Chapter 5
Radio-navigation, sensors, actuators
The previous chapters provide a solid basis for the construction of a simulation model
of the aircraft and its working environment. This model allows us to extract all
incorporated flight data without any restrictions, which is very useful for the analysis
of flight dynamics and which allows us to build elaborate control systems. However,
it should be noted that this transparency is actually an idealised representation of
reality.
In real flight, the relevant flight data is obtained through a limited array of sensors
of limited accuracy and bandwidth. Some data can only be obtained indirectly;
for instance, signals emitted by radio-navigation beacons on the ground are often
used to derive the position of the airplane. Similarly, control inputs made by the
(auto-) pilot do not represent the actual control surface deflections, as there is a system
of cables and actuators (again of limited accuracy and bandwidth) in between.
In short: we have to take into account the interface between the aircraft and the
flight-deck. This chapter will provide a starting point by presenting some additional
models, primarily focussing on the mathematical representation of radio-navigation
systems that are still commonly used for approach guidance and short-range navigation.
At the end of the chapter, a short overview about additional sensor models,
actuator models, and other navigation equipment will be given, thus identifying
room for future expansion of the model library (and this documentation).
5.1 The Instrument Landing System
The Instrument Landing System (ILS) has been the standard aid for non-visual precision
approaches to landing since the 1940’s, and is still being used worldwide. Under
certain circumstances it can provide guidance data of such integrity that fully coupled
approaches and landings may be achieved. The system, which is ground-based,
broadcasts very precise directional signals, providing a lateral and vertical path to
the runway up to a distance of approximately 40 kilometers from the runway.
The system comprises three distinct parts of on-ground equipment: (i) the localizer
transmitter, which gives guidance in the horizontal plane, (ii) the glideslope (or
glide-path) transmitter, which provides vertical guidance, and (iii) one, two, or three
marker beacons situated on the approach line, which serve as checkpoints for the
pilot to verify the distance to the runway (these markers are increasingly often being
56 Chapter 5. Radio-navigation, sensors, actuators
x
loc
xgs
y
gs
Runway
Localizer
antenna
Glideslope
antenna
Landing
direction
Figure 5.1: Positioning of ILS ground equipment
3.9 NM
3000 ft
Localizer
transmitter
Join glide-path
and commence
final descent
Runway
Glide-path
transmitter
Alt. 300 ft
Alt. 1200 ft
Aircraft
track
Alt. 2500 ft
Middle marker
Outer
marker
Figure 5.2: Lay-out of the approach path
replaced by the use of distance measuring equipment for direct distance-to-threshold
measurements). In this section we will analyze the localizer and glideslope signals
in more detail.
5.1.1 Nominal ILS signals
Figures 5.1 and 5.2 show the lay-out of the ILS ground equipment and the approach
path. The localizer signal is emitted by an antenna, situated beyond the up-wind end
of the runway. Operating at a frequency within the 108.0 to 112 MHz frequency band,
it radiates a signal modulated by 90 and 150 Hz tones, in which 90 Hz predominates
to the left-hand side of the approach path and 150 Hz predominates to the right, as
seen from an aircraft flying on final approach course. Figure 5.3 shows the required
minimum coverage of the localizer signals according to ICAO guidelines [2].
The glideslope antenna is located some 300 meters beyond the runway threshold
(approximately adjacent to the touch-down point) and at about 120 to 150 m from
the runway centerline. The frequency of the glideslope signal lies within the 328.6 to
5.1. The Instrument Landing System 57
Front beam area Back beam area
Runway
10 NM
17 NM
25 NM
10o
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