Thanks, but I am still confused.
Here in Oz most Transponders are Mode C capable - it's just that some of the pilots arent! (I put it down to training and telling them what ALT means on the KT76 or whatever.)
The situation we usually get is aircraft on descent to an IAF for the ILS, say thru 5 or 6000 FT, and getting traffic on a Mode A target. Traffic passed, not much else to say.
What I am wondering is why that same Mode A target does not become traffic for an overflying or departing aircraft at say 10000FT? i,e,, as discussed in the above posts, if the TCAS unit does not know the ALT of the intruder, how does it filter it out? And yes, I have asked the TCAS aircraft if anything observed on TCAS - "Negative".
Thanks for the replies on the over/under/normal. Not sure if this applies to the aircraft in question (mainly 737/A320/A330/767/747), although we also have a few DHC8-400's.