Data Collection for Radar-Camera Calibration

Calibration Target:

Our current method for radar-camera calibration uses a checkerboard along with a trihedral corner reflector as the calibration target.

Please note that:

1. The checkerboard can be of any size and can have different internal corners. The checkerboard cannot be made of metal such as aluminum because it will block radar signals. Other non-metal materials such as foam board, wood, or cardboard is fine.

2. The trihedral corner reflector can be of any length.

Eg: You can print the attached pdf file on a foam board at 1.0 m x 0.6 m. Most print shops can print this. It has 5 internal corners horizontally and 9 internal corners vertically. Each square size is 10 cms. The distance from the left-most corner is 10cms, the distance from the right-most corner is 10 cms, the distance from the top-most corner is 10 cms, the distance from the right-most corner is 10cms.

https://drive.google.com/file/d/1mTR8HTpvROE1Pv0rmXEBVLSxs_yMDnvf/view?usp=sharing

The trihedral corner reflector is taped to the back of the checkerboard, as shown in the image below.

Note: The tip of the trihedral corner reflector must be in line with the internal corner of the checkerboard that is exactly in the middle of the pattern.

Data for radar-camera calibration:

Place the checkerboard at roughly 3 m - 10 m from the camera. For the closest position, the closer, the better, but it should be far enough so that all the edges of the board are visible from the camera and radar. For radar that can detect elevation, the height of the calibration target should be varied above/below the radar level. Otherwise, keep the calibration target at a levelled ground. The checkerboard should not be occluded in the camera or radar view.

The size of the checkerboard squares should be about 10 cm. The side of the squares must be parallel to the edge of the checkerboard.

The target and all sensors should be static while collecting the data. In order to avoid time-synchronization problems, please keep the board and the sensors stationary for at least 10 seconds while collecting each set of calibration data.

For example, these are the steps to collect one set of calibration data:

1. Orient the camera and radar toward the calibration target. Start recording. Wait for 10 seconds (Don't move/rotate your car/robot/sensors). Stop recording. You must have a recording of images and radar data for 10 seconds. Extract one image from the camera and one frame of radar data captured 5 seconds after recording has started (e.g. if you start recording at 3:00:00, you stop recording at 3:00:10. We need a frame captured at 3:00:05) and save them.

2. Change the checkerboard's location and orientation. Start recording. Wait for 10 seconds (Don't move/rotate your car/robot). Stop recording. Again, you must have a recording of images and radar data for 10 seconds. Extract one image from the camera and one frame of radar data captured 5 seconds after recording has started and save them.

3. In the same manner, collect data for different positions of the calibration target.

Note: To achieve good results, collect at least 6-7 pairs of image and radar data.

Each calibration dataset supports only one pair of radar-camera sensors. To calibrate a new pair of radar-camera sensors, follow the above steps and create a new dataset.