STUDY OF AN INTEGRATED ATTITUDE SENSING SYSTEM AND DEVELOPMENT OF A PROTOTYPE

A gyro based attitude control system is described for geostationary satellites requiring high accuracy (0.05 deg. in pitch/roll, 0.1 deg. in yaw). The system is based on high quality strapdown rate integrating gyro updated by earth and sun sensors via an estimation procedure implemented in an on-board digital computer. Two system mechanizations are studied: – A single yaw gyro with digital sun sensor updates for yaw reference and two axes infra red earth sensor for pitch and roll, – two gyros in gyro-compass mode with sun sensor and roll earth sensor updates for roll/yaw and earth sensor for pitch. Control system is based on reaction wheels (normal pointing) and hydrazine thrusters (station keeping and momentum undoad). The calibration filter is a discrete KALMAN filter that estimates gyro error sources. An efficient approach permits separation of low frequency and high frequency sources. In the calibration filter only low frequency error sources are estimated : gyro integral error and gyro drift. For the purpose of real 3-axis dynamic performance test of the two concepts, a representative prototype of envisaged future systems of attitude sensing and filtering has been developped. During this system test, control loops, spacecraft dynamics and kinematics are done by a computer, driving an air bearing platform (servoed table) on which all detectors are mounted. The key design features of the prototype (optical sensors and gyropackage) are detailed, with emphasis on gyro configuration selection, gyro rebalance loop, ground effects reduction concepts, sensors simulation. System test implementation and procedures are described, to provide necessary background to results announced during conference. As a conclusion, a brief discussion about interest and future developments of modular attitude estimator systems is made.