The Effect Of Long Surface Waves And Illuminated Area On Microwave Backscatter From The Ocean

Abstract

During SAXON-FPN which took place in the' North Sea last November, we mounted two coherent, CW microwave systems on an elevator to measure the effect of long surface waves on microwave backscatter when surface areas of various sizes were illuminated. The two systems operated at x- and Ka-Band. This paper presents initial results from these experiments on the impact of long surface waves on cross section, Doppler bandwidth, and cross section variance for different illuminated surface areas. The systems operated looking to the north of the tower at incidence angles ranging from 10 to 55 degrees. The elevator was able to vary the height of the antennas above mean sea level from 5 to 28 meters. We were careful to design the systems such that we were in the far field of both antennas at all heights. The output of the two microwave systems consisted of in-phase and quadrature baseband signals for HH and VV polarization. These signals were sampled at frequencies high enough to avoid aliasing the Doppler spectra. Selected sets of these high-frequency signals were recorded. The primary method of processing the data for storage, however, consisted of forming estimates of the mean cross section, cross section variance, mean doppler offset, and mean Doppler bandwidth over 125 msec intervals and recording data from consecutive intervals on optical disc. This method preserved the information essential for examining variations of cross sections and Doppler bandwidths with long wave phase while reducing the capacity requirements on the data storage medium. It also allowed the examination of the variance of the cross section as a function of illuminated area. In this paper, we present our preliminary results on a subset of these effects. The variations in cross section and Doppler bandwidth caused by long surface waves are examined in terms of the standard modulation transfer function and a similar function defined for bandwidth. That is, both the cross section and bandwidth are cross correlated with variations of surface velocity deduced from the Doppler offset. These correlations then allow us to examine the amplitude and phase of the variations in these quantities with respect to the long wave phase. Modulation of the Doppler bandwidth can occur either due to the effect of long wave acceleration or as a result of the modulation of intermediate scale waves by the longer waves. We use our data to determine the relative importance of these effects and to attempt to assess whether their combination accounts for the observed bandwidth modulation. Changes in the variance of the cross section with illuminated area are examined for a variety of wind conditions. Theory predicts that thi.s variance will decrease with increasing illuminated area in a manner which depends on the modulation of the cross section by long surface waves. No theory is available, however, to predict the dependence of the variance level on wind speed. We rely on our data to attempt to determine this dependence.