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

shane-dated-1234940584 at csy.ca shane-dated-1234940584 at csy.ca
Wed Feb 11 07:16:54 UTC 2009


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.

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?


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