An Aircraft Ocean Altimetry, Wind Speed and Wind Direction Measurement Using GPS Reflections

We present a simultaneous aircraft ocean altimetry, wind-speed and wind-direction measurement using GPS signals reflected from the ocean's surface. An assessment of the potential science return from aircraft and spacecraft measurements similar to or scaled to this result will be presented. The measurement data were collected near the Harvest oil platform, a Topex/Poseidon calibration site, where extensive ground-truth information is available. This measurement is the second GPS aircraft ocean altimetry measurement ever made, and the first made under controlled conditions. Preliminary results show that, when the geometry is approximately constant, the altimetric error is dominated by speckle. Combining measurements from several simultaneous satellite observations having different geometries show additional systematic errors, presumably due to mis-modeling of the fit waveforms, which will likely be improved with further work. Preliminary altimetric precision and accuracy will be presented for both the GPS C/A and Y-code data. The altimetric measurement requires a wind vector determination in order to use the best model waveforms. These high-quality data show large systematic errors in the C/A-code waveforms caused by sidelobes in the C/A-code auto-correlation function. This results in erroneous wind-speed retrievals; previous GPS-reflections wind vector measurements have not had the resolution to see these effects. Techniques used to mitigate these problems will be presented. We will also present the first Y-code wind-vector measurement, along with precision and accuracy estimates of our wind-vector retrievals.