Malaysia MH370: SATCOMS 101 (Part Three)

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This shows the southern tracks for a ground speed of 400 and 450 knots ground speed. It should be noted that further work is required to determine the aircraft speed and final position.

British satellite business Inmarsat has continued to refine the analysis of routine automatic communications between one of its satellites and the missing Malaysia Airlines Flight 370.

Together with the UK’s Air Accident Investigation Branch (AAIB), Inmarsat presented its most recent findings earlier this week in a bid to determine whether indeed the aircraft flew along the southern – or northern – corridor.

Aircraft are able to communicate with ground stations via satellite. If the ground station has not heard from an aircraft for an hour it will transmit a ‘log on / log off’ message, sometimes referred to as a ‘ping’, using the aircraft’s unique identifier. If the aircraft receives its unique identifier it returns a short message indicating that it is still logged on. This process has been described as a “handshake” and takes place automatically.

From the ground station log Inmarsat was established that after ACARS stopped sending messages, six complete handshakes took place.

The position of the satellite is known, and the time that it takes the signal to be sent and received, via the satellite, to the ground station can be used to establish the range of the aircraft from the satellite. This information was used to generate arcs of possible positions from which the northern and southern corridors were established.

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The blue line is the burst frequency offset measured at the ground station for MH370. The green line is the predicted burst frequency offset for the southern route, which over the last 6 handshakes show close correlation with the measured values for MH370. The red line is the predicted burst frequency offset for the northern route, which over the last 6 handshakes does not correlate with the measured values for MH370.

More recently Inmarsat has 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 Boeing 777 aircraft flying on the same day in various directions. There was a good level of agreement.

While on the ground at Kuala Lumpur airport, and during the early stage of the flight, Flight 370 transmitted several messages. At this stage the location of the aircraft and the satellite were known, so it was possible to calculate system characteristics for the aircraft, satellite, and ground station.

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This diagram shows the Doppler contributions to the burst frequency offset.

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. The Malaysian transport ministry pointed out however that this is not the final position of the aircraft.

There is evidence of a partial handshake between the aircraft and ground station at 0019 UTC. At this time this transmission is not understood and is subject to ongoing work.

No response was received from the aircraft at 0115 UTC, when the ground earth station sent the next log on / log off message. This indicates that the aircraft was no longer logged on to the network.

Therefore, some time between 0011 UTC and 0115 UTC the aircraft was no longer able to communicate with the ground station. This is consistent with the maximum endurance of the aircraft.

Inmarsat told The Wall Street Journal that the cause of the partial ping could have several possible explanations including electrical system fluctuations and flight manoeuvres, but that it did not alter the determination of the likely end location of Flight 370. “We’re not looking at this [partial ping] as someone trying to turn on the system and communicate,” Inmarsat said.

The latest analysis by Inmarsat forms the basis for further study to attempt to determine the final position of the aircraft. Accordingly, the Malaysian investigation has set up an international working group, comprising agencies with expertise in satellite communications and aircraft performance, to take this work forward.

Read More:
Malaysian MH370: SATCOMS 101 (Part One)
Malaysian MH370: SATCOMS 101 (Part Two)
This entry was posted in Communications, Features.

One Response to Malaysia MH370: SATCOMS 101 (Part Three)

  1. Lewy Bodden says:

    The Doppler Effect is not mentioned in the Inmarsat Patent.
    There is information that would indicate that the receiver introduces errors in timing and frequency.
    The following information is from the Inmarsat Patent;
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    Inmarsat Global Limited
    Multiple Access Method and Apparatus
    Patent Application number: US 13/140,797
    Publication number: US 20120051273 A1
    ABSTRACT
    A wireless multiple access method allows multiple transmitters to access the same channels so that substantial interference occurs. The transmitters use low rate turbo coding and the receiver uses multi-user detection to separate and decode the transmissions. Different propagation and/or transmission characteristics between the transmitters help to distinguish the transmissions at the receiver. The transmitters may add random variation to their transmission timing, power and/or frequency to assist with decoding.

    Small time and frequency errors between the simultaneous transmissions and multipath fading are different for each mobile terminal helping the receiver to discriminate the various transmissions.

    Each transmission travels via a different propagation channel PC1 . . . PCn which introduces random propagation characteristics such as different delay, attenuation, frequency shift, multipath fading and/or phase noise. The system has no specific means for ensuring signal orthogonality. However, it should be noted that each channel introduces some signature that is likely to be independent or different for each user by affecting at least some (if not all) of the previously mentioned signal parameters. This unique signature allows the bursts to be decoded separately at the receiver.

    Due to the unscheduled nature of return transmissions and the satellite long round trip, transmission coordination in terms of timing, frequency and power may not be possible.

    Hence, in contrast with conventional methods, system performance may be improved by frequency and timing errors.
    —-
    Please explain how the Doppler Effect can be extrapolated from the signals from the aircraft?