In-flight Monitoring of TerraSAR-X Radar Instrument Stability

Abstract

For calibrating and monitoring the radiometric stability, the radar instrument of TerraSAR-X hosts a passive calibration network coupling into an additional port of the T/R modules. Three different types of calibration pulses are applied, whereby sets of these pulses are needed at the start and end of each data acquisition. Additionally, TerraSAR-X has been equipped with an additional calibration mode based on the so-called "PN Gating method". The PN Gating mode allows operating the T/R modules under most realistic conditions with the advantage that all modules can be characterised simultaneously. The individual characteristics are found by assigning special calibration pulse sequences with orthogonal codes to the T/R modules. During on-ground characterisation of the TerraSAR-X instrument this technique has established as a crucial diagnostic tool for functional checks as well as T/R module drift and failure monitoring. For the first time ever, this innovative method of PN Gating measurements will be applied for in-flight monitoring of a satellite SAR instrument. Starting with the TerraSAR-X commissioning phase the instrument behaviour can be accurately characterised to guarantee the radiometric stability of the system throughout the mission lifetime. In this paper, the latest results from the in-orbit calibration of TerraSAR-X are shown and compared to on-ground measurements of pre-launch characterisations. The techniques described above are also applicable for characterisation and calibration of other advanced SAR systems coping with multiple mode radar instruments.