[time-nuts] MH370 Doppler

David I. Emery die at dieconsulting.com
Mon Aug 18 18:14:28 EDT 2014


On Mon, Aug 18, 2014 at 04:18:30PM -0400, Brian, WA1ZMS wrote:

> I thought that Inmarsat terminals had AFC to the sat's down-link.  Not
> to the degree of true phase-lock like DSN has but enough so that the
> sat's abillity to do doppler correction on the uplink is valid to help
> with BER, etc... Otherwise the doppler correction would be of no help
> and not be needed.

	I beleive most Aero Classic terminals use a fairly good
OCXO.   Somewhere I may have a limit spec on stability, but those
docs are not immediately handy.

	Normally a demod in the terminal is kept tuned to one of the
continuous L band control channels which I believe may be Doppler
compensated in the ground uplink transmitter for the 6 Ghz C band uplink
Doppler and LO drift on the satellite so it is correctly on frequency as
radiated on the L band downlink.   This could supply a frequency
reference to the terminal that could be used to AFC the terminal
frequency standard so it is close to right on.   Doing this would
require terminal firmware to determine estimated Doppler at the L band
control channel downlink frequency from the satellite based on some
estimate of the planes position, satellite position and relative
velocities.

	The QPSK DSP modems used at both ends would be easily able to
supply estimated frequency offset, both on the ground at ground earth
station and in the plane. It is presumably true that this measurement is
corrected on the ground end for the Doppler due to movement of the
satellite relative to the ground station on the C band downlink relaying
the L band uplinks from the plane so it reflects frequency error as seen
at the satellite on the L band uplink with the downlink and satellite LO
drift terms removed.   I presume this is what INMARSAT is reporting, but
am not sure.

	IIRC the plane is expected to adjust its burst uplink frequency
and timing to come out right at the satellite receive antenna... thus 
compensating for the uplink Doppler at L band and the time delay too.
But I do remember that the ground supplies feedback on the control
channel as to how much the plane is off so it can adjust...

	Guess it might be time to dig out the docs again.

-- 
  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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