What frequency does the TCAS system reply on?

The TCAS (Traffic Alert and Collision Avoidance System) operates primarily on a frequency of 1090 MHz for its transponder replies. This frequency is part of the secondary surveillance radar system used in aviation. TCAS utilizes two frequencies: it receives interrogations on 1030 MHz and responds on 1090 MHz. The 1090 MHz frequency is crucial because it allows the TCAS-equipped aircraft to communicate their altitude, position, and other relevant data back to ground systems and nearby aircraft, facilitating situational awareness and enhancing safety in air traffic management.

The frequency of 1030 MHz is used for receiving signals, not replying, and other options such as 1200 MHz and 950 MHz do not align with the standards used in TCAS operations. Thus, the correct identification of 1090 MHz as the reply frequency underscores its importance in ensuring ongoing communication between aircraft and ground systems to maintain safe distances and prevent collisions in busy airspaces.