Landedoutagain,
Contrary to previous replies - you do NOT need mode S for TCAS II to work, you will get RA's if both aircraft are fitted with mode C transponders only.
You don't need mode S in the intruder. You do need it in the TCAS aircraft.
What you get with Mode S is complementary RA's, ie, there is no possibility of both aircraft being given the same instruction.
Not true either. It's TCAS in both aircraft that provides this. If aircraft 1 is Mode S + TCAS and aircraft 2 is just Mode S, the RA's won't be complementary... because there will only be one of them.
Under Mode C only, it is possible for both aircraft to be given a climb or descent, but its rare and even then will resolve itself quickly as when the relative positions change one of the aircraft will then change its command.
No. If the intruder is non TCAS (but altitude reporting) then an RA could be generated for the TCAS aircraft that coincidentally happened to be in the same sense as a spontaneous manoeuvre by the intruder. In this case the RA would update.
I wonder what the source of your information is because you seem to have the wrong end of the stick about the architecture of this system.
In me experience such confusion is usually caused by teaching material that does not clearly show whether it is talking about the equipment fit for the TCAS aircraft or the equipment fit for the intruder.
pb