[time-nuts] time-nuts Digest, Vol 188, Issue 32

[Demetrios Matsakis]

 I have cut two quotes from this posting, which I would like to comment on.

One is this:
> 
> 
> Btw.: The GPS system delivers an? in-official uncertainty, because the 
> D.O.D. clock is not participating in the S.I. representation of the UTC.
> 
and  the other is this:

"This is not true. While BIPM only allows a single NMI per country to
contribute to TAI/EAL these days, this wasn't case in the past. And
for historic reasons there are a few countries where two entities contribute
to TAI/EAL. The USNO, master over GPS time, is one of those non-NMI
entities contributing. They also used to be in the past the one single
organization that had the most atomic clocks running, though that's slowly
changing now. They still are one of of the organisations that have the most
stable clock ensambles contributing to EAL, though, and will stay so for the
forseeable future"

***

With regards to the first quote, I have argued in published papers that UTC(USNO) via GPS can be used to establish traceability because the difference between that and UTC is published by the BIPM with an uncertainty of 10 ns.  (It used to be in the Circular T itself, but not is in one of the supplementary files you can download).   I am told some meteorologists believe this doesn’t count, and that the 10 ns is only a nominal value. That is something for the auditor to decide, I guess, but either way, you can still use GPS for traceability by using NIST web pages, with NIST uncertainties, to correct your GPS data to UTC(NIST), and then if you like and are willing to wait a month, you can correct for UTC-UTC(NIST) using the Circular T.  (My coauthors were from NIST, but this was just a minor point of the paper.)

Responding to the second quote, at least until I retired last year, the BIPM will allow any laboratory to contribute to UTC and realize UTC so long as the national signatory requests it.  For the USA, that signatory is NIST, and they have permitted three other labs to participate - USNO, Naval Research Lab, and Applied Physic Lab of Johns Hopkins University.  The name of the game, for precise timing labs, is no longer the size of the ensemble.  It is the number of atomic fountains you have.  Several labs have them, among the first to get them being Germany’s PTB and France’s OP.  If you compare their fountain-based realizations of time to the USNO’s, the difference is very small.   When optical clocks come on line, the game might change again.  But maybe not, as the timescale on which optical standards really gain over atomic fountains, in the presence of time transfer noise, might be longer than the timescale with which the participating labs steer their realizations to UTC.