On Thu, Nov 16, 2023 at 07:58:33AM -0500, Greg Troxel wrote:
> > As for stability and accuracy... using just the USB data alone will
> > yield very poor results, as has been mentioned. That can, however, be
> > used for a quick test with the NEMA driver that ntpd has, just don't be
> > impressed by it. By adding the digital PPS signal into the mix that
> > will deal with the USB problems and you will get a good result once the
> > device and ntpd stabilizes. My modules present their NEMA output as
> > digital tty (uart) signals that I hook to a FTDI chip and into a USB
> > port... so the effect is very simular to what you are probably doing. I
> > also use ntpd which can deal with both a /dev/ttyXX NEMA device and
> > /dev/gpioppsX PPS device at the same time. In this arrangement, you
> > won't be using shared memory and your output would look something like
> > this:
>
> It is true that using USB PPS has 1 ms of fuzz. However, people say
> "stratum 1" and make varying assumptions about what they care about.
>
> If the concern is to keep time sync when the Internet is down, 1 ms of
> fuzz is ok. If you are trying to build something to distribute time to
> other people, and especially to be a public stratum 1, then it's not ok.
Sure; my feeling (but I may be wrong) is that is this case the 1s NMEA messages
may be good enough for NTP to sync, and the PPS may not bring much.
I have a setup where I only use the NMEA message with gpsd and ntpd, no PPS.
ntpd has to problem to sync but I don't know how acurate it is (this host
is not connected to internet). For this use case 1s acuracy is good enough :)
--
Manuel Bouyer <bou...@antioche.eu.org>
NetBSD: 26 ans d'experience feront toujours la difference
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