Refer to figure.
Today’s higher traffic densities and greater speed differences have generated a need for an Airborne Collision Avoidance System.

The system is designed to provide an additional margin of safety and keep commercial aircraft clear of conflict, independently of Air Traffic Control. An aircraft must carry a transponder and have the facility to interrogate other aircraft transponders.

TCAS II detects intruders in the TCAS aircraft’s vicinity, assesses the collision risk and presents warnings to the crew in the form of TAs and Resolution Advisories (RAs).

“Climb” “Increase Climb” “Descend” “Increase Descent” “Monitor Vertical Speed” “Decrease Climb” “Decrease Descent”

RAs offer manoeuvring advice in the vertical plane to resolve conflict.

If your aircraft and the intruder both have Mode S data link transponders the system will co-ordinate the RAs to provide complementary vertical avoidance instructions.