Spacecraft Attitude Control using NSTSM and RW Desaturation via Magnetorquer

In this paper, the problem of controlling the orientation of a rigid spacecraft in the presence of external disturbances is examined. Non-singular terminal sliding mode (NSTSM) based control has been derived using the backstepping framework giving the system asymptotic stability with continuous control inputs. NSTSM with fixed controller gains can only reject matched disturbances, therefore, the controller gains are made adaptive to adjust according to the magnitude of unmatched external disturbance. Reaction wheels (RWs) are taken as primary actuators and modeled as first order system. An intuitive RW desaturation strategy using magnetorquer has been proposed. The proposed desaturation method requires minimum magnetic moment for a given angular momentum dumping rate. Finally, the effectiveness of the control algorithm along with desaturation strategy is demonstrated by numerical simulations.