SRTM X-SAR motion compensation: concept and first assessment of the interferometric observation geometry

The space shuttle Endeavour that flew from 11 until 22 February carried the radar systems for the Shuttle Radar Topography Mission (SRTM). In the course of this project the first space born single pass interferometer has mapped the Earth's topography. Two different radar systems operated on board of the space shuttle: the C-band radar of the American NASA/JPL and the X-band radar of the German DLR and the Italian ASI. It is the objective of this mission to generate a digital elevation model (DEM) of the Earth with an accuracy that has never been reached before on this global scale. The inevitable technically caused oscillation of the secondary antenna turns this task into a scientific and technological challenge. The Synthetic Aperture Radar (SAR) observation is influenced by a resulting sensor motion in the line of sight. The consequence of this effect on the secondary SAR scene and subsequently on the DEM is described in this paper. To obtain the requested height accuracy in spite of the antenna oscillation, the principle of motion compensation is applied. The concept of this key algorithm in the X-SAR SRTM project is explained in detail. The essential parameters of the interferometric observation geometry were recorded by the Attitude and Orbit Determination Avionics (AODA) system of the JPL. These AODA data are employed to investigate the movement of the secondary antenna.