[time-nuts] 1 pps Accuracy in two locations

[Dana Whitlow]

If anybody knows how to do this, I bet it would be the people who do
long-baseline
interferometry at millimeter-wave frequencies, such as at the eVLA in New
Mexico.

Dana


On Wed, Jan 15, 2020 at 4:35 PM Attila Kinali <attila at kinali.ch> wrote:

> On Wed, 4 Dec 2019 09:40:34 -0000
> <martyn at ptsyst.com> wrote:
>
> > I'm always being asked to provide equipment that can produce two 1 pps
> > outputs aligned to each other to within a few ps.
> >
> > These two 1 pps pulses are not in the same location and could be 100
> metres
> > to a few km away.
>
> As others have written, getting down to a few ps is not feasible, at least
> not with the amount of money your customers are likely willing to pay.
> To get down to these levels you will need to pull fibres from one location
> to another and using special circuitry to activly compensate variation
> in length due to temperature changes and vibration, even for burried
> fibres.
> Just to put into perspective what your customers are asking for: in 1ps
> light travels 300µm in vacuum/air or ~150µm in fibre/coax. This means,
> to get better than 10ps, you have to control the length of everything down
> to better than 1mm. Thermal expansion coefficient is somewhere between
> 1e-6 to 1e-4 for solids. Assuming you have 1e-6 and want to bridge 1km
> you would need to keep the temperature of the whole fibre stable to 1°C
> in order to keep the timing variation due to length change below 10ps.
> And that's not yet accounting for changes in refraction index (i.e. speed
> of light) due to temperature changes or a myriad other effects that you
> will have to deal with.
>
> Even white rabbit, which is probably the most advanced system you will
> likely get your hands on (does two-way timing to compensate for length
> of fibre and length variation), gives you about 200ps of uncertainty
> from one node to the next (but <10ps rms jitter). Each WR switch/node
> costs ~3k€ so isn't exactly cheap either, but you get GBit ethernet
> ontop of the time transfer as well.
>
> > So they are asking for two of my GNSS frequency standards with 1 pps
> > outputs.
>
> State of the art time transfer using GPS, I am aware of, is what BIPM has
> demonstrated two years ago using iPPP and, IIRC a base line of a few 100km
> of 200ps uncertainty. But that's using calibrated GPS receivers in a
> temperature controlled environment with lots of post-processing.
> I.e. it's not real-time. For shorter baselines, I expect the uncertainty
> to come down a bit. But I would not assume for it to go below 100ps without
> verifying first, even at very short baseliens of a few 10s of meters.
>
> If you have money to spare and line of sight between the locations,
> you could employ something like the NIST free space laser system
>
> https://www.nist.gov/programs-projects/optical-two-way-time-frequency-transfer
> But beware: it uses multiple frequency combs which cost as much as a very
> nice car. Each of them. But they get to sub-100fs stability, which means
> you could get to low-ps uncertainty, if done correctly.
>
> Rule of thumb: if you need time transfer better than 1ns you need to think
> about what you are doing, even within the same room. If you leave the
> building, going below 1ns is going to be hard. Doing better than 1ns within
> a neighborhood, while possible, will need some serious equipment and proper
> planing. Any factor of 10 better will drive up your cost by a factor 100 to
> 1000.
>
> Rule of thumb #2: if you need to control lengths to better than 10µm, your
> mechanics guy will throw a fit as he will most likely be unable to
> manufacture
> to that precision, unless you are building something tiny.
>
>                         Attila Kinali
>
> --
> Science is made up of so many things that appear obvious
> after they are explained. -- Pardot Kynes
>
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