Note: Descriptions are shown in the official language in which they were submitted.
20~J5~'
P ~2 o6 3z7.2
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 (TLS), mainly far
carrying out so°called Category III landings.
Twa-frequency instrument landing systems are described,
for example, in a book by E.Kramar, "Funksysteme fur
Ortung and Navigation", VerCag Berliner Union GmbH,
Stuttgart, and Verlag W. Kohlhammer GmbH, Stuttgart,
Berlin, KBIn, Mainz, 1973, particularly in Section 5.9.2,
pp. 196 et seq.
The ground equipment of a two-frequency instrument land-
ing system consists of a localizes portion far guiding
an aircraft to an airport and for providing azimuth guid-
ance during landing, a glide-slope portion for providing
vertical guidance unfit touchdown on the runway, and two
marker beacons for transmitting coarse distance informa-
tion. At least the localizes portion consists of two
separate transmitters operating with a slight difference
in their carrier frequencies ttwo-frequency system).
Frequently, the glide-slope portion is also designed as
ZPL/S-P/Ke/Lo
25.01.93 G. Greying - W. Poschadel 10-13
2~~~~~~
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such a two-frequency system.
According to the regulations of the International Civil
Aviation Organization CIGAO), one of the localiz.ers of
two-frequency localizes equipment radiates a so-called
clearance signal of a predetermined minimum field strength
within ~35° from the (extended) runway centerline, and
the other radiates a sharply defined course signal in
the direction of 'the runway centerline. The two signals
differ slightly in carrier frequency and are each modu-
lated at two audio frequencies (90 Hz., 150 Hz..). The
audio frequencies used for modulation are equal for the
clearance and course signals and are generally in phase.
Their depths of modulation are initially equal. The trans-
mitting 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,
sa 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 located, and which additionally indicates the distance
to this plane within a small angular range near this
plane.
In the receiver, the slight difference between the carrier
frequencies of the clearance signal and course signal
causes the respective stronger incoming signal to dis-
proportionately suppress the weaker incoming signal,
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 ICAO
rules, must not 'fall below a value of 10 de along the
runway centerline.
The capture effect allows the radiation of the course
signal 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 tram obstacles located on both
sides of the runway, will be suppressed. In phactice,
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 of 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 no longer impossible even in
the case of 'two-frequency LLS installations that the
values required by LCAO 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 spec ify a glide path, reflections
of the signal radiated into the wider angular range below
~ _
an elevation plane containing the glide path firom. Large
natural or artifiicial obstacles on the ground may re-
duce the field-strength ratio required to utilize the
capture effect Ccapture ratio) to the point that a re-
liable specification of the glide path is endangered by
excessive DDM distortions.
To improve the suppression of reflected signals, British
Patent 1,0b2,551, p.2, right column, line 91 et seq.,
for example,praposes for localizer transmitting apparatus
to place equal audio frequencies C90 tlz. and 150 Elz) of the
clearance signal and course signal, which are used for
modulation, in quadrature, i.e., to shift their phases
by 900 with respect to each other,
Such a phase shift ofi +900 or -900 would contravene the
regulations of the ICAO, which, to ensure undisturbed
operation of arbitrary receiver types, require common
passage of both modulation frequencies through zero in
the same direction.
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 shifits of the modulation frequencies,
corresponding to the phase shift of a common fundamental
frequency, a 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
~~~~~?~:~
a corresponding suppression of reflections of the glide-
path signal radiated near the ground into the wider angular
range is achieved along the glide path if 'the phase
shift is introduced in glide-slope transmitting apparatus.
The respective suppression acts in addition to the cap-
ture effect.
According to claim 2, the phase sh ift is optimizable
by measuring the disturbing influence of a transmitted sig-
nal thus modulated with out-of-phase audio°~frequency
signals while changing the phase shift.
Such measurements yielded the phase shifts given in
claim 3 as values of minimum interference.
Values given in claim 4 have an added advantage over the
other values given in claim 3 in that the influence of
deviations from the predetermined optimum phase-shift
angle is least there.
The invention will now be described in detail using a
localizes transmitting apparatus as an example.
Fig. 1 is a block diagram of a test setup, and
Fig. 2 is a graph representing a typical test
result.
Fig. 1 shows schematically the far-end portion of a run-
way RW with a test receiving antenna TA located on the
runway centerline CL. The test receiving antenna is
connected to a test receiver TE followed by an output
device PL. Located beyond the far end of the runway is
a localizes antenna LA for a two-frequency instrument
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landing system which - unlike in conventional fieed sys-
tems - is fled 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 Ce.g.,
35° on both sides of the runway centerline), and part
of the energy is reflected from a hangar N,located in
the vicinity of the runway~toward the runway centerline,
as indicated by an arrow RR. Part of the clearance sig-
nal is also radiated directly in the direction KR.
Since, in twa-frequency instrument landing systems, there
is 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 higher than that afi the clear-
ance transmitter, the so-called capture efifect 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 effiect i.s not
sufficient to guarantee that a stable localizes course
is specified along the runway centerline. The clear-
ance signal will interfere with the course signs l
which results in one component of the course being weakened
or strengthened relative to the other, thus causing a change it
the depth of modulation of one audio frequency with re-
spect to that of the other audio frequency after demodu-
lation (DDM distortion). Instead of a straight, stable
localizes course, a distorted course tine wilt thus be
communicated to the aircraft which does not permit a
landing in poor visibility according 'to TCAO regulations.
The inventors have found out that such distortions of
the course line can be greatly reduced by shifting the
phase of the audio-frequency signals used to modulate the
clearance-signal transmitter with respect to 'the respec-
tive identical audio-frequency signals used to modulate
the course transmitter. The phase shift must be dif-
ferent for each audio frequency (90 Hz and 150 Hz) and
must correspond to the same phase angle of a common
fundamental frequency (30 Hz) of 'the two audio frequencies.
For a system with audio frequencies of 90 Hz and 150 Hz,
a 15o phase shift of the 30-Fiz fundamental frequency,
for example, corresponds to a 45° phase shift of the
90°Hz audio frequency and to a 75o phase shift of the
150-Hz audio frequency.
In Fig. 2, distortions of the localizes course (DDM
distortions 6,DDM) in such a two-frequency instrument
landing system, which are measurable at a point of the
runway centerline, are plotted as a function of the
phase shift ~30 of the 30-Hz fundamental Frequency for
a field-strength ratio (capture ratio) of 10 dB and a
_ g _
DDM basic value of 200 ~ A for the clearance signal. To
adjust the phase shifts necessary for the '90-tiz and
150-Hz audio frequencies, dig ital modulators in the
two transmitters S1, S2 were so driven in a manner
known per se that the phase shifts C3 Xf~30 and 5 Xfp30)
corresponding to 'the currently desired phase shift of
the fundamental frequency was obtained for the two audio
frequencies. The phase shifts of the two audio frequencies
can also be produced, of course, if only one transmitter
is employed. This only necessitates giving up ttie rigid
coupling existing in currently available transmitters,
which operate without phase shift, between the equal
audio-frequency signals used to modulate the eLearance
signal and course signal, and making available the auclio-
frequency signals separately w ith the desired phase shift.
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 F15, ~50,
ø90, *130, and ~165 degrees. At these zeros, DDM dis-
tortions caused by extreme reflections of the clearance
signal are reduced to values far below the limit values
prescribed by the ICAO for so-called category III
landings (~S~A on the runway).
Fig, 2 also shows that in the case of phase shifts cor-
responding to 15o and 165° of 'the 30-Hz fundamental fre-
quency, the curve goes through zero less steeply than
at the other zeros. At these points, deviations from the
adjusted phase shift~as may be caused, 'for example, by
slight disturbances of synchronism and of the modulator
toterances~resutt in a smaller increase of DDN!
_ g _
distortions than at points where the curve crosses the
zero line very steeply. The greatest tolerance sensi-
tivity would be obtained at t~30 = 90°.
A phase shift corresponding to one of the above angles
of the fundamental 'frequency, which represent distortion
zeros, eliminates the need for many of the conventional,
generally expensive or otherwise disadvantageous measures
for distortion suppression, and offers a number of addi-
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 minimum value 'for the clearance signal is possible
without increasing the risk of intolerable DDM dis-
tortions of the course signal.
The design of the transmitting antennas need not be
adapted to the terrain. Since a higher transmitting power
of the clearance signal is possible, a greater range of
the clearance signal at interference minima resulting
from the reflections in the far field is achieved. DDM distor-
tions (DDM dips) are also suppressed in the far field
of the clearance-signal transmitter through the possible
increase in the DDM of the clearance signal.
