GB2253744A - Scanning electromagnetic beam systems - Google Patents

Abstract

A scanning electromagnetic beam system comprising a source of electromagnetic energy 25, a linear antenna array comprising a plurality of radiator elements 31-34, beamformer means 26 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 from 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. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SCANNING ELECTROMAGNETIC 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 that 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 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, beamformer 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 from 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 effective to change the centroid of the amplitude distribution so that it differs for the successive TO and FRO scans of each pair.

Furthermore the modulator means may be additionally arranged to change the centroid of the amplitude distribution 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 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 5, 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, Figure 8 is a block schematic diagram of one part of the system as shown in Figure 5 in greater detail; and, Figure 9 is a block schematic diagram of another part of the system as shown in Figure 5 in greater detail.

Referring now to the drawings, a known MLS is shown in Figure 1 which comprises a microwave signal source 1 which feeds a beamformer 2. 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 and 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 shifters and radiator elements would normally be provided as would be required to define a suitable MLS 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 2a 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 with this more sophisticated system is that RF path lengths involved in the 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 US Patent Serial Number 4811022 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 multipath 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 1800 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 this 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 arranged 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 20, 21, 22 and 23 respectively. Signals from the beam former 15 are fed to the phase shifter 16, 17, 18 and 19 via a switch matrix 24 which serves to switch the aperture distribution between TO and FRO scans. As shown in Figures 4a and 4b respectively, the linear array comprises N2 radiator elements and the switch matrix 24 is operated such that elements N1 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 N1 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.

Moreover 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 multipath interference is known.

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 arranged 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 32. In operation of the system shown in Figure 5, the transmitter control modules 27, 28, 29 and 30 are controlled by the 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 51 a and 51b respectively of the amplitude distribution may be changed between the TO and FRO scan of each pair as shown 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 readily 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 Figure 9, the function 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 decoder 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 control unit 31.

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 34 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 42, 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 from the elements of the linear array to provide the required modulation function may be utilised.

Claims (6)

1. A scanning electromagnetic beam system comprising a source of electromagnetic energy, a linear antenna array comprising a plurality of radiator elements, beamformer 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 from 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.

2. A system as claimed in Claim 1 wherein the modulator means is arranged to be effective to change the centroid of the amplitude distribution so that it differs for successive TO and FRO scans of each pair.

3. A system as claimed in Claim 1 or Claim 2 wherein the modulator means is arranged to change the centroid of the amplitude distribution at least once during each TO scan and at least once during each FRO scan.

4. A system as claimed in any preceding claim wherein the amplitude control means comprises a plurality of gain control units one for each radiator element via which signals for each element are supplied and wherein the modulator means comprises an aperture distribution processor which serves to control operation of the gain control units thereby to effect a predetermined amplitude distribution of signals applied to the radiator elements.

5. A system as claimed in any preceding claim wherein the phase control means comprises a controllable phase shifter arranged one in series with each gain control amplifier and operated under the control of a beam steering computer whereby the phase of signals applied to each radiator element is controlled to effect a predetermined beam steering function.

6. A system as claimed in Claim 1 and substantially as hereinbefore described with reference to Figure 5, Figure 6a and Figure 6b, Figure 7a and Figure 7b, Figure 8 and Figure 9 of the accompanying drawings.