[time-nuts] MH370 Doppler

[Philip Koh, Ph.D.]

Joe Leikhim <jleikhim at ...> writes:

> 
> I raised this on the Duncan Steel website and was pretty much blown 
off.
> 
> "Oh there is a nice stable OCXO aboard" etc.
> 
> Well DUHH yes there is an OCXO aboard and if it is good to -20 to 
+75C, 
> or just -20 to +60C and there is a huge fire raging around it for an 
> hour, and then perhaps later the plane decompresses at 32,000 feet and 
> ice forms inside the aircraft that all has to be a factor to consider.
> 
> 

Funny, I also raised this issue on the Duncan Steel website and was also 
blown off.  

I think you are dead on about the response of an OCXO to temperature 
shocks.  I designed a satcom frequency converter for 14 GHz that was 
phase locked to an internal OCXO.  When we were trying to make phase 
noise measurements at 100 and 10 Hz offsets, we had to allow the units 
to warm up and sit in still air for 20 minutes so the frequency would 
stabilize enough so that the carrier would remain on the spectrum 
analyzer screen when set at 1 Hz per division.
  I also recall that when the equipment lid was opened, you could blow 
on the outside of the OCXO case and watch in real time the carrier shift 
side to side on the spectrum analyzer screen.  
   I think people are seeing the "Oven-Controlled" phrase and assuming 
this means they are immune to temperature effects.  In reality, the oven 
is what gets a crystal from the 1E-5 stability range down to 1E-7 or 1E-
8, but at 1600 MHz that is still 16 to 160 Hz.
   Unless the OCXO is double-ovened, my experience was that the OCXO 
specified stability is really achieved in steady-state conditions, when 
all the internal parts of the OCXO are at their nominal temperature 
gradients.  During temperature transients, especially fast changes, the 
heater circuit may respond with overshoot, undershoot, etc.  Plus, the 
case-to-crystal temperature gradients are all different than the 
conditions in which it is calibrated.
    Similarly, during a loss of power and reset, all bets are off, and 
our OCXOs would way overshoot as the oven heater circuits suddenly 
kicked in full blast.  The number of minutes to gradually re-converge on 
stable operation may depend on many factors such as OCXO size and 
thermal inertia.
   I think given all the unknowns, all kinds of situations like power 
outages and sudden temperature changes could have occurred right before 
any of the hourly pings.  Comparisons to BFOs from other flights that 
were completely normal level flight with nearly zero temperature changes 
may not apply.
   I can't find any schematics of what equipment was inside the MH370 
aero classic terminal; hard to guess if the OCXO is a compact, low cost 
OCXO or some super-performing NIST marvel.
    In general, all the discussion of BFO talking so confidently about 
7Hz frequency variations at 1600 MHz, and the inherent assumption of 4 
parts per billion frequency stability under possible temperature changes 
just feels unrealistic to me.