The development and application of GPS drifter for ocean current measurement

A radio system to transmit data of the Global Positioning System (GPS) was applied to observe ocean currents. The platform has been designed and tested with success. Ideally, the control system at a base station can simultaneously monitor any number of drifters to measure current at a depth from just beneath the surface to dozens of meters. Horizontally, a land base at 100 m altitude can monitor over a region of at least 50 km radius with a radio power of 35 watts. Power supply may last 3-4 days for a transmitting interval of 1 minute. In conjunction with the monitoring of the base station on a ship which deploys and retrieves drifters, the whole system proves to be satisfactory for the purpose of studying the dynamics of very shallow coastal environment. In the authors' field experiments, a land base was set in a lighthouse to monitor three GPS drifters continuously. Their trajectories well depicted the various patterns of coastal tidal currents. The deviation of GPS C/A code data from its true value derived from a synchronous GPS recording at the land base was used to calibrate drifter data, indicating that the DGPS (differential GPS) process can effectively smooth out the zigzag trajectories of drifters. The statistics of the data from an attached current meter shows a mean slip of about 4 cm/sec of the tube-like drifter equipped with a 7 m holysock drogue at 10 m depth in the presence of wave height of about 1 m and wind speed of about 5 m/sec.