The analysis of communications between Malaysia Airlines Flight 370 and Inmarsat's satellite telecommunication network provide the primary 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 MYT (16:41 UTC, 7 March) 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.
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 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 ram air turbine started generating electrical power.
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 to .
Background
Malaysia Airlines Flight 370
Malaysia Airlines Flight 370 departed Kuala Lumpur International Airport at 00:41 Malaysia Standard Time (MYT) on 8 March 2014 (16:41 UTC, 7 March), bound for Beijing Capital International Airport. At 1:19, Malaysian air traffic control (ATC) initiated a hand-off to Ho Chi Minh area ATC. The captain responded "Good night Malaysian Three Seven Zero", after which no further communications were made with the pilots. At 01:21, the aircraft disappeared from the radar of air traffic control after passing navigational waypoint IGARI () in the South China Sea between Malaysia and Vietnam. The aircraft continued to be tracked by Malaysian military radar, which recorded that Flight 370 deviated from its planned flight path, turning around and crossing the Malay Peninsula. Flight 370 left the range of Malaysian military radar at 02:22 and was last located northwest of Penang. Flight 370 was expected to arrive in Beijing at 06:30 China Standard Time (CST) on 8 March (06:30 MYT; 22:30 UTC, 7 March). At 07:24 MYT/CST, Malaysia Airlines issued a media statement that Flight 370 was missing.
Satellite datalink The datalink for Malaysia Airline's avionics communications at the time of the incident was supplied by SITA, which contracted with Inmarsat to provide a satellite communication link using Inmarsat's Classic Aero service. Aeronautical satellite communication (SATCOM) systems are used to transmit messages from the aircraft cockpit as well as automated messages from on-board systems using the ACARS communications protocol, but may also be used to transmit FANS and ATN messages and provide voice, fax, and data links using other protocols. An appropriate comparison of ACARS' relationship to the SATCOM system is that of a messaging application to a smartphone; the smartphone functions and will remain registered on a mobile phone network even if the messaging application is closed.
The data/messages from the aircraft are transmitted by the aircraft's Satellite Data Unit (SDU) and relayed via satellite to a ground station, where they are routed to other communication networks to reach their destination. Messages may also be sent to the aircraft, in reverse order. When passing through the satellite, the signals are amplified and translated in frequency—mixed with the signal from an oscillator in the satellite, leaving the satellite at the combined frequency. Transmissions from the aircraft are made on one of several channels (frequencies) near 1.6 GHz, combined with frequency of the satellite's oscillator, and transmitted to the GES at the combined frequency (one of several channels near 3.6 GHz). The ground station then translates the received signal before it reaches equipment to be processed. The ground station keeps a log of transmissions and some data about them.
When the SDU tries to connect with the Inmarsat network, it will transmit a log-on request, which the ground station acknowledges. This is, in part, to determine that the SDU belongs to an active service subscriber and also used to determine how to route messages to the SDU. After connecting, if a ground station hasn't received any contact from a terminal for one hour, the ground station will transmit a "Log-on Interrogation" (LOI) message—informally referred to as a "ping"; an active terminal automatically responds. The entire process of interrogating the terminal is referred to as a "handshake".
Equipment at Inmarsat's Perth ground station had been upgraded in 2013 with additional storage capacity and new software to record an expanded data set for transmissions, including the addition of Burst Frequency Offset (BFO) and Burst Timing Offset (BTO) values. Without the additional data values, it would have been impossible to determine the aircraft's distance from the satellite at each handshake. The expanded data values were prompted by Inmarsat's involvement in the search for Air France Flight 447, which disappeared over the Atlantic Ocean in 2009. According to Mark Dickinson, Inmarsat's vice-president of satellite operations, the company did not know what the additional data might be used for, but had a "hunch" and upgraded their hardware.
Emergency locator transmitters The aircraft was equipped with four emergency locator transmitters (ELTs): a fixed ELT on the aft fuselage which is activated by sudden deceleration, a portable ELT in a cabinet located in the front of the aircraft which must be activated by moving a switch, and two ELTs attached to slide rafts which are armed when the rafts are inflated and activated by water immersion Once activated, the ELTs emit a radio signal which can be detected by the satellites of the International Cospas-Sarsat Programme. The ELTs are designed to work at or near the water's surface. Damage during a crash, shielding by aircraft wreckage or terrain, and submersion in deep water are all factors which may prevent the signal's detection. In a review of accident records maintained by ICAO over the past 30 years, there were 173 accidents involving aircraft over equipped with ELTs; of these, an effective ELT detection was made in only 39 accidents. No signals from ELTs aboard Flight 370 were detected.
Communications from Flight 370 The SDU on 9M-MRO (the aircraft used for Flight 370) logged onto the Inmarsat network at midnight MYT. In the 30 minutes before take-off, seventeen messages were exchanged between the SDU and the Inmarsat network. An additional three messages were exchanged between take-off and the time Flight 370 disappeared from secondary radar. The final message to use the ACARS protocol was sent at 01:07; ACARS reports expected at 01:37 and 02:07 were not received. At 02:03 and 2:05, messages from the ground station went unanswered, indicating that the link was lost at some point between 01:07 and 02:03.
After last contact by primary radar west of Malaysia, the following records were recorded in the log of Inmarsat's ground station at Perth, Western Australia (HH:MM:SS; UTC times 7–8 March):