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required for simulations (e.g. for assessing automatic cat. III landing systems) an actual
calibration of the localizer and glideslope signals for the site in question should
be used.
1Note: the expressions for ILS noise and atmospheric turbulence given in refs.[1] and [19] have an
additional term p in the denominator. The Dryden spectra from ref.[27] do not contain this term, due
to a slightly different definition of the Fourier transform. In this report, the definition of the Dryden
filters from ref.[27] has been used, and due to the similarity of the expressions for ILS noise the term p
will be omitted here too. See also the footnote on page 48.
The definitions of the Fourier transform and the inverse Fourier transform used in ref.[27] are:
X(w) = F{x(t)} =
Z ¥
−¥
x(t)e−jwt dt; x(t) = F−1{X(w)} = 1
2p
Z ¥
−¥
X(w)ejwt dw
5.1. The Instrument Landing System 65
Distance to threshold, [m]
-600 0 600 1050 7410
2.5
5
7.5
10
15
s
μA
gs
loc
[ ]
(cat. III)
s s loc
sgs
gss
sloc
sloc
(cat. II, III)
(cat. I)
(cat. II, III)
(cat. I)
,
Figure 5.10: Maximum allowable ILS localizer and glideslope noise
With Taylor’s hypothesis it is possible to substitute w = WV. Then the ILS noise can
be modelled as a white-noise signal which is sent through a linear forming filter in
the same way as the derivations for atmospheric turbulence shown in figure 4.4. The
resulting filters are:
Hloc(w) = sloc
r
2Lloc
V
1
1 + Lloc
V jw
(5.17)
Hgs(w) = sgs
r
2Lgs
V
1
1 + Lgs
V jw
(5.18)
Alternative shapes of the power spectral density functions for localizer and glideslope
noise are given in ref.[22]. These expressions were based upon average power
spectral density plots of beam noise, observed at several airports:
Sloc = |Hloc(w)|2 = 25 (1.5 + jw)2
(0.35 + jw)2(10 + jw)2
h
μA2rad−1s
i
(5.19)
Sgs = |Hgs(w)|2 = 15.9
(0.25 + jw)2
h
μA2rad−1s
i
(5.20)
The filters for these spectral density functions are:
Hloc(w) = ±
5 (1.5 + jw)
(0.35 + jw)(10 + jw)
(5.21)
Hgs(w) = ±
3.9875
0.25 + jw
(5.22)
Both ILS noise models have been implemented in the FDC toolbox. The maximum
allowable values of the standard deviations of localizer and glideslope noise, according
to ICAO standards [2] have been given in figure 5.10.
66 Chapter 5. Radio-navigation, sensors, actuators
In addition to the ILS noise and steady-state errors, specific deterministic interference
patterns may occur due to signal reflections from aircraft in the vicinity of the
glideslope and/or localizer transmitters. These disturbances may be quite severe
and should be taken into account for the evaluation of automatic landing systems. It
is possible to construct relatively simple models of these interference effects, but that
goes beyond the scope of this report. Ref.[1] provides more details about these types
of disturbances.
5.2 The VOR navigation system
Another radio-navigation system that is still commonly used today is the Very-high
frequency Omnidirectional Radio-range (VOR) system. The system was developed
after World War II, and has become the backbone of the air navigational system
since the 1950’s. It provided more accurate signals than the Low-Frequency beacons
used at that time (the LF Non-Directional Beacon or NDB system is still in use today,
mainly to augment instrument approach and departure procedures), and it was less
prone to interference from thunderstorms.
Distance measuring equipment (DME) was added to many VOR transmitters and
receivers, allowing the distance between the station and the aircraft to be shown in
the cockpit. The VOR system can be used for navigating between airports, following
so-called ‘airways’, and it can also be used for final approach guidance if the
pilot flies along a reference VOR bearing to the runway, using a timed descent or
an altitude-versus-distance table (the latter procedure requires both VOR and DME
signals).
However, unlike the ILS system, the VOR system is considered a non-precision
landing aid, because it is not accurate enough and does not provide enough information
to allow a pilot to land. Because of this, VOR/DME approaches require considerably
higher minimum values of visibility and cloud-ceiling than ILS approaches.
5.2.1 Nominal VOR signals
The VOR system uses the 108-118 MHz frequency radio range (VHF). The VOR
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