The aircraft transponder replies to ground interrogations at what frequency?

The aircraft transponder replies to ground interrogations at what frequency?

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

Answer: (D)

In secondary surveillance radar, the interrogation and the transponder reply use different frequencies to keep the system clear and unambiguous. The ground-based interrogator sends its signal at 1030 MHz, and the aircraft transponder replies on 1090 MHz with encoded data such as identity (Mode A), altitude (Mode C), or the selective address/data of Mode S. This separation prevents the transmitter on the aircraft from interfering with the ground receiver and lets air traffic control match replies to the correct aircraft. Therefore, the transponder replies at 1090 MHz. The other frequencies listed are not the standard transponder reply frequency for SSR.

In secondary surveillance radar, the interrogation and the transponder reply use different frequencies to keep the system clear and unambiguous. The ground-based interrogator sends its signal at 1030 MHz, and the aircraft transponder replies on 1090 MHz with encoded data such as identity (Mode A), altitude (Mode C), or the selective address/data of Mode S. This separation prevents the transmitter on the aircraft from interfering with the ground receiver and lets air traffic control match replies to the correct aircraft. Therefore, the transponder replies at 1090 MHz. The other frequencies listed are not the standard transponder reply frequency for SSR.