This paper proposes an adaptive fault tolerant atti- tude controller, based on variable structure control, to address tracking control problem of spacecraft in the presence of un- known actuator failure, control input saturation and external dis- turbances. In contrast to traditional robust fault tolerant attitude controllers, the proposed controller is capable of controlling the transient response of the closed loop system by means of a dedi- cated parameter. Also this method does not require exact knowl- edge of the actuator faults and is implemented with uncertain value of fault information. By adjusting a simple parameter dynamically and using Lyapunov direct method and the properties of quater- nion representation, the ultimate boundedness of attitude and an- gular velocity errors in presence of external disturbances is proven. Numerical simulations used to prove the success of proposed con- troller in achieving high attitude performance in the presence of external disturbances, inexact knowledge of fault information, ac- tuator failures and control input saturation.

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