Globally stable adaptive tracking control design for spacecraft under input saturation

We present a globally stable adaptive tracking control algorithm for spacecraft in the presence of control input saturation, parametric uncertainty, and external disturbances. The control algorithm is based on a variable structure control design, and has the following properties: (i) fast and accurate response in the presence of bounded disturbances and parametric uncertainty; (ii) explicit accounting for control input saturation; and (iii) computational simplicity and straightforward tuning. We include a detailed stability analysis for the resulting closed-loop system. It is shown that global stability of the overall system is guaranteed even in the presence of bounded disturbances and parametric uncertainty. The stability proof is based on a Lyapunov-like analysis and the properties of the quaternion representation of spacecraft dynamics. One of the main features of the proposed design is that it establishes a straightforward relationship between the magnitudes of the available control inputs and those of the desired trajectories and disturbances. Numerical simulations are included to illustrate the spacecraft performance obtained using the proposed controller.