[time-nuts] 1 pps Accuracy in two locations

[Magnus Danielson]

Hi,

On 2019-12-04 16:55, Tom Van Baak wrote:
> Martyn,
>
> > I'm always being asked to provide equipment that can produce two 1
> > pps outputs aligned to each other to within a few ps.
>
> They should look at their best 1PPS on a 'scope. You can get ns with
> care; I doubt ps is possible. I mean, that's THz BW isn't it?
No, you do not need THz BW for ps level timing precision, you can
achieve it in the microwave region for sure. Stabilizing it to that will
be a pain regardless, but doable given that the media and environmental
conditions is good enough.
>
> Can you share with us what their application is?
This is indeed a good question. What is the actual requirements and
conditions.
>
>
> > So they are asking for two of my GNSS frequency standards with 1 pps
> > outputs.
> >
> > The 1 pps outputs being derived from the rubidium oscillator (which
> > is aligned to GPS/GNSS)
> >
> > The best I think I can achieve is in the low ns range.
>
> Right. It will be ns, not ps. Forget about using GNSS for ps level
> timing.
Sub ns is possible but painful, but you are not deep sub ns. This is a
very well studied issue for national timing laboratories and part of the
fundament keeping these labs tied to the major labs to compare for the
full EAL/TAI/UTC time-scale contribution. Major labs also have two-way
satellite links, which is better.
>
>
> > Does anyone know how this can be achieved?
>
> Google for papers by high-end national timing laboratories. Words
> like: active temperature stabilized (phase stabilized) bidirectional
> optical fiber links.
>
> Very possible, very expensive, quite common now. I'd guess most of the
> timing centers in Europe are linked this way.

Well, Europe has come further on the optical links than US, but not a
dominant feature, even if the ambition is there. The map shown regularly
looks somewhat more connected than reality gives. I go to these
conferences, and see the progress. Also, there is two types of links,
the frequency links (now stable down to 1E-19), and the time-links. For
time you end up with lots of issues that you can just ignore as you do
frequency, essentially you need to re-learn. For distances like these,
look at White Rabbit for sure. It is being run for some links on long
distances and it is pretty good, but for big-scale usage it does not
make economical sense.

US/North America has extremely little links being done. The only one I
know was really reported was from GPS(MC) to UTC(NIST) local to
Colorado. As I recall it, that was only a temporary setup.

Cheers,
Magnus