Demonstrating GPS Coastal Altimetry from the Harvest Oil Platform

Measurements of GPS differential phase between direct and ocean-reflected signals were performed from the Arguello Inc. Harvest Platform on 30 July through 1 August 2001, bracketing a TOPEX/POSEIDON overflight. The ultimate goal of the experiment is to determine the degree to which fixed-receiver GPS altimetry can achieve TOPEX/POSEIDON open-ocean accuracy by direct comparison of results near Platform Harvest. If successfully demonstrated, GPS coastal altimetry will substantially improve on that obtained within 20 km of the coast where spaceborne sensors are innaccurate due to shore effects. The experiment was conducted about 17 km off Pt. Conception, California. Measurements of the GPS phase difference between the direct and reflected signals were taken at both polarizations with a single directional antenna at about 40 m altitude above the surface, at elevations from 0-15 degrees. Low-elevation observations are desired because they enable monitoring of the ocean 4-20 km out, from receivers at reasonable altitudes 40-200 m. Low-elevation observations minimize the effects of surface roughness, and thereby enable the use of the GPS differential phase between the direct and reflected signals, which is how few-cm accuracy may be realized. Preliminary results suggest coherent phase determination at 2-degree elevations, with few-cm precision realized with a few seconds of data. The accuracy of the altimetry and comparison with TOPEX/POSEIDON may require modeling ocean-surface and tropospheric effects.