[time-nuts] Testing frequency using NTP

Bruce Griffiths 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?
>   Thanks,
>   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 
comparison somewhat.

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 
noise levels.
However there are a large number of effects that have to be taken into 
account and data reduction and correction is very complex.


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