Why don't parking sensors interfere with each other?

[jamieb]

The parking sensors on the MS and other manufacturers' cars appear very similar, suggesting to me that they're made by only a handful of vendors. As I understand it, they are small sonar units that emit a pulse of ultrasound and listen for an echo, a modern version of the big sonar rangefinder on the old Polaroid SX-70 camera (I'm dating myself).

Sitting in traffic behind another car with an array of similar parking sensors tonight, I got to wondering how they all don't interfere with each other (both between different cars and even among the different sensors on one car causing cross-interference).

I couldn't find a decent explanation online. Frequency hopping? I thought I'd post this (admittedly geeky) question to my fellow geeks on TMC to see if someone knows the answer who can explain.

 

[woof]

They DO interfere with each other...but that can be worked around. The link below has a good explanation as to how they work and what it takes to work around interference:

Because of spurious noise in the environment it's best to double verify objects instead of relying on a single ping. There is lots of junk in the environment the generates ultrasonic energy. Therefore, ping the object a second time to verify it's location. The odds that a noise source being seen at the exact same distance are slim.

http://www.instructables.com/id/Hacking-Automotive-Ultrasonic-Sensors/step3/Decoding-the-Results/

So in essence, send out ping, await the echo. The time from ping to echo is distance (based on speed of sound). Then do it again (after a random interval). If the time is the same the second time, odds are the object is really there. If the original "echo" came from another sensor that just happened to be sending a ping (i.e. the detected noise was not an echo of the transmitted ping) then sending another ping should result in a different response the second time. If one randomizes the transmit interval, then two systems right next to each other should rarely interfere, even when on the same frequency.

Fancier systems can modulate a unique code into the ultrasonic audio transmitted, and thus can tell their own echo as it contains that code. That's unlikely in automotive grade equipment. Systems like this: http://pub.uni-bielefeld.de/luur/download?func=downloadFile&recordOId=2288504&fileOId=2407281

 

[jamieb]

Thank you! Great explanation and links.