Help from above: Satellite signals can confirm a plane's identity
CNN) -- There's a reason why Malaysian officials are so confident it was Flight 370 that sent signals to a satellite many hours after the plane disappeared from radar March 8. That's because CNN has learned signals from commercial aircraft to Inmarsat satellites always include a code confirming the identity of the plane. An Inmarsat official, while declining to discuss specifics of Flight 370, tells CNN the satellite system is highly reliable, that each signal to an aircraft is met by a return signal and that those signals always contains a code verifying the identity of the aircraft.
It is "virtually impossible" to change an aircraft's identifying code or to confuse one aircraft with another, the Inmarsat official said.
Further, after a satellite link is established at the beginning of a plane's flight, it makes automatic, periodic checks until the end of the flight -- helping investigators determine the duration of the flight, if not its location.
That could explain why Malaysian authorities now say they have a "high degree of confidence" that Flight 370 continued flying well after it disappeared from civilian radar screens.
Government officials now believe the plane continued flying until at least 8:11 a.m. -- almost seven hours after disappearing from radar at 1:21 a.m.
Malaysian officials, citing "satellite information" but giving scant details, this weekend refocused the search for the missing Boeing 777, moving attention to massive arcs on both sides of the equator.
Malaysian authorities believe someone disabled several communications systems, perhaps to conceal the plane's location. One of those systems was a digital data system known as ACARS, which uses the satellite to relay messages to the ground.
But while it is possible for someone in the cockpit to turn off ACARS, the system's powered antenna remained on, receiving and responding to hourly checks from a ground station, via the satellite.
Inmarsat technicians continue to help, the company said.
"Our experts have been pulled into the investigation. We've had people in Kuala Lumpur," said Inmarsat subject expert David Coiley. "We are putting everything into this to assist the investigation as best we can, because it seems there's no other data set."
How Inmarsat works
Inmarsat, which is prohibited from discussing details of the Malaysia Flight 370 investigation, was able to provide CNN with a detailed explanation about how its system works.
The
London-based satellite communications company owns and operates 10 satellites, all in geostationary orbit
some 22,200 miles above the equator.
Since a single satellite can see one-third of the Earth, multiple satellites are needed to provide seamless coverage and provide redundancy and reliability, the company said.
Among other services, Inmarsat provides satellite communications for the ACARS, the acronym for Aircraft Communications Addressing and Reporting System. It's a digital datalink for short messages between an aircraft and an airline operations center, air traffic controllers and others.
ACARS can be used to send messages and data of all types, including text messages from pilots to dispatchers, or automatically generated data on the health of the plane.
When a plane is over land, ACARS messages typically are sent via VHF radio. But when a plane is in remote regions, or over water and out of range of VHF radio, the signal is seamlessly switched to satellite. "The pilot doesn't have to do anything," Coiley said.
At its most fundamental, the satellite is a simple relay, transferring signals from ground stations to the aircraft,and back again, not unlike a cell phone tower, Coiley said
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