[time-nuts] What time is it anyway?

Steve Rooke sar10538 at gmail.com
Mon Mar 29 12:10:03 UTC 2010


So there are 250 clocks, presumably, spread around the World and owned
by the member countries of the BIPM. Their time is somehow compared
centrally and an "absolute" time is determined from them. Each clock
will then have a delta to apply to it's own time to provide the BIPM
"absolute" time which then allows each member country to use its own
clock for timing.

This sounds like a logistic and bureaucratic nightmare,  I will have
to look further as to how this is achieved. Any pointers gratefully
accepted.

On 29 March 2010 01:51, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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
>
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-- 
Steve Rooke - ZL3TUV & G8KVD
A man with one clock knows what time it is;
A man with two clocks is never quite sure.




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