Sensor-enhancing software helps detect diesel submarines.

U.S. military planners have become concerned that rogue states or terrorist groups may acquire Russian Kilo-class, diesel-electric submarines and equip them with "air independent propulsion."

Diesel-electric submarines using air independent propulsion can remain submerged for extended periods of up to two or three weeks, and unlike nuclear submarines, diesel-electric submarines can "bottom out" or rest on the ocean floor.

Since shallow coastal regions are complex and noisy, the detection of such diesel-electric submarines is not possible using traditional acoustic methods.

With air-independent propulsion, the fuel source and reused exhaust gases are combined in a closed loop to generate a submarine's power. Current power plants include closed-cycle diesel engines and, more recently, fuel cells.

This form of propulsion, while not used by the United States, Britain or Russia, is used by smaller navies. It is considered ideal for small vessels, is cheaper to operate and makes the submarine more difficult to detect.

Diesel-electric submarines can provide a formidable challenge to current surveillance systems. The high ambient noise levels from local shipping traffic and marine life make passive sonar detection almost impossible in littoral waters.

The challenge of detecting diesel-electric submarines is best illustrated by an example from the Falklands War. During that conflict, the British Royal Navy could not defeat a single Argentinean diesel-electric submarine, although the British were highly experienced and released more than 150 weapons with no hits scored.

Low-frequency active sonar technology, such as the surveillance towed-array sensor system, may be an effective alternative to passive detection, but is politically disadvantaged because it has been reported to harm marine mammals.

Detecting submarines in littoral waters is comparable to locating landmines. It requires fusing data from a variety of sensors to pull the hidden signal out of the noise.

One of the technologies now being used by the German and Swedish navies to counter the threat of quiet submarines is a software architecture called the scalable generic signal processor, or scalable GSP.

The German Navy selected the scalable GSP technology for its new underwater acoustic analysis system, to be located at the Federal German Navy Hydroacoustic Analysis Centre.

The analysis system was designed to analyze the sonar records of the new U212A submarines, the U206A submarine, maritime patrol aircraft and fleet vessels.

The scalable GSP technology is built on an open-architecture cluster of parallel, off-the-shelf, general-purpose processors using Linux, an open-source operating system. The term "scalable" describes the ability of the control software to recognize and utilize the available processors, whether one, 10 or hundreds.

The data-processing rate may be increased with the addition of processors, without a need to re-write or recompile the software. Conversely, single processor failures simply reduce processing speed. Users can add their own software modules to meet specific needs, such as the introduction of classified algorithms.

The scalable GSP employs a "Lego" style technique that allows users to graphically modify existing applications or to construct new applications with building blocks of processing functions.

The technology has been adopted internationally for sonar applications, and has been used in synthetic aperture radar processing.

Brian Markle is the chief technology officer of Array Systems Computing Inc., a supplier of scalable generic signal processor technology, based in Toronto, Canada.