Jaime Hueso Gonzalez

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.

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.

Final results of the efficient TerraSAR-X calibration method

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 in-orbit operation of the entire SAR system can be ensured. The paper describes both 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.

Results of the TanDEM-X Baseline Calibration

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 2 m and 10 m absolute. In order to achieve this mission goal, the radial and cross-track distance between both satellites, the so called baseline has to be known with high accuracy. 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.

In-orbit calibration of the TanDEM-X system

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.

Unexpected height offsets in TanDEM-X: Explanation and correction

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 Einsteins 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.

TanDEM-X commissioning phase status

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.