Re: NTP with only GPS/PPS flags PPS as falseticker

[Greg Troxel]

My $0.02 of advice

  Long ago, in 2008, I set up gpsd and ntpd on a mac notebook on a
  computer with no internet, and that worked fine, and it provided sync
  to about 100 other computers with no internet, 50 of which were
  connected by an ad hoc radio network under development.  Were we 10 ms
  off from true UTC?  Maybe, but we didn't care about that last 10 ms.

  Set fudge on the non-PPS GPS time (which is based on mesage arrival).
  It is typically off a bit, and I dimly remember 100 ms late (on one
  computer, running a different OS, with a different receiver -- just
  mentioning it that you should not be upset by 20-300 ms of fudge being
  needed).  This is basically calibrating the delays, and I found that
  variance was not that bad (a few ms) once done.

  Read about minsane.  If you want to sync to just GPS, then you need to
  accept a single source.  ntpd by default does not do this.

  Turn on ntpd debugging and figure out why it's doing what it's doing.

  If you have an ntpd problem, ask them :-)




My ntp.conf, for 3 USB GPS mice, NetBSD, base system ntpd, gpsd:

# GR-601W
#fudge 127.127.28.0 time1 0.110 refid GPS
# VFAN (ublox 8)
# u-blox AG - www.u-blox.com (0x1546) u-blox GNSS receiver (0x1a8)
#fudge 127.127.28.0 time1 0.048 refid GPS
# VIK (ublox 7, I think)
fudge 127.127.28.0 time1 0.072 refid GPS

Not that you should take my numbers, but basically if when on the net
the gpsd time is slow, increase, and if fast, decrease, and then you'll
be dialed in where it's pretty good.   Again if you care about sub-ms
from this you are out of luck, but getting < 5 ms should be easy, and
1ish probably doable.