Distributed arrays of acoustic sensors with elements less capable than the mobile UUVs suggested above are also of great interest for littoral or shallow water (100 to 500 m) area surveillance. Deployed like sonobuoys and drifting or moored in place, each node is envisioned to possess considerable on-board signal and data-processing resources and to be capable of passive automatic detection and classification as well as active signal processing. These individual capabilities, complemented by the networked acoustic/RF communications, would permit them to operate as a single large and very capable coherent distributed sonar. A typical multistatic configuration might consist of a few active sources and as many as 10 to 100 “smart” sonobuoy-like nodes distributed broadly over the area under surveillance.
In addition to the loosely coupled multistatic systems of active sources and smart sonobuoys, various physically connected drifting or moored arrays are under consideration. The autonomous drifting line array would consist of a very-low-frequency passive array of up to 100 hydrophone elements, drifting freely in the ocean currents. Data from the linear, but almost certainly not straight, aperture would be processed on board the array through battery-powered computer resources and would report detection/classification information back to the decision makers via an RF link as necessary.
Other concepts envision similar autonomous arrays moored in shallower water. Key to the success of all these concepts is the availability of significant local on-board sensor processing supported by long-life batteries.
In spite of our technology, animal sonar, as utilized by bats and porpoises, far exceeds our capabilities for precise location and target classification. Operating in complicated environments, in the presence of perhaps dozens of competing individuals, these animals emit complex, very broadband sonar pulses that they change rapidly as they move from detection to classification and final capture of prey, apparently adapting their internal signal-matched filter on a pulse-to-pulse basis, easily sorting out their own signals from those of other bats or porpoises.
Since the early 1990s, building on pioneering university studies of bat sonar, the Navy has sponsored efforts to develop a biologically inspired, ultra-wideband sonar, using multioctave signals and multichannel nonlinear processing with coherent recombination. Computer studies applying such processing to existing narrowband field data have already demonstrated an encouraging reduction in false alarms. The development of transducers capable of emitting the desired ultra-wideband signal is under way, and it is hoped that soon we will be able to duplicate in the littorals at least some of the extraordinary capabilities common in nature.