Active microwave measurement from space of sea-surface winds

Radar backscatter measurements from the ocean were made at 13.9 GHz from Skylab. The radar signal increased rapidly with wind speed over the entire range of winds encountered, and for angles of incidence of
30\deg
and larger. Signals observed were normalized to a nominal incidence angle (from values within
\pm2\deg
of the nominal) and to a nominal upwind observation direction, using a theoretical model that has been verified as approximately true with aircraft experiments. The wind speed was regressed against the resulting scattering coefficients
\sigma^{0}
and the values of
\beta
in wind
\propto \sigma^{0\beta}
were obtained for incident angles of
1\deg , 17\deg , 32\deg , 43\deg,
and
50\deg
, and for vertical, horizontal, and cross polarizations. For the three larger angles,
\beta
varies from 0.3 to 0.6. Observations during the summer and winter Skylab missions were treated separately because of possible differences caused by an accident to the antenna between the two sets of observations. The results are in general agreement with the theory [26] in all cases, with the winter and cross-polarized agreement somewhat better than that for summer like-polarized data. The "objective analysis" method used for determining "surface-truth" winds in the Skylab experiment was tested by comparing results obtained at weather ships (using all other ship reports to produce the analysis) with the observations made by the weather ships themselves. In most cases, the variance about the regression line between objective analysis and weather-ship data actually exceeded that about the regression line between objective analysis and backscattcr data!