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

[Miroslav Lichvar]

On Wed, Aug 10, 2016 at 10:28:54AM -0700, Gary E. Miller wrote:
> Roger Oberholtzer <address@hidden> wrote:
> > The Garmin 18x-5Hz LVC seems to provide 'five-pulse-per-second'
> > information. I am guessing this related to the 5Hz NMEA records.
> 
> Only vaguuely related.  The rate of each can be independently selected.

If the rate of NMEA messages is equal to the PPS rate, I think it will
be easier to assign reference time to the pulses. If the PPS rate is
higher (as with some u-blox modules that allow rates up to 1000 Hz),
gpsd will need to count pulses between NMEA messages.

> > Is this any problem for gpsd and time synchronization?  
> 
> On theory, no.  In practice, never tested.

gpsd supports detection of 5 Hz PPS, but there is a problem that it
always sets the fractional part of the timestamp to zero, so with 5 Hz
PPS the offsets have an error of 0.0, 0.2, 0.4, 0.6 and 0.8 seconds.

There was actually a patch that tried to fix this couple years ago,
but there was no interest back then.

-- 
Miroslav Lichvar