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.
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.
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
Related articles
Patrick Rael: Yes, it would be possible to “simulate” the Doppler shifts in the manner you describe. But the satellite-aircraft distance would show a net decrease each hour, while the claim is it increased.
It’s important to remember that propagation time and Doppler shift both measure only in the radial direction (line of sight from satellite to aircraft). The first gives distance, the second velocity (directly convertible to distance if you assume uniform motion in the same direction for the hour). So the first gives an integrated motion over the hour, the second an instantaneous motion at the transmission time each hour.
So it’s clear the two data sets are inconsistent. No explanation of this has been forthcoming.
Interestingly, the data don’t seem to be posted on the INMARSAT site any longer. The graphics have disappeared:
http://www.inmarsat.com/wp-content/uploads/2014/03/Doppler-correction_contribution.jpg
Obviously, their desire to discuss this is negative.
I notice the news services are getting bored with the whole missing plane story and so soon the heat will be off these agencies to answer questions. Mission successful.
Chris, the data are still on the Inmarsat website. See http://www.inmarsat.com/news/malaysian-government-publishes-mh370-details-uk-aaib/
Both the propagation times and the Doppler measurements pertain only to the radial (light of sight motion) between satellite and aircraft. The propagation times give distance, that is the integrated radial motion for each hour. Doppler gives instantaneous velocity – convertible to distance if we assume uniform motion over the hour.
The two data sets as plotted are inconsistent. The Doppler velocities could be created as Patrick describes, but the distances would show up as decreasing, not increasing as claimed.
INMARSAT website no longer displays this information – it has apparently been removed.
It is hard not to think of these plots as a fabrication. For what purpose? For whose purpose?
I’ll post this information again and hopefully someone will comment.
There are two INMARSAT satellites that would have received the ‘pings’.
Inmarsat – 3 F1 also known as 64E IOR and
Inmarsat – 3 F3 also known as 178E POR
64E IOR would have been approximately 5km to 10km West of the aircraft depending on the flight path taken.
178E POR would have been approximately 11km to 16km East of the aircraft depending on flight path taken.
Also the doppler effect would be much more pronounced from the 178E POR satellite as it would have appeared at a lower altitude in relation to the plane.
Using information from both these satellites would almost allow a
triangulation of the flight course due to overlapping of the arcs being created.
However only information from 64E IOR has been made available. Why is that?
What don’t they want us to know?
Is it possible that the data they are using is actually off the 178E POR satellite?
Could they have made a mistake? Perhaps a deliberate mistake which they can correct later as a ‘get out of jail’ card?
This whole investigation is a shambles!
The JEWS did it!!! LOON LIB American Conspiracy Theorist On Iranian State TV: Jews Likely Behind Flight MH370 Disappearance… I’m SHOCKED by Iran!! American political commentator Dr. Kevin Barrett recently said that he believed that Israel may have been behind the Malaysian plane mystery in “another planes-into-buildings deception like 9/11.” Speaking on the Iranian English-language Press TV channel on March 30, 2014, Barrett raised another theory: that British banker Jacob Rothschild was behind the disappearance of Malaysian flight MH370. http://weaselzippers.us/181585-american-conspiracy-theorist-on-iranian-state-tv-jews-likely-behind-flight-mh370-disappearance/
After looking closely at the doppler shift explanation for MH370 and
plotting values in several different models I think it is an unreliable
means of determining the direction this plane took.
The effective
change in speed due to the satellite being between 25 – 30 km away from
the plane and having an offset angle of between 10and 20 degrees from
the vertical is extremely small.
Then without knowing the planes
altitude for each ping it becomes impossible to calculate accurate
values. This is all assuming the electronics are perfect and no other
atmospheric influences are taking place.
A nice theory which would work if the satellite was only 10km above the earth instead of 35km.