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[gpsd-users] Large ntp.conf time1 offset needed with GPSD shared memory

From: Jon Brase
Subject: [gpsd-users] Large ntp.conf time1 offset needed with GPSD shared memory driver
Date: Tue, 09 Aug 2016 03:02:16 -0500
User-agent: Opera Mail/12.16 (Linux)

Just a quick question: Is it typical for the data that GPSD hands to NTP through the shared memory driver to arrive with a ~500 ms offset from top-of-second as shown by PPS (where NTP is pulling the PPS data directly) or by external timeservers? The hardware is a Raspberry Pi running Raspbian Jessie with an Adafruit GPS breakout, with the following server configuration in /etc/ntp.conf:


#coarse time from GPSD
server minpoll 4 maxpoll 4 prefer
fudge time1 0.47 refid GPS

#PPS ref-clock for top-of-second
server minpoll 4 maxpoll 4
fudge refid PPS

# maps to about 1000 low-stratum NTP servers. Your server will # pick a different set every time it starts up. Please consider joining the
# pool: <>
server iburst
server iburst
server iburst
server iburst


I found that I needed a time1 offset in the .45 to .5 range (currently .47, as shown above), to get the coarse GPS time to line up with the PPS / external timeserver time. Is this typical? I'm just wanting to make sure that it doesn't indicate some gross misconfiguration, as half a second seems like a rather large fudge. If everybody here thinks everything is fine, though, I think I will be ready to take leave of this list, as this is the last outstanding GPSD-specific issue I have with my GPS timeserver project.

On that note, what's the most appropriate forum for discussing general issues with setting up a GPS timeserver (for example, some of the output I'm getting from ntpq doesn't match what the references I used for setup lead me to expect, and I suspect misconfiguration or other problems with ntpd)?

Jon Brase

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