What is Instrument Landing System & Its Working

In 1929, an Instrument Landing System (ILS) was designed to guide the airline pilots during precision and landing approaches under poor visibility conditions. It provides horizontal, vertical, course, and altitude guidance to the pilot to execute a precise runway approach to land. It is an instrument-based precision runway and landing approach with two radio transmitter beams. This article gives a brief description of the working of the Instrument Landing System and its operating categories.

What is Instrument Landing System?

The ILS is a standard ICAO ( International Civil Aviation Organization) precision landing assistance that is utilized to provide precise guidance descent and azimuth signals that assist the aircraft to land on the runway under adverse and normal weather conditions.

It navigates the pilot to the runway in IFR conditions with high accuracy. This electronic system provides the necessary horizontal and vertical assistance to the aircraft with a precision approach. The Instrument Landing System diagram is shown below.

Instrument Landing System
Instrument Landing System

It consists of a localizer, glides path, and marker beacons. Both glide path and localizer are located at the aircraft nose. They receive and transmit the signals to the ILS indicator in the cockpit.

The VHF signal is transmitted by the ground localizer antenna in the opposite direction of the runway to guide the aircraft horizontally to the centreline of the runway.

The UHF signal is transmitted by the ground glide path/glide slope antenna in the vertical direction. It assists the aircraft vertically to the touchdown point.


These transmitted signals activate the horizontal and vertical needles of an instrument landing system indicator (ILS). This assists the pilot to run the aircraft either up/down or to left/right.

When both the needles are kept in the center position, the pilot guides the aircraft down to the end of the landing runway that is aligned with the runway centerline. This moves the aircraft towards the touchdown point.

Instrument Landing System Block Diagram

A ground-based radio beam transmitter that guides the pilot during the aircraft’s approach and landing on the runway, is called an Instrument Landing System (ILS). The receiver frequency of the approaching aircraft is tuned to the frequency of the ILS to receive guidance from it. The below figure illustrates the ILS block diagram.

Instrument Landing System or ILS
Instrument Landing System or ILS

Working of Localizer Component (LOC)

The Localizer component of the ILS transmits two signals that are overlapping at the center and can operate at the VHF range of 108MHz to 117MHz. The on-track signal is provided by the overlap area.

During the aircraft’s approach towards the runway centerline, the modulation frequency from the left side will be 90Hz and the right side will be 150Hz.

To align the aircraft to the runway centerline, the difference in depth of modulation is considered. The old Amplitude Modulation (AM) is used by all the ILS transmitters.

Working of Glide Path or Glide Slope (GP/GS)

The glide path is the vertical antenna that generates two signals in the vertical plane. It is fixed at one side of the runway of about 300m to the runway end. It operates in the UHF range of 329.15MHz to 335MHz.

The modulation frequency of the upper (aircraft approach to runway is too high) will be 90Hz and the lower will be 150Hz.

The horizontal needle of the instrument landing system indicator (ILS indicator) is the difference in depth of modulation. Hence, the pilot directs the aircraft to go down.

Working of Marker Beacons

Marker beacon is one of the ILS components, which operates at a frequency of 75MHz. When it receives the transmission, the indicator is activated on the instrument panel of the pilot. When the signal is received in an aircraft, the height of it should be precise. It helps to indicate the distance between the aircraft and the runway.

Instrument Landing System Components

If all the Instrument Landing System components are available together with an approved approach mechanism, then the precision approach is executed by the pilot.

The ILS components are of two categories. They are Ground Installation components and Airborne equipment.

The ground installation component consists of the localizer, glide path, Marker Beacons, DME, approach Light system, Runway Visual Range.


It is the main component of the ILS, which provides lateral guidance. The antenna and the transmitter are located on the centerline at the runway opposite end from the approach threshold.

Glide Path Or Glide Slope

During the approach, this component provides vertical guidance to the pilot. It is positioned at 750 to 1250 feet down to the runway from the threshold as shown in the above instrument landing system diagram. Offset 400 to 600 feet from the midline of the runway.

Markers Beacons (MB)

This component of the instrument landing system is divided into two types. They are,

OM (Outer Marker)

If the outer marker is installed, then it is positioned at 3 to 6 NM from the threshold within 250 feet of the extended centerline of the runway. Thus, it helps the pilot to create a positive position fix on the localizer. It provides distance, height, and functioning of equipment to check the intermediate and final approach of aircraft.

MM (Middle Marker)

If the middle marker component of the ILS is installed, then it is located at nearly 0.5 to 0.8 NM distance from the threshold on the extended centerline of the runway.

DME (Distance Measuring Instrument)

This instrument landing system component is located with a glide path to provide slope/slant distance to the aircraft regarding touchdown point.

Approach Lighting System

There are several lighting systems, which work as an essential part of the Instrument Landing System (ILS) to assist the pilot in landing the aircraft. The lighting systems that can provide this facility are ALS (Approach Lighting System), SFL (Sequenced Flash Lighting), TZD (Touchdown Zone Lighting), and CLL (Centreline Lights needed for operations of Category II and III).

Runway Visual Range

To land the aircraft easily, the pilot should have the ability to observe the relevant visual aids before the arrival at the DH (Distance Height) or the MAP (Missed Approach Point)

There are two types of equipment for Airborne ILS equipment.

  • LOC / LLZ (Localizer) and GP (Glide Path) are located on the nose of the aircraft. While the aircraft indicator is inside the cockpit.
  • Marker Beacons indicators and antennas are also inside the cockpit

Instrument Landing System Categories

The instrument landing system categories of precision approach and landing operations are divided based on the applied DH (Decision Height/Altitude) and visibility range or RVR (Runway Visual Range). There are 3 categories of ILS precision approach and landing approach, which are shown in the following table.

  • Category DH (Decision Height) Runway Visual Range (RVR) Visibility Range
  • CAT I 200 ft (not lower than 60 m) Not <550 m 800m
  • CAT II 200 ft (lower than 60m), 100 ft (not lower than 30 m) Not <350 m –
  • CAT IIIA 100 ft or lower than 30m or NO DH Not <200 m –
  • CAT IIIB 50 ft or lower than 15m or No DH Less than 200 m but not less than 50 m –
  • CAT IIIC No DH No limitation of RVR –
  • For conducting an autoland aircraft, category D is used with a Runway Visual Range of 300m.
  • CAT II and CAT III operations of instrument precision approach and landing operations are not allowed unless the information of RVR is obtained.
  • The altitude specified in the instrument precision approach is represented in height, which is above the runway threshold elevation. This is called Decision Height (DH), which aids the pilot to decide or continue the approach or to operate the missed approach.


A few advantages of Instrument Landing System are as follows

  • This type of system approach is very accurate
  • It is a landing aid system that is used by airlines.
  • It is used to help the pilot during the landing and approach of the aircraft.
  • It helps the pilot to watch the runway and the airports when there is finite visibility.
  • It provides a precision final approach and runway touchdown approach to the aircraft.
  • It helps to increase situational awareness and safety measures.
  • It guides to the aircraft runway in both vertical and horizontal planes.


The disadvantages of the Instrument Landing System are given below

  • Due to the localized, glide slope beams moving objects, vehicles, and other big reflecting objects produces interference
  • This generated interference reduces the directional signal’s strength.

Thus, this is all about an overview of Instrument Landing System (ILS) definition, Diagram, Block Diagram, Components, Categories, Advantages, and Disadvantages. The purpose of ILS is to guide the pilot safe landing under any weather conditions. The signals of ILS are very essential to enable the Auto-Land in aircraft. Here is a question for you, “What are the limitations of the Instrument Landing System?”