Navy using optical sonar sensors to enhance submarine detection and alleviate maintenance problems

Sonar experts from the Northrop Grumman Navigation Systems Division in Woodland Hills, Calif., envision a vast ocean-floor optical sensor array that can detect and track some of the world's quietest submarines through minute phase shifts of light.

By John Keller

WOODLAND HILLS, Calif. — Sonar experts from the Northrop Grumman Navigation Systems Division in Woodland Hills, Calif., envision a vast ocean-floor optical sensor array that can detect and track some of the world's quietest submarines through minute phase shifts of light.

This array, which U.S. Navy leaders call the All Optical Underwater Segment — or AO-UWS — is to be deployed in strategic ocean areas that either funnel heavy submarine traffic, or where pinpointing hostile submarines is crucial.

The AO-UWS could go on line as early as 2004, says Jim Andersen, director of business development for fiber optic acoustic systems at Northrop Grumman Navigation Systems.

Northrop Grumman engineers are designing the AO-UWS optical sensor array under terms of an $8.9 million contract from the U.S. Space and Naval Warfare Systems Command (SPAWAR) in San Diego. The 24-month development program will culminate in an at-sea demonstration of the system sometime in 2003.

Among the chief advantages of underwater optical sensors are sensitivity and reliability, Andersen says. Optical sensors are to augment or replace existing underwater arrays of electronic hydrophones that are part of the U.S. Navy's Integrated Undersea Surveillance System — better known as the IUSS.

"This system is for deep water, so you could deploy it anywhere," Andersen says. "It will be large fields of sensors — hundreds to thousands of sensors — that are used for ASW- [anti-submarine warfare] type detection."

The sensor array and signal-transmission media will be all-optical components — manufactured from plastic or silica, which is not susceptible to the corrosive influences of saltwater, Andersen says. Electronic components, on the other hand, can suffer corrosion or short-circuits in seawater, which present the Navy with a persistent maintenance headache.

"The advantage is there is no electronics in the water, or on the 'wet end', so the stuff we put in the water is very reliable" Andersen explains. "All the electronics is on shore or on a ship, which can be mounted in COTS [commercial off-the-shelf] versions; you don't have to package electronics for the water, and you can rely much more on standard COTS electronics."

The AO-UWS sensor array detects phase shifts in light waves caused by the sounds of submarines. "We put a carrier frequency on the light beam, and at the sensor the acoustic pressure signal will shift the phase of the light," Andersen says. "It is these phase shifts that we pick up."

The AO-UWS works by connecting arrays of optical sensors with optical fiber. These interconnected arrays, in turn, connect via optical fiber with signal-processing gear on shore, aboard ships, or inside submarines.

"Once we convert the optical signal back into a voltage signal, we format it to whatever someone might want, such as fast Ethernet," Andersen says. "We rely on the signal-processing people to process it however they want. We just give it to them in the right format for the processing."

Two subcontractors are working with Northrop Grumman to develop the AO-UWS — MariPro Inc. of Goleta, Calif., which is concentrating on mechanical wet-end junctions, terminations, and packaging; and Digital System Resources (DSR) Inc. of Fairfax, Va., which concentrates on interfacing to existing Navy sonar processing systems.

The AO-UWS is to become part of the Navy's Integrated Undersea Surveillance System, which includes the Sound Surveillance System (SOSUS), Fixed Surveillance System (FSS), the Fixed Distributed System (FDS), the Surveillance Towed Array Sensor System (SURTASS), and the Advanced Deployable System (ADS).

Northrop Grumman sonar experts envision the AO-UWS as a complement or replacement for the Fixed Distributed System, Andersen says. The FDS is a fork-shaped array of ocean-floor sensors that not only detect the sounds of submarines, but also help analysts use triangulation to gather range and bearing information.

Northrop Grumman is also using optical sonar sensors on submarines such as the future Virginia-class new attack submarine to enhance submarine-detection capabilities, and also to reduce maintenance tasks.

With optical sensors on submarines, "the beauty is the previous system had electronics external to the pressure hull," Andersen explains. "It is difficult to maintain them, even in port. We had to dry-dock them. Now we have optical fiber outboard and the electronic maintenance issues involve simply changing out a printed circuit card. They can do that even underway."

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