OK - what the hell is Mode A + C without Altitude reporting?
OK, both mode A and mpde C replies contain a some pulses, the timing of which marks whether the reply is mode A or Mode C. These also serve to provide a frame inside which data can be sent.
So a transponder without Altitude Reporting can still reply to a mode C interrogation by sending the pulse frame. The pulse frame itself will just not contain any data. But since there
is a response, a radar can do the time delay related to speed of light calculation to work out the range.
Since TCAS want to know the altitude of the intruder it needs to interrogate Mode C. It has no interest in the 4096 ident, so to avoid congestion on the SSR frequencies it
does not interrogate Mode A , at all.
The basic problem here is that when folks say they are mode A, what they usually mean is "Mode A + C without Altitude reporting". When they say they are mode C what they mean is "Mode A + C with Altitude reporting".
Note the 2nd and 3rd quotes in mr ripleys post. There is no mention of A or C, only altitude reporting or non-altitude reporting.
OK, some might say I'm being picky. Certainly the distinction between mode C and altitude reporting is one that many people do not follow, but if you are going to go delving into the guts of TCAS technology understanding the difference will make it easier to understand.
Strictly speaking, references that say TCAS can give a TA against mode A are incorrect.
CPB