[time-nuts] What time is it anyway?

[Magnus Danielson]

Arnold Tibus wrote:
> The answer looks to me a bit difficult reading the USNO definition :
>
> INTERNATIONAL TIME SCALES AND THE B.I.P.M. 
> http://tycho.usno.navy.mil/bipm.html
>
> citing:
> "...the U.S. Naval Observatory timescale, UTC(USNO), 
> and its real-time implementation,  Master Clock #2  (MC #2), 
> are kept within a close but unspecified tolerance of the
>  international atomic timescale published 
> by the  Bureau International des Poids et Mesures  
> (International Bureau of Weights and Measures [BIPM])
>  in Sevres, France."
>
> "...Hence, all these atomic timescales are called 
> Coordinated Universal Time (UTC), of which USNO's 
> version is UTC(USNO)."
>
> "...The difference between UTC (computed by BIPM) and any other 
> timing center's UTC only becomes known after computation and 
> dissemination of UTC, which occurs about two weeks after the fact. 
> This difference is presently limited mainly by the long-term frequency
>  instability of UTC. 
> UTC(USNO) has been kept within 26 nanoseconds of UTC during 
> the past year through frequency steering of our Master Clocks to our 
> extrapolation of UTC."
>
> So I do understand that BIPM is the world's time keeper, but there 
> may be a difference between the UTCs  of up to 26ns?
>   
First of all, that comment related only to USNOs performance, not any 
other lab like NIST, PTB or SP. Historically, it may be 100s of ns away 
if they fluke it. This is the difference in should and will supply 
stability.
> "...Since synchronization is never perfect, we provide the latest data 
> below on the differences between UTC and the UTC of other timing 
> centers, including USNO,..."
> and
> "...All of our reference clocks are real-time approximations of UTC(USNO),
>  and as such are denoted UTC(USNO,MC). Master Clock #2 (MC #2) is 
> our official reference clock..." 
>
> So I understand this as, that the USA do refer to the time reference 
> of USNO - and the rest of the world to BIPM directly? 
>   
No. First of all, you should consider that there is two bodies in the 
USA, NIST and USNO. NIST is the primary body for all BIPM stuff, but 
USNO is a complementary body in regards to time.

Then, both NIST and USNO refer back to BIPM, just as PTB in Germany 
refers back to BIPM and SP in Sweden refers back to BIPM. The national 
laboratories represent the first link in the traceability chain down 
from BIPM. In this regard NIST and USNO is just that, the US national 
laboratories. If I where to get my traceability through my national 
laboratory SP and I also follow the calibration and reporting rules, set 
forth in ISO 17025 and related, then that measurement is traceable to 
BIPM and let's say HP in their Santa Clara lab does the same, they 
relate to NIST (not USNO really) then by mutual agreements these 
measurements is being recognised and agreed upon.

This aspect of recognised is highly related to trade and trust in trade.
> Since dec. 2009  the PTB in Braunschweig, Germany (with the new 
> CSF2) and the BIPM in Sevre are the only countries running 4 of the
>  most precise primary Cs fountain clocks, if I am informed correctly. 
> Together they should run quite close to the time defined by BIPM I think, 
> and according our law our official time is transmitted by the PTB. 
>   
Cesium fountains is not a good measure of lab stability. Only a small 
fraction of all the clocks (250 is a number frequently occuring) being 
used. Each individual clock is being measured and weighted in.  Most of 
them is HP (now Symmetricom) 5071A clocks with normal or 
high-performance tubes. The stability of these clocks is being compared 
and a optimum stable paper-clock called EAL is created. This is then 
corrected into TAI and with the decissions of IERS is corrected into UTC.

TAI is being somewhat of a paper-time, but several labs realize their 
variant of it, referred to as TA(x) where x is replaced with lab name.
> Now, How do I have to interprete the readout of GPSDOs like 
> Trimble's Thunderbolt and others PPS difference in ns to UTC?
>
> To which UTC? I suppose to the time transmitted by the US GPS SATs.
>   
The traceability of GPS time becomes:

BIPM -> USNO -> GPS-time -> Sat time -> User equipment

USNO provides measurements into BIPM, they then realize UTC(USNO) and 
steer it towards the BIPM for long-term stability.

GPS time is separately maintained and is being supervised by USNO, the 
UTC(GPS) is then steered towards UTC(USNO). USNO provides the 
calibration values being transmitted over GPS such that the 
UTC(GPS)-UTC(USNO) difference can be estimated and corrected for by user 
equipment. Precision users use other methods.

The satellite time is being measured by the GPS monitoring stations and 
the Kalman filters they employ will estimate ephemeris and time errors 
and provide the correction values which is then being linked up to the 
satellites and then transmitted. The 50 Wing space command will 
regularly provide corrections of orbit and steer the onboard clocks. The 
user equipment is able to convert the satellite time into GPS system 
time, do the navigation part and then correct the GPS time into UTC 
being traceable to BIPM... if only the user equipment itself will meet 
the standards of a measurement device and also provide means for 
completing the traceability.
> Is there another difference in UTC to BIPM included? 
>   
UTC(USNO)-UTC
> What is with Glonass (and will be later perhaps with Galileo)?
>   
Glonass: Refers back to UTC(SU) which is traceable to BIPM.
Galileo: Will work similarly.
> In fact the use of GPS is already spreaded all over the world and 
> in use in many technical applications, meaning that the world does 
> refer to UTC given by US GPS !?
>   
Nope. It may look like that. From a legal standingpoint, the UTC time 
out of any GPS receiver is not provide a traceable time, but for most 
practical use, it is good enough if one only ensures that the 
installation is good (GPS visibility, good receiver etc).
> Does it make sense under this circumstance as Time Nut to go 
> below the Xns (26ns ?) frontier as absolute measure? 
> Will this ever be possible? (Everything is relative...)
>   
The 26 ns number is taken out of context and doesn't relate to anything 
else.
> But excuse me in case I do miss and misunderstand something 
> fully...
There are several things behind the scene that you didn't look for. I 
hope I have given you a few hints.

Two useful links:
http://www.bipm.org/en/publications/guides/vim.html
http://www.bipm.org/en/publications/guides/gum.html

If you look up traceable in VIM and look for related definitions, you 
get the terminology right. There is references out from there to all 
kinds of material.

Cheers,
Magnus