[ntp:questions] how stable is GPS after startup

unruh unruh at invalid.ca
Tue Feb 14 21:36:06 UTC 2012

On 2012-02-14, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> 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.

That should not make a difference.

> 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.

Since you are using the end of the sentences as the timing mark, that
sentence and could be occuring at differnt times. Eg if there is an
extra character (byte) at 9600dB that is about 1ms difference. So if
there were a whole extra sentence being insterted every once in a while
( eg your gps receiver every hour say reports extra sentences) it could
explain that. That is why gps is better using PPS. 

Note that there could be manyreasons for your system jumping in time if
you restart.

> 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?

It would have to out in its fix by 300km ( 200 miles) to make the time
be out by 1ms. Not likely.

> Sincerely,
> Ron

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