[time-nuts] Frequency Ensemble

[Bob kb8tq]

Hi

There are a variety of GPS devices that put out a PPS. uBlox makes some, there 
are a number of other companies that do so. The PPS comes out modulo the local
timebase. On a precision part, there is a “sawtooth correction” message that also
comes out to further quantify the best guess time of that pulse. 

Noise wise, even with correction you are lucky to get a one second ADEV in the 
1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like the F9P,
you might do a bit better than that. 

The ADEV of your other sources at short tau will be much better than the GPS PPS
noise. As you average out over long periods, the GPS will win the race.

Bob

> On Mar 18, 2019, at 9:56 AM, Rodger via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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