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This paper describes the results of a sea level measurement test conducted off La Jolla, California, in November of 1991. The purpose of this test was to determine accurate sea level measurements using a Global Positioning System (GPS) equipped buoy. These measurements were intended to be used as the sea level component for calibration of the EI•S i satellite altimeter. Measurements were collected on November 25 and 28 when the EP•S i satellite overflew the calibration area. Two different types of buoys were used. A waverider design was used on November 25 and a spar design on November 28. This provided the opportunity to examine how dynamic effects :øf'•he measurement platform might affect the sea level accuracy. The two buoys were deployed at locations approximately 1.2 km apart and about 15 km west of a reference GPS receiver located on the rooftop of the Institute of Geophysics and Planetary Physics at the Scripps Institute of Oceanography. GPS solutions were computed for 45 minutes on each day and used to produce two sea level time series. An estimate of the mean sea level at both locations was computed by subtracting tide gage data collected at the Scripps Pier from the GPS-determined sea level measurements and then filtering out the high-frequency components due to waves and buoy dynamics. In both cases the GPS estimate differed from l•app's mean altimetric surface by 0.06 m. Thus the gradient in the GPS measurements matched the gradient in l•app's surface. These results suggest that accurate sea level can be determined using GPS on widely differing platforms as long as care is taken to determine the height of the GPS antenna phase center above water level. Application areas include measurement of absolute sea level, of temporal variations in sea level, and of sea level gradients (dominantly the geoid). Specific applications would include ocean altimeter calibration, monitoring of sea level in remote regions, and regional experiments requiring spatial and temporal resolution higher than that available from altimeter data. Paper number 93JC03355. 0148-0227/94/93JC-03355505.00 ceivers can be used to compute the relative position of a moving G PS antenna, as a function of time, relative to a fixed reference G PS antenna to an accuracy of centimeters over baselines of several tens of kilometers [Rocken et yd., 1990]. These position measurements can be expressed in terms of the latitude, longitude, and geodetic height of the antenna phase center. If the moving G PS antenna is mounted to an ocean buoy and the buoy antenna phase center above sea level is accurately calibrated, a sea level time series c&n be measured. If the buoy is horisontally constrained and measurements are made at the appropriate frequency, then mean sea level, sea level change due to tides, and 7951 7952 KELECY ET AL.: GLOBAL POSITIONING SYSTEM SEA LEVEL MEASUREMENTS