Post #2958989
2026-03-26 15:54 UTC
First, it's not "GPS" that you're having Chrony (or NTPd) syncing to. It's 1 Pulse-Per-Second (PPS), which is why my screenshot says PPS is my reference clock.
GPS delivers "NMEA stanzas" over a serial connection. Which means we're talking over (usually slow) serial baud rates, over which you receive "the time is now X", along with many other messages and have to guess at how long ago that timestamp was generated, since it takes many milliseconds to receive.
2/x
Replies (1)
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@rune@social.intothecloud.net 2026-03-26 15:54
You need a GPS that can also deliver PPS on a GPIO pin, or separate serial port. PPS uses a voltage rise (that causes a kernel interrupt, providing accuracy) once per second to get a measure of accurate time, in nanoseconds range, which is zero point seconds (compared to milliseconds accuracy for GPS). That gets the time into GPSd. Getting the time into Chrony (or NTPd) was also quite painful (for me, at least). 3/x