Instruments Landing System (ILS) - Principles.

Arrangement of Subsystems.

The basic subsystems belonging to the ILS system, namely

• the Localizer (LLZ)

• the Glide Path (GP)

• and the Markers, and in addition

• a Distance Measuring Equipment (DME) (optional)

• and a Far Field Monitor for the localizer (optional)

are arranged on the runway as shown in Figure below. This arrangement is valid for the single and dual frequency (1F, 2F) installations described in further detail in the next chapters.

The LLZ antenna is located 200 to 360 m beyond the end of the runway on the extended centre line. The associated LLZ transmitter is in a shelter near the antenna.

The GP antenna is located 120 to 180 m from the runway centre line. The reference height for the glide path has been fixed at 15 m above the runway threshold. The dimension "D" (286 to 344 m) between the GP antenna mast and the runway threshold is calculated from this height and the glide angle, the latter being determined on the basis of local circumstances. The associated GP transmitter is in a shelter in near the antenna.

The inner marker (IM) is 75 to 450 m ahead of the runway threshold on the extended centre line, the middle marker (MM) is 1050 m ahead, and the outer marker (OM) is 7200 m ahead. In most cases only the middle and the outer marker are used.

When a DME systems is used to supplement the marker beacons there are a number of installation
alternatives, for example:

• DME antenna on the GP mast (DME transponder in the GP shelter),

• DME antenna on the roof of the LLZ shelter (DME transponder in the LLZ shelter),

• DME transponder in a separate shelter with the DME antenna on its roof,

• DME transponder in a separate shelter and DME antenna on a separate mast.

The latter two configurations are preferable, since they permit the runway to be approached from both directions. Running time differences between touchdown and the DME installation are always included, so that touchdown is always at exactly 0 m.