The Extended Timing Annotation Dataset (ETAD) product provides timing corrections for Sentinel-1 (S-1) single-look complex (SLC) data for improved geometric accuracy. Given the close relationship between path delay and radar signal phase, it also has the potential to serve as the basis for phase corrections in SAR interferometry (InSAR) applications with Sentinel-1. The provided timing corrections may be converted to phase offsets, which can be used to derive interferometric phase screens between pairs of repeat-pass acquisitions. In this work we explore the capabilities and limitations of ETAD to provide accurate interferometric phase corrections and introduce new features currently under investigation in the context of S1-ETAD scientific evolution study.

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.

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.

Over the last years the TanDEM-X mission acquired data for a second global digital elevation model (DEM) the TanDEM-X Change DEM. This new DEM is temporally independent of the former global TanDEM-X DEM and therefore yields the possibility of change detection. In order to decrease the phase noise level the interferometric processing for the Change DEM has been upgraded. This also allows a more accurate change detection. Currently, the processing of the global data is performed operationally. It includes the detection of terrain changes and first examples of detected terrain changes can be presented.

The “TanDEM-X Change DEM” will be a new DEM consisting of the data globally acquired by the TanDEM-X mission from 2017 until 2020. This new DEM aims to characterize terrain changes which occurred between the acquisition of the TanDEM-X global DEM, acquired between 2010 and 2015, and the new temporally independent and up-to-date data set. The new data will mostly contain only one global coverage. Therefore, necessary improvements in the acquisition planning process as well as in the interferometric processing were made. Specifically, a new adaptive filtering approach is presented in this paper as well as its influence on the interferometric phase and the DEM for test sites over Germany and Chile.

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.

Synthetic Aperture Radar (SAR) satellites observe range and azimuth geometric accuracies at the low centimeter level. The accuracy of geolocation is driven by several aspects, e.g. orbit determination, SAR image processing, or atmospheric error correction. Our paper concentrates on the Sentinel-1 mission and the compensation of the platform motion effects in the geolocation, which were found to limit the best possible geolocation capabilities of Sentinel-1. The key to advance the geolocation results is the rigorous compensation of the bistatic effect in azimuth, and the correction of the Doppler-induced shifts in range. First results for Sentinel-1 at the Australian reflector array consisting of 40 Corner Reflector (CR) show consistent improvement in the geolocation$(1\sigma)$to 6 cm in range and 28 cm in azimuth for both spacecrafts when using the Interferometric Wide swath (IW) product.

In the past years we have exploited the TanDEM-X elevation model for impact crater research. This is the first high-resolution global DEM which permits accessing all confirmed impact structures. We demonstrate that the high horizontal and vertical accuracies, coupled with the dense pixel grid, allow studying the morphologies of simple and complex craters over a wide diameter range.

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 TanDEM-X mission is the first free flying bistatic SAR mission. It has the primary objective to generate within a short time frame a global digital elevation model (DEM) of 10-m absolute vertical accuracy and 2-m relative height accuracy. For that, the whole land mass has been mapped at least twice with different baselines. The success of the mission depends on the accuracy of the final DEM and therefore on the reliability of the phase unwrapping (PU) algorithm. Hence, a robust and versatile PU method, which is in accordance with the acquisition concept, is necessary. This paper presents a new method that combines bistatic high-resolution interferometric data in order to perform an accurate PU on a huge amount of data. The dual-baseline PU correction (DB-PUC) framework addresses this challenge by correcting errors that occurred during the single-baseline PU procedure. It benefits from the additional information available through the differential interferogram and the stereo-radargrammetric phase, which are used to correct region-wise the ambiguity bands of the misestimated unwrapped phases to be less sensitive to noise and possible temporal changes. The multilevel of the DB-PUC approach makes it flexible, computationally efficient, and well adapted to deal with the various PU error scenarios. This framework is used operationally for the processing of the data of the TanDEM-X mission.

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.

The next milestone in the TanDEM-X mission is the generation of high-resolution DEMs for selected areas. New scenes with lower heights of ambiguity are being acquired to fulfill the goal on the relative height accuracy of 0.8 m. To enable a reliable phase unwrapping in a finite time, a processing concept based on the usage of the final global TanDEM-X DEM has been developed. Since the global DEM provides an up-to-date measurement of the terrain height with an unprecedented high accuracy, it is of great help for the phase unwrapping procedure. This paper proposes an adaption of the Integrated TanDEM-X Processor, which is used operationally for the generation of the global DEM data, so that it can process DEMs of higher resolution from interferometric bistatic data with very low height of ambiguity and better resolution by making good use of the finalized global DEM. The final paper will focus more particularly on the assessment of the unwrapping results.

This paper evaluates the applicability and achievable SAR accuracy for octahedrons - a combination of eight corner reflectors with a common phase centre. In an experiment at the observatory in Wettzell from July to November 2015 these cost-efficient and mobile radar targets were measured with TerraSAR-X Staring Spotlight and High-Resolution Spotlight. Applying the geodetic stereo SAR concept, octahedrons are very robust for absolute 3D positioning through their backscattering in multiple directions. Using octahedrons with as size of 47 cm, we achieve 3σ standard deviations of about 3 cm for east, north and height components. For individual measurements in Staring Spotlight the standard deviation shows 1.4 cm in range and 3.2 cm in azimuth.

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.

SAR interferometry is a well-known technique to produce digital surface models (DSM). In the global scale, the quality of these models significantly increased with the use of bistatic systems as for the TanDEM-X mission. DSM uncertainty can be measured in two scales: a global and a local one. A global scale analysis involves measures for the complete take, characterizing the whole correctness with a single parameter. A local scale analysis enters instead in the single elevation model cell, evaluating the quality obtained by different terrains and media present in the take. A coherence analysis is a typical measure for InSAR models. Globally, a simple average gives an overall quality measure hint. Locally, it provides a measure of the DSM pixel height error. Unfortunately, this measure lacks in several aspects as not considering unwrapping errors and not being valid for certain types of scattering. In this paper, various strategies for assessing elevation accuracy methods and demonstrate the different capabilities of the bistatic TanDEM-X system are proposed for variegated terrain types.

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.

In this letter, the suitability of alternating bistatic (AB) TanDEM-X data for tomographic processing is demonstrated. The AB acquisition mode allows to acquire data from three different phase centers at the same time by means of a single pass of the TSX-1 and TDX-1 satellites. The channels of an AB acquisition present no temporal decorrelation within them. In addition, the atmospheric delay can be considered equal for all the channels. This letter analyzes the use of a single acquisition in the AB mode for tomographic processing. The theoretical analysis and the experimental result show that it is possible to separate different scattering mechanisms within a resolution cell under certain constraints.

Due to the indirect measurement principle of radar on base of signal travel time, a very precise calibration of the sensor's internal clock with regard to clock rate and potential time offsets relative to the Coordinated Universal Time is an essential prerequisite for accurate pixel localization in Synthetic Aperture Radar. Giving special considerations on this aspect, we developed two algorithms that improve the already high localization accuracy of TerraSAR-X: The either one very precisely determines the true oscillator clock rate, while the other one effectively refines the accuracy of time annotation. Experimental analyses evaluate the obtained improvements in localization accuracy from both techniques.

This paper deals with the fusion of TanDEM-X raw DEMs in ascending and descending pass over Mumbai test area and enhance its quality. Before applying fusion method, a robust layover and shadow map has been calculated in ITP using TanDEM-X DEM and the corresponding slant range image. The selection of optimum weights for fusion has been based on height error map calculated from interferometric coherence. Results show a substantial reduction in number of invalid pixels after fusion. In the fused DEM, invalid is only 1.2%, while ascending and descending pass DEMs have 6.7% and 5.7% respectively. The improvement in accuracy of the DEM is very slight in this case which is due to the coarse resolution of the SRTM DEM used as reference.

The unprecedented geometric accuracy of the German SAR satellites TerraSAR-X and TanDEM-X raises a new perspective in SAR geolocation. The focus of the present paper is to exploit this unprecedented SAR geolocation accuracy for change detection applications where is not possible to use any reference. This paper presents three specific applications. The first application takes advantage of the along track (AT) baseline of TanDEM-X bistatic data to derive ships 2D velocity. The second application is related to monitor the stability of isolated offshore platforms using TerraSAR-X repeat-pass data. The last one consists on glacier monitoring using a specific deployed Top-Hat reflectors.

The German satellites TerraSAR-X/TanDEM-X are high resolution imaging SARs with a resolution in the meter and even sub-meter range. The original product specification document stated an absolute geometric pixel localization accuracy in the same order of magnitude i.e. 1 meter. It was shown by several teams that this accuracy requirement is easily met and even surpassed [1] by the operational products. During the last years the remaining residual error sources could be further studied and attributed mainly to tropospheric delay variations [2] geodynamic effects such as solid earth tides [3] and plate tectonics to refraction in the ionosphere and to technical limitations in the satellite [4]. Our team investigated all these contributions developed correction and calibration methods and validated them in experiments with corner reflectors at different locations. Furthermore we explore novel applications of this high geolocation accuracy that range from classical earth surface displacement measurements to the localization of scatterers in 3D space with a precision comparable to GNSS. This paper provides a summary of latest measurement results of our globally distributed corner reflectors which show consistently a high accuracy better than 2 cm in range and azimuth. Furthermore we report on new developments concerning tropospheric correction using ECMWF and numerical weather models (WRF). Using the best available compensation techniques we can demonstrate accurate 3D localization of corner reflectors and of other objects. Current investigations aim to further improve the orbital accuracy to sub-centimeter level by improved modelling of air-drag and solar radiation pressure on the spacecraft.

InSAR Processing and especially phase unwrapping is a crucial step in the realization of the global TanDEM-X DEM. After three years of operation, the InSAR chain has delivered more than 350,000 RawDEMs, which mostly fulfill already the high accuracy requirements. At the same time, it has been also optimized and improved to cope with input data of various quality. Special efforts have been made in the dual-baseline interferometric chain to deal with CoSSCs with larger ranges of height of ambiguity and time spans. This paper presents the key elements of the InSAR chain and analyses the dual-baseline phase unwrapping results.

Spectral estimation is considered in the paper as an additional instrument towards a better understanding of the physical phenomena behind the layover scattering decomposition. A super-resolution technique is employed to derive the fringe frequencies characterizing the layover portion. Due to the limited estimation support, only the dominant frequency is found to be reliable information. The non-linear relationship with slopes is employed to derive a principal slope map. A bistatic interferometric scenario is tested. It is found that for the majority of the detections the facade contribution is the prevailing one due to the presence of targets with a high backscattered signal return at the vertical slope. The number of layover contributors is assessed prior to the spectral estimation. It has been estimated that the signal return is dominated by a single contribution for the majority of the layovers.

This paper addresses the November 2013 Balochistan Earthquake. A co-seismic TerraSAR-X pair acquired in wide-swath ScanSAR mode has been used to derive two-dimensional deformation measurements (radar line-of-sight and azimuth direction) of the eastern part of the main M7.7 earthquake and the large M6.8 aftershock by correlating SAR amplitude images. Atmospheric and solid Earth tide corrections have been considered to achieve accuracy in the order of several centimeters. Correlation measurements from Landsat-8 images have been additionally estimated. The intention is to isolate vertical and horizontal components in order to obtain three-dimensional deformation measurements. Interferometric processing issues of ScanSAR data for non-stationary scenarios, specifically co-registration, are additionally discussed.

The aim of this work is to show two possible applications for Alternating Bistatic TanDEM-X data. The first application is a tomographic processing while the second is a 2D ship velocity estimation. Results over real data are presented for both applications.

A procedure to simulate the effects of the ionosphere on Synthetic Aperture Radar (SAR) images is presented. The propagation delay errors induced by the ionosphere have to be compensated to millimeter level in order to meet the scientific requirements for an L-band mission dedicated to deformation measurements, which are summarized in [1]. The simulator presented in this paper can be used to study the effects of an arbitrary ionospheric state on SAR images and to generate disrupted raw and focused images starting from ionospherefree real SAR images and use them to validate ionosphere estimation methods.

This paper presents the processing results of the dual-baseline phase unwrapping correction chain of the Integrated TanDEM-X Processor. Second year data are operationally processed since August 2012 with the help of supporting CoSSCs. They are shifted of half a swath width with respect to the first year data. Supporting CoSSCs are the first year scenes. The first and second year scenes have different height of ambiguity but are taken from the same look angle. The chain has to deal with huge amount of data of different quality and from every part of the world landmass. A success rate of more than 97% is achieved overall.

The german TerraSAR-X and TanDEM-X satellites are able to acquire images operationally in ScanSAR mode and experimentally in TOPS mode, the future Sentinel-1 Interferometric Widesath and Extra Wideswath modes. This paper gives an overview of the interferometric processing steps of burst-mode acquisitions and present the current status in preparation for future algorithm development towards PSI. Interferometric results of TerraSAR-X repeat-pass images in ScanSAR and TOPS mode are shown.

The German SAR (synthetic aperture radar) satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1), launched in June 2007 and June 2010 respectively, provide an unprecedented geometric accuracy. Previous studies showed an absolute pixel localization for both sensors at the centimeter level [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a location accuracy of even a few millimeters seems to be attainable. While on a long-term scale, we observed a slow variation of subsequent measurements; on a short-term scale, they coincided to within a few millimeters. The measurement series will be continued. The cause of the long-term variation is the subject of current investigation.

In order to obtain the accurate ground deformation from SAR interferograms on volcanic areas, the signal propagation delay through the atmosphere needs to be considered. In this paper numerical weather model data are used to compensate the stratified atmospheric delay in test site El Hierro (Canary Islands). Three GPS stations, installed on the nearby Island, are selected for validating and evaluating the weather model data. Atmospheric phase screen from PS-estimation shows a good agreement with turbulence induced atmospheric effect.

Our paper reports about temporal changes of the Earth's topography found in first samples of TanDEM-X data when compared to SRTM Digital Elevation Models collected ten years earlier. We give an accuracy prediction and explain the methods to avoid systematic errors from biases and phase unwrapping ambiguities. Our paper provides examples from different Earth surface domains such as glaciers, volcanoes, mining activity and urban areas. We demonstrate and conclude that the higher the resolution of a DEM gets, the more frequent updates are required to keep the DEM valid with time.

The operational dual-baseline phase unwrapping algorithm for the TanDEM-X mission is based on a combination of separate single-baseline phase unwrapping and a correction procedure of different levels of complexity. It benefits from all the information available from the two TanDEM-X acquisitions by computing a differential interferogram to obtain a more reliable unwrapped phase. This may still be prone to phase unwrapping errors, but these can be mitigated using a stereo-radargrammetric measurement. Hence, the resulting dual-baseline phase unwrapping algorithm outperforms a single-baseline one.

Started in June 2010, the TanDEM-X satellite joined the TerraSAR-X satellite in space to perform the conjoint interferometric TanDEM-X mission to acquire a truly global Digital Elevation Model (DEM) of unprecedented accuracy [1]. Since the very first interferometric acquisitions, the Integrated TanDEM-X Processor (ITP) delivered operationally “Raw”-DEMs and complex products of mono- and bistatic data. The RawDEMs are scenes of about 50 km × 30 km, generated for a dedicated DEM Mosaicking and Calibration Processor (MCP) which produces the final DEM. The so-called Coregistered Single-look Slant-range Complex (CoSSC) products are provided for each of these scenes in different flavors for production internal purposes and system performance monitoring as well as for scientific use. The capabilities of the ITP go far beyond the primary mission objective of DEM generation alone: it also provides the operationally available end-user products from different experimental modes as e.g. pursuit monostatic, dual polarization bistatic data, alternating bistatic in single and dual polarization and different bistatic and alternating bistatic spotlight modes. This paper focuses on the accuracy of the generated products, the ITPs contribution to the achieved accuracy of the data and the direct effect of it on the use and interpretation of RawDEM heights for temporal change detection. Also the basic characteristics of the operational experimental products are introduced..

During the second operational year of the TanDEM-X mission, a second coverage of the whole land mass is acquired in order to produce a high accurate and high resolution DEM from a combination of both data sets. This paper presents the dual-baseline interferometric processing chain. Its main steps consist of coregistering the different interferograms (having different baselines), of unwrapping the phases and of comparing them to eliminate the possible unwrapping errors.

In this letter, we report on two major improvements with respect to the results presented in our recent paper on imaging geodesy: the atmospheric delay estimation using numerical weather model data and the continental drift adjustment from different geodetic coordinate systems. First, we demonstrate that the GPS-based zenith delay estimates used in our recent paper to correct synthetic aperture radar (SAR) data can be replaced by estimates calculated from numerical weather model data. This leads to a much wider applicability of our approach and even improves the accuracy from 3.8 cm to 3.2 cm. Second, for measuring the absolute position within the SAR image, the coordinate system used has to be carefully chosen, otherwise continental drift affects the results. We show that correct consideration of the reference frames eliminates residual offsets by up to 3.1 cm.

The German SAR (synthetic aperture radar) satellites TSX-1 and TDX-1 outperform former civilian space-borne radar sensors in terms of geometric accuracy. Recently, both satellites constitute the first bistatic SAR system in space: TanDEM-X. With the high geometric accuracy of both satellites, the geolocation of ground targets has to accurately consider signal propagation effects and geodynamic effects which formerly were negligible. The prediction of expected radar positions is additionally complicated by the bistatic acquisition geometry of TanDEM-X. Here, we present our approach to cope with these demanding requirements. With the application of these techniques, measurements of the geometric accuracy of TSX-1 and TDX-1 reveal that their pixel localization accuracy is significantly better than in previous studies.

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.

Since July 2010, TerraSAR-X and TanDEM-X jointly acquire interferometric data. Starting their common commissioning phase with a so called pursuit monostatic configuration with 3 seconds time lag between the two passes, they were later put in a close formation in October 2010, acting since then as the first freely configurable bistatic SAR interferometer in space. All operational acquisitions were processed from instrument raw data to DEMs from day one of the data taking on by one single processing system: the Integrated TanDEM-X Processor (ITP) (see [1],[2]). Data take analysis, common parameter calculation, synchronization, bistatic focusing, filtering, co-registration, phase unwrapping and geocoding are all performed in one sequence inside this processor. This approach allows a high precision processing by passing all applied corrections and determined parameters from one step to the next. Specifically the geometric & phase accuracy and stability of the instruments, the processor and the auxiliary data (i.e. the millimetric precision of the baseline products) provide an unprecedented level of relative and absolute geometric accuracy in the bistatic operation. While many challenges of bistatic processing of the TanDEM-X data are encountered, the benefits of this single pass acquisition mode can be used to derive additional information from the data itself for further processing and calibration. In this paper, we will outline the bistatic interferometric processing steps of the ITP and focus on the aspects of geometric accuracy in DEM generation.

Ship detection is an important application of global monitoring of environment and security. In order to overcome the limitations by other systems, surveillance with satellite synthetic aperture radar (SAR) is used because of its possibility to provide ship detection at high resolution over wide swaths and in all weather conditions. A new X-band radar onboard the TerraSAR-X (TS-X) satellite gives access to spatial resolution as fine as 1 m. In this paper, first results on the combined use of TS-X ship detection, automatic identification system (AIS), and satellite AIS (SatAIS) is presented. The AIS system is an effective terrestrial method for tracking vessels in real time typically up to 40 km off the coast. SatAIS, as a space-based system, allows almost global coverage for monitoring of ships since not all ships operate their AIS and smaller ships are not equipped with AIS. The system is considered to be of cooperative nature. In this paper, the quality of TS-X images with respect to ship detection is evaluated, and a first assessment of its performance for ship detection is given. The velocity of a moving ship is estimated using complex TS-X data. As test cases, images were acquired over the North Sea, Baltic Sea, Atlantic Ocean, and Pacific Ocean in Stripmap mode with a resolution of 3 m at a coverage of 30 km$\times$100 km. Simultaneous information on ship positions was available from TS-X and terrestrial as well as SatAIS. First results on the simultaneous superposition of SatAIS and high-resolution radar images are presented.

In this paper, we report on experiments to measure large-scale Earth surface displacements, such as those caused by solid Earth tides, with centimeter-level accuracy using TerraSAR-X radar images. With two totally different approaches, corner reflectors and image correlation techniques, we show the clear interrelation between the radar range measurements and the projection of the solid Earth tide motion vector onto the radar line of sight. Pixel location accuracies of up to 2.6-cm standard deviation can be achieved after a single calibration. We further demonstrate that solid Earth tides and tropospheric water vapor variations are the largest sources of ranging error if not compensated for. Alternatively, tropospheric water vapor can be estimated with centimeter accuracy using our proposed technique of synthetic aperture radar (SAR) image correlation and solid Earth motion compensation by the existing models. We also consider ionospheric delays which improve the results marginally in the X-band. Our results show the best ranging accuracies so far reported for spaceborne radar amplitude images and make TerraSAR-X-together with our simple compensation methodology-suitable for the imaging of centimeter-level Earth displacements. Absolute measurements of volcanoes or glaciers are possible without the use of ground equipment and without the use of SAR interferometry, thus avoiding the associated problems of phase ambiguity, phase unwrapping, and reference points.

On June 21st, 2010, the German radar satellite TanDEM-X was launched and successfully placed in an orbit approaching the TerraSAR-X satellite until both systems will fly in close formation and will establish the only available bi-static interferometer in space. The primary TanDEM-X mission goal is to generate a global Digital Elevation Model (DEM) with a relative point-to-point height accuracy of 2 meters for moderate terrain at 12 m posting. For that purpose interferometric SAR data will be acquired over a period of 3 years in parallel to the operational running TerraSAR-X mission. Systematic processing of SAR raw data to so-called Raw-DEMs is performed by one single processing system, the Integrated TanDEM Processor (ITP). The final global DEM is then calibrated and mosaicked by a second system, the Calibration and Mosaicking Processor (MCP). The scope of this paper is to present an overview of ITP functionalities and to summarize the first processing results.

We report on a method and experiments to achieve about 3 centimeters pixel location accuracy of TerraSAR-X SAR images. The method is based on atmospheric refraction corrections, Earth tide compensation and image correlation or the use of corner reflectors. Our method does not exploit the SAR phase and is therefore not subject to phase unwrapping problems or unknown phase offsets.

The purpose of this paper is to investigate the estimation of surface currents directly on stripmap TerraSAR-X data, as an alternative to Along Track Interferometry. The algorithm relies on efficient baseband magnitude-based Doppler estimation, preventing the estimate from possible biases like azimuth ambiguities and strong target reflections. The validation of the algorithm is made with acquisitions over the Elbe estuary river in Germany and the Eyjafjallajökull volcano in Iceland.

The purpose of this paper is to provide an algorithmic overview of the interferometric processing embedded in the Integrated TanDEM-X Processor (ITP), settled to the generation of the raw digital elevation model (DEM). The main processing blocks are described, with a focus on the spectral matching of the azimuth spectra, the high-precision coregistration, the dual-baseline phase unwrapping and the geocoding of the products. The robustness of the algorithms is demonstrated through a dual-pass TerraSAR-X scenario.

Synthetic aperture radar (SAR) image intensity is disturbed by additive system noise. During SAR focusing, pattern corrections that are adapted to the characteristics of the wanted signal, but not to the characteristics of the noise, influence the spatial distribution of the noise power. Particularly in the case of ScanSAR, a distinct residual noise pattern in low backscatter areas results. This necessitates a noise-adapted radiometric correction of the focused image for almost all applications except interferometry. In this paper, we thoroughly investigate this topic. Based on signal theoretical and stochastic considerations, we develop a radiometric correction scheme. Simulations and the application of the algorithm to TerraSAR-X datatakes support the theoretical results.

The TerraSAR-X mission was launched in June 2007. After successful completion of the commissioning phase, the mission entered its operational phase in January 2008. Since that time, TerraSAR-X provides the scientific remote sensing community and commercial customers with high-quality spaceborne synthetic aperture radar (SAR) data products. The intention of this paper is to present the SAR data processing concept and the comprehensive portfolio of products reflecting the instrument's diverse imaging capabilities together with options of processing and achieved product quality as well as the essentials of SAR processing. Furthermore, it shall also provide details on how to fully exploit the precision of the TerraSAR-X products.

In 2009, the German radar satellite TerraSAR-X will be supplemented with the TanDEM-X satellite to form the first bi-static single pass interferometer in space. TanDEM-X will fly close to TerraSAR-X in a controlled helix configuration for 3 years to jointly acquire interferometric SAR data in bistatic mode. The primary TanDEM-X mission goal is to generate a global Digital Elevation Model (DEM) with a relative point-to-point height accuracy of 2 meters for moderate terrain at 12 m posting. This paper outlines the SAR data workflow from quality check screening through to bistatic focusing and interferometric processing to raw DEM generation.

TerraSAR-X, the first national German radar satellite, was launched in June 2007. It carries an X-band high-resolution synthetic aperture radar instrument featuring Stripmap, ScanSAR and, particularly, Spotlight imaging in a variety of different polarization modes. The mission completed its commissioning phase (CP) in December 2007, the provision of the SAR products for both the scientific and commercial user community was started in January 2008. One central TerraSAR-X element on ground is the pay-load ground segment PGS. From the beginning of the mission, PGS was nominally operated. About ten thousand data takes were already acquired and processed in 2007, not only for SAR verification and calibration purposes, but also for the operational ground segment validation. This paper provides the commissioning and early operational phase results from the SAR payload data processing perspective addressing data reception and SAR processing. Specifically the tuning and adjustment of the TerraSAR-X multi-mode SAR processor TMSP to meet the in-orbit data characteristics and to optimize the SAR focusing results is addressed. Relevant issues are the high-bandwidth chirp replica processing, side lobe suppression, Doppler frequency determination, processor normalization and phase-preservation.

TerraSAR-X data are operationally available to the public since January 2008. This paper shows selected TerraSAR-X interferometric examples with an emphasis on challenges introduced by the high resolution in spotlight mode and with 300 MHz.

In February 2007 the German TerraSAR-X satellite will be launched and the TerraSAR-X mission will enter its approximately 5 months commissioning phase. At that time, the challenging developments on both sides, the advanced high- resolution multi-mode SAR instrument on the one hand and the corresponding sophisticated TerraSAR-X ground segment on the other hand will prove correct interaction and functioning. Screening and processing of the SAR data is the task of the DLR developed TerraSAR Multi Mode SAR Processor TMSP. Preceded by data reception, transcription including decryption and followed by archiving, cataloguing and product delivery, processing of the data by the TMSP is the central part of the SAR data workflow implemented in the Payload Ground Segment PGS. Space and ground segment have been subject to intense complete system testing on ground. Here, the compatibility of SAR instrument commanding, SAR instrument operations and subsequent SAR data processing has been successfully proven for the various acquisition modes of the sensor. Compliance of specified and measured product performance has been investigated as far as possible utilizing simulated point target SAR data. However, the real challenge will be the screening and SAR processing of TerraSAR-X data acquired in orbit and linked down to the receiving station. Therefore, the complete reception and processing chain will be properly tuned and adjusted to the properties of the received TerraSAR-X payload data. The TMSP algorithms have to be configured, e.g. thresholds for calibration pulse analysis, estimation window sizes for SAR data analysis, parameterization of estimation algorithms. Also the configuration of product variants with respect to resolution and radiometric quality will be checked and refined. This paper presents the very first experiences in reception, transcription, screening and processing of TerraSAR-X data with respect to performance, throughput and quality. During the TMSP checkout phase the compatibility of instrument commanding and SAR processing have to be verified and the accordance of SAR performance prediction and the obtained product performance and quality have to be investigated. First characteristics of the SAR data with respect to raw data statistics, calibration pulse analysis and Doppler centroid measurements will be shown. As far as available examples of SAR image products featuring the different image modes, Stripmap, ScanSAR and Spotlight at different incidence angles and polarizations will be displayed and a first estimate of product performance parameters will be given.

The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission will start in 2009 with the aim of generating a global Digital Elevation Model with high accuracy corresponding to HRTI-3 specifications (12 m posting, 2 m relative point-to-point height accuracy for flat terrain). To achieve this goal, a second satellite similar to TerraSAR-X will fly close to TerraSAR-X in a controlled Helix configuration for 3 years to jointly acquire interferometric SAR data in bistatic mode. According to the current mission concept, there will be at least two complete coverages of the global land surface, each one running one year. The different coverages will have different heights of ambiguity to allow multi-baseline phase unwrapping. For the sake of a homogenous data quality the second acquisition will be shifted by half the swath width with respect to the first coverage. Finally difficult terrain will be covered two more times with different acquisition geometries (i.e. different look direction and/or incidence angles). This paper presents first study results of phase unwrapping algorithms foreseen to process SAR data from the bistatic TanDEM-X configuration.

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.

TerraSAR-X is a German satellite to be launched towards end of the year 2006. The SAR instrument built by EADS/Astrium has a multipolarized active phased array antenna and is programmable in wide ranges. This allows to operate imaging modes such as e.g. stripmap, spotlight, ScanSAR and dual-polarization modes. By further dividing the antenna in halves with different position or different polarization, experimental modes for ground moving target detection (GMT) or Quad-Polarization are possible. As a consequence of the various sensor capabilities the tree of SAR products offered by the DLR Ground Segment contains many different variants of polarization, resolution and scene size. Even more product variants are caused by different processing parameters selectable by the user. Some product performance parameters, such as the radiometric accuracy as a function of geometric resolution or the image resolution as a function of incidence angle depend on both, sensor settings (PRF, pulse duration) and on processing parameters (bandwidth, weighting, multi-looking). The processing parameters for the products have been designed by DLR in a way so that valuable products with maximum information content are offered to the user. In order to generate the specified products, the TerraSAR Multi Mode SAR Processor TMSP has been designed by DLR. This processor will be operated at the DLR payload ground segment in Neustrelitz for commercial and scientific customers. The processor will also be operated at additional direct access partner stations of Infoterra GmbH, who will serve private and public customers.

Ionospheric propagation effects have a significant impact on the signal properties of low-frequency synthetic aperture radar (SAR) systems. Range delay, interferometric phase bias, range defocusing, and Faraday rotation are the most prominent ones. All the effects are a function of the so-called total electron content (TEC). Methods based on two-frequency global positioning system observations allow measuring TEC in the ionosphere with coarse spatial resolution only. In this letter, the potential of broadband L-band SAR systems for ionospheric TEC mapping is studied. As a basis, the dispersive nature of the ionosphere and its effects on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices in mapping small-scale ionospheric TEC disturbances. A differential TEC estimator that separates ionospheric from tropospheric contributions is proposed

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