Instrument Landing System (ILS)

Instrument Landing System (ILS) 

An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow.

An instrument landing system (ILS) consists of two independent sub-systems, one providing lateral guidance (Localizer), the other vertical guidance (Glide slope or Glide Path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.

 Localizer: 

A localizer is one of the components of an Instrument Landing System (ILS), and it provides runway centerline guidance to aircraft. In some cases, a course projected by localizer is at an angle to the runway (usually due to obstructions around the airport). It is then called a Localizer Type Directional Aid (LDA). Localizers also exist in stand-alone instrument approach installations and are not always part of an ILS. The localizer is placed about 1,000 feet from the far end of the approached runway. 

Two signals are transmitted on one out of 40 ILS channels between the carrier frequency range 108.10 MHz and 111.95 MHz (with the 100 kHz digit always odd). One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each the left of the runway center line, the other to the right. Antenna transmits a narrow beam, one slightly to the left of the runway center line, the other to the right.

 Specifics of the Localizer:

1. The localizer antenna is located at the far end of the runway.
2. The approach course of the localizer is called the front course.
3. The course line in the opposite direction to the front course is called the back course.
4. The localizer signal is normally usable 18 NM from the field.

5. The Morse code Identification of the localizer consists of a three-letter identifier preceded.  

Glide slope:

A glide slope (GS) or glide path (GP) antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 329.15and 335 MHz using a technique similar to that of the localizer. The centerline of the glide slope signal is arranged to define a glide slope of approximately 3° above horizontal (ground level)The beam is 1.4° deep; 0.7° below the glide slope centerline and 0.7° above the glide slope center line.

Marker beacons:

Marker beacons are used to alert the pilot by audio and visual cues. It gives the distance from threshold point on the extended center line of the runway, at a particular height. ILS contain three marker beacons: inner, middle and outer. The inner marker is used only for Category II operations. The marker beacons are located at specified intervals on the extended center line. All marker beacons operate on a frequency of 75 MHz.