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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