Station keeping of an Autonomous Surface Vehicle in surf zone

Abstract

A small-size Autonomous Surface Vehicle (ASV) is designed to perform autonomous survey missions mainly in river and coastal areas. To do station keeping in surf zone areas, the ASV is designed to have a good heading performance in a specific range of tidal currents period. GPS signals are utilized to navigate the ASV into a way-point area, and then a wave crest measurement system using stereo cameras on the ASV are developed to monitor the wave fields surrounding the ASV. To facilitate the wave field monitoring, stereo images were calibrated and rectified. The corresponding epipolar lines become collinear and parallel to the image scanning lines. CIALAB and k-means techniques are adopted to find the wave crest shadow. Wave fronts are found by the Hough transform from the images. Normalized cross correlation is then used to find the correspondence between left and right images. Stereo visions of the wave crest are then transformed by the space relation to the coordinate of the ASV. Information of sea wave crest is taken into account for the ASV as the vehicle is inside the way-point area drifting with currents. Finally, the wave crest detection and the navigation strategies are verified by experimental data using the ASV station-kept in a coastal surf zone area. The feasibility to reduce ASV drifting rate considering the wave drifting effect as the vehicle is under station keeping mode in the way-point area are discussed.