An ocean-altimetry measurement using reflected GPS signals observed from a low-altitude aircraft

The authors present the first airborne ocean-altimetry measurement made using reflected GPS signals. Data were collected from a Cessna airplane flying off the coast of Santa Barbara, California, where the direct and ocean-reflected GPS L1 signals were downconverted and recorded onboard at 20.456 MHz. In addition, the direct signals were processed with a TurboRogue GPS receiver and used to derive accurate receiver positioning. The 20 MHz samples were processed with a "software GPS receiver", developed for this task, to extract the direct and reflected signal waveforms, using both the C/A and Y-codes. These waveforms were fit to obtain a direct/reflected reception delay difference, each second. These delays were fit for ocean height and clock offset. First results for the C/A-code waveforms show the mean of the measured 1-sec values agrees well with altimetry database values. The residual height RMS was 3.1 m during straight, level flight, and 4.7 m for all data and all PRNs. Further analysis refinements, including more accurate waveform modeling and fitting, C/A-code sidelobe elimination, atmospheric modeling, and the use of Y-code waveforms, will likely improve these results.