nkawtg said:
The S in Mode S X-ponders stands for "selective" so groundstations (ATC) can interrogate the X-ponder onboard flying Aircraft, retrieving data like Flt Lvl, Grndspd, Squawk, A/C type without any fltcrew action. TCAS is also using data but the main goal of mode S was reducing ATC comms and stay well informed in highly crowded airspaces.
Close. Groundspeed is computed by the ATC computers based on the radar return or transponder return. Mode S provides ATC ground stations and other aircraft altitude, discrete address and transponder code. The discrete address is the aircraft ID, normally its side number. When providing TCAS II Resolution Advisory escape maneuver recommendations, the Mode S transponder will also tell the other aircraft it's planned avoidance maneuver (i.e. climb or descent). The other aircraft's TCAS II than gives it's pilots the opposite maneuver (very simplified version of the events). The TCAS II / Mode S transponders continue to coordinate escape maneuvers until the aircraft are satisfactorily deconflicted. ATC does not participate in this conflict resolution and it's computers do not get any of the advisory alerts or escape recommendations. However the pilot is required by FAR to follow this guidance, we just tell ATC over the radio either while it's happening or after it's over. From what I have been able to gather from talking to ATC folks, they don't really care if an aircraft has Mode C or S. Mode S gives a little better altitude readout and allows them to ID aircraft if there is a violation. That's about it and it really doesn't effect their day-to-day controlling. The FAA mandates Mode S because of the TCAS II requirement.
There was supposed to be a follow on TCAS III where the systems would add the horizontal plane to the escape maneuvers. The Mode S data link is capable of transmitting this info too with the proper software change. So far this has not happened.
ATC's goal with Mode S was originally a system where they could provide headings, altitudes, etc over the data link vice voice. It will be a long time before this ever happens if at all.