Robust adaptive fixed-time 6 DOF tracking control for spacecraft non-cooperative rendezvous

In this paper, the problem of adaptive fixed-time 6 degree-of-freedom (DOF) tracking control for spacecraft noncooperative rendezvous in presence of the parameter uncertainties and external disturbances is investigated. Firstly, a new multilayer fixed-time sliding mode surface (MFSMS) is designed, and the setting time of the proposed surface can be estimated without the knowledge of the initial conditions. Subsequently, an adaptive fixed-time nonsingular terminal sliding mode (AFNTSM) control law is proposed, which is continuous and can eliminate the chattering phenomenon. Moreover, the proposed AFNTSM control law does not require the precise information of the mass and inertia matrix of the chaser spacecraft and the disturbances. Further, it is proved that the fixed-time reachability of the relative position and attitude into a small region containing the origin can be guaranteed by the proposed control law. Finally, numerical simulations are performed to demonstrate the effectiveness of the proposed control scheme.