CRYOSAT-2 - SIRAL FBR, STACK & L1B Calibration with Transponder
Garcia-Mondéjar, Albert1; Fornari, Marco2; Reche, Mercedes3; García-Arnaud, Pablo1; Roca, Mònica4
1isardSAT, SPAIN; 2ESA, ITALY; 3Pildo, SPAIN; 4isardSAT, POLAND

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 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 phase (or angle of arrival) to meet the missions requirements [2]. 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 [3]. 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.

In these calibrations, we are using 3 different types of data: the raw Full Bit Rate data, the stack of Doppler beams before they are multi-looked (stack data) in the Level 1b processor, and the Level 1b data itself [4]. In addition, data from the CryoSat thermistors have also been analysed, with the purpose of correlating the thermal evolution of the platform with the evolution of the interferometric phase error.

Due to equipment degradation over time, the transponder can have internal delay uncertainties from its initial specifications so it needs to be calibrated. The way of doing this calibration is by retrieving the range with RA-2 EnviSat data, as it has a well-known absolute range bias, so the source of the error obtained will come from the delay in the transponder.

This paper presents the analysis and results of all these calibrations. 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.

[1] C.R. Francis, ''CryoSat Mission and Data Description'', CS-RP-ESA-SY-0059.
[2] CryoSat Science and Mission Requirements Document, CS-RS-UCL-SY-001.
[3] SIRAL2 Calibration using TRP: Detail Processing Model - DPM; ISARD_ESA_CR2_TRP_CAL_DPM.
[4] D.J.Wingham, et al.: ''CryoSat: A mission to determine the fluctuations in Earth’s land and marine ice fields'', Advances in Space Research 37 (2006) 841-871.