Hardware

A very old Raspberry Pi 0 W v1.1
A GT-U7 GPS receiver https://www.amazon.com/dp/B0CWL6QZN8

Software

An 8gb MicroSD running raspberry pi os 32-bit lite
Install all updates apt update && apt upgrade -y
Install additional packages apt install -y pps-tools gpsd gpsd-clients python3-gps ntpsec
Update firmware with rpi-update, no need to restart yet
Configure overlays in /boot/firmware/config.txt:

# the next 4 lines are for GPS PPS signals and serial UART
dtoverlay=pps-gpio,gpiopin=18
dtoverlay=disable-bt
enable_uart=1
init_uart_baud=9600

and load the pps-gpio module at boot /etc/modules:

pps-gpio

Use raspi-config interface options, no login shell and yes hardware enabled
Power off

Hardware Again

GPS to PI

VCC to pin 4 (5V)
GND to pin 6 (GND)
RX to pin 8 (GPIO 14 TXD)
TX to pin 10 (GPIO 15 RXD)
PPS to pin 12 (GPIO 18 PCM_CLK)

Thankfully this is just a straight 5-pin wiring. Could even solder them straight together as long as the antenna is far enough away.

Software Again

Verify module is loaded: lsmod |grep pps
Monitor the pps device manually: ppstest /dev/pps0
Add devices and flags in /etc/default/gpsd

DEVICES="/dev/ttyAMA0 /dev/pps0"
GPSD_OPTIONS="-n -s 9600"

Set gpsd to start on boot systemctl enable gpsd
Start gpsd: systemctl start gpsd
Verify gps fix: gpsmon may take a while to get a fix, cgps is another tool to consider
Verify that gpsd is sending clock data to ntp via shm: ntpshmmon -o and find the ID of the source that is high precision (lowest offset)
Edit /etc/ntpsec/ntp.conf to add PPS and GPS sources:

driftfile /var/lib/ntpsec/ntp.drift
leapfile /usr/share/zoneinfo/leap-seconds.list
refclock shm unit 2 prefer refid PPS #NTP2 from ntpshmmon PPS
refclock shm unit 0 refid GPS #NTP0 from ntpshmmon GPS
tos maxclock 11
tos minclock 4 minsane 3
pool 0.debian.pool.ntp.org iburst #keep these in case the gps fix is lost
pool 1.debian.pool.ntp.org iburst
pool 2.debian.pool.ntp.org iburst
pool 3.debian.pool.ntp.org iburst
restrict default kod nomodify nopeer noquery limited notrap #let us be a time server
restrict -6 default kod nomodify nopeer noquery limited notrap #let us be a time server
restrict 127.0.0.1
restrict -6 ::1

Restart everything

Is it working?

ntpq -pn output shows the current state of ntp.

Find the entry that starts with an asterisk. That's the one your server is going to use. + are the ones it likes but not likes the best.

remote                                   refid      st t when poll reach   delay   offset   jitter
=======================================================================================================
*SHM(2)                                  .PPS.            0 l    4   64  377   0.0000   0.0683   0.0447
xSHM(0)                                  .GPS.            0 l    1   64  377   0.0000 -133.252   1.2762
0.debian.pool.ntp.org                   .POOL.          16 p    -   64    0   0.0000   0.0000   0.0019
1.debian.pool.ntp.org                   .POOL.          16 p    -  256    0   0.0000   0.0000   0.0019
2.debian.pool.ntp.org                   .POOL.          16 p    -  256    0   0.0000   0.0000   0.0019
3.debian.pool.ntp.org                   .POOL.          16 p    -  256    0   0.0000   0.0000   0.0019
-216.66.48.42                            .PPS.            1 u    2  128  377  56.2374  -0.8680  20.8313
-64.79.100.196                           199.102.46.70    2 u   51   64  377  14.8558   2.1709   8.0471
+199.195.248.88                          17.253.2.253     2 u   44   64  377  31.5927   2.8269   5.0408
+162.159.200.1                           10.142.8.5       3 u   57   64  377  11.6038   1.1217  11.7965
+23.186.168.2                            243.50.127.182   2 u   28   64  337  23.7234   2.9758   6.8061

Check it with nagios plugins too, just for fun:

check_ntp_peer -H localhost -w 0.2 -c 0.5 -j 0.2 -k 0.3 -m @4 -n @2 -C 1

Bibliography

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