Measuring the Pitch of CryoSat-2 Using the SAR Mode of the SIRAL Altimeter

Abstract

This letter describes the retrieval of the pitch of the CryoSat-2 satellite from the full bit rate (FBR) SAR mode data that are collected over the ocean by the SAR/Interferometric Radar Altimeter (SIRAL). Starting with the FBR SAR mode data, we form beams at the forward and backward look angles with respect to the nadir direction. If the satellite is flying pitched, the asymmetrical weighting of the echo power in the forward-looking and backward-looking beams that resulted from the along-track antenna gain pattern is used to measure the pitch of the satellite. The method depends on accurate knowledge of the orbit and the altimeter echoes only. In consequence, we are able to compare the results with the contemporaneous pitch value that is measured by the onboard star trackers, and particularly, we are able to determine a bias in the star tracker pitch measurements. We find that the star trackers' pitch measurements are biased, with a constant offset of 0.055 ° ± 0.0073 °. Because the star tracker attitude is used to inform the onboard control system, this bias implies that CryoSat-2 is flying nose up, with an average pitch of about 0.055 °.