Ferran Gibert (isardSAT S.L., Spain); Gorka Moyano (isardSAT S.L., Spain); Marco Fornari (European Space Agency, Netherlands); Albert Garcia-Mondéjar (isardSAT S.L., Spain); Mònica Roca i Aparici (isardSAT S.L., Spain)

Radar altimeter processors require knowledge of platform and radar antenna attitude to provide accurate range and datation measurements. Although the attitude solution generated at real-time by the on-board computer may be accurate enough for ground applications, it is actually of interest to reprocess the star-trackers raw measurements on-ground with consolidated orbit data for validation purposes. Additionally, having more than one star-tracker on-board allows to derive independent Roll-Pitch-Yaw attitude estimates as observed from each star-tracker, what is of interest to identify and characterise eventual mechanical distortions between the star-trackers and the platform, such as thermo-elastic distortion due to the long-term variable thermal loads. In this contribution we present the star-tracker processor that has been implemented by isardSAT for the Sentinel-6 mission as part of the ESA Sentinel-6 Poseidon-4 (P4) L1 Ground Prototype Processor (GPP) to address the points raised before. We describe and discuss the overall performance of the star-tracker processor implemented. Then, we provide a comparison of the pitch measured by the star-trackers with an independent estimate provided by the P4 L1 GPP, allowing to determine the residual bias in pitch between the star-trackers measurements and the actual radar antenna pointing. Finally, we present a thermal analysis addressing how this residual bias evolves during the long-term thermal variation of the spacecraft.