Raspberry Pi stratum 0 NTP server

HW

Raspberry Pi 3
SD card (200MB will do, yes, 200MB!)
Waveshare MAX-M8Q GNSS HAT for Raspberry Pi
Raspberry Pi official case for 3B+

SW - OS and basic settings

Download OpenWRT 24.10.5 openwrt-24.10.5-bcm27xx-bcm2710-rpi-3-squashfs-factory.img.gz, flash it on the flash.

I find the squashfs more stable than the ext4 one, and it will cause less wear on the SD card.

Modify the /boot/config.txt (config.txt in the boot partition):

...
[all]
# Place your custom settings here.
dtoverlay=pps-gpio,gpiopin=18
...

Disable the serial console (we need the serial port for the GPS) in /boot/dline.txt
```
console=tty1 root=PARTUUID=c8dca970-02 rootfstype=squashfs,ext4 rootwait
```
Assemble all the HW, and boot. The raspberry will have the 192.168.1.1 address, t I prefer it to DHCP.
```
uci set network.lan.proto=dhcp
uci commit
/etc/init.d/network restart
```

Since this machine is not a router :

/etc/init.d/firewall disable
/etc/init.d/odhcpd disable
/etc/init.d/dnsmasq disable
reboot

Setup a root password

root@OpenWrt:~# passwd
Changing password for root
New password:
Bad password: too weak
Retype password:
passwd: password for root changed by root

Setup authorized_keys and disable root password login

Configure hostname

 uci set system.@system[0].hostname=pim-ntp
 uci commit
 echo pim-ntp > /proc/sys/kernel/hostname

Install the tools

opkg update
uci set system.ntp.enable='0'
uci commit
/etc/init.d/sysntpd disable
/etc/init.d/sysntpd stop
/etc/init.d/ntpd disable
/etc/init.d/ntpd stop
opkg install chrony kmod-pps-gpio gpsd gpsd-clients \
nano rsync umdns coreutils-who zabbix-agentd python3-light kmod-usb-net-rtl8152
/etc/init.d/chronyd enable

SW - GPSD, chrony and PPS

Configure gpsd

uci set gpsd.core.device='/dev/ttyAMA0'
uci set gpsd.core.enabled='1'
uci commit
/etc/init.d/gpsd enable
/etc/init.d/gpsd start

Check GPS connectivity : cgps -s -u m should show Status 3D FIX

Configure chronyd for using country specific servers in /etc/config/chrony:

config pool
	       option hostname '0.ch.pool.ntp.org'
        option iburst 'yes'
...

Add the GPS and PPS sources at the end of : /etc/chrony/chrony.conf
refclock PPS /dev/pps0 refid PPS poll 0

local stratum 10 orphan …
(Re-) Start chronyd
```
/etc/init.d/chronyd restart
```

Check GPS and PPS functionality :

chronyc sources

MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
#- GPS                           0   2   377     4  +1157us[+1157us] +/-  100ms
#* PPS                           0   1   377     1   -114ns[ -209ns] +/-  669ns
^- redacted1.example.com         3  10   377  1028  -9345us[-9381us] +/-   52ms
^- redacted2.example.com         2  10   377   824  -4990us[-5019us] +/-   11ms
^- redacted3.example.com         2  10   377   174  -3748us[-3755us] +/-   35ms
^- redacted4.example.com         1  10   377  1023  -3986us[-4022us] +/- 8996us
^- router.lan                    2   6   377    33  -3157us[-3157us] +/-   40ms
^? pim-ntp.lan                   0   8     0     -     +0ns[   +0ns] +/-    0ns

Configure NTP pool DNS and DHCP on a OpenWRT router

Setup all your NTP servers in a DNS pool (/etc/config/dhc)

...
config domain
    option name 'ntp'
    option ip 'XXX.XXX.XXX.XXX' # address of the router with a NTP server

config domain
    option name 'ntp'
    option ip 'XXX.XXX.XXX.YYY' # address of the ntp server (from this post)
...

Setup the DHCP to announce NTP servers:

uci add_list dhcp.lan.dhcp_option='42,XXX.XXX.XXX.XXX,XXX.XXX.XXX.YYY'
uci commit

Restart dnsmasq
```
/etc/init.d/dnsmasq restart
```
Result

A small animation : watch chronyc sources (opkg install procps-ng-watch)

(Optional) Optimisation for timing

The backup battery on the Waveshare MAX-M8Q GNSS HAT is mandatory. Whitout it, the module will lose it’s configuration every power loss. The symptom will be NO FIX, since the module will go back to 9600 bauds.

set jumpers on A
connect to your favorite Linux host using the USB cable

start gpsd on the usb port

 ubxtool -s 9600 -S 115200
 ubxtool -p MODEL,2
 ubxtool -p CFG-PMS,0
 ubxtool -p SAVE
 ubxtool -p COLDBOOT # validate the settings are OK

re-set jumpers on B

configure 11500 bauds using -s 115200 in /etc/init.d/gpsd:

 ...
 [ "$enabled" = "0" ] && return 1

  	    procd_open_instance
  	    procd_set_param command "$PROG" -N -n -s 115200

  	    [ "$listen_globally" -ne 0 ] && procd_append_param command -G
  	    procd_append_param command -S "$port"
 ...

~~~

Question, remark, bug? Don't hesitate to contact me or report a bug.

HW

SW - OS and basic settings

SW - GPSD, chrony and PPS

Configure NTP pool DNS and DHCP on a OpenWRT router

Result