Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS IN OR RELATING TO SCANNING
ELECTROMAGi~ETIC: BEAM SYSTEMS
This invention relates to scanning electromagnetic beam
systems as may be used for guidance or tracking and more especially
but not exclusively it relates to scanning beam microwave landing
systems ~MLS) for aircraft.
Scanning beam MLS comprise ground based microwave
transmission apparatus which is adapted to produce a narrow beam
of microwave energy, which beam is scanned TO and FRO to define a
predetermined angular sector in space between scan limits, a receiver
carried by an aircraft being used to detect the transmitted beam as it
is scanned past the aircraft, for the purpose of establishing the
aircraft's position relative to a glide path corresponding to the centre
line of the scanned angular sector.
Such systems are well known and the detailed construction and
mode of operation of such systems will not therefore be described
herein except in so far as is necessary to facilitate an understanding
of the present invention.
One of the problems associated with scanning beam MLS is the
susceptibility of such systems to multipath interference and in
particular to interference due to the reflection of transmitted
sidelobes of the scanned beam from buildings for example so tha~
they are delayed so as to be received by an aircraft
contemporaneously with unreflected signals in the main lobe of the
scanned beam. Multipath interference of the kind as just before
described gives rise to guidance errors which should be minimised in
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guidance systems and which it is especially important to minimise in
microwave landing systems.
It is an object of the present invention to provide a scanning
electromagnetic beam guidance or tracking system such as a scanning
beam microwave landing system for example, in which the adverse
effects of multipath interference are obviated or at least reduced.
According to the present invention a scanning electromagnetic
beam system comprises a source of electromagnetic energy, a linear
antenna array comprising a plurality of radiator elements, beam-
former means via which energy is fed from the source to the radiator
elements of the array, phase control means for selectively controlling
the phase of signals fed ~rom the source via the beamformer means to
the elements of the array, thereby to effect successive TO and FRO
scans of the beam across a predetermined angular sector, amplitude
control means for selectively controlling the amplitude of signals fed
from the source via the beamformer means to the elements of the
array thereby to effect a predetermined amplitude distribution across
the array and modulator means effective to change the centroid of the
distribution in successive TO/FRO scan pairs.
The modulator means may additionally be ef~ective to change
the centroid of the amplitude distribution so that it di~fers for the
successive TO and ~RO scans of each pair.
Furthermore the modulator means may be additionally
arranged to change the centroid of the amplitude dislribution at least
once during each TO scan and at least once during each FRO scan.
It will be appreciated that modulation applied in accordance
with the invention serves to randomise the effect of multipath
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spurious signals whereby ,guidance errors which include 'path
following' error caused thereby is reduced at the expense of an
acceptable 'control motion' noise level increase, which noise level
increase is occasioned by effective decorrelation of the multipath
interference due to the random nature of the modulation.
Some embodiments of the invention will now be described by
way of example with reference to the accompanying drawings in
which;
Figure 1 is a block schematic of a known scanning beam MLS,
Figures 2a and 2b comprise two graphs which represent
aperture distribution on TO and FRO scans respectively of the system
of Figure 1,
Figure 3 is a block schematic diagram of a further known
scanning beam MLS,
Figures 4a and 4b comprise two graphs which represent
aperture distribution on TO and FRO scans respectively of the system
of Figure 3,
Figure 5 is a block schematic diagram of a scanning beam MLS
according to the present invention,
Figures 6a and 6b comprise two graphs which represent
aperture distribution on successive TO/FRO scan pairs respectively
which obtain in accordance with one implementation of the system of
Figure ~,
Figures 7a and 7b comprise two graphs which represent
aperture distribution on the TO and FRO scans respectively of one pair
which obtain in accordance with an alternative implementation of the
system of Figure 5,
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Figure 8 is a block schematic diagram of one part of the system
~s shown in Figure 5 in greater detail; and,
Figure 9 is a block schematic diagram of another par~ of the
system as shown in Figure 5 in greater detail.
Referring now to the drawings, a known MLS is shown in Figure
I which comprises a microwave signal source 1 which feeds a beam-
former ? The beamformer serves to distribute energy from the
microwave signal source 1 to a plurality of phase shifters 3, 4, 5 and
6. The phase shifters are arranged to feed radiator elements 7~ 8, 9
~nd 10 respectively. Only four phase shifters and four radiator
elements are shown in the present example for simplicity, but it will
be appreciated that a number of phase shif~ers and radiator elements
would normally be provided as would be required to define a suitable
l~ILS linear array. The relative phase of signals applied to the
elements is controlled by means of the phase shifters 3, 4, 5 and 6 to
effect a beam scanning function and an aperture taper as shown in
Figures ~a and 2b on TO and FRO scans respectively is achieved by
amplitude weighting which is fixed and afforded by the beamformer
2 which serves to control the relative amplitude of the signals fed to
the phase shifters 3~ 4, 5 and 6.
A more sophisticated development of the arrangement shown in
Figure 1 comprises the use of a limited scan array antenna system
with a sharply cut off element pattern which serves to reduce the
level of radiation outside the scanning region of the antenna whereby
multipath interference as may be a problem with the arrangement
shown in Figure 1 is reduced. One significant disadvantage wi~h this
more sophisticated system is that RF path lengths involved in the
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beamformer are comparatively long with the result that undesirable
temperature and frequency sensitivity obtains.
A further known technique as shown schematically in Figure 3
has been described by Cafarelli and Adams in IJS Patent Serial
Number 481 1022 entitled Phase Centre Diversity (PCD). The system
described by Cafarelli and Adams in this US patent has for an object
to reduce the effect of mul~ipath radiation caused by sidelobe
radiation rather than to reduce the actual level of sidelobe radiation.
This is achieved by switching the electrical phase centre of the array
between TO and FRO scans to effect a phase shift of 180 between
direct and multipath signals. This switched phase shift introduces
equal and opposite contamination of the desired signal on the TO and
FRO scans whereby guidance error introduced by the spurious
multipath signals is cancelled out. The benefits of ~his known
switching system can also be achieved if the phase centre is switched
between TO and FRO scan pairs. In this latter case the receiver is
used to average the guidance angle data whereby equal and opposite
error is introduced into adjacent TO/FRO scan pairs whereby the
effects of multipath interference are cancelled.
A simplified block schematic diagram illustrating a system
which utilises this known switching system is shown in Figure 3 and
comprises;
A microwave signal source 14 which is alTanged to feed a beam
former 15. Signals from the beam former 15 are fed to phase shifters
16, 17, 18 and 19 which serve radiator elements 2~, 21, 22 and 23
respectively. Signals ~rom the beam former 15 are fed ~o the phase
shifter 16, 17, 18 and 19 via a switch matrix 24 which serves to
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switch the aperture distribution between TO and FRO scans. As
shown in Figures ~a and 4b respectively, the linear array comprises
N2 radiator elements and the switch matrix 24 is operated such that
elements Nl to N2 only are used as shown in Figure 4a during the TO
scan. During the FRO scan however, as shown in Figure 4b, elements 1
to N3 are used where N1 = N2- N3. Thus a switching of the phase
centre of the antenna is achieved whereby the effect of multipath
interference as hereinbefore described tends to be cancelled.
The systems just before described has however a number of
practical drawbacks. A significant disadvantage is that the
movement of the antenna phase centre between two discreet points
will provide optimum error cancellation at a single guide slope only,
whereas MLS is required to operate over a wide range of guide slopes.
Additionally movement of the antenna phase centre produced
by switching the electromagnetic energy to selected radiator
elements, particularly the end elements requires that an extra Nl
radiator elements must be incorporated into the linear array in order
to maintain antenna bandwidth and effective radiated power. This
can be seen from Figure 4a. The exact number of additional radiator
elements required depends upon the guide slope angle selected for
optimum error reduction or cancellation.
~ oreover this known switching system does not allow the
benefits afforded by PCD to be obtained except on a flat or uniformly
sloped terrain or, in the case of an azimuth antenna, maximum benefit
may only be achieved when the source of multipa~h in~erference is
known .
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Perhaps the most significant shortcoming of the switching
system technique is that TO and FRO scans should be identical in
order to achieve optimum sidelobe improvement. This would not of
course be the case where other means such as phase shifter cycling is
used which results in a different sidelobe distribution on the TO scan
as compared with the FRO scan.
Referring now to Figure 5, one embodiment of the present
invention comprises a microwave signal source 25 alTanged to feed a
beamformer 26 which serves a plurality of transmission control
modules 27, 28, 29 and 30 only 4 of which are shown for simplicity.
The transmission control modules feed radiator elements 31, 32, 33
and 34. The transmitter control modules 27, 28, 29 and 30 are
controlled by means of a transmitter control unit 31 via a control bus
3~. In operation of the system shown in Figure 5, the transmitter
control modules 27, 28, 29 and 30 are controlled by ~he transmitter
control unit 31 via the control bus 32 to effect a beam steering
function by controlling the relative phase of signals radiated from the
elements 31, 32, 33 and 34. Modulation of the aperture distribution
on the other hand is effected by controlling the relative amplitude of
the signals radiated from the elements 31, 32, 33 and 34. Thus an
aperture distribution as shown in Figures 6a and 6b may be produced
wherein the centroids 50a and 50b respectively of the distribution in
successive TO/FRO scan pairs is changed. Thus the distribution in one
TO/FRO scan pair as shown in Figure 6a is changed with respect to the
next successive TO/FRO scan pair as shown in Figure 6b. Additionally
the centroids 51a and 51b respectively of the amplitude distribution
may be changed between the TO and FRO scan of each pair as shown
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in Figures 7a and 7b. Moreover although not specifically illustrated
herein it will be appreciated that the centroid of the amplitude
distribution may be changed at least once during each TO scan and at
least once during each FRO scan. It will be readlly appreciated that
by applying modulation to control aperture taper as just before
described. a randomising effect is produced which serves to
randomise the effect of multipath spurious signals whereby 'path
following' error caused thereby is reduced at the expense of an
acceptable 'control motion' noise level increase, which noise level is
occasioned by effective decorrelation of the multipath interference
due to the random nature of the modulation.
Referring now to Figures 8 and Fi~ure 9, the func~ion of the
transmitter control unit 31 and the transmission modules 27, 28, 29
and 30 will now be described in greater detail. Referring firstly to
Figure 8, each of the transmission modules 27, 28, 29 and 30
comprises a phase shifter 33, a gain control unit 34 and a power
amplifier 35. Input signals from the beam former are fed to the
phase shifter 33 on a line 36 and output signals to a radiator element
such as the radiator element 31 are fed on a line 37 from the power
amplifier 35. Control of the phase shifter 33, and the gain control unit
34 is effected by a bus deGoder unit 38 which is coupled to the phase
shifter 33, and the gain control unit 34 via lines 39 and 40,
respectively. The bus decoding unit 38 receives control signals via
the control bus 32, as shown in Figure 5, from the transmitter con~ol
unit 3 1.
The transmitter control unit as shown in Figure 9 comprises an
aperture distribution processor 42 and a beam steering processor 43,
which feed a bus encoder unit 44 which provides signals for the
control bus 32. The aperture distribution processor is in effect a
computer which provides appropriate signals for the gain control unit
3~ and the power amplifier unit 35 as shown in Figure 8 whereby the
aperture taper is varied to produce appropriate modulation. The
beam steering processor 43 on the other hand comprises a computer
which determines the phase of signals required to effect a
predetermined beam steering function and provides appropriate
signals for the bus encoder unit 44 which are in due course decoded
by the bus decoder unit 38 before application to the phase shifter 33
such that the appropriate phase is applied to signals radiated from an
associated radiator element coupled to line the 37.
It will be appreciated that the aperture distribution processor
4~ operates to effect a modulation function of the transmitted signal
whereas the beam steering processor 43 serves a beam steering
function.
Various modifications may be made to the arrangement just
before described without departing from the scope of the invention
and for example any suitable means for controlling the phase or
amplitude of signals radiated ~rom the elemen~s of the linear a~ray to
provide the required modulation function may be utilised.
