Abstract
A six degree of freedom nonlinear control law for the trajectory tracking of marine vehicles that operate in the presence of unknown time-varying disturbances, input saturation and actuator rate limits is developed using a disturbance observer and nonlinear backstepping. The disturbance observer provides estimates of the unknown time-varying disturbances and a continuously differentiable function is employed to model input saturation. The uniform ultimate boundedness of all signals in the closed-loop control system is proved. Trajectory-tracking simulations of an autonomous underwater vehicle demonstrate the performance of the proposed controller.