GPS ocean altimetry from aircraft using the P(Y) code signal

A bistatic global positioning system (GPS) altimeter has been tested onboard a NOAA WP-3D research aircraft. The first altimetric data were obtained with this equipment above the sea surface to study the accuracy of such measurements using the P(Y)-encrypted GPS signal. The flight was conducted in the northeast Gulf of Mexico in September 2006 at about 1500 m altitude over a 150-by-150 km area under quiescent sea-state conditions. The ocean waves were a low, short swell of about 20 cm significant wave height (SWH) and 20 m dominant wavelength, providing minimal effect of sea-surface roughness. The data obtained over approximately a three-hour period consist of direct and reflected waveforms and the time delays between them. Smoothing of the raw waveforms was used to better identify the leading edge and the maximum of the reflected signal. Comparison of GPS-derived altitudes and altitudes obtained by the aircraft altimeter showed that the former followed most of the aircraft´s altitude variations. The rms of the difference in height between the smoothed aircraft altimeter data and the GPS height determination based on a 0.4-s - averaged sea-surface - reflected waveform was on the order of 1 m. Additional averaging over 1 min reduced the rms to about 20 cm.