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

From: David J Taylor
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Sun, 14 Aug 2016 09:14:37 +0100

Dear Gary, dear Frank
You convinced me that the uputronics is the better choice over the
adafruit. I read the specs. The Adafruit is limited to 112 kbaud. The
uputronics is four times as fast.
I tried to make a standalone time server with GPS, gpsd and NTPd. The
approach of David Taylor (who uses a network connected system) of giving
the gpsd coarse clock a high stratum and the pps stratum 0 did not work
because you cannot get a lock this way. But when you have two stratum 0
clocks and there is jitter on the "coarse" one you don't get best
results either.


I have tested using /just/ a GPS/PPS Uputronics module to provide an NTP server with no Internet connection and it works just as expected. It's on my Web site:

The serial support on the Raspberry Pi changed with model 3, and with the Jessie OS, and may still be changing so be prepared to fiddle around a little. You need the "prefer" flag against the serial "time of day" source, not against the PPS source (as that had no time-of-day information). In ntp.conf:

# Kernel-mode PPS reference-clock for the precise seconds
server minpoll 4 maxpoll 4
fudge refid kPPS

# Coarse time reference-clock - nearest second
server minpoll 4 maxpoll 4 iburst prefer
fudge time1 +0.105 flag1 1 refid GPSD stratum 1

The older Garmin GPS18 LVC needs to be outdoors as it has a relatively insensitive receivers, but the GPS18x LVC works OK for me located indoors on the top storey of a building near a south-facing wall.

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