[time-nuts] generalization of three cornered hat

Bob kb8tq kb8tq at n1k.org
Sun Jun 16 13:21:33 UTC 2019


Hi

…….. and if you want to dig into ensembles of “non similar” clocks, you want to look into
David Allan’s Smart Clock stuff….

http://www.allanstime.com/Publications/DWA/ <http://www.allanstime.com/Publications/DWA/>

There’s also a lot of stuff there on clock ensembles in general. 

Bob

> On Jun 16, 2019, at 7:22 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> 
> 
>> Note that when you consider transitive measurement noise, you may find 
>> that making N simultaneous measurements against a single independent LO 
>> is simpler and better than making N(N-1)/2 measurements of all possible 
>> clock pairs. Think of star vs. ring vs. mesh networks.
> 
> I think there is a fundamental assumption that needs to be stated clearly here:
> 
> 3-cornered hat and it's extensions are only productive with homogenous,
> and preferably identical, sources.  Ie: 3 identical OCXO's, 3 HP5071As.
> 
> If you do a 3CH on a raw GPS-PPS, an Rb and a H-maser, you get a really
> shitty result, because it attributes the worst properties of each source
> to the "virtual source".
> 
> For heterogenous sources, pretty much all proposed time-scale
> algorithms degenerate to 1st or 2nd order PLLs with suitably chosen
> parameters and filters.
> 
> Arguably, the most important property of any timescale algorithm
> is the ability to alert and exclude false-tickers as early as
> possible, which makes Kalman filters obvious, despite their quirks.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
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