Transponder requirement confusion

In article qAPEg.4109$SZ3.3107@dukeread04, Ray Andraka
wrote:

That's correct. Mode C only transmits either the squawk code or the
encoded altitude, depending on the specific interrogation.

Mode 3A provides the code, 3C provides the altitude.

,
TCAS is an active system that sends an interrogation and listens for the
reply. It uses the time delay between the interrogation it sends and
the reply it recieves back to accurately measure range. Many use an
antenna array and beamforming techniques to determine azimuth as well.

TCAS II systems MUST use at least one directional antenna (the top one).
Normally both the top and bottom ones are directional.

--
Bob Noel
Looking for a sig the
lawyers will hate