[time-nuts] Three-cornered hat on timelab?

Magnus Danielson magnus at rubidium.dyndns.org
Mon Apr 17 18:28:38 EDT 2017


Bob,

First of all, there is a first degree of compensation from the GPS 
transmitted Klobuchar ionspheric model. There is a limit to how well 
those would match the actual values at the time and behavior for your 
spot on the globe. The GPS models this to a fair fit for the globe.
Use of WAAS/EGNOS or even DGPS would allow for a better correction.

Second, these changes is slow, so you better measure them compared to a 
cesium or maybe rubidium rather than the GPS itself or another GPS. A 
GPS tracks in these deviations, so it will only be visible when compared 
to an independent source. The frequency error and drift of the reference 
clock can be compensated, but the remainder will dominantly be remaining 
delay variations.

You can fair better if you have a double-frequency GPS setup, as it can 
first-degree measure and compensate the ionospheric shifts, which allows 
for a benefit over L1 CA only receiver.

Cheers,
Magnus

On 04/17/2017 10:38 PM, Bob Stewart wrote:
> Hi Tom,
> The reason I express so much confusion over this is because I don't see the wild phase excursions on my GFS units that people insist will happen due to ionospheric effects.  Is this because they are rare events, and I just haven't been saving data during a bad time?  I notice in your example page, you aren't seeing them, either during your 8+ day capture of the Tbolt.
>
> Bob
>
>
>       From: Tom Van Baak <tvb at LeapSecond.com>
>  To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>  Sent: Monday, April 17, 2017 3:05 PM
>  Subject: Re: [time-nuts] Three-cornered hat on timelab?
>
> Bob S,
>
> Here's an example of a one week GPSDO run:
>
> http://leapsecond.com/pages/tbolt-8d/
>
> IIRC, this was a default, untuned, self-surveyed TBolt. You can see some level of daily variations -- probably a mix of sky view, survey error, ionosphere, multi-path, sidereal effects [1], temperature (antenna, cable, GPSDO, reference), etc. It takes some time and equipment to sort out which is which, but even a simple test like this can give you an upper bound.
>
> /tvb
>
> [1] Fun GPS orbit stuff here:
> http://leapsecond.com/pages/sidereal/index.htm
> http://leapsecond.com/pages/sidereal/14years.htm
> http://leapsecond.com/pages/sidereal/sv.htm
> http://leapsecond.com/pages/gps-orbit/
>
>
> ----- Original Message -----
> From: "Bob Stewart" <bob at evoria.net>
> To: "Bob kb8tq" <kb8tq at n1k.org>; "Discussion of Precise Time and Frequency Measurement" <time-nuts at febo.com>
> Sent: Monday, April 17, 2017 9:33 AM
> Subject: Re: [time-nuts] Three-cornered hat on timelab?
>
>
> Hi Bob,
> OK, thanks. I've kicked off a 7 day run of a GFS against the PRS-45A. That should be long enough to separate out the GFS from the PRS' drift direction from the ionosphere.
>
> Bob
>
>
>
>       From: Bob kb8tq <kb8tq at n1k.org>
>  To: Bob Stewart <bob at evoria.net>
> Cc: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>  Sent: Monday, April 17, 2017 11:28 AM
>  Subject: Re: [time-nuts] Three-cornered hat on timelab?
>
> Hi
> The ionosphere is the culprit in terms of the daily swing. The swing is a function of the goodness of fit between the GPS broadcast dataand the ionosphere as it impacts the satellites you are using. There is no rime or reason to it beyond that. If you get “lucky” things don’t move much. If you live in exciting times, things move quite a bit. Unless you go to something like an L1/L2 receiver, the GPS module you use has little to do with it (unless it’s broke ….). Yes there are some fiddly little qualifiers relating to being at the north or south pole and GPS coverage (along with space weather impacts). Very few of us do our runs at either location :) Just for reference, the area of concern also hasat least one day each year where the sun sets for < 1 hours.
> Bob
>
>
>
>
> On Apr 17, 2017, at 11:33 AM, Bob Stewart <bob at evoria.net> wrote:
> Hi Bob,
> Oh, I had completely forgotten about the many runs you gifted us with back then. Fortunately, I kept all of them in my email archive. I can't compare like for like, of course, but I think I can work up something that compares at the larger taus where the 5370 doesn't dominate.
> I'm going to run another long term test of my GFS unit against my PRS-45A. The problem, the issue that made me ask for data is that everything from phase plots to ADEV plots of my unit are just so much better than the KS. In addition, I don't see the large ionospheric swings on my GFS unit that you and Bruce and others have spoken about. This bothers me a lot. Could it be my location here in Houston? Could it be the Ublox LEA-6T compared to the much older Motorola in the KS?
>
> Bob
>
>       From: Bob kb8tq <kb8tq at n1k.org>
>  To: Bob Stewart <bob at evoria.net>
> Cc: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>  Sent: Monday, April 17, 2017 7:55 AM
>  Subject: Re: [time-nuts] Three-cornered hat on timelab?
>
> Hi
> The data I have on the KS boxes was posted to the list back when they were.a hot topic. I’m sure it is still in the archives.I’m guessing it’s not quite what you are after.
>
> The closer the devices are to each other the better the technique works. A simple way to look at it is as an attenuation. If it knocks noise down 10:1, the worst unit should be no more than 10X noise than the best unit. How much things are knockeddown is a function of the length of the runs compared to the longest tau. For a 10:1 ratio of tau to run, attenuation of noise by 10:1 is very optimistic. You usually need something beyond 100:1 to get that sort of performance. A lot depends on the noise involved. Some types of behavior simply don’t work well with the technique.
> The KS box goes from “better than” to “worse than” and back to “better than” most atomic standards you would compare it to overa range of tau from 0.1 S to 1,000,000 seconds. To get the 1,000,000 second data accurately, you would need a 100,000,000 secondrun. The simple answer there is that nobody has that kind of time or that reliable a setup. Even the three month run to get good100,000 second data is a challenge. None of that relates to three corner hat stuff, it’s just the confidence bars on ADEV. It givesyou another (say) 100:1 wait on top of the three corner stuff.
> Now toss in the basics of GPS. Depending on the day, you will get <10 ns to >100 ns swing over a 24 hour period. Today may or may not be the same as tomorrow. That’s with a “perfect” L1 setup. The variation comes from the ionosphere and the fact thatthe GPS data does not allow you to fully correct for it. In addition, you will get some interesting bumps related to constellations and your local antenna setup. Any GPSDO that is quartz based will happily follow the 24 hour swing in the GPS from the ionosphere. At 100,000 seconds, a 100 ns swing is 1x10^-12. That’s a lot of disruption. It most certainly is not the sort of thing that ADEV expects to pop up in the middle of a run.
> The simple answer to all this is “don’t go there”. Three corner hat is fine for short term stuff. It’s a mess for long term runs. Getting datathat is good enough for a long term ADEV run out of a three corner setup is a major struggle. The time for the correlation to knock downthe noise on top of the time to get good ADEV data gets you into impractically long runs.
> Bob
>
>
>
>
> On Apr 16, 2017, at 10:16 PM, Bob Stewart <bob at evoria.net> wrote:
> Hi Bob,
> OK, I give up. Try as I might, I can't fudge things enough to make any sense. I did run a set of 1 hour tests that seemed to confirm what I can infer from the phase plots in timelab, but anything longer than that and the curves either contradict the phase plots, or there are large gaps in at least one trace, or a trace is even missing entirely. Oh well. I seem to remember reading that the 3c-hat was only useful in comparing similar devices. The KS just isn't close enough to what I'm trying to compare it to, I guess.
>
> If you or anyone else has an ADEV plot of the KS against some local standard (for any length of time, any standard, even just a bare OCXO that is not a Trimble 34310-T) could you please share it with me? I'm looking for relative peformance, not a definitive test. Of course if you also have one of a 34310-T against the same standard, that would be great!
>
> Bob
>
>       From: Bob kb8tq <kb8tq at n1k.org>
>  To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Cc: John Miles <john at miles.io>
>  Sent: Thursday, April 13, 2017 7:19 PM
>  Subject: Re: [time-nuts] Three-cornered hat on timelab?
>
> Hi
>
> There are a number of papers from the 70’s and 80’s digging into three corner hat data. The net result often
> would turn out to be “less than zero” noise on one of the DUT’s. Since that’s physically impossible the technique
> got a bit of “attention”. The Cliff Notes version of the results is that simultaneous measurements were the key
> to getting decent results. The closer to “same time” (as in microseconds or nanoseconds) the better. Even with very careful
> data collection, odd things can still happen. Phase noise pops up at crazy low levels or ADEV goes to bizarre
> numbers. In many ways a TimePod (or other ADC based setup) is ideal for getting the data synchronized. Running
> all three devices on one is by far the best way I have seen to make the technique work. It still can have problems,
> but less so that other ways of doing it.
>
> Bob
>
>
>
>> On Apr 13, 2017, at 7:16 PM, Bob Stewart <bob at evoria.net> wrote:
>>
>> Hi John,
>> I had a chance to think about this some more after I pressed the send key. The ionospheric effects are certainly going to be different if the distance in time between tests is large. And, of course, there is the fact that the KS has a pretty old receiver compared the Ublox I use, so that even the reaction to the ionosphere is likely to be different. So, I thought I'd experiment with some runs with both GPSDOs in holdover to see if that would even the score, so to speak. Of course then I have the temperature variable, so it's never going to be perfect.
>> Anyway, thanks for the help. If I get anything that seems useful out of this, I'll post links to the data.
>> Bob
>>
>> From: John Miles <john at miles.io>
>> To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com>
>> Sent: Thursday, April 13, 2017 6:01 PM
>> Subject: Re: [time-nuts] Three-cornered hat on timelab?
>>
>> Longer runs would be better to the extent that they give you smaller error bars in your tau range of interest, certainly. But any effects that influence one of your runs but not the others will render the 3-cornered hat solution questionable, if not outright invalid. Only through many repeated runs can you learn to tell the bogus data from the good stuff. So I'd make shorter runs at first, until you're sure you know what you're looking at.
>>
>>
>>
>> It doesn't matter which source is applied to the start versus stop channel, as long as the assignments are consistent with the source labels you apply. I would use frequency-count mode to simplify things, at least at first. This is already a very challenging measurement for all the reasons mentioned.
>>
>>
>>
>> -- john, KE5FX
>>
>> Miles Design LLC
>>
>>
>>
>>
>>
>> From: Bob Stewart [mailto:bob at evoria.net]
>> Sent: Thursday, April 13, 2017 8:41 AM
>> To: Discussion of precise time and frequency measurement; John Miles
>> Subject: Re: [time-nuts] Three-cornered hat on timelab?
>>
>>
>>
>> Hi John,
>>
>>
>>
>> Thanks! With lesser equipment, such as the 5370A, would longer runs be better? I used a set of 1 hr runs and the result wasn't quite what I had expected. However, it may be that I had mislabeled the files, and thus got the sources confused. Of course, it may be that the ionospheric effect was grossly different between the three tests. So, with a 5370, Source A would be the START input and Source B would be the STOP input, right? For my testing, the sources are all 10MHz signals, and I'm driving the EXT input with 1PPS from a GPSDO.
>>
>>
>>
>>
>>
>> Bob
>>
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