The Naval Research Laboratory (NRL) and the National Geodetic Survey recently collaborated on an experiment to test interferometric mode Global Positioning System (GPS) tracking as a source of altitude and vertical acceleration for airborne gravity measurement reduction. A Texas Instruments TI-4100 GPS receiver and an extremely precise radar altimeter were installed in an NRL P-3 Orion aircraft. A second TI-4100 was located at a fixed base station of known location. Carrier phase data from four GPS satellites were recorded from the fixed and airborne receivers and postprocessed to produce three-dimensional displacements of the airborne station relative to the base station. Differences in GPS and radar altimeter coordinate systems were modeled and removed: the resulting GPS and radar altitudes were compared. The two unfiltered 1-Hz data sets differed by an rms of 20 cm when aircraft dynamics were controlled by autopilot in straight and level flight. During hand-flown maneuvers such as turns, climbs, and descents, rms differences varied from 40 to 80 cm. A significant portion of the differences can be attributed to remaining coordinate system discrepancies and to uncompensated geometric effects due to aircraft motion. We have also examined vertical acceleration time series produced by doubly differencing both data sets. By applying a low-pass filter which allows resolution of features with wavelengths of 20 km and greater (at nominal aircraft speeds of 100 m/s) the rms difference between the radar and GPS derived vertical accelerations was less than 2 mGal (2¿10-5 m/s2) during straight and level flight. It should be possible to improve this accuracy and wavelength resolution with future improvements in measurement and processing techniques. The results of this experiment show that the incorporation of interferometric mode GPS tracking into the airborne gravity measurement system will make possible rapid and relatively inexpensive gravity surveys over any type of terrain while simultaneously providing accurate measurements of ground height profiles beneath the aircraft. ¿ American Geophysical Union 1989