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Experimental data on radar sea clutter are described and discussed with reference to the physical principles involved. It is shown that the scattering elements are illuminated by a combination of direct and indirect fields, the latter being "reflected" by surface elements closer to the radar. At small depression angles these interfere destructively, giving rise to the so-called "critical angle," polarization dependence, "spikiness," and steeper frequency dependence. A theory is developed in which the elemental scatterers are taken to be the small patches, or "facets," of the surface. It is found that the facets which back scatter most effectively at small depression angles are those whose dimensions are of the order of the wavelength, and that the frequency dependence of the scattering parameter Â¿Â° at small depression angles is determined by the size distribution of the facets. If the size and slope distributions of the facets are independent of each other, then, below a high-frequency limit, Â¿Â° has essentially the same frequency dependence at low angles and high angles. The high-angle variation of Â¿Â° with angle is determined mainly by the slope distribution of the facets. With the slope distribution determined from optical measurements by Cox and Munk, Â¿Â° is approximately proportional to wind speed at small depression angles, but inversely proportional to wind speed at vertical incidence. Using measured values of Â¿Â°, the facet mechanism accounts for scattering by the entire surface of the sea.