[time-nuts] NPR Story I heard this morning

Attila Kinali attila at kinali.ch
Tue Nov 4 15:42:54 EST 2014

On Tue, 4 Nov 2014 11:04:58 -0800
Peter Monta <pmonta at gmail.com> wrote:

> Let me rephrase what I'm after.  The geoidal uncertainty sets a hard limit
> on clock comparison performance on the Earth's surface (for widely-spaced
> clocks).  At some point, as Chris Albertson noted, the clocks will measure
> the potential and not the other way around.  (It should be possible to
> express this geoidal uncertainty as an Allan variance and include it in
> graphs with the legend "Earth surface performance limit".)

Actually, currently the limit of clock comparison is not the geoid
uncertainties, but the comparison itself. Common view GPS does not
cut it at all. TWSTFT might be enough, if more second/third order
effects are compensated for and the orbit is measured with a higher
precision [1,2]. For "short" distances (up to 1000km or so), temperature
and delay-variation compensated fibers seem to be the way to go [3,4].

Also, frequency transfer down to very low numbers seems to be easier
(depending on system and distance in the range of 1e-12 to 1e-16)
than accurate time transfer (around a couple 100ps seems to be the limit
for any non-trival distance, at the moment)

[1] "Two-way Satellite Time and Frequency Transfer:Overview, Recent
Developments and Application", by Wu et al, 2014

[2] "Practical Evaluation of Relativistic Effects in Two-Way
Satellite Time and Frequency Transfer", by Shemar, Delva, Lamb, 2014

[3] "Optical Frequency Transfer with a 1284 km Coherent Fiber Link",
by Calonico et al, 2014

[4] "Novel Techniques for Optical Fiber Links beyond Current Practice",
by Calosso et al, 2014

And these are just a few of the presentation given at EFTF this year
on that topic. 

			Attila Kinali

I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
			-- Sophie Scholl

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