Re: [gpsd-dev] Meaning of negative PPS offset?

[Gary E. Miller]

Yo Greg!

On Tue, 26 Aug 2014 15:52:46 -0400
Greg Troxel <address@hidden> wrote:

> 
> "Gary E. Miller" <address@hidden> writes:
> 
> >> which is I think what you are saying.   So the bug is that in
> >> cases we are saying.
> >> 
> >>    n - N = -40ms
> >
> > Which makes no sense.  The NMEA at time X never preceeds the PPS
> > for time X.
> 
> As I understand it, this discussion is happening because of a device
> which is sending NMEA in advance of the PPS edge.

I finally got that far back in my email.  The GR-601W, which has a known
kernel driver glitch.  It should be special cased as it is kernel driver
specific.  No other known USB device, of any driver, support PPS.

> >>  Or perhaps even to only capture PPS when it's within 200 ms
> >> of the system clock.
> >
> > Which fails for older and slower GPS.
> 
> I meant system clock, not NMEA.

So did I.

>  So it would work when the device was
> synced via NTP coarsely.

Which was removed as a contraint with the latest patches.

> (I didn't quote most of your message, because I basically am in
> agreement with it - I knew my example times were not really quite
> right, but that they weren't critical to the point.)

Fair enough, except similar constants have been tried in the code
and failed, so best to be explicit they are WAGs.

> So the real issue is to find out if this device that seems to have
> premature NMEA really does do that.

GR-601W, which I have right here.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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