Now we know for sure "there's no way it went north," said Inmarsat Senior Vice President Chris McLaughlin.
Malaysian Prime Minister Najib Razak said Monday that the plane was last tracked over the middle of the Indian Ocean, west of Perth, Australia. Malaysian Airlines has informed passengers' relatives that "all lives are lost," a relative told CNN.
Monday's announcement brings new questions about the mystery that has captivated the planet for more than two weeks. It also provoked a call that all airliners be constantly tracked.
The mathematics-based process used by Inmarsat and the UK's Air Accidents Investigation Branch (AAIB) to reveal the definitive path was described by McLaughlin as "groundbreaking."
"We've done something new," he said.
Here's how the process works in a nutshell: Inmarsat officials and engineers were able to determine whether the plane was flying away or toward the satellite's location by expansion or compression of the satellite's signal.
What does expansion or compression mean? You may have heard about something called the Doppler effect.
"If you sit at a train station and you listen to the train whistle -- the pitch of the whistle changes as it moves past. That's exactly what we have," explained CNN Meteorologist Chad Myers, who has studied Doppler technology. "It's the Doppler effect that they're using on this ping or handshake back from the airplane. They know by nanoseconds whether that signal was compressed a little -- or expanded -- by whether the plane was moving closer or away from 64.5 degrees -- which is the longitude of the orbiting satellite."
Each ping was analyzed for its direction of travel, Myers said. The new calculations, McLaughlin said, underwent a peer review process with space agency experts and contributions by Boeing.
It's possible to use this analysis to determine more specifically the area where the plane went down, Myers said. "Using trigonometry, engineers are capable of finding angles of flight."
Experts said they weren't surprised by the news that the flight traveled along the southern track -- one of two possible paths revealed by satellite data last week. The possible northern track toward Pakistan would have been heavily monitored by radar. Pakistan had said it found no evidence of Flight 370 on its radar systems. "It was very difficult to believe that no watch captain" along the possible northern path "would've seen a burning or distressed aircraft in the sky during the course of their watch," said McLaughlin.
Is the more pinpointed flight path now focused enough to increase the chances of finding wreckage from the plane?
If the flight definitely ended far from land, does that support the theory that the plane was not hijacked? It's just one question of many that investigators likely will be pondering in the coming days.
Hours before the Prime Minister's announcement, Australian officials said they had spotted two objects in the southern Indian Ocean that could be related to the flight, which has been missing since March 8.
One object is "a gray or green circular object," and the other is "an orange rectangular object," the Australian Maritime Safety Authority said.
"This is obviously a major tragedy," McLaughlin said. "The only thing you can hope is that from this, just as the Titanic resulted in (new safety legislation), that from this, there will be a mandate that all aircraft should be constantly tracked."