A two-scale model of rough surfaces is considered which permits theoretical interpretation of the features of very high frequency scattering from such surfaces (say, from a sea surface). The scattering snrface is a m e d to be a superposition of small-scale ripple and large waves (swell). Reflection from the latter may be considered by the Kirchhoff approximation. The spatial spectrum of corrugations is taken into consideration; the calculations based on this model help to give an explanation of the behavior of the scattered intensity as a function of the angle of incidence 4, and to establish which factors affect this dependence at various values of 9. Theoretically predicted dependence of the scattered intensity upon radio wavelength 1, depolarization of the scattered s i g n a l , and other features of the scattered radiation are in good agreement with the experimental data obtained from direct measurements.Frequency spectra of the backscattered s i 4 were also investigated (experimental measurements were carried out at wavelengths 3.2 cm, 10 cm, 50 cm, 1.5 m, and 4 m). Observed shifts of the central frequency agree with results of other authors (for the range of a=3 cm to 200 m).The measured values of the spectrmn width appeared approximately twice those theoretically calculated. This may be explained by the M uence of dissipative processes and of fluctuations of the skin-deep layer drift velocities. Space correlation of the backscattered signal was also investigated (both theoretically and experimentally). 50 percent decorrelation occurs at distances compared with dimensions of a wave slope.

Measured results are presented of the amplitude and spectral characteristics of the envelopes of vertically and horizontally polarized and cross-polarized 3.2-cm radar backscatter signals from the surface of the sea at grazing angles • < 8 ø with fine range resolution. The features of scattering on vertical polarization are explained completely by the 'two-scale' modele For horizontal polarization, relatively stable backscatter signals, called 'bursts,' are observed in addition to the signals scattered by the capillary waves. The results are also used to explain features of the fading spectra of the signals for the different polarizations. The characteristics 0f radio wave backscattering from the sea surface have been experimentally studied in detail. A model of the sc•Rering surface previously developed [Bass et al., 1968; Wright, 1968; Barrick and Peake, 1968] permitted explanation and calculation of many characteristics of scattered signals. In this model the water waves with lengths equal (at small grazing angles for the radio waves) to half a radio wavelength are the reflecting elements; i.e., in the centimeter range, such an element is a capillary wave or 'ripple.' The effect of large wave components of the sea surface is to modulate the amplitude and phase of the radio signals scattered by the ripple. The following characteristics of the scattering proved not to be explained by the two-scale model. For a slight sea the differential backscattering cross section for horizontal polarization o'nn ø exceeds that for vertical polarization •roo ø, although in accordance with the model, it should be •roo ø > •rnn ø. Another phenomenon previously observed [Pidgeon, Measurements were made with variation of the measuring direction relative to the wind over a 180 ø angular range. The most interesting measurements, and those primarily reported here, were in the upwind direction. The typical fetch of the waves was 200 km; i.e., the waves developed across the entire width of the Caspian Sea. Figures 1-5 show typical changes of the envelope of the recorded signal as a function of time within one large wave while we observed toward the waves (upwind) at •b = 3.5 ø. Below the instrumental recordings are shown 3 graphs: (1) the running average of signal E (in the same relative units for both components), (2) the degree of depolarization •t = 20 1ogao (Et•/E.) (1) (i andj correspond to the polarizations of radiation and reception),' and (3) the spectral widths F of the amplitude fluctuations calculated for a time interval/xt much shorter than the period of the large wave. Other indices in the figure denote 1968; Mel'nichuk and Chernikov, 1971] but not explained by polarization of radiation (Tro is vertical polarization of the the model is the difference in frequency spectra of signals of transmitter) and polarization of received signals (oh is a different polarizations. In some papers, attempts to explain horizontally received component for vertical transmission, and this last phenomenon called into question the validity of t...

Tbis paper considers the scattering of very high frequency 0 electromagnetic waves from a random weakly corrugated surface by the perturbation method. The calculations show that the scattering has a resonant nature, i.e., only certain Fourier components of the surface shape are responsible for scattering in every given direction. Experiments carried out in a water basin confkmed the results of the calculations. The backscattered intensity is proportional to the spectral density of those Fourier components of the surface oscillation that have a resonant space period. In these experiments, resonant maxima of the reflected s i , d corresponding to the second-ordw approximation of the perturbation method were also observed.The frequency spectrum of the scattered electromagnetic field is also investigated. It is shown that the spectrum of the scattered radiation is shifted from the incident frequency by a certain value related to the phase velocity of the resonantly scattering Fourier component of the surface shape. The experimentally observed dependence of the scattered intensity on frequency and the theoretically predicted one are very much alike.

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