page_title: Malaysia Airlines Flight 370 satellite communications
section_title: Burst timing offset
hierarchical_section_title: Malaysia Airlines Flight 370 satellite communications / Analysis by the Joint Investigation Team / Burst timing offset
caption_attribution_description: English: The BTO arc (highlighted in red) of the sixth handshake (00:11&#160;UTC / 08:11&#160;MYT) between Malaysia Airlines Flight 370 and the Inmarsat satellite communications network. The chart on the right lists the elevation angle of the satellite from the aircraft at several of the transmissions. A version of this map was publicly released by the Malaysian government on 15 March 2015, with the arc divided into two arcs (the Northern and Southern Corridors), extending from Kazakhstan to Laos/Thailand and from south of Java into the southern Indian Ocean (roughly where the bottom edge of this map). This graphic was released in October after further analysis, but the elevation angle of the sixth handshake remains unchanged.
context_page_description: 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 2:22 Malaysia Standard Time on 8 March 2014, 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. 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.
context_section_description: For system efficacy and reliability, aircraft transmissions made in response to a signal from a satellite are sent in timed slots referenced to the time the signal from the satellite arrived, using the slotted ALOHA protocol. The time that the signal is sent from the ground station begins the time slot. The burst timing offset (BTO) is the time difference between the start of the time slot and the start of the transmission received from the aircraft; it equals twice the distance (for the ground station's signal and the aircraft's response) from the ground station to the satellite to the aircraft plus the time the aircraft's SDU takes between receiving the signal and responding (the SDU bias) and the delay between the time the signal arrives at the ground station and the time it is processed (when the BTO value is logged; the ground station bias). The satellite's location is known, thus the distance from the satellite to the ground station can be calculated, while the combined bias of the SDU and ground station is relatively constant and can be calculated from signals exchanged earlier in the flight while it was on the ground at KLIA, thus leaving the distance between the aircraft and satellite as the only variable.
  The combined SDU and ground station bias was calculated from 17 signals exchanged between the ground station and the aircraft during a 30-minute period before take-off, when the aircraft's location was known (at Kuala Lumpur International Airport). To establish the accuracy of their calculations, the bias value was used to calculate the distance from the aircraft to the satellite during the time it was on the ground at KLIA, with errors of <1 km-8.85 km (<0.6 mi-5.5 mi). The distance from the satellite to the aircraft was also calculated while the aircraft was in-flight and at a known location shortly after take-off, showing similar accuracy. The distance between the satellite and aircraft could thus be calculated for signals exchanged between 02:25–08:19, after Flight 370 disappeared from radar. However, this could only produce a ring on Earth's surface that is equidistant from the satellite at the calculated value, adjusted to account for the aircraft flying at 10,000 m (33,000 ft). The ring could also be reduced to an arc by taking into consideration the maximum range of the aircraft if it flew at its maximum speed.
  The BTO value was added to the ground station's data set to assist in geo-locating an aircraft following the Air France Flight 447 accident in 2009 and the initial BFO analysis—relating the BFO value to the elevation angle between the aircraft and satellite—was based on methods developed during the Flight 447 investigation. The first and seventh handshakes gave anomalous results and were excluded from the initial analysis, but the issue was later resolved. The initial analysis, accurate to approximately 1°, determined that elevation angle between the aircraft and satellite at the 08:11 handshake was 40°. When this was publicly disclosed by Malaysian officials, the arc was broken into two arcs—dubbed the "northern corridor" and "southern corridor." The first and seventh handshakes were later determined to be part of a log-on sequence, as opposed to the other handshakes that were log-on interrogation messages. The bias value during the logon sequence is different and was calculated using historical data for the aircraft's SDU. This allowed the distance between the satellite and aircraft to be determined at these times.
caption: The BTO arc of the sixth handshake and a table of elevation angles of transmissions from the plane.
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