[ntp:questions] Re: Please demistify the SHM driver

Jerome Oufella oufella at SPAMBLOCKchez.com
Thu Sep 25 01:49:41 UTC 2003

Paul Croome wrote:
> (.....)
> Well... 4.625 is not a very good value for the jitter of a GPS refclock.
> My GPS (Motorola Oncore) is currently doing:
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> *GPS_ONCORE(0)   .GPS.            0 l    9   16  377    0.000   -0.069   0.004
> (...more stuff snipped)
> But, much more important: If you have a GPS refclock, why the **** are you
> using the local clock driver? That's very much a last resort for NTP nets
> that don't have access to anything better. And why is your "remote host"
> also configured with the local clock driver? There should be at most one
> NTP server in the net that syncs to its local clock...
 > Paul

The LOCAL driver is only used as fallback, if the serial GPS (rare but 
possible) or network link ever fall down (this often happens).
My GPS clock sends an RS232 frame once every minute. The frame 
informations give date, hour, minute and seconds only (not msec, nor 
usec). I just hope its internal usec and msec are constant when the 
RS232 frame is emitted. Maybe not, this may explain the jitter.

The previous 4.625 value was acquired after only some dozens of minutes 
of work.

The actual values (after some days up) are :

remote      refid    st t when poll reach delay   offset jitter
  LOCAL(1)   LOCAL(1) 9  l 17   64   377   0.000   0.000  0.008
*SHM(1)     SHM(1)   10 l 11   64   377   0.000   1.349  0.044

This seems better.
Thanks again.

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