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.

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

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

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

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.

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

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.

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.

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

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.

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

Correcting for Radio Frequency Interference (RFI) has become a critical aspect of current and future Synthetic Aperture Radar (SAR) missions. While most RFI mitigation methods suffer significant drawbacks (e.g., loss of resolution) or require individual filter parameters for each acquisition, spatial filtering methods only rely on system design parameters, such as antenna size. Further, combining spatial filtering with digital beamforming (DBF) allows for the removal of in-swath interference while the instantaneous SAR and RFI signal arrive from different directions. One option to apply a DBF-based RFI mitigation on the ground in post processing are auxiliary beams. Hereby, the RFI is measured with simultaneous auxiliary beams and this information is downlinked at the cost of a small increase of data volume. This paper presents an on-board implementation of RFI detection, which can then be used to select the best auxiliary beams.

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

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

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

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.

Spatial resolution and swath width are fundamental quality parameters for spaceborne synthetic aperture radar (SAR) products. They have driven the research on new spaceborne SAR system concepts in the last decades, and a new generation of SAR systems is emerging. Main feature of these future systems is the use of multiple digital channels and new SAR processing techniques. A further characteristic is that they can resort to a large radar signal bandwidth. In this letter, a novel SAR operational mode is presented, denoted as multifrequency subpulse (MFSP). The MFSP exploits the available radar signal bandwidth to increase the imaged swath extension, without the emergence of range ambiguities. The proposed approach is explained both theoretically and by a design example, based on the future German X-band SAR system, HRWS. Moreover, the achievable SAR imaging quality is investigated.

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.

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

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

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.

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

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

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

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.

This paper presents a methodology to design a spaceborne dual-beam along-track synthetic aperture radar interferometer to retrieve ocean surface velocity vectors. All related aspects and necessary tradeoffs are identified and discussed or reviewed, respectively. This includes a review of the measurement principle and the relation between baseline and sensitivity, the relation between wind and radar backscatter, a discussion of the observation geometry, including the antenna concept, polarization diversity, and all main error contributions. The design methodology consists of a sensitivity-based derivation of explicit instrument requirements from scientific requirements. In turn, this derivation is based on a statistical model for the interferometric phase error. This allows a quantitative, wellgrounded instrument design offering an additional degree of freedom to the approach, which we call “noise-equivalent-sigmazero requirement space.” Crucial tradeoffs for the system design, such as the resolution, the number of independent looks, the minimum wind speed, and the coherence and ambiguities, are pointed out and discussed. Finally, this paper concludes with a single platform system concept operating in Ku-band, which provides the measurement quality needed to achieve a surface velocity estimation accuracy of 5 cm/s, 200-km swath coverage, for 4×4 km2L2-product resolution and winds starting at 3 m/s.

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.

SAR instruments with multiple transmit/receive channels allow for a variety of operation modes. The trade-space includes the instrument and antenna parameters designed to enable various operation modes. These are utilized to achieve a required SAR performance derived from the mission and user requirements. Finding a solution in this multi-dimensional trade space is not trivial and often involves the compromise of parameters. This paper addresses the topic of SAR instrument design tailored to a set of operation modes. It explains and compares the SAR techniques and gives the performance.

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.

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.

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

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

This paper presents a spaceborne SAR mission concept in Ku-band for the measurement of the two-dimensional ocean surface current. The measurement concept is based on dual-beam along-track interferometry, which requires two highly squinted antennas each looking simultaneously in fore and aft direction. A single side-looking geometry and a ScanSAR imaging mode is chosen to complete the observational concept in order to achieve the requirements on revisit-time and coverage. The applied hybrid polarization is intended to increase the information level about different ocean scattering mechanisms. Furthermore a methodology of instrument requirement derivation is briefly discussed, ensuring the ocean current measurement accuracy requirements. Finally the instrument concept, including digital beamforming, and the SAR performance are presented.

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

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

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

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

Future spaceborne Synthetic Aperture Radar (SAR) systems will be based on multiple transmit/receive channels and use Digital Beam Forming (DBF) technique and on board processing to provide high coverage and high resolution simultaneously. Due to the necessary on board processing and required alignment between the signals from different channels by the multi-channel operation, a big challenge in the system calibration arises. To manage that issue, new techniques have to be identified and verified. This paper presents the analysis of the effect of system and calibration errors on the SAR image quality for an example multi-channel spaceborne SAR system, which should give a starting point in the calibration accuracy considerations. Further, some ideas for the error correction in the data processing are given and computation results shown.

This paper presents a new evolution of SIGNAL's instrument concept that uses novel Digital Beamforming (DBF) techniques to further extend the coverage of the mission. In addition, by introducing two phase centers in azimuth, which are needed to achieve the swath extension, the system recovers the desired along-track interferometric capabilities.

The term MIMO SAR is used for Synthetic Aperture Radar (SAR) systems utilizingmultiple-receive andmultiple-transmit channels (Multiple-Input Multiple Output: MIMO). The trade-space for such SAR systems includes the instrument and antenna parameters taking into consideration the techniques, i.e. the operation modes of the radar. The purpose of exploring this trade-space is to improve and optimize the performance of the system, described through the performance parameters. The aim of this paper is to show examples of how MIMO SAR offers new and unexpected ways of trading the system and performance parameters versus each other. Adequately exploring the trade-space yields a high flexibility and allows to simultaneously improve multiple performance parameters; a feature not available in conventional single channel SAR systems. The content of this paper is considered novel in the sense that it deals with highly advance SAR techniques.

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

Over the past years, the use of multiple antenna apertures combined with digital beamforming (DBF) has been spotlighted as a promising solution for the fundamental restriction for high-resolution and wide-swath spaceborne synthetic aperture radar (SAR) imaging. In this paper, we present the first spaceborne experiment of a DBF technique on receive, using the TerraSAR-X dual receive antenna mode. For this experiment, we implemented a DBF module, which includes the reconstruction filter as a digital beam former and associated signal processing for calibration and channel balancing. The experimental results exhibit the successful ambiguity suppression capability of the DBF and validate the high potential of the DBF both for advanced future and current SAR systems.

The High-Resolution Wide-Swath (HRWS) is the spaceborne Synthetic Aperture Radar system, founded by the German Aerospace Centre, conceived to obtain simultaneously high spatial resolution and width swaths. Its main distinctive features are the multichannel architecture, the use of Digital Beamforming on receive and Scan on Receive algorithm. The HRWS allows for a spatial resolution of 1 m over swaths 70 km wide in the basic Stripmap mode, and a resolution of 8 m over a swath of 360 km in the advanced Ultra-Wide ScanSAR mode. In this paper a performance investigation in the novel operational mode, denoted as Multisubpulse, is presented: design constraints, proposed solutions and achievable SAR performance are explained and analyzed.

The paper presents the first spaceborne experiment of digital beamforming (DBF) in synthetic aperture radar (SAR) and its result. The DBF in future spaceborne SAR will be used to suppress azimuth ambiguities and to reconstruct Doppler spectrum from multiple spectra distorted by spectral aliasing due to spatial sub-sampling. TerraSAR-X dual receive antenna (DRA) mode provides two parallel datasets, which are the sum and difference signals of two receive antennas. For the DBF experiment, firstly, we recover each receive antenna signal from those datasets and carry the DBF experiment out with the recovered data. Therefore, this experiment contains the each channel data recovery and channel balancing as well as SAR imaging with DBF. The experimental results show the prominent performance improvement in the azimuth ambiguity level, compared to a conventional single antenna mode.

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

This paper provides an overview of the BIOMASS Mission End-to-End simulator (BEES) and of the mission performance analysis performed with it. The end-to-end performance, in terms of biomass estimates error, is close to the 20% error goal set for the mission. The main sources of errors are temporal decorrelation and the limited available bandwidth, while system induced errors have a negligible impact on the final performance.

Synthetic Aperture Radar (SAR) based on the Digital Beam-Forming (DBF) concept belongs to the family of Multi-Modal Radar Systems (MMRS) offering higher operational flexibility and improved performance compared to conventional radar systems using analog beam steering. DBF SAR, in particular, overcomes the fundamental limitation of classical SAR systems and can deliver high resolution and simultaneously wide swath images. The purpose of this paper is to present a laboratory prototype of such a MMRS architecture - a reconfigurable radar demonstrator based on the DBF architecture. A hardware configuration of the radar as well as advanced concepts to be experimentally verified using it are considered and discussed.

During the past ten years a number of research groups around the world have been investigating the use of digital beamforming for spaceborne SAR instruments. The separate optimization of the effective transmit and receive aperture sizes together with the splitting of the receive side into multiple receive sub-apertures which can be combined on board by means of digital beamforming allows for a number of new SAR modes. With this it is for example possible to overcome the principle swath width and azimuth resolution limitation known in classical SAR instrument designs. ESA has performed and initiated a number of systems studies to identify the critical technologies needed for this type of instrument. Based on these system studies a number of technology development activities have been started. This paper gives an overview on the studies and technology developments performed in this context.

Synthetic Aperture Radar (SAR) operating at Ka-band frequency is increasingly being considered for spaceborne applications. The benefits are seen in the compactness of the instruments, the high bandwidth and resolution. Besides this the high carrier frequency facilitates single-pass interferometry by allowing for an accommodation of all transmitters and receivers on the same platform. This paper presents the instrument architecture and SAR performance for a system utilizing an advanced operational mode enabled through digital beamforming techniques.

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.

The current generation of spaceborne synthetic aperture radar (SAR) systems suffers from a tradeoff between the achievable spatial resolution and swath width. This has motivated intensive research both on more flexible SAR systems, using multiple transmit/receive channels, and on techniques for removing the ambiguities. Among these techniques, the azimuth phase coding (APC), recently proposed to suppress range ambiguities in conventional SAR systems, stands out for its negligible implementation complexity and its effectiveness for point and distributed ambiguities. This paper investigates the possibility of applying the APC technique to the new, forthcoming generation of multichannel SAR systems, based on digital beamforming on receive. The extension of APC to multichannel SAR systems is mathematically described. Specific merit figures are defined to quantify the APC performance. A numerical analysis is developed to characterize the influence on the APC behaviors of the main SAR system parameters. Finally, an example of APC performance is provided, by considering two multichannel SAR systems based on a planar and a reflector antenna.

The Azimuth Phase Coding (APC) technique, proposed to suppress range ambiguities in conventional SAR systems, stands out for its low implementation complexity and its effectiveness for point and distributed ambiguities. This paper investigates the possibility of applying the APC to the new, forthcoming generation of multichannel SAR systems, for high resolution and wide swath imaging, based on Digital Beamforming on receive. The extension of APC to multichannel SAR systems is mathematically described. A new metric is defined to quantify the APC performance. A numerical analysis is developed to characterize the influence on the APC behaviors of the main SAR system parameters. Finally, an example of APC performance is provided, by considering two multichannel SAR systems based on a planar and a reflector antenna.

The German TanDEM-X mission flies the two satellites TerraSAR-X (TSX) and TanDEM-X (TDX) in a close orbit formation establishing a bistatic interferometer in space. A major challenge in bistatic SAR data processing is the synchronization of the satellites' oscillators. The paper at hand summarizes the synchronization concept, provides in-orbit verification and performance figures, describes the compensation approach within the SAR processing workflow and finally quantifies the synchronization performance in terms of achieved DEM accuracy.

This paper discusses the implementation of an end- to-end simulator for the BIOMASS mission. An overview of the system architecture is provided along with a functional description of the modules that comprise the simulator.

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

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

To cope with the mission requirements for monitoring Earth's dynamic processes, future Synthetic Aperture Radar (SAR) systems will need to meet different performance requirements. The driver is to increase the acquisition frequency and to provide capabilities to monitor large areas. In addition, the imaging parameters show a high degree of heterogeneity depending on the application. In Terms of SAR instrument performance this translates into a large swath and access range to increase the coverage and a lower power consumption to increase the orbit duty cycle. Further, the instrument must be capable of operating in various modes. In this context the use of large reflectors combined with digital feed arrays for SAR is receiving increased interest. This paper suggests a spaceborne SAR system utilizing a deployable reflector together with a digital feed array, shortly states its design and addresses the various performance optimization issues.

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

Tandem-L is a proposal for an innovative interferometric and polarimetric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important mission objectives are global forest height and biomass inventories, large scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify the dynamics of a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics.

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

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.

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

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

The TerraSAR-X satellite is a high-resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low, TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna (DRA) mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad-Pol). Fore and aft channel reconstruction is necessary if ground moving target indication (GMTI) algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X DRA mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and first examples on the GMTI capability of the TerraSAR-X DRA mode are given.

This paper provides an overview of the TerraSAR-X commissioning phase (CP). The overall CP planning and preparation is presented. The strategy for data-take (DT) command generation is discussed, and statistical reports summarize the acquired CP DTs. An overview summary on the main results in the different characterization and verification areas is provided together with synthetic aperture radar image examples.

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters—ambiguities, impulse-response function, noise, and radiometric resolution—is discussed. ScanSAR and Spotlight particularities are described.

Intensive research is currently ongoing in the field of Smart Multi-Aperture Radar Techniques (SMART) for high-resolution wide-swath Synthetic Aperture Radar (SAR) imaging. This work investigates the possibility of applying direction of arrival estimation methods to spaceborne SMART SAR systems, that employ receive beam steering. In particular, a new algorithm based on the actual spatial distribution of the received signal power is proposed. The performance of the algorithm is evaluated by Monte Carlo simulations and compared with that of the conventional scan-on-receive approach, in different operational scenarios. The Cramer Rao Lower Bound is also reported as a benchmark on the performance.

TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite which provides the option to split the antenna in along-track direction and sample two physical channels separately. This so-called dual receive antenna (DRA) mode is implemented by use of a Magic-T hybrid junction which provides the sum and difference channel data instead of the fore and aft channel data. One way to exploit this spatial degree of freedom is to reconstruct the fore and aft channel data in the processing. The critical issue with the fore and aft channel reconstruction is the fact, that for space-based radar systems very high precision calibration is required. This is even more difficult since the wide bandwidth pulses employed by the radar makes the receive hardware transformation frequency dependent. In this paper a fore and aft channel reconstruction approach is described based on an estimation of the receive hardware transformation matrix by use of internal calibration pulses and calibration beams. The quality gain of the fore and aft channel reconstruction is demonstrated with experimental data and results on the ground moving target indication (GMTI) capability are shown.

The trend in the conception of future spaceborne radar remote sensing is clearly towards the use of digital beam-forming techniques. These systems will comprise multiple digital channels, where the analog-to-digital converter is moved closer to the antenna. This dispenses the need for analog beam steering and by this the used of transmit/receive modules for phase and amplitude control. Digital beam-forming will enable Synthetic Aperture Radar (SAR) which overcomes the coverage and resolution limitations applicable to state-of-the-art systems. On the other hand, new antenna architectures, such as reflectors, already implemented in communication satellites, are being considered for SAR applications. The paper compares the system architecture and SAR performance of a planar and a reflector antenna SAR.

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

In this paper, we present the experimental results of a Digital Beam Forming (DBF) Synthetic Aperture Radar (SAR) performance on the purpose of the High-Resolution Wide-Swath (HRWS) SAR concept. A ground-based SAR system successfully demonstrated the DBF SAR operation. The demonstrator acquired SAR raw data with very dense spatial sampling rate in order to obtain various sampling rates. We evaluate DBF performance with respect to the image quality factor with two different types of the beam former, a fixed-beam former and an adaptive beam former. The results show that an adaptive DBF algorithm offers a wide range of the selection of the pulse repetition frequency (PRF). In addition we evaluate the noise performance compared to a reference mono-static SAR system on the same condition on single target experiment.

The paper presents selected results from the TerraSAR-X Commissioning Phase from instrument performance, SAR system performance and command generation.

The paper summarizes the results of the TerraSAR-X commissioning phase. The overall schedule and the planning tool are presented. The strategy for data take (DT) command generation and a statistic about all acquired data takes are discussed. An overview about the characterization/verification results is provided.

The paper deals with the performance of next generation SAR sensors, here referred to as SMART (Smart Multi Aperture Radar Techniques) equipped with digital beamforming capabilities. Apart from representing a technological jump, these sensors offer the flexibility of actually deciding on the (possibly hybrid) mode(s) of operation on-ground after the data have been acquired. The paper presents the performance of SMART system configurations and modes of operation for a digital beamforming SAR which covers a large swath with a high resolution.

Data compression is crucial for modern synthetic aperture radar systems where high resolution or large coverage may result in huge amounts of raw data. Modern satellite systems, such as TerraSAR-X, give complete flexibility in choosing between various compression levels. This, however, results in additional effort to decide on the suitable compression level used, which may depend on the operation mode, polarization, scene backscatter, etc. The paper describes the approach used in the case of TerraSAR-X and shows the result of analyzing the data acquired during the commissioning phase. The methology is considered novel in the sense that it combines SAR measured data analysis with theoretical, i.e. model based simulations, results and later combines theory and measured data to extract optimum compression levels.

TerraSAR-X is the first German Radar satellite for scientific and commercial applications. The project is a publicprivate partnership between DLR and EADS Astrium GmbH. TerraSAR-X consists of a high resolution Synthetic Aperture Radar at X-Band. The radar antenna is based on active phased array technology that allows the control of many different instrument parameters and operational modes (Stripmap, ScanSAR and Spotlight) with various polarizations. Following the TerraSAR-X launch, it is planned a six month Commissioning Phase covering the characterization and verification of the SAR mission. Within this phase, the Overall SAR System Performance takes care of the correct working and interaction of all SAR system elements essential for obtaining an optimum SAR Performance. The paper covers the first in-orbit characterization and verification results of the SAR system performance for TerraSAR-X operational and experimental modes. This characterization is divided into four phases: Initial Characterization, Scene Characterization -both mostly based on basic and experimental products-, and Verification of TerraSAR- X Instrument Command Generation. The different optimization strategies and performance trade-offs are investigated and discussed, including very first TerraSAR-X images. The result of the real SAR data analysis determines the final system baseline and thus the final image quality, e.g. Temperature compensation, Total Zero Doppler Steering, Up/down chirp toggling, transmitted bandwidth, timing interferences, etc.

The paper discusses the areas covered by the TerraSAR-X Calibration/Verification-Plan. By means of a commissioning phase planning tool the data take requests required for characterization, calibration and verification are sequentially arranged. The status of the individual data takes is tracked from request planning to the final image analysis. All data take requests and results are supervised and summarized in a so-called verification matrix. In the end the paper gives an overview about the commissioning phase schedule and the repeat cycle based planning.

The paper describes the synchronization link of the two satellites of the TanDEM-X mission. The synchronization link is established to track the oscillator phase noise difference which- after appropriate processing - can be used to compensate the effect of oscillator phase noise. A signal model based on the synchronization link hardware is presented which is then used to derive an analytical expression for the compensation phase including effects such as instrument drift, Doppler, antennas, and receiver noise. Specifically the influence of the synchronization link RF hardware on the quality of the derived compensation signal is crucial for the performance. Therefore the synchronization link RF hardware of the TerraSAR-X satellite is characterized to obtain realistic data. These measurements will be used for the calibration of the synchronization link, i.e. to remove systematic errors due to temperature drifts of the instrument.

The factorization method (FM) has been applied to measurement data from a multistatic ground-penetrating radar operating in close proximity to the ground, which was used in a measurement campaign on the Joint Research Centre mine test lane in Ispra, Italy. This paper is targeted toward a future hand-held demining system. The according space limits restrict an independent positioning of transmit and receive antennas. Hence, very small multistatic datasets are obtained, representing a difficult case for the reconstruction with the FM

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative spaceborne radar interferometer that is based on two TerraSAR-X radar satellites flying in close formation. The primary objective of the TanDEM-X mission is the generation of a consistent global digital elevation model (DEM) with an unprecedented accuracy, which is equaling or surpassing the HRTI-3 specification. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new radar imaging techniques and applications. This paper gives a detailed overview of the TanDEM-X mission concept which is based on the systematic combination of several innovative technologies. The key elements are the bistatic data acquisition employing an innovative phase synchronization link, a novel satellite formation flying concept allowing for the collection of bistatic data with short along-track baselines, as well as the use of new interferometric modes for system verification and DEM calibration. The interferometric performance is analyzed in detail, taking into account the peculiarities of the bistatic operation. Based on this analysis, an optimized DEM data acquisition plan is derived which employs the combination of multiple data takes with different baselines. Finally, a collection of instructive examples illustrates the capabilities of TanDEM-X for the development and demonstration of new remote sensing applications.

This letter addresses the impact of limited oscillator stability in bistatic and multistatic synthetic aperture radars (SARs). Oscillator noise deserves special attention in distributed SAR systems since there is no cancellation of low-frequency phase errors as in a monostatic SAR, where the same oscillator signal is used for modulation and demodulation. It is shown that the uncompensated phase noise may cause a time-variant shift, spurious sidelobes, and a broadening of the impulse response, as well as a low-frequency phase modulation of the focused SAR signal. Quantitative estimates are derived analytically for each of these errors based on a system-theoretic model taking into account the second-order statistics of the oscillator phase noise

Oscillator phase noise can dictate the performance of bistatic and multistatic synthetic aperture radar imaging. In this letter, the use of a dedicated synchronization link to quantify and compensate oscillator phase noise is investigated. Different synchronization schemes are presented, and their performance is analyzed. The error contribution of the synchronization link itself, which may suffer from receiver noise, aliasing, interpolation, and filter mismatch, is included in the analysis. The synchronization link performance is given in a frequency-domain closed integral form

We appreciate that authors Kerr et al. concur that our paper [ibid., vol.42, no.7, p.1387-98 (2004)] provides the right approach to the analysis of potential interference from anthropogenic sources to remote sensing satellites. The potential for such interference is likely to grow as new active systems are developed, necessitating acceptable procedures for interference analysis based on accepted scientific knowledge and engineering principles. While some simple clarifications are in order to improve the acceptability of our procedure, we suggest, however, that Kerr et al. have not studied our paper in detail and misinterpret several points.

The automotive industry is currently considering the introduction of Short Range Radars operating near 24 GHz for improving road traffic safety. Short Range Radars are intended to observe the full azimuthal space cover around a vehicle using up to eight sensors. The sensors would operate in an ultra wideband (UWB) mode, occupying 3 to 5 GHz of bandwidth. Interference from SRR transmitters with passive microwave remote sensing satellites could occur as the result of several coupling mechanisms, including direct coupling via the transmit antenna beam and scattering and diffraction of the transmitted signals from leading vehicles, buildings, and other nearby objects. In this study, we estimate the amount of coupling anticipated to occur from SRRs, including the direct and scattered contributions. The calculations are based on bistatic scattering measurements at of a typical automobile and ray optical simulations of reflection and propagation in an urban environment.

The automotive industry is currently considering the introduction of short-range radars (SRR) operating near 24 GHz for improving road traffic safety. SSRs are intended to observe the full azimuthal space cover around a vehicle using up to eight sensors. The sensors would operate in an ultrawideband (UWB) mode, occupying 3-5 GHz of bandwidth. Interference from SRR transmitters with passive microwave remote sensing satellites used for weather and climate monitoring could occur as the result of several coupling mechanisms, including direct coupling via the transmit antenna beam and scattering and diffraction of the transmitted signals from leading vehicles, buildings, and other nearby objects. In this study, we estimate the amount of coupling anticipated to occur from SRRs, including the direct and scattered contributions. The calculations are based on bistatic scattering measurements of a typical automobile and ray optical simulations of reflection and propagation in an urban environment. Using these calculations, the maximum allowable SRR transmitted power for interference levels acceptable for meteorological and climatological remote sensing applications are quantified. The study provides criteria for SRR operation with the Earth Exploration Satellite Service on a noninterference basis.

The rapid progression in digital hardware and signal processing capabilities stimulates the development of radar systems. The tendency is to move the digital interface toward the antenna, replacing, whenever possible, analog RF-hardware. Based on software codes, these digital systems are more flexible and easier to reconfigure than RF-hardware. This letter illustrates the general concept for digital beamforming (DBF) in synthetic aperture radar systems and investigates their principle capabilities, limitations, and performance parameters. It is shown that using DBF a simultaneous improvement in azimuth coverage and resolution can be achieved.

This paper proposes multistatic measurements to increase the information content of ground penetrating radar data. Three different acquisition strategies are distinguished. To obtain measurement data, a flexible setup has been realized. Tomographic backpropagation and the nonlinearized factorization method are discussed with regard to their applicability to multistatic data.

The IEEE Geoscience and Remote Sensing Society (GRSS) Technical Committee on Frequency Allocation in Remote Sensing (FARS) is charged with providing recommendations and responses to queries on interference and frequency allocation issues in passive and active microwave remote sensing. In response to questions stemming from proposals to develop ultra-wideband (UWB) vehicular radar systems operating in the 22-27 GHz frequency range a technical assessment on the potential for radio frequency interference to passive Earth remote sensing activities was prepared. The study suggests that interference to the passive services at power levels several orders of magnitude above threshold levels is likely from commercial deployment of vehicular UWB radar and telecommunications systems.

Cryosat is an approved ESA mission. Its objective is to determine fluctuations in the Earth's land mass and marine ice fields. The mission utilizes a completely new SAR/interferometric altimeter concept, which combines a nadir-looking configuration with pulse-limited altimetry. The concept has been verified by simulations, but no dedicated experimental validation has been performed so far. For this reason, an Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) is currently under development in the frame of an ESA contract. Due to the large bandwidth of the system, several components, i.e. the chirp generator, are critical and therefore require innovative solutions. A dedicated software performs SAR- and interferometric radar data processing and, in addition, supports internal instrument calibration. ASIRAS is intended to be tested, verified and optimized on a DO 228/100 aircraft carrier during a measurement campaign scheduled fall 2002 in the area of Fram Street north of Greenland. The paper presents mission objectives, the instrument concept of ASIRAS, and critical building blocks.

The rapid progression in digital hardware systems and signal processing capabilities is increasingly advantageous for the development of radar systems. The trend is to move the digital hardware towards the antenna front-end replacing, whenever possible, RF-hardware. Based on software codes these digital systems are more flexible and easier to reconfigure than RF-hardware. This paper investigates capabilities and limitations of synthetic aperture radar systems utilising digital beamforming.

A novel numerical algorithm based on the radiative transfer equation (RTE) is presented, which allows the calculation of the scattering and penetration of isolated vegetation specimens such as, e.g., single trees, bushes, etc., at mm-wave frequencies. For this purpose, the RTE is stated in a discrete form, and the vegetation structure is discretized in elementary volumes. A step-by-step solution is proposed to solve for the desired attenuation, and indications on the influence of the different parameters on the propagation characteristics are given. A qualitative justification to the approach is presented and finally, a comparison with scattering measurements of ficus trees at 75 GHz is shown.