Added guide to run both Galmon and a Stratum 1 NTP server
Stijn Jonker
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# GalMon and Stratum 1 clock with an ArduSimple simpleRTK2B
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Author: [Stijn Jonker](mailto: sjcjonker+gm@sjc.nl) - Version: 1
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## Introduction
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When I started with GalMon I ordered an [ArduSimple simpleRTK2B](https://www.ardusimple.com/product/simplertk2b-basic-starter-kit-ip65/) starter kit. This board contains a [u-blox ZED-F9P](https://www.u-blox.com/en/product/zed-f9p-module) receiver / GGNS module. I joined because I wanted to learn about satellite based location systems and associated items **AND** for my home network/lab to setup a [NTP stratum 1](https://en.wikipedia.org/wiki/Network_Time_Protocol) server. As an accurate time is imported, but also time with all the aspects like timezone, daylight saving, leap seconds and all crazy things is one of the most complex topics to master and not make mistakes when troubleshooting / investigating in my day job which is in the telco area.
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Anyway I got GalMon/ubxtool running quite quickly via the USB port only then to find out that you cannot run the [gpsd](https://gpsd.gitlab.io/gpsd/index.html) and [Chrony](https://chrony.tuxfamily.org) (a modern NTP daemon) at the same time in the default setup with the simpleRTK2B by just connecting via the USB interface.
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So initially I opened a [issue](https://github.com/ahupowerdns/galmon/issues/102) on GalMon to expose an [Pulse Per Second](https://en.wikipedia.org/wiki/Pulse-per-second_signal) (PPS) signal next to an [SHM driver](https://github.com/ahupowerdns/galmon/issues/100) for ntpd / Chrony. The feedback on my issue was that there is also a hardware way. This short write-up is to show how to do this.
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To allow your Raspberry PI to function as an NTP stratum 1 clock its best to have multiple time sources, simply said to keep them in balance and assure the right time is propagated. Via the [gpsd](https://gpsd.gitlab.io/gpsd/index.html) daemon together with a PPS signal you achieve high time accuracy within your time synchronisation software. Nowadays most systems use the [NTP protocol](https://en.wikipedia.org/wiki/Network_Time_Protocol) to sync the time and stay accurate. This is often done by ntpd or Chrony, whereby Chrony is an attempt to provide a more accurate and secure version. to ntpd.
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This writeup shows how to connect the TimePulse PPS and [UART](https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter) (simply said an serial port) to an Raspberry PI and allow this RPI to run as an stratum 1 NTP source for your home network.
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::NOTE:::if you still need to order your simpleRTK2B then I'm **guessing** you can connect the simpleRTK2B to the Raspberry PI without soldering.
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## Hardware setup
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The ZED-F9P on the simpleRTK2B contains besides the exposed USB connection also an UART and an TimePulse, when you connect those to the right GPIO connectors on a Raspberry PI you can receive input for gpsd as a serial port and a PPS signal for Chrony.
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We need to connect some wires to the simpleRTK2B via soldering to connect it to the Raspberry PI GPIO and one wire on the simpleRTK2B board to select the voltage level at 3.3V.
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### simpleRTK2B / Raspberry PI connectors/wires
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- UART1 TX of the ZED-F9P to Raspberry UART TX is shown in green
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- UART1 RX of the ZED-F9P to Raspberry UART RX is shown in yellow
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- GND of the simpleRTK2B to Raspberry GND is shown in grey
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- TimePulsse of the ZED-F9P to Raspberry PCM_CLK is coloured blue
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- The IOREF path on the simpleRTK2B is shown in orange, this selects 3.3V as voltage. Connecting it **wrongly** to the 5V output could damage your Raspberry PI.
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## Software setup
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Now that the physical hardware part is done, the Raspberry PI itself need to be configured to disable Bluetooth, as the Bluetooth module uses the UART we just connected to. For this one should edit the config.txt, which can be found in `/boot/config.txt`.
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Under a section [all] there should be the following entries:
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```
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# Enable the UART for the use with the simpleRTK2B
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enable_uart=1
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# Disable Bluetooth, as this used the uart exclusively
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dtoverlay=disable-bt
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# Enable the pps signal and indicate it's on GPIO PIN 18.
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dtoverlay=pps-gpio,gpiopin=18
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```
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Now to make sure this is parsed and not overruled by a later statement, the safest way is to add this to the end of the file.
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Next for the module for the pps singal to be loaded, for this edit `/etc/modules` and make sure on a single line there is:
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```
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# Load pps_gpio for the simpleRTK2B TimePulse signal.
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pps_gpio
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```
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Then reboot the Raspberry PI; to disable Bluetooth and configure the PPS settings. Once rebooted check if the uart and pps is correctly loaded and working.
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### PPS signal check
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The linux kernel messages can be read from it's ring buffer with dmesg, to find the pps related message run: `dmesg | grep pps` then on my raspberry the output is shown as:
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```
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root@raspberrypi:/boot# dmesg | grep pps
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[ 4.177390] pps_core: LinuxPPS API ver. 1 registered
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[ 4.178898] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
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[ 4.190231] pps_ldisc: PPS line discipline registered
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[ 5.455744] pps pps0: new PPS source pps@12.-1
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[ 5.455833] pps pps0: Registered IRQ 166 as PPS source
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```
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Now to be sure please install the package `pps-tools` if not yet installed with the command: `apt intall pps-tools` part of this package is `ppstest` which is listed as "ppstest: PPSAPI interface tester".
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Now you can check whether the pps signal is detected correctly:
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```
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root@raspberrypi:/boot# ppstest /dev/pps0
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trying PPS source "/dev/pps0"
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found PPS source "/dev/pps0"
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ok, found 1 source(s), now start fetching data...
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source 0 - assert 1582286566.999999077, sequence: 65588 - clear 0.000000000, sequence: 0
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source 0 - assert 1582286567.999998716, sequence: 65589 - clear 0.000000000, sequence: 0
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source 0 - assert 1582286569.000001741, sequence: 65590 - clear 0.000000000, sequence: 0
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^C
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```
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### uart output check via gpsd
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To verify the uart is now enabled and available this can also be done with `dmesg`, you can use: `dmesg | grep serial` on my Raspberry PI it shows as:
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```
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root@raspberrypi:/boot# dmesg | grep serial
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[ 0.814261] uart-pl011 3f201000.serial: cts_event_workaround enabled
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[ 0.815793] 3f201000.serial: ttyAMA0 at MMIO 0x3f201000 (irq = 81, base_baud = 0) is a PL011 rev2
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[ 90.037958] uart-pl011 3f201000.serial: no DMA platform data
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```
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Checking the uart output can be done via various terminal applications, however most Raspberry PI's don't have these installed, so the best approach could be to do this by configuring gpsd, install gpsd via: `apt install gpsd`
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The serial port is shown above as ttyAMA0, as such we can include this in the gpsd configuration in `/etc/default/gpsd`,
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```
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# Default settings for the gpsd init script and the hotplug wrapper.
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# Start the gpsd daemon automatically at boot time
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START_DAEMON="true"
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# Use USB hotplugging to add new USB devices automatically to the daemon
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USBAUTO="false"
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# Devices gpsd should collect to at boot time.
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# They need to be read/writeable, either by user gpsd or the group dialout.
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DEVICES="/dev/ttyAMA0"
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# Other options you want to pass to gpsd
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GPSD_OPTIONS="-n -b"
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```
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For the `GPSD_OPTIONS` I decided amongst the lines of better safe then sorry to not allow gpsd to configure the u-blox GPS module by specifying `-b` the `-n` option tells gpsd to start getting a fix and sync before any client connects to gpsd, which seems like a nobrainer to me.
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The next step is to enable and start gpsd on the Raspberry PI via the below command:
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```
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root@raspberrypi:/boot# systemctl enable --now gpsd
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Synchronizing state of gpsd.service with SysV service script with /lib/systemd/systemd-sysv-install.
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Executing: /lib/systemd/systemd-sysv-install enable gpsd
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Created symlink /etc/systemd/system/multi-user.target.wants/gpsd.service ? /lib/systemd/system/gpsd.service.
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Created symlink /etc/systemd/system/sockets.target.wants/gpsd.socket ? /lib/systemd/system/gpsd.socket.
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```
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In case an error occurs, the startup error can most likely be seen with: `journalctl -u gpsd`
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Most likely there is no fix yet on the GPS module, but you can check the status of the gpsd daemon and GPS module with the `cgps` command. If you want to see all messages received over the serial port, then you can just run `cgps` and the raw output is also printed, below is the output of `cgps -s` to just show the status:
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```
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┌───────────────────────────────────────────┐┌─────────────────────────────────┐
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│ Time: 2020-02-21T12:19:21.000Z ││PRN: Elev: Azim: SNR: Used: │
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│ Latitude: 52.27193166 N ││ 124 65 076 00 Y │
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│ Longitude: 4.61930849 E ││ 205 12 120 00 N │
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│ Altitude: 4.100 m ││ 208 06 066 39 N │
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│ Speed: 0.01 kph ││ 210 12 057 34 N │
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│ Heading: 2.1 deg (mag) ││ 211 11 023 43 N │
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│ Climb: 0.00 m/min ││ 213 00 087 00 N │
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│ Status: 3D FIX (11 secs) ││ 219 41 268 00 N │
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│ Longitude Err: +/- 5 m ││ 220 26 208 00 N │
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│ Latitude Err: +/- 7 m ││ 222 18 323 00 N │
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│ Altitude Err: +/- 18 m ││ 223 13 080 00 N │
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│ Course Err: n/a ││ 227 05 148 00 N │
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│ Speed Err: n/a ││ 228 25 101 00 N │
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│ Time offset: 1.001 ││ │
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│ Grid Square: JO22hg ││ │
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└───────────────────────────────────────────┘└─────────────────────────────────┘
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```
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Once you see this cgps output whilst ubxtool is also correctly running then you have both the GalMon monitoring active and all pieces ready to also run the NTP stratum 1 clock.
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## Configuring Chrony daemon
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As this guide is using chrony, please make sure ntp is removed and chrony is installed by running: `apt delete ntpd` and `apt install chrony` depending ont he state of your Raspberry PI it could be this is already in the correct state.
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The chrony daemon is configured via `/etc/chrony/chrony.conf` the majority of the config can remain as default, then add the following to the file:
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```
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# SHM0 is a shared memory driver / connection from gpsd
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refclock SHM 0 delay 0.5 refid NEMA
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# PPS is from the /dev/pps0 device.
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refclock PPS /dev/pps0 refid PPS
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## Now add your network ranges here to allow syncing to your chrony instance
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allow 192.0.2.0/24
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allow 2001:DB8:DB8::/48
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```
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Enable and start chrony with the following command:
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```
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root@raspberrypi:~# systemctl enable --now chrony
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Synchronizing state of chrony.service with SysV service script with /lib/systemd/systemd-sysv-install.
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Executing: /lib/systemd/systemd-sysv-install enable chrony
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```
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Just let is all run for some time to get a Galileo / BeiDou / Glonass / GPS etc fix and then chronyc should show something along the lines of:
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```
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root@raspberrypi:~# chronyc sources -v
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210 Number of sources = 6
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.-- Source mode '^' = server, '=' = peer, '#' = local clock.
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/ .- Source state '*' = current synced, '+' = combined , '-' = not combined,
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| / '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
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|| .- xxxx [ yyyy ] +/- zzzz
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|| Reachability register (octal) -. | xxxx = adjusted offset,
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|| Log2(Polling interval) --. | | yyyy = measured offset,
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|| \ | | zzzz = estimated error.
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|| | | \
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MS Name/IP address Stratum Poll Reach LastRx Last sample
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===============================================================================
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#- NEMA 0 4 377 14 +178ms[ +178ms] +/- 254ms
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#* PPS 0 4 377 15 -331ns[ -345ns] +/- 555ns
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^- ntp0.example.com 2 6 377 41 +410us[ +410us] +/- 13ms
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^- ntp1.example.com 2 6 377 42 +68us[ +68us] +/- 41ms
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^- ntp2.example.com 2 6 377 42 +321us[ +321us] +/- 41ms
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^- ntp3.example.com 2 6 377 41 +131us[ +131us] +/- 38ms
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```
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Please note the number of network based sources (prefixed with ^) can differ (and obviously I substituted the actual hostnames). Here you see we have 2 local sources, based on # in front of the sources "NEMA" and "PPS" whereby the system is synchronised on the second pulse received presently.
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That's it folks, you now have a Stratum 1 NTP server at your disposal, point the other hosts in your network to this Raspberry PI's IP address or hostname (if you run DNS locally).
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I would recommend to always have more then one time source, so I recommend adding your new server. For chrony or ntpd add a line:
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`server time.dev.sjc.nl prefer` to your `chrony.conf` or `ntp.conf`on those systems they should report it as a Stratum 1 server, see the output of Chrony and ntpd on some of my other servers:
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**Chrony based output:**
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```
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[root@otherlinuxserver1 ~]# chronyc sources -v
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210 Number of sources = 9
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.-- Source mode '^' = server, '=' = peer, '#' = local clock.
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/ .- Source state '*' = current synced, '+' = combined , '-' = not combined,
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| / '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
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|| .- xxxx [ yyyy ] +/- zzzz
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|| Reachability register (octal) -. | xxxx = adjusted offset,
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|| Log2(Polling interval) --. | | yyyy = measured offset,
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|| \ | | zzzz = estimated error.
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MS Name/IP address Stratum Poll Reach LastRx Last sample
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===============================================================================
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^* 192.0.2.123 1 9 343 392 -59us[ -78us] +/- 575us
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---SNIP---
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```
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**ntpd based output:**
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```
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root@linuxserver2:~# ntpq -n -c peers
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remote refid st t when poll reach delay offset jitter
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==============================================================================
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*192.0.2.123 .PPS. 1 u 3 64 1 0.366 0.224 0.094
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---SNIP---
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```
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That's all folks! Good luck
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## Thanks and Links
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First of all thanks to bert@hubertnet.nl or [@PowerDNS_Bert](https://twitter.com/PowerDNS_Bert) for setting up GalMon and the rest of the community. Additionally to [amontefusco](https://github.com/amontefusco) for feedback on the issue and his pictures.
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- amontefusco's page on his setup for NTP with his simpleRTK2B: https://www.montefusco.com/ntpgnss/
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- Raspberry PI page to enable the UART (and disable bluetooth) : https://www.raspberrypi.org/documentation/configuration/uart.md
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- SatSignal quick start page on a Raspberry PI as Stratum 1 server: https://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html
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- SatSignal describing in more detail using the Raspberry PI as Stratum 1 server: http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
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