When the TCAS interrogates a target, the reply signal from the target usually contains the airplane altitude and any information requested by TCAS.
The TCAS computer uses the round-trip time of the reply signal to calculate the range and range rate of the target. It uses the direction of the reply to calculate bearing of the target. If the TCAS computer receives altitude data, it calculates these parameters of the traffic airplane:
· altitude
· altitude rate
· relative altitude.
The TCAS computer uses the target data to calculate if the target is on a possible collision or near collision course with its flight path. If it is, the TCAS computer calculates traffic coordination maneuver data. This data and the position data go to the DEUs to make the TCAS displays. If TCAS advises that a maneuver be made it also sends an aural command to the flight crew.