Re: [chrony-users] gpsd, pps and chrony

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Mon, Mar 21, 2011 at 09:16:50AM -0700, Bill Unruh wrote:
> Maybe an NMEA source should not be regarded as a stratum 0 clock but rather as
> a stratum 1. Its error ( many ms) is far greater than the PPS. Although even
> with that error that does not make up for the huge (300 ms or greater) delay
> from the nmea.

I'm not sure how would increasing stratum help here. The falseticker
detection ignores the stratum of the sources. Also, chrony doesn't know
if it's an NMEA source or not, the SHM protocol doesn't provide such
information.

> Certainly pairing the nmea with the PPS is the only proper thing to do,
> although apparently the GPS18x has a firmware bug that causes the nmea to come
> out over a second after the PPS, which screws up that association.

Yes, that's unfortunate. I'm keeping an older firmware in mine until
a fixed one is released. In gpsd it's not possible to adjust for the
large delay, so SHM 0 refclock with adjusted offset + PPS refclock is
probably the only reliable configuration with the broken firmware.

BTW, the soon to be released gpsd supports kernel PPS timestamping and
also the chrony SOCK protocol, which means that to get PPS samples
with nanosecond resolution the PPS refclock won't be necessary.

-- 
Miroslav Lichvar

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