- U.S. Navy commander: Initial optimism over pulse signals becomes more cautious
- Search officials call new pulse signals the best indication so far they are on the right track
- Without wreckage from the missing plane, little is certain
- Finding an answer will take time, officials warn
More than a month after Malaysia Airlines Flight 370 disappeared, searchers say pulse signals detected in the Indian Ocean provide their best hope yet for finding it and unlocking clues to what may have happened.
However, those same officials warn it will take time to determine whether the sonar pings come from the missing plane.
The information raises questions about what the pulse signals mean and what happens now.
Is this it?
An Australian ship using high-tech U.S. equipment has twice detected signals along the suspected flight path of the airliner off its country's western coast.
Angus Houston, who heads the rescue effort, told reporters that a device called a towed pinger locator being pulled by the vessel Ocean Shield detected signals similar to those emitted by flight data and cockpit voice recorders.
The first detection, which occurred over the weekend, lasted more than two hours before the ship lost contact, Houston said. A second detection several hours later lasted 13 minutes, and included two separate signals audible to the locator device, he said.
Two signals could mean that one came from each of the so-called black boxes.
"It's probably the best information that we have had," said Houston, who quickly added, "We haven't found the aircraft yet; we need further confirmation."
Why this may be it
The signals were near the standard 37.5 kHz frequency used by the recorders, a frequency chosen to limit the possibility that it would be confused with other ocean noises, officials said.
They were detected along the missing plane's estimated flight path, which was calculated based on its direction and fuel capacity and which has helped narrow the search area.
"With the acoustic events that we're getting in the area, we are encouraged that we're very close to where we need to be," Houston said. "This is quite an extraordinary set of circumstances that we're now in a very well-defined search area, which hopefully will eventually yield the information that we need to say MH370 might have entered the water just here."
On Saturday, a Chinese ship reported that it detected a single pulse signal more than 300 miles farther south, also near the most recently projected flight path. Houston said the distance between the two reports made it unlikely the Chinese ship and Australian vessel detected the same signal, but added "in deep water, funny things happen with acoustic signals."
Why this may not be it
Officials have said that, without wreckage, the odds of finding the plane would be slim.
For now, the pulse signals are the most promising leads, said Houston, who cautioned that he was not certain that they were from the plane.
"What I'd like to see now is us find some wreckage because that will basically help solve the mystery," Houston said. "Without wreckage, we can't say it's definitely here."
Oceanographers note that the ocean is full of sounds that may include whale calls and locator signals emitted by research equipment left on the bottom. While the frequency used by the black box signals is intended to be unique, it can still be confused with other ocean sounds, they note.
"Unlike in air, where sound travels in a straight line, acoustic energy -- sound through the water -- is greatly affected by temperature, pressure and salinity," said Peter Leavy, commander of the military task force conducting the search. "And that has the effect of attenuating, bending -- sometimes through 90 degrees -- sound waves. So it is quite possible and very hard to predict -- it's quite possible for sound to travel great distances laterally but be very difficult to hear near the surface of the ocean, for instance."
On Monday, U.S. Navy Commander William Marks told CNN from the search operation that the failure of Ocean Shield's pinger locator to find the signals again -- more than a day since the last reception sparked optimism -- had led him to become "more and more cautious."
The Ocean Shield and its towed pinger locator continued Tuesday to search the area where it had detected the signals. If they do hear it again, searchers would send out a Bluefin-21 autonomous underwater vehicle with a more accurate sonar and possibly a camera for mapping the ocean floor, Leavy said.
"At the moment that's not deployed," he told reporters. "The focus is on trying to reacquire the acoustic signal that they had."
Meanwhile, the website MarineTraffic.com indicated Monday that three ships were searching a spot farther south, where the Haixun 01, a Chinese patrol boat, reported having detected pulse signals on Saturday.
Another Chinese vessel and the HMS Echo of Britain's Royal Navy joined the Haixun 01 in the area, according to the website, which has been reliable in reporting the movements of search vessels.
Officials had said they would send additional resources to help the Haixun 01 try to find the source of pulse signals it detected on the same 37.5 kHz frequency used by airplane recorders.
A looming question is how long the batteries in the recorders will last. They are certified to last 30 days once activated by immersion in salt water. Tuesday was day 32.
"We hope that it keeps going for a little bit longer," Houston said.
Confirmation that the signal comes from the Boeing 777-200ER's locator beacon would mean "the possibility of recovering the plane -- or at least the black boxes -- goes from being one in a million to almost certain," said Simon Boxall, a lecturer in ocean and earth science at the University of Southampton.
Houston, a retired Australian Air chief marshal who is chief coordinator of the Joint Agency Coordination Centre, warned against expecting a quick resolution.
"It could take some days before the information is available to establish whether these detections can be confirmed as being from MH370," he said. "In very deep oceanic water, nothing happens fast."