Chrony and GPSD

2021-03-13T19:38:02Z

LordMike: Replace incorrect acronym NEMA with correct NMEA

Sources such as:

* http://www.rjsystems.nl/en/2100-ntpd-garmin-gps-18-lvc-gpsd.php
* http://lists.ntp.org/pipermail/questions/2005-November/007878.html

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 [http://gpsd.berlios.de/gpsd.html 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
{{Cat|/etc/modules|pps_ldisc}}
* If the GPS 18 LVC is on /dev/ttyS1, then:
{{Cat|/etc/conf.d/gpsd|<nowiki>
# Copyright 1999-2010 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2

# Config file for gpsd server
# 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

GPSD_OPTIONS="-n -b"
DEVICES="/dev/ttyS1"
GPSD_SOCKET="/var/run/gpsd.sock"
BAUDRATE="4800"

# 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
</nowiki>
}}

{{Note|As of v3.11, the init.d and conf.d files have been merged from gentoo.
The purpose is to allow gpsd to start without editing /etc/conf.d/gpsd.
Note that while gpsd will now start without editing the conf.d file, it will do so with no gps devices attached.

Also note that if you are using a serial device it is still recommended to add the stty and setserial commands as noted in the example configuration above.}}

* This shows all four methods of getting time from gpsd:
{{Cat|/etc/chrony/chrony.conf|<nowiki>
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 NMEA data at 4800bps, so is not very accurate
refclock SHM 0 delay 0.5 refid NMEA

# 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 NMEA+

# 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

# PPS is from the /dev/pps0 device. Note that
# chronyd creates the /var/run/chrony.ttyS1.sock device, but
# gpsd creates the /dev/pps0 device
# openrc rules start gpsd /after/ chronyd, so /dev/pps0
# is not created until after chronyd is started
# If you want to use pps0, either edit the openrc rules
# or add this source after gpsd is started

# refclock PPS /dev/pps0 refid PPS

# 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
</nowiki>
}}

== Chronyc ==

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 NMEA 0 4 377 14 +684ms[ +684ms] +/- 252ms
#+ PPS 0 4 377 13 +2040ns[+2060ns] +/- 50ms
#* SOCK 0 4 377 7 +225ns[ +245ns] +/- 2217ns
^? tick.tock.com 2 10 377 318 -5144us[-5144us] +/- 72ms
^? time.keeper.net 3 10 377 720 -4571us[-4567us] +/- 139ms
^? dead.time.server.org 0 10 0 10y +0ns[ +0ns] +/- 0ns

* NMEA (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.

== Notes ==

* In /etc/chrony/chrony.conf, there are four possible time sources:
** SHM 0 (NMEA serial data)
** SHM 1 (NMEA with PPS)
** SOCK (PPS 'proprietary' gpsd/chrony interface)
** PPS (PPS only from the serial interface pulse)
* You only need 1 of them, although 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 (NMEA 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.

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[[Category:Networking]]

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Chrony and GPSD