[time-nuts] MH370 Doppler

David I. Emery die at dieconsulting.com
Tue Aug 19 19:21:31 EDT 2014


On Tue, Aug 19, 2014 at 12:08:24PM -0400, Joe Leikhim wrote:
> My understanding is that the BFO and BTO values are self reported from the 
> SDU (Terminal in the aircraft) and they represent adjustments made by the 
> SDU. If so the BFO value would be the AFC adjustment relative to the OCXO 
> onboard. My contention is that if the investigators are assuming the OCXO 
> is 2Hz high and reporting an +88Hz offset as 86Hz Doppler, what if in fact 
> the OCXO is 10Hz high? Then the doppler is 78Hz and that means the velocity 
> and location at each of the pings is way off.

	Clearly they have a history of the MH370 Aero Clasic terminal
measured burst frequency at the ground earth station and the BFO value
the SDU reports it used WHEN the aircraft was on the ground before it
took off and WHEN it was being tracked by radar/mode-s/ads-b and was in
a known position going at a known velocity on a known heading.    This
should presumably allow determination of the baseline OCXO long term
error, and some indication of its short term drift as well.

	Whether that particular SDU attempts to use any form of EFC of
its OCXO based on measured satellite L band downlink frequency error
corrected for doppler or not I do not know.   It is quite possible  that
any correction for OCXO error is just a value factored into computing
the BFO to use and not used for actually correcting the standard with a
EFC DAC.

	If that is true then the drift should be presumably be pretty
typical of the class of OCXO used in the SDU which I suspect should be
fairly small once it warms up - over a 6-8 hour period after warmup.
And there may be some history of that particular terminal from previous
flights to validate this.

	Of course if environment significantly changes the drift
performance of that particular OCXO it is possible that temperature, or
pressure or power conditions were so different on the fatal flight that
the drift might be larger and unknown in character... not sure.  It is
an error to consider of course.   Not clear to me how carefully it has
been or what possible factors have been considered.   But surely the
folks doing the analysis know about these issues.


-- 
  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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