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Detector Helps Crash Investigation

Photonics Spectra
Dec 1998
Stephanie A. Weiss

Following the Sept. 2 crash of Swissair Flight 111 just off Nova Scotia, Canadian aviation investigators immediately turned to sonar imaging systems to highlight areas that might contain crash debris. The sonar images indicated large areas of dense wreckage but did not provide enough resolution to determine immediate priorities for the divers who would be combing the debris for clues to the cause of the accident.

Mounted in a torpedo-shaped "tow fish," laser-scanning technology developed for underwater mine detection instead aided recovery of Swissair Flight 111 wreckage by retrieving images from the murky ocean floor.

Therefore, shortly after the crash, the Office of Naval Research called the US Navy's Coastal Systems Station in Panama City to ask for a favor: Could the Canadians borrow the high-resolution laser line-scan imager that the station was developing in its Very Shallow Water Advanced Sensors program? "They were in a hurry for us to get up there," recalled John Lathrop, the program's chief scientist.

Coastal Systems Station is the Navy's mine warfare laboratory, doing research on mine countermeasures. The requested device is a trio of 21-in.-diameter imagers: a low-resolution forward-scan sonar, a higher-resolution side-scan sonar and a high-resolution (up to 1/4 × 1/4 in.) laser line-scan imager. All components are housed in a torpedo-shaped "tow fish" that is submerged and dragged behind a moving vessel.

"In mine countermeasures, we would use the sonar first, and when we find a target that's of interest, we can immediately return to it and take a laser line-scan image," Lathrop said.

Ideal wavelength

In the Flight 111 situation, the Canadians already had done the sonar scan and needed only the laser imager. Rather than disassemble the "fish" to bring only the laser section, researchers packed the whole thing onto an Air Force C-17 transport for the trip to the Nova Scotia crash site.

The laser line-scan system uses a frequency-doubled Nd:YAG laser that operates at 532 nm to illuminate a spot on the ocean floor. This wavelength is ideal because light between 450 and 550 nm transmits through water with the least attenuation. The laser spot size determines the system resolution.

Four rotating mirrors scan the spot across the bottom of the ocean, and a photomultiplier tube detects the amount of reflected light. As the fish is towed through the water, the imager's forward movement creates a two-dimensional image much the same way as a computer printer does: one thin slice at a time.

When Raytheon Corp. in Tewksbury, Mass., was designing the system, the main constraint was size. The device had to fit into a cylinder that measures 21 in. in diameter and 30 in. long so that Navy crews could install it in many different types of towed or remotely operated craft.

"We've designed everything so that we can mix and match them and put them in vehicles of all kinds," Lathrop said. "It would be possible to run the laser line-scanner section by itself if we wanted something particularly compact."

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