Which of the following is not a function of TCAS?

The closest point of approach is not a direct function of the Traffic Alert and Collision Avoidance System (TCAS). Instead, TCAS primarily focuses on providing pilots with information about nearby aircraft to help prevent mid-air collisions. Its core functions include issuing traffic advisories, which alert pilots to nearby aircraft, and resolution advisories, which give specific instructions on how to maneuver to avoid potential collisions.

Vertical speed monitoring is also part of TCAS's capabilities, as it assists in determining whether vertical maneuvers are necessary for safe separation from other aircraft. In contrast, while TCAS may indicate relative proximity to other aircraft, it does not explicitly calculate the closest point of approach, which is a concept used more in flight planning and situational awareness rather than a specific function of TCAS.