Bragg Scattering of Electromagnetic Waves from the Air/Sea Interface

The advent of radar during World War II had consequences for research in air/sea interactions which were far from obvious in the face of the immediate wartime need to detect enemy aircraft. When the newly-discovered tool was put to use to detect targets on or near the surface of the sea, these targets were often obscured by strong echos from the ocean itself. This “sea echo” constituted a considerable nuisance to those engaged in locating enemy vessels so investigations into the nature of this unwanted return were launched. Early theoretical work concentrated on explaining sea echo in terms of either return from the sea surface itself or from the spray and bubbles above the surface. Methods of modelling electromagnetic scattering which had proved useful to the characterization of many types of targets were adapted for use modelling sea return. Specular reflection seemed to explain the characteristics of sea echo for small incidence angles but not for larger ones. The standard method of carrying out such calculations utilized the “Kirchoff principle,” also known as physical optics or the tangent plane method, which assumes that the surface is smooth in the sense that the radius of curvature is large compared to the electromagnetic wavelength. In addition to yielding disturbingly low return at large incidence angles, the method was unable to account for the observation that vertically polarized return was generally stronger than horizontal at such angles. Since interference between radiation directly incident on bubbles above the surface and radiation incident on the bubbles following reflection from the surface could in principal yield such polarization differences, theories based on Rayleigh scattering by the bubbles were also proposed.