The answer is ACAS, which means airborne collison avoidance sytem. But today we are going to mention about TCAS (Traffic Collision Avoidance System) that is using on commercial airliners.
The traffic alert and collision avoidance system (TCAS) helps the flight crew
maintain safe air traffic conditions with other ATC transponder equipped
TCAS sends interrogation signals to nearby airplanes. In traffic conditions, there is an airplane which response the interrogation signals.
TCAS uses these response signals to calculate the range, relative bearing, and altitude of the responding airplane. 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
If the separation at CPA is within certain limits, TCAS provides advisory
messages to the flight crew. TCAS also communicates with other airplanes that have TCAS to coordinate the flight movement to prevent a collision.