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[1] 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 À2740 Hz and 497 Hz, respectively from that expected from their respective specular reflection points, or À2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the À2900 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.

An analysis of spaceborne Global Positioning System reflectometry (GPS‐R) data from the TechDemoSat‐1 (TDS‐1) satellite is carried out to image the ocean sea surface height (SSH). An SSH estimation algorithm is applied to GPS‐R delay waveforms over two regions in the South Atlantic and the North Pacific. Estimates made from TDS‐1 overpasses during a 6 month period are aggregated to produce SSH maps of the two regions. The maps generally agree with the global DTU10 mean sea surface height. The GPS‐R instrument is designed to make bistatic measurements of radar cross section for ocean wind observations, and its altimetric performance is not optimized. The differences observed between measured and DTU10 SSH can be attributed to limitations with the GPS‐R instrument and the lack of precision orbit determination by the TDS‐1 platform. These results represent the first observations of SSH by a spaceborne GPS‐R instrument.

The ability of spaceborne Global Navigation Satellite System (GNSS) bistatic radar receivers to sense changes in soil moisture is investigated using observations from the low Earth orbiting UK TechDemoSat‐1 satellite (TDS‐1). Previous studies using receivers on aircraft or towers have shown that ground‐reflected GNSS signals are sensitive to changes in soil moisture, though the ability to sense this variable from space has yet to be quantified. Data from TDS‐1 show a 7 dB sensitivity of reflected signals to temporal changes in soil moisture. If the effects of surface roughness and vegetation on the reflected signals can be quantified, spaceborne GNSS bistatic radar receivers could provide soil moisture on relatively small spatial and temporal scales.

[1] We present the first two aircraft Global Positioning System (GPS)-reflection altimetry measurements, the most precise GPS ocean-altimetry measurement, and demonstrate the altimetric precision and spatial resolution necessary to map mesoscale eddies. Our first experiment demonstrated a 14-cm precision single-satellite ocean altimetry measurement while our more recent experiment demonstrates 5 cm altimetric precision with 5-km spatial resolution. The new results show significant improvement over our previous effort, due to improved modeling, greater aircraft altitudes and velocities, improved receiver positioning, and better experimental control. Plans to further reduce speckle and refine models to obtain 5-cm altimetric accuracy are presented.