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Re: PPS in GPSD vs Chrony

From: Miroslav Lichvar
Subject: Re: PPS in GPSD vs Chrony
Date: Tue, 30 Jun 2020 08:31:28 +0200

On Mon, Jun 29, 2020 at 03:57:54PM +0800, Dennis Law wrote:
> 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.
> Is this understanding correct? Also are there any benefits of using one
> over the other?

They are quite similar and should perform similarly well in the
typical case. SOCK has a small advantage over SHM+PPS that there is no
delay due to polling, so the clock update quickly follows the edge of
the pulse.

In general, gpsd is a better place for the PPS processing as it can
identify the device and use a database for things like the expected
message delay and whether the pulse comes after the message or before.
In chronyd these things needs to be configured for each refclock
manually. gpsd could also apply a sawtooth correction that some GPS
units support.

As for the PPS rate, in gpsd it is detected automatically, but if I
remember correctly it can select only from few different rates like
0.5, 1, 5Hz. In chrony it needs to be set with the rate option, but it
supports any value. For example, I set the PPS on my ublox units to
32 Hz to get more samples and improve the stability.

Miroslav Lichvar

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