The portrayal of body shape by a sonar or radar system

When acoustic waves are incident upon a rigid body or electromagnetic waves are incident upon a perfectly conducting body, the amplitude of the scattered radiation usually varies in a very complicated manner with direction of incidence, with direction of observation and with frequency. Even if consideration is restricted to back-scattering, the resulting scattering directivity curves appear to offer no clue to the shape of the scattering body. Yet the visual identification of objects by scattered radiation is an everyday experience. This paper endeavours to clarify the mechanism whereby the eye perceives shape and to see how this is applicable to methods of display in sonar and radar equipment. The analysis, which is qualitative, is based upon a recent theory of the mechanism of scattering of short wavelength acoustic radiation by a rigid body immersed in a fluid medium, and its electromagnetic counterpart.