It's been done, and is in widespread use by gliders (just not in the
US or Canada, thanks to lawyers). It's called FLARM. Won't keep that
Airbus from hitting you, however...

One of the problems with transponders is that they don't tell you
where other traffic is - they just tell others where you are. So you
still need TCAS or TPAS capablility if you like being actively
involved in avoiding a collision.

Bottom line - make yourself visible (xponder helps), know where the
threat is likely to be (TPAS helps) and look out the window (clean
canopy helps).

Kirk
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