Magnetic attitude control for Earth-pointing satellites in the presence of gravity gradient

A sliding mode control law is applicable to a variety of nonlinear problems and can adapt itself to disturbance torques and parameter uncertainties in real time. In this work, an adaptive sliding mode control law for purely magnetic actuated Earth-pointing satellite with a gravity gradient boom is presented. This control law has been developed for the BUAA-SAT microsatellite, which is designed and manufactured by Beihang University, Beijing. The new control law is a modification of the general sliding mode strategy that enables the satellite to achieve an Earth-pointing attitude and deal with the uncertainty of the principal moment of inertia, which is caused by the incomplete deployment of the gravity gradient boom. In addition, a B-dot algorithm is employed for the detumbling before the boom is deployed. The performance and applicability of the proposed methods have been analyzed and demonstrated by implementing the control law in the BUAA-SAT mission simulator and compared with the proportional derivative-like controller. Presented results show good performance in terms of acquisition and stability of the satellite rotation rate and Earth-pointing attitude.