Peter G. Cable
BBN Systems and Technologies, Union Station, New London, CT 06320
Under certain conditions the multipath time delay and frequency spread of signals propagating in shallow water can be described usefully with the ocean modeled as a randomly time-varying linear filter. Starting with an explicit physical model for the time-varying transfer function of a shallow water channel, an alternative system function, the delay-Doppler spread function, can be defined and its autocorrelation determined. A particularly simple form of the spread function autocorrelation, the scattering function, results when the propagation multipaths are uncorrelated and locally stationary in the wide sense. When the scattering function description is valid, signal propagation input--output relations can be expressed in terms of the medium scattering function and the input signal ambiguity function. The scattering function formulation will be reviewed and shown to be most useful for channels supporting many boundary-interacting paths (e.g., shallow water) at mid or high frequency. Using the scattering function the influence of the medium on different sonar signal types will be demonstrated and methods for measuring channel characteristics indicated. Channel characteristics determined from data obtained during ACT II on the New Jersey shelf under downward refracting propagation conditions will be presented and discussed in terms of the scattering function description. [Work supported in part by ARPA.]