Radar calibration techniques including propagation effects

Abstract

A general description of meaning and goals of calibration procedures concerning SAR measurements is followed by an overview on calibration concepts of some realized and planned spaceborne SAR missions. By studying these concepts lacks have been found with respect to an adequate treatment of the propagation medium and the in-flight antenna pattern measurement. Attempts to measure the in-flight antenna pattern in the L-band have been made by DFVLR and the University of Stuttgart in connection with the first SIR-B mission. Within the framework of SIR-B reflight and X-SAR/SIR-C mission, these institutions intend to perform investigations in the mentioned problem areas by calibrated on-ground receiver arrays covering the X-, C- and L-band mission frequencies placed adequately within the selected test site. For each mission frequency, the array comprises one high quality receiver and several low-cost receivers. Furthermore, there is a need of on-board sensor stability control, transmit power measurement and on-ground passive or active reference targets with known RCSs. Such an experiment allows to measure independently the power density by means of test site receivers and the backscattered power of the reference targets received by the Shuttle sensors. Both measured quantities combined result in the separation of the actual antenna gains respective patterns and the transmission medium losses and admit comparison of these losses between X-, C- and L-band frequencies. Under ground located low-cost receivers are useful to measure the penetration depth especially in snow and ice covered regions. Additionally, polarisation investigations are informative with respect to propagation medium and earth surface properties.