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.

The German SAR satellites TerraSAR-X and TanDEM-X are in orbit since 2007 and 2010, respectively. This paper gives an overview about the challenges faced by such long-enduring missions, recent measures taken to preserve the satellites and to continue the mission, and major lessons learned for future SAR missions.

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

The TanDEM-X mission acquired a global Digital Elevation Model (DEM) between 2011 and 2014 and generated the final DEM product until 2016. Based on the success and high quality of the first DEM a second complete coverage of the Earth was acquired between 2017 and 2021. This DEM is called “TanDEM-X DEM 2020”. Its processing is based on an edited version of the first global TanDEM-X DEM at 30 m resolution. This edited DEM is required to enable the phase unwrapping and significantly accelerates the DEM processing. The paper in hand describes the automatic global editing process for the edited DEM. It explains the detection and discrimination of DEM gaps and the different methods applied to fill these void areas with external information or derived values. Furthermore, for the editing of water bodies different techniques are described which are used to flatten oceans, lakes and rivers respectively. At last, an overview on the status of the global editing process and further improvements are presented.

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

The Earth is a very dynamic system and the topographic height of its landmass changes over time, especially in forested areas, glaciers, permafrost regions or where human activities take place. After the TanDEM-X mission provided a first global DEM of unprecedented quality in 2016, a new complete coverage of the Earth's landmass was acquired mainly between 2017 and 2020. This data is used to create another global DEM. In addition to providing more up-to-date elevation information, these new acquisitions also provide a great dataset to show the changes that have occurred in the few years between the two global datasets. The new product - the TanDEM-X DEM Change Maps - will be produced in 30m and 90m postings and will focus on showing these changes between the first global TanDEM-X DEM and the newly acquired time-tagged DEM scenes. It will also include the in-house automatically edited TanDEM-X DEM.

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

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

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

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

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

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.

In this paper, we present a novel approach to monitor the evolution of deforested areas in the Amazon rainforest, by combining Sentinel-1 and TanDEM-X SAR data. The idea is firstly to exploit the large coverage and short revisit time provided by the constellation of Sentinel-1 satellites in order to cover the entire arch of deforestation about ones per month. The goal is here to discriminate forest/non- forest by exploiting C-band backscatter signatures in dual polarization together with the behavior of the interferometric coherence in time, in order to identify the so-called deforestation hotspots: local areas characterized by a significant amount of on-going deforestation activities. Secondly, high-resolution time series of bistatic TanDEM-X data can be acquired over these hot spots with a repeat-cycle of 11 days, and used to track fast changes at small scales, aiming at identifying specific on-going deforestation activities. In the final paper, we intent to present more consolidated results, supported by a large scale acquisition scenario.

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

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.

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

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

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements)is an interferometric SAR mission flying two radar satellites in close orbit formation. Its primary goal is the production of a homogeneous global digital elevation model (DEM) of unprecedented accuracy. Since 2010 all land surfaces have been mapped at least twice and difficult terrain even up to four times. While data acquisition for the DEM generation will be concluded in August 2014 it is expected to complete the processing of the global DEM by the end of 2015. This paper gives a status update on the current acquisition planning and presents quality results from a huge data base of more than 400,000 single DEM scenes and 1700 final DEM products.

In the frame of the GMES program, the main objective of the Sentinel-1 mission is to ensure the continuity of SAR data acquisitions in C-band for global earth monitoring. Sentinel-1A is the first of two C-band satellites launched in April 2014. In addition to the commissioning of Sentienl-1A executed by ESA, an independent verification of the system calibration is executed for the first time by an external institution. For this purpose, the complete calibration chain was developed and established by DLR, starting with an efficient calibration concept, a detailed in-orbit calibration plan, the SW-tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. Based on an efficient calibration strategy, this paper describes the different activities performed by DLR and presents first results obtained from real in-orbit data. As Sentinel-1A had not achieved the reference orbit at the time of uploading the full paper, a discussion of radiometric and geometric calibration results will be presented at the conference on July 2014.

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

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

TanDEM-X is a spaceborne SAR mission with the goal to derive a global Digital Elevation Model (DEM) with unprecedented accuracy. This paper describes the way how the global DEM is acquired, explains the changes that have been applied during the first almost 1.5 years in orbit and shows the actual status of the global acquisitions. In the second part, the calibration of the bi-static interferometer is explained. The implications to correct the different effects coming from the baseline, the internal delays in the instrument and the phase measurements themselves enabling the derivation of the height are described and the actual results are presented.

The TanDEM-X mission based on two satellites provides a radar interferometer in space with the goal to derive a global Digital Elevation Model (DEM) with never achieved quality for a global coverage: a global DEM with a relative height accuracy of 2m and 10m absolute. In order to achieve this mission goal, the distance between both satellites, the so called baseline has to be known extreme precisely. Only then, systematic baseline errors can be detected and compensated for, i.e. an accurate calibration of the global DEM can be ensured. The paper describes the procedure and the results of calibrating the baseline, verifying the outstanding accuracy of this calibration procedure.

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

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

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in space borne radar remote sensing [1]. A single-pass SAR interferometer with adjustable baselines in across- and in along-track directions is formed by adding a second (TDX), almost identical spacecraft to TerraSAR-X (TSX) and flying the two satellites in a closely controlled formation. With typical across-track baselines of 200–400m a global Digital Elevation Model (DEM) with 2m relative height accuracy at a 12 m posting will be generated. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new SAR techniques and applications. The TDX satellite was launched on June 21st, 2010 from Baikonur. After finishing the Commissioning Phase early December 2010 data acquisition for the global DEM commenced.

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.

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

This paper recalls the essential elements of the joint TerraSAR-X and TanDEM-X ground segment. It elaborates on some topics which are usually not in the primary focus from a pure SAR technical point of view, e.g. the flight formation. Both commissioning and early routine phase results from operating the joint TerraSAR-X and TanDEM-X ground segment are given.

In the frame of the GMES program, the main objective of the Sentinel-1 mission is to ensure the continuity of SAR data acquisitions for SAR applications in C-band for global earth monitoring. But in contrast to SAR systems already existing in C-band like ASAR/ENVISAT or RADARSAT-2, high demands on the radiometric accuracy are made. Thus, product quality is of paramount importance and the success or failure of the mission depends essentially on the method of calibrating the entire Sentinel-1 system in an efficient way. This paper describes the strategy and the method of calibrating Sentinel-1.

ALOS, an enhanced successor of the Japanese Earth Resources Satellite 1 (JERS-1), was launched from JAXA's Tanegashima Space Center in January 2006. An important contribution to the ALOS mission is the verification of PALSAR products to be distributed by the European ADEN node using the PALSAR processor developed by JAXA. A total of 28 ALOS PALSAR products have been analysed with respect to radiometric, geometric and polarimetric quality (including effects of Faraday rotation caused by the ionosphere) and a summary of the results is shown in this paper.

The TanDEM-X mission [1] comprises two fully active synthetic aperture radar satellites operating in X-band. The primary goal of this mission is the derivation of a high-precision global Digital Elevation Model (DEM) according to HRTI level 3 quality [2]. This requires accurate calibration of the interferometric system parameters. Content of this paper is the development of a general concept for this calibration, which comprises the determination of instrument and baseline errors, an adjustment concept and the distribution of control points. This concept has a key incidence on mission aspects like the data acquisition plan and the data take adjustment procedure.

In this paper, we will present well controlled experimental bistatic X-band measurements of rough surfaces, which have been recorded in the Bistatic Measurement Facility (BMF) at the DLR Oberpfaffenhofen, Microwaves and Radar Institute. The bistatic measurement sets are composed of soils with different statistical roughness and different moistures controlled by a TDR (Time Domain Reflectivity) system. The BMF has been calibrated using the Isolated Antenna Calibration Technique (IACT). The validation of the calibration was achieved by measuring the reflectivity of fresh water. In the second part, the first validation of the specular algorithm by estimating the soil moisture of two surfaces with different roughness scales will be reported. Additionally, a new technique using the coherent term of the Integral Equation Method (IEM) to estimate the soil roughness will be presented, as well as evaluation of the sensitivity of phase and reflectivity with regard to moisture variation in the specular direction.

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

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

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

The TerraSAR-L system, currently being designed in a Phase B definition study, provides ESA with its most powerful radar-imaging programme to date. The platform is optimized for and built around the 11 mtimes2.9 m active phased array antenna of the L-band Synthetic Aperture Radar (L-SAR). The L-SAR features, on top of standard Stripmap and ScanSAR operations, full polarimetric capabilities, repeat-pass ScanSAR interferometry and a Wave Mode. Specification of the L-SAR has been guided by a careful analysis of the product requirements resulting in a robust baseline design with considerable margins. Besides, a major contribution to applications in areas of climate change and oceanography, the TerraSAR-L design responds specifically to requirements from interferometric applications. One key element of the TerraSAR-L operations strategy is a long-term systematic and repetitive acquisition scenario to ensure consistent data archives and to maximize the scientific and commercial exploitation of this SAR system. The other important factor is a systematic processing of all acquired data to Single-look Slant-range Complex (SSC) products to facilitate higher level product generation and services based on these products. This paper describes the TerraSAR-L basic product tree and explains the rationale behind. The nominal TerraSAR-L imaging modes are introduced and their performance characteristics are described. The generation concept of multi-look detected products based consistently on complex products - generated either at an intermediate stage during ground segment processing or serving as input product into a stand-alone tool - is introduced. A modular SAR processor design using the same complex product generation algorithms as far as possible independent of the underlying imaging mode is addressed

The TerraSAR-L system, currently in Phase B definition carried out by a European/Canadian consortium lead by EADS Astrium Ltd., provides ESA with its most powerful radar imaging programme to date. Key mission characteristics are a 5-year mission lifetime, global coverage from a 14-day repeat 635-km orbit, more than 20 minutes per orbit of SAR data acquisition in L-band, in both right-looking (nominal) and left-looking configurations, and a launch in late 2008. TerraSAR-L first mission priority is serving commercial applications focused on agriculture, forestry and marine applications. Joint products from TerraSAR-L and the German national programme TerraSAR-X use the complementary properties of the backscattering in L and X bands for high levels of classification performance necessary for crop monitoring, forest inventory and cartographic maps of different thematic content and scale. Scientific and institutional applications are defined focused on solid Earth monitoring relying on differential interferometry. L-band penetration of vegetation cover facilitates these applications also over vegetated surfaces. The high coherence of L-band assists the monitoring of ice sheet and glacier dynamics. Sea ice classification is supported with full polarimetric measurements. Large-scale land cover change detection, global forest biomass estimation, permafrost monitoring, wetland inventory, soil moisture retrieval and flood monitoring are also supported. A dedicated Wave Mode provides measurements of directional ocean surface wave spectra. Additionally for single-pass interferometry and generation of a global DEM, a complementary cartwheel system with different micro satellite constellations flying in close formation with TerraSAR-L has been investigated. The spacecraft is built around an active phased array antenna and provides full polarimetric capabilities, maximum bandwidth within the 85 MHz allocation for Earth observation in L-band, and repeat-pass ScanSAR interferometry. The platform is based on a novel Snapdragon configuration. The TerraSAR-L Ground Segment provides the operational link between the spacecraft, the TerraSAR Exploitation and Service Infrastructure and the users. A high-performance modular architecture is designed to fulfil the high requirements on throughput, quality and accessibility of data, as well as the reuse of existing infrastructure elements and the interoperability with other missions, especially with the companion TerraSAR-X Ground Segment

This paper presents the main characteristics of the advanced synthetic aperture radar (ASAR) instrument on board ENVISAT, ASAR products, the challenges in the ASAR calibration and product validation, the methodology used to perform the sensor performance monitoring and product calibration based on the special ASAR features and dedicated calibration sites and finally a summary on the product quality status will also be provided.

This paper presents the approach for the in-flight calibration of the ENVISAT-1 ASAR and the verification of the ground processing facility PF-ASAR during the commissioning phase. The philosophy presented is a logical progression from the experience gained during calibration of the ERS SARs. The ASAR has a comprehensive internal calibration loop, which is described distinctly from the external calibration and characterisation. The antenna patterns of the various beams have been fully measured during on-ground flight-model testing and have been used for initial performance predictions. In-flight characterisation of the main beams is performed over the South American rainforest. As for ERS, absolute gain calibration is achieved using three fixed and one transportable precision calibration transponders situated in the Netherlands. These transponders are also capable of recording the azimuth beam patterns and supporting the external characterisation mode of ASAR.

The on-orbit alignment of the antenna beams of both the X-band and C-band radar systems during operations of the shuttle radar topography mission/X-band synthetic aperture radar (SRTM/X-SAR) was a key requirement for achieving best interferometric performance. In this paper, we consider the X-SAR antenna beam alignment in azimuth. For a single-pass cross-track SAR interferometer, we establish the relation between yaw and pitch misalignment of the antenna beams and the resulting relative shift of the Doppler frequency bands. This relation is used to provide solutions for the mechanical adjustments of the outboard antenna and electronic beam steering to correct for azimuth misalignment. Furthermore, the effects of the X-SAR effective outboard antenna pattern on the azimuth beam alignment are analyzed. As a result, a so-called "relaxing" factor is derived, which increases the limit for the difference in antenna azimuth angle with respect to the requirement on spectral overlap, and hence spatial interferogram resolution. However, we also show that the alignment requirement is driven by the constraint on decreasing the azimuth ambiguity-to-signal ratio (AASR) for the effective outboard antenna pattern to reduce the resulting additional height error. The strategy for misalignment determination and correction is presented, and results of the analysis of the in-flight X-SAR antenna beam alignment are discussed.

In April and October, 1994 the X-SAR was flown as part of the SIR-C/X-SAR space radar laboratory missions (SRL-1/2) on the Space Shuttle. Amongst other activities DLR is responsible for the calibration of all X-SAR data products and is running the German Processing and Archiving Facility (D-PAF). Calibration activities included three major parts. Before the first mission, the authors performed a detailed analysis of the overall system to localize the main error sources and developed algorithms and procedures to correct these errors. During the missions they concentrated their efforts on calibration campaigns at the Oberpfaffenhofen super test site. Post mission activities included the determination of the antenna pattern and the absolute calibration factor as well as detailed performance analyses. This paper describes the overall approach to radiometrically calibrate the X-SAR and provides information on system performance and data quality to users in the different application fields.>

In mapping nonflat regions of the Earth using airborne synthetic aperture radar, (SAR), terrain height variations cause two problems in radiometric calibration: the first being that the local incidence angle for any pixel may vary from that given by the flat (or curved) Earth assumption, the second, being that the wrong elevation angle may be used in correcting for the radiometric variation of the antenna pattern. In tracking radars, simultaneous amplitude or phase measurements made by the same radar antenna, but modulated differently, are compared to determine the angular position of targets. This is known as the monopulse principle. In the present paper, the authors show how polarimetric SAR data can be used in a novel application of the monopulse principle to determine the elevation angle and thus, the height at the different parts of the image. The authors' approach begins with the observation that, provided like- and cross-polarized backscatter are uncorrelated, then the algorithm described in van Zyl (1990) for calculating antenna crosstalk yields a measurable quantity whose amplitude (and phase) depends only on elevation angle (or off-boresight angle). Thus, if one determines the crosstalk for a given point in the image, one can relate that measurement to the elevation angle appropriate to that point. Knowledge of the slant range to the point then allows determination of the height of the platform above it. This operation, repeated at many locations throughout the image, allows a topographic map of the height of the aircraft above each location to be built up. The approach described in this paper gives sufficient resolution in elevation to allow the antenna pattern radiometric correction to be done properly, since it allows the determination of elevation angles at a grid of points in the image. Knowledge of the slant range to each point and the height of the aircraft then allows a grid of height estimates for the imaged area to be built up. Unfortunately, the spatial resolution of the grid was not sufficient to turn the height maps into usable maps of local incidence angle. The approach may be improved upon, using an active, phased array antenna.>

Terrain height variations in mountainous areas cause problems in the radiometric correction of SAR images. The authors propose a novel SAR system which exploits the monopulse principle to determine the elevation angle and thus the height at the different parts of the image. From the ratios of images radiometrically modulated by the difference and sum antenna pattern in cross-track direction, the authors can calculate the appropriate elevation angle at any point in the image. In this paper, they present design considerations for an array antenna for DLR's airborne X-band SAR system and give estimates of the error due to system noise and azimuth ambiguities as well as the expected performance and precision in topographic mapping.>

A comparative study of data acquired by two different airborne synthetic aperture radar (SAR) sensors from the same site is presented. External, ground-target-based calibration has been performed on the NASA/JPL DC-8 SAR C-band data and the DLR E-SAR C-band data collected over the DLR test site in Oberpfaffenhofen. The consistency of both the polarimetric and radiometric calibration parameters derived from different tracks indicates the stability of DC-8 SAR system during the campaign. Radar cross-sections and backscattering coefficients measured from different tracks under different incidence angles emphasize this stability.>

An approach is presented for determining the inflight antenna pattern in the cross-track direction for air- and spaceborne synthetic aperture radar (SAR) systems. In the 1991 Oberpfaffenhofen DC-8/E-SAR calibration campaign, ground-based measurement, equipment comprising 18 precision calibration receivers and nine polarimetric active radar calibrators, all operating in C-band, were tested. These instruments are capable of handling various pulse lengths and repetition frequencies, and they have a very high dynamic range. Together with precise internal clocks, these instruments are suitable for recording the actual radar transmit pulse shape for the later evaluation of the desired inflight antenna pattern. Lining up these devices in the cross-track direction, each receiver yields an azimuth cut of the three-dimensional antenna pattern. The elevation pattern was then obtained by time correlation of these azimuth cuts. Further results concerning pulse shapes, squint angles, and H-V pattern misalignment are presented.>