The most popular model of microwave backscatter from rough water surfaces at
mid-incidence angles (20 degrees < theta i < 70 degrees ) is
composite surface theory. This theory holds that the backscattered return is directly
proportional to the spectral density of centimetric, Bragg-resonant water waves which are
tilted and advected by longer waves. A stringent test of this theory is to measure,
independently and from the same surface area, the normalized microwave cross section
(sigma 0) and the Bragg wave spectral density, and compare them using the
theory. In this paper, we use a calibrated optical slope imaging system in a wind-wave
tank to measure the two-dimensional wavenumber spectrum of short waves. From these
spectra, we calculate both the pure Bragg scattering sigma0 which neglects
longwave effects and the more complex composite surface sigma0. The results are
compared with sigma0 obtained from backscatter measurements at X band (10 GHz)
and Ka band (35 GHz) made between 28 degrees and 68 degrees incidence angle. We
find that composite surface theory generally shows better agreement with experiment at
both frequencies than pure Bragg scattering theory. The agreement seems best for friction
velocities above 40 cm/s-1. For all friction velocities up to 70 cm/s-1,
however, composite surface theory somewhat underpredicts the actual sigma0 in a
majority of the cases. This is especially true for horizontal polarization at large
incidence angles. We conclude that while composite surface theory accounts for much of the
backscatter at both frequencies in the incidence angle range we examined, the discrepancy
between the predicted and measured cross sections is sufficiently large that contributions
from other scattering processes cannot be ruled out.
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