The search for the missing Malaysia Airlines plane has been narrowed to the southern Indian Ocean by what experts say is an "unprecedented" use of satellite technology.
Satellites detected the possible route for Flight 370 after losing contact with ground controllers. And satellites have spotted possible debris, offering leads to planes and ships searching an area larger than West Virginia.
After British satellite company Inmarsat refined its findings to say the plane likely headed over the remote ocean, Malaysian Prime Minister Najib Razak announced the Boeing 777 crashed and all 239 occupants were dead.
But no wreckage has been found from the plane. The more than two-week search has been difficult because the transponder and routine maintenance equipment stopped signaling about 40 minutes into the flight March 8 from Kuala Lumpur to Beijing. Radar coverage in the area was patchy.
An Inmarsat satellite orbiting above the Indian Ocean received hourly signals from the plane that officials called a "ping" or a "handshake" from Flight 370, in case it needed to reach a satellite. The signals that lasted seven hours suggested how far away the plane was from the satellite, which gave a long arc of possibilities to search.
In its further detective work, Inmarsat studied the so-called Doppler effect, the name for what makes the noise from an approaching vehicle sound different from one speeding away from a listener.
Inmarsat studied the compression and expansion of waves from the plane to determine whether it was moving toward or away from the satellite, and from there determined it was heading over the Indian Ocean, Chris McLaughlin, Inmarsat's senior vice president for external affairs, told CNN.
That's what suggested the plane headed south, rather than north over land. McLaughlin called the technique "ground-breaking" for satellite signals but said it relied on traditional math and a process reviewed by experts at Boeing and in space research.
John Hansman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology, said the case is unusual because there is usually a lot more information to track or plane – or none at all.
Typically, a plane's transponder will send out signals about its location, and airliners have maintenance systems that relay information about their engines and other equipment to their airlines or manufacturers. But in this case, those two systems stopped signaling, either because they were turned off or damaged during the flight.
"I think that it is remarkable that they have been able to piece as much together as they have from the very limited Inmarsat data," Hansman said. "I think that this case is essentially unprecedented."
The high-tech search has also benefited from satellites from Australia, China and France that spotted apparent debris floating on the ocean. But planes and ships navigating sometimes bad weather have not found debris from the pictures, which are often days old.
NASA has contributed to the search since March 11, when Administrator Charles Bolden assigned the Earth-Observing-1 satellite and the ISERV camera aboard the International Space Station to scan the Indian Ocean. Both objects pass over the region during their orbits, rather than hovering overhead like the Inmarsat satellite.
"They're really not meant to look for a missing aircraft, and obviously NASA isn't a lead agency in this effort," said spokesman Allard Beutel. "But we've been trying to support the search, if possible."
The search reignited interest in better tracking for planes over oceans, after the 2009 crash of Air France Flight 447 over the Atlantic.
Robert Mann, an industry consultant and former airline executive at R.W. Mann and Co., said planes should provide automatic GPS information about their position every minute by satellite over water and VHF radio signal over land.
He argued that such messages would be a marginal cost to airlines facilitating e-mail, Twitter feeds and videos by broadband satellite connections.
But there has been no international agreement to adopt satellite tracking.
"That could be done tomorrow with the existing technology that we have," McLaughlin said. "But the mandate is not there globally."