6-DOF integrated adaptive backstepping control for spacecraft proximity operations

Relative motion control with 6 degrees of freedom (6 DOF) is investigated for a chaser spacecraft with parametric uncertainties to approach an unknown tumbling space target. Unlike the conventional relative motion model described in the target orbital frame, the relative motion model formulated in the chaser's body-fixed frame can simplify modeling and control design for spacecraft proximity operations, while the chaser's thrust misalignment and the natural couplings between relative translation and relative rotation are considered in the 6-DOF integrated dynamics. After the coupled relative motion dynamics are modeled, a 6-DOF integrated state feedback controller is designed by combining the classical backstepping technique with a simple norm-estimation adaptive method to achieve good control performance and decrease the computational burden. The chaser's and target's unknown inertial parameters, unknown thrust misalignment, and upper bound of unknown disturbances are estimated by online adaptive laws. Ultimately, uniformly bounded convergence of the relative position and relative attitude is proved via Lyapunov analysis. The performance of the proposed controller is demonstrated through numerical simulations.