What is the main function of a transponder in the context of TCAS?

The main function of a transponder in the context of Traffic Alert and Collision Avoidance System (TCAS) is to relay collision avoidance information. Transponders are devices installed in an aircraft that send out information regarding the aircraft's identity, altitude, and velocity. When integrated with TCAS, these transponders allow for the detection of nearby aircraft and the assessment of potential collision threats.

TCAS utilizes the data from transponders of other aircraft to determine their proximity and relative altitude. This enables TCAS to provide pilots with timely advisories to avoid mid-air collisions. As such, the ability to relay vital collision avoidance information makes the transponder an essential component of the TCAS functionality, enhancing flight safety.