Minimum switching thruster control for spacecraft precision pointing

Maintaining the attitude of a spacecraft precisely aligned to a given orientation is crucial for commercial and scientific space missions. The problem becomes challenging when on/off thrusters are employed instead of momentum exchange devices due to, e.g., wheel failures or power limitations. In this case, the attitude control system must enforce an oscillating motion about the setpoint, so as to minimize the switching frequency of the actuators, while guaranteeing a pre-defined pointing accuracy and rejecting the external disturbances. This paper develops a three-axis attitude control scheme for this problem, accounting for the limitations imposed by the thruster technology. The proposed technique is able to track both the period and the phase of periodic oscillations along the rotational axes, which is instrumental to minimize the switching frequency in the presence of input coupling. Two simulation case studies of a geostationary mission and a low Earth orbit mission are reported, showing that the proposed controller can effectively deal with both constant and time-varying disturbance torques.