ESA GNC Conference Papers Repository
CryoSat-2 Terma Star Tracker In-orbit Experiences
The ESA icy mission CryoSat-2 was launched from Baikonur on April 8, 2010 into a non-sun-synchronous 717 km polar orbit, with the primary purpose of measuring the thickness of floating sea ice with centimetre accuracy for a minimum of 3 years. Three Terma HE-5AS star trackers provide the orientation knowledge of the SAR/Interferometric Radar Altimeter (SIRAL) main payload with arc-second precision essential for mission success. This paper presents the status of the Terma star trackers on-board CryoSat-2 following the very successful Commissioning Results Review on October 22, 2010. The emphasis of the paper will be on the following subjects: The star tracker switch-on and tracking during LEOP. The attitude tracking stability over the first 12 months of operation and the recovery from two software anomalies comprising attitude outliers observed during LEOP in approx. 1 out of 30,000 attitude samples, and timestamp irregularities first observed after 27 days of continuous operation. The baffle performance comprising verification of Sun (30 deg) and Moon (25 deg) exclusion angles. Three in-flight calibrations have been performed with the primary purpose of optimizing key operational parameters describing the optical model. The calibrations were separated in time by 5 months, allowing assessment of both short- time and long-time thermo-mechanical stability of the focal plane. Full resolution (1 Mpixel, 22 deg FOV) raw image dumps have been carried out, providing an excellent input for various CCD health assessments. Having three star trackers on-board CryoSat-2 makes an ideal situation for assessing the attitude accuracy by comparing the output from two simultaneously operating sensors. This paper will present the attitude accuracy from this approach taking into consideration the influence from thermal variations of the star tracker sensors.