Fixed-Time Synchronized Attitude and Orbit Control for Spacecraft Flyaround with Asymmetric Input Saturation and Output Constraints

This paper investigates the problem of fixed-time synchronized attitude and orbit control for spacecraft flyaround mission in the presence of parametric uncertainties, external disturbances, asymmetric output constraints, and asymmetric input saturation. A fixed-time disturbance observer and a fixed-time anti-windup compensator are first developed, leveraging a variable-exponent fixed-time stability theorem, to accurately estimate lumped uncertainties and mitigate saturation effects. Subsequently, a fixed-time sliding mode manifold is constructed based on an equivalent error derived from the spacecraft pose tracking errors and the compensator states. To enforce constraints on the sliding mode variables, an adaptive barrier Lyapunov function is introduced, whose boundary is dynamically adjusted according to the real-time level of input saturation. This key feature prevents the control signals from becoming unbounded when the system states approach the constraint boundaries, thus avoiding further aggravation of saturation and ensuring system stability. The practical fixed-time stability of the closed-loop system is rigorously established through Lyapunov analysis. Finally, numerical simulations demonstrate the effectiveness of the proposed control strategy.