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THE PORT OF NEW HAMPSHIRE, a critical regional commerce center and home to the Portsmouth Naval Shipyard, plans this summer to install the country’s first underwater intrusion detection system at a civilian port. The system is intended to greatly enhance the port’s ability to deter or detect any terrorist activity beneath the surface.

The system’s setup costs are covered by a Department of Homeland Security (DHS) Port Security Program grant of $1.18 million. In 2006, Deputy Chief Harbormaster Grant Nichols initially sought funding only for a small, video-enabled remotely operated sub (ROV), and a 35-foot landing craft from which to operate it. Administrators planned to use the sub for random or threat-based inspections of vessels, bridges, and dock infrastructure to spot weapons, such as explosive devices, and smuggled goods, especially narcotics.

But then Nichols met Harvey Woodsum, president and CEO of the Bedford, New Hampshire-based Bayshore Technology Labs, who was then developing surveillance applications for sophisticated sonar arrays. Nichols submitted a revised request for added grant funding to enable it to buy the sonar array. The New Hampshire Division of Ports and Harbors won the grant, with enough funding for Bayshore’s array, the patrol vessel, ROV, and a wireless connection between the sonar system and port stakeholders.

As with most sonar arrays, the system offers both passive mode, in which hydrophones listen for suspicious noise in the water, and active mode, in which the system emits sound and the hydrophones listen for an echo. The system finds the object by determining the distance the echo traveled.

Should Bayshore’s array detect a suspicious object moving under water, it can transmit a focused beam of sound for more precise tracking. The sound’s relatively low frequency offers an effective range of up to 1.2 statute miles; the signals are easily read without special training.

Perhaps most remarkable is the array’s capability as a vocal annunciator. The same technology that sends the tracking beam can transmit an alarm and verbal warning, audible underwater to the suspect object, which may be a diver or divers.

The system is also easy to use. In the past, reading sonar displays was the sole domain of trained experts. With the new system, suspicious objects will appear on cockpit displays in real time on standard, electronic nautical charts of the harbor. Data will be transmitted wirelessly to terminals on stakeholder vessels, and be accessible via a secure Web-based portal.

The 30-foot landing craft and ROV will serve as a rapid response vehicle for threats detected by the system. It can also be used in standard searches.

DHS consistently highlights the threat of underwater attacks, but some terrorism experts question the likelihood of trained terrorist scuba divers executing a sneak attack on a harbor.

Nichols calls the array’s price tag a “drop in the bucket” next to the cost of a successful attack on the commercial port, the naval shipyard, or the Piscataqua River Bridge, which carries Interstate 95 over the harbor.