What frequency does the TCAS system interrogate on?

The TCAS (Traffic Alert and Collision Avoidance System) primarily utilizes a frequency of 1030 MHz for its interrogations. This frequency is specifically allocated for active mode operations, where TCAS units on an aircraft transmit signals to interrogate transponders in nearby aircraft. The choice of 1030 MHz is essential for ensuring effective communication and response from transponders, enabling TCAS to receive necessary information regarding the proximity and altitude of other aircraft.

The 1090 MHz frequency is related to the transponder reply signals, which means it is involved in the communication process but is not the interrogation frequency. The other frequencies mentioned, 920 MHz and 850 MHz, are not relevant to TCAS operations and are not used in the aviation industry for this purpose. Thus, recognizing that TCAS specifically interrogates at 1030 MHz provides insight into its operational framework and how it maintains air traffic safety.