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Portable radio equipment, which transmits the glide path signals
tor the instrument landing system. is placed about 7:;0 fcet upwind
from the runwav's approach end and about 400 feet to one side.
Because the equipment that has made successful instrun.1ent
landings possible is strictly a radio system, a description
of the component parts and their operation necessarily
must be semi-technical.
Fundamentally, the system includes a localizer to hring
the plane in line with the runway, a glide path transmitter
to guide the plane dO\Y11at the proper rate of descent, and
markers or check points O\Tr which the pilot gets a radio
fix including distance from end of runwav.
,The localizer is similar to an ordinary radio range. It
differs in two respects in that it indicates the course line to
the pilot visually (on instrument panel) rather than aurullv
and it operates at ultra-high frequencies where it is out of
the range of ordin.irv static.
The localizer projects tin> tone-modulated radio patterns
which intersect as a line or plane in space, thereby provicling
the pilot with a center-line to the middle of the runway
from as far away as 75 miles at 8,000 feet.
On the instrument panel is a cross pointer meter that
indicates the position of the airplane in relation to the
localizer line, whether it is to the left or to the righ t.
One side of the instrument is vcllow, the other blue. At
all times, irrespective of heading: the localizer indicates the
color of the sector in which the airplane is flying. However,
the localizer needle points toward the course line when the
aircraft is approaching the fllI1Way from the proper direction,
\\'hen the aircraft is f1}'ing in the opposite direction,
the needle points '1I\'ay from the course line.
Since the localizer is a special form of radio beam or
range, it must be flown by "bracketing." \Vith the aid of
the directional gyro, heading corrections of onlv a fell' degrees
need be made because of the sharpness of the localizer
course. So sensitive is the indicator needle that a deviation
of 3° from the course line will be indicated by a complete
swing of the needle to its scale maximum. This scnsitivitv
is essential to assure proper alignment of the airplane with
the cen tcr of the runwav.
Three markers, 75-nlegaeycle transmitters, are placed
along the localizer course, one at 4 Y2 miles from the field
(flashing two dashes per second}, one at one mile from the
field (flashing six dashes per sccond) and the boundary
marker approximately 200 feet from the end of the runway
(projecting a constant light).
The third component is the glide path, a precise radio
beam that descends at a low angle to the runwav. As seen
by the pilot, it starts from the landing point on the fllnll'ay
at an angle of 21/2° and projects to a distance of 15 miles
or more. A cross-pointer instrument on the panel indicates
whether the plane is above or below the glide path; when
the two needles intersect, the plane is on the proper flight
path.
The 2 Y20 glide path angle represents a rate of descent of
approximately 400 feet per minute at a ground speed of
100 mph. The glide path also may be used for rough inclications
of distance from the runwuv since each 200 feet
of altitude approximates one mile in cfishmce from the field.
If the pilot's altimeter reads 1.000 feet when he is descending
on the glide path, he is approxim.itclv five miles from
the end of the runwav.
Ground equipment includes the 10C