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Rasbian NTP Server

Setup guide for a Stratum 1 NTP server using GPS receiver & Raspberry Pi.

Materials

Substitutions may apply. Your mileage may vary. Additional equipment required for setup (i.e. keyboard, mouse, monitor).

Assembly

This diagram applies to the newer Pi Model 3 B+, and the original hardware used for this project, the Pi Model 1 B+:

Hardware diagram

TODO add diagram for clock display components, and include usage of prototyping board

Software Setup

Previously, this project used ntpd on Raspbian Jessie. Instructions are still provided for reference purposes here.

The latest version of this project is based on Ubuntu Mate 16.04 LTS with chrony and gpsd.

Enable serial port

The stock configuration for Raspberry Pi Model 3 B+ hardware is to present a login over the hardware serial port, and to use the hardware uart to support Bluetooth connections To enable the hardware serial port for use with the GPS receiver, first disable the login:

sudo raspi-config

Interfacing Options > Serial
  • Would you like a login shell to be accessible over serial? No
  • Would you like the serial port hardware to be enabled? Yes

Enable required kernel modules

sudo nano /boot/config.txt

 ...
+
+# Reclaim hardware UART for hardware serial port
+dtoverlay=pi3-miniuart-bt
+
+## pps-gpio
+##     Enable kernel support for GPS receiver pulse-per-second (PPS) input
+dtoverlay=pps-gpio
sudo reboot

Test the GPS connection

First, the serial port data stream:

sudo cat /dev/serial0

Next, the PPS input connection:

Ensure the GPS has signal lock (slow ~15sec LED flashes) because it will not provide a PPS signal without full signal lock.

sudo apt-get install pps-tools -y

sudo 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 1455208600.181885044, sequence: 480 - clear  0.000000000, sequence: 0
source 0 - assert 1455208601.265220834, sequence: 481 - clear  0.000000000, sequence: 0
source 0 - assert 1455208602.348548499, sequence: 482 - clear  0.000000000, sequence: 0

Configure GPS module

The Adafruit Ultimate GPS Breakout v3 sends 5 NMEA sentences by default. These extra sentences introduce unnecessary processing and increase jitter.

To disable all but the Recommended Minimum GPS data sentence (GPRMC), issue the PMTK314 command as described in the command packet (pg 12).

sudo bash -c "echo -e '$PMTK314,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29\r\n' > /dev/gps0"

Optionally ensure any future re-configuration is handled automatically by adding a daily crontab entry:

sudo crontab -e

 ...
+# send modem configuration string everyday at noon (@reboot simply does not work)
+0 12 * * * echo -e '$PMTK314,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29\r\n' > /dev/serial

Remove DHCP hook

To prevent the Pi from getting NTP configuration from any DHCP servers, remove ntp-servers from the end of the request block:

sudo nano /etc/dhcp/dhclient.conf

 ...
 request subnet-mask, broadcast-address, time-offset, routers,
         ...
-        rfc3442-classless-static-routes, ntp-servers;
+        rfc3442-classless-static-routes;
 ...

Install gpsd

sudo apt-get install gpsd gpsd-clients -y

Update config file:

sudo nano /etc/default/gpsd

-DEVICES=""
+DEVICES="/dev/serial0"

 # Other options you want to pass to gpsd
-GPSD_OPTIONS=""
+GPSD_OPTIONS="-n"

Reboot, then test:

gpsmon -n

Install chrony

In latest versions of Raspbian, ntp is replaced by a client-only systemd NTP implementation. Disable it and install chrony instead:

sudo systemctl stop prefer-timesyncd.service
sudo systemctl disable prefer-timesyncd.service
sudo apt-get install chrony -y

And configure for use with GPS device:

sudo nano /etc/chrony/chrony.conf

+refclock SHM 0 offset 0 delay 0 refid GPS noselect
+refclock PPS /dev/pps0 lock GPS refid GPPS
 pool 2.debian.pool.ntp.org offline iburst

From the chrony.conf docs:

  • refclock specifies a hardware reference clock
    • SHM is the shared memory driver, which is utilized by gpsd
    • PPS is for pulse-per-second signals, read from /dev/pps0
  • lock is used to link PPS samples to another refclock
  • noselect is an optional flag used to signal the GPS shouldn't be used directly
  • refid is the tracking label, "GPPS" indicates a PPS-enabled GPS source
  • offset and delay are values which will be tuned after running overnight

At this point, test the time synchronization using:

  • chronyc tracking
  • chronyc sources -v
  • chronyc sourcestats -v

If necessary, apply a step-change to system clock:

The -a argument is required in older versions of chronyc to prevent an error.

sudo chronyc -a makestep

Immediately after starting, the sources may not provide a good time signal:

Tracking data during initial warm-up

After running overnight, with a good GPS signal, you should obtain a lock to the GPPS refclock:

Tracking data from the next morning

Fine Tuning

After warming up, the GPS refclock showed an Offset value consistently between 400-500ms, generally on the higher end. This value can be incorporated directly to improve the source:

sudo nano /etc/chrony/chrony.conf

-refclock SHM 0 offset 0 delay 0 refid GPS noselect
+refclock SHM 0 offset 0 delay 0.5 refid GPS noselect
 refclock PPS /dev/pps0 lock GPS refid GPPS

Although recommended, the makestep directive is not included in the default chrony configuration file. Manually add it:

sudo nano /etc/chrony/chrony.conf

 ...
+# Step the clock, if necessary, for a short window after starting
+#
+
+makestep 1 5

Serving time to clients

Finally, to enable this device as an NTP server, add allow directives to /etc/chrony/chrony.conf as appropriate. See the user documentation for more details.

Example, to permit all clients from local subnet 192.168.1.x:

sudo nano /etc/chrony/chrony.conf

 ...
 # This directive designates subnets (or nodes) from which NTP clients are allowed
 # to access to `chronyd'.

+# allow everyone on home network
+allow 192.168.1.0/24

 #allow foo.example.net
 #allow 10/8
 ...

Enable standalone mode

In case the server loses GPS and Internet access, but network clients are still attempting to sync, use the local hardware clock as a last resort source:

sudo nano /etc/chrony/chrony.conf

-#local stratum 10
+local stratum 10

References