How Do Parking Sensors Work?
Reverse parking sensors are available using one of two different types of technology; Ultrasonics or Electromagnetics.
Sound Wave Parking Sensors
Ultrasonics is the use of sound waves to detect the presence of obstacles as the emitted sound reflects off surfaces behind. Ultrasonic systems use a number of sensors, much like speakers, which transmit a cone of sound which is not spherical but generally at angles. When these sound waves bounce off other cars and parking hazards behind you they are reflected back to the sensors which can then calculate the time it took for the sound wave to travel out and back again and by comparing against the speed of sound, approximately 770mph, they can then give you a reading of the distance travelled to the object which is how much room you have left behind you to safely reverse.
Electromagnetic Parking Sensors
Electromagnetism works by creating an electromagnetic field behind the car which is then monitored to see if this field is disturbed. As you reverse, physical masses such as cars or lamp posts will distort the electromagnetic field and the location of this distortion can be calculated to give you a reading to the distance to this interference. This is the same principle as used in microwave car alarms, although they just sensor for a disturbance rather than using it to calculate a distance.
Parking Safety Precautions
When using parking sensor systems to park, regardless of the technology you go for we would still suggest that you use them as a guide in addition to your own sight and judgement rather than rely on them without also looking where you are going. This is because the technology still relies on eyesight to ensure the path ahead is clear.
The speed of sound is not actually a fixed number and varies. The standard speed given is that as measured at sea level at approximately 21°C (70°F) as it varies according to temperature, pressure and the medium the sound is passing through. Sound travels faster through water for example. In reality this means the technology cannot be used to measure down to the last centimetre and so ultrasonic parking sensor systems will generally give the warning to stop reversing at about 40cm, giving you time to react and a margin to account for the variation in the speed of sound.
Electromagnetic Reversing Accuracy
The electromagnetic technology driven systems can be more accurate in this respect and will generally let you get down to less than 10cm from an object before giving you the warning to stop reversing. In very inclement weather however, such as sleet or snow, this can give rise to the odd false reading if its heavy enough to distort the electromagnetic field generated. The ultrasonic systems keeps firing out lots of pulses of sound every second and ignores the disruption from such weather.
As electromagnetic systems are constantly reporting a change in the terrain behind a vehicle, if you stop reversing with one of these systems then the unit will stop alerting you. This is because since you are stationery there will be no change in the location and distance of objects behind you - unless someone walks past etc. As soon as you start moving again, however slowly, it will start alerting again. This is slightly different to the way in which the ultrasonic systems works since they are calculating the distance to objects behind they will still alert you to the distance even when you're stationary.