Velocity-Free Saturated Control for Hovering Over an Asteroid With Disturbance Rejection

Hovering over an asteroid is challenging due to both the large gravitational uncertainty and the strong external disturbance. When a spacecraft is not equipped with velocity sensors to simplify the navigation system and reduce the mass and costs, or the velocity sensors malfunction, the situation may be worse. In this paper, an extended state observer (ESO) is devised to estimate the velocity and the lumped disturbance simultaneously. The estimate errors are proved to be uniformly ultimately bounded with the assumption that the change rate of the lumped disturbance is bounded. To cope with control input saturation, a backstepping controller in conjunction with an auxiliary system is proposed for the hovering. Combining the controller with the ESO, we develop a disturbance rejection control scheme without velocity feedback. The stability of the whole closed-loop system is analyzed via the Lyapunov theory, showing that the control scheme can drive a spacecraft to the neighborhood of the desired hovering state. The numerical simulations of both body-fixed hovering and inertial hovering are conducted to demonstrate the effectiveness of the proposed scheme.