[ntp:questions] how stable is GPS after startup

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Feb 14 19:53:51 UTC 2012

Hi all,

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.)

Ron Frazier
timekeepingdude AT c3energy.com

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