Initial Method On 13 March, British satellite company Inmarsat indicated that routine automatic communications between one of its sa...
Initial Method
On 13 March, British satellite company Inmarsat indicated that routine automatic communications between one of its satellites and the Flight MH370 could be used to determine several possible flight paths.
The conclusion was based on the pings received by its satellite network. 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. From the ground station log it was established that after ACARS stopped sending messages, 6 complete handshakes took place. 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.
The UK Air Accidents Investigation Branch (AAIB) then presented Inmarsat’s most recent findings on 24 March, indicating the southern corridor.
Latest Method
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.
The Burst Frequency Offset changes depending on the location of the aircraft on an arc of possible positions, its direction of travel, and its speed.
In order to establish confidence in its theory, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions. There was good agreement.
While on the ground at Kuala Lumpur airport, and during the early stage of the flight, MH370 transmitted several messages. At this stage the location of the aircraft and the satellite were known, so it was possible to calculate system characteristics for the aircraft, satellite, and ground station.
During the flight the ground station logged the transmitted and received pulse frequencies at each handshake. Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each arc the calculated burst frequency offset fit best.
The analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor, and depending on the ground speed of the aircraft it was then possible to estimate positions at 0011 UTC, at which the last complete handshake took place.
There is evidence of a partial handshake between the aircraft and ground station at 0019 UTC.
At this time this transmission is not understood and is subject to further ongoing work. No response was received from the aircraft at 0115 UTC, 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. Therefore, some time between 0011 UTC and 0115 UTC the aircraft was no longer able to communicate with the ground station.
This is consistent with the maximum endurance of the aircraft. This analysis by Inmarsat forms the basis for further study to attempt to determine the final position of the aircraft.
The Malaysian government statement emphasised that the last handshake should not be interpreted as the final position of the aircraft because of evidence of a partial handshake between the aircraft and ground station at 00:19 UTC.
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