Radio frequency interference (RFI) in synthetic aperture radar (SAR) is a daunting challenge, affecting both sensing reliability and image quality. To ensure that SAR remains a powerful tool for Earth observation, this letter presents a 2-D variable attenuation space (azimuth)-frequency filtration (VASFF) method. This framework leverages the time-frequency characteristics of Level-0 SAR data, the RFI power profile, estimated RFI signal parameters, and the SAR antenna pattern to design a novel variable filter. Signal power localization estimates the interference source’s relative position, facilitating filter application. Simulated results, obtained using our open-source emulator, SEMUS, to generate both clean and interference-contaminated raw SAR data, demonstrate that the proposed filter achieves a 2 dB improvement over traditional notch filtering. The framework is further tested on real-life interference events on TerraSAR-X revealing previously obscured image details, validating the framework’s effectiveness.

The TanDEM-X twin satellites have been acquiring bistatic SAR data for 13 years. The main mission goal was achieved in 2016: the generation of a global DEM of unprecedented homogeneous quality and resolution. For the processing of the second global DEM, the TanDEM-X DEM 2020, a gap-free DEM was required. Therefore, we developed a fully automated edition to fill the gaps corresponding to about 0.1% of the land mass and to flatten the water bodies. External reference DEM datasets were used to fill the gaps. The Reference Elevation Model of Antarctica (REMA) was used for editing the Antarctic continent. Here we found large overlapping gaps within both REMA DEM and the TanDEM-X DEM. We developed a new algorithm to patch these regions in the REMA DEM in order to edit the TanDEM-X DEM with a single editing approach. This method considers the recursive calibration of new InSAR-derived DEMs in a mosaicking process. In this paper we present the fully automatic editing approach used for the released version of the TanDEM-X 30m Edited DEM, the challenges faced and the future activities related to this topic.

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

The 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.

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 concurrent imaging technique enables parallel acquisitions with different beams or modes, e.g., a wide area Stripmap mode with a High-Resolution Spotlight mode. Such a concurrent Stripmap/Spotlight imaging technique is investigated for TerraSAR-X. This technique employs a pulse-to-pulse interleaving scheme to acquire two acquisitions-even of disjunctive areas-at the same time, offering products with different resolution and coverage portfolios. This capability is especially interesting for customers interested in an overview of a larger area but at the same time observing an area of interest with higher resolution, e.g., for infrastructure monitoring or reconnaissance applications. The basic concepts, as well as the driving system parameters, are discussed in detail, together with a coverage analysis revealing the high availability rate of the mode combinations on a global scale. A processing approach re-using a substantial part of the existing infrastructure is described and exemplary acquisitions are shown, together with a detailed performance analysis with respect to resolution and ambiguities.

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

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.

In 2017, the TanDEM-X Mission decided to generate a second - more recent - global DEM. The acquisitions took place from 2017 till mid 2020 and represent a new global coverage of the whole Earth's landmass. This global dataset is well separated in time from the data used for the first global TanDEM-X DEM. Recent terrain height information can be delivered globally again with similar accuracy and consequently, terrain changes can be monitored. Currently, this data is being processed by the Integrated TanDEM-X Processor (ITP) into pre-calibrated single scenes. A reference DEM is a pre-requisite to enable a correct interferometric processing.

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.

With the global TanDEM-X DEM generation being finished in 2016, the mission is now acquiring a new dataset to provide an independent DEM, the so-called "TanDEM-X Change DEM". It is based on a completely new dataset acquired from 2017 - 2019 in contrast to the acquisitions for the global DEM between 2010 and 2015 in the aim to characterise terrain changes. It benefits from improvements in the acquisition planning process and in the data processing which enable to achieve reliable DEM data of high accuracy with fewer acquisitions. For this goal, the use of an edited TanDEM-X DEM as "starting point" for the processing is mandatory. Detectable 3d elevation changes are presented exemplarily in Indonesian forest and on an outlet glacier in Antarctica.

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

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.

Remote sensing of Antarctic sea ice is required to characterize properties of the vast sea ice cover to understand its long-term increase in contrast to the decrease of Arctic sea ice. For this objective, the OIB/TanDEM-X Coordinated Science Campaign (OTASC) was successfully conducted in 2017 to obtain contemporaneous and collocated remote sensing data from NASA's Operation IceBridge (OIB) and the German Aerospace Center (DLR) TanDEM-X Synthetic Aperture Radar (SAR) system at X band together with Sentinel-1 and RADARSAT-2 SARs at C band in conjunction with WorldView satellite spectral sensors, surface measurements, and field observations. The Weddell Sea and the Ross Sea were two primary regions while SAR data were also collected over six other regions in the Southern Ocean. Satellite SAR data included both polarimetric and interferometric capabilities to infer snow and sea ice information in three dimensions (3D), while OIB/P-3 aircraft data include snow radar together with altimeter data for snow and sea ice observations in 3D over the Weddell Sea. Across the Ross Sea, IcePOD and AntNZ/York-University flights were carried out together with satellite SAR data acquisitions.

Tandem-L is a proposal for a spaceborne L-band SAR mission for the systematic observation of dynamic processes on the Earth's surface with hitherto unparalleled quality and resolution. The enormous amount of data to be acquired will be used to provide information concerning dynamic processes in the biosphere, geosphere, cryosphere, and hydrosphere [1]. Within the scope of this mission, several operational phases have been established and corresponding flight configurations have been designed. Subsequently an observation concept has been developed in order to fulfill all the scientific requirements. A sophisticated planning and optimization process has been implemented. It combines the predetermined satellite resources, ground station network, and the scientific requirements to generate an acquisition timeline on individual data take level. The resulting timeline fulfills all requirements for the various scientific applications and can be used to analyze different aspects of the mission like data volumes and orbit usage.

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.

TanDEM-X is a spaceborne SAR mission with the goal to derive a global Digital Elevation Model (DEM). This paper gives an overview on the acquisition planning and data analysis after completion of two global coverages. The first part summarizes the DEM acquisition strategy including the satellite formation evolution, coverage status, and the planning concept for further interferometric measurements over difficult terrain. In the second part of the paper, the single acquisitions are analyzed for their interferometric quality, such as coherence and relative height errors. After calibration of systematic baseline offsets and instrument internal effects, the monitoring status of absolute DEM height errors is presented, too.

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.

TanDEM-X is an interferometric SAR (InSAR) mission acquiring bistatic images with two satellites. Systematic mapping of the Earth's land masses will provide individual interferometric data sets which will be mosaicked and calibrated into a global Digital Elevation Model (DEM). The concept of InSAR and DEM quality monitoring throughout the acquisition and processing phase is presented in this paper.

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.

This letter presents an investigation on scalloping correction in the Terrain Observation by Progressive Scan (TOPS) imaging mode for synthetic aperture radar systems with electronically steered phased array antennas. A theoretical simulation of the scalloping is performed, and two correction methods are introduced. The simulation is based on a general cardinal sine (sinc) antenna model as well as on the TerraSAR-X (TSX) antenna model. Real TSX data acquired over rainforest are used for demonstration and verification of the scalloping simulation and correction. Furthermore, a calibration approach, taking into account the special TOPS imaging mode properties, is introduced.

This paper reports on several experiments performed with the TerraSAR-X (TSX) and the TanDEM-X (TDX) satellites. The experiments consist in the generation of improved image products by coherently combining the images acquired individually by both satellites, hence the name distributed imaging. In the literature it has already been suggested the use of two or more satellites in close formation not only to have interferometric capabilities, but also to improve the azimuthal or range resolutions by performing the acquisitions simultaneously (no temporal decorrelation). This idea can be carried out by acquiring different portions of the spectrum and adding them coherently afterwards. Another possibility is to synthesize quad-pol acquisitions using dual-pol ones, an idea already carried out with airborne systems. Such experiments have been performed with TSX and TDX and are described in this paper.

In addition to the first satellite TSX already in-flight since 2007 [1], the second satellite TDX of the TanDEM-X system could be successfully launched in 2010 [2]. The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale. But in addition to this DEM acquisition based on a bistatic satellite constellation, nominal TerraSAR-X operation shall be available anymore, i.e. the bistatic TanDEM-X mission and the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX had to achieve the same accuracy and performance as those of the first satellite TSX. Based on a short overview of the different calibration procedures the paper discusses the calibration results achieved for the whole TanDEM-X system, successfully in-flight since June 2010.

After the recent launch of the TanDEM-X satellite, the twin of TerraSAR-X, its demanding commissioning phase has started. On the one hand, it has to ensure the same monostatic operation performance as TerraSAR-X. On the other hand, the bistatic aspects have to be verified, which are essential for the acquisition of the global digital elevation model (DEM). This has to be done in a limited time period in order to keep the required nominal operation duration. This paper shows the summary of the commissioning phase activities and its running status.

The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale [1]. But in addition to this bistatic TanDEM-X mission the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX, successfully launched in June 2010, has to achieve the same accuracy and performance as those of the first satellite TSX, already in-flight since 2007 [2]. Thus, the monostatic calibration of the second satellite TDX is performed according to the same strategy based on effective and exact calibration techniques successfully demonstrated by the first satellite TSX [3]. The paper discusses the calibration results of the first satellite TSX derived two years after launch by an extended re-calibration campaign executed in summer 2009, and presents first results of the second satellite TDX. But it has to be mentioned, the main calibration activities of TDX will start three weeks after launch, i.e. after uploading of this paper.

The high flexibility and tight accuracy requirements of modern spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process SAR images. To perform accurate pattern correction during SAR processing, an antenna model can be used to derive the multitude of different antenna beams generated by active antenna steering. The application of such an antenna model could be successfully demonstrated for the TerraSAR-X mission, launched in 2007. The methodology and the results of the in-orbit verification with an achieved accuracy of better than ±0.2 dB are reviewed in this paper in detail, showing its outstanding accuracy. Additionally, the results of the antenna pattern long-term monitoring are described, pointing out the high stability of the system.

Spaceborne remote sensing with synthetic aperture radar (SAR) has become an essential source of high-resolution and continuous Earth observation. Modern satellites like the German TerraSAR-X system provide state-of-the-art radar images with respect to operating flexibility and imaging quality. The outstanding performance of TerraSAR-X image products is achieved by an innovative calibration approach that minimizes systematic antenna and instrument characteristics. The active phased array X-band antenna is fed by 384 transmit/receive modules for electronic beam steering and shaping in the azimuth and elevation direction. The flexible radar instrument hosts an internal calibration system which guarantees the high radiometric stability of all SAR products. New techniques for antenna performance control have been successfully implemented, setting a high standard for next-generation SAR missions. This paper summarizes all essential calibration results of TerraSAR-X that cover internal instrument behavior. Furthermore, we give an outlook on the required bistatic calibration techniques for the future TanDEM-X mission that faces additional performance challenges when calibrating two TerraSAR-X satellites flying in close formation.

The TanDEM-X mission, result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, opens a new era in spaceborne radar remote sensing. The first bistatic satellite synthetic aperture radar mission is formed by flying TanDEM-X and TerraSAR-X in a closely controlled helix formation. The primary mission goal is the derivation of a high-precision global digital elevation model (DEM) according to High-Resolution Terrain Information (HRTI) level 3 accuracy. The finite precision of the baseline knowledge and uncompensated radar instrument drifts introduce errors that may compromise the height accuracy requirements. By means of a DEM calibration, which uses absolute height references, and the information provided by adjacent interferogram overlaps, these height errors can be minimized. This paper summarizes the exhaustive studies of the nature of the residual-error sources that have been carried out during the development of the DEM calibration concept. Models for these errors are set up and simulations of the resulting DEM height error for different scenarios provide the basis for the development of a successful DEM calibration strategy for the TanDEM-X mission.

The TanDEM-X satellite synthetic aperture radar (SAR) mission, which is the result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, has the goal to deliver a high-precision global digital elevation model (DEM). The X-band SAR interferometry-derived DEMs contain absolute and relative height errors that have to be minimized with the help of height references in order to achieve the specified accuracies. ICESat laser altimetry data are suited for this task, due to their accuracy and global distribution. In order to gain experience in the comparison between a radar-derived DEM and ICESat GLA14 elevation data, an X-band DEM was acquired over a test region with the experimental airborne radar system of DLR in Oberpfaffenhofen. Additionally, a laser DEM of the area was used to verify the height accuracy claimed by previously published ICESat studies over different terrain types and after applying different selection threshold criteria. The analyses described in this paper are the basis for the definition of a suitable global ICESat selection strategy and include the computation of the density of selected ICESat samples over the Earth. These aspects are crucial for a successful TanDEM-X DEM generation.

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band synthetic aperture radar (SAR) instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. This paper describes the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures.

Modern synthetic aperture radars (SARs) are equipped with active phased array antennas to electronically generate various antenna beams. The TerraSAR-X satellite is a high resolution SAR system launched in June 2007. Its active phased array X-band antenna hosts 384 transmit/receive modules (TRMs) for controlling the electronic beam steering in azimuth and elevation direction. The precise modeling of the antenna performance is only possible if the actual characteristics of each individual TRM are monitored. TerraSAR-X has been equipped with an innovative characterization mode based on a coding technique, which is the so-called pseudonoise gating method. The individual and simultaneous characterization of all TRMs is realized under most realistic conditions with power supply loads like in nominal radar operation. For the first time, this novel technique has been applied on a spaceborne SAR system.

The TanDEM-X mission has the goal to deliver a digital elevation model (DEM) that fulfils HRTI-3 quality requirements on a global scale. The interferometric height is determined by the phase difference between the two acquired images and the spatial geometry. Baseline errors intrinsic of the bi-static SAR configuration combined with errors and drifts of the radar instrument introduce phase inaccuracies in the interferogram. Therefore, an accurate calibration of the interferometric system parameters and independent height references are required. In order to validate the DEM calibration concept with real interferometric data, a measurement campaign was carried out with the experimental airborne radar system (E-SAR) of the German Aerospace Center (DLR) in Oberpfaffenhofen. This paper will present some of the various results from these interferometric experiments, stressing on the quality assessment of the ICESat GLAS14 elevation data, which will be a key aspect in a successful TanDEM-X DEM generation.

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band SAR instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct operation of the SAR system can be ensured. The paper describes the method of calibrating TerraSAR-X and final results derived from all calibration procedures.

The TanDEM-X mission will derive a global digital elevation model (DEM) with satellite SAR interferometry. The aimed accuracies are an absolute height error of 10 m and a relative height error of 2 m for 90% of the data. This requires a correction of the elevation heights after interferometric processing by residual systematic DEM errors. The estimation and correction of these errors is called DEM calibration. This paper gives an overview of the DEM calibration strategy within the TanDEM-X mission. First, the error sources and their influence on the DEM are determined by a functional description. Then, a strategy for a new block adjustment of DEMs is set up and evaluated with simulated and real DEMs.

TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite due for launch in 2007. Its active phased array X-Band antenna hosts 384 transmit/receive modules controlling the beam steering in azimuth and elevation direction. Precise modelling of the antenna is only possible if the actual characteristics of each individual transmit/receive module are known. TerraSAR-X has been equipped with an innovative characterisation mode based on the so-called PN Gating method. Individual and simultaneous characterisation of all transmit/receive modules is realised under most realistic conditions with the same power loads like in the nominal mode. This paper shows the results of PN Gating measurements on a satellite SAR system.

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.

As TerraSAR-X, due for launch in 2007, will be an operational scientific mission with commercial potential, product quality is of crucial importance. The success or failure of the mission essentially depends on the calibration of the TerraSAR- X system ensuring the product quality and the correct in-orbit operation of the entire SAR system. The paper describes the in- orbit calibration procedure for TerraSAR-X and dedicated activities performed during the commissioning phase. First results could not be described because TerraSAR-X was not launched up to the time of uploading the full paper.

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.