SATELLITE ACTIVE MAGNETIC ATTITUDE CONTROL SYSTEM IN SAFE HOLD MODE

摘要

Explored here is the active magnetic attitude control system of a satellite on a polar orbit in the safe hold mode(SHM). Usually, the satellite stays in Normal Mode, An alarm or a ground request could involve a mode exit towards the Safe Hold Mode. In this mode, in order to simplify the hardware of the attitude control system(ACS), thrusters and sophisticated equipment (for instance gyros) are not used. Coarse sun sensors(CSS) and three-axis magnetometers(TAM) provide attitude measurement, and magnetic torquers generate torque. This mode consists in 2 main phases: rate damping phase (RDP), sun pointing phase (SPP). The aim of this mode consists in reaching autonomously a safe attitude with the -Ys satellite axis pointed to the Sun and with the mean three-axis angular rate equal to -0.2 deg/s. When the angular rates are not measured directly, they can be estimated from knowledge of the quaternion vector or the direction cosine matrix, which are obtained from attitude position measurements. Based on a recently introduced method for angular rate estimation of a geostationary satellite from sun direction observations, a new method is presented for estimation of the angular rate in this paper. The estimator consists of an Unscented Kalman Filter(UKF). First, a deterministic algorithm generates a coarse estimate of the satellite angular rate from the sun vector measurements and the readings of a three-axis magnetometer. This rate then used to initialize an Unscented Kalman Filter, thus rendering the resulting estimate highly accurate and robust. Then the controllers of rate damping phase and sun pointing phase are designed. The Regulators are Minus-Dot-B and PID control respectively. Stability and control analysis are presented. Finally, a close-loop simulation is done. Detailed simulation results are presented to demonstrate the performance of the algorithm. This paper has shown that the designed system can meet the technology demand of the safe hold mode.