MH370: Probe discloses search focus details

An Australian investigation team has disclosed some of the crucial details which helped the multi-national search for missing Malaysia Airlines Flight 370 to select a specific zone on which to focus its efforts.

The information released by the Australian Transport Safety Bureau indicates that the flight path of MH370 had three distinct sections; one under secondary radar in which the aircraft transponder was operational and ACARS messages were being transmitted, a primary radar section during which the aircraft was being tracked solely by air defence radar systems and the final stage for which the only information available was the satellite communications log data.

It is not however the raw satellite ping data that the Malaysian authorities promised the relatives of those on board the missing flight last week and which will be released to the wider public at some point this week.

In a joint press statement UK satellite firm Inmarsat and DCA, the civil aviation authority of Malaysia last week said: “In moving forward, it is imperative for us to provide helpful information to the next of kin and general public which will include the data communication logs as well as relevant explanation to enable the reader to understand the data provided,” adding, “It must also be noted that the data communication logs is just one of the many elements of the investigation information.”

The two partners said they were working towards greater transparency in releasing the data communication logs and the technical description of the analysis for public consumption.

ATSB STATEMENT

ACARS and radar data

The final ACARS transmission was at 17:07 UTC and provided location reports from the initial stage of the flight as well as a recording of the aircraft fuel remaining. The final secondary radar point was at approximately 17:22 UTC. The final primary radar point was at 18:22 UTC. Figure 1 shows the first and second sections of the flight.

MH370 Flight path derived from primary and secondary radar data Fig 1_MH370 Flight Path
Source: NTSB/Google
 

Satellite communications (SATCOM) data

Following the loss of primary radar, the only available information was from satellite signalling messages, also referred to as ‘handshakes’, between the ground station, the satellite and the aircraft’s satellite communication system.

For each transmission to the aircraft, the ground station recorded the burst timing offset (BTO) and the burst frequency offset (BFO).

Satellite communications  Figure 2: Satellite communications schematic
Source: Inmarsat

Burst Timing Offset (BTO)

The BTO is a measure of the time taken for a transmission round trip (ground station to satellite to aircraft and back) and allows a calculation of the distance between the satellite and the aircraft. Based on this measure, a possible location ring can be mapped on the surface of the earth (see below). An analysis of SATCOM system parameters showed that the accuracy of the rings was ± 10 km. This analysis was validated using recorded BTO values from the initial stage of the flight when the aircraft’s position was known.

Satellite ring derivation Figure 3: Satellite ring derivation
Source: Inmarsat

There were 7 handshakes between the ground station and the aircraft after the loss of primary radar data. The location rings calculated from the recorded BTO values are shown below.

MH370 timing (UTC) with corresponding rings arrowed

Figure 4: MH370 timing (UTC) with corresponding rings arrowed

Source: Inmarsat/Boeing /Google

The information from the BTO places the aircraft somewhere on each ring at the corresponding time. By taking the maximum speed of the aircraft into account, the rings can be reduced in length to arcs – there are some areas of the rings it simply could not have reached.

Burst Frequency Offset (BFO)

The BFO is the measure of the difference between the expected frequency of the transmission and the frequency received at the ground station. This difference is attributed to various sources including the Doppler Effect from the motion of the satellite and the aircraft, as well as some processing effects. Once the known components that contribute to the BFO are resolved, the remainder can be used to estimate the speed and direction of the aircraft. There are a large number of speeds and headings that can be consistent with a BFO recording. These are limited, however, by the operational constraints of the aircraft.

Candidate paths of different speeds were created which met the BTO ring location/time constraints and the predicted BFO values of these paths have been compared with the recorded values. The better the match, the higher the probability that the path was close to that of MH370.

Final handshake message at 00:19 (7th arc)

The 00:19 signalling message (7th arc) was a logon request from the aircraft.  This is consistent with the satellite communication equipment on the aircraft powering up following a power interruption. The interruption in electrical supply may have been caused by fuel exhaustion.

Note on the satellite communication

The satellite’s normal function is essentially communication and it was never initially intended to have the capability to track an aircraft. Following the Air France 447 accident, Inmarsat engineers began recording the BTO in order to provide another potential means of geo-locating aircraft in the event of a similar accident.

Aircraft Performance Calculations

Estimates of fuel consumption were calculated from the time of the last recorded fuel quantity, using a range of flight paths and speeds. The results of these calculations were consistent with fuel exhaustion occurring close to the 7th arc.

Validation

Several teams independently provided both satellite communications and performance analysis as part of the validation process. The location of 9M-MRO on previous flights as well as the locations of other aircraft in the air at the same time were all used to validate the techniques.

Other information

Surface search

An international air and maritime force conducted a surface search of drifted regions along the 7th arc from 18 March to 28 April 2014. A drifted region is created by modelling the movement of an area of water over the time period when the surface search is conducted. During this time, no debris was identified to be likely from MH370.

Underwater search

Acoustic detections possibly related to underwater locator beacons were made by two vessels in the refined probability area from 5 – 8 April 2014. To further investigate these signals, a search of the ocean floor around the detections was performed by a number of vessels. To date no further sign of MH370 has been detected.

Hydrophones

Low frequency hydroacoustic signals present in the Indian Ocean are being examined to determine whether they can provide any information to help define the search area. These signals are recorded by hydrophones as part of the United Nations Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) or the Integrated Marine Observing System (IMOS).

Use of waypoints

Comparison of possible flight paths with tracks using waypoints is also under consideration.

Air Routes

There is only one published north-south air route in the south-eastern Indian Ocean. Air route M641 connects Cocos Island to Perth and has four waypoints. The air route crosses the area where the four acoustic signals were detected.

Shape of the search area

At the time MH370 reached the 7th arc, the aircraft is considered to have been descending. A study completed after the Air France 447 accident concluded that the majority of aircraft in loss of control accidents were found within 20 nautical miles (32 km) of their last known position. This provides a reasonable limitation for the size of the search area across the arc.

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