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ILS (Instrument Landing System) Basics

instrument landing system ils aviation navigation aircraft

The ILS, or Instrument Landing System, is an essential avionic system used as a navigational aid for aircraft approach and landing. As the name suggests, it’s an instrument landing system, meaning it guides aircraft using instruments, allowing for precise landings even in low visibility conditions.

The ILS system provides aircraft with guidance along a straight flight path for landing. It’s primarily utilized in Instrument Flight Rule (IFR) precision approaches, guiding aircraft from the Final Approach Fix (FAF) all the way down to the touchdown point.

The system is designed to ensure an ideal landing by guiding the aircraft to the precise intersection of:

  • The runway centerline
  • The Localizer (LOC) beam
  • The Glide Slope (GS) beam

ILS,Instrument Landing System

How ILS Works

The ILS operates with the help of three primary transmitting components:

  • LOC-Tx (Localizer Transmitter): This system provides horizontal alignment with the runway centerline. Think of it as guiding the aircraft left or right.
    • Operates in the VHF band (108 to 112 MHz) with 20 channels.
    • Provides horizontal guidance.
    • Has a range of approximately 40 km (25 miles).
  • GS-Tx (Glide Slope Transmitter): This system ensures a fixed descent rate. It essentially tells the aircraft how steeply to descend.
    • Operates in the UHF band (329 to 335 MHz) with 20 channels.
    • Provides vertical guidance.
    • Has a vertical range of approximately 1 km (0.6 miles).
  • MB-Tx (Marker Beacon Transmitter): These provide crew indication of the aircraft’s specific location along the approach path. Think of them as checkpoints.
    • Operates in the VHF band at 75 MHz.
    • Provides horizontal guidance with discrete signals at specific distances from the runway.

Advantages of ILS

  • ILS is a highly effective system for providing precise landing guidance, particularly in poor weather.

Disadvantages of ILS

  • LOC and GS signals can be distorted (“bent”) due to site and terrain effects, impacting accuracy.
  • GS signals are susceptible to interference from snow, moisture on the airport grounds, etc.
  • The number of available frequency channels (20) for LOC and GS use is limited.
  • Installation and maintenance can be expensive.