First of all, big thanks to Patrick O'Keeffefor the base of this fork.
Why do we need a dedicated guide for U-blox ZED-F9R? I ran into some pitfalls when trying to use the original guide, and have updated it accordingly. The main difference is the way the configuration of the GNSS-receiver takes place. In this guide, setup of baud rate and configuration of messages is done with systemd before and after the startup of gpsd
In the end, this guide will end in a Stratum 1 NTP server using U-blox ZED-F9R GNSS receiver & Raspberry Pi.
Substitutions may apply. Your mileage may vary. Additional equipment required for setup (i.e. keyboard, mouse, monitor).
- Raspberry Pi (Pi Zero not recommended)
- a quality micro SD card (class 10 recommended)
- a quality power supply
- a case (to protect, and dampen temperature fluctuations)
- SparkFun GPS-RTK Dead Reckoning pHAT for Raspberry Pi
- SMA to uFL/u.FL/IPX/IPEX RF Adapter Cable
- GPS Antenna, External, Active, 3-5V 28dB 5m SMA
- Soldering iron
- Dupoint female connector
Assembling the ZED-F9R pHAT is as easy as stacking it on top of the Raspberry Pi GPIO comb, but in order to get the PPS-signal, some smaller modifications needs to be done. Solder a wire with a dupoint-female plug on, from the PPS-output and connect to pin12_GPIO18. In the picture below, this is shown with the green wire.
The latest version of this project is based on Raspberry Pi OS Lite
with chrony and gpsd.
The stock configuration for Raspberry Pi Model 4 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-configInterfacing 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
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-gpiosudo rebootFirst, the serial port data stream:
sudo cat /dev/serial0If this gives a lot of rubish text on the screen, no worries, it probably works fine and it's only a matter of changing baud rate which will be done in Configure gpsd.
Ensure a good GNSS fix and check that the PPS-light blinks every one second. This indicates that the GNSS-receiver has sufficient with satelites to give a good timestamp.
Start by installing the pps-tools and test that you are getting a new line every one second.
sudo apt-get install pps-tools -ysudo ppstest /dev/pps0trying 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
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;
...sudo apt-get install gpsd gpsd-clients -yIn latest versions of Raspbian, ntp is replaced by a client-only
systemd NTP implementation. When installing chrony, this client is disable automatically
sudo apt-get install chrony -yUpdate config file with the receiver and PPS-signal:
sudo nano /etc/default/gpsd-DEVICES=""
+DEVICES="/dev/serial0 /etc/pps0"
# Other options you want to pass to gpsd
-GPSD_OPTIONS=""
+GPSD_OPTIONS="-n"
# if having several receivers connected that you don't want to use with gpsd, comment this one out:
-USBAUTO="true"
+#USBAUTO="true"Since the ZED-F9Rs serial interface is default set to a baud rate of 38400, it needs to be specified before attaching the receiver to gpsd. This is easiest done by running stty in systemd before starting gpsd.service:
sudo nano /etc/systemd/system/gpsd.serviceAdd the following content:
[Unit]
Description=GPS (Global Positioning System) Daemon
Requires=gpsd.socket
# Needed with chrony SOCK refclock
After=chronyd.service
[Service]
# set up serial0 with baud rate 38400
ExecStartPre=/usr/bin/stty -F /dev/serial0 speed 38400 parodd
EnvironmentFile=-/etc/default/gpsd
EnvironmentFile=-/etc/sysconfig/gpsd
ExecStart=/usr/sbin/gpsd -N $GPSD_OPTIONS $DEVICES
ExecStartPost=/bin/sleep 2
# Disable all NMEA messages from the device
ExecStartPost=/usr/bin/ubxtool -d NMEA --device /dev/serial0
ExecStartPost=/bin/sleep 2
# Enable the bare minimum of message, the NMEA RMC message
ExecStartPost=/usr/bin/ubxtool -z CFG-MSGOUT-NMEA_ID_RMC_UART1,1 --device /dev/serial0
ExecStartPost=/bin/sleep 2
# Disable all ubx messages
ExecStartPost=/usr/bin/ubxtool -d BINARY --device /dev/serial0
[Install]
WantedBy=multi-user.target
Also=gpsd.socketAfter updating the systemd file for gpsd.service
sudo systemctl daemon-reload
sudo rebootWhen the Raspberry pi is up and running again, test it with:
gpsmonSomething like this should show:
If this for some reason doesn't work, start by looking in the logs of systemd for the gpsd.service:
systemctl status gpsd.service And configure for use with GPS device:
sudo nano /etc/chrony/chrony.conf+# Add PPS:
+refclock SHM 0 offset 0 delay 0 refid GPS noselect
+refclock SOCK /run/chrony.pps0.sock lock GPS refid GPPS
pool 2.debian.pool.ntp.org iburstFrom the
chrony.confdocs:
refclockspecifies a hardware reference clock
SHMis the shared memory driver, which is utilized by gpsdPPSis for pulse-per-second signals, read from/dev/pps0SOCKis a UNIX domain socket, similar to SHM
lockis used to link PPS samples to another refclock
noselectis an optional flag used to signal the GPS shouldn't be used directly
refidis the tracking label, "GPPS" indicates a PPS-enabled GPS source
offsetanddelayare values which will be tuned after running overnight
At this point, test the time synchronization using:
chronyc trackingchronyc sources -vchronyc sourcestats -v
If necessary, apply a step-change to system clock:
The
-aargument is required in older versions of chronyc to prevent an error.
sudo chronyc -a makestepImmediately after starting, the sources may not provide a good time signal:
After running overnight, with a good GPS signal, you should obtain a lock to the GPPS refclock:
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 GPPSAlthough 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 5Finally, 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
...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