CryoSat: Siral calibration and performance

CryoSat's Synthetic Interferometric Radar Altimeter (SIRAL) [1] is a Ku-band pulsewidth limited radar altimeter that transmits pulses at a high pulse repetition frequency thus making the received echoes phase coherent and suitable for De-lay/Doppler processing [2]. Moreover SIRAL takes advantage of two antennas mounted across-track for interferometric capability, in order to determine the across-track direction from which the echo is received [3]. The calibration strategy for SIRAL includes both internal calibrations and external calibration [1]. Due to the fact that SIRAL is an interferometric phase coherent pulse-width limited radar altimeter, a proper calibration approach has been developed. In this paper we will describe as first the internal calibration strategy and then the different calibration corrections that are applied to science data. The internal calibration results over more than three years of mission will be presented, analysing their temporal evolution in order to highlight the stability of the instrument over its life. Finally, the external calibration measurements for SIRAL will be presented.