Hybrid robust and optimal control for pointing a staring-mode spacecraft

This paper addresses the problem of optimally repointing the optical axis of a spacecraft to align with a target. A new metric defining the repointing error is proposed where the corresponding kinematic equations provide a simple and convenient form for control design. A control is proposed that integrates a Control Lyapunov Function (CLF) approach with a sliding mode controller which guarantees both optimality and robustness of the closed-loop system. Moreover, a CLF based control scheme ensures that the state optimally converges to a sliding surface, while a fixed-time non-singular terminal sliding mode controller provides robust convergence to the origin while on this surface. The region of attraction and convergence time is analyzed. Finally, numerical investigations are conducted to verify the effectiveness and superiority of the proposed algorithm with respect to the classical CLF method and an integral sliding mode based CLF approach.