[ntp:questions] Adjusting PPS offset

David Woolley david at ex.djwhome.demon.invalid
Mon Sep 5 07:50:30 UTC 2011

Maarten Wiltink wrote:

> "In general" PPS will be off by anything up to half a second in either
> direction, distributed uniformly.

The sort of PPS source that ntpd normally uses is synchronised to the 
UTC second.

A GPS source showing an apparent offset of 2ms is much more likely to be 
doing that than just randomly being that close.

Your Caesium example is a pure frequency standard, but GPS needs an 
accurate common time across the satellites to provide a proper spatial 
solution. It needs very good relative time between satellites to work at 
all, and it needs quite good absolute time in order to exactly locate 
the fast moving satellites.  In practice, it is marketed as a time 
standard as well as a position one.


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