ESA GNC Conference Papers Repository
AOCS solution to Euclid's highly stringent pointing and stability performance requirements
Euclid is an ESA medium class cosmology mission dedicated to the investigation of Dark Energy and Dark Matter The mission will operate in a large amplitude Lissajous orbit around the Sun-Earth Lagrangian point L2, and requires an unprecedented pointing and stability performance. The Euclid instrument is a 1.2m diameter wide field (0.5 deg²) telescope with a high resolution visible imaging channel, and a co-aligned near-IR channel for lower resolution broad-band photometry and spectroscopy. The highly demanding pointing stability requirements include a relative pointing error (RPE) of 75 milli-arcsec (99.7% confidence level) within a period of 700 seconds. A dedicated state-of-the-art attitude sensor, the so-called Fine Guidance Sensor (FGS)- to be mounted into the Focal Plane of the telescope -, is being developed specifically for this mission in order to achieve this RPE performance. It resembles a star tracker with a very narrow field of view necessary to achieve the desired measurement performance. In conjunction with the FGS, a Micro Propulsion System (MPS) with variable thrust level in the range of 1µN to 1mN, is used as actuator to generate the control torques during science. The reaction wheels (RWLs) are not used during the science observations in order to avoid micro-vibrations. However, they provide a pivotal role as they are used to perform small slews between observation fields in combination with high performance Gyroscopes. Two types of small slew repointing exist, which are the dither slews in the range of 100 arcsec and the field steps in the range of up to 1.2 deg. These slew manoeuvres have as distinctive feature that they operate the RWLs under very specific conditions; they are commanded from stand-still and return to stand-still after each slew. Apart from the technical needs associated with the RWL hardware operation and reliability aspects, it also incorporates significant challenges from a GNC design perspective, e.g. in coping with the control issues caused by internal wheel friction, in particular stiction, and wheel zero-crossings, which are to be minimised and best avoided whenever possible. A summary of the overall design of the Euclid AOCS is presented to provide an overview of the mission and the context in which the science is performed. The paper concentrates on the Science Mode (SCM) as the mode of highest performance and thus of major interest. A large emphasis has been put on designing a SCM submode architecture that can successfully perform the complex sequences that are required by the science observation operations (and the idiosyncrasies of the FGS) while meeting the stringent pointing requirements. The SCM submode architecture is presented where it is explained how the various submodes, each having their own dedicated controllers, are employed to perform the science observation sequence. The combined management of SCM submodes and FGS states are of special relevance for obtaining a high level of autonomy. Relevant technical issues, such as the compensation of the disturbances caused by the motion of internal mechanisms, are highlighted and relevant high-fidelity simulations results obtained during the design phase are shown.