Disturbance-observer-based fixed-time surge–heading control of USVs with explicit actuator dynamics

This paper presents a surge–heading control framework for unmanned surface vessels (USVs) with explicit propulsion-actuator dynamics. A control-oriented model retains the first-order propeller dynamics and separates environmental disturbances, unmodeled hydrodynamics, and parameter uncertainties from the actuator-lag-induced input-realization mismatch. A fixed-time disturbance observer (FTDO) uses the realized propulsion input reconstructed from the actual propeller states and is combined with a backstepping-based fixed-time sliding mode controller (FTSMC). Lyapunov analysis establishes regional practical fixed-time attraction of the observer residuals, sliding variables, and physical surge and heading errors to compact residual sets, with an initial-condition-independent convergence-time bound in the prescribed actuator-feasible region. Simulations, benchmark comparisons, actuator-sensitivity tests, and a structured current–wind–JONSWAP environment demonstrate the closed-loop performance, while a real-vessel waypoint-transition test provides preliminary onboard validation.