ATTITUDE CONTROL SYSTEM OF THE X-RAY OBSERVATORY ASTRO-D

ASTRO-D is a Japan's fourth X-ray observation satellite to be launched in the begining of 1993. The satellite is equipped with high-throughput X-ray telescope which provides a large effective area over a wide energy range. The attitude determination and control system of the satellite consist of a pair of star trackers, inertial gyroscopes, attitude control electronics based on a microcomputer, and four momentum wheels in a skewed configuration, and magnetic torquers. In this paper, the following unique features of the attitude control system design to satisfy the mission requirements under the resource restrictions imposed by the launch vehicle capability are described: 1 A new initial momentum-transfer and attitude-acquisition control-law using skewed wheels. 2 Unique attitude maneuver-strategy and pointing control schemes for the bias momentum system with four skewed wheel configuration. 3 A large angle maneuver strategy for the biased multi-wheel system. 4 A new scheme of momentum management by magnetic torquing, which takes into account not only wheel unloading but also the safe-hold attitude in contingencies.