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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Mon Jul 19 06:32:02 UTC 2010

"David Lord" <snews at lordynet.org> wrote in message 
news:mhpbh7-ug9.ln1 at p4x2400c.home.lordynet.org...
> The serial RX data for my Garmin seems to have an offset > 500ms
> and also vary by at least 20ms. Once PPS synchronises the GPS
> offset comes more or less in line with PPS but unless that fudge
> time is used PPS will never be used. That was a known problem
> that has been fixed but not yet made it to NetBSD I use.

Strange, I've never seen that problem with the smaller serial offset in 
the GPS-18 LVC, and I have had to add no fudge lines for either:

GPS-18 LVC on:
FreeBSD 5.4, type 20 ref.clock, NTP version - as supplied ~2006
FreeBSD 8.0, type 20 ref.clock, NTP version - as supplied: 4.2.4p5-a (1)
Windows, type 20 ref.clock, NTP version 4.2.6-o Dec 09

GPS-18x LVC on:
Windows-7, type 20 ref.clock, NTP version 4.2.6p2-RC5

PPS seems to come up more or less at the same time as GPS, even without a 
~200ms or 500ms+ offset being coded in the config file.

I wonder what the specification is for the NTP ref.clock drivers, in terms 
of maximum delay of serial data compared to the PPS signal?


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