What is Instrument Landing System? How does it work?

The ILS gives lateral and vertical guidance during an approach to a runway. Therefore pilots use this system whenever he makes a landing at their destination airport.

The ILS consists of a localizer, glide slope, and a marker beacon.

The localizer ground station transmits the signals to give the lateral deviation from the runway centerline.

The glide slope ground station transmits the signals to give the vertical deviation from the glide slope down to the runway.

It is a system that helps the pilot for precise landing, especially in poor visibility conditions. It consists of 2 subsystems:

Localizer (LOC):

  • It ensures that landing on the airport center line;
  • It provides lateral guidance.

Glide slope (G/S):

  • It ensures that landing in the 1st third of the airport;
  • It provides vertical guidance.

Operating Frequency Range:

  • LOC band: 108.00 to 111.00 MHz (odd numbers);
  • VOR band: 108.00 to 111.00 MHz (even numbers)

(Common band between ILS & VOR – 50 kHz channel spacing)

Glideslope band: 329 to 335 MHz with 150 kHz channel spacing.

No. of ILS systems: There are 2 ILS systems on most aircraft.

Marker beacon.

The marker beacon system is a radio navigation aid that determines the distance between the aircraft and the runway threshold.

The system provides visual and aural indications of the passage of the aircraft over the marker transmitters on the ground.

How does Instrument Landing System work?

ILS RX antenna (glide slope and localizer)

  • 90 Hz & 150 Hz directional waves transmit from the ILS antenna;
  • They are in phase;
  • The pilot tries to reach the centerline on the runway;
  • The Rx measures the relative strength of the two audio signals;
  • Selecting the LOC frequency will automatically select the glideslope frequency.

ILS Indication using marker beacon. How does a marker beacon work?

  • MKR beacon system consists of 3 transmitters on the ground (outer, middle, and inner markers) and a receiver at the aircraft.
  • The three marker transmitters on the ground are at specific distances from the runway threshold.

All marker stations transmit on a radio frequency carrier of 75 MHZ. The thing that distinguishes one from the other is the audio tone frequency, which modulates the carrier frequency (AM modulation).

  • The outer marker transmits 400 HZ tone;
  • The middle marker conveys 1300 HZ tone;
  • The inner marker transmits 3000 HZ tone

The system operates when the aircraft passes through the marker beam and the modulating frequency is detected. Then, the system provides aural and visual indications to the flight crew to indicate passage over a marker beacon transmitter. Appropriate light will flash, and the pilot can hear a notification audio.

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