[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

David J Taylor david-taylor at blueyonder.co.uk.invalid
Wed Mar 14 14:41:11 UTC 2012


> Hi David,
>
> OK.  You asked for it.  8-)

Well, I actually suggested /all/ the Internet servers being enabled, 
allowing NTP to make its best choice.

> When I'm through testing, I'll open up the other internet servers as a 
> backup in case the GPS fails.  For now, I'm just running with one clock 
> source at a time.  Still trying to document and chase down this 
> wandering effect.
>
> I ran with NY NIST as the only selectable clock source and monitoring 
> the GPS for comparison all night.  The results were horrible.  My 
> offsets from NIST time were in the + 65 ms / - 75 ms range.  I had the 
> polling interval set to start at 1 minute and go up to 4 minutes.  There 
> is way too much clock wander to even think about testing the accuracy of 
> the GPS.  I've gone back to polling the GPS every 8 seconds as the sole 
> selectable clock source and monitoring the internet servers for 
> comparison.  Over the short term, minutes to hours, my GPS, even with 
> NMEA only, is by far the most accurate time source I have.  Even if the 
> NMEA signal wanders 70 ms either way over the course of a few days, it 
> won't get any further off than I did using the internet server, and the 
> clock will be much more consistent over shorter time frames.
>
> Here are the graphs.
>
> http://dl.dropbox.com/u/9879631/nynist01.jpg
> http://dl.dropbox.com/u/9879631/nynist02.jpg
>
> Sincerely,
>
> Ron

I'm not surprised that using a single Internet server is worse than the 
GPS/USB, but that's not how NTP is designed to work.  With two or more 
Internet servers active, your GPS 50-second glitch would not have affected 
your PC's timekeeping anything like as severely, when you have the GPS/USB 
included to help improve the offset and more like the narrow band (about 
15 milliseconds wide) shown in:

  http://dl.dropbox.com/u/9879631/drifting01-peerstats.20120312.jpg

Cheers,
David 



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