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Project 1: GPS-based NTP server

Source: Microsecond accurate NTP with a Raspberry Pi and PPS GPS

Note: Seems the NTP server has an easier time starting if the GPS is NOT LOCKED before booting, so leave antenna unplugged UNTIL Pi has finished booting!

Info

This page describes setting up a GPS module with a Pi to act as a Stratum1Stratum 1 NTP server.

The GPS module mustI purchased is from the NEO-xM line (6M, 7M, 8M). The board looks similar to this:

nmeo-8m.jpg

Things of note: GPIO header, USB, included antenna module, and external antenna port:

  • The GPIO header should have a 3-5V input, with serial AND PPS outputs (5the pins minimum).labelled Example:on Adafruitone Ultimateside of the board, preferrably in the same order as the pins on the Pi, so the GPS Breakout.

    board

    Cheapercan option:plug searchdirectly forinto NEO-6M/7M/8Mit.

    • based
  • The GPSmodule modules.can Mostbe haveconnected anto antennaa builtPC via USB, but that is not covered in (thethis guide.
  • The large white and tan ceramic block withon tanthe edging),bottom it will be bypassed if you use an external antenna. Remote mountingof the module tois utilisea GPS receiver antenna. It can be used, but then the built-inmodule antennamust isbe notplaced reccommended,remotely asfrom the longPi, wirewhich maycan throw thecause timing offissues forwith the PPS signal.line.
    • It is recommended to use an
  • An external GPS antenna andis mountrecommended, which plugs into the GPSSMA moduleport. directlyThe toon-board theantenna Piis header.

    disabled in this case.

PPS = Pulse Per Second. While the serialserial/USB data from the GPS module contains the actual time & date information required for the NTP server, the PPS input is required to keep sub-millisecond accuracy. the PPS output putssends outa EXACTLY 1signal pulse perat the start of each second, +/- a few nanoseconds. The PPStime keeping software running on the Pi uses this.this second input to keep precise time.

Setup:

  1. Install packages:

    sudo apt install pps-tools gpsd gpsd-clients gpsd-tools chrony

    ('pps/gpsd' packages are for interperating GPS data, chrony is anthe actual NTP server)

  2. Add these lines to the end of /boot/config.txt

    # the next 3 lines are for GPS & PPS signals
dtoverlay=pps-gpio,gpiopin=18
enable_uart=1
init_uart_baud=9600

    These lines initialise pins on the Pi's GPIO header to enable the serial port, and set the PPS pin as an input.

  3. Add this text to the end of /etc/modules: to enable the PPS module:

    pps-gpio

  • These programs and config changes enable PPS time sync inputs on GPIO 18 (headern 12), and initializes the serial COM port with a baud rate of 9600.

    1. disable system handling of the COM port:port (allows the GPS software to keep control of the port):

      sudo systemctl mask serial-getty@ttyS0.service

    This prevents the system from taking control of the port. Check this command worked after rebooting:rebooting the Pi: /dev/ttyS0 should be owned by root:dialout and have permissions crw-rw----.

    • Wire up the GPS module:

    Pinout:

    1. GPS PPS to RPi pin 12 (GPIO 18)
    2. GPS VIN to RPiheader pin 2 or 4 (+5Vbus)
    3. GPS GND to RPi pin 6 (GND)
    4. GPS RX to RPi pin 8 (Pi TX)
    5. GPS TX to RPi pin 10 (Pi RX)
    6. GPS PPS to header pin 12 (GPIO 18)

    Picture reference (noteall Pi's have the same header): raspberry-pi-zero-5-1536x768.png Note that all the Pirequired pins on the Pi header are seqential):sequential. raspberry-pi-zero-5-1536x768.pngMost GPS modules with headers are pin-compatible with the Pi, however some swap the positions of the TX and RX pins (The 'TX' of the GPS module must go the the 'RX' pin on the header, and vice-versa.) If the GPS is not detected when you plug it into the header, try extending the header with jumper wires and crossing over the TX/RX pins.

    Check GPS functionality

    The GPS device should put out either 3.3V or 5V on the antenna connector to power the antenna. With the GPS module powered,powered via the Pi, The on-board LED will light solid.a solid color. The LED will start blinking once it has a GPS lock.lock, Theand modulewill spitsstart sending GPS data to(called "NMEA" data) via the serial lineslines. atOnce allthe times,Pi therehas booted, check that the GPS module is no bi-directional communication!working:

    1. Check that the pps serice is running:

       lsmod | grep pps

      Should return at least the service pps_core., and sometimes other services as well, they can be ignored.

    2. Check the PPS input for PPSgood pulses (after GPS has a lock)lock, ie the indicator light is blinking):

      sudo ppstest /dev/pps0
###
      Output:

      Example output:

      trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1655253832.999996389, sequence: 966 - clear  0.000000000, sequence: 0
source 0 - assert 1655253834.000004254, sequence: 967 - clear  0.000000000, sequence: 0
source 0 - assert 1655253835.000001120, sequence: 968 - clear  0.000000000, sequence: 0
source 0 - assert 1655253836.000000985, sequence: 969 - clear  0.000000000, sequence: 0
source 0 - assert 1655253836.999996852, sequence: 970 - clear  0.000000000, sequence: 0
source 0 - assert 1655253838.000001719, sequence: 971 - clear  0.000000000, sequence: 0
source 0 - assert 1655253839.000002586, sequence: 972 - clear  0.000000000, sequence: 0
source 0 - assert 1655253840.000001453, sequence: 973 - clear  0.000000000, sequence: 0
### ...etc

    If there is a timout, then there is likely not a good GPS lock yet.

    Set up software:

    Enable the GPS decoder software and the NTP server:

    1. Edit /etc/default/gpsd:
      • change GPSD_OPTIONS=”” to GPSD_OPTIONS=”-n”
      • change START_DAEMON="false" to START_DAEMON="true"
      • change DEVICES=”” to DEVICES=”/dev/ttyS0 /dev/pps0″
    2. edit /etc/chrony/chrony.conf file, add this block of code to the top: 
      ### GPS TIME SYNC INFO
# 'delay'GPS describesreference the accuracy drift of the time source, in seconds. Larger numbers deprioritizes the source. NMEA Source needs at least some delay else chrony loses sync.
# 'offset' adjusts source arrival time forwarddefines and backward. Use to sync GPS and PPS signals (time in [brackets] in chrony sources list).adjustments:
refclock SHM 0 delay 0.1 offset 0.1165 refid NMEA
refclock PPS /dev/pps0 refid PPS

# Allow all LAN IP Ranges so NTP server is network-agnostic (can be used on any LAN):
allow 10.0.0.0/8   
allow 192.168.0.0/16
allow 172.16.0.0/12

### END GPS TIME SYNC INFO

      Notes:

      • 'delay 0.1' describes the accuracy of the serial time source, in seconds. Larger numbers deprioritizes the source (sources with smaller delays have higher priority). NMEA Source needs a non-zero delay, else chrony refuses to use it.
      • 'offset' adjusts the fixed offset delay on the NMEA source. Use to sync GPS and PPS signals for higher accuracy.

    3. Due to an inconsistency with the Pi Zero, the gpsd service often starts in an active (disabled) state. This can be solved by restarting the service. To restart automatically, add the following line to the file /etc/rc.local:
    /usr/sbin/service gpsd restart

    4) Reboot the Pi.
  • Check:

    5) Check NMEA function: after rebooting, run the program gpsmon`gpsmon`, it shouldmay sit on a blank screen for up to 60 seconds, but afterwards will display a window similar to this: gps-monitor.png
    • [![gps-monitor.png](https://wiki.jank.tech/uploads/images/gallery/2022-06/scaled-1680-/dUAcDKLK5jLHI3Rr-gps-monitor.png)](https://wiki.jank.tech/uploads/images/gallery/2022-06/dUAcDKLK5jLHI3Rr-gps-monitor.png)

    This confirms that the software is decoding the GPS info properly.

    1. Check Chrony is selecting GPS as a time source; run chronyc sources 
      ###

      
Output:

      MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
#- NMEA                          0   4   377    15  +3794us[+3794us] +/-  470us
#* PPS                           0   4   377    16   +351ns[ +515ns] +/- 3000ns
^- time.cloudflare.com           3   6   377    70  -2009us[-2008us] +/-   16ms
^- smtp.us.naz.com               2   6   377     2    -26ms[  -26ms] +/-  105ms
^- hc-007-ntp1.weber.edu         2   6   377     5  +3131us[+3131us] +/-   72ms
^- dns2.kcweb.net                2   6   377     6  +1296us[+1296us] +/-   88ms

      
More info here under the header 'Time Sources'. TLDR: 'PPS' should have a '*' next to it, indicating it is the primary time source (may take up to 5 minutes to update the primary source after GPS is locked), and the [bracketed] time in the 'NMEA' row should be less than ~5msec. change the offset'offset' value (step 2) in the chrony.conf file to adjust this bracketed value, and restart chrony with the command sudo systemctl restart chrony. A "Reach" of '377' indicates source was polled sucessfully all 8 of the last 8 tries, a higher reach value for a source marks it as more trustworthy, and ups trustworthiness.the source's priority. Lower numbers mean polls have been missed, an the source is less reliable.

    Set up other machines to use the Pi as an NTP server (Linux):

    1. Install ntp package on the machine
    2. Edit /etc/ntp.conf and add server [pi.local.ip] true to the list of servers
    3. start the ntp service (on Arch: sudo systemctl start ntpd)
    4. Check the NTP sources with ntpq -p: 
      remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 *192.168.1.137   .PPS.            1 u   35   64    1    1.851  +144501   0.001
 +time.walb.tech  50.205.244.21    3 u   34   64    1   82.802  +144501   0.001
 -li1187-193.memb 132.163.96.3     2 u   30   64    1  179.522  +144501   0.001
 +time-dfw.0xt.ca 68.166.61.255    2 u   33   64    1  106.817  +144501   0.001
 +LAX.CALTICK.NET 17.253.26.253    2 u   32   64    1  118.527  +144501   0.001
      Check back in about 20 minutes, after which one source should have a '*' next to it to indicate that server is the chosen server. Remove default NTP servers and restart the ntp service if the pi is not selected. Note: true added after the pi's IP in the config indicates it is more "trustworthy" than other sources, and is more likely to be picked.
    5. If NTP source is registered correctly, and you are ready to use NTP, enable the ntp service (on Arch: sudo systemctl enable ntpd)

    Set up other machines to use the Pi as an NTP server (Windows):

    1. Install Dimension 4.
    2. add the pi as an SNTP source, set the sync time to every 5 minutes.
    3. In the advanced settings menu, force the program to only sync the selected server to force it to use the pi.
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