TanDEM-X Bistatic SAR Processing

In June, 2010 the German SAR satellite TanDEM-X (TerraSAR-X-Add-on for Digital Elevation Measurements) will be launched. Together with TerraSAR-X, launched June 15, 2007, it will form the first spaceborne bistatic SAR platform. Usually one of the satellite is transmitting (active satellite), while both are receiving. As both satellites fly in a helix orbit constellation, during a recording a satellite has to be passive, if the other one is close to the line of sight to the observation target at the moment of observation in order not to destroy the instruments of the other satellite, in other words they change their roles along the orbit. Even tiny differences in the instruments of both satellites can yield huge effects on final products. Within this paper we present the approaches of correcting thus differences. In the first part we explain, what variances in the satellite hardware are expected and how they are determined. In the second part we show, how the SAR processing copes with those differences. 1 Hardware Differences Differences between TerraSAR-X and TanDEM-X hardware, even though intending to build an identical second satellite, might come just from slight variation in hardware production. Athough very small they might have strong impacts on the final bistatic SAR product. Thus they have to be examined in detail and corrected as far as possible. Without having access to an absolute time reference, we use the active satellite as reference and evaluate the timing of the passive satellite relative to it. In so doing, all corrections which relate to timing are applied on the imaging channel of the passive satellite. Variations in the antenna hardware yield to different antenna patterns in both signal gain and phaseshift. For the interferogram, a fully corrected image phase is needed. Thus according to the well known antenna pattern of a certain imaging mode and look angle, the phase pattern is projected to the ground range area of the scene according to timing, orbit, attitude and coarse (input) DEM information. This projected phase antenna patterns are sampled on a ground grid for further usage within the processing chain. The projection is already included in the newest version of the TerraSAR multimode SAR processor (TMSP) [3]. Figure 1: Phase antenna pattern of a TerraSAR-X scene Figure 1 shows an exemplary phase antenna pattern for HH polarization1 of a Quadpol TerraSAR-X scene in stripmap beam “stripFar_008” recorded on April 24, 2009 in southern Bavaria, Germany in ascending orbit direction. One can easily see the effects of the mountains (Alps) at the early azimuth (front part) of the plot. Besides the antenna hardware another electronic part has impact on the image phase. The ultra stable oscillators used for all signal generation and timing of the satellites might slightly differ in frequency due to unavoidable manufacturing tolerances. These differences might even vary or drift within a datatake. Thus several effects might occur: 1this polarization is recorded on the antenna half at the backside of the satellite according to flight direction EUSAR 2010