Airborne measurement of absolute sea‐surface heights

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Abstract
Topographic profiles may be measured from an aircraft by differencing precise radar or laser altitudes with ellipsoidal heights obtained by means of kinematic interferometric‐mode GPS. Over the ocean, the difference between the two altitudes is the geoid height plus dynamic oceanographic effects such as tides, currents, eddies, etc. Aircraft might provide, on a local scale, altimetry measurements similar to those from GEOSAT and ERS‐1. Such airborne measurements could be applied to the determination of a local high‐resolution geoid or for oceanographic studies.Several long over‐water altimetry profiles were obtained during the 1991 field campaign of the Greenland Aerogeophysics Project (GAP91). Kinematic interferometric GPS between multiple short or zero‐baseline airborne receivers and several sets of stationary GPS receivers on the ground provided precise three‐dimensional positioning for airborne gravity, magnetic and topographic profiling sensor systems mounted aboard a P‐3 Orion aircraft. The logistics of surveying a large region (long GPS baselines and poor satellite coverage) limit the accuracy of the GPS positioning, but the geoid signal is clearly visible in the over‐water profiles. In spite of GPS baseline lengths of up to 1000 km, differences between modeled geoid heights and measured sea‐surface heights ranged from about 1–2 meters r.m.s. Although the differences are primarily due to errors in the GPS positions, some portion is due to errors in the model geoid, uncompensated tides, sea‐ice contamination of the radar data and other errors in the radar. We believe that this represents the first airborne measurement of absolute sea‐surface heights.