[time-nuts] Testing frequency using NTP Bruce GPS ps
Mike Monett
XDE-L2G3 at myamail.com
Sat Oct 4 02:28:13 UTC 2008
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> Mike
> They actually use an augmented form of GPS common view for which
> the GPS PPS signal and its timing variations are largely common to
> both locations and thus largely cancel when comparing the
> frequencies at the customer site with the standards at NIST.
> With a good timing receiver (and antenna location) the sawtooth
> corrected PPS signal timing noise can be as low as a few nanosec.
Is that like the CNSC02-O1 High Performance PPS system? From their
web site, they state:
"Provides for dynamic hardware correction of the 1PPS quantization
("sawtooth") error. This reduces the noise on the 1PPS pulses from
(typically) +/-27 nsec (15 nsec 1-sigma) to +/- 11 nsec (3 nsec
1-sigma). More importantly, eliminates periods of bias error caused
by the quanization error going through a "zero beat" period that can
last one to two minutes about every 10 minutes or so."
http://www.cnssys.com/cnsclock/CNSClockII.html
That paragraph is very interesting. The "zero beat" error is
particularly nasty. I tried to get more information, but it wasn't
really clear how their correction method fixes this problem. Could
you explain it a bit more?
I am working on a new technique that might give one or two orders of
magnitude improvement locking to the 1PPS signal. It might give
sub-nanosecond locking in a very low cost system. But the zero beat
is a serious problem, so I am interested to learn as much as
possible how other solutions work. Maybe the simplest is to just
apply some jitter to the GPS crystal oscillator to keep it off zero
beat.
> The "all in view" technique will reduce the noise contribution to
> the comparison somewhat.
That term is a bit confusing. Does it mean using all satellites that
are in view, or does it mean the NIST receivers see the same ones
that the user sees?
> The timestamp resolution of better than 30ps or so ensures that
> time stamp quantisation noise is negligible.
> It also allows, in principle at least, standalone 3 cornered hat
> comparisons of the frequency instabilities of the 3 sources
> connected to the customer instrument.
Another term I must study and learn how to do: "3 cornered hat". It
sounds like a very powerful technique, ideally suited for a fairly
simple program and some data logging.
> For more detail see:
> http://tf.nist.gov/timefreq/service/fms.htm
Thanks very much for the link. It is curious they don't seem to
spend much effort on correcting the user's frequency errors. They
just want to report how much they are off.
Why is that? You'd think they would perform a more valuable service
by applying advanced techniques to adjust the user's equipment to
minimize the error, then report and certify the actual result. Is
there some reason they want to leave the user's equipment
free-running?
> Only carrier phase GPS techniques are potentially capable of
> picosecond noise levels.
Now we are getting very interesting. How do you do that?
> However there are a large number of effects that have to be taken
> into account and data reduction and correction is very complex.
What if we just want to stabilize an oscillator frequency, without
caring much what the exact phase offset is from the USNO? The high
carrier frequency should make it much easier to lock a rubidium or
crystal oscillator, and it should give much lower phase noise.
Of course, auroras and other disturbances would be more significant,
but here I'm mainly interested in getting a good lab reference to
measure and compare the performance of other commercial oscillators.
This is a separate topic, but as long as we are talking about
precision signals, do you happen to know what kind of distribution
amplifiers are used at the USNO to distribute the signals from their
cesium and hydrogen standards?
www.usno.navy.mil/
> Bruce
Thanks very much for your help. It might seem like a lot of
questions, but I'm pretty sure you will be very pleased with the
results. I think I can beat just about all the other methods except
carrier phase techniques.
Best Regards,
Mike Monett
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