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35
o
35
o
10o
Localizer
transmitter
Figure 5.3: Required coverage of localizer signal
Idealized glide-path
(straight line)
Actual glide-path
(hyperbola) Conical
surface
Glide-path
antenna
Localizer
plane
Figure 5.4: Hyperbolic intersection of localizer and glideslope reference planes
58 Chapter 5. Radio-navigation, sensors, actuators
8o 8o
10 NM
Glideslope
transmitter Localizer
transmitter
Runway
Figure 5.5: Required coverage of glideslope signal compared to localizer coverage
335.0 MHz band. The signal is modulated by 90 and 150 MHz tones, in which 90 Hz
is predominant above the desired glide-path and 150 Hz dominates below the glidepath.
Due to the position of the glideslope antenna, the intersection of the localizer
reference plane and the glideslope reference cone is actually a hyperbola which is
located a small distance above the idealized straight glide-path. This is shown in
figure 5.4. The minimum coverage of the glideslope signals, required by ICAO [2] is
shown in figure 5.5.
An ILS installation is said to belong to a certain performance category, representing
the meteorological conditions under which it is to be used. These conditions
have been summarized in figure 5.6. An ILS installation of category I is intended to
provide guidance down to an altitude of 200 ft, a category II installation provides
guidance down to 100 ft, and an installation of category III must provide guidance
down to the runway surface. Only cat. III signals can be used for fully automatic
landings. If the aircraft is making an approach under cat. I conditions, the pilot
should either see the runway lights at an altitude of 200 ft or cancel the final approach
and go-around.1
The localizer and glideslope signals are received on board the aircraft and displayed
in an appropriate form to the pilot. In addition, these signals may be fed directly to
an automatic pilot for automatic localizer and glideslope tracking. The nominal ILS
signals on board the aircraft are expressed in terms of the currents which are supplied
to the pilot’s cockpit instrument.
The magnitude of the localizer current iloc is proportional to the localizer deviation
angle Gloc, shown in figures 5.7 and 5.8, while the glideslope current igs is proportional
to the glideslope deviation angle #gs, shown in figure 5.8. From these figures,
1The altitude at which the runway lights should be visible is called the decision altitude or decision
height. Some states and some individual airlines require larger values for the decision height than the
numbers shown in figure 5.6. Local circumstances, such as special terrain features or radio-interference
patterns, may also dictate higher minimums.
5.1. The Instrument Landing System 59
-
-
?
?
?
?
0 ? ? ?
100
200
1000 800 600 400 200 50 0
Cat. I
Cat. II
Cat. III
A B C
Runway visual range [m] !
"
Decision
height [ft]
Cat. I : Operation down to minima of 200 ft decision height and runway visual range
of 800 m with a high probability of approach success
Cat. II : Operation down to minima below 200 ft decision height and runway visual
range of 800 m, and as low as 100 ft decision height and runway visual range of
400 m with a high probability of approach success
Cat. III A: Operation down to and along the surface of the runway, with external visual
reference during the final phase of the landing down to runway visual range
minima of 200 m
Cat. III B: Operation to and along the surface of the runway and taxiways with visibility
sufficient only for visual taxiing comparable to runway visual range in the order
of 50 m
Cat. III C: Operation to and along the surface of the runway and taxiways without exter-
nal visual reference
Figure 5.6: ILS performance categories
we can deduce that Gloc is defined as the the angle between the localizer reference
plane and a vertical plane that passes through the localizer antenna, and #gs is the
angle between the reference glidepath and the line that connects the aircraft to the
glideslope antenna. Since these angles are supposed to remain zero during the final
approach, we will call them ‘localizer deviation angle’ and ‘glideslope deviation
angle’, respectively.
For the simulation of ILS-approaches, we will use the runway-fixed reference
frame FRW, which has been defined in appendix A. At time t = 0 the position of
the aircraft’s center of gravity coincides with the origin of the Earth-fixed reference
frame FE, hence, at that moment xe = 0, ye = 0, and H = H0. The position of the
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