Burst Timing Offset
'Burst Timing Offset' and MH370
After Malaysia Airlines flight MH370 went missing on Saturday, 8 March 2014 the company which provided satellite communications services to the Malaysia Airlines fleet, INMARSAT, analysed the log files for transmissions between the aircraft 9M-MRO and the Inmarsat ground station in Perth, Western Australia.
The SATCOM link between MH370 and Inmarsat's satellite above the Indian Ocean was lost at around the same time as MH370 diverted west. Consequently, the subsequent log files contain no useful information like position data.
However, after the SATCOM link was partially restored at 0225:27 MYT there were seven short bursts of data to or from the aircraft which Inmarsat engineers were able to analyse. A timing difference indicated the aircraft distance from the satellite and a frequency shift indicated the direction of the aircraft relative to the satellite.
This metadata is referred to as the Burst Timing Offset and the Burst Frequency Offset respectively.
Satellite Communications Link
The diagram below illustrates the communications path between an aircraft and a Ground Earth Station (GES) via a satellite, referred to as a Satellite Communications link or SATCOM Link.
Transmissions from the Ground Earth Station up to the satellite is an Uplink.
Transmissions from the aircraft via the satellite down to the Ground Earth Station is a Downlink.
Burst
Each satellite can cope with a limited number of radio transmissions concurrently. There may be many aircraft, and ships, logged on to the SATCOM system, so it must be shared.
Instead of long continuous transmissions it is typical for SATCOM systems to use short bursts of data.
In each burst the data is preceded by metadata such as the date, time, source address, destination address, etc., and is followed by metadata identifying the end of the transmission.
Time
Each aircraft using SATCOM is equipped with a satellite data unit (SDU). This device accurately maintains the current time in a method similar to a mobile phone synchronising to a network time.
Data transmissions via SATCOM include time metadata such as the start time.
Time Differences
Radio transmissions propagate at the speed of light so although exceptionally quick it is not instantaneous.
The total time taken for a transmission burst to be received depends on factors such as the distance from the satellite plus delays caused within the equipment itself.
The difference between time metadata - time received minus time sent - can be called a time offset.
Burst Timing Offset (BTO)
In the context of MH370, the Burst Timing Offset is a measure of the time taken for a transmission from the ground station to the aircraft via the satellite, and a response from the SDU on the aircraft to return via the satellite to the ground station.
To be useful, known signal processing delays have to be considered.
Distance
From the velocity (speed of light) and the measure of Burst Timing Offset it is possible to calculate the total distance travelled.
The Ground Earth Station (GES) is absolutely stationary and a satellite in geo-synchronous orbit is supposedly stationary, so the distance from the earth Station to the satellite can be calculated and subtracted from the total to give the distance from the satellite to the aircraft.
When these calculations were done for MH370 an adjustment had to be made because the satellite was actually moving around a bit and it's relative location had to be known for each transmission!
Handshakes
Communications between the ground station and the aircraft occurred seven times without any useful data being transmitted.
On several occasions the ground station automatically contacted the aircraft, called a Log-on Interrogation. The aircraft SDU responded to five of these requests, each counted as a handshake. There was no response to a similar request at 0915:56 MYT.
In total, there were seven 'handshakes'.
BTO Rings
The BTO metadata for each handshake provided the distance from the satellite, not a location:- the aircraft could be anywhere on a circle or ring at that distance from the satellite.
The calculations were repeated for each of the seven handshakes so the diagram below shows seven rings. Each ring is labelled with the time when each handshake occurred. The time is UTC. Malaysian time is UTC + 8 hours. The final ring represents a handshake at 08:19 AM MYT.
Figure 18: BTO ring solutions for 9M-MRO
Source: MH370 - Definition of Underwater Search Areas, Australian Transport Safety Bureau, 2014
ATSB-RPT-111
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Arcs and Corridors
The world was shocked to learn that MH370 had continued flying for hours after it went missing. The BTO calculations produced a nice diagram but needed further refinement.
The aircraft left Malaysia at a known time, diverted west at a known time, was last sighted by radar at 0222:12 MYT, had a known quantity of fuel on board and known rate of fuel consumption, and had known maximum and typical cruising speeds.
Realistically, the aircraft could only be on a short section of each ring, an Arc, at each of the seven 'handshake' times. Arcs were numbered to correspond with the sequence of 'handshakes'. The Serventh Arc is the most publicised because it was derived from the last, final handshake.
Flight paths could be suggested by joining locations on one arc to the next where the distance travelled was based on the time of each handshake and best guess at the speed of the aircraft.
On Saturday, 15 March 2014 Prime Minister Najib Razak made an announcement that, based on raw satellite data (provided by Inmarsat), "the aviation authorities of Malaysia and their international counterparts have determined that the plane's last communication with the satellite was in one of two possible corridors: a northern corridor stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand, or a southern corridor stretching approximately from Indonesia to the southern Indian ocean. PMO-MS-008
The arcs are not shown in the diagrams below. The arcs lie within the outer red lines which define each 'corridor'. The missing aircraft would be located within either the northern corridor, or the southern corridor. See the Search Summary 18 March to 23 March 2014
Figure III: Northern Corridor
Source: MH370 Search and Rescue Operations and Lesson Learnt
ICAO-RPTMY-324
Which Corridor?
Malaysia Airlines flight MH370 left Kuala Lumpur at 12:41 AM on Saturday, 8 March 2014 and was expected to land at Beijing at 6:30 AM. It never arrived.
Burst Timing Offset metadata enabled investigators to 'narrow down' the search for MH370 to one of two corridors. But in which direction? North or South?
Evidence for the Southern Corridor came from two sources:-
- All of the countries in the northern corridor reported that they had no radar sighting of the aircraft and no evidence of a landing or crash site.
- Inmarsat used Burst Frequency Offset metadata in calculations to indicate that MH370 had flown south.