ESA GNC Conference Papers Repository
The Sentinel-2 Satellite Attitude Control System - Challenges and Solutions
Sentinel-2 is a European polar satellite system built on a constellation of two similar satellites for the provision of operational land services based on an optical Earth observation payload. Sentinel-2 is being built by Airbus Defence and Space in the framework of the European Space Agency's Global Monitoring for Environment and Security program, now termed Copernicus, and is currently in an advanced phase D status. Sentinel-2 operates in a sun-synchronous orbit with 786 km mean altitude and Earth-oriented attitude in all operational modes. While the required pointing performance is moderate, the crucial design driver for the attitude control system is the attitude knowledge required for precise geo-location of the images taken by the Multi Spectral Instrument payload. The first two satellites are currently undergoing completion at Airbus Defence and Space in Friedrichshafen, Germany, with Sentinel-2A scheduled for launch in Q2 2015 and Sentinel-2B roughly one year later. After on overview of the Sentinel-2 mission and satellite design, the attitude control system architecture is presented. The preeminent features of the attitude control system are the Earth-pointing safe mode and the high accuracy on-board attitude and position estimation. High-accuracy attitude estimation is provided by a constant-gain Kalman filter using the measurements of the multi-head high performance star tracker and a fibre-optical four axes gyro. Specific design solutions are employed to minimise thermal distortion. A dual-frequency GPS receiver provides the orbit state vector. The Earth-pointing safe mode ensures easy transition back to normal mode and stable thermal conditions for platform and payload. The main sensor is the Airbus Defence and Space developed Coarse Earth and Sun Sensor, actuation is performed by magnetic torquers and thrusters. Floquet theory has proven to be a valuable tool for the tuning of the controller with proper consideration of the periodic nature of the magnetic actuation. Elastomer wheel isolators and solar array drive micro stepping are employed to avoid image distortion caused by micro-vibrations. A detailed solar array drive model, whose parameters have been identified from test data, is used for performance analysis. For the implementation of the attitude control algorithms software, a model-based approach has been successfully employed, which allows continuous simulation and testing from the start of the development until pre-validation of the algorithms before specification for coding. Airbus Defence and Space has recently developed a complete software development process based on automatic code generation directly from the algorithms model as natural extension of the model-based development applied for Sentinel-2.