A guidance and control approach is proposed in the present work for autonomous under-actuated marine surface vessels. The dynamic model considers the six rigid body degrees of freedom of the ship and one degree-of-freedom for the rudder dynamics. The formulation accounts for the effects of coriolis and centripetal accelerations, retardation forces, wave excitations forces, linear damping forces, nonlinear restoring forces, wind and current loads. The guidance scheme is based on the concepts of the variable radius line-of-sight (LOS) and the acceptance radius. It can handle large cross-track error while aiding the controller to quickly converge the ship to its desired trajectory. The control strategy consists of a two-layer controller, which is designed based on the sliding mode methodology. The simulation results demonstrate the robust performance of the integrated guidance and control system in spite of significant modeling uncertainties and environmental disturbances.
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