A Fixed-time Stable Output Feedback Non-singular Fast Terminal Sliding Mode Design for Attitude Control of a Rigid Spacecraft Considering External Disturbances and System Uncertainties

This study introduces a novel spacecraft attitude control framework designed to operate reliably under uncertainties and external disturbances. At its core lies a dual-component strategy: a controller and a disturbance observer, both engineered for fixed-time convergence. Unlike conventional finite-time methods, where stabilization speed depends on initial conditions, this approach guarantees a preset stabilization window, tunable via controller parameters giving engineers precise command over performance timelines. By modeling spacecraft attitude dynamics using modified Rodriguez parameters (MRP), the system is cast into a second-order nonlinear structure. A non-singular terminal sliding surface is developed to enforce fixed-time convergence, enabling the derivation of robust control inputs. To address real-world unpredictability, a universally robust exact differentiator (URED) observer estimates and neutralizes disturbances within the same fixed timeframe, ensuring seamless compensation. Stability is validated using Lyapunov theory, while comparative simulations against existing methods demonstrate that the proposed framework achieves faster convergence and superior tracking accuracy for both controller and observer. These advancements set it as a promising solution for critical space missions requiring rapid, precise attitude control, such as satellite docking or deep-space exploration.