[ntp:questions] Slow convergence of NTP with GPS/PPS

Richard B. Gilbert rgilbert88 at comcast.net
Sat Oct 25 03:34:23 UTC 2008

Hal Murray wrote:
>> It's the poll interval of ntpd.  Ntpq does not poll!  The poll interval 
>> varies between 2^MINPOLL and 2^MAXPOLL.  You have set MINPOLL=MAXPOLL=4 
>> giving a poll interval of 2^4 or 16 seconds.  This is usually the 
>> correct choice for a GPS receiver.
> Why do you say that?  

Because the GPS time signal is extremely accurate!

> Or let me ask it another way, how would you
> decide what the right polling interval is?

NTPD uses much longer poll intervals over the internet or even over a 
local network because of the variable delays introduced by the network. 
  No two packets are guaranteed the same path between two points unless 
there IS only one path.  In addition to the variations in travel time 
due to differing paths, there are also variable queuing delays!

Ntpd does not know how long any particular packet was delayed, it only 
knows what the typical delay is.  The longer polling intervals smooth 
out the errors caused by variable delays.

See Dave's book for a rigorous mathematical explanation.  Or, if your 
math is as poor as mine, you'll just have to take Dave's word for it.

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