
How British satellite company Inmarsat tracked down MH370 - pedrocr
http://www.telegraph.co.uk/technology/news/10719304/How-British-satellite-company-Inmarsat-tracked-down-MH370.html
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cynwoody
The Doppler stuff is new (as of the 24th).

What I had understood before was that the airplane carried an Inmarsat
satellite transceiver which remained active, even though certain subsystems
that use it (notably ACARS) were disabled. While active, the transceiver
communicates with the satellite once an hour (the "pings" at 11 minutes past),
even if it has no traffic to send. Failing to disable the transceiver in
addition to the other systems would have been an oversight on the part of the
plane's hijackers, assuming it was hijacked.

Apparently, Inmarsat had available accurate ping timing data. Combined with
knowledge of turnaround delays, this would enable them to calculate the
plane's distance from the satellite during each ping. That defines an
imaginary sphere in space, which intersects with the earth's surface to form a
circular locus of where the plane could have been at the time. Presumably,
there were eight such ping interactions, ending with the one at 0811 Kuala
Lumpur time. There was no 0911 ping, meaning the plane had either crashed or
landed and powered off before that time.

And so we had the northern and southern arcs. These were obtained by cropping
the circle based on factors such as how far the plane could have flown in the
available time, whether or not military radars, if any, had detected the
plane, etc.

Now comes the Doppler data. Apparently, Inmarsat also has an accurate read of
the plane's transmit frequency. The extent to which this frequency differs
from the nominal frequency would indicate the plane's velocity towards or away
from the satellite at the time of the ping.

Today, Chris McLaughlin revealed that Inmarsat had studied the Doppler
patterns of normal flights in the same region and concluded that the Doppler
data for MH370 indicated that the southern arc was the right one. Hopefully,
Inmarsat has it right, and the black boxes will be found before their beacon
batteries run out.

~~~
brador
Is the data on the black boxes dependent on continual power? That seems
strange.

~~~
Shivetya
the second biggest issue with the data is that it will have written over some
of the information that would have occurred at the time the flight deviated
from the assigned course.

It used to be that voice recorders with 30 minute loops, I am not sure if that
is still the case.

~~~
masklinn
Depends on the model (and how much you pay for it). Honeywell's SSVCR[0] is
available in 30 minutes and 120 minutes.

[0] [http://www.sea-
avionics.com/lc/cart.php?target=productDetail...](http://www.sea-
avionics.com/lc/cart.php?target=productDetails&model=SSCVR&substring=sscvr)

~~~
Fuxy
That's odd with today's technology we should be able to record hours of voice
on to an a usb stick.

Granted that is a lot more fragile then a plane black box but still. There
should be a way to record a lot more then 120 minutes.

~~~
roc
I suspect the answer is similar to "why don't they stream FDR-data, live, for
all flights?".

That being: the airlines feel the cases in which extra measures matter are
only a small subset (flights with problems in incredibly remote areas) of a
small subset (flights that have any problems at all) and thus not worth the
extra cost/complexity/risk to all the other flights.

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wallflower
A pretty detailed explanation from Reddit user XenonOfArcticus.

[http://www.reddit.com/r/MH370/comments/218i36/how_the_satell...](http://www.reddit.com/r/MH370/comments/218i36/how_the_satellite_company_inmarsat_tracked_down/cgapbao?context=1)

~~~
ynniv
A better one:
[http://www.reddit.com/r/MH370/comments/218i36/how_the_satell...](http://www.reddit.com/r/MH370/comments/218i36/how_the_satellite_company_inmarsat_tracked_down/cgb7b6j)

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zaroth

      engineers were able to narrow the location of the plane down
      with an initial analysis of the Doppler effect on the signal
      from the flight’s pings and the plane’s approximate altitude.
    

How and why would they have a record of this to be able to study it?

~~~
Serow225
I'm curious about that too, it seems very unlikely that the actual digitized
RF waveform was available for analysis. My guess, based on working with
somewhat similar systems in the past, is that the ping packet content was also
tagged by the satellite with the measured carrier frequency (or offset from
nominal) and this value had enough precision that it let them calculate the
Doppler effect. The reason I would say this is because I've done similar
things in the past for telemetry applications, it can be really handy for
debugging when things aren't working right.

~~~
chronomex
Inmarsat is a "bent pipe" system, meaning that the satellite just amplifies
and sends down to the base station everything that it receives. So the
operator's RF fanciness is limited only by the noise factor in the satellite's
amplifiers.

~~~
rz2k
Ah, that makes a lot more sense, though nevertheless impressive precision on
the part of the base station.

However, given the corridors, it still seems like the suggestion is that the
Doppler measurements said it was moving rightward or leftward, when it should
only say how much the aircraft was getting closer or getting further away.
Looking at the map alone, it doesn't seem like that information would help
distinguish which corridor it was in.

~~~
Tuna-Fish
The satellite was also moving (south or north), and the measured
redshift/blueshift could be used to determine if the plane was north or south
of the satellite.

~~~
rz2k
I thought it was geostationary rather than polar orbiting.

~~~
Tuna-Fish
Geostationary satellites don't actually stay completely stationary for any
length of time without active stationkeeping. They very quickly develop a
north-south motion which makes them trace out an analemma, a sort of figure
eight on the ground.

~~~
rz2k
I first wrote geosynchronous and approximately geostationary, then decided it
was too cumbersome. What sort of magnitude does the analemma have? Also is the
effect amplified by it being a "bent pipe"?

Even if it isn't intentional, in scenarios such as measuring a Doppler shift I
can see how it can be a useful tool. Almost analogous to microsaccades we make
with our eyes.

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Patient0
In the actual video he seems to have misunderstood what the Doppler effect
actually is. He gives the example of a train whistle "getting louder as it
comes towards you and fainter as it moves away" but he should have said
something along the lines of "the same way that a train whistle has a higher
pitch as it moves towards you and a lower pitch as it moves away from you".
The intensity of the signal is much harder to measure/calibrate than the
_frequency_ of the signal.

~~~
jacoplane
Chris McLaughlin (the guy in the video) is a PR guy, not a scientist.

~~~
mkonecny
We learned what the doppler effect was in Grade 11 physics - it's literally
high school material.

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awalton

        ** is thought to have tracked down.
    

They still haven't actually found the plane, they've just got a very well
reasoned assumption of the area the plane should be in.

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Tloewald
...and by the way, we offer a service that only costs $1/h/plane and we'd like
governments to force everyone to pay us. Now, I have no idea whether
$1/h/plane is reasonable (is that to cover the satellite comms, or is that on
top of them?) but it is kind of stunning that planes aren't tracked
automatically and as a matter of course the way, say, most people with cell
phones are.

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topbanana
As an interesting aside, Inmarsat are based on the Old Street roundabout in
London. This is reputedly the centre of the startup scene.

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sixQuarks
Call me a conspiracy nut, but here's what I believe may have happened:

It's almost a certainty that the US government has spy satellites video
recording the entire planet at all times, at a level good enough to
distinguish individual aircraft. The US spy agencies probably knew where the
plane was right away, but they can't just tell everybody or the jig is up.

So they may have just contacted this satellite company, told them where the
plane is, and came up with a technical explanation of how it was located.

~~~
ewoodrich
The US certainly has a bloated defense budget, but I strongly doubt that we
have satellites constantly trained on the typically empty oceans that cover
70% of the Earth.

~~~
marshray
While I don't subscribe to GP's view that satellites have bandwidth for "video
recording the entire planet at all times", it seems plausible that SIGINT
satellites would have a relatively constant bandwidth available for
observation. As a (non-geostationary) satellite passes over relatively
unpopulated areas (e.g., the southern Indian Ocean), it will be "bored" and
thus far more likely to notice a single low-power transmission.

On the other hand, if it is storage and downlink bandwidth that is limited,
sats would probably be programmed to ignore or discard uninteresting signals.
This seems likely to me.

So the question may be whether commercial airliner pings from the southern
Indian Ocean area were considered 'interesting' at the time and, if not, did
anyone realize there was a rogue 777 in time to re-task the satellites?

Regardless, I'd bet those signals are considered interesting going forward.

~~~
dingaling
The other complication is that Inmarsat up-link is a steered spot-beam ( the
antenna on the aircraft is stabilsed and driven, or phased-array in recent
implemenations ). So the SIGINT sat would have to be passing through the up-
link 'beam' precisely as it was transmitting.

Down-links are ( depending on generation and usage ) either from one of an
array of narrow-beam antennae on the Inmarsat or from the wide-area general
broadast antenna; much easier to intercept, you and I can do it with a sat-TV
dish and an RTL dongle.

~~~
oakwhiz
With the right equipment it may be feasible for an eavesdropper to try to
listen to information from one of the satellite antenna's sidelobes.

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sebcat
What people write in the article's comment section is certainly an interesting
read. Irrational at best.

