VHF Radar Measurements of Ocean Surface Currents from a Moving Ship

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. To relieve the geometric constraints imposed by two shore sites and allow coverage of more coastal areas, necessitates at least one sea-based system (Skop et al., 1994). 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 near shore and deep water area coverage is the subject of this report. In December 1993 and July 1994 a near shore, ship-based data set 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 data sets 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, two vector current maps off north and south Key Biscayne a obtained. Using the same methodology, the vectorization of the deep water radials produces two more vector current maps from on 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 pseudo-currents 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-induce Doppler currents from the measured currents. (MM)