How does a car reversing radar determine distance?

[McJayden]

Car reversing radar determines distance by: judging the distance through sound. The alarm emitted by the reversing radar is a beeping sound at a certain frequency, and the frequency increases as the obstacle gets closer. When the beeping frequency becomes faster and is accompanied by a continuous sound, it indicates that the vehicle is getting closer to the obstacle. The reversing radar mainly consists of ultrasonic sensors, a controller, and a buzzer. Its functions are: 1. Accurately measuring the distance between the rear of the car and the obstacle; 2. Emitting rapid warning sounds to alert the driver to brake when reversing to a dangerous distance; 3. Repeatedly emitting voice warnings to remind pedestrians to pay attention.

[AmirFitz]

I previously studied the principle of reverse parking sensors, which actually rely on ultrasonic distance measurement. The radar probe emits ultrasonic pulses, similar to how we can hear echoes when shouting into a cave. When the ultrasonic waves hit an obstacle behind the vehicle and reflect back, the controller calculates the distance by multiplying the time difference between sending and receiving the sound waves by the speed of sound. Typically, it emits over 30 pulses per second, with different distances corresponding to different frequencies of beeping sounds: 1.5 meters triggers intermittent beeps, 1 meter changes to continuous beeping, and within 0.5 meters, a long warning sound is emitted. If encountering sloped obstacles or thin railings, the reflected waves may weaken, leading to inaccurate distance measurement. In such cases, it's safer to rely on the rearview mirror.

[LeDelaney]

The beeping sound you hear when reversing is the radar at work! Each black dot on the rear of the car is an ultrasonic sensor, emitting ultrasonic waves like a bat's echolocation. Recently, I noticed an interesting phenomenon: when mud splashes onto the sensor in rainy weather, it can trigger false alarms. Once, my car displayed an obstacle within 1 meter, but when I got out to check, there was actually more than half a parking space left. The mechanic told me that high-quality radars can filter out interference signals from raindrops. Factory-installed radars typically use 40kHz high-frequency ultrasonic waves, which are much more accurate than the 20kHz ones used in aftermarket shops. The distance measurement error is generally within 5cm, but low obstacles below 30cm can easily be overlooked.

[MaddoxRose]

Distance judgment relies entirely on the travel time of sound waves. The radar probe emits a 'beep' sound wave, which bounces back as a 'reply' sound upon hitting the rear wall, and the controller calculates this interval by timing. With the speed of sound at 340 m/s, a 0.01-second delay indicates an obstacle is 1.7 meters away. Modern radars are smarter—my car features dynamic guideline display, where the rear camera feed shows colored distance markers: the green safe zone is about 1 meter, while the red zone, down to 30 cm, signals it's time to brake. When applying a car wrap, ensure not to cover the probes, as it would muffle the echo like wearing ear muffs.