GPS tides: A project to determine todal datums with the Global Positioning System

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The recent development of on-the-fly ambiguity resoltuion techniques with Global Positioning System (GPS) observations has created a powerful tool for the study of dynamic time series, such as tidal analysis or the study of water level heights. For this project a GPS receiver was successfully operated on board a Canadian Coast Guard navigation buoy. Ten days of data, at a one-second sample interval, were collected at a station in the open waters of the Bay of Fundy, followed by ten days from an inshore mooring. The buoy data were transmitted in real time to a reference station on shore where antenna heights were computed in real time. Subsequent processing reduced the 1-Hz antenna height series to a 15-minute water surface height series for comparison to two nearby "conventional" gauges. These were the Saint John Harbour permanent gauge and a temporary Socomar pressure gauge. Mean tide heights computed from the inshore data are all within 1 centimetre. The means, computed from the inshore series, differed by 10 centimeters between the buoy and the "conventional gauges," indicating a differential slope between the water surface and the ellipsoid. In addition to the water surface measurements, about 10 days of GPS data were collected from a static baseline to characterize the performance of the on-the-fly GPS system, without the influence of a moving platform. From the 20 days of data, 97 percent is within 3 cm of truth, while the mean daily heights are about 0.3 cm below the truth. This project has demonstrated that the "GPS buoy" is a viable method od tidal datum determination and it should be possible to use the ellipsoid as the basis for water surface datums.