The traffic alert and collision avoidance system (TCAS) helps the flight crew maintain safe air traffic separation from other ATC transponder equipped airplanes. TCAS is an airborne system and operates independently of the ground-based ATC system.
TCAS sends interrogation signals to nearby airplanes. These airplanes which are equipped with an air traffic control radar beacon system (ATCRBS) transponder or an air traffic control (ATC) mode S transponder respond to these interrogations. TCAS uses these response signals to calculate the range, relative bearing, and altitude of the responding airplane. If a responding airplane does not report altitude, TCAS cannot calculate the altitude of that airplane. Airplanes tracked by TCAS are called targets.
Using the information from the response signals and altitude of own airplane, TCAS calculates the relative movement between own airplane and the target. TCAS then calculates how close the target will be to own airplane at the closest point of approach (CPA).
Targets are classified as one of these four types depending on the separation at CPA and the time it will take until CPA occurs:
· Other traffic
· Proximate traffic
· Intruders
· Threats.
Each type of target has a different symbol on the display.
If the separation at CPA is within certain limits, TCAS provides advisory messages to the flight crew. TCAS provides two levels of advisories to the flight crew, traffic advisory (TA) and resolution advisory (RA). The type of advisory is determined by a combination of altitude, the time to CPA, and the separation at CPA. The TA shows for relatively longer times to CPA and relatively larger separation at CPA and is for intruder targets. The RA shows for relatively shorter times to CPA and relatively smaller separation at CPA and is for threat targets.
The TA shows the range, bearing, and relative altitude (if relative altitude is known) of the intruder target. The RA also gives visual and aural commands to the flight crew to make sure there is safe vertical separation from the threat target. TCAS also communicates with other airplanes that have TCAS to coordinate the flight movement to prevent a collision.