CA1221442A - Monopulse detection systems - Google Patents
Monopulse detection systemsInfo
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- CA1221442A CA1221442A CA000441414A CA441414A CA1221442A CA 1221442 A CA1221442 A CA 1221442A CA 000441414 A CA000441414 A CA 000441414A CA 441414 A CA441414 A CA 441414A CA 1221442 A CA1221442 A CA 1221442A
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- difference
- monopulse
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Abstract
ABSTRACT (Figures 2,4) A planar array (13) of radar antenna elements (B) used in a monopulse detection systems includes means to apply an appropriate phase ? by the phase delay (4) on transmission and reception so as to define only two beams.
Received signals are added to give a sum beam (22), and for a normal beam perpendicular to the plane of the array appropriately weighted, signals are added in phase in a resistor network F1. A single difference beam (27) is generated by added receiver outputs in a second resistor network F2 with each output phase delayed (23) by a phase angle equal to the angular position of the detector measured from the array centre with respect to the array centre line (14). Then a target position can be defined by taking the ratio of the amplitudes and the relative phases of signals received by the sum and difference beams. Beam steering is done by applying additional phase delays to both beams to tilt the phase planes of the array as desired.
Received signals are added to give a sum beam (22), and for a normal beam perpendicular to the plane of the array appropriately weighted, signals are added in phase in a resistor network F1. A single difference beam (27) is generated by added receiver outputs in a second resistor network F2 with each output phase delayed (23) by a phase angle equal to the angular position of the detector measured from the array centre with respect to the array centre line (14). Then a target position can be defined by taking the ratio of the amplitudes and the relative phases of signals received by the sum and difference beams. Beam steering is done by applying additional phase delays to both beams to tilt the phase planes of the array as desired.
Description
a~,~z Improvements in or Relatinq to Monopulse Detection Systems . . . _ The invention relates to detector arrays used in active or passive detection systems for producing monopulse tracking.
Conventional monopulse tracking systems require three channels to define the positions of targets within the beams of an array of detectors. Two signal difference channels are required to give information on the target offset in two perpendicular directions and a sum channel gives amplitude information. Comparison of the magnitude and sign of these difference signals with the sum signals then gives a target's position relative to the centres of the beams.
The object of the present invention is to provide an alternative form of monopulse detection system which requires only two channels to determine the position of targets and is particularly relevant to circular arrays.
The invention provides a monopulse system for detecting energy received from a target comprising:
(i) a planar array of detectors responsive to the received energy;
(ii) a sum channel for adding together outputs from all detectors;
(iii) a difference channel both for applying to the output of each detector a fixed polar phase delay which is linear in the range O - 2 ~! or an integral multiple thereof, in dependence on the angular position of each detector in the plane of the array and for adding together the product signals formed thereby; and (iv) means for comparison of the outputs from the two channels to derive the position of the target.
The invention thus requires only two channels one : . .
of which produces a sum beam and 1:he other a difference beam wherein the relative amplitucles determine the target's angular off-set and the relative phases determine the direction of this off-set as projected on the plane of the array.
The array may be a uniform distribution of detectors as for example by placing detectors at the interstices of a rectangular grid. However in order to reduce the overall system complexity the invention may also be applied to a thinned array wherein selected detectors are omitted from the uniform array.
In the preferred arrangement the array consists of waveguide apertures, dipoles or similar elements for the radiation and reception of r.f. energy. During reception each element may be connected to a receiverwhich amplifies and/or converts to a lower frequency the energy emitted or reflected by targets. The separate outputs are then preferably passed through selectable phase delays employed for beam steering. The separate phase delayed outputs are added directly to form the sum beam and are also added after applying the fixed polar phase delays to form the difference beamO
With the selectable phase delays set to zero the sum and difference beams are circularly symmetric about the normal to ~h~ plane of the array with a central peak and a central null respectively. By applying phase delays linearly dependent on the positions of the elements in the array, the phase plane may be tiltedrelative to the plane of the array, thereby steering the beams.
Circuitry may be included in the system to automatically steer the sum and difference beams to maintain a target on the axis of the sum beam.
Side lobes in the radiation patterns of both the sum and the difference beams may be attenuated by carrying out L~ 4 ~2 the addition oE the respective signals in the two channels in weighting circuits, wherein each detector output i.s weighted as a function of the distance of the detector from the centre of the array.
The fixed polar O - 2 ~ phase delays applied to the receiver outputs in the difference channel may be introduced by appropriate lengths of transmission line connecting the receivers to the difference channel addition circuit.
In order that these features may be more fully appreciated the in`vention will now be described by way of example only with reference to the accompanying drawings of which:
Fig. 1 is a schematic arrangement to illustrate a monopulse radar tracking antenna;
Fig. 2 is is a functional circuit diagram of a monopulse tracking radar;
Fig. 3 is a circuit diagram of signal processing circuits in a conventional monopulse radar;
Fig. 4 is a circuit diagram of signal processing circuits according to the invention for producing a polar coordinate monopulse radar;
Fig. 5 is a geometrical representation of the planar array used-to define the polar coordinate monopulse radar; and Fig. 6 illustrates the radiation patterns (response curves) of the ~wo beams of the polar monopulse ra~ar.
Monopulse systems started with the use of four horn aerials in a square formation feeding a reflector. A typical arrangement is shown in Figure 1 wherein the four rectangular horns A,B,C and D are placed on the axis of a reflector R. The respective signals received from the horns A to D may be respectively added and subtracted to give a sum signal S and two difference signals ~x and ~y as follows:
S = A + B + C + D
~x = (A + D) - (B + C) ~y = (A + B) - (C + D) ~Z~ 4;2 By comparing the magnitude and sign of the difference signals ~x and ~ y with the sum signal S an azimuth and an elevation servo motor connected to the aerial system can be driven in the appropriate directions to reduce the di~ference signals to zero and so keep track oE a target.
This sytem of separate ~x and ~ y signal channels had been adopted in conventional aerial arrays employing a centralised transmitter and receivers with r.f. feeder networks to isolate the sum and the two difference signals. Since the feeder networks are normally disposed horizontally and vertically the provision of separate rectangular coordinate ~x and ~y difference signal channels is a logical arrangement. However this is no longer necessary in an electronically-scanned active array in which each element or small group of elements within the array is associated with a separate receiver. It is now possible to provide phase shifts by IF or digital processing.
With this type of processing two or more outputs of any desired phase can be obtained from each receiver with no loss of signal to noise ratio.
Figure 2 shows a planar antenna array A, circular or near-circular in shape, which is fully or partially filled with antenna elements B. Each of the elements B is connected to a transmitter 1 or a receiver 2 according to whether the associated switches S1,S~ and S3 are respectively in the lower position (as shown) or the upper position.
During transmission, power from the transmitter 1 is fed via a divider 3 to all the radiating antenna elements B via respective phase shifters 4. The output from each phase shifter 4 may be fed via an optional amplifier 5 to the antenna element B. During reception, the divider3 is used to feed power for a local oscillator 6 via the respective phase shifters 4 and inputs 7 to rf mixers in the receivers 2. Signals received in the antenna elements B are fed to inputs 8 in the respective receivers 2 where they are down-converted to an IF frequency L~4~
signal at the receiver output ~. For both transmission and reception the direction of maximum gain - the main beam -is controlled by the settings of the phase shif-te.rs ~.
In the conventional system the output signal from each receiver 2 is processed as shown in Figure 3 to provide three IF outputs 10, 11 and 12. For the sum signal output 11 (equivalent to S) all the receiver outputs 9 have equal path lengths to the resistor network F, where they are summed. In most cases signals received by antenna elements 13 towards the edge of the array A have additional resistive attenuation 14 (say) in order to suppress side-lobes in the antenna sensitivity pattern. For the azimuth signal output 10 (equivalent to ~x) the signals from all the antenna elements to the left of the centre line 14 of the array A are summed in anti-phase to those on the right in the network F2. Thus the signal from each receiver output 9 is fed to the network F2 via a phase delay 15 set to O or ~
radians depending upon whether the associated antenna element B
is respecti.vely to the left or right of the array centre line 14.
Similarly for the elevation signal output 12 (equivalent to ~y) signals from antenna elements above the array centre line 16 are summed in antiphase to those below in the network F3. Signals from each receiver output 9 are fed via a phase delay 17 to the network F3 with the phase delay set to O or ~ radians depending upon whether the associated antenna elements are respectively above or below the line 16. In both of these dif~erence outputs 10 and 12 low side-lobes are achieved by attenuating or weighting the individual signal amplitudes by an appropriate choice of resistor 17 or 18 depending on the distances of the individual elements from the respective centre lines 14 and 16 as well as from the centre of the array.
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The signals in the three IF outputs 10-12 can then be processed in three identical channels. Finally after separation in known manner by ranye gates the phase and amplitude of the signals in the difference channels are compared with phase and amplitude of the signal in the sum channel to determine the angular offset of targets in the horizontal and vertical directions.
According to the present invention the circuit arrangement shown in Figure 4 is used in co-operation with the antenna array A and circuitry shown in Figure 2. The invention replaces the conventional rectangular cartesian co-ordinate system by applying polar coordinate dependent phases to pro-duce a polar monopulse radar system.
As before the outputs 9 from the receivers 2 as-sociated with each antenna element B in the array A are sum-med in a weighting network F, of resistors 19-21 (of which only three are shown) to produce a conventional sum signal (~) at the output 22 from the network Fl. The output 9 from each receiver 2, however, is also applied via a fixed polar phase delay 23 ( fh) to a second weighting network F4 of resistors 24-26 to produce a signal weighted output difference signal 27 (~). The fixed polar phase delay 23 introduces a phase shift for each antenna element B equal to the.angle ~ of the element measured in a clockwise sense from the array centre line 14. This is achieved by using appropriate lengths of cable to connect the receivers 2 to the network F4, The polar monopulse thus requires only two channels with two weighting networks F1 and F4 compared with the three net-works required in the conventional Figure 3 arrangement.
In addition, amplitude weighting in the F4 network of the difference channel depends only on the distance of the antenna element from the centre of the array i.e. there i5 circular symmetry for this channel as well as for the sum channel. The sum output 22 (~) and the difference out-put 27 (~) are then combined to determine the angular position of the target uniquely by taking the amplitude ratio ~/~
3~Z~ 42 to give the angular offset of the target from the boresight direction, and the phase difference ~ between the two outputs to give the radial direction of the target relative to the centre line 1~.
In a circular array of detectors a phase mode can be selected such that the phase of the individual elements caries linearly with their angular position, with total phase of 2~ n per 360, where n is an integer. It is then possible by suitably phased addition o:E the various modes to generate a bean or a null in particular directions in the plane of the -5a-.
array. This considexation is extended to a circular planar array in the present invention by considering what happens in planes normal to the array. Figure 5 indicates a typical antenna element 28 in a planar array located at an angle ~ from the array centre line 29 and a distance r from the centre 30 of the array.
The amplitude of excitation of the array is a function of the radial distance x only and is represented by:
Amplitude = f(r) If n = 0, there is a constant phase over the face of the array and a beam is formed normal to it. For the sum (~ ) pattern n is therefore chosen to be zero. The cross-section of the resulting beam is indicated by the pattern 31 in Figure 6 as a symmetrical beam centred on the bore-sight 32 (normal to the plane of the array, through the array centre 30). For the polar difference (~) beam n is chosen to be 1. In this case the amplitude excitation of the array as shown by the pattern 33 is symmetrical with a null 34 in the bore-sight direction 32 surrounded by an annular peak 35, constant in amplitude, but with a 360 change of phase around it.
Thus, if a target is within the sum (~ ) beam 31 the ratio of the amplitudes of the signal return~ in the sum 31 and polar difference 33 beams indicates the angular distance of the target off bore-sight. Comparison of the phases of the two returns (~ ) then indicates the radial direction of the target. It is also possible to choose integer values of n greater than 1. This gives higher angular resolution at the expense of ambiguity.
Although not shown in Figure 6, for the purpose of simplicity, a uniform amplitude distribution in a difference pattern generally leads to high side-lobes as well as a lower monopulse angular sensitivityO In the usual rectangular coordinate monopulse system lower side-lobes are greater angular sensitivity are obtained by applying a linear taper in ampli-tude proportional to the distance of the antenna element from the centre of the array. This is also true for the polar coordinate monopulse system. It can be shown in the latter case that this tapering results in a difference radiation pattern of circular symmetry with a cross-section identical to that of the conventional difference pattern, though with a doubling of receiver noise.
The side-lobes can be further lowered by reducing the amplitude close to the edge of the array, but in this case the polar monopulse does not correspond exactly with the conventional difference pattern.
With a uniform phase across the array as described above the sum beam is aligned on the bore-sight i.e. along the array axis perpendicular to the plane of the array. In order to steer the beams to some other direction a uniform phase slope has to be applied across the face of the array. The uniform phase slope is applied in known manner by setting the phase delays in the phase shifters 4 so as to be linearly dependent on the positions of the respective detectors in the array. In the case of the polar difference beam the same phase slope must be added to the phase mode.
The annular peak of the difference beam will remain undistorted in terms of sin ~ (where ~ is the angle from bore-sight), however in terms o~ ~ it will appear somewhat egg-shaped.
It may therefore be preferable to perform the calculation of the position of a target offset from the null in terms of sin ~ space and then convert to actual angles. In general the phase shifts applied to the individual antenna elements will be quantised (as for the conventional monopulse) and this quantisation introduces an angular error. The angular error will also be increased by reducing the number of antenna elements as in conventional thinned arrays. It can be shown that the angular accuracy of the polar monopulse is independent of direction, and equal to that which the conventional monopulse achieves in directions at 45 to the centre line axis 14 of array.
The polar difference beam oE the present invention is more difEicult to set up than the conventional monopulse since it involves radial amplitude and circumferential phase changes across the face of the array. By comparison each conventional monopulse difference beam involves amplitude changes in two dimensions but with a single phase reversal at the central axis.
The polar monopulse however requires only two channels instead of three for tracking. This gives a saving of one third in the cost of signal processing and in such auxiliaries as 3amming cancellation loops. Apart from the example described above the invention is applicable to any detectic-~n system in which both the phase and amplitude of the individual elements is preserved. The combining networks can be at rf, if, or lf or alternatively digital signals including phase information can be combined in a computer. In a further modification the outputs from small groups or sub-arrays of antenna elements can be combined as for the individual antenna elements, described.
Further modifications within the scope of the invention described will be apparent to those skilled in the art.
Conventional monopulse tracking systems require three channels to define the positions of targets within the beams of an array of detectors. Two signal difference channels are required to give information on the target offset in two perpendicular directions and a sum channel gives amplitude information. Comparison of the magnitude and sign of these difference signals with the sum signals then gives a target's position relative to the centres of the beams.
The object of the present invention is to provide an alternative form of monopulse detection system which requires only two channels to determine the position of targets and is particularly relevant to circular arrays.
The invention provides a monopulse system for detecting energy received from a target comprising:
(i) a planar array of detectors responsive to the received energy;
(ii) a sum channel for adding together outputs from all detectors;
(iii) a difference channel both for applying to the output of each detector a fixed polar phase delay which is linear in the range O - 2 ~! or an integral multiple thereof, in dependence on the angular position of each detector in the plane of the array and for adding together the product signals formed thereby; and (iv) means for comparison of the outputs from the two channels to derive the position of the target.
The invention thus requires only two channels one : . .
of which produces a sum beam and 1:he other a difference beam wherein the relative amplitucles determine the target's angular off-set and the relative phases determine the direction of this off-set as projected on the plane of the array.
The array may be a uniform distribution of detectors as for example by placing detectors at the interstices of a rectangular grid. However in order to reduce the overall system complexity the invention may also be applied to a thinned array wherein selected detectors are omitted from the uniform array.
In the preferred arrangement the array consists of waveguide apertures, dipoles or similar elements for the radiation and reception of r.f. energy. During reception each element may be connected to a receiverwhich amplifies and/or converts to a lower frequency the energy emitted or reflected by targets. The separate outputs are then preferably passed through selectable phase delays employed for beam steering. The separate phase delayed outputs are added directly to form the sum beam and are also added after applying the fixed polar phase delays to form the difference beamO
With the selectable phase delays set to zero the sum and difference beams are circularly symmetric about the normal to ~h~ plane of the array with a central peak and a central null respectively. By applying phase delays linearly dependent on the positions of the elements in the array, the phase plane may be tiltedrelative to the plane of the array, thereby steering the beams.
Circuitry may be included in the system to automatically steer the sum and difference beams to maintain a target on the axis of the sum beam.
Side lobes in the radiation patterns of both the sum and the difference beams may be attenuated by carrying out L~ 4 ~2 the addition oE the respective signals in the two channels in weighting circuits, wherein each detector output i.s weighted as a function of the distance of the detector from the centre of the array.
The fixed polar O - 2 ~ phase delays applied to the receiver outputs in the difference channel may be introduced by appropriate lengths of transmission line connecting the receivers to the difference channel addition circuit.
In order that these features may be more fully appreciated the in`vention will now be described by way of example only with reference to the accompanying drawings of which:
Fig. 1 is a schematic arrangement to illustrate a monopulse radar tracking antenna;
Fig. 2 is is a functional circuit diagram of a monopulse tracking radar;
Fig. 3 is a circuit diagram of signal processing circuits in a conventional monopulse radar;
Fig. 4 is a circuit diagram of signal processing circuits according to the invention for producing a polar coordinate monopulse radar;
Fig. 5 is a geometrical representation of the planar array used-to define the polar coordinate monopulse radar; and Fig. 6 illustrates the radiation patterns (response curves) of the ~wo beams of the polar monopulse ra~ar.
Monopulse systems started with the use of four horn aerials in a square formation feeding a reflector. A typical arrangement is shown in Figure 1 wherein the four rectangular horns A,B,C and D are placed on the axis of a reflector R. The respective signals received from the horns A to D may be respectively added and subtracted to give a sum signal S and two difference signals ~x and ~y as follows:
S = A + B + C + D
~x = (A + D) - (B + C) ~y = (A + B) - (C + D) ~Z~ 4;2 By comparing the magnitude and sign of the difference signals ~x and ~ y with the sum signal S an azimuth and an elevation servo motor connected to the aerial system can be driven in the appropriate directions to reduce the di~ference signals to zero and so keep track oE a target.
This sytem of separate ~x and ~ y signal channels had been adopted in conventional aerial arrays employing a centralised transmitter and receivers with r.f. feeder networks to isolate the sum and the two difference signals. Since the feeder networks are normally disposed horizontally and vertically the provision of separate rectangular coordinate ~x and ~y difference signal channels is a logical arrangement. However this is no longer necessary in an electronically-scanned active array in which each element or small group of elements within the array is associated with a separate receiver. It is now possible to provide phase shifts by IF or digital processing.
With this type of processing two or more outputs of any desired phase can be obtained from each receiver with no loss of signal to noise ratio.
Figure 2 shows a planar antenna array A, circular or near-circular in shape, which is fully or partially filled with antenna elements B. Each of the elements B is connected to a transmitter 1 or a receiver 2 according to whether the associated switches S1,S~ and S3 are respectively in the lower position (as shown) or the upper position.
During transmission, power from the transmitter 1 is fed via a divider 3 to all the radiating antenna elements B via respective phase shifters 4. The output from each phase shifter 4 may be fed via an optional amplifier 5 to the antenna element B. During reception, the divider3 is used to feed power for a local oscillator 6 via the respective phase shifters 4 and inputs 7 to rf mixers in the receivers 2. Signals received in the antenna elements B are fed to inputs 8 in the respective receivers 2 where they are down-converted to an IF frequency L~4~
signal at the receiver output ~. For both transmission and reception the direction of maximum gain - the main beam -is controlled by the settings of the phase shif-te.rs ~.
In the conventional system the output signal from each receiver 2 is processed as shown in Figure 3 to provide three IF outputs 10, 11 and 12. For the sum signal output 11 (equivalent to S) all the receiver outputs 9 have equal path lengths to the resistor network F, where they are summed. In most cases signals received by antenna elements 13 towards the edge of the array A have additional resistive attenuation 14 (say) in order to suppress side-lobes in the antenna sensitivity pattern. For the azimuth signal output 10 (equivalent to ~x) the signals from all the antenna elements to the left of the centre line 14 of the array A are summed in anti-phase to those on the right in the network F2. Thus the signal from each receiver output 9 is fed to the network F2 via a phase delay 15 set to O or ~
radians depending upon whether the associated antenna element B
is respecti.vely to the left or right of the array centre line 14.
Similarly for the elevation signal output 12 (equivalent to ~y) signals from antenna elements above the array centre line 16 are summed in antiphase to those below in the network F3. Signals from each receiver output 9 are fed via a phase delay 17 to the network F3 with the phase delay set to O or ~ radians depending upon whether the associated antenna elements are respectively above or below the line 16. In both of these dif~erence outputs 10 and 12 low side-lobes are achieved by attenuating or weighting the individual signal amplitudes by an appropriate choice of resistor 17 or 18 depending on the distances of the individual elements from the respective centre lines 14 and 16 as well as from the centre of the array.
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, '1 2 ~
The signals in the three IF outputs 10-12 can then be processed in three identical channels. Finally after separation in known manner by ranye gates the phase and amplitude of the signals in the difference channels are compared with phase and amplitude of the signal in the sum channel to determine the angular offset of targets in the horizontal and vertical directions.
According to the present invention the circuit arrangement shown in Figure 4 is used in co-operation with the antenna array A and circuitry shown in Figure 2. The invention replaces the conventional rectangular cartesian co-ordinate system by applying polar coordinate dependent phases to pro-duce a polar monopulse radar system.
As before the outputs 9 from the receivers 2 as-sociated with each antenna element B in the array A are sum-med in a weighting network F, of resistors 19-21 (of which only three are shown) to produce a conventional sum signal (~) at the output 22 from the network Fl. The output 9 from each receiver 2, however, is also applied via a fixed polar phase delay 23 ( fh) to a second weighting network F4 of resistors 24-26 to produce a signal weighted output difference signal 27 (~). The fixed polar phase delay 23 introduces a phase shift for each antenna element B equal to the.angle ~ of the element measured in a clockwise sense from the array centre line 14. This is achieved by using appropriate lengths of cable to connect the receivers 2 to the network F4, The polar monopulse thus requires only two channels with two weighting networks F1 and F4 compared with the three net-works required in the conventional Figure 3 arrangement.
In addition, amplitude weighting in the F4 network of the difference channel depends only on the distance of the antenna element from the centre of the array i.e. there i5 circular symmetry for this channel as well as for the sum channel. The sum output 22 (~) and the difference out-put 27 (~) are then combined to determine the angular position of the target uniquely by taking the amplitude ratio ~/~
3~Z~ 42 to give the angular offset of the target from the boresight direction, and the phase difference ~ between the two outputs to give the radial direction of the target relative to the centre line 1~.
In a circular array of detectors a phase mode can be selected such that the phase of the individual elements caries linearly with their angular position, with total phase of 2~ n per 360, where n is an integer. It is then possible by suitably phased addition o:E the various modes to generate a bean or a null in particular directions in the plane of the -5a-.
array. This considexation is extended to a circular planar array in the present invention by considering what happens in planes normal to the array. Figure 5 indicates a typical antenna element 28 in a planar array located at an angle ~ from the array centre line 29 and a distance r from the centre 30 of the array.
The amplitude of excitation of the array is a function of the radial distance x only and is represented by:
Amplitude = f(r) If n = 0, there is a constant phase over the face of the array and a beam is formed normal to it. For the sum (~ ) pattern n is therefore chosen to be zero. The cross-section of the resulting beam is indicated by the pattern 31 in Figure 6 as a symmetrical beam centred on the bore-sight 32 (normal to the plane of the array, through the array centre 30). For the polar difference (~) beam n is chosen to be 1. In this case the amplitude excitation of the array as shown by the pattern 33 is symmetrical with a null 34 in the bore-sight direction 32 surrounded by an annular peak 35, constant in amplitude, but with a 360 change of phase around it.
Thus, if a target is within the sum (~ ) beam 31 the ratio of the amplitudes of the signal return~ in the sum 31 and polar difference 33 beams indicates the angular distance of the target off bore-sight. Comparison of the phases of the two returns (~ ) then indicates the radial direction of the target. It is also possible to choose integer values of n greater than 1. This gives higher angular resolution at the expense of ambiguity.
Although not shown in Figure 6, for the purpose of simplicity, a uniform amplitude distribution in a difference pattern generally leads to high side-lobes as well as a lower monopulse angular sensitivityO In the usual rectangular coordinate monopulse system lower side-lobes are greater angular sensitivity are obtained by applying a linear taper in ampli-tude proportional to the distance of the antenna element from the centre of the array. This is also true for the polar coordinate monopulse system. It can be shown in the latter case that this tapering results in a difference radiation pattern of circular symmetry with a cross-section identical to that of the conventional difference pattern, though with a doubling of receiver noise.
The side-lobes can be further lowered by reducing the amplitude close to the edge of the array, but in this case the polar monopulse does not correspond exactly with the conventional difference pattern.
With a uniform phase across the array as described above the sum beam is aligned on the bore-sight i.e. along the array axis perpendicular to the plane of the array. In order to steer the beams to some other direction a uniform phase slope has to be applied across the face of the array. The uniform phase slope is applied in known manner by setting the phase delays in the phase shifters 4 so as to be linearly dependent on the positions of the respective detectors in the array. In the case of the polar difference beam the same phase slope must be added to the phase mode.
The annular peak of the difference beam will remain undistorted in terms of sin ~ (where ~ is the angle from bore-sight), however in terms o~ ~ it will appear somewhat egg-shaped.
It may therefore be preferable to perform the calculation of the position of a target offset from the null in terms of sin ~ space and then convert to actual angles. In general the phase shifts applied to the individual antenna elements will be quantised (as for the conventional monopulse) and this quantisation introduces an angular error. The angular error will also be increased by reducing the number of antenna elements as in conventional thinned arrays. It can be shown that the angular accuracy of the polar monopulse is independent of direction, and equal to that which the conventional monopulse achieves in directions at 45 to the centre line axis 14 of array.
The polar difference beam oE the present invention is more difEicult to set up than the conventional monopulse since it involves radial amplitude and circumferential phase changes across the face of the array. By comparison each conventional monopulse difference beam involves amplitude changes in two dimensions but with a single phase reversal at the central axis.
The polar monopulse however requires only two channels instead of three for tracking. This gives a saving of one third in the cost of signal processing and in such auxiliaries as 3amming cancellation loops. Apart from the example described above the invention is applicable to any detectic-~n system in which both the phase and amplitude of the individual elements is preserved. The combining networks can be at rf, if, or lf or alternatively digital signals including phase information can be combined in a computer. In a further modification the outputs from small groups or sub-arrays of antenna elements can be combined as for the individual antenna elements, described.
Further modifications within the scope of the invention described will be apparent to those skilled in the art.
Claims (6)
OR PRIVILEGE AS CLAIMED ARE DEFINED AS FOLLOWS:
1. A radar monopulse detection system for locating a target in azimuth and elevation by means of energy received from the target comprising:
(a) a planar array of spaced identical antenna elements arranged to form a rectangular lattice array;
(b) a sum channel formed by connecting together in phase the outputs from each antenna element;
(c) a difference channel formed by connecting together the outputs from each antenna element via a respective phase delay which lies between 0 and 2.pi., or an integral multiple thereof, in linear dependence on the angular position of said element in the plane of the array relative to a reference axis of symmetry of the array; and (d) means for comparing the amplitudes and phases of the output signals from the two channels to derive the position of the target.
(a) a planar array of spaced identical antenna elements arranged to form a rectangular lattice array;
(b) a sum channel formed by connecting together in phase the outputs from each antenna element;
(c) a difference channel formed by connecting together the outputs from each antenna element via a respective phase delay which lies between 0 and 2.pi., or an integral multiple thereof, in linear dependence on the angular position of said element in the plane of the array relative to a reference axis of symmetry of the array; and (d) means for comparing the amplitudes and phases of the output signals from the two channels to derive the position of the target.
2. A monopulse detection system according to claim 1 wherein during reception each element may be connected to a receiver which amplifies and/or converts to a lower frequency the energy emitted or reflected by targets.
3. A monopulse detection system according to claim 1 wherein the output from each element is also connected to a selectable phase delay employed for beam steering, the outputs from the selectable phase delays being added directly to form the sum beam and being also added after applying the respective phase delays to form the difference beam.
4. A monopulse detection system according to claim 3 further including circuitry to automatically steer the sum and difference beams to maintain a target on the axis of the sum beam.
5. A monopulse detection system according to claim 3 wherein side lobes in the radiation patterns of both the sum and the difference beams are attenuated by carrying out the addition of the respective signals in the two channels in weighting circuits, wherein each element output is weighted as a function of the distance of the element from the centre of the array.
6. A monopulse detection system according to claim 1 wherein the respective 0-2.pi. phase delays applied to receiver outputs in the difference channel are introduced by connecting appropriate lengths of transmission line between the receivers and a difference channel addition circuit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000441414A CA1221442A (en) | 1983-11-17 | 1983-11-17 | Monopulse detection systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000441414A CA1221442A (en) | 1983-11-17 | 1983-11-17 | Monopulse detection systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1221442A true CA1221442A (en) | 1987-05-05 |
Family
ID=4126549
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000441414A Expired CA1221442A (en) | 1983-11-17 | 1983-11-17 | Monopulse detection systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1221442A (en) |
-
1983
- 1983-11-17 CA CA000441414A patent/CA1221442A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |