[ntp:questions] Maximum time2 fudge value for NMEA refclock?

David Lord snews at lordynet.org
Sun Jul 18 10:52:06 UTC 2010

jimmyterrence wrote:
> Is there a maximum fudge value above which the NMEA refclock throws
> away input? Both my 18xLVC GPS receivers output serial data about 550
> ms after the PPS signal. That works fine with gpsd, but I've been
> trying to set up the NMEA driver with pps compiled in the 2.6.34
> kernel and it doesn't seem to work.I thought I read somewhere that it
> accepts a max of 400 ms but I can't seem to find that anywhere. Could
> anybody tell me what the value is, and maybe point me to the line in
> refclock_nmea.c (if that's where it is) that governs that?

I don't have a Linux system handy but I've used my Garmin
no problem from both serial and with relatively poor
performance from USB. This will have been on Ubuntu 9.04
and up. I just created symlinks /dev/gps0 => /dev/tty00
and /dev/pps0 => /dev/tty00.

On NetBSD I've been trying parallel port device and hit a
problem due to a bug in ntpd (corrected in later versions
but these not available for NetBSD). For ntpd 4.2.4p6-o
ntp.conf has

tos mindist 0.8


server mode 1 minpoll 6 maxpoll 8 prefer
fudge time1 0.680 refid GPSb

Note that in my docs it states that time2 is not used

Without the fudge time1 the PPS doesn't kick in as time
from GPS without serial DCD is too far out.

PPSb is currently at offset 0.019 and has been getting
more deviation from 0.000 during summer with growth of
nearby trees. Even a mast of several metres is not
likely to give any benefit for more than a couple of
years at most. Most sats I've had in view when checked
is three and I don't think that's enough.


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