Details are now emerging as to how London-based Inmarsat managed to track the signals sent from Malaysia Airlines flight MH-370, helping to narrow down its possible location.
In the end it used the Doppler effect and time delays from the aircraft’s signals to help it plot the likely last known locations of the aircraft.
A UK Air Accidents Investigation Branch (AAIB) report states that the calculations were made using the automatic ‘pings’ sent to the satellite via the ground station and the aircraft after it vanished.
The aircraft was fitted with equipment capable of transmitting over the Inmarsat I-3/I-4 satellite network.
Inmarsat’s “classic” Aero H, Aero H+ and Aero I services can provide packet data with speeds of up to 9.6kbps at latitudes of up to around 80 degrees north and south.
Inmarsat explained that if the ground station does not hear from an aircraft for an hour it will transmit a ‘log on/log off’ message – a ‘ping’ – and the aircraft automatically returns a short message indicating that it is still logged on, a process described as a ‘handshake’.
The ground station log recorded six complete handshakes after ACARS, the aircraft’s operational communications system, stopped sending messages.
Inmarsat was then able to calculate the range of the aircraft from the satellite, by using the time it took the signal to be sent and received, to generate two arcs of possible positions – a northern and a southern corridor.
This would have been done by noting the time delay between the time the signal was sent and the time it was received. Given that radio waves (in a vacuum) travel at 186,000 miles per second (300,000 kilometres a second) this would give an “arc” along which the aircraft may have been located.
The report goes on to explain that Inmarsat developed a second innovative technique that took into account the velocity of the aircraft relative to the satellite and the resulting change in signal frequency, known as the Doppler effect.
The Inmarsat technique analysed the difference between the frequency that the satellite expected to receive and the one actually measured, known as the Burst Frequency Offset.
That is, the small difference between the two would indicate if the aircraft was moving towards or away from the satellite.
Some news sources have said that they looked at the “speed” of the signals, but that is not true. All electromagnetic signals travel at the speed of light – it is the frequency that changes if the transmitter and receiver are moving relative to each other, similar to the red shift seen with galaxies that are moving away from us.
The Doppler frequency shift, albeit small, shows whether the aircraft was moving towards the satellite (a shift upwards) or away (a shift downwards).
That is, was it moving towards the equator or away? At the equator you would probably expect no Doppler shift at all.
To check its theory, Inmarsat compared its predictions with six other Boeing B777 aircraft flying on the same day in various directions, which resulted in a good match.
The analysis fitted well with the southern corridor, according to the report, and depending on the ground speed of the aircraft, it was then possible to estimate positions at 00:11 UTC – when the last complete handshake took place.
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.
No response was received from the aircraft at 01:15 UTC, when the ground earth station sent the next log on/log off message, indicating that the aircraft was no longer logged on to the network, it added.
Therefore, at some time between 00:11 UTC and 01:15 UTC the aircraft was no longer able to communicate with the ground station – consistent with the maximum time the aircraft was able to fly, the article concluded.
The Malaysian investigation has now set up an international working group, including agencies with expertise in satellite communications and aircraft performance, to take this work forward.This may have been based on the signal strength rather than the angle – a certain signal strength then giving a possible “circle of interest” in the satellite’s footprint.