Mode S and TCAS

[FlyingForFun]

Imagine the followign hypothetical situation:

Two aircraft are on respective courses which will result in them becoming very close to each other. One is a public transport aircraft, fitted with TCAS. The second has a Mode S transponder, but not TCAS (for example, a General Aviation aircraft of the near future, if Mode S is made mandatory).

Will the first aircraft's TCAS be able to give a RA? Or will it just give a TA, exactly as if the second aircraft just had Mode C?

Thanks!

FFF
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[ETOPS]

Just like to correct an apparent misunderstanding here. TCAS equipped aircraft will receive RA's when in conflict with non TCAS traffic so long as that aircraft has a functioning mode "C" transponder. If mode "C" is not present then an TA will be given.

The full TCAS operating environment has the added advantage of RA's being co-ordinated where both aircraft are equipped but the system still funtions if only one aircraft is. That is why ATC sometimes get low level VFR traffic to stop mode "C" if they pass under a busy approach or get the TCAS aircraft to select TA only..........

[RadarContact]

Plus a mode S transponder wouldn't really make sense without the appendant TCAS-system since there would be no coordinated RA's, which are the only real advantage of the mode S...

[FlyingForFun]

TCAS equipped aircraft will receive RA's when in conflict with non TCAS traffic so long as that aircraft has a functioning mode "C" transponderOh dear. I'd better dig those books out and do some revision then, because that's not the way I recall it from my studies!

That minor mistake aside, though, it seems I am correct in thinking that having Mode S would give no advantage, from a TCAS point of view, over having Mode C.

Thanks.

FFF
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[RadarContact]

Well, obviously the aircraft with mode C only will get no warning of the other traffic.
The aircraft equipped with mode S will receive
- traffic advisory (TA) for targets equipped with mode A upwards,
- resolution advisorys for targets equipped with mode C upwards
- coordinated RAs for targets with mode S (coupled with a functional TCAS that is).

To get back to your original question there would be no benefit to neither aircraft if one was equipped with mode S but no TCAS rather than mode C only, since this would make the coordination function of mode S useless (you won't ever get to know "your" part of the resolution), basically making it a more expensive mode C model.