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 1992 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. ¿ American Geophysical Union 1993