Spacecraft line-of-sight nonlinear control using two wheels

When an underactuated spacecraft using two wheels has non-zero total angular momentum, it cannot reach an arbitrary stable state unless it meets some constraint conditions. In this paper, with the constraint on pointing direction of the line-of-sight of the spacecraft, we obtain the stable state that the spacecraft can be reached and give a judging criteria. Then, a nonlinear singular controller is presented. This controller can make the line-of-sight move and be stabilized to the desired point, that is, it can minimize the angle between the line-of-sight and the target axis, and satisfy the conservation of system angular momentum as well. This controller is simple in formation, not sensitive to initial conditions of attitude, and can bring the spacecraft's line-of-sight to desired point without any transient oscillations.