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Re: PPS in GPSD vs Chrony

From: Gary E. Miller
Subject: Re: PPS in GPSD vs Chrony
Date: Mon, 29 Jun 2020 21:07:44 -0700

Yo Dennis!

On Tue, 30 Jun 2020 10:44:05 +0800
Dennis Law <> wrote:

> We have actually been using the SHM0 output from GPSD "locked" with
> the PPS clock in Chrony.
> As per our current understanding, SHM0 takes the time directly from
> the NMEA without consideration for PPS.

Yes, which means it can easily be off a second.

> On the other hand, the SHM1 or Sock output from GPSD already has the
> full complete timestamp with PPS considerations.


> Therefore, is it correct to say that the SHM0 locked with PPS in
> Chrony is somewhat similar to the SHM1 or Sock output from GPSD?

Nope.  That is backwardsd   The SHM1 from gpsd is locked by gpsd to

> If both of these mechanisms are indeed similar, are there any
> performance benefits of one over the other?

SHM0 +/- 300 ms.  SNM1 +/- 1 us

> We had a look at the PPS code in GPSD and Chrony and found the GPSD's
> version to be a lot more advanced (eg. handling of different PPS
> frequencies).

Yes.  But rarely used.

> Therefore, we are wondering perhaps using GPSD's PPS would have some
> sort of performance benefit over Chrony's PPS.

Unlikely.  Only way to know is to test.

> Also, the reason we are asking is because we are trying to achieve a
> high accuracy of 0.1ms in our system clock by using both GPSD +
> Chrony.

NMEA will never get to 0.1ms.

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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