Fully Magnetic Attitude Control for Spacecraft Subject to Gravity Gradient

This paper presents stability and control analysis of a satellite on a near polar orbit subject to the gravity torque. The satellite is actuated by a set of mutually perpendicular coils. The concept is that interaction between the Earth’s magnetic field and a magnetic field generated by the coils results in a mechanical torque used for attitude corrections. Magnetic actuation due to its simplicity and power efficiency is attractive on small, inexpensive satellites. This principle is however inherently nonlinear, and difficult to use since the control torque can only be generated perpendicular to the geomagnetic field vector. This paper shows that three-axis control can be achieved with magnetorquers as sole actuators in a low Earth near polar orbit. It considers the problem from a time-varying, nonlinear system point of view and suggests controllers for three-axis stabilization. A stability analysis is presented, and detailed simulation results show convincing performance over the entire envelope of operation of the Danish Ørsted satellite.