

British satellite business Inmarsat has continued to refine the analysis of routine automatic communications between one of its satellites and the missing Malaysia Airlines Flight 370.

Together with the UK’s Air Accident Investigation Branch (AAIB), Inmarsat presented its most recent findings earlier this week in a bid to determine whether indeed the aircraft flew along the southern – or northern – corridor.

Aircraft are 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.

From the ground station log Inmarsat was established that after ACARS stopped sending messages, six 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.



More recently Inmarsat has developed a second innovative technique which 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. This is called the Doppler effect. 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 Boeing 777 aircraft flying on the same day in various directions. There was a good level of agreement.

While on the ground at Kuala Lumpur airport, and during the early stage of the flight, Flight 370 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. The Malaysian transport ministry pointed out however that this is not the final position of the aircraft.

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 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.

Inmarsat told The Wall Street Journal that the cause of the partial ping could have several possible explanations including electrical system fluctuations and flight manoeuvres, but that it did not alter the determination of the likely end location of Flight 370. “We’re not looking at this [partial ping] as someone trying to turn on the system and communicate,” Inmarsat said.

The latest analysis by Inmarsat forms the basis for further study to attempt to determine the final position of the aircraft. Accordingly, the Malaysian investigation has set up an international working group, comprising agencies with expertise in satellite communications and aircraft performance, to take this work forward.

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