[ntp:questions] Sure GPS - Very High Jitter and Offset

Miroslav Lichvar mlichvar at redhat.com
Tue Aug 16 14:53:37 UTC 2011


On Tue, Aug 16, 2011 at 09:42:09AM -0500, Ken Link wrote:
> I wrote a script to compare the previous/current assert timestamp
> (ignoring the seconds), and this is what I get when NTP is *not*
> running:
> 
> -.000004357
> -.000007328
> -.000008905
> -.000007969
> -.000007593
> -.000009174
> -.000006566
> -.000008152
> -.000007500
> -.000006854

That looks good.

> I tried setting flag3 to 0 but it didn't appear to make a difference.
> Also, I would have expected NTP to mark the clock as a PPS source with
> 'o', but when I let it sync it seems to stick with '*' instead:
> 
> $ ntpq -p
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
>  LOCAL(0)        .LOCL.          12 l  232   64   10    0.000    0.000   0.002
> *GPS_NMEA(0)     .GPS.            0 l    7   16  377    0.000   20.012  26.423

It seems the GPS driver is not getting or is ignoring the PPS signal.
I think there were some issues fixed recently in it. I'd try the ATOM
driver (22) first to verify ntpd was compiled with PPS support and is
able to use it.

-- 
Miroslav Lichvar



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