TerraSAR-X Calibration - First Results

Abstract

As TerraSAR-X, due for launch in February 2007, will be an operational scientific mission with commercial potential, product quality is of paramount importance. The success or failure of the mission is essentially dependent on the calibration of the TerraSAR-X system ensuring the product quality and the correct in-orbit operation of the entire SAR system. The essential task of calibrating TerraSAR-X is to estimate and correct systematic error contributions throughout the complete SAR system and to tie-down image information (magnitude and phase) to reference units in geophysical terms. The quality of this calibration process is dependent on the inherent stability of the radar system and the capability to determine and monitor the radiometric and geometric characteristics. In addition to the geometric calibration the main calibration activities are defined by the radiometric accuracy and the radiometric stability specified for the operational modes. Due to the high degree of flexibility of the instrument, e.g. the large number of operation modes (StripMap, Spotlight, ScanSAR, left/right looking, etc.) and possible antenna beams, the in-orbit calibration procedure of TerraSAR-X is based on a novel antenna model approach. In addition to the complexity of TerraSAR-X and the tight performance requirements, the main goal after launch of the satellite is to provide calibrated and verified SAR data products as soon as possible. Thus, a strategy for an efficient but robust in-orbit calibration procedure has been developed. For this purpose, the novel antenna model developed and validated on ground, is verified in orbit by measuring only a number of selected beams using deployed ground receivers and natural targets (e.g. the rain forest). Absolute calibration, yielding an absolute calibration factor by measuring the radar system against reference ground targets, requires precise and sophisticated ground equipment. To account for the restricted time of calibration campaigns the number of passes and places of test sites is optimised versus cost and time effort by calibrating several beams and polarisation modes with the same test site. Therefore, corresponding test site configurations have been developed. The evaluation of measurements performed during the calibration campaigns yields the different calibration parameters like the antenna pattern or the absolute calibration factor. The paper describes the in-orbit calibration procedure and the different activities performed during the commissioning phase of TerraSAR-X and discuss first results.