Long Term Water Level Measurements Using GPS on a Buoy

The National Geodetic Survey (NGS) has undertaken several projects to demonstrate the operational feasibility of determining the height of GPS antennas deployed on buoys. The first effort was a 6-month occupation on a buoy moored in the Chesapeake Bay near Annapolis, MD. Other efforts use buoys moored in San Francisco Bay and in Lake Huron. These projects have already gathered about 6 months worth of data and are still continuing. In all cases, continuous and renewable power is provided by a combination of batteries and solar panels. The data from the buoy is delivered to nearby shore stations by a radio modem at a 5-second rate. Kinematic GPS solutions are obtained using the NGS KARS program. The precision of the height measurements is estimated to be about 2-3cm. For comparison with nearby tide gauges, the 5-second GPS measurements have been combined into 6-minute averages. These GPS and tide gauge measurements are highly correlated but do show systematic differences in amplitude and susceptibility to sea state. These comparisons will be presented along with descriptions of the equipment used. These efforts are intended to investigate the reliability of unattended GPS operation including the reliability of renewable power sources and data downloading by radio modem. In each case the range from the base station to the buoy has been less than 10 km. Our experiences have shown that, except for rare power loss during extended periods of cloudy weather, the entire system, from data collection to data processing, has performed extremely well. NGS is considering plans for additional operations at extended ranges.