Uniform practical output-feedback stabilization of spacecraft relative rotation

In this paper we present a solution to the problem of tracking relative rotation in a leader-follower spacecraft formation using feedback from relative attitude only. The controller incorporates a linear approximation filter to achieve knowledge of angular velocity, and the controller structure renders the equilibrium points of the closed-loop system uniformly practically asymptotically stable (UPAS). That is, the state errors in the closed-loop system are proved to converge from any initial conditions in a region of attraction to a ball in close vicinity of the origin in a stable way, and this ball can be diminished arbitrarily by increasing the gains in the control law. The controller assumes boundedness of angular velocities of spacecraft relative to an inertial frame. Simulation results of a leader-follower spacecraft formation using the proposed controller structure are also presented.