AVL Focus - Issue 2023

THE AVL MOBILITY TREND MAGAZINE - No. 1 2023

automated and connected mobility

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N O . 1 2 0 2 3

THE PROCESS FOR GROUND TRUTH SENSOR

CALIBRATION

Three process steps are necessary for calibrating a GT ref-

erence system consisting of cameras, LiDARs and GNSS

(Global Navigation Satellite System):

1.) Intrinsic camera calibration: Removes the distortion

in the camera picture resulting from the specific “lens

– aperture” combination of the camera

2.) Extrinsic calibration of all sensors of the reference

system: This calibration step defines the orientation

and the position of each sensor in the GT reference

system with regard to one reference coordinate sys-

tem (in our case the central LiDAR).

3.) Extrinsic calibration of the reference system with re-

spect to the vehicle coordinate system: As the AVL GT

system is exchangeable between arbitrary vehicles in

the shortest time (thanks to plug-and-play function-

ality), a calibration routine is requested that matches

the reference coordinate system of the GT system to

that of the vehicle.

THE AVL SENSOR CALIBRATION FACILITY

For achieving the requirements and process steps men-

tioned above, the decision for a calibration bench (facto-

ry calibration) with defined target objects has been taken

in an early stage of development. In comparison to on-

line and self-calibration routines the achievable accura-

cy is higher and more revisable due to the well-known

target objects and their defined positions. Additional-

ly, a monitoring algorithm supervises the validity of the

sensor calibration during vehicle fleet operation. For in-

trinsic camera calibration and extrinsic sensor calibration

(step 1 and 2) the GT system (rooftop box) is mounted

on a turntable and turned through a defined scenery of

target objects (checkerboards).

Intrinsic camera calibration is carried out with respect to the known liter-

ature based on checkerboards with 7 rows and 10 lines. As quality criteria

for the intrinsic calibration routine a reprojection error of < 0.4 pixel at a

target object coverage rate between 70 % – 80 % is defined.

The “indirect” extrinsic calibration of the sensors using target objects is

necessary, as sensor production tolerances have to be considered. Thus a

“direct” measurement of the sensor position and orientation using a coor-

dinate measuring machine (CMM) is not feasible. For a pairwise matching

of the sensor’s coordinate systems (= extrinsic calibration) to the central

LiDAR (= reference coordinate system) the normal vectors for each de-

tected checkerboard are being calculated. A numerical optimization algo-

rithm aligns these normal vectors for each sensor pair and computes ro-

tation and translation matrix accordingly. In this way the extrinsic sensor

calibration of the four cameras as well as the two side LiDAR to the cen-

tral lidar are carried out.

GT SYSTEM PLUS ACCURATE CALIBRATION ENABLES PLUG-

AND-PLAY ADAS/AD VALIDATION AND HOMOLOGATION

AVL’s Ground Truth reference system provides a highly precise 360° view

of the environment around the vehicle. The sensor system comes with

a­ccurate sensor calibration. This offers a great advantage compared to

chassis-­mounted reference sensors and also provides plug-and-play func-

tionality to our customers. Together with AVL’s worldwide on-site pres-

ence and network of experts our customers receive the best support in their

global data recording and sensor validation campaigns.  

Extrinsic calibration routine: Numerical optimization

algorithm aligns the two normal vectors (e.g. red

from central lidar, blue from front camera)