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Chrony Time Sync

Written by Max GardenswartzUpdated 2026-11-06Autonomy Park

At a glance

Every computer that runs our ROS 2 nodes syncs its clock with chrony to the discovery server, which serves time to the Autonomy Park network. How to set up the server and the clients.

Every computer that runs ROS 2 nodes for the lab runs chrony: the Homebrew Jetsons, the ground station computers, and the discovery server. The discovery server (192.168.1.201 on the Autonomy Park network) is the time server. It syncs to the internet and serves time to the network. Everything else syncs to it.

This page is adapted from TIME_SYNC.md in homebrew_docker.

Why

  • Message ages are computed across machines. MAVROS stamps PX4 data with the Jetson's clock. A node on a ground computer compares that stamp with its own clock, so any offset between the two clocks goes straight into the computed age.
  • Comparing times to the millisecond. To measure the timestamps an offboard program receives, anywhere but on the companion computer, you need chrony. Without it, the times on different computers differ by milliseconds over ROS. That can hide problems such as stale odometry, and it makes experiments harder to replicate.
  • One shared reference. When every machine syncs to the same server, their clocks agree with each other even if the internet is slow or down.
  • No clock jumps after boot. chrony steps the clock only in its first updates after starting (makestep 1 3 in Ubuntu's default config), and slews it gradually afterwards. A jump while MAVROS runs makes it reset its time sync. After a battery pull, a Jetson boots with its clock at 1970 (homebrew05, 2026-10-14: chrony stepped it forward 56.8 years, 22.6 s after boot), so the homebrew_docker container's entrypoint waits until the clock agrees with the time server before starting ROS.
  • Ubuntu's server documentation recommends chrony for serving NTP. systemd-timesyncd, the default client, implements SNTP only and cannot serve time.

Containers need nothing. A container cannot have its own wall clock, so the homebrew_docker container uses the Jetson's clock.

chrony and systemd-timesyncd Conflict

On Ubuntu, installing one removes the other. All the configuration below is new files in /etc/chrony/conf.d/. Ubuntu's chrony.conf includes that directory, and the packaged chrony.conf is never edited.

The Discovery Server

Run these on the discovery server computer.

  1. Check the Ubuntu version and whether the firewall is active:

    head -2 /etc/os-release
sudo ufw status

  2. Install chrony:

    sudo apt-get update && sudo apt-get install -y chrony

  3. Check that Ubuntu's chrony.conf includes the conf.d directory:

    grep -n confdir /etc/chrony/chrony.conf

    It must print confdir /etc/chrony/conf.d. If it prints nothing, stop: the files below would be ignored.

  4. Serve time to the network, and keep serving it if the internet is lost:

    printf 'allow 192.168.1.0/24\nlocal stratum 10\n' | sudo tee /etc/chrony/conf.d/lan-server.conf

  5. Restart chrony:

    sudo systemctl restart chrony
    • allow 192.168.1.0/24 serves time to the network.
    • local stratum 10 keeps the server answering when the internet is lost. Internet sources are stratum 1 to 3, and chrony's documentation says the local stratum should be larger than the largest stratum expected in the network.

  6. Only if ufw status said active, allow the network to reach the time port:

    sudo ufw allow from 192.168.1.0/24 to any port 123 proto udp

  7. About a minute after the restart, check that it works:

    chronyc sources
chronyc tracking
sudo ss -ulpn | grep ':123 '
    • chronyc sources: one line starts with ^*, the selected internet source.
    • chronyc tracking: Stratum 2 (or 3), a Reference ID naming an internet server, and Leap status : Normal. Reference ID 7F7F0101 with Stratum 10 means the server is on its own clock. That is normal for a few seconds after a restart, and it is what the server does when the internet is down.
    • ss: chronyd is listening on UDP port 123.

The Homebrew Jetsons

This is part of the first-time setup in the homebrew_docker README (step 1.8). The commands are the same as for a ground computer, below.

Ground Computers

Do this on any other computer that runs ROS 2 nodes.

  1. Install chrony:

    sudo apt-get install -y chrony

  2. Point it at the discovery server:

    printf 'server 192.168.1.201 iburst prefer\n' | sudo tee /etc/chrony/conf.d/lab-time-server.conf

  3. Restart chrony and check the sources after 30 seconds:

    sudo systemctl restart chrony
sleep 30
chronyc sources
    • iburst sends a burst of requests at startup, so the first sync takes seconds, not minutes.
    • prefer uses the lab server over the internet pools from Ubuntu's default configuration. The pools stay as a fallback if the lab server is unreachable, for example when the computer is away from the lab network.

    The 192.168.1.201 line must start with ^*. The internet pools are listed below it with ^- (usable, not selected) or ^? (not reachable yet). Both are normal.

  4. Check the offset:

    chronyc tracking | grep -E "Reference ID|Stratum|System time"

    Expect Reference ID : C0A801C9 (192.168.1.201) and Stratum 3, one more than the server. System time is this computer's estimated offset from the server.

Clocks a few tens of milliseconds off when chrony is installed are corrected gradually, not stepped. On 2026-10-14, homebrew05 started 24 ms behind the server and was within 0.2 ms about a minute later, with no step in the chrony log (journalctl -u chrony), while its container kept running.

The Indoor Lab

The Indoor Lab does not use chrony. Nothing indoors needs it today, because the Bebops are the only indoor quadcopters. It should be used there in the future, when PX4 quadcopters with companion computers are bought for the Indoor Lab.

Undo

Undo the server last. Clients that still point at it fall back to the internet pools, but their clocks no longer share one reference.

A Jetson or Ground Computer

Go back to Ubuntu's default systemd-timesyncd, which removes chrony:

sudo rm -f /etc/chrony/conf.d/lab-time-server.conf
sudo apt-get install -y systemd-timesyncd
timedatectl timesync-status

timesync-status should show a Server: line within a minute.

The Discovery Server

  1. Remove the server configuration and list the firewall rules:

    sudo rm -f /etc/chrony/conf.d/lan-server.conf
sudo ufw status numbered

  2. If the list has a rule for port 123, delete it by its number:

    sudo ufw delete <number>

  3. Go back to systemd-timesyncd:

    sudo apt-get install -y systemd-timesyncd
timedatectl timesync-status