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 4467 Articles
David Cenciotti is a freelance 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.

11 Comments

  1. Does the satellite log the frequency delta itself from the target receiver frequency? Or, does it log start time and end time of the burst allowing them to calculate a frequency from the known burst data length?

    • The Inmarsat 3’s were designed more than 20 years ago with minimal hardware served as relays for communications. Over the years, they have been upgraded with many software protocols for the modern communication enterprise.

      http://en.wikipedia.org/wiki/Inmarsat
      So, according to the first figure posted in the blog, some ‘genius’ was able to figure out the over-all Doppler effect of the packets sent by the aircraft after been converted from L band to C band.
      I am still waiting for the same ‘genius’ to claim that these positive Doppler results come from the fact that the satellite time would run so much faster than the aircraft.

  2. To accurately measure doppler you need to know the EXACT expected frequency of the received signal. Any error in that frequency will otherwise be measured as doppler shift.

    Its why radar speed detectors can use doppler on their own reflected signal to determine speed. They know the true frequency because they sent it (even if it is not the exact frequency in theory due to variances in the oscillators).

    If the suggestion is that the planes oscillators, which generate the burst frequency, are of such high precision that they do not have measurable error which would interfere with any doppler calculation on the signal then prove it with a part number and oscillator circuit diagram.

    If they do not know the exact (not average) frequency being sent then they know nothing about the doppler shift of it. Crystal Oscillaotrs have very good long term accuracy but very poor short term accuracy.

    Just look at their published chart, it varies wildly even in the part they are predicting, but apparently because a line could be fabricated which matches a presupposed assumption then thats what it must have done? where is the variance in the second half of the line like the first half?

    Oh and what happened to page 2 from http://www.malaysiaairlines.com/content/dam/mas/master/en/pdf/Annex_I_images.pdf

    Page 1 and 3 are for general consumption but page 2 is not disclosed????

    • Hi
      Re what’s called doppler radar or “speed radar” are quite simple devices. Let’s say you use a 1GHz (half wavelength 0.15m) carrier which you are pointing against a target moving towards you.
      It moves at 100km/hour (27.77m/s)
      You take a small part of the transmitted signal and mix that with the reflected signal giving you a differential signal “tone” in this case of 185Hz (27.77/0.15=185.13Hz)
      So the mixed frequency received is an exact measure of the speed of the moving target. Even if the carrier frequency is not stable let say it varies +/-0.1% the error in the resulting frequency is just 0.2 Hz. The frequency is always positive as the resulting frequency after mixing is the difference between the two so it doesn’t matter if the target moves away or towards you.

      Now the big question?? As the diagram of the pings only shows positive values could it be that they use a similar principle for what they call the dopplershift? As there are several carriers of the “pings” (end to end) I can’t see how they can calculate the actual frequency variation as you have the errors in the oscillators etc. that will destroy the accuracy.
      In my example above I mixed the outgoing carrier frequency with the reflected one. But if I have a modulated signal, a pulse train or sinus wave I can do the same measurement on that signal by mixing the outgoing signal with the incoming. If the pulse-train is shorter than the total travel-time the outgoing pulse can be held for an exact time before mixing with the incoming The beauty of it is that it doesn’t matter how many transponders the signal goes through as we can assume that the delay is constant for each measurement.
      If the modulated signal is e.g. 60KHz each 360 degree phase shift (=1Hz) measured is +/-5km of target movement.
      I have used this method to accurately measure distances to communication satellites using very simple equipment.

    • It turns out my browser was messing up the link and not showing me page 2, its still published.

    • All they’re looking for is relative changes in Doppler. Not absolute values so much.

  3. Hi,
    I have been following this story intensely from day one. people are correct to question the doppler explanation for MH370. All the information being released seems to be designed to support their flight path theory. To me this shows they are covering up something.
    I believe the plane continued flying north towards Beijing and may have circled at some time too.
    To prove that the plane flew north into the South China Sea area the data from the INMARSAT 178E PDR satellite should be accessed. This satellite covers this area and the plane would have communicated with it if it was there.

  4. Some points that are puzzling me:-

    1) Other information published states that the pings occurred at 11 mins past the hour until the last full one at 00:11 UTC. However the graph shows different arrival times for the pings.

    2) At the point on the graph labelled “Possible Turn” there are three data points closely grouped in time. That is inconsistent with the report of a single data point per hour after ACAS was switched off.

    3) A doppler shift of 150Hz in 1.5GHz equates to 0.1 ppm which is about the accuracy of a temperature controlled xtal oscillator. To give confidence that the doppler trend observed is real the system error bounds need to be shown on the plot.

    4) At this frequency the geometry of the path from just north of Malaysia to the Southern Indian Ocean would suggest a wider range of doppler shifts than on the plot perhaps ranging from +500Hz in the north down to -1500Hz in the south.

    • > A doppler shift of 150Hz in 1.5GHz equates to 0.1 ppm which is about the accuracy of a temperature controlled xtal oscillator.

      Assuming the oscillator is a 1.5ghz oscillator and not a lower frequency clock multiplied, in which case the ppm error is likely much worse.

      • The error bar in Doppler effect is actually independent of frequency but strictly of velocity. At 550MPH incidental to the geosynchronous satellite, the Doppler effect is about 50 parts per million or 50 PPM (speed of aircraft divided by the speed of light times the cosine of the incident angle with the satellite times the cosine of the angle off course from the exact east-west trajectory, roughly speaking). However, the transponder of the satellite introduces its own error. This satellite is almost 15 years old towards its end of life, and the drift must be close to its maximum tolerance from the initial deployment. On top of that, the ground station also introduces its own error.

        Despite all that, some “genius” was able to figure out a broadband signal which fluctuate all over the place and determine its Doppler signature. From my own experience, any broadband signal can account for the reported Doppler signature. Maybe this “genius” has very conveniently discovered
        a very clever algorithm to determine the exact Doppler signature just over the past few days.

        However, the whole thing smells fishy to me. The episode is played out
        with false clue after another as if very deliberate to guide further away from
        the truth. On top of that, the obvious employment of crisis actors behaving so dramatically outrageous from any other scenario, including the recent mud slide tragedy in Washington, become very suspicious. Of course, I can never be sure, but if you want me to bet on something, I would have to say the whole thing is an elaborate hoax where the red eye flight MH370 never existed in the first place. That is how an imaginary airplane can just vanish like that. Of course, I do not have enough information or smarts to figure out the motive if my hypothesis is correct.

  5. A solution to flight MH370. I am the Director of the Intractable Studies Institute in New Mexico, USA. This discussion of the missing flight MH370 seems technical enough that I think the commenters can understand what I’m about to demonstrate. This mystery seems hard to solve, but once we understand the assumptions, and change them, then it becomes simpler.

    It is simple to fly a plane West to the Maldives, yet once hourly a ping-check frequency doppler shift reports the plane receding from the satellite. How? Once an hour at the exact minute, turn and fly eastward, the doppler shift will show the plane receding from the satellite, throwing investigators entirely off track. This will require the person(s) flying the plane to know the pings arrive hourly and exactly to the minute, and this is an intentional maneuver. So the plane flies 59 minutes West, 1 minute East, eventually reaching the Maldives where it was reported flying extremely low, but dismissed likely incorrectly. A simple maneuver is to fly a circle every hour, known in submarines tactics as a crazy Ivan.

    Regarding the time of round-trip-times to estimate distance to satellite and thus arcs, I suspect that is not reliable unless it is a physical mirror. If not a physical mirror, processing response times surely can be changed where a computer or circuit is involved.

    So, there is the hypothesis, shoot it down if you can technically.

    Sincerely, Patrick Rael, Director, Intractable Studies Institute
    IntractableStudiesInstitute.org. motto: Think Outside The Box.

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