The upcoming ROSE-L synthetic aperture radar (SAR) mission employs a large array antenna with multiple azimuth channels on receive, enabling the usage of different beamforming algorithms on ground. Recent analyses have shown the feasibility of retrieving along-track deformation with the ROSE-L system using two-look ScanSAR, however with limited accuracy towards the burst edges due to reduced antenna gain and increased azimuth ambiguities. With beamforming being a state-of-the-art technique to adapt the characteristics of antenna arrays, this letter presents and analyses a cascaded beamforming approach to improve the retrieval performance. It therefore analyses azimuth ambiguities and system noise and their impact on the performance. It is shown that the increased SNR leads to an increased interferometric coherence for scenes with low SNR, thus improving the accuracy of the retrieved along-track deformation in this case.

This article provides an overview of the state-of-the-art and future developments in spaceborne synthetic aperture radar (SAR). Today, we are experiencing a golden age of spaceborne SAR, with the number of satellites in orbit increasing rapidly. This article presents novel technologies and mission concepts associated with innovative imaging modes, which are required to meet the stringent user requirements for improved performance, global coverage, faster revisit times, enhanced spatial resolution, and increased information content in SAR imagery. It is shown that key technologies and mission concepts, based on digital beamforming (DBF), bistatic and multistatic SARs, distributed SAR, and multiple-input multiple-output SAR (MIMO-SAR), will boost the performance and capabilities of the future generation of spaceborne SAR systems. In addition, this article addresses dedicated mission designs, technologies, and orbit concepts that are required to fulfill the specific user requirements. Two main streams in the spaceborne SAR development are presented alongside a tradeoff analysis: 1) NewSpace SAR, consisting of small satellites in the 85-kg to 250-kg weight class with the highest-resolution imagery and very short revisit times and 2) full-fledged SAR satellites with global coverage, high performance, and enhanced capabilities. It is shown that NewSpace and full-fledged SAR satellites are complementary in terms of user requirements, and that combining both mission concepts enables novel mission concepts, such as multistatic SAR, opening the door to a wealth of new applications. This article concludes with an outlook on the future of spaceborne SAR.

High-resolution wide swath (HRWS) synthetic aperture radar (SAR) systems are normally designed to have identical antenna patterns in each receive channel. Nevertheless, due to different factors, this condition might not be satisfied, resulting in a multichannel radar system with different antenna patterns. This letter studies the impact of these relative antenna differences on the performance of a state-of-the-art azimuth motion-adaptive image reconstruction for a real airborne SAR sensor with multiple azimuth channels on receive. The performance of the reconstruction algorithm is evaluated both when these relative differences are neglected and when they are accounted for in the multichannel reconstruction process. Additionally, the range dependence of the antenna pattern as a function of wavenumber and the use of range block processing to minimize the impact of this dependence are analyzed. The results confirm the relevance of including these additional steps in the reconstruction, extending the understanding of SAR systems using digital beamforming (DBF) in azimuth.

The effectiveness of swath and Doppler spectrum reconstruction algorithms in cross-track and along-track multistatic synthetic aperture radar (SAR) formations heavily relies on the precise relative positioning of each spacecraft. Ideally, a static array configuration is desired, where the satellites are evenly distributed in an Earth-fixed frame. However, maintaining this ideal configuration is not feasible without constant active flight control to counter the satellites’ relative dynamics. Alternatively, one can design natural trajectories that result in the desired formation geometry within an acceptable margin for a significant fraction of the orbital period. In this paper, we present a relative motion description in an Earth-fixed geometry for distributed SAR concepts. We derive a general expression for the transformation matrix from the Hill-Clohessy-Wiltshire (HCW) frame to the SAR-appropriate zero-Doppler (ZD) frame, and propose a general optimization method to fit the best natural non-drifting formation solution for any desired formation configuration. By applying the developed technique to optimize along-track and cross-track relative positioning requirements, we obtain conforming baselines over extended orbit stretches. We present SAR imaging examples for along-track distributed formations and formations with small cross-track baselines, using distributed platforms spanning a 72° latitude range. The demonstrated configurations achieve orbit duty cycles exceeding 20%, with baseline errors kept below 5% relative to the ideal array element positions. These results indicate the feasibility of these concepts using natural orbit solutions.

This paper addresses the critical issue of phase synchronization in multistatic SAR. We present the experimental validation of a GNSS-based synchronization technique planned for use in ESA’s upcoming Earth Explorer mission, Harmony. In this technique, the radar payload and GNSS receiver utilize the same master oscillator, and radar synchronization is achieved through post-processing of carrier phase data from the GNSS receiver and precise baseline determination outputs. The paper presents an experimental procedure that serves as a general proof-of-concept of the technique, a method for assessing the achievable synchronization accuracy for a given GNSS receiver, and a method to estimate the covariance matrix to optimize the weighting between the various carrier phase observables. We present point-to-point estimation and smoothing approaches. The technique achieved in a lab environment relative synchronization errors below 515 fs (1 σ), or 1 degree for a 5.4 GHz radar system, in a zero-baseline scenario, and below 1.5 degrees at 5.4 GHz in a short baseline scenario, in which the systems are physically separated.

A new generation of spaceborne synthetic aperture radar (SAR) systems with digital beamforming capabilities is currently under development. In particular, multiple azimuth channels provide access to a large Doppler bandwidth, which can be used to improve the azimuth resolution. In the ScanSAR mode, however, the resolution is given by the burst time. But the exploitation of a large Doppler bandwidth leads to a wider azimuth coverage and, therefore, to a larger overlap between consecutive bursts. This overlap can be exploited interferometrically in order to retrieve an improved measurement of the azimuth differential shift between two acquisitions. The retrieval performance is hereby limited by the antenna pattern, which can severely deteriorate the image quality towards the edges of the extended bursts. The reduced quality leads to a decreased deformation retrieval accuracy in these parts of the acquisition. We analyze the performance of the along-track deformation retrieval using a two-look ScanSAR processing for the particular case of the upcoming ROSE-L mission. For this purpose we analyze the influences of thermal noise and azimuth ambiguities on the performance and show their impact on the retrieval. The derived performance is validated with simulations using distributed targets and realistic scenes. The results show that the retrieval is subject to a tradeoff between spatial resolution and retrieval accuracy. However, even for low multilooking factors, the azimuth displacement is retrieved with a better accuracy than single-look approaches. The retrieval accuracy is also shown to be sufficient in most cases in order to decouple the along-track and across-track components of the deformation at medium resolutions. In this way, the phase jumps between bursts usually observed in interferograms of non-stationary scenes are significantly mitigated.

A significant level of range ambiguity disturbance may affect high-resolution wide-swath spaceborne synthetic aperture radar (SAR) systems with multiple elevation receive beams. To overcome this limitation, a promising approach, based on the higher order blind source separation (BSS), was recently proposed. This method, denoted as range ambiguity BSS (RABSS), demonstrates outstanding range ambiguity suppression performance when applied to heterogeneous scenes. Nevertheless, a degradation is expected for relatively homogeneous scenes. In this letter, the RABSS method is extended by introducing a novel mixing matrix calibration procedure. The robustness of the proposed approach towards different types of imaged surfaces is numerically analyzed by means of realistic simulations, which rely on the backscattering coefficients of TerraSAR-X images and the simulated antenna patterns of a large array-fed reflector antenna employing multiple elevation beams.

High Earth orbit Synthetic Aperture Radar (SAR) systems offer high temporal sampling and moderate spatial resolution on a global scale, potentially outperforming conventional Low Earth Orbit (LEO) systems in revisit times. However, this requires complex system architectures such as burst operation modes with multiple subswaths, large antennas, and digital beamforming. Similar temporal sampling and coverage enhancements can be realized with constellations of classical monostatic SAR instruments in LEO. This letter compares the complexity of such equivalent monostatic LEO-SAR constellations to complex high-altitude SAR systems and provides design numbers for two Medium Earth Orbit (MEO)-SAR mission examples and their LEO counterparts.

Advanced multichannel spaceborne synthetic aperture radar (SAR) systems with multiple elevation beams allow to obtain high-resolution and wide-swath SAR images. However, the degradation of the SAR image quality due to range ambiguities is still an issue. In this paper, the range-ambiguity problem is analytically modeled and solved by employing a range-time and Doppler-frequency dependent mixing matrix which describes the linear superposition of multiple mutually range-ambiguous radar echoes. The overall framework can be regarded as a generalization of the cocktail party problem where each listener has to separate one speech signal out of a linear superposition of multiple voices. For the first time, the two-dimensional dependence of the antenna radiation pattern in the elevation and azimuth directions is considered by accounting for both the real-time beamsteering in elevation and the systematic variations of the mixing coefficients with Doppler frequency. A novel solution, based on higher-order blind source separation, is presented. The performance of the proposed method is numerically analyzed, with reference to an array-fed reflector antenna SAR system, by simulating a realistic acquisition scenario. To this aim, real SAR data and the actual antenna patterns of the Tandem-L mission proposal are considered.

The use of airborne synthetic aperture radar (SAR) to demonstrate high-resolution wide swath (HRWS) operational modes in spaceborne SAR missions has supported the development of advanced digital beamforming (DBF) techniques. In doing so, one of the challenges to overcome is the temporal variation of the antenna phase centers in the airborne DBF SAR scenario, which significantly degrades the performance of the azimuth reconstruction. Multiple motion compensation (MoCo) solutions have been explored to correct these inconsistencies. However, the compensation of residual phase errors in the Doppler domain remains unresolved when the multichannel data has an aliased azimuth spectrum. This article proposes an algorithm that exploits the properties of the DBF azimuth reconstruction to correct these residual motion inconsistencies, although the channels are undersampled. The algorithm modifies the input range-compressed multichannel data by using an innovative MoCo technique to compensate the phase components coming from undesired 3-D time-variant baselines between the different apertures. Furthermore, a two-step azimuth reconstruction configuration is implemented to account for the polychromatic nature of SAR signals. To test the performance of the algorithm, point target simulations were carried out, in which the impact of a realistic across-track motion, inaccuracies in the digital elevation model (DEM), and variable velocity are analyzed. The results confirm the efficacy of the proposed technique in azimuth ambiguity suppression, where excellent ambiguity suppression is observed after applying the proposed MoCo technique. Finally, the outcome of the simulations is validated with real multichannel data acquired by the German Aerospace Center (DLR) airborne DBFSAR system.

The recently proposed frequency scanning (F-Scan) technique, together with the new International Telecommunication Union (ITU) allocation of 1200 MHz in X-band, enables the improvement of important performance parameters of synthetic aperture radar (SAR) acquisitions, such as the swath width, the signal-to-noise ratio, and the range ambiguity-to-signal ratio. The concurrent imaging technique, in turn, increases the flexibility of the radar system by allowing the simultaneous imaging of two or more areas with independent imaging modes. The integration of both techniques, therefore, enables the already valuable concurrent mode to achieve much better performance. Furthermore, motivated by the high-resolution wide-swath (HRWS) mission proposal in X-band, the displaced phase center antenna (DPCA) technique is considered to improve the azimuth resolution by using multiple receive (Rx) channels in azimuth. In this article, we discuss the design and performance of a new concurrent imaging mode that is enhanced by the F-Scan and DPCA techniques to make simultaneous imaging more flexible and powerful. Special attention is given to timing, range ambiguities, and availability aspects. The capability to simultaneously acquire two high-quality images with noteworthy flexibility is innovative and of great value, not only in daily operational applications but especially in extraordinary and crisis situations.

The feasibility of single-pass interferometric or multistatic SAR mission concepts largely relies on the ability to achieve the matching of the carrier phase of the different elements of the constellations within a few degrees. We put forward a phase synchronization scheme in which the GNSS receiver and the radar payload share the same oscillator; the estimation of the synchronization phase follows from the combined evaluation of the navigation raw data and the precise orbit and baseline determination solutions. The paper presents a discussion on accuracy, an error analysis, and an evaluation of its viability using a system-oriented simulation of the navigation data. The results suggest the proposed approach is capable of delivering reliable estimates of carrier frequency and phase errors for low-and medium-frequency systems in the absence of strong baseline velocity deviations if multipath and other systematic errors are successfully suppressed or calibrated.

The effectiveness of swath reconstruction algorithms in cross-track multistatic synthetic aperture radar (SAR) constellations heavily relies on the precise positioning of individual spacecrafts. Ideally, a static array configuration is desired, where the satellites are evenly distributed in a Local Vertical Local Horizontal (LVLH) frame, emulating a fixed antenna. However, achieving this ideal configuration is not feasible without the implementation of constant active control to counter the relative dynamics between the satellites. Alternatively, it is possible to design natural trajectories that yield the desired formation geometry for a significant portion of the orbital period.In this paper, we present a comprehensive analysis of the feasibility of a continuous control concept for building a static array in space. Additionally, we propose suitable formation strategies that eliminate the need for continuous control.

Geosynchronous synthetic aperture radar (SAR) missions offer the advantage of conducting multiple sub-daily interferometric acquisitions using a single platform. However, this comes at the expense of increased fuel and power budgets, due to the significant distance to Earth’s surface. Consequently, such missions are suited for delivering SAR products with relatively lower spatial resolutions, typically in the range of hundreds of meters, over large localized areas spanning more than one million square kilometers. Alternatively, a constellation of small SAR satellites operating in low Earth orbits (LEO) can also deliver similar interferometric products. In LEO, the reduced free-space propagation losses and simpler orbit insertion can be exploited to realize and operate a larger constellation of LEO-SAR satellites. This paper provides an equivalence framework between the two alternatives in terms of interferometric lags and coverage. It further outlines an example mission concept, equivalent to Hydroterra, one of ESA’s Earth Explorer 10 candidate missions.

In the last decade we entered into a golden age for spaceborne synthetic aperture radar (SAR) systems with the number of satellites increasing in a quasi-exponential way. The reason is obvious: SAR data utilization and associated services are making a most important contribution in addressing societal challenges of global dimension since spaceborne SAR is the only sensor technology that is able to provide high-resolution images on a global scale independent of weather conditions and sunlight illumination. This paper provides a short overview on the latest developments including new technologies and progress on digital beamforming in azimuth and elevation, bistatic and multistatic mission concepts as well as NewSpace SAR satellites.

This paper provides an overview of the state of the art and an outlook on future developments of spaceborne Synthetic Aperture Radar (SAR) systems with multi-baseline imaging capability, such as 3D differential SAR interferometry (3D-DinSAR), polarimetric SAR interferometry (Pol-InSAR), tomography (TomoSAR), and holography (HoloSAR). The goal is to fill the multidimensional data space with additional information from images with different spatial and/or temporal baselines.

Synthetic aperture radar (SAR) interferometry is an essential remote sensing technique with a wide range of applications. Studying and monitoring dynamic processes on the Earth’s surface requires digital elevation models (DEMs) at short time intervals, making distributed and multi-static SAR systems a very promising solution to this need. This work introduces a concept for distributed SAR interferometry using a cluster of smallsats with small antenna apertures operating with a pulse repetition frequency much smaller than the Doppler bandwidth and capable of single-pass DEM generation with high accuracy and robustness to phase unwrapping errors. The novelty of the proposed method is that the DEM is extracted from a tomogram formed using all available data without requiring as an intermediate step the formation of SAR images. This allows reducing the number of required satellites and obtaining increased baseline diversity at the same time. A cluster of smallsats is a very attractive low-cost solution for the implementation of future interferometric SAR missions.

We present in this paper an algorithm for the estimation of synchronisation phase errors in bistatic and multistatic SAR systems with azimuth diversity, such as multi-channel architectures or azimuth constellations, solely based on the evaluation of the received bistatic data. The suggested algortihm working with unfocussed SAR data shows more robustness against processing errors when compared to the current literature. The relevance of the approach is enhanced by the current interest in both multi-channel SAR architectures and companion SAR missions.

We present in this paper a sensitivity analysis of swath reconstruction algorithms for multistatic SAR constellations, mainly focused on geometric aspects of the formation, such as the irregularity of the sampling and the tilt.

Spaceborne synthetic aperture radar (SAR) is an essential tool for Earth observation. The combination of multiple SAR images taken from different viewing angles allows forming accurate digital elevation models and high-resolution tomograms that unveil the three-dimensional structure of vegetation, ice, and dry soil. Whereas nowadays such images are mostly acquired sequentially, compromising product quality and hindering the monitoring of fast dynamics, multi-static SAR systems enable the simultaneous acquisition of all required data, paving the way for effective and powerful monitoring of our planet. One fundamental challenge is the conception of multi-static concepts that allow the generation of high-quality digital elevation models and tomograms from large sets of noisy and undersampled radar data acquired by clusters of smallsats with small antenna apertures. Swarms of drones, equipped with radars and localization systems, represent an affordable option to test multi-static concepts and acquire multi-static data with sub-hour temporal resolution.

The Venus Interferometric Synthetic Aperture Radar (VISAR) is one of two instruments carried by the VERITAS Discovery Mission to Venus that was selected by NASA in 2021 [1] , [2] . VERITAS (Venus Emissivity, Radio Science, Insar, Topography And Spectroscopy) is a partnership between scientists and engineers at NASA/JPL in an international cooperation with the Germany Aerospace Center (DLR), the Italian Space Agency (ASI) and the French Space Agency (CNES). VISAR aims to be the first to image Venus at 30 m resolution on a global scale and deliver a digital elevation model at 6 m height accuracy in addition to proving interferometric deformation maps of activity on another planet. The VISAR instrument has several interesting features and challenging aspects. The focus is put on SAR performance and the options for the radar operation and imaging mode parameters given the constraints inherent to a planetary mission.

Highly accurate digital elevation models (DEMs) from spaceborne synthetic aperture radar (SAR) interferometry are often affected by phase unwrapping errors. These errors can be resolved by the use of additional interferograms with different baselines, but this requires additional satellites in a single-pass configuration, resulting in higher cost and system complexity, or additional passes of the satellites, which affects mission planning and makes the system less suitable for monitoring fast-changing phenomena. This work proposes augmenting a bistatic SAR interferometer with one or more receive-only CubeSats, whose images are used to form an additional interferogram with a small baseline, making the system robust to unwrapping errors. In spite of the lower quality of the CubeSat images due to their small antenna aperture, this additional information can be used to detect and resolve phase unwrapping errors in the DEM without impacting its resolution or accuracy. A processing scheme for the phase unwrapping correction is presented along with a theoretical model for its performance. Finally, a design example is presented and discussed along with a simulation based on TanDEM-X data. It is also shown that CubeSat add-ons allow further increasing the baseline and thus improving the accuracy of DEMs. This concept represents a cost-effective solution for the generation of highly accurate, robust DEMs and paves the way to distributed SAR interferometric concepts based on CubeSats.

Radio Frequency Interference (RFI) is a growing problem for future Synthetic Aperture Radar (SAR) missions, resulting in data loss, image artifacts and undetected biases. A new approach for mitigating RFI is digital beamforming (DBF), which is possible with the next generation of multichannel SAR systems and allows for a spatial filtering of signals from different directions. While on-board RFI removal with DBF is challenging for spaceborne systems due to the computational load, past publications have shown that this problem can be overcome with DBF-based auxiliary beams by moving most of the processing to the ground without requiring the down-linking of all channels. A remaining problem of measuring RFI information with auxiliary beams is determining the interferer position. This paper shows the performance simulation of a series of digital square-law detectors which can be used to overcome this issue. It is shown that a series of digital square-law detectors provides the opportunity to simultaneously determine if RFI is present and under which direction, while maintaining a low system complexity. This is possible because the detectors provide a good probability of detection and false alarm rate even if the data rate is decimated. The simulated system performs well for a decimation factor of 110.

The paper provides an up-to-date overview of the German TanDEM-X satellite mission status and its ongoing science activities. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. In addition, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The acquisition for the global change layer was completed by mid-2020. Since then a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions are ongoing.

In March 2021 a field campaign was conducted in the Austrian Alps exploring the application potential and performance of snow mass (snow water equivalent, SWE) retrievals by means of repeat-pass radar interferometry (RP-InSAR). Multiple repeat-pass acquisitions were obtained with an airborne polarimetric C-band and L-band SAR, covering days without snowfall, as well as two snowfall events of different intensity. SWE maps, deduced from the observed change of the RP-InSAR phase delay in the snowpack, show good agreement with field measurements. Whereas L-band data are well applicable for SWE mapping of both snowfall events, the 2 π phase ambiguity and low coherence prevent the use of C-band data for the snowfall event of high intensity. The analysis of the campaign data is going on, addressing among other topics the application of RP-InSAR for SWE monitoring within the planned L-band SAR mission ROSE-L of the European Copernicus program and by means of geostationary C-band SAR as proposed for the Hydroterra mission.

An overview of the German spaceborne X-Band radar program will be presented in this paper. A view into the wide heritage of the German synthetic aperture radar (SAR) instruments and the paramount results obtained with the German X-band Missions will be provided. This paper will cover the Shuttle Imaging Radar missions (SIR-C/X-SAR) from 1994 and the Shuttle Radar Topography Mission (SRTM) from 2000, followed by the current missions TerraSAR-X from 2007 and TanDEM-X launched in 2010. Since then, the TanDEM-X mission is providing high-resolution X-band images for scientific, commercial and governmental applications. The milestones of the TanDEM-X mission, like the generation of a global, high-resolution digital elevation model of the Earth surface, will be presented. The paper will also discuss the challenges faced by the current mission under planning: High Resolution Wide Swath (HRWS), which is a very ambitious multi static SAR system exploiting the formation flight of one active main satellite and three smaller passive companion satellites. With HRWS, the MirrorSAR concept will be implemented in space for the first time. Finally, the future of the German SAR Program will be discussed.

An intrinsic challenge in the geophysical interpretation of low-frequency synthetic aperture radar (SAR) imagery of semitransparent media, such as ice sheets, is the position ambiguity of the scattering structures within the glacial volume. Commonly tackled by applying interferometric and tomographic techniques, their spaceborne implementation exhibits by orders higher complexity compared to missions relying on single SAR images, making them cost expensive or, in the context of planetary missions, even impossible due to limited navigation capability. Besides, even these sophisticated techniques are commonly biased due to inaccurate permittivity estimates, leading to geometric distortions up to several meters. We present a novel inversion procedure to estimate volume parameters of ice sheets, namely, the depth of the scattering layer within the glacial volume and the dielectric permittivity of the ice, based on single-image single-polarization SAR acquisitions. The information is inherent in the processed SAR data as phase errors on the azimuth signals resulting from uncompensated nonlinear propagation of the radar echoes through ice. We suggest a local map-drift autofocus approach to quantify and spatially resolve the phase errors and an inversion model to relate them to the penetration depth and permittivity. Testing the proposed technique using P-band SAR data acquired using DLR’s airborne sensor F-SAR during the ARCTIC15 campaign in Greenland shows promising results and good agreement with tomographic products of the analyzed test site.

In the above article[1], references [21] and [29] incorrectly provide the publication years. Reference [21] was published in 2020; reference [29] was published in 2021. The references appear in full here:

Radio frequency interference (RFI) is becoming a major concern for future synthetic aperture radar (SAR) missions due to the increased user demand for frequency occupation in a number of applications. Each occurrence of interference introduces artifacts in the radar imagery, biasing the measurements and leading to erroneous results. In addition to conventional techniques, the use of multichannel SAR for RFI mitigation has been proposed, because its digital beamforming (DBF) capability allows for a spatial filtering of the received signals. Thereby, it becomes possible to remove RFI that arrives from a different direction than the SAR signal. Past publications on the topic presented highly flexible spatial filtering techniques. Those methods require either additional on-board processing or a substantial increase in the downlink capacity. This article shows that by slightly reducing the flexibility of the spatial filtering, DBF can be utilized for RFI mitigation without either drawbacks: the processing is performed on-ground after downlinking the data and the data volume remains manageable. This is achieved with auxiliary beams. Their concept and limitations are discussed in detail in this article and are supported with simulated RFI mitigation results. Furthermore, it is shown that the information collected with an auxiliary beam can also be used to filter the RFI signal when it is spatially nonorthogonal to the SAR signal.

This article reports the Phase A study results of the interferometric extension of the high-resolution wide-swath (HRWS) mission with three MirrorSAR satellites. According to the MirrorSAR concept, small, low-cost, transponder-like receive-only satellites without radar signal demodulation, digitization, memory storage, downlink, and synchronization are added to the planned German X-band HRWS mission. The MirrorSAR satellites fly a triple helix orbit in close formation around the HRWS orbit and span multiple single-pass interferometric baselines. A comprehensive system engineering and performance analysis is provided that includes orbit formation, MirrorLink, Doppler steering, antenna pattern and swath design, multi-static echo window timing, SAR performance, height performance, and coverage analysis. The overall interferometric system design analysis of Phase A is presented. The predicted performance of the global digital elevation model (DEM) is improved by one order of magnitude compared to presently available global DEM products such as the TanDEM-X DEM.

In the constant strive for improving the capacity of next-generation spaceborne SARs while containing the increase in system complexity, distributed along-track constellations operated below Nyquist appear as one of the most promising solutions for delivering metric resolution imagery over swaths of hundreds of kilometers. In the operation of multistatic SAR constellations, however, sampling singularities typically result in dramatic noise scaling and the impossibility to recover the entire Doppler bandwidth unambiguously. We investigate in this article the likelihood of these singularities in the case of along-track distributed constellations and justify the use of irregular sampling schemes to reduce the percentage of invalid samples of an acquisition. This article also presents a bistatic polychromatic reconstruction algorithm, which is used for the evaluation of the performance of along-track distributed constellations. With all these elements, the performance of along-track multistatic systems is assessed by means of a Monte Carlo analysis and conclusions on the scaling numbers of the constellations are drawn. Based on the performance investigations, an exemplary distributed L-band SAR constellation is proposed as a corollary of this article.

Synthetic aperture radar (SAR) interferometry is a well-established technique for producing high-resolution digital elevation models (DEMs) of the Earth’s surface and measuring displacements on different time scales. Observations of SAR interferograms, however, show that azimuth ambiguities can be coherently imaged and may lead to phase biases and coherence losses that significantly degrade the interferometric performance. Whereas imposing very low ambiguity levels may represent a severe design constraint for a spaceborne SAR system, a slight variation of the pulse repetition interval (PRI) is a new, simple, yet effective technique to decorrelate ambiguities, which in turn reduces the phase biases and coherence losses without substantially affecting the imaged swath width. An additional benefit of the PRI variation is that range ambiguities also become decorrelated. This paper addresses two cases: For the repeat-pass case, slightly different pulse repetition frequencies (PRFs) can be used for the two acquisitions and the minimum required PRF difference can be analytically derived resorting to the power spectral density of the ambiguous signals; For the single-pass case, a slight variation of the PRI during the common acquisition is an effective solution, in case an along-track baseline is present. In particular, a square wave PRI variation scheme outperforms sinusoidal or random ones. Finally, simulations using TanDEM-X data are presented to show the improvement in interferogram and DEM quality resulting from ambiguity decorrelation. This work is relevant for the design of future spaceborne interferometric SAR systems and for the enhanced exploitation of current ones.

The synthetic aperture radar (SAR) imaging mode described in this paper utilizes the available signal bandwidth to form a narrow frequency-scanning transmit antenna beam illuminating the swath of interest from far to near range. The imaging technique is named frequency Scan for Time-of-Echo Compression (f-STEC), because, for a proper choice of mode parameters, the radar echo duration is reduced, i.e., compressed. The paper provides a detailed analysis of the f-STEC imaging technique; derives the operational and performance parameters as well as — to the authors knowledge — for the first time, analytic expressions for the f-STEC timing constraints. Further, the performance and trade-space is reported and compared to both conventional and modern, i.e., digital beamforming imaging modes. The f-STEC imaging technique is shown to be specifically advantageous for SAR systems operating at higher carrier frequencies or an attractive add-on for state-of-the-art SAR instruments.

Hydroterra has been considered as one of three candidates for ESA's Earth Explorer 10 mission, and consists of a geosynchronous Synthetic Aperture Radar to measure key processes of the diurnal water cycle. These measurements would improve the prediction of intense rainfall, flooding and landslides. This concept has been the subject of Phase 0 studies, in which the mission requirements were consolidated and initial system design possibilities were investigated. This paper summarizes some of the elements of the mission concept contributing to the requirement consolidation process, namely the orbit design aspects, with focus on potential sub-sidereal interferometric capabilities, and the optimization performed in the bandwidth/integration time trade-space to define the operational scenarios and coverage zones, which are key inputs for the mission design.

High-resolution and wide swath are known to be inherently incompatible requirements for spaceborne synthetic aperture radar (SAR) imaging systems. Extensive research shows that advanced instrument modes and architectures employing multiple receive channels in elevation and/or azimuth represent a feasible solution for this conflict. Currently, high SAR performance alternatives include ScanSAR systems with multiple channels in azimuth and Staggered-SAR systems with Scan-on-Receive (SCORE). The first are subject to Doppler spectral gaps and the undesirable scalloping effect. Staggered-SAR shows the considerable advantage of a complete Doppler spectrum, but at the cost of non-uniform sampling. This drives up the sampling rate and as a consequence the complexity, as on-board processing becomes necessary for data-rate reduction. This paper discusses a method to achieve high-resolution imaging using complementary coverage of a wide swath with two (interleaved) constant-PRI sequences, thus without incurring Doppler spectral gaps but also without paying the system-complexity penalty of irregular sampling.

This paper provides an overview of the German spaceborne radar program starting with the X-band synthetic aperture radar (SAR) instrument on board the Shuttle Imaging Radar missions (SIR-C/X-SAR) in 1994, followed by the Shuttle Radar Topography Mission (SRTM) in 2000. The German national satellite radar program began in 2007 with the launch of the satellite TerraSAR-X, which is providing since then high-resolution X-band images for scientific, commercial and governmental applications. TanDEM-X, an almost identical twin, joined TerraSAR-X in 2010 to form the first bistatic SAR interferometer consisting of two satellites in close formation flight. The TanDEM-X mission generated a global, high-resolution digital elevation model of the Earth surface with unprecedented accuracy. The High-Resolution Wide-Swath (HRWS) mission has been approved for realization at the end of 2020. It is a very ambitious multistatic SAR mission exploiting the formation flight of one active main satellite and three smaller passive companion satellites. HRWS implements for the first time in space the novel MirrorSAR concept. As for the current planning, the launch is expected in 2027.

The idea of the former Earth Explorer 10 mission Hydroterra is to place a SAR system into a geosynchronous orbit to understand rapid processes of the diurnal water cycle over regions of interest in Europe and Africa. The DLR Microwaves and Radar Institute conducted an airborne SAR campaign to support the Hydroterra mission. Airborne data were collected to simulate the long integration time of a geosynchronous SAR image and to evaluate the potential to detect diurnal moisture changes. In this paper the airborne campaign is described and the achieved results are presented.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. In June 2020 TanDEM-X reached his 10 years of successful operation - a view forward will be presented in the final paper. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The data collection for the global change layer will be ready by the end of 2020. Currently, a new science phase of TanDEM-X is running with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

This paper discusses multistatic SAR, highlighting the potential of constellations to deliver scalable performance, which allows for the simplification and reduction of the single units in terms of size and power. In particular, the paper illustrates the impact of technical challenges such as navigation control, clock synchronization, and data processing. The paper also includes the discussion of the main applications of multistatic SAR, including the measurement of single-pass interferometric and tomographic signatures as part of the calibration process. The paper is closed with examples of possible constellations offering comparable performance to SAR missions of the present and next generations.

The development of synthetic aperture radar (SAR) instruments at higher carrier frequency is motivated, among others, by the relative wavelength scaling, which provides an increased bandwidth and reduced physical dimensions of the RF hardware, for the same coverage. The later causes a reduced area and a high length-to-height aspect ratio of the antenna, which is unfavorable, since the resulting design compromise leads to systems of small swath width and low signal-to-noise ratio.

In the context of spaceborne synthetic aperture radar (SAR) imaging, high resolution and wide swath are inherently conflicting requirements. These may, however, be simultaneously satisfied by advanced imaging modes with multichannel architectures in elevation and/or azimuth. This article elaborates on a new mode based on multiple elevation beams and a simple pulse repetition interval (PRI) variation scheme which allows high-resolution wide-swath imaging. It is shown to use the illumination time more efficiently than ScanSAR and yet to be simpler than staggered SAR. Good SAR imaging performance is achieved with a rather compact antenna design. The proposed imaging mode is suitable for spaceborne SAR systems with planar and reflector antennas. In order to improve the imaging performance, a reflector antenna architecture with a multichannel feed in both elevation and azimuth is considered.

Modern synthetic aperture radar (SAR) systems for Earth observation from space employ innovative hardware concepts. The key idea is to digitize the output of a multielement antenna almost immediately after the receiver and to dynamically process these data either onboard the radar satellite in real time or on the ground. This article addresses the performance of such digital beamforming (DBF) systems in the presence of phase and magnitude errors in the digital channels. For this, analytic expressions for the sensitivity and range ambiguity performance are derived. These equations are kept general so that they are valid for both planar array antennas and array-fed reflector antennas. It is an important objective of this article to compare these two antenna types to each other. A major conclusion from this analysis is that direct-radiating phased arrays are inherently more susceptible to random phase and magnitude errors compared with array-fed reflector antenna-based systems. This manifests itself in a more rapid degradation of the imaging performance with phased array antennas.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. In June 2020 TanDEM-X will reach his 10 years of successful operation - a view back and forward will be presented in the final paper. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The data collection for the global change layer will be ready by the end of 2020. By that time, it is planned to start a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

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.

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 15λ over swaths of hundreds of kilometers. The validity of our approach has been tested using point targets in two- and six-receiver multistatic configurations.

Synthetic aperture radar (SAR) concepts based on digital beamforming (DBF) in elevation and multiple receive beams, for example, staggered SAR, allow high-resolution imaging of wide swaths and are, therefore, well-suited for the systematic observation of dynamic processes on the Earth's surface. While these concepts are compatible with quad-polarimetric (quad-pol) operation, due to severe ambiguity constraints, the achievable swath in the quad-pol mode is narrower than in the single- (or dual-) pol mode. Therefore, either the single- (or dual-) pol data over a wider swath or the quad-pol data over a narrower swath can be acquired. This article proposes a novel SAR acquisition mode, based on pulse-to-pulse alternation of the antenna pattern on transmit, which delivers at the same time single- (or dual-) pol data over a wider swath and quad-pol data over a narrower swath at the expense of an acceptable degradation of the ambiguity performance. This technique yields remarkable benefits for the design and operation of future SAR systems and represents an effective solution to manage and resolve conflicting user requirements.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The global change layer will be available by 2020. By that time, it is planned to start a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

This paper discusses the relevance and use of efficient focusing in the time domain in spaceborne SAR. With the increasing number of space-variant corrections required by higher resolution and wide swath observations, the efficiency of SAR focusing algorithms in the Fourier domain drops significantly. The main sources of space variance to be faced in future spaceborne SAR missions and their implications on the complexity of the focusing process are discussed. The paper also presents a direct comparison between time-domain and frequency-domain approaches, providing an estimate of the computational burden of a fast-factorized backprojection algorithm compared to a sophisticated numerical ωk, identifying the operation areas where the the former may show an efficiency advantage.

A multi-aperture Synthethic Aperture Radar (SAR) system offers to exceed the capabilities of the conventional SARs in various ways, one of which is to realize high-resolution wide-swath mode imaging when operated under the Nyquist frequency. Due to the low operational pulse repetition frequency, the Doppler spectrum of the received data appears strongly aliased in such systems and needs to be resolved via azimuth reconstruction. Our aim in this paper is to suggest an accurate reconstruction strategy that is applicable to multi-aperture SAR point target simulation in a two-spacecraft constellation. We suggested a two-step reconstruction method and validated it with point target simulation in two-channel multistatic constellation configurations with large along-track baselines and high resolution. The results show that our method is capable of accommodating both the polychromatic and range-variant nature of the azimuth reconstruction.

Synthetic aperture radar (SAR) concepts based on digital beamforming (DBF) in elevation and either multiple azimuth channels or multiple receive beams, e.g., staggered SAR, allow high-resolution imaging of wide swaths and are therefore well suited for the systematic observation of dynamic processes on the Earth's surface. While these concepts are compatible with quad-polarimetric (quad-pol) operation, due to severe ambiguity constraints, the achievable swath in quad-pol mode is narrower than in single- (or dual-) pol mode. Therefore, either single- (or dual-) pol data over a wider swath or quad-pol data over a narrower swath can be acquired. This paper proposes a novel SAR acquisition mode, based on pulse-to-pulse alternation of the antenna pattern on transmit, that delivers at the same time single- (or dual-) pol data over a wider swath and quad-pol data over a narrower swath at the expense of an acceptable degradation of the ambiguity performance. This technique yields remarkable benefits for the design and operation of future SAR systems and represents an effective solution to manage and resolve conflicting user requirements.

Multistatic or distributed satellite systems offer new and unique capabilities necessary for Earth observation with high spatial and temporal resolution. This letter describes a multistatic synthetic aperture radar (SAR) experiment employing the satellites TerraSAR-X and TanDEM-X. In order to demonstrate distributed SAR imaging from space, special data acquisitions with a dedicated geometry were performed. The data evaluation approach is outlined and azimuth profiles over a region with high-contrast backscatter are used to evaluate the azimuth signal reconstruction performance. The results are verified with simulations performed with a flexible SAR simulator reproducing the acquired scene. Finally, the effect of target motion on the reconstruction is analyzed and discussed based on the experimental data.

Burst-mode acquisition schemes achieve wide coverage at the expense of a degraded azimuth resolution, reducing, therefore, the performance on the retrieval of ground displacements in the azimuth direction, when interferometric acquisitions are combined. Moreover, the azimuth varying line of sight can induce discontinuities in the interferometric phase when local azimuth displacements are present, e.g., due to ground deformation. In this contribution, we propose the interferometric 2-look terrain observation by progressive scans (TOPS) mode, a sustaining innovation, which records bursts of radar echoes of two separated slices of the Doppler spectrum. The spectral separation allows to exploit spectral diversity techniques, achieving sensitivities to azimuth displacements better than with StripMap and eliminating discontinuities in the interferometric phase. Moreover, some limitations of the TOPS mode to compensate ionospheric perturbations, in terms of data gaps or restricted sensitivity to azimuth shifts, are overcome. The design of 2-look TOPS acquisitions will be provided, taking the TerraSAR-X system as reference to derive achievable performances. The methodology for the retrieval of the azimuth displacement is exposed for the case of using pairs of images, as well as for the calculation of mean azimuth velocities when working with stacks. We include results with experimental TerraSAR-X acquisitions demonstrating its applicability for both scenarios.

In the design of spacebome synthetic aperture radar (SAR) imaging systems, high-resolution and wide swath are inherently incompatible requirements. As shown by extensive research in this field, advanced instrument modes and architectures employing multiple receive channels in elevation and/or azimuth represent a feasible solution for this conflict. This paper addresses a novel high-resolution wide-swath SAR imaging mode that uses multiple elevation and azimuth beams in combination with a scheme of slow variation of the pulse repetition interval (PRI). It is shown that this new mode enables a high-performance SAR system with a rather compact antenna. As an example, the design of an L-band system is presented that is capable of imaging a 400-km wide swath with an azimuth resolution of 5 m in single polarization. A detailed analysis reveals that an excellent performance is achieved by combining a digital feed with 3 azimuth and 30 elevation channels using an unfurlable reflector with a diameter of 12 m. In addition, the same architecture enables the operation in a multichannel staggered SAR mode, which allows for the mapping of a 200-km wide swath with an even higher azimuth resolution of less than 2.5 m in a fully polarimetric mode.

Tandem-L is a proposal for an innovative L-band SAR mission for the systematic observation of dynamic processes on the Earth's surface. The mission concept is based on two SAR satellites flying in close formation featuring latest digital beamforming techniques in combination with a large deployable reflector for increasing the swath width and imaging resolution. This enables innovative operation modes such as polarimetric SAR tomography for determining the vertical structure of vegetation and ice. With novel imaging and processing techniques and the vast recording capacity of up to 8 Terabyte/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere and will make an essential contribution for a better understanding of the Earth system and its dynamics. This paper gives an overview of the Tandem-L mission project which has successfully passed the intermediate system requirements review of Phase B1.

This paper presents first the performance analysis of the state-of-the-art multi-channel SAR signal reconstruction algorithms that suffer from the geometrical approximations. Then, it proposes a two-step signal reconstruction method for spaceborne large along-track baseline multistatic SAR constellations operated with a pulse repetition frequency (PRF) under the Nyquist rate. The suggested algorithm first applies a bulk polychromatic reconstruction in wavenumber domain and then compensates the residual range variance in range-Doppler domain. This simple modification helps to reduce overall phase errors in the reconstruction process of the analysed case with respect to the state-of-the-art algorithms by about one order of magnitude. The performance of the algorithm is verified by showing the results in point target simulations for a multistatic X-band constellation with a resolution of about 15 times the carrier wavelength.

In the context of spaceborne synthetic aperture radar (SAR) for remote sensing, multichannel system architectures coupled with digital beamforming techniques are deemed a necessary technological advancement to fulfill the requirements for near-future spaceborne radar missions. Calibration of such systems is an important topic, since channel imbalances may lead to considerable degradation of their performance. This letter analyzes the impact of residual errors in a SAR system with multiple channels in azimuth and derives an analytical model for the resulting performance degradation, which may be used in system design as an aid to establish requirements in an error budget analysis.

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 15λ. Finally, we put forward a generalized range-Doppler strategy to accommodate the geometry of distributed along-track constellations in an accurate manner.

Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is, therefore, very attractive for the systematic observation of dynamic processes on the earth's surface. However, as a consequence of the pulsed operation and side-looking geometry of SAR, nadir echoes may significantly affect the SAR image quality, if the pulse repetition frequency is not conveniently constrained in the design of the SAR system. As the nadir interference constraint typically limits both the swath width and the ambiguity performance of the SAR system, the investigation of novel concepts for nadir echo removal is of great interest. This letter describes how to design an SAR system without the nadir interference constraint and how to remove (not only smear) the nadir echoes by means of waveform diversity on transmit and appropriate postprocessing. The proposed technique yields improved image quality and can be exploited in a similar manner for range ambiguity suppression with remarkable benefits for the design of novel SAR systems.

State-of-the-art and future spaceborne synthetic aperture radar (SAR) systems increasingly often face the requirement of providing high-resolution images with reduced revisit times, requiring coverage of wide swaths. Since these are contradicting drivers in terms of system design, different alternatives for high-resolution wide-swath SAR imaging have been investigated, relying on digital beamforming and the use of multiple receiver channels, both in elevation and azimuth dimensions. In this context, staggered SAR, which operates with a pulse repetition frequency (PRF) variation, using a single channel in azimuth proves itself as a promising alternative for covering wide continuous swaths with moderate azimuth resolution, whereas the use of multiple azimuth receiver channels bears the potential of improving the azimuth resolution over a given swath, but has yet only been applied to systems with a fixed PRF. This paper introduces and analyzes in detail processing techniques suitable for the combination of these techniques, leading to novel multichannel staggered SAR imaging modes with the potential for very fine azimuth resolution over ultra-wide swaths. A system concept with 2-m azimuth resolution over a 400-km swath in quad-pol is provided as an example.

Tandem-L is proposed as a spaceborne synthetic aperture radar (SAR) mission developed and operated by the German Aerospace Center in cooperation with several Helmholtz research centers and the German space industries. The mission concept comprises two fully polarimetric radar satellites providing monostatic and bistatic SAR imagery. A key feature of these SAR sensors is the employment of large lightweight unfurlable mesh reflectors fed by digital feed arrays. The main advantage of this new SAR system concept is the provision of large antenna apertures in space and flexible operation via reconfigurable feed electronics. By this, it becomes possible to map, for the first time, a continuous 350-km wide swath with a 7-m azimuth resolution with excellent noise equivalent sigma zero and ambiguity suppression. This paper shall give an overview on the technical aspects of the Tandem-L SAR instrument and antenna design. In particular, after a short review of the SAR system requirements, the concept of reflector SAR systems is outlined and the operation principle is presented. General guidelines for the design of array-fed reflector antennas with application to SAR imaging are given. Then, the optimization approach of the feed array design is detailed with a specific emphasis on a fixed beamforming concept in azimuth. In this context, also the problem of cross-pol pattern mitigation is addressed. These optimization steps are shown to be crucial for achieving the performance requirements in quad-pol acquisitions. Beamforming in elevation is performed onboard the spacecraft via digital hardware. This paper presents the beamforming architecture on receive for Tandem-L, which would apply in general for instance also to planar multielevation beam SAR antennas with Scan-On-Receive capabilities. Tandem-L is operated as a staggered SAR, which means varying the pulse repetition interval from pulse to pulse. In this context, the major design challenges are presented. Moreover, the impact of pulse staggering on the imaging performance is discussed. Tandem-L's SAR performance is presented by means of numerical simulations showing that the performance requirements imposed by the scientific user community could be met. The final part of this paper addresses options for high azimuth resolution imaging as well as a beamforming method for enhanced range ambiguity suppression.

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.

This paper introduces the concept of a fractionated MirrorSAR which is based on a set of mutually separated transmitter and receiver satellites. As opposed to previously published bi- and multistatic SAR systems, the receiver satellites are considerably simplified, as their main functionality is reduced to a kind of microwave mirror (or space transponder) which routes the radar echoes towards the transmitter. The forwarded radar signals are then coherently demodulated within the transmitter by using the same oscillator that had been used for radar pulse generation. This avoids the necessity of a bidirectional phase synchronization link as currently employed in TanDEM-X. Since the needs for fully equipped radar receivers, on-board memory and downlink are also overcome, the weight and costs of the receiver satellites can be significantly reduced. This allows for a scaling of their number without cost explosion, thereby paving the way for novel applications like multi-baseline SAR interferometry and single-pass tomography. Several additional opportunities make the MirrorSAR concept even more attractive. First, the separation of the transmitter and receiver front-ends reduces not only RF losses by avoiding switches and circulators, but it may also lower the peak power in the transmitter satellite by employing a frequency-modulated continuous wave (FMCW) illumination. This simplifies the design of the high-power amplifier and increases its efficiency. Second, the opportunity for continuous radar data collection enables new modes for the imaging of ultra-wide swaths with very high resolution, thereby overcoming an inherent limitation of conventional monostatic SAR systems. Third, the joint availability of all receiver signals in a centralized node offers new opportunities for efficient data compression, as the multistatic radar signals from close satellite formations are characterized by a high degree of mutual redundancy. Fourth, the use of a sufficiently separated transmitter satellite can avoid the risk for mutual illumination, which challenges the design and operation of fully-active multistatic SAR systems. Further advantages arise from the scalability and reconfigurability, which support new redundancy concepts and pave at the same time the way to new modes like MIMO-SAR tomography.

This paper provides a compact overview of SESAME, a mission concept in which two receive-only small Synthetic Aperture Radar (SAR) satellites flying in close formation would allow single-pass interferometric observations using Sentinel-1 as transmitter.

This contribution is dedicated to present tomographic investigations on 3D vegetation imaging for future spaceborne SAR missions. The main problem to tackle when performing tomography via repeat-pass spaceborne data is that the temporal decorrelation between acquisitions can be very severe making it difficult to achieve reliable results. In this context, if two or more sensors are available to perform the surveys, a set of quasi-simultaneous data can be achieved for a certain time instant. It is understood that for such data the temporal decorrelation effect as well as the atmospheric artefacts will be strongly mitigated. By varying the acquisition geometry, it is in principle now possible to achieve cross-range resolution and retrieve the vertical profile via SAR tomography. The present paper focuses on a two-satellite scenario like TanDEM-X [1], Tandem-L [2], SAOCOM-CS [3]. In particular, TanDEM-X data, acquired in a pursuit monostatic mode, is employed to perform the demonstration over boreal as well as tropical forest.

Companion SAR missions offer a cost-effective solution to enhance the added value of existing SAR satellites. In particular, they offer new possibilities to extend the observation space and provide one or several single-pass interferometric channels for the acquisitions. The paper aims to provide a description of an end-to-end performance simulation for companion SAR missions, including the evaluation of two exemplary configurations under realistic conditions and with the incorporation of the major error sources affecting these systems. A critical discussion of the results will be included of the final version of the paper drawing from the analysis relevant conclusions for the design of payload and concepts for future companion SAR missions.

SESAME (SEntinel-1 SAR companion Multistatic Explorer) is a passive SAR satellite mission proposed for the ESA Earth Explorer Program. SESAME comprises two receive-only C-band SAR satellites flying in close formation to build a single-pass SAR interferometer (SP-InSAR) using the active signal of the European Sentinel-1 satellite. The SESAME mission addresses applications in geoscience and climate research that require repeat measurements of high precision elevation data over land surfaces including ice covered areas and forests, exploiting the SP-InSAR and multistatic observation geometry of the satellite formation. The objectives, the measurement approach and geo-biophysical products of the mission are described.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an Earth observation radar mission that consists of a SAR interferometer built by two almost identical satellites flying in close formation [1]-[4]. With a typical separation between the satellites of 120 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at 12 m posting has been generated. While the main mission phase for DEM data acquisition has been finished in 2014, the processing of the global TanDEM-X DEM was concluded in September 2016. Final DEMs are well within specifications and feature an extremely low percentage of void areas. Following the DEM data acquisition the capabilities of this unique mission for new scientific application have been demonstrated. Satellite resources allow for a continuation of the joint TerraSAR-X/TanDEM-X mission for several years. Beyond improvements of the global DEM the mission will be dedicated to the generation of a global 3D information change layer and of the corresponding DEM updates as a demonstration of the future climate research and environmental monitoring mission Tandem-L.

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.

Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is therefore very attractive for the systematic observation of dynamic processes on the earth's surface. Conventional SAR systems are, however, limited in that a wide swath can only be imaged at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive subapertures displaced in along-track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along-track is available, it is still possible to map a wide area: multiple subswaths can simultaneously be imaged using digital beamforming in elevation, but “blind ranges” are present between adjacent swaths, as the radar cannot receive while it is transmitting. Staggered SAR overcomes the problem of blind ranges by continuously varying the pulse repetition interval (PRI). A proper selection of the PRIs, together with moderate oversampling in azimuth, allows an accurate interpolation of the nonuniformly sampled raw data into a uniform grid, so that resampled data can then be focused with a conventional SAR processor. This approach thereby allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple subapertures. In this paper, the performance of staggered SAR is thoroughly discussed and novel methods for the evaluation of the range and azimuth ambiguity-to-signal ratios in staggered SAR are proposed. An L-band design example based on a reflector antenna with multiple feeds shows that outstanding ambiguity performance is obtained, provided that data are moderately oversampled in azimuth. As an additional benefit, the energy of range and azimuth ambiguities is spread over large areas: ambiguities therefore appear in the image as a noise-like disturbance rather than localized artifacts. The impact of staggered SAR operation on image quality is furthermore assessed with experiments using real data. As the first step, highly oversampled F-SAR airborne data have been used to generate equivalent staggered SAR data sets and to evaluate the performance for different oversampling factors and interpolation methods. Then, the radar instrument of the German satellite TerraSAR-X has been commanded to acquire data over the lake Constance in staggered SAR mode. Measurements on these data show very good agreement with predictions from simulations. Staggered SAR is currently being considered as the baseline acquisition mode for Tandem-L, a proposal for a polarimetric and interferometric spaceborne SAR mission to monitor dynamic processes on the earth's surface with unprecedented accuracy and resolution.

Quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) is a well-established technique which has numerous applications in remote sensing. However, spaceborne quad-pol SAR systems are often constrained by severe range and azimuth ambiguities. A deep understanding of the nature of ambiguities in conventional, hybrid, and ±π/4 quad-pol SAR allows a correct evaluation of the ambiguity-to-signal ratio and the design of systems optimized for ambiguity suppression. In that respect azimuth phase coding and merging data available from both cross-polarized channels play an important role. The results of this paper are relevant not only for the design of future spaceborne quad-pol SAR systems, including staggered SAR ones, but also for the optimal exploitation of quad-pol SAR systems currently in operation.

With the relentless increase in population density, the anthropogenic expansion into natural terrestrial hazard zones has become irreversible resulting in ever more catastrophic disasters, not only in the Asia-Pacific region more so within the entire tropical belt engulfing Mother Earth. Thus not only the Indonesian Pacific Islands, so also South America, Africa and back via the Islands of the Indian Ocean to Asia-Pacific, these natural events like volcano eruptions, earthquakes with emerging tsunami, cyclones and severe down pours, humidity and haze have caused havoc, loss of lives, destruction of infrastructure and above all intentional manmade interference resulting in the deterioration of pristine tropical jungle forests. Matters have become so bad that proposals are forthcoming for equating oil-palm and tropical-fruit orchard mono-cultures with pristine tropical jungle habitat by greedy developers mostly exterior to local environmental regions suffering helplessly from it.

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

This paper puts forward a reverse backprojection algorithm for the moderately efficient generation of raw data of natural scenes acquired with general SAR geometries; in particular, we are interested in monostatic and bistatic geosynchronous (GSO) geometries. The backprojection algorithm has the advantage of being arbitrarily precise for all acquisition modes and systems, and allows an exact accommodation of space-variant effects introduced by topography and the propagation through the atmosphere. Its exactness allows both to simulate any real scenarios and understand further optimization potentials. The reverse backprojection algorithm is presented and analyzed in the following pages. The algorithm is also validated using both simulated clutter and data from the Sentinel-1 mission.

This paper provides an overview of the future development of spaceborne SAR systems with multi-baseline imaging capability like polarimetric SAR interferometry (PolInSAR), tomography (TomoSAR) and holography (HoloSAR). The goal is to fill the multi-dimensional data space with additional information from acquisitions having different spatial or temporal baselines. Multi-baseline imaging opens the door for a new class of image products in spaceborne SAR. Well-known examples are across-track and along-track interferometry, which allow the measurement of surface topography, ground deformation, ocean currents as well as glacier movements. While across-track and along-track interferometry are well established techniques and have been widely used by current spaceborne SAR systems, PolInSAR, TomoSAR and HoloSAR are emerging techniques which are shaping the future development of spaceborne SAR. New mission concepts for multi-static SAR configurations with distributed and sparse arrays will pave the way for this development.

Companion SAR missions offer a cost-effective solution to boost the performance and capabilities of existing SAR missions. With the use of companion satellites, a whole suite of coherent single-pass observations become feasible, allowing for the interferometric and tomographic exploitation of the received data. Interferometric and tomographic measurements are however very sensitive to even the slightest degradations in the phase of the received data, and impose stringent requirements on the calibration concept of the mission. Weakly-synchronised companion SAR missions may have significant residual clock errors, which challenge the attainment of this requirements. We put forward an innovative calibration concept based on autonomous (data-based) calibration at different product levels and present an estimation of the expected performance for ESA's SAOCOM/CS mission.

Correlating SAR (CoSAR) has been recently proposed as a geosynchronous remote sensing mission capable of delivering continental coverage of ocean surfaces with a high repeat cycle. The main specific measurements provided by CoSAR are estimates of the normalised radar cross section (NRCS), Doppler shifts, and surface topography with higher resolution than microwave radiometers and larger coverage than state-of-the-art LEO SAR constellations. This paper analyses the specific geometrical characteristics of CoSAR surveys and discusses the challenges of efficient CoSAR image formation approaches. The space-variance of CoSAR surveys is expected to be small, hence CoSAR image formation can benefit from the available knowledge in efficient bistatic and multistatic SAR image formation approaches.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an Earth observation radar mission that consists of a SAR interferometer built by two almost identical satellites flying in close formation [1]-[4]. With a typical separation between the satellites of 120 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at 12 m posting is being generated. While the main mission phase for DEM data acquisition has been finished in 2014, the processing of the global TanDEM-X DEM will be concluded in autumn 2016. Final DEMs are well within specifications and feature a low percentage of void areas. Following the DEM data acquisition the capabilities of this unique mission for new scientific application have been demonstrated. In the meantime the continuation of the joint TerraSAR-X/TanDEM-X mission was approved and will be dedicated to the generation of DEMs with even higher accuracy for selected areas and further scientific experiments.

High-resolution wide-swath synthetic aperture radar (SAR) systems are very attractive for the observation of dynamic processes on the Earth's surface, but they require the downlink of a huge volume of data. In order to comply with azimuth ambiguity requirements, in fact, a pulse repetition frequency much higher than the required processed Doppler bandwidth is often desirable. The volume of downlinked data, however, can be drastically reduced by performing Doppler filtering and decimation on board. A finite-impulse-response filter with a relatively small number of taps suffices to suppress the additional ambiguous components and to recover the original impulse response. This strategy is of special relevance for staggered SAR systems, which are typically characterized by a high oversampling factor. The proposed data reduction technique is also baseline for Tandem-L, where onboard Doppler filtering, resampling, and decimation will be jointly implemented.

Rapid advances in synthetic aperture radar (SAR) technologies have brought challenges in image interpretation toward the development of new Earth observation applications. Both novel scattering-imaging models and intelligent inversion techniques are required for advanced information retrieval and interpretation of high-resolution multidimension and multimode SAR data. As an example, scene reconstruction attempts to transfer the SAR image to human-understandable representation of man-made targets and natural environment. In this letter, a framework for scene reconstruction is outlined. It includes three key elements: a dictionary of parametric scatterer model, a method for scatterer recognition and parameter estimation, and a method for target reconstruction. A preliminary case is presented, where a simulated 3-D SAR image of a simple target is successfully reconstructed to a solid geometry. It uses a novel surface extension method to connect isolated scatterers to form a complete target geometry.

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.

This paper proposes the wrapped staring spotlight (WSS) SAR imaging mode, which is a new method to extend the azimuth steering capability for phased array SAR to achieve either an improved azimuth geometric or radiometric resolution. It investigates the utility of steering directions with main lobe gains that are smaller than that of the grating lobes and exposes how these directions can be exploited. Furthermore, two methods are proposed to reduce the speckle and the image noise at once, i.e., the Look-Normalized Pattern Correction and the Ω-weighting. Based on two example TerraSAR-X WSS acquisitions, the image performance of extended and point targets is discussed.

In the last few years, interest in multicircular synthetic aperture radar (SAR) acquisitions has arisen as a consequence of the potential achievement of full 3-D reconstructions at very high resolution over 360° azimuth angle variation. In particular, SAR systems at low frequencies are sensitive to volumetric backscattering of semi-transparent media, and they allow the imaging of internal structures, such as forests. To achieve a full 3-D reconstruction, a 2-D synthetic aperture is required, consisting of a circular (azimuthal) and a vertical component. This 3-D capability can be understood as the result of the combination of holographic and tomographic techniques. In this paper, both techniques will be presented to establish the concept of holographic SAR tomography (HoloSAR). As a further investigation, this paper also presents an analytical expression of the 3-D impulse response function (IRF) of targets in and off the center of the illuminated area. The IRF is characterized by its spatial resolution and sidelobe power, both being a function of the radar resolution capabilities and the geometric acquisition. The second part of this paper presents a polarimetric analysis of HoloSAR tomograms. In particular, the polarimetric signature of scatterers in forested areas is investigated for three different focusing approaches, namely coherent imaging, incoherent imaging, and the generalized likelihood ratio test (GLRT). The three algorithms use the fast-factorized back-projection (FFBP) for individual circular trajectories, and the latter two use in addition compressive sensing (CS) to retrieve the complex reflectivity in elevation. The IRF is validated using a polarimetric L-band HoloSAR survey, which consists of 19 circular passes conducted by the German Aerospace Center's airborne F-SAR sensor over a test site in Kaufbeuren, Germany. The same data set is used for the analysis of the backscattering of forests. Results show a significant horizontal resolution improvement for distributed targets with the coherent imaging approach, whereas a better sidelobe suppression in the direction perpendicular to the line of sight is achieved for the incoherent imaging and the GLRT.

In this paper we propose a new hybrid technique to suppress range ambiguities in spaceborne SAR systems with multiple elevation beams. First, conventional scan-on-receive (SCORE) is performed in real-time onboard the satellite by employing a set of dispersive beams that maximize the collected signal energy for each transmitted pulse. The range ambiguities are then removed in a second step by a joint processing of the signals collected by the multiple elevation beams. The suggested two-stage approach has the advantage that a more robust range ambiguity suppression, which may involve advanced nulling techniques to account for local topography as well as satellite attitude and instrument phase errors, can be performed on ground without tremendously increasing the onboard processing demands or the data downlink volume.

This paper analyzes the impact of a spatially inhomogeneous ionosphere on spaceborne single-pass SAR interferometry. For this, linear TEC gradients and higher-order irregularities are considered. It is shown that TEC gradients as low as 0.01 TECU/km may already noticeably affect the accuracy of an L-band cross-track interferometer, causing, e.g., horizontal and vertical offsets in the order of 1-2 m. Higher-order perturbations of the electron plasma lead to additional errors that vary nonlinearly with the length of the interferometric baseline. To predict these errors, we model the ionospheric irregularities as the product of a vertical profile and a second-order stationary stochastic process with a 3-D power-law spectrum. The interferometric errors are then derived via a set of projection integrals that express the expected phase error variance as a function of the baseline length and the angular extent of the synthetic aperture. With this model, we show that the phase errors of an L-band single-pass SAR interferometer may reach several tens of degrees under medium turbulence conditions. Since these phase errors are highly correlated among neighboring resolution cells, they cannot be reduced by multi-looking, thereby posing a possible challenge for multiple baseline interferometry and SAR tomography.

ALOS-Next/Tandem-L is a proposal for a highly innovative L-band SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unparalleled quality and resolution. It is based on a collaboration between DLR and JAXA which started with a pre-phase A study in 2013 and is currently undergoing a phase A study. Thanks to the novel imaging techniques and the vast recording capacity with up to 8 Tbytes/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere. By this, the new L-band SAR mission will make an essential contribution for a better understanding of the Earth system and its dynamics. ALOS-Next/Tandem-L will, moreover, open new opportunities for risk analysis, disaster management and environmental monitoring by employing especially designed acquisition modes and techniques in combination with a reconfigurable tandem satellite configuration and an L-band SAR instrument with advanced digital beamforming techniques.

This paper presents an assessment of the backscattering of glaciers in the holographic SAR tomography (HoloSAR) mode. The motivation of HoloSAR in the cryosphere is mainly driven by recent investigations in the biosphere that have shown the capabilities of this mode to reconstruct volumes with 3-D imaging reconstructions over 360 ° at very high resolution. To that end, an HoloSAR campaign was conducted at L-band by the DLR's F-SAR sensor over the Findel glacier, Monte Rosa, Switzerland. The first part of this study shows a polarimetric analysis of the resulting images of a single circular flight, where the arc- and circular patterns of the 2-D images in the (x, y) plane already indicate wave penetration. The second part presents 3-D images obtained by the combination of the circular and vertical synthetic apertures, which enable an estimation of the vertical profile of snow, ice sheets and bedrock.

The first radar has been patented 110 years ago. Meanwhile the applications became numerous and the system concepts have been adopted to the available technologies. Typical applications are speed control, air traffic control, synthetic aperture radar, airborne and spaceborne missions, military applications and remote sensing. Research for medical radar applications is well progressing for breast cancer detection and tumor localization. Automobile radar for save and autonomous driving are meanwhile produced in millions per year. In the next years the state-of-the-art radar system concepts will experience almost a revolution. Despite the significant advancements, the radar system technology did not develop like communications or other technologies during the last 20 years. Some of these new technologies will within a few years penetrate radar and revolutionize radar system concepts. This will then allow for new radar features and radar signal processing approaches.

The signal-to-noise ratio of a radar system is described through the well-known radar equation, which can be adapted to apply to imaging Synthetic Aperture Radar (SAR). Extension to multi-channel SAR systems utilizing digital beam-forming turns out to be treacherous; the pivotal question being, if and how to include the gain of the multiple antenna apertures. This paper puts an effort into answering this question in a simple and comprehensible, yet mathematically rigorous way.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an Earth observation radar mission that consists of a SAR interferometer built by two almost identical satellites flying in close formation [1]-[4]. With a typical separation between the satellites of 120 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at 12 m posting is being generated. While the main mission phase for DEM data acquisition has been finished in 2014, the processing of the global TanDEM-X DEM will be concluded mid-2016. Final DEMs for more than 65% of all land masses are already available for scientific and commercial applications. A 15-month science phase of the TanDEM-X mission started in October 2014 which offers the opportunity to explore the generation of DEMs with even higher accuracy for selected areas, and to demonstrate the capabilities of this unique mission for new scientific applications.

One of the benefits of synthetic aperture radar (SAR) systems utilizing digital beamforming is the ability to increase the receive power. The relevant SAR technique is known as SCan-On-REceive (SCORE), which steers the receive antenna beam such that it follows the radar pulse echo traversing the ground. This allows the use of a narrow receive beam in elevation, and therefore, the height of the receive antenna can be increased, resulting in a higher gain, which explains the higher receive power. Although advantageous, this technique has some pitfalls, which impose an upper limit on the antenna size and constrain the selection of SAR operation parameters. These limitations (which are often neglected in the system conception) are caused by the pulse extent on the ground and the way it is modulated by the receive antenna pattern. This letter addresses and quantifies the effects caused by the transmit pulse length (here denoted as pulse extension loss) through a rigorous analysis, with the purpose of introducing an important SAR performance figure. Closed expressions are derived for the simplified case of a uniform linear antenna array.

In this paper, we introduce a novel multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) concept for multimodal operation. The proposed system employs waveforms based on the orthogonal frequency division multiplexing (OFDM) technique and digital beamforming (DBF) on receive. Thereby, it becomes feasible to maximize spatial degrees of freedom, which are necessary for the multimodal operation. The proposed MIMO SAR system produces multiple high-resolution wide-swath SAR imageries that are used for coherent postprocessing. Through this paper, we aim to open up a new perspective of using MIMO concept for a wide-swath SAR imaging with high resolution in interferometric and polarimetric modes, based on OFDM and DBF techniques. Therefore, this paper encompasses broad theoretical backgrounds and general system design issues as well as specific signal processing techniques and aspects.

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.

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.

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.

In this paper, we provide a systematic overview on bistatic SAR image formation for arbitrary platform trajectories. Bistatic SAR may offer improved flexibility, performance and cost-efficiency if compared to monostatic SAR systems certainly at the expense of increased operational complexity. In the previous years, the road from experimental to operational bistatic radar systems has been paved with the launch of TanDEM-X [1], the first bistatic SAR in space. Future bistatic SAR systems will encompass multistatic constellations and formations with large baselines capable of delivering novel Earth observation products with improved performance and coverage. In this context, efficiency and accuracy in bistatic SAR image formation becomes an important challenge, asking for the development of suitable signal processing algorithms. This paper describes first the relevant developments on bistatic SAR image formation. The validity and shortcomings of previously suggested approaches for different bistatic geometries is systematically discussed. An overview on bistatic SAR image formation is provided, stressing those key techniques and algorithms which, in our opinion, are applicable in future operational systems and missions; the theoretical background is complemented by results from multiple pioneering airborne, spaceborne, and hybrid air-/spaceborne bistatic SAR experiments conducted at DLR over the past decade.

Tandem-L is a proposal for an innovative interferometric radar mission to monitor the Earth system and its intricate dynamics. Important mission objectives are global inventories of forest height and above-ground biomass, largescale measurements of Earth surface deformations due to plate tectonics, erosion and anthropogenic activities, observations of glacier movements and 3-D structure changes in land and sea ice, and the monitoring of ocean surface currents. A detailed description of the mission goals can be found in [1]. The mission concept is based on co-flying two fully-polarimetric L-band SAR satellites in a close formation. Tandem-L employs new techniques and advanced technologies to achieve its ambitious mission goals. The feasibility of a joint DLR/JAXA mission is currently being investigated in the scope of a pre-phase A study. This paper provides an overview of the Tandem-L instrument design concept and its performance predictions. Innovative aspects like the employment of advanced digital beamforming techniques and operation in a variety of imaging modes are detailed.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in spaceborne radar remote sensing [1]-[5]. A single-pass SAR interferometer with adjustable baselines in across- and in along-track directions was formed by adding a second spacecraft (TDX), almost identical to TerraSAR-X (TSX), and flying the two satellites in a closely controlled formation. With typical across-track baselines of 150 to 500 m a global Digital Elevation Model (DEM) is being generated. To reach this level of DEM quality, all land surfaces have to be mapped at least twice, difficult mountainous terrain requires additional coverage. While data acquisition will be finished by the end of 2014, the processing to the global TanDEM-X DEM is expected to last until the end of 2015. Currently, final DEMs for most of Australia, larger parts of North America, Siberia and South Africa are already available.

Multicircular synthetic aperture radar (SAR) (MCSAR) is an extension of circular SAR (CSAR) characterized by the formation of a synthetic aperture in elevation with several circular flights. This imaging mode allows an improved resolution in the plane perpendicular to the line of sight ( LOS⊥), thus suppressing the 3-D cone-shaped sidelobes that are formed when focusing with CSAR. This letter presents the first polarimetric MCSAR airborne experiment acquired at P-band by the German Aerospace Center (DLR)'s F-SAR system over a forested area in Vordemwald, Switzerland. This letter also includes a phase calibration method based on the singular value decomposition (SVD) using ground signatures to estimate constant phase offsets within a stack of 2-D images. Focusing methods, such as fast-factorized back projection (FFBP), beamforming (BF), and compressive sensing (CS), described in previous publications are used to solve the complex reflectivity in the (x, y, z) space.

Staggered synthetic aperture radar (SAR) is an innovative concept, where the pulse repetition interval (PRI) is continuously varied. This, in combination with digital beamforming (DBF) on receive, allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple apertures. However, staggered-SAR systems require a mean pulse repetition frequency (PRF) much larger than the signal Doppler bandwidth and allow the use of transmitted pulses of limited length. This letter proposes a novel processing strategy for staggered SAR data, which allows a reduction of the mean PRF and the use of longer transmitted pulses. The performance obtained with the proposed novel strategy is evaluated and compared with a conventional SAR system operating with constant PRI.

This paper presents the first fully polarimetric high-resolution 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.

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.

Synthetic aperture radar (SAR) is a remote sensing technique, capable of providing high-resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earth's surface. However, conventional SAR systems are limited in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive apertures, displaced in along track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along track is available, it is still possible to map a wide area: Multiple swaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but “blind ranges” are present between adjacent swaths. This paper considers an innovative concept, staggered SAR, where the pulse repetition interval (PRI) is continuously varied. This concept allows the imaging of a wide continuous swath without the need for a long antenna with multiple apertures. The choice of the sequence of PRIs and the preprocessing of the raw data are discussed in detail, showing how the staggered SAR is even less affected by ambiguities of pointlike or extended targets with respect to a system with constant PRI, which simultaneously maps multiple swaths. Some system design examples are finally presented and compared.

Missing echoes in the raw data introduce artifacts that might be noticeable in processed SAR images. As a consequence, recovery methods must be applied to ensure the high quality of the images. Although spectral estimation based interpolators are suitable for the recovery of point-like targets, their efficiency is poor for distributed ones. In this paper we propose a new reconstruction method for interferometric systems, which uses the common information in one image of a coherent pair to recover the other and vice versa. Since a typical example of missing data happens when acquiring with the bistatic mode of TanDEM-X, the proposed method is verified with data from a bistatic TanDEM-X survey in addition to simulated data. The results show that the proposed reconstruction can significantly improve the accuracy of the resulting interferograms.

This paper presents the first 3-D polarimetric holographic tomograms at L-band using fully polarimetric multi-circular SAR (MCSAR) over forested areas. This concept offers an innovative way of analyzing the Earth, in particular volume scatterers such as forested areas, ice and dry soils. Holographic tomograms are formed by low-frequency radar in L-band and by the acquisition of multi-angular measurements with MCSAR over 360°. MCSAR consists of the combination of CSAR with an additional synthetic aperture in height (z). The analysis of the impulse response function (IRF) of MCSAR shows 3-D sidelobe reduction and an improvement in the resolution along the z direction. A fully polarimetric MCSAR campaign using the DLR's F-SAR airborne system at L-band over the region of Kaufbeuren, Germany was carried out. Images in 3-D were focused with beamforming (BF) and compressive sensing (CS). Results depict advantages of MCSAR such as shadow reduction, low temporal decorrelation and a better understanding of the coherent 3-D radar backscattering.

This paper presents an analytical analysis of the impulse response function (IRF) of multi-circular SAR (MCSAR) as a function of the system bandwidth and the number of tracks, thereby optimizing the acquisition geometry to get the best 3-D resolution and sidelobe suppression. Moreover, the wide range of spectrum measurements provided by MCSAR allows the information retrieval over 360 °, thus understanding the anisotropic signature of scatterers, e.g. by their polarimetric signature. The second part of this paper presents a polarimetric analysis of the resulting holographic SAR tomograms of forested areas by means of the generalized likelihood ratio test (GLRT) algorithm, as well as by coherent and incoherent imaging. The IRF and the anisotropic analysis are validated with a polarimetric MCSAR campaign conducted at L-band by the DLR's F-SAR sensor over a forested region in Kauf-beuren, Germany.

This paper proposes a method for extracting the tropospheric time delay in a single-pass using SAR images acquired in squinted geometries. The required accuracies and error sources are analysed and a satellite configuration providing single-pass measurements of the tropospheric delay is proposed. Moreover, a joint topography-tropospheric delay estimation using very-high resolution SAR images is also suggested for cases where an inaccurate DEM is available. A performance of the approach with current TerraSAR-X staring spotlight like parameters is also computed.

The possibility of combining data acquired with different platforms to generate a single SAR image results in an advantageous relaxation of the timing and sampling requirements of the system which can be used to improve in different ways its performance. We report about a generalised approach for distributed SAR image formation which is based on a platform dedicated compensation of the observation geometry followed by a differential calibration of the data. The validity of the approach on simulated data is presented. Further tests with actual data from a TanDEM-X dedicated campaign are being conducted.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in spaceborne radar remote sensing [1]-[3], [9], [12]. A single-pass SAR interferometer with adjustable baselines in across- and in along-track directions was formed by adding a second (TDX), almost identical spacecraft to TerraSAR-X (TSX) and flying the two satellites in a closely controlled formation. With typical across-track baselines of 150 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at a 12-m posting is being generated.

In this letter, we present a new waveform technique for the use of multiple transmitters in synthetic aperture radar (SAR) data acquisition. This approach is based on the principle of the orthogonal-frequency-division-multiplexing technique. Unlike multiple subband approaches, the proposed scheme allows the generation of multiple orthogonal waveforms on common spectral support and thereby enables to exploit the full bandwidth for each waveform. This letter introduces the modulation and the demodulation processing in regard to typical spaceborne SAR receive signals. The proposed processing techniques are verified by a simulation for the case of pointlike targets.

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.

This correction refers to Eqs. (6), (7), and (19) in the above paper (ibid., vol. 48, no. 7, pp. 2994-3008, Jul. 2010).

This paper introduces the bidirectional synthetic aperture radar (BiDi SAR) imaging mode, i.e., the simultaneous imaging of two directions by one antenna into one receiving channel, and presents short-term time series of images and interferograms acquired by the TerraSAR-X and TanDEM-X satellites. A comparison to alternative approaches for the acquisition of short-term time series is provided. The BiDi acquisition geometry is defined, and a TerraSAR-X BiDi antenna pattern is analyzed. BiDi raw data are simulated, sampled with different pulse repetition frequency values, and compared with real BiDi raw data. The spectral separation of simultaneously acquired forward- and backward-looking images is explained. This paper presents the image results of BiDi acquisitions with TerraSAR-X and TanDEM-X satellites flying with 20-km along-track separation. This pursuit configuration allowed for the acquisition of up to six short-term repeated images and up to three interferograms in a single pass. An overview of potential applications for the new BiDi SAR imaging mode concludes this paper.

In the recent years, there has been increased interest in using synthetic aperture radar (SAR) to study and monitor glaciers and ice sheets for glaciological and climate change research. This paper describes the estimation of ice extinctions through the modeling of polarimetric interferometric SAR (Pol-InSAR) coherences as a combination of a surface contribution (from the snow-firn interface and wind-induced sastrugi features) and a volume response. Ground-to-volume scattering ratios derived from a novel polarimetric decomposition are used in conjunction with Pol-InSAR coherence magnitudes to invert the extinction of the ice layer. The inversion is performed for experimental airborne Pol-InSAR data at L-band and P-band acquired by the German Aerospace Center's (DLR) E-SAR system over the Austfonna ice cap in Svalbard, Norway, as part of the 2007 IceSAR campaign. Extinction dependences on frequency and glacier facie are investigated, and validation is performed comparing ground-penetrating radar data to SAR backscatter and extinction values. Best extinction results are obtained at shallow incidence angles with small wavenumbers and for low ground-to-volume scattering ratios. For swath areas in mid range to far range, accuracies of 25% in extinction are anticipated when averaging over 100 effective looks for a four-baseline inversion constraining solutions to vertical wavenumbers of 0.01 ≤kz≤ 0.1. To allow inversion using single-baseline Pol-InSAR, the proposed model is of limited complexity. Suggested extensions for a more realistic scattering scenario include incorporating multiple englacial ice scattering layers and improving the way multiple baselines are combined.

This paper reviews radar architectures that employ multiple transmit and multiple receive channels to improve the performance of synthetic aperture radar (SAR) systems. These advanced architectures have been dubbed multiple-input multiple-output SAR (MIMO-SAR) in analogy to MIMO communication systems. Considerable confusion arose, however, with regard to the selection of suitable waveforms for the simultaneous transmission via multiple antennas. In this paper, it is shown that the mere use of orthogonal waveforms is insufficient for the desired performance improvement in view of most SAR applications. As a solution to this fundamental MIMO-SAR problem we had previously suggested to exploit the special data acquisition geometry of a side-looking imaging radar equipped with multiple receiver channels in addition to appropriately designed waveforms transmitted by multiple antennas. Here, we extend this approach to a more general set of radar waveforms with special correlation properties that satisfy a short-term shift-orthogonality condition. We show that the echoes from simultaneously transmitted pulses can be separated if the short-term shift orthogonality is combined with digital beamforming on receive in elevation. This enables the implementation of a fully functional MIMO-SAR without correlation noise leakage for extended scattering scenarios.

This paper presents an initial analysis of the possibilities for velocity and acceleration measurement with the Bi-Directional SAR imaging mode (BiDi). It comprises single satellite single path acquisitions as well as a constellation of two satellites. The translational velocity components into azimuth and range directions are simulated. A BiDi approach for measuring the azimuth velocity from one satellite with one receiving channel is proposed. Image examples acquired with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites show velocity and acceleration effects on ships and the proposed BiDi velocity approach is verified. Interferometric fringes were observed on anchoring ships. A first approach into the understanding of rotational effects was achieved by simulation of rotational fringes.

Three-dimensional reconstruction and sub-wavelength resolution can be achieved in Circular SAR (CSAR) mode with a single pass. However, circles or arcs are present when the assumed height during focussing is not that of the target. In 3-D imaging these artifacts are seen as cone-shaped sidelobes due to the poor resolution in the direction perpendicular to the line of sight. In this work, a multicircular fully polarimetric SAR experiment using DLR's F-SAR system at P-band is presented. The purpose is to enhance the resolution while focusing 3-D images by retrieving the backscattering profile in elevation. A phase calibration method using the Singular Value Decomposition (SVD) is presented. Processing methods such as Fast Factorized Back Projection for circular tracks, and reconstruction techniques with coherent averaging with beamforming and compressive sensing for 3-D are used. Finally, the first holographic SAR tomogram at P-Band of a forested area is presented.

This paper addresses several innovative steps needed in the chirp scaling and extended chirp scaling (ECS) algorithms in order to process staring Spotlight TerraSAR-X (TSX) images with 21 cm azimuth resolution and 300 MHz range bandwidth. The aspects that need of special consideration are the 2D phase truncation in frequency domain of ECS, the element pattern of the antenna array, the curved orbit, the stop-and-go approximation, and the troposphere. All these aspects are expounded in detail and a solution is given for each of them. The suggested corrections are applied at raw data level, hence easing the integration within the existing TSX processor. Real data acquired by TSX in the experimental staring Spotlight mode are used to validate the proposed methodology.

Launched in June 2010, TanDEM-X is an interferometric mission with the main goal of providing a high-resolution, global and unprecedentedly accurate digital elevation model (DEM) of the Earth by means of single-pass X-band 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 was operated in pursuit monostatic mode. During that time, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes were conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this article includes results of the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focussing on the analysis of the experimental results. Even in the absence of essential synchronisation and calibration information, bistatic images and interferogramswith similar quality to pursuit monostatic have been obtained. Some months later, with TanDEM-X already in its operational DEM-acquisition phase a further challenging bistatic acquisition carried out in cooperation with DLR's airborne radar F-SAR has been carried out. The objective was to acquire fully polarimetric interferometric data with a high range of available bistatic angles. A dedicated commanding of both TanDEM-X and F-SAR was required, including irregular sampling schemes, partially missing bistatic echo reception and bistatic synchronisation, which pose a number of technological challenges in SAR data processing before the calibrated bistatic SAR images are obtained. This article reports about these two set of experiments.

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).

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.

This paper discusses cross-track SAR interferometry under crossing orbits. The underlaying theory is briefly outlined, showing that a crossing angle between the ground tracks needs to be compensated by applying different squint angles in order to have overlap in the the ground-spectral domain. Two sets of crossing orbits InSAR experiments are described. The results of the first experiment, are discussed.

Tandem-L is a mission proposal for an innovative interferometric L-band radar instrument that enables the systematic monitoring of dynamic Earth processes using advanced techniques and technologies. The mission is science driven aiming to provide a unique data set for climate and environmental research, geodynamics, hydrology and oceanography. Important application examples are global forest height and biomass inventories, measurements of Earth deformation due to tectonic processes and/or anthropogenic factors, observations of ice/glacier velocity field and 3-D structure changes, and the monitoring of soil moisture and ocean surface currents. The Tandem-L mission concept consists of two cooperating satellites flying in close formation. The Pol-InSAR and repeat-pass acquisition modes provide a unique data source to observe, analyse and quantify a wide range of mutually interacting processes in the bio-, litho-, hydroand cryosphere. The systematic observation of these processes benefits from the high data acquisition capacity and the novel high-resolution wide-swath SAR imaging modes that combine digital beamforming with a large reflector antenna. This paper provides an overview of the Tandem-L mission concept and its main application areas. It is planned to realise the Tandem-L mission in cooperation with NASA/JPL. The mission concept was developed in detail in a joint two-year pre-phase A study and it will be further studied in the next 18 months. This will allow a cost-effective implementation, whereby each partner contributes its predevelopments and experience. According to current planning, the Tandem-L satellites could be launched in 2019.

TanDEM-X is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. De spite 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 non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this paper reports about the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X. Even in the absence of essential synchronisation and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

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.

In recent years, there has been increased interest in using synthetic aperture radar (SAR) to study and monitor glaciers and ice sheets for glaciological and climate change research. However, due to the medium's complexity, SAR backscattering from ice remains poorly understood, including the relative importance of scattering from surface and volume layers and also dependences on frequency and glacier zone. Extreme weather conditions can result in quickly changing surface conditions influencing backscatter signatures while leaving the underlying volume of interest unchanged. Surface and volume components must thus be separated in order to infer information regarding the properties of the ice volume. This paper describes a three-component scattering model to decompose polarimetric SAR (PolSAR) images of glacier ice. Total backscatter is modeled as the incoherent summation of surface, volume, and sastrugi (wind-induced feature) components. The proposed model adapts and extends the Freeman and Durden decomposition for an ice volume scenario in which the volume is a dielectric medium. Forms of the model for both random and oriented volumes are considered, and a new oriented sastrugi component is introduced which is able to explain backscatter behavior between different winter scenes. Validation is performed with airborne PolSAR data at L- and P-band collected using the E-SAR system of the German Aerospace Center over the Austfonna ice cap in Svalbard, Norway, as part of the ICESAR campaign.

SIGNAL is an innovative earth exploration mission proposal with the main objective to estimate accurately and repeatedly topography and topographic changes associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. Elevation measurements are complemented with glacier velocity measurements, providing valuable additional information for a better understanding of the hydrology of glacierized basins and of the Arctic and Antarctic water cycle. SIGNAL is capable of monitoring all critical regions with a high spatial resolution and an adequate revisit time. This paper gives an overview about the actual mission design status and provides a brief description of the topography (DEM - digital elevation map) self-calibration strategy and the estimated global interferometric performance.

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 reviews advanced multi-channel SAR system concepts for the imaging of wide swaths with high resolution. Several novel system architectures employing both direct radiating arrays and reflector antennas fed by a digital array are introduced and compared to each other with regard to their imaging performance. In addition, innovative SAR imaging modes are proposed which enable the mapping of ultra-wide swaths with high azimuth resolution. The new techniques and technologies have the potential to enhance the imaging performance of future SAR systems by one order of magnitude if compared to state of the art SAR sensors like TerraSAR-X, ALOS, Radarsat-2 or Sentinel-1.

On behalf of the IEEE Geoscience and Remote Sensing Society (IEEE GRS-S), I would like to warmly welcome you to IGARSS 2010 in Honolulu! This year is a very special one, since we are commemorating the 30th anniversary of IGARSS. Since the first IGARSS in Washington, DC, a revolution has taken place in the technologies, data processing and applications of remote sensing. With the Earth Observing System (EOS) satellites in orbit, the upcoming Global Monitoring for Environmental and Security (GMES) satellites, the National Polar Orbiting Operational Environmental Satellite System (NPOESS), the new set of U.S. Earth observing missions as recommended by the U.S. National Research Council (the so-called U.S. Decadal Survey missions), Global Earth Observation System of Systems (GEOSS) initiative, etc., remote sensing data play a most important and indispensable role in providing reliable information to address environmental issues, sustainable development, study of global climate change, geoscience research and the monitoring of natural disasters. Our Society is proud to have actively contributed to shaping these exciting developments in the remote sensing field. IGARSS, our annual symposium, is recognized today as a premier event in remote sensing and provides an ideal forum for obtaining up-to-date information about the latest developments, exchanging ideas, identifying future trends in your research area and making contacts with the international remote sensing community.

The success of current spaceborne Synthetic Aperture Radar (SAR) is boosting the performance requirement of next generation systems. In order to cope with the evolution of SAR the design of the new systems will need to meet higher requirements for spatial and radiometric resolution together with an increased availability. This tendency is recognized nearly independently of the application area and manifests itself through several study programs initiated by space agencies aiming at the design of future SAR systems. In this context the use of large reflectors combined with digital feed arrays for SAR is considered a possible alternative to planar array antennas. This paper suggests an X-band spaceborne SAR system utilizing a deployable reflector together with a digital feed array, analyzes its performance and highlights its advantages compared to other systems based on direct radiating arrays.

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 outlook on future interferometric and tomographic concepts and developments, including multistatic SAR systems with multiple receivers, is provided.

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.

The 34 papers in this special issue provide an overview of the TerraSAR-X mission, commissioning phase, calibration, data processing, and applications.

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.

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.

This paper presents the very first assessment of the permanent scatterer (PS) technique for airborne data. A data set of 14 SAR images at L-band, acquired over the Oberpfaffenhofen area on the same day with the E-SAR system of the German Aerospace Center (DLR), is used for the first airborne time series analysis with PS. The paper shows the importance of mitigating the residual motion errors through the use of precise motion compensation strategy before PS analysis. The target velocity and DEM error results are obtained by a periodogram-based estimation considering the linear displacement model. Due to the small number of images in our data set, the displacement velocity and DEM error results are presented on a PS basis. Target structures related to selected reliable PSs are shown and the corresponding periodograms highlighted.

The objective of this paper is to make a review of the current status of the PolSARpro v4.0 Software (Polarimetric SAR Data Processing and Educational Toolbox), developed under contract to ESA by a consortium comprising I.E.T.R at the University of Rennes 1, AELc, DLR-HR and Dr mark Williams from Adelaide. The objective of this current project is to provide Educational Software that offers a tool for self-education in the field of Polarimetric SAR data analysis at University level and a comprehensive suite of functions for the scientific exploitation of fully and partially polarimetric multi-data sets and the development of applications for such data. The PolSARpro v4.0 Software establishes a foundation for the exploitation of Polarimetric techniques for scientific developments and stimulates research and applications developments using PolSAR and PolInSAR data.

This paper presents an efficient phase preserving processor for the focusing of data acquired in sliding spotlight, TOPS (Terrain Observation by Progressive Scans) and ScanSAR 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 for the first two modes, and due to the burst mode in the ScanSAR case. 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 an azimuth scaling approach is presented to perform the azimuth processing, whose kernel is the same for all three modes. Data acquired by TerraSAR-X in sliding spotlight, TOPS and ScanSAR modes are used to validate the developed algorithm.

Following the success of the first bistatic spaceborne-airborne experiment between TerraSAR-X and F-SAR carried out in November 2007, DLR has performed a second bistatic experiment in July 2008 with new challenging acquisitions. Furthermore, the existing bistatic processing chain has been updated with two significant improvements: a) clock offset synchronisation is now performed without the use of reference targets, and b) SAR imaging is done using a fast focussing technique. The new SAR imaging algorithm, based on the fast factorised backprojection algorithm, has proved very good focussing qualities while dramatically reducing (up to a factor 100 with respect to direct backprojection) the overall computational load. The new processing chain is tested using the image of the first TerraSAR-X experiment. Results of a dualpol acquisition performed during the second TerraSAR-X/F-SAR experiment and showing the first dual-pol bistatic spaceborne-airborne images are also presented in this paper.

This IEEE Transactions on Geoscience and Remote Sensing (TGRS) special issue is dedicated to the advances in synthetic aperture radar (SAR) techniques and applications presented at the 5th International Symposium on Retrieval of Bio- and Geophysical Parameters from SAR Data for Land Applications (BioGeoSAR), which was organized by Consiglio Nazionale delle Ricerche, with active support from the European Space Agency, and held in Bari, Italy, on September 25-28, 2007. The purpose of the symposium has been to provide a forum for researchers to review current trends in the field of retrieval of quantitative information from SAR data and to present and discuss new ideas and new research directions. The BioGeoSAR symposia are usually held every three years in Europe with worldwide participation.

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.

Multi-channel synthetic aperture radar (SAR) systems enable high-resolution wide-swath imagery thus overcoming the inherent limitation of conventional SAR. A possible realization based on the combination of multi-aperture SAR signal reconstruction in azimuth with digital beamforming on receive in elevation is given in [1]. The present paper turns focus to advanced concepts for the imaging of even wider swaths while still providing high azimuth resolution [2]. In this regard, the operation of multi-channel SAR systems in burst modes like ScanSAR or TOPS is introduced and aspects of applying the multi-aperture reconstruction algorithm to burst mode data are analyzed. The influence of the digital processing network on performance parameters as signal-to-noise-ratio and azimuth ambiguity-to-signal-ratio in multi-channel burst mode systems is considered and embedded in the design example of a ScanSAR system that allows for the imaging of a 400 km wide swath with a geometric resolution of 5 m. Finally, first results for a multi-channel TOPS system are presented and an optimized TOPS processing approach is introduced.

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.

The objective of this paper is to make a review of the current status of the PolSARpro v3.3 Software (Polarimetric SAR Data Processing and Educational Toolbox), developed under contract to ESA by a consortium comprising I.E.T.R at the University of Rennes 1, AELc, DLR-HR and Dr mark Williams from Adelaide. The objective of this current project is to provide Educational Software that offers a tool for selfeducation in the field of Polarimetric SAR data analysis at University level and a comprehensive suite of functions for the scientific exploitation of fully and partially polarimetric multi-data sets and the development of applications for such data. The PolSARpro v3.3 Software establishes a foundation for the exploitation of Polarimetric techniques for scientific developments and stimulates research and applications developments using PolSAR and PolInSAR data.

Following an original proposal by the authors to the TerraSAR-X (TSX) scientific coordination board, a spaceborne-airborne bistatic experiment was successfully performed early November 2007. TSX was used as transmitter and DLR's new airborne radar system, F-SAR, as receiver; due to the capability of the latter to acquire data quasi-continuously, no echo window synchronisation is needed. Monostatic data were also recorded during the acquisition. This paper includes description and results of the spaceborne-airborne bistatic experiment, with special focus on data processing and image comparison. Given the acquisition scenario, with two-channel sampling and transmitter and receiver clocks operating independently, data processing must necessarily follow a three-step strategy: 1) channel balancing, 2) data synchronisation and 3) bistatic SAR processing. Since neither absolute range nor Doppler references are available in the bistatic data set, synchronisation is done with the help of calibration targets on ground and based on the analysis of the acquired data compared to expected data. Due to the variant nature of the bistatic acquisition and the required precision for the processing, data are processed using a bistatic backprojection approach.

This paper introduces the innovative concept of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new generation of SAR systems with improved performance and flexible imaging capabilities. Examples are high-resolution wide-swath radar imaging with compact antennas, enhanced sensitivity for applications like alongtrack interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy hitherto incompatible user requirements. Implementation-specific issues are discussed and performance examples demonstrate the potential of the new technique for different remote sensing applications.

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.

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.

The paper deals with the problem of reconstructing the height of forests from polarimetric/multi-baseline SAR data. The approach consists of optimizing an objective functional defined as the distance between the measured data and the data predicted by the model at the actual estimate of the unknowns. We indicate the role of global optimization on the performance of the forest height reconstruction algorithm. As global optimizer, a multilevel single-linkage method, which incorporates a local optimization into the global search, is exploited, thus offering computational efficiency and reliability. The performance of the method are illustrated against numerically simulated data.

This paper introduces and analyses the innovative paradigm of multidimensional waveform encoding for space- borne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new class of highly performant SAR systems employing novel and highly flexible radar imaging modes. Examples are adaptive high-resolution wide-swath SAR imaging with compact antennas, enhanced parameter estimation sensitivity for applications like along-track interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy the hitherto incompatible user requirements for frequent monitoring and detailed mapping. Implementation specific issues will be discussed and examples demonstrate the potential of the new technique for different remote sensing applications.

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-strip- map 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 range- dependent 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 independently for each image. Repeat-pass data of the E-SAR system of the German Aerospace Center (DLR) are used to demonstrate the performance of the proposed approach.

This paper presents an efficient phase preserving processor for TOPS (Terrain Observation by Progressive Scans) imaging mode. TOPS has been proposed as a new wide swath imaging mode, which solves the problems of scalloping and azimuth-varying ambiguities introduced by the conventional ScanSAR mode by means of steering the antenna along the azimuth direction. An algorithm based on ECS (Extended Chirp Scaling) is proposed, which uses sub-apertures and a new azimuth scaling step. The proposed solution is also efficient in the sense that it allows selecting the final azimuth image spacing by means of azimuth scaling, hence easing the forthcoming mosaicking of the different sub swaths. Simulations with point targets are used to validate the processor.

This paper presents airborne differential SAR results using a stack of 14 images, which were acquired by the experimental SAR (E-SAR) system of the German Aerospace Center during a time span of only two and a half hours. 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 (inaccuracies in the navigation system in the order of 1-5 cm), and the accommodation of the topography and the aperture during processing. The SAR focusing chain to process the data is also presented, together with the modifications in the differential processor to deal with the remaining baseline error. The detected deformation of a corner reflector and of several agricultural fields allows validating the proposed techniques.

TanDEM-X opens a new era in space borne radar remote sensing. The first bistatic SAR mission, is formed by adding a second, almost identical spacecraft, to TerraSAR-X and flying the two satellites in a closely controlled formation with typical distances between 250 and 500 m. Primary mission objective is the generation of a consistent global digital elevation model with an unprecedented accuracy according to the HRTI-3 specifications. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new SAR techniques and applications. This paper gives an overview of the TanDEM-X mission concept, summarizes the capabilities of the system, illustrates the achievable performance, and provides some examples for new imaging modes and applications. The mission has been approved for full implementation by the German Space Agency with a planned launch in spring 2009.

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.

Multi-aperture synthetic aperture radar (SAR) systems in combination with an appropriate coherent processing of the individual aperture signals enable high resolution wide swath (HRWS) SAR imaging [1]-[8]. An innovative reconstruction algorithm for such a digital beamforming on receive was presented in [9]-[12] that allows for HRWS even in case of a non-uniformly sampled data array in azimuth. This paper will compare this algorithm to different azimuth processing strategies regarding their performance in dependency of the overall sampling. Further, optimization strategies are discussed to maximize the system's performance by pattern tapering on transmit and "Pre-Beamshaping on Receive" networks that allow for pattern tapering on receive and adaptively adjust the virtual sample positions.

This paper presents airborne DInSAR results using a stack of 14 images, which were acquired by the Experimental SAR (E-SAR) system of the German Aerospace Center (DLR) during a time span of only three hours and fifteen minutes. An advanced differential technique is used to retrieve the error in the digital elevation model (DEM) and the temporal evolution of the deformation for every coherent pixel in the image. Furthermore, some modifications in the differential processing chain are included to deal with the existence of the so-called residual motion errors, which play a similar role as atmospheric artifacts in the spaceborne case. The detected deformation of a corner reflector and of some agricultural fields allows to validate the proposed techniques to measure deformation phenomena with an airborne platform.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is an innovative radar interferometry mission to generate a global, consistent and highly accurate digital elevation model (DEM) and to provide a configurable SAR interferometry platform for demonstrating new SAR techniques and applications. This paper summarizes the mission concept starting from the user requirements, the HELIX orbit and TanDEM-X operational modes to the expected height performance. Examples of new SAR techniques are presented.

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.

Abstract —The EU-sponsored research and training network AMPER deals with state of the art research in the area of multi-parameter polarimetry, duly emphasizing radar engineering, scattering theory, inversion methods, polarimetric theory, propagation and applications. This paper summarises some of the network’s scientific achievements. The AMPER partners include: M. Chandra and Alberto Moreira (DLR, Germany), D.H.O Bebbington (Uni. Essex, UK), E. Pottier (Uni. Rennes, France), E. Krogager (DDRE, Denmark), F. Molinet (MOTHESIM, France), J. Fortuny (JRC, Italy), X. Fabregas (UPC, Spain), G. Wanielik and U. Neubert (Uni. Chemnitz, Germany), U. Benz (DEFINIENs, Germany). The EU project officer is Gordana Popovic and the network is coordinated by the lead author (DLR /U. Chemnitz) . Keywords: multiprameter polarimetry, inversion methods, propagation effects, sensor development and calibration I. I NTRODUCTION calibration, multistatic-multipath polarimetric signals in Network activities fall into three main areas dealing with the sensor systems and base data, the underlying physics and scattering models and parameter retrieval and product generation. Specific objectives for each research area are identified as follows: Systems - Devising simple, data-based methods for calibrating multi-static polarimetric sensors and enhancing their information-carrying capacity. The approach taken, exploits the use of polarimetric invariants and absolute self-calibrating physical and statistical properties. Physics - Innovation is sought in various topics: 1. Identification of polarimetric transformation invariants in multistatic mode, 2. Polarimetric scattering models of extended and distributed random target taking into account the coherency properties of the targets, 3. Correcting for propagation-induced errors. Applications - Optimization of inversion methodology is the main challenge which is addressed by seeking appropriate and new decompositions that also relate to the statistical features arising from eigenvalue analysis. II. R

The displaced phase centre (DPC) technique enables a wide swath SAR with high azimuth resolution. In classic DPC systems, the pulse repetition frequency (PRF), sensor velocity and antenna length underlie a stringent timing requirement and any deviation will result in a non-uniform sampling of the syn­thetic aperture. This restriction can be overcome by the use of a recon­struction algorithm, which enables a recovery of the unambiguous Doppler spectrum also in case of a non-uniform sampling. This paper ex­plains limitations of the reconstruction imposed by the processing and the impacts on the ambiguity suppression. The potential of the algorithm when applied to DPC systems like TerraSAR-X with its split receive antenna is demon­strated and the benefits re­sulting from the additional receive ap­ertur e regarding swath width and resolution are pointed out. Further on, the impact of ad­ditive and phase noise on the recon­struction and its sensitivity against these perturbations are shown.

This paper discusses the capabilities of multistatic SAR satellite configurations for different applications like high resolution DEM generation using multibaseline single-pass crosstrack interferometry, 3-D vegetation mapping and layover solution with spaceborne SAR tomography, high resolution wide swath SAR imaging with sparse satellite arrays, multibaseline velocity estimation of moving objects and scatterer fields, spatial and radiometric resolution enhancement in SAR images, and multistatic imaging for improved detection and classification. Furthermore, some major challenges like phase and time synchronisation, multistatic SAR processing, satellite orbit selection, and relative position sensing will be addressed.

We address the problem of reconstructing parameters \nregarding the vertical structure of forests from multibaseline \nSAR data. The attention is focused on the retrieval of the \nforest height so that the case of vanishing ground scattering is \nconsidered. \nThe cross-spectral density of the scattered field is taken as datum \nof the problem and the forest height is searched for by tackling \nan optimization problem. An analytical and numerical analysis \nof the involved functional is performed by taking due care to the \ndiscrete nature of the scatterers.

In this paper the detection of point-like coherent scatterers (CSs) in single and quad-pol images is addressed and applied on polarimetric and interferometric high-resolution SAR data. For the detection of CSs two different approaches are described, both based on spectral correlation. The effect of multi acquisitions on the number and quality of CSs is verified and the results are discussed. The role of polarimetry on the detection is evaluated and the potential of polarimetric based information extraction as the estimation of the orientation angle – is addressed.

The displaced phase centre (DPC) technique enables 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 results in a non-uniform 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 also has a great potential for multistatic satellite constellations as well as the dual receive antennas in Radarsat II and TerraSAR-X

In this paper, inversion of synthetic aperture radar data is dealt with and, in particular, the problem of reconstructing forest height is addressed. The forest is modelled as a random volume of particles over a rough ground. Under this model, the forest height turns out to be related to the bandwidth of the scattered field. The inversion is cast as an optimization one. Preliminary results using DLR-ESAR experimental SAR data are shown

TanDEM-X is a mission proposal for a TerraSAR-X add-on satellite for high-resolution single-pass SAR interferometry. This mission proposal has been selected for a Phase A study within the scope of a Call for Proposals for a next German Earth Observation Mission to be launched in 2008/2009. The mission has the goal of generating a global Digital Elevation Model (DEM) with an accuracy corresponding to the DTED-3 specifications (12 m posting, 2 m relative height accuracy for flat terrain). This goal will be achieved by means of a second, TerraSAR-X like satellite (TanDEM-X) flying in a close orbit configuration with TerraSAR-X. This paper describes the mission concept and requirements, including several innovative aspects like operation modes, orbit selection and maintenance as well as PRF and phase synchronization. Results from a detailed performance estimation show the achievable DEM accuracy. Finally, an overview of the potential of the TanDEM-X mission for several scientific applications is presented.

The displaced phase center (DPC) technique will enable a wide-swath synthetic aperture radar (SAR) with high azimuth resolution. In a classic DPC system, the pulse repetition frequency (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 letter derives an innovative reconstruction algorithm and shows that an unambiguous reconstruction of a SAR signal is possible for nonuniform sampling of the synthetic aperture. This algorithm will also have great potential for multistatic satellite constellations as well as the dual receive antenna mode in Radarsat 2 and TerraSAR-X.

In this overview, reasons are provided on why we do need to place multi-modal, multi-band single and multiple pass POLinSAR monitoring platforms into air and space. The questions "on what POLinSAR monitoring can provide that POL-SAR and IN-SAR by themselves cannot accomplish" is assessed; whereupon facts and justifications on placing POL-IN-BISAR satellite clusters into space are presented. Reasons for this technology becoming a basic requirement for current, near-future and much more so for future all day & night year-round monitoring of the terrestrial covers are analyzed in view of the un-abating and uncontrollable terrestrial population explosion, which has, does and for ever will result in unavoidable conflicts deteriorating unfortunately at times into terrorism. The pertinent questions on how to reduce the exorbitant cost for initiating this "home-globe security protection" technology are therefore also broached, and the expected benefits are laid out. The pertinent National and International airborne and space borne multi-modal, multi-band SAR remote sensing and security conflict surveillance support agencies are herewith invited for co-sponsoring our proposal, which is timely and fleets of orbiting multi-band POLinSAR platforms are urgently required to be placed into space.

The performance and capabilities of a bistatic spaceborne synthetic aperture radar (SAR) are analyzed, where the transmitter and receiver are in different orbits. Such a configuration may be optimized for a broad range of applications like frequent monitoring, wide swath imaging, single-pass cross-track interferometry, along-track interferometry, resolution enhancement or radar tomography.

Digital beamforming on receive is an innovative concept to gather additional information about the direction of scattered radar echoes during the acquisition of synthetic aperture radar (SAR) images. This additional information can be used to suppress range and azimuth ambiguities, to improve geometric and/or radiometric resolution, and to increase the unambiguous swath width. It is shown that this technique is especially promising in combination with bistatic SAR constellations, where digital beamforming on the receive will allow for more compact antenna structures. Furthermore, an extension of digital beamforming to multistatic SAR formations will enable a reduced antenna aperture for each receiver, thereby allowing cost-effective and powerful SAR missions with wide swath coverage in the future.

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.

In this work the potential of using the interferometric coherence at different polarisations over surface scatterers in order to extract information about surface parameters is investigated. The sensitivity of the individual coherence contributions to surface roughness and moisture conditions is discussed and simulated using a polarimetric surface scattering model. Finally, experimental airborne SAR data are used to demonstrate the discussed effects.

The results of an interferometric performance analysis for spaceborne parasitic SAR configurations are presented. The analysis includes errors due to limited SNR, block adaptive quantization, range and azimuth ambiguities, and geometric decorrelation for both flat surfaces and random volumes. Relative height accuracies for three spaceborne configurations with PALSAR, ASAR and TerraSAR-X as illuminators are derived and compared.

Among the family of exponential distributions the normal or Gaussian distribution plays a fundamental role in natural environments-above all-due to its symmetrical properties. In distributed terrain the size, position and orientation of scatterers within the ground resolution cell of a SAR system is Gaussian distributed as well as their backscatter over a series of radar pulses; particularly when the size of scatterers is of similar magnitude as the radar wavelength and their number is greater then 10. Now, the motion of the SAR system influences the statistics of backscatter in such a way that its symmetrical properties are lost to asymmetry. The more the structure of terrain changes from homogeneous to heterogeneous (from pasture to forest) and from heterogeneous to extremely heterogeneous (from forest to urban) the more the statistics of SAR backscatter deviates from Gaussian. Hence, another distribution, the inverse Gaussian, arises most naturally for backscatter of distributed targets under the assumption that the single scatter cell keeps its Gaussian behavior. (The term inverse refers to the cumulant generating function of this distribution being the inverse of the Gaussian one.).

Recently, several passive interferometric configurations consisting of two or more receiving only micro-satellites operating in a fully polarimetric mode positioned ahead or behind a master satellite have been proposed as a cost efficient option for future single-pass interferometric spaceborne SAR systems. In this paper the potential performance of such a passive polarimetric and interferometric micro-satellite concept for the estimation of forest structure parameters in terms of polarimetric interferometry is investigated.

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.

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.

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.

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.

Presents 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.

Proposes new algorithms for synthetic aperture radar (SAR) raw data compression and compares the resulting image quality with the quality achieved by commonly used methods. The compression is carried out in time and frequency domain, with statistic, crisp, and fuzzy methods. The algorithms in the time domain lead to high resolution and a good signal-to-noise ratio, but they do not optimize the performance of the compression according to the frequency envelope of the signal power in both range and azimuth directions. The hardware requirements for the compression methods in the frequency domain are significant, but a higher performance is obtained. Even with a data rate of 3 bits/sample, a satisfactory phase accuracy is achieved which is an essential parameter for polarimetric and interferometric applications. Preliminary analysis concerning the suitability of the proposed algorithms for different SAR applications shows that the compression ratio should be adaptively selected according to the specific application.>

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/spl deg/ 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.>

A method for removing the azimuth ambiguities from synthetic aperture radar (SAR) images is proposed. The basic idea is to generate a two-dimensional reference function for SAR processing which provides, in addition to the matched filtering for the unaliased part of the received signal, the deconvolution of the azimuth ambiguities. This approach corresponds to an ideal filter concept, where an ideal impulse response function is obtained even in the presence of several phase and amplitude errors. Modeling the sampled azimuth signal shows that the absolute phase value of the ambiguities cannot easily be determined due to their undersampling. The concept of the ideal filter is then extended to accommodate the undefined phase of the ambiguities and also the fading of the azimuth signal. Raw data from the E-SAR system have been used to verify the improvement in image quality obtained by the new method. It has a substantial advantage in enabling the pulse-repetition frequency (PRF) constraints in the SAR system design to be relaxed and also for improving SAR image quality and interpretation.>

A time-domain subaperture approach that is suitable for real-time synthetic aperture radar (SAR) processing and that produces high-quality, full-resolution images is presented. The real-time subaperture algorithm is based on an approximation of the phase history correction in each subaperture with a simple linear correction, which can be carried out by an up/down-conversion of the received signal followed by a moving average operation. The characteristics of the impulse response function are improved considerably by means of a frequency overlap of the subapertures and become comparable to a conventional matched filter response. Special algorithms for high-quality SAR processing, such as autofocus, Doppler centroid estimation, and range migration correction, can be efficiently implemented. High flexibility is achieved for multilook processing and no secondary range compression is needed, even for high squint angles.>

The multilook processing used in SAR (synthetic aperture radar) image formation to reduce the speckle noise is considered. Two multilook techniques are proposed for improving the radiometric resolution without altering the geometric resolution and the characteristics of the impulse response. These techniques are based on the formation of looks with different bandwidths. The final image is formed by giving each look a proper size and weighting, and by adding them incoherently. The looks with wide bandwidth contribute to an improvement of the overall geometric resolution, while the looks with narrow bandwidth improve the overall radiometric resolution. The equivalent number of looks is more than 2.3 times the number of independent looks and is superior to conventional multilook processing with overlapping. Finally, using the proposed techniques, an algorithm for efficient SAR and ScanSAR processing is presented, and its validity is proved by image comparison and analysis.>