Chrony and GPSD

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

pps_ldisc

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, http://catb.org/gpsd/upstream-bugs.html#tiocmwait 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 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org

initstepslew 30 0.pool.ntp.org 1.pool.ntp.org 2.pool.ntp.org

# 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

Notes

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.