GPS/GNSS Synchronization DELAY

[HaroldMurcia]

Describe your question
Hello,
In the current configuration with my LiDAR I want to synchronise several sensors. The LiDAR is taking the timestamp directly from the GPS/GNSS with the PPS and serial-NMEA lines. Then I use rosbag record /ouster/metadata /ouster/imu_packets /ouster/lidar_packets and process the generated bag file with ouster/replay.launch in my computer.

On the computer a chrony service synchronises the clock with the same GNSS clock using gpsd with a second serial connection. When I determine the time difference (online) between the ouster received messages and the ROS time, there is a time difference of < 0.05 secs. approximately:

ros::Subscriber ouster_sub = nh.subscribe("/ouster/points", 10, &MyROSNode::ousterCallback, this);

void ousterCallback(const sensor_msgs::PointCloud2ConstPtr& pointcloud_msg) {
        int64_t last_lidar_timestamp_ = pointcloud_msg->header.stamp.toNSec();
        int64_t last_lidar_ros_timestamp_ = ros::Time::now().toNSec();
        int64_t LiDAR_rostime_timestamp = last_lidar_timestamp_ - last_lidar_ros_timestamp_;
        . . .

However, when evaluating the rosbag imu/linear_acceleration and imu/twist responses from the /ouster/imu rostopic with respect to other sensors in my system (rostime based), I notice a time difference around -0.27 secs

1. Any idea how to correct or reduce this time difference ?
2. Is PTP a better option, considering that my on-board GPS is SERIAL connection? (The example configuration is for an ethernet GPS grandmaster clock.

Screenshots
/ouster/imu/twist/z = red signal

/ouster/imu/linear acceleration = green signal

Platform:

• Ouster Sensor= OS1-128 REV 7
• Ouster Firmware Version=v3.0.1
• Programming Language= C++, ROS1 Noetic
• Operating System= Linux Ubuntu 20.04
• Machine Architecture= x64

Catkin makefile:

all : debug release

debug:
        catkin_make --cmake-args -DCMAKE_BUILD_TYPE=Debug

release:
        catkin_make --cmake-args -DCMAKE_BUILD_TYPE=Release -DCATKIN_WHITELIST_PACKAGES="ouster_ros"

Roslauch:

<include file="$(find ouster_ros)/launch/sensor.launch"> 
            <arg name="sensor_hostname" value="os-122313000337.local"/> 
            <arg name="udp_profile_lidar" value="RNG19_RFL8_SIG16_NIR16_DUAL"/> 
            <arg name="lidar_mode" value="2048x10"/> 
           <arg name="timestamp_mode" value="TIME_FROM_SYNC_PULSE_IN" /> 
            <arg name="viz" value="false"/>
            <arg name="tf_prefix" value=""/>
 </include>

get_sensor_info {"prod_line": "OS-1-128", "prod_pn": "860-105010-07", "prod_sn": "122313000337", "image_rev": "ousteros-image-prod-bootes-v3.0.1+20230209044733", "build_rev": "v3.0.1", "build_date": "2023-02-09T04:49:03Z", "status": "RUNNING", "initialization_id": 1629346}

get_time_info {"timestamp": {"time": 1696851520.9933181, "mode": "TIME_FROM_SYNC_PULSE_IN", "time_options": {"ptp_1588": 1651175671, "sync_pulse_in": 1696851520, "internal_osc": 7923}}, "sync_pulse_in": {"locked": 1, "polarity": "ACTIVE_LOW", "diagnostics": {"last_period_nsec": 0, "count": 7923, "count_unfiltered": 7056}}, "multipurpose_io": {"mode": "INPUT_NMEA_UART", "sync_pulse_out": {"polarity": "ACTIVE_HIGH", "frequency_hz": 1, "angle_deg": 360, "pulse_width_ms": 10}, "nmea": {"locked": 1, "polarity": "ACTIVE_LOW", "ignore_valid_char": 0, "baud_rate": "BAUD_9600", "leap_seconds": 0, "diagnostics": {"decoding": {"utc_decoded_count": 6291, "date_decoded_count": 6010, "not_valid_count": 281, "last_read_message": "GPRMC,113840.00,A,4333.7201301,N,00128.6298792,E,0.255,230.1,091023,0.0,E,D*37"}, "io_checks": {"start_char_count": 6742, "char_count": 508001, "bit_count": 1469122, "bit_count_unfiltered": 1469121}}}}}