Oz Lander writes:

OK, so TCAS reads transponder codes. I thought it was like a radar.

TCAS I (the kind often used on small aircraft) reads Mode C replies from other
transponders and builds a picture of the airspace from that, with moderate
accuracy. TCAS II (the kind mandated on large aircraft) also listens to other
TCAS systems to build a much more accurate picture of the airspace (if lots of
other aircraft are similarly equipped).

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