[time-nuts] Airraft Ping Timing

David I. Emery die at dieconsulting.com
Tue Mar 25 18:48:34 EDT 2014

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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