Configure NTPd with PPS. Based on Raspbian Stretch.

Notes:

• Could not get PPS to work with the new refclock driver 46, GPSD NG client protocol. Switched to PPS (22) and SHM (28).
• GPSd did not auto start at boot, added the driver above with noselect to get GPSd to start.

Sources:

• https://www.eecis.udel.edu/~mills/ntp/html/refclock.html
• https://www.slsmk.com/how-to-setup-a-gps-pps-ntp-time-server-on-raspberry-pi/

Based on Raspbian Stretch

• Disable hardware; Wifi, Bluetooth, HDMI.
• Disable services; avahi, triggerhappy, wpa_supplicant
• Add user, remove pi user.
• Set correct timezone.
• Update packages.

• Disable Bluetooth:

  echo "dtoverlay=pi3-disable-bt" >> /boot/config.txt

• Disable the HCI UART:

  sudo systemctl disable hciuart

• Disable getty:

  systemctl mask serial-getty@serial0.service

• Remove console=serial0,115200 from /boot/cmdline.txt
• Reboot
• Test GPS output: cat /dev/serial0

• Install PPS tools:

  apt install pps-tools setserial

• Configure GPIO for PPS:

  echo "dtoverlay=pps-gpio,gpiopin=18" >> /boot/config.txt

• Configure udev rules in /etc/udev/rules.d/gps.rules

  KERNEL=="ttyAMA0", RUN+="/bin/setserial /dev/ttyAMA0 low_latency"
  KERNEL=="ttyAMA0", SYMLINK+="gps0"
  KERNEL=="pps0", GROUP="dialout", MODE="0660"

• Reboot
• There should be a /dev/pps0 device
• Check for pulse

  # ppstest /dev/pps0
  trying PPS source "/dev/pps0"
  found PPS source "/dev/pps0"
  ok, found 1 source(s), now start fetching data...
  source 0 - assert 1552847111.000139273, sequence: 152 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847112.000135096, sequence: 153 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847113.000135337, sequence: 154 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847114.000134886, sequence: 155 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847115.000135615, sequence: 156 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847116.000137416, sequence: 157 - clear  0.000000000, sequence: 0

• Install packages:

  apt install gpsd gpsd-clients

• Configure devices and options in /etc/default/gpsd

  DEVICES="/dev/ttyAMA0"
  GPSD_OPTIONS="-n"

• Reboot
• Test GPS output with cgps or gpsmon.

• Disable systemd timesyncd:

  systemctl stop systemd-timesyncd
  systemctl disable systemd-timesyncd

• Install required packages:

  sudo apt install libcap-dev

• Install NTP:

  # Download latest from http://www.ntp.org/downloads.html
  curl -OL http://www.eecis.udel.edu/~ntp/ntp_spool/ntp4/ntp-4.2/ntp-4.2.8p13.tar.gz
  tar zxvf ntp-4.2.8p13.tar.gz
  cd ntp-4.2.8p13
  ./configure --enable-linuxcaps
  make -j4
  sudo make install
  
  sudo cp /home/user/ntp-4.2.8p13/scripts/rc/ntpd /etc/init.d/
  # Modify ntpd location in /etc/init.d/ntpd, and add lines below above variables.
  
  ### BEGIN INIT INFO
  # Provides:             ntpd
  # Default-Start:        2 3 4 5
  # Default-Stop:
  # Short-Description:    NTPD
  ### END INIT INFO
  
  
  groupadd ntp
  useradd -d /var/lib/ntp -g ntp -s /bin/false ntp
  printf 'PATH=${PATH}:/usr/local/bin\n' > /etc/profile.d/ntp-path.sh
  install -v -o ntp -g ntp -d /var/lib/ntp
  
  systemctl enable ntpd
  

• Add to /etc/ntp.conf

  # PPS
  server 127.127.22.0 minpoll 4 maxpoll 4 true
  fudge 127.127.22.0 flag3 1 refid PPS
  
  # GPS Serial, GPSD SHM
  server 127.127.28.0 minpoll 4 maxpoll 4 iburst prefer
  fudge 127.127.28.0 flag1 1 time1 0.200 refid SHM0 stratum 1
  
  # PPS, GPSD SHM
  server 127.127.28.2 minpoll 4 maxpoll 4
  fudge 127.127.28.2 flag1 1 refid SHM2
  
  # GPSD JSON driver, to auto start GPSD
  server 127.127.46.0 minpoll 4 maxpoll 4 noselect
  
  # Fix False tickers
  tos mindist 0.5

• Enable and start ntpd:

  systemctl enable ntp
  systemctl start ntp

• Comment ntp option in /etc/dhcpcd.conf

  #option ntp_servers

• Reboot