[ntp:questions] Local time sources

David J Taylor david-taylor at blueyonder.co.uk.invalid
Mon May 10 06:58:37 UTC 2010

> However, what is the recommended minimum number for systems that only
> work with local time sources, such as GPS devices and time signal radio
> receivers, that each supply their own timing information as well? Is one
> always enough, or are there still advantages other than redundancy in
> having more than one?
> Thanks,
> Jaap


I suspect that depends on the precision required.  If you /need/ the 
precision which the GPS provides, and an inaccurate time is unacceptable, 
then add no other sources.  If serving time at a reduced accuracy is 
acceptable, add four or five external servers for redundancy.  In addition 
to redundancy, another advantage might be that you get a check that you 
aren't working on the wrong edge of the GPS pulse, by looking at the 
output of ntpq -p, and seeing that the offset of the backup sources is 

BTW: depending on the version of NTP you are using, you may need to force 
the external servers to have minpoll and maxpoll set to 10, so that the 
external servers are not polled at one minute intervals (or less!).  I 
think 4.2.6 fixes that.


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