Since the bombing of the USS Cole in October 2000 and the attacks of 9/11, border and transportation security have become a priority for the Navy and the Department of Homeland Security, especially for the U.S. Coast Guard. One of the most challenging aspects of port security is providing the means to protect against threats from under the surface of the water as well as surface threats, such as vessels of all sizes, including small, fast boats. In particular, it is felt that a significant terrorist threat might be posed to domestic harbors in the form of an explosive device delivered underwater by a diver using an underwater breathing apparatus, such as SCUBA gear.
Existing hydro-acoustic detection systems may use either active or passive SONAR. Limitations on active SONAR include a high level of false alarms produced by scattering from fish and marine mammals, or the bottom in shallow water, the possibility of being detected by an intruder, the possibility of being harmful to marine mammals, and a possible need to be large and heavy (hundreds of pounds) to be effective. Some passive systems may utilize hundreds of sensors in an array, and may also need to be large and heavy to work effectively, or may need to be deployed more or less permanently over a widespread area where the area of interest is known a priori, and may cover wide reaching areas, but not smaller areas, such as a harbor or breakwater entry, a ship berth or mooring, a remote beach being defended or monitored, etc.