Measurements of the normalized radar cross section of the sea at K_u band at an incidence angle of 10 degree were performed from a manned airship off the Oregon coast in September and October 1993. The cross section at this incidence angle is often assumed to have little dependence on wind speed and direction. Our measurements, however, indicate that at wind speeds below 6 to 7 m/s, the cross section is, in fact, dependent on these quantities and that the azimuthal modulation can reach values on the order of 5-8 dB. Comparisons of the measured values with the predictions of the quasi-specular scattering model are presented. The theory is shown to be accurate in predicting the azimuthal modulation and the strength of the backscatter either if effects of swell are included or if measured wind directions are ignored and the upwind direction is forced to be near the maximum cross section. Values of mean-square wind-wave slope and effective reflection coefficient required to obtain these fits are very close to each other and to those obtained by previous investigators. In particular, mean-square wind-wave slopes are about 70 to 80\% of those of Cox and Munk (1954) because the radar responds only to facets larger than about 10 cm, smaller ripples acting to reduce the reflection coefficient. If swell is included, we find that mean-square slopes in the direction of the swell of as much as 10 times the measured swell slopes are required to fit the model to the cross section data at low winds speeds. We suggest that this may be due to high-order effects of the hydrodynamic modulation of short waves by the swell. We believe that this explanation is more likely than assuming that wind directions were incorrectly measured.
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