TCAS
flyingwsiyig
I can only talk for the Skywatch that I've got in my plane.
It generates secondary radar interrogation pulses which
trigger local transponders in other aircraft to respond as
if the trigger were from a radar installation.
The Skywatch has a pair of aerials which triangulate the
incoming responses, stir in the response signal time
of flight, to get a picture of the aircraft (with transponders)
in the immediate volume of the source aircraft.
Over time (a few seconds) the TCAS builds up an idea
of where everyone is going and whether any aircraft
are on closing paths with the source aircraft.
It then issues alerts to the pilot to allow them to take
avoiding action.
Airline quality kit will talk to the TCAS in the other aircraft
to ensure that their respective avoiding actions will not
make the situation worse - as long as the crew obey the
TCAS instructions...
-- Andrew