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

[Attila Kinali]

Good morning Hal,

On Thu, 24 Nov 2022 22:31:05 -0800
Hal Murray <halmurray at sonic.net> wrote:

> 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.

This is exactly how it works.

If you look at the ADEV plot of the iMaser3000 (see attachment) you see
that it goes down to a few parts in 1e-16 at about a day. The medium
term drift (a day to a few weeks) of hydrogen masers is very predictable
and usually follows a linear or low order polynomial (see [1]). Hence it
is easy to measure and correct for, even when measurements are done
only every few days/weeks. A 5071's medium term behaviour is a bit more
chaotic, but still quite stable.
 
The reason this works is because, for a lot if not most frequency standards,
once they hit the flicker frequency floor, their behaviour is not domiated
by a noise process, but by various drift processes. Hence their behaviour
is correlated in time and thus becomes, to some extend, predictable. But
in our normal way of analysis, this gets hidden in the ADEV plot, or rather
we do usually not really care what the source of random walk frequency is.

> 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?

As far as I am aware of, they used a similar technique of using some
flywheel standards (either crystal oscillators or commercial caesium
beam standards) that would run continuously to fill the gaps when the
primary standard wasn't running. I am not aware what the uptime percentage
for those old standards was. I know that beam standards are less finicky
than fountains, but I do not know how the uptime of the old beam standards
relate to our modern fountains. 

> Crystals are known to have jumps.  Do boxes based on atomic properties also 
> jump?

Yes, but the crystals that end up in atomic clocks pass to some stringend
screening to ensure that they either do not exhibit jumps or have a very
low rate. Besides, the loop bandwidth for atomic clocks is usually relatively
high, somewhere between 10Hz and 1kHz. So these jumps, while they do degrade
the performance, do not have a significant effect on the medium and long term
performance.


> > 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.

In most juristictions GPS alone is not considered legaly tracable.
Depending on where you live you also have to have your receiver certified
and regularly calibrated.
 
> 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.

There it's mostly an issue of time-stamping. High frequency trading depends
on who gets their request processed first. And this is depends on who's
request arrived first. Thus time-stamping is used to ensure a proper ordering. 
And because there is a lot of money involved, regulators demand that the
time-stamping devices are all synchronized to UTC with low uncertainty and 
traceability.

				Attila Kinali

[1] "Medium-Term Frequency Stability of Hydrogen Masers as Measured
by a Cesium Fountain", by T.E. Parker, S.R. Jefferts and T.P. Heavner, 2010
https://doi.org/10.1109/CCA.1995.555631
https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=906021
-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes
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