Estimating absolute current velocities by merging shipboard doppler current profiler data with LORAN-C data

The output of a shipboard-mounted Doppler Current Profiler (DCP) is a profile of water velocity, in various depth bins, relative to the ship. To obtain absolute water velocities, some independent measure of the absolute ship\´s velocity must be used. The most common measure of ship\´s velocity is derived from LORAN-C navigation. Unfortunately, LORAN-C derived velocities are not as accurate as Doppler estimates. A process involving a reference layer concept and numerical filtration to obtain estimates of absolute water velocities is described. The cost of using this process is the loss of information concerning the variation of absolute water velocities at periods less than 1/2 hours. Once the problem of inaccurate navigation is solved, there are still two problems that can affect the quality of absolute water velocity estimates. These problems arise from small errors in the heading ascribed to the ship and to the speed calibration of the Doppler system.