Velocity-free control of spacecraft body-fixed hovering around asteroids

This paper addresses the control of spacecraft hovering about uniformly rotating asteroids without translational velocity measurements. An extended state observer is designed to estimate the spacecraft velocity and uncertain perturbations simultaneously. It ensures ultimately bounded estimation errors irrespective of the control law applied. A velocity-free hovering controller is then obtained by driving a fullstate feedback controller with estimates from the observer. The observer and controller design accounts for both measurement errors and unknown perturbations, and Lyapunov analysis shows that the resultant control scheme globally stabilizes the spacecraft trajectory to the vicinity of the desired hovering state. Furthermore, body-fixed hovering operations, implemented by either continuous or on-off thrusters are simulated with sensor noise and multiple unknown disturbances to demonstrate the effectiveness of the proposed methods.