I believe TCAS use different frequencies too.
TCAS is an add-on to the ATC SSR transponder. That had various modes that could give info to the ATC operator such as altitude (Mode C) and there were unused Modes that were for 'further developement'. The interrogator is still on 1030MHz and the replies on 1090MHz.
When TCAS was in the development stages, a lot of aircraft were going from dead reckoning and ground-based radio aids to INS and GPS and ultimately a combination of all navigation inputs. (RNAV) It was a comparatively easy job to put the aircraft's position into Mode S. Then all you had to do was to get the transponder to react to interrogation from not only radar but from other aircraft. So the interrogating aircraft sends out his signal via a pair of omni-directional antenna and receives replies from nearby aircraft on another pair of directional "crossed dipoles" antenna. The computer has a heap of information to decode now - altitude, position, bearing, other aircraft's flight number and has to decide if the approaching aircraft is a threat - if so it displays it's position and utlimately advises the pilot of suitable avoiding action. If there is no threat, nothing is displayed. Now we throw EGPWS into the mix and really get complicated!
So all I want to do is receive the aircraft's replies which contain all that data and display the position, altitude and flight number.
Can't be that hard, can it?
Yes, it can!