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Re: [gpsd-users] TOFF and PPS values from gpspipe

From: Gary E. Miller
Subject: Re: [gpsd-users] TOFF and PPS values from gpspipe
Date: Tue, 3 Sep 2019 19:24:48 -0700

Yo Kaya!

On Wed, 4 Sep 2019 03:08:27 +0100
Kaya Saman <address@hidden> wrote:

> Just wanted to let you know that I'm pretty much done with getting
> the information from gpsd onto my LCD :-)


> In the meantime I got advised to publish the code on Github so here's 
> the repo:

I'll check it out.

> There's still the remaining issue of the TOFF which I asked about 
> initially. Basically what's happening is the gpsmon is coming up with
> a different value from the one published in the TOFF sentence.

Yes.  gpsmon is an odd duck, not well maintained.  Always better to
use cgps if possible.

In the past the TOFF sentence was a compile time to gpsd.  Now it is
always enabled.  So gpsmon computed its own.

In any case, TOFF is horrible, only useful when things go wrong...

> gpsmon shows: 0.709454834
> while the value received in the sentence is this: 0.587889426

Calcualted differently, but well within the typical jitter of TOFF.

if .2 sec matters to you, do not use TOFF.

> I don't know if it's relevant but I do have a 'fudge' value in my
> ntp.conf:

Relevant if you also run NTP.  And the TOFF is useless if you
do not run NTP.  But a bad idea if you have PPS.
> fudge__ time1 0.536 refid NMEA

ntpmon will tell you if that is good.

But, sort of circular.  If TOFF is calculated from NTP and you are
using TOFF to set NTP then you are chasing your tail.

> Though I don't see how that can relate to the offset between 0.709
> and 0.587??

You can't.  Different ways of calculating one thing.  Neither useful
to get a real fudge.

> Unfortunately nothing worked :-( Maybe there's a simple solution
> which I'm just missing?

Yeah, don't do it.  :-)

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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