Problem with GPSD refclock

Rich Wales richw at
Mon Jan 30 03:31:10 UTC 2017

Gary E. Miller wrote:

> No, setting a good fudge on the NMEA time is a bad thing.  You only want
> ntpd to use SHM(1) as a last resort.  If you know you have good network
> time as a backup then set SHM(1) to noselect.

I'm confused here.

First of all, did you really mean to say "SHM(1)" in the above?  My
understanding was/is that SHM(1) is the GPS's NMEA time info combined
with the PPS signal (and will therefore normally be highly accurate),
and SHM(0) is raw NMEA without PPS (very inaccurate with high jitter).

Thus, I would normally want to use the SHM(1) refclock, and I would only
want to consider using SHM(0) as a last resort if my GPS reception went
bad *and* I also lost my Internet connection at the same time.  Right?
Or am I still mixed up?

Rich Wales
richw at

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