First Spaceborne Observation of an Earth-Reflected GPS Signal

We present the first spaceborne observation of a Global Positioning System (GPS) signal reflected from the Earth’s surface, specifically from the Pacific Ocean. This result is scaled to obtain the expected voltage signal-to-noise ratio (SNR) and altimetric accuracy for a generic GPS-reflections altimetry mission, and the current SAC-C and CHAMP missions. Cross-correlating a three-parameter phase model with both a 1-second and 4-second segment of Spaceborne Imaging Radar-C (SIR-C) calibration data, recorded before and after a GalapagosIslands imaging pass, results in beam-limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: • The signals’ temporal shapes agree closely with that predicted using a detailed scattering model, at two different observation geometries, and differ significantly from the expected direct signal shapes. • The signal in the 4-second data has a measured coherence time of 1.0 msec which agrees closely with that expected for a reflected signal, and is completely inconsistent with the direct signal’s coherence properties. • The 1 and 4-second signals’ voltage SNR is maximized by shifting the model frequency -2740 Hz and 497 Hz, respectively, from that expected from their respective specular reflection points, or -2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the -2900 Hz and 510 Hz Doppler frequency shifts expected from those points on the surface corresponding to the antenna’s pointing direction, thus illustrating beam-steering effects on the surface. • Plausible hypotheses for the detected waveform being a corrupted direct signal, including second-order mismodeling effects, Shuttle multipath effects, or a bandpass cutoff of the GPS spread spectrum, are shown to be inconsistent with the data. Space-based observations of reflected GPS signals, like the ones presented here, may enable a new class of ocean-topography measurements unavailable from traditional altimeters, such as TOPEX/Poseidon, and perhaps surface wind-vector measurements. Making such observations with sufficient SNR will require unusually large, high-gain antennas. The measurement presented here is scaled to assess the expected SNR for those applications. Because this result lies in a non-linear scaling regime, the correct scaling equations are presented and the expected signal strength from a generic GPS-reflections altimetry mission is derived to illustrate the most important contributions to the signal SNR. An SNR estimate is also derived for the SAC-C and CHAMP missions which are expected to make GPS-reflection measurements in the near future. Finally, a qualitative wind-speed determination is extracted from the observed signal.