Evidence for non-Rayleigh characteristics in ship underwater acoustic signatures.

Abstract

This work examines ship underwater acoustic signature amplitude statistics and statistical distributions and explores the hypothesis that ship signatures exhibit amplitude fluctuations that are different from Rayleigh-distributed ambient ocean noise. Signature measurements from a small oceanographic ship conducting a variety of maneuvers are examined, focusing on those conditions where propeller cavitation and broadband signal modulation occur, specifically during maximum speed runs and turning maneuvers. A key difference for a ship signature is the amplitude modulation generated by propeller cavitation, and this is found to be associated with super-Rayleigh signal characteristics. Cyclostationary processing techniques are used to estimate propeller shaft- and blade-rate modulation. Under conditions of clearly observable propeller modulation, time-series statistics scintillation index and skewness showed values significantly in excess of Rayleigh-distributed values. Ship signature amplitude probability density functions were found to be better matched by a K-distribution model with small shape factor, indicating clearly non-Rayleigh behavior. Evidence for a secondary, high-amplitude, lognormally distributed sound generation mechanism was observed during maximum helm turns.