[ntp:questions] how stable is GPS after startup
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Tue Feb 14 19:53:51 UTC 2012
I have my USB GPS running pretty well. However, I have noticed a few
occasions when it goes wonky. Those are:
a) Sometimes it appears to produce more consistent loopstats charts
running the process on Above Normal priority (in Windows) rather than
RealTime priority. I have to do further testing and documentation on this.
b) Sometimes, it appears to destabilize after about 3 days of operation,
and offsets jump by a factor of 10 to over 100 ms. I have to do more
testing on this too, but I have noticed that unplugging and replugging
the GPS appears to fix the problem. This has prompted me on some
occasions to unplug and replug the GPS before restarting NTPD after
making any configuration changes.
c) I have sometimes noticed a large offset on the order of 50 ms TO THE
NIST SERVERS after restarting NTPD even though the PC was closely
synchronized to under 5 ms just a minute or two before. Note, the GPS
is the preferred clock, AND, I have the GPS time fudged so it is in very
close agreement with NIST. So, normally, both the NIST servers and the
GPS are reporting less than 5 ms of offset.
So, my main question for this thread is about item c). I know it has
been said that NTPD takes a while to stabilize, but the phenomenon I
mention doesn't always occur. Sometimes, the very first report I get
after restarting NTPD says I'm within 5 ms or so of GPS time and the
NIST servers. I was originally thinking NTPD was at fault, or possibly
NTPQ or possibly the Meinberg server monitor.
I'm now wondering if the GPS receiver is the problem. I want to know if
this theory is likely. I have noted on an old hand held GPS I have,
that, when it gets a fix, it will first say, for example, that it's
accuracy is 150 ft, then later, maybe 70 ft, then, after a while, maybe
23 feet, and occasionally, 15 feet or so. So, I'm wondering it my unit
here is doing the same thing. Maybe, when I first plug it in, it
doesn't have an accurate position fix, and possibly, is not outputting
the time as accurately. Maybe that's why I see immediate offsets to all
the NIST servers of 50 ms or more. Then, later, after the GPS has been
running 20 minutes or so, maybe it's position and time fix is much more
accurate, so then I see my normal offsets to NIST of under 5 ms. Does
that make sense?
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
timekeepingdude AT c3energy.com
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