Vector Wind Estimation From GPS Sea Reflections

Recently, there has been considerable interest in the possibility of estimating various parameters of geophysical interest by measuring the signals from the Global Positioning Satellite (GPS) constellation reflected from the ocean surface. Since the original suggestion by Martín-Neira [1993], it has been further argued [Garrison et al., 1998] that information on the local wind field may be contained in the shape of the waveform of the reflected signal. The basic idea is to make use of the time-coding already present for standard GPS navigation to measure the time delay between reception of the direct-path signal from a particular GPS satellite and reception of the signal reflected from the ocean surface. This time delay may be accurately computed using the geometry of the GPS transmitter and the position of the receiver (determined by GPS navigation in the usual manner). The leading edge of the reflected signal emanates from the so-called "specular point" from which a perfectly flat surface would reflect the transmitted signal to the receiver. In contrast to the direct-path signal, which is received at a precise time depending on the location of the transmitter-receiver pair, the power in the reflected signal is spread in time depending on the inherent roughness of the reflecting ocean surface. Since the surface roughness is related to the local wind field (among other things), the width of the reflected waveform can provide information about this quantity and other geophysical parameters. The GSP satellites transmit at L-band (1.2-1.5 GHz) with right-hand circular polarization (RHCP). Due to the phase shift at reflection, the surface-bounce signal (at least from regions near the specular point) is expected to become left-hand circularly-polarized (LHCP) on reflection. In this presentation, the basic physics that governs the scattering of GPS signals from a wind-roughened sea surface will be briefly reviewed. We will then describe the results of recent experimental campaigns in which reflected GPS signals (at both LHCP and RHCP) have been recorded in the vicinity of coastal buoys or research vessels where independent measurements of the surface wind vector were available. By correlating the measured waveforms with the known surface conditions and the underlying physics, we will assess the sensitivity of the measurements to various geophysical parameters including surface wind. This assessment will set the stage for more specific discussions of GPS oceanography to be presented in the other talks in the session.