Attitude stabilization of inertial pointing spacecraft using magnetic actuators

We revisit attitude stabilization for inertial pointing spacecraft endowed with magnetic actuators. Under mild assumptions on the time-variability of the geomagnetic field, it was proven that a quaternion-based PD-like controller can guarantee global convergence to the desired attitude provided that the gains are sufficiently small, thus posing an intrinsic limiation to the achievable performance. In this paper we propose a geometric projection-based controller which guarantees almost-global stability of the desired equilibrium, thereby avoiding the so-called "unwinding phenomenon" that affects the quaternion-based controller. Furthermore, simulation examples show that the proposed design, combined with suitable state-dependent time-varying gains, provides better results than the PD-like controller both in terms of convergence rate and of stabilization error when environmental disturbances and actuators saturation are included in the model.