Malaysia Airlines Flight 370 satellite communications

Parts of this article (those related to add information from the latest report MH370 – Definition of underwater search areas (released 3 December 2015) and book Bayesian Methods in the Search for MH370 (pre-publication draft made available by JACC on 30 November 2015)) need to be updated. Please help update this article to reflect recent events or newly available information. (December 2015)

The analysis of communications between Malaysia Airlines Flight 370 and Inmarsat's satellite telecommunication network provide the primary^[1][a] source of information about Flight 370's location and possible in-flight events after it disappeared from military radar coverage at 02:22 Malaysia Standard Time (MYT) on 8 March 2014 (17:22 UTC, 7 March), one hour after communication with air traffic control ended and the aircraft departed from its planned flight path while over the South China Sea.

Flight 370 was a scheduled commercial flight with 227 passengers and 12 crew which departed Kuala Lumpur, Malaysia at 0:41 and was scheduled to land in Beijing, China at 6:30 China Standard Time (6:30 MYT; 22:30 UTC, 7 March). Malaysia has worked in conjunction with the Australian Transport Safety Bureau to co-ordinate the analysis, which has also involved the UK's Air Accidents Investigation Branch, Inmarsat, and US National Transportation Safety Board. Other groups have also made efforts to analyse the satellite communications, albeit challenged by a lack of publicly available information for several months after the disappearance. On 29 July 2015, debris was discovered on Réunion Island which was later confirmed to have come from Flight 370; it is the first physical evidence that Flight 370 ended in the Indian Ocean.^[2]

During flight, the aircraft maintains a datalink with a satellite communication network for data and telephone calls. The datalink connects the aircraft and a ground station via satellite, which translates (changes) the signal's frequency and amplifies the signal; the ground station is connected to telecommunication networks which allows messages to be sent to and received from other locations, such as the airline's operations centre. Normal communications from Flight 370 were last made at 1:07 MYT. The datalink between the aircraft and satellite telecommunication network was lost at some point between 1:07 and 2:03, when the aircraft did not acknowledge a message sent from the ground station. Three minutes after the aircraft left the range of radar coverage—at 2:25—the aircraft's satellite data unit (SDU) transmitted a log-on message, which investigators believe occurred when the SDU restarted after a power interruption. Between the 2:25 message and 8:19, the SDU acknowledged two ground-to-aircraft telephone calls, which were not answered, and responded to automated, hourly requests from the ground station that were made to determine whether the SDU was still active. None of the communications from 2:25-8:19 contain explicit information about the aircraft's location. The aircraft's final transmission at 8:19 was a log-on message; the aircraft did not respond to a message from the ground station at 9:15. Investigators believe the 8:19 log-on message was made when the SDU was restarting after the aircraft ran out of fuel and the aircraft's auxiliary power unit was started.

The search for Flight 370 was launched in Southeast Asia near the location of the last verbal and radar contact with air traffic control. The day after the disappearance, staff at Inmarsat reviewed the log of communications between their satellite network and Flight 370 and discovered that they continued for several hours after contact with air traffic control was lost. On 11 March, they provided a preliminary analysis to investigators based on recorded burst timing offset (BTO) values. Relatively simple calculations can be made from BTO values to determine the distance between the aircraft and satellite at each transmission. When these distances are plotted on Earth, they result in rings which are then further reduced to arcs, due to the limited flying range of the aircraft. Another value—burst frequency offset (BFO)—was analysed to determine the movement of the aircraft relative to the satellite, based on the Doppler shift of the signals, which provides the location of the aircraft along the BTO-derived arcs. Initial analysis of the BFO values showed a strong correlation with a track south into the southern Indian Ocean, west of Australia. On 24 March, Malaysia's Prime Minister cited this analysis to conclude that Flight 370 ended in the southern Indian Ocean with no survivors. After the initial analysis, the BFO calculations were later adjusted to account for a wobble in the satellite's orbit and thermal changes in the satellite which affected the recorded BFO values. Further analysis considered the BTO and BFO calculations together with the aircraft flight dynamics, such as possible and probable aircraft speeds, altitudes, and autopilot modes. Two statistical analyses were made and combined with calculations of Flight 370's maximum range to determine the most probable location of Flight 370 at the time of the 8:19 transmission, which is along the 8:19 BTO arc from approximately 38°18′S 88°00′E / 38.3°S 88°E / -38.3; 88 (Southwest corner of the area of interest along the 8:19 BTO arc, ATSB Flight Path Analysis Update (October 2014)) to 33°30′S 95°00′E / 33.5°S 95°E / -33.5; 95 (Southwest corner of the area of interest along the 8:19 BTO arc, ATSB Flight Path Analysis Update (October 2014)).

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