Fine Gyroless Attitude Control: The Sax Experience

Abstract

When a serious AOCS failure occurs in orbit the task of the designer is made, more complex. To achieve the minimum performance drop, managing a control system gravely degraded in its capabilities, can seem an impossible challenge. The experience of SAX Extended Science Mode (ESM) is the demonstration that exploiting all the features of the still working equipment and applying clever strategies and algorithms, it is possible to discover hidden failure tolerance degrees and permit the successful continuation of a mission apparently compromised. ESM project has been developed in teo steps. The former SW release (ESM1) was up-linked in August '97 to recover as fast as possible scientific functionality of SAX. it relies on a single gyro and is still working as no more gyro collapses occurred after May '97. When only one star tracker is available, the AOCS filter combines the co-ordinates of a star in the FOV with the unique available incremental angle data guaranteeing an accuracy compliant with the requirement. The latter version has been thought to be completely gyroless. It will permit extreme exploiting of satellite equipment up to usage, during scientific observations, of a sensor set nominally dedicated to safe mode (QSS and magnetometer). Those are the so-called bridinng phases that require a target recovery: a precise attitude reacquisition and the change of guide stars up to completion of the manoeuvre.