Components and Characteristics of the Instrument Landing System (ILS)
T-45A UJPT & E2-C2 INav-04
Point of equal overlap is the center of the extended centerline
of the runway indicating on course
Shielded localizer array (antenna) is normally located 1,000 ft
beyond downwind end of the runway centerline
Actual signal generated by transmitter from a building that is
offset a minimum of 250 ft from center of antenna array
Optimum signal strength and minimum possible signal
Primary approach to a localizer is called "Front Course"
NOTE: Localizer front course beam is adjusted at the antenna
array for a 700-ft width at the runway threshold. The beam, as
displayed on the aircraft instruments, has a width of 5 degrees
allowing for 2-1/2 degrees left-full deflection and 2-1/2 degrees
right-full deflection. A full deflection of the CDI at runway threshold
would place the aircraft 350 ft left or right of the runway centerline
while tracking on the localizer.
The localizer provides course guidance for descent path:
(1) 18 nm from the antenna
(2) 1,000 ft above the highest terrain along course line
(3) 4,500 ft above antenna elevation site
(4) Signal acquisition and off-course indications are provided
Fig 2: Localizer Signal
through angular areas of the operational service volume:
(a) To 10 degrees either side of the course along a radius
of 18 nm from the antenna
(b) To 35 degrees either side of the course along a radius
of 10 nm from the antenna
NOTE: After the missed approach point, any localizer signal
indication received should be disregarded unless it is
designated part of the published missed approach procedure.
Any signal that is not part of the procedure should be
considered unreliable and disregarded. Some fields will have
functional back course localizers (BAC LOC) and are
designated as such.
NOTE: Loss of the localizer information during the approach
will render the approach procedure unusable. The approach
should be terminated or another type of an approach should be
selected, weather permitting.