Robust attitude control of spacecraft with magnetic actuators

Magnetic torquers are frequently adopted as actuators for attitude control of small satellites in low Earth orbit. Such actuators generate a magnetic dipole which, in turn, lead to control torques thanks to the interaction with the magnetic field of the Earth. The main difficulty in the design of attitude control laws based on magnetic torquers is that the torques they generate are instantaneously constrained to lie in the plane orthogonal to the local direction of the geomagnetic field vector, which varies according the current orbital position of the spacecraft. This implies that the attitude regulation problem is formulated over a time-varying model. In this paper the problem of robust design of control laws for magnetically actuated spacecraft is considered and two approaches are proposed and compared; the former is based on linear time-periodic models while the latter relies on linear parameter-varying models. The results obtained by applying both approaches to the problem are presented and discussed.