Frequency scanning non linear sonar performance modelling

Non-linear sonar allows high resolution sidelobe-free bathymetric surveillance over an exceptionally large frequency bandwidth. When one fixed and one frequency-scanned channel are transmitted simultaneously, mixing in the water produces a low, scanned, difference frequency signal. In a system being developed, the technique produces an ocean floor reflection signature with more than a decade bandwidth. The difference-frequency source level varies in an extremely non-linear manner with both frequency and primary-frequency power. This variation must be known in order to correctly interpret the frequency-scanned returned signals. Consequently a source level prediction model was developed. In addition to predicting the frequency variation of source level, the model reveals an optimum primary frequency power at which the non-linear conversion efficiency is maximum. The benefits and limitations of operation near the optimum power are discussed, together with the predicted signal-to-noise ratio versus secondary frequency, for various ocean depths.