[ntp:questions] Odd offset for PPS DCD w/ Garmin GPS 18x LVC

Dave Hart davehart at gmail.com
Tue Mar 22 16:51:45 UTC 2011


On Tue, Mar 22, 2011 at 15:30 UTC, lellis <larry.ellis at gmail.com> wrote:
> On Win XP, I am seeing an odd positive offset of about 128ms on the
> Atom driver, relative to other sources:
>
>
>     remote           refid      st t when poll reach   delay
> offset  jitter
> ==============================================================================
> *GPS_NMEA(1)     .GPS.            0 l    6 16 377 0.000
> 0.373   0.919
> xPPS(1)          .PPS.            0 l    5 16 377 0.000
> 125.447   0.830
> -clock.team-cymr 172.16.65.22     2 u   39   64   77   12.989
> -41.044 44.960
> +ntp.okstate.edu .USNO.           1 u   42   64   77   28.906
> -18.762  25.784
> +navobs1.wustl.e .GPS.            1 u   38   64   77   19.133
> 1.819  19.599
>
> Normally, I just use NMEA with fudge, but that was also showing the
> odd offset so I removed fudge and added the Atom driver to help debug
> this.
>
> (Normally the status shows oPPS(1) but xPPS(1) is seen above for the
> obvious reason.

I think your PPS is probably right on, while your NMEA is off by
nearly a second.  Make sure either the GPS is generating only the
sentence you want each second, or use the mode option to the NMEA
driver to select a sentence.  Try using the same value for fudge time1
and fudge time2 for NMEA (one is used to offset NMEA end-of-line
times, the other to offset PPS time.  Thanks to the user PPS hack in
Windows ntpd, the "end of line" times should actually be PPS times
with tens of microseconds offset and a bit more jitter than PPSAPI
timestamps.

Cheers,
Dave Hart



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