Note: Descriptions are shown in the official language in which they were submitted.
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Two-Frequency Transmitting Apparatus with
Tone-Modulation Phasing for an Instrument
Landing System
The present invention relates to two-frequency transmitting
apparatus as set forth in the preamble of claim 1 and as
is used in instrument landing systems ~ILS), ma;nly for
carrying out so-called Category III landings.
Two-frequency instrument landing systems are described,
for example, in a book by E.Kramar, "Funksysteme fur
Ortung und Navigat;on", Verlag Berliner Union GmbH,
Stuttgart, and Verlag W. Kohlhammer GmbH, Stuttgart,
~erlin, Koln, Mainz, 1973, particularly ;n Sect;on 5.9.2,
pp. 196 et seq.
The ground equipment of a two-frequency instrument land-
ing system consists of a localizer portion for guiding
an aircraft to an airport and for providing azimuth guid-
ance during Landing, a glide-slope portion for prov;ding
vertical guidance until touchdown on the runway, and two
marker beacons for transmitting coarse distance informa-
tion. At least the localizer portion consists of two
separate transmitters operating ~ith a slight difference
;n their carrier frequencies (two-frequency system).
Frequently, the glide-slope portion is aLso designed as
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such a two-frequency system.
Accord;ng to the regulations of the International Civil
Aviation Organization (ICAO), one of the Localizers in the
two-frequency localizer equipment rad;ates a so-called
clearance signal of a predeterm;ned m;nimum field strength
within -35 from the (extended) runway centerline, and the other
localizer radiates a sharply defined course signal ;n
the direction of the runway centerline. The two signals
differ slightly in carrier frequency and are each modu-
lated with two audio frequencies t90 Hz, 150 Hz). The
audio frequencies used for modulat;on are equal for the
clearance and course signals and are generally in phase.
Their respective depths of modulation are initially equal. The
transmitt;ng antennas are so designed, however, that the radia-
tion fields formed on both sides of the centerline contain
one or the other modulation frequency in a higher measure,
so that along the centerline and its extension, a ver-
tical plane is defined along which the modulation compon-
ents of the two audio frequencies are equal, so that
their difference becomes zero. On both sides of this plane,
a receiver, by comparing the modulation components, can
derive a criterion ~DDM = difference in depth of modulation)
which indicates on which side of the plane the receiver is lo-
cated, and which add;t;onally indicates the angular d;stance
to this plane within a small angular range near this
plane.
In the receiver, the slight difference bet~een the carrier
frequencies of the clearance signal and course signal
causes the respective stronger incoming signal to dis-
proportionately suppress the weaker incoming signal, which is
the so-called capture effect. The field-strength ratio
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between clearance signal and course signal is referred
to as "capture ratio" and, according to the current ICA0
rules, must not fall below a value of 10 d~ along the
runway centerline.
The capture effect allows the radiation of the course
s;gnal to be restricted to a narrow, obstacle-free angu-
lar range on both sides of the centerline and to increase
the radiated field strength to the point that interference
signals, which may be caused, for exampLe, by reflections
of the clearance signal from obstacles located on either
side of the runway, will be suppressed. In practice,
however, the increase in the power of the course-signal
transmitter is limited by the transmitter technology used
and by the requirement that interference with the ILS
installations at other airports due to nonstandard propa-
gation should be avoided.
With the use of larger aircraft and the construction of
larger hangars for such aircraft, on the one hand, and
because of the frequent lack of space, which forces air-
port planners to place buildings closer to the runway,
on the other hand, it is quite possible that euen with
the use of two-frequency ILS installations, the values
required bylthe ICA0 for category III cannot be met,
so that a possibly important airport cannot be approved
for category III landings.
Interference due to reflection may, in principle, aLso
occur along the glide path. If two-frequency transmitting
apparatus is used to specify a glide path, reflections
of the signal radiated into the wîder angular range below
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an elevation plane conta;ning the glide path from large
natural or artif;cial obstacles on the ground may re-
duce the field-strength ratio required to utilize the
capture effect (capture ratio) to the point that a re-
liable specif;cation of the glide path is endangered by
excessive DDM distortions.
To improve the suppression of reflected signals, ~ritish
Patent 1,062,551, page 2, right-hand column, line 91 et seq.,
for example,proposes that the localizer transmitting apparatus
uses equal audio frequencies (90 Hz and 150 Hz) of the
clearance s;gnal and course signal, which are employed for
modulation, in quadrature, i.e., that their phases are
sh;fted by 90 with respect to each other.
Such a phase shift of +90 or -90 contravenes the
regulations of the ICA0, wh;ch, to ensure undisturbed
operation of arbitrary receiver types, require common
passage of both modulation frequencies through zero in
the same direct;on.
It is the object of the invention to provide an improved
two-frequency transmitting apparatus which is also insen-
sitive to strong reflection-induced interference and
meets the relevant regulations.
An apparatus which attains the object of the invention
is described by the features set forth in claim 1.
By the different phase differences between the modulation fre-
quencies, corresponding to the phase shift of a common funda-
mental frequency, suppression of reflected clearance signals
is achieved in the region of the runway centerline if the phase
shift is introduced in localizer transmitting apparatus, and
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a correspond;ng suppression of reflect;ons o~ the glide-
path signal rad;ated near the ground ;nto the w;der angular
range ;s achieved along the glide path if the phase shift ;s
introduced in glide-slope transmitting apparatus. The respec-
tive suppression acts in add;tion to the capture effect. r
Accord;ng to claim 2, the phase sh;ft ;s opt;mizable
by measuring the disturbing influence of a transm;tted
signal thus modulated with out-of-phase audio-frequency
s;gnals wh;le changing the phase shift.
Such measurements yielded the phase shifts given in
claim 3 as values of minimum interference.
Values g;ven ;n claim 4 have an added advantage over the
other values g;ven ;n cla;m 3 ;n that the ;nfluence of
deviations from the predetermined optimum phase-sh;ft
angle ;s least there.
Clajm 5 relates to a transmitting apparatus suitable for
quick adaptation to different interference situations.
The~jnvention will now be described in detail using a
localizer transm;tting apparatus as an example.
F;g~ 1 ;s a block diagram of a test setup, and
Fig. 2 is a graph represent;ng a typ;cal test
result.
F;g. 1 shows scnematically the far-end port;on of a run-
way RW with a test rece;v;ng antenna TA located on the
runway centerline CL. The test receiving antenna ;s
connected to a test rece;ver TE followed by an output
device PL. Located beyond the far end of the runway ;s
a localizer antenna LA for a two-frequency instrument
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landing system which - unlike in conuentional feed sys-
tems - is fed here by two separate transmitters S1, S2
which provide the course signal K and the clearance
signal R, respectively.
The direction of radiation KR of the sharply focused
course signal is the direction of the runway centerline.
The clearance signal is radiated in a wider angle (e.g.,
35 on both sides of the runway centerline), and part
of the energy is reflected from a hangar H,located in
the vicinity of the runway,toward the runway centerl;ne,
as indicated by an arrow RR. Part of the clearance sig-
nal is also radiated directly ;n the direct;on KR.
Since, in two-frequency instrument landing systems, there
;s a slight difference between the carrier frequencies
of the course transmitter and the clearance transmitter,
and the field strength of the course transmitter along
the runway centerline is h;gher than that of the clear-
ance transmitter, the so-called capture effect normally
becomes effective, wherein the course signal nearly com-
pletely suppresses the clearance signal.
It has turned out, however, that in extreme cases - e.g.,
if the clearance signal is reflected from large metallic
buildings or large aircraft parked near the runway -
superpositions of directly radiated and reflected com-
ponents of the clearance signaL may occur which de-
teriorate the capture ratio, i.e., the field-strength
ratio of clearance signal to course signal, in these
superposition regions to the point that the suppression
of the clearance signal by the capture effect is not
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sufficient to guarantee that a stable localizer course
is specified along the runway centerline. The clear-
ance signal will interfere with the course si9nal, which
results in one component of the course signal being weakened
or strengthened relative to the other, thus causing a change in
the depth of modulation of one audio frequency with re-
spect to that of the other audio frequency after demodu-
lation (DDM distort;on). Instead of a stra;ght, stable
local;zer course, a d;stQrted course line will thus be
communicated to the aircraft which does not permit a
land;ng ;n poor visibility according to ICA0 regulat;ons.
The ;nventors have found out that such d;stort;ons of
the course l;ne can be greatly reduced by sh;ft;ng the
phase of the audio-frequency s;gnals used to modulate the
clearance-signal transm;tter with respect to the respec-
t;ve ident;cal audio-frequency s;gnals used to modulate the
course-s;gnal transm;tter. The phase shift must be d;f-
ferent for each aud;o frequency (90 Hz and 150 Hz) and
must correspond to the same phase angle of a common
fundamental frequency (30 Hz) of the t~o audio frequenc;es.
For a system with audio frequencies of 90 Hz and 150 Hz,
an18 phase shift of the 30-Hz fundamental frequency,
for example, corresponds to a 54 phase shift of the
90-Hz audio frequency and to a 90 phase shift of the
150-Hz audio frequency.
In F;g. 2, distortions of the ~ocalizer course (DDM
d;stortions aDDM) in such a two-frequency instrument
landing system, which are measurable at a point on the
runway centerline, are plotted as a function of the
phase shift ~30 of the 30-Hz fundamental frequency for
a field-strength rat;o (capture ratio) of 10 d8 and a
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DDM basic value of 200 mA for the clearance signaL. To
adjust the phase sh;fts necessary for the 90-Hz and
150-Hz audio frequencies, digital modu~ators in the two
transmitters S1, S2 were driven in a convent;onal manner,
nameLy so that the phase shifts (3 X ~30 and 5 X ~30)
corresponding to the currently desired phase shift of
the fundamental frequency was obtained for the two audio
frequencies. The phase sh;fts of the two aud;o frequencies
can also be produced, of course, if only one transmitter
is employed. This only necessitates giving up the rigid
coupLing existing in currently available transmitters,
which operate w;thout phase sh;ft, between the equal
aud;o-frequency s;gna~s used to modu~ate the clearance
signal and course signal, and making available the audio-
frequency signals separately w;th the des;red phase shift.
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Fig. 2 clearly shows that depending on the phase shift
of the fundamental frequency, the DDM distortions
(curve M) assume different values, and repeatedly the
value zero. The zeros of the curve are at about -18, - 50,
-90, -130, and -163 degrees.
It has be~en found that the positions of these zeros also
depend on the location of the interfering obstacles with
respect to the runway centerline, namely whether they
are located on the side on which the ~50-Hz modulation
predominates, as in the case underlying the curve of
Fig. 2, or on the side on which the 90-Hz modulation pre-
dominates. For the latter case, zeros are at phase shifts
of the fundamental frequency of -32, +90, and -150
~not shown ;n F;g. 2).
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At these zeros, DDM distortions caused by reflections
of the clearance signal will, even in an extreme case,
be reduced to values far below the limit values pre-
scribed by the ICA0 for so-called category ~II landings
(+5 mA on the runway).
Fig. 2 also shows that w;th phase shifts corresponding
to 18 and 163 of the 30-Hz fundamental frequency and
obstacles on the 150-Hz s;de of the runway centerline,
the curve goes through zero less steeply than at the
other zeros. At these points, deviations from the
phase-sh;ft setting, as may be caused, for example, by
slight out~of-sync condit;ons and variat;ons of the
modulator toLerances, resuLt in a smaller ;ncrease of
DDM distortions than at points where the curve crosses
the zero line very steeply ~e.g., p30 = 90)~ For ob-
stacles on the 90-Hz side, minimally steep zero crossings
are observed at 32 and 150 (not shown in the figure).
A phase shift corresponding to one of the above angles
of the fundamental frequency, which represent distortion
zeros, el;minates the need for many of the convent;onal,
generally expensive or otherw;se d;sadvantageous measures
for distort;on suppression, and offers a number of add;-
tionaL advantages:
For exampLe, a reduction in transmitter power to in-
crease the capture ratio or a reduction of the dif-
ference in depth of modulation (DDM) for the clearance
signal can be dispensed with. Even an increase in the
DDM m;nimum ~alue for the clearance signal is possible
without increasing the risk of intolerable DDM dis-
tortions of the course signal along the runway.
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The design of the transmitting antennas need not be
adapted to the terrain. Since a higher transmitting power
of the clearance s;gnal is possible, a greater range of
the clearance signal at interference min;ma result;ng
from the reflections in the far field is achieved. DDM
distortions (DDM dips) are also reduced in the far field
of the clearance-s;gnal transmitter through the possible
;ncrease in the DDM of the clearance s;gnal.
S;nce strong interference sources (large obstacles re-
flect;ng the clearance s;gnal) are seldom located on
both sides of the runway centerline, optimum ;nterference
suppression can be achieved by sett;ng a phase difference
wh;ch corresponds, for example, to a 18 phase shift of
the 30-Hz fundamental frequency in case of ;nterferences
;ncident from the 150-Hz s;de, and to a -150 phase
sh;ft of the 30-Hz fundamental frequency in case of
interferences ;ncident from the 90-Hz side. -50 and -163
phase sh;fts of the 30-Hz fundamental frequency for inter-
ferences from the 150-Hz s;de and -32 phase shifts for
interferences from the 90-Hz side are also effect;ve
for suppressing interference in the area of the runway,
but they present problems in the transition region be-
t~een course signal and clearance signal which make
them less suitable than the phase shifts specified
above.
If, ;n except;onal cases, strong interference sources
should be located on both s;des of the runway centerl;ne,
a phase difference correspond;ng to a -90 phase sh;ft
o~ the 30-Hz fundamental frequency can prov;de non-opt;-
mum, but suff;c;ent interference suppression which is
equally well effectiue for reflections from both
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interference sources.
The observed phase-shift values of the common fundamental
frequency, which provide DDM interference minima (zeros
;n F;g. 2), apply for the case where exact synchronism
exists between the two modulation frequencies~ Even
slight inaccuracies ;n synchron;zat;on may change the
opt;mum phase-shift values by a few degrees upwards or
downwards. Another factor which slightly influences
the pos;tions of the opt;mum phase-sh;ft values ;s
the ;ntens;ty of the ;nc;dent ;nterference s;g-
nal.
Preferably, the transmitting apparatus according to the
invent;on ;s so des;gned that besides a sett;ng without
phase sh;ft, prepared phase sh;fts are selectable which
correspond to phase shifts of the common 30-Hz funda-
mental frequency of 18, 90, and 150.
