I have been running ntp.yaojun.sg for NTP service on a Raspberry PI for a while. This is service is part of pool.ntp.org and responds to sg.pool.ntp.org and asia.pool.ntp.org. However, my SD card decided to give up recently when I wanted to explore SNMP tools for monitoring purposes.

The GPS module I am using is from HAB. I gotten this particular module late last year built specifically for the older Raspberry Pi Model B. My NTP service was made public around March 2014 using a ready made image. However, just looking at how 2014 turn out, it is important to keep both base system and NTP patched. In addition, it is not advisable to just run a “BlackBox” because you never know what’s going on inside it. So here I am restoring the service from a clean Raspbian image.

Do note that the module requires a GPS 3D lock for PPS to work, so remember to connect your antenna. I was scratching my head on why my PPS was not working and had to contact the friendly vendor to ask if I have a dead card.

Installing Raspbian into Raspberry Pi is a straight forward process. After the basic configuration (resizing the File System, locale, timezone and etc), remember to configure Rasbian to disable the serial shell under the advanced settings. Also update and patch the system.

apt-get update
apt-get dist-upgrade
rpi-update

Recent development in Raspbian allows the use of PPS devices without recompiling the kernel. We need to configure the system to use the PPS on the GPIO port.

Edit /boot/config.txt add the following line,

dtoverlay=pps-gpio,gpiopin=18

Add the following line to /etc/modules,

pps-gpio

Install the GPS and PPS tools

apt-get install pps-tools libcap-dev gpsd gpsd-clients python-gps

Configure gpsd to use the GPS sensor at /dev/ttyAMA0 and enable -n as a option using dpkg-reconfigure to allow ntpd to use the GPS clock and remember to enable the gpsd daemon

systemctl enable gpsd.socket
dpkg-reconfigure gpsd

and you will be able to see your GPS data using

cgps -s

Check that your PPS clock is working

ppstest /dev/pps0

Remove default NTP client (which does not work with the GPS and PPS)

apt-get remove ntp

Install NTP dependencies to ensure successful build of NTP from source

apt-get install libbsd-dev libssl-dev

Install checkinstall to compile source to binaries safely

apt-get install checkinstall

Get the latest NTP source code from www.ntp.org, untar and compile. This is not a fancy high speed CPU so compiling will take some time (estimate 30 minutes).

./configure --enable-linuxcaps --with-NMEA --with-ATOM
make 
checkinstall

Follow through checkinstall wizard.

Symbolically link compiled binaries to well defined paths,

ln -s /usr/local/bin/ntp* /usr/bin/
ln -s /usr/local/sbin/ntp* /usr/sbin/

Edit /etc/init.d/ntp to use the compiled binaries,

#PATH=/sbin:/bin:/usr/sbin:/usr/bin
PATH=/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

#DAEMON=/usr/sbin/ntpd
DAEMON=/usr/local/sbin/ntpd

Edit /etc/ntp.conf to use PPS and GPS as Stratum-0 and Stratum-1. This will make PPS disciplined by GPS and verified by other NTP servers.

# pps-gpio on /dev/pps0
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 refid PPS
fudge 127.127.22.0 flag3 1  # enable kernel PLL/FLL clock discipline

# gpsd shared memory clock
server 127.127.28.0 minpoll 4 maxpoll 4 prefer  # PPS requires at least one preferred peer
fudge 127.127.28.0 refid GPSD
fudge 127.127.28.0 time1 +0.150 stratum 1 # coarse processing delay offset

Lock the ntp binaries to prevent apt from overwriting from packages

apt-make hold ntp