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gps time

the pi keeps time from a MediaTek GPS module on the serial port (gpsd identifies it as MTK-3301), with its PPS output on GPIO 4. gpsd reads it and chrony uses it. the serial data gives the time to the second, and the PPS pulse marks the start of each second.

gpsd

/etc/default/gpsd:

START_DAEMON="true"
DEVICES="/dev/ttyAMA0 /dev/pps0"
GPSD_OPTIONS="-n"
USBAUTO="false"
  • -n makes gpsd talk to the GPS straight away rather than waiting for a client, which chrony needs
  • USBAUTO="false" stops gpsd grabbing the USB radios, which are serial devices too

gpsd starts from gpsd.socket (enabled), and chrony pulls gpsd.service in.

chrony

i added these lines to the end of /etc/chrony/chrony.conf. the rest is Debian's default:

# GPS refclocks
refclock SHM 0 refid NMEA precision 1e-1 offset 0.2 delay 0.2 noselect
refclock PPS /dev/pps0 refid PPS lock NMEA precision 1e-7 prefer

# serve time to the LAN
allow 192.168.0.0/16
allow 2001:db8:1000::/56
allow fe80::/10
  • NMEA is gpsd's time from the serial data, via shared memory. it is only good to a fraction of a second, so noselect stops chrony using it on its own. offset 0.2 corrects for the time message arriving a little after the second it describes, so it lines up with the pulse
  • PPS is the pulse. lock NMEA pairs each pulse with the NMEA time to know which second it belongs to, and prefer makes it the source when it is working

  • the default pool servers stay in, so when there is no GPS fix chrony carries on from the internet

  • allow turns chrony from a client into an NTP server for those ranges. without any allow line it serves nobody

/etc/systemd/system/chrony.service.d/override.conf starts chrony after gpsd:

[Unit]
After=gpsd.service
Wants=gpsd.service

clients use it as ntp.mydomain.com, a DNS CNAME to the pi. check the CNAME target carefully. a typo in the domain points your clocks at someone else's domain, which goes unnoticed until somebody registers the name.

checking it

chronyc sources         # #* PPS means the pulse is the source, #? means no usable samples
gpspipe -w | grep TPV   # "mode":3 is a 3D fix, 2 is 2D, 1 is no fix
sudo ppstest /dev/pps0  # prints a line per second when pulses arrive

most GPS modules only pulse once they have a fix. with no fix there are no pulses, and chrony shows both GPS sources as #? while it keeps time from the pool. a fix needs sky view. mine had none until the antenna was fitted.

from any other machine, the pi should answer as stratum 1 with reference PPS. mine does over IPv4 and IPv6, a few milliseconds from the LAN.