Applying geodetic SAR with TerraSAR-X and TanDEM-X

Within the project “High-resolution Geodetic Earth Observation” funded by the Helmholtz Association DLR@Uni, our research group composed of German Aerospace Center (DLR) and Technical University of Munich (TUM) has considerably extended the concept of geometric spaceborne Synthetic Aperture Radar (SAR), and developed new applications for this approach now termed Geodetic SAR. Its core element is the use of range and azimuth as observation quantities, which are encoded in a SAR image through the fundamental SAR image formation process. Perturbations caused by atmospheric path delay (ionosphere, troposphere) and signals of the dynamic Earth (solid earth tides, ocean loading effects ...) are well known in the field of geodesy, and applying the geodetic correction principles to TerraSAR-X revealed its potential of cm-level radar observations [1]. During our research, we have examined the geometric processing chain of TerraSAR-X and developed prototypes for improving its elements, namely the precise orbit determination [2], the image generation by the TerraSAR-X Multimode SAR processor (TMSP) [3], and the geodetic corrections [4]. The tests using corner reflectors (CRs) with known reference coordinates demonstrated that TerraSAR-X/TanDEM-X range and azimuth data are accurate within 2-3 cm. Moreover, our rigorous solution of the range-Doppler equations enables absolute 3-D positioning of point targets by means of stereo SAR at a similar accuracy level, if the precise orbit and the corrected observations are combined [4]. Persistent Scatterers or CRs localized by TerraSAR-X may not only serve as reference points for the phase-based methods (Persistent Scatterer Interferometry, TomoSAR) [5], but can also be used to observe wide-area displacement signals affecting the Radar scene as whole. An example is the post glacial rebound (PGR), leading to uplift signals of up to 1.5 cm/y. For the city of Oulu, Finland, located in the region of the Scandinavian PGR, TerraSAR-X and TanDEM-X have repeatedly acquired high-resolution spotlight images from four different geometries, i.e. two ascending and two descending tracks. The acquisitions started in July 2014 and based on this data set, we will highlight different aspects of Geodetic SAR (image formation process, Orbit determination, the geodetic corrections, the 3-D positioning ...), and present first results of our attempt to detect the PGR with TerraSAR-X/TanDEM-X imagery.