SATELLITE ATTITUDE CONTROL BY CENTER-OF-MASS SHIFTING

This paper introduces a novel control technique that uses active variation of the aerodynamic torque through center of mass shifting, together with reaction wheels and magnetic torquers. The variation of the center of mass position is obtained by moving three shifting masses, which causes variation of the distances between the spacecraft's center of mass and the center of pressures of the spacecraft external surfaces. Therefore, the total aerodynamic torque acting on the satellite changes in both magnitude and direction. The main limitation of this method is related to the impossibility of generate a torque around the direction of the velocity vector. In this paper, we demonstrate how to use the shifting masses as actuators so that the aerodynamic disturbance torque can be utilized for attitude control purposes. Two control methods are introduced: one using aerodynamic torque and one reaction wheel, and the other using aerodynamic torque, and three magnetic torquers. Notably, the second method allows to compensate the well known residual oscillation error related to the use of magnetic control for spacecraft three-axes stabilization. The control authority of such system is proportional to the value of the external disturbance force acting on the satellite. For this reason, the proposed method is more efficient in an environment with high disturbance torques such as low Earth orbits. Furthermore, this method is based on adaptive nonlinear feedback control, where the system stability has been analyzed through the Lyapunov stability theory.