On polarization features of radio signals scattered from the sea surface at small grazing angles

Abstract

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...

What are the mechanisms for non‐Bragg scattering from water wave surfaces?

“…His observation, reported in volume 13 of the MIT Radiation Laboratory Series, is probably the earliest documentation of superevents observed in microwave backscatter. From the mid-1960s to the mid-1970s, Pidgeon [1968] documented the separation of VV and HH Doppler peaks; Mel'nichuk and Chernikov [1971] noted the failure of the Bragg scattering mechanism to explain small grazing angle backscatter; Long [1974] correlated return spikes with wave breaking; Leykin et al [1975] noted that at small grazing angles, the horizontal polarization returns are mainly due to backscatter from the crest region of larger waves and stated that breaking wave crests contributed significantly to the horizontal returns; and Kalmykov and Pustovoytenko [1976] documented the observation of spiking and superevents. In 1991, Jessup et al [1991] observed superevents but tentatively attributed the signals to wedge scattering, "random fluctuations in the measurements or the effects also the dominant non-Bragg scattering mechanism for fast scatterers at small grazing angles.…”

Space‐time properties of radar sea spikes and their relation to wind and wave conditions

“…Field observations of low-grazing angle backscatter and its potential relation to wave breaking date back over 30 years [see Pidgeon, 1968;Kalmykov and Pustovoytenko, 1976;Trizna, 1991]. Many observations have focused on characterizing the amplitude distribution of sea clutter for the purpose of rejecting it in surveillance radar applications.…”

Investigation of Low-Grazing-Angle Microwave Backscattering From Threedimensional Breaking Sea Waves

“…According to the photographs and data taken in the field experiments [14,24], the single sharply crested unbroken wave can be approximately simulated with the dihedral wedge of finite length shown in Figure 1. Different from the infinite two-dimensional wedge in the previous literatures, the dihedral wedge is characterized by internal angle β, length d, width 2l and height h. Since the size of the wave crest is related to incidence microwave, d and 2l are presented in wavelength of incidence wave λ.…”

“…According to the measured data and the photographs of the sea surface at LGA, Kalmykov and Pustovoytenko assumed that the wedges could approximate the sharp crests, which led to the increase of polarization ratio (HH/V V ) [24]. Kwoh and Lake also suggested that the dominant scattering from the wave crest could be approximated by wedge scattering and analyzed the relationship of scattering fields from capillary wave and the wedge-shaped crest with small perturbation method (SPM) and geometrical theory of diffraction (GTD) respectively [25].…”