Electromagnetic Parking Sensors
Rather than bouncing sound signals off obstacles, an electromagnetic system creates an electrostatic force-field around the rear of the car, in much the same way some car alarm systems work, and when this force-field is interrupted by the physical mass of an object whether it's a person, car or bollard, the system will sound a warning. Furthermore it can sound the warning depending on the extent of the intrusion, and therefore distance to the object, with beeps of increasing frequencies; much like the more traditional ultrasonic parking sensors.
Protecting Your Car from Bumps & Scrapes
Please note however that electromagnetic parking sensors only alert when an object is getting closer, so you only get alerts when moving and if you stop reversing, the alerts stop. Secondly this is not an easy solution for a car-DIY amateur. You will need some knowledge to remove the bumper to fit these alarms. These parking sensors are more suitable for situations where drilling the bumper is not an option and for those who are reasonable at parking but need more guidance at closer range, as the warning starts at a shorter distance. These are excellent where parking is very tight and every inch counts.