Originally Posted by
11K-AVML
Thanks punkalouver, but I have to side with Ziggys post; TCAS uses Mode C for communication and can also interpret the presence of Mode A codes in an a/c's vicinity.
TCAS does not interrogate in Mode A. It uses Mode C, even if the target transponder is incapable of encoding altitude (and the content of the response is therefore empty).
BTW, I think you misunderstood punkalouver's post. TCAS II can detect Mode A/C transponders (by interrogating using Mode C) but the aircraft using the TCAS II must have Mode S.