  Field trials of a 4 meter long, 180 kilogram, unmanned surface vehicle (USV)
have been conducted to evaluate the performance of station-keeping heading and
position controllers in an outdoor marine environment disturbed by wind and
current. The USV has a twin hull configuration and a custom-designed propulsion
system, which consists of two azimuthing thrusters, one for each hull.
Nonlinear proportional derivative, backstepping and sliding mode feedback
controllers were tested in winds of about 4-5 knots, with and without wind
feedforward control. The controllers were tested when the longitudinal axis of
the USV was aligned with the mean wind direction and when the longitudinal axis
was perpendicular to the mean wind direction. It was found that the sliding
mode controller performed best overall and that the addition of wind
feedforward control did not significantly improve its effectiveness. However,
wind feedforward control did substantially improve the performance of the
proportional derivative and backstepping controllers when the mean wind
direction was perpendicular to the longitudinal axis of the USV. An analysis of
the length scales present in the power spectrum of the turbulent speed
fluctuations in the wind suggests that a single anemometer is sufficient to
characterize the speed and direction of the wind acting on the USV.
