ESA GNC Conference Papers Repository

This paper describes the architecture, design and expected performances of the AOCS for the SEOSAT mission. This is a 2.5 m resolution optical Earth observation satellite which is being developed by Astrium CASA Espacio for the Spanish Government under the technical supervision of ESA. The satellite avionics (including the AOCS) is based on the AS250 platform of EADS Astrium. The system introduces the gyroless architecture relying on a synchronous multi-head star tracker for attitude determination. Such architecture conveys challenges on the normal mode (such as angular rate estimation and attitude propagation in case of temporary star tracker outage) which are successfully addressed by a dynamic-stellar estimation. This allows the AOCS to satisfy all the mission pointing requirements, including a level 1b geolocation of 50 m which is significantly improved by the system design. The architecture also features a redundant GPS unit that allows autonomous navigation, along with the reaction wheels option of the AS250 platform. The safe mode is gyroless and it is based on robust equipment and an Astrium patented version of the B-dot law. After a successful review of the AOCS design at system PDR, tuning and detailed simulations have been conducted with satisfactory results.