Based on the hourly connections with the plane, described by a U.S. official as a "handshake," the satellite knows at what angle to tilt its antenna to be ready to receive a message from the plane should one be sent. Using that antenna angle, along with radar data, investigators have been able to draw two vast arcs, or "corridors" — a northern one from northern Thailand through to the border of the Central Asian countries Kazakhstan and Turkmenistan, and a southern one from Indonesia to the southern Indian Ocean. The plane is believed to be somewhere along those arcs.
Air crash investigators have never used this kind of satellite data before to try to find a missing plane, but after pursing other leads it's the best clue left.
"The people that are doing this are thinking outside the box. They're using something that wasn't designed to be used this way, and it seems to be working," said William Waldock, who teaches accident investigation at the Embry-Riddle Aeronautical University in Arizona. "In terms of search and rescue, they're probably going to have rewrite the book after this."