1 Two Years of Tandem - X Baseline Determination

The TanDEM-X mission is a German dual satellite formation with the task to acquire a global digital elevation model (DEM) by bistatic interferometric synthetic aperture radar (SAR) data takes. Therefore the two satellites are kept in a close helixformation with a distance of less than 1 km. In order to reach the intended DEM accuracy, the baseline vector between the two spacecraft needs to be determined with an accuracy of 1 mm. To achieve this goal, both satellites are equipped with high grade dual-frequency GPS receivers. The baseline vector between the two satellites is determined by GFZ and the German Space Operations Center (GSOC/DLR) using independent software packages. The GSOC/DLR baseline solution is processed with the FRNS software (Filter for Relative Navigation of Spacecraft). The underlying concept is to achieve a higher accuracy for the relative orbit between two spacecraft by making use of differenced GPS observations, than by simply differencing two independent precise orbit determination (POD) results. The use of single-differenced code and carrier phase observations rigorously eliminates GPS clock offset uncertainties and largely reduces the impact of GPS satellite orbit and phase pattern errors. Double differences are used for the integer ambiguity resolution of the carrier phase observations. In studies prior to the TanDEM-X mission, comparisons between independent software packages showed biases of a few millimeters. In order to ensure the highest accuracy, a baseline calibration and combination process has been installed. The baseline products are validated by dedicated baseline calibration data takes over test sites, where the DEM is well known. Using those DEMs as a reference, height differences in the TanDEM-X scenes are estimated. Taking into account the incident angle and the height of ambiguity, these height differences can then be used to infer errors of the baseline products.. The resultant offset parameters are then applied in the baseline calibration process. The analyses show that the derived offset parameters are in the range of few millimeters. Finally the different solutions are merged to a combined product.