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

John Pettitt jpp at cloudview.com
Mon Mar 28 00:29:26 UTC 2005

I'm trying to understand the interaction of PPS, Kernel PPS and the
time1 fudge and it's not making sense to me.

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.

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).

Is this making sense?  What I want to do is fudge the PPS disciplined
time from my GPS and I don't seem to be able to do it.


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