The spectrum of X-band radiation backscattered from the sea surface

Abstract

A coherent radar was used to measure the ‘sea clutter’ or backscattering of X‐band electromagnetic energy from the sea surface. More than 200 recorded samples of clutter were analyzed to give power spectra of the clutter. Each spectrum was displayed as a function of frequency and of position on the water surface and was also averaged to give the mean spectrum of patches 3750 feet long. Five of the samples showed an anomalous downwind displacement of the clutter by as much as 7 knots. The displays indicate again, as in earlier Control Systems Laboratory measurements, that the upwind edge of the clutter spectrum is smooth for all wind speeds observed, but that the downwind edge, for sea state 3 or above, is broadened in an irregular fashion as a function of range. This irregular broadening implies a considerable variability, from patch to patch, in the downwind side of the probability distribution of velocity of scatterers on the sea surface. The width at half‐power of a mean spectrum is proportional, for reasonable assumptions, to the width at half‐maximum of the probability distribution of scatterer velocities. The variation of the latter width, Δ0, with sea state can be represented by the equation (expressed in consistent units) Δ0=11H1/3/Tm where the numerical factor is dimensionless, H1/3 is the significant wave height, and Tm is the period corresponding to the maximum of the energy spectrum for the water waves themselves when this spectrum is plotted as a function of frequency. This equation fits the experimental data within about 10 per cent for bandwidths in the range of 2 to 5 knots (64 to 160 cps) and wind speeds in the range of 8 to 19 knots. The bandwidth of the clutter is found to be approximately proportional to the wind speed. The relationships of clutter bandwidths to wave and whitecap velocities, radar depression angle, and wind direction are also discussed.

Very high frequency radiowave scattering by a disturbed sea surface Part II: Scattering from an actual sea surface

“…L. Hicks et ut [34]. empirically established the proportionality of the spectrum width to H/T, while in our paper it follows from the theory; numerical comparison with the experimental data shows that in order to adequately explain the total spectrum width, it is necessary to take into account the fluctuations of wind-drift velocity.…”

Scattering of Electromagnetic Waves From a Slightly Rough Surface Moving With Uniform Velocity

“…The mean Doppler frequencies for horizontal polarization were greater than those for vertical polarization for the same sea conditions. Pidgeon's measurements for horizontal polarization strengthened the explanation for seaecho, that Hicks et al (1960) used in their paper. However, the sea-echo for vertical polarization, must then be due to a different mechanism.…”

“…From the measured Doppler specrra, he concluded that the return could be explained in terms of trains of waves and not by a continuous wave (thus he found the sea is not homogeneous). Hicks et al (1960) obtained with an X-band radar, horizontally polarized, that sea-echoes seemed to come from two sets of scatterers: a set of sea waves of unbroken water, of a period of one-half the wavelength of the radar frequency (slightly rough surface scattering), that produced a strong and symmetrical Doppler spectrum with a mean Doppler frequency equivalent to the particle motion of larger sea waves and a second set of scatterers, of wind driven irregularities, composed of broken water (white caps) which contributed an asymmetrical Doppler spectrum which was only half as strong in amplitude as that from the previous set.…”

The effect of temporal correlation with K and KK-distributed sea-clutter

“…Regarding the latter, Hicks et al [17] has shown experimentally that the Doppler spread at high grazing angles is proportional to the wave height divided by the wave period. This relationship has been verified with the Ingara data in a previous publication, [18].…”