[time-nuts] Re: The SI second and the ease of realization (was: leap seconds finally being retired?)

[Magnus Danielson]

Hi Hal,

On 2022-11-25 07:31, Hal Murray via time-nuts wrote:
> Thanks to Magnus and Attila for a wonderful discussion.
Thanks. I saw the opportunity to go into aspects that we rarely touch 
on, and there is plenty of aspects in it that we should be talking to. I 
saw the oppertunity to potentially raise the knowledge. In this thread, 
we now use the primary reference roughly in accordance with it's defined 
meaning in VIM, and it's practical operations. This alone is different 
from the commercialized form of "primary reference" to be that of a 
cesium clock. Wonderful marketing trick. I think we can do better and we 
do that by learning more about how things actually work. Also, as things 
is being redefined, it is worthwhile to consider what goes into a 
definition. Turns out that it's a fairly complex issue, and there are 
many facettes to it that comes into a decision. All we can do is to 
contribute to it by doing the exercise to help sketch up some of it. 
Others will contribute their insights and at the end a more robust line 
of arguments will make clear if the decision is mature enough and wise 
enough.
>
> Attila Kinali said:
>> I would like to add here, that we already have this problem. If you look at
>> the current list of primary standards contributing to TAI https://
>> webtai.bipm.org/database/show_psfs.html you see that it's only a few labs.
>> And it was just SYRTE, PTB, NIST and INRIM 20 years ago. Also note the huge
>> gaps most of the primary standards have. I.e. very few are run once a month,
>> much less continuous. And this is a technology that's quite mature and well
>> understood.[1]
> I'm missing a key step.  If the primary standards are only run once a month,
> how can they contribute to TAI?
>
> I'm guessing that they are used to calibrate non-primary standards AND that
> the non-primary standards are known to drift slowly relative to how often the
> primary standards are run.
Yes, more or the less that. I wrote a separate reply to Attila that 
sketches the EAL-TAI-UTC process.
> Does that mean that back in the early days of primary standards, they were run
> for long-enough to get good data on the non-primary standards?
Recall that what is used as primary standards is the cutting edge of 
it's time, and that shifts. The primary standards tends to be hand-built 
devices. They are tested to the other clocks and also to the TAI/UTC 
through the traceability work and their performance develop that way. If 
they show the needed performance and long-term behaviour, clocks can be 
considered to contribute as primary standard rather than being just 
considered for secondary standards. Even contribution to the secondary 
standard in EAL/TAI means evaluation over time to ensure both the lab, 
test-link and clock stability.
>
> Crystals are known to have jumps.  Do boxes based on atomic properties also
> jump?
Well, not if well designed. However, crystal jumps to cause output to 
deviate until tracked in. As you lock a crystal oscillator to an atomic 
reference, the phase-response of the crystal will be high-pass filtered. 
A frequency jump will cause a phase-ramp to start from the previous 
locked state, but as the integrator updates the correction the frequency 
error hunts in and a remaining phase-shift often being the end result, 
looking like a phase-step. A higher degree compensation may track in the 
phase-step. Do look at Gardner book.
>
>> Yes, this means that any time-nut with a GPS disciplined Rb gets to within
>> 1-2 orders of magnitude of an average NMI. And yes, I find this incredible!
> +1  :)
>
>
>> Sure, there is no legal traceability for a time-nuts lab, but who needs that
>> anyways?
> Is there legal traceability to GPS?  I thought somebody offcial (in a legal
> sense) published the offset between GPS and UTC.

You can achieve it, but it requires "unbroken chain of calibrations". 
Just steering up a clock to track GPS will not suffice. That will not be 
a traceable replica of GPC(MC) or UTC(USNO). Traceability is defined in 
Vocabulary of International Metrology (VIM) as an unbroken chain of 
calibrations, and calibration is then the measure of a device to a 
traceable reference to establish it's deviation with known documented 
uncertainty. Further reading I recommend both VIM and Guide to 
Uncertainty in Measurement (GUM). Both VIM and GUM is available for free 
download from BIPM.

> I guess my real question is what does "legal traceability" mean to the US
> court system?
There is a paper trail that points out the legal support for 
traceability and realization of national measures in most countries to 
their NMI. The "legal traceability" ties to VIM and GUM, points to the 
NMI and the connectivity to the internaltional agreements for the SI 
system. For the US this points to NIST, so NIST traceability has that 
context. For military the USNO fills the role for time and frequency. 
Once you shown traceability to any lab k, you can use it's traceability 
record to show relationship to international agreements, but also to 
another lab l. This is the strengths of the traceability system, the 
agreed system establish the traceability for compareable of measures.
> In the stock markets there are rules I don't understand involving time.  I
> think the typical computers involved get their time via NTP from a GPS box.
> These days, they probably use PTP to shave a bit on the error bars.

The requirement for legal traceability is there in SOX for the US and 
MIFID II for EU. The traceability need is there to meet the needs to be 
able to review the logs and essentially the traceable time-stamps become 
measures for evidence. I recall MIFID II require traceability to be 
within +/- 100 us of UTC.

It's fairly well understood and realized on regular basis.

Cheers,
Magnus