The dependence of chirp centre frequency on sonar processor output stability when operating in an estuarine channel

Abstract

In hydroacoustic systems it is desirable to maximise the stability of the receiver processor output in the face of a fluctuating channel. Such systems are particularly vulnerable to the rapid fluctuations of littoral oceanographic environments. In this paper we allude to the likely existence of many ray-paths when operating in shallow waters with complex bathymetries. Experimental data collected from the ForthBase hydroacoustic test range are presented which illustrate the degree of output fluctuations for a coherent processor in a one-way propagation configuration. Modelling the constructive/destructive interference of closely arriving multipath components phenomenologically, we show the dependence of processor output stability on the chosen centre frequency of a linear FM transmit waveform for fluctuating path propagation times. Through this experimental/modelling comparison, we suggest that this temporal instability of individual multipaths is the prime cause of the observed processor output fluctuations.