DEM Calibration Concept for TanDEM-X

Abstract

The TanDEM-X mission comprises two fully active synthetic aperture radar satellites operating in X-band. The primary goal of this mission is the derivation of a highprecision global DEM according to HRTI level 3 quality. The required baselines are in the order of 300m-500m, which require a close formation. A formation, which enables such baselines, is the HELIX formation with adequate e/i-vector separations, which simultaneously includes collision avoidance of the satellites. This proposed mission has passed a Phase A study successfully in 2005. The generation of DEMs according to HRTI-3 standard requires accurate calibration of the interferometric system parameters. The interferometric height is determined by the phase difference between the two images and the spatial geometry. For the spatial geometry, the three dimensional baseline vector, connecting the phase centres of the two SAR antennae, is the crucial factor. The baseline is derived from the orbit and attitude products, pre-flight alignment measurements of the spacecraft, and characterisation of the SAR antenna phase centre position. Its determination accuracy is limited due to e.g. GPS measurements, attitude errors, applied co-ordinate transformations, change of the centre of mass of the satellites due to their respective fuel consumptions, and gain and phase variations in the T/R modules. The error of the interferometric phase is determined by the system s signal-to-noise ratio, inaccuracy in the phase synchronisation link, and limitations in the internal calibration. Starting from a detailed analysis of the spatial and temporal behaviour of the different error sources, a calibration concept for the operational processing chain will be developed. Relative corrections can be derived from bundle block adjustment (crossing orbits) and/or swath overlap. Absolute height calibration requires e.g. tie points, GPS ground tracks, reference DEMs, and/or altimeter measurements.