Components and Characteristics of the Instrument Landing System (ILS)
T-45A UJPT & E2-C2 INav-04
II.
Range (position) Systems
The following systems are commonly used to identify range (position) when
flying an ILS/LOC approach.
*
Marker beacons
*
Distance Measuring Equipment (DME)
*
Cross Radial
*
Compass Locator
The position of the aircraft on ILS/LOC approaches is very important due to
the precise nature of the approach, lower minimums, and obstacle
clearances associated with them. The pilot relies on this information to
execute specific portions of the approach and for position in relation to the
glidepath.
A. Marker beacons
Fig 1: FAA Instrument
Landing System (ILS)
1.
Low power transmitters (3 watts, 75 Mhz)
2.
Produces a signal in an elliptical pattern
a.
1,000 ft above the antenna
b.
2,400 ft in width and 4,200 ft in length
3.
Elliptical pattern (fan pattern) placement is centered and
perpendicular to the FAC
4.
Normally, two marker beacons are associated with an ILS
approach, Outer Marker (OM) and Middle Marker (MM). However,
Category II and III ILS approaches may also have an Inner Marker
(IM) beacon
a.
Outer Marker (OM)
(1) Indicates aircraft, if at appropriate altitude, will intercept the
glidepath
(2) Identified by a coded audio tone (continuous dashes) and
flashing blue cockpit marker light
b.
Middle Marker (MM)
(1) Intersects the glidepath at approximately 200 ft AGL and
identifies the approximate missed approach point for
Category I approaches
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