Time-varying multipath interference of broad-band signals over a 7-NM range in the Florida Straits

Abstract

The frequency response function of a propagation channel is formulated by the coherent (vectorial) summation of CW signals arriving via many ray paths between source and receiver. Six months of continuous measurements of water temperature from a moored thermistor string in the Florida Straits are analyzed to determine the temporal variation of the sound speed profile. The observed average profile and its temporal variations are shown to be closely modeled by fitting the data with two layers of linear sound speed gradient. In order to investigate the effects on propagation of temporal variations of the two layers, a model of Refracted Bottom-Reflected (RBR) propagation is developed for the particular case of a bottomed source and receiver. Model computations determine the relative influence of the parameters of the two layers upon the interference of broad-band signals. The general features of multipath interference which are observed in the Florida Straits are presented and discussed. Model simulations of multipath interference are used to interpret experimental results. Some characteristics of the propagation channel (e.g., bandwidth of an interference fade) are shown to depend on average values of the model parameters, while others (e.g., CW amplitude) depend on small fluctuations about the averages and thus cannot be estimated with idealized propagation models. For typical profiles the gradient near the bottom determines the average properties of the channel frequency response function.