Relationship between GPS signals reflected from sea surfaces and surface winds: Modeling results and comparisons with aircraft measurements

Theoretical calculations of global positioning system (GPS) signals reflect ed from rough sea surfaces are discussed and compared with the aircraft mea surements. The theoretical model is based on the assumption that rough sea surfaces are composed of facets, and the reflection of GPS signals from the facets can be calculated on the basis of geometric optics. Ln-order to det ermine the slopes of the sea surface facets, the statistical model of Cox a nd Munk [1954] for ocean surface slopes is used. Since much of the sea surf ace roughness observed is at scales much less than the GPS wavelength, the dependence of the mean square slopes on frequency is taken into considerati on. Model results agree well with aircraft measurements: the differences be tween model results and observations are within the level of experimental e rrors. For calm sea surfaces (near-sea-surface wind speeds < 1 m s(-1)), th e correlation powers of the GPS signals are narrow functions of time delay with peak values of similar to 0 - -3 dB. In stronger wind cases (wind spee ds > 7 m s(-1)), the correlation powers decrease their peak values to simil ar to -4 - -8 dB, significantly widen time delay responses, and shift towar d the delays corresponding to path lengths longer than those of specular po ints. Sensitivity tests show that there is considerable potential for curre nt or advanced GPS receiving systems to estimate Pear-sea-surface wind spee ds: the errors in the wind speed estimates could be smaller than 2 m s(-1). For near-sea-surface wind directions, the maximum changes in the correlati on powers of the GPS signals as a function of azimuth appear to be similar to 0.5 dB for current GPS receiving systems, which may be too small for win d direction estimation. New designs of the GPS receivers or a higher signal -to-noise ratio than that of current aircraft receivers may be needed.