What is the frequency range of the marker beacon system used to indicate an aircraft's position during an Instrument Landing System (ILS) approach to an airport's runway?

• A. The outer, middle, and inner marker beacons' carrier frequencies are unique for each ILS equipped airport to provide unambiguous frequency-protected reception areas in the 329.15 to 335.00 MHz range.
• B. The outer marker beacon's carrier frequency is 400 MHz, the middle marker beacon's carrier frequency is 1300 MHz, and the inner marker beacon's carrier frequency is 3000 MHz.
• C. The outer, the middle, and the inner marker beacon's carrier frequencies are all 75 MHz but the marker beacons are 95% tone-modulated at 400 Hz (outer), 1300 Hz (middle), and 3000 Hz (inner).
• D. The outer, marker beacon's carrier frequency is 3000 kHz, the middle marker beacon's carrier frequency is 1300 kHz, and the inner marker beacon's carrier frequency is 400 kHz.

Answer: (D)

Marker beacons used with an ILS rely on a single, fixed carrier frequency: 75 MHz. All three markers (outer, middle, inner) share this same 75 MHz carrier, and they are differentiated by their audio tone modulations: 400 Hz for the outer marker, 1300 Hz for the middle marker, and 3000 Hz for the inner marker. So when you’re asked about the frequency range, it’s not a range of frequencies but a single carrier at 75 MHz, with the marker identity conveyed by the distinct tones. The other options mix in frequencies in the hundreds or thousands of kilohertz or megahertz, which do not match how marker beacons operate.