Quaternion observer-based model-independent attitude tracking control of spacecraft

Based on the separation property for a rigid-body tracking control problem established by Seo and Akella, this work explores a model-independent and observer-based attitude tracking control method in which not only the unavailability of direct/accurate measurement of quaternion attitude is considered, but the inevitable factors of external disturbances and uncertain (or even time-varying) system parameters as well as actuation saturation are also explicitly addressed. The proposed control scheme is essentially model-independent in that the system dynamic model (valid for any rigid-body spacecraft) is used for stability analysis only, but not actually needed for setting up and implementing the proposed control strategy. As such, there is no need for extensively redesigning or reprogramming the control algorithms, even if unexpected external disturbances occur or system parameters/dynamics change during system operation. Those features, highly desirable in practice, are conformed and verified via theoretical analysis and numerical simulations.