Preserving Spacecraft Attitude Control Accuracy Using Theta-D Controller Subject to Reaction Wheel Failures

Abstract

Satellite mission life, especially for commercial Geostationary Earth Orbit (GEO) missions, has recently been aimed at extending its nominal 15 years life of services to an additional 5 to 8 years via the in-orbit refueling concept. This goal, driven by customers’ demand and competitive market, has challenged Attitude Control Subsystem (ACS) engineers to develop adequate ACS algorithms to accommodate this extended life of services in the presence of limited and aging hardware components, i.e., ACS sensors (e.g., star Trackers and IRUs) and actuators (i.e., Reaction Wheel Assembly) since thrusters alone will not provide adequate pointing accuracy due to its intended design for station keeping and orbit maintenance only. This paper investigates an attractive ACS scheme, which is designed using the nonlinear state dependent factorization modeling approach coupled with the  D  controller as an efficient on-line solver (which offers many advantages over the traditional Riccati solver), to maintain the satellite pointing accuracy subject to loss of one or two reaction wheels out of its four wheels assembly mounted in a pyramid configuration. The high fidelity reaction wheel model and 3 degrees of freedom attitude dynamics are used in the simulation to demonstrate the robust performance characteristics of the  D  based adaptive controller where the baseline PID controller fails to provide sufficient robustness.