TerraSAR-X and TanDEM-X Mission Status
Buckreuss, Stefan; Zink, Manfred; Hajnsek, Irena
DLR, Microwaves and Radar Institute, GERMANY

The missions TerraSAR-X and TanDEM-X jointly share the same space segment consisting of two almost identical satellites orbiting in close formation and are operated using a common ground segment, that was originally developed for TerraSAR-X and that has been extended for the TanDEM-X mission. A key issue in operating both missions jointly is the different acquisition scenarios: whereas TerraSAR-X requests are typically single scenes for individual scientific and commercial customers, the global DEM requires a global mapping strategy.

Thus the TerraSAR-X mission goal is retained and served by both satellites, whose performance is almost identical. Due to the excellent calibration the final image products are not distinguishable from the user’s point of view. The TerraSAR-X mission provides the 'classic' product portfolio containing Stripmap, ScanSAR and Spotlight mode images in single and dual polarization. The portfolio will be upgraded by two new products, which are currently under development: a Wide-ScanSAR product with an extended swath width of ca. 200 km and a Staring Spotlight product with a geometric resolution of 1 m and considerably improved radiometric resolution.

Meanwhile TerraSAR-X, launched in June 15, 2007, has reached its nominal lifetime end of 2012. Yet the current status of the spacecraft resources allow at least two years extra lifetime until end of 2014, providing four years of joint operation with its twin satellite in order to accomplish the TanDEM-X mission.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in spaceborne radar remote sensing. A single-pass SAR interferometer with adjustable baselines in across- and in along-track directions is formed by adding a second, almost identical spacecraft to TerraSAR-X and flying the two satellites in a closely controlled formation. With typical across-track baselines of 200 m – 400 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at a 12 m posting will be generated.

After the launch of the TanDEM-X satellite in June 2010 the monostatic commissioning phase was dedicated to calibration and performance verification and revealed calibration accuracies and overall performance of the TanDEM-X SAR system and its products as good as for TerraSAR-X. After comprehensive testing of the various safety measures close formation was achieved mid October 2010 and the operations at typical distances between 150 and 500 m is running remarkably smooth and stable since then.

The subsequent bistatic commissioning phase of the TanDEM-X mission concentrated on checking out the complete bistatic chains from acquisition planning to bistatic and interferometric processing and generation of so-called raw DEMs. Global DEM acquisitions have started in December 2010 and the first global coverage (except Antarctica) was completed in January 2012. Parallel to the first month of operational data acquisition the team concentrated its efforts on the calibration of the bistatic interferometer. Correction of differential delays between TSX and TDX was necessary to facilitate the utilization of radargrammetry for resolving the 2π-ambiguity band. A comprehensive monitoring system has been established to evaluate the performance of each individual data take and provide feedback to the TanDEM-X acquisition planning for additional acquisitions. In the meantime phase, delay and baseline calibration have reached such an accuracy level, that more than 90% of all so-called Raw DEMs are within ±10 m of DEM derived from SRTM/ICESat data already before the final calibration step using ICESat data as reference heights.

Efforts currently concentrate on making the final calibration and mosaicking chain fully operational. Based on the first global acquisition so-called intermediate DEMs are being produced for larger regions. The second global coverage will be completed in the first quarter of 2013. Planning for the third or even fourth acquisitions over difficult terrain is in preparation. This period will also include Antarctica during local winter conditions in summer 2013. It is anticipated to finalise data acquisitions by the end of 2013. First parts of the global TanDEM-X DEM will become available in early 2014.

After the standard DEM acquisitions, DEMs of even higher accuracy will be produced for selected areas. In addition, interferometric acquisitions with even larger baselines can be adjusted for the exploration and demonstration of scientific experiments.