Optimal Momentum Unloading of Reaction Wheels in the Presence of Attitude Control Errors

The attitude of low earth orbiting (LEO) satellites is typically controlled by reaction wheels, which exchange angular momentum with the platform without consuming propellant. However, the momentum of wheels is known to build up and reach wheel speed saturation under the effect of external disturbance torques such as gravity gradient torques, aerodynamics torques, solar radiation torques and erroneous control torques from thrusters and magnetic torquers. As a consequence, the control torque required for attitude stabilization can no longer be generated by the wheels. To prevent this phenomenon, the momentum of the reaction wheels has to be rapidly unloaded by magnetic torquers and/or thrusters. The momentum desaturation must be fast but energy efficient. In this work, we consider different optimal control approaches to achieve this momentum dumping task.