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 Nonlinearit ies-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 b ackscatter from the sea surface on illuminated area. The two systems o perated at X and ka bands (10 and 35 GHz) and collected HH and VV pola rized backscattered signals simultaneously. The elevator system allowe d us to vary the altitude of the two microwave systems above the sea s urface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45-degrees incidence angle, which implied th at 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 m . We examined the dep endence of the normalized radar cross section (sigma0), its variance, and the bandwidth of the Doppler spectrum on illuminated areas. We wer e unable to detect any dependence of sigma0 on area but found a defini te decrease in its variance as area increased. At X band the variance divided by the square of sigma0, the normalized variance, decreased fr om 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 de pendence of the normalized variance to available theory, we deduce tha t correlation lengths are on the order of 10 times the microwave wavel ength at both X and Ka band. Values for the normalized variance of an elementary scattering facet were also inferred and are presented in th is paper. From the Doppler bandwidths we obtained radial velocity spre ads over the illuminated areas and found that they agreed well at X an d Ka band. These velocity spreads, which axe inversely proportional to the correlation time of the backscatter, increased rapidly with illum inated 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 so mewhat larger values for small areas. The dependence of the velocity s pread was found to be well explained by theory if an intrinsic velocit y spread of 0.07 m s-1 was used to represent scatterer lifetime effect s.