TRANSPONDER CALIBRATION OF CRYOSAT-2 DATATION, RANGE AND INTERFEROMETRIC PHASE ERRORS

摘要

The CryoSat 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 inteferometric mode (SARin). This radar is named SIRAL: Synthetic aperture interferometer radar altimeter [1]. Transponders (TRP) are commonly used to calibrate absolute range from conventional altimeter waveforms because of it 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 project (a refurbished ESA transponder developed for the ERS-1 altimeter calibration). It is deployed at the KSAT Svalbard station: SvalSat. We are using the ESA CryoSat transponder to calibrate SIRAL’s range, datation, and interferometric baseline (angle of arrival) to meet the missions requirements [2]. In these calibrations, we are using 3 different types of data: the raw Full Bit Rate data (FBR), the stack beams before they are multi-looked (stack data) in the Level 1b processor, and the Level 1b data itself [3]. 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 the instrument is introducing when performing its measurement [4]. When this error can be assumed to be constant regardless the conditions, it will provide the bias of the instrument. And if the measurements can be repeated after a certain period of time, it can also provide an indication of the instrument drift. This paper presents the analysis and results of this calibration. The work presented here has been carried out under an ESTEC/ESA contract, to calibrate CryoSat-2 during the Commissioning phase. It was later extended with an ESRIN/ESA contract, for continue monitoring and including further analysis.