The buoys each have a hydrophone listening device that dangles about 300 meters (1,000 feet) below the surface and their data are sent via radio back to a plane, Royal Australian Navy Commodore Peter Leavy said.
The Ocean Shield was still towing its pinger locator to try to find additional signals on Friday, and the Orions were continuing their hunt, Houston said. The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the ocean floor, about the size of the city of Los Angeles.
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"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active," Houston said in a statement.
The searchers are trying to pinpoint the exact location of the source of the signals so they can send down a robotic submersible to look for wreckage. Houston said Friday that a decision to send the sub could be "some days away."
The Bluefin 21 submersible takes six times longer to cover the same area as the pinger locator being towed by the Ocean Shield and would take six weeks to two months to canvass the current underwater search zone.
Complicating matters is the depth of the seabed in the search area. The signals are emanating from 4,500 meters (15,000 feet) below the surface, which is the deepest the Bluefin can dive. The search coordination center said it was considering options in case a deeper-diving sub is needed.
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Meanwhile, the center said the surface area to be searched for floating debris had been narrowed to 46,713 square kilometers (18,036 square miles) of ocean extending from 2,300 kilometers (1,400 miles) northwest of Perth. Up to 15 planes and 13 ships were conducting the visual search Friday, west of the underwater search based on expected drift from the suspected crash site.
Investigators believe the plane went down in the southern Indian Ocean based on a flight path calculated from its contacts with a satellite and analysis of its speed and fuel capacity.
Separately, a Malaysian government official said Thursday that investigators have concluded the pilot spoke the last words to air traffic control, "Good night, Malaysian three-seven-zero," and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.
The senior government official spoke on condition of anonymity because he wasn't authorized to speak to the media.