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Re: [gpsd-dev] Meaning of negative PPS offset?

From: Miroslav Lichvar
Subject: Re: [gpsd-dev] Meaning of negative PPS offset?
Date: Wed, 27 Aug 2014 09:39:44 +0200
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Aug 26, 2014 at 07:39:52AM -0400, Eric S. Raymond wrote:
> There is a comment in ppsthread.c that reads:
>              * FIXME! The GR-601W at 38,400 or faster can send the
>              * serial fix before PPS by about 10 mSec!
> And, indeed, when I run gosmon live at 9600 on a GR-601W I observe a 
> small negative offset. At the moment I write it is -0.003518431.
> This puzzles me.  It makes me fear we may have a design or
> implementation bug.
> The offset is the delta in fractional seconds between the clock time
> of the last PPS and the second part of the last GPS timestamp seen, minus
> one second.

Right, it's the offset between PPS and system clock. A negative offset
means the clock is ahead. When the clock is synchronized by chronyd or
ntpd, the offset should be close to zero and alternate between
positive and negative values.

Should it be something else?

Miroslav Lichvar

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