TanDEM-X: A Global Mapping Mission

Abstract

TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is an innovative spaceborne earth observation mission based on two synthetic aperture radar satellites operating in close formation as a single-pass interferometer and allowing flexible baseline selection. The mission is currently prepared and implemented in the frame of a public-private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH. The launch of TanDEM-X is planned for September 2009. Focus of this paper is the mission’s primary goal, the derivation of a global, consistent Digital Elevation Model (DEM) in accordance with the HRTI level 3 specification. First of all, the mission scenario and data acquisition strategy is presented which shall ensure global mapping within the limited mission time. The mission scenario is based on a large number of bi-static radar image acquisitions in stripmap mode with all land surfaces being covered at least twice with different satellite formation parameters (baselines) to facilitate interferogram generation (phase unwrapping). To meet the stringent requirements on DEM vertical accuracy a comprehensive calibration concept has been developed. As the interferometric height of the terrain is derived from the phase difference between two bi-static images, all phase errors and drifts of the SAR instruments will directly lead to degraded height information. In addition to internal and external calibration of the radar instruments inter-satellite synchronization is required to reduce this type of errors. Apart from instrument phase, the precision of height measurements also depends on signal noise and the precision of baseline determination, i.e. relative orbit as well as attitude of the two satellites. Some of these inaccuracies can be reduced in the process of assembling the single raw DEMs (mosaicking) by exploitation of available height reference data and by combining multiple image acquisitions and image overlaps.