Hi!
On 10/24/2011 10:02 PM, Tom Van Baak wrote:
I have a more basic time-nut question. Why is it a problem at all?
How can the time uncertainty between two known and fixed locations be
that large?
In principal it's simple. But the logistics of equipment and cables
and rooms and labs is quite complicated. Read a few documents
at http://www.ohwr.org/projects/cngs-time-transfer/wiki and you'll
get a sense. All the numbers should add up. But it didn't.
The stakes are really high so everyone is double checking their
piece of the pie more carefully than a moon landing.
I think I have essentially all the equipment needed for both sides.
Double frequency receivers with antennas, counters, cesiums etc. Lacking
5071As but...
What I don't have is the time to build two complete sets with software
etc. Not that I am not trying, as being the time-nut that I am.
I think a few other time-nuts such as Tom would also be able to provide
the equipment. Again the time to build a pair of functional sets.
If they know they have a 70ns uncertainty in time, that would suggest
that their time measurement is known to be varying at one or both places.
Is this just from a spec or do they see a true variation in time
between something, and if so compared to what?
It's not varying. It was a 3 year experiment and there appears to
be a fixed discrepancy of 60 ns in time or 18 meters in distance.
Remember it's not certain that the error is in the timing at all; it
could just as well be in the distance. Or in the start trigger, or the
stop signal, etc.
Just re-validating the distance, time and time-distribution would be fun
and time-consuming.
Is this time difference or variation between several difference timing
devices at each end or is it variation when compared to time of flight
of the supposedly same neutrinos?
The latter.
I can not say anything about the accuracy of my absolute time, but the
difference and uncertainly comparing the phase difference between
different external Osc Tbolts at the same location is way way under 70ns.
Yeah, this is true for most GPS receivers, which is why it's hard
to imagine the problem has anything to do with their GPS timing
set up. Their error budget is a couple of ns, stable over years,
which is why they use atomic clocks along with dual-frequency
carrier-phase receivers in common view, and calibration visits by
more than one national UTC lab, etc. It would appear they really
did their homework.
It's unfortunate that GPS even got brought into the whole neutrino
situation because GPS brings with it all sort of UTC and physics
and relativity baggage. The experiment could have done relative
timing without using GPS at all. It's not about GPS; it's about having
synchronized clocks at two locations. There are many ways to
achieve that. And when the stakes are high, then one must do it
in multiple independent ways.
This is also why the preliminary PTB report is a bit unfortunate, as it
leaves bits and pieces out which the professional should need, but also
the larger picture of bias compensations intended for the
non-professional on GPS based time-transfer systems. There are many
enhancements to be done to be able to present to physics people. The
necessary backlog of articles to read is quite high.
So that's why there's talk of direct fiber links, radio links, satellite
links (non-GPS), traveling clocks, etc.
Indeed. PTB should have used a traveling clock alongside their GPS
receiver system.
Sure lots of BASIC things to do to make sure the two Tbolts are set
the same so that their oscillator's phase do they agree, such as using
the same type antenna and same cable and length, and getting the
antenna's location correct, etc, etc,
but basic stuff and seems like if using the same basic GPS system at
two different locations, what would the additional problems be except
to make sure both ends are syncing on the same 100ns 10MHz cycle.
I was under the impression that getting down to ns uncertainly
differences (and staying there) at theses distances is old stuff using
common view GPS.
So what are the problems that cause their large timing uncertainty?
Yes, it is very old hat to those in the timing community. It just takes
time for the rest of the physics community to catch up. Many of us
amateurs have better timing at home than most physics laboratories.
Indeed. It's strange that a few private time-nuts have the capability to
do this, but it is fully possible. There is a few things like recording
RINEX data, get it post-processed alongside some other measurement stuff.
So, to come back to the original question, I agree with TvB that the way
to go would be to get the assistance from NIST. I think we are a few
time-nuts that would be "happy to assist" as a fun treat. :)
However, I think that one should combine traveling clock (traveling both
directions) with double frequency GPS receivers exercise. It would
assist in providing consistency.
Cheers,
Magnus
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