Passive acoustic tracking using a library of nearby sources of opportunity.

A method of localizing unknown acoustic sources using data derived replicas from ships of opportunity has been reported previously by Verlinden, Sarkar, Hodgkiss, Kuperman, and Sabra [J. Acoust. Soc. Am, 138(1), EL54-EL59 (2015)]. The method is similar to traditional matched field processing, but differs in that data-derived measured replicas are used in place of modeled replicas and, in order to account for differing source spectra between library and target vessels, cross-correlation functions are compared instead of comparing acoustic signals directly. The method is capable of localizing sources in positions where data derived replicas are available, such as locations previously transited by ships tracked using the Automatic Identification System, but is limited by the sparsity of ships of opportunity. This paper presents an extension of this localization method to regions where data derived replicas are not available by extrapolating the measured cross-correlation function replicas onto a larger search grid using waveguide invariant theory. This new augmentation provides a method for continuous tracking.