Re: [gpsd-dev] Disturbing numbers

[Eric S. Raymond]

Andy Walls <address@hidden>:
> > I do notice a curious thing, though. While I've been composing this reply 
> > my PPS offset has crept up from 2 to 13 mSec, and seems to be steadily 
> > rising.  What does this indicate?
> 
> If just (re)started without a drift file, ntpd has a start up
> calibration period on the order of minutes while it ponders things.
> 
> My guess is that your system clock is drifting with respect to UTC and
> the PPS while ntpd is calibrating and letting things free-run.
> 
> If this is the case, ntpd could take up to 15 minutes for ntpd to kick
> in, step the clock, and start adjusting the frequency and offset of your
> system clock. 

This is exactly the sort of thing that needs to go in the Time Service HOWTO!

The 15 minutes fits pretty exactly.  A couple of minutes after I sent my 
reply the offset started dropping again, bottomed out at about 2mSec and
is slowly rising again.

Is a long-period sawtooth what I should expect to see?

Yay, visualization tools.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>