A GPS-tracked surface drifter for harbor and estuary applications

Surface current information in harbors, bays, and estuaries is important to navigation, environmental management, numerical hydrodynamic model verification, and other scientific studies. However, measurements in these areas are rather difficult due to lack of adequate sensors and the presence of frequent boat/ship traffic. Several emerging remote sensing techniques appear promising, but require in-situ sensors to verify their measurements. Existing GPS-tracked drifters are mostly designed for coastal and deep ocean applications which emphasize long term survivability and do not require very high position accuracy. This paper describes the development of a precision GPS-tracked surface drifter that could provide accurate surface current measurements in restricted estuarine waters. An Original Equipment Manufacture (OEM) GPS receiver card with a horizontal circular error probable (CEP) of 40 cm is housed in a flow-following drogue assembly to measure the vertically integrated velocity of the top 1 meter surface layer. Data are stored in a miniature data logger. The data and shore-based reference GPS data are collected for post processing in the Differential GPS mode (DGPS). This paper describes the system design and results of the tests and evaluations