Each ILS is categorized according to the performance capability of the ground
equipment. A Category I ILS is capable of providing acceptable guidance information
down to a decision height (DH) (the point where a missed approach is made if the pilot
cannot complete the approach visually) of not less than 200 feet. A Category II ILS is
capable of providing acceptable guidance down to a decision height of not less than 100
feet. A Category III ILS is capable of providing acceptable guidance information without
decision height minima.
Localizer Transmitter
The localizer transmitter operates on one of 40 ILS channels within the frequency range
of 108.10 to 111.95 MHz. Identification is in Morse code and consists of a three-letter
identifier preceded by the letter "I". The signal provides the pilot with course guidance to
the runway centerline. The localizer antenna is sited at the far end of the runway so that
the center of the antenna is in line with the centerline of the runway. The localizer
provides course guidance throughout the descent path to the runway threshold for a
distance of 18 nm from the antenna between an altitude of 1,000 feet above the highest
terrain along the course line and 4,500 feet above the elevation of the antenna site. The
course line along the extended centerline in the opposite direction to the front course is
called the back course.
Glide Slope Transmitter
The glide slope transmitter operates on one of 40 ILS channels within the frequency
range of 329.15 to 335.00 MHz. The glide slope transmitter is located between 750 and
1,250 feet from the approach end of the runway and is offset 250 to 650 feet from the
runway centerline. The glide path feature of the ILS is what makes it a precision
approach. The principle of operation is similar to that of the localizer; however, the glide
path transmitter provides precision descent information along the final approach course
at the desired degree of glide slope. The glide path projection angle is normally adjusted
to 3 degrees above horizontal so that it intersects the middle marker (MM) at 200 feet
and the outer marker (OM) at about 1,400 feet above the runway elevation. The glide
slope is normally usable to a distance of 10 nm. Since the glide path signal is provided
for the front course only, the back course is not a precision approach and will have
higher minimums.
Marker Beacons
ILS marker beacons have a rated power output of 3 watts or less and are located along
the ILS approach course. Ordinarily, there are two marker beacons associated with an
ILS: the OM and the MM. Airports with a Category II or Category III ILS will also have an
inner marker (IM).
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