You've wandered a bit off the thread here but just to answer 411A's comment that not all TCAS equipped aircraft have mode 'S' transponders.
TCAS equipped aircraft have mode "S" Transponders, that's how TCAS works. The TCAS creates a 'protected area' around the aircraft by interrogating nearby transponders operating in modes A, C or S. The TCAS antennae are direction sensitive but are accurate to only about 10 degrees or so. In combination with the range data obtained from the transponder replies the TCAS Processor derives a closure rate, and determines the threat level. If the TCAS determines that a threat exists, it tries contacting the target aircraft via the mode 'S' transponder data link. If the target is also TCAS equipped the two TCAS cooperate via mode 'S' to calculate a joint avoidance manouvre with one TCAS advising a climb or hold and the other a descend or hold manouvre. If the target aircraft has only mode A/C then a cooperative response is not possible and TCAS assumes that the target aircraft will continue its present flight path. TCAS then computes an avoidance manouvre based on no cooperation from the target.
To emphasise. By definition, ALL aircraft that have TCAS have mode 'S' transponders.
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