PRECISE MEAN SEA-LEVEL MEASUREMENTS USING THE GLOBAL POSITIONING SYSTEM

This paper describes the results of a sea level measurement test condu cted off La Jolla, California, in November of 1991. The purpose of thi s test was to determine accurate sea level measurements using a Global Positioning System (GPS) equipped buoy. These measurements were inten ded to be used as the sea level component for calibration of the ERS 1 satellite altimeter. Measurements were collected on November 25 and 2 8 when the ERS 1 satellite overflew the calibration area. Two differen t 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 ex amine how dynamic effects of the measurement platform might affect the sea level accuracy. The two buoys were deployed at locations approxim ately 1.2 km apart and about 15 km west of a reference GPS receiver lo cated on the rooftop of the Institute of Geophysics and Planetary Phys ics at the Scripps Institute of Oceanography. GPS solutions were compu ted 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 comput ed by subtracting tide gage data collected at the Scripps Pier from th e GPS-determined sea level measurements and then filtering out the hig h-frequency components due to waves and buoy dynamics. In both cases t he GPS estimate differed from Rapp's mean altimetric surface by 0.06 m . Thus the gradient in the GPS measurements matched the gradient in Ra pp's surface. These results suggest that accurate sea level can be det ermined using GPS on widely differing platforms as long as care is tak en to determine the height of the GPS antenna phase center above water level. Application areas include measurement of absolute sea level, o f temporal variations in sea level, and of sea level gradients (domina ntly the geoid). Specific applications would include ocean altimeter c alibration, monitoring of sea level in remote regions, and regional ex periments requiring spatial and temporal resolution higher than that a vailable from altimeter data.