[ntp:questions] Garmin 18 LVC: whether to fudge

Unruh unruh-spam at physics.ubc.ca
Wed Feb 11 23:55:51 UTC 2009

shane-dated-1234940584 at csy.ca writes:


>I'm using a Garmin 18 LVC connected using LinuxPPS with mostly good results. 
>I am curious about one thing though.  The offset reported by the GPS18
>differ from the public NIST servers by around 1.7-1.9MS.  as shown in the
>offset numbers of ntpq below.
>     remote           refid      st t when poll reach   delay   offset  jitter
>*GPS_NMEA(0)     .GPS.            0 l    4   16  377    0.000   -0.448   0.189
>+bigben.cac.wash .USNO.           1 u   34 1024  377   11.134    0.283   2.645

>All of the server's internet peers are ahead by around that same value so
>I'm guessing that when the GPS18 loses sync, ntpd would have to bring the
>clock up by 2MS and reverse when signal is reacquired.

Sorry, where is that 1.7-1.9ms? I see a difference of .7ms, which, given
the 11 ms delay is pretty good. 

>Is there any way to know whether it's our internet link (ADSL) causing the
>internet servers to appear off or does the GPS18 need a time1 fudge to bring
>it in line with the others?  That is, is there a 2MS lag in processing the
>interrupt for PPS?

If there is a 2ms delay is processing the interrupt, throw away your system
and buy a new one. It is severely damaged. 

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