PCAS is a passive, transponder-based detection system.

Passive refers to the sensing method. Passive systems listen for replies from transponders to interrogations. In comparison, active systems send the interrogation to “spark” the replies from aircraft within the detection area. Passive systems are extremely effective in most areas because of the abundance of interrogations from other active systems. Ground-based RADAR and airborne TCAS systems are examples of active systems. PCAS is able to receive interrogation responses sparked by both of these methods. In fact, in an area of average coverage, PCAS rejects 50% or more of the incoming signals because of redundancy.