The air traffic control (ATC) ground stations interrogate the airborne ATC system. The ATC transponder replies to the interrogations in the form of coded information that the ground station uses.
The ATC transponder also replies to mode S interrogations from the traffic alert and collision avoidance systems (TCAS) of other airplanes or ground stations.
When a ground station or a TCAS computer from another airplane interrogates the ATC system, the transponder transmits a pulse-coded reply signal. The reply signal identifies and shows the altitude of the airplane.
· Maximum airspeed
· TCAS coordinating signals
· TCAS status
· Altitude
· Mode A identity code
· Twenty-four bit address.