When radar signals penetrate snow and ice, they experience additional delays and directional changes due to the higher refractive index compared to that of air. These propagation effects should be taken into account accurately when processing, simulating, or geocoding radar data. Travel time computation is straightforward when the refractive index is constant, but it becomes challenging in heterogeneous media. This letter introduces novel methods based on the Eikonal equation and Fermat’s principle for efficiently computing radar signal travel times in heterogeneous snow and ice volumes. These approaches can accommodate nearly arbitrary refractive index distributions, ensuring precise handling of propagation effects in radar remote sensing applications.

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.

This paper presents an image formation algorithm for the focusing of SAR data with space-variant impulse response functions caused by eccentric orbits around small high-curvature surfaces, such as is encountered in stable orbits around Saturn’s moon Enceladus, a potential target for future SAR missions such as the Nightingale mission concept under development at JPL. Due to the extreme geometry, the range history shows a significant dependence on the target’s azimuth position within time scales significantly shorter than the synthetic aperture duration. Therefore, additional steps are needed in order to compensate for this effect and minimize image degradation. In this context, the present contribution evaluates the space variance of the geometry for SAR surveys over Enceladus and proposes a processing flow to account for it. Point target simulations using the proposed processing algorithm are shown to verify the approach.

In this letter, we discuss the estimation of the location of ionospheric irregularities exploiting the appearance of intensity scintillations in ALOS-2 images, as they are semifocused at different heights. The intensity scintillations (stripes) are not always visible in the single-look complex (SLC) images. However, they start to be visible, as the image is semifocused at different heights with a peak of contrast where electron density irregularities are found (ionospheric height). The slant range between the satellite and the ionospheric plane can be estimated and converted directly into the ionospheric height by autofocusing the stripes. The observations show good agreement with the height of maximum ionization estimated by the International Reference Ionosphere (IRI). Furthermore, we perform an alternative geometric validation based on feature tracking by comparing the shifts between azimuth sublooks. With both methods for the presented dataset, the height of the ionospheric irregularities was estimated to be 330 km.

This letter presents an algorithm for the introduction of ionospheric disturbances into synthetic aperture radar (SAR) simulations in an aperture-dependent manner using subapertures. Its suitability is compared with other methods that follow the beam-center approximation. The method can be generalized to the injection of all kinds of disturbances, and its two main benefits are the accuracy of the squint angle accommodation inside the synthetic aperture and the possibility of neglecting the static ionosphere assumption. For example, realistic ionospheric disturbance maps (phase and intensity scintillation) are introduced into clutter images simulated for the Biomass mission. In this case, with a typical ionospheric irregularity height of 350 km, the limiting azimuth resolution of the irregularities to be injected is around 337 m.

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.

The motion of the synthetic aperture radar (SAR) sensor during the transmit and receive events introduces a space-invariant polychromatic phase, known as intrapulse (IP) effect, which may degrade the impulse response function (IRF) if left uncorrected. A straightforward solution is already available in the literature to compensate for this effect for conventional SAR systems using a single waveform during the acquisition. However, there is currently no solution in the case the SAR system transmits an interleaved up/down sequence. If left uncorrected, besides the impulse response degradation in terms of azimuth and range resolution loss, additional azimuth ambiguities will appear. This letter proposes an azimuth-reconstruction-based approach to mitigate the IP effect for both single-receiver and multireceiver SAR systems with up- and down-chirp (UDC) modulation. The performance of the approach is demonstrated with point-target simulations.

Spaceborne differential SAR interferometry (D-InSAR) has been demonstrated to potentially allow snow water equivalent (SWE) change measurements for dry snow on a spatial scale, resolution, and accuracy unprecedented by other sensor concepts. However, its operational use is hindered mainly because of: 1) low coherence areas resulting from temporal decorrelation, complicating a robust phase unwrapping and 2) an unknown phase offset due to the$2\pi $ambiguity of the interferometric measurement and therefore a strongly biased SWE change estimate. Furthermore, conventional D-InSAR does not provide a direct measurement of the snow density, which is used in the phase-to-SWE inversion and is an important snow parameter. This article presents strategies to potentially overcome these shortcomings by exploiting simultaneously acquired interferograms with different squint angles. The different lines of sight result in differential phase delays introduced by a SWE change. The phase difference between the interferograms may be exploited to produce a low-resolution SWE estimate without the need for phase unwrapping and to resolve the$2\pi $phase ambiguity of the single interferogram. In addition to that, the ratio between the interferograms is a measure of the dielectric permittivity of the snow and can be related to the snow density. The theoretical performance and functionality of the strategies are analyzed for the planned Harmony mission (ESA’s Earth Explorer 10) based on simulated data, indicating great potential of the approach given the large squint diversity of the Harmony constellation.

Single-pass InSAR elevation measurements of dry snow, firn, and ice are known to be substantially biased downward due to a partial penetration of the radar signals into the medium, resulting in a phase center location within the volume. The so-called penetration bias, i.e., the elevation difference between surface and InSAR phase center, can be estimated from the contribution of the volume to the interferometric coherence and may be used to retrieve the surface elevation. In this paper, we show that both an additional elevation bias and a horizontal shift occur in the InSAR processing for natural media with a dielectric constant different to the one of air, originating from an uncompensated stretch of the vertical wavenumber in the medium and refraction effects at the surface. This geolocation error depends on the magnitude of the penetration bias, the dielectric constant, and the acquisition geometry. It may reach up to few meters for X- and C-band frequencies and more for lower frequencies and therefore may significantly affect cryospheric elevation products from past (SRTM), current (TanDEM-X), and future (e.g., Harmony, Tandem-L) SAR interferometers. In this paper, the geolocation error is assessed and an adapted interferometric processing allowing for an accurate geolocation (i.e., surface elevation measurement) is presented.

Low-frequency Synthetic Aperture Radar (SAR) images are affected by trans-ionospheric propagation of the radar waves. 2D Total Electron Content maps can be obtained as a product of the ionospheric corrections, allowing for imaging of ionospheric irregularities at very high resolution and broader coverage compared to other sensing technologies (like GNSS, ionosondes, etc). We analyze in this paper the case of the Biomass mission and characterize the errors in the imaging of the ionosphere resulting from the calibration algorithms foreseen for its Ground Processor Prototype: a Faraday rotation-based and an autofocus-approach. The analysis relies on a turbulent power law Rino model for the perturbation of the ionosphere and the spectral behavior of the calibration algorithms. We also discuss the suitability of both approaches for image correction in different scenarios (varying Signal-to-Noise-Ratio and geographic location).

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.

Plenty of data records demonstrate that differential InSAR acquisitions of snow covered areas are often affected by severe temporal decorrelation, complicating the estimation of snow physical parameters such as the snow water equivalent. The decorrelation effect is commonly attributed to a change in the underlying scattering center distribution due to melting/refreezing, compacting of snow, or redistribution of underlying vegetation. We demonstrate that a mere change of the dielectric constant of a dry snow cover may lead to severe decorrelation, even without a change in scatterer distribution, which provides additional opportunities for the estimation of snow parameters. In this paper, a first discussion of the snow-induced decorrelation effect is provided and the derived model is evaluated against Sentinel-1 12-day coherence data using SWE measurements provided by the Copernicus Global Land Service.

Over the last decades, repeat-pass SAR interferometry (InSAR) for deformation measurement and topographic mapping has revolutionized our understanding of many geophysical processes on Earth. A new mission concept, currently in development at the Jet Propulsion Laboratory (JPL) and Caltech, aims at using orbital repeat-pass InSAR for deformation and topography mapping of Saturn’s ice-covered and geologically active moon Enceladus. In this paper, we present an initial performance assessment of the system and the suggested SAR processing approach, along with simulated InSAR acquisitions using a DLR in-house End-to-End performance simulator.

ESA’s BIOMASS is the seventh Earth Explorer mission. It will be devoted to the global monitoring of: 1) above ground forest biomass and biomass change maps with an accuracy90%, a 50-200 m spatial resolution, and 2-12 months temporal resolution; and 3) global forest height maps with an accuracy of 20-30%, 100 m spatial resolution, and 12 months temporal resolution.BIOMASS is based on a P-band SAR (438 MHz center frequency, 6 MHz bandwidth) orbiting in a dawn-dusk, Sun-synchronous orbit at 674 km, that will systematically acquire fully- (quad-) polarized image data in an interferometric mode over all major forested areas on the globe and a tomographic phase (7 images) to retrieve forest vertical structure information. At P-band ionospheric effects are very important and need to be corrected for. This paper describes the current status of the Ionospheric module for the Biomass End-to-end Performance Simulator (BEEPS-IOM).

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.

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.

This paper presents our new SAR processing framework known as PRISM (Processor for Interferometric SAR Missions). This flexible approach allows the efficient and accurate processing of SAR data independent of the sensor and the acquisition mode. The two main PRISM components (the focusing and the interferometric chains) are described in this paper together with the philosophy of the software architecture. Experimental results are presented and discussed based on the impulse response function analysis of simulated data as well as the focusing and interferometric results using real TerraSAR-X data.

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.

Phase synchronization is a crucial requirement for bistatic and multistatic SAR missions. One possible solution for this problem could be to use the same master oscillator for the radar payload and the GNSS receiver, which could allow the recovery of the oscillator phase noise from the carrier phase measurements. This solution would be cost-effective and straightforward, but its effectiveness depends on whether the GNSS receiver would maintain the spectral purity of the master oscillator internally. We executed zero and short baseline experiments to evaluate this technique with a commercial receiver. Despite some unexpected signatures on the carrier phase single-difference measurements, we achieved frequency synchronization of less than 10 ppm and phase synchronization of less than one degree at the L1 carrier frequency after applying a moving average filter of one second at a 10 Hz sampling rate. The results suggest the feasibility of the GNSS-based phase synchronization concept, but further experiments are necessary to evaluate it thoroughly.

Differential SAR interferometry for deformation measurement and coherent change detection has revolutionized our understanding of many geophysical processes on the Earth since its first demonstration in the 1980’s. The recently selected NASA Discovery mission VERITAS aims to be the first planetary mission to use differential radar interferometric techniques for measuring surface deformation and for coherent change detection measurements. These measurements provide the most sensitive means for detecting and measuring present day geologic activity on Venus. Differential SAR interferometric measurements at Venus by the VERITAS mission has several interesting and unique features including the atmosphere and orbit considerations that are described in this paper.

In a conventional synthetic aperture radar (SAR), good azimuth resolution and wide coverage pose contradicting requirements for the pulse repetition frequency (PRF). With a PRF tailored to a desired resolution, range ambiguities is then the main obstacle to a wide swath imaging. This paper proposes a method, based on a minimum variance distortion-less response (MVDR) beam former, to suppress range ambiguities, in range Doppler domain, via a coherent combination of echoes from multiple satellites maneuvering in a close formation. Numerical results in L band confirm validity of the approach. Limits of the technique and main difficulties in improving suppression performance are also discussed.

At lower frequencies, synthetic aperture radar (SAR) images can be significantly affected by ionospheric scintillations. This letter proposes an approach to estimate ionospheric artifacts in single SAR images by combining Faraday rotation (FR) estimates and autofocus methods by using a Wiener filter. This novel approach achieves superior performance compared to using these techniques independently. This improvement is demonstrated by injecting randomly simulated ionospheric phase screens into simulated point targets and airborne data adjusted to the parameters of the Biomass system.

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:

Observations of wind and ocean surface velocity vectors by along-track interferometry (ATI) with the synthetic aperture radar (SAR) are not only important for direct applications but also to increase understandings of ocean upper layer mixing, air–ocean interactions, and mapping submesoscale (1–10 km) structures. An experimental bidirectional (BiDi) ATI acquisition mode of TanDEM-X observes with two squinted beams separated by an angle of approximately 13.2° in azimuth on the ground. The baseline is very short, and the along-track interferometric phase (ATI phase) of the ocean surface in the line-of-sight direction of the beams can be interpreted as a total Doppler velocity. The 2-D Doppler velocity field will thus include wind-wave detected motions. In this article, Doppler velocity fields acquired from this experimental acquisition mode are presented. The sequential retrieval of wind vector and total surface current vectors (TSCV) is demonstrated on the BiDi TanDEM-X data. The retrieval algorithm builds on existing geophysical model functions (GMFs) of normalized radar cross section (NRCS) and Doppler velocity. XMOD2 and a GMF based on the Elfouhaily ocean wave spectrum coupled with a Kirchhoff approximation (EOWS&KA) are used. The retrieved wind fields are generally consistent with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5. While the ATI phase errors are small, the retrieved TSCV field looks promising. Acquisitions were located at sea over the tip of the Novaya Zemlya in Russia and over an area near Tromsø, Norway.

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.

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.

ESA's BIOMASS Earth Explorer mission aims at providing global above ground forest biomass. Its payload is a UHF synthetic aperture radar. At UHF ionospheric effects, notably the group delay and phase advance, the Faraday rotation, and the intensity and phase scintillations (or rapid fluctuations) are significant and have to be properly modelled to be compensated, to understand the limitations of the proposed technique, and to select the optimum observation conditions. This paper summarizes the structure and main characteristics of the ionospheric module of the BIOMASS end-to-end performance simulator (BEEPS-IOM).

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.

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.

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.

Ambiguities in short-baseline ATI interferometry need to be treated not as noise that lowers the coherence, but as a source of bias. A mathematical formulation of the interferometric ambiguity model is given, and an approach to correct ambiguities is proposed and illustrated with simulation results.

The NASA-ISRO SAR (NISAR) mission will map the Earth's surface every 12 days using a reflector-feed system with scan-on-receive (SweepSAR) capability. An alternative for continuous swath coverage is to employ staggered SAR operation, i.e., to vary the pulse repetition intervals over time. In this way, the blocked (missing) samples are spread in range and azimuth and can be interpolated prior to focusing. However, the relatively low azimuth oversampling intended for NISAR in combination with the strong non-uniformity of the staggered data grid forbids the use of conventional interpolation kernels. This paper discusses a processing approach for the NISAR staggered data and validates the strategy with simulated data. Moreover, the impact of the staggered operation for contrast-based and interferometric applications is discussed with examples.

This paper addresses the aspects of image formation and interferometric processing in the frame of the STEREOID Earth Explorer 10 candidate mission. In particular, the large along-track baseline configuration (approx. 250 km), which results in a high Doppler centroid, will be addressed in terms of bistatic SAR image formation and interferometric processing. The proposed focusing algorithm is validated with point-target simulations, while the interferometric investigations are performed using raw data generated with a bistatic end-to-end simulator.

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.

Utilizing digital multi-channel technology, spacebome synthetic aperture radar instruments are capable of imaging swath widths of hundreds of kilometers at fine azimuth resolution. The main benefit follows through the extension of the trade space and the use of new digital beam-forming techniques facilitated through the multi-channel instrument architecture. This is truly a quantum leap as the performance of these systems will be orders of magnitude better than current in-orbit and state-of-the art systems. One of the basic restrictions applicable to spaceborne platforms hosting both the transmitter and receiver is the "blinding" of the receiver during the transmit time instances, which manifests itself through imaging gaps. One of the main challenges the instrument designers are faced with, is to circumvent these gaps, requiring the use of dedicated instrument operation modes. An alternative approach is multi-beam imaging, i.e. to allow the gaps in the single SAR acquisition, while using an appropriate mission design for filling the blind gaps. This paper explores the trade space options for high-resolution wide-swath SAR imaging. The comparison of multi-beam and gapless imaging from an instrument design and performance point of view is elaborated.

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 this paper, the potential of very-high resolution multi-baseline (MB) SAR images acquired in Staring Spotlight mode and Pursuit Monostatic configuration of the TanDEM-X mission during the Science Phase will be evaluated for series of InSAR acquisitions over a natural scene. In particular, this work focuses on the use of very-high resolution and baseline diversity to help to characterize X-band backscattering in the 3-D space, e.g. in terms of penetration in forest scenarios. At the same time, the application of classical MB InSAR techniques to identify geometry-related issues such as layover in (mainly) surface areas with rough topography is preliminarily addressed.

Augmenting traditional spaceborne SAR sensors with additional receive-only satellites in close formation enhances the observation space, allowing for single-pass interferomtry in along-and/or across-track with flexible baselines and the potential to build tomographic stacks with reduced temporal decorrelation properties. The feasibility of such add-ons is presently investigated by ESA and other national space agencies and for a variaty of master satellites operating from X-to L-band. This paper presents a simulation framework for complete scenes dedicated specifically to the analysis of the additional technical requirements imposed by the bistatic SAR imaging geometry with relatively large along-track separation of illuminating master/chief and the receive-only slaves/deputies.

In this contribution, we propose the use of 2-looks Synthetic Aperture Radar data to retrieve azimuth velocities of moving targets with a single platform. The established technique to retrieve velocities of moving targets is Along-track Inter-ferometry (ATI), which provides a measurement of the velocity in the radar line of sight direction by employing two or more phase centers separated in the along-track direction. The use of 2-looks data allows to observe targets at two different instants of time, with a Doppler separation, enabling the retrieval of the target velocity in the azimuth direction. We introduce the 2-looks Terrain Observation by Progressive Scans (TOPS) mode, presenting the performance that can be achieved with the TerraSAR-X system. Moreover, we present experimental results with real data acquired with TerraSAR-X over coastal areas to retrieve velocities of vessels. A validation of the results with Automatic Identification System (AIS) data (ground truth) provides accuracies below 1 m/s.

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.

This paper addresses the performance in the retrieval of 3-D mean deformation maps by exploiting simultaneous or quasi-simultaneous squinted synthetic aperture radar (SAR) interferometric acquisitions in a repeat-pass scenario. In multisatellite or multibeam low earth observation missions, the availability of two (or more) lines of sight (LOSs) allows the simultaneous acquisition of SAR images with different squint angles, hence improving the sensitivity to the north-south component of the deformation. Due to the simultaneity of the acquisitions, the troposphere will be highly correlated and, therefore, will tend to cancel out when performing the differential measurement between the interferograms obtained with the different LOSs, hence resulting in a practically troposphere-free estimation of the along-track deformation measurement. In practice, however, the atmospheric noise in the differential measurement will increase for increasing angular separations. This paper expounds the mathematical framework to derive the performance by properly considering the correlation of the atmospheric delays between the simultaneous acquisitions. To that aim, the hybrid Cramér-Rao bound is exploited making use of the autocorrelation function of the troposphere. Some performance examples are presented in the frame of future spaceborne SAR missions at C and L band.

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.

Companion missions provide a path to add single-pass interferometric capabilities to existing monostatic SAR missions. Large baselines, only achievable through formation flying, are necessary both to achieve the required performance in many interferometric applications and to enable SAR tomography. Some of the main challenges include meeting tight formation flying requirements, often calling for multiple-companion solutions, and oscillator synchronization.

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.

The contribution focuses on the technical aspects related to the focusing and interferometric processing of bistatic data acquired by companion satellite (CS) SAR missions. In particular, the processing aspects related to the large along-track baseline configuration will be addressed, for the processing needs to properly consider a potential high squint angle. The technical challenges encompass synchronization, image formation and coregistration. On the other hand, the availability of a large squint angle has some advantages in terms of the sensitivity to the north-south component of the deformation for interferometric applications. The theoretical derivations are complemented with point-target simulations and Monte-Carlo simulations.

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.

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.

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.

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.

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.

SAOCOM-CS denotes a satellite formation based on SAOCOM, Argentina's L-band SAR satellite, and a small passive companion satellite (CS) of the European Space Agency (ESA). The realization of this mission is currently under evaluation by ESA. A possible acquisition geometry of particular interest for its scientific and innovative value is a large baseline bistatic (LBB) configuration. This paper discusses the impact of the LBB geometry on the SAR imaging performance computation and presents the expected performance results in a reference scenario based on a stripmap, full polarization operational mode.

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.

This contribution presents two experiments performed with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites working in the pursuit monostatic configuration. Their objective is to estimate the along-track component of the motion in the scene in repeat-pass scenarios with an accuracy better than the one given by the stripmap azimuth resolution. Such performance is possible by exploiting the angular diversity of the bidirectional (BiDi) SAR mode and the π-shifted (or staggered) TOPS.

The orbit for Earth observation satellite synthetic aperture radar (SAR) missions is maintained within an Earth-fixed orbital tube to ensure ground-track coverage repeatability and, consequently, to enable repeat-pass interferometric compatibility between data takes. In this letter, it is shown that the size of the orbital tube may affect the interferometric performance in terms of azimuth spectral decorrelation and azimuth coregistration accuracy under the presence of squint. These effects require special consideration for SAR burst modes, such as ScanSAR or TOPS (i.e., Terrain Observation by Progressive Scans). This letter presents and analyzes these aspects in the frame of the Sentinel-1 mission.

This letter discusses some aspects of the implementation of Delta-k methods for shift estimation with synthetic aperture radar images. In particular, it shows that a common Delta-k algorithm, which postpones the multilooking to the differential interferogram and is therefore robust to the presence of interferometric fringes in the averaging window, does not reach the maximum possible performance and should be better considered as a variant of incoherent cross-correlation. A small adaptation, retaining some multilooking at interferogram level, can significantly improve the efficiency.

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.

The spaceborne SAR geometry is space-variant due to the curved orbit and Earth's rotation. A first effect is the azimuth variance, which in current missions is accommodated by processing the data in azimuth blocks larger than the synthetic aperture and updating the processing parameters of the kernel. However, such option is not valid when the variation occurs within the observation time, as it happens in high-resolution modes. A second effect occurs due to the topography dependence of the geometry, i.e., targets at the same zero-Doppler slant-range distance but different altitudes have slightly different range curvatures. This topography dependence is neglected in current SAR missions. However, it has been recently shown that in very high-resolution spaceborne SAR image formation (e.g., resolutions a factor 10 of the wavelength or smaller) the azimuth- and topography-dependence become critical. This paper analyses first these two effects and then proposes a processing scheme to efficiently handle the geometry. The suggested approach is validated with simulations and TerraSAR-X staring spotlight data.

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.

This contribution addresses the efficient evaluation of Fourier-based kernels for synthetic aperture radar (SAR) image formation. The goal is to evaluate the quality of the focused impulse response function and the residual phase errors of the kernel without having to implement the processor itself nor perform a costly point-target simulation followed by the processing. The proposed methodology is convenient for situations where the assumption of a hyperbolic range history does not hold anymore, and hence a compact analytic expression of the point target spectrum is not available. Examples where the hyperbolic range history does not apply include very high-resolution spaceborne SAR imaging or bistatic SAR imaging. The approach first computes numerically the two-dimensional (2D) spectrum of a point target and then uses the transfer function of the focusing kernel to match it, and hence obtain the impulse response function (IRF). The methodology is validated by comparing the matched IRFs with the ones obtained using point-target simulations.

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) systems construct the equivalent to a very long antenna array by coherently combining radar echoes of a scene acquired along the trajectory of a space- or airborne radar. A key underlying assumption, or requirement, is that the observed scene stays coherent during the acquisition. This is true for static scenes, in particular when the acquisition time is kept short. For decorrelating targets, the azimuth resolution achievable by a SAR system is limited by the coherence time of the targets, τcoh, which limits the length of the synthetic aperture to the product of τcohand the azimuth velocity of the system. Typical cases where this can be an issue is for the observation of water surfaces, where the coherence times at microwave frequencies are typically well below 100 ms, which makes them short compared to the typically associated integration times.

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 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.

The distortions caused in conventionally focused TOPS SAR images which originate from the azimuth-variant Doppler centroid within a burst are presented and analyzed. In particular, the azimuth distortions due to topography mismatch in focusing stages and the range distortions due to the assumption of validity for the stop-and-go approximation are expounded in detail. Compensation strategies to accommodate the two effects in an accurate and precise manner are discussed and validated with TOPS data acquired with TerraSAR-X.

This paper reports on systematic height offsets that have been observed in TanDEM-X by evaluating a large number of digital elevation model (DEM) acquisitions. Besides the expected instrument and baseline offsets, which are compensated in the calibration chain, two unexpected external error sources have been identified that apply to any formation flying bistatic SAR interferometer. The first contribution is due to relativistic effects and can be well explained within Einstein's special theory of relativity. The second effect is due to differential delays in the troposphere. It is shown that the theoretic predictions are in good agreement with the observed offsets. All necessary corrections have in the meantime been integrated in the operational TanDEM-X processing chain.

The cooperative nature of the TanDEM-X mission bistatic operation allows the periodically exchange of information between transmitter and receiver over the SAR acquisition. With the information gathered during the synchronization (sync) events it is possible to perform the phase and time referencing necessary for the calibration of the bistatic SAR image. However, given that transmitter and receiver are dedicated to the link during the sync events, the reception of SAR signal is periodically interrupted leading to gaps in the SAR raw data. Consequently, the processed SAR image will be corrupted, especially when acquiring close to the Nyquist rate. The paper addresses different approaches based on parametric and non-parametric spectral estimation to reconstruct the missing data and presents results with SAR data from TerraSAR-X and TanDEM-X.

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.

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 describes the implementation of an efficient implementation of a Fast Factorized Back Projection (FFBP) algorithm for Circular SAR (CSAR) trajectories for real air- borne data. Unlike Fourier-domain based focusing processors, this approach considers the azimuth variance and topography changes with high accuracy, while improving significantly the computational time factor in comparison with the direct Back Projection (BP). To further accelerate the focusing, the circular FFBP was implemented also on a Graphics Processor Unit (GPU). In the second part of the document is shown a fully polarimetric image of the region of Kaufbeuren, Germany, (acquired by the DLR's E-SAR system) focused with this method to the theoretical limit of ~λ/4 j. Thus, the efficiency, accuracy and performance of the circular FFBP is demonstrated, as well as the potential of CSAR when focusing over 360 °.

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.

TanDEM-X is a high-resolution interferometric radar mission with the main goal of providing a global digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. It is, moreover, the first genuinely bistatic spaceborne SAR mission, and, independently of its usual quasi-monostatic configuration, includes many of the peculiarities of bistatic SAR. An experimental, versatile, and flexible interferometric chain has been developed at DLR Microwaves and Radar Institute for the scientific exploitation of TanDEM-X data acquired in non-standard configurations. The paper describes the structure of the processing chain and focusses on some essential aspects of its bistatic part. Some experimental results performed with TerraSAR-X demonstrate the flexibility of the implemented processor.

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.

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.

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.

Bistatic configurations have awakened a great deal of interest in the recent past among the SAR research community for both airborne and spaceborne sensing. However, bistatic SAR campaigns remain a technical challenge. In this context, current experiments are helping to understand real engineering problems concerning bistatic SAR data acquisitions. In February 2003, DLR and ONERA carried out successfully [1] one of the very first bistatic airborne campaigns. The E-SAR and RAMSES systems were flown in two basic geometrical configurations, both involving parallel flights at the same speed: one defined by along-track acquisitions and the other by across-track acquisitions. These bistatic configurations share with monostatic SAR the invariancy in the along-track dimension, which allows and suggests the using of Doppler domain focussing techniques. \n \nThis paper puts forward an extension of airborne SAR monostatic focussing algorithms to the azimuth-invariant, bistatic case. A derivation of the point response in range-Doppler domain is presented; the differences between azimuth-invariant, bistatic and monostatic point responses are outlined. Using these results, bistatic focussing solutions based on range-Doppler (RD) [2] and extended chirp scaling (ECS) [3] algorithms are analysed. Particular implementation problems, like Doppler parameter estimation and bistatic motion compensation [4] strategies are also presented. Finally, the algorithms are tested with real data from the DLR-ONERA bistatic airborne campaign.

This paper presents a radiometric resolution optimization strategy which can be used in new generation of Synthetic Aperture Radar (SAR) systems. An expression, which allows optimization depending on application, has been developed for this purpose. In particular, special effort has been made for improving the radiometric resolution keeping the geometric resolution constant. Optimization examples have been carried out with realistic parameters taken from TerraSAR-X. The objective is to provide an effective and realistic way to tradeoff the instrument parameters for optimal exploitation of SAR images. Furthermore, a precise SNR formulation has been derived including processing gain and noise distribution