Detection enhancement using multiple time-reversed guide sources in shallow water

Detection in a monostatic, broadband, active sonar system is degraded by propagation-induced spreading. To mitigate the problem, a technique using multiple guide sources is investigated to improve probability of detection in noisy environments without explicit environmental knowledge. The approach is similar to that used in astronomy for detecting faint stars by observing the atmospheric aberration of brighter "guide" stars. In ocean acoustics, the guide sources can be sonobuoys or explosive charges. The technique was applied to echo detection during the 2004 Naval Research Laboratory Time-Reversal Experiment performed south of the Hudson Canyon off the coast of New Jersey. In this test, 0.25 sec LFMs were transmitted with 500 Hz bandwidths chosen over a 0.5-3.5 kHz range using the NRL 64 element source-receiver array. The transmissions were then echo-repeated by a ship at a range varying between 0.5-5 km. The echo-repeating ship also transmitted one-way, 1 sec, 1 kHz bandwidth LFMs that were used as the guide-source signals. As the ship drifts, multiple guide source signals are received over a volume. These guide-source signals, which contain environmental information and characterize the propagation-induced spreading, form the basis of the technique for improving detection without having explicit environmental knowledge. Using an empirical-orthogonal-function representation of the set of monostatic guide-source signals, echoes are convolved with the time-reversed orthogonal functions as part of a filter bank. In this study, the probability of detection of noisy echoes using multiple guide-source (MGS) signals is compared with a baseline probability of detection using matched filtering. ROC curve improvement using the MGS Alters is obtained using two different detectors. Significant gains in signal-to-noise ratio are obtained for echoes originating outside the volume where guide-source signals were transmitted and at significantly later transmission times than the guide source signals