Burst Frequency Offset
'Burst Frequency 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 the Burst Timing Offset and the Burst Frequency Offset respectively.
Background
The article Burst Timing Offset explains a number of terms related to satellite communications or SATCOM, the role of Inmarsat as a service provider to Malaysia Airlines, and how metadata from satellite communication logs recorded by Inmarsat helped define the location of the missing aircraft on an 'Arc', derived from the distance of the aircraft from the satellite at the time of each communications 'handshake'.
Also, these arcs lay within 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
But which corridor? Did the aircraft fly north, or south?
Inmarsat then used Burst Frequency Offset metadata in calculations to indicate that MH370 had flown south.
Frequencies and Channels
The Inmarsat satellite above the Indian Ocean is simply an expensive solar-powered microwave repeater.
Transmissions from the Land Earth Station up to the satellite is an Uplink.
Transmissions from the aircraft via the satellite down to the Land Earth Station is a Downlink.
Transmissions occur in short bursts at a time allocated by the satcom system and on a radio frequency allocated by the satcom system.
The specific frequency corresponds to a channel and when the satellite receives a transmission on an uplink channel it amplifies the signal and repeats it on a corresponding channel to the aircraft or ship. The difference between the in and outgoing frequencies is fixed. The system works in reverse too.
When the satellite receives a downlink transmission from an aircraft it amplifies the signal and repeats it on a corresponding channel which is received by the Land Earth Station.
Frequency Variation
If the aircraft is stationary at an airport and if the satcom system on-board is warmed up and stable the frequency of any satcom transmission should be correct for the selected channel.
In flight the frequency of the satcom transmission should also be correct for the selected channel.
However, when the transmission is received at the satellite the frequency may be higher or lower. This is caused by motion of the aircraft relative to the satellite. The speed and direction of the aircraft, and any movement of the satellite at the same time, affects the frequency as measured at the satellite.
This variation is known a Doppler Shift or Doppler Effect.
Doppler Effect
The Austrian physicist Christian Doppler described this frequency change back in 1842. In his era there were no high-speed trains, racing cars or aircraft, sirens or horns on moving vehicles - all of which are used now to describe how sound waves appear to change pitch (or frequency) - raised as the object approaches, lowering as it passes.
Instead, his theory was based on the colour of stars which may be moving relative to us. Light and radio transmissions are both forms of electromagnetic radiation and Doppler's calculations can now be applied to the perceived frequency shift in radio waves.[1]
Burst Frequency Offset
Inmarsat is a British company. To communicate their findings with the Malaysian Government, Inmarsat first provided their data to the UK Air Accidents Investigation Branch (AAIB).
In a document for the Malaysian Government, the AAIB explained how the Burst Frequency Offset metadata is the result of the Doppler effect:-
In recent days Inmarsat developed a second innovative technique which considers the velocity of the aircraft relative to the satellite. Depending on this relative movement, the frequency received and transmitted will differ from its normal value, in much the same way that the sound of a passing car changes as it approaches and passes by. This is called the Doppler effect. The Inmarsat technique analyses the difference between the frequency that the ground station expects to receive and that actually measured. This difference is the result of the Doppler effect and is known as the Burst Frequency Offset.
The Burst Frequency Offset changes depending on the location of the aircraft on an arc of possible positions, its direction of travel, and its speed. In order to establish confidence in its theory, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions. There was good agreement.
. . .
During the flight the ground station logged the transmitted and received pulse frequencies at each handshake. Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each arc the calculated burst frequency offset fit best.
The analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor, and depending on the ground speed of the aircraft it was then possible to estimate positions at 0011 UTC, at which the last complete handshake took place. AAIB-DOC-018
Prime Minister Najib Razak's Press Statement on MH370 - 24 March 2014
The information provided by Inmarsat and the AAIB was so profound it was consequently communicated to the world by the Prime Minister of Malaysia. The most heart-breaking part of his statement is shown here:-
Based on their new analysis, Inmarsat and the AAIB have concluded that MH370 flew along the southern corridor, and that its last position was in the middle of the Indian Ocean, west of Perth.
This is a remote location, far from any possible landing sites. It is therefore with deep sadness and regret that I must inform you that, according to this new data, flight MH370 ended in the southern Indian Ocean. PMO-MS-017
AAIB and Inmarsat Documents
At a press briefing on Tuesday, 25 March 2014 Hishammuddin Hussein, Minister of Defence and Acting Minister of Transport, provided more detail and a good explanation of the Doppler Effect and the Burst Frequency Offset metadata.
Two documents were linked from the Press Briefing by Hishammuddin Hussein:-
INFORMATION PROVIDED TO MH370 INVESTIGATION BY UK AIR ACCIDENTS INVESTIGATION BRANCH (AAIB) 25/03/14
Annex 1: Doppler Correction Contributions (Information provided to the UK AAIB by Inmarsat)
Both documents are presented here: Attachments.
Southern Corridor
Following the conclusion that 'flight MH370 ended in the southern Indian Ocean', the search for MH370 was moved to the Southern Corridor and focussed on the 'seventh arc'.
From 18 March to 28 April 2014 - 42 days - the search for MH370 was coordinated by the Australian Maritime Safety Authority and included ships and aircraft from many countries.
No debris associated with MH370 was identified by the surface search.
However, on Wednesday, 29 July 2015 a flaperon was discovered on Réunion Island. French authorities later confirmed it was from MH370.
Debris confirmed to be from Malaysia Airlines Boeing 777 9M-MRO which went missing on flight MH370 has been found in Réunion Island, Mozambique, Tanzania, South Africa, Madagascar and Mauritius.
Each of the items is believed to have been transported from the southern Indian Ocean to these locations by a current known as the Indian Ocean Gyre, confirming that flight MH370 did end in the southern Indian Ocean, and will be found on or near the seventh arc within the southern corridor, a remote location, far from any possible landing sites.
Conclusion
Using satellite communication logs for flight MH370 the British company Inmarsat was able to use Burst Timing Offset metadata to determine the distance between the aircraft and their satellite at each of seven 'handshakes' which placed the aircraft on corresponding arcs; and Burst Frequency Offset metadata to determine that MH370 did indeed fly south. Although debris was not located during a surface search and wreckage was not located during the underwater search, or subsequently by American company Ocean Infinity, the arrival of a flaperon on the coast of Réunion Island in 2015 and other debris in Mauritius and east African coastline confirms the validity of Inmarsat's calculations.
References
- ↑ Source: The Doppler Effect