Measurements of Ocean Surface Currents from a Moving Ship Using VHF Radar

Shore-based, high-frequency (HF) and very high frequency (VHF) radar systems for measuring ocean surface currents have been well detailed in the literature. The use of these systems has been limited to coastal ocean applications since both of the required radar sites must be positioned ashore. The geometric constraints imposed by two shore sites can be relieved by using at least one sea-based system, thus allowing coverage of more coastal areas. A reduction in the number of required sites to one can be achieved by placing the site on a platform that transits between transmit–receive cycles to generate an overlapping field of coverage. Reducing the number of required sites to one increases the ocean area that can be measured. The feasibility of using a single VHF radar system based on a moving ship for both nearshore and deep-water area coverage is the subject of this paper. In December 1993 and July 1994 a nearshore, ship-based dataset was taken off the east coast of Key Biscayne, Florida, with overlapping coverage by a shore-based site. The July 1994 experiment also saw deep water datasets acquired as the ship described a square pattern to obtain overlapping grid coverage from the ship itself. The measurement and reduction of ship-induced contamination in the returned Doppler spectra are detailed. By combining ship and shore radial data, vector current maps off Key Biscayne are obtained. Using the same methodology, the vectorization of the deep water radials produces additional vector current maps from one transmit–receive ship-based site. The deep water surface current maps are dominated by the high speed, northerly flowing Florida Current and are quite accurate. The Key Biscayne surface current maps are less accurate, being dominated by pseudocurrents that are introduced through GPS errors in ship position. These GPS errors lead to inaccurate calculations of the ship velocity and, hence, to inaccurate removal of the ship-induced Doppler currents from the measured currents.