Stratum‑1 NTP Server on Raspberry Pi 5

John Burns

This guide explains how to transform a fresh installation of Raspberry Pi OS into a stratum‑1 NTP server. The project leverages a GPIO‑based GPS module with PPS, the PTP hardware timestamping capability of the Ethernet interface, Chrony for NTP synchronization, and Telegraf with InfluxDB for monitoring.

1. Overview

In this project you’ll build a stratum‑1 NTP server using:

  • Raspberry Pi 5 as the host.
  • A GPS module (via the serial UART port) for an absolute time reference.
  • A PPS (Pulse Per Second) signal from the same GPS for sub-microsecond timing accuracy.
  • The PTP hardware timestamping on the Ethernet interface for improved precision on PTP packets.
  • Chrony for NTP time synchronization.
  • gpsd to interface with your GPS receiver.
  • Telegraf for monitoring various metrics (including Chrony statistics) sent to an InfluxDB backend.

2. Prerequisites and Required Hardware

Before you begin, ensure you have the following:

  • Raspberry Pi 5 running a fresh install of Raspberry Pi OS.
  • A GPS module with serial (UART) output and PPS capability.
  • An external GPIO connection for the PPS signal (typically connected to a dedicated GPIO pin, e.g. GPIO4).
  • An Ethernet interface with PTP hardware timestamping support.
  • A monitoring server running InfluxDB (or you can install InfluxDB locally if desired).

3. Software Packages to Install

For a fully functional stratum‑1 NTP server, you’ll need to install these packages:

  • chrony – Provides the NTP daemon with support for hardware timestamping and refclock interfaces.
  • gpsd – Daemon to interact with your GPS device.
  • gpsd-clients – Utilities to query the GPS data.
  • telegraf – For gathering system and application metrics and sending them to InfluxDB.
  • ethtool – For inspecting and verifying hardware timestamping support on the Ethernet interface.
  • (Optionally) pps-tools – To assist with troubleshooting PPS signals.

Install these packages using:

sudo apt update
sudo apt install chrony gpsd gpsd-clients telegraf ethtool pps-tools

4. Kernel and Firmware Configuration

To properly use the GPIO PPS and enable the UART for your GPS, update your boot configuration and load the required kernel modules.

4.1. Modify the Boot Configuration

Edit the /boot/firmware/config.txt file to enable the UART and configure the PPS overlay (using GPIO4):

[all]
dtparam=uart0=on
dtoverlay=pps-gpio,gpiopin=4
dtoverlay=disable-wifi

4.2. Ensure Kernel Modules Load at Boot

Edit /etc/modules to load the necessary kernel modules:

 #/etc/modules: kernel modules to load at boot time.
pps-gpio
i2c-dev

This configuration ensures the PPS GPIO module and I2C support (if needed) are loaded during boot.

5. Configuring gpsd

Configure gpsd to start at boot and use the correct devices. Edit /etc/default/gpsd as follows:

# Devices gpsd should collect to at boot time.
DEVICES="/dev/ttyAMA0 /dev/pps0"

# Other options you want to pass to gpsd
GPSD_OPTIONS="-n"

# Automatically hot add/remove USB GPS devices via gpsdctl
USBAUTO="true"

This configuration tells gpsd to open the UART device (for NMEA data from the GPS) and the PPS device for precise pulse timing.

6. Configuring Chrony

Chrony will serve as your NTP daemon. Its configuration utilizes the GPS and PPS refclocks and leverages PTP hardware timestamping on the Ethernet interface.

Edit /etc/chrony/chrony.conf to include:

confdir /etc/chrony/conf.d
server 0.us.pool.ntp.org iburst
server 1.us.pool.ntp.org iburst
server 2.us.pool.ntp.org iburst
server 3.us.pool.ntp.org iburst
keyfile /etc/chrony/chrony.keys
driftfile /var/lib/chrony/chrony.drift
ntsdumpdir /var/lib/chrony
#log tracking measurements statistics
logdir /var/log/chrony
maxupdateskew 100.0
rtcsync
makestep 1 3
leapsectz right/UTC

refclock SOCK /run/chrony.ttyAMA0.sock refid GPS precision 1e-3 noselect
refclock SOCK /run/chrony.pps0.sock refid PPS precision 1e-7 lock GPS
allow 192.168.109.0/24
allow fd79:605:8025:a109::/64
ptpport 319
hwtimestamp *

Key Points:

  • Refclock SOCK entries: These specify the Unix sockets where Chrony receives data from gpsd (for the serial GPS) and the PPS signal.
  • PTP support: The ptpport 319 and hwtimestamp * lines enable PTP hardware timestamping, which improves timing accuracy.
  • Network access: The allow lines configure which subnets may query your NTP server.

7. Verifying PTP Hardware Timestamping

To verify that your Ethernet interface supports hardware timestamping, run:

ethtool -T eth0

The output should indicate support for hardware transmit/receive timestamps and include details similar to:

PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
    off
    on
    onestep-sync
Hardware Receive Filter Modes:
    none
    all

This confirms that your interface is correctly configured for precise timestamping.

8. Configuring Telegraf for Monitoring

Telegraf collects and sends system metrics to your InfluxDB instance. Edit /etc/telegraf/telegraf.conf to monitor Chrony statistics, GPS data, and general system metrics:

## Minimal telegraf.conf for monitoring a Raspberry Pi with gpsd, chrony, and full system metrics

[agent]
  interval = "10s"
  round_interval = true
  metric_batch_size = 1000
  metric_buffer_limit = 10000
  collection_jitter = "0s"
  flush_interval = "10s"
  flush_jitter = "0s"
  debug = false
  quiet = false

[[outputs.influxdb]]
  ## URL of your InfluxDB instance using HTTPS
  urls = ["https://192.168.109.7:8086"]
  ## Ignore certificate chain and host verification (not recommended for production)
  insecure_skip_verify = true
  ## The target database for metrics.
  database = "ntpserver"
  retention_policy = ""
  write_consistency = "any"
  timeout = "5s"
  ## Uncomment and set credentials if authentication is required.
  username = "ntp"
  password = "022510"

## Plugin for Chrony time synchronization data
[[inputs.chrony]]
  server = "udp://localhost:323"
  timeout = "5s"
  metrics = ["tracking", "activity", "serverstats", "sources", "sourcestats"]
  socket_group = "chrony"
  socket_perms = "0660"

## CPU metrics: monitors CPU usage across cores and total CPU usage.
[[inputs.cpu]]
  percpu = true
  totalcpu = true
  fielddrop = ["time_*"]

## Memory metrics: tracks usage, free memory, and swap.
[[inputs.mem]]

## Disk metrics: monitors disk usage; ignore temporary filesystems.
[[inputs.disk]]
  ignore_fs = ["tmpfs", "devtmpfs"]

## Disk I/O metrics: measures read/write activity.
[[inputs.diskio]]

## Network metrics: captures network interface statistics.
[[inputs.net]]

## System metrics: collects system-level stats such as uptime and load.
[[inputs.system]]

## Optional: Raspberry Pi specific temperature metric.
[[inputs.exec]]
  commands = ["vcgencmd measure_temp"]
  name_override = "rpi_temp"
  interval = "10s"
  timeout = "5s"
  data_format = "value"
  data_type = "float"

9. Testing and Verification

After rebooting your Pi to apply all changes, verify that everything is working correctly.

9.1. Check Chrony Status

Run the following command to check Chrony’s synchronization status:

chronyc tracking

A sample output might look like:

Reference ID    : 50505300 (PPS)
Stratum         : 1
Ref time (UTC)  : Sat Mar 29 05:35:12 2025
System time     : 0.000000086 seconds fast of NTP time
Last offset     : +0.000000175 seconds
RMS offset      : 0.000000729 seconds
Frequency       : 1.945 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.012 ppm
Root delay      : 0.000000001 seconds
Root dispersion : 0.000019379 seconds
Update interval : 16.0 seconds
Leap status     : Normal

This output indicates that Chrony is synchronizing correctly with your GPS/PPS source.

9.2. Verify NTP Sources

To view the status of the NTP sources, run:

chronyc sources -v

This command displays the current state of all NTP sources, including the GPS and PPS inputs along with fallback servers.

10. Configuring PTP as a Grandmaster Clock

To use your Raspberry Pi as a PTP grandmaster clock, you’ll need to run the PTP daemon (ptp4l) in master mode so it broadcasts precise timing over the network. Follow these steps:

10.1. Install Linux PTP

Make sure the Linux PTP package is installed:

sudo apt update
sudo apt install linuxptp

10.2. Create a PTP Configuration File

Create a configuration file for ptp4l (for example, /etc/linuxptp/ptp4l.conf) with the following content:

[global]
masterOnly 1
priority1 128
priority2 128
logAnnounceInterval 1
logSyncInterval 0
logMinDelayReqInterval 0
time_stamping hardware

Key Points:

  • masterOnly 1: Forces the PTP daemon to run in grandmaster mode.
  • time_stamping hardware: Uses the Ethernet interface’s hardware timestamping for best accuracy.
  • The priority settings help determine your device’s preference as a master if multiple grandmasters exist.

10.3. Start ptp4l in Grandmaster Mode

You can test the configuration by running:

sudo ptp4l -f /etc/linuxptp/ptp4l.conf -i eth0 -m

Replace eth0 with the name of your Ethernet interface if different.

10.4. (Optional) Create Systemd Service Files

To ensure ptp4l starts automatically at boot as a grandmaster, create a systemd service file.

Create the ptp4l Service File

Create /etc/systemd/system/ptp4l.service with the following content:

[Unit]
Description=PTP Grandmaster Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/sbin/ptp4l -f /etc/linuxptp/ptp4l.conf -i eth0 -m
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

Reload the systemd daemon and enable the service:

sudo systemctl daemon-reload
sudo systemctl enable ptp4l.service
sudo systemctl start ptp4l.service

11. Verifying the PTP Grandmaster Status

After configuring ptp4l in master mode, you can verify that your Raspberry Pi is functioning as the PTP grandmaster.

11.1. Use the PMC Tool

The pmc utility helps check PTP status. Run:

sudo pmc -u -b 0 'GET CURRENT_DATA_SET'

A successful output might look similar to:

s o m t f e e f a p s n s e P R t a e F t m r h o o D v m e e M l d a a s y t e r 0 0 2 0 0 0 . 0
  • stepsRemoved: Should be 0, indicating the device is not acting as a slave.
  • offsetFromMaster: Near 0 when operating as a grandmaster.
  • meanPathDelay: Shows the average network delay measured.

11.2. Monitor ptp4l Logs

Check the service logs for any errors:

sudo journalctl -u ptp4l.service -f

This helps ensure that ptp4l is correctly broadcasting time over your network.

12. Conclusion

In this guide, you learned how to build stratum‑1 NTP server by:

  • Installing essential packages like chrony, gpsd, telegraf, and others.
  • Configuring the kernel and boot settings to enable GPS and PPS support.
  • Setting up gpsd for interfacing with the GPS module.
  • Configuring Chrony to use both GPS and PPS for time synchronization, with the additional benefit of PTP hardware timestamping.
  • Using Telegraf to monitor system and synchronization metrics, sending data to an InfluxDB instance.
  • NTP Service: Using Chrony to provide precise NTP time synchronization with GPS/PPS support.
  • PTP Grandmaster: Running ptp4l in master mode to offer precise time distribution via PTP on the local network.

By following these steps, you can build a highly accurate time server that not only ensures precise network synchronization but also offers detailed monitoring of its performance.

Happy time syncing!

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