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Re: [gpsd-dev] Disturbing numbers

From: Andy Walls
Subject: Re: [gpsd-dev] Disturbing numbers
Date: Sun, 03 Nov 2013 14:16:45 -0500

On Sun, 2013-11-03 at 14:07 -0500, Eric S. Raymond wrote:
> Andy Walls <address@hidden>:

> I thought of this, but since I wasn't running ntp it follows that gpsd
> cannot have been feeding it funky time.
> And I see by running gpsmon that my offset from GPS time has dripped
> to around 2mSec. OK, it was not having ntpd running.  We can scratch that
> bug, then.
> (File under "Why I wanted gpsmon to be able to see PPS".)
> 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. 


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