An aircraft transponder answers an interrogation by transmitting a reply pulse train signal back to the ground station at a frequency of

• A. 1030 MHz
• B. 1215 MHz
• C. 1090 MHz
• D. 915 MHz

Answer: (C)

In secondary surveillance radar, the ground station interrogates the aircraft on 1030 MHz, and the aircraft transponder replies on 1090 MHz with a pulse train that carries the identification and altitude data. This separate reply frequency (1090 MHz) is what distinguishes the transponder’s response from the uplink interrogation. The other options aren’t the transponder’s reply: 1030 MHz is the interrogation frequency, not the response; 1215 MHz and 915 MHz aren’t the designated SSR reply frequencies. Therefore, 1090 MHz is the correct reply frequency.