Coordinate system#

Overview#

The commonly used coordinate systems include the world coordinate system, the vehicle coordinate system, and the sensor coordinate system.

The world coordinate system is a fixed coordinate system that defines the physical space in the environment where the vehicle is located.

The vehicle coordinate system is the vehicle's own coordinate system, which defines the vehicle's position and orientation in the world coordinate system.

The sensor coordinate system is the sensor's own coordinate system, which is used to define the sensor's position and orientation in the vehicle coordinate system.

How coordinates are used in Autoware#

In Autoware, coordinate systems are typically used to represent the position and movement of vehicles and obstacles in space. Coordinate systems are commonly used for path planning, perception and control, can help the vehicle decide how to avoid obstacles and to plan a safe and efficient path of travel.

Transformation of sensor data

In Autoware, each sensor has a unique coordinate system and their data is expressed in terms of the coordinates. In order to correlate the independent datas between different sensors, we need to find the position relationship between each sensor and the vehicle body. Once the installation position of the sensor on the vehicle body is determined, it will remain fixed during running, so the offline calibration method can be used to determine the precise position of each sensor relative to the vehicle body.
ROS TF2

The TF2 system maintains a tree of coordinate transformations to represent the relationships between different coordinate systems. Each coordinate system is given a unique name and they are connected by coordinate transformations. How to use TF2, refer to the TF2 tutorial.

TF tree#

In Autoware, a common coordinate system structure is shown below:

graph TD
    /earth --> /map
    /map --> /base_link
    /base_link --> /imu
    /base_link --> /lidar
    /base_link --> /gnss
    /base_link --> /radar
    /base_link --> /camera_link
    /camera_link --> /camera_optical_link

earth: earth coordinate system describe the position of any point on the earth in terms of geodetic longitude, latitude, and altitude. In Autoware, the earth frame is only used in the GnssInsPositionStamped message.

map: map coordinate system is used to represent the location of points on a local map. Geographical coordinate system are mapped into plane rectangular coordinate system using UTM or MGRS. The map frame`s axes point to the East, North, Up directions as explained in Coordinate Axes Conventions.

base_link: vehicle coordinate system, the origin of the coordinate system is the center of the rear axle of the vehicle.

imu, lidar, gnss, radar: these are sensor frames, transfer to vehicle coordinate system through mounting relationship.

camera_link: camera_link is ROS standard camera coordinate system .

camera_optical_link: camera_optical_link is image standard camera coordinate system.

Estimating the `base_link` frame by using the other sensors#

Generally we don't have the localization sensors physically at the base_link frame. So various sensors localize with respect to their own frames, let's call it sensor frame.

We introduce a new frame naming convention: x_by_y:

x: estimated frame name
y: localization method/source

We cannot directly get the sensor frame. Because we would need the EKF module to estimate the base_link frame first.

Without the EKF module the best we can do is to estimate Map[map] --> sensor_by_sensor --> base_link_by_sensor using this sensor.

Example by the GNSS/INS sensor#

For the integrated GNSS/INS we use the following frames:

flowchart LR
    earth --> Map[map] --> gnss_ins_by_gnss_ins --> base_link_by_gnss_ins

The gnss_ins_by_gnss_ins frame is obtained by the coordinates from GNSS/INS sensor. The coordinates are converted to map frame using the gnss_poser node.

Finally gnss_ins_by_gnss_ins frame represents the position of the gnss_ins estimated by the gnss_ins sensor in the map.

Then by using the static transformation between gnss_ins and the base_link frame, we can obtain the base_link_by_gnss_ins frame. Which represents the base_link estimated by the gnss_ins sensor.

References:

https://www.ros.org/reps/rep-0105.html#earth

Coordinate Axes Conventions#

We are using East, North, Up (ENU) coordinate axes convention by default throughout the stack.

X+: East
Y+: North
Z+: Up

The position, orientation, velocity, acceleration are all defined in the same axis convention.

Position by the GNSS/INS sensor is expected to be in earth frame.

Orientation, velocity, acceleration by the GNSS/INS sensor are expected to be in the sensor frame. Axes parallel to the map frame.

If roll, pitch, yaw is provided, they correspond to rotation around X, Y, Z axes respectively.

Rotation around:
  X+: roll
  Y+: pitch
  Z+: yaw