Nonlinear Attitude Control of an Underactuated Spacecraft Subject to Disturbance Torques

We propose a procedure to correct and stabilize the orientation of a spacecraft subjected to an external disturbance torque, using only two available control torques. A corrective maneuver that best satisfies the objectives of the mission is first determined, and a nonlinear controller is developed to stably implement the corrective maneuver. Then, using a modified form of the backstepping method, for systems with nonlinear effort-state relation, the respective torques are determined to achieve the prescribed angular velocities and have the system converge to the desired equilibrium state. Numerical simulations were performed to illustrate the performance of the controller.