[time-nuts] Airraft Ping Timing

J. Forster jfor at quikus.com
Tue Mar 25 19:00:39 EDT 2014

Certainly, if it's a bent-pipe repeater, that makes extracting the Dopplar
a whole lot easier. Furthermore, since it's unlikely that the missing
plane was the only signal, you can essentially do a differential Dopplar
measurement against other sorces, stationary or moving in a know



> On Mon, Mar 24, 2014 at 06:15:57PM -0700, 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.
> 	Perhaps some of the readers here are unaware that the INMARSAT
> F3 in question is a bent pipe repeater in both directions.   It takes a
> C band uplink from the ground and turns it around to L band, and turns L
> band uplinks around to a C band downlink.
> 	It has 8 spot beams, and one regional beam.   Channelization of
> the uplink and downlink bandwidth and an board switch matrix allows
> various allocations of frequencies and bandwidth to the 9 beams varying
> with load and demand.
> 	There is no on satellite signal demodulation/modulation or
> protocol processing  for the classic AERO signals to/from the plane ...
> that is ALL done on ground at the GES (in Perth Australia AFAIK).
> 	This would make it possible for INMARSAT (and others in the
> region tasked with monitoring such things) to capture the actual
> repeated RF from the plane and digitize it - this happens in the ground
> equipment as part of the normal processing anyway - and dumping it to a
> disk array somewhere is certain to be going on, either both inside
> INMARSAT at the GES or at least at other (less public)  sites such as
> Alice Springs.   The C band downlinks are global beams BTW and can
> be received anywhere that sees the satellite.
> 	As such the quality of the recovered Doppler and other signal
> parameters is very much a function of the stability of the various LOs
> (and sample clocks)  involved, which I believe can correctly be presumed
> to be really high grade both in space and certainly on the ground. AES
> (plane) timing and frequency may be less good, but it is more or less
> locked to the L band downlink timing and frequency signals as reference.
> 	The newer INMARSAT F4 birds do have DSP processing on the
> satellite, but apparently NOT used for demodulating and processing the
> various control channel signals on the satellite - but just for doing
> beam forming and power allocation for the 120 spot beams these birds
> support.   This of course would impact delay through the satellite
> for precision timing and ranging.
> 	But so far there are no reports that the F4 POR satellite was
> involved. The high gain antennas on the AES (plane) are fairly
> directional and if they were in use there might not be a lot of signal
> seen on the POR bird.   Not sure if those pings would have been sent
> via a low gain antenna on the AES, but I suspect normally not.
> --
>   Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass
> 02493
> "An empty zombie mind with a forlorn barely readable weatherbeaten
> 'For Rent' sign still vainly flapping outside on the weed encrusted pole -
> in
> celebration of what could have been, but wasn't and is not to be now
> either."
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