Anti-Unwinding Terminal Sliding Mode Attitude Control via Fully Actuated System Approach

This paper investigates high-precision attitude pointing control for spacecraft within the fully actuated system (FAS) framework. Firstly, a second-order nonlinear attitude model based on error quaternions is established. Building upon this model, a novel terminal sliding-mode manifold and its corresponding control law are designed to guarantee finite-time convergence. By leveraging the inherent structural properties of the FAS framework, the nominal nonlinearities are directly compensated, which allows the sliding-mode gain to be significantly reduced while preserving robust performance against model uncertainties and external disturbances. Furthermore, an auxiliary system is synthesized to actively counteract the risks of control saturation. Rigorous stability analysis proves the stability of the closed-loop system and its global anti-unwinding property. Numerical simulations demonstrate that the proposed scheme achieves rapid and high-precision attitude pointing maneuvers without unwinding.