[time-nuts] GPS discipline oscillator vs phase lock

Attila Kinali attila at kinali.ch
Fri Jun 16 18:21:54 EDT 2017

On Fri, 16 Jun 2017 17:46:45 -0400
Bob kb8tq <kb8tq at n1k.org> wrote:

> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> radio link at the level you are looking for.

Well, we still don't know what the requirements are.
Though I doubt that a GPSDO cannot deliver the needed stability.
Unless the goal is to build a multi-static radar that can locate
a plane down to sub-cm range, ps level stabilty is not required.
Even something as demanding as VLBI is in the couple ns range.
(But they measure over long intervals (hours) which puts higher
demands on the local oscillator)

Another reference value: Time transfer using calibrated GPS receivers
achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
several 10km with single frequency receivers. Dual frequency receivers
have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).

			Attila Kinali

[1] "Comparing a GPS time link calibration with an optical fibre
self-calibration with 200ps accuracy", by Jlang et al 2015

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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