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Time Synchronization Setup and Verification

This document describes how to configure GPS PPS on a Raspberry Pi and verify end-to-end time synchronization across the system, from GPS to Chrony, PHC, and PTP.

Raspberry Pi PPS Configuration

Configure the PPS GPIO line by appending the following to /boot/config.txt on the Raspberry Pi:

# GPS PPS configuration
dtoverlay=pps-gpio,gpiopin=18
enable_uart=1
init_uart_baud=9600

Reboot the Raspberry Pi after making these changes.

Time Synchronization Verification Steps

Follow the steps below in order to verify that time synchronization is functioning correctly.

1. Start IMU Driver and Acquire GPS Lock

Launch the sensor platform and wait until the IMU has a valid GPS lock and UTC time.

ros2 launch sensor_bringup sensor_platform_launch.py

2. Verify NMEA Topic Publishing

Confirm that NMEA messages are being published in ROS 2.

ros2 topic echo /nmea

3. Verify UDP NMEA Output and gpsd Input

Ensure the ROS node is subscribing to the NMEA topic and forwarding UDP NMEA strings (including constructed GPRMC sentences).

Check that gpsd is running and receiving raw NMEA data:

sudo systemctl status gpsd.service
gpspipe -r

4. Verify Chrony Is Using GPS Time

Confirm that gpsd is correctly feeding time to Chrony.

gpsmon
cgps
chronyc sources

Chrony should show GPS as a valid time source.

5. Verify PPS Discipline of Chrony

Check that the PPS signal is active and disciplining Chrony.

sudo ppstest /dev/pps0
chronyc sources

The PPS source should appear with low offset and high precision.

6. Verify PHC Discipline via phc2sys

Ensure that phc2sys is disciplining the Ethernet hardware clock using the system clock (already disciplined by Chrony).

sudo systemctl status phc2sys
sudo phc_ctl eth0 cmp

The offset between the system clock and the PHC should be minimal.

7. Verify PTP Distribution to External Devices

Confirm that ptp4l is running and disciplining downstream devices using the hardware clock.

sudo systemctl status ptp4l
curl -i -X GET http://<lidar_ip_address>/api/v1/time/ptp
curl -i -X GET http://10.42.0.28/api/v1/time/ptp

The device should report an active PTP lock and synchronized time.

8. Start the Lidar Driver

ros2 launch sensor_bringup lidar_launch.py

9. Check the timestamps of the messages

bash scripts/monitor_topics.sh

Summary

This setup establishes the following time chain:

GPS → PPS → Chrony (system clock) → PHC (phc2sys) → PTP (ptp4l) → External devices

If any step fails, verify that all upstream components are correctly synchronized before proceeding downstream.

Recording Data

Steps:

1. Mount the drive

Check for drive then mount it with -t flag if not not sda1.

lsblk -f # look for drive

bash scripts/mount_drive.sh # -t /dev/sdb1

2. Check the time sync

bash scripts/monitor_topics.sh

3. Record data

bash scripts/record.sh

Enter the name of the folder This will copy the config folder into the bag once the recording is complete

4. Unmount the drive

bash scripts/mount_drive.sh -u