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