Investigators compared subtle changes in electromagnetic signal frequency, caused by the Doppler effect, to conclude that Malaysia Airlines’ missing Boeing 777-200ER routed south after contact was lost on 8 March.
Doppler shift is the apparent shift in frequency that arises from relative movement between the source of the signal and the receiver. The frequency becomes higher if the two are moving towards one another, and lower if they are moving apart.
Analysts have been able to examine the six complete routine log-in messages transmitted by a satellite ground station, to which flight MH370 replied, following the loss of ACARS communications from the aircraft.
This data had already enabled investigators to collate all the possible positions of the aircraft at the time of the final log-in message at 00:11UTC, or 08:11 Malaysian time.
After considerations of the aircraft’s range, and other factors, these collective positions formed two arcs – one stretching across central Asia, the other over the Indian Ocean – but left investigators with little evidence as to which of the two reflected the direction, north or south, the aircraft had taken.
Doppler analysis, which takes into account the motion of the satellite as well as the aircraft, appears to have broken the geometric symmetry.
The analysis model used six other 777 flights, conducted on the same day, against which data could be cross-checked, as well as baseline information from MH370 obtained while it was still parked at Kuala Lumpur.
Examination of the frequency shift – which amounted to some 150Hz – at a typical cruise airspeed of 450kt showed that it was much closer to that expected from a southern course than a northern.
“The analysis showed poor correlation with the northern [arc], but good correlation with the southern,” said acting Malaysian transport minister Hishammuddin Hussein, speaking at a briefing on 25 March.
Hishammuddin revealed that the investigation had found evidence of a partial log-in ‘handshake’ message which occurred about 8min after the final complete transmission. This signal, at 00:19UTC, has yet to be explained.
An expected transmission at 01:15 was not received, indicating that all communication with the aircraft had ceased by that point. This timeframe is consistent with the aircraft’s maximum endurance. Malaysia Airlines has confirmed that MH370 was carrying 49,100kg of fuel for the 6h service to Beijing.
“Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each [search] arc the calculated burst frequency offset fit best,” says Hishammuddin.
Investigators are analysing the Inmarsat data further to try to narrow the search field for the aircraft.
The information has enabled the inquiry to draw up possible southern tracks based on cruise speeds covering 400-450kt. This has enabled the search teams not only to abandon the northern arc entirely but also the northern section of the Indian Ocean zone in the vicinity of Indonesia.
But although this gives a better estimate of the aircraft’s course, the last log-in handshake at 00:11UTC does not provide a final position for MH370. Investigators have yet to determine how much further the aircraft travelled before coming down – either because its fuel was exhausted or for reasons unknown.
While the analysis has enabled an 80% reduction in the search area, Hishammuddin stressed that the size of the zone being covered – around 1.6 million km2 – is still vast.