ESA GNC Conference Papers Repository

The ESA CryoSat-2 was launched from Baikonur on April 8, 2010, with the primary purpose of measuring the thickness of floating sea ice with centimetre accuracy for a minimum of 3 years. More than 6 years later, CryoSat-2 is still operational and a further lifetime extension to 2018 has been approved. To facilitate this extension, the performance of the three aging Terma HE-5AS star trackers, which provide the mission essential orientation knowledge of the SAR/Interferometric Radar Altimeter (SIRAL) main payload, has been significantly improved by a series of changes implemented in the on-board software algorithms for initial attitude acquisition and tracking. This paper presents the in-orbit improvements of the Terma star trackers on-board CryoSat-2. The overall aim of the improvements has been to provide more robustness, especially during South Atlantic Anomaly (SAA) passes, and accuracy during warm periods. The emphasis of the paper will be on the following subjects. The algorithm improvements and related trade-offs: - The initial attitude acquisition algorithm was improved with an optional alternative method for image background estimation based on reference image subtraction. - The attitude tracking algorithm was modified in several of its steps. The major changes include the introduction of a pixel specific background to allow for Dark Signal Non-Uniformity correction, a clustering algorithm for finding spots in opened windows and a validation check of the identifies stars based on their individual expected distances. Implementation and on-ground testing: - The software changes needed to fit into the existing design, so a number of constraints existed on both processing/timing and memory usage. - On-ground testing on system level was achieved by feeding simulated image sequences into a test program embedding the attitude determination part of the on-board software. The results from the testing will be presented. In-orbit Validation: - An extensive test campaign was jointly defined by ESA, Airbus and Terma. The in-orbit results will be presented. - The ESA Lisa Pathfinder is planned to update its Terma star trackers in January 2017 with the improvements designed for CryoSat-2. The in-orbit results will, if possible, be presented.