ESA GNC Conference Papers Repository
AOCS design for the Athena X-ray telescope: challenges and solutions
The ATHENA - Advanced Telescope for High-ENergy Astrophysics - mission is currently assessed in a phase A feasibility study as L-class mission in ESA's Cosmic Vision 2015-2025 plan, with a launch foreseen in 2028. Primary goal of the mission is the mapping of hot gas structures and the determination of their physical properties to search for supermassive black holes. ATHENA is a X-ray telescope with a focal length of 12 m. It has a mass of ~5500 kg and it is about 15 m high with a diameter of 3 m. The main mass is distributed to the service module with the mirror on the one side of the spacecraft and to the focal plane module with instruments on the other side of the spacecraft. In order to achieve its science goals ATHENA performs a sky survey with precision LoS pointing requirements, i.e. AKE < 2 arcsec and APE < 10 arcsec at 95% confidence level, that are very demanding for large X-ray telescopes. In addition to the precision pointing requirements ATHENA cannot violate a certain sun exclusion zone to prevent any stray light falling onto the instruments, as it would immediately destroy them. The sky survey is defined by an observation plan that is demanding in terms of availability and thus spacecraft agility. The pointing and agility requirements and the fact that ATHENA is a spacecraft with high mass and volume introduce several AOCS design challenges. The aim of this paper is to present those challenges, corresponding solutions and preliminary results, which have been achieved during the phase A study led by Airbus Defence & Space in Friedrichshafen, Germany. In particular the following AOCS design topics will be addressed: autonomous and agile large angle slew manoeuvres with exclusion zones, time-varying spacecraft inertia due to the moving instrument mass (1500 kg), achievable performance with hexapod and relative state metrology in the control loop, and precision LoS determination and analysis during the design process using the ESA Pointing Error Engineering Tool (PEET). In addition, the analysis approach w.r.t. a mock observation plan defined by ESA will be presented with the achievable availability. The control design is based on a precision control design tool that has been further developed for ATHENA needs. The tool has originally been developed in an ESA networking partnering initiative (NPI) together with ESA, the University of Stuttgart and Airbus DS in Friedrichshafen to address these types of missions.