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 <t...@leapsecond.com>
To: Discussion of precise time and frequency measurement <time-nuts@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" <b...@evoria.net>
To: "Bob kb8tq" <kb...@n1k.org>; "Discussion of Precise Time and Frequency
Measurement" <time-nuts@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 <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 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 <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>
To: Bob Stewart <b...@evoria.net>
Cc: Discussion of precise time and frequency measurement <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 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 <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>
To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency
measurement <time-nuts@febo.com>
Cc: John Miles <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> 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>
To: 'Discussion of precise time and frequency measurement' <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]
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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