High accuracy spacecraft attitude measurement: the Euclid Fine Guidance Sensor

Achievement of the scientific objectives of ESA's Euclid mission requires a spacecraft (S/C) capable of guaranteeing very stable pointing once the target attitude has been reached. Pointing stability at the level of a few tens of milliarcseconds is guaranteed by the implementation of a high accuracy attitude sensor (Fine Guidance Sensor) in the focal plane of the main telescope. The sensor is based on four full frame CCD devices in the proximity of the detectors used for science purposes. The use of the telescope as optics lead to a small detector field of view allowing a very precise measurement of star position and is challenging in terms of star coverage over the entire sky. Each 2 second FGS is able to provide a measurement of attitude variation (relative attitude) and, thanks to a star catalogue developed on purpose, also an accurate measurement of absolute attitude. These measures are combined by Attitude and Orbit Control Subsystem (AOCS) SW with the ones from Star Tracker and Gyroscopes to improve the spacecraft attitude estimation and reach the requested performance.