomography, vegetation mapping, wavelet transforms, DLR, E-SAR sensor, German Aerospace Center, covariance fitting, data adaptive compressed sensing approach, data adaptive orthonormal basis, forested areas, fully polarimetric L-band data, multiple looks, multiple polarimetric channels, partial scatterers, polarimetric SAR tomography, superresolution imaging, Distributed compressed sensing (DCS), Kronecker basis, polarimetry, wavelets, E-SAR, F-SAR, L-Band, Airborne SAR.
    Abstract: Super-resolution imaging via compressed sensing (CS)-based spectral estimators has been recently introduced to synthetic aperture radar (SAR) tomography. In the case of partial scatterers, the mainstream has so far been twofold, in that the tomographic reconstruction is conducted by either directly working with multiple looks and/or polarimetric channels or by exploiting the corresponding single-channel second-order statistics. In this letter, we unify these two methodologies in the context of covariance fitting. In essence, we exploit the fact that both vertical structures and the unknown polarimetric signatures can be approximated in a low-dimensional subspace. For this purpose, we make use of a wavelet basis in order to sparsely represent vertical structures. Additionally, we synthesize a data-adaptive orthonormal basis that spans the space of polarimetric signatures. Finally, we validate this approach by using fully polarimetric L-band data acquired by the E-SAR sensor of the German Aerospace Center (DLR).
    [bibtex-key = aguileraNanniniReigberGRSL2013] [bibtex-entry]


  401. Esteban Aguilera, Matteo Nannini, and Andreas Reigber. Wavelet-Based Compressed Sensing for SAR Tomography of Forested Areas. IEEE Trans. Geosci. Remote Sens., 51(12):5283-5295, December 2013. Keyword(s): SAR Processing, SAR Tomography, Covariance matrix, Estimation, Power distribution, Sensors, Synthetic aperture radar, Tomography, Wavelet transforms, Compressed sensing (CS), compressive sensing, CS, forest structure, synthetic aperture radar (SAR) tomography, wavelets, E-SAR, F-SAR, L-Band, Airborne SAR.
    Abstract: Synthetic aperture radar (SAR) tomography is a 3-D imaging modality that is commonly tackled by spectral estimation techniques. Thus, the backscattered power along the cross-range direction can be readily obtained by computing the Fourier spectrum of a stack of multibaseline measurements. In addition, recent work has addressed the tomographic inversion under the framework of compressed sensing, thereby recovering sparse cross-range profiles from a reduced set of measurements. This paper differs from previous publications, in that it focuses on sparse expansions in the wavelet domain while working with the second-order statistics of the corresponding multibaseline measurements. In this regard, we elaborate on the conditions under which this perspective is applicable to forested areas and discuss the possibility of optimizing the acquisition geometry. Finally, we compare this approach with traditional nonparametric ones and validate it by using fully polarimetric L-band data acquired by the Experimental SAR (E-SAR) sensor of the German Aerospace Center (DLR).
    [bibtex-key = aguileraNanniniReigberTGRS2013CSTomo] [bibtex-entry]


  402. F. Alshawaf, T. Fuhrmann, B. Heck, S. Hinz, A. Knöpfler, X. Luo, M. Mayer, A. Schenk, A. Thiele, and M. Westerhaus. Integration of InSAR and GNSS observations for the determination of atmospheric water vapour. Lecture Notes in Geoinformation and Cartography, 0(199659):147-162, 2013. Note: Cited By 3. [bibtex-key = Alshawaf2013147] [bibtex-entry]


  403. Fabrizio Argenti, Alessandro Lapini, Tiziano Bianchi, and Luciano Alparone. A Tutorial on Speckle Reduction in Synthetic Aperture Radar Images. IEEE Geoscience and Remote Sensing Magazine, 1(3):6-35, Sep. 2013. Keyword(s): Teaching.
    Abstract: Speckle is a granular disturbance, usually modeled as a multiplicative noise, that affects synthetic aperture radar (SAR) images, as well as all coherent images. Over the last three decades, several methods have been proposed for the reduction of speckle, or despeckling, in SAR images. Goal of this paper is making a comprehensive review of despeckling methods since their birth, over thirty years ago, highlighting trends and changing approaches over years. The concept of fully developed speckle is explained. Drawbacks of homomorphic filtering are pointed out. Assets of multiresolution despeckling, as opposite to spatial-domain despeckling, are highlighted. Also advantages of undecimated, or stationary, wavelet transforms over decimated ones are discussed. Bayesian estimators and probability density function (pdf) models in both spatial and multiresolution domains are reviewed. Scale-space varying pdf models, as opposite to scale varying models, are promoted. Promising methods following non-Bayesian approaches, like nonlocal (NL) filtering and total variation (TV) regularization, are reviewed and compared to spatial- and wavelet-domain Bayesian filters. Both established and new trends for assessment of despeckling are presented. A few experiments on simulated data and real COSMO-SkyMed SAR images highlight, on one side the costperformance tradeoff of the different methods, on the other side the effectiveness of solutions purposely designed for SAR heterogeneity and not fully developed speckle. Eventually, upcoming methods based on new concepts of signal processing, like compressive sensing, are foreseen as a new generation of despeckling, after spatial-domain and multiresolution-domain methods.
    [bibtex-key = argentiEtAlIEEEGRSM2013TutorialOnSpeckleReductionInSARImages] [bibtex-entry]


  404. E Bleszynski, M Bleszynski, and T Jaroszewicz. Autofocus algorithm for synthetic aperture radar imaging with large curvilinear apertures. Inverse Problems, 29(5):054004, April 2013. Keyword(s): SAR Processing, Autofocus, Airborne SAR, Gotcha SAR Data.
    Abstract: An approach to autofocusing for large curved synthetic aperture radar (SAR) apertures is presented. Its essential feature is that phase corrections are being extracted not directly from SAR images, but rather from reconstructed SAR phase-history data representing windowed patches of the scene, of sizes sufficiently small to allow the linearization of the forward- and backprojection formulae. The algorithm processes data associated with each patch independently and in two steps. The first step employs a phase-gradient-type method in which phase correction compensating (possibly rapid) trajectory perturbations are estimated from the reconstructed phase history for the dominant scattering point on the patch. The second step uses phase-gradient corrected data and extracts the absolute phase value, removing in thisway phase ambiguities and reducing possible imperfections of the first stage, and providing the distances between the sensor and the scattering point with accuracy comparable to the wavelength. The features of the proposed autofocusing method are illustrated in its applications to intentionally corrupted small-scene 2006 Gotcha data. The examples include the extraction of absolute phases (ranges) for selected prominent point targets. They are then used to focus thescene and determine relative target-target distances.
    [bibtex-key = bleszynskiBleszynskiJaroszewiczInverseProblems2013AutofocusForCurvilinearSAR] [bibtex-entry]


  405. A. E. Albright Blomberg, A. Austeng, Roy E. Hansen, and S. A. V. Synnes. Improving Sonar Performance in Shallow Water Using Adaptive Beamforming. IEEE Journal of Oceanic Engineering, 38(2):297-307, April 2013. Keyword(s): Synthetic Aperture Sonar, SAS, adaptive signal processing, array signal processing, interference suppression, optimisation, sonar imaging, Hanning window, LCA beamformer, MVDR beamformer, SNR, adaptive beamforming, adaptive minimum variance distortionless response beamformer, bottom-imaging sonars, dense receiver array, four-element receivers, low complexity adaptive beamformer, low sidelobe levels, multipath interference avoidance, multipath propagation, narrow mainlobe width, optimization criteria, second-order multipaths, shallow-water environments, sidelobe suppression, signal-to-noise ratio, sonar performance, spatial smoothing, vertical array, Array signal processing, Arrays, Receivers, Scattering, Sea surface, Signal to noise ratio, Sonar, Adaptive beamforming, multipath reduction, shallow-water imaging, sonar.
    Abstract: Multipath propagation degrades the performance of active, bottom-imaging sonars in shallow-water environments. One way to avoid multipath interference is to use a vertical array with a narrow enough angular response to separate the direct bottom return from the multipath. However, this requires a large array and is often infeasible for practical reasons. In this study, we focus on the use of adaptive beamforming on the receiver side to reduce multipath interference and hence improve the signal-to-noise ratio (SNR). Using a small, dense receiver array, we apply classical and adaptive beamformers to real data collected by the NATO Undersea Research Centre in a shallow-water environment. Our results show that the adaptive minimum variance distortionless response (MVDR) beamformer offers an improvement in the estimated SNR compared to a conventional beamformer in most cases. However, the MVDR beamformer is suboptimal when the receiver consists of only a few elements. We propose using the low complexity adaptive (LCA) beamformer, which is based on the same optimization criteria as the MVDR beamformer, but is robust in a coherent environment without the need for spatial smoothing. For two to 4-element receivers, we observe an improvement of about 0.5-2.5 dB in the estimated SNR when using the LCA beamformer. In cases where the model indicates that the direct bottom return and the dominating multipath arrive from nearly the same angle, little or no improvement is observed. This is typically the case for first- or second-order multipaths reflected off the seafloor toward the receiver. The results from this study also show that with a small vertical array, a narrow mainlobe width is more important than low sidelobe levels, in terms of maximizing the SNR. Consequently, an unweighted conventional beamformer performs better than a conventional beamformer with a Hanning window applied for sidelobe suppression.
    [bibtex-key = blombergAustengHansenSynnesJOE2013ImprovingSonarPerformanceUsingAdaptiveBeamforming] [bibtex-entry]


  406. A. E. A. Blomberg, C. I. C. Nilsen, A. Austeng, and Roy E. Hansen. Adaptive Sonar Imaging Using Aperture Coherence. IEEE Journal of Oceanic Engineering, 38(1):98-108, January 2013. Keyword(s): Synthetic Aperture Sonar, SAS, array signal processing, sonar imaging, speckle, DAS beamformer, HISAS 1030 sonar, SNR scenarios, SWiP, Wiener postfilter, adaptive sonar imaging, aperture-coherence-based methods, backscattered energy, coherence factor, computationally efficient adaptive imaging methods, delay-and-sum beamformer, image resolution, medical ultrasound imaging, noise suppression capabilities, receiver array, signal-to-noise ratio, speckle environment, Apertures, Arrays, Coherence, Imaging, Noise, Sonar, Speckle, Adaptive beamforming, Wiener postfilter, aperture coherence, array signal gain, coherence factor (CF), sonar imaging.
    Abstract: A class of computationally efficient adaptive imaging methods originating from the coherence factor (CF) has been proposed for improved medical ultrasound imaging. These methods are based on the idea that when steering the receiver toward a point of interest, the backscattered energy from this point exhibits a high degree of aperture coherence, while random noise, multipath interference, and sidelobe energy do not. Aperture coherence can be understood as a normalized measure of the degree of signal variability across the receiver array. This paper presents a study of the use of aperture-coherence-based methods for improved sonar imaging, with particular emphasis on a recently introduced robust implementation known as the scaled Wiener postfilter (SWiP). We show that while the CF has strong noise suppression capabilities and performs well on point targets, it lacks robustness in low signal-to-noise ratio (SNR) scenarios and introduces undesirable artifacts in speckle scenes. The SWiP is closely related to the CF, but contains a single-user-defined parameter, which allows the method to be tuned to suit the application needs. The SWiP can be tuned to offer robustness in a speckle environment such as when imaging the seafloor, or for strong noise suppression capabilities. This makes it a promising method for a wide range of sonar applications. We base our conclusions on simulated data from a constructed speckle scene as well as experimental data from the SX90 fish-finding sonar and sidescan data from the HISAS 1030 sonar. Our results show that the SWiP offers improved edge and shadow definitions and reduced sidelobe levels when compared to the conventional delay-and-sum (DAS) beamformer. These improvements do not compromise the image resolution, and they come at a low computational cost.
    [bibtex-key = blombergNilsenAustengHansenJOE2013AdaptiveSonarUsingApertureCoherence] [bibtex-entry]


  407. F. Bovenga, V.M. Giacovazzo, A. Refice, and N. Veneziani. Multichromatic Analysis of InSAR Data. IEEE Trans. Geosci. Remote Sens., 51(9):4790-4799, September 2013. Keyword(s): SAR Processing, airborne radar, geophysical image processing, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, AES-1 airborne sensor, InSAR data multichromatic analysis, MCA absolute phase measurement, MCA experimental validation, MCA processing parameters, absolute optical path difference inference, central carrier frequency, height estimation performance, height information retrieval, interferometric SAR image pairs, parametric analysis, pixel phase trend, spatial phase unwrapping, wideband SAR single pass interferometric data set, Accuracy, Bandwidth, Distance measurement, Estimation, Noise, Synthetic aperture radar, Thyristors, synthetic aperture radar interferometry (InSAR).
    Abstract: The multichromatic analysis (MCA) uses interferometric pairs of SAR images processed at range subbands and explores the phase trend of each pixel as a function of the different central carrier frequencies to infer absolute optical path difference. This approach allows retrieving unambiguous height information on selected pixels, potentially solving the problem of spatial phase unwrapping, which is instead critical in the standard monochromatic processing. The method, based on concepts originally introduced by Madsen and Zebker, has been developed in previous work both theoretically and through simulations. This paper presents the first MCA experimental validation of the procedure, through application to a wideband SAR single-pass interferometric data set acquired by the AES-1 airborne sensor. An evaluation of the impact of the MCA processing parameters on the height estimation performances is obtained through a parametric analysis. The results confirm the indications derived by the theoretical analysis, demonstrating the feasibility of the MCA absolute phase measurement, provided that a sufficient bandwidth is available.
    [bibtex-key = bovengaGiacovazzoReficeVenezianiTGRS2013] [bibtex-entry]


  408. A. Capozzoli, C. Curcio, and A. Liseno. FAST GPU-BASED INTERPOLATION FOR SAR BACKPROJECTION. Progress In Electromagnetics Research, 133:259-283, 2013. Keyword(s): SAR Processsing, Time-Domain Back-Projection, TDBP, Back-Projection, Non-Uniform FFT, NUFFT, CUDA, GPU, NVIDIA, Parallelized Processing, Synthetic Aperture Radar (SAR), motion compensation, Airborne SAR, Topography-dependent motion compensation, Motion Compensation, MoComp.
    Abstract: We introduce and discuss a parallel SAR backprojection algorithm using a Non-Uniform FFT (NUFFT) routine implemented on a GPU in CUDA language. The details of a convenient GPU implementation of the NUFFT-based SAR backprojection algorithm, amenable to further generalizations to a multi-GPU architecture, are also given. The performance of the approach is analyzed in terms of accuracy and computational speed by comparisons to a ``standard'', parallel version of the backprojection algorithm exploiting FFT+interpolation instead of the NUFFT. Different interpolators have been considered for the latter processing scheme. The NUFFT-based backprojection has proven significantly more accurate than all the compared approach, with a computing time of the same order. An analysis of the computational burden of all the different steps involved in both the considered approaches (i.e., standard and NUFFT backprojections) has been also reported. Experimental results against the Air Force Research Laboratory (AFRL) airborne data delivered under the ``challenge problem for SAR-based Ground Moving Target Identification (GMTI) in urban environments" and collected over circular flight paths are also shown.
    [bibtex-key = capozzoliCurcioLisenoPIER2013CUDAGPUBackprojection] [bibtex-entry]


  409. Michele Crosetto, J. A. Gili, Oriol Monserrat, M. Cuevas-González, J. Corominas, and D. Serral. Interferometric SAR monitoring of the Vallcebre landslide (Spain) using corner reflectors. Natural Hazards and Earth System Sciences, 13(4):923-933, 2013. [bibtex-key = crosettoGiliMonserratCuevasGonzalezCorominasSerral2013InSARLandslide] [bibtex-entry]


  410. Francesco De Zan, Gerhard Krieger, and Paco López-Dekker. On Some Spectral Properties of TanDEM-X Interferograms Over Forested Areas. IEEE Geoscience and Remote Sensing Letters, 10(1):71-75, January 2013. Keyword(s): Analytical models, Coherence, Interferometry, Predictive models, Remote sensing, Synthetic aperture radar, Synthetic aperture radar (SAR) interferometry, Tandem-X.
    Abstract: This letter reports about some observations over rainforests (in Brazil and Indonesia), where the spectra of TanDEM-X interferograms show distinct features, almost a signature, which is explained and modeled in terms of the scattering properties. Supported by comparisons with simulations, the observations exclude any homogeneous horizontally layered forest; instead, they are compatible with a model with point scatterers clustered in clouds. Such a model, with high extinction and large gaps that allow significant penetration, is able to explain to a good degree the observations.
    [bibtex-key = deZanKriegerLopezDekkerGRSL2013TandemXCoherence] [bibtex-entry]


  411. A. Elsherbini and K. Sarabandi. ENVELOP Antenna: A Class of Very Low Profile UWB Directive Antennas for Radar and Communication Diversity Applications. IEEE_J_AP, 61(3):1055-1062, March 2013. Keyword(s): airborne radar, dipole antennas, directive antennas, diversity reception, horn antennas, magnetic moments, radar antennas, ultra wideband antennas, ultra wideband communication, ENVELOP antenna, airborne radars, antenna radiation characteristics, communication diversity application, crossed electric dipole, impedance matching performance, inductive loop antenna, magnetic dipoles, polarization diversity, radar application, sheet loop, short TEM horn, very low profile UWB directive antenna, Antenna radiation patterns, Bandwidth, Directive antennas, Horn antennas, Radar antennas, Ultra wideband antennas, Monopole antenna, UWB antenna, radar antenna. [bibtex-key = Elsherbini2013] [bibtex-entry]


  412. Gianfranco Fornaro, Diego Reale, and Simona Verde. Bridge Thermal Dilation Monitoring With Millimeter Sensitivity via Multidimensional SAR Imaging. IEEE Geosci. Remote Sens. Lett., 10(4):677-681, July 2013. Keyword(s): SAR Processing, SAR interferometry, Interferometry, differential interferometry, InSAR, DInSAR, Persistent Scatterer Interferometry, PSI, SAR Tomography, Tomography, Spaceborne SAR, TerraSAR-X, X-band, Bridges, Imaging, Interferometry, Monitoring, Remote sensing, Synthetic aperture radar, Temperature measurement, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, Italy, Musmeci bridge, Potenza, SAR processing chains, SAR sensors, TerraSAR-X data set, accurate monitoring capabilities, bridge static structure, bridge thermal dilation monitoring, interferometric approaches, man-made structures, millimeter sensitivity, multidimensional SAR imaging, multidimensional imaging approach, spatial resolution, synthetic aperture radar, system sensitivity, thermal coefficient, Multidimensional SAR imaging (MDI-SAR), SAR tomography, TerraSAR-X (TSX), synthetic aperture radar (SAR), thermal dilation;.
    Abstract: The new generation of synthetic aperture radar (SAR) sensors is providing images with very high spatial resolution, improved up to the meter scale. Such a resolution increase allows more accurate monitoring capabilities by means of interferometric approaches. The use of higher frequency enhances the sensitivity of the system even to minute changes, such as thermal dilations. This phenomenon has an impact on the interferometric products, particularly on the deformation velocity maps, if not properly handled. Man-made structures, such as steel core bridges and specific buildings, may be very sensible to thermal dilation effects. By extending the multitemporal differential interferometry SAR processing chains, in our case based on the multidimensional imaging (MDI) approach, an additional parameter related to temperature differences at acquisition instants, the thermal coefficient, can be accurately estimated. This parameter provides interesting perspectives in application to infrastructure monitoring: It brings information about the thermal behavior of the imaged objects. In this letter, we investigate the thermal response of the Musmeci bridge (Potenza, Italy), by experimenting the extended MDI approach on a real TerraSAR-X data set. Results highlight the possibility of such a technique to obtain measurements of the motion that is highly correlated with temperature, thus providing useful information about the static structure of bridges.
    [bibtex-key = fornaroRealeVerdeGRSL2013] [bibtex-entry]


  413. Othmar Frey, Maurizio Santoro, Charles L. Werner, and Urs Wegmuller. DEM-based SAR pixel area estimation for enhanced geocoding refinement and radiometric normalization. IEEE Geosci. Remote Sens. Lett., 10(1):48-52, January 2013. Keyword(s): SAR Processing, Geocoding, radiometric calibration, terrain-based radiometric normalization.
    Abstract: Precise terrain-corrected georeferencing of synthetic aperture radar (SAR) images and derived products in range-Doppler coordinates is important with respect to several aspects, such as data interpretation, combination with other geodata products, and transformation of, e.g., terrain heights into SAR geometry as used in differential interferometric SAR (DInSAR) applications. For georeferencing, a lookup table is calculated and then refined based on a coregistration of the actual SAR image to a simulated SAR image. The impact of using two different implementations of such a simulator of topography-induced radar brightness, 1) an approach based on angular relationships and 2) a pixel-area-based method, is discussed in this letter. It is found that the pixel-area-based method leads to considerable improvements with regard to the robustness of georeferencing and also with regard to radiometric normalization in layover-affected areas.
    [bibtex-key = freySantoroWegmullerWernerGRSL2012] [bibtex-entry]


  414. Ruben Iglesias and Jordi J. Mallorqui. Side-Lobe Cancelation in DInSAR Pixel Selection With SVA. IEEE Geosci. Remote Sens. Lett., 10(4):667-671, 2013. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, DInSAR, Differential Interferometry, geophysical image processing, geophysical techniques, image enhancement, radar imaging, synthetic aperture radar, DInSAR pixel selection algorithm, SAR images, SAR systems, SVA filtering, amplitude stability, bidimensional sinc function shape, complex data, complex scattering scenarios, cosine-on-pedestal weighting functions, differential interferometric synthetic aperture radar, nonlinear filter, original image resolution, pixel cluster, pixel position, point spread function, resolution enhancement, side-lobe cancelation, single cell, spatially variant apodization, strong scatterer main lobe, strong scatterer side lobe, synthetic aperture radar, total side-lobe cancelation, Azimuth, Dispersion, Image resolution, Indexes, Interferometry, Standards, Synthetic aperture radar, Differential interferometric synthetic aperture radar (SAR) (DInSAR), SAR, interferometry, spatially variant apodization (SVA).
    Abstract: Synthetic aperture radar (SAR) systems are inherently band limited in both range and azimuth, and hence, the point spread function (PSF) has the shape of a bidimensional sinc function. In addition, all SAR images are slightly oversampled, and as a consequence, the contribution of a single target extends to more than a single cell. The main lobe and the side lobes of strong scatterers are sometimes clearly visible in the images. This characteristic of the SAR images must be considered when applying differential interferometric synthetic aperture radar (DInSAR) pixel selection algorithms. For persistent scatterers, the properties, for instance, the amplitude stability, are preserved in both redundant information around the main lobe and side lobes. For this reason, a cluster of pixels rather than just the pixel position corresponding to the exact location of the target will be detected. Spatially variant apodization (SVA) is a nonlinear filter based on cosine-on-pedestal weighting functions able to achieve a total side-lobe cancelation without degrading the original image resolution. When working with complex data under complex scattering scenarios, the PSF moves away from the ideal bidimensional sinc, and the SVA performance worsens. The amplitude and phase of the original images could be distorted by the SVA filtering compromising the pixel selection and the quality of the final DInSAR results. In this letter, SVA is used to method locate in the image the side lobes of high-power scatterers and generate a mask while preserving the amplitude and phase of the original images.
    [bibtex-key = iglesiasMallorqui2013GRSLPSISVA] [bibtex-entry]


  415. J.M.L. King, R. Kelly, A. Kasurak, C. Duguay, G. Gunn, and J.B. Mead. UW-Scat: A Ground-Based Dual-Frequency Scatterometer for Observation of Snow Properties. IEEE Geosci. Remote Sens. Lett., 10(3):528-532, May 2013. Keyword(s): geophysical equipment, snow, AD 2009 to 2011, Canada, Canadian Subarctic, Churchill, UW-Scat scatterometer, University of Waterloo scatterometer, angular width, azimuth user-programmable range, elevation angle, frequency 17.2 GHz, frequency 9.6 GHz, frequency-modulated continuous-wave radars, ground-based dual-frequency scatterometer, harsh environmental conditions, ice property observation, radar beam, snow property observation, terrestrial snow, two-axis positioning system, Backscatter, Calibration, Ice, Radar measurements, Radio frequency, Snow, Spaceborne radar, Radar polarimetry, scatterometer, snow.
    Abstract: The University of Waterloo scatterometer, which is a system developed for observation of snow and ice properties, is described. The system is composed of two frequency-modulated continuous-wave radars operating at center frequencies of 17.2 and 9.6 GHz. A field-deployable platform allows a rapid setup and observation at remote sites under harsh environmental conditions. A two-axis positioning system moves the radar beam across a user-programmable range of azimuth (+/-180 deg) and elevation angles (15-105 deg). Typical azimuth scans of 60deg angular width generate between 21 and 586 independent samples, depending on the wavelength and the elevation angle. The backscatter response of terrestrial snow in the Canadian Subarctic is demonstrated with two experiments conducted in Churchill, MB, Canada, between 2009 and 2011.
    [bibtex-key = kingKellyKasurakDuguayGunnMeadGRSL2013XandKuScatSnow] [bibtex-entry]


  416. Seung-Kuk Lee, Florian Kugler, Konstantinos P. Papathanassiou, and Irena Hajnsek. Quantification of Temporal Decorrelation Effects at L-Band for Polarimetric SAR Interferometry Applications. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 6(3):1351-1367, June 2013. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, airborne radar, data acquisition, decorrelation, estimation theory, radar polarimetry, spaceborne radar, synthetic aperture radar, BioSAR 2007, L-band E-SAR repeat-pass data acquisition, Pol-InSAR forest parameter inversion, TempoSAR 2008, TempoSAR 2009, ground temporal decorrelation effect, height inversion error, interferometric repeat-pass mode, multibaseline Pol-InSAR data acquisition, polarimetric SAR interferometry application, quantitative estimation, rain-induced dielectric change, satellite airborne SAR system, temporal baseline estimation, time 10 min to 54 day, velocity 2 m/s, volume temporal decorrelation effect, Height inversion, polarimetric synthetic aperture radar interferometry (Pol-InSAR), temporal baseline, temporal decorrelation.
    Abstract: Temporal decorrelation is the most critical issue for the successful inversion of polarimetric SAR interferometry (Pol-InSAR) data acquired in an interferometric repeat-pass mode, typical for satellite or lower frequency airborne SAR systems. This paper provides a quantitative estimation of temporal decorrelation effects at L-band for a wide range of temporal baselines based on a unique set of multibaseline Pol-InSAR data. A new methodology that allows to quantify individual temporal decorrelation components has been developed and applied. Temporal decorrelation coefficients are estimated for temporal baselines ranging from 10 min to 54 days and converted to height inversion errors caused by them. The temporal decorrelations (volume temporal decorrelation) and (ground temporal decorrelation) depend not only on the wind-induced movement but also strongly on the rain-induced dielectric changes in volume and on the ground at temporal baseline on the order of day or longer. At temporal baselines on the order of minutes, the wind speed is a critical parameter and the speed of 2 m/s already hampers the application of Pol-InSAR forest parameter inversion. The approach is supported and validated by using L-band E-SAR repeat-pass data acquired in the frame of three dedicated campaigns, BioSAR 2007, TempoSAR 2008, and TempoSAR 2009.
    [bibtex-key = leeKuglerPapathanassiouHajnsekJSTARS2013TempDecorrelationLBand] [bibtex-entry]


  417. Deren Li, Yuxiao Qin, Daniele Perissin, and Ling Lei. The Design and Experiments on Corner Reflectors for Urban Ground Deformation Monitoring in Hong Kong. International Journal of Antennas and Propagation, 2013:191685, 2013. Keyword(s): Corner reflector, trihedral corner reflector, radar cross section, RCS, signal-to-clutter ratio.
    Abstract: PSInSAR technology has been proved to be a powerful tool for monitoring urban ground displacement information to a millimetric accuracy. When it comes to the validation of PS-derived ground deformation, artificial corner reflector (CR) can be very useful due to its relative stability and high signal-to-noise ratio (SNR). In this paper, we will evaluate some general criteria for designing and setting up corner reflectors, including the shape, size, material, location, and others. An ideal prototype known as the rectangular trihedral with special designs is brought up in this paper, and validation experiments were conducted in Hong Kong to demonstrate the ability of the proposed prototype. The field data agreed with theoretical analysis, bringing up an economical and applicable approach for CR application in urban ground deformation monitoring.
    [bibtex-key = liQinPerissinLeiHINDAWIIJAP2013DesignAndExperimentOnCOrnerReflectorsForUrbanDeformationHongKong] [bibtex-entry]


  418. Fabrizio Lombardini, Francesco Cai, and Davide Pasculli. Spaceborne 3-D SAR tomography for analyzing garbled urban scenarios: single-look superresolution advances and experiments. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 6(2):960-968, 2013. [bibtex-key = lombardiniCaiPasculliJSTARS2013TomoUrban] [bibtex-entry]


  419. R. Magagi, A. A. Berg, K. Goita, S. Belair, T. J. Jackson, B. Toth, A. Walker, H. McNairn, P. E. O'Neill, Mahta Moghaddam, I. Gherboudj, A. Colliander, M. H. Cosh, Mariko S. Burgin, J. B. Fisher, S. B. Kim, I. Mladenova, N. Djamai, L. P. B. Rousseau, J. Belanger, J. Shang, and A. Merzouki. Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10): Overview and Preliminary Results. IEEE Transactions on Geoscience and Remote Sensing, 51(1):347-363, January 2013. Keyword(s): land surface temperature, remote sensing, soil, vegetation, AD 2010, AD 2010 05 31 to 2010 06, AMSR-E soil moisture, CanEx-SM10, Canada, Canadian experiment, National Snow and Ice Data Center, SMOS data, SMOS mission, SMOS soil moisture product, Saskatchewan, active microwave measurement, agricultural area, airborne L-band brightness temperatures, airborne L-band radiometer data, airborne platform, boreal forested area, ocean salinity, passive microwave measurement, radio frequency, reflected soil moisture measurements, satellite platform, soil condition, soil ground-based measurements, soil moisture, soil temperature profiles, vegetation characteristics, vegetation condition, Moisture measurement, Satellites, Soil measurements, Soil moisture, Temperature measurement, Vegetation mapping, Agricultural and boreal forested areas, Soil Moisture and Ocean Salinity (SMOS), brightness temperature, soil moisture, validation.
    Abstract: The Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10) was carried out in Saskatchewan, Canada, from 31 May to 16 June, 2010. Its main objective was to contribute to Soil Moisture and Ocean Salinity (SMOS) mission validation and the prelaunch assessment of the proposed Soil Moisture Active and Passive (SMAP) mission. During CanEx-SM10, SMOS data as well as other passive and active microwave measurements were collected by both airborne and satellite platforms. Ground-based measurements of soil (moisture, temperature, roughness, bulk density) and vegetation characteristics (leaf area index, biomass, vegetation height) were conducted close in time to the airborne and satellite acquisitions. Moreover, two ground-based in situ networks provided continuous measurements of meteorological conditions and soil moisture and soil temperature profiles. Two sites, each covering 33 km x 71 km (about two SMOS pixels) were selected in agricultural and boreal forested areas in order to provide contrasting soil and vegetation conditions. This paper describes the measurement strategy, provides an overview of the data sets, and presents preliminary results. Over the agricultural area, the airborne L-band brightness temperatures matched up well with the SMOS data (prototype 346). The radio frequency interference observed in both SMOS and the airborne L-band radiometer data exhibited spatial and temporal variability and polarization dependency. The temporal evolution of the SMOS soil moisture product (prototype 307) matched that observed with the ground data, but the absolute soil moisture estimates did not meet the accuracy requirements (0.04 m3/m3) of the SMOS mission. AMSR-E soil moisture estimates from the National Snow and Ice Data Center more closely reflected soil moisture measurements.
    [bibtex-key = magagiEtAlTGRS2013CanadianSoilMoistureExperiment] [bibtex-entry]


  420. N. J. Marechal, S. S. Osofsky, and R. M. Bloom. Demonstration of W-band SAR Imagery with a Ground Based System Having 7.5 GHz of Bandwidth Obtained with a Stepped Chirp Waveform. IEEE Transactions on Aerospace and Electronic Systems, 49(4):2522-2532, October 2013. Keyword(s): SAR Processing, W-Band, airborne radar, ground penetrating radar, microwave imaging, millimetre wave imaging, millimetre wave radar, radar antennas, radar detection, radar imaging, radar interferometry, radar resolution, synthetic aperture radar, terrain mapping, airborne high resolution imaging, azimuth resolution, bandwidth 7.5 GHz, data collection, frequency 95 GHz, ground-based security application, ground-based synthetic aperture radar, mapping system, radar antenna, radar detection, repeat pass interferometric SAR, stepped chirp waveform, terrain mapping, w-band SAR imagery, Azimuth, Bandwidth, Image resolution, Radar imaging, Rails, Synthetic aperture radar.
    Abstract: Imagery formed from data collected by a ground-based synthetic aperture radar (SAR) operating at W-band (95 GHz) with a bandwidth of 7.5 GHz is presented. The synthetic aperture is generated by translating the radar antenna along a precision rail mounted atop a two story building which illuminates targets at ground level below. The azimuth resolution achieved is 1.4 cm. The 7.5 GHz of range bandwidth is created as a union of sub-bands using a stepped chirp waveform providing a range resolution of 2.5 cm. Repeat pass interferometric SAR for terrain mapping and change detection are also demonstrated. The results are of particular importance for the development of airborne high resolution imaging and mapping systems, and ground-based security applications.
    [bibtex-key = marechalOsofskyBloomTAES2013WBandSAR] [bibtex-entry]


  421. Mauro Mariotti d' Alessandro, Stefano Tebaldini, and Fabio Rocca. Phenomenology of Ground Scattering in a Tropical Forest Through Polarimetric Synthetic Aperture Radar Tomography. IEEE Trans. Geosci. Remote Sens., 51(8):4430-4437, August 2013. Keyword(s): SAR Processing, Biomass, SAR Tomography, Tomography, Polarimetry, Polarimetric synthetic aperture radar, SAR Interferometry, Interferometry, Multibaseline Interferometry, digital signal processing, forest, longer wavelength radar, P-Band, Airborne SAR, Paracou, French Guyana, ONERA, SETHI, TropiSAR, ESA.
    Abstract: This paper aims at characterizing the scattering mechanisms occurring at the ground level in a tropical forest illuminated by a P-band synthetic aperture radar (SAR). The analysis is carried out based on the multibaseline, fully polarimetric, data set collected by ONERA over Paracou, French Guyana, in the frame of the European space agency campaign TropiSAR. The favorable baseline distribution of this data set results in the possibility of removing most contributions from the vegetation layer by tomographic techniques, thus allowing the generation of a new fully polarimetric single look complex SAR image relative to scattering contributions from the ground level only. Such a ground layer image is then analyzed by considering the variation of its polarimetric signature with respect to terrain local slope and Radar look angle. Two major conclusions are drawn: 1) double bounce scattering from trunk-ground interactions is observed to be the dominant scattering mechanism at the ground level on flat terrains, whereas it rapidly tends to vanish as the topographic slope increases, and 2) the characteristic parameter that rules trunk-ground scattering is not the tree height, but rather the available free path facing the tree, as a result of the presence of nearby trees, undulating topography, or understory preventing double bounce scattering from taking place whenever the ground bounce occurs too far away from the considered tree. The mean free path length resulting from the analysis of this data-set is found to be L approx. 7 m. Finally, we discuss how the concept of free path length can be accounted for in simple terms by assuming an equivalent extinction model characterized by a variation along the horizontal dimension.
    [bibtex-key = mariottidAlessandroTebaldiniRoccaTGRS2013] [bibtex-entry]


  422. Kyra Moon and David G. Long. A New Factorized Backprojection Algorithm for Stripmap Synthetic Aperture Radar. Positioning, 4:42-56, 2013. Keyword(s): SAR Processing, Azimuth Focusing, Time-domain back-projection, TDBP, SAR focusing, motion compensation, radar signal processing, synthetic aperture radar, 3D motion compensation, SAR processing scheme, agile SAR platforms, time domain backprojection processing, Fast-Factorized Back-Projection, FFBP, Fast Back-Projection, Synthetic aperture radar.
    Abstract: Factorized backprojection is a processing algorithm for reconstructing images from data collected by synthetic aperture radar (SAR) systems. Factorized backprojection requires less computation than conventional time-domain backprojection with minimal loss in accuracy for straight-line motion. However, its implementation is not as straightforward as direct backprojection. This paper provides a new, easily parallelizable formulation of factorized backprojection designed for stripmap SAR data that includes a method of implementing an azimuth window as part of the factorized backprojection algorithm. We compare the performance of windowed factorized backprojection to direct backprojection for simulated and actual SAR data.
    [bibtex-key = moonLongPositioning201FFBP] [bibtex-entry]


  423. Alberto Moreira, Pau Prats-Iraola, Marwan Younis, Gerhard Krieger, Irena Hajnsek, and Konstantinos P. Papathanassiou. A tutorial on synthetic aperture radar. IEEE Geoscience and Remote Sensing Magazine, 1(1):6-43, March 2013. Keyword(s): SAR Processing, Tutorial, SAR Focusing, Azimuth Focusing, SAR Interferometry, SAR Polarimetry, polarimetric interferometry, PolInSAR, InSAR, SAR Tomography, geophysical techniques, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, Earth remote sensing, bistatic configurations, differential interferometry, digital beamforming, holographic tomography, multiple-input multiple-output, multistatic configurations, polarimetric SAR interferometry, polarimetry, synthetic aperture radar, Radar imaging, Remote sensing, Scattering, Spaceborne radar, Synthetic aperture radar, Tutorial, Teaching Material, Teaching.
    Abstract: Synthetic Aperture Radar (SAR) has been widely used for Earth remote sensing for more than 30 years. It provides high-resolution, day-and-night and weather-independent images for a multitude of applications ranging from geoscience and climate change research, environmental and Earth system monitoring, 2-D and 3-D mapping, change detection, 4-D mapping (space and time), security-related applications up to planetary exploration. With the advances in radar technology and geo/bio-physical parameter inversion modeling in the 90s, using data from several airborne and spaceborne systems, a paradigm shift occurred from the development driven by the technology push to the user demand pull. Today, more than 15 spaceborne SAR systems are being operated for innumerous applications. This paper provides first a tutorial about the SAR principles and theory, followed by an overview of established techniques like polarimetry, interferometry and differential interferometry as well as of emerging techniques (e.g., polarimetric SAR interferometry, tomography and holographic tomography). Several application examples including the associated parameter inversion modeling are provided for each case. The paper also describes innovative technologies and concepts like digital beamforming, Multiple-Input Multiple-Output (MIMO) and bi- and multi-static configurations which are suitable means to fulfill the increasing user requirements. The paper concludes with a vision for SAR remote sensing.
    [bibtex-key = moreiraPratsYounisKriegerHajnsekPapathanassiouGRSMagazine2013] [bibtex-entry]


  424. K. Morrison, J. C. Bennett, and M. Nolan. Using DInSAR to Separate Surface and Subsurface Features. IEEE_J_GRS, 51(6):3424-3430, June 2013. Keyword(s): radar imaging, radar interferometry, soil, synthetic aperture radar, C-band VV SAR images, DInSAR scheme, Ground-Based SAR Microwave Measurement Facility, SAR DInSAR, airborne platform, differential interferometric synthetic aperture radar, drying soil, moisture content, phase soil moisture, signal phase record, soil dielectric properties, spaceborne platform, volumetric soil moisture, Radar imaging, Soil measurements, Soil moisture, Spaceborne radar, Synthetic aperture radar, Dielectric materials, ground-penetrating radar, moisture measurement, permittivity measurement, radar signal analysis, radar signature, soil measurements, synthetic aperture imaging, synthetic aperture radar (SAR). [bibtex-key = Morrison2013] [bibtex-entry]


  425. Keith Morrison, John Bennett, and Svein Solberg. Ground-based C-band tomographic profiling of a conifer forest stand. International Journal of Remote Sensing, 34(21):7838-7853, September 2013. [bibtex-key = morrisonBennettSolbergIJRS2013GroundbasedCBandTomographicProfilingOfForestStand] [bibtex-entry]


  426. S. Perna, V. Zamparelli, A. Pauciullo, and G. Fornaro. Azimuth-to-Frequency Mapping in Airborne SAR Data Corrupted by Uncompensated Motion Errors. Geoscience and Remote Sensing Letters, IEEE, 10(6):1493-1497, November 2013. Keyword(s): remote sensing by radar, synthetic aperture radar, MOCO algorithm, airborne SAR raw data, azimuth-to-frequency mapping, precise aperture-dependent procedure, precise topography-dependent procedure, standard two-step motion compensation, synthetic aperture radar, uncompensated motion errors, Airborne SAR, SAR motion compensation, synthetic aperture radar (SAR).
    Abstract: Airborne synthetic aperture radar (SAR) raw data are affected by motion errors. These are commonly accounted for via standard two-step motion compensation (MOCO) algorithm followed by the Precise Topography- and Aperture-dependent (PTA) procedure proposed some years ago. In this letter, we show how the azimuth-to-frequency mapping used by the PTA approach should be modified to fully account for the presence of uncompensated motion errors.
    [bibtex-key = pernaZamparelliPauciulloFornaroGRSL2013] [bibtex-entry]


  427. Luca Pipia, Xavier Fabregas, Albert Aguasca, and Carlos Lopez-Martinez. Polarimetric Temporal Analysis of Urban Environments With a Ground-Based SAR. IEEE Trans. Geosci. Remote Sens., 51(4):2343-2360, 2013. Keyword(s): SAR Processing, Ground-based SAR, GBSAR, DInSAR, Differential SAR Interferometry, X-Band, Time series, Polarimetry, SAR Polarimetry, Backscatter, Geologic measurements, Monitoring, Scattering, Stability analysis, Synthetic aperture radar, Urban areas, Ground-based synthetic aperture radar (gbSAR) systems, polarimetric entropy, radar polarimetry.
    Abstract: Revisiting time constitutes a key constraint for continuous monitoring activities based on space- and airborne synthetic aperture radar (SAR) acquisitions. Conversely, the employment of terrestrial platforms overcomes this limitation and makes it possible to perform time-continuous observations of small space-scale phenomena. New research lines of SAR dealing with the backscattering evolution of different types of scenarios become hence possible through the analysis of ground-based SAR (gbSAR) data collections. The Remote Sensing Laboratory of the Universitat Politecnica de Catalunya drove a one-year measurements campaign in the village of Sallent, northeastern Spain, using its X-Band gbSAR sensor. The field experiment aimed at studying the subsidence phenomenon induced by the salt mining activity carried out in this area during the past decades. In this paper, the polarimetric behavior of an urban environment is investigated at different time scales. After a brief description of the test site and the measurement campaign, the analysis is focused on the stability on man-made structures at different time scales. PolSAR data monthly acquired from June 2006 to July 2007 are employed to stress the presence of nonstationary backscattering processes within the urban scene and the effect they have on differential phase information. Then, a filtering procedure aiming at reducing backscattering randomness in one-day and long-term data collections is then put forward. The improvements provided by the proposed technique are assessed using a new polarimetric descriptor, the time entropy. In the end, the importance of preserving the interferometric phase information from nonstationary backscattering contaminations using fully polarimetric data is discussed.
    [bibtex-key = pipiaFabregasAquascaLopezMartinez2013GBSAR] [bibtex-entry]


  428. F. Prieur and Roy E. Hansen. Theoretical Improvements When Using the Second Harmonic Signal in Acoustic Doppler Current Profilers. IEEE Journal of Oceanic Engineering, 38(2):275-284, April 2013. Keyword(s): Synthetic Aperture Sonar, SAS, Doppler shift, acoustic pulses, acoustic signal processing, acoustic wave velocity, autonomous underwater vehicles, harmonic generation, signal generators, underwater sound, Doppler frequency shift, Doppler log transmission, acoustic Doppler current profilers, floating particles, frequency 153.6 kHz, geometrical properties, geometry, harmonic signal generation, mainlobe-to-sidelobe ratio, perturbations, power 250 W, pulse transmission, sea bottom, second harmonic signal, size 400 m, velocity estimation error, velocity logs, vessel relative, water currents, Acoustic beams, Doppler shift, Estimation, Harmonic analysis, Transducers, Vectors, Current profiler, Doppler, harmonic, nonlinearity.
    Abstract: Acoustic Doppler current profilers and velocity logs are devices that compute the Doppler frequency shift undergone by a pulse after reflection by floating particles in water or by the seafloor. Using this Doppler shift, the velocity of the water currents carrying the reflecting particles or the speed of a vessel relative to the sea bottom can be estimated. The attainable performance of Doppler logs in terms of range and velocity estimate error are directly linked to the physical dimensions and geometry of the transceivers as well as the nature of the pulse transmitted. Beyond a certain transmitted power, distortion of the transmitted pulse due to nonlinear effects is significant. The second harmonic signal generated in that case can be used to estimate velocity in conjunction with the fundamental signal. It has a narrower mainlobe and a higher mainlobe-to-sidelobe ratio compared to the fundamental signal. Such geometrical properties contribute to a more localized velocity determination with less perturbations coming from scatterers away from the region of interest. Combining the velocity estimates using the fundamental and second harmonic signals also helps decreasing the velocity estimate error. For a Doppler log transmitting a pulse at 153.6 kHz using 250-W input power with a nominal range of 400 m, the attainable range for the second harmonic signal is estimated to around 221 m.
    [bibtex-key = prieurHansenJOE2013SASImprovementsUsingSecondHarmonicInAcousticDopplerCurrentProfilers] [bibtex-entry]


  429. Diego Reale, Gianfranco Fornaro, and Antonio Pauciullo. Extension of 4-D SAR imaging to the monitoring of thermally dilating scatterers. IEEE Transactions on Geoscience and Remote Sensing, 51(12):5296-5306, 2013. [bibtex-key = realeFornaroPauciulloTGRS2013Tomo4DThermal] [bibtex-entry]


  430. Andreas Reigber, Rolf Scheiber, Marc Jäger, Pau Prats-Iraola, Irena Hajnsek, Thomas Jagdhuber, Konstantinos P. Papathanassiou, Matteo Nannini, Esteban Aguilera, S. Baumgartner, Ralf Horn, Anton Nottensteiner, and Alberto Moreira. Very-High-Resolution Airborne Synthetic Aperture Radar Imaging: Signal Processing and Applications. Proceedings of the IEEE, 101(3):759-783, March 2013. Keyword(s): Applications, high resolution, signal processing, synthetic aperture radar (SAR);.
    Abstract: During the last decade, synthetic aperture radar (SAR) became an indispensable source of information in Earth observation. This has been possible mainly due to the current trend toward higher spatial resolution and novel imaging modes. A major driver for this development has been and still is the airborne SAR technology, which is usually ahead of the capabilities of spaceborne sensors by several years. Today's airborne sensors are capable of delivering high-quality SAR data with decimeter resolution and allow the development of novel approaches in data analysis and information extraction from SAR. In this paper, a review about the abilities and needs of today's very high-resolution airborne SAR sensors is given, based on and summarizing the longtime experience of the German Aerospace Center (DLR) with airborne SAR technology and its applications. A description of the specific requirements of high-resolution airborne data processing is presented, followed by an extensive overview of emerging applications of high-resolution SAR. In many cases, information extraction from high-resolution airborne SAR imagery has achieved a mature level, turning SAR technology more and more into an operational tool. Such abilities, which are today mostly limited to airborne SAR, might become typical in the next generation of spaceborne SAR missions.
    [bibtex-key = reigberEtAlProcIEEE2012] [bibtex-entry]


  431. Maxwell L Rudolph, Manoochehr Shirzaei, M Manga, and Y Fukushima. Evolution and future of the Lusi mud eruption inferred from ground deformation. Geophysical Research Letters, 40(6):1089-1092, 2013. [bibtex-key = Rudolph2013] [bibtex-entry]


  432. T. O. Saebo, S. A. V. Synnes, and Roy E. Hansen. Wideband Interferometry in Synthetic Aperture Sonar. IEEE Transactions on Geoscience and Remote Sensing, 51(8):4450-4459, August 2013. Keyword(s): Synthetic Aperture Sonar, SAS, autonomous aerial vehicles, geophysical image processing, image resolution, oceanographic equipment, oceanographic techniques, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, Goldstein branch cut phase unwrapping algorithm, HISAS wideband interferometric SAS, HUGIN autonomous underwater vehicle, SAS interferometry, absolute phase difference, advanced phase unwrapping techniques, complex cross correlation technique, direct phase difference estimation, final phase estimate, high resolution images, interferometric data processing, misregistrated areas, multiband split spectrum technique, multichromatic approach, narrowband interferometry processing, phase ambiguities, seafloor, synthetic aperture sonar interferometry, topographic maps, wideband interferometry techniques, Delay effects, Interferometry, Maximum likelihood estimation, Synthetic aperture sonar, Wideband, Complex cross correlation, multichromatic analysis, split spectrum algorithm, synthetic aperture radar (SAR), synthetic aperture sonar (SAS), wideband interferometry.
    Abstract: Synthetic aperture sonar (SAS) interferometry can provide very high resolution images and topographic maps of the seafloor over large swaths. Processing of interferometric data to retrieve reliable depth estimates of the seafloor is, however, nontrivial. Traditional narrowband interferometry processing relies on advanced phase unwrapping techniques, constraints, and prior knowledge to resolve phase ambiguities. These methods all have dependencies throughout the dataset, e.g., a small error in the assumption may cause a global error in the final phase estimate. For wideband systems, there are alternative techniques to estimate the absolute (or ambiguity-free) phase difference directly. We consider four different wideband interferometry techniques for direct phase difference estimation: complex cross correlation, split-spectrum algorithm, and the multichromatic approach. In addition, we develop a weighted split-spectrum algorithm, where the weights minimize the variance in the absolute phase estimate. We benchmark these techniques on simulated data and demonstrate the techniques on real data from the HISAS wideband interferometric SAS on a HUGIN autonomous underwater vehicle. We have found the following: the cross correlation technique always outperforms the other techniques in misregistrated areas caused by severe topographic changes. The split spectrum techniques are substantially faster than the cross correlation technique. The multichromatic approach has similar performance as the multiband split spectrum technique for the same choice of bands. We demonstrate that all the wideband interferometry techniques outperform a standard Goldstein's Branch cut phase unwrapping algorithm on real data from a complicated scene containing an elevated target and severe layover.
    [bibtex-key = saeboSynnesHansenTGRS2013WidebandInterferometryInSyntheticApertureSonar] [bibtex-entry]


  433. Manoochehr Shirzaei. A wavelet-based multitemporal DInSAR algorithm for monitoring ground surface motion. IEEE Geoscience and Remote Sensing Letters, 10(3):456-460, 2013. [bibtex-key = Shirzaei2013a] [bibtex-entry]


  434. Manoochehr Shirzaei and Roland Bürgmann. Time-dependent model of creep on the Hayward fault from joint inversion of 18 years of InSAR and surface creep data. Journal of Geophysical Research: Solid Earth, 118(4):1733-1746, 2013. [bibtex-key = Shirzaei2013] [bibtex-entry]


  435. Manoochehr Shirzaei, R Bürgmann, J Foster, TR Walter, and BA Brooks. Aseismic deformation across the Hilina fault system, Hawaii, revealed by wavelet analysis of InSAR and GPS time series. Earth and Planetary Science Letters, 376:12-19, 2013. [bibtex-key = Shirzaei2013b] [bibtex-entry]


  436. Manoochehr Shirzaei, Thomas R Walter, and Roland Bürgmann. Coupling of Hawaiian volcanoes only during overpressure condition. Geophysical Research Letters, 40(10):1994-1999, 2013. [bibtex-key = Shirzaei2013d] [bibtex-entry]


  437. Luigi Tosi, Pietro Teatini, and Tazio Strozzi. Natural versus anthropogenic subsidence of Venice. Scientific Reports, 3:2710-, September 2013. [bibtex-key = tosiTeatiniStrozziScientificRepNature2013SubsidenceVenice] [bibtex-entry]


  438. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. Fast Time-Domain Algorithms for UWB Bistatic SAR Processing. IEEE Trans. Aerosp. Electron. Syst., 49(3):1982-1994, July 2013. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Bistatic SAR, Bistatic Fast-Factorized Back-Projection, BiFFBP, radar cross-sections, radar imaging, synthetic aperture radar, time-domain analysis, ultra wideband radar, BiFFBP, CARABAS-II, UWB bistatic SAR, bistatic fast factorized backprojection, radar echo, radar signal processing, subaperture, subimage basis, synthetic aperture radar, time-domain algorithm, ultra wideband radar, ultra widebeam radar, Radar imaging, Receivers, Signal processing algorithms, Synthetic aperture radar, Time-domain analysis, Transmitters.
    Abstract: Two fast time-domain algorithms are introduced for ultrawideband-ultrawidebeam (UWB) bistatic synthetic aperture radar (SAR) processing; they are bistatic fast backprojection (BiFBP) and bistatic fast factorized backprojection (BiFFBP). Both algorithms process radar echoes on a subaperture and subimage basis in order to minimize processing time. They are shown to work with any configuration of bistatic SAR. They also own time-domain characteristics, which are essential for UWB radar signal processing. BiFBP and BiFFBP are experimented successfully on the CARABAS-II simulated data.
    [bibtex-key = vuSjogrenPetterssonTAES2013BiFFBP] [bibtex-entry]


  439. Viet Thuy Vu, Thomas K. Sjogren, and Pettersson. Phase Error Calculation for Fast Time-Domain Bistatic SAR Algorithms. IEEE Trans. Aerosp. Electron. Syst., 49(1):631-639, January 2013. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, TDBP, SAR image processing, Fast Factorized Back-Projection, FFBP, Bistatic SAR, Phase Error, Bistatic Fast Factorized Back-Projection, Motion Compensation, MoComp, Airborne SAR radar imaging, synthetic aperture radar, BiFFBP, SAR image quality, bistatic fast factorized backprojection, monostatic SAR, phase error calculation, synthetic aperture radar, time domain bistatic SAR, Apertures, Radar tracking, Receivers, Synthetic aperture radar, Target tracking, Transmitters CARABAS, LORA.
    Abstract: In synthetic aperture radar (SAR) processing, there is a trade-off between accuracy and speed. The approximations in an algorithm help to increase the algorithm's speed but cause deterministic phase errors which directly affect the SAR image quality. This paper discusses the phase error calculations for bistatic fast backprojection (BiFBP) and bistatic fast factorized backprojection (BiFFBP) which are essential for setting their parameters. The phase error calculation principle for bistatic SAR in comparison to monostatic SAR is presented. This principle is used to derive the maximum phase error equation.
    [bibtex-key = vuSjogrenPettersson2013BiFFBP] [bibtex-entry]


  440. Enes Yigit, Sevket Demirci, Caner Ozdemir, and Mustafa Tekbas. Short-range ground-based synthetic aperture radar imaging: performance comparison between frequency-wavenumber migration and back-projection algorithms. Journal of Applied Remote Sensing, 7(1):1-13, 2013. Keyword(s): GBSAR, SAR Processing, Ground-based SAR, Synthetic aperture radar, TDBP, Time-Domain Back-Projection, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm.
    Abstract: Two popular synthetic aperture radar (SAR) reconstruction algorithms, namely the back-projection (BP) and the frequency wavenumber (omega-k) algorithms, were tested and compared against each other, especially for their use in ground-based (GB) SAR applications directed to foreign object debris removal. For this purpose, an experimental setup in a semi-anechoic chamber room was accomplished to obtain near-field SAR images of objects on the ground. Then, the 90 to 95 GHz scattering data were acquired by using a stepped frequency continuous-wave radar operation. The performances of the setup and the imaging algorithms were then assessed by exploiting various metrics including point spread function, signal-to-clutter ratio, integrated side-lobe ratio, and computational complexity. Results demonstrate that although both algorithms produce almost accurate images of targets, the BP algorithm is shown to be superior to the omega-k algorithm due to its some inherent advantages specifically suited for short-range GB-SAR applications.
    [bibtex-key = yigitDemirciOzdemirTekbasJARS2013GBSAR] [bibtex-entry]


  441. Bing Yu, Guoxiang Liu, Zhilin Li, Rui Zhang, Hongguo Jia, Xiaowen Wang, and Guolin Cai. Subsidence detection by TerraSAR-X interferometry on a network of natural persistent scatterers and artificial corner reflectors. Computers & Geosciences, 58:126-136, 2013. [bibtex-key = yuGuoxiangLiZhangJiaWangCai2013SubsidenceTerraSARX] [bibtex-entry]


  442. Lei Zhang, Hao-lin Li, Zhi-Jun Qiao, Meng-Dao Xing, and Zheng Bao. Integrating Autofocus Techniques With Fast Factorized Back-Projection for High-Resolution Spotlight SAR Imaging. IEEE Geosci. Remote Sens. Lett., 10(6):1394-1398, November 2013. Keyword(s): SAR Processing, Fast-Factorized Back-Projection, FFBP, Time-Domain Back-Projection, TDBP, Fourier transforms, geophysical techniques, remote sensing by radar, synthetic aperture radar, BP coordinate, BP imagery, FFBP SA images, FFBP recursion, FFBP sub-aperture images, Fourier transform, MAMD algorithm, MAMD recursion, conventional autofocus techniques integration, fast factorized back-projection, high efficiency FFBP, high precision FFBP, high-resolution SAR imaging, high-resolution spotlight SAR imaging, high-resolution synthetic aperture radar imaging, novel multiple aperture map drift algorithm, phase correction, phase error function, range-compressed phase history data, raw data, real data experiments, well-focused imagery, Apertures, Azimuth, Doppler effect, Fourier transforms, History, Signal processing algorithms, Synthetic aperture radar, Autofocus, back-projection (BP), fast factorized back-projection (FFBP), multiple aperture map drift (MAMD), synthetic aperture radar (SAR).
    Abstract: Back-projection (BP) is considered as an ideal methodology for the high-resolution synthetic aperture radar (SAR) imaging. However, applying conventional autofocus techniques to BP imagery requires a special consideration and is usually difficult to implement. In this letter, we present a scheme to compatibly blending a novel multiple aperture map drift (MAMD) algorithm with fast factorized back-projection (FFBP). Through an elaborate BP coordinate, we construct the Fourier transform relationship between FFBP sub-aperture (SA) images and the corresponding range-compressed phase history data. The phase error function is achieved by the MAMD within FFBP recursions, and well-focused imagery is obtained by phase correction on the range-compressed phase history data. The proposed scheme inherits the advantages of high precision and efficiency of the FFBP, and is suitable for high-resolution spotlight SAR imaging with raw data. Real data experiments guarantee the effectiveness of our proposed scheme.
    [bibtex-key = zhangLiQiaoXingBaoGRSL2013AutofocusFFBP] [bibtex-entry]


  443. D. Zhu, R. Jiang, X. Mao, and Z. Zhu. Multi-Subaperture PGA for SAR Autofocusing. IEEE Transactions on Aerospace and Electronic Systems, 49(1):468-488, January 2013. Keyword(s): SAR Processing, Autofocus, synthetic aperture radar, SAR autofocusing, spotlight mode synthetic aperture radar, full-aperture phase gradient autofocus, PGA algorithm, high-order phase error, residual range cell migration, RCM, coherent processing interval, stripmap data, multisubaperture PGA algorithm, map drift technique, subaperture phase error, PGA-MD, Electronics packaging, Synthetic aperture radar, Azimuth, Image resolution, Accuracy, Polynomials, Estimation.
    Abstract: For spotlight mode synthetic aperture radar (SAR) autofocusing, the traditional full-aperture phase gradient autofocus (PGA) algorithm might suffer from performance degradation in the presence of significant high-order phase error and residual range cell migration (RCM), which tend to occur when the coherent processing interval (CPI) is long. Meanwhile, PGA does not perform satisfactorily when applied directly on the stripmap data. To address these shortcomings, we present a multi-subaperture PGA algorithm, which takes advantage of the map drift (MD) technique. It smoothly incorporates the estimation of residual RCM and combines the subaperture phase error (SPE) estimated by PGA in a very precise manner. The methodology and accuracy of PGA-MD are investigated in detail. Experimental results indicate the effectiveness of PGA-MD in both the spotlight and the stripmap modes.
    [bibtex-key = zhuJiangMaoZhuTAES2013MultiSubaperturePGAforSARAutofocusing] [bibtex-entry]


  444. Esteban Aguilera, Matteo Nannini, and Andreas Reigber. Multisignal Compressed Sensing for Polarimetric SAR Tomography. IEEE Geosci. Remote Sens. Lett., 5(9):871-875, September 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressed sensing, Remote sensing, Sensors, Tomography, Vectors, compressed sensing, compressive sensing, CS, geometry, image reconstruction, image sensors, radar imaging, radar polarimetry, synthetic aperture radar, tomography, 3D imaging, DCS, E-SAR sensor, German Aerospace Center, azimuth-range pixel, data collection processing, distributed compressed sensing, multisignal compressed sensing, parallel track, polarimetric L-band data, polarimetric SAR sensor, polarimetric SAR tomography, polarimetric channel, polarimetric synthetic aperture radar sensor, repeat-pass acquisition geometry, signal reconstruction, temporal baseline, tomographic signal, vertical reflectivity function, Compressed sensing (CS), distributed compressed sensing (DCS), polarimetry, synthetic aperture radar (SAR) tomography, L-band, E-SAR, F-SAR.
    Abstract: In recent years, 3-D imaging by means of polarimetric synthetic aperture radar (SAR) sensors has become a field of intensive research. In SAR tomography, the vertical reflectivity function for every azimuth /range pixel is usually recovered by processing data collected using a defined repeat-pass acquisition geometry. The most common approach is to generate a synthetic aperture in the elevation direction through imaging from a large number of parallel tracks. This imaging technique is appealing, since it is very simple. However, it has the drawback that large temporal baselines can severely affect the reconstruction. In an attempt to reduce the number of parallel tracks, we propose a new approach that exploits structural correlations between neighboring azimuth-range pixels and/or polarimetric channels. As a matter of fact, this can be done under the framework of distributed compressed sensing (CS) (DCS), which stems from CS theory, thus also exploiting sparsity in the tomographic signal. Finally, results demonstrating the potential of the DCS methodology will be validated by using fully polarimetric L-band data acquired by the E-SAR sensor of the German Aerospace Center (DLR).
    [bibtex-key = aguileraNanniniReigberGRSL2012] [bibtex-entry]


  445. J. N. Ash. An Autofocus Method for Backprojection Imagery in Synthetic Aperture Radar. IEEE Geoscience and Remote Sensing Letters, 9(1):104-108, January 2012. Keyword(s): SAR Processing, Autofocus, autoregressive processes, convolution, digital elevation models, geophysical image processing, geophysical techniques, radar imaging, synthetic aperture radar, convolution backprojection, autofocus method, backprojection imagery, autofocus routine, spotlight-mode synthetic aperture radar data, image sharpness, imaging geometry, wide-angle aperture, digital elevation map, image-quality-based autofocus approach, natural geometric interpretation, optimal single-pulse phase correction, quartic polynomial, sequential imaging application, autoregressive backprojection, Synthetic aperture radar, Imaging, Apertures, Measurement, Optimization, History, Azimuth, Autofocus, autoregressive backprojection (ARBP), convolution BP (CBP) imaging, synthetic aperture radar (SAR).
    Abstract: In this letter, we present an autofocus routine for backprojection imagery from spotlight-mode synthetic aperture radar data. The approach is based on maximizing image sharpness and supports the flexible collection and imaging geometries of BP, including wide-angle apertures and the ability to image directly onto a digital elevation map. While image-quality-based autofocus approaches can be computationally intensive, in the backprojection setting, we demonstrate a natural geometric interpretation that allows for optimal single-pulse phase corrections to be derived in closed form as the solution of a quartic polynomial. The approach is applicable to focusing standard backprojection imagery, as well as providing incremental focusing in sequential imaging applications based on autoregressive backprojection. An example demonstrates the efficacy of the approach applied to real data for a wide-aperture backprojection image.
    [bibtex-key = ashGRSL2012AutofocusForBackprojectionImageryInSAR] [bibtex-entry]


  446. A. E. A. Blomberg, A. Austeng, and Roy E. Hansen. Adaptive Beamforming Applied to a Cylindrical Sonar Array Using an Interpolated Array Transformation. IEEE Journal of Oceanic Engineering, 37(1):25-34, January 2012. Keyword(s): Synthetic Aperture Sonar, SAS, array signal processing, interpolation, sonar arrays, sonar signal processing, SX90 fish finding sonar, adaptive beamforming, adaptive minimum variance, amplitude estimation, azimuth resolution, cylindrical sonar array, interference rejection, interpolated array transformation, phase estimation, virtual linear array, Array signal processing, Arrays, Covariance matrix, Signal resolution, Sonar applications, Vectors, Adaptive signal processing, array signal processing, beam steering, imaging, sonar, spatial resolution.
    Abstract: In applications such as fishery sonar and navigation, cylindrical or spherical arrays are often used because of the need for a 360deg field of view. However, adaptive beamforming methods, known for their high angular resolution and interference rejection capabilities, often rely on a Vandermonde structure of the steering vectors. This is generally not the case for nonlinear arrays. In this paper, we use an interpolated array transformation to map the data to a virtual linear array before adaptive beamforming. We evaluate the performance of two different adaptive beam- formers using simulations as well as experimental data from the SX90 fish finding sonar. We show that the adaptive minimum variance (MV) and amplitude and phase estimation of a sinusoid (APES) beamformers offer a significant improvement in azimuth resolution compared to the conventional delay-and-sum (DAS) beamformer. The APES beamformer offers slightly more reliable amplitude estimates in the direction of interest compared to the MV beamformer, at the cost of a somewhat lower azimuth resolution. When applied to data from the SX90 fish finding sonar, the MV beamformer offers a 40%-50% improvement in resolution, while the APES beamformer offers an improvement of 20%-30%.
    [bibtex-key = blombergAustengHansenJOE2012AdaptiveBeamformingCylindricalSonarArray] [bibtex-entry]


  447. Federica Bordoni, Marwan Younis, and Gerhard Krieger. Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems. IEEE Trans. Geosci. Remote Sens., 50(2):617-629, February 2012. Keyword(s): SAR Processing, APC technique, ambiguity suppression, azimuth phase coding, digital beamforming, flexible SAR systems, multichannel SAR systems, multiple transmit-receive channels, numerical analysis, reflector antenna, spaceborne SAR systems, spaceborne synthetic aperture radar systems, array signal processing, numerical analysis, phase coding, radar signal processing, remote sensing by radar, synthetic aperture radar.
    Abstract: The current generation of spaceborne synthetic aperture radar (SAR) systems suffers from a tradeoff between the achievable spatial resolution and swath width. This has motivated intensive research both on more flexible SAR systems, using multiple transmit/receive channels, and on techniques for removing the ambiguities. Among these techniques, the azimuth phase coding (APC), recently proposed to suppress range ambiguities in conventional SAR systems, stands out for its negligible implementation complexity and its effectiveness for point and distributed ambiguities. This paper investigates the possibility of applying the APC technique to the new, forthcoming generation of multichannel SAR systems, based on digital beamforming on receive. The extension of APC to multichannel SAR systems is mathematically described. Specific merit figures are defined to quantify the APC performance. A numerical analysis is developed to characterize the influence on the APC behaviors of the main SAR system parameters. Finally, an example of APC performance is provided, by considering two multichannel SAR systems based on a planar and a reflector antenna.
    [bibtex-key = bordoniYounisKrieger2012] [bibtex-entry]


  448. A. Budillon, A. Evangelista, and G. Schirinzi. GLRT Detection of Moving Targets via Multibaseline Along-Track Interferometric SAR Systems. IEEE Geosci. Remote Sens. Lett., 9(3):348-352, May 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, ATI, MTI, Along-track interferometry, Moving target indication, Multibaseline InSAR, InSAR, SAR Interferometry.
    Abstract: Along-track interferometric synthetic aperture radar systems can be used for ground moving target indication. We analyze a scheme for detecting moving targets with unknown parameters (velocity and signal-to-clutter ratio) and with constant false-alarm rates, based on the generalized likelihood ratio test (GLRT), and adopting a Gaussian model for target and clutter signals. We compare its performance with the one obtained in the ideal case of known target parameters applying the likelihood ratio test (LRT). A closed form for the LRT receiver operating characteristic is derived and used as reference for GLRT performance assessment. The analysis is carried out on simulated TerraSAR-X data.
    [bibtex-key = 6058583] [bibtex-entry]


  449. Hermann Bähr and Ramon F. Hanssen. Reliable estimation of orbit errors in spaceborne SAR interferometry. Journal of Geodesy, 86(12):1147-1164, 2012. Keyword(s): SAR Processing, Interferometry, Baseline Errors, orbital errors, SAR Interferometry, Spaceborne SAR.
    Abstract: An approach to improve orbital state vectors by orbit error estimates derived from residual phase patterns in synthetic aperture radar interferograms is presented. For individual interferograms, an error representation by two parameters is motivated: the baseline error in cross-range and the rate of change of the baseline error in range. For their estimation, two alternatives are proposed: a least squares approach that requires prior unwrapping and a less reliable gridsearch method handling the wrapped phase. In both cases, reliability is enhanced by mutual control of error estimates in an overdetermined network of linearly dependent interferometric combinations of images. Thus, systematic biases, e.g., due to unwrapping errors, can be detected and iteratively eliminated. Regularising the solution by a minimum-norm condition results in quasi-absolute orbit errors that refer to particular images. For the 31 images of a sample ENVISAT dataset, orbit corrections with a mutual consistency on the millimetre level have been inferred from 163 interferograms. The method itself qualifies by reliability and rigorous geometric modelling of the orbital error signal but does not consider interfering large scale deformation effects. However, a separation may be feasible in a combined processing with persistent scatterer approaches or by temporal filtering of the estimates.
    [bibtex-key = baehrHanssenJournalOfGeodesy2021ReliableEstimationOfOrbitErrorsInSpaceborneSARInterferometry] [bibtex-entry]


  450. Karlus A. Câmara de Macedo, Christian Wimmer, T.L.M. Barreto, Dieter Lubeck, J.R. Moreira, L.M.L. Rabaco, and W.J. de Oliveira. Long-Term Airborne DInSAR Measurements at X- and P-Bands: A Case Study on the Application of Surveying Geohazard Threats to Pipelines. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 5(3):990-1005, June 2012. Keyword(s): SAR Processing, Deformation measurements, displacement measurements, DInSAR, differential interferometry, Airborne SAR, X-band, P-Band, OrbiSAR, Orbisat, BRADAR, geohazards, hazards, motion measurement, pipelines, radar interferometry, remote sensing by radar, surveying, synthetic aperture radar, time series, Brazil, OrbiSAR system, OrbiSat, P-band airborne DInSAR, PROTRAN, Petrobras, Sao Sebastiao, X-band airborne DInSAR, airborne DInSAR data, geodynamic processes, geohazard risk monitoring, geohazard risks, land movement measurement, long term airborne DInSAR measurements, pipeline geohazard threat surveying, pipeline structural damage, residual motion errors, time series analysis, Antenna measurements, Antennas, Global Positioning System, Interferometry, Pipelines, Radar tracking, Synthetic aperture radar, Airborne, P-band, X-band, differential interferometry (DInSAR), geohazards.
    Abstract: Recently some experiments demonstrated that reliable DInSAR measurements can be achieved at any band and that further time-series analyses can be applied to airborne data. However, most of the airborne DInSAR results, including the time-series, published so far have focused on short-term analyses (baselines within hours or few days). This paper presents the first long-term airborne DInSAR survey at P- and X-band with temporal baselines of 1 year and 3 months. The survey was performed by the OrbiSAR system of OrbiSat under contract with Petrobras (PROTRAN), as part of a project to investigate the potential of DInSAR technique to identify, prior to structural damage, geohazards threats to the oil/gas pipelines in Sao Sebastiao-SP, Brazil. After 1 year and 3 months, the P-band data have interferometric coherence equal or greater than 0.3 for approximately 80% of the imaged dense vegetated areas. At X-band, we achieved coherence equal or greater than 0.3 in the urban area. After removing all residual motion errors with proper up-to-date processing, we derived, for the P-band data, land movements with absolute accuracy in the order of centimeters. For X-band we derived land movement measurements with absolute accuracy in the order of millimeters. Through field work evidences, the paper analyses the causes of these centimeter to millimeter land movements, and how they are related to geodynamic processes and geohazard risks. A comparison between the DInSAR and in-loco inclinometer measurements is presented. The paper suggests some possible operational scenarios and discusses on the potential of the airborne DInSAR for geohazard risk monitoring.
    [bibtex-key = deMacedoWimmerBarretoLubeckMoreiraRabacoOliveiraJSTARS2012AirborneDINSAR] [bibtex-entry]


  451. A. Elsherbini and K. Sarabandi. Compact Directive Ultra-Wideband Rectangular Waveguide Based Antenna for Radar and Communication Applications. IEEE_J_AP, 60(5):2203-2209, May 2012. Keyword(s): UHF antennas, antenna feeds, baluns, microwave antennas, planar antennas, radar antennas, ultra wideband antennas, ultra wideband communication, ultra wideband radar, waveguide antennas, VSWR, aperture field distribution, balun structure, communication applications, compact directive ultra-wideband rectangular waveguide based antenna, cross-polarization performance, directive ultra-wideband antenna, fabrication tolerances, frequency 1.08 GHz to 4.9 GHz, integrated balun design, planar antenna aperture, polarizer, radar applications, rectangular waveguide antenna feeding structure, rectangular waveguide aperture antennas, standard balun, waveguide mode, Apertures, Directive antennas, Feeds, Impedance matching, Radar antennas, Ultra wideband antennas, Aperture antennas, UWB antennas, directive antennas. [bibtex-key = Elsherbini2012a] [bibtex-entry]


  452. Gianfranco Fornaro, Diego Reale, and Simona Verde. Potential of SAR for monitoring transportation infrastructures: an analysis with the multi-dimensional imaging technique. Journal of Geophysics and Engineering, 9(4):S1, 2012. Keyword(s): SAR Processing, SAR interferometry, Interferometry, differential interferometry, InSAR, DInSAR, Persistent Scatterer Interferometry, PSI, SAR Tomography, Tomography, Spaceborne SAR, TerraSAR-X, X-band.
    Abstract: Differential interferometric synthetic aperture radar (SAR) has proven to be effective for accurate localization and monitoring of the displacement of ground targets. The high accuracy and spatial density of the measurements make this technique cost effective compared to the classical geodetic techniques typically used in the risk monitoring context. Ground infrastructure monitoring is typically carried out with in situ sensors. The new generation of high-resolution SAR sensors, however, allows one to acquire data sets with a spatial resolution reaching metric/submetric values. Here we investigate the application of a multi-dimensional SAR imaging technique, which is an extension of classical differential interferometric techniques, to very high resolution TerraSAR-X data in order to demonstrate the potential of this technology for monitoring of transportation infrastructures.
    [bibtex-key = fornaroRealeVerde2012] [bibtex-entry]


  453. P. Frolind, A. Gustavsson, M. Lundberg, and L. M. H. Ulander. Circular-Aperture VHF-Band Synthetic Aperture Radar for Detection of Vehicles in Forest Concealment. IEEE Transactions on Geoscience and Remote Sensing, 50(4):1329-1339, April 2012. Keyword(s): focusing, image resolution, radar imaging, synthetic aperture radar, VHF devices, circular-aperture VHF-band synthetic aperture radar, vehicles, dense forest concealment, image resolution, object information, signal processing, imaging geometry, high-quality image focusing, contrast optimization, false-alarm rate, detection probability, frequency 20 MHz to 90 MHz, Focusing, Vehicles, Apertures, Radar tracking, Image resolution, Geometry, Circular aperture, forest concealment, synthetic aperture radar (SAR), target detection, very high frequency (VHF).
    Abstract: Circular-aperture synthetic aperture radar (SAR) imaging has been evaluated using the airborne very high frequency (VHF) band Coherent All RAdio BAnd Sensing (CARABAS)-II system (20-90 MHz). Images, as well as results, from detection of vehicles in dense forest concealment have been compared with linear-aperture SAR. Circular-aperture SAR imaging provides higher image resolution and increased object information, but complexity of signal processing and requirements on imaging geometry accuracy increases. The latter is, however, partly mitigated by using low frequencies in the VHF band. A high-quality digital elevation model is used to ensure high-quality image focusing and to avoid distorting object shape. Contrast optimization is used to reduce global focusing errors. The image resolution is observed to be about 1 m2 in agreement with theoretical predictions. Detection performance has been evaluated using image data from a full circular synthetic aperture, i.e., 360 deg of aspect angle variation. Results, both for single-pass detection (SPD) and change detection (CD), show a considerable advantage compared with detection based on linear-aperture SAR. The detection probability for SPD increases from 0.4 to 0.8 at a false-alarm rate (FAR) of 30/km2. For CD, the detection probability increases from 0.7 to 0.9 at a FAR of 2/km2.
    [bibtex-key = frolindGustavssonLundbergUlanderTGRS2012CircularVHFSARForDetectionofVehiclesInForest] [bibtex-entry]


  454. S. Gernhardt, Xiaoying Cong, M. Eineder, S. Hinz, and R. Bamler. Geometrical Fusion of Multitrack PS Point Clouds. IEEE Geoscience and Remote Sensing Letters, 9(1):38-42, January 2012. Keyword(s): COSMO-SkyMed satellite, TerraSAR-X satellite, geocoded PS geometrical fusion, high-resolution spotlight data, multitrack PS point clouds, nonurban areas, persistent scatterer positions, radar satellites, synthetic aperture radar images, geophysical image processing, remote sensing by radar, synthetic aperture radar;.
    Abstract: Recent radar satellites like TerraSAR-X and COSMO-SkyMed deliver very high resolution synthetic aperture radar images at a spatial resolution of less than 1 m. Persistent scatterer (PS) positions obtained from stacks of high-resolution spotlight data show very much details of buildings and other structures in 3-D due to the enormous amount of PS obtainable from data of this resolution class. As soon as more than one stack covering the same area is available, a combination of the results is eligible. However, geocoded PSs cannot be simply united due to residual offsets in their absolute positions which stem from unknown absolute height values of the different reference points chosen when processing the individual stacks independently. In this letter, two different methods for a geometrical fusion of geocoded PSs from stacks acquired at different aspect and incidence angles are presented. The algorithms are applied to PS interferometry results of both urban and nonurban areas.
    [bibtex-key = gernhardtCongEinederHinzBamler2012] [bibtex-entry]


  455. Kanika Goel and Nico Adam. An advanced algorithm for deformation estimation in non-urban areas. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):100 - 110, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, DInSAR, Adaptive spatial phase filtering, Distributed scatterer (DS), L1-norm minimization, Singular Value Decompostion, SVD, L2-norm minimization, Small Baseline Subset Algorithm, SBAS, TerraSAR-X, Spaceborne SAR, X-band.
    Abstract: This paper presents an advanced differential SAR interferometry stacking algorithm for high resolution deformation monitoring in non-urban areas with a focus on distributed scatterers (DSs). Techniques such as the Small Baseline Subset Algorithm (SBAS) have been proposed for processing DSs. SBAS makes use of small baseline differential interferogram subsets. Singular value decomposition (SVD), i.e. L2 norm minimization is applied to link independent subsets separated by large baselines. However, the interferograms used in SBAS are multilooked using a rectangular window to reduce phase noise caused for instance by temporal decorrelation, resulting in a loss of resolution and the superposition of topography and deformation signals from different objects. Moreover, these have to be individually phase unwrapped and this can be especially difficult in natural terrains. An improved deformation estimation technique is presented here which exploits high resolution SAR data and is suitable for rural areas. The implemented method makes use of small baseline differential interferograms and incorporates an object adaptive spatial phase filtering and residual topography removal for an accurate phase and coherence estimation, while preserving the high resolution provided by modern satellites. This is followed by retrieval of deformation via the SBAS approach, wherein, the phase inversion is performed using an L1 norm minimization which is more robust to the typical phase unwrapping errors encountered in non-urban areas. Meter resolution TerraSAR-X data of an underground gas storage reservoir in Germany is used for demonstrating the effectiveness of this newly developed technique in rural areas
    [bibtex-key = goelAdamISPRSJ2012] [bibtex-entry]


  456. Jaime Hueso González, John Mohan Walter Antony, Markus Bachmann, Gerhard Krieger, Manfred Zink, Dirk Schrank, and Marco Schwerdt. Bistatic system and baseline calibration in TanDEM-X to ensure the global digital elevation model quality. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):3-11, 2012. Keyword(s): SAR Processing, Accuracy, Calibration, Interferometry, SAR Interferometry, InSAR, DInSAR, TerraSAR-X, Spaceborne SAR, X-band.
    Abstract: TanDEM-X is an operational satellite mission with the goal of generating a high quality global digital elevation model (DEM) based on synthetic aperture radar (SAR) interferometry in X-band. In order to ensure the quality of the DEM, the differential range measurements and knowledge of the interferometric baseline have to be extremely accurate. In this paper, the bistatic system calibration strategy implemented in TanDEM-X to achieve the desired DEM quality will be described, focusing on the baseline calibration procedure. The results of the tests, which were performed in parallel to the operational DEM acquisition, verify the suitability of this approach
    [bibtex-key = HuesoGonzalezEtAlISPRSJ2012] [bibtex-entry]


  457. Astrid Gruber, Birgit Wessel, Martin Huber, and Achim Roth. Operational TanDEM-X DEM calibration and first validation results. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):39 - 49, 2012. Keyword(s): SAR Processing, SAR interferometry, Interferometry, InSAR, Block adjustment, DEM, Calibration, TanDEM-X, Spaceborne SAR, X-band.
    Abstract: In June 2010, the German TanDEM-X satellite was launched. Together with its twin satellite TerraSAR-X it flies in a close formation enabling single-pass SAR interferometry. The primary goal of the TanDEM-X mission is the derivation of a global digital elevation model (DEM) with unprecedented global accuracies of 10m in absolute and 2m in relative height. A significant calibration effort is required to achieve this high quality world-wide. In spite of an intensive instrument calibration and a highly accurate orbit and baseline determination, some systematic height errors like offsets and tilts in the order of some meters remain in the interferometric DEMs and have to be determined and removed during the TanDEM-X DEM calibration. The objective of this article is the presentation of an approach for the estimation of correction parameters for remaining systematic height errors applicable to interferometric height models. The approach is based on a least-squares block adjustment using the elevation of ICESat GLA14 data as ground control points and connecting points of adjacent, overlapping DEMs as tie-points. In the first part its implementation in DLR's ground segment is outlined. In the second part the approach is applied and validated for two of the first TanDEM-X DEM test sites. Therefore, independent reference data, in particular high resolution reference DEMs and GPS tracks, are used. The results show that the absolute height errors of the TanDEM-X DEM are small in these cases, mostly in the order of 1-2m. An additional benefit of the proposed block adjustment method is that it improves the relative accuracy of adjacent DEMs
    [bibtex-key = gruberWesselHuberRothISPRSJ2012] [bibtex-entry]


  458. Andrew Hooper, David Bekaert, Karsten Spaans, and Mahmut Arikan. Recent advances in SAR interferometry time series analysis for measuring crustal deformation. Tectonophysics, 514-517:1-13, 2012. Keyword(s): Time series InSAR, PS-InSAR, Small baseline InSAR, Crustal deformation, Eyjafjallajoekull, Guerrero slow slip.
    Abstract: Synthetic aperture radar (SAR) interferometry is a technique that permits remote detection of deformation at the Earth's surface, and has been used extensively to measure displacements associated with earthquakes, volcanic activity and many other crustal deformation phenomena. Analysis of a time series of SAR images extends the area where interferometry can be successfully applied, and also allows detection of smaller displacements, through the reduction of error sources. Here, we review recent advances in time series SAR interferometry methods that further improve accuracy. This is particularly important when constraining displacements due to processes with low strain rates, such as interseismic deformation. We include examples of improved algorithms applied to image deformation associated with the 2010 eruption of Eyjafjallajoekull volcano in Iceland, slow slip on the Guerrero subduction zone in Mexico, and tectonic deformation in western Anatolia, Turkey.
    [bibtex-key = hooperBekaertSpaansArikan2012InSARTimeSeriesAnalysisMeasuringCrustalDeformation] [bibtex-entry]


  459. Yue Huang, Laurant Ferro-Famil, and Andreas Reigber. Under-Foliage Object Imaging Using SAR Tomography and Polarimetric Spectral Estimators. IEEE Trans. Geosci. Remote Sens., 50(6):2213-2225, June 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography.
    Abstract: This paper addresses the imaging of objects located under a forest cover using polarimetric synthetic aperture radar tomography (POLTOMSAR) at L-band. High-resolution spectral estimators, able to accurately discriminate multiple scattering centers in the vertical direction, are used to separate the response of objects and vehicles embedded in a volumetric background. A new polarimetric spectral analysis technique is introduced and is shown to improve the estimation accuracy of the vertical position of both artificial scatterers and natural environments. This approach provides optimal polarimetric features that may be used to further characterize the objects under analysis. The effectiveness of this novel technique for POLTOMSAR is demonstrated using fully polarimetric L-band airborne data sets acquired by the German Aerospace Center (DLR)'s E-SAR system over the test site in Dornstetten, Germany.
    [bibtex-key = huangFerroFamilReigberTGRS2012UnderFoliageObjectImagingUsingSARTomoAndPolSpectralEstimators] [bibtex-entry]


  460. Susanne Kunis and Stefan Kunis. The nonequispaced FFT on graphics processing units. PAMM, 12(1):7-10, 2012. Keyword(s): Non-equispaced Fourier Transform, Non-equispaced FFT, NFFT, Fast Fourier Transform, FFT, CUDA, GPU, CUNFFT.
    Abstract: Without doubt, the fast Fourier transform (FFT) belongs to the algorithms with large impact on science and engineering. By appropriate approximations, this scheme has been generalized for arbitrary spatial sampling points. This so called nonequispaced FFT is the core of the sequential NFFT3 library and we discuss its computational costs in detail. On the other hand, programmable graphics processing units have evolved into highly parallel, multithreaded, manycore processors with enormous computational capacity and very high memory bandwidth. By means of the so called Compute Unified Device Architecture (CUDA), we parallelized the nonequispaced FFT using the CUDA FFT library and a dedicated parallelization of the approximation scheme.
    [bibtex-key = kunisKunisPAMM2012NFFTCUDA] [bibtex-entry]


  461. Hengxing Lan, Langping Li, Hongjiang Liu, and Zhihua Yang. Complex Urban Infrastructure Deformation Monitoring Using High Resolution PSI. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 5(2):643-651, April 2012. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, China, building infrastructures, complex urban infrastructure deformation monitoring, economic center, engineering geological settings, engineering structures, geometries, ground surface subsidence, high resolution PSI, intensive infrastructure development, persistent scatterer interferometry technology, spotlight mode TerraSAR-X images, thermal dilation, transportation infrastructures, urban vulnerability, building, computerised monitoring, deformation, geophysical image processing, remote sensing;.
    Abstract: The rising concern on the urban vulnerability to the intensive infrastructure development requires enabling technologies offering a prompt and accurate monitoring of urban infrastructures deformation. Urban infrastructures vary dramatically both spatially and temporally and their deformation characteristics are complex. We evaluated the potential of high resolution Persistent Scatterer Interferometry (PSI) technology using coherent stacks of Spotlight mode TerraSAR-X images in monitoring the deformations of different types of infrastructures in a new economics center in China. The high density of Persistent Scatterers (PSs) was identified and therefore facilitates analyzing the deformation character of individual structures. All PSs weer categorized by identifying their corresponding ground object so as to enable to characterize deformation pattern of certain type of urban infrastructure. The spatial and temporal varying patterns of the deformations of typical building infrastructures and transportation infrastructures are revealed. They are strongly related to the interactive effects between the types, engineering structures, geometries, engineering geological settings and various loading scenarios. Besides subsidence of the ground surface, thermal dilation of the infrastructure itself might be another factor accounting for the observed deformation of infrastructure. Although the interpretation for the observed deformation patterns could be quite site-specific, high resolution PSI is shown to have the potential to reveal detailed deformation characteristics of complex urban infrastructures at a relatively large scale.
    [bibtex-key = lanLiLiuZang2012PSI] [bibtex-entry]


  462. Yake Li, Chang Liu, Yanfei Wang, and Qi Wang. A Robust Motion Error Estimation Method Based on Raw Data. IEEE Trans. Geosci. Remote Sens., 50(7):2780-2790, 2012. Keyword(s): SAR Processing, Autofocus, SAR Autofocus, MoComp, Motion Compensation, curve fitting, geophysical image processing, least squares approximations, motion compensation, radar imaging, remote sensing by radar, synthetic aperture radar, RCMC, aircraft reference track deviations, curve fitting, double phase gradient estimation, filtering method, high order motion errors, high precision navigation system, high resolution airborne SAR systems, high resolution imagery, image processing, image quality, large swath mode, light aircraft SAR platform, motion compensation, motion error estimation method, range cell migration correction, range dependent phase errors, range resolution improvement, raw data, synthetic aperture radar, weighted total least square method, Aircraft, Azimuth, Electronics packaging, Error analysis, Estimation, Robustness, Trajectory, Autofocus, motion error estimation, phase gradient filtering, synthetic aperture radar (SAR), weighted total least square (WTLS) method.
    Abstract: High-resolution airborne synthetic aperture radar (SAR) systems are very sensible to deviations of the aircraft from the reference track. In high-resolution imagery, the improvement of range resolution increases the difficulty of implementing range cell migration correction (RCMC), while a wider synthetic aperture increases the cumulative time of motion errors which will affect the image quality. To enable accurate motion compensation in image processing, a high-precision navigation system is needed. However, in many cases, due to the limit of accuracy of such systems, motion errors are hard to be compensated correctly, causing mainly the resolution decrease in final image. Moreover, in large swath mode, the range-dependent phase errors are difficult to be compensated by using the conventional autofocus algorithm only. In this paper, we propose a robust motion error estimation method based on raw SAR data. To apply this estimation method, we first estimate the double phase gradients in subaperture. Second, a filtering method based on curve fitting was proposed to reduce the phase estimation errors caused by low signal-to-clutter ratio (SCR). Finally, we propose a weighted total least square method to calculate the motion errors using the filtered phase gradients. Because the proposed algorithm is nonparametric, it can estimate high-order motion errors. This is very important for the airborne SAR, particularly the light aircraft SAR platform, due to their more complicated movement in air turbulence. The versatility that the proposed method can be used in any imaging algorithms is another advantage. The processing of large number of raw SAR data shows that the algorithm is as robust and practical as phase gradient autofocus and can generate better focused images.
    [bibtex-key = liLiuWangWang2012Autofocus] [bibtex-entry]


  463. Z. W. Li, W. B. Xu, G. C. Feng, J. Hu, C. C. Wang, X. L. Ding, and J. J. Zhu. Correcting atmospheric effects on InSAR with MERIS water vapour data and elevation-dependent interpolation model. Geophysical Journal International, 189(2):898-910, 2012. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Displacement, Surface Displacement, Atmosphere, APS, MERIS, MERIS water vapour data, water vapour, Image processing, Satellite geodesy, Radar interferometry, Creep and deformation, Wave propagation.
    Abstract: The propagation delay when radar signals travel from the troposphere has been one of the major limitations for the applications of high precision repeat-pass Interferometric Synthetic Aperture Radar (InSAR). In this paper, we first present an elevation-dependent atmospheric correction model for Advanced Synthetic Aperture Radar (ASAR: the instrument aboard the ENVISAT satellite) interferograms with Medium Resolution Imaging Spectrometer (MERIS) integrated water vapour (IWV) data. Then, using four ASAR interferometric pairs over Southern California as examples, we conduct the atmospheric correction experiments with cloud-free MERIS IWV data. The results show that after the correction the rms differences between InSAR and GPS have reduced by 69.6 per cent, 29 per cent, 31.8 per cent and 23.3 per cent, respectively for the four selected interferograms, with an average improvement of 38.4 per cent. Most importantly, after the correction, six distinct deformation areas have been identified, that is, Long Beach, Santa Ana Basin, Pomona, Ontario, San Bernardino and Elsinore basin, with the deformation velocities along the radar line-of-sight (LOS) direction ranging from 20 mm/yr to 30 mm/yr and on average around 25 mm/yr and Santa Fe Springs and Wilmington, with a slightly low deformation rate of about 10 mm/yr along LOS. Finally, through the method of stacking, we generate a mean deformation velocity map of Los Angeles over a period of 5 yr. The deformation is quite consistent with the historical deformation of the area. Thus, using the cloud-free MERIS IWV data correcting synchronized ASAR interferograms can significantly reduce the atmospheric effects in the interferograms and further better capture the ground deformation and other geophysical signals.
    [bibtex-key = liXuFengHuWangDingZhuGJI2012InSARAtmosphereMERIS] [bibtex-entry]


  464. F. Lombardini and M. Pardini. Superresolution Differential Tomography: Experiments on Identification of Multiple Scatterers in Spaceborne SAR Data. IEEE Trans. Geosci. Remote Sens., 50(4):1117-1129, April 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography.
    Abstract: Interest is growing in the application of coherent processing of synthetic aperture radar (SAR) data to the monitoring of complex urban or infrastructure areas. However, such scenarios are characterized by the layover phenomenon, in the presence of which conventional interferometric SAR techniques degrade or cannot operate. As a consequence, to monitor reliably a high number of ground structures, the identification, i.e., the detection and height and deformation velocity estimation, of both single and multiple scatterers interfering in the same SAR cell can be a key step. This issue is addressed here by means of differential tomography (Diff-Tomo), a recent multibaseline-multitemporal generalized interferometric framework which allows to resolve multiple moving scatterers at different heights in the same cell. In particular, superresolution adaptive Diff-Tomo is extensively tested and augmented with a new information extraction algorithm for the automated identification of the multiple scatterers. Experiments have been carried out with real C-band spaceborne data over urban areas; corresponding results are shown and discussed.
    [bibtex-key = lombardiniPardiniTGRS2011] [bibtex-entry]


  465. Mauro Mariotti d'Alessandro and Stefano Tebaldini. Phenomenology of P-Band Scattering From a Tropical Forest Through Three-Dimensional SAR Tomography. IEEE Geosci. Remote Sens. Lett., 9(3):442-446, May 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, P-Band, Airborne SAR, Paracou, French Guyana, ONERA, SETHI, TropiSAR, ESA.
    Abstract: The aim of this letter is to discuss recent results from the tomographic analysis of the P-band synthetic aperture radar multibaseline data set acquired by ONERA over French Guyana, in the frame of the European Space Agency campaign TropiSAR. Such a data set is characterized by a vertical resolution of about 20 m, whereas forest height ranges from 20 to 40 m. These features make it possible to map the 3-D distribution of the scene complex reflectivity in up to three independent layers by coherent focusing, i.e., without assuming any physical model or employing superresolution techniques. The most relevant features within the observed results are the presence of dihedral-like scattering in the ground layer, which is hardly noticeable in the original single-look complex data, and the substantial invariance of the innermost forest layer to topographic slopes.
    [bibtex-key = mariottidAlessandroTebaldiniGRSL2012] [bibtex-entry]


  466. Michele Martone, Benjamin Bräutigam, Paola Rizzoli, Carolina Gonzalez, Markus Bachmann, and Gerhard Krieger. Coherence evaluation of TanDEM-X interferometric data. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):21 - 29, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Synthetic Aperture Radar (SAR), Bistatic SAR, Coherence, Digital elevation model, DEM, TanDEM-X, Spaceborne SAR, X-band.
    Abstract: The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission comprises two nearly identical satellites: TerraSAR-X (TSX, launched in 2007), and TanDEM-X (TDX, launched in June 2010). The primary objective of the mission is to generate a worldwide and consistent digital elevation model (DEM) with an unprecedented accuracy. During the first 3 months after its launch, the TDX satellite was tested and calibrated in monostatic configuration with both satellites flying in 20km along-track distance, and it was proven that the system and acquisition performance is almost identical to TSX. Both satellites were then brought into close formation of a few hundred meters distance to begin the bistatic commissioning phase. Since then, TSX and TDX have acted as a large single-pass radar interferometer, which overcomes the limitations imposed by repeat-pass interferometry and allow the acquisition of highly accurate cross- and along-track interferograms. In December 2010, TanDEM-X began with operational global acquisition: bistatic and monostatic SAR images are simultaneously acquired in stripmap mode and processed to interferograms, from which a global DEM is derived. The key parameter in estimating interferometric performance is the coherence, which is deeply evaluated in this paper. The impact of different decorrelation sources as well as the performance stability over time is investigated by means of statistical analyses and dedicated acquisitions on defined test sites, demonstrating the outstanding interferometric capabilities of the TanDEM-X mission
    [bibtex-key = MartoneEtAlISPRSJ2012] [bibtex-entry]


  467. B. Minchew, C.E. Jones, and B. Holt. Polarimetric Analysis of Backscatter From the Deepwater Horizon Oil Spill Using L-Band Synthetic Aperture Radar. Geoscience and Remote Sensing, IEEE Transactions on, 50(10):3812-3830, October 2012. Keyword(s): AD 2010 06 23, Bragg scattering mechanism, DWH slick, Gulf of Mexico, L-band synthetic aperture radar, backscatter polarimetric analysis, coherency matrix eigenvalue, deepwater horizon, deepwater horizon oil spill, dielectric constant, entropy parameters, fully-polarimetric uninhabited aerial vehicle, ocean wave spectral components, oil slick, oil volumetric concentration, radar backscatter, sea water, slick detection method, substantial variation parameter, surface scattering analysis, synthetic aperture radar data, backscatter, eigenvalues and eigenfunctions, entropy, marine pollution, matrix algebra, ocean chemistry, ocean waves, oceanographic regions, oceanographic techniques, permittivity, radar interferometry, remote sensing by radar, seawater, synthetic aperture radar;.
    Abstract: We analyze the fully-polarimetric Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on June 23, 2010, from two adjacent, overlapping flight tracks that imaged the main oil slick near the Deepwater Horizon (DWH) rig site in the Gulf of Mexico. Our results show that radar backscatter from both clean water and oil in the slick is predominantly from a single surface scatterer, consistent with the tilted Bragg scattering mechanism, across the range of incidence angles from 26 #x00B0; to 60 #x00B0;. We show that the change of backscatter over the main slick is due both to a damping of the ocean wave spectral components by the oil and an effective reduction of the dielectric constant resulting from a mixture of 65-90% oil with water in the surface layer. This shows that synthetic aperture radar can be used to measure the oil volumetric concentration in a thick slick. Using the H/A/ #x03B1; parameters, we show that surface scattering is dominant for oil and water whenever the data are above the noise floor and that the entropy (H) and #x03B1; parameters for the DWH slick are comparable to those from the clean water. The anisotropy, A, parameter shows substantial variation across the oil slick and a significant range-dependent signal whenever the backscatter in all channels is above the instrument noise floor. For slick detection, we find the most reliable indicator to be the major eigenvalue of the coherency matrix, which is approximately equal to the total backscatter power for both oil in the slick and clean sea water.
    [bibtex-key = 6166389] [bibtex-entry]


  468. Ricardo D. Monleone, Matteo Pastorino, Joaquim Fortuny-Guasch, Andrea Salvade, Thomas Bartesaghi, Giovanni Bozza, Manuela Maffongelli, Andrea Massimini, Andrea Carbonetti, and Andrea Randazzo. Impact of Background Noise on Dielectric Reconstructions Obtained by a Prototype of Microwave Axial Tomograph. IEEE Transactions on Instrumentation and Measurement, 61(1):140-148, January 2012. Keyword(s): Tomography, Axial Tomography, dielectric measurement, error analysis, interference (signal), microwave imaging, tomography, background noise, dielectric object inspection, dielectric reconstruction, error parameter, interference signal, measurement environment, microwave axial tomograph, Anechoic chambers, Dielectrics, Image reconstruction, Laboratories, Permittivity, Permittivity measurement, Electromagnetic Interference, imaging systems, inverse problems, microwave sensor, microwave tomography, nondestructive testing.
    Abstract: This paper investigates the influence of noise on a microwave axial tomograph developed by some of the authors for the inspection of dielectric objects. In particular, the impact of the measurement environment is considered and the images obtained from data measured in a controlled and an uncontrolled environment are presented and compared. Moreover, the effects of interference signals are considered. Accordingly, several experimental results are reported and discussed in terms of proper error parameters.
    [bibtex-key = monleonePastorinoFortunyGuaschSalvadeBartesaghiBozzaMaffongelliMassiminiCarbnettiRandazzoIEEETIM2012MicroWaveAxialTomography] [bibtex-entry]


  469. Matteo Nannini, Rolf Scheiber, Ralf Horn, and Alberto Moreira. First 3-D Reconstructions of Targets Hidden Beneath Foliage by Means of Polarimetric SAR Tomography. IEEE Geosci. Remote Sens. Lett., 9(1):60-64, January 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, 3-D reconstructions, AD 2006 09, Dornstetten test site, E-SAR system, German Aerospace Center, Germany, L-band airborne data, imaging technique, multiple phase center separation, polarimetric SAR tomography, tomographic campaign, volume structure information, geophysical image processing, geophysical techniques, image reconstruction, synthetic aperture radar;.
    Abstract: SAR tomography (SARTom) is an imaging technique that allows multiple phase center separation in the vertical direction, leading to a 3-D reconstruction of the imaged scene. The retrieval of volume structure information (e.g., for forest classification) and the solution of the layover problem are two of the most promising applications. In this letter, SARTom, in combination with polarimetry (PolSARTom), is exploited to image and to extract characteristic features (e.g., shape and height) of targets hidden beneath foliage. This analysis is applied to L-band airborne data acquired by the E-SAR system of the German Aerospace Center (DLR) during a tomographic campaign that took place in September 2006 on the test site of Dornstetten (Germany).
    [bibtex-key = nanniniScheiberHornMoreira2012] [bibtex-entry]


  470. Victor D. Navarro-Sanchez and Juan M. Lopez-Sanchez. Improvement of Persistent-Scatterer Interferometry Performance by Means of a Polarimetric Optimization. IEEE Geosci. Remote Sens. Lett., 9(4):609-613, July 2012. Keyword(s): SAR Processing, persistent scatterer interferometry, PSI, urban area, deformation monitoring, polarimetric optimization, Spain, TerraSAR-X, optimization, phase quality, pixel identification, polarimetry, polarization image, airborne radar, artificial satellites, electromagnetic wave scattering, geophysical image processing, optimisation, polarisation, radar imaging, radar interferometry, radar polarimetry, synthetic aperture radar, spaceborne SAR.
    Abstract: This letter is aimed at presenting results confirming the contribution of polarimetry to improve the performance of persistent-scatterer interferometry. The improvement is provided by the identification of more pixels with good phase quality, under criteria commonly employed in this context, after a search in the available polarimetric space. The ground deformation results obtained with a series of 41 dual-polarization images acquired by TerraSAR-X over the metropolitan area of Murcia, Spain, have been used to illustrate this approach.
    [bibtex-key = navarroSanchezLopezSanchez2012] [bibtex-entry]


  471. Antonio Pauciullo, Diego Reale, Antonio De Maio, and Gianfranco Fornaro. Detection of Double Scatterers in SAR Tomography. IEEE Transactions on Geoscience and Remote Sensing, 50(9):3567-3586, September 2012. Keyword(s): SAR Processing, double scatterers, Bayesian Information Criterion, Generalized Likelihood Ratio Test, SAR Tomography, differential tomography, double scatterers, ground scatterers, high resolution radar systems, multi-dimensional SAR imaging, scatterers detection, SAR Interferometry, Multidimensional system, Spaceborne SAR, X-Band, Urban, Persistent Scatterer Interferometry, PSI, time series, geophysical image processing, image reconstruction, image resolution, radar imaging, radar interferometry, radar resolution, synthetic aperture radar, 3D reconstruction.
    Abstract: Synthetic aperture radar (SAR) tomography is a technique that extends the concept of SAR interferometry for the accurate localization and monitoring of ground scatterers. Data that are being acquired by the new high resolution SAR sensors offer new perspectives in the 3-D reconstruction and monitoring of urban areas and, particularly, of individual buildings. SAR tomography allows increasing the density of measurements by handling situations where multiple stable scatterers interfere in the same resolution cell. The detection of reliable, i.e., persistent, scatterers is however a challenging issue. In this paper, we investigate three detection approaches: The first is based on a modification of information theoretical criteria; the last two are based on the generalized likelihood ratio test. Theoretical performances are analyzed in details on simulated data, and results of the application to real data from both medium and very high resolution sensors are also provided.
    [bibtex-key = pauciulloRealeDeMaioFornaroTGRS2009TomoDoubleScatterer] [bibtex-entry]


  472. D. Perissin and Teng Wang. Repeat-Pass SAR Interferometry With Partially Coherent Targets. IEEE Trans. Geosci. Remote Sens., 50(1):271-280, January 2012. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR, Interferometry, Coherence, Decorrelation, Dispersion, Indexes, Noise, Pixel, Spatial coherence, displacement measurement, electromagnetic wave scattering, feature extraction, radar imaging, radar interferometry, radar target recognition, spaceborne radar, synthetic aperture radar, 3D location estimation, PS technique, SAR images, SAR interferometry, displacement estimation, electromagnetic signature, extraurban areas, information extraction, multitemporal analysis, partially coherent targets, permanent scatterer technique, radar imaging, spaceborne radar, synthetic aperture radar, Deformation monitoring, digital elevation models (DEMs), synthetic aperture radar interferometry (InSAR), time-series analysis;.
    Abstract: By means of the permanent scatterer (PS) technique, repeated spaceborne synthetic aperture radar (SAR) images with relatively low resolution (about 25 m #x00D7; 5 m for the European Remote Sensing (ERS) and Envisat satellites) can be used to estimate the displacement (1-mm precision) and 3-D location (1-m precision) of targets that show an unchanged electromagnetic signature. The main drawback of the PS technique is the limited spatial density of targets that behave coherently during the whole observation span (hundreds of PSs per square kilometer in urban site and up to few points in vegetated areas). In this paper, we describe a new approach for multitemporal analysis of SAR images that also allows extracting information from partially coherent targets. The basic idea is to loosen the restrictive conditions imposed by the PS technique. The results obtained in different test sites allowed to increase significantly the spatial coverage of the estimate of height and deformation trend, particularly in extraurban areas.
    [bibtex-key = perissinWangTGRS2012] [bibtex-entry]


  473. Daniele Perissin, Zhiying Wang, and Hui Lin. Shanghai subway tunnels and highways monitoring through Cosmo-SkyMed Persistent Scatterers. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):58-67, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Persistent Scatterer Interferometry, PSI, COSMO-SkyMed, X-Band, Spaceborne SAR, Urban subsidence monitoring, Subways, Highways, Shanghai.
    Abstract: Synthetic Aperture Radar Interferometry (InSAR) is an alternative technique to obtain measurements of surface displacement providing better spatial resolution and comparable accuracy at an extremely lower cost per area than conventional surveying methods. InSAR is becoming more and more popular in monitoring urban deformations, however, the technique requires advanced tools and high level competence to be successfully applied. In this paper we report important results obtained by analyzing new high resolution SAR data in the Shanghai urban area. The data used in this work have been acquired by the Italian X-band sensor Cosmo-SkyMed. About 1.2 million of individual and independent targets have been detected in 600 sq km, revealing impressive details of the ground surface deformation. Using the SARPROZ InSAR tool and integrating the results with Google Earth, we were able to track subway tunnels recently excavated and several highways. Tunnels are visible due to very localized subsidence of the above surface along their path. On the other hand, highways, standing over the ground, in most cases show higher stability than the surrounding areas. The density of targets is so high to allow studying the profile of the tunnel subsidence, which is very useful to predict building damage. Finally, the identification of targets on high buildings helps checking the stability of high constructions along the subway lines, highlighting possible risky situations.
    [bibtex-key = perissinWangLinISPRSJ2012] [bibtex-entry]


  474. P. Prats-Iraola, R. Scheiber, L. Marotti, S. Wollstadt, and A. Reigber. TOPS Interferometry With TerraSAR-X. Geoscience and Remote Sensing, IEEE Transactions on, 50(8):3179-3188, August 2012. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, Doppler radar, geophysical image processing, image registration, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, Doppler centroid, SAR interferometry, TOPS imaging mode, TOPS interferometry, TerraSAR-X, Terrain Observation by Progressive Scans imaging mode, accurate interferometric products, azimuth coregistration performance, burst edges, coregistration accuracy, repeat pass TOPS data, squint effects, stripmap interferometric data, Accuracy, Azimuth, Doppler effect, Electrostatic discharges, Estimation, Orbits, Thyristors, Coregistration, SAR interferometry, TOPS interferometry, synthetic aperture radar (SAR), terrain observation by progressive scans (TOPS). [bibtex-key = pratsScheiberMarottiWollstadtReigberTGRS2012TOPSINSAR] [bibtex-entry]


  475. G. Quin and P. Loreaux. Submillimeter Accuracy of Multipass Corner Reflector Monitoring by PS Technique. IEEE Transactions on Geoscience and Remote Sensing, PP(99):1-9, 2012. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI.
    Abstract: This paper presents the results of an experiment that is performed with a network of bidirectional corner reflector (CR) multipass scattering equipment (MUSE), which enable 3-D-displacement measurements. We describe the results of an experiment which was designed to assess the precision of the measurements of ground displacement using MUSE CRs and the permanent scatterer (PS) technique. The CR displacements are applied by micrometric vernier controls during the acquisition of a TerraSAR-X time series of ten images. The relative displacements are estimated between each date, using a PS technique. This paper shows that the relative displacements between the reflectors are estimated with a precision of 0.48 mm along the line of sight. This precision is defined as the standard deviation of the difference between the measured and the applied displacements along the time series. The linear displacement rates of the reflectors are then estimated using the spatiotemporal unwrapping network algorithm, with a 0.4-mm/year precision. We finally show that the experimental results are well predicted by theorical simulations.
    [bibtex-key = quinLoreaux2012] [bibtex-entry]


  476. S.N. Riddick, D.A. Schmidt, and N.I. Deligne. An analysis of terrain properties and the location of surface scatterers from persistent scatterer interferometry. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):50 - 57, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Persistent Scatterer Interferometry, PSI, StaMPS, Cascades, Three Sisters, LiDAR, Geology, Land cover, Vegetation, Volcanoes.
    Abstract: Standard interferometry poses a challenge in heavily vegetated areas due to decorrelation of the radar signal. To alleviate this problem, we implement StaMPS, a persistent scatterer (PS) technique, to obtain a more spatially complete signal in the Cascade Range of the Pacific Northwest. In addition to comparing the spatial extent of the signal from standard Interferometric Synthetic Aperture Radar (InSAR) and StaMPS, we further analyze the selection of scatterers over several terrain types in the Cascades, and systematically vary StaMPS parameters to minimize the selection of false positives and negatives. Utilizing the best parameters, we correlate the location of persistent scatterers to geologic units, and vegetation density derived from Light Detection and Ranging (LiDAR) data. Our findings indicate that persistent scatterers most frequently occur on young, rough basaltic to andesitic lava flows and to a lesser extent on older, reworked basaltic andesitic lava flows exposed as boulder fields in the forests. Very few or no scatterers were found over water, permanent snowfields, evergreen forest, or unconsolidated pyroclastics. Over 90 percent of the scatterers are located in areas with no or very sparse vegetation cover. Based on surface roughness and the percentage of bare earth within the radar footprint, we are able to predict where PS InSAR is most likely to be successful on natural terrains.
    [bibtex-key = riddickSchmidtDeligneISPRSJ2012] [bibtex-entry]


  477. Paola Rizzoli, Benjamin Bräutigam, Thomas Kraus, Michele Martone, and Gerhard Krieger. Relative height error analysis of TanDEM-X elevation data. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):30-38, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Digital elevation model, Relative height error, TanDEM-X, Spaceborne SAR, X-band.
    Abstract: The primary objective of the TanDEM-X mission is the generation of a global high resolution digital elevation model (DEM) with single-pass SAR interferometry. Within the mission, the Earth's land masses will be mapped at least twice to achieve relative vertical accuracies in the order of two meters. This paper presents an analysis of the mission performance in terms of the relative height error showing first results obtained from TanDEM-X interferometric data. For critical areas characterized by strong volume decorrelation phenomena or mountainous terrain, different approaches to improve the final height error are investigated as well
    [bibtex-key = RizzoliBrautigamKrausMartoneKrieger2012ISPRSJ2012] [bibtex-entry]


  478. Marc Rodriguez-Cassola, Pau Prats, D. Schulze, N. Tous-Ramon, U. Steinbrecher, L. Marotti, M. Nannini, M. Younis, P. Lopez-Dekker, M. Zink, A. Reigber, G. Krieger, and A. Moreira. First Bistatic Spaceborne SAR Experiments With TanDEM-X. IEEE Geoscience and Remote Sensing Letters, 9(1):33-37, January 2012. Keyword(s): SAR Processing, Bistatic SAR, calibration, digital elevation models, geophysical equipment, remote sensing by radar, synthetic aperture radar, bistatic spaceborne SAR experiments, TerraSAR-X Add-on for Digital Elevation Measurements, high-resolution interferometric mission, TanDEM-X mission, digital elevation model, Earth surface, SAR interferometry, X-band synthetic aperture radar, quasimonostatic configuration, bistatic SAR system, monostatic commissioning phase, monostatic mode, synchronization information, calibration information, bistatic images, Satellites, Synchronization, Spaceborne radar, Azimuth, Radar imaging, Coherence, Bistatic radar, bistatic SAR processing, spaceborne SAR missions, synthetic aperture radar (SAR), time and phase synchronization.
    Abstract: TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model of the Earth surface by means of single-pass X-band synthetic aperture radar (SAR) interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in nonnominal modes have been conducted with the main purpose of validating the performance of both space and ground segments in very demanding scenarios. In particular, this letter reports about the first bistatic acquisition and the first single-pass interferometric (mono-/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focusing on the analysis of the experimental results. Even in the absence of essential synchronization and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.
    [bibtex-key = rodriguezCassolaEtAlGRSL2012FirstBistaticSARWithTanDEMX] [bibtex-entry]


  479. Cristian Rossi, Fernando Rodriguez Gonzalez, Thomas Fritz, Nestor Yague-Martinez, and Michael Eineder. TanDEM-X calibrated Raw DEM generation. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):12 - 20, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, TanDEM-X, DEM, Raw DEM, SAR stereo-radargrammetry, radargrammetry, Absolute phase offset estimation, DEM calibration.
    Abstract: The TanDEM-X mission successfully started on June 21st 2010 with the launch of the German radar satellite TDX, placed in orbit in close formation with the TerraSAR-X (TSX) satellite, and establishing the first spaceborne bistatic interferometer. The processing of SAR raw data to the Raw DEM is performed by one single processor, the Integrated TanDEM-X Processor (ITP). The quality of the Raw DEM is a fundamental parameter for the mission planning. In this paper, a novel quality indicator is derived. It is based on the comparison of the interferometric measure, the unwrapped phase, and the stereo-radargrammetric measure, the geometrical shifts computed in the coregistration stage. By stating the accuracy of the unwrapped phase, it constitutes a useful parameter for the determination of problematic scenes, which will be resubmitted to the dual baseline phase unwrapping processing chain for the mitigation of phase unwrapping errors. The stereo-radargrammetric measure is also operationally used for the Raw DEM absolute calibration through an accurate estimation of the absolute phase offset. This paper examines the interferometric algorithms implemented for the operational TanDEM-X Raw DEM generation, focusing particularly on its quality assessment and its calibration
    [bibtex-key = RossiEtAlISPRSJ2012] [bibtex-entry]


  480. A. Rucci, A. Ferretti, A. Monti Guarnieri, and F. Rocca. Sentinel-1 SAR interferometry applications: The outlook for sub millimeter measurements. Remote Sensing of Environment, 120:156 - 163, 2012. Note: The Sentinel Missions - New Opportunities for Science. Keyword(s): Synthetic Aperture Radar (SAR), INSAR, Permanent scatterers, Ground deformation.
    Abstract: Optical leveling campaigns, tiltmeters, GPS and InSAR are geodetic techniques used to detect and monitor surface deformation phenomena. In particular, InSAR data from satellite radar sensors are gaining increasing attention for their cost-effectiveness and unique technical features, making possible the monitoring of large areas, even revisiting the past. Moreover, more advanced InSAR techniques (PSInSAR?, SqueeSAR?) developed in the last decade are capable of providing millimeter precision, comparable to optical leveling, and a high spatial density of displacement measurements, over long periods of time without need of installing equipment or otherwise accessing the study area. Thanks to the high density and quality of the measurements the PSInSAR data can be successfully used in geophysical inversion, to measure the permeability of oil reservoirs and/or to evaluate the possibilities and risks due to seismic faulting in the sequestration of CO2. In these cases, the precision, the sub weekly frequency of the measurements and the time required for the data to be available are the most important aspects, more relevant than the spatial resolution. Until recently, the main limitation to the application of InSAR was the relatively long revisiting time (24 or 35days) and the quite long waiting period for the delivery of the acquired data. The new Sentinel-1 mission, based on a constellation of two satellites, is expected to reduce such limitations guaranteeing a revisit cycle of 6days on a global scale and in particular over Europe and Canada and providing a high level of service reliability with near-real-time delivery of data within 24h, important for risk management applications. The new X band satellite SAR constellations like Cosmo Skymed and TerraSAR X have also a short revisiting time, from 4 to 11days. However, their coverage is limited to well definite areas, and an expensive decision has to be made if to initiate the observations on any target. Sentinel 1, instead, yields global and costless observations and thus, after the end of the commissioning phase, will always produce present and past ground motion for any target. It's important to underline that the millimeter accuracy, applying the InSAR analysis with Sentinel-1, will be achieved within a shorter observation time frame, thanks to the increased number of acquired images per year (Attema et al. 2010, De Zan et al., 2008). Results from ground based radar show that this improved precision is indeed achievable from C to Ku band, provided that an accurate model of the delay due to atmospheric water vapor is available or that precise reference points are close by.
    [bibtex-key = rucciFerrettiMontiGuarnieriRoccaRSE2012Sentinel1InSARAppMillimeterMeasurements] [bibtex-entry]


  481. Alexander Schunert and Uwe Soergel. Grouping of Persistent Scatterers in high-resolution SAR data of urban scenes. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):80 - 88, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Persistent Scatterer Interferometry, PSI, TerraSAR-X, Spaceborne SAR, Urban, Grouping.
    Abstract: Persistent Scatterer Interferometry (PSI) is a technique to simultaneously estimate surface deformation and 3D structure from stacks of SAR images. It was proposed first about one decade ago to monitor preferably urban areas, where in general the highest numbers of PS are found. At that time no high-resolution satellite SAR data were available. Instead, for example, stacks of ERS imagery were used providing ground range resolution of about 25m. In data of such kind only the strongest PS can be detected, which are usually caused by corner reflectors built by orthogonal building and road planes of considerable size, whereas smaller structures causing weaker ones signal are averaged by clutter or mutually interfere with others in the same resolution cell. Thus, if any, only a few or even just one single PS are found per building. The advent of a new senor generation of systems like TerraSAR-X and COSMO-Skymed in 2007 led to a significant improvement of spatial resolution of about one order of magnitude. This comes along with a dramatic rise of PS density: In some cases tens to hundreds are detected at large buildings, which offers the possibility to monitor even individual urban objects. In addition, especially at building facades the distribution of those PS is often quite regular. A reason for that is the usually rectilinear arrangement of facade structures inducing PS like windows or balconies. Those patterns contain a lot of information about the objects under investigation, which is mostly ignored in current PSI processing schemes. For example, consider a regular structure of windows on a certain fcade of a multi-story building. Assuming the same kind of structure generates one single PS at each window, the phase centers of all scatterers caused by windows of each floor share the same height. This means, we may benefit from such kind of redundancy, for instance, to improve the height estimate by averaging over PS having the same elevation. In this work, we first discuss the regular appearance of PS at urban facades for an urban test scene in TerraSAR-X spotlight mode data. Then, we show how PS analysis could benefit by exploitation of the redundancy due to repetitive patterns of man-made objects. Finally, we propose a PS grouping scheme based on a production system and discuss first results achieved for the test area.
    [bibtex-key = schunertSoergelISPRSJ2012] [bibtex-entry]


  482. Manoochehr Shirzaei and R Bürgmann. Topography correlated atmospheric delay correction in radar interferometry using wavelet transforms. Geophysical Research Letters, 39(1), 2012. [bibtex-key = Shirzaei2012] [bibtex-entry]


  483. Manoochehr Shirzaei, R Bürgmann, O Oncken, TR Walter, P Victor, and O Ewiak. Response of forearc crustal faults to the megathrust earthquake cycle: InSAR evidence from Mejillones Peninsula, Northern Chile. Earth and Planetary Science Letters, 333:157-164, 2012. [bibtex-key = Shirzaei2012a] [bibtex-entry]


  484. Marc Simard, Scott Hensley, Marco Lavalle, Ralph Dubayah, Naiara Pinto, and Michelle Hofton. An Empirical Assessment of Temporal Decorrelation Using the Uninhabited Aerial Vehicle Synthetic Aperture Radar over Forested Landscapes. Remote Sensing, 4(4):975-986, 2012. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, Differential Interferometry, DInSAR, SAR Interferometry, Coherence, Airborne SAR, UAVSAR, L-Band.
    Abstract: We present an empirical assessment of the impact of temporal decorrelation on interferometric coherence measured over a forested landscape. A series of repeat-pass interferometric radar images with a zero spatial baseline were collected with UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar), a fully polarimetric airborne L-band radar system. The dataset provided temporal separations of 45 minutes, 2, 7 and 9 days. Coincident airborne lidar and weather data were collected. We theoretically demonstrate that UAVSAR measurement accuracy enables accurate quantification of temporal decorrelation. Data analysis revealed precipitation events to be the main driver of temporal decorrelation over the acquisition period. The experiment also shows temporal decorrelation increases with canopy height, and this pattern was found consistent across forest types and polarization.
    [bibtex-key = simardHensleyLavalleDubayahPintoHofton2012TempDecorrelation] [bibtex-entry]


  485. Thomas K. Sjogren, Viet Thuy Vu, Mats I. Pettersson, A. Gustavsson, and Lars M. H. Ulander. Moving Target Relative Speed Estimation and Refocusing in Synthetic Aperture Radar Images. IEEE Trans. Aerosp. Electron. Syst., 48(3):2426-2436, July 2012. Keyword(s): SAR Processing, Moving Target indication, MTI, compensation, frequency-domain analysis, motion compensation, motion estimation, object detection, radar clutter, radar detection, radar imaging, radar tracking, synthetic aperture radar, ultra wideband radar, SAR imaging, UWB, cell migration, chirp estimator, clutter, frequency domain analysis, moving target refocusing, moving target relative speed estimation, moving target signal, phase compensation, synthetic aperture radar, target acceleration, ultra wideband, Approximation methods, Azimuth, Clutter, Estimation, Focusing, Image resolution, Synthetic aperture radar.
    Abstract: In this paper, a method for moving target relative speed estimation and refocusing based on synthetic aperture radar (SAR) images is derived and tested in simulation and on real data with good results. Furthermore, an approach on how to combine the estimation method with the refocusing method is introduced. The estimation is based on a chirp estimator that operates in the SAR image and the refocusing of the moving target is performed locally using subimages. Focusing of the moving target is achieved in the frequency domain by phase compensation, and therefore makes it even possible to handle large range cell migration in the SAR subimages. The proposed approach is tested in a simulation and also on real ultrawideband (UWB) SAR data with very good results. The estimation method works especially well in connection with low frequency (LF) UWB SAR, where the clutter is well focused and the phase of the smeared moving target signal becomes less distorted. The main limitation of the approach is target accelerations where the distortion increases with the integration time.
    [bibtex-key = sjogrenVuPetterssonGustavssonUlander2012MTI] [bibtex-entry]


  486. Tazio Strozzi, Charles L. Werner, Andreas Wiesmann, and Urs Wegmuller. Topography Mapping With a Portable Real-Aperture Radar Interferometer. IEEE Geosci. Remote Sens. Lett., 9(2):277-281, March 2012. Keyword(s): SAR Processing, Radar antennas, Radar imaging, Radar remote sensing, Rocks, Spaceborne radar, Surfaces, digital elevation models, glaciology, radar interferometry, rocks, terrain mapping, topography (Earth), AD 2009 08 to 2010 03, Grabengufer rock glacier, Switzerland, digital elevation model, height error standard deviation, ice surface change, instrument design, interferometric phase shift, massive Rhone glacier thinning, portable real-aperture radar interferometer, portable terrestrial radar interferometer, rock glacier destabilization, surface topography, topography mapping, Digital elevation model (DEM), glacier, radar, radar interferometry;.
    Abstract: In this letter, the requirements to derive topography from a portable terrestrial radar interferometer are introduced, the instrument design and the relationship between interferometric phase shift and surface topography are explained, and two examples of topographic maps from measurements at the Rhone glacier and Grabengufer rock glacier in Switzerland are presented. In the first case, an external digital elevation model was used to assess the error of topography mapping with the portable radar interferometer and to analyze ice surface changes of the glacier in the last 14 years. We found that the height error standard deviation is about 3 m within a distance of 2 km from the sensor and observed massive thinning of the Rhone glacier. In the second case, we used the terrestrial radar interferometer in order to measure the height difference between August 2009 and March 2010 over the rock glacier as a consequence of its destabilization.
    [bibtex-key = strozziWernerWiesmannWegmullerGRSL2012GPRITopo] [bibtex-entry]


  487. Alireza Tabatabaeenejad, Mariko S. Burgin, and Mahta Moghaddam. Potential of L-Band Radar for Retrieval of Canopy and Subcanopy Parameters of Boreal Forests. IEEE Transactions on Geoscience and Remote Sensing, 50(6):2150-2160, June 2012. Keyword(s): calibration, data analysis, forestry, remote sensing by radar, simulated annealing, soil, vegetation mapping, AD 2010 06, Jet Propulsion Laboratory, L-band radar, National Aeronautics and Space Administration, SMAP mission, Uninhabited Aerial Vehicle Synthetic Aperture Radar, absolute retrieval error analysis, calibration method, canopy parameter retrieval, central Canada boreal forests, discrete scatterer radar model, forest structure, forward scattering model, global optimization scheme, inversion method, old black spruce site, old jack pine forests, optimization algorithm, optimization problem, relative retrieval error, scattering mechanisms, simulated annealing, soil moisture information, subcanopy parameter retrieval, surface soil moisture retrieval, synthetic data, vegetation parameters, young jack pine forests, Backscatter, Dielectric constant, L-band, Radar, Scattering, Soil moisture, Vegetation mapping, Allometric relationships, Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10), L-band radar, Soil Moisture Active and Passive (SMAP), boreal forest, canopy, inverse problem, microwave scattering.
    Abstract: In this paper, we study the radar retrieval of soil moisture as well as canopy parameters in a range of boreal forests. The retrieval is formulated as an optimization problem where the difference between data and prediction of a forward scattering model is minimized. The forward model is a discrete scatterer radar model, and the optimization algorithm is a global optimization scheme known as simulated annealing. The inversion method is first applied to synthetic data assuming hypothetical allometric relationships to make the retrieval possible by reducing the number of unknown vegetation parameters. The inversion algorithm is then validated using the data acquired with the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL) Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) in June 2010 in central Canada boreal forests in support of the prelaunch calibration and validation activities of NASA's Soil Moisture Active and Passive (SMAP) mission. The inversion results for synthetic data show that the absolute retrieval error in soil moisture and relative retrieval error in canopy height are small, while the relative output error in trunk density could be large. The inversion results for actual field data show a great accuracy in soil moisture retrieval for Old Jack Pine and Young Jack Pine forests but show large retrieval errors for many of the radar pixels in the Old Black Spruce site. This paper shows that L-band radar is capable of retrieving surface soil moisture in forests with a high biomass where the forest structure allows soil moisture information to be carried by scattering mechanisms.
    [bibtex-key = tabatabaeenejadBurginMoghaddamTGRS2012LBandCanopySubCanopyParamRetrieval] [bibtex-entry]


  488. Stefano Tebaldini and Fabio Rocca. Multibaseline Polarimetric SAR Tomography of a Boreal Forest at P- and L-Bands. IEEE Trans. Geosci. Remote Sens., 50(1):232-246, January 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, ESA campaign BioSAR, Krycklan River catchment, L-band analysis, Northern Sweden, P-band analysis, PolInSAR approach, algebraic synthesis technique, boreal forest, direct volume backscattering process, ground-volume interaction process, multibaseline polarimetric SAR tomography, multipolarimetric data acquisition, polarimetric analysis, remote sensing, single-baseline data analysis, tomographic analysis, tomographic imaging method, vegetation layer analysis, backscatter, data acquisition, forestry, radar interferometry, radar polarimetry, remote sensing by radar, rivers, synthetic aperture radar, tomography, vegetation mapping;.
    Abstract: Longer wavelength synthetic aperture radars (SARs) are precious in the remote sensing of forested areas, being sensitive to contributions from the whole vegetation layer and from the ground below. The electromagnetic properties of such contributions are retrieved from multipolarimetric acquisitions, whereas their vertical structure is retrieved from multibaseline acquisitions through tomographic imaging. Combining baseline and polarization diversity provides most information, allowing the decomposition of the SAR signal into ground- and volume-only contributions. A formal treatment of this problem is provided with the algebraic synthesis technique, which extends the concepts of PolInSAR. The decomposition, however, is shown to be ambiguous in that different solutions are equally consistent with the data. The main goal of this paper is to discuss this topic in light of the experimental results from a tomographic and polarimetric analysis of the boreal forest within the Krycklan River catchment, Northern Sweden, investigated at P- and L-bands during the ESA campaign BioSAR 2008. Different solutions to the decomposition problem will be discussed by examining the corresponding vertical structures accessible through tomographic techniques. Elements are shown supporting the idea that ground-volume interactions play a nonnegligible role at P-band, and a solution is proposed to isolate contributions from direct volume backscattering. The retrieval of forest top height is discussed as well, leading to the conclusion that such parameter is robust against erroneous choices in the identification of volume-only contributions, thus corroborating the PolInSAR approach for the analysis of single-baseline data.
    [bibtex-key = tebaldiniRoccaTGRS2012MultibaselinePolTomoSARBorealForestPBandAndLBand] [bibtex-entry]


  489. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. On Synthetic Aperture Radar Azimuth and Range Resolution Equations. IEEE Trans. Aerosp. Electron. Syst., 48(2):1764-1769, April 2012. Keyword(s): SAR Processing, image resolution, integration, radar imaging, radar resolution, synthetic aperture radar, transient response, ultra wideband radar, HPBW, IRF-NSAR-sinc function, IRF-USAR, NB SAR imaging, UWB SAR imaging, half power beamwidth, impulse response function, integration angle, intensity interval, narrow-band-narrow-beam SAR, range resolution equation, signal fractional bandwidth, spatial resolution equation, synthetic aperture radar azimuth, ultrawide-band-ultrawide-beam synthetic aperture radar, Approximation methods, Azimuth, Bandwidth, Equations, Niobium, Spatial resolution.
    Abstract: This paper discusses spatial resolutions for ultrawide-band-ultrawide-beam (UWB) synthetic aperture radar (SAR) in comparison to narrow-band-narrow-beam (NB) SAR. The study shows that in the intensity interval from -6 dB to 0 dB, the behavior of the impulse response function in NB SAR imaging (IRF-NSAR)-sinc function-and the impulse response function in UWB SAR imaging (IRF-USAR) in azimuth and range are similar. This similarity is utilized in a derivation of new spatial resolution equations for UWB SAR based on -3 dB width or half power beamwidth (HPBW). Signal fractional bandwidth and associated integration angle are shown to affect the behavior of IRF-USAR. The effects to HPBW are described by the so-called HPBW narrowing/broadening factors.
    [bibtex-key = vuSjogrenPettersson2012RgAziResolutionEq] [bibtex-entry]


  490. Ling Wang, M. Cheney, and B. Borden. Multistatic Radar Imaging of Moving Targets. IEEE Transactions on Aerospace and Electronic Systems, 48(1):230 -242, jan. 2012. Keyword(s): Doppler SAR, filtered backprojection, inverse synthetic aperture radar, linearized imaging theory, matched filtering, moving target, moving target tomography, multistatic radar imaging, narrowband waveform, object distribution, phase space imaging distribution, phase-space imaging formula, phase-space point-spread function, range-Doppler imaging, scattered wave spectral aspect, speed of light, undergoing linear motion, Doppler radar, geometry, image sensors, phase space methods, radar imaging, scattering, synthetic aperture radar, target tracking, tomography;.
    Abstract: We develop a linearized imaging theory that combines the spatial, temporal, and spectral aspects of scattered waves. We consider the case of fixed sensors and a general distribution of objects, each undergoing linear motion; thus the theory deals with imaging distributions in phase space. We derive a model for the data that is appropriate for narrowband waveforms in the case when the targets are moving slowly relative to the speed of light. From this model, we develop a phase-space imaging formula that can be interpreted in terms of filtered backprojection or matched filtering. For this imaging approach, we derive the corresponding phase-space point-spread function (PSF). We show plots of the phase-space point-spread function for various geometries. We also show that in special cases, the theory reduces to: 1) range-Doppler imaging, 2) inverse synthetic aperture radar (ISAR), 3) synthetic aperture radar (SAR), 4) Doppler SAR, and 5) tomography of moving targets.
    [bibtex-key = 6129632] [bibtex-entry]


  491. Yuanyuan Wang, Xiao Xiang Zhu, and Richard Bamler. Retrieval of phase history parameters from distributed scatterers in urban areas using very high resolution SAR data. ISPRS Journal of Photogrammetry and Remote Sensing, 73(0):89 - 99, 2012. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Persistent Scatterer Interferometry, PSI, Phase history, Distributed scatterer, Covariance matrix, TerraSAR-X, Spaceborne SAR.
    Abstract: In a recent contribution Ferretti and co-workers (Ferretti, A., Fumagalli, A., Novali, F., Prati, C., Rocca, F., Rucci, A., 2011. A new algorithm for processing interferometric data-stacks: SqueeSAR IEEE Transactions on Geoscience and Remote Sensing 49(9), pp. 3460-3470) have proposed the SqueeSAR method, a way to exploit temporally coherent distributed scatterers in coherent SAR data stacks. Elevation and deformation or subsidence estimates are obtained with accuracy similar as in the well known persistent scatterer interferometry (PSI). In this paper we propose an alternative approach and provide a first demonstration of the optimal estimation of distributed scatterers' phase histories in urban areas. Different to SqueeSAR, we derive phase histories for each distributed scatterer pixel rather than for groups of pixels. We use the Anderson-Darling statistical test to identify neighboring samples of the same distribution. Prior to covariance matrix estimation required for maximum likelihood estimation we apply a multi-resolution defringe technique. By using TerraSAR-X high resolution spotlight data, it is demonstrated that we are able to retrieve reliable phase histories and motion parameter estimates from distributed scatterers with signal-to-noise-ratio far below the common range.
    [bibtex-key = wangZhuBamlerISPRSJ2012] [bibtex-entry]


  492. Urs Wegmuller, Tazio Strozzi, Andreas Wiesmann, Charles L. Werner, Othmar Frey, Rafael Caduff, and Andrew Kos. Hangrutschungskartierung mittels Radar Interferometrie. Geomatik Schweiz, 110(9), 2012.
    Abstract: Insbesondere durch die ERS-1, ERS-2 und ENVISAT ASAR Sensoren der Europ{\"a}ischen Raumfahrtsagentur ESA hat sich in den letzten 20 Jahren die SAR Interferometrie Methode sehr stark entwickelt. Ein wichtiger Anwendungsbereich ist die Kartierung von Gel{\"a}ndebewegungen. Innerhalb der Schweiz wird die SAR Interferometrie Methodevor allem f{\"u}r die Kartierung von Hangrutschungen eingesetzt. Hangrutschungsinformation wird momentan f{\"u}r die Erstellung der Gefahrenhinweiskarten durch die Kantone ben{\"o}tigt. Angesichts einer wahrscheinlichen Zunahme der Hangrutschungs- undFelssturzgefahren durch die Klimaerw{\"a}rmung und daraus resultierenden sekund{\"a}renVer{\"a}nderungen, wie der Reduktion des Alpinen Permafrosts und des Gletscherr{\"u}ckzugs, kann mit einem zunehmenden Informationsbedarf gerechnet werden.
    [bibtex-key = wegmullerStrozziWiesmannWernerFreyCaduffKosGEOMATIK2012DInSAR] [bibtex-entry]


  493. Steffen Wollstadt, Pau Prats, Markus Bachmann, Josef Mittermayer, and Rolf Scheiber. Scalloping Correction in TOPS Imaging Mode SAR Data. IEEE Geosci. Remote Sens. Lett., 9(4):614-618, July 2012. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, antenna phased arrays, beam steering, calibration, geophysical image processing, phased array radar, radar antennas, radar imaging, synthetic aperture radar, terrain mapping, TOPS imaging mode, TSX, TerraSAR-X antenna model, calibration, electronic beam steering, general cardinal sine antenna model, phased array antenna, scalloping correction, scalloping simulation, synthetic aperture radar, terrain observation by progressive scan, Antennas, Arrays, Azimuth, Beam steering, Calibration, Imaging, Quantization, Electronic beam steering, TOPS calibration, TerraSAR-X (TSX), Terrain Observation by Progressive Scan (TOPS), phased array antenna, scalloping.
    Abstract: This letter presents an investigation on scalloping correction in the Terrain Observation by Progressive Scan (TOPS) imaging mode for synthetic aperture radar systems with electronically steered phased array antennas. A theoretical simulation of the scalloping is performed, and two correction methods are introduced. The simulation is based on a general cardinal sine (sinc) antenna model as well as on the TerraSAR-X (TSX) antenna model. Real TSX data acquired over rainforest are used for demonstration and verification of the scalloping simulation and correction. Furthermore, a calibration approach, taking into account the special TOPS imaging mode properties, is introduced.
    [bibtex-key = wollstadtPratsBachmannMittermayerScheiberGRSL2012TOPScalloping] [bibtex-entry]


  494. S. Xilong, Y. Anxi, D. Zhen, and L. Diannong. Three-Dimensional SAR Focusing via Compressive Sensing: The Case Study of Angel Stadium. IEEE Geosci. Remote Sens. Lett., PP(99):1-5, 2012.
    Abstract: Recently, a synthetic aperture radar (SAR) tomograhic focusing method based on compressive sensing was proposed. This focusing method can reduce the required number of measurements and achieve satisfying elevation resolving ability. First, we briefly review this novel focusing method and prove the applicability of compressed sensing (CS) for SAR tomography theoretically using the latest improvement of CS. Then, we apply this focusing method to the 3-D reconstruction of Angel Stadium with Envisat-ASAR data. Both the theoretical analysis and satisfying results of real data processing confirmed the applicability of this SAR tomograhic focusing method.
    [bibtex-key = 6156739] [bibtex-entry]


  495. Yajing Yan, M.P. Doin, P. Lopez-Quiroz, F. Tupin, B. Fruneau, V. Pinel, and E. Trouve. Mexico City Subsidence Measured by InSAR Time Series: Joint Analysis Using PS and SBAS Approaches. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 5(4):1312-1326, August 2012. Keyword(s): AD 2002 11 to 2007 03, ENVISAT images, Gamma-IPTA chain, InSAR time series, Mexico City subsidence rate, adhoc SBAS approach, ground displacement rates, high-pass filtered difference maps, joint analysis method, low-pass filtered difference maps, multitemporal InSAR processing, permanent scatterer approach, small baseline subset approach, subsidence measurement method, subsidence pattern, subsoil compaction analysis, surface drying process, compaction, feature extraction, geophysical image processing, geophysical techniques, high-pass filters, low-pass filters, radar interferometry, soil, synthetic aperture radar, time series;.
    Abstract: In multi-temporal InSAR processing, both the Permanent Scatterer (PS) and Small BAseline Subset (SBAS) approaches are optimized to obtain ground displacement rates with a nominal accuracy of millimeters per year. In this paper, we investigate how applying both approaches to Mexico City subsidence validates the InSAR time series results and brings complementary information to the subsidence pattern. We apply the PS approach (Gamma-IPTA chain) and an ad-hoc SBAS approach on 38 ENVISAT images from November 2002 to March 2007 to map the Mexico City subsidence. The subsidence rate maps obtained by both approaches are compared quantitatively and analyzed at different steps of the PS processing. The inter-comparison is done separately for low-pass (LP) and high-pass (HP) filtered difference maps to take the complementarity of both approaches at different scales into account. The inter-comparison shows that the differential subsidence map obtained by the SBAS approach describes the local features associated with urban constructions and infrastructures, while the PS approach quantitatively characterizes the motion of individual targets. The latter information, once related to the type of building foundations, should be essential to quantify the relative importance of surface loads, surface drying and drying due to aquifer over-exploitation, in subsoil compaction.
    [bibtex-key = yanDoinLopeyQuiroyTupinFruneauPinelTrouve2012] [bibtex-entry]


  496. L. Zhang, Z. Qiao, M. Xing, L. Yang, and Z. Bao. A Robust Motion Compensation Approach for UAV SAR Imagery. IEEE Trans. Geosci. Remote Sens., 50(8):3202-3218, August 2012. Keyword(s): autonomous aerial vehicles, geophysical image processing, geophysical techniques, maximum likelihood estimation, motion compensation, remote sensing by radar, synthetic aperture radar, robust motion compensation approach, UAV SAR imagery, unmanned aerial vehicle, synthetic aperture radar, remote sensing application, atmospheric turbulence, range invariant motion error, weighted phase gradient autofocus, nonsystematic range cell migration function, range dependent phase error, maximum likelihood WPGA algorithm, subaperture phase error, inertial navigation system, Electronics packaging, Estimation, Trajectory, Robustness, Navigation, Thyristors, Geometry, Local maximum-likelihood (LML), motion compensation (MOCO), phase gradient autofocus (PGA), synthetic aperture radar (SAR), unmanned aerial vehicle (UAV), weighted phase gradient autofocus (WPGA).
    Abstract: Unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) is an essential tool for modern remote sensing applications. Owing to its size and weight constraints, UAV is very sensitive to atmospheric turbulence that causes serious trajectory deviations. In this paper, a novel databased motion compensation (MOCO) approach is proposed for the UAV SAR imagery. The approach is implemented by a three-step process: 1) The range-invariant motion error is estimated by the weighted phase gradient autofocus (WPGA), and the nonsystematic range cell migration function is calculated from the estimate for each subaperture SAR data; 2) the retrieval of the range-dependent phase error is executed by a local maximum-likelihood WPGA algorithm; and 3) the subaperture phase errors are coherently combined to perform the MOCO for the full-aperture data. Both simulated and real-data experiments show that the proposed approach is appropriate for highly precise imaging for UAV SAR equipped with only low-accuracy inertial navigation system.
    [bibtex-key = zhangQiaoXingYangBaoTGRS2012RobustMoCompForUAVsarImagery] [bibtex-entry]


  497. Xiao Xiang Zhu and Richard Bamler. Demonstration of Super-Resolution for Tomographic SAR Imaging in Urban Environment. IEEE Trans. Geosci. Remote Sens., 50(8):3150-3157, August 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry, differential SAR interferometry, Persistent Scatterer Interferometry, PSI, DInSAR, Buildings, Image resolution, Optical imaging, Optical scattering, Signal resolution, Strontium, Tomography, geophysical equipment, geophysical image processing, radar imaging, SL1MMER algorithm, SR power, TerraSAR-X real data, classical linear estimators, differential SAR tomography, elevation aperture size, geometric analysis, meter-resolution spaceborne SAR systems, spectral analysis problem, statistical analysis, super-resolution demonstration, super-resolution reconstruction algorithms, super-resolving algorithm, synthetic aperture radar, tomographic SAR imaging, tomographic SAR inversion, tomographic elevation resolution, urban environment, urban infrastructure monitoring, Compressive sensing, SL1MMER, TerraSAR-X, sparse reconstruction, super-resolution, synthetic aperture radar, tomographic SAR inversion.
    Abstract: Tomographic synthetic aperture radar (SAR) inversion, including SAR tomography and differential SAR tomography, is essentially a spectral analysis problem. The resolution in the elevation direction depends on the elevation aperture size, i.e., on the spread of orbit tracks. Since the orbits of modern meter-resolution spaceborne SAR systems, such as TerraSAR-X, are tightly controlled, the tomographic elevation resolution is at least an order of magnitude lower than in range and azimuth. Hence, super-resolution (SR) reconstruction algorithms are desired. Considering the sparsity of the signal in elevation, a compressive sensing based super-resolving algorithm, named Scale-down by L1 norm Minimization, Model selection, and Estimation Reconstruction (SL1MMER, pronounced slimmer), was proposed by the authors in a previous paper. The ultimate bounds of the technique on localization accuracy and SR power were investigated. In this paper, the essential role of SR for layover separation in urban infrastructure monitoring is indicated by geometric and statistical analysis. It is shown that double scatterers with small elevation distances are more frequent than those with large elevation distances. Furthermore, the SR capability of SL1MMER is demonstrated using TerraSAR-X real data examples. For a high rise building complex, the percentage of detected double scatterers is almost doubled compared to classical linear estimators. Among them, half of the detected double scatterer pairs have elevation distances smaller than the Rayleigh elevation resolution. This confirms the importance of SR for this type of applications.
    [bibtex-key = zhuBamlerTGRS2012b] [bibtex-entry]


  498. Xiao Xiang Zhu and Richard Bamler. Super-Resolution Power and Robustness of Compressive Sensing for Spectral Estimation With Application to Spaceborne Tomographic SAR. IEEE Trans. Geosci. Remote Sens., 50(1):247-258, January 2012. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry, Persistent Scatterer Interferometry, PSI, TerraSAR-X, X-band, Estimation, Image resolution, Minimization, Noise, Robustness, Strontium, Tomography, Fourier analysis, data acquisition, geophysical techniques, least squares approximations, maximum likelihood estimation, minimisation, probability, remote sensing by radar, spaceborne radar, synthetic aperture radar, tomography, Fourier domain sample, Rayleigh resolution analysis, SL1MMER algorithm, TomoSAR algorithm, compressive sensing robustness analysis, generic super-resolution problem, maximum likelihood parameter estimation, nonlinear least-squares estimation, numerical simulation, probability, spaceborne SAR tomography, sparse spectral estimation, spectral estimation method, super-resolution power, uniformly distributed phase difference analysis, Compressive sensing (CS), SAR tomography (TomoSAR), SL1MMER, spectral estimation, super-resolution (SR).
    Abstract: We address the problem of resolving two closely spaced complex-valued points from N irregular Fourier domain samples. Although this is a generic super-resolution (SR) problem, our target application is SAR tomography (TomoSAR), where typically the number of acquisitions is N = 10-100 and SNR = 0-10 dB. As the TomoSAR algorithm, we introduce Scale-down by L1 norm Minimization, Model selection, and Estimation Reconstruction (SL1MMER), which is a spectral estimation algorithm based on compressive sensing, model order selection, and final maximum likelihood parameter estimation. We investigate the limits of SLIMMER concerning the following questions. How accurately can the positions of two closely spaced scatterers be estimated? What is the closest distance of two scat- terers such that they can be separated with a detection rate of 50% by assuming a uniformly distributed phase difference? How many acquisitions N are required for a robust estimation (i.e., for separating two scatterers spaced by one Rayleigh resolution unit with a probability of 90%)? For all of these questions, we provide numerical results, simulations, and analytical approximations. Although we take TomoSAR as the preferred application, the SLIMMER algorithm and our results on SR are generally applicable to sparse spectral estimation, including SAR focusing of point-like objects. Our results are approximately applicable to nonlinear least-squares estimation, and hence, although it is derived experimentally, they can be considered as a fundamental bound for SR of spectral estimators. We show that SR factors are in the range of 1.5-25 for the aforementioned parameter ranges of N and SNR.
    [bibtex-key = zhuBamlerTGRS2012CS] [bibtex-entry]


  499. S. Auer, S. Gernhardt, and Richard Bamler. Ghost Persistent Scatterers Related to Multiple Signal Reflections. IEEE Geosci. Remote Sens. Lett., 8(5):919-923, September 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, 3D SAR simulation methods, SAR data stack, TerraSAR-X, building structures, ghost persistent scatterers, multiple signal reflections, persistent scatterer interferometry, reflection levels, synthetic aperture radar, geophysical image processing, geophysical techniques, remote sensing by radar, synthetic aperture radar;.
    Abstract: Persistent scatterer interferometry using stacks of very high resolution synthetic aperture radar (SAR) data reveals that single or even patterns of scatterers representing building structures may wrongly be localized below the ground level. In this letter, a case study on a test building model is presented using 3-D SAR simulation methods in order to explain the underlying localization problem. The case study indicates that Ghost-PSs are likely to be related to reflection levels that are higher than three. Moreover, the temporal stability of the amplitude of fivefold bounce signals is confirmed for a SAR data stack.
    [bibtex-key = auerGernhardtBamler2011] [bibtex-entry]


  500. A. Budillon, A. Evangelista, and G. Schirinzi. Three-Dimensional SAR Focusing From Multipass Signals Using Compressive Sampling. IEEE Trans. Geosci. Remote Sens., 49(1):488 -499, jan. 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, 3D SAR data imaging, SAR tomography, compressive sampling, image formation, multipass SAR data, multipass signals, optimization problem, spaced acquisition orbits, three-dimensional synthetic aperture radar, tomographic imaging, truncated singular value decomposition technique, image sampling, optimisation, radar imaging, synthetic aperture radar, tomography;.
    Abstract: Three-dimensional synthetic aperture radar (SAR) image formation provides the scene reflectivity estimation along azimuth, range, and elevation coordinates. It is based on multipass SAR data obtained usually by nonuniformly spaced acquisition orbits. A common 3-D SAR focusing approach is Fourier-based SAR tomography, but this technique brings about image quality problems because of the low number of acquisitions and their not regular spacing. Moreover, attained resolution in elevation is limited by the overall acquisitions baseline extent. In this paper, a novel 3-D SAR data imaging based on Compressive Sampling theory is presented. It is shown that since the image to be focused has usually a sparse representation along the elevation direction (i.e., only few scatterers with different elevation are present in the same range-azimuth resolution cell), it suffices to have a small number of measurements to construct the 3-D image. Furthermore, the method allows super-resolution imaging, overcoming the limitation imposed by the overall baseline span. Tomographic imaging is performed by solving an optimization problem which enforces sparsity through #x2113;1-norm minimization. Numerical results on simulated and real data validate the method and have been compared with the truncated singular value decomposition technique.
    [bibtex-key = 5549892] [bibtex-entry]


  501. Mariko S. Burgin, D. Clewley, R. M. Lucas, and Mahta Moghaddam. A Generalized Radar Backscattering Model Based on Wave Theory for Multilayer Multispecies Vegetation. IEEE Transactions on Geoscience and Remote Sensing, 49(12):4832-4845, December 2011. Keyword(s): backscatter, radar polarimetry, remote sensing by radar, vegetation, AIRSAR data, ALOS PALSAR, Advanced Land Observing Satellite, Airborne Synthetic Aperture Radar data, Australia, NASA JPL, NASA Jet Propulsion Laboratory, Phased Arrayed L-band Synthetic Aperture Radar data, Queensland, distorted Born approximation, generalized radar backscattering model, microwave interaction, multilayer multispecies vegetation, polarimetric radar backscattering coefficients, single species discrete scatterer model, soil moisture, structurally complex vegetation, surface model, surface roughness parameterization, two layer crown trunk models, wave theory, wooded savanna sites, Backscatter, Data models, Mathematical model, Scattering, Synthetic aperture radar, Vegetation, Forest scattering, multispecies vegetation, synthetic aperture radar (SAR) backscattering, wave theory.
    Abstract: A generalized radar scattering model based on wave theory is described. The model predicts polarimetric radar backscattering coefficients for structurally complex vegetation comprised of multiple species and layers. Compared to conventional two-layer crown-trunk models, modeling of actual forests has been improved substantially, allowing better understanding of microwave interaction with vegetation. The model generalizes an existing single-species discrete scatterer model and, by including scattering and propagation effects through judiciously defined vegetation layers, enables its application to an arbitrary number of species types. The scatterers within each layer are modeled as finite cylinders or disks having arbitrary size, density, and orientation, as in the predecessor model. The distorted Born approximation is used to represent the propagation through each layer, while scattering from each is modeled as a linear superposition of scattering from its respective random collection of scatterers. Interactions of waves within and between each layer and direct scattering from the ground are accounted for. Validation of the model is presented based on its application to 23 wooded savanna sites located in Queensland, Australia, and comparison with Advanced Land Observing Satellite (ALOS) Phased Arrayed L-band Synthetic Aperture Radar (PALSAR) and National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) Airborne Synthetic Aperture Radar (AIRSAR) data. Results indicate good agreement between simulated and actual backscattering coefficients, particularly at HH and VV polarizations. More discrepancies are found at HV polarizations and can be explained by uncertainties in the knowledge of input parameters, such as inaccuracies in the surface model, surface roughness parameterization, and soil moisture.
    [bibtex-key = burginClewleyLucasMoghaddamTGRS2011RadarBackscatterModelBasedonWaveTheoryForMultilayerMultispeciesVeg] [bibtex-entry]


  502. E.J. Candes and Y. Plan. A Probabilistic and RIPless Theory of Compressed Sensing. IEEE Transactions on Information Theory, 57(11):7235-7254, November 2011. Keyword(s): Fourier coefficients, Gaussian model, RIPless theory, compressed sensing, frequency measurements, probabilistic theory, probability distribution, restricted isometry property, signal random model, sparse signals, Fourier analysis, data compression, random processes, signal reconstruction, statistical distributions;.
    Abstract: This paper introduces a simple and very general theory of compressive sensing. In this theory, the sensing mechanism simply selects sensing vectors independently at random from a probability distribution F; it includes all standard models-e.g., Gaussian, frequency measurements-discussed in the literature, but also provides a framework for new measurement strategies as well. We prove that if the probability distribution F obeys a simple incoherence property and an isotropy property, one can faithfully recover approximately sparse signals from a minimal number of noisy measurements. The novelty is that our recovery results do not require the restricted isometry property (RIP) to hold near the sparsity level in question, nor a random model for the signal. As an example, the paper shows that a signal with s nonzero entries can be faithfully recovered from about s logn Fourier coefficients that are contaminated with noise.
    [bibtex-key = 5967912] [bibtex-entry]


  503. Emmanuel J. Candès, Xiaodong Li, Yi Ma, and John Wright. Robust Principal Component Analysis. J. ACM, 58(3), June 2011. Keyword(s): Principal components, sparsity, robustness vis-a-vis outliers, L1-norm minimization, nuclear-norm minimization, duality, low-rank matrices, video surveillance.
    Abstract: This article is about a curious phenomenon. Suppose we have a data matrix, which is the superposition of a low-rank component and a sparse component. Can we recover each component individually? We prove that under some suitable assumptions, it is possible to recover both the low-rank and the sparse components exactly by solving a very convenient convex program called Principal Component Pursuit; among all feasible decompositions, simply minimize a weighted combination of the nuclear norm and of the L1 norm. This suggests the possibility of a principled approach to robust principal component analysis since our methodology and results assert that one can recover the principal components of a data matrix even though a positive fraction of its entries are arbitrarily corrupted. This extends to the situation where a fraction of the entries are missing as well. We discuss an algorithm for solving this optimization problem, and present applications in the area of video surveillance, where our methodology allows for the detection of objects in a cluttered background, and in the area of face recognition, where it offers a principled way of removing shadows and specularities in images of faces.
    [bibtex-key = candesLiMaWrightJACM2011RobustPCA] [bibtex-entry]


  504. Hubert M.J. Cantalloube and Carole E. Nahum. Multiscale Local Map-Drift-Driven Multilateration SAR Autofocus Using Fast Polar Format Image Synthesis. IEEE Trans. Geosci. Remote Sens., 49(10):3730-3736, 2011. Keyword(s): SAR Processing, Autofocus, SAR Autofocus, MoComp, Motion Compensation, Map-Drift Autofocus, Multiscale Local Map-Drift, geophysical image processing, geophysical techniques, image registration, remote sensing by radar, synthetic aperture radar, SAR high-resolution imaging, autofocus method, bistatic errors, bistatic synthetic aperture radar autofocus, clock drift errors, coarse-to-fine resolution, fast polar format image synthesis, frequency-domain polar format algorithm, local images, multilateration, range-clipped Doppler low-filtered profiles, target points, Doppler effect, Equations, Image resolution, Optical transmitters, Receivers, Synthetic aperture radar, Trajectory, Airborne radar, bistatic synthetic aperture radar (SAR), focusing.
    Abstract: This new autofocus method is based on multilateration by ranging to small target areas at independent directions on the ground. Range-clipped Doppler low-filtered profiles around target points are used to compute local images using frequency-domain polar format algorithm. Images obtained from adjacent subapertures are registered, and the displacements yield elevation, trajectory, or clock drift (bistatic case) errors. To alleviate the insensitivity of map drift to error fluctuation faster than subaperture duration, the algorithm is reiterated with coarse-to-fine resolution, yielding high to low frequency errors. This allowed true bistatic synthetic aperture radar (SAR) autofocus (without monostatic image), autofocus in circular SAR on remote areas, and, as a side product, our first successful air-to-air inverse SAR high-resolution imaging.
    [bibtex-key = cantalloubeNahumTGRS2011Autofocus] [bibtex-entry]


  505. M. C. Cuenca, A. J. Hooper, and Ramon F. Hanssen. A New Method for Temporal Phase Unwrapping of Persistent Scatterers InSAR Time Series. IEEE Trans. Geosci. Remote Sens., 49(11):4606-4615, November 2011. Keyword(s): Bayesian theory, InSAR time series, Southern Netherlands, persistent scatterer technique, probability density function, radar time series, spatially correlated information, temporal algorithm, temporal phase unwrapping, Bayes methods, geophysical image processing, radar imaging, radar interferometry, spatiotemporal phenomena, synthetic aperture radar, time series;.
    Abstract: The analysis of radar time series with persistent scatterer techniques usually relies on temporal unwrapping, because phase behavior can be often described by simple models. However, one of the major limitations of temporal algorithms is that they do not take advantage of spatially correlated information. Here, we focus on two types of information that can be spatially estimated, namely, observation precision and the probability density function of the model parameters. We introduce them in phase unwrapping using Bayesian theory. We test the proposed method using simulated data. We also apply them to a small area in the southern Netherlands and compare with conventional temporal unwrapping methods.
    [bibtex-key = cuencaHooperHanssen] [bibtex-entry]


  506. Francesco De Zan and Paco López-Dekker. SAR Image Stacking for the Exploitation of Long-Term Coherent Targets. IEEE Geosci. Remote Sens. Lett., 8(3):502-506, May 2011. Keyword(s): SAR Processing, SAR interferometry, Interferometry, differential interferometry, InSAR, DInSAR, Coherence, Decorrelation, Estimation, Interferometry, Noise, Stacking, Synthetic aperture radar, covariance matrices, filtering theory, radar imaging, synthetic aperture radar, SAR image stacking, TerraSAR-X data, coherent temporal filtering, covariance matrix, long-term coherent targets, long-term interferograms, signal to noise ratio, synthetic aperture radar images, Synthetic aperture radar interferometry;.
    Abstract: This letter shows that in a repeat-pass data set of synthetic aperture radar (SAR) images, a long-term coherent component, when present, can be recovered by coherent temporal filtering of the SAR images and can successively form interferograms with higher signal-to-noise ratio. The validity of the idea is confirmed through simulations and one example with real TerraSAR-X data. The theoretical necessity of using long-term interferograms is also discussed and linked to autoregressive processes, starting from the observation that the optimal weighting is given by the inverse of the covariance matrix.
    [bibtex-key = deZanLopezDekkerGRSL2011InSARStackingLongTimeSeries] [bibtex-entry]


  507. M. Eineder, C. Minet, P. Steigenberger, Xiaoying Cong, and T. Fritz. Imaging Geodesy: Toward Centimeter-Level Ranging Accuracy With TerraSAR-X. IEEE Transactions on Geoscience and Remote Sensing, 49(2):661-671, February 2011. Keyword(s): TerraSAR-X radar image, corner reflector technique, geodesy imaging method, glacier measurement, image correlation technique, large-scale Earth surface displacement, solid Earth tide motion vector, spaceborne radar amplitude image, synthetic aperture radar image correlation technique, tropospheric water vapor variation, volcano measurement, geodesy, geophysical image processing, geophysical techniques, ionosphere, radar imaging, remote sensing by radar, synthetic aperture radar, tides, troposphere.
    Abstract: In this paper, we report on experiments to measure large-scale Earth surface displacements, such as those caused by solid Earth tides, with centimeter-level accuracy using TerraSAR-X radar images. With two totally different approaches, corner reflectors and image correlation techniques, we show the clear interrelation between the radar range measurements and the projection of the solid Earth tide motion vector onto the radar line of sight. Pixel location accuracies of up to 2.6-cm standard deviation can be achieved after a single calibration. We further demonstrate that solid Earth tides and tropospheric water vapor variations are the largest sources of ranging error if not compensated for. Alternatively, tropospheric water vapor can be estimated with centimeter accuracy using our proposed technique of synthetic aperture radar (SAR) image correlation and solid Earth motion compensation by the existing models. We also consider ionospheric delays which improve the results marginally in the X-band. Our results show the best ranging accuracies so far reported for spaceborne radar amplitude images and make TerraSAR-X-together with our simple compensation methodology-suitable for the imaging of centimeter-level Earth displacements. Absolute measurements of volcanoes or glaciers are possible without the use of ground equipment and without the use of SAR interferometry, thus avoiding the associated problems of phase ambiguity, phase unwrapping, and reference points.
    [bibtex-key = einederMinetSteigenbergerXiaoyingCongFritz2011] [bibtex-entry]


  508. A. Elsherbini and K. Sarabandi. Dual-Polarized Coupled Sectorial Loop Antennas for UWB Applications. IEEE_J_AWPL, 10:75-78, 2011. Keyword(s): antenna feeds, antenna radiation patterns, directive antennas, electromagnetic wave polarisation, loop antennas, radar antennas, ultra wideband antennas, ultra wideband communication, ultra wideband radar, communications applications, compact dual-polarized UWB antennas, coupled sectorial loop antenna concept, crossed dipole configuration, directional dual-polarized cavity-backed asymmetric CSLA, dual-polarized coupled sectorial loop antennas, integrated balun feed, planar CSLA, radar applications, Antenna measurements, Cavity resonators, Directive antennas, Feeds, Impedance matching, Ultra wideband antennas, Dual-polarized antenna, radar antenna, ultrawideband (UWB) antenna. [bibtex-key = Elsherbini2011f] [bibtex-entry]


  509. Geir Engen and Yngvar Larsen. Efficient Full Aperture Processing of TOPS Mode Data Using the Moving Band Chirp Z -Transform. IEEE Trans. Geosci. Remote Sens., 49(10):3688-3693, October 2011. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, Z transforms, artificial satellites, satellite communication, European Space Agency, Sentinel-1 operational satellite, TOPS mode data, Terrain Observation by Progressive Scans, azimuth aperture, full aperture processing, imaging mode, moving band chirp Z-transform, signal transform, wavenumber domain processor, Antennas, Azimuth, Bandwidth, Chirp, Doppler effect, Focusing, Time frequency analysis, SAR processing, synthetic aperture radar (SAR), terrain observation by progressive scans (TOPS).
    Abstract: The main operational mode of the European Space Agency's upcoming Sentinel-1 operational satellite will be the Terrain Observation by Progressive Scans (TOPS) imaging mode. This paper presents a very efficient wavenumber domain processor for the processing of TOPS mode data. In particular, a novel signal transform, called a moving band chirp Z-transform, is introduced in order to allow the entire azimuth aperture to be focused simultaneously without any need for temporary unaliasing, which requires upsampling, or subaperture processing.
    [bibtex-key = engenLarsenTGARS2011TOPSwithChirpZTransform] [bibtex-entry]


  510. A. Ferretti, A. Fumagalli, F. Novali, C. Prati, F. Rocca, and A. Rucci. A New Algorithm for Processing Interferometric Data-Stacks: SqueeSAR. IEEE Trans. Geosci. Remote Sens., 49(9):3460-3470, September 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Alpine area, PSInSAR, SqueeSAR, city, coherence matrix, coherent radar targets, distributed scatterers, geophysical parameters, high phase stability, interferograms, interferometric data-stacks processing algorithm, man-made objects, permanent scatterer SAR interferometry, point-wise objects, statistical behavior, data analysis, geophysical techniques, radar interferometry, radar signal processing, remote sensing by radar, synthetic aperture radar;.
    Abstract: Permanent Scatterer SAR Interferometry (PSInSAR) aims to identify coherent radar targets exhibiting high phase stability over the entire observation time period. These targets often correspond to point-wise, man-made objects widely available over a city, but less present in non-urban areas. To overcome the limits of PSInSAR, analysis of interferometric data-stacks should aim at extracting geophysical parameters not only from point-wise deterministic objects (i.e., PS), but also from distributed scatterers (DS). Rather than developing hybrid processing chains where two or more algorithms are applied to the same data-stack, and results are then combined, in this paper we introduce a new approach, SqueeSAR, to jointly process PS and DS, taking into account their different statistical behavior. As it will be shown, PS and DS can be jointly processed without the need for significant changes to the traditional PSInSAR processing chain and without the need to unwrap hundreds of interferograms, provided that the coherence matrix associated with each DS is properly squeezed to provide a vector of optimum (wrapped) phase values. Results on real SAR data, acquired over an Alpine area, challenging for any InSAR analysis, confirm the effectiveness of this new approach.
    [bibtex-key = ferrettiFumagalliNovaliPratiRoccaRucci2011PSISqueeSAR] [bibtex-entry]


  511. G. Fornaro, A. Pauciullo, and D. Reale. A Null-Space Method for the Phase Unwrapping of Multitemporal SAR Interferometric Stacks. IEEE Transactions on Geoscience and Remote Sensing, 49(6):2323-2334, June 2011. Keyword(s): SAR Processing, DInSAR, SAR Interferometry, Phase unwrapping, 2-D azimuth-range domain, Earth surface displacement, MCF algorithm, data analysis, full 3-D unwrapping method, interferogram generation scheme, interferogram spatial structure, multitemporal SAR interferometric stack analysis, multitemporal differential interferometric synthetic aperture radar analysis, null-space method, optimization technique, phase unwrapping method, spatial baseline domain, time series, unrestricted phase signal reconstruction, data analysis, geophysical techniques, radar interferometry, synthetic aperture radar, time series, topography (Earth).
    Abstract: Multitemporal differential interferometric synthetic aperture radar analysis is of fundamental importance in the monitoring of Earth surface displacements. In this context, a key role for the reconstruction of the deformation maps and time series is played by the phase unwrapping (PhU) that reconstructs the unrestricted phase signals starting from the measured wrapped versions, i.e., the interferograms. PhU is typically carried out independently for each interferogram in the 2-D azimuth-range domain via the efficient minimum cost flow (MCF) optimization technique. Recently, it has been proposed a two-step (TS) strategy that exploits both the temporal and the spatial structures of the available interferograms. The MCF algorithm is applied in this case also in the temporal/spatial baseline domain, and this step is combined with the classical 2-D space unwrapping. However, the restriction on the use of the MCF algorithm in the baseline domain poses limitations on the interferogram generation scheme. We present a formulation which makes use of the overdetermined nature of the operator that relates the phase differences to the absolute phase values: the problem is addressed in a more general framework that can cope with the 3-D (2-D space and time) nature of the data. This formulation is derived with reference to the sequential (TS) approach to overcome its restrictions on the interferogram generation. The new algorithm is validated on both simulated and real data. Moreover, the use of this new formulation for a full 3-D unwrapping is also addressed.
    [bibtex-key = fornaroPauciulloRealeTGRS2011PhaseUnwrapInSARStacks] [bibtex-entry]


  512. Othmar Frey and Erich Meier. 3-D Time-Domain SAR Imaging of a Forest Using Airborne Multibaseline Data at L- and P-Bands. IEEE Trans. Geosci. Remote Sens., 49(10):3660-3664, October 2011. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: In this paper, a time-domain back-projection based tomographic processing approach to a 3-D reconstruction grid is detailed, with the focusing in the third dimension being either modified versions of multilook standard beamforming, robust Capon beamforming, or multiple signal classification. The novel feature of the proposed approach compared to previous synthetic aperture radar (SAR) tomography approaches is that it allows for an approximation-free height-dependent calculation of the sample covariance matrix by exploiting the azimuth-focused data on the 3-D reconstruction grid. The method is applied to experimental multibaseline quad-pol SAR data at L- and P-bands acquired by German Aerospace Center's (DLR) E-SAR sensor: Tomographic images of a partially forested area, including a 3-D voxel plot that visualizes the very high level of detail of the tomographic image, are shown, and an analysis of the focusing performance is given for the full as well as reduced synthetic aperture in the normal direction.
    [bibtex-key = freyMeier2010:TGARS] [bibtex-entry]


  513. Othmar Frey and Erich Meier. Analyzing Tomographic SAR Data of a Forest With Respect to Frequency, Polarization, and Focusing Technique. IEEE Trans. Geosci. Remote Sens., 49(10):3648-3659, October 2011. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, scattering, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: Forest canopies are semitransparent to microwaves at both L- and P-bands. Thus, a number of scattering sources and different types of scattering mechanisms may contribute to a single range cell of a synthetic aperture radar (SAR) image. By appropriately combining the SAR data of multiple parallel flight paths, a large 2-D aperture is synthesized, which allows for tomographic imaging of the 3-D structure of such semitransparent media and the underlying ground. A separate paper deals with the actual tomographic imaging part that leads to the 3-D data cube. In particular, three focusing techniques are described and analyzed: multilook beamforming, robust Capon beamforming, and multiple signal classification beamforming. In this paper, the resulting data products obtained by tomographically focusing two airborne multibaseline SAR data sets of a partially forested area, one at L-band and another at P-band, are subject to a detailed analysis with respect to the location and the type of backscattering sources. In particular, the following aspects are investigated: 1) The forest structure, as obtained from the vertical profiles of intensities at sample plot locations within the forest, is compared to the height distribution of the top of the forest canopy, as derived from airborne laser scanning data, and profiles are presented for all polarimetric channels and focusing techniques, as well as at both frequencies; 2) the type and location of scattering mechanisms are analyzed as functions of height for the two frequencies, namely, L- and P-bands, and using the polarimetric channels, as well as the Pauli and Cloude-Pottier decompositions thereof; and 3) the accuracy of the ground elevation estimation obtained from the different focusing techniques and the two frequencies is assessed with the help of a lidar-derived digital elevation model.
    [bibtex-key = freyMeier2010DataAnalysis:TGARS] [bibtex-entry]


  514. Guido Gatti, Stefano Tebaldini, Mauro Mariotti d'Alessandro, and Fabio Rocca. ALGAE: A Fast Algebraic Estimation of Interferogram Phase Offsets in Space-Varying Geometries. IEEE Trans. Geosci. Remote Sens., 49(6):2343-2353, June 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, ALGAE, DLR, E-SAR airborne system, ESA BIOSAR 2008 campaign, European Space Agency, Experimental SAR airborne system, German Aerospace Center, P-band data set, airborne multipass interferometric campaigns, data stack phase locking, incidence angle sensitivity, interferogram phase offset algebraic estimation, interferogram phase offset compensation, multipass InSAR analysis, null space component identification, space varying geometries, synthetic aperture radar interferometry, system geometry space varying nature, system geometry variation, terrain topography estimation, terrain topography retrieval, geophysical signal processing, linear algebra, radar interferometry, radar signal processing, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth);.
    Abstract: This paper deals with the estimation of terrain topography from multipass synthetic aperture radar (SAR) interferometry (InSAR), focusing on the case where variation of the system geometry within the imaged swath is relevant. A typical case is represented by airborne multipass interferometric campaigns where, due to the closeness between the radar sensor and the targets, the incidence-angle sensitivity undergoes a dramatic increase with respect to the spaceborne case, resulting in a high spatial variability of the normal baselines. The space-varying nature of the system geometry gives rise to a major issue in multipass InSAR analyses in that it prevents from compensating for the presence of interferogram phase offsets by simply phase locking the data stack to a reference point, therefore hindering the retrieval of terrain topography. To cope with this issue properly, we propose a novel approach that exploits the algebraic properties of the problem. Such an approach allows casting the problem in terms of identification of a null-space component for terrain topography after which both topography and the interferogram phase offsets are quickly obtained without exploiting calibration points. Experimental results are shown based on a P-band data set acquired by the Experimental SAR (E-SAR) airborne system, operated by the German Aerospace Center (DLR), in the framework of the European Space Agency (ESA) campaign BIOSAR 2008.
    [bibtex-key = gattiTebaldiniMariottidAlessandroRocca2010ALGAE] [bibtex-entry]


  515. K. Goel and N. Adam. Three-Dimensional Positioning of Point Scatterers Based on Radargrammetry. IEEE Trans. Geosci. Remote Sens., PP(99):1-9, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography.
    Abstract: This paper presents a new technique for the retrieval of 3-D point scatterer (PS) location and resolution cell configuration exploiting only the intensity of synthetic aperture radar (SAR) images. The implemented method is based on the principle of radargrammetry and makes use of Bayesian inference, wherein, directed graphs are utilized to represent dependencies of random variables and probability density functions are modeled by particle filter representations. Compared to published radargrammetric work, the newly developed algorithm optimally exploits stacks of acquisitions taken with at least three different incidence angles. Due to the large angular diversity, the method is insensitive to atmospheric propagation effects and motion of the scatterers in contrast to the coherent phase-based techniques such as persistent scatterer interferometry and SAR Tomography. Additionally, the method estimates absolute height, which is a big advantage compared to the relative estimates provided by the phase-based techniques, which moreover encounter phase unwrapping errors and temporal decorrelation. High-resolution spotlight TerraSAR-X data of Berlin central station is used as a processing example for this technique. The test case demonstrates the unambiguous absolute height estimation of PSs and resolving of complicated scattering situations (e.g., layover) in urban areas.
    [bibtex-key = 6087374] [bibtex-entry]


  516. Roy E. Hansen, H. J. Callow, T. O. Sabo, and S. A. V. Synnes. Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar. IEEE Transactions on Geoscience and Remote Sensing, 49(10):3677-3687, October 2011. Keyword(s): Synthetic Aperture Sonar, SAS, bathymetry, oceanographic equipment, radar imaging, radar interferometry, seafloor phenomena, synthetic aperture sonar, underwater sound, HISAS 1030 interferometric SAS, acoustic signals, aided inertial navigation, bathymetry, centimeter resolution, coherence, grating lobes, image focusing, nonstraight tracks, ocean environment, real aperture interferometry, sea surface, seafloor imaging, seafloor mapping, shallow waters, sonar positioning, sound velocity, synthetic aperture sonar, vehicle instability, Apertures, Image resolution, Imaging, Sonar navigation, Synthetic aperture sonar, Vehicles, Interferometry, multiple reflections, navigation, nonlinear tracks, seafloor imaging, sound velocity errors, synthetic aperture radar (SAR), synthetic aperture sonar (SAS), topography errors.
    Abstract: Synthetic aperture sonar (SAS) is emerging as an imaging technology that can provide centimeter resolution over hundreds-of-meter range on the seafloor. Although the principle of SAS has been known for more than 30 years, SAS systems have only recently become commercially available. The success of SAS is critically dependent on overcoming several challenges related to the ocean environment. The sonar has to be positioned with accuracy better than a fraction of a wavelength along the synthetic aperture. We use the sensor itself for navigation, in combination with aided inertial navigation. The sound velocity has to be accurately estimated for successful focusing of SAS images. We calculate a simple rule of thumb for tolerance and show the effect of incorrect sound velocity. For nonstraight synthetic apertures, the bathymetry must be estimated. We use real aperture interferometry to map the scene before SAS processing. We calculate the required bathymetry accuracy and show the effects of insufficient mapping. Vehicle instability and nonstraight tracks, in combination with insufficient navigation accuracy, can cause grating lobes in the SAS images, which is not common in single-channel synthetic aperture radars. We show example imagery with severe grating lobes. In shallow waters, the acoustic signals will interact with the sea surface, possibly causing multipath. This will reduce the SAS quality. We use coherence to map the signal to multipath and, thereby, the valid sensor range. This paper illustrates the different challenges using examples from the HISAS 1030 interferometric SAS.
    [bibtex-key = hansenCallowSaboSynnesTGRS2011ChallengesInSeafloorImaginingWithSASonar] [bibtex-entry]


  517. Lorenzo Iannini and Andrea Monti Guarnieri. Atmospheric Phase Screen in Ground-Based Radar: Statistics and Compensation. IEEE Geoscience and Remote Sensing Letters, 8(3):537-541, May 2011. Keyword(s): atmospheric electromagnetic wave propagation, atmospheric humidity, atmospheric pressure, atmospheric techniques, atmospheric temperature, radiowave propagation, remote sensing by radar, APS removal, Bolzano, Italy, atmospheric humidity, atmospheric phase screen evaluation, atmospheric phase screen removal, atmospheric pressure, atmospheric temperature, compensation approach, ground based radar, initial calibration step, meteorological parameters, time varying delay statistics, Atmospheric modeling, Calibration, Coherence, Delay, Humidity, Radar, Refractive index, Atmospheric artifact compensation, differential interferometry, ground-based radar (GB-RADAR).
    Abstract: In this letter, we face one of the main issues in ground-based radar applications, i.e., the evaluation and removal of the atmospheric phase screen (APS). The time-varying delay statistics are assessed by means of both radars and meteo simulated data sets and are critically interpreted with particular reference to the entailed compensation issues. A compensation approach based on the available on-site meteo parameters (pressure, temperature, and humidity) is then investigated. The technique proposes an initial calibration step on humidity which leads to significant improvements in the APS removal. The results of the technique are discussed in the case of a real campaign data set (Bolzano, Italy) that covers a temporal baseline of about one week.
    [bibtex-key = ianniniMontiGuarnieriGRSL2011AtmosphereStatisticsGBSAR] [bibtex-entry]


  518. M.A. Janssen, A. Le Gall, and L.C. Wye. Anomalous radar backscatter from Titan's surface?. Icarus, 212(1):321-328, 2011. Keyword(s): Titan, Satellites, Surfaces, Radio observations, Radar observations.
    Abstract: Since Cassini arrived at Saturn in 2004, its moon Titan has been thoroughly mapped by the RADAR instrument at 2-cm wavelength, in both active and passive modes. Some regions on Titan, including Xanadu and various bright hummocky bright terrains, contain surfaces that are among the most radar-bright encountered in the Solar System. This high brightness has been generally attributed to volume scattering processes in the inhomogeneous, low-loss medium expected for a cold, icy satellite surface. We can test this assumption now that the emissivity has been obtained from the concurrent radiometric measurements for nearly all the surface, with unprecedented accuracy (Janssen et al., and the Cassini RADAR Team [2009]. Icarus 200, 222-239). Kirchhoff's law of thermal radiation relates the radar and radiometric properties in a way that has never been fully exploited. In this paper we examine here how this law may be applied in this case to better understand the nature of Titan's radar-bright regions. We develop a quantitative model that, when compared to the observational data, allows us to conclude that either the reflective characteristics of the putative volume scattering subsurface must be highly constrained, or, more likely, organized structure on or in the surface is present that enhances the backscatter.
    [bibtex-key = janssenLeGallWyeICARUS2011AnomalousRadarBackscatterFromTitansSurface] [bibtex-entry]


  519. Tom R. Lauknes, Howard A. Zebker, and Y. Larsen. InSAR Deformation Time Series Using an L1-Norm Small-Baseline Approach. IEEE Trans. Geosci. Remote Sens., 49(1):536-546, January 2011. Keyword(s): SAR Processing, InSAR deformation time series, L1-norm small-baseline approach, displacement phase, land displacement monitoring, reweighted least squares algorithm, satellite data, satellite synthetic aperture radar interferometry, sparse data set, surface displacement, two-dimensional unwrapping, unwrapped interferogram, least squares approximations, radar interferometry, spaceborne radar, synthetic aperture radar, time series;.
    Abstract: Satellite synthetic aperture radar interferometry (InSAR) is an invaluable tool for land displacement monitoring. Improved access to time series of satellite data has led to the development of several innovative multitemporal algorithms. Small baseline (SB) is one such time-series InSAR method, based on combining and inverting a set of unwrapped interferograms for surface displacement. Two-dimensional unwrapping of sparse data sets is a challenging task, and unwrapping errors can lead to incorrectly estimated deformation time series. It is well known that L1-norm is more robust than L2-norm cost function minimization if the data set has a large number of outlying points. In this paper, we present an L1-norm-based SB method using an iteratively reweighted least squares algorithm. We show that the displacement phase of both synthetic data, as well as a real data set that covers the San Francisco Bay area, is recovered more accurately than with L2-norm solutions.
    [bibtex-key = lauknesZebkerLarsen2011] [bibtex-entry]


  520. Yun Lin, Wen Hong, Weixian Tan, Yanping Wang, and Yirong Wu. Interferometric Circular SAR Method for Three-Dimensional Imaging. IEEE Geosci. Remote Sens. Lett., 8(6):1026-1030, November 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, circular apertures, coherence, interferometric CSAR method, phase cycle ambiguities, phase difference, three dimensional imaging, tomographic imaging capability, coherence, radar imaging, remote sensing by radar, synthetic aperture radar;.
    Abstract: The aperture of 360 #x00B0; gives circular synthetic aperture radar (SAR) (CSAR) the capability to detect hidden target when its orientation is unknown. Subwavelength resolution can also be achieved when the target in the spotted area is observed under a complete circular aperture. Furthermore, the aspect angle diversity inherent to the circular trajectory makes possible a 3-D target reconstruction. However, the latter two potentials require certain target reflectivity homogeneity. For a highly directive scatterer, it has no resolving ability in the direction normal to the data collection plane. In this letter, a new interferometric CSAR method is presented to enhance the tomographic imaging capability for highly directive scatterers without sacrificing other scatterers' resolutions. This method takes advantage of the coherence and the phase difference between a pair of 3-D SAR images formed from data collected at two separate circular apertures to eliminate targets that focused at a wrong elevation. In addition, it uses two different transmit frequencies to solve the problem of phase cycle ambiguities. Finally, simulation results validate this new approach.
    [bibtex-key = linHongTanWangWu2011] [bibtex-entry]


  521. Guoxiang Liu, Hongguo Jia, Rui Zhang, Huixin Zhang, Hongliang Jia, Bing Yu, and Mingzhi Sang. Exploration of Subsidence Estimation by Persistent Scatterer InSAR on Time Series of High Resolution TerraSAR-X Images. Selected Topics in Applied Earth Observations and Remote Sensing, IEEE Journal of, 4(1):159 -170, march 2011. Keyword(s): China, Jinghai County, Jinghu high-speed railway, Tianjin, X-band radar sensor, buildings, data reduction procedures, engineering risk assessment, ground subsidence, groundwater, high resolution TerraSAR-X images, in situ data, land use planning, manhole covers, persistent scatterer InSAR, radar line-of-sight direction, street lamps, subsidence estimation, time series, wavelength 3.1 cm, geomorphology, geophysical techniques, radar interferometry, remote sensing by radar, synthetic aperture radar;.
    Abstract: Ground subsidence is a major concern for land use planning and engineering risk assessment. This paper explores subsidence detection by the persistent scatterer (PS) interferometric synthetic aperture radar (InSAR) technique using the multitemporal high resolution spaceborne SAR images. We first describe the mathematical models and the data reduction procedures of the PS solution. The experiments of subsidence detection are then carried out over the Jinghai County in Tianjin (China) which has been sinking due to overuse of groundwater. The time series of high resolution SAR images collected by the X-band radar sensor onboard the satellite TerraSAR-X (TSX) are utilized for the PS detection, PS networking and subsidence estimation. The experimental results demonstrate that the high resolution of TSX SAR images can dramatically increase the PSs' density and coverage extent, especially in the built-up areas. Subsidence values can be extracted on the individual objects like buildings, street lamps and manhole covers, and on the linear engineering structures like the Jinghu high-speed railway. The PS InSAR with short radar wavelength (3.1 cm) is quite sensitive to ground displacement in the radar line-of-sight direction, and the derived subsidence measurements are in good agreement with the in situ data taken by optical leveling.
    [bibtex-key = 5560692] [bibtex-entry]


  522. Paco López-Dekker, Pau Prats, F. De Zan, D. Schulze, G. Krieger, and Alberto Moreira. TanDEM-X First DEM Acquisition: A Crossing Orbit Experiment. IEEE Geoscience and Remote Sensing Letters, PP(99):1-943, 2011. Keyword(s): SAR Processing, InSAR, Interferometry, SAR Interferometry, TerrSAR-X, TanDEM-X, Spaceborne SAR, Single-Pass.
    Abstract: This letter describes the first interferometric acquisitions and results obtained by the TerraSAR-X add-on for Digital Elevation Measurements mission. Due to the large along-track separation between the two satellites during the approaching maneuver and the Earth's rotation, useful interferometric acquisitions were only possible at high latitudes. This resulted in a crossing angle between the ground tracks whose impact was corrected by acquiring the two synthetic-aperture radar images with an opposite squint. The still very large 2-km cross-track baseline resulted in a 3.8-m interferometric height of ambiguity, producing extremely detailed images of the topography of the target area. Results acquired over the October Revolution Island, Russia, are shown and discussed.
    [bibtex-key = lopezDekkerPratsDeZanSchulzeKriegerMoreira2011:TanDEM] [bibtex-entry]


  523. O. Monserrat, Michele Crosetto, M. Cuevas, and B. Crippa. The Thermal Expansion Component of Persistent Scatterer Interferometry Observations. IEEE Geosci. Remote Sens. Lett., 8(5):864 -868, September 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Barcelona metropolitan area, PSI analysis, PSI products, Spain, TerraSAR-X spaceborne sensor, X-band SAR interferometric phases, deformation velocity maps, imaged objects, persistent scatterer interferometry observations, spaceborne radar, standard two-parameter PSI model, synthetic aperture radar acquisitions, thermal dilation parameter, thermal dilation phase component, thermal expansion component, thermal map, very high resolution X-band StripMap SAR data, geophysical image processing, geophysical techniques, radar interferometry, spaceborne radar, synthetic aperture radar, thermal expansion;.
    Abstract: This letter focuses on the thermal expansion component of persistent scatterer (PS) interferometry (PSI), which is a result of temperature differences in the imaged area between synthetic aperture radar (SAR) acquisitions. This letter is based on very high resolution X-band StripMap SAR data captured by the TerraSAR-X spaceborne sensor. The X-band SAR interferometric phases are highly influenced by the thermal dilation of the imaged objects. This phenomenon can have a strong impact on the PSI products, particularly on the deformation velocity maps, if not properly handled during the PSI analysis. In this letter, we propose a strategy to deal with the thermal dilation phase component, which involves further developing the standard two-parameter PSI model (deformation velocity and residual topographic error) with a third unknown parameter called the thermal dilation parameter, which is estimated for each PS. The map obtained from plotting this parameter for all PSs of a given area is hereafter called thermal map. This letter describes the proposed model and outlines the issue of parameter estimability. In addition, the potential of exploiting the thermal maps is analyzed by illustrating two examples of the Barcelona (Spain) metropolitan area. Thermal maps provide two types of information: The first one is the coefficient of thermal expansion of the observed objects, while the second one, which is related to the pattern of the thermal dilation parameter, gives information about the static structure of these objects. Two important aspects that influence the exploitation of thermal maps are discussed in the last section of this letter: the line-of-sight nature of the derived estimates and the achievable precision in the estimation of the coefficient of thermal expansion.
    [bibtex-key = monserratCrosettoCuevasCrippa2011PSI] [bibtex-entry]


  524. Daniele Perissin and Teng Wang. Time-Series InSAR Applications Over Urban Areas in China. Selected Topics in Applied Earth Observations and Remote Sensing, IEEE Journal of, 4(1):92-100, March 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR, Interferometry, geomorphology, radar interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, time series, Badong, Dragon project, European Space Agency, NRSCC, National Remote Sensing Center of China, PS-InSAR, Permanent Scatterers, Quasi-PS InSAR, SAR interferometry, Shanghai, Three Gorges Dam, Tianjin, big man-made structures, cities growth, coherent-uncoherent analysis, landslide monitoring, subsidence monitoring, terrain movement, time-series InSAR, urban areas, urban development, Synthetic aperture radar interferometry (InSAR), Three Gorges Project (TGP), time-series analysis, urban development;.
    Abstract: In this study, we present the results achieved within the Dragon project, a cooperation program between the European Space Agency (ESA) and the National Remote Sensing Center of China (NRSCC), about monitoring subsidences and landslides in urban areas, analyzing cities growth and measuring the deformation of big man-made structures. Among the processed areas, we report here the main results we obtained in the test sites of Shanghai, Tianjin, Badong, and Three Gorges Dam. The techniques that have been used to process the data are original SAR interferometry (InSAR), Permanent Scatterers (PS-InSAR), Quasi-PS InSAR (QPS-InSAR), and a combination of coherent-uncoherent analysis. The results show that time-series InSAR techniques allow us to extract ground information with high spatial density and thus help us understanding the impact of urban development on terrain movements.
    [bibtex-key = perissinWangJSTARS2011] [bibtex-entry]


  525. Diego Reale, Gianfranco Fornaro, A. Pauciullo, Xiao Xiang Zhu, and Richard Bamler. Tomographic Imaging and Monitoring of Buildings With Very High Resolution SAR Data. IEEE Geosci. Remote Sens. Lett., 8(4):661-665, July 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, InSAR, SAR Interferometry, Interferometry, DInSAR, persistent scatterer interferometry, PSI, 3-D structure, 4-D imaging, TerraSAR-X spotlight acquisitions, buildings, high-resolution sensors, impressive reconstruction, medium-resolution SAR data tomographic techniques, scatterer density, steep topography, synthetic aperture radar, tomographic imaging, urban environment, geophysical image processing, image reconstruction, image resolution, radar resolution, remote sensing by radar, synthetic aperture radar.
    Abstract: Layover is frequent in imaging and monitoring with synthetic aperture radar (SAR) areas characterized by a high density of scatterers with steep topography, e.g., in urban environment. Using medium-resolution SAR data tomographic techniques has been proven to be capable of separating multiple scatterers interfering (in layover) in the same pixel. With the advent of the new generation of high-resolution sensors, the layover effect on buildings becomes more evident. In this letter, we exploit the potential of the 4-D imaging applied to a set of TerraSAR-X spotlight acquisitions. Results show that the combination of high-resolution data and advanced coherent processing techniques can lead to impressive reconstruction and monitoring capabilities of the whole 3-D structure of buildings.
    [bibtex-key = realeFornaroPauciulloZhuBamler2011] [bibtex-entry]


  526. D. Reale, D. O. Nitti, D. Peduto, R. Nutricato, F. Bovenga, and G. Fornaro. Postseismic Deformation Monitoring With the COSMO/SKYMED Constellation. IEEE Geoscience and Remote Sensing Letters, PP(99):696-700, 2011. Keyword(s): SAR Processing, DInSAR, Deformation monitoring, InSAR, SAR Interferometry.
    Abstract: COSMO/SKYMED is currently the unique constellation of synthetic aperture radar (SAR) sensors operative, which is also for civilian use. On April 6, 2009, an Mw 6.3 earthquake struck the city of l'Aquila in Central Italy. The constellation acquired data stacks over the hit area at an unprecedented temporal rate. In this letter, the results obtained by processing several data set via two independent multitemporal differential interferometric SAR techniques are presented to demonstrate the capability of this constellation in postseismic deformations monitoring.
    [bibtex-key = 5710028] [bibtex-entry]


  527. A. Refice, A. Belmonte, F. Bovenga, and G. Pasquariello. On the Use of Anisotropic Covariance Models in Estimating Atmospheric DInSAR Contributions. IEEE Geoscience and Remote Sensing Letters, 8(2):341-345, March 2011. Keyword(s): DInSAR techniques, anisotropic atmospheric signals, anisotropic covariance models, atmospheric phase delays, atmospheric phase estimation, atmospheric phase screen field, automated methods, computational cost, correlation function parameters, critical sampling conditions, differential interferometric synthetic aperture radar data, kriging reconstruction approaches, persistent scatterer interferometry, sampling density, sparse-grid point-target DInSAR applications, spatial structure, stochastic models, synthetic aperture radar interferograms, atmospheric techniques, radar interferometry, stochastic processes, synthetic aperture radar;.
    Abstract: Stochastic models are often used to describe the spatial structure of atmospheric phase delays in differential interferometric synthetic aperture radar (DInSAR) data. Synthetic aperture radar interferograms often exhibit anisotropic atmospheric signals. In view of this, the use of anisotropic models for atmospheric phase estimation is increasingly advocated. However, anisotropic models lead to increased computational complexity in estimating the correlation function parameters with respect to the isotropic case. Moreover, the performance is degraded when dealing with DInSAR techniques involving only a few sparse points usable for computations, as in the case of persistent scatterer interferometry applications, particularly when this estimation has to be done in an automated way on many interferograms. In the present work, we propose some observations about the actual advantage given by anisotropic modeling of atmospheric phase in the case of sparse-grid point-target DInSAR applications. Through analysis of simulated data, we observe that an improvement in the performances of kriging reconstruction approaches can be obtained only when sufficient sampling densities are available. In critical sampling conditions, automated methods with reasonable computational cost may improve their performance if external information on the atmospheric phase screen field is available.
    [bibtex-key = reficeBelmonteBovengaPasquariello2011] [bibtex-entry]


  528. Xiaozhen Ren, Jiantao Sun, and Ruliang Yang. A New Three-Dimensional Imaging Algorithm for Airborne Forward-Looking SAR. IEEE Geosci. Remote Sens. Lett., 8(1):153-157, January 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Fourier transform, airborne forward-looking SAR, along-track processing, echo signal, imaging geometry, multiplication operations, nonlinear frequency modulation scaling, point scatterers, synthetic aperture radar, three-dimensional imaging algorithm, Fourier transforms, airborne radar, modulation, radar imaging, synthetic aperture radar;.
    Abstract: In this letter, a new 3-D imaging algorithm is proposed for forward-looking synthetic aperture radar based on the imaging geometry and the characteristic of the echo signal. The key point of the proposed algorithm is the introduction of the nonlinear frequency modulation scaling in along-track processing to obtain accurate focusing. As the method needs only Fourier transform and multiplication operations, it is computationally efficient. Simulations with point scatterers are used to validate the method.
    [bibtex-key = 5535115] [bibtex-entry]


  529. Angel Ribalta. Time-Domain Reconstruction Algorithms for FMCW-SAR. IEEE Geosci. Remote Sens. Lett., 8(3):396-400, May 2011. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, frequency-modulated continuous-wave, Ka-band, SAR, airborne SAR, FMCW, COBRA, Fraunhofer, FGAN, X-Band, W-Band, 10 GHz, 35 GHz, 94 GH, 220 GHz, 4 GHz Bandwidth at 35 and 94 GHz, 8 GHz Bandwidth at 220 GHz.
    Abstract: In this letter, we develop time-domain reconstruction algorithms for frequency-modulated continuous wave synthetic aperture radar (FMCW-SAR). The algorithms considered here are the time-domain correlation algorithm, and two versions of the backprojection algorithm: the standard one based on the start-stop approximation, and a modified version that takes into account the movement of the sensor during the transmission of the pulse. Numerical simulations illustrate the performance of the algorithms, showing that the start-stop approximation may not be valid for FMCW-SAR, whereas the modified backprojection algorithm works very well here.
    [bibtex-key = ribaltaGRSL2011TDBPforFMCW] [bibtex-entry]


  530. Eric J. Rignot, J. Mouginot, and Bernd Scheuchl. Ice Flow of the Antarctic Ice Sheet. Science, 333(6048):1427-1430, 2011.
    Abstract: We present a reference, comprehensive, high-resolution, digital mosaic of ice motion in Antarctica assembled from multiple satellite interferometric synthetic-aperture radar data acquired during the International Polar Year 2007 to 2009. The data reveal widespread, patterned, enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent. This view of ice sheet motion emphasizes the importance of basal-slip-dominated tributary flow over deformation-dominated ice sheet flow, redefines our understanding of ice sheet dynamics, and has far-reaching implications for the reconstruction and prediction of ice sheet evolution.
    [bibtex-key = rignotMouginotScheuchlScience2011] [bibtex-entry]


  531. Marc Rodriguez-Cassola, Pau Prats, Gerhard Krieger, and Alberto Moreira. Efficient Time-Domain Image Formation with Precise Topography Accommodation for General Bistatic SAR Configurations. IEEE Transactions on Aerospace and Electronic Systems, 47(4):2949-2966, October 2011. Keyword(s): SAR Processing, Bistatic SAR, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Fast Back-Projection, Back-Projection, Doppler information, German Aerospace Center, TerraSAR-X/F-SAR bistatic data, TerraSAR-X, F-SAR, nonstationary bistatic acquisitions, phase-preserving bistatic focusing, synchronization algorithm, airborne radar, backscatter, calibration, data acquisition, geophysical signal processing, radar signal processing, remote sensing by radar, spaceborne radar, synchronisation, synthetic aperture radar.
    Abstract: Due to the lack of an appropriate symmetry in the acquisition geometry, general bistatic synthetic aperture radar (SAR) cannot benefit from the two main properties of low-to-moderate resolution monostatic SAR: azimuth-invariance and topography-insensitivity. The precise accommodation of azimuth-variance and topography is a real challenge for efficent image formation algorithms working in the Fourier domain, but can be quite naturally handled by time-domain approaches. We present an efficient and practical implementation of a generalised bistatic SAR image formation algorithm with an accurate accommodation of these two effects. The algorithm has a common structure with the monostatic fast-factorised backprojection (FFBP), and is therefore based on subaperture processing. The images computed over the different subapertures are displayed in an advantageous elliptical coordinate system capable of incorporating the topographic information of the imaged scene in an analogous manner as topography-dependent monostatic SAR algorithms do. Analytical expressions for the Nyquist requirements using this coordinate system are derived. The overall discussion includes practical implementation hints and a realistic computational burden estimation. The algorithm is tested with both simulated and actual bistatic SAR data. The actual data correspond to the spaceborne-airborne experiment between TerraSAR-X and F-SAR performed in 2007 and to the DLR-ONERA airborne experiment carried out in 2003. The presented approach proves its suitability for the precise SAR focussing of the data acquired in general bistatic configurations.
    [bibtex-key = rodriguezCassolaPratsKriegerMoreiraTAES2011] [bibtex-entry]


  532. Olivier Ruault du Plessis, Jean-Francois Nouvel, Remi Baque, Gregory Bonin, Philippe Dreuillet, Colette Coulombeix, and Helene Oriot. ONERA SAR facilities. IEEE Aerospace and Electronic Systems Magazine, 26(11):24-30, November 2011.
    Abstract: This provides an overview of the airborne SAR systems developed by the French Aerospace Lab ONERA over the past five years. The first system, called SETHI, is presented in the Sethi Sensor section. It has been developed according to the standard FAR25 applied to civil applications. The main improvement compared to the previous ONERA airborne radar system, RAMSES is that the antennas are located in two pods compatible with small aircrafts like the Falcon 20. This pod-based configuration allows the easy integration of any kind of payloads under the single certification of the pods by authorities. The Busard Platform section describes the BUSARD concept, dedicated to UAV-like payloads. Special attention is set on its Ka-band radar payload. Finally, we describe the RAMSES NG project in the self-titled section with a new installation on-board a Falcon 20 aircraft and we expose the latest X-band developments. The three systems are summarized on overview tables.
    [bibtex-key = ruaultEtAlAESMag2011OneraSARfacilitiesSETHI] [bibtex-entry]


  533. S. Samsonov and K. Tiampo. Polarization Phase Difference Analysis for Selection of Persistent Scatterers in SAR Interferometry. IEEE Geoscience and Remote Sensing Letters, 8(2):331-335, March 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, HH channel, SAR Interferometry, San Francisco region, VV channel, amplitude dispersion threshold, data acquisition, deformation map, normalized PPD, odd bounce scattering properties, polarization phase difference analysis, quadpol RADARSAT-2 SAR images, standard PSI technique, standard persistent scatterer interferometry, synthetic aperture radar images, geophysical image processing, geophysical techniques, radar interferometry, synthetic aperture radar;.
    Abstract: In this letter, we propose a technique for selecting persistent scatterers (PSs) based on their polarization phase difference (PPD). We analyze a normalized PPD between HH and VV channels averaged over a temporal set of images and select pixels that demonstrate predominantly even or odd bounce scattering properties. We compare selected scatterers to PSs selected by applying an amplitude dispersion threshold as suggested by a standard PS interferometry (PSI) approach and show that both methods are complementary. However, the proposed approach can be potentially used on a small set of synthetic aperture radar (SAR) images, which can be beneficial in the early stage of data acquisition. We apply the proposed technique to produce a deformation map for the San Francisco region from six quad-pol RADARSAT-2 SAR images acquired during 2008-2009. The coverage and the precision of the produced deformation map are higher than if it was calculated with the standard PSI technique applied to the same data set.
    [bibtex-key = samsonovTiampo2011] [bibtex-entry]


  534. Stefan Sauer, Laurent Ferro-Famil, Andreas Reigber, and Eric Pottier. Three-Dimensional Imaging and Scattering Mechanism Estimation Over Urban Scenes Using Dual-Baseline Polarimetric InSAR Observations at L-Band. IEEE Trans. Geosci. Remote Sens., PP(99):1-14, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Capon, MUSIC, multiple signal classification, Polarimetry, Multibaseline SAR, InSAR, Interferometry, SAR Interferometry, Urban Remote Sensing.
    Abstract: This paper introduces new polarimetric algorithms for generating 3-D images and estimating scattering mechanisms from polarimetric multibaseline (MB) interferometric synthetic aperture radar (SAR) measurements. First, an MB interferometric SAR signal model is generalized to the fully polarimetric configuration, establishing the notion of polarimetric reflectivity. Subsequently, polarimetric beamforming, Capon, and MUSIC methods that determine optimal polarization combinations for height estimation are developed. These new techniques allow for extracting the height of reflectors, the associated scattering mechanisms, and the polarimetric (pseudo)reflectivities. By means of polarimetric dual-baseline interferometric SAR observations of an urban environment, the performance of the conceived algorithms is examined in detail. Producing 3-D images of a building layover, the quality of the approaches is compared in terms of refined resolution and lowered side lobes. Furthermore, the scattering processes occurring in urban scenes are investigated thoroughly by analyzing the optimal reflection types. The algorithms are validated using dual-baseline polarimetric SAR interferometric data at L-band acquired by German Aerospace Center's experimental SAR system over Dresden city.
    [bibtex-key = sauerFerroFamilReigberPottier2011:TomoUrban] [bibtex-entry]


  535. D.J. Sego, H. Griffiths, and M.C. Wicks. Waveform and aperture design for low-frequency RF tomography. IET Radar, Sonar Navigation, 5(6):686-696, July 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, RF spectral environment, aperture Fourier surface, frequency selection, high vertical resolution circular SAR, image quality metrics, image resolution, image sidelobe, low-frequency RF tomography, spectrally sparse narrowband waveforms, ultrawideband waveforms, image resolution, radar imaging, synthetic aperture radar, tomography;.
    Abstract: Multiple applications would benefit from low frequency, three-dimensional (3-D), high resolution, imagery, among them remote archeological survey through foliage, and detecting voids in collapsed structures and underground. High vertical resolution circular SAR requires ultrawideband waveforms, a problematic aspect in the modern RF spectral environment, particularly at lower frequencies. RF tomography offers the potential to yield high, 3-D resolution using spectrally sparse, narrowband waveforms simultaneously with operation at frequencies that have demonstrated favourable penetration through intervening dielectric media. In this paper, we explore this potential; evaluating minimal spatial support tomographic apertures, combining diverse narrowband signals with 2- and 3-dimensional monostatic and bistatic apertures. Results are presented in terms of image quality metrics: resolution and sidelobe levels. It is shown that, generally, frequency selection is a soft constraint in terms of the achievable resolution and sidelobe levels, that the tomographic aperture with spatial sampling that is linearly continuous and substantially less than hemispherical yields high spatial resolution, and that there is interaction between the tomographic aperture and the waveform set as evidenced in the point spread function. Additionally the bistatic contribution of the collection aperture Fourier surface is demonstrated to enhance image quality when combined in a hybrid monostatic-bistatic aperture.
    [bibtex-key = 5940386] [bibtex-entry]


  536. Piyush Shanker Agram, F. Casu, Howard A. Zebker, and R. Lanari. Comparison of Persistent Scatterers and Small Baseline Time-Series InSAR Results: A Case Study of the San Francisco Bay Area. IEEE Geosci. Remote Sens. Lett., 8(4):592-596, July 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Small Baseline Subset Algorithm, SBAS, Interferometry, InSAR, SAR Interferometry, Deformation, Deformation Monitoring.
    Abstract: Time-series interferometric synthetic aperture radar (InSAR) methods estimate the spatiotemporal evolution of deformation over large areas by incorporating information from multiple SAR interferograms. Persistent scatterer (PS) and small baseline (SB) methods, which identify areas where the surface is least affected by geometric and temporal decorrelation, represent two families of time-series InSAR techniques to study successfully a wide spectrum of ground deformation phenomena worldwide. However, little is known comparatively about the performance of PS and SB techniques applied to the same region. Here, we compare quantitatively and cross validate the time-series InSAR results generated using two representative algorithms -- the maximum likelihood PS method and the small baseline subset algorithm -- in selected test sites, over the San Francisco Bay Area imaged by European Remote Sensing (ERS) sensors during 1995-2000. We present line of sight (LOS) velocities and deformation time series using both techniques and show that the root mean squared differences of the estimated mean velocities and deformation from each method are about 1 mm/year and 5 mm, respectively. These values are within expected noise levels and a characteristic of the pixel selection parameters for both the time-series techniques. We validate our deformation estimates against creep measurements from alignment arrays along the Hayward Fault and show that our estimates agree to within 0.5 mm/year LOS velocity and 1.5 mm LOS displacement.
    [bibtex-key = shankerAgramCasuZebkerLanari2011:PSI_SBAS] [bibtex-entry]


  537. Manoochehr Shirzaei and Thomas R Walter. Estimating the effect of satellite orbital error using wavelet-based robust regression applied to InSAR deformation data. IEEE Transactions on Geoscience and Remote Sensing, 49(11):4600-4605, 2011. [bibtex-key = Shirzaei2011] [bibtex-entry]


  538. Manoochehr Shirzaei, TR Walter, HR Nankali, and EP Holohan. Gravity-driven deformation of Damavand volcano, Iran, detected through InSAR time series. Geology, 39(3):251-254, 2011. [bibtex-key = Shirzaei2011a] [bibtex-entry]


  539. David Small. Flattening Gamma: Radiometric Terrain Correction for SAR Imagery. IEEE Trans. Geosci. Remote Sens., 49(8):3081-3093, August 2011. Keyword(s): SAR Processing, ASAR, ERS-1, Earth ellipsoid model, Earth terrain-model, PALSAR, RADARSAT-1, SAR sensors, TerraSAR-X, beta nought SAR radiometry, elevation models, gamma nought SAR radiometry, image geolocation accuracy, radar cross section, radar imaging, radar measurements, radar scattering, radar terrain factors, radiometric calibration, radiometric normalisation standard, sigma nought SAR radiometry, terrain variations, radar cross-sections, radar imaging, radiometry, synthetic aperture radar, terrain mapping.
    Abstract: Enabling intercomparison of synthetic aperture radar (SAR) imagery acquired from different sensors or acquisition modes requires accurate modeling of not only the geometry of each scene, but also of systematic influences on the radiometry of individual scenes. Terrain variations affect not only the position of a given point on the earth's surface but also the brightness of the radar return as expressed in radar geometry. Without treatment, the hill-slope modulations of the radiometry threaten to overwhelm weaker thematic land cover induced backscatter differences, and comparison of backscatter from multiple satellites, modes, or tracks loses meaning. The ASAR and PALSAR sensors provide state vectors and timing with higher absolute accuracy than was previously available, allowing them to directly support accurate tie-point-free geolocation and radiometric normalization of their imagery. Given accurate knowledge of the acquisition geometry of a SAR image together with a digital height model (DHM) of the area imaged, radiometric image simulation is applied to estimate the local illuminated area for each point in the image. Ellipsoid-based or sigma naught $(sigma^{0})$ based incident angle approximations that fail to reproduce the effect of topographic variation in their sensor model are contrasted with a new method that integrates terrain variations with the concept of gamma naught $(gamma^{0})$ backscatter, converting directly from beta naught $(beta^{0})$ to a newly introduced terrain-flattened $gamma^{0}$ normalization convention. The interpretability of imagery treated in this manner is improved in comparison to processing based on conventional ellipsoid or local incident angle based $sigma^{0}$ normalization.
    [bibtex-key = small2011:TGRSRadiometricCalibration] [bibtex-entry]


  540. Guangcai Sun, Mengdao Xing, Yong Wang, Yufeng Wu, YiRong Wu, and Zheng Bao. Sliding Spotlight and TOPS SAR Data Processing Without Subaperture. IEEE Geosci. Remote Sens. Lett., 8(6):1036-1040, November 2011. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, data acquisition, synthetic aperture radar, Doppler domain, TOPS SAR data processing, azimuth bandwidth, azimuth signal aliasing, data acquisition, imaging algorithm, instantaneous bandwidth, progressive scan, pulse repetition frequency, sliding spotlight, subaperture method, synthetic aperture radar, terrain observation, Azimuth, Bandwidth, Chirp, Focusing, Signal processing algorithms, Synthetic aperture radar, Sliding spotlight synthetic aperture radar (SAR), subaperture, terrain observation by progressive scan (TOPS) SAR.
    Abstract: During the data acquisition of a sliding spotlight or terrain observation by progressive scan (TOPS) synthetic aperture radar (SAR), the steering of the antenna main beam increases the azimuth bandwidth but could result in the azimuth signal aliasing in the Doppler domain. To remove the aliasing, one has used a subaperture method. In this letter, we show a focusing scheme without the use of the subaperture for both sliding spotlight and TOPS SARs. In doing so, we eliminated the obvious increase in data volume or the subaperture division by choosing the pulse repetition frequency that is only 20% greater than the instantaneous bandwidth. The method was incorporated with an available imaging algorithm and then used to process simulated and collected data of the sliding spotlight and TOPS SARs. Well-focused results without aliasing were obtained.
    [bibtex-key = sunXingWangWuWuBaoGRSL2011TOPSwithoutSubaperture] [bibtex-entry]


  541. Xiaoqing Wu, K.C. Jezek, E. Rodriguez, S. Gogineni, F. Rodriguez-Morales, and A. Freeman. Ice Sheet Bed Mapping With Airborne SAR Tomography. IEEE Trans. Geosci. Remote Sens., 49(10):3791 -3802, oct. 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, 2D image formation, 3D tomographic ice sounding method, AD 2006 05, AD 2008 07, airborne SAR tomography, ice sheet bed mapping, ice sheet surface topography, ice thickness, multiple-phase-center VHF radar system, nadir depth sounder tracks, swath measurements, time-domain subaperture method, very high frequency radar data, geophysical image processing, glaciology, hydrological techniques, radar imaging, remote sensing by radar, synthetic aperture radar;.
    Abstract: We develop and then demonstrate a 3-D tomographic ice sounding method applied to very high frequency (VHF) radar data that produces swath measurements of ice sheet surface topography, ice thickness, and radar reflectivity of both the surface and bed of the ice sheet. First, we formulate the ice sheet imaging problem as a problem of estimating signal arrival angles and illustrate how the method resolves ambiguous echoes arriving simultaneously from the left and right sides of the aircraft, as well as from the surface and base of the ice sheet. We then discuss why we chose the time-domain subaperture method for 2-D image formation for ice sounding. We apply the tomographic technique to the data that we collected in May 2006 and again in July 2008 from a multiple-phase-center VHF radar system. We present 3-D images of the upper and lower surfaces of the ice sheet and compare the estimated surface topography with Ice, Cloud, and land Elevation Satellite altimeter nadir track measurements and the measured swath ice thickness with independent nadir depth sounder tracks. We achieved a 5-m surface topography accuracy and a 14-m ice thickness accuracy.
    [bibtex-key = 5766033] [bibtex-entry]


  542. Wei Xu, Pingping Huang, Yunkai Deng, Jiantao Sun, and Xiuqin Shang. An Efficient Approach With Scaling Factors for TOPS-Mode SAR Data Focusing. IEEE Geosci. Remote Sens. Lett., 8(5):929-933, Sept 2011. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, geophysical techniques, synthetic aperture radar, Doppler spectrum, ScanSAR, TOPS-mode SAR data focusing, TOPS-mode synthetic aperture radar data, Terrain Observation by Progressive Scans mode, azimuth baseband scaling operation, azimuth scaling factors, extended chirp scaling processing procedure, full-aperture imaging approach, limited azimuth-data extension, residual TOPS raw-data focusing, sliding spotlight SAR data focusing, spaceborne imaging mode, two-step focusing technique, wide-swath coverage, Azimuth, Bandwidth, Doppler effect, Focusing, Image resolution, Remote sensing, Aliasing, Terrain Observation by Progressive Scans (TOPS), deramp, extended chirp scaling, synthetic aperture radar (SAR).
    Abstract: The Terrain Observation by Progressive Scans (TOPS) mode is a novel spaceborne imaging mode which can be used to obtain wide-swath coverage and overcome major drawbacks in conventional ScanSAR. An efficient full-aperture imaging approach, which takes advantage of the two-step focusing technique and the azimuth baseband scaling operation, is presented for processing the TOPS-mode synthetic aperture radar (SAR) data. First, the proposed two-step focusing technique for spotlight and sliding spotlight SAR data focusing is adopted to resolve the aliased Doppler spectrum. Afterward, the following extended chirp scaling processing procedure with azimuth scaling factors is used to implement the residual TOPS raw-data focusing. Since the use of subapertures is avoided and only a limited azimuth-data extension is required, this algorithm is highly efficient. Simulation results validate the proposed imaging approach.
    [bibtex-key = xuHuangDengSunShangGRSL2011TOPS] [bibtex-entry]


  543. Dochul Yang and Sean M. Buckley. Estimating High-Resolution Atmospheric Phase Screens From Radar Interferometry Data. IEEE Transactions on Geoscience and Remote Sensing, 49(8):3117 -3128, August 2011. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Arizona, HiRAPS algorithm, InSAR deformation measurement, Phoenix, Radarsat-1 image, USA, bubbles, deformation time series, height error difference, high-resolution atmospheric phase screen estimation, high-spatial-frequency atmospheric effects, multiinterferogram phase correlation, nonlinear deformation, persistent scatterer algorithm, pixel density, radar interferometry data, root mean square error, satellite-orbit repeat cycle, short-period interferogram, singular value decomposition, spatiotemporal filtering, time series algorithm, time-linear deformation, topographic phase contribution, geophysical image processing, geophysical techniques, mean square error methods, radar imaging, radar interferometry, remote sensing by radar, singular value decomposition, synthetic aperture radar, time series;.
    Abstract: Radar interferometry (InSAR) deformation measurements are afflicted by artifacts associated with the atmosphere and errors in removing the topographic phase contribution. We present a new time series algorithm that eliminates high-spatial-frequency atmospheric effects (bubbles) not removed with existing advanced InSAR approaches applied to measurements of smoothly varying deformation through time. Our High-Resolution Atmospheric Phase Screen (APS) (HiRAPS) algorithm initially uses a connected set of short-period interferograms, each spanning no more than three satellite-orbit repeat cycles. We estimate height error differences between a pixel and its neighbors within a radius chosen to be significantly smaller than a bubble. The height errors are unwrapped and removed from those pixels with high values of a newly defined multi-interferogram phase correlation. We then create a deformation time series for the pixels using singular value decomposition. The high-resolution APS are estimated from a dense set of pixels using spatiotemporal filtering. We evaluate the HiRAPS algorithm on simulated data consisting of realistic time-linear and nonlinear deformation, height errors, and bubbles. The root mean square error between all simulated and estimated APS pixels is 0.26 rad with the HiRAPS algorithm and 0.39 rad with a persistent scatterer (PS) algorithm. We also apply the HiRAPS algorithm to 66 Radarsat-1 images of Phoenix, AZ. Our HiRAPS approach results in an 18-fold increase in APS pixel density over PS processing. After removing the HiRAPS and PS APS from PS interferograms, we find that HiRAPS provides an 18 percent increase in the number of final PS detected.
    [bibtex-key = zangBuckley2011PSIAtmo] [bibtex-entry]


  544. Xiao Xiang Zhu and Richard Bamler. Let's Do the Time Warp: Multicomponent Nonlinear Motion Estimation in Differential SAR Tomography. IEEE Geosci. Remote Sens. Lett., 8(4):735-739, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Estimation, Persistent Scatterer Interferometry, PSI, Motion estimation, Pixel, Remote sensing, Signal to noise ratio, Synthetic aperture radar, Tomography, motion estimation, radar imaging, synthetic aperture radar, time warp simulation, tomography, D-TomoSAR system model, TerraSAR-X spotlight, complex motion model, differential synthetic aperture radar tomography, linear motion, motion history, multicomponent generalization, multicomponent nonlinear motion estimation, nonlinear motion, periodic motion component, spectral estimation, time warp method, user-defined motion model order, Differential synthetic aperture radar tomography (D-TomoSAR), TerraSAR-X (TS-X), multicomponent nonlinear motion, synthetic aperture radar (SAR), time warp.
    Abstract: In the differential synthetic aperture radar tomography (D-TomoSAR) system model, the motion history appears as a phase term. In the case of nonlinear motion, this phase term is no longer linear and, hence, cannot be retrieved by spectral estimation. We propose the time warp method that rearranges the acquisition dates such that a linear motion is pretended. The multicomponent generalization of time warp rewrites the D-TomoSAR system model to an (M + 1)-dimensional standard spectral estimation problem, where M indicates the user-defined motion model order and, hence, enables the motion estimation for all possible complex motion models. Both simulations and real data (from TerraSAR-X spotlight) examples demonstrate the applicability of the method and show that linear and periodic (seasonal) motion components can be separated and retrieved.
    [bibtex-key = zhuBamlerGRSL2011] [bibtex-entry]


  545. Stefano Tebaldini and Andrea Monti Guarnieri. Methods and Performances for Multi-Pass SAR Interferometry, chapter 18, pages 329-356. InTech, 2010. Keyword(s): SAR Processing, Modelling Interferogram Stacks, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, D-InSAR, InSAR, SAR Interferometry, Interferometry, Decorrelation, Temporal Decorrelation, C-band measurement, DInSAR, ERS-1 data, Italy, Rome, agricultural areas, differential interferometric SAR, distributed targets, geometrical decorrelation, interferogram stack modeling, permanent scatterers, progressive ground motion, progressively decorrelating targets, sinusoidal ground motion, synthetic aperture radar interferometry, temporal decorrelation, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: Thanks to the several space missions accomplished since ERS-1, the scientific community has been provided with a huge amount of data suitable for interferometric processing. The innovation was the availability of multiple compatible images of the same areas. Such images, achieved by looking from slightly different point of view different orbits, and/or by different frequencies, and/or at different times, has largely extended the capabilities of InSAR with respect to the traditional dual image case. The advantage granted by the possibility to form multiple interferograms, instead than just one, is two folded. On the one hand, the estimation of the parameters of interest, be them related to the DEM or the terrain deformations, is driven by a larger data set, resulting in more accurate estimates. On the other hand, new parameters may be added to the set of the unknowns, allowing to study complex phenomena, such as the temporal evolution of the atmospheric and deformation fields. A major issue with multi-image InSAR is that targets are, in general, affected by temporal and spatial decorrelation phenomena, which hinders the exploitation of large spatial and/or temporal baselines. For this reason, most of literature about multi-image InSAR has focused mainly on targets that stay coherent in all the acquisitions, which has resulted in a substantial lack of a systemic approach to deal with decorrelating targets in the field of InSAR. The aim of this chapter is to propose a general approach to exploit all the available information, that is the stack of interferometric SAR images, and that formally accounts for the impact of target decorrelation. This approach is based on the optimal estimate of the data in a statistical sense. The basic idea is to split the estimation process into two steps. In the first step, a maximum likelihood (ML) estimator is used that jointly exploits all the N x (N-1)/2 interferograms available with N acquisitions, in order to yield the best estimates of the N-1 phases that correspond to the optical path differences between the target and the sensors. Target decorrelation is accounted for by properly weighting each interferogram in dependence on the target statistics. The estimated phases will be referred to as Linked Phases, to remind that these terms are the result of the joint processing of all the N(N-1)/2 interferograms. Once the first estimation step has yielded the estimates of the interferometric phases, the second step is required to separate the contributions of the APSs and the decorrelation noise from the parameters of interest, such as the Line of Sight Deformation Field (LDF) and the topography.
    [bibtex-key = tebaldiniMontiGuarnieriInBook2010MethodsPerformancesMultiPassSARInterferometry] [bibtex-entry]


  546. Joong-Sun Won Chang-Wook Lee, Zhong Lu, Hyung-Sup Jung and Daniel Dzurisin. Surface Deformation of Augustine Volcano, 1992?2005, from Multiple-Interferogram Processing Using a Refined Small Baseline Subset (SBAS) Interferometric Synthetic Aperture Radar (InSAR) Approach. In J.A. Power, M.L. Coombs, and J.T. Freymueller, editors, The 2006 Eruption of Augustine Volcano, Alaska, number 1769, chapter 18. U.S. Geological Survey, 2010. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, DInSAR, SBAS, Small Baseline Subset, Volcano Monitoring, Surface Deformation, Deformation Monitoring, Geology.
    Abstract: Augustine Volcano is an active stratovolcano located in southwestern Cook Inlet, about 280 kilometers southwest of Anchorage, Alaska. The volcano produced six significant explosive eruptions between 1812 and 1986. Augustine eruptions typically have an explosive onset followed by dome building. The most recent eruption began on January 11, 2006. We applied the small baseline subset (SBAS) interferometric synthetic aperture radar (InSAR) technique to measure ground surface deformation during 1992?2005 with the use of European Remote Sensing Satellites 1 and 2 (ERS?1 and ERS?2) radar imagery. Through a multiple-interferogram approach, atmospheric delay artifacts, which hinder conventional InSAR measurements, are significantly reduced by spatial and temporal filtering. This allows us to retrieve time-series deformation over coherent points at millimeter-scale accuracy. The deformation results from two independent satellite tracks agree with each other, suggesting 2 to 8 cm wholesale uplift of Augustine Volcano from 1992 to 2005. Global Positioning System (GPS) data acquired in September 2004 and October 2005 confirm the SBAS InSAR results. A preliminary model consisting of a contracting source at 2 to 4 km depth and an inflating source at 7 to 12 km depth fits the observed deformation reasonably well. We interpret the deeper source as a long-term magma storage zone and the shallower source as a subsidiary reservoir that was tapped during the 2006 eruption. The shallow source corresponds approximately to the location of the volcano-tectonic earthquakes that preceded and followed the 1976 and 2006 eruptions, respectively.
    [bibtex-key = leeLuJungWonDzurisin2010_SBAS_PSI_p1769_chapter18] [bibtex-entry]


  547. R.G. Baraniuk, E. Candes, M. Elad, and Yi Ma. Applications of Sparse Representation and Compressive Sensing [Scanning the Issue]. Proceedings of the IEEE, 98(6):906-909, june 2010. Keyword(s): NP-hard, compressive sensing, convex optimization, greedy methods, high-dimensional data, linear combination, overcomplete dictionary, sparse representation, computational complexity, signal processing, sparse matrices;.
    Abstract: Sparse representation and compressive sensing establishes a more rigorous mathematical framework for studying high-dimensional data and ways to uncover the structures of the data, giving rise to a large repertoire of efficient algorithms. A sparse signal is a signal that can be represented as a linear combination of relatively few base elements in a basis or an overcomplete dictionary. A sufficiently sparse linear representation can be correctly and efficiently computed by greedy methods and convex optimization (i.e., the l1-l0 equivalence), even though this problem is extremely difficult-NP-hard in the general case.
    [bibtex-key = 5466604] [bibtex-entry]


  548. B. Brautigam, J.H. Gonzalez, M. Schwerdt, and M. Bachmann. TerraSAR-X Instrument Calibration Results and Extension for TanDEM-X. IEEE Transactions on Geoscience and Remote Sensing, 48(2):702-715, February 2010. Keyword(s): TerraSAR-X, TanDEM-X, Earth observation, German TerraSAR-X system, TanDEM-X, TerraSAR-X image products, TerraSAR-X instrument calibration results, active phased array X-band antenna, antenna performance control, bistatic calibration techniques, electronic beam steering, radar images, radiometric stability, satellite flying formation, spaceborne remote sensing, synthetic aperture radar, antenna phased arrays, calibration, phased array radar, radar antennas, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: Spaceborne remote sensing with synthetic aperture radar (SAR) has become an essential source of high-resolution and continuous Earth observation. Modern satellites like the German TerraSAR-X system provide state-of-the-art radar images with respect to operating flexibility and imaging quality. The outstanding performance of TerraSAR-X image products is achieved by an innovative calibration approach that minimizes systematic antenna and instrument characteristics. The active phased array X-band antenna is fed by 384 transmit/receive modules for electronic beam steering and shaping in the azimuth and elevation direction. The flexible radar instrument hosts an internal calibration system which guarantees the high radiometric stability of all SAR products. New techniques for antenna performance control have been successfully implemented, setting a high standard for next-generation SAR missions. This paper summarizes all essential calibration results of TerraSAR-X that cover internal instrument behavior. Furthermore, we give an outlook on the required bistatic calibration techniques for the future TanDEM-X mission that faces additional performance challenges when calibrating two TerraSAR-X satellites flying in close formation.
    [bibtex-key = brautigamGonzalesSchwerdtBachmann2010] [bibtex-entry]


  549. E.J. Candes and Y. Plan. Matrix Completion With Noise. Proceedings of the IEEE, 98(6):925-936, june 2010. Keyword(s): compressed sensing, convex optimization problem, data constraints, low rank matrices, matrix completion, nuclear norm minimization, data integrity, matrix algebra, minimisation, noise, signal sampling;.
    Abstract: On the heels of compressed sensing, a new field has very recently emerged. This field addresses a broad range of problems of significant practical interest, namely, the recovery of a data matrix from what appears to be incomplete, and perhaps even corrupted, information. In its simplest form, the problem is to recover a matrix from a small sample of its entries. It comes up in many areas of science and engineering, including collaborative filtering, machine learning, control, remote sensing, and computer vision, to name a few. This paper surveys the novel literature on matrix completion, which shows that under some suitable conditions, one can recover an unknown low-rank matrix from a nearly minimal set of entries by solving a simple convex optimization problem, namely, nuclear-norm minimization subject to data constraints. Further, this paper introduces novel results showing that matrix completion is provably accurate even when the few observed entries are corrupted with a small amount of noise. A typical result is that one can recover an unknown matrix of low rank from just about log noisy samples with an error that is proportional to the noise level. We present numerical results that complement our quantitative analysis and show that, in practice, nuclear-norm minimization accurately fills in the many missing entries of large low-rank matrices from just a few noisy samples. Some analogies between matrix completion and compressed sensing are discussed throughout.
    [bibtex-key = 5454406] [bibtex-entry]


  550. E.J. Candes and T. Tao. The Power of Convex Relaxation: Near-Optimal Matrix Completion. IEEE Transactions on Information Theory, 56(5):2053-2080, May 2010. Keyword(s): collaborative filtering, convex relaxation, free probability, information theoretic limit, matrix completion problem, near-optimal matrix completion, nuclear norm minimization, random matrices, random matrix theory, semidefinite programming, convex programming, information theory, random processes;.
    Abstract: This paper is concerned with the problem of recovering an unknown matrix from a small fraction of its entries. This is known as the matrix completion problem, and comes up in a great number of applications, including the famous Netflix Prize and other similar questions in collaborative filtering. In general, accurate recovery of a matrix from a small number of entries is impossible, but the knowledge that the unknown matrix has low rank radically changes this premise, making the search for solutions meaningful. This paper presents optimality results quantifying the minimum number of entries needed to recover a matrix of rank r exactly by any method whatsoever (the information theoretic limit). More importantly, the paper shows that, under certain incoherence assumptions on the singular vectors of the matrix, recovery is possible by solving a convenient convex program as soon as the number of entries is on the order of the information theoretic limit (up to logarithmic factors). This convex program simply finds, among all matrices consistent with the observed entries, that with minimum nuclear norm. As an example, we show that on the order of nr log(n) samples are needed to recover a random n x n matrix of rank r by any method, and to be sure, nuclear norm minimization succeeds as soon as the number of entries is of the form nr polylog(n).
    [bibtex-key = 5452187] [bibtex-entry]


  551. M. Crosetto, O. Monserrat, R. Iglesias, and B. Crippa. Persistent Scatterer Interferometry: potential, limits and initial C- and X-band comparison. Photogrammetric Engineering and Remote Sensing, 76(9):1061-1069, 2010. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, ground deformation time series, DInSAR, remote sensing.
    Abstract: This paper is focused on Persistent Scatterer Interferometry (PSI), a powerful remote sensing technique used to measure and monitor deformation phenomena. It only refers to satellite-based PSI techniques, focusing in particular on the most important sources of C-band SAR data: ERS-1/2 and Envisat. In addition, it compares C- and X-band results, considering data from the high resolution TerraSAR-X sensor. The paper begins with a concise description of the main characteristics of PSI. It then discusses the most important PSI products and their performances, analysing in detail their spatial sampling, the so-called residual topographic error and PSI geocoding, the average displacement rates and the deformation time series. As C-band products are concerned, the paper reports some relevant PSI validation results, which come from the ESA- funded Terrafirma Validation Project. Regarding the X-band, it describes the results obtained over the city of Barcelona by processing 13 StripMap TerraSAR-X images. The last part of the paper discusses the main limits of PSI.
    [bibtex-key = crosettoMoserratIglesiasCrippaPSI2010] [bibtex-entry]


  552. D. D'Aria, A. Ferretti, A.M. Guarnieri, and S. Tebaldini. SAR Calibration Aided by Permanent Scatterers. IEEE Trans. Geosci. Remote Sens., 48(4):2076-2086, April 2010. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, C band spaceborne SAR, Ku band ground based SAR, PS based normalisation, SAR calibration, absolute calibrated devices, corner reflectors, interferometric SAR image stack, iterative maximum likelihood method, permanent scatterers, radiometric stability, repeated SAR acquisitions, stable targets, synthetic aperture radar, calibration, electromagnetic wave scattering, iterative methods, maximum likelihood estimation, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: We propose a calibration method suitable for a set of repeated synthetic aperture radar (SAR) acquisitions that uses both absolute calibrated devices (such as corner reflectors) and stable targets identified in the scene [the permanent scatterers (PSs)]. Precisely, the role of the PS is to extend the initial calibration sequence by monitoring the radiometric stability of the system throughout the whole mission life span. At a first step, this paper approaches the problem of PS-based normalization by an iterative maximum-likelihood method that exploits the stack of complex interferometric SAR images. Two solutions are given based on different assumptions on the PS phases. As a second step, the merging of these estimates with the available calibration information is discussed. Results achieved by experimental acquisitions are shown in two different SAR systems: 1) a C-band spaceborne SAR and 2) a Ku-band ground-based SAR.
    [bibtex-key = dAriaFerrettiMontiGuarnieriTebaldini2010CalibPSI] [bibtex-entry]


  553. Lin Du, Jian Li, and Petre Stoica. Fully Automatic Computation of Diagonal Loading Levels for Robust Adaptive Beamforming. IEEE Transactions on Aerospace and Electronic Systems, 46(1):449-458, January 2010. Keyword(s): Capon, array signal processing, covariance matrices, fully automatic computation, diagonal loading levels, robust adaptive beamforming, covariance matrix estimate, Capon beamforming formulation, Robustness, Array signal processing, Covariance matrix, Degradation, Councils, Uncertainty, Guidelines, Spatial filters, Signal to noise ratio, Calibration.
    Abstract: The main drawback of the conventional diagonal loading (DL) approaches is that there is no clear guideline on how to choose the DL level reliably or how to select user parameters appropriately. An algorithm that can be used to compute the DL level completely automatically from the given data without the need of specifying any user parameter is considered. In this algorithm an enhanced covariance matrix estimate obtained via a shrinkage method, instead of the sample covariance matrix, is used in the standard Capon beamforming formulation. The performance of the resulting beamformer is illustrated via numerical examples, and it is compared with several other adaptive beamformers.
    [bibtex-key = duLiStoicaTAES2010FullyAutomaticDiagonalLoadingForRobustCAPONBeamforming] [bibtex-entry]


  554. A. Elsherbini and K. Sarabandi. Mapping of Sand Layer Thickness in Deserts Using SAR Interferometry. IEEE_J_GRS, 48(9):3550-3559, September 2010. Keyword(s): electromagnetic wave scattering, geophysical image processing, geophysical techniques, radar interferometry, remote sensing by radar, sand, synthetic aperture radar, terrain mapping, topography (Earth), Ka InSAR, SAR interferometry, Saudi Arabia, aperture radar system, bedrock topography, desert area, groundwater exploration, inversion algorithm, oil field, radar imaging, sand layer thickness, sand topography, sensitivity analysis, subsurface imaging, Costs, Explosives, Laser radar, Light scattering, Optical scattering, Petroleum, Radar scattering, Seismic waves, Surfaces, Synthetic aperture radar interferometry, Interferometric synthetic aperture radar (InSAR). [bibtex-key = Elsherbini2010a] [bibtex-entry]


  555. Joachim H.G. Ender. On compressive sensing applied to radar. Signal Processing, 90(5):1402 - 1414, 2010. Note: Special Section on Statistical Signal and Array Processing. Keyword(s): Compressive Sensing, Compressed Sensing, Radar, Sparse arrays, Pulse compression, Radar imaging, ISAR, Airspace surveillance, DOA estimation.
    Abstract: Compressive sensing (CS) techniques offer a framework for the detection and allocation of sparse signals with a reduced number of samples. Today, modern radar systems operate with high bandwidth demanding high sample rates according to the Shannon-Nyquist theorem and a huge number of single elements for phased array antennas. Often only a small amount of target parameters is the final output, arising the question, if CS could not be a good mean to reduce data size, complexity, weight, power consumption and costs of radar systems. There is only a small number of publications addressing the application of CS to radar, leaving several open questions. This paper addresses some aspects as a further step to CS-radar by presenting generic system architectures and implementation considerations. It is not the aim of this paper to investigate numerically efficient algorithms but to point to promising applications as well as arising problems. Three possible applications are considered: pulse compression, radar imaging, and air space surveillance with array antennas. Some simulation results are presented and enriched by the evaluation of real data acquired by an experimental radar system of Fraunhofer FHR.
    [bibtex-key = Ender2010CompressiveSensing] [bibtex-entry]


  556. G. Fornaro, G. Franceschetti, F. Lombardini, A. Mori, and M. Calamia. Potentials and Limitations of Moon-Borne SAR Imaging. IEEE Transactions on Geoscience and Remote Sensing, 48(7):3009-3019, July 2010. Keyword(s): SAR Processing, Earth observation, Moon-borne SAR imaging, artificial Earth orbit satellites, interferometry, natural Earth satellite, synthetic aperture radar system, Moon, artificial satellites, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: Moon exploitation is among the next space mission priorities. Earth observation (EO), which is traditionally implemented on artificial lower Earth orbit satellites, can be, in principle, extended to the platform constituted by the natural Earth satellite. With this regard, we investigate the features related to the EO by a possible Moon-borne synthetic aperture radar system in terms of imaging characteristics and potential applications, as well as of expected limitations.
    [bibtex-key = 5443570] [bibtex-entry]


  557. G. Fornaro, F. Serafino, and D. Reale. 4-D SAR Imaging: The Case Study of Rome. IEEE Geoscience and Remote Sensing Letters, 7(2):236 -240, April 2010. Keyword(s): SAR Processing, SAR Tomography, 4-D SAR imaging, Italy, Rome, differential SAR tomography, differential interferometric SAR processing, ground-scatterer monitoring, multibaseline SAR processing, multitemporal SAR processing, permanent-scatterer interferometry, spaceborne data, geophysical image processing, geophysical techniques, radar interferometry, synthetic aperture radar.
    Abstract: Four-dimensional synthetic aperture radar (SAR) imaging, also known as differential SAR tomography, is a new research topic in the framework of coherent multitemporal/multibaseline SAR processing that extends the interferometry concept. Four-dimensional SAR imaging-based processing could improve the capability of ground-scatterer monitoring with respect to classical differential interferometric SAR processing. The first results on the applicability of such an advanced tomographic SAR processing to real spaceborne data were recently discussed in the literature. In this letter, we present the results of an experiment with a data set that demonstrates the potentialities of this new technique for monitoring complex targets, such as infrastructures.
    [bibtex-key = fornaroSerafinoRealeTGRS2010] [bibtex-entry]


  558. Martina Gabele, Benjamin Brautigam, Daniel Schulze, Ulrich Steinbrecher, Nuria Tous-Ramon, and Marvan Younis. Fore and Aft Channel Reconstruction in the TerraSAR-X Dual Receive Antenna Mode. IEEE Trans. Geosci. Remote Sens., 48(2):795-806, February 2010. Keyword(s): TerraSAR-X, Quad-Pol, TerraSAR-X dual receive antenna mode, TerraSAR-X satellite, aft channel reconstruction, along-track direction, along-track interferometric data, antenna splitting, fore channel reconstruction, ground moving target indication, hardware transformation matrix estimation, internal calibration pulses, redundant receiver unit, sum and difference channel data, synthetic aperture radar, geophysical techniques, radar antennas, radar target recognition, synthetic aperture radar.
    Abstract: The TerraSAR-X satellite is a high-resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low, TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna (DRA) mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad-Pol). Fore and aft channel reconstruction is necessary if ground moving target indication (GMTI) algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X DRA mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and first examples on the GMTI capability of the TerraSAR-X DRA mode are given.
    [bibtex-key = gabeleBrautigamSchulzeSteinbrecherTousRamonYounis2010] [bibtex-entry]


  559. Nicolas Gebert, Gerhard Krieger, and Alberto Moreira. Multichannel Azimuth Processing in ScanSAR and TOPS Mode Operation. IEEE Trans. Geosci. Remote Sens., 48(7):2994-3008, July 2010. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, geophysical signal processing, geophysical techniques, synthetic aperture radar, SAR missions, SAR signal processing, TOPS mode operation, Terrain Observation by Progressive Scans system, azimuth antenna length, conventional synthetic aperture radar, digital beamforming algorithms, digital signal processing network, geometric resolution, high-resolution wide-swath SAR imaging, multichannel ScanSAR systems, multichannel azimuth processing, multichannel burst-mode operation, multichannel stripmap mode, multiple azimuth channels, multiple receive channels, staircase multichannel processing, stripmap operation, ultrawide-swath imaging, ultrawide-swath synthetic aperture radar imaging, High-resolution ultrawide-swath synthetic aperture radar (SAR) imaging, ScanSAR, TOPS, multichannel azimuth processing, multichannel burst-mode operation.
    Abstract: Due to a system-inherent limitation, conventional synthetic aperture radar (SAR) is incapable of imaging a wide swath with high geometric resolution. This restriction can be overcome by systems with multiple receive channels in combination with an additional digital signal processing network. So far, the application of such digital beamforming algorithms for high-resolution wide-swath SAR imaging has been restricted to multichannel systems in stripmap operation. However, in stripmap mode, the overall azimuth antenna length restricts the achievable swath width, thus preventing very wide swaths as requested by future SAR missions. Consequently, new concepts for ultrawide-swath imaging are needed. A promising candidate is a SAR system with multiple azimuth channels being operated in burst mode. This paper analyzes innovative ScanSAR and Terrain Observation by Progressive Scans (TOPS) system concepts with regard to multichannel azimuth processing. For this, the theoretical analyses, performance figures, and SAR signal processing, which had previously been derived for multichannel stripmap mode, are extended to systems operating in burst modes. The investigations reveal that multichannel ScanSAR systems enable the imaging of ultrawide swaths with high azimuth resolution and compact antenna lengths. These considerations are embedded in a multichannel ScanSAR system design example to demonstrate its capability to image an ultrawide swath of 400 km with a high geometric resolution of 5 m. In a next step, this system is adapted to TOPS mode operation, including an innovative staircase multichannel processing approach optimized for TOPS.
    [bibtex-key = gebertKriegerMoreiraTGRS2010TOPSmultichannel] [bibtex-entry]


  560. Stefan Gernhardt, Nico Adam, Michael Eineder, and Richard Bamler. Potential of very high resolution SAR for persistent scatterer interferometry in urban areas. Annals of GIS, 16(2):103-111, 2010. [bibtex-key = gernhardtAdamEinederBamler2010PSIUrban] [bibtex-entry]


  561. J.H. Gonzalez, M. Bachmann, G. Krieger, and H. Fiedler. Development of the TanDEM-X Calibration Concept: Analysis of Systematic Errors. IEEE Transactions on Geoscience and Remote Sensing, 48(2):716-726, February 2010. Keyword(s): TerraSAR-X, TanDEM-X, Astrium GmbH, DEM calibration, DEM height error, DLR, German Aerospace Center, HRTI level 3 accuracy, High-Resolution Terrain Information, TanDEM-X calibration concept, TerraSAR-X, bistatic satellite synthetic aperture radar mission, height accuracy requirements, helix formation, high-precision global digital elevation model, residual-error sources, spaceborne radar remote sensing, systematic error analysis, calibration, digital elevation models, measurement errors, radar altimetry, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: The TanDEM-X mission, result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, opens a new era in spaceborne radar remote sensing. The first bistatic satellite synthetic aperture radar mission is formed by flying TanDEM-X and TerraSAR-X in a closely controlled helix formation. The primary mission goal is the derivation of a high-precision global digital elevation model (DEM) according to High-Resolution Terrain Information (HRTI) level 3 accuracy. The finite precision of the baseline knowledge and uncompensated radar instrument drifts introduce errors that may compromise the height accuracy requirements. By means of a DEM calibration, which uses absolute height references, and the information provided by adjacent interferogram overlaps, these height errors can be minimized. This paper summarizes the exhaustive studies of the nature of the residual-error sources that have been carried out during the development of the DEM calibration concept. Models for these errors are set up and simulations of the resulting DEM height error for different scenarios provide the basis for the development of a successful DEM calibration strategy for the TanDEM-X mission.
    [bibtex-key = 5353668] [bibtex-entry]


  562. G. Herrera, R. Tomás, F. Vicente, J. M. Lopez-Sanchez, Jordi J. Mallorquì, and J. Mulas. Mapping ground movements in open pit mining areas using differential SAR interferometry. International Journal of Rock Mechanics and Mining Sciences, 47(7):1114-1125, 2010. [bibtex-key = herreraTomasVicenteLopezSanchezMallorquiMulas2010] [bibtex-entry]


  563. Michael Jehle, Othmar Frey, David Small, and Erich Meier. Measurement of Ionospheric TEC in Spaceborne SAR Data. IEEE Trans. Geosci. Remote Sens., 48(6):2460-2468, June 2010. Keyword(s): SAR Processing, Total Electron Content Estimation, TEC Estimation, Ionospheric TEC, Faraday Rotation, Path Delay, Autofocus, TEC Autofocus, Spaceborne SAR, L-Band, ALOS, Phased Array L-band SAR, PALSAR, P-Pand, Simulation, Calibration.
    Abstract: The propagation of spaceborne radar signals operating at L-band frequency or below can be seriously affected by the ionosphere. At high states of solar activity, Faraday rotation (FR) and signal path delays disturb radar polarimetry and reduce resolution in range and azimuth. While these effects are negligible at X-band, FR and the frequency-dependent path delays can become seriously problematic starting at L-band. For quality assurance and calibration purposes, existing L-band or potential spaceborne P-band missions require the estimation of the ionospheric state before or during the data take. This paper introduces two approaches for measuring the ionospheric total electron content (TEC) from single-polarized spaceborne SAR data. The two methods are demonstrated using simulations. Both methods leverage knowledge of the frequency-dependent path delay through the ionosphere: The first estimates TEC from the phase error of the filter mismatch, while the second gauges path-delay differences between up and down chirps. FR, mean (direct current) offsets, and noise contributions are also considered in the simulations. Finally, possibilities for further methodological improvements are discussed.
    [bibtex-key = jehleFreySmallMeier2010:IonTECfromSAR] [bibtex-entry]


  564. S. Jun, Z. Xiaoling, Y. Jianyu, and W. Chen. APC Trajectory Design for One-Active Linear-Array Three-Dimensional Imaging SAR. IEEE Transactions on Geoscience and Remote Sensing, 48(3):1470-1486, March 2010. Keyword(s): SAR Processing, W-Band, antenna phased arrays, geophysical signal processing, geophysical techniques, image resolution, radar imaging, stereo image processing, synthetic aperture radar, 3D ambiguity function, 3D spatial resolution, Lagrange multiplier method, W-band wave carrier, along-track resolution, antenna phase center function, antenna phase center trajectory design, cross-track resolution, millimeter wave carrier, minimum variance criterion, one-active linear-array 3D imaging SAR, parabolic distribution, switching mode, one-active linear-array 3-D imaging SAR (LASAR), 3-D imaging SAR, Antenna phase center trajectory (APCT) design, Lagrange multiplier method, synthetic aperture radar (SAR).
    Abstract: This paper discusses the antenna phase center trajectory (APCT) design for the "one-active" linear-array 3D imaging SAR (LASAR). First, we discuss the principle of the one-active LASAR and demonstrate its feasibility by experiment. To describe the 3D spatial resolution of the one-active LASAR, the relationship between the 3D ambiguity function (AF) of the one-active LASAR and the system parameters is discussed in detail. Based on the analysis, we divide the APCT design into three topics: the direction of the linear array, the length of the linear array, and the switching mode of the active element [named as antenna phase center function (APCF)]. On the first topic, we conclude that, when the range, along-track, and cross-track directions are orthogonal to each other, the ambiguity region of the one-active LASAR attains minimum, and the 3D spatial resolution can be separated into the range, along-track, and cross-track resolutions. On the second topic, we find that the cross-track resolution is determined by the length of the linear array and the frequency of the carrier. To ensure that the length of the linear array is acceptable, the carrier should be W-band wave or millimeter wave. On the third topic, the effect of APCF is researched, and we find that both the periodic APCF and the pseudorandom APCF can produce 3D resolution, except for the periodic rectangle APCF. For the pseudorandom APCF and the periodic APCF with short period, the cross-range 2D AF is or can be approximated as the product of two 1D AFs in the along- and cross-track directions. Finally, the distribution of the pseudorandom APCF is optimized by the Lagrange multiplier method under the minimum variance criterion, and we find that, when the pseudorandom APCF obeys the parabolic distribution, the cross-range 2D AF is optimal.
    [bibtex-key = junXiaolingJianyuChenTGARS2010WBANDRadar] [bibtex-entry]


  565. G. Krieger, Irena Hajnsek, Konstantinos P. Papathanassiou, M. Younis, and A. Moreira. Interferometric Synthetic Aperture Radar (SAR) Missions Employing Formation Flying. Proceedings of the IEEE, 98(5):816-843, May 2010. Keyword(s): SAR Processing, Tandem-L, Tandem-X, Formation Flying, Spaceborne SAR, SAR Tomography, Tomography.
    Abstract: This paper presents an overview of single-pass interferometric Synthetic Aperture Radar (SAR) missions employing two or more satellites flying in a close formation. The simultaneous reception of the scattered radar echoes from different viewing directions by multiple spatially distributed antennas enables the acquisition of unique Earth observation products for environmental and climate monitoring. After a short introduction to the basic principles and applications of SAR interferometry, designs for the twin satellite missions TanDEM-X and Tandem-L are presented. The primary objective of TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy as the basis for a wide range of scientific research as well as for commercial DEM production. This goal is achieved by enhancing the TerraSAR-X mission with a second TerraSAR-X like satellite that will be launched in spring 2010. Both satellites act then as a large single-pass SAR interferometer with the opportunity for flexible baseline selection. Building upon the experience gathered with the TanDEM-X mission design, the fully polarimetric L-band twin satellite formation Tandem-L is proposed. Important objectives of this highly capable interferometric SAR mission are the global acquisition of three-dimensional forest structure and biomass inventories, large-scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The sophisticated mission concept and the high data-acquisition capacity of Tandem-L will moreover provide a unique data source to systematically observe, analyze, and quantify the dynamics of a wide range of additional processes in the bio-, litho-, hydro-, and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics. Enabling technologies and techniques are described in detail. An ou- tlook on future interferometric and tomographic concepts and developments, including multistatic SAR systems with multiple receivers, is provided.
    [bibtex-key = KriegerHajnsekPapathanassiouYounisMoreira2010:FormationFlyingSpaceborneMissions] [bibtex-entry]


  566. Dawei Liu, Guoqing Sun, Zhifeng Guo, K.J. Ranson, and Yang Du. Three-Dimensional Coherent Radar Backscatter Model and Simulations of Scattering Phase Center of Forest Canopies. IEEE Trans. Geosci. Remote Sens., 48(1):349-357, January 2010. Keyword(s): 3D coherent radar backscatter model, InSAR signals, SAR interferometric data, canopy height, canopy spatial structure, forest canopies, forest stand, forest structural parameters, ground surface backscattering, interferometric SAR, scattering phase center simulations, synthetic aperture radar, time delay, backscatter, geophysical signal processing, radar interferometry, remote sensing by radar, synthetic aperture radar, vegetation;.
    Abstract: A 3-D coherent radar backscatter model for forest canopies was developed and used to improve the understanding of synthetic aperture radar (SAR) interferometric data. The model was based on a realistic 3-D spatial structure of a forest stand, in which every scatterer has its deterministic location. A backscattering signal from a scatterer was mapped into a pixel according to its range or signal time delay. The range or the time delay also determines the phase of the scattered field. All scattering matrices within a pixel were coherently added to yield the total backscattering field of the pixel. The coherent radar backscatter model takes into account not only the scattering contribution from the scatterers in the forest canopy but also the direct backscattering of the ground surface. Forest stands with three different spatial structures were simulated using L-system and field measurements. The number and sizes of trees in these forest stands were identical, but the 2-D arrangements of the trees were different. The interferometric SAR (InSAR) signals of these scenes were simulated using the 3-D coherent SAR model, and the heights of scattering phase centers were estimated from the simulated InSAR data. The results reported in this paper show that the spatial structures of vegetation play an important role in the location of the scattering phase center. The height of scattering phase center depends on canopy height, attenuation of canopy, and the gaps within the canopy. This paper shows that the spatial structure needs to be considered when the InSAR data are used for the estimation of forest structural parameters.
    [bibtex-key = liuSunGuoRansonDu2010] [bibtex-entry]


  567. Paco Lopez-Dekker and Jordi. J. Mallorqui. Capon- and APES-Based SAR Processing: Performance and Practical Considerations. IEEE Transactions on Geoscience and Remote Sensing, 48(5):2388-2402, May 2010. Keyword(s): SAR Processing, APES, Amplitude and Phase EStimation, Capon, minimum variance distortionless response, MVDR, Capon Beamforming, Beamforming, geophysical techniques, spectral analysis, synthetic aperture radar, minimum variance method, APES-based SAR processing, spectral-estimation algorithms, synthetic aperture radar, Capon processing chain, chip-image size, resampling factor, diagonal loading, joint-processing approach, Monte Carlo simulations, RADARSAT-2 quad-polarization data, Barcelona, adaptive processing, 2D spectral analysis, Spain, Azimuth, Backpropagation algorithms, Focusing, Covariance matrix, Phase estimation, Tomography, Remote sensing, Laboratories, Microwave communication, Array signal processing, Adaptive processing, synthetic aperture radar, 2-D spectral analysis.
    Abstract: This paper discusses the use of Capon's minimum-variance method (MVM) and Amplitude and Phase EStimation (APES) spectral-estimation algorithms to synthetic aperture radar range-azimuth focusing. The rationale of the algorithms is discussed. An implementation of a Capon or APES processing chain is explained, and processing parameters such as chip-image size, resampling factor, and diagonal loading are discussed. For multichannel cases, a joint-processing approach is presented. A set of Monte Carlo simulations are described and used to benchmark Capon- and APES-based processing against conventional matched-filter-based approaches. Both methods improve the resolution and reduce sidelobes. APES yields generally better estimates of amplitude and phase than Capon but with worse resolution. Results with RADARSAT-2 quad-polarization data over Barcelona are used to qualitatively study the real-life performance of these algorithms.
    [bibtex-key = lopezDekkerMallorqui2010CaponAndAPESbasedSARProcessing] [bibtex-entry]


  568. G. Margarit, J. J. Mallorqui, and L. Pipia. Polarimetric Characterization and Temporal Stability Analysis of Urban Target Scattering. IEEE_J_GRS, 48(4):2038-2048, April 2010. Keyword(s): geophysical image processing, radar polarimetry, synthetic aperture radar, vegetation mapping, GRaphical Electromagnetic Computing SAR data, RADARSAT-2, TerraSAR-X, geometrical configuration, geometry-scattering, high resolution images, land classification, nonprobabilistic models, polarimetric capabilities, polarimetric characterization, polarimetric-dispersion properties, quasideterministic scattering behavior, synthetic aperture radar images, temporal stability analysis, urban target scattering, urban-image postprocessing, Analytical models, Computational modeling, Electromagnetic modeling, Electromagnetic scattering, Geometry, Image analysis, Radar scattering, Solid modeling, Stability analysis, Synthetic aperture radar, Polarimetry, synthetic aperture radar (SAR) simulation, urban scattering. [bibtex-key = Margarit2010] [bibtex-entry]


  569. Adriano Meta, Joseph Mittermayer, Pau Prats, Rolf Scheiber, and Ulrich Steinbrecher. TOPS Imaging With TerraSAR-X: Mode Design and Performance Analysis. IEEE Trans. Geosci. Remote Sens., 48(2):759-769, February 2010. Keyword(s): SAR Processing, TOPS, geophysical techniques, radar antennas, radar imaging, radar interferometry, synthetic aperture radar, DTAR deterioration, ScanSAR modes, TOPS imaging, TSX, TerraSAR-X, Terrain Observation by Progressive Scan, antenna steering, burst acquisition, complete azimuth antenna pattern, imaging mode design, interferometry, performance analysis, squinted angles, wide-swath TOPS, ScanSAR, synthetic aperture radar (SAR), terrain observation by progressive scan (TOPS), wide-swath SAR. [bibtex-key = metaMittermayerPratsScheiberSteinbrecherTGARS2010TOPS] [bibtex-entry]


  570. J. Mittermayer, Marwan Younis, R. Metzig, S. Wollstadt, J. Marquez Martinez, and Adriano Meta. TerraSAR-X System Performance Characterization and Verification. IEEE Transactions on Geoscience and Remote Sensing, 48(2):660-676, February 2010. Keyword(s): calibration, geophysical techniques, optimisation, remote sensing by radar, spaceborne radar, synthetic aperture radar, ScanSAR, Spotlight, TerraSAR-X system, acquisition geometry, ambiguities, block adaptive quantization setting, commissioning phase, elevation beam definition, impulse-response function, noise, optimization, performance characterization, radiometric resolution, range timing, receiving gain, synthetic aperture radar, systems verification, Commissioning phase, SAR system performance, TerraSAR-X (TS-X), synthetic aperture radar (SAR).
    Abstract: This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters-ambiguities, impulse-response function, noise, and radiometric resolution-is discussed. ScanSAR and Spotlight particularities are described.
    [bibtex-key = mittermayerYounisMetzigWollstadtMarquezMartinezMetaTGRS2010TerraSARXSystemPerformance] [bibtex-entry]


  571. Andrea Monti Guarnieri and Silvia Scirpoli. Efficient Wavenumber Domain Focusing for Ground-Based SAR. IEEE Geoscience and Remote Sensing Letters, 7(1):161-165, January 2010. Keyword(s): SAR Processing, omega-k, Wavenumver Domain Algorithm, Range Migration Algorithm, GB-SAR, SAR Focusing, Azimuth Focusing, computational complexity, focusing, geophysical signal processing, geophysical techniques, radar signal processing, remote sensing by radar, synthetic aperture radar, computational complexity, efficient wavenumber domain focusing, ground-based SAR, kernel nonseparability, modified Stolt interpolation, polar formatting algorithm, range-angular domain, space-varying resampling, synthetic aperture radar, Focusing, ground-based (GB) synthetic aperture radar (SAR), wavenumber domain algorithm.
    Abstract: We present an efficient focusing algorithm for synthetic aperture radar (SAR) data acquired in short bursts by a geometry with a large range spread, i.e., the case of ground-based (GB) SAR. The usual approach for focusing GB SAR data is the polar formatting algorithm, whose computational complexity, however, is quite relevant due to the nonseparability of the kernel. In this letter, we introduce a different format for the focused data, namely, the range-angular domain. Such format keeps the benefit of the polar format that samples data close to the resolution, but allows for the design of a separable kernel in the wavenumber domain. The proposed kernel exploits a modified Stolt interpolation and an efficient space-varying resampling. Results of processing on both simulated and real data are presented.
    [bibtex-key = montiGuarnieriScirpoliGRSL2010EfficientOmegaKFocusingGBSAR] [bibtex-entry]


  572. Andrea Monti-Guarnieri and Stefano Tebaldini. ML-Based Fringe-Frequency Estimation for InSAR. IEEE Geoscience and Remote Sensing Letters, 7(1):136-140, January 2010. Keyword(s): SAR Processing, InSAR, SAR Interferometry, ENVISAR, ASAR.
    Abstract: This letter focuses on estimating the local fringe frequency of the interferometric phase, under the hypothesis of superficial scattering. Starting from the formulation of the maximum-likelihood estimator, a new simplified estimator is derived. Due to computational efficiency and robustness versus model errors, the resulting estimator is suited for large data processing in the presence of model uncertainty. Furthermore, such an estimator can be straightforwardly extended to the multibaseline case, resulting in the possibility to estimate the terrain slope with great accuracy. An application to real data is presented, based on a multibaseline ENVISAT data set.
    [bibtex-key = montiGuarnieriTebaldiniGRSL2010:InSARFringeFreqEstim] [bibtex-entry]


  573. V. D. Navarro-Sanchez, J. M. Lopez-Sanchez, and F. Vicente-Guijalba. A Contribution of Polarimetry to Satellite Differential SAR Interferometry: Increasing the Number of Pixel Candidates. IEEE Geosci. Remote Sens. Lett., 7(2):276-280, April 2010. Keyword(s): geophysical techniques, radar interferometry, radar polarimetry, synthetic aperture radar, TerraSAR-X, advanced differential SAR interferometry, amplitude dispersion index, amplitude dispersion selection, average coherence, copolar linear channels, dual-pol images, interferograms, pixel candidates, polarimetric information, satellite sensors, single-pol data, urban area, Differential synthetic aperture radar (SAR) interferometry, polarimetry.
    Abstract: This letter presents a general method for increasing the number of pixel candidates, those selected for processing in advanced differential SAR interferometry, by means of the exploitation of the polarimetric information provided by new satellite sensors. The algorithm is formulated for two different criteria of selection: the average coherence over the stack of interferograms and the amplitude dispersion index of the stack of images. Experimental results obtained with dual-pol images of TerraSAR-X over an urban area have demonstrated the expected improvement. The number of pixel candidates for an arbitrary threshold is 60% higher than that for single-pol data when using the average coherence and three times higher when using the dispersion index. The approach has also been compared to a selection based on a set of conventional channels (the copolar linear channels and the first two Pauli ones), showing a slight improvement for coherence selection and an important one for amplitude dispersion selection.
    [bibtex-key = navarroSanchezLopezSanchezVicenteGuijalbaGRSL2010PolarimetrySatelliteDInSAR] [bibtex-entry]


  574. Maxim Neumann, Laurent Ferro-Famil, and Andreas Reigber. Estimation of Forest Structure, Ground, and Canopy Layer Characteristics From Multibaseline Polarimetric Interferometric SAR Data. IEEE Trans. Geosci. Remote Sens., 48(3):1086-1104, March 2010. Keyword(s): SAR Processing, Multibaseline SAR, Germany, PolInSAR, RVoG, vertical structure, Traunstein test site, airborne SAR, L-band, angular distribution, canopy layer heights, differential extinction, double-bounce ground-trunk interactions, forest layer heights, forest parameter retrieval, forest structure estimation, forest vegetation, ground topography, ground-to-volume ratio, ground-truth measurements, interferometric coherence, particle scattering anisotropy, polarimetric Synthetic Aperture Radar interferometry, polarimetric decomposition, polarimetric scattering media model, polarization orientation randomness, random-volume-over-ground PolInSAR parameter inversion, repeat-pass configuration, root-mean-square error, surface scattering, temporal decorrelation, tree morphology, volume coherency matrices, volumetric canopy, volumetric understory scattering, wave attenuation, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: This paper concerns forest parameter retrieval from polarimetric interferometric synthetic aperture radar (PolInSAR) data considering two layers, one for the ground under the vegetation and one for the volumetric canopy. A model is designed to combine a physical model-based polarimetric decomposition with the random-volume-over-ground (RVoG) PolInSAR parameter inversion approach. The combination of a polarimetric scattering media model with a PolInSAR RVoG vertical structure model provides the possibility to separate the ground and the volume coherency matrices based on polarimetric signatures and interferometric coherence diversity. The proposed polarimetric decomposition characterizes volumetric media by the degree of polarization orientation randomness and by the particle scattering anisotropy. Using the full model enhances the estimation of the vertical forest structure parameters by enabling us to estimate the ground-to-volume ratio, the temporal decorrelation, and the differential extinction. For forest vegetation observed at L-band, this model accounts for the ground topography, forest and canopy layer heights, wave attenuation in the canopy, tree morphology in the form of the angular distribution and the effective shapes of the branches, and the contributions from the ground level consisting of surface scattering and double-bounce ground-trunk interactions, as well as volumetric understory scattering. The parameter estimation performance is evaluated on real airborne L-band SAR data of the Traunstein test site, acquired by the German Aerospace Center (DLR)'s E-SAR sensor in 2003, in both single- and multibaseline configurations. The retrieved forest height is compared with the ground-truth measurements, revealing, for the given test site, an average root-mean-square error (rmse) of about 5 m in the repeat-pass configuration. This implies an improvement in rmse by over 2 m in comparison to the pure coherence-based RVoG PolInSAR parameter inversion.
    [bibtex-key = neumannFerroFamilReigber2010:ForestPolInSAR] [bibtex-entry]


  575. Pau Prats, Rolf Scheiber, Josef Mittermayer, Adriano Meta, and Alberto Moreira. Processing of Sliding Spotlight and TOPS SAR Data Using Baseband Azimuth Scaling. IEEE Trans. Geosci. Remote Sens., 48(2):770-780, February 2010. Keyword(s): data acquisition, geophysical image processing, radar imaging, synthetic aperture radar, Doppler centroid, SAR processing, ScanSAR mode, TOPS SAR Data, TerraSAR-X, azimuth processing, baseband azimuth scaling, data focusing, phase preserving processor, sliding spotlight, synthetic aperture radar, terrain observation, Azimuth scaling, SAR processing, ScanSAR, Terrain Observation by Progressive Scans (TOPS), spotlight, synthetic aperture radar (SAR).
    Abstract: This paper presents an efficient phase preserving processor for the focusing of data acquired in sliding spotlight and Terrain Observation by Progressive Scans (TOPS) imaging modes. They share in common a linear variation of the Doppler centroid along the azimuth dimension, which is due to a steering of the antenna (either mechanically or electronically) throughout the data take. Existing approaches for the azimuth processing can become inefficient due to the additional processing to overcome the folding in the focused domain. In this paper, a new azimuth scaling approach is presented to perform the azimuth processing, whose kernel is exactly the same for sliding spotlight and TOPS modes. The possibility to use the proposed approach to process data acquired in the ScanSAR mode, as well as a discussion concerning staring spotlight, is also included. Simulations with point targets and real data acquired by TerraSAR-X in sliding spotlight and TOPS modes are used to validate the developed algorithm.
    [bibtex-key = pratsScheiberMittermayerMetaMoreiraTGARS2010TOPS] [bibtex-entry]


  576. B. Rabus, H. Wehn, and M. Nolan. The Importance of Soil Moisture and Soil Structure for InSAR Phase and Backscatter, as Determined by FDTD Modeling. IEEE_J_GRS, 48(5):2421-2429, May 2010. Keyword(s): backscatter, geophysical techniques, radar interferometry, soil, spaceborne radar, surface roughness, synthetic aperture radar, FDTD Modeling, SAR phase, air-filled-void content, backscatter data, backscatter inversions, backscatter responses, bulk moisture content, correlation length, finite-difference time domain, finite-difference time-domain simulator, interferometric phase, mean bulk moisture, microwave backscatter, near-surface soil-structure parameters, phase changes, phase differences, soil structures, spaceborne InSAR techniques, spaceborne interferometric synthetic aperture radar, uniform soil-moisture distributions, vertical moisture gradient, Finite-difference time domain (FDTD), interferometric synthetic aperture radar (InSAR), soil moisture, synthetic aperture radar (SAR). [bibtex-key = Rabus2010] [bibtex-entry]


  577. M. Rodriguez-Cassola, S.V. Baumgartner, G. Krieger, and A. Moreira. Bistatic TerraSAR-X/F-SAR Spaceborne-Airborne SAR Experiment: Description, Data Processing, and Results. IEEE Trans. Geosci. Remote Sens., 48(2):781-794, February 2010. Keyword(s): SAR Processing, Bistatic SAR, Time-Domain Back-Projection, TDBP, Doppler information, German Aerospace Center, TerraSAR-X/F-SAR bistatic data, backscatter, bistatic back-projection algorithm, bistatic synthetic aperture radar, calibration, data processing, first X-band spaceborne-airborne SAR, nonstationary bistatic acquisitions, phase-preserving bistatic focusing, synchronization algorithm, airborne radar, backscatter, calibration, data acquisition, geophysical signal processing, radar signal processing, remote sensing by radar, spaceborne radar, synchronisation, synthetic aperture radar.
    Abstract: We report about the first X-band spaceborne-airborne bistatic synthetic aperture radar (SAR) experiment, conducted early November 2007, using the German satellite TerraSAR-X as transmitter and the German Aerospace Center's (DLR) new airborne radar system F-SAR as receiver. The importance of the experiment resides in both its pioneering character and its potential to serve as a test bed for the validation of nonstationary bistatic acquisitions, novel calibration and synchronization algorithms, and advanced imaging techniques. Due to the independent operation of the transmitter and receiver, an accurate synchronization procedure was needed during processing to make high-resolution imaging feasible. Precise phase-preserving bistatic focusing can only be achieved if time and phase synchronization exist. The synchronization approach, based on the evaluation of the range histories of several reference targets, was verified through a separate analysis of the range and Doppler contributions. After successful synchronization, nonstationary focusing was performed using a bistatic backprojection algorithm. During the campaign, stand-alone TerraSAR-X monostatic as well as interoperated TerraSAR-X/F-SAR bistatic data sets were recorded. As expected, the bistatic image shows a space-variant behavior in spatial resolution and in signal-to-noise ratio. Due to the selected configuration, the bistatic image outperforms its monostatic counterpart in almost the complete imaged scene. A detailed comparison between monostatic and bistatic images is given, illustrating the complementarity of both measurements in terms of backscatter and Doppler information. The results are of fundamental importance for the development of future nonsynchronized bistatic SAR systems.
    [bibtex-key = RodriguezCassolaBaumgartnerKriegerMoreira2007:BiStaticBackProjection] [bibtex-entry]


  578. Helmut Rott, Simon H. Yueh, Donlad W. Cline, Claude Duguay, Richard Essery, Christian Haas, Florence Heliere, Michael Kern, Giovanni Macelloni, Eirik Malnes, Thomas Nagler, Jouni Pulliainen, H. Rebhan, and A. Thompson. Cold Regions Hydrology High-Resolution Observatory for Snow and Cold Land Processes. Proceedings of the IEEE, 98(5):752-765, May 2010. Keyword(s): glaciology, ice, remote sensing by radar, sea ice, snow, synthetic aperture radar, water, climate models, cold land processes, cold regions hydrology high-resolution observatory satellite mission, cryosphere, frequency 17.2 GHz, frequency 9.6 GHz, glaciers, global snow observations, global water cycle, lake ice, land snow, scattering signal decomposition, sea ice, synthetic aperture radar, water cycle variability prediction, Hydrology, Ice surface, Lakes, Land surface, Observatories, Satellites, Sea ice, Sea surface, Snow, Water resources, Climate research, earth observation satellite, glaciers, snow cover, synthetic aperture radar, water resources.
    Abstract: Snow is a critical component of the global water cycle and climate system, and a major source of water supply in many parts of the world. There is a lack of spatially distributed information on the accumulation of snow on land surfaces, glaciers, lake ice, and sea ice. Satellite missions for systematic and global snow observations will be essential to improve the representation of the cryosphere in climate models and to advance the knowledge and prediction of the water cycle variability and changes that depend on snow and ice resources. This paper describes the scientific drivers and technical approach of the proposed Cold Regions Hydrology High-Resolution Observatory (CoReH2O) satellite mission for snow and cold land processes. The sensor is a synthetic aperture radar operating at 17.2 and 9.6 GHz, VV and VH polarizations. The dual-frequency and dual-polarization design enables the decomposition of the scattering signal for retrieving snow mass and other physical properties of snow and ice.
    [bibtex-key = rottEtAlProcIEEE2010CoReH20] [bibtex-entry]


  579. Sabine Rödelsperger, Gwendolyn Läufer, Carl Gerstenecker, and Matthias Becker. Monitoring of displacements with ground-based microwave interferometry: IBIS-S and IBIS-L. Journal of Applied Geodesy, 4(1):41 - 54, 2010. [bibtex-key = roedelspergerEtAl2010DisplacementWithGBInSARIBISSandIBISL] [bibtex-entry]


  580. P. Samczynski and K.S. Kulpa. Coherent MapDrift Technique. IEEE Trans. Geosci. Remote Sens., 48(3):1505-1517, 2010. Keyword(s): SAR Processing, Autofocus, SAR Autofocus, MoComp, Motion Compensation, Map-Drift Autofocus, Coherent Map-Drift Autofocus, geophysical signal processing, radar signal processing, remote sensing by radar, synthetic aperture radar, target tracking, Earth imaging, MapDrift principles, coherent MapDrift technique, flight parameter estimation, moving target indication, parametric autofocus technique, real time processing, strip mode SAR systems, synthetic aperture radar, Autofocus, MD, coherent MapDrift (CMD), moving-target indication (MTI), multilook, subaperture, synthetic aperture radar (SAR).
    Abstract: A new parametric autofocus technique with a high accuracy of flight-parameter estimation dedicated to strip-mode synthetic aperture radar (SAR) systems is presented. Most of the known autofocus techniques require high-reflectivity targets (man-made targets) to obtain a properly focused SAR image. The technique proposed in this paper allows flight parameters to be estimated effectively, even for a low-contrast scene (e.g., forests, fields, small paths, etc.). The autofocus technique is based on well-known MapDrift (MD) principles. The presented technique is a coherent one, which allows flight parameters to be estimated more precisely than in the other well-known parametric technique referred to as classical MD. The presented technique allows flight parameters to be estimated with accuracy that is independent of the initial velocity error. It can be used for real-time processing for both Earth imaging and moving-target indication.
    [bibtex-key = samczynskiKulpaTGRS2010] [bibtex-entry]


  581. E. Sansosti, F. Casu, M. Manzo, and R. Lanari. Space-borne radar interferometry techniques for the generation of deformation time series: An advanced tool for Earth's surface displacement analysis. Geophys. Res. Lett., 37(20):L20305, October 2010. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, ground deformation time series, DInSAR, remote sensing, Mathematical Geophysics, Time series analysis, Remote sensing, Satellite geodesy.
    Abstract: This work is focused on advanced differential SAR interferometry (DInSAR) techniques for the generation of deformation time series from sequences of SAR images. We first present the basic rationale of these techniques providing some details of the most well known algorithms. Subsequently, through the analysis of selected case studies focused on the available C-band SAR data archives, we show the relevance of the retrieved spatially dense deformation time series for the comprehension of several geophysical phenomena. We finally introduce, again via a real case study, the advances brought in by the new generation X-band space-borne SAR sensors, highlighting new investigation possibilities for fast varying deformation phenomena.
    [bibtex-key = sansostiCasuManzoLanari2010PSI] [bibtex-entry]


  582. Piyush Shanker Agram and Howard Zebker. Edgelist phase unwrapping algorithm for time series InSAR analysis. J. Opt. Soc. Am. A, 27(3):605-612, March 2010. Keyword(s): SAR Processing, Three-dimensional image processing, Interferometry, InSAR, SAR Interferometry, Synthetic aperture radar, Phase, Phase unwrapping.
    Abstract: We present here a new integer programming formulation for phase unwrapping of multidimensional data. Phase unwrapping is a key problem in many coherent imaging systems, including time series synthetic aperture radar interferometry (InSAR), with two spatial and one temporal data dimensions. The minimum cost flow (MCF) [IEEE Trans. Geosci. Remote Sens. 36, 813 (1998)] phase unwrapping algorithm describes a global cost minimization problem involving flow between phase residues computed over closed loops. Here we replace closed loops by reliable edges as the basic construct, thus leading to the name ``edgelist.'' Our algorithm has several advantages over current methods---it simplifies the representation of multidimensional phase unwrapping, it incorporates data from external sources, such as GPS, where available to better constrain the unwrapped solution, and it treats regularly sampled or sparsely sampled data alike. It thus is particularly applicable to time series InSAR, where data are often irregularly spaced in time and individual interferograms can be corrupted with large decorrelated regions. We show that, similar to the MCF network problem, the edgelist formulation also exhibits total unimodularity, which enables us to solve the integer program problem by using efficient linear programming tools. We apply our method to a persistent scatterer-InSAR data set from the creeping section of the Central San Andreas Fault and find that the average creep rate of 22 mm/Yr is constant within 3 mm/Yr over 1992-2004 but varies systematically with ground location, with a slightly higher rate in 1992-1998 than in 1999-2003.
    [bibtex-key = shankerAgramZebkerJOSAA2010:PhaseUnwrapping] [bibtex-entry]


  583. Manoochehr Shirzaei and TR Walter. Time-dependent volcano source monitoring using interferometric synthetic aperture radar time series: A combined genetic algorithm and Kalman filter approach. Journal of Geophysical Research: Solid Earth, 115(B10), 2010. Keyword(s): SAR Processing, Deformation, Deformation Monitoring, DInSAR, SAR Interferometry, Differential Interferometry, Kalman Filter.
    Abstract: Modern geodetic methods allow continuous monitoring of deformation fields of volcanoes. The acquired data contribute significantly to the study of the dynamics of magmatic sources prior to, during and after eruptions and intrusions. In addition to advancing the monitoring techniques, it is important to develop suitable approaches to deal with deformation time series. Here, we present, test and apply a new approach for time-dependent, nonlinear inversion using a combination of a genetic algorithm (GA) and a Kalman filter (KF). The GA is used in the form presented by Shirzaei and Walter (2009), and the KF implementation now allows for the treatment of monitoring data as a full time series rather than as single time steps. This approach provides a flexible tool for assessing unevenly sampled and heterogeneous time series data and explains the deformation field using time-consistent dislocation sources. Following synthetic tests, we demonstrate the merits of time-consistent source modeling for interferometric synthetic aperture radar (InSAR) data available between 1992 and 2008 from the Campi Flegrei volcano in Italy. We obtained multiple episodes of linear velocity for the reservoir pressure change associated with a parabolic surface deformation on the volcano. These data may be interpreted via differential equations as a linear flux to the shallow reservoir that provides new insight into how both the shallow and deep reservoirs communicate beneath Campi Flegrei. The synthetic test and case study demonstrate the robustness of our approach and the ability to track and monitor the source of systems with complex dynamics. It is applicable to time-dependent optimization problems in volcanic and tectonic environments in other tectonic environments in other areas and allows understanding of the spatiotemporal extent of a physical process in quantitative manner.
    [bibtex-key = Shirzaei2010] [bibtex-entry]


  584. Stefano Tebaldini. Single and Multipolarimetric SAR Tomography of Forested Areas: A Parametric Approach. IEEE Trans. Geosci. Remote Sens., 48(5):2375-2387, May 2010. Keyword(s): SAR Processing, SAR Tomography, Tomography, Forest, Forestry, Fourier spectrum, Remningstorp, Sweden, backscattered powers, covariance matching estimation technique, effective scattering centers, forested areas, multibaseline data, multiple synthetic aperture radar observations, multipolarimetric SAR tomography, multipolarimetric acquisition, parameter estimation, polarimetric data set, radar signal processing, real P-band multibaseline, resolution cell, geophysical signal processing, geophysical techniques, optical tomography, radar polarimetry, synthetic aperture radar;.
    Abstract: In this paper, a technique is described for the tomographic characterization of forested areas through multiple synthetic aperture radar (SAR) observations, based on either single or multipolarimetric acquisitions. This technique is based on the idea of characterizing the Fourier spectrum of the multibaseline data as being constituted by two effective scattering centers displaced along the vertical direction, plus the associated decorrelation terms. As a result, SAR tomography will be formulated as the problem of detecting the number of scattering centers within the resolution cell, estimating the parameters that describe their spatial structure, and evaluating the associated backscattered powers. Parameter estimation is carried out through the covariance matching estimation technique, which provides an asymptotically optimal solution. The results of an experiment performed on a real P-band multibaseline fully polarimetric data set relative to the forested site of Remningstorp, Sweden, are reported.
    [bibtex-key = tebaldiniTGRS2010:TOMOFOREST] [bibtex-entry]


  585. Stefano Tebaldini and Andrea Monti-Guarnieri. On the Role of Phase Stability in SAR Multibaseline Applications. IEEE Transactions on Geoscience and Remote Sensing, 48(7):2953-2966, July 2010. Keyword(s): SAR Processing, InSAR, Interferometry, SAR Interferometry, Monte Carlo simulations, Neyman-Pearson theory, P-band, Remningstorp, SAR multibaseline applications, Sweden, atmospheric disturbances, multibaseline synthetic aperture radar interferometry, phase calibration, phase stability, propagation disturbances, residual platform motion, signal-to-noise ratio, statistical analysis, SAR Tomography, Tomography, Monte Carlo methods, geophysical techniques, radar interferometry, remote sensing by radar, statistical analysis, synthetic aperture radar;.
    Abstract: This paper is meant to present a statistical analysis of the role of propagation disturbances (PDs), such as those due to atmospheric disturbances or to residual platform motion, in multibaseline synthetic aperture radar (SAR) interferometry (InSAR) and tomography (T-SAR) applications. The analysis will consider both pointlike and distributed targets in such a way as to cover all the cases that are relevant in the applications. In order to provide a tool for the evaluation of the impact of PDs on the analysis of an arbitrary scenario, a definition of signal-to-noise ratio (SNR) will be introduced that accounts for both the presence of PDs and the characteristics of the imaged scene. In the case of pointlike targets, it will be shown that such definition of SNR allows reusing well known results following after the Neyman-Pearson theory, thus providing a straightforward tool to asses phase-stability requirements for the detection and localization of multiple pointlike targets. In the case of distributed targets, instead, it will be provided a detailed analysis of the random fluctuations of the reconstructed scene as a function of the extent of the PDs, of the vertical structure of the imaged scene, and of the number of looks that are employed. Results from Monte Carlo simulations will be presented that fully support the theoretical developments within this paper. The most relevant conclusion of this paper is that the impact of PDs is more severe in the case where the imaged scene is characterized by a complex vertical structure or when multiple pointlike targets are present. As a consequence, it follows that the T-SAR analyses require either a higher phase stability or a more accurate phase calibration with respect to InSAR analyses. Finally, an example of phase-stability analysis and phase calibration of a real data set will be shown, based on a P-band data set relative to the forest site of Remningstorp, Sweden.
    [bibtex-key = tebaldiniMontiGuarnieri2010TomoPhaseStab] [bibtex-entry]


  586. R Tomás, G Herrera, J Delgado, Juan Manuel Lopez-Sanchez, JJ Mallorquì, and J Mulas. A ground subsidence study based on DInSAR data: calibration of soil parameters and subsidence prediction in Murcia City (Spain). Engineering geology, 111(1):19-30, 2010. [bibtex-key = tomasHerreraDelgadoLopezSanchezMallorquiMulasEnginGeology2010SubsidenceDInSAR] [bibtex-entry]


  587. G. Vilardo, R. Isaia, G. Ventura, P. De Martino, and C. Terranova. InSAR Permanent Scatterer analysis reveals fault re-activation during inflation and deflation episodes at Campi Flegrei caldera. Remote Sensing of Environment, 114(10):2373-2383, 2010. [bibtex-key = vilardoIsaiaVenturaDeMartinoTerranovaRSE2010] [bibtex-entry]


  588. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. Ultrawideband Chirp Scaling Algorithm. IEEE Geoscience and Remote Sensing Letters, 7(2):281-285, April 2010. Keyword(s): SAR Processing, Chirp Scaling, Chirp Scaling Algorithm, CSA, UCS, Azimuth Focusing, Focusing Algorithm, CARABAS-II, LORA, UWB SAR, UWB, antenna beamwidth, fractional bandwidth, ground-moving-target-indication SAR systems, high pulse repetition frequency, integration angle, synthetic aperture radar imaging algorithms, ultrawideband chirp scaling algorithm, geophysical image processing, geophysical techniques, synthetic aperture radar;.
    Abstract: A new version of chirp scaling (CS), the so-called ultrawideband (UWB) CS (UCS), is proposed in this letter. UCS aims at UWB synthetic aperture radar (SAR) systems utilizing large fractional bandwidth and wide antenna beamwidth associated with a wide integration angle. Furthermore, it is also valid for SAR systems with special characteristics such as ground-moving-target-indication SAR systems with a very high pulse repetition frequency.
    [bibtex-key = vuSjogrenPettersson2010UWCSA] [bibtex-entry]


  589. Viet Thuy Vu, Thomas K. Sjogren, M.I. Pettersson, A. Gustavsson, and Lars M.H. Ulander. Detection of Moving Targets by Focusing in UWB SAR: Theory and Experimental Results. IEEE Trans. Geosci. Remote Sens., 48(10):3799 -3815, October 2010. Keyword(s): SAR Processing, UWB SAR, azimuth focusing, moving target detection, multichannel SAR data, reference system, reliable detection, signal-to-clutter noise ratio, single-channel SAR data, ultrawideband synthetic aperture radar, object detection, synthetic aperture radar, target tracking, ultra wideband radar;.
    Abstract: Moving-target detection in ultrawideband (UWB) synthetic aperture radar (SAR) is associated with long integration time and must accommodate azimuth focusing for reliable detection. This paper presents the theory on detection of moving targets by focusing and experimental results on single-channel SAR data aimed at evaluating the detection performance. The results with respect to both simulated and real data show that the ability to detect moving targets increases significantly when applying the proposed detection technique. The improvement in signal-to-clutter noise ratio, which is a basic requisite for evaluating the performance, reaches approximately 20 dB, using only single-channel SAR data. This gain will be preserved for the case of multichannel SAR data. The reference system for this study is the airborne UWB low-frequency SAR Coherent All RAdio BAnd Sensing II.
    [bibtex-key = vuSjogrenPetterssonGustavssonUlander2010] [bibtex-entry]


  590. Viet Thuy Vu, Thomas K. Sjogren, Mats I. Pettersson, L. Hakansson, A. Gustavsson, and Lars M. H. Ulander. RFI Suppression in Ultrawideband SAR Using an Adaptive Line Enhancer. IEEE Geosci. Remote Sens. Lett., 7(4):694-698, October 2010. Keyword(s): SAR Processing, RFI, radio frequency interference, RFI mitigation, interference suppression, least mean squares methods, radiofrequency interference, synthetic aperture radar, ultra wideband radar, RFI suppression, UWB low-frequency SAR data, adaptive line enhancer, least mean square algorithm, radiofrequency interference, ultrawideband low-frequency synthetic aperture radar, Adaptive control, Interference suppression, Least mean square algorithms, Line enhancers, Programmable control, Proposals, Radio frequency, Radiofrequency interference, Synthetic aperture radar, Ultra wideband technology, Adaptive line enhancer (ALE), CARABAS-II, normalized least mean square (LMS) (NLMS), radio-frequency interference (RFI), synthetic aperture radar (SAR), ultrawideband (UWB).
    Abstract: In this letter, we propose an approach to suppress radio-frequency interference (RFI) in ultrawideband (UWB) low-frequency synthetic aperture radar (SAR). According to the proposal, RFI is suppressed by using an adaptive line enhancer controlled by the normalized least mean square algorithm. The approach is tested successfully on real UWB low-frequency SAR data. In order to keep the computational burden down, possible ways to integrate the RFI suppression approach into SAR imaging algorithms are also suggested.
    [bibtex-key = vuSjogrenPetterssonHakanssonGustavssonUlander2010RFI] [bibtex-entry]


  591. Viet Thuy Vu, Thomas K. Sjogren, Mats I. Pettersson, and H. Hellsten. An Impulse Response Function for Evaluation of UWB SAR Imaging. IEEE Transactions on Signal Processing, 58(7):3927-3932, July 2010. Keyword(s): SAR Processing, impulse response function evaluation, Resolution, Range Resolution, Azimuth Resolution, IRF, UWB, object detection, radar imaging, synthetic aperture radar, transient response, ultra wideband radar, IRF-SAR, UWB SAR imaging, image quality measurement, impulse response function, narrowband-narrowbeam SAR systems, point target, spatial resolution estimation, two-dimensional sine function, Impulse response function in SAR imaging (IRF-SAR), Sinc, impulse response function in UWB SAR imaging (IRF-USAR), synthetic aperture radar (SAR), ultrawideband-ultrawidebeam (UWB).
    Abstract: Based on analysis of a point target imaged by different synthetic aperture radar (SAR) systems, the commonly used impulse response function in SAR Imaging (IRF-SAR)-a two-dimensional (2-D) sinc function-is shown to be inappropriate for ultrawideband-ultrawidebeam (UWB) SAR systems utilizing a large fractional signal bandwidth and a wide antenna beamwidth. As a consequence, the applications of the 2-D sinc function such as image quality measurements and spatial resolution estimations are limited to narrowband-narrowbeam (NB) SAR systems exploiting a small fractional signal bandwidth and a narrow antenna beamwidth. In this paper, a more general IRF-SAR, which aims at UWB SAR systems, is derived with an assumption of flat two-dimensional (2-D) Fourier transform (FT) of a SAR image and called IRF-USAR. However, the derived IRF-USAR is also valid for NB SAR systems.
    [bibtex-key = vuSjogrenPetterssonHellsten2010IRFEvaluationUWBSAR] [bibtex-entry]


  592. Teng Wang, Mingsheng Liao, and Daniele Perissin. InSAR Coherence-Decomposition Analysis. IEEE Geosci. Remote Sens. Lett., 7(1):156-160, January 2010. Keyword(s): SAR Processing, geophysical signal processing, radar interferometry, radar signal processing, radar theory, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping, topography (Earth), Badong, China, European Remote Sensing satellite tandem data, InSAR coherence decomposition analysis, acquisition geometry, classification algorithms, distributed target, geometrical coherence, geometrical decorrelation, mountain areas, phase coherence, pointlike target identification, sensor acquisition parameters, synthetic aperture radar interferometry, temporal coherence estimation, temporal decorrelation, terrain temporal changes, topographic model, Coherence estimation, synthetic aperture radar interferometry (InSAR);.
    Abstract: The phase coherence in synthetic aperture radar interferometry is often used in classification algorithms to detect possible temporal changes of the imaged terrain. However, in mountain areas, the interferometric coherence is also sensitive to the slight variations of the acquisition geometry. In this letter, we propose a very simple but effective method to separate the temporal decorrelation from the geometrical one. Assuming the imaged terrain can be modeled as a distributed target, the geometrical coherence can be estimated by exploiting a topographic model and the sensor acquisition parameters. The discrepancy between the geometrical coherence and the observed one can then be ascribed to temporal changes. Moreover, in presence of pointlike targets, the hypothesis of distributed terrain is no longer valid, and higher values of the observed coherence with respect to the synthetic geometrical one can be used to detect such targets. The proposed approach allows then in mountain areas the following conditions: (1) a simple and very fast rough estimation of the temporal coherence, and (2) the identification of pointlike targets using just two images. The method has been applied and tested in the Badong (China) site using European Remote Sensing satellite tandem data.
    [bibtex-key = wangLiaoPerissinGRSL2010] [bibtex-entry]


  593. R. Wang, O. Loffeld, Y.L. Neo, H. Nies, I. Walterscheid, T. Espeter, J. Klare, and J. Ender. Focusing Bistatic SAR Data in Airborne/Stationary Configuration. IEEE Trans. Geosci. Remote Sens., 48(1):452-465, January 2010. Keyword(s): SAR Processing, Bistatic SAR, BiSAR data, Forschungsgesellschaft fur Angewandte Naturwissenschaften, PAMIR, Transall C-160, airborne SAR system, azimuth modulation, bistatic SAR airborne-stationary configuration, bistatic SAR data, bistatic point-target reference spectrum, frequency domain based focusing algorithm, interpolation free wavenumber domain algorithm, range offset, range variant interpolation, stationary X-band transmitter, synthetic aperture radar, target azimuth position, target range migration trajectory, airborne radar, geophysical signal processing, interpolation, radar signal processing, remote sensing by radar, synthetic aperture radar.
    Abstract: This paper presents a frequency-domain-based focusing algorithm for the bistatic synthetic aperture radar (BiSAR) data in airborne/stationary configuration. In this bistatic configuration, only the moving platform contributes to the azimuth modulation, whereas the stationary platform introduces a range offset (RO) to the range migration trajectories of targets at the same range. The offset is determined by the azimuth position of different targets with respect to the stationary platform. Since the RO is position dependent, monostatic SAR imaging algorithms are not able to focus the bistatic data collected in this configuration. In this paper, an analytical bistatic point-target reference spectrum is derived, and then, a frequency-domain-based algorithm is developed to focus the bistatic data. It uses an interpolation-free wavenumber-domain algorithm as a basis and performs a range-variant interpolation to correct the position-dependent RO in the image domain after coarse focusing. The proposed algorithm is validated by the simulated data and the real BiSAR data acquired by the Forschungsgesellschaft f{\"u}r Angewandte Naturwissenschaften's airborne SAR system, PAMIR, in December 2007. In this BiSAR experiment, an X-band transmitter was stationary operated on a hill with PAMIR as the receiver mounted on a Transall C-160.
    [bibtex-key = WangLoffeldNeoNiesWalterscheidEspeterKlareEnder2010:BiStatic] [bibtex-entry]


  594. U. Wegmuller, D. Walter, V. Spreckels, and Charles L. Werner. Nonuniform Ground Motion Monitoring With TerraSAR-X Persistent Scatterer Interferometry. IEEE Trans. Geosci. Remote Sens., 48(2):895 -904, February 2010. Keyword(s): mining, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, PSI, TerraSAR-X persistent scatterer interferometry, deep-level mining, nonuniform ground motion monitoring, Differential SAR interferometry (DINSAR), TerraSAR-X, land deformation, mining, persistent scatterer interferometry (PSI);.
    Abstract: In the past, the application of Persistent Scatterer Interferometry (PSI) was primarily possible in the case of slow (less than a few centimeters per year) uniform movements. In this paper, we show how PSI permits the monitoring of relatively fast (including rates up to gt; 50 cm/year) and nonuniform movements using TerraSAR-X repeat observations over deep-level mining. To enable this, parts of the PSI methodology were adapted to the special characteristics of the example studied. Apart from a description of the methodology used and the result achieved, error considerations and a validation of the result with in situ measurements are included.
    [bibtex-key = wegmullerWalterSpreckelsWernerTGRS2010NonuniformDefPSI] [bibtex-entry]


  595. Meng Wei and David T. Sandwell. Decorrelation of L-Band and C-Band Interferometry Over Vegetated Areas in California. IEEE Trans. Geosci. Remote Sens., 48(7):2942-2952, July 2010. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, Earth crust, faulting, geomorphology, radar interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping, C-band European Remote Sensing Satellite interferograms, Coast Range area, ERS interferogram, Imperial Valley, L-band Advanced Land Observation Satellite interferograms, Northern California forests, Parkfleld, San Andreas Fault system, Southern California, crustal deformation, decorrelation behavior, decorrelation time, fault creep, interferometric synthetic aperture radar, interseismic deformation, local uplifting signal, near-fault interseismic deformation, sandy surfaces, seasonal acquisitions, signal-to-noise ratio, spatial baseline, spatial baseline lost correlation, temporal baseline, temporal baseline lost correlation, vegetated areas, Correlation, crustal deformation, interferometry, synthetic aperture radar (SAR).
    Abstract: Temporal decorrelation is one of the main limitations for recovering interseismic deformation along the San Andreas Fault system using interferometric synthetic aperture radar. To assess the improved correlation properties of L-band with respect to C-band, we analyzed L-band Advanced Land Observation Satellite (ALOS) interferograms with a range of temporal and spatial baselines over three vegetated areas in California and compared them with corresponding C-band European Remote Sensing Satellite (ERS) interferograms. Over the highly vegetated Northern California forests in the Coast Range area, ALOS remains remarkably well correlated over a 2-year period, whereas an ERS interferogram with a similar temporal and spatial baseline lost correlation. In Central California near Parkfield, we found a similar pattern in decorrelation behavior, which enabled the recovery of a fault creep and a local uplifting signal at L-band that was not apparent at C-band. In the Imperial Valley in Southern California, both ALOS and ERS have low correlation over farmlands. ALOS has lower correlation over some sandy surfaces than ERS, probably due to low signal-to-noise ratio. In general, L-band interferograms with similar seasonal acquisitions have higher correlation than those with dissimilar season. For both L- and C-band, correlation over vegetated areas decreases with time for intervals less than 1 year and then remains relatively constant at longer time intervals. The decorrelation time for L-band is more than 2 years in the forest in California whereas that for C-band is less than 6 months. Overall, these results suggest that L-band interferograms will reveal near-fault interseismic deformation once sufficient data become available.
    [bibtex-key = weiSandwellTGRS2010TempDecorrelationLBandCBand] [bibtex-entry]


  596. Howard A. Zebker, Scott Hensley, Piyush Shanker Agram, and C. Wortham. Geodetically Accurate InSAR Data Processor. IEEE Trans. Geosci. Remote Sens., 48(12):4309-4321, December 2010. Keyword(s): SAR Processing, InSAR data processor, focus correction phase terms, image distortion, interferometric synthetic aperture radar, motion-compensation techniques, orbit tracking, radar echoes, radar interferometry, topography-corrected interferograms, motion compensation, radar interferometry, synthetic aperture radar;.
    Abstract: We present a new interferometric synthetic aperture radar (InSAR) processing approach that capitalizes on the precise orbit tracking that is available with modern radar satellites. Our method uses an accurate orbit information along with motion-compensation techniques to propagate the radar echoes to positions along a noninertial virtual orbit frame in which the location and focusing equations are particularly simple, so that images are focused without requiring autofocus techniques and are computed efficiently. Motion compensation requires two additional focus correction phase terms that are implemented in the frequency domain. If the images from an interferometric pair or stack are all computed along the same reference orbit, flat-Earth topographic correction is not needed, and image coregistration is simplified, obviating many difficulties that are often encountered in InSAR processing. We process several data sets collected by the ALOS PALSAR instrument and find that the geodetic accuracy of the radar images is 10-20 m, with up to 20 m of additional image distortion needed to align 100 km #x00D7; 100 km scenes with reference digital elevation models. We validated the accuracy by using both known radar corner reflector locations and by the registration of the interferograms with digital maps. The topography-corrected interferograms are free from all geometric phase terms, and they clearly show the geophysical observables of crustal deformation, atmospheric phase, and ionospheric phase.
    [bibtex-key = zebkerHensleyShankerAgramWortham2010:GeodeticallyAccurateInSARProc] [bibtex-entry]


  597. Xiao Xiang Zhu and Richard Bamler. Tomographic SAR Inversion by $L_1$-Norm Regularization --- The Compressive Sensing Approach. IEEE Trans. Geosci. Remote Sens., 48(10):3839-3846, 2010. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry, Anisotropic magnetoresistance, Azimuth, Control systems, Image reconstruction, Radar tracking, Reconstruction algorithms, Signal resolution, Spaceborne radar, Synthetic aperture radar, Tomography, image reconstruction, image resolution, radar imaging, radar resolution, remote sensing by radar, spaceborne radar, synthetic aperture radar, 3D imaging, 3D tomographic resolution element, L1-norm regularization, azimuth-range cell, compressive sensing, elevation direction, point localization, spaceborne SAR systems, super-resolution reconstruction algorithm, synthetic aperture radar, tomographic SAR inversion, tomographic elevation resolution, Compressive sensing (CS), TerraSAR-X, differential synthetic aperture radar tomography (D-TomoSAR), urban mapping.
    Abstract: Synthetic aperture radar (SAR) tomography (TomoSAR) extends the synthetic aperture principle into the elevation direction for 3-D imaging. The resolution in the elevation direction depends on the size of the elevation aperture, i.e., on the spread of orbit tracks. Since the orbits of modern meter-resolution spaceborne SAR systems, like TerraSAR-X, are tightly controlled, the tomographic elevation resolution is at least an order of magnitude lower than in range and azimuth. Hence, super-resolution reconstruction algorithms are desired. The high anisotropy of the 3-D tomographic resolution element renders the signals sparse in the elevation direction; only a few pointlike reflections are expected per azimuth-range cell. This property suggests using compressive sensing (CS) methods for tomographic reconstruction. This paper presents the theory of 4-D (differential, i.e., space-time) CS TomoSAR and compares it with parametric (nonlinear least squares) and nonparametric (singular value decomposition) reconstruction methods. Super-resolution properties and point localization accuracies are demonstrated using simulations and real data. A CS reconstruction of a building complex from TerraSAR-X spotlight data is presented.
    [bibtex-key = zhuBamlerTGRS2010L1NormCS] [bibtex-entry]


  598. Xiao Xiang Zhu and Richard Bamler. Very High Resolution Spaceborne SAR Tomography in Urban Environment. IEEE Trans. Geosci. Remote Sens., 48(12):4296-4308, December 2010. Keyword(s): SAR Processing, SAR Tomography, Tomography, Singular Value Decomposition, SVD, InSAR, SAR Interferometry, Interferometry, Apertures, Floors, High-resolution imaging, Image reconstruction, Radar scattering, Reflectivity, Spaceborne radar, Spectral analysis, Tomography, Urban areas, parameter estimation, radar imaging, singular value decomposition, spaceborne SAR, spectral analysis, synthetic aperture radar, tomography, 3D imaging, COSMO-Skymed, TerraSAR-X, TomoSAR, Wiener-type regularization, azimuth-range pixel, elevation direction, maximum a posteriori estimator, meter-resolution spaceborne SAR systems, parametric estimation algorithms, radar reflectivity, reconstructions, singular-value decomposition method, spaceborne SAR data, spectral analysis, synthetic aperture principle, synthetic aperture radar tomography, tomographic reconstruction, urban environment, very high resolution spaceborne SAR tomography, Differential synthetic aperture radar tomography (D-TomoSAR), TerraSAR-X, spotlight SAR, urban mapping;.
    Abstract: Synthetic aperture radar tomography (TomoSAR) extends the synthetic aperture principle into the elevation direction for 3-D imaging. It uses stacks of several acquisitions from slightly different viewing angles (the elevation aperture) to reconstruct the reflectivity function along the elevation direction by means of spectral analysis for every azimuth-range pixel. The new class of meter-resolution spaceborne SAR systems (TerraSAR-X and COSMO-Skymed) offers a tremendous improvement in tomographic reconstruction of urban areas and man-made infrastructure. The high resolution fits well to the inherent scale of buildings (floor height, distance of windows, etc.). This paper demonstrates the tomographic potential of these SARs and the achievable quality on the basis of TerraSAR-X spotlight data of urban environment. A new Wiener-type regularization to the singular-value decomposition method-equivalent to a maximum a posteriori estimator-for TomoSAR is introduced and is extended to the differential case (4-D, i.e., space-time). Different model selection schemes for the estimation of the number of scatterers in a resolution cell are compared and proven to be applicable in practice. Two parametric estimation algorithms of the scatterers' elevation and their velocities are evaluated. First 3-D and 4-D reconstructions of an entire building complex (including its radar reflectivity) with very high level of detail from spaceborne SAR data by pixelwise TomoSAR are presented.
    [bibtex-key = zhuBamlerTGRS2010a] [bibtex-entry]


  599. A. Zymnis, S. Boyd, and E. Candes. Compressed Sensing With Quantized Measurements. IEEE Signal Processing Letters, 17(2):149-152, February 2010. Keyword(s): Gaussian noise, compressed sensing, convex function, first order method, numerical simulation, quantized measurement, sparse signal estimation, Gaussian noise, quantisation (signal), signal processing;.
    Abstract: We consider the problem of estimating a sparse signal from a set of quantized, Gaussian noise corrupted measurements, where each measurement corresponds to an interval of values. We give two methods for (approximately) solving this problem, each based on minimizing a differentiable convex function plus an l 1 regularization term. Using a first order method developed by Hale et al, we demonstrate the performance of the methods through numerical simulation. We find that, using these methods, compressed sensing can be carried out even when the quantization is very coarse, e.g., 1 or 2 bits per measurement.
    [bibtex-key = 5306135] [bibtex-entry]


  600. Nico Adam, Alessandro Parizzi, Michael Eineder, and Michele Crosetto. Practical persistent scatterer processing validation in the course of the Terrafirma project. Journal of Applied Geophysics, 69(1):59-65, 2009. [bibtex-key = adamParizziEinederCrosetto2009] [bibtex-entry]


  601. Richard Bamler, Michael Eineder, Nico Adam, Xiaoxiang Zhu, and Stefan Gernhardt. Interferometric Potential of High Resolution Spaceborne SAR. Photogrammetrie - Fernerkundung - Geoinformation, 2009(5):407-419, 2009. Keyword(s): SAR Processing, SAR Interferometry, SAR Tomography, Tomography, Persistent Scatterer Interferometry, PSI, Spotlight SAR, TERRASAR-X, SAR TOMOGRAPHY.
    Abstract: The new class of high resolution spaceborne SAR systems, like TerraSAR-X and COSMO-Skymed opens new possibilities for SAR interferometry. The 1m resolution is particularly helpful when 2D, 2.5D, 3D, or 4D (space-time) imaging of buildings and urban infrastructure is required, where the non-interferometric interpretation of SAR imagery is difficult. Structure and deformation of individual buildings can be mapped, rather than only coarse deformation patterns of areas. The paper demonstrates several new developments in high resolution SAR interferometry using TerraSAR-X as an example. Of particular interest is the very high resolution spotlight mode, which requires some care in interferometric processing. Results from interferometry, Persistent Scatterer Interferometry (PSI), and tomographic SAR in urban environment are presented. The high resolution of TerraSAR-X also supports accurate speckle and feature tracking. An example of glacier monitoring is shown and discussed.
    [bibtex-key = bamlerEinederAdamZhuGernhardtPFG2009] [bibtex-entry]


  602. Antonio De Maio, Gianfranco Fornaro, and Antonio Pauciullo. Detection of Single Scatterers in Multidimensional SAR Imaging. IEEE Transactions on Geoscience and Remote Sensing, 47(7):2284-2297, July 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, radar imaging, radar interferometry, synthetic aperture radar, CFAR detection scheme, Rao test, SAR imaging, SAR interferometry, Wald test, constant false alarm rate, differential interferometry, generalized likelihood ratio test, multi-interferogram complex coherence, multidimensional synthetic aperture radar, scatterer interferometry, single scatterers detection, space deformation-velocity analysis.
    Abstract: Multidimensional synthetic aperture radar (SAR) imaging is a technique based on coherent SAR data combination for space (full 3-D) and space deformation-velocity (4-D) analysis. It is an extension of the concepts of SAR interferometry and differential interferometry SAR and offers new options for the analysis and monitoring of ground scenes. In this paper, we consider the problem of detecting single scatterers for localization and monitoring issues. To this end, we resort to a constant false alarm rate (CFAR) detection scheme which can be synthesized according to three different design criteria: generalized likelihood ratio test, Rao test, and Wald test. At the analysis stage, the performance of the aforementioned detector is compared to that of a previously proposed CFAR scheme, based on the multi-interferogram complex coherence and widely used in persistent scatterer interferometry. The analysis is conducted both on simulated and on real SAR data, acquired by ERS-1/2 satellites. Finally, Cramer-Rao lower bounds for the estimation of the scatterer elevation and velocity are provided.
    [bibtex-key = deMaioFornaroPauciullo2009] [bibtex-entry]


  603. M.-P. Doin, C. Lasserre, G. Peltzer, O. Cavalié, and C. Doubre. Corrections of stratified tropospheric delays in SAR interferometry: Validation with global atmospheric models. Journal of Applied Geophysics, 69(1):35-50, September 2009. Note: Advances in SAR Interferometry from the 2007 Fringe Workshop. Keyword(s): Radar interferometry, Phase propagation delay, Global climate model, Stratified atmosphere, InSAR, Troposphere, Tropospheric Delay, SAR Interferometry, Interferometry, Spaceborne SAR.
    Abstract: The main limiting factor on the accuracy of Interferometric SAR measurements (InSAR) comes from phase propagation delays through the troposphere. The delay can be divided into a stratified component, which correlates with the topography and often dominates the tropospheric signal, and a turbulent component. We use Global Atmospheric Models (GAM) to estimate the stratified phase delay and delay-elevation ratio at epochs of SAR acquisitions, and compare them to observed phase delay derived from SAR interferograms. Three test areas are selected with different geographic and climatic environments and with large SAR archive available. The Lake Mead, Nevada, USA is covered by 79 ERS1/2 and ENVISAT acquisitions, the Haiyuan Fault area, Gansu, China, by 24 ERS1/2 acquisitions, and the Afar region, Republic of Djibouti, by 91 Radarsat acquisitions. The hydrostatic and wet stratified delays are computed from GAM as a function of atmospheric pressure P, temperature T, and water vapor partial pressure e vertical profiles. The hydrostatic delay, which depends on ratio P/T, varies significantly at low elevation and cannot be neglected. The wet component of the delay depends mostly on the near surface specific humidity. GAM predicted delay-elevation ratios are in good agreement with the ratios derived from InSAR data away from deforming zones. Both estimations of the delay-elevation ratio can thus be used to perform a first order correction of the observed interferometric phase to retrieve a ground motion signal of low amplitude. We also demonstrate that aliasing of daily and seasonal variations in the stratified delay due to uneven sampling of SAR data significantly bias InSAR data stacks or time series produced after temporal smoothing. In all three test cases, the InSAR data stacks or smoothed time series present a residual stratified delay of the order of the expected deformation signal. In all cases, correcting interferograms from the stratified delay removes all these biases. We quantify the standard error associated with the correction of the stratified atmospheric delay. It varies from one site to another depending on the prevailing atmospheric conditions, but remains bounded by the standard deviation of the daily fluctuations of the stratified delay around the seasonal average. Finally we suggest that the phase delay correction can potentially be improved by introducing a non-linear dependence to the elevation derived from GAM.
    [bibtex-key = doinLasserrePeltzerCavalierDoubreJAG2009CorrectionsOfStratifiedTroposphericDelaysInSARInterferometry] [bibtex-entry]


  604. Esra Erten, Andreas Reigber, Olaf Hellwich, and Pau Prats. Glacier Velocity Monitoring by Maximum Likelihood Texture Tracking. IEEE Transactions on Geoscience and Remote Sensing, 47(2):394-405, Feb. 2009. Keyword(s): SAR Processing, glaciology, hydrological techniques, image processing, maximum likelihood estimation, remote sensing by radar, spaceborne radar, synthetic aperture radar, Asia, ENVISAT-ASAR data acquisition, Inyltshik glacier, Kyrgyzstan, Maximum Likelihood Texture Tracking, alpine glacier systems, classical intensity tracking technique, glacier velocities measurement, glacier velocity monitoring, ice flows, intensity-based matching algorithm, melting, multiplicative speckle/noise model, remotely sensed data, signal-to-noise ratio, snowfall, spatial dynamics, speckle decorrelation, statistical description, synthetic aperture radar data, temporal dynamics, temporal speckle structure, tracking algorithm.
    Abstract: The performance of a tracking algorithm considering remotely sensed data strongly depends on a correct statistical description of the data, i.e., its noise model. The objective of this paper is to introduce a new intensity tracking algorithm for synthetic aperture radar (SAR) data, considering its multiplicative speckle/noise model. The proposed tracking algorithm is discussed regarding the measurement of glacier velocities. Glacier monitoring exhibits complex spatial and temporal dynamics including snowfall, melting, and ice flows at a variety of spatial and temporal scales. Due to these complex characteristics, most traditional methods based on SAR suffer from speckle decorrelation that results in a low signal-to-noise ratio. The proposed tracking technique improves the accuracy of the classical intensity tracking technique by making use of the temporal speckle structure. Even though a new intensity-based matching algorithm is proposed, particularly for incoherent data sets, the analysis of the proposed technique was also performed for correlated data sets. As it is demonstrated, the velocity monitoring can be continuously performed by using the maximum likelihood (ML) texture tracking without any assumption concerning the correlation of the data set. The ML texture tracking approach was tested on ENVISAT-ASAR data acquired during summer 2004 over the Inyltshik glacier in Kyrgyzstan, representing one of the largest alpine glacier systems of the world. It will be demonstrated that the proposed technique is capable of robustly and precisely detecting the surface velocity field and velocity changes in time.
    [bibtex-key = ertenReigberHellwichPrats2009:GlacierVelTextureTrack] [bibtex-entry]


  605. Gianfranco Fornaro and Antonio Pauciullo. LMMSE 3-D SAR Focusing. IEEE Trans. Geosci. Remote Sens., 47(1):214-223, January 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, data acquisition, remote sensing by radar, singular value decomposition, synthetic aperture radar, LMMSE, synthetic aperture radar imaging systems, antenna SAR sensor, atmospheric phase miscalibration, atmospheric residual miscalibration, beamforming, data correlation properties, data integration, linear minimum mean square error method, multistatic data acquisition, satellite technology, singular values decomposition inversion, stochastic process.
    Abstract: Three-dimensional synthetic aperture radar (SAR) imaging, a technique also known as SAR tomography, uses multiple views to extend the capability of SAR systems to 3-D imaging by achieving a profiling of the scattering power at different heights. Multiple views are obtained with the current satellite technology via successive passes of a single antenna SAR sensor over the same scene, but next-generation sensor formations are foreseen to acquire multistatic data. Conventional processing, such as the beamforming, or singular values decomposition inversion is based on geometrical derivations and, hence, assumes the accurate phase calibration and the absence of target decorrelation. This paper analyzes the effects of phase miscalibration due to residual uncompensated atmospheric contribution and temporal decorrelation and proposes a 3-D imaging technique based on a linear minimum mean square error approach. The resulting algorithm extends the possibilities of the conventional processing by carrying out an integration of data that accounts for the a priori data correlation properties. Hence, it allows handling of the presence of additional stochastic contributions such as: temporal coherence losses and atmospheric phase miscalibration. Moreover, with reference to future bistatic and multistatic systems, it permits an improved coherent integration of data acquired by simultaneous antenna in repeated passes.
    [bibtex-key = fornaroPauciullo2009:SARTOMO] [bibtex-entry]


  606. G. Fornaro, D. Reale, and F. Serafino. Four-Dimensional SAR Imaging for Height Estimation and Monitoring of Single and Double Scatterers. IEEE Transactions on Geoscience and Remote Sensing, 47(1):224-237, January 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical signal processing, geophysical techniques, height measurement, radar signal processing, remote sensing by radar, synthetic aperture radar4D SAR imaging application, 4D space-velocity imaging, SAR signal postprocessing, differential SAR tomography, double scatterer monitoring, ground scatterers, height estimation, interfering target separation, mean deformation velocity, multipass SAR interferometry, nonlinear temporal deformations, single scatterer monitoring, slow deformation velocity, synthetic aperture radar, target contribution superposition.
    Abstract: The superposition of contributions from different stable targets within the same pixel is a phenomenon that may impair the imaging and monitoring of ground scatterers via the multipass synthetic aperture radar (SAR) interferometry technique. Three-dimensional SAR imaging, also known as SAR tomography, uses multiple views to profile the scattering power at different heights. This technique has been shown to be capable of separating interfering target responses on real data. Differential SAR tomography has been recently proposed as a technique that extends the potentialities of SAR tomography to the target deformation monitoring. It performs a 4-D space-velocity imaging that enables not only separating interfering targets in elevation but also distinguishing their single slow deformation velocities. This work addresses for the first time the application of 4-D SAR imaging to real data to determine the height and mean deformation velocity of single scatterers and double-scattering mechanisms interfering at high resolution in the same pixel. It also discusses the postprocessing steps required to identify the presence of stable single and double scatterers after elevation-velocity focusing. Moreover, it proposes a technique for the extraction of time series from interfering targets to measure possible nonlinear temporal deformations.
    [bibtex-key = fornaroRealeSerafino2009:SARTomo] [bibtex-entry]


  607. Joaquim Fortuny-Guasch. A Fast and Accurate Far-Field Pseudopolar Format Radar Imaging Algorithm. IEEE Transactions on Geoscience and Remote Sensing, 47(4):1187-1196, April 2009. Keyword(s): SAR Processing, GB-SAR, GB-SAR Focusing, Focusing, Azimuth Focusing, Terrestrial radar, Terrestrial SAR, CW radar, fast Fourier transforms, geophysical signal processing, radar imaging, remote sensing by radar, synthetic aperture radar, 2D fast Fourier transforms, frequency domain backscatter, frequency modulated CW radar, interpolation free series expansion, linear radar aperture, pseudopolar format radar imaging algorithm, stepped frequency continuous wave, synthetic aperture radar systems, ultrawideband radar, Antenna arrays, far field, radar data processing, radar imaging, synthetic aperture radar (SAR).
    Abstract: A novel imaging algorithm to be used under the condition of having an image scene in the far field of a linear radar aperture is presented. This is an application scenario that is drastically different from those of spaceborne and airborne synthetic aperture radar (SAR) systems, which has not been properly addressed to date. The technique is particularly tailored for a stepped-frequency continuous wave (CW) or frequency-modulated CW radar. The radar aperture must be linear and can be formed either with a physical or synthetic array. With the suggested method, the radar reflectivity of the image scene is obtained through an interpolation-free series expansion, where only 2-D fast Fourier transforms of the frequency-domain backscatter data are required. The resulting image is sampled on a polarlike or pseudopolar grid, which is introduced to simplify the formulation. The main advantages of this method are its extremely low computational cost and the high accuracy of the resulting imagery. The technique is extensively validated both with numerical simulations and two ground-based SAR data sets. Last but not least, numerical simulations show that this technique can be used with an ultrawideband radar of 1 GHz of bandwidth.
    [bibtex-key = fortunyGuaschTGRS2009GBSARPseudoPolarFocusing] [bibtex-entry]


  608. Othmar Frey, Christophe Magnard, Maurice Rüegg, and Erich Meier. Focusing of Airborne Synthetic Aperture Radar Data from Highly Nonlinear Flight Tracks. IEEE Trans. Geosci. Remote Sens., 47(6):1844-1858, June 2009. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, TDBP, Non-Linear Flight Tracks, Curvilinear SAR, Extended Chirp Scaling, ECS, Mosaicking, Geocoding, Azimuth Focusing, Integrated Focusing and Geocoding, Georeferencing, mapping, corridor mapping, E-SAR, L-Band, digital elevation model, Airborne SAR.
    Abstract: Standard focusing of data from synthetic aperture radar (SAR) assumes a straight recording track of the sensor platform. Small non-linearities of airborne platform tracks are corrected for during a motion compensation step while maintaining the assumption of a linear flight path. This paper describes the processing of SAR data acquired from non-linear tracks, typical of sensors mounted on small aircraft or drones flying at low altitude. Such aircraft do not fly along straight tracks, but the trajectory depends on topography, influences of weather and wind, or the shape of areas of interest such as rivers or traffic routes. Two potential approaches for processing SAR data from such highly non-linear flight tracks are proposed: a patchwise frequency-domain processing and mosaicking technique, as well as a time-domain back-projection based technique. Both are evaluated with the help of experimental data featuring tracks with altitude changes, a double bend, a 90-degree curve and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared, impulse response functions of a point target are analyzed, and the coherence is evaluated. The experimental data was acquired by the German Aerospace Center's E-SAR L-band system.
    [bibtex-key = freyMagnardRueeggMeier08TGRS:Tracks] [bibtex-entry]


  609. Franck Garestier, Pascale C. Dubois-Fernandez, Dominique Guyon, and Thuy Le Toan. Forest Biophysical Parameter Estimation Using L- and P-Band Polarimetric SAR Data. IEEE Transactions on Geoscience and Remote Sensing, 47(10):3379-3388, October 2009. Keyword(s): SAR Processing, Biomass, Biophysical Parameters, Forst, L-Band, P-Band, PolSAR, SAR Polarimetry, Airborne SAR, RAMSES, ONERA.
    Abstract: L- and P-band airborne polarimetric synthetic aperture radar (SAR) data acquired by the RAMSES system over different height maritime pine (Pinus Pinaster Ait.) stands of the Nezer forest (Landes, France) have been evaluated for forest biophysical parameter estimation. A pseudolinear correlation has been brought to evidence at P-band between polarimetric anisotropy and mean tree height, which is also linked to other biophysical parameters in the Nezer forest, meaning that SAR polarimetry constitutes a promising tool for forest parameter retrieval at low frequency. The spatial conditions have been evaluated through the quantification of the impact of signal-to-noise ratio diminution and resolution degradation on the forest height inversion. It has been shown that the inversion accuracy remains acceptable for $NEsigma_{0}$, representing the noise level of the SAR image, which is lower than $-$15 dB, and for spatial resolution increasing up to 15 m.
    [bibtex-key = garestierDuboisGuyonLeToan2009:BioPhysPolSARLandPBand] [bibtex-entry]


  610. Scott Goetz, Alessandro Baccini, Nadine Laporte, Tracy Johns, Wayne Walker, Josef Kellndorfer, Richard Houghton, and Mindy Sun. Mapping and monitoring carbon stocks with satellite observations: a comparison of methods. Carbon Balance and Management, 4(1):2, 2009. Keyword(s): Carbon Balance, Carbon stock, DESDynl mission, Biomass mission, biomass, ecosystem function, ecosystem structure, integrated L-band InSAR, Lidar, vegetation structure, optical, synthetic aperture radar, vegetation mapping.
    Abstract: Mapping and monitoring carbon stocks in forested regions of the world, particularly the tropics, has attracted a great deal of attention in recent years as deforestation and forest degradation account for up to 30 percent of anthropogenic carbon emissions, and are now included in climate change negotiations. We review the potential for satellites to measure carbon stocks, specifically aboveground biomass (AGB), and provide an overview of a range of approaches that have been developed and used to map AGB across a diverse set of conditions and geographic areas. We provide a summary of types of remote sensing measurements relevant to mapping AGB, and assess the relative merits and limitations of each. We then provide an overview of traditional techniques of mapping AGB based on ascribing field measurements to vegetation or land cover type classes, and describe the merits and limitations of those relative to recent data mining algorithms used in the context of an approach based on direct utilization of remote sensing measurements, whether optical or lidar reflectance, or radar backscatter. We conclude that while satellite remote sensing has often been discounted as inadequate for the task, attempts to map AGB without satellite imagery are insufficient. Moreover, the direct remote sensing approach provided more coherent maps of AGB relative to traditional approaches. We demonstrate this with a case study focused on continental Africa and discuss the work in the context of reducing uncertainty for carbon monitoring and markets.
    [bibtex-key = GoetzBacciniLaporteJohnsTracyWalkerKellndorferHoughtonSun2009] [bibtex-entry]


  611. J. Groen, Roy E. Hansen, H. J. Callow, J. C. Sabel, and T. O. Sabo. Shadow Enhancement in Synthetic Aperture Sonar Using Fixed Focusing. IEEE Journal of Oceanic Engineering, 34(3):269-284, July 2009. Keyword(s): Synthetic Aperture Sonar, SAS, image enhancement, sonar imaging, sonar target recognition, synthetic aperture sonar, HUGIN autonomous underwater vehicle, fixed focus shadow enhancement, shadow clarity, synthetic aperture sonar imaging, target recognition, widebeam synthetic aperture imaging systems, Acoustics, beamforming, focusing, imaging, shadow, synthetic aperture sonar (SAS).
    Abstract: A shadow cast by an object on the seafloor is important information for target recognition in synthetic aperture sonar (SAS) images. Synthetic aperture imaging causes a fundamental limitation to shadow clarity because the illuminator is moved during the data collection. This leads to a blend of echo and shadow, or geometrical fill-in in the shadow region. The fill-in is most dominant for widebeam synthetic aperture imaging systems. By treating the shadow as a moving target and compensating for the motion during the synthetic aperture imagery, we avoid the geometrical shadow fill-in. We show this to be equivalent to fixing the focus at the range of the shadow caster. This novel technique, referred to as fixed focus shadow enhancement (FFSE) can be used directly as an imaging method on hydrophone data or as a postprocessing technique on the complex SAS image. We demonstrate the FFSE technique on simulated data and on real data from a rail-based SAS, and on two different SAS systems operated on a HUGIN autonomous underwater vehicle.
    [bibtex-key = groenHansenCallowSabelSaboHOE2009ShadowEnhancementInSAS] [bibtex-entry]


  612. Irena Hajnsek, Thomas Jagdhuber, Helmut Schön, and Konstantinos P. Papathanassiou. Potential of Estimating Soil Moisture Under Vegetation Cover by Means of PolSAR. IEEE Transactions on Geoscience and Remote Sensing, 47(2):442-454, Feb. 2009. Keyword(s): SAR Processing, agriculture, crops, moisture, radar polarimetry, remote sensing by radar, soil, synthetic aperture radarAgriSAR campaign, L-band, PolSAR images, PolSAR, agricultural vegetation, canonical scattering components, crop types, dihedral components, polarimetric SAR acquisitions, scattering processes, soil moisture estimation, synthetic aperture radar, vegetation cover, ESAR, Airborne SAR.
    Abstract: In this paper, the potential of using polarimetric SAR (PolSAR) acquisitions for the estimation of volumetric soil moisture under agricultural vegetation is investigated. Soil-moisture estimation by means of SAR is a topic that is intensively investigated but yet not solved satisfactorily. The key problem is the presence of vegetation cover which biases soil-moisture estimates. In this paper, we discuss the problem of soil-moisture estimation in the presence of agricultural vegetation by means of L-band PolSAR images. SAR polarimetry allows the decomposition of the scattering signature into canonical scattering components and their quantification. We discuss simple canonical models for surface, dihedral, and vegetation scattering and use them to model and interpret scattering processes. The performance and modifications of the individual scattering components are discussed. The obtained surface and dihedral components are then used to retrieve surface soil moisture. The investigations cover, for the first time, the whole vegetation-growing period for three crop types using SAR data and ground measurements acquired in the frame of the AgriSAR campaign.
    [bibtex-key = hajnsekJagdhuberSchoenPapathanassiou2009:SoilMoisture] [bibtex-entry]


  613. Michael Jehle, Maurice Rüegg, Lukas Zuberbühler, David Small, and Erich Meier. Measurement of Ionospheric Faraday Rotation in Simulated and Real Spaceborne SAR Data. IEEE Trans. Geosci. Remote Sens., 47(5):1512-1523, May 2009. Keyword(s): SAR Processing, Ionosphere, ALOS PALSAR, Advanced Land Observing Satellite, Earth's magnetic field, PALSAR data, Phased Array L-band Synthetic Aperture Radar, focused radar images, frequency-modulated electromagnetic wave traverse, ionospheric Faraday Rotation measurement, radar polarimetry, range-compressed, signal chirp bandwidth effects, signal path delays, spaceborne SAR data, spaceborne synthetic aperture radar, total electron content, TEC, Faraday effect, ionospheric electromagnetic wave propagation, radar polarimetry, radar signal processing, remote sensing by radar, synthetic aperture radar.
    Abstract: The influence of the atmosphere on a frequency-modulated electromagnetic wave traversing the ionosphere is becoming increasingly important for recent and upcoming low-frequency and wide-bandwidth spaceborne synthetic aperture radar (SAR) systems. The ionized ionosphere induces Faraday rotation (FR) at these frequencies that affects radar polarimetry and causes signal path delays resulting in a reduced range resolution. The work at hand introduces a simulation model of SAR signals passing through the atmosphere, including both frequency-dependent FR and path delays. Based on simulation results from this model [proven with real Advanced Land Observing Satellite Phased Array L-band Synthetic Aperture Radar (PALSAR) data], estimation of FR in quad-polarized SAR data using the given approach is shown for raw, range-compressed, and focused radar images. Path delays and signal chirp bandwidth effects are considered. Investigations discuss the suitability of raw and compressed data versus combination of total electron content maps with the Earth's magnetic field for FR estimation and deduced from a large number of analyzed PALSAR data sets.
    [bibtex-key = JehleRueggZuberbuhlerSmallMeier2009:MeasurementofIonoFaradayRot] [bibtex-entry]


  614. Wang Jinfeng and Pi Yiming. SAR tomography imaging via higher-order spectrum analysis. Journal of Systems Engineering and Electronics, 20(4):748-754, August 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography.
    Abstract: To deal with the non-Gaussian noise in standard 2-D SAR images, the deramped signal in imaging plane, and the possible symmetric distribution of complex noise, the fourth-order cumulant of complex process is introduced into SAR tomography. With the estimated AR parameters of ARMA model of noise through Yule-Walker Walker equation, the signal series of height is pre-filtered. Then, through ESPRIT, the spectrum is obtained and the aperture in height direction is synthesized. Finally, the SAR tomography imaging of scene is achieved. The results of processing on signal with non-Gaussian noise demonstrate the robustness of the proposed method. The tomography imaging of the scenes shows that the higher-order spectrum analysis is feasible in the application.
    [bibtex-key = 6074498] [bibtex-entry]


  615. Jong-Sen Lee, Jen-Hung Wen, T.L. Ainsworth, Kun-Shan Chen, and A.J. Chen. Improved Sigma Filter for Speckle Filtering of SAR Imagery. IEEE Transactions on Geoscience and Remote Sensing, 47(1):202-213, January 2009.
    Abstract: The Lee sigma filter was developed in 1983 based on the simple concept of two-sigma probability, and it was reasonably effective in speckle filtering. However, deficiencies were discovered in producing biased estimation and in blurring and depressing strong reflected targets. The advancement of synthetic aperture radar (SAR) technology with high-resolution data of large dimensions demands better and efficient speckle filtering algorithms. In this paper, we extend and improve the Lee sigma filter by eliminating these deficiencies. The bias problem is solved by redefining the sigma range based on the speckle probability density functions. To mitigate the problems of blurring and depressing strong reflective scatterers, a target signature preservation technique is developed. In addition, we incorporate the minimum-mean-square-error estimator for adaptive speckle reduction. Simulated SAR data are used to quantitatively evaluate the characteristics of this improved sigma filter and to validate its effectiveness. The proposed algorithm is applied to spaceborne and airborne SAR data to demonstrate its overall speckle filtering characteristics as compared with other algorithms. This improved sigma filter remains simple in concept and is computationally efficient but without the deficiencies of the original Lee sigma filter.
    [bibtex-key = leeWenAinsworthChenChenTGRS2009ImprovedSigmaFilterForSpeckleFilteringOfSARImagery] [bibtex-entry]


  616. Guoxiang Liu, Sean M. Buckley, Xiaoli Ding, Qiang Chen, and Xiaojun Luo. Estimating Spatiotemporal Ground Deformation With Improved Persistent-Scatterer Radar Interferometry. IEEE Trans. Geosci. Remote Sens., 47(9):3209-3219, September 2009. Keyword(s): AD 1992 to 2000, Arizona, C-band SAR images, EMD approach, European Remote Sensing 1-2 satellites, PS-InSAR technique, PS-neighborhood networking approach, Phoenix, USA, atmospheric artifacts, atmospheric signals, empirical mode decomposition approach, least squares method, linear deformation rates, nonlinear deformation, persistent-scatterer interferometric synthetic aperture radar technique, singular value decomposition, spatial-temporal decorrelation, subsidence, synthetic aperture radar images, temporally coherent radar targets, time series, topographic errors, geophysical techniques, radar interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth);.
    Abstract: Synthetic aperture radar interferometry has been applied widely in recent years to ground deformation monitoring although difficulties are often encountered when applying the technology, among which the spatial and temporal decorrelation and atmospheric artifacts are the most prominent. The persistent-scatterer interferometric synthetic aperture radar (PS-InSAR) technique has overcome some of the difficulties by focusing only on the temporally coherent radar targets in a time series of synthetic aperture radar (SAR) images. This paper presents an improved PS-InSAR technique by introducing PS-neighborhood networking and empirical mode decomposition (EMD) approaches in the PS-InSAR solution. Linear deformation rates and topographic errors are estimated based on a least squares method, while the nonlinear deformation and atmospheric signals are computed by singular value decomposition and the EMD method. An area in Phoenix, AZ, is used as a test site to determine its historical subsidence with 39 C-band SAR images acquired by European Remote Sensing 1 and 2 satellites from 1992 to 2000.
    [bibtex-key = liuBuckleyDingChenLuo2009] [bibtex-entry]


  617. F. Lombardini, M. Pardini, G. Fornaro, F. Serafino, L. Verrazzani, and M. Costantini. Linear and adaptive spaceborne threedimensional SAR tomography: a comparison on real data. IET Radar, Sonar Navigation, 3(4):424-436, August 2009. Keyword(s): ERS satellite urban data, SAR interferometry concept, adaptive beamforming, adaptive spaceborne three-dimensional SAR tomography, linear spaceborne three-dimensional SAR tomography, singular value decomposition, synthetic aperture radar imaging, array signal processing, geophysical signal processing, radar imaging, remote sensing by radar, singular value decomposition, spaceborne radar, synthetic aperture radar, tomography.
    Abstract: SAR Processing, SAR Tomography, Tomography, Three-dimensional (3-D) synthetic aperture radar (SAR) imaging is a recent technique, based on coherent SAR data combination, and aims to obtain a full 3-D analysis in space. It is a multibaseline extension of the SAR interferometry concept and offers new options for the analysis and monitoring of ground scenes by means of the capability of separating the scattering phenomena along the height dimension. In this work, the authors summarise and extend the results obtained by processing real ERS satellite urban data characterised by a long time span of acquisition and non-uniformly spaced satellite passes, comparing the performance in height focusing obtained with a singular value decomposition (SVD)-based method and adaptive beamforming.
    [bibtex-key = 5210026] [bibtex-entry]


  618. R. L. Morrison, M. N. Do, and D. C. Munson. MCA: A Multichannel Approach to SAR Autofocus. IEEE Transactions on Image Processing, 18(4):840-853, April 2009. Keyword(s): SAR Processing, Autofocus, SAR autofocus, defocusing operation, focused image, image restoration, linear algebraic formulation, multichannel autofocus, multichannel redundancy, sharpness metric optimization, synthetic aperture radar, vector-space formulation, image restoration, linear algebra, synthetic aperture radar;.
    Abstract: We present a new noniterative approach to synthetic aperture radar (SAR) autofocus, termed the multichannel autofocus (MCA) algorithm. The key in the approach is to exploit the multichannel redundancy of the defocusing operation to create a linear subspace, where the unknown perfectly focused image resides, expressed in terms of a known basis formed from the given defocused image. A unique solution for the perfectly focused image is then directly determined through a linear algebraic formulation by invoking an additional image support condition. The MCA approach is found to be computationally efficient and robust and does not require prior assumptions about the SAR scene used in existing methods. In addition, the vector-space formulation of MCA allows sharpness metric optimization to be easily incorporated within the restoration framework as a regularization term. We present experimental results characterizing the performance of MCA in comparison with conventional autofocus methods and discuss the practical implementation of the technique.
    [bibtex-key = morrisonDoMunsonMCA2009] [bibtex-entry]


  619. Matteo Nannini, Rolf Scheiber, and Alberto Moreira. Estimation of the Minimum Number of Tracks for SAR Tomography. IEEE Trans. Geosci. Remote Sens., 47(2):531-543, February 2009. Keyword(s): SAR Processing, SAR Tomography, Capon, MUSIC, image reconstruction, airboren SAR, image representation, radar interferometry, synthetic aperture radar3D representation, German Aerospace Center, DLR, L-band, SAR interferometry, SARTom, data acquisition, equivalent targets, experimental SAR system, minimum tomographic aperture, spheroidal wave functions, subspace superresolution methods, synthetic aperture radar tomography, tracks minimum number determination, volumetric source, ESAR.
    Abstract: Synthetic aperture radar tomography (SARTom) is the natural extension of SAR interferometry to solve for multiple phase centers within a resolution cell and obtain the 3-D representation of a scene. This paper deals with the determination of the minimum number of tracks required to perform SARTom. Through the prolate spheroidal wave functions, the number of equivalent targets of a volumetric source is derived, and from it, the minimum number of observations required to apply subspace superresolution methods is computed. The minimum tomographic aperture length is also investigated. The results are validated on real data acquired in L-band by the experimental SAR system of the German Aerospace Center.
    [bibtex-key = nanniniScheiberMoreiraTGRS2009:SARTom] [bibtex-entry]


  620. S. Ozsoy and A.A. Ergin. Pencil Back-Projection Method for SAR Imaging. IEEE Transactions on Image Processing, 18(3):573-581, March 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, SAR imaging, forward-backward total least squares bandpass matrix pencil method, pencil back-projection method, projected target reflectivity density function, synthetic aperture radar, tomographic image reconstruction, image reconstruction, least squares approximations, matrix algebra, radar imaging, synthetic aperture radar, Algorithms, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Radar, Reproducibility of Results, Sensitivity and Specificity, Tomography;.
    Abstract: We present a high-resolution method for spotlight mode SAR imaging that utilizes parametric modeling of projected target reflectivity density function and tomographic reconstruction. The method requires no polar-to-cartesian interpolation in spectral domain. Utilization of forward-backward total least squares bandpass matrix pencil method allows super resolution to be achieved in range for a single imaging angle. Hence, the quality of the image reconstructed by convolution back-projection is also improved. It is shown that the method is very resistant to noise and can generate images down to very low SNR values. Direct formulation in terms of physical quantities such as electric field and current density is another contribution of this paper.
    [bibtex-key = 4770154] [bibtex-entry]


  621. Luca Pipia, Xavier Fabregas, Albert Aguasca, Carlos Lopez-Martinez, Sergi Duque, Jordi J. Mallorqui, and Jordi Marturia. Polarimetric Differential SAR Interferometry: First Results With Ground-Based Measurements. IEEE Geosci. Remote Sens. Lett., 6(1):167-171, January 2009. Keyword(s): SAR Processing, GBSAR, Ground-based SAR, deformation monitoring, geomorphology, geophysical signal processing, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, topography (Earth), AD 2006 06 to 2007 03, PolSAR, Remote Sensing Laboratory, Sallent, Universitat Politecnica de Catalunya, coherent pixels technique, ground-based synthetic aperture radar sensor, high-quality deformation maps, northeastern Spain, polarimetric differential SAR interferometry, subsidence information, troposphere changes, zero-baseline polarimetric SAR, Ground-based synthetic aperture radar (GBSAR) sensor, SAR differential interferometry (DInSAR), SAR polarimetry.
    Abstract: The Remote Sensing Laboratory of the Universitat Politecnica de Catalunya carried out a one-year measuring campaign in the village of Sallent, northeastern Spain, using a polarimetric ground-based synthetic aperture radar (SAR) sensor. The objective was to study the subsidence phenomenon induced by the salt mining activity conducted in this area up to the middle of the last century. Zero-Baseline polarimetric SAR (PolSAR) data were gathered at X-band in nine different days, from June 2006 to March 2007. In this letter, the problem of extracting subsidence information from fully PolSAR acquisitions for the retrieval of high-quality deformation maps is addressed. After compensating for the atmospheric artifacts caused by troposphere changes, the linear component of the deformation process is estimated separately for each polarization channel with the Coherent Pixels Technique (CPT). Afterward, a novel polarimetric approach mixing the differential-phase information of each polarization channel is proposed. The results obtained in the two cases are quantitatively compared, and the advantages provided by the polarimetric acquisitions are finally stressed.
    [bibtex-key = pipiaFabregasAguascaLopezMartinezDuqueMallorquiMarturiaGRSL2009PolGBSAR] [bibtex-entry]


  622. Pau Prats, Rolf Scheiber, Andreas Reigber, Christian Andres, and Ralf Horn. Estimation of the Surface Velocity Field of the Aletsch Glacier Using Multibaseline Airborne SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 47(2):419-430, Feb. 2009. Keyword(s): SAR Processing, InSAR, DInSAR, airborne SAR, glaciology, hydrological techniques, SAR Interferometry, Interferometry, remote sensing by radar, synthetic aperture radar, Aletsch Glacier, Alps, German Aerospace Center, L-band, airborne interferometric synthetic aperture radar, data acquisition, differential interferometry, experimental SAR system, line-of-sight displacement, multisquint approach, navigation system, residual motion errors, surface velocity field estimation, temperate glaciers, ESAR.
    Abstract: This paper presents a methodology to process airborne interferometric synthetic aperture radar (SAR) data to measure surface velocity fields (SVFs) of temperate glaciers, and applies it to data acquired over the Aletsch glacier. The first part of this paper deals with the main limitation in airborne interferometric SAR to retrieve reliable interferometric products, namely, the existence of the so-called residual motion errors - inaccuracies on the order of a few centimeters in the navigation system. An extended multisquint approach is proposed for their estimation in the case of nonstationary scenes. The second part of this paper expounds an efficient methodology to derive SVFs with airborne systems, where the line-of-sight displacement is estimated using differential interferometry and the along-track component by estimating the azimuth coregistration offsets. The necessary steps to finally obtain the 3-D SVF are also presented, as well as the possibility of combining different acquisition geometries. Airborne interferometric SAR data acquired by the Experimental SAR system of the German aerospace center over the Aletsch glacier, located in the Swiss Alps, are used to evaluate the performance of the proposed approach. The motion of the corner reflectors deployed in the scene is retrieved with an accuracy between 1 and 5 cm/day using L-band data.
    [bibtex-key = pratsScheiberReigberAndresHorn2009:DInSARAletsch] [bibtex-entry]


  623. S. Sauer, L. Ferro-Famil, A. Reigber, and E. Pottier. Polarimetric Dual-Baseline InSAR Building Height Estimation at L-Band. IEEE Geoscience and Remote Sensing Letters, 6(3):408-412, July 2009. Keyword(s): SAR Procerssing, PolInSAR, Polarimetry, InSAR, SAR Interferometry, Dual-Baseline, Multi-baseline SAR, SAR Tomography, Tomography, ESAR, Airborne SAR, Building Height.
    Abstract: This letter generalizes a multibaseline interferometric synthetic aperture radar (InSAR) signal model to the polarimetric scenario. Based on this formulation, two high-performance spectral analysis techniques are adapted to process multibaseline Pol-InSAR observations. These new methods enhance the height estimation of scatterers by calculating optimal polarization combinations and allow the determination of their physical characteristics. Applying the proposed algorithms to urban environments, the building layover problem is analyzed by means of polarimetric dual-baseline InSAR measurements: the ground and building height are estimated. The techniques are validated using dual-baseline Pol-InSAR data acquired by DLR's Experimental SAR (E-SAR) system over Dresden city.
    [bibtex-key = SauerFerroFamilReigberPottier2009:POlInSARBuildingHeight] [bibtex-entry]


  624. Piyush Shanker Agram and Howard A. Zebker. Sparse Two-Dimensional Phase Unwrapping Using Regular Grid Methods. IEEE Geosci. Remote Sens. Lett., 6(3):519-522, July 2009. Keyword(s): SAR Processing, Delaunay triangulation, image reconstruction, interferometric synthetic aperture radar, nearest neighbor interpolation scheme, regular grid methods, sparse data, sparse two-dimensional phase unwrapping, image reconstruction, radar imaging, radar interferometry, synthetic aperture radar;.
    Abstract: Phase unwrapping is usually defined as the reconstruction of a function sampled on a spatial grid given its value modulo 2pi. Phase unwrapping is a key step in image reconstruction in many imaging techniques including interferometric synthetic aperture radar (InSAR). In recent years, many new methods have been developed to exploit the presence of coherent or persistent scattering points for extracting deformation signatures in regions where conventional InSAR fails. These techniques often yield measurements that are only poorly sampled spatially, yet these sparse data must still be unwrapped if we are to be able to extract useful geophysical information. The conventional well-sampled 2-D phase unwrapping problem based on phase residues is fairly well understood and many novel techniques involving geometry and network flow concepts have been implemented successfully to date. For sparse data, residues may be computed over the Delaunay triangulation of the data points, but published algorithms meet with limited success when the sparse data are unwrapped. The advantages of modern unwrapping methods applicable to well-sampled data are often lost when sparse data are analyzed. In this letter, we show that a nearest neighbor interpolation scheme allows powerful and existing 2-D solvers to be applied to sparse data. We present results using both simulated and real data sets to illustrate our method.
    [bibtex-key = shankerAgramZebkerGRSL2009:] [bibtex-entry]


  625. Manoochehr Shirzaei and TR Walter. Randomly iterated search and statistical competency as powerful inversion tools for deformation source modeling: Application to volcano interferometric synthetic aperture radar data. Journal of Geophysical Research: Solid Earth, 114(B10), 2009. [bibtex-key = Shirzaei2009] [bibtex-entry]


  626. Stefano Tebaldini. Algebraic Synthesis of Forest Scenarios From Multibaseline PolInSAR Data. IEEE Trans. Geosci. Remote Sens., 47(12):4132-4142, December 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, E-SAR, P-Band, algebra, geophysical techniques, radar polarimetry, remote sensing by radar, synthetic aperture radar, vegetationBioSAR, E-SAR airborne system, Kronecker products sum, P-band data set, Remningstorp, SAR surveys, Sweden, algebraic synthesis, forest scenarios, forested areas, least square solution, multibaseline PolInSAR data, multipolarimetric multibaseline synthetic aperture radar, single-baseline polarimetric SAR interferometry, statistical uncorrelation, temporal coherence losses, volumetric coherence losses.
    Abstract: In this paper, a new methodology is proposed for the analysis of forested areas basing on multipolarimetric multibaseline synthetic aperture radar (SAR) surveys. Such a methodology is based on three hypotheses: 1) statistical uncorrelation of the different scattering mechanisms (SMs), such as ground, volume, and ground-trunk scattering; 2) independence of volumetric and temporal coherence losses of each SM on the choice of the polarimetric channel; and 3) invariance (up to a scale factor) of the average polarimetric signature of each SM with respect to the choice of the track. Under these hypotheses, the data covariance matrix can be expressed as a Sum of Kronecker Products, after which it follows that K SMs are uniquely identified by K (K - 1) real numbers. This result provides the basis to perform SM separation by employing not only model-based approaches, generally retained in literature but also model-free and hybrid approaches, while yielding the best Least Square solution given the hypothesis of K SMs. It will be shown that this approach to SM separation is consistent with the inversion procedures usually exploited in single-baseline polarimetric SAR interferometry. Experimental validation of this methodology is provided on the basis of the P-band data set relative to the forest site of Remningstorp, Sweden, acquired by German Aerospace Center's E-SAR airborne system in the framework of the European Space Agency campaign BioSAR.
    [bibtex-key = tebaldiniTGRS2009:Tomo] [bibtex-entry]


  627. R. N. Treuhaft, B. D. Chapman, J. R. dos Santos, F. G. Gonçalves, L. V. Dutra, P. M. L. A. Graça, and J. B. Drake. Vegetation profiles in tropical forests from multibaseline interferometric synthetic aperture radar, field, and lidar measurements. J. Geophys. Res., 114:1-16, December 2009. Keyword(s): SAR Processing, SAR Tomography, tropical forest structure, InSAR, interferometric SAR, lidar, Ecosystems, structure and dynamics, Remote sensing, Atmospheric Composition and Structure, Biosphere atmosphere interactions, Trace gases.
    Abstract: This paper addresses the estimation of vertical vegetation density profiles from multibaseline interferometric synthetic aperture radar (InSAR) data from the AirSAR aircraft at C band over primary, secondary, and abandoned-pasture stands at La Selva Biological Station, Costa Rica in 2004. Profiles were also estimated from field data taken in 2006 and lidar data taken with the LVIS, 25 m spot instrument in 2005. After motivating the study of tropical forest profiles based on their role in the global carbon cycle, ecosystem state, and biodiversity, this paper describes the InSAR, field, and lidar data acquisitions and analyses. Beyond qualitative agreement between profiles from the 3 measurement techniques, results show that InSAR and lidar profile-averaged mean height have RMS scatters about field-measured means of 3.4 m and 3.2 m, 16% and 15% of the average mean height, respectively. InSAR and lidar standard deviations of the vegetation distribution have RMS scatters about the field standard deviations of 1.9 m and 1.5 m, or 27% and 21%, respectively. Dominant errors in the profile-averaged mean height for each measurement technique were modeled. InSAR inaccuracies, dominated by ambiguities in finding the ground altitude and coherence calibration, together account for about 3 m of InSAR error in the mean height. The dominant, modeled error for the field measurements was the inaccuracy in modeling the trees as uniformly filled volumes of leaf area, inducing field errors in mean height of about 3 m. The dominant, modeled lidar error, also due to finding the ground, was 2 m.
    [bibtex-key = treuhaftChapmanDosSantosGoncalvesDutraGracaDrake2009:SARVegetationProfiles] [bibtex-entry]


  628. R. Wang, O. Loffeld, H. Nies, and J. Ender. Focusing Spaceborne/Airborne Hybrid Bistatic SAR Data Using Wavenumber-Domain Algorithm. IEEE Trans. Geosci. Remote Sens., 47(7):2275-2283, July 2009. Keyword(s): SAR Processing, Bistatic SAR, SAR data processing, azimuth time-bandwidth product, azimuth-dependent range-cell-migration terms, azimuth-frequency dependence, bistatic formula, bistatic point-target reference spectrum, bistatic synthetic aperture radar, bistatic-deformation, hybrid spaceborne/airborne simulation experiment, inverse scaled Fourier transformation, inverse sliding-spotlight mode, sliding-spotlight mode, spaceborne/airborne hybrid bistatic configuration, total Doppler spectrum, wavenumber-domain algorithm, weighting factor, Fourier transforms, airborne radar, spaceborne radar, synthetic aperture radar.
    Abstract: This paper focuses on the bistatic synthetic aperture radar (SAR) data processing in a spaceborne/airborne hybrid bistatic configuration. Due to the extreme differences in platform velocities and slant ranges, the airborne system operates in the inverse sliding-spotlight mode, while the spaceborne system works in the sliding-spotlight mode to achieve a tradeoff between azimuth scene size and azimuth resolution. In this extreme bistatic configuration, our original bistatic formula shows a limitation of accurately describing the bistatic point-target reference spectrum, owing to the assumption of equal contributions of transmitter and receiver to the total Doppler spectrum. We extend our previous formula using the weighting operation where the weighting factor is the ratio of the azimuth time-bandwidth product (TBP) of the platform to the total azimuth TBP. In this paper, the bistatic-deformation and azimuth-dependent range-cell-migration terms were removed with phase multiplications performed blockwise in range-azimuth subsections. The remaining quasi-monostatic term shows the characteristic of the conventional monostatic SAR besides an additional azimuth-scaling term. For the monostatic characteristic, any precision monostatic SAR processing algorithms can handle. In this paper, we prefer the wavenumber-domain algorithm (also known as Omega-K), since it can accurately correct the range dependence of the range-azimuth coupling, as well as the azimuth-frequency dependence. For the azimuth-scaling term, an inverse scaled Fourier transformation is performed to correct it. Finally, a hybrid spaceborne/airborne simulation experiment is conducted to validate the proposed processing procedure.
    [bibtex-key = WangLoffeldNiesEnder2009:WavenumberDomainBistatic] [bibtex-entry]


  629. R. Wang, O. Loffeld, H. Nies, S. Knedlik, and J. Ender. Chirp-Scaling Algorithm for Bistatic SAR Data in the Constant-Offset Configuration. IEEE Trans. Geosci. Remote Sens., 47(3):952-964, March 2009. Keyword(s): SAR Processing, Bistatic SAR, Chirp Scaling Algorithm, ECS, CSA processor, Doppler phase parameters, Loffeld bistatic formula, SAR receiver velocity vector, SAR transmitter velocity vector, azimuth invariant configuration, azimuth stationary configuration, bistatic SAR data processing method, bistatic deformation term linearisation, bistatic motion error model, bistatic slant range displacement, chirp scaling algorithm, constant offset configuration, monostatic motion compensation technique, quasimonostatic term linearisation, trajectory deviation compensattion, zero Doppler plane, Doppler radar, chirp modulation, geophysical signal processing, motion compensation, radar receivers, radar signal processing, radar transmitters, remote sensing by radar, synthetic aperture radar.
    Abstract: This paper discusses the processing method for bistatic SAR data in the constant-offset configuration. The constant-offset configuration is also known as the azimuth stationary or invariant configuration where transmitter and receiver follow each other, moving on identical velocity vector. In this paper, the proposed processing method for bistatic SAR data is based on Loffeld's bistatic formula that consists of two terms, i.e., the quasi-monostatic (QM) term and bistatic-deformation (BD) term. Our basic idea is to linearize the aforementioned two terms and then incorporate the BD term into the QM term to obtain an analogous monostatic spectrum. Based on the new spectrum, any efficient 2-D frequency or range-Doppler domain processor can easily be employed to process the bistatic data, where the Doppler phase parameters of the processor need to be adjusted. In this paper, we concentrate on the application of chirp-scaling-algorithm (CSA) processor. In addition, a bistatic-motion error model is developed where the position deviations of the two platforms are simplified as the bistatic slant-range displacement in the zero Doppler plane. Using this model, the monostatic motion-compensation technique is applied and integrated into CSA to compensate the trajectory deviations of transmitter and receiver. Finally, real and simulated data are used to validate the proposed processing method.
    [bibtex-key = WangLoffeldNiesKnedlikEnder2009:BistaticConstOffset] [bibtex-entry]


  630. Mengdao Xing, Xiuwei Jiang, Renbiao Wu, Feng Zhou, and Zheng Bao. Motion Compensation for UAV SAR Based on Raw Radar Data. IEEE Transactions on Geoscience and Remote Sensing, 47(8):2870-2883, August 2009. Keyword(s): SAR Processing, Motion Compensation. MoComp, 3D MOCO method, 3D motion error analysis, Doppler rate estimate, UAV SAR, Airborne SAR, aircraft properties, atmospheric turbulence, forward velocity, inertial navigation system, line-of-sight direction displacement, motion parameters extraction, raw radar data, synthetic aperture radar systems, unmanned aerial vehicle, UAV, error analysis, geophysical techniques, inertial navigation, radar imaging, remotely operated vehicles, synthetic aperture radar.
    Abstract: Unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) is very important for battlefield awareness. For SAR systems mounted on a UAV, the motion errors can be considerably high due to atmospheric turbulence and aircraft properties, such as its small size, which makes motion compensation (MOCO) in UAV SAR more urgent than other SAR systems. In this paper, based on 3-D motion error analysis, a novel 3-D MOCO method is proposed. The main idea is to extract necessary motion parameters, i.e., forward velocity and displacement in line-of-sight direction, from radar raw data, based on an instantaneous Doppler rate estimate. Experimental results show that the proposed method is suitable for low- or medium-altitude UAV SAR systems equipped with a low-accuracy inertial navigation system.
    [bibtex-key = xingJiangWuZhouBaoTGRS2009MoCompUAVSAR] [bibtex-entry]


  631. Simon H. Yueh, Steve J. Dinardo, Ahmed Akgiray, Richard West, Donald W. Cline, and Kelly Elder. Airborne Ku-Band Polarimetric Radar Remote Sensing of Terrestrial Snow Cover. IEEE Transactions on Geoscience and Remote Sensing, 47(10):3347-3364, October 2009. Keyword(s): snow, ku-band, airborne radar, backscatter, hydrological techniques, ice, radar cross-sections, radar polarimetry, remote sensing by radar, snow, vegetation, AD 2006 to 2008, CLPX-II, Cold Land Processes Experiment, Colorado, HH/VV backscatter ratio, Ku-band polarimetric scatterometer, POLSCAT data acquisition, USA, airborne Ku-band polarimetric radar, biomass, freeze-thaw cycles, ice crust layers, ice lenses, radar echoes, radar signals, radiative transfer scattering model, remote sensing, snow-grain size, snow-water-equivalent accumulation, snowpack change, surface hoar growth, terrestrial snow cover, vegetation types, Microwave remote sensing, radar, snow.
    Abstract: Characteristics of the Ku-band polarimetric scatterometer (POLSCAT) data acquired from five sets of aircraft flights in the winter months of 2006-2008 for the second Cold Land Processes Experiment (CLPX-II) in Colorado are described in this paper. The data showed the response of the Ku-band radar echoes to snowpack changes for various types of background vegetation in the study site in north central Colorado. We observed about 0.15-0.5-dB increases in backscatter for every 1 cm of snow-water-equivalent (SWE) accumulation for areas with short vegetation (sagebrush and pasture). The region with the smaller amount of biomass, signified by the backscatter in November, seemed to have the stronger backscatter response to SWE in decibels. The data also showed the impact of surface hoar growth and freeze/thaw cycles, which created large snow-grain sizes, ice crust layers, and ice lenses and consequently increased the radar signals by a few decibels. The copolarized HH/VV backscatter ratio seems to indicate double-bounce scattering between the ground surface and snow or vegetation. The cross-polarized backscatter [vertical-horizontal (VH)] showed not only the influence of vegetation but also the strong response to snow accumulation. The observed HV/VV ratio suggests the importance of multiple scattering or nonspherical scattering geometry of snow grain in the dense-media radiative transfer scattering model. Comparison of the POLSCAT and QuikSCAT data was made and confirmed the effects of mixed terrain covers in the coarse-resolution QuikSCAT data.
    [bibtex-key = yuehDinardoAkgirayWestClineElderTGRS2009KuBandSnowCover] [bibtex-entry]


  632. Evan C. Zaugg and David G. Long. Generalized Frequency-Domain SAR Processing. IEEE Transactions on Geoscience and Remote Sensing, 47(11):3761-3773, November 2009. Keyword(s): SAR Processing, Azimuth Focusing, Chirp Scaling Algorithm, CSA, Extended Chirp Scaling, ECS, range-Doppler algorithm, omega-k, wavenumber domain algorithm, range migration algorithm, frequency-domain analysis, geophysical techniques, synthetic aperture radar, 2D frequency domain analysis.
    Abstract: The range-Doppler algorithm and the chirp-scaling algorithm (CSA) process synthetic aperture radar (SAR) data with approximations to ideal SAR processing. These approximations are invalid for data from systems with wide beamwidths, large bandwidths, and/or low center frequencies. While simple and efficient, these frequency-domain methods are thus limited by the SAR parameters. This paper explores these limits and proposes a generalized chirp-scaling approach for extending the utility of frequency-domain processing. We demonstrate how different order approximations of the SAR signal in the 2-D frequency domain affect image focusing for varying SAR parameters. From these results, a guideline is set forth, which suggests the required order of approximation terms for proper focusing. A proposed generalized frequency-domain processing approach is derived. This method is an efficient arbitrary-order CSA that processes the data using the appropriate number of approximation terms. The new method is demonstrated using simulated data.
    [bibtex-key = zauggLongTGRS2009:GeneralizedFreqDomainProcessing] [bibtex-entry]


  633. Lei Zhang, Cheng-Wei Qiu, Mengdao Xing, and Zheng Bao. Azimuth preprocessing for monostatic and bistatic spotlight synthetic aperture radar maging based on spectral analysis convolution. Journal of Applied Remote Sensing, 3(1):1-20, January 2009. Keyword(s): SAR Processing, Bistatic SAR, SPECAN, Azimuth Focusing, Spotlight SAR, Spotlight-mode data.
    Abstract: Dechirping is a technique widely used to reduce sampling rate. It is well suited for the illumination of small scenes. In this paper, we extend this idea to mono/bistatic spotlight synthetic aperture radar (SAR) imaging. An azimuth preprocessor based on the spectral analysis (SPECAN) convolution is presented. The convolution overcomes the Doppler aliasing of echoed signals, while the wavenumber analytic formula keeps unchanged. Since the spatial characteristic of the signal is preserved, the preprocessing is well compatible with conventional focusing approaches, such as chirp scaling algorithm and frequency scaling algorithm. The proposed method is validated by simulations in both monostatic and bistatic cases.
    [bibtex-key = zhangQiuXingBao2009BistaticSpotlight] [bibtex-entry]


  634. Lei Zhang, Meng-dao Xing, Cheng-Wei Qiu, and Zheng Bao. Two-Dimensional Spectrum Matched Filter Banks for High-Speed Spinning-Target Three-Dimensional ISAR Imaging. IEEE Geoscience and Remote Sensing Letters, 6(3):368-372, July 2009. Keyword(s): SAR Processing, 3D SAR, ISAR, Spinning Targets, Rotating Targets.
    Abstract: In this letter, a 3-D inversed synthetic aperture radar imaging algorithm for targets in high-speed spinning is proposed based on 2-D spectrum matched filter (MF) banks. Each spectrum MF bank yields a focused slice for its corresponding scatterers. By extracting the spatial parameters from all slices, the 3-D image of the target can be constructed. Numeric simulation confirms the validity of the algorithm.
    [bibtex-key = ZhangXingQiuBao2009:] [bibtex-entry]


  635. Zhu Zhengwei and Zhou Jianjiang. Optimum selection of common master image for ground deformation monitoring based on PS-DInSAR technique. Systems Engineering and Electronics, Journal of, 20(6):1213 -1220, December 2009. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR.
    Abstract: Considering the joint effects of various factors such as temporal baseline, spatial baseline, thermal noise, the difference of Doppler centroid frequency and the error of data processing on the interference correlation, an optimum selection method of common master images for ground deformation monitoring based on the permanent scatterer and differential SAR interferometry (PS-DInSAR) technique is proposed, in which the joint correlation coefficient is used as the evaluation function. The principle and realization method of PS-DInSAR technology is introduced, the factors affecting the DInSAR correlation are analysed, and the joint correlation function model and its solution are presented. Finally an experiment for the optimum selection of common master images is performed by using 25 SAR images over Shanghai taken by the ERS-1/2 as test data. The results indicate that the optimum selection method for PS-DInSAR common master images is effective and reliable.
    [bibtex-key = zhengweiJianjiang2009] [bibtex-entry]


  636. R.G. Baraniuk, E. Candes, R. Nowak, and M. Vetterli. Compressive Sampling [From the Guest Editors]. IEEE Signal Processing Magazine, 25(2):12 -13, March 2008.
    Abstract: The ten articles in this special section provide the reader with specific insights into the basic theory, capabilities, and limitations of compressed sensing (CS). The papers are summarized here.
    [bibtex-key = 4472238] [bibtex-entry]


  637. F. Berizzi, M. Martorella, A. Cacciamano, and A. Capria. A Contrast-Based Algorithm For Synthetic Range-Profile Motion Compensation. IEEE Transactions on Geoscience and Remote Sensing, 46(10):3053-3062, October 2008. Keyword(s): motion compensation, radar signal processing, synthetic aperture radarSAR image reconstruction, SNR loss, acceleration distortion effects, asymmetric smearing, contrast based algorithm, contrast optimization, estimation error analysis, low PRF radars, motion compensation technique, radar pulse repetition frequency, range shift, range-profile distortions, stepped frequency radar, stepped frequency waveform, symmetric spreading, synthetic aperture radar, synthetic range profile cost function, synthetic range-profile motion compensation, target motion, target radial acceleration, target radial velocity.
    Abstract: In stepped-frequency radar, target motions produce range-profile distortions. Range shift, signal-to-noise ratio loss, and symmetric spreading are produced by target radial velocity, whereas target radial acceleration is mainly responsible for asymmetric smearing. Acceleration-distortion effects are usually negligible when a high Pulse Repetition Frequency (PRF) is used, although this is not the case for low-PRF radars. In this paper, a new motion-compensation technique based on contrast optimization is proposed. The innovative contributions of this paper are as follows: (1) A theoretical analysis of the distortions produced by target motions on the reconstruction of synthetic aperture radar is provided; (2) the proposed technique compensates both phase terms, which are due to target radial velocity and acceleration; therefore, synthetic range profiles can be focused by processing low-PRF radar returns; (3) a new cost function for the synthetic range profiles (namely, contrast) is defined and used for motion compensation; (4) the proposed technique can be applied to any kind of stepped-frequency waveforms; and (5) an estimation error analysis is performed, first theoretically and then by means of both simulations and real data.
    [bibtex-key = 4637925] [bibtex-entry]


  638. Pablo Blanco-Sáchez, Jordi J. Mallorquì, Sergi Duque, and Daniel Monells. The Coherent Pixels Technique (CPT): An Advanced DInSAR Technique for Nonlinear Deformation Monitoring. Pure and Applied Geophysics, 165(6):1167-1193, 2008. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Spaceborne SAR, InSAR, DInSAR, Interferometry, Coherernt Pixel Technique, CPT, Orbital SAR, differential interferometry, deformation monitoring.
    Abstract: This paper shows the potential applicability of orbital Synthetic Aperture Radar (SAR) Differential Interferometry (DInSAR) with multiple images for terrain deformation episodes monitoring. This paper is focused on the Coherent Pixels Technique (CPT) developed at the Remote Sensing Laboratory (RSLab) of the Universitat Politecnica de Catalunya (UPC). CPT is able to extract from a stack of differential interferograms the deformation evolution over vast areas during wide spans of time. The former is achieved thanks to the coverage provided by current SAR satellites, like ESA's ERS or ENVISAT, while the latter due to the large archive of images acquired since 1992. An interferogram is formed by the complex product of two SAR images (one complex conjugate) and its phase contains information relative to topography, terrain deformation and atmospheric conditions among others. The goal of differential interferometric processing is to retrieve and separate the different contributions. The processing scheme is composed of three main steps: firstly, the generation of the best interferogram set among all the available images of the zone under study; secondly, the selection of the pixels with reliable phase within the employed interferograms and, thirdly, their phase analysis to calculate, as the main result, their deformation time series within the observation period. In this paper, the Coherent Pixels Technique (CPT) is presented in detail as well as the result of its application in different scenarios. Results reveal its practical utility for detecting and reproducing deformation episodes, providing a valuable tool to the scientific community for the understanding of considerable geological process and to monitor the impact of underground human activity.
    [bibtex-key = blancoSanchezMallorquiDuqueMonells2008PSI] [bibtex-entry]


  639. Andreas .R. Brenner and Ludwig Roessing. Radar Imaging of Urban Areas by Means of Very High-Resolution SAR and Interferometric SAR. IEEE Trans. Geosci. Remote Sens., 46(10):2971-2982, Oct. 2008. Keyword(s): SAR Processing, InSAR, Interferometry, SAR Interferometry, X-Band, Repeat-Pass Interferometry, Repeat-Pass, Single-Pass, Airborne SAR, PAMIR, Autofocus, Residual Motion Errors, Motion Compensation, MoComp, earthquakes, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth), Forschungsgesellschaft fur Angewandte Naturwissenschaften, Germany, Research Institute for High Frequency Physics and Radar Techniques, Wachtberg, X-band demonstrator, building recognition, building reconstruction, earthquake damage mapping, interferometric SAR sensor, phased array multifunctional imaging radar, radar-based urban analysis, remote-sensing applications, structural image analysis, subdecimeter resolution features, urban area monitoring, urban elevation models.
    Abstract: In remote-sensing applications, the monitoring of urban areas by means of synthetic aperture radar (SAR) sensors has grown into a valuable and indispensable tool. Although SAR imaging with a spatial resolution down to 1 m is widespread, a resolution as fine as 10 cm and below is offered only by very few SAR sensors worldwide. In this paper, the potential of very high-resolution radar imaging of urban areas by means of SAR and interferometric imaging will be demonstrated and discussed. Results of urban SAR imaging down to subdecimeter resolution will be shown. Even though the immanent layover situation in urban areas is an obstacle to simple image understanding, a remedy can be found by using interferometric SAR imaging. Interferometric results based on very high-resolution SAR images acquired over urban areas, partially with a severe layover situation, will be presented. The corresponding data was acquired with the phased array multifunctional imaging radar (PAMIR), the X-band demonstrator of the Research Institute for High Frequency Physics and Radar Techniques (FHR), Forschungsgesellschaft fur Angewandte Naturwissenschaften (FGAN), Wachtberg, Germany. It can be stated that high-resolution interferometric SAR will be an important basis for upcoming radar-based urban analysis.
    [bibtex-key = brennerRoessing2008:InSARPAMIR] [bibtex-entry]


  640. Jehanzeb Burki and Christopher F. Barnes. Slant Plane CSAR Processing Using Householder Transform. IEEE Transactions on Image Processing, 17(10):1900-1907, October 2008. Keyword(s): SAR Processing, Circular SAR, Azimuth Focusing, Fourier transforms, radar imaging, synthetic aperture radar, Fourier analysis-based focusing, Householder transform, ground plane circular SAR signal, linear shift-varying system, slant plane CSAR processing, synthetic aperture radar, Adaptive optics, Fourier transforms, History, Image analysis, Image reconstruction, Optical imaging, Optical sensors, Optical signal processing, Radar polarimetry, Synthetic aperture radar, 2-D aperture synthesis, Circular aperture, Fourier optics, Fourier transform, Householder transform, circular synthetic aperture radar (CSAR), synthetic aperture radar (SAR), Algorithms, Fourier Analysis, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Radar, Reproducibility of Results, Sensitivity and Specificity.
    Abstract: Fourier analysis-based focusing of synthetic aperture radar (SAR) data collected during circular flight path is a recent advancement in SAR signal processing. This paper uses the Householder transform to obtain a ground plane circular SAR (CSAR) signal phase history from the slant plane CSAR phase history by inverting the linear shift-varying system model, thereby circumventing the need for explicitly computing a pseudo-inverse. The Householder transform has recently been shown to have improved error bounds and stability as an underdetermined and ill-conditioned system solver, and the Householder transform is computationally efficient.
    [bibtex-key = burkiBarnesTIP2008CircularSARProcessingUsingHouseholderTransform] [bibtex-entry]


  641. Emmanuel J. Candes. The restricted isometry property and its implications for compressed sensing. Comptes Rendus Mathematique, 346(9-10):589-592, 2008. Keyword(s): Compressive Sensing, Compressed Sensing. [bibtex-key = Candes2008IsometryPropertyCS] [bibtex-entry]


  642. E.J. Candes and M.B. Wakin. An Introduction To Compressive Sampling. IEEE Signal Processing Magazine, 25(2):21-30, March 2008. Keyword(s): Relatively few wavelet, compressed sensing, compressive sampling, data acquisition, image recovery, sampling paradigm, sensing paradigm, signal recovery, data acquisition, image processing, signal processing equipment, signal sampling;.
    Abstract: Conventional approaches to sampling signals or images follow Shannon's theorem: the sampling rate must be at least twice the maximum frequency present in the signal (Nyquist rate). In the field of data conversion, standard analog-to-digital converter (ADC) technology implements the usual quantized Shannon representation - the signal is uniformly sampled at or above the Nyquist rate. This article surveys the theory of compressive sampling, also known as compressed sensing or CS, a novel sensing/sampling paradigm that goes against the common wisdom in data acquisition. CS theory asserts that one can recover certain signals and images from far fewer samples or measurements than traditional methods use.
    [bibtex-key = 4472240] [bibtex-entry]


  643. D. Cerutti-Maori, J. Klare, A.R. Brenner, and Joachim H. G. Ender. Wide-Area Traffic Monitoring With the SAR/GMTI System PAMIR. IEEE Trans. Geosci. Remote Sens., 46(10):3019-3030, Oct. 2008. Keyword(s): road traffic, synthetic aperture radar, target trackingGround Moving Target Indication mode, SAR-GMTI system, airborne radar sensor PAMIR, positioning accuracy, radial velocity, scan-MTI mode, signal-to-noise ratio, vehicles detection, wide area traffic monitoring experiment.
    Abstract: This paper presents a wide area traffic monitoring experiment under real conditions, using the scan-MTI mode of the airborne radar sensor PAMIR. This flexible GMTI (Ground Moving Target Indication) mode was designed in order to rapidly monitor wide areas for moving targets. The scan operation enables the detection of targets from different aspect angles with a high revisit rate. The parameters (e.g., radial velocity, signal-to-noise ratio, and positioning accuracy) of the detected vehicles are investigated and compared to the expected theoretical GMTI performance. It will be shown that the scan-MTI mode is particularly adapted to perform an efficient wide-area traffic monitoring.
    [bibtex-key = 4637928] [bibtex-entry]


  644. Karlus A. Câmara de Macedo, Rolf Scheiber, and Alberto Moreira. An Autofocus Approach for Residual Motion Errors With Application to Airborne Repeat-Pass SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 46(10):3151-3162, October 2008. Keyword(s): SAR Processing, Autofocus, Residual Motion Errors, WPCA, Weighted PCA, Weighted Phase Curvature Autofocus, Phase Curvature Autofocus, PCA, Phase Gradient Autofocus, PGA, Repeat-Pass Interferometry, Interferometry, InSAR, D-InSAR, Differential SAR Interferometry, E-SAR, airborne SAR, Baseline Calibration, Tomography, SAR Tomography, deformation, geophysical techniques, synthetic aperture radar, topography (Earth)E-SAR system, German Aerospace Center, airborne repeat-pass SAR Interferometry, autofocus algorithm, autofocus techniques, high-precision navigation system, image processing, interferometric-phase accuracy, phase curvature autofocus, residual motion errors, synthetic-aperture-radar, terrain deformations measurement, weighted least squares phase estimation.
    Abstract: Airborne repeat-pass SAR systems are very sensible to subwavelength deviations from the reference track. To enable repeat-pass interferometry, a high-precision navigation system is needed. Due to the limit of accuracy of such systems, deviations in the order of centimeters remain between the real track and the processed one, causing mainly undesirable phase undulations and misregistration in the interferograms, referred to as residual motion errors. Up to now, only interferometric approaches, as multisquint, are used to compensate for such residual errors. In this paper, we present for the first time the use of the autofocus technique for residual motion errors in the repeat-pass interferometric context. A very robust autofocus technique has to be used to cope with the demands of the repeat-pass applications. We propose a new robust autofocus algorithm based on the weighted least squares phase estimation and the phase curvature autofocus (PCA) extended to the range-dependent case. We call this new algorithm weighted PCA. Different from multisquint, the autofocus approach has the advantage of being able to estimate motion deviations independently, leading to better focused data and correct impulse-response positioning. As a consequence, better coherence and interferometric-phase accuracy are achieved. Repeat-pass interferometry based only on image processing gains in robustness and reliability, since its performance does not deteriorate with time decorrelation and no assumptions need to be made on the interferometric phase. Repeat-pass data of the E-SAR system of the German Aerospace Center (DLR) are used to demonstrate the performance of the proposed approach.
    [bibtex-key = deMacedoScheiberMoreira2008:WPCA] [bibtex-entry]


  645. Othmar Frey, Felix Morsdorf, and Erich Meier. Tomographic Imaging of a Forested Area By Airborne Multi-Baseline P-Band SAR. Sensors, Special Issue on Synthetic Aperture Radar, 8(9):5884-5896, September 2008. Keyword(s): SAR Processing, SAR Tomography, Tomographic Processing, Multi-Baseline SAR, Time-Domain Back-Projection, Back-Projection, E-SAR, P-Band, Forestry.
    Abstract: In recent years, various attempts have been undertaken to obtain information about the structure of forested areas from multi-baseline synthetic aperture radar data. Tomographic processing of such data has been demonstrated for airborne L-band data but the quality of the focused tomographic images is limited by several factors. In particular, the common Fourierbased focusing methods are susceptible to irregular and sparse sampling, two problems, that are unavoidable in case of multi-pass, multi-baseline SAR data acquired by an airborne system. In this paper, a tomographic focusing method based on the time-domain back-projection algorithm is proposed, which maintains the geometric relationship between the original sensor positions and the imaged target and is therefore able to cope with irregular sampling without introducing any approximations with respect to the geometry. The tomographic focusing quality is assessed by analysing the impulse response of simulated point targets and an in-scene corner reflector. And, in particular, several tomographic slices of a volume representing a forested area are given. The respective P-band tomographic data set consisting of eleven flight tracks has been acquired by the airborne E-SAR sensor of the German Aerospace Center (DLR).
    [bibtex-key = freyMorsdorfMeier08:SensorsTomo] [bibtex-entry]


  646. M. Ge, G. Gendt, M. Rothacher, C. Shi, and J. Liu. Resolution of GPS carrier-phase ambiguities in Precise Point Positioning (PPP) with daily observations. Journal of Geodesy, 82(7):389-399, 2008. Keyword(s): GNSS, lobal Navigation Satellite Systems, GPS, Global Positioning System, Carrier phase, ambiguities, carrier-phase ambiguities, Precise Point Positioning, PPP.
    Abstract: Precise Point Positioning (PPP) has been demonstrated to be a powerful tool in geodetic and geodynamic applications. Although its accuracy is almost comparable with network solutions, the east component of the PPP results is still to be improved by integer ambiguity fixing, which is, up to now, prevented by the presence of the uncalibrated phase delays (UPD) originating in the receivers and satellites. In this paper, it is shown that UPDs are rather stable in time and space, and can be estimated with high accuracy and reliability through a statistical analysis of the ambiguities estimated from a reference network. An approach is implemented to estimate the fractional parts of the single-difference (SD) UPDs between satellites in wide- and narrow-lane from a global reference network. By applying the obtained SD-UPDs as corrections to the SD-ambiguities at a single station, the corrected SD-ambiguities have a naturally integer feature and can therefore be fixed to integer values as usually done for the double-difference ones in the network mode. With data collected at 450 stations of the International GNSS Service (IGS) through days 106 to 119 in 2006, the efficiency of the presented ambiguity-fixing strategy is validated using IGS Final products. On average, more than 80% of the independent ambiguities could be fixed reliably, which leads to an improvement of about 27% in the repeatability and 30% in the agreement with the IGS weekly solutions for the east component of station coordinates, compared with the real-valued solutions.
    [bibtex-key = geGendtRothacherShiLiuG2008ResolutionOfGPSCarrierPhaseAmbiguitiesInPPP] [bibtex-entry]


  647. José-Tomás González-Partida, Pablo Almorox-González, Mateo Burgos-Garcìa, and Blas-Pablo Dorta-Naranjo. SAR System for UAV Operation with Motion Error Compensation beyond the Resolution Cell. Sensors, Special Issue on Synthetic Aperture Radar, 8(5):3384-3405, 2008. Keyword(s): SAR Processing, Motion Compensation, MoComp, Airborne SAR, UAV, Unmanned Airborne Vehicle, LFM-CW, Continuous Wave SAR, Phase Gradient Autofocus, Autofocus, PGA, Range Alignment, Residual Motion Errors, mmW SAR, mmW, Ka-Band SAR.
    Abstract: This paper presents an experimental Synthetic Aperture Radar (SAR) system that is under development in the Universidad Politecnica de Madrid. The system uses Linear Frequency Modulated Continuous Wave (LFM-CW) radar with a two antenna configuration for transmission and reception. The radar operates in the millimeter-wave band with a maximum transmitted bandwidth of 2 GHz. The proposed system is being developed for Unmanned Aerial Vehicle (UAV) operation. Motion errors in UAV operation can be critical. Therefore, this paper proposes a method for focusing SAR images with movement errors larger than the resolution cell. Typically, this problem is solved using two processing steps: first, coarse motion compensation based on the information provided by an Inertial Measuring Unit (IMU); and second, fine motion compensation for the residual errors within the resolution cell based on the received raw data. The proposed technique tries to focus the image without using data of an IMU. The method is based on a combination of the well known Phase Gradient Autofocus (PGA) for SAR imagery and typical algorithms for translational motion compensation on Inverse SAR (ISAR). This paper shows the first real experiments for obtaining high resolution SAR images using a car as a mobile platform for our radar.
    [bibtex-key = gonzalezPartidaAlmoroxGonzalezBurgosGarciaDortaNaranjo2008:UAVMoCo] [bibtex-entry]


  648. Andrew Hooper. A multi-temporal InSAR method incorporating both persistent scatterer and small baseline approaches. Geophysical Research Letters, 35(16), 2008. Keyword(s): MT-InSAR, PSI, SBAS.
    Abstract: Synthetic aperture radar (SAR) interferometry is a technique that provides high-resolution measurements of the ground displacement associated with many geophysical processes. Advanced techniques involving the simultaneous processing of multiple SAR acquisitions in time increase the number of locations where a deformation signal can be extracted and reduce associated error. Currently there are two broad categories of algorithms for processing multiple acquisitions, persistent scatterer and small baseline methods, which are optimized for different models of scattering. However, the scattering characteristics of real terrains usually lay between these two end-member models. I present here a new method that combines both approaches, to extract the deformation signal at more points and with higher overall signal-to-noise ratio than can either approach alone. I apply the combined method to data acquired over Eyjafjallaj\"okull volcano in Iceland, and detect time-varying ground displacements associated with two intrusion events.
    [bibtex-key = hooperGRL2008MultitemporalInSARWithPSIandSBAS] [bibtex-entry]


  649. S. Jaruwatanadilok and A. Ishimaru. Electromagnetic Coherent Tomography Array Imaging in Random Scattering Media. IEEE Antennas and Wireless Propagation Letters, 7:524-527, 2008. Keyword(s): SAR Processing, SAR Tomography, Tomography, Monte Carlo method, array processing method, discrete random media, electromagnetic coherent tomography array imaging, optical coherence tomography, random scattering media, Monte Carlo methods, optical tomography, random media;.
    Abstract: Imaging through discrete random media is an important problem in several applications such as medicine, remote sensing, and security. Discrete random media create scattering which deteriorates the quality of the image and there have been several efforts to mitigate the problem. We propose an array processing method called coherence tomography array (CTA) whose algorithm is derived from that of optical coherence tomography. We use the Monte Carlo method to simulate the imaging scenarios in random scattering media. The results show that CTA can be used to alleviate the clutter effects from discrete random scatterers.
    [bibtex-key = 4623153] [bibtex-entry]


  650. Michael Jehle, Donat Perler, David Small, Adrian Schubert, and Erich Meier. Estimation of Atmospheric Path Delays in TerraSAR-X Data using Models vs. Measurements. Sensors, 8(12):8479-8491, 2008. Keyword(s): SAR Processing, Ionosphere, TEC, Total Electron Content, Troposphere, Path Delay. [bibtex-key = jehlePerlerSmallSchubertMeier2008:EstimPathDelay] [bibtex-entry]


  651. Shi Jun, Xiaoling Zhang, and Jianyu Yang. Principle and Methods on Bistatic SAR Signal Processing via Time Correlation. IEEE Trans. Geosci. Remote Sens., 46(10):3163-3178, Oct. 2008. Keyword(s): fast Fourier transforms, geophysical signal processing, radar imaging, radar signal processing, remote sensing by radar, synthetic aperture radar3D scene space ambiguity problem, AVME, RVME, SAR image shifting, SAR image space bases, absolute velocity measurement error, ambiguity region, bistatic SAR 2D PSF, bistatic SAR angular velocity direction, bistatic SAR image space, bistatic SAR signal processing, inverse fast Fourier transform, motion measurement error effects, perspective line, perspective operator, point spread function, range Doppler algorithm, relative velocity measurement error, scaled IFFT, space truncation error, synthetic aperture radar, time correlation radar signal processing, translational variant bistatic SAR imaging method.
    Abstract: In this paper, we discuss the mapping between the 3-D scene space and the bistatic synthetic aperture radar (SAR) image space and show that when the direction of the angular velocity of the bistatic SAR remains constant, the process of bistatic SAR imaging can be approximately modeled as a perspective operator from the 3-D scene space to the 2-D image space, and the perspective line is perpendicular to the plane determined by the composition direction of the T/R line of sight and the composition direction of the angular velocity of the T/R platform. Then, we show that the 2-D point spread function of the bistatic SAR is determined not only by the range and ldquoazimuthrdquo resolutions but also by the geometry of the bistatic SAR and the bases of the SAR image space, and the concept ldquoambiguity regionrdquo is introduced to describe the ambiguity problem in the 3-D scene space. Then, the range-Doppler algorithm is discussed, and a new translational-variant bistatic SAR imaging method is proposed, which uses the scaled inverse fast Fourier transform (IFFT) technique to eliminate the translational-variant feature of the SAR space resolution. The space truncation error of this new algorithm is discussed to analyze the depth of focus of the scaled IFFT bistatic SAR imaging algorithms, and we find that the upper bounce of the space truncation error is proportional to the square of the distance from the scatterer to the T/R platforms. Last, the effects of motion measurement errors are discussed in detail, and, through theoretical analysis and numerical experiments, we show that the absolute position measurement error, the baseline measurement error, the perpendicular (vertical) component of the absolute velocity measurement error (AVME), and the perpendicular component of the relative velocity measurement error (RVME) cause SAR image shifting in the image space mainly, and the parallel component of the AVME and the parallel component of the RVME cause the SAR image to s- - everely defocus.
    [bibtex-key = junZhangYang2008:Bistatic] [bibtex-entry]


  652. Jong-Sen Lee, T.L. Ainsworth, J.P. Kelly, and C. Lopez-Martinez. Evaluation and Bias Removal of Multilook Effect on Entropy/Alpha/Anisotropy in Polarimetric SAR Decomposition. IEEE Trans. Geosci. Remote Sens., 46(10):3039-3052, Oct. 2008. Keyword(s): Monte Carlo methods, geophysical techniques, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, vegetationGerman Aerospace Research Center, JPL, Jet Propulsion Laboratory, L-band Advanced Land Observing Satellite, Monte Carlo simulation, airborne X-band polarimetric SAR, airborne interferometric SAR, alpha estimation, anisotropy estimation, bias removal algorithm, entropy estimation, forest, geophysical parameter estimation, grassland, multilook processing, phased array type L-band SAR, polarimetric SAR decomposition, scattering mechanisms, urban returns.
    Abstract: Entropy, alpha, and anisotropy (H/alpha/A) of the polarimetric target decomposition have been an effective and popular tool for polarimetric synthetic aperture radar (SAR) image analysis and for a geophysical parameter estimation. However, multilook processing can severely affect the values of these parameters. In this paper, a Monte Carlo simulation is used to evaluate and remove the bias generated by the multilook effect on these parameters for various media composed of grassland, forest, and urban returns. Due to insufficient averaging, entropy is underestimated, and anisotropy is overestimated. We also found that the bias in the alpha angle can be either underestimated or overestimated depending on scattering mechanisms. Based on simulation results, efficient bias removal procedures have been developed. In particular, the entropy bias can be precisely corrected, and the amount of correction is independent of the radar frequency and SAR systems. Data from L-band Advanced Land Observing Satellite/phased array type L-band SAR, German Aerospace Research Center (DLR)/enhanced SAR, Jet Propulsion Laboratory (JPL)/airborne SAR, and X-band polarimetric and interferometric SAR are used for demonstration in this paper.
    [bibtex-key = 4637955] [bibtex-entry]


  653. Lianlin Li and Fang Li. Ionosphere tomography based on spaceborne SAR. Advances in Space Research, 42(7):1187-1193, October 2008. Keyword(s): SAR Processing, Ionosphere tomography, Spaceborne SAR, Electron density isolines, Inverse scattering technique for multi-layered random surfaces, Method of moment, MoM, TEC, Total Electron Content, CT, computerized tomography.
    Abstract: Two models of ionosphere tomography based on spaceborne SAR (Synthetic Aperture Radar) are proposed. For HF-SAR the signal with sweeping frequency lower than the characteristic frequency of ionosphere will be scatted during the ionosphere propagation and completely reflected at a corresponding height. The ionospheric electron density isolines looked as series of random surfaces can be reconstructed from the HF-SAR echoes by using the inverse scattering technique for layered rough surfaces and the method of moment (MoM). The numerical simulation show that due to the MoM can provide a full wave solution, the ionosphere tomography with high resolution can be obtained as long as enough sampling data of HF-SAR echoes are used. For VHF/UHF/P/L-band SAR the TEC (Total Electron Content) can be obtained from the SAR echoes scattered by some strong point targets (such as the calibrators, etc.) appeared in the SAR imaged ground region, and the ionosphere tomography can be performed by CT technique.
    [bibtex-key = lili2008:IonoTomoSAR] [bibtex-entry]


  654. Fabrizio Lombardini and Matteo Pardini. 3-D SAR Tomography: The Multibaseline Sector Interpolation Approach. IEEE Geoscience and Remote Sensing Letters, 5(4):630-634, Oct. 2008. Keyword(s): SAR Processing, Tomography, SAR Tomography, Multi-baseline SAR, Interpolation, Sector Interpolation, 3-D imaging, SAR Interferometry, Interferometry, InSAR, Spectral Analysis, Electromagnetic Tomography, Signal Sampling.
    Abstract: Multibaseline (MB) synthetic aperture radar (SAR) tomography is a promising mode of SAR interferometry, allowing full 3-D imaging of volumetric and layover scatterers in place of a single elevation estimation capability for each SAR cell . However, Fourier-based MB SAR tomography is generally affected by unsatisfactory imaging quality due to a typically low number of baselines with irregular distribution. In this paper, we improve the basic elevation focusing technique by reconstructing a set of uniform baselines data exploiting in the interpolation step the ancillary information about the extension of a height sector which contains all the scatterers. This a priori information can be derived from the knowledge of the kind of the observed scenario (e.g., forest or urban). To demonstrate the concept, an imaging enhancement analysis is carried out by simulation.
    [bibtex-key = lombardiniPardini2008:Tomo] [bibtex-entry]


  655. Alberto Martinez-Vazquez and Joaquim Fortuny-Guasch. A GB-SAR Processor for Snow Avalanche Identification. IEEE Trans. Geosci. Remote Sens., 46(11):3948-3956, November 2008. Keyword(s): SAR Processing, GBSAR, ground-based radar, ground-based SAR, feature extraction, geomorphology, geophysical techniques, geophysics computing, image classification, image segmentation, risk management, snow, synthetic aperture radar, features extraction, ground-based SAR processor, image thresholding, morphological filters, object classification, risk assessment, snow avalanches detection, snow avalanches identification, synthetic aperture radar images, Event detection, Feature extraction, Filters, Monitoring, Object detection, Radar detection, Snow, Statistics, Synthetic aperture radar, Testing, Classification, coherence, ground-based synthetic aperture radar (GBSAR), snow avalanche.
    Abstract: An algorithm for the automatic detection and classification of snow avalanches has been developed and assessed through the archive of synthetic aperture radar (SAR) images acquired during six winter campaigns with a ground-based linear SAR. The scheme, based on the following classic steps: 1) detection; 2) features extraction; and 3) object classification, is fully described in this paper, representing the first attempt to implement a semiautomatic near-real-time operational snow avalanche monitoring tool by SAR. Detection of possible avalanche events is performed by the combined application of thresholding and morphological filters to the differential coherence of consecutive images. Classification of events likely to be snow avalanches is based on statistics extracted from the whole image and features associated to the single regions. Tests have been conducted over more than 60 000 images. Results show a drastic reduction on the images to manually supervise (2.2%). With a 9% false-negative rate and 60% accuracy over the 2.2% of images to examine, the processor represents an interesting support tool for the daily operations of avalanche risk assessment in a commercial ski resort.
    [bibtex-key = martinezVazquezFortunyGuaschTGRS2008GBSARSnowAvalanche] [bibtex-entry]


  656. Franz J. Meyer and J.B. Nicoll. Prediction, Detection, and Correction of Faraday Rotation in Full-Polarimetric L-Band SAR Data. IEEE Trans. Geosci. Remote Sens., 46(10):3076-3086, Oct. 2008. Keyword(s): Faraday effect, electromagnetic wave polarisation, ionospheric disturbances, ionospheric electromagnetic wave propagation, ionospheric techniques, radar polarimetry, radiowave propagation, remote sensing by radar, spaceborne radar, synthetic aperture radarAdvanced Land Observing Satellite, Faraday rotation correction, Faraday rotation detection, Faraday rotation estimation, Faraday rotation prediction, PALSAR, SAR data quality degradation, data continuity, full polarimetric L-band SAR data, geophysical parameter recovery accuracy, kilometer scale ionospheric disturbances, spaceborne L-band SAR instrument, synthetic aperture radar.
    Abstract: With the synthetic aperture radar (SAR) sensor PALSAR onboard the Advanced Land Observing Satellite, a new full-polarimetric spaceborne L-band SAR instrument has been launched into orbit. At L-band, Faraday rotation (FR) can reach significant values, degrading the quality of the received SAR data. One-way rotations exceeding 25 deg are likely to happen during the lifetime of PALSAR, which will significantly reduce the accuracy of geophysical parameter recovery if uncorrected. Therefore, the estimation and correction of FR effects is a prerequisite for data quality and continuity. In this paper, methods for estimating FR are presented and analyzed. The first unambiguous detection of FR in SAR data is presented. A set of real data examples indicates the quality and sensitivity of FR estimation from PALSAR data, allowing the measurement of FR with high precision in areas where such measurements were previously inaccessible. In examples, we present the detection of kilometer-scale ionospheric disturbances, a spatial scale that is not detectable by ground-based GPS measurements. An FR prediction method is presented and validated. Approaches to correct for the estimated FR effects are applied, and their effectiveness is tested on real data.
    [bibtex-key = meyerNicoll2008:FaradayRotation] [bibtex-entry]


  657. Andrea Monti-Guarnieri and Stefano Tebaldini. On the Exploitation of Target Statistics for SAR Interferometry Applications. IEEE Transactions on Geoscience and Remote Sensing, 46(11):3436-3443, November 2008. Keyword(s): SAR Processing, Phase Linking, SAR Tomography, geophysical techniques, geophysics computing, image processing, radar interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth)ENVISAT images, Monte Carlo simulations, SAR interferometry applications, decorrelation models, interferometric phases, line-of-sight displacement, line-of-sight motion, multiimage synthetic aperture radar interferometry, physical parameters, residual topography, target statistics.
    Abstract: This paper focuses on multiimage synthetic aperture radar interferometry (InSAR) in the presence of distributed scatterers, paying particular attention to the role of target decorrelation in the estimation process. This phenomenon is accounted for by splitting the analysis into two steps. In the first step, we estimate the interferometric phases from the data, whereas in the second step, we use these phases to retrieve the physical parameters of interest, such as line-of-sight (LOS) displacement and residual topography. In both steps, we make the hypothesis that target statistics are at least approximately known. This approach is suited both to derive the performances of InSAR with different decorrelation models and for providing an actual estimate of LOS motion and topography. Results achieved from Monte Carlo simulations and a set of repeated pass ENVISAT images are shown.
    [bibtex-key = montiGuarnieriTebaldiniTGRS2008] [bibtex-entry]


  658. Felix Morsdorf, Othmar Frey, Erich Meier, Klaus I. Itten, and Britta Allgöwer. Assessment of the Influence of Flying Altitude and Scan Angle on Biophysical Vegetation Products Derived from Airborne Laser Scanning. International Journal of Remote Sensing, 29(5):1387 - 1406, March 2008.
    Abstract: Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical vegetation properties such as tree height, crown width, fractional cover and leaf area index (LAI). It is expected that the conditions of data acquisition, such as viewing geometry and sensor configuration influence the value of these parameters. In order to gain knowledge about these different conditions, we test for the sensitivity of vegetation products for viewing geometry, namely flying altitude and scanning (incidence) angle. Based on two methodologies for single tree extraction and derivation of fractional cover and LAI previously developed and published by our group, we evaluate how these variables change with either flying altitude or scanning angle. These are the two parameters which often need to be optimized towards the best compromise between point density and area covered with a single flight line, in order to reduce acquisition costs. Our test-site in the Swiss National Park was sampled with two nominal flying altitudes, 500 and 900 m above ground. Incidence angle and local incidence angle were computed based on the digital terrain model using a simple backward geocoding procedure. We divided the raw laser returns into several different incident angle classes based on the flight path data; the TopoSys Falcon II system used in this study has a maximum scan angle of 7.15 deg . We compared the derived biophysical properties from each of these classes with field measurements based on tachymeter measurements and hemispherical photographs, which were geolocated using differential GPS. It was found that with increasing flying height the well-known underestimation of tree height increases. A similar behaviour can be observed for fractional cover; its respective values decrease with higher flying height. The minimum distance between first and last echo increases from 1.2 metres for 500 m AGL to more than 3 metres for 900 m AGL, which does alter return statistics. The behaviour for incidence angles is not so evident, probably due to the small scanning angle of the system used. fCover seems to be most affected by incidence angles, with significantly higher differences for locations further away from nadir. As expected, incidence angle appears to be of higher importance for vegetation density parameters than local incidence angle.
    [bibtex-key = morsdorfFreyMeierIttenAllgoewer08:Lidar] [bibtex-entry]


  659. Maxim Neumann, Laurent Ferro-Famil, and Andreas Reigber. Multibaseline Polarimetric SAR Interferometry Coherence Optimization. IEEE Geosci. Remote Sens. Lett., 5(1):93-97, January 2008. Keyword(s): coherence, optimisation, radar interferometry, radar polarimetry, synthetic aperture radar, coherence optimization, multibaseline polarimetric SAR interferometry, scattering mechanism, synthetic aperture radar sensor, Interferometry, L-band, Optimization methods, Phase estimation, Polarimetric synthetic aperture radar, Polarization, Radar scattering, Samarium, Signal resolution, Uncertainty, Coherence optimization, multibaseline (MB), polarimetric synthetic aperture radar interferometry (PolInSAR).
    Abstract: This letter analyzes different approaches for polarimetric optimization of multibaseline (MB) interferometric coherences. Two general methods are developed to simultaneously optimize coherences for more than two data sets. The first method provides every data set with a distinct dominant scattering mechanism (SM). The second optimization method is constrained to use equal SMs at all data sets. As the experimental results indicate, MB coherence optimization does improve the accuracy in the estimation of dominant SMs and the associated interferometric phases. Both methods are evaluated on real data acquired by the German Aerospace Agency (DLR)'s enhanced synthetic aperture radar sensor (ESAR) at L-band.
    [bibtex-key = neumannFerroFamilReigberGRSL2008MBPolInterferoCoherenceOptimization] [bibtex-entry]


  660. Matteo Pardini, Fabrizio Lombardini, and Fabrizio Gini. The Hybrid Cramér -- Rao Bound on Broadside DOA Estimation of Extended Sources in Presence of Array Errors. IEEE Transactions on Signal Processing, 56(4):1726-1730, April 2008. Keyword(s): SAR Processing, SAR Tomography, Tomography, Residual Motion Errors, InSAR, SAR Interferometry, Interferometry, antenna arrays, direction-of-arrival estimation, DOA estimation, hybrid Cramer-Rao bound, multibaseline interferometers, randomly perturbed arrays, remote sensing systems, signal direction of arrival, synthetic aperture radar.
    Abstract: In this correspondence we derive explicit expressions for the hybrid Cramer-Rao lower bound (HCRB) on the estimation accuracy of signal direction of arrival (DOA) from data collected by randomly perturbed arrays. The presence of a wavefront spatial decorrelation, which is modeled as a multiplicative correlated noise, has also been taken into account in the data model, since it is typical in those applications involving extended sources. In particular, we consider perturbations in sensor positions. Existing approaches to DOA HCRB calculation do not consider the presence of multiplicative noise and are referred to the assumption of small perturbations only, still not being worked out explicitly. Here, we assume that the impinging wavefronts are coming from broadside or more generally from a narrow DOA sector, allowing the explicit derivation of the HCRB for any variance of the sensor positioning errors in the line-of-sight direction. This scenario corresponds to the typical operative condition of remote sensing systems such as synthetic aperture radar (SAR) multibaseline interferometers, for which a few HCRB sample curves are reported.
    [bibtex-key = pardiniLombardiniGini2008:Tomo] [bibtex-entry]


  661. Daniele Perissin. Validation of the Submetric Accuracy of Vertical Positioning of PSs in C-Band. IEEE Geosci. Remote Sens. Lett., 5(3):502-506, July 2008. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR, Interferometry, digital elevation models, geophysical signal processing, photogrammetry, radar interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, C-band, DEM, Envisat images, European Remote Sensing satellite, European Space Agency, Italy, Milan, Shuttle Radar Topography Mission, digital elevation models, digital terrain models, height estimate, multitrack PS DTM, permanent scatterers technique, photogrammetric technique, radar target displacement, spaceborne synthetic aperture radar interferometry, street level, submetric accuracy, target localization capability, urban area, vertical positioning, Interferometry, remote sensing, synthetic aperture radar (SAR);.
    Abstract: The permanent scatterers (PSs) technique is an operational tool in the context of spaceborne synthetic aperture radar interferometry for monitoring the displacement of radar targets with millimetric accuracy. Recently, the target localization capability of the PS technique has been subject of study, and the possibility of generating digital elevation models (DEMs) and digital terrain models (DTMs) by means of the height of a sparse set of points has been evaluated. In this letter, for the first time, the PS height estimate has been validated by exploiting about 250,000 spot heights at street level derived from photogrammetric techniques in the urban area around Milan, Italy. The very high correlation between the two independent measurements confirms the theoretical submetric accuracy of vertical positioning. A multitrack PS DTM has then been generated and compared to the spot heights together with the corresponding Shuttle Radar Topography Mission (SRTM) DEM, showing the very high improvement given by the PS technique to the freely available topographic data. The results have been obtained by processing about 300 European Space Agency (ESA) European Remote Sensing (ERS) satellite and Envisat images acquired from two descending tracks and an ascending one over Milan.
    [bibtex-key = perissinGRSL2008PSI] [bibtex-entry]


  662. S. Perna, C. Wimmer, J. Moreira, and G. Fornaro. X-Band Airborne Differential Interferometry: Results of the OrbiSAR Campaign Over the Perugia Area. IEEE Trans. Geosci. Remote Sens., 46(2):489-503, February 2008. Keyword(s): SAR Processing, BFNT, Backward-Forward to the Nominale Track, Airborne SAR, D-InSAR, differential SAR interferometry, Interferometry, OrbiSAR, X-Band, Motion Compensation, Residual Motion Errors, Autofocus, airborne radar, motion compensation, radar imaging, synthetic aperture radar airborne SAR images, digital elevation model inaccuracies, motion compensation errors, phase errors.
    Abstract: Differential synthetic aperture radar interferometry (DInSAR) is a remote sensing technique that allows monitoring ground deformation with accuracy of the order of fractions of the radiated wavelength, by means of proper combination and processing of repeat-pass data. In contrast to the satellite case, application of such a technique to airborne data is not, today, a well-established task. Several airborne campaigns, involving mainly C/L-band data, have been planned in the last years to exploit the potentialities of these more flexible platforms for deformation monitoring. In this paper, we show the results of an airborne DInSAR X-band experiment carried out over the Perugia area (center of Italy) by using the OrbiSAR system. We discuss the processing chain applied to the acquired data, which allows achieving a satisfactory compromise between accuracy and efficiency. Eleven repeated passes were carried out in two days; two corner reflectors were located on the ground in a hilly region. One corner reflector was vertically moved between the two days to evaluate the system detection capability. Moreover, we carry out an analysis of all possible differential interferograms for a region 2 x 4 km wide.
    [bibtex-key = pernaWimmerMoreiraFornaro2008AirborneDInSARXBand] [bibtex-entry]


  663. L. Pipia, X. Fabregas, A. Aguasca, and C. Lopez-Martinez. Atmospheric Artifact Compensation in Ground-Based DInSAR Applications. IEEE Geosci. Remote Sens. Lett., 5(1):88-92, January 2008. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, atmospheric humidity, atmospheric pressure, atmospheric techniques, atmospheric temperature, radar interferometry, synthetic aperture radar, AD 2005 06, Barcelona, Collserola Park, Spain, Universitat Politecnica de Catalunya, atmosphere variations, atmospheric artifact compensation, coherence-based technique, differential interferometry Synthetic Aperture Radar, ground-Based DInSAR applications, heterogeneous environment, interferometric information, polarimetric measurements, Atmosphere, Atmospheric measurements, Atmospheric modeling, Information retrieval, Interferometry, Layout, Sensor phenomena and characterization, Sensor systems, Synthetic aperture radar, Testing, Differential interferometric SAR (DInSAR), GB-SAR sensor, polarimetric SAR (PolSAR), synthetic aperture radar (SAR). [bibtex-key = pipiaFabregasAguascaLopezMartinezGRSL2008APSCompensationInGBDInSAR] [bibtex-entry]


  664. Pau Prats, J. J. Mallorqui, Andreas Reigber, Rolf Scheiber, and Alberto Moreira. Estimation of the Temporal Evolution of the Deformation Using Airborne Differential SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 46(4):1065-1078, April 2008. Keyword(s): SAR Processing, DInSAR, InSAR, Interferometry, digital elevation models, error analysis, motion compensation, MoComp, radar interferometry, Multi-Baseline SAR, synthetic aperture radar, topography (Earth)DLR, Experimental SAR system, E-SAR, Airborne SAR, German Aerospace Center, agricultural fields, airborne differential synthetic aperture radar interferometry, baseline error, corner reflector, deformation, differential interferometry processor, digital elevation model, image coregistration, residual motion errors, temporal evolution, topography.
    Abstract: This paper presents airborne differential synthetic aperture radar (SAR) interferometry results using a stack of 14 images, which were acquired by the Experimental SAR system of the German Aerospace Center (DLR) during a time span of 2.5 h. An advanced differential technique is used to retrieve the error in the digital elevation model and the temporal evolution of the deformation for every coherent pixel in the image. The two main limitations in airborne SAR processing are analyzed, namely, the existence of residual motion errors (RMEs) (inaccuracies in the navigation system on the order of 1-5 cm) and the accommodation of the topography and the aperture dependence on motion errors during the processing. The coupling between them is also addressed, showing that the estimation of the differential RME, i.e., baseline error, can be biased when using techniques based on the coregistration between interferometric looks. The SAR focusing chain to process the data is also presented together with the modifications in the differential interferometry processor to deal with the remaining baseline error. The detected motion of a corner reflector and the measured deformation in several agricultural fields allows one to validate the proposed techniques.
    [bibtex-key = pratsReigberMallorquiScheiberMoreira2008:DInSAR] [bibtex-entry]


  665. Xiaolan Qiu, Donghui Hu, and Chibiao Ding. An Improved NLCS Algorithm With Capability Analysis for One-Stationary BiSAR. IEEE Trans. Geosci. Remote Sens., 46(10):3179-3186, Oct. 2008. Keyword(s): geophysical techniques, synthetic aperture radarBiSAR imaging problem, NLCS algorithm, azimuth perturbation, compensation methods, differential range cell migration correction, local fit method, nonlinear chirp scaling algorithm, one-stationary bistatic SAR, range chirp scaling function.
    Abstract: This paper deals with the imaging problem of one-stationary bistatic SAR (BiSAR) with large bistatic angle. An improved nonlinear chirp scaling (NLCS) algorithm is proposed for this BiSAR. The main work here includes three aspects. First, a range chirp scaling function for correcting the differential range cell migration correction is derived. Then, the azimuth perturbation is generated by local fit method, which makes the NLCS algorithm suitable for the large bistatic angle case. Furthermore, the negative effects introduced by the perturbation (including phase error and locality error) are discussed, and some compensation methods are proposed to enhance the capability of the algorithm. The simulating results exhibited at the end of this paper validate the correctness of the analysis and the feasibility of the algorithm.
    [bibtex-key = QiuHuDing2008:NLCS] [bibtex-entry]


  666. Eric J. Rignot, Jonathan L. Bamber, Michiel R. van den Broeke, Curt Davis, Yonghong Li, Willem Jan van de Berg, and Erik van Meijgaard. Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Nature Geosci, 1(2):106-110, February 2008. [bibtex-key = rignotBamberVanDenBroekeDavisLiVanDeBergVanMeijgaardNatureGeoscience2008AntarcticIceMassLoss] [bibtex-entry]


  667. Christian Ruckstuhl, Rolf Philipona, Klaus Behrens, Martine Collaud Coen, Bruno Dürr, Alain Heimo, Christian Mätzler, Stephan Nyeki, Atsumu Ohmura, Laurent Vuilleumier, and others. Aerosol and cloud effects on solar brightening and the recent rapid warming. Geophysical Research Letters, 35(12), 2008. [bibtex-key = Ruckstuhl2008] [bibtex-entry]


  668. T.K. Sjogren, V.T. Vu, and M.I. Pettersson. A comparative study of the polar version with the subimage version of Fast Factorized Backprojection in UWB SAR. International Radar Symposium, pp 1-4, May 2008. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast Back-Projection, Back-Projection, Fast Factorized Back-Projection, Comparison of Algorithms, interpolation, radar imaging, synthetic aperture radar, time-domain analysis, ultra wideband radar, UWB SAR, interpolation method, phase error, polar version, subimage version, time domain SAR algorithm, Factorized Back-Projection.
    Abstract: This paper presents a comparative study of the polar and the subimage based variants of the time domain SAR algorithm Fast Factorized Backprojection. The difference between the two variants with regard to the phase error, which causes defocusing in the image, is investigated. The difference between the algorithms in interpolation between stages is also discussed. To investigate the sidelobes in azimuth, the paper gives simulation results for a low frequency UWB SAR system for both algorithms. How the algorithms differ with regard to amount of beams and length of beams is also discussed.
    [bibtex-key = sjoerenVuPetterson2008:FFBPComparison] [bibtex-entry]


  669. Robert Wang, Otmar Loffeld, Qurat Ul-Ann, Holger Nies, Amaya Medrano Ortiz, and Ashraf Samarah. A Bistatic Point Target Reference Spectrum for General Bistatic SAR Processing. IEEE Trans. Geosci. Remote Sens.L, 5(3):517-521, July 2008. Keyword(s): SAR Processing, Bistatic SAR, Loffeld bistatic formula, airborne configuration, azimuth time-bandwidth products, bistatic point target reference spectrum, bistatic synthetic aperture radar, general bistatic SAR processing, spaceborne configuration, total azimuth modulation, total azimuth phase, geophysical signal processing, radar signal processing, synthetic aperture radar.
    Abstract: A bistatic point target reference spectrum (BPTRS) based on Loffeld's bistatic formula (LBF) is derived in this letter. For LBF, the same contributions of the transmitter and receiver to the total azimuth modulation are assumed. This assumption results in the failure of LBF in the extreme configuration (i.e., spaceborne/airborne configuration). For general bistatic configurations, the azimuth modulations are unequal for the transmitter and receiver due to the different slant ranges and velocities. Therefore, the azimuth time-bandwidth products (TBPs) from the transmitter and receiver are different; in some cases (e.g., spaceborne/airborne case), one of them might be very small, which might even result in a serious error of the principle of stationary phase. This letter uses TBP to weight the azimuth phase modulation contributions of the transmitter and receiver to the common azimuth spectrum to approximately obtain the point of stationary phase of the total azimuth phase history. Simulations show that the proposed BPTRS can work well for spaceborne/airborne configurations.
    [bibtex-key = WangLoffeldUlAnnNiesOrtizSamarah2008:Bistatic] [bibtex-entry]


  670. Evan C. Zaugg and David G. Long. Theory and Application of Motion Compensation for LFM-CW SAR. IEEE Transactions on Geoscience and Remote Sensing, 46(10):2990-2998, Oct. 2008. Keyword(s): SAR Processing, LFM-CW, LFM-CW SAR, FMCW, MoComp, motion compensation, CSA, ECS, Chirp Scaling, Extended Chirp Scaling, FSA, Frequency Scaling Algorithm, Range-Doppler Algorithm, synthetic aperture radar, Brigham Young University, muSAR system, LFM-CW signal model, SAR image quality, aircraft, atmospheric turbulence, high-resolution synthetic aperture radar systems, linear frequency-modulated continuous-wave signal, motion correction algorithms, unmanned aerial vehicle, Airborne SAR, geophysical techniques.
    Abstract: Small low-cost high-resolution synthetic aperture radar (SAR) systems are made possible by using a linear frequency-modulated continuous-wave (LFM-CW) signal. SAR processing assumes that the sensor is moving in a straight line at a constant speed, but in actuality, an unmanned aerial vehicle (UAV) or airplane will often significantly deviate from this ideal. This nonideal motion can seriously degrade the SAR image quality. In a continuous-wave system, this motion happens during the radar pulse, which means that existing motion compensation techniques that approximate the position as constant over a pulse are limited for LFM-CW SAR. Small aircraft and UAVs are particularly susceptible to atmospheric turbulence, making the need for motion compensation even greater for SARs operating on these platforms. In this paper, the LFM-CW SAR signal model is presented, and processing algorithms are discussed. The effects of nonideal motion on the SAR signal are derived, and new methods for motion correction are developed, which correct for motion during the pulse. These new motion correction algorithms are verified with simulated data and with actual data collected using the Brigham Young University muSAR system.
    [bibtex-key = zauggLongTGRS2008:MocompLFMCWSAR] [bibtex-entry]


  671. Mark Simons and Paul A. Rosen. Interferometric Synthetic Aperture Radar Geodesy. In Gerald Schubert, editor, Treatise on Geophysics, pages 391-446. Elsevier, Amsterdam, 2007. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, DInSAR, Crustal deformation, Geodesy, GPS, Radar interferometry, Displacement Mapping, Deformation Mapping, Teaching Material, Tutorial, Baseline Errors, orbital errors, Spaceborne SAR, Teaching.
    Abstract: Satellite-based interferometric synthetic aperture radar (InSAR) provides a synoptic high spatial resolution perspective of Earth's deforming surface, permitting one to view large areas quickly and efficiently. We review basic InSAR theory for geodetic applications and attempt to provide an overview of what processing and analysis schemes are currently used and a glimpse of what the future may hold. As part of this discussion, we present a biased view of what constitutes best practices for use of InSAR observations in geodetic modeling. Finally, we provide a basic primer on the ties between different mission design parameters and their relationship to the character of the resulting observations.
    [bibtex-key = simonsRosenBookSection2007InterferometricSARGeodesy] [bibtex-entry]


  672. R. Bamler, F. Meyer, and W. Liebhart. Processing of Bistatic SAR Data From Quasi-Stationary Configurations. IEEE Trans. Geosci. Remote Sens., 45(11):3350-3358, November 2007. Keyword(s): SAR Processing, Bistatic SAR, Earth surface, NuSAR approach, bistatic SAR data processing, curved orbit, equivalent velocity approximation, hyperbolic range function, quasistationarity restriction, quasistationary configuration, radar receiver, radar transmitter, range Doppler domain, synthetic aperture radar, time-domain post focusing, time-domain prefocusing, transfer functions, velocity vector, data acquisition, geophysical signal processing, geophysical techniques, radar signal processing, remote sensing by radar, synthetic aperture radar, transfer functions.
    Abstract: Standard synthetic aperture radar (SAR) processing algorithms use analytically derived transfer functions in the 2D frequency and range/Doppler domains. These rely on the assumption of hyperbolic range histories of monostatic SARs with straight flight paths. For bistatic SARs, the range histories are no longer hyperbolic, and simple analytic transforms do not exist. This paper offers two solutions for bistatic SAR data processing under the restriction of quasi-stationarity, i.e., sufficiently equal velocity vectors of transmitter and receiver. 1) Moderately bistatic configurations can be handled satisfactorily by using hyperbolic range functions with a modified velocity parameter, which is a solution already well known for the accommodation of curved orbits in the monostatic case. This equivalent velocity approach is shown to be of surprising range of validity even for pronounced bistatic situations. It is not to be confused with the equivalent monostatic flight path approximation, which is shown to be inapplicable for any practical case. 2) With increasing separation of transmitter and receiver, the equivalent velocity approximation deteriorates. To cope with extreme bistatic configurations, a general approach named NuSAR is proposed, where the involved transfer functions are replaced by numerically computed ones. This paper describes how the transfer functions are computed from the given orbits and the shape of the Earth surface. In any of these two cases, the bistatic SAR data can be processed by standard SAR processors; only the conventional transfer functions need to be replaced. Neither are there time-domain prefocusing or post focusing steps required nor complicated mathematical expansions involved. The presented algorithms are also applicable to very high resolution wide-swath (or squinted) SARs on curved orbits.
    [bibtex-key = BamlerMeyerLiebhart2007:BiStaticNumericSAR] [bibtex-entry]


  673. C. Castillo-Rubio, S. Llorente-Romano, and M. Burgos-Garcia. Robust SVA method for every sampling rate condition. IEEE Transactions on Aerospace and Electronic Systems, 43(2):571 -580, April 2007. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, SVA, SAR system, finite signal sampling rate condition, nonlinear filtering, robust SVA method, sidelobe levels signal resolution, synthetic aperture radar, target detection capability, two-dimensional generalization, variant filter, image resolution.
    Abstract: Linear apodization, or data weighting, is the traditional procedure to improve sidelobe levels in a finite sampled signal at the expense of resolution. New apodization methods, such as spatially variant apodization (SVA), apply nonlinear filtering to the signal in order to completely remove sidelobes without any loss of resolution. However, the results are strongly influenced by signal sampling rate. Some variations which improve results have been previously published, but sidelobe cancellation gets worse since sampling frequency is not settled at Nyquist (or a multiple). This paper presents a new and efficient technique based on SVA that drastically reduces sidelobe levels for every sampling rate condition. The algorithm is, essentially, a parameter optimization of a variant filter for each pixel of the image. A one-dimensional case and a two-dimensional generalization are presented, as well as some applications to target detection capability in a synthetic aperture radar (SAR) system.
    [bibtex-key = castilloRubioLlorenteRomanoBurgosGarcia2007:SpatiallyVariantApodization] [bibtex-entry]


  674. C.F. Castillo-Rubio, S. Llorente-Romano, and C.M. Burgos-Garcia. Spatially Variant Apodization for Squinted Synthetic Aperture Radar Images. IEEE Transactions on Image Processing, 16(8):2023-2027, August 2007. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, Nyquist rate, bidimensional finite impulse response filter, nonlinear sidelobe reduction technique, spatially variant apodization, squinted synthetic aperture radar image, synthetic aperture radar, FIR filters, radar imaging, synthetic aperture radar, Algorithms, Image Enhancement, Image Interpretation, Computer-Assisted, Information Storage and Retrieval, Radar, Reproducibility of Results, Sensitivity and Specificity.
    Abstract: Spatially variant apodization (SVA) is a nonlinear sidelobe reduction technique that improves sidelobe level and preserves resolution at the same time. This method implements a bidimensional finite impulse response filter with adaptive taps depending on image information. Some papers that have been previously published analyze SVA at the Nyquist rate or at higher rates focused on strip synthetic aperture radar (SAR). This paper shows that traditional SVA techniques are useless when the sensor operates with a squint angle. The reasons for this behaviour are analyzed, and a new implementation that largely improves the results is presented. The algorithm is applied to simulated SAR images in order to demonstrate the good quality achieved along with efficient computation.
    [bibtex-key = castilloRubioLlorenteRomanoBurgosGarcia2007TIP:SpatiallyVariantApodization] [bibtex-entry]


  675. Shane R. Cloude. Dual-Baseline Coherence Tomography. IEEE Geoscience and Remote Sensing Letters, 4(1):127-131, January 2007. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar applications, singular value decomposition, vegetation, EMSL, European Microwave Signature Laboratory, dual-baseline coherence tomography, maize plants, matrix singular value decomposition, numerical stability, regularization technique, scattering mechanisms, tomograms.
    Abstract: In this letter, we consider a dual-baseline formulation of coherence tomography and show how practical application of the method is limited by numerical stability. To help reduce this, we propose a regularization technique based on a matrix singular value decomposition to stabilize the inversion. We then apply the new dual-baseline algorithm to ground-based radar data from the European Microwave Signature Laboratory. We consider a sample of maize plants and employ dual-baseline interferometric data to reconstruct vertical tomograms through the vegetation as a function of frequency. We use these reconstructions to interpret the primary scattering mechanisms and their polarization dependence.
    [bibtex-key = cloude2007:DualBaselineCoherentTomo] [bibtex-entry]


  676. Ian G. Cumming and S. Li. Adding Sensitivity to the MLBF Doppler Centroid Estimator. IEEE Transactions on Geoscience and Remote Sensing, 45(2):279-292, Feb. 2007. Keyword(s): SAR Processing, Multilook Beat Frequency, MLPF, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Doppler radar, fast Fourier transforms, frequency estimation, radar signal processing, remote sensing by radar, sensitivity, synthetic aperture radarDoppler ambiguity, Doppler centroid estimator, FFT, Fourier transform, MLBF algorithm, RADARSAT-1, ScanSAR, cross beating, estimation sensitivity, multilook beat frequency algorithm, range migration, target trajectory.
    Abstract: The multilook beat frequency (MLBF) algorithm is the Doppler centroid estimator most commonly used in practice to solve the Doppler ambiguity. However, it still makes errors, notably in medium- or low-contrast scenes. In this paper, we present two ways in which the estimation sensitivity of the MLBF algorithm can be improved. First, we give a more thorough frequency-domain explanation of how the MLBF algorithm works and explain how cross beating and range migration cause estimation difficulties. The first improvement to the algorithm replaces the fast Fourier transform (FFT)-based beat frequency estimator with a more accurate one that uses phase increments. It avoids the FFT limitations of resolution and quantization, especially when the signal is discontinuous in one range cell due to range cell migration or burst mode operation (ScanSAR). A second improvement uses range cell migration correction to straighten the target trajectories before the beat frequency estimator is applied. This has the effect of narrowing the bandwidth of the beat signal and reducing the effect of cross beating. Finally, experiments with RADARSAT-1 data are used to illustrate the improved estimation accuracy of the modified algorithm
    [bibtex-key = cummingLi2007:MLBFExtended] [bibtex-entry]


  677. Davide D'Aria and Andrea Monti-Guarnieri. High-Resolution Spaceborne SAR Focusing by SVD-Stolt. IEEE Geoscience and Remote Sensing Letters, 4(4):639-643, October 2007. Keyword(s): Stolt interpolation, geophysical application, high-resolution spaceborne SAR focusing, microwave SAR, motion compensation, orbit curvature, singular value decomposition-Stolt approach, synthetic aperture radar, focusing, geophysical techniques, interpolation, motion compensation, radar resolution, remote sensing by radar, singular value decomposition, spaceborne radar, synthetic aperture radar.
    Abstract: In spaceborne synthetic aperture radars (SARs), the orbit curvature may prevent the use of the processor, causing artifacts that depend on both the extent of the orbit arc and the slant range interval. A viable solution has been derived by extending the singular value decomposition-Stolt approach that was proposed for geophysical applications to microwave SAR. The resulting processor has the same simple scheme as the approach but a different (numerical) computation of both the reference and the Stolt interpolation.
    [bibtex-key = DAriaGuarnieri2007] [bibtex-entry]


  678. A. Ferretti, G. Savio, R. Barzaghi, A. Borghi, S. Musazzi, F. Novali, C. Prati, and F. Rocca. Submillimeter Accuracy of InSAR Time Series: Experimental Validation. IEEE Trans. Geosci. Remote Sens., 45(5):1142-1153, May 2007. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Validation, reflector, deformation, displacement, ground motion, Envisat acquisition, GPS data, InSAR time series, Radarsat acquisition, dihedral reflectors, interferometric synthetic aperture radar, submillimeter accuracy, radar interferometry, remote sensing by radar, synthetic aperture radar, time series.
    Abstract: This paper presents the results of a blind experiment that is performed using two pairs of dihedral reflectors. The aim of the experiment was to demonstrate that interferometric synthetic aperture radar (InSAR) measurements can indeed allow a displacement time series estimation with submillimeter accuracy (both in horizontal and vertical directions), provided that the data are properly processed and the impact of in situ as well as atmospheric effects is minimized. One pair of dihedral reflectors was moved a few millimeters between SAR acquisitions, in the vertical and east-west (EW) directions, and the ground truth was compared with the InSAR data. The experiment was designed to allow a multiplatform and multigeometry analysis, i.e., each reflector was carefully pointed in order to be visible in both Envisat and Radarsat acquisitions. Moreover, two pairs of reflectors were used to allow the combination of data gathered along ascending and descending orbits. The standard deviation of the error is 0.75 mm in the vertical direction and 0.58 mm in the horizontal (EW) direction. GPS data were also collected during this experiment in order to cross-check the SAR results
    [bibtex-key = ferrettiSavioBarzaghiBorghiMusazziNovaliPratiRocca2007] [bibtex-entry]


  679. Jeffrey A. Fessler. On NUFFT-based gridding for non-Cartesian MRI. Journal of Magnetic Resonance, 188(2):191-195, 2007. Keyword(s): MRI imaging, NUFFT, Spiral trajectory, Non-Cartesian sampling, Gridding.
    Abstract: For MRI with non-Cartesian sampling, the conventional approach to reconstructing images is to use the gridding method with a Kaiser-Bessel (KB) interpolation kernel. Recently, Sha et al. [L. Sha, H. Guo, A.W. Song, An improved gridding method for spiral MRI using nonuniform fast Fourier transform, J. Magn. Reson. 162(2) (2003)] proposed an alternative method based on a nonuniform FFT (NUFFT) with least-squares (LS) design of the interpolation coefficients. They described this LS_NUFFT method as shift variant and reported that it yielded smaller reconstruction approximation errors than the conventional shift-invariant KB approach. This paper analyzes the LS_NUFFT approach in detail. We show that when one accounts for a certain linear phase factor, the core of the LS_NUFFT interpolator is in fact real and shift invariant. Furthermore, we find that the KB approach yields smaller errors than the original LS_NUFFT approach. We show that optimizing certain scaling factors can lead to a somewhat improved LS_NUFFT approach, but the high computation cost seems to outweigh the modest reduction in reconstruction error. We conclude that the standard KB approach, with appropriate parameters as described in the literature, remains the practical method of choice for gridding reconstruction in MRI.
    [bibtex-key = fesslerJMR2007NUFFT] [bibtex-entry]


  680. Andrew Hooper, P Segall, and Howard Zebker. Persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis, with application to Volcán Alcedo, Galápagos. Journal of Geophysical Research: Solid Earth, 112(B7), 2007. [bibtex-key = hooperSegallZebkerJGR2007PSI] [bibtex-entry]


  681. G. Krieger, A. Moreira, H. Fiedler, I. Hajnsek, M. Werner, M. Younis, and M. Zink. TanDEM-X: A Satellite Formation for High-Resolution SAR Interferometry. IEEE Trans. Geosci. Remote Sens., 45(11):3317-3341, November 2007. Keyword(s): HRTI-3 specification, TanDEM-X mission concept, TerraSAR-X add-on for Digital Elevation Measurement, TerraSAR-X radar satellites, bistatic data acquisition, global digital elevation model, high-resolution SAR interferometry, radar applications, radar imaging techniques, remote sensing applications, satellite formation flying concept, spaceborne radar interferometer, synchonization, synthetic aperture radar, data acquisition, digital elevation models, geophysical techniques, radar imaging, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: TerraSAR-X, TanDEM-X ,TanDEM-X (TerraSAR-X add-on for digital elevation measurements) is an innovative spaceborne radar interferometer that is based on two TerraSAR-X radar satellites flying in close formation. The primary objective of the TanDEM-X mission is the generation of a consistent global digital elevation model (DEM) with an unprecedented accuracy, which is equaling or surpassing the HRTI-3 specification. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new radar imaging techniques and applications. This paper gives a detailed overview of the TanDEM-X mission concept which is based on the systematic combination of several innovative technologies. The key elements are the bistatic data acquisition employing an innovative phase synchronization link, a novel satellite formation flying concept allowing for the collection of bistatic data with short along-track baselines, as well as the use of new interferometric modes for system verification and DEM calibration. The interferometric performance is analyzed in detail, taking into account the peculiarities of the bistatic operation. Based on this analysis, an optimized DEM data acquisition plan is derived which employs the combination of multiple data takes with different baselines. Finally, a collection of instructive examples illustrates the capabilities of TanDEM-X for the development and demonstration of new remote sensing applications.
    [bibtex-key = kriegerMoreiraFiedlerHajnsekWernerYounisZink2007:TandemXInSAR] [bibtex-entry]


  682. Riccardo Lanari, Francesco Casu, Mariarosaria Manzo, and Paul Lundgren. Application of the SBAS-DInSAR technique to fault creep: A case study of the Hayward fault, California. Remote Sensing of Environment, 109(1):20-28, 2007. [bibtex-key = lanariCasuManzoLundgrenRSE2007SBASDInSAR] [bibtex-entry]


  683. Riccardo Lanari, Francesco Casu, Mariarosaria Manzo, Giovanni Zeni, Paolo Berardino, Michele Manunta, and Antonio Pepe. An Overview of the Small BAseline Subset Algorithm: a DInSAR Technique for Surface Deformation Analysis. Pure and Applied Geophysics, 164(4):637-661, April 2007.
    Abstract: We present an overview of the Differential SAR Interferometry algorithm referred to as Small BAseline Subset (SBAS) technique, that allows us to detect surface deformation and to analyze their space-time characteristics. Following the description of the main theoretical aspects of the algorithm, we present several results obtained by applying the SBAS approach in selected case studies relevant to phenomena affecting volcanic areas (Campi Flegrei caldera and Somma-Vesuvio complex, Italy), aquifers (Santa Clara area within the San Francisco bay, California) and landslides (Maratea Valley, Italy). The overall analysis is carried out by using multilook DInSAR interferograms with a spatial resolution of the order of 100 { exttimes} 100 m, computed from SAR data acquired by the ERS-1 and ERS-2 sensors. In particular, we highlight the peculiarities of the SBAS technique and its surface deformation retrieval capability for what concerns both large-scale deformation phenomena and more localized displacement effects.
    [bibtex-key = lanariEtAlPureAppliedGeophysics2007SBASOverviewArticle] [bibtex-entry]


  684. R. Linnehan, D. Brady, J. Schindler, L. Perlovsky, and M. Rangaswamy. On the design of SAR apertures using the Cramer-Rao bound. Aerospace and Electronic Systems, IEEE Transactions on, 43(1):344 -355, january 2007. Keyword(s): Cramer-Rao bound, SAR, likelihood function, scatterer parameter vector, synthetic aperture radar, military radar, synthetic aperture radar;.
    Abstract: The Cramer-Rao inequality is applied to the likelihood function of the synthetic aperture radar (SAR) scatterer parameter vector to relate the choice of flight path to estimation performance. Estimation error bounds for the scatterer parameter vector (including height) are developed for multi-dimensional synthetic apertures, and quantify the performance enhancement over a limited sector of the image plane relative to standard-aperture single-pass SAR missions. An efficient means for the design and analysis of SAR waveforms and flight paths is proposed using simulated scattering models that are limited in size. Comparison of the error bounds to those for standard-aperture SAR show that estimates of scatterer range and cross-range positions are accurate for multi-dimensional aperture SAR, even with the additional estimator for height. Furthermore, multi-dimensional SAR is shown to address the layover problem
    [bibtex-key = 4194776] [bibtex-entry]


  685. R. M. C. Lopes, K. L. Mitchell, E. R. Stofan, J. I. Lunine, R. Lorenz, F. Paganelli, R. L. Kirk, C. A. Wood, S. D. Wall, L. E. Robshaw, A. D. Fortes, C. D. Neish, J. Radebaugh, E. Reffet, S. J. Ostro, Charles Elachi, M. D. Allison, Y. Anderson, R. Boehmer, G. Boubin, P. Callahan, P. Encrenaz, E. Flamini, G. Francescetti, Y. Gim, G. Hamilton, Scott Hensley, M. A. Janssen, W. T. K. Johnson, K. Kelleher, D.O. Muhleman, G. Ori, R. Orosei, G. Picardi, F. Posa, L.E. Roth, R. Seu, S. Shaffer, L.A. Soderblom, B. Stiles, S. Vetrella, R.D. West, L. Wye, and Howard A. Zebker. Cryovolcanic features on Titan's surface as revealed by the Cassini Titan Radar Mapper. Icarus, 186(2):395-412, 2007. Keyword(s): Cassini, Cassini Radar, Titan, Volcanism, Satellites of Saturn.
    Abstract: The Cassini Titan Radar Mapper obtained Synthetic Aperture Radar images of Titan's surface during four fly-bys during the mission's first year. These images show that Titan's surface is very complex geologically, showing evidence of major planetary geologic processes, including cryovolcanism. This paper discusses the variety of cryovolcanic features identified from SAR images, their possible origin, and their geologic context. The features which we identify as cryovolcanic in origin include a large (180 km diameter) volcanic construct (dome or shield), several extensive flows, and three calderas which appear to be the source of flows. The composition of the cryomagma on Titan is still unknown, but constraints on rheological properties can be estimated using flow thickness. Rheological properties of one flow were estimated and appear inconsistent with ammonia-water slurries, and possibly more consistent with ammonia-water-methanol slurries. The extent of cryovolcanism on Titan is still not known, as only a small fraction of the surface has been imaged at sufficient resolution. Energetic considerations suggest that cryovolcanism may have been a dominant process in the resurfacing of Titan.
    [bibtex-key = lopesEtAlIcarus2007CassiniRadarTitanMapper] [bibtex-entry]


  686. Alexander Löw and Wolfram Mauser. Generation of geometrically and radiometrically terrain corrected SAR image products. Remote Sensing of Environment, 106(3):337-349, February 2007. Keyword(s): SAR Processing, Radiometric Calibration, Calibration, Image enhancement, Image processing, Synthetic aperture radar, Terrain mapping, SAR geocoding, Radiometric normalization.
    Abstract: Terrain undulations affect the geometric and radiometric quality of synthetic aperture radar images. The correction of these effects becomes indispensable when quantitative image analysis is performed with respect to the derivation of geo- and biophysical parameters. The paper presents a rigorous approach for geometric and radiometric correction of SAR images. Using a digital elevation model, the imaging geometry is reconstructed and is used to perform geometric and radiometric correction of terrain induced distortions. The importance of a stringent radiometric correction based on the integration of the image brightness is emphasized. The approach guarantees that the energy contained in the image data is preserved throughout the geocoding process. The resulting backscattering images are fully terrain corrected and can be used for further quantitative investigations and may also improve qualitative studies as e.g. land cover classifications. The technique is applicable for different sensor types and image products, including already geocoded SAR images. The effect of different resolutions of digital elevation models used for the correction of the backscattering coefficient is investigated.
    [bibtex-key = LoewMauser2007:GeoRadiometricCorr] [bibtex-entry]


  687. Jean-Michel Martinez and Thuy Le Toan. Mapping of flood dynamics and spatial distribution of vegetation in the Amazon floodplain using multitemporal SAR data. Remote Sensing of Environment, 108(3):209 - 223, 2007. Keyword(s): Wetland, Floodplain, Vegetation, Flood, Temporal dynamic, Radar, Classification.
    Abstract: This paper presents the use of time series of SAR images to map the flood temporal dynamics and the spatial distribution of vegetation over a large Amazonian floodplain. The region under study (3500 km2) presents a diversity of landscape units with open lakes, bogs, large meadows, savannahs, alluvial forests and terra firma forest, covered by 21 images acquired by J-ERS between 1993 and 1997. Ground data include in situ observations of vegetation structure and flood extent as well as water level records. Image analysis demonstrates that temporal variations of the radar backscatter can be used to monitor efficiently the flood extent regardless of the landscape units. Also, analysis of the backscatter temporal variation greatly reduces the confusion between smooth surfaces (e.g. open water bodies, bare soils) inherent to L-band backscatter. The mapping method is based on decision rules over two decision variables: 1) the mean backscatter coefficient computed over the whole time series; 2) the total change computed using an Absolute Change estimator. The first variable provides classification into rough vegetation types while the second variable yields a direct estimate of the intensity of change that is related to flood dynamics. The classifier is first applied to the whole time series to map the maximum and minimum flood extent by defining 3 flood conditions: never flooded (NF); occasionally flooded (OF); permanently flooded (PF). It also furnishes the broad land cover type: open water/bare soils/low vegetation/forest. The accuracy of the flood extent mapping shows a kappa value of 0.82. Then, the classifier is run iteratively on the OF pixels to monitor flood stages during which the occasionally flooded areas get submerged. The mapping accuracy is assessed on one intermediate flood stage, showing a precision in excess of 90%. The importance of the time sampling for flood mapping is discussed along with the influence of SAR backscatter accuracy and the number of images. Then general guidelines for floodplain mapping are presented. By combining water level reports with maps of different flood stages the flooding pattern can be retrieved along with the vegetation succession processes. It is shown that the spatial distribution of vegetation communities is governed by flood stress and can be modelled as a function of the mean annual exposure to floods.
    [bibtex-key = martinezLeToanRSE2007FloodMappingUsingMultitemporalSARData] [bibtex-entry]


  688. Adriano Meta, Peter Hoogeboom, and L. P. Ligthart. Signal Processing for FMCW SAR. IEEE Transactions on Geoscience and Remote Sensing, 45(11):3519-3532, November 2007. Keyword(s): SAR Processing, SAR Focusing, Azimuth Focusing, FMCW, FMCW SAR, geophysical signal processing, geophysical techniques, image sensors, radar imaging, synthetic aperture radar, Delft University of Technology, FMCW technique, Netherlands, SAR technique, conventional stop-and-go approximation, data collection, frequency-modulated continuous-wave technology, high resolution image sensor, pulse-radar algorithms, signal model, signal processing, signal transmission, synthetic aperture radar, Frequency, High-resolution imaging, Image resolution, Image sensors, Limiting, Radar signal processing, Signal processing, Signal processing algorithms, Signal resolution, Synthetic aperture radar, Doppler frequency correction, frequency-modulated continuous wave (FMCW), nonlinearity correction, synthetic aperture radar (SAR).
    Abstract: The combination of frequency-modulated continuous-wave (FMCW) technology and synthetic aperture radar (SAR) techniques leads to lightweight cost-effective imaging sensors of high resolution. One limiting factor to the use of FMCW sensors is the well-known presence of nonlinearities in the transmitted signal. This results in contrast- and range-resolution degradation, particularly when the system is intended for high-resolution long-range applications, as it is the case for SAR. This paper presents a novel processing solution, which solves the nonlinearity problem for the whole range profile. Additionally, the conventional stop-and-go approximation used in pulse-radar algorithms is not valid in FMCW SAR applications under certain circumstances. Therefore, the motion within the sweep needs to be taken into account. Analytical development of the FMCW SAR signal model, starting from the deramped signal and without using the stop-and-go approximation, is presented in this paper. The model is then applied to stripmap, spotlight, and single-transmitter/multiple-receiver digital-beamforming SAR operational mode. The proposed algorithms are verified by processing real FMCW SAR data collected with the demonstrator system built at the Delft University of Technology.
    [bibtex-key = metaHoogeboomLigthartTGRS2007FMCWSARFocusing] [bibtex-entry]


  689. Andrea Monti-Guarnieri and Stefano Tebaldini. Hybrid Cramer-Rao Bounds for Crustal Displacement Field Estimators in SAR Interferometry. IEEE Signal Processing Letters, 14(12):1012-1015, December 2007. Keyword(s): SAR Processing, APS variance, SAR interferometry, atmospheric phase screen, crustal displacement field estimator, deterministic parameter, error analysis, hybrid Cramer-Rao bounds, line-of-sight crustal deformation, radar signal analysis, spaceborne synthetic aperture radar interferometry, stochastic variable, system configuration, target decorrelation, decorrelation, error analysis, interferometry, radar signal processing, radar tracking, spaceborne radar, stochastic processes, synthetic aperture radar;.
    Abstract: This letter focuses on the performance achievable by spaceborne synthetic aperture radar interferometry (InSAR) in the estimation of line-of-sight crustal deformations from acquisitions over a distributed scatterer. Our model is suited for exploiting the hybrid Cramer-Rao bound (HCRB), where the unknowns are both deterministic parameters and stochastic variables. We take into account both target decorrelation and atmospheric phase screen (APS). This approach leads to a viable evaluation of InSAR performance as a function of system configuration, target decorrelation, and APS variance.
    [bibtex-key = montiGuarnieriTebaldini2007] [bibtex-entry]


  690. Robert. L. Morrison, Minh N. Do, and David C. Munson. SAR Image Autofocus By Sharpness Optimization: A Theoretical Study. IEEE Transactions on Image Processing, 16(9):2309-2321, September 2007. Keyword(s): SAR Processing, Autofocus, SAR image autofocus, intensity-squared metric, point-targets image, sharpness optimization, synthetic aperture radar, image processing, optimisation, synthetic aperture radar, Algorithms, Artificial Intelligence, Computer Simulation, Image Enhancement, Image Interpretation, Computer-Assisted, Models, Theoretical, Pattern Recognition, Automated, Reproducibility of Results, Sensitivity and Specificity;.
    Abstract: Synthetic aperture radar (SAR) autofocus techniques that optimize sharpness metrics can produce excellent restorations in comparison with conventional autofocus approaches. To help formalize the understanding of metric-based SAR autofocus methods, and to gain more insight into their performance, we present a theoretical analysis of these techniques using simple image models. Specifically, we consider the intensity-squared metric, and a dominant point-targets image model, and derive expressions for the resulting objective function. We examine the conditions under which the perfectly focused image models correspond to stationary points of the objective function. A key contribution is that we demonstrate formally, for the specific case of intensity-squared minimization autofocus, the mechanism by which metric-based methods utilize the multichannel defocusing model of SAR autofocus to enforce the stationary point property for multiple image columns. Furthermore, our analysis shows that the objective function has a special separble property through which it can be well approximated locally by a sum of 1-D functions of each phase error component. This allows fast performance through solving a sequence of 1-D optimization problems for each phase component simultaneously. Simulation results using the proposed models and actual SAR imagery confirm that the analysis extends well to realistic situations.
    [bibtex-key = morrisonDoMunsonTIP2007SARAutofocusSharpnessOptimization] [bibtex-entry]


  691. Ross F. Nelson, Peter Hyde, Patrick Johnson, Bomono Emessiene, Marc L. Imhoff, Robert Campbell, and Wilson Edwards. Investigating RaDAR-LiDAR synergy in a North Carolina pine forest. Remote Sensing of Environment, 110(1):98-108, September 2007. Keyword(s): SAR Processing, Biomass, Forest, VHFRaDAR, profiling LiDAR, biomass, RaDAR-LiDAR synergy, VHFSAR DATA, SMALL-FOOTPRINT LIDAR, AIRBORNE LASER DATA, STEM VOLUME, STAND CHARACTERISTICS, AERIAL-PHOTOGRAPHY, VEGETATION BIOMASS, CONIFEROUS FOREST, BOREAL FORESTS, SCANNER DATA.
    Abstract: A low frequency (80-120 MHz) VHF RaDAR, BioSAR, specifically designed for forest biomass estimation and a profiling LiDAR, PALS, were flown over loblolly pine plantations in the southeastern United States. LiDAR-only, RaDAR-only, and joint LiDAR-RaDAR linear models were developed to determine if returns from two sensors could be used to estimate pine biomass more accurately and precisely than returns from either sensor alone. The best five-variable RaDAR model explained 81.8\% (R (2)) of the stem green biomass variability, with a regression RMSE of 57.5 t/ha. The best one-variable LiDAR model explained 93.3\% of the biomass variation (RMSE=33.9 t/ha). Combining the RaDAR normalized volumetric returns with the profiling LiDAR ranging measurements did little to improve the best LiDAR-only model. The best LiDAR-RaDAR model explained 93.8\% of the biomass variation (RSME=32.7 t/ha). Cross-validation and training/test validation procedures demonstrated (1) that all models are unbiased and (2) the increased precision of the LiDAR-only and LiDAR-RaDAR models. The results of this investigation and a companion study indicate that there is little to be gained combining VHF-RaDAR volumetric returns and profiling LiDAR ranging measurements in pine forests; a LiDAR ranging system is sufficient for accurate, precise biomass estimation.
    [bibtex-key = nelsonHydeJohnsonEmessieneImhoffCampbellEdwards2007:RaDARLiDARSynergy] [bibtex-entry]


  692. Jean-Marie Nicolas, Gabriel Vasile, Michel Gay, Florence Tupin, and Emmanuel Trouvé. SAR processing in the temporal domain: application to direct interferogram generation and mountain glacier monitoring. Canadian Journal of Remote Sensing, 33(1):52-59, 2007. Keyword(s): SAR Processing, Time-Domain Back-Projection, InSAR, Interferometry, SAR Interferometry, Glacier Displacement Measurement, ERS, Spaceborne SAR.
    Abstract: Synthetic aperture radar (SAR) interferometry has the potential to measure temperate glacier displacement with a large coverage of the surface compared with pointwise terrestrial ground measurements. The significant topographic relief in mountainous areas, however, where most alpine glaciers are located, makes the use of SAR imagery rather difficult. Among the difficulties, when the resolution increases, the focusing of satellite SAR images, usually performed in the frequency domain with a constant-height hypothesis, becomes a critical issue. SAR processing in the temporal domain is a different approach that enables the use of information such as the local topography. In this paper, we present an original method to perform this temporal domain focusing by modeling the relative motion of the satellite and Earth points. The method allows production of SAR single look complex (SLC) images directly in ground geometry and reduces the need for resampling and phase correction to obtain interferograms. A tandem pair of European remote sensing (ERS) SAR images acquired over the Mont-Blanc area is used to illustrate the proposed approach. The results are presented with amplitude images and interferograms measuring glacier 1 day displacements and are compared with the results from the differential interferometric automated process applied to survey of nature (DIAPASON) and repeated orbit interferometry package (ROI-PAC) conventional SAR processors.
    [bibtex-key = NicolasVasileGayTupinTrouve2007:TDBPInSAR] [bibtex-entry]


  693. D. Pastina, F. Colone, and P. Lombardo. Effect of Apodization on SAR Image Understanding. IEEE Transactions on Geoscience and Remote Sensing, 45(11):3533-3551, November 2007. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, SVA, SAR images, apodization effect, homogeneous regions, main lobe resolution preservation, pixel complex amplitude, probability density function, radar cross sections, sidelobe level reduction, statistical moments, statistical properties, supervised classification, synthetic aperture radar images, geophysical techniques, image classification, radar cross-sections, synthetic aperture radar.
    Abstract: This paper investigates the effect of apodization on the statistical properties of synthetic aperture radar (SAR) images and its impact on the capability of extracting information from homogeneous regions of apodized SAR images. The statistical model for the pixel complex amplitude of the apodized image is derived in terms of both probability density function and statistical moments. Knowledge of the statistical properties is then used to develop appropriate schemes for parameter estimation and supervised classification of homogeneous regions with different radar cross sections in apodized SAR images. The performance analysis shows that the new techniques (properly derived for the apodized case) provide information extraction capabilities only slightly worse than those provided by the conventional techniques applied to the nonapodized case. This allows us to conclude that the use of nonlinear apodization yields sidelobe level reduction and main lobe resolution preservation that can be traded with the small losses above. A full characterization of the estimation and classification performance of these new techniques shows that nonlinear apodization globally introduces a performance degradation comparable to a reduction of the number of looks of a factor of 1.455 for a homogeneous region.
    [bibtex-key = pastinaColoneLombardo2007:Apodization] [bibtex-entry]


  694. D. Perissin and A. Ferretti. Urban-Target Recognition by Means of Repeated Spaceborne SAR Images. IEEE Trans. Geosci. Remote Sens., 45(12):4043 -4058, December 2007. Keyword(s): SAR Processing, Doppler radar, Image recognition, Image resolution, Image sensors, Polarization, Radar imaging, Radar scattering, Spaceborne radar, Synthetic aperture radar, Target recognition, airborne radar, geophysical signal processing, geophysical techniques, object recognition, radar interferometry, radar polarimetry, synthetic aperture radar, Doppler centroid acquisition geometry, automatic scattering structure recognition, elevated backscatterers, extended resonating targets, ground level backscatterers, normal baseline acquisition geometry, radar signal amplitude, radar signal polarization diversity, repeated spaceborne SAR images, resonating dihedrals, resonating poles, resonating trihedrals, simple dihedrals, simple poles, simple trihedrals, spaceborne C band SAR, synthetic aperture radar, urban target recognition, Interferometry, polarimetry, radar target recognition, synthetic aperture radar (SAR), urban areas;.
    Abstract: The relative low resolution (~25 m times 5 m on the ground) of spaceborne C-band synthetic aperture radar (SAR) data as acquired, for example, by European Space Agency sensors ERS and Envisat, can be significantly increased (up to submeter precisions) by processing coherently long series of images. Moreover, by analyzing the amplitude of the radar signal and by exploiting polarization diversity, the main radar characteristics of urban targets can be estimated, and a system for automatic recognition of a set of scattering structures can be developed. In this paper, we study the variation of the amplitude of the received radar signal as a function of the acquisition geometry [normal baseline and Doppler centroid (DC)] to retrieve the extension of the targets in range and azimuth. The dependence of the radar amplitude on temperature at the time of acquisition has been discovered to be very useful to identify extended resonating targets. Dihedrals are discriminated from specular or trihedral reflectors through the phase of Envisat alternating polarization (AP) acquisitions. By means of all gathered radar measurements, the bases for the development of a system for the automatic recognition of six main typologies of urban SAR targets (ground-level and elevated backscatterers, simple and resonating dihedrals, poles and trihedrals) have been laid. Radar data are then combined with in situ surveys and aerial photos, allowing a first assessment of the methodology in urban area.
    [bibtex-key = perissinFerrettiTGRS2007] [bibtex-entry]


  695. P. Prats, Karlus A. Câmara de Macedo, A. Reigber, R. Scheiber, and J. J. Mallorqui. Comparison of Topography- and Aperture-Dependent Motion Compensation Algorithms for Airborne SAR. IEEE Geosci. Remote Sens. Lett., 4(3):349-353, 2007. Keyword(s): SAR Processing, Motion Compensation, PTA-Algorithm, Precise Topography- and Aperture-Dependent (PTA) Algorithm, SATA, Subaperture Topography- and Aperture-dependent (SATA) Algorithm, Frequency Division (FD) Algorithm, Topography-Based Motion Compensation, ESAR, L-Band Calibration, image registration, interferometry, motion compensation (MoCo), synthetic aperture radar (SAR), Comparison, Comparsion of Motion Compensation Algorithms, InSAR, Airborne SAR.
    Abstract: This letter presents a comparison between three Fourier-based motion compensation (MoCo) algorithms for airborne synthetic aperture radar (SAR) systems. These algorithms circumvent the limitations of conventional MoCo, namely the assumption of a reference height and the beam-center approximation. All these approaches rely on the inherent time-frequency relation in SAR systems but exploit it differently, with the consequent differences in accuracy and computational burden. After a brief overview of the three approaches, the performance of each algorithm is analyzed with respect to azimuthal topography accommodation, angle accommodation, and maximum frequency of track deviations with which the algorithm can cope. Also, an analysis on the computational complexity is presented. Quantitative results are shown using real data acquired by the Experimental SAR system of the German Aerospace Center (DLR).
    [bibtex-key = PratsMacedoReigberScheiberMallorqui2007:MoComp] [bibtex-entry]


  696. Fabio Rocca. Modeling Interferogram Stacks. IEEE Trans. Geosci. Remote Sens., 45(10):3289-3299, October 2007. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, D-InSAR, InSAR, SAR Interferometry, Interferometry, Decorrelation, Temporal Decorrelation, C-band measurement, DInSAR, ERS-1 data, Italy, Rome, agricultural areas, differential interferometric SAR, distributed targets, geometrical decorrelation, interferogram stack modeling, permanent scatterers, progressive ground motion, progressively decorrelating targets, sinusoidal ground motion, synthetic aperture radar interferometry, temporal decorrelation, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: Synthetic aperture radar interferometry is limited by temporal and geometrical decorrelation. Permanent scatterers (PSs) are helpful to overcome these problems, but their density in agricultural and out-of-town areas is not always sufficient. The forthcoming availability of satellite platforms with thinner orbital tubes and shorter revisit times will enhance the use of interferogram stacks, which are usable for distributed and progressively decorrelating targets, like those found in agricultural areas. To estimate the possibilities of the interferogram stack technique, a Markovian model for the temporal decorrelation is considered. ERS-1 data measured in C-band over Rome with a three-day repeat cycle are used to identify the parameters for this model, namely, the decorrelation time (estimated as 40 days) and the short-term coherence (estimated as 0.6). In the hypothesis of small deviations from a model of the motion, the optimal weights to be used to combine a sequence of interferograms taken at intervals that are shorter than the decorrelation time are calculated in the cases of progressive and sinusoidal ground motion. The dispersion of the optimal estimate of the motion is then determined. This model is extended to frequencies other than C-band. These evaluations are compared with the known results obtained for PSs. As an example, the case of a time interval between the takes of T = 12 days is considered. With N consecutive images, interferogram stack results are equivalent to PSs if the pixel count in the window used to smooth the interferograms grows with N2.
    [bibtex-key = rocca2007TGRS_PSI] [bibtex-entry]


  697. T. O. Saebo, Roy E. Hansen, and A. Hanssen. Relative Height Estimation by Cross-Correlating Ground-Range Synthetic Aperture Sonar Images. IEEE Journal of Oceanic Engineering, 32(4):971-982, October 2007. Keyword(s): Synthetic Aperture Sonar, SAS, oceanographic techniques, seafloor phenomena, sonar imaging, synthetic aperture sonar, Elba Island, InSAS-2000 experiment, Italy, SAS processing, along-track resolution, azimuth resolution, broadband signals, coherent frequency-dependent scattering, correction technique, cross correlations, geometrical decorrelation effect, image acquisition, mathematical model, moving platform, seafloor relative height, signal-to-noise ratio, sonar images beamforming, synthetic aperture sonar imaging, time-delay estimates, vertically displaced receivers, Azimuth, Coherence, Decorrelation, Frequency estimation, Mathematical model, Scattering, Sea floor, Signal resolution, Signal to noise ratio, Synthetic aperture sonar, Bathymetry, coherence, cross correlation, height estimation, interferometry, synthetic aperture sonar (SAS), time-delay estimation.
    Abstract: The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS processing improves the along-track (or azimuth) resolution, as well as the signal-to-noise ratio (SNR), which in turn improves the estimated height accuracy. We show that the shift of the effective center frequency induced by coherent, frequency-dependent scattering affect the time-delay estimates from complex cross correlations, and we propose a correction technique for broadband signals with uneven magnitude spectra. To reduce the effect of geometrical decorrelation and increase the coherence between the images, we beamform the sonar images onto an a priori estimate of the seafloor height before correlating. We develop a mathematical model for the imaging geometry. Finally, we demonstrate our proposed estimators by providing relative seafloor height estimates from real aperture and SAS images, obtained during the InSAS-2000 experiment at Elba Island in Italy. In particular, we demonstrate that the SAS image quality is significantly improved by inclusion of the height estimates as a priori information.
    [bibtex-key = saeboHansenHanssenJOE2007RelativeHeightEstimationCrossCorrelatingGroundRangeSASonar] [bibtex-entry]


  698. T. J. Schulz. Optimal Sharpness Function for SAR Autofocus. IEEE Signal Processing Letters, 14(1):27-30, January 2007. Keyword(s): error correction, error statistics, maximum likelihood estimation, radar imaging, synthetic aperture radar, image-domain sharpness function, autofocusing, phase-error correction, synthetic aperture radar imagery, SAR, optimization, statistical estimation, maximum-likelihood estimation, maximum-posterior estimation, intensity-squared sharpness, Layout, Phase estimation, Optimization methods, Synthetic aperture radar, Discrete Fourier transforms, Maximum likelihood estimation, Reflectivity, Error correction, Computer errors, Signal processing, MAP estimation, synthetic aperture radar (SAR).
    Abstract: Phase-error correction through the optimization of an image-domain sharpness function provides one method for the auto-focus of synthetic aperture radar (SAR) imagery. Several ad-hoc sharpness functions have been proposed, implemented, and analyzed, but none of these has been motivated as the solution to a well-defined statistical estimation problem. In this letter, a SAR sharpness function is derived that induces the solution to maximum-likelihood and maximum-posterior estimation for idealized SAR data. Furthermore, a limiting form of the optimal sharpness function is shown to be the popular intensity-squared sharpness
    [bibtex-key = schulzIEEESPL2007OptimalSharpnessFuntionSARAutofocus] [bibtex-entry]


  699. T. Sparr, Roy E. Hansen, H. J. Callow, and J. Groen. Enhancing target shadows in SAR images. Electronics Letters, 43(5):69-70, March 2007. Keyword(s): SAR Processing, image enhancement, radar imaging, synthetic aperture radar, SAR images, fixed focus shadow enhancement, radar image, sharp target image, synthetic aperture radar, target shadow enhancement.
    Abstract: A method called fixed focus shadow enhancement is proposed. The purpose is to sharpen target shadows in synthetic aperture radar images. The method applies a fixed focus range to a given target, resulting in a sharp target image and a sharp target shadow. Other parts of the radar image, including the background against which the shadow is seen, are smeared
    [bibtex-key = sparrHansenCallowGroenElectrLett2007EnhancingTargetShadowsInSARImages] [bibtex-entry]


  700. Z. Su, Y. Peng, and X. Wang. Feature-Independent Aperture Evaluator for the Curvilinear SAR. IEEE Geosci. Remote Sens. Lett., 4(2):191-195, 2007. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, 3D Feature Extraction, Feature Extraction, Curvilinear SAR.
    Abstract: Curvilinear synthetic aperture radar (SAR), as a more practicable 3-D SAR imaging system, utilizes parametric target feature estimates extracted from the received data to reconstruct the target image. The reconstructed image quality is then impacted by the estimation accuracy of the features. In this letter, through discussing the correlation between the system parameters and the estimation performance of the curvilinear SAR, a conclusion can be drawn on how the overall location accuracy of a target is determined by the correlation between the azimuth and elevation coordinates of the flight path, compactly characterizing the curvilinear aperture. Consequently, a new index, determined only with the aperture parameters, is proposed as an aperture evaluator, which is referred to as the feature-independent aperture evaluator (FAE). FAE can be used for guiding the operational aperture design.
    [bibtex-key = suPengWang2007:NonLinearSARTOMO] [bibtex-entry]


  701. Ridha Touzi. Target Scattering Decomposition in Terms of Roll-Invariant Target Parameters. IEEE Trans. Geosci. Remote Sens., 45(1):73-84, January 2007. Keyword(s): SAR Processing, Polarimetric Decomposition, Canada, Cloude-Pottier incoherent target decomposition, Cloude-Pottier scattering type, Kennaugh-Huynen scattering matrix condiagonalization, Ontario, Pauli basis, Ramsar Mer Bleue wetland site, asymmetric scattering, coherent target scattering, entropy, maximum polarization parameters, polarimetric Convair-580 airborne SAR, polarization basis invariant representation, roll invariant target parameters, scattering vector model, symmetric scattering magnitude, symmetric scattering phase, synthetic aperture radar, target orientation angle, target phase parameters, target scattering decomposition, S-matrix theory, airborne radar, geophysical techniques, radar theory, remote sensing by radar, synthetic aperture radar.
    Abstract: The Kennaugh-Huynen scattering matrix con-diagonalization is projected into the Pauli basis to derive a new scattering vector model for the representation of coherent target scattering. This model permits a polarization basis invariant representation of coherent target scattering in terms of five independent target parameters, the magnitude and phase of the symmetric scattering type introduced in this paper, and the maximum polarization parameters (orientation, helicity, and maximum return). The new scattering vector model served for the assessment of the Cloude-Pottier incoherent target decomposition. Whereas the Cloude-Pottier scattering type alpha and entropy H are roll invariant, beta and the so-called target-phase parameters do depend on the target orientation angle for asymmetric scattering. The scattering vector model is then used as the basis for the development of new coherent and incoherent target decompositions in terms of unique and roll-invariant target parameters. It is shown that both the phase and magnitude of the symmetric scattering type should be used for an unambiguous description of symmetric target scattering. Target helicity is required for the assessment of the symmetry-asymmetry nature of target scattering. The symmetric scattering type phase is shown to be very promising for wetland classification in particular, using polarimetric Convair-580 synthetic aperture radar data collected over the Ramsar Mer Bleue wetland site to the east of Ottawa, Ontario, Canada
    [bibtex-key = Touzi2007:PolDecomp] [bibtex-entry]


  702. Marijke Vandewal, Rainer Speck, and Helmut Süss. Efficient and Precise Processing for Squinted Spotlight SAR through a Modified Stolt Mapping. EURASIP Journal on Advances in Signal Processing, 2007:1-7, 2007.
    Abstract: Processing of squinted SAR spotlight data is a challenge because of the significant range migration effects of the raw data over the coherent aperture time. Although in theory the (omega, k)-algorithm takes care of these aspects, its digital implementation requires a time-consuming interpolation step. Moreover, the limited precision of this interpolation can introduce distortions at the edges of the final image especially for squinted geometries. A wave number domain processing using a modified Stolt mapping will be developed and analyzed to enhance the quality of the final SAR image. Additionally, the proposed algorithm has a decreased computational load compared to the original (omega,k)-algorithm. Simulation results will validate the focusing and efficiency performances of the modified wave number domain algorithm.
    [bibtex-key = vandewalSpeckSuessModifiedStoltMapping2007] [bibtex-entry]


  703. J-P Wigneron, Yann Kerr, Philippe Waldteufel, Kauzar Saleh, M-J Escorihuela, Philippe Richaume, Paolo Ferrazzoli, Patricia De Rosnay, Robert Gurney, J-C Calvet, and others. L-band microwave emission of the biosphere (L-MEB) model: Description and calibration against experimental data sets over crop fields. Remote Sensing of Environment, 107(4):639-655, 2007. [bibtex-key = Wigneron2007] [bibtex-entry]


  704. Thomas L. Ainsworth, Laurent Ferro-Famil, and Jong-Sen Lee. Orientation angle preserving a posteriori polarimetric SAR calibration. IEEE Trans. Geosci. Remote Sens., 44(4):994-1003, April 2006. Keyword(s): SAR Processing, Polarimetry, Polarimetric Calibration, calibration, radar polarimetrybackscatter, calibration, covariance matrices, geophysical techniques, radar polarimetry, remote sensing by radar, synthetic aperture radar, SAR data analysis, a posteriori polarimetric SAR calibration, anechoic chamber data, orientation angle, polarimetric channels, polarimetric covariance matrix, polarimetric distortion, polarimetric fidelity, polarimetric synthetic aperture radar, radar polarimetry, scattering reciprocity, Calibration, Costs, Covariance matrix, Data analysis, Equations, Parameter estimation, Polarimetric synthetic aperture radar, Radar scattering, Soil moisture, Synthetic aperture radar, Calibration, radar polarimetry.
    Abstract: Fully polarimetric synthetic aperture radar (SAR) data analysis has found wide application for terrain classification, land-use, soil moisture, and ground cover classification. Critical to all analyses and applications is accurate calibration of the relative amplitudes of and phases between the various polarimetric channels. Here we propose an a posteriori method imposing only the weakest of constraints, scattering reciprocity, on the polarimetric data. Calibration parameters are self-consistently estimated from full 4x4 polarimetric covariance matrices. Whilst the complete set of calibration parameters is underdetermined, we give several reasonable heuristic methods to provide a complete calibration. Stronger constraints reduce the number of independent parameters and provide an overdetermined set of equations but at a cost - the loss of polarimetric fidelity when the underlying assumptions are violated. Without recourse to in situ calibration targets, the extent of the polarimetric distortion that results from polarimetric calibration remains unknown. We apply our new method to simulated data, anechoic chamber data and polarimetric SAR imagery. We also present comparisons with alternate calibration methods and different approximate solutions of the new technique.
    [bibtex-key = ainsworthFerroFamilLeeTGRS2006PolCalibration] [bibtex-entry]


  705. Noa B. D. Bechor and Howard A. Zebker. Measuring two-dimensional movements using a single InSAR pair. Geophysical Research Letters, 33(16):L16311 (1-5), 2006. Keyword(s): SAR Processing, ionospheric techniques, radar interferometry, synthetic aperture radar, azimuth shift, interferometric SAR ionosphere correction, ionospheric geometric distortion, Interferometry, Ionosphere, L-band, Synthetic aperture radar.
    Abstract: We present a new method to extract along-track displacements from InSAR data, based on split-beam InSAR processing, to create forward- and backward-looking interferograms. The phase difference between the two modified interferograms provides the along-track displacement component. Thus, from each conventional InSAR pair we extract two components of the displacement vector: one along the line of sight, the other in the along-track direction. We analyze the precision of the new method by comparing our solution to GPS and offset-derived along-track displacements in interferograms of the 1999, Hector Mine earthquake. The RMS error between GPS displacements and our results ranges from 5 to 8.8cm. Our method is consistent with along-track displacements derived by pixel-offsets, which are limited to 12-15 cm precision. The theoretical precision of the new method depends on SNR and coherence. For a signal to noise ratio of 30 the expected precisions are 3, 11 cm for coherence of 0.8, 0.4, respectively.
    [bibtex-key = bechorZebkerGRL2006MeasuringMovementsUsingASingleInSARPair] [bibtex-entry]


  706. Karl-Heinz Bethke, S. Baumgartner, M. Gabele, D. Hounam, E. Kemptner, D. Klement, G. Krieger, and R. Erxleben. Air- and spaceborne monitoring of road traffic using SAR moving target indication--Project TRAMRAD. ISPRS Journal of Photogrammetry and Remote Sensing, Theme Issue: Airborne and Spaceborne Traffic Monitoring, 61(3-4):243-259, December 2006. Keyword(s): SAR Processing, TRAMRAD, Traffic Monitoring, TerraSAR-X, STAP, Space-Time Adaptive Processing, MTI, Moving Target Indication, SAR Interferometry, Interferometry, ATI, Along-Track SAR Interferometry.
    Abstract: To ensure mobility, future road traffic management urgently needs actual and reliable information about the road traffic over wide areas in order to keep the traffic moving and the travel time short. Nowadays, outside of motorways the actual traffic situation is almost unknown due to the lack of sensor installations. This is the background for the TRAMRAD (TrafficMonitoring with Radar) project. TRAMRAD aims at profiting from research and development in earth observation and advances in radar remote sensing techniques to define realizable future air- or spaceborne sensor systems for the wide area monitoring of road traffic. The project comprises the necessary theoretical and practical investigations, as well as experimental campaigns with airborne instruments. The paper describes the requirements for the systems and the concepts being investigated. In particular, it discusses the complex detection conditions, the requirements on the radar instrument and the methods for processing the data. Possible system concepts are described and their capabilities are discussed.
    [bibtex-key = bethkeBaumgartnerGabeleHounamKemptnerKlementKriegerErxleben:TRAMRAD] [bibtex-entry]


  707. E.J. Candes, J. Romberg, and T. Tao. Robust uncertainty principles: exact signal reconstruction from highly incomplete frequency information. IEEE Transactions on Information Theory, 52(2):489-509, feb. 2006. Keyword(s): Fourier coefficient, convex optimization, discrete-time signal, image reconstruction, incomplete frequency information, linear programming, minimization problem, nonlinear sampling theorem, piecewise constant object, probability value, robust uncertainty principle, signal reconstruction, sparse random matrix, trigonometric expansion, Fourier analysis, convex programming, image reconstruction, image sampling, indeterminancy, linear programming, minimisation, piecewise constant techniques, probability, signal reconstruction, signal sampling, sparse matrices;.
    Abstract: This paper considers the model problem of reconstructing an object from incomplete frequency samples. Consider a discrete-time signal f isin;CN and a randomly chosen set of frequencies Omega;. Is it possible to reconstruct f from the partial knowledge of its Fourier coefficients on the set Omega;? A typical result of this paper is as follows. Suppose that f is a superposition of |T| spikes f(t)= sigma; tau; isin;Tf( tau;) delta;(t- tau;) obeying |T| le;CM middot;(log N)-1 middot; | Omega;| for some constant CM>0. We do not know the locations of the spikes nor their amplitudes. Then with probability at least 1-O(N-M), f can be reconstructed exactly as the solution to the #8467;1 minimization problem. In short, exact recovery may be obtained by solving a convex optimization problem. We give numerical values for CM which depend on the desired probability of success. Our result may be interpreted as a novel kind of nonlinear sampling theorem. In effect, it says that any signal made out of |T| spikes may be recovered by convex programming from almost every set of frequencies of size O(|T| middot;logN). Moreover, this is nearly optimal in the sense that any method succeeding with probability 1-O(N-M) would in general require a number of frequency samples at least proportional to |T| middot;logN. The methodology extends to a variety of other situations and higher dimensions. For example, we show how one can reconstruct a piecewise constant (one- or two-dimensional) object from incomplete frequency samples - provided that the number of jumps (discontinuities) obeys the condition above - by minimizing other convex functionals such as the total variation of f.
    [bibtex-key = 1580791] [bibtex-entry]


  708. Emmanuel J. Candes, Justin K. Romberg, and Terence Tao. Stable signal recovery from incomplete and inaccurate measurements. Communications on Pure and Applied Mathematics, 59(8):1207-1223, 2006. Keyword(s): Compressive Sensing, Compressed Sensing.
    Abstract: Suppose we wish to recover a vector x0 element of R (e.g., a digital signal or image) from incomplete and contaminated observations y = A x0 + e; A is an n times m matrix with far fewer rows than columns (n << m) and e is an error term. Is it possible to recover x0 accurately based on the data y? To recover x0, we consider the solution x# to the L1-regularization problem $${\rm \min}\; |x\|_{\ell_1} \quad {\rm subject \; to } \; \|Ax-y\|_{\ell_2} \leq \epsilon,$$ where epsilon is the size of the error term e. We show that if A obeys a uniform uncertainty principle (with unit-normed columns) and if the vector x0 is sufficiently sparse, then the solution is within the noise level $$\|x^\sharp - x_0\|_{\ell_2} \leq C \cdot \epsilon.$$ As a first example, suppose that A is a Gaussian random matrix; then stable recovery occurs for almost all such A's provided that the number of nonzeros of x0 is of about the same order as the number of observations. As a second instance, suppose one observes few Fourier samples of x0; then stable recovery occurs for almost any set of n coefficients provided that the number of nonzeros is of the order of n/(log m)^6. In the case where the error term vanishes, the recovery is of course exact, and this work actually provides novel insights into the exact recovery phenomenon discussed in earlier papers. The methodology also explains why one can also very nearly recover approximately sparse signals.
    [bibtex-key = CPA:CPA20124] [bibtex-entry]


  709. Hubert-M.J. Cantalloube and Pascale Dubois-Fernandez. Airborne X-band SAR imaging with 10 cm resolution: technical challenge and preliminary results. IEE Proceedings - Radar, Sonar and Navigation, 153(2):163-176, April 2006. Keyword(s): SAR Processing, Doppler radar, airborne radar, antenna radiation patterns, frequency-domain synthesis, image resolution, microwave antennas, radar antennas, radar cross-sections, radar imaging, radar resolution, radar tracking, synthetic aperture radar, 1.2 GHz, Ku-band, RAMSES, bandwidth, X-band, Airborne SAR, antenna pattern compensation, carrier trajectory, cross-range resolution, deterministic motion, fast-frequency domain synthesis, isotropic point-like echo, phase-tracking, Autofocus, Residual Motion Errors, Motion Compensation, MoComp, Time-Domain Back-Projection, TDBP, temporal-domain back-projection synthesis.
    Abstract: The bandwidth of RAMSES, an airborne synthetic aperture radar (SAR) system was recently increased to 1.2 GHz in X- and Ku-bands, yielding (unweighted) 3 dB range resolution of 11 cm. The synthesis of SAR images with matching cross-range resolution requires long integration times, and this disqualifies the temporal-domain back-projection synthesis algorithm as impractically slow. The wider relative bandwidth also disqualifies simplified range/Doppler types of algorithms, as the hypothesis of proportionality between Doppler and squint is no longer valid. Therefore the authors implemented a fast-frequency domain synthesis algorithm (\u03a9-k or range-migration algorithm) and designed a new deterministic motion and antenna pattern compensation method for it. As the required accuracy on carrier trajectory exceeded the performance of the differential GPS-hybridised inertial navigation unit, the authors implemented an autofocus based on the phase-tracking of several isotropic point-like echoes.
    [bibtex-key = cantalloubeDuboisFernandez:OneraHighResXBand] [bibtex-entry]


  710. Shane R. Cloude. Polarization coherence tomography. Radio Sci., 41, August 2006. Keyword(s): SAR Processing, SAR Tomography, radar tomography, polarimetric interferometry, radar polarimetry, Remote sensing, Inverse scattering, Random media and rough surfaces, Interferometry, InSAR, SAR Interferometry, PolInSAR.
    Abstract: In this paper we introduce a new radar-imaging technique, called polarization coherence tomography (PCT), which employs variation of the interferometric coherence with polarization to reconstruct a vertical profile function in penetrable volume scattering. We first show how this profile function can be efficiently represented as a Fourier-Legendre series, with tomographic reconstruction reducing to estimation of the unknown coefficients of this series from coherence data. We then show that we can linearize this inversion by using a priori knowledge of two parameters, namely, volume depth and topographic phase. We further propose a new algorithm based on polarimetric interferometry to estimate these two from the data itself. To assess stability, we investigate both the single- and dual-baseline conditioning of the associated matrix inversion and then concentrate on the single-baseline case to demonstrate that for sufficient multilooking (around 50), stable retrievals of profiles can be obtained in the presence of coherence noise. Finally, we apply the technique to simulated L band coherent radar data to demonstrate its potential for new applications in radar remote sensing.
    [bibtex-key = Cloude2006] [bibtex-entry]


  711. E. Colin, C. Titin-Schnaider, and W. Tabbara. An interferometric coherence optimization method in radar polarimetry for high-resolution imagery. IEEE Trans. Geosci. Remote Sens., 44(1):167-175, January 2006. Keyword(s): backscatter, data acquisition, geophysical signal processing, geophysical techniques, radar imaging, radar polarimetry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, X-band interferometric SAR image, X-band polarimetric SAR image, anechoic chamber, data acquisition, high-resolution imagery, interferometric coherence optimization, point scatterer separation, radar polarimetry, single-mechanism coherence, synthetic aperture radar, target measurements, Anechoic chambers, Antenna measurements, Decorrelation, Electromagnetic scattering, Image resolution, Optimization methods, Polarization, Radar polarimetry, Radar scattering, Synthetic aperture radar interferometry, Coherence optimization, interferometry, polarimetry.
    Abstract: This paper investigates to what extent a new interferometric coherence optimization in radar polarimetry allows the separation of point scatterers located in the same resolution cell according to their interferometric phases. An interferometric coherence definition called the single-mechanism coherence is introduced, and the corresponding optimization method is briefly discussed. This method was first validated theoretically when no volume decorrelation occurs. Then, it has been applied to simple target measurements acquired in an anechoic chamber, and to an X-band polarimetric and interferometric synthetic aperture radar image containing man-made targets. In both cases, the single-mechanism coherence optimization enables to resolve the interferometric phases of several scattering centers inside the same resolution cell.
    [bibtex-key = colinTitinSchnaiderTabbaraTGRS2006InSARCoherenceOptimization] [bibtex-entry]


  712. Ian G. Cumming and Shu Li. Improved slope estimation for SAR Doppler ambiguity resolution. IEEE Trans. Geosci. Remote Sens., 44(3):707-718, 2006. Keyword(s): SAR Processing, Doppler Centroid Estimation, Doppler radar, Radon transforms, radar resolution, remote sensing by radar, synthetic aperture radar, Doppler Ambiguity Resolver, DAR, Radon transform, SAR, antenna pointing angle, look displacement algorithm, satellite beam pointing angle, slope estimation.
    Abstract: The idea of using the Radon transform to measure the alignment of linear features in synthetic aperture radar (SAR) data has breathed new life into the look displacement class of Doppler ambiguity resolution algorithms. In these algorithms, the slope of target energy is estimated to obtain the satellite beam pointing angle accurately enough to resolve the Doppler ambiguity. After explaining the method and adding some minor improvements, it is shown how it can work well on satellite SAR data. Then, an alternate method is developed that combines the ideas of the Radon and look displacement algorithms to obtain a computationally simpler and more accurate algorithm. In addition, the quality checks of the spatial diversity approach are used to increase the robustness of the algorithm. Even though the algorithm was conceived for high-contrast scenes, it works remarkably well in low to medium contrast scenes as well.
    [bibtex-key = cummingLi2006:DopcenRadon] [bibtex-entry]


  713. Francesco De Zan and Andrea Monti Guarnieri. TOPSAR: Terrain Observation by Progressive Scans. IEEE Trans. Geosci. Remote Sens., 44(9):2352-2360, Sept 2006. Keyword(s): array signal processing, remote sensing by radar, synthetic aperture radar, terrain mapping, SPOT, TOPSAR, azimuth-varying ambiguities, focusing technique, radar beam, scalloping ambiguities, scanning synthetic aperture radar, terrain observation with progressive scan, Azimuth, Focusing, Frequency, Geometry, Interferometry, Low earth orbit satellites, Polarization, Radar antennas, Radar signal processing, Spaceborne radar, Array signal processing, interferometry, scanning antennas, synthetic aperture radar (SAR).
    Abstract: In this paper, a novel (according to the authors' knowledge) type of scanning synthetic aperture radar (ScanSAR) that solves the problems of scalloping and azimuth-varying ambiguities is introduced. The technique employs a very simple counterrotation of the radar beam in the opposite direction to a SPOT: hence, the name terrain observation with progressive scan (TOPS). After a short summary of the characteristics of the ScanSAR technique and its problems, TOPSAR, which is the technique of design, the limits, and a focusing technique are introduced. A synthetic example based on a possible future system follows
    [bibtex-key = deZanMontiGuarnieriTGARS2006TOPS] [bibtex-entry]


  714. P. M. L. Drezet and S. Quegan. Environmental effects on the interferometric repeat-pass coherence of forests. IEEE_J_GRS, 44(4):825-837, April 2006. Keyword(s): backscatter, dielectric properties, forestry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping, C-band one-day tandem coherence measurement, SAR, dielectric backscattering coefficient, environmental effects, forest canopy coherence, interferometric repeat-pass coherence, moisture conditions, moisture fluctuations, soil moisture, soil-vegetation-atmosphere transfer model, synthetic aperature radar, vegetation, wind conditions, Backscatter, Coherence, Coupled mode analysis, Dielectric measurements, Fluctuations, Moisture measurement, Predictive models, Satellites, Soil measurements, Wind, dielectric, dynamic, forest, multitemporal, synthetic aperature radar (SAR). [bibtex-key = Drezet2006] [bibtex-entry]


  715. C. Elachi, S. Wall, M. Janssen, E. Stofan, R. Lopes, R. Kirk, R. Lorenz, J. Lunine, F. Paganelli, L. Soderblom, C. Wood, L. Wye, H. Zebker, Y. Anderson, S. Ostro, M. Allison, R. Boehmer, P. Callahan, P. Encrenaz, E. Flamini, G. Francescetti, Y. Gim, G. Hamilton, S. Hensley, W. Johnson, K. Kelleher, D. Muhleman, G. Picardi, F. Posa, L. Roth, R. Seu, S. Shaffer, B. Stiles, S. Vetrella, and R. West. Titan Radar Mapper observations from Cassini's T3 fly-by. Nature, 441(7094):709-713, June 2006. [bibtex-key = elachiEtAlNature2006CassiniRadarTitanMapper] [bibtex-entry]


  716. Qianqian Fang, P.M. Meaney, and K.D. Paulsen. The Multidimensional Phase Unwrapping Integral and Applications to Microwave Tomographical Image Reconstruction. IEEE Transactions on Image Processing, 15(11):3311-3324, November 2006. Keyword(s): SAR Processing, SAR Tomography, Tomography, Gauss-Newton iterative microwave image reconstruction, MRI, closed path integral, dynamic phase unwrapping, iteratively modified property distributions, microwave tomographical image reconstruction, multidimensional phase unwrapping integral, optical confocal microscopy, phase singularities, singularity location, spatial unwrapping, synthetic aperture radar interferometry, time varying electromagnetic fields, Newton method, computerised tomography, electromagnetic fields, image reconstruction, medical image processing;.
    Abstract: Spatial unwrapping of the phase component of time varying electromagnetic fields has important implications in a range of disciplines including synthetic aperture radar (SAR) interferometry, MRI, optical confocal microscopy, and microwave tomography. This paper presents a fundamental framework based on the phase unwrapping integral, especially in the complex case where phase singularities are enclosed within the closed path integral. With respect to the phase unwrapping required when utilized in Gauss-Newton iterative microwave image reconstruction, the concept of dynamic phase unwrapping is introduced where the singularity location varies as a function of the iteratively modified property distributions. Strategies for dynamic phase unwrapping in the microwave problem were developed and successfully tested in simulations and clinical experiments utilizing large, high contrast targets to validate the approach
    [bibtex-key = 1709977] [bibtex-entry]


  717. S. Feng and J. Chen. Low-angle reflectivity modeling of land clutter. IEEE Geoscience and Remote Sensing Letters, 3(2):254-258, April 2006. Keyword(s): least squares approximations, radar clutter, reflectivity, remote sensing by radar, terrain mapping, complex system design, ground based radar design, inductive reasoning, least squares method, low angle radar targets, low angle reflectivity, mean reflectivity model, radar land clutter modeling, radar surface clutter backscattering, Backscatter, Frequency, Parameter estimation, Polarization, Predictive models, Radar clutter, Radar scattering, Reflectivity, Rough surfaces, Surface roughness, Least squares (LS) methods, radar clutter, radar scattering, radar terrain factors.
    Abstract: In this letter, a practical mean reflectivity model of radar land clutter (LC) for the complex system design of ground-based radars involved in low-angle targets on some typical terrains is studied using the inductive reasoning method. The functional relationships between the radar parameters and radar surface clutter backscattering are analyzed. Following the recent research work in the area of reflectivity modeling of LC in the literature, the least squares method is employed to estimate the model parameters. The model is validated using reliable practical data and shown to outperform other models in accuracy.
    [bibtex-key = fengChenGRSL2006LowAngleLandClutterModelling] [bibtex-entry]


  718. G. Fornaro, G. Franceschetti, and S. Perna. On center-beam approximation in SAR motion compensation. IEEE Geoscience and Remote Sensing Letters, 3(2):276 - 280, april 2006. Keyword(s): airborne SAR, airborne raw data focusing, center beam approximation, geometrical analysis, motion compensation, synthetic aperture radar, airborne radar, motion compensation, remote sensing by radar, synthetic aperture radar.
    Abstract: This work provides a geometrical analysis to assess the effects of center-beam approximation, crucial for efficient airborne raw data focusing, on the final image.
    [bibtex-key = fornaroFranceschettiPernaGRSL2006BeamCenterApproxInMoComp] [bibtex-entry]


  719. G. Fornaro, A. Monti Guarnieri, A. Pauciullo, and Francesco De-Zan. Maximum likelihood multi-baseline SAR interferometry. IEE Proceedings - Radar, Sonar and Navigation, 153(3):279-288, June 2006. Keyword(s): SAR Processing, InSAR, Interferometry.
    Abstract: A technique to provide interferometry by combining multiple images of the same area is proposed. This technique exploits all the images jointly and performs an optimal spectral shift pre- processing to remove most of the decorrelation for distributed targets. Its applications are mainly for DEM generation at centimetric accuracy, and for differential interferometry. The major requirement is that targets are coherent over all the images: this may be the case for current multi-pass over desert areas, or better for the case of images coming from future short revisit time systems (constellations, cart-wheel, geosynchronous SAR etc.).
    [bibtex-key = fornaroMontiGuarnieriPauciulloDeZanIEERSN2005MaxLieklihoodMultiBaselineSARInterferometry] [bibtex-entry]


  720. G. Fornaro and F. Serafino. Imaging of Single and Double Scatterers in Urban Areas via SAR Tomography. IEEE Trans. Geosci. Remote Sens., 44(12):3497-3505, 2006. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, remote sensing by radar, synthetic aperture radar, tomography, topography (Earth), 3D synthetic aperture radar tomography, European Remote Sensing 1 satellite, European Remote Sensing 2 satellite, Italy, Naples, Napoli, SAR sensors, data calibration, microwave scattering, residual topography, spatial-differencing technique, surface deformation, urban areas, Multibaseline coherent synthetic aperture radar (SAR) processing, SAR tomography, three-dimensional (3-D) SAR focusing.
    Abstract: Microwave scattering is a rather complex mechanism, especially in urban areas. Three-dimensional (3-D) synthetic aperture radar (SAR) tomography is a technique that uses multiple views to map the scattering power at different heights, thus extending the capability of SAR sensors to fully image the scene in the 3-D space. This paper presents a first validation of spaceborne long-term SAR tomography by demonstrating the capability to resolve a simple layover case, i.e., to separate single- and double-scattering mechanisms within imaged pixels. Results obtained with real data acquired by the European Remote Sensing 1 and 2 (ERS-1 and ERS-2) satellites over the urban area of Napoli are presented. As an additional contribution, an innovative algorithm estimating residual topography and surface deformation, called the spatial-differencing technique, is also discussed in detail at the data calibration stage
    [bibtex-key = fornaroSerafino06:Tomo] [bibtex-entry]


  721. G. Franceschetti, A. Iodice, S. Perna, and D. Riccio. SAR Sensor Trajectory Deviations: Fourier Domain Formulation and Extended Scene Simulation of Raw Signal. IEEE Transactions on Geoscience and Remote Sensing, 44(9):2323-2334, September 2006. Keyword(s): SAR Processing, Simulation, SAR RAW Signal Simulation, Fourier transforms, geophysical signal processing, remote sensing by radar, synthetic aperture radar, 1D azimuth Fourier domain processing, 2D Fourier domain SAR raw signal simulator, Fourier domain formulation, airborne SAR systems, computation efficiency, extended scene simulation, fast Fourier transform algorithms, inversion algorithm design, mission planning, narrow beam-slow deviation assumption, nominal straight-line path, processing algorithm testing, range time-domain integration, sensor trajectory deviations, synthetic aperture radar raw signal simulation, system design, Algorithm design and analysis, Computational modeling, Fast Fourier transforms, Layout, Process planning, Signal design, Signal processing, Synthetic aperture radar, System testing, Two dimensional displays, Motion compensation, simulation, synthetic aperture radar (SAR).
    Abstract: Synthetic aperture radar (SAR) raw signal simulation is a useful tool for SAR system design, mission planning, processing algorithm testing, and inversion algorithm design. A two-dimensional (2-D) Fourier domain SAR raw signal simulator, exploiting the efficiency of fast Fourier transform algorithms, has been presented some years ago and is able to generate the raw signal corresponding to extended scenes. However, it cannot account for the effects of sensor trajectory deviations with respect to the nominal straight-line path. This paper explores the possibility of extending the efficient Fourier domain simulation approach to the case of sensor trajectory deviations, which is more realistic for airborne SAR systems. We first of all obtain a general and compact Fourier domain formulation of the SAR raw signal in the presence of arbitrary trajectory deviations, and show that in this general case no efficient simulation scheme can be devised. However, we demonstrate that, if a narrow beam and slow trajectory deviation assumption is made, a full 2-D Fourier domain simulation can be used. This approach can be applied only to some SAR systems and/or trajectory deviations, but it has the advantage that processing time is practically not increased with respect to the nominal trajectory case. The validity limits of the approach are analytically evaluated. Some simulation results are finally presented in order to verify the effectiveness of the proposed simulation scheme. In another paper, which is the second part of this work, it will be shown that the narrow beam-slow deviation assumption can be relaxed, at the expense of computation efficiency, if a one-dimensional azimuth Fourier domain processing followed by a range time-domain integration is used
    [bibtex-key = franceschettiIodicePernaRiccioTGRS2006SARTrajectoryDeviationFFTSimulationOfRawSignal] [bibtex-entry]


  722. J.L. Gomez-Dans, S. Quegan, and J.C. Bennett. Indoor C-band polarimetric interferometry observations of a mature wheat canopy. IEEE Trans. Geosci. Remote Sens., 44(4):768-777, April 2006. Keyword(s): SAR Processing, SAR Tomography, Tomography, 2D scan, HH polarization, VH polarization, VV polarization, crop height retrieval, ground-based synthetic aperture radar, incidence angle, indoor C-band polarimetric interferometry observation, polarimetric coherence optimization, polarimetric tomography, polarization synthesis, unconstrained coherence optimization, vegetation monitoring, wheat canopy, crops, radar polarimetry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: We present results from experiments carried out in the ground-based synthetic aperture radar (GB-SAR) facility at the University of Sheffield to ascertain the role of polarimetric interferometry in crop height retrieval. To this end, a mature wheat canopy, grown in outdoor conditions, was reassembled inside the GB-SAR chamber and imaged at C-band using a two-dimensional scan. This allowed fully polarimetric tomography and interferometry. Interferometry using the VV, HH, and VH polarization states shows that the HH and VH interferograms retrieve a height close to the top of the soil layer for all angles of incidence considered, whereas the height retrieved from the VV interferogram increases with angle of incidence. The use of a Pauli basis gives poor results, due to the different location of the scattering phase centers in the VV and HH channels. The use of arbitrary polarization states shows that the top of the soil can be very accurately estimated using left-circular polarization, whereas, for angles of incidence close to 45 deg;, a polarization state similar to VV can be used to retrieve the top of the canopy; hence crop height can be recovered as the difference of these two interferometric heights. Polarimetric coherence optimization techniques are also studied. Unconstrained coherence optimization gives very unstable results, due to the small number of available samples. Constrained optimization results in stable retrieved heights, and the retrieved polarization states agree well with the polarization synthesis results.
    [bibtex-key = 1610813] [bibtex-entry]


  723. Franz Meyer, Richard Bamler, N. Jakowski, and Thomas Fritz. The Potential of Low-Frequency SAR Systems for Mapping Ionospheric TEC Distributions. IEEE_J_GRSL, 3(4):560-564, October 2006. Keyword(s): SAR Processing, electron density, ionosphere, ionospheric techniques, microwave propagation, synthetic aperture radar, SAR signal properties, broadband L-band SAR, broadband microwave radiation, correlation technique, dispersive media, group delay, interferometric technique, ionospheric TEC distribution mapping, ionospheric propagation effects, low-frequency SAR systems, phase advance, total electron content, two-frequency global positioning system observations, Delay systems, Dielectrics, Dispersion, Electrons, Ionosphere, L-band, Position measurement, Refractive index, Signal mapping, Synthetic aperture radar, Atmospheric effects, L-band SAR, SAR interferometry, correlation, synthetic aperture radar (SAR), total electron content (TEC).
    Abstract: Ionospheric propagation effects have a significant impact on the signal properties of low-frequency synthetic aperture radar (SAR) systems. Range delay, interferometric phase bias, range defocusing, and Faraday rotation are the most prominent ones. All the effects are a function of the so-called total electron content (TEC). Methods based on two-frequency global positioning system observations allow measuring TEC in the ionosphere with coarse spatial resolution only. In this letter, the potential of broadband L-band SAR systems for ionospheric TEC mapping is studied. As a basis, the dispersive nature of the ionosphere and its effects on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices in mapping small-scale ionospheric TEC disturbances. A differential TEC estimator that separates ionospheric from tropospheric contributions is proposed.
    [bibtex-key = meyerBamlerJakowskiFritzGRSL2006PotentialLowFreqSARForIonoTEC] [bibtex-entry]


  724. Daniele Perissin, Claudio Prati, Marcus E. Engdahl, and Y.L. Desnos. Validating the SAR Wavenumber Shift Principle With the ERS-Envisat PS Coherent Combination. IEEE Trans. Geosci. Remote Sens., 44(9):2343-2351, September 2006. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR, Interferometry, Decorrelation, Frequency, Geometry, Interferometry, Master-slave, Radar imaging, Radar scattering, Remote sensing, Satellites, Synthetic aperture radar, data acquisition, image registration, remote sensing by radar, synthetic aperture radar, ERS-Envisat PS coherent combination, European Remote Sensing Satellite Synthetic Aperture Radar archive by means Envisat Advanced SAR data, SAR wavenumber shift principle, ground resolution cell, images coregistration step, permanent scatterers, slant-range position, spectral shift principle, subcell accuracy, Interferometry, synthetic aperture radar (SAR);.
    Abstract: Continuity of the European Remote Sensing Satellite Synthetic Aperture Radar (ERS SAR) archive by means of Envisat Advanced SAR (ASAR) data acquired from March 2002 has introduced the problem of the coherent combination of images coming from sensors with slightly different frequencies. The spectral shift principle states that in case of extended distributed targets, the frequency shift is equivalent to a change of looking angle. In this paper, the same principle is exploited to analyze the behavior of permanent scatterers (PSs) with an extension that is smaller than the ground resolution cell. The conditions under which the PSs identified by ERS can be continued by Envisat are then theoretically determined and experimentally validated. Moreover, this analysis shows that acquisitions characterized by different frequencies can be used to identify the slant-range position of scatterers with high subcell accuracy (tens of centimeters). From the processing side, a very precise images coregistration step is required to get the results described in this paper
    [bibtex-key = perissinPratiEngdahlDesnosTGRS2006] [bibtex-entry]


  725. Daniele Perissin and Fabio Rocca. High-Accuracy Urban DEM Using Permanent Scatterers. IEEE Trans. Geosci. Remote Sens., 44(11):3338-3347, November 2006. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, SAR Interferometry, InSAR, DInSAR, Interferometry, Area measurement, Cities and towns, Decorrelation, Digital elevation models, Interferometry, Monitoring, Radar detection, Radar scattering, Synthetic aperture radar, Volcanoes, building, interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), Milan, building profile, city street level, digital elevation model, ground deformations monitoring, high-accuracy urban DEM, interferometry, permanent scatterers, slow-landslide detection, spatial density grid, subsidence, synthetic aperture radar, terrain mapping, urban elevation maps, volcano monitoring, Interferometry, synthetic aperture radar (SAR), terrain mapping, urban areas;.
    Abstract: The permanent scatterers (PS) technique is a powerful operational tool that exploits a long series of synthetic aperture radar data for monitoring ground deformations with millimeter accuracy on a high spatial density grid of pointwise targets. The technique has been applied successfully to a number of applications, from subsidence and volcano monitoring to slow-landslide detection. This paper aims to analyze and demonstrate the positioning capability of the PS technique applied to the generation of urban elevation maps. The problem of the univocal identification of the PS position (discarding pixel-dependent sidelobes, both far and near) is addressed, and an easy and efficient solution is proposed. The results obtained in the Milan site allow the appreciation of the very high quality of an urban digital elevation model retrieved with the PS technique. The ground level of the city is identified with submeter accuracy, and elevated targets, where present, reveal building profiles. The estimated city street level (ranging plusmn3m in 16times18 km2) is then compared to those obtained with the same technique using a descending parallel track and an ascending one. Furthermore, the estimated PS elevation with respect to the ground has been connected to temperature-dependent elongations of high structures
    [bibtex-key = perissinRoccaTGRS2006PSI] [bibtex-entry]


  726. A. Refice, F. Bovenga, and R. Nutricato. MST-based stepwise connection strategies for multipass Radar data with application to coregistration and equalization. IEEE Trans. Geosci. Remote Sens., 44(8):2029-2040, 2006. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical signal processing, image registration, interferometry, radar imaging, remote sensing by radar, synthetic aperture radar, European Remote Sensing Satellite, InSAR coherence, MST-based stepwise connection strategy, data processing, distance function, geometric shift, image coregistration, image equalization, interferometric SAR, interferometric coherence, minimum spanning tree, multipass radar data, radiometric calibration, radiometric equalization, Data processing, minimum spanning tree (MST), multipass synthetic aperture radar (SAR) interferometry.
    Abstract: This paper proposes a unified framework for predicting optimized pairing strategies for interferometric processing of multipass synthetic aperture radar data. The approach consists in a minimum spanning tree (MST) structure based on a distance function encoding an a priori model for the interferometric quality of each image pair. Using a distance function modeled after the interferometric coherence allows reproducing many small baseline strategies presented in the recent literature. A novel application of the method to the processing steps of image coregistration and equalization is illustrated, using a test European Remote Sensing Satellite dataset. Widespread methods used for these two operations rely on the computation of the amplitude cross correlation over a large number of corresponding tie patches distributed over the scene. Geometric shift and radiometric equalization parameters are estimated over the patches and used, respectively, within a polynomial warp model and a radiometric correction scheme. The number of reliable patches available behaves similarly to the interferometric synthetic aperture radar (InSAR) coherence with respect to the baselines, and can be assimilated to a quality figure for the derivation of the MST. Results show an improvement in the quality of the stepwise (SW)-processed image stack with respect to the classical single-master procedure, confirming that the SW approach is able to provide better conditions for the estimation of correlation-related InSAR parameters.
    [bibtex-key = reficeBovengaNutricato06:Tomo] [bibtex-entry]


  727. Andreas Reigber, E. Alivizatos, A. Potsis, and Alberto Moreira. Extended wavenumber-domain synthetic aperture radar focusing with integrated motion compensation. Radar, Sonar and Navigation, IEE Proceedings, 153(3):301-310, 2006. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Extended omega-k, EOK, Extended Wavenumber Domain Algorithm, Motion Compensation, Squinted SAR, Airborne SAR, E-SAR, ECS, Extended Chirp Scaling.
    Abstract: Modern synthetic aperture radar (SAR) systems are continually developing in the direction of higher spatial resolution. This requires the usage of high range bandwidths combined with long azimuth integration intervals. High-quality SAR processing methods, which are able to deal with such sensor parameters, are necessary for focusing the raw data of such sensors. Wavenumber-domain (omegak) processing is commonly accepted as the ideal solution to the SAR focusing problem. However, it is only applicable to spaceborne SAR data where a straight sensor trajectory is given. In the case of airborne data, wavenumber-domain processing is limited because of its inability to perform high-precision motion compensation. Here, the extended chirp scaling (ECS) algorithm has proven to be very powerful, although it has certain limitations concerning long aperture syntheses and highly squinted geometries. In the paper, a new stripmap SAR data-processing algorithm, called extended omega-k (EOK), is analytically derived. The EOK algorithm aims to combine the high focusing accuracy of the wavenumber-domain algorithm with the high-precision motion compensation of the ECS algorithm. The new EOK algorithm integrates a three-step motion compensation correction in the general formulation of the wavenumber-domain algorithm, leading to a new airborne SAR processing scheme, which is also very robust in the cases of long synthetic apertures and high squint angles. As demonstrated, it offers the possibility of processing wide-band, low-frequency airborne SAR data up to nearwavelength resolution. The performance and accuracy of the new EOK SAR data-processing algorithm are demonstrated using simulated data in different data collection scenarios and geometries as well as using interferometric data acquired by the airborne experimental SAR system of DLR at L-band (Horn, 1996; Scheiber, 1999).
    [bibtex-key = reigberAlivizatosPotsisMoreira06:EOK] [bibtex-entry]


  728. A. Reigber, P. Prats, and J. J. Mallorqui. Refined estimation of time-varying baseline errors in airborne SAR interferometry. IEEE Geoscience and Remote Sensing Letters, 3(1):145-149, January 2006. Keyword(s): SAR Processing, Airborne SAR, calibration, Interferometry, L-Band, Baseline refinement, interferometry, InSAR, Motion Compensation, repeat-pass interferometry, Residual Motion Errors, RME, Squinted SAR, Multi-Squint Processing, ESAR, geophysical signal processing, geophysical techniques, motion compensation, remote sensing by radar, synthetic aperture radar SAR focusing, airborne SAR data, airborne SAR interferometry, digital elevation model, multisquint processing approach, navigation system, repeat-pass systems, residual motion errors, synthetic aperture radar, time-varying baseline errors.
    Abstract: The processing of airborne synthetic aperture radar (SAR) data requires a precise compensation of the deviations of the platform movement from a straight line. This is usually carried out by recording the trajectory with a high-precision navigation system and correcting them during SAR focusing. However, due to the lack of accuracy in current navigation systems, residual motion errors persist in the images. Such residual motion errors are mainly noticeable in repeat-pass systems, where they are causing time-varying baseline errors, visible as artefacts in the derived phase maps. In this letter, a refined method for the estimation of time-varying baseline errors is presented. An improved multisquint processing approach is used for obtaining robust estimates of higher order baseline errors over the entire scene, even if parts of the scene are heavily decorrelated. In a subsequent step, the proposed method incorporates an external digital elevation model for detection of linear and constant components of the baseline error along azimuth. Calibration targets in the scene are not necessary.
    [bibtex-key = reigberPratsMallorqui2006RefinedBaseLineEstim] [bibtex-entry]


  729. E. Sansosti, P. Berardino, M. Manunta, F. Serafino, and G. Fornaro. Geometrical SAR image registration. IEEE Transactions on Geoscience and Remote Sensing, 44(10):2861-2870, October 2006. Keyword(s): SAR Processing, image registration, coregistration, InSAR, Interferometry, ENVISAT, European Remote Sensing, acquisition flight tracks, baseline spans, digital elevation model, image registration, multichannel SAR processing, satellite systems, sensitivity analysis, steep topography regions, subpixel registration, synthetic aperture radar images, two-pass interferometry, warping functions, artificial satellites, geophysical signal processing, image registration, remote sensing by radar, sensitivity analysis, synthetic aperture radar, topography (Earth).
    Abstract: Accurate subpixel registration of synthetic aperture radar (SAR) images is an issue that is again growing interest since its initial developments related to two-pass interferometry. Recent progress in coherent (multichannel) SAR processing raises the need for accurate registration of data takes acquired with large baseline spans, high temporal coverage, and with different frequency and/or operational modes. In this paper, we discuss a SAR image-registration procedure, based on the use of external measures which allows obtaining a very accurate alignment of SAR images. The presented technique makes use of a digital elevation model and of the precise information about the acquisition flight tracks, to compute the warping functions that map the position of each pixel in the different takes, thus avoiding any approximation. The resulting algorithm is simple, robust, precise, and very efficient; as a matter of fact, it may achieve high accuracy even in critical areas, such as steep topography regions. Moreover, the availability of an analytical and exact model allows performing a detailed sensitivity analysis that can be useful in evaluating the applicability of this technique even to future high-precision satellite systems. Extensive testing, carried out on several real European Remote Sensing and ENVISAT datasets, clearly shows the effectiveness of such algorithm in registering critical SAR images
    [bibtex-key = 1704979] [bibtex-entry]


  730. Martin Stangl, Rolf Werninghaus, B. Schweizer, C. Fischer, Michael Brandfass, Josef Mittermayer, and Helko Breit. TerraSAR-X Technologies and First Results. Radar, Sonar & Navigation, 153(2):86 - 95, April 2006. Keyword(s): SAR Processing, TerraSAR-X, Spaceborne SAR, X-Band, Instrument Description, Imaging Modes, Stripmap, ScanSAR, Spotlight SAR, TerraSAR-X multimode SAR processor, TMSP, Chirp Scaling Algorithm, CSA, SPECAN Algorithm, SPECAN.
    Abstract: The TerraSAR-X Satellite which is currently being built in the framework of a public private partnership between the German Aerospace Centre DLR and Astrium GmbH Germany carries an X-Band SAR instrument which is equipped with an active phased array antenna. Its operational flexibility allows the use of the instrument for scientific and commercial applications. High amplitude and phase stability of the radar instrument is achieved by a Carbon Fibre Reinforced Plastic (CFRP) slotted waveguide radiator and high precision Transmit/Receive Module. Additionally internal calibration hardware allows for determination of the residual drifts. A precise Central Electronics controls the radar instrument, provides an arbitrary transmit chirp and receives the radar echo with selectable bandwidth and raw data compression ratio. The ground stations multimode SAR processor is supported by a novel satellite steering law to reduce the attitude dependent mean Doppler shift. This article summarises the EUSAR 2004 TerraSAR-X contributions.
    [bibtex-key = stanglWerninghausSchweizerFischerBrandfassMittermayerBreit2006:TerraSARX] [bibtex-entry]


  731. H. C. Stankwitz and S. P. Taylor. Advances in non-linear apodization. IEEE Aerospace and Electronic Systems Magazine, 21(1):3-8, January 2006. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, SVA, NonLinear Apodization, European Soil Moisture and Ocean Salinity system, GeoSTAR, Y-type synthetic aperture radiometers, antenna size, bandwidth extrapolation, impulse response performance, irregularly-shaped apertures, microwave radar systems, microwave remote sensing, nonlinear apodization, parse coherent apertures, sidelobe control, sparse aperture filling property, superresolution algorithms, superresolution techniques, synthetic aperture radar systems, synthetic multiple aperture radar technology, microwave measurement, radar antennas, radar resolution, remote sensing by radar, synthetic aperture radar.
    Abstract: Selected new methods and applications of non-linear apodization for irregularly-shaped and parse coherent apertures and arrays are presented. The benefits include unproved impulse response performance, i.e., reduced peak sidelobes and integrated sidelobe power, along with improved mainlobe resolution, compared to classic windowing techniques. Nonlinear apodization (NLA) techniques can also serve as powerful engines for effective superresolution and bandwidth extrapolation of coherent data for filling sparse apertures. The sparse aperture filling property of superresolution algorithms for radar data forms the basis for a new concept which is introduced here: synthetic multiple aperture radar technology (SMART). Increased swath and/or reduced antenna size are some of the benefits postulated for SMART applied to synthetic aperture radar (SAR) systems. The benefits of these new methods and applications for nonlinear apodization are then demonstrated for two specific applications: 1) sidelobe control for Y-type synthetic aperture radiometers, such as the European Soil Moisture and Ocean Salinity (SMOS) system (Kerr et al.) and JPL's proposed GeoSTAR (Lambrigsten) concept; and, 2) filling of sparse synthetic aperture radar data by exploiting the bandwidth extrapolation properties of nonlinear apodization based superresolution techniques. The methods that have been developed and demonstrated herein have potential application to a wide range of passive and active microwave remote sensing and radar systems.
    [bibtex-key = stankwitzTaylor2006:NonLinearApodization] [bibtex-entry]


  732. Z. Su, Y. Peng, and X. Wang. Evaluation of the Aperture in the Curvilinear SAR. Radar, 2006. CIE'06. International Conference on, pp 1-4, 2006. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR.
    Abstract: The curvilinear synthetic aperture radar (SAR), as a more practicable three-dimensional (3-D) SAR imaging system, has to utilize the parametric methods to extract the target features from its received data, the sparse data in 3-D frequency space. With the obtained feature estimates, these parametric methods, who can efficiently remove the effects of the sidelobes, reconstruct the target image whose quality is affected by the estimation accuracy of the features. By minimizing the Cramer-Rao bounds (CRBs) of the target features, we show, in this paper, the different effects on the estimation accuracy of different aperture parameters. A feature-independent aperture evaluator is also given for the curvilinear SAR system.
    [bibtex-key = suPengWang2006:NonLinearSARTomo] [bibtex-entry]


  733. L. Thirion, E. Colin, and C. Dahon. Capabilities of a forest coherent scattering model applied to radiometry, interferometry, and polarimetry at P- and L-band. IEEE Transactions on Geoscience and Remote Sensing, 44(4):849 - 862, April 2006. Keyword(s): L-band, P-band, SAR image, backscattering, electromagnetic study, forest coherent scattering model, forested areas, radar configuration, radar image, radar interferometry, radar polarimetry, radiometry, synthetic aperture radar, temperate forests, tropical forests, backscatter, forestry, microwave measurement, radar imaging, radar polarimetry, radiometry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: The interpretation of radar data would ideally require extensive and numerous observations. However, the number of observations is limited by the difficulty and the cost of acquiring ground truth and radar data. On the other hand, numerical models can provide a wide range of situations, both in inputs and in outputs. More precisely, they have to provide radiometric, polarimetric, and interferometric simulations and be applicable to various forested areas (high density, high/low moisture, inhomogeneous area, etc.) and radar configurations (low/high frequency, bistatic observation, etc.). This paper is dedicated to the presentation of the capabilities of a descriptive coherent scattering model (COSMO) applied to the electromagnetic study of the backscattering by forested areas. Improvements have been implemented in order to produce in output a radar image, which can be treated with the same polarimetric and interferometric tools as those applied to real synthetic aperture radar images. Thus, comparisons are possible. COSMO has been widely tested from P- to L- bands, over temperate and tropical forests and applied to radiometry, polarimetry, and interferometry. It appears finally as an efficient simulating tool to carry out parametric studies and to analyze how the total scattered field is built from canonical mechanisms and individual scatterer contributions.
    [bibtex-key = thirionColinDahon2006] [bibtex-entry]


  734. W. L. Van Rossum, M. P. G. Otten, and R. J. P. Van Bree. Extended PGA for range migration algorithms. IEEE Transactions on Aerospace and Electronic Systems, 42(2):478-488, April 2006. Keyword(s): SAR Proceedings, Autofocus, Phase Gradient Autofocus, PGA, gradient methods, radar imaging, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Spotlight SAR, synthetic aperture radar.
    Abstract: The phase gradient autofocus (PGA) algorithm is extended to work for synthetic aperture radar (SAR) spotlight images processed with range migration (w-k) algorithms. Several pre-processing steps are proposed for aligning the range-compressed phase-history data needed for successful autofocusing of the data. The proposed algorithm gave good results for both data with large point targets and data without point targets.
    [bibtex-key = vanRossumOttenVanBree2006:PGAAutofocus] [bibtex-entry]


  735. I. Walterscheid, J.H.G. Ender, A.R. Brenner, and O. Loffeld. Bistatic SAR Processing and Experiments. IEEE Trans. Geosci. Remote Sens., 44(10):2710-2717, October 2006. Keyword(s): SAR Processing, Bistatic SAR, Airborne Experimental Radar II, Forschungsgesellschaft fur Angewandte Naturwissenschaften, Phased Array Multifunctional Imaging Radar, X-band SAR systems, antenna pointing, bistatic SAR, bistatic reflectivity, flight coordination, forward-looking SAR imaging, motion compensation, radar cross section, range migration algorithm, receive gate timing, synthetic aperture radar, transmit pulse, airborne radar, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar.
    Abstract: Bistatic synthetic aperture radar (SAR) uses a separated transmitter and receiver flying on different platforms to achieve benefits like exploitation of additional information contained in the bistatic reflectivity of targets, reduced vulnerability for military applications, forward-looking SAR imaging, or increased radar cross section. Besides technical problems such as synchronization of the oscillators, involved adjustment of transmit pulse versus receive gate timing, antenna pointing, flight coordination, and motion compensation, the development of a bistatic focusing algorithm is still in progress and not sufficiently solved. As a step to a numerically efficient processor, this paper presents a bistatic range migration algorithm for the translationally invariant case, where transmitter and receiver have equal velocity vectors. In this paper, the algorithm was successfully applied to simulated and real bistatic data. The real bistatic data have been acquired with the Forschungsgesellschaft fur Angewandte Naturwissenschaften (FGAN)'s X-band SAR systems, namely the Airborne Experimental Radar II and the Phased Array Multifunctional Imaging Radar, in October 2003
    [bibtex-key = WalterscheidEnderBrennerLoffeld2006:Bistatic] [bibtex-entry]


  736. L. Xiangle and Y. Ruliang. Study of Composite Mode Curvilinear SAR. Radar, 2006. CIE'06. International Conference on, pp 1-4, 2006. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR.
    Abstract: A new mode of curvilinear SAR called composite mode curvilinear SAR(CCLSAR) is proposed in this paper. In CCLSAR, the SAR operates in strip mode in azimuth and range dimension but spotlight mode in the height dimension. CCLSAR can synthesize a very large aperture in the direction of height, which cause very high resolution in the height dimension. Compared with the conventional curvilinear SAR, CCLSAR can acquire the imaging of large terrain quickly and immediately. Firstly the CCLSAR scheme was proposed and then the primary three-dimensional imaging in the case of full aperture of CCLSAR was discussed in this paper.
    [bibtex-key = xiangleRulinag2006:NonLinearSARTomo] [bibtex-entry]


  737. Y. Yamaguchi, Y. Yajima, and H. Yamada. A four-component decomposition of POLSAR images based on the coherency matrix. IEEE Geosci. Remote Sens. Lett., 3(3):292 -296, July 2006. Keyword(s): POLSAR image decomposition, coherency matrix, covariance matrix, four-component decomposition, nonreflection symmetric scattering, polarimetric SAR, synthetic aperture radar, covariance matrices, geophysical techniques, image processing, radar polarimetry, remote sensing by radar, synthetic aperture radar;.
    Abstract: A four-component decomposition scheme of the coherency matrix is presented here for the analysis of polarimetric synthetic aperture radar (SAR) images. The coherency matrix is used to deal with nonreflection symmetric scattering case, which is an extension of covariance matrix approach. The same decomposition results have been obtained. The advantage of this approach is explicit expressions of four scattering powers in terms of scattering matrix elements, which serve the interpretation of polarimetric SAR data quantitatively
    [bibtex-key = yamaguchiYajimaYamada2006] [bibtex-entry]


  738. Rafael Zandoná-Schneider, K.P. Papathanassiou, Irena Hajnsek, and Alberto Moreira. Polarimetric and interferometric characterization of coherent scatterers in urban areas. IEEE Trans. Geosci. Remote Sens., 44(4):971-984, 2006. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Selection of point target candidates, backscatter, radar imaging, radar polarimetry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, L-band radar, SAR interferometry, SAR polarimetry, airborne SAR data, orientation angle, point-like coherent scatterers, quadpolarized images, synthetic aperture radar, urban areas, Azimuth, Data mining, Interferometry, Object detection, Polarimetry, Radar detection, Radar scattering, Synthetic aperture radar, Urban areas, Coherent scatterers, SAR interferometry, SAR polarimetry, orientation angle, polarimetric SAR interferometry, synthetic aperture radar (SAR), urban areas.
    Abstract: In this paper the concept of point-like coherent scatterers (CSs) in urban areas is introduced. The detection of CSs in single and quad-polarized images is addressed and applied on polarimetric and interferometric high-resolution airborne SAR data at L-band. For the detection of CSs two different approaches are proposed and their individual performance is analyzed. The properties of the detected CSs and their polarimetric and interferometric characteristics are assessed. The role of polarimetry on the detection of the CSs is evaluated and the potential of extracting the orientation and dielectric properties of individual CSs is finally addressed.
    [bibtex-key = zandonaSchneiderPapathanassiouHajnsekMoreira2006] [bibtex-entry]


  739. J. J. M. de Wit, Adriano Meta, and Peter Hoogeboom. Modified range-Doppler processing for FM-CW synthetic aperture radar. IEEE Geoscience and Remote Sensing Letters, 3(1):83-87, January 2006. Keyword(s): SAR Processing, SAR focusing, Azimuth Focusing, FMCW, FMCW SAR, Doppler shift, airborne radar, geophysical signal processing, remote sensing by radar, synthetic aperture radar, Doppler frequency shift, SAR processing, echo, frequency-modulated continuous-wave radar, modified range-Doppler processing, radar Earth observation, range migration compensation, remote sensing, synthetic aperture radar, Airborne radar, Doppler radar, Earth, Frequency, High-resolution imaging, Radar imaging, Radar remote sensing, Radar signal processing, Remote monitoring, Synthetic aperture radar, Frequency-modulated continuous-wave (FM-CW) radar, radar Earth observation, remote sensing, synthetic aperture radar (SAR).
    Abstract: The combination of compact frequency-modulated continuous-wave (FM-CW) technology and high-resolution synthetic aperture radar (SAR) processing techniques should pave the way for the development of a lightweight, cost-effective, high-resolution, airborne imaging radar. Regarding FM-CW SAR signal processing, the motion during the transmission of a sweep and the reception of the corresponding echo were expected to be one of the major problems. In FM-CW SAR, the so-called stop-and-go approximation is no longer valid due to the relatively long sweeps that FM-CW radars transmit. The main effect of the continuous motion is a Doppler frequency shift throughout the SAR observation time. This Doppler frequency shift can be compensated for by modifying the range migration compensation.
    [bibtex-key = deWitMetaHoogeboomTGRS2006ModRangeDopplerForFMCWSAR] [bibtex-entry]


  740. Richard Bamler and Michael Eineder. Accuracy of differential shift estimation by correlation and split-bandwidth interferometry for wideband and delta-k SAR systems. IEEE Geoscience and Remote Sensing Letters, 2(2):151-155, April 2005. Keyword(s): SAR Processing, split-bandwidth, geophysical signal processing, image registration, radar imaging, radar interference, remote sensing by radar, speckle, synthetic aperture radar, Cramer-Rao bound, Delta-k interferometry, clutter, coherent speckle correlation, differential delay estimation, differential shift estimation, digital elevation model, ground motion mapping, image coregistration, image correlation, incoherent speckle correlation, macro-scale shift estimation, phase cycle ambiguities, phase unwrapping, point scatterers, speckle tracking, split-bandwidth interferometry, synthetic aperture radar, Bandwidth, Clutter, Digital elevation models, Equations, Interferometry, Motion estimation, Radar scattering, Speckle, Synthetic aperture radar, Wideband, Cramer-Rao bound (CRB), Delta-k interferometry, differential delay estimation, image correlation, speckle tracking, split-bandwidth interferometry, synthetic aperture radar (SAR) image coregistration.
    Abstract: Estimation of differential shift of image elements between two synthetic aperture radar (SAR) images is the basis for many applications, like digital elevation model generation or ground motion mapping. The shift measurement can be done nonambiguously on the macro scale at an accuracy depending on the range resolution of the system or on the micro scale by employing interferometric methods. The latter suffers from phase cycle ambiguities and requires phase unwrapping. Modern wideband high-resolution SAR systems boast resolutions as small as a few tens of a wavelength. If sufficiently many samples are used for macro-scale shift estimation, the accuracy can be increased to a small fraction of a resolution cell and even in the order of a wavelength. Then, accurate absolute ranging becomes precise enough to support phase unwrapping or even make it obsolete. This letter establishes a few fundamental equations on the accuracy bounds of shift estimation accuracy for several algorithms: coherent speckle correlation, incoherent speckle correlation, split-band interferometry, a multifrequency approach, and correlation of point scatterers in clutter. It is shown that the performance of split-band interferometry is close to the Cramer-Rao bound for a broad variety of bandwidth ratios. Based on these findings, Delta-k systems are proposed to best take advantage of the available radar bandwidth.
    [bibtex-key = bamlerEinederGRSL2005] [bibtex-entry]


  741. Federica Bordoni, Fabrizio Lombardini, Fulvio Gini, and A. Jakobsson. Multibaseline cross-track SAR interferometry using interpolated arrays. IEEE Transactions on Aerospace and Electronic Systems, 41(4):1473-1482, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, Layover, Array Interpolation.
    Abstract: This work deals with the problem of interferometric radar phase estimation in the presence of layover. The focus here is on multichannel interferometric synthetic aperture radar (InSAR) systems with a low number of phase centers and nonuniform array geometry. An interpolated array (IA) approach is proposed in order to apply parametric spectral estimation techniques designed for uniform linear arrays (ULAs). In particular, the interpolated MUSIC and weighted subspace fitting (WSF) algorithms are considered and compared with conventional methods. Performance analysis under different InSAR scenarios is carried out based on Monte Carlo simulations. The Cramer-Rao lower bound (CRLB) for the nonuniform interferometric array is derived and reported as a benchmark on the estimation accuracy.
    [bibtex-key = bordoniLombardiniGiniJakobsson05:Tomo] [bibtex-entry]


  742. E.J. Candes and T. Tao. Decoding by linear programming. IEEE Transactions on Information Theory, 51(12):4203-4215, December 2005. Keyword(s): Gaussian random matrix, basis pursuit, linear code decoding, linear programming, minimization problem, natural error correcting problem, simple convex optimization problem, sparse solution, uncertainty principle, Gaussian processes, convex programming, decoding, error correction codes, indeterminancy, linear codes, linear programming, minimisation, random codes, sparse matrices;.
    Abstract: This paper considers a natural error correcting problem with real valued input/output. We wish to recover an input vector f isin;Rn from corrupted measurements y=Af+e. Here, A is an m by n (coding) matrix and e is an arbitrary and unknown vector of errors. Is it possible to recover f exactly from the data y? We prove that under suitable conditions on the coding matrix A, the input f is the unique solution to the #8467;1-minimization problem (||x|| #8467;1:= Sigma;i|xi|) min(g isin;Rn) ||y - Ag|| #8467;1 provided that the support of the vector of errors is not too large, ||e|| #8467;0:=|{i:ei ne; 0}| le; rho; middot;m for some rho;>0. In short, f can be recovered exactly by solving a simple convex optimization problem (which one can recast as a linear program). In addition, numerical experiments suggest that this recovery procedure works unreasonably well; f is recovered exactly even in situations where a significant fraction of the output is corrupted. This work is related to the problem of finding sparse solutions to vastly underdetermined systems of linear equations. There are also significant connections with the problem of recovering signals from highly incomplete measurements. In fact, the results introduced in this paper improve on our earlier work. Finally, underlying the success of #8467;1 is a crucial property we call the uniform uncertainty principle that we shall describe in detail.
    [bibtex-key = 1542412] [bibtex-entry]


  743. Karlus A. Câmara de Macedo and Rolf Scheiber. Precise topography- and aperture-dependent motion compensation for airborne SAR. IEEE Geosci. Remote Sens. Lett., 2(2):172-176, 2005. Keyword(s): SAR Processing, PTA-MoComp, Postprocessing, Motion Compensation, Topography-Based Motion Compensation, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, fast Fourier transform-based postprocessing methodology, FFT, D-InSAR, German Aerospace Center, DLR, airborne repeat-pass interferometry, differential interferometry, geometric fidelity, motion errors, phase accuracy, residual phase errors, topographic heights, Topography, DEM, Terrain, wide beamwidth, Airborne SAR, ESAR, P-Band, Interferometry.
    Abstract: Efficient synthetic aperture radar (SAR) processing algorithms are unable to exactly implement the aperture- and topography-dependent motion compensation due to the superposition of the synthetic apertures of several targets having different motion errors and potentially different topographic heights. Thus, during motion compensation, a reference level is assumed, resulting in residual phase errors that impact the focusing, geometric fidelity, and phase accuracy of the processed SAR images. This letter proposes a new short fast Fourier transform-based postprocessing methodology capable of efficient and precise compensation of these topography- and aperture-dependent residual phase errors. In addition to wide beamwidth (very high resolution) SAR systems, airborne repeat-pass interferometry especially benefits from this approach, as motion compensation can be significantly improved, especially in areas with high topographic changes. Repeat-pass interferometric data of the E-SAR system of the German Aerospace Center (DLR) are used to demonstrate the performance of the proposed approach.
    [bibtex-key = deMacedoScheiber05:DEMBasedMoComp] [bibtex-entry]


  744. M. Eineder and N. Adam. A maximum-likelihood estimator to simultaneously unwrap, geocode, and fuse SAR interferograms from different viewing geometries into one digital elevation model. Geoscience and Remote Sensing, IEEE Transactions on, 43(1):24 - 36, jan. 2005. Keyword(s): SAR interferogram fusion, SAR interferogram geocoding, SAR interferogram unwrapping, SRTM, Shuttle Radar Topography Mission, critical phase-unwrapping, digital elevation model, geometric baseline error estimates, heterogeneous synthetic aperture radar interferograms, incidence angle, interferometric multiangle observations, interferometric multibaseline observations, map geometry, maximum-likelihood algorithm, maximum-likelihood estimation, periodic likelihood function, radar baseline, radar heading angle, radar wavelength, rugged terrain, scatterer height, terrain mapping, viewing geometries, geophysical signal processing, maximum likelihood estimation, radar signal processing, radiowave interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping;.
    Abstract: This paper presents theory, algorithm, and results of a maximum-likelihood algorithm that is capable to fuse a number of heterogeneous synthetic aperture radar interferograms into a single digital elevation model (DEM) without the need for the critical phase-unwrapping step. The fusion process takes place in the object space, i.e., the map geometry, and considers the periodic likelihood function of each individual interferometric phase sample. The interferograms may vary regarding their radar wavelength, their baseline, their heading angle (ascending or descending), and their incidence angle. Geometric baseline error estimates and a priori knowledge from other estimates like existing DEMs are incorporated seamlessly into the estimation process. The presented approach significantly differs from the standard DEM generation method where each interferogram is first phase-unwrapped individually, then geocoded into a common map geometry, and finally averaged with DEMs generated from other interferograms. By avoiding the phase-unwrapping step, the proposed algorithm does not depend on gradients between samples and is therefore capable to reconstruct the arbitrary height of each single scatterer. Because the height of each DEM sample is determined individually, spatial propagation of phase-unwrapping errors is avoided. The algorithm is targeted to fuse an ensemble of interferometric multiangle or multibaseline observations in areas of rugged terrain or highly ambiguous data where algorithms based on phase unwrapping may fail. The algorithm is explained, and examples with real data from the Shuttle Radar Topography Mission are given. Conditions of future missions are simulated, and optimization criteria for the viewing geometry are discussed.
    [bibtex-key = 1381616] [bibtex-entry]


  745. C. Elachi, S. Wall, M. Allison, Y. Anderson, R. Boehmer, P. Callahan, P. Encrenaz, E. Flamini, G. Franceschetti, Y. Gim, G. Hamilton, S. Hensley, M. Janssen, W. Johnson, K. Kelleher, R. Kirk, R. Lopes, R. Lorenz, J. Lunine, D. Muhleman, S. Ostro, F. Paganelli, G. Picardi, F. Posa, L. Roth, R. Seu, S. Shaffer, L. Soderblom, B. Stiles, E. Stofan, S. Vetrella, R. West, C. Wood, L. Wye, and H. Zebker. Cassini Radar Views the Surface of Titan. Science, 308(5724):970-974, 2005. Keyword(s): SAR Processing, Cassini Radar, Saturn, astronomical instruments, planetary satellites, radar applications, radioastronomy, space vehicles, Cassini Saturn Mission, Cassini Titan Radar Mapper, Titan, antenna configuration, design constraints, multimode radar, radar modes, surface imaging, topographic mapping, Instruments, Laser radar, Moon, Optical design, Payloads, Probes, Radar antennas, Radar imaging, Saturn, Surface topography.
    Abstract: The Cassini Titan Radar Mapper imaged about 1% of Titan's surface at a resolution of ~0.5 kilometer, and larger areas of the globe in lower resolution modes. The images reveal a complex surface, with areas of low relief and a variety of geologic features suggestive of dome-like volcanic constructs, flows, and sinuous channels. The surface appears to be young, with few impact craters. Scattering and dielectric properties are consistent with porous ice or organics. Dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons.
    [bibtex-key = elachiEtAlScience2005CassiniRadarImagesTitan] [bibtex-entry]


  746. Alessandro Ferretti, Marco Bianchi, Claudio Prati, and Fabio Rocca. Higher-Order Permanent Scatterers Analysis. EURASIP Journal on Advances in Signal Processing, 2005(20):609604, 2005. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, SAR Tomography, Permanent scatterers, geodesy, geophysical techniques, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth)InSAR, SAR interferometry, atmospheric disturbance, atmospheric phase screen, differential interferometry, geometrical decorrelation, geophysical measurement technique, land surface topography, permanent scatterer, radar remote sensing, stable natural reflector, surface deformation monitoring, topographic profile reconstruction.
    Abstract: The permanent scatterers (PS) technique is a multi-interferogram algorithm for DInSAR analyses developed in the late nineties to overcome the difficulties related to the conventional approach, namely, phase decorrelation and atmospheric effects. The successful application of this technology to many geophysical studies is now pushing toward further improvements and optimizations. A possible strategy to increase the number of radar targets that can be exploited for surface deformation monitoring is the adoption of parametric super-resolution algorithms that can cope with multiple scattering centres within the same resolution cell. In fact, since a PS is usually modelled as a single pointwise scatterer dominating the background clutter, radar targets having cross-range dimension exceeding a few meters can be lost (at least in C-band datasets), due to geometrical decorrelation phenomena induced in the high normal baseline interferograms of the dataset. In this paper, the mathematical framework related to higher-order SAR interferometry is presented as well as preliminary results obtained on simulated and real data. It is shown how the PS density can be increased at the price of a higher computational load.
    [bibtex-key = ferrettiBianchiPratiRoccaEURASIP2005HigherOrderPSI] [bibtex-entry]


  747. Gianfranco Fornaro, Giorgio Franceschetti, and Stefano Perna. Motion compensation errors: effects on the accuracy of airborne SAR images. IEEE Transactions on Aerospace and Electronic Systems, 41(4):1338-1352, October 2005. Keyword(s): SAR Processing, Motion Compensation, Residual Motion Errors, Autofocus, Airborne SAR, airborne radar, interferometry, motion compensation, radar imaging, synthetic aperture radar airborne SAR images, digital elevation model inaccuracies, motion compensation errors, phase errors, positioning measurement instrument, repeat pass airborne interferometry, residual uncompensated motion errors, synthetic aperture radar.
    Abstract: This work addresses the study of the effect of residual uncompensated motion errors due to positioning measurement instrument and digital elevation model inaccuracies on the accuracy of airborne synthetic aperture radar (SAR) images. It is shown that these not only introduce phase errors following pure geometric considerations, but they also cause additional aberrations related to their interaction with the SAR processing procedure. Extension to the repeat pass airborne interferometry is also included to show their impact on the resulting interferograms.
    [bibtex-key = fornaroFranceschettiPernaTGRS2005MoCompErrorsOnInSAR] [bibtex-entry]


  748. Gianfranco Fornaro, Fabrizio Lombardini, and Francesco Serafino. Three-dimensional multipass SAR focusing: experiments with long-term spaceborne data. IEEE Trans. Geosci. Remote Sens., 43(4):702-714, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, Interferometry, airborne SAR, Spaceborne SAR, stereo image processing, three-dimensional multipass SAR focusing, ERS data, ERS multipass tomography, European Remote Sensing satellite, SAR interferometry, atmospheric variations, azimuth-range resolution cell, critical nonlinear inversion steps, ground pixel, height profile reconstruction, limited spatial-coverage, limited time-span, multibaseline 3D SAR focusing, multiple targets, radiation penetration, scattering mechanisms, scene deformations, side-looking geometry, space-varying phase calibration, tomographic processing, Multibaseline, multipass, three-dimensional focusing.
    Abstract: Synthetic aperture radar (SAR) interferometry is a modern efficient technique that allows reconstructing the height profile of the observed scene. However, apart for the presence of critical nonlinear inversion steps, particularly crucial in abrupt topography scenarios, it does not allow one to separate different scattering mechanisms in the elevation (height) direction within the ground pixel. Overlay of scattering at different elevations in the same azimuth-range resolution cell can be due either to the penetration of the radiation below the surface or to perspective ambiguities caused by the side-looking geometry. Multibaseline three-dimensional (3-D) SAR focusing allows overcoming such a limitation and has thus raised great interest in the recent research. First results with real data have been only obtained in the laboratory and with airborne systems, or with limited time-span and spatial-coverage spaceborne data. This work presents a novel approach for the tomographic processing of European Remote Sensing satellite (ERS) real data for extended scenes and long time span. Besides facing problems common to the airborne case, such as the nonuniformly spaced passes, this processing requires tackling additional difficulties specific to the spaceborne case, in particular a space-varying phase calibration of the data due to atmospheric variations and possible scene deformations occurring for years-long temporal spans. First results are presented that confirm the capability of ERS multipass tomography to resolve multiple targets within the same azimuth-range cell and to map the 3-D scattering properties of the illuminated scene.
    [bibtex-key = fornaroLombardiniSerafino05:MultipassTomo3D] [bibtex-entry]


  749. G. Fornaro, A. Pauciullo, and E. Sansosti. Phase difference-based multichannel phase unwrapping. IEEE Transactions on Image Processing, 14(7):960-972, July 2005. Keyword(s): image processing, maximum likelihood phase unwrapping algorithm, phase difference-based multichannel phase unwrapping algorithm, image processing, maximum likelihood estimation, Algorithms, Artificial Intelligence, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Information Storage and Retrieval, Pattern Recognition, Automated, Radar, Subtraction Technique.
    Abstract: This work addresses the derivation of the phase difference-based maximum likelihood (ML) phase unwrapping algorithm. To this end, we derive the joint statistics of the phase differences on a two-dimensional grid for the multichannel case, where several scaled wrapped phase values are available. Subsequently, we determine and study the structure of the phase difference-based ML estimator and compare it to known phase unwrapping techniques. This work allows us to frame single and multichannel algorithms in a common formulation. Moreover, among the known single-channel phase difference-based procedures, we identify those attaining an ML solution. We also show that multichannel phase difference-based and, recently proposed, phase-based ML algorithms achieve equivalent solutions.
    [bibtex-key = fornaroPauciulloSansostiTIP2005PhaseUnwrapping] [bibtex-entry]


  750. F. Gini and F. Lombardini. Multibaseline Cross-Track SAR Interferometry: A Signal Processing Perspective. IEEE Aerospace and Electronic Systems Magazine, 20(8):71-93, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, AWGN, array signal processing, radar signal processing, speckle, surface topography measurement, synthetic aperture radar, InSAR, MB SAR tomography, SAR signal processing, ULA, XTI-SAR, additive white Gaussian noise, multibaseline cross-track SAR interferometry, multicomponent signal detection, multiplicative noise, natural layover area reflectivity, nonparametric spectral estimation, nonperfectly calibrated arrays, parametric spectral estimation, radar imaging speckle, semitransparent volume scattering layers, synthetic aperture radar interferometry, uniform linear array, Teaching.
    Abstract: Synthetic aperture radar interferometry (InSAR) is a powerful and increasingly expanding technique for measuring the topography of a surface, its changes over both short- and long-time scale, and other changes in the detailed characteristics of the surface. We provide a tutorial description of recent results on multibaseline (MB) InSAR processing. The main focus is on the problem of retrieving both heights and radar reflectivities of natural layover areas by means of a cross-track InSAR (XTI-SAR) system with a uniform linear array (ULA). It is formulated as the problem of detecting and estimating a multicomponent signal corrupted by multiplicative noise - the speckle in the radar imaging jargon - and by additive white Gaussian noise. Application to the InSAR problem of both nonparametric and parametric modern spectral estimation techniques is described. The problem of estimating the number of signal components in the presence of speckle is also addressed. Finally, a brief mention is given to recent research trends on robust methods for nonperfectly calibrated arrays, on processing for non-ULA configurations, and on MB SAR tomography, which is an extension of MB SAR interferometry for the full 3D mapping of semitransparent volume scattering layers. The state of the art of other advanced multichannel interferometric techniques is also briefly recalled.
    [bibtex-key = giniLombardini05:Tomo] [bibtex-entry]


  751. J.A. Given and W.R. Schmidt. Generalized ISAR - part I: an optimal method for imaging large naval vessels. Image Processing, IEEE Transactions on, 14(11):1783 -1791, November 2005. Keyword(s): Doppler rate, fixed linear combination, generalized ISAR, high-accuracy three-dimensional location, inverse synthetic aperture radar, large naval vessel imaging, localized scatterer, moderate intensity ship roll, optimal method, radar cross section, radar signal processing, electromagnetic wave scattering, military radar, radar cross-sections, radar imaging, ships, synthetic aperture radar, Algorithms, Artificial Intelligence, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Information Storage and Retrieval, Pattern Recognition, Automated, Radar, Ships;.
    Abstract: We describe a generalized inverse synthetic aperture radar (ISAR) process that performs well under a wide variety of conditions common to the naval ISAR tests of large vessels. In particular, the generalized ISAR process performs well in the presence of moderate intensity ship roll. The process maps localized scatterers onto peaks on the ISAR plot. However, in a generalized ISAR plot, each of the two coordinates of a peak is a fixed linear combination of the three ship coordinates of the scatterer causing the peak. Combining this process with interferometry will then provide high-accuracy three-dimensional location of the important scatterers on a ship. We show that ISAR can be performed in the presence of simultaneous roll and aspect change, provided the two Doppler rates are not too close in magnitude. We derive the equations needed for generalized ISAR, both roll driven and aspect driven, and test them against simulations performed in a variety of conditions, including large roll amplitudes.
    [bibtex-key = givenSchmidt2005partI] [bibtex-entry]


  752. J.A. Given and W.R. Schmidt. Generalized ISAR-part II: interferometric techniques for three-dimensional location of scatterers. Image Processing, IEEE Transactions on, 14(11):1792 -1797, nov. 2005. Keyword(s): aspect angle, diagnostic inverse synthetic aperture radar, interferometric ISAR optimization, naval vessel, radio-frequency scatterer, roll angle, ship orientation, ship position, specialized signal processing, substantial Doppler return, three-dimensional location, Doppler radar, electromagnetic wave scattering, military radar, radar resolution, radiowave interferometry, ships, synthetic aperture radar, Algorithms, Artificial Intelligence, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Information Storage and Retrieval, Interferometry, Pattern Recognition, Automated, Radar, Ships;.
    Abstract: This paper is the second part of a study dedicated to optimizing diagnostic inverse synthetic aperture radar (ISAR) studies of large naval vessels. The method developed here provides accurate determination of the position of important radio-frequency scatterers by combining accurate knowledge of ship position and orientation with specialized signal processing. The method allows for the simultaneous presence of substantial Doppler returns from both change of roll angle and change of aspect angle by introducing generalized ISAR ates. The first paper provides two modes of interpreting ISAR plots, one valid when roll Doppler is dominant, the other valid when the aspect angle Doppler is dominant. Here, we provide, for each type of ISAR plot technique, a corresponding interferometric ISAR (InSAR) technique. The former, aspect-angle dominated InSAR, is a generalization of standard InSAR; the latter, roll-angle dominated InSAR, seems to be new to this work. Both methods are shown to be efficient at identifying localized scatterers under simulation conditions.
    [bibtex-key = givenSchmidt2005partII] [bibtex-entry]


  753. Young-Kyun Kong, Byung-Lae Cho, and Young-Soo Kim. Ambiguity-free Doppler centroid estimation technique for airborne SAR using the Radon transform. IEEE Trans. Geosci. Remote Sens., 43(4):715-721, 2005. Keyword(s): SAR Processing, Doppler Centroid Estimation, Doppler effect, Radon transforms, airborne radar, geophysical signal processing, geophysical techniques, radar clutter, synthetic aperture radar, Doppler Ambiguity Resolver, DAR, Radon transform, SAR image, SAR signal processing, airborne SAR, ambiguity-free Doppler centroid estimation technique, clutter-lock, geometric distortion, geometry-based Doppler estimator, radiometric error, signal-to-noise ratio, squint angle, Clutter-lock.
    Abstract: In synthetic aperture radar (SAR) signal processing, the Doppler centroid estimation technique, called the ``clutter-lock'', is important because it is related to the signal-to-noise ratio, geometric distortion, and radiometric error of the final SAR image. Conventional algorithms have either ambiguity problems or somewhat high computational load. Using the fact that the Doppler centroid and the squint angle are directly related, we propose an ambiguity-free Doppler centroid estimation technique using Radon transform, named geometry-based Doppler estimator. The proposed algorithm is computationally efficient and shows good performance of estimating the absolute Doppler centroid.
    [bibtex-key = kongChoKim2005:DopCenEstimRadon] [bibtex-entry]


  754. Fabrizio Lombardini. Differential tomography: a new framework for SAR interferometry. IEEE Trans. Geosci. Remote Sens., 43(1):37-44, 2005. Keyword(s): SAR Processing, backscatter, data acquisition, geophysical signal processing, geophysical techniques, multidimensional signal processing, radiowave interferometry, remote sensing by radar, spectral analysis, synthetic aperture radar, Tomography, SAR interferometry, baseline-time acquisition patterns, bidimensional baseline-time spectral analysis, data-dependent bidimensional spectral estimation, SAR Tomography, differential SAR tomography, differential tomography, electromagnetic tomography, elevation-velocity resolution, interferometric technique, joint baseline-time processing, layover scatterers, motion conditions, multibaseline SAR tomography, multiple scatterers, sparse sampling.
    Abstract: A new interferometric mode crossing the differential synthetic aperture radar (SAR) interferometry and multibaseline SAR tomography concepts, that can be termed differential SAR tomography, is proposed. Its potentials, coming from the joint elevation-velocity resolution capability of multiple scatterers, are discussed. Processing is cast in a bidimensional baseline-time spectral analysis framework, with sparse sampling. The use of a modern data-dependent bidimensional spectral estimator is proposed for joint baseline-time processing. Simulated results are reported for different baseline-time acquisition patterns and two motion conditions of layover scatterers, showing that this new challenging interferometric technique is promising.
    [bibtex-key = lombadini05:DiffTomo] [bibtex-entry]


  755. Fabrizio Lombardini and Fulvio Gini. Model order selection in multi-baseline interferometric radar systems. EURASIP Journal on Advances in Signal Processing, 2005(20):1-14, 2005. [bibtex-key = LombardiniGiniEURASIPSignal2005ModelOrderSelection] [bibtex-entry]


  756. L. Noferini, M. Pieraccini, D. Mecatti, G. Luzi, C. Atzeni, A. Tamburini, and M. Broccolato. Permanent scatterers analysis for atmospheric correction in ground-based SAR interferometry. IEEE Trans. Geosci. Remote Sens., 43(7):1459-1471, July 2005. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, Ground-based SAR, Citrin Valley, Italy, atmospheric correction, atmospheric disturbance, atmospheric effects, building structural change detection, ground-based SAR interferometry, landslide monitoring, permanent scatterers analysis, spaceborne SAR interferometry, synthetic aperture radar, terrain monitoring, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth);.
    Abstract: Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, complementary or alternative to spaceborne SAR interferometry, for terrain monitoring, and for detecting structural changes in buildings. It has been noted that, in spite of the very short range, compared with the satellite configuration, in GB-SAR measurement the disturbances due to atmospheric effects cannot be neglected either. The analysis of the interferometric phases of very coherent points, called permanent scatterers (PSs), allows the evaluation of the atmospheric disturbance and the possibility of removing it. In this paper, the PS analysis is carried out both on a test site facility and on a real campaign (Citrin Valley, Italy) that provided data with a temporal baseline of about ten months.
    [bibtex-key = noferiniPieracciniMecattiLuziAtzeniTamburiniBroccolato2005PSIAtmoGroundbased] [bibtex-entry]


  757. Pau Prats, Andreas Reigber, and Jordi J. Mallorqui. Topography-dependent motion compensation for repeat-pass interferometric SAR systems. IEEE Geosci. Remote Sens. Lett., 2(2):206-210, 2005. Keyword(s): SAR Processing, Motion Compensation, Topography-Based Motion Compensation, ESAR, L-Band, Airborne SAR, radar imaging, remote sensing by radar, synthetic aperture radar, Topography, German Aerospace Center E-SAR, DLR, SAR data processing, airborne L-band repeat-pass interferometric data, Interferometry, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, azimuth compression, azimuth coregistration errors, external digital elevation model, DEM, image enhancement, image registration, impulse response degradation, phase artifacts, repeat-pass interferometric SAR systems, Calibration, repeat-pass interferometry.
    Abstract: This letter presents a new motion compensation algorithm to process airborne interferometric repeat-pass synthetic aperture radar (SAR) data. It accommodates topography variations during SAR data processing, using an external digital elevation model. The proposed approach avoids phase artifacts, azimuth coregistration errors, and impulse response degradation, which usually appear due to the assumption of a constant reference height during motion compensation. It accurately modifies phase history of all targets before azimuth compression, resulting in an enhanced image quality. Airborne L-band repeat-pass interferometric data of the German Aerospace Center experimental airborne SAR (E-SAR) is used to validate the algorithm.
    [bibtex-key = pratsReigberMallorqui05TopographyDepMoCo] [bibtex-entry]


  758. A. Reigber and L. Ferro-Famil. Interference suppression in synthesized SAR images. IEEE Geosci. Remote Sens. Lett., 2(1):45-49, 2005. Keyword(s): SAR Processsing, filtering theory, interference suppression, radar imaging, radar interference, synthetic aperture radar, German Aerospace Agency, L-Band, ESAR, SAR imaging, data processing, experimental SAR system, focused SAR images, SLC, image degradation, interferometric coherence, interferometric repeat-pass data, polarimetric descriptors, RFI Suppression, sensitive parameters estimation, synthetic aperture radar imaging.
    Abstract: Radio interferences are becoming more and more an important source for image degradation in synthetic aperture radar (SAR) imaging. Especially at longer wavelengths, interferences are often very strong, and their suppression is required during data processing. However, at shorter wavelengths, interferences are often not obvious in the image amplitude, and filtering is not performed in an operational way. Nevertheless, interferences might significantly degrade the image phase, and the estimation of sensitive parameters like interferometric coherence or polarimetric descriptors becomes imprecise. Interference suppression is usually performed on the raw data, which are in most cases not available to the end-user. In this letter, a new interference suppression method for focused SAR images is proposed. Its performance is tested on interferometric repeat-pass data acquired by the German Aerospace Agency's experimental SAR system (E-SAR) at L-band.
    [bibtex-key = reigberFerrofamil2005:RFISuppInSLC] [bibtex-entry]


  759. Brian D. Rigling and Randolph L. Moses. Taylor expansion of the differential range for monostatic SAR. Aerospace and Electronic Systems, IEEE Transactions on, 41(1):60-64, 2005. Keyword(s): SAR Processing, polynomial approximation, radar imaging, synthetic aperture radar, Taylor expansion, differential range, linear approximation, monostatic SAR, Polar Format Algorithm, second-order Taylor series approximation, Spotlight SAR.
    Abstract: The polar format algorithm (PFA) for spotlight synthetic aperture radar (SAR) is based on a linear approximation for the differential range to a scatterer. We derive a second-order Taylor series approximation of the differential range. We provide a simple and concise derivation of both the far-field linear approximation of the differential range, which forms the basis of the PFA, and the corresponding approximation limits based on the second-order terms of the approximation.
    [bibtex-key = riglingMoses05:diffRange] [bibtex-entry]


  760. Petre Stoica, Zhisong Wang, and Jian Li. Extended derivations of MUSIC in the presence of steering vector errors. IEEE Transactions on Signal Processing, 53(3):1209-1211, March 2005. Keyword(s): SAR Processing, MUSIC, MUltiple SIgnal Classifier, direction-of-arrival estimation, DOA, robustness to steering vector errors, extended derivation, steering vector error, SAR Tomography, Tomography, multi-baseline SAR, InSAR.
    Abstract: We present two extensions of MUSIC to the case in which the steering vector is imprecisely known and show that the extended methods lead to the same direction-of-arrival (DOA) estimates as MUSIC. This somewhat surprising result provides a more general motivation of MUSIC than those currently available and shows that MUSIC possesses a certain degree of inherent robustness to steering vector errors.
    [bibtex-key = stoicaWangLi2005:musicDOARobust] [bibtex-entry]


  761. Tazio Strozzi, Paolo Farina, Alessandro Corsini, Christian Ambrosi, Manfred Thüring, Johannes Zilger, Andreas Wiesmann, Urs Wegmuller, and Charles L. Werner. Survey and monitoring of landslide displacements by means of L-band satellite SAR interferometry. Landslides, 2(3):193-201, 2005. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Landslides, Displacement, Landslide displacement investigation, SAR interferometry, L-band, Swiss and Italian Alps.
    Abstract: This paper illustrates the capabilities of L-band satellite SAR interferometry for the investigation of landslide displacements. SAR data acquired by the L-band JERS satellite over the Italian and Swiss Alps have been analyzed together with C-band ERS-1/2 SAR data and in situ information. The use of L-band SAR data with a wavelength larger than the usual C-band, generally considered for ground motion measurements, reduces some of the limitations of differential SAR interferometry, in particular, signal decorrelation induced by vegetation cover and rapid displacements. The sites of the Alta Val Badia region in South Tyrol (Italy), Ruinon in Lombardia (Italy), Saas Grund in Valais (Switzerland) and Campo Vallemaggia in Ticino (Switzerland), representing a comprehensive set of different mass wasting phenomena in various environments, are considered. The landslides in the Alta Val Badia region are good examples for presenting the improved performance of L-band in comparison to C-band for vegetated areas, in particular concerning open forest. The landslides of Ruinon, Saas Grund, and Campo Vallemaggia demonstrate the strength of L-band in observing moderately fast displacements in comparison to C-band. This work, performed with historical SAR data from a satellite which operated until 1998, demonstrates the capabilities of future planned L-band SAR missions, like ALOS and TerraSAR-L, for landslide studies.
    [bibtex-key = strozziEtAlLandslidesJERS2005] [bibtex-entry]


  762. Y. Yamaguchi, T. Moriyama, M. Ishido, and H. Yamada. Four-component scattering model for polarimetric SAR image decomposition. IEEE Trans. Geosci. Remote Sens., 43(8):1699-1706, August 2005. Keyword(s): SAR Processing, polarimetric decomposition, Yamaguchi decomposition, asymmetric covariance matrix, cloud, co-pol correlation, cross-pol correlation, dipole scatterers, double bounce scattering, four-component scattering model, helix scattering power, image decomposition, nonreflection symmetric scattering, polarimetric SAR, probability density function, radar polarimetry, reflection symmetry condition, scattering contribution decomposition, surface scattering, symmetric covariance matrix, synthetic aperture radar, three-component decomposition method, urban area scattering, volume scattering, covariance matrices, geophysical signal processing, probability, radar polarimetry, remote sensing by radar, synthetic aperture radar.
    Abstract: A four-component scattering model is proposed to decompose polarimetric synthetic aperture radar (SAR) images. The covariance matrix approach is used to deal with the nonreflection symmetric scattering case. This scheme includes and extends the three-component decomposition method introduced by Freeman and Durden dealing with the reflection symmetry condition that the co-pol and the cross-pol correlations are close to zero. Helix scattering power is added as the fourth component to the three-component scattering model which describes surface, double bounce, and volume scattering. This helix scattering term is added to take account of the co-pol and the cross-pol correlations which generally appear in complex urban area scattering and disappear for a natural distributed scatterer. This term is relevant for describing man-made targets in urban area scattering. In addition, asymmetric volume scattering covariance matrices are introduced in dependence of the relative backscattering magnitude between HH and VV. A modification of probability density function for a cloud of dipole scatterers yields asymmetric covariance matrices. An appropriate choice among the symmetric or asymmetric volume scattering covariance matrices allows us to make a best fit to the measured data. A four-component decomposition algorithm is developed to deal with a general scattering case. The result of this decomposition is demonstrated with L-band Pi-SAR images taken over the city of Niigata, Japan.
    [bibtex-key = yamaguchiMoriyamaIshidoYamada2005:PolDecomp] [bibtex-entry]


  763. Urs Wegmuller, Charles L. Werner, Tazio Strozzi, and Andreas Wiesmann. Multi-temporal interferometric point target analysis. In Paul Smits and Lorenzo Bruzzone, editors, Analysis of multi-temporal remote sensing images, volume 3 of Series in Remote Sensing, pages 136-144. World Scientific, 2004. Keyword(s): SAR Processing, SAR Interferometry, InSAR, Persistent Scatterer Interferometry, PSI, Interferometric Point Target Analysis, IPTA.
    Abstract: Interferometric Point Target Analysis (IPTA) is a method to exploit the temporal and spatial characteristics of interferometric signatures collected from point targets to accurately map surface deformation histories, terrain heights, and relative atmospheric path delays. In this contribution the IPTA methodology is presented. Furthermore, based on examples, its applicability for stacks of ERS and JERS SAR data is validated.
    [bibtex-key = wegmullerWernerStrozziWiesmannMultiTempBook2014IPTA] [bibtex-entry]


  764. Nico Adam, Bert Kampes, and Michael Eineder. Development of a Scientific Permanent Scatterer System: Modifications for Mixed ERS/Envisat Time Series. Proceedings of the 2004 Envisat and ERS Symposium (ESA SP-572), September 2004. Keyword(s): SAR Processing, Persistent Scatter Interferometry, PSI, phase standard deviation, signal-to-clutter ratio, point targets, phase standard deviation as a function of signal-to-clutter ratio, Corner reflector, trihedral corner reflector, radar cross section, RCS. [bibtex-key = adamKampesEinederConf2004DevelopmentOfAPSIsystem] [bibtex-entry]


  765. Sune R.J. Axelsson. Beam characteristics of three-dimensional SAR in curved or random paths. IEEE Trans. Geosci. Remote Sens., 42(10):2324-2334, Oct. 2004. Keyword(s): SAR Processing, SAR Tomography, Tomography, Non-Linear Flight Path, Non-linear SAR.
    Abstract: Interferometric synthetic aperture radar (InSAR) provides average height information by combining data from two parallel paths. True three-dimensional (3-D) SAR also detects the height distribution, which is of significant interest in airborne reconnaissance, forest inventory, and subsurface or wall-penetrating sensing applications as examples. In this paper, the beam performance of 3-D SAR is studied and compared for different curved line paths, such as circles, ellipses, and spirals ending up with random sampling. Curved path geometry reduces the ambiguity in height angle of traditional multipass SAR, and random path variation further improves the sidelobe suppression. The poor sidelobe suppression of a single circle path is significantly improved in near-range geometry in combination with high range resolution. By introducing a window function dependent on focus point and path position, high sidelobe suppression was achieved in an extended ground area below the circle path.
    [bibtex-key = Axelsson2004a] [bibtex-entry]


  766. Ciro Cafforio, Pietro Guccione, and Andrea Monti-Guarnieri. Doppler Centroid Estimation for ScanSAR Data. IEEE Transactions on Geoscience and Remote Sensing, 42(1):14-23, January 2004. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Clutterlock, Satellite SAR, RADARSAT 1, ENVISAT, ScanSAR.
    Abstract: We introduce a novel accurate technique to estimate the Doppler centroid (DC) in ScanSAR missions. The technique starts from the ambiguous DC measures in the subswaths and uses a method alternative to standard unwrapping to undo the jumps in estimates induced by modulo pulse repetition frequency (PRF) measures. The proposed alternative is less error prone than the usual unwrapping techniques. Doppler Ambiguity is then solved by implementing a maximum-likelihood estimate that exploits the different PRFs used in different subswaths. An azimuth pointing of the antenna that does not change with subswaths, or that changes in a known way, is assumed. However, if the PRF diversity is strong enough, unknown small changes in azimuth pointing are tolerated and accurately estimated. This estimator is much simpler and more efficient, than those in the literature. Results achieved with both RADARSAT 1 and ENVISAT ScanSAR data are reported.
    [bibtex-key = CaffGuccMonti04:Doppler] [bibtex-entry]


  767. Boon Leng Cheong, Michael W. Hoffman, Robert D. Palmer, Stephen J. Frasier, and F. J. López-Dekker. Pulse pair beamforming and the effects of reflectivity field variations on imaging radars. Radio Science, 39(3), 2004. Note: RS3014. Keyword(s): Radio Science: Remote sensing, Radio Science: Signal processing, Radio Science: Tomography and imaging, Radio Science: Instruments and techniques, pulse pair beamforming, gradient variations, imaging radars.
    Abstract: Coherent radar imaging (CRI), which is fundamentally a beamforming process, has been used to create images of microscale, reflectivity structures within the resolution volume of atmospheric Doppler radars. This powerful technique has the potential to unlock many new discoveries in atmospheric studies. The Turbulent Eddy Profiler (TEP) is a unique 915 MHz boundary layer radar consisting of a maximum of 91 independent receivers. The TEP configuration allows sophisticated CRI algorithms to be implemented providing significant improvement in angular resolution. The present work includes a thorough simulation study of some of the capabilities of the TEP system. The pulse pair processor, used for radial velocity and spectral width estimation with meteorological radars, is combined with beamforming technique, in an efficient manner, to the imaging radar case. By numerical simulation the new technique is shown to provide robust and computationally efficient estimates of the spectral moments. For this study, a recently developed atmospheric radar simulation method is employed that uses the ten thousand scattering points necessary for the high resolution imaging simulation. Previous methods were limited in the number of scatterers due to complexity issues. Radial velocity images from the beamforming radar are used to estimate the three-dimensional wind field map within the resolution volume. It is shown that a large root mean square (RMS) error in imputed three-dimensional wind fields can occur using standard Fourier imaging. This RMS error does not improve even as SNR is increased. The cause of the error is reflectivity variations within the resolution volume. The finite beamwidth of the beamformer skews the radial velocity estimate, and this results in poor wind field estimates. Adaptive Capon beamforming consistently outperforms the Fourier method in the quantitative study and has been demonstrated to enhance the performance compared to the Fourier method.
    [bibtex-key = CheongHoffmanPalmerFrasierLopezDekker2004] [bibtex-entry]


  768. Ian G. Cumming. A Spatially Selective Approach to Doppler Estimation for Frame-Based Satellite SAR Processing. IEEE Transactions on Geoscience and Remote Sensing, 42(6):1135-1148, June 2004. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Satellite SAR, Geometry Models, Quality Metrics, Global Surface Fit.
    Abstract: When Doppler centroid estimators are applied to satellite SAR data, biased estimates are often obtained because of anomalies in the received data. Typical anomalies include areas of low SNR, strong discrete targets and radiometric discontinuities. In this paper, a new method of Doppler centroid estimation is presented that takes advantage of principles such as spatial diversity, estimator quality checks, geometric models, and the fitting of a global estimate over a wide area of a SAR scene. In the proposed scheme, Doppler estimates are made over small blocks of data covering a whole frame, so that all parts of the scene are potentially represented. The quality of each block estimate is examined using data statistics or estimator quality measures. Poor estimates are rejected, and the remaining estimates are used to fit a surface model of the Doppler centroid versus the range and azimuth extent of the scene. A physical model that relates the satellite's orbit, attitude and beam-pointing-direction to the Doppler centroid is used to get realistic surface fits and to reduce the complexity (dimensionality) of the estimation problem. The method is tested with RADARSAT-1 and Shuttle Radar Topography Mission X-band SAR (SRTM/X-SAR) spaceborne data and is found to work well with scenes that do have radiometric anomalies, and in scenes where attitude adjustments cause the Doppler to change rapidly.
    [bibtex-key = Cumming04:DopCentrEst] [bibtex-entry]


  769. Knut Eldhuset. Ultra high resolution spaceborne SAR processing. Aerospace and Electronic Systems, IEEE Transactions on, 40(1):370-378, 2004. Keyword(s): SAR Processing, radar signal processing, spaceborne radar, synthetic aperture radar, transfer functions, EETF4, SAR geometry, X-band radar, azimuth signal accuracy requirements, erroneous Doppler parameters, fourth-order extended exact transfer function, spaceborne SAR processing, squint, ultra high resolution SAR.
    Abstract: The accuracy requirements of the azimuth signal for spaceborne synthetic aperture radar (SAR) processing is analysed. Degradations of simulated extended scenes and point targets are studied at high squint or if erroneous Doppler parameters are used. At a resolution less than 1 m for X-band it is shown that the azimuth signal for spaceborne SAR geometry should be used. It is found that the fourth-order extended exact transfer function (EETF4) is a good candidate for demanding SAR processing.
    [bibtex-key = eldhuset04:EETF4] [bibtex-entry]


  770. Jos Groot. River dike deformation measurement with airborne SAR. IEEE Geosci. Remote Sens. Lett., 1(2):94-97, April 2004. Keyword(s): SAR Processing, DInSAR, repeat-pass interferometry, InSAR, SAR interferometry, deformation measurement, displacement, subsidence, airborne SAR, deformation, remote sensing by radar, rivers, synthetic aperture radar, 5.5 day, airborne SAR, deformation measurement, interferometry, reflector analysis, river dikes, synthetic aperture radar, water levels, Atmospheric measurements, Interferometry, Levee, Radar tracking, Rivers, Synthetic aperture radar, Testing.
    Abstract: Due to extremely high water levels or extremely dry periods, river dikes can deform and eventually burst. The deformation can be measured with repeat-pass synthetic aperture radar (SAR) interferometry. Two flights (5.5-day interval) with an airborne C-band SAR were carried out to verify a particular case. During the second flight, the dike was deliberately deformed. Interferogram analyses showed that the dike deformed by at most 2 mm on this day, in approximate agreement with tachymeter measurements. Corner reflector analysis showed a deformation of about 1 cm during the 5.5-day interval, in good agreement with ground measurements.
    [bibtex-key = grootGRSL2004DikeDInSAR] [bibtex-entry]


  771. Andrew Hooper, Howard Zebker, Paul Segall, and Bert Kampes. A new method for measuring deformation on volcanoes and other natural terrains using InSAR persistent scatterers. Geophysical research letters, 31(23), 2004. [bibtex-key = hooperZebkerSegallKampesGRL2004DINSARPSIVolcanoDef] [bibtex-entry]


  772. Bert M. Kampes and Ramon F. Hanssen. Ambiguity resolution for permanent scatterer interferometry. IEEE Trans. Geosci. Remote Sens., 42(11):2446-2453, November 2004. Keyword(s): LAMBDA method, ambiguity decorrelation, ambiguity resolution, bootstrap estimation, carrier phase observations, global positioning systems, integer least squares estimation, multivariate normal distribution, nonlinear parameter inversion algorithm, permanent scatterer interferometry, phase cycle ambiguity estimation, synthetic aperture radar interferometry, system of equations, variance-covariance matrix, geophysical signal processing, least squares approximations, normal distribution, parameter estimation, radiowave interferometry, remote sensing, remote sensing by radar, synthetic aperture radar;.
    Abstract: In the permanent scatterer technique of synthetic aperture radar interferometry, there is a need for an efficient and reliable nonlinear parameter inversion algorithm that includes estimation of the phase cycle ambiguities. Present techniques make use of a direct search of the solution space, treating the observations as deterministic and equally weighted, and which do not yield an exact solution. Moreover, they do not describe the quality of the estimated parameters. Here, we use the integer least squares estimator, which has the highest probability of correct integer estimation for problems with a multivariate normal distribution. With this estimator, the propagated variance-covariance matrix of the estimated parameters can be obtained. We have adapted the LAMBDA method, part of an integer least squares estimator developed for the ambiguity resolution of carrier phase observations in global positioning systems, to the problem of permanent scatterers. Key elements of the proposed method are the introduction of pseudo-observations to regularize the system of equations, decorrelation of the ambiguities for an efficient estimation, and the combination of a bootstrap estimator with an integer least squares search to obtain the final integer estimates. The performance of the proposed algorithm is demonstrated using simulated and real data.
    [bibtex-key = kampesHanssen2004PSI] [bibtex-entry]


  773. R. Lanari, O. Mora, M. Manunta, J. J. Mallorqui, P. Berardino, and E. Sansosti. A small-baseline approach for investigating deformations on full-resolution differential SAR interferograms. IEEE Trans. Geosci. Remote Sens., 42(7):1377-1386, July 2004. Keyword(s): radar resolution, radiowave interferometry, remote sensing by radar, singular value decomposition, synthetic aperture radar, terrain mapping, topography (Earth), DIFSAR interferograms, European Remote Sensing satellites, atmospheric phase artifacts, differential SAR interferometry, digital elevation model, geodetic measurements, ground deformation, large-scale deformations, multilook data, residual phase components, single-look data, singular value decomposition, small-baseline subset, spatial resolutions, synthetic aperture radar, temporal evolution, topographic errors, Buildings, Digital elevation models, Interferometry, Large-scale systems, Phase detection, Phase estimation, Singular value decomposition, Spatial resolution, Synthetic aperture radar, Testing, Ground deformations, SAR, SAR interferometry, synthetic aperture radar.
    Abstract: This paper presents a differential synthetic aperture radar (SAR) interferometry (DIFSAR) approach for investigating deformation phenomena on full-resolution DIFSAR interferograms. In particular, our algorithm extends the capability of the small-baseline subset (SBAS) technique that relies on small-baseline DIFSAR interferograms only and is mainly focused on investigating large-scale deformations with spatial resolutions of about 100×100 m. The proposed technique is implemented by using two different sets of data generated at low (multilook data) and full (single-look data) spatial resolution, respectively. The former is used to identify and estimate, via the conventional SBAS technique, large spatial scale deformation patterns, topographic errors in the available digital elevation model, and possible atmospheric phase artifacts; the latter allows us to detect, on the full-resolution residual phase components, structures highly coherent over time (buildings, rocks, lava, structures, etc.), as well as their height and displacements. In particular, the estimation of the temporal evolution of these local deformations is easily implemented by applying the singular value decomposition technique. The proposed algorithm has been tested with data acquired by the European Remote Sensing satellites relative to the Campania area (Italy) and validated by using geodetic measurements.
    [bibtex-key = lanariMoraManuntaMallorquiBerardinoSansosti2004TGRSSBASFullResDInSAR] [bibtex-entry]


  774. F. Lombardini, F. Bordoni, F. Gini, and L. Verrazzani. Multibaseline ATI-SAR for robust ocean surface velocity estimation. IEEE Transactions on Aerospace and Electronic Systems, 40(2):417-433, April 2004. Keyword(s): Bragg frequency, Bragg scatterers, Doppler centroid measurements, Doppler resolution, MB estimators, along-track interferometry, multibaseline ATI-SAR, ocean surface currents, robust data inversion, robust ocean surface velocity estimation, scatterer propagation velocity, synthetic aperture radar, wind direction, Doppler radar, Doppler shift, oceanographic techniques, radiowave interferometry, remote sensing by radar, synthetic aperture radar;.
    Abstract: An open problem of along-track interferometry (ATI) for synthetic aperture radar (SAR) sensing of ocean surface currents is the need of ancillary wind information for inversion of Doppler centroid measurements, that have to be compensated for the propagation velocity of advancing and/or receding Bragg scatterers. We propose three classes of estimators which exploit multibaseline (MB) ATI acquisition and Doppler resolution for robust data inversion under different degrees of a priori information about the wind direction and the value of the characteristic Bragg frequency. Performance analysis and comparison with conventional ATI show that the proposed MB estimators can produce accurate velocity estimates in the absence of detailed ancillary data.
    [bibtex-key = lombardiniBordoniGiniVerrazzani2004] [bibtex-entry]


  775. G. Luzi, M. Pieraccini, D. Mecatti, L. Noferini, G. Guidi, F. Moia, and C. Atzeni. Ground-based radar interferometry for landslides monitoring: atmospheric and instrumental decorrelation sources on experimental data. IEEE Trans. Geosci. Remote Sens., 42(11):2454-2466, November 2004. Keyword(s): Decorrelation, Temporal Decorrelation, Adaptive optics, Decorrelation, Image analysis, Instruments, Monitoring, Optical distortion, Optical interferometry, Radar interferometry, Synthetic aperture radar, Terrain factors, data acquisition, decorrelation, geomorphology, geophysical signal processing, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), 5.725 GHz, AD 2002, C-band ground-based equipment, Civita di Bagnoregio, Italy, SAR image acquisition, ancient town, atmospheric decorrelation sources, coherent synthetic aperture radar, geometric distortion, ground-based radar interferometry, instrumental decorrelation sources, interferometric data, landslide monitoring, mechanical stability, optical photogrammetry, spatial decorrelation, temporal decorrelation, terrain movements, 65, Decorrelation, SAR, differential interferometry, ground-based synthetic aperture radar, phase stability, synthetic aperture radar;.
    Abstract: The application of ground-based radar interferometry for landslide monitoring is analyzed: a case study based on an experimental campaign carried out in Italy during 2002 is discussed. Interferometric data obtained from coherent synthetic aperture radar (SAR) images acquired by means of C-band ground-based equipment are analyzed. The campaign was aimed at retrieving potential terrain movements of a small landslide observed hundreds of meters away. Critical aspects related to spatial and temporal decorrelation are discussed: the use of optical photogrammetry as a technique for evaluating mechanical stability and correcting geometric distortion is presented. Results also confirmed that the application of ground-based radar interferometry can be attractive and effective if the acquired SAR images maintain an adequate coherence on different dates.
    [bibtex-key = luziPieracciniMecattiNoferiniGuidiMoiaAtzeniTGRS2004GroundBasedSAR] [bibtex-entry]


  776. Andrew S. Milman. The Hyperbolic Geometry of SAR Imaging. Submitted to RADIO SCIENCE, 0(0):0-0, 2004. Keyword(s): SAR Processing, Hyperbolic Functions, Fourier-Hankel Inversion, Hankel Transform, Abel Transform, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Stolt Mapping.
    Abstract: This paper shows how we can use hyperbolic functions to write an exact mathematical representation of SAR imaging. This problem is primarily a geometric one, that of accounting for curved wavefronts: the spirit of this paper is to emphasize these geometrical properties over electromagnetic ones. This gives us a new and fruitful way to think about SAR imaging. Within this framework, I show how to correct for deviations of a radar from a straight flight path. This method will work even in situations where the curvature of the wavefronts is very large, where traditional methods do not. The image-formation algorithm, called omega-k migration, that results from this analysis of SAR imaging is simpler and faster than polar formatting, especially for radars with very large beamwidths as they will at very low frequencies. As an added benefit, omega-k migration is surprisingly simple to derive.
    [bibtex-key = milmanSubmitted:Hyperbolic] [bibtex-entry]


  777. G. Nico, D. Leva, G. Antonello, and D. Tarchi. Ground-based SAR interferometry for terrain mapping: theory and sensitivity analysis. IEEE_J_GRS, 42(6):1344-1350, June 2004. Keyword(s): radar theory, radiowave interferometry, remote sensing by radar, sensitivity analysis, synthetic aperture radar, terrain mapping, GB-SAR interferometry, GB-SAR system, InSAR, digital elevation model, ground-based SAR interferometry, ground-based radar, phase-to-height relationship, Analytical models, Aperture antennas, Focusing, Radar antennas, Radar imaging, Rails, Sensitivity analysis, Synthetic aperture radar, Synthetic aperture radar interferometry, Terrain mapping, DEM, Digital elevation model, GB, SAR, ground-based, radar, synthetic aperture radar interferometry. [bibtex-key = Nico2004] [bibtex-entry]


  778. Mats I. Pettersson. Detection of Moving Targets in Wideband SAR. IEEE Transactions on Aerospace and Electronic Systems, 40(3):780-796, July 2004. Keyword(s): SAR Processing, Fast Back-Projection, Factorized Back-Projection, Time-Domain Back-Projection, TDBP, Back-Projection, Moving Target Indication, Ultra-Wideband SAR, Wideband SAR, VHF SAR, UHF SAR, CARABAS, LORA, Airborne SAR, C-Band.
    Abstract: A likelihood ratio is proposed for moving target detection in a wideband (WB) synthetic aperture radar (SAR) system. WB is defined here as any systems having a large fractional bandwidth, i.e., an ultra wide frequency band combined with a wide antenna beam. The developed method combines time-domain fast backprojection SAR processing methods with moving target detection using space-time processing. The proposed method reduces computational load when sets of relative speeds can be tested using the same clutter-suppressed subaperture beams. The proposed method is tested on narrowband radar data.
    [bibtex-key = Pettersson04:BackprojMTI] [bibtex-entry]


  779. Pau Prats, Andreas Reigber, and Jordi J. Mallorqui. Interpolation-free coregistration and phase-correction of airborne SAR interferograms. IEEE Geosci. Remote Sens. Lett., 1(3):188-191, 2004. Keyword(s): SAR Processing, Airborne SAR, calibration, Interferometry, L-Band, synthetic aperture radar, airborne L-band repeat-pass interferometry, airborne SAR interferograms, azimuth registration errors, interpolation-free coregistration, navigation system, phase azimuth undulations, residual motion error correction, spectral diversity technique, Calibration, SAR, image registration, interferometry, Motion Compensation, repeat-pass interferometry.
    Abstract: This letter discusses the detection and correction of residual motion errors that appear in airborne synthetic aperture radar (SAR) interferograms due to the lack of precision in the navigation system. As it is shown, the effect of this lack of precision is twofold: azimuth registration errors and phase azimuth undulations. Up to now, the correction of the former was carried out by estimating the registration error and interpolating, while the latter was based on the estimation of the phase azimuth undulations to compensate the phase of the computed interferogram. In this letter, a new correction method is proposed, which avoids the interpolation step and corrects at the same time the azimuth phase undulations. Additionally, the spectral diversity technique, used to estimate registration errors, is critically analyzed. Airborne L-band repeat-pass interferometric data of the German Aerospace Center (DLR) experimental airborne SAR is used to validate the method.
    [bibtex-key = pratsReigberMallorqui04InterpolFreeCoregistration] [bibtex-entry]


  780. B.D. Rigling and R.L. Moses. Flight path strategies for 3-D scene reconstruction from bistatic SAR. Radar, Sonar and Navigation, IEE Proceedings -, 151(3):149-157, 2004. Keyword(s): SAR Processing, Non-linear Flight Path, SAR Tomography, Curvilinear SAR, Bistatic SAR, airborne radar, image reconstruction, parameter estimation, radar imaging, synthetic aperture radar, 3-D scene reconstruction, bistatic SAR, data collection strategy, flight path strategies, nonlinear receiver trajectory, nonparametric scene reconstructions, phase history, receiver flight paths, three-dimensional information, three-dimensional scattering centre location parameter estimates, three-dimensional scene information.
    Abstract: Proper design of receiver flight paths allows three-dimensional information to be encoded in sensed bistatic SAR phase history data. The authors show how the flight path taken by the receiving platform determines the resolution of nonparametric scene reconstructions and determines the variance of three-dimensional scattering centre location parameter estimates. Based on these relationships, a bistatic SAR data collection strategy can be designed, involving a nonlinear receiver trajectory that allows preservation of three-dimensional scene information.
    [bibtex-key = riglingMoses2004:BistaticNonLinearSARTomo] [bibtex-entry]


  781. Brian D. Rigling and Randolph L. Moses. Polar format algorithm for bistatic SAR. Aerospace and Electronic Systems, IEEE Transactions on, 40(4):1147-1159, 2004. Keyword(s): SAR Processing, backpropagation, computational complexity, matched filters, radar imaging, synthetic aperture radar, Back-Projection, Time-Domain Back-Projection, Fast Back-Projection, Bistatic SAR, bistatic far-field assumption, matched filtering, monostatic SAR image formation, phase history data, Polar Format Algorithm.
    Abstract: Matched filtering (MF) of phase history data is a mathematically ideal but computationally expensive approach to bistatic synthetic aperture radar (SAR) image formation. Fast backprojection algorithms (BPAs) for image formation have recently been shown to give improved O(N/sup 2/ log/sub 2/N) performance. An O(N/sup 2/ log/sub 2/N) bistatic polar format algorithm (PFA) based on a bistatic far-field assumption is derived. This algorithm is a generalization of the popular PFA for monostatic SAR image formation and is highly amenable to implementation with existing monostatic image formation processors. Limits on the size of an imaged scene, analogous to those in monostatic systems, are derived for the bistatic PFA.
    [bibtex-key = riglingMoses04:PFABackproj] [bibtex-entry]


  782. B.D. Rigling and R.L. Moses. Polar format algorithm for bistatic SAR. IEEE_J_AES, 40(4):1147-1159, October 2004. Keyword(s): SAR Processing, Bistatic SAR, Time-Domain Back-Projection, TDBP, back-projection, bistatic far-field assumption, matched filtering, monostatic SAR image formation, phase history data, polar format algorithm, synthetic aperture radar, backpropagation, computational complexity, matched filters, radar imaging, synthetic aperture radar.
    Abstract: Matched filtering (MF) of phase history data is a mathematically ideal but computationally expensive approach to bistatic synthetic aperture radar (SAR) image formation. Fast backprojection algorithms (BPAs) for image formation have recently been shown to give improved O(N2 log2N) performance. An O(N2 log2N) bistatic polar format algorithm (PFA) based on a bistatic far-field assumption is derived. This algorithm is a generalization of the popular PFA for monostatic SAR image formation and is highly amenable to implementation with existing monostatic image formation processors. Limits on the size of an imaged scene, analogous to those in monostatic systems, are derived for the bistatic PFA.
    [bibtex-key = RiglingMoses2004:BiStaticPolarFormat] [bibtex-entry]


  783. Z. Su, Y. Peng, and X. Wang. Efficient algorithm for three-dimensional target feature extraction via CLSAR. Electronics Letters, 40(15):965-966, 2004. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR, fast Fourier transforms, feature extraction, radar imaging, synthetic aperture radar, Cramer-Rao bounds, LODIPS algorithm, RELAX algorithm, SNR, computational load, curvilinear synthetic aperture radar system, lower dimensional FFT, lower dimensional position searching, three dimensional target feature extraction.
    Abstract: A lower dimensional position searching (LODIPS) algorithm is proposed for three-dimensional (3-D) target feature extraction via the curvilinear synthetic aperture radar (CLSAR) system. Compared with another similarly structured algorithm RELAX, the LODIPS algorithm dramatically reduces the computational load through the utilisation of lower dimensional FFTs. Simulation results show that the new algorithm can reach CRB at low SNR.
    [bibtex-key = suPengWang2004:NonLinearSARTomo] [bibtex-entry]


  784. Robert N. Treuhaft, Beverly E. Law, and Gregory P. Asner. Forest Attributes from Radar Interferometric Structure and Its Fusion with Optical Remote Sensing. BioScience, 54(6):561-571, 2004. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Forest, remote sensing, carbon cycle, LiDAR, optical remote sensing, forest ecology, SAR Tomography.
    Abstract: The possibility of global, three-dimensional remote sensing of forest structure with interferometric synthetic aperture radar (InSAR) bears on important forest ecological processes, particularly the carbon cycle. InSAR supplements two-dimensional remote sensing with information in the vertical dimension. Its strengths in potential for global coverage complement those of lidar (light detecting and ranging), which has the potential for high-accuracy vertical profiles over small areas. InSAR derives its sensitivity to forest vertical structure from the differences in signals received by two, spatially separate radar receivers. Estimation of parameters describing vertical structure requires multiple-polarization, multiple-frequency, or multiple-baseline InSAR. Combining InSAR with complementary remote sensing techniques, such as hyperspectral optical imaging and lidar, can enhance vertical-structure estimates and consequent biophysical quantities of importance to ecologists, such as biomass. Future InSAR experiments will supplement recent airborne and spaceborne demonstrations, and together with inputs from ecologists regarding structure, they will suggest designs for future spaceborne strategies for measuring global vegetation structure.
    [bibtex-key = treuhaftLawAsner2004:ForestAttributeFromInSAR] [bibtex-entry]


  785. D.A. Yocky, D.E. Wahl, and C.V. Jakowatz. Terrain elevation mapping results from airborne spotlight-mode coherent cross-track SAR stereo. IEEE Transactions on Geoscience and Remote Sensing, 42(2):301-308, February 2004. Keyword(s): Terrain mapping, Synthetic aperture radar, Spatial resolution, Radar scattering, Electromagnetic scattering, Image resolution, Earth, Convergence, Spaceborne radar, Geometry.
    Abstract: Coherent cross-track synthetic aperture radar (SAR) stereo is shown to produce high-resolution three-dimensional maps of the Earth surface. This mode utilizes image pairs with common synthetic apertures but different squint angles allowing automated stereo correspondence and disparity estimation using complex correlation calculations. This paper presents two Ku-band, coherent cross-track stereo collects over rolling and rugged terrain. The first collect generates a digital elevation map (DEM) with 1-m posts over rolling terrain using complex SAR imagery with spatial resolution of 0.125 m and a stereo convergence angle of 13.8/spl deg/. The second collect produces multiple DEMs with 3-m posts over rugged terrain utilizing complex SAR imagery with spatial resolutions better than 0.5 m and stereo convergence angles greater than 40/spl deg/. The resulting DEMs are compared to ground-truth DEMs and relative height root-mean-square, linear error 90-percent confidence, and maximum height error are reported.
    [bibtex-key = Yocky2004] [bibtex-entry]


  786. V. Alberga and M. Chandra. Volume decorrelation resolution in polarimetric SAR interferometry. Electronics Letters, 39(3):314-315, February 2003. Keyword(s): buildings, distributed scatterer height, forests, interferometric coherence images, man-made artifacts, polarimetric SAR data, polarimetric SAR interferometry, random volumes, stationary distributed targets, synthetic aperture radar, volume decorrelation resolution, decorrelation, radar imaging, radar polarimetry, radiowave interferometry, synthetic aperture radar, target tracking;.
    Abstract: Volume decorrelation is a phenomenon affecting the synthetic aperture radar (SAR) interferometric coherence images in the presence of forests or buildings. 'Resolving' this effect could lead to an estimation of the height of the observed distributed scatterers. An empirical method is suggested to distinguish volume decorrelation due to random volumes (such as trees) and stationary distributed targets (such as man-made artifacts) when polarimetric SAR data are used.
    [bibtex-key = 1179472] [bibtex-entry]


  787. I. Baran, M. P. Stewart, B. M. Kampes, Z. Perski, and P. Lilly. A modification to the Goldstein radar interferogram filter. IEEE Transactions on Geoscience and Remote Sensing, 41(9):2114-2118, Sep. 2003. Keyword(s): radar theory, radiowave interferometry, synthetic aperture radar, Goldstein radar interferogram filter modification, adaptive Goldstein radar interferogram filter, radar interferometry products, synthetic aperture radar, SAR, phase filtering, phase statistics, coherence, incoherent areas, coherent areas, loss of signal, noise level reduction, Adaptive filters, Filtering, Australia, Radar interferometry, Noise reduction, Monitoring, Decorrelation, Noise level, Statistics, Synthetic aperture radar.
    Abstract: We present a modification to the adaptive Goldstein radar interferogram filter which improves the quality of interferometry products. The proposed approach makes the Goldstein filter parameter alpha dependent on coherence, such that incoherent areas are filtered more than coherent areas. This modification minimizes loss of signal while still reducing the level of noise.
    [bibtex-key = baranStewartKampesPerskiLillyTGRS2003ModifiedGoldsteinWernerInSARFilter] [bibtex-entry]


  788. Silvia Cimmino, Giorgio Franceschetti, Antonio Iodice, Daniele Riccio, and Giuseppe Ruello. Efficient Spotlight SAR Raw Signal Simulation of Extended Scenes. IEEE Transactions on Geoscience and Remote Sensing, 41(10):2329-2337, October 2003. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator, Spotlight SAR.
    Abstract: Synthetic aperture radar (SAR) raw signal simulation is a powerful tool for designing new sensors, testing processing algorithms, planning missions, and devising inversion algorithms. In this paper, a spotlight SAR raw signal simulator for distributed targets is presented. The proposed procedure is based on a Fourier domain analysis: a proper analytical reformulation of the spotlight SAR raw signal expression is presented. It is shown that this reformulation allows us to design a very efficient simulation scheme that employs fast Fourier transform codes. Accordingly, the computational load is dramatically reduced with respect to a time-domain simulation and this, for the first time, makes spotlight simulation of extended scenes feasible.
    [bibtex-key = CimFrancesIodiceRiccio03:Simulation] [bibtex-entry]


  789. C. Colesanti, A. Ferretti, F. Novali, C. Prati, and F. Rocca. SAR monitoring of progressive and seasonal ground deformation using the permanent scatterers technique. IEEE Trans. Geosci. Remote Sens., 41(7):1685-1701, July 2003. Keyword(s): SAR Processing, Persistent Scatterer Interferometry, PSI, California, Fremont, SAR, San Jose, Santa Clara Valley, Southern Bay Area, USA, United States, coseismic displacement, differential radar interferometry, geodesy, geodetic deformation, geophysical measurement technique, land surface, permanent scatterer, phase decorrelation, postseismic displacement, progressive deformation, radar remote sensing, seasonal ground deformation, spaceborne radar, subsidence, synthetic aperture radar, terrain mapping, geodesy, geophysical techniques, radar theory, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping;.
    Abstract: Spaceborne differential radar interferometry has proven a remarkable potential for mapping ground deformation phenomena (e.g., urban subsidence, volcano dynamics, coseismic and postseismic displacements along faults, as well as slope instability). However, a full operational capability has not been achieved yet due to atmospheric disturbances and phase decorrelation phenomena. These drawbacks can often be-at least partially-overcome by carrying out measurements on a subset of image pixels corresponding to natural or artificial stable reflectors (permanent scatterers (PS) ) and exploiting long temporal series of interferometric data. This approach allows one to push the measurement precision very close to its theoretical limit (in the order of sim;1 mm for C-band European Remote Sensing (ERS)-like sensors). In this paper, the detection of both time-uniform and seasonal deformation phenomena is addressed, and a first assessment of the precision achievable by means of the PS Technique is discussed. Results highlighting deformation phenomena occurring in two test sites in California are reported (Fremont in the Southern Bay Area and San Jose in the Santa Clara Valley).
    [bibtex-key = colesantiFerrettiNovaliPratiRocca2003] [bibtex-entry]


  790. F.M. Dickey, L.A. Romero, J.M. DeLaurentis, and A.W. Doerry. Super-resolution, degrees of freedom and synthetic aperture radar. Radar, Sonar and Navigation, IEE Proceedings -, 150(6):419-429, 2003. Keyword(s): SAR Processing, SAR Tomography, Tomography, Fourier transforms, eigenvalues and eigenfunctions, extrapolation, inverse problems, mathematical operators, radar imaging, radar resolution, synthetic aperture radar, Fourier transform extrapolation, SAR images, degrees of freedom, eigenvalue spectrum, ill posed problem, operator inversion problem, super-resolution.
    Abstract: Super-resolution concepts offer the potential of resolution beyond the classical limit. This great promise has not generally been realised. The potential application of super-resolution concepts to synthetic aperture radar is investigated. The analytical basis for super-resolution theory is discussed. The application of the concept to synthetic aperture radar is investigated as an operator inversion problem. Generally, the operator inversion problem is ill posed. However, it is shown that it is the particular form of the eigenvalue spectrum of the associated operator that precludes any significant resolution enhancement of synthetic aperture radar images. A criterion for judging super-resolution processing of an image is presented.
    [bibtex-key = dickeyRomeroDeLaurentisDoerry03:Tomo] [bibtex-entry]


  791. Fred M. Dickey, Louis A. Romero, and Armin W. Doerry. Window functions for imaging radar: a maximum energy approach to contiguous and notched spectrums. Optical Engineering, 42(7):2113-2128, 2003. Keyword(s): SAR Processing, radar imaging, synthetic aperture radar, sidelobes, window functions, filtering, notched spectrum.
    Abstract: Imaging systems such as synthetic aperture radar collect band-limited data from which an image of a target scene is rendered. The band-limited nature of the data generates sidelobes, or spilled energy, most evident in the neighborhood of bright point-like objects. It is generally considered desirable to minimize these sidelobes, even at the expense of some generally small increase in system bandwidth. This is accomplished by shaping the spectrum with window functions prior to inversion or transformation into an image. A window function that minimizes sidelobe energy can be constructed based on prolate spheroidal wave functions. A parametric design procedure allows doing so, even with constraints on allowable increases in system bandwidth. This approach is extended to accommodate spectral notches or holes, although the guaranteed minimum sidelobe energy can be quite high in this case. Interestingly, for a fixed bandwidth, the minimum mean-squared-error image rendering of a target scene is achieved with no windowing at all (rectangular or boxcar window).
    [bibtex-key = DickeyRomeroDoerry2003] [bibtex-entry]


  792. Michael Eineder. Efficient simulation of SAR interferograms of large areas and of rugged terrain. IEEE Transactions on Geoscience and Remote Sensing, 41(6):1415-1427, June 2003. Keyword(s): SAR Processing, DEM generation, InSAR, SAR Interferometry, Interferometry, Tomography, SAR Tomography, digital elevation model generation, interferometric synthetic aperture radar, large areas, remote sensing, rugged terrain, surface motion mapping, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping;.
    Abstract: Interferometric synthetic aperture radar (InSAR) techniques are today applied in many areas of remote sensing, ranging from digital elevation model (DEM) generation to surface motion mapping and InSAR tomography. To enhance the understanding of the InSAR mapping process and to test new algorithms, accurate tools for the simulation of the topographic InSAR phase are necessary. Whereas the equations for the interferometric phase of a given DEM are well known, the actual implementation is tedious. Furthermore, a straightforward implementation would take far more computation time than all the other InSAR processing steps put together. This paper presents a novel algorithm for the efficient simulation of the InSAR phase, taking into account the special problems in mountainous terrain. Simulation results are compared to and illustrated with real data from the European Remote Sensing satellite (ERS-1/2) tandem mission and the Shuttle Radar Topography Mission (SRTM). Accuracy estimates for the phase simulation are given for different terrain types. The algorithm is described in enough detail that it can be implemented as a general-purpose tool for the accurate simulation of interferograms with virtually unlimited size, taking no more processing time than other InSAR processing steps. The algorithm in the presented form is used operationally within the interferometry software GENESIS to support the processing of SRTM/X-SAR data at the German Aerospace Center (DLR).
    [bibtex-key = 1220250] [bibtex-entry]


  793. Jeffrey A. Fessler and B. P. Sutton. Nonuniform fast Fourier transforms using min-max interpolation. IEEE Transactions on Signal Processing, 51(2):560-574, February 2003. Keyword(s): fast Fourier transforms, frequency-domain analysis, interpolation, minimax techniques, multidimensional signal processing, signal sampling, FFT, Kaiser-Bessel function, frequency domain, min-max interpolation, multidimensional signals, nonuniform FT, nonuniform fast Fourier transforms, nonuniform sampling, signal processing, worst-case approximation error, Approximation error, Fast Fourier transforms, Frequency domain analysis, Image reconstruction, Interpolation, Iterative methods, Magnetic resonance imaging, Multidimensional signal processing, Multidimensional systems, Nonuniform sampling, NFFT, nonuniform FFT, nonequispaced FFT.
    Abstract: The fast Fourier transform (FFT) is used widely in signal processing for efficient computation of the FT of finite-length signals over a set of uniformly spaced frequency locations. However, in many applications, one requires nonuniform sampling in the frequency domain, i.e., a nonuniform FT. Several papers have described fast approximations for the nonuniform FT based on interpolating an oversampled FFT. This paper presents an interpolation method for the nonuniform FT that is optimal in the min-max sense of minimizing the worst-case approximation error over all signals of unit norm. The proposed method easily generalizes to multidimensional signals. Numerical results show that the min-max approach provides substantially lower approximation errors than conventional interpolation methods. The min-max criterion is also useful for optimizing the parameters of interpolation kernels such as the Kaiser-Bessel function.
    [bibtex-key = fesslerSuttonIEEETSiP2003NonUniformFFT] [bibtex-entry]


  794. J. R. Fienup and J. J. Miller. Aberration correction by maximizing generalized sharpness metrics. J. Opt. Soc. Am. A, 20(4):609-620, April 2003. Keyword(s): SAR Processing, Autofocus, Active or adaptive optics, Image reconstruction techniques, Image reconstruction-restoration, Phase retrieval, Image quality assessment, Phased-array imaging systems, Synthetic aperture radar, Aberration correction, Adaptive optics, Discrete Fourier transforms, Fast Fourier transforms, Image quality, Synthetic aperture radar.
    Abstract: The technique of maximizing sharpness metrics has been used to estimate and compensate for aberrations with adaptive optics, to correct phase errors in synthetic-aperture radar, and to restore images. The largest class of sharpness metrics is the sum over a nonlinear point transformation of the image intensity. How the second derivative of the point nonlinearity varies with image intensity determines the effects of various metrics on the imagery. Some metrics emphasize making shadows darker, and other emphasize making bright points brighter. One can determine the image content needed to pick the best metric by computing the statistics of the image autocorrelation or of the Fourier magnitude, either of which is independent of the phase error. Computationally efficient, closed-form expressions for the gradient make possible efficient search algorithms to maximize sharpness.
    [bibtex-key = fienupMillerJOptSoc2003AutofocusAberationCorrectionMaximizingSharpnessMetrics] [bibtex-entry]


  795. G. Fornaro, F. Serafino, and F. Soldovieri. Three-dimensional focusing with multipass SAR data. IEEE Trans. Geosci. Remote Sens., 41(3):507-517, 2003. Keyword(s): SAR Processing, SAR Tomography, Tomography, focusing, synthetic aperture radar, 3D tomography reconstruction, linear inverse problem, multipass SAR data, multipass synthetic aperture radar data, nonuniform orbit separation, processing technique, scattering mechanism, singular value decomposition, volumetric scattering.
    Abstract: Deals with the use of multipass synthetic aperture radar (SAR) data in order to achieve three-dimensional tomography reconstruction in presence of volumetric scattering. Starting from azimuth- and range-focused SAR data relative to the same area, neglecting any mutual interaction between the targets, and assuming the propagation in homogeneous media, we investigate the possibility to focus the data also in the elevation direction. The problem is formulated in the framework of linear inverse problem and the solution makes use of the singular value decomposition of the relevant operator. This allows us to properly take into account nonuniform orbit separation and to exploit a priori knowledge regarding the size of the volume interested by the scattering mechanism, thus leading to superresolution in the elevation direction. Results obtained on simulated data demonstrate the feasibility of the proposed processing technique.
    [bibtex-key = fornaroSerafinoSoldovieri03:Tomo] [bibtex-entry]


  796. Jong-Sen Lee, Shane R. Cloude, Konstantinos P. Papathanassiou, M. R. Grunes, and I. H. Woodhouse. Speckle filtering and coherence estimation of polarimetric SAR interferometry data for forest applications. IEEE Trans. Geosci. Remote Sens., 41(10):2254-2263, October 2003. Keyword(s): SAR Processing, Forest, Forest parameters, biomass, averaging, boxcar filter, coherence estimation, forest applications, forest heights, ground topography, interferometric coherence, neighboring pixels, polarimetric SAR interferometry data, polarimetric interferometry matrix, polarimetric synthetic aperture radar interferometry, random volume, scattering characteristics, speckle filtering, adaptive filters, forestry, radar imaging, radar polarimetry, radiowave interferometry, remote sensing by radar, speckle, synthetic aperture radar.
    Abstract: Recently, polarimetric synthetic aperture radar (SAR) interferometry has generated much interest for forest applications. Forest heights and ground topography can be extracted based on interferometric coherence using a random volume over ground coherent mixture model. The coherence estimation is of paramount importance for the accuracy of forest height estimation. The coherence (or correlation coefficient) is a statistical average of neighboring pixels of similar scattering characteristics. The commonly used algorithm is the boxcar filter, which has the deficiency of indiscriminate averaging of neighboring pixels. The result is that coherence values are lower than they should be. In this paper, we propose a new algorithm to improve the accuracy in the coherence estimation based on speckle filtering of the 6 times;6 polarimetric interferometry matrix. Simulated images are used to verify the effectiveness of this adaptive algorithm. German Aerospace Center (DLR) L-Band E-SAR data are applied to demonstrate the improved accuracy in coherence and in forest height estimation.
    [bibtex-key = leeCloudePapthanassiouGrunesWoodhouse2003:PolInSARForestApplications] [bibtex-entry]


  797. D. Leva, G. Nico, D. Tarchi, J. Fortuny-Guasch, and A.J. Sieber. Temporal analysis of a landslide by means of a ground-based SAR Interferometer. IEEE Trans. Geosci. Remote Sens., 41(4):745-752, April 2003. Keyword(s): Feeds, Global Positioning System, Image sequence analysis, Instruments, Interferometry, Layout, Monitoring, Morphology, Position measurement, Terrain factors, geomorphology, geophysical techniques, radiowave interferometry, synthetic aperture radar, terrain mapping, 16.70 to 16.78 GHz, 700 to 1150 m, Austria, D-InSAR, GBSAR interferometer, GPS receivers, Schwaz, Tyrol, accumulation zone, debris flow, deformation map, differential synthetic aperture radar interferometry, ground-based synthetic aperture radar interferometer, high-resolution images, interferogram sequence, landslide monitoring, landslides, phase image analysis, slowly deforming upper scarp, terrain morphology, velocity field;.
    Abstract: A ground-based synthetic aperture radar (GB-SAR) interferometer is used to retrieve the velocity field of a landslide. High-resolution images are obtained by means of a time domain SAR processor. An in-depth analysis of the sequence of SAR interferograms enables the recognition of a slowly deforming upper scarp in the scene, and a debris flow that feeds the accumulation zone of the landslide, where a fast change in terrain morphology is observed. The estimated deformation map is in agreement with the available measurements obtained by means of Global Positioning System receivers. Results show that GB-SAR interferometry is a cost-effective solution for the monitoring of landslides. The proposed method is shown to be a valid complement to space- and airborne SAR and to the traditional geodetic instruments.
    [bibtex-key = levaNicoTarchiFortunyGuaschSieberTGRS2003GroundBasedSAR] [bibtex-entry]


  798. Jian Li, Petre Stoica, and Zhisong Wang. On robust Capon beamforming and diagonal loading. Signal Processing, IEEE Transactions on [see also Acoustics, Speech, and Signal Processing, IEEE Transactions on], 51(7):1702-1715, 2003. Keyword(s): SAR Processing, Capon, Spectral Estimation, Beamforming, array signal processing, interference suppression, parameter estimation, signal resolution, SOI power estimation, SOI steering vector, adaptive arrays, array steering vector, data-independent beamformer, diagonal loading, interference rejection, robust Capon beamforming, signal of interest, standard Capon beamformer, uncertain steering vectors, uncertainty set.
    Abstract: The Capon (1969) beamformer has better resolution and much better interference rejection capability than the standard (data-independent) beamformer, provided that the array steering vector corresponding to the signal of interest (SOI) is accurately known. However, whenever the knowledge of the SOI steering vector is imprecise (as is often the case in practice), the performance of the Capon beamformer may become worse than that of the standard beamformer. Diagonal loading (including its extended versions) has been a popular approach to improve the robustness of the Capon beamformer. We show that a natural extension of the Capon beamformer to the case of uncertain steering vectors also belongs to the class of diagonal loading approaches, but the amount of diagonal loading can be precisely calculated based on the uncertainty set of the steering vector. The proposed robust Capon beamformer can be efficiently computed at a comparable cost with that of the standard Capon beamformer. Its excellent performance for SOI power estimation is demonstrated via a number of numerical examples.
    [bibtex-key = LiStoicaWang2003:RobustCapon] [bibtex-entry]


  799. D. Lloyd and I. D. Longstaff. Ultra-wideband multistatic SAR for the detection and location of landmines. IEE Proceedings - Radar, Sonar and Navigation, 150:158-164(6), June 2003. Keyword(s): location, small mine-like objects, ground coordinates, target strengths, signal to clutter ratio, bistatic antenna configurations, ultra-wideband multistatic SAR, reference depression angle, detection, target-to-clutter ratios, thresholding, images, amplitude distribution, landmines;.
    Abstract: Techniques for improving the signal to clutter ratio of an ultra-wideband SAR designed to detect small mine-like objects in the surface of the ground were investigated. In particular, images were collected using different bistatic antenna configurations in an attempt to decorrelate the clutter with respect to the targets. The images were converted to a reference depression angle, summed, and then converted to ground coordinates. The resulting target strengths were then compared with the amplitude distribution of the ground clutter to show the improvement obtained. While some improvement was demonstrated, this was for the relatively easy scenario of targets on the surface partially obscured by grass. Detection based on thresholding the raw RF signal (the bipolar response) rather than the envelope (baseband I2+Q2) was also considered to further enhance target-to-clutter ratios.
    [bibtex-key = lloydLongstaffIEEProcRadSonNav2003UltraWidebandMultistaticSARForLandMineDetection] [bibtex-entry]


  800. F. Lombardini, M. Montanari, and F. Gini. Reflectivity estimation for multibaseline interferometric radar imaging of layover extended sources. IEEE Transactions on Signal Processing, 51(6):1508-1519, June 2003. Keyword(s): SAR Processing, Tomography, SAR Tomography, Multi-baseline SAR, Interferometry, SAR Interferometry, AWGN, Monte Carlo methods, amplitude estimation, radar imaging, radiowave interferometry, synthetic aperture radar APES, Capon's approach, InSAR, Monte Carlo simulation, RELAX, SAR, additive white Gaussian noise, complex orography, component sinusoidal signal, hybrid spectral estimators, layover extended sources, multibaseline interferometric radar imaging, multibaseline operation, multiplicative complex correlated noise, nonparametric estimators, parametric estimators, reflectivity estimation, spatial frequency estimation, synthetic aperture radar interferometry, terrain height, undistorted multibaseline image formation.
    Abstract: In recent years, there has been great interest in exploiting the advanced multibaseline operation of synthetic aperture radar interferometry (InSAR) for solving layover effects from complex orography, which can degrade both SAR and InSAR imagery of terrain radar reflectivity and height. In this work, the problem of retrieving radar reflectivity of layover areas is addressed. It is formulated as the problem of estimating a multicomponent sinusoidal signal corrupted by multiplicative complex correlated noise and additive white Gaussian noise. Application of nonparametric [e.g., Capon, amplitude and phase estimation filter (APES)], parametric [least squares, modern parametric RELAXation spectral estimator (RELAX)], and hybrid spectral estimators for amplitude estimation is investigated for a multilook scenario. In particular, the multilook extensions of RELAX and APES are applied to the interferometric problem. Performance analysis is investigated through a Cramer-Rao lower bound calculation and Monte Carlo simulation. The method of least squares, coupled with Capon's approach to spatial frequency estimation, multilook APES, and multilook RELAX turn out to provide accurate reflectivity estimates for undistorted multibaseline image formation of layover areas.
    [bibtex-key = lombardiniMontanariGini2003:Tomo] [bibtex-entry]


  801. A. Luckman and W. Grey. Urban building height variance from multibaseline ERS coherence. IEEE Transactions on Geoscience and Remote Sensing, 41(9):2022-2025, September 2003. Keyword(s): Cardiff, ERS images, European Remote Sensing satellite, InSAR, UK, Wales, central business district, coherence maps, interferometric synthetic aperture radar coherence images, multibaseline ERS coherence, surface roughness, urban building height variance, urban decorrelation models, vertical scatterer distributions, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping;.
    Abstract: Multibaseline European Remote Sensing (ERS) interferometric synthetic aperture radar coherence images from Cardiff, U.K. are investigated with respect to urban form. A model of spatial coherence, taking into account the vertical distribution of scatterers, is inverted to allow urban building height variance to be retrieved. Sixty-nine coherence maps are employed, generated from 20 ERS images. No a priori information is required in the analysis. However, realistic vertical scatterer distributions are retrieved, and Cardiff's central business district is automatically identified. This analysis demonstrates the utility of the urban decorrelation models employed.
    [bibtex-key = luckmanGrey2003] [bibtex-entry]


  802. Oscar Mora, Jordi J. Mallorquì, and Antoni Broquetas. Linear and nonlinear terrain deformation maps from a reduced set of interferometric SAR images. IEEE Trans. Geosci. Remote Sens., 41(10):2243-2253, October 2003. Keyword(s): SAR Processing, geophysical signal processing, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), DEM, Delauney triangulation, atmospheric artifacts, coherence level, digital elevation model, filtering techniques, interferometric SAR images, linear terrain deformation maps, low spatial resolution interferograms, nonlinear terrain deformation maps, nonuniform mesh, remote sensing, synthetic aperture radar, topographic terms, Atmospheric modeling, Coherence, Digital elevation models, Information filtering, Information filters, Information retrieval, Remote sensing, Spatial resolution, Synthetic aperture radar, Testing, PSI, Persistent Scatterer Interferometry.
    Abstract: In this paper, an advanced technique for the generation of deformation maps using synthetic aperture radar (SAR) data is presented. The algorithm estimates the linear and nonlinear components of the displacement, the error of the digital elevation model (DEM) used to cancel the topographic terms, and the atmospheric artifacts from a reduced set of low spatial resolution interferograms. The pixel candidates are selected from those presenting a good coherence level in the whole set of interferograms and the resulting nonuniform mesh tessellated with the Delauney triangulation to establish connections among them. The linear component of movement and DEM error are estimated adjusting a linear model to the data only on the connections. Later on, this information, once unwrapped to retrieve the absolute values, is used to calculate the nonlinear component of movement and atmospheric artifacts with alternate filtering techniques in both the temporal and spatial domains. The method presents high flexibility with respect to the required number of images and the baselines length. However, better results are obtained with large datasets of short baseline interferograms. The technique has been tested with European Remote Sensing SAR data from an area of Catalonia (Spain) and validated with on-field precise leveling measurements.
    [bibtex-key = moraMallorquiBroquetasTGARS2003] [bibtex-entry]


  803. Clifford J. Nolan and Margaret Cheney. Synthetic Aperture Inversion for Arbitrary Flight Paths and Nonflat Topography. IEEE Transactions on Image Processing, 12(9):1035-1043, September 2003. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, FOPEN, Non-Flat Topography, Non-Linear Flight Path.
    Abstract: This paper considers synthetic aperture radar (SAR) and other synthetic aperture imaging systems in which a backscattered wave is measured from positions along an arbitrary (known) flight path.We assume a single-scattering model for the radar data, and we assume that the ground topography is known but not necessarily flat. We focus on cases in which the antenna footprint is so large that the standard narrow-beam algorithms are not useful.We showthat certain artifacts can be avoided if the antenna and antenna footprint avoid particular relationships with the ground topography. In this case, we give an explicit backprojection imaging algorithm that corrects for the ground topography, flight path, antenna beam pattern, source waveform, and other geometrical factors. For the case of a nondirectional antenna, the image produced by the above algorithm contains artifacts. For this case, we analyze the strength of the artifacts relative to the strength of the true image. The analysis shows that the artifacts can be somewhat suppressed by increasing the frequency, integration time, and the curvature of the flight path.
    [bibtex-key = nolan:SARInversion2] [bibtex-entry]


  804. M. Nolan and D. R. Fatland. Penetration depth as a DInSAR observable and proxy for soil moisture. IEEE_J_GRS, 41(3):532-537, March 2003. Keyword(s): ground penetrating radar, moisture measurement, radiowave interferometry, remote sensing by radar, soil, synthetic aperture radar, C-band, L-band, X-band, agriculture, attenuation, clay rich soils, clay swelling, deformation signals, differential interferometric synthetic aperture radar, digital elevation models, microwave penetration depth, remote sensing, soil moisture, surface deformation, synthetic aperture radar microwaves, terrain mapping, volumetric water content, Interferometry, Microwave theory and techniques, Moisture measurement, Radar detection, Remote sensing, Soil measurements, Soil moisture, Surface topography, Synthetic aperture radar. [bibtex-key = Nolan2003] [bibtex-entry]


  805. Christine Pielmeier and Martin Schneebeli. Developments in the Stratigraphy of Snow. Surveys in Geophysics, 24(5-6):389-416, 2003. Keyword(s): Snow, Snow Stratigraphy, mechanical properties, physical properties, profile, snow cover, snow properties.
    Abstract: Snow on the ground can be regarded as aeolian sediment with rapidly changing properties. We explore the Swiss (Alpine) history of stratigraphy of snow to show the trends and developments. The observation of snow stratigraphy starts in the 18th and 19th century with a geologic focus, descriptions are superficial and only verbal. In the early 20th century, the scientific interest in snow stratigraphy increases. Detailed descriptions and drawings become available. Slope scale geomorphologic features and surface processes were observed and documented. Starting from the 1940s, a shift of interest to the physical and mechanical properties in ''homogeneous'' layers takes place, from a slope-centred approach to a sample-centred approach. Stratigraphic description becomes one-dimensional, and the concept of well-defined layers gets accepted and still predominates today. However, all physical and mechanical processes are strongly related to the spatial variability of the snow mechanical properties. New instrumental developments show that the perceived strict layering may be a too simple model. The requirements for modern snow stratigraphy, integrating different scales and modern technology, is discussed from an international viewpoint.
    [bibtex-key = pielmeierSchneebeli2013DevelopmentsStratigraphyOfSnow] [bibtex-entry]


  806. Pau Prats and Jordi J. Mallorqui. Estimation of azimuth phase undulations with multisquint processing in airborne interferometric SAR images. IEEE Trans. Geosci. Remote Sens., 41(6):1530-1533, 2003. Keyword(s): SAR Processing, Squinted SAR, airborne radar, radar imaging, synthetic aperture radar, ESAR, DLR experimental airborne SAR, Germany, Oberpfaffenhofen test site, azimuth phase undulations, calibration, image pairs, interferometric airborne synthetic aperture radar systems, multisquint processing, phase error correction, phase error detection, single-pass interferometrie data, squint angles, technique, InSAR, Interferometry.
    Abstract: Presents a technique to detect and correct phase errors appearing in interferometric airborne synthetic aperture radar (SAR) systems due to the lack of precision in the navigation system. The technique is based on a multisquint processing approach, i.e. by processing the same image pairs with different squint angles we can combine the information of different interferograms to obtain the desired phase correction. Airborne single-pass interferometric data from the Deutsches Zentrum fuer Luft- und Raumfahrt (DLR) experimental airborne SAR is used to validate the method.
    [bibtex-key = pratsMallorqui03ESAR] [bibtex-entry]


  807. A. Reigber and R. Scheiber. Airborne differential SAR interferometry: first results at L-band. IEEE Trans. Geosci. Remote Sens., 41(6):1516-1520, 2003. Keyword(s): SAR Processing, airborne radar, radiowave interferometry, remote sensing by radar, synthetic aperture radar, vegetation mapping, Earth surface, Germany, L-Band, Oberpfaffenhofen test site, agricultural areas, airborne differential SAR interferometry, airborne sensors, atmospheric effects, centimetre-scale deformations, critical region monitoring, data acquisition intervals, forested areas, glacier flows, interferometric repeat-pass mode, landslides, long-term decorrelation, motion compensation, motion errors, phase artifacts, ESAR, Motion Compensation, Interferometric SAR, Interferometry, D-InSAR.
    Abstract: In recent years, differential interferometry using spaceborne synthetic aperture radar (SAR) sensors has become an established technique for detecting and monitoring centimetre-scale deformations of the Earth's surface, as well as glacier flows and landslides. Although often very efficient, the use of spaceborne SAR data has several drawbacks, namely phase artifacts caused by atmospheric effects and very low coherence due to long data acquisition intervals and the short radar wavelength of the sensor. Most important, current spaceborne sensors are not able to ensure flexible monitoring of critical regions. Airborne sensors may overcome most of the problems mentioned above, but up to now, the operational use of airborne differential SAR interferometry has been prevented by insufficiently accurate motion compensation of the platform. In this letter, first results of airborne differential interferometry using the German Aerospace Center (DLR) experimental SAR system (E-SAR) in the interferometric repeat-pass mode are addressed. This includes an analysis of long-term decorrelation behavior in L-band and, particularly, the correction of residual motion errors in heavily decorrelated interferograms. A first differential interferogram of agricultural and forested areas is presented and analyzed.
    [bibtex-key = reigberScheiber03:DiffSARLBand] [bibtex-entry]


  808. David A. Schmidt and Roland Bürgmann. Time-dependent land uplift and subsidence in the Santa Clara valley, California, from a large interferometric synthetic aperture radar data set. J. Geophys. Res., 108(B9):2416, September 2003. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, DInSAR, Groundwater hydrology, Water supply, Physical Properties of Rocks, Permeability and porosity, Space geodetic surveys.
    Abstract: We invert 115 differential interferograms derived from 47 synthetic aperture radar (SAR) scenes for a time-dependent deformation signal in the Santa Clara valley, California. The time-dependent deformation is calculated by performing a linear inversion that solves for the incremental range change between SAR scene acquisitions. A nonlinear range change signal is extracted from the ERS InSAR data without imposing a model of the expected deformation. In the Santa Clara valley, cumulative land uplift is observed during the period from 1992 to 2000 with a maximum uplift of 41 +/- 18 mm centered north of Sunnyvale. Uplift is also observed east of San Jose. Seasonal uplift and subsidence dominate west of the Silver Creek fault near San Jose with a maximum peak-to-trough amplitude of approx. 35 mm. The pattern of seasonal versus long-term uplift provides constraints on the spatial and temporal characteristics of water-bearing units within the aquifer. The Silver Creek fault partitions the uplift behavior of the basin, suggesting that it acts as a hydrologic barrier to groundwater flow. While no tectonic creep is observed along the fault, the development of a low-permeability barrier that bisects the alluvium suggests that the fault has been active since the deposition of Quaternary units.
    [bibtex-key = schmidtBuergmann2003JGR_PSI] [bibtex-entry]


  809. Jean-Claude Souyris, Caroline Henry, and Frédéric Adragna. On the use of complex SAR image spectral analysis for target detection: assessment of polarimetry. IEEE Trans. Geosci. Remote Sens., 41(12):2725-2734, December 2003. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Interferometric Point Target Analysis, IPTA, Spectral coherence, Selection of point target candidates, radar detection, radar imaging, radar polarimetry, remote sensing by radar, spectral analysis, synthetic aperture radar, 2L-HIP derivation, azimuth, clutter, complex SAR image spectral analysis, magnitude, phase, point target detection, polarimetry, range spectra, synthetic aperture radar polarimetric image, target detection, two-looks internal Hermitian product, Azimuth, Image analysis, Object detection, Polarization, Radar clutter, Radar imaging, Radar polarimetry, Speckle, Spectral analysis, Synthetic aperture radar.
    Abstract: The objective of this paper is to assess the joint use of the magnitude and the phase of a synthetic aperture radar (SAR) polarimetric image for point target detection and analysis. We first consider a single-look complex (SLC), single polarized radar image including point targets embedded in clutter. A series of sublooks are generated from this SLC image, both in azimuth and in range in order to analyze the inherent speckle effects. The two-looks internal Hermitian product (2L-IHP) is defined and is further shown to qualitatively increase the target/environment contrast. The processing of azimuth and range spectra preliminary to the 2L-IHP derivation (spectral whitening, generation and overlapping of sublooks) is described. A simulation tool is developed to model a point target behavior. Then, the polarimetric extension of the 2L-IHP is proposed, and the optimized polarimetric 2L-IHP is defined. The gain is twofold: in comparison with single polarization, polarimetry is shown to enhance detection capabilities, but also to provide additional information for target analysis.
    [bibtex-key = souyrisHenryAdragnaTGRS2003] [bibtex-entry]


  810. Petre Stoica, Zhisong Wang, and Jian Li. Robust Capon beamforming. IEEE Signal Processing Letters, 10(6):172-175, June 2003. Keyword(s): SAR Processing, Capon, Capon Beamforming, Robust Capon beamforming, Beamforming, Spectral Estimation, array signal processing, interference suppression, signal resolution adaptive arrays, adaptive beamforming, array steering vector, interference rejection capability, resolution, signal of interest, uncertain steering vectors.
    Abstract: The Capon beamformer has better resolution and much better interference rejection capability than the standard (data-independent) beamformer, provided that the array steering vector corresponding to the signal of interest (SOI) is accurately known. However, whenever the knowledge of the SOI steering vector is imprecise (as is often the case in practice), the performance of the Capon beamformer may become worse than that of the standard beamformer. We present a natural extension of the Capon beamformer to the case of uncertain steering vectors. The proposed robust Capon beamformer can no longer be expressed in a closed form, but it can be efficiently computed. Its excellent performance is demonstrated via a number of numerical examples.
    [bibtex-key = stoicaWangLi2003:Capon] [bibtex-entry]


  811. Tazio Strozzi, Urs Wegmuller, Charles L Werner, Andreas Wiesmann, and Volker Spreckels. JERS SAR interferometry for land subsidence monitoring. IEEE Trans. Geosci. Remot Sens., 41(7):1702-1708, 2003. Keyword(s): SAR Processing, SAR Interferometry, Interferometry. [bibtex-key = strozziWegmullerWernerWiesmannSpreckelsTGRS2003JERSInSARSubsidence] [bibtex-entry]


  812. Dario Tarchi, Nicola Casagli, Sandro Moretti, Davide Leva, and Alois J. Sieber. Monitoring landslide displacements by using ground-based synthetic aperture radar interferometry: Application to the Ruinon landslide in the Italian Alps. Journal of Geophysical Research: Solid Earth, 108(B8), 2003. Note: 2387. Keyword(s): GBSAR, SAR Processing, Ground-based SAR, Synthetic aperture radar, SAR interferometry, differential SAR interferometry, DInSAR, interferometric stacking, deformation monitoring, subsidence monitoring, deformation measurements, Electromagnetics: Instrumentation and techniques, Control surveys, Geomorphology: general, Geomorphology: fluvial, Geomorphology: hillslope, Instruments and techniques: modeling, Instruments and techniques: monitoring, General or Miscellaneous: New fields (not classifiable under other headings), remote sensing, interferometry, radar, landslide, monitoring, ground-based DInSAR. [bibtex-key = tarchiCasagliMorettiLevaSieberJGRB2003GBSARLISAJRC] [bibtex-entry]


  813. G. Tavernier, J. P. Granier, C. Jayles, P. Sengenes, and F. Rozo. The current evolutions of the DORIS system. Advances in Space Research, 31(8):1947-1952, 2003. Keyword(s): DORIS, Orbit, Precise Orbit, Accuracy, ENVISAT, ASAR.
    Abstract: DORIS was developed for precise orbit determination and precise positioning on Earth. Three new satellites fitted out with dual-channel second-generation receivers have been recently launched. Jason-1, ENVISAT and SPOT-5 acquired a real autonomy thanks to DIODE real time on-board orbit determination software. Today the DORIS system has built up a global network of 55 stations. In order to reach new accuracy goals for Jason-1 and ENVISAT, it was decided to improve the long-term stability of the antennas when necessary. Third-generation beacons deployed from the end of 2001 offer new features and greater reliability. The satellites relay acquired and stored data at regular intervals to SSALTO, the new DORIS mission control center. DORIS data from the different satellites are currently available in the two Data Centers and used by the International DORIS Service Analysis groups.
    [bibtex-key = tavernierGranierJaylesSengenesRozo03:DORIS] [bibtex-entry]


  814. Lars M. H. Ulander, Hans Hellsten, and Gunnar Stenström. Synthetic-Aperture Radar Processing Using Fast Factorized Back-Projection. IEEE Transactions on Aerospace and Electronic Systems, 39(3):760-776, July 2003. Keyword(s): SAR Processing, Fast Factorized Back-Projection, Time-Domain Back-Projection, TDBP, Back-Projection, Fast Back-Projection, Factorized Back-Projection, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR.
    Abstract: Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude.
    [bibtex-key = ulanHellSten:FastBackpro] [bibtex-entry]


  815. Marwan Younis, Christian Fischer, and Werner Wiesbeck. Digital beamforming in SAR systems. IEEE Trans. Geosci. Remote Sens., 41(7):1735-1739, July 2003. Keyword(s): SAR, digital beamforming, digital hardware, digital interface, geophysical measurement technique, land surface, radar remote sensing, signal processing, software codes, software-defined radar sensors, synthetic aperture radar, terrain mapping, geophysical signal processing, geophysical techniques, radar signal processing, remote sensing by radar, synthetic aperture radar, terrain mapping.
    Abstract: The rapid progression in digital hardware and signal processing capabilities stimulates the development of radar systems. The tendency is to move the digital interface toward the antenna, replacing, whenever possible, analog RF-hardware. Based on software codes, these digital systems are more flexible and easier to reconfigure than RF-hardware. This letter illustrates the general concept for digital beamforming (DBF) in synthetic aperture radar systems and investigates their principle capabilities, limitations, and performance parameters. It is shown that using DBF a simultaneous improvement in azimuth coverage and resolution can be achieved.
    [bibtex-key = younisFischerWiesbeck2003DBF] [bibtex-entry]


  816. Andrea Bellettini and M. A. Pinto. Theoretical accuracy of synthetic aperture sonar micronavigation using a displaced phase-center antenna. IEEE Journal of Oceanic Engineering, 27(4):780-789, October 2002. Keyword(s): Synthetic Aperture Sonar, SAS, navigation, synthetic aperture sonar, 118 to 182 kHz, Cramer-Rao lower bound, array gain, beampattern specification, displaced phase-center antenna, DPCA, ping-to-ping cross-track displacement, sway, synthetic aperture sonar micronavigation, yaw, Antenna theory, Aperture antennas, Backscatter, Computational modeling, Counting circuits, Helium, Predictive models, Principal component analysis, Sonar navigation, Synthetic aperture sonar.
    Abstract: The Cramer-Rao lower bounds on the cross-track translation and rotation of a displaced phase-center antenna (DPCA) in the slant range plane between two successive pings (known as DPCA sway and yaw in what follows) are computed, assuming statistically homogeneous backscatter. These bounds are validated using experimental data from a 118-182-kHz sonar, showing an accuracy of the order of 20 microns on the ping-to-ping cross-track displacements. Next, the accuracy required on the DPCA sway and yaw in order to achieve a given synthetic aperture sonar (SAS) beampattern specification, specified by the expected SAS array gain, is computed as a function of the number P of pings in the SAS. Higher accuracy is required when P increases to counter the accumulation of errors during the integration of the elementary ping-to-ping estimates: the standard deviation must decrease as P-12/ for the DPCA sway and P-32/ for the yaw. Finally, by combining the above results, the lower bounds on DPCA micronavigation accuracy are established. These bounds set an upper limit to the SAS length achievable in practice. The maximum gain Q in cross-range resolution achievable by a DPCA micronavigated SAS is computed as a function of the key SAS parameters. These theoretical predictions are compared with simulations and experimental results.
    [bibtex-key = bellettiniPintoJOE2002TheoreticalAccuracySASonarMicronavigationDPCA] [bibtex-entry]


  817. P. Berardino, G. Fornaro, R. Lanari, and E. Sansosti. A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Transactions on Geoscience and Remote Sensing, 40(11):2375-2383, November 2002. Keyword(s): SAR Processing, SBAS, small baseline differential SAR interferometry, DInSAR, InSAR, SAR Interferometry, Campi Flegrei caldera, European Remote Sensing satellites, Italy, Naples, SAR interferometry, atmospheric phase artifacts filtering, differential synthetic aperture radar interferometry algorithm, ground deformations, independent SAR acquisition datasets, singular value decomposition method, spatial information, surface deformation monitoring algorithm, temporal evolution, temporal information, Earth crust, radiowave interferometry, remote sensing by radar, synthetic aperture radar, tectonics, volcanology.
    Abstract: We present a new differential synthetic aperture radar (SAR) interferometry algorithm for monitoring the temporal evolution of surface deformations. The presented technique is based on an appropriate combination of differential interferograms produced by data pairs characterized by a small orbital separation (baseline) in order to limit the spatial decorrelation phenomena. The application of the singular value decomposition method allows us to easily link independent SAR acquisition datasets, separated by large baselines, thus increasing the observation temporal sampling rate. The availability of both spatial and temporal information in the processed data is used to identify and filter out atmospheric phase artifacts. We present results obtained on the data acquired from 1992 to 2000 by the European Remote Sensing satellites and relative to the Campi Flegrei caldera and to the city of Naples, Italy, that demonstrate the capability of the proposed approach to follow the dynamics of the detected deformations.
    [bibtex-key = berardinoFornaroLanariSansostiTGARS2002SBAS] [bibtex-entry]


  818. H.L. Chan and T.S. Yeo. Non-iterative quality phase-gradient autofocus (QPGA) algorithm for spotlight SAR imagery. IEEE Trans. Geosci. Remote Sens., 40(11):2517, November 2002. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, Quality Phase Gradient Autofocus, QPGA.
    Abstract: The quality phase-gradient autofocus (QPGA) technique was proposed to speed up the estimation convergence of phase-gradient autofocus by selectively increasing the pool of quality synchronization sources instead of selecting the brightest pixels within the image. It is now found hat the QPGA, with its inherent scatter growing- concept and target-filtering procedure, is also able to focus in environments with stationary and moving targets.
    [bibtex-key = Chan2002] [bibtex-entry]


  819. Curtis W. Chen and Howard A. Zebker. Phase unwrapping for large SAR interferograms: statistical segmentation and generalized network models. IEEE Trans. Geosci. Remote Sens., 40(8):1709-1719, August 2002. Keyword(s): SAR Processing, phase unwrapping, SNAPHU, geophysical signal processing, image segmentation, optimisation, radar imaging, radiowave interferometry, remote sensing by radar, statistical analysis, synthetic aperture radar, InSAR data, SNAPHU, a posteriori probability, central Alaska, full-size interferograrn, generalized network models, interferometric input data, interferometric synthetic aperture radar data, large SAR interferograms, network-flow, nonlinear solver, phase unwrapping, secondary optimization problem, statistical models, statistical segmentation, statistical-cost network-flow phase-unwrapping algorithm, tile-unwrapping stage, tiles, topographic interferogram, Algorithm design and analysis, Data analysis, Geophysics computing, Partitioning algorithms, Phase estimation, Probability, Synthetic aperture radar interferometry, Two dimensional displays.
    Abstract: Two-dimensional (2-D) phase unwrapping is a key step in the analysis of interferometric synthetic aperture radar (InSAR) data. While challenging even in the best of circumstances, this problem poses unique difficulties when the dimensions of the interferometric input data exceed the limits of one's computational capabilities. In order to deal with such cases, we propose a technique for applying the statistical-cost, network-flow phase-unwrapping algorithm (SNAPHU) of Chen and Zebker (2001) to large datasets. Specifically, we introduce a methodology whereby a large interferogram is partitioned into a set of several smaller tiles that are unwrapped individually and then divided further into independent, irregularly shaped reliable regions. These regions are subsequently assembled into a full unwrapped solution, with the phase offsets between regions computed in a secondary optimization problem whose objective is to maximize the a posteriori probability of the final solution. As this secondary problem assumes the same statistical models as employed in the initial tile-unwrapping stage, the technique results in a solution that approximates the solution that would have been obtained had the full-size interferogram been unwrapped as a single piece. The secondary problem is framed in terms of network-flow ideas, allowing the use of an existing nonlinear solver. Applying the algorithm to a large topographic interferogram acquired over central Alaska, we find that the technique is less prone to unwrapping artifacts than more simple tiling approaches.
    [bibtex-key = chenZebkerTGRS2002SNAPHUPhaseUnwrappingForLargeSARInterferograms] [bibtex-entry]


  820. G. Fornaro and A.M. Guarnieri. Minimum mean square error space-varying filtering of interferometric SAR data. IEEE Transactions on Geoscience and Remote Sensing, 40(1):11 -21, January 2002. Keyword(s): SAR Processing, SAR Interferometry, DEM, Doppler centroid decorrelation, IFSAR, InSAR, algorithm, azimuth direction, digital elevation model, geometrical decorrelation, geophysical measurement technique, imaging, interferometric SAR, land surface, minimum mean square error method, nonplanar topography, radar remote sensing, scene topography, space varying filtering, space-variant, synthetic aperture radar, terrain mapping, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping.
    Abstract: This paper addresses the problem of filtering interferometric synthetic aperture radar (IFSAR) signals in presence of nonplanar topography to mitigate geometrical decorrelation effects. The problem is space-variant. The authors assume knowledge about the scene topography and derive an optimal, minimum mean square error (MMSE), filtering procedure. The algorithm is flexible and, beside the standard stripmap-stripmap interferometry, it may be applied to IFSAR data acquired in any operative mode. For instance, in scan-scan, scan-strip, and scan-spot interferometry. The scene topography contribution may be either derived from an external rough digital elevation model (DEM) or directly estimated from the SAR data. The filtering technique is extended to the azimuth direction to account for possible Doppler centroid decorrelation. Experimental results carried out on real data confirm the validity of the theory and show that this filtering procedure allows the authors to obtain a reduction of the interferometric noise content. Its gain is particularly marked in the cases of steep topography, where application of the standard common band filters could deteriorate the signal quality, or for large Doppler centroid shifts between the two acquisitions
    [bibtex-key = 981345] [bibtex-entry]


  821. G. Fornaro, E. Sansosti, R. Lanari, and M. Tesauro. Role of processing geometry in SAR raw data focusing. IEEE Transactions on Aerospace and Electronic Systems, 38(2):441-454, April 2002. Keyword(s): SAR Processing, 2D frequency SAR processing, SAR raw data focusing, airborne remote sensing, conical reference systems, geometric artifacts, high-resolution images, high-resolution microwave images, interferometric SAR, microwave remote sensing, phase aberrations, processing geometry role, received backscattered echoes, space-invariant component, space-variant component, spaceborne remote sensing, squinted geometries, squinted raw data acquisitions, stripmap mode, airborne radar, image registration, radar imaging, radar resolution, remote sensing by radar, sensor fusion, spaceborne radar, synthetic aperture radar.
    Abstract: Synthetic aperture radar (SAR) systems require that a focusing operation be performed on the received backscattered echoes (raw data) to generate high-resolution microwave images. Either due to platform attitude instabilities, or to Earth rotation effects, the SAR raw data may be acquired squinted geometries, i.e., with the radar beam directed with an offset angle (squint angle) from the broadside direction. This research investigates the impact of the focusing operation carried out on squinted raw data acquisitions performed by SAR sensors operating in the stripmap mode. To this end the 2D frequency SAR processing approach is generalized with respect to conical, i.e., nonorthogonal, reference systems. This allows analysis of the geometric, spectral, and phase aberrations introduced in the images by the chosen processing geometry with respect to the acquisition, and identification of the focusing procedure that minimizes these aberrations. The whole theory is validated by experimental results carried out on simulated data. Moreover, the extension of this analysis to the interferometric case where these aberrations may have a significant role is also investigated
    [bibtex-key = fornaroSansostiLanariTesauro2002:ProcessingGeometrySAR] [bibtex-entry]


  822. Per-Olov Frölind and Lars M. H. Ulander. Digital Elevation Map Generation Using VHF-Band SAR Data in Forested Areas. IEEE Transactions on Geoscience and Remote Sensing, 40(8):1769-1776, August 2002. Keyword(s): SAR Processing, Interferometry, DEM Generation, Time-Domain Back-Projection, Backprojection, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR.
    Abstract: The paper investigates digital elevation model (DEM) generation based on data from the ultra wideband coherent all radio band sensing (CARABAS) very high frequency (VRF)-band synthetic aperture radar (SAR). The results show excellent capability to penetrate forest areas, i.e., the generated DEMs are found to be close to the true ground height. A conventional DEM, based on stereo photography and surveying, and additional phase differential Global Positioning System (GPS) measurements have been used for comparison. The results in heavily vegetated areas (stem volume up to 600 m^3/ha) show a mean height difference of less than 1.5 m and a root-mean-square (rms) error of less than 1.0 in compared to the conventional DEM. Stable backscattering properties allows us to use large baselines in order to obtain high height sensitivity. However, the amount of poor data due to low coherence increases with the increase of the baseline. The optimum baseline which balances these two effects is found to correspond to an incidence angle difference of 4 deg - 8 deg.
    [bibtex-key = froelindUlander02:Interfero] [bibtex-entry]


  823. Dirk Geudtner, Manfred Zink, Christoph Gierull, and Scott Shaffer. Interferometric Alignment of the X-SAR Antenna System on the Space Shuttle Radar Topography Mission. IEEE Transactions on Geoscience and Remote Sensing, 40(5):995-1006, May 2002. Keyword(s): SAR Processing, Antenna Beam Alignment, Interferometry, SRTM, X-SAR.
    Abstract: The on-orbit alignment of the antenna beams of both the X-band and C-band radar systems during operations of the shuttle radar topography mission/X-band synthetic aperture radar (SRTM/X-SAR) was a key requirement for achieving best interferometric performance. In this paper, we consider the X-SAR antenna beam alignment in azimuth. For a single-pass cross-track SAR interferometer, we establish the relation between yaw and pitch misalignment of the antenna beams and the resulting relative shift of the Doppler frequency bands. This relation is used to provide solutions for the mechanical adjustments of the outboard antenna and electronic beam steering to correct for azimuth misalignment. Furthermore, the effects of the X-SAR effective outboard antenna pattern on the azimuth beam alignment are analyzed. As a result, a so-called relaxing factor is derived, which increases the limit for the difference in antenna azimuth angle with respect to the requirement on spectral overlap, and hence spatial interferogram resolution. However, we also show that the alignment requirement is driven by the constraint on decreasing the azimuth ambiguity-to-signal ratio (AASR) for the effective outboard antenna pattern to reduce the resulting additional height error. The strategy for misalignment determination and correction is presented, and results of the analysis of the in-flight X-SAR antenna beam alignment are discussed.
    [bibtex-key = geudtnerZink02:SRTM] [bibtex-entry]


  824. Christoph H. Gierull and Ishuwa C. Sikaneta. Estimating the effective number of looks in interferometric SAR data. IEEE Trans. Geosci. Remote Sens., 40(8):1733-1742, August 2002. Keyword(s): SAR Processing, ENL, effective number of looks, maximum likelihood estimation, method of moments, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar, CFAR detection thresholds, MoM, adjacent pixels, compensation, complex correlation coefficient, constant false alarm rate detection thresholds, filtering, interferometric SAR data, maximum-likelihood estimator, moving-target detection, multilook interferometric phase, pdf, probability density function, statistical dependence, synthetic aperture radar images, Clutter, Interferometry, Layout, Maximum likelihood estimation, Pixel, Probability density function, Radar detection, Speckle, Statistics.
    Abstract: The probability density function (pdf) of the multi-look interferometric phase between two complex synthetic aperture radar (SAR) images is parameterized by the number of looks and the complex correlation coefficient. In practice, adjacent pixels in a real SAR interferogram, are statistically dependent due to filtering, and hence, the number of looks is usually smaller than the number of samples averaged. It has been shown that compensation with an effective number of looks, rather than an intractable rederivation of the pdf, can account for the statistical dependence. This paper addresses the challenge of how to determine a suitable value for the effective number of looks. It is shown that an optimum value can be found via a maximum-likelihood estimator (MLE) based on the interferometric phase pdf. However, since such an MLE is computationally intensive and numerically unstable, an estimator based on the method of moments (MoM) possessing similar fidelity is proposed. MoM is fast and robust and can be used in operational applications, such as determining constant false alarm rate (CFAR) detection thresholds for moving-target detection in SAR along-track interferometry.
    [bibtex-key = gierullSikanetaTGRS2002EffectiveNumberOfLooks] [bibtex-entry]


  825. F. Gini, F. Lombardini, and M. Montanari. Layover solution in multibaseline SAR interferometry. Aerospace and Electronic Systems, IEEE Transactions on, 38(4):1344-1356, 2002. Keyword(s): SAR Processing, SAR Tomography, Tomography, Monte Carlo methods, airborne radar, radiowave interferometry, remote sensing by radar, synthetic aperture radar, MUSIC, Cramer-Rao lower bounds, Interferometry, InSAR, M-RELAX, Monte Carlo simulations, RELAX algorithm, baseline diversity, discontinuities, height contributions, layover solution, multibaseline SAR interferometry, multichannel interferometric synthetic aperture radar, multiplicative noise, natural targets, nonparametric techniques, parametric techniques, range-azimuth resolution cell, spectral estimation techniques, terrain slopes.
    Abstract: In this work, spectral estimation techniques are used to exploit baseline diversity of a multichannel interferometric synthetic aperture radar (SAR) system and overcome the layover problem. This problem arises when different height contributions collapse in the same range-azimuth resolution cell, due to the presence of strong terrain slopes or discontinuities in the sensed scene. We propose a multilook approach to counteract the presence of multiplicative noise, which is due to the extended nature of natural targets; to this purpose we extend the RELAX algorithm to the multilook data scenario (M-RELAX). A thorough performance analysis of nonparametric (beamforming and Capon) and parametric (root MUSIC and M-RELAX) techniques is carried out based on Monte Carlo simulations and Cramer-Rao lower bounds (CRLB) calculation. The results suggest the superiority of parametric methods over nonparametric ones.
    [bibtex-key = giniLombardiniMontanari02:LayoverMultibaseline] [bibtex-entry]


  826. F. Gustrau and A. Bahr. W-band investigation of material parameters, SAR distribution, and thermal response in human tissue. IEEE Transactions on Microwave Theory and Techniques, 50(10):2393-2400, October 2002. Keyword(s): SAR Processing, W-Band, bioelectric phenomena, biological effects of microwaves, biological tissues, biothermics, dosimetry, eye, finite difference time-domain analysis, health hazards, infrared imaging, skin, 3 to 100 GHz, 77 GHz, Gunn oscillator, SAR distribution, W-band dielectric properties, analytical method, dosimetry, electromagnetic field, eye tissue, finite-difference time-domain method, horn antenna, human eye, human tissue, layered skin model, maximum local SAR values, maximum temperature increase, millimeter-wave irradiation, plane-wave exposure, porcine eye, safety guidelines, skin, specific absorption rate, superficial tissue, temperature changes, thermal bio-heat-transfer simulation, thermal infrared imaging system, thermal response, Antenna measurements, Biological materials, Biological system modeling, Biological tissues, Dielectric materials, Electromagnetic fields, Humans, Millimeter wave measurements, Skin, Temperature.
    Abstract: This investigation is divided into three parts. First, the W-band dielectric properties of different biological tissues are determined. Then, the electromagnetic field in the human eye and skin is simulated for plane-wave exposure. An analytical method is used to investigate the specific absorption rate (SAR) inside a layered model of the human skin between 3-100 GHz. Furthermore, the SAR inside a detailed model of the human eye is investigated numerically by the finite-difference time-domain method for a frequency of 77 GHz. Maximum local SAR values of 27.2 W/kg in skin tissue and 45.1 W/kg in eye tissue are found for 77 GHz and an incident power density of 1 mW/cm2. In the third part of the investigation, the temperature changes of superficial tissue caused by millimeter-wave irradiation are measured by a thermal infrared imaging system. The exposure setup is based on a horn antenna with a Gunn oscillator operating at 15.8-dBm output power. The measurements showed a maximum temperature increase of 0.7 deg C for a power density of 10 mW/cm2 and less than 0.1 deg C for 1 mW/cm2, both in human skin (in vivo), as well as in porcine eye (in vitro). The comparison of the temperature measurements with a thermal bio-heat-transfer simulation of a layered skin model showed a good agreement.
    [bibtex-key = gustrauBahrTMTT2002WBandSAR] [bibtex-entry]


  827. Jürgen Holzner and Richard Bamler. Burst-Mode and ScanSAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 40(9):1917-1934, September 2002. Keyword(s): SAR Processing, Burst-mode, ScanSAR, Interferometry, pack-and-go algorithm, ENVISAT, RADARSAT, SRTM.
    Abstract: ScanSAR interferometry is an attractive option for efficient topographic mapping of large areas and for monitoring of large-scale motions. Only ScanSAR interferometry made it possible to map almost the entire landmass of the earth in the 11-day Shuttle Radar Topography Mission. Also the operational satellites RADARSAT and ENVISAT offer ScanSAR imaging modes and thus allow for repeat-pass ScanSAR interferometry. This paper gives a complete description of ScanSAR and burst-mode interferometric signal properties and compares different processing algorithms. The problems addressed are azimuth scanning pattern synchronization, spectral shift filtering in the presence of high squint, Doppler centroid estimation, different phase-preserving ScanSAR processing algorithms, ScanSAR interferogram formation, coregistration, and beam alignment. Interferograms and digital elevation models from RADARSAT ScanSAR Narrow modes are presented. The novel pack-and-go algorithm for efficient burst-mode range processing and a new time-variant fast interpolator for interferometric coregistration are introduced.
    [bibtex-key = HolzBaml:burstmode] [bibtex-entry]


  828. Lance M. Kaplan, James H. McClellan, and Seung-Mok Oh. Prescreening During Image Formation for Ultrawideband Radar. IEEE Transactions on Aerospace and Electronic Systems, 38(1):74-88, January 2002. Keyword(s): SAR Processing, Back-Projection, Ultra-Wideband SAR, Prescreening, Quadtree Processing, Divide and Conquer Methods.
    Abstract: Standard radar image formation techniques waste computational resources by full resolving all areas of the scene, even regions of benign clutter. We introduce a multiscale prescreener algorithm that runs as part of the image formation processing step for ultrawideband (UWB) synthetic aperture radar (SAR) systems. The prescreener processes intermediate radar data generated by a quadtree backprojection image former. As the quadtree algorithm iterates, it is resolving increasingly finer subpatches of the scene. After each quadtree stage, the prescreener makes an estimate of the signal-to-background ratio of each subpatch and applies a constant false alarm rate (CFAR) detector to decide which ones might contain a target of interest. Whenever the prescreener determines that a subpatch is not near a detection, it cues the image former to terminate further processing of that subpatch. Using a small database of UWB radar field data, we demonstrate that the prescreener is able to decrease the overall computational load of the image formation process. We also show that the new multiscale prescreener method produces fewer false alarms than the conventional two-parameter CFAR prescreener applied to the completely formed image.
    [bibtex-key = KaplanEtAl02:Backproj] [bibtex-entry]


  829. Erik G. Larsson and Petre Stoica. Fast Implementation of Two-Dimensional APES and CAPON Spectral Estimators. Multidimensional Systems and Signal Processing, 13(1):35-53, 2002. Keyword(s): 2D Imaging, Capon, APES, Fast Implementation, Spectral Estimation, Beamforming.
    Abstract: The matched-filterbank spectral estimators APES and CAPON have recently received considerable attention in a number of applications. Unfortunately, their computational complexity tends to limit their usage in several cases --a problem that has previously been addressed by different authors. In this paper, we introduce a novel approach to the computation of the APES and CAPON spectra, which leads to a computational method that is considerably faster than all existing techniques. The new implementations of APES and CAPON are called fast APES and fast CAPON, respectively, and are developed for the two-dimensional case, with the one-dimensional case as a special case. Numerical examples are provided to demonstrate the application of APES to synthetic aperture radar (SAR) imaging, and to illustrate the reduction in computational complexity provided by our method.
    [bibtex-key = larssonStoica2002FastImplementationsOf2DAPESandCapon] [bibtex-entry]


  830. Christian Matzler. MATLAB functions for Mie scattering and absorption, version 2. IAP Res. Rep, 8:1-24, 2002. [bibtex-key = Maetzler2002] [bibtex-entry]


  831. Christian Matzler. Relation between grain-size and correlation length of snow. Journal of Glaciology, 48(162):461-466, February 2002.
    Abstract: In the past it has often been difficult to compare results of different types of snow-structural information. Grain-size and correlation length are such parameters of granular media, and there exist different definitions and different measurement methods for both of them. The relation between these parameters is analyzed from theoretical and from experimental points of view, considering optical and microwave properties. For spherical ice grains the connecting formulas are simple, but for other shapes the two parameters are not directly related. Care must be taken in the measurement procedure. Especially if grain-size is regarded as the maximum extent of connected ice particles, the results are likely to lead to extreme overestimates. Therefore it is concluded that grain-size should be complemented by an additional size parameter, namely, the surface-to-volume ratio of equivalent spheres, i.e.a measure of the correlation length. Methods to determine this quantity in the laboratory have been known for a long time. Methods to obtain such measurements in the field are described here.
    [bibtex-key = matzlerJournalofGlaciology2002RelationGrainSizeCorrelationLength] [bibtex-entry]


  832. Jean-Luc Starck, E.J. Candes, and D.L. Donoho. The curvelet transform for image denoising. IEEE Transactions on Image Processing, 11(6):670-684, June 2002. Keyword(s): Cartesian samples, Fourier space, Fourier-domain, approximate digital Radon transform, approximate digital implementations, concentric squares geometry, curvelet coefficients, curvelet transform, decimated wavelet transforms, exact reconstruction, filter bank, frequency domain, image denoising, interpolation, low computational complexity, overcomplete wavelet pyramid, pseudo-polar sampling set, rectopolar grid, ridgelet transform, stability, tree-based Bayesian posterior mean methods, trous wavelet filters, undecimated wavelet transforms, visual performance, wavelet-based image reconstruction, white noise, Fourier transforms, Radon transforms, channel bank filters, filtering theory, image reconstruction, interpolation, wavelet transforms, white noise;.
    Abstract: We describe approximate digital implementations of two new mathematical transforms, namely, the ridgelet transform and the curvelet transform. Our implementations offer exact reconstruction, stability against perturbations, ease of implementation, and low computational complexity. A central tool is Fourier-domain computation of an approximate digital Radon transform. We introduce a very simple interpolation in the Fourier space which takes Cartesian samples and yields samples on a rectopolar grid, which is a pseudo-polar sampling set based on a concentric squares geometry. Despite the crudeness of our interpolation, the visual performance is surprisingly good. Our ridgelet transform applies to the Radon transform a special overcomplete wavelet pyramid whose wavelets have compact support in the frequency domain. Our curvelet transform uses our ridgelet transform as a component step, and implements curvelet subbands using a filter bank of a grave; trous wavelet filters. Our philosophy throughout is that transforms should be overcomplete, rather than critically sampled. We apply these digital transforms to the denoising of some standard images embedded in white noise. In the tests reported here, simple thresholding of the curvelet coefficients is very competitive with state of the art techniques based on wavelets, including thresholding of decimated or undecimated wavelet transforms and also including tree-based Bayesian posterior mean methods. Moreover, the curvelet reconstructions exhibit higher perceptual quality than wavelet-based reconstructions, offering visually sharper images and, in particular, higher quality recovery of edges and of faint linear and curvilinear features. Existing theory for curvelet and ridgelet transforms suggests that these new approaches can outperform wavelet methods in certain image reconstruction problems. The empirical results reported here are in encouraging agreement
    [bibtex-key = 1014998] [bibtex-entry]


  833. Curtis W. Chen and Howard A. Zebker. Two-dimensional phase unwrapping with use of statistical models for cost functions in nonlinear optimization. J. Opt. Soc. Am. A, 18(2):338-351, February 2001. Keyword(s): SAR Processing, phase unwrapping, SNAPHU, SAR Interferometry, InSAR, Synthetic aperture radar, Phase.
    Abstract: Interferometric radar techniques often necessitate two-dimensional (2-D) phase unwrapping, defined here as the estimation of unambiguous phase data from a 2-D array known only modulo 2$\pi$ rad. We develop a maximum a posteriori probability (MAP) estimation approach for this problem, and we derive an algorithm that approximately maximizes the conditional probability of its phase-unwrapped solution given observable quantities such as wrapped phase, image intensity, and interferogram coherence. Examining topographic and differential interferometry separately, we derive simple, working models for the joint statistics of the estimated and the observed signals. We use generalized, nonlinear cost functions to reflect these probability relationships, and we employ nonlinear network-flow techniques to approximate MAP solutions. We apply our algorithm both to a topographic interferogram exhibiting rough terrain and layover and to a differential interferogram measuring the deformation from a large earthquake. The MAP solutions are complete and are more accurate than those of other tested algorithms.
    [bibtex-key = chenZebkerJOptSoc2001SNAPHU2DPhaseUnwrappingCostFunctionNonlinearOptimization] [bibtex-entry]


  834. A. Ferretti, C. Prati, and F. Rocca. Permanent scatterers in SAR interferometry. IEEE Trans. Geosci. Remote Sens., 39(1):8-20, 2001. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, Permanent scatterers, geodesy, geophysical techniques, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth)InSAR, SAR interferometry, atmospheric disturbance, atmospheric phase screen, differential interferometry, geometrical decorrelation, geophysical measurement technique, land surface topography, permanent scatterer, radar remote sensing, stable natural reflector, surface deformation monitoring, topographic profile reconstruction.
    Abstract: Temporal and geometrical decorrelation often prevents SAR interferometry from being an operational tool for surface deformation monitoring and topographic profile reconstruction. Moreover, atmospheric disturbances can strongly compromise the accuracy of the results. The authors present a complete procedure for the identification and exploitation of stable natural reflectors or permanent scatterers (PSs) starting from long temporal series of interferometric SAR images. When, as it often happens, the dimension of the PS is smaller than the resolution cell, the coherence is good even for interferograms with baselines larger than the decorrelation one, and all the available images of the ESA ERS data set can be successfully exploited. On these pixels, submeter DEM accuracy and millimetric terrain motion detection can be achieved, since atmospheric phase screen (APS) contributions can be estimated and removed. Examples are then shown of small motion measurements, DEM refinement, and APS estimation and removal in the case of a sliding area in Ancona, Italy. ERS data have been used
    [bibtex-key = ferrettiPratiRocca2001:PermanentScat] [bibtex-entry]


  835. T. Guneriussen, K.A. Hogda, H. Johnsen, and I. Lauknes. InSAR for estimation of changes in snow water equivalent of dry snow. IEEE Transactions on Geoscience and Remote Sensing, 39(10):2101-2108, October 2001. Keyword(s): snow, DInSAR, snow water equivalent, SWE, radar, SAR interferometry, InSAR.
    Abstract: This paper describes the theoretical relation between interferometric phase and changes in snow water equivalent (SWE) and show results from experiments using ERS- 1/2 tandem data. The main scattering contribution from a dry snow cover is from the snow-ground interface. However, the radar wave will be refracted in the snow. Thus, only small changes in the snow properties between two interferometric synthetic aperture radar (SAR) images will change the interferometric phase. This phase change is shown to introduce a significantly increase in the digital elevation model (DEM) height error, although no effects are observed on the degree of coherence. The phase change is also shown to affect the differential interferometric results and may wrongly be interpreted as range displacement. The presented theory and results implies that light snowfall and/or small changes in snow properties between interferometric SAR (InSAR) image acquisitions, may introduce significant height errors in DEM derived from glaciers, ice sheets, or bare ground, even in the case of high degree of coherence. Thus, meteorological observations in addition to degree of coherence must be considered when generating DEM in areas covered with snow or where snow fall is likely to have occurred.
    [bibtex-key = guneriussenHogdaJohnsenLauknesTGRS2001InSARforEstimationOfChangesInSWEofDrySnow] [bibtex-entry]


  836. R. Lanari, M. Tesauro, E. Sansosti, and G. Fornaro. Spotlight SAR data focusing based on a two-step processing approach. IEEE Transactions on Geoscience and Remote Sensing, 39(9):1993 -2004, September 2001. Keyword(s): SPECAN algorithm, azimuth spectral folding, data focusing, data-focusing algorithm, deramping, geophysical measurement technique, high bandwidth transmitted chirp signals, land surface, radar remote sensing, space-invariant azimuth filtering, space-variant characteristics, spaceborne radar, spectral analysis, spotlight SAR, stripmap focusing, synthetic aperture radar, terrain mapping, two-step process, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping.
    Abstract: The authors present a new spotlight SAR data-focusing algorithm based on a two-step processing strategy that combines the advantages of two commonly adopted processing approaches: the efficiency of SPECAN algorithms and the precision of stripmap focusing techniques. The first step of the proposed algorithm implements a linear and space-invariant azimuth filtering that is carried out via a deramping-based technique representing a simplified version of the SPECAN approach. This operation allows the authors to perform a bulk azimuth raw data compression and to achieve a pixel spacing smaller than (or equal to) the expected azimuth resolution of the fully focused image. Thus, the azimuth spectral folding phenomenon, typically affecting the spotlight data, is overcome, and the space-variant characteristics of the stripmap system transfer function are preserved. Accordingly, the residual and precise focusing of the SAR data is achieved by applying a conventional stripmap processing procedure requiring a minor modification and implemented in the frequency domain. The extension of the proposed technique to the case of high bandwidth transmitted chirp signals is also discussed. Experiments carried out on real and simulated data confirm the validity of the presented approach, which is mainly focused on spaceborne systems
    [bibtex-key = 951090] [bibtex-entry]


  837. R. Lanari, S. Zoffoli, E. Sansosti, G. Fornaro, and F. Serafino. New approach for hybrid strip-map/spotlight SAR data focusing. IEE Proceedings - Radar, Sonar and Navigation, 148(6):363 -372, December 2001. Keyword(s): SAR data processing, azimuth convolution, azimuth data filtering, azimuth resolution, chirp signal, focused SAR images, generalised processing code, hybrid acquisition mode, hybrid strip-map/spotlight SAR, microwave image generation, spacebome systems, spotlight SAR data processing, strip-map processing, two-step focusing technique, convolution, filtering theory, image resolution, radar imaging, radar resolution, spaceborne radar, synthetic aperture radar.
    Abstract: A new algorithm is presented for processing SAR data acquired in the hybrid strip-map/spotlight SAR configuration whose acquisition mode allows the generation of microwave images with an azimuth resolution better than that obtained in the strip-map case and with an imaged area larger than that achieved in the spotlight operation. The algorithm extends the focusing capability of conventional strip-map processing techniques to the hybrid SAR data; this result is achieved by generalising a previously proposed two-step focusing technique for spotlight SAR data processing, the spotlight mode being a particular case of the more general hybrid one. The key point of the procedure is the first filtering step which implements an azimuth convolution between the raw data and a chirp signal whose rate is selected, in the generalised approach, depending on the characteristics of the hybrid acquisition mode. Following this stage, standard strip-map processing procedures can be used to implement the second processing step, leading to fully focused SAR images. The algorithm, mainly oriented to spaceborne systems, is simple and efficient and allows the design of a generalised processing code suitable for data acquired in the hybrid strip-map/spotlight SAR configuration including the strip-map and spotlight modes as special cases. Experiments carried out on simulated data clarify the rationale of the method and confirm its validity
    [bibtex-key = 980764] [bibtex-entry]


  838. Fabrizio Lombardini and Hugh D. Griffiths. Optimum and Suboptimum Estimator Performance for Multibaseline InSAR. EUSAR 2000 Special Issue, Frequenz, Zeitschrift für Telekommunikation (Journal of Telecommunications), 55:114-118, March 2001. Keyword(s): SAR Processing, Interferometry, Multibaseline InSAR, Model-Based Interferometric Processing.
    Abstract: Multibaseline methods have been recently proposed to reduce problems of phase ambiguity and data noise in SAR interferometry for topographic mapping. Both phase-only interferometry and direct processing of the complex multibaseline data have been considered. In this paper, two non-model based multibaseline estimators and two recently proposed model based algorithms are discussed and compared in terms of statistical accuracy of the reduced-ambiguity phase estimate. lt is shown that model-based processing of the complex multibaseline data can be significantly more efficient than the other methods.
    [bibtex-key = lombardiniFrequenz01:MultibaselineInSAR] [bibtex-entry]


  839. X. Luo, Lars M. H. Ulander, J. Askne, G. Smith, and Per-Olov Frölind. RFI Suppression in Ultra-Wideband SAR Systems Using LMS Filters in Frequency Domain. Electronics Letters, 37(4):241-243, February 2001. Keyword(s): SAR Processing, RFI Suppression, Back-Projection, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR.
    Abstract: A least-mean-square algorithm in the frequency domain, with amplitude normalisation, is proposed to remove narrowband radio frequency interference in low-frequency ultra-wideband synthetic aperture radar systems. The performance of the algorithm was tested on experimental data acquired with the CARABAS system
    [bibtex-key = LuoUlandAskSmiFro01:RFI] [bibtex-entry]


  840. Andrea Monti-Guarnieri and Pietro Guccione. Optimal Focusing for Low Resolution ScanSAR. IEEE Transactions on Geoscience and Remote Sensing, 39(3):479-491, March 2001. Keyword(s): SAR Processing, ScanSAR, Focusing.
    Abstract: This paper deals with the focusing of low resolution ScanSAR data, for both detected amplitude images and interferometric applications. The SAR reference is exploited to achieve ScanSAR focusing in conventional techniques. Such techniques provide quite effective compensation of the azimuth antenna pattern (AAP) (e.g., no scalloping) when the azimuth time-bandwidth product (TBP ) of the ScanSAR echo is large, but fail to do so as the burst shortens, being reduced to an ineffective weighting of the output. The result is an azimuth varying distortion of the focused impulse responses, a distortion that is partly compensated for in the multilook average (not available for interferometric applications) at the price of a reduction in the processed Doppler bandwidth. This paper proposes quite a different approach. A set of short kernels, each suitable for focusing at a specific azimuth bin, has been optimized to reconstruct source reflectivity in the minimum mean square error (MMSE) sense. That pseudoinversion converges to the conventional focusing when the burst extent is large and for short bursts, edge effects are accounted for. These azimuth-varying kernels can be suitably tuned to meet constraints in the resolution/sidelobes trade-off and have proved capable of providingfairly undistorted output and fine resolution. They better exploit the available Doppler bandwidth, maximizing the number of looks and the interferometric quality. A decomposition is suggested that implements the inverse operator as a fast preprocessing to be followed by a conventional ScanSAR processor.
    [bibtex-key = monti01:optFocus] [bibtex-entry]


  841. A. Potsis, A. Reigber, J. Mittermayer, A. Moreira, and N. Uzunoglou. Sub-aperture algorithm for motion compensation improvement in wide-beam SAR data processing. Electronics Letters, 37(23):1405-1407, 2001. Keyword(s): SAR Processing, error compensation, Motion Compensation, radar imaging, Airborne SAR, synthetic aperture radar, motion compensation improvement, residual motion error compensation, strong motion errors, sub-aperture algorithm, synthetic aperture radar processing, wide-beam SAR data processing, wide-beam azimuth processing, ESAR, P-Band.
    Abstract: The effects of strong motion errors in wide-beam azimuth synthetic aperture radar (SAR) processing are analysed and discussed, using simulated data, as well as data collected by the airborne experimental SAR system of the Deutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR) (E-SAR). A new sub-aperture approach for residual motion error compensation in wide-beam azimuth processing is proposed.
    [bibtex-key = potsisReigberMittermayerMoreiraUzunoglou01:SubApertureMocoAlgo] [bibtex-entry]


  842. A. Reigber. Correction of residual motion errors in airborne SAR interferometry. Electronics Letters, 37(17):1083-1084, 2001. Keyword(s): SAR Processing, airborne radar, error correction, motion compensation, radar interference, radiowave interferometry, synthetic aperture radar, airborne SAR interferometry, airborne repeat pass interferometric SAR data, interferometric phase, residual motion compensation errors, residual motion error correction, uncompensated motion errors, Motion Compensation, Interferometry, interferometric SAR.
    Abstract: The main limitation for a wider applicability of airborne repeat pass interferometric SAR data is the presence of small uncompensated motion errors. The effect of residual motion compensation errors is addressed and a new technique to minimise their influence on the interferometric phase is proposed.
    [bibtex-key = reigber01:ResMotionComp] [bibtex-entry]


  843. Andreas Reigber, Konstantinos P. Papathanassiou, Shane R. Cloude, and Alberto Moreira. SAR Tomography and Interferometry for the Remote Sensing of Forested Terrain. EUSAR 2000 Special Issue, Frequenz, Zeitschrift für Telekommunikation (Journal of Telecommunications), 55:119-122, March 2001. Keyword(s): SAR Processing, Tomography, SAR Tomography, Interferometry, Multi-Antenna Interferometry, Multi-Baseline Interferometry, 3D imaging, Airborne SAR, L-Band, E-SAR.
    Abstract: SAR interferometry and tomography are two potential techniques for resolving the scattering behaviour of volume scatterers. Therefore, they can be used to investigate the vertical structure of forested terrain. ln this paper we contrast the principles and the performance of the two approaches and show experimental results achieved using airborne SAR data acquired by the experimental SAR systemof DLR (E-SAR).
    [bibtex-key = ReigberPapathanassiouCloudeMoreira01:Tomography] [bibtex-entry]


  844. Ludwig Rössing and Joachim H.G. Ender. Multi-Antenna SAR Tomography Using Superresolution Techniques. EUSAR 2000 Special Issue, Frequenz, Zeitschrift für Telekommunikation (Journal of Telecommunications), 55:123-128, March 2001. Keyword(s): SAR Processing, Tomography, SAR Tomography, Interferometry, Multi-Antenna Interferometry, 3D imaging, superresolution, a posteriori probability density function, Airborne SAR, X-Band, Multi-Channel X-Band, AER-II, FGAN.
    Abstract: Classical SAR interferometry provides a two-dimensional image of the surface together with a pixel-to-pixel measurement of the elevation angle of that pixel with respect to the radar antenna. The elevation angle can be transformed to a height estimate for each pixel yielding a digital elevation map. This assumes that the scattering points are distributed over a two dimensional surface and that for each azimuth-range resolution cell there is only one elevation angle. In this paper, we look for solutions for those cases, in which this assumption is invalid, i.e. where there are multiple point-like scatterers with distinct elevation angles in a single resolution cell. Using a parametric approach these distinct elevation angles can be estimated simultaneously. Since for single-pass interferometry the height resolution is mostly poor because of the limited baseline, we want to apply superresolution to get an appropriate height resolution. To achieve imaging also along the third dimension we propose a calculation of an a posteriori marginal distribution of the elevation angle. This approach is illustrated by simulation results and with airborne SAR data from the multi-channel X-band system 'AER-II'.
    [bibtex-key = RoessingTomogrFrequenz01:Tomography] [bibtex-entry]


  845. Mehrdad Soumekh. Wavefront-Based Synthetic Aperture Radar Signal Processing. EUSAR 2000 Special Issue, Frequenz, Zeitschrift für Telekommunikation (Journal of Telecommunications), 55:99-113, March 2001. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Back-Projection, Moving Target Indication, Range Stacking Algorithm, Wavefront Reconstruction, Range Migration Algorithm, Wavenumber Domain Algorithm, omega-k, Comparison of Algorithms, Wideband SAR, Squinted SAR, FOPEN, Motion Compensation.
    Abstract: This paper is concerned with the processing of Synthetic Aperture Radar (SAR) data, using Gabor's theory of wavefront reconstruction [9]. In the framework of this theory, multidimensional digital signal processing algorithms have been developed for accurate and computationally-efficient analysis of SAR data via a single or multi processor computer. This paper exhibits the utility of the SAR wavefront signal theory and its associated digital algorithms in addressing the practical information processing issues that are encountered in high-resolution and/or specialized SAR systems. We present results from two modern SAR systems. One system is the United States Navy P-3 ultra wideband UHF stripmap SAR that is intended for imaging man-made targets hidden under foliage (FOliage PENetrating, FOPEN, SAR). The other system is a high-resolution X-band spotlight SAR that is operated in an along-track monopulse, mode (single transmitter and dual receivers) for Ground Moving Target Indication (GMTI).
    [bibtex-key = soumekhFrequenz01:Wavefront] [bibtex-entry]


  846. Mehrdad Soumekh, David A. Nobles, Michael C. Wicks, and Gerard J. Genello. Signal Processing of Wide Bandwidth and Wide Beamwidth P-3 SAR data. IEEE Transactions on Aerospace and Electronic Systems, 37(4):1122-1141, October 2001. Keyword(s): SAR Processing, P-Band, Ultra-Wideband SAR, Time-Domain Back-Projection, TDBP, Back-Projection, RFI Suppression.
    Abstract: This research is concerned with multidimensional signal processing and image formation with FOliage PENetrating (FOPEN) airborne radar data which were collected by a Navy P-3 ultra wideband (UWB) radar in 1995. The digital signal processors that were developed for the P-3 data commonly used a radar beamwidth angle that was limited to 35 deg. Provided that the P-3 radar beamwidth angle (after slow-time FIR filtering and 6:1 decimation) was 35 deg, the P-3 signal aperture radar (SAR) system would approximately yield alias-free data in the slow-time Doppler domain. We provide an analysis here of the slow-time Doppler properties of the P-3 SAR system. This study indicates that the P-3 database possesses a 50 deg beamwidth angle within the entire [215, 730] MHz band of the P-3 radar. We show that the 50-degree beamwidth limit is imposed by the radar (radial) range swath gate; a larger beamwidth measurements would be possible with a larger range swath gate. The 50-degree beamwidth of the P-3 system results in slow-time Doppler aliasing within the frequency band of [444, 730] MHz. We outline a slow-time processing of the P-3 data to minimize the Doppler aliasing. The images which are formed via this method are shown to be superior in quality to the images which are formed via the conventional P-3 processor. In the presentation, we also introduce a method for converting the P-3 deramped (range-compressed) data into its alias-free baseband echoed data; the utility of this conversion for suppressing radio frequency interference signals is shown
    [bibtex-key = soumNoblWicGen01:PBand] [bibtex-entry]


  847. Falk Amelung, Sigurjon Jonsson, Howard Zebker, and Paul Segall. Widespread uplift and trapdoor faulting on Galapagos volcanoes observed with radar interferometry. Nature, 407(6807):993-996, October 2000. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Deformation Mapping, Deformation Monitoring, ERS-1, ERS-2, Displacement, Surface Displacement, Surface Deformation, Spaceborne SAR, C-band, Volcano, Galapagos, deflation, Galapagos Islands, Cerro Azul, Fernandina, co-eruptive deflation, dyke intrusion.
    Abstract: Volcanic uplift, caused by the accumulation of magma in subsurface reservoirs, is a common precursor to eruptions. But, for some volcanoes, uplift of metres or more has not yet led to an eruption. Here we present displacement maps of volcanoes in the Galapagos Islands, constructed using satellite radar interferometry, that might help explain this dichotomy. We show that all but one of the seven volcanoes on the islands of Isabela and Fernandina deformed during 1992-99. Cerro Azul and Fernandina erupted during the observation period and show evidence of inflation, co-eruptive deflation and shallow dyke intrusion. In contrast, the largest volcano, Sierra Negra, has not erupted, yet exhibits spatially and temporally variable deformation, with a maximum uplift of 2.7 m between 1992 and 1999, which can be modelled by a shallow inflating sill. Inflation during 1997-98, however, was accompanied by 'trapdoor' faulting on a steeply dipping fracture system within the caldera. Repeated trapdoor faulting over geological time has formed an arcuate intra-caldera ridge within Sierra Negra and may have acted to relax stresses above the magma chamber, inhibiting summit eruptions. Similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes.
    [bibtex-key = amelungJonssonZebkerSegallNature2000DInSARGalapagosVolcanoes] [bibtex-entry]


  848. Marc Bara, Rolf Scheiber, Antoni Broquetas, and Alberto Moreira. Interferometric SAR signal analysis in the presence of squint. IEEE Transactions on Geoscience and Remote Sensing, 38(5):2164-2178, September 2000. Keyword(s): SAR Processing, InSAR, Interferometry, SAR Interferometry, Airborne SAR, Phase Ramp, Impulse Response Function, IRF Analysis, Phase Plots, E-SAR, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), geophysical measurement technique, high squint angle, impulse response function, interferometric SAR, land surface, misregistration, phase bias, phase ramp, radar remote sensing, squint, topography.
    Abstract: This paper develops an analysis of the SAR impulse response function from the interferometric point of view, with the intention of studying its phase behavior in the presence of high squint angle values. It will be pointed out that in this case, a phase ramp is present in the range direction, which, in combination with a certain degree of misregistration between the two images induces an offset in the generated interferometric phase. This behavior, if not compensated, imposes strong limits on the performance of the interferometric techniques in a squinted case, especially for airborne SAR systems. The article proposes two new techniques, which are appropriate to correct the phase bias coming from this source. The first one is based on a modification of the azimuth compression filter, which cancels the phase ramp of the range impulse response function for one specific squint value. In case the SAR processing is performed with variable squint over range, the authors propose a second method oriented to estimating the expected misregistration and thus, the phase bias by means of an iterative approach. Simulated data as well as real corner reflector responses are used to show that the correct topography can be recovered precisely even in the presence of phase bias coming from the squinted geometry.
    [bibtex-key = baraScheiberBroquetasMoreira2000:InSARSquinted] [bibtex-entry]


  849. Roland Bürgmann, Paul A. Rosen, and Eric J. Fielding. Synthetic Aperture Radar Interferometry to Measure Earth's Surface Topography and Its Deformation. Annual Review of Earth and Planetary Sciences, 28(1):169-209, 2000. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, Differential Interferometry, DInSAR, SAR Interferometry, Coherence, Synthetic aperture radar, SAR interferometry, differential SAR interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, deformation measurements.
    Abstract: Synthetic aperture radar interferometry (InSAR) from Earth-orbiting spacecraft provides a new tool to map global topography and deformation of the Earth's surface. Radar images taken from slightly different viewing directions allow the construction of digital elevation models of meter-scale accuracy. These data sets aid in the analysis and interpretation of tectonic and volcanic landscapes. If the Earth's surface deformed between two radar image acquisitions, a map of the surface displacement with tens-of-meters resolution and subcentimeter accuracy can be constructed. This review gives a basic overview of InSAR for Earth scientists and presents a selection of geologic applications that demonstrate the unique capabilities of InSAR for mapping the topography and deformation of the Earth.
    [bibtex-key = buergmannRosenFielding2000InSARReviewPaperApplicationOriented] [bibtex-entry]


  850. Curtis W. Chen and Howard A. Zebker. Network approaches to two-dimensional phase unwrapping: intractability and two new algorithms. J. Opt. Soc. Am. A, 17(3):401-414, March 2000. Keyword(s): SAR Processing, phase unwrapping, SNAPHU, Interferometry, InSAR, SAR Interferometry, Synthetic aperture radar, Phase.
    Abstract: Two-dimensional (2-D) phase unwrapping, that is, deducing unambiguous phase values from a 2-D array of values known only modulo 2$\pi$, is a key step in interpreting data acquired with synthetic aperture radar interferometry. Noting the recent network formulation of the phase unwrapping problem, we apply here some well-established ideas of network theory to formalize the problem, analyze its complexity, and derive algorithms for its solution. It has been suggested that the objective of phase unwrapping should be to minimize the total number of places where unwrapped and wrapped phase gradients differ. Here we use network constructions to show that this so-called minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly. Therefore we must instead find approximate solutions; we present two new algorithms for doing so. The first uses the network ideas of shortest paths and spanning trees to improve on the Goldstein et al. residue-cut algorithm \[Radio Sci.23, 713 (1988)\]. Our improved algorithm is very fast, provides complete coverage, and allows user-defined weights. With our second algorithm, we extend the ideas of linear network flow problems to the nonlinear L0 case. This algorithm yields excellent approximations to the minimum L0 norm. Using interferometric data, we demonstrate that our algorithms are highly competitive with other existing algorithms in speed and accuracy, outperforming them in the cases presented here.
    [bibtex-key = chenZebkerJOptSoc2000SNAPHUNetworkApproachesTo2DPhaseUnwrapping] [bibtex-entry]


  851. Marina Dragosevic and Burkhard Plache. Doppler Tracker for a Spaceborne ScanSAR System. IEEE Transactions on Aerospace and Electronic Systems, 36(3):907-924, 2000. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Doppler Tracker, Attitude Angles, Doppler Ambiguity Resolver, DAR, Satellite SAR.
    Abstract: This paper presents a methodology for tracking the Doppler parameters in long swaths of ScanSAR signals. The approach is entirely developed around the physical model of the Doppler effect, parameterized in terms of the spacecraft attitude angles. A new, combined algorithm is designed to estimate yaw and pitch and resolve the PRF (pulse repetition frequency) ambiguity using all available, current, and past, return samples. It is shown that the variance of the Doppler centroid (DC) estimates can be brought down to the low single-digit Hz level with computationally simple estimation algorithms
    [bibtex-key = dragosevic00:DopCentrEst] [bibtex-entry]


  852. A. Ferretti, C. Prati, and F. Rocca. Nonlinear subsidence rate estimation using permanent scatterers in differential SAR interferometry. IEEE Trans. Geosci. Remote Sens., 38(5):2202 -2212, September 2000. Keyword(s): SAR Processing, Persistent, Scatterer Interferometry, PSI, deformation measurement, displacement measurement, InSAR, absidence, atmospheric phase contribution, differential SAR interferometry, downlift, geodesy, geophysical measurement technique, nonlinear motion, nonlinear subsidence rate estimation, permanent scatterer, phase unwrapping, radar remote sensing, subsidence, synthetic aperture radar, tectonics, terrain mapping, uplift, urban area, geodesy, geophysical techniques, remote sensing by radar, synthetic aperture radar, tectonics, terrain mapping;.
    Abstract: Discrete and temporarily stable natural reflectors or permanent scatterers (PS) can be identified from long temporal series of interferometric SAR images even with baselines larger than the so-called critical baseline. This subset of image pixels can be exploited successfully for high accuracy differential measurements. The authors discuss the use of PS in urban areas, like Pomona, CA, showing subsidence and absidence effects. A new approach to the estimation of the atmospheric phase contributions, and the local displacement field is proposed based on simple statistical assumptions. New solutions are presented in order to cope with nonlinear motion of the targets
    [bibtex-key = ferrettiPratiRocca2000PSINonLinear] [bibtex-entry]


  853. A. Freeman, W.T.K. Johnson, B. Huneycutt, R. Jordan, S. Hensley, P. Siqueira, and J. Curlander. The myth of the minimum SAR antenna area constraint. IEEE Transactions on Geoscience and Remote Sensing, 38(1):320-324, January 2000. Keyword(s): SAR, Antenna, Spaceborne SAR, minimum SAR antenna constraint.
    Abstract: A design constraint traceable to the early days of spaceborne synthetic aperture radar (SAR) is known as the minimum antenna area constraint for SAR. In this paper, it is confirmed that this constraint strictly applies only to the case in which both the best possible resolution and the widest possible swath are the design goals. SAR antennas with area smaller than the constraint allows are shown to be possible, have been used on spaceborne SAR missions in the past, and should permit further, lower-cost SAR missions in the future.
    [bibtex-key = freemanJohnsonHuneycuttJordanHensleySiqueiraCurlanderTGRS2000MythOfMinimumSARAntennaAreaConstraint] [bibtex-entry]


  854. A. Laurence Gray, Karim E. Mattar, and George Sofko. Influence of ionospheric electron density fluctuations on satellite radar interferometry. Geophysical Research Letters, 27(10):1451-1454, 2000. Keyword(s): SAR Processing, ionospheric techniques, radar interferometry, synthetic aperture radar, azimuth shift, interferometric SAR ionosphere correction, ionospheric geometric distortion, Interferometry, Ionosphere, L-band, Synthetic aperture radar.
    Abstract: Evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry (SRI) obtained with the RADARSAT, ERS and JERS-1 satellites. Fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar (SAR) imagery, which can be detected using SRI. Measurements of azimuth shift in SRI can help to differentiate ionospheric from tropospheric propagation problems, and to understand better the impact of the ionosphere on spaceborne SAR. Further, SRI azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances.
    [bibtex-key = grayMattarSofkoGRL2000IonosphericTEConInSAR] [bibtex-entry]


  855. Jong-Sen Lee, D.L. Schuler, and T.L. Ainsworth. Polarimetric SAR data compensation for terrain azimuth slope. IEEE Transactions on Geoscience and Remote Sensing, 38(5):2153-2163, 2000. Keyword(s): geophysical techniques, radar imaging, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mapping, POLSAR, azimuth slope variation, data compensation, effective scattering pixel area, geophysical measurement technique, land surface, polarimetric SAR, polarization, radar cross section, radar remote sensing, radiometric slope correction.
    Abstract: This paper addresses the problem of polarimetric SAR (POLSAR) data
    [bibtex-key = LeeSchulerAinsworth00:PolSARTerrainSlope] [bibtex-entry]


  856. Juan M. Lopez-Sanchez and Joaquim Fortuny-Guasch. 3-D Radar Imaging Using Range Migration Techniques. IEEE Transactions on Antennas and Propagation, 48(5):728-737, May 2000. Keyword(s): SAR Processing, Near Field, SAR Tomography, Tomography, Radar Imaging, Range Migration Algorithm, Wavenumber Domain Algorithm, omega-k, 3D imaging algorithm, Stationary Phase Method.
    Abstract: An imaging system with three-dimensional (3-D) capability can be implemented by using a stepped frequency radar which synthesizes a two-dimensional (2-D) planar aperture. A 3-D image can be formed by coherently integrating the backscatter data over the measured frequency band and the two spatial coordinates of the 2-D synthetic aperture. This paper presents a near-field 3-D synthetic aperture radar (SAR) imaging algorithm. This algorithm is an extension of the 2-D range migration algorithm (RMA). The presented formulation is justified by using the method of the stationary phase (MSP). Implementation aspects including the sampling criteria, resolutions, and computational complexity are assessed. The high computational efficiency and accurate image reconstruction of the algorithm are demonstrated both with numerical simulations and measurements using an outdoor linear SAR system.
    [bibtex-key = lopez00:statphase] [bibtex-entry]


  857. Christian Matzler and A Standley. Technical note: Relief effects for passive microwave remote sensing. 2000. [bibtex-key = Maetzler2000] [bibtex-entry]


  858. Richard Rau and James H. McClellan. Analytic Models and Postprocessing Techniques for UWB SAR. IEEE Transactions on Aerospace and Electronic Systems, 36(4):1058-1074, October 2000. Keyword(s): SAR Processing, Back-Projection, Ultra-Wideband SAR, TDBP, Time-Domain Back-Projection.
    Abstract: The latest generation of fully polarimetric, ultra wideband (UWB), wide angle, low frequency, foliage and ground-penetrating synthetic aperture radars (SARs) record huge amounts of data that must be processed to focus high quality images. At the same time only a few small subimages contain important information. We investigate the relationship between the focused image and the reflectivity profile and show that a linear shift-invariant model adequately represents the overall process when the focusing is done with a backprojection algorithm over a constant integration angle. The derived signal model can be used to replace many computationally expensive processing techniques, previously performed on the raw data, with equivalent postprocessing algorithms which can be applied selectively to subimages.
    [bibtex-key = RauMcClellan00:UWB] [bibtex-entry]


  859. A. Reigber and A. Moreira. First Demonstration of Airborne SAR Tomography Using Multibaseline L-Band Data. IEEE Transactions on Geoscience and Remote Sensing, 38(5):2142-2152, 2000. Keyword(s): SAR Processing, airborne radar, geophysical techniques, radar imaging, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), InSAR, L-Band, SAR interferometry, SAR Tomography, Tomography, UHF, aperture synthesis, geophysical measurement technique, height ambiguity, land surface, multibaseline imaging geometry, multibaseline method, phase difference, radar remote sensing, tomographic imaging, topography, undersampled spatial distribution, ESAR.
    Abstract: In synthetic aperture radar (SAR) interferometry, the phase differences between two different sensor positions are used to estimate the terrain topography. Although it is possible in this way to find a three-dimensional (3D) surface representation, the distribution of the different scatterers in the height direction at a fixed range and azimuth position remains unknown. Contrary to this, tomographic techniques enable a real geometric resolution capability in the height direction and introduce new possibilities for many applications and inversion problems. Even misinterpretations in SAR images caused by layover and foreshortening effects can be solved by the tomographic processing. In this paper, the successful experimental realization of polarimetric airborne SAR tomography is demonstrated for the first time. The authors present the concept of aperture synthesis for tomographic imaging for the case of a multibaseline imaging geometry and discuss the constraints arising from the limited number of flight tracks. They propose a method for reduction of the height ambiguities associated to the irregular and undersampled spatial distribution of the imaging positions. Finally, they address the experimental requirements for polarimetric airborne SAR tomography and show experimental results using a multibaseline data set acquired in L-band by DLR's experimental SAR (E-SAR) of a test-site near Oberpfaffenhofen, Germany.
    [bibtex-key = reigberMoreira00:TomoLBand] [bibtex-entry]


  860. Paul A. Rosen, Scott Hensley, Ian R. Joughin, Fuk K. Li, Soren N. Madsen, Ernesto Rodriguez, and Richard M. Goldstein. Synthetic aperture radar interferometry. Proceedings of the IEEE, 88(3):333-382, March 2000. Keyword(s): Review Paper, SAR Processing, SAR Interferometry, differential SAR Interferometry, DInSAR, InSAR, deformation mapping, surface deformation, surface displacement, cartography, geodesy, geophysical techniques, radiowave interferometry, remote sensing by laser beam, surface topography measurement, synthetic aperture radar, cartography, coherent radar signal phase, geodesy, land cover characterization, natural hazards, radar remote sensing, surface topography, synthetic aperture radar interferometry, Extraterrestrial measurements, Geophysical measurements, Hazards, Radar antennas, Radar imaging, Remote sensing, Spaceborne radar, Surface topography, Synthetic aperture radar interferometry, Vegetation mapping, Interferometry, Baseline Errors, orbital errors, Spaceborne SAR.
    Abstract: Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristic of the surface. By exploiting the phase of the coherent radar signal, interferometry has transformed radar remote sensing from a largely interpretive science to a quantitative tool, with applications in cartography, geodesy, land cover characterization, and natural hazards. This paper reviews the techniques of interferometry, systems and limitations, and applications in a rapidly growing area of science and engineering.
    [bibtex-key = rosenHensleyJoughinFukMadsenRodriguezGoldsteinProcOfIEEE2000SyntheticApertureRadarInterferometryReviewPaper] [bibtex-entry]


  861. K. Sarabandi and Y.-C. Lin. Simulation of interferometric SAR response for characterizing the scattering phase center statistics of forest canopies. IEEE Trans. Geosci. Remote Sens., 38(1):115-125, January 2000. Keyword(s): InSAR, Monte Carlo simulation, SAR, backscatter, coherent scattering model, equivalent scatterer, forest, forest canopy, forestry, fractal, geophysical measurement technique, interferometric SAR, physical parameters, radar remote sensing, radar theory, scatterer collection, scattering phase center statistics, simulation, synthetic aperture radar, vegetation mapping, Monte Carlo methods, backscatter, forestry, fractals, geophysical techniques, radar cross-sections, radar theory, remote sensing by radar, synthetic aperture radar, vegetation mapping;.
    Abstract: A coherent scattering model for tree canopies is employed in order to characterize the sensitivity of an interferometric SAR (INSAR) response to the physical parameters of forest stands. The concept of an equivalent scatterer for a collection of scatterers within a pixel, representing the vegetation particles of tree structures, is used for identifying the scattering phase center of the pixel whose height is measured by an INSAR. Combining the recently developed coherent scattering model for tree canopies and the INSAR Delta;k-radar-equivalence algorithm, accurate statistics of the scattering phase-center location of forest stands are obtained numerically for the first time. The scattering model is based on a Monte Carlo simulation of scattering from fractal-generated tree structures, and therefore is capable of preserving the absolute phase of the backscatter. The model can also account for coherent effects due to the relative position of individual scatterers and the inhomogeneous extinction experienced by a coherent wave propagating through the random collection of vegetation particles. The location of the scattering phase center and the correlation coefficient are computed using the Delta;k-radar equivalence simply by simulating the backscatter response at two slightly different frequencies. The model is successfully validated using the measured data acquired by JPL TOPSAR over a selected pine stand in Raco, MI. A sensitivity analysis is performed to characterize the response of coniferous and deciduous forest stands to a multifrequency and multipolarization INSAR in order to determine an optimum system configuration for remote sensing of forest parameters
    [bibtex-key = 823906] [bibtex-entry]


  862. Rolf Scheiber and Alberto Moreira. Coregistration of interferometric SAR images using spectral diversity. IEEE Transactions on Geoscience and Remote Sensing, 38(5):2179-2191, September 2000. Keyword(s): SAR Processing, Spectral Diversity, Coregistration, Image Coregistration, InSAR, SAR Interferometry, Airborne SAR, Spaceborne SAR, geophysical signal processing, geophysical techniques, image registration, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mappingInSAR, complex SAR signal, geophysical measurement technique, land surface, radar remote sensing, relative misregistration, terrain mapping.
    Abstract: This article presents a technique for the determination of the relative misregistration between two interferometric SAR images. The proposed technique is based on the spectral properties of the complex SAR signal. Unlike conventional coregistration methods, the proposed technique does not need any interpolation nor cross-correlation procedures and also no coherence or fringe optimization must be performed. Instead, the phase information of different spectral looks is evaluated giving misregistration information on a pixel by pixel basis. The proposed technique is at least as accurate as the conventional algorithms and its implementation is very simple. Airborne repeat-pass interferometric data and simulated ScanSAR data are used to illustrate the operation of the proposed technique
    [bibtex-key = scheiberMoreira2000:InSARCoregistrationSpectralDiversity] [bibtex-entry]


  863. Ari Sihvola. Mixing Rules with Complex Dielectric Coefficients. Subsurface Sensing Technologies and Applications, 1(4):393-415, 2000. Keyword(s): Microwave, Mixing Rules, Dielectric Property, Complex Permittivity, Dielectric Response, depolarization factors, depolarization factors for an ellipsoid.
    Abstract: This article discusses the determination of effective dielectric properties of hetereogeneous materials, in particular media with lossy constituents that have complex permittivity parameters. Several different accepted mixing rules are presented and the effects of the structure and internal geometry of the mixture on the effective permittivity are illustrated. Special attention is paid to phenomena that the mixing process causes in the character of the macroscopic dielectric response of the mixture when the losses of one or several of the components are high or when there is a strong dielectric contrast between the component permittivities.
    [bibtex-key = sihvola2000MixingRulesWithComplexDielectricCoefficients] [bibtex-entry]


  864. Robert N. Treuhaft and Paul R. Siqueira. Vertical structure of vegetated land surfaces from interferometric and polarimetric radar. Radio Science, 35:141-178, 2000. Keyword(s): SAR Processing, InSAR, Interferometry, Pol-InSAR, Multi-Baseline SAR, Vegetation Parameters, Parameter Extraction, Topography, SAR Tomography.
    Abstract: This paper describes the estimation of parameters characterizing the vertical structure of vegetated land surfaces, from combined interferometric and polarimetric radar data. Physical models expressing radar observations in terms of parameters describing vegetated land surfaces are the foundation for parameter estimation techniques. Defining a general complex cross correlation enables the unified development of models for interferometry and polarimetry, including polarimetric interferometry. Three simple physical models in this paper express this complex cross correlation in terms of vegetation parameters: (1) a randomly oriented volume, (2) a randomly oriented volume with a ground return, and (3) an oriented volume. For the first two models the parameters include vegetation height, extinction coefficient, underlying topography, and another parameter depending on ground electrical properties and roughness. For the oriented volume, additional parameters depend on the refractivity, extinction coefficients, and backscattering characteristics of waves propagating along eigenpolarizations of the vegetation volume. The above models show that the interferometric cross-correlation amplitude and the polarimetric {HHHH/VVVV} ratio both change by about 1% per meter of vegetation height change, for experimental conditions typical of airborne and spaceborne interferometric radars. These vertical-structure sensitivities prompt a parameter estimation demonstration with two-baseline TOPSAR interferometric and zero-baseline polarimetric data from the Boreal Ecosystem-Atmosphere Study (BOREAS) Southern Study Area in Prince Albert National Park, Saskatchewan, Canada. The demonstrations show the feasibility of measuring vegetation height to better than 4.2 m, underlying topography to better than 6.5 m, and the ratio of ground-to-volume power to better than 10%, using interferometry and polarimetry, coupled with parameter-constraining assumptions, concerning the degree of surface roughness. This paper suggests that single-baseline and multibaseline fully polarimetric interferometry have the potential to obviate the need for such assumptions, thereby making parameter estimation more robust, accurate, and realistic.
    [bibtex-key = treuhaftSiqueira2000:InSARVegetation] [bibtex-entry]


  865. C. Wimmer, R. Siegmund, M. Schwabisch, and João Moreira. Generation of high precision DEMs of the Wadden Sea with airborneinterferometric SAR. IEEE Trans. Geosci. Remote Sens., 38(5):2234-2245, 2000. Keyword(s): SAR Processing, InSAR, Interferometry, airborne radar, bathymetry, geodesy, geophysical techniques, oceanographic regions, remote sensing by radar, seafloor phenomena, synthetic aperture radar, terrain mapping, topography (Earth), AeS-1, Bremerhaven, DEM, German Bight, Germany, North Sea, Wadden Sea, airborne interferometric radar, coast, digital elevation model, geophysical measurement technique, interferometric SAR, intertidal zone, land surface topography, ocean, radar remote sensing, seafloor, verification, Airborne SAR, Doppler Centroid Estimation.
    Abstract: This paper describes how high-precision digital elevation models(DEMs) are obtained over the Wadden Sea using the AeS-1 airborneinterferometric radar. The Wadden Sea is an intertidal zone along thecoast having height variations less than 5 m over 30 km and is free ofvegetation. The resulting DEM has a grid spacing of 2.5 m and anabsolute height accuracy of 5 cm root mean square (rms), as verified bytheodolite measurements. This paper describes the radar system, theprocessing techniques, the test area, the results, and the verificationprocedure
    [bibtex-key = wimmerSiegmundSchwaebischMoreira2000:DEMGen] [bibtex-entry]


  866. S. Albrecht and I. Cumming. Application of momentary Fourier transform to SAR processing. IEE Proceedings - Radar, Sonar and Navigation, 146(6):285-297, December 1999. Keyword(s): SAR Processing, SPECAN, Modified SPECAN, discrete Fourier transforms, fast Fourier transforms, inverse problems, radar signal processing, synthetic aperture radar, DFT, FFT/IFFT algorithms, IDFT, SAR processing, SIFFT method, general matrix transforms, image processing, inverse momentary Fourier transform, momentary Fourier transform, MFT, recursive momentary Fourier transform, signal processing, windowing.
    Abstract: A common technique in signal and image processing is to extract a portion of the signal by windowing, and then perform the DFT on the window contents. The momentary Fourier transform (MFT) applies to the particular case where the window is moved one data sample along the signal between successive transforms. An alternative derivation of the recursive form of the MFT using general matrix transforms is given. How DFTs and IDFTs are used in the SPECAN (spectral analysis) and SIFFT (short IFFT) methods of synthetic aperture radar (SAR) processing is described. The MFT and inverse MFT are applied to those methods and the advantages and disadvantages they have compared to the FFT/IFFT algorithms are shown
    [bibtex-key = albrechtCumming1999:MFTSPECAN] [bibtex-entry]


  867. S.R. Cloude, J. Fortuny, J.M. Lopez-Sanchez, and A.J. Sieber. Wide-band polarimetric radar inversion studies for vegetation layers. IEEE Transactions on Geoscience and Remote Sensing, 37(5):2430-2441, September 1999. Keyword(s): backscatter, forestry, geophysical techniques, image classification, radar cross-sections, radar polarimetry, remote sensing by radar, synthetic aperture radar, vegetation mappingbackscatter, canopy, complex volume scattering, entropy-alpha target decomposition scheme, ficus tree, fig, fir tree, forest, geophysical measurement technique, image classification scheme, inversion algorithm, parametric inversion, polarimetric radar inversion, radar scattering, radar theory, random particle cloud model, small anisotropic particles, two-parameter model, vegetation layer, vegetation mapping, wide band method.
    Abstract: The authors show how the entropy-alpha target decomposition scheme may be used for parametric inversion studies on random particle cloud models for vegetation layers. The decomposition is detailed first and then applied to a two-parameter model for backscatter from a random cloud of small anisotropic particles. The two main parameters used are the mean particle shape and the mean orientation angle of the cloud. An inversion algorithm is presented and applied to broad-band polarimetric radar data from the European Microwave Signature Laboratory (EMSL), Joint Research Center, Ispra, Italy. The results have been obtained from measurements of a fir tree and a ficus tree. They show a wavelength scale dependence of the shape and distribution of scatterers, which reflects the complex volume scattering nature of such problems. Moreover, the values and trends from these two trees as a function of the frequency are different, as expected from their physical structures. Consequently, this algorithm has the potential to be useful in the construction of classification schemes for vegetation
    [bibtex-key = cloudeFortunyLopezSanchezSieber1999:PolSARDecompVegetatioInversion] [bibtex-entry]


  868. D.L. Evans. Applications of imaging radar data in Earth science investigations. Electronics Communication Engineering Journal, 11(5):227 -234, October 1999. Keyword(s): Earth science investigations, Earth's surface, LightSAR mission, SAR interferometric measurements, biodiversity, biomass, fine resolution, forest regrowth, glacier ice velocity measurement, imaging radar data, interferometric data acquisition, land cover, large-scale surface change, long-term observations, natural hazards, polarimetric data acquisition, receive polarisation, resource assessment, scatterer cross-sections, spaceborne SAR systems, surface topographic change monitoring, synthetic aperture radar, topographic data sets, transmit polarisation, vegetation, data acquisition, electromagnetic wave scattering, health hazards, image resolution, radar applications, radar cross-sections, radar imaging, radar polarimetry, radar resolution, radiowave interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth);.
    Abstract: Synthetic aperture radar (SAR) data provide unique information about the Earth's surface and biodiversity, including critical data for natural hazards and resource assessments. The ability to calculate the cross-section of a scatterer for any transmit and receive polarisation combination provides detailed information about vegetation for assessing changes in land cover, biomass and forest regrowth. Unique SAR interferometric measurements, predominantly large-scale surface change at fine resolution, are used to generate topographic data sets, monitor surface topographic change, and measure glacier ice velocity. The LightSAR mission, planned for launch in 2002 will be optimised for polarimetric and interferometric data acquisition in order to provide long-term observations of the Earth's changing conditions
    [bibtex-key = 810214] [bibtex-entry]


  869. Gianfranco Fornaro. Trajectory Deviations in Airborne SAR: Analysis and Compensation. IEEE Transactions on Aerospace and Electronic Systems, 35(3):997-1009, July 1999. Keyword(s): SAR Processing, Motion Compensation, Airborne SAR, Residual Motion Errors, Non-Linear Flight Path, Non-linear SAR.
    Abstract: This paper concerns the analysis and compensation of trajectory deviations in airborne synthetic aperture radar (SAR) systems. Analysis of the received data spectrum is carried out with respect to the system geometry in the presence of linear, sinusoidal, and general aircraft displacements. This shows trajectory deviations generally produce spectral replicas along the azimuth frequency that strongly impair the quality of the focused image. Based on the derived model, we explain the rationale of the motion compensation (MOCO) strategy that must be applied at the SAR processing stage in order to limit the resolution loss. To this end aberration terms are separated into range space invariant and variant components. The former can be accounted for either in a preprocessing step or efficiently at range compression stage. The latter needs a prior accommodation of range migration effect. We design the procedure for efficient inclusion of the MOCO within a high precision Scaled FT based SAR processing algorithm. Finally, we present results on simulated data aimed at validating the whole analysis and the proposed procedure.
    [bibtex-key = fornaroTGRS1999TrajectoryDeviationsAirborneSAR] [bibtex-entry]


  870. G. Fornaro and E. Sansosti. A two-dimensional region growing least squares phase unwrapping algorithm for interferometric SAR processing. IEEE Transactions on Geoscience and Remote Sensing, 37(5):2215 -2226, September 1999. Keyword(s): 2-D phase unwrapping algorithm, boundary phase values, discrete domain case, finite element method, interferometric SAR processing, reconstructed phase, two-dimensional region growing least squares phase unwrapping algorithm, wrapped phase image, geophysical signal processing, least squares approximations, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar.
    Abstract: This paper presents a new two-dimensional (2-D) phase unwrapping (PhU) algorithm based on a least squares (LS) region growing strategy: the wrapped phase image is partitioned in different regions that are sequentially unwrapped via a LS algorithm. Reliable regions are dealt with at the beginning of the procedure, while critical areas are unwrapped in the final steps, thus avoiding error propagation from critical to reliable areas. A conditioned least squares formulation of the phase unwrapping problem is the core of the proposed procedure: this allows the solution to be tied to ldquo;some rdquo; known boundary phase values, thus guaranteeing the correct joining of the reconstructed phase in between the different regions and preventing them from being independently unwrapped. The application of the finite element method allows a straightforward implementation of the algorithm in the discrete domain case. Experimental results, carried out on simulated and real interferometric SAR data, show the effectiveness of the proposed algorithm and the improved performances with respect to existing unwrapping procedures
    [bibtex-key = 789618] [bibtex-entry]


  871. Ramon F. Hanssen, Tammy M. Weckwerth, Howard A. Zebker, and Roland Klees. High-Resolution Water Vapor Mapping from Interferometric Radar Measurements. Science, 283(5406):1297-1299, 1999. Keyword(s): Troposphere, Water Vapor, InSAR.
    Abstract: Spaceborne radar interferometric delay measurements were used to infer high-resolution maps of integrated atmospheric water vapor, which can be readily related to meteorological phenomena. Maps of the water vapor distribution associated with a precipitating cloud, a partly precipitating cold front, and horizontal convective rolls reveal quantitative measures that are not observed with conventional methods, and suggest that such radar observations can be used for forecasting and to study atmospheric dynamics.
    [bibtex-key = hanssenWeckwerthZebkerKleesSCIENCE1999] [bibtex-entry]


  872. Xiaotao Huang and Diannong Liang. Gradual RELAX Algorithm for RFI Suppression in UWB-SAR. Electronics Letters, 35(22):1916-1917, October 1999. Keyword(s): SAR Processing, RFI Suppression, Ultra-Wideband SAR, Gradual RELAX Algorithm.
    Abstract: Parametric methods of radio frequency interference (RFI) suppression in ultra-wideband synthetic aperture radar (UWB-SAR) often outperform their non-parametric counterparts at the expense of computational complexity. The authors present a parametric algorithm that gradually applies the RELAX algorithm and results in greatly improved computational efficiency and stability
    [bibtex-key = HuangLiang99:RFI] [bibtex-entry]


  873. Guy Indebetouw and Prapong Klysubun. Space-time digital holography: A three-dimensional microscopic imaging scheme with an arbitrary degree of spatial coherence. Applied Physics Letters, 75(14):2017-2019, October 1999. Keyword(s): Fresnel diffraction, holographic interferometry, image reconstruction, light coherence, optical correlation, optical images, optical microscopy;.
    Abstract: An on-line, spatiotemporal, digital holographic method is described and demonstrated experimentally. Using interferometric imaging, each scatterer of a three-dimensional object is encoded as a temporally modulated Fresnel pattern, and recorded on a charge-coupled device. Temporal heterodyning of the signal from each pixel results in a single-sideband, on-line holographic record in digital form. Reconstruction of an image focused on a chosen transverse plane in the object is done by digital correlation with a reconstruction function matched to that plane. The method circumvents most of the drawbacks of both coherent and incoherent holography, and may find applications in three-dimensional imaging and microscopy. #xa9; 1999 American Institute of Physics.
    [bibtex-key = 4901834] [bibtex-entry]


  874. Jong-Sen Lee, M.R. Grunes, T.L. Ainsworth, Li-Jen Du, D.L. Schuler, and Shane R. Cloude. Unsupervised classification using polarimetric decomposition and the complex Wishart classifier. IEEE Transactions on Geoscience and Remote Sensing, 37(5):2249-2258, September 1999. Keyword(s): geophysical signal processing, geophysical techniques, image classification, radar imaging, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mappingSAR, complex Wishart classifier, complex Wishart distribution, covariance matrix, entropy-alpha plane, geophysical measurement technique, initial classification map, iteration, land surface, man-made object, maximum likelihood classifier, polarimetric decomposition, polarimetric target decomposition, polarization, radar remote sensing, terrain mapping, terrain type, training, unsupervised classification.
    Abstract: The authors propose a new method for unsupervised classification of terrain types and man-made objects using polarimetric synthetic aperture radar (SAR) data. This technique is a combination of the unsupervised classification based on polarimetric target decomposition, S.R. Cloude et al. (1997), and the maximum likelihood classifier based on the complex Wishart distribution for the polarimetric covariance matrix, J.S. Lee et al. (1994). The authors use Cloude and Pottier's method to initially classify the polarimetric SAR image. The initial classification map defines training sets for classification based on the Wishart distribution. The classified results are then used to define training sets for the next iteration. Significant improvement has been observed in iteration. The iteration ends when the number of pixels switching classes becomes smaller than a predetermined number or when other criteria are met. The authors observed that the class centers in the entropy-alpha plane are shifted by each iteration. The final class centers in the entropy-alpha plane are useful for class identification by the scattering mechanism associated with each zone. The advantages of this method are the automated classification, and the interpretation of each class based on scattering mechanism. The effectiveness of this algorithm is demonstrated using a JPL/AIRSAR polarimetric SAR image
    [bibtex-key = leeGrunesAinsworthDuSchulerCloude1999:PolSARDecompForClassification] [bibtex-entry]


  875. J. Li, Z. Bi, and Z.-S. Liu. Autofocus and feature extraction in curvilinear SAR via a relaxation-based algorithm. Radar, Sonar and Navigation, IEE Proceedings -, 146(4):201-207, 1999. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR, error analysis, feature extraction, focusing, radar imaging, synthetic aperture radar, 3D target features, AUTORELAX, CLSAR, SAR imaging, aperture errors compensation, curvilinear SAR, curvilinear synthetic aperture radar, data model, estimation accuracy, experimental results, relaxation-based algorithm, relaxation-based autofocus algorithm, simulation results, target parameters.
    Abstract: The paper presents a relaxation-based autofocus (AUTORELAX) algorithm that can be used to compensate for the aperture errors in curvilinear synthetic aperture radar (CLSAR) and to extract three-dimensional target features. A self-contained detailed derivation of the data model for the autofocus problem in CLSAR is presented. Experimental and simulation results show that AUTORELAX can be used to significantly improve the estimation accuracy of the target parameters.
    [bibtex-key = liBiLiu1999:NonLinearSARTomo] [bibtex-entry]


  876. Richard T. Lord and Michael R. Inggs. Efficient RFI suppression in SAR using LMS adaptive filter integrated with range/Doppler algorithm. Electronics Letters, 35(8):629-630, 1999. Keyword(s): SAR Processing, Doppler radar, adaptive filters, interference suppression, least mean squares methods, radar imaging, radar interference, synthetic aperture radar, Doppler algorithm, LMS adaptive filter, RFI Suppression, SAR image processing, radiofrequency interference, range compression.
    Abstract: Radio frequency interference (RFI) suppression in SAR images often requires a great amount of computation. The authors describe how significant computational savings can be achieved by integrating the RFI suppression stage, implemented with a least-mean-squared (LMS) adaptive filter, with the range compression stage of the range/Doppler SAR processing algorithm
    [bibtex-key = lordInggsElLetters99:RFI] [bibtex-entry]


  877. Christian Matzler and Andreas Wiesmann. Extension of the Microwave Emission Model of Layered Snowpacks to Coarse-Grained Snow. Remote Sensing of Environment, 70(3):317-325, December 1999. Keyword(s): MEMLS, Snow, Microwave, Microwave emission model of lalayer snowpacks, Dielectric Properties of Dry Snow, relative permittivity, snow density.
    Abstract: The microwave emission model of layered snowpacks (MEMLS) is a multilayer and multiple-scattering radiative transfer model developed for dry winter snow using an empirical parametrization of the scattering coefficient (se the copanion article). A limitation is in the applicable range of frequencies and correlation lengths. In order to extend the model, a physical determination of the volume-scattering coefficients, describing the coupling between the six fluxes, is developed here, based on the improved Born approximation. An exponential spatial autocorrelation function was selected. With this addition, MEMLS obtains a complete physical basis. The extended model is void of free parameters. The validation was done with two types of experiments made at the alpine test site, Weissfluhjoch: 1) radiometry at 11 GHz, 21 GHz, 35 GHz, 48 GHz, and 94 GHz of winter snow samples on a blackbody and on a metal plate, respectively, and 2) radiometric monitoring at 4.9 GHz, 10.4 GHz, 21 GHz, 35 GHz, and 94 GHz of coarse-grained crusts growing and decaying during melt-and-refreeze cycles. Digitized snow sections were used to measure snow structure in both experiments. The coarsest grains were found in the refrozen crusts with a correlation length up to 0.71 mm; the winter snow samples had smaller values, from 0.035 mm for new snow to about 0.33 mm for depth hoar. Good results have been obtained in all cases studied so far.
    [bibtex-key = matzlerWiesmannRSE1999MEMLSRef2ExtensionForCoarseGrainSnow] [bibtex-entry]


  878. Josef Mittermayer, Alberto Moreira, and Otmar Loffeld. Spotlight SAR data processing using the frequency scaling algorithm. IEEE Trans. Geosci. Remote Sens., 37(5):2198-2214, September 1999. Keyword(s): SAR Processing, Spotlight SAR, dechirp, dechirp-on-receive, Doppler radar, geophysical signal processing, radar imaging, remote sensing by radar, spectral analysis, synthetic aperture radarazimuth processing, azimuth scaling, chirp convolution, frequency scaling algorithm, Chirp Scaling Algorithm, nonchirped SAR signals, nonchirped raw data, range Doppler domain, range cell migration correction, residual video phase, RVP, spectral analysis approach, spotlight SAR data processing, stripmap raw data, subaperture approach.
    Abstract: This paper presents a new processing algorithm for spotlight SAR data processing. The algorithm performs the range cell migration correction for non-chirped raw data without interpolation by using a novel frequency scaling operation. The azimuth processing is based on a spectral analysis approach which is made highly accurate by azimuth scaling. In almost all processing stages, a subaperture approach is introduced for efficient azimuth processing. In this paper, the complete derivation of the algorithm is presented. A very useful formulation for non-chirped SAR signals in the range Doppler domain is also proposed where the residual video phase is expressed by a chirp convolution. The algorithm performance is shown by several simulations. A spotlight image, which has been extracted from stripmap raw data of the experimental SAR system of DLR, shows the validity of the frequency scaling algorithm
    [bibtex-key = mittermayerMoreiraLoffeld1999:SpotlightCS] [bibtex-entry]


  879. Andreas Reigber. Range dependent spectral filtering to minimize the baseline decorrelation in airborne SAR interferometry. 3:1721-1723, 1999. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, Range Spectral Filter, Spectral Filter, adaptive signal processing, airborne radar, geophysical signal processing, geophysical techniques, radar imaging, radar theory, remote sensing by radar, synthetic aperture radar, terrain mapping, E-SAR, L-band, SAR, Airborne SAR interferometry, Airborne SAR, Baseline Decorrelation, fixed bandwidth filtering, geometric resolution, geophysical measurement technique, interferogram coherence, interferometric SAR, land surface, radar remote sensing, range dependent spectral filtering, repeat-pass, spectral misalignment.
    Abstract: This paper discusses two methods to solve the problem of a range-dependent baseline decorrelation occurring especially in airborne repeat-pass SAR interferometry. The first approach allows a fixed bandwidth filtering for the whole range avoiding the spectral misalignment of the standard method. The second approach enables a real adaptive filtering of the range-dependent baseline decorrelation and allows one also to obtain the best geometric resolution for each range position without decreasing the interferogram coherence. The efficiency of the two methods is demonstrated by using interferograms obtained from DLR's E-SAR in the L-band repeat-pass-mode
    [bibtex-key = reigberIGARSS1999:InSARRangeSpecFilt] [bibtex-entry]


  880. Ridha Touzi, Armand Lopes, Jérôme Bruniquel, and Paris W. Vachon. Coherence estimation for SAR imagery. IEEE Trans. Geosci. Remote Sens., 37(1):135-149, January 1999. Keyword(s): SAR Processing, SAR imagery, coherence, coherence estimation, coherence map resolution, decorrelation, temporal decorrelation, cross-channel correlation, dual channel method, geophysical measurement technique, land surface, multiple-channel, radar imaging, radar remote sensing, synthetic aperture radar, terrain mapping, unbiased coherence estimate, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping;.
    Abstract: In dual- or multiple-channel synthetic aperture radar (SAR) imaging modes, cross-channel correlation is a potential source of information. The sample coherence magnitude is calculated over a moving window to generate a coherence magnitude map. High-resolution coherence maps may be useful to discriminate fine structures. Coarser resolution is needed for a more accurate estimation of the coherence magnitude. In this study, the accuracy of coherence estimation is investigated as a function of the coherence map resolution. It is shown that the space-averaged coherence magnitude is biased toward higher values. The accuracy of the coherence magnitude estimate obtained is a function of the number of pixels averaged and the number of independent samples per pixel (i.e., the coherence map resolution). A method is proposed to remove the bias from the space-averaged sample coherence magnitude. Coherence magnitude estimation from complex (magnitude and phase) coherence maps is also considered. It is established that the magnitude of the averaged sample coherence estimate is slightly biased for high-resolution coherence maps and that the bias reduces with coarser resolution. Finally, coherence estimation for nonstationary targets is discussed. It is shown that the averaged sample coherence obtained from complex coherence maps or coherence magnitude maps is suitable for estimation of nonstationary coherence. The averaged sample (complex) coherence permits the calculation of an unbiased coherence estimate, provided that the original signals can be assumed to be locally stationary over a sufficiently coarse resolution cell
    [bibtex-key = touziLopesBruniquelVachon1999TGRS_Coherence] [bibtex-entry]


  881. R. N. Treuhaft and S. R. Cloude. The structure of oriented vegetation from polarimetric interferometry. IEEE Trans. Geosci. Remote Sens., 37(5):2620-2624, September 1999. Keyword(s): SAR Processing, Forest, Forest parameters, biomass, forest canopy, forestry, geophysical measurement technique, height, oriented object, oriented vegetation, oriented-vegetation volume, polarimetric interferometry, radar polarimetry, radar remote sensing, randomly oriented volume, single-baseline polarimetric interferometry, underlying topography, vegetated land surface, vegetation mapping, geophysical techniques, radar polarimetry, remote sensing by radar, vegetation mapping.
    Abstract: Polarimetric radar interferometry is much more sensitive to the distribution of oriented objects in a vegetated land surface than either polarimetry or interferometry alone. This paper shows that single-baseline polarimetric interferometry can be used to estimate the heights of oriented-vegetation volumes and underlying topography, while at least two baselines are needed for randomly oriented volumes. Single-baseline, calculated vegetation-height accuracies are in the range of 2-8 m for reasonable levels of vegetation orientation in forest canopies
    [bibtex-key = treuhaftCloude1999:PolINSAROrientedVegetation] [bibtex-entry]


  882. Andreas Wiesmann and Christian Matzler. Microwave emission model of layered snowpacks. Remote Sensing of Environment, 70(3):307-316, 1999. Keyword(s): MEMLS, Snow, Microwave, Microwave emission model of lalayer snowpacks, Dielectric Properties of Dry Snow, relative permittivity, snow density.
    Abstract: A thermal microwave emission model of layered snowpacks (MEMLS) was developed for the frequency range 5-100 GHz. It is based on radiative transfer, using six-flux theory to describe multiple volume scattering and absorption, including radiation trapping due to total reflection and a combination of coherent and incoherent superpositions of reflections between layer interfaces. The scattering coefficient is determined empirically from measured snow samples, whereas the absorption coefficient, the effective permittivity, refraction, and reflection at layer interfaces are based on physical models and on measured ice dielectric properties. The number of layers is only limited by computer time and memory. A limitation of the empirical fits and thus of MEMLS is in the range of observed frequencies and correlation lengths (a measure of grain size). First model validation for dry winter snow was successful. An extension to larger grains is given in a companion article (Matzler and Wiesmann, 1999). The objective of the present article is to describe and illustrate the model and to pave the way for further improvements. MEMLS has been coded in MATLAB. It forms part of a combined land-surface-atmosphere microwave emission model for radiometry from satellites (Pulliainen et al., 1998).
    [bibtex-key = wiesmannMatzlerRSE1999MEMLSReferencePaper] [bibtex-entry]


  883. Wei Ye, Tat Soon Yeo, and Zheng Bao. Weighted least-squares estimation of phase errors for SAR/ISAR autofocus. IEEE Transactions on Geoscience and Remote Sensing, 37(5):2487-2494, September 1999. Keyword(s): SAR Processsing, Autofocus, Weighted Least-Squares Estimation, WLS, Residual Motion Errors, geophysical techniques, measurement errors, remote sensing by radar, synthetic aperture radar, terrain mapping, ISAR, SAR, geophysical measurement technique, inverse SAR, land surface, phase error, phase error estimation, phase errors, radar remote sensing.
    Abstract: A new method of phase error estimation that utilizes the weighted least-squares (WLS) algorithm is presented for synthetic aperture radar (SAR)/inverse SAR (ISAR) autofocus applications. The method does not require that the signal in each range bin be of a certain distribution model, and thus it is robust for many kinds of scene content. The most attractive attribute of the new method is that it can be used to estimate all kinds of phase errors, no matter whether they are of low order, high order, or random. Compared with other methods, the WLS estimation is optimal in the sense that it has the minimum variance of the estimation error. Excellent results have been obtained in autofocusing and imaging experiments on real SAR and ISAR data
    [bibtex-key = yeYeoBao1999:Autofocus] [bibtex-entry]


  884. Richard Bamler and Philipp Hartl. Synthetic Aperture Radar Interferometry. Inverse Problems, 14:1-54, August 1998. Keyword(s): Review Paper, SAR Processing, Interferometry, SAR Interferometry, differential SAR Interferometry, DInSAR, InSAR, deformation mapping, surface deformation, surface displacement, cartography, geodesy, geophysical techniques, radiowave interferometry, coherence.
    Abstract: Synthetic aperture radar (SAR) is a coherent active microwave imaging method. In remote sensing it is used for mapping the scattering properties of the Earth's surface in the respective wavelength domain. Many physical and geometric parameters of the imaged scene contribute to the grey value of a SAR image pixel. Scene inversion suffers from this high ambiguity and requires SAR data taken at different wavelength, polarization, time, incidence angle, etc. Interferometric SAR (InSAR) exploits the phase differences of at least two complex-valued SAR images acquired from different orbit positions and/or at different times. The information derived from these interferometric data sets can be used to measure several geophysical quantities, such as topography, deformations (volcanoes, earthquakes, ice fields), glacier flows, ocean currents, vegetation properties, etc. This paper reviews the technology and the signal theoretical aspects of InSAR. Emphasis is given to mathematical imaging models and the statistical properties of the involved quantities. Coherence is shown to be a useful concept for system description and for interferogram quality assessment. As a key step in InSAR signal processing two-dimensional phase unwrapping is discussed in detail. Several interferometric configurations are described and illustrated by real-world examples. A compilation of past, current and future InSAR systems concludes the paper.
    [bibtex-key = BamlerHartl98] [bibtex-entry]


  885. Hian Lim Chan and Tat Soon Yeo. Noniterative quality phase-gradient autofocus (QPGA) algorithm for spotlight SAR imagery. IEEE Trans. Geosci. Remote Sens., 36(5):1531-1539, September 1998. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus.
    Abstract: The phase-gradient autofocus (PGA) technique is robust over a wide range of imagery and phase error functions, but the convergence usually requires four-six iterations. It is necessarily iterative in an attempt to converge on a dominant target against clutter interference, while sufficiently capturing the blur function. In this paper, we propose to speed the estimation convergence by selectively increasing the pool of quality synchronization sources and not be limited by the range pixels of the SAR map. This is highly probable since each range bin contains more than one prominent scatterer across the integration aperture. It is also highly probable that the least-brightest selected scatterer in a range gate may turn out to be of higher energy as compared to the maximum brightest scatterer of another gate. With appropriate target filtering to final select the quality scatterers out of the large pool and with higher order phase error measurement tool, the new algorithm achieves near-convergence focusing quality without iteration. We named this solution the quality PGA (QPGA) algorithm.
    [bibtex-key = Chan1998] [bibtex-entry]


  886. Hyeokho Choi and David C. Munson, Jr.. Direct-Fourier Reconstruction in Tomography and Synthetic Aperture Radar. International Journal of Imaging Systems and Technology, 9(1):1-13, 1998. Keyword(s): SAR Processing, Tomography, Computed Tomography, CT, Convolution Back-Projection, CBP, Direct-Fourier Image Reconstruction, DF, Interpolator, Fourier-Domain Interpolator.
    Abstract: We investigate the use of direct-Fourier (DF) image reconstruction in computed tomography and synthetic aperture radar (SAR). One of our aims is to determine why the convolution-backprojection (CBP) method is favored over DF methods in tomography, while DF methods are virtually always used in SAR. We show that the CBP algorithm is equivalent to DF reconstruction using a Jacobian-weighted two-dimensional periodic sinc-kernel interpolator. This interpolation is not optimal in any sense, which suggests that DF algorithms using optimal interpolators may surpass CBP in image quality. We consider use of two types of DF interpolation: a windowed sinc kernel, and the least-squares optimal Yen interpolator. Simulations show that reconstructions using the Yen interpolator do not possess the expected visual quality, because of regularization needed to preserve numerical stability. Next, we show that with a concentric-squares sampling scheme, DF interpolation can be performed accurately and efficiently, producing imagery that is superior to that obtainable by other algorithms. In the case of SAR, we show that the DF method performs very well with interpolators of low complexity. We also study DF reconstruction in SAR for trapezoidal grids. We conclude that the success of the DF method in SAR imaging is due to the nearly Cartesian shape of the sampling grid.
    [bibtex-key = choiMunson98:CTSAR] [bibtex-entry]


  887. S.R. Cloude and Konstantinos P. Papathanassiou. Polarimetric SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing, 36(5):1551-1565, September 1998. Keyword(s): SAR Processing, PolInSAR, geophysical techniques, radar imaging, radar polarimetry, radar theory, remote sensing by radar, synthetic aperture radarInSAR, coherence optimization problem, elevated forest canopy, general formulation, geophysical measurement technique, interferogram, interferometric SAR, interferometric coherence, land surface, linear combinations, maximization, polarimetric SAR interferometry, polarimetric basis transformation, radar remote sensing, scalar interferometry, stochastic scattering model, strong polarization dependency, synthetic aperture radar, terrain mapping, vector wave interferometry, SAR Tomography.
    Abstract: The authors examine the role of polarimetry in synthetic aperture radar (SAR) interferometry. They first propose a general formulation for vector wave interferometry that includes conventional scalar interferometry as a special case. Then, they show how polarimetric basis transformations can be introduced into SAR interferometry and applied to form interferograms between all possible linear combinations of polarization states. This allows them to reveal the strong polarization dependency of the interferometric coherence. They then solve the coherence optimization problem involving maximization of interferometric coherence and formulate a new coherent decomposition for polarimetric SAR interferometry that allows the separation of the effective phase centers of different scattering mechanisms. A simplified stochastic scattering model for an elevated forest canopy is introduced to demonstrate the effectiveness of the proposed algorithms. In this way, they demonstrate the importance of wave polarization for the physical interpretation of SAR interferograms. They investigate the potential of polarimetric SAR interferometry using results from the evaluation of fully polarimetric interferometric shuttle imaging radar (SIR)-C/X-SAR data collected during October 8-9, 1994, over the SE Baikal Lake Selenga delta region of Buriatia, Southeast Siberia, Russia
    [bibtex-key = cloudePapathanassiou1998:PolInSAR] [bibtex-entry]


  888. Mario Costantini. A novel phase unwrapping method based on network programming. IEEE Trans. Geosci. Remote Sens., 36(3):813-821, May 1998. Keyword(s): SAR Processsing, phase unwrapping, SAR Interferometry, InSAR, SAR, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, SAR interferometry, function reconstruction, geophysical measurement technique, global minimization problem, interferometric SAR, land surface, neighboring pixel, network programming, network structure, phase difference, phase unwrapping method, radar imaging, radar remote sensing, synthetic aperture radar, terrain mapping, Costs, Fast Fourier transforms, Functional programming, Interferometry, Performance evaluation, Phase estimation, Robustness, Synthetic aperture radar, Testing, Two dimensional displays.
    Abstract: Phase unwrapping is the reconstruction of a function on a grid given its values mod 2 pi. Phase unwrapping is a key problem in all quantitative applications of synthetic aperture radar (SAR) interferometry, but also in other fields. A new phase unwrapping method, which is a different approach from existing techniques, is described and tested. The method starts from the fact that the phase differences of neighboring pixels can be estimated with a potential error that is an integer multiple of 2 pi. This suggests the formulation of the phase unwrapping problem as a global minimization problem with integer variables. Recognizing the network structure underlying the problem makes for an efficient solution. In fact, it is possible to equate the phase unwrapping problem to the problem of finding the minimum cost flow on a network, for the solution of which there exist very efficient techniques. The tests performed on real and simulated interferometric SAR data confirm the validity of the approach
    [bibtex-key = costantiniTGRS1998PhaseUnwrappingNetworkProgramming] [bibtex-entry]


  889. Stuart R. DeGraaf. SAR imaging via modern 2-D spectral estimation methods. Image Processing, IEEE Transactions on, 7(5):729-761, 1998. Keyword(s): SAR Processing, Spectral Estimation, adaptive estimation, adaptive signal processing, estimation theory, image resolution, interference suppression, radar imaging, radar interference, radiowave interferometry, speckle, spectral analysis, synthetic aperture radar, 2-D spectral estimation methods, ASR, MVM, RRMVM, SVA, adaptive SAR imaging, adaptive nulling, adaptive sidelobe reduction, adaptive spectral estimation, averaging, height estimates, interferometric height, minimum variance method, multichannel variants, polarimetric displaced-aperture interferometric data, power spectrum estimation methods, reduced-rank MVM, reflectivity intensity, resolution, scattering intensity, sidelobe artifacts, space variant apodization, synthetic aperture radar imaging, MUSIC.
    Abstract: Discusses the use of modern 2D spectral estimation algorithms forsynthetic aperture radar (SAR) imaging. The motivation for applyingpower spectrum estimation methods to SAR imaging is to improveresolution, remove sidelobe artifacts, and reduce speckle compared towhat is possible with conventional Fourier transform SAR imagingtechniques. This paper makes two principal contributions to the field ofadaptive SAR imaging. First, it is a comprehensive comparison of 2Dspectral estimation methods for SAR imaging. It provides a synopsis ofthe algorithms available, discusses their relative merits for SARimaging, and illustrates their performance on simulated and collectedSAR imagery. Some of the algorithms presented or their derivations arenew, as are some of the insights into or analyses of the algorithms.Second, this work develops multichannel variants of four relatedalgorithms, minimum variance method (MVM), reduced-rank MVM (RRMVM),adaptive sidelobe reduction (ASR) and space variant apodization (SVA) toestimate both reflectivity intensity and interferometric height frompolarimetric displaced-aperture interferometric data. All of theseinterferometric variants are new. In the interferometric contest,adaptive spectral estimation can improve the height estimates through acombination of adaptive nulling and averaging. Examples illustrate thatMVM, ASR, and SVA offer significant advantages over Fourier methods forestimating both scattering intensity and interferometric height, andallow empirical comparison of the accuracies of Fourier, MVM, ASR, andSVA interferometric height estimates
    [bibtex-key = deGraaf98ModernSpectralEstim] [bibtex-entry]


  890. Knut Eldhuset. A new fourth-order processing algorithm for spaceborne SAR. Aerospace and Electronic Systems, IEEE Transactions on, 34(3):824-835, 1998. Keyword(s): SAR Processing, digital simulation, radar theory, signal processing, spaceborne radar, synthetic aperture radar, transfer functions, azimuth lines, extended ETF, fourth-order EETF, fourth-order processing algorithm, fourth-order signal aperture radar, high quality images, integration times, phase corrections, phase preservation, range-variant phase corrections, satellite-Earth relative motion, spaceborne SAR, spatial resolution, two-dimensional exact transfer function.
    Abstract: A new fourth-order signal aperture radar (SAR) processingalgorithm has been developed for a general satellite-Earth relativemotion. The two-dimensional exact transfer function (ETF) is calculatedand range-variant phase corrections have been calculated in order toprocess many azimuth lines per block. The ETF together with the phasecorrections has been called the fourth-order EETF (extended ETF). It isalso shown that a fourth-order EETF is necessary to process high qualityimages from spaceborne SAR with long integration times with spatialresolution around 1 m. The algorithm is fast and is anticipated to havegood phase preservation properties
    [bibtex-key = eldhuset98:EETF] [bibtex-entry]


  891. Giorgio Franceschetti, Antonio Iodice, Maurizio Migliaccio, and Daniele Riccio. A Novel Across-Track SAR Interferometry Simulator. IEEE Transactions on Geoscience and Remote Sensing, 36(3):950-962, May 1998. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator, Interferometry, Interferometry Simulator.
    Abstract: A novel across-track interferometric synthetic aperture radar (SAR) raw signal simulator is presented. It is based on an electromagnetic backscattering model of the scene and an accurate description of the SAR system impulse response function. A set of meaningful examples are also presented. They show that the proposed simulator is structurally consistent and correctly simulates the decorrelation effect, both in the mean and in the distribution sense.
    [bibtex-key = francescIodMigliaRic98:Simulation] [bibtex-entry]


  892. Anthony Freeman and Stephen L. Durden. A three-component scattering model for polarimetric SAR data. IEEE Trans. Geosci. Remote Sens., 36(3):963-973, May 1998. Keyword(s): SAR Processing, Polarimetric Decomposition, Freeman-Durden Decomposition, Bragg scatter, backscatter, canopy scatter, composite scattering model, double-bounce scatter, flooding, forest, geophysical measurement technique, inundation, land surface, orthogonal surface, polarimetric SAR, radar polarimetry, radar remote sensing, radar scattering, randomly oriented dipoles, rough surface, synthetic aperture radar, terrain mapping, three-component scattering model, tropical rain forest, vegetation mapping, backscatter, forestry, geophysical techniques, radar cross-sections, radar polarimetry, radar theory, remote sensing by radar, synthetic aperture radar;.
    Abstract: An approach has been developed that involves the fit of a combination of three simple scattering mechanisms to polarimetric SAR observations. The mechanisms are canopy scatter from a cloud of randomly oriented dipoles, evenor double-bounce scatter from a pair of orthogonal surfaces with different dielectric constants and Bragg scatter from a moderately rough surface. This composite scattering model is used to describe the polarimetric backscatter from naturally occurring scatterers. The model is shown to describe the behavior of polarimetric backscatter from tropical rain forests quite well by applying it to data from NASA/Jet Propulsion Laboratory's (JPLs) airborne polarimetric synthetic aperture radar (AIRSAR) system. The model fit allows clear discrimination between flooded and nonflooded forest and between forested and deforested areas, for example. The model is also shown to be usable as a predictive tool to estimate the effects of forest inundation and disturbance on the fully polarimetric radar signature. An advantage of this model fit approach is that the scattering contributions from the three basic scattering mechanisms can be estimated for clusters of pixels in polarimetric SAR images. Furthermore, it is shown that the contributions of the three scattering mechanisms to the HH, HV, and VV backscatter can be calculated from the model fit. Finally, this model fit approach is justified as a simplification of more complicated scattering models, which require many inputs to solve the forward scattering problem
    [bibtex-key = freemanDurden1998:PolDecomp] [bibtex-entry]


  893. Richard M. Goldstein and Charles L. Werner. Radar interferogram filtering for geophysical applications. Geophysical Research Letters, 25(21):4035-4038, 1998. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, Goldstein-Werner filter, adaptive filter, adaptive interferogram filtering.
    Abstract: The use of SAR interferometry is often impeded by decorrelation from thermal noise, temporal change, and baseline geometry. Power spectra of interferograms are typically the sum of a narrow-band component combined with broad-band noise. We describe a new adaptive filtering algorithm that dramatically lowers phase noise, improving both measurement accuracy and phase unwrapping, while demonstrating graceful degradation in regions of pure noise. The performance of the filter is demonstrated with SAR data from the ERS satellites over the Jakobshavns glacier of Greenland.
    [bibtex-key = goldsteinWernerGRL1998GoldsteinWernerFilterInSAR] [bibtex-entry]


  894. Aruna Gunawardena and Dennis Longstaff. Wave Equation Formulation of Synthetic Aperture Radar (SAR) Algorithms in the Time-Space Domain. IEEE Transactions on Geoscience and Remote Sensing, 36(6):1995-1999, November 1998. Keyword(s): SAR Processing, Time-Space Domain Algorithm, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Comparison of Algorithms.
    Abstract: In this paper, we propose an alternative wave equationbased time-space domain synthetic aperture radar (SAR) algorithm. The proposed algorithm can be interpreted as the exact time-space domain counterpart of the wave equation-based omega-k domain SAR algorithms proposed in recent years. Links to conventional SAR and seismic migration algorithm are also established.
    [bibtex-key = gunawardenaLongstaff98:SAR] [bibtex-entry]


  895. R. Lanari, S. Hensley, and P.A. Rosen. Chirp z-transform based SPECAN approach for phase-preserving ScanSAR image generation. Radar, Sonar and Navigation, IEE Proceedings -, 145(5):254-261, October 1998. Keyword(s): SAR Processing, Modified SPECAN, SPECAN, Spectral Analysis, Z transforms, airborne radar, image resolution, radar imaging, radar resolution, InSAR, SAR Interferometry, radiowave interferometry, spaceborne radar, synthetic aperture radar, time-domain analysis, transient response, ScanSAR, airborne platform, algorithm, azimuth focusing, chirp z-transform, chirp-z, experiments, high resolution microwave images, image impulse response, interferometric ScanSAR systems, modified SPECAN algorithm, phase analysis, phase-preserving ScanSAR image generation, real data, scan mode synthetic aperture radar, simulated data, spaceborne platform, standard range-Doppler approach, time domain.
    Abstract: The scan mode synthetic aperture radar (ScanSAR) image impulse response is derived in the time domain, and particular attention is given to the analysis of the phase, which is important for several applications, and especially in interferometric ScanSAR systems. A new algorithm for phase-preserving azimuth focusing of ScanSAR data, that extends the basic SPECAN procedure, is presented. The proposed algorithm avoids the interpolation step needed to achieve a constant azimuth pixel spacing by replacing the standard Fourier transform used in the SPECAN procedure with an appropriate chirp z-transform. The relationship between the modified SPECAN algorithm and the standard range-Doppler approach is also discussed. Experiments on real and simulated data are carried out to validate the theory
    [bibtex-key = lanariHensleyRosen1998Long:ModifiedSPECAN] [bibtex-entry]


  896. Zheng-She Liu and Jian Li. Synthetic-aperture-radar motion compensation and feature extraction by means of a relaxation-based algorithm. J. Opt. Soc. Am. A, 15(3):599-610, 1998. Keyword(s): SAR Processing, Autofocus, Motion Compensation, MoComp, Residual Motion Errors, Airborne SAR, Phase Gradient Autofocus, PGA, RELAX, MCRELAX, Cramer-Rao Bound, Parametric Estimation. [bibtex-key = liuLi98:Autofocus] [bibtex-entry]


  897. Zheng-She Liu, Hongbin Li, and Jian Li. Efficient implementation of Capon and APES for spectral estimation. IEEE Transactions on Aerospace and Electronic Systems, 34(4):1314-1319, October 1998. Keyword(s): Capon, MVDR, minimum variance distortionless reponse, spectral analysis, adaptive estimation, adaptive filters, matched filters, statistical analysis, filtering theory, Capon, APES, spectral estimation, matched-filterbank spectral estimators, narrow spectral peaks, sidelobe levels, statistical performance, Finite impulse response filter, Frequency, Fast Fourier transforms, Two dimensional displays, Application software, Synthetic aperture radar, Adaptive filters, Filtering, Noise level, Interference.
    Abstract: Both the Capon and APES estimators can be shown to belong to the class of matched-filterbank spectral estimators and can be used to obtain complex spectral estimates that have more narrow spectral peaks and lower sidelobe levels than the fast Fourier transform (FFT) methods. It can also be shown that APES has better statistical performance than Capon. In this paper, we address the issue of how to efficiently implement Capon and APES for spectral estimation.
    [bibtex-key = liuLiLiTAES1998EffientCaponAndAPESforSpectralEstimation] [bibtex-entry]


  898. Didier Massonnet and Kurt L. Feigl. Radar interferometry and its application to changes in the Earth's surface. Reviews of Geophysics, 36(4):441-500, 1998. Keyword(s): Permeability and porosity.
    Abstract: Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (100 pixels/km), a competitive precision (1 cm), and a useful observation cadence (1 pass /month). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS-1, ERS-2, JERS-1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS-1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.
    [bibtex-key = massonnetFeiglReviewsGeophysics1998DINSARReviewPaper] [bibtex-entry]


  899. Christian Matzler. Improved Born approximation for scattering of radiation in a granular medium. Journal of Applied Physics, 83(11):6111-6117, 1998. [bibtex-key = Maetzler1998] [bibtex-entry]


  900. Johan J. Mohr, Niels Reeh, and Soren N. Madsen. Three-dimensional glacial flow and surface elevation measured with radar interferometry. Nature, 391(6664):273-276, January 1998. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Glacier velocity, glacial flow.
    Abstract: Outlet glaciers which serve to drain ice from ice sheets, seem to be dynamically less stable in North Greenland than in South Greenland. Storstrommen, a large outlet glacier in northeastern Greenland which surged between 1978 and 1984, has been well studied. In general, neither glacier surge mechanisms nor the geographical distribution of the surges are well known. Conventional satellite radar interferometry can provide large-scale topography models with high resolution, and can measure the radar line-of-sight component of ice-flow vectors5, but cannot map full vector flow fields. Here we present an interferometry method that combines observations from descending and ascending satellite orbits which, assuming ice flow parallel to the topographic surface, allows us to use the differing view angles to estimate full three-dimensional surface flow patterns. The accuracy of our technique is confirmed by the good agreement between our radar-based flow model and in situ Global Positioning System (GPS) reference data at Storstrommen. Radar measurements such as these, made regularly and at high spatial density, have the potential to substantially enhance our understanding of glacier dynamics and ice-sheet flow, as well as improve the accuracy of glacier mass-balance estimates.
    [bibtex-key = mohrReehMadsenNature1998InSARGlacierFlowAndDEM] [bibtex-entry]


  901. Eric J. Rignot. Fast Recession of a West Antarctic Glacier. Science, 281(5376):549-551, 1998.
    Abstract: Satellite radar interferometry observations of Pine Island Glacier, West Antarctica, reveal that the glacier hinge-line position retreated 1.2 +/- 0.3 kilometers per year between 1992 and 1996, which in turn implies that the ice thinned by 3.5 +/- 0.9 meters per year. The fast recession of Pine Island Glacier, predicted to be a possible trigger for the disintegration of the West Antarctic Ice Sheet, is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters.
    [bibtex-key = rignotScience1998AntarticGlacier] [bibtex-entry]


  902. David T. Sandwell and Evelyn J. Price. Phase gradient approach to stacking interferograms. Journal of Geophysical Research: Solid Earth, 103(B12):30183-30204, 1998. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, Inteferogram Stacking, DInSAR, Displacement, Surface Displacement, Deformation Mapping, Deformation Monitoring, Oceanic structures, Space geodetic surveys, Instruments and techniques, Tectonophysics: Dynamics, seismotectonics.
    Abstract: The phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping. Our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms. The standard approach requires phase unwrapping, scaling the phase according to the ratio of the perpendicular baseline, and finally forming the average or difference; however, unique phase unwrapping is usually not possible. Since the phase gradient due to topography is proportional to the perpendicular baseline, phase unwrapping is unnecessary prior to averaging or differencing. Phase unwrapping may be needed to interpret the results, but it is delayed until all of the largest topographic signals are removed. We demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m. Cross-spectral analysis of the difference between two Tandem interferograms provides estimates of spatial resolution, which are used to design prestack filters. A wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage. Outside of mountainous areas the topography has a relative accuracy of better than 2 m. Residual interferograms (single interferogram minus stack) have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth, reflecting both orbit error and atmospheric delay. Smaller-scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves. A few Global Positioning System (GPS) control points within a frame could increase the precision to 20 mm for a single interferogram; further improvements may be achieved by stacking residual interferograms.
    [bibtex-key = sandwellPriceJGRB1998InterferogramStackingPhaseGradient] [bibtex-entry]


  903. Beat Schmid, Paul R Spyak, Stuart F Biggar, Christoph Wehrli, Jörg Sekler, Thomas Ingold, Christian Mätzler, and Niklaus Kämpfer. Evaluation of the applicability of solar and lamp radiometric calibrations of a precision Sun photometer operating between 300 and 1025 nm. Applied Optics, 37(18):3923-3941, 1998. [bibtex-key = Schmid1998] [bibtex-entry]


  904. Martin Schneebeli and Jerome B. Johnson. A constant-speed penetrometer for high-resolution snow stratigraphy. Annals of Glaciology, 26:107-111, 1998. Keyword(s): snow, snow micro pen, snow stratigraphy.
    Abstract: A new constant-speed penetrometer for field and laboratory measurement has been developed. The initially independent work of SFISAR and CRREL has been brought together, and a portable field device is now in an advanced stage of testing. The new penetrometer has high rigidity and a high-resolution large dynamic range force sensor. It uses a much smaller sensing head (5mm) than previous designs and has a constant-speed drive. With this construction, the penetration resistance of very fine layers and the influence of the bonding strength between snow grains can be more accurately determined than is possible with the rammsonde or Pandalp. Artificial foam layers as thin as 2 mm and thin layers in snow have been detected by the penetrometer. Thin snow layers detected from penetration-resistance profiles have been correlated to fine layering as determined from plane-section microphotographs of samples taken adjacent to the profile. The instrument's measurements are highly repeatable and the lack of subjective decisions when operating the penetrometer makes the penetration resistance a quantitative measure of snow stratigraphy.
    [bibtex-key = schneebeliJohnson1998SnowMicroPen] [bibtex-entry]


  905. Tazio Strozzi and Christian Matzler. Backscattering measurements of alpine snowcovers at 5.3 and 35 GHz. IEEE Transactions on Geoscience and Remote Sensing, 36(3):838-848, May 1998. Keyword(s): backscatter, hydrological techniques, radar cross-sections, radar polarimetry, remote sensing by radar, snow, 35 GHz, 5.3 GHz, AD 1993, AD 1994, AD 1995, AD 1996, Austria, Austrian Alp, C-band, EHF, Ka-band, SHF, Switzerland, hydrology, measurement technique, network-analyzer based scatterometer, radar backscatter, radar polarimetry, radar remote sensing, radar scattering, refrozen crust thickness, snow cover, snowcover, volumetric liquid water content, Backscatter, Frequency, Instruments, Millimeter wave measurements, Performance evaluation, Physics, Polarization, Radar measurements, Snow, Testing.
    Abstract: This paper describes two network-analyzer (NA)-based scatterometers at 5.3 (C-band) and 35 GHz (Ka-band) as well as snowcover measurements made in the Swiss and Austrian Alps between December 1993 and January 1996. First, the setup and the mode of operation of the scatterometers are discussed. Both instruments measure the backscattering coefficients gamma at hh, vv, vh, and vh polarizations and for incidence angles ranging from 0 to 70deg. The accuracy of gamma is generally better than +/-1.8 dB, and the scatterometers are well suited for signature studies of natural surfaces. During the two years, the authors performed many backscattering measurements of natural, strongly layered snowcovers and the authors investigated relationships between gamma and physical parameters of the snowcover. All measurements were collected in a signature catalogue. They report on results at 40deg incidence angle. They found that the combined use of active sensors at 5.3 and 35 GHz allows the discrimination of various snowcover situations, if multitemporal information is available. In addition, they observed a relationship of gamma at 5.3 GHz with the integrated column height of liquid water and dependencies of gamma at 35 GHz on the height of the dry snow, on the volumetric liquid water content at the snow surface, and on the thickness of the refrozen crust at the snow surface
    [bibtex-key = strozziMatzlerTGRS1998Snow] [bibtex-entry]


  906. Lars M. H. Ulander and Per-Olov Frölind. Ultra-Wideband SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 36(5):1540-1550, September 1998. Keyword(s): SAR Processing, Interferometry, DEM Generation, Fourier-Hankel Inversion, Hankel Transform, Abel Transform, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR.
    Abstract: We introduce ultra-wideband synthetic aperture radar (SAR) interferometry as a new technique for topographic height retrieval. It is based on using a SAR system with large relative bandwidth that acquires data along two parallel tracks with a separation of the same order of magnitude as the flight altitude. The complex SAR image data are resampled onto a common reference surface, filtered, and followed by a Hermitian multiplication. The resulting interferogram is shown to have a finite depth-of-focus (DOF) in terms of phase coherence. The achieved height precision is controlled by the ambiguity height, which is shown to scale to the DOF as the relative bandwidth. This means that only one fringe is within the DOF as the resolution approaches the fundamental wavelength limit; i.e., the phase is unambiguously related to topographic height. The topography may thus be determined by changing the reference surface and retrieving the height at each step. The technique is successfully demonstrated to generate fringes based on VHF-band data acquired by the CARABAS airborne SAR system. Temporal decorrelation is not a problem due to the long wavelengths nor is the effect of tropospheric delay on the retrieved height.
    [bibtex-key = ulanderfroelind98:Interfero] [bibtex-entry]


  907. Andreas Wiesmann, Christian Matzler, and Thomas Weise. Radiometric and structural measurements of snow samples. Radio Science, 33(2):273-289, 1998.
    Abstract: The interaction of microwaves with the natural snow cover strongly depends on the complex structure of the snowpack. In order to quantify this dependency, dedicated experiments were performed with homogeneous slabs of dry, natural snow samples measured over a frequency range from 11 to 94 GHz. A new method introduced by M{\"a}tzler and Wegm{\"u}ller [1995] and Weise [1996a] for determining the scattering and absorption behavior of test samples was applied and further developed by application of a multiple scattering model. Homogeneous samples of dry snow were (1) investigated using a set of portable, linearly polarized Dicke radiometers at frequencies of 11, 21, 35, 48 and 94 GHz, (2) characterized by temperature, grain size and shape, density and permittivity, and (3) structurally analyzed by digitized snow sections in order to obtain statistical information of the snow structure i.e. the autocorrelation function. During the winters 1994/1995 and 1995/1996 additional measurements of snow samples were made to extend the variability of the investigated snow types. Up to now, 20 samples, representing alpine snow in winter (that is, without melt metamorphism) have been collected during three winter campaigns. Here, we present the method and the radiative transfer model and show how it can be inverted to obtain scattering and absorption coefficients. A first assessment of the snow sample data is also presented. The results show good agreement between the measured and the theoretical absorption coefficient. The scattering coefficient turns out to be a strong function of frequency and correlation length as expected from Rayleigh scattering. However, distinct differences can be noted.
    [bibtex-key = wiesmannMatzlerWeiseRadioScience1998RadiometricStructuralMeasurementOfSnowSamples] [bibtex-entry]


  908. Simon Williams, Yehuda Bock, and Peng Fang. Integrated satellite interferometry: Tropospheric noise, GPS estimates and implications for interferometric synthetic aperture radar products. Journal of Geophysical Research: Solid Earth, 103(B11):27051-27067, 1998. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Displacement, Surface Displacement, Atmosphere, APS, Geodesy and Gravity, Instruments and techniques, GPS, GNSS.
    Abstract: Interferometric synthetic aperture radar (INSAR), like other astronomic and space geodetic techniques, is limited by the spatially and temporally variable delay of electromagnetic waves propagating through the neutral atmosphere. Statistical analysis of these variations, from a wide variety of instruments, reveals a power law dependence on frequency that is characteristic of elementary (Kolmogorov) turbulence. A statistical model for a major component of the delay fluctuations, the ''wet'' component, has previously been developed by Treuhaft and Lanyi [1987] for very long baseline interferometry. A continuous Global Positioning System (GPS) network is now in place in southern California that allows estimation of, along with geodetic parameters, the total delay due to the atmosphere above each site on a subhourly basis. These measurements are shown to conform to the Treuhaft and Lanyi (TL) statistical model both temporally and spatially. The TL statistical model is applied to the problem of INSAR and used to produce the covariance between two points separated in time and/or space. The error, due to the atmospheric variations, for SAR products such as topography and surface deformation is calculated via propagation of errors. There are two methods commonly cited to reduce the effect of atmospheric distortion in products from SAR interferometry, stacking and calibration. Stacking involves averaging independent interferograms to reduce the noise. Calibration involves removing part (or all) of the delay using data from an independent source such as total zenith delay estimates from continuous GPS networks. Despite the relatively poor spatial density of surface measurements, calibration can be used to reduce noise if the measurements are sufficiently accurate. Reduction in tropospheric noise increases with increasing number of measurement points and increasing accuracy up to a maximum of sqrt(N), where N is the number of points. Stacking and calibration are shown to be complementary and can be used simultaneously to reduce the noise to below that achievable by either method alone.
    [bibtex-key = williamsBockFangJGRBInSARandGPSTroposphere] [bibtex-entry]


  909. Shane R. Cloude and Eric Pottier. An entropy based classification scheme for land applications of polarimetric SAR. IEEE Trans. Geosci. Remote Sens., 35(1):68-78, January 1997. Keyword(s): SAR Processing, Polarimetric Decomposition, Cloude-Pottier Decomposition, Polarimetry, PolSAR, Entropy, Anisotropy, Alpha, H-A-alpha, S-matrix theory, geophysical signal processing, geophysical techniques, image classification, radar imaging, radar polarimetry, radar theory, remote sensing by radar, synthetic aperture radar, average target scattering matrix parameters, coherency matrix, eigenvalue analysis, entropy based classification, geophysical measurement technique, land surface, land use, parameterization, polarimetric SAR, polarimetric scattering problem, quantitative analysis, radar remote sensing, scattering entropy, terrain mapping, three-level Bernoulli statistical model, unsupervised classifier.
    Abstract: The authors outline a new scheme for parameterizing polarimetric scattering problems, which has application in the quantitative analysis of polarimetric SAR data. The method relies on an eigenvalue analysis of the coherency matrix and employs a three-level Bernoulli statistical model to generate estimates of the average target scattering matrix parameters from the data. The scattering entropy is a key parameter is determining the randomness in this model and is seen as a fundamental parameter in assessing the importance of polarimetry in remote sensing problems. The authors show application of the method to some important classical random media scattering problems and apply it to POLSAR data from the NASA/JPL AIRSAR data base
    [bibtex-key = cloudePottier1997:EntropyAalpha] [bibtex-entry]


  910. G. Fornaro, G. Franceschetti, R. Lanari, D. Rossi, and M. Tesauro. Interferometric SAR phase unwrapping using the finite element method. IEE Proceedings - Radar, Sonar and Navigation, 144(5):266-274, October 1997. Keyword(s): IFSAR images, computations speedup, efficiency, experiments, finite element method, interferometric SAR phase unwrapping, least-squares solution, multigrid technique, noise robust method, phase unwrapping, precision, real ERS-I data, simulated patterns, time domain, two-dimensional algorithm, weighting functions, finite element analysis, least squares approximations, radar imaging, radiowave interferometry, synthetic aperture radar, time-domain analysis.
    Abstract: The authors present a new two-dimensional algorithm for interferometric synthetic aperture radar phase unwrapping based on the finite element method. The proposed technique allows an efficient least-squares solution carried out in the time domain. Weighting functions can be introduced without increasing the computational requirements. The possibility to speed up the computations via a multigrid technique is discussed. A noise-robust extension of the proposed technique is also investigated. The presented experiments on real ERS-I data, as well as on simulated patterns, demonstrate the precision and efficiency of the procedure
    [bibtex-key = 635858] [bibtex-entry]


  911. G. Fornaro, V. Pascazio, and G. Schirinzi. Synthetic aperture radar interferometry using one bit coded raw and reference signals. IEEE Transactions on Geoscience and Remote Sensing, 35(5):1245 -1253, September 1997. Keyword(s): SAR Processing, InSAR, Signum Coded, geodesy, geophysical measurement technique, interferometric SAR, interferometric phase pattern generation, interferometric phase patterns, land surface topography, one bit coded raw signal, quick-look DEM, radar imaging, radar remote sensing, reference function, reference signal, signal processing, spaceborne radar, synthetic aperture radar interferometry, terrain mapping, geodesy, geophysical signal processing, geophysical techniques, radar imaging, radar signal processing, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth).
    Abstract: This paper is concerned about the generation of interferometric phase patterns using synthetic aperture radar (SAR) images obtained by processing the raw data and reference function both quantized at one bit (Signum Coded). Such processing technique involves one-bit coded (i.e., binary) sequences, and can be efficiently implemented in real time using very simple and low cost hardware. It is shown that the proposed SC processing technique preserves, besides the image intensities, also interferometric phase patterns, before and after phase unwrapping. To test the performance of the proposed technique, experiments have been carried out on real data relative to the ERS-1 mission. Quantitative comparison between the results of conventional and SC processing clearly show that the presented method can be used for quick-look DEMs generation. Moreover, in accordance with the SC-SAR theory, an upsampling has also been performed on the signals to be processed to obtain higher quality patterns. This produce a noticeable improvement of the obtained results, so that the SC techniques can be considered a valid alternative to the conventional ones, still preserving the advantages in terms of real time
    [bibtex-key = 628791] [bibtex-entry]


  912. H. Israelsson, L. M. H. Ulander, J. L. H. Askne, J. E. S. Fransson, P.-O. Frölind, A. Gustavsson, and H. Hellsten. Retrieval of forest stem volume using VHF SAR. IEEE Trans. Geosci. Remote Sens., 35(1):36-40, January 1997. Keyword(s): SAR Processing, Forest, Forest parameters, biomass, 28 to 60 MHz, CARABAS, HF radar, Oland, SAR, Sweden, VHF radar, airborne radar, backscattering coefficient, biomass, coherent all radio band sensing, deciduous mixed forest, dense forest, discrimination, forest stem volume, forestry, geophysical measurement technique, horizontal dipole array, radar imaging, radar remote sensing, radar scattering, tree trunk, vegetation mapping, wood, airborne radar, forestry, geophysical techniques, radar cross-sections, radar imaging, remote sensing by radar, synthetic aperture radar.
    Abstract: The ability to retrieve forest stem volume using CARABAS (coherent all radio band sensing) SAR images (28-60 MHz) has been investigated. The test site is a deciduous mixed forest on the island of Oland in southern Sweden. The images have been radiometrically calibrated using an array of horizontal dipoles. The images exhibit a clear discrimination between the forest and open fields. The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies. The backscatter increases with increasing radar frequency. This work shows an advantage compared to higher frequencies for stem volume estimation in dense forests
    [bibtex-key = israelssonUlanderAskneFranssonFrolindGustavssonHellsten1997:BiomassBackscatter] [bibtex-entry]


  913. Riccardo Lanari and Gianfranco Fornaro. A short discussion on the exact compensation of the SAR range-dependent range cell migration effect. IEEE Transactions on Geoscience and Remote Sensing, 35(6):1446-1452, November 1997. Keyword(s): SAR Processing, Motion Compensation, Comparison of Algorithms, Chirp Scaling Algorithm, Range Cell Migration, Range Migration, FM radar, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, RCM, RDRCM, SAR, chirp z-transform, exact compensation, geophysical measurement technique, land surface, radar remote sensing, range-dependent range cell migration effect, terrain mapping.
    Abstract: Efficient and precise compensation of the range cell migration (RCM) effect is a key point for a fast and accurate synthetic aperture radar (SAR) data processor. In particular the range-dependent nature of the range cell migration effect complicates the compensation operation. It has been recently shown that an exact compensation of the range-dependent RCM (RDRCM) phenomenon can be carried out either by applying the chirp scaling algorithm or the chirp z-transform procedure. This paper investigates the relationship between the two methods. In particular, it is shown that the chirp z-transform based approach represents a particular implementation of the chirp scaling algorithm. A final discussion is dedicated to show how the chirp z-transform and the chirp scaling procedure can be applied within a SAR data processing algorithm.
    [bibtex-key = LanariFornaro1997:MoCoDiscussion] [bibtex-entry]


  914. J. Li, Z. Bi, Z.S. Liu, and K. Knaell. Use of curvilinear SAR for three-dimensional target feature extraction. Radar, Sonar and Navigation, IEE Proceedings-, 144(5):275-283, 1997. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Feature Extraction, Target Feature Extraction, Curvilinear SAR. [bibtex-key = liBiLiuKnaell1997:NonLinearSARTomo] [bibtex-entry]


  915. J. Li, Z.-S. Liu, and Petre Stoica. 3-D target feature extraction via interferometric SAR. Radar, Sonar and Navigation, IEE Proceedings -, 144(2):71-80, 1997. Keyword(s): SAR Processing, Tomography, SAR Tomography, InSAR, fast Fourier transforms, feature extraction, least squares approximations, parameter estimation, radar cross-sections, radar signal processing, radiowave interferometry, signal resolution, synthetic aperture radar, 3D target feature extraction, Capon, Cramer-Rao bounds, FTF, IFRELAX, IFSAR, MUSIC, data model, interferometric SAR, interferometric synthetic aperture radar, nonlinear least squares parameter estimation method, nonparametric methods, parameter estimates, parameter identifiability conditions, performance, point scatterers, two-dimensional feature extraction methods, windowed FFT.
    Abstract: The authors consider 3-D (three-dimensional) target feature extraction via an interferometric synthetic aperture radar (IFSAR). The targets of interest are relatively small and consist of a small number of distinct point scatterers. Since using IFSAR to extract the features of such targets has not been addressed before, a self-contained detailed derivation of the data model is presented. A set of sufficient parameter identifiability conditions on the data model and the Cramer-Rao bounds (CRBs) on the parameter estimates are also derived. Four existing two-dimensional feature extraction methods (FFT, windowed FFT, Capon and MUSIC) are extended to estimate the 3-D parameters of the target scatterers. A new nonlinear least squares parameter estimation method, referred to as IFRELAX, is also derived to extract the target features. Finally, numerical examples are presented to compare the performances of the presented methods with each other and with the corresponding CRBs.The authors show by means of numerical examples that, among the three nonparametric methods (FFT, windowed FFT, and Capon), Capon has the best resolution. The parametric methods MUSIC and IFRELAX can have much better resolution and provide much more accurate parameter estimates than the nonparametric methods. It is shown IFRELAX can be faster and provide much better parameter estimates than MUSIC.
    [bibtex-key = liLiuStoica97:Tomography] [bibtex-entry]


  916. T. Miller, L. Potter, and John W. McCorkle. RFI suppression for ultra wideband radar. Aerospace and Electronic Systems, IEEE Transactions on, 33(4):1142-1156, 1997. Keyword(s): SAR Processing, interference suppression, least squares approximations, radar imaging, radar interference, radiofrequency interference, synthetic aperture radar, Army Research Laboratory, RFI Suppression, SAR imagery, UWB rail SAR, bias, estimate-and-subtract algorithm, estimated targets, foliage-penetrating imaging, ground-penetrating imaging, radio frequency interference, real-time digital suppression, sidelobe levels, signal model, target energy loss, ultra wideband radar, ultrawideband synthetic aperture radar.
    Abstract: An estimate-and-subtract algorithm is presented for the real-time
    [bibtex-key = millerPotterMcCorkle97:RFI] [bibtex-entry]


  917. Andrea Monti-Guarnieri and C. Prati. SAR interferometry: A ``Quick and dirty'' coherence estimator for data browsing. IEEE Transactions on Geoscience and Remote Sensing, 35(3):660-669, May 1997. Keyword(s): SAR Processing, Coherence, Coherence Estimation, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, speckle, synthetic aperture radarERS, SAR interferometry, coherence estimator, coherence map, data browsing, fast algorithm, geophysical measurement technique, land surface, quick method, radar remote sensing, speckle similarity, synthetic aperture radar, terrain mapping.
    Abstract: Usual coherence estimation in SAR interferometry is a time consuming task since an accurate estimation of the local frequency of the interferometric fringes is required. This paper presents a fast algorithm for generating coherence maps, mainly intended for data browsing. The proposed estimator is based on the speckle similarity of coherent SAR data, and is thus independent of fringe frequency. The following advantages, with respect to the usual estimates, are achieved: (a) The estimator is more than 100 times faster, achieved at the cost of a reduced statistical confidence. (b) The estimator is not affected by possible local frequency estimation errors. (c) The estimator can be directly applied to single look detected images. The theoretical derivation of the statistical properties of the frequency independent estimator is carried out in the stationary case. The nonstationary case is then analyzed on real ERS SAR images
    [bibtex-key = montiGuarnieriPrati97:Coherence] [bibtex-entry]


  918. Tazio Strozzi, Andreas Wiesmann, and Christian Matzler. Active microwave signatures of snow covers at 5.3 and 35 GHz. Radio Science, 32(2):479-495, 1997. Keyword(s): microwave signatures, Snow, Remote Sensing of Snow, C-band, Ka-band, Electromagnetics.
    Abstract: During the last 3 years we performed in situ backscattering measurements on various natural snow covers at 5.3 and 35 GHz (C and Ka band, respectively). Most of the measurements were made at a test site above Davos in the Swiss Alps at 2540 m above sea level (asl), but studies were conducted also at other test sites in Switzerland and Austria at heights between 500 and 2200 m asl. The backscattering coefficient gamma was measured at hh, vv, hv and vh polarization, together with physical parameters of the snow cover, like snow height, stratification, temperature, density, and permittivity. On the basis of ground information and on a simple distribution of gamma, we identified signatures of object classes. The signatures were used in order to evaluate the capability of active microwave sensors at 5.3 and 35 GHz for the classification of snow covers. In addition, semiempirical algorithms for the retrieval of physical parameters of the snow cover, such as water equivalent, liquid water content, and thickness of the refrozen crust, were defined.
    [bibtex-key = strozziWiesmannMatzlerRADIOSCIENCE1997Snow] [bibtex-entry]


  919. Howard A. Zebker, Paul A. Rosen, and Scott Hensley. Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps. Journal of Geophysical Research: Solid Earth, 102(B4):7547-7563, 1997. Keyword(s): Remote sensing, Instruments and techniquesInstruments and techniques.
    Abstract: Interferogram images derived from repeat-pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor. Other tropospheric variations, such as pressure and temperature, also induce distortions, but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term. Spatial and temporal changes of 20% in relative humidity lead to 10 cm errors in deformation products, and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries. In wet regions such as Hawaii, these are by far the dominant errors in the Spaceborne Imaging Radar-C and X Band Synthetic Aperature Radar (SIR-C/X-SAR) interferometric products. The unknown time delay from tropospheric distortion is independent of frequency, and thus multiwavelength measurements, such as those commonly used to correct radar altimeter and Global Positioning System (GPS) ionospheric biases, cannot be used to rectify the error. In the topographic case, the errors may be mitigated by choosing interferometric pairs with relatively long baselines, as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline. For the SIR-C/X-SAR Hawaii data we found that the best (longest) baseline pair produced a map supporting 100 m contouring, whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval. In the case of deformation map errors the result is either independent of baseline parameters or else very nearly so. Here the only solution is averaging of independent interferograms, so in order to create accurate deformation products in wet regions many multiple passes may be required. Rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are (1) to use the longest radar wavelengths possible, within ionospheric scintillation and Faraday rotation limits, (2) for topography, maximize interferometer baseline within decorrelation limits and (3) for surface deformation, use multiple observations and average the derived products. Following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions, such as Hawaii.
    [bibtex-key = zebkerRosenHensleyJGRB1997AtmosphereDInSAR] [bibtex-entry]


  920. Regine Bolter, Margrit Gelautz, and Franz Leberl. SAR Speckle Simulation. International Archives of Photogrammetry and Remote Sensing, 21:20-25, 1996. Keyword(s): SAR Processing, Simulation, Speckle, Speckle Simulation.
    Abstract: After a short introduction to the principles of SAR speckle generation and its statistical properties, we give a review of different speckle simulation methods described in literature. Then, the implementation of some selected algorithms is described, and their performance is tested on simulated ERS-1 images. Special attention is paid to the modeling of multiple looks, and the differences between image pixel size and original radar ground resolution. A chi-square distribution and a Rayleigh distribution with multiple file averaging were found to produce the most realistic results.
    [bibtex-key = BoltGelaLeb96:specklesim] [bibtex-entry]


  921. P.S. Chang, James B. Mead, E.J. Knapp, G.A. Sadowy, R.E. Davis, and R.E. McIntosh. Polarimetric backscatter from fresh and metamorphic snowcover at millimeter wavelengths. IEEE Transactions on Antennas and Propagation, 44(1):58-73, January 1996. Keyword(s): Radar, Polarimetry, Polarimetric radar, snow, fresh snow, metamorphic snow, remote sensing, radar, millimeter wavelength, 35GHz, 95GHz, 225GHz.
    Abstract: This paper presents 35, 95, and 225 GHz polarimetric radar backscatter data from snowcover. It compares measured backscatter data with detailed in situ measurements of the snowcover including microstructural anisotropies within the snowpack. Observations of backscatter mere made during melt-freeze cycles, and measurable differences in the normalized radar cross section between older metamorphic snow and fresh low-density snow were observed. In addition, these data show that the average phase difference between the copolarized terms of the scattering matrix, S/sub vv/and S/sub hh/, is nonzero for certain snow types. This phase difference was found to be related to snowpack features including anisotropy, wetness, density, and particle size. A simple backscatter model based on measured particle size and anisotropy is found to predict the Mueller matrix for dry snowcover with reasonable accuracy.
    [bibtex-key = changMeadEtAlTAP1996PolarimetricBackscatterFromFreshAndMetamorphicSnowCoverAtMMWavelength] [bibtex-entry]


  922. Shane R. Cloude and Eric Pottier. A review of target decomposition theorems in radar polarimetry. IEEE Transactions on Geoscience and Remote Sensing, 34(2):498-518, March 1996. Keyword(s): S-matrix theory, backscatter, covariance matrices, eigenvalues and eigenfunctions, geophysical signal processing, matrix decomposition, radar cross-sections, radar imaging, radar polarimetry, remote sensing by radar, reviews, Mueller matrix, Stokes vector, coherency matrix, coherent decomposition, covariance matrix, eigenvector analysis, scattering matrix, target decomposition theorems, terrain, transformation theory.
    Abstract: In this paper, we provide a review of the different approaches used for target decomposition theory in radar polarimetry. We classify three main types of theorem; those based on the Mueller matrix and Stokes vector, those using an eigenvector analysis of the covariance or coherency matrix, and those employing coherent decomposition of the scattering matrix. We unify the formulation of these different approaches using transformation theory and an eigenvector analysis. We show how special forms of these decompositions apply for the important case of backscatter from terrain with generic symmetries
    [bibtex-key = cloudePottier1996:ReviewOfPolSARDecomp] [bibtex-entry]


  923. G.W. Davidson, Ian G. Cumming, and M.R. Ito. A chirp scaling approach for processing squint mode SAR data. IEEE_J_AES, 32(1):121-133, January 1996. Keyword(s): SAR Processing, C-band, L-band, chirp scaling algorithm, image degradations, image formation, nonlinear FM chirp scaling, phase modulation, range-variant filtering, resolution width, secondary range compression, sidelobe level, squint mode SAR data, FM radar, data compression, filtering theory, image resolution, phase modulation, radar imaging, synthetic aperture radar.
    Abstract: Image formation from squint mode synthetic aperture radar (SAR) is limited by image degradations caused by neglecting the range-variant filtering required by secondary range compression (SRC). Introduced here is a nonlinear FM chirp scaling, an extension of the chirp scaling algorithm, as an efficient and accurate approach to range variant SRC. Two methods of implementing the approach are described. The nonlinear FM filtering method is more accurate but adds a filtering step to the chirp scaling algorithm, although the extra computation is less than that of a time domain residual compression filter. The nonlinear FM pulse method consists of changing the phase modulation of the transmitted pulse, thus avoiding an increase in computation. Simulations show both methods significantly improve resolution width and sidelobe level, compared with existing SAR processors for squint angles above 10 deg for L-band and 20 deg for C-band.
    [bibtex-key = DavidsonCummingIto1996:ChirpScalingForHigherSquint] [bibtex-entry]


  924. G. Fornaro, G. Franceschetti, and R. Lanari. Interferometric SAR phase unwrapping using Green's formulation. IEEE Transactions on Geoscience and Remote Sensing, 34(3):720 -727, May 1996. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Phase Unwrapping, Green's first identity, Green's formulation, ISAR, SAR, algorithm, geodesy, geophysical measurement technique, interferometric SAR phase unwrapping, land surface topography, radar imaging, synthetic aperture radar, terrain mapping, unwrapped phase value, geophysical techniques, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth).
    Abstract: Any method that permits retrieving full range (unwrapped) phase values starting from their (- pi;, pi;) determination (wrapped phase) can be defined as a phase unwrapping technique. This paper addresses a new procedure for phase unwrapping especially designed for interferometric synthetic aperture radar applications. The proposed algorithm is based on use of Green's first identity. Results on simulated as well as on real data are presented. They both confirm the excellent performance of the procedure
    [bibtex-key = 499751] [bibtex-entry]


  925. Michael Y. Jin. Optimal Range and Doppler Centroid Estimation for a ScanSAR System. IEEE Transactions on Geoscience and Remote Sensing, 34(2):479-488, March 1996. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, ScanSAR, SIR-C, ERS.
    Abstract: This paper presents a new range and Doppler centroid estimation algorithm for a ScanSAR system. This algorithm is based on processing the image data in the overlapped region of two bursts of the same beam or adjacent beams. It leads to highly accurate radar pointing angles that are paramount to achieving good radiometric performance in ScanSAR imagery. The achievable accuracy is derived theoretically and verified by tests performed using SIR-C ScanSAR data and ERS data. This algorithm is computationally efficient and easy to implement. The proposed Doppler centroid estimation algorithm is also an excellent candidate for a strip mode SAR system.
    [bibtex-key = Jin96:Doppler] [bibtex-entry]


  926. Guoliang Jin and Dajun Tang. Uncertainties of differential phase estimation associated with interferometric sonars. Oceanic Engineering, IEEE Journal of, 21(1):53 -63, January 1996. Keyword(s): Applied Physics Laboratory, Benthic Acoustic Measurement System, University of Washington, baseline decorrelation, depth measurement, differential phase estimation, differential phase technique, equivalent noise source, error, excess deviation, flat sandy bottom, grazing angles, instantaneous insonified area, interferometric sonars, interferometric system, multiple bottom scatterers, noise interference, numerical simulations, signal-arrival-angle, speckle phenomena, uncertainties, acoustic noise, acoustic wave interferometry, acoustic wave scattering, bathymetry, correlation theory, digital simulation, measurement errors, phase estimation, sonar;.
    Abstract: The differential phase technique has been widely used in various sonar systems; however, uncertainties associated with the estimation of scatterer depths are not completely understood. Numerical simulations for multiple bottom scatterers are performed, and they show that the uncertainties of depth measurements, in the absence of noise interferences, are much greater than the amount explainable by the uncertainty associated with the signal-arrival-angle within an instantaneous insonified area. The cause of the excess deviation is analyzed, particularly for the two-scatterer case. This kind of error is referred to as ldquo;baseline decorrelation rdquo; which is related to the speckle phenomena and can be considered as an equivalent noise source. Experimental data obtained by a particular high-frequency (40 kHz) interferometric system, the Benthic Acoustic Measurement System (BAMS) developed by the Applied Physics Laboratory, University of Washington, at a flat sandy bottom off the coast of Panama City, FL, were analyzed. Both analytical formulas and a numerical model are given to estimate the measurement uncertainty caused by the baseline decorrelation, as well as noise interferences based on the parameters of the BAMS, in order to understand uncertainties of the differential phase estimation. It is found that baseline decorrelation is the main source of error for the BAMS for grazing angles greater than 12 deg;. The measurement uncertainties at this grazing angle interval are in agreement with the theoretical predictions
    [bibtex-key = jinTang1996] [bibtex-entry]


  927. R. Kwok and M.A. Fahnestock. Ice sheet motion and topography from radar interferometry. IEEE Trans. Geosci. Remote Sens., 34(1):189-200, January 1996. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Displacement, Surface Displacement, glaciology, hydrological techniques, radar applications, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, Greenland, SAR method, flow, geophysical measurement technique, glaciology, hydrology, ice sheet motion, motion, polar ice sheet, radar interferometry, radar remote sensing, repeat pass ERS-1 interferogram, surface displacement field, topography, velocity field, Data mining, Ice surface, Laboratories, Propulsion, Radar interferometry, Satellites, Space technology, Streaming media, Surface topography, Synthetic aperture radar.
    Abstract: Both topography and motion information are present in repeat pass ERS-1 interferograms over ice sheets. The authors demonstrate that the topography is separable from the surface displacement field when a sequence of radar images are available. If the velocity field is constant over the time span of observation, the topography can be derived from differential interferograms formed from sequential observations. With this measurement, a pure displacement field can then be obtained by removal of the topographic contribution to the interferometric phase at each pixel. Further, they discuss how the vertical and horizontal components of displacement affect the interferometrically-derived motion field. They illustrate their approach with four successive (3-day repeat) ERS-1 images of a flow feature in northeastern Greenland
    [bibtex-key = kwokFahnestockTGRS1996DINSARicesheetmotion] [bibtex-entry]


  928. R. Lanari, G. Fornaro, D. Riccio, M. Migliaccio, Konstantinos P. Papathanassiou, J.R. Moreira, M. Schwabisch, L. Dutra, G. Puglisi, G. Franceschetti, and M. Coltelli. Generation of digital elevation models by using SIR-C/X-SAR multifrequency two-pass interferometry: the Etna case study. IEEE Transactions on Geoscience and Remote Sensing, 34(5):1097-1114, September 1996. Keyword(s): C-band, Etna, Italy, Kalman filter, L-band, SAR, SHF, SIR, SIR-C, Sicily, UHF, X-SAR, X-band, algorithm, digital elevation model, geology, geophysical measurement technique, land surface, microwave radar, multifrequency two-pass interferometry, radar imaging, radar remote sensing, spaceborne radar, synthetic aperture radar, terrain mapping, topography, unwrapped pattern, unwrapping performance, volcano, Kalman filters, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth), volcanology.
    Abstract: The authors exploit the interferometric multifrequency potentiality of the SIR-C/X-SAR system which is equipped with an L-, C-, and X-band sensor. They present a solution to improve the unwrapping performance of the C- and X-band data by considering the L-band unwrapped pattern. A new algorithm for the generation of a single digital elevation model (DEM) combining L-, C-, and X-band information is presented. This solution is based on the fusion of the unwrapped phase patterns by using a Kalman filter. The proposed fusion operation also accounts for the coherence characteristics of the three data sets. The selected test site is the Mt. Etna region in Italy which is very interesting from the volcanological and geological point of view. Numerical assessments of the achieved results are provided by evaluating the height accuracy with respect to a reference DEM
    [bibtex-key = 536526] [bibtex-entry]


  929. Christian Matzler. Microwave permittivity of dry snow. IEEE Transactions on Geoscience and Remote Sensing, 34(2):573-581, 1996. Keyword(s): UHF measurement, hydrological equipment, permittivity, snow, -10 to 0 degC, 1 GHz, Austrian Alps, Swiss Alps, average axial ratio, coaxial sensor, density, destructive metamorphism, dry snow, ice grains, ice volume fraction, liquid-like surface layer, microwave permittivity, oblate spheroids, physical mixing theory, prolate spheroids, relative dielectric constant, relative permittivity, resometer, resonator, sintering, Coaxial components, Dielectric constant, Dielectric measurements, Frequency, Ice, Instruments, Measurement standards, Permittivity measurement, Snow, Testing.
    Abstract: The relative dielectric constant, or relative permittivity, \epsilon of dry snow, is independent of frequency from about 1 MHz up to the microwave range of at least 10 GHz. New measurements of with improved accuracy were made with a specially designed resonator operating near 1 GHz. The coaxial sensor accurately defines the sample volume whose actual mass can be determined to give the density of the snow sample. A special electronic instrument, called a resometer, enabled accurate and rapid measurements under field conditions. Some 90 measurements of different kinds of dry snow (fresh, old, wind-pressed snow, depth hear, and refrozen crusts) were made at test sites in the Swiss and Austrian Alps. The data indicate that \epsilon is a function of snow density only, given that the standard deviation of 0.006 from the fitted curve is just due to the expected measurement errors. The interpretation of these data in terms of physical mixing theory favors the effective medium formula of Polder and van Santen (1946). The data allow to relate the average axial ratio X as a function of ice volume fraction. Both prolate and oblate spheroids can explain the data. Independent reasoning gives preference to oblate particles. In both cases, the axial ratio increases with increasing fraction up to a critical value of 0.33, followed by a decrease at still higher fractions. The destructive metamorphism of slowly compacting snow explains the increase of X, while the following decrease might be due to sintering. So far, no effect on \epsilon by a liquid-like surface layer on the ice grains at temperatures between -10 deg C and 0 deg C has been observed
    [bibtex-key = matzlerTGRS1996MicrowavePermittivityDrySnow] [bibtex-entry]


  930. Andrea Monti-Guarnieri. Residual SAR Focusing: An Application to Coherence Improvement. IEEE Transactions on Geoscience and Remote Sensing, 34(1):201-211, January 1996. Keyword(s): SAR Processing, Interferometry, Residual Azimuth Compression, Coherence Improvement.
    Abstract: The focusing quality of a SAR processor greatly depends on the accuracy of the system geometry estimate. Sometimes ancillary data do not provide enough accuracy, therefore autofocusing has to be performed to get the finest quality possible. A residual azimuth compression is introduced to show how a defocused image can be compensated by means of a monodimensional local operator. The residual transfer function that generates defocusing is then derived. The effects of the defocusing are shown on both a complex single SAR image and a SAR interferogram. SAR interferograms, however, are much more sensitive to defocusing than the single SAR image. Two algorithms have been developed to estimate, and compensate for, the defocusing in both the single SAR image and SAR interferometric cases. The processors select data suitable for estimating focusing parameters from the whole images by exploring Kurtosis (for single image focusing) or coherence (for interferometric autofocusing). The residual, short time-domain operator is then exploited to retrieve the focusing parameter values and, finally, to get the focused image. The limitations and accuracy of the algorithm in terms of parameter estimation are investigated. Experimental results, obtained from different SAR missions, are presented
    [bibtex-key = monti:resSARfoc] [bibtex-entry]


  931. Alberto Moreira, Josef Mittermayer, and Rolf Scheiber. Extended Chirp Scaling Algorithm for Air- and Spaceborne SAR Data Processing in Stripmap and ScanSAR Imaging Modes. IEEE Transactions on Geoscience and Remote Sensing, 34(5):1123-1136, September 1996. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, Spaceborne SAR, Airborne SAR, ScanSAR, Automatic Azimuth Coregistration, Azimuth Scaling, Squinted SAR, Interferometry, Phase-Preserving Processing, Range Scaling Formulation, Stripmap SAR, Subaperture Processing, Terrain Mapping.
    Abstract: This paper resents a generalized formulation of the extended chirp scaling (ECS) approach for high precision processing of air- and spaceborne SAR data. Based on the original chirp scaling function, the ECS algorithm incorporates a new azimuth scaling function and a subaperture approach, which allow an effective phase-preserving processing of ScanSAR data without interpolation for azimuth geometric correction. The azimuth scaling can also be used for automatic azimuth coregistration of interferometric image pairs which are acquired with different sampling distances. Additionally, a novel range scaling formulation is proposed for automatic range coregistration of interferometric image pairs or for improved robustness for the processing of highly squinted data. Several simulation and processing results of air- and spaceborne SAR data are presented to demonstrate the validity of the proposed algorithms
    [bibtex-key = moreiraMittermayerScheiber96:Extended] [bibtex-entry]


  932. Kamal Sarabandi and Tsen-Chieh Chiu. Optimum corner reflectors for calibration of imaging radars. IEEE Transactions on Antennas and Propagation, 44(10):1348-1361, October 1996. Keyword(s): calibration, electromagnetic wave reflection, geometrical optics, microwave measurement, optimisation, physical optics, radar cross-sections, radar equipment, 1-dB RCS beamwidths, 9.5 GHz, RCS, SHF, UHF, backscatter measurements, calibration, coherent interaction, edge length, geometrical optics, ground plane, imaging radar, low microwave frequencies, near-field physical optics, optimum corner reflector, optimum corner reflectors, panel geometry, polygonal self-illuminating corner reflectors, radar cross section, self-illuminating corner reflectors, total surface area, Azimuth, Calibration, Geometrical optics, Microwave frequencies, Microwave imaging, Optical imaging, Physical optics, Radar cross section, Radar imaging, Uncertainty.
    Abstract: Trihedral corner reflectors are widely used as calibration targets for imaging radars because of their large radar cross section (RCS) and extremely wide RCS pattern. An important source of uncertainty in the RCS of a trihedral sitting on a ground plane is the coherent interaction of the ground plane with the trihedral. At UHF and low microwave frequencies the large physical size of corner reflectors become a limiting factor in regard to difficulties in field deployment and deviation of their RCS from the expected values. In this paper, a general class of corner reflectors with high-aperture efficiency referred to as self-illuminating corner reflectors, is introduced whose coherent interaction with the surrounding terrain is minimized and their total surface area is two-thirds of that of a triangular corner reflector having the same maximum RCS. Analytical expressions based on geometrical optics and a new numerical solution based on near-field physical optics for the RCS of two simple self-illuminating corner reflectors are presented and compared with backscatter measurements. Also the panel geometry for an optimum corner reflector which has the shortest edge length among polygonal self-illuminating corner reflectors is obtained. High-aperture efficiency is achieved at the expense of azimuth and elevation beamwidth. It is shown that the 1-dB RCS beamwidths of the optimal corner reflectors, both in azimuth and elevation directions, are about 16 deg. RCS measurements of corner reflectors in the presence of a ground plane show that the RCS of self-illuminating corner reflectors are less affected by the coherent ground interaction
    [bibtex-key = sarabandiChiuTGARS1996OptimumCornerReflectorsForCalibration] [bibtex-entry]


  933. M. Soumekh. Reconnaissance with slant plane circular SAR imaging. Image Processing, IEEE Transactions on, 5(8):1252-1265, 1996. Keyword(s): SAR Processing, Non-Linear Flight Path, Fourier analysis, Fourier transforms, Green's function methods, image reconstruction, image resolution, inverse problems, radar imaging, synthetic aperture radar, Green's function, SAR data inversion, SAR system, aspect angle, circular flight path, full rotation, ground penetrating UHF radar signature, imaging system, partial segment, reconnaissance, reconstruction algorithm, simulated target, slant plane circular SAR imaging, slant plane data, slant plane linear SAR, target scene, three-dimensional imaging, ultrawideband foliage penetrating radar signature.
    Abstract: This paper presents a method for imaging from the slant plane data collected by a synthetic aperture radar (SAR) over the full rotation or a partial segment of a circular flight path. A Fourier analysis for the Green's function of the imaging system is provided. This analysis is the basis of an inversion for slant plane circular SAR data. The reconstruction algorithm and resolution for this SAR system are outlined. It is shown that the slant plane circular SAR, unlike the slant plane linear SAR, has the capability to extract three-dimensional imaging information of a target scene. The merits of the algorithm are demonstrated via a simulated target whose ultra wideband foliage penetrating (FOPEN) or ground penetrating (GPEN) ultrahigh frequency (UHF) radar signature varies with the radar's aspect angle
    [bibtex-key = Soumekh1996] [bibtex-entry]


  934. Robert N. Treuhaft, Soren N. Madsen, Mahta Moghaddam, and Jakob van Zyl. Vegetation characteristics and underlying topography from interferometric radar. Radio Science, 31(6):1449-1485, 1996. Keyword(s): SAR Processing, InSAR, Interferometry, Vegetation Parameters, Parameter Extraction, Topography, SAR Tomography.
    Abstract: This paper formulates and demonstrates methods for extracting vegetation characteristics and underlying ground surface topography from interferometric synthetic aperture radar (INSAR) data. The electromagnetic scattering and radar processing, which produce the INSAR observations, are modeled, vegetation and topographic parameters are identified for estimation, the parameter errors are assessed in terms of INSAR instrumental performance, and the parameter estimation is demonstrated on INSAR data and compared to ground truth. The fundamental observations from which vegetation and surface topographic parameters are estimated are (1) the cross-correlation amplitude, (2) the cross-correlation phase, and (3) the synthetic aperture radar (SAR) backscattered power. A calculation based on scattering from vegetation treated as a random medium, including the effects of refractivity and absorption in the vegetation, yields expressions for the complex cross correlation and backscattered power in terms of vegetation characteristics. These expressions lead to the identification of a minimal set of four parameters describing the vegetation and surface topography: (1) the vegetation layer depth, (2) the vegetation extinction coefficient (power loss per unit length), (3) a parameter involving the product of the average backscattering amplitude and scatterer number density, and (4) the height of the underlying ground surface. The accuracy of vegetation and ground surface parameters, as a function of INSAR observation accuracy, is evaluated for aircraft INSAR, which is characterized by a 2.5 m baseline, an altitude of about 8 km, and a wavelength of 5.6 cm. It is found that for ~0.5% accuracy in the INSAR normalized cross-correlation amplitude and ~5% accuracy in the interferometric phase, few-meter vegetation layer depths and ground surface heights can be determined from INSAR for many types of vegetation layers. With the same observational accuracies, extinction coefficients can be estimated at the 0.1-dB/m level. Because the number of parameters exceeds the number of observations for current INSAR data sets, external extinction coefficient data are used to demonstrate the estimation of the vegetation layer depth and ground surface height from INSAR data taken at the Bonanza Creek Experimental Forest in Alaska. This demonstration shows approximately 5 m average ground truth agreement for vegetation layer depths and ground-surface heights, with a clear dependence of error on stand height. These errors suggest refinements in INSAR data acquisition and analysis techniques which will potentially yield few-meter accuracies. The information in the INSAR parameters is applicable to a variety of ecological modeling issues including the successional modeling of forested ecosystems.
    [bibtex-key = treuhaftMadsenMoghaddamVanZyl96:.pdf] [bibtex-entry]


  935. Lars M. H. Ulander. Radiometric slope correction of synthetic-aperture radar images. IEEE Transactions on Geoscience and Remote Sensing, 34(5):1115-1122, 1996. Keyword(s): SAR Processing, Radiometric Correction, Topography, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, SAR, algorithm, calibration equation, fringe frequencies, geophysical measurement technique, land surface, maximum-likelihood estimator, radar remote sensing, radiometric slope correction, synthetic aperture radar image, terrain mapping, topographic height variation.
    Abstract: The brightness in a SAR image is affected by topographic height
    [bibtex-key = Ulander96:RadiometricSlopeCorection] [bibtex-entry]


  936. Frank Wong and Ian G. Cumming. A Combined SAR Doppler Centroid Estimation Scheme Based Upon Signal Phase. IEEE Transactions on Geoscience and Remote Sensing, 34(3):696-707, May 1996. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Multilook Cross Correlation, MLCC, Multilook Beat Frequency, MLBF, Clutterlock, Doppler Ambiguity Resolver, DAR.
    Abstract: This paper describes a complete end-to-end Doppler centroid estimation scheme, which determines the fractional PRF part of the Doppler centroid. It also resolves the Doppler ambiguity. Experiments show that the scheme works successfully over various terrain types, including land, water, and ice, and that it requires only a modest amount of SAR data to perform reliably. The proposed scheme has an added advantage that it is directly applicable to RADARSAT and ENVISAT ScanSAR data. The scheme uses two complementary Doppler estimation algorithms, both utilizing the phase information embedded in the radar signal. In each algorithm, upper and lower parts of the available bandwidth of the received signal are extracted to form two range looks. The first algorithm, called multilook cross correlation (MLCC), computes the average cross correlation coefficient between adjacent azimuth samples for each of the two looks and then takes the difference between the angles of the two coefficients. The Doppler ambiguity is determined from the angle difference. The fractional pulse repetition frequency (PRF) part is also determined from the cross correlation coefficients. In the second algorithm, called multilook beat frequency (MLBF), the two looks are multiplied together to generate a beat signal. The beat frequency is then estimated and the Doppler ambiguity determined from the beat frequency. The MLCC algorithm performs better with low contrast scenes while the MLBF works better with high contrast ones. Although each algorithm works well on its own with sufficient averaging, it is also possible to use quality measures to select the best result from either algorithm. In this way, scenes of different content or contrast can be handled reliably. This paper presents the analysis of the two algorithms, explaining why their performance is affected by scene contrast, which is confirmed by experimental results with ERS-1 and JERS-1 data.
    [bibtex-key = wongCum:DopCentrEstim] [bibtex-entry]


  937. G. Fornaro and G. Franceschetti. Image registration in interferometric SAR processing. IEE Proceedings - Radar, Sonar and Navigation, 142(6):313-320, December 1995. Keyword(s): SAR Processing, InSAR, SAR Interferometry, IFSAR applications, common reference output system, image registration, interferometric SAR processing, linear stretch, phase difference, raw data processing stage, received signal, scaling compensation, shifting compensation, spectral shift, geophysical signal processing, image registration, radar imaging, radiowave interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: In interferometric synthetic aperture radar (IFSAR) applications image registration is needed to extract the correct phase difference between the two received signals. The authors present a new image registration procedure implemented at the raw data processing stage; the two complex SAR images are generated with respect to a common reference output system. The registration task is achieved via scaling and shifting compensation that can be efficiently and easily included in a standard SAR processing code. Examples on real as well as on simulated data validate the procedure. An algorithm to estimate the correct processing parameters, whenever the orbital information is inaccurate, is also presented and is based on the relation between spectral shift and linear stretch of the two images
    [bibtex-key = 487697] [bibtex-entry]


  938. Giorgio Franceschetti, Riccardo Lanari, and E. S. Marzouk. Efficient and High Precision Space-Variant Processing of SAR Data. IEEE Transactions on Aerospace and Electronic Systems, 31(1):227-237, January 1995. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Range-Doppler Algorithm, Space-Variance Compensation, Space-Variant Processing, Earth Rotation Effect, Nonstandard Fourier Transform.
    Abstract: We investigate the space-variance of the synthetic aperture radar (SAR) transfer function due to focus depth variation and Earth rotation effect. We introduce a procedure for efficient space-variance compensation which is based on the use of a nonstandard Fourier transform (FT). A number of experiments confirming theoretical results are presented
    [bibtex-key = francescLanaMar95:Processing] [bibtex-entry]


  939. Richard M. Goldstein. Atmospheric limitations to repeat-track radar interferometry. Geophysical Research Letters, 22(18):2517-2520, 1995. Keyword(s): SAR Processing, Interferometry, SAR interferometry, Atmospheric Composition and Structure: Troposphere-composition and chemistry, Geodesy and Gravity: Space geodetic surveys, Geodesy and Gravity: Instruments and techniques, Radio Science: Remote sensing.
    Abstract: In its recent radar imaging mission, the Shuttle Imaging Radar satellite (SIR-C) devoted three days to repeat-track interferometry. We have analyzed the data from a test site in the Mojave desert of California. Although good topography (+/-10 m on 21 m postings) was obtained, most of the error was caused by turbulent water vapor in the lower atmosphere. Spatial structure of 6 km and all smaller sizes was observed. The RMS, one-way time delay was found to be 0.24 cm. Essentially identical results were obtained at two wavelengths, 24 and 5.7 cm.
    [bibtex-key = goldsteinGRL1995AtmosphereInSAR] [bibtex-entry]


  940. J.O. Hagberg, L. M. H. Ulander, and J. Askne. Repeat-pass SAR interferometry over forested terrain. IEEE Trans. Geosci. Remote Sens., 33(2):331-340, March 1995. Keyword(s): SAR Processing, SAR interferometry, forest, forested terrain, forestry, general system model, geophysical method, interferometry, land surface topography, measurement technique, radar remote sensing, repeat pass method, scene scattering, synthetic aperture radar, topographic map, vegetation mapping, forestry, geophysical techniques, radar applications, radar imaging, remote sensing, remote sensing by radar, synthetic aperture radar, topography (Earth).
    Abstract: Repeat-pass synthetic aperture radar (SAR) interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images. However, the usefulness of such maps and images depends on our understanding of how different types of terrain affect the radar measurements. It is essential that the scene coherence between passes is sufficient. In this paper, the authors derive a general system model including both radar system and scene scattering properties. The model is used to interpret measurements over a forested area where the scene coherence varies between 0.2 and 0.5. The coherence is found to be sensitive to temperature changes around 0 deg Celsius but surprisingly insensitive to wind speed. The interferometric height discontinuity at the forest to open-field boundary shows good agreement with in situ tree height measurements. For a dense boreal forest, but is observed to decrease for a less dense forest. This suggests the possibility of estimating bole volume from the interferometric tree height and a ground DEM. The decrease of scene coherence over a dense forest with increasing baseline is also used to estimate the effective scattering layer thickness
    [bibtex-key = HagbergUlanderAskne1995] [bibtex-entry]


  941. Marc L. Imhoff. Radar backscatter and biomass saturation: ramifications for global biomass inventory. IEEE Transactions on Geoscience and Remote Sensing, 33(2):511-518, March 1995. Keyword(s): SAR Processing, backscatter, forestry, geophysical techniques, radar applications, radar cross-sections, radar imaging, remote sensing by radar, synthetic aperture radar, 0.44 to 5.3 GHz, C-band, Hawaii, L-band, P-band, SAR, UHF SHF microwave, biomass saturation, biome, broadleaf evergreen forest, canopy, coniferous forest, geophysical measurement technique, global biomass inventory, land surface, phytomass, radar backscatter, radar remote sensing, vegetated surface, vegetation mapping, Space-borne SAR, SAR Tomography, Tomography.
    Abstract: Two SAR and biomass data sets of forests with different canopy architectures were examined for commonalties regarding backscatter/biomass saturation. The SAR data were collected using the NASA/JPL AIRSAR at incidence angles between 40 deg and 50 deg for tropical broadleaf evergreen forests in Hawaii and coniferous forests in North America and Europe. Radar signal saturation limits with respect to biomass for both forest types were determined to be ~100 tons/ha for P-band (0.44 GHz), ~40 tons/ha for L-band (1.25 GHz), and ~20 tons/ha for C-band (5.3 GHz). The effect of the saturation limits on making global biomass inventories with SAR sensors was assessed by comparing the biomass saturation limits to a global vegetation type and biomass data base. C-band can be used to measure biomass in biomes covering 25% of the world's total ice-free vegetated surface area accounting for 4% of Earth's store of terrestrial phytomass. L- and P-band can be used to measure biomass in biomes covering 37% and 62% of the total vegetated surface area accounting for 8% and 19% of Earth's pool of terrestrial phytomass respectively. Biomes occupying approximately 38% of Earth's vegetated surface area containing 81% of the estimated total terrestrial phytomass have biomass densities above the saturation limit of current SAR systems (>100 tons/ha for P-band). Since P-band radar systems cannot currently operate effectively from orbit, the use of SAR sensors for biomass surveys may be limited even further to the L-band threshold. Emphasis should be shifted toward using SAR to characterize forest regeneration and development up to the saturation limits shown in this article rather than attempting to measure biomass directly in heavy forests. The development of new and innovative technologies for measuring biomass in high density vegetation is encouraged
    [bibtex-key = imhoff95:RadarBackscatterAndBiomass] [bibtex-entry]


  942. C.V. Jakowatz and P.A. Thompson. A new look at spotlight mode synthetic aperture radar as tomography: imaging 3-D targets. Image Processing, IEEE Transactions on, 4(5):699-703, 1995. Keyword(s): SAR Processing, SAR Tomography, Tomography, Fourier transforms, image reconstruction, radar imaging, synthetic aperture radar, tomography, 3D Fourier transform, 3D tomographic formulation, demodulated radar return data, layover, projection effect, radar target reflectivities, reconstructed 2D SAR image, spotlight mode synthetic aperture radar.
    Abstract: A new 3D tomographic formulation of spotlight mode synthetic aperture radar (SAR) is developed. This extends the pioneering work of Munson et al. (1983), who first formally described SAR in terms of tomography but who made the simplifying assumption that the target scene was 2D. The present authors treat the more general and practical case in which the radar target reflectivities comprise a 3D function. The main goal is to demonstrate that the demodulated radar return data from a spotlight mode collection represent a certain set of samples of the 3D Fourier transform of the target reflectivity function and to do so using a tomographic paradigm instead of traditional range-Doppler analysis. They also show that the tomographic approach is useful in interpreting the reconstructed 2D SAR image corresponding to a 3D scene. Specifically, the well-known SAR phenomenon of layover is easily explained in terms of tomographic projections and is shown to be analogous to the projection effect in conventional optical imaging
    [bibtex-key = jakowatzThompson95Tomo] [bibtex-entry]


  943. K. K. Knaell and G. P. Cardillo. Radar tomography for the generation of three-dimensional images. IEE Proceedings - Radar, Sonar and Navigation, 142(2):54-60, April 1995. Keyword(s): SAR Processing, SAR Tomography, TomoSAR, computerised tomography, Fourier transforms, microwave imaging, radar computing, radar cross-sections, radar imaging, radar tomography, three-dimensional images generation, computer-aided tomography, 2D cross-sectional image, radar imaging, microwave energy reflection, radar cross-section density, 1D projection, Fourier slice theorem states, backprojection algorithm, sampled transform function, 1D range profiles, point spread functions, Tomography, Fourier transforms, Microwave imaging, Radar data processing, Radar cross sections, Radar imaging.
    Abstract: Computer-aided tomography is normally a process by which a 2D cross-sectional image of an object is obtained by illuminating it from many different directions in a plane. For the case of radar imaging, microwave energy reflected by the object is processed to produce an image which maps the object's radar cross-section (RCS) density into the image plane. Each observation provides a 1D projection of the RCS density. The Fourier slice theorem states that the Fourier transform (FT) of each projection is equal to the functional value of the 2D FT of the RCS density along a related projection. By accumulating the FT of many 1D projections, it is possible to accumulate a sample representation of the FT of the RCS density. The image can then be obtained using the backprojection algorithm by taking the inverse FT of the sampled transform function. The authors extend the tomographic technique to the generation of 3D images from 1D range profiles. It is seen that the Fourier slice theorem, the backprojection image generation algorithm, and the backprojected function are useful concepts in the interpretation of 3D imagery. Point spread functions (PSFs) for various radar and observation parameters are illustrated.
    [bibtex-key = knaellCardilloIEEProcRadSonNav1995SARTomography] [bibtex-entry]


  944. Nick Marechal. Tomographic formulation of interferometric SAR for terrain elevation mapping. IEEE Transactions on Geoscience and Remote Sensing, 33(3):726-739, May 1995. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, Spotlight SAR, SAR Tomography, geophysical signal processing, geophysical techniques, radar applications, radar imaging, remote sensing by radar, synthetic aperture radar, topography (Earth)SAR image, SAR image model, baseline separation, correlation, equations, geophysical measurement technique, image pair decorrelation, interferometric SAR, phase difference, radar remote sensing, spotlight synthetic aperture radar, terrain elevation mapping, terrain mapping, tomographic formulation, tomography, topographic height error variance model.
    Abstract: Topographic mapping with spotlight synthetic aperture radar (SAR) using an interferometric technique is studied. Included is a review of the equations for determination of terrain elevation from the phase difference between a pair of SAR images formed from data collected at two differing imaging geometries. This paper builds upon the systems analysis of Li and Goldstein in which image pair decorrelation as a function of the \u201cbaseline\u201d separation between the receiving antennas was first analyzed. In this paper correlation and topographic height error variance models are developed based on a SAR image model derived from a tomographic image formation perspective. The models are general in the sense that they are constructed to analyze the case of single antenna, two-pass interferometry with arbitrary antenna line of sight, and velocity vector directions. Correlation and height error variance sensitivity to SAR system parameters and terrain gradients are studied
    [bibtex-key = marechal1995:TomoFormulationForInSAR] [bibtex-entry]


  945. Didier Massonnet, Pierre Briole, and Alain Arnaud. Deflation of Mount Etna monitored by spaceborne radar. Nature, 375(6532):567-570, June 1995. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Deformation Mapping, Deformation Monitoring, ERS-1, Displacement, Surface Displacement, Surface Deformation, Volcano Monitoring, Etna, Mount Etna, Deflation, Spaceborne SAR, C-band.
    Abstract: Ground-based measurements of volcano deformation can be used to assess eruptive hazard, but require the costly (and often hazardous) installation and maintenance of an instrument network. Here we show that spaceborne radar interferometry, which has already shown its utility in mapping earthquake-related deformation1, can be used to monitor long-term volcano deformation. Two families of synthetic aperture radar images, acquired from ascending and descending orbits by the satellite ERS-1, and looking at Mount Etna from opposite sides, cover the time period from 17 May 1992 to 24 October 1993, and include the second half of Etna's most recent eruption. Despite artefacts of the interferometric technique, we can observe a volcano-wide deflation, which is an expected consequence of the eruption, but which had not previously been appreciated. We quantify it using a simple model based on the change of pressure in a sphere located in an elastic half-space; the modelled deformation increases linearly with time until the end of the eruption. Our results show that it will be possible to use this technique to detect the inflation of volcanic edifices that usually precedes eruptions.
    [bibtex-key = massonnetBrioleArnaudNature1995DInSAREtnaVolcanoDeflation] [bibtex-entry]


  946. J. Moreira, M. Schwabisch, G. Fornaro, R. Lanari, R. Bamler, D. Just, U. Steinbrecher, H. Breit, M. Eineder, G. Franceschetti, D. Geudtner, and H. Rinkel. X-SAR interferometry: first results. IEEE Transactions on Geoscience and Remote Sensing, 33(4):950 -956, July 1995. Keyword(s): SAR Processing, InSAR, SAR interferometry, SIR-C, Space Shuttle, X-SAR, X-band SHF microwave, digital elevation model, geodesy, geophysical measurement technique, ground range geometry, land surface topography, radar remote sensing, repeat pass interferometry, spaceborne radar, synthetic aperture radar, terrain mapping, geophysical techniques, radar applications, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth).
    Abstract: Repeat-pass interferometry data were acquired during the first and second SIR-C/X-SAR missions in April and October 1994. This paper presents the first results from X-SAR interferometry at four different sites. The temporal separations were one day and six months. At two sites the coherence requirements were met, resulting in high quality interferograms. A digital elevation model in ground range geometry has been derived. The limitations of the X-SAR interferometry are discussed
    [bibtex-key = 406681] [bibtex-entry]


  947. M. Soumekh. Reconnaissance with ultra wideband UHF synthetic aperture radar. IEEE Signal Processing Magazine, 12(4):21-40, July 1995.
    Abstract: The author addresses the problem of detecting and identifying stationary and moving targets with foliage penetrating UHF synthetic aperture radar (SAR). The role of a target's coherent SAR signature, which varies with the radar's frequency and aspect angle, in forming the Fourier space of the SAR signal is analyzed. The resultant relationship is the basis of an algorithm which, after extracting (digital spotlighting) the target's coherent SAR signature in the reconstruction domain, could be used to differentiate man-made structures from foliage. Methods for blind-velocity moving target indication are discussed. The main tool of the work is a signal theory based analysis of SAR signal via Fourier transform. However, the theory is at most as good as the collected SAR data.
    [bibtex-key = soumekhIEEESigProcMag1995ReconnaissanceWithUltraWidebandUHFSAR] [bibtex-entry]


  948. H. C. Stankwitz, R. J. Dallaire, and J. R. Fienup. Nonlinear apodization for sidelobe control in SAR imagery. IEEE Transactions on Aerospace and Electronic Systems, 31(1):267-279, January 1995. Keyword(s): SAR Processing, Apodization, Nonlinear Apodization, Dual Apodization, Complex Dual Apodization, Multiapodization, Spatially Variant Apodization, DA, CDA, SVA, Fourier transform, SAR imagery, antenna design, complex-valued SAR imagery, digital filtering, mainlobe resolution, nonlinear apodization techniques, sequential nonlinear operations, ships. sidelobe control, sonar, spectral weighting, synthetic aperture radar imagery, Nyquist diagrams, fast Fourier transforms, frequency-domain analysis, synthetic aperture radars.
    Abstract: Synthetic aperture radar (SAR) imagery often requires sidelobe control, or apodization, via weighting of the frequency domain aperture. This is of particular importance when imaging scenes containing objects such as ships or buildings having very large radar cross sections. Sidelobe improvement using spectral weighting is invariably at the expense of mainlobe resolution presented here is a class of nonlinear operators which significantly reduce sidelobe levels without degrading mainlobe resolution implementation is via sequential nonlinear operations applied to complex-valued (undetected) SAR imagery. SAR imaging is used to motivate the concepts developed in this work. However, these nonlinear apodization techniques have potentially broad and far-ranging applications in antenna design, sonar, digital filtering etc., i.e., whenever data can be represented as the Fourier transform of a finite-aperture signal.<>
    [bibtex-key = stankwitzDallaireFienup1995:NonLinearApodization] [bibtex-entry]


  949. Petre Stoica and M. Cedervall. On LP-MUSIC. IEEE Transactions on Signal Processing, 43(2):552-555, February 1995. Keyword(s): SAR Processing, MUSIC, SAR Tomography, frequency estimation, noise, prediction theory, signal processing, LP-MUSIC, consistency properties, frequency estimates, linear prediction algorithm, linear prediction-MUSIC, noisy data, signal subspace, spatial frequency estimation, temporal frequency estimation.
    Abstract: Studies the consistency properties of a method recently proposed for temporal or spatial frequency estimation from noisy data. The method in question is a MUSIC technique that makes use of a linear prediction algorithm to determine the signal subspace. It is shown that the signal subspace determined by the subject linear prediction-MUSIC (LP-MUSIC) algorithm can collapse in certain scenarios and. Hence, that the LP-MUSIC frequency estimates are not always consistent. The difficulties LP-MUSIC may encounter in some cases are illustrated by means of numerical examples
    [bibtex-key = stoicaCedervall1995:LPMUSIC] [bibtex-entry]


  950. T. M. Calloway and G. W. Donohoe. Subaperture autofocus for synthetic aperture radar. IEEE Transactions on Aerospace and Electronic Systems, 30(2):617-621, 1994. Keyword(s): SAR Processing, Autofocus.
    Abstract: A subaperture autofocus algorithm for synthetic aperture radar (SAR) partitions range-compressed phase-history data collected over a full aperture into equal-width subapertures. Application of a one-dimensional Fourier transform to each range bin converts each subaperture data set into a full-scene image (map). Any linear phase difference, or phase ramp, between a pair of subapertures expresses itself as cross-range drift in their maps. A traditional autofocus algorithm fits a polynomial to inferred equal-width phase ramps. If the true phase error function contains significant high-order components, then polynomial regression generates a poor estimate of the phase error function. Instead of filling a polynomial, we fit a sinusoidal function through the inferred phase ramps. An example with a degraded SAR image shows how a sinusoidal correction improves image quality. We compare lower bounds on mean squared error (MSE) for polynomial and sinusoidal parameterizations. Sinusoidal parameterization reduces MSE significantly for model orders greater than five.
    [bibtex-key = callowayDonohoeTAES1994SubapertureAutofocusForSyntheticAperureRadar] [bibtex-entry]


  951. Giorgio Franceschetti, Maurizio Migliaccio, and Daniele Riccio. SAR Raw Signal Simulation of Actual Ground Sites Described in Terms of Sparse Input Data. IEEE Transactions on Geoscience and Remote Sensing, 32(6):1160-1169, November 1994. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator, Fractals, Sparse Input Data, SARAS.
    Abstract: Deals with the simulation of synthetic aperture radar (SAR) raw signal of actual ground sites described in terms of sparse input data. Since in most cases the input data reference system does not match SAR requirements, it is necessary to adopt appropriate interpolation schemes. The authors focus their attention on elevation input data, noting that natural surfaces exhibit fractal properties. Fractal and nonfractal interpolation schemes are discussed and applied. Simulated images are shown and compared to actual examples. Subjective and objective tests validate the simulation and support the fractal-based elevation interpolation.
    [bibtex-key = francescMigliaRiccio94:Simulation] [bibtex-entry]


  952. Fabio Gatelli, Andrea Monti-Guarnieri, Francesco Parizzi, Paolo Pasquali, Claudio Prati, and Fabio Rocca. The wavenumber shift in SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing, 32(4):855-865, July 1994. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, Range Spectral Filter, geophysical techniques, remote sensing by radar, synthetic aperture radar, TINSAR, decorrelation reduction, Baseline Decorrelation, geophysical measurement technique, ground wavenumber spectra, land surface, local slope, low noise interferogram, off-nadir angle, phase unwrapping, quick-look interferogram, radar imaging, range resolution enhancement, relative shift, remote sensing, spectral shift, terrain mapping, wavenumber shift.
    Abstract: SAR surveys from separate passes show relative shifts of the ground wavenumber spectra that depend on the local slope and the off-nadir angle. The authors discuss the exploitation of this spectral shift for different applications: 1) generation of \u201clow noise\u201d interferograms benefiting phase unwrapping, 2) generation of quick-look interferograms, 3) decorrelation reduction by means of tunable SAR systems (TINSAR), 4) range resolution enhancement, and 5) the combination of SAR data gathered by different platforms (airborne and satellite) for a \u201clong-time coherence\u201d study
    [bibtex-key = gatelliMontiGuarnieriParizziPasqualiPratiRocca1994:InSARWavenumberShift] [bibtex-entry]


  953. Dieter Just and Richard Bamler. Phase Statistics of Interferograms with Applications to Synthetic Aperture Radar. Applied Optics, 33(20):4361-4368, July 1994. Keyword(s): SAR Processing, InSAR, Interferometry, SAR Interferometry, Phase Statistics, Decorrelation, Aberrations, Wavenumber Shift, Coregistration.
    Abstract: Interferometric methods are well established in optics and radio astronomy. In recent years, interferometric concepts have been applied successfully to synthetic aperture radar (SAR) and have opened up new possibilities in the area of earth remote sensing. However interferometric SAR applications require thorough phase control through the imaging process. The phase accuracy of SAR images is affected by decorrelation effects between the individual surveys. We analyze quantitatively the influence of decorrelation on the phase statistics of SAR interferograms. In particular, phase aberrations as they occur in typical SAR processors are studied in detail. The dependence of the resulting phase bias and variance on processor parameters is presented in several diagrams.
    [bibtex-key = justBamler94] [bibtex-entry]


  954. J. S. Lee, M. R. Grunes, and Ron Kwok. Classification of multi-look polarimetric SAR imagery based on complex Wishart distribution. International Journal of Remote Sensing, 15(11):2299-2311, 1994.
    Abstract: Multi-look polarimetric SAR (synthetic aperture radar) data can be represented either in Mueller matrix form or in complex covariance matrix form. The latter has a complex Wishart distribution. A maximum likelihood classifier to segment polarimetric SAR data according to terrain types has been developed based on the Wishart distribution. This algorithm can also be applied to multifrequency multi-look polarimetric SAR data, as well as 10 SAR data containing only intensity information. A procedure is then developed for unsupervised classification. The classification error is assessed by using Monte Carlo simulation of multilook polarimetric SAR data, owing to the lack of ground truth for each pixel. Comparisons of classification errors using the training sets and single-look data are also made. Applications of this algorithm are demonstrated with NASA/JPL P-, L- and C-band polarimetric SAR data.
    [bibtex-key = leeGrunesKwokIJRS1994ClassificationMLPolarimetricSARImagery] [bibtex-entry]


  955. Jong-Sen Lee, K. W. Hoppel, S. A. Mango, and A. R. Miller. Intensity and phase statistics of multilook polarimetric and interferometric SAR imagery. IEEE Trans. Geosci. Remote Sens., 32(5):1017-1028, September 1994. Keyword(s): feature extraction, geophysical techniques, geophysics computing, image coding, image recognition, remote sensing by radar, synthetic aperture radar, complex correlation coefficient, data compression, decorrelation effects, feature classification, feature extraction, geophysical measurement technique, image classification, intensity statistics, interferometric SAR imagery, land surface imaging, multilook phase difference, multilook polarimetry, phase statistics, probability density function, radar remote sensing, scattering matrix, signal processing, speckle reduction, synthetic aperture radar, Covariance matrix, Decorrelation, Density functional theory, NASA, Phase measurement, Radar polarimetry, Radar scattering, Sea measurements, Speckle, Statistics.
    Abstract: Polarimetric and interferometric SAR data are frequently multilook processed for speckle reduction and data compression. The statistical characteristics of multilook data are quite different from those of single-look data. The authors investigate the statistics of their intensity and phase. Probability density function (PDF's) of the multilook phase difference, magnitude of complex product, and intensity and amplitude ratios between two components of the scattering matrix are derived, and expressed in closed forms. The PDF's depend on the complex correlation coefficient and the number of looks. Comparisons of these theoretically derived PDF's are made to measurements from NASA/JPL AIRSAR data. The results of this paper can be applied to feature classification using polarimetric SAR and to the estimation of decorrelation effects of the interferometric SAR
    [bibtex-key = leeHoppelMangoMillerTGRS1994IntensityAndPhaseStatisticsMLPolarimetricAndInSARImagery] [bibtex-entry]


  956. V.I. Lytle and K.C. Jezek. Dielectric permittivity and scattering measurements of Greenland firn at 26.5-40 GHz. IEEE Transactions on Geoscience and Remote Sensing, 32(2):290-295, March 1994. Keyword(s): Radar, Radar Remote Sensing, Microwave Remote Sensing, Dielectric permittivity, scattering measurements, Greenland firn, firn, Greenland, 26.5-40 GHz, Ka-band, snow, remote sensing of snow.
    Abstract: Three blocks of snow approximately 33 cm on each side were harvested from the Greenland ice sheet at depths ranging from 0.3 to 2.7 m below the surface and brought back to the lab. A step frequency radar operated from 26.5 to 40.0 GHz was used to measure the wave velocity through the snow in three orthogonal directions. From these measurements, the relative permittivity was calculated; it varied with density from 1.69 to 1.91 and agreed well with other measurements on terrestrial snow. It was also found that the permittivity of the two deeper firn blocks was anisotropic by 4-7%. This anisotropy is explained by the elongation of the snow grains in the vertical direction. Extinction losses of about 5 dB/m were estimated by measuring the change in amplitude of signals propagating through the blocks. Although the estimated losses were highly variable, a minimum penetration depth of 87 cm into the firn was computed.<>
    [bibtex-key = lytleJezekTGRS1994DielectricPermittivityAndScatteringMeasurementsOfGreenlandFirnAtKaBand] [bibtex-entry]


  957. V.K. Madisetti. A fast spotlight-mode synthetic aperture radar imaging system. IEEE Transactions on Communications, 42(234):873-876, February 1994. Keyword(s): SAR Processing, Back-Projection, Tomography, Radon Transform, Spotlight SAR.
    Abstract: The author reformulates the standard spotlight mode synthetic aperture radar (SAR) problem as a ( tau;,p) Radon transform. This results in a new algorithm for SAR that uses a linear chirp FM source signal that is a function of the geometry between the source and the object. While the close relation between computer-aided-tomography (CAT) and SAR has been understood for over a decade, no accompanying computational advantages could be realized. Recently, Kelley and Madisetti (1991), have proposed a new approach to image reconstruction from projections using the so-called fast Radon transform (FRT). He derives, in the ( tau;,p) domain, a FRT-based solution to the SAR problem. The FRT offers implicit interpolation and parallel processing advantages not found in conventional back projection operations. He proposes its efficient application in the computationally intensive problems in telecommunications and remote sensing, especially in military applications
    [bibtex-key = 580192] [bibtex-entry]


  958. Didier Massonnet, F. Adragna, and M. Rossi. CNES general-purpose SAR correlator. IEEE Trans. Geosci. Remote Sens., 32(3):636-643, 1994. Keyword(s): SAR Processing, Presumming, geophysical techniques, geophysics computing, image processing, remote sensing, remote sensing by radar, synthetic aperture radar, CNES general-purpose SAR correlator, constant phase reference, contrast seeker, data presumming, fast Fourier transform, frequency azimuth frequency domain, geophysical measurement technique, interferometry, land surface, migration compensation, multilooking, polynomial law generator, quick look, radar remote sensing, radiometric equalization map, signal processing, software, standard single look complex product, terrain mapping.
    Abstract: CNES has designed a new SAR correlator to process both airborne
    [bibtex-key = massonnetAdragnaRossi94:Presumming] [bibtex-entry]


  959. Didier Massonnet, Kurt Feigl, Marc Rossi, and Frederic Adragna. Radar interferometric mapping of deformation in the year after the Landers earthquake. Nature, 369(6477):227-230, May 1994. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Deformation Mapping, Deformation Monitoring, ERS-1, Displacement, Surface Displacement, Surface Deformation, Spaceborne SAR, C-band, Earthquake, Landers earthquake, Fault Slip.
    Abstract: Altough the 1992 Landers, California, earthquake sequence occurred in an area well sampled by geodetic networks, the postseismic deformation in the months following the earthquake has been measured at only 15 geodetic stations. Another shortcoming in the geodetic coverage occurs west of the primary rupture, where the existing geodetic observations suggest, but cannot resolve, sympathetic slip on secondary faults. Such measurements, which are needed to place the Landers earthquake sequence in the context of a recurring seismic cycle in California, can be obtained with the dense spatial coverage provided by satellite radar interferometry. Here we present radar maps of the surface deformation field which reveal features that would otherwise have been poorly sampled, particularly if the earthquake had occurred in a less accessible area. We see triggered slip at the level of several centimetres as far as 100 km from the primary rupture, and can resolve the geodetic signal of at least one small (magnitude 5) aftershock. The amount of surface slip following the main shock is less than a decimetre, and is consistent with an exponential decay time of several months for the postseismic deformation.
    [bibtex-key = massonnetFeiglRossiAdragnaNature1994DINSARLandersEQuake] [bibtex-entry]


  960. Christian Matzler. Microwave (1-100 GHz) dielectric model of leaves. IEEE Transactions on Geoscience and Remote Sensing, 32(4):947-949, 1994. [bibtex-key = Matzler1994] [bibtex-entry]


  961. Alberto Moreira and Yonghong Huang. Airborne SAR Processing of Highly Squinted Data Using a Chirp Scaling Approach with Integrated Motion Compensation. IEEE Transactions on Geoscience and Remote Sensing, 32(5):1029-1040, September 1994. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, Motion Compensation, Motion Estimation, Squinted SAR, Cubic Phase Term, Airborne SAR, Automatic Azimuth Coregistration, Azimuth Scaling, Interferometry, Phase-Preserving Processing, Range Scaling Formulation, Stripmap SAR, Subaperture Processing, Terrain Mapping.
    Abstract: Proposes a new approach for high-resolution airborne SAR data processing, which uses a modified chirp scaling algorithm to accommodate the correction of motion errors, as well as the variations of the Doppler centroid in range and azimuth. By introducing a cubic phase term in the chirp scaling phase, data acquired with a squint angle up to 30? can be processed with no degradation of the impulse response function. The proposed approach is computationally very efficient, since it accommodates the variations of Doppler centroid without using block processing. Furthermore, a motion error extraction algorithm can be incorporated into the proposed approach by means of subaperture processing in azimuth. The new approach, denoted as extended chirp scaling, is considered to be a generalized algorithm suitable for the high-resolution processing of most airborne SAR systems.
    [bibtex-key = moreiraHuang94:ChirpScaling] [bibtex-entry]


  962. Shaun Quegan. A unified algorithm for phase and cross-talk calibration of polarimetric data-theory and observations. IEEE Trans. Geosci. Remote Sens., 32(1):89-99, January 1994. Keyword(s): SAR Processing, Polarimetry, Polarimetric Calibration, calibration, geophysical techniques, polarimetry, remote sensing, remote sensing by radar, MAESTRO campaign, agriculture, cross-polarized backscattering coefficient, cross-talk calibration, geophysical measurement technique, land surface SAR radar imaging, linear distortion model, minimum least squares, observation, phase, polarimetry, polarisation, scattering matrix, synthetic aperture radar, theory, unified algorithm, Backscatter, Calibration, Covariance matrix, Crosstalk, Data mining, Equations, Layout, Phase distortion, Scattering, System testing.
    Abstract: A unified approach to phase and cross-talk calibration of polarimetric data which can be applied to calibrating scattering matrix data or to extraction of the descriptors of distributed targets is described. It relies on the scene being dominated by targets with uncorrelated like and cross-polarized backscattering coefficients, but provides cross-talk calibration of targets for which this is not true. The algorithm needs unsymmetrized data, but uses only quantities derived from the covariance matrix of large areas. It makes no assumptions about system reciprocity, permits ready interpretation of the terms in the calibration procedure, allows comparison of the relative magnitude of the system-induced mixing of terms in the observed covariance matrix, is noniterative, and produces indicators which allow testing of whether it meets its own underlying assumptions. The linear distortion model is shown to lead to an inconsistent system of equations; this inconsistency can be removed by introducing an extra parameter which has properties expected of system noise. The modulus of the copolarized correlation coefficient, which is important in polarimetric classification and as a phase descriptor, is shown to be invariant under all effects embodied in the linear distortion model. Calibration of the scattering matrix data is based on a minimum least squares principle. This suggests that current methods of symmetrization are not optimal. The same analysis shows that estimates of parameters needed to form an equivalent reciprocal system are also nonoptimal. The method is more general than the well-known van Zyl algorithm for cross-talk removal, and permits an analysis of the conditions under which the van Zyl algorithm will yield valid results. Correction of phase distortion induced by channel imbalance Is treated as an optional extra step relying on a known HH-VV phase difference in some region of the image. Results from the algorithm are discussed using scattering matrix data from the 1989 MAESTRO campaign
    [bibtex-key = queganTGRS1994PolCalibration] [bibtex-entry]


  963. R. Keith Raney, Hartmut Runge, Richard Bamler, Ian G. Cumming, and Frank Wong. Precision SAR Processing Using Chirp Scaling. IEEE Transactions on Geoscience and Remote Sensing, 32(4):786-799, July 1994. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Phase Preserving, Range-Doppler Algorithm, Comparison of Algorithms.
    Abstract: A space-variant interpolation is required to compensate for the migration of signal energy through range resolution cells when processing synthetic aperture radar (SAR) data, using either the classical range/Doppler (R/D) algorithm or related frequency domain techniques. In general, interpolation requires significant computation time, and leads to loss of image quality, especially in the complex image. The new chirp scaling algorithm avoids interpolation, yet performs range cell migration correction accurately. The algorithm requires only complex multiplies and Fourier transforms to implement, is inherently phase preserving, and is suitable for wide-swath, large-beamwidth, and large-squint applications. This paper describes the chirp scaling algorithm, summarizes simulation results, presents imagery processed with the algorithm, and reviews quantitative measures of its performance. Based on quantitative comparison, the chirp scaling algorithm provides image quality equal to or better than the precision range/Doppler processor. Over the range of parameters tested, image quality results approach the theoretical limit, as defined by the system bandwidth.
    [bibtex-key = RaneyRunBamCummWong94:Precision] [bibtex-entry]


  964. Andrew Reilly, Gordon Frazer, and Boualem Boashash. Analytic Signal Generation-Tips and Traps. Signal Processing, IEEE Transactions on [see also Acoustics, Speech, and Signal Processing, IEEE Transactions on], 42(11):3241-3245, 1994. Keyword(s): Analytic Signal, Hilbert Transform, filtering theory, frequency estimation, parameter estimation, signal processing, signal synthesis, time-frequency analysis, Hilbert transform filters, analytic signal generation, discrete analytic signal, discrete real-valued signal, instantaneous-frequency estimation, modem designs, negative frequencies, positive frequencies, quadrature filters, time-frequency signal analysis.
    Abstract: In this correspondence we discuss methods to produce the discreteanalytic signal from a discrete real-valued signal. Such an analyticsignal is complex and contains only positive frequencies. Its projectiononto the real axis is the same as the original signal. Our use stemsfrom instantaneous-frequency estimation and time-frequency signalanalysis problems. For these problems the negative frequency componentof real signals causes unwanted interference. The task of designing afilter to produce an approximation to the ideal analytic signal is notas simple as its formulation might suggest. Our result is that thedirect methods of zeroing the negative frequencies, or using Hilberttransform filters, have undesirable defects. We present an alternative which is similar to the quadrature filters used in modemdesigns
    [bibtex-key = reillyFrazerBoashash94:AnalyticSignal] [bibtex-entry]


  965. Sasan S. Saatchi, D. M. Le Vine, and R. H. Lang. Microwave backscattering and emission model for grass canopies. IEEE Trans. Geosci. Remote Sens., 32(1):177-186, January 1994. Keyword(s): atmospheric techniques, atmospheric temperature, geophysical techniques, hydrological techniques, radiometry, remote sensing, remote sensing by radar, soil, temperature measurement, 1.4 GHz, 4.75 GHz, C-band, L-band, UHF SHF, distorted Born approximation, elliptical disc, emission model, emissivity, geophysical measurement technique, grass canopies, grassland, hydrology, land surface, microwave backscattering, model, radar cross section, radar remote sensing, soil moisture, thatch layer, vegetation, vegetation canopy, wet Konza prairie, Backscatter, Electromagnetic heating, L-band, Microwave measurements, Microwave radiometry, Moisture measurement, Radar cross section, Radar measurements, Soil measurements, Soil moisture.
    Abstract: Microwave radar and radiometer measurements of grasslands indicate a substantial reduction in sensor sensitivity to soil moisture in the presence of a thatch layer. When this layer is wet it masks changes in the underlying soil, making the canopy appear warm in the case of passive sensors (radiometer) and decreasing backscatter in the active case (scatterometer). A model for a grass canopy with thatch is presented in order to explain this behavior and for comparison with observations. The canopy model consists of three layers: grass, thatch, and the underlying soil. The grass blades are modeled by elongated elliptical discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneous and flat. The distorted Born approximation is used to compute the radar cross section of this three layer canopy and the emissivity is computed from the radar cross section using the Peake formulation for the passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moisture, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-band radiometer measurements at these grasslands
    [bibtex-key = saatchiLeVineLangTGRS1994] [bibtex-entry]


  966. D.E. Wahl, P.H. Eichel, D.C. Ghiglia, and C.V. Jakowatz. Phase gradient autofocus-a robust tool for high resolution SAR phase correction. IEEE Transactions on Aerospace and Electronic Systems, 30(3):827-835, July 1994. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus.
    Abstract: The phase gradient autofocus (PGA) technique for phase error correction of spotlight mde synthetic aperture radar (SAR) imagery is examined carefully in the context of four fundamental signal processing steps that constitute the algorithm We demnstrate that excellent results over a wide variety of scene content, and phase error function structure are obtained if and only if all of these steps are included in the processing. Finally, we show that the computational demands of the full PGA algorithm do not represent a large fraction of the total image formation problem, when mid to large size images are involved.
    [bibtex-key = wahlEichelGhigliaJakowatz94] [bibtex-entry]


  967. Howard A. Zebker, Paul A. Rosen, Richard M. Goldstein, Andrew Gabriel, and Charles L. Werner. On the derivation of coseismic displacement fields using differential radar interferometry: The Landers earthquake. Journal of Geophysical Research: Solid Earth, 99(B10):19617-19634, 1994. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Deformation Mapping, Deformation Monitoring, ERS-1, Displacement, Surface Displacement, Surface Deformation, Spaceborne SAR, C-band, Earthquake, Landers earthquake, Fault Slip, Remote sensing, Seismic instruments and networks, Earthquake source observations, Earthquake interaction, forecasting, and prediction.
    Abstract: We present a map of the coseismic displacement field resulting from the Landers, California, June 28, 1992, earthquake derived using data acquired from an orbiting high-resolution radar system. We achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques. Data from the ERS 1 synthetic aperture radar instrument acquired in April, July, and August 1992 are used to generate a high-resolution, wide area map of the displacements. The data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30-m resolution element in a 113 km by 90 km image. Our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake. Fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing. Comparison of these data with Global Positioning System and electronic distance measurement survey data yield a correlation of 0.96; thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format. The technique we use is (1) more automatic, (2) more precise, and (3) better validated than previous similar applications of differential radar interferometry. Since we require only remotely sensed satellite data with no additional requirements for ancillary information, the technique is well suited for global seismic monitoring and analysis.
    [bibtex-key = zebkerRosenGoldsteinGabrielWernerJGR1994DInSARLandersEQuakeCoseismicDisplacement] [bibtex-entry]


  968. S. El Assad, I. Lakkis, and J. Saillard. Holographic SAR image formation by coherent summation of impulse response derivatives. IEEE Transactions on Antennas and Propagation, 41(5):620-624, May 1993. Keyword(s): SAR Processing, SAR Tomography, Tomography, Peugeot 504 automobile, SAR, back-projected range responses, coherent summation, complex impulse responses, hologram, holographic image formation, impulse response derivatives, linear synthetic-aperture radar, monostatic backscattered field, near-field, physical optics approximation, radar image, target image, tomographic reconstruction, two-dimensional Fourier transform, wheat field, image reconstruction, microwave holography, physical optics, radar theory, synthetic aperture radar;.
    Abstract: Using the physical optics approximation, the radar image of a target can be constructed from a knowledge of the monostatic backscattered field or hologram for all frequencies and all aspects angles. The target image is the two-dimensional Fourier transform of the hologram. This is based on the same principle as conventional holography. In the near-field the image is computed by the coherent summation of back-projected range responses which are derived from complex impulse responses. Consequently, the image can be interpreted as a tomographic reconstruction. If the target is within the antenna beam at each radar position in the linear synthetic-aperture radar (SAR) geometry, then the algorithm for the coherent summation of impulse response derivatives (IRDs) can be applied. Experimental results for the near-field of a wheat field and a Peugeot 504 automobile are presented to verify the effectiveness of the method
    [bibtex-key = 222281] [bibtex-entry]


  969. Richard M. Goldstein, Hermann Engelhardt, Barclay Kamb, and Richard M. Frolich. Satellite Radar Interferometry for Monitoring Ice Sheet Motion: Application to an Antarctic Ice Stream. Science, 262(5139):1525-1530, 1993. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Displacement, Surface Displacement, glaciology, hydrological techniques, radar applications, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, Greenland, SAR method, flow, geophysical measurement technique, glaciology, hydrology, ice sheet motion.
    Abstract: Satellite radar interferometry (SRI) provides a sensitive means of monitoring the flow velocities and grounding-line positions of ice streams, which are indicators of response of the ice sheets to climatic change or internal instability. The detection limit is about 1.5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction. The grounding line, detected by tidal motions where the ice goes afloat, can be mapped at a resolution of ~0.5 kilometer. The SRI velocities and grounding line of the Rutford Ice Stream, Antarctica, agree fairly well with earlier ground-based data. The combined use of SRI and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior.
    [bibtex-key = goldsteinEngelhardtKambFrolichScience1993DInSARIceSheetMonitoring] [bibtex-entry]


  970. Charles V. Jakowatz and Daniel E. Wahl. Eigenvector method for maximum-likelihood estimation of phase errors in synthetic-aperture-radar imagery. J. Opt. Soc. Am. A, 10(12):2539-2546, 1993. Keyword(s): SAR Processing, Autofocus, Motion Compensation, MoComp, Residual Motion Errors, Airborne SAR, Phase Gradient Autofocus, PGA, Eigenvector Method, Maximum Likelihood Estimation. [bibtex-key = JakowatzJrWahl1993:Autofocus] [bibtex-entry]


  971. M. D. Macleod. Fast Interpolation by FFT with Greatly Increased Accuracy. Electronics Letters, 29(13):1200-1201, June 1993. Keyword(s): Interpolation, Interpolation by FFT, Fast Fourier Transforms, FFT, Nyquist limit, RMS error, Sampling Rate Conversion, Upsampling, Error Analysis, Block Edges.
    Abstract: When the FFT (or DCT or DST) is used for fast interpolation, errors arise due to the assumed periodicity in the data, and the resulting implied discontinuities of the function and its derivatives at the block edges. By adding simple functions before interpolation and subtracting them again afterwards, these interpolation errors can be removed.
    [bibtex-key = macleod93:Interpolation] [bibtex-entry]


  972. S.N. Madsen, H.A. Zebker, and J. Martin. Topographic mapping using radar interferometry: processing techniques. IEEE Transactions on Geoscience and Remote Sensing, 31(1):246-256, January 1993. Keyword(s): SAR Processing, InSAR, Interferometry, Interferometric SAR, Airborne SAR, cartography, geophysical techniques, remote sensing by radar, topography (Earth), NASA, JPL, TOPSAR, SAR, height maps, land surface, mapping, measurement, motion compensation, phase ambiguity, processing algorithm, radar interferometry, technique, terrain, three-dimensional target location algorithm, topographic radar mapper, topography.
    Abstract: A new processing algorithm for the NASA JPL TOPSAR topographic radar mapper is described. It incorporates extensive motion compensation features as well as accurate three-dimensional target location algorithm. The processor applies an algorithm to resolving the absolute phase ambiguity. This allows rectified height maps to be generated without any use of ground reference points. The processor was tested using data acquired with extreme aircraft motion so that performance could be evaluated under adverse conditions. The topographic maps generated by the radar were compared to digital elevation models (DEMs) derived using conventional optical stereo techniques. In one region, the RMS elevation deviations measured were less than the specified DEM accuracy, and, in the region covered by the more accurate DEM, errors varied from 2.2 m RMS in relatively flat terrain up to 5.0 m in mountainous area. The RMS difference between radar and DEM elevation over the 6.5-km by 22-km area covered by the more accurate DEM was 3.6 m
    [bibtex-key = madsenZebkerMartin1993:InSAR] [bibtex-entry]


  973. Didier Massonnet and Thierry Rabaute. Radar interferometry: limits and potential. IEEE Trans. Geosci. Remote Sens., 31(2):455-464, 1993. Keyword(s): SAR Processing, electromagnetic wave interferometry, geophysical techniques, image processing, remote sensing by radar, topography (Earth), accuracy, differential interferometry, digital terrain modeling, height restitution, image pair, image processing, orbital geometry, phase ambiguity, radar interferometry, topography, Geometrical optics, Instruments, Laser radar, Optical interferometry, Optical sensors, Radar antennas, Radar imaging, Radar interferometry, Spaceborne radar, Synthetic aperture radar.
    Abstract: The contribution of radar interferometry to the field of digital terrain modeling is important because this technique offers specific features which optical instruments cannot attain. However, the complexity of the height restitution and the accuracy of the result strongly depend on the orbital geometry at the time of the data takes. The present study aims at assessing the potential of a given image pair with regard to interferometry and at automatically reducing the phase ambiguity intrinsic to such processing. Particular applications of differential interferometry are also discussed in order to estimate their requirements and prepare future experiments
    [bibtex-key = massonnetRabauteTGRS1993InSAROverview] [bibtex-entry]


  974. Didier Massonnet, Marc Rossi, Cesar Carmona, Frederic Adragna, Gilles Peltzer, Kurt Feigl, and Thierry Rabaute. The displacement field of the Landers earthquake mapped by radar interferometry. Nature, 364(6433):138-142, July 1993. Keyword(s): SAR Processing, Interferometry, SAR interferometry, differential SAR interferometry, DInSAR, Deformation Mapping, Deformation Monitoring, ERS-1, Displacement, Surface Displacement, Surface Deformation, Spaceborne SAR, C-band, Earthquake, Landers earthquake, Fault Slip.
    Abstract: Geodetic data, obtained by ground- or space-based techniques, can be used to infer the distribution of slip on a fault that has ruptured in an earthquake. Although most geodetic techniques require a surveyed network to be in place before the earthquake, satellite images, when collected at regular intervals, can capture co-seismic displacements without advance knowledge of the earthquake's location. Synthetic aperture radar (SAR) interferometry, first introduced in 1974 for topographic mapping can also be used to detect changes in the ground surface, by removing the signal from the topography. Here we use SAR interferometry to capture the movements produced by the 1992 earthquake in Landers, California1. We construct an interferogram by combining topographic information with SAR images obtained by the ERS-1 satellite before and after the earthquake. The observed changes in range from the ground surface to the satellite agree well with the slip measured in the field, with the displacements measured by surveying, and with the results of an elastic dislocation model. As a geodetic tool, the SAR interferogram provides a denser spatial sampling (100 m per pixel) than surveying methods and a better precision (~3 cm) than previous space imaging techniques
    [bibtex-key = massonnetRossiCarmonaAdragnaPeltzerFeiglRabauteNatureLett1993LandersDINSAR] [bibtex-entry]


  975. Andrew S. Milman. SAR Imaging by Omega-K Migration. International Journal of Remote Sensing, 14(10):1965-1979, 1993. Keyword(s): SAR Processing, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Stolt Mapping, Stationary Phase Method.
    Abstract: A new method of processing synthetic aperture radar (SAR) data was originally developed by a group of geophysicists, who use seismic migration to generate seismic images of the Earth. Called here omega-k migration, it has several advantages over more traditional methods of forming SAR images; they all arise because this method provides a mathematically exact solution to the imaging problem, in that the emitted waves are not being treated as plane waves. This paper discusses how migration processing can be applied to chirped radars; how it applies to spotlight data; and how to correct for non-uniform motions of the SAR platform.
    [bibtex-key = milman93:omegak] [bibtex-entry]


  976. Claudio Prati and Fabio Rocca. Improving slant-range resolution with multiple SAR surveys. IEEE Transactions on Aerospace and Electronic Systems, 29(1):135-143, January 1993. Keyword(s): aerospace instrumentation, microwave imaging, radar systems, synthetic aperture radar, SAR interferometry, across-track resolution, data rate constraints, microwave imaging, multiple SAR surveys, multiple surveys, off-nadir angles, slant-range resolution, spaceborne synthetic aperture radar, spaceborne trajectories, Antenna measurements, Azimuth, Extraterrestrial measurements, Image resolution, Pulse measurements, Pulse modulation, Radar antennas, Signal resolution, Spaceborne radar, Synthetic aperture radar.
    Abstract: Across-track resolution of a spaceborne synthetic aperture radar (SAR) system is limited by power and data rate constraints. The authors derive and discuss a new technique for increasing the across-track resolution of objects that do not change with time, using multiple surveys of the same area from different off-nadir angles. Precise information on the spaceborne trajectories are not requested since they can be derived from SAR interferometry. Simulated data show that theoretical derivations are in good agreement with practice
    [bibtex-key = pratiRoccaTAES1993WavenumberShiftImprovedSlantRange] [bibtex-entry]


  977. Harish Subbaram and Ken Abend. Interference suppression via orthogonal projections: a performance analysis. Antennas and Propagation, IEEE Transactions on, 41(9):1187-1194, 1993. Keyword(s): SAR Processsing, RFI Suppression, orthogonal projections, subspace separation, principle component analysis, antenna phased arrays, antenna radiation patterns, array signal processing, interference suppression, jamming, adaptive jammer suppression, additive noise, average residual interference, computer simulations, jammer plus noise power, jammer snapshots, mainbeam, performance analysis, performance parameters, performance predictions, phased array antennas, sample matrix inversion algorithm, sidelobe levels, vector spaces, weight vector.
    Abstract: Several recent studies indicate the promise of subspace separation principles when applied to adaptive jammer suppression in phased arrayantennas. This paper theoretically analyzes the performance of a subspace separation technique based on orthogonal projections (OP) foradaptively suppressing interference in phased arrays; the theoretical performance predictions are validated using computer simulations. Thisanalysis holds for the case when it is possible to differentiate between the vector spaces spanned by jammers and additive noise. The performance parameters used are (a) the average residual interference (jammer plusnoise) power at the output of the adapted array as a function of the number of jammer snapshots used for calculating the weight vector, and (b) the similarity of the adapted array pattern to the design pattern away from the jammer locations. The performance of the OP-based subspace separation technique is compared with the sample matrix inversion (SMI) algorithm. It is shown that the weight vector calculated using OP converges more quickly to the optimal solution (infinite number of interference snapshots) than the SMI weight vector. Further, in contrast to the SMI adapted pattern, which exhibits large sidelobe levels away from the jammer locations, the OP adapted pattern closely follows the design pattern both in the main beam and in the sidelobe region away from the jammer locations.
    [bibtex-key = subbaramAbend93:RFI] [bibtex-entry]


  978. D.E. Wahl. Towed array shape estimation using frequency-wavenumber data. IEEE Journal of Oceanic Engineering, 18(4):582-590, October 1993. Keyword(s): Frequency estimation, Sensor arrays, Signal processing algorithms, Array signal processing, Shape measurement, Sea measurements, Degradation, Maximum likelihood estimation, Acoustic sensors, Sensor systems.
    Abstract: Towed array beamforming algorithms require accurate array shape information in order to perform properly. Very often, these algorithms assume the array is linear. Unfortunately, the mechanical forces on the array due to ship motion and sea dynamics can change the shape of the array, which degrades the performance of the beamforming algorithm. A data-driven approach to estimating the relative shape of a nominally linear array is presented. The algorithm is robust in that it optimally combines information contained in a wide band of frequencies and source bearings. At the heart of the algorithm is a maximum-likelihood (ML) estimation scheme. The Cramer-Rao lower bound is derived and compared to the performance of the ML estimator. The utility of the algorithm is verified using both simulated and actual towed array data experiments.<>
    [bibtex-key = wahl1993TowedArrayShapeEstimationUsingFrequencyWavenumberData] [bibtex-entry]


  979. Jakob van Zyl, Bruce D. Chapman, Pascale Dubois, and Jiancheng Shi. The effect of topography on SAR calibration. IEEE Trans. Geosci. Remote Sens., 31(5):1036-1043, September 1993. Keyword(s): SAR Processing, Radiometric Normalization, AIRSAR, Austria, Austrian Alps, C-band ERS-1 spaceborne radar system, NASA/JPL airborne SAR system, Oetztal, SAR calibration, Tombstone, United States, airborne systems, antenna pattern removal, calibration errors, digital elevation model, high relief area, local slopes, moderate relief area, radiometric corrections, scattering area removal, spaceborne SAR, synthetic aperture radar, topographic variations, calibration, geophysical techniques, remote sensing by radar, synthetic aperture radar, topography (Earth).
    Abstract: During normal synthetic aperture radar (SAR) processing, a flat Earth is assumed when performing radiometric corrections such as antenna pattern and scattering area removal. The authors examine the effects of topographic variations on these corrections. Local slopes will cause the actual scattering area to be different from that calculated using the flat Earth assumption. It is shown that this effect may easily cause calibration errors larger than a decibel. Ignoring the topography during antenna pattern removal may also introduce errors of several decibels in the case of airborne systems. The effect of topography on antenna pattern removal is expected to be negligible for spaceborne SARs. The authors show how these effects can be taken into account if a digital elevation model is available for the imaged area. The errors are quantified for two different types of terrain, a moderate relief area near Tombstone, AZ, and a high relief area near Oetztal in the Austrian Alps. The authors show errors for two well-known radar systems, the C-band ERS-1 spaceborne radar system and the three frequency NASA/JPL airborne SAR system (AIRSAR)
    [bibtex-key = vanZylChapmanDuboisShi1993:RadiometricNormalization] [bibtex-entry]


  980. Richard Bamler. A Comparison of Range-Doppler and Wavenumber Domain SAR Focusing Algorithms. IEEE Transactions on Geoscience and Remote Sensing, 30(4):706-713, July 1992. Keyword(s): SAR Processing, Range-Doppler Algorithm, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Secondary Range Compression, Comparison of Algorithms.
    Abstract: Focusing of SAR data requires a space-variant two-dimensional correlation. Different algorithms are compared with each other in terms of their focusing quality and their ability to handle the space-variance of the correlation kernel: the range-Doppler approach with and without secondary range compression, modified range-Doppler algorithms, and four versions of the wavenumber domain processor. The phase aberrations of the different algorithms are given in analytic form. Numerical examples are presented for Seasat and ERS-1. A novel systems theoretical derivation of the wavenumber domain algorithm is presented.
    [bibtex-key = bamler92:Comparison] [bibtex-entry]


  981. C. Y. Chang and John C. Curlander. Application of the Multiple PRF Technique to Resolve Doppler Centroid Estimation Ambiguity for Spaceborne SAR. IEEE Transactions on Geoscience and Remote Sensing, 30(5):941-949, September 1992. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Multiple PRF Technique, Range Cross-Correlation Technique, Clutterlock, SIR-C.
    Abstract: Estimation of the Doppler centroid ambiguity is a necessary element of the signal processing for synthetic aperture radar (SAR) systems with large antenna pointing errors. Without proper resolution of the Doppler centroid estimation (DCE) ambiguity, the image quality will be degraded in the system impulse response function and the geometric fidelity. Two techniques for resolution of DCE ambiguity for the spaceborne SAR are presented in this paper. They include a brief review of the range cross-correlation technique and presentation of a new technique using multiple pulse repetition frequencies (PRFs). We formulate an algorithm employing simple integer arithmetic for radar systems, such as moving target indicator radar (MTIR) systems, where the PRFs contain a large common divisor. For SAR systems, where other performance factors control selection of the PRFs, an algorithm is devised to resolve the ambiguity that uses PRFs of arbitrary numerical values. The performance of this multiple PRF technique is analyzed based on a statistical error model. An example is presented that demonstrates for the Shuttle Imaging Radar-C (SIR-C) C-band SAR, the probability of correct ambiguity resolution is higher than 95 percent for antenna attitude errors as large as 3?.
    [bibtex-key = ChangCurl92:Doppler] [bibtex-entry]


  982. C. Y. Chang, M. Jin, and John C. Curlander. SAR Processing Based on the Exact Two-Dimensional Transfer Function. IGARSS '92, International Geoscience and Remote Sensing Symposium, pp 355-359, May 1992. Keyword(s): SAR Processing, Azimuth Processing, Range-Doppler Algorithm, Range Migration Algorithm, Wavenumber Domain Algorithm, omega-k, Point Target Transfer Funtion, Two-Dimensional Fourier Transform, Doppler Centroid, Doppler Rate Estimation, Autofocus.
    Abstract: The two-dimensional transfer funtions of several synthetic aperture radar (SAR) focussing algorithms are derived considering the spaceborne SAR environments. The formulation includes the factors of the earth rotation and the antenna squint angles. The resultant functions are explicitly expressed in terms of Doppler centroid frequency and Doppler frequency rate, which can be conveniently and accurately estimated from the SAR data. Point target simulation results show that the algorithm based on the two-dimensional Fourier transformation out-performs the one-dimensional one for processing data acquired from high squint angles. The two-dimensional Fourier transformation approach appears to be a viable and simple solution for the processor design of future spceborne SAR systems.
    [bibtex-key = Cur92:SARProc] [bibtex-entry]


  983. Jorgen Dall. A Fast Autofocus Algorithm for Synthetic Aperture Radar Processing. IEEE International Conference on Acoustics, Speech, and Signal Processing ICASSP, 3:5-8, September 1992. Keyword(s): SAR Processing, Doppler Rate Estimation, Autofocus, Airborne SAR.
    Abstract: High-resolution synthetic aperture radar (SAR) imaging requires the motion of the radar platform to be known very accurately. Otherwise, phase errors are induced in the processing of the raw SAR data, and bad focusing results. In particular, a constant error in the measured along-track velocity or the cross-track acceleration leads to a phase error that varies quadratically over the synthetic aperture. The process of estimating this quadratic phase error directly from the radar data is termed autofocus. A novel autofocus algorithm with a computational complexity which is at least an order of magnitude lower than that of other algorithms providing comparable accuracies is presented. The algorithm has been tested on data from the Danish Airborne SAR, and the performance is compared with that of the traditional map drift algorithm.
    [bibtex-key = Dal92:Fast] [bibtex-entry]


  984. Mita D. Desai and W. Kenneth Jenkins. Convolution Backprojection Image Reconstruction for Spotlight Mode Synthetic Aperture Radar. IEEE Transactions on Image Processing, 1(4):505 - 517, October 1992. Keyword(s): SAR Processing, Spotlight SAR, Back-Projection, Convolution Back-Projection.
    Abstract: Synthetic aperture radar (SAR) image reconstruction falls into the class of inverse (deconvolution) problems. A spotlight mode SAR system obtains line integrals (projections) of the ground reflectivity at various look angles as the radar platform progresses along the flight trajectory. The image of the ground area is then reconstructed from this set of projections. Conventionally, the SAR image has been produced by a direct Fourier reconstruction algorithm referred to here as the 2-D inverse FFT method. This method has two major problems: 1) due to the batch processing nature of the FFT, all returns must be recorded before the image processing can begin, and 2) a polar-to-cartesian interpolation, which is computationally intensive and error prone due to interpolation inaccuracies, is necessary before a 2-D inverse FFT can be performed. In this paper, a method based on a convolution backprojection (CBP) algorithm is presented. CBP is a widely used technique in computer-aided tomography (CAT). The CBP algorithm has been modified and applied to image reconstruction from SAR data. A quantitative evaluation using computer simulation of the CBP algorithm for spotlight mode SAR is presented. Its performance is then compared with the 2-D inverse FFT method with respect to the multiplicative noise ratio (MNR). Conclusions are supported by a reconstruction example on real SAR data collected by the Lincoln Laboratory's high resolution (0.3 m) radar.
    [bibtex-key = DesaiJenkins92:Backprojection] [bibtex-entry]


  985. M. C. Dobson, F. T. Ulaby, T. LeToan, A. Beaudoin, E. S. Kasischke, and N. Christensen. Dependence of radar backscatter on coniferous forest biomass. IEEE Trans. Geosci. Remote Sens., 30(2):412-415, March 1992. Keyword(s): SAR Processing, Forest, Forest parameters, biomass, C-band, Duke, France, L-band, Landes, North Carolina, P-band, SAR data, United States, age, coniferous forest biomass, maritime pines, plantations, radar backscatter, remote sensing, backscatter, ecology, forestry, remote sensing by radar.
    Abstract: Two independent experimental efforts have examined the dependence of radar backscatter on above-ground biomass of monospecie conifer forests using polarimetric airborne SAR data at P-, L- and C-bands. Plantations of maritime pines near Landes, France, range in age from 8 to 46 years with above-ground biomass between 5 and 105 tons/ha. Loblolly pine stands established on abandoned agricultural fields near Duke, NC, range in age from 4 to 90 years and extend the range of above-ground biomass to 560 tons/ha for the older stands. These two experimental forests are largely complementary with respect to biomass. Radar backscatter is found to increase approximately linearly with increasing biomass until it saturates at a biomass level that depends on the radar frequency. The biomass saturation level is about 200 tons/ha at P-band and 100 tons/ha at L-band, and the C-band backscattering coefficient shows much less sensitivity to total above-ground biomass
    [bibtex-key = DobsonUlabyLeToanBeaudoinKasischkeChristensen1992:BiomassBackscatter] [bibtex-entry]


  986. Duane T Eppler, L Dennis Farmer, Alan W Lohanick, Mark R Anderson, Donald J Cavalieri, Josefino Comiso, Per Gloersen, Caren Garrity, Thomas C Grenfell, Martti Hallikainen, and others. Passive microwave signatures of sea ice. Microwave remote sensing of sea ice, pp 47-71, 1992. [bibtex-key = Eppler1992] [bibtex-entry]


  987. Giorgio Franceschetti, Maurizio Migliaccio, Daniele Riccio, and Gilda Schirinzi. SARAS: A Synthetic Aperture Radar (SAR) Raw Signal Simulator. IEEE Transactions on Geoscience and Remote Sensing, 30(1):110-123, January 1992. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator.
    Abstract: A SAR simulator of an extended three-dimensional scene is presented. It is based on a facet model for the scene, asymptotic evaluation of SAR unit response, and a two-dimensional fast Fourier transform code for the data processing. Prescribed statistics of the model account for a realistic speckle of the image. The simulator is implemented in Synthetic Aperture Radar Advance Simulators (SARAS), whose performance is described and illustrated by a number of examples.
    [bibtex-key = FMRS92:SARAS] [bibtex-entry]


  988. A. Freeman. SAR calibration: an overview. IEEE Trans. Geosci. Remote Sens., 30(6):1107-1121, November 1992. Keyword(s): calibration, polarimetry, radiometry, reviews, synthetic aperture radar, airborne data, Earth surface, synthetic-aperture radar, spaceborne SAR image data, radar backscatter, image quality, radiometric calibration, polarimetric radar calibration, phase calibration, interferometric SAR, Calibration, Spaceborne radar, Synthetic aperture radar, Radar imaging, Radar polarimetry, Radar measurements, Backscatter, Image quality, Radiometry, Equations.
    Abstract: Progress in synthetic-aperture radar, (SAR) calibration is reviewed. The difficulties of calibrating both airborne and spaceborne SAR image data are addressed. The quantities measured by a SAR, i.e. radar backscatter, are defined and mathematical formulations for the three basic types of SAR image are developed. The difficulties in establishing science requirements for calibration are discussed. The measurement of SAR image quality is briefly addressed. The problem of radiometric calibration is introduced via the SAR form of the radar equation, with both internal and external calibration approaches considered. The development of algorithms for polarimetric radar calibration is reviewed and the problems involved in phase calibration of interferometric SAR are discussed. Future challenges in the field of SAR calibration are considered.<>
    [bibtex-key = freemanTGRS1992SARCalibrationOverview] [bibtex-entry]


  989. Ute Christina Herzfeld. Least-squares collocation, geophysical inverse theory and geostatistics: a bird's eye view. Geophysical Journal International, 111(2):237-249, 11 1992.
    Abstract: Geodetical least-squares prediction and collocation, geophysical inverse theory, and geostatistics (the theory of regionalized variables) provide methods that from a theoretical point of view solve a common problem: a continuously defined spatial model is supported by only finitely many observations. The synopsis focuses on common methodological aspects as well as on differences in the underlying mathematical assumptions and their geological implications. Introductory examples familiarize with approaches specific to three earth science applications (least-squares collocation of gravity anomalies, geostatistical estimation of mineral resources, and spectral interpolation of marine magnetic anomalies).
    [bibtex-key = herzfeldGJI1992LeastSquaresCollocationGeophysicalInverseTheoryAndGeostatistics] [bibtex-entry]


  990. Claudio Prati and Fabio Rocca. Focusing SAR Data With Time-Varying Doppler Centroid. IEEE Transactions on Geoscience and Remote Sensing, 30(3):550-559, May 1992. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock.
    Abstract: SAR data spatially sampled at the Nyquist limit can be correctly processed if the Doppler centroid is precisely known. Whenever the Doppler centroid shows rapid variations either with range or azimuth, more care is required in order to take advantage of the computational efficiency of frequency domain techniques. In this paper it is shown that such focusing techniques can still be exploited, provided that SAR raw data are previously modified and a space-varying nondimensional filter is applied to the focused image. The computational cost increases, but it is still smaller than time-space domain processing. Results obtained with simulated SIR-C/X-SAR data and SPOTlight geometries are presented.
    [bibtex-key = pratiRocca92:Doppler] [bibtex-entry]


  991. R. Keith Raney. An exact wide field digital imaging algorithm. International Journal of Remote Sensing, 13:991-998, March 1992. Keyword(s): SAR Processing, Chirp Scaling, Extended Chirp Scaling, ECS.
    Abstract: A new imaging algorithm is presented for Synthetic Aperture Radar (SAR) that is exact in the sense that it is capable of producing a complex image with excellent geometrical, radiometrical and phase fidelity. No interpolations or significant approximations are required, yet the method accomplishes range curvature correction over the complete range swath. The key to the approach is a quadratic phase perturbation of the range linearly frequency modulated signals while in the range signal, azimuth frequency transform (Doppler) domain. Range curvature correction is completed by a phase multiply in the two-dimensional frequency domain. Other operations required are relatively conventional. The method is generalizable to imaging geometries encountered in squint and spotlight SAR, inverse SAR, seismics, sonar, and tomography.
    [bibtex-key = raney92:ChirpScaling] [bibtex-entry]


  992. Ernesto Rodriguez and J. M. Martin. Theory and design of interferometric synthetic aperture radars. IEE Proceedings - Radar and Signal Processing, 139(2):147-159, April 1992. Keyword(s): SAR Processing, InSAR, interferometry, radar interferometry, SAR interferometry, electromagnetic wave interferometry, geophysical equipment, Monte Carlo methods, radar systems, radar theory, remote sensing by radar, topography (Earth), geophysical technique, remote sensing, design, interferometric synthetic aperture radars, high spatial resolution, height accuracy, signal statistics, optimal estimator, interferometric phase, height-error budget, high-resolution global topographic mapping, Monte Carlo simulation, InSAR system, Interferometry, Geophysical measurements, Monte Carlo methods, Radar, Radar theory, Radar imaging, Remote sensing.
    Abstract: SAR Processing, InSAR, SAR interferometry,Interferometric synthetic aperture radar (InSAR) is a method which may provide a means of estimating global topography with high spatial resolution and height accuracy. The authors present a derivation of the signal statistics, an optimal estimator of the interferometric phase, and the expressions necessary to calculate the height-error budget. These expressions are used to derive methods of optimising the InSAR-system parameters, and are then used in a specific design example for a system to perform high-resolution global topographic mapping with a one-year mission lifetime, subject to current technological constraints. A Monte Carlo simulation of this InSAR system is performed to evaluate its performance for realistic topography. The results indicate that this system has the potential to satisfy the stringent accuracy and resolution requirements for geophysical use of global topographic data.
    [bibtex-key = rodriguezMartinIEEProc1992TheoryAndDesignOfInSAR] [bibtex-entry]


  993. Mehrdad Soumekh. A System Model and Inversion for Synthetic Aperture Radar Imaging. IEEE Transactions on Image Processing, 1(1):64-76, January 1992. Keyword(s): SAR Processing, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm.
    Abstract: A system model and its corresponding inversion for synthetic aperture radar (SAR) imaging are presented. The system model incorporates the spherical nature of a radar's radiation pattern at far field. The inverse method based on this model performs a spatial Fourier transform (Doppler processing) on the recorded signals with respect to the available coordinates of a translational radar (SAR) or target (inverse SAR). It is shown that the transformed data provide samples of the spatial Fourier transform of the target's reflectivity function. The inverse method can be modified to incorporate deviations of the radar's motion from its prescribed straight line path. The effects of finite aperture on resolution, reconstruction, and sampling constraints for the imaging problem are discussed.
    [bibtex-key = soumekh92Process] [bibtex-entry]


  994. A. G. Stove. Linear FMCW radar techniques. IEE Proceedings F - Radar and Signal Processing, 139(5):343-350, October 1992. Keyword(s): SAR Processing, FMCW, FMCW radar, frequency modulation, military systems, radar systems, automotive radars, frequency modulated continuous wave, linear FMCW radar, low probability of intercept waveform, moving target indication, naval tactical navigation radars, reflected power canceller, solid-state transmitters, Frequency modulation, Radar.
    Abstract: Frequency modulated continuous wave (FMCW) radar uses a very low probability of intercept waveform, which is also well suited to make good use of simple solid-state transmitters. FMCW is finding applications in such diverse fields as naval tactical navigation radars, smart ammunition sensors and automotive radars. The paper discusses some features of FMCW radar which are not dealt with in much detail in the generally available literature. In particular, it discusses the effects of noise reflected back from the transmitter to the receiver and the application of moving target indication to FMCW radars. Some of the strengths and weaknesses of FMCW radar are considered. The paper describes how the strengths are utilised in some systems and how the weaknesses can be mitigated. It also discusses a modern implementation of a reflected power canceller, which can be used to suppress the leakage between the transmitter and the receiver, a well known problem with continuous wave radars.
    [bibtex-key = stove1992LFMCW] [bibtex-entry]


  995. H.A. Zebker and J. Villasenor. Decorrelation in interferometric radar echoes. IEEE Trans. Geosci. Remote Sens., 30(5):950-959, September 1992. Keyword(s): SAR Processing, geophysical techniques, radiowave interferometry, remote sensing by radar, synthetic aperture radar, Decorrelation, Temporal Decorrelation, topography (Earth), Oregon, United States, backscatter, decorrelation, digital terrain model generation, echo correlation statistics, forested area, global digital terrain map, interferometric radar echoes, radar interferometric technique, remote sensing, single synthetic aperture radar, surficial change, topographic mapping, unvegetated lava flows, vegetated surfaces, Decorrelation, Digital elevation models, Noise level, Propulsion, Radar antennas, Space technology, Spaceborne radar, Surface topography, Synthetic aperture radar, Terrain mapping.
    Abstract: A radar interferometric technique for topographic mapping of surfaces, implemented utilizing a single synthetic aperture radar (SAR) system in a nearly repeating orbit, is discussed. The authors characterize the various sources contributing to the echo correlation statistics, and isolate the term which most closely describes surficial change. They then examine the application of this approach to topographic mapping of vegetated surfaces which may be expected to possess varying backscatter over time. It is found that there is decorrelation increasing with time but that digital terrain model generation remains feasible. The authors present such a map of a forested area in Oregon which also includes some nearly unvegetated lava flows. Such a technique could provide a global digital terrain map
    [bibtex-key = zebkerVillasenor1992DecorrelationInSAR] [bibtex-entry]


  996. J. J. van Zyl and C. F. Burnette. Bayesian classification of polarimetric SAR images using adaptive a priori probabilities. International Journal of Remote Sensing, 13(5):835-840, 1992.
    Abstract: Most implementations of Bayesian classification assume fixed a priori probabilities. These implementations can be placed into two general categories: (1) those that assume equal a priori probabilities and (2) those that assume unequal but fixed a priori probabilities. We report here on results of classifying polarimetric SAR images using a scheme in which the classification is done iteratively. The first classification is done assuming fixed (but not necessarily equal) a priori probabilities. The results of this first classification are then used in successive iterations to change the a priori probabilities adaptively. The results show that only a few iterations are necessary to improve the classification accuracy dramatically.
    [bibtex-key = vanZylBurnetteIJRS1992BaysianClassificationPolSARImages] [bibtex-entry]


  997. Richard Bamler. Doppler Frequency Estimation and the Cramer-Rao Bound. IEEE Transactions on Geoscience and Remote Sensing, 29(3):385-390, May 1991. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Cramer-Rao Bound, Correlation-Based Estimator, Maximum-Likelihood Estimator, Accuracy Comparison, Clutterlock.
    Abstract: This paper addresses the problem of Doppler frequency estimation in the presence of speckle and receiver noise. An ultimate accuracy bound for Doppler frequency estimation is derived from the Cramer-Rao inequality. It is shown that estimates based on the correlation of the signal power spectra with an arbitrary weighting function are approximately Gaussian-distributed. Their variance is derived in terms of the weighting function. It is shown that a special case of a correlation-based estimator is a maximum-likelihood estimator that reaches the Cramer-Rao bound. These general results are applied to the problem of Doppler centroid estimation from SAR data. Different estimators known from the literature are investigated with respect to their accuracy. Numerical examples are presented and compared with experimental results.
    [bibtex-key = bamler91:Doppler] [bibtex-entry]


  998. Richard Bamler and Hartmut Runge. PRF-Ambiguity Resolving by Wavelength Diversity. IEEE Transactions on Geoscience and Remote Sensing, 29(6):997-1003, November 1991. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Wavelength Diversity, Clutterlock.
    Abstract: For high-precision synthetic aperture radar (SAR) processing, the determination of the Doppler centroid is indispensable. The Doppler frequency estimated from azimuth spectra, however, suffers from the fact that the data are sampled with the pulse repetition frequency (PRF) and an ambiguity about the correct PRF band remains. A new algorithm to resolve this ambiguity is proposed. It uses the fact that the Doppler centroid depends linearly on the transmitted radar frequency for a given antenna squint angle. This dependence is not subject to PRF ambiguities. It can be measured by Fourier transforming the SAR data in the range direction and estimating the Doppler centroid at each range frequency. The achievable accuracy is derived theoretically and verified with Seasat data of different scene content. The algorithm works best with low contrast scenes, where the conventional look correlation technique fails. It needs no iterative processing of the SAR data and causes only low computational load.
    [bibtex-key = BamlRunge91:Doppler] [bibtex-entry]


  999. Stefan Buckreuss. Motion errors in an airborne synthetic aperture radar system. European Transactions on Telecommunications, 2(6):655-664, 1991. Keyword(s): SAR Processing, Airborne SAR, Motion Compensation, MoComp, motion errors.
    Abstract: Abstract The Institute for Radio Frequency Technology of the German Aerospace Research Establishment (DLR) in Oberpfaffenhofen operates an experimental synthetic aperture radar, E-SAR. The Radar configuration was developed by DLR [1, 2] and consists of a L/C/X-band SAR System and antennas with a wide beamwidth in azimuth and elevation. The system is installed in a small Dornier DO 228 aircraft, which operates at low altitudes between 1000 m - 3000 m above ground. Due to atmospheric turbulences, the aircraft is displaced from its nominal flight path, causing a variation of the phase history of the returned radar signal. This eventually results in a degraded image quality and therefore a highly accurate motion compensation becomes necessary. It is intended to integrate an inertial measurement unit (IMU) within the E-SAR system to support the used motion compensation algorithm during data processing [3]. The required relative accuracy of the IMU can be determined by means of statistics i.e. an approach is demonstrated to estimate the power spectral density (PSD) of the tolerable uncompensated aircraft motion. The performance of an IMU motion compensation system has been evaluated by analysis, as well as by computer simulation.
    [bibtex-key = buckreussEToT1991MotionErrorsInAirborneSAR] [bibtex-entry]


  1000. Ciro Cafforio, Claudio Prati, and Fabio Rocca. SAR Data Focusing Using Seismic Migration Techniques. IEEE Transactions on Aerospace and Electronic Systems, 27(2):194-207, March 1991. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Stationary Phase Method.
    Abstract: The focusing of synthetic-aperture-radar (SAR) data using migration techniques quite similar to those used in geophysics is treated. The algorithm presented works in the omega-kx domain. Because time delays can be easily accommodated with phase shifts that increase linearly with omega, range migration poses no problem. The algorithm is described in plane geometry first, where range migration and phase history can be exactly matched. The effects of the sphericity of the Earth, of the Earth's rotation, and of the satellite trajectory curvature are taken into account, showing that the theoretically achievable spatial resolution is well within the requirements of present day and near future SAR missions. Terrestrial swaths as wide as 100 km can be focused simultaneously with no serious degradation. The algorithm has been tested with synthetic data, with Seasat-A data, and with airplane data (NASA-AIR). The experimental results fully support the theoretical analysis.
    [bibtex-key = cafforio:rangemigr] [bibtex-entry]


  1001. Charles Elachi, Eastwood Im, Ladislav E. Roth, and Charles L. Werner. Cassini Titan Radar Mapper. Proceedings of the IEEE, 79(6):867-880, June 1991. Keyword(s): SAR Processing, Cassini Radar, Saturn, astronomical instruments, planetary satellites, radar applications, radioastronomy, space vehicles, Cassini Saturn Mission, Cassini Titan Radar Mapper, Titan, antenna configuration, design constraints, multimode radar, radar modes, surface imaging, topographic mapping, Instruments, Laser radar, Moon, Optical design, Payloads, Probes, Radar antennas, Radar imaging, Saturn, Surface topography.
    Abstract: The Cassini Titan Radar Mapper is a multimode radar instrument designed to probe the optically inaccessible surface of Titan, Saturn's largest moon. The instrument is to be included in the payload of the Cassini Saturn Mission, scheduled for launch in 1995. The individual modes of Cassini Radar Mapper will allow topographic mapping and surface imaging at few hundred meters resolution. The requirements that lay behind the design are briefly discussed, and the configuration and capability of the instrument are described. The present limited knowledge of Titan's surface and the measurement requirements imposed on the radar instrument are addressed. Also discussed are the Cassini mission and the projected orbits, which imposed another set of design constraints that led to the multitude of modes and to an unconventional antenna configuration. The antenna configuration and the different radar modes are described
    [bibtex-key = elachiImRothWernerProcIEEE1991CassiniRadarTitanMapper] [bibtex-entry]


  1002. J.W. McCoy, N. Magotra, and B.K. Chang. Coherent Doppler tomography---a technique for narrow band SAR. IEEE Aerospace and Electronic Systems Magazine, 6(2):19-22, February 1991. Keyword(s): SAR Processing, SAR Tomography, Tomography, coherent Doppler tomography, coherent processing, computational overhead, high-resolution global mapping, imaging, multiple discrete frequencies, narrow band SAR, narrowband synthetic aperture radar, noncoherent subaperture processing, point spread function, radar platform, satellite geometry configuration, sidelobe level, simulation, single-frequency signal, spaceborne applications, Doppler effect, aerospace computing, aerospace instrumentation, computerised picture processing, digital simulation, radar theory;.
    Abstract: Some concerns regarding a technique of narrowband synthetic aperture radar (N-SAR) imaging called coherent Doppler tomography (CDT), which may be a good candidate for spaceborne applications, are addressed. Using a single-frequency signal, are addressed. Using a single-frequency signal, resolution of two tenths of a wavelength can be achieved in the point spread function if the radar platform circles the ground path to be imaged. However, the high sidelobe level of -8-dB in the point spread function results in an unacceptable dynamic range. To reduce the sidelobe level, two approaches are presented: coherent processing using multiple discrete frequencies and noncoherent subaperture processing. Simulation results demonstrate that the sidelobe level is substantially reduced by both methods. However, the resolution is degraded and the computational overhead is greatly increased for noncoherent subaperture processing. Also presented is a possible satellite geometry configuration that could utilize N-SAR processing to provide high-resolution global mapping capability.<>
    [bibtex-key = 67844] [bibtex-entry]


  1003. Claudio Prati, Fabio Rocca, Yuval Kost, and Elvio Damonti. Blind Deconvolution for Doppler Centroid Estimation in High Frequency SAR. IEEE Transactions on Geoscience and Remote Sensing, 29(6):934-941, November 1991. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Clutterlock.
    Abstract: For high quality SAR processing, the Doppler centroid frequency is needed. However, SAR data are sampled along the azimuth direction at the Pulse Repetition Frequency (PRF); the estimation of the Doppler centroid frequency by means of spectral analysis techniques may produce ambiguous results due to aliases. The mathematical expression of the residual error that occurs when SAR data are focused with an incorrect alias of the PRF is thus derived. Then, a blind deconvolution technique is used to estimate the actual PRF replica from the focused image. Squinted X-band data, corresponding to those that will be generated by the SIR-C mission, have benn generated from the JPL-AirSAR L and C band data by means of an inversion of the focusing process. Even if the real data may show differences with respect to the simulated data, the blind deconvolution method appears to be more precise and robust than the other conventional techniques tested.
    [bibtex-key = pratRocKosDam:DopCentrEstim] [bibtex-entry]


  1004. A. M. Smith. A new approach to range-Doppler SAR processing. International Journal of Remote Sensing, 12(2):235-251, 1991. Keyword(s): SAR Processing, Range-Doppler Algorithm, RDA, Azimuth Focusing, Spaceborne SAR, Ambiguity Analysis, Stripmap Mode.
    Abstract: In this paper we present a general analysis of frequency domain SAR processing based on the relationship between the phase of the two-dimensional Fourier transform of a point response to its range-time history. The paper demonstrates how this provides an appropriate basis for the design of a coherent strip-mode processor, free of geometric or phase distortion and artefacts and without excessive computational cost. In consequence the paper is highly relevant to the real-time, precision, processing of SAR data. The paper comments on the relevance of the analysis to ambiguity estimation and the processing of very long integration time SAR data.
    [bibtex-key = smith1991] [bibtex-entry]


  1005. Lars M. H. Ulander. Accuracy of Using Point Targets for SAR Calibration. IEEE Transactions on Aerospace and Electronic Systems, 27(1):139-148, January 1991. Keyword(s): SAR Processing, Radiometric Calibration, calibration, measurement errors, radar, radiometry, remote sensing by radar, RMS errors, SAR calibration, equivalent rectangle system resolution, impulse response, integral method, nonlinear phase errors, reference point targets, synthetic aperture radar, system focus.
    Abstract: The peak and integral methods for radiometric calibration of a synthetic aperture radar (SAR) using reference point targets are analyzed. Both calibration methods are shown to be unbiased, but the peak method requires knowledge of the equivalent rectangle system resolution which is sensitive to system focus. Exact expressions for the RMS errors of both methods are derived. It is shown that the RMS error resulting from the peak method is always smaller than or equal to that from the integral method for a well-focused system. However, for robust radiometric calibration of SAR, or when nonlinear phase errors are present, the integral method is recommended, because it does not require detailed knowledge of the impulse response and the resulting RMS error is not dependent on system focus.
    [bibtex-key = ulander1991:MoCoCalibration] [bibtex-entry]


  1006. Michael W. Whitt, Fawwaz T. Ulaby, Paul Polatin, and Valdis V. Liepa. A general polarimetric radar calibration technique. IEEE Transactions on Antennas and Propagation, 39(1):62-67, January 1991. Keyword(s): Polarimetric Calibration, calibration, polarimetry, radar systems, distortion matrices, general polarimetric radar calibration technique, received waves, scattering matrices, single-antenna radar, transmitted waves, Calibration, Distortion measurement, Layout, Phase measurement, Polarization, Radar antennas, Radar imaging, Radar measurements, Radar polarimetry, Radar scattering.
    Abstract: A polarimetric radar calibration procedure is introduced and verified with experimental results. The procedure requires measurements of three known targets in order to determine the distortion matrices that characterize the effect of the measurement system on the transmitted and received waves. The scattering matrices for the known targets can be of any form, provided that a limited set of constraints is satisfied. A special case, wherein the transmit and receive distortion matrices are the transpose of each other, is considered. This case is useful for some single antenna systems and has the advantage that only two known targets are required
    [bibtex-key = whittUlabyPolatinLiepaTAP1991PolarimetricCalibration] [bibtex-entry]


  1007. E. Krogager. New decomposition of the radar target scattering matrix. Electronics Letters, 26(18):1525-1527, August 1990. Keyword(s): decomposition, radar target scattering matrix, S-matrix theory, radar theory;.
    Abstract: A decomposition of the complex radar target scattering matrix resolves a 2*2 complex scattering matrix into three components that provide a clearer picture of the physical mechanisms behind the scattering and, thereby, a clearer picture of the target itself.
    [bibtex-key = krogager1990] [bibtex-entry]


  1008. Fuk K. Li and R. M. Goldstein. Studies of Multibaseline Spaceborne Interferometric Synthetic Aperture Radars. IEEE Transactions on Geoscience and Remote Sensing, 28(1):88-97, January 1990. Keyword(s): SAR Processing, SAR Interferometry, Interferometry, InSAR, Multi-Baseline SAR, SAR Tomography, geophysical techniques, radar applications, radar measurement, radiowave interferometry, remote sensing, topography (Earth)INSAR design, Seasat, baseline separations, geophysical technique, multibaseline spaceborne interferometric synthetic aperture radars, performance, phase measurement error model, signal-to-noise ratios, topography measurement capability.
    Abstract: The authors have utilized a set of Seasat synthetic aperture radar (SAR) data that were obtained in nearly repeat ground-track orbits to demonstrate the performance of spaceborne interferometric SAR (INSAR) systems. An assessment of the topography measurement capability is presented. A phase measurement error model is described and compared with the data obtained at various baseline separations and signal-to-noise ratios. Finally, the implications of these results on future spaceborne INSAR design are discussed
    [bibtex-key = liGoldstein1990:MultiBaselineInsAR] [bibtex-entry]


  1009. João Moreira. A New Method Of Aircraft Motion Error Extraction From Radar Raw Data For Real Time Motion Compensation. IEEE Transactions on Geoscience and Remote Sensing, 28(4):620-626, July 1990. Keyword(s): SAR Processing, Autofocus, Motion Compensation, MoComp, Residual Motion Errors, Airborne SAR, ESAR.
    Abstract: Presented is a new solution for real-time motion compensation. The main idea is to extract all the necessary motions of the aircraft from the radar backscatter signal using a new radar configuration and new methods for evaluating the azimuth spectra of the radar signal. Hence an inertial navigation system becomes unnecessary for many applications. The motion compensation parameters for realtime motion error correction are the range delay, the range dependent phaseshift, and the pulse repetition frequency. The motions of the aircraft to be extracted are the displacement in line-of-sight (LOS) direction, the aircraft\u2019s yaw and drift angle, and the forward velocity. Results show that a three-look image with an azimuth resolution of 3 m in L-band using a small aircraft is achievable, and the implementation of this method in real time using an array processor is feasible.
    [bibtex-key = moreiraJoao1990:MoComp] [bibtex-entry]


  1010. Claudio Prati, Fabio Rocca, Andrea Monti-Guarnieri, and Elvio Damonti. Seismic Migration For Sar Focusing: Interferometrical Applications. IEEE Transactions on Geoscience and Remote Sensing, 28(4):627-640, July 1990. Keyword(s): SAR Processing, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Interferometry.
    Abstract: Conventional techniques for Synthetic Aperture Radar (SAR) image focusing use the matched filter concept and convolve the data with a reference phase signal which changes with range. The resulting algorithm is space-variant and its frequency-domain implementation is cumbersome. SAR data, however, can be focused using migration techniques identical to those used in seismic signal processing for oil prospecting. The implications of the higher precision achieved with migration as regards the phases of the synthesized radar returns is discussed. Two interferometrical applications where the phases of the returns are essential for the recovery of interesting parameters of the observed scene are presented: the determination from a satellite of the altitude map of the terrain and the determination from an airplane of the attitude of the sensor. In both cases the precision achieved is satisfactory.
    [bibtex-key = pratiRoccaMontiDamonti90:Migration] [bibtex-entry]


  1011. Kamal Sarabandi and Fawwaz T. Ulaby. A convenient technique for polarimetric calibration of single-antenna radar systems. IEEE Transactions on Geoscience and Remote Sensing, 28(6):1022-1033, November 1990. Keyword(s): Polarimetric Calibration, calibration, polarimetry, radar antennas, radar systems, X-band radar, channel imbalances, conducting sphere, crosscoupling, crosspolarization channels, crosstalk factors, effective polarization isolation, error, four-port passive network, metal sphere, polarimetric calibration, single-antenna radar systems, technique, trihedral corner reflector, Antenna measurements, Backscatter, Calibration, Crosstalk, Passive networks, Polarization, Radar antennas, Radar polarimetry, Receiving antennas, Transmitting antennas.
    Abstract: A practical technique for calibrating single-antenna polarimetric radar systems is introduced. This technique requires only a single calibration target such as a conducting sphere or a trihedral corner reflector to calibrate the radar system, both in amplitude and phase, for all linear polarization configurations. By using a metal sphere, which is orientation independent, error in calibration measurement is minimized while simultaneously calibrating the crosspolarization channels. The antenna system and two orthogonal channels (in free space) are modeled as a four-port passive network. Upon using the reciprocity relations for the passive network and assuming the crosscoupling terms of the antenna to be equal, the crosstalk factors of the antenna system and the transmit and receive channel imbalances can be obtained from measurement of the backscatter from a metal sphere. For an X-band radar system with cross polarization isolation of 25 dB, comparison of values measured for a sphere and a cylinder with theoretical values shows agreement within 0.4 dB in magnitude and 5 deg in phase. An effective polarization isolation of 50 dB is achieved using this calibration technique
    [bibtex-key = sarabandiUlabyTGRS1990PolarimetricCalibration] [bibtex-entry]


  1012. Kamal Sarabandi, Fawwaz T. Ulaby, and M. Ali Tassoudji. Calibration of Polarimetric Radar Systems With Good Polarization Isolation. IEEE Trans. Geosci. Remote Sens., 28(1):70-75, January 1990. Keyword(s): SAR Processing, polarimetric calibration, polarimetry, calibration, electromagnetic wave polarisation, radar measurement, radar systems, remote sensing, remote sensing, calibration, polarization isolation, single-antenna polarimetric radar systems, metal sphere, cross-polarized radar cross section, radar transfer function, scattering matrix, tilted cylinder, X-band, radar overall cross-polarization isolation, alignment, Calibration, Radar polarimetry, Polarization, Radar scattering, Particle scattering, Radar cross section, Radar antennas, Phase measurement, Radar measurements, Laboratories.
    Abstract: A practical technique is proposed for calibrating single-antenna polarimetric radar systems using a metal sphere plus any second target with a strong cross-polarized radar cross section. This technique assumes perfect isolation between antenna ports. It is shown that all magnitudes and phases (relative to one of the like-polarized linear polarization configurations) of the radar transfer function can be calibrated without knowledge of the scattering matrix of the second target. Comparison of the values measured (using this calibration technique) for a tilted cylinder in the X-band with theoretical values shows agreement within +or-0.3 dB in magnitude and +or-5 degrees in phase. The radar overall cross-polarization isolation was 25 dB. The technique is particularly useful for calibrating a radar under field conditions, because it does not require the careful alignment of calibration targets.<>
    [bibtex-key = sarabandiUlabyTassoudjiTGRS1990PolarimetricCalibrationForSystemsWithGoodPolIsolation] [bibtex-entry]


  1013. Mehrdad Soumekh. Echo imaging using physical and synthesized arrays. Optical Engineering, 29(5):545-554, 1990. Keyword(s): image reconstruction, echo imaging, arrays, synthetic aperture, Imaging systems, Fourier transforms, Synthetic aperture radar, Systems modeling, Imaging arrays, Signal processing, Image resolution, Reflectivity, Spatial frequencies, Spherical lenses.
    Abstract: This paper addresses the problem of imaging from (inverting) the data collected by a physical or synthesized array. A scattering model describing the source/object interaction is presented. The inversion based on this model is a computationally manageable method that incorporates the radiation pattern of each element on the array at the transmit and receive modes. This study indicates that a physical array and its synthesized counterpart possess the same resolution despite the fact that the synthesized array's signal subspace is a subset of the much larger signal subspace for the physical array.
    [bibtex-key = soumekhOpticalEngineering1990EchoImagingUsingPhysicalAndSynthesizedArrays] [bibtex-entry]


  1014. Petre Stoica and A. Nehorai. MUSIC, maximum likelihood, and Cramer-Rao bound: further results and comparisons. IEEE Transactions on Acoustics, Speech and Signal Processing, 38(12):2140-2150, December 1990. Keyword(s): SAR Processing, MUSIC, SAR Tomography, parameter estimation, signal detection, signal processing, statistical analysisCramer-Rao bound, covariance matrix, direction-of-arrival, maximum likelihood estimator, multiple signal characterization, narrowband plane waves, noisy measurements, performance comparisons, statistical performance, superimposed signals, uniform linear sensor arrays, unweighted MUSIC estimator, weighted MUSIC estimators.
    Abstract: The problem of determining the direction-of-arrival of narrowband plane waves using sensor arrays and the related problem of estimating the parameters of superimposed signals from noisy measurements are studied. A number of results have been recently presented by the authors on the statistical performance of the multiple signal characterization (MUSIC) and the maximum likelihood (ML) estimators for the above problems. This work extends those results in several directions. First, it establishes that in the class of weighted MUSIC estimators, the unweighted MUSIC achieves the best performance (i.e. the minimum variance of estimation errors), in large samples. Next, it derives the covariance matrix of the ML estimator and presents detailed analytic studies of the statistical efficiency of MUSIC and ML estimators. These studies include performance comparisons of MUSIC and MLE with each other, as well as with the ultimate performance corresponding to the Cramer-Rao bound. Finally, some numerical examples are given which provide a more quantitative study of performance for the problem of finding two directions with uniform linear sensor arrays
    [bibtex-key = stoicaNehorai1990:MUSIC] [bibtex-entry]


  1015. M. W. Whitt and F. T. Ulaby. A polarimetric radar calibration technique with insensitivity to target orientation. Radio Science, 25(6):1137-1143, 1990. Keyword(s): Polarimetric Calibration, Measurement and standards, Electromagnetics: Instrumentation and techniques, Techniques applicable in three or more fields.
    Abstract: A convenient polarimetric radar calibration technique is presented and used to calibrate an X band polarimetric radar. The technique uses a distortion matrix model for the errors introduced by the transmitter and receiver, and it is applicable to any polarimetric radar system where the concept of an invariant distortion matrix is valid. A sphere and two arbitrary passive targets are used to calibrate the radar with respect to the unknown polarization transmitted when the v-polarized channel is energized. The unknown transmitted polarization is then recovered by measuring any nondepolarizing target. Because knowledge of the scattering matrices for the two arbitrary targets is not required, the technique is insensitive to errors in the orientation of calibration targets. Experimental results are presented, and they indicate magnitude and relative phase measurement errors of less than 0.2 dB and 2 deg, respectively.
    [bibtex-key = whittUlabyRadioScience1990PolarimetricCalibration] [bibtex-entry]


  1016. Jakob J. van Zyl. Calibration of polarimetric radar images using only image parameters and trihedral corner reflector responses. IEEE Trans. Geosci. Remote Sens., 28(3):337-348, May 1990. Keyword(s): SAR Processing, Polarimetry, Polarimetric Calibration, calibration, polarimetry, radar measurement, remote sensing, C-band, L-band, Stokes matrix, absolute amplitude, calibration, compressed data, image parameters, natural targets, polarimetric radar images, radar return, receive matrices, relative amplitude, relative phase, remote sensing, scattering matrix, system crosstalk, terrain, transmit matrix, trihedral corner reflector responses, Calibration, Crosstalk, Frequency estimation, Image coding, Parameter estimation, Pixel, Radar imaging, Radar polarimetry, Radar scattering, Transmitters.
    Abstract: A technique that uses the radar return from natural targets and at least one trihedral corner reflector to calibrate compressed polarimetric radar data is described. Calibration for relative amplitude, relative phase, absolute amplitude, and system crosstalk is addressed. The crosstalk calibration method is based on the theoretical result that for natural targets with azimuthal symmetry the copolarized and crosspolarized components of the scattering matrix are uncorrelated, and the method does not require any external calibration targets to be deployed before imaging. Because compressed data are used, one is forced to model the transmitting and receiving systems as reciprocal. Even though the inferred transmit and receive matrices are not each simply related to the physical transmitter and receiver, the true Stokes matrix for each pixel in an image can be accurately determined by this approach. The method is illustrated by estimating the crosstalk parameters of the NASA/JPL aircraft for different types of terrain and for two frequencies. For the C-band system, the crosstalk is less than -20 dB for all ranges in the images. The crosstalk of the L-band system is a function of range, however, and may be as poor as -10 dB in the near range, leading to a noticeable distortion of the polarization signatures
    [bibtex-key = vanZylTGRS1990PolCalibration] [bibtex-entry]


  1017. J. L. Bauck and W. K. Jenkins. Convolution-Backprojection Image Reconstruction For Bistatic Synthetic Aperture Radar With Correction For Wavefront Curvature And Propagation Attenuation. Proc.SPIE, 1101:1-8, 1989. Keyword(s): SAR Processing, Bistatic SAR, Back-Projection, bistatic synthetic aperture radar, Azimuth Focusing, convolution back-projection, elliptical-arc projections, final reconstructed image, ground patch, image resource, pixel, weighting, radar cross-sections, radar theory, Spotlight mode, Airborne SAR, Tomographic Processing, Tomography, Wavefront Curvature.
    Abstract: Signal processing for image reconstruction in synthetic aperture radar (SAR) historically has been based on Fourier transform techniques. One reason for this is the fact that, at the time when SAR was invented in the early 1950's and for some time after that, the only way to process the huge amounts of data in a reasonably expeditious manner was to use optical techniques, such processors being based, at least in part, on Fourier optical principles. With the advent of digital processing in recent years, the existence of efficient algorithms for the computation of the discrete Fourier transform has continued to offer compelling reasons to use Fourier-type inversion methods. Additionally, geometrically-related simplifications in most analyses have engendered the assumption of plane waves being present over the ground patch being imaged, again encouraging the use of Fourier techniques. In applications where the distance of the radar from the ground patch is very large compared to the size of the ground patch to be imaged, such processing is appropriate, though still an approximation. In other cases, the wavefront curvature cannot be ignored, and other steps must be taken in order to yield high-quality imagery. Recent investigations into the use of convolution-backprojection algorithms modified from computer-aided tomography have proved fruitful in correcting for wavefront curvature in monostatic SAR. This paper reports on similar success in bistatic SAR. There appear to be other applications that could benefit from other adaptations of the convolution-backprojection idea.
    [bibtex-key = bauckJenkinsProcSPIE1989BistaticConvolutionBackprojection] [bibtex-entry]


  1018. D. Blacknell, A. Freeman, S. Quegan, I.A. Ward, I.P. Finley, C.J. Oliver, R.G. White, and J.W. Wood. Geometric accuracy in airborne SAR images. Aerospace and Electronic Systems, IEEE Transactions on, 25(2):241-258, March 1989. Keyword(s): SAR Processing, Airborne SAR, Motion Compensation, Motion Errors, Residual Motion Errors, aircraft instrumentation, microwave imaging, position measurement, radar, X-band, airborne SAR images, autofocus, azimuth processing, azimuthal positioning accuracy, defocusing, geometric accuracy, range positioning accuracy, synthetic aperture radar.
    Abstract: Uncorrected across-track motions of a synthetic aperture radar (SAR) platform can cause both a severe loss of azimuthal positioning accuracy in, and defocusing of, the resultant SAR image. It is shown how the results of an autofocus procedure can be incorporated in the azimuth processing to produce a fully focused image that is geometrically accurate in azimuth. Range positioning accuracy is also discussed, leading to a comprehensive treatment of all aspects of geometric accuracy. The system considered is an X-band SAR
    [bibtex-key = blacknellFreemanQueganWardFinleyOliverWhiteWood1989] [bibtex-entry]


  1019. P. H. Eichel, D. C. Ghiglia, and C. V. Jakowatz. Speckle processing method for synthetic-aperture-radar phase correction. Opt. Lett., 14(1):1, 1989. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, PGA.
    Abstract: Uncompensated phase errors present in synthetic-aperture-radar data can have a disastrous effect on reconstructed image quality. We present a new iterative algorithm that holds promise of being a robust estimator and corrector for arbitrary phase errors. Our algorithm is similar in many respects to speckle processing methods currently used in optical astronomy. We demonstrate its ability to focus scenes containing large amounts of phase error regardless of the phase-error structure or its source. The algorithm works extremely well in both high and low signal-to-clutter conditions without human intervention.
    [bibtex-key = eichelGhigliaJakowatz1989:PGAutofocus] [bibtex-entry]


  1020. P. H. Eichel and C. V. Jakowatz. Phase-gradient algorithm as an optimal estimator of the phase derivative. Opt. Lett., 14(20):1101, 1989. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, PGA.
    Abstract: The phase-gradient algorithm represents a powerful new signal-processing technique with applications to aperturesynthesis imaging. These include, for example, synthetic-aperture-radar phase correction and stellar-image reconstruction. The algorithm combines redundant information present in the data to arrive at an estimate of the phase derivative. We show that the estimator is in fact a linear, minimum-variance estimator of the phase derivative.
    [bibtex-key = eichelJakowatz1989b:PGAutofocus] [bibtex-entry]


  1021. Donald Fraser. Interpolation by the FFT Revisited - an Experimental Investigation. IEEE Transactions on Acoustics, Speech, and Signal Processing, 37(5):665-675, May 1989. Keyword(s): Interpolation, Interpolation by FFT, Fast Fourier Transforms, FFT, Nyquist limit, RMS error, Sampling Rate Conversion, Upsampling, Sinusoidal Test Signal.
    Abstract: Interpolation by the FFT has become a practial proposition in many new areas, such as image resampling, with the recent emergence of extremly fast FFT and microcircuits. This paper desctibes a numerical investigation into the accuracy of interpolation by fast Fourier transform (FFT) using a sinusoidal test signal. The method is precisely defined, including a previously unnoticed detail which makes a significant difference to the accuracy of the result. The experiments show that, with no input windowing, the accuracy of interpolation is almost independent of sinusoidal wavelength very close to the Nyquist limit. The resulting RMS error is inversely proportional to input sequence length and is very low for sequence lengths likely to be encountered in practice. As wavelength passes through the Nyquist limit, there is a sudden increase in error, as is expected from sampling theory. If the sequence ends are windowed by short, cosine half-bells, accuracy is further improved at longer wavelengths. In comparison, small-kernel convolution methods, such as linear interpolation and cubic convolution, perform badly at wavelengths anywhere near the Nyquist limit
    [bibtex-key = fraser89:Interpolation] [bibtex-entry]


  1022. Andrew K. Gabriel, Richard M. Goldstein, and Howard A. Zebker. Mapping small elevation changes over large areas: Differential radar interferometry. Journal of Geophysical Research: Solid Earth, 94(B7):9183-9191, 1989. Keyword(s): Review Paper, SAR Processing, Interferometry, SAR Interferometry, differential SAR Interferometry, DInSAR, InSAR, deformation mapping, surface deformation, surface displacement, Topographic Mapping, Planetology: Solid Surface Planets and Satellites: Surfaces, Remote sensing, Radar astronomy.
    Abstract: A technique that uses synthetic aperture radar (SAR) images to measure very small (1 cm or less) surface motions with good resolution (10 m) over large swaths (50 km) is presented along with experimental results. The method could be used for accurate measurements of many geophysical phenomena, including swelling and buckling in fault zones, residual displacements from seismic events, and prevolcanic swelling. The method is based on SAR interferometry, where two images are made of a scene by simultaneously flying two physically separated antennas. Then the phases of corresponding pixels are differenced, and altitude formation is deduced from some simple computation and image rectification. It is also possible to use one antenna flown twice over the same scene; then, if the second flight exactly duplicates the track of the first, an interesting possibility occurs. There would be no phase changes between the images at all unless there was a physical change in the scene, such as ground swelling, that would alter the distance from some resolution element to the antenna. Since the phase changes all occur at the short carrier wavelength, the basic limitation on sensitivity is only the phase noise in the system. When the two imaging passes are made from flight tracks that are separated (which is the case with the Seasat images used here), it is no longer possible to distinguish surface changes from the parallax caused by topography. However, with some additional computation, a third image made at some other baseline may be used to remove the topography and leave only the surface changes. This method was applied using Seasat data to an imaging site in Imperial Valley, California, where motion effects were observed that were ascribed to the expansion of water-absorbing clays. Phase change images of this area are shown, along with associated ground truth about the presence of water. Problems with the technique are explored, along with a discussion of future experimental possibilities on upcoming SAR missions like Earth Observing System (EOS), Earth Resources Satellite (ERS 1), SIR-C, and the Venus imaging radar, Magellan.
    [bibtex-key = gabrielGoldsteinZebkerJGRB1989DInSAR] [bibtex-entry]


  1023. Dennis C. Ghiglia and Gary A. Mastin. Two-dimensional phase correction of synthetic-aperture-radar imagery. Opt. Lett., 14(20):1104-1106, October 1989. Keyword(s): SAR processing, Autofocus, Atmospheric turbulence, Coherent systems, Fourier transforms, Phase estimation, Point spread function, Speckle imaging.
    Abstract: A two-dimensional synthetic-aperture-radar (SAR) phase-correction algorithm is described as a natural extension of a one-dimensional technique developed previously. It embodies some similarities of phase-gradient speckle imaging and incorporates improvements in phase estimation. Diffraction-limited performance has been obtained on actual SAR imagery regardless of scene content or phase-error structure. The algorithm is computationally efficient, robust, and easily implemented on a general-purpose computer or special-purpose hardware.
    [bibtex-key = ghigliaMastinOptLett1989TwoDimensionalPhaseCorrectionForSARAutofocus] [bibtex-entry]


  1024. Soren N. Madsen. Estimating the Doppler Centroid of SAR Data. IEEE Transactions on Aerospace and Electronic Systems, 25(2):134-140, 1989. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Sign Doppler Estimator, SDE, Correlation Doppler Estimator, CDE, delta E Estimator, Satellite SAR, SEASAT.
    Abstract: After reviewing frequency-domain techniques for estimating the Doppler centroid of synthetic-aperture radar (SAR) data, the author describes a time-domain method and highlights its advantages. In particular, a nonlinear time-domain algorithm called the sign-Doppler estimator (SDE) is shown to have attractive properties. An evaluation based on an existing SEASAT processor is reported. The time-domain algorithms are shown to be extremely efficient with respect to requirements on calculations and memory, and hence they are well suited to real-time systems where the Doppler estimation is based on raw SAR data. For offline processors where the Doppler estimation is performed on processed data, which removes the problem of partial coverage of bright targets, the delta_E estimator and the CDE (correlation Doppler estimator) algorithm give similar performance. However, for nonhomogeneous scenes it is found that the nonlinear SDE algorithm, which estimates the Doppler-shift on the basis of data signs alone, gives superior performance.
    [bibtex-key = madsen89:DopCentrEst] [bibtex-entry]


  1025. R.L. Mitchell. Creating complex signal samples from a band-limited real signal. Aerospace and Electronic Systems, IEEE Transactions on, 25(3):425-427, 1989. Keyword(s): quadrature demodulation, demodulation, digital filters, filtering and prediction theory, radar theory, signal processing, FIR filter, band-limited real signal, filter, finite-duration impulse response, image band rejection.
    Abstract: A very efficient method of creating complex signal samples from a band-limited real signal is presented. Because the method employs a simple mixer followed by one analog-to-digital (A/D) converter, plus a finite-duration impulse response (FIR) filter for image band rejection, there is no phase distortion in the resulting sampled signal. The method is more efficient than competing methods based on infinite-duration impulse response (IIR) filters.
    [bibtex-key = mitchell89:demod] [bibtex-entry]


  1026. David C. Munson and Robert L. Visentin. A signal processing view of strip-mapping synthetic aperture radar. IEEE Transactions on Acoustics, Speech, and Signal Processing, 37(12):2131-2147, December 1989. Keyword(s): SAR Processing, SAR Focusing, Azimuth Focusing, radar theory, signal processing, Doppler effect, SAR, airborne, imaging equations, polar-format, pulse compression, radar return signal, radar theory, signal processing, spaceborne, spotlight-mode, strip-mapping synthetic aperture radar, Doppler effect, Equations, Geometry, Image resolution, Pulse compression methods, Radar signal processing, Signal analysis, Signal resolution, Spaceborne radar, Synthetic aperture radar.
    Abstract: The authors derive the fundamental strip-mapping SAR (synthetic aperture radar) imaging equations from first principles. They show that the resolution mechanism relies on the geometry of the imaging situation rather than on the Doppler effect. Both the airborne and spaceborne cases are considered. Range processing is discussed by presenting an analysis of pulse compression and formulating a mathematical model of he radar return signal. This formulation is used to obtain the airborne SAR model. The authors study the resolution mechanism and derive the signal processing relations needed to produce a high-resolution image. They introduce spotlight-mode SAR and briefly indicate how polar-format spotlight processing can be used in strip-mapping SAR. They discuss a number of current and future research directions in SAR imaging
    [bibtex-key = munsonVisentinTASS1989SignalProcViewStripMapSAR] [bibtex-entry]


  1027. Petre Stoica, Randolph L. Moses, Benjamin Friedlander, and Torsten Söderström. Maximum likelihood estimation of the parameters of multiple sinusoids from noisy measurements. Acoustics, Speech, and Signal Processing [see also IEEE Transactions on Signal Processing], IEEE Transactions on, 37(3):378-392, 1989. Keyword(s): RFI Suppression, filtering and prediction theory, spectral analysis, Cramer-Rao bound covariance matrix, initial estimates, maximum-likelihood, maximum-likelihood estimator, MLE, multiple sinusoids, noisy measurements.
    Abstract: The problem of estimating the frequencies, phases, and amplitudesof sinusoidal signals is considered. A simplified maximum-likelihoodGauss-Newton algorithm which provides asymptotically efficient estimatesof these parameters is proposed. Initial estimates for this algorithmare obtained by a variation of the overdetermined Yule-Walker method andperiodogram-based procedure. Use of the maximum-likelihood Gauss-Newtonalgorithm is not, however, limited to this particular initializationmethod. Some other possibilities to get suitable initial estimates arebriefly discussed. An analytical and numerical study of the shape of thelikelihood function associated with the sinusoids-in-noise processreveals its multimodal structure and clearly sets the importance of theinitialization procedure. Some numerical examples are presented toillustrate the performance of the proposed estimation procedure.Comparison to the performance corresponding to the Cramer-Rao lowerbound is also presented, using a simple expression for the asymptoticCramer-Rao bound covariance matrix derived in the paper
    [bibtex-key = stoicaMosesFriedlanderSoederstroem89:RFI] [bibtex-entry]


  1028. P. Stoica and Arye Nehorai. MUSIC, maximum likelihood, and Cramer-Rao bound. IEEE Transactions on Acoustics, Speech and Signal Processing, 37(5):720-741, May 1989. Keyword(s): SAR Processing, MUSIC, SAR Tomography, radio direction-finding, signal processing, Cramer-Rao bound, MUSIC estimator, covariance matrix, direction finding, maximum likelihood method, plane waves, statistical efficiency, uniform linear array.
    Abstract: The performance of the MUSIC and ML methods is studied, and their statistical efficiency is analyzed. The Cramer-Rao bound (CRB) for the estimation problems is derived, and some useful properties of the CRB covariance matrix are established. The relationship between the MUSIC and ML estimators is investigated as well. A numerical study is reported of the statistical efficiency of the MUSIC estimator for the problem of finding the directions of two plane waves using a uniform linear array. An exact description of the results is included
    [bibtex-key = stoicaNehorai1989:MUSICMLCramerRao] [bibtex-entry]


  1029. Jakob van Zyl. Unsupervised classification of scattering behavior using radar polarimetry data. IEEE Trans. Geosci. Remote Sens., 27(1):36-45, January 1989. Keyword(s): imaging radar polarimeter, land surface measurement, ocean, radar polarimetry, remote sensing technique, scattering behavior, unsupervised classification, urban, vegetation, electromagnetic wave polarisation, electromagnetic wave scattering, geophysical techniques, oceanographic techniques, radar applications, remote sensing;.
    Abstract: The use of an imaging radar polarimeter data for unsupervised classification of scattering behavior is described by comparing the polarization properties of each pixel in an image to that of simple classes of scattering such as even number of reflections, odd number of reflections, and diffuse scattering. For example, when this algorithm is applied to data acquired over the San Francisco Bay area in California, it classifies scattering by the ocean as being similar to that predicted by the class of odd number of reflections, scattering by the urban area as being similar to that predicted by the class of even number of reflections, and scattering by the Golden Gate Park as being similar to that predicted by the diffuse scattering class. It also classifies the scattering by a lighthouse in the ocean and boats on the ocean surface as being similar to that predicted by the even number of reflections class, making it easy to identify these objects against the background of the surrounding ocean
    [bibtex-key = vanZyl1989] [bibtex-entry]


  1030. B.D. Carlson. Covariance matrix estimation errors and diagonal loading in adaptive arrays. IEEE Transactions on Aerospace and Electronic Systems, 24(4):397-401, July 1988. Keyword(s): SAR Processing, Beamforming, array processing, antenna phased arrays, antenna theory, digital simulation, eigenvalues and eigenfunctions, estimation theory, matrix algebraadapted antenna patterns, adaptive arrays, covariance matrix sample size, diagonal loading, distorted mainbeams, eigenvector decomposition, estimation errors, low-level interference, nulling, sample matrix inversion, sidelobes.
    Abstract: Simulations were used to investigate the effect of covariance matrix sample size on the system performance of adaptive arrays using the sample matrix inversion (SMI) algorithm. Inadequate estimation of the covariance matrix results in adapted antenna patterns with high sidelobes and distorted mainbeams. A technique to reduce these effects by modifying the covariance matrix estimate is described from the point of view of eigenvector decomposition. This diagonal loading technique reduces the system nulling capability against low-level interference, but parametric studies show that it is an effective approach in many situations
    [bibtex-key = carlson1988:DiagonalLoading] [bibtex-entry]


  1031. Richard M. Goldstein, Howard A. Zebker, and Charles L. Werner. Satellite radar interferometry: Two-dimensional phase unwrapping. Radio Science, 23(4):713-720, 1988. Keyword(s): SAR Processing, Phase Unwrapping, Branch-Cut, Branch-Cut Phase Unwrapping, SAR Interferometry, Spaceborne SAR, InSAR, DInSAR, geophysical image processing, geophysical techniques, radar interferometry, synthetic aperture radar.
    Abstract: Interferometric synthetic aperture radar observations provide a means for obtaining high-resolution digital topographic maps from measurements of amplitude and phase of two complex radar images. The phase of the radar echoes may only be measured modulo 2pi; however, the whole phase at each point in the image is needed to obtain elevations. We present here our approach to unwrapping the 2pi ambiguities in the two-dimensional data set. We find that noise and geometrical radar layover corrupt our measurements locally, and these local errors can propagate to form global phase errors that affect the entire image. We show that the local errors, or residues, can be readily identified and avoided in the global phase estimation. We present a rectified digital topographic map derived from our unwrapped phase values.
    [bibtex-key = goldsteinZebkerWernerBranchCutPhaseUnwrapping1988] [bibtex-entry]


  1032. Mehrdad Soumekh. Band-Limited Interpolation from Unevenly Spaced Sampled Data. IEEE Transactions on Acoustics, Speech, and Signal Processing, 36(1):110-122, January 1988. Keyword(s): Interpolation, Band-Limited, Reconstruction, Unevenly Spaced Data.
    Abstract: This paper adresses the problem of reconstructing a band-limited signal from a finite number of unevenly spaced sampled data. A Fourier analysis of the available unevenly spaced sampled data is presented. The result is utilized to develop an interpolation scheme from the available data. Conditions for accurate reconstruction are examined. Algorithms to implement the reconstruction scheme are discussed. The method's application in one-dimensional and two-dimensional reconstruction problems are shown.
    [bibtex-key = soumekh88:interpol] [bibtex-entry]


  1033. Alan di Cenzo. A New Look at Nonseparable Synthetic Aperture Radar Processing. IEEE Transactions on Aerospace and Electronic Systems, 24(3):218-223, May 1988. Keyword(s): SAR Processing, Template Correlation, 2D Filter, Nonseparable SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm.
    Abstract: The author examines the nonseparable, template correlation approach to digital-strip-mode synthetic-aperture radar (SAR) phase history processing and concludes that it can now outperform the traditional separable approaches in the areas of speed, image quality, control simplicity, and flexibility. A working nonseparable frequency-domain SAR processor is described and evaluated. The image quality is seen to be superior to that resulting from the separable approaches. Based on the working intermediate hardware implementation, the author strongly suggests that future VHSIC and other advanced implementation will provide extremely fast (real time), high-quality, small, flexible SAR processors.
    [bibtex-key = DiCenzo88:NonseparableProc] [bibtex-entry]


  1034. Jan Askne and H. Nordius. Estimation of tropospheric delay for microwaves from surface weather data. Radio Science, 22(3):379-386, 1987.
    Abstract: We have developed two methods to determine the tropospheric path delay for microwaves. One uses a closed form model of the atmosphere in which two parameters are used to describe the decrease in temperature with height and the relation between total pressure and water vapor partial pressure, respectively. The other method uses numerical integration of refractivity profiles, which are computed from temperature and humidity profiles obtained by statistical regression. The two methods have been tested on two sets of observations and compared with simultaneous radio soundings. The best results show an improvement of 10\% in one case and 30\% in the other compared with a presently used model. Possibilities to improve the statistical model further are discussed.
    [bibtex-key = askneNordiusRadioScience1987TroposphericDelayForMicrowaves] [bibtex-entry]


  1035. M. Borgeaud, R.T. Shin, and J.A. Kong. Theoretical Models For Polarimetric Radar Clutter. Journal of Electromagnetic Waves and Applications, 1(1):73-89, 1987.
    Abstract: The Mueller matrix and polarization covariance matrix are described for polarimetric radar systems. The clutter is modelled by a layer of random permittivity, described by a three-dimensional correlation function, with variance, and horizontal and vertical correlation lengths. This model is applied, using the wave theory with Born approximations carried to the second order, to find the backscattering elements of the polarimetric matrices. It is found that 8 out of 16 elements of the Mueller matrix are identically zero, corresponding to a covariance matrix with four zero elements. Theoretical predictions are matched with experimental data for vegetation fields.
    [bibtex-key = borgeaudShinKong1987] [bibtex-entry]


  1036. Hans Hellsten and Lars E. Andersson. An inverse method for the processing of synthetic aperture radar data. Inverse Problems, 3(1):111-124, 1987. Keyword(s): SAR Processing, Back-Projection, Hankel Transform, Fourier-Hankel Inversion, Abel Transform.
    Abstract: An inverse method for the processing of synthetic aperture radar signals is presented. The method is independent of the relative bandwidth of the radar signal and should thus work when, for instance, the frequency of the emitted radiation is of the same order as the bandwidth, i.e. in low frequency, high resolution applications. It is shown that the obtained relations reduce to the correlation integrals of conventional SAR image processing in the case that the relative bandwidth of the signal is small. Range attenuation and antenna illumination effects may be fully compensated for.
    [bibtex-key = hellstenAndersson87:Backprojection] [bibtex-entry]


  1037. Christian Matzler. Applications of the interaction of microwaves with the natural snow cover. Remote sensing reviews, 2(2):259-387, 1987. [bibtex-key = Maetzler1987] [bibtex-entry]


  1038. Christian Matzler and Urs Wegmuller. Dielectric properties of freshwater ice at microwave frequencies. Journal of Physics D: Applied Physics, 20(12):1623, 1987.
    Abstract: So far, knowledge about the dielectric properties, especially the loss factor, of ice at microwave frequencies has been unsatisfactory. In this work the authors report on new measurements made over the frequency range from 2 to 100 GHz by a resonator method (2-10 GHz) and a radiometer method (10-100 GHz). Measurements were made with pure and with slightly saline (10 to 13 p.p.m.) ice. The results agree with the assumption of a single minimum of the dielectric loss at 2 to 4 GHz. For pure ice the data are a natural link between the measurements of Westphal made below 1 GHz and the far-infrared spectrum. The influence of small impurities on the dielectric loss is compared with the behaviour of sea ice, and it is found that the same linear relationship with salinity can be applied to both cases.
    [bibtex-key = matzlerWegmuller1987] [bibtex-entry]


  1039. Christian Mätzler. Applications of the interaction of microwaves with the natural snow cover. Remote Sensing Reviews, 2(2):259-387, 1987. [bibtex-key = matzler1987ApplicationsOfTheInteractionOfMicrowavesWithNaturalSnowCover] [bibtex-entry]


  1040. Fabio Rocca. Synthetic Aperture Radar: a New Application for Wave Equation Techniques. Stanford Exploration Project SEP-56, pp 167-189, 1987. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Seismic Migration Techniques.
    Abstract: The techniques of downward continuation and imaging invented for seismic waves can be applied to other types of waves. In this paper, we see how this application can be made in the case of electromagnetic surveys conducted with Synthetic Aperture Radar (SAR). The algorithms used closely follow those used for seismic waves. Differences are induced by alternate wavelengths, wave velocities, distances between sources and reflectors etc. In the case that we analyze in detail, a survey carried out using a satellite, difficulties arise from the fact that the orbit of the satellite cannot be approximated with a simple straight line if the spatial resolution of the survey is high. We determine appropriate techniques for the correction of the distortion induced by the latter and we delimit the resolution of the observed data, as seen from a satellite. Finally we show examples of the application of the technique of seismic migration to satellite data that were irradiated to earth during the short but productive life of SEASAT.
    [bibtex-key = rocca87:SAR] [bibtex-entry]


  1041. Einar Svendsen, Christian Matzler, and Thomas C Grenfell. A model for retrieving total sea ice concentration from a spaceborne dual-polarized passive microwave instrument operating near 90 GHz. International Journal of Remote Sensing, 8(10):1479-1487, 1987. [bibtex-key = Svendsen1987] [bibtex-entry]


  1042. Martti T. Hallikainen, Fawwaz Ulaby, and Mohamed Abdelrazik. Dielectric properties of snow in the 3 to 37 GHz range. IEEE Transactions on Antennas and Propagation, 34(11):1329-1340, November 1986. Keyword(s): Dielectric measurements, Density measurement, Dielectric measurements, Frequency, Predictive models, Scattering, Shape measurement, Size measurement, Snow, Temperature distribution, Volume measurement.
    Abstract: Microwave dielectric measurements of dry and wet snow were made at nine frequencies betweeo 3 and 18 GHz, and at 37 GHz, using two free-space transmission systems. The measurements were conducted during the winters of 1982 and 1983. The following parametric ranges were covered: 1) liquid water content, 0 to 12.3 percent by volume; 2) snow density, 0.09 to 0.42 g cm-3; 3) temperature, 0 to -5 \deg C and -15\deg C (scattering-loss measurements); and 4) crystal size, 0.5 to 1.5 mm. The experimental data indicate that the dielectric behavior of wet snow closely follows the dispersion behavior of water. For dry snow, volume scattering is the dominant loss mechanism at 37 GHz. The applicability of several empirical and theoretical mixing models was evaluated using the experimental data. Both the Debye-like semi-empirical model and the theoretical Polder-Van Santen mixing model were found to describe adequately the dielectric behavior of wet snow. However, the Polder-Van Santen model provided a good fit to the measured values of the real and imaginary parts of wet snow only when the shapes of the water inclusions in snow were assumed to be both nonsymmetrical and dependent upon snow water content. The shape variation predicted by the model is consistent with the variation suggested by the physical mechanisms governing the distribution of liquid water in wet snow.
    [bibtex-key = hallikainenUlabyAbdelrazik1986DielectricPropertiesSnow] [bibtex-entry]


  1043. Michael Y. Jin. Optimal Doppler Centroid Estimation for SAR Data from a Quasi-Homogeneous Source. IEEE Transactions on Geoscience and Remote Sensing, 24(6):1022-1025, 1986. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Satellite SAR.
    Abstract: In synthetic aperture radar (SAR) signal processing, an accurate Doppler centroid is required for most applications involving target motion estimation and antenna pointing direction estimation. In some cases the Doppler centroid can be sufficiently determined using available information regarding the terrain topography, the relative motion between the sensor and the terrain, and the antenna pointing direction. But most often, a highly accurate Doppler centroid value has to be derived by analyzing the received SAR signal itself. This kind of signal processing is referred to as Doppler centroid estimation (DCE). This correspondence briefly describes two DCE algorithms, provides a performance summary for these algorithms, and presents the experimental results. These algorithms include a previously reported one and a newly developed one that is optimized for quasi-homogeneous sources. The performance enhancement achieved by the optimal DCE algorithm is clearly demonstrated by the experimental results.
    [bibtex-key = jin86:DopCentrEst] [bibtex-entry]


  1044. Jong-Sen Lee. Speckle Suppression and Analysis for Synthetic Aperture Radar Images. Optical Engineering, 25(5):636 - 643, 1986. Keyword(s): Synthetic aperture radar, Speckle, Algorithm development, Detection and tracking algorithms, Radar, Radar imaging, Statistical analysis, Statistical modeling, Speckle Filter, Lee filter, SAR.
    Abstract: Speckle appearing in synthetic aperture radar (SAR) images is generated by coherent interference of radar echoes from target scatters. Basically, speckle noise has the nature of a multiplicative noise. In this paper procedures for defining and verifying a statistical noise model are developed, and two multiplicative noise-smoothing algorithms are pre-sented. These two algorithms are computationally efficient and have the potential of achieving real-time or near-real-time processing. Several SEASAT SAR and SIR-B (Shuttle Imaging Radar) images are used for illustration.
    [bibtex-key = leeOpticalEng1986SpeckleSuppressionSyntheticApertureRadarImages] [bibtex-entry]


  1045. Ralph O. Schmidt. Multiple emitter location and signal parameter estimation. IEEE Transactions on Antennas and Propagation, 34(3):276-280, March 1986. Keyword(s): MUSIC, Multiple Signal Classification, null Adaptive arrays, DOA estimation, Direction-of-arrival estimation, Parameter estimation, Signal processing antennas.
    Abstract: Processing the signals received on an array of sensors for the location of the emitter is of great enough interest to have been treated under many special case assumptions. The general problem considers sensors with arbitrary locations and arbitrary directional characteristics (gain/phase/polarization) in a noise/interference environment of arbitrary covariance matrix. This report is concerned first with the multiple emitter aspect of this problem and second with the generality of solution. A description is given of the multiple signal classification (MUSIC) algorithm, which provides asymptotically unbiased estimates of 1) number of incident wavefronts present; 2) directions of arrival (DOA) (or emitter locations); 3) strengths and cross correlations among the incident waveforms; 4) noise/interference strength. Examples and comparisons with methods based on maximum likelihood (ML) and maximum entropy (ME), as well as conventional beamforming are included. An example of its use as a multiple frequency estimator operating on time series is included.
    [bibtex-key = schmidt1986:MUSICOrig] [bibtex-entry]


  1046. Alan di Cenzo. A Comparison of Resolution for Spotlight Synthetic-Aperture Radar and Computer-Aided Tomography. Proceedings of the IEEE, 74:1165-1166, August 1986. Keyword(s): SAR Processing, Tomography, Resolution, Comparison of Algorithms, Spotlight SAR.
    Abstract: It is shown that the difference between computer-aided tomographic (CAT) resolution and spotlight synthetic-aperture radar (SAR) resolution for narrow apertures is a consequence of the magnification in Fourier bandwidth induced by the SAR offset carrier frequency. Implications for CAT are discussed.
    [bibtex-key = DiCenzo86:Comp] [bibtex-entry]


  1047. B. C. Barber. Theory of Digital Imaging from Orbital Synthetic-Aperture Radar. International Journal of Remote Sensing, 6(7):1009-1057, 1985. Keyword(s): SAR Processing, Range Migration, Range Compression, Range Polynomial, Azimuth Processing, Upsampling, 2D Filter, Time-Domain Correlation, Satellite SAR.
    Abstract: Digital synthetic-aperture radar (SAR) imaging techniques have previously only been reported in the literature in a fragmentary manner. This article presents a comprehensive review of the theory of digital SAR imaging from Earth-orbiting satellites. The digital SAR imaging process is explained, including a discussion of various aspects which are specific to satellite-borne SAR. A number of relevant digital-processing techniques are reviewed and it is shown how these techniques may be applied to the processing of digital SAR data. The range migration problem is discussed and various techniques for overcoming it are presented. The paper should be useful not only to the designer of SAR processors, but also to the user of digital SAR data and images.
    [bibtex-key = barber85:SAR] [bibtex-entry]


  1048. John C. Curlander, Benjamin Holt, and Kevin J. Hussey. Determination of sea ice motion using digital SAR imagery. Oceanic Engineering, IEEE Journal of, 10(4):358-367, 1985. Keyword(s): SAR Processing, Image analysis, motion, Image motion analysis, Sea ice, Sea surface electromagnetic scattering, Synthetic-aperture radar.
    Abstract: Using digital SEASAT synthetic aperture radar (SAR) imagery, high-precision densely sampled maps of ice motion have been derived by tracking ice features to determine the small-scale spatial variability of ice deformation. The digital SAR imagery was processed to remove geometric distortions and located on the Earth to an accuracy of about 100 m utilizing an algorithm based on the spacecraft orbital data and the characteristics of the SAR data collection system, independent of attitude information or ground reference points. Radiometric enhancement of the imagery using a variable linear stretch algorithm was performed to remove a system-related gradient and improve the identification of sea ice features. Using ice features common to an overlapping pair of images, vector plots of ice motion were then produced. Examples of ice motion are shown in the marginal ice zone and in the central ice pack where mean displacements of 15.3 km/day and 5.0 km/day were measured, respectively. Considering errors in Earth location with those in feature identification an overall error of 150-200 m in displacement measurements was estimated. The ice motion vector plots indicate a high degree of spatial deformation, demonstrating the potential value of spaceborne SAR data for production of precision large-scale maps of ice displacement with a spatial resolution of ice deformation on scales much less than 100 km.
    [bibtex-key = curlanderHoltHussey85:SeaIce] [bibtex-entry]


  1049. Fuk-Kwok Li, Daniel N. Held, John C. Curlander, and Chialin Wu. Doppler Parameter Estimation for Spaceborne Synthetic-Aperture Radars. IEEE Transactions on Geoscience and Remote Sensing, 23(1):47-56, 1985. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, Clutterlock, Satellite SAR, SEASAT, Doppler Rate Estimation, Autofocus.
    Abstract: Problems in the determination of Doppler parameters for spaceborne synthetic-aperture radar (SAR) data processing are examined. The degradations in image quality due to errors in these parameters are summarized. We show that these parameters can be estimated using accurate spacecraft ancillary data. In cases where such data are not available, we propose two techniques to estimate these parameters using the coherent radar return. These techniques were tested with the Seasat SAR data and the test results demonstrate that the accuracies achieved exceed the system performance requirements. Possible applications of these techniques in other areas of SAR data utilization are briefly discussed.
    [bibtex-key = li85:DopCentrEst] [bibtex-entry]


  1050. Hans J. Liebe. An updated model for millimeter wave propagation in moist air. Radio Science, 20(5):1069-1089, 1985. Keyword(s): Radio oceanography, Radio wave propagation, Remote sensing. [bibtex-key = RDS:RDS2523] [bibtex-entry]


  1051. Matthew N. O. Sadiku. Refractive index of snow at microwave frequencies. Appl. Opt., 24(4):572-575, February 1985. Keyword(s): Microwaves, Snow, refractive index, refraction, birefringence.
    Abstract: A systematic procedure for calculating the refractive index of snow at microwave frequencies is presented. The refractive index of snow at 0{ extdegree}C was calculated for different snow types (classified in terms of snow wetness as dry, most, wet, and watery) and microwave frequencies. For the sake of completeness, the refractive indices of water and ice were also calculated for the same frequencies.
    [bibtex-key = Sadiku1985RefractiveIndexOfSnowTables] [bibtex-entry]


  1052. Carl A. Wiley. Synthetic Aperture Radars. IEEE Trans. Aerosp. Electron. Syst., AES-21(3):440-443, May 1985. Keyword(s): SAR Processing, Azimuth Focusing, Synthetic Aperture Radar, Adaptive optics, Airborne radar, Aircraft, Doppler radar, Laser radar, Optical signal processing, Radar imaging, Radar signal processing, Railway engineering, Synthetic aperture radar.
    Abstract: This is the 1984 Pioneer Award story-told in the author's own words.
    [bibtex-key = wileyTAAS1985SyntheticApertureRadar] [bibtex-entry]


  1053. N. H. Farhat, Charles L. Werner, and T. H. Chu. Prospects for three-dimensional projective and tomographic imaging radar networks. Radio Science, 19(5):1347-1355, 1984. Keyword(s): SAR Processing, Radar, Tomography, SAR Tomography, Holographic Imaging, 3-D Imaging.
    Abstract: Results of a study demonstrating the feasibility of broadband and speckle-free projective imaging of a complex shaped scattering object in the 6-17 GHz range with centimeter resolution are presented. We show how angular, spectral, and polarization diversities can be combined in the data acquisition process, with a unique target-derived reference technique to access the three-dimensional Fourier space of the scatterer cost-effectively. Image retrieval algorithms, based on the projection slice theorem and knowledge of object symmetry, are utilized in obtaining images of a model aircraft with near optical resolution. The implications for high-resolution three-dimensional tomographic imaging radar networks are discussed.
    [bibtex-key = farhatWernerChuRadioScienceTOMO1984] [bibtex-entry]


  1054. Christian Matzler and Erwin Schanda. Snow mapping with active microwave sensors. Remote Sensing, 5(2):409-422, 1984. [bibtex-key = Maetzler1984] [bibtex-entry]


  1055. Earl R. Ferrara and Terry M. Parks. Direction finding with an array of antennas having diverse polarizations. IEEE Transactions on Antennas and Propagation, 31(2):231-236, March 1983. Keyword(s): SAR Processing, Antenna arrays, Estimation, Radio direction-finding, maximum-likelihood (ML) estimation, SAR Tomography, Tomography.
    Abstract: The advantages of using diversely polarized antennas to determine bearings of multiple cochannel narrow-band signals are shown. Three bearing estimation algorithms-maximum likelihood (ML), adapted angular response (AAR), and Music (multiple signal classification)-are extended to handle antenna arrays with diverse polarizations; the maximum entropy method does not readily extend. The proposed algorithms are applicable to arbitrary antenna locations and directional characteristics and arbitrary noise correlations between the antenna outputs. The algorithms are compared on the basis of multiple signal resolution and bearing accuracy in the presence of noise. The Music algorithm exhibits superior performance at moderate to low signal-to-noise ratio (SNR).
    [bibtex-key = ferraraParksPolDOA1983] [bibtex-entry]


  1056. David C. Munson, Jr., James Dennis O'Brien, and W. Kenneth Jenkins. A tomographic formulation of spotslight-mode synthetic aperture radar. Proceedings of the IEEE, 71(8):917-925, August 1983. Keyword(s): SAR Processing, Polar Format Algorithm, Convolution Back-Projection, Spotlight SAR, Spotlight mode, Tomographic formulation.
    Abstract: Spotlight-mode synthetic aperture radar (spotlight-mode SAR) synthesizes high-resolution terrain maps using data gathered from multiple observation angles. This paper shows that spotlight-mode SAR can be interpreted as a tomographic reeonstrution problem and analyzed using the projection-slice theorem from computer-aided tomograpy (CAT). The signal recorded at each SAR transmission point is modeled as a portion of the Fourier transform of a central projection of the imaged ground area. Reconstruction of a SAR image may then be accomplished using algorithms from CAT. This model permits a simple understanding of SAR imaging, not based on Doppler shifts. Resolution, sampling rates, waveform curvature, the Doppler effect, and other issues are also discussed within the context of this interpretation of SAR.
    [bibtex-key = munsonOBrienJenkins1983:TomoFormulationSpotlightSAR] [bibtex-entry]


  1057. Erwin Schanda, Christian Matzler, and Klaus Kunzi. Microwave remote sensing of snow cover. International Journal of Remote Sensing, 4(1):149-158, 1983. [bibtex-key = Schanda1983] [bibtex-entry]


  1058. Christian Matzler, Erwin Schanda, and Walter Good. Towards the definition of optimum sensor specifications for microwave remote sensing of snow. IEEE Transactions on Geoscience and Remote Sensing, (1):57-66, 1982. [bibtex-key = Matzler1982] [bibtex-entry]


  1059. Chialin Wu, K.Y. Liu, and M. Jin. Modeling and a Correlation Algorithm for Spaceborne SAR Signals. IEEE_J_AES, 18(5):563-575, September 1982. Keyword(s): SAR Processing, Range-Doppler Algorithm, RDA, Spaceborne SAR.
    Abstract: A mathematical model of a spaceborne synthetic aperture radar (SAR) response is presented. The associated SAR system performance, in terms of the resolution capability, is also discussed. The analysis of spaceborne SAR target response indicates that the SAR correlation problem is a two-dimensional one with a linear shift-variant response function. A new digital processing algorithm is proposed here in order to realize an economical digital SAR correlation system. The proposed algorithm treats the two-dimensional correlation by a combination of frequency domain fast correlation in the azimuth dimension and a time-domain convolver type of operation in the range dimension. Finally, digitally correlated SEASAT satellite SAR imagery is used in an exemplary sense to validate the SAR response model and the new digital processing technique developed.
    [bibtex-key = wuLiuJin1982:RDA] [bibtex-entry]


  1060. Robert G. Keys. Cubic convolution interpolation for digital image processing. IEEE Transactions on Acoustics, Speech, and Signal Processing, 29(6):1153-1160, December 1981. Keyword(s): Interpolation, Boundary conditions, Convolution, Digital images, Image converters, Image processing, Image sampling, Interpolation, Kernel, Sampling methods, Signal processing algorithms.
    Abstract: Cubic convolution interpolation is a new technique for resampling discrete data. It has a number of desirable features which make it useful for image processing. The technique can be performed efficiently on a digital computer. The cubic convolution interpolation function converges uniformly to the function being interpolated as the sampling increment approaches zero. With the appropriate boundary conditions and constraints on the interpolation kernel, it can be shown that the order of accuracy of the cubic convolution method is between that of linear interpolation and that of cubic splines. A one-dimensional interpolation function is derived in this paper. A separable extension of this algorithm to two dimensions is applied to image data.
    [bibtex-key = keysTASSP1981CubicConvolutionInterpolation] [bibtex-entry]


  1061. Jong-Sen Lee. Refined filtering of image noise using local statistics. Computer Graphics and Image Processing, 15(4):380-389, 1981. Keyword(s): Speckle Filter, Refined Lee filter, Speckle, Synthetic aperture radar, SAR.
    Abstract: An effective algorithm for digital image noise filtering is presented in this paper. Most noise filtering techniques such as Kalman filter and transform domain methods require extensive image modeling and produce filtered images with considerable contrast loss. The algorithm proposed in this paper is an extension of Lee's local statistics method modified to utilize local gradient information. It does not require image modeling, and it will not smear edges and subtle details. For both the additive and multiplicative noise cases, the local mean and variance are computed from a reduced set of pixels depending on the orientation of the edge. Consequently, noise along the edge is removed, and the sharpness of the edge is enhanced. For practical applications when the noise variance is spatially varying and unknown, an adaptive filtering algorithm is developed. Experiments show its good potential for processing real-life images. Examples on images containing 256�256 pixels are given to substantiate the theoretical development.
    [bibtex-key = leeCGIP1981RefinedFilteringOfImageNoiseUsingLocalStatistics] [bibtex-entry]


  1062. Jack L. Walker. Range-Doppler Imaging of Rotating Objects. IEEE Transactions on Aerospace and Electronic Systems, AES-16(1):23-52, January 1980. Keyword(s): SAR Processing, Polar Format Algorithm, PFA.
    Abstract: During the integration time required to obtain fine Dopplerfrequency resolution in a range-Doppler imaging radar, a point on a rotating object may move through several range and Doppler resolution cells and produce a smeared image. This motion can be compensated by storing the appropriately processed return pulse, and the angular coordinates are determined by the angular coordinates of the radar antenna. The resulting stored data represents the three-dimensional Fourier transform of the object reflectivity density, and hence can be processed by an inverse Fourier transformation. Also included is an analysis of the three-dimensional radar/object geometry with separate source and receiver locations. The effects of various system aberrations are investigated and experimental results from a microwave test range which demonstrate the image improvement are presented.
    [bibtex-key = walker1980:PolarFormatAlgorithm] [bibtex-entry]


  1063. John W. Brewer. Kronecker products and matrix calculus in system theory. IEEE Transactions on Circuits and Systems, 25(9):772-781, September 1978. Keyword(s): Algebra, Algebraic and geometric techniques, Linear systems, time-invariant continuous-time, Matrix functions, Parameter identification, Algebra, Calculus, Feedback, Helium, Matrices, Sensitivity analysis, Steady-state, Stochastic systems, Subspace constraints, Sufficient conditions.
    Abstract: The paper begins with a review of the algebras related to Kronecker products. These algebras have several applications in system theory including the analysis of stochastic steady state. The calculus of matrix valued functions of matrices is reviewed in the second part of the paper. This calculus is then used to develop an interesting new method for the identifiication of parameters of lnear time-invariant system models.
    [bibtex-key = brewer1978KroneckerProducts] [bibtex-entry]


  1064. R. H. Stolt. Migration by Fourier Transform. Geophysics, 43(1):23-48, February 1978. Keyword(s): SAR Processing, Migration, Wavenumber Domain Algorithm, omega-k, Range Migration Algorithm, Stolt Mapping.
    Abstract: Wave equation migration is known to be simpler in principle when the horizontal coordinates are replaced by their Fourier conjugates. Two practical migration schemes utilization this concept are developed in this paper. One scheme extends the Claerbout finite difference method, greatly reducing dispersion problems usually associated with this method at higher dips and frequencies. The second scheme effects a Fourier transform in both space and time; by using the full scalar wave equation in the conjugate space, the method eliminates (up to the aliasing frequency) dispersion altogether. The second method in particular appears adaptable to three-dimensional migration and migration before stack.
    [bibtex-key = stolt78:Migration] [bibtex-entry]


  1065. J. W. Goodman. Some fundamental properties of speckle. J. Opt. Soc. Am., 66(11):1145-1150, November 1976. Keyword(s): Laser light, Laser speckle, Polarization, Spatial frequency, Speckle patterns, Wavefronts.
    Abstract: A speckle pattern formed in polarized monochromatic light may be regarded as resulting from a classical random walk in the complex plane. The resulting irradiance fluctuations obey negative exponential statistics, with ratio of standard deviation to mean (i.e., contrast) of unity. Reduction of this contrast, or smoothing of the speckle, requires diversity in polarization, space, frequency, or time. Addition of M uncorrelated speckle patterns on an intensity basis can reduce the contrast by 1/{ extsurd}M. However, addition of speckle patterns on a complex amplitude basis provides no reduction of contrast. The distribution of scale sizes in a speckle pattern (i.e., the Wiener spectrum) is investigated from a physical point of view.
    [bibtex-key = goodmanJOptSocAm1976FundamentalPropertiesOfSpeckle] [bibtex-entry]


  1066. John C. Kirk. Motion Compensation for Synthetic Aperture Radar. IEEE Transactions on Aerospace and Electronic Systems, AES-11(3):338-348, May 1975. Keyword(s): SAR Processing, Airborne SAR, Motion Compensation, MoComp, Stripmap SAR, Spotlight SAR.
    Abstract: A generalized motion compensation approach applicable to all SAR modes, i.e., strip mapping (side-looking or squint), spotlight (or telescope) mapping, and Doppler beam sharpened mapping (DBS), is described. The basic concept is the formation for unit vector and the slaving of the real illuminating antenna and the processed synthetic antenna to this unit vector. The amount of motion compensation which is required is developed in terms of transfer curves for the main motion reduction paths, i.e., translational, rotational (lever arm), and real antenna stabilization. The transfer curves are obtained by dividing the expected motion spectrum by the required sensitivity spectrum. The most critical motion reduction path for typical parameters is shown to be the translational path. The lever arm and real antenna stabilization paths are less critical, but must also be implemented.
    [bibtex-key = kirk1975MoCo] [bibtex-entry]


  1067. Leroy C. Graham. Synthetic interferometer radar for topographic mapping. Proceedings of the IEEE, 62(6):763-768, June 1974. Keyword(s): SAR Processing, SAR Interferometry, InSAR, Radar imaging, Radar antennas, Position measurement, Radar measurements, Optical films, Production, Airborne radar, Clouds, Aircraft navigation, Coordinate measuring machines.
    Abstract: The production of topographic maps requires two kinds of information. First, the detail to be placed on the map sheet must be identified. Second, the positions of the various objects and features must be measured in three dimensions. Current airborne radar technology provides the means to satisfy both of these requirements in adverse weather and at any time, day or night. Radar used specifically for this purpose employs synthetic-aperture techniques to obtain fine resolution measurement in two dimensions and interferometry to obtain the third measurement.
    [bibtex-key = grahamProcIEEE1974SyntheticInterferometerRadarForTopographicMapping] [bibtex-entry]


  1068. W. M. Brown, G. G. Houser, and R. G. Jenkins. Synthetic aperture processing with limited storage and presumming. IEEE Transactions on Aerospace and Electronic Systems, 9:166-176, 1973. Keyword(s): SAR Processing, Presumming, Limited Storage.
    Abstract: Presumming of data prior to correlation is considered as a means for achieving the desired performance in a side-looking radar with a minimum amount of digital storage. A presummer with optimal resolution is derived for this purpose, and the optimal apportionment of the available storage capacity between the presumming and correlation operations is determined. The optimal presumming coefficients are given for the condition that the illumination pattern of the antenna illuminates uniformly a Doppler bandwidth equal to the PRF of the radar. The obtainable optimum resolution is given as a function of total storage and of the number of transmitted pulses in the received Doppler history. Expressions are also given for responses with presumming for linearly frequency-modulated signals and their matched filters.
    [bibtex-key = BrownHouserJenkins73:Presumming] [bibtex-entry]


  1069. Georges Matheron. The Intrinsic Random Functions and Their Applications. Advances in Applied Probability, 5(3):439-468, 1973. Keyword(s): Kriging, intrinsic random functions, IRF, linear intrinsic estimator, best linear intrinsic estimator (BLIE), BLIE.
    Abstract: The intrinsic random functions (IRF) are a particular case of the Guelfand generalized processes with stationary increments. They constitute a much wider class than the stationary RF, and are used in practical applications for representing non-stationary phenomena. The most important topics considered are: existence of a generalized covariance (GC) for which statistical inference is possible from a unique realization; theory of the best linear intrinsic estimator (BLIE) used for contouring and estimating problems; the turning bands method for simulating IRF; and the models with polynomial GC, for which statistical inference may be performed by automatic procedures.
    [bibtex-key = matheron1973] [bibtex-entry]


  1070. J. Saastamoinen. Contributions to the theory of atmospheric refraction: Part II. Refraction corrections in satellite geodesy. Bulletin Géodésique (1946-1975), 107(1):13-34, 1973. [bibtex-key = saastamoinen1973AtmosphericRefractionPartIIRefractionCorrectionsInSatelliteGeodesy] [bibtex-entry]


  1071. J. Saastamoinen. Atmospheric Correction for the Troposphere and Stratosphere in Radio Ranging Satellites, pages 247-251. American Geophysical Union, 1972. Keyword(s): Meteorology, Radio ranging of satellites, Refractive index of air, Stratosphere, Troposphere.
    Abstract: Since the barometer measures the weight of the overlying atmosphere, it follows by the law of Gladstone and Dale that the height integral int (n - 1)dr of the atmospheric refractivity for radio microwaves, taken from ground level up to the top of the stratosphere, is in a dry atmosphere directly proportional to ground pressure. The refractivity integral, therefore, can be determined without detailed knowledge of the height distribution of the refractive index, which not only simplifies the derivation of refraction formulas in which atmospheric models have been used hitherto, but also improves their accuracy. The range correction for troposphere and stratosphere to be subtracted from the observed microwave distance will be given by the formula ds (meters) = 0.002277 sec z [p + (1255/T + 0.05)e - 1.16 tan^2 z] where z is the zenith distance, p is the total barometric pressure and e is the partial pressure of water vapor, both in millibars, and T is the absolute temperature in degrees Kelvin.
    [bibtex-key = saastamoinen1972AtmosphericPathDelay] [bibtex-entry]


  1072. J. Saastamoinen. Contributions to the theory of atmospheric refraction. Bulletin Géodésique (1946-1975), 105(1):279-298, 1972.
    Abstract: Since the barometer measures the weight of the overlying atmosphere, it follows by the law of Gladstone and Dale that the height integral of the atmospheric refractivity for light, taken from ground level up to the top of the atmosphere, is directly proportional to ground pressure. The refractivity integral, therefore, can be determined without detailed knowledge of the height distribution of the refractive index, which not only simplifies the derivation of refraction formulas in which atmospheric models have been used hitherto, but also improves their accuracy. For zenith distances not exceeding about 75 degrees, the correction for astronomical refraction will be given by the standard formula$$\begin{gathered} \Delta z''_0 = 16''.271 tan z\left[ {1 + 0.0000394 tan^2 z\left( {\frac{{p - 0.156e}}{T}} \right)} \right]\left( {\frac{{p - 0.156e}}{T}} \right) - \hfill \\ - 0''.0749 (tan^3 z + tan z)\left( {\frac{p}{{1000}}} \right) \hfill \\ \end{gathered} $$wherez is the apparent zenith distance,p is the total pressure ande is the partial pressure of water vapour, both in millibars, andT is the absolute temperature in degrees Kelvin Part II of the paper contains further applications of the theory to refraction problems in satellite geodesy, including the photogrammetric refraction and the atmospheric corrections in the ranging of artificial satellites.
    [bibtex-key = saastamoinen1972AtmosphericRefractionPart1] [bibtex-entry]


  1073. H. S. Hopfield. Tropospheric Effect on Electromagnetically Measured Range: Prediction from Surface Weather Data. Radio Science, 6(3):357-367, 1971.
    Abstract: Knowledge of the height integral of atmospheric refractivity (n-1), where n is the refractive index, is essential for prediction of atmospheric range effect at any elevation angle. Observed values of the height integral for the lower, nonionized atmosphere can be obtained from weather balloon ascent data. Year-long collections of data from widely separated locations were used to relate this integral to surface data. Although (n-1) at any point in a dry atmosphere depends on both pressure and temperature (the ratio P/T), the height integral of the observed dry part of (n-1) is a linear function of surface pressure only, not of temperature. This is theoretically correct since P/T is equivalent to density, and the integral of density with height yields surface pressure. By application of this finding, the equivalent height for a (theoretically justified) quartic (n-1) model (dry part) should be found to vary directly as surface temperature; the value obtained (least-squares fit to observed data) is 40.1 km for surface T=0 degree C with a height expansion coefficient of 0.149 km per surface degree C. This would reduce the equivalent height to zero near 0 degree Kelvin. This theoretical model matches observed height integrals with an rms error of a few millimeters out of 2.3 meters (far less than 1\%). Agreement between stations is excellent. A study of the more variable but much smaller wet part is in progress. The wet part is significant at radio but not at optical frequencies.
    [bibtex-key = hopfieldRadioScience1971TroposphericEffectOnElectromagneticallyMeasuredRange] [bibtex-entry]


  1074. Jack Capon. High-resolution frequency-wavenumber spectrum analysis. Proceedings of the IEEE, 57(8):1408-1418, August 1969. Keyword(s): SAR Processing, Capon, Capon beamforming, SAR Tomography, Tomography, super-resolution.
    Abstract: The output of an array of sansors is considered to be a homogeneous random field. In this case there is a spectral representation for this field, similar to that for stationary random processes, which consists of a superposition of traveling waves. The frequency-wavenumber power spectral density provides the mean-square value for the amplitudes of these waves and is of considerable importance in the analysis of propagating waves by means of an array of sensors. The conventional method of frequency-wavenumber power spectral density estimation uses a fixed-wavenumber window and its resolution is determined essentially by the beam pattern of the array of sensors. A high-resolution method of estimation is introduced which employs a wavenumber window whose shape changes and is a function of the wavenumber at which an estimate is obtained. It is shown that the wavenumber resolution of this method is considerably better than that of the conventional method. Application of these results is given to seismic data obtained from the large aperture seismic array located in eastern Montana. In addition, the application of the high-resolution method to other areas, such as radar, sonar, and radio astronomy, is indicated.
    [bibtex-key = capon1969:CaponBeamforming] [bibtex-entry]


  1075. H. E. Rauch, F. Tung, and T.C. Striebel. Maximum Likelihood Estimates of Linear Dynamic Systems. AIAA JOURNAL, 3(8):1445-1450, August 1965. Keyword(s): INS/GNSS, INS/GNSS integration, Rauch-Tung-Striebel Smoothing, RTS, Kalman Filter.
    Abstract: This paper considers the problem of estimating the states of linear dynamic systems in the presence of additive Gaussian noise. Difference equations relating the estimates for the problems of filtering and smoothing are derived as well as a similar set of equations relating the covariance of the errors. The derivation is based on the method of maximum likelihood and depends primarily on the simple manipulation of the probability density functions. The solutions are in a form easily mechanized on a digital computer. A numerical example is included to show the advantage of smoothing in reducing the errors in estimation. In the Appendix the results for discrete systems are formally extended to continuous systems.
    [bibtex-key = rauchTungStriebelAIAA1965MLEstimatesOfLinearDynamicSystems] [bibtex-entry]


  1076. E.K. Smith and S. Weintraub. The Constants in the Equation for Atmospheric Refractive Index at Radio Frequencies. Proceedings of the IRE, 41(8):1035-1037, August 1953. Keyword(s): Atmosphere, Atmospheric measurements, Equations, Humidity, Microwave measurements, Microwave theory and techniques, Radio frequency, Radio propagation, Refractive index, Temperature distribution.
    Abstract: Recent improvements in microwave techniques have resulted in precise measurements which indicate that the conventional constants K1 = 79 K/mb and K2' =4,800 K in the expression for the refractivity of air, N=(n-1) 10^6 = [K1/T](p+ K2'(e/T)] should be revised. Various laboratories appear to have arrived at this conclusion independently. In much of radio propagation work the absolute value of the refractive index of the atmosphere is of small moment. However, in some work it is important and it seems highly desirable to decide upon a particular set of constants. Through consideration of the various recent experiments this paper arrives at a relation 77.6 e N = ~ p + 4,810-T T where p=total pressure in millibars e=partial pressure of water vapor in millibars T=absolute temperature= (degree C + 273). This expression is considered to be good to 0.5 per cent in N for frequencies up to 30,000 mc and normally encountered ranges of temperatures, pressure and humidity.
    [bibtex-key = smithWeintraub1953AtmosphericRefractiveIndex] [bibtex-entry]


  1077. L. Essen and K. D. Froome. The Refractive Indices and Dielectric Constants of Air and its Principal Constituents at 24,000 Mc/s. Proceedings of the Physical Society. Section B, 64(10):862, 1951.
    Abstract: The refractive indices of air and its principal constituents have been measured at a frequency of 24000 Mc/s. with a precision comparable with that obtained in the optical range. The method is based on the measurement of the resonant frequency of a cavity resonator first when it is filled with the gas and then when it is evacuated. The source of oscillations used is a Pound-stabilized velocity-modulated oscillator, and its frequency is measured by reference to a high harmonic of a quartz standard, with a precision of 1 part in 10^8 . The cavity is provided with a tuning plunger, which changes the resonant frequency through a range of about 10 Mc/s, and is calibrated to an accuracy of within 1 kc/s. Most of the measurements were made by the frequency-change method and the plunger was used in a narrow region only for a precise setting to resonance. It is possible by a larger movement of the plunger to compensate for the whole frequency change due to the removal of the gas and thus to work at a fixed frequency. The following results were obtained for ( n - 1)10 6 where n is the refractive index at 0 deg C., 760 mm Hg: dry CO2 - free air, 288.15 +/- 0.1; nitrogen, 294.1 +/-0 1; oxygen, 266.4 +/-0.2; argon, 277.8 +/- 0.2; carbon dioxide, 494 +/- 1; and the value for water vapour at 20 deg C, 10 mm. Hg pressure was 60.7 +/- 0.1. The dielectric constants can be calculated from the relationship mu epsilon = n^2 , where mu is the magnetic permeability and epsilon is the dielectric constant, the values of (mu - 1) 10^6 being taken as 0.4 for air, 1.9 for oxygen and zero for the other gases. Accurate formulae are given for obtaining the refractive index of moist air at different atmospheric conditions, and are reduced to the following simple formula which is applicable for normal atmospheric conditions: (n_t,p - 1)*10^6 = (103.49/T)*p_1 + (177.4/T)*p_2 + (86.26/T)(1+5748/T)*p_3, where p_1, p_2, p_3 are the partial pressures of dry air, carbon dioxide and water vapour, t is the temperature in degrees C., and T = 273 + t is the absolute temperature. A value of 1.839+/-0.002 * 10^-18 E S.U. was derived for the dipole moment of the water vapour molecule.
    [bibtex-key = essenFroome1951RefractiveIndexAir] [bibtex-entry]


  1078. Frits Zernike. The concept of degree of coherence and its application to optical problems. Physica, 5(8):785 - 795, 1938. Keyword(s): van Cittert-Zernike.
    Abstract: The maximum visibility of the interferences obtainable from two points in a wave field is defined as their degree of coherence gamma. By a simple statistical method general formulae are found for deducing gamma from illumination data. For any extended lightsource gamma is found equal to the amplitude in a certain diffraction image. It does not change by the use of a condensing lens, but depends only on the aperture of the illuminating cone. These properties are applied to the microscopic observation of objects in transmitted light.
    [bibtex-key = zernikePHYSICA1938] [bibtex-entry]


  1079. Pieter Hendrik van Cittert. Die wahrscheinliche Schwingungsverteilung in einer von einer Lichtquelle direkt oder mittels einer Linse beleuchteten Ebene. Physica, 1(1-6):201-210, 1934. Keyword(s): van Cittert-Zernike.
    Abstract: Es wird die wahrscheinliche Schwingungsverteilung in einer von einer Lichtquelle entweder direkt oder mittels einer Linse beleuchteten Ebene untersucht. Es zeigt sich, dass diese Verteilung in beiden F\"allen v\"ollig identisch ist und nur abh\"angt von dem \"Offnungswinkel des Lichtb\"undels, welches die Ebene beleuchtet. Die Korrelation zwischen den Schwingungsvektoren in verschiedenen Punkten der Ebene wird bestimmt durch eine Funktion, welche mit der Beugungsfunktion der Linse identisch ist.
    [bibtex-key = vanCittertPHYSICA1934] [bibtex-entry]


  1080. Car-borne and UAV-borne SAR interferometry with a compact L-band FMCW radar for mobile mapping of surface displacements. In preparation for submission to IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, . [bibtex-key = FreyEtAl2021CarAndUAVBorneSARInterferometryGammaLBandSARMobileMappingOfSurfaceDisplacements] [bibtex-entry]


  1081. O. Antropov, J. Miettinen, T. Hame, R. Yrjo, L. Seitsonen, R.E. McRoberts, M. Santoro, O. Cartus, N.M. Duran, M. Herold, M. Pardini, K. Papathanassiou, and I. Hajnsek. Intercomparison of Earth Observation Data and Methods for Forest Mapping in the Context of Forest Carbon Monitoring. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5777-5780, July 2022. [bibtex-key = Antropov2022] [bibtex-entry]


  1082. D. R. Boyd, A. M. Alam, M. Kurum, A. C. Gurbuz, and B. Osmanoglu. Preliminary Snow Water Equivalent Retrieval of SnowEX20 SWESARR Data. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 3927-3930, July 2022.
    Abstract: This paper explores the retrieval of snow water equivalent (SWE) through the use of machine learning techniques and active radar data collected over the 2020 SnowEx campaign. The retrieval makes use of active radar measurements provided by NASA's SWESARR instrument for direct sensing of snowpack sensitivity to SWE. The example results show that an RMSE of 1.93 cm can be obtained through a combined use of SAR data with sufficient ancillary data. Such results may indicate successful SWE estimation by means of pairing spaceborne SAR measurements with sufficient auxiliary information.
    [bibtex-key = boydAlamKurumGurbuzOsmanogluIGARSS2022PreliminarySWERetrievalOfSnowEX20SweSARRData] [bibtex-entry]


  1083. Othmar Frey, Charles Werner, and Rafael Caduff. Dual-frequency car-borne DInSAR at L-band and Ku-band for mobile mapping of surface displacements. In Proc. of EUSAR 2022 - 14th European Conference on Synthetic Aperture Radar, pages 489-492, July 2022. VDE Verlag GmbH. Keyword(s): SAR Processing, mobile mapping, surface displacements, mobile mapping of surface displacements, landslide, geohazard mapping, car-borne SAR, Interferometry, SAR Interferometry, repeat-pass Interferometry, differential interferometry, DInSAR, Gamma L-band SAR, L-band, Ku-band, Gamma Portable Radar Interferometer, GPRI, INS, GNSS, Honeywell, Honeywell HGuide n580, deformation, displacement, monitoring, UAV, Time-Domain Back-projection, TDBP, GPU, NVIDIA, CUDA.
    Abstract: We present our recent developments and experimental results on car-borne mobile mapping of ground-surface displacementswith our in-house-developed SAR systems. Recently, we have successfully demonstrated car-borne andUAV-borne DInSAR with the Gamma L-band SAR system. Meanwhile we have upgraded our car-borne measurementconfiguration that now permits acquiring simultaneously at L-band and at Ku-band. We show first interferometricresults with short temporal baselines from simultaneous acquisitions at both frequencies and in particular we discussthe complementary aspects of the two frequencies in terms of sensitivity to line-of-sight displacements and temporaldecorrelation in typical measurement scenarios.
    [bibtex-key = freyWernerCaduffEUSAR2022DualFrequencyCarborneDInSARatLBandAndKuBandForMobileMappingOfSurfaceDisplacements] [bibtex-entry]


  1084. M. Lavalle, C. Telli, N. Pierdicca, U. Khati, O. Cartus, and J. Kellndorfer. Global Sentinel-1 InSAR Coherence: Opportunities for Model-Based Estimation of Land Parameters. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1133-1136, July 2022. [bibtex-key = Lavalle2022] [bibtex-entry]


  1085. Kaiyu Liu, Robert Wang, Heng Zhang, Dacheng Liu, Naiming Ou, Yafeng Chen, Haixia Yue, Weidong Yu, Yunkai Deng, Da Liang, Yuanbo Jiao, Jili Wanga, and Wei Yu. LuTan-1: An Innovative L-band Spaceborne SAR Mission. In Proc. of EUSAR 2022 - 14th European Conference on Synthetic Aperture Radar, pages 614-618, 2022. VDE. Keyword(s): SAR Mission, LuTan-1, LT-1, L-band, Tandem, formation flying, bistatic, bistatic SAR, spaceborne SAR, synchronization link.
    Abstract: The LuTan-1 (referred as LT-1) mission is the first bistatic spaceborne SAR mission for civil applications in China, which contains two full-polarimetric L-band SAR satellites operating with flexible flight configurations. The life time of the individual satellites is specified as 8 years, and the two data acquisition stages are planned. In phase I, two satellites fly in a formation with a variable baseline, and the bistatic InSAR stripmap mode is utilized to acquire the global digital elevation and terrain models with high accuracy and spatial resolution. In phase II, two satellites shall share the common reference orbit with a 180-degree orbital phasing difference. The repeat cycle will be decreased to 4 days and topographic variations with millimetre accuracy at large scale can be measured using the differential InSAR technique. Many meth-odologies and technologies of high degree of innovation are proposed and employed. Both satellites have been success-fully launched in Jan.26, and Feb. 28, 2022, respectively. In the following lifetime, LuTan-1 will continually provide observation data of high-quality which could provide new possibilities, advances and relevant information for the land dynamics monitoring.
    [bibtex-key = liuaEtAlEUSAR2022LuTan1AnInnovativeLBandSpaceborneSARMission] [bibtex-entry]


  1086. Robert Siegmund, Ramon Brcic, Paul Kotzerke, and Michael Eineder. The European Ground Motion Service EGMS - Processing Central Europe with First Results on Quality and Point Densities. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5105-5108, July 2022.
    Abstract: We report on first results of the European Ground Motion Service from the perspective of GAF AG and DLR who are responsible for specific tasks and areas of the overall project. The EGMS products and processing chain are shortly described and first results on point density and data quality are presented.
    [bibtex-key = siegmundBrcicKotzerkeEinederIGARSS2022EGMSFirstResultsOnQualityAndPointDensities] [bibtex-entry]


  1087. Marcel Stefko, Othmar Frey, and Irena Hajnsek. Snow Characterization at Ku-Band with a Bistatic Polarimetric Ground-Based Radar. In IGARSS 2022 - 2022 IEEE International Geoscience and Remote Sensing Symposium, pages 4256-4259, July 2022.
    Abstract: The Ku-band provides opportunities for investigations of snow morphology through radar observations, since it exhibits a relatively high amount of scattering even from snow layers of limited depth, while maintaining low absorption. Due to technological and practical challenges, the bistatic parameter space of Ku-band radar observations of natural media such as snow, has been relatively unexplored. We present radar measurements of snow cover obtained with KAPRI, a bistatic polarimetric Ku-band radar system. In August 2021 and March 2022, we carried out time series observations of the Aletsch glacier in the Swiss Alps, acquiring a fully-polarimetric interferometric time series of both monostatic and simultaneous bistatic observations of the glacier's accumulation zone. This dataset will serve as a test-bed to investigate new snow parameter inversion methods based on bistatic Ku-band radar data. The bistatic polarimetric measurement configuration, as well as preliminary results of the analysis of radar backscatter, are presented.
    [bibtex-key = stefkoFreyHajnsekIGARSS2022SnowCharacterizationAtKuBandWithABistaticPolarimetricGroundBasedRadar] [bibtex-entry]


  1088. Tazio Strozzi, Rafael Caduff, Nina Jones, Andrea Manconi, and Urs Wegm�ller. L-Band StripMap-ScanSAR Persistent Scatterer Interferometry in Alpine Environments with ALOS-2 PALSAR-2. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1644-1647, 2022. [bibtex-key = Strozzi2022a] [bibtex-entry]


  1089. Haokui Xu, Leung Tsang, and Xiaolan Xu. Tomography imaging of terrestrial snow for SWE retrieval using frequency-angular correlation functions and asymmetrical distorted Born's approximation. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 4557-4558, July 2022.
    Abstract: Stratification in terrestrial snow is a key factor in the retrieval of snow water equivalence (SWE) due to the different snow volume fractions and particle sizes. In studying the layered structure of snow, radar tomography has been used and multiple ground based experiments are performed. The conventional back projection method has been used to construct the image based on radar measurements at different incident angles and frequencies. However, the conventional back projection method based on Born's approximation would show deformation in the final snow image. In this paper, we use the asymmetrical distorted Born's approximation to correct the deformation in the image.
    [bibtex-key = xuTsangXuIGARSS2022TomographyImagingOfTerrestrialSnowForSWERetrieval] [bibtex-entry]


  1090. Davide Castelletti, Gordon Farquharson, Craig Stringham, Michael Duersch, and Duncan Eddy. Capella Space First Operational SAR Satellite. In 2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, pages 1483-1486, July 2021.
    Abstract: Capella Space launched its first commercial Synthetic Aperture Radar (SAR) satellite in August 2020. After commissioning phase, Capella started commercial operations in January 2021, deploying a fully-functional SAR system that demonstrates three engineering milestones: an 8-m2 deployable reflector mounted on small and agile satellite; the capability to process and timely deliver 0.5 m resolution spotlight images with 9 looks; and an automated tasking and delivery system that simplifies the ordering of high-quality SAR imagery for expert and novice users. In this paper, we present results from the commissioning and calibration/validation operations. We also compare images collected with the variety of imaging modes that Capella systems can collect.
    [bibtex-key = castellettiEtAlIGARSS2021CapellaSpaceFirstOperationalSARSatellite] [bibtex-entry]


  1091. C. Derksen, J. King, S. Belair, C. Garnaud, V. Vionnet, V Fortin, J. Lemmetyinen, Y. Crevier, P. Plourde, B. Lawrence, H. van Mierlo, G. Burbidge, and P. Siqueira. Development of the Terrestrial Snow Mass Mission. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 614-617, July 2021. IEEE. Keyword(s): Radar Remote Sensing, snow, Terrestrial Snow Mass Mission, TSMM, Ku-band, Ka-band.
    Abstract: For northern countries like Canada, seasonal snow cover is a key component of the water cycle and a commodity of high importance to public safety, economic sustainability, and ecosystem function. Despite this importance, snow water equivalent (SWE - the amount of water stored by snow) information from existing surface observing networks and satellite data does not adequately address most user needs. To address this gap, a new synthetic aperture radar (SAR) mission capable of providing information on terrestrial SWE at previously unrealized spatial resolution is currently under development. The Terrestrial Snow Mass Mission (?TSMM?) will provide moderate resolution (500m) dual frequency (13.5/17.25 GHz) Ku-band radar measurements across all northern hemisphere snow covered areas every 7 days. Data from this mission will be used at Environment and Climate Change Canada to (1) provide a new level of information on the temporal/spatial variability in SWE in support of climate services, and (2) feed into environmental prediction and analysis systems to improve weather and hydrological forecasts.
    [bibtex-key = derksenEtAl2021DevelopmentOfTheTerrestrialSnowMassMissionTSMM] [bibtex-entry]


  1092. Gordon Farquharson, Davide Castelletti, Craig Stringham, and Duncan Eddy. An Update on the Capella Space Radar Constellation. In EUSAR 2021; 13th European Conference on Synthetic Aperture Radar, pages 1-4, March 2021.
    Abstract: We present an update on the Capella synthetic aperture radar constellation. Analysis of the performance of the radar is discussed and sample imagery collected is shown. We find that the imagery meets or exceeds pre-launch spatial resolution and radiometric goals.
    [bibtex-key = farquharsonCastellettiStringhamEddyEUSAR2021UpdateOnCapellaSpaceRadarConstellation] [bibtex-entry]


  1093. Othmar Frey and Charles L. Werner. UAV-borne repeat-pass SAR interferometry and SAR tomography with a compact L-band SAR system. In Proc. Europ. Conf. Synthetic Aperture Radar, EUSAR, pages 181-184, March 2021. VDE. Keyword(s): SAR Processing, UAV, SAR Tomography, Time-Domain Back-projection, TDBP, GPU, mobile mapping, surface displacements, mobile mapping of surface displacements, landslide, geohazard mapping.
    Abstract: In this contribution, we present SAR image focusing, interferometric, and first tomographic processing results computed from repeat-pass SAR data sets acquired on-board of a vertical-take-off-and-landing (VTOL) unmanned aerial vehicle (UAV): the data was acquired using a novel compact FMCW L-band SAR system in two repeat-pass SAR campaigns flown on 2019-02-13 and 2019-03-28, respectively. In these demonstration campaigns, the Gamma L-band SAR system was deployed and operated on Aeroscout's VTOL UAV Scout B1-100. Repeat-pass interferograms and coherence maps with a temporal baseline of up to 43 days are presented and a tomographic profile obtained from short-term repeat-pass measurements is shown. The results demonstrate the feasibility of UAV-borne repeat-pass SAR interferometry and SAR tomography at L-band
    [bibtex-key = freyWernerEUSAR2021UAVborneRepeatPassSARInterferometryAndSARTomography] [bibtex-entry]


  1094. Othmar Frey, Charles L. Werner, Andrea Manconi, and Roberto Coscione. Measurement of surface displacements with a UAV-borne/car-borne L-band DInSAR system: system performance and use cases. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 628-631, 2021. IEEE. Keyword(s): SAR Processing, SAR interferometry, mobile mapping, car-borne SAR, UAV, airborne SAR, surface displacements, landslide, geohazard, monitoring, terrestrial radar interferometer, back- projection, GPU, CUDA, interferometry, L-band, INS, GNSS.
    Abstract: In this paper, we present examples of DInSAR-based measurement of surface displacements using a novel compact L-band SAR system that can be mounted on mobile mapping platforms such as a UAV or a car. The good DInSAR system performance is demonstrated and, particularly, we also show a use case in which a car-borne system setup is employed to map surface displacements of a fast-moving landslide and the surrounding area in Switzerland. Our results show that car-borne and UAV-borne interferometric displacement measurements at L-band are feasible with high quality over various natural terrain. This novel compact DInSAR system for agile platforms complements existing terrestrial, airborne, and space-borne radar interferometry systems in terms of its new combination of (1) radar wavelength (sensitivity to displacement/decorrelation properties), (2) spatial resolution, (3) (near-) terrestrial observation geometry, and (4) mobile mapping capability.
    [bibtex-key = freyEtAlIGARSS2021UAVandCarborneDinSARwithGammaLbandSAR] [bibtex-entry]


  1095. Silvan Leinss, Shiyi Li, and Othmar Frey. Measuring Glacier Velocity by Autofocusing Temporally Multilooked SAR Time Series. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5493-5496, July 2021. IEEE.
    Abstract: ABSTRACT SAR offset tracking, applied on areas with strong temporal decorrelation, requires relatively large image templates for cross-correlation to compensate for incoherent radar speckle. Template edge lengths of 64-12 pixels are common. Furthermore, velocity maps are often incomplete because weakly visible features are obscured by uncorrelated speckle. To improve SAR offset tracking, we propose a new robust method which can significantly enhance both the spatial completeness and the resolution of velocity products by assuming a stationary velocity field. The method minimizes the motion blur of moving features which occurs when SAR backscatter time se- ries are multilooked in time. Our velocity-adaptive temporal multilooking strongly reduces speckle without losing spatial resolution which makes the cross-correlation much more ro- bust even for template sizes as small as 30 x 30 pixels. We demonstrate the method by generating a high resolution velocity map of Great Aletsch Glacier in Switzerland.
    [bibtex-key = leinssLiFreyIGARSS2021GlacierVelocityByAutofocusingTemporallyMultilookedSARTimeSeries] [bibtex-entry]


  1096. Andrew Rittenbach and John Paul Walters. Demonstration of a fully neural network based synthetic aperture radar processing pipeline for image formation and analysis. In Sachidananda R. Babu, Arnaud Hélière, and Toshiyoshi Kimura, editors, Sensors, Systems, and Next-Generation Satellites XXV, volume 11858, pages 98 - 109, 2021. International Society for Optics and Photonics, SPIE. Keyword(s): SAR Processing, synthetic aperture radar, onboard processing, SAR image formation and analysis, deep learning based image formation, AI, machine learning, azimuth focusing.
    Abstract: Synthetic Aperture Radar (SAR) imaging systems operate by emitting radar signals from a moving object, such as a satellite, towards the target of interest. Reflected radar echoes are received and later used by image formation algorithms to form a SAR image. There is great interest in using SAR images in computer vision tasks such as classification or automatic target recognition. Today, however, SAR applications consist of multiple operations: image formation followed by image processing. In this work, we train a deep neural network that performs both the image formation and image processing tasks, integrating the SAR processing pipeline. Results show that our integrated pipeline can output accurately classified SAR imagery with image quality comparable to those formed using a traditional algorithm, showing that fully neural network based SAR processing pipeline is feasible.
    [bibtex-key = rittenbachWaltersSPIE2021NeuralNetworkBasedSARProcessingForImageFormationAndAnalysis] [bibtex-entry]


  1097. Marcel Stefko, Othmar Frey, Charles Werner, and Irena Hajnsek. KAPRI: a Bistatic Full-Polarimetric Interferometric Real-Aperture Radar System for Monitoring of Natural Environments. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1950-1953, 2021. IEEE. Keyword(s): SAR Processing, WBSCAT, Wide-band Scatterometer, ESA, European Space Agency, Snow, ESA Snowlab, Wideband Scatterometer, WBScat, microwave scatterometer, aperture synthesis, time series, polarimetry, tomography, SAR tomography. [bibtex-key = stefkoFreyWernerHajnsekIGARSS2021KAPRIBistaticPolInterferoRealApertureSystem] [bibtex-entry]


  1098. Dario Tagliaferri, Marco Rizzi, Stefano Tebaldini, Monica Nicoli, Ivan Russo, Christian Mazzucco, Andrea Virgilio Monti-Guarnieri, Claudio Maria Prati, and Umberto Spagnolini. Cooperative Synthetic Aperture Radar in an Urban Connected Car Scenario. In 2021 1st IEEE International Online Symposium on Joint Communications Sensing (JC S), pages 1-4, February 2021. Keyword(s): Image resolution, Bandwidth, Radar imaging, Radar polarimetry, Sensors, Synthetic aperture radar, Automotive engineering, Multi-vehicle SAR, Cooperative SAR, Environment Mapping, Joint Communication and Sensing.
    Abstract: With the raising interest in automated driving, onboard perception systems are required to perform a wide range of functionalities, from basic emergency braking to mapping of the surroundings. However, current automotive radars are known to suffer from low angular/range resolution and might be unable to meet the high-definition mapping required by high levels of automation. Synthetic Aperture Radar (SAR) systems allow to improve the angular resolution of standard automotive radars, but their imaging performance is constrained by the available bandwidth. In Joint Communication and Sensing (JCS), where the on-board radio is used also to inter-connect road users, cooperation enables SAR performance augmentation thanks to the improved geometric factor, especially for near-isotropic targets (poles, pedestrians, etc.). Furthermore, the combination of communication and sensing allows to trade between bandwidth, cooperation and synthetic aperture so as to optimize the overall JCS performance. In this paper, we propose a cooperative SAR (C-SAR) system for automotive scenarios, where single ego-vehicle SAR images are exchanged with neighboring vehicles thus combined to enhance SAR performance. Preliminary simulations support the proposed idea, enabling SAR imaging improvements in low bandwidth scenarios.
    [bibtex-key = tagliaferriEtAlConf2021CooperativeSARinUrbanConnectedCarScenario] [bibtex-entry]


  1099. Charles Werner, Othmar Frey, Reza Naderpour, Andreas Wiesmann, Martin Süss, and Urs Wegmuller. Aperture Synthesis and Calibration of the WBSCAT Ground-Based Scatterometer. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1947-1950, 2021. IEEE. Keyword(s): SAR Processing, WBSCAT, Wide-band Scatterometer, ESA, European Space Agency, Snow, ESA Snowlab, Wideband Scatterometer, WBScat, microwave scatterometer, aperture synthesis, time series, polarimetry, tomography, SAR tomography. [bibtex-key = wernerFreyNaderpourWiesmannSussWegmullerIGARSS2021ApertureSynthesisAndCalibrationOfWBSCAT] [bibtex-entry]


  1100. Christine A. Bischoff, Alessandro Ferretti, Fabrizio Novali, Andrea Uttini, Chiara Giannico, and Francesco Meloni. Nationwide deformation monitoring with SqueeSAR using Sentinel-1 data. In Proceedings of IAHS, volume 382, pages 31-37, April 2020. Copernicus GmbH.
    Abstract: Subsidence can now be routinely mapped on a national scale thanks to ESA's Sentinel-1 sensors and advanced scalable SqueeSAR processing. In order to be integrated into existing monitoring programmes, the SqueeSAR datasets can be calibrated with GNSS measurements. The dense spatial coverage of SqueeSAR deformation maps captures local deformation phenomena, and with appropriate calibration, can advance the understanding of regional deformation trends. The regular and reliable SAR image acquisitions by Sentinel-1, as well as significant improvements in the scalability of SqueeSAR processing allow regular updates of deformation maps on a national scale. Filtering the large amount of data for relevant information is achieved by using an algorithm to detect changes in displacement trends.
    [bibtex-key = bischoffFerrettiNovaliUttiniGiannicoMeloni2020NationwideDeformationMonitoringWithSqueeSARusingSentinel1Data] [bibtex-entry]


  1101. Davide Castelletti, Gordon Farquharson, Craig Stringham, and Duncan Eddy. Operational Readiness of the Capella Space SAR System. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 3571-3573, Sep. 2020.
    Abstract: We summarize the key elements of the operational readiness of Sequoia, the first commercially-operational satellite SAR system built and launched by Capella Space. We provide an overview the radar system, and the space operations and commissioning plan. Sample imagery collected with Capella radar hardware on an airborne platform is used to demonstrate the imaging capability of Sequoia.
    [bibtex-key = castellettiFarquharsonStringhamEddyIGARSS2020OperationalReadinessOfCapellaSAR] [bibtex-entry]


  1102. Othmar Frey, Charles Werner, Andrea Manconi, and Roberto Coscione. Measuring surface displacements using a novel UAV/car-borne radar interferometer: including a case study on a fast-moving landslide in Brinzauls. In Swiss Geoscience Meeting 2020: Symposium 20. Remote Sensing of the Spheres, Zurich, Switzerland, November 2020. [bibtex-key = freyWernerManconiCoscioneSGM2020UAVandCARSARmobilemappingDisplacements] [bibtex-entry]


  1103. Othmar Frey, Charles Werner, Andrea Manconi, and Roberto Coscione. Mobile Mapping of Surface Displacements Using a Novel Compact UAV-Borne / Car-Borne InSAR System. In American Geophysical Union, Fall Meeting 2020, 2020.
    Abstract: Flexible mobile mapping of surface displacements with repeat-pass interferometry from moving platforms such as cars and UAVs has been a rather unexplored field. In this contribution we address this topic comprehensively: we demonstrate InSAR-based measurement of surface displacements with our novel car-borne and UAV-borne L-band SAR system setup at three different test sites in Switzerland. The reduced temporal decorrelation at L-band is an important advantage and a complementary property as compared to high-frequency (quasi-)stationary systems. While the sensitivity to line-of-sight displacements is lower, the longer wavelength permits to acquire longer interferometric time intervals also in natural terrain and in adverse conditions, in which the decorrelation time at X- or Ku-band (the frequencies of many stationary terrestrial radar interferometers) can be in the order of minutes or less. Terrestrial synthetic aperture radar acquisitions from a car driving on a road or acquisitions from a UAV allow to obtain synthetic aperture lengths of 100m and more which yields high-resolution SAR imagery also at lower frequency such as L-band. At the same time the view geometry can be chosen to offer line-of-sight views to landslides that complement the view geometries available from spaceborne SAR systems. Then, using a time-domain back-projection image focusing approach, it is ensured that even for curvilinear paths (e.g. a car driving along a curved road) high-quality SAR images and interferograms with good spatial resolution are obtained. Based on these properties we show that such a mobile InSAR system fills a current gap in terms of available InSAR systems for displacement monitoring. We show the potential and discuss the challenges and the limitations of this novel InSAR-based mobile mapping system. We do so with the help of three repeat-pass interferometry showcases (see also attached image): 1) car-borne mapping of surface displacements of fast-moving land slide and surrounding area, 2) car-borne mapping of surface displacements of a glacier, 3) UAV-borne mapping of surface displacements of a steep slope with various land covers. The three test cases show that UAV-borne and car-borne interferometric displacement measurements at L-band are feasible with high quality over various natural terrain.
    [bibtex-key = freyWernerManconiCoscioneAGU2020UAVandCARSARmobilemappingDisplacements] [bibtex-entry]


  1104. Silvan Leinss, Shiyi Li, Philipp Bernhard, and Othmar Frey. Temporal Multi-Looking of SAR Image Series for Glacier Velocity Determination and Speckle Reduction. In EGU General Assembly 2020, volume EGU2020-3643, May 2020.
    Abstract: The velocity of glaciers is commonly derived by offset tracking using pairwise cross correlation or feature matching of either optical or synthetic aperture radar (SAR) images. SAR images, however, are inherently affected by noise-like radar speckle and require therefore much larger images patches for successful tracking compared to the patch size used with optical data. As a consequence, glacier velocity maps based on SAR offset tracking have a relatively low resolution compared to the nominal resolution of SAR sensors. Moreover, tracking may fail because small features on the glacier surface cannot be detected due to radar speckle. Although radar speckle can be reduced by applying spatial low-pass filters (e.g. 5x5 boxcar), the spatial smoothing reduces the image resolution roughly by an order of magnitude which strongly reduces the tracking precision. Furthermore, it blurs out small features on the glacier surface, and therefore tracking can also fail unless clear features like large crevasses are visible. In order to create high resolution velocity maps from SAR images and to generate speckle-free radar images of glaciers, we present a new method that derives the glacier surface velocity field by correlating temporally averaged sub-stacks of a series of SAR images. The key feature of the method is to warp every pixel in each SAR image according to its temporally increasing offset with respect to a reference date. The offset is determined by the glacier velocity which is obtained by maximizing the cross-correlation between the averages of two sub-stacks. Currently, we need to assume that the surface velocity is constant during the acquisition period of the image series but this assumption can be relaxed to a certain extend. As the method combines the information of multiple images, radar speckle are highly suppressed by temporal multi-looking, therefore the signal-to-noise ratio of the cross-correlation is significantly improved. We found that the method outperforms the pair-wise cross-correlation method for velocity estimation in terms of both the coverage and the resolution of the velocity field. At the same time, very high resolution radar images are obtained and reveal features that are otherwise hidden in radar speckle. As the reference date, to which the sub-stacks are averaged, can be arbitrarily chosen a smooth flow animation of the glacier surface can be generated based on a limited number of SAR images. The presented method could build a basis for a new generation of tracking methods as the method is excellently suited to exploit the large number of emerging free and globally available high resolution SAR image time series.
    [bibtex-key = leinssEtAlEGU2020Stacking] [bibtex-entry]


  1105. A. Manconi, R. Caduff, T. Strozzi, O. Frey, Werner. C., and U. Wegmuller. Monitoring displacements of complex landslide with broadband multiplatform radar techniques. In Swiss Geoscience Meeting 2020: Symposium 20. Remote Sensing of the Spheres, Zurich, Switzerland, 2020. [bibtex-key = Manconi2020] [bibtex-entry]


  1106. J. Mittermayer, G. Krieger, and A. Moreira. Concepts and Applications of Multi-static MirrorSAR Systems. In 2020 IEEE Radar Conference (RadarConf20), pages 1-6, Sep. 2020. Keyword(s): data acquisition, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, transponders, MirrorLink, radar signal, space transponder manner, transmit satellite, system complexity, data acquisition, SAR product quality, interferometric SAR application example, multistatic MirrorSAR systems, multistatic SAR acquisitions, minimal functionality, receive satellites, Spaceborne radar, Radar, Satellites, Satellite broadcasting, Synthetic aperture radar, Radar antennas, Synchronization, Synthetic Aperture Radar (SAR), MirrorSAR, Multiple Baselines, Synchronization, MirrorLink.
    Abstract: The paper describes the basic components of MirrorSAR and explains how bi- and multistatic SAR acquisitions are achieved by shifting only minimal functionality to the receive satellites. It shows that synchronization is not required or reduced in complexity by a MirrorLink. The MirrorLink forwards the ground reflected radar signal from the receiving satellites in a space transponder manner to the transmit satellite. Several options for the MirrorLink are discussed. In MirrorSAR, a number of satellites enable the acquisition of dual- or multi-baselines in a single-pass. The paper discusses several application examples where MirrorSAR eases the overall system complexity, the data acquisition and improves the SAR product quality. An interferometric SAR application example is discussed in more detail.
    [bibtex-key = mittermayerKriegerMoreiraIEEERadarCon2020MirrorSAR] [bibtex-entry]


  1107. I. Walterscheid, P. Berens, M. Caris, S. Sieger, O. Saalmann, D. Janssen, G. El-Arnauti, A. Ribalta, D. Henke, and E. M. Dominguez. First results of a joint measurement campaign with PAMIR-Ka and MIRANDA-94. In 2020 IEEE Radar Conference (RadarConf20), pages 1-6, September 2020. Keyword(s): Radar, Radar imaging, Synthetic aperture radar, Radar polarimetry, Aircraft, Bandwidth, Radar antennas, Multi-dimensional radar imaging, Synthetic Aperture Radar, SAR, Multi-look SAR, Multi-aspect SAR, Polarimetric SAR, PAMIR-Ka, MIRANDA-94.
    Abstract: Fraunhofer FHR has participated in the international measurement campaign of the NATO research task group SET-250 with two airborne SAR systems in July 2019. The general objective of the trials was to investigate the use of multidimensional radar to increase the performance of radar imaging systems. The first system PAMIR-Ka is a multi-channel pulsed radar system operating at 34 GHz with a very high bandwidth of up to 8 GHz. The second system MIRANDA-94 is a multichannel frequency modulated continuous wave (FMCW) radar with up to 3 GHz at 94 GHz center frequency with a dual polarized antenna. The paper introduces the systems, explains the data collection, and presents first results with respect to multi-look, multi-frequency, multi-polarization, and multi-aspect radar imaging of a test site with military targets.
    [bibtex-key = walterscheidBerensCarisSiegerSaalmannJanssenElArnautiRibaltaHenkeDominguezIEEERadarCon2020PamirAndMIRANDA94] [bibtex-entry]


  1108. Evan Zaugg, A. Margulis, M. Margulis, J. Bradley, A. Kozak, and J. Budge. Next-Generation Software Defined Radar: First Results. In 2020 IEEE International Radar Conference (RADAR), pages 749-754, April 2020. Keyword(s): aerospace testing, airborne radar, SlimSDR, flight testing, ARTEMIS SlimSAR, next-generation software defined radar, slim software defined radar.
    Abstract: ARTEMIS, Inc. has begun flight testing a new radar system called the SlimSDR (for Slim, Software Defined Radar). This successor to the ARTEMIS SlimSAR began test flights in September 2019. Like the SlimSAR, it is a compact radar system, but provides additional capabilities and flexibility. As a software defined radar, the SlimSDR is modular, multi-frequency, and applicable to multiple applications. This paper details the design and development process of the SlimSDR and shows initial results from the first flight tests of the system.
    [bibtex-key = zauggEtAl2020SoftwareDefinedRadioRadar] [bibtex-entry]


  1109. Roberto Coscione, Irena Hajnsek, and Othmar Frey. Trajectory Uncertainty in Repeat-Pass SAR Interferometry: A Case Study. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 338-341, 2019. Keyword(s): SAR Processing, Synthetic aperture radar (SAR), SAR interferometry, mobile mapping, car-borne SAR, UAV, airborne SAR, terrestrial radar interferometer, repeat-pass interferometry, differential interferometry, DInSAR, SAR imaging, INS, GNSS, GPS, Trajectory Uncertainty.
    Abstract: In the context of differential synthetic aperture radar interferometry (DInSAR), precise trajectory estimation of the SAR platform is necessary to minimize residual phase errors induced by inaccurate knowledge of the 3D acquisition geometry. Inertial navigation systems (INS) and global navigation satellite system (GNSS) are usually employed to track the position of the platform. However, their unavoidable inaccuracies lead to motion estimation errors that negatively affect the quality of the processed radar data.To assess the positioning performance in a repeat-pass scenario, we used a navigation-grade INS/GNSS system to precisely track the position and the attitude of a platform moving along a rail and carrying a SAR sensor. We analyse the performance of the positioning solution for different scenarios relevant to repeat-pass DInSAR. Since the position of the platform is nearly perfectly repeated at every pass (zero interferometric baseline), the precision of the estimated position can be assessed and the interferometric performance evaluated.
    [bibtex-key = coscioneHajnsekFreyIGARSS2019INSGNSSNavAndDInSAR] [bibtex-entry]


  1110. C. De Luca, G. Onorato, F. Casu, R. Lanari, and M. Manunta. A Genetic Algorithm for Phase Unwrapping Errors Correction in the SBAS-DInSAR Approach. In IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium, pages 266-269, July 2019.
    Abstract: The Differential Synthetic Aperture Radar Interferometry (DInSAR) remote sensing technique permits to investigate the temporal behaviour of the detected displacements through the generation of the deformation time-series. In this scenario the Phase Unwrapping (PhU) is a crucial point where several errors could occur since the problem is intrinsically ill-posed. In this paper we present a technique to correct unavoidable PhU errors and limit their impact in the final deformation time-series. The proposed approach works pixel-by-pixel and is based on the combination of an L1-norm inversion for the identification of the possible PhU errors and a genetic algorithm (GA) for the search of the best fitting solution. We include the technique in the Small Baseline Subset (SBAS) DInSAR processing chain to correct the results of the Extended Minimum Cost Flow algorithm, but in principle it can be used as a correction step after a generic PhU procedure used in SBAS. Results from MonteCarlo simulations are shown in this paper while real data cases will be presented at the conference.
    [bibtex-key = 8900515] [bibtex-entry]


  1111. Othmar Frey, Charles L. Werner, and Roberto Coscione. Car-borne and UAV-borne mobile mapping of surface displacements with a compact repeat-pass interferometric SAR system at L-band. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 274-277, 2019. Keyword(s): SAR Processing, Synthetic aperture radar (SAR), SAR interferometry, mobile mapping, car-borne SAR, UAV, airborne SAR, terrestrial radar interferometer, repeat-pass interferometry, differential interferometry, DInSAR, SAR imaging, focusing, back-projection, Time-Domain Back-Projection, TDBP, GPU, CUDA, interferometry, L-band, INS, GNSS, GPS.
    Abstract: In this paper, we present first results of carborne and UAV-borne mobile mapping of potential surface displacements with a compact repeat-pass interferometric FMCW SAR system at L-band: (1) glacier-flow-induced displacements were measured at Stein glacier in the Swiss alps in car-borne mode along a slightly curved road section; (2) a valley slope was observed repeatedly using the vertical-take-off-and-landing (VTOL) UAV Scout B1-100 flown by Aeroscout. The SAR raw data were focused directly to an image grid in map coordinates, involving a digital elevation model and accurate GNSS/INS navigation data, by using a time-domain back-projection (TDBP) approach. These geocoded complex SAR images then allow to directly form differential interferograms in map coordinates. The feasibility of repeat-pass interferometry using our novel FMCW L-band SAR on mobile platforms such as a car or a UAV is successfully demonstrated with several data examples.
    [bibtex-key = freyWernerCoscioneIGARSS2019CARandUAVborneDInSARLBand] [bibtex-entry]


  1112. Brian P. Hawkins and Wayne Tung. UAVSAR Real-Time Embedded GPU Processor. In IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium, pages 545-547, July 2019. Keyword(s): SAR Processing, GPU, real-time, azimuth focusing, Jetson, CUDA, SAR imaging.
    Abstract: Synthetic aperture radar (SAR) can provide high-resolution imagery regardless of cloud cover or lighting conditions. These qualities make SAR potentially well-suited for informing response efforts to natural and man-made disasters, but such applications require data products with minimal latency. To meet this challenge, we implemented a real-time SAR processor capable of producing 10 m imagery using an NVIDIA Jetson TX2 embedded GPU module. With its low mass (87 g module) and power consumption under 8 W, the system also holds promise for spaceborne applications.
    [bibtex-key = hawkinsTungIGARSS2019UAVSARRealTimeEmbeddedGPUProcessor] [bibtex-entry]


  1113. Daniel Henke, Max Frioud, Julian Fagir, Sebastien Guillaume, Michael Meindl, Alain Geiger, S. Sieger, D. Janssen, F. Kloppel, M. Caris, S. Stanko, M. Renker, and Peter Wellig. Miranda35 Experiments in Preparation for Small UAV-Based SAR. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 8542-8545, July 2019. Keyword(s): airborne radar, autonomous aerial vehicles, CW radar, FM radar, Global Positioning System, image motion analysis, radar imaging, radar receivers, synthetic aperture radar, units (measurement), frequency-modulated continuous-wave synthetic aperture radar, energy efficiency, navigation data, inertial measurement unit, IMU, SAR image quality, airborne platform, SAR autofocus, small UAV-based SAR systems, FMCW SAR, Miranda35 experiments, moving baseline differential GPS, optical structure-from-motion-based localization, FHR FMCW MIRANDA35 sensor, Synthetic aperture radar, Global Positioning System, Radar polarimetry, Cameras, Optical sensors, SAR autofocus, navigation, synthetic aperture radar, small UAV.
    Abstract: Technological advances in frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) and the associated miniaturization and energy efficiency make it increasingly possible to transfer SAR systems from traditional airborne platforms to small UAVs. An important factor to successfully achieve high-quality imaging from SAR systems mounted on small drones is the precise knowledge of the platform's navigation data in best case by avoiding the use of an expensive and heavy inertial measurement unit (IMU). In this paper, we test different concepts and discuss the impact on SAR image quality using FHR's FMCW MIRANDA35 sensor. To compare several methods simultaneously on one platform and to have an IMU as reference, first preparatory steps were carried out on an airborne platform. Specifically, we present and evaluate solutions based on SAR autofocus, moving baseline differential GPS and optical structure-from-motion-based localization. SAR autofocus shows the best performance in our preliminary investigations.
    [bibtex-key = henkeEtALIGARSS2019UAVMiranda35SAR] [bibtex-entry]


  1114. Laila Moreira, F. Castro, J. A. Goes, L. Bins, B. Teruel, J. Fracarolli, V. Castro, M. Alcantara, G. Ore, Dieter Luebeck, L. P. Oliveira, L. Gabrielli, and H. E. Hernandez-Figueroa. A Drone-borne Multiband DInSAR: Results and Applications. In 2019 IEEE Radar Conference (RadarConf), pages 1-6, April 2019. Keyword(s): SAR Processing, agriculture, cartography, geophysical techniques, radar imaging, radar interferometry, remote sensing, remote sensing by radar, synthetic aperture radar, tomography, change-detection maps, precision agriculture, subsurface tomography, cartography, low weight drone-borne SAR, foreseen applications, drone-borne multiband DInSAR, Synthetic Aperture Radar, powerful remote sensing tool, relevant products, cartographic system, Synthetic aperture radar, Radar antennas, Drones, Airborne radar, Agriculture, Tomography, SAR, remote sensing, drone-borne SAR.
    Abstract: Synthetic Aperture Radar (SAR) has become a powerful remote sensing tool during the last 25 years. Most relevant products are three dimensional and projected on a cartographic system: topographic, thematic and change-detection maps. Starting with the requirements of precision agriculture, subsurface tomography, subsidence and cartography a low weight drone-borne SAR was designed. It operates in P, L and C-bands with cross-track (InSAR) and differential Interferometry (DInSAR). The requirements of the foreseen applications, concept, design, results and validation from regular surveys and ground truths are presented.
    [bibtex-key = moreiraEtAlRadarCon2019DroneBorneDInSAR] [bibtex-entry]


  1115. R. Rincon, B. Osmanoglu, P. Racette, Q. Bonds, M. Perrine, L. Brucker, S. Seufert, and C. Kielbasa. Tri-Frequency Synthetic Aperture Radar for the Measurements of Snow Water Equivalent. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 8653-8655, July 2019. Keyword(s): airborne radar, hydrological equipment, hydrological techniques, radiometers, snow, synthetic aperture radar, airborne synthetic aperture radar system, snow water equivalent, SWE, radiometer, active passive microwave system, frequency bands, successful system performance, tri-frequency synthetic aperture radar, SWESARR instrument, dual polarization radar, AD 2019 11, frequency 9.65 GHz, frequency 13.6 GHz, frequency 200.0 MHz, frequency 17.25 GHz, Snow, Spaceborne radar, Radar antennas, Synthetic aperture radar, Instruments, Microwave radiometry, Snow, SAR, SWE.
    Abstract: A new airborne synthetic aperture radar (SAR) system was recently developed for the estimation of snow water equivalent (SWE). The radar is part of the SWESARR (Snow Water Equivalent Synthetic Aperture Radar and Radiometer) instrument, an active passive microwave system specifically designed for the accurate estimation of SWE. The dual polarization (VV, VH) radar operates at three frequency bands (9.65 GHz, 13.6 GHz, and 17.25 GHz), with bandwidths of up to 200 MHz. The radar flew its first flight campaign in November 2019, along with SWESARR's -already operational - radiometer. The radar collected comprehensive data sets over various terrains that show a successful system performance. The instrument is slated to participate in future SnowEx campaigns.
    [bibtex-key = rinconOsmanogluRacetteBondsPerrineBruckerSeufertKielbasaIGARSS2019SWESARRtriFreqSARforSWE] [bibtex-entry]


  1116. E. Schreiber, A. Heinzel, M. Peichl, M. Engel, and Werner Wiesbeck. Advanced Buried Object Detection by Multichannel, UAV/Drone Carried Synthetic Aperture Radar. In 2019 13th European Conference on Antennas and Propagation (EuCAP), pages 1-5, March 2019.
    Abstract: The great innovations of Synthetic Aperture Radar (SAR) technology during the past years stimulated new applications in several areas. While in the past SAR was primarily operated for airborne and spaceborne applications, novel operations for quite low altitude like surveillance of cities, local agricultural applications, or even buried object detection, are of new interest. For such operations the well-known and established SAR system concepts should apply similarly, while the technology has to be transferred to the state of the art and new platforms like unmanned aerial vehicles (UAVs) or drones. The paper addresses the special concept for a SAR to detect buried mines. It combines a novel DLR approach based on multistatic observation with the capability to create nearly arbitrary azimuth sampling trajectories. By providing very high resolution it is possible to even identify man-made objects like landmines in the SAR image by their spatial radar-cross section (RCS) distribution. These capabilities allow advanced detection capabilities and satisfy the ultimate demand for buried object identification, both being great improvement in landmine detection and related activities. Beside this concept the paper addresses an idea on robust high-precision positioning, which is indispensable for a successful SAR system. It is based on a photogrammetric method providing the trajectory, the orientation of the UAV and the three-dimensional (3D) ground surface as a side effect as well. Measurement results are shown confirming the feasibility of the proposed approach.
    [bibtex-key = schreiberHeinzelPeichlEngelWiesbeckEuCAP2019UAVDroneSARMineDetection] [bibtex-entry]


  1117. Muhammad Adnan Siddique, Karina Wilgan, Tazzio Strozzi, Alain Geiger, Irena Hajnsek, and Othmar Frey. A Comparison of Tropospheric Path Delays estimated in PSI Processing against Delays Derived from a GNSS Network in the Swiss Alps. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 342-345, 2019. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas, alpine, rugged terrain, atmospheric phase, atmospheric phase screen, APS, mitigation of atmospheric phase, turbulent atmospheric phase in alpine areas, Cosmo-SkyMed, Zermatt, Mattertal, Matter valley, Switzerland, multi-baseline interferometry, GNSS, GPS, Comparison, tropospheric path delay, Collocation, Kriging.
    Abstract: This paper reports the first results of a comparative study of tropospheric delays retrieved by means of PSI processing of an interferometric stack of SAR images against those derived independently from a permanent GNSS network. The stack comprises 33 Cosmo-SkyMed stripmap images acquired in the summers between 2008-13 over the Matter Valley in the Swiss Alps. The long-term objective of the study is to explore whether GNSS-derived delays from existing networks (i.e., not deployed specifically for a test site) in Swiss Alpine regions can aid in tropospheric phase corrections in SAR data, or rather the phase corrections derived within the PSI processing being at a higher spatial resolution might be appropriate to build upon the GNSS products by improving their resolution.
    [bibtex-key = siddiqueWilganStrozziGeigerHajnsekFreyIGARSS2019ComparisonTropoGNSSandInSAR] [bibtex-entry]


  1118. Craig Stringham, Gordon Farquharson, Davide Castelletti, Eric Quist, Lucas Riggi, Duncan Eddy, and Scott Soenen. The Capella X-band SAR Constellation for Rapid Imaging. In IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium, pages 9248-9251, July 2019.
    Abstract: The Capella constellation of 36 X-band SAR Satellites will provide global hourly imaging opportunities for the entire earth and InSAR collection intervals of 4 hours. With the successful launch of the first satellite last year, Capella is prepared to launch the first operational this year and the first incrementally launch 36 satelites over two years. In addition to providing short revisit times, Capella will provide game-changing image request and delivery services where users can request imagery on-demand and receive it in a matter of minutes not days or weeks.
    [bibtex-key = stringhamEtAlIGARSS2019CapellaSpaceXbandSARConstellation] [bibtex-entry]


  1119. Dennis Valuyskiy, Sergey Vityazev, and Vladimir Vityazev. Resolution Improvement in Ground-Mapping Car-Borne Radar Imaging Systems. In 2019 IEEE International Conference on Imaging Systems and Techniques (IST), pages 1-5, December 2019. Keyword(s): SAR processing, carborne SAR, terrestrial radar interferometry, autofocus, phase gradient autofocus, image resolution, motion compensation, radar imaging, radar resolution, road vehicle radar, motion compensation technique, data acquisition, radar system, ground-mapping car-borne radar imaging systems, resolution improvement, Radar imaging, Electronics packaging, Data acquisition, Motion compensation, Radar antennas, Image resolution, radar imaging, PGA, autofocus, car-borne radar imaging, motion compensation, resolution improvement.
    Abstract: The problem of radar imaging is considered in this paper. An automobile is used as a platform for radar system mounting. It produces some special requirements for the data acquisition and processing, including the necessity for motion compensation. A motion compensation technique is offered in the paper and applied to the real-life data. The results demonstrate the efficiency of the suggested processing technique.
    [bibtex-key = valuyskiyVityazevVityazevConf2019CarborneSARPhaseGradientAutofocus] [bibtex-entry]


  1120. S. Wang, W. Feng, K. Kikuta, G. Chernyak, and M. Sato. Ground-Based Bistatic Polarimetric Interferometric Synthetic Aperture Radar System. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 8558-8561, July 2019. Keyword(s): radar imaging, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, ground-based bistatic polarimetric interferometric synthetic aperture radar system, novel ground-based bistatic polarimetric synthetic aperture radar system, Optical Electric Field Sensor, trihedral corner reflector, polarimetric capability, monostatic SAR images, bistatic polarimetric SAR images, 2D displacement estimation, displaceable corner reflector, Radar polarimetry, Estimation, Two dimensional displays, Synthetic aperture radar, Optical interferometry, Bistatic radar, Adaptive optics, Bistatic radar, radar polarimetry, radar interferometry, two-dimensional displacement, OEFS.
    Abstract: A novel ground-based bistatic polarimetric synthetic aperture radar (SAR) system using an Optical Electric Field Sensor (OEFS) as the receiver was developed for environmental studies. Fundamental experiments were carried out with a trihedral corner reflector (CR) as a target, showing the polarimetric capability of the system. Different polarization signatures from the monostatic SAR images were found by analyzing the bistatic polarimetric SAR images. Meanwhile, this system has the capability to estimate the two-dimensional (2D) displacement of the targets in the field of view. The simulation shows the feasibility and effectiveness of the 2D displacement estimation. The experimental results demonstrate that the accuracy along the x and y direction to the line of sight (LOS) based on the designed bistatic synthetic aperture radar system can reach millimeter level for the displaceable corner reflector.
    [bibtex-key = wangFengKikutaChernyakSatoIGARSS2019TerrestrialBistaticPolInSARSystem] [bibtex-entry]


  1121. Charles L. Werner, Martin Suess, Othmar Frey, and Andreas Wiesmann. The ESA Wideband Microwave Scatterometer (WBSCAT): Design and Implementation. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 8339-8342, 2019. Keyword(s): ESA Snowlab, SnowScat, Wideband Scatterometer, WBScat, snow, microwave scatterometer, aperture synthesis, time series, polarimetry, tomography, SAR tomography.
    Abstract: WBSCAT is a new terrestrial 1-40 GHz polarimetric scatterometer. This instrument, built for the European Space Agency, with additional support from ETH WSL, is currently an element of the ESA SnowLab project for continuous microwave measurements of snowpack in Davos-Laret Switzerland. WBSCAT is based on a compact Vector Network Analyzer (VNA), combined with calibration standards and low-noise amplifiers to increase sensitivity. A pan/tilt positioner provides the angular and spatial diversity required for measurement of radar cross-section, 3D tomographic imaging, and measurements of interferometric coherence. The instrument can apply angular diversity and aperture synthesis to increase radiometric accuracy and suppress clutter. In Davos-Laret, WBSCAT is suspended from a 2.2-meter linear rail positioner that provides additional spatial diversity for high-resolution 3D tomographic imaging.
    [bibtex-key = wernerSuessWegmullerFreyWiesmannIGARSS2019ESAWBScat] [bibtex-entry]


  1122. Andreas Wiesmann, Rafael Caduff, Charles L. Werner, Othmar Frey, Martin Schneebeli, Henning Löwe, Matthias Jaggi, Mike Schwank, Reza Naderpour, and Thorsten Fehr. ESA Snowlab Project: 4 Years of Wide Band Scatterometer Measurements of Seasonal Snow. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5745-5748, 2019. Keyword(s): ESA Snowlab, SnowScat, Wideband Scatterometer, WBScat, snow, microwave scatterometer, aperture synthesis, time series, polarimetry, tomography, SAR tomography.
    Abstract: The aim of the ESA SnowLab project is to provide a comprehensive multi-frequency, multi-polarisation, multitemporal dataset of active microwave measurements over snow-covered grounds to investigate the relationship between effective snow- and ground parameters and the resultant signals detected by microwave radar. An important part for the development of microwave models is the microstructural characterisation. This characterisation can only be done by repeated measurements by SnowMicroPen and more completely, but also much more expensive, by X-ray micro-tomography. Within this project we complemented the microwave measurements of Alpine snow in Switzerland with extensive effective snow- and ground parameters and meteorological data. Microwave backscatter measurements were conducted using the 9-18 GHz ESA SnowScat instrument and since December 2018 the recently built ESA WBScat instrument. WBScat allows to extend the spectral coverage to 1-40 GHz.
    [bibtex-key = wiesmannCaduffWernerFreySchneebeliLoeweJaggiSchwankNaderpourFehrIGARSS2019ESASnowlabOverview] [bibtex-entry]


  1123. Simone Baffelli, Othmar Frey, and Irena Hajnsek. Geostatistical Analysis and Mitigation of Atmosphere Induced Phase in Terrestrial Radar Interferometric Observations of an Alpine Glacier. In Proc. of EUSAR 2018 - 12th European Conference on Synthetic Aperture Radar, pages 626-631, 2018. Keyword(s): Radar Interferometry, Terrestrial Radar Interferometry, TRI, Ground-based radar, Interferometry, glacier velocity, atmospheric phase, mitigation of atmospheric phase, APS, Bisgletscher, Radar time series, GPRI, Gamma Portable Radar Interferometer.
    Abstract: Terrestrial Radar Interferometry is used to map surface displacement velocites with high temporal resolution, irrespective of sunlight and cloud cover. The main factor limiting estimation accuracy are variations in the atmospheric refractive index, observed as atmospheric phase screens (APS). A statistical model for APS assuming a separable spatio-temporal covariance structure is described. It facilitates the extrapolation of the APS from observations at persistent scatterers (PS) using regression-Kriging, which is followed by a timeseries inversion to estimate the surface velocity. A statistical analysis of the APS is performed using a Ku-Band radar timeseries of Bisgletscher, a glacier in the Southwestern Swiss Alps. The results show that, while some non-stationarity in the covariance structure is observed at large timescales, the covariance models obtained assuming separability perform well in APS mitigation using regression-Kriging.
    [bibtex-key = baffelliFreyHajnsekEUSAR2018GeostatsMitigationTerrestrialRadarInterferometryBisgletscher] [bibtex-entry]


  1124. Simone Baffelli, Othmar Frey, and Irena Hajnsek. Geostatistical Analysis and Mitigation of Atmospheric Phase Screens in Ku-Band Terrestrial Radar Interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 6504-6507, 2018. Keyword(s): Radar Interferometry, Terrestrial Radar Interferometry, TRI, Ground-based radar, Interferometry, glacier velocity, atmospheric phase, mitigation of atmospheric phase, APS, Bisgletscher, Radar time series, GPRI, Gamma Portable Radar Interferometer.
    Abstract: A geostatistical analysis of the atmospheric phase screen (APS) affecting Ku-Band terrestrial radar interferometric (TRI) observations of a fast-flowing alpine glacier is made assuming a separable spatio-temporal covariance structure. Such a structure facilitates the mitigation of APS: the atmospheric phase affecting individual interferograms can be extrapolated form a set of persistent scatterers (PS) using regression-Kriging. After removing this estimate the residual APS is only correlated in time; its effect on surface displacement estimation is mitigated with a generalized least squares (GLS) inversion employing an estimate of the temporal covariance of the APS. The applicability of a separable covariance structure and the performance of the APS correction method are assessed on a TRI timeseries of Bisgletscher, a glacier in the southwestern Swiss Alps.
    [bibtex-key = baffelliFreyHajnsekIGARSS2018GeostatisticalMitigationOfAPSinTerrestrialRadarInterferometry] [bibtex-entry]


  1125. Roberto Coscione, Irena Hajnsek, and Othmar Frey. An experimental car-borne SAR System: measurement setup and positioning error analysis. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 6364-6367, 2018. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, dechirp-on-receive, FMCW, Frequency-modulated continous wave, Ground-based SAR, car-borne SAR, CARSAR, InSAR, DInSAR, geophysical techniques, ground-based SAR system, radar interferometry, synthetic aperture radar, GAMMA Portable Radar Interferometer (GPRI), GPRI, GPRI-II, interferometric technique, Coherence, Correlation, Interferometry, agile platform, airborne SAR, Inertial Naviation System (INS), Global Navigation Satellite System (GNSS), INS/GNSS, iMAR.
    Abstract: Repeat-pass differential SAR interferometry (DInSAR) using spaceborne SAR data or stationary terrestrial radar data is an established technique to measure surface displacements. However, repeat-pass DInSAR from agile platforms (airborne/car-borne) is challenging due to residual motion errors. This is particularly true for high-frequency radar where motion errors of few millimeters represent a non-negligible fraction of the wavelength. In this paper, an experimental car-borne SAR system is presented. Such a system is complementary to the existing solutions (namely spaceborne, airborne, and terrestrial systems) in terms of geometry of acquisition, and flexibility in the selection of temporal baselines and location of the acquisitions. To meet the need of consistent and precise trajectory information, proper postprocessing procedures must be applied to the raw positioning data collected from the inertial navigation system (INS) and the global positioning system (GNSS). A viable procedure is here presented and first results discussed.
    [bibtex-key = coscioneHajnsekFreyIGARSS2018CarborneSARPositioningErrorAnalysis] [bibtex-entry]


  1126. Othmar Frey, Charles L. Werner, Rafael Caduff, and Andreas Wiesmann. Tomographic profiling with SnowScat within the ESA SnowLab Campaign: Time Series of Snow Profiles Over Three Snow Seasons. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 6512-6515, 2018. Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: As part of the ESA SnowLab campaign the SnowScat device, a terrestrial stepped-frequency continuous wave (SFCW) scatterometer which supports fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz, was operated in tomographic profiling mode. In this tomographic profiling mode the SnowScat device is subsequently displaced in elevation direction to obtain a high-resolution not only in range direction but also along elevation. This leads to two-dimensional vertical profiles of a snowpack, which means that radar backscatter, co-polar phase difference, interferometric phase and coherence can be distinguished also along the vertical dimension of the snowpack. In this paper, we provide a summary and a few examples of a time series of tomographic measurements of snow obtained within the ESA SnowLab campaign at two different locations in the Swiss Alps during three snow seasons.
    [bibtex-key = freyWernerCaduffWiesmannIGARSS2018SnowScatTomoTimeSeries] [bibtex-entry]


  1127. Othmar Frey, Charles L. Werner, Irena Hajnsek, and Roberto Coscione. A car-borne SAR system for interferometric measurements: development status and system enhancements. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 6508-6511, 2018. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, dechirp-on-receive, FMCW, Frequency-modulated continous wave, Ground-based SAR, car-borne SAR, CARSAR, InSAR, DInSAR, geophysical techniques, ground-based SAR system, radar interferometry, synthetic aperture radar, GAMMA Portable Radar Interferometer (GPRI), GPRI, GPRI-II, interferometric technique, Coherence, Correlation, Interferometry, agile platform, airborne SAR.
    Abstract: Terrestrial radar systems are used operationally for area-wide measurement and monitoring of surface displacements on steep slopes, as prevalent in mountainous areas or also in open pit mines. One limitation of these terrestrial systems is the decreasing cross-range resolution with increasing distance of observation due to the limited antenna size of the real aperture radar or the limited synthetic aperture of the quasi-stationary SAR systems. Recently, we have conducted a first experiment using a car-borne SAR system at Ku-band, demonstrating the time-domain back-projection (TDBP) focusing capability for the FMCW case and single-pass interferometric capability of our experimental Ku-band car-borne SAR system. The cross-range spatial resolution provided by such a car-based SAR system is potentially independent from the distance of observation, given that an adequate sensor trajectory can be built. In this paper, we give (1) an overview of the updated system hardware (radar setup and high-precision combined INS/GNSS positioning and attitude determination), and (2) present SAR imagery obtained with the updated prototype Ku-band car-borne SAR system.
    [bibtex-key = freyWernerHajnsekCoscioneIGARSS2018CarborneSARforInSARDevelopmentAndEnhancements] [bibtex-entry]


  1128. C. Magnard, U. Wegmuller, C. Werner, F. Bonvin, and E. Meier. Planning tool for SAR missions. In Proc. European Conf. Synthetic Aperture Radar, pages 1052-1057, 2018. Keyword(s): Image segmentation, Optical variables measurement, Radar imaging, Image quality parameters, Incidence angles, Planning tools, Region of interest, Robust planning, Scattering char-acteristics, Sensor parameter, User-friendly-software, Synthetic aperture radar.
    Abstract: A methodology was developed to assist in the planning of SAR acquisitions, supported by a user-friendly software. Using sensor parameters, the position and topography of the region of interest, and a proposed flight path, this planning tool simulates backscatter images, maps of scattering characteristics and image quality parameters such as the local incidence angle and the noise equivalent sigmaO. It produces flight and ground profiles and lists of key parameters. These outputs inform about expected SAR image characteristics and can be employed for optimization of the planning. Using this tool, a non-specialist can perform a robust planning of SAR acquisitions.
    [bibtex-key = magnardWegmullerWernerBonvinMeierEUSAR2018SARPlanningTool] [bibtex-entry]


  1129. Muhammad Adnan Siddique, Tazio Strozzi, Irena Hajnsek, and Othmar Frey. A case study on the correction of atmosphere-induced phase disturbances for SAR tomography in mountainous areas. In Proc. of EUSAR 2018 - 12th European Conference on Synthetic Aperture Radar, pages 1412-1416, 2018. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas, alpine, rugged terrain, atmospheric phase, atmospheric phase screen, APS, mitigation of atmospheric phase, turbulent atmospheric phase in alpine areas, Cosmo-SkyMed, Zermatt, Mattertal, Matter valley, Switzerland, multi-baseline interferometry.
    Abstract: The estimation of the atmosphere-induced phase delay variations is often more involved in mountainous areas due to strong spatial variations of the local atmospheric conditions and propagation paths through the troposphere. Height dependent phase delay variation owing to vertical stratification of the atmosphere within the same range-azimuth resolution cell cannot be ignored. We propose a regression kriging-based data-driven method whereby phase corrections are applied for differential tomographic focusing at each 3D point of interest along the elevation axis. Experiments are performed on an interferometric stack comprising 32 Cosmo-SkyMed stripmap images acquired between 2008-2013 over the Matter Valley in the Swiss Alps.
    [bibtex-key = siddiqueStrozziHajnsekFreyEUSAR2018PSITomoAlpineAtmoCaseStudy] [bibtex-entry]


  1130. Muhammad A. Siddique, Tazio Strozzi, Irena Hajnsek, and Othmar Frey. SAR tomography for spatio-temporal inversion of coherent scatterers in villages of alpine regions. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 6099-6102, 2018. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas, alpine, rugged terrain, atmospheric phase, atmospheric phase screen, APS, mitigation of atmospheric phase, turbulent atmospheric phase in alpine areas, Cosmo-SkyMed, Zermatt, Mattertal, Matter valley, Switzerland, multi-baseline interferometry.
    Abstract: Differential synthetic aperture radar (SAR) tomography allows separation of multiple coherent scatterers interfering in the same range-azimuth resolution cell as well as the estimation of the deformation parameters of each scatterer. In this way, the spatio-temporal tomographic inversion serves as a means to resolve the layover and simultaneously improve deformation sampling. Compared to metropolitan regions with several man-made structures, the prevalence of coherent scatterers in the villages of alpine regions is generally low, while at the same time layovers are widespread due to the ruggedness of the terrain. Moreover, the drastic height variations in the imaged scene necessitate height-dependent compensation of the atmospheric phase delay variations within the tomographic inversion. This paper addresses these concerns while performing experiments on an interferometric stack comprising 33 Cosmo-SkyMed strimap images acquired in the summers between 2008-13 over Matter Valley in the Swiss Alps. The results show improved deformation sampling along the layover-affected mountainside.
    [bibtex-key = siddiqueStrozziHajnsekFreyIGARSS2018PSITomoAlpineAtmo] [bibtex-entry]


  1131. F. Viviani, A. Michelini, L. Mayer, and F. Conni. IBIS-ArcSAR: an Innovative Ground-Based SAR System for Slope Monitoring. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1348-1351, July 2018. Keyword(s): digital elevation models, geomorphology, geophysical equipment, mining, radar imaging, radar interferometry, synthetic aperture radar, vertical height, digital elevation model, computationally fast interferometric processing algorithm, simple fast interferometric processing algorithm, Arc-scanning SAR systems, 2D SAR Image, Multiple input Multiple Output, MIMO Radar channels, recent IBIS-ArcSAR product, acquisition time, IBIS-Rover product, horizontal coverage, IDS GeoRadar company, IBIS-FM product, elliptical shape, azimuth coverage, relevant limits, open pit mines application, open pit mines scenarios, efficient tool, GB-SAR, decade Ground-Based SAR technology, slope monitoring, innovative ground-based SAR system, time 1.0 min, Synthetic aperture radar, Azimuth, Image reconstruction, Monitoring, Focusing, Two dimensional displays, Groun d-based Synthetic Aperture Radar, Interferometry, MIMO Radar, Digital Elevation Models, Radar Imaging.
    Abstract: In the last decade Ground-Based SAR (GB-SAR) technology has been demonstrated to be one of the most efficient tool for slope monitoring in open pit mines scenarios. However, especially for open pit mines application, one of the relevant limits is the azimuth coverage, being the mine often of elliptical shape. In this context, the IBIS-FM product of IDS GeoRadar company can reach about 80deg of horizontal coverage, and can be overcome by IBIS-Rover product that can reach 270deg of horizontal coverage but increasing the acquisition time. The recent IBIS-ArcSAR product developed by IDS GeoRadar can overcome these limits, offering a full 360deg of horizontal coverage in less than 1 minute of acquisition time, with additional feature of a constant angular resolution. Moreover, by exploiting the new capability of MIMO (Multiple input Multiple Output) Radar channels, the vertical height or Digital Elevation Model (DEM) of the mine can be reconstructed through simple and computationally fast interferometric processing algorithms, together with an enhanced focusing of 2D SAR Image that is an open issue in Arc-scanning SAR systems.
    [bibtex-key = vivianiEtalIGARSS2018IBISArcSAR] [bibtex-entry]


  1132. X. Xu, C. A. Baldi, J. De Bleser, Y. Lei, S. Yueh, and D. Esteban-Fernandez. Multi-Frequency Tomography Radar Observations of Snow Stratigraphy at Fraser During SnowEx. In IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium, pages 6269-6272, July 2018. Keyword(s): CW radar, FM radar, geophysical signal processing, hydrological techniques, radar polarimetry, radar signal processing, remote sensing by radar, snow, signal processing algorithm, snow free image, snow density, ground snow pit measurement, radar design, snow layer, multiple viewing positions, radar echo, tomography technique, radar channel, three-dimensional variability, fully polarimetric frequency-modulated continuous-wave radar, triple-frequency, 2017 NASA SnowEx campaign, Earth's terrestrial snow-covered regions, multiyear airborne snow campaign, Fraser, snow stratigraphy, multifrequency tomography radar observations, frequency 17.2 GHz, size 30.0 cm, frequency 9.6 GHz, frequency 13.5 GHz, Snow, Tomography, Synthetic aperture radar, Radar polarimetry, Spaceborne radar, Radar imaging, tomography, snow, FMCW radar, SWE.
    Abstract: SnowEx is a multi-year airborne snow campaign led by NASA. The purpose of SnowEx is to figure out how much water is stored in Earth's terrestrial snow-covered regions. As part of the 2017 NASA SnowEx campaign, we deployed a portable triple-frequency (9.6GHz, 13.5GHz and 17.2GHz) and fully polarimetric frequency-modulated continuous-wave (FMCW) radar at Fraser, Colorado. The radar was installed on a 60cmx60cm frame to enable a full reconstruction of the three-dimensional variability per each radar channel. The tomography technique uses the radar echo from the multiple viewing positions and provides a unique access to the vertical structure of the snow layer. With current setup, the range resolution is 30cm. In this paper, we will review the radar design and signalprocessing algorithm - time domain back projection. The generated vertical images show the snow stratigraphy, which is consistent with ground snow pit measurement. The continuous operation demonstrates diurnal thawing and refreezing process. The snow density is retrieved by comparing to the snow free image.
    [bibtex-key = xuBaldiDeBleserLeiYuehEstebanFernandezIGARSS2018TomographyRadarObservationsofSnowSnowEx] [bibtex-entry]


  1133. Ludovic Brucker, Christopher Hiemstra, Hans-Peter Marshall, Kelly Elder, Roger De Roo, Mohammad Mousavi, Francis Bliven, Walt Peterson, Jeffrey Deems, Peter Gadomski, Arthur Gelvin, Lucas Spaete, Theodore Barnhart, Ty Brandt, John Burkhart, Christopher Crawford, Tri Datta, Havard Erikstrod, Nancy Glenn, Katherine Hale, Brent Holben, Paul Houser, Keith Jennings, Richard Kelly, Jason Kraft, Alexandre Langlois, Daniel McGrath, Chelsea Merriman, Noah Molotch, Anne Nolin, Chris Polashenski, Mark Raleigh, Karl Rittger, Chago Rodriguez, Alexandre Roy, McKenzie Skiles, Eric Small, Marco Tedesco, Chris Tennant, Aaron Thompson, Liuxi Tian, Zach Uhlmann, Ryan Webb, and Matt Wingo. A first overview of SnowEx ground-based remote sensing activities during the winter 2016-2017. In 2017 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), pages 1391-1394, July 2017.
    Abstract: NASA SnowEx's goal is estimating how much water is stored in Earth's terrestrial snow-covered regions. To that end, two fundamental questions drive the mission objectives: (a) What is the distribution of snow-water equivalent (SWE), and the snow energy balance, among different canopy and topographic situations? and (b) What is the sensitivity and accuracy of different SWE sensing techniques among these different areas? In situ, ground-based and airborne remote sensing observations were collected during winter 2016-2017 in Colorado to provide the scientific community with data needed to work on these key questions. An intensive period of observations occurred in February 2017 during which over 30 remote sensing instruments were used. Their observations were coordinated with in situ measurements from snowpits (e.g. profiles of stratigraphy, density, grain size and type, specific surface area, temperature) and along transects (mainly for snow depth measurements). Both remote sensing and in situ data will be archived and publicly distributed by the National Snow and Ice Data Center at nsidc.org/data/snowex.
    [bibtex-key = bruckerEtAl2017OverviewSnowExGroundBasedRemoteSensingInWinter20162017] [bibtex-entry]


  1134. Ning Cao, Hyongki Lee, Evan Zaugg, Ramesh Shrestha, William E. Carter, Craig Glennie, Zhong Lu, and Juan Carlos Fernandez Diaz. Evaluation of an airborne SAR system for deformation mapping: A case study over the slumgullion landslide. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1684-1687, July 2017. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, InSAR, Differential SAR Interferometry, DInSAR, Repeat-Pass Interferometry, deformation monitoring, subsidence monitoring, Displacement, Earth, Global Positioning System, Laser radar, Spaceborne radar, Strain, Synthetic aperture radar, Terrain factors, GPS, Landslide, LiDAR, SAR.
    Abstract: In this study, we present a case study of the Slumgullion landslide conducted in July 2015 to demonstrate the feasibility of deformation mapping with an airborne synthetic aperture radar (SAR) system known as ARTEMIS SlimSAR, which is a compact, modular, and multi-frequency radar system. For this study, the L-band SlimSAR was installed on a Cessna 206 aircraft and data were collected on July 3, 7, and 10 of 2015 and processed using the time-domain backprojection algorithm. Airborne light detection and ranging (LiDAR) campaign, GPS surveys and spaceborne InSAR analysis using COSMO-SkyMed images were also conducted to verify the performance of the airborne SAR system. The airborne InSAR results showed satisfying agreement with the GPS and spaceborne InSAR results. A 3-D deformation map over Slumgullion landslide was also generated, which displayed distinct correlation between the landslide motion and topography.
    [bibtex-key = caoLeeZauggShresthaCarterGlennieLuDiazIGARSS2017TDBPAirborneDInSAR] [bibtex-entry]


  1135. Elias J. Deeb, Hans-Peter Marshall, Richard R. Forster, Cathleen E. Jones, Christopher A. Hiemstra, and Paul R. Siqueira. Supporting NASA SnowEx remote sensing strategies and requirements for L-band interferometric snow depth and snow water equivalent estimation. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1395-1396, July 2017.
    Abstract: The objectives of this research are to (1) address remote sensing strategies and requirements for estimating snow depth and snow water equivalent (SWE) using existing L-Band interferometric data sets in coordination with field-based observations and modeling frameworks and, with this information, (2) inform the Next Generation Cold Land Processes Experiment (SnowEx) toward articulating the appropriate science and research questions for a single motivating science plan. As proposed, SnowEx is a multi-year airborne snow campaign with a primary goal of exploring multimodal sensor observations in coordination with field campaigns to inform the next generation snow remote sensing satellite platform. Based on limitations of satellite-based optical and LiDAR instruments operating in regions of the globe with consistent cloud-cover, the fact that many snow-dominated regions are at more northerly latitudes (limited solar illumination in the middle of winter), and these snow-dominated regions often experience periods of prolonged cloud cover (due to synoptic precipitation events), a microwave remote sensing platform may be the most viable path to space for a dedicated snow remote sensing mission. Specifically, L-Band radar interferometry has shown some unique promise with an archive of historical and contemporary satellite collections from JAXA's PALSAR-1 and PALSAR-2 instruments, respectively. Moreover, with the expected NISAR (NASA-ISRO Synthetic Aperture Radar) mission launch in 2020 and the unprecedented availability of dedicated global interferometric L-Band products every 12-days, as well as what is in essence a NISAR airborne simulator in JPL's UAVSAR platform, the L-Band interferometric approach to estimating snow depth and snow water equivalent (SWE) requires further investigation within the context of in-situ observations and modeling frameworks.
    [bibtex-key = deebEtAl2017SupportingNASASnowExForLBandinterferometricSnowDepthAndSWE} ] [bibtex-entry]


  1136. M. Faisal, M. A. H. Chowdhury, M. A. I. Bhuyan, N. H. M. Bhuyan, and A. Matin. Development of a polarimetric interferometric GB-SAR and perform measurement for a fixed target. In Proc. Computer and Communication Engineering (ECCE) 2017 Int. Conf. Electrical, pages 285-289, February 2017. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, horn antennas, radar antennas, radar interferometry, radar target recognition, synthetic aperture radar, SAR system, antenna positioner, building vehicles, data compression, double ridge guide horn antenna, fixed target, ground vehicles, ground-based synthetic aperture radar system, innovative monitoring technique, perform measurement, polarimetric & interferometric GB-SAR, vector network analyzer, Antenna measurements, Apertures, Frequency-domain analysis, Image reconstruction, Monitoring, Synthetic aperture radar, Time-domain analysis, Global Backprojection (GBP), Ground-Based SAR (GB-SAR), Interferometry, Low loss-high frequency cable, Vector Network Analyzer (VNA). [bibtex-key = Faisal2017] [bibtex-entry]


  1137. Othmar Frey, Charles L. Werner, Rafael Caduff, and Andreas Wiesmann. Inversion of Snow Structure Parameters from Time Series of Tomographic Measurements With SnowScat. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 2472-2475, 2017. Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar, SWE, Snow Water Equivalent, Autofocus, Time-Domain Back-Projection, TDBP, Backprojection.
    Abstract: SnowScat is a terrestrial stepped-frequency continuous-wave (SFCW) scatterometer which supports fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. Recently, the hardware has been upgraded by adding a tomographic profiling mode. This tomographic approach allows to retrieve high-resolution information about a snowpack via observables, such as radar backscatter, co-polar phase difference, interferometric phase and coherence. Since the tomographic imaging itself is also affected by the refraction occurring at the air-snow interface and within the snowpack the two problems, 1) the production of well-focused and correctly located tomographic profiles, and 2) the retrieval of snow structure parameters are inherently linked. In this contribution, a tomographic inversion scheme to retrieve the refractive index of snow through an autofocus approach is presented. The current autofocus-based retrieval relies on using an aluminium sphere of a test target deployed in the scene. The refractive indices and accompanying snow density measurements obtained at four dates during a cold period in January during the ESA SnowLab 2016/2017 campaign are compared to an empirical model by Matzler and Wiesmann that describes the relation between snow density and the real part of the relative permittivity for dry snow.
    [bibtex-key = freyWernerCaduffWiesmannIGARSS2017SnowScatTomoInversionSWE] [bibtex-entry]


  1138. Scott Hensley. An analytic expression for the phase noise properties of the Goldstein-Werner power spectral filter. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 3802-3805, July 2017. Keyword(s): Fourier transforms, filters, geophysical signal processing, radar interferometry, remote sensing by radar, spectral analysis, synthetic aperture radar, Fourier transform, Goldstein-Werner power spectral filter, interferogram filtering, maximal noise reduction, phase noise properties, radar interferometric applications, repeat pass radar interferometry, Correlation, Noise reduction, Phase noise, Radar, Signal to noise ratio, Transfer functions, Goldstein-Werner filter, filtering, interferogram, phase noise, power spectral filter. [bibtex-key = hensleyIGARSS2017AnalyticExpressionGoldsteinWernerFilter] [bibtex-entry]


  1139. Zahra Sadeghi, M. J. Valadan Zoej, A. Hooper, and J. M. Lopez-Sanchez. An Enhanced Polarimetric Persistent Scatterer Interferometry Method to Increase Number and Quality of Selected Pixels. In Proc. FRINGE 2017, Helsinki, Finland, June 2017. [bibtex-key = sadeghiValadanZoejHooperLopezSanchezFRINGE2017] [bibtex-entry]


  1140. M. Schuetz, M. Oesterlein, C. Birkenhauer, and M. Vossiek. A custom lightweight UAV for radar remote sensing: Concept design, properties and possible applications. In 2017 IEEE MTT-S International Conference on Microwaves for Intelligent Mobility (ICMIM), pages 107-110, March 2017. Keyword(s): embedded systems, Global Positioning System, ground penetrating radar, MIMO radar, radar interferometry, remote sensing, synthetic aperture radar, custom lightweight UAV, radar remote sensing, concept design, tailor-made UAV-based sensor platform, radar remote sensing, radar baseband processing, differential GPS system, synthetic apertures, interferometric remote sensing, MIMO radar system, side-looking-airborne-radar, full interferometric SAR processing, radar altimeter measurement, altitude estimators, ultrasonic sensors, barometric sensors, Radar remote sensing, Synthetic aperture radar, Radar antennas, Baseband, Antenna arrays, Clocks, multicopter, UAV, remote sensing, system design, radar system, SAR, interferometry.
    Abstract: In this paper, we introduce a novel, tailor-made UAV-based sensor platform for radar remote sensing. It is controlled by a redundantly implemented, customized flight control and equipped with a comprehensive embedded system for radar baseband processing. Both systems are synchronized with a common, low-phase noise and temperature-stable reference clock, which allows coherent acquisition and continuous recording of sensor data. A commercial differential GPS system is used for positioning. The platform potentially allows interferometric remote sensing with synthetic apertures. In a first case study, it is equipped with a bistatic 24GHz MIMO radar system to create a side-looking-airborne-radar, allowing full interferometric SAR processing. As a proof-of-concept, a radar altimeter measurement is presented, which replaces and outperforms common altitude estimators, such as barometric or ultra sonic sensors.
    [bibtex-key = schuetzOsterleinBirkenhauerVossiekConf2017LightweightUAVforRADARremotesensing] [bibtex-entry]


  1141. M. Adnan Siddique, Irena Hajnsek, and Othmar Frey. A Case Study on the Use of Differential SAR Tomography for Measuring Deformation in Layover Areas in Rugged Alpine Terrain. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5850-5853, 2017. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne SAR, Cosmo Skymed, interferometric stack, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Alpine Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: Differential SAR tomography is a means to resolve layover of temporally coherent scatterers while simultaneously estimating their elevation and average deformation. In alpine regions, drastic height variations result in frequent layovers which are rejected during typical persistent scatterer interferometric (PSI) analyses. In this paper, we explore the potential of tomographic techniques to improve deformation sampling in an alpine region of interest relative to a PSI-based deformation assessment. The mitigation of the atmospheric phase contributions, as required for both tomography and PSI, is often more involved in alpine regions due to strong spatial variations of the local atmospheric conditions and propagation paths through the troposphere. We assume a linear multivariate dependence of atmospheric phase on the spatial location and height of the scatterers, estimate it using universal/regression kriging and subsequently incorporate it within the tomographic focusing. Experiments are performed on an interferometric stack comprising of 32 Cosmo-SkyMed strimap images acquired in the summers of 2008-2013 over Mattervalley in the Swiss Alps.
    [bibtex-key = siddiqueHajnsekFreyIGARSS2017PSITomoAlpine] [bibtex-entry]


  1142. Hui Wang, Yongjiang Xia, Man Jiang, and Lingzhen Kong. Research on W-band FMCW Rail-SAR system with high resolution. In 2017 18th International Radar Symposium (IRS), pages 1-6, June 2017. Keyword(s): SAR Processing, W-Band, FM radar, millimetre wave radar, radar imaging, radar resolution, synthetic aperture radar, RD imaging algorithm, W-band FMCW rail-SAR system, high resolution imaging, small target imaging, system parameter simulation, Antenna arrays, Azimuth, Imaging, Rails, Signal resolution, Simulation, Synthetic aperture radar.
    Abstract: Combined with short wavelength of the w-band and small volume and low cost advantage of the FMCW SAR, much attention has been paid to the application of W band FMCW SAR with high resolution imaging to close small target imaging. This paper introduces a W-band Rail FMCW SAR used to small target imaging in near-distance. Based on the system parameters simulation and design, the design of the system scheme is also given. In addition, using the modified RD imaging algorithm to get the point target simulation result. The result proves the feasibility and reliability of the system.
    [bibtex-key = wangXiaJiangKong2017FMCWSARWBAND] [bibtex-entry]


  1143. Simone Baffelli, Othmar Frey, Charles L. Werner, and Irena Hajnsek. System Characterization and Polarimetric Calibration of the Ku-Band Advanced Polarimetric Interferometer. In Proc. of EUSAR 2016 - 11th European Conference on Synthetic Aperture Radar, pages 735-740, June 2016. Keyword(s): Terrestrial Radar Interferometry, TRI, Ground-based radar, Interferometry, Polarimetry, Calibration, Pol-GPRI.
    Abstract: This paper addresses the system characterization and the polarimetric calibration of the Ku-Band Advanced Polarimetric Interferometer (KAPRI). KAPRI is an FMCW ground-based real aperture radar system that uses slotted waveguide antennas. The rotation of the antennas introduces undesired phase ramps in azimuth. We present a geometrical model to account for this phase, and propose a method to correct it. Experimental data with a set of trihedral corner reflectors (TCR) in the scene was acquired with the system. A linear phase variation of 30 degrees was observed over the TCR which was geometrically modeled and successfully corrected.
    [bibtex-key = baffelliFreyWernerHajnsekEUSAR2016PolGPRI] [bibtex-entry]


  1144. T. J. Czernuszewicz, J. W. Homeister, M. C. Caughey, B. Y. Huang, E. R. Lee, C.A. Zamora, M. A. Farber, J. J. Fulton, P. F. Ford, W. A. Marston, R. Vallabhaneni, T. C. Nichols, and C. M. Gallippi. Carotid plaque characterization with ARFI imaging: Blinded reader study. In Proc. IEEE Int. Ultrasonics Symp., pages 1-4, September 2016. Keyword(s): acoustic imaging, biomedical ultrasonics, blood vessels, calcium, feature extraction, medical image processing, proteins, regression analysis, sensitivity analysis, ARFI Imaging, AUC, CEA, acoustic radiation force impulse imaging, area under the ROC curve, blinded reader study, carotid endarterectomy, carotid plaque characterization, carotid vasculature, collagen, fibrous cap thickness measurements, histologic examination, intraplaque hemorrhage, linear regression, necrotic core, noninvasive elastography imaging technique, parametric 2D ARFI images, peak displacement, plaque feature detection, plaque risk assessment, receiver operating characteristic curve analysis, surgery, thromboembolic events, vulnerable atherosclerotic plaque, Acoustics, Atherosclerosis, Calcium, Hemorrhaging, Imaging, In vivo, Thickness measurement, ARFI, acoustic radiation force, atherosclerosis, plaque characterization, stroke. [bibtex-key = Czernuszewicz2016] [bibtex-entry]


  1145. Othmar Frey, Charles L. Werner, Rafael Caduff, and Andreas Wiesmann. A time series of SAR tomographic profiles of a snowpack. In Proc. of EUSAR 2016, pages 726-730, June 2016. Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: Recently, the SnowScat hardware - a tower-mounted fully-polarimetric scatterometer at X-/Ku-band - has been enhanced to also provide a tomographic profiling mode which allows to obtain high-resolution 2-D vertical profiles that may provide further insights into the electromagnetic interaction within layered snowpacks. In winter 2014/2015, a first test campaign was carried out yielding a successful proof of concept of the hardware, tomographic measurement, and basic processing concept. As a follow-up, in Nov/Dec 2015, the SnowScat device was installed at a test site on 1700m altitude close to the Grimsel pass in Switzerland. Since then it has been acquiring a time series of tomographic profiles of a snow pack. In this paper, we present and discuss first results of this new time series.
    [bibtex-key = freyWernerCaduffWiesmannEUSAR2016SnowScatTomo] [bibtex-entry]


  1146. Othmar Frey, Charles L. Werner, Rafael Caduff, and Andreas Wiesmann. A time series of tomographic profiles of a snow pack measured with SnowScat at X-/Ku-Band. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 17-20, July 2016. Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: The SnowScat device is a ground-based stepped-frequency continuous-wave (SFCW) scatterometer supporting fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. It was originally designed to support the investigation and validation of Snow Water Equivalent (SWE) retrieval algorithms in the context of the development of the deselected COld REgions Hydrology High-resolution Observatory (CoReH20) candidate Earth Explorer 7 mission. Recently, the SnowScat hardware has been enhanced to also provide a tomographic profiling mode which allows to obtain high-resolution 2-D vertical profiles that may provide further insight into the electromagnetic interaction within layered snow packs. In winter 2014/2015, a first test campaign was carried out yielding a successful proof of concept of the enhanced hardware, tomographic measurement, and basic processing concept. In Nov/Dec 2015, the SnowScat device was then installed as a part of the SnowLab experiment at a test site on 1700m altitude close to the Grimsel pass in Switzerland. A comprehensive time series of tomographic profiles of a snow pack was acquired until end of March, 2016. In this paper, we present and discuss first results of this new time series of tomographic profiles including 2-D vertical profiles of backscatter, phase difference between the copolar channels, and interferometric phase difference.
    [bibtex-key = freyWernerWiesmannIGARSS2016SnowScatTomo] [bibtex-entry]


  1147. G. Gomba, F. De Zan, and A. Parizzi. Ionospheric Phase Screen and Ionospheric Azimuth Shift Estimation Combining the Split-Spectrum and Multi-Squint Methods. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-4, June 2016. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, ionospheric electromagnetic wave propagation, synthetic aperture radar, L-band interferograms, L-band synthetic aperture radar interferometric pairs, SAR interferograms, advanced land observing satellite phased-array, differential ionospheric path delay, geophysical processes, ground deformation signals, ionospheric effects operational compensation, ionospheric phase, split-spectrum method, tropospheric path delay, Accuracy, Azimuth, Coherence, Delays, Estimation, Ionosphere, Synthetic aperture radar, Interferometric synthetic aperture radar (InSAR), ionosphere estimation, split spectrum, synthetic aperture radar (SAR) ionospheric effects. [bibtex-key = Gomba2016] [bibtex-entry]


  1148. U. Herter, H. Schmaljohann, and T. Fickenscher. Autofocus performance on multi channel SAS images in the presence of overlapping phase centers. In OCEANS 2016 MTS/IEEE Monterey, pages 1-6, September 2016. Keyword(s): geophysical image processing, image restoration, optical focusing, sonar imaging, synthetic aperture sonar, DPCA, RPC, SPGA focusing, autofocus performance, blurred images, data driven micronavigation, displaced phase center antenna, echo signals, multichannel SAS images, navigation data, overlapping phase centers, raw echo data, redundant phase centers, residual phase errors, side lobe levels, strip-map phase gradient autofocus, synthetic aperture images, synthetic aperture sonar, Apertures, Approximation algorithms, Sonar navigation, Synthetic aperture sonar, Transmitters, aperture sonar, autofocus, multi-channel, strip-map.
    Abstract: Synthetic aperture sonar (SAS) suffers from a fundamental problem: The navigation data accuracy required for coherent summation of the echo signals is not directly achievable. Unamended this issue leads to heavily blurred images. Fortunately, under certain conditions, algorithms like displaced phase center antenna (DPCA) can be used to fine tune the navigation from the raw echo data itself. In contrary to DPCA, strip-map phase gradient autofocus (SPGA) is an algorithm which extracts residual phase errors from processed synthetic aperture images. The results can be used to correct and reprocess the raw data yielding images with reduced blurring. In the presence of overlapping phase centers in raw data, as required for data driven micro navigation like DPCA, some assumptions of SPGA are violated and can cause degradation of it's results. In this paper we investigate the effect of overlapping (redundant) phase centers (RPC) in raw data on the performance of SPGA. Results indicate that the use of redundant data during image formation can bias the phase information in SAS imagery and therefore affects SPGA focusing. In this case, side lobe levels in the focused images are increased compared to images processed without RPCs. Suitable selection schemes should be employed prior to processing to avoid biasing of the phase information due to data collected at redundant phase centers if SPGA processing on top of DPCA is required.
    [bibtex-key = 7761082] [bibtex-entry]


  1149. G. Krieger, T. Rommel, and A. Moreira. MIMO-SAR Tomography. In Proceedings of EUSAR 2016: 11th European Conference on Synthetic Aperture Radar, pages 1-6, June 2016.
    Abstract: MIMO SAR employs multiple transmit and receive channels to improve the imaging performance and to acquire novel geoinformation products. One example is SAR tomography, where the simultaneous transmission and reception with multiple antennas can provide a large number of baselines with a small number of antennas. In the limit, an appropriately designed MIMO-SAR configuration with NTx transmitters and NRx receivers can provide in total NTx . NRx independent phase centers and therefore NTx . NRx - 1 independent baselines for SAR tomography. The other extreme is provided by uniform linear arrays with co-located transmitters and receivers. Such configurations are characterized by a large number of overlapping effective phase centers and are therefore regarded as highly redundant. In this paper, we will show that such a redundancy is nevertheless well suited to resolve an inherent challenge of conventional SAR tomography, which is limited in providing unambiguous 3-D scatterer position estimates in case of multiple scattering. For this, we show that redundant MIMO arrays allow not only an a posteriori beamforming on receive, but, at the same time, also a comparable a posteriori (i.e. after data acquisition) beamforming on transmit. This means that one can emulate, from one and the same recorded MIMO-SAR data set, different illumination scenarios on transmit and receive. By evaluating the 2-D spectrum provided by the independent Tx and Rx beams, it becomes then possible to differentiate between single- and multiplebounce scattering. The separation between single- and double-bounce scattering has also been successfully demonstrated in a ground-based radar experiment and is presented in another EUSAR paper.
    [bibtex-key = kriegerRommelMoreiraEUSAR2016MIMOSARTomography] [bibtex-entry]


  1150. H. Liao and F. J. Meyer. A combined estimator for Interferometric SAR ionosphere correction. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 6499-6501, July 2016. Keyword(s): Faraday effect, ionospheric techniques, radar interferometry, radar polarimetry, synthetic aperture radar, Faraday rotation based method, InSAR differential ionosphere, InSAR ionospheric effect correction, azimuth shift based method, interferometric SAR ionosphere correction, interferometric synthetic aperture radar ionosphere correction, ionospheric geometric distortion, multiple aperture interferometric azimuth shift, polarimetric property, split spectrum InSAR technique, Earthquakes, Interferometry, Ionosphere, L-band, Synthetic aperture radar. [bibtex-key = Liao2016a] [bibtex-entry]


  1151. H. Liao and F. J. Meyer. Ionospheric effect correction of ice motion mapping using interferometric synthetic aperture radar. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 6502-6504, July 2016. Keyword(s): adaptive filters, geophysical signal processing, glaciology, ice, ionosphere, radar interferometry, remote sensing by radar, synthetic aperture radar, AD 1990, Antarctica, C-band ERS1-2, Envisat ASAR, Greenland, InSAR-based ionospheric correction, L-band ALOS 1-2 PALSAR SAR data, Radarsat-1-2, Sentinel-1, X band TerraSAR-X, adaptive filter technique, automatic phase unwrapping error correction, coregistration technique, differential ionospheric phase signal, error correction algorithm, filter-based method, ice mass balance, ice motion analysis, ice motion mapping, ice motion monitoring, ice sheet, ice velocity, interferogram, interferometric synthetic aperture radar, ionospheric effect correction, ionospheric error, ionospheric phase delay, sea level rise, split spectrum technique, Ice, Ionosphere, L-band, Monitoring, Sea level, Synthetic aperture radar. [bibtex-key = Liao2016] [bibtex-entry]


  1152. P. Lopez-Dekker, F. Q. d. Almeida, M. Rodriguez-Cassola, P. Prats, O. Ponce, and M. Younis. End-to-end simulation of reflector based DBF SAR Systems. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-5, June 2016. [bibtex-key = Lopez-Dekker2016] [bibtex-entry]


  1153. T. Marston and J. Kennedy. TomoSAS images of acoustically penetrable objects. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-4, June 2016. [bibtex-key = Marston2016] [bibtex-entry]


  1154. T. Marston and J. Kennedy. TomoSAS in bathymetrically complex environments. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-4, June 2016. [bibtex-key = Marston2016a] [bibtex-entry]


  1155. A. Moreira, O. Ponce, M. Nannini, M. Pardini, P. Prats, A. Reigber, K. Papathanassiou, and G. Krieger. Multi-baseline spaceborne SAR imaging. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 1420-1423, July 2016. Keyword(s): geophysical image processing, holography, radar imaging, radar interferometry, radar polarimetry, remote sensing by radar, spaceborne radar, synthetic aperture radar, HoloSAR, PolInSAR, TomoSAR, across-track interferometry, along- track interferometry, glacier movement, ground deformation, holography SAR, multi-baseline imaging capability, multi-dimensional data space, multibaseline spaceborne SAR imaging, multistatic SAR configuration, ocean current, polarimetric SAR interferometry, spaceborne SAR development, spaceborne SAR image product, sparse array, surface topography measurement, tomography SAR, Interferometry, Radar imaging, Radar polarimetry, Spaceborne radar, Synthetic aperture radar, Tomography, Multi-Baseline Imaging, Polarimetric SAR Interferometry, Polarimetry, SAR Holography, SAR Tomography, Synthetic Aperture Radar (SAR). [bibtex-key = Moreira2016] [bibtex-entry]


  1156. Stephan Palm, Rainer Sommer, A. Hommes, Nils Pohl, and Uwe Stilla. Mobile Mapping by FMCW Synthetic Aperture Radar Operating at 300 GHZ. In Int. Arch. Photogramm. Remote Sens. Spatial Inf. Sci., XLI-B1, volume XLI-B1, pages 81-87, June 2016. Copernicus GmbH. Keyword(s): SAR Processing, Miranda, Miranda 300, FMCW, FMCW SAR, Mobile Radar Mapping, Car-borne SAR, Street Mapping, Ultra-High Resolution SAR, Subcentimeter Resolution.
    Abstract: While optical cameras or laser systems are widely used for mobile mapping low attention was payed for radar systems. Due to new semiconductor technologies, compact and leight weight SAR systems based on the Frequency Modulated Continuous Wave (FMCW) principle in the millimeter wave domain can serve for mobile radar mapping on cars. For mapping of long stripes along roads in close range a special strategy for focusing of SAR images was developed. Hereby local adapted planes for processing are used considering the IMU data of the sensor. An experimental system was designed for high resolution radar mapping of urban scenes in close range geometry. This small and leight weighted system has a bandwidth of 30 GHz (5 mm resolution) and operates with 300 GHz in the lower terahertz domain. Experiments with a van in an urban scenario were carried out for proof of applicability of an operating SAR system resolving objects in the subcentimeter domain. The results show that narrow cracks in the asphalt of the road are visible and the measuring of small metallic objects placed in the scene is possible. Based on this mobile mapping techniques a first result from an acquisition of vertical facade structure is shown.
    [bibtex-key = palmSommerHommesPohlStillaISPRS2016Miranda300GHzFMCW] [bibtex-entry]


  1157. M. Pieraccini. Polarimetrie RotoSAR. In Proc. IEEE Radar Conf. (RadarConf), pages 1-5, May 2016. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar antennas, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, ground based inteferometric synthetic aperture radar systems, linear mechanical guide, polarimetric RotoSAR, radar head, radar images, remote sensing instruments, rotating antenna GB-SAR, Monitoring, Radar antennas, Radar imaging, Radar polarimetry, Radar remote sensing, Synthetic aperture radar, SAR, radar. [bibtex-key = Pieraccini2016] [bibtex-entry]


  1158. O. Ponce, H. Joerg, R. Scheiber, P. Prats, I. Hajnsek, and A. Reigber. First study on holographic SAR tomography over agricultural crops at C-/X-band. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 7403-7406, July 2016. Keyword(s): crops, geophysical image processing, image reconstruction, radar polarimetry, remote sensing by radar, synthetic aperture radar, 2D polarimetric image, 3D crop field backscattering, 3D forest backscattering distribution, 3D imaging reconstruction, 3D polarimetric image, C-band SAR, DLR F-SAR sensor, Germany, HoloSAR campaign, HoloSAR imaging mode, HoloSAR tomography, Wallerfing, X-band SAR, agricultural crop, azimuthal aspect angle, holographic SAR, scattering mechanisms, tomographic constellation, Tomography, Agricultural crops, Fast Factorized Back-Projection (FFBP), Holographic SAR Tomography (HoloSAR), Polarimetric Synthetic Aperture Radar (PolSAR). [bibtex-key = Ponce2016] [bibtex-entry]


  1159. O. Ponce, R. Scheiber, P. Prats, I. Hajnsek, and A. Reigber. Multi-dimensional airborne holographic SAR tomography reconstruction for glaciers at L-/P-band. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 9-12, July 2016. Keyword(s): geophysical image processing, glaciology, hydrological techniques, image reconstruction, remote sensing by radar, solid modelling, synthetic aperture radar, 2-D image arc-pattern, 2-D image circular pattern, 3-D imaging reconstructions, Greenland, HoIoSAR campaign, HoloSAR mode, K-Transect, circular synthetic aperture, cryosphere, fast factorized back-projection, fully polarimetric data, glacier structures, glacier vertical profile, ice sheet vertical profile, ice structures, multidimensional airborne holographic SAR tomography reconstruction, vertical synthetic aperture, Apertures, Ice, Image resolution, L-band, Synthetic aperture radar, Tomography, Cryosphere, Fast Factorized Back-Projection (FFBP), Glaciers, Holographic SAR Tomography (HoloSAR), Polarimetric Synthetic Aperture Radar (PolSAR). [bibtex-key = Ponce2016a] [bibtex-entry]


  1160. R. Que, O. Ponce, S. V. Baumgartner, and R. Scheiber. Multi-mode Real-Time SAR On-Board Processing. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-6, June 2016. [bibtex-key = Que2016] [bibtex-entry]


  1161. R. Que, Octavio Ponce, Rolf Scheiber, and Andreas Reigber. Real-time processing of SAR images for linear and non-linear tracks. In Proc. 17th Int. Radar Symp. (IRS), pages 1-4, May 2016. Keyword(s): airborne radar, backpropagation, radar imaging, synthetic aperture radar, DLR's F-SAR sensor, GPU, SAR images, airborne SAR, direct backprojection, distributed real-time processing, fast factorized back-projection algorithms, linear tracks, multiprocessors multicore CPU, nonlinear tracks, real-time computation, Containers, Graphics processing units, Instruction sets, Interpolation, Radar tracking, Real-time systems, Synthetic aperture radar. [bibtex-key = quePonceScheiberReigber2016a] [bibtex-entry]


  1162. M. Adnan Siddique, Urs Wegmuller, Irena Hajnsek, and Othmar Frey. SAR tomography as an add-on to PSI for improved deformation sampling in urban areas: a quality assessment. In Proc. of EUSAR 2016 - 11th European Conference on Synthetic Aperture Radar, pages 669-612, June 2016. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, Barcelona, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne TerraSAR-X data, interferometric stack, high-rise building vertical tomographic slice, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: Persistent scatterer interferometry (PSI) typically rejects layovers. Therefore, layover-affected urban areas may suffer from inadequate deformation sampling. SAR tomography, when used as an add-on to PSI, reveals additional deformation samples by resolving layovers. In this paper we quantify the relative gain in deformation sampling, while taking into account the quality of the additional (double) scatterers in terms of root-mean-square (RMS) phase deviation. We experiment on an interferometric stack of 50 TerraSAR-X stripmap images acquired over the city of Barcelona. The results show a tradeoff between the gain and the quality of the detected scatterers. For the observed urban area, we obtain a gain of 9.8% while the RMS phase deviation for 99% of the detected double scatterers is less than 1.1 radians.
    [bibtex-key = siddiqueWegmullerHajnsekFreyEUSAR2016PSITomo] [bibtex-entry]


  1163. Muhammad Adnan Siddique, Urs Wegmuller, Irena Hajnsek, and Othmar Frey. SAR tomography as an add-on to PSI: gain in deformation sampling vis-a-vis quality of the detected scatterers. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1452-1455, July 2016. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, Barcelona, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne TerraSAR-X data, interferometric stack, high-rise building vertical tomographic slice, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: SAR tomography can be used as an add-on to persistent scatterer interferometry (PSI) to increase deformation sampling in urban areas by resolving the frequently occurring layovers that are by definition rejected in the PSI processing. This paper, while focusing on the case of a typical highrise building in layover, quantitatively assesses the potential gain in deformation sampling achieved by the added use of an advanced SAR tomographic technique relative to a PSI approach. At the same time, the quantity of the detected scatterers is weighed against their quality, as assessed on the basis of root-mean-square (RMS) phase deviation between the measurements and the model fit. The quality of the scatterers is also compared with the quality of the persistent scatterers as identified with a PSI approach. The experiments are performed on an interferometric stack of 50 TerraSAR-X stripmap mode images. The results show that although there is a trade-off between the quantity and the quality of the scatterers, SAR tomography effectively resolves the layovers for the test site and provides an improvement in deformation sampling.
    [bibtex-key = siddiqueWegmullerHajnsekFreyIGARSS2016PSITomo] [bibtex-entry]


  1164. Urs Wegmuller, Charles L. Werner, Andreas Wiesmann, Penelope Kourkouli, and Othmar Frey. Time-series analysis of Sentinel-1 interferometric wide swath data: techniques and challanges. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 3898-3901, July 2016. Keyword(s): SAR Processing, SAR Interferometry, InSAR, differential SAR interferometry, D-InSAR, TOPS interferometry, TOPS mode, Sentinel-1, Spaceborne SAR, C-Band. [bibtex-key = wegmullerWernerWiesmannStrozziKourkouliFreyIGARSS2016Sentinel1TimeSeries] [bibtex-entry]


  1165. T. M. d. Hoyo and O. Ponce. Understanding Spaceborne Missions for TomoSAR Imaging with Multi-Angular Acquisitions. In Proc. EUSAR 2016: 11th European Conf. Synthetic Aperture Radar, pages 1-4, June 2016. [bibtex-key = Hoyo2016] [bibtex-entry]


  1166. W. M. Boerner, G. Krieger, A. Reigber, I. Hajnsek, C. C. Schmullius, A. Moreira, M. Eineder, R. Bamler, F. J. Meyer, S. Hensley, J. J. van Zyl, M. Neumann, M. Shimada, M. Ohki, J. T. S. Sumantyo, K. Hattori, F. J. Ocampo-Torres, O. Ponce, J. Moreira, J. Campos, L. Yi-Long, P. Dubois-Fernandez, E. Pottier, T. LeToan, C. Surussavadee, V. C. Koo, T. S. Lim, R. H. Triharjanto, W. Hasbi, S. Mohan, and G. Singh. Development of new multi-band equatorially orbiting POLinSAR satellite sensors system configurations for varying latitudinal coverage within total tropical belt: Invited group presentation for establishing an associated Consortium. In Proc. IEEE 5th Asia-Pacific Conf. Synthetic Aperture Radar (APSAR), pages 342-345, September 2015. Keyword(s): meteorological radar, radar interferometry, radar polarimetry, remote sensing by radar, energy resources, global weather phenomena, latitudinal coverage, local environmental deterioration, mineral extraction, multiband equatorially orbiting POLINSAR satellite sensors, polarimetric POLINSAR satellite, stable food base, total tropical belt, Belts, Hazards, Orbits, Remote sensing, Satellites, Sensors, Synthetic aperture radar, Disaster assessment and reduction, Environmental remote sensing, Equatorial orbiting satellite sensors, Geophysical monitoring, Natural and manmade hazard detection, Polarimetric Synthetic Aperture Radar (SAR), Polarization radar, Surveillance, Tropical Equatorial Belt (TEB). [bibtex-key = Boerner2015] [bibtex-entry]


  1167. T. J. Czernuszewicz, J. W. Homeister, M. C. Caughey, M. A. Farber, J. J. Fulton, P. F. Ford, W. A. Marston, R. Vallabhaneni, T. C. Nichols, and C. M. Gallippi. In vivo carotid plaque stiffness measurements with ARFI ultrasound in endarterectomy patients. In Proc. IEEE Int. Ultrasonics Symp. (IUS), pages 1-4, October 2015. Keyword(s): biological tissues, biomechanics, biomedical ultrasonics, diseases, ultrasonic imaging, ARFI ultrasound, acoustic radiation force impulse ultrasound, atherosclerotic plaque, calcification, carotid endarterectomy, carotid plaque stiffness measurements, elasticity imaging technique, endarterectomy patients, fibrosis, fibrotic-calcified areas, fibrous cap, hemorrhage, lipid-necrotic areas, necrotic core, Atherosclerosis, Biomedical imaging, Computed tomography, Hemorrhaging, Thickness measurement, Ultrasonic imaging, ARFI, CEA, acoustic radiation force, atherosclerosis, plaque characterization, stroke. [bibtex-key = Czernuszewicz2015] [bibtex-entry]


  1168. Othmar Frey, Charles L. Werner, Martin Schneebeli, Amy Macfarlane, and Andreas Wiesmann. Enhancement of SnowScat for tomographic observation capabilities. In Proc. FRINGE 2015, ESA SP-731, March 2015. Keyword(s): SAR Processing, SAR Tomography, Snow, Snowpack, X-band, Ku-band, SnowScat, ESA, European Space Agency.
    Abstract: The SnowScat device, a tower-mounted fully polarimetric scatterometer for measurements of the radar cross-section of snow at X-band up to Ku-band, has recently been enhanced to also support a tomographic profiling mode. The new tomographic profiling capability of SnowScat allows for performing high-resolution observations providing further insights into the complex electromagnetic interaction within snowpacks. In this paper, we present first results obtained from a series of tomographic profiles of a snowpack acquired with the enhanced SnowScat device at a test site of SLF in Davos, Switzerland, between Dec. 2014 and March 2015.
    [bibtex-key = freyWernerSchneebeliMacfarlaneWiesmannFRINGE2015SnowScatTomo] [bibtex-entry]


  1169. Othmar Frey, Charles L. Werner, and Andreas Wiesmann. SnowScat tomography: first experimental results. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 1-4, July 2015. Note: Abstract.Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: The SnowScat device is a fully-polarimetric scatterometer that has originally been designed to measure the radar cross-section of snow at a frequency range from 9.2 to 17.8 GHz (X-band to Ku-band). Recently, a modification of the measurement setup was implemented an tested that extends the capabilities of the SnowScat device towards tomographic profiling of snowpacks. This extension aims at enhancing the SnowScat device in order to better respond to the ESAC recommendations which were made on the deselected CoReH2O candidate following the User Consultation meeting in March 2013 for the 7 Earth Explorer mission. Such new capability allows for performing high-resolution tomographic profiling observations providing further insights into the complex electromagnetic interaction within snowpacks. In this contribution, we (1) describe the tomographic mode of the SnowScat device including the experimental setup at the test site in Davos, Switzerland, as well as the tomographic processing approach. (2) First results are presented that validate the tomographic measurement concept with the help of a tomographic test target.
    [bibtex-key = freyWernerWiesmannIGARSS2015SnowScatTomo] [bibtex-entry]


  1170. Othmar Frey, Charles L. Werner, and Andreas Wiesmann. Tomographic Profiling of the Structure of a Snow Pack at X-/Ku-Band Using SnowScat in SAR Mode. In Proc. EuRAD 2015 - 12th European Radar Conference, pages 21-24, September 2015. Keyword(s): SAR Processing, SAR Tomography, Tomographic profiling, SnowScat, ESA, European Space Agency, X-Band, Ku-Band, Polarimetry, ground-based radar, Snow, Snowpack, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: The SnowScat device, a fully-polarimetric scatterometer originally designed to measure the radar cross-section of snow at a frequency range from 9.2 to 17.8 GHz (X-band to Ku-band), has recently been extended towards a high-resolution tomographic measurement mode. Such tomographic profiling observations provide further insights into the complex electromagnetic interaction within snowpacks, e.g., by revealing different layers, such as melt-freeze crusts, inside the snowpack. In this contribution, we report first results from an initial tomographic measurement campaign carried out at a test site in Davos, Switzerland, in winter 2014/2015.
    [bibtex-key = freyWernerWiesmannEURAD2015SnowScatTomo] [bibtex-entry]


  1171. M. T. Ghasr, K. P. Ying, and R. Zoughi. Wideband millimeter wave interferometer for high-resolution 3D SAR imaging. In 2015 IEEE International Instrumentation and Measurement Technology Conference (I2MTC) Proceedings, pages 925-929, May 2015. Keyword(s): SAR Processing, W-Band, holographic interferometry, image resolution, millimetre wave imaging, millimetre wave radar, radar imaging, radar interferometry, radar resolution, reflectometers, synthetic aperture radar, high-resolution 3D SAR imaging, nondestructive testing, phase referencing, portable millimeter wave imaging system, synthetic aperture radar image resolution, three-dimensional millimeter wave holographic image production, wideband millimeter wave interferometer, Apertures, Imaging, Millimeter wave technology, Ports (Computers), Rubber, Three-dimensional displays, Wideband, high-resolution, holographical images, millimeter wave imaging, synthetic aperture radar, wideband interferometer.
    Abstract: Portable millimeter wave imaging systems are desired in many nondestructive testing and imaging applications. Interferometry-based instruments have shown to produce three-dimensional holographic images when proper phase referencing is implemented. This paper introduces the design of an interferometry-based wideband, and low-cost reflectometer capable of producing holographic three-dimensional millimeter wave images. Analysis of sources of image errors, and three-dimensional synthetic aperture radar (SAR) image examples are presented at Ka-band (26.5-40 GHz), V-band (50-75 GHz) and W-band (75-110 GHz).
    [bibtex-key = ghasrYingZoughi2015WBandfor3DSARImaging] [bibtex-entry]


  1172. G. Gomba, X. Y. Cong, and M. Eineder. Correction of ionospheric and tropospheric path delay for L-band interferograms. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 310-313, July 2015. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, ionospheric electromagnetic wave propagation, ionospheric techniques, refractive index, remote sensing by radar, synthetic aperture radar, tropospheric electromagnetic wave propagation, weather forecasting, L-band interferograms, SAR data, differential atmospheric path delay, direct integration method, error source, geophysical processes, ground deformation signal, height-dependent tropospheric effects, ionospheric path delay correction, nominal value, numerical weather prediction data, radio wave delay, radio wave propagation, refractivity index variation, slant range distance, split-spectrum method, stratified delay, topography signal, tropospheric path delay correction, Atmospheric measurements, Delays, Dispersion, Ionosphere, L-band, Synthetic aperture radar, InSAR, SAR ionospheric effects, ionosphere estimation. [bibtex-key = gombaCongEineder2015IonoSplitSpectrumInSAR] [bibtex-entry]


  1173. G. Gomba and F. De Zan. Estimation of ionospheric height variations during an aurora event using multiple semi-focusing levels. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 4065-4068, July 2015. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, aurora, ionospheric electromagnetic wave propagation, ionospheric techniques, remote sensing by radar, synthetic aperture radar, SAR images, SAR interferograms, aurora event, integrated-azimuth-shifts method, ionosphere scintillation, ionosphere vertical profile, ionospheric effects, ionospheric height variation estimation, ionospheric phase screen, multiple semifocusing levels, normal ionospheric state, Azimuth, Estimation, Hafnium, Ionosphere, Satellites, InSAR, SAR ionospheric effects, ionosphere estimation. [bibtex-key = Gomba2015a] [bibtex-entry]


  1174. Marc Jäger, Muriel Pinheiro, Octavio Ponce, Andreas Reigber, and Rolf Scheiber. A Survey of novel airborne SAR signal processing techniques and applications for DLR's F-SAR sensor. In Proc. 16th Int. Radar Symp. (IRS), pages 236-241, June 2015. Keyword(s): airborne radar, image filtering, radar detection, radar imaging, radar interferometry, radar polarimetry, radar resolution, synthetic aperture radar, DLR F-SAR sensor, SAR imaging capability, advanced filtering, airborne SAR signal processing technique, dual-frequency SAR interferometry, high resolution circular SAR imaging, large baseline, polarimetric change detection, precise DEM generation, Covariance matrices, Image resolution, Imaging, Interferometry, Radar polarimetry, Speckle, Synthetic aperture radar. [bibtex-key = jaegerPinheiroPonceReigberScheiber2015OverviewAirborneSAR] [bibtex-entry]


  1175. A. Martinez, M. Lort, A. Aguasca, and A. Broquetas. Submillimetric motion detection with a 94 GHZ ground based synthetic aperture radar. In IET International Radar Conference 2015, pages 1-5, October 2015. Keyword(s): SAR Processing, W-Band, CW radar, FM radar, motion compensation, radar imaging, radar interferometry, synthetic aperture radar, CW-FM radar, W band, frequency 94 GHz, ground based synthetic aperture radar, high resolution imaging, radar interferometry, submillimetric motion detection, Band, CW-FM Radar, Ground Based SAR.
    Abstract: The paper presents the validation and experimental assessment of a 94 GHz (W-Band) CW-FM Radar that can be configured as a Ground Based SAR for high resolution imaging and interferometry. Several experimental campaigns have been carried out to assess the capability of the system to remotely observe submillimetric deformation and vibration in infrastructures.
    [bibtex-key = martinezLortAguascaBroquetasIRC2015WBandDeformation] [bibtex-entry]


  1176. A. Moreira, O. Ponce, M. Nannini, M. Pardini, and P. Prats. Multi-baseline imaging: A vision for spaceborne SAR. In Proc. 16th Int. Radar Symp. (IRS), pages 20-29, June 2015. Keyword(s): radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth), across-track interferometry, along-track interferometry, glacier movement, ground deformation measurement, holography SAR, image product, multibaseline imaging, multidimensional data space, multistatic SAR configuration, ocean current measurement, polarimetric SAR interferometry, spaceborne SAR system development, sparse array, surface topography measurement, tomography SAR, Imaging, Interferometry, Radar imaging, Radar polarimetry, Spaceborne radar, Synthetic aperture radar. [bibtex-key = Moreira2015] [bibtex-entry]


  1177. M. Pieraccini, N. Agostini, F. Papi, and S. Rocchio. A rotating antenna ground-based SAR. In Proc. European Radar Conf. (EuRAD), pages 493-496, September 2015. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar interferometry, synthetic aperture radar, RotoSAR, ground based linear SAR, imagery quality performances, in-field tests, rotating antenna GB-SAR, rotating antenna ground-based SAR, Monitoring, Radar antennas, Radar imaging, Radar remote sensing, Spaceborne radar, Synthetic aperture radar, SAR, radar. [bibtex-key = Pieraccini2015b] [bibtex-entry]


  1178. M. Pieraccini, F. Papi, and S. Rocchio. SAR imagery by RotoSAR. In Proc. Antennas and Electronic Systems (COMCAS) 2015 IEEE Int. Conf. Microwaves, Communications, pages 1-5, November 2015. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar antennas, radar imaging, synthetic aperture radar, RotoSAR, SAR imagery, imagery performances, in-field measurements, rotating antenna, Monitoring, Radar antennas, Radar imaging, Radar remote sensing, Spaceborne radar, Synthetic aperture radar, SAR, radar. [bibtex-key = Pieraccini2015] [bibtex-entry]


  1179. O. Ponce, P. Prats, R. Scheiber, A. Reigber, I. Hajnsek, and A. Moreira. Polarimetric 3-D imaging with airborne holographic SAR tomography over glaciers. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 5280-5283, July 2015. Keyword(s): airborne radar, glaciology, hydrological techniques, radar polarimetry, synthetic aperture radar, DLR F-SAR sensor, Findel glacier, HoloSAR campaign, L-band, Monte Rosa, Switzerland, airborne holographic SAR tomography mode, arc-pattern, bedrock vertical profile, biosphere, circular pattern, circular synthetic aperture, cryosphere, glacier backscattering, ice sheet vertical profile, polarimetric 3-D imaging reconstruction, polarimetric analysis, single circular flight, snow vertical profile, vertical synthetic aperture, Apertures, Ice, Image resolution, Radar imaging, Synthetic aperture radar, Tomography, Compressive Sensing (CS), Cryosphere, Fast Factorized Back-Projection (FFBP), Holographic SAR Tomography (HoloSAR), Polarimetric Synthetic Aperture Radar (PolSAR). [bibtex-key = Ponce2015] [bibtex-entry]


  1180. Badreddine Rekioua, Matthieu Davy, and Laurent Ferro-Famil. Snowpack characterization using SAR tomography: experimental results of the AlpSAR campaign. In Radar Conference (EuRAD), 2015 European, pages 33-36, Sept 2015. Keyword(s): SAR Processing, Tomography, SAR tomography, Antennas, Ice, Lenses, Refractive index, Snow, Ground-based SAR.
    Abstract: In this paper, we present experimental results of 3D characterization of snowpack layers using Ground Based SAR (GB-SAR) data. The data have been acquired using a GB-SAR system operating at X and Ku frequency bands. We process the acquired data using the Back Projection Algorithm (BPA). The results of this processing show strong backscattering contributions from interfaces between different layers of the snowpack. We notice also geometrical distortions due to the assumption of free space propagation medium during the processing. We introduce in this paper a correction process in order to recover the true thickness of snow layers and the corresponding refractive indexes. The obtained results are discussed according to the in situ measurement relating grain size, density and snow particle shapes description at different depths of the snowpack.
    [bibtex-key = rekiouaDavyFerroFamilEuRAD2015TomoSnowAlpSAR] [bibtex-entry]


  1181. Muhammad Adnan Siddique, Irena Hajnsek, Urs Wegmuller, and Othmar Frey. Investigating the Combined Use of Differential SAR Tomography and PSI for Spatio-Temporal Inversion. In Proc. Joint Urban Remote Sensing Event - JURSE, pages 1-4, March 2015. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, thermal dilation, urban, urban remote sensing, buildings, estimation, remote sensing, synthetic aperture radar, thermal expansion, tomography, urban areas.
    Abstract: Persistent Scatterer Interferometry (PSI) inherently assumes a single temporally coherent scatterer inside a range-azimuth resolution cell. This restriction leads to the rejection of numerous persistent scatterer (PS) candidates, particularly in urban areas where layovers occur frequently. Moreover, in case of high-rise buildings, it is necessary to compensate the phase associated with thermal expansion in an iterative way. It is worthwhile to approach tomographic techniques to address these concerns. SAR tomography has the potential to separate scatterers in elevation, thus resolving layover. Differential SAR tomography additionally allows retrieval of deformation parameters, including a possible thermal expansion term. In this paper, we investigate the combined use of SAR tomographic approaches and PSI for elevation and deformation estimation. Results are presented for an interferometric time-series of 50 TerraSAR-X stripmap images acquired over Barcelona city. Spatio-temporal inversion of scatterers along the facade of a high-rise building is presented as a special case.
    [bibtex-key = siddiqueHajnsekWegmullerFreyJURSE2015TomoTSXBarca] [bibtex-entry]


  1182. Muhammad Adnan Siddique, Irena Hajnsek, Urs Wegmuller, and Othmar Frey. Towards the integration of SAR tomography and PSI for improved deformation assessment in urban areas. In Proc. FRINGE 2015, ESA SP-731, March 2015. Keyword(s): SAR Processing, SAR Tomography, persistent scatterer interferometry, PSI, DInSAR, multibaseline interferometry, interferometric stacking, deformation monitoring, subsidence monitoring, thermal dilation, urban, urban remote sensing.
    Abstract: Persistent scatterer interferometry (PSI) typically rejects the range-azimuth pixels containing multiple scatterers, such as in a layover scenario. Since layovers occur frequently in urban areas, a significant number of candidates may get rejected. SAR tomography allows for resolving layover and has thus the potential to extend the spatial sampling of deformation measurements to layoveraffected areas. Using extended phase models, also taking into account temperature, an improved simultaneous estimation of elevation, deformation velocity, and temperature-induced scatterer displacement is possible. This paper explores the combined use of PSI and SAR tomography for deformation analysis in urban areas, using a multibaseline and multitemporal interferometric stack of stripmap TerraSAR-X images acquired over the city of Barcelona.
    [bibtex-key = siddiqueHajnsekWegmullerFreyFRINGE2015TomoTSXBarca] [bibtex-entry]


  1183. Muhammad Adnan Siddique, Urs Wegmuller, Irena Hajnsek, and Othmar Frey. SAR tomography for spatio-temporal inversion of point-like scatterers in urban areas. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 5272-5275, July 2015. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, Barcelona, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne TerraSAR-X data, interferometric stack, high-rise building vertical tomographic slice, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: Persistent scatterer interferometry (PSI) assumes the presence of a single temporally coherent scatterer in a range-azimuth pixel. Multiple scatterers interfering in the same pixel, as for the case of a layover, are typically rejected. Conventional SAR tomography (3D SAR) is a means to separate the individual scatterers in layover. Advanced tomographic inversion approaches employing extended phase models additionally allow simultaneous retrieval of scatterer elevation and deformation parameters. In this way, SAR tomography can increase deformation sampling and thereby complement a PSI-based analysis. This paper investigates the use of tomography as an add-on to PSI for spatio-temporal inversion of single and double scatterers in urban areas. Results are provided on an interferometric stack of 50 stripmap TerraSAR-X images acquired over the city of Barcelona.
    [bibtex-key = siddiqueWegmullerHajnsekFreyIGARSS2015PSITomo] [bibtex-entry]


  1184. Urs Wegmuller and Charles L. Werner. Mitigation of thermal expansion phase in persistent scatterer interferometry in an urban environment. In 2015 Joint Urban Remote Sensing Event (JURSE), pages 1-4, 2015. IEEE. [bibtex-key = wegmullerWernerJURSE2015PSIThermalExpansion] [bibtex-entry]


  1185. Urs Wegmuller, Charles L. Werner, Tazio Strozzi, Andreas Wiesmann, and Othmar Frey. Sentinel-1 support in the GAMMA Software. In Proc. FRINGE 2015, ESA SP-731, pages 1-6, March 2015. Keyword(s): SAR Processing, Sentinel-1, Spaceborne SAR, C-band, European Space Agency, ESA, Terrain Observation by Progressive Scans, TOPS, TOPSAR.
    Abstract: First results using the new Sentinel-1 SAR look very promising but the special interferometric wide-swath data acquired in the TOPS mode makes InSAR processing challenging. The steep azimuth spectra ramp in each burst results in very stringent co-registration requirements. Combining the data of the individual bursts and sub-swaths into consistent mosaics requires careful book-keeping in the handling of the data and meta data and the large file sizes and high data throughputs require also a good performance. Considering these challenges good support from software is getting increasingly important. In this contribution we describe the Sentinel-1 support in the GAMMA Software, a high-level software package used by researchers, service providers and operational users in their SAR, InSAR and PSI work.
    [bibtex-key = wegmullerWernerStrozziWiesmannFreyFRINGE2015Sentinel1Gamma] [bibtex-entry]


  1186. Urs Wegmuller, Charles L. Werner, Tazio Strozzi, Andreas Wiesmann, Othmar Frey, and Maurizio Santoro. Sentinel-1 IWS mode support in the GAMMA software. In IEEE 5th Asia-Pacific Conference on Synthetic Aperture Radar (APSAR), pages 431-436, September 2015. Keyword(s): SAR Processing, TOPS, TOPS mode, Terrain Observation by Progressive Scans, Sentinel-1, Interferometry, SAR Interferometry, Spaceborne SAR, geophysics computing, image registration, meta data, radar computing, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, Fhe steep azimuth spectra ramp, GAMMA software, IWS data, InSAR processing, PSI, Sentinel-1 IWS mode support, Sentinel-1 SAR, TOPS, high-level software package, interferometrie wide-swath data, meta data, synthetic aperture radar, Apertures, Conferences, Decision support systems, DINSAR, GAMMA Software, Nepal earthquake, PSI, Sentinel-1 TOPS IWS, ionospheric effects, offset-tracking, spectral diversity, split-beam interferometry.
    Abstract: First results using the new Sentinel-1 SAR look very promising, but the special interferometrie wide-swath (IWS) data acquired in the TOPS mode makes InSAR processing challenging. The steep azimuth spectra ramp in each burst results in very stringent co-registration requirements. Combining the data of the individual bursts and sub-swaths into consistent mosaics requires careful bookkeeping in the handling of the data and meta data and the large file sizes and high data throughputs require also a good performance. Considering these challenges good support from software is getting increasingly important. In this contribution we describe the Sentinel-1 support in the GAMMA Software, a high-level software package used by researchers, service providers and operational users in their SAR, InSAR and PSI work.
    [bibtex-key = wegmullerWernerStrozziWiesmannFreySantoro2015] [bibtex-entry]


  1187. Urs Wegmuller, Charles L. Werner, Andreas Wiesmann, Tazio Strozzi, and Othmar Frey. Wide-area persistent scatterer interferometry with Sentinel-1 TOPS mode data. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 1-4, July 2015. Note: Abstract.Keyword(s): SAR Processing, Sentinel-1, TOPS, Terrain Observation by progressive scans, ESA, European Space Agency, C-Band, Spaceborne SAR, Persistent Scatterer Interferometry, PSI, Interferometry, SAR Interferometry, geophysical signal processing, radar polarimetry, synthetic aperture radar.
    Abstract: In 2014 the Sentinel-1A satellite was launched as part of the EU/ESA Copernicus Program. One of the novelties of the Sentinel 1 SAR (S1) mission is that the satellite is mainly operated in the so-called TOPS mode [1]. TOPS stands for Terrain Observation with Progressive Scans in azimuth, but the word is also the reverse of SPOT and actually the beam scanning done is the opposite of the scanning done in spotlight mode. One of the strengths of TOPS mode is that wide areas can be covered. In the in IWS (Interferometric Wide-Swath) mode of S1 the width of the strips is about 250km. S1 is operated at C-band with an orbit repeat cycle of 12 days. The orbital tube is very narrow (of the order of 100m) and the TOPS mode bursts are almost perfectly synchronized. As a result S1 IWS data are well suited for interferometric SAR (InSAR). First very promising S1 interferograms were presented by several authors at two workshops organized by ESA in September and December 2014. An example of a S1 IWS differential interferogram is shown in Figure 1. Such results confirm on one hand the usefulness of S1 TOPS mode data for interferometric techniques and on the other hand the adequacy of the processing techniques applied. In particular the co-registration accuracy requirements are very stringent [2]. Because of the strong Doppler Centroid variation within each burst even a very tiny azimuth co-registration error of 0.01 pixel lead to significant phase jumps between adjacent bursts in the resulting interferogram. Spectral diversity techniques [3] permit refining the co-registration to the required level considering in the burst overlap area the difference between the interferometric phases of the two overlapping bursts. Being confident about the co-registration and interferometric processing we investigated as a next step how well Persistent Scatterer Interferometry (PSI) can be applied. Regular IWS acquisitions started in October 2014. Thanks to the 12 day repeat cycle consistent data stacks suited for PSI are now becoming available.
    [bibtex-key = wegmullerWernerWiesmannStrozziFreyIGARSS2015Sentinel1InSAR] [bibtex-entry]


  1188. Alex Coccia, Stefano Tebaldini, and Adriano Meta. Preliminary results from most recent SAR airborne campaigns by MetaSensing. In Proc. SPIE, volume 9245, pages 92450L-92450L-7, 2014.
    Abstract: MetaSensing B.V.1 is a Dutch company producing and operating Synthetic Aperture Radar (SAR) sensors at high resolution and different frequency bands, including P-, L-, C-, X- and Ku. By operating its most recent SAR sensors in the framework of different projects MetaSensing showed how diverse SAR imagery techniques can be applied to different areas of remote sensing, offering an effective tool for monitoring and mapping purposes. The present paper gives an overview about the last achievements of MetaSensing during a recent airborne measurement mission within the AlpTomoExpcampaign, also showing some preliminary results. A fully-polarimetric L-band radar system has been successfully operated in March 2014 within the framework of the future SAOCOM+ mission currently under investigation by the European Space Agency (ESA). At these low frequencies penetration capabilities for several meters is possible on dry snow. More than 40 fully polarimetric images have been focused by MetaSensing of a glacier of the Otzal Alps, in Austria, within the framework of the AlpTomoExp campaign. Thanks to the good interferometric coherence of the acquired images, further tomographic processing has been possible for 3D images generation showing the vertical profile of the monitored scenario.
    [bibtex-key = cocciaTebaldiniMetaSPIE2014AirborneSARTomoSAR] [bibtex-entry]


  1189. T. J. Czernuszewicz, J. W. Homeister, M. C. Caughey, M. A. Farber, J. J. Fulton, P. F. Ford, W. A. Marston, R. Vallabhaneni, T. C. Nichols, and C. M. Gallippi. In vivo characterization of atherosclerotic plaque of human carotid arteries with histopathological correlation using ARFI ultrasound. In Proc. IEEE Int. Ultrasonics Symp, pages 1794-1797, September 2014. Keyword(s): bioacoustics, biological tissues, biomedical ultrasonics, blood vessels, haemodynamics, phantoms, ultrasonic imaging, ARFI ultrasound, acoustic radiation force impulse imaging, atherosclerotic pigs, calcification, carotid atherosclerotic plaques, clinically-indicated carotid endarterectomy, dense fibrosis, histopathological correlation, human carotid arteries, inflammation, mechanical properties, mild intraplaque hemorrhage, necrotic cores, plaque rupture, spatially-matched histopathology, stiff tissue, ultrasound imaging plane, Acoustics, Atherosclerosis, Carotid arteries, Force, Imaging, In vivo, Ultrasonic imaging, ARFI, CEA, acoustic radiation force, atherosclerosis, carotid endarterectomy, plaque characterization, stroke. [bibtex-key = Czernuszewicz2014] [bibtex-entry]


  1190. Laurent Ferro-Famil, Stefano Tebaldini, Matthieu Davy, and Frederic Boute. 3D SAR imaging of the snowpack at X- and Ku-Band: results from the AlpSAR campaign. In Proc. of EUSAR 2014 - 10th European Conference on Synthetic Aperture Radar, pages 1-4, June 2014. Keyword(s): SAR Processing, SAR Tomography, Snow, Snowpack, X-band, Ku-band.
    Abstract: In this paper we present results obtained by processing the ground based SAR (GBSAR) data collected in february 2013 on the Austrian Alps in the frame of the ESA campaign AlpSAR. The GBSAR was operated at X- and Ku-Band with a bandwidth of 4 GHz and employing a 2D synthetic aperture, resulting in 3D resolution capabilities at a resolution of few centimeters. Images produced at two different sites reveal the presence of multiple layers within the snowpack. The strongest backscatter contributions have been observed to correspond to bottom layers, that dominate the ones from the snow/air interface and the near subsurface. GBSAR data are observed to provide sensitivity to the propagation velocity into the snowpack, as revealed by the apparent depth variation with respect to the incidence angle.
    [bibtex-key = ferroFamilTebaldiniDavyBouteEUSAR2014SnowTomo] [bibtex-entry]


  1191. Laurent Ferro-Famil, Stefano Tebaldini, Matthieu Davy, and Frederic Boute. 3D SAR imaging of the snowpack in presence of propagation velocity changes: Results from the AlpSAR campaign. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 3370-3373, July 2014. Keyword(s): SAR Processing, SAR Tomography, Snow, Snowpack, X-band, Ku-band, geophysical image processing, image resolution, radar imaging, snow, synthetic aperture radar, terrain mapping, 2D synthetic aperture, 3D SAR imaging, 3D resolution capabilities, AD 2013 02, AlpSAR campaign, ESA campaign, GBSAR data, Ku-Band, X-band, apparent depth variation, backscatter contributions, propagation velocity, propagation velocity change, snow-air interface, snowpack, Communication channels, Equations, Scattering, Sensitivity, Snow, Synthetic aperture radar, Three-dimensional displays.
    Abstract: In this paper we present results relative to the 3D GBSAR surveys acquired in february 2013 on the Austrian Alps as a part of the ESA campaign AlpSAR. The GBSAR was operated at X- and Ku-Band with a bandwidth of 4 GHz and employing a 2D synthetic aperture, resulting in 3D resolution capabilities at a resolution of few centimeters. Images produced at two different sites reveal the presence of multiple layers within the snowpack. The strongest backscatter contributions have been observed to correspond to bottom layers, that dominate the ones from the snow/air interface and the near subsurface. GBSAR data are observed to provide sensitivity to the propagation velocity into the snowpack, as revealed by the apparent depth variation with respect to the incidence angle.
    [bibtex-key = ferroFamilTebaldiniDavyBouteIGARSS2014SnowTomo] [bibtex-entry]


  1192. Othmar Frey, Irena Hajnsek, Urs Wegmuller, and Charles L. Werner. SAR tomography based 3-D point cloud extraction of point-like scatterers in urban areas. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1313-1316, July 2014. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, Barcelona, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne TerraSAR-X data, interferometric stack, high-rise building vertical tomographic slice, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: SAR tomography as an extension to persistent scatterer interferometry (PSI) approaches has the potential to improve the level of detail of retrievable information, in particular, in the case of layover scenarios in urban areas. In this paper, a processing approach is sketched that eventually allows for retrieving a 3-D point cloud of point-like scatterers based on subsequent PSI and SAR tomography processing of an interferometric stack of high-resolution spaceborne TerraSAR-X data acquired over the city of Barcelona between the years 2008 and 2012. Experimental results are presented in the form of (1) vertical tomographic slices of high-rise buildings and (2) a 3-D point cloud of a larger district of the city of Barcelona retrieved from the tomograms.
    [bibtex-key = freyHajnsekWegmullerWernerIGARSS2014TomoTSXBarca] [bibtex-entry]


  1193. Othmar Frey, Muhammad Adnan Siddique, Irena Hajnsek, Urs Wegmuller, and Charles L. Werner. Combining SAR tomography and a PSI approach for highresolution 3-D imaging of an urban area. In Proc. of EUSAR 2014 - 10th European Conference on Synthetic Aperture Radar, pages 1-4, June 2014. Keyword(s): SAR Processing, SAR tomography, Synthetic aperture radar (SAR), SAR Interferometry, InSAR, interferometric stacking, persistent scatterer interferometry, PSI, spaceborne SAR radar interferometry, spaceborne radar, X-Band, TerraSAR-X, synthetic aperture radar, tomography, 3-D point cloud retrieval, Barcelona, SAR tomography based 3-D point cloud extraction, high-resolution spaceborne TerraSAR-X data, interferometric stack, high-rise building vertical tomographic slice, layover scenario case, persistent scatterer interferometry, PSI, point-like scatterer, processing approach, Urban Remote Sensing, Spaceborne radar, Synthetic aperture radar, Three-dimensional displays, Tomography, 3-D point cloud, SAR interferometry.
    Abstract: Combining persistent scatterer interferometry (PSI) and SAR tomography approaches has the potential to overcome layover scenarios in urban areas and may thus increase the level of detail of differential interferometric measurements of displacements in such environments. In this paper, we report the current status and results of our efforts to integrate SAR tomography into an operational interferometric point target analysis (IPTA) processing tool as an extension to the conventional persistent scatterer interferometry approach. In particular, the PSI and the SAR tomography processing approaches applied are highlighted. In addition, details in the form of tomographic slices of two high-rise buildings are presented as well as a 3-D point cloud reconstruction of parts of the city of Barcelona are shown, as extracted from an interferometric stack of high-resolution stripmap-mode SAR data at X-band acquired by the TerraSAR-X spaceborne SAR sensor.
    [bibtex-key = freySiddiqueHajnsekWegmullerWernerEUSAR2014TomoTSXBarca] [bibtex-entry]


  1194. Othmar Frey, Charles L. Werner, and Urs Wegmuller. GPU-based parallelized time-domain back-projection processing for agile SAR platforms. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1132-1135, July 2014. Keyword(s): SAR Processing, Azimuth Focusing, Time-Domain Back-Projection, TDBP, SAR focusing, GPU, GPU-based parallelized TDBP, graphics processing units, motion compensation, parallel processing, radar signal processing, synthetic aperture radar, 3D motion compensation, GPU based backprojection processing, NVIDIA CUDA GPU computing framework, SAR processing scheme, agile SAR platforms, car borne SAR data set, nonlinear sensor trajectories, parallelized backprojection processing, single look complex SAR images, slant azimuth geometry, slant range geometry, synthetic aperture, time domain backprojection processing, Azimuth, Focusing, Graphics processing units, Remote sensing, Synthetic aperture radar, Time-domain analysis, Azimuth focusing, CARSAR, CUDA, GPU, Nonlinear Sensor Trajectory, Parallelization, SAR imaging, SAR interferometry, Synthetic aperture radar (SAR), car-borne SAR, ground-based SAR system.
    Abstract: Agile SAR platforms such as an automobile require a flexible SAR processing scheme to account for nonlinear sensor trajectories during the synthetic aperture. In this contribution, a parallelized implementation of a time-domain back-projection SAR focusing algorithm based on NVIDIA's CUDA GPU computing framework is presented and discussed using a car-borne SAR data set. The processing performance is assessed using different hardware. In addition, a pre-processing scheme is described that allows for full 3-D motion compensation, yet staying conveniently in conventional slant-range/azimuth geometry of single-look complex SAR images.
    [bibtex-key = freyWernerWegmullerIGARSS2014GPUbasedTDBP] [bibtex-entry]


  1195. G. Gomba, M. Eineder, A. Parizzi, and R. Bamler. High-resolution estimation of ionospheric phase screens through semi-focusing processing. In Proc. IEEE Geoscience and Remote Sensing Symp, pages 17-20, July 2014. Keyword(s): SAR Processing, split-spectrum, split-spectrum interferometry, split-band, split-band interferometry, geophysical techniques, ionospheric electromagnetic wave propagation, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, SAR images, blurring, coherence, decorrelation, delta-k split-band interferometry method, high-resolution estimation, ionosphere irregularities, ionospheric azimuth effect, ionospheric phase screens, semifocusing processing, synthetic aperture generate shift, turbulent ionosphere, Apertures, Azimuth, Coherence, Estimation, Focusing, Ionosphere, Synthetic aperture radar, InSAR, SAR ionospheric effects, ionosphere estimation, ionosphere scintillation.
    Abstract: Ionosphere irregularities along the synthetic aperture generate shifts and blurring that cause decorrelation. In this paper it is shown how, by partially focusing SAR images to the height of the ionosphere, it is possible to reduce the ionospheric azimuth effects and increase the coherence. This permits, even in case of turbulent ionosphere, to obtain better accuracies when separating the deformations phase from the ionospheric phase using the delta-k split-band interferometry method.
    [bibtex-key = gombaEinederParizziBamler2014IonoPhaseEstimation] [bibtex-entry]


  1196. O. Ponce, P. Prats, R. Scheiber, A. Reigber, and A. Moreira. Study of the 3-D Impulse Response Function of Holographic SAR Tomography with Multicircular Acquisitions. In Proc. EUSAR 2014; 10th European Conf. Synthetic Aperture Radar, pages 1-4, June 2014. [bibtex-key = Ponce2014a] [bibtex-entry]


  1197. O. Ponce, T. Rommel, M. Younis, P. Prats, and A. Moreira. Multiple-input multiple-output circular SAR. In Proc. 15th Int. Radar Symp. (IRS), pages 1-5, June 2014. Keyword(s): MIMO radar, radar imaging, synthetic aperture radar, 3-D reconstruction, CSAR, HPBW, MCRA, MIMO techniques, PRF, SCORE, circular synthetic aperture radar, holoSAR imaging, minimum half-power beamwidth, multichannel reconstruction in azimuth, multicircular SAR acquisitions, multiple-input multiple-output circular SAR, orthogonal waveforms, pulse repetition frequency, quadpolarized systems, range ambiguity, range-azimuth directions, scan-on-receive, spotlighted region, subwavelength resolution, Apertures, Azimuth, Imaging, MIMO, Radar imaging, Synthetic aperture radar, Circular SAR, digital beam forming (DBF), multicircular SAR, multiple-input multiple-output (MIMO), synthetic aperture radar (SAR). [bibtex-key = Ponce2014] [bibtex-entry]


  1198. Marc Rodriguez-Cassola, Pau Prats-Iraola, Paco López-Dekker, Andreas Reigber, Gerhard Krieger, and Alberto Moreira. Autonomous time and phase calibration of spaceborne bistatic SAR systems. In EUSAR 2014; 10th European Conference on Synthetic Aperture Radar; Proceedings of, pages 1-4, June 2014.
    Abstract: Bistatic and multistatic SAR constellations offer increased performance at the expense of increased operational complexity. Due to geometric or cost constraints, multistatic SAR constellations might be forced to operate in a partially cooperative manner, i.e., without a direct synchronisation link. In demanding scenarios, like high-resolution bistatic SAR imaging or cross-platform SAR interferometry or tomography, the data need undergo a calibration step to compensate the lack of synchronisation between transmitter and receiver master clocks. Autonomous synchronisation, based on the inversion of the phase and positioning errors of the bistatic SAR images caused by the lack of synchronisation, is used to calibrate the time and phase references of the system with the sole help of the received radar data, which drastically reduces the requirements on the hardware of the system.
    [bibtex-key = 6856780] [bibtex-entry]


  1199. Sabine Rödelsperger and Adriano Meta. MetaSensing's FastGBSAR: ground based radar for deformation monitoring. In Proc. SPIE, volume 9243, pages 924318-924318-8, 2014. Keyword(s): SAR Processing, FastGBSAR, GBSAR, InSAR, Ground-based radar, Ground-based SAR, deformation measurement, displacement, subsidence, terrestrial radar interferometry, mass movements, surface deformation, Dike monitoring, Dike failure.
    Abstract: The continuous monitoring of ground deformation and structural movement has become an important task in engineering. MetaSensing introduces a novel sensor system, the Fast Ground Based Synthetic Aperture Radar (FastGBSAR), based on innovative technologies that have already been successfully applied to airborne SAR applications. The FastGBSAR allows the remote sensing of deformations of a slope or infrastructure from up to a distance of 4 km. The FastGBSAR can be setup in two different configurations: in Real Aperture Radar (RAR) mode it is capable of accurately measuring displacements along a linear range profile, ideal for monitoring vibrations of structures like bridges and towers (displacement accuracy up to 0.01 mm). Modal parameters can be determined within half an hour. Alternatively, in Synthetic Aperture Radar (SAR) configuration it produces two-dimensional displacement images with an acquisition time of less than 5 seconds, ideal for monitoring areal structures like dams, landslides and open pit mines (displacement accuracy up to 0.1 mm). The MetaSensing FastGBSAR is the first ground based SAR instrument on the market able to produce two-dimensional deformation maps with this high acquisition rate. By that, deformation time series with a high temporal and spatial resolution can be generated, giving detailed information useful to determine the deformation mechanisms involved and eventually to predict an incoming failure. The system is fully portable and can be quickly installed on bedrock or a basement. The data acquisition and processing can be fully automated leading to a low effort in instrument operation and maintenance. Due to the short acquisition time of FastGBSAR, the coherence between two acquisitions is very high and the phase unwrapping is simplified enormously. This yields a high density of resolution cells with good quality and high reliability of the acquired deformations. The deformation maps can directly be used as input into an Early Warning system, to determine the state and danger of a slope or structure. In this paper, the technical principles of the instrument are described and case studies of different monitoring tasks are presented.
    [bibtex-key = roedelspergerMetaSPIE2014FastGBSARDikeDemo] [bibtex-entry]


  1200. Jan Torgrimsson, Patrik Dammert, Hans Hellsten, and Lars M. H. Ulander. Autofocus and analysis of geometrical errors within the framework of fast factorized back-projection. In Edmund Zelnio and Frederick D. Garber, editors, Proc. SPIE, volume 9093, pages 909303-909303-16, 2014. International Society for Optics and Photonics, SPIE. Keyword(s): SAR Processing, Fast-Factorized Back-Projection, FFBP, Time-Domain Back-Projection, TDBP, Autofocus, SAR Autofocus, Geometrical Autofocus, Airborne SAR, CARABAS, factorized geometrical autofocus algorithm, fast factorized back-projection chain, gain 3 dB, intensity correlation, object function, peak-to-sidelobe ratio measurement, radar imaging, residual space variant range cell migration, ultrawideband synthetic aperture radar processing, Apertures, Electronics packaging, Geometry, Radar tracking, Synthetic aperture radar, Transforms, Autofocus, back-projection, phase gradient algorithm, PGA, synthetic aperture radar (SAR).
    Abstract: This paper describes a Fast Factorized Back-Projection (FFBP) formulation that includes a fully integrated autofocus algorithm, i.e. the Factorized Geometrical Autofocus (FGA) algorithm. The base-two factorization is executed in a horizontal plane, using a Merging (M) and a Range History Preserving (RHP) transform. Six parameters are adopted for each sub-aperture pair, i.e. to establish the geometry stage-by-stage via triangles in 3-dimensional space. If the parameters are derived from navigation data, the algorithm is used as a conventional processing chain. If the parameters on the other hand are varied from a certain factorization step and forward, the algorithm is used as a joint image formation and autofocus strategy. By regulating the geometry at multiple resolution levels, challenging defocusing effects, e.g. residual space-variant Range Cell Migration (RCM), can be corrected. The new formulation also serves another important purpose, i.e. as a parameter characterization scheme. By using the FGA algorithm and its inverse, relations between two arbitrary geometries can be studied, in consequence, this makes it feasible to analyze how errors in navigation data, and topography, affect image focus. The versatility of the factorization procedure is demonstrated successfully on simulated Synthetic Aperture Radar (SAR) data. This is achieved by introducing different GPS/IMU errors and Focus Target Plane (FTP) deviations prior to processing. The characterization scheme is then employed to evaluate the sensitivity, to determine at what step the autofocus function should be activated, and to decide the number of necessary parameters at each step. Resulting FGA images are also compared to a reference image (processed without errors and autofocus) and to a defocused image (processed without autofocus), i.e. to validate the novel approach further.
    [bibtex-key = torgrimssonDammertHellstenUlanderSPIE2014AutofocusAnalysisGeometricalErrorsFFBackprojection] [bibtex-entry]


  1201. Mariantonietta Zonno, Luigi Mascolo, Pietro Guccione, Giovanni Nico, and Andrea Di Pasquale. Impact of focusing of Ground Based SAR data on the quality of interferometric SAR applications. In Proc. of SPIE, volume 9243, pages 1-10, 2014. International Society for Optics and Photonics.
    Abstract: A Ground-Based Synthetic Aperture Radar (GB-SAR) is nowadays employed in several applications. The processing of ground-based, space and airborne SAR data relies on the same physical principles. Nevertheless specific algorithms for the focusing of data acquired by GB-SAR system have been proposed in literature. In this work the impact of the main focusing methods on the interferometric phase dispersion and on the coherence has been studied by employing a real dataset obtained by carrying out an experiment. Several acquisitions of a scene with a corner reflector mounted on a micrometric screw have been made; before some acquisitions the micrometric screw has been displaced of few millimetres in the Line-of-Sight direction. The images have been first focused by using two different algorithms and correspondently, two different sets of interferograms have been generated. The mean and standard deviation of the phase values in correspondence of the corner reflector have been compared to those obtained by knowing the real displacement of the micrometric screw. The mean phase and its dispersion and the coherence values for each focusing algorithm have been quantified and both the precision and the accuracy of the interferometic phase measurements obtained by using the two different focusing methods have been assessed.
    [bibtex-key = zonnoMascoloGuccioneNicoDiPasqualeSPIE2014GBSAR] [bibtex-entry]


  1202. Kerry E. Dungan, LeRoy A. Gorham, and Linda J. Moore. SAR digital spotlight implementation in MATLAB. In Proc. SPIE, volume 8746, pages 1-11, 2013. Keyword(s): SAR Processing, Azimuth Focusing, Digital Spotlighting, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, MATLAB.
    Abstract: Legacy synthetic aperture radar (SAR) exploitation algorithms were image-based algorithms, designed to exploit complex and/or detected SAR imagery. In order to improve the efficiency of the algorithms, image chips, or region of interest (ROI) chips, containing candidate targets were extracted. These image chips were then used directly by exploitation algorithms for the purposes of target discrimination or identification. Recent exploitation research has suggested that performance can be improved by processing the underlying phase history data instead of standard SAR imagery. Digital Spotlighting takes the phase history data of a large image and extracts the phase history data corresponding to a smaller spatial subset of the image. In a typical scenario, this spotlighted phase history data will contain much fewer samples than the original data but will still result in an alias-free image of the ROI. The Digital Spotlight algorithm can be considered the first stage in a ``two-stage backprojection'' image formation process. As the first stage in two-stage backprojection, Digital Spotlighting filters the original phase history data into a number of ``pseudo''-phase histories that segment the scene into patches, each of which contain a reduced number of samples compared to the original data. The second stage of the imaging process consists of standard backprojection. The data rate reduction offered by Digital Spotlighting improves the computational efficiency of the overall imaging process by significantly reducing the total number of backprojection operations. This paper describes the Digital Spotlight algorithm in detail and provides an implementation in MATLAB.
    [bibtex-key = dunganGorhamMooreSPIE2013DigitalSpotlightWithMatlabCode] [bibtex-entry]


  1203. Othmar Frey, Irena Hajnsek, and Urs Wegmuller. Spaceborne SAR Tomography in Urban Areas. In Proc. IEEE Int. Geosci. Remote Sens. Symp., Melbourne, AUS, pages 69-72, July 2013. Keyword(s): SAR Processing, SAR Tomography, Tomography, SVD, truncated SVD, tikhonov regularization, PSI, Persistent Scatterer Interferometry, Spaceborne SAR, InSAR, DInSAR, geophysical image processing, geophysical techniques, radar interferometry, synthetic aperture radar, Bucarest, PanGEO, ENVISAT, ASAR, ENVISAT ASAR, interferometric technique, Differential SAR Interferometry, single polarization SAR, terrain deformation phenomena, Coherence, Correlation, Interferometry, Synthetic aperture radar, Vectors, Coherence, Terrain deformation, multi-baseline.
    Abstract: Persistent scatterer interferometry relies on the assumption that only one dominant scatterer is present per range-azimuth resolution cell. If this criterion is not met the point target candidate is discarded during the iterative processing sequence. This one-scatterer assumption contrasts with the fact that in urban scenarios layover is an ubiquitous phenomenon, and, therefore two or even more scatterers per resolution cell occur frequently. SAR tomography has the potential to support persistent scatterer interferometry in urban areas by providing a means to identify and separate two scatterers in elevation direction. In this paper, we explore an interferometric stack consisting of 25 ENVISAT/ASAR SLC images over Bucharest using SAR tomography approaches combined with interferometric point target processing. Elevation profiles are extracted using beamforming and truncated singular value decomposition focusing approaches.
    [bibtex-key = freyHajnsekWegmullerIGARSS2013Tomo] [bibtex-entry]


  1204. Othmar Frey, Charles L. Werner, Urs Wegmuller, Andreas Wiesmann, Daniel Henke, and Christophe Magnard. A car-borne SAR and InSAR experiment. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 93-96, 2013. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, dechirp-on-receive, FMCW, Frequency-modulated continous wave, Ground-based SAR, car-borne SAR, CARSAR, InSAR, DInSAR, geophysical techniques, ground-based SAR system, radar interferometry, synthetic aperture radar, GAMMA Portable Radar Interferometer (GPRI), GPRI, GPRI-II, PanGEO, interferometric technique, Coherence, Correlation, Interferometry, agile platform.
    Abstract: In this contribution, a car-borne SAR and InSAR experiment is described. The slope of a valley was imaged by means of a single-pass InSAR system mounted on a car driving on roads along the bottom of the valley. The GAMMA portable radar interferometer {GPRI-II} hardware with a modified antenna configuration was used for data acquisition. The experimental setup (1), SAR imagery focused along a slightly curved sensor trajectory (2), and first interferometric results (3) obtained using this configuration are presented.
    [bibtex-key = freyWernerWegmullerWiesmannHenkeMagnardIGARSS2013CARSAR] [bibtex-entry]


  1205. Giorgio Gomba, Michael Eineder, Thomas Fritz, and Alessandro Parizzi. Simulation of ionospheric effects on L-band synthetic aperture radar images. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 4463-4466, July 2013. Keyword(s): deformation, geophysical techniques, ionospheric electromagnetic wave propagation, radar imaging, radiowave propagation, remote sensing by radar, synthetic aperture radar, L-band mission, L-band synthetic aperture radar images, arbitrary ionospheric state, deformation measurements, disrupted raw images, focused images, ionosphere effect simulation, ionosphere estimation methods, ionosphere-free real SAR images, propagation delay errors, Abstracts, Ice, Ionosphere, Monitoring, Ionospheric effects simulation, SAR Interferometry errors.
    Abstract: A procedure to simulate the effects of the ionosphere on Synthetic Aperture Radar (SAR) images is presented. The propagation delay errors induced by the ionosphere have to be compensated to millimeter level in order to meet the scientific requirements for an L-band mission dedicated to deformation measurements, which are summarized in [1]. The simulator presented in this paper can be used to study the effects of an arbitrary ionospheric state on SAR images and to generate disrupted raw and focused images starting from ionospherefree real SAR images and use them to validate ionosphere estimation methods.
    [bibtex-key = gombaEinederFritzParizzi2013IonoSARSimulation] [bibtex-entry]


  1206. Stephan Palm, Anika Maresch, and Uwe Stilla. Investigation on Circular Mapping by FMCW-SAR on Small Airplanes. In International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XL-1/W1, ISPRS Hannover Workshop, 2013.
    Abstract: The evaluation of local damages after natural disasters by using remote sensing demands for flexible platforms as well as sensor systems,which guarantee both weather- and daylight-independence. Due to the fact of small energy consumption, small size and lightweight millimeter-wave FMCW radar sensors on small airplanes are very promising for this task. Especially in urban environments theside looking SAR geometry causes shadowing and foreshortening effects, which lead to gaps in the reconstructed scene and misinterpretation. Multiple views from different aspect angles can reduce the shadowing effects but especially in unknown areas the best lineof sight cannot be investigated in advance. This is achievable by performing circular flight trajectories. However, as millimeter-waveFMCW radar sensors typically have very narrow beams and small airplanes are sensitive to air turbulences, a beam stabilization method is necessary to provide full illumination of the target scene. In this paper the calculations for system specific optimal flight parameters are demonstrated and validated by simulations. The impact of air turbulences, causing angular deviations to the sensor and thus misplacementof the main beam lobe on the ground are explored by experimental data performed with our SUMATRA system. The effects of a potential mechanical beam stabilization are visualized and requirements for such a system are formulated. Our experiments showthat for typical flight conditions a stabilized platform is well suitable to stabilize a narrow radar beam in order to keep a target sceneconstantly illuminated over a full circular trajectory. Typically these stabilized platforms can handle angular corrections in all threegeometries (pitch, roll, squint) of up to 12 deg - 15 deg by a speed of 15 deg per second. Therefore a more cost intensive full gimbal systemwhich is known to be used in optical applications and which can handle a full 360 deg tracking is not neccessarily needed.
    [bibtex-key = palmMareschStillaISPRS2013FMCWCircularSAR] [bibtex-entry]


  1207. O. Ponce, P. Prats, R. Scheiber, A. Reigber, and A. Moreira. Analysis and optimization of multi-circular SAR for fully polarimetric holographic tomography over forested areas. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. - IGARSS, pages 2365-2368, July 2013. Keyword(s): geophysical image processing, holography, radar imaging, radar polarimetry, synthetic aperture radar, vegetation, 3D resolution, DLR F-SAR sensor, GLRT algorithm, Germany, IRF, Kauf-beuren, L-band, acquisition geometry, anisotropic analysis, forested areas, fully polarimetric holographic tomography, generalized likelihood ratio test, holographic SAR tomograms, impulse response function, incoherent imaging, multicircular SAR analysis, multicircular SAR optimization, polarimetric MCSAR campaign, scatterers, sidelobe suppression, system bandwidth, Apertures, Bandwidth, Geometry, Image resolution, Imaging, Synthetic aperture radar, Anisotropy, compressive sensing (CS), fast factorized back-projection (FFBP), holographic tomography, multi-circular synthetic aperture radar (MCSAR), polarimetric synthetic aperture radar (PolSAR). [bibtex-key = Ponce2013] [bibtex-entry]


  1208. O. Ponce, P. Prats, R. Scheiber, A. Reigber, and A. Moreira. First demonstration of 3-D holographic tomography with fully polarimetric multi-circular SAR at L-band. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. - IGARSS, pages 1127-1130, July 2013. Keyword(s): airborne radar, array signal processing, compressed sensing, geophysical image processing, holography, optical focusing, optical tomography, radar imaging, radar polarimetry, radar resolution, synthetic aperture radar, transient response, 3D holographic tomography, 3D polarimetric holographic tomogram, 3D sidelobe reduction, BF, CS, DLR F-SAR airborne system, Earth analysis, IRF, Kaufbeuren Germany, L-band, MCSAR, beamforming, coherent 3D radar backscattering, compressive sensing, dry soil, forested area, fully polarimetric multicircular SAR, ice, impulse response function, multiangular measurement acquisition, temporal decorrelation, volume scatterer, Apertures, Bandwidth, Image resolution, Imaging, Synthetic aperture radar, Vegetation, Circular synthetic aperture radar (CSAR), compressive sensing (CS), fast factorized back-projection (FFBP), holographic tomography, polarimetric synthetic aperture radar (PolSAR). [bibtex-key = Ponce2013a] [bibtex-entry]


  1209. Marc Rodriguez-Cassola, Pau Prats-Iraola, Francesco De Zan, Rolf Scheiber, and Andreas Reigber. Doppler-related focusing aspects in the TOPS imaging mode. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 2043-2046, July 2013. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, geophysical techniques, synthetic aperture radar, Doppler spectrum, ScanSAR, TOPS-mode SAR data focusing, TOPS-mode synthetic aperture radar data, Terrain Observation by Progressive Scans mode, azimuth baseband scaling operation, azimuth scaling factors, extended chirp scaling processing procedure, full-aperture imaging approach, limited azimuth-data extension, residual TOPS raw-data focusing, sliding spotlight SAR data focusing, spaceborne imaging mode, two-step focusing technique, wide-swath coverage, Azimuth, Bandwidth, Doppler effect, Focusing, Image resolution, Remote sensing, Aliasing, Terrain Observation by Progressive Scans (TOPS), deramp, extended chirp scaling, synthetic aperture radar (SAR).
    Abstract: The distortions caused in conventionally focused TOPS SAR images which originate from the azimuth-variant Doppler centroid within a burst are presented and analyzed. In particular, the azimuth distortions due to topography mismatch in focusing stages and the range distortions due to the assumption of validity for the stop-and-go approximation are expounded in detail. Compensation strategies to accommodate the two effects in an accurate and precise manner are discussed and validated with TOPS data acquired with TerraSAR-X.
    [bibtex-key = rodriguezCassolaPratsDeZanScheiberReigberIGARSS2013TOPS] [bibtex-entry]


  1210. Stefano Tebaldini and Laurent Ferro-Famil. High resolution three-dimensional imaging of a snowpack from ground-based SAR data acquired at X and Ku Band. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 77-80, July 2013. Keyword(s): SAR Processing, SAR Tomography, Snow, Snowpack, X-band, Ku-band, remote sensing by radar, snow, synthetic aperture radar, 2D synthetic array, 3D imaging, AD 2010 12, Col de Porte, French Alps, GB SAR campaign, Ground Based SAR data, MeteoFrance, ice layer, snowpack, Backscatter, Focusing, Ice, Snow, Synthetic aperture radar.
    Abstract: In this paper we present experimental results relative to the vertical structure of a 60 cm snow-pack as sensed with X- and Ku-Band microwaves. The available data are from a Ground Based (GB) SAR campaign carried out by the University of Rennes I in December 2010 at Col de Porte, in the French Alps, in collaboration with Meteo-France. The data have been acquired by moving a VNA along two orthogonal directions, so as to obtain a two dimensional synthetic array. This allowed to focus the signal in the three dimensional space, thus providing a direct imaging of the vertical structure of the snow-pack at a resolution of few centimeters. Results revealed the presence of strong backscattering contributions from beneath the snow layer, that appear to be linked to the presence of an ice layer.
    [bibtex-key = tebaldiniFerroFamil2013SnowTomo] [bibtex-entry]


  1211. Thomas M. Benson, Daniel P. Campbell, and Daniel A. Cook. Gigapixel spotlight synthetic aperture radar backprojection using clusters of GPUs and CUDA. In 2012 IEEE Radar Conference, pages 0853-0858, May 2012. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, CUDA, GPU, SAR Focusing, Azimuth Focusing, fast Fourier transforms, graphics processing units, parallel architectures, radar computing, radar imaging, resource allocation, synthetic aperture radar, CUDA Clusters, GPU Clusters, SAR image formation, computing nodes, fast Fourier transforms, gigapixel scale data set, gigapixel spotlight synthetic aperture radar backprojection, graphics processing units, image formation algorithms, image formation framework, nonplanar surfaces, wavefront planarity, Graphics processing unit, History, Interpolation, Kernel, Scalability, Sparse matrices, Synthetic aperture radar.
    Abstract: Synthetic aperture radar (SAR) image formation via backprojection offers a robust mechanism by which to form images on general, non-planar surfaces, without often restrictive assumptions regarding the planarity of the wavefront at the locations being imaged. However, backprojection presents a substantially increased computational load relative to other image formation algorithms that typically depend upon fast Fourier transforms. In this paper, we present an image formation framework for accelerated SAR backprojection that utilizes a cluster of computing nodes, each with one or more graphics processing units (GPUs). We address the parallelization of the backprojection process among multiple nodes and the scalability thereby obtained, several optimization approaches, and performance as a function of both allocated resources and desired precision. Finally, we demonstrate the achieved performance on a simulated gigapixel-scale data set.
    [bibtex-key = bensonCampbellCook2012GigaPixelNERFFTBackprojectionCUDA] [bibtex-entry]


  1212. O.O. Bezvesilniy, I. M. Gorovyi, and D. M. Vavriv. Estimation of phase errors in SAR data by Local-Quadratic map-drift autofocus. In Proc. Int. Radar Symp., pages 376-381, 2012. Keyword(s): SAR Processing, Autofocus, SAR Autofocus, MoComp, Motion Compensation, Map-Drift Autofocus, airborne radar, radar imaging, synthetic aperture radar, SAR data, SAR images, X-band airborne SAR system, arbitrary residual phase error, local-quadratic map-drift autofocus, phase error estimation, quadratic errors, small data blocks, uncompensated phase errors, Antennas, Azimuth, Bandwidth, Doppler effect, Measurement uncertainty, Radar, Trajectory, autofocus, map-drift, motion compensation, motion errors, synthetic aperture radar.
    Abstract: Uncompensated phase errors lead to quality degradation of SAR images what is especially critical for high-resolution systems. In the paper, a novel approach to the stripmap autofocus is proposed. The idea of the method is to estimate the local quadratic phase errors by processing small data blocks. The conventional map-drift autofocus (MDA) algorithm is used for such estimation. Then, by a double integration of the estimated quadratic errors, an arbitrary residual phase error for large data blocks is evaluated. The performance of the proposed method is demonstrated with data obtained with an X-band airborne SAR system.
    [bibtex-key = bezvesilniyGorovyiVavrivIRS2012Autofocus] [bibtex-entry]


  1213. T. R. Clem, D. D. Sternlicht, J. E. Fernandez, J. L. Prater, R. Holtzapple, R. P. Gibson, J. P. Klose, and T. M. Marston. Demonstration of advanced sensors for underwater unexploded ordnance (UXO) detection. In Proc. Oceans, pages 1-4, October 2012. Keyword(s): inspection, oceanographic equipment, sensors, Office of Naval Research Small Synthetic Aperture MineHunter, REMUS BMI system, REMUS buried mine identification system, SSAM, UXO detection, UXO removal, advanced sensor demonstration, advanced sensors, clutter rejection, commercial-off-the-shelf sensors, diver inspections, hydronalix surfsense microUSV, hydronalix surfsense microunmanned surface vessel, man-made objects, military test ranges, seabed unexploded ordnance, subsurface UXO sized objects, underwater unexploded ordnance detection, Clutter, Inspection, Magnetic resonance imaging, Magnetoacoustic effects, Magnetometers, Sensors, Synthetic aperture sonar. [bibtex-key = Clem2012] [bibtex-entry]


  1214. H. Essen, W. Johannes, S. Stanko, R. Sommer, A. Wahlen, and J. Wilcke. High resolution W-band UAV SAR. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5033-5036, July 2012. Keyword(s): SAR Processing, W-Band, SUMATRA, Airborne SAR, UAV, Fraunhofer, geophysical equipment, remote sensing by radar, synthetic aperture radar, FM-CW principle, MIRANDA, SAR imaging, W-band UAV SAR, average transmit power, fibre optic gyroscope, frequency 94 GHz, high quality SAR processing, miniaturised millimetre wave radar, ordinary analogue data link, quick-look processing, real-time SAR-focusing algorithm, synthetic aperture radar, unmanned helicopter, Bandwidth, Chirp, Radar antennas, Real-time systems, Sensors, Synthetic aperture radar, High Resolution, Synthetic Aperture Radar, UAV, mm- waves.
    Abstract: A miniaturised millimetre wave radar, MIRANDA, to be used as Synthetic Aperture Radar onboard a small UAV was designed, built and tested onboard of an unmanned helicopter. The design followed the FM-CW principle, to get the highest possible average transmit power and thus the best range performance. The experiments described here were conducted at a centre frequency of 94 GHz. An inertial system of high quality, based upon a fibre optic gyroscope maintained the necessary precision, to allow high resolution SAR imaging. The raw data are transmitted to the ground station using an ordinary analogue data link, where they are A/D converted, preprocessed and finally undergo a real-time SAR-focusing algorithm. Additionally to the quick-look processing the data are stored to be able to apply further high quality SAR processing. The paper describes the design principles and gives results from the flight tests.
    [bibtex-key = EssenEtAlIAGRSS2012wbandSUMATRASARforUAV] [bibtex-entry]


  1215. Othmar Frey, Erich Meier, and Irena Hajnsek. Towards a more reliable estimation of forest parameters from polarimetric SAR tomography data. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 3110-3113, 2012. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, scattering, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: n this contribution, tomographic imagery obtained from estimating the 3-D localization and the polarimetric signature of backscattering sources within a forest environment from fully-polarimetric multibaseline SAR data at L- and P-bands are presented. Both, polarization diversity and spatial diversity were exploited jointly within the tomographic focusing to estimate the targets' localization and polarimetric signature. We thereby ex- tend our recently proposed time-domain back-projection (TDBP)-based tomographic focusing approach. The tomo- graphic slices obtained from polarimetric spectral estima- tion using MLBF, Capon, and MUSIC are opposed to the results previously obtained by tomographic focusing of the individual polarization channels. The results are briefly discussed with respect to potential advancements towards more reliable estimation of forest parameters from SAR tomography data.
    [bibtex-key = freyMeierHajnsekIGARSS2012Tomo] [bibtex-entry]


  1216. Othmar Frey, Erich Meier, and Irena Hajnsek. Tweaking baseline constellations for airborne SAR tomography and InSAR: an experimental study at L- and P-bands. In Proc. EUSAR 2012 - 9th European Conference on Synthetic Aperture Radar, pages 215-218, 2012. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, scattering, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: A notable obstacle hindering widespread application of SAR tomography for 3D mapping of vegetation is the relatively large number of acquisitions that are needed to obtain a high resolution and a good rejection of spurious responses in the direction perpendicular to the line of sight. In this paper, we discuss the impact of different baseline constellations on 3-D mapping of vegetation volumes and the underlying topography in terms of tomographic focusing as well as classical single-baseline repeat-pass interferometry. The effects are studied using two airborne tomography data sets at L- and P-bands.
    [bibtex-key = freyMeierHajnsekEUSAR2012Tomo] [bibtex-entry]


  1217. Othmar Frey, Urs Wegmuller, and Charles L. Werner. Terrain motion measurements over European urban areas using persistent scatterer interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 7565-7568, 2012. Keyword(s): Spaceborne radar, Persistent Scatterer Interferometry, PSI, PanGeo, C-Band, Terrain motion, subsidence, interferometry, InSAR, SAR Interferometry, Remote Sensing, synthetic aperture radar, SAR, scattering, Urban Areas.
    Abstract: In this contribution Persistent Scatterer Interferometry (PSI) processings done in the framework of the FP7 Project PanGEO over a number of European sites are presented. After a short introduction of the PanGEO project, we present the data used, the processing done and the results of the PSI analysis. The influence of specifics as the presence of large non-urban areas, significant topography, seasonal effects, special deformation characteristics, and the size of the available data stacks on the processing are highlighted.
    [bibtex-key = freyWegmullerWernerIGARSS2012PanGeo] [bibtex-entry]


  1218. Luca Marotti, Pau Prats, Rolf Scheiber, Steffen Wollstadt, and Andreas Reigber. Differential SAR interferometry with TerraSAR-X TOPS data: Mexico city subsidence results. In Synthetic Aperture Radar, 2012. EUSAR. 9th European Conference on, pages 677-680, April 2012. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, Azimuth, Decorrelation, Estimation, Interferometry, Signal resolution, Synthetic aperture radar, Time frequency analysis.
    Abstract: In this paper we investigate the possibility to exploit TOPS (Terrain Observation by Progressive Scan) data to measure ground displacement movements by means of Differential SAR Interferometry (DInSAR). Several critical points (i.e. coregistration) have to be faced during the data processing since, due to the TOPS signal characteristics, the interferometric chain is very sensitive to small implementation errors. The obtained results will be compared with the ones conventionally retrieved when applying DInSAR on stripmap data. Finally, the potential of DInSAR measurements, when combining TOPS and stripmap data by means of point-like scatterers, will be also analysed.
    [bibtex-key = marottiPratsScheiberWollstadtReigberEUSAR2012TOPSINSAR] [bibtex-entry]


  1219. T. Marston. A correlation-based autofocus algorithm for coherent circular synthetic aperture sonar. In Proc. EUSAR 2012; 9th European Conf. Synthetic Aperture Radar, pages 66-69, April 2012. Keyword(s): Apertures, Correlation, Fourier transforms, Manganese, Navigation, Synthetic aperture sonar. [bibtex-key = Marston2012] [bibtex-entry]


  1220. D. Monells, R. Iglesias, Jordi J. Mallorqui, X. Fabregas, and C. Lopez-Martinez. Phase quality optimization in Orbital Differential SAR Interferometry with fully polarimetric data. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1864-1867, 2012. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Spaceborne SAR, InSAR, DInSAR, Polarimetry, geophysical image processing, geophysical techniques, optimisation, radar interferometry, radar polarimetry, synthetic aperture radar, Barcelona, DInSAR processing, Fine Quad-Pol Radarsat-2 acquisitions, Radarsat-2, Spain, TerraSAR-X, analytical techniques, fully polarimetric data, interferometric technique, orbital Differential SAR Interferometry, phase quality optimization, polarimetric technique, single polarization SAR, terrain deformation phenomena, Coherence, Correlation, Histograms, Interferometry, Optimization, Synthetic aperture radar, Vectors, Coherence optimization, Pol-DInSAR, Terrain deformation, multi-baseline.
    Abstract: Orbital Differential SAR Interferometry (DInSAR) is a well-known technique to retrieve terrain deformation phenomena from wide areas with high resolution. Historically its application has been limited to single polarization SAR, mainly due to the unavailability of polarimetric data. Lately, the launch of several satellites with polarimetric capabilities, such as Radarsat-2 or TerraSAR-X, allows merging polarimetric and interferometric techniques in order to improve the results obtained in the DInSAR processing. This work will explore the existent analytical techniques in order to optimize the quality of the subsidence results. The dataset used contains 35 Fine Quad-Pol Radarsat-2 acquisitions over the city of Barcelona (Spain).
    [bibtex-key = monellsIglesiasMallorquiFabregasLopezMartinez2012DINSARPolarimetric] [bibtex-entry]


  1221. O. Ponce, P. Prats, R. Scheiber, A. Reigber, and A. Moreira. Multibaseline 3-D circular SAR imaging at L-band. In Proc. EUSAR 2012; 9th European Conf. Synthetic Aperture Radar, pages 113-116, April 2012. Keyword(s): Apertures, Image reconstruction, Image resolution, Imaging, L-band, Synthetic aperture radar, Three dimensional displays. [bibtex-key = Ponce2012a] [bibtex-entry]


  1222. O. Ponce, P. Prats, R. Scheiber, A. Reigber, and A. Moreira. Polarimetric 3-D reconstruction from multicircular SAR at P-band. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, pages 3130-3133, July 2012. Keyword(s): array signal processing, calibration, image reconstruction, image resolution, radar imaging, radar polarimetry, singular value decomposition, synthetic aperture radar, 3D imaging, CSAR mode, DLR F-SAR system, SVD, backscattering profile, beamforming, compressive sensing, cone-shaped sidelobes, fast factorized back projection, holographic SAR tomogram, multicircular fully polarimetric SAR experiment, phase calibration method, polarimetric 3D reconstruction, polarimetric three-dimensional reconstruction, subwavelength resolution, Array signal processing, Calibration, Focusing, Image reconstruction, Image resolution, Synthetic aperture radar, Tomography, Circular SAR, PolSAR, holographic tomography, multicircular SAR. [bibtex-key = Ponce2012] [bibtex-entry]


  1223. M. A. Remy, Karlus A. Camara de Macedo, and Joao R. Moreira. The first UAV-based P- and X-band interferometric SAR system. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5041-5044, July 2012. Keyword(s): geophysical equipment, radar interferometry, remote sensing, synthetic aperture radar, UAV-based P-band interferometric SAR system, UAV-based X-band interferometric SAR system, OrbiSat system, Brazil, compact InSAR system, flexible InSAR system, topographic survey, differential interferometry, unmanned air vehicle, Synthetic aperture radar, Global Positioning System, Remote sensing, Interferometry, Radar antennas, UAV, SAR, P, X, interferometry.
    Abstract: In this paper we present the design overview of the UAV-based P- (repeat-pass) and X-band (single-pass) interferometric SAR system of OrbiSat, Brazil. The main objective is to built a compact and flexible InSAR system capable of performing accurate topographic survey and differential interferometry. To our knowledge, this is the first UAV-based In-SAR system with such characteristics, multi-band and fixed-baseline (X-band), to be projected. The status, and some main project results and solution are described and depicted.
    [bibtex-key = remyDeMacedoMoreiraIGARSS2012FirstUAVbasedPandXbandSARSystem] [bibtex-entry]


  1224. Angel Ribalta. Optimizing the factorisation parameters in the fast factorized backprojection algorithm. In Proc. EUSAR 2012 - 9th European Conference on Synthetic Aperture Radar, pages 356-359, April 2012. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Optimization, Factorization Parameter Optimization.
    Abstract: An explicit formula for optimizing the factorisation parameters in the fast factorized backprojection algorithm is given. It is shown that the optimal factorisation basis is given by b = 3, whereas the optimal number of factorisation levels can be easily computed a priori. A numerical example illustrates the effectivity of the proposed choice.
    [bibtex-key = ribaltaFFBPeusar2012] [bibtex-entry]


  1225. S. Rödelsperger, A. Coccia, D. Vicente, and A. Meta. Introduction to the new metasensing ground-based SAR: Technical description and data analysis. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 4790-4792, July 2012. Keyword(s): data analysis, geophysical equipment, geophysical image processing, synthetic aperture radar, metasensing ground-based SAR, data analysis, MetaSensing FastGBSAR, high sampling rate, submillimeter accuracy, Synthetic aperture radar, Sensors, Monitoring, Instruments, Microwave imaging, Microwave theory and techniques, Accuracy.
    Abstract: This paper introduced the novel MetaSensing FastGBSAR with the ability to map deformations of a large area in real-time with a high sampling rate of less then 10 seconds per image. Therefore, the sensor allows the reliable measurement of deformations with sub-millimeter accuracy in a variety of applications. The first campaign with a prototype is currently in progress. The analysis and results will be shown at the conference.
    [bibtex-key = roedelspergerCocciaVicenteMetaIGARSS2010FastGBSAR] [bibtex-entry]


  1226. Urs Wegmuller, Othmar Frey, and Charles L. Werner. Point Density Reduction in Persistent Scatterer Interferometry. In Proc. EUSAR 2012 - 9th European Conference on Synthetic Aperture Radar, pages 673-676, 2012. Keyword(s): SAR processing, PSI, Persistent Scatterer Interferometry, interferometry, InSAR, multibaseline InSAR, point density, point density reduction.
    Abstract: Persistent Scatterer Interferometry (PSI) is widely used to determine ground deformation rates and histories. The data sets processed are increasing in size due to larger areas considered, higher resolution, and higher point den- sities. Working with lists of several million points and related networks reduces the computational efficiency. In this paper a method to adaptively reduce the size of the point list is presented. Considering the local point density and a point quality measure points are removed such that the density is significantly reduced in areas of very high density while maintaining the available density in areas of lower density. The main PSI processing is then done for the reduced point list. Later on the result obtained for the reduced list is expanded to the full point list. In sev- eral cases this methodology improves the processing efficiency significantly.
    [bibtex-key = wegmullerFreyWernerEUSAR2012PointDensity] [bibtex-entry]


  1227. Urs Wegmuller, Tazio Strozzi, and Charles Werner. Ionospheric path delay estimation using split-beam interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 3631-3634, July 2012. Keyword(s): SAR Processing, Ionosphere, Ionospheric Path Delay, split-beam interferometry, SBI, ionospheric electromagnetic wave propagation, ionospheric techniques, radar interferometry, remote sensing by radar, L-band split beam interferograms, along track ground displacement estimation, azimuth spectrum band pass filtering, directional scattering identification, ionospheric path delay estimation, long baseline pair coherence estimation, split beam interferometry, Azimuth, Band pass filters, Coherence, Delay, Ionosphere, L-band, Time series analysis, Split-beam interferometry, ionosphere, ionospheric path delay.
    Abstract: Azimuth spectrum band-pass filtering has been applied successfully for estimation of along-track ground displacement [1] as well as for other applications, such as the identification of directional scattering [2] and the coherence estimation for long-baseline pairs [3]. Particularly L-band split-beam interferograms have shown another phase component related to along-track variations in the ionospheric path delay. In our work we present methodologies to identify and quantify ionospheric path delays affecting an interferogram using the corresponding split-beam interferogram.
    [bibtex-key = wegmullerStrozziWernerIGARSS2012SplitBeamInterferometryIonosphericPathDelay] [bibtex-entry]


  1228. Charles L. Werner, Andreas Wiesmann, Tazio Strozzi, Andrew Kos, Rafael Caduff, and Urs Wegmuller. The GPRI multi-mode differential interferometric radar for ground-based observations. In Proc. EUSAR 2012, pages 304-307, April 2012. Keyword(s): SAR Processing, Interferometry, Radar, Radar interferometry, Surface deformation;.
    Abstract: We describe the Gamma Portable Radar Interferometer (GPRI), an instrument designed to perform ground-based radar interferometry to measure surface deformation with an accuracy < 0.2 mm and generate digital elevation models (DEMs). Ground-based interferometric observations are complementary to spaceborne or airborne SAR based measurements because they can be acquired continuously in order to track rapid deformation and mitigate the effects of temporal decorrelation and atmospheric phase variability. Results from test cases are shown including the Aletsch glacier, Aletschwald, and Kornhaus bridge in Bern.
    [bibtex-key = wernerWiesmannStrozziKosCaduffWegmullerEUSAR2012GPRI2] [bibtex-entry]


  1229. J. Abril, E. Nova, A. Broquetas, A. Aguasca, J. Romeu, and L. Jofre. Deforming and relief interferometric SAR imaging at W-band. In 2011 International Conference on Infrared, Millimeter, and Terahertz Waves, pages 1-2, October 2011. Keyword(s): SAR Processing, W-Band, backpropagation, deformation, millimetre wave antennas, radar antennas, radar imaging, radar interferometry, receiving antennas, reflectivity, rough surfaces, synthetic aperture radar, L-shape target, W-band, backpropagation algorithm, image local deformation, interferogram coherence, interferometric phase difference, interferometric synthetic aperture radar, metallic rough surface deformation test, micrometric displacement, radar reflectivity, relief interferometric SAR imaging, scattered field, surface random change, surface relieve retrieval, Antenna measurements, Image resolution, Imaging, Radar imaging, Rough surfaces, Surface roughness, Surface topography.
    Abstract: Interferometry at W Band can be exploited to form images of small deformations with precisions in the order of tenths of microns and retrieve surface relieves. An Interferometric Synthetic Aperture Radar setup is proposed in which the scattered field is focused using a combination of range compression and backpropagation algorithm. Interferometric images involving micrometric displacements have been obtained showing a good agreement between real and measured displacements. A metallic rough surface deformation test has been first performed, by imaging the phase difference between acquisitions before and after the displacement. The interferometric phase difference between acquisitions can be used to image local deformations from a range of several meters. Furthermore the interferogram coherence shows the degree of decorrelation of the radar reflectivity which provides valuable information for surface random change. Likewise the relief of the surface with an L-shape target is retrieved mapping the phase difference between the two receiving antennas, in this case an static scenario is required.
    [bibtex-key = abrilNovaBroquetasAguascaRomeuJofre2011WBANDSARDeformationAndRelief] [bibtex-entry]


  1230. Esteban Aguilera, Matteo Nannini, and Andreas Reigber. Multi-signal compressed sensing for polarimetric SAR tomography. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1369-1372, July 2011. Keyword(s): Airborne SAR, SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, Distributed Compressive Sensing, DCS methodology, E-SAR sensor, SAR image, azimuth-range pixel, distributed compressed sensing, elevation direction, fully polarimetric L-band data, multiplicative speckle noise, multisignal compressed sensing, pass acquisition geometry, polarimetric SAR tomography, polarimetric channel, space-borne radar platform, synthetic aperture radar, three dimensional imaging, tomographic focusing approach, tomographic signal, vertical reflectivity function, image reconstruction, image resolution, image sensors, radar imaging, radar polarimetry, radar resolution, spaceborne radar, speckle, synthetic aperture radar, tomography;.
    Abstract: In recent years, three-dimensional imaging by means of SAR tomography has become a field of intensive research. In SAR tomography, the vertical reflectivity function for every azimuth-range pixel is usually recovered by processing data collected using a defined repeat pass acquisition geometry. The most common approach is to generate a synthetic aperture in the elevation direction through imaging from a large number of parallel tracks. This imaging technique is appealing, since it is very simple. However, it has the draw back that large temporal baselines, which is the case for space-borne platforms, can severely affect the reconstruction. In an attempt to reduce the number of parallel tracks, we propose a new tomographic focusing approach that trades number of SAR images for correlations between neighboring azimuth-range pixels and polarimetric channels. As a matter of fact, this can be done under the framework of Distributed Compressed Sensing (DCS), which stems from Compressed Sensing (CS) theory, thus also exploiting sparsity in our tomographic signal. In addition, we address the problem of measurements affected by additive as well as multiplicative speckle noise. Results demonstrating the potential of the DCS methodology will be validated by using fully polarimetric L-band data acquired by the E-SAR sensor of DLR.
    [bibtex-key = aguileraNanniniReigber2011] [bibtex-entry]


  1231. R. Brcic, A. Parizzi, M. Eineder, R. Bamler, and F. Meyer. Ionospheric effects in SAR interferometry: An analysis and comparison of methods for their estimation. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1497-1500, July 2011. Keyword(s): ionospheric electromagnetic wave propagation, ionospheric techniques, radar interferometry, synthetic aperture radar, C-band system, L-band ALOS-PALSAR acquisitions, L-band system, P-band system, SAR signal, X-band frequencies, dispersive effects, ionospheric compensation, ionospheric effects, phase error, repeat-pass SAR interferometry, spaceborne SAR systems, topographic retrieval, wideband interferometry, Azimuth, Bandwidth, Delay, Estimation, Interferometry, Ionosphere, Synthetic aperture radar, delta-k, ionosphere, split-spectrum.
    Abstract: For spaceborne SAR (Synthetic Aperture Radar) systems, the dispersive effects of the ionosphere on the propagation of the SAR signal can be a significant source of phase error. While at X-band frequencies the effects are small, current and future P-, Land C-band systems would benefit from ionospheric compensation to avoid errors in topographic retrieval. In this paper the focus is on the effects of the ionosphere on repeat-pass SAR interferometry from Pthrough X-bands and methods for their estimation which are demonstrated on L-band ALOS-PALSAR acquisitions.
    [bibtex-key = brcicParizziEinederBamlerMeyerIGARSS2011IonoInSARComparisonOfMethods] [bibtex-entry]


  1232. A. Elsherbini and K. Sarabandi. Image distortion effects in subsurface SAR imaging of deserts and their correction technique. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, pages 1075-1078, July 2011. Keyword(s): geophysical image processing, iterative methods, radar imaging, radar interferometry, remote sensing by radar, synthetic aperture radar, topography (Earth), arid regions, conventional SAR focusing techniques, correction technique, deserts, dual frequency InSAR, geometric distortion, image degradation, image distortion effects, interferometric coherence, interferometric synthetic aperture radar, iterative approach, nonplanar top surface, point spread function dispersion, scaled model measurements, subsurface SAR imaging, subsurface region imaging, subsurface topography estimation, Azimuth, Focusing, History, Optical surface waves, Solid modeling, Surface topography, Interferometric Synthetic Aperture Radar (InSAR), Radar Imaging, Subsurface Imaging, Terrain Mapping. [bibtex-key = Elsherbini2011a] [bibtex-entry]


  1233. A. Elsherbini and K. Sarabandi. Subsurface topography mapping in deserts using two frequency SAR interferometry. In Proc. XXXth URSI General Assembly and Scientific Symp, pages 1-4, August 2011. Keyword(s): radar interferometry, synthetic aperture radar, terrain mapping, topography (Earth), InSAR inversion, Ka-InSAR, SAR image, VHF-InSAR, arid region, desert, height estimation, image distortion, scaled model measurement, subsurface topography mapping, top interface topography, two frequency SAR interferometry, Antennas, Azimuth, Extraterrestrial measurements, Focusing, Scattering, Surface topography. [bibtex-key = Elsherbini2011] [bibtex-entry]


  1234. H. Essen, S. Stanko, R. Sommer, W. Johannes, A. Wahlen, J. Wilcke, and S. Hantscher. Millimetre wave SAR for UAV operation. In Proc. Asia-Pacific Microwave Conference, pages 963-966, December 2011. Keyword(s): SAR Processing, W-Band, SUMATRA, Airborne SAR, UAV, Fraunhofer, CW radar, FM radar, Global Positioning System, airborne radar, autonomous aerial vehicles, high electron mobility transistors, millimetre wave amplifiers, millimetre wave field effect transistors, millimetre wave radar, synthetic aperture radar, Fraunhofer spin-off, UAV operation, W-band, advanced front-end technique, environmental monitoring, frequency 94 GHz, highly miniaturized FM-CW radar, low noise HEMT amplifiers, military reconnaissance, millimetre wave SAR system, millimetre wave technology, miniaturized radars, off-the-shelf GPS, sensor package, small airborne sensing aircraft, synthetic aperture radar, transmission equipment, Aircraft, Bandwidth, Chirp, Radar imaging, Sensors, Synthetic aperture radar, FM-CW waveform, UAV, millimetre waves, synthetic aperture radar.
    Abstract: Most recent developments in millimetre wave technology at W-band, especially the advance of medium power and low noise HEMT amplifiers, allow the set-up of miniaturized radars, operating with a wide RF bandwidth, achieving high range resolution. A highly miniaturized FM-CW radar at 94 GHz was developed and tested for SAR applications onboard small UAVs and within a sensor package to be used with a small airborne sensing aircraft. Advanced front-end technique was combined with off-the-shelf GPS and transmission equipment which is readily available. The main goal of the research project is to show, that new front-end technology in combination with achievable back-and components can lead to a SAR system, which is capable to serve a wide field of applications, as environmental monitoring, border control, agricultural applications and equally well can serve the needs of military reconnaissance applications. The paper describes the current state of the research project and gives the perspective of a later prototype production by a Fraunhofer Spin-off.
    [bibtex-key = EssenEtAlAPMW2011SUMATRAWBandSARforUAV] [bibtex-entry]


  1235. Gianfranco Fornaro, A. Pauciullo, D. Reale, Xiao Xiang Zhu, and Richard Bamler. Peculiarities of urban area analysis with very high resolution interferometric SAR data. In Proc. Joint Urban Remote Sensing Event (JURSE), pages 185-188, 2011. [bibtex-key = fornaroPauciulloRealeZhuBamler2011] [bibtex-entry]


  1236. Othmar Frey and Erich Meier. Characterizing the Backscattering Properties of a Forest by Polarimetric SAR Tomography at L- and P-Bands. In Proc. PolInSAR, 5th Int. Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry, ESA SP-695, Frascati, Italy, January 2011. [bibtex-key = freyMeierPolInsAR2011:Tomo] [bibtex-entry]


  1237. Othmar Frey, Erich Meier, and Irena Hajnsek. On the sensitivity of measured backscattering properties to variations of incidence angle and baselines in tomographic SAR data. In Proc. Int. Asia-Pacific Conf. on Synthetic Aperture Radar (APSAR), pages 1-4, September 2011. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, sensitivity analysis, synthetic aperture radar, SAR, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: SAR tomography at L- and P-bands reveals 3-D structural information of forested areas. A drawback, however, are the large number of samples, i.e. overflights, typically used for such configurations. Based on two fully-polarimetric tomographic SAR data sets, at L- and P-bands, we analyze the sensitivity of backscattering from a forest volume as measured by means of SAR tomography with respect to (1) a reduction of the total baseline by subsequently reducing the number of baselines, and (2), with respect to a variation of the incidence angle. In this paper, an excerpt of this sensitivity analysis is presented and discussed.
    [bibtex-key = freyMeierHajnsekAPSAR2011] [bibtex-entry]


  1238. Othmar Frey, Maurizio Santoro, Charles L. Werner, and Urs Wegmuller. DEM-based SAR pixel area estimation for enhanced geocoding refinement and radiometric normalization. In Proc. FRINGE 2011, ESA SP-697, Frascati, Italy, September 2011. Keyword(s): SAR Processing, Geocoding, radiometric calibration, terrain-based radiometric normalization, Coregistration.
    Abstract: Precise terrain-corrected georeferencing of SAR images and derived products in range-Doppler coordinates is important with respect to several aspects, such as data interpretation, combination with other geodata products, and transformation of, e.g., terrain heights into SAR geometry as used in DInSAR applications. For georeferencing a look-up table is calculated and refined based on a coregistration of the actual SAR image to a simulated SAR image. The impact of using two different implementations of such a simulator of topography-induced radar brightness, an approach based on angular relationships and a pixel-area based method are discussed in this paper. It is found that the pixel-area-based method leads to considerable improvements with regard to the robustness of georeferencing and also with regard to radiometric normalization in layover-affected areas
    [bibtex-key = freySantoroWegmullerWernerFRINGE2011] [bibtex-entry]


  1239. D. S. Goshi, Y. Liu, K. Mai, L. Bui, and Y. Shih. A W-band interferometric real-beam scanning FMCW imaging radar. In 2011 IEEE MTT-S International Microwave Symposium, pages 1-4, June 2011. Keyword(s): SAR Processing, W-Band, CW radar, FM radar, image enhancement, radar imaging, radar interferometry, synthetic aperture radar, 2D-based image enhancement, 3D volumetric sensor, RF characteristics, W-band interferometric real beam scanning FMCW imaging radar, frequency 10 Hz, height estimation, interferometric SAR radar system, interferometric technique, long post processing time, phase difference measurement, phase estimation measurement, real beam radar image, real time imaging system, Antenna measurements, Apertures, Imaging, Phase measurement, Radar antennas, Radar imaging, FMCW, imaging, interferometric, radar.
    Abstract: This paper presents an interferometric real-beam scanning FMCW imaging radar. As opposed to the more commonly discussed interferometric SAR radar system that requires a finite capture interval as well as traditionally long post-processing times, the proposed system is a real-time imaging system that generates a 10Hz real-beam radar image. The interferometric technique is proposed as a means to enhance the raw 2D-based image result by providing target height estimation from phase difference measurements, paving a foundation for a 3D volumetric sensor. The general concepts of the system will be discussed followed by details on the RF characteristics and performance. Lastly, the initial validation results of phase estimation measurements will be presented.
    [bibtex-key = goshiLiuMaiBuiShih2011WBandInterferometricFMCWRadar] [bibtex-entry]


  1240. Paco López-Dekker, Francesco De Zan, Thomas Borner, Marwan Younis, K. Papathanassiou, T. Guardabrazo, V. Bourlon, S. Ramongassie, N. Taveneau, Lars Ulander, D. Murdin, N. Rogers, Shaun Quegan, and R. Franco. BIOMASS end-to-end mission performance simulator. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 4249-4252, July 2011. Keyword(s): BIOMASS, P-band synthetic aperture radar, end-to-end mission performance simulator, fully quad-polarized image data acquistion, interferometric mode, module functional description, system architecture, data acquisition, radar imaging, radar interferometry, radar polarimetry, remote sensing by radar, vegetation, vegetation mapping.
    Abstract: This paper discusses the implementation of an end- to-end simulator for the BIOMASS mission. An overview of the system architecture is provided along with a functional description of the modules that comprise the simulator.
    [bibtex-key = lopezDekkerEtalBiomassIGARSS2011] [bibtex-entry]


  1241. T. M. Marston, J. L. Kennedy, and P. L. Marston. Coherent and semi-coherent processing of limited-aperture circular synthetic aperture (CSAS) data. In Proc. OCEANS'11 MTS/IEEE KONA, pages 1-6, September 2011. Keyword(s): backscatter, radar imaging, synthetic aperture sonar, 360 degree scattering information, acoustic targets, backscattered echoes, circular synthetic aperture sonar, destructive interference, image clarity, image masking process, imaging algorithm, limited-aperture circular synthetic aperture data, remote environmental monitoring units, semi-coherent processing, sonar platform, time-domain information, unexploded ordnance objects, unmanned underwater vehicle, Acoustics, Aluminum, Apertures, Image reconstruction, Imaging, Synthetic aperture sonar, Tires, classification, semicoherent imaging, subaperture, unexploded ordinance. [bibtex-key = Marston2011] [bibtex-entry]


  1242. O. Ponce, P. Prats, M. Rodriguez-Cassola, R. Scheiber, and A. Reigber. Processing of Circular SAR trajectories with Fast Factorized Back-Projection. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, pages 3692-3695, July 2011. Keyword(s): geophysical techniques, remote sensing by radar, Circular SAR trajectories, Germany, Kaufbeuren region, azimuth variance, computational time factor, fast factorized back-projection, graphics processor unit, Accuracy, Apertures, Focusing, Geometry, Graphics processing unit, Image resolution, Trajectory, Circular SAR (CSAR), Fast Back Projection (FBP), Graphics Processor Unit (GPU), focusing, polarimetry. [bibtex-key = Ponce2011] [bibtex-entry]


  1243. Ricardo Portillo, Sarala Arunagiri, Patricia J. Teller, Song J. Park, Lam H. Nguyen, Joseph C. Deroba, and Dale Shires. Power versus performance tradeoffs of GPU-accelerated backprojection-based synthetic aperture radar image formation. In Proc. SPIE, volume 8060, pages 1-21, 2011. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Back-Projection, Non-Linear Flight Tracks, Curvilinear SAR, GPU, GPGPU, Graphics Processing Unit, Focusing, Azimuth Focusing, Airborne SAR, Motion Compensation. [bibtex-key = portilloArunagiriTellerParkNguyenDerobaShires2011:TDBPandGPU] [bibtex-entry]


  1244. J. L. Prater and T. M. Marston. Partial synthetic aperture beamformer. In Proc. OCEANS'11 MTS/IEEE KONA, pages 1-5, September 2011. Keyword(s): array signal processing, autonomous underwater vehicles, image resolution, sonar imaging, synthetic aperture sonar, AUV technology, SAS, autonomous underwater vehicle, nonlinear trajectories, partial synthetic aperture beamformer, Apertures, Array signal processing, Image resolution, Synthetic aperture sonar, Trajectory, Vehicles, autonomatic target recognition, beamforming. [bibtex-key = Prater2011] [bibtex-entry]


  1245. Kenneth Ranney, Lam Nguyen, Francois Koenig, Getachew Kirose, Anthony Martone, Gregory Mazzaro, Kelly Sherbondy, Chi Tran, and Karl Kappra. Side-looking image formation with a maneuvering vehicle-mounted antenna array. In Proc. SPIE, volume 8021, pages 80211W, 2011. SPIE.
    Abstract: Researchers at the U.S. Army Research Laboratory (ARL) designed and fabricated the Synchronous Impulse REconstruction (SIRE) radar system in an effort to address fundamental questions about the utilization of low frequency, ultrawideband (UWB) radar. The SIRE system includes a receive array comprising 16 receive channels, and it is capable of operating in either a forward-looking or a side-looking mode. When operated in side-looking mode, it is capable of producing high-resolution Synthetic Aperture Radar (SAR) data. The SAR imaging algorithms, however, initially operated under the assumption that the vehicle followed a nearly linear trajectory throughout the data collection. Under this assumption, the introduction of vehicle path nonlinearities distorted the processed SAR imagery. In an effort to mitigate these effects, we first incorporated segmentation routines to eliminate highly non-linear portions of the path. We then enhanced the image formation algorithm, enabling it to process data collected from a non-linear vehicle trajectory. We describe the incorporated segmentation approaches and compare the imagery created before and after their incorporation. Next, we describe the modified image formation algorithm and present examples of output imagery produced by it. Finally, we compare imagery produced by the initial segmentation algorithm to imagery produced by the modified image-formation algorithm, highlighting the effects of segmentation parameter variation on the final SAR image.
    [bibtex-key = ranneyNguyenKoenigKiroseMartoneMazzaroSherbondyTranKappra2011] [bibtex-entry]


  1246. Stephan Stanko, Winfried Johannes, Rainer Sommer, Alfred Wahlen, Jörn. Wilcke, Helmut Essen, Axel Tessmann, and Ingmar Kallfass. SAR with MIRANDA - millimeterwave radar using analog and new digital approach. In Proc. European Radar Conference, pages 214-217, October 2011. Keyword(s): SAR Processing, FMCW, Frequency-Modulated Continuous-Wave, Ka-Band, W-Band, Airborne SAR, millimetre wave radar, radar imaging, synthetic aperture radar, IMU, MIRANDA, SAR, SAR-processing, UAV, analog approach, digital approach, frequency generation module, image scenes, inertial measurement unit, microlight, millimeterwave radar, synthetic aperture radar system, unmanned aerial vehicles, Chirp, Radar antennas, Radar imaging, Radar measurements, Synthetic aperture radar, Wavelength measurement.
    Abstract: The capability to image scenes from unmanned aerial vehicles (UAVs) even through bad weather conditions or dust clouds gets more and more important. In addition, modern synthetic aperture radar (SAR) systems giving very high resolution have to be light and small in size to fit in small aircrafts. In this paper we show a newly designed SAR system using a high flexible frequency generation module. The SAR- system was flown using a microlight and the radar raw data was transmitted to the ground and processed in real-time, so in an operational scenario the user can have an online image of the measurement. One crucial part of SAR-processing is the accuracy of the used inertial measurement unit (IMU). In this project we tested different kinds of these systems and determined the minimum requirements.
    [bibtex-key = stankoJohannesSommerWahlenWilckeEssenTessmannKallfassEURAD2011MirandaFMCWSAR] [bibtex-entry]


  1247. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. Fast backprojection algorithm for UWB bistatic SAR. In Proc. IEEE Radar Conf., pages 431-434, May 2011. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast Factorized Back-Projection, FFBP, Back-Projection, UWB SAR, bistatic SAR, Motion Compensation, MoComp, UWB bistatic SAR, beamforming, bistatic fast backprojection algorithm, ground image plane, motion compensation, subaperture basis, subimage basis, time-domain characteristics, ultrawideband ultrawidebeam bistatic synthetic aperture radar, array signal processing, motion compensation, radar imaging, synthetic aperture radar, time-domain analysis, ultra wideband radar.
    Abstract: The paper introduces an algorithm for Ultrawide band Ultrawide-beam (UWB) bistatic Synthetic Aperture Radar (SAR). The algorithm works in time-domain and therefore inherits time-domain characteristics such as unlimited scene size, local processing and manageable motion compensation. The proposed algorithm is not limited by any configuration of bistatic SAR. The algorithm processes the UWB bistatic SAR data on a subaperture and subimage basis. This means, instead of backprojecting directly the SAR data to a ground image plane, the algorithm handles the data in two stages: beam forming and local backprojection. The algorithm is named Bistatic Fast Backprojection (BiFBP) and has been tested successfully with the simulated UWB bistatic SAR data.
    [bibtex-key = vuSjogrenPetterssonRADARCON2011BistaticFFBP] [bibtex-entry]


  1248. Viet Thuy Vu, Thomas K. Sjogren, and Mats I. Pettersson. Fast factorized backprojection algorithm for UWB SAR image reconstruction. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 4237-4240, July 2011. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast Factorized Back-Projection, FFBP, Back-Projection, UWB SAR, bistatic SAR, Motion Compensation, MoComp, migration handling, one beam forming stage, real time processing, synthetic aperture radar, time-domain characteristics, unlimited scene size, geophysical image processing, remote sensing by radar, synthetic aperture radar.
    Abstract: The fast factorized backprojection for bistatic SAR algorithm (BiFFBP), which processes the bistatic SAR data in more than one beam forming stage, is presented in this paper. The algorithm is developed on the bistatic fast backprojection algorithm SAR (BiFBP) and inherits time-domain characteristics such as unlimited scene size, real time processing, local processing, manageable motion compensation, and large range migration handling. Also, the algorithm works well with different kinds of bistatic configuration. For these reason, it is very suitable for UWB bistatic SAR processing. The pro- posed algorithm is tested successfully with simulated UWB bistatic SAR data.
    [bibtex-key = vuSjogrenPetterssonIGARSS2011BistaticFFBP] [bibtex-entry]


  1249. Yuanyuan Wang, Xiao Xiang Zhu, and R. Bamler. Advanced coherence stacking technique using high resolution TerraSAR-X spotlight data. In Proc. Joint Urban Remote Sensing Event (JURSE), pages 233-236, 2011. [bibtex-key = Wang2011a] [bibtex-entry]


  1250. Yuanyuan Wang, Xiao Xiang Zhu, and R. Bamler. Optimal estimation of distributed scatterer phase history parameters from meter-resolution SAR data. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 3468-3471, 2011. [bibtex-key = Wang2011] [bibtex-entry]


  1251. Urs Wegmuller, Charles L. Werner, Othmar Frey, Tazio Strozzi, and Maurizio Santoro. Multi-pass ERS-ENVISAT cross-interferometry methods and results. In Proc. FRINGE 2011, ESA SP-697, Frascati, Italy, September 2011. Keyword(s): SAR Processing, Interferometry, InSAR, SAR Interferometry, Multi-pass InSAR, ERS, ENVISAT, ASAR, ERS-ENVISAT, C-Band, Spaceborne SAR.
    Abstract: ERS-ENVISAT Tandem (EET) cross-interferometry (CInSAR) pairs are characterized by long 2km baselines and short 28 minute time intervals. Over some sites multiple pairs are available. In our work we discuss multi-pass interferometric techniques and investigate for several applications, including DEM generation, mapping of fast motions and grounding line mapping for Antarctic ice sheets, the use of multiple EET pairs.
    [bibtex-key = wegmullerWernerFreyStrozziSantoro2011] [bibtex-entry]


  1252. Charles L. Werner, Urs Wegmuller, Othmar Frey, and Maurizio Santoro. Interferometric processing of PALSAR Wide-Beam SCANSAR Data. In Proc. FRINGE 2011, ESA SP-697, Frascati, Italy, September 2011. Keyword(s): SAR Processing, Interferometry, InSAR, SAR Interferometry, PALSAR, ALOS PALSAR, ScanSAR, L-Band, Spaceborne SAR.
    Abstract: Processing of ScanSAR data for interferometric applications requires careful attention to the phase and position accuracy to obtain interferometric products with high correlation and continuous phase across the bursts. We describe an interferometric processing system developed for ScanSAR data acquired by the ALOS PALSAR instrument able to produce differential interferometric products with 350 km swath width that are without visible phase discontinuities between ScanSAR beams.
    [bibtex-key = wernerWegmullerFreySantoro2011] [bibtex-entry]


  1253. Xiao Xiang Zhu and R. Bamler. A fundamental bound for super-resolution --- with application to3D SAR imaging. In Proc. Joint Urban Remote Sensing Event (JURSE), pages 181-184, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2011f] [bibtex-entry]


  1254. Xiao Xiang Zhu and R. Bamler. Multi-component nonlinear motion estimation in differential SAR tomography - the time-warp method. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 2409-2412, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2011a] [bibtex-entry]


  1255. Xiao Xiang Zhu and R. Bamler. Sparse reconstrcution techniques for SAR tomography. In Proc. 17th Int Digital Signal Processing (DSP) Conf, pages 1-8, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2011e] [bibtex-entry]


  1256. Xiao Xiang Zhu and R. Bamler. Within the resolution cell: Super-resolution in tomographic SAR imaging. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 2401-2404, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2011c] [bibtex-entry]


  1257. Xiao Xiang Zhu, Xuan Wang, and Richard Bamler. Compressive sensing for image fusion - with application to pan-sharpening. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 2793-2796, 2011. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2011] [bibtex-entry]


  1258. R. Brcic, A. Parizzi, M. Eineder, R. Bamler, and F. Meyer. Estimation and compensation of ionospheric delay for SAR interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 2908-2911, July 2010. Keyword(s): compensation, data acquisition, ionospheric electromagnetic wave propagation, radar interferometry, radiowave propagation, remote sensing by radar, spaceborne radar, synthetic aperture radar, L-band ALOS-PALSAR acquisition, L-band system, SAR interferometry, SAR signal propagation, X-band frequency, dispersive effect, ionospheric delay compensation, ionospheric delay estimation, phase error, spaceborne SAR system, synthetic aperture rada, Azimuth, Bandwidth, Delay, Estimation, Frequency estimation, Ionosphere, L-band, delta-k, ionosphere, split-spectrum, wideband interferometry.
    Abstract: For spaceborne SAR (Synthetic Aperture Radar) systems, the dispersive effects of the ionosphere on the propagation of the SAR signal can be a significant source of phase error. While at X-band frequencies the effects are small, current and future L-band systems would benefit from ionospheric compensation. We consider two ways to estimate the ionospheric delay in SAR signals and evaluate them on L-band ALOS-PALSAR acquisitions.
    [bibtex-key = brcicParizziEinederBamlerMeyerIGARSS2010IonoDelayInSAR] [bibtex-entry]


  1259. E. Candes, Xiaodong Li, Yi Ma, and J. Wright. Robust principal component analysis?: Recovering low-rank matrices from sparse errors. In Proc. IEEE Sensor Array and Multichannel Signal Processing Workshop, pages 201-204, October 2010. Keyword(s): bioinformatics, computer vision, low rank data matrix, nonvanishing fraction, nuclear norm, positive fraction, robust principal component analysis, simple convex program, weighted combination, matrix algebra, principal component analysis;.
    Abstract: The problem of recovering a low-rank data matrix from corrupted observations arises in many application areas, including computer vision, system identification, and bioinformatics. Recently it was shown that low-rank matrices satisfying an appropriate incoherence condition can be exactly recovered from non-vanishing fractions of errors by solving a simple convex program, Principal Component Pursuit, which minimizes a weighted combination of the nuclear norm and the #x2113;1 norm of the corruption. Our methodology and results suggest a principled approach to robust principal component analysis, since they show that one can efficiently and exactly recover the principal components of a low-rank data matrix even when a positive fraction of the entries are corrupted. These results extend to the case where a fraction of entries are missing as well.
    [bibtex-key = 5606734] [bibtex-entry]


  1260. N. Chamberlain, H. Ghaemi, L. Giersch, L. Harcke, R. Hodges, J. Hoffman, W. Johnson, R. Jordan, B. Khayatian, P. Rosen, G. Sadowy, S. Shaffer, Y. Shen, L. Veilleux, and P. Wu. The DESDynI synthetic aperture radar array-fed reflector antenna. In Proc. IEEE Int. Symposium on Phased Array Systems and Technology (ARRAY), pages 381-386, October 2010. Keyword(s): DESDynl, Earth-orbit remote sensing, NASA, SweepSAR, active switched array, array-fed reflector antenna, lidar instruments, patch antenna, radar imaging, synthetic aperture radar antenna, active antenna arrays, aperture antennas, microstrip antenna arrays, radar antennas, reflector antenna feeds, synthetic aperture radar.
    Abstract: DESDynI is a mission being developed by NASA with radar and lidar instruments for Earth-orbit remote sensing. This paper focuses on the design of a large-aperture antenna for the radar instrument. The antenna comprises a deployable reflector antenna and an active switched array of patch elements fed by transmit / receive modules. The antenna and radar architecture facilitates a new mode of synthetic aperture radar imaging called `SweepSAR'. A system-level description of the antenna is provided, along with predictions of antenna performance.
    [bibtex-key = chamberlainGhaemiGierschHarckeEtAl2010DESDynI] [bibtex-entry]


  1261. G. C. Eastland, T. M. Marston, and P. L. Marston. Evolution of acoustic feature timing and imaging for different cylinder exposures and applications of reversible SAS filtering. In Proc. OCEANS 2010 MTS/IEEE SEATTLE, pages 1-4, September 2010. Keyword(s): acoustic wave scattering, filtering theory, sonar imaging, synthetic aperture sonar, acoustic feature imaging, acoustic feature timing, acoustic paths, acoustic scattering, cylinder exposures, reversible SAS filtering, timing slope, Geometry, Imaging, Receivers, Scattering, Synthetic aperture sonar, Timing, Transducers. [bibtex-key = Eastland2010] [bibtex-entry]


  1262. A. L. Espana, K. L. Williams, S. G. Kargl, M. Zampolli, T. M. Marston, and P. L. Marston. Measurements and modeling of the acoustic scattering from an aluminum pipe in the free field and in contact with a sand sediment. In Proc. OCEANS 2010 MTS/IEEE SEATTLE, pages 1-5, September 2010. Keyword(s): acoustic wave scattering, finite element analysis, sediments, sonar imaging, spatial filters, synthetic aperture sonar, FE calculation, SAS images, acoustic scattering measurement, acoustic scattering modeling, aluminum pipe, elastic contribution, elastic response, finite element model, flattened sand sediment, free field, fresh water pond, monostatic scattering, spatial filter boundary, specular response, synthetic aperture sonar techniques, target strength, Aluminum, Arrays, Finite element methods, Rails, Scattering, Sediments, Synthetic aperture sonar. [bibtex-key = Espana2010] [bibtex-entry]


  1263. Othmar Frey. SAR Imaging in the Time-Domain for Nonlinear Sensor Trajectories and SAR Tomography. In Proc. CEOS SAR Calibration and Validation Workshop, Zurich, Switzerland, August 2010. Note: Abstract.[bibtex-key = freyCEOSAbstract2010:Tomo] [bibtex-entry]


  1264. Othmar Frey and Erich Meier. 3D SAR Imaging of a Forest Using Airborne MB-SAR Data at L- and P-Band: Data Processing and Analysis. In Proc. EUSAR 2010 - 8th European Conference on Synthetic Aperture Radar, pages 166-169, 2010. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: Using a time-domain back-projection based focusing algorithm in combination with three tomographic focusing techniques (multilook standard beamforming, robust Capon beamforming, and MUSIC) a 3D volume containing a forested area has been tomographically imaged at L- and P-band. In this paper, we present further results and insights obtained by processing and analyzing these data sets with respect to the localization of the scattering sources using the three different focusing techniques, as well as for both, the two frequency bands and the different polarimetric channels.
    [bibtex-key = freyMeierEUSAR2010:Tomo] [bibtex-entry]


  1265. Othmar Frey and Erich Meier. Analyzing Tomographic SAR Data of a Forest With Respect to Frequency, Polarization, and Focusing Techniques. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 150-153, July 2010. Note: Invited Paper. Keyword(s): Airborne radar, Array signal processing, Capon, Capon beamformer, L-band, P-band, SAR processing, SAR tomography, beamforming, Focusing, forestry, interferometry, InSAR, multibaseline, multiple signal classification, MUSIC, polarimetry, Remote Sensing, synthetic aperture radar, SAR, three-dimensional imaging, 3-D imaging, time-domain back-projection, TDBP, tomography, Vegetation.
    Abstract: In this paper, two fully-polarimetric tomographic SAR data sets of a forested area, at L-band and P-band, are analyzed with respect to the localization of scattering sources and scattering mechanisms. In particular, the 3D SAR data is examined regarding the performance of three different tomographic focusing techniques multilook standard beamforming, robust Capon beamforming, and MUSIC, as well as for both, the two frequency bands and the different polarimetric channels.
    [bibtex-key = freyMeier2010Igarss:Tomo] [bibtex-entry]


  1266. LeRoy A. Gorham and Brian D. Rigling. Dual format algorithm for monostatic SAR. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 7699, pages 769905, 2010. SPIE. Keyword(s): SAR Processing, Dual Format Algorithm, DFA, Polar Format Algorithm, PFA, Spotlight SAR, Spotlight-mode data. [bibtex-key = gorhamRiglingDualFormatAlgorithm2010] [bibtex-entry]


  1267. S. Hantscher, S. Lang, M. Hägelen, and H. Essen. 94 GHz person scanner with circular aperture as part of a new sensor concept on airports. In International Radar Symposium, pages 1-4, June 2010. Keyword(s): SAR Processing, W-Band, Fraunhofer, Airports, Apertures, Radar imaging, Radar tracking, Security, Synthetic aperture radar, Airport security, Radar, Synthetic Aperture Radar.
    Abstract: In the following paper, a new concept for increasing the aiport security is proposed. As a part of this innovative system, a W-band person scanner with a circular aperture for 360 deg scans has been built up. The usage of the Synthetic Aperture Radar (SAR) principle allows the detection of concealed objects with a resolution of 3mm in azimuth and 3 cm in range direction.
    [bibtex-key = hantscherLangHaegelenEssenIRS2010WBANDPersonScanner] [bibtex-entry]


  1268. Leif Harcke, L. Weintraub, Sang-Ho Yun, R. Dickinson, E. Gurrola, Scott Hensley, and N. Marechal. Spotlight-mode synthetic aperture radar processing for high-resolution lunar mapping. In Proc. IEEE Radar Conference, pages 1260-1264, May 2010. Keyword(s): AD 2008 to 2009, Goldstone Solar System Radar, NASA LCROSS mission impact site, autofocus technique, time-domain back-projection, bistatic time-domain backprojection technique, bistatic SAR, change detection techniques, fast-time Doppler removal, focus-plane motion, geolocation, high-resolution lunar mapping, local topography, lunar poles, phase errors, polar format algorithm, preprocessing system, radar interferometry, radar mapping, resolution cells, spotlight imaging techniques, spotlight-mode synthetic aperture radar processing, topographic mapping, geophysical equipment, radar interferometry, remote sensing by radar, synthetic aperture radar.
    Abstract: During the 2008-2009 year, the Goldstone Solar System Radar was upgraded to support radar mapping of the lunar poles at 4 m resolution. The finer resolution of the new system and the accompanying migration through resolution cells called for spotlight, rather than delay-Doppler, imaging techniques. A new pre-processing system supports fast-time Doppler removal and motion compensation to a point. Two spotlight imaging techniques which compensate for phase errors due to (i) out of focus-plane motion of the radar and (ii) local topography, have been implemented and tested. One is based on the polar format algorithm followed by a unique autofocus technique, the other is a full bistatic time-domain backprojection technique. The processing system yields imagery of the specified resolution. Products enabled by this new system include topographic mapping through radar interferometry, and change detection techniques (amplitude and coherent change) for geolocation of the NASA LCROSS mission impact site.
    [bibtex-key = harckeWeintraubYunDickinsonGurrolaHensleyMarechal2010] [bibtex-entry]


  1269. Hans Hellsten, Patrick Dammert, and Anders Ahlander. Autofocus in fast factorized backprojection for processing of SAR images when geometry parameters are unknown. In Proc. IEEE Radar Conf., pages 603-608, May 2010. Keyword(s): SAR Processing, Autofocus, Time-Domain Back-Projection, TDBP, FFBP, SAR image processing, antenna path parameters, autofocus, fast factorized backprojection, radar imaging, synthetic aperture radar.
    Abstract: This paper introduces a new autofocus method for high-resolution SAR systems. The new method relies on varying antenna path parameters, i.e. the cause of the focusing problem. The variation and determination of antenna path parameters is computed and structured by incorporating the method into the framework of fast factorized backprojection, and thus also blending deterministic focus and autofocus into one method. The new autofocus has been tested with wavelength-resolution SAR data with good results.
    [bibtex-key = hellstenDammertAhlander2010AutofocusFFBP] [bibtex-entry]


  1270. Scott Hensley, E. Gurrola, Leif Harcke, M. Slade, K. Quirk, M. Srinivasan, C. Lee, Sang-Ho Yun, J. Jao, B. Wilson, E. De Jong, N. Marechal, L. Weintraub, R. Dickinson, R. Bloom, G. Karamyan, and A. Lilje. Lunar topographic mapping using a new high resolution mode for the GSSR radar. In Proc. IEEE Radar Conference, pages 464-469, May 2010. Keyword(s): Doppler image formation technique, Earth based radar interferometric measurement, Goldstone Solar System Radar, Moon topography mapping, autofocusing method, bandwidth 40 MHz, geodetic control, high resolution mode, interferograms, lidar topography maps, lunar topographic mapping, lunar topographic maps, range image formation technique, spotlight mode processing, Doppler radar, Moon, astronomical image processing, image resolution, optical radar, radar imaging, radar interferometry, radar resolution.
    Abstract: Mapping the Moon's topography using Earth based radar interferometric measurements by the Goldstone Solar System Radar (GSSR) has been done several times since the mid 1990s. In 2008 we reported at this conference the generation of lunar topographic maps having approximately 4 m height accuracy at a horizontal posting of 40 m. Since then GSSR radar has been improved to allow 40 MHz bandwidth imaging and consequently obtained images and interferograms with a resolution of about 4 m in range by 5 m in azimuth. The long synthetic aperture times of approximately 90 minutes in duration necessitated a migration from range/Doppler image formation techniques to spotlight mode processing and autofocusing methods. The improved resolution imagery should permit the generation of topographic maps with a factor of two better spatial resolution with about same height accuracy. Coupled the with the recent availability of new lidar topography maps of the lunar surface made by orbiting satellites of Japan and the United States the geodetic control of the radar generated maps products can be improved dramatically. This paper will discuss the hardware and software improvements made to the GSSR and present some of the new high resolution products.
    [bibtex-key = hensleyGurrolaHarckeSladeEtAl2010LunarTopoMapping] [bibtex-entry]


  1271. Yue Huang, Laurent Ferro-Famil, and Andreas Reigber. Under foliage object imaging using SAR tomography and polarimetric spectral estimators. In Proc. EUSAR, pages 1-4, June 2010. Keyword(s): SAR Processing, SAR Tomography, Forestry, FOPEN, Tomography.
    Abstract: This paper addresses the imaging of objects located under a forest cover using Polarimetric SAR Tomography (POLTOM-SAR) at L band. High-Resolution (HR) spectral estimators, able to accurately separate multiple scattering centers in the vertical direction, are used to separate the response of objects and vehicles embedded in a volumetric background. Polarimetric spectral analysis techniques are introduced, and are shown to both improve the estimation accuracy of the vertical position of artificial scatterers and natural environments and provide optimal polarimetric features that may be used to further characterize the media under analysis. The effectiveness of the proposed approaches is demonstrated using tomograms derived from fully polarimetric L-band airborne data acquired by DLR's E-SAR system over the test site of Dornstetten, Germany.
    [bibtex-key = huangFerroFamilReigberEUSAR2010:Tomo] [bibtex-entry]


  1272. Charles V. Jakowatz, Daniel E. Wahl, and David A. Yocky. A beamforming algorithm for bistatic SAR image formation. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 7699, pages 769902, 2010. SPIE. Keyword(s): SAR Processing, Bistatic SAR, Bistatic Spotlight-mode SAR, Autofocus, Autofocus in the TDBP Framework, Back-projection, Time-Domain Back-Projection, TDBP, Fast Back-projection, Fast Factorized Back-Projection, FFBP, Spotlight SAR, Spotlight-mode data, Beamforming. [bibtex-key = jakowatzWahlYockyBeamformingBistatic2010] [bibtex-entry]


  1273. Fabrizio Lombardini, Francesco Cai, and Matteo Pardini. Tomographic Analyses of Non-stationary Volumetric Scattering. In Proc. EUSAR, pages 1-4, June 2010. Keyword(s): SAR Processing, SAR Tomography, Forestry, Tomography.
    Abstract: Much interest is continuing to grow in the advanced differential tomography framework which crosses differential SAR interferometry with 3D multibaseline tomography, producing `space-time' signatures of scattering dynamics in the SAR cell. In this paper, the important effect of temporal decorrelation during the repeat pass multibaseline acquisition is analyzed on superresolution (MUSIC) tomography. Moreover, a differential tomographic processor is proposed to get the new functionalities of forest volume tomography robust to temporal decorrelation, and of subcanopy ground subsidence monitoring. To this aim, a generalized MUSIC method matched to non line spectra is adapted to the 2-D domain. Simulated and first airborne real data results prove the concept.
    [bibtex-key = lombardiniCaiPardiniEUSAR2010:Tomo] [bibtex-entry]


  1274. T. M. Marston, P. L. Marston, and K. L. Williams. Scattering resonances, filtering with reversible SAS processing, and applications of quantitative ray theory. In Proc. OCEANS 2010 MTS/IEEE SEATTLE, pages 1-9, September 2010. Keyword(s): acoustic wave scattering, deconvolution, elasticity, feature extraction, geophysical image processing, image classification, sonar imaging, spatial filters, synthetic aperture sonar, SAS image processing, acoustic spectral content, hybrid processing technique, quantitative ray theory, scattering resonance, spatial filter, spectral feature, structural resonance, Aluminum, Approximation methods, Backscatter, Feature extraction, Scattering, Synthetic aperture sonar. [bibtex-key = Marston2010] [bibtex-entry]


  1275. Josef Mittermayer, Pau Prats, Davide D'Aria, Riccardo Piantanida, S. Sauer, Andrea Monti Guarnieri, Evert Attema, and Paul Snoeij. TOPS Sentinel-1 and TerraSAR-X Processor Comparison based on Simulated Data. In Synthetic Aperture Radar (EUSAR), 2010 8th European Conference on, pages 1-4, June 2010. Keyword(s): SAR Processing, TOPS, Terrain Observation by Progressive Scans, Antennas, Azimuth, Chirp, Floors, Image resolution, Noise, Remote sensing.
    Abstract: The paper reports about the comparison of the Sentinel-1 Prototype TOPS Processor with the Experimental TerraSAR-X TOPS processor. The comparison is based on simulated raw data generated from TerraSAR-X and Sentinel-1 parameter scenarios. Fundamental impulse response parameters were investigated in point target scenarios. Scenarios with point targets on top a noise floor allowed for comparison of burst images by means of a cross-interferogram. The comparison shows good accordance between the processing results from both processors.
    [bibtex-key = mittermayerPratsdAriaPiantanidaSauerMontiGuarnieriAttemaSnoeijEUSAR2010TOPSSentinelVsTerraSARX] [bibtex-entry]


  1276. D. Reale, G. Fornaro, A. Pauciullo, Xiao Xiang Zhu, N. Adam, and R. Bamler. Advanced techniques and new high resolution SAR sensors for monitoring urban areas. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 1800-1803, 2010. [bibtex-key = Reale2010] [bibtex-entry]


  1277. Paul A. Rosen, Scott Hensley, and Curtis Chen. Measurement and mitigation of the ionosphere in L-band Interferometric SAR data. In 2010 IEEE Radar Conference, pages 1459-1463, May 2010. Keyword(s): SAR Processing, cartography, image registration, ionosphere, ionospheric measuring apparatus, radar imaging, radar interferometry, synthetic aperture radar, ALOS PALSAR data, L-band InSAR data, PALSAR spectral band, bandwidth 28 MHz, deformation maps, differential TEC estimation, dispersive medium, earth changing surface, image registration, ionosphere measurement, ionosphere mitigation, multifrequency split-spectrum processing technique, nondispersive effects, pixel-by-pixel observation, radar waveform, relative phase change measurement, satellite-based repeat-pass interferometric synthetic aperture radar, split spectrum technique, subtle deformation signatures, synoptic high spatial resolution, Atmosphere, Atmospheric measurements, Delay effects, Dispersion, Earth, Ionosphere, L-band, Phase measurement, Spatial resolution, Synthetic aperture radar interferometry.
    Abstract: Satellite-based repeat-pass Interferometric Synthetic Aperture Radar (InSAR) provides a synoptic high spatial resolution perspective of Earth's changing surface, permitting one to view large areas quickly and efficiently. By measuring relative phase change from one observation to the next on a pixel-by-pixel basis, maps of deformation and change can be derived. Variability of the atmosphere and the ionosphere leads to phase/time delays that are present in the data that can mask many of the subtle deformation signatures of interest, so methods for mitigation of these effects are important. Many of these effects have been observed in existing ALOS PALSAR data, and studies are underway to characterize and mitigate the ionosphere using these data. Since the ionosphere is a dispersive medium, it is possible in principle distinguish the ionospheric signatures from the non-dispersive effects of deformation and the atmosphere. In this paper, we describe a method for mapping the ionosphere in InSAR data based on a multi-frequency split-spectrum processing technique. We examine a number of PALSAR data sets, including fully polarimetric and single-polarization 28 MHz bandwidth data, where anomalous effects in phase, amplitude and image registration have been observed. We demonstrate the estimation of the ionosphere by means of the split spectrum technique for estimating differential TEC, whereby a radar waveform is transmitted over the full PALSAR spectral band and widely separated portions of the receive spectrum are processed independently and compared for dispersive effects, and quantify its performance.
    [bibtex-key = rosenHensleyChenIEEERadarCon2010IonosphereMitigationInSAR] [bibtex-entry]


  1278. David Small, Nuno Miranda, Lukas Zuberbühler, Adrian Schubert, and Erich Meier. Terrain-corrected Gamma: Improved thematic land-cover retrieval for SAR with robust radiometric terrain correction. In Proc. ESA Living Planet Symp., ESA SP-686, Bergen, Norway, July 2010. [bibtex-key = smallMirandaZuberbuehlerSchubertMeier2010:RadiometricNormalization] [bibtex-entry]


  1279. Charles L. Werner, Andreas Wiesmann, Tazio Strozzi, Martin Schneebeli, and Christian Matzler. The SnowScat ground-based polarimetric scatterometer: Calibration and initial measurements from Davos Switzerland. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 2363-2366, July 2010. Keyword(s): SnowScat, KuScat, calibration, geophysical equipment, radar polarimetry, snow, COld REgions Hydrology High-resolution Observatory Mission, Davos Switzerland, Ku-Band, SNOWSCAT ground-based polarimetric scatterometer, SnowScat, VH polarization, VV polarization, X-Band, calibration, dry snow, dual frequency radar, ground-based coherent polarimetric scatterometer, snow water equivalent retrieval algorithms, Antenna measurements, Antennas, Calibration, Frequency measurement, Instruments, Radar measurements, Snow.
    Abstract: The COld REgions Hydrology High-resolution Observatory (CoReH2O) Mission proposes a dual frequency radar operating at 9.6 and 17 GHz utilizing VV and VH polarization [1]. By combining Xand Ku-Band with both coand cross-polarization diversity it is possible to estimate the Snow Water Equivalent of dry snow. To support this proposed mission, ESA has sponsored the development of a ground-based coherent polarimetric scatterometer operating over the 9-18 GHz frequency range. ESA is supporting campaigns to acquire and process data using SnowScat for validation of Snow Water Equivalent (SWE) retrieval algorithms.
    [bibtex-key = wernerWiesmannStrozziSchneebeliMatzlerIGARSS2010] [bibtex-entry]


  1280. Andreas Wiesmann, Charles L. Werner, Tazio Strozzi, Christian Matzler, Thomas Nagler, Helmut Rott, Martin Schneebeli, and Urs Wegmuller. SnowScat, X- to Ku-Band Scatterometer Development. In Proc. ESA Living Planet Symposium, June 2010. Keyword(s): SnowScat, KuScat, backscatter, hydrological techniques, radiometry, remote sensing by radar, snow, spaceborne radar, C-band SAR satellite systems, ESA CoRe-H2O mission, Ku-band scatterometer, Swiss Alps, X-band scatterometer, backscatter information, backscattering signal, dry snow cover, dual frequency radar, frequency 18 GHz, frequency 9 GHz, mobile scatterometer, snow coverage, snow liquid water content, snow structure, spaceborne active microwave remote sensing, Backscatter, Frequency, Ground support, Radar measurements, Remote sensing, Satellites, Signal generators, Snow, Spaceborne radar, Water storage, Scatterometer, Snow, backscatter, snow grain.
    Abstract: The CoreH2O mission was selected, in May 2006, for pre-phase A and in 2009 for Phase-A mission feasibility study following a call for Earth Explorer Core mission ideas. Though various components of the cryosphere have been observed by non-dedicated satellite missions for years, and a dedicated European mission, CryoSat, is in preparation to observe fluctuations of the masses of ice sheets and sea ice, still large gaps in spatially detailed observation of key parameters and processes of global snow and ice masses and high latitude environment remain. The ESA CoRe-H2O mission aims at closing these gaps, providing high resolution data on extent, mass, and metamorphic state of snow and ice and on surface water extent at regular repeat intervals over extended areas. The CoRe-H2O mission objectives lead to a set of observational requirements for seasonal snow cover, glaciers and ice-sheets, fresh-water ice, sea ice and surface water. These requirements shall be achieved using a dual frequency X- (9.6 GHz), and Ku-band (17.2) SAR with two polarizations VV, VH, operated in ScanSAR mode. To some degree retrieval methods based on these frequencies have been demonstrated for the observation requirements. Recently, two parallel activities were undertaken for developing scattering and propagation models of snow-pack at Ku-band to improve the understanding of the scattering behavior of targets at this frequency. However, there is a lack of validation data to compare and validate these models. From field work and airborne campaigns many backscatter data sets of snow are available at X- band. In recent years measurements have also been acquired at Ku-band in the 14 to 15 GHz range. However, there is almost a complete lack of X- and Ku-band data over snow. Here we will present the work conducted within ESA/ESTEC project 20716/07/NL/EL whose purpose was to help close this important gap through the development of a ground-based scatterometer covering X and Ku-band, and test the instrument during a dedicated campaign to demonstrate the performance of the instrument in cold and harsh winter environment. The proposed and implemented solution was to build a coherent stepped frequency scatterometer covering 9 to 18 GHz. The instruments allows not only to cover the CoRe-H2O relevant channels but also the frequency band used for Cryosat. Another advantage of the wide bandwidth is that it allows to investigate the scattering behavior of the snowpack in detail by investigating the time domain data. The instrument has an embedded system controller and data storage, so that it runs autonomously also during communication link outages and can be controlled ober the Internet. The system was successfully tested during a dedicated field campaign at the high alpine test site Weissfluhjoch Davos in Switzerland. For 3 months the system was continuously operating from a 12m mast. To get comparable snow free measurements, the mesasurements were repeated at the same location during summer at snow free conditions. The instrument proved to withstand temperatures as low as 30 deg C, and wind up to 80 km/h. Detailed ground information complements the acquired radar data. To improve the data quality a campaign was conducted in the anechoic chamber at ESTEC. The campaign included detailed antenna measurements and calibration target measurements.
    [bibtex-key = wiesmannEtAl2010SnowScat] [bibtex-entry]


  1281. Xiao Xiang Zhu and Richard Bamler. Compressive sensing for high resolution differential SAR tomography - the SL1MMER algorithm. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. (IGARSS), pages 17-20, 2010. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2010c] [bibtex-entry]


  1282. Nico Adam, Xiao Xiang Zhu, and Richard Bamler. Coherent stacking with TerraSAR-X imagery in urban areas. In Proc. Joint Urban Remote Sensing Event, pages 1-6, 2009. [bibtex-key = AdamZhuBamler2009CohStacking] [bibtex-entry]


  1283. Nico Adam, Xiao Xiang Zhu, C. Minet, W. Liebhart, M. Eineder, and R. Bamler. Techniques and examples for the 3D reconstruction of complex scattering situations using TerraSAR-X. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, 2009. [bibtex-key = Adam2009a] [bibtex-entry]


  1284. S. Auer, Xiao Xiang Zhu, S. Hinz, and R. Bamler. 3D analysis of scattering effects based on Ray Tracing techniques. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, 2009. [bibtex-key = auerZhuHinzBamler2009] [bibtex-entry]


  1285. Federica Bordoni, Marwan Younis, and Gerhard Krieger. Calibration Issue in SMART Synthetic Aperture Radar Based on Scan-On-Receive. In Advanced RF Sensors for Earth Observation (ARSI), Noordwijk, The Netherlands, 2009. ESA ESTEC. Keyword(s): SAR Processing, Scan-on-Receive, SCORE, Synthetic Aperture Radar, Digital Beam-Forming, DBF, Multichannel Receiver.
    Abstract: A new spaceborne Synthetic Aperture Radar (SAR) system based on SCan-On-REceive (SCORE) algorithm has been recently proposed in order to overcome the trade-off between spatial resolution and swath wide of current SAR systems. The compound architecture of the receiver, which employs multiple channels and Digital Beam-Forming technique, places new challenges to spaceborne SAR internal calibration (Cal) and requires the definition of a new Cal approach. In this paper a novel method for onboard internal Cal of the multichannel receiver of a SAR system based on SCORE is proposed and numerically analyzed.
    [bibtex-key = BordoniYounisKriegerARSI2009] [bibtex-entry]


  1286. Federica Bordoni, Marwan Younis, E.M. Varona, and Gerhard Krieger. Adaptive scan-on-receive based on spatial spectral estimation for high-resolution, wide-swath Synthetic Aperture Radar. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 64-67, July 2009. Keyword(s): SAR Processing, Cramer Rao Lower Bound, Monte Carlo simulations, SCORE, scan-on-receive, Smart Multi-Aperture Radar Techniques, adaptive scan-on-receive, beam steering, high-resolution wide-swath synthetic aperture radar, signal power, spaceborne SMART SAR systems, spatial spectral estimation, beam steering, geophysical techniques, synthetic aperture radar.
    Abstract: Intensive research is currently ongoing in the field of Smart Multi-Aperture Radar Techniques (SMART) for high-resolution wide-swath Synthetic Aperture Radar (SAR) imaging. This work investigates the possibility of applying direction of arrival estimation methods to spaceborne SMART SAR systems, that employ receive beam steering. In particular, a new algorithm based on the actual spatial distribution of the received signal power is proposed. The performance of the algorithm is evaluated by Monte Carlo simulations and compared with that of the conventional scan-on-receive approach, in different operational scenarios. The Cramer Rao Lower Bound is also reported as a benchmark on the performance.
    [bibtex-key = bordoniYounisVaronaKrieger2009] [bibtex-entry]


  1287. Antonio De Maio, Gianfranco Fornaro, Antonio Pauciullo, and Diego Reale. Detection of double scatterers in SAR Tomography. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 172-175, July 2009. Keyword(s): SAR Processing, double scatterers, Bayesian Information Criterion, Generalized Likelihood Ratio Test, SAR Tomography, differential tomography, double scatterers, ground scatterers, high resolution radar systems, multi-dimensional SAR imaging, scatterers detection, SAR Interferometry, Multidimensional system, Spaceborne SAR, X-Band, Urban, Persistent Scatterer Interferometry, PSI, time series, geophysical image processing, image reconstruction, image resolution, radar imaging, radar interferometry, radar resolution, synthetic aperture radar, 3D reconstruction.
    Abstract: Multi-Dimensional (3D/4D) SAR imaging (SAR Tomography and Differential SAR Tomography) allows the localization and monitoring of ground scatterers, even interfering in the same azimuth-range pixel. Indeed, the presence of multiple scatterers has shown to affect even the performances of high resolution radar systems. In this paper we discuss two strategies for the detection of interfering scatterer pairs. The first one is based on the extension of the GLRT test already proposed for the detection of single scatterers, the second one is based on the BIC criteria commonly used in the contest of model order selection. Performances of the two decision schemes are evaluated on simulated data.
    [bibtex-key = deMaioFornaroPauciulloRealeIGARSS2009TomoDoubleScatterer] [bibtex-entry]


  1288. A. Elsherbini and K. Sarabandi. Topography of sand covered bedrock using two-frequency airborne interferometric SAR measurements. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, volume 2, pages II-250-II-253, July 2009. Keyword(s): airborne radar, geophysical prospecting, radar interferometry, remote sensing by radar, rocks, sand, synthetic aperture radar, terrain mapping, topography (Earth), InSAR processing, iterative algorithm, oil field search, sand covered bedrock, sand dunes, subsurface imaging, topography, two-frequency airborne interferometric SAR, Backscatter, Bandwidth, Frequency, Iterative algorithms, Petroleum, Receiving antennas, Signal design, Signal to noise ratio, Surface topography, Synthetic aperture radar interferometry, Interferometric synthetic aperture radar (InSAR), Subsurface Imaging, radar imaging. [bibtex-key = Elsherbini2009] [bibtex-entry]


  1289. H. Essen, M. Bräutigam, R. Sommer, A. Wahlen, W. Johannes, J. Wilcke, M. Schlechtweg, and A. Tessmann. SUMATRA, a W-band SAR for UAV application. In 2009 International Radar Conference Surveillance for a Safer World (RADAR 2009), pages 1-4, October 2009. Keyword(s): SAR Processing, W-Band, SUMATRA, Airborne SAR, UAV, Fraunhofer, airborne radar, aircraft, millimetre wave radar, remotely operated vehicles, synthetic aperture radar, HEMT amplifiers, IF amplifiers, LNA, SUMATRA-94, UAV application, data transmission equipment, frequency 94 GHz, miniaturized GPS, mixer, remotely piloted aircraft, synthetic aperture unmanned millimeterwave airborne test radar, w-band SAR, Airborne radar, Aircraft, Data communication, Global Positioning System, HEMTs, Hardware, Low-noise amplifiers, Power system modeling, Remote sensing, Unmanned aerial vehicles, HEMTAmplifiers, Millimeterwaves, SAR, UAV.
    Abstract: Based upon most recent advances in millimeterwave technology, especially monolithic integrated low noise or medium power HEMT amplifiers and an integrated receiver containing an LNA, Mixer and IF Amplifier, a miniaturized experimental radar at 94 GHz was designed with the aim to be used on board of a remotely piloted model aircraft. This highly advanced front-end technique was combined with of-the-shelf model aircraft hardware and miniaturized GPS and data transmission equipment which is readily available. Goal of the project is to demonstrate, that using modern 94-GHz front-end technique combined with achievable back-end components it is possible to set up a versatile SAR system usable for a wide range of remote sensing applications at medium range. The paper describes the current state of the research project SUMATRA-94 and gives some perspectives for future applications.
    [bibtex-key = EssenEtAlIEEERADARCONwbandSUMATRAARforUAV2009] [bibtex-entry]


  1290. Othmar Frey and Erich Meier. A Comparative Tomographic Evaluation of Airborne Multi-Baseline SAR Data at P- and L-Band. In IEEE Int. Geosci. and Remote Sens. Symp., 2009. Note: Invited Talk, Abstract. [bibtex-key = freyMeier09:IGARSSInvited] [bibtex-entry]


  1291. Leif J. Harcke. Time-domain backprojection for precise geodetic coding of spaceborne SAR imagery. In Radar Conference, 2009 IEEE, pages 1-3, May 2009. Keyword(s): SAR Processing, TDBP, Time-Domain Back-Projection, ALOS/PALSAR instrument, D-InSAR, WGS-84 Cartesian system, backprojection image formation, differential radar interferometry, geocoded image production, geodetic coding, position measurement, precise-orbit determination, reflector, space-borne SAR, spaceborne SAR imagery, time-domain back-projection, geodesy, position measurement, radar imaging, radar interferometry, spaceborne radar, synthetic aperture radar.
    Abstract: A new era of precise-orbit determination for space-borne SAR permits time-domain backprojection of the data for accurate geocoded image production. In this work, time-domain backprojection is applied to Level 1.0 data from the ALOS/PALSAR instrument to form imagery at two sites in southern California. The accuracy of the backprojection is verified by comparing the measured position of a corner reflector at a calibration site to its position in the formed SAR imagery. The observed offset of the corner reflector is lt;2 m in the range direction and 12 m in the cross-range or along track direction. Images backprojected in an absolute WGS-84 Cartesian system onto 1/3 arc second or 10 m posting digital elevation data exhibit no gross registration errors. This indicates that the backprojection image formation method may be useful for processing differential radar interferometry (D-InSAR) products, where topography terms must first be removed.
    [bibtex-key = harcke2009TDBP] [bibtex-entry]


  1292. Scott Hensley, T. Michel, M. Simard, C. Jones, R. Muellerschoen, C. Le, H. Zebker, and B. Chapman. Residual motion estimation for UAVSAR: Implications of an electronically scanned array. In 2009 IEEE Radar Conference, pages 1-5, May 2009. Keyword(s): SAR Processing, Global Positioning System, antenna arrays, motion estimation, radar interferometry, synthetic aperture radar, UAVSAR, along-track offsets, electronically scanned antenna array, repeat pass radar interferometry, residual motion estimation, surface deformation, Antenna measurements, Azimuth, Data engineering, Earth, Geophysical measurements, Instruments, L-band, Motion estimation, Radar interferometry, Solids.
    Abstract: The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in the ESTO Instrument Incubator Program and after 3 years of development has begun collecting engineering and science data. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In order to make geophysically useful repeat pass interferometric measurements it is necessary to reconstruct the repeat pass baseline with millimeter accuracy, however onboard motion metrology systems only achieve 5-15 cm accuracy. Thus it is necessary to recover the residual motion from the data itself. Algorithms for recovering the motion based on along-track offsets between the repeat pass interferometric pair of images were described in [3], [1] and [4]. Later these techniques were extended to use azimuth subbanded differential interferograms in [5]. This paper provides a derivation for the formula for the along-track offsets (or corresponding the subbanded differential phase), i.e. the relative displacement between two SAR images in a interferometric pair in the along track direction, as a function of baseline for systems employing an electronically scanned antenna. The standard formula for systems not employing electronically scanned antenna for the along-track offsets, Deltas, has the form in given equation where bc is the cross-track baseline, bh is the vertical baseline, thetaslscr is the look angle, thetasaz is the azimuth or squint angle, rho is the range and d = 1 for left looking systems and d = -1 for right looking systems. A key feature of this formula is the along-track offsets only range dependency is from the derivatives of the baseline with respect to along-track position. In the electronically scanned case this in no longer true and - an additional range dependency arises that is a function of the electronic steering angle.
    [bibtex-key = hensleyMichelSimardJonesMuellerschoenLeZebkerChapmanRadarCon2009UAVSARResidualMotionfromDPCA] [bibtex-entry]


  1293. S. Hensley, H. Zebker, C. Jones, T. Michel, R. Muellerschoen, and B. Chapman. First deformation results using the NASA/JPL UAVSAR instrument. In Proc. Asian-Pacific Conference on Synthetic Aperture Radar, pages 1051-1055, October 2009. Keyword(s): active antennas, radar interferometry, synthetic aperture radar, deformation results, NASA/JPL UAVSAR instrument, L-band actively electronically scanned antenna, ESTO instrument incubator program, solid earth applications, radar interferometry, Ka-band single-pass radar interferometer, NASA Gulfstream III aircraft, NASA Dryden Flight Research Center, repeat-pass radar interferometric applications, 1NU measurement data, NASA, Instruments, L-band, Antenna measurements, Testing, Aircraft, Airborne radar, Solids, Earth, Radar interferometry.
    Abstract: The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in NASA's ESTO Instrument Incubator Program and after 4 years of development has begun collecting science data for investigators. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In fact a Ka-band single-pass radar interferometer for making high precision topographic maps of ice sheets was developed and tested based to a large extent on components of the UAVSAR L-band radar. By designing the radar to be housed in an external unpressurized pod, it has the potential to be readily ported to many platforms. Initial testing is being carried out with the NASA Gulfstream III aircraft, which has been modified to accommodate the radar pod and has been equipped with precision autopilot capability developed by NASA Dryden Flight Research Center. With this the aircraft can fly within a 10 m diameter tube on any specified trajectory necessary for repeat-pass radar interferometric applications. To maintain the required pointing for repeat-pass interferometric applications we have employed an actively scanned antenna steered using INU measurement data. This paper presents a brief overview of some of the initial deformations made by the UAVSAR instrument.
    [bibtex-key = hensleyZebkerJonesMichelMuellerschoenChapman2009FirstDeformationResultsUAVSAR] [bibtex-entry]


  1294. F. Hélière, C.C. Lin, F. Fois, M. Davidson, A. Thompson, and P. Bensi. BIOMASS: A P-band SAR Earth explorer core mission candidate. In Proc. IEEE Radar Conf., pages 1-6, May 2009. Keyword(s): BIOMASS mission, Earth explorer core mission candidate, P-band, forest area, forest biomass, forest disturbances, frequency 425 MHz, global maps, synthetic aperture radar, terrestrial carbon cycle, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: The greatest uncertainties in the global carbon cycle involve estimating how carbon dioxide is taken up by land. The BIOMASS mission aims to improve the present assessment and future projection of the terrestrial carbon cycle by providing consistent global maps of forest biomass and forest area, forest disturbances and recovery with time, and the extent and evolution of the forest flooding. The BIOMASS primary objectives can be achieved through P-band (435 MHz) synthetic aperture radar (SAR) observations of global forest cover. Two parallel industrial studies at phase 0 level were awarded respectively to Astrium GmbH and Thales Alenia Space Italy. This paper presents the resulting system concepts as elaborated by the industrial teams. The result of the phase 0 was presented to the user community in January 2009 in Lisbon together with other Earth Explorer candidates for further down-selection. If successfully selected after phase 0 and phase A, BIOMASS will be launched during 2016.
    [bibtex-key = HeliereLinFoisDavidsonThompsonBensi2009:BIOMASSmission] [bibtex-entry]


  1295. Charles V. Jakowatz and Daniel E. Wahl. Considerations for autofocus of spotlight-mode SAR imagery created using a beamforming algorithm. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 7337, pages 73370A, 2009. SPIE. Keyword(s): SAR Processing, Autofocus, Autofocus in the TDBP Framework, Back-projection, Time-Domain Back-Projection, TDBP, Fast Back-projection, Fast-Factorized Back-Projection, FFBP, Spotlight SAR, Spotlight-mode data, Beamforming. [bibtex-key = jakowatzWahlAutofocusBeamformingSpotlight2009] [bibtex-entry]


  1296. W.T.K. Johnson, P.A. Rosen, S. Hensley, and A. Freeman. Radar designs for the DESDynI mission. In IEEE Radar Conference, pages 1-3, May 2009. Keyword(s): DESDynI mission, InSAR, Lidar, SweepSAR, dual polarization, ecosystem structure, ice dynamics, interferometric radar design, near-polar orbit, quadpolarization, solid Earth, spacecraft, vegetation, electromagnetic wave polarisation, geophysical techniques, ice, optical radar, radar interferometry, synthetic aperture radar.
    Abstract: The interferometric radar (InSAR) design has undergone several iterations in the years that this mission has been under consideration. In the most recent proposal the InSAR is combined with a Lidar and the mission is called deformation, ecosystem structure, and dynamics of ice (DESDynI). This spacecraft would be in a near-polar orbit around the Earth and repeatedly collect data to monitor changes in the solid Earth, vegetation, and ice. The radar is a repeat pass interferometric SAR with dual and quad polarization capability. This paper examines the state of the configuration of the radar including a ldquoSweepSARrdquo method that has been incorporated as the present baseline.
    [bibtex-key = JohnsonRosenHensleyFreeman2009:DESDynIRadarDesign] [bibtex-entry]


  1297. G. Krieger, I. Hajnsek, Konstantinos P. Papathanassiou, M. Eineder, M. Younis, F. De Zan, P. Prats, S. Huber, M. Werner, H. Fiedler, A. Freeman, P. Rosen, S. Hensley, W. Johnson, L. Veilleux, B. Grafmueller, R. Werninghaus, R. Bamler, and A. Moreira. The Tandem-L mission proposal: Monitoring Earth's dynamics with high resolution SAR interferometry. In Proc. IEEE Radar Conf., pages 1-6, May 2009. Keyword(s): Earth's Dynamics Monitoring, Tandem-L mission, advanced digital beamforming techniques, biomass inventories, glacier movement observations, global forest height, high data acquisition, high resolution SAR interferometry, innovative interferometric radar mission, millimetric displacements measurements, polarimetric radar mission, synthetic aperture radar, tectonic shifts, data acquisition, glaciology, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, tectonics, vegetation.
    Abstract: Tandem-L is a proposal for an innovative interferometric and polarimetric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important mission objectives are global forest height and biomass inventories, large scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify the dynamics of a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics. This paper provides an overview of the Tandem-L mission concept and its main application areas. Performance predictions show the great potential of Tandem-L to acquire a wide range of bio- and geophysical parameters with high accuracy on a global scale. Innovative aspects like the employment of advanced digital beamforming techniques to improve performance and coverage are discussed in detail.
    [bibtex-key = KriegerEtAl_DLR_and_JPL:tandemL] [bibtex-entry]


  1298. F. Lombardini and M. Pardini. Detection of scatterer multiplicity in spaceborne SAR tomography with array errors. In Radar Conference, 2009 IEEE, pages 1-6, May 2009. [bibtex-key = Lombardini2009] [bibtex-entry]


  1299. Antonio Moccia and Alfredo Renga. Hybrid space-airborne bistatic SAR geometric resolutions. In Lorenzo Bruzzone, Claudia Notarnicola, and Francesco Posa, editors, , volume 7477, pages 74771Y, 2009. SPIE. Keyword(s): SAR Processing, Bistatic SAR, Geometric Resolution, Geometric Bistatic Resolution, Bistatic Resolution, Resolution, Airborne SAR, Spaceborne SAR. [bibtex-key = mocciaRengaBistaticResolutions2010] [bibtex-entry]


  1300. Alberto Moreira, Gerhard Krieger, Irena Hajnsek, Kostas Papathanassiou, Michael Eineder, Francesco De Zan, Marwan Younis, and Marian Werner. Tandem-L: Monitoring the Earth's Dynamics with InSAR and Pol-InSAR. In Proc. PolInSAR, Frascati, Italy (ESA SP-668), January 2009. Keyword(s): Earth's Dynamics Monitoring, Tandem-L mission, advanced digital beamforming techniques, biomass inventories, glacier movement observations, global forest height, high data acquisition, high resolution SAR interferometry, innovative interferometric radar mission, millimetric displacements measurements, polarimetric radar mission, synthetic aperture radar, tectonic shifts, data acquisition, glaciology, radar interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, tectonics, vegetation.
    Abstract: Tandem-L is a proposal for an innovative interferometric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important application examples are global forest height and biomass inventories, measurements of Earth deformations due to tectonic processes, observations of 3-D structure changes in ice, and the monitoring of ocean surface currents. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its climate dynamics. The Tandem-L mission concept relies on a systematic data acquisition strategy using a pair of cooperating L-band SAR satellites flying in close formation. The satellite system is operated in two basic data acquisition modes: 1) The 3-D structure mode employs fully-polarimetric single-pass SAR interferometry (Pol-InSAR) to acquire structural parameters and quasi-tomographic images of volume scatterers like vegetation, sand, and ice. 2) The deformation mode employs repeat-pass interferometry (InSAR) in an ultra-wide swath mode to measure small shifts on the Earth surface with millimetric accuracy and short repetition intervals. This paper provides an overview of the Tandem-L mission concept and its main application areas. Performance predictions show the great potential of Tandem-L to acquire a wide range of bio- and geophysical products with high accuracy on a global scale. Furthermore, innovative technical aspects like for example the use of digital beamforming to improve performance and coverage will be presented.
    [bibtex-key = moreiraKriegerHajnsekPapathanassiouEinederDeZanYounisWerner:tandemLatPOLINSAR2009] [bibtex-entry]


  1301. Alessandro Parizzi, Xiaoying Cong, and Michael Eineder. First Results from Multifrequency Interferometry. A comparison of different decorrelation time constants at L-, C-, and X-Band. In Proc. FRINGE 2009, Frascati, Italy, pages 1-5, September 2009. Keyword(s): SAR Processing, Decorrelation, Temporal Decorrelation, Differential Interferometry, DInSAR, SAR Interferometry, Coherence.
    Abstract: The measurement of displacement from SAR images, using either interferometric or correlation techniques, is always carried out from a comparison between two or more acquisitions separated in time. It is therefore necessary to evaluate the level of phase decorrelation between the two acquisitions in order to understand the quality of the measures that can be obtained. The change of radar resolution cells in time is known to be the main responsible of the gradual decorrelation of the interferometric phase. However, a model that physically describes this process, is at the moment not available. Exploiting the test sites from GITEWS and Exupery, German projects focussed on determination of geo-risks, an analysis of the decorrelation time constants was performed on SAR data from PALSAR, ASAR and TerraSAR-X. Time series of the coherence have been computed and compared with simple statistical models. The time constants were analyzed as a measure of temporal decorrelation, in order to forecast the precision of displacement measures also for future missions. The extracted parameters were finally compared with the type of land covering using optical data and land use maps in order to determine a qualitative relationship between them and extend their validity in a more global context.
    [bibtex-key = parizziCongEinederFRINGE2009TempDecorrelation] [bibtex-entry]


  1302. L. Pipia, X. Fabregas, A. Aguasca, C. López-Martìnez, and J. J. Mallorquì. Polarimetric coherence optimization for interferometric differential applications. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, volume 5, pages 146-149, July 2009. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, data acquisition, optimisation, radar interferometry, radar polarimetry, DInSAR, X-band ground-based PolSAR acquisitions, deformation phase information, differential interferometric SAR, interferometric differential applications, optimized differential phase, polarimetric coherence optimization, polarimetric coherence-optimization techniques, synthetic PolSAR data, urban environment, zero-baseline fully-polarimetric data sets, Amplitude estimation, Analytical models, Convergence, Deformable models, Information retrieval, Interferometry, Optimization methods, Remote sensing, Scattering, Synthetic aperture radar, PolInSAR. [bibtex-key = Pipia2009a] [bibtex-entry]


  1303. R.K. Raney. DESDynI adopts hybrid polarity SAR architecture. In Radar Conference, 2009 IEEE, pages 1-4, May 2009. Keyword(s): DESDynI mission, L-band synthetic aperture radar, The National Research Council's Earth Science 2007 Decadal Survey, circular polarization, forest biomass, hybrid-polarity dual-polarised SAR, quadrature-polarimetric SAR mode, terrain surface deformation, polarisation, synthetic aperture radar.
    Abstract: DESDynl-Deformation, Ecosystem Structure, and Dynamics of Ice-is one of the first-tier missions recommended in The National Research Council's Earth Science 2007 Decadal Survey. DESDynI's L-band synthetic aperture radar (SAR) is designed to measure terrain surface deformation and forest biomass, hence its quadrature-polarimetric SAR mode. The objective to provide quantitative information on a global scale imposes severe requirements on the radar to maximize coverage and to sustain reliable operational calibration. These requirements are best served by the hybrid-polarity architecture, in which the radar transmits in circular polarization, and receives on two orthogonal linear polarizations, coherently, retaining their relative phase. This architecture offers many significant advantages over conventional all-linearly-polarized SARs. This paper reviews those advantages, summarizes key attributes of hybrid-polarity dual- and quadrature-polarized SARs including conditions under which the signal-to-noise ratio is conserved, and describes the evolution of this architecture from first principles.
    [bibtex-key = Raney2009:DESDynIHybridPolaritySAR] [bibtex-entry]


  1304. David Small, Nuno Miranda, and Erich Meier. A revised radiometric normalisation standard for SAR. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 4, pages 566-569, July 2009. Keyword(s): SAR Processing, ASAR, ERS-1, Earth ellipsoid model, Earth terrain-model, PALSAR, RADARSAT-1, SAR sensors, TerraSAR-X, beta nought SAR radiometry, elevation models, gamma nought SAR radiometry, image geolocation accuracy, radar cross section, radar imaging, radar measurements, radar scattering, radar terrain factors, radiometric calibration, radiometric normalisation standard, sigma nought SAR radiometry, terrain variations, radar cross-sections, radar imaging, radiometry, synthetic aperture radar, terrain mapping.
    Abstract: Improved geometric accuracy in SAR sensors implies that more complex models of the Earth may be used not only to geometrically rectify imagery, but also to more robustly calibrate their radiometry. Current beta, sigma, and gamma nought SAR radiometry conventions all assume a simple \E5\C0flat as Kansas\E5\C0 Earth ellipsoid model. We complement these simple models with improved radiometric calibration that accounts for local terrain variations. In the era of ERS-1 and RADARSAT-1, image geolocation accuracy was in the order of multiple samples, and tiepoint-free establishment of the relationship between radar and map geometries was not possible. Newer sensors such as ASAR, PALSAR, and TerraSAR-X all support accurate geolocation based on product annotations alone. We show that high geolocation accuracy, combined with availability of high-resolution accurate elevation models, enables a more robust radiometric calibration standard for modern SAR sensors that is based on gamma nought normalised using an Earth terrain-model.
    [bibtex-key = smallMirandaMeier2009:RadiometricNormalization] [bibtex-entry]


  1305. David Small, Nuno Miranda, and Erich Meier. Local incidence angle considered harmful. In Proc. CEOS SAR Cal/Val Workshop, Pasadena, CA, Nov. 17\D019 2009. [bibtex-key = smallMirandaMeierCEOS2009:RadiometricNormalization] [bibtex-entry]


  1306. Stefano Tebaldini and Fabio Rocca. On the impact of propagation disturbances on SAR Tomography: Analysis and compensation. In IEEE Radar Conference, pages 1-6, May 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry.
    Abstract: This paper is meant to discuss the role of propagation disturbances, such as those due to atmospheric disturbances or to residual platform motion, in SAR Tomography applications. It will be shown that phase stability requirements vary as a function of the number of targets within the system resolution cell, from which it follows that SAR Tomography applications require a higher phase stability, or a more accurate phase calibration, with respect to SAR Interferometry. Phase calibration will be discussed by comparing two approaches. The first is the traditional approach from Permanent Scatterers Interferometry, based on the assumption that it is possible to find stable targets in the imaged scene. The second is a new technique based on the Sum of Kronecker Products Decomposition, recently introduced in literature, which allows to isolate ground and volume contributions within the data provided the availability of multi-polarimetric acquisitions. As such, the latter approach is suited to carry out phase calibration in presence of volume scattering, as it is the case of forested areas. Experimental results will be provided basing on the analysis of the P-Band data-set relative to the forest site of Remningstorp, Sweden.
    [bibtex-key = tebaldiniRoccaRADARConf2009:TomoPropgationDisturbance] [bibtex-entry]


  1307. Stefano Tebaldini and Fabio Rocca. Polarimetric Options for SAR Tomography of Forested Areas. In Proc. of PolInSAR - 4th Int. Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry, Frascati, Italy, January 2009. ESA SP-668. Keyword(s): SAR Processing, SAR Tomography, Tomography, E-SAR, P-Band.
    Abstract: The aim of this paper is to discuss the potentialities of SAR Tomography to infer information about the vertical structure of forested areas, basing on the availability of either multi-polarimetric or single-polarimetric data. The core of the analysis is represented by a model of the second order statistics of the multi-baseline, multi-polarimetric data that describes the imaged scene as the superposition of two distributed scattering mechanisms. In this framework, the well posing of the Tomographic problem can be discussed in a simple fashion, by considering the number of equations that arise from the data-set with respect to the number of the unknowns required to solve the model. As a result, it follows that through the exploitation of strongly structured models the vertical structure can be retrieved basing on single-polarimetric data as well, provided that a sufficient number of baselines is available. Still, the availability of multi-polarimetric and multi-baseline data offers the possibility to face the problem of the retrieval of the vertical structure of each scattering mechanism from a very general point of view, allowing to relax many of the assumptions that are otherwise mandatory. In other words, the availability of multipolarimetric data makes it possible to exploit not only model based approaches, but also model free, and hybrid approaches. As a result of this analysis, a new general methodology will be presented for the processing of multi-polarimetric and multibaseline data, that is consistent the inversion procedures usually exploited in single-baseline PolInSAR in the case where only single baseline data are available. Experimental results will be shown basing on a data-set of 9 P-Band SAR images of the forest site of Remningstorp, Sweden, acquired under the framework of the ESA campaign BioSAR 2007.
    [bibtex-key = tebaldiniRocca2009PolInSAR:Tomo] [bibtex-entry]


  1308. A. Wyholt and Lars M. H. Ulander. Evaluating VHF-band SAR autofocus algorithms using a forest backscatter model. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 4, pages 9-12, July 2009. Keyword(s): SAR Processing, Autofocus, Time-Domain Back-Projection, TDBP, FFBP, SAR subimages, VHF-band SAR autofocus algorithms, fast factorized back-projection algorithm, forest backscatter model, forest clutter model, matching error, residual displacement errors, subimage matching, backscatter, focusing, geophysical image processing, image matching, remote sensing by radar, synthetic aperture radar.
    Abstract: The objective of this paper is to assess the accuracy of an autofocus method developed for the Fast Factorized Back-Projection (FFBP) algorithm in simulated scenarios. We specifically address the question whether correlation measurements between subimages will suffice in the focusing process in one arbitrary merging step. A forest clutter model is used together with a model of the impulse responses to simulate two SAR sub-images of a forest. Correlation is used for sub-image matching and residual displacement errors are compiled using simulation. We conclude that the matching error increases with increased number of trees per resolution cell but can be restored with a larger image size in the correlation measurements. We also conclude that the autofocus method will be successful.
    [bibtex-key = wyholtUlander2009AutofocusFFBP] [bibtex-entry]


  1309. Ping Zhang, Jian Shang, and Ruliang Yang. A New algorithm improving SAR resolution based on SVA. In Radar Conference, 2009 IET International, pages 1-4, 2009. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, SVA.
    Abstract: Resolution enhancement techniques in radar imaging have attracted considerable interest in recent years. The paper develops a new technique enhancing SAR resolution as well as suppressing sidelobes based on SVA technique. Spatially variant apodization (SVA) is a nonlinear sidelobe reduction method without lose the resolution of mainlobe. The signal bandwidth is extrapolated after SVA, which is the basis of resolution enhancement. An inverse weight function is used to equalize the SVA spectrum. A modified noninteger Nyquist spatially variant apodization (SVA) formulation is used to suppress sidelobes after extrapolation. Examples of 1D case and 2D case demonstrate enhanced image resolution with sidelobe reduction.
    [bibtex-key = zhangShangYang2009:SpatiallyVariantApodization] [bibtex-entry]


  1310. Xiao Xiang Zhu, N. Adam, and R. Bamler. Space-borne high resolution tomographic interferometry. In Proc. Geoscience and Remote Sensing Symp.,2009 IEEE Int.,IGARSS 2009, volume 4, 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2009] [bibtex-entry]


  1311. Xiao Xiang Zhu, Nico Adam, Ramon Brcic, and Richard Bamler. Space-borne high resolution SAR tomography: experiments in urban environment using TS-X Data. In Proc. Joint Urban Remote Sensing Event, pages 1-8, May 2009. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Xiao2009] [bibtex-entry]


  1312. Marcelo Albuquerque, Pau Prats, and Rolf Scheiber. Applications of Time-Domain Back-Projection SAR Processing in the Airborne Case. In European Conference on Synthetic Aperture Radar (EUSAR), pages 4, June 2008. VDE Verlag GmbH. Keyword(s): SAR Processsing, Time-Domain Back-Projection, TDBP, Back-Projection, Synthetic Aperture Radar (SAR), motion compensation, tomography, Airborne SAR, E-SAR, Topography-dependent motion compensation, Motion Compensation, MoComp, Interferometry, Non-Linear SAR, Non-Linear Flight Tracks.
    Abstract: The Back-Projection Algorithm is a SAR processing approach that uses time-domain convolution of the SAR data in order to perform SAR focusing. Some benefits of this approach are exact inversion, ideal motion compensation including topography information and handling of general aperture geometries. The implementation of the Back-Projection Algorithm was done focusing on the parallelization aspects. Applications of the algorithm are presented with respect to topography adaptive processing, direct generation of map projections and consideration of non linear trajectories.
    [bibtex-key = albuquerquePratsScheiberEUSAR08:TDBP] [bibtex-entry]


  1313. Michael Brandfass and Luis Fernando Lobianco. Modified Fast Factorized Backprojection as Applied to X-Band Data for Curved Flight Paths. In European Conference on Synthetic Aperture Radar (EUSAR), pages 4, June 2008. VDE Verlag GmbH. Keyword(s): SAR Processsing, Time-Domain Back-Projection, TDBP, Back-Projection, Fast Factorized Back-Projection, FFBP, Fast Back-Projection, Synthetic Aperture Radar (SAR), motion compensation, tomography, Airborne SAR, X-Band, Motion Compensation, MoComp, Non-Linear SAR, Non-Linear Flight Tracks.
    Abstract: A Fast Factorized Backprojection scheme modified to X-band frequencies and applicable to small aperture beamwidths is presented to compute SAR images from real and synthetic airborne data sets. The numerical complexity and memory consumption of the algorithm is verified and compared to ordinary Backprojection. The modified Fast Factorized Backprojection scheme is investigated for exceedingly curved flight paths and compared to an \u03c9-k algorithm in combination with a motion error correction. Excellent SAR image focusing results were found for the modified Fast Factorized Backprojection approach while keeping the numerical complexity to O(N2log(N)).
    [bibtex-key = brandfassLobiancoEUSAR2008:FFBPforXBand] [bibtex-entry]


  1314. Honglei Chen and D. Kasilingam. Performance Analysis of Multivariate Super-resolution Processing of Polarimetric Synthetic Aperture Radar Tomography. In Geoscience and Remote Sensing Symposium, 2008. IGARSS 2008. IEEE International, volume 4, pages 169-172, July 2008. [bibtex-key = Chen2008] [bibtex-entry]


  1315. A. Donnellan, P. Rosen, J. Graf, A. Loverro, A. Freeman, R. Treuhaft, R. Oberto, Marc Simard, Eric J. Rignot, R. Kwok, Xiaoqing Pi, J.B. Blair, W. Abdalati, J. Ranson, H. Zebker, B. Hager, H. Shugart, M. Fahnestock, and R. Dubayah. Deformation, Ecosystem Structure, and Dynamics of Ice (DESDynI). In Proc. IEEE Aerospace Conf., pages 1-13, March 2008. Keyword(s): DESDynl mission, National Research Council Earth Science Decadal Survey, Earth Science Applications from Space, biomass, cryosphere objectives, ecosystem function, ecosystem structure, ice dynamics, integrated L-band InSAR, multibeam Lidar mission, solid Earth, surface deformation, topography, vegetation structure, deformation, optical radar, synthetic aperture radar, topography (Earth), vegetation mapping.
    Abstract: The National Research Council Earth Science Decadal Survey, Earth Science Applications from Space, recommends that DESDynl (Deformation, Ecosystem Structure, and Dynamics of Ice), an integrated L-band InSAR and multibeam Lidar mission, launch in the 2010- 2013 timeframe. The mission will measure surface deformation for solid Earth and cryosphere objectives and vegetation structure for understanding the carbon cycle. InSAR has been used to study surface deformation of the solid Earth and cryosphere and more recently vegetation structure for estimates of biomass and ecosystem function. Lidar directly measures topography and vegetation structure and is used to estimate biomass and detect changes in surface elevation. The goal of DESDynl is to take advantage of the spatial continuity of InSAR and precision and directness of Lidar. There are several issues related to the design of the DESDynl mission, including combining the two instruments into a single platform, optimizing the coverage and orbit for the two techniques, and carrying out the science modeling to define and maximize the scientific output of the mission.
    [bibtex-key = DonnellanEtAl2008:DESDynI] [bibtex-entry]


  1316. A. Donnellan, P. Rosen, J. Ranson, and H. Zebker. Deformation, Ecosystem Structure, and Dynamics of Ice (DESDynI). In IEEE Int. Geoscience and Remote Sensing Symposium, IGARSS 2008, volume 3, pages 5-8, July 2008. Keyword(s): DESDynl mission, Deformation, Ecosystem Structure, and Dynamics of Ice, Earth Science Decadal Survey, National Research Council, biomass estimation, carbon cycle, cryosphere objectives, ecosystem function, integrated L-band InSAR, multibeam Lidar mission, solid Earth surface deformation, surface elevation changes, topography measure, vegetation structure, deformation, optical radar, radar interferometry, remote sensing by radar, topography (Earth), vegetation.
    Abstract: The National Research Council Earth Science Decadal Survey, Earth Science Applications from Space, recommends that DESDynI (Deformation, Ecosystem Structure, and Dynamics of Ice), an integrated L-band InSAR and multibeam Lidar mission, launch in the 2010-2013 timeframe. The mission will measure surface deformation for solid Earth and cryosphere objectives and vegetation structure for understanding the carbon cycle. InSAR has been used to study surface deformation of the solid Earth and cryosphere and more recently vegetation structure for estimates of biomass and ecosystem function. Lidar directly measures topography and vegetation structure and is used to estimate biomass and detect changes in surface elevation. The goal of DESDynI is to take advantage of the spatial continuity of InSAR and the precision and directness of Lidar. There are several issues related to the design of the DESDynI mission, including combining the two instruments into a single platform, optimizing the coverage and orbit for the two techniques, and carrying out the science modeling to define and maximize the scientific output of the mission.
    [bibtex-key = DonnellanRosenRansonZebker2008:DESDynI] [bibtex-entry]


  1317. H. Essen, T. Brehm, M. Haegelen, and H. Schimpf. Remote Sensing at Millimetre Waves with the MEMPHIS Synthetic Aperture Radar. In Proc. European Conf. Synthetic Aperture Radar, pages 1-4, June 2008. Keyword(s): SAR Processing, W-Band, SUMATRA, Airborne SAR, UAV, Fraunhofer, Acceleration, Image resolution, Radar imaging, Remote sensing, Synthetic aperture radar.
    Abstract: Synthetic Aperture Radar (SAR) is a tool for a broad community and a range of different applications. Especially for remote sensing applications data with high resolution preferably in all three dimensions are requested to provide the users with images of the earth's surface in high quality at any time regardless of cloud cover or time of the day. This is possible within quick reaction times, if an aircraft is taken as sensor platform. For the mapping of smaller areas with high precision, millimetre wave SAR offers best opportunities, combining all advantages of SAR imaging with ease of processing and the capability to be operated from small and light aircrafts, even remotely piloted. To show the advantages of these radar bands for remote sensing applications and especially to allow simultaneous comparative measurements at both relevant millimetre wave atmospheric windows, Ka- and W-band, the experimental SAR sensor MEMPHIS (Millimeterwave experimental multifrequency polarimetric high resolution imaging sensor) was developed and has been improved and extended in its capability over recent years. The paper gives a description of the present state of the system and presents representative examples for remote sensing applications.
    [bibtex-key = EssenEtAlEUSAR2008MEMPHISWBandKaBand] [bibtex-entry]


  1318. G. Fornaro, A. Pauciullo, F. Lombardini, and M. Pardini. Detection of Single and Multiple Scatterers in Multibaseline Multitemporal SAR Data. In Geoscience and Remote Sensing Symposium, 2008. IGARSS 2008. IEEE International, volume 2, pages 453-456, July 2008. [bibtex-key = Fornaro2008] [bibtex-entry]


  1319. Othmar Frey, Christophe Magnard, Maurice Rüegg, and Erich Meier. Focusing SAR Data Acquired From Non-Linear Sensor Trajectories. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 415-418, 2008. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Nonlinear Flight Tracks, Curvilinear SAR, Extended Chirp Scaling, ECS, Mosaicking, Geocoding, Integrated Focusing and Geocoding, Georeferencing, mapping, corridor mapping, E-SAR, L-Band, digital elevation model, Airborne SAR.
    Abstract: Standard focusing of SAR data assumes a straight recording track of the sensor platform. Small non-linearities of airborne platform tracks are corrected for during a motion compensation step while keeping the assumption of a linear flight path. In the following, the processing of SAR data from nonlinear tracks is discussed as may originate from small aircraft or drones flying at low altitude. They fly not a straight track but one dependent on topography, influences of weather and wind, or dependent on the shape of dedicated areas of interest such as rivers or traffic routes. A time-domain backprojection based technique, is proposed and evaluated with the help of experimental data featuring a drop in height, a double bend, a 90-degree curve and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared and the coherence is evaluated. The experimental data was acquired by the German Aerospace Center's E-SAR L-band system.
    [bibtex-key = freyMagnardRueeggMeier08Igarss:Tracks] [bibtex-entry]


  1320. Othmar Frey, Christophe Magnard, Maurice Rüegg, and Erich Meier. Non-Linear SAR Data Processing By Time-Domain Back-Projection. In Proc. EUSAR 2008 - 7th European Conference on Synthetic Aperture Radar, pages 165-168, 2008. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Non-Linear Flight Tracks, Curvilinear SAR, Extended Chirp Scaling, ECS, Mosaicking, Geocoding, Integrated Focusing and Geocoding, Georeferencing, mapping, corridor mapping.E-SAR, L-Band, digital elevation model, Airborne SAR.
    Abstract: Focusing of conventional stripmap SAR data assumes a straight recording track of the sensor platform. Small deviations from that linear trajectory are corrected by motion compensation steps while keeping the assumption of a linear acquisition path. In the following, the processing of SAR data from non-linear tracks is discussed as may originate from small aircraft or drones flying at low altitude. They fly not a straight track but one dependent on topography, influences of weather and wind, or dependent on the shape of dedicated areas of interest such as rivers or traffic routes. Experimental data featuring a drop in height, a double bend and a 90-degree curve have been processed using a time-domain back-projection approach. The data was acquired by the German Aerospace Center's E-SAR L-band system.
    [bibtex-key = freyMagnardRueeggMeier08Eusar:Tracks] [bibtex-entry]


  1321. Othmar Frey and Erich Meier. Combining Time-Domain Back-Projection and Capon Beamforming for Tomographic SAR Processing. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 445-448, 2008. Keyword(s): SAR Processing, SAR Tomography, Tomographic Processing, Multi-Baseline SAR, Time-Domain Back-Projection, Back-Projection, E-SAR, P-Band, Forestry, Capon, Capon Beamforming, Superresolution.
    Abstract: Various tomographic processing methods have been investigated in recent years. The quality of the focused tomographic image is usually limited by several factors. In particular, Fourier-based focusing methods are susceptible to irregular and sparse sampling, two problems that are unavoidable in case of multi-pass, multi-baseline SAR data acquired by an airborne system. Neither time-domain back-projection (TDBP) processing, although providing a very accurate processing framework, is able to overcome the problem of ambiguous target detection in the tomographic image. In this paper, a possible extension of the TDBP approach to multilooking based tomographic focusing methods like standard beamforming and Capon beamforming is discussed. Preliminary results obtained with a simulated and a real airborne tomographic P-band data set are shown.
    [bibtex-key = freyMeier08Igarss:Tomo] [bibtex-entry]


  1322. Othmar Frey and Erich Meier. Tomographic Focusing by Combining Time-Domain Back-Projection and Multi-Looking Based Focusing Techniques. In Proc. EUSAR 2008 - 7th European Conference on Synthetic Aperture Radar, pages 73-76, 2008. Keyword(s): SAR Processing, SAR Tomography, Tomographic Processing, Multi-Baseline SAR, Time-Domain Back-Projection, Back-Projection, E-SAR, P-Band, Forestry, Capon, Capon Beamforming, Superresolution.
    Abstract: Various tomographic processing methods have been investigated in recent years. The quality of the focused tomographic image is usually limited by several factors. In particular, Fourier-based focusing methods are susceptible to irregular and sparse sampling, two problems that are unavoidable in case of multi-pass, multi-baseline SAR data acquired by an airborne system. Neither time-domain back-projection (TDBP) processing, although providing a very accurate processing framework, is able to overcome the problem of ambiguous target detection in the tomographic image. In this paper, a possible extension of the TDBP approach to multi-looking based tomographic focusing methods like standard beamforming and Capon beamforming is discussed.
    [bibtex-key = freyMeier08Eusar:Tomo] [bibtex-entry]


  1323. M. Haegelen, G. Briese, H. Essen, and A. Tessmann. Millimetre Wave Near Field SAR Scanner for Concealed Weapon Detection. In 7th European Conference on Synthetic Aperture Radar, pages 1-4, June 2008. Keyword(s): SAR Processing, W-Band, Apertures, Geometry, Radar imaging, Security, Sensors, Weapons.
    Abstract: A light weight, transportable measurement system for the inspection of suspicious humans and of luggage in a threat situation has been developed, based upon miniaturized millimetre wave radar modules operating at W-band. To allow a sufficient detection capability of objects hidden under the clothing or within a piece of luggage within buildings at a sufficiently short scanning time, a radar approach rather than a passive radiometer technique was chosen. It is generally recognized, that only synthetic aperture is able to supply a sufficient geometrical resolution at high imaging speed. Moreover a radar can also deliver a three dimensional description for the position and the shape of a concealed object using high range resolution techniques. The paper describes the measurement set-up and the evaluation algorithm. Typical examples for different set-ups are presented.
    [bibtex-key = haegelenBrieseEssenTessmannEUSAR2008wbandCloseRangeSAR] [bibtex-entry]


  1324. Charles V. Jakowatz, Daniel E. Wahl, and David A. Yocky. Beamforming as a foundation for spotlight-mode SAR image formation by backprojection. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 6970, pages 69700Q, 2008. SPIE. Keyword(s): SAR Processing, Back-projection, Time-Domain Back-Projection, TDBP, Fast Back-projection, Fast Factorized Back-Projection, FFBP, Spotlight SAR, Spotlight-mode data, Beamforming. [bibtex-key = jakowatzWahlYockyBeamformingTDBPSpotlightMode2008] [bibtex-entry]


  1325. F. Lombardini, G. Fornaro, M. Pardini, D. Reale, F. Serafino, F. Soldovieri, and M. Costantini. SAR tomography for scene elevation and deformation reconstruction: Algorithms and potentialities. In Radar Conference, 2008. RADAR '08. IEEE, pages 1-7, May 2008. [bibtex-key = Lombardini2008] [bibtex-entry]


  1326. Christophe Magnard, Othmar Frey, Maurice Rüegg, and Erich Meier. Improved Airborne SAR Data Processing by Blockwise Focusing, Mosaicking and Geocoding. In Proc. EUSAR 2008 - 7th European Conference on Synthetic Aperture Radar, pages 375-378, 2008. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Non-Linear Flight Tracks, Curvilinear SAR, Extended Chirp Scaling, ECS, Mosaicking, Geocoding, Integrated Focusing and Geocoding, Georeferencing, mapping, corridor mapping.E-SAR, L-Band, digital elevation model, Airborne SAR.
    Abstract: Standard focusing of SAR data assumes a straight recording track of the sensor platform. Small non-linearities of airborne platform are corrected for during a motion compensation step while keeping the assumption of a stripmap geometry. In the case of high resolution and high frequency SAR systems, the navigation data may not be accurate enough to perform such a motion compensation; SAR systems mounted on small aircrafts or drones flying at low altitude do not follow a straight track but one dependent on topography and atmospheric conditions. We present a blockwise focusing, mosaicking and geocoding method which allows processing such data. For the experiments, MEMPHIS and E-SAR data were used.
    [bibtex-key = magnardFreyRueeggMeier08Eusar:Tracks] [bibtex-entry]


  1327. G. Margarit, Jordi J. Mallorqui, I. Corney, and C. Lopez-Martinez. A Public Database of Simulated Multidimensional SAR Data for Techniques Validation. In Geoscience and Remote Sensing Symposium, 2008. IGARSS 2008. IEEE International, volume 2, pages 601-604, July 2008. [bibtex-key = Margarit2008] [bibtex-entry]


  1328. Thierry Michel and Scott Hensley. Wavenumber domain focusing of squinted SAR data with a curved orbit geometry. In Proc. 42nd Asilomar Conf. on Signals, Systems and Computers, pages 492-496, October 2008. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Curvilinear SAR, airborne SAR, SAR data, curved orbit geometry, squinted geometry, synthetic aperture radar, wavenumber domain focusing, synthetic aperture radar.
    Abstract: Synthetic Aperture Radar systems provide raw data that need focusing to achieve full-resolution imaging. Current SAR applications, including interferometry, require accurate, phase-preserving, and precisely co-registered coherent images over large ground swaths with the highest achievable resolution. In addition to these challenges, stripmap SAR data may be acquired with an off-broadside (squinted) geometry, either by design or through platform motion. The precise batch focusing of these large aperture and wide bandwidth data sets is known to require a 2D frequency processing approach. The standard wave domain focusing algorithm, however, is only exact for data acquired on a rectilinear trajectory. We investigate a generalization of the standard omega-k focusing formulation that allows curved data acquisition tracks. The new formulation can be used in conjunction with a known extension for conical, squinted imaging grids. The approximations necessary to allow the generalized geometry are analysed to determines the range of applicability of the proposed algorithm. The theory is validated using data simulated with parameters similar to the UAVSAR L-band SAR system.
    [bibtex-key = michelHensley2008] [bibtex-entry]


  1329. Matteo Nannini, Rolf Scheiber, and Alberto Moreira. On the Minimum Number of Tracks for SAR Tomography. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 2, pages 441-444, July 2008. Keyword(s): SAR Processing, SAR Tomography, Capon, MUSIC, image reconstruction, airboren SAR, image representation, radar interferometry, synthetic aperture radar3D representation, German Aerospace Center, DLR, L-band, SAR interferometry, SARTom, data acquisition, equivalent targets, experimental SAR system, minimum tomographic aperture, spheroidal wave functions, subspace superresolution methods, synthetic aperture radar tomography, tracks minimum number determination, volumetric source, ESAR.
    Abstract: The main drawback of SAR Tomography (SARTom) is the considerable number of tracks required to achieve the 3-dimensional (3D) representation of a viewed scene. The key point concerns the trade-off between the vertical resolution and the control on ambiguities phenomena. This paper deals with the problem of the determination of the minimum number of required tracks when super-resolution subspace methods are applied. The results are validated on real data acquired in L-band by the E-SAR system of the German Aerospace Centre.
    [bibtex-key = nanniniScheiberMoreira2008:SARTom] [bibtex-entry]


  1330. P.A. Rosen, S. Hensley, and C. Le. Observations and mitigation of RFI in ALOS PALSAR SAR data: Implications for the DESDynI mission. In IEEE Radar Conference, pages 1-6, May 2008. Keyword(s): DESDynI mission, L-band polarimetric radar, RFI, SAR data, radio frequency interference, synthetic aperture radar, radiofrequency interference, synthetic aperture radar.
    Abstract: Initial examination of ALOS PALSAR synthetic aperture radar (SAR) data has indicated significant radio frequency interference (RFI) in several geographic locations around the world. RFI causes significant reduction in image contrast, introduces periodic and quasi-periodic image artifacts, and introduces significant phase noise in repeat-pass interferometric data reduction. The US National Research Council Decadal Survey of Earth Science has recommended DESDynI, a Deformation, Ecosystem Structure, and Dynamics of Ice satellite mission comprising an L-band polarimetric radar configured for repeat-pass interferometry. There is considerable interest internationally in other future L-band and lower frequency systems, as well. Therefore, the issues of prevalence and possibilities of mitigation of RFI in these crowded frequency bands are of considerable interest. RFI is observed in ALOS PALSAR in California and Hawaii, USA, and in southern Egypt in data examined to date. Application of several techniques for removing it from the data prior to SAR image formation, ranging from straight-forward spectral normalization to time-domain, multi-phase filtering techniques, are considered. Considerable experience has been gained from the removal of RFI from P-band acquired by the GeoSAR system. These techniques applied to the PALSAR data are most successful when the bandwidth of any particular spectral component of the RFI is narrow. Performance impacts for SAR imagery and interferograms are considered in the context of DESDynI measurement requirements.
    [bibtex-key = RosenHensleyLe2008:DESDynIandRFI] [bibtex-entry]


  1331. Stefano Tebaldini. Forest SAR tomography: A covariance matching approach. In IEEE Radar Conference, 2008. RADAR '08., pages 1-6, May 2008. Keyword(s): SAR Processing, SAR Tomography, Tomography, E-SAR, P-Band, radar imaging, radar interferometry, radar polarimetry, radar resolution, synthetic aperture radar, tomography, P-band SAR images, covariance matching approach, forest SAR tomography, multipolarimetric channel data, single polarimetric channel data, single target interferometric analysis, synthetic aperture radar interferometry, system resolution cell.
    Abstract: In this paper a technique to conduct tomographic analyses of forested areas through multiple, coherent SAR images is presented. This technique differs from other methods existing in literature in that it may be regarded as a natural extension of SAR interferometry to the case where several distributed targets are present within the system resolution cell. In this way, it is possible to estimate the position of each target in the resolution cell with virtually the same accuracy obtainable by a single target interferometric analysis. Furthermore, this approach is suited to both single and multi polarimetric channel data. As a validation, this paper reports the results of a tomographic analysis of the forest site of Remningstorp, Sweden, basing on a data-set of 9 P-band SAR images acquired by DLRpsilas E-SAR.
    [bibtex-key = tebaldiniRADAR2008:Tomo] [bibtex-entry]


  1332. Stefano Tebaldini, Fabio Rocca, and A Monti-Guarnieri. Model Based SAR Tomography of Forested Areas. In IEEE International Geoscience and Remote Sensing Symposium, volume 2, pages 593-596, July 2008. Keyword(s): SAR Processing, SAR Tomography, Tomography, E-SAR, P-Band.
    Abstract: In this paper a technique is described for the tomographic characterization of forested areas through multiple SAR observations. This technique is based on a model of the second order statistics of the multi baseline, multi polarimetric, data which accounts for the presence of multiple distributed targets within the system resolution cell. The results of an experiment performed on a real P-band, multi-baseline, fully polarimetric data set relative to the forested site of Remningstorp, Sweden, are reported. Such results show the feasibility of performing a model based tomographic analysis of forests, resulting in a characterization of both the ground and the canopy in terms of elevation, spatial structure, and scattered power.
    [bibtex-key = tebaldiniRoccaGuarnieri2008:Tomo] [bibtex-entry]


  1333. Daniel E. Wahl, David A. Yocky, and Charles V. Jakowatz. An implementation of a fast backprojection image formation algorithm for spotlight-mode SAR. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 6970, pages 69700H, 2008. SPIE. Keyword(s): SAR Processing, Back-projection, Time-Domain Back-Projection, TDBP, Fast Back-projection, Fast Factorized Back-Projection, FFBP, Spotlight SAR, Spotlight-mode data. [bibtex-key = wahlYockyJakowatzFastBackprojectionSpotlight2008] [bibtex-entry]


  1334. Yanping Wang, Bin Wang, Wen Hong, Lei Du, and Yirong Wu. Imaging Geometry Analysis of 3D SAR using Linear Array Antennas. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2008, volume 3, pages 1216-1219, July 2008. Keyword(s): SAR Processing, SAR Tomography, Tomography, linear antenna arrays, radar imaging, radar interferometry, synthetic aperture radar, 3D SAR, elevation direction, imaging geometry analysis, linear array antennas, signal model.
    Abstract: Linear array antennas SAR has a resolving capability in the elevation direction, and can get the 3D image of the target. In this paper, we derive the signal model of 3D SAR using a linear array antenna, and get the 3D resolutions and 3D point spread function of array antenna SAR, at the same time the sampling space of array antennas is given. The variance of the resolution in the elevation direction with array antenna angle and referenced look angle is studied. The geometry to reach the best resolution in the elevation direction is analyzed. Meanwhile the resolution for horizontal and vertical antenna array are calculated and compared.
    [bibtex-key = wangWangHongDuWu2008:SARTom] [bibtex-entry]


  1335. Charles L. Werner, Tazio Strozzi, Andreas Wiesmann, and Urs Wegmuller. A Real-Aperture Radar for Ground-Based Differential Interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 210-213, July 2008. Keyword(s): SAR Processing, Antenna measurements, Instruments, Interferometry, Monitoring, Radar antennas, Radar imaging, Receiving antennas, Satellites, Spaceborne radar, Terrain factors, antennas, geomorphology, geophysical equipment, radar interferometry, remote sensing by radar, rocks, spaceborne radar, synthetic aperture radar, GAMMA Portable Radar Interferometer, GPRI, frequency 17.2 GHz, ground-based differential interferometry, height measurements, image acquisition, in-situ radar imaging system, landslides, line-of-sight displacements, phase differences, photogrammetry systems, real-aperture radar, receiving antennas, rock fall warning system, slope stability, space-borne INSAR, space-borne SAR interferometry, surface topography, GAMMA, RAR, Radar Interferometer, glacier, landslide;.
    Abstract: Satellite interferometry has been used extensively for ground-motion monitoring with good success. In the case of landslides, for example, space-borne SAR interferometry has a good potential to get an overview on the slope stability. The role of a space-borne INSAR as an element in a landslide or rock fall warning system is constrained by the specific space-borne SAR imaging geometry, the typical multiple-week repeat-interval, and uncertainties in the data availability. Most of these limitations can be overcome with an in-situ radar imaging system. GAMMA has developed a portable radar interferometer that utilizes real-aperture antennas to obtain high azimuth resolution. Images are acquired line by line while rotating the transmitting and receiving antennas about a vertical axis. Phase differences between successive images acquired from the same location are used to determine line-of-sight displacements. The instrument operates at 17.2 GHz and has measurement sensitivity better than 1 mm. The instrument uses two receiving antennas with a short baseline to form an interferometer. Phase differences between simultaneous acquisitions by these antennas are used to calculate the precise look angle relative to the baseline, permitting derivation of the surface topography. Expected statistical noise in the height measurements is on the order of 1 meter. In this contribution the design, measurement principles and characteristics of GAMMA's Portable Radar Interferometer (GPRI) are presented.
    [bibtex-key = wernerStrozziWiesmannWegmullerIGARSS2008GPRI] [bibtex-entry]


  1336. Andreas Wiesmann, Charles L. Werner, Christian Matzler, Martin Schneebeli, Tazio Strozzi, and Urs Wegmuller. Mobile X- to Ku-band Scatterometer in Support of the CoRe-H2O Mission. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 5, pages 244-247, July 2008. Keyword(s): SnowScat, KuScat, backscatter, hydrological techniques, radiometry, remote sensing by radar, snow, spaceborne radar, C-band SAR satellite systems, ESA CoRe-H2O mission, Ku-band scatterometer, Swiss Alps, X-band scatterometer, backscatter information, backscattering signal, dry snow cover, dual frequency radar, frequency 18 GHz, frequency 9 GHz, mobile scatterometer, snow coverage, snow liquid water content, snow structure, spaceborne active microwave remote sensing, Backscatter, Frequency, Ground support, Radar measurements, Remote sensing, Satellites, Signal generators, Snow, Spaceborne radar, Water storage, Scatterometer, Snow, backscatter, snow grain.
    Abstract: Information on snow coverage, structure, and liquid water content are important for many applications including avalanche warning, numerical weather prognosis (NWP), and snow pack water storage estimate. Active microwave remote sensing from space has an excellent potential to address these needs. However, the current generation of C-band SAR satellite systems are not well suited for snow related applications because of the small impact of the dry snow cover on the backscattering signal at 5.3 GHz. In order to be more sensitive to snow properties the envisaged ESA CoReH2O mission proposes a dual-frequency radar operating at 9 and 18 GHz (X- and Ku-band). In support of this mission, a ground based 9 to 18 GHz scatterometer is being developed to help to address the lack of simultaneous backscatter information of snow in this frequency range. We are also preparing a dedicated field campaign in the Swiss Alps to evaluate the performance of the system and acquire first data.
    [bibtex-key = wiesmannWernerMatzlerSchneebeliStrozziWegmullerIGARSS2008SnowScat] [bibtex-entry]


  1337. E.C. Zaugg and D.G. Long. Along-Track Resolution Enhancement Forwide-Bandwidth, Low-Frequency SAR by Accounting for the Wavelength Change over the Bandwidth. In Geoscience and Remote Sensing Symposium, 2008. IGARSS 2008. IEEE International, volume 4, pages 1272-1275, July 2008. Keyword(s): SAR Processing. [bibtex-key = Zaugg2008] [bibtex-entry]


  1338. A. Ahlander, H. Hellsten, K. Lind, J. Lindgren, and B. Svensson. Architectural Challenges in Memory-Intensive, Real-Time Image Forming. In Int. Conf. on Parallel Processing, pages 35-35, Sept. 2007. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, Fast-Factorized Back-Projection, FFBP, GBP, interpolation, parallel algorithms, parallel architectures, radar imaging, real-time systems, storage management, synthetic aperture radarcomplex memory access pattern, computer architecture, flight path error compensation, memory-intensive real-time image forming, parallel algorithm, performance-intensive data interpolation, synthetic aperture radar system.
    Abstract: The real-time image forming in future, high-end synthetic aperture radar systems is an example of an application that puts new demands on computer architectures. The initial question is whether it is at all possible to meet the demands with state-of-the-art technology or foreseeable new technology. It is therefore crucial to understand the computational flow, with its associated memory, bandwidth and processing demands. In this paper we analyse the application in order to, primarily, understand the algorithms and identify the challenges they present on a basic architectural level. The processing in the radar system is characterized by working on huge data sets, having complex memory access patterns, and doing real-time compensations for flight path errors. We propose algorithm solutions and execution schemes in interplay with a two-level (coarse-grain/fine-grain) system parallelization approach, and we provide approximate models on which the demands are quantified. In particular, we consider the choice of method for the performance- intensive data interpolations. This choice presents a trade-off problem between computational performance and size of working memory. The results of this study will serve as a basis for further, more detailed architecture studies.
    [bibtex-key = ahlanderHellstenLindLindgrenSvensson2007:TDBPRealTime] [bibtex-entry]


  1339. Christian Andres, Torben Keil, Raik Herrmann, and Rolf Scheiber. A multiprocessing framework for SAR image processing. In , pages 524-527, July 2007. Keyword(s): SAR Processing, Airborne SAR, application program interfaces, geophysics computing, grid computing, image processing, multiprocessing systems, synthetic aperture radar, workstation clusters, F-SAR, SAR image processing, airborne SAR sensor, application programming interface, cluster grids, different architectures, multiprocessing framework, source code, workstation cluster.
    Abstract: This paper introduces a framework developed for image processing of synthetic aperture radar (SAR) images. It encapsulates features of modern hardware architectures, including symmetric and asymmetric multiprocessing, within an easy and intuitive to use application programming interface (API). The multiprocessing part is designed for unified usage of different architectures reaching from multicore processors to cluster of workstations to grids of clusters. So an application using the framework can be ported from one architecture to another without any changes in the source code. The framework builds the bottom layer of the processing system developed for the German Aerospace Center's (DLR) new airborne SAR sensor, the F-SAR.
    [bibtex-key = andresKeilHerrmannScheiber2007:SARMultiProcessingFrame] [bibtex-entry]


  1340. Wang Bin, Wang Yanping, Hong Wen, and Wu Yirong. Simulation research of parametric methods for multi-baseline SAR tomography. In Synthetic Aperture Radar, 2007. APSAR 2007. 1st Asian and Pacific Conference on, pages 203-206, November 2007. [bibtex-key = Bin2007] [bibtex-entry]


  1341. E. Candes, N. Braun, and M. Wakin. SPARSE SIGNAL AND IMAGE RECOVERY FROM COMPRESSIVE SAMPLES. In Biomedical Imaging: From Nano to Macro, 2007. ISBI 2007. 4th IEEE International Symposium on, pages 976-979, april 2007. Keyword(s): compressive sampling, data acquisition, image recovery, magnetic resonance imaging, medical imaging, model-based framework, random measurements, random noise-like basis, signal reconstruction, sparse signal recovery, biomedical MRI, biomedical measurement, data acquisition, image coding, image reconstruction, image sampling, medical image processing, random noise, sparse matrices;.
    Abstract: In this paper we present an introduction to compressive sampling (CS), an emerging model-based framework for data acquisition and signal recovery based on the premise that a signal having a sparse representation in one basis can be reconstructed from a small number of measurements collected in a second basis that is incoherent with the first. Interestingly, a random noise-like basis will suffice for the measurement process. We will overview the basic CS theory, discuss efficient methods for signal reconstruction, and highlight applications in medical imaging
    [bibtex-key = 4193451] [bibtex-entry]


  1342. Hubert M. J. Cantalloube, Élise Colin-Koeniguer, and Hélène Oriot. High resolution SAR imaging along circular trajectories. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2007, pages 850-853, July 2007. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, PGA, Autofocus by Deterministic Trajectory Triangulation Technique, data acquisition, Motion Compensation, MoComp, Residual Motion Errors, radar antennas, radar polarimetry, remote sensing by radar, synthetic aperture radar, L-band radar, P-band radar, Sweden, X-band sensor, Airborne SAR, aircraft attitude fluctuation, circular trajectories, Non-Linear Flight Track, high resolution SAR imaging, joint FOI-ONERA campaign, narrower antenna pattern, polarimetric full circle radar acquisition, steerable antenna.
    Abstract: After a first series of full circle SAR acquisitions in L- and P-bands during a 2004 joint FOI-ONERA campaign in Sweden, ONERA experimented in 2006 high resolution (15 cm) polarimetric, full circle acquisitions in France and Germany using its X-band sensor. In order to cope with narrower antenna pattern and aircraft attitude fluctuations, a steerable antenna was used. Furthermore, an experimental setup for retrieving high accuracy trajectory was installed. This paper describes the processing of this signals.
    [bibtex-key = cantalloubeColinKoeniguerOriot2007:NonLinearFlightTracks] [bibtex-entry]


  1343. Karlus A. Câmara de Macedo, Rolf Scheiber, and Alberto Moreira. An autofocus approach for residual motion errors with application to airborne repeat-pass SAR interferometry. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2007, Barcelona, Spain, pages 4886 - 4889, July 2007. Keyword(s): SAR Processing, Autofocus, Residual Motion Errors, WPCA, Weighted PCA, Weighted Phase Curvature Autofocus, Phase Curvature Autofocus, PCA, Phase Gradient Autofocus, PGA, Repeat-Pass Interferometry, Interferometry, E-SAR, airborne SAR, Baseline Calibration, Tomography, SAR Tomography.
    Abstract: Airborne repeat-pass SAR data are very sensible to sub-wavelength deviations from the reference track. To enable repeat-pass interferometry a high-precision navigation system is needed. Due to the limit of accuracy of such systems, deviations in the order of centimeters remain between the nominal and the processed reference track causing mainly undesirable phase undulations and misregistration in the interferograms, referred as residual motion errors. Up to now only interferometric approaches, as multi-squint, are used to estimate those deviations to compensate for such residuals. In this paper we present for the first time the use of the Autofocus technique for residual motion errors. A very robust autofocus technique has to be used since the accuracy of the estimated motion has to be at millimeter scale. Because we deal with low-altitude-stripmap mode data we propose a new robust autofocus technique based on the WLS (Weighted Least-Squares) phase estimation and Phase Curvature Autofocus (PCA) extended to the rangedependent case. We call this new technique WPCA (Weighted PCA). While the multi-squint approach is only able to estimate the baseline variation from coregistered images, the autofocus approach has the advantage of being able to estimate motion deviations independendtly for each image. Repeat-pass data of the E-SAR system of the German Aerospace Center (DLR) are used to demostrate the performance of the proposed approach.
    [bibtex-key = deMacedoScheiberMoreira2007:Autofocus] [bibtex-entry]


  1344. John M. Dow, Ruth E. Neilan, and Chris Rizos. The International GNSS Service (IGS): Preparations for the Coming Decade. In 20th Int. Tech. Meeting of the Satellite Division of the US Inst. of Navigation, 2007. Keyword(s): GNSS, Global Navigation Satellite System, International GNSS Service, IGS, Zenith Tropospheric Delay, ZTD.
    Abstract: The International GNSS Service (IGS) is an international activity with more than 200 participating organisations in over 80 countries with a track record of more than 13 years of successful service. The IGS is a service of the International Association of Geodesy (IAG) and primarily supports the scientific research based on, and the analysis of, long-term, highly precise and accurate Earth observations using the technologies of Global Navigation Satellite Systems (GNSS), primarily the U.S. Global Positioning System (GPS). The mission of the IGS, recently revised at the IGS Strategic Planning Meeting held in December 2006, is``to provide the highest-quality GNSS data and products in support of the terrestrial reference frame, Earth rotation, Earth observation and research, positioning, navigation and timing and other applications that benefit society''. The IGS will continue to support the IAG's initiative to coordinate cross-technique global geodesy for the next decade - via the development of the Global Geodetic Observing System (GGOS), which focuses on the needs of global geodesy at the mm-level. The IGS activities are fundamental to scientific disciplines related to climate, weather, sea level change, and space weather. However, the IGS also supports many other applications, including precise navigation, machine automation, and surveying and mapping. This paper will discuss the IGS Strategic Plan and future directions of the globally-coordinated ~400 station IGS network, tracking data and information products, and outline the concerns of a few of its numerous working groups and pilot projects as the world anticipates a truly multi-system GNSS in the coming decade.
    [bibtex-key = dowNeilanRizosGNSS2007IGSpreparationsForComingDecades.pdf] [bibtex-entry]


  1345. H. Essen, H. H. Fuchs, and A. Pagels. High resolution millimeterwave SAR for the remote sensing of wave patterns. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 963-966, July 2007. Keyword(s): SAR Processing, W-Band, airborne radar, clutter, ocean waves, oceanographic equipment, radar polarimetry, remote sensing by radar, sea level, statistical analysis, synthetic aperture radar, Ka-band clutter statistics, MEMPHIS, Multi-frequency Experimental Monopulse High-resolution Interferometric SAR, W-band clutter statistics, frequency 35 GHz, frequency 94 GHz, high resolution millimeterwave SAR, high resolution ocean swell imaging, nonimaging statistical methods, polarimetric millimetric SAR, south Spanish Atlantic coast, synthetic aperture radar, wave pattern remote sensing, Bandwidth, Chirp, Frequency, High-resolution imaging, Polarization, Radar imaging, Remote sensing, Sea measurements, Sea surface, Synthetic aperture radar, K-distribution wave pattern, Millimetre waves, SAR, polarimetry, resolution.
    Abstract: High resolution imaging of the ocean swell was performed using data collected with the polarimetric millimetre wave synthetic aperture radar MEMPHIS. The data, representative for a region off the south Spanish Atlantic coast in spring, have been evaluated using imaging and non-imaging statistical methods. The influence of high resolution processing on the clutter statistics for the Ka- and the W-band is discussed.
    [bibtex-key = essenFuchsPagelsIGRARSS2007] [bibtex-entry]


  1346. G. Fornaro, F. Lombardini, M. Pardini, F. Serafino, F. Soldovieri, and M. Costantini. Spaceborne multi-dimensional SAR imaging: Current status and perspectives. In Geoscience and Remote Sensing Symposium, 2007. IGARSS 2007. IEEE International, pages 5277-5280, July 2007. [bibtex-key = Fornaro2007] [bibtex-entry]


  1347. Othmar Frey, Felix Morsdorf, and Erich Meier. Tomographic Processing of Multi-Baseline P-Band SAR Data for Imaging of a Forested Area. In Proc. IEEE Int. Geosci. Remote Sens. Symp., 2007. Keyword(s): SAR Processing, SAR Tomography, Tomographic Processing, Multi-Baseline SAR, Time-Domain Back-Projection, Back-Projection, E-SAR, P-Band, Forestry.
    Abstract: Recently, various attempts have been undertaken to obtain information about the structure of forested areas from multi-baseline synthetic aperture radar data. Tomographic processing of such data has been demonstrated but the quality of the focused tomographic image is limited by several factors. In particular Fourier-based focusing methods are susceptible to irregular and sparse sampling, two problems, that are unavoidable in case of multi-pass, multi-baseline SAR data acquired by an airborne system. We propose a tomographic focusing method based on the time-domain back-projection algorithm, which maintains the geometric relationship between the original sensor positions and the imaged target and is therefore able to cope with irregular sampling without introducing any approximations with respect to the geometry. We assess the tomographic focusing quality with the help of the impulse response of simulated point targets and an in-scene corner reflector. And, in particular, preliminary results obtained with the newly acquired P-band tomographic data set consisting of eleven flight tracks are presented.
    [bibtex-key = freyMorsdorfMeier07IGARSS:Tomo] [bibtex-entry]


  1348. Othmar Frey, Felix Morsdorf, and Erich Meier. Tomographic SAR Imaging of a Forested Area by Time-Domain Back-Projection. In M.E. Schaepman, S. Liang, N.E. Groot, and M. Kneubühler, editors, 10th Intl. Symposium on Physical Measurements and Spectral Signatures in Remote Sensing, volume XXXVI, 2007. Intl. Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Keyword(s): SAR Processing, SAR Tomography, Tomographic Processing, Multi-Baseline SAR, Time-Domain Back-Projection, Back-Projection, E-SAR, P-Band, Forestry.
    Abstract: Recently, various attempts have been undertaken to retrieve information about the three-dimensional structure of vegetation from multibaseline synthetic aperture radar data. Although tomographic processing of such data has been demonstrated, yet, there are still several problems that limit the focusing quality. In particular, the frequency-domain based focusing methods are susceptible to irregular and sparse sampling, two problems, which are unavoidable in case of multi-pass, multi-baseline radar data acquired by an airborne system. We propose a time-domain back-projection algorithm, which maintains the original geometric relationship between the original sensor positions and the imaged target and is therefore able to cope with irregular and sparse sampling without introducing any geometric approximations. Preliminary results obtained with a newly acquired P-band tomographic data set consisting of eleven flight tracks are shown and discussed.
    [bibtex-key = freyMorsdorfMeierISPMSRS2007:Tomo] [bibtex-entry]


  1349. F. Gumbmann, H. P. Tran, J. Weinzierl, and L. P. Schmidt. Optimization of a fast scanning millimetre-wave short range SAR imaging system. In Proc. European Radar Conference, pages 24-27, October 2007. Keyword(s): SAR Processing, W-Band, frequency response, millimetre wave imaging, radar imaging, synthetic aperture radar, waveguide couplers, SAR algorithms, W-band short range imaging system, active imaging system, broadband frequency response, fast scanning millimetre-wave short range SAR imaging system, high directivity waveguide coupler, monostatic virtual antenna concept, nondestructive testing applications, quasioptical broadband diplexer, receiver channel, security applications, synthetic aperture radar, transmitter channel, unfocused measurement setup, Antenna measurements, Focusing, Frequency measurement, Frequency response, Nondestructive testing, Pixel, Radar polarimetry, Security, System testing, Transmitters.
    Abstract: This paper presents the development of a W-band short range imaging system for security and non-destructive testing applications, which uses an unfocused measurement setup employing SAR algorithms in order to focus the image numerically. The active imaging system offers the possibility to get a broadband frequency response from each pixel in the measurement plane. For real-time applications a fast scanning, monostatic virtual antenna concept has been developed and optimized. The monostatic setup is demonstrated in two different configurations, on the one hand using a quasioptical, broadband diplexer with high decoupling between transmitter and receiver channel and on the other hand a very compact setup using a commercially available high directivity waveguide coupler. The measurement setups are compared with special respect to their RF-performance. Experimental results on specific test objects are presented to prove the performance of the fast short range SAR imaging system.
    [bibtex-key = gumbmannTranWeinzierlSchmidtEuRAD2007WBandRadar] [bibtex-entry]


  1350. Wang Jinfeng, Zhou Peng, and Pi Yiming. SAR tomography imaging based on high-order spectrum analysis. In Synthetic Aperture Radar, 2007. APSAR 2007. 1st Asian and Pacific Conference on, pages 364-367, November 2007. [bibtex-key = Jinfeng2007] [bibtex-entry]


  1351. M. Lachaise, Michael Eineder, and Thomas Fritz. Multi baseline SAR acquisition concepts and phase unwrapping algorithms for the TanDEM-X mission. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 5272-5276, July 2007. Keyword(s): TerraSAR-X, TanDEM-X, AD 2009, HRTI-3 specifications, TanDEM-X mission, TerraSAR-X add-on for Digital Elevation Measurement, bistatic mode interferometric SAR data, controlled helix two satellite configuration, digital elevation model, global DEM, global land surface, multibaseline SAR acquisition concept, multibaseline SAR phase unwrapping algorithm, artificial satellites, data acquisition, digital elevation models, geophysical signal processing, radar interferometry, radar signal processing, remote sensing by radar, spaceborne radar, synthetic aperture radar.
    Abstract: The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission will start in 2009 with the aim of generating a global Digital Elevation Model with high accuracy corresponding to HRTI-3 specifications (12 m posting, 2 m relative point-to-point height accuracy for flat terrain). To achieve this goal, a second satellite similar to TerraSAR-X will fly close to TerraSAR-X in a controlled Helix configuration for 3 years to jointly acquire interferometric SAR data in bistatic mode. According to the current mission concept, there will be at least two complete coverages of the global land surface, each one running one year. The different coverages will have different heights of ambiguity to allow multi-baseline phase unwrapping. For the sake of a homogenous data quality the second acquisition will be shifted by half the swath width with respect to the first coverage. Finally difficult terrain will be covered two more times with different acquisition geometries (i.e. different look direction and/or incidence angles). This paper presents first study results of phase unwrapping algorithms foreseen to process SAR data from the bistatic TanDEM-X configuration.
    [bibtex-key = lachaiseEinederFritz2007] [bibtex-entry]


  1352. F. Lombardini. New potentials of differential SAR tomography: Volumetric differential interferometry and robust DEM generation. In Geoscience and Remote Sensing Symposium, 2007. IGARSS 2007. IEEE International, pages 5281-5284, July 2007. [bibtex-key = Lombardini2007b] [bibtex-entry]


  1353. Fabrizio Lombardini, Matteo Pardini, and Fulvio Gini. Sector interpolation for 3D SAR imaging with baseline diversity data. In Waveform Diversity and Design Conference, 2007. International, pages 297-301, June 2007. [bibtex-key = Lombardini2007a] [bibtex-entry]


  1354. Fabrizio Lombardini, Ludwig Rössing, Joachim H. G. Ender, and F. Viviani. Towards a Complete Processing Chain of Multibaseline Airborne InSAR Data for Layover Scatterers Separation. In Urban Remote Sensing Joint Event, 2007, pages 1-6, April 2007.
    Abstract: Interest is continuing to grow in exploiting the advanced multibaseline operation of synthetic aperture radar interferometry (InSAR) to solve layover effects, that can degrade conventional InSAR topographic mapping. In this work we report about experiments of the functionality of layover-free or higher-order interferometry with the dual-baseline single-pass SAR interferometer AER-II. Estimation of the number of multiple layover scatterers, i.e. of the interferometric order, and model-based spatial spectral estimation are integrated to process the three-antenna non uniform array data. Results are discussed for a bridge over the valley test site.
    [bibtex-key = LombardiniRoessingEnderViviani2007a:Tomo] [bibtex-entry]


  1355. Andrea Monti-Guarnieri and Stefano Tebaldini. A new framework for multi-pass SAR interferometry with distributed targets. In IEEE International Geoscience and Remote Sensing Symposium, pages 5289-5293, July 2007. Keyword(s): Monte Carlo methods, digital elevation models, radar interferometry, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth)DEM, InSAR, LOS displacement, Monte Carlo simulation, distributed scattering, multipass spaceborne SAR interferometry, repeated pass ENVISAT images, residual topography, synthetic aperture radar interferometry.
    Abstract: This paper focuses on multi-pass spaceborne synthetic aperture radar interferometry (InSAR) in presence of distributed scattering, paying particular attention to the role of target decorrelation in the estimation process. This phenomenon is accounted for by splitting the analysis into two steps. In the first step we estimate the interferometric phases from the data, while in the second step we use these phases to retrieve the physical parameters of interest, such as LOS displacement and residual topography. This approach is suited both to derive the performances of InSAR with different decorrelation models and for providing an actual estimate of LOS motion and DEM. Results achieved from Monte-Carlo simulations and a set of repeated pass ENVISAT images are shown.
    [bibtex-key = montiGuarnieriTebaldiniIGARSS2007:Tomo] [bibtex-entry]


  1356. Keith Morrison, Helmut Rott, Thomas Nagler, Helge Rebhan, and Patrick Wursteisen. The SARALPS-2007 measurement campaign on X- and Ku-Band Backscatter of snow. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1207-1210, July 2007. Keyword(s): snow, radar interferometry, radar polarimetry, snow, synthetic aperture radar, AD 2006 to 2007, Austrian Alps, InSAR mode, Innsbruck, SARALPS-2007 measurement campaign, active microwave sensors, polarimetric backscatter, snow parameters, snow water equivalent, spaceborne scatterometry, Backscatter, Frequency, Hydrologic measurements, Hydrology, Microwave sensors, Radar measurements, Snow, Spaceborne radar, Synthetic aperture radar, System testing, Synthetic aperture radar, ice, snow.
    Abstract: The retrieval of snow parameters, and snow water equivalent in particular, are key parameters in hydrology and climate research. Theory, ground-based signature research and analysis of spaceborne scatterometry suggests that the high- frequency combination of Ku- and X-band active microwave sensors is an excellent tool for the retrieval of snow physical properties. In order to validate this, a snow measurement campaign was carried out with the University of Cranfield's portable Ground-Based Synthetic Aperture Radar (GB-SAR) System during the winter of 2006/7 at two test-sites in the Austrian Alps close to Innsbruck. Fully polarimetric X-and Ku-band backscatter signatures were acquired over a range of incidence angles (~20deg-70deg), with the active sensor operating predominately in SAR mode, but occasionally also in InSAR mode. Microwave signatures and snow properties were measured on seven different dates. Detailed complementary meteorological and snow metamorphic conditions were also recorded.
    [bibtex-key = morrisonRottNaglerRebhanWursteisenIGARSS2007XandKuBandBackscatterSnow] [bibtex-entry]


  1357. Matteo Nannini and Rolf Scheiber. Height dependent motion compensation and coregistration for airborne SAR tomography. In International Geoscience and Remote Sensing Symposium, 2007. IGARSS 2007, pages 5041-5044, July 2007. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical signal processing, image registration, Motion Compensation, radar imaging, remote sensing by radar, synthetic aperture radar2D SAR repeat-pass processing, E-SAR system, L-Band, German Aerospace Centre, SAR imaging, airborne SAR tomography, data acquisition, height dependent motion compensation, image coregistration, multiple phase center separation, processing correction, tomogram quality, volumetric target.
    Abstract: SAR tomography (SARTom) is an imaging technique that allows multiple phase centers separation in the vertical (height) direction. It is performed after standard 2D SAR repeat-pass processing and operates on a stack of coregistered SAR images. Theoretically, the coregistration between two images is height dependent and the use of a reference height (or a DEM) is needed, although not ideal in the case of volumetric target (multiple phase centers in one resolution cell). In this paper, the drawbacks related to the choice of this reference in a tomographic context are analysed and a height dependent coregistration approach is proposed. In order to do this, it is also necessary to remove processing corrections related to the reference height, such as motion compensation, and make them height dependent. The inclusion of the height dependency during the tomographic SAR processing results in a better quality of the final tomograms in terms of pseudo-power and phase centers separation. The results of the proposed approach are validated on real data acquired by the E-SAR system of the German Aerospace Centre - DLR.
    [bibtex-key = nanniniScheiber2007:Tomo] [bibtex-entry]


  1358. C. Neumann, G. Weiss, A. Wahlen, and T. Brehm. Ground surveillance with mmW radar for border control and camp protection applications. In Proc. European Microwave Conference, pages 700-703, October 2007. Keyword(s): SAR Processing, W-Band, Airborne SAR, UAV, Fraunhofer, military radar, search radar, terrorism, automatic detection, automatic recognition, border control, camp protection application, ground surveillance, light weight miniature sensors, millimetre wave region, mmW radar, moving hostile objects, peace enforcing missions, peacekeeping missions, portable 35-GHz radar, terrorism, Bandwidth, Costs, Meteorological radar, Protection, Radar applications, Radar clutter, Signal resolution, Snow, Surveillance, Terrorism.
    Abstract: In a world where terrorism is threatening as well civilian people as the forces involved in peacekeeping and peace enforcing missions, the protection of borderlines of regions of interest as well as military camps is a necessary demand. To maintain day- and night performance under adverse weather conditions especially with the ability to cope with dust, fog and haze or snow, radar sensors are under development. Especially the millimetre wave region offers the additional advantage of small, light weight miniature sensors with the capability to accommodate a high signal bandwidth and thus high range resolution. To demonstrate these capabilities for both millimetre wave regions, Ka- and W-band, experiments were conducted jointly by EADS, Ulm, and FGAN-FHR, Wachtberg. A low cost and easily portable 35-GHz radar, originally designed for SAR applications in small UAVs, is used for automatic detection and recognition of moving hostile objects like persons and vehicles at distances of several kilometres in a strong ground clutter environment. A similar FM-CW radar operating at 94 GHz has been used under comparable environmental conditions to gather data at this radar band. It could be shown that possible foes will be recognized and exactly localized in range, azimuth and velocity. Possible applications of such sensors embedded in protection system concepts are presented as well as signature examples, algorithm approaches and results from outdoor experiments.
    [bibtex-key = neumannWeissWahlenBrehmEuMW2007WBandRadar] [bibtex-entry]


  1359. L. Pipia, X. Fabregas, A. Aguasca, C. Lopez-Martinez, J. J. Mallorqui, and O. Moraline. Polarimetric temporal information for urban deformation map retrieval. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, pages 192-195, July 2007. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, radar interferometry, radar polarimetry, synthetic aperture radar, town and country planning, PolSAR, Salient village, UPC RSLab, X-Band ground- based SAR sensor, differential interferometric applications, full scattering matrix, polarimetric persistent scatterers, polarimetric temporal information, urban deformation map retrieval, Fasteners, Geometry, Information retrieval, Interferometry, Laboratories, Polarization, Remote monitoring, Remote sensing, Scattering, Sensor phenomena and characterization. [bibtex-key = Pipia2007] [bibtex-entry]


  1360. Pau Prats, Christian Andres, Rolf Scheiber, Karlus A. Câmara de Macedo, Jens Fischer, and Andreas Reigber. Glacier displacement field estimation using airborne SAR interferometry. In IEEE International Geoscience and Remote Sensing Symposium, 2007. IGARSS 2007., pages 2098-2101, July 2007. Keyword(s): SAR Processing, SAR interferometry, InSAR, DInSAR, Spectral Diversity, Residual Motion Errors, Motion Compensation, MoComp, Airborne SAR, ESAR, airborne radar, glaciology, hydrological techniques, radar interferometry, synthetic aperture radar2D displacement map, Aletsch glacier, E-SAR, Experimental SAR, German Aerospace Center, Swiss Alps, across-track displacement, airborne SAR data, airborne, along-track displacement, extended multisquint approach, glacier displacement field estimation, slant-range geometry.
    Abstract: This paper deals with the methodology in the processing of airborne SAR data to measure glacier displacement fields. The possibility to retrieve a 2D displacement map of the deformation in slant-range geometry with an airborne platform is discussed. A new extended multisquint approach is proposed to simultaneously estimate residual motion errors and the along-track displacement of the glacier, while the across-track displacement is obtained by means of differential interferomatry. Experimental results are shown with data acquired by the Experimental SAR (E-SAR) of the German Aerospace Center over the Aletsch glacier in the Swiss Alps.
    [bibtex-key = pratsAndresScheiberdeMacedoFischerReigber2007:DInSARAletsch] [bibtex-entry]


  1361. Rolf Scheiber, Pau Prats, Matteo Nannini, Karlus A. Câmara de Macedo, Christian Andres, Jens Fischer, and Ralf Horn. Advances in airborne SAR interferometry using the experimental SAR system of DLR. In European Radar Conference, EuRAD 2007., 2007. Keyword(s): SAR Processing, SAR Tomography, Tomography, InSAR, Interferometry, PTA, SATA, Residual Motion Errors, Motion Compensation, DInSAR, Differential InSAR, Differential Interferometry, E-SAR.
    Abstract: During recent years the experimental SAR (E-SAR) system of the German Aerospace Center (DLR) has continuously been operated to acquire data for different scientific applications in forestry, agriculture and glaciology. In this context, the data where mainly used to develop new scattering models in preparation of future space-borne missions posing increasing accuracy requirements especially for the processing of repeat-pass interferometric data. Also direct model-free methods like differential SAR interferometry for measuring displacements in the order of the wavelength or SAR tomography for real 3D microwave imaging of scattering volumes determined the development of advanced and very accurate motion compensation techniques. This paper first gives an overview of the updates of the E-SAR system performed during the last couple of years and then focusses on the recently developed airborne interferometric processing methods and their applications.
    [bibtex-key = scheiberPratsNanninideMacedoAndresFischerHorn2007:ESAR] [bibtex-entry]


  1362. Philipp Thompson, Matteo Nannini, and Rolf Scheiber. Target separation in SAR image with the MUSIC algorithm. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2007., pages 468-471, July 2007. Keyword(s): SAR Processing, SAR Tomography, Tomography, radar imaging, synthetic aperture radar, DLR, E-SAR system, Fourier SAR image, MUSIC, MUSIC algorithm, Spectral Estimation, X-band, corner reflectors, post-processing implementation, super-resolution experiment, target separation, targets geometry.
    Abstract: The aim of this work is to exploit the MUSIC algorithm performance in order to enhance target separability in range and azimuth, i.e. achieve point targets separation inside a resolution cell. Simulations have been done in order to plan and check the feasibility of a super-resolution experiment that took place in September 2006 on the test site of Oberpfaffenhofen (Germany). The data set has been acquired with the E-SAR system of the DLR in X-band. The targets to be separated were seven small corner reflectors that have been placed in a way that their response falls in one or, at maximum, two resolution cells of the standard Fourier SAR image. A post-processing implementation of the MUSIC algorithm has been proposed allowing, in the already focused SAR image, to retrieve the targets geometry. Conditions and analysis of the results have been carried out.
    [bibtex-key = thompsonNanniniScheiberTomo2007:Music] [bibtex-entry]


  1363. Tan Weixian, Wang Yanping, Hong Wen, Wu Yirong, Li Nanjing, Hu Chufeng, and Zhang Linxi. SAR Three-Dimensional Imaging Experiments with Microwave Anechoic Chamber SAR Data. In Microwave Conference, 2007. APMC 2007. Asia-Pacific, pages 1-4, December 2007. [bibtex-key = Weixian2007] [bibtex-entry]


  1364. Andreas Wiesmann, Tazio Strozzi, Charles L. Werner, Urs Wegmuller, and Maurizio Santoro. Microwave remote sensing of alpine snow. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 1223-1227, July 2007. Keyword(s): SnowScat, KuScat, microwave measurement, remote sensing by radar, snow, ASSIST, Alpine Safety, Security and Information Services and Technologies, Alpine snow, C-band SAR, CoReH2O mission, SnowScat project, avalanche maps, avalanche warning, flood management, liquid water content, microwave remote sensing, snow coverage, snow structure, Content management, Ecosystems, Information security, Knowledge management, Microwave measurements, Remote sensing, Safety, Satellites, Snow, Space technology, ASSIST, CoReH20, SnowScat, avalanche, snow.
    Abstract: In the alpine zone snow is a dominant factor for more than half of the year and has strong influence on the ecosystem and economy. The knowledge of snow coverage, structure, liquid water content etc. is important and useful for many applications ranging from flood management to avalanche warning. Remote sensing from space has good potential to address these needs. Within ASSIST, Alpine Safety, Security and Information services and Technologies, these topics are also of interest. Two snow related products were identified that can be produced on an operational base with the available satellite systems to be ingested into the ASSIST service. Avalanche maps, mapping the contours of avalanches, and snow cover maps, mapping the snow covered area. The produced products are in good agreement with validation data. Unfortunately the current available satellite systems (mainly c-band SAR that is applicable) are not very well suited for snow related applications due to the small influence of the dry snow on the microwave signal at C- band. To overcome this limitation the CoReH2O mission was designed. With its X- and Ku-band system and repeat rates of 3 and 15 days it has high potential for alpine snow applications. Additional microwave signature measurements at these frequencies with standardized and reproducible snow characterization information will be needed for model development and validation. Recent developments allow a more quantitative snow characterizations and will be considered in the SnowScat project in combination with traditional snow characterization methods.
    [bibtex-key = wiesmannEtAlIGARSS2007SnowScat] [bibtex-entry]


  1365. Tan Wei Xian, Hong Wen, Wang Yan-ping, and Wu Yi-rong. A novel imaging approach for multi-baseline SAR tomography. In Synthetic Aperture Radar, 2007. APSAR 2007. 1st Asian and Pacific Conference on, pages 423-426, November 2007. [bibtex-key = Xian2007] [bibtex-entry]


  1366. Evan C. Zaugg and David G. Long. Full motion compensation for LFM-CW synthetic aperture radar. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2007., pages 5198-5201, July 2007. Keyword(s): SAR Processing, LFM-CW, LFM-CW SAR, MoComp, motion compensation, CSA, ECS, Chirp Scaling, Extended Chirp Scaling, FSA, Frequency Scaling Algorithm, Range-Doppler Algorithm, synthetic aperture radar, Brigham Young University, muSAR system, LFM-CW signal model, SAR image quality, aircraft, atmospheric turbulence, high-resolution synthetic aperture radar systems, linear frequency-modulated continuous-wave signal, motion correction algorithms, unmanned aerial vehicle, Airborne SAR, geophysical techniques. [bibtex-key = Zaugg2007] [bibtex-entry]


  1367. Howard Zebker, Pius Shankar, and Andy Hooper. InSAR Remote Sensing Over Decorrelating Terrains: Persistent Scattering Methods. In IEEE Radar Conference, pages 717-722, April 2007. Keyword(s): SAR Processing, decorrelation, Temporal Decorrelation, filtering theory, radar interferometry, radar signal processing, remote sensing by radar, synthetic aperture radar, terrain mapping, topography (Earth), vegetation mapping, InSAR remote sensing, San Francisco Bay segments, bare-Earth topography, decorrelation, detection theory, displacement measurement, echoes, filtering algorithms, information theoretic estimation, interferometric synthetic aperture radar, motion mapping, persistent scatterer detection, persistent scattering method, phase unwrapping, radar images, radar signals, spaceborne satellite data, stable true-ground scattering point identification, subtle surface motion mapping, surface deformation, terrain mapping, vegetation mapping, visual geodetic technique, Decorrelation, Displacement measurement, Radar imaging, Radar remote sensing, Radar scattering, Remote sensing, Spaceborne radar, Synthetic aperture radar interferometry, Terrain mapping, Vegetation mapping, InSAR, persistent scattering, radar remote sensing, surface deformation.
    Abstract: Interferometric synthetic aperture radar, or InSAR, is a visual geodetic technique permitting detailed mapping of motion over wide areas. InSAR has been limited to regions without much vegetation, which shields the ground from the radar signals and contributes random motions to the observed deformation. The resulting "decorrelation" of the echoes precludes accurate displacement measurements in these areas. Decorrelation also occurs in interferograms with acquisitions separated too far in the sky. Yet certain points, denoted persistent scatterers, in a radar image are stable, do not decorrelate, and form a network of fiducial points that allow measurements in otherwise poor-quality interferograms. We have generalized an algorithm to find networks of stable points in natural terrain, rather than in urban areas, and applied the method to spaceborne satellite data. Using modern information theory to optimize persistent scatterer detection, we can now find many, many more such points than previously possible. We have applied this improved algorithm to the San Francisco Bay segments of the San Andreas and Hayward faults, and in both cases find that a large number of stable points are seen in the vegetated areas that have to date resisted InSAR analysis. Our method of integrating information theoretic estimation and detection theory to all parts of the method, improves the identification, filtering, and phase unwrapping of the observations. Identification of stable true-ground scattering points permits mapping of subtle surface motions and deformations and also of "bare-Earth" topography.
    [bibtex-key = zebkerShankarHooperRadCon2007TempDecorrelation] [bibtex-entry]


  1368. E. Candes and J. Romberg. Robust Signal Recovery from Incomplete Observations. In Proc. IEEE Int. Conf. Image Processing, pages 1281-1284, October 2006. Keyword(s): convex optimization program, linear measurement, sparse signal reconstruction, convex programming, signal reconstruction;.
    Abstract: Recently, a series of exciting results have shown that it is possible to reconstruct a sparse signal exactly from a very limited number of linear measurements by solving a convex optimization program. If our underlying signal f can be written as a superposition of B elements from a known basis, it is possible to recover f from a projection onto a generic subspace of dimension about B log N. Moreover, the procedure is robust to measurement error; adding a perturbation of size isin to the measurements will not induce a recovery error of more than a small constant times isin. In this paper, we will briefly overview these results, and show how the recovery via convex optimization can be implemented in an efficient manner, and present some numerical results illustrating the practicality of the procedure.
    [bibtex-key = 4106771] [bibtex-entry]


  1369. H. Chen and D. Kasilingam. Auto-Regressive Aperture Extrapolation for Multibaseline SAR Tomography. In Geoscience and Remote Sensing Symposium, 2006. IGARSS 2006. IEEE International Conference on, pages 3743-3745, July 2006. [bibtex-key = Chen2006] [bibtex-entry]


  1370. Karlus A. Câmara de Macedo, Rolf Scheiber, and Alberto Moreira. First Evaluations of Airborne InSAR Time-Series. In Proc. of EUSAR 2006 - 6th European Conference on Synthetic Aperture Radar, 2006. Keyword(s): SAR Processing, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, Permanent Scatterers, Phase Gradient Autofocus, PS-PGA, Autofocus, Motion Compensation, Residual Motion Error, Repeat-Pass Interferometry, Phase Calibration, Baseline Calibration, Tomographic Baseline Calibration, Airborne SAR, E-SAR.
    Abstract: To allow time-series analysis of airborne SAR images using PSs (Permanent Scatterers), this paper has two main objectives. The first is to show, in a quantitative way, that there is a compromise between the number of images used to detect PSs, their probability of being detected and their stability. This tradeoff is derived based on estimation and detection theories. The second objective is to investigate the possibility of the use of permanent scatterers to estimate undesired phase undulations in airborne data due to residual motion errors. A new technique is proposed, the so-called PS-PGA, where we apply the Phase Gradient algorithm on the PSs in order to obtain sub-wavelength estimations of residual motion errors for both master and slaves, separately, differently from current approaches. Compensation of these residual errors will lead to more reliable airborne D-InSAR measurements.
    [bibtex-key = deMacedoScheiberMoreira2006:InSAR] [bibtex-entry]


  1371. An Daoxiang, Huang Xiaotao, and Wang Liang. Contrast Optimized PGA Algorithm for P-band UWB SAR. In Radar, 2006. CIE '06. International Conference on, pages 1-4, Oct. 2006. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, Contrast Optimization.
    Abstract: The Phase Gradient Autofocus (PGA) technique has been proved to be a superior method for higher order autofocus. However, given the different nature of P-band Ultra-Wideband Synthetic Aperture Radar (UWB SAR) imagery and very low signal-to-clutter ratio in the data, conventional PGA algorithm usually fail to yield robust focusing result on raw data without distortion. We proposed the modified PGA algorithm that combines the traditional PGA with contrast optimization principle and reference scatterer coregistration. Our method is successfully applied on raw P-band UWB SAR image and excellent results are attained.
    [bibtex-key = Daoxiang2006] [bibtex-entry]


  1372. Cao Fang, Hong Wen, and Wu Yirong. An improved Cloude-Pottier decomposition using H/alpha/SPAN and complex Wishart classifier for polarimetric SAR classification. In International Conference on Radar, 2006. CIE '06., pages 1-4, October 2006. Keyword(s): image classification, radar imaging, radar polarimetry, synthetic aperture radarCloude-Pottier decomposition, H/alpha/SPAN, IHSL transform, Wishart classifier, polarimetric SAR classification, synthetic aperture radar.
    Abstract: An improvement is proposed for the Cloude-Pottier decomposition using H/alpha/SPAN and IHSL transform. Based on this decomposition, an unsupervised classification with SPAN is also given in this paper. The main advantages of this decomposition are that it uses SPAN to maintain the space information for further polarimetric analysis and provides a straight way to present the result. Our experiments show that this decomposition method provides better results than the general Cloude-Pottier method and the corresponding Wishart H/alpha/SPAN classification also achieves better performance than the current Wishart H/alpha/A method
    [bibtex-key = fangWenYirong2006:EntropyAlphaSpan] [bibtex-entry]


  1373. Othmar Frey, Erich Meier, and Daniel Nüesch. An Integrated Focusing and Calibration Procedure for Airborne SAR Data. In Proc. EUSAR 2006 - 6th European Conference on Synthetic Aperture Radar, 2006. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Terrain Correction, Radiometry, Radiometric Calibration, Radiometric Correction, Terrain, Topography, DEM Geocoding, Geocoding, DLR, E-SAR, Airborne SAR, L-Band.
    Abstract: Topography-induced variation of radar brightness still poses a problem in terms of radiometric calibration of SAR data, which is undoubtedly an indispensable step in order to deduce bio- or geophysical parameters from amplitude images. We propose an integrated focusing and calibration procedure for airborne SAR data based on the time-domain backprojection technique. With the help of sensor position and attitude data as well as a digital elevation model (DEM) the true acquisition geometry is maintained throughout the combined focusing and calibration step. The effectiveness of the method is evaluated by means of an E-SAR L-band data set acquired over undulating terrain.
    [bibtex-key = freyMeierNueschEusar2006:TDBP] [bibtex-entry]


  1374. Per-Olov Frölind and Lars M. H. Ulander. Evaluation of angular interpolation kernels in fast back-projection SAR processing. In IEE Proceedings -- Radar, Sonar and Navigation, volume 153, pages 243-249, June 2006. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Fast Factorized Back-Projection, Comparison of Algorithms, interpolation, interpolation kernels, angular interpolation kernels, radar imaging, synthetic aperture radar, ultra wideband radar, UWB SAR, interpolation method, phase error, polar version, subimage version, time domain SAR algorithm, Factorized Backprojection.
    Abstract: This paper presents a comparative study of the polar and the subimage based variants of the time domain SAR algorithm Fast Factorized Backprojection. The difference between the two variants with regard to the phase error, which causes defocusing in the image, is investigated. The difference between the algorithms in interpolation between stages is also discussed. To investigate the sidelobes in azimuth, the paper gives simulation results for a low frequency UWB SAR system for both algorithms. How the algorithms differ with regard to amount of beams and length of beams is also discussed.
    [bibtex-key = froelindUlander2006:FFBP] [bibtex-entry]


  1375. Charles V. Jakowatz and Neall Doren. Comparison of polar formatting and back-projection algorithms for spotlight-mode SAR image formation. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 6237, pages 62370H, 2006. SPIE. Keyword(s): SAR Processing, Polar Format Algorithm, PFA, Convolution Backprojection Algorithm, CPB, Comparison of Algorithms, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzDorenPFABackprojectionComparison2006] [bibtex-entry]


  1376. Charles V. Jakowatz and Daniel E. Wahl. Correction of propagation-induced defocus effects in certain spotlight-mode SAR collections. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 6237, pages 62370I, 2006. SPIE. Keyword(s): SAR Processing, Polar Format Algorithm, PFA, Atmospheric Influence, Phase Gradient Algorithm, PGA, Autofocus, Residual Phase Errors, Residual Error, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzWahlPGASpotlightmode2006] [bibtex-entry]


  1377. Bert M. Kampes and Nico Adam. The STUN algorithm for persistent scatterer interferometry. In Fringe 2005 Workshop, volume 610, 2006. [bibtex-key = kampesAdamFRINGE2006STUNalgoPSI] [bibtex-entry]


  1378. Fabrizio Lombardini, Ludwig Rössing, Joachim H. G. Ender, and F. Cai. Interferometric Model Order Selection: Validation of ITC Methods with Airborne Three-antenna SAR Data. In Geoscience and Remote Sensing Symposium, 2006. IGARSS 2006. IEEE International Conference on, pages 2565-2568, July 2006. [bibtex-key = Lombardini2006] [bibtex-entry]


  1379. J.M. Munoz-Ferreras, J. Calvo-Gallego, F. Perez-Martinez, A. Blanco-del-Campo, A. Asensio-Lopez, and B.P. Dorta-Naranjo. Motion compensation for ISAR based on the shift-and-convolution algorithm. In IEEE Conference on Radar, pages 1-5, April 2006. Keyword(s): ISAR. [bibtex-key = Munoz-Ferreras2006] [bibtex-entry]


  1380. Matteo Nannini and Rolf Scheiber. A Time Domain Beamforming Algorithm for SAR Tomography. In Proc. of EUSAR 2006 - 6th European Conference on Synthetic Aperture Radar, 2006. Keyword(s): SAR Processing, SAR Tomography, Tomography, Time-Domain Beamforming, Phase Calibration, Baseline Calibration, Tomographic Baseline Calibration, Airborne SAR, E-SAR.
    Abstract: Interest on 3D imaging in a remote sensing frame has grown in the recent years and it finds in SAR Tomography (TomSAR) a natural way to resolve for targets in the third dimension. In this paper we compare the performance of a pure beamforming technique with the SpecAn algorithm. This comparison has the goal to estabilish if the time domain beamforming (TDB) performance are efficient in order to perform tomographic focusing. A tomographic baseline calibration is also presented. The TDB is applied on simulated and real airborne data in L-band. The real data have been acquired during a tomographic campaign in May 1998 on the test site of Oberpfaffenhofen (Germany) with the E-SAR system of the German Aerospace Center (DLR).
    [bibtex-key = nanniniScheiber06:TimeDomainTomo] [bibtex-entry]


  1381. P. Prats, A. Reigber, Jordi J. Mallorqui, P. Blanco, and A. Moreira. Estimation of the Deformation Temporal Evolution Using Airborne Differential SAR Interferometry. In Geoscience and Remote Sensing Symposium, 2006. IGARSS 2006. IEEE International Conference on, pages 1894-1897, July 2006. Keyword(s): SAR Processing, D-InSAR, ESAR, Airborne SAR, L-Band, differential interferometry, Interferometry, Motion Compensation, RME, Residual Motion Errors, deformation measurement. [bibtex-key = PratsReigberMallorquiBlancoMoreira2006a:DInSAR] [bibtex-entry]


  1382. Brian D. Rigling. Multistage entropy minimization for SAR image autofocus. In Edmund G. Zelnio and Frederick D. Garber, editors, Algorithms for Synthetic Aperture Radar Imagery XIII, volume 6237, pages 150 - 159, 2006. International Society for Optics and Photonics, SPIE. Keyword(s): SAR Processing, SAR, ground map, autofocus.
    Abstract: This paper discusses a multistage approach to entropy minimization for SAR image autofocus. The new algorithm is compared to existing approaches, including point based autofocus, sub-aperture based autofocus, and hybrid methods. Monte Carlo statistical results are presented for simulated clutter scenes and point target scenes. The new minimum entropy autofocus provides improved speed and accuracy in correcting azimuth phase errors in both scenarios.
    [bibtex-key = riglingSPIE2006MultiStageEntropyMinimizationSARImageAutofocus] [bibtex-entry]


  1383. J. Sanz-Marcos, Jordi J. Mallorqui, A. Aguasca, and P. Prats. First ENVISAT and ERS-2 Parasitic Bistatic Fixed Receiver SAR Images Processed with the Subaperture Range-Doppler Algorithm. In Geoscience and Remote Sensing Symposium, 2006. IGARSS 2006. IEEE International Conference on, pages 1840-1843, August 2006. [bibtex-key = SanzMarcosMallorquiAguascaPrats2006:Bistatic] [bibtex-entry]


  1384. H.P. Tran, F. Gumbmann, J. Weinzierl, and L.P. Schmidt. A Fast Scanning W-Band System for Advanced Millimetre-Wave Short Range Imaging Applications. In Proc. European Radar Conference, pages 146-149, September 2006. Keyword(s): SAR Processing, W-Band, broadband antennas, focusing, frequency response, millimetre wave antennas, millimetre wave imaging, millimetre wave measurement, radar antennas, radar imaging, scanning antennas, synthetic aperture radar, SAR, antenna, broadband frequency response, conical horn, dielectric lens, focused bistatic measurement setup, free space millimetre-wave imaging setup, planar test object, scanning W-band system, synthetic aperture radar algorithm, unfocused measurement setup, Antenna measurements, Costs, Dielectric losses, Dielectric measurements, Distortion measurement, Electromagnetic measurements, Focusing, Lenses, Microwave imaging, Thickness measurement.
    Abstract: The paper presents the development of a fast scanning W-Band (75-100 GHz) system and a suitable antenna concept for advanced imaging applications. The goal is to obtain a broadband frequency response from a large target area within a short time period. Two typical approaches for the design of a free space millimetre-wave imaging setup will be discussed. The first approach consists of a focused bistatic measurement setup that includes a combination of a conical horn and a dielectric lens in order to focus the beam. The basic idea of the second approach is to use an unfocused measurement setup employing synthetic aperture radar (SAR) algorithms in order to focus the image numerically. Both measurement setups will be discussed with respect to the capability for fast scanning mm-wave imaging systems. Experimental results on planar test objects demonstrate the performance of the developed W-Band system compared to a commercial vector network analyzer (VNA)
    [bibtex-key = tranGumbmannWeinzierlSchmidtEURAD2006WBandRadar] [bibtex-entry]


  1385. U. Wegmuller, C. Werner, T. Strozzi, and A. Wiesmann. Ionospheric Electron Concentration Effects on SAR and INSAR. In 2006 IEEE International Symposium on Geoscience and Remote Sensing, pages 3731-3734, July 2006. Keyword(s): SAR Processing, Ionosphere, Ionospheric Path Delay, TEC, Azimuth, Earth, Electrons, Frequency, Ionosphere, L-band, Layout, Synthetic aperture radar, Synthetic aperture radar interferometry, Tracking. [bibtex-key = wegmullerWernerStrozziWiesmannIGARSS2006SARIonospericTEC] [bibtex-entry]


  1386. D. Yocky and D. Wahl. Minimum-Latency Polar Format Algorithm. In Proc. IEEE Int. Geosci. Remote Sens. Symposium, pages 3177-3179, July 2006. Keyword(s): SAR Processing, Real-Time, Real-Time Processing, Real-Time SAR, Video SAR, Polar Format Algorithm, PFA.
    Abstract: The polar format algorithm (PFA) is a computationally efficient image formation method for high-resolution spotlight-mode SAR data collections. Yet, its usual real-time implementation waits until the entire synthetic aperture has been collected before beginning image formation. This results in a latency time. This paper presents a new approach to PFA that performs range and azimuth interpolation and range compression as the aperture is collected. Thus, the remaining latency is approximately the azimuth compression time. Performing image formation during aperture collection makes minimum-latency PFA a viable real-time image formation algorithm.
    [bibtex-key = yockyWahlIEEE2006:RealTimePolarFormat] [bibtex-entry]


  1387. D. Yocky, D. Wahl, and C. V. Jakowatz Jr.. Spotlight-Mode SAR Image Formation Utilizing the Chirp Z-Transform in Two Dimensions. In Proc. IEEE Int. Geosci. Remote Sens. Symposium, pages 4180-4182, July 2006. Keyword(s): SAR Processing, SPECAN-like Processing, SPECAN, Chirp-Z Transform, Chirp-Z Transform-based Focusing, Spotlight SAR, Spotlight-mode data.
    Abstract: A new spotlight-mode synthetic aperture radar image formation approach is presented that directly utilizes the reflected electromagnetic returns collected on a polar grid. This approach eliminates polar-to-rectangular grid interpolation by employing chirp z-transforms in two dimensions. Since the chirp z-transform can be implemented via fast Fourier transforms (FFTs), this image formation algorithm consists almost entirely of FFTs allowing it to exploit computationally-efficient FFT engines for fast image formation speeds. This paper presents the algorithm, and compares the operation counts and execution time between the new algorithm and traditional polar formatting, which employs interpolation. The paper also presents conditions for which the chirp z-transform in two dimensions is advantageous.
    [bibtex-key = yockyWahlJakowatzSpotlightChirpZIEEE2006] [bibtex-entry]


  1388. E.C. Zaugg, D.L. Hudson, and D.G. Long. The BYU SAR: A Small, Student-Built SAR for UAV Operation. In Proc. IEEE Int. Geosci. Remote Sens. Symp., pages 411-414, July 2006. Keyword(s): SAR Processing, BYU SAR, UAV. [bibtex-key = Zaugg2006] [bibtex-entry]


  1389. X. Zheng, W. Yu, and Z. Li. A Novel Algorithm for Wide Beam SAR Motion Compensation Based on Frequency Division. In IEEE International Geoscience and Remote Sensing Symposium, 2006. IGARSS 2006., pages 3160-3163, August 2006. Keyword(s): SAR Processing, Motion Compensation, MoComp, P-Band, Wide Beamwidth, Airborne SAR, Frequency Division, Frequency-Division Motion Compensation, FD-MOCO.
    Abstract: This paper proposes a novel motion compensation algorithm for wide beam Synthetic Aperture Radar (SAR) based on frequency division. It applies motion compensation in time domain and corrects both low- and high- frequency motion errors. The rationale and procedure of this algorithm are introduced in detail. Point scatterers of a P-band airborne SAR with different motion errors are tested to validate this algorithm. Compared with traditional narrow beam motion compensation and available wide beam compensation, the proposed algorithm has better performance.
    [bibtex-key = zhengYuLi2006:MoComp] [bibtex-entry]


  1390. M. Blom and P. Follo. VHF SAR image formation implemented on a GPU. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS '05., volume 5, pages 3352-3356, July 2005. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, GPU, Graphic Processing Unit, GPU Processing, 3D Graphics Card, VHF, Airborne SAR, FOI, CARABAS.
    Abstract: This paper will describe how off-the-shelf 3D graphics cards can be used for scientific computation like SAR processing. In particular, a highly efficient one-dimensional FFT and a fast direct (global) backprojection implementation will be presented and analyzed.
    [bibtex-key = blomFollo2005:TDPBonGPU] [bibtex-entry]


  1391. F. Bordoni, A. Jakobsson, F. Gini, and F. Lombardini. On the Effects of Nonuniform Sampling for Interferometric Phase Estimation in the Presence of Layover. In Statistical Signal Processing, 2005 IEEE/SP 13th Workshop on, pages 645-650, July 2005. [bibtex-key = Bordoni2005] [bibtex-entry]


  1392. Karlus A. Câmara de Macedo, Christian Andres, and Rolf Scheiber. On the requirements of SAR processing for airborne differential interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 4, pages 2693-2696, July 2005. Keyword(s): SAR Processing, PTA-MoComp, Postprocessing, Motion Compensation, Topography-Based Motion Compensation, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, fast Fourier transform-based postprocessing methodology, FFT, D-InSAR, German Aerospace Center, DLR, airborne repeat-pass interferometry, differential interferometry, geometric fidelity, motion errors, phase accuracy, residual phase errors, topographic heights, Topography, DEM, Terrain, wide beamwidth, Airborne SAR, ESAR.
    Abstract: Airborne Differential SAR Interferometry (DInSAR) is still a challenging task when compared to the spaceborne case due to the fact that airborne platforms are unable to describe a stable flight track. For that reason a very precise motion compensation which includes the correction of topographic-induced phase errors has to be performed in the airborne SAR data. This paper presents the required steps of phase correction to achieve accurate airborne D-InSAR data. The latest airborne D-InSAR processing chain of the E-SAR system is shown. Differential interferograms results using the proposed processing chain are also shown.
    [bibtex-key = macedoAndresScheiber2005:DInSAR] [bibtex-entry]


  1393. Armin W. Doerry. Autofocus correction of SAR images exhibiting excessive residual migration. In Robert N. Trebits and James L. Kurtz, editors, Proceedings of SPIE Vol. 5788, Radar Sensor Technology IX, volume 5788, pages 34-45, 2005. SPIE. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus.
    Abstract: Relatively small motion measurement errors manifest themselves principally as a phase error in Synthetic Aperture Radar (SAR) complex data samples, and if large enough become observable as a smearing, blurring, or other degradation in the image. The phase error function can be measured and then deconvolved from the original data to compensate for the presumed motion error, ultimately resulting in a well-focused image. Techniques that do this are termed ?autofocus? algorithms. A very popular autofocus algorithm is the Phase Gradient Autofocus (PGA) algorithm. The nearly universal, and typically reasonable, assumption is that the motion errors are less than the range resolution of the radar, allowing solely a phase correction to suffice. Very large relative motion measurement errors manifest themselves as an unexpected additional shifting or migration of target locations beyond any deterministic migration during the course of the synthetic aperture. Degradation in images from data exhibiting errors of this magnitude are substantial, often rendering the image completely useless. When residual range migration due to either real or apparent motion errors exceeds the range resolution, conventional autofocus algorithms fail. Excessive residual migration is increasingly encountered as resolutions become finer, less expensive inertial sensors are used, and operating ranges become longer (due to atmospheric phenomena). A new migration-correction autofocus algorithm has been developed that estimates the excessive residual migration and applies phase and frequency corrections to properly focus the image. This overcomes the conventional constraint that motion errors not exceed the SAR range resolution.
    [bibtex-key = Doerry2005Autofocus] [bibtex-entry]


  1394. Armin W. Doerry, Dale F. Dubbert, Martin Thompson, and Vivian D. Gutierrez. A portfolio of fine resolution Ka-band SAR images: part I. In Robert N. Trebits and James L. Kurtz, editors, Proc. of SPIE Vol. 5788, Radar Sensor Technology IX, number 1, pages 13-24, 2005. SPIE. Keyword(s): SAR Processing, High Resolution, Fine Resolution, Sandia National Laboratoiries, Ka-Band.
    Abstract: Sandia National Laboratories designs and builds Synthetic Aperture Radar (SAR) systems capable of forming highquality exceptionally fine resolution images. During the spring of 2004 a series of test flights were completed with a Ka-band testbed SAR on Sandia?s DeHavilland DHC-6 Twin Otter aircraft. A large data set was collected including real-time fine-resolution images of a variety of target scenes. This paper offers a sampling of high quality images representative of the output of Sandia?s Ka-band testbed radar with resolutions as fine as 4 inches. Images will be annotated with descriptions of collection geometries and other relevant image parameters.
    [bibtex-key = DoerryDubbertThompsonGutierrez2005:PartOne] [bibtex-entry]


  1395. Armin W. Doerry, Dale F. Dubbert, Martin Thompson, and Vivian D. Gutierrez. A portfolio of fine resolution Ka-band SAR images: part II. In Robert N. Trebits and James L. Kurtz, editors, Proc. of SPIE Vol. 5788, Radar Sensor Technology IX, number 1, pages 185-196, 2005. SPIE. Keyword(s): SAR Processing, High Resolution, Fine Resolution, Sandia National Laboratoiries, Ka-Band.
    Abstract: Sandia National Laboratories designs and builds Synthetic Aperture Radar (SAR) systems capable of forming highquality exceptionally fine resolution images. During the spring of 2004 a series of test flights were completed with a Ka-band testbed SAR on Sandia?s DeHavilland DHC-6 Twin Otter aircraft. A large data set was collected including real-time fine-resolution images of a variety of target scenes. This paper offers a sampling of high quality images representative of the output of Sandia?s Ka-band testbed radar with resolutions as fine as 4 inches. Images will be annotated with descriptions of collection geometries and other relevant image parameters.
    [bibtex-key = DoerryDubbertThompsonGutierrez2005:PartTwo] [bibtex-entry]


  1396. Othmar Frey, Erich Meier, and Daniel Nüesch. A Study on Integrated SAR Processing and Geocoding by Means of Time-Domain Backprojection. In Proc. Int. Radar Symp., 2005. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Geocoding, Radiometry, Calibration, Radiometric Calibration, ENVISAT/ASAR, Spaceborne SAR.
    Abstract: Geocoded products of synthetic aperture radar data are of great interest for many applications. The conventional processing chain, which leads to geographically referenced synthetic aperture data consists of two main steps: first, the raw data are focused and, in a second step, the resulting single look complex image is geocoded to the favoured coordinate system. We investigate a time-domain backprojection approach that replaces the two steps, focusing and geocoding, by one algorithm leading directly to terrain-geocoded images. The technique is evaluated with ENVISAT/ASAR image mode data. We assess the geolocation accuracy and the radiometric performance of dedicated point targets such as transponders and a corner reflector. In addition, we compare our findings with results from corresponding level 1 products processed at the European Space Agency (ESA), which were validated within the scope of ENVISAT/ASAR Cal/Val activities.
    [bibtex-key = freyMeierNueschIRS05:StudyIntegratedBackproj] [bibtex-entry]


  1397. Othmar Frey, Erich Meier, and Daniel Nüesch. Processing SAR data of rugged terrain by time-domain back-projection. In SPIE Vol. 5980: SAR Image Analysis, Modeling, and Techniques X, 2005. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Terrain Correction, Radiometry, Radiometric Calibration, Radiometric Correction, Terrain, Topography, DEM Geocoding, ENVISAT/ASAR, Spaceborne SAR.
    Abstract: Processing of SAR images of rugged terrain deserves special care because the topography affects the focused image in a number of ways. In order to obtain geometrically and radiometrically corrected SAR images of mountainous areas additional knowledge about the topography and the sensor's trajectory and attitude has to be included in the processing or post-processing steps. Various well-known focusing techniques are available to transform SAR raw data into a single look complex image such as the range-Doppler, the chirp scaling or the omega-k algorithm. While these algorithms perform the azimuth focusing step in the frequency domain the time-domain back-projection processing technique focuses the data geometrically, i.e., in the time domain. In contrast to the frequency-domain techniques, time-domain back-projection maintains the entire geometric relationship between the sensor and the illuminated area. This implies a couple of advantages: a stringent, terrain-based correction for the elevation antenna gain pattern may be implemented and topography-induced variation of radar brightness can be eliminated in a single step. Further, the SAR image is focused directly onto an arbitrary reconstruction grid and in the desired geodetic reference frame without requiring any additional processing steps. We discuss the influence of rugged terrain on the radiometric properties of focused SAR data and demonstrate how the time-domain back-projection approach accounts for these effects within one integrated processing framework by incorporating both a correction for terrain slope induced variation of radar brightness and a stringent correction for the elevation antenna gain pattern. The algorithm is evaluated for ENVISAT/ASAR image mode data of a mountainous area.
    [bibtex-key = freyMeierNueeschSPIE2005:Backprojection] [bibtex-entry]


  1398. U. Gebhardt, Ottmar Loffeld, H. Nies, S. Knedlik, and Joachim H. G. Ender. Bistatic airborne/spaceborne hybrid experiment: basic considerations. In Roland Meynart, Steven P. Neeck, and Haruhisa Shimoda, editors, Proc. of SPIE Vol.5978, Sensors, Systems, and Next-Generation Satellites IX, number 1, 2005. SPIE. Keyword(s): SAR Processing, Bistatic SAR, Hybrid Bistatic SAR, Bistatic Airborne/Spaceborne SAR, Airborne SAR, Spaceborne SAR, Hybrid, Spotlight Mode, Sliding Spotlight Mode, Simulation, TerraSAR-X.
    Abstract: Collecting data using different sensors mounted on different platforms is the challenge of multisensorics. Applications in Synthetic Aperture Radar (SAR) normally lead to extreme bi- or multistatic constellations in the multisensorial case. This paper describes basic considerations concerning the geometry, especially the antenna steering for a bistatic SAR experiment. Using the TerraSAR-X as a transmitter and a SAR system mounted on a plane as a receiver we want to record experimental raw data for further processing. Because of the high difference between the velocity of the transmitter platform and that of the receiver platform relative to a point target, stripmap mode is not useful in this case. By operating the transmitter in sliding spotlight or spotlight mode and using antenna steering to provide footprint chasing on the side of the receiving system, a useful scene extension in azimuth can be achieved. This is of course at the cost of a shorter time interval in which the point target is both illuminated by the transmitter and seen by the receiver. First simulations of a point target response will show that nevertheless we can expect a useful Doppler bandwidth and thus an adequate resolution in azimuth.
    [bibtex-key = GebhardtLoffeldNiesKnedlikEnder2005] [bibtex-entry]


  1399. Stéphane Guillaso and Andreas Reigber. Polarimetric SAR Tomography (POLTOMSAR). In Proceedings of POLINSAR'05, Frascati, Italy, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, MUSIC, Beamforming, Capon, Modified MUSIC, DoA, Direction of Arrival, Fourier, Pol-InSAR, L-Band, ESAR.
    Abstract: In this paper, different approaches of airborne SAR tomography are presented. A SAR tomographic data acquisition system can be represented like an antenna array. The use of high-resolution methods is indicated to the concept of aperture synthesis for 3D-imaging using SAR data. Techniques presented are the standard Fourier-, Capon-based beamforming methods to improve the resolution quality. In order to estimate the nature of retrieved target, a polarimetric approach is introduced based on the modified high-resolution MUSIC algorithm. Finally, experimental results are shown using a multibaseline data set acquired in L-band by DLR?s experimental SAR (E-SAR) on a test site near Oberpfaffenhofen / Germany.
    [bibtex-key = guillasoReigber05:TomoSAR] [bibtex-entry]


  1400. S. Guillaso and A. Reigber. Scatterer characterisation using polarimetric SAR tomography. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 4, pages 2685-2688, July 2005. [bibtex-key = Guillaso2005a] [bibtex-entry]


  1401. S. Guillaso, A. Reigber, and L. Ferro-Famil. Evaluation of the ESPRIT approach in polarimetric interferometric SAR. In Proc. Geosci. Remote Sens. Symp., volume 1, pages 1-4, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, airborne radar, data acquisition, radar imaging, radar polarimetry, radiowave interferometry, remote sensing by radar, stereo image processing, synthetic aperture radar, tomography, vegetation mapping, 3D images, ESPRIT approach, German Aerospace Center, SAR tomography, airborne L-band repeat-pass interferometric data, experimental airborne SAR, polarimetric interferometric SAR, tomographic image, vegetation, volumetric area, E-SAR.
    Abstract: This paper presents a first evaluation of the ESPRIT approach in polarimetric interferometric SAR. This evaluation is carried out by using 3D images obtained by SAR tomographic like an alternative to the acquisition of ground-truth data, which is an extremely complex task in the case of volume areas. All parameters over a volumetric area are directly visible in a tomographic image and can, therefore, be employed to validate the ESPRIT approach by comparing parameters generated by ESPRIT and the SAR tomography approach. This allows to identify the principal deficiencies of the ESPRIT method, which occur over high vegetation areas, where there is a misinterpretation of the ESPRIT results. Whereas, the ESPRIT approach is useful for building characterisation, identifying a good applicability area. Airborne L-band repeat-pass interferometric data of the German Aerospace Center (DLR) experimental airborne SAR are used to perform this evaluation.
    [bibtex-key = guillasoReigberFerroFamil05:Tomo] [bibtex-entry]


  1402. T. Hamasaki, L. Ferro-Famil, E. Pottier, and M. Sato. Applications of polarimetric interferometric ground-based SAR (GB-SAR) system to environment monitoring and disaster prevention. In Proc. EURAD 2005. European Radar Conf, pages 29-32, October 2005. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, disasters, environmental management, monitoring, radar polarimetry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, coniferous tree, differential interferometry, disaster prevention, environment monitoring, natural phenomena, polarimetric interferometric ground-based SAR, resources management, scattering mechanisms, Aperture antennas, Frequency, Information analysis, Interferometry, Optical scattering, Radar detection, Radar scattering, Remote monitoring, Spaceborne radar, Synthetic aperture radar. [bibtex-key = Hamasaki2005] [bibtex-entry]


  1403. J. Kolman. PACE: an autofocus algorithm for SAR. In Proc. IEEE Int. Radar Conference, pages 310-314, May 2005. Keyword(s): SAR Processing, Autofocus, Phase Adjustment by Contrast Enhancement, PACE, Azimuth, Focusing, Error correction, Synthetic aperture radar, Phase measurement, Pixel, Flexible printed circuits, Hardware, History, Error analysis. [bibtex-key = kolmanIEEERadarCon2005PhaseAdjustmentByContrastEnhancementAutofocus] [bibtex-entry]


  1404. V. C. Koo, T. S. Lim, and H. T. Chuah. A Comparison of Autofocus Algorithms for SAR Imagery. In Progress In Electromagnetics Research Symposium, volume 1, Hangzhou, China, pages 16-9, 2005. Keyword(s): SAR Processing, Autofocus, Motion Compensation, MoComp, Residual Motion Errors, Comparion of Algorithms, Comparison of Autofocus Algorithms, Airborne SAR, Phase Gradient Autofocus, PGA, Eigenvector Method, Maximum Likelihood Estimation.
    Abstract: A challenge in SAR system development involves compensation for nonlinear motion errors of the sensor platform. The uncompensated along-track motions can cause a severe loss of geometry accuracy and degrade SAR image quality. Autofocus techniques improve image focus by removing a large part of phase errors present after conventional motion compensation. It refers to the computer-automated error estimation and subsequent removal of the phase errors. Many autofocus algorithms have been proposed over the years, ranging from quantitative measurement of residual errors to qualitative visual comparison. However, due to the fact that different data sets and motion errors were employed, it is difficult to perform comparative studies on various algorithms. This paper compares and discusses some practical autofocus algorithms by using a common data set. Standard focal quality metrics are defined to measure how well an image is focused. Their implementation schemes and performance are evaluated in the presence of various phase errors, which include polynomial-like, high frequency sinusoidal, and random phase noise.
    [bibtex-key = kooLimChuah2005:AutofocusComparison] [bibtex-entry]


  1405. Shu Li and Ian Cumming. Improved beat frequency estimation in the MLBF Doppler ambiguity resolver. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 5, pages 3348-3351, 2005. Keyword(s): SAR Processing, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, MLBF, Multilook Beat Frequency, Improved Multilook Beat Frequency.
    Abstract: Among the current Doppler ambiguity resolvers, the Multi-Look Beat frequency (MLBF) algorithm proves to be the most reliable one, especially in high contrast areas. The existing MLBF algorithm uses FFTs to measure the central frequency of the beat signal but the estimation accuracy is limited by quantization errors. This paper proposes an improved method of estimating the beat frequency in the MLBF algorithm that is based on phase increments. In our work, we examined five established frequency estimators and found that the Iterative Linear Prediction (ILP) method has the best performance. The experimental results on RADARSAT-1 data show that the new MLBF algorithm using ILP can obtain the correct ambiguity number in a higher percentage of blocks and that the RMS error of the results is less than half that of the existing method.
    [bibtex-key = liCumming2005:DopAmb] [bibtex-entry]


  1406. F. Lombardini. Analysis of non-gaussian speckle statistics in high-resolution SAR images. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 2, pages 1337-1340, July 2005. [bibtex-key = Lombardini2005] [bibtex-entry]


  1407. F. Lombardini and G. Fornaro. First trials of fourier and adaptive tomo-doppler sar imaging. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 4, pages 2656-2659, July 2005. [bibtex-key = Lombardini2005a] [bibtex-entry]


  1408. C. Lopez-Martinez and E. Pottier. Topography independent InSAR coherence estimation in a multiresolution scheme. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS'05, volume 4, pages 2689-2692, July 2005. Keyword(s): SAR Processing, Coherence, Coherence Estimation, InSAR, Interferometry, SAR Interferometry, Speckle Noise, Wavelet Transform. [bibtex-key = lopezMartinezPottier2005:Coherence] [bibtex-entry]


  1409. Andreas Muschinski, Fred M. Dickey, and Armin W. Doerry. Possible effects of clear-air refractive-index perturbations on SAR images. In Robert N. Trebits and James L. Kurtz, editors, Proc. of SPIE Vol. 5788, Radar Sensor Technology IX, number 1, pages 25-33, 2005. SPIE. Keyword(s): SAR Processing, Atmospheric Modelling, clear-air radar, thin-lens approximation, clear-air refractive index, sheets, gravity waves, diffraction patterns.
    Abstract: Airborne synthetic aperture radar (SAR) imaging systems have reached a degree of accuracy and sophistication that requires the validity of the free-space approximation for radio-wave propagation to be questioned. Based on the thin-lens approximation, a closed-form model for the focal length of a gravity wave-modulated refractive-index interface in the lower troposphere is developed. The model corroborates the suggestion that mesoscale, quasi-deterministic variations of the clear-air radio refractive-index field can cause diffraction patterns on the ground that are consistent with reflectivity artifacts occasionally seen in SAR images, particularly in those collected at long ranges, short wavelengths, and small grazing angles.
    [bibtex-key = MuschinskiDickeyDoerry2005] [bibtex-entry]


  1410. L. Pipia, A. Aguasca, X. Fabregas, J. J. Mallorqui, and C. Lopez-Martinez. Temporal decorrelation in polarimetric differential interferometry using a ground-based SAR sensor. In Proc. IEEE Int. Geoscience and Remote Sensing Symp. IGARSS '05, volume 6, pages 4108-4111, July 2005. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, Azimuth, Decorrelation, Temporal Decorrelation, Interferometry, Monitoring, Polarization, Satellites, Semiconductor device measurement, Testing, Urban areas, Vegetation. [bibtex-key = Pipia2005] [bibtex-entry]


  1411. P. Prats, A. Reigber, and J. J. Mallorqui. Topography accommodation during motion compensation in interferometric repeat-pass SAR images. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 1, pages 1-4, July 2005. Keyword(s): SAR Processing, Motion Compensation, Topography-Based Motion Compensation, ESAR, L-Band, Airborne SAR, radar imaging, synthetic aperture radar, Topography, German Aerospace Center E-SAR, DLR, SAR data processing, airborne L-band repeat-pass interferometric data, Interferometry, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, azimuth compression, azimuth coregistration errors, external digital elevation model, DEM, image enhancement, image registration, impulse response degradation, phase artifacts, repeat-pass interferometric SAR systems, Calibration, repeat-pass interferometry. [bibtex-key = PratsReigberMallorqui2005a:MoComp] [bibtex-entry]


  1412. Andreas Reigber, Maxim Neumann, Stephane Guillaso, Stefan Sauer, and Laurent Ferro-Famil. Evaluating PolInSAR parameter estimation using tomographic imaging results. In Proc. European Radar Conf., pages 189-192, 2005. Keyword(s): SAR Processing, SAR Tomography, Tomography, forestry, matrix algebra, radar imaging, radar polarimetry, radiowave interferometry, remote sensing by radar, synthetic aperture radar, tomography, vegetation mapping, PolInSAR parameter estimation, canopy, forest height, ground topography estimation, polarimetric SAR interferometry, tomographic imaging results.
    Abstract: This paper concentrates on the forest height and ground topography estimation by means of polarimetric SAR interferometry and tomography. In polarimetric SAR interferometry, one of the most important methods described in literature is the line-fitting approach in the complex unitary circle (S.R. Cloude and K.P. Papathanassiou, 2003). Although it has shown their principal potential, an open issue is still the precise validation of the estimated parameters, as ground-truth collection is an extremely complex task in the case of forest parameters. SAR tomography is an alternative technique, which generates a fully three-dimensional representation of the imaged scene through coherent combination of a greater number of tracks (A. Reigber and A. Moreira, 2000) (S. Guillaso and A. Reigber, 2005). Forest ground and canopy are directly visible in a tomographic image; a tomographic image can therefore be used as an ideal validation base for PolInSAR forest parameter estimation. This paper compares high-resolution polarimetric SAR tomograms with PolInSAR forest height estimations, both derived from the same data set. This allows to identify areas of good applicability, as well as principal deficiencies of the different PolInSAR approaches.
    [bibtex-key = reigberNeumannGuillasoSauerFerroFamil05:Tomo] [bibtex-entry]


  1413. A. Reigber, P. Prats, and J. J. Mallorqui. Refined estimation of time-varying baseline errors in airborne SAR interferometry. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 7, pages 4799-4802, July 2005. Keyword(s): SAR Processing, Airborne SAR, calibration, Interferometry, L-Band, Baseline refinement, interferometry, InSAR, Motion Compensation, repeat-pass interferometry, Residual Motion Errors, RME, Squinted SAR, Multi-Squint Processing. [bibtex-key = ReigberPratsMallorqui2005:MoComp] [bibtex-entry]


  1414. J. Sanz-Marcos, P. Prats, and J. J. Mallorqui. Bistatic fixed-receiver parasitic SAR processor based on the back-propagation algorithm. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 2, pages 1056-1059, July 2005. [bibtex-key = SanzMarcosPratsMallorqui2005:Bistatic] [bibtex-entry]


  1415. F. Serafino, F. Soldovieri, F. Lombardini, and G. Fornaro. Singular value decomposition applied to 4D SAR imaging. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 4, pages 2701-2704, July 2005. [bibtex-key = Serafino2005] [bibtex-entry]


  1416. G. Shippey, S. Banks, and J. Pihl. SAS image reconstruction using Fast Polar Back Projection: comparisons with Fast Factored Back Projection and Fourier-domain imaging. In Oceans 2005 - Europe, volume 1, pages 96-101, June 2005. Keyword(s): SAR Processing, Time-Domain Back-Projection, TDBP, Fast-Factorized Back-Projection, FFBP, Fast Polar Back-Projection, FPBP, fast Fourier transforms, image reconstruction, radar imaging, sonar imaging, synthetic aperture radar, synthetic aperture sonar FFBP, Fast Factored Back Projection, Fast Polar Back Projection, Fourier-domain imaging, SAS image reconstruction, Synthetic Aperture Radar, Synthetic Aperture Sonar, autopositioning purposes, azimuth sidelobe level, computation time reduction, intermediate physical aperture images, multielement sonar arrays, nonlinear platform trajectories, preset approximation error, review, standard FFT-based method, time-domain methods, ultra-wideband airborne SAR, wide bandwidths, wide swaths.
    Abstract: Fast Polar Back-Projection (FPBP) is a variant of the Fast-Factored Back-Projection (FFBP) algorithm, originally developed for ultra-wideband airborne Synthetic Aperture Radar (SAR), but since applied with success to Synthetic Aperture Sonar (SAS). The paper outlines the FPBP and FFBP algorithms, comparing computation time and memory requirements for the two methods. Processing time comparisons with a standard FFT-based method are also given. Since FFBP and FPBP are both approximation methods, computation time also depends on the preset approximation error, which particularly affects azimuth sidelobe level. The paper provides an opportunity to review speed and accuracy estimates made in previous literature. However reduction in computation time is not the decisive advantage of these time-domain methods. The difference from the FFT-based methods lies in the flexibility with which nonlinear platform trajectories, wide swaths, wide bandwidths, and multielement sonar arrays can be handled. It is also straightforward to obtain a set of intermediate physical aperture images for autopositioning purposes.
    [bibtex-key = ShippeyBanksPihl2005:FastBackprojection] [bibtex-entry]


  1417. Zhigang Su, Yingning Peng, and Xiutan Wang. Non-Iterative Imaging Algorithm for CLSAR. In Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on, volume 2, pages 577-580, 2005. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR.
    Abstract: Curvilinear synthetic aperture radar (CLSAR), which aperture is formed via a curvilinear trajectory, is considered as a more practical three-dimensional (3-D) imaging system. The 3-D images obtained by using non-parametricmethods, however, have little practical use because the data collected by CLSAR is sparse in the 3-D frequency space. Some parametric methods have been successfully applied into CLSAR for imaging but have expensive computational cost since they are iteration methods. In this paper, a non-iterative imaging (NII) algorithm is proposed. The new algorithm estimates the range parameters of all scatterers via modern spectrum method, and then using these range estimates and the received data to form the two-dimensional (2-D) data slices, from which the cross-range parameters are estimated. Once the position (range and cross-range) estimates are obtained, the radar cross section (RCS) can be calculated from the data. Simulation results show that the new algorithm can efficiently form the target's 3-D image via CLSAR.
    [bibtex-key = suPengWang2005b:NonLinearSARTomo] [bibtex-entry]


  1418. Zhigang Su, Yingning Peng, and Xiutan Wang. Three-dimensional target features extraction in curvilinear SAR with aperture errors. In Communications and Information Technology, 2005. ISCIT 2005. IEEE International Symposium on, volume 2, pages 1227-1230, 2005. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, feature extraction, radar imaging, synthetic aperture radar, aperture errors compensation, curvilinear SAR, curvilinear synthetic aperture radar, inverse SAR, phase compensation technique, phase information, range information, reference bins, three-dimensional target features extraction.
    Abstract: In curvilinear synthetic aperture radar (SAR), it is difficult to compensate the curvilinear aperture errors. The algorithm proposed in this paper, based on the phase compensation technique in inverse SAR (ISAR), compensates the aperture errors by using the range and phase information in the reference bins. Consequently, the scatterers' three-dimensional (3-D) features are extracted from the compensated data. Simulation results show that the distribution obtained via the new algorithm, compared with the original distribution, only shifts in three-dimensional position without structure changing. So, the new algorithm is a novel 3-D features extraction algorithm for curvilinear SAR.
    [bibtex-key = suPengWang2005:NonLinearSARTomo] [bibtex-entry]


  1419. Qulin Tan, Zhou Fu, Zhengjun Liu, and Jiping Hu. An experiment for high resolution airborne SAR imaging based on phase gradient autofocus. In Geoscience and Remote Sensing Symposium, 2005. IGARSS '05. Proceedings. 2005 IEEE International, volume 5, pages 3322-3324, July 2005. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Tan2005] [bibtex-entry]


  1420. G.J. Vigurs, M.S. Wood, and M.L. Jarrett. Non-linear synthetic aperture radar techniques. In Radar Conference, 2005. EURAD 2005. European, pages 13-16, October 2005. Keyword(s): SAR Processing, Non-Linear Flight Path, Simulation, MTI, Moving Target Indication.
    Abstract: The Non-Linear Synthetic Aperture Radar (SAR) technique uses a combination of platform manoeuvre and novel processing to separate the effects of a target's radial velocity and cross-range displacement, giving accurate estimates of both. The technique provides high resolution images free from the image distortion caused in conventional SAR imagery by moving targets, and allows the accurate target location of both stationary and moving objects. The technique also allows the platform to fly a wide range of planned and unplanned manoeuvres, improving platform survivability in potentially hostile environments.
    [bibtex-key = vigursWoodJarrett2005:NonLinearSAR] [bibtex-entry]


  1421. A. Aguasca, A. Broquetas, J. J. Mallorqui, and X. Fabregas. A solid state L to X-band flexible ground-based SAR system for continuous monitoring applications. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 2, pages 757-760, September 2004. Keyword(s): GB-SAR, ground-based SAR, terrestrial SAR, airborne radar, interferometry, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mapping, 100 MHz, L-X band ground-based SAR system, Synthetic Aperture Radar, airborne/satellite SAR systems, continuous terrain change monitoring, differential interferometry, optional polarimetric capability, ruggedized structure, Bandwidth, Buildings, Chirp, Frequency synthesizers, Interferometry, Monitoring, Solid state circuits, Spaceborne radar, Synthetic aperture radar, Transmitters. [bibtex-key = Aguasca2004] [bibtex-entry]


  1422. Emmanouil Alivizatos, Athanasios Potsis, Andreas Reigber, Alberto Moreira, and Nikolaos K. Uzunoglou. SAR Processing with Motion Compensation using the Extended Wavenumber Algorithm. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, pages 157-160, 2004. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Extended omega-k, Extended Wavenumber Domain Algorithm, Motion Compensation, Squinted SAR, Airborne SAR.
    Abstract: Modern Synthetic Aperture Radar (SAR) systems are continuously developing into the direction of higher spatial resolution and new modes of operation. This requires the use of high bandwidths, combined with wide azimuthal integration intervals. For focusing such data, a high quality SAR processing method is necessary, which is able to deal with more general sensor parameters. Wavenumber domain (Omega-K) processing is commonly accepted to be an ideal solution of the SAR focusing problem [1]. However, it is mostly applicable on spaceborne SAR data where a straight sensor trajectory is given. In case of airborne SAR data, wavenumber domain processing has certain limitations in performing high-precision motion compensation. In this paper, a detailed description of the motion errors in the wavenumber domain, as well as a motion compensation technique in this domain is formulated. The correction of the motion errors in the two dimensional spectral domain can result in very accurate second order motion compensation. This procedure can also be combined with a 2D sub-aperture technique, which results in a fully azimuth-frequency adaptive block processing scheme. The reason why the wavenumber MoCo can be very critical especially in low frequency-widebeam and high squinted SAR data, is that in these cases wavelength dependent corrections become mandatory.
    [bibtex-key = AlivizatosReigberMoreiraUzunoglu04:ExtendedOmegaK] [bibtex-entry]


  1423. Elke Boerner, Hauke Fiedler, Gerhard Krieger, and Josef Mittermayer. A new Method for Total Zero Doppler Steering. In IGARSS '04, International Geoscience and Remote Sensing Symposium, September 2004. Keyword(s): SAR Processing, Steering, Yaw Steering, Attitude Steering, Doppler Centroid, Zero Doppler Steering, Total Zero Doppler Steering.
    Abstract: This paper describes a new method to perform zero Doppler steering, namely Total Zero Doppler Steering. It is developed for spaceborne synthetic aperture radar (SAR) systems. This new method combines the yaw-steering with an additional pitch-steering, resulting in a Doppler centroid of theoretically zero Hertz over the whole desired range of incidence angles for the whole orbit and simultaneously for left and right looking geometry.
    [bibtex-key = BoernerFiedlerKriegerMittermayer04:Doppler] [bibtex-entry]


  1424. F. Bordoni, A. Jakobsson, F. Lombardini, and F. Gini. Layover solution in multibaseline InSAR using interpolated arrays. In Proceedings of the Fourth IEEE International Symposium on Signal Processing and Information Technology, 2004, pages 175-178, 2004. Keyword(s): SAR Processing, Interferometry, SAR Tomography, Tomography, Layover, Array Interpolation, MUSIC, WSF.
    Abstract: This work deals with the problem of direction of direction of arrival estimation of interferometric synthetic aperture radar (InSAR) signals in presence of layover. The focus here is on realistic acquisition systems with a low number of phase centres and nonuniform array geometry. An interpolated array approach is proposed, in order to apply spectral estimation techniques designed for uniform linear arrays. In particular, interpolated MUSIC and weighted subspace fitting (WSF) algorithms are developed and investigated.
    [bibtex-key = bordoniJakobssonLombardiniGini04:] [bibtex-entry]


  1425. Knut Eldhuset. Raw signal simulation for very high resolution SAR based on polarimetric scattering theory. In Proc. IEEE Int. Geosci. Remote Sens. Symp., September 2004. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator, Polarimetric Scattering Theory.
    Abstract: A method for raw signal simulation for extended SAR scenes with very high resolution is described. This simulator shall handle resolution better than 1 m, squinted geometry, elliptical orbit motion and use a polarimetric reflectivity matrix. Classical polarimetric scattering theory is based on electromagnetic harmonic fields using Maxwell s equations, Green's function, Huygens principle and Kirchhoff's approximation. Here, the scattering theory is modified for a chirp field. Calculated expressions for the scattered chirp field or the reflectivity matrix are presented. Such a reflectivity matrix can then be input to the inverse-EETF4 for raw data generation.
    [bibtex-key = Eldhuset04:Simulation] [bibtex-entry]


  1426. Joachim H. G. Ender, I. Walterscheid, and Andreas R. Brenner. New aspects of bistatic SAR: processing and experiments. In IGARSS '04, International Geoscience and Remote Sensing Symposium, September 2004. Keyword(s): SAR Processing, Bistatic SAR, Bistatic Processing, Back-Projection, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Range-Doppler Algorithm, Airborne SAR.
    Abstract: The interest in bistatic synthetic aperture radar, using separated transmitter and receiver flying on different platforms, has been increasing rapidly over the last years. The reason for this are specific advantages, like the reduced vulnerability in military systems, forward looking SAR imaging, additional information about the target, or increased RCS (see e.g. [1]). Nevertheless, besides technical problems (see [2]) - like the synchronisation of the oscillators, the involved adjustment of transmit pulse versus receive gate timing, antenna pointing, flight coordination, double trajectory measurement and motion compensation - the processing of bistatic radar data is still not sufficiently solved. Some of the possibilities and problems will be discussed. The second part of this paper deals with a bistatic experiment performed in November 2003: Two SAR systems of FGAN have been flown on two different airplanes, the AER-II system has been used as a transmitter and the PAMIR system as a receiver. Different spatially invariant flight geometries have been tested. High resolution bistatic SAR images were generated successfully.
    [bibtex-key = EnderWalterscheidBrenner04:bistaticSAR] [bibtex-entry]


  1427. G. Fornaro, Giorgio Franceschetti, and S. Perna. Motion Compensation of Squinted Airborne SAR Raw Data: Role of Processing Geometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., September 2004. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Extended omega-k, Extended Wavenumber Domain Algorithm, Motion Compensation, Squinted SAR, Airborne SAR.
    Abstract: We discuss the role of processing geometry and the problem of motion compensation for non zero squint in airborne SAR processing.
    [bibtex-key = franceschettiPernaFornaro04:Moco] [bibtex-entry]


  1428. G. Fornaro and F. Serafino. Spaceborne 3D SAR Tomography: experiments with ERS data. In Geoscience and Remote Sensing Symposium, 2004. IGARSS '04. Proceedings. 2004 IEEE International, volume 2, pages 1240-1243, 2004. Keyword(s): SAR Processing, SAR Tomography, Tomography, data acquisition, image processing, radar cross-sections, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, tomography, 3D back-scattering property, ERS data, European Remote Sensing, multibaseline SAR experiment, spaceborne 3D SAR tomography.
    Abstract: This paper presents the first results of a multibaseline SAR experiments for the reconstruction of the 3D back-scattering properties of ground scenes by using ERS data.
    [bibtex-key = fornaroSerafino04:Tomo] [bibtex-entry]


  1429. Othmar Frey, Erich Meier, Daniel Nüesch, and Achim Roth. Geometric Error Budget Analysis for TerraSAR-X. In Proc. EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, Ulm, Germany, pages 513-516, May 2004. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, Error Budget Analysis, TerraSAR-X, Atmospheric Correction, Atmospheric Path Delay, Ionosheric Path Delay, Antenna Gain Pattern Correction.
    Abstract: The impact of potential error sources on geocoded products has been investigated with respect to the high resolution capabilities of the TerraSAR-X sensor. Datum shift parameters, maps, digital terrain and surface models have been identified as external error sources. The accuracy of the geocoded products depends heavily on the quality and availability of this information, which underlies regional variations. Error sources closely related to the sensor are its position, sampling window start time and Doppler centroid frequency. Another error source is given by atmospheric refraction. Ionospheric and atmospheric path delays have a considerable impact. Appropriate modeling can mitigate this effect. Further, high requirements on radiometric accuracy ask for an improved antenna gain pattern correction, which depends on the actual elevation angle and the terrain height.
    [bibtex-key = freyMeierNueeschRoth04:ErrorBudget] [bibtex-entry]


  1430. F. Gini and F. Lombardini. Multibaseline post-processing for SAR interferometry. In Sensor Array and Multichannel Signal Processing Workshop Proceedings, 2004, pages 20-29, 2004. Keyword(s): SAR Processing, SAR Tomography, Tomography, radar detection, radar imaging, radiowave interferometry, synthetic aperture radar, 3D mapping, XTI-SAR, additive white Gaussian noise, cross-track InSAR system, multibaseline InSAR processing, multicomponent signal corruption estimation, multiplicative noise, radar reflectivity retrieval, semitransparent volume scattering layer, signal detection, tomography.
    Abstract: In this paper we provide a tutorial description of recent results of the research activity at the University of Pisa on multibaseline (MB) InSAR processing. The main focus is on the problem of retrieving both heights and radar reflectivities of natural layover areas by means of a cross-track InSAR (XTI-SAR) system. It is formulated as the problem of detecting and estimating a multicomponent signal corrupted by multiplicative noise and by additive white Gaussian noise. The problem of estimating the number of signal components in the presence of speckle is also addressed. Finally, a brief mention is given to recent research trends on robust methods for non-perfectly calibrated arrays and on MB-SAR tomography, which is an extension of MB-InSAR for full 3D mapping of semitransparent volume scattering layers.
    [bibtex-key = giniLombardini04:Tomo] [bibtex-entry]


  1431. B. Hallberg, G. Smith, A. Olofsson, and Lars M. H. Ulander. Performance Simulation of Spaceborne P-band SAR for Global Biomass Retrieval. In IGARSS '04, International Geoscience and Remote Sensing Symposium, September 2004. Keyword(s): SAR Processing, Simulation, P-Band, Biomass Retrieval, Forest.
    Abstract: This paper evaluates the use of a spaceborne lowfrequency synthetic aperture radar (SAR) for forest biomass retrieval. Airborne radar data are used as input to a SAR simulator in which SAR system parameters of the assumed spaceborne system and propagation effects in the ionosphere (primarily scintillation and Faraday rotation) are modelled. The simulations are performed for different iononospheric perturbation states. Some simulated spaceborne low-frequency SAR images over boreal forest are shown and their usefulness for forest biomass retrieval are studied and discussed. The results indicate that it is possible to separate boreal forest into three classes assuming a moderate distorted ionosphere.
    [bibtex-key = HallbergSmithOlofssonUlander04:Simulation] [bibtex-entry]


  1432. Charles V. Jakowatz, Daniel E. Wahl, David A. Yocky, Brian K. Bray, Wallace J. Bow, and John A. Richards. Comparison of algorithms for use in real-time spotlight-mode SAR image formation. In Edmund G. Zelnio and Frederick D. Garber, editors, , volume 5427, pages 108-116, 2004. SPIE. Keyword(s): SAR Processing, Real-Time, Real-Time Processing, Real-Time SAR, Video SAR, Polar Format Algorithm, PFA, Range Migration Algorithm, RMA, omega-k, Comparison of Algorithms, Comparison of Focusing Algorithms, overlapped subaperture algorithm, OSA, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzWahlYockyBrayBowRichardsSpotlightComparisonOfAlgorithms2004] [bibtex-entry]


  1433. Michael Jehle, Othmar Frey, David Small, Erich Meier, and Daniel Nüesch. Improved Knowledge of SAR Geometry through Atmospheric Modelling. In Proc. EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, Ulm, Germany, pages 909-911, May 2004. Keyword(s): Geocoding, Geometry, Atmospheric Path Delay, Ionosheric Path Delay, Tropospheric Path Delay, Atmospheric Modelling.
    Abstract: Satellites observing and measuring the Earth s surface with electromagnetic waves are subject to atmospheric path delays. These atmospheric effects on radar signal propagation modify the signal velocity and direction and can be considered by simple modeling. In order to increase the geolocation accuracy of spaceborne SAR applications we developed a software tool that accounts for atmospheric path delays. Well-calibrated spaceborne ENVISAT-ASAR data are used to investigate improvements to knowledge of the geometry of the scene.
    [bibtex-key = jehleFreySmallMeierNueesch04:Atmosphere] [bibtex-entry]


  1434. David Kettler, Doug Gray, and Nick Redding. The Point Spread Function for UWB SAR Imaging using Inversion of the Circular Radon Transform. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, Ulm, Germany, pages 175-178, May 2004. Keyword(s): SAR Processing, Fourier-Hankel Inversion, Hankel Transform, Abel Transform, Circular Radon Transform, CRT, Radon Transform, Ultra-Wideband SAR, Airborne SAR.
    Abstract: This paper summarises how SAR data collection can be viewed as taking the Circular Radon Transform of the ground reflectivity and outlines how image formation can be achieved by inverting the CRT via a Fourier-Hankel transform. An expression for the point spread function (PSF) of the imaging process is arrived at by means of an analytic inversion for a Gaussian. This PSF shows the effect on the image of the finite synthetic aperture. Illustrations of the PSF to show its range and aperture dependence are given.
    [bibtex-key = kettlerGrayRedding04:Hankel] [bibtex-entry]


  1435. Hiroshi Kimura, Toshiyuki Mizuno, Konstantinos P. Papathanassiou, and Irena Hajnsek. Improvement of polarimetric SAR calibration based on the Quegan algorithm. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 187, September 2004. Keyword(s): SAR Processing, Polarimetry, Polarimetric Calibration, airborne radar, crosstalk, geophysical signal processing, radar polarimetry, synthetic aperture radar, CR response analysis, JAXA/NICT airborne Pi-SAR, L-band polarimetric SAR data, Quegan algorithm, cross-polarized channel, like/cross-polarized observed echo, natural/artificial target, noise imbalance, polarimetric SAR calibration, polarimetric classification, polarimetric cross-talk calibration algorithm, Calibration, Chromium, Crosstalk, L-band, Parameter estimation, Radar polarimetry, Radar scattering, Spaceborne radar, Stability analysis, Synthetic aperture radar.
    Abstract: The polarimetric cross-talk calibration algorithm proposed by Quegan, which is more general than the one proposed by van Zyl, is modified to improve polarimetric SAR calibration. L-band polarimetric SAR data acquired by the JAXA/NICT airborne Pi-SAR, including a variety of natural and artificial targets, are calibrated introducing noise imbalance of cross-polarized channels into the Quegan algorithm and deriving the cross-talk parameters from areas with low correlation between like and cross-polarized observed echoes. The achieved enhancement in stability of cross-talk parameter estimation and removal over is demonstrated in terms of CR response analysis and polarimetric classification result
    [bibtex-key = kimuraMizunoPapathanassiouHajnsekIGARSS2004PolCalibration] [bibtex-entry]


  1436. John C. Kirk, Don Woods, and Joe Salzman. Efficient Motion-Tolerant Fopen SAR Processing. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, volume 1, Ulm, Germany, pages 179-182, May 2004. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Motion Compensation, Thinned Range Migration Algorithm, Airborne SAR.
    Abstract: The Range Migration Algorithm (RMA) is an efficient approach to process strip map data with large range cell migration. However, it is sensitive to cross track motion of the radar. To provide for a degree of motion tolerance, an applicable approach is to retain only a portion of the full processed image around a central reference point (CRP) where motion compensation is applied. This results in throwing away that part of the image that is potentially defocused with the result that this discarded data area has to be reprocessed with additional CRPs. This results in a processing in-efficiency that can greatly drive up the processing requirements. An efficient algorithm is being developed which alleviates this wasted data processing in-efficiency problem. This new algorithm, called tyhe Thinned RMA, operates by filtering the data to just the area to be processed and then resampling the data at a much lower rate. The RMA then proceeds normally, but now operates on a much reduced cross track data size. This approach can also provide a much greater tolerance to cross track motion. To date the algorithm as been developed and tested on simulated data and recorded phase history data. Processing speed-ups of approximately 4 to 1 are being achieved compared to a full RMA algorithm that retains a small patch about the CRP. Ultimately the algorithm will progress to a two stage version that will provide the optimum in motion tolerance and processing efficiency.
    [bibtex-key = KirkWoodsSalzman04:Processing] [bibtex-entry]


  1437. Gerhard Krieger, Nicolas Gebert, and Alberto Moreira. SAR Signal Reconstruction from Non-Uniform Displaced Phase Centre Sampling. In IGARSS '04, International Geoscience and Remote Sensing Symposium, September 2004. Keyword(s): SAR Processing, Bistatic SAR, Bistatic Processing, Dual Receive Antenna.
    Abstract: The displaced phase centre (DPC) technique will enable a wide swath SAR with high azimuth resolution. In a classic DPC system, the PRF has to be chosen such that the SAR carrier moves just one half of its antenna length between subsequent radar pulses. Any deviation from this PRF will result in a nonuniform sampling of the synthetic aperture. This paper shows that an unambiguous reconstruction of the SAR signal is also possible in case of such a non-optimum PRF. For this, an innovative reconstruction algorithm is derived, which enables a recovery of the unambiguous Doppler spectrum also in case of a non-uniform sampling of the synthetic aperture. This algorithm will also have a great potential for multistatic satellite constellations as well as the dual receive antennas in Radarsat II and TerraSAR-X.
    [bibtex-key = KriegerGebertMoreira04:DisplacedPhase] [bibtex-entry]


  1438. J. Li and P. Stoica. Versatile robust Capon beamforming: theory and applications. In Sensor Array and Multichannel Signal Processing Workshop Proceedings, 2004, pages 38-42, July 2004. Keyword(s): SAR Processing, SAR Tomography, Tomography, Capon, Robust Capon Beamforming, RCB, acoustic imaging, array signal processing, beam steering, channel bank filters, covariance matrices, ground penetrating radar, image resolution, interference (signal), landmine detection, radar imaging CBRCB, CPRCB, DCRCB, FLGPR imaging system, RCF, SCB, constant-beamwidth RCB, constant-powerwidth RCB, covariance fitting formulation, doubly constrained robust Capon beamformer, forward-looking ground penetrating radar, interference rejection, rank-deficient robust Capon filter-bank approach, resolution, spectral estimation, standard Capon beamformer, steering vector.
    Abstract: The standard Capon beamformer (SCB) has better resolution and much better interference rejection capability than the data-independent beamformer provided that the array steering vector corresponding to the signal-of-interest (SOI) is accurately known. However, whenever the knowledge of the SOI steering vector is imprecise (as is often the case in practice), the performance of the Capon beamformer may become worse than that of the data-independent beamformer. Most of the early suggested robust adaptive methods are rather ad hoc in that the choice of their parameters are not directly related to the uncertainly of the steering vector. In this paper we provide a review of the recently proposed robust Capon beam-former (RCB) and doubly constrained robust Capon beamformer (DCRCB), which directly address the uncertainty of the steering vector and naturally extend the covariance fitting formulation of SCB to the case of uncertain steering vectors by enforcing a double constraint on the steering vector, viz. a constant norm constraint and an uncertainty set constraint. We also present several extensions and applications of RCB including constant-powerwidth RCB (CPRCB) and constant-beamwidth RCB (CBRCB) for acoustic imaging, rank-deficient robust Capon filter-bank (RCF) approach for spectral estimation, and rank-deficient RCB for landmine detection using forward-looking ground penetrating radar (FLGPR) imaging systems. The excellent performances of RCB, DCRCB, and the various extensions of RCB are demonstrated by simulated and experimental examples.
    [bibtex-key = liStoica2004:RobustCapon] [bibtex-entry]


  1439. M. Limbach, B. Gabler, and Ralf Horn. Fine Resolution, fully Polarimetric P-band subsystem for E-SAR -- Technique and Results. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, pages 275-278, 2004. Keyword(s): SAR Processing, ESAR, P-Band, Airborne SAR, Polarimetry, Polarimetric P-Band.
    Abstract: The design of the P-band subsystem of DLR's E-SAR system is presented in this paper. With a new microstrip patch antenna and other key hardware components a bandwidth of more than 28% at a lower centre frequency is achieved. The system parameters correspond to a range resolution better than 2.1m. Measurements of system gain, cross polarisation suppression, system sensitivity and some antenna parameters are shown.
    [bibtex-key = limbachGablerHorn04:ESARPBand] [bibtex-entry]


  1440. F. Lombardini, Joachim H. G. Ender, L. Rössing, M. Galletto, and L. Verrazzani. Experiments of interferometric layover solution with the three-antenna airborne AER-II SAR system. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS '04, volume 5, pages 3341-3344, September 2004. Keyword(s): SAR Processing, Capon, MUSIC, airborne radar, antenna arrays, radar imaging, radiowave interferometry, remote sensing by radar, synthetic aperture radar, terrain mapping, InSAR, SAR imagery, SAR Interferometry, advanced multibaseline operation, airborne experimental radar, calibration, dual-baseline single-pass SAR interferometer, electromagnetic tomography, higher-order interferometry, hybrid spatial spectral estimation, interferometric layover solution, layover-free interferometry, multichannel operation, nonparametric spectral estimation, parametric spectral estimation, spectral analysis, three-antenna airborne AER-II SAR System, three-antenna nonuniform array data.
    Abstract: Interest is recently growing in exploiting the advanced multibaseline operation of synthetic aperture radar interferometry (InSAR) to solve layover effects, that can degrade conventional SAR and InSAR imagery. In this work we report about: experiments of the functionality of layover-free or higher-order interferometry with the dual-baseline single-pass SAR interferometer AER-II. Non-parametric, parametric and hybrid spatial spectral estimators are applied to process the three-antenna non uniform array data. Calibration issues, first real data results and impact of order selection are discussed for a bridge over the valley scene
    [bibtex-key = lombardiniEnderRoessingGallettoVerrazzani2004:AERLayoverSolution] [bibtex-entry]


  1441. José Márquez-Martìnez and Josef Mittermayer. Analysis of Range Ambiguity Suppression methods in SAR by using a Novel Range Ambiguity Raw Data Simulator. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, volume 1, Ulm, Germany, pages 593-596, May 2004. Keyword(s): SAR Processing, Simulation, SAR Simulator, Raw Data Simulator, Range Ambiguity Suppression, ESAR, Airborne SAR.
    Abstract: This paper presents a novel range ambiguity raw data simulator (RAS). The simulator was used to investigate range ambiguity suppression obtained by means of up and down chirp coding for point targets and realistic scenes including extended targets. A pre-processing technique for up and down-chirp processing has also been developed. Different applications are investigated using data from the experimental SAR system from DLR (ESAR), either oriented to point target detection or to extended target analysis. Several image processing results with different range ambiguities ratios are presented.
    [bibtex-key = MarquezMittermayer04:Simulator] [bibtex-entry]


  1442. Yibo Na, Hongbo Sun, Yee Hui Lee, Ling Chiat Tai, and Hian Lim Chan. Performance evaluation of back-projection and range migration algorithms in foliage penetration radar imaging. In Image Processing, 2004. ICIP '04. 2004 International Conference on, volume 1, pages 21-24, 2004. Keyword(s): SAR Processing, Back-Projection, omega-k, Comimage reconstruction, radar imaging, synthetic aperture radar, FOPEN SAR imaging, back-projection algorithm, computer-aided tomography, foliage penetration radar imaging, image reconstruction, range migration algorithm, seismic migration techniques.
    Abstract: In this paper, two relatively novel synthetic aperture radar (SAR) imaging techniques, namely the back-projection algorithm and range migration algorithm, are discussed. The back-projection algorithm originates from the medical imaging reconstruction technique called computer-aided tomography whereas the range migration algorithm is derived from seismic migration techniques. In this paper, both the back-projection and range migration algorithms are applied to foliage penetration (FOPEN) SAR imaging and performance comparisons are made. The simulations and experimental data processing results show that both algorithms are suitable for FOPEN radar imaging and that theoretical performances can be achieved.
    [bibtex-key = naSunLeeTaiChan04:Backp] [bibtex-entry]


  1443. Lam H. Nguyen, Marc Ressler, and Mehrdad Soumekh. Signal Processing and Image Formation Using Low-Frequency Ultra-Wideband Radar Data. In Russell S. Harmon, J. Thomas Broach, and John H. Holloway(Jr.), editors, Proceedings of SPIE: Detection and Remediation Technologies for Mines and Minelike Targets IX, volume SPIE 5415, pages 1053-1064, 2004. Keyword(s): SAR Processing, Ultra-Wideband SAR, Back-Projection, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, Image Formation, RFI Suppression, Digital Spotlighting.
    Abstract: In support of the U.S. Army Night Vision And Electronic Sensors Directorate (NVESD), the U.S. Army Research Laboratory (ARL) has developed infrastructures, tools, and algorithms to evaluate the data set. This paper focuses on the signal processing and image formation using data from a low-frequency ultrawideband sensor. We examine various issues that are associated with this class of SAR databases such as radio frequency interference (RFI), the effects of spectral notches, and errors in motion measurement to image quality. We show the pre-processing steps such as frequency and phase calibration, radio frequency interference extraction. We also show the application of digital spotlight technique to correct motion errors introduced by the measurement system. Finally, we show the resulting SAR imagery of various minefields.
    [bibtex-key = NguyenResslerSoumekh04:LowFreqUWB] [bibtex-entry]


  1444. Lam H. Nguyen, Marc Ressler, D. Wong, and Mehrdad Soumekh. Enhancement of backprojection SAR imagery using digital spotlighting preprocessing. In Radar Conference, 2004. Proceedings of the IEEE, pages 53-58, 2004. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, Digital Spotlighting, Boom-SAR, Self-Induced Resonance Suppression, SIR Suppression, RFI Suppression, Doppler effect, antialiasing, image enhancement, radar imaging, synthetic aperture radar, ARL boom-SAR data, Doppler aliasing suppression, PRF, SAR data filtering scheme, SAR imagery enhancement, azimuth-compressed SAR data, back-projection SAR imagery, digital spotlighting preprocessing, image fidelity improvement, radar radiation pattern, side lobe artifacts.
    Abstract: This paper examines signal processing methods for improving the fidelity of backprojection SAR imagery using a preprocessing method that suppresses Doppler aliasing as well as other side lobe artifacts that are introduced by the radar radiation pattern. The algorithm, known as digital spotlighting, imposes a filtering scheme on the azimuth-compressed SAR data, and manipulates the resultant spectral data to achieve a higher PRF to suppress the Doppler aliasing. The merits of the algorithm are studied using the ARL boom-SAR data.
    [bibtex-key = nguyenResslerWongSoumekh04:Backproj] [bibtex-entry]


  1445. Lam H. Nguyen, Tuan Ton, David Wong, and Mehrdad Soumekh. Adaptive coherent suppression of multiple wide-bandwidth RFI sources in SAR. In Edmund G. Zelnio and Frederick D. Garber, editors, Algorithms for Synthetic Aperture Radar Imagery XI, volume 5427, pages 1-16, 2004. SPIE. Keyword(s): SAR Processing, RFI Suppression, Ultra-Wideband SAR, Wide-Bandwidth RFI, Boom-SAR, Self-Induced Resonance Suppression, SIR Suppression.
    Abstract: This paper is concerned with suppressing multiple wide-bandwidth radio frequency interference (RFI) sources in SAR systems. A coherent processing of passive radar (sniff) data is presented to diminish the effects of wide-bandwidth as well as narrow-bandwidth RFI sources in the active radar data that are collected by a SAR system. The approach is based on a two-dimensional adaptive filtering of the active SAR data using the passive sniff data as the reference signal. A similar mathematical (signal) model and processing is also utilized to suppress self-induced resonance (SIR) signals that are generated by the interaction of the radar-carrying platform and the transmitted radar signal. Results are shown using the Army Research Laboratory (ARL) low-frequency, ultra-wideband (UWB) imaging radar (Boom-SAR).
    [bibtex-key = nguyenTonWongSoumekh04:RFI] [bibtex-entry]


  1446. David Small, Michael Jehle, Erich Meier, and Daniel Nüesch. Radiometric terrain correction incorporating local antenna gain. In Proc. of EUSAR 2004 - 5th European Conference on Synthetic Aperture Radar, Ulm, Germany, pages 929-932, May 2004. Keyword(s): SAR Geocoding, Radiometric Correction, Radiometric Terrain Correction, Radiometric Calibration, Calibration, Local Antenna Gain.
    Abstract: Radiometric terrain correction consists of normalising a SAR image for well-understood backscatter contributions in order to amplify less easily apparent influences (e.g. thematic land cover variance). A rigorous modelling of the SAR image formation process includes consideration of how foreshortening and layover create ambi guity when connecting map geometry grid points to and from counterparts in radar geometry (slant or ground range vs. azimuth). A radar amplitude image simulation is formed by iterating through a facetted DEM, calcu lating the accumulated illuminated area at every range and azimuth coordinate in radar geometry. We show how DEM-based image simulations gain further realism by incorporating knowledge of the SAR antenna's elevation an-tenna gain pattern (AGP). Although typical AGP corrections assume an ellipsoidal Earth, the AGP is actually draped upon the Earth's terrain. We quantify differences between estimates of local antenna gain and illuminated area performed using (a) the typical ellipsoid assumption, (b) a DEM. We demonstrate application of local antenna gain knowledge within the image simulation process using ENVISAT ASAR images acquired over Switzerland. We introduce a weighted resolution approach for robust combination of multiple radiometrically normalised terrain geocoded backscatter maps.
    [bibtex-key = smallJehleMeierNueesch04:TerrainCorr] [bibtex-entry]


  1447. David Small, Erich Meier, and Daniel Nüesch. Robust radiometric terrain correction for SAR image comparisons. In IGARSS '04, International Geoscience and Remote Sensing Symposium, volume 3, pages 1730-1733, 2004. Keyword(s): backscatter, radiometry, remote sensing by radar, terrain mapping, AGP, ENVISAT ASAR images, Earth ellipsoid, Earth rolling terrain, SAR imagery, Switzerland, composite radar map, ideal flat terrain, local antenna gain pattern, mountainous terrain, normalized images, radar backscatter coefficient, radar brightness coefficient, radar equation, radiometric errors, radiometrically calibrated images, robust radar image simulation, robust radiometric terrain correction, robust technique, terrain geocoded images, terrain induced modulations, terrain variations.
    Abstract: We demonstrate a robust technique for radiometric terrain correction, whereby terrain-induced modulations of the radiometry of SAR imagery are modelled and corrected. The resulting normalized images may be more easily compared with other data sets acquired at different incidence angles, even opposing look directions. We begin by reviewing the radar equation, pointing out simplifications often made to reduce the complexity of calculating the backscatter coefficient, normalized either by ground area sigma0, or illuminated area projected into the look direction gamma0 . The integral over the illuminated area is often approximated by a scale factor modelling a simple planar slope, departing only slightly from ideal flat terrain: for gamma0, the radar brightness beta is normalized via modulation with the tangent of the local incidence angle. We quantify the radiometric errors introduced by ignoring terrain variations, comparing results based on (a) a robust radar image simulation-based approach properly modelling variations in local illuminated area, and (b) an ellipsoidal Earth assumption. A second simplification often made in solving for backscatter using the radar equation is the assumption that the local antenna gain does not vary significantly from a simple model draping the antenna gain pattern (AGP) across an Earth ellipsoid, returning the local antenna gain as a function of slant range alone. In reality, the AGP is draped across the Earth's rolling terrain retrieval of properly calibrated backscatter values should model these variations and compensate for them: although smaller than the errors caused by not property modelling variations in local illuminated area, they can be significant. We use well-calibrated and annotated ENVISAT ASAR images acquired over Switzerland to show how robust radiometric terrain correction, incorporating models for the variations of local illuminated area with terrain enables calibrated mixture of imagery acquired at differing incidence angles. Only robust retrieval of backscatter values enables such inter-mode comparisons - a capability that significantly reduces the required revisit time for monitoring changes to the radar backscatter. In conclusion, we describe a techn- ique for combining a set of terrain-geocoded and radiometrically calibrated images derived from ascending and descending passes and multiple incidence angles to create composite radar backscatter maps. At each point, the contribution of each image to the composite is weighted according to its local resolution. The resulting composite image manifests relatively uniform high ground resolution, even in highly mountainous terrain.
    [bibtex-key = smallMeierNueesch04:TerrainCorr] [bibtex-entry]


  1448. David Small, Betlem Rosich, Erich Meier, and Daniel Nüesch. Geometric Calibration and Validation of ASAR Imagery. In CEOS SAR Workshop 2004, Ulm, May 2004. Keyword(s): SAR Geocoding, Calibration, Validation, Quality Assessment, ASAR, ENVISAT.
    Abstract: We describe work conducted to calibrate and then validate the geometry of ENVISAT ASAR products. A systematic error in range location was observed in ASAR products during the commissioning phase. A careful and complete analysis has been performed to establish the precise error. It has been compensated by updating the range gate bias (or sampling window start time bias). Validation of the absolute location accuracy of most ASAR products was performed subsequently. The location of surveyed targets is predicted using the satellite state vectors and ancillary timing information via the range and Doppler equations. The prediction's accuracy is affected by instrument bias, ionospheric and atmospheric path delay, as well as target survey errors. Transponders are in addition subject to internal delay uncertainty. The positions of the strong transponder and corner reflector targets in the images are measured to sub-sample accuracy by employing large oversampling factors. Initial and residual bias determinations were made using image acquisitions covering transponders and corner reflectors in the Netherlands, Canada, and Switzerland. Using a large number of independent targets helps reduce the influence of their independent survey errors. The highest resolution slant range single look complex (SLC) products (IMS, APS) were mainly used for testing. In addition, absolute location error was also measured on selected ground range products (IMP, APP, IMM, APM, WSM). Some ground range products also require treatment of multiple slant/ground range polynomials - proper handling is validated. For all test cases processed with precise orbits to date, the residual bias in the slant range direction has been smaller than the size of a single range sample. Predictability of target image location within ASAR image products is very high - better than experience with ERS-1/2, JERS-1, and RADARSAT-1. This result is encouraging, as it opens possibilities for ground control point (GCP) free terrain-geocoding and simplified interferometric processing.
    [bibtex-key = SmallRosichMeierNueesch04:ASARGeoloc] [bibtex-entry]


  1449. David Small, Betlem Rosich, Adrian Schubert, Erich Meier, and Daniel Nüesch. Geometric Validation of Low and High-Resolution ASAR Imagery. In Proc. of the 2004 Envisat & ERS Symposium, ESA SP-572, Salzburg, Sep. 2004. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, Validation, Geometric Validation, ASAR, ENVISAT. [bibtex-key = smallRosichSchubertMeierNueesch04:SubmittedBistatic] [bibtex-entry]


  1450. Junfeng Wang and Xingzhao Liu. SAR Automatic Range-Migration Correction. In IGARSS '04, International Geoscience and Remote Sensing Symposium, September 2004. Keyword(s): SAR Processing, range-Doppler Algorithm, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Range-Migration Correction.
    Abstract: A new idea is presented to correct range migration in SAR imaging. In the range-Doppler domain, all the samples at a given Doppler frequency constitute a Doppler slice. Different Doppler slices are found to have similar envelopes. According to this similarity, the Doppler slices are shifted in range to correct range migration. This technique applies even without the prior information about the relative motion between the radar and the target.
    [bibtex-key = WangLiu04:RangeMigration] [bibtex-entry]


  1451. Nico Adam, Bert M. Kampes, Michael Eineder, Jirathana Worawattanamateekul, and Michaela Kircher. The Development of a Scientific Permanent Scatterer System. In ISPRS Workshop High Resolution Mapping from Space, Hannover, Germany, pages 1-6, 2003. Keyword(s): SAR Processing, SAR Interferometry, D-InSAR, Differential SAR Interferometry, Persistent Scatterer Interferometry, PSI, Deformation measurement, Displacement measurement. [bibtex-key = adamEtAl2003ScientificPSISystem] [bibtex-entry]


  1452. W. - M. Boerner. Recent Advances in Extra-Wide-Band Polarimetry, Interferometry and Polarimetric Interferometry in Synthetic Aperture Remote Sensing and its Applications. In IEE Proceedings - Radar, Sonar and Navigation, volume 150, pages 113-124, June 2003. Keyword(s): SAR Processing, Interferometry, Pol-InSAR, RFI Suppression, Extra-WideBand SAR, WideBand SAR.
    Abstract: The development of radar polarimetry and radar interferometry is advancing rapidly, and these novel radar technologies are revamping 'synthetic aperture radar imaging' decisively. The successive advancements are sketched beginning with the fundamental formulations and highlighting the salient points of these diverse remote sensing techniques. Whereas with radar polarimetry the textural fine-structure, target orientation and shape, symmetries and material constituents can be recovered with considerable improvements above that of standard 'amplitude-only polarisation radar'; with radar interferometry the spatial (in depth) structure can be explored. In 'polarimetric-interferometric synthetic aperture radar (POL-IN-SAR) imaging' it is possible to recover such co-registered textural plus spatial properties simultaneously. This includes the extraction of 'digital elevation maps (DEM)' from either 'fully polarimetric (scattering matrix)' or 'interferometric (dual antenna) SAR image data takes' with the additional benefit of obtaining co-registered three-dimensional 'POL-IN-DEM' information. Extra-wide-band POL-IN-SAR imaging - when applied to 'repeat-pass image overlay interferometry' - provides differential background validation and measurement, stress assessment, and environmental stress-change monitoring capabilities with hitherto unattained accuracy, which are essential tools for improved global biomass estimation and also for wetland assessment and monitoring. More recently, by applying multiple parallel repeat-pass EWB-POL-D(RP)-IN-SAR imaging along stacked (altitudinal) or displaced (horizontal) flight-lines will result in 'tomographic (multi-interferometric) polarimetric SAR stereo-imaging', including foliage- and ground-penetrating capabilities. In addition, various closely related topics of (i) acquiring additional and protecting existing spectral windows of the 'natural electromagnetic spectrum (NES)' pertinent to remote sensing; and (ii) mitigation against common 'radio frequency interference (RFI)' and intentional 'directive jamming of airborne and spaceborne POL-IN-SAR imaging platforms' are appraised.
    [bibtex-key = Boerner03:EWB] [bibtex-entry]


  1453. H.J. Callow, M.P. Hayes, and P.T. Gough. Autofocus of stripmap SAS data using the range-variant SPGA algorithm. In OCEANS 2003 Proceedings, volume 5, pages 2422-2426, September 2003. Keyword(s): SAR Processing, Autofocus, Phase Curvature Autofocus, Phase Gradient Autofocus, PGA, SPGA, Stripmap Phase Gradient Algorithm, Synthetic Aperture Sonar, SAS. [bibtex-key = Callow2003a] [bibtex-entry]


  1454. H.J. Callow, M.P. Hayes, and P.T. Gough. Stripmap phase gradient autofocus. In OCEANS 2003. Proceedings, volume 5, pages 2414-2421, September 2003. Keyword(s): SAR Processing, Autofocus, Phase Curvature Autofocus, Phase Gradient Autofocus, PGA, SPGA, Stripmap Phase Gradient Algorithm, Synthetic Aperture Sonar, SAS.
    Abstract: Current sonar autofocus techniques for blur removal originate in the radar community but have not provided a complete solution for Synthetic Aperture Sonar (SAS) imagery. The wide-beam, wide-band nature of SAS imagery makes implementation of Synthetic Aperture Radar (SAR) autofocus techniques difficult. This paper describes a generalisation of the standard Phase Gradient Antofocus (PGA) algorithm used in spotlight SAR that allows operation with stripmap SAS geometries. PGA uses prominent points within the target scene to estimate image blurring and phase errors. We show how PGA can be generalised to work with wide-band, wide-heam stripmap geometries. The SPGA method works by employing wavenumher domain 2D phase estimation techniques. The 2D phase errors are related to aperture position errors using the wavenumber transform. Robust sway estimates are obtained by using redundancy over a number of target points. We also present an improved Phase Curvature Autofocus (PCA) algorithm using the wavenumher transform. Preliminary results from the two algorithms (both on field-collected and simulated data sets) are presented and related to those obtained using previous methods. A discussion of SPGA's benefits over traditional algorithms and the limitations of the SPGA algorithm. The SPGA algorithm was found to perform better than 2-D PCA on both simulated and field-collected data sets. Further testing on a variety of target scenes and imagery is required to investigate avenues of autofoeus improvement.
    [bibtex-key = Callow2003] [bibtex-entry]


  1455. Hubert-M.J. Cantalloube and Pascale Dubois-Fernandez. Airborne X-band SAR imaging with 10 cm resolution - technical challenge and preliminary results. In , volume 1, pages 185-187, July 2003. Keyword(s): SAR Processing, Motion Compensation, Autofocus, radar cross-sections, radar imaging, radar resolution, remote sensing by radar, synthetic aperture radar 2 1/2 D surface modelling, Doppler algorithms, Ku bands, RAMSES, X-Band, X-band SAR imaging, Airborne SAR, antenna pattern compensation method, back-injection synthesis algorithm, carrier trajectory, clutter appearance, differential GPS-hybridized inertial navigation unit, high resolution clutters, isotropic echoes, matching cross-range resolution, optical surface modelling, phase tracking, point-like echoes, Range Migration Algorithm, resolution cell, synthetic aperture radar, TDBP, Time-Domain Back-Projection, temporal-domain synthesis algorithm, texture simulations, omega-k algorithm.
    Abstract: RAMSES airborne SAR system bandwith was recently increased to 1.2 GHz in X and Ku bands, yielding (unweighted) 3 dB range resolution of 11 cm. Synthesis of SAR images with matching cross-range resolution, requires long integration time thus disqualifies temporal-domain back-injection synthesis algorithm as impractically slow. The wider relative bandwidth also disqualifies simplified range/Doppler types of algorithms because the hypothesis of proportionality between Doppler and squint is no more valid. Therefore, we implemented a fast frequency-domain synthesis algorithm omega-k or range-migration algorithm) and designed a new deterministic motion and antenna pattern compensation method for it. Since the required accuracy on carrier trajectory exceeded the performance of our differential GPS-hybridized inertial navigation unit, we implemented an autofocus based on the phase tracking of several isotropic point-like echoes. Since the resolution cell is only a few wavelength wide, clutter appearance and statistics is unusual. We present here some typical examples for high resolution clutters and compare with texture simulations from optical 2 1/2 D surface modelling.
    [bibtex-key = cantalloubeDuboisFernandez2003:HiResAutofocus] [bibtex-entry]


  1456. Shane R. Cloude and Konstantinos P. Papathanassiou. Three-stage inversion process for polarimetric SAR interferometry. In , volume 150, pages 125-134, June 2003. Keyword(s): SAR Processing, decorrelation, electromagnetic wave scattering, inverse problems, parameter estimation, radar imaging, radar polarimetry, remote sensing by radar, synthetic aperture radar geometrical approach, ground topography, interferograms, inversion accuracy, mean extinction estimation, model structure, multiple polarisation channels, parameter estimates, polarimetric SAR interferometry, random canopy, simulated vector coherent SAR data, single frequency sensor, temporal decorrelation, three-stage inversion process, two-layer coherent scattering model, vegetation height, vertical tree structure.
    Abstract: The authors provide a new geometrical approach for the inversion of a two-layer coherent scattering model, widely used for the interpretation of polarimetric interferometric SAR data. It has been shown in several recent publications that, by using interferograms in multiple polarisation channels, estimation of vegetation height, underlying ground topography and mean extinction is possible. Furthermore, this can be achieved with a single frequency sensor without the need for a separate reference DEM, other a priori information or the use of data-specific regression formulas. The authors first review the details of this approach and then develop a three-stage inversion procedure to illustrate the steps involved in parameter estimation. They then consider several possible sources of error in the inversion. In particular, they concentrate on the effects of vertical tree structure and on the effects of temporal decorrelation on inversion accuracy. It is shown that the former leads to errors, mainly in the extinction estimation, while the latter does not change the model structure but reduces the available parameter set and increases the variance of the parameter estimates. Finally, the new algorithm is applied to simulated vector coherent SAR data for a random canopy.
    [bibtex-key = cloudePapathanassiou2003:3StageInversionPolInSAR] [bibtex-entry]


  1457. C. Colesanti, A. Ferretti, C. Prati, and F. Rocca. Multi-image satellite SAR interferometry: state of the art and future trends. In Proc. Int. Radar Conf., pages 239-244, September 2003. Keyword(s): SAR Processing, InSAR, SAR Interferometry, PSI, Persistent Scatterer Interferometry, Differential SAR Interferometry, electromagnetic wave scattering, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, ground deformation data, high precision elevation data, multi-image satellite SAR interferometry, permanent scatterers technique, spaceborne interferometric SAR images, sparse privileged point-wise radar target grid.
    Abstract: In this paper, we wish to review briefly the principles underlying a recently developed approach, known as the permanent scatterers (PS) technique and aimed at the joint exploitation of a series of spaceborne interferometric SAR images for the retrieval of high precision elevation and ground deformation data on a sparse grid of privileged point-wise radar targets.
    [bibtex-key = colesantiFerrettiPratiRocca2003:MBInSAR] [bibtex-entry]


  1458. Ian G. Cumming, Y. L. Neo, and Frank Wong. Interpretations of the Omega-K Algorithm and Comparisons with other Algorithms. In IGARSS '03, International Geoscience and Remote Sensing Symposium, 2003. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Stolt Mapping, Chirp Scaling Algorithm, Range-Doppler Algorithm, Comparison of Algorithms.
    Abstract: This paper presents a Fourier interpretation of the Omega-k SAR processing algorithm that helps explain the key Stolt mapping operation. An approximate form of the algorithm is sometimes used, and we explain how both forms of the Omega-k compare with the range Doppler and the chirp scaling algorithms. Finally, a brief discussion is given on which radar parameters allow the accurate use of each algorithm.
    [bibtex-key = CumNeoWong:omegaK] [bibtex-entry]


  1459. M. De Stefano and Andrea Monti-Guarnieri. Robust Doppler Centroid estimate for ERS and ENVISAT. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 6, pages 4062-4064, 2003. Keyword(s): SAR Processing, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, ENVISAT, ERS, MLBF, RADARSAT, Wavelength Diversity, WDAR, Multilook Beat Frequency, fine polynomial estimation, higher order technique, robust Doppler centroid estimation, robustness, second order statistic estimator, unambiguous Doppler estimation.
    Abstract: The algorithm presented is capable of retrieving the correct DC ambiguity and to fit a fine polynomial estimate both on uniform and contrasted scenes. The core of the algorithm exploits a block wise processing: in each block a coarse unambiguous estimate is provided by exploiting both a second order statistic estimator (WDAR) and a higher order technique (MLBF). The final, fine estimate of the unambiguous Doppler is achieved by jointly exploiting the coarse unambiguous estimate with a fine, ambiguous one. The proposed algorithm accounts carefully for large variation of DC with range, like for recent Emergency Backup Mode of ERS and RADARSAT. The final estimate and its confidence is provided by a weighted average of the block measures. Tuning of the weights and additional check ensure robustness. The estimate of the offset frequency constant is then approached and a solution for calibrating its value is provided.
    [bibtex-key = deStefanoMontiGuarinieri03:dopCen] [bibtex-entry]


  1460. Joachim H. G. Ender. SAR/MTI with Multi-Subaperture Phased Arrays. In Proceedings of the Tyrrhenian International Workshop on Remote Sensing TIWRS, pages 313-331, September 2003. Keyword(s): SAR, Multi-Channel SAR, MTI, AER-II, PAMIR, Subaperture Processing, Phased Array Radar, SAR Processing, Multi-Channel SAR, MTI, AER-II, PAMIR, Subaperture Processing, Phased Array Radar, Tomography, SAR Tomography.
    Abstract: SAR systems equipped with a phased array antenna and several receiver channels offer additional flexibility and extended target signatures. Besides of the electronic beam-steering permitting interesting operational modes, the partitioning into subarrays with parallel receiving channels opens the possibility to receive multichannel signals containing much more information about the scene than in the classical single channel case. In this paper, we start from a general signal model for wideband multi-channel data. This signal model serves as a basis for the development of algorithms for some special array configurations and applications. The approach is done from a signal theoretical point of view including statistical analysis. Most of the methods apply also to small groups of classical antennas, so the considerations are not limited to phased arrays. The airborne experimental multi-channel SAR systems AER and PAMIR serve as demonstrators for extended possibilities of imaging radars equipped with active phased arrays and parallel receiving channels. The presented examples have been achieved with these demonstrators.
    Comments: + Much und MTI, but topics like SAR tomography are covered as well. Anything that is multi-channel.
    [bibtex-key = ender03:SARMTITomo] [bibtex-entry]


  1461. Joachim H.G. Ender and Andreas R. Brenner. PAMIR - a wideband phased array SAR/MTI system. In IEE Proceedings - Radar, Sonar and Navigation, number 3, pages 165-172, June 2003. Keyword(s): SAR Processing, PAMIR, MTI, GMTI, Time-Domain Back-Projection, Back-Projection, Spotlight SAR, FGAN, X-Band, InSAR 1.8 GHz, ISAR, IfSAR, Phased Array Multifunctional Imaging Radar, X-band radar, airborne imaging radar, electronically steerable phased array, ground moving objects, ground moving target indication, ground-moving target indication, inverse SAR, long-range imaging capabilities, multichannel capability, operational modes, receive channels, reconfigurable phased array antenna, reconnaissance tasks, resolution, signal bandwidth, single-pass interferometric SAR, space-time adaptive processing, spaceborne imaging radar, subapertures, surveillance, synthetic aperture radar, wideband phased array SAR/MTI system, wideband system design.
    Abstract: Air- and spaceborne imaging radar systems in forthcoming surveillance and reconnaissance tasks have to meet increasingly severe demands. The next generation of top-level synthetic aperture radar (SAR) systems will comprise, among others, high resolution and long-range imaging capabilities, highly sensitive ground moving target indication and a multitude of sophisticated operational modes. The variety of tasks can be fulfilled only by the use of a reconfigurable phased array antenna together with a comprehensive wideband system design and a multichannel capability. At FGAN a new experimental X-band radar has been conceived, which will possess in its final upgrade an electronically steerable phased array consisting of 16 autonomous and reconfigurable subapertures, five independent receive channels, and a total signal bandwidth of about 1.8 GHz. The sensor is called PAMIR (Phased Array Multifunctional Imaging Radar). It is envisaged to demonstrate SAR imaging at a very high resolution and for a long range. The fine resolution will also be achieved with inverse SAR (ISAR) imaging of ground moving objects. Furthermore, the number of receive channels will allow ground-moving target indication (GMTI) by space?time adaptive processing and single-pass interferometric SAR (IfSAR) with a very high 3-D resolution. In its current stage of extension PAMIR is operable with one receive channel and a mechanically steerable antenna array. The system design and the intended capabilities of PAMIR are described. Ground-based and airborne experimental results concerning high-resolution SAR and ISAR imaging are also presented.
    [bibtex-key = enderBrennerIEE2003:PAMIR] [bibtex-entry]


  1462. Tuo Fu, Meiguo Gao, and Yuan He. An improved scatter selection method for phase gradient autofocus algorithm in SAR/ISAR autofocus. In Neural Networks and Signal Processing, 2003. Proceedings of the 2003 International Conference on, volume 2, pages 1054-1057, December 2003. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Fu2003] [bibtex-entry]


  1463. C. Henry, J. Souyris, and P. Marthon. Target detection and analysis based on spectral analysis of a SAR image: a simulation approach. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 2005-2007, July 2003. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Interferometric Point Target Analysis, IPTA, Analytical models, Azimuth, Image analysis, Layout, Object detection, Radar imaging, Radiometry, Speckle, Spectral analysis, Synthetic aperture radar.
    Abstract: In a previous study, the joint use of Synthetic Aperture Radar (SAR) image magnitude and phase has been assessed in the context of target detection. The two-looks Internal Hermitian Product (2L-IHP), based on a correlation between sub-looks of a single look complex (SLC) radar image, was introduced. It has been applied on an airborne radar image including deterministic targets deployed in a natural environment. The 2L-IHP was shown to increase the target environment contrast.
    [bibtex-key = henrySouyrisMarthonIGARSS2003TargetDetectionSpectralAnalysis] [bibtex-entry]


  1464. A. Jakobsson, F. Gini, and F. Lombardini. Layover solution in multibaseline INSAR using robust beamforming. In Signal Processing and Information Technology, 2003. ISSPIT 2003. Proceedings of the 3rd IEEE International Symposium on, pages 328-331, December 2003. [bibtex-key = Jakobsson2003] [bibtex-entry]


  1465. A. Jakobsson, F. Lombardini, and F. Gini. Weighted subspace fitting of interferometric phases for multibaseline SAR interferometry. In Signal Processing and Its Applications, 2003. Proceedings. Seventh International Symposium on, volume 1, pages 321-324, July 2003. [bibtex-key = Jakobsson2003a] [bibtex-entry]


  1466. J.S. Lee, D.L. Schuler, T.L. Ainsworth, and W.-M. Boerner. Polarization orientation estimation and applications: a review. In IEEE International Geoscience and Remote Sensing Symposium, 2003. IGARSS '03., volume 1, pages 428-430, July 2003. Keyword(s): SAR Processsing, calibration, covariance matrices, data acquisition, radar polarimetry, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping DEM generation, AIRSAR, L-band, polarimetric SAR images, P-band, SIR-C, circular polarization covariance matrix, estimation algorithms, ocean surface feature characterization, polarimetric SAR data compensation, polarimetric calibration, polarization orientation angle shifts, polarization orientation estimation, radar frequency, scattering media, terrain slopes, Airborne SAR.
    Abstract: We review estimation algorithms and applications of polarization orientation angle shifts induced by terrain slopes. We develop a unified analysis of estimation algorithms based on circular polarization covariance matrix. The effect of radar frequency, scattering media, and polarimetric calibration will also be discussed. Applications to DEM generation, polarimetric SAR data compensation and ocean surface feature characterization will be mentioned. SIR-C, and JPL AIRSAR L-band and P-band polarimetric SAR images are used for demonstration.
    [bibtex-key = leeSchulerAinsworthBoerner2003:PolSAR] [bibtex-entry]


  1467. F. Lombardini. Differential tomography: a new framework for SAR interferometry. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 2, pages 1206-1208, July 2003. [bibtex-key = Lombardini2003] [bibtex-entry]


  1468. F. Lombardini and F. Gini. Multiple reflectivities estimation for multibaseline InSAR imaging of layover extended sources. In Radar Conference, 2003. Proceedings of the International, pages 257-263, September 2003. [bibtex-key = Lombardini2003b] [bibtex-entry]


  1469. Fabrizio Lombardini and Andreas Reigber. Adaptive spectral estimation for multibaseline SAR tomography with airborne L-band data. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS '03., volume 3, pages 2014-2016, 2003. Keyword(s): SAR Processing, Tomography, SAR Tomography, Capon spectral estimator, adaptive Capon spectral estimator, spectral estimation, multibaseline 3D SAR focusing, multi-baseline SAR, InSAR, Multibaseline InSAR, Forestry.
    Abstract: In the recent years there has been growing interest in exploiting multibaseline (MB) SAR interferometry in a tomographic framework, to produce full 3D imaging e.g. of forest layers. However, Fourier-based MB SAR tomography is generally affected by unsatisfactory imaging quality due to a typically low number of baselines and their irregular distribution. In this work, we apply the more modern adaptive Capon spectral estimator to the vertical image reconstruction problem, using real airborne MB data. A first demonstration of possible imaging enhancement in real-world conditions is given.
    [bibtex-key = lombardiniReigber03:TomoCapon] [bibtex-entry]


  1470. A. Löw and W. Mauser. Generation of geometrically and radiometrically terrain corrected ScanSAR images. In IEEE International Geoscience and Remote Sensing Symposium, 2003. IGARSS '03, volume 6, pages 3995-3997, 2003. Keyword(s): SAR Processing, SAR Geocoding, Radiometric Calibration, Calibration, Radiometric Correction, backscatter, microwave imaging, radiometry, synthetic aperture radar, terrain mapping, topography (Earth), ENVISAT, ASAR, RADARSAT, biophysical parameters, geometrically terrain corrected ScanSAR images, ScanSAR, geophysical parameters, microwave imagery, quantitative image analysis, radiometrically terrain corrected ScanSAR images, surface topography, synthetic aperture imagery.
    Abstract: Inclined surface topography diminishes the geometric and radiometric quality of synthetic aperture imagery. The correction of these effects becomes indispensable when quantitative image analysis is performed with respect to the derivation of geo- and biophysical parameters. Due to their spatial extent and frequent availability, ScanSAR image products extend the operative range of microwave imagery and have a high potential for numerous operational applications over larger areas. The study presents a procedure for a pre-operational terrain correction of ScanSAR imagery as acquired by RADARSAT and ENVISAT ASAR.
    [bibtex-key = loewMauser03:RadiometricCalibration] [bibtex-entry]


  1471. P. Prats, J. J. Mallorqui, and A. Broquetas. Calibration of interferometric airborne SAR images using a multisquint processing approach. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 7, pages 4353-4355, July 2003. Keyword(s): SAR Processing, Motion Compensation, ESAR, L-Band, X-Band, Airborne SAR, Squinted SAR, Interferometry, azimuth phase undulations, calibration, image pairs, interferometric airborne synthetic aperture radar systems, InSAR, multisquint processing, phase error correction, phase error detection, single-pass interferometrie data. [bibtex-key = PratsMallorquiBroquetas2003a:MoComp] [bibtex-entry]


  1472. A. Reigber, A. Potsis, E. Alivizatos, N. Uzunoglu, and A. Moreira. Wavenumber domain SAR focusing with integrated motion compensation. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 3, pages 1465-1467, 2003. Keyword(s): SAR Processing, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Extended omega-k, Extended Wavenumber Domain Algorithm, Motion Compensation, Squinted SAR, Airborne SAR.
    Abstract: In this paper a new SAR data processing algorithm denoted with Extended Omega-K (EOK) is analytically presented and formulated. EOK algorithm combines the advantages of the high accurate focusing of the wavenumber domain algorithms with high precision motion compensation. The new EOK algorithm integrates a two-step range adaptive motion compensation correction in the general formulation of the wavenumber domain algorithm, leading to a new SAR processing scheme, which is much more robust concerning long synthetic apertures and squint angle than for example the chirp-scaling method. Additionally it offers the possibility of processing wideband low-frequency airborne SAR data up to near-wavelength resolution. The performance and the accuracy of the new EOK SAR data processing algorithm is demonstrated using simulated data.
    [bibtex-key = reigberPotsisAlivizatosUzunogluMoreira03:ExtendedOmegaK] [bibtex-entry]


  1473. A. Reigber, P. Prats, R. Scheiber, and J. J. Mallorqui. Options for high-precision motion compensation for airborne differential SAR interferometry. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 7, pages 4356-4358, July 2003. Keyword(s): SAR Processing, D-InSAR, InSAR, ESAR, Airborne SAR, L-Band, differential interferometry, Interferometry, Motion Compensation, RME, Residual Motion Errors, Spectral Diversity, Multi-Squint Processing, Digital Elevation Model, DEM. [bibtex-key = ReigberPratsScheiberMallorqui2003:MoComp] [bibtex-entry]


  1474. M. Rombach and João Moreira. Description and applications of the multipolarized dual band OrbiSAR-1 InSAR sensor. In Radar Conference, 2003. Proceedings of the International, volume 5, pages 245-250, 2003. Keyword(s): electromagnetic wave reflection, electromagnetic wave scattering, radar polarimetry, remote sensing by radar, Spaceborne SAR, synthetic aperture radar, terrain mapping, topography (Earth), vegetation mapping, OrbiSAR-1 InSAR sensor, area mapping, bald earth height information, biomass, canopy top foliage scattering, digital elevation, forest-classification, ground elevation, interferometric SAR, multipolarized dual band InSAR sensor, permanent cloud covered tropical areas, soil reflection, surface elevation, topographic maps, trunk reflection, vegetation density, vegetation height, vegetation/microwave interaction.
    Abstract: In the last decade, interferometric SAR (InSAR) has reached a wide acceptance as being a suitable tool to generate high-precision digital elevation models. Especially in tropical areas, with nearly permanent cloud coverage, InSAR provides a cost-efficient means for mapping large areas in short time periods. However, the interaction of microwaves with vegetation is strongly dependant on their frequency, demanding a careful interpretation of the extracted information. Short waves like X-band are mainly scattered back from the top of the canopy, whereas P-band penetrates the foliage and gets reflected from trunk and soil, thus carrying the phase information (and therefore the height information as well) from bald earth. For the generation of topographic maps, generally the ground elevation rather than the surface elevation is required, whereas the surface and ground elevation together enable the estimation of additional physical parameters like vegetation height, density, or biomass.
    [bibtex-key = RombachMoreira03:OrbiSAR] [bibtex-entry]


  1475. R. Scheiber. A three-step phase correction approach for airborne repeat-pass interferometric SAR data. In Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International, volume 2, pages 1190-1192, July 2003. [bibtex-key = Scheiber2003] [bibtex-entry]


  1476. David Small, Jürgen Holzner, Hannes Raggam, Detlef Kosmann, and Adrian Schubert. Geometric performance of ENVISAT ASAR products. In IGARSS '03, International Geoscience and Remote Sensing Symposium, volume 2, pages 1121-1123, 2003. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, synthetic aperture radar, terrain mapping, topography (Earth), ASAR geolocation accuracy, ASAR images, ASAR slant range products, DEM, ENVISAT, ASAR, ENVISAT ASAR products, ESA, alternating polarization, corner reflectors, derivative geocoded products, ellipsoid-geocoded products, estimation techniques, geometric performance, ground control points, ground range precision, ground range products, ground range transformation, map features, medium resolution products, multiple validation, nominally geocoded GTC locations, radar geometry image products, radar image, single look complex, topographic maps, transponders, wide swath mode acquisitions, zero-Doppler iteration.
    Abstract: We describe validation measurements of the geometric accuracy of ASAR images, measured redundantly via independent methods. Our tests include image (IM), alternating polarization (AP), and wide swath (WS) mode acquisitions over a variety of test sites. ASAR's slant range products (IMS/APS) require a slightly different validation methodology than ground range precision (IMP, APP) and medium resolution products (IMM, APM, WSM). A third approach is required for ellipsoid-geocoded products (IMG, APG). The most highly accurate validation is possible with single look complex (SLC) data (IMS and APS products), as all other product types lose resolution during multilooking. For a library of ground control points (GCPs) including map features such as bridges or road intersections, as well as (where available) transponders and corner reflectors, we use surveyed or map-measured position information (together with the delay value in the case of transponders) to solve the zero-Doppler iteration and predict the position of the GCP as an azimuth and slant range coordinate in the radar image. In the case of ground range products (e.g. IMP, APP, IMM, APM, WSM) the predicted slant range value is additionally transformed by a slant to ground range transformation tro determine the predicted image coordinate. The GCP feature is then either measured by inspection of a detected image, or localized automatically within the neighborhood of the prediction. GCPs are measured within the radar geometry image products, derivative geocoded products, and topographic maps, providing their measured map, radar geometry, and nominally geocoded GTC locations. Radar image locations are compared to map reference values and statistics of differences are tabulated. We compare the accuracies of the estimates achievable using transponders and map GCPs. Based on the suite of products (and accompanying orbit information) available to us, we establish a methodology for estimating a preliminary sampling window start time bias. The multiple validation and estimation techniques used ensure robust determination of ASAR geolocation accuracy.
    [bibtex-key = smallHolznerRaggamKosmannSchubert03:ASARGeometricPerformance] [bibtex-entry]


  1477. Gordon C. Staples and Joost van der Sanden. RADARSAT-2 Polarimetry Applications. In Anais XI SBSR, Simposio Brasileiro de Sensoriamento Remoto 2003, INPE, Belo Horizonte, Brasil, volume 1, pages 2383-2389, 2003. Keyword(s): Radarsat-2, Polarimetry, Applications.
    Abstract: RADARSAT-2, planned for a mid 2004 launch, is an advanced polarimetric SAR satellite. Key features of RADARSAT-2 are high resolution (3 m), polarimetric modes, enhanced ground system providing rapid satellite tasking and near-real time data processing, improved image location accuracy, and on-board solid state recorders. The focus of this paper is on the RADARSAT-2 polarimetric applications including agriculture, cartography, disaster management, forestry, geology, hydrology, oceans, and sea ice.
    [bibtex-key = staples:radsat2pol] [bibtex-entry]


  1478. B. Subiza, E. Gimeno-Nieves, J.M. Lopez-Sanchez, and J. Fortuny-Guasch. An Approach to SAR Imaging by Means of Non-Uniform FFTs. In IEEE International Geoscience and Remote Sensing Symposium, 2003. IGARSS '03., volume 6, pages 4089-4091, July 2003. Keyword(s): SAR Processing, Range Migration Algorithm, RMA, omega-k, NUFFT, Non-Uniform Fast Fourier Transform, Fast Fourier Transform, FFT, geophysical techniques, interpolation, radar imaging, synthetic aperture radar, SAR imaging algorithm, Stolt interpolation, computation time, computational efficiency, numerical simulations, seismic migration SAR processing.
    Abstract: In this study, the potential use of the non-uniform FFT (NUFFT) in SAR imaging is analyzed. The main objective has been the improvement of the computational efficiency and image accuracy of seismic migration SAR processing. Different NUFFT methods have been implemented and tested in order to choose an adequate technique for the imaging problem. Our approach consists in substituting both the Stolt interpolation and the final range inverse FFT, in the omega-k algorithm, by a single NUFFT. Numerical simulations illustrate the performance of the new method and the influence of the selection of NUFFT parameters in the precision and computation time of the SAR imaging algorithm.
    [bibtex-key = subizaGimenoNievesLopezSanchezFortunyGuasch2003:NUFFT] [bibtex-entry]


  1479. T.J. Sutton, H.D. Griffiths, S.A. Chapman, R. Crook, and M. Way. Optimizing a three-stage autofocus system for synthetic aperture imaging using a UUV. In OCEANS 2003. Proceedings, volume 5, pages 2433-2437, September 2003. [bibtex-key = Sutton2003a] [bibtex-entry]


  1480. T.J. Sutton, H.D. Griffiths, A.P. Hetet, Y. Perrot, and S.A. Chapman. Experimental validation of autofocus algorithms for high-resolution imaging of the seabed using synthetic aperture sonar. In Radar, Sonar and Navigation, IEE Proceedings -, volume 150, pages 78-83, April 2003. [bibtex-key = Sutton2003] [bibtex-entry]


  1481. L. R. Varshney and D. Thomas. Sidelobe reduction for matched filter range processing. In Proc. IEEE Radar Conf., pages 446 - 451, 2003. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, Dual Apodization, leakage energy minimization, linear frequency modulation, matched filtering, nonlinear frequency modulation, pulse compression ratio, range sidelobes, sidelobe control, sidelobe reduction, chirp modulation, frequency modulation, matched filters, minimisation, nonlinear filters, radar detection, radar interference, radar signal processing.
    Abstract: Linear frequency modulation (LFM) matched filtering results in range sidelobes. These sidelobes are often objectionable because they may mask small targets or may be mistaken for targets themselves. Various methods of sidelobe control are investigated and their performance is measured. The methods of sidelobe reduction include dual apodization, spatially variant apodization, and leakage energy minimization. Nonlinear frequency modulation (NLFM) matched filtering is also investigated. A simulation was run to compare LFM with sidelobe control and NLFM, all using moderately low pulse compression ratios. Results suggest that generally, NLFM matched filtering has better detection and estimation characteristics than LFM with sidelobe control.
    [bibtex-key = varshneyThomas2003:Apodization] [bibtex-entry]


  1482. Charles L. Werner, Urs Wegmuller, Tazio Strozzi, and Andreas Wiesmann. Interferometric point target analysis for deformation mapping. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 7, pages 4362-4364, 2003. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Interferometric Point Target Analysis, IPTA, synthetic aperture radar, terrain mapping, ERS-1 data, ERS-2 data, atmospheric path delays, deformation mapping, interferometric point target analysis, surface deformation, Atmospheric modeling, Decorrelation, Delay, History, Ice, Interferometry, Land surface, Phase noise, Remote sensing, Volcanic activity.
    Abstract: Interferometric Point Target Analysis (IPTA) is a method to exploit the temporal and spatial characteristics of interferometric signatures collected from point targets to accurately map surface deformation histories, terrain heights, and relative atmospheric path delays. In this contribution the IPTA concept is introduced, including the point selection criteria, the phase model and the iterative improvement of the model parameters. Intermediate and final results of an IPTA example using a stack of ERS-1 and ERS-2 data, confirm the validity of the concept and indicate a high accuracy of the resulting products.
    [bibtex-key = wernerWegmullerStrozziWiesmann2003] [bibtex-entry]


  1483. Charles Werner, Urs Wegmuller, Andreas Wiesmann, and Tazio Strozzi. Interferometric point target analysis with JERS-1 L-band SAR data. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 7, pages 4359-4361, July 2003. IEEE. Keyword(s): SAR Processing, Interferometry, SAR Interferometry, Persistent Scatterer Interferometry, PSI, Interferometric Point Target Analysis, IPTA, L-band, Atmospheric modeling, Deformable models, Phase estimation, Atmosphere, Testing, Surface topography, Azimuth, Remote sensing, Sensor phenomena and characterization, topography, synthetic aperture radar, spaceborne radar, remote sensing by radar, radiowave interferometry, geophysical techniques, subsidence, JERS-1 L-band SAR data, long-term coherence, surface deformation, Koga, Japan.
    Abstract: Interferometric Point Target Analysis (IPTA) is a method that exploits the temporal and spatial characteristics of interferometric signatures collected from point targets that exhibit long-term coherence to map surface deformation. This paper demonstrates the viability of this technique for L-band data collected by the JERS-1 sensor during the time period 1992-1998. A data set covering Koga, Japan is used for demonstration and indicates regions of substantial subsidence.
    [bibtex-key = wernerWegmullerWiesmannStrozziIGARSS2003PSIIPTAJERS] [bibtex-entry]


  1484. David A. Yocky and Charles V. Jakowatz. Automated wide-angle SAR stereo height extraction in rugged terrain using shift-scaling correlation. In Andrew G. Tescher, editor, , volume 5203, pages 10-20, 2003. SPIE. Keyword(s): SAR Processing, Stereo SAR, Rugged Terrain, Polar Format Algorithm, PFA, Spotlight SAR, Spotlight-mode data. [bibtex-key = yockyJakowatzStereoHeightExatraction] [bibtex-entry]


  1485. P. Berardino, G. Fornaro, R. Lanari, E. Sansosti, F. Serafino, and F. Soldovieri. Multi-pass synthetic aperture radar for 3-D focusing. In Proc. IEEE Int. Geosci.Remote Sens. Symp., volume 1, pages 176-178, 2002. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar imaging, radar theory, remote sensing by radar, synthetic aperture radar, terrain mapping, 3D focusing, SAR, SVD, geophysical measurement technique, land surface, multipass method, penetration depth, radar remote sensing, radar tomography, singular value decomposition.
    Abstract: In the area of tomographic synthetic aperture radar processing wepresent a new technique that makes use of the singular value decomposition method to improve the resolution limits by including a-priori information about the radiation penetration depth.
    [bibtex-key = bernardinoFornaroLanariSansostiSerafinoSoldovieri2002:Tomo] [bibtex-entry]


  1486. P. Berens. Estimation of Carrier Track For High Precision SAR Imaging Using Active Reference Reflectors. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, pages 241-244, 2002. Keyword(s): SAR Processing, Active Reflector, Transponder, Autofocus, Carrier Track Estimation, Flight Track Estimation, Kalman Filter, PAMIR, X-Band. DGPS, INS.
    Abstract: Synthetic aperture radar (SAR) processing needs precise information about the path of the radar sensor. Modern DGPS and INS systems are commonly used. However, for highest resolution, additional auto focus algorithms have to be implemented. The success of these algorithms depends strongly on the scene. For a reliable estimation of the flight path, a new idea is presented: active reference reflectors (transponders) within the scene receive the pulses of the radar system and reradiates them with an additional amplitude modulation. The echoes from these transponders can be separated from the echoes of the scene and offer an excellent basis for the estimation of the antenna track. The paper describes the processing steps to separate the echoes of the transponders from the scene echoes. The range histories from the antenna to the transponders can be determined very precisely afterwards. A Kalman filter combines the measured ranges and motion information given by a DGPS system to estimate the carrier track.
    [bibtex-key = berensEUSAR2002:EstimFlightTrackPamir] [bibtex-entry]


  1487. Andreas R. Brenner. DISTRIBUTED SAR PROCESSING IN THE TIME DOMAIN. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, 2002. Keyword(s): SAR Processing, Back-projection, Time-Domain Back-Projection, PAMIR, Distributed Processing, Parallel Processing.
    Abstract: The next generation airborne SAR sensors will comprise among others high resolution imaging capabilities (< 1 dm) and long range surveillance (> 100 km). This wide band and wide angle scenario requires an accurate modeling for SAR image formation with respect to motion compensation and focusing. This paper reports on the feasibility to jointly realise two approaches: First, a non-approximative time domain based SAR processor was developed. Second, because of its high computational burden, a distributed implementation on a heterogeneous workstation cluster by means of message passing interfaces was carried out. The evaluation of the processor on simulated data as well as an assessment of the distributed implementation is presented. The distributed time domain processor is successfully applied to data acquired with the new very wideband SAR sensor PAMIR of FGAN-FHR as well.
    [bibtex-key = brennerEUSAR2002:TDBP] [bibtex-entry]


  1488. Andreas R. Brenner and Joachim H. G. Ender. First Experimental Results Achieved With The New Very Wideband SAR System PAMIR. In Proc. of EUSAR 2002 - 4rd European Conference on Synthetic Aperture Radar, pages 81-86, 2002. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, PAMIR, Phased Array Multifunctional Imaging Radar, AER-II, X-Band, High Resolution, Wideband SAR.
    Abstract: Imaging radar systems in forthcoming surveillance and reconnaissance tasks have to meet increasingly severe demands. The next generation airborne SAR should comprise high resolution imaging capabilities (< 1 dm), long range surveillance (> 100 km), moving target indication (< 1 m/s) and a multitude of complex operational modes like MultipleSpotlight, ScanMTI and ISAR imaging of ground moving targets. At FGAN-FHR, where the experimental SAR system AER-II is successfully operational since 1996, a new experimental X-band system was conceived, which will possess in its final stage of realisation an electronically steerable phased array, five independent receive channels, a total signal bandwidth of about 1.8 GHz and will support novel multifunctional radar modes. The system is termed PAMIR (Phased Array Multifunctional Imaging Radar) and operates in its current realization with one channel and two horn antennas. In this paper, the first experimental results concerning calibration, synthetic bandwidth, motion compensation and high resolution image formation are presented.
    [bibtex-key = brennerEnderEUSAR2002:PAMIR] [bibtex-entry]


  1489. Yu Ding and David C. Munson, Jr.. A fast back-projection algorithm for bistatic SAR imaging. In Proc. Int. Conf. on Image Processing, volume 2, pages 449-452, 2002. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-projection, Bistatic SAR, image reconstruction, integral equations, radar imaging, synthetic aperture radar, tomography 2D interpolation, FFT, Fourier domain, Fourier domain data, bistatic SAR imaging, computational cost reduction, direct Fourier reconstruction, fast back-projection algorithm, image formation algorithms, near-field imaging, nonCartesian grid, simulation results, tomography.
    Abstract: Using a far-field model, bistatic synthetic aperture radar (SAR) acquires Fourier data on a rather unusual, non-Cartesian grid in the Fourier domain. Previous image formation algorithms were mainly based on direct Fourier reconstruction to take advantage of the FFT, but the irregular coverage of the available Fourier domain data and the 2-D interpolation in the Fourier domain may adversely affect the accuracy of image reconstruction. Back-projection techniques avoid Fourier-domain interpolation, but ordinarily have huge computational cost. We present a fast back-projection algorithm for bistatic SAR imaging, motivated by a fast back-projection algorithm previously proposed for tomography. It has a reduced computational cost, on the same order as that of direct Fourier reconstruction. Furthermore, this approach can be used for near-field imaging. Simulation results verify the performance of this new algorithm.
    [bibtex-key = dingMunson2002:BistaticFastBackp] [bibtex-entry]


  1490. Armin W. Doerry, Fred M. Dickey, Louis A. Romero, and John M. DeLaurentis. Difficulties in Superresolving Synthetic Aperture Radar Images. In Edmund G. Zelnio, editor, Proc. of SPIE Vol. 4727, Algorithms for Synthetic Aperture Radar Imagery IX, number 1, pages 122-133, 2002. SPIE. Keyword(s): SAR Processing, Superresolution, Super Resolution, High Resolution, Fine Resolution, Spectral Estimation.
    Abstract: The ability to resolve Synthetic Aperture Radar (SAR) images to finer resolutions than the system bandwidths classically allow is a tantalizing prospect. Seemingly superresolution offers something for nothing, or at least something better than the system was designed for if only we process enough or right. Over the years this has proved to be a rather popular area of investigation, generating a wide variety of algorithms and corresponding claims of performance. Nevertheless, the literature on the fundamental underlying principles of superresolution as applied to SAR has been rather anemic. This paper addresses the following questions: What exactly is superresolution? and What is not really superresolution, but perhaps more aptly described as image enhancement? Is true superresolution possible? and to what degree? What constrains superresolution? and very importantly, How should we objectively test whether an image is in fact superresolved? Whereas superresolution concepts offer the potential of resolution beyond the classical limit, this great promise has not generally been realized. That is not to say that many reported algorithms have no useful effect on images. True superresolution is defined herein as the recovery of true scene spectrum, that allows more accurate scene rendering. The analytical basis for superresolution theory is outlined, and the application to SAR is then investigated as an operator inversion problem, which is generally ill posed. Noise inherent in radar data tends to severely inhibit significant enhancement of image resolution. A criterion for judging superresolution processing of an image is presented.
    [bibtex-key = DoerryDickeyRomeroDeLaurentis2002] [bibtex-entry]


  1491. P. Dubois-Fernandez, O. R. du Plessis, D. le Coz, J. Dupas, B. Vaizan, X. Dupuis, H. Cantalloube, C. Coulombeix, C. Titin-Schnaider, P. Dreuillet, J. M. Boutry, J. P. Canny, L. Kaisersmertz, J. Peyret, P. Martineau, M. Chanteclerc, L. Pastore, and J. P. Bruyant. The ONERA RAMSES SAR system. In IEEE International Geoscience and Remote Sensing Symposium, volume 3, pages 1723-1725 vol.3, June 2002. Keyword(s): SAR Processing, W-Band, airborne radar, geophysical signal processing, geophysical techniques, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mapping, 0.43 to 95 GHz, C-band, EHF, InSAR, K-band, Ka-band, Ku-band, L-band, ONERA, P-band, RAMSES, S-band, SAR, SHF, TDRI, UHF, W-band, X-band, airborne radar, algorithm, geophysical measurement technique, instrument, land surface, radar polarimetry, radar remote sensing, synthetic aperture radar, target detection recognition and identification, terrain mapping, Calibration, Frequency, Interferometry, Object detection, Radar applications, Radar imaging, Space technology, Synthetic aperture radar, System testing, Target recognition.
    Abstract: The ONERA RAMSES system (Radar Aeroporte Multi-spectral d'Etude des Signatures) is a flexible SAR system in constant evolution developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). This paper gives an overview of the system and its recent upgradings.
    [bibtex-key = DuboisFernandezEtAlIGARSS2002OneraRAMSESSARWBAND] [bibtex-entry]


  1492. Joachim H.G. Ender and Andreas R. Brenner. PAMIR - A Wideband Phased Array SAR/MTI System. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, pages 157-162, 2002. Keyword(s): SAR Processing, PAMIR, MTI, GMTI, Time-Domain Back-Projection, Back-Projection, Spotlight SAR, FGAN, X-Band, InSAR 1.8 GHz, ISAR, IfSAR, Phased Array Multifunctional Imaging Radar, X-band radar, airborne imaging radar, electronically steerable phased array, ground moving objects, ground moving target indication, ground-moving target indication, inverse SAR, long-range imaging capabilities, multichannel capability, operational modes, receive channels, reconfigurable phased array antenna, reconnaissance tasks, resolution, signal bandwidth, single-pass interferometric SAR, space-time adaptive processing, spaceborne imaging radar, subapertures, surveillance, synthetic aperture radar, wideband phased array SAR/MTI system, wideband system design.
    Abstract: Future air- and spacebased reconnaissance systems will be equipped with long range radar platforms of high flexibility, very high resolution in the order of one decimetre, covering a large angular sector and operating in sophisticated modes using multi channel signal processing. This variety of tasks can be fulfilled only by use of a phased array antenna. The need for a large bandwidth rises a lot of problems to be solved. To study the achievable performance in practice, FGAN-FHR has decided to build up an experimental system: PAMIR, the Phased Array Multifunctional Imaging Radar. Now, the system has come to a first stage allowing to gather preliminary radar data using a simple horn antenna. The phased array antenna is planned to be available in 2003.
    [bibtex-key = enderBrennerEUSAR2002:PAMIR] [bibtex-entry]


  1493. Irena Hajnsek, Konstantinos P. Papathanassiou, Alberto Moreira, and Shane R. Cloude. Surface parameter estimation using interferometric and polarimetric SAR. In Proc. IEEE Int. Geoscience and Remote Sensing Symp, volume 1, pages 420-422 vol.1, 2002. Keyword(s): backscatter, geophysical techniques, hydrological techniques, radar cross-sections, radar polarimetry, radar theory, remote sensing by radar, synthetic aperture radar, terrain mapping, geophysical measurement technique, hydrology, interferometric SAR, interferometric coherence, land surface, moisture, polarimetric SAR, radar remote sensing, radar scattering, soil moisture, surface parameter estimation, surface roughness, surface scattering model, Anisotropic magnetoresistance, Coherence, Decorrelation, Parameter estimation, Polarization, Reflectivity, Rough surfaces, Scattering, Surface roughness, Surface topography. [bibtex-key = Hajnsek2002] [bibtex-entry]


  1494. Andrea Monti-Guarnieri. Processing Strategies for Phase Unwrapping for InSAR Applications. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, volume 1, Ulm, Germany, pages 349-352, May 2002. Keyword(s): SAR Processing, Interferometry, Phase Unwrapping, Multifractal Modelling of Earth Topography.
    Abstract: We exploit the terrain statistics derived from multifractal modelling of earth topography to derive some statistical properties of SAR interferogram, with particular reference to the gradient of the unwrapped phase.
    [bibtex-key = monti02:phaseUnWrap] [bibtex-entry]


  1495. R.L. Morrison, Jr. and David C. Munson, Jr.. An experimental study of a new entropy-based SAR autofocus technique. In Image Processing. 2002. Proceedings. 2002 International Conference on, volume 2, pages 441-444, September 2002. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Morrison2002] [bibtex-entry]


  1496. Clifford J. Nolan and Margaret Cheney. Synthetic Aperture Inversion For Non-Flat Topography. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, volume 1, pages 105-108, 2002. Keyword(s): SAR Processing, Back-Projection, Non-Flat Topography, Time-Domain Back-Projection, TDBP.
    Abstract: This paper considers Synthetic Aperture Radar and other synthetic aperture imaging systems in which a backscattered wave is measured from positions along a single flight track. We assume that the ground topography is known but not necessarily flat. We consider two cases, corresponding to the degree of directionality of the antenna. For the high-directivity case, we propose an imaging algorithm involving backprojection and a spatially varying filter that corrects for the antenna beam pattern, source waveform, and other geometrical factors. We give conditions on the relationship between the flight track and the topography to avoid artifacts. We show that the algorithm correctly reproduces certain features of the scene. For the case of an antenna with poor directionality, the image produced by the above algorithm contains artifacts. For this case, we analyze the strength of the artifacts relative to the strength of the true image. The analysis of this paper shows that the artifacts can be somewhat suppressed by increasing the curvature of the flight track and by keeping the desired target in view for as long as possible.
    [bibtex-key = nolancheney:SARInversion] [bibtex-entry]


  1497. Mats I. Pettersson. Detection of Moving Target in Wideband SAR Using Fast Time Backprojection Processing. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, volume 1, pages 217-220, 2002. Keyword(s): SAR Processing, Back-Projection, Fast Back-Projection, TDBP, Time-Domain Back-Projection, Moving Target Indication, Ultra-Wideband SAR.
    Abstract: A likelihood ratio test is proposed for moving target detection in an ultra wide frequency band and wide antenna beam (wide band) SAR system. The developed method combines time domain fast backprojection SAR processing methods with moving target detection. It saves computational load when all relative speeds can be tested using the same clutter suppressed sub-aperture beams. The proposed method is tested on narrow band radar data.
    [bibtex-key = pettersson:backproj] [bibtex-entry]


  1498. Athanasios Potsis, Andreas Reigber, Emmanouil Alivizatos, Alberto Moreira, and Nikolaos K. Uzunoglou. Comparison of Chirp Scaling and Wavenumber Domain Algorithms for Airborne Low-Frequency SAR. In Francesco Posa, editor, SAR Image Analysis, Modeling, and Techniques V, volume 4883, pages 25-36, March 2002. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Extended Chirp Scaling Algorithm, omega-k, Range Migration Algorithm, Wavenumber Domain Algorithm, Comparison of Algorithms, P-Band, Wideband SAR, Airborne SAR.
    Abstract: In recent years a new class of Synthetic Aperture Radar (SAR) systems, using low frequencies, have emerged. The combination of low frequencies with high bandwidths allows a variety of new applications. Several new fields arise in forestry, biomass estimation and in archaeological and geological exploration. The P-band SAR technology benefits from technological advances in antenna design, low noise amplifiers, band pass filters, digital receiver technology, as well as new processing algorithms. For all the new applications of an airborne P-band SAR system, the high-resolution imaging is an important parameter, but it cannot be easily achieved with conventional processing techniques. In this paper, the performance and limitations of the Extended Chirp Scaling (ECS) algorithm and wavenumber domain Omega-K processing algorithm are analysed and discussed. Additionally, modifications of both algorithms are proposed, which optimise the respective algorithm for processing low frequency, wide-beam and wide-band SAR data. Despite of the inherent limitations of the above mentioned processing algorithms, a deterministic phase error, called ``digital phase error'', due to digital signal processing characteristics is formulated and its effect to the processed SAR data is analytically described. The analysis is carried out, using simulated low frequency airborne SAR data.
    [bibtex-key = PotsisReigAliMorUzun02:Comparison] [bibtex-entry]


  1499. Rolf Scheiber and V. M. Bothale. Application of Multi-Look Techniques for Interferometric SAR Data. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, Cologne, Germany, pages 77-80, June 2002. Keyword(s): SAR Processing, Extended Chirp Scaling, ECS, Chirp Scaling, CS, Azimuth Focusing, Multi-Look Processing, Non-Linear Flight Paths, Non-Linear SAR, Interferometry, InSAR, E-SAR, Airborne SAR, SAR interferometry, azimuth registration, geophysical measurement technique, image registration, interferometric SAR, land surface, motion compensation, multi-look method, multi-pass airborne method, multilook method, radar remote sensing, residual motion errors, spectral domain multi-look approach, synthetic aperture radar, terrain mapping.
    Abstract: Two different multi-look techniques for interferometric SAR data are investigated in the first part of this paper. The first one uses the box-car type of filter on the complex interferogram in the spatial domain, whereas the second one performs look-wise filtering of the individual images in the spectral domain with subsequent coherent addition of the look-wise interferograms. Next, it is shown that the flexibility of the spectral domain multi-look technique can be used for improved motion compensation and further for precisely updated estimation of azimuth misregistration offsets. This leads finally to the compensation of residual motion errors in case of multi-pass SAR interferometry. Investigations using data of the DLR owned airborne ESAR system are presented.
    [bibtex-key = scheiberBothale2002:multiLookProcessing] [bibtex-entry]


  1500. Rolf Scheiber and V.M. Bothale. Interferometric multi-look techniques for SAR data. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS '02, volume 1, pages 173-175, 2002. Keyword(s): SAR Processing, Extended Chirp Scaling, ECS, Chirp Scaling, CS, Azimuth Focusing, Multi-Look Processing, Non-Linear Flight Paths, Non-Linear SAR, Interferometry, InSAR, E-SAR, Airborne SAR, airborne radar, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping, SAR interferometry, azimuth registration, geophysical measurement technique, image registration, interferometric SAR, land surface, motion compensation, multi-look method, multi-pass airborne method, multilook method, radar remote sensing, residual motion errors, spectral domain multi-look approach.
    Abstract: This paper addresses the benefits of the spectral domain multi-look approach for SAR interferometry. A comparison with the wide spread spatial averaging filter is included in the beginning. Next, it is shown that the flexibility of the spectral domain multi-look technique can be used for improved motion compensation and further for precisely updated estimation of azimuth misregistration offsets. This leads finally to the compensation of residual motion errors in case of multi-pass airborne SAR interferometry, e.g. for the E-SAR system of DLR.
    [bibtex-key = scheiberBothaleIgarss2002:multiLookProcessing] [bibtex-entry]


  1501. H. Schimpf, H. Essen, S. Boehmsdorff, and T. Brehm. MEMPHIS-a fully polarimetric experimental radar. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 1714-1716, June 2002. Keyword(s): SAR Processing, W-Band, MEMPHIS, Airborne SAR, UAV, Fraunhofer, calibration, geophysical equipment, geophysical techniques, radar polarimetry, remote sensing by radar, synthetic aperture radar, terrain mapping, 5 to 36 GHz, 75 to 110 GHz, EHF, InSAR, Ka-band, MEMPHIS, SHF, W-band, X-band, across-track interferometry, calibration, forward squinted Doppler beam sharpening mode, geophysical measurement technique, land surface, polarimetric SAR, radar polarimetry, radar remote sensing, side-looking mode, synthetic aperture radar, technical description, terrain mapping, Calibration, Chirp, Frequency, Polarization, Radar antennas, Radar polarimetry, Receiving antennas, Satellite broadcasting, Synthetic aperture radar, Transmitting antennas.
    Abstract: The MEMPHIS polarimetric SAR is able to operate simultaneously at X-, Ka- and W-band. After a detailed technical description, several examples are given to demonstrate its capabilities in the side-looking mode, in forward squinted Doppler beam sharpenung (DBS) mode and with across-track interferometry. Of great importance is the polarimetric calibration which is described in some detail.
    [bibtex-key = schimpfEssenBoehmsdorffBrehmIGARSS2002MemphisWBandKaBandSAR] [bibtex-entry]


  1502. David Small, Detlev Kosmann, Jürgen Holzner, Hannes Raggam, Mauro Pirri, Adrian Schubert, Urs Krüttli, Wolfgang Hummelbrunner, and Martina Franke. ASAR Level 1 Geolocation. In Huguette Sawaya-Lacoste, editor, Proceedings of the Envisat Calibration Review, 2002. Keyword(s): SAR Geocoding, Calibration, Validation, Quality Assessment, ASAR, ENVISAT.
    Abstract: The localisation of ASAR products is vital to the ground segment, as overlays with independent information sources (typically in a map geometry) are only possible when the transformation between radar and map geometry is well calibrated. In this paper we describe calibration and validation steps undertaken to ensure that the transformations from radar to map geometry and back again are as accurate as possible. The ground segment of every new system must validate its geocoding chain to ensure that all parameters are treated consistently and are compatible with the product specifications. Special attention is devoted to the range and azimuth timing, as well as the orbit quality, cartographic and geodetic parameters describing the reference map projections. ASAR IMS and APS products are in the radar's native slant-range geometry. IMP, APP, IMM, APM, and WSM products are arranged in ground-range geometry. IMG and APG products are ellipsoid-geocoded (no terrain corrections applied), and delivered in map geometry. Each product type requires a slightly different calibration and validation methodology.
    [bibtex-key = SmallKosHolRagPirSchuKruHumFra02:ASARGeoloc] [bibtex-entry]


  1503. David Small, Adrian Schubert, Urs Krüttli, Erich Meier, and Daniel Nüesch. Preliminary Validation of ASAR Geometric Accuracy. In Proceedings of ENVISAT Validation Workshop, ESA-ESRIN, Frascati, Dec. 2002. Keyword(s): SAR Processing, SAR Geocoding, Geometric Accuracy, Geometric Calibration, Calibration, Validation, Quality Assessment, ASAR, ENVISAT.
    Abstract: We describe preliminary validation experiments performed to validate the geometric accuracy of ENVISAT ASAR data acquired in image (IM) and alternating polarisation (AP) modes. ESA?s ASAR transponders in The Netherlands were used primarily as reference locations. Corner reflectors were deployed at test sites in Switzerland for comparison, and conventional ground control points such as bridges and road intersections were also used. The location of the reference points in radar geometry was predicted based upon the reflector?s geographical position (and delay term in the case of transponders) and compared with the actual measured location in the image products. We form tentative conclusions on the residual error sources.
    [bibtex-key = smallSchubertKruettliNuesch02:PrelASARGeomAccuracy] [bibtex-entry]


  1504. M. Weiss and P. Berens. Motion compensation of wideband synthetic aperture radar with a new transponder technique. In IEEE International Geoscience and Remote Sensing Symposium, 2002. IGARSS '02., volume 6, pages 3649-3651, June 2002. Keyword(s): SAR Processing, Motion Compensation, calibration, Autofocus, phased array radar, radar imaging, synthetic aperture radar, transponder, PAMIR, SAR, active transponders, coherent integration, echoes, large synthetic aperture, phased array multifunctional imaging radar, radar pulses, transponder technique, wideband synthetic aperture radar.
    Abstract: A high resolution synthetic aperture radar (SAR) system called phased array multifunctional imaging radar (PAMIR) is currently under development at FGAN. This system uses a very high bandwidth and performs a coherent integration along a large synthetic aperture. Problems in the area of calibration and motion compensation arise which can't be solved using common tools like corner reflectors. This paper describes the construction of active transponders which modulate and re-radiate radar pulses, discusses the advantages for calibration and shows how the echoes can be used for motion compensation in the SAR processing.
    [bibtex-key = weissBerens2002:MoCoAutofocusTransponder] [bibtex-entry]


  1505. Charles L. Werner, Urs Wegmuller, and Tazio Strozzi. Processing Strategies for Phase Unwrapping for InSAR Applications. In Proc. of EUSAR 2002 - 4th European Conference on Synthetic Aperture Radar, volume 1, pages 353-356, 2002. Keyword(s): SAR Processing, Interferometry, Phase Unwrapping, Minimum Cost Flow, Branch Cut.
    Abstract: One of the most challenging aspects in the successful application of SAR interferometry (INSAR) is unwrapping the interferometric phase. The difficulties arise in attempting to find global optimization procedures with the best possible cost criteria for data that are both noisy and incomplete. Recent progress in this problem includes introduction of network flow optimization, and the use of triangular irregular networks for sparse data. Interferograms differ greatly in the difficulty to unwrap depending on the interferogram fringe complexity and correlation. We examine the characteristics of these types and present phase unwrapping strategies for each of these.
    [bibtex-key = WernWegStroz02:phaseUnWrap] [bibtex-entry]


  1506. Xiaojian Xu and R. M. Narayanan. SAR image enhancement using noninteger Nyquist SVA technique. In Proc. IEEE Antennas and Propagation Society International Symposium, volume 4, pages 298-301, 2002. Keyword(s): SAR Processing, Apodization, Spatially Variant Apodization, SVA, ISAR, image enhancement, image resolution, inverse SAR imaging, iterative super SVA procedure, noninteger Nyquist SVA, nonlinear filtering, sidelobe level reduction, spatially variant apodization, synthetic aperture radar, iterative methods, nonlinear filters.
    Abstract: In SAR and inverse SAR (ISAR) imaging, conventional Fourier transform (FT) based image reconstruction techniques result in images with limited resolution. The down-range and cross-range resolutions of these algorithms are inversely proportional to the radar signal waveform bandwidth and to the synthetic aperture size, respectively. On the other hand, when modem spectral estimation methods are applied to radar imaging, these nonlinear techniques, usually called super resolution algorithms, offer improved resolution, better contrast, and reduced speckle. Spatially variant apodization (SVA) is a nonlinear filtering operation which significantly reduces the sidelobe levels without degrading mainlobe resolution of the sinc impulse response. In this work, we propose a modified version of noninteger Nyquist SVA and develop an iterative super SVA procedure for SAR and ISAR image enhancement. The proposed technique was successfully applied to various SAR/ISAR images.
    [bibtex-key = xiaojianNarayanan2002:Apodization] [bibtex-entry]


  1507. Richard Abrahamsson, Jian Li, Petre Stoica, and Gunnar Thordarson. Sensitivity of two autofocus algorithms to spatially variant phase errors. In E. G. Zelnio, editor, Proceedings of SPIE Vol. 5788, volume 4382 of Presented at the Society of Photo-Optical Instrumentation Engineers (SPIE) Conference, pages 29-40, August 2001. Keyword(s): SAR Processing, Phase Gradient Autofocus, PGA, AUTOCLEAN, CLEAN, Polar Format Algorithm, Autofocus, Residual Motion Errors, Motion Errors, Motion Compensation, MoComp, Motion Through Resolution Cells, Spatially Variant Phase Errors, Airborne SAR.
    Abstract: In this paper we study the performance of two existing autofocus algorithms in a difficult SAR scenario. One algorithm is the well known phase gradient autofocus (PGA) algorithm and the other is the more recent AUTOCLEAN. The latter was introduced particularly with ISAR autofocus of a small target in mind and has been shown to outperform the PGA when range misalignment is present. This was expected as AUTOCLEAN, as opposed to PGA, has a built-in ability to compensate for range misalignment. In most available studies of the above autofocus algorithms spatially variant phase errors are absent or insignificant. The data used here is far-field SAR data collected over a large range of aspect angles. The target area is large, hence significant motion through resolution cells (MTRC) occurs due to target scene rotation. The polar format algorithm (PFA) is applied prior to autofocus to handle MTRC and compensate for off-track platform motion. However, the platform motion measurements used in PFA are not precise enough to compensate for the off-track motion and left after PFA are phase errors corrupting the data. These phase errors are spatially variant due to the large target scene and this violates the models for the autofocus algorithms above. This in contrast with the previously mentioned studies. We show that the performances of the autofocus algorithms considered are much deteriorated by the presence of spatially variant phase error but in different ways since the averaging of the phase error estimates is made differently in the two algorithms. Based on our numerical study of the two autofocus methods we try to rank them with respect to their sensitivity to spatially variant phase errors.
    [bibtex-key = abrahamssonLiStoicaThordarson2001:PGAandAUTOCLEAN] [bibtex-entry]


  1508. Arnold Barmettler, Erich Meier, and Daniel Nüesch. Development of an Ultra-Wideband SAR Processor. In CEOS SAR Workshop 2001, April 2001. Keyword(s): SAR Processing, Time-Domain Back-Projection, Back-Projection, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR.
    Abstract: Ultra-wideband SAR (UWB) has a high potential for applications because it makes high-resolution low-frequency imaging radar feasible. In combination with other SAR frequency bands, topographic or even tomographic mapping will be possible and geophysical parameters determinable. RSL has its own SAR processor for frequency bands from P- to X-band for various space- and airborne SAR sensors. For the processing of VHF UWB data, a new module for azimuth focusing was developed, due to the special system requirements at these wavelengths. We present first results from RSL's VHF processing chain based upon rangecompressed data from the Swedish CARABAS sensor system.
    [bibtex-key = BarmettMeierNuesch01:Backproj] [bibtex-entry]


  1509. Matthew C. Cobb and James H. McClellan. Omega-k Quadtree UWB SAR Focusing. In Proceedings of the 2001 IEEE Radar Conference, pages 311-314, May 2001. Keyword(s): SAR Processing, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k Quadtree Processing, omega-k, Quadtree Processing, Back-Projection, Ultra-Wideband SAR, Ghost Target Elimination, FOPEN.
    Abstract: A variation on the quadtree algorithm for ultra- wideband, wide-angle (UWB-WA) SAR imaging that uses the omega-k algorithm for final stage focusing is introduced. Several signal processing techniques appropriate to the requirements of UWB-WA SAR, and that prevent errors and artifacts in the omega-k focusing will be reviewed. These techniques include spatial and temporal shifts for spotlight data, elimination of ghost targets, and adjustments needed to account for the virtual sampling of the quadtree algorithm. In order to justify these techniques, a simple Fourier-based model of the imaging problem appropriate to UWB-WA SAR is presented.
    [bibtex-key = CobbMcClellan01:Quadtree] [bibtex-entry]


  1510. Ian G. Cumming. Model-Based Doppler Estimation for Frame-Based SAR Processing. In IGARSS '01, International Geoscience and Remote Sensing Symposium, volume 6, pages 2645-2647, 2001. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock.
    Abstract: This paper presents a new method of Doppler centroid estimation whereby estimates are made over small blocks of data covering a whole frame of data, and examined for strong SNR and lack of bias. Poor estimates are rejected, and the remaining estimates are used to fit a surface model of the Doppler centroid vs. range and azimuth. The method is applied to both the fractional and integer PRF part of the centroid. A geometric model is used to constrain the model to allowable roll, pitch and yaw values of satellite attitude. The method is tested with RADARSAT-1 and SRTM/X-SAR data.
    [bibtex-key = Cumming:DopCentrEst] [bibtex-entry]


  1511. Joachim H. G. Ender. The meaning of k-space for classical and advanced SAR techniques. In International Symposium Physics in Signal and Image Processing, PSIP 2001, Marseille, pages 23-38, January 2001. Keyword(s): SAR Processing, k-Space, Airborne SAR, Bistatic SAR, Teaching.
    Abstract: Synthetic aperture radar (SAR) has proven to be a powerful technique for imaging the surface of the earth and other celestial bodies with high resolution in the microwave region. As sensor a radar is used which is mounted on a moving platform - an airplane or satellite. By its motion a synthetic aperture is formed; the image is processed from the radar raw data using focusing algorithms of high complexity. Similar to SAR imaging is the technique of inverse SAR (ISAR), which allows to image turning or moving objects with a fixed radar. The k-space as the domain of the spatial Fourier transform presents an important mathematical tool for general imaging problems. As an example, in biomedical ultrasonic imaging, k-space techniques are widely used, for instance for the analysis of imaging systems and evaluation of data collection strategies, see e.g. [33]. In the field of radar imaging, the k-space can serve as a system analysis tool as well as the basis for reconstruction algorithms: Many of the phenomena arising for SAR and ISAR can be explained by analysis in the k-space. Some of the algorithms like polar reformatting are based on k-space formulation. To get a deeper insight into physical effects, it is also worthwile to look at advanced techniques like bistatic SAR in the light of k-space. On the other hand, the k-space is also a domain for the synthesis of imaging conditions: Thanks to the fast development of technical components, it will be more and more possible to create flexible waveforms and geometries, i.e.: to design the measurement configuration in the k-space opening new exciting possibilities. In this tutorial, a unified view of such techniques in terms of k-space is presented. Here, the aim of this tutorial is not to give a comprehensive summary of the numerous focussing algorithms, but to give aspects of the way of thinking in the three-dimensional Fourier domain. Though it is also possible to handle the k-space in terms of electromagnetic theory (e.g.[20]), we will concentrate to the signal-theoretical view.
    [bibtex-key = ender2001:kspace] [bibtex-entry]


  1512. F. Gini, F. Lombardini, P. Matteucci, and L. Verrazzani. System and estimation problems for multibaseline InSAR imaging of multiple layovered reflectors. In Geoscience and Remote Sensing Symposium, 2001. IGARSS '01. IEEE 2001 International, volume 1, pages 115-117, July 2001. [bibtex-key = Gini2001] [bibtex-entry]


  1513. Xiaotao Huang, Zhimin Zhou, and Diannong Liang. Effects of RFI on UWB-SAR Using LFM Waveforms. In CIE International Conference on Radar, 2001, pages 631-633, October 2001. Keyword(s): SAR Processing, RFI Suppression, Ultra-Wideband SAR, VHF SAR.
    Abstract: The dual requirement of ultra-wide band synthetic aperture radar (UWB-SAR) for high range resolution and low frequency penetration proposes the problem of radio frequency interference (RFI) suppression. Although careful design of the receiver hardware can reduce this difficulty, a great amount of RFI will still distort the actual target returns. The effects of RFI on both of the design of hardware and the task of signal processing are studied. We focus our analysis on the basic pulse compression in linear frequency modulated (LFM) UWB-SAR. The output of a typical sinusoidal RFI after pulse compression is derived and proved to be closely related to the frequency of the RFI. Computer simulations successfully verify our conclusions.
    [bibtex-key = XiaoZhimDian01:RFI] [bibtex-entry]


  1514. C.V. Jakowatz and D.E. Wahl. Three-dimensional tomographic imaging for foliage penetration using multiple-pass spotlight-mode SAR. In Conference Record of Thirty-Fifth Asilomar Conference on Signals, Systems and Computers (Cat.No.01CH37256), volume 1, pages 121-125 vol.1, November 2001. Keyword(s): Tomography, Radar imaging, History, Synthetic aperture radar, Focusing, Laboratories, Tree graphs, Graphics, Fourier transforms, Layout.
    Abstract: In this paper we demonstrate how spotlight-mode synthetic aperture radar (SAR) imagery can be collected and processed using tomographic techniques to produce three-dimensional images. The technique is particularly useful for the application of foliage penetration (FOPEN), wherein it is desired to separate the radar returns corresponding to tree leaves and branches from the objects of interest that may lie beneath them. We present a mathematical framework for spotlight-mode SAR three-dimensional tomographic imaging and show with real Ku-band SAR data collections the effectiveness of the technique for "seeing" beneath trees.
    [bibtex-key = Jakowatz2001] [bibtex-entry]


  1515. Roger R.-Y. Lee, James S. Verdi, and Mehrdad Soumekh. Enhancements of NP-3 UHF Image Quality Using Digital Spotlighting Technique. In Proceedings of the 2001 IEEE Radar Conference, pages 1-6, May 2001. Keyword(s): SAR Processing, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, RFI Suppression, Subaperture Processing, Digital Spotlighting, Slow-Time Upsampling, Alias-free Processing, Quadband SAR, P-Band, X-Band, L-Band, C-Band, Airborne SAR.
    Abstract: This paper is concerned with signal processing issues that are associated with foliage penetrating (FOPEN) USA Navy NP-3 ultra-wideband (UWB) synthetic aperture radar. The digital signal processors that were developed for the NP-3 data commonly used a radar beamwidth angle that was limited to 35 degrees. Provided that the NP-3 radar beamwidth angle was 35 degrees, the NP-3 SAR system would approximately yield alias-free data in the slow-time Doppler domain. We show that the NP-3 data possess a 50-degree beamwidth angle within the entire 215-730 MHz band of the NP-3 radar that is imposed by the radar (radial) range swath gate. The 50-degree beamwidth of the NP-3 system results in slow-time Doppler aliasing within the frequency band of 444-730 MHz. We outline a slow-time processing of the NP-3 data, that we refer to as digital spotlighting and PRF upsampling, to minimize the Doppler aliasing. The digital spotlighting is also used for in-scene target calibration
    [bibtex-key = LeeVerdiSoumekh01:Spotlighting] [bibtex-entry]


  1516. F. Lombardini, F. Gini, and P. Matteucci. Application of array processing techniques to multibaseline InSAR for layover solution. In Radar Conference, 2001. Proceedings of the 2001 IEEE, pages 210-215, May 2001. [bibtex-key = Lombardini2001] [bibtex-entry]


  1517. F. Lombardini, F. Gini, and P. Matteucci. Multibaseline ATI-SAR for robust ocean surface velocity estimation in presence of bimodal Doppler spectrum. In Geoscience and Remote Sensing Symposium, 2001. IGARSS '01. IEEE 2001 International, volume 1, pages 578-580, July 2001. [bibtex-key = Lombardini2001a] [bibtex-entry]


  1518. Jordi J. Mallorqui, I. Rosado, and M. Bara. Interferometric calibration for DEM enhancing and system characterization in single pass SAR interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 404-406, July 2001. [bibtex-key = Mallorqui2001] [bibtex-entry]


  1519. Seung-Mok Oh and James H. McClellan. Multiresolution Imaging with Quadtree Backprojection. In The Record of the Thirty-Fifth Asilomar Conference on Signals, Systems and Computers, 2001, volume 1, pages 105-109, November 2001. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, Quadtree Processing, Divide and Conquer Methods.
    Abstract: The quadtree backprojection is an efficient space-time synthetic aperture radar (SAR) imaging algorithm that is based on the spectral decomposition of SAR data. Normally the quadtree backprojection is represented as a multiple stage imaging process that performs the space-time domain imaging over a number of sub-patches separately at each stage. By representing the contents of each sub-patch with a predefined energy function, it is possible to form a sequence of multi-resolution images. In this paper, we discuss various applications where this quadtree imaging can be applied to provide the multiresolution imaging. These include SAR, tomographic medical imaging and beamforming.
    [bibtex-key = SeungMcClellan01:Backproj] [bibtex-entry]


  1520. Tim Payne. Phase analysis for the limitations of the tomographic paradigm on a 3D scene. In Geoscience and Remote Sensing Symposium, 2001. IGARSS '01. IEEE 2001 International, volume 7, pages 3030-3032, 2001. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar theory, remote sensing by radar, synthetic aperture radar, terrain mapping, 3D Fourier transform, 3D scene, SAR, complex surface reflectivity, demodulated pulses, geophysical measurement technique, land surface, phase analysis, phase errors, spotlight mode, three dimensional scene.
    Abstract: The tomographic paradigm argues that the demodulated pulses from a spotlight mode SAR system trace a 2D slice of the 3D Fourier transform of the complex surface reflectivity. This paper derives the phase errors that result from imaging a 3D surface from a non planar collection geometry and shows how correct projection to the true surface can eliminate many of the errors. The response from an ideal scatterer is derived and then approximated to simplify the expression into a manageable and meaningful form and so that insight can be gained into the artifacts produced. The theory indicates that warping an image by distorting the final image to correct for layover doesn't eliminate the second order blurring terms produced by the relief and that both the layover and these blurring affects can be properly eliminated through correct projection to the real ground plane
    [bibtex-key = payne01:Tomo] [bibtex-entry]


  1521. Mats I. Pettersson. Moving Target Detection in Wide Band SAR. In CIE International Conference on Radar, 2001, pages 614-618, October 2001. Keyword(s): SAR Processing, Back-Projection, Fast Back-Projection, Time-Domain Back-Projection, TDBP, Moving Target Indication, Ultra-Wideband SAR.
    Abstract: A likelihood ratio test is proposed for moving target detection in a ultra wide frequency band and wide antenna beam (wide band) SAR systems. The developed method combines time domain fast backprojection SAR processing methods with moving target detection. It saves the computational load when all relative speeds can be tested using the same clutter suppressed sub-aperture beams. The proposed method is tested on narrow band radar data
    [bibtex-key = Pettersson01:BackprojTarget] [bibtex-entry]


  1522. M. Preiss, D. Gray, and N.J.S. Stacy. The effect of polar format resampling on uncompensated motion phase errors and the phase gradient autofocus algorithm. In Geoscience and Remote Sensing Symposium, 2001. IGARSS '01. IEEE 2001 International, volume 3, pages 1442-1444, July 2001. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Preiss2001] [bibtex-entry]


  1523. Rolf Scheiber and P. Robert. Origin and correction of phase errors in airborne repeat-pass SAR interferometry. In IEEE Int. Geosci. Remote Sens. Symp., volume 7, pages 3114-3116, Jul. 2001. Keyword(s): SAR Processing, SAR Interferometry, Phase Errors, Airborne SAR, E-SAR, DLR, geophysical techniques, terrain mapping, airborne radar, remote sensing by radar, synthetic aperture radar, motion error, geophysical measurement technique, land surface, terrain mapping, radar remote sensing, phase error, airborne radar, repeat pass method, SAR interferometry, InSAR, synthetic aperture radar, residual motion error, L-band, UHF, quantitative analysis, Error correction, Radar tracking, Radio interferometry, Space technology, Radio frequency, L-band, Aircraft, Global Positioning System, Geometry.
    Abstract: Airborne SAR surveys are often subject to severe motion errors. As the different tracks of a repeat-pass interferometric data set are acquired successively, any residual motion error has strong influences on the accuracy of the derived interferometric phase. In this paper we analyze the different error sources and present suitable compensation methods. Interferometric data acquired in L-band by the DLR's E-SAR system are used for quantitative analysis and for the demonstration of the proposed correction methods.
    [bibtex-key = scheiberRobertIGARSS2001PhaseErrorsInAirborneRepeatPassSARInterferometry] [bibtex-entry]


  1524. Lars M. H. Ulander, Per-Olov Frölind, A. Gustavsson, H. Hellsten, T. Jonsson, B. Larsson, and G. Stenstrom. Performance of the CARABAS-II VHF-Band Synthetic Aperture Radar. In IGARSS '01, International Geoscience and Remote Sensing Symposium, volume 1, pages 129 - 131, Jul. 2001. Keyword(s): SAR Processing, RFI Suppression, Back-Projection, Ultra-Wideband SAR, Time-Domain Back-Projection, TDBP, VHF SAR, CARABAS, Airborne SAR.
    Abstract: CARABAS-II is an airborne SAR operating in the 20-90 MHz band. The low operating frequency enables detection of concealed objects in dense forests as well as mapping of forest stem volume. A number of calibration experiments have recently been conducted to evaluate system performance. In this paper, we report on some of the results from the analysis. Spatial resolution, measured using 5-m trihedrals, is typically 2.5 m in both slant range and azimuth. The right-left ambiguity ratio, measured using 5-m trihedrals on both sides of the flight track, is about 10 dB for a single antenna element on receive. The noise level varies in the images and includes both multiplicative (integrated sidelobe ratio, right-left ambiguity ratio) and additive (radio-frequency interference, receiver noise) terms. Analysis of images from a recent campaign in northern Sweden shows that the additive noise term is less than -20 dB (noise-equivalent beta ?) for slant ranges less than 14 km.
    [bibtex-key = UlaFroGustHelJonLarsSten01:CARABAS] [bibtex-entry]


  1525. Lars M. H. Ulander, Hans Hellsten, and Gunnar Stenström. Performance analysis of fast backprojection for synthetic-aperture radar processing. In Edmund G. Zelnio, editor, Proc. of SPIE Vol. 4382, Algorithms for Synthetic Aperture Radar Imagery VIII, number 1, pages 13-21, 2001. SPIE. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, Fast Back-Projection, inversion.
    Abstract: Exact SAR inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, backprojection in time domain may be used which can also handle general curved apertures. In the past, however, backprojection has seldom been used due to its heavy computational burden. We show in the paper that the backprojection method can be formulated as an exact recursive method based on factorization of the aperture. By sampling the backprojected data in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms.
    [bibtex-key = UlanderHellstenStenstroem2001] [bibtex-entry]


  1526. M. Bara, A. Broquetas, and J. Closa. Precise geometry simulation of interferometric SAR signal for air and spaceborne sensors. In Geoscience and Remote Sensing Symposium, 2000. Proceedings. IGARSS 2000. IEEE 2000 International, volume 2, pages 746-748, July 2000. [bibtex-key = Bara2000] [bibtex-entry]


  1527. T. Ekman, A. Jakobsson, and Petre Stoica. On the efficient implementation of the Capon spectral estimator. In 2000 10th European Signal Processing Conference, pages 1-4, Sep. 2000. Keyword(s): Capon, Spectral estimation, beamforming, Beamforming, Amplitude and phase preserving Capon, Covariance matrices, Frequency estimation, Estimation, Signal resolution, Signal to noise ratio, Vectors, Spectral analysis.
    Abstract: We present an efficient implementation of the Amplitude Spectrum Capon (ASC) estimator. The implementation is based on the FFT and an efficient computation of the Cholesky-factor for the inverse covariance matrix. The Cholesky-factor is obtained from the linear prediction coefficients as computed by the modified covariance method. The implementation is significantly simpler than previous implementations, and it will yield spectral estimates of a similar quality as these. A short discussion on the differences between different Capon estimators is also included.
    [bibtex-key = ekmanJakobssonStoica2000EfficientImplementationOfCaponSpectralEstimator] [bibtex-entry]


  1528. D. G. Falconer. Radar imaging using statistical orthogonality. In E. G. Zelnio, editor, Proc. SPIE Vol. 4053, p. 10-19, Algorithms for Synthetic Aperture Radar Imagery VII, Edmund G. Zelnio; Ed., volume 4053 of Presented at the Society of Photo-Optical Instrumentation Engineers (SPIE) Conference, pages 10-19, August 2000. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR. [bibtex-key = falconer2000:NonLinearSAR] [bibtex-entry]


  1529. Guoyongmei, Chenhao, Hongwen, and Maoshiyi. Resample in the first order motion compensation of real-time SAR processor. In 5th International Conference on Signal Processing Proceedings, 2000. WCCC-ICSP 2000, volume 3, pages 1830-1833, 2000. Keyword(s): SAR Processsing, Airborne SAR, Motion Compensation, MoComp, airborne radar, error compensation, image sampling, motion compensation, radar imaging, real-time systems, synthetic aperture radarSAR image, azimuth focus, first order motion compensation, image defocus, image distortion, motion error model, phase errors, real-time SAR processor, resample problem, signal amplitude, synthetic aperture radar.
    Abstract: The resample problem in the first order motion compensation of real-time SAR processor is presented. The motion error model is properly established, and the simulation shows that the formation of the motion error with liner or second phase term gives rise to the image defocus and distortion. The magnitude of motion error poses changes in the range gate drift. When the motion error exceeds a range bin during synthetic time, resample should be taken into consideration; the resample inaccuracy within one bin does not have impact on azimuth focus, but the signal amplitude, so resample is necessary and important in motion compensation
    [bibtex-key = guoyongmeiChenhaoHongwenMaoshiyi2000:MoComp] [bibtex-entry]


  1530. Leif J. Harcke, Howard A. Zebker, R.F. Jurgens, and M.A. Slade. Delay-Doppler radar imaging of ``overspread'' planets. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 284-286, 2000. Keyword(s): delay-Doppler radar imaging, high resolution imaging, measurement technique, overspread planet, planetary radar astronomy, processing method, radar astronomy, radar signal processing, radioastronomy, rapidly rotating object, signal design, Doppler radar, asteroids, planetary surfaces, planets, radar applications, radar imaging, radar signal processing, radioastronomical techniques.
    Abstract: The authors review the current state of signal design and processing methods for planetary radar astronomy. Extensions to these techniques required for high resolution imaging of rapidly rotating solar system objects are discussed
    [bibtex-key = harckeZebkerJurgensSlade2000] [bibtex-entry]


  1531. Lance M. Kaplan, Seung-Mok Oh, Matthew C. Cobb, and James H. McClellan. Error Analysis for Quadtree Image Formation. In International Conference on Image Processing, ICIP 2000, volume 1, pages 717-720, September 2000. Keyword(s): SAR Processing, Back-Projection, Quadtree Processing, Error Analysis, Ultra-Wideband SAR.
    Abstract: The quadtree image formation technique is a computationally efficient approximation to standard backprojection. Where the computational load of backprojection is O(N^3) for N sensors forming an N?N image, the quadtree method uses a divide-and-conquer strategy similar to the fast Fourier transform (FFT) to reduce the computational load down to O(N^2*log(N)). However, the quadtree introduces errors in the relative time shifts used to focus pulses. These errors reduce the signal gain in the mainlobe response for isotropic point-like targets. In addition, the oscillations of the sidelobes increase from stage to stage. This paper develops performance bounds for the mainlobe losses under far field conditions and relates these bounds to the slow-time Nyquist rate
    [bibtex-key = kaplanEtAllErrorAnaly00:Backproj] [bibtex-entry]


  1532. Lance M. Kaplan, Seung-Mok Oh, and James H. McClellan. Detection of Broadside Targets During Image Formation Using a Quadtree Approach. In The Record of the 2000 IEEE Radar Conference, pages 104-109, May 2000. Keyword(s): SAR Processing, Back-Projection, Quadtree Processing, Ultra-Wideband SAR, Boom-SAR, Multiscale Detection.
    Abstract: The military is interested in using ultra-wideband (UWB) synthetic aperture radar (SAR) systems to detect ground targets. Standard automatic target detection methods search the entire scene for regions of interest (ROI) after image formation. In order to save computations, we introduce a multiscale detection algorithm that uses partially processed radar data during the intermediate stages of a quadtree-based backprojection image formation algorithm. When the detector accrues enough information to determine that a patch of ground is free of potential targets, it then cues the image former to terminate the processing that would further resolve that patch. The detector combines a feature that estimates the coherent signal to noise ratio with another feature that exploits the broadside flash scattering phenomenon. The new approach is evaluated over a measured database generated by the ARL Boom-SAR radar.
    [bibtex-key = kaplanEtAllDet00:Backproj] [bibtex-entry]


  1533. Yunjin Kim and Jakob van Zyl. Overview of Polarimetric Interferometry. In Aerospace Conference Proceedings, 2000 IEEE, volume 3, pages 231-236, 2000. Keyword(s): SAR Processing, radar polarimetry, radiowave interferometry, synthetic aperture radar, SAR interferometry, InSAR, SAR polarimetric interferometry, SAR polarimetry, Pol-InSAR.
    Abstract: SAR (Synthetic Aperture Radar) interferometry has enabled twoimportant science applications: surface change detection and topographicmapping. SAR interferometry is sensitive to the location of the imagedarea and the scattering geometry. SAR polarimetry makes use of thepolarization dependent scattering response of each pixel within theimaged area. The polarimetric response is highly sensitive to thescattering mechanism of a pixel. S.R. Cloude and K.P. Papathanassioufirst published the formulation of polarimetric interferometry thatcombines both SAR interferometry and SAR polarimetry. The main purposeof using polarimetric interferometry is to extract scattering mediuminformation that may be difficult to obtain from scalar interferometry.Even though the formulation and initial demonstrations appear to be verypromising, potential applications of polarimetric interferometry canonly be verified by comparing polarimetric interferometry signatureswith ground truth data. In this talk, we present the theory andimplementation of SAR polarimetric interferometry. Especially, we reviewSAR polarimetry, SAR interferometry, and SAR polarimetric interferometryin a unified manner. In addition, a new calibration technique suitablefor polarimetric interferometry is suggested in this paper
    [bibtex-key = vanZylKim00:Polarimetry] [bibtex-entry]


  1534. J. C. Kirk, R. Lefevre, R. Van Daalen Wetters, D. Woods, and B. Sullivan. Signal based motion compensation (SBMC). In Record of the IEEE 2000 International Radar Conference, pages 463-468, May 2000. Keyword(s): SAR Processing, Autofocus, motion compensation, radar theory, radar signal processing, aircraft navigation, jamming, air traffic control, signal based motion compensation, SAR, down range compensation, cross range compensation, radar signal, platform motion, aircraft navigation, autofocus performance, navigation system performance degradation, component failure, GPS jamming, Motion compensation, Costs, Robustness, Motion measurement, Radar measurements, Airborne radar, Aircraft navigation, System performance, Degradation, Global Positioning System.
    Abstract: Signal based motion compensation (SBMC) is an enabling technology for low cost SAR applications as well as an enhancement for more robust SAR applications. SBMC involves down range and cross range compensation for platform motion with motion compensation signals derived entirely from radar signal based measurements. It is an alternative to motion measurement sensor (MMS)-based motion compensation and a next step growth from autofocus. A very robust SBMC algorithm has been developed and demonstrated on simulated data and real radar data provided by actual radars. The algorithm is tuned to a particular radar and adapted to any level of aircraft navigation system. This SBMC approach will enable a low cost SAR capability to be provided for budget minded applications. Alternately, it can also be applied to high performance SAR systems for improved capability. It will extend current MMS motion compensation and autofocus performance and provide a backup capability to compensate for navigation system performance degradation, such as from component failure or GPS jamming.
    [bibtex-key = kirkLefevreVanDaalenWettersWoodsSullivanIEEERadarConf2000SignalBasedMotionCompensation] [bibtex-entry]


  1535. J.M. Lopez-Sanchez, J. Fortuny, A.J. Sieber, L. Sagues, M. Bara, X. Fabregas, and A. Broquetas. Experimental comparison of different scattering mechanism selections for vegetation height retrieval by POLINT. In Geoscience and Remote Sensing Symposium, 2000. Proceedings. IGARSS 2000. IEEE 2000 International, volume 1, pages 138-140, July 2000. [bibtex-key = Lopez-Sanchez2000] [bibtex-entry]


  1536. Jordi J. Mallorqui, M. Bara, and A. Broquetas. Sensitivity equations and calibration requirements on airborne interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 6, pages 2739-2741, July 2000. [bibtex-key = Mallorqui2000] [bibtex-entry]


  1537. David Small, Stefan Biegger, and Daniel Nüesch. Automated tie point retrieval through heteromorphic image simulation for spaceborne SAR sensors. In Proc. ESA ERS-ENVISAT Symp., Gothenburg, Sweden, 2000. Keyword(s): SAR Processing, Radiometric Calibration, Automated Tiepoint Retrieval. [bibtex-key = smallBieggerNuesch2000:AutomaticTiePointRetrieval] [bibtex-entry]


  1538. David Small, Stefan Biegger, and Daniel Nüesch. The Topology of SAR Imagery in Rough Terrain. In Proc. of EUSAR 2000 - 3rd European Conference on Synthetic Aperture Radar, 2000. Keyword(s): SAR Processing, SAR, Calibration, Terrain, DEM, Image Simulation, Terrain-Geocoding, Topology, Heteromorphism. [bibtex-key = smallBieggerNueschEUSAR2000:ImgRoughTerrain] [bibtex-entry]


  1539. Mehrdad Soumekh, Gernot Gunther, Mark Linderman, and Ralph Kohler. Digitally-Spotlighted Subaperture SAR Image Formation Using High Performance Computing. In Edmund G. Zelnio, editor, Algorithms for Synthetic Aperture Radar Imagery VII, volume SPIE 4053, pages 260-271, 2000. Keyword(s): SAR Processing, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, RFI Suppression, Subaperture Processing, Parallel Processing, Digital Spotlighting, Slow-Time Upsampling, Alias-free Processing, Real-Time Processing, High Performance Computing, FFTW.
    Abstract: This paper is concerned with the implementation of the SAR wavefront reconstruction algorithm on a high performance computer. For this purpose, the imaging algorithm is reformulated as a coherent processing (spectral combination) of images that are formed from a set of subapertures of the available synthetic aperture. This is achieved in conjunction with extracting the signature of a specific target region (digital spotlighting). Issues that are associated with implementing the algorithm on SMP-HPCs and DMP-HPCs are discussed. The results using the FOPEN P-3 SAR data are provided.
    [bibtex-key = SoumekhGuntherLindermanKohler00:Subaperture] [bibtex-entry]


  1540. Mehrdad Soumekh, Steve Worrell, Edward G. Zelnio, and Brett Keaffaber. SAR Wavefront Reconstruction Using Motion Compensated Phase History (Polar Format) Data and DPCA-Based GMTI. In Edmund G. Zelnio, editor, Algorithms for Synthetic Aperture Radar Imagery VII, volume SPIE 4053, pages 64-75, 2000. Keyword(s): SAR Processing, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, Polar Format Algorithm, X-Band, Squinted SAR, Motion Compensation, Monopulse SAR, MTI, GMTI.
    Abstract: This paper address the problem of processing an X-band SAR database that was originally intended for processing via a polar format imaging algorithm. In our approach, we use the approximation-free SAR wavefront reconstruction. For this, the measured and motion compensated phase history (polar format) data are processed in a multi-dimensional digital signal processing algorithm that yields alias-free slow-time samples. The resultant database is used for wavefront image formation. The X-band SAR system also provides a two channel along-track monopulse database. The alias-free monopulse SAR data are used in a coherent signal subspace algorithm for Ground Moving Target Indication (GMTI). Results are provided.
    [bibtex-key = SoumekhWorrellZelnioKeaffaber00:Wavefront] [bibtex-entry]


  1541. David A. Yocky and Charles V. Jakowatz. Two-target height effects on interferometric synthetic aperture radar coherence. In Edmund G. Zelnio, editor, , volume 4053, pages 102-108, 2000. SPIE. Keyword(s): SAR Processing, SAR Tomography, InSAR, SAR Interferometry, Polar Format Algorithm, PFA, Spotlight SAR, Spotlight-mode data. [bibtex-key = yockyJakowatzTwoTargetsInSAR2000] [bibtex-entry]


  1542. Alain Arnaud. Ship detection by SAR interferometry. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 5, pages 2616-2618 vol.5, June 1999. Keyword(s): ships, radar detection, synthetic aperture radar, radar imaging, marine radar, naval radar, ship detection, SAR interferometry, InSAR, small ship, agitated sea surface, low incidence angle, method, coherence, phase, phase coherence, Marine vehicles, Interferometry, Synthetic aperture radar, Satellites, Electromagnetic scattering, Radar detection, Sea surface, Backscatter, Azimuth, Phase detection. [bibtex-key = arnaudIGARSS1999ShipDetectionBySARInterferometry] [bibtex-entry]


  1543. Güner Arslan, Magesh Valliappan, and Brian L. Evans. Quality Assessment of Compression Techniques for Synthetic Aperture Radar Images. In International Conference on Image Processing, ICIP 1999, volume 3, pages 857-861, October 1999. Keyword(s): Data Compression, Quality Assessment, Quality Measures, Edge Correlation Quality Measure.
    Abstract: Synthetic aperture radar (SAR) systems are mounted on airplanes and satellites, which have limited downlink and storage capacity, yet SAR image sequences may be produced at rates of several Gbps. Compression is difficult because SAR images contain significant high-frequency information, such as terrain boundaries and terrain texture. In assessing the quality of compressed images, peak signal-to-noise ratio and men-squared error are inadequate because they assume that distortion is solely due to image-independent additive noise. In this paper, we provide objective measures to assess the visual quality of SAR images compressed by JPEG and SPIHT coders. The human visual system responds differently to linear distortion and noise injection (nonlinear distortion plus additive noise). Our key contributions are that we first decouple and quantify the linear distortion and noise injection in JPEG and SPIHT coders, and second introduce a new edge correlation quality measure which we use to quantify nonlinear distortion
    [bibtex-key = ArslVallEvans99:Quali] [bibtex-entry]


  1544. M. Bara, J. Monne, and A. Broquetas. Navigation systems requirements for airborne interferometric SAR platforms. In Geoscience and Remote Sensing Symposium, 1999. IGARSS '99 Proceedings. IEEE 1999 International, volume 4, pages 2158-2160, July 1999. [bibtex-key = Bara1999] [bibtex-entry]


  1545. M. Bara, O. Mora, M. Romero, and A. Broquetas. Generation of precise wide-area geocoded elevation models with ERS SAR data. In Geoscience and Remote Sensing Symposium, 1999. IGARSS '99 Proceedings. IEEE 1999 International, volume 4, pages 1924-1926, July 1999. [bibtex-key = Bara1999a] [bibtex-entry]


  1546. Svante Björklund and David Rejdemyhr. A MATLAB Toolbox for Radar Array Processing. In ISSPA '99, International Symposium on Signal Processing and its Applications, pages 547-550, 1999. Keyword(s): Radar Array Processing, MATLAB Toolbox for Radar Array Processing.
    Abstract: This paper describes the design, implemented possibilities and usage of a MATLAB Toolbox for radar signal processing. The Toolbox is especially suited for processing in the spatial dimension using signals from an antenna array. Both simulated and measured signals can be used. Both conventional processing, e.g. conventional beamforming, and model based processing is possible.
    [bibtex-key = Bjorklund:MatlabToolbox] [bibtex-entry]


  1547. G. Connan, H. D. Griffiths, P. V. Brennan, and R. Garello. W-band radar measurements of laboratory water waves. In OCEANS '99 MTS/IEEE. Riding the Crest into the 21st Century, volume 3, pages 1333-1337 vol.3, 1999. Keyword(s): SAR Processing, W-Band, backscatter, ocean waves, oceanographic techniques, radar cross-sections, remote sensing by radar, synthetic aperture radar, 75 to 110 GHz, 94 GHz, EHF, FMCW radar, SAR, W-band, backscattering, internal wave, measurement technique, millimetre wave radar, ocean wave, radar remote sensing, radar scattering, synthetic aperture radar, wave-tank experiment, Chirp modulation, Frequency modulation, Laboratories, Radar antennas, Radar imaging, Radar measurements, Sea surface, Signal resolution, Surface waves, Synthetic aperture radar.
    Abstract: The paper presents results on millimetre wave radar backscattering from laboratory water waves. Firstly, in a linear wave-tank, the radar has sensed mechanically generated surface waves of varying frequency (1 to 4 Hz) and amplitude (0.5 to 10 cm). Finally, in a 13 m diameter wave-tank, scaled versions of particular internal wave phenomena have been set-up under mechanically generated surface waves, and the resulting wave-field has been sensed by the radar in SAR mode. Both series of experiments have been carried out at the Laboratoire des Ecoulements Geophysiques et Industriels, Grenoble, France, within the EC project Mesoscale Ocean Radar Signature Experiments. First, the role of the radar and its operating mode are briefly presented, and the experiments are described. Then, the paper focuses on the data analysis and draws some conclusions on the backscattering mechanisms
    [bibtex-key = connanGriffithsBrennanGarelloOCEANS1999FMCWSARWBAND] [bibtex-entry]


  1548. Ian G. Cumming, Frank Wong, and Bob Hawkins. RADARSAT-1 Doppler Centroid Estimation Using Phase-Based Estimators. In CEOS SAR Workshop 1999, 1999. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Multilook Cross Correlation, MLCC, Multilook Beat Frequency, MLBF, Clutterlock, Doppler Ambiguity Resolver, DAR, Satellite SAR.
    Abstract: Doppler Centroid (DOPCEN) estimation continues to be an important and sometimes overlooked component of SAR processing. This is especially true in the case of ScanSAR, where the estimate must be accurate to approximately 5 Hz in order to avoid radiometric artifacts in the processed images. In the last 10 years, a new class of estimator has been developed based on the phase of the received signal, rather than on the spectral amplitude. The concepts were developed by Madsen, Bamler and Runge, and more recently by Wong and Cumming. It is generally acknowledged that the phase-based estimators can be more accurate than the amplitude-based estimators, provided their limitations are understood, and they are applied properly. We consider the DLR (Bamler & Runge), the MLCC and the MLBF (the latter two both Wong & Cumming). We show how their performance differs as a function of radiometric discontinuities, partially-exposed targets, noise levels, scene contrast and radar squint angle. The 3 algorithms provide different estimation accuracies with respect to each of these data attributes. RADARSAT data has tighter DOPCEN estimation requirements, because of ScanSAR operation, and because of its higher noise equivalent sigma naught. We have made improvements to the existing phase-based estimators and tested their performance on RADARSAT data. In the paper, we will review the operation of the DOPCEN algorithms, compare their performance, and explain why it is advantageous to use an algorithm which combines features of more than one of the 3 algorithms. Finally, we describe our recommendations for a reliable, combined algorithm.
    [bibtex-key = cum:DopCentrEst] [bibtex-entry]


  1549. Marina Dragosevic. On Accuracy of Attitude Estimation and Doppler Tracking. In CEOS SAR Workshop 1999, 1999. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Doppler Tracker, Attitude Angles, Doppler Ambiguity Resolver, DAR, Satellite SAR.
    Abstract: A precise physical model of the Doppler effects is based on the spacecraft state vectors, earth model and spacecraft attitude. Thus, the problem of Doppler tracking can be posed as the problem of adaptive estimation of the satellite attitude. The same physical model and general approach to Doppler/attitude tracking can be applied to ERS and RADARSAT. However, in the case of RADARSAT there are two additional problems: 1) Since RADARSAT is not zero-Doppler steered, attitude estimation must be combined with PRF (pulse repetition frequency) ambiguity resolution. An efficient and reliable method that achieves this will be presented and discussed. 2) For RADARSAT there is a beam peak dislocation in the azimuth direction and the amount of this dislocation depends on the elevation angle. This is especially significant for the wide beams and for ScanSAR due to the large elevation aperture of the combined beams. It is shown how this effect can be modeled as equivalent elevation-dependent yaw and pitch in addition to the ordinary (very small) RADARSAT attitude angles. It is also shown how these equivalent yaw and pitch values can be incorporated in the generic Doppler centroid model. This representation of the beam peak dislocation feature is very useful because it facilitates the verification of the existing (or new) characterizations of the RADARSAT beam peak dislocation. Ideally, when the elevation-dependent yaw and pitch is taken into account, all beams should provide the same value for the true attitude (via independent Doppler centroid measurements). Thus, a mismatch can be used to correct the dislocation model. Based on these geometric/kinematic models, a complete procedure for very accurate Doppler and attitude tracking for ERS, RADARSAT single beam and RADARSAT ScanSAR modes will be outlined. Results regarding theoretical estimation accuracy and observed variability will be compared. The paper will include comparison with other similar published methods and results. For ScanSAR, all beams are used simultaneously and the sensitivity of the attitude estimates to the proper beam dislocation modeling will be considered.
    [bibtex-key = dragosevic99:DopCentrEst] [bibtex-entry]


  1550. Gerard J. Genello(Jr.), Michael C. Wicks, and Mehrdad Soumekh. Alias-free Processing of P-3 Data. In Edmund G. Zelnio, editor, Algorithms for Synthetic Aperture Radar Imagery VI, volume SPIE 3721, pages 189-200, 1999. Keyword(s): SAR Processing, Back-Projection, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, RFI Suppression, Digital Spotlighting, Slow-Time Upsampling, Alias-free Processing, P-Band, Ultra-Wideband SAR, FOPEN, Airborne SAR.
    Abstract: This paper is concerned with multidimensional signal processing and image formation with FOliage PENetrating (FOPEN) airborne radar data which were collected by a Navy P-3 ultra wideband (UWB) radar in 1995 [Raw]. A commonly-used assumption for the processing of the P-3 data is that the beamwidth angle of the radar is limited to 35 degrees [Bes],[Goo]; provided that this assumption is valid, the PRF of the P-3 SAR system yields alias-free data in the slow-time Doppler domain. However, controlled measurements with the P-3 radar have indicated a beamwidth which exceeds 35 degrees [Raw]. In this paper, we examine a method for processing of the P-3 data in which the incorrect assumption that its radar beamwidth angle is limited to 35 degrees is not imposed. In this approach, a SAR processing scheme which enables the user to extract the SAR signature of a specific target area (digital spotlighting) is used to ensure that the resultant reconstructed SAR image is not aliased [S94], [S95], [S99]. The images which are formed via this method with 8192 pulses are shown to be superior in quality to the images which are formed via the conventional P-3 processor with 16386 pulses which was developed at the MIT Lincoln Laboratory [Bes]. In the presentation, we also introduce a method for converting the P-3 deramped data into its alias-free baseband echoed data; the signature of the Radio Frequency Interference (RFI) signals in the two-dimensional spectral domain of the resultant data is examined.
    [bibtex-key = GenelloWicksSoumekh99:Aliasfree] [bibtex-entry]


  1551. Rüdiger Gens. On Phase Unwrapping Based on Minimum Cost Flow Networks. In FRINGE '99, Advancing ERS SAR Interferometry from Applications towards Operations, 1999. Keyword(s): SAR Processing, Interferometry, Phase Unwrapping, Minimum Cost Flow.
    Abstract: Phase unwrapping is a key step in the SAR interferometric processing chain as it converts the phase information derived from an interferometric image pair into valuable height information. Many algorithms have been developed to solve the phase unwrapping problem. None of the algorithms implemented so far has met with all the requirements for an optimal solution. Recently, a very promising approach has been introduced by Costantini (1998). The new method formulates the phase unwrapping problem as a global minimisation problem which can be solved by using minimum cost flow (MCF) networks. These MCF networks in general have been well studied and efficient algorithms exist. However, application of the MCF for phase unwrapping is a new approach and requires further research. This paper deals with the investigation of this particular algorithm and focuses on the optimisation of the cost function used to define the MCF network.
    [bibtex-key = gens99:phaseUnWrap] [bibtex-entry]


  1552. Harry Jackson, Ian Sinclair, and Sebastian Tam. ENVISAT/ASAR Precision Transponders. In CEOS SAR Workshop 1999, Toulouse, Oct. 1999. Keyword(s): Transponders, ASAR, ASAR Transponders, ENVISAT, ENVISAT Transponders.
    Abstract: MPB Technologies Inc. is currently building the Radio Frequency (RF) and control units for a suite of three transponders for the European Space Agency. The transponders are instrumental in the external characterization of the Advanced Synthetic Aperture Radar (ASAR) mounted on board ENVISAT. The prototype transponder was designed and built at ESTEC [1]. This paper presents the operation of the transponders, and describes the final design, integration and testing of the production RF and control transponder units. Like the earlier ERS-1/2 and RADARSAT-1 transponders [2,3], these units provide a constant RCS mode, with constant gain radiometric response, and an azimuth pattern mode to allow for amplitude recording of the satellite pass. In the ASAR transponders, two further modes have been added - a characterization mode to assist in the commissioning and monitoring of ASAR, and an experimental phase-stable mode to provide point target echoes with stabilized phase response.
    [bibtex-key = JackSincTam99:ASARTransponders] [bibtex-entry]


  1553. Richard T. Lord and Michael R. Inggs. Efficient RFI Suppression in SAR Using a LMS Adaptive Filter with Sidelobe Suppression Integrated with the Range-Doppler Algorithm. In IGARSS '99, International Geoscience and Remote Sensing Symposium, volume 1, pages 574-576, June 1999. Keyword(s): SAR Processing, RFI Suppression, P-Band, Range-Doppler Algorithm.
    Abstract: The LMS adaptive filter has been used successfully to suppress radio frequency interference (RFI) from SAR images. This paper describes a method to efficiently implement this filter by integrating it with the range-Doppler algorithm. A technique to reduce the sidelobes created by the filter is described and illustrated on simulated data and on real P-band data
    [bibtex-key = LordInggs99:RFI] [bibtex-entry]


  1554. Andrea Monti-Guarnieri, Fabio Rocca, Pietro Guccione, and Ciro Cafforio. Optimal Interferometric ScanSAR Focusing. In IGARSS '99, International Geoscience and Remote Sensing Symposium, volume 3, pages 1718-1720, 1999. Keyword(s): SAR Processing, Interferometry, ScanSAR, Focusing.
    Abstract: This paper deals with phase preserving focusing for very low resolution ScanSAR. Conventional techniques get ScanSAR focusing by exploiting the SAR matched reference, and compensate scalloping by an inverse antenna weighting. Yet, this approach introduces a space-variant distortion in the focused impulse response (IRF). A rather different focusing technique is then proposed, where the set of space-variant focusing kernels is computed by means of Wiener deconvolution. They perform ScanSAR focusing and descalloping at one time, achieving the finest resolution and without distorting the impulse response.
    [bibtex-key = monti01:optInterfFocus] [bibtex-entry]


  1555. A. Potsis, A. Reigber, and K.P. Papathanassiou. A phase preserving method for RF interference suppression in P-band. In Geoscience and Remote Sensing Symposium, 1999. IGARSS '99 Proceedings. IEEE 1999 International, volume 5, pages 2655-2657, 1999. Keyword(s): SAR Processing, interference suppression, radar interference, radar theory, radiowave interferometry, synthetic aperture radar, ESAR, P-Band, Airborne SAR, P-band synthetic aperture radar interferometric data, RFI Suppression, Solothurn, Switzerland, interferometric SAR data applications, least mean square method, phase preserving method, phase preserving notch filter, polarimetric interferometric SAR data.
    Abstract: Addresses the least mean square method for estimation and coherent subtraction of the RF interference in interferometric SAR data applications. The authors also compare the results with a phase preserving notch filter. For this purposes the authors use polarimetric interferometric SAR data from a test site in Solothum/Switzerland collected by the DLR's Experimental SAR (ESAR)
    [bibtex-key = potsisReigberPapathanassiou99:RFI] [bibtex-entry]


  1556. Richard Rau and James H. McClellan. Data Efficient Implementation of UWBWA SAR Algorithms. In ICASSP '99, International Conference on Acoustics, Speech, and Signal Processing, volume 6, pages 3525-3528, 1999. Keyword(s): SAR Processing, Back-Projection, Ultra-Wideband SAR, Fan Filter, Quincunx Grid, TDBP, Time-Domain Back-Projection.
    Abstract: It is shown that the particular form of the frequency support of raw data and focused imagery obtained from an ultra-wideband, wide beamwidth synthetic aperture radar system can be exploited in nonseparable sampling schemes to reduce the overall amount of raw data samples and image pixels that need to be stored and computed. Furthermore, it is demonstrated that the constant integration angle backprojection (CIAB) image former implicitly applies a fan filter that interpolates raw data sampled on a quincunx grid back onto the underlying rectangular grid. This subtle property of the CIAB has not been exploited so far. It leads to higher quality images with less computational complexity.
    [bibtex-key = RauMcClellan99:] [bibtex-entry]


  1557. Z. She, D.A. Gray, R.E. Bogner, and J. Homer. Three-dimensional SAR imaging via multiple pass processing. In Geoscience and Remote Sensing Symposium, 1999. IGARSS '99 Proceedings. IEEE 1999 International, volume 5, pages 2389-2391, 1999. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, terrain mapping, topography (Earth), DFT, InSAR, SAR, SAR image, beamforming, eigenvector method, elevation, geophysical measurement technique, image registration, land surface topography, multiple pass processing, phase correction, radar remote sensing, subspace method, superresolution, three-dimensional imaging.
    Abstract: This paper develops a novel approach to reconstruct a three-dimensional (3D) SAR image with multiple pass processing. It involves image registration, phase correction and beamforming in elevation. An eigenvector method is proposed for the phase correction and the beamforming in elevation is carried out by a DFT or a subspace method for superresolution. 3D SAR images are demonstrated by processing ERS-1 real data with the proposed approach
    [bibtex-key = sheGrayBognerHomer99:Tomo] [bibtex-entry]


  1558. D.G. Thompson, J.S. Bates, and D.V. Arnold. Extending the phase gradient autofocus algorithm for low-altitude stripmap mode SAR. In The Record of the 1999 IEEE Radar Conference, pages 36-40, April 1999. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, Strip-map, Range-Dependent. [bibtex-key = thompsonBatesArnold1999a:PGA] [bibtex-entry]


  1559. D.G. Thompson, J.S. Bates, D.V. Arnold, and David G. Long. Extending the phase gradient autofocus algorithm for low-altitude stripmap mode SAR. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS '99, volume 1, pages 564-566, July 1999. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, Strip-map, Range-dependent. [bibtex-key = thompsonBatesArnold1999b:PGA] [bibtex-entry]


  1560. P. Tsakalides and C.L. Nikus. A new phase gradient autofocus technique for high resolution image formation based on fractional lower-order statistics. In Electronics, Circuits and Systems, 1999. Proceedings of ICECS '99. The 6th IEEE International Conference on, volume 2, pages 667-670, September 1999. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Tsakalides1999] [bibtex-entry]


  1561. Lars M. H. Ulander and Per-Olov Frölind. Precision Processing of CARABAS HF/VHF-Band SAR Data. In IGARSS '99, International Geoscience and Remote Sensing Symposium, volume 1, pages 47-49, June 1999. Keyword(s): SAR Processing, RFI Suppression, Back-Projection, Time-Domain Back-Projection, TDBP, Ultra-Wideband SAR, VHF SAR, CARABAS, Airborne SAR, Motion Compensation.
    Abstract: A stream-lined precision processor has been developed for the CARABAS-II HF/VHF-band SAR system. The authors describe the basic system characteristics, the normal waveform used, and the signal processing techniques to form images. In particular, challenges related to the stepped-frequency waveform, the radio-frequency interference environment, and widebeam motion-compensation are discussed and processing solutions are devised.
    [bibtex-key = UlanderForlind99:RFI] [bibtex-entry]


  1562. Urs Wegmuller. Automated terrain corrected SAR geocoding. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 1712-1714, 1999. Keyword(s): SAR, SAR intensity image, automated terrain corrected method, cross-correlation, digital elevation model, geocoding, geocoding transformation, geophysical measurement technique, image coding, land surface, radar imaging, radar remote sensing, synthetic aperture radar, terrain mapping, geophysical signal processing, geophysical techniques, image coding, radar imaging, remote sensing by radar, synthetic aperture radar, terrain mapping.
    Abstract: A technique for automated terrain corrected SAR geocoding is presented. Instead of operator selected control points the presented method uses cross-correlation with a SAR intensity image simulated from the digital elevation model to refine the geocoding transformation
    [bibtex-key = wegmuller1999:TerrainGeocoding] [bibtex-entry]


  1563. Ali F. Yegulalp. Fast Backprojection Algorithm for Synthetic Aperture Radar. In The Record of the 1999 IEEE Radar Conference, pages 60-65, 1999. Keyword(s): SAR Processing, Back-Projection, Convolution Back-Projection, FOPEN, Ultra-Wideband SAR, Image Formation, Focusing, Motion Compensation, Time-Domain Back-Projection, TDBP.
    Abstract: We introduce a new algorithm for time-domain backprojection of synthetic aperture radar (SAR) data. The algorithm reproduces images generated by standard backprojection pixel-for-pixel to any required tolerance, but it runs roughly ?N times faster for an N by N pixel image. Fast backprojection retains the advantages of standard backprojection: perfect motion compensation for any flight path, low artifact levels, unlimited scene size, perfect focus for arbitrarily wide bandwidths and integration angles, and strictly local processing (i.e., pulses can be processed as they are collected without along-track buffering or corner turns). The new algorithm also makes it possible to store the image in progress on disk (rather than in memory) with only a mild penalty in processing speed.
    [bibtex-key = yegulalp:fourier] [bibtex-entry]


  1564. P. Zavattero. Distributed target SAR image de-blurring using phase gradient autofocus. In Radar Conference, 1999. The Record of the 1999 IEEE, pages 246-249, April 1999. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus.
    Abstract: A new analysis of errors in blur function estimate formed by the real-time Phase Gradient Autofocus (PGA) algorithm is presented for synthetic aperture radar images of distributed targets in correlated noise clutter. It is shown that the PGA algorithm, like the Attia-Steinberg and Vachon-Raney focusing algorithms, can estimate a translation-invariant blur function when no point reflectors are present. The analysis shows that simulation evaluations of PGA performance which do not include sufficient simulated clutter can tend to underestimate the performance of the algorithm in initial iterations. Implications of the error analysis for performance optimization of real-time PGA implementations are presented for the algorithm steps that involve range bin selection, circular shifting, and windowing. It is shown that range bins selected for processing should be widely spaced if possible. If distributed targets are present which cause locally spatially correlated imagery, then it is desirable that the circular shifting segment of the algorithm maintain maximum decorrelation of the intermediate windowed and aligned images used for iterative phase error estimation.
    [bibtex-key = Zavattero1999] [bibtex-entry]


  1565. Stefan Buckreuss and Ralf Horn. E-SAR P-band SAR subsystem design and RF-interference suppression. In Geoscience and Remote Sensing Symposium Proceedings, 1998. IGARSS '98. 1998 IEEE International, volume 1, pages 466-468, 1998. Keyword(s): SAR Processing, airborne radar, interference suppression, microstrip antenna arrays, radar interference, radar polarimetry, synthetic aperture radar, 450 MHz, E-SAR P-band SAR subsystem design, Germany, Institut fur Hochfrequenztechnik, P-Band, P-band radar front-end, RFI Suppression, RF-interference suppression, Weilheim, carrier frequency, experimental airborne synthetic aperture radar, microstrip patch array antenna, polarimetric radar, signal bandwidth.
    Abstract: Since the beginning of 1994 the Institut f{\"u}r Hochfrequenztechnik of the German Aerospace Center (DLR) has been operating an experimental airborne SAR E-SAR with a P-band radar front-end. The microstrip patch array antenna allows a signal bandwidth of slightly more than 10% of the 450 MHz carrier frequency. The radar is polarimetric on a pulse-to-pulse basis.
    [bibtex-key = buckreussHorn98:RFI] [bibtex-entry]


  1566. N. Doren and D.E. Wahl. Implementation of SAR interferometric map generation using parallel processors. In IGARSS '98. Sensing and Managing the Environment. 1998 IEEE International Geoscience and Remote Sensing. Symposium Proceedings. (Cat. No.98CH36174), volume 5, pages 2640-2642 vol.5, July 1998. Keyword(s): Pixel, Layout, Geometry, Laboratories, Synthetic aperture radar interferometry, Phase estimation, Parallel processing, Reflectivity, Azimuth, Postal services.
    Abstract: Interferometric fringe maps are generated by accurately registering a pair of complex SAR images of the same scene imaged from two very similar geometries, and calculating the phase difference between the two images by averaging over a neighborhood of pixels at each spatial location. The phase difference (fringe) map resulting from this IFSAR operation is then unwrapped and used to calculate the height estimate of the imaged terrain. Although the method used to calculate interferometric fringe maps is well known, it is generally executed in a post-processing mode well after the image pairs have been collected. In that mode of operation, there is little concern about algorithm speed and the method is normally implemented on a single processor machine. This paper describes how the interferometric map generation is implemented on a distributed-memory parallel processing machine. This particular implementation is designed to operate on a 16 node Power-PC platform and to generate interferometric maps in near real-time. The implementation is able to accommodate large translational offsets, along with a slight amount of rotation which may exist between the interferometric pair of images. If the number of pixels in the IFSAR image is large enough, the implementation accomplishes nearly linear speed-up times with the addition of processors.
    [bibtex-key = Doren1998] [bibtex-entry]


  1567. Francesco Holecz, Paolo Pasquali, João Moreira, and Daniel Nüesch. Rigorous radiometric calibration of airborne AeS-1 InSAR data. In IGARSS '98, Geoscience and Remote Sensing Symposium, volume 5, pages 2442-2444, 1998. Keyword(s): SAR Processing, Radiometry, Radiometric Calibration, Airborne SAR, Topography, Interferometry, calibration, geophysical techniques, remote sensing by radar, synthetic aperture radar, 9.6 GHz, AeS-1, InSAR, X-band, airborne radar, elevation data, geophysical measurement technique, interferometric SAR, land surface, radar remote sensing, radiometric calibration, terrain mapping, topographic data.
    Abstract: The processing of synthetic aperture radar images usually does not
    [bibtex-key = HoleczPasqualiMoreiraNuesch98:RadiometricCalibrationAes1Data] [bibtex-entry]


  1568. Charles V. Jakowatz, Daniel E. Wahl, and Paul H. Eichel. Refocus of constant-velocity moving targets in synthetic aperture radar imagery. In Edmund G. Zelnio, editor, , volume 3370, pages 85-95, 1998. SPIE. Keyword(s): SAR Processing, Moving Target Indication, MTI. [bibtex-key = jakowatzWahlEichelRefocusMovingTargets] [bibtex-entry]


  1569. Bruno Juhel, Yannick Chevalier, Marc Le Goff, Emmanuel Legros, and Georges Vezzosi. Experimental Ultra Wide Band SAR Images of Canonical Targets. In IGARSS '98, International Geoscience and Remote Sensing Symposium, volume 2, pages 1153-1155, 1998. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, Ultra-Wideband SAR.
    Abstract: This paper describes one technique for ultra wide band (UWB) time domain radar signal processing. Usually a radar is a narrow band system, but in this case transmitted signals are nanosecond short pulses without carrier which have a spectral content from 100 MHz to 1 GHz. Time domain backprojection is used to focus SAR images. The algorithm is tested with experimental data measured with an UWB prototype radar. The configuration of this radar is described and some UWB SAR images of canonical targets are presented
    [bibtex-key = JuhelEtAl98:Backproj] [bibtex-entry]


  1570. Bruno Juhel, Georges Vezzosi, and Marc Le Goff. Radio Frequency Interferences Suppression for Noisy Ultra Wide Band SAR Measurements. In Ultra-Wideband Short-Pulse Electromagnetics 4, 1998, pages 387-393, June 1998. Keyword(s): SAR Processing, Back-Projection, Ultra-Wideband SAR, RFI Suppression.
    Abstract: A back projection algorithm dedicated to Ultra Wide Band (UWB) signals for synthetic aperture radar (SAR) imaging is described. The time domain aspect is all the more important that the transient nature of UWB signal supplies a vast amount of information on the electromagnetic scattering mechanisms. But, during UWB measurements, radio frequency interferences (RFI) can appear and obscure target detection. We have described how RFI can be an important noise source in UWB system and how they can be suppressed. The Least Mean Square (LMS) method for extracting RFI give good results, which are quantified with simulated data. Next, we would like to improve our sinusoidal model of the RFI to include a more complete description of the RFI signal and to apply the LMS algorithm to measured data
    [bibtex-key = JuhelVezzosiLeGoff98:Backproj] [bibtex-entry]


  1571. Kenneth Knaell. Progress in three-dimensional SAR from curvilinear apertures. In William J. Miceli, editor, , volume 3462, pages 110-121, 1998. SPIE. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, Curvilinear SAR. [bibtex-key = knaell:110] [bibtex-entry]


  1572. Riccardo Lanari, Scott Hensley, and Paul Rosen. Modified SPECAN algorithm for ScanSAR data processing. In IEEE International Geoscience and Remote Sensing Symposium, volume 2, pages 636-638, July 1998. Keyword(s): SAR Processing, Modified SPECAN, SPECAN, Spectral Analysis, geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radarSAR, ScanSAR, ScanSAR image, chirp z-transform, chirp-z, geophysical measurement technique, land surface, modified SPECAN algorithm, phase-preserving algorithm, radar remote sensing, range-dependent correction factor, synthetic aperture radar, terrain mapping.
    Abstract: The authors present a new phase-preserving algorithm for ScanSAR data processing that extends the SPECAN procedure. The proposed technique allows one to avoid the range dependent scaling of the azimuth pixel dimension, obtained by applying the standard SPECAN approach; this result is achieved by replacing the standard Fourier transform included in the SPECAN algorithm with a chirp z-transform, whose kernel includes a range-dependent correction factor. ScanSAR images generated via the proposed procedure have a constant azimuth pixel spacing whose dimension can be selected according to the application requirements
    [bibtex-key = lanariHensleyRosen1998Short:ModifiedSPECAN] [bibtex-entry]


  1573. Li Liwei, Asif Raza, and Mao Shiyi. Improvement of rank one phase estimation (ROPE) autofocusing technique. In Signal Processing Proceedings, 1998. ICSP '98. 1998 Fourth International Conference on, volume 2, pages 1461-1464, October 1998. [bibtex-key = Liwei1998] [bibtex-entry]


  1574. F. Lombardini, H.D. Griffiths, and F. Gini. A ML multichannel ATI-SAR technique for measuring ocean surface velocities. In OCEANS '98 Conference Proceedings, volume 2, pages 778-782, September 1998. [bibtex-key = Lombardini1998] [bibtex-entry]


  1575. F. Lombardini and P. Lombardo. A ML thinned array SAR interferometric sensor for high accuracy absolute phase retrieval. In Radar Conference, 1998. RADARCON 98. Proceedings of the 1998 IEEE, pages 263-268, May 1998. [bibtex-key = Lombardini1998a] [bibtex-entry]


  1576. Richard T. Lord and Michael R. Inggs. Approaches to RF interference suppression for VHF/UHF synthetic aperture radar. In Communications and Signal Processing, 1998. COMSIG '98. Proceedings of the 1998 South African Symposium on, pages 95-100, 1998. Keyword(s): SAR Processing, adaptive filters, adaptive signal processing, interference suppression, least mean squares methods, radar imaging, radar interference, synthetic aperture radar, LMS adaptive filter, P-Band, RF interference suppression, RFI Suppression, SAR imagery, VHF/UHF SAR applications, VHF/UHF synthetic aperture radar, coherent subtraction algorithms, filter approaches, image quality, interference power, least-mean-squared filter, radio frequency interference, receiver noise, spectral estimation.
    Abstract: An increasing amount of interest has developed in VHF/UHF SAR applications. Unfortunately the VHF-UHF portion of the spectrum is already in heavy use by other services, such as television and mobile communications. Even in remote locations the interference power often exceeds receiver noise by many dB, becoming the limiting factor on system sensitivity and severely degrading the image quality. This paper addresses the problem of radio frequency (RF) interference and its impact on SAR imagery. Several RF interference suppression methods are described and discussed. These include spectral estimation and coherent subtraction algorithms, as well as various filter approaches. The least-mean-squared (LMS) adaptive filter is described in detail, and its effectiveness in suppressing RF interference is demonstrated on simulated data and on real P-band data.
    [bibtex-key = lordInggs98:RFI] [bibtex-entry]


  1577. Carole E. Nahum. Autofocusing using multiscale local correlation. In Proc. SPIE, volume 3497, pages 21-30, 1998. Keyword(s): SAR Processing, Autofocus, SAR Autofocus, MoComp, Motion Compensation, Map-Drift Autofocus, airborne radar, radar imaging, synthetic aperture radar, SAR data, SAR images, X-band airborne SAR system, arbitrary residual phase error, local-quadratic map-drift autofocus, phase error estimation, quadratic errors, small data blocks, uncompensated phase errors, Antennas, Azimuth, Bandwidth, Doppler effect, Measurement uncertainty, Radar, Trajectory, autofocus, map-drift, motion compensation, motion errors, synthetic aperture radar.
    Abstract: Ground imaging using a Synthetic Aperture Radar (SAR) requires the knowledge of the antenna trajectory with a relative accuracy of a fraction of the wavelength upon an integration time of a few seconds. This information is not always available from inertial navigation unit especially in the airborne framework and must be recovered from the radar signal itself using the techniques known as autofocus. We describe in this paper an original method designed at ONERA for the registration of looks. We show how it applies to autofocusing. It improves the map-drift algorithm making it time variant thus allowing for the correction of low frequency trajectory errors, and not only velocity bias.
    [bibtex-key = nahumSPIE1998Autofocus] [bibtex-entry]


  1578. Stefan Nilsson and Lars-Erik Andersson. Application of Fast Backprojection Techniques for some Inverse Problems of Synthetic Aperture Radar. In Edmund G. Zelnio, editor, Algorithms for Synthetic Aperture Radar Imagery V, volume SPIE 3370, pages 62-72, 1998. Keyword(s): SAR Processing, Back-Projection, Fast Back-Projection, Quadtree Processing, Time-Domain Back-Projection, TDBP, Circular Averages, Wideband SAR, Focusing, Motion Compensation.
    Abstract: In certain radar imaging applications one encounters the problem of reconstructing a reflectivityfunction from information about its averages over circles with center on a straight line. A robust inversion method is afiltered backprojection method, similar to the one used in medical tomography. We will present a fast algorithm for this backprojection operator. Numerical examples are given.
    [bibtex-key = NilssonAndersson98:Backprojection] [bibtex-entry]


  1579. Richard Rau and James H. McClellan. A Directional Image Decomposition for Ultra-Wideband SAR. In ICASSP '98, International Conference on Acoustics, Speech, and Signal Processing, volume 5, pages 2877-2880, May 1998. Keyword(s): SAR Processing, Back-Projection, Ultra-Wideband SAR, Directional Filterbanks, TDBP, Time-Domain Back-Projection.
    Abstract: This paper presents a theoretical analysis of the structure of wide angle, ultra-wideband SAR images formed by a constant integration angle backprojection image former. It is shown that the effects of the image former can be modeled as a filtering operation on the original data. Furthermore, SAR images for different squint angles can be obtained from the original images by directional filtering. As a result, it is shown that perfect reconstructing directional filterbanks can be used as a unitary transform between SAR images and a 3-D representation containing additional aspect-angle information. It is demonstrated, how this new representation can be used to enhance targets.
    [bibtex-key = RauMcClellan98:Backproj] [bibtex-entry]


  1580. Andreas Reigber and Andreas Ulbricht. P-band repeat-pass interferometry with the DLR experimental SAR. In Geoscience and Remote Sensing Symposium Proceedings, 1998. IGARSS '98. 1998 IEEE International, volume 4, pages 1914-1916, 1998. Keyword(s): SAR Processing, airborne radar, geophysical techniques, remote sensing by radar, synthetic aperture radar, DLR experimental SAR, ESAR, P-Band, RF interference removal, RFI Suppression, SAR interferometry, Interferometry, UHF, geophysical measurement technique, ground parameters, land surface, radar remote sensing, repeat-pass interferometry, repeat-pass mode, surface topography, terrain mapping.
    Abstract: SAR interferometry is a powerful tool for the determination of surface topography and for estimation of ground parameters. In particular the analysis of different frequencies has the capability to provide a broad set of useful information. Therefore long wavelengths like the P-band of great interest, because they often show different interactions with scatterers than shorter wavelengths like the C-band. The authors address the implementation of a P-band repeat-pass mode for the DLR's experimental SAR (ESAR), including the problematic removal of RF-interferences in the data. Recent results on a test site in Solothurn/Switzerland are shown.
    [bibtex-key = reigberUlbricht98:InterferoRFI] [bibtex-entry]


  1581. Olle Seger, Magnus Herberthson, and Hans Hellsten. Real time SAR processing of low frequency ultra wide band radar data. In Proc. of EUSAR '98 - European Conference on Synthetic Aperture Radar, pages 489-492, May 1998. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, Local Back-Projection, Real Time Operation, Ultra-Wideband SAR, FOPEN, Image processing, Ground Penetrating Radar, Low-Frequency SAR, Image Reconstruction, Radar Resolution, Parallel Processing.
    Abstract: CARABAS (Coherent All RAdio BAnd Sensing) is a synthetic aperture radar with high relative bandwidth. Furthermore, CARABAS operates at radio band frequencies, which enables the radar to penetrate foliage. In this paper, we propose and analyze a reconstruction algorithm suitable for parallel implementation. The core idea of the method is a subdivision of the radar raw data into subapertures, from which coarse resolution subimages can be reconstructed. These subimages are then distributed to the nodes of a powerful parallel computer in order to achieve fine resolution at real-time rate.
    [bibtex-key = segerHerberthsonHellsten98:LocalBackprojection] [bibtex-entry]


  1582. David Small, Francesco Holecz, Erich Meier, and Daniel Nüesch. Absolute radiometric correction in rugged terrain: A plea for integrated radar brightness. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 1, pages 330-332, 1998. Keyword(s): geophysical signal processing, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, absolute radiometric correction, biomass estimation, geophysical measurement technique, integrated radar brightness, integrative faceted heteromorphic approach, land surface, multimode SAR imagery, multimode image comparison, radar remote sensing, radiometric normalisation, rugged terrain, terrain mapping, terrain-induced variation.
    Abstract: Rigorous intercomparison of multimode SAR imagery requires not
    [bibtex-key = SmallHoleczMeierNuesch98:RadiometricCorrectionIGARSS] [bibtex-entry]


  1583. David Small, Francesco Holecz, Erich Meier, and Daniel Nüesch. Radiometric Normalization for Multimode Image Comparison. In Proc. of EUSAR '98 - European Conference on Synthetic Aperture Radar, pages 191-194, 1998. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, Calibration, Radiometric Calibration, Radiometric Correction, Digital Elevation Model, Image Simulation, Terrain-Geocoding.
    Abstract: Intercomparison of backscatter collected by SAR sensors at heterogeneous radar look angles gives rise to highly variable ground areas being associated with each pixel location within a radar geometry (slant or ground range) image. Many elements within a digital elevation model (in map geometry) can be mapped to a single location in the radar image (range / Doppler coordinates). An image simulation technique uses a faceted high resolution elevation model to integrate all backscatter returned to each range and Doppler location in the radar image (incorporating knowledge of local radar shadow). Modelling the imaging process in this manner, a map of illuminated area is produced in radar geometry, and used to normalize the true backscatter returned by the radar sensor. Although radar shadow must be considered specially, no extraordinary treatment is required of layover regions, as they are correctly accounted for by integration of the illuminated area. The image simulation approach improves on the conventional consideration of 2D incidence angles, as the 3D configuration defining the illuminated area (in both the range and azimuth dimensions) is captured. RADARSAT images acquired over Switzerland are used to demonstrate the benefit of such normalization for thematic interpretation. A high resolution digital elevation model (DEM) with 25m pixel spacing is used as input to the image simulation. The deterioration of the normalization with progressively poorer input DEMs is studied empirically to gauge the required DEM resolution for acceptable normalization of images acquired over pre-alpine topography.
    [bibtex-key = smallHoleczMeierNueesch98:RadiometricNorm] [bibtex-entry]


  1584. Mehrdad Soumekh. Range Stacking: An Interpolation-free SAR Reconstruction Algorithm. In Edmund G. Zelnio, editor, Algorithms for Synthetic Aperture Radar Imagery V, volume SPIE 3370, pages 13-24, 1998. Keyword(s): SAR Processing, Range Stacking Algorithm, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, Back-Projection, Time-Domain Back-Projection, TDBP, Wideband SAR, Squinted SAR, FOPEN, Motion Compensation.
    Abstract: A method for digital image formation in Synthetic Aperture Radar (SAR) systems is presented. The proposed approach is based on the wavefront reconstruction theory for SAR imaging systems. However, this is achieved without image formation in the spatial frequency domain of the target function which requires interpolation. The proposed method forms the target function at individual range points within the radar range swath; this is referred to as range stacking. The range stacking reconstruction method is applicable in stripmap and spotlight (broadside and squint) SAR systems. Results using a wide-beamwidth FOliage PENetrating (FOPEN) SAR database are provided, and the effect of beamwidth filtering on the signature of moving targets in the imaging scene is shown.
    [bibtex-key = Soumekh98:] [bibtex-entry]


  1585. D.G. Thompson, J.S. Bates, D.V. Arnold, David G. Long, and A. Robertson. Range dependent phase gradient autofocus. In IEEE International Geoscience and Remote Sensing Symposium Proceedings, IGARSS '98, volume 5, pages 2634-2636, July 1998. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, PGA, Range-dependent.
    Abstract: The Phase Gradient Autofocus (PGA) algorithm has been widely used in Spotlight Synthetic Aperture Radar (SAR) to remove motion-induced blurs in the images. The PGA algorithm has been proven to be a superior autofocus method. PGA assumes a narrow beam, which is valid for most SAR systems. However, lower altitude SAR have large range dependencies that cannot be ignored. A new phase estimator for PGA is introduced and extended to allow range dependence. An ERS-1 image of Death Valley is used in simulations comparing the new estimator to the widely used maximum likelihood approach and in demonstrating the range-dependent PGA algorithm.
    [bibtex-key = thompsonBatesArnoldLong1998:PGA] [bibtex-entry]


  1586. F. Berizzi, G. Corsini, M. Diani, F. Lombardini, and G. Pinelli. Simulation model and performance analysis of a three-antenna InSAR system. In Radar 97 (Conf. Publ. No. 449), pages 119-123, October 1997. [bibtex-key = Berizzi1997] [bibtex-entry]


  1587. Tsz-King Chan, Y. Kuga, and A. Ishimaru. Feasibility study on localized subsurface imaging using circular synthetic aperture radar and angular correlation function measurement. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume 3, pages 1138-1140 vol.3, August 1997. Keyword(s): SAR Processing, W-Band, airborne radar, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, terrestrial electricity, 3D imaging, 7 to 13 GHz, 75 to 100 GHz, CSAR, EHF, SAR, SHF, W-band, X-band, airborne radar, angular correlation function, buried object detection, circular angular memory effect, circular flight path, circular synthetic aperture radar, confocally focused plane, down-looking spotlight mode, geoelectric method, geology, geophysical measurement technique, ground penetrating radar, localized subsurface imaging, radar imaging, radar remote sensing, subsurface structure, terrestrial electricity, High-resolution imaging, Image reconstruction, Image resolution, Laboratories, Pixel, Radar antennas, Radar applications, Radar imaging, Signal resolution, Signal to noise ratio.
    Abstract: The traditional linear synthetic aperture radar (LSAR) flying along a linear flight path is modified to circular synthetic aperture radar (CSAR) flying along a circular flight path. When operating in down-looking spotlight mode for imaging purposes, CSAR, in comparison with LSAR, can provide higher pixel resolutions on any confocally focused plane within the volume illuminated by the antenna beams. By combining this 3D imaging technique with the circular angular memory effect, it is possible to reconstruct volume radar images in localized subsurface imaging applications where poor signal-to-noise ratio (SNR) is a critical issue. Laboratory experiments at X-band (7-13 GHz) and W-band (75-100 GHz) frequencies were conducted to illustrate the capability of CSAR to perform 3D imaging, and the analytical basis of circular angular memory effect due to random surface scattering based on the first-order Kirchhoff approximation
    [bibtex-key = chanKugaIshimaruIGARSS1997WBandSAR] [bibtex-entry]


  1588. G. Connan, H. D. Griffiths, and P. V. Brennan. FMCW-SAR development for internal wave imaging. In OCEANS '97. MTS/IEEE Conference Proceedings, volume 1, pages 73-78 vol.1, October 1997. Keyword(s): SAR Processing, W-Band, CW radar, FM radar, electromagnetic wave scattering, oceanographic equipment, oceanographic techniques, radar cross-sections, radar equipment, radar imaging, radar polarimetry, remote sensing by radar, synthetic aperture radar, 94 GHz, EHF, FMCW radar, MORSE, Mesoscale Ocean Radar Signature Experiments, SAR, W-band, internal wave, measurement technique, millimetre wave radar, millimetric radar, mm wave, model, ocean dynamics, radar imaging, radar polarimetry, radar remote sensing, synthetic aperture imaging, synthetic aperture radar, ultra-high-resolution SAR, Azimuth, Hydrodynamics, Laboratories, Laser radar, Radar antennas, Radar imaging, Sea surface, Spaceborne radar, Surface topography, Surface waves.
    Abstract: The observation of internal waves in SAR images is not new but the imaging mechanisms involved are not fully understood. The Mesoscale Ocean Radar Signature Experiments (MORSE) project aims to provide models of these processes, both by theoretical and experimental means. As part of this work, a laboratory-based advanced ultra-high-resolution SAR is being developed to obtain images of the water surface in scaled experiments. The radar, which has a polarimetric capability, operates at a frequency of 94 GHz and uses the FMCW technique, with a range resolution of 5 cm and an azimuth resolution of 1 cm for an imaged area of 5x4 m2. This SAR is to be used in a set of experiments in LEGI, France, first in a flume, and then in a wave tank in which scaled versions of particular internal wave phenomena can be set-up under laboratory conditions. This paper will particularly focus on the development of this radar
    [bibtex-key = connanGriffithsBrennanOCEANS1997FMCWSARWBAND] [bibtex-entry]


  1589. G. Corsini, M. Diani, F. Lombardini, and G. Pinelli. Reduction of the phase-unwrapping drawbacks by the three-antenna interferometric SAR system. In Geoscience and Remote Sensing, 1997. IGARSS '97. 'Remote Sensing - A Scientific Vision for Sustainable Development'., 1997 IEEE International, volume 4, pages 1536-1538, August 1997. [bibtex-key = Corsini1997] [bibtex-entry]


  1590. N.E. Doren, C.V. Jakowatz, D.E. Wahl, and P.A. Thompson. General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering. In Proceedings of International Conference on Image Processing, volume 1, pages 861-864 vol.1, October 1997. Keyword(s): Layout, Fourier transforms, Image reconstruction, Radar imaging, Filters, Frequency, History, US Department of Energy, Reflectivity, Microwave theory and techniques.
    Abstract: We introduce a general formulation for wavefront curvature correction in spotlight-mode SAR images formed using the polar-formatting algorithm (PFA). This correction is achieved through the use of an efficient, image domain space-variant filter which is applied as a post-processing step to the PFA. Wavefront curvature defocus effects occur in certain SAR collection modes that include imaging at close range, using a low center frequency, and/or imaging very large scenes. Our formulation is general in that it corrects for wavefront curvature in broadside as well as squinted collection modes, with no computational penalty for correcting squint-mode images. Algorithms such as the range migration technique (also known as seismic migration), and an enhancement known as frequency domain replication, FReD, have been developed to accommodate these wavefront curvature effects. However, they exhibit no clear computational advantage over space-variant post-filtering in conjunction with polar formatting (PF2). This paper presents the basic concepts of the formulation, and provides computer results demonstrating the capabilities of space-variant post-filtering.
    [bibtex-key = Doren1997] [bibtex-entry]


  1591. C.V. Jakowatz, D.E. Wahl, and P.A. Thompson. Three-dimensional SAR imaging using cross-track coherent stereo collections. In Conference Record of the Thirty-First Asilomar Conference on Signals, Systems and Computers (Cat. No.97CB36136), volume 2, pages 1199-1203 vol.2, November 1997. Keyword(s): Radar polarimetry, Aircraft, Synthetic aperture radar, Lighting, Optical imaging, Optical films, Layout, Geometry, Laboratories, Speckle.
    Abstract: We describe a new method for creating three-dimensional images using pairs of synthetic aperture radar (SAR) images obtained from a unique collection geometry. This collection mode involves synthetic apertures that have a common center. In this sense the illumination directions for the two SAR images are the same, while the slant planes are at different spatial orientations. The slant plane orientations give rise to cross-range layover (fore-shortening) components in the two images that are of equal magnitude but opposite directions. This differential cross-range layover is therefore proportional to the elevation of a given target, which is completely analogous to the situation in stereo optical imaging, wherein two film planes (corresponding to the two slant planes) result in elevation-dependent parallax. Because the two SAR collections are coherent in this particular collection mode, the images have the same speckle patterns throughout. As a result the images may be placed into stereo correspondence via calculation of correlations between micro-patches of the complex image data. The resulting computed digital stereo elevation map can be quite accurate. Alternatively, an analog anaglyph can be displayed for 3-D viewing, avoiding the necessity of the stereo correspondence calculation.
    [bibtex-key = Jakowatz1997] [bibtex-entry]


  1592. Charles V. Jakowatz, Daniel E. Wahl, Paul A. Thompson, and Neall E. Doren. Space-variant filtering for correction of wavefront curvature effects in spotlight-mode SAR imagery formed via polar formatting. In Edmund G. Zelnio, editor, , volume 3070, pages 33-42, 1997. SPIE. Keyword(s): SAR Processing, Polar Format Algorithm, PFA, Range Migration Algorithm, RMA, omega-k, correction of wavefront curvature effects, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzWahlThompsonDorenSpotlightPolarFormat1997] [bibtex-entry]


  1593. Michael Y. Jin. High Quality Spotlight SAR Processing Algorithm Designed for LightSAR Mission. In IGARSS '97, International Geoscience and Remote Sensing Symposium, volume 29, pages 477-480, August 1997. Keyword(s): SAR Processing, Subband Images, LightSAR, SIR-C, Spotlight SAR.
    Abstract: A high quality spotlight SAR processing algorithm is presented. In this algorithm, subband images of the radar illuminated spot are processed using well known strip mode processing algorithms. A full resolution image is then formed by (1) merging the spectra of subband images into a full band spectrum, and (2) taking an inverse FFT. Advantages of this algorithm include (1) higher image quality, (2) higher processing throughput rate, and (3) lower S/W development cost. The image quality achieved by this algorithm is better than that previously achieved by the backprojection algorithm. The fine image quality is demonstrated by the SIR-C spotlight SAR images.
    [bibtex-key = Jin97:LightSAR] [bibtex-entry]


  1594. Chan Hian Lim and Yeo Tat Soon. Non-iterative spotlight SAR autofocusing using a modified phase-gradient approach. In Geoscience and Remote Sensing, 1997. IGARSS '97. 'Remote Sensing - A Scientific Vision for Sustainable Development'., 1997 IEEE International, volume 1, pages 484-486, August 1997. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Lim1997] [bibtex-entry]


  1595. P. Lombardo and F. Lombardini. Multi-baseline SAR interferometry for terrain slope adaptivity. In Radar Conference, 1997., IEEE National, pages 196-201, May 1997. [bibtex-key = Lombardo1997] [bibtex-entry]


  1596. S. Lawrence Marple, Jr.. Computing the discrete-time analytic signal via FFT. In Signals, Systems and Computers, 1997. Conference Record of the Thirty-First Asilomar Conference on, volume 2, pages 1322-1325, 1997. Keyword(s): Analytic Signal, discrete time systems, fast Fourier transforms, frequency-domain analysis, signal sampling, FFT, complex-valued decimated N/2-point discrete-time analytic signal, complex-valued interpolated NM-point discrete-time analyticsignal, complex-valued standard N-point discrete time analytic signal, discrete-time analytic signal, frequency-domain algorithms, real-valued N-point discrete-time signal, sample rate, transform end points, Hilbert Transform.
    Abstract: Starting with a real-valued N-point discrete-time signal,frequency-domain algorithms are provided for computing (1) thecomplex-valued standard N-point discrete time `analytic' signal of thesame sample rate, (2) the complex-valued decimated N/2-pointdiscrete-time analytic signal of half the original sample rate, and(3) the complex-valued interpolated NM-point discrete-time `analytic'signal of M times the original sample rate. Special adjustment oftransform end points is shown to generate proper discrete-time`analytic' signals
    [bibtex-key = marple97:AnalyticSignal] [bibtex-entry]


  1597. H. Rudolf, D. Tarchi, and A.J. Sieber. Combination of linear and circular SAR for 3-D features. In Geoscience and Remote Sensing, 1997. IGARSS '97. 'Remote Sensing - A Scientific Vision for Sustainable Development'., 1997 IEEE International, volume 4, pages 1551-1553, 1997. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar imaging, remote sensing by radar, synthetic aperture radar, InSAR, LISA, buried object detection, circular SAR, circular aperture, geophysical measurement technique, ground penetrating radar, interferometric SAR, land surface, linear SAR, linear SAR outdoor system, mine detection, radar remote sensing, terrain mapping, three dimensional feature, vegetation mapping.
    Abstract: An economic and fast change of an existing radar system with alinear synthetic aperture into a system with 3D capabilities ispresented. For actual and future fields of radar in remote sensing, suchas high precision mine detection, highly accurate vegetationmeasurements and interferometric SAR applications the importance ofresolution in the third dimension is growing fast. Until now a such aradar required an expensive phased array antenna, a costly narrow-beamscanning system, or a combination of both of them. In order to overcomethese problems, the combination of an existing linear SAR outdoor system(LISA) with a circular aperture for vertical resolution led to a fastand economic additional 3rd dimension feature. A test experiment hasbeen performed to validate the system and the implemented SAR processor.The very promising results are presented.
    [bibtex-key = rudolfTarchiSieber97:Tomography] [bibtex-entry]


  1598. Shunhua Wang and Xiaotao Huang. Autofocus techniques for reducing phase errors in UWB-SAR. In IEEE National Aerospace and Electronics Conference, NAECON 1997, volume 2, pages 1009-1014, July 1997. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus, PGA, Residual Motion Errors, UWB SAR. [bibtex-key = wangHuang1997:Autofocus] [bibtex-entry]


  1599. Weidong Yu and Zhaoda Zhu. Comparison of Doppler Centroid Estimation Methods in SAR. In IEEE National Aerospace and Electronics Conference NAECON, volume 2, pages 1015-1018, 1997. Keyword(s): SAR Processing, Doppler Centroid Estimation, Energy Balancing, Matched Correlation Estimator, Maximum Likelyhood Estimator, Sign Doppler Estimator, SDE, Correlation Doppler Estimator, CDE, ERS.
    Abstract: This paper compares five Doppler centroid estimation methods which are: energy balancing, matched-correlation maximum likelihood, correlation Doppler estimator (CDE) and sign Doppler estimator (SDE). Their estimation performances in raw data domain and image domain are studied. The computer simulation results are presented. ERS-1 raw data are also used to test the performances of every method.
    [bibtex-key = yuZhu97:DopCen] [bibtex-entry]


  1600. G. Cazzaniga and Andrea Monti-Guarnieri. Removing RF interferences from P-band airplane SAR data. In Geoscience and Remote Sensing Symposium, 1996. IGARSS '96. 'Remote Sensing for a Sustainable Future.', International, volume 3, pages 1845-1847, 1996. Keyword(s): SAR Processing, geophysical signal processing, geophysical techniques, interference, interference (signal), interference filters, notch filters, radar imaging, radar interference, remote sensing by radar, synthetic aperture radar, MUSIC, P-Band, RFI Suppression, RF interference removal, UHF radar, adaptive signal processing, airborne radar, airborne SAR, geophysical measurement technique, in-phase subtraction, land surface, notch filtering, radar remote sensing, terrain mapping, urban area.
    Abstract: This paper approaches the problem of canceling the disturbances due to RF interferences in P-band, airborne SAR missions. Two techniques are introduced: one exploits MUSIC to estimate the interferences' frequencies, and then performs notch filtering at that frequencies; whereas the other adaptively estimate the interference contributions and cancel them by means of in-phase subtraction. Both techniques have been successfully tested on the data acquired by the DLR E-SAR sensor over urban areas.
    [bibtex-key = cazzanigaMontiGuarnieri96:RFI] [bibtex-entry]


  1601. F. Gatelli, Andrea Monti-Guarnieri, Claudio Prati, and Fabio Rocca. Medium resolution efficient phase preserving focusing for interferometry. In IGARSS '96, International Geoscience and Remote Sensing Symposium, volume 1, pages 671-673, 1996. Keyword(s): SAR Processing, Presumming, Interferometry, geophysical signal processing, geophysical techniques, image processing, image resolution, radar imaging, radar signal processing, synthetic aperture radar, SAR imagery, SAR interferometry, Unix Workstations, algorithm, coherence map, geophysical measurement technique, geophysics computing, image pair, land surface, medium resolution efficient phase preserving focusing, radar remote sensing, real time method, strip-map SAR interferogram, terrain mapping.
    Abstract: A real time technique to get strip-map SAR interferograms and coherence maps with common Unix Workstations is presented. For the ERS mission, the ?real time? throughput corresponds to approximately 1/8 of PRF: e.g. ~4 min for processing a 100?100 km image pair. The proposed algorithm achieves that goal on a medium cost 160 Mflops/s Unix Workstation. The output is a 5 looks averaged interferogram, with a geometric resolution of 50?50.
    [bibtex-key = gatelliMontiGuarnieriPratiRocca96:QuicklookPresumming] [bibtex-entry]


  1602. H. Hellsten, Lars M. H. Ulander, A. Gustavsson, and B. Larsson. Development of VHF CARABAS II SAR. In Proc. SPIE, volume 2747, pages 48-60, June 1996. [bibtex-key = hellstenUlanderGustavsson1996:CARABASII] [bibtex-entry]


  1603. J. Homer, I.D. Longstaff, and G. Callaghan. High resolution 3-D SAR via multi-baseline interferometry. In IEEE International Geoscience and Remote Sensing Symposium, IGARSS'96, volume 1, pages 796-798, May 1996. Keyword(s): SAR Processing, SAR Tomography, Tomography, geophysical techniques, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, topography (Earth)3D imagery, InSAR, Multi-baseline Interferometry, SAR imaging, azimuth resolving capability, geophysical measurment technique, interferometric SAR, land surface topography, node aperture, normal-to-slant-range direction, procedural outline, radar remote sensing, terrain height, terrain mapping, theoretical analysis, three dimensional SAR method.
    Abstract: The ability of interferometric SAR (InSAR) to provide terrain height estimation can be interpreted as being due to the baseline (of the two SAR imaging flight paths) acting as an aperture in the normal-to-slant-range (n&oarr;) direction. However, the aperture, because it consists of only two nodes, has effectively no resolving power. The authors introduce and examine a technique which synthesises an N>2 node aperture in the n&oarr; direction from N-1 connected baselines. This, together with the slant-range and azimuth resolving capability of SAR imaging systems, enables the generation of high resolution 3D imagery. A theoretical analysis and procedural outline of the proposed technique are presented
    [bibtex-key = homerLongstaffCallaghan1996:Tomo] [bibtex-entry]


  1604. Charles V. Jakowatz, Daniel E. Wahl, and Paul A. Thompson. Ambiguity resolution in SAR interferometry by use of three phase centers. In Edmund G. Zelnio and Robert J. Douglass, editors, , volume 2757, pages 82-91, 1996. SPIE. Keyword(s): SAR Processing, InSAR, SAR Interferometry, multibaseline, Multibaseline InSAR, Three phase centers, Terrain Mapping, Topography, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzWahlThompsonAmbiguityResolutionInSARByThreeAntennas] [bibtex-entry]


  1605. Kenneth Knaell. Three-dimensional SAR from curvilinear apertures. In Radar Conference, 1996., Proceedings of the 1996 IEEE National, pages 220-225, 1996. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, curvilinear SAR, image enhancement, interference suppression, maximum likelihood estimation, radar antennas, radar clutter, radar cross-sections, radar imaging, synthetic aperture radar, IMP algorithm, aperture configuration, artifacts, coherent CLEAN algorithm, curvilinear apertures, dynamic range, image sidelobes, image size, imaging techniques, likelihood extremization, scatterers, sidelobe leakage effects, three-dimensional SAR.
    Abstract: Three-dimensional information content in radar data taken from suitably curved aperture paths is sufficient to allow useful 3D images to be produced by appropriate imaging techniques. The coherent CLEAN algorithm, the IMP algorithm, and maximum likelihood estimation have been used to produce 3D images from data obtained for two such scenarios. The IMP algorithm in conjunction with likelihood extremization produces images free of sidelobe leakage effects found in the CLEAN algorithm. The CLEAN and IMP algorithms find use initiating likelihood extremization on or near its global maximum. Dynamic range of such images appears dependent upon levels at which artifacts appear although valid scatterers are indicated below such levels. Artifacts appear to be a function of the image sidelobes determined by the aperture configuration. Reduction of this problem and methods to increase image size will extend the general usefulness of this technique
    [bibtex-key = knaell1996:NonLinearSARTomo] [bibtex-entry]


  1606. Jung Ah C. Lee, Orhan Arikan, and David C. Munson, Jr.. Formulation of a General Imaging Algorithm for High-Resolution Synthetic Aperture Radar. In ICASSP '96, International Conference on Acoustics, Speech, and Signal Processing, volume 4, pages 2092-2095, May 1996. Keyword(s): SAR Processing, Backprojection, Convolution Backprojection.
    Abstract: We consider the application of an alternative imaging algorithm to the inversion of strip-mapping synthetic aperture radar (SAR) data. The algorithm was originally developed and successfully applied in the area of geophysics to estimate the conductivity distribution from wellbore induction measurements. The SAR measurement relation satisfies the same form of integral equation describing the wellbore problem. By exploiting the form of the measurement kernel, we derive a SAR image formation algorithm involving deconvolution-backprojection. Unlike correlation-based SAR image formation, our approach is more general, without simplifying assumptions on the range function, and is robust to measurement noise, at the expense of increased computational complexity. Simulation results are presented that demonstrate the effectiveness of the proposed algorithm.
    [bibtex-key = JungArikMuns96:Backproj] [bibtex-entry]


  1607. F. Lombardini. Absolute phase retrieval in a three-element synthetic aperture radar interferometer. In Radar, 1996. Proceedings., CIE International Conference of, pages 309-312, October 1996. [bibtex-key = Lombardini1996] [bibtex-entry]


  1608. F. Lombardini and P. Lombardo. Maximum likelihood array SAR interferometry. In Digital Signal Processing Workshop Proceedings, 1996., IEEE, pages 358-361, September 1996. [bibtex-key = Lombardini1996a] [bibtex-entry]


  1609. John W. McCorkle and Martin Rofheart. Order N^2 log(N) Backprojector Algorithm for Focusing Wide-Angle Wide-Bandwidth Arbitrary-Motion Synthetic Aperture Radar. In Gerald S. Ustach, editor, Radar Sensor Technology, volume SPIE 2747, pages 25-36, 1996. Keyword(s): SAR Processing, Back-Projection, Fast Back-Projection, Quadtree Processing, Time-Domain Back-Projection, Wideband SAR, Focusing, Motion Compensation.
    Abstract: A new, fast algorithm for synthetic aperture radar (SAR) image formation is introduced. The algorithm is based on a decomposition of the time domain backprojection technique. It inherits the primary advantages of time domain backprojection: simple motion compensation, simple and spatially unconstrained propagation velocity compensation, and localized processing artifacts. The computational savings are achieved by using a divide-and-conquer strategy of decomposition, and exploiting spatial redundancy in the resulting sub-problems. The decomposition results in a quadtree data structure that is readily parallelizable and requires only limited interprocessor communications. For a SAR with N aperture points and an N by N image area, the algorithm is seen to achieve O(N^2 logN) complexity. The algorithm allows a direct trade between processing speed and focused image quality.
    [bibtex-key = McCorkle:N2logNBackproj] [bibtex-entry]


  1610. B. Walker, G. Sander, M. Thompson, B. Burns, R. Fellerhoff, and D. Dubbert. A high-resolution, four-band SAR Testbed with real-time image formation. In Geoscience and Remote Sensing Symposium, 1996. IGARSS '96. 'Remote Sensing for a Sustainable Future.', International, volume 3, May 1996. [bibtex-key = Walker1996] [bibtex-entry]


  1611. Zhu Zhaoda, Qiu Xiaohui, and She Zhishun. Modified Doppler centroid tracking method for phase compensation in ISAR. In Radar, 1996. Proceedings., CIE International Conference of, pages 751-754, October 1996. Keyword(s): ISAR. [bibtex-key = Zhaoda1996] [bibtex-entry]


  1612. Zhaoda Zhu, Xiaohui Qiu, and Zhishun She. ISAR motion compensation using modified Doppler centroid tracking methods. In Aerospace and Electronics Conference, 1996. NAECON 1996., Proceedings of the IEEE 1996 National, volume 1, pages 359-363, May 1996. Keyword(s): ISAR. [bibtex-key = Zhu1996] [bibtex-entry]


  1613. Kenneth Abend and John W. McCorkle. Radio and TV interference extraction for ultrawideband radar. In Dominick A. Giglio, editor, Algorithms for Synthetic Aperture Radar Imagery II, Dominick A. Giglio; Ed., volume 2487, pages 119-129, 1995. SPIE. Keyword(s): SAR Processing, RFI Suppression, Ultra-Wideband SAR, Airborne SAR.
    Abstract: This paper describes a computationally efficient, high-performance, UWB radar interference suppression algorithm. An adaptive FIR (finite impulse response) prediction-error noise- whitening filter exhibits minimal computational complexity and achieves 30 dB interference reduction per pulse (1 microsecond(s) long) with 16-bit simulated interference. Using measured interference data digitized to 8-bits with a 6.5 effective bit digitizer, collected just north of Washington, DC at the Army Research Laboratory, the technique achieved 20 to 27 dB of reduction. To minimize the computational load, the filter weights are periodically determined from data collected during a fraction of a radar range sweep. These weights are found to be effective for hundreds of subsequent radar pulses. Previous work on an estimate-and-subtract, tone-extraction technique resulted in 20 dB average interference reduction on the same measured data with a computational load linearly related to the number of tones extracted. The adaptive filtering approach uses an over-determined system producing an FIR filter with N taps, independent of the number of interference signals. An iterative technique to reduce the range sidelobes caused by the filter's impulse response is also presented. The computational load of this iterative stage is, at worst, linearly related to the number of targets whose sidelobes are extracted. It is shown that, with a small reduction in performance, the sidelobe reduction can be accomplished with a relatively small increase in the overall computational load. The computational complexity of the proposed technique relative to the estimate-and- subtract technique depends on the signal and interference environment and on the acceptable sidelobe level. A comprehensive radio and TV interference simulator was developed to test the interference suppression algorithm. It avoids difficulties in memory requirements and code complexity typically encountered in high-sample rate, long duration, and UWB simulations. Data was generated for various population densities, sampling rates, and quantization levels. Results using the simulation data showed that the performance of the algorithm was related to the quantization level with more bits producing better results.
    [bibtex-key = abendMccorkle95:RFI] [bibtex-entry]


  1614. A. Beaudoin, N. Stussi, D. Troufleau, N. Desbois, L. Piet, and M. Deshayes. On the use of ERS-1 SAR data over hilly terrain: Necessity of radiometric corrections for thematic applications. In Geoscience and Remote Sensing Symposium, 1995. IGARSS '95, volume 3, pages 2179-2182, 1995. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, Radiometric Calibration, Radiometric Correction, Calibration, DEM, ERS-1, Mediterranean forest, acquisition geometry, empirical cosine model, geophysical measurement technique, geophysical techniques, hilly terrain mapping, land surface, quantitative use, radar imaging, radar remote sensing, radiometric correction, remote sensing by radar, spaceborne radar, synthetic aperture radar, thematic applications, vegetation mapping.
    Abstract: A radiometric correction method for the quantitative use of ERS SAR data acquired over hilly terrain is illustrated over a Mediterranean forest test-site. It is based on a DEM coupled to the SAR acquisition geometry. It is shown that uncorrected ?0 information is mainly related to the ground scattering area (8 dB variations), and the useful thematic information (2-3 dB) is therefore masked. Once calibrated, pixel intensity decreases with local incidence angle (variations up to 1.5 dB) and this variability can be reduced using an empirical cosine model. Then, the thematic benefits of such corrections are illustrated for surface discrimination and parameter estimation, along with a discussion on applicability and limitations
    [bibtex-key = beaudoinStussiTroufleauDesboisPietDeshayes95:Radiometry] [bibtex-entry]


  1615. Fulvio Gini and Fabrizio Lombardini. A novel linear-quadratic technique for coherent radar detection in mixed clutter environment. In European Microwave Conference, 1995. 25th, volume 1, pages 360-364, Oct. 1995. [bibtex-key = Gini1995a] [bibtex-entry]


  1616. Fulvio Gini, Fabrizio Lombardini, and Lucio Verrazzani. A robust approach to decentralized CFAR detection in spiky clutter. In European Microwave Conference, 1995. 25th, volume 1, pages 584-588, Oct. 1995. [bibtex-key = Gini1995] [bibtex-entry]


  1617. Ron S. Goodman, Sreenidhi Tummala, and Walter G. Carrara. Issues in Ultra-Wideband, Widebeam SAR Image Formation. In The Record of the 1995 IEEE Radar Conference, pages 479-485, May 1995. Keyword(s): SAR Processing, Range Migration Algorithm, Wavefront Reconstruction, Wavenumber Domain Algorithm, omega-k, RFI Suppression, Motion Compensation, Autofocus Techniques, Ultra-Wideband SAR, Sidelobe Control, P-Band, FOPEN, Airborne SAR.
    Abstract: The formation of low-frequency, ultra-wideband/widebeam (UWB/WB) SAR imagery faces inherent challenges not present in conventional SAR systems operating at higher frequencies. Small angle approximations and other shortcuts taken in conventional SAR processors may be inappropriate for the UWB/WB SAR processor. The severe range migration associated with fine resolution UWB/WB SAR poses a significant problem for the image formation algorithm. The combination of a long synthetic aperture and a wide azimuth beamwidth has important implications for motion compensation implementation and performance. We survey the major challenges to UWB/WB SAR image formation. The emphasis is on the formation of fine resolution digital imagery from low frequency UWB/WB data collected in the stripmap mode by an airborne SAR system. We examine the differences in image formation and data processing between UWB/WB systems and conventional SAR systems. We illustrate aspects of UWB/WB image formation with imagery from the P3 ultra-wideband radar system - a 0.3 m resolution SAR which operates in the VHF/UHF frequency bands
    [bibtex-key = GoodmanTummalaCarrara95:UWB] [bibtex-entry]


  1618. Francesco Holecz, Anthony Freeman, and Jakob van Zyl. Topographic effects on the antenna gain pattern correction. In IGARSS '95, Geoscience and Remote Sensing Symposium, volume 1, pages 587-589, 1995. Keyword(s): SAR Processing, SAR Geocoding, Geocoding, SIR-C, X-SAR, antenna gain pattern correction, antenna radiation patterns, radiometric calibration, calibration, Radiometric Correction, remote sensing by radar, satellite antennas, Spaceborne SAR, terrain effect, terrain mapping, topographic effect, ERS-1, ERS-2, JERS-1, RADARSAT.
    Abstract: The purpose of this paper is to analyse and quantify the topographic effects on the antenna gain pattern correction of existing spaceborne Synthetic Aperture Radar systems, namely ERS-1, JERS-1, SIR-C, and X-SAR. Simulations and real SAR data of a test site are used. The corrections are carried out taking into account the local surface topography and compared with the standard method based on a reference ellipsoid. Results show that elevation variations in the ERS-1 and JERS-1 cases do not. affect significantly the antenna gain pattern correction. For extreme topographic differences, greater than 3000 m, a reference altitude for the radiometric calibration is suggested. On the other hand, for the low-orbit SRL-1J2 terrain information is strongly recommended, particularly, if relief differences within the image are significant, namely greater than 1000 m. Furthermore, it will be shown that in the SIR-C case, even if the polarizations of the am tenna gain patterns are slightly different, polarimetric calibration errors due to relief variations can be neglected. Finally, implications for forthcoming spaceborne SAR systems, i.e. ERS-2 and RADARSAT, are discussed.
    [bibtex-key = holeczFreemanVanZyl95:AGP] [bibtex-entry]


  1619. P. Pasquali, C. Prati, F. Rocca, M. Seymour, J. Fortuny, E. Ohlmer, and A.J. Sieber. A 3-D SAR experiment with EMSL data. In Geoscience and Remote Sensing Symposium, 1995. IGARSS '95. 'Quantitative Remote Sensing for Science and Applications', International, volume 1, pages 784-786, 1995. Keyword(s): SAR Processing, Tomography, SAR Tomography, radar applications, radar imaging, remote sensing by radar, synthetic aperture radar, topography (Earth), EMSL, European Microwave Signature Laboratory o, JRC, geodesy, geophysical measurement technique, horizontal slice, interferometric SAR image, land surface topography, multi-baseline SAR, radar remote sensing, target reconstruction, terrain mapping, three dimensional structure.
    Abstract: In usual SAR systems the image spectrum is regularly sampled along azimuth and slant range. A third dimension is found in the baseline of interferometric SAR images. An experiment on the use of multi-baseline SAR images far 3D target reconstruction is presented. The data have been collected in the European Microwave Signature Laboratory of JRC. The volume is presented as horizontal slices of the target at different depths.
    [bibtex-key = pasqualiPratiRoccaSeymourFortunyOhlmerSieber95:3DSAR] [bibtex-entry]


  1620. Matthew Braunstein, James M. Ralston, and David A. Sparrow. Signal processing approaches to radio frequency interference (RFI) suppression. In Dominick A. Giglio, editor, Algorithms for Synthetic Aperture Radar Imagery, volume 2230, pages 190-208, 1994. SPIE. Keyword(s): SAR Processing, RFI Suppression, Ultra-Wideband SAR, Airborne SAR.
    Abstract: Ultra-wideband radar (UWB) has been shown to be among the most powerful techniques available for underground and obscured object detection. The value of such systems is that they combine the penetration enhancement associated with VHF/UHF (and lower) frequencies with the resolution of wide absolute bandwidth. Such systems necessarily make use of much of the frequency spectrum already in heavy use by other services, such as television and mobile communications. Although this spectral overlap provides occasion for adverse consequences in both directions, to date the principal consequence has been often-severe impact on UWB radar measurements. Even in remote locations, the average interference power often exceeds receiver noise by many dB, becoming the limiting factor on system sensitivity. Nor are UWB radar designers free to overcome this interference by increasing radar power, since regulatory sanction for UWB operation will depend on maintaining sufficiently low spectral power densities to assure that other, prior, services are not appreciably degraded. Given the importance of radio frequency interference (RFI) on practical ultrawide band ground penetrating radar systems, it is important to consider how and to what extent the effects of RFI noise may be reduced. The overall problem of RFI and its impacts will be described and several signal processing approaches to removal of RFI will be discussed. These include spectral estimation and coherent subtraction algorithms and various filter approaches, which have been developed and applied by the signal processing community in other contexts. These methods will be applied to several different real-world experimental data sets, and quantitative measures of the effectiveness of each of these algorithms in removing RFI noise will be presented. Although computationally-intensive, most of the techniques to be described achieve substantial increases in S/RFI without requiring concomitant increases in radar average power.
    [bibtex-key = braunsteinRalstonSparrow94:RFI] [bibtex-entry]


  1621. D.C. Ghiglia and D.E. Wahl. Interferometric synthetic aperture radar terrain elevation mapping from multiple observations. In Proceedings of IEEE 6th Digital Signal Processing Workshop, pages 33-36, October 1994. Keyword(s): Synthetic aperture radar interferometry, Terrain mapping, Robustness, Pixel, Rain, Optical interferometry, Spatial resolution, Phase noise, Poisson equations, Laboratories.
    Abstract: All prior interferometric SAR imaging experiments to date dealt with pairwise processing. Simultaneous image collections from two antenna systems or two-pass single antenna collections are processed as interferometric pairs to extract corresponding pixel by pixel phase differences which encode terrain elevation height. The phase differences are wrapped values which must be unwrapped and scaled to yield terrain height. We propose two major classes of techniques that hold promise for robust multibaseline (multiple pair) interferometric SAR terrain elevation mapping. The first builds on the capability of a recently published method for robust weighted and unweighted least-squares phase unwrapping, while the second attacks the problem directly in a maximum likelihood (ML) formulation. We will provide several examples (actual and simulated SAR imagery) that illustrate the advantages and disadvantages of each method.<>
    [bibtex-key = Ghiglia1994] [bibtex-entry]


  1622. Charles V. Jakowatz, Daniel E. Wahl, Paul H. Eichel, and Paul A. Thompson. New formulation for interferometric synthetic aperture radar for terrain mapping. In Dominick A. Giglio, editor, , volume 2230, pages 411-418, 1994. SPIE. Keyword(s): SAR Processing, InSAR, SAR Interferometry, Terrain Mapping, Topography, Spotlight SAR, Spotlight-mode data. [bibtex-key = jakowatzWahlEichelThompsonInSAR1994] [bibtex-entry]


  1623. Alberto Moreira and Rolf Scheiber. Doppler parameter estimation algorithms for SAR processing with the chirp scaling approach. In Geoscience and Remote Sensing Symposium, 1994. IGARSS '94. 'Surface and Atmospheric Remote Sensing: Technologies, Data Analysis and Interpretation'., International, volume 4, pages 1977-1979, 1994. Keyword(s): SAR Processing, Doppler Centroid Estimation, Doppler Ambiguity Resolver, DAR, SDE, CDE, Sign Doppler Estimator, Correlation Doppler Estimator, Doppler radar, FM radar, airborne radar, geophysical signal processing, geophysical techniques, radar applications, radar imaging, remote sensing by radar, spaceborne radar, synthetic aperture radar, Doppler parameter estimation algorithm, Doppler parameters, Doppler rate, SAC, SAR imaging, SAR processing, SPECAN, autocorrelation function, Chirp Scaling Algorithm, chirp scaling approach, correlation method, geophysical measurement technique, image processing, land surface, range-frequency centroid, remote sensing, shift and correlate algorithm, signal processing, spaceborne SAR, terrain mapping.
    Abstract: This paper presents several correlation based methods for estimation of the Doppler parameters from SAR raw data in connection with the chirp scaling algorithm. For the estimation of the Doppler rate, a modified approach of the shift and correlate (SAC) algorithm is proposed. In this case, the auto-correlation function of each data set and the standard deviation of the velocity calculations are used in order to monitor the validity of the estimations. For resolving the PRF ambiguity, a new approach is proposed, which is based on the estimation of the range-frequency centroid as a function of the azimuth frequency. Several results of the Doppler parameters estimation are presented for airborne and spaceborne SAR data
    [bibtex-key = moreiraScheiber94:DopplerParam] [bibtex-entry]


  1624. Rolf Scheiber and Alberto Moreira. Extension of the Correlation Doppler Estimator for Determination of the Doppler Rate and for Resolving the PRF-Ambiguity. In Giorgio Franceschetti, editor, SAR Data Processing for Remote Sensing, volume SPIE 2316, pages 33-41, 1994. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Correlation Doppler Estimator, CDE, Sign Doppler Estimator, SDE, Doppler Ambiguity Resolver, DAR, Doppler Rate Estimation, Autofocus.
    Abstract: Recently the chirp scaling algorithm has been proposed for high quality SAR processing. The algorithm requires first a transformation of the range uncompressed SAR raw data into the range-Doppler domain, which does not permit conventional techniques for the estimation of the Doppler parameters to be introduced efficiently into the processing. This paper first reviews the so called 'correlation Doppler estimator' (CDE), which was proposed for the estimation of the Doppler centroid in the time domain. This estimation algorithm is further extended in order to allow also the estimation of the Doppler rate. To perform this only bright targets are considered. By continuous calculation of the first coefficient of the auto-correlation function, the slope of the frequency history of the Doppler signal is determined, giving an exact estimate of the Doppler rate. to obtain also an estimation of the Doppler rate for scenes without bright targets but with some contrast, a modified version of the SAC algorithm is presented. Further a new method is presented, which can solve the PRF-ambiguity by means of an evaluation of the signal envelope skew in the range-Doppler domain. This technique does not require any additional range FFT, since it is based on the estimation of the variation of a range centroid as a function of the azimuth frequency. Several results are presented, which show the performance of the proposed new approaches. Some remarks are made regarding the inclusion of the described methods into the extended chirp scaling algorithm.
    [bibtex-key = scheiberMoreira94:DopCentrEst] [bibtex-entry]


  1625. D.E. Wahl, C.V. Jakowatz, and P.A. Thompson. New approach to strip-map SAR autofocus. In Digital Signal Processing Workshop, 1994., 1994 Sixth IEEE, pages 53-56, October 1994. Keyword(s): SAR Processing, Autofocus, Phase Curvature Autofocus, Phase Gradient Autofocus.
    Abstract: This paper demonstrates how certain concepts from the Phase Gradient Autofocus (PGA) algorithm for automated refocus of spotlight mode SAR imagery may be used to design a similar algorithm that applies to SAR imagery formed in the conventional strip-mapping mode. The algorithm derivation begins with the traditional view of strip-map image formation as convolution (compression) using a linear FM chirp sequence. The appropriate analogies and modifications to the spotlight mode case are used to describe a working algorithm for strip-map autofocus.
    [bibtex-key = wahlJakowatzThompson1994:PhaseCurvatureAutofocus] [bibtex-entry]


  1626. Pierre Duhamel, Mohsen Montazeri, and Katia Hilal. Classical adaptive algorithms (LMS, RLS, CMA, decision directed)seen as recursive structures. In Acoustics, Speech, and Signal Processing, 1993. ICASSP-93., 1993 IEEE International Conference on, volume 3, pages 496-499, 1993. Keyword(s): RFI Suppression, adaptive filters, computational complexity, correlation theory, least squares approximations, recursive functions, FIR algorithm, adaptive algorithms, block algorithms, constant modulus algorithm, correlation coefficients, decision-directed algorithm, equilibrium states, finite impulse response, least mean square, LMS, recursive least squares, RLS, recursive structures.
    Abstract: Any finite impulse response (FIR) adaptive algorithm has aninherent recursive structure, since the error at time n is fedback into the algorithm to provide the next tap vector. The authors explicitly provide this recursive structure and show that a least meansquare (LMS) adaptive algorithm can be expressed in terms of a recursivefilter, the coefficients of this recurrence being the correlation coefficients of the input signal. Corresponding structures are proposed for the recursive least squares (RLS) algorithm, the constant modulus algorithm (CMA), and the decision-directed (DD) algorithm. These schemes provide block algorithms that are equivalent to the corresponding sample-by-sample algorithm, while requiring fewer computations. The corresponding structures are easily used for studying equilibrium states of the adaptive algorithms.
    [bibtex-key = duhamelMontazeriHilal93:LMS] [bibtex-entry]


  1627. A. Martinez and Jean L. Marchand. Implementation and Quality Analysis of a CSA SAR Processor. In IGARSS '93, International Geoscience and Remote Sensing Symposium, volume 3, pages 1179-1181, August 1993. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Quality Assessment, Quality Measures, ISLR, PSLR.
    Abstract: Recently, a new SAR processing algorithm has been proposed. The basis of the method is to equalize the range migration trajectories of the whole image with respect to a reference range. During azimuth compression, the range migration at the reference range can be exactly corrected, and its effect is extended to all ranges. In order to assess the quality of the resulting image, a number of quality criteria are evaluated. They include classical measurements, such as spatial resolution, ISLR and PSLR, as well as refined ones for better analysis of the pulse shape. The stability and variation of the quality criteria when changing several key input parameters (Doppler centroid, FM rate acid reference range) are also studied. The tests have been performed using real ERS-1 raw data. The selected scene is the Flevoland site where three transponders and several corners reflectors are deployed, thus allowing precise quality analysis to be performed.
    [bibtex-key = MartMarch93:Quali] [bibtex-entry]


  1628. B.L. Robertson and David C. Munson, Jr.. Motion errors in ISAR imaging of approaching targets. In Acoustics, Speech, and Signal Processing, 1993. ICASSP-93., 1993 IEEE International Conference on, volume 5, pages 449-452, April 1993. Keyword(s): ISAR. [bibtex-key = Robertson1993] [bibtex-entry]


  1629. Ian G. Cumming, Frank Wong, and R. Keith Raney. A SAR Processing Algorithm With No Interpolation. In IGARSS '92, International Geoscience and Remote Sensing Symposium, pages 376-379, May 1992. Keyword(s): SAR Processing, Differential Range Deramp - Frequency Domain Algorithm, DRD-FD Algorithm, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Comparison of Algorithms.
    Abstract: Current SAR processing algorithms incorporate interpolators to perform key functions. It turns out that the interpolators are difficult to implement, and are one of the largest sources of error in the processing. In this paper, we introduce a new algorithm which eliminates the use of the interpolation operation, yet achieves accurate range migration correction over the full range swath. The algorithm can handle large apertures and large squints, and has noticeably better phase and geometry accuracy than current algorithms, even when the apertures and squints are high. The new algorithm is called Differential Range Deramp - Frequency Domain (DRD-FD) Algorithm, because its key operation is to use the linear-FM property of the range chirp to differentially shift the range energy as a function of azimuth frequency, and then to do the remaining range cell migration correction in the two-dimensional frequency domain. In this paper, the new algorithm is described, and simulation results are given to demonstrate its focusing, phase and geometric performance with squinted SAR data. In addition, an image is shown made from SEASAT data.
    [bibtex-key = CummWongRaney92:Processing] [bibtex-entry]


  1630. Didier Dendal and Jean L. Marchand. Omega-k Techniques Advantages and Weaker Aspects. In IGARSS '92, International Geoscience and Remote Sensing Symposium, pages 366-368, May 1992. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Comparison of Algorithms.
    Abstract: Wave equation techniques and the omega-k algorithm are very attractive for future SAR space missions with on board reconstruction. The aim of the present work is to point out the stronger advantages and the weaker aspects of this algorithm compared to current SAR processors. Theoretical assumptions and approximations, practical limitations and drawbacks of the method will be outlined in contradistinction to its more positive aspects. A sensitivity analysis, with some positive conclusions, has also been performed. It will appear that there is no great revolution with regard to the traditional algorithms and that the major problems and chief restrictions are always the same, as well as the unavoidable antagonism between processing speed and reconstruction precision, even if some secondary effects are more easily handled there.
    [bibtex-key = DendalMarchand92:Processing] [bibtex-entry]


  1631. P.H. Eichel, D.C. Ghiglia, C.V. Jakowatz, and D.E. Wahl. Phase Gradient Autofocus for SAR Phase Correction: Explanation and Demonstration of Algorithmic Steps. In Digital Signal Processing workshop, 1992. The, September 1992. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Eichel1992] [bibtex-entry]


  1632. Ralf Horn and Erich Meier. A Refined Procedure To Generate Calibrated Imagery From Airborne Synthetic Aperture Radar Data. In Geoscience and Remote Sensing Symposium, 1992. IGARSS '92. International, pages 406-408, 1992. Keyword(s): SAR Processing, AGC, Automatic Gain Control, STC, Sensitivity Time Control, Calibration, Radiometry, Radiometric Calibration, Radiometric Correction, APG, Antenna Gain Pattern, ESAR, E-SAR.
    Abstract: The paper desciibes a procedure realized at DLR to generate calibrated imagery from synthetic aperture radar systems installed on board of small aircrafts. It has been especially developed for the DLR experimental radar system E-SAR, which uses antennas fixed directly to the body of a DO 228 aircraft. A receiver gain control system (STC) is implemented. The correction of the STC variable receiver gain, the compensation of the translational and rotational motion errors of the aircraft and the calibration of the radar data in the SAR processor are described. First preliminary results obtained from a recent experiment in Switzerland are shown.
    [bibtex-key = hornMeier92:STC] [bibtex-entry]


  1633. Alberto Moreira, Artur Brodscholl, Jacob Dom, Frank Kochsiek, and Winfried Poetzsch. Airborne Real-time SAR Processing Activities at DLR. In Geoscience and Remote Sensing Symposium, 1992. IGARSS '92. International, pages 412-414, 1992. Keyword(s): SAR Processing, Real-Time SAR Processing, Subaperture Processing, Real-Time Subaperture Processing, Hardware Processor, STC, Sensitivity Time Control. [bibtex-key = moreiraBrodschollDomKochsiekPotzsch92:realTimeSARProc] [bibtex-entry]


  1634. R. Keith Raney. A New and Fundamental Fourier Transform Pair. In IGARSS '92, International Geoscience and Remote Sensing Symposium, volume 1, pages 106-107, 1992. Keyword(s): SAR Processing, Wavenumber Domain Algorithm, omega-k, Stationary Phase Method, Closed Form Fourier Transform.
    Abstract: A closed form Fourier transform relationship between the range signal, azimuth signal domain and the range signal, azimuth frequency (or Doppler) domain is presented. The derivation assumes the presence of a large time-bandwidth pulse modulation in range. The expression is valid over a wide angular field of view, and expressed using the hyperbolic range equation. The resulting Fourier transform pair is of general utility in SAR, tomography, active seismics and related imaging problems, and allows more insight into the imaging process in the presence of range curvature than is normally available.
    [bibtex-key = raney:fourier] [bibtex-entry]


  1635. Hartmut Runge and Richard Bamler. A Novel High Precision SAR Focussing Algorithm Based On Chirp Scaling. In IGARSS '92, International Geoscience and Remote Sensing Symposium, pages 372 - 375, May 1992. Keyword(s): SAR Processing, Chirp Scaling Algorithm, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Range-Doppler Algorithm, Comparison of Algorithms, Squinted SAR.
    Abstract: Azimuth compression of synthetic aperture radar data is an inherently two-dimensional problem because the SAR reference function migrates through several range resolution cells. This effect is referred to as range cell migration. For perfect focusing of the image the fact has to be accounted for that the amount of range cell migration varies with range. In classical range-Doppler SAR processors as well as with two-dimensional frequency domain processors a space-variant interpolation is required in order to compensate for this effect. In general, interpolation will degrade the final image quality and is computation time consuming. The proposed new algorithm avoids any interpolation step. It is essential for the algorithm that the SAR data are transformed to the range-Doppler domain prior to range compression. In this domain each range line is premultiplied with a particular phase function which is designed to perfectly straighten the range cell migration trajectories even for arbitrarily wide swath by the subsequent range compression step. This new method for range migration correction is the key element of a new SAR processing algorithm described in detail. The proposed method is inherently phase preserving. Its focusing quality is neither limited by high squint nor by wide swath. The implementation is simple because only multiplications and Fourier transforms are required. The paper presents a detailed derivation of the algorithm theory and illustrates possible implementations.
    [bibtex-key = RungeBamler92:Processing] [bibtex-entry]


  1636. Richard Bamler. A Systematic Comparison of SAR Focussing Algorithms. In IGARSS '91, International Geoscience and Remote Sensing Symposium, volume 2, pages 1005-1009, 1991. Keyword(s): SAR Processing, Range-Doppler Algorithm, Wavenumber Domain Algorithm, omega-k, Secondary Range Compression, Comparison of Algorithms.
    Abstract: Focussing of SAR data is an inherently space-variant two-dimensional correlation. Six different algorithms are compared with each other in terms of their focussing quality and their ability to handle the space-variance of the correlation kernel: range-Doppler with and without secondary range compression, a modified range-Doppler algorithm, and three versions of the wavenumber domain processor. Quantitative examples are given for SEASAT and ERS-1.
    [bibtex-key = bamler:compare] [bibtex-entry]


  1637. Terry M. Calloway, Charles V. Jakowatz, Paul A. Thompson, and Paul H. Eichel. Comparison of synthetic-aperture radar autofocus techniques: phase gradient versus subaperture. In Franklin T. Luk, editor, , volume 1566, pages 353-364, 1991. SPIE. Keyword(s): SAR Processing, Autofocus, Comparison of Algorithms, Comparison of Autofocus Algorithms, Phase Gradient Algorithm, PGA, Subaperture-based Autofocus, subaperture correlation, look-misregistration autofocus, Map Drift. [bibtex-key = callowayJakowatzThompsonEichelAutofocusComparisonPGASubaperture1991] [bibtex-entry]


  1638. M. Cocard, A. Geiger, E. Meier, and D. Nuesch. X-SAR Geometric Error Budget Analysis. In Proc. IEEE Int. Geosci. Remote Sens. Symp., volume I, pages 247-251, 1991. [bibtex-key = cocardGeigerMeierNuesch91:XSARGEBA] [bibtex-entry]


  1639. Jorgen Dall. A new frequency domain autofocus algorithm for SAR. In IGARSS '91, Geoscience and Remote Sensing Symposium, volume 2, pages 1069 - 1072, 1991. Keyword(s): SAR Processing, Autofocus, SAC, Shift and Correlate Algorithm, Doppler Rate Estimation, Map Drift, Airborne SAR.
    Abstract: A new autofocus algorithm with a very high computational efficiency is presented. Although based on a completely different principle, this 'shift-and-correlate' algorithm has some similarity with the traditional map drift algorithm. Theory and preliminary tests indicate that the two algorithms have comparable accuracies. However, with the new algorithm the number of arithmetic operations is reduced by a factor of about 50. This, in combination with its non-iterative nature, makes it very suitable for real-time processing.
    [bibtex-key = dall91:Autofocus] [bibtex-entry]


  1640. Tapan Misra and Alberto Moreira. A New Method For Generation Of Optimum Matched Filter From Replica Data For Pulse Compression. In Geoscience and Remote Sensing Symposium, 1991. IGARSS '91. 'Remote Sensing: Global Monitoring for Earth Management'., International, volume 2, pages 1011-1014, 1991. Keyword(s): SAR Processing, Range Compression, Pulse Compression, Matched Filter, Replica, Chirp Replica, Optimum Matched Filter. [bibtex-key = misraMoreira91:OptimMatchedFilt] [bibtex-entry]


  1641. T.E. Scheuer and Frank Wong. Comparison of SAR Processors Based on a Wave Equation Formulation. In IGARSS '91, International Geoscience and Remote Sensing Symposium, volume 2, pages 635-639, June 1991. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Range-Doppler Algorithm, Comparison of Algorithms.
    Abstract: In this paper, we utilize a wave equation framework to describe SAR processing. Within this framework, various conventional and new processing techniques can be reliably compared. In particular, we compare wave equation implementations of the conventional range-Doppler algorithm and the seismic/SAR algorithm recently proposed. The range-Doppler algorithm is a matched filter solution of the Kirchhoff integral in range-time and Doppler-wavenumber space, while the seismic/SAR algorithm is a frequency-wavenumber or plane wave solution to the wave equation using the measured data as a boundary condition. Thus, both processors can be viewed as boundary value solutions to the electromagnetic wave equation. Point spread function analysis, of simulated ERS-1 data and of actual SEASAT data, shows that both processors provide very accurate relative amplitude and phase results. Nevertheless, we expect that future SAR processors may incorporate features of both techniques.
    [bibtex-key = ScheuerWong91:Comparison] [bibtex-entry]


  1642. Carmine Tarantino. Comparison Between Different Doppler Centroid Estimation Methods For Spaceborne SAR Processing. In Geoscience and Remote Sensing Symposium, 1991. IGARSS '91. 'Remote Sensing: Global Monitoring for Earth Management'., International, volume 2, pages 1047-1049, 1991. Keyword(s): SAR Processing, Doppler Centroid Estimation, Spectra Correlation Estimator, SIR-B.
    Abstract: High accuracy estimation of Doppler centroid values is an important input for precision SAR processing. This paper describes the resutls of the numberical comparison between two different algorithms for Doppler centroid estimation. The Spectra Correlation Estimator and the Clutterlock Algorithm Estimator are tested by processing real SIR-B SAR data. The comparison of these two algorithms according to the required accuracy, the scene type and the processing time is proposed and results are analyzed. Test resutls indicate that an estimation accuracy of a few Hz is obtained.
    [bibtex-key = tarantino91:dopCen] [bibtex-entry]


  1643. Daniel E. Wahl, Charles V. Jakowatz, Dennis C. Ghiglia, and Paul H. Eichel. Relationships between autofocus methods for SAR and self-survey techniques for SONAR. In Andrew G. Tescher, editor, , volume 1567, pages 32-40, 1991. SPIE. Keyword(s): SAR Processing, Autofocus, Phase Gradient Algorithm, PGA, Subaperture-based Autofocus, subaperture correlation, look-misregistration autofocus, Map Drift, SONAR. [bibtex-key = wahlJakowatzGhiliaEichelAutofocusSARandSONAR1991] [bibtex-entry]


  1644. Terry M. Calloway, Paul H. Eichel, and Charles V. Jakowatz. Iterative registration of SAR imagery. In Andrew G. Tescher, editor, , volume 1349, pages 412-420, 1990. SPIE. Keyword(s): SAR Processing, Image Registration, Basic Geocoding, Iterative Image Registration. [bibtex-key = callowayEichelJakowatzIterativeRegistration1990] [bibtex-entry]


  1645. Daniel E. Wahl, Paul H. Eichel, and Charles V. Jakowatz. Implementation of the phase-gradient algorithm. In Franklin T. Luk, editor, , volume 1348, pages 528-535, 1990. SPIE. Keyword(s): SAR Processing, Autofocus, Phase Gradient Algorithm, PGA. [bibtex-key = wahlEichelJakowatzPGAImplementation1990] [bibtex-entry]


  1646. J.L. Bauck and W. K. Jenkins. Convolution-backprojection image reconstruction for bistatic synthetic aperture radar. In Proc. IEEE Int. Symp. on Circuits and Systems, volume 3, pages 1512-1515, May 1989. Keyword(s): SAR Processing, Bistatic SAR, Back-Projection, bistatic synthetic aperture radar, Azimuth Focusing, convolution back-projection, elliptical-arc projections, final reconstructed image, ground patch, image resource, pixel, weighting, radar cross-sections, radar theory, Spotlight mode, Airborne SAR, Tomographic Processing, Tomography, Wavefront Curvature.
    Abstract: The algorithm presented accounts for the elliptical nature of the wavefronts over the ground patch (resulting in elliptical-arc projections) and is based on the convolution-backprojection (CBP) algorithm of computer tomography. Essentially, three changes were made to the CBP algorithm. First, instead of backprojection along straight lines, the backprojection is along the same elliptical arcs from which the data were taken. Second, each pixel in the image, during each backprojection, receives a weighting depending on its position in the image. Third, each projection receives an additional overall weighting depending on the positions of the transmitter and the receiver for the corresponding projection. As with CBP, each projection is convolved with a specified function before backprojection, and all of the backprojections are accumulated to form the final reconstructed image.
    [bibtex-key = BauckJenkins1989:BackProjectionBiStatic] [bibtex-entry]


  1647. C. Y. Chang and John C. Curlander. Doppler Centroid Estimation Ambiguity For Synthetic Aperture Radars. In IGARSS '89, International Geoscience and Remote Sensing Symposium, volume 4, pages 2567-2571, July 1989. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Doppler Ambiguity Resolver, DAR, Range Cross-Correlation Technique, Multiple PRF Technique, SIR-C.
    Abstract: A technique for estimation of the Doppler centroid of synthetic aperture radar (SAR) in the presence of a large antenna boresight pointing uncertainty is described. Also investigated is the image degradation resulting from data processing using an ambiguous centroid. Two approaches for Doppler centroid estimation (DCE) ambiguity resolution are presented: The range cross-correlation technique and the multiple PRF technique. For the multiple PRF technique, since other design factors control the selection for SAR, a generalized algorithm is derived for PRFs not containing a common divisor. An example using the Shuttle Imaging Radar (SIR-C) parameters illustrates that this algorithm is capable of resolving the C-band DCE ambiguity for antenna pointing uncertainties of 2? ~ 3?.
    [bibtex-key = ChanCurl89:Doppler] [bibtex-entry]


  1648. P.H. Eichel, D.C. Ghiglia, C.V. Jakowatz, G.A. Mastin, L.A. Romero, and D.E. Wahl. Applications of phase gradient autofocus to aperture synthesis imaging. In Multidimensional Signal Processing Workshop, 1989., Sixth, pages 57-58, Sept. 1989. Keyword(s): SAR Processing, Autofocus, Phase Gradient Autofocus. [bibtex-key = Eichel1989] [bibtex-entry]


  1649. João Moreira and Winfried Poetzsch. Results Of The Real-time Adaptive Radiometric Correction Implemented In The Dfvlr L/C-band Sar. In Geoscience and Remote Sensing Symposium, 1989. IGARSS'89. 12th Canadian Symposium on Remote Sensing., 1989 International, volume 4, pages 2232-2234, 1989. Keyword(s): SAR Processing, AGC, Automatic Gain Control, STC, Sensitivity Time Control, Calibration, Radiometry, Radiometric Calibration, Radiometric Correction, APG, Antenna Gain Pattern, ESAR, E-SAR. [bibtex-key = moreiraPoetzsch89:AGCSTC] [bibtex-entry]


  1650. R. Keith Raney and Paris W. Vachon. A Phase Preserving SAR Processor. In IGARSS '89, International Geoscience and Remote Sensing Symposium, volume 4, pages 2588-2591, July 1989. Keyword(s): SAR Processing, Phase Preserving, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm.
    Abstract: Synthetic aperture radar (SAR) image phase information is necessary to support many advanced SAR applications. The phase information in the complex image for conventional range-Doppler processors is not a robust estimate of scene phase. A SAR processor specifically designed to preserve phase information is being developed at the Canada Centre for Remote Sensing (CCRS). In addition to preserving vital phase information, this processor can support large degrees of range curvature and range migration. Therefore, it is possible, in principle, to use this processor for satellite SAR data, high resolution airborne SAR data, and for both squint mode and spotlight mode SAR data. This paper summarizes the theory and presents early results.
    [bibtex-key = raneyVachon89:PhasePreserving] [bibtex-entry]


  1651. Hartmut Runge and Richard Bamler. PRF Ambiguity Resolving for SAR. In IGARSS '89, International Geoscience and Remote Sensing Symposium, volume 4, pages 2572-2575, July 1989. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, Doppler Ambiguity Resolver, DAR, Look Correlation, Comparison of Algorithms, SIR-C, X-SAR.
    Abstract: For high precision SAR (Synthetic Aperture Radar) processing, the determination of the absolute Doppler centroid is indispensable. The Doppler frequency estimated from azimuth spectra, however, suffers from the fact that the data are sampled with the PRF and an ambiguity about the correct PRF-band remains. Five methods for ambiguity resolving are proposed and discussed together with the already known technique of look correlation. None of these methods have a requirement on the mission schedule. It is shown that the following effect can be used to measure the absolute Doppler frequency: the Doppler shift of range spectra, range migration, image geometric misregistration and the use of multifrequency radar data.
    [bibtex-key = RungeBaml89:Doppler] [bibtex-entry]


  1652. J. Siewerth. Theory And Quantitative Comparison Of Doppler Centroid Estimation Methods. In Geoscience and Remote Sensing Symposium, 1989. IGARSS'89. 12th Canadian Symposium on Remote Sensing. 1989 International, volume 4, pages 2576-2578, 1989. Keyword(s): SAR Processing, Doppler Centroid Estimation, Energy Balancing, Sign Doppler Estimator, SDE, Correlation Doppler Estimator, CDE, ERS.
    Abstract: The purpose of this paper is to describe the theory and implementation of three different Doppler centroid estimation methods and to present the first results of currently performed quantitative investigations. The Doppler centroid shift caused by the relative velocity between the sensor platform and the targets is derived by analysing the recieved SAR data. In contrast to the conventionally used Delta-E method (also called energy balancing), which is a frequency approach, the two other methods, the Correlation Doppler Estimator (CDE) and the Sign Doppler Estimator (SDE), are both performed in the time domain.
    [bibtex-key = siewerth89:dopCen] [bibtex-entry]


  1653. J. L. Bauck and W. K. Jenkins. Tomographic Processing of Spotlight-Mode Synthetic Aperture Radar Signals with Compensation for Wavefront Curvature. In ICASSP '88, International Conference on Acoustics, Speech, and Signal Processing, volume 2, pages 1192-1195, April 1988. Keyword(s): SAR Processing, Bistatic SAR, Back-Projection, bistatic synthetic aperture radar, Azimuth Focusing, convolution back-projection, elliptical-arc projections, final reconstructed image, ground patch, image resource, pixel, weighting, radar cross-sections, radar theory, Spotlight mode, Airborne SAR, Tomographic Processing, Tomography, Wavefront Curvature.
    Abstract: An algorithm is introduced which corrects for problems which arise in spotlight-mode synthetic aperture radar (SAR) when a significant amount of wavefront curvature is present. Wavefront curvature can be problematic when it is desired to reconstruct high-resolution images of ground patches which subtend a large angle at the radar. The proposed, non-Fourier, algorithm uses mappings to modify the convolution-backprojection method of computed tomography, providing reconstructions from circular-arc projections that are of the same high quality as those made from standard straight-line projections using the unmodified algorithm. In addition, other problems in high-resolution SAR imaging are identified which have solutions involving tomographic concepts, including situations which give rise to fan-beam-like projections. The new algorithm can be modified to deal with these problems as well.
    [bibtex-key = BauckJenkins88:Backproj] [bibtex-entry]


  1654. D.J. DiFilippo, G.E. Haslam, and W.S. Widnall. Evaluation of a Kalman filter for SAR motion compensation. In IEEE Position Location and Navigation Symposium, PLANS '88, pages 259-268, December 1988. Keyword(s): SAR Processing, Motion Compensation, Residual Errors, Kalman filters, filtering and prediction theory, military systems, radar antennas, Kalman filter, SAR motion compensation, SARMCS, air turbulence, airborne SAR, aircraft maneuvers, inertial measurement unit, low-cost strapdown IMU, radar antenna, synthetic aperture radar.
    Abstract: A synthetic aperture radar motion compensation system (SARMCS) is being developed at the Defence Research Establishment Ottawa to compensate an airborne SAR for spurious motions of the radar antenna that may be caused by air turbulence of aircraft maneuvers. A Kalman filter has been developed as part of this SAR motion compensation system which uses a low-cost strapdown IMU (inertial measurement unit), to measure antenna motion. The function of the Kalman filter is to control misalignments of the strapdown analytical platform, since analysis has indicated that these errors are dominant contributors to motion compensation error. Representative results from processing raw recorded flight data have verified the proper operation of all aspects of the Kalman filter and have indicated that the filter performance is consistent with the motion compensation requirements. Some examples of actual SAR strip-map imagery are shown in order to demonstrate the enhancement provided by the SARMCS.
    [bibtex-key = DiFilippoHaslamWidnall88:MoCoResidualErrors] [bibtex-entry]


  1655. James E. Harris, Randall S. Ostler, Douglas M. Chabries, and Richard W. Christiansen. Quality Measures for SAR Images. In IEEE International Conference on Acoustics, Speech, and Signal Processing ICASSP, volume 2, pages 1064-1067, April 1988. Keyword(s): SAR Processing, Quality Assessment, Quality Measures.
    Abstract: Synthetic aperture radar (SAR) contains information features different from those of infrared images and conventional photography. Image quality measures specifically created for SAR are necessary to measure SAR processor performance especially in the context of lossy image data compression. The quality measures discussed include several existing measures and newly developed measures created to better gauge specific features of SAR imagery. Each quality performance measure is evaluated subjectively to determine the applicability to SAR image fidelity.
    [bibtex-key = HarrOstlChabChris88:Quali] [bibtex-entry]


  1656. Ian G. Cumming, P. F. Kavanagh, and M. R. Ito. Resolving the Doppler Ambiguity for Spaceborne Synthetic Aperture Radar. In IGARSS '86, International Geoscience and Remote Sensing Symposium, volume 3, pages 1639-1643, 1986. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Azimuth Look Correlation, Clutterlock, Doppler Ambiguity Resolver, DAR, Doppler Rate Estimation, Autofocus.
    Abstract: In spaceborne SAR systems, the radar beam pointing angle must be known to approximately one half the beamwidth in order to resolve the Doppler centroid ambiguity and provide accurate data processing. This constraint may place a heavy burden on the beam pointing and measurement error budget, unless an alternate means can be provided to estimate the beam pointing angle or Doppler centroid. In this paper, a new method is presented for estimating the Doppler centroid directly from the received radar data, during the image formation process. The algorithm has been programmed into the GSAR processor, and encouraging test results have been obtained.
    [bibtex-key = cum86:DopCentrEst] [bibtex-entry]


  1657. Ian G. Cumming and John R. Bennett. Digital processing of SEASAT SAR data. In Proc. Rec. IEEE Int. Conf. Acoust., Speech Signal, volume 4, Washington, DC, pages 710-718, April 1979. Keyword(s): SAR Processing, Seasat, Spaceborne SAR, Range Compression, Azimuth Compression, Azimuth Focusing.
    Abstract: The Synthetic Aperture Radar (SAR), on board the Seasat-A satellite, provides an all-weather imaging capability which should prove useful in a number of remote sensing applications. Unlike optical (Landsat) data, the SAR data requires extensive two-dimensional, space variant signal processing before an image is formed. This paper describes the signal processing operations in a digital processor which has been built to produce images from the Seasat-A SAR data. It describes the operations of real-to-complex data conversion, range compression via fast convolution, matrix transformation of 40 MB disk arrays, range cell migration correction, look extraction via bandpass filtering, azimuth compression via fast convolution, interpolation and detection.
    [bibtex-key = CummingBennett1979:RDA] [bibtex-entry]


  1658. ESA. Report for Mission Assessment: Earth Explorer 10 Candidate Mission Hydroterra. Technical report, ESA ESTEC, 2020. Keyword(s): Hydroterra, G-Class, geosynchronous SAR, spaceborne SAR, ground motion, atmospheric water waper, troposphere, persistent scatterer interferometry. [bibtex-key = ESA2020HydroterraGClassReportForMissionAssessmentEarthExplorer10CandidateMission] [bibtex-entry]


  1659. Malcolm Davidson, Marco Chini, Wolfgang Dierking, Samuel Djavidnia, Joerg Haarpaintner, Guillaume Hajduch, Gaia Vaglio Laurin, Marco Lavalle, Carlos Lopez Martinez, Thomas Nagler, Nazzarreno Pierdicca, and Bob Su. Copernicus L-band SAR Mission Requirements Document (MRD). Technical report 2.0, ESA/ESTEC, October 2019. Note: ESA-EOPSM-CLIS-MRD-3371. Keyword(s): L-band, ROSE-L, Copernicus, Copernicus L-band SAR, ESA, spaceborne SAR, SAR Interferometry, deformation, displacement, ground motion, geohazards, sea ice types, detection of icebergs, forest, monitoring, changes in global forest carbon stocks, carbon stocks, Agriculture, food security, mapping of water availability, water use, soil moisture, moisture, glacier, ice sheets, climate change, maritime surveillance.
    Abstract: By filling important observation gaps in the current Copernicus satellite constellation, the Copernicus L-band SAR mission supports key European policy objectives and provides enhanced continuity for a number of Copernicus services and down-stream commercial and institutional users. Due to the longer wavelength, L-Band SAR observations from space provide additional information that cannot be gathered by other means benefiting a variety of services and applications. A high-level mapping between specific European policy objectives and the unique information provided by the mission is provided below. In particular the mission will contribute inter alia to: (1) The safety of European Citizens by greatly extending the monitoring of geohazards linked with surface motion such as landslides, subsidence and earthquake/volcanic phenomena into vegetated areas which are inaccessible to current Copernicus satellites and will be critical to the nascent European Ground Motion Service (EUGMS) (2) The European Arctic policy and the sustainable economic development of the Arctic region by providing new information sea ice types and detection of icebergs critical to safe navigation and building of infrastructure in Arctic areas; (3) Forestry and maintaining biodiversity through the continuous high-resolution monitoring of changes in global forest carbon stocks and their spatial distribution; (4) Agriculture and food security by providing reliable high-resolution soil moisture information to support improved management of water use, enhances weatherindependent land cover and crop information, feeding meteorological and hydrological forecast models; (5) EU Water Framework Directive [2] through mapping of water availability and water use particularly for agriculture; (6) Climate change policy through the enhanced monitoring of glaciers and ice sheets, forest carbon stocks and changes with time and water availability; (7) The European Union Integrated Maritime Policy [3] by extending the capacity to monitor our marine ecosystem and by increasing our maritime surveillance abilities. This document details the Mission Requirements for the candidate L-band SAR Copernicus Expansion Mission. It summarises available information on the background of the mission, details and justifies the mission objectives and provides the mission requirements that guide the technical design of the mission. Chapter 1 of the MRD contains a brief outline of the document and provides a high-level view of the contribution of the mission to European policies. Chapter 2 provides details on the background and justification for an L-band SAR mission within the EU and the Copernicus Programme. Chapter 3 presents the mission objectives product requirements that address the user requirements provided by the European Commission (EC). It also highlights related European and international policy needs and their applications. Chapter 4 contains the specific observational requirements (Level-1 and 2), which provide guidance and a concrete basis for the mission implementation. Chapter 5 provides system concepts and mission scenarios. Chapter 6 gives a qualitative description of the data products at Level-1 and Level- 2, the operational data processing requirements and accompanying requirements for (static and dynamic) auxiliary data, e.g. digital elevation model (DEM), and meteorological data. Chapter 6 also includes a high-level description of the required Level-1 to Level-3 retrieval algorithms. Chapter 7 provides a brief description of synergies between satellite systems, and provides the international context. Chapter 8 provides a list of references. Please note that this Mission Requirements Document (MRD) is evolving with time as the user and mission requirements are matched to the capabilities and technical design of the mission following an iterative process. [1] https://eeas.europa.eu/arctic-policy/eu-arctic-policy_en (accessed 10 Oct 2019) [2] https://ec.europa.eu/environment/water/water-framework/index_en.html (accessed on 10 Oct 2019) [3] https://ec.europa.eu/maritimeaffairs/policy_en (accessed 10 Oct 2019)
    [bibtex-key = davidsonEtal2018ROSELESAMissionRequirementsDocumentMRD] [bibtex-entry]


  1660. NISAR. NASA-ISRO SAR (NISAR) Mission ScienceUsers' Handbook.. Technical report, NASA Jet Propulsion Laboratory, 2018. Keyword(s): L-band, NASA, ISRO, NISAR, spaceborne SAR, SAR Interferometry, deformation, displacement, ground motion, geohazards, sea ice types, detection of icebergs, forest, monitoring, changes in global forest carbon stocks, carbon stocks, Agriculture, food security, mapping of water availability, water use, soil moisture, moisture, glacier, ice sheets, climate change, maritime surveillance.
    Abstract: NISAR PROVIDESA MEANS OF DISENTANGLING AND CLARIFYING SPATIALLY AND TEMPORALLY COMPLEX PHENOMENA, RANGING FROM ECOSYSTEM DISTURBANCES, TO ICE SHEET COLLAPSE AND NATURAL HAZARDS INCLUDING EARTHQUAKES, TSUNAMIS, VOLCANOES, AND LANDSLIDES. NISAR WILL BE THE FIRST NASA RADAR MISSION TO SYSTEMATICALLY AND GLOBALLY STUDY SOLID EARTH, ICE MASSES, AND ECOSYSTEMS. The NASA-ISRO Synthetic Aperture Radar (SAR), or NISAR mission, is a multidisciplinary radar mission to make integrated measurements to understand the causes and consequences of land surface changes. NISAR will make global measurements of the causes and consequences of land surface changes for integration into Earth system models. NISAR provides a means of disentangling and clarifying spatially and temporally complex phenomena, ranging from ecosystem disturbances, to ice sheet collapse and natural hazards including earthquakes, tsunamis, volcanoes, and landslides. The purpose of this handbook is to prepare scientists and algorithm developers for NISAR by providing a basic description of the mission and its data characteristics that will allow them to take full advantage of this comprehensive data set when it becomes available. NISAR is a joint partnership between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO). Since the 2007 National Academy of Science ''Decadal Survey'' report, ''Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond,'' NASA has been studying concepts for a Synthetic Aperture Radar mission to determine Earth change in three disciplines: ecosystems (vegetation and the carbon cycle), deformation (solid Earth studies), and cryospheric sciences (primarily as related to climatic drivers and effects on sea level). In the course of these studies, a partnership with ISRO developed, which led to a joint spaceborne mission with both L-band and S-band SAR systems onboard. The current 2018 Decadal Survey, ''Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space,'' confirms the importance of NISAR and encourages the international partnership between NASA and ISRO. The Earth Science Division (ESD) within the Science Mission Directorate (SMD) at NASA Headquarters has directed the Jet Propulsion Laboratory (JPL) to manage the United States component of the NISAR project. ESD has assigned the Earth Science Mission Program Office (ESMPO), located at Goddard Space Flight Center (GSFC), the responsibility for overall program management. The NISAR mission is derived from the Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) radar mission concept, which was one of the four Tier 1 missions recommended in the 2007 Decadal Survey. To satisfy requirements of three distinct scientific communities with global perspectives, as well as address the potentials of the system for new applications, the NISAR system comprises a dual frequency, fully polarimetric radar, with an imaging swath greater than 240 km. This design permits complete global coverage in a 12-day exact repeat to generate interferometric time series and perform systematic global mapping of the changing surface of the Earth. The recommended lidar component of DESDynI will be accomplished with the GEDI mission (Global Ecosystem Dynamics Investigation Lidar). NISAR's launch is planned for January 2022. After a 90-day commissioning period, the mission will conduct a minimum of three full years of science operations with the L-band SAR in a near-polar, dawn-dusk, frozen, sun-synchronous orbit to satisfy NASA's requirements; ISRO requires five years of operations with the S-band SAR. If the system does not use its fuel reserved excess capacity during the nominal mission, it is possible to extend mission operations further for either instrument. NISAR's science objectives are based on priorities identified in the 2007 Decadal Survey and rearticulated in the 2010 report on NASA's Climate-Centric Architecture. NISAR will be the first NASA radar mission to systematically and globally study solid Earth, ice masses, and ecosystems. NISAR will measure ice mass and land surface motions and changes, ecosystem disturbances, and biomass, elucidating underlying processes and improving fundamental scientific understanding. The measurements will improve forecasts and assessment of changing ecosystems, response of ice sheets, and natural hazards. NASA also supports use of the NISAR data for a broad range of applications that benefit society, including response to disasters around the world. In addition to the original NASA objectives, ISRO has identified a range of applications of particular relevance to India that the mission will address, including monitoring of agricultural biomass over India, monitoring and assessing disasters to which India responds, studying snow and glaciers in the Himalayas, and studying Indian coastal and near-shore oceans. All NISAR science data (L- and S-band) will be freely available and open to the public, consistent with the long-standing NASA Earth Science open data policy. With its global acquisition strategy, cloud-penetrating capability, high spatial resolution, and 12-day repeat pattern, NISAR will provide a reliable, spatially dense time series of radar data that will be a unique resource for exploring Earth change (Table 1-1). Anticipated scientific results over the course of the mission include: (1) Comprehensive assessment of motion along plate boundaries that cross land, identifying areas of increasing strain, and capturing signatures of several hundred earthquakes that will contribute to our understanding of fault systems; (2) Comprehensive inventories of global volcanoes, their state of activity and associated risks; (3) Comprehensive biomass assessment in low biomass areas where dynamics are greatest, and global disturbance assessments, agricultural change, and wetlands dynamics, informing carbon flux models at the most critical spatial and temporal scales; (4) In combination with GEDI and other missions, comprehensive global biomass to set the decadal boundary conditions for carbon flux models; (5) Complete assessments of the velocity state of Greenland's and Antarctica's ice sheets, each month over the mission life, as a key boundary condition for ice sheet models; (6) Regular monitoring of the world's most dynamic mountain glaciers; (7) Comprehensive mapping of sea ice motion and deformation, improving our understanding of ocean-atmosphere interaction at the poles; (8) A rich data set for exploring a broad range of applications that benefit from fast, reliable, and regular sampling of areas of interest on land or ice. These include infrastructure monitoring, agriculture and forestry, disaster response, aquifer utilization, and ship navigability.
    [bibtex-key = NISAR2018NISARMissionScienceUsersHandbook] [bibtex-entry]


  1661. ESA. Report for Mission Selection : CoReH2O (ESA SP-1324/2). Technical report, European Space Agency (ESA), 2012. Keyword(s): CoReH20, Earth Explorer 7 Candiate, X-band, Ku-band, Spaceborne SAR, Mission Concept, Snow, Snow Water Equivalent, SWE, snow mass, glaciology, ice, remote sensing by radar, sea ice, snow, synthetic aperture radar, water, climate models, cold land processes, cold regions hydrology high-resolution observatory satellite mission, cryosphere, frequency 17.2 GHz, frequency 9.6 GHz, glaciers, global snow observations, global water cycle, lake ice, land snow, scattering signal decomposition, sea ice, synthetic aperture radar, water cycle variability prediction, Hydrology, Ice surface, Lakes, Land surface, Observatories, Satellites, Sea ice, Sea surface, Snow, Water resources, Climate research, earth observation satellite, glaciers, snow cover, synthetic aperture radar, water resources. [bibtex-key = ESA2012CoReH2OEarthExplorer7CandReportForMissionSelection] [bibtex-entry]


  1662. Shaun Quegan, Thuy Le Toan, Jérôme Chave, Jorgen Dall, Konstantinos P. Papathanassiou, Fabio Rocca, Lars Ulander, and Mathew Williams. Report for Mission Selection: Biomass. Technical report SP-1324/1 (3 volume series), ESA, Noordwijk, The Netherlands, May 2012. Keyword(s): BIOMASS, BIOMASS MISSION, ESA, European Space Agency, P-Band, Spaceborne SAR, SAR.
    Abstract: The Earth Explorer Biomass mission will provide the scientific community with the first accurate maps of tropical, temperate and boreal forest biomass, including height and disturbance patterns. This information is urgently needed to improve our understanding of the global carbon cycle and to reduce uncertainties in the calculations of carbon stocks and fluxes associated with the terrestrial biosphere. Biomass addresses one of the most fundamental questions in our understanding of the land component in the Earth system, namely the status and the dynamics of forests, as represented by the distribution of biomass and how it is changing. Gaining accurate and frequent information on forest properties at scales that allow changes to be observed will mean that the scientific community is equipped to address a range of critical issues with far-reaching benefits for science and society. Moreover, Biomass will greatly improve our knowledge of the size and distribution of the terrestrial carbon pool, and provide much-improved estimates of terrestrial carbon fluxes. In addition, the mission responds to the pressing need for biomass observations in support of global treaties such as the United Nations Framework Convention on Climate Change initiative for the Reduction of Emissions due to Deforestation and Forest Degradation. These mission objectives respond directly to the specific scientific challenges in ESA's Living Planet Programme. The emission of carbon dioxide to the atmosphere by human activity has been recognised as the major driver in climate change. Terrestrial ecosystems play an important role, both in the release of carbon through land use and deforestation and in the sequestration of carbon through vegetation growth processes. There is strong evidence that the terrestrial biosphere has acted as a net carbon sink over the last 30 years, removing from the atmosphere approximately one third of the carbon dioxide emitted from the combustion of fossil fuel. Nevertheless, terrestrial ecosystems are the largest source of uncertainty in the global carbon budget. Uncertainties lie in the spatial distribution of carbon stocks and carbon exchange, and in the estimates of carbon emissions resulting from human activity and natural processes. A central parameter in the terrestrial carbon budget is forest biomass, which is a proxy for carbon. Despite its crucial role in the terrestrial carbon budget, forest biomass in most parts of the world is poorly quantified owing to the difficulties in taking measurements from the ground and the lack in consistency when aggregating measurements across scales. Biomass will be implemented as a P-band Synthetic Aperture Radar (SAR) mission. It will exploit the unique sensitivity of P-band SAR together with advanced retrieval methods to measure forest biomass, height and disturbance across the entire biomass range every six months. The resolution and accuracy of the Biomass products will be compatible with the needs of international reporting on carbon stocks and terrestrial carbon models. In addition, Biomass will provide the first opportunity to explore Earth's surface using the P-band wavelength. The data are also expected to be used for monitoring glacier and ice sheet velocities, mapping subsurface geology in deserts and mapping the topography of forest floors. Additional products and applications are likely to emerge and be evaluated during the life of the mission...
    [bibtex-key = queganLeToanChaveDallPapathanassiouRoccaUlanderWilliamsESA2012BIOMASSReportforMissionSelection] [bibtex-entry]


  1663. Report of the DESDynI Applications Workshop. Technical report, Version 1, April 2009. Keyword(s): DESDynI mission, BIOMASS mission, Earth explorer core mission candidate, P-band, forest area, forest biomass, forest disturbances, global maps, synthetic aperture radar, terrestrial carbon cycle, remote sensing by radar, spaceborne radar, DESDynl mission, Deformation, Ecosystem Structure, and Dynamics of Ice, Earth Science Decadal Survey, National Research Council, biomass estimation, carbon cycle, cryosphere objectives, ecosystem function, integrated L-band InSAR, multibeam Lidar mission, solid Earth surface deformation, surface elevation changes, topography measure, vegetation structure, deformation, optical radar, radar interferometry, remote sensing by radar, topography (Earth), vegetation. [bibtex-key = DESDynIApplicationsWorkshopReport] [bibtex-entry]


  1664. Candidate Earth Explorer Core Mission BIOMASS - Report for Assessment. Technical report, ESA SP-1313/2, November 2008. Keyword(s): BIOMASS mission, Earth explorer core mission candidate, P-band, forest area, forest biomass, forest disturbances, frequency 425 MHz, global maps, synthetic aperture radar, terrestrial carbon cycle, remote sensing by radar, spaceborne radar, synthetic aperture radar. [bibtex-key = BIOMASS_ESA_EARTH_EXPLORER_MISSION2008] [bibtex-entry]


  1665. Mehrdad Soumekh. Time Domain Non-Linear SAR Processing. Technical report, Department of Electrical Engineering,State University of New York, 2006. Keyword(s): SAR Processing, Back-Projection, Time-Domain Back-Projection, TDBP, Non-Linear Flight Path, Matlab, MPI, MatlabMPI, Video SAR, Parallel Processing, Wavefront Reconstruction, omega-k Algorithm, omega-k, Range Migration Algorithm. [bibtex-key = soumekhReport2006:TDBP] [bibtex-entry]


  1666. Betlem Rosich and Peter Meadows. Absolute Calibration of ASAR Level 1 Products Generated with PF-ASAR. Technical report Iss. 1 rev. 5, ESA, Oct. 2004. Keyword(s): ASAR, ENVISAT, Calibration, Validation, Product Calibration, Quality Measures, Quality Assessment, Level 1 Products, SAR.
    Abstract: The aim of this document is to describe the absolute calibration of high rate ASAR Level 1 products generated by ESA using the ASAR processing Facility (PF-ASAR). ESA ASAR level 1 products are generated at the Processing and Archiving Centres (PACs) and at the acquisitions stations: D-PAC, I-PAC, UK-PAC, PDHS-K (Kiruna), PDHS-E (Esrin). Since the same processor is used in all facilities, a unique methodology is described here, which is applicable to any product regardless of where it has been generated. The document is organised as follows: Section 3 describes the derivation of sigma and gamma nought over distributed targets. Section 4 presents the estimation of point targets Radar Cross Section. Annex A provides a procedure to derive the elevation angle for each image pixel, which is a key parameter for the absolute product calibration. Annex B describes how to perform the elevation antenna pattern correction, which is required for complex products calibration. Annex C defines the parameters required for the above operations. Annex D is provided for reference. It presents the evolution of the elevation antenna pattern since Aug. 2002.
    Comments: Reference: ENVI-CLVL-EOPG-TN-03-0010, Issue 1, revision 5, 07. Oct 2004
    [bibtex-key = RosichMeadows04:ASAR] [bibtex-entry]


  1667. Michiel Otten and Henno Boomkamp. Estimation of the Absolute Orbit Accuracy of Envisat. Technical report, European Space Operations Centre (ESOC), Robert-Bosch Strasse 5, D-64283 Darmstadt, 2003. Keyword(s): ENVISAT, DORIS, Precise Orbit, Orbit, Orbit Accuracy, Accuracy, ASAR.
    Abstract: This paper presents the results of the orbit comparison campaign conducted at the European Space Operations Centre (ESOC) in May 2003. The goal of the comparison is to better quantify the absolute orbit accuracy of ENVISAT. For this comparison cycle 12 of ENVISAT was selected which corresponds to a period of 35 days starting at 10.12.2002 and ending at 13.01.2003. Two days, 18 and 19 December, were excluded within this period from the comparison. These days were selected because of the large inclination manoeuvre on 18 December and the resulting loss of DORIS and SLR tracking data for nearly a day. Six different centers contributed their POD solution to the comparison. The CNES POD solution used in this comparison is the same solution as the one that is found on the ENVISAT altimeter GDR. The JPL-DORIS solution only uses the DORIS tracking dataset and is based on the EGM- 96 gravity field where all the other centers use DORIS and SLR data and the GRIM5-C1 field. Three complementary analysis methods where applied to all contributions, namely pair-wise orbit comparison, SLR tracking data analysis and altimeter crossover analysis.
    [bibtex-key = ottenBoomkamp03:DorisAbsAccuracy] [bibtex-entry]


  1668. ASAR-Cal-Val-Team. Quality Measurements Definition for ASAR Level 1 Products. Technical report Iss. 1, ESA, Mar. 2002. Keyword(s): ENVISAT, ASAR, Product Calibration, Calibration, Quality Assessment, Quality Measures, ISLR, PSLR, SSLR, Level 1 Products, SAR, ASAR, ENVISAT, Calibration, Validation, Quality Measures.
    Abstract: The activities to verify and calibrate ASAR products during the ENVISAT Commissioning Phase (C.P.) will be carried out by the ASAR CAL/VAL team members at different centres and in some cases, using different product analysis tools. It is therefore important to establish the methodology for deriving all the quality parameters so that results from different team members are completely consistent. This document describes the procedures to be followed during the C.P. for measuring the ASAR product quality parameters on Level I products.
    [bibtex-key = qualityMeasuresASAR] [bibtex-entry]


  1669. Kenneth Knaell. Three-Dimensional SAR from Curvilinear Apertures. Technical report, Carerock Division, Naval Surface Warfare Center, 1994. Keyword(s): SAR Processing, Non-Linear Flight Path, SAR Tomography, 3D Feature Extraction, Target Feature Extraction, Curvilinear SAR. [bibtex-key = knaellReport1994:NonLinearSARTomo] [bibtex-entry]


  1670. Fabio Rocca, Claudio Prati, and Andrea Monti-Guarnieri. New Algorithms for Processing of SAR Data. ESA Contract Report, ESRIN Contract no. 7998/88/F/FL(SC), 1989. Keyword(s): SAR Processing, Range Migration Algorithm, omega-k, Wavenumber Domain Algorithm, Range-Doppler Algorithm, Secondary Range Compression, Comparison of Algorithms. [bibtex-key = roccaPratiMontiGuarnieri89:omegak] [bibtex-entry]


  1671. Othmar Frey, Charles Werner, Andrea Manconi, and Roberto Coscione. High-resolution mobile mapping of slope stability with car- and UAV-borne InSAR systems, 2022. Note: EGU General Assembly 2022; Conference Location: Vienna, Austria; Conference Date: May 23-27, 2022; Conference lecture held on May 25, 2022.
    Abstract: Terrestrial radar interferometry (TRI) has become an operational tool to measure slope surface displacements [1,2]. The day-and-night and all-weather capability of TRI together with the ability to measure line-of-sight displacements in the range of sub-centimeter to sub-millimeter precision are strong assets that complement other geodetic measurement techniques and devices such as total stations, GNSS, terrestrial laser scanning, and close/mid-range photogrammetric techniques.(Quasi-)stationary TRI systems are bound to relatively high frequencies (X- to Ku-band or even higher) to obtain reasonable spatial resolution in azimuth and yet the azimuth resolution is typically only in the order of tens of meters for range distances beyond a few kilometers. These aspects are limiting factors to obtain surface displacement maps at high spatial resolution for areas of interest at several kilometers distance and also for (slightly) vegetated slopes due to the fast temporal decorrelation at high frequencies. Recently, we have implemented and demonstrated car-borne and UAV-borne repeat-pass interferometry-based mobile mapping of surface displacements with an in-house-developed compact L-band FMCW SAR system which we have deployed 1) on a car and 2) on VTOL UAVs (Scout B1-100 and Scout B-330) by Aeroscout GmbH [3,4]. The SAR imaging and interferometric data processing is performed directly in map coordinates using a time-domain back-projection (TDBP) approach [5,6] which precisely takes into account the 3-D acquisition geometry.We have meanwhile further consolidated our experience with the repeat-pass SAR interferometry data acquisition, SAR imaging, interferometricprocessing, and surface displacement mapping using the car-borne and UAV-borne implementations of our InSAR system based on a number of repeat-pass interferometry campaigns. In our contribution, we present the capabilities of this new InSAR-based mobile mapping system and we discuss the lessons learned from our measurement campaigns. References: [1] Caduff, R., Schlunegger, F., Kos, A. & Wiesmann, A. A review of terrestrial radar interferometry for measuring surface change in the geosciences. Earth Surface Processes and Landforms 40, 208-228 (2015). [2] Monserrat, O., Crosetto, M. & Luzi, G. A review of ground-based SAR interferometry for deformation measurement. ISPRS Journal of Photogrammetry and Remote Sensing 93, 40-48 (2014). [3] O. Frey, C. L. Werner, and R. Coscione, Car-borne and UAV-borne mobile mapping of surface displacements with a compact repeat-pass interferometric SAR system at L-band, in Proc. IEEE Int. Geosci. Remote Sens. Symp., 2019, pp. 274-277. [4] O. Frey, C. L. Werner, A. Manconi, and R. Coscione, Measurement of surface displacements with a UAV-borne/car-borne L-band DInSAR system: system performance and use cases, in Proc. IEEE Int. Geosci. Remote Sens. Symp.IEEE, 2021, pp.628-631. [5] O. Frey, C. Magnard, M. R\"uegg, and E. Meier, Focusing of airborne synthetic aperture radar data from highly nonlinear flight tracks, IEEE Trans. Geosci. Remote Sens., vol. 47, no. 6, pp. 1844-1858, June 2009. [6] O. Frey, C. L. Werner, and U. Wegmuller, GPU-based parallelized time-domain back-projection processing for agile SAR platforms, in Proc. IEEE Int. Geosci. Remote Sens. Symp., July 2014, pp. 1132-113.
    [bibtex-key = freyWernerManconiCoscioneEGU2022Abstract] [bibtex-entry]


  1672. Achille Capelli, Franziska Koch, Christoph Marty, Patrick Henkel, and Jürg Schweizer. Snow water equivalent measurements with low-cost GNSS receivers along a steep elevation gradient in the Eastern Swiss Alps, 2020. Keyword(s): Data set with DOI, SLF, Snow Water Equivalent, SWE, GNSS. [bibtex-key = swe-measurements-gnss-along-a-steep-elevation-gradient-2020] [bibtex-entry]


  1673. IEEE Std 686-2017 (Revision of IEEE Std 686-2008), Sep. 2017. Keyword(s): IEEE standards, radar, IEEE standard, radar terminology, IEEE Standards, Radar, Aerospace electronics, Dictionaries, Terminology, IEEE 686(TM), radar, terminology.
    Abstract: The promotion of clarity and consistency in the use of radar terminology is the purpose of the definitions provided in this guide. The consensus of a panel of radar experts are represented in the definitions herein.
    [bibtex-key = IEEEStandard686y2017forRadarDefinitions2017] [bibtex-entry]


  1674. FRINGE 2011 Sorted Recommendations, September 2011. [bibtex-key = FRINGE2011SortedRecommendations] [bibtex-entry]


  1675. Xiao Xiang Zhu and Richard Bamler. Super-resolution for 4-D SAR Tomography via Compressive Sensing, 2010. Note: Synthetic Aperture Radar (EUSAR), 2010 8th European Conference on. Keyword(s): SAR Processing, SAR Tomography, Tomography, Compressive Sensing, CS, InSAR, SAR Interferometry, Interferometry. [bibtex-key = Zhu2010] [bibtex-entry]


  1676. PolInSAR 2009 Sorted Recommendations, January 2009. [bibtex-key = PolInSAR2009SortedRecommendations] [bibtex-entry]


  1677. A. Colettaa, D. De Lisle, A. Moreira, A. Freeman, M. Shimada, T. Ainsworth, Konstatinos Papathanassiou, S. R. Cloude, P. Dubois-Fernandez, P. Lombardo, G. Trianni, K. Raney, A. Minchella, F. Charbonneau, L. Ferro-Famil, D. Floricioiu, S. Lehner, Irena Hajnsek, Ridha Touzi, Wolfgang Martin Boerner, Eric Pottier, and Fabio Rocca. Summaries and Recommendations of the POLInSAR 2009 Workshop, January 2009. [bibtex-key = PolInSAR2009Recommendations] [bibtex-entry]


  1678. Andreas Reigber. Multimodale Verarbeitung hochauflösender SAR Daten, February 2008. Note: Habilitationsschrift an der Fakultät IV -Elektrotechnik un Informatik - der Technischen Universität Berlin. Keyword(s): SAR Processing, airborne SAR, omega-k, Range Migration Algorithm, Wave Number Domain Algorithm, Extended Chirp Scaling, ECS, SAR Interferometry, Interferometry, InSAR, Residual Motion Errors, Residual Errors, Motion Compensation, MoComp, PolInSAR, Polarimetry.
    Abstract: Abbildende Radartechnik ist ein Fernerkundungsverfahren, welches das Ziel hat, von einer beobachteten Gegend eine hochaufgel\"oste Reflektivit\"atskarte im Mikrowellenbereich zu erzeugen. Erreicht wird dies durch Abstrahlung und Empfang von elektromagnetischer Strahlung im Mikrowellenbereich, typischerweise durch Sensoren, die auf Flugzeugen oder Satelliten montiert sind. Unter einer ganzen Reihe von Mikrowellensensoren hat sich in den letzten Jahren ein besonderes Interesse in Radar mit synthetischer Apertur (SAR) herausgebildet. Der Grund hierf\"ur ist, dass das SAR als einziger Mikrowellensensor eine fl\"achige Abbildung mit einer hohen r\"aumlichen Aufl\"osung, die durchaus mit der optischer Systeme vergleichbar ist, erm\"oglicht. Die Entwicklungsgeschichte des Radars mit synthetischer Apertur begann bereits vor \"uber 50 Jahren mit der Idee, die Doppler-Verschiebung des Radarsignals zu nutzen, um die Azimutaufl\"osung des damals aktuellen side-looking airborne radar (SLAR) zu verbessern [215]. Zur Prozessierung der Daten war man, bis in die 1970er Jahre hinein, auf die Verwendung optischer, holographischer Verfahren angewiesen; erst danach war man in der Lage, mittels digitaler Datenverarbeitung hochaufgel\"oste SAR Aufnahmen in hoher Qualit\"at zu erzeugen [10],[216]. Seitdem entwickelte sich die Fernerkundung mit SAR Sensoren rasant weiter, hin zu immer h\"oheren Aufl\"osungen und Aufnahmemodi. In den letzten 10 Jahren gewannen dabei vor allem die mehrkanalige SAR Modi stark an Bedeutung, wie z.B. multispektrales SAR [66], SAR Interferometrie [8] und SAR Polarimetrie [13]. Diese Arbeit besch\"aftigt sich vor allem mit speziellen Datenverarbeitungstechniken solcher mehrkanaliger SAR Daten. SAR Sensoren arbeiten im Mikrowellenbereich des elektromagnetischen Spektrums bei Wellenl\"angen zwischen wenigen Millimetern und mehreren Metern. Betrachtet man das Transmissionsspektrum der Erdatmosph\"are, so stellt man fest, dass bei Wellenl\"angen gr\"osser als etwa 1cm praktisch keine nennenswerte Absorption mehr auftritt. Dies gilt sowohl f\"ur die Luft selbst als auch f\"ur Wolken und kleinere Wassertropfen. SAR Aufnahmen lassen sich daher praktisch unabh\"angig von den aktuell herrschenden Wetterbedingungen generieren, wohingegen Wolken und Nebel f\"ur optische Systeme oft eine grosse Einschr\"ankung darstellen. Als aktives System, das seine eigene Beleuchtung mitbringt, besteht weiterhin keinerlei Abh\"angigkeit von der jeweiligen Tageszeit. Zusammengenommen f\"uhren diese Punkte dazu, dass sich SAR Sensoren besonders gut f\"ur verl\"assliche und regelm\"assige Beobachtungen eignen. Der Informationsgehalt von Radaraufnahmen ist deutlich anders gelagert als der von optischen oder Infrarotsystemen. W\"ahrend im optischen Bereich vor allem die molekulare Zusammensetzung des Objekts f\"ur die charakteristische Reflektivit\"at des Objekts verantwortlich zeichnet, sind im Mikrowellenbereich vor allem die geometrische Form sowie die dielektrischen Eigenschaften f\"ur die St\"arke der R\"uckstreuung von Bedeutung. In Radaraufnahmen tritt daher das Relief und morphologische Strukturen besonders deutlich hervor. Auch \"Anderungen in der Leitf\"ahigkeit, z.B. durch unterschiedliche Bodenfeuchte, k\"onnen so beobachtet werden. Aufgrund der Sensitivit \"at auf dielektrische Eigenschaften k\"onnen im Prinzip sogar Informationen \"uber den Vegetationszustand gesammelt werden.
    [bibtex-key = ReigberHabil2008:MultimodalSAR] [bibtex-entry]


  1679. Daniel Svensson and Jan Johansson. Suppression of Radio Frequency Interference in Low Frequency SAR. Master's thesis, Chalmers University of Technology, 2004. Keyword(s): SAR Processing, RFI Suppression, CARABAS, Airborne SAR, Analogue TV, VHF SAR.
    Abstract: To acquire high resolution in airborne radar imaging, either minuscule wavelengths or incredibly large antennas must be used. Another way of solving that issue is to utilise moving radar to realise the effect of a large aperture; a technique called synthetic aperture radar (SAR). CARABAS is a low frequency SAR system, operating in the band of 20 ? 90 MHz, designed to minimise the influence from speckle by using wavelengths of the same order as the resolution cells of the ground image. Radio frequency interference (RFI) is, however, highly prevalent in that frequency band, and must be filtered before the SAR image formation is initialised. The purpose of this thesis is to investigate a proposed method of suppressing RFI, particularly from analogue TV, with a technique based on Doppler compression filtering, to conclude if that technique is applicable. The technique is based on the fact that temporal variations in the TV signal can be predicted. Thus, by synchronising the radar transmission to the line synchronisation pulse of the TV signal, the interference after pulse compression will be concentrated to a component at DC and components at the field rate and its harmonics, in the slow time unfolding of radar data, which enables efficient notch filtering in the range and Doppler domains. The thesis has been carried out at Ericsson Microwave Systems AB in M?lndal, under the supervision of Dr. Hans Hellsten, the originator of CARABAS. The thesis work comprised thorough literature studies, profound measurements of DVD signals, representing ordinary TV signals, implementation of the proposed RFI suppression system in MATLAB, and analysis of the system, regarding attained suppression for different parameter sets. From the results of the thesis it can be concluded that the suppression technique is indeed applicable, but that not all parameter sets provide the required mitigation of 30 dB. That requirement was only fulfilled for two cases: for one second of integration time, together with 4 Hz null-to-null notch widths, and for two seconds of integration time, together with 2 Hz notch widths ? both with a PRF of 15 kHz and a complete Doppler band-pass filtering outside the interval of ?100 to 100 Hz.
    [bibtex-key = svenssonJohansson04:RFI] [bibtex-entry]


  1680. Betlem Rosich. Preliminary Doppler Analysis on ASAR Products, 2002. Keyword(s): SAR Processing, Doppler Centroid, Doppler Centroid Estimation, Clutterlock, ASAR, ENVISAT.
    Abstract: This paper summarised the different Doppler estimation algorithms used in PF-ASAR for the different modes, how this information appears on the ASAR products and provides a first comparisons between measured and expected Doppler as well as a preliminary estimation of the in-flight antenna miss-pointing based on wave mode data products.
    [bibtex-key = rosich:asarDoppler] [bibtex-entry]


  1681. David T. Sandwell. SAR Image Formation: ERS SAR Processor Coded in Matlab. Note: Lecture Notes - Radar and Sonar Interferometry, 2002. Keyword(s): SAR, SAR Processing, Digitizing, ERS, MATLAB, Range-Doppler, Range Compression, Range Migration, Azimuth Processing, SLC, Raw Data, Parameter Files, Squinted SAR, Example Processor. [bibtex-key = San02d:SAR] [bibtex-entry]


  1682. Clint Slatton. Improving Segmented INSAR Processing Using Presumming. , 1998. Keyword(s): SAR Processing, InSAR, TOPSAR, Presumming, Range-Doppler Algorithm, Azimuth Processing, omega-k Algorithm.
    Abstract: Literature Survey
    Comments: +/- Not much on presumming, but generally a good introduction to some SAR concepts.
    [bibtex-key = Sla98:Improving] [bibtex-entry]


  1683. Carl A. Wiley. Pulsed doppler radar methods and apparatus, July 1965. Note: US Patent: US3196436 A. Filing date: Aug 13, 1954. Publication date: Jul 20, 1965. [bibtex-key = wiley1965PatentUS3196436_SAR] [bibtex-entry]


  1684. Groundprobe: https://www.groundprobe.com. [bibtex-key = goundprobeUrl] [bibtex-entry]


  1685. IDS Georadar: https://idsgeoradar.com. [bibtex-key = idsgeoradarUrl] [bibtex-entry]


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Last modified: Fri Feb 24 14:25:37 2023
Author: Othmar Frey, Earth Observation and Remote Sensing, Institute of Environmental Engineering, Swiss Federal Institute of Technology - ETH Zurich .

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