How to NTP a Raspberry Pi 4 via GPS and PPS

This also works on Pi 3.

  • Hardware: A GPS with serial connected to the Pi’s serial port, and PPS connected to gpio 18 (physical pin 12 on the GPIO connector)
  • If possible, configure GPS to output GPZDA at 9600 baud. GPSD will find other baud rates, but takes approximately 8 minutes to get to 115200, and 9600 is the first one it tries.

sudo raspi-config, disable serial console login, enable hardware serial port
sudo apt install pps-tools gpsd gpsd-clients python-gps chrony
edit /etc/modules, add:


edit /boot/firmware/config.txt (or /boot/config.txt), add:


edit /etc/default/gpsd, set device to “/dev/ttyS0”, options to “-n -b”
sudo systemctl enable gpsd

edit /etc/chrony/chrony.conf, comment out all servers and pools, add:

refclock SHM 0 poll 0 refid GPS precision 1e-1 offset 0.055
refclock PPS /dev/pps0 lock GPS poll 0 refid PPS precision 1e-9

sudo systemctl restart chrony

Monitor using chronyc sources, or chronyc sourcestats