the instrument landing system (ILS)

In nearly 60 years of use ILS systems were never found at fault in an airplane Crash. This flawless record is a direct result of the training given to Navaid technicians. While training, technicians are told that the equipment cannot be allowed to operate outside the established tolerances. Any doubt and the equipment is shut down pending re-evaluation and recertification.

Brief description

The ILS usually consists of a Localizer, Glide Path, and Markers(OM, MM, & IM).

Localizer: This equipment provides lateral guidance to the runway centreline from about 5nm out.(five nautical miles).

Glide Path: This equipment provides the aircraft with a glide angle - usually 3 degrees. The Localizer and Glide Path combine to bring the aircraft to a point where the aircraft is 50 feet high at the runway threshold (decision point).

1. The Outer Marker at approximately 5nm helps the a/c adjust its course and height.
2. The Middle Marker is located at approximately 3500 feet and used similarly.
3. The Inner Maker at 1000 feet is used only for Category II operations.

There are always exceptions and here are some main exceptions.
1. DME & GP (Distance Measuring Equipment & Glide Path) when it is impossible to have Markers.
2. DME & Localizer when there is no GP for whatever reason.
3. Offset Localizer. In this case the Localizer is not on the runway centreline, but offset and lined up to bring the aircraft over the threshold at decision height. Decision height is 50 feet at threshold.

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The localizer is used to provide lateral guidance to the aircraft and thus allows for tracking the extended runway centreline. The localizer information is typically displayed on a course deviation indicator (CDI) which is used by the pilot until visual contact is made and the landing completed. The localizer radiates on a carrier frequency between 108 to 112 MHz with 50 kHz channel spacing. This carrier is modulated with audio tones of 90 Hz, 150 Hz, and 1020 Hz. The 1020 Hz tone is used for facility identification.

Normal limits of localizer coverage

  1. Glide Slope
    1. Frequency range: 329.15 - 335 MHz.
    2. Housed in a building next to the runway.
    3. 2.5 - 3 degrees above horizon.
    4. Used on front course only.
    5. Signal is 1.4 degrees wide.
    6. Automatically received with LOC frequency.
  2. Marker Beacons
    1. ILS - outer and middle.
    2. LOC BC -- at FAF (BCM - back course marker)
    3. Outer marker (OM)
      1. Located 4-7 miles from runway.
      2. Indicates approximately where aircraft will intercept the glide slope when aircraft is at the proper altitude.
      3. Signal -- continuous dashes (2 per second).
      4. Purple light.
    4. Middle marker (MM)
      1. Signal -- alternate dot/slash.
      2. 3500' from landing threshold.
      3. Amber light.
    5. Back course marker (BCM)
      1. Signal -- 2 dots.
      2. White light.
      3. Located at FAF of LOC BC approach.
    6. Inner marker (IM)
      1. Signal -- 2 dots.
      2. White light.
      3. Located between middle marker and landing threshold.

The localizer antenna array radiates two different signals, carrier plus sideband (CSB) and suppressed carrier sideband only (SBO). The CSB signal consists of the RF carrier amplitude modulated (AM) with equal amplitudes of 90 Hz and 150 Hz tones. The SBO signal is similar except that the carrier is suppressed. The localizer radiation patterns are normally arranged so that the course sector of the proportional guidance sector is symmetrical around the runway centreline (see figure).

If the aircraft on approach is aligned with the runway centreline, the CDI will display no difference in the depth of modulation (DDM) between the 90 Hz and 150 Hz audio tones; therefore, the CDI needle is centred.

If the aircraft is to the right of the centreline, the 150 Hz modulation will exceed that of the 90 Hz and produce a deflection on the CDI towards the left. Conversely, if the aircraft is to the left of the centreline, the 90 Hz modulation will exceed that of the 150 Hz and produce a similar but opposite deflection. This deflection corresponds to the direction the pilot must fly to be aligned with runway centreline and is proportional to the angular displacement from centreline.

The CDI has a full-scale deflection of 150 microamperes where the DDM equals 0.155 in both the 90 Hz and 150 Hz directions. The angular displacement, or proportional guidance sector, that corresponds to this full scale deflection is known as the localizer course width. This width is typically tailored for a full-scale CDI deflection to occur at 350 feet from runway centreline at threshold.

When the aircraft is outside this course guidance sector, the CDI is required to provide full scale deflection. This region is known as the clearance sector. The FAA requires that this region extend from the localizer course edge out to 35 degrees on both sides of centreline.

Reflected or scattered signals that come from hangars and buildings historically and today pose the greatest concern for establishing a localizer. These reflected signals cause quality derogation of the on-course signal as seen by the aircraft. To minimize these reflections, the common technique is to use larger array apertures that narrow the localizer course beam and thus reduce the quantity of signals incident on the reflecting surface.

In some cases, to maintain the 35-degree clearance coverage, a separate RF carrier, offset from the course frequency by 8 kHz, is radiated. Since these two signals fall within the passband of the ILS receiver, the stronger of the signals is "captured" by the receiver and is used for the guidance. These two-frequency localizer arrays are called dual-frequency and are primarily used to support Category II/III operations.

Glide Path

The glide slope provides the pilot with vertical guidance. This signal gives the pilot information on the horizontal needle of the CDI to allow the aircraft to descend at the proper angle to the runway touchdown point. The glide slope radiates on a carrier frequency between 329 and 335 MHz and is also modulated with 90 Hz and 150 Hz tones. The glide slope frequencies are paired with the localizer, meaning the pilot has to tune only one receiver control.  

The radiation patterns of a typical glide slope system are similar to those of the Localizer - if you remember to rotate the pattern so that it is vertical instead of horizontal. The null in the sideband-only (SBO) signal produces essentially a straight glide path angle for the aircraft. The patterns are arranged so that 90 Hz modulation predominates above the glide path and the 150 Hz modulation predominates below.

The glide path angle is normally referenced at 3 degrees. If the aircraft is on this three-degree glide path, equal amounts of the 90 Hz and 150 Hz are received and the CDI will be centred. If the aircraft is above the glide path, the 90 Hz modulation exceeds that of the 150 Hz and produces a deflection on the CDI downwards. If the aircraft is below the established glide path, the 150 Hz modulation predominates and produces a similar but opposite deflection. This deflection corresponds to the direction the pilot must fly to intercept the glide path and is proportional to the angular displacement from the glide path angle. As with the localizer, the full scale deflection is 150 microamperes. Typically, the glide slope sensitivity is set so that the full-scale indications occur at approximately 2.3 and 3.7 degrees elevation.

The FAA presently maintains five types of glide slope systems. They are the null-reference, sideband-reference, capture-effect, endfire, and waveguide. The null-reference, sideband-reference, and capture-effect glide slope systems use the terrain in front of the antenna mast to double, effectively, the vertical aperture of the radiating system and produce the path in space. These three systems are typically referred to as image glide slope systems. Where the ground plane in front of the glide slope mast is irregular or absent, endfire or waveguide types are used. These systems are called non-image and do not rely on the terrain to form the path in space.

Marker Beacons

Inner, Middle, and Outer

Marker beacons are used to alert the pilot that an action (e.g., altitude check) is needed. This information is presented to the pilot by audio and visual cues. The ILS may 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 along the ILS approach and are identified by discrete audio and visual characteristics (see Table 1). All marker beacons operate on a frequency of 75 MHz.

Marker Beacon Characteristics
Marker Beacon Pilot Alert Distance to Threshold Modulated frequency Audio Keying
Outer Glide Path Intercept 4 to 7nm 400Hz ------
Middle Category 1 Decision Height 3500 ft 1300Hz .-.-.-
Inner Category 2 Decision Height 1000 ft 3000Hz ......



Table table 1

The marker beacon coverage provides adequate signal laterally throughout the localizer proportional guidance sector. Marker beacons produce cone or fan-shaped radiated patterns directed upward and, therefore, pose very few siting problems. The majority of problems in locating the marker beacon are the availability of real estate and access to utilities. If an acceptable site for the outer marker cannot be found, an alternative is to collocate a Distance Measurement Equipment (DME) transponder with the localizer. This DME then provides the range indication to the aircraft. The ILS models do not provide any information with regard to marker beacon performance.