How Do Reverse Parking Sensors Work?

Looking closely at the reverse indicator on a gear shift housing.

If you have wondered how reverse parking sensors work, you are not alone. These little technological marvels allow you to back your vehicle out of a driveway or parking space with more confidence and make driving in reverse safer in general. They detect objects in your vehicle's path and emit a sound or beep that lets you know you need to stop and check behind you before proceeding. And, like most technology, it is much easier to add a sensor to your car or troubleshoot and repair one if you know what you're getting into.

How They Work

Parking sensors are labelled by various vehicle manufacturers under many different brand names, such as Park Distance Control, Park Assist, Parktronic, or EPS, but they only come in two varieties: ultrasonic and electromagnetic.

Ultrasonic Sensors

These types emit radio or ultrasonic waves that bounce off of objects behind the vehicle, much like some animals use echolocation. The returning waves are then registered and analyzed by a computer inside your vehicle. By measuring the time it took the wave to return to the sensor as well as any changes in the length or spectrum of the wave, the computer can determine the size and relative proximity of the object behind you. If the wave returns quickly, then the computer knows that something is close behind the vehicle and emits an alarm signal. Most computers and sensors are actually set up to not register signals that indicate an object is more than a certain distance away (usually five to 15 meters depending on the way the system is set up).

Since this system relies on the use of sound waves, there are cases when the sensor cannot properly detect objects behind you. For example, some surface types can interfere with how sound waves reflect, or, if an object is too narrow or small to reflect sound waves, your sensor will not see it. Also, since an ultrasonic system works by using four to six individual sensors mounted on the outside of the car's bumper, there are cases in which blind spots and excess dirt and grime on the sensors themselves will contribute to a lack of detection.

Electromagnetic Sensors

As indicated by the name, these sensors operate using electromagnetic waves. A transceiver strip generates an elliptical near field behind the car, and when objects that meet a certain mass requirement disturb that field, a detector picks up the voltage change and sends the information to a computer in the car. The computer analyzes the data to determine the distance to the object and then alerts the driver with a series of tones that grow progressively louder or faster as their vehicle gets closer to whatever is behind it.

Most, if not all, of these sensors mount on the inside of your vehicle's bumper, so there is no issue with dirt interfering with the system's detection capability. These are also the more discreet design; however, they may prove to be a more complicated install for some DIYers.

Detecting Moving Objects

Some parking sensors can detect moving objects as well, but they will react in much the same fashion regardless. In the case of an ultrasonic system, once a sensor receives a signal or wave that the computer determines is close, all sensors on the rear of the vehicle will then send a signal or wave simultaneously. Depending on which sensor receives the returned wave, the computer can determine if the object is moving or not. However, the system will still sound the alarm the entire time the object is within the sensory area.

Electromagnetic sensors will also register a moving obstruction in the same manner as a non-moving one if it's close enough. These sensors actually only react to objects approaching the bumper, so fixed accessories like tow bars don't perpetually set off the alarm, so a moving object, like a person, will set off the alarm until it leaves the field if it is in close enough proximity.