Malaysia Airlines plane

Missing MH370: How satellites communicate with a plane and determine its location

Investigators say the missing Malaysia Airlines plane sent signals to satellites from air or ground as late as 8.11am on March 8. Five points on how satellites communicate with a plane and determine its location.

Satellite data shows that the electrical system of Flight MH370, the Malaysia Airlines plane that went missing on March 8, was still functioning more than six hours after it lost contact with air traffic control.

Investigators now believe the plane's communication systems were deliberately switched off and it was diverted. They have said the aircraft sent signals to satellites from air or ground as late as 8.11am on March 8.

News reports say the satellite communications came from the plane's Aircraft Communications Addresssing and Reporting System (ACARS), which is a digital datalink for short messages between a plane and airline operations, air traffic controllers and other authorised parties. ACARS allows pilots to send simple short text messages transmitted through radio signals and satellites. ACARS also sends data about engine performance to its manufacturers, and automatically generated data on the technical condition of the plane.

Although the Malaysian authorities believe someone on Flight MH370 deliberately switched off its communication systems including ACARS, perhaps to hide its location, its powerful antenna remained operational, and was receiving and responding to checks from a satellite once an hour. London-based Inmarsat, which operates 10 satellites in geostationary orbit about 35,780km above Earth, said on March 14 that its network had picked up “routine, automated signals” from Flight MH370.

How satellites communicate with a plane and determine its location

1. When an aircraft powers up, the plane automatically sends a signal logging onto the communication network. After that, CNN reported, the ground station sends "polling signals" to the satellite, which relays them to the aircraft. When the plane responds, it is known as a "handshake".

2. The data transmitted during these communications contain a unique code identifying the plane. The satellite sends a return signal back to the plane, which also contains a similar code. An Inmarsat official told CNN it is "virtually impossible" to change a plane's identifying code or to confuse an aircraft with another.

3. When a plane is flying over land, ACARS messages are normally sent via VHF radio, an Inmarsat subject expert David Colley told CNN. But when it is flying over remote regions, over water or out of range of VHF radio, the back-and-forth signals between the plane and ground stations are transmitted automatically via satellite.

4. Through the hourly "handshakes", the satellite can determine the approximate location of the plane so that it can relay messages efficiently. A plane that is flying directly under a satellite would be at a 90-degree angle to the satellite. An aircraft flying at the poles would be at 0 degrees, the CNN report said. The last message sent by MH370 was at 40 degrees.

5. A satellite can “see” in an arc that stretches north and south of its fixed position, but without global positioning system (GPS) on the plane, the satellite can say only how far away the electronic "ping" from the aircraft is, not where it is coming from, a source told The New York Times.

The source said the older satellite communications box fitted on MH370 had no GPS. But investigators have calculated the distance between the final "ping” from the plane and a stationary Inmarsat-3 satellite: One of two air corridors stretching as far as Kazakhstan to the north, or the southern Indian Ocean – around western Australia – to the south.

By that time, if still in the air, the plane would probably have been almost out of fuel. But the fact that it was still transmitting signals means it was still operational, as it needs electrical power to do so.