Malaysia on Tuesday released details on how an analysis of satellite data came to the conclusion that the missing Malaysia Airlines has ended up in the southern Indian Ocean. We look at how this was done:
How an aircraft communicates via satellite
- An aircraft is able to communicate with ground stations via satellite.
- If the ground station has not heard from an aircraft for an hour it will transmit a "log on/log off" message, sometimes referred to as a "ping", using the aircraft's unique identifier.
- If the aircraft receives its unique identifier, it returns a short message indicating that it is still logged on. This process has been described as a "handshake" and takes place automatically.
When was MH370's last "handshake"
- Six complete handshakes took place between MH370 and the ground station, after the plane's Aircraft Communications Addressing and Reporting System (ACARS), which transmits key information on a plane's condition to the ground, stopped sending messages.
- MH370's last complete "handshake" took place 0011 UTC (8.11am Singapore time).
- There is evidence of a partial handshake between the aircraft and ground station at 0019 UTC (8.19am). But further analysis is needed to determine what this means.
- No response was received from the aircraft at 0115 UTC (9.15am), when the ground earth station sent the next log on / log off message. This indicates that the aircraft was no longer logged on to the network.
How data was used to identify MH370's possible location
- The position of the satellite is known, and the time that it takes the signal to be sent and received, via the satellite, to the ground station can be used to establish the range of the aircraft from the satellite.
- This information was used to generate arcs of possible positions from which the northern and southern corridors were established.
- British satellite company Inmarsat then turned to an old theory, about the Doppler effect, to further analyse the data.
- The theory considers the velocity of the aircraft relative to the satellite. Depending on this relative movement, the frequency received and transmitted will differ from its normal value, in much the same way that the sound of a passing car changes as it approaches and passes by.
- The Inmarsat technique analyses the difference between the frequency that the ground station expects to receive and that actually measured. This difference is the result of the Doppler effect and is known as the Burst Frequency Offset.
- To verify the data, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions.
- Inmarsat's analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor.
How Malaysia came to the conclusion MH370 has crashed
- Some time between 0011 UTC and 0115 UTC (8.11am - 9.15am), MH370 was no longer able to communicate with the ground station.
- This is consistent with the maximum endurance of the aircraft.
- MH370, which left Kuala Lumpur for Beijing at 12.41am on March 8, had enough fuel to fly only up until 8.30am.