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Re: [gpsd-users] Large ntp.conf time1 offset needed with GPSD shared mem

From: George Sexton
Subject: Re: [gpsd-users] Large ntp.conf time1 offset needed with GPSD shared memory driver
Date: Tue, 9 Aug 2016 11:19:37 -0600
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

On 8/9/2016 2:02 AM, Jon Brase wrote:
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:

I'm using Jessie with instructions here:

and a Garmin GPS16X-HVS. My clock line is:

server mode 49 minpoll 4 maxpoll 4 iburst true prefer
server time2 0.45 flag1 1 refid GPS

So, yes I would say it's normal. Without the offset, I wouldn't get a PPS lock because the time differential was too big.


#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)?

George Sexton
MH Software, Inc.
Voice: 303 438 9585

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