Fault-tolerant spacecraft attitude control system

Spacecraft perform a variety of useful tasks in our day-to-day life. These are such that spacecraft need to function properly without interruptions for 7 to 15 years in space without any maintenance. Though most spacecraft have redundant systems to serve as back-ups in case of failures, they greatly depend on human assistance through ground stations for failure analysis, remedial actions and redundancy management, resulting in interruption in services rendered. There is, therefore, need for a fault-tolerant system that functions despite failures and takes remedial action, without human assistance/intervention, autonomously on board the spacecraft. Commonly used techniques for fault-tolerance in computers cannot be directly used for fault-tolerance in sensors and actuators of a closed loop control system. Further, for space applications fault-tolerance needs to be achieved without much penalty in weight and computational requirements. This paper describes briefly the attitude control system (ACS) of a spacecraft and highlights the essential features of a fault-tolerant control system. Schemes for fault tolerance in sensors and actuators are presented with an analysis on various failure modes and their effects. Newly developed fault-detection, identification and reconfiguration (FDIR) algorithms for various elements of ACS are described in detail. Also an optimum symmetrically skewed configuration for the attitude reference system using dynamically tuned gyros is developed. Some of the schemes have already been used in Indian Spacecraft. As future Indian space missions will directly cater to various applications on an operational basis, the ultimate objective is to have a totally fault-tolerant 'intelligent' autonomous spacecraft