TCAS transmits these two types of interrogation signals:
· Whisper-shout for ATCRBS transponders
· Mode S.
The TCAS finds and monitors all of the airplanes within range that have ATCRBS or mode S transponders. The TCAS operates with its mode S transponders to interrogate traffic airplanes and calculate if they are a threat. The TCAS does not track airplanes that do not have a transponder. The TCAS tracks airplanes with transponders that do not reply with mode C information. These airplanes send framing pulses in response to a mode C interrogation. The TCAS uses the framing pulse replies to give range and bearing to the target. TCAS cannot give RAs for targets that do not report altitude.
The TCAS can calculate maneuver commands to prevent
possible collisions with target airplanes.
If the traffic airplane also has TCAS, the two airplanes can agree on maneuvers that give a safe distance between them.