Sensor Suite

Interchagable Payload Unit

Sensing suite is desinged as an interchagable playload unit between Bell 412 Advanced Research Systems Aircraft (ASRA) and DJI M600 hexacopter drone platforms. The payload unit is capable of running visual-inertial-LiDAR odometery and mapping algorithms real-time while collecting data.

Sensors

IMU

The IMU publish the following ROS topics.

Message Type	Topic Name	Rate [Hz]	Description
sensor msgs/Imu	/imu/data	400	raw data
sensor msgs/Imu	/imu/data stamped	400	raw data with trigger hardware timestamp
geometry msgs/Vector3Stamped	/imu/mag	100	raw magnetometer data
sensor msgs/TimeReference	/imu/time_ref	400	hardware timestamp at IMU trigger
sensor msgs/TimeReference	/imu/time_ref_cam	20	hardware timestamp at camera trigger
sensor msgs/TimeReference	/imu/time_ref_pps	1	hardware timestamp at PPS receive

Cameras

The two cameras are publising the following ROS topics.

Message Type	Topic Name	Rate [Hz]	Description
sensor msgs/Image	/camera/image_mono	20	Mono8 format down camera image
sensor msgs/Image	/camera/image_color	20	RGB8 format down camera image
sensor msgs/Image	/front_camera/image_mono	20	Mono8 format front camera image
sensor msgs/Image	/front_camera/image_color	20	RGB8 format front camera image

LiDAR

LiDAR publish the following ROS topics.

Message Type	Topic Name	Rate [Hz]	Description
sensor msgs/PointCloud2	/velodyne_points	10Hz	raw velodyne pointcloud
sensor msgs/TimeReference	/time_ref_scan	10Hz	hardware timestamp at pcd trigger

GNSS Receriver

GNSS module publishes the following ROS topics.

Message Type	Topic Name	Rate [Hz]	Description
sensor msgs/NavSatFix	/fix	5	raw RTK-GNSS
sensor msgs/NavSatFix	/fix_ppk	5	PPK GNSS
nmea msgs/Sentence	/nmea_sentence	5	nmea formatted GNSS
nav msgs/Odometry	/fix_frl	5	6DOF pose ground-truth

GNSS Base Station

Reach RS2 Based station is used during data capture. The real-time NMEA stream for the simpleRTK2B was provided by Reach RS2 base station. RS2 communicates with the ZED-F9P using a ultra high frequncy (UHF) LoRa radio. The Reach RS2 also provides a post processed kinematics (PPK) solution upto 100km by logging data at 10Hz.

Jetson Xavier AGX GPU

The Xavier is running on Ubuntu 18.04 and NVIDIA L4T 32.3.1 with Cuda 10.1 support, and ROS Melodic is installed on it. Additionally, the Xavier board is equipped with a 1GB SSD to store the ROS bags and a Wi-Fi module for data visualization purposes.

Hardware Time-Synchronization Scheme

The IMU publishes a $/imu/time\_ref$ topic, including the hardware timestamp of the IMU’s internal clock for the published data. The point cloud acquisition hardware time and the camera trigger pulse hardware timestamp are pub- lished on $/time\_ref\_scan`$ and $/imu/time\_ref\_cam`$ topics, respectively. The timestamp of the received PPS message from the GNSS receiver is published on the $/imu/time\_ref\_pps`$ topic. These hardware timestamps can be used to accurately time-sync sensor messages. The block diagram of the hardware synchronization is shown below.