[time-nuts] Testing frequency using NTP
bruce.griffiths at xtra.co.nz
Thu Oct 2 16:28:56 EDT 2008
Mike Monett wrote:
> Hello Bruce,
> This is my first attempt to post so I'm not sure if it will work.
> I have been researching the forum archive and find it is by far the
> best resource on time and frequency available anywhere. Also, your
> posts are extremely helpful to a newcomer. Thanks!
> I am curious about the NIST GPS timing receiver that has a
> resolution of a few ps. It doesn't seem possible to get that kind of
> precision with conventional GPS methods that produce a 1pps signal.
> Can you describe more how this is done, and do you have any web
> links that give more information?
> Mike Monett
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.
The "all in view" technique will reduce the noise contribution to the
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.
For more detail see:
Only carrier phase GPS techniques are potentially capable of picosecond
However there are a large number of effects that have to be taken into
account and data reduction and correction is very complex.
More information about the time-nuts