Physical Model Studies of Bistatic Surface Scattering.

Abstract : An extensive series of measurements of the scattering strength of a wind driven surface at an acoustic frequency of 1300 kHz were made in a model tank. The measurement were made using short, relatively narrow band probing signals under computer control. Data were converted to scattering strength using a flat surface reference and compensation for transducer beam patterns and data acquisition timing. A number of results were reported by this study in the form of observational conclusions regarding the magnitude and parametric sensitivity of scattering strength at short wavelenghts. A subsequent investigation concerned itself with the measurement of frequency spreading in forward scattering. A new technique of channel probing using wideband test signals transmitted at frequent regular intervals was used to obtain estimates of frequency spread at a variety of wavelengths simultaneously. Rayleigh roughness parameters varying from 0.7 to 12. were studied both crosswind and downwind. The most dramatic result of the study was the demonstration under controlled conditions that spreading under conditions of exactly symmetric geometry is not necessarily symmetric about the insonification frequency. This report contains some additional frequency spreading measurements which correct errors in the originally reported data and test the sensitivity of the data to small alignment errors.