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.

Across-track synthetic aperture radar (SAR) interferometry is a well-established technique to obtain the topographic information using a pair of SAR images, whose coherence influences the topographic height accuracy. Volume decorrelation occurs in the presence of semi-transparent media, such as forests and vegetation, and is usually modeled using the vertical scattering profile, thus providing a link between the coherence and the physical parameters of the medium. This has been successfully exploited in polarimetric SAR interferometry and SAR tomography to retrieve 3-D structure information of the scene. Future SAR systems are expected to be equipped with larger bandwidths, which also allow for interferometric acquisitions with larger baselines and enhanced height accuracies. In these cases, a significant amount of co-registration decorrelation may occur within the volume, which should be considered in the model of volume decorrelation. This letter presents a novel volume decorrelation model, which accounts for co-registration effects and accurately predicts the coherence for interferometric acquisitions with large bandwidths and/or baselines. Simulation examples are shown for typical scenarios to prove the validity of the model, which is capable of predicting the phase of the complex coherence of a semi-transparent medium with subdegree accuracy. The proposed model will be a crucial tool to analyze semi-transparent media in future spaceborne SAR missions and drone-borne SAR systems.

The performance tradeoffs required for a small satellite synthetic aperture radar (SAR) system designed to measure surface deformations using persistent scatterer interferometry (PSI) are investigated. Existing X-band satellite data is systematically deteriorated to account for the increased range resolution and noise-equivalent sigma zero (NESZ). It is found that with an NESZ below 0 dB, the deformation signal of a selected region of interest (ROI) can be captured with mean absolute errors of 5 and 15 mm, for ground range resolutions of 5 and 20 m, respectively. This analysis is used to develop preliminary SAR system designs suitable for small satellites.

Synthetic aperture radar (SAR) interferometry (InSAR) is a well-established remote sensing technique capable of providing accurate topographic information of an area. Multiple scattering occurring at different heights within a single resolution cell, however, cannot be resolved using a single baseline and results in a degradation of the height accuracy. A method was recently proposed to retrieve the three-dimensional structure of semi-transparent media using wide fractional bandwidth InSAR. The frequency dependency of the InSAR coherence was exploited to retrieve the three-dimensional structure, assuming that signals with wide fractional bandwidth are available, as it is the case for unmanned aerial vehicles (UAVs) or drones. This paper demonstrates the usage of wide-fractional bandwidth InSAR to estimate the frequency-dependent penetration depths using repeat-pass InSAR data acquired from a radar mounted on a drone platform.

One relevant application of synthetic aperture radar (SAR) is ship monitoring. Conventional SAR modes, however, are limited in their ability to detect small ships over wide swaths (> 100 km) due to their low azimuth resolution. A novel SAR mode is presented in which the pulse repetition interval (PRI) is continuously varied and is much smaller than the minimum PRI imposed by the swath width, producing images that are corrupted by strong range ambiguities. For the same observed swath, this mode has significantly better detection performance than ScanSAR due to higher azimuth resolution and ambiguity blurring effect due to the PRI variation. Waveform alternation can be moreover explored to further blur ambiguities and increase the mapped swath. Spaceborne demonstrations using experimental TerraSAR-X acquisitions are presented. The idea of SAR systems affected by SAR ambiguities is then extended to the case of a cluster of small satellites, whose acquired data are jointly processed to create an accurate, high-resolution digital elevation model (DEM) of the observed scene in a single pass. This technique is demonstrated through a dedicated multi-baseline airborne campaign, carried out using DLR’s F-SAR airborne sensor.

Synthetic aperture radar interferometry (InSAR) is a technique that exploits the phase difference between two synthetic aperture radar (SAR) images to generate digital elevation models (DEMs) of the imaged terrain. InSAR has a well-established theoretical background, but approximations for narrowband signals are often made. To unwrap the interferometric phase, dual-baseline techniques are also commonly employed. Modern InSAR systems will use wider bandwidths to improve the accuracy and resolution of the DEMs. This letter generalizes the expressions of the baseline decorrelation and the critical baseline for InSAR systems with wide bandwidths and large baselines, where the spectral shrinkage is not negligible. It is also shown that radargrammetry offers high potential to unwrap the interferometric phase already on a pixel-by-pixel basis. The proposed approach can also exploit acquisitions with multiple baselines or frequency bands. The results show that a single wideband interferometric acquisition may suffice to perform phase unwrapping for high interferometric coherences. The concepts discussed in this letter will help to design and enhance the performance of future wideband and multiband InSAR missions to deliver a new generation of DEMs with unprecedented resolution and accuracy.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swaths and high resolution. This enables monitoring of wide areas with high detection probabilities and low false alarm rates at short time intervals. Ambiguous SAR modes have proven effective for ship monitoring in remote offshore regions. They deliver high-resolution SAR images over wide swaths that can be effectively exploited for ship detection, even though they are corrupted by significant ambiguities of ships and sea clutter. Building upon the successful demonstration of the staggered ambiguous mode using TerraSAR-X, this letter presents the results of a further experimental acquisition, carried out near the Dutch coast, where an ultrawide ground swath of 160 km is imaged at an azimuth resolution of 2.2 m by introducing in the staggered ambiguous mode alternation of up- and down-chirp waveforms; 79 small ships were detected in the dataset, some of which even near the coast and despite the presence of first-order range ambiguities caused by land scatterers, as these ambiguities are very blurred and manifest as noise like disturbances. These results are of fundamental importance for incorporating the ambiguous modes into the existing and future SAR systems as an efficient additional mode for ship monitoring, suitable for both open sea and coastal surveillance.

The article proposes a novel approach for in-orbit satellite antenna pattern measurement by means of a dedicated small measurement satellite (MES) that flies in Double-Cross-Helix formation. For establishing and maintaining the quality of a satellite mission and its products, antenna pattern measurement while the satellite is in orbit is crucial. In remote sensing missions like synthetic aperture radar (SAR), the pattern is measured using calibration targets on the Earth’s surface. Such on-ground measurements are costly, time-consuming, and only offer 1-D azimuth or elevation patterns within constrained angular ranges. The novel approach provides a 2-D in-orbit measurement that covers the full angular range. The 2-D pattern is obtained from numerous central cuts resulting from small modifications of the MES’s orbit parameters that establish the Double-Cross-Helix formation. The measurement is performed in free space with a single free-flying MES. This avoids all distortions from atmosphere, ionosphere, ground clutter and multipath effects, ambiguities, and volume scattering. The approach is of great value for future satellite missions that are increasingly based on a huge number of electronically steered antenna beams and/or digital beamforming. The Dual-Cross-Helix approach provides faster, cheaper, and more accurate pattern measurements. The article discusses a high-level system concept of a feasible in-orbit pattern measurement of a SAR satellite similar to TerraSAR-X by means of a passive MES that is equipped with a reflecting sphere. Orbit simulations, measurement gain, and angular sampling accuracy analyses verify that the Double-Cross-Helix approach is feasible and advantageous for in-orbit antenna pattern measurement.

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.

PERTEO is a mission proposal intended to provide real-time EO services to reduce Natural Disaster impact by proposing a heterogeneous small satellite constellation and performing in-orbit processing. For that aim, this mission takes advantage of the enhanced capabilities of AI edge processing by enabling on-demand user services through the "satellite as a service" concept. The design of the constellation includes three types of sensors: SAR, Hyperspectral and Multispectral imager VHR combining their capabilities to achieve almost real-time due to their in-orbit distances. Six spacecrafts are considered in each orbit organized in pairs (180° phased platforms) mounting the same instrument. The proposed solution is responsive achieving almost real-time latencies ( Ex: ≲ 1 min, down to seconds is achieved in the first observation product). Persistence is achieved with a low revisit time (≲ 1 hour any payload; ≲ 3 hour s any specific payload choice).

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.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swath and high resolution. This allows frequent monitoring of large areas with high detection probability and low false alarm rate. Conventional SAR modes are, however, limited in that a wide swath can only be imaged at the expense of a reduced azimuth resolution. Ambiguous SAR modes, based on low pulse repetition frequency or continuous variation of short pulse repetition intervals (staggered ambiguous mode), overcome this limitation and allow imaging a wide swath with high resolution for specific ship monitoring applications without the need for digital beamforming or multiple receive apertures. This paper reports on the demonstration of the staggered ambiguous mode via an experimental acquisition with the TerraSAR-X satellite over the North Sea. In spite of technical limitations in the SAR instrument, a ground range swath of 110 km was imaged with an azimuth resolution of 2.2 m, i.e., with a resolution improvement of a factor of eight with respect to TerraSAR-X ScanSAR mode. Despite the higher disturbance level resulting from the presence of range ambiguities of the sea clutter a detection probability higher than 0.8 was achieved for small ships of 21 m × 6 m size. Range ambiguities of the ships were furthermore identified based on their position and signature. The detected ships were validated using maritime positioning data from their automatic identification system. These results motivate the adoption of ambiguous SAR modes in existing and future SAR systems and missions.

This paper reviews the recent activities in the field of NewSpace synthetic aperture radar (SAR) with particular attention to the instrument aspects and serves as the introductory paper of the “NewSpace SAR Instruments” Invited Session of the 42ndIEEE International Geoscience and Remote Sensing Symposium (IGARSS).

The paper details a SAR imaging mode named frequency SCan On Receive and Transmit (fSCORT) where, a narrow, frequency-scanning transmit antenna beam is formed illuminating the swath of interest from far to near range. Similarly, the frequency-scanning receive antenna beam collects the return echo signal reflected from the ground. It is shown that the imaging technique trades (sacrifices) range resolution for (improved) signal-to-noise ratio, azimuth resolution, and swath width. It is shown that the (transmit) pulse duty cycle may be significantly increased over what is common for pulsed SAR systems, allowing for a lower peak-to-average transmit power ratio. A comparison to classical stripmap operation mode shows an improvement of several decibels. The imaging mode is especially suitable for spaceborne SAR systems operating at high carrier frequencies where other advanced digital beam-forming techniques may not be feasible due to technology limitations, while, on the other hand, a large system bandwidth may be available. The final paper provides a simple mathematical model describing fSCORT operation modes. Closed expressions for the mode performance parameters, such as range resolution, echo window length, signal-to-noise ratio, etc.\ are derived. The performance of fSCORT is compared to a SAR operating in stripmap and SCORE mode.

Synthetic aperture radar (SAR) provides high-resolution images for remote-sensing applications regardless of sunlight and weather conditions. The pulsed operation of SAR may lead to an occurrence of nadir echoes in SAR images that significantly affect the image quality in case the pulse repetition frequency (PRF), which is not properly constrained within the SAR system design. As an alternative, pulse-to-pulse variation of the transmitted waveform and dual-focus postprocessing can be exploited to remove the nadir echo and alleviate the PRF constraints (also in ScanSAR operation). This work provides a demonstration of the latter concept through an experimental acquisition of the TerraSAR-X satellite. The experiment is designed by selecting the scene and the acquisition parameters in order to have the nadir echo appearing in the SAR image. The waveform variation is achieved by alternating up- and down-chirps on transmit. The analysis of the results shows the effectiveness of dual-focus postprocessing for nadir echo suppression.

The letter proposes a systematic derivation and description of dispersed synthetic aperture radar (SAR) for uniform intersatellite separation, points out some of its major challenges, that is, gapless sampling and power consumption, and provides solutions for them. Using a reference stripmap scenario, the improvements introduced by dispersed SAR using either fixed or alternating transmit satellites are discussed. It is shown that dispersed configurations allow for both a transmit power suited for small satellites and safe intersatellite separations.

Synthetic aperture radar (SAR) is a class of high-resolution imaging radar particularly suitable for satellite remote sensing with diverse applications, such as biomass and ice monitoring, generation of digital elevation models, and measuring of subsidence. Staggered SAR is a novel mode of operation under consideration for the next-generation SAR missions such as Tandem-L and NASA-ISRO SAR. It uses digital beamforming and a continuous variation of the pulse repetition interval (PRI) to achieve high azimuth resolution over a much wider, continuous swath than is traditionally possible. This PRI variation renders infeasible use of existing methods to avoid nadir echoes, which might impair the quality of staggered SAR images. This work proposes processing techniques that mitigate the impact of nadir echoes in staggered SAR through localization and thresholding-and-blanking of these echoes in range-compressed data and recovery of part of the underlying useful signal through interpolation. The performance of these processing techniques is evaluated through simulations using real TerraSAR-X data. The proposed technique can be implemented as an optional stage in the processing chain of future staggered SAR missions and leads to improved image quality at a reasonable additional computational cost.

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.

Multiple-elevation-beam synthetic aperture radar (SAR) is a concept based on digital beamforming (DBF) in elevation and simultaneous recording of the echoes of multiple transmitted pulses. It enables high-resolution imaging of wide areas and is therefore ideal for the systematic observation of dynamic processes on the Earth’s surface. Furthermore, if the pulse repetition interval (PRI) is continuously varied (staggered SAR), it is possible to map a wide continuous swath rather than multiple subswaths separated by “blind” ranges. Within the design of multiple-elevation-beam SAR, however, it is fundamental to consider how nadir echoes affect the mapping capabilities of systems with constant PRI and the image quality of staggered SAR systems, where nadir echoes are intrinsically smeared due to the PRI variation. This article addresses the characterization of nadir echoes in multiple-elevation-beam SAR with constant and variable PRI by presenting a parametric model for the nadir echo profile based on real radar measurements, a formulation of the nadir echo location and smearing in staggered SAR, and realistic simulations based on TerraSAR-X data, which show that nadir echo are likely to be barely visible in staggered SAR images. The results of this work are relevant to both the design of future SAR systems and the interpretation of the acquired data.

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.

Synthetic aperture radar (SAR) systems have unique imaging capabilities that are suitable for several Earth observation applications. This paper proposes a new concept for the design of a low-cost SAR for a single dedicated application, namely illegal vessel detection, that can be implemented using a small satellite and is characterized by reduced transmit power and high resolution. Minimum requirements in terms of noise-equivalent sigma zero (NESZ) and resolution that ensure acceptable detection performance are derived, by using TerraSAR-X data to retrieve the intensity distribution of typical ships. One peculiarity of the design is that a pulse repetition frequency (PRF) much smaller than the nominal Doppler bandwidth is selected to ensure a wider swath but still guaranteeing a high azimuth resolution. A design example of a system in X band imaging a 50-km ground swath and characterized by an average transmit power of only 15 W is presented.

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.

Staggered synthetic aperture radar (SAR) is an innovative SAR acquisition concept which exploits digital beamforming (DBF) in elevation to form multiple receive beams and continuous variation of the pulse repetition interval to achieve high-resolution imaging of a wide continuous swath. Staggered SAR requires an azimuth oversampling higher than an SAR with constant pulse repetition interval (PRI), which results in an increased volume of data. In this article, we investigate the use of linear predictive coding, which exploits the correlation properties exhibited by the nonuniform azimuth raw data stream. According to this, the prediction of each sample is calculated onboard as a linear combination of a set of previous samples. The resulting prediction error is then quantized and downlinked (instead of the original value), which allows for a reduction of the signal entropy and, in turn, of the onboard data rate achievable for a given target performance. In addition, the a priori knowledge of the gap positions can be exploited to dynamically adapt the bit rate allocation and the prediction order to further improve the performance. Simulations of the proposed dynamic predictive block-adaptive quantization (DP-BAQ) are carried out considering a Tandem-L-like staggered SAR system for different orders of prediction and target scenarios, demonstrating that a significant data reduction can be achieved with a modest increase of the system complexity.

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 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.

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.

In this letter, a novel method for onboard data reduction for multichannel synthetic aperture radar (SAR) (MC-SAR) systems is presented. Such systems allow for high-resolution imaging of a wide swath but, on the other hand, require for their operation the acquisition and downlink of a huge amount of data: together with the intrinsic requirement related to resolution and swath width, this is due to the use of a pulse repetition frequency (PRF) typically higher than the processed Doppler bandwidth (PBW), which introduces a certain oversampling in the azimuth raw data. In this context, we propose a convenient data reduction strategy, named multichannel block-adaptive quantization (MC-BAQ), which exploits the existing correlation between subsequent azimuth samples by performing a discrete Fourier transform (DFT) of the MC-SAR data block. Then, a variable-bit quantization is applied which allows for the optimization of the resulting performance and data rate. Simulations have been carried out on scenes with distributed scatterers showing different backscatter characteristics to demonstrate that the proposed MC-BAQ allows for a significant reduction of the data volume to be downlinked to the ground at the cost of a modest increase of onboard computational effort.

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.

The increasing complexity of multi-channel SAR sensors and the real-time on-board phase/amplitude correction requirement pose new challenges for the calibration, which cannot rely on state-of-the-art calibration techniques. On the other hand, the digital hardware utilized in multi-channel SAR systems, offer new opportunities for the calibration such as on-board error correction and digital calibration. This paper addresses the internal calibration strategy for future digital beamforming SAR instruments and details the implementation of a dedicated calibration simulator software.

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.

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.

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.

A calibration scheme for future SAR systems utilizing digital beamforming, is suggested. These systems pose new challenges for the calibration, as the increased complexity of multi-channel SAR does not allow for the extrapolation of current calibration techniques. Further, depending on the operation mode, real-time error measurement and correction is unavoidable, which requires a reconsideration to current calibration approaches. On the other hand, the digital hardware already present in multi-channel SAR systems offer new possibilities for novel calibration such as on-board error correction and digital calibration. This paper presents a calibration scheme suitable for multi-channel digital beamforming SAR.

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.

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.

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.

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.

Thermal noise affects polarimetric measurements; but a first order noise correction can be applied if the noise variance (or power) is known. This letter deals with the estimation of the noise variance and the signal-to-noise ratio (SNR) of the cross-polarized channels in polarimetric synthetic aperture radar (SAR) data. Cramér-Rao lower bounds (CRLB) and maximum likelihood (ML) estimators are derived. The ML noise variance estimator is unbiased and efficient, while the ML SNR estimator is biased, but an unbiased SNR estimator can be derived from the biased one. It is also shown that commonly used noise variance and SNR estimators are biased. The results are finally validated using TerraSAR-X fully polarimetric data.

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.

An innovative technique to estimate the local azimuth ambiguity-to-signal ratio (AASR) in synthetic aperture radar (SAR) images is presented. Unlike the backscatter-based (BB) technique, the proposed one, which is based on the spectral properties of the image, does not require that the areas responsible for the ambiguity lie within the focused image. Analysis of real TerraSAR-X data shows that the estimates of the proposed technique are consistent with the BB ones. Moreover, according to simulations, the proposed technique seems to provide more accurate estimates than the BB method, especially for high values of the local AASR.

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.

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 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.

Azimuth ambiguities affect the interferometric performance of SAR systems, causing a bias in the interferometric phase and a modulation of the interferometric coherence, as also visible in some TanDEM-X interferograms. This paper provides an explanation for this phenomenon and derives the analytical expressions for the phase bias and the coherence, resorting to the interferogram statistics for jointly circular Gaussian processes. The impact of azimuth ambiguities on the overall system performance is then considered. Plots are provided, which display the standard deviation of the phase bias, as well as the expected value and the standard deviation of the coherence loss component due to azimuth ambiguities. These plots can be useful for interferometric performance analysis.

The impact of azimuth ambiguities on interferometric performance in terms of phase bias and standard deviation of the interferometric phase is analyzed, resorting to the interferogram statistics for jointly circular Gaussian processes. The theoretical results are validated through simulation and compared with measurements on a TanDEM-X interferogram, affected by azimuth ambiguities.

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.