On 3/14/2012 10:41 AM, David J Taylor wrote:
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
OK. Here are the loopstats from another computer for 7 days (in the
chart). I don't have any peerstats for it. It has the same server
list. One is preferred. All servers are active. Performance is no better.
http://dl.dropbox.com/u/9879631/TAZ%20loopstats%202012-03-07%20to%202012-03-14.jpg
http://dl.dropbox.com/u/9879631/ntp.conf-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120313-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120314-TAZ
Sincerely,
Ron
--
(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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