Trajectory tracking and ocean current estimation for marine underactuated vehicles

In this work, a guidance system for 2-D straight-path tracking applications of underactuated marine vessels exposed to unknown ocean currents is developed. A relative velocity kinematic model is considered, hence making the method suitable for underwater vehicles where absolute velocities might not be available. The total position error vector has two components, the along-track error (tangent to the path) and the cross-track error (normal to the path). Two adaptive nonlinear observers are designed in order to estimate the current components w.r.t. the path-fixed frame and the origin of the error dynamics is shown to be globally κ-exponentially stable. The guidance algorithm uses this information and generates appropriate relative surge speed reference trajectories for minimizing the along-track error, while an augmented version of the line-of-sight (LOS) guidance is designed for minimization of the cross-track error. Moreover, the estimates from the nonlinear observers can be used to compute the ocean current vector w.r.t. the inertial frame.