Malaysia's prime minister announced earlier that the Inmarsat analysis of flight MH370's path placed its last position in remote waters off Australia's west coast, meaning it can only have run out of fuel above the southern Indian Ocean.
Inmarsat explained how they plotted models of the flight's route by measuring the Doppler effect of satellite pings, giving corridors arcing north and south along which the plane could have flown for at least five hours.
Despite the plane's communication systems being switched off, satellite pings were still bouncing back from the aircraft, which vanished on March 8 with 239 people on board while flying from Kuala Lumpur to Beijing.
The pings are sent from a ground station to a satellite, then onto the plane, which automatically sends a ping back to the satellite and down to the ground station.
They do not include global positioning system (GPS) data, time or distance information.
So the British satellite operator measured the amount of time it took for the pings to be returned.
"We looked at the Doppler effect, which is the change in frequency due to the movement of a satellite in its orbit," Chris McLaughlin, Inmarsat's senior vice president of external affairs, told Britain's Sky News television.
"What that then gave us was a predicted path for the northerly route and a predicted path the southerly route."
"We don't know whether the plane stayed at a constant speed; we don't know whether its headings changed subsequently," he explained.
Therefore, "we applied the autopilot speeds - about 350 knots. We applied what we knew about the fuel and range of the aircraft to hit the series of ping information we had.
"Normally you'd want to triangulate, often you'd have GPS. But because aircraft in that region are not mandated to send out signals of their location we were working from blind, so this is very much a unique approach - the first time it's been done."
Ran out of fuel
They then compared those figures to data from other Malaysia Airlines planes and similar flight routes, which definitively showed the plane could only have been going down the southern corridor, and would eventually have run out of fuel.
They established an "extraordinary matching" between Inmarsat's predicted southern path and readings from other planes on such routes.
The BBC reported that as far as could could be worked out, the plane was flying at a cruising height, above 30,000 feet (9,100 metres). They found no evidence of fluctuating heights.
Inmarsat handed over new information to Britain's Air Accidents Investigation Branch on Sunday for checking.
"By yesterday they were able to definitively say that the plane had undoubtedly taken the southern route," said McLaughlin.
He called for all commercial aircraft to be fitted with existing technology that would mean a plane cannot go missing.