TanDEM-X: A Challenging Radar Mission for Generating a Next-Generation Earth’s Topography Manfred Zink and Alberto Moreira German Aerospace Center (DLR) Microwaves and Radar Institute (HR) 82230 Weßling/Oberpfaffenhofen, Germany Manfred.Zink@dlr.de Abstract— TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is a German Earth observation radar mission that consists of a synthetic aperture radar interferometer with two almost identical satellites flying in a closely controlled formation. With a typical separation between the satellites of 120 to 500 m interferometric data sets are acquired and processed to a global Digital Elevation Model (DEM) with 2 m relative height accuracy at 12 m posting. While the main mission phase for DEM data acquisition has been finished by mid-2014, the processing of the global (100% global land surface coverage including the poles) TanDEM-X DEM will be concluded in 2016. Final DEMs for more than 60% of all land masses are already available for scientific and commercial applications. The results achieved so far are well within the expected performance for the global DEM. A 15-month science phase of the TanDEM-X mission started in October 2014 which offers the opportunity to generate DEMs with even higher accuracy for selected areas, and to explore and demonstrate the new scientific applications of this unique mission. This paper provides an overview of the TanDEM-X and summarizes the current status and the quality of the available DEMs. Index Terms—Synthetic Aperture Radar (SAR), Spaceborne SAR, Interferometry, Digital Elevation Model (DEM). I. INTRODUCTION TanDEM-X is the first radar interferometer in space that employs two satellites operating in a closely controlled formation flight. TerraSAR-X (TSX), the first satellite of the formation, was launched in June 2007 and is providing high- resolution X-band radar images for scientific [1], [2], [2] and commercial [4] applications. TanDEM-X (TDX), an almost identical satellite to TerraSAR-X, was launched in June 2010 [5] and is equipped with an additional 36 kg cold gas tank to allow precise maneuvers for keeping the formation flight. In addition, TanDEM-X has a larger solid-state mass memory with 768 Gbit (twice the size of TerraSAR-X) to support the on-board data buffering and downlink of the large data amount during interferometric acquisitions. While both radar satellites are operated during the interferometric acquisition mode, either the TerraSAR-X or the TanDEM-X satellite is used for acquiring high-resolution 2-D radar images. The primary objective of the TanDEM-X mission is the generation of a global Digital Elevation Model (DEM) of the Earth’s surface with unprecedented accuracy as the basis for a wide range of commercial applications as well as for scientific research. It is expected that this data set will become a new reference in geosciences and remote sensing applications since its 3-D information content is ca. 30 times more accurate than the presently available global scale DEM data set. The first DEM data sets are available since January 2014 and the global DEM will be available by mid-2016. TanDEM-X has an ambitious time schedule to reach its main mission goal. After the commissioning phase, the first four years were dedicated to the global DEM acquisitions. To facilitate dual-baseline phase unwrapping all land masses were covered at least twice in the same looking direction but with different baselines. Difficult mountainous terrain required additional acquisitions viewing from the opposite direction to allow filling gaps due to shadow and layover. The main mission phase dedicated to DEM acquisition has been finished by mid-2014. By the end of 2014 final DEMs for approx. 35% of the landmasses will be available. The data quality assessment shows that all specifications for the DEM accuracy are being met. The baseline geometry in these first 4 years was optimized for DEM performance. A limited number of scientific Fig. 1 – Comparison of SRTM 1 arcsec DEM (left, acquired in 2000) and TanDEM-X DEM (right, 2011) of a coal mining area in north-western Germany clearly showing the improved quality and the changes due to ten years of mining activity.