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

From: Jon Brase
Subject: Re: [gpsd-users] Large ntp.conf time1 offset needed with GPSD shared memory driver
Date: Wed, 10 Aug 2016 02:11:26 -0500
User-agent: Opera Mail/12.16 (Linux)

On Tue, 09 Aug 2016 16:13:55 -0500, Gary E. Miller <address@hidden> wrote:

Yo Jon!

On Tue, 09 Aug 2016 15:23:04 -0500
Jon Brase <address@hidden> wrote:

In my case I'm using GPSD for coarse time, but not for PPS, because I
have not been able to get PPS working with GPSD on the Raspberry Pi
(but ntp picks it up just fine).

Many problems with that.  Not the least of which is that ntpd does not
know when PPS is valid or not.

I have three RasPi's.  A B, a 2 and a 3.  Each with a different PPS/GPS.
They all work great.

Have you looked at the RasPi HOWTO?

Which RasPi HOWTO are you referring to. Several turn up on Google searches and I've seen several mentioned on this list, plus one mentioned by a member offlist. Different ones recommend different setups. Anything that recommended using 28.0 and 28.1 has come to grief trying to get GPSD to pick up PPS on the Pi (I haven't even gotten as far as configuring NTPD with that setup). Using 28.0 and 22.0 seems to work best of everything I've tried (28.0 + 28.1 fails on getting GPSD to recognize PPS input, 20.0 alone with flag1 set goes out half a second as soon as PPS skips a beat and then comes back on the wrong second when PPS returns, and 28.0 + 22.0 intermittently ends up on the wrong second if I don't offset it, but works fine with a half-second offset).

Thus I'm using 28 for coarse time,
22 for PPS.

Which makes it real easy to get the PPS on the wrong second.  Or worse,
following PPS when the PPS is not valid.

But that's the point of the offset fudge on the coarse data, right? To bring it close enough to external time sources that there's enough agreement on what second it is that each PPS pulse can be assigned to the right second?

I'm needing to offset the coarse time by nearly half a
second,  and am just wondering if this is normal.

Yes, that is normal.  But that brings it's own problems.  You need to
set 'prefer' on the PPS and 'noselect' on the NMEA time.  And of course
at least 3 other chimers.

Yes, pretty much everything recommended other chimers.

And one of the guides I was referencing said that prefer has to be marked on something *other* than PPS because PPS doesn't include a second count and only indicates top of second.

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