Re: PPS in GPSD vs Chrony

[Gary E. Miller]

Yo Dennis!

On Mon, 29 Jun 2020 15:57:54 +0800
Dennis Law <cmlaw1993@gmail.com> wrote:

> I noticed that the PPS correction used by GPSD is rather similar to
> that in Chrony.

Yup.  The PPS errors are before gpsd passes the time data to any
daemon (ntpd, chronyd ,etc.)

> In GPSD, the PPS correction is performed on top of the NMEA input and
> output to Chrony via SHM1 or Sock.

No correction based on NMEA.  All the NMEA does is tell gpsd what second
the PPS pulse refers to.

> If I am not mistaken, Chrony does the same correction when combining
> a SHM refclock and a PPS refclock. This is achieved through the
> "lock" feature in the configuration file.

Do not use "lock" with data from gpsd.  gpsd sends chronyd, and ntpd,
a full and complete timestamp.  That needs no reference, or "lock", to
anything else.

You use "lock" when you have chronyd directly read the PPS pulse, which
has no seconds part, and chronyd needs the NMEA time for the seconds part.

> Is this understanding correct?

Nope.

> Also are there any benefits of using
> one over the other?

If you do not need the other features of gpsd, then no benefit to gpsd.
But there are a lot of benefits that gpsd provides, like you lat/lon,
skyview, etc.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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