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- Subject: Re: [chrony-users] Multi-PPS+RTC Setup Questions
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Tue, 29 Sep 2020 18:20:11 +0200
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On Tue, Sep 29, 2020 at 12:02:40PM -0400, Matt Corallo wrote:
> In theory the GPS should be able to provide a precision, certainly looking
> at the NMEA stream *should*, and definitely looking at u-blox's proprietary
> messages does, though as far as I can tell gpsd doesn't make any effort to
> expose it in a useful way (and I'm not sure how much effort should be
> required to do so in a vaguely cross-device manner). Even if it did, I don't
> see a great way to feed it into chrony, eg there's no way to pass it in via
> the socket interface (AFAICT, I was hoping to do this for the RTC to track
> its drift over time and feed it into chrony) and chrony appears to ignore
> the precision field in ntpshm (not that anything sets it usefully).
That wouldn't help with the other errors, e.g. due to the serial port,
interrupts, etc, which are typically much larger than the GPS
time/position error.
> My PPS interrupts are serial-DCD, which has some noise, but in my case
> u-blox appears to be garbage and the two devices are off persistently by
> 30-ish us (ie an order of magnitude or so more than noise), and somehow its
> tied to the u-blox devices themselves (the usual swapping of all
> ports/antennas/etc doesn't change the fact that the two devices just have
> different concepts of the current time, across nearly every setting
> including different tick rates so they tick at different times).
30 microseconds between different GPS receivers is a lot. Something
has to be wrong. I think largest I have seen was with the old 18x LVC,
that was off by about 1 microsecond, but that was still within its
spec. Between different u-blox receivers I see at most few tens or
maybe a hundred of nanoseconds.
Have you tried factory reset? Are they the same model? Bought on ebay?