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Instrument Landing System
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Pilots use various radio aids to help guide them to the runway for landing, particularly in poor weather or other low-visibility conditions. Collectively, these aids are referred to as the Instrument Landing System (ILS). ILS utilizes 75 MHz, 108.1-111.95 MHz, and 329.15-335.0 MHz. According to NTIA, "there is international agreement within the International Civil Aviation Organization (ICAO) establishing the ILS as a standard landing system, and the ILS is used extensively worldwide."
Localizer
The localizer helps establish the proper horizontal path for an approach to the runway. The localizer transmission consists of two signals, one modulated with a 90 Hz tone and transmitted with a beam pointed along the left side of the approach, and another modulated with a 150 Hz tone and transmitted with a beam pointed along the right side of the approach. The plane's ILS receiver compares the strength of the two modulated tones and can determine whether the plane is aligned too far left (too much 90 Hz tone), too far right (too much 150 Hz tone), or along the center of the runway (equal strengths for both tones).
The localizer signal is useful for a distance of approximately 18 nm from the runway.
The localizer and glide slope frequencies are paired, so that the pilot need only select one, and the other is set automatically. The localizer channels are in the 108.1-111.95 MHz band. Note that the localizer channels are interspersed with channels for VHF Omnidirectional Range (VOR) signals in this band. VORs are used for enroute navigation, as opposed to precision navigation to the runway. See the related links for a full list of the ILS channel plan.
Glide Slope
The glide slope indicator works similarly to the localizer signal, but instead of indicating proper horizontal position, it indicates the proper vertical path to the runway (typically a 3 deg slope down to the runway). The 90 Hz tone is transmitted pointed above the proper path, while the 150 Hz modulated signal is pointed below the proper path. Comparison of the strength of the two tones informs the ILS receiver (and pilot) whether the plane is on the correct vertical path.
The glide slope signal is useful out to a distance of about 10 nm from the runway.
The glide slope and localizer frequencies are paired, so that the pilot need only select one, and the other is set automatically. The glide slope channels are in the 329.15-335.0 MHz band.
See the related links for a full list of the ILS channel plan.
Marker Beacons
The Instrument Landing System (ILS) marker beacons are located at varying distances along the approach to a runway to indicate the approximate distance to the runway. Marker beacons are typically used when an airport does not have Distance Measurement Equipment (DME) capabilities.
Outer markers are located between about 4-7 miles from the end of the runway. The antenna system, typically two yagis in a V configuration with the open part of the V pointing upwards, creates a narrow vertical beam that the pilots receive when they fly over. The outer marker transmits an AM signal at 75 MHz with a 400 Hz modulated tone.
The middle marker is typically about 2000 ft from the end of the runway, and transmits a 1 kHz modulated tone. The middle marker beacon is often a simple three-element yagi pointed straight up.
The inner marker is typically 700-800 ft from the end of the runway and transmits a 3 kHz modulated tone.
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Frequencies |
| Frequency | Bandwidth | Use | Service | Table |
| 75 MHz | 400 kHz | Instrument Landing System outer, middle, and inner markers | Aeronautical Radionavigation | - |
Frequency Bands |
| Band | Use | Service11 | Table |
| 108.1 - 111.95 MHz | Instrument Landing Systemn localizer signal | Aeronautical Radionavigation | - |
| 329.15 - 335 MHz | Instrument Landing System glide slope signal | Aeronautical Radionavigation | - |
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