[ntp:questions] Re: Interaction of PPS and fudge time1 - how to fudge a GPS?

David Schwartz davids at webmaster.com
Mon Mar 28 02:17:08 UTC 2005

"John Pettitt" <jpp at cloudview.com> wrote in message 
news:GdI1e.12435$m31.127222 at typhoon.sonic.net...

> What I know so far is that if I run my GPS_NMEA without the kernel PPS
> flag then the time1 fudge seems to work in that the system syncs to
> GPS time + fudge factor.   However if I enable the kernel PPS flag
> then the system locks to the PPS and ntpq shows the GPS with an offset
> equal to the fudge factor.

    Right, so don't enable the kernel PPS flag.

> My assumption is that the kernel PPS (FreeBSD 5.4-BETA1) knows nothing
> about the fudge factor - is this correct?


> If this is the case how can
> I get ntpd to look at PPS but without the kernel being involved - if I
> just don't set the flag then ntptime shows PPSSIGNAL but not PPSTIME
> or PPSFREQ (I notice in the NMEA driver source enable_pps is only set
> if the kernel pps flag is given).

    Do not enable hardpps. You'll suffer a slight reduction in accuracy, but 
the PPS input will still be used by NTP to compute time offsets, just not by 
the kernel directly.


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