Measurement of ocean surface currents using a ship towed log

Abstract

A ship towed log for use on ships-of-opportunity to measure ocean surface currents was built and tested over the past two years. The technique used is one of the oldest known to navigators. The ship's dead reckoned position is calculated from the speed and heading as measured by the towed log. This is then compared to the ship's true position as obtained from a reference navigation system (Loran, satellites, etc.) and the difference is attributed to the currents encountered by the ship. The system was used on six sea cruises and was successfully towed over 11,000 miles. While it i s not capable of making high precision current measurements as would be obtained from moored current meters, it can distinguish features on the order of 20 to 30 em/sec. over a large horizontal scale in the upper ocean.