[time-nuts] GPS discipline oscillator vs phase lock

Bob kb8tq kb8tq at n1k.org
Fri Jun 16 18:29:31 EDT 2017


> On Jun 16, 2017, at 6:21 PM, Attila Kinali <attila at kinali.ch> wrote:
> 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.

…. well, we do. A requirement of 0.1 degree at 100 MHz was stated earlier on.
That’s where the ps stuff comes in.


> 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
> http://dx.doi.org/10.1088/0026-1394/52/2/384
> -- 
> 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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