Markus Bachmann

TanDEM-X: The New Global DEM Takes Shape

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative formation flying radar mission that opens a new era in spaceborne radar remote sensing. The primary objective is the acquisition of a global Digital Elevation Model (DEM) with unprecedented accuracy (12 m horizontal resolution and 2 m relative height accuracy). This goal is achieved by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X like satellite TanDEM-X (TDX) flying in close formation with TerraSAR-X (TSX). The resulting large single-pass SAR interferometer features flexible baseline selection enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. Beyond the global DEM, several secondary mission objectives based on alongtrack interferometry as well as new bistatic and multistatic SAR techniques have been defined. Since 2010 both satellites have been operated in close formation to map all land surfaces at least twice and difficult terrain even up to four times. While data acquisition for DEM generation will be concluded in the second half of 2014 it is expected to complete the processing of the global DEM by the end of 2015. This paper provides an overview of the TanDEM-X mission and summarizes its actual status as well as the performance of the system and the first final DEMs. Up to now the mission driver was the DEM generation and scientific experiments have been limited to the pre-defined DEM formation geometries. This paper also outlines the current planning for a dedicated science phase starting in the last quarter of this year.

Development of the TanDEM-X Calibration Concept: Analysis of Systematic Errors

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 mission: A satellite formation for high-resolution SAR interferometry

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative spaceborne radar interferometer mission that was approved for full implementation by the German Space Agency in spring 2006. This paper gives an overview of the TanDEM-X mission concept, summarizes the basic products, illustrates the achievable performance, and provides some examples for new imaging modes and applications.

Definition of ICESat Selection Criteria for Their Use as Height References for TanDEM-X

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 Calibration Results

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.

Individual T/R module characterisation of the TerraSAR-X active phased array antenna by calibration pulse sequences with orthogonal codes

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.

TerraSAR-X Instrument Calibration Results and Extension for TanDEM-X

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.

TanDEM-X: DEM acquisition in the third year era

TerraSAR-X add-on for digital elevation measurement (TanDEM-X) is a space-borne X-band SAR mission designed to derive a digital elevation model (DEM) of the Earth’s land surface with an unprecedented relative vertical accuracy of 2 m at a 12 m posting. To achieve this goal, the two satellites fly in close controlled formation with the opportunity to adjust flexible along- and across-track baselines. A combination of multiple acquisitions with different baseline lengths is required to allow stable phase unwrapping and to achieve the high height accuracy. This paper provides an overview of the TanDEM-X acquisition plan for the third year over Antarctica, mountainous areas and deserts. Critical aspects of the areas to be acquired are presented and the acquisition strategy is illustrated. Moreover, the formation flight and the baselines are compared to the ones of the previous years in terms of their impact on the height of ambiguity and on the performance with respect to the interferometric quality.

Accurate Antenna Pattern Modeling for Phased Array Antennas in SAR Applications - Demonstration on TerraSAR-X

The high flexibility and tight accuracy requirements of todays spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process the SAR images. To perform accurate pattern correction during SAR processing, an Antenna Model is 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 inorbit verification with an achieved accuracy of better than ± 0 . 2  dB is reviewed in this paper in detail showing its outstanding accuracy.

The TerraSAR-X Antenna Model Approach

TerraSAR-X is a highly flexible X-band radar satellite. Its primary objective is the acquisition of high quality SAR images in a multitude of possible acquisition modes. The great amount of antenna patterns needed for image acquistion requires an antenna model accurately describing all beams. To guarantee the required image quality, the model has to be verified on-ground and validated in-orbit. The results of the verification will be described here as well as the validation approach.