[ntp:questions] SIRF time output wobble, GPS or NTP fault?
David J Taylor
david-taylor at blueyonder.co.uk.invalid
Thu Mar 8 08:36:03 UTC 2012
"Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message
news:4F57CC17.3020708 at c3energy.com...
> I have an adapter based on the Prolific chipset which is supposed to
> pass the handshaking signals. It's the Trendnet TU-S9. I'm going to
> try that, but I'll certainly keep the one you mentioned in mind.
If you're lucky, that should work on Windows to get offsets within the 400
microsecond level, and jitter I saw as ~45 microseconds using Windows XP.
> That is the $ 20,000 question. From our prior discussion, it looks like
> you're saying the Garmin has a jitter in the start time of the NMEA
> sentence of about 170 ms. I appear to be observing a similar phenomenon
> here, but I'm not totally convinced it's real.
For the Garmin GPS 18x with 3.70 firmware, it's real. I could see
something similar on the 'scope here. Whether other GPSs have similar
values I don't know.
> I think the monitor program may be lying to me about the offsets of the
> internet servers. If it is real, the variation is very slow, and it
> takes a day or two to swing from about + 80 ms from NIST to - 80 ms from
> NIST. I have nothing to explain that 900 ms jump I mentioned. Is this
> "wobble" effect something that has been observed with other GPS units?
Be aware that servers such as NIST's may well be overloaded. Choose ones
with a short network path and, ideally, one with little load. My gut
feeling is that until you have PPS, you may get better performance from a
selection of Internet servers than a serial-only GPS,
>> Are you using that version or later?
> On Windows, I'm running 4.2.7p259. On Ubuntu Linux, when I boot into
> it, I'm running the version from the Ubuntu repositories which is about
> 2 years old. I did a manual upgrade to try to troubleshoot some abrupt
> jumps in the reported offsets, but that didn't solve the problem. I
> retrograded back to the one that's in the repositories as doing a manual
> install breaks several things. It's better to use the package manager.
For use with the Sure, I suggest the latest NTP on Linux as well. I had
to ask here how to update FreeBSD and, apart from the time taken to
recompile everything under the sun, it went well.
>> May I suggest using the $GPGGA sentence instead even if it is a little
>> longer? The $GPZDG sentence appears to be the only other one with a
>> "valid" indicator.
> I think the satellite lock was good at the time but cannot prove it. It
> looks to me like the Meinberg monitor is reporting the server offsets
I very much doubt that.
> I'm presuming it uses the ntpq command internally to get that data but I
> don't know.
Doesn't it read the loopstats files for its graphs, though?
> I'm currently experimenting with all internet servers noselected to see
> what happens. I have a variation from my GPS time going from 11 of 12
> internet servers within 9.99 ms (single digit before the decimal point)
> to 4 of 12 internet servers within 9.99 ms. I have not observed any
> more 950 ms offsets to the internet servers. I just checked while
> typing this and 8 of 12 internet servers are within 9.99 ms.
The 950 ms sounds like the GPS getting the wrong second - slipping sync
compared to the clock.
> It doesn't look like GPGGA has any validation in it.
The Fix Quality field. 0 = Invalid, 1 = GPS fix, 2 = DGPS fix. I would
hope that NTP rejects any data with 0 in this field, but I don't read the
sources at all well.
> I am reluctant to switch to another sentence for a few reasons, assuming
> NMEA only is ultimately usable at all for a few reasons:
Noted, and I see your reasons, but using only $GPZDA concerns me as it
lacks any validity check.
> This is the part of my ntp.conf file in windows related to stats. Do I
> need to add anything else? I think all the stats are already on in
> enable stats
> statsdir "C:\NTP Service\NTP\etc\"
> statistics loopstats
> Thanks for the help.
You may want to add peerstats:
statistics loopstats peerstats
so that you can look at the 5th column of the results, and read the offset
for each clock.
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