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toward more precise characterization of acoustic signatures using passive acoustic receivers in very lono arrays. very orecise frequency analysis of received signals, and ~ _ _ _ _ _ _ , _ , , , , , , , , _ long integration times. The principal objective has been to maximize the signal to noise ratio of steacly narrow band signals. Receiver arrays placed on ship hulls, towed arrays, and sonobuoys released from ships or aircraft all shared these characteristics, with optimal band widths ranging from ten to a few hundred Hz. Passive systems were complemented by active systems using frequencies of a few kHz, which were almost exclusively reserved for final localization of the foreign submarines. During this period, use and support of operational systems for environmental science has also evolved. Transmission, reverberation, propagation, and reflection studies have been conducted over much of the northern hemisphere using frequency bands similar to those used by the Na\ry. ASW RESEARCH AND DEVELOPMENT Speakers at the symposium emphasized that the traditional approach described above has been rapidly altered (to the extent permitted by available systems) in the last few years, in response to the changed threat from acoustic quieting. Both operational Navy personnel and system developers have recently concentrated on low frequency passive, low frequency active, and full spectrum detection. ASW research, development, and operations still emphasize acoustics and will continue to do so, given the governing laws of physics. The dramatic change in acoustic signatures of foreign submarines, however, has prompted an interest in monitoring submarines by non-acoustic methods. Academic speakers identified major research and development efforts that have been made in the following areas: characterizing the full spectrum of submarine acoustic emissions, creating a national capability in structural acoustics to help in submarine design as well as analysis of target reflections, understanding the limits of passive long-range detection (centered on the Navy High Gain Initiative), understanding the performance of very loud (220+ db) low frequency active signals, and providing improved environmental predictive and analytical tools and models that help operators understand system performance. 2

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As identified by Navy speakers, new major systems under development include fixed distributive systems, tactical and surveillance multi-scale low frequency active sonars, new ASW patrol procedures, automated detection systems, and tactical environmental forecasting (TESS). - . Research examining the effect of the environment on acoustic signals has particularly emphasized: . Sub-bottom propagation of very low frequency energy, Bohom, surface, and volume reverberation at 50-1 00 Hz, Matched-field processing, and accompanying descriptions of transmission characteristics, Mesoscale environmental predictive models and accompanying acoustic models, Tomography, a potential validation too! for the models and a technique for determining propagation characteristics over large areas, and Fast acoustic performance prediction models. The naval officers were optimistic about their improved detection ability, particularly their ability to cletect new signal types. They emphasized that young, inexperienced operators must be able to use tactical oceanographic systems in a complex and confusing environment. Tactical oceanography products must be in summary form and readily accessible (through personal computers or simple graphs) to a busy non-scientist. ACADEMIC-NAVY COOPERATION Navy laboratory and academic representatives spoke at length about the extensive shore-based environmental measurement and prediction systems and about large-scale oceanographic experiments. These experiments are important in producing the data for Navy operations as well as for defining system characteristics. Although symposium participants were reminded that there was more to the Navy than ASW, and more to ASW than acoustics, virtually all the talks reflected the importance of ASW in Navy operations. Many speakers reiterated that the changed Soviet threat has prompted greater interaction between acoustic and oceanographic research communities than there has been for two to three decades, but this collaboration could still be improved. Ocean physical and acoustic characteristics are 3