X-SAR/SRTM instrument phase error calibration

Before starting the operational processing of all the acquired X-SAR interferometric SAR data from the SRTM mission, the calibration of deterministic phase errors as far as possible is necessary. Besides the phase errors caused by the shuttle and mast dynamic variations and the position and attitude inaccuracy, there are phase errors originating from the radar instrument itself that may be compensated. Temperature variations and radar parameter changes like gain settings are the major contributors to these phase errors. During the mission we monitored and recorded telemetry data indicating periodic temperature variation along with the orbit, during a data-take and from mission day one to the end. Temperature variations in microwave devices and cables cause a phase variation, which contributes to the overall phase error and degrades the accuracy of the interferometric height measurement. The phase errors have been measured directly, like for the long mast cables, or derived from the calibration tone phase analysis or simply by correcting the known phase to temperature characterization values of the microwave parts with the measured mission temperature data. The lessons learned from that mission and the results of the analysis and the improvement of the height accuracy is presented in this paper.