Re: [time-nuts] MH370 Doppler
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.
The ATSB (Australian NTSB) report is mute on this as well.
Plus the Doppler reports are only every hour or so, so there isn't much
of a trendline. But some interesting excursions.
I was surprised no time-nuts have ventured over to that blog.
"David I. Emery" wrote:
"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."
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
407-982-0446
The total Doppler in this case is on the order of 100 Hz. The tiny
frequency shifts of an out of spec OCXO is just to small to measure.
The data says at UTC 18:30 the shift was in the mid range and was
about 175Hz. Assume the OCXO drifts 10 parts per million. That
is a lot for an OCXO. But maybe the effect is only about 50 feet on
the ground.
The OCXO error of even 1E-5 is just not very important as it does not
move the aircrafts ground track enough to matter.
On Tue, Aug 19, 2014 at 6:26 PM, Joe Leikhim jleikhim@leikhim.com wrote:
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.
The ATSB (Australian NTSB) report is mute on this as well.
Plus the Doppler reports are only every hour or so, so there isn't much of a
trendline. But some interesting excursions.
I was surprised no time-nuts have ventured over to that blog.
"David I. Emery" wrote:
"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."
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
407-982-0446
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--
Chris Albertson
Redondo Beach, California
Joe Leikhim <jleikhim@...> 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.