NTP Server from scratch

raspi-config: Advanced > Serial = No

/boot/config.txt:
# GPS PPS, Adafruit ultimate GPS hat PPS on pin 4
dtoverlay=pps-gpio,gpiopin=4
enable_uart="1"

Build gpsd from source with timeservice=yes

Edit /lib/systemd/system/gpsd.service, change "ExecStart" to /usr/local/sbin/gpsd

Create /etc/default/gpsd, add:
DEVICES="/dev/ttyAMA0 /dev/pps0"
GPSD_OPTIONS="-n" # Update clock even when no clients are active.

systemctl enable gpsd
reboot

gps should be running, check with "gpsmon".

If PPS is functioning, "PPS offset" lines will appear.

Build chrony from source