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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Wed Mar 14 13:42:38 UTC 2012


On 3/14/2012 2:46 AM, David J Taylor wrote:
>> Hi David,
>>
>> OK, you've convinced me.  I've put noselect on the GPS and taken it 
>> away on the New York NIST server.  The NY NIST server is now 
>> preferred and polling from 1 - 4 minutes.  Hopefully they won't ban 
>> me for hitting it too often.  I think the NIST server will have more 
>> jitter, particularly as the polling interval increases, but we'll see 
>> what happens.
>>
>> Sincerely,
>>
>> Ron
>
> It will be interesting to see what difference that makes, Ron, and I 
> look forward to seeing the graphs.
>
> Personally, I saw little to choose between the various Internet 
> servers you had, and I would have allowed four or five to be selected 
> rather than just one, as that would not then leave NTP open to failure 
> of the one server, and it would allow NTP to select what It believes 
> to be the best.
>
> Cheers,
> David
>

Hi David,

OK.  You asked for it.  8-)

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


PS - By the way, I've changed my ntp.conf a few times since I originally 
started this thread, including the one in dropbox.  What's in there now 
is my current ntp.conf, not the one that was in there when I started the 
thread.



-- 

(PS - If you email me and don't get a quick response, don't be concerned.
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Ron Frazier
timekeepingdude AT c3energy.com



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