Speaking of ADS-B transponders, the guy in the next cube from me at the office is a pilot and has one that is a little ~2" cube he can attach to his iPad. Amazing what the non-commercial pilots have at their fingertips these days in portable electronics.
On a similar concept, in the VHF marine bands you will find AIS, a ship based packet broadcast intended to reduce collisions and near misses on the high seas, and around ports.
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