Configuring NTP

Network Time Protocol, or NTP, is a network protocol that allows computers on a network to synchronize their clocks with each other. This is essential for ROS, as many types of data are time-sensitive.

Chrony

Clearpath uses a Linux package called chrony to provide clock synchronization across multiple devices inside a robot. The robot's primary PC acts as the main NTP source for other computers inside the robot.

To (re-)configure chrony, edit the file /etc/chrony/chrony.conf. The following shows the default configuration for the primary PC:

# Welcome to the chrony configuration file. See chrony.conf(5) for more
# information about usuable directives.

# This will use (up to):
# - 4 sources from ntp.ubuntu.com which some are ipv6 enabled
# - 2 sources from 2.ubuntu.pool.ntp.org which is ipv6 enabled as well
# - 1 source from [01].ubuntu.pool.ntp.org each (ipv4 only atm)
# This means by default, up to 6 dual-stack and up to 2 additional IPv4-only
# sources will be used.
# At the same time it retains some protection against one of the entries being
# down (compare to just using one of the lines). See (LP: #1754358) for the
# discussion.
#
# About using servers from the NTP Pool Project in general see (LP: #104525).
# Approved by Ubuntu Technical Board on 2011-02-08.
# See http://www.pool.ntp.org/join.html for more information.
pool ntp.ubuntu.com        iburst maxsources 4
pool 0.ubuntu.pool.ntp.org iburst maxsources 1
pool 1.ubuntu.pool.ntp.org iburst maxsources 1
pool 2.ubuntu.pool.ntp.org iburst maxsources 2

# This directive specify the location of the file containing ID/key pairs for
# NTP authentication.
keyfile /etc/chrony/chrony.keys

# This directive specify the file into which chronyd will store the rate
# information.
driftfile /var/lib/chrony/chrony.drift

# Uncomment the following line to turn logging on.
#log tracking measurements statistics

# Log files location.
logdir /var/log/chrony

# Stop bad estimates upsetting machine clock.
maxupdateskew 100.0

# This directive enables kernel synchronisation (every 11 minutes) of the
# real-time clock. Note that it can’t be used along with the 'rtcfile' directive.
rtcsync

# Step the system clock instead of slewing it if the adjustment is larger than
# one second, but only in the first three clock updates.
makestep 1 3

# Configure this host to act as the NTP source for the rest of the devices in this robot
local stratum 10
allow 192.168.131/24

For secondary PCs inside the robot, the chrony configuration file is very similar, but adds a new server directive after the pool, and omits the final local and allow sections:

# Welcome to the chrony configuration file. See chrony.conf(5) for more
# information about usuable directives.

# This will use (up to):
# - 4 sources from ntp.ubuntu.com which some are ipv6 enabled
# - 2 sources from 2.ubuntu.pool.ntp.org which is ipv6 enabled as well
# - 1 source from [01].ubuntu.pool.ntp.org each (ipv4 only atm)
# This means by default, up to 6 dual-stack and up to 2 additional IPv4-only
# sources will be used.
# At the same time it retains some protection against one of the entries being
# down (compare to just using one of the lines). See (LP: #1754358) for the
# discussion.
#
# About using servers from the NTP Pool Project in general see (LP: #104525).
# Approved by Ubuntu Technical Board on 2011-02-08.
# See http://www.pool.ntp.org/join.html for more information.
pool ntp.ubuntu.com        iburst maxsources 4
pool 0.ubuntu.pool.ntp.org iburst maxsources 1
pool 1.ubuntu.pool.ntp.org iburst maxsources 1
pool 2.ubuntu.pool.ntp.org iburst maxsources 2

# Add the robot's primary PC as an NTP source
server 192.168.131.1 offline minpoll 8

# This directive specify the location of the file containing ID/key pairs for
# NTP authentication.
keyfile /etc/chrony/chrony.keys

# This directive specify the file into which chronyd will store the rate
# information.
driftfile /var/lib/chrony/chrony.drift

# Uncomment the following line to turn logging on.
#log tracking measurements statistics

# Log files location.
logdir /var/log/chrony

# Stop bad estimates upsetting machine clock.
maxupdateskew 100.0

# This directive enables kernel synchronisation (every 11 minutes) of the
# real-time clock. Note that it can’t be used along with the 'rtcfile' directive.
rtcsync

# Step the system clock instead of slewing it if the adjustment is larger than
# one second, but only in the first three clock updates.
makestep 1 3

Work-around for no RTC

Most computers have a realtme clock (RTC) that keeps time while the computer is otherwise powered-off. This clock has a replaceable battery, referred to either as the RTC backup battery or CMOS battery. If you are using a computer that does not have an RTC backup battery, or does not have a RTC at all, you may see unusual behaviour. Typically the clock will reset to a default date (often January 1, 1970) when the computer loses power. The clock will then jump ahead to the present as soon as a connection with an NTP server is established.

Under most circumstances this is fine, but some ROS nodes will behave incorrectly if the clock suddenly jumps ahead by 50+ years. To mitigate this, if you are configuring a computer that does not have a reliable RTC, modify the file /lib/systemd/system/ros.service to add an ExecStartPre command to the service:

[Service]
ExecStart=/usr/sbin/ros-start
ExecStartPre=/usr/sbin/ros-wait-clock

Then create the file /usr/sbin/ros-wait-clock by running

sudo nano /usr/sbin/ros-wait-clock

and entering the following:

#!/bin/bash

hw_time="$(hwclock -r -f /dev/rtc1)"

# wait at most 15s
for i in {1..15};
do
  if [[ "$hw_time" == 19* ]];
  then
    echo "HW clock says it's ${hw_time}. Waiting for clock-sync..."
    sleep 1
    hw_time="$(hwclock -r -f /dev/rtc1)"
  else
    echo "HW clock says it's now ${hw_time}. Clock appears synchronized with reality"
    exit 0
  fi
done

echo "Wait for clock sync timed out. Moving ahead with possibly bad clock."

Save the file, close the editor, and make the file executable by running

sudo chmod +x /usr/sbin/ros-wait-clock