First spaceborne observation of an Earth‐reflected GPS signal

Abstract

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‐s and 4‐s segment of spaceborne imaging radar‐C (SIR‐C) calibration data, recorded before and after a Galapagos Islands imaging pass, results in beam‐limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: (1) 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. (2) The signal in the 4‐s data has a measured coherence time of 1.0 ms, which agrees closely with that expected for a reflected signal and is completely inconsistent with the direct signal's coherence properties. (3) The 1‐ and 4‐s 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. (4) Plausible hypotheses for the detected waveform being a corrupted direct signal, including second‐order mismodeling effects, shuttle multipath effects, or a band‐pass 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 nonlinear 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.