[time-nuts] Airraft Ping Timing
jimlux at earthlink.net
Mon Mar 24 22:00:19 EDT 2014
On 3/24/14 6:15 PM, Chris Albertson wrote:
> Yes, word is that they were able to determine the Doppler shift in the
> plane's signal. I'm surprised this was even recorded but it must have been
> in the satellite's telemetry downlink. Projecting radial velocity and
> constraining it to be close to the earth's surface, I guess determines one
> path and the direction on it.
> The key they said was getting the doppler shift
> I used to work in the telemetry business. The experts (not me) would be
> able to pull information that you'd never think possible from it.
And sometimes you have to just be lucky..
For instance, most receivers in space have a "static phase error"
telemetry which is basically the voltage going to the VCO in the carrier
tracking loop (or the digital equivalent). That can be used to infer
doppler, assuming you've taken out all the other things (temperature,
etc.). Comparing SPE among multiple signals, with some of them known,
would be one way.
I'm not saying this is what they did, but it's the kind of thing that if
you get lucky, and you happen to have the right telemetry, and you have
someone who can figure this stuff out, you can do it.
INMARSAT birds are pretty sophisticated, RF wise. They have broad
coverage but also some spot beams, so one might be able to do all sorts
of things that aren't originally thought of.
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