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This must work on the same principle as my garage light system which tells the wife to stop before going too close to wall but my system works with lights...
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An alignment aid in a garage, like the safety system which stops a garage door from closing if something is in the way, might be based on a
light source and sensor in a fixed location: when something "breaks the beam" between them, that's the signal that the designated location has been reached.
The backup warning systems which have become common as new-car options (the $300 referred to in the Odyssey forum) are set of ultrasonic proximity sensors: they send out ultrasonic "pings" and listen for the echo, just like a crude sonar system. They can measure how far the object is from the sensor, and the alarm is just a comparison with a set distance. You could have a reading (in centimetres or inches or whatever) from these things if you wanted... they're not triggered by a very specific location.
The product described here looks like an ultrasonic proximity system.