Doppler effect analysis on Satellite pings disclosed MH370’s final route

Even if the aircraft’s crash position could not be determined, Doppler effect analysis on SATCOM pings enabled INMARSAT to determine MH370’s final route over South Indian Ocean until a final, “partial ping,” received 8 minutes after the last known one.

Whereas search for debris from the missing Malaysia Airlines MH370’s Boeing 777 continues in a wide area located about 2,300 kilometers to the southwest of Perth, Australia, INMARSAT released new details to explain how the British satellite telecommunications company was able to exclude the so-called “north route” and focus on the southern one, pointing towards the South Pole.

As already explained on a previous post, hourly SATCOM system pings continued for more than 7 hours since the Loss Of Contact with MH370, until 08.11 AM LT.

Based on the round-trip times of such pings, two arcs made of all the possible positions located at the same distance from the INMARSAT satellite were drawn.

But it was further analysis, on Doppler Effect, as well as correlation between the “signature” of other B777s, that clearly indicated the aircraft southbound route.

Doppler Effect

The Doppler effect is something we are familiar without even knowing it. The sound of the ambulance’s siren or the train whistle are among the most common examples of how Doppler Effect works: the high pitch of the siren of an approaching ambulance suddenly drops as the vehicle passes you. Even if the source wavelength and speed do not change, movement of the source alters the wavelength and frequency of the sound.

You can use several online tools to calculate the frequency change induced by motion.

Since satellite pings are carried on a radio wave, the sensed wavelength, frequency increase or decrease depending on the fact the aircraft is moving towards or away from the satellite.

The difference between the expected received frequency and the actual measured one due to Doppler Effect is known as Burst Frequency Offset.

By comparing the Burst Frequency Offset due to Doppler on MH370 against the predicted one based on six B777s flying on the same day, INMARSAT could determine close correlation for the southern route and eliminate the northern one.

BFO MH370

Here’s an excerpt from UK Air Accidents Investigation Branch (AAIB) release that explains how INMARSAT calculated the route.

INFORMATION PROVIDED TO MH370 INVESTIGATION BY UK AIR ACCIDENTS INVESTIGATION BRANCH (AAIB)
25/03/14
[…]
As you have heard, an aircraft is 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 it was established that after ACARS stopped sending messages, 6 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.

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.
While on the ground at Kuala Lumpur airport, and during the early stage of the flight, MH370 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.
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. I must emphasise that this is not the final position of the aircraft.

 

Here below is an INMARSAT image which shows the southern tracks for a ground speed of 400 and 450 knots ground speed.

 

MH370 tracks

 

Last “Partial” Ping

Noteworthy, INMARSAT collected  evidence of a partial handshake between the aircraft and ground station at 00:19 UTC, 8 minutes since the last acknowledged response. This partial ping is currently being investigated: there are several different theories, including the one that the final handshake was attempted outside of the hourly window, possibly at fuel starvation because of power fluctuations.

At 0115 UTC, when the ground earth station sent the next log on / log off message, no response was sent by the plane, indicating that the MH370 was no longer logged on to the network (because already crashed).

Lessons Learned

1) Pilots have the power to make aircraft almost invisible to radars. This will have to be addressed in some way, with some system capable to track the plane regardless of the aircrew’s willingness.

2) Black Box data have to be streamed via satellite and stored for the shortest time possible (until the next flight, then automatically erased) somewhere (for instance, in a Cloud Network architecture, to save money and have it immediately available, should the need arise).

Image credit: INMARSAT via AAIB

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About David Cenciotti
David Cenciotti is a journalist based in Rome, Italy. He is the Founder and Editor of “The Aviationist”, one of the world’s most famous and read military aviation blogs. Since 1996, he has written for major worldwide magazines, including Air Forces Monthly, Combat Aircraft, and many others, covering aviation, defense, war, industry, intelligence, crime and cyberwar. He has reported from the U.S., Europe, Australia and Syria, and flown several combat planes with different air forces. He is a former 2nd Lt. of the Italian Air Force, a private pilot and a graduate in Computer Engineering. He has written five books and contributed to many more ones.

9 Comments

  1. There is a big assumption here that the airplane was traveling at full speed in a straight line; the southern track is favored as it is the most likely fit for a straight trajectory based on this Doppler data. However, what would be the best fit if the airplane were to make multiple turns or was circling over the Indian Ocean at a slow/holding pattern rate of speed? Is this possible that the plane slowly circled? It seems like it could be…

  2. To compute Doppler shift in frequency one must first project the airplane’s velocity to the line of sight of the satellite. The projection of the airplane’s velocity (~ 800 km/hr) to the line of sight of the Inmartsat satellites reduces it by a factor ~1/35 (1000 km /satellite orbit altitude 35000 km). The resulting Doppler shift is about 0.0001 GHz for an L band signal (the frequency of the Inmarsat SatCom signal), or 0.1 MHz, or 100,000 Hz. I note that the plot of the burst frequency offset has a unit in Hz. This implies that the values shown in the plot are in the range of a few hundred Hz, which are wrong and in fact too small to be detected in telecommunications. I wonder about the noise level in frequency detection with the Inmarsat L band system? Is 0.1 MHz in fact above the noise level for detection?

    • I made a mistake earlier. The actual Doppler shift should be only in the range of a few tens of Hz. Can the telecommunication system detect such a small shift in frequency??

      • And then you have to factor in transient response of the transmitter PLL and consider thermal drift. I think they are grabbing at straws making the data fit a curve.

  3. I still do not understand how you can have such a catastrophic failure–one that does not permit the pilots to communicate the situation–and still have the plane continue flight for such a prolonged period of time.

    It doesn’t add up. At this point, if none of the alleged debris is linked to the aircraft, it might be wise to assume that whoever deviated the craft knew precisely how officials would try to locate the missing plane, and maybe even took steps to intentionally deceive investigators. I know that this thought seems farfetched, but this whole ordeal has been beyond belief already.

    • Fire decompression hypoxia. Google- Payne Stuart and then Captain Button A10 warthog for examples.

    • I would add that the sighting by oil rig worker Mike McKay was quite credible and spot on. he said he saw an aircraft on fire west of Songa Mercur oil rig, but the fire went out after 10-15 seconds. I have seen an unpublished email by Mike McKay to Denise Wong in which he added that the flames were bright orange down the left side of the aircraft.

      Naked flames can’t exist at 35,000ft therefore any fire had to have started inside the cabin and then vented outside through a melt hole in the skin. Once pressure had equalised then such a fire would naturally self-extinguish.

      http://i257.photobucket.com/albums/hh212/727Kiwi/MH370/Egyptair667_zpscbe3c44e.jpeg

      This was a hole melted in the side of Egyptair flight 667 in 2011 after an electrical fire melted the co-pilot’s oxygen hose. It took just 20 seconds to melt this hole before the co-pilot, who was out of his seat fighting the flames, reported seeing daylight through the cockpit wall.

      The resulting damage was not enough to cause structural failure, but quite enough to kill or incapacitate the crew had this been MH370. The Boeing 777 is designed to withstand power interruptions on autopilot and the aircraft is also designed to manage electrical power failures and restore systems autonomously without human intervention.

      MH370 most likely suffered an electrical failure and lost on board navigation displays before the fire broke out, then pilots tried to use an old fashioned radio beacon at Con Son island not far from the oil rig as a reference point to turn back for Singapore. I would speculate that pilots changed heading on the autopilot before fire engulfed their cockpit. From Con Son island MH370 had exactly 5.5 hours flight time before the last satellite contact.

      If you trace that line from Con Son over Singapore to where it intersects the southern arc at 5.5 hours you get this position: (31.3147S, 91.3128E) and from that I predict the approximate location of MH370:

      http://i257.photobucket.com/albums/hh212/727Kiwi/Crashsite_zpse026e02a.jpg

  4. Hi, where is the graph showing the doppler shift for each ping from MH370 ?

    Google is displaying that image for this page, and that’s why I’m here.

    Very informative graph. Why is it not here any longer ?

  5. The only published hard information seems to be that contained in three
    diagrams:

    http://www.malaysiaairlines.com/content/dam/mas/master/en/pdf/Annex_I_images.pdf

    The plots are inconsistent on two points:

    1) All Doppler frequency offsets are positive, implying the distance from Inmarsat-3 F1 and
    the aircraft was always increasing: the plane could not have flown in
    a westerly direction. I have communicated this inconsistency with
    Inmarsat headquarters in London, but received no answer.

    2) Indonesian authorities insist MH370 never entered their
    airspace:

    http://www.thejakartapost.com/news/2014/03/26/govt-insists-mh370-did-not-fly-indonesian-airspace.html

    If that’s true, the track shown cannot be correct.

    Why are the nations wasting millions searching in areas that don’t make
    sense?

    Because the Malaysian authorities are successfully distracting them.

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