Chrony and GPSD

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Sources such as:

Describe how to wire a Garmin gps 18 lvc to a serial port to grab the PPS (pulse-per-second) signal to create a Stratum 1 timesource. Other sources show using ntpd, and the gpsd man page provides config snippets.

Alpine Linux gpsd package 3.9-r1 and higher has the necessary pps code to interface with chrony. This page lists all of the files for a complete, working example:

  • /etc/modules needs to list the pps_ldisc module - you'll need to manually load it if not doing a reboot

Contents of /etc/modules

  • If the GPS 18 LVC is on /dev/ttyS1, then:

Contents of /etc/conf.d/gpsd

# The GPS device (/dev/ttyUSB0, /dev/ttyS0, ...) DEVICE="/dev/ttyS1" BAUDRATE="4800" # Optional arguments # Options include: # -b = bluetooth-safe: open data sources read-only # -n = don't wait for client connects to poll GPS # -N = don't go into background # -F sockfile = specify control socket location # -G = make gpsd listen on INADDR_ANY # -D integer (default 0) = set debug level # -S integer (default 2947) = set port for daemon ARGS="-n -b" # Serial setup # # For serial interfaces, options such as low_latency are recommended # Also, recommends # setting the baudrate with stty # Uncomment the following lines if using a serial device: # /bin/stty -F ${DEVICE} ${BAUDRATE} /bin/setserial ${DEVICE} low_latency
  • This shows all three methods of getting time from gpsd:

Contents of /etc/chrony/chrony.conf

server server server initstepslew 30 # SHM0 from gpsd is the NEMA data at 4800bps, so is not very accurate refclock SHM 0 delay 0.5 refid NEMA # SHM1 from gpsd (if present) is from the kernel PPS_LDISC # module. It includes PPS and will be accurate to a few ns refclock SHM 1 offset 0.0 delay 0.1 refid PPS # SOCK protocol also includes PPS data and # it also provides time within a few ns refclock SOCK /var/run/chrony.ttyS1.sock delay 0.0 refid SOCK # If you see something in ns... its good. # 1 second = # 1000 ms = # 1000000 us = # 1000000000 ns logchange 0.5 local stratum 10 logdir /var/log/chrony keyfile /etc/chrony/chrony.keys commandkey 10 dumpdir /var/log/chrony driftfile /var/log/chrony/chrony.drift allow all


If everything is working correctly, a chronyc 'sources' command should look like this:

chronyc> sources
210 Number of sources = 6
MS Name/IP address         Stratum Poll Reach LastRx Last sample
#x NEMA                          0   4   377    14   +684ms[ +684ms] +/-  252ms
#+ PPS                           0   4   377    13  +2040ns[+2060ns] +/-   50ms
#* SOCK                          0   4   377     7   +225ns[ +245ns] +/- 2217ns
^?                 2  10   377   318  -5144us[-5144us] +/-   72ms
^?               3  10   377   720  -4571us[-4567us] +/-  139ms
^?          0  10     0   10y     +0ns[   +0ns] +/-    0ns
  • NEMA (SHM0) is "x" - its really unstable
  • PPS (SHM1) is good, but the delay of 0.1 makes it the not-preferred master
  • SOCK is the preferred master
  • The other NTP servers are only used if the local server goes down.


  • In /etc/chrony/chrony.conf, there are three possible time sources:
    • SHM 0 (NEMA serial data)
    • SHM 1 (NEMA with PPS)
    • SOCK (PPS 'proprietary' gpsd/chrony interface)
  • You only need 1 of them, although all 3 are shown above
  • In the example above, SHM1 is specified with a delay of 0.1 to prevent it from competing with the SOCK protocol
    • If you prefer to use the SHM1 source instead of SOCK, then either:
      • comment out the SOCK protocol line
      • or reverse the delay values.
  • Note that the SHM0 source (NEMA only, no PPS) is set to a higher delay - don't use it if you have PPS available.
    • An example of where you would want to use SHM0 is a USB based gps receiver - they don't have the PPS line
  • Chrony creates the SOCK interface; chrony should be started before gpsd. If you restart chronyd for some reason, make sure you restart gpsd after.
  • The SHM0 interface can be used with USB based gps devices. In this case, use /dev/ttyUSBX in the /etc/conf.d/gpsd file, and leave the stty and setserial lines commented out.