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marker at an altitude of roughly 1400 ft (above runway elevation). Sectorised antennas
are used, with powers roughly similar to airport VORs (i.e. 50W).
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. All marker
beacons operate on a frequency of 75 MHz (OM: 400 Hz, MM: 1300 Hz, IM: 3000 Hz).
The beams radiate vertically with a diameter of approximately 3500ft at a height of 1000ft
(evidently, the value increases as the altitude increases). Power is usually 3W or less
(although note that the Outer Marker may be replaced by an NDB, which uses a power of
around 25W).
Distance Measuring Equipment (DME) is increasingly co-located with ILS to replace the
marker beacons.
ILS suffers from Course Distortion. This is disturbances to localiser and glide slope
courses when surface vehicles (e.g. aircraft that taxi) operate near the localiser or glide
slope antennas. Areas where this occurs are called ILS Critical Areas. It is the Air Traffic
Controller’s responsibility to ensure that no interference exists when ILS approaches are
in progress.
Operational Aspects
The visibility conditions at the airport have an impact on the approach and landing phase
of a flight; that is the main reason why for landing operations, visibility conditions on the
runway have been classified into several categories namely NPA, NPV, CAT I, CAT II and
CAT III.
• Non-precision approach operations basic (NPA). An instrument approach
which does not rely on the utilisation of vertical guidance relative to a glide path.
• Non-precision approach operations with vertical guidance (NPV). An
instrument approach which utilises guidance relative to a glide path and does not
meet requirements established for precision approach and landing operations.
• Precision approach and landing operations (PA). An instrument approach
and landing using precision lateral and vertical guidance relative to a glide path
with minima as determined by category of operation. Precision approach is
classified into three operational categories.
The three groups are defined in terms of the Runway Visual Range (RVR) and Decision
Height. The Decision Height is the lowest height above the runway where pilots make the
decision to continue the landing or to abort. It is based on the ability of pilots to obtain
guidance from visual cues on the ground rather than from instruments in the cockpit. If
pilots are unable to see a sufficient number of visual cues at the decision height, the
landing must be aborted.
The International Civil Aviation Organisation (ICAO) defines three categories of visibility
for landing civil aircraft with the aid of an Instrument Landing System:
Category I: The decision height (height at which the pilot must have established visual
contact with the runway) is not lower than 200 ft. If the runway is not clearly visible to the
pilot at the Decision Height, then a Missed Approach must be executed. The Runway
Visual Range is not less than 1800 ft (using some kind of runway lighting). The RVR is
measured on the ground and communicated to the Pilot by ATC.
Category II: The decision height is not lower than 100 ft. The Runway Visual Range is
not less than 700 ft. The aircraft must carry a dual ILS receiver to perform a CAT II
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