Abstract-This paper presents an extensive analysis of the integral line-of-sight (ILOS) guidance method for path following tasks of underactuated marine vehicles, operating on and below the sea surface. It is shown that due to the embedded integral action, the guidance law makes the vessels follow straight lines by compensating for the drift effect of environmental disturbances such as currents, wind and waves. The ILOS guidance is first applied to a 2D model of surface vessels that includes the underactauted sway dynamics of the vehicle as well as disturbances in the form of constant irrotational ocean currents and constant dynamic, attitude dependent, forces. The actuated dynamics are not taken into account at this point. A Lyapunov closed loop analysis yields explicit bounds on the guidance law gains to guarantee uniform global asymptotic stability (UGAS) and uniform local exponential stability (ULES).The complete kinematic and dynamic closed loop system of the 3D ILOS guidance law is analyzed next, hence extending the analysis to underactuated AUVs for 3D straight-line path following applications in the presence of constant irrotational ocean currents. The actuated surge, pitch and yaw dynamics are included in the analysis where the closed loop system forms a cascade, and the properties of UGAS and ULES are shown. The 3D ILOS control system is a generalization of the 2D ILOS guidance. Finally, results from simulations and experiments are presented to validate and illustrate the theoretical results, where the 2D ILOS guidance is applied to the CART and the LAUV vehicles.

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Unmanned marine crafts constitute a priority area within several fields of study, and there are still many challenges related to making such vessels autonomous. A basic task of an autonomous marine craft is to follow a general path in the presence of unknown ocean currents. This paper presents a method to achieve this for surface vessels. The results are an extension of the results in [1] regarding path following of space curves when no ocean currents are present, and introduce a virtual Serret-Frenet reference frame that is anchored in and propagates along the desired path. The closed-loop system consists of an ocean current observer, a guidance law, a controller and an update law to drive the Serret-Frenet frame along the path, and is shown to be UGAS. Simulation results are presented to verify the theoretical results.

The integral line-of-sight guidance law for path following applications of autonomous surface vessels is presented in a unified manner, merging intuitive and theoretical aspects of this valuable control technique. Straight line path following scenarios of underactuated surface vessels in the presence of unknown constant irrotational ocean currents are considered. The integral line-of-sight guidance and two feedback controllers are combined into a cascaded configuration where the integral effect in the line-of-sight guidance is introduced to counteract the disturbance. The chosen integration law is defined to reduce the risk of wind-up effects, and it is shown that the integral action in the line-of-sight guidance law performs a vectorial sum between the vessel relative velocity and the unknown current velocity to compensate for the drift. Moreover, only relative velocities are used in the feedback loop since the ocean current is assumed constant and irrotational. Redefining the vessel model with relative velocities significantly simplifies the control system compared to the approach based on absolute velocities. Closed-loop uniform local exponential stability is achieved for path following of straight line paths. Furthermore, in steady state, the presented guidance law paired with measurements of the absolute speed and the relative speed of the vessel yields to an estimation of the ocean current. Simulations are presented to support the theoretical results.