ESA GNC Conference Papers Repository
Using a Star-tracker in degraded modes: outputs of the STEAM R&D study
Star trackers have become ubiquitous on space missions. Their versatility is unparalleled, while their track record belies their apparent internal complexity. One counter-intuitive benefit arising from said complexity is that they have no trivial failure mode in which an invalid attitude measurement can be wrongly flagged as valid. This allows to trust them enough to include STR measurements in the mix of AOCS observables in safe mode. The approach inspiring the STEAM study was to investigate the possibility of using star trackers as the backbone for a generic safe mode architecture (which can be instantiated with or without the STR). We will be presenting the main outcomes of this study: - Verification of robustness of real STR HW to high angular rate and high radiation levels with optical stimulation test benches - Architecture definition of a generic safe mode, with a versatile attitude determination module that can incorporate STR measurements seamlessly in the overall attitude determination mix, while still capable of sun-pointing convergence in the absence of STR or gyro data. - Detailed performance simulations for the safe mode, when instantiated for 2 very different and very challenging science mission safe modes: an Ariel-like and an Envision-like use case. - Real-time closed-loop tests with the STR in the loop confirming the good behaviour. A main conclusion of the study was that all of the perceived obstacles to the use of STR measurements in safe mode have been successfully addressed and overcome, and that future projects could greatly benefit from the added flexibility and functionality.