Hi One way to check what’s going on is to watch for 12 or 24 hour “blips” in the data. Unless you have a very unusual antenna setup, you should see some of them. They aren’t the ionosphere, but they are normally there in most datasets.
Bob > On Apr 17, 2017, at 12:33 PM, Bob Stewart <b...@evoria.net> wrote: > > 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 <kb...@n1k.org> > To: Bob Stewart <b...@evoria.net> > Cc: Discussion of precise time and frequency measurement <time-nuts@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 data > and 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 has > at least one day each year where the sun sets for < 1 hours. > > Bob > > > > >> On Apr 17, 2017, at 11:33 AM, Bob Stewart <b...@evoria.net >> <mailto:b...@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 <kb...@n1k.org <mailto:kb...@n1k.org>> >> To: Bob Stewart <b...@evoria.net <mailto:b...@evoria.net>> >> Cc: Discussion of precise time and frequency measurement <time-nuts@febo.com >> <mailto:time-nuts@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 knocked >> down 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 over >> a 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 second >> run. The simple answer there is that nobody has that kind of time or that >> reliable a setup. Even the three month run to get good >> 100,000 second data is a challenge. None of that relates to three corner hat >> stuff, it’s just the confidence bars on ADEV. It gives >> you 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 that >> the 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 data >> that 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 down >> the 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 <b...@evoria.net >>> <mailto:b...@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 <kb...@n1k.org <mailto:kb...@n1k.org>> >>> To: Bob Stewart <b...@evoria.net <mailto:b...@evoria.net>>; Discussion of >>> precise time and frequency measurement <time-nuts@febo.com >>> <mailto:time-nuts@febo.com>> >>> Cc: John Miles <j...@miles.io <mailto:j...@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 <b...@evoria.net >>> > <mailto:b...@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 <j...@miles.io <mailto:j...@miles.io>> >>> > To: 'Discussion of precise time and frequency measurement' >>> > <time-nuts@febo.com <mailto:time-nuts@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:b...@evoria.net <mailto:b...@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 >>> > >>> > _______________________________________________ >>> > time-nuts mailing list -- time-nuts@febo.com <mailto:time-nuts@febo.com> >>> > To unsubscribe, go to >>> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> > <https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> >>> > and follow the instructions there. >>> >>> > >>> > >>> > >>> > _______________________________________________ >>> > time-nuts mailing list -- time-nuts@febo.com <mailto:time-nuts@febo.com> >>> > To unsubscribe, go to >>> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> > <https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> >>> > and follow the instructions there. >>> >>> >> >> >> > > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
