This is a project to create a Network Time Protocol (NTP) server using a Raspberry Pi 3 Model B and a u-blox CAM-M8C GNSS module.

Yes, there are a lot of existing web pages describing how to setup a Raspberry PI to be a GPS based NTP server, but none of them had the level of detail I wanted.

• GPS time pulse output for Pulse Per Second (PPS) synchronization.
• A full Linux server OS to support additional services on the Raspberry Pi.
• U-blox based chipset (I have used them for projects at a previous employer).
• Multiple independent interfaces for the GPS chip so it can be used for multiple purposes.
• Reasonable cost as this is a hobby project.

I should note that a Raspberry Pi can be used as an NTP server without GPS integration by simply synchronizing to NTP server pools available on the internet. However, this project was to setup GNSS synchronized stratum 1 NTP server.

I started out using gpsd and chrony for the NTP service, but found both to be fairly opinionated and added unnecessary layers to the software stack which added latency. It was a useful learning experience, but I ended up removing those packages and installing NTPsec instead. Both gpsd and chrony are useful for simple deployments and for intermittently connected clients, but they did not let me control the u-blox module the way I want to.

Chrony does have one advantage, which is kernel or hardware timestamping when synchronizing over the network. However, the Raspberry Pi 3 network interface does not support hardware timestamping. I plan to setup linuxptp to provide Precision Time Protocol (PTP) on my network in the future. This will leverage the timestamping features of the kernel and/or network interface. I expect this will require a Raspberry Pi 4, possibly with a USB network interface that supports hardware timestamping. See the Future Work section for more information.

I selected parts that I already had available or were familiar with. As mentioned in the introduction, I have used u-blox modules before and have been impressed with their capabilities. I also already had a Raspberry Pi 3-B hosting my other network services.

• Raspberry Pi 3 model B
• CanaKit setup for the Raspberry Pi 3 (Note: Some of these kits include the Raspberry Pi 3 model B board)
  • Heatsinks
  • Power supply
  • Case
• 128GB "high endurance" microSD card
• OzzMaker BerryGPS-IMUv3 (contains the u-blox CAM-M8C module)
• Active GPS antenna with the required uFL to SMA adapter

In addition, I already had the required USB keyboard, USB mouse, HDMI, microSD to USB adapter, and network cables required to set this up.

I have created an Amazon "Idea List" with the list of parts here. (Note: I do not get any compensation if you order from this list, it's just there for your convenience)

You may be able to source these parts cheaper or substitute alternate parts.

I selected the OzzMaker BerryGPS-IMUv3 over the other GPS offerings for a few reasons:

• It uses a modern u-blox module.
• It pulls the UART, I²C, and PPS pins out from the u-blox module.
• Supports an external antenna.

In the end, the BerryGPS-IMUv3 has more capability in the IMU than I need and is a bit more expensive than some options, but I am happy with the board and the responsiveness I got from OzzMaker when I had a question about the board.

I am recommending a "high endurance" microSD purely due to the write load running Linux can have. Specifically, the logging and other actions under /var. Unfortunately, most microSD cards have very low write endurance and most do not provide information on the endurance of a device or if the device includes write wear leveling. Purchasing a microSD card that is larger than you need may help extend the life of the card if write wear leveling is in use. Below I will make some recommendations about ways to reduce the write load for this use case.

Someone was very kind by gifting me a Raspberry Pi 4 board. Thank you! I have created a update section below discussing what I have found with the Raspberry Pi 4.

Another very kind person gifted me one of the OzzMaker BerryGPS-IMUv4 boards. Thank you!

This is an update to the BerryGPS-IMUv3 which adds a Super Cap that allows the GPS module to maintain the ephemeris data for up to four hours if the module loses power. This means a faster time-to-first-fix if power is restored within the four hour window. After four hours the ephemeris data will be stale and should be refreshed or reacquired from the satellites.

The BerryGPS-IMUv4 also has slightly different IMU sensors and QWIIC connectors.

The BerryGPS-IMUv4 is fully compatible with the BerryGPS-IMUv3 board.

1. Solder the 5x2 header onto the BerryGPS-IMUv3 board. The header should be on the back of the board so that it will join with the Raspberry Pi 40 pin header.
2. Solder a wire from the "T_PULSE" pin hole on the BerryGPS-IMUv3 to a free GPIO ping on the Raspberry Pi. I selected the GPIO 21 pin. You can use a jumper cable with a female connector for the GPIO pin if you have one, but make sure it is a tight fit with the pin. Make note of the pin you selected as you will need it later during the kernel configuration step.
3. Optionally, connect the u-blox module I²C bus to the BerryGPS-IMUv3 I²C bus.
   • Solder a wire from the SDA pin hole to the uSDA pin hole.
   • Solder a wire from the SCL ping hole to the uSCL pin hole.

I chose to use Ubuntu Server on my Raspberry Pi for this project. It is a complete Linux distribution that includes all of the packages I needed for this project but also the other services I want to run on the Raspberry Pi. They also regularly release patches and updates which is nice.

At the time of this writing, version 19.10.1 (Eoan Ermine) of Ubuntu Server was available. I plan to upgrade this device to 20.04 LTS (Focal Fossa) when it is released.

1. Download the 64-bit version of Ubuntu Server for the Raspberry Pi 3 from the Ubuntu Raspberry Pi page.

2. Follow the instructions on this page to flash the microSD card with the downloaded image.

   Note

   If you use the Win32Diskimager tool, it will not run if you have any RAM disks mounted in Windows. This is listed in the release notes, but you have to look to find it. I use a RAM disk as a temporary cache and got blocked by this issue for a bit. Simply unmounting the RAM disk allows the application to start.

3. Make sure the BerryGPS-IMUv3 is not yet attached to the Raspberry Pi. The NEMA codes from the GPS module UART will halt the Raspberry Pi from booting.

4. Install the microSD card in the Raspberry Pi and power it up.

5. You will see the normal Linux kernel boot sequence on the screen. On first boot, give it some time before attempting to login. The cloud-init on the first run will take some time even after the login prompt is up. The default image username and password of "ubuntu" will not work until cloud-init is finished. Once you see the kernel booting, it is a good time to get a beverage.

6. Once you are logged in, do your updates:

   sudo apt-get update
   sudo apt-get dist-upgrade

7. You will also need some additional packages:

   sudo apt-get install pps-tools rng-tools ntpsec cpufrequtils dkms python3

8. Configure the kernel command line:

   • Edit the /boot/firmware/nobtcmd.txt
     • Remove the "console=ttyAMA0,115200" section. This stops the kernel from using the Raspberry Pi UART as a serial console. We will be using it for the u-blox UART.
     • Add "nohz=off" to the command line. This causes the kernel to never omit scheduling clock ticks.

9. Configure the kernel device tree:

   • Edit the /boot/firmware/syscfg.txt
     • Add "dtparam=i2c_arm=off". This disables the broken I²C bus on the Broadcom chip used in the Raspberry Pi 3 model B. The hardware I²C bus on the Broadcom chip does not support clock stretching used by the u-blox module.
     • Add "dtparam=spi=off". This disables the SPI bus on the Raspberry Pi. We don't need it.
     • Add "dtoverlay=pps-gpio,gpiopin=21". This sets the GPIO pin on the Raspberry Pi that is connected to the "T_PULSE" or PPS pin hole on the BerryGPS-IMUv3. If you used a GPIO pin other than 21 in the soldering section above, replace the "21" on this line with the correct pin number.
     • Add "dtoverlay=pi3-disable-bt". This disables the Bluetooth device on the Raspberry Pi. This is optional, but I don't need it so I am going to disable it.
     • Add "dtoverlay=pi3-disable-wifi". This disables the WiFi device on the Raspberry Pi. This is optional, but I don't need it so I am going to disable it.
     • Add "dtoverlay=i2c-gpio,bus=3,i2c_gpio_sda=02,i2c_gpio_scl=03,i2c_gpio_delay_us=2". This enables the software I²C driver using GPIO pins on the Raspberry Pi. This approach avoids the corruption that occurs with the hardware Raspberry Pi I²C bus due to clock stretching. This is optional and only required if you intend to use the I²C bus on the BerryGPS-IMUv3.

10. Enable the pps-gpio kernel module at boot:

    echo "pps-gpio" | sudo tee -a /etc/modules-load.d/pps-gpio.conf

11. Shutdown getty on the ttyAMA0 device:

    sudo systemctl stop [email protected]
    sudo systemctl disable [email protected]

12. Setup udev to disable echo on the ttyAMA0 device:

    sudo cp udev/09.ttyAMA0.rules /etc/udev/rules.d

    If you do not disable echo on the tty device, you will see garbage in your NMEA message stream from the ttyAMA0 device and GNTXT NMEA messages with "More than 100 frame errors, UART RX was disabled" in them.

13. Set the CPU frequency governor to "performance":

    echo 'GOVERNOR="performance"' | sudo tee -a /etc/default/cpufrequtils

14. Reboot and disable the uboot boot delay to stop the GPS messages from aborting the boot process.

    • Run "sudo reboot"
    • When you see text, after the Raspberry Pi color gradient, start hitting the "enter" key until you get a uboot prompt.
    • Enter "setenv bootdelay -2". This disables the uboot delay so that NMEA messages from the u-blox UART do not interrupt the boot sequence.
    • Enter "saveenv". This saves the above setting so it is in effect on each boot.

15. Attach the BerryGPS-IMUv3 board to the Raspberry Pi:

    • Power off the Raspberry Pi.
    • Install the plastic support pins included with the BerryGPS-IMUv3. Only two line up for me.
    • Attach the BerryGPS-IMUv3 to the Raspberry Pi by lining up the 5x2 header with the top GPIO pins (1 and 2) on the Raspberry Pi 3.
    • Attach the antenna to the BerryGPS-IMUv3.
    • Be sure to set the antenna switch to "EXT" to use the external antenna.
    • Attach the PPS (T_PULSE) wire to the GPIO pin (21 in my case) if you have not already done so.

1. Power up the Raspberry Pi. It should boot back to the login prompt if the previous steps were completed correctly.

2. Login and run a test on the PPS source (ctrl-c to exit):

   sudo ppstest /dev/pps0

   This should show similar output to this example:

   trying PPS source "/dev/pps0"
   found PPS source "/dev/pps0"
   ok, found 1 source(s), now start fetching data...
   source 0 - assert 1578164816.999990228, sequence: 966890 - clear  0.000000000, sequence: 0
   source 0 - assert 1578164817.999992699, sequence: 966891 - clear  0.000000000, sequence: 0
   

3. Check that the NMEA messages are streaming on the ttyAMA0 device (ctrl-c to exit):

   sudo cat /dev/ttyAMA0

   This should show similar output to this example:

   $GNRMC,193854.00,V,0000.00000,N,00000.00000,W,0.015,,040120,,,A*71
   
   $GNZDA,193855.00,04,01,2020,00,00*7E
   

   You should not see any non-ascii characters in this stream. Note: I have zeroed out the coordinates and marked the message as 'V', invalid, here for privacy reasons. Your RMC message will likely have an 'A' and actual coordinates.

4. If these steps all check out ok, you have successfully completed the above steps and can now move on to configuring the NTP service on your Raspberry Pi.

5. If not, go back through the initial steps and make sure you didn't miss a step. Also, double check you solder work. Adafruit has an excellent Common Soldering Problems guide that may help.

1. Allow the ntpd process access to the devices:

   echo '@{NTPD_DEVICE}="/dev/ttyAMA0" "/dev/pps0"' | sudo tee -a /etc/apparmor.d/tunables/ntpd
   sudo apparmor_parser -r /etc/apparmor.d/usr.sbin.ntpd

2. Configure ntpsec:

   • Create the /etc/ntpsec/ntp.d/refclock.conf file.
     • Add "refclock nmea flag1 1 path /dev/ttyAMA0 ppspath /dev/pps0 baud 9600". This enables the NMEA driver with a PPS source.

   Note

   You may want to add "flag4 1" to this string if your NTP service will be accessible from untrusted systems. This will mask the GPS antenna location information from being avialable in the logs or via "ntpq -c clockvar <server>".

3. Optionally update the network NTP pool configuration:

   • Edit the /etc/ntpsec/ntp.conf file.
     • Modify the "pool" configuration lines to reflect network NTP pool you would like to use. By default, Ubuntu configures these for "ubuntu.pool.ntp.org" pools. See the NTP Pool Project for more information about available pools.

   Note

   If you don't define any additional time sources, ntpsec will not select the PPS source and set the system clock. This is because the default configuration file includes a "tos" "minsane" configuration that requires multiple servers. You can comment out this line if you will only be using the NMEA and PPS source from the CAM-M8C module.

4. Restart the ntp service to load the new configuration:

   sudo systemctl restart ntpsec

5. Check the NTP server peer status:

   sudo ntpq -np

   You should see output similar to this:

   remote           refid           st t when poll reach   delay   offset   jitter
   ===============================================================================
   oNMEA(0)         .GPS.            0 l   37   64  377   0.0000   0.0129   0.0014
   

   It will take a few minutes before the 'o' appears in front of the NMEA word. This 'o' means that the NTP service is receiving NMEA messages and has synced to the PPS time pulses from the kernel.

The NTPsec package we are using for the NTP service on Linux has an optional package called ntpsec-ntpviz. ntpviz reads the statistics files produced by ntpsec and generates HTML pages with graphs of the ntp service performance.

To use ntpviz, you will need to install a few more packages:

sudo apt-get install gnuplot-nox ntpsec-ntpviz

Note

I used gnuplot-nox here because if you don't specify this, installing the ntpsec-ntpviz package will pull in the X windows versions of gnuplot, which installs the full X windows environment on the Raspberry Pi.

The ntpsec-ntpviz package will automatically configure ntpsec to write out the required statistics files and will enable cron jobs to generate the HTML pages. The default configuration will produce daily and weekly summaries. This package will also enable the /ntpviz path in Apache if it is installed.

The ntpsec package includes a cron job that will automatically rotate the stats files.

Flash storage devices have a limited number of program/erase (P/E), or write, cycles they can tolerate before wearing out. This is especially true of microSD cards. Unfortunately, most microSD manufactures do not provide a specification for the number of P/E cycles their device is expected to handle.

Some flash devices use write wear leveling to increase the overall life of a flash device by using extra un-used space on the device to write new data before resorting to re-writing. Unfortunately, like the expected P/E cycles, most flash vendors do not disclose if their device has wear leveling capabilities.

Due to this limitation of flash storage, and the lack of good data about the endurance of the microSD card, I have recommended getting an oversized "high endurance" microSD card.

Beyond that, we can take some steps to reduce the amount of wear we put on the microSD card in our Raspberry Pi.

Linux based systems need to write data to storage on a regular basis. This includes everything from logs, socket files, process ID files, and other configuration data. Most of these writes occur under the /var file path, with the highest write files typically writing to files under /var/log.

Normal logging does not produce a lot of regular writes, but the ntpviz package we installed above does. Reducing the Linux filesystem write wear is beyond the scope of this document, but I will provide some ideas to reduce the wear from ntpviz.

The ntpsec-ntpviz package enables the following statistics logging: loopstats, peerstats, and clockstats. Each of these can write hundreds of thousands of lines a data per day and will later be re-written to disk in compressed form. Finally, they will be expired out and deleted after a week. On top of this, ntpviz will rewrite the graphs and HTML content every hour.

Since this data is purely for monitoring, and does not impact the performance of the ntp service, I would recommend storing these in RAM instead of on the microSD flash. This means they will not persist across reboots, but they will also not increase the wear on the flash storage. After each reboot, the graphs will start over as if you just installed ntpsec-ntpviz.

To store these files in RAM, we need to setup these paths on tmpfs:

• Configure the fstab to mount the directories on tmpfs:

  echo "tmpfs   /var/log/ntpsec tmpfs   rw,size=5M,nodev,nosuid,noexec,uid=ntpsec,gid=ntpsec,mode=755 0 0" | sudo tee -a /etc/fstab
  echo "tmpfs   /var/lib/ntpsec/ntpviz  tmpfs   rw,size=10M,nodev,nosuid,noexec,uid=root,gid=root,mode=755 0 0" | sudo tee -a /etc/fstab

• Reboot to make sure all of the ntpviz processes are using the new filesystem:

  sudo reboot

Optionally, you can clear out the already stored data in these directories before the reboot. Even if you do not, the old data will not be used.

By default, ntpviz will graph the temperature reading from the Raspberry Pi processor as "ZONE0" using the "ntplogtemp" program. ntplogtemp has built in support for pulling temperature readings from alternate sources, one of which is using a command called "temper-poll".

The BerryGPS-IMUv3 includes a temperature sensor that is attached to the I²C bus and I have created a python script that is compatible with the ntplogtemp use of "temper-poll" that can be used to capture the temperature from the BerryGPS-IMUv3 called "get-imu-temp.py". This can be installed and symbolic linked to the name "temper-poll" and ntplogtemp will automatically start using it to collect the "TEMPER0" temperature reading from the BerryGPS-IMUv3. I have found this temperature reading to be much closer to the ambient temperature than the reading from the Raspberry Pi CPU.

To enable the BerryGPS-IMUv3 temperature reading:

1. Install the required python module:

   sudo apt-get install python3-smbus

2. Copy the get-imu-temp.py application into /usr/local/bin:

   sudo cp -p get-imu-temp/get-imu-temp.py /usr/local/bin

3. Link the "temper-poll" name to get-imu-temp.py:

   sudo ln -s /usr/local/bin/get-imu-temp.py /usr/local/bin/temper-poll

After the next ntpviz daily graph run, you should see the "TEMPER0" label appear on the "Local Frequency/Temp" daily graph produced by ntpviz. By default, this runs once an hour.

You can also verify the "TEMPER0" temperature polling by looking at the /var/log/ntpsec/temps file. After about five minutes, you should see a temperature reading for "TERMER0" in addition to the "ZONE0" readings.

By default, all temperature values are in centigrade.

In general, the u-blox GNSS chips are highly configurable. This includes settings that can enhance the stability of your NTP service.

One of the nice things about u-blox is that they provide a graphical tool that allows you to see how your u-blox module is performing and configure it. This software is called u-center. You can download u-center from the u-blox website for free.

The u-blox u-center software supports connecting to the u-blox module over a network.

To connect u-center to the Raspberry Pi, you will need to install the ser2net package and make sure it doesn't automatically start on boot:

sudo apt-get install ser2net
sudo systemctl disable ser2net

Configure ser2net for u-center connection:

echo "6000:raw:600:/dev/ttyAMA0:9600 NONE 1STOPBIT 8DATABITS XONXOFF LOCAL -RTSCTS" | sudo tee -a /etc/ser2net.conf

If you have configured NTPsec to use the I²C interface, you do not need to stop the NTP service to use u-center. However, if you are not using the I²C interface for NTPsec, you will need to stop NTPsec before starting the ser2net service:

sudo systemctl stop ntpsec

Now that you have ser2net prepared you can start the ser2net service:

sudo systemctl start ser2net

Connect the u-center application to the Raspberry Pi:

• From the top menu, select Receiver.

• Select Connection from the Receiver menu.

• Select Network Connection from the Connection menu.

• Select New from the Network Connection menu.

• In the Address field, enter the URL to the Raspberry Pi:

  tcp://<ip address>:6000
  

• In the ser2net configuration we used port 6000, so I have indicated that in this above example.

At this point you should see satellites populating in the satellite level history window.

Once you are done using u-center, be sure to shut down ser2net as it does not have any access control.

sudo systemctl stop ser2net

If you cannot run the u-center software, you can still build a custom configuration using the u-blox protocol specification document.

To configure the u-blox module:

• Select the View menu.
• From the View menu, select Configuration View.

This will open the Configure window. It will show you the current configuration values on the CAM-M8C module. At the bottom of the window, there is a Poll button that allows you to query the module to load the current configuration.

Along the left side of the window is the list of possible configuration categories. Not all of these categories apply to the CAM-M8C module.

On the right side of the window are the configuration settings in the selected category. If you make a change to one of these settings, you must click the Send button at the bottom of the window for the configuration settings to be applied to the module.

In this section I will go over the u-center configuration categories and make recommendations on settings that may improve the timing stability.

This section allows the configuration of the Global Navigation Satellite System (GNSS) the module will track and use for time synchronization.

• Confirm that GPS is enabled, with a minimum of 8 and maximum of 16.
• Disable the SBAS. This is recommended in the u-blox protocol specification document, Time Pulse section 19.2.
• Enable Galileo with a minimum of 4 and maximum of 8.
• Confirm QZSS is enabled, with a minimum of 0 and maximum of 3. This is recommended in the u-blox protocol specification document, GNSS system configuration section 32.10.9.1.
• Confirm GLONAAS is enabled, with a minimum of 8 and a maximum of 14.
• All other GNSS systems should be disabled.
• Click the Send button at the bottom.

This section configures which messages the u-blox module will send out which communications port. The NTPsec NMEA driver only requires one of the following messages to synchronize the time: $GPRMC, $GPGLL, $GPGGA, or $GPZDA. The default settings for the u-blox module send many additional messages used for navigation.

We can reduce the latency of the required messages and reduce the processing power that NTPsec will use by limiting the messages sent from the u-blox device. This is optional configuration as NTPsec can successfully operate with the default message settings.

• If you are only using the UART interface (ttyAMA0) and want status and navigation messages in addition to the timing messages:

  • Leave the Messages defaults.

• If you are only using the UART interface (ttyAMA0) and are only using the u-blox module for NTPsec:

  1. Select "F0-00 NMEA GxGGA" from the drop down, uncheck "UART1" On box, click the Send button at the bottom.
  2. Select "F0-01 NMEA GxGLL" from the drop down, uncheck "UART1" On box, click the Send button at the bottom.
  3. Select "F0-02 NMEA GxGSA" from the drop down, uncheck "UART1" On box, click the Send button at the bottom.
  4. Select "F0-03 NMEA GxGSV" from the drop down, uncheck "UART1" On box, click the Send button at the bottom.
  5. Select "F0-05 NMEA GxVTG" from the drop down, uncheck "UART1" On box, click the Send button at the bottom.
  6. Select "F0-05 NMEA GxZDA" from the drop down, check "UART1" On box, click the Send button at the bottom.

  At this point you should only see $GNRMC and $GNZDA messages being output over the UART (ttyAMA0) device.

• If you have enabled the I²C device (ttyUBLX0) and would like to use the I²C device for NTPsec (Please see the I²C Warning):

  1. Leave all of the "UART1" settings using the defaults.
  2. Select "F0-00 NMEA GxGGA" from the drop down, uncheck "I2C" On box, click the Send button at the bottom.
  3. Select "F0-01 NMEA GxGLL" from the drop down, uncheck "I2C" On box, click the Send button at the bottom.
  4. Select "F0-02 NMEA GxGSA" from the drop down, uncheck "I2C" On box, click the Send button at the bottom.
  5. Select "F0-03 NMEA GxGSV" from the drop down, uncheck "I2C" On box, click the Send button at the bottom.
  6. Select "F0-05 NMEA GxVTG" from the drop down, uncheck "I2C" On box, click the Send button at the bottom.
  7. Select "F0-05 NMEA GxZDA" from the drop down, check "I2C" On box, click the Send button at the bottom.

  At this point you should only see $GNRMC and $GNZDA messages being output over the I²C device (ttyUBLX0) and multiple message types over the UART (ttyAMA0) device.

This section configures how the u-blox module navigation engine interprets the measurements.

• From the "Dynamic Model" drop down, select the "2 - Stationary" setting.
• Click the Send button at the bottom.

This is the recommended setting for timing applications in the u-blox protocol specification document section 8.1.

This section configures the NMEA protocol output from the u-blox module.

• From the "NMEA Version" drop down, select "4.1".
• Click the Send button at the bottom.

This will enable the output of the Galileo satellites.

This section configures the u-blox module output interfaces.

• If you are only using the UART interface (ttyAMA0):

  1. Select "0 - I2C' from the "Target" drop down.
  2. Select "none" in the "Protocol in" drop down.
  3. Select "none" in the "Protocol out" drop down.
  4. Click the Send button at the bottom.
  5. Select "3 - USB' from the "Target" drop down.
  6. Select "none" in the "Protocol in" drop down.
  7. Select "none" in the "Protocol out" drop down.
  8. Click the Send button at the bottom.

  This will disable the I²C and USB interfaces on the u-blox module, leaving just the UART1 interface enabled.

• If you are using both the UART (ttyAMA0) and the I²C (ttyUBLX0) interfaces:

  1. Select "0 - I2C' from the "Target" drop down.
  2. Select "none" in the "Protocol in" drop down.
  3. Select "1 - NMEA" in the "Protocol out" drop down.
  4. Click the Send button at the bottom.
  5. Select "3 - USB' from the "Target" drop down.
  6. Select "none" in the "Protocol in" drop down.
  7. Select "none" in the "Protocol out" drop down.
  8. Click the Send button at the bottom.

  This will configure the I²C interface to only output NMEA messages and will disable the USB interface. It will also leave the default setting for the UART1 interface to support UBX and NMEA messages.

This section configures the time pulse output on the Pulse Per Second (PPS) pin.

The only setting we need to configure here is the cable delay.

The formula to calculate the cable delay is:

D = \frac{L \cdot C}{V}

You can find the nominal velocity of propagation from the cable datasheet provided by the manufacturer.

For example, my cable is RG316 which has a nominal velocity of propagation of 69.5.

The cable delay for my antenna is 15.16637681 ns.

• To configure your antenna cable delay:
  1. Calculate the cable delay in nanoseconds.
  2. Enter this value in the "Cable Delay" box. Using my value, I enter "15".
  3. Click the Send button at the bottom.

Once you have configured the module, you can save this configuration to a file that can be used to configure the module on boot.

To save the configuration from u-center:

• From the top menu, select Tools.
• On the Tools menu, select Receiver Configuration.
• In the Load/Save Receiver Configuration window, specify your configuration file save location in the Configuration File field.
• Click the Transfer GNSS -> File button to start the configuration save process.

I have included a simple python3 application that will load a u-center saved configuration file into a u-blox module called u-blox-cfg-loader.py. We can use this to configure the u-blox module when the Raspberry Pi boots.

• Copy the u-blox-cfg-loader.py into /usr/local/bin on your Raspberry Pi:

  sudo cp -p u-blox-cfg-loader.py /usr/local/bin

• Copy your u-center configuration file into /etc on your Raspberry Pi:

  sudo cp u-blox.cfg /etc/u-blox.cfg
  sudo chmod 644 /etc/u-blox.cfg
  sudo chown root.root /etc/u-blox.cfg

• Configure udev to run the u-blox-cfg-loader.py on boot:

  sudo cp udev/10.u-blox-cfg-loader.rules /etc/udev/rules.d

• Run the u-blox-cfg-loader.py tool to load your configuration without requiring a reboot:

  sudo /usr/local/bin/u-blox-cfg-loader.py --port /dev/ttyAMA0 --file /etc/u-blox.cfg

On future reboots of the Raspberry Pi, the u-blox-cfg-loader.py will be run by udev automatically.

As mentioned above in the GNSS (GNSS Config) section note, u-blox recommends a cold start after changing the GNSS settings. We can accomplish this by creating another u-blox configuration file and setting up another udev rule:

echo "CFG-RST - 06 04 04 00 FF B9 02 00" | sudo tee -a /etc/u-blox-rst.cfg
sudo cp udev/60-u-blox-cfg-loader-rst.rules /etc/udev/rules.d

You can configure the u-blox UART1 interface to run at a higher baud rate than the default of 9600. This will not improve the accuracy of the time but will reduce the chance of a transmit buffer overflow in the u-blox module if you enable additional messages on the UART1 (ttyAMA0) interface. To change the baud rate of the UART1 interface on the u-blox module:

• Append the configuration line to the u-center configuration file:

  echo "CFG-PRT - 06 00 14 00 01 00 00 00 C0 08 00 00 00 C2 01 00 07 00 03 00 00 00 00 00" | sudo tee -a /etc/u-blox.cfg

• Run the u-blox-cfg-loader.py tool to load your configuration without requiring a reboot:

  sudo /usr/local/bin/u-blox-cfg-loader.py --port /dev/ttyAMA0 --file /etc/u-blox.cfg

• Update your NTPsec configuration to use 115200 baud:

  sudo sed -i 's/9600/115200/g' /etc/ntpsec/ntp.conf

• Restart the NTPsec service:

  sudo systemctl restart ntpsec

• Update your ser2net configuration to use 115200 baud:

  sudo sed -i 's/ttyAMA0:9600/ttyAMA0:115200/g' /etc/ser2net.conf

• Update the cold start udev rule:

  sudo sed -i 's/u-blox-rst.cfg/u-blox-rst.cfg --speed 115200/g' /etc/udev/rules.d/60-u-blox-cfg-loader-rst.rules

The Raspberry Pi includes a hardware watchdog device that can be used to reset the Raspberry Pi should the software freeze (such as a kernel panic).

1. Enable the watchdog hardware device:

   • Edit the /boot/firmware/syscfg.txt
     • Add "dtparam=watchdog=on". On reboot, this will enable the watchdog device.

2. Install the watchdog system service:

   sudo apt-get update
   sudo apt-get install watchdog

3. Configure the watchdog service:

   • Edit the /etc/watchdog.conf file
     • Add "watchdog-device = /dev/watchdog". This will set the location of the hardware watchdog device file.
     • Add "watchdog-timeout = 15". This sets the time, in seconds, the hardware device will wait for an update before triggering a hardware reset.
     • Add "max-load-1 = 24". This is the one-minute load average threshold at which the watchdog service will reboot the device. The one-minute load average is the first "load average" number when you run the "uptime" command. Twenty-four is a large number, approximately six times the load a four core Raspberry Pi can normally process.
     • Add "interface = eth0". This will cause the watchdog process to watch the "eth0" network interface to make sure it is receiving traffic.
     • Add "temperature-sensor = /sys/class/thermal/thermal_zone0/temp". This is the file where the Raspberry Pi core temperature is reported. Note, it is reported in thousandths of a degree Celsius.
     • Add "max-temperature = 82". This sets the watchdog service temperature threshold to eighty-two degrees Celsius. This is the temperature the Raspberry Pi will start throttling the CPU.
     • Add "min-memory = 25000". This sets a minimum available memory threshold for the watchdog process. This value is in memory pages, which is 4096 on the Raspberry Pi (getconf PAGESIZE). A value of twenty-five thousand will set a low memory threshold of one hundred megabytes of available memory.

4. Enable the watchdog service:

   sudo systemctl enable watchdog

5. Reboot the Raspberry Pi to enable the watchdog device:

   sudo reboot

6. Verify the watchdog service started successfully:

   sudo systemctl status watchdog | less

   The output should show that the service is active (running).

The u-blox CAM-M8C module on the BerryGPS-IMUv3 provides multiple data interfaces that allow access to the NMEA and UBX protocols. Above we configured and used the UART interface over the Raspberry Pi hardware serial port (UART). In addition to the UART interface on the u-blox module, it also supports an I²C compatible Display Data Channel (DDC) interface and a Serial Peripheral Interface (SPI). On the CAM-M8C module, if the SPI is enabled, the UART and DDC/I²C interfaces cannot be used as they share pins on the u-blox module. Since I want to use the UART and I²C interfaces, I will not be discussing how to use SPI with the u-blox module.

By enabling the I²C interface and making it available to the Raspberry Pi we can have two, independent, interfaces on the u-blox module. This allows one interface to be configured to support only the messages required for our NTP service, and the other can be used to monitor and configure the u-blox module.

1. Copy over the ublox6-gps-i2c dkms directory:

   sudo mkdir /usr/src/ublox6-gps-i2c-1.0
   sudo cp -a ublox6-gps-i2c/* /usr/src/ublox6-gps-i2c-1.0

2. Add the module to dkms so that it will be built for future kernel updates:

   sudo dkms add -m ublox6-gps-i2c -v 1.0

3. Build and install the module for the current kernel:

   sudo dkms install -m ublox6-gps-i2c -v 1.0

4. Enable the ublox6-gps-i2c kernel module for boot:

   echo "ublox6-gps-i2c" | sudo tee -a /etc/modules-load.d/ublox6-gps-i2c.conf

5. Setup udev to enable the ublox_gps I²C driver:

   sudo cp udev/10.ubox_i2c.rules /etc/udev/rules.d

   Currently the driver doesn't support auto loading the I²C driver so, I am working around this by setting up a udev rule that detects the kernel module loading and tells the I²C bus there is a new device. Maybe in the future I will update the driver to auto load for this i2c bus number and the u-blox I²C address. However, that would be unsafe as the u-blox module doesn't have any ID registers available to query on the I²C bus to validate it is the device we want.

6. Enable the u-blox i2c device without the need to reboot:

   sudo udevadm control --reload
   sudo modprobe ublox6-gps-i2c

   These steps happen automatically on reboot.

7. Optionally update NTPsec to use the I²C device:

   • Edit the /etc/ntpsec/ntp.d/refclock.conf file.

     • Change the /dev/ttyAMA0 to /dev/ttyUBLX0 on the "refclock" line.

       sudo sed -i 's/ttyAMA0/ttyUBLX0/g' /etc/ntpsec/ntp.d/refclock.conf
       sudo sed -i 's/ baud 9600//g' /etc/ntpsec/ntp.d/refclock.conf

   • Edit the /etc/apparmor.d/tunables/ntpd file.

     • Change the "/dev/ttyAMA0" to "/dev/ttyUBLX0" on the @{NTP_DEVICE} line.

       sudo sed -i 's/ttyAMA0/ttyUBLX0/g' /etc/apparmor.d/tunables/ntpd

   • Update the apparmor configuration:

     sudo apparmor_parser -r /etc/apparmor.d/usr.sbin.ntpd

   • Restart the ntp service to load the new configuration:

     sudo systemctl restart ntpsec

The very kind person that gifted me the BerryGPS-IMUv4 board inquired about supporting u-blox Multi GNSS AssistNow to allow a quicker time-to-first-fix on startup. I was more than happy to add support for AssistNow to this project.

After following these installation instructions, AssistNow will be used to load the almanac and ephemeris data into the u-blox module after the it is configured and the cold start is issued. In my testing, this dropped the time-to-first-fix to about two minutes when I enabled all of the data types and provided the receiver location settings.

The u-blox Multi GNSS AssistNow Online is a free service, but it does require a token to access the service. You can follow the instructions provided in the AssistNow getting started guide to get your free token.

• Copy the u-blox-assistnow-loader.py into /usr/local/bin on your Raspberry Pi:

  sudo cp -p u-blox-assistnow/u-blox-assistnow-loader.py /usr/local/bin

• Copy the configuration file into /etc on your Raspberry Pi:

  sudo cp -p u-blox-assistnow/u-blox-assistnow.conf /etc

• Configure udev to run the u-blox-assistnow-loader.py on boot:

  sudo cp -p udev/65-u-blox-assistnow.rules /etc/udev/rules.d

At a minimum you must configure the token in the u-blox-assistnow.conf file. If this is not set, the u-blox AssistNow loader will skip the AssistNow data loading.

If you know the exact position of the receiver, you can enable the "pos" datatype and fill in the position information in the configuration file. An easy way to get an accurate location is to run the receiver for a while and once is has established a stable location, connect to the receiver using the u-center application. Then, from the Tools menu, select AssistNow Online and AssistNow Online, GNSS. From the Multiple GNSS AssistNow Online window set Reference Position to Manual and click the Use Current button. You can then copy those values into the u-blox-assistnow.conf file.

Once you have completed these steps, on the next reboot, the u-blox module will get AssistNow data loaded and the time-to-first-fix should be reduced.

Someone was kind enough to gift me a Raspberry Pi 4 (Thank you again!). Here is what I have learned about the Raspberry Pi 4 so far:

U-boot on Ubuntu 19.10 (eoan) is broken. You cannot do the "setenv bootdelay -2" trick to stop the GPS serial port from aborting the boot sequence. This was caused by two issues: my USB keyboard is not detected by u-boot and the saved environment file is corrupt.

I worked around the USB keyboard issue by using my Raspberry Pi 3 serial port to access the u-boot serial console on the Raspberry Pi 4.

The issue with the corrupt environment file was a bigger problem. Not only did it save out without my boot delay change, but it would not load at boot. I later found out there is an issue in this version with the size of the u-boot code and the environment data.

In the end, I resorted to building a custom version of u-boot that sets the autoboot delay and stop strings in u-boot. It would be super nice if Ubuntu set these by default in the u-boot-rpi package.

1. Download the source files by searching for the correct u-boot-rpi package on https://packages.ubuntu.com. There are three files: u-boot_2019.07+dfsg-1ubuntu3.dsc, u-boot_2019.07+dfsg.orig.tar.xz, and u-boot_2019.07+dfsg-1ubuntu3.debian.tar.xz.

2. Unpack the u-boot_2019.07+dfsg.orig.tar.xz file:

   tar xJf u-boot_2019.07+dfsg.orig.tar.xz

3. Go into the new u-boot-2019.07 directory and unpack the debian directory.

   cd u-boot-2019.07
   tar xJf ../u-boot_2019.07+dfsg-1ubuntu3.debian.tar.xz

4. Make the required changes to enable the delay and stop strings:

   echo "#define CONFIG_AUTOBOOT_KEYED" >> include/configs/rpi.h
   echo "#define CONFIG_AUTOBOOT_DELAY_STR \"delay\"" >> include/configs/rpi.h
   echo "#define CONFIG_AUTOBOOT_STOP_STR \"stop\"" >> include/configs/rpi.h

5. Update the package to include a new patch file for the changes:

   dpkg-source --commit

   This will ask for a patch name, I used "rpi4-autoboot-strings". It will then open your favorite editor (vim right?) where you can put in a description for the patch. Update as you see fit since you will not be distributing it.

6. Build the new u-boot packages:

   dpkg-buildpackage -us -uc

   This will take a long time as it rebuilds all of the u-boot packages.

7. Install the newly built package:

   sudo dpkg --install ../u-boot-rpi_2019.07+dfsg-1ubuntu3_arm64.deb

I am sad to report that the I²C bus clock stretching issue that the Raspberry Pi 3 model B suffers from is still present on the Raspberry Pi 4. I will continue to use the software/GPIO I²C driver on the Raspberry Pi 4.

Unfortunately the Raspberry Pi 4 ethernet chip does not support IEEE 1588 hardware timestamping. The ethtool output:

$ ethtool -T eth0
Time stamping parameters for eth0:
Capabilities:
     software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
     software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
     software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

For those of you that might be curious about the other offloading capability on the Raspberry Pi 4, here is default offload settings on Ubuntu 19.10:

$ ethtool -k eth0
Features for eth0:
rx-checksumming: off
tx-checksumming: off
     tx-checksum-ipv4: off
     tx-checksum-ip-generic: off [fixed]
     tx-checksum-ipv6: off
     tx-checksum-fcoe-crc: off [fixed]
     tx-checksum-sctp: off [fixed]
scatter-gather: off
     tx-scatter-gather: off
     tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: off
     tx-tcp-segmentation: off [fixed]
     tx-tcp-ecn-segmentation: off [fixed]
     tx-tcp-mangleid-segmentation: off [fixed]
     tx-tcp6-segmentation: off [fixed]
udp-fragmentation-offload: off
generic-segmentation-offload: off [requested on]
generic-receive-offload: on
large-receive-offload: off [fixed]
rx-vlan-offload: off [fixed]
tx-vlan-offload: off [fixed]
ntuple-filters: off [fixed]
receive-hashing: off [fixed]
highdma: off [fixed]
rx-vlan-filter: off [fixed]
vlan-challenged: off [fixed]
tx-lockless: off [fixed]
netns-local: off [fixed]
tx-gso-robust: off [fixed]
tx-fcoe-segmentation: off [fixed]
tx-gre-segmentation: off [fixed]
tx-gre-csum-segmentation: off [fixed]
tx-ipxip4-segmentation: off [fixed]
tx-ipxip6-segmentation: off [fixed]
tx-udp_tnl-segmentation: off [fixed]
tx-udp_tnl-csum-segmentation: off [fixed]
tx-gso-partial: off [fixed]
tx-sctp-segmentation: off [fixed]
tx-esp-segmentation: off [fixed]
tx-udp-segmentation: off [fixed]
fcoe-mtu: off [fixed]
tx-nocache-copy: off
loopback: off [fixed]
rx-fcs: off [fixed]
rx-all: off [fixed]
tx-vlan-stag-hw-insert: off [fixed]
rx-vlan-stag-hw-parse: off [fixed]
rx-vlan-stag-filter: off [fixed]
l2-fwd-offload: off [fixed]
hw-tc-offload: off [fixed]
esp-hw-offload: off [fixed]
esp-tx-csum-hw-offload: off [fixed]
rx-udp_tunnel-port-offload: off [fixed]
tls-hw-tx-offload: off [fixed]
tls-hw-rx-offload: off [fixed]
rx-gro-hw: off [fixed]
tls-hw-record: off [fixed]

I would like to try setting this up on the Raspberry Pi 4 platform. Specifically, to see if I get additional stability out of the 4.

Beyond the Raspberry Pi 4 interests I would like to compare my results on the u-blox CAM-M8C with other u-blox modules.

U-blox ships the RCB-F9T timing board that should be fairly straight forward to integrate with a Raspberry Pi. It includes the ZED-F9T "high accuracy timing" module. I am curious to see the stability improvement this module may bring.

There are also boards available with the ZED-F9P module which is considered a "high precision GNSS" module.

I am pretty sure that the antenna I am using now is limiting the channels I am receiving from the GNSS systems. I think this antenna, like most currently available, filter for the L1 band fairly tightly. U-blox sells a multi-band external antenna, the ANN-MB-00, that supports the L1 and L2 bands and is tailored to this usecase. I would be curious to see if this also improves the stability by using multiple frequencies with different interference/noise.

If you would like to gift me hardware, I have an Amazon gift wish list available.

• Raspberry Pi is a trademark of the Raspberry Pi Foundation
• OzzMaker and BerryGPS-IMUv3 are likely marks owned by OzzMaker
• u-blox is a registered trademark of u-blox Holding AG
• Ubuntu is a registered trademark of Canonical Ltd.
• Broadcom is a registered trademark of Broadcom Inc.
• Adafruit is a registered trademark of Adafruit Industries.
• I did not get compensation from any of these companies for this project.
• This document comes without any warranty of any kind.
• Not intended for safety of life applications.
• The code provided in this repository is licensed under the GNU General Public License v3.0. See the included LICENSE for terms.
• This document is Copyright 2020-2022 Michael Johnson
• This document is licensed under the Creative Commons Attribution-ShareAlike 4.0 International Public License