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

Richard B. Gilbert rgilbert88 at comcast.net
Wed Feb 11 13:16:57 UTC 2009

shane-dated-1234940584 at csy.ca wrote:
> Hello,
> 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?
> Best,
> Shane

My bet would be that there is an asymmetry in your ADSL link!   If I'm 
not mistaken, the "A" in ADSL stands for asymmetric!  Your GPS PPS 
output is probably within 50 nanoseconds of the exact time!  The process 
of getting that signal into your computer is going degrade the accuracy 
by an amount that is somewhere between difficult and impossible to measure.

More information about the questions mailing list