[time-nuts] Frequency Ensemble

[rodger_adams at yahoo.com]

Tom,

Thanks for the explanation of clock ensembles.  That answered a few
questions I've had for a while.
Regarding your comments on collecting raw time data from GPS and post
processing it.  Can you provide any reference info, links, etc. with more
detail on that topic?
Clearly I'd need a GPS that outputs the proper raw messaging and the
software for processing it.  I'm somewhat familiar with the techniques
involved to improve GPS position data, but hadn't thought about it as much
for timing.

Thanks,

Rodger

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Tom Van Baak
Sent: Sunday, March 17, 2019 3:00 PM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Subject: Re: [time-nuts] Frequency Ensemble

> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 
> Rb clock together and take the average to control 1 "master clock" and
have better stability ?
> like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much 
> advantage I get from some clock that I already have.Thank You Anton

Anton,

The rule-of-thumb is that, *under the right conditions*, N clocks will
perform sqrt(N) better than 1 clock.

So yes, NIST, USNO, PTB, BIPM -- all the big boys -- use ensemble
techniques. But the key is that they mostly use cesium clocks, not OCXO or
Rb clocks from eBay. Laboratory cesium standards don't suffer from frequency
drift. The other key is that the clocks are independent. Under these
conditions one can obtain sqrt(N) advantage.

The problem with using cheap OCXO or Rb clocks is that they drift, and this
drift may depend on make / model / environment; all of which are possibly
common mode for you. This means the full sqrt(N) assumption is likely not
valid.

The problem with using GPSDO is that they are not independent clocks. In
fact, they aren't clocks at all: they are just noisy radio receivers,
implementing "time transfer" from the USNO GPS master clock, which is
related to but not equal to UTC(USNO) which is related to but not equal to
UTC itself. There's a lot of common mode error amongst a set of GPSDO. This
means the full sqrt(N) assumption is likely not valid.

Those who use GPS for highest accuracy tend not to use GPSDO. Instead they
just collect raw timing information and post-process it some hours to weeks
later. That is, they want to know
    what time-it-was-precisely
rather than
    what time-it-is-approximately.
A GPSDO only does the latter.

/tvb


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