If you’ve ever used a self-parking system, then the car was probably fitted with ADAS ultrasonic sensors, fitted over the wheel wells and on the front and rear bumper covers. This type of sensor is also used for the more common “parking assist” function which tells you how close you are to other objects when you park.
Ultrasonic sensors work by bouncing high-frequency sound waves off objects near the car (like radar sensors, except that radar uses electromagnetic waves, not sound waves). The on-board computer uses the reflected waves to instantly calculate the distance of surrounding objects and alert the driver to potential points of impact.
An advantage of ultrasonic sensors is that they do not require calibration. However, they are designed to be fitted in very precise positions in the bumper cover or bodywork over the wheel well. This is why most automakers do not approve the use of aftermarket or reconditioned body parts for repairs if they have to accommodate ultrasonic sensors.