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The CryoSat-2 mission is designed to determine fluctuations in the mass of the Earth’s land and the marine ice fields. Its primary payload is a radar altimeter that operates in different modes optimised depending on the kind of surface: Low resolution mode (LRM), SAR mode (SAR) and SAR interferometric mode (SARIn). This instrument is named SIRAL: SAR Interferometric Radar Altimeter. Transponders are commonly used to calibrate absolute range from conventional altimeter waveforms because of its characteristic point target radar reflection. The waveforms corresponding to the transponder distinguish themselves from the other waveforms resulting from natural targets, in power and shape. ESA has deployed a transponder available for the CryoSat-2 project (a refurbished ESA transponder developed for the ERS-1 altimeter calibration). It is deployed at the KSAT Svalbard station: SvalSAT. Another transponder was deployed in Greece Technical University of Crete for the Sentinel-3 calibration, and later moved to West Crete in a permanent position. A new transponder is located in Gavdos, in another island close to Crete, and was deployed during the Commissioning phase of Sentinel-6. We are using these transponders to calibrate SIRAL’s range, datation, and interferometric baseline (or angle of arrival) to meet the mission requirements. In these calibrations, we are using three different types of data: raw Full Bit Rate data, stack beams before they are multi-looked (stack data) in the Level 1B processor, and the Level 1B data itself. Ideally the comparison between (a) the theoretical value provided by the well-known target, and (b) the measurement by the instrument to be calibrated provides us with the error that the instrument is introducing when performing its measurement. When this error can be assumed to be constant regardless the conditions, it will provide the bias of the instrument. And if measurements can be repeated after a certain period of time, an indication of the instrument drift can also be provided. |
