Development and Implementation of Global Positioning System Buoy in San Francisco
There are several Physical Oceanographic Real-Time Systems (PORTS) installed in U. S. harbors. PORTS was developed by National Ocean Service (NOS) scientists and engineers. The system measures currents, water levels, and other physical conditions at specific locations in the harbor, i.e., Golden Gate, Alameda, and Richmond in San Francisco Bay. By providing information about actual conditions at specific sites, PORTS allows ship pilots and masters to use the Bay's channels to their fullest extent, while simultaneously increasing safety and minimizing the need for additional dredging. Mariners now can access PORTS information before entering the Bay. However, they can only obtain actual water-level values at the specific PORT location; all other water-level values are based on models.
One key ingredient in providing good water-level nowcasts (i.e., real-time predictions at hundreds of locations throughout the Bay) is obtaining real-time water level data at strategic locations in the harbor that represent the water level signal propagating throughout the port, i.e., wind-induced water-level changes. Actual water-level measurements obtained at nearshore sites strategically location throughout the harbor compared with the forecast water levels can be used to validate and improve the model. Improved water-level models mean that the pilot of the ship will more accurately know the separation between the keel and the bottom of the channel. Water-level data from a buoy equipped with a Global Positioning System (GPS) receiver can provide these data needed to improve and validate the water-level model used in U.S. harbors.
With the availability of high-accuracy, differential GPS results in real-time, it is possible to use GPS to accurately measure the position of a buoy equipped with GPS. The National Geodetic Survey (NGS) has transferred this technology to the maritime community. NGS has installed a GPS buoy system in San Francisco Bay. The vertical positions of the buoy in the Bay were continuously determined with a precision of 2-3 cm. The solutions showed excellent agreement with a nearby tide gauge and clearly proved the feasibility of remote GPS operations with renewable power and radio modems. Applications for obtaining real-time water heights in shipping channels or off-shore seem possible. This project will enhance the efficiency and effectiveness of future water measurements in U.S. harbors and the Great Lakes, as well as other navigable waterways.
The results of the project will be used to improve the design of the GPS buoy system in order to measure actual water-level values. These values will be used to validate water-level models, provide data for testing a new continuous water-level zoning technique for hydrographic surveys (and other applications), and investigate the use of buoys equipped with GPS for detecting real-time directional changes in water currents.
NGS provided the logistics, two GPS receivers, batteries, solar panels, a computer, and modems, and the United States Coast Guard provided a buoy, assisted in modifications of the buoy to equip it with the GPS system, and deployed the buoy. NGS personnel processed the GPS data and prepared height values, plots, and statistics which will be provided to U.S. Geological Survey personnel to compare GPS buoy results with their San Francisco Bay water-level models.