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Re: [gpsd-users] Garmin 18X-5Hz

From: Miroslav Lichvar
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Fri, 12 Aug 2016 11:52:34 +0200
User-agent: Mutt/1.6.2 (2016-07-01)

On Fri, Aug 12, 2016 at 09:20:52AM +0000, Roger Oberholtzer wrote:
> > From: Miroslav Lichvar address@hidden
> > Another issue with ntpd is that it reads from SHM only once per second
> > (and this can't be easily fixed), so in order to take advantage of a
> > higher rate PPS it needs to use some mechanism based on sockets or
> > pipes (maybe the GPSD JSON driver?).
> In fact I don't really want the higher rate of pulses. What I want are the 
> NMEA messages at the highest rate for this receiver (5Hz), and the PPS at one 
> per second. The question is if this could be done with this receiver. We have 
> one on order, so we will see.

If I'm reading the specification for 18x-5Hz correctly, the rate of
NMEA messages and PPS rate are both fixed to 5 Hz. The only thing that
seems configurable is the width of the pulse.

I think a fix in gpsd will be needed for reliable operation. As ntpd
reads SHM only once per second, the error in the offset might be
stable, but I guess it will depend on what time ntpd is started and
there is a 4 in 5 chance it will be off :).

Miroslav Lichvar

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