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Re: PPS in GPSD vs Chrony

From: Gary E. Miller
Subject: Re: PPS in GPSD vs Chrony
Date: Thu, 2 Jul 2020 15:45:27 -0700

Yo Miroslav!

On Thu, 2 Jul 2020 09:57:49 +0200
Miroslav Lichvar <> wrote:

> There is a median filter (in chrony, ntp and ntpsec I presume)

I see no such thing in refclock_pps.c in ntpsec.  Pretty much the same
code as i NTP Classic.

Sounds like a good idea.

> When the PPS rate is increased, and the
> driver can be configured to collect all those samples (I'm not sure
> about ntp/ntpsec), the median becomes more stable and as a result the
> synchronized clock should be more stable too.

AFAIK ntpd does no averaging/filtering.  Might be a good idea now that
our time sources are far more accurate than the measurement precision.

> > Hmm, how does the time daemon (gpsd, chonryd, ntpd) know which of
> > the 32 pulses is the top of the second?  NMEA time is very unlikely
> > to be better than 30 ms.  
> Yeah, that's tricky. The binary protocols should be more stable.

Even then you would need a very fast serial port, which modern receivers
can do, just not by default.

> In my
> case it was unreliable with a 32Hz PPS, so I rely on (close) NTP
> servers to get the PPS refclock running.

Classic chicken and egg problem.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  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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