The dependence of microwave backscatter from the sea on illuminated area: Correlation times and lengths

During the Synthetic Aperture Radar (SAR) and X band Ocean Nonlinearities-Forschungsplattform Nordsee experiment, we mounted two continuous wave microwave systems on an elevator on the German Research Platform Nordsee for the purpose of investigating the dependence of microwave backscatter from the sea surface on illuminated area. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH and VV polarized backscattered signals simultaneously. The elevator system allowed us to vary the altitude of the two microwave systems above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45° incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We examined the dependence of the normalized radar cross section (σ0), its variance, and the bandwidth of the Doppler spectrum on illuminated areas. We were unable to detect any dependence of σ0 on area but found a definite decrease in its variance as area increased. At X band the variance divided by the square of σ0, the normalized variance, decreased from values near 12 for small areas to values near 2 for large areas. At Ka band, corresponding values were 40 and 2. The normalized variance was always slightly larger for HH polarization. By fitting the area dependence of the normalized variance to available theory, we deduce that correlation lengths are on the order of 10 times the microwave wavelength at both X and Ka band. Values for the normalized variance of an elementary scattering facet were also inferred and are presented in this paper. From the Doppler bandwidths we obtained radial velocity spreads over the illuminated areas and found that they agreed well at X and Ka band. These velocity spreads, which are inversely proportional to the correlation time of the backscatter, increased rapidly with illuminated area for small areas but tended to level off to values of about 0.5 m s−1 at large areas. This implies a decorrelation time for large illuminated areas of about 10 ms at X band and 3 ms at Ka band but somewhat larger values for small areas. The dependence of the velocity spread was found to be well explained by theory if an intrinsic velocity spread of 0.07 m s−1 was used to represent scatterer lifetime effects.