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Searchers looking for the missing Malaysia Airlines plane have discovered signals consistent with those emitted by black boxes in the Indian Ocean, but they may still be a long way from finding the first piece of wreckage. Here's a look at what they've accomplished, and what remains to be done:
The Australian navy vessel Ocean Shield picked up the signals using a U.S. Navy device called a towed pinger locator. It's essentially a long cable with a listening device, or hydrophone, attached to the end. It's pulled behind the boat at a depth of 3 kilometers (1.9 miles).
The pinger locator is designed to detect signals at a range of 1.8 kilometers, meaning it would need to be almost on top of the black boxes to detect them if they were on the ocean floor, which is 4.5 kilometers down.
The first signal was picked up Sunday and lasted two hours and 20 minutes before it was lost as the ship moved forward. The ship then turned around and a few hours later picked up a second signal that lasted for 13 minutes.
In an earlier development, the crew of a Chinese ship told Australian authorities they had picked up possible signals using a less sophisticated hydrophone dangled over the side of a small boat some 555 kilometers (345 miles) away from the Ocean Shield. Another ship is separately investigating further.
U.S. Navy Capt. Mark Matthews said on both occasions the Ocean Shield detected sounds, the pings were emitted about 1 second apart - consistent with those that could be sent from the black boxes: the flight data recorder and cockpit voice recorder.
The signals were broadcast at 33.3 kilohertz rather than the 37.5 kilohertz frequency at which black boxes are supposed to broadcast pings. Matthews said that according to the manufacturer, the frequency of the black boxes can drift near the end of the device's shelf life, which is about six years.
Matthews said the signals are man-made. He said there's always the possibility they're getting a false reading, for instance from a device on their own ship, but they are careful to avoid that.
He said the two signal detections came from points estimated to be about two kilometers apart. Matthews said that water temperature and water layers can distort and refract signals, so they may in fact be from the same area.