[time-nuts] WWV Doppler Shift

[Magnus Danielson]

Hi Bob,

On 11/21/18 6:43 PM, Bob kb8tq wrote:
> Hi
> 
> Gravity is not the only thing you need to “standardize” if you are building a Cs clock from scratch
> in your basement. Magnetic field also quickly gets its nasty fingers into things as well. There are other
> environmental impacts, even on a Cs standard. For “best” performance you do indeed need to sweat
> what seem like really minor details. 

Actually, the SI definition is for zero magnetic field, so that
standardized.

Three shields of mumetal and degaussing the remaining internal magnetic
field is first line of defense.

However, technically this is not achievable since you have 7 different
peaks that would interfere with each other. They split with magnetic
field and separates quadratic to the magnetic field strength while the
central peak shifts slightly linearly. It's the central peak you want to
measure.

Old analog cesiums of industrial kind just makes the separation and you
have to tweak the magnetic field to make it match the expected
miss-tuning, because you have to expect miss-tuning. This doesn't make a
real primary standard, but a stable secondary standard. Good enough for
folk music.

The very analog trimming of that the magnetic field isn't very stable.
You can monitor the nearest side-lobes, which has much higher
sensitivity to the magnetic field, and then servo the magnetic field to
keep the side-peaks in a fixed relationship and thus the magnetic field,
this stabilizes the frequency of the central peak further.

Then it still remains to validate the correction for offset, which you
can do in laboratory clocks, you intentionally vary the magnetic field
to monitor the slope of the frequency shift with magnetic field and you
can get the needed corrections to correctly estimate the offset from 0
magnetic field conditions. This is however more research since when you
know how magnetic field affects the side-peaks and central peaks, the
relationship can be maintained, since other effects start to dominate to
shift perfection.

So well, the magnetic part is an engineering challenge, but not
completely uncovered by the generations of clock developers.

Then there is doppler, DC Stark, AC Stark, microwave lensing, black body
raditiation shift to name a few other imperfections you also need to
cancel out.

> This ultimately gets back to a never ending debate about depending on one design for all of your standards. 
> Even if a *really* good job was done - how can you be sure? Having multiple this and that in your comparison
> “pool” is the answer to that concern. 

The repeatability of primary designs and comparing these is critical
important aspects. That we finally got rid of the kilogram prototype as
the definition helps. The actual lump of metal will still be important
reference points but now as more secondary measurements but also after
the fact comparisons to see how the kilogram drift in the international
systems was actually performing.

There is some debates relating to fountains where as I recall it only
one institute does not compensate for the lensing effect.

However, with the optical clocks, that particular debate may be settled
as optical standards is now pushing two degrees of order more stable and
makes it potential to secondary measurements of the actual performance
of the various fountains. Ultimately we will redefine the SI second in
terms of optical transitions, which will not make the cesium fountains
useless, but we can get a better knowledge of their real behavior after
them being the primary reference.

Cheers,
Magnus