Abstract
A third order sliding mode disturbance observer, coupled with a modified super twisting controller, is proposed for the trajectory tracking of fully-actuated marine vehicles in the presence of unknown, time-varying disturbances. The observer-controller is formulated in a general vector form that can be applied to marine vehicles with configuration spaces corresponding to three, four or six degrees of freedom. The effectiveness of the observer-controller is demonstrated using simulations of an unmanned surface vessel tracking a square-shaped trajectory.