Stellar reference unit for CASSINI mission

For the CASSINI mission, Officine Galileo are presently developing, under Jet Propulsion Laboratory (JPL) contract, a CCD based star tracker, named Stellar Reference Unit (SRU). The SRU will be used in a new attitude determination approach developed by JPL, in which 'gyroless' operation can be implemented. The unit has a wide angle (15 degree(s) full cone) Field of View (FOV) and a maximum sensitivity up to +6 Visual Magnitude for an integration time of 400 msec. Exposure time can be commanded via Attitude and Articulation Control Subsystem Flight Computer (AFC) from 0 to 65535 msec. Optics characteristics and centroiding algorithm for star position determination, are defined by Officine Galileo, to achieve required performance (total accuracy: 53 (mu) rad, 3(sigma) ). The defined centroiding algorithm will be then implemented on the AFC. In this paper, the SRU architecture, characteristics, and main technological areas are described. Expected performance is reported.