[lightswitch05]

Good question, it absolutely syncs without any network connectivity. For better or for worse, I have it setup NOT to sync to anything else- being a stratum 1 source is enough. Now… I will admit I’m just a hobbyist with this stuff. Anyways, the way I got around it trying to sync to another source when using the PPS was to define the GPS as two separate sources. One for the PPS signal and another for the NMEA sentences:

    refclock pps ppspath /dev/gpspps0 prefer time1 0.004 minpoll 3 maxpoll 4 iburst 
    refclock nmea baud 57600 time2 0.068 minpoll 3 maxpoll 4 prefer iburst

To get around issues with sync being slow, I have NTPD configured with some extra flags to allow large time jumps at startup:

    NTPD_OPTS="-ggg -N"

I have two raspberry pies setup this way- which I don’t use for much else. My primary computers have these two servers configured as time sources- but also have external sources- so as long as network connections are available, they can determine if they are providing a sane time or are false tickers. As for accuracy, you can view the ntpviz output for each setup in the hamburger menu. For catwoman, its regularly within 4 microseconds.

[hcfman]

Ah okay. Because I found that the default start order of gpsmon and chrony was incorrect meaning that the shared memory communications wasn't working but there were no error messages stating this. So sometimes, it would sync (slowly) and then chrony sources would say that it was having microsecond sync time but when I connected a switch (On/Off type) to the same gpio on two separate Pi's with linked earth connections and had a python program output the system time in nano seconds I found that despite telling you it was keeping time accurate to microseconds it was in fact many milliseconds out. The only thing I can conclude is that despite the setup indicating that it should use the PPS source that use the interrupt, this was in fact not happening and it was somehow getting it's time from the serial line only. Without indicating that this was happening.

When I switched the startup of gpsmon and the chrony daemon to the other way around time sync was achieved within 1/2 minute to a minute (Real fast) and the reported time on two separate Pi's with the joined up gpio's (So you can get very close to triggering a time check at the same time) was then sometimes as low as 30 ns's from each other, which is damn good considering that most of the difference would be jitter.

And none of the how-to's that I read talked about changing the start order of chronyd and gpsmon so I can only assume they all might be suffering from the same problem. It's important to note that the diagnostic commands from chrony will not indicate this and the only way you will notice if this is happening if you setup a rig like I mentioned above. I noticed because my sound localization calculations were showing errors that didn't make sense.

[lightswitch05]

I must admit I've never used chrony, so I'm unfamiliar with how to configure it. I've read a lot say saying gpsmon's shared-memory-segment is a really great interface and to go with it instead of NTP's own GPS drivers, but I cannot say that lines up with my experience. I do have one computer setup this way, and it works, but I found on the PPS configurations, it seemed to me to have better accuracy and less jitter using NTP's own GPS drivers and taking gpsmon out of the equation. Its quite possible I just don't know the right configuration combination, but ntpsec's own GPS drivers work great for me.