Satellite data led British company Inmarsat to conclude that Malaysia Airlines Flight 370 went down in the south Indian Ocean, but how did Inmarsat track the missing flight's route? About two weeks ago, it picked up on "pings" from the aircraft—despite the fact that the Boeing 777's transponders and communication systems were shut down, its satellite terminal was still sending out the automated "pings" in an attempt to stay connected to a satellite, the Los Angeles Times reports. The hourly "pings" are typically used to "synchronize timing information," the Telegraph reports. Then Inmarsat used the Doppler effect to figure out how fast the plane was going relative to the satellite—a technique that has "never been done before," an Inmarsat VP says.
Reuters explains that the Doppler effect is the reason "the sound of a police car siren changes as it approaches and then overtakes an observer." The company determined two possible flight path arcs. "The arcs were based on time shift—how long the ping took to reach the satellite—which gave a distance and thus an arc," an expert explains. "By looking at speed—and comparing to other planes at the same distance from the satellite—it was possible to determine the plane was not in the Northern Hemisphere and pin down the track with a high probability," concluding that the plane took the southern route. As for figuring out where exactly it went down, the VP explains that the company knew where the last ping came from, and that the plane must have run out of fuel before the next ping would have gone out, which helped it to narrow down the search area—but there's still a lot of uncertainty related to how fast the plane was going at the time and whether it "plunged or glided." The focus of the search, meanwhile, has turned to the black boxes.