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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.

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