In article . com,
wrote:

wrote:
john smith wrote:

but a TCAS II equipped aircraft can see you.

Only if you have a Mode-S transponder.

Nope, all they need is mode C.

Actually, they don't even need mode C, now that I think about it.
You get different levels of service depending on what the target has.

If they've got Mode A you'll get an callout.
If they've got Mode C then you'll get an RA (climb / descend)
If they've got Mode S and they've also got TCAS, the two TCAS's
will boogie with each other to coordinate the RA.

So really mode S by itself does nothing for TCAS II unless you've
also got a TCAS as well.

My understanding is that the Mode-S provides the communication
information. Mode C may contain altitude, but there is no distance or
direction information.