GB2279529A

GB2279529A - Radar

Info

Publication number: GB2279529A
Application number: GB8911380A
Authority: GB
Inventors: Bryan Stephen Rickett
Original assignee: Plessey Co Ltd; Siemens Plessey Electronic Systems Ltd
Current assignee: Plessey Co Ltd; BAE Systems Defence Systems Ltd
Priority date: 1989-05-18
Filing date: 1989-05-18
Publication date: 1995-01-04
Anticipated expiration: 2009-05-18
Also published as: ITMI910291A0; GB2279529B; IT9021752A1; GB8911380D0; DE4042022C2; IT1267962B1; ITMI910291A1; IT9021752A0; DE4042022A1

Abstract

A method of guiding an object 28 by means of a radar information field, wherein a field is created comprising a beam which has a generally annular cross sectional region defining a central null in the field, controlling the beam so that the null is directed towards a destination or target 26, and positioning said object in or at a defined position relative to the null, said object including an antenna and receiver means responsive to the signal strength difference between the annular region of the information field and the null in order to guide the object to the destination or target. The radar field is generated by a four horn feed system, the homes being energised in a time multiplexed manner to produce in turn side lobes A, C, side lobes B, D, whereby an average side lobes A, B, C, D define the annular field with central null, and central tracking beam T. <IMAGE>

Description

RADAR This invention relates to radar, in particular to the generation of a radar information field which is employed for the guidance of an object which is responsive to the field. The present invention has primary application to the guidance of a missile to its target but it could equally well be used for other applications such as a landing system for guiding an aircraft onto an air field or to the guidance of a robotic truck in a warehouse.

The problem of guiding a missile to a target is well known. One common method is to track the target by a first radar system and to track the missile by a second radar system, the missile being issued with control instructions via an RF communication link for guiding the missile to the target. Such an arrangement is clearly expensive in terms of hardware required, and another well known system which has been developed is known as "beam riding". In this arrangement a single radar beam is provided tracking the target. This beam is of a narrow defined shape as illustrated in figure 1, figure 1 showing a radar dish 2 generating a beam 4 tracking a target 6 and a missile 8 which requires guidance. Missile 8 has no direct control from the base station but contains an antenna and receiver for sensing the presence of the radar beam.As the missile approaches the edge of the beam. it will detect a sharp fall in field strength and the missile guidance system will be issued with an instruction to change direction towards the centre of the beam. The missile thus tends to follow a zig-zag path as indicated at 12 veering between the edges of the beam. When the missile approaches the vicinity of the target, it will detect radar signals reflected from the target by a further antenna system and this will enable the missile to hit the target.

Such systems are described for example in "Guidance", Arthur S.

LOCKE, D. Van Nostrand Co. Inc., 1958, page 446 et seq. A significant problem with this arrangement is that as the target is approached, the strength of the beam is diminished considerably and the missile has difficulty in detecting the edge of the beam.

In order to improve missile miss distances, radar tracking and guidance RTG techniques must be improved beyond the capabilities of such beam riding, or command to line of sight systems. Also. the cost of the missiles or munitions must be reduced in order to allow for higher rates of fire in situations where the target is an oncoming missile. It has been proposed to use a phased array system where the radar frequency is swept in order to sweep a narrow beam across the Radar Information Field. The system monitors the time delay with respect to a reference to obtain position fixes. This proposal is thought to suffer from problems related to: (a) the production of a suitable frequency scan array at the required millimetric frequencies.

(b) multipath owing to the relatively large angle over which the RIF exists, causing reflections from land, sea surfaces.

(c) the high precision with which the munition will have to measure the antenna beam shape in order to determine sub beam positions. Optimum tracking is not provided, since it is necessary for accurate tracking to measure a 0.1db signal change, which is a vert difficult problem, since simple receivers in missiles typically respond to 3db signal strength changes.

It is an object of the invention to improve the accuracy with which a missile or other object may be guided to its target with a relatively simple and inexpensive system.

The present invention recognises that radar beams are frequently generated by a monopulse system in which usually four separate channels of radar energy are generated in a four horn system. This arrangement is indicated schematically in figure 3.

Generation of radar energy in each of the horns or channels (a. b. c, d) simultaneously will provide the required narrow tracking beam (T). However if two pairs of adjacent horns are energised in antiphase then the two beams created will be vector added to produce so-called difference beams (A, B, C, D) each of which is located to one side of the main beam and occupies roughly one quadrant of the main beam, as indicated in figure 3. The table accompanying figure 3 shows the combination of horns energised which will produce the beams indicated. The generation of difference beams is a known technique for enabling the missile to be tracked in arrangements where the missile does not ride the main beam.

The concept of the present invention is to generate the difference beams in a time multiplexed manner with the main beam.

The missile or other object is arranged to "ride" the null in the centre (or a defined or programmed position relative to the null) of the four difference beams which coincides with the centre of the main beam.

This arrangement has the advantage that the relative signal strengths between the boundaries of the difference beams and the null are very much higher than the signal strength differences at the edge of the main beam. Thus by arranging the missile control system so that its receiver responds to signal strength differences of the difference beams, it is possible to arrange that the missile rides the null between the difference beams or defined positions relative to the null until it reaches the target.

Accordingly the present invention provides a method of guiding an object by means of radar information field, wherein a field is created comprising a plurality of lobes closely positioned about a central null in the field, controlling the beam so that the null is directed towards a destination or target, and positioning said object in the null, said object including an antenna and receiver means responsive to the signal strength difference between the lobes of the information field and the null in order to guide the object to th- destination or target.

As preferred a separate tracker radar beam is generated in order to track the destination or target, and the radar information field is arranged to follow the direction of the tracker beam. Mg preferred, a so called monopulse guidance system is provided comprising a multi-horn feed system preferably a four horn feed system, in which separate channels of radar wave energy are fed to a radar reflector dish. When the radar pulses in each feed channel are in phase. the narrow target beam is created for detecting the target.

However. when only a pair of adjacent channels are energised in antiphase. then side lobes are created. By energising all adjacent pairs in antiphase in a time multiplex manner, there is provide4 a plurality of side lobes forming a ring with a central null position coaxially with the tracker beam. By positioning the- missile or other object in the null, and arranging for the missile to detect the signal strength difference between the side lobes and the null, it is possible for the missile to ride the null until it is close to the object.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein; Figure 1 is a schematic view of a conventional beam-riding guided missile; Figure 2 is a schematic view of the system in accordance with the invention; Figure 3 is a schematic view of the manner in which the various beams are created in accordance with the invention; Figure 4 is a circuit diagram of the microwave circuits described for achieving the radar beam configurations of Figure 3; and, Figures 5 and 6 are schematic views of the RF receiving systems within a missile guided in accordance with the invention.

Referring now to Figure 2, there is shown a radar system in accordance with the invention comprising a monopulse radar generating system having a main parabolic reflector dish 22, which is steerable to track a target 26, having at its centre a four horn feed system 24 which generates four separate channels of radar wave energy to provide a beam array as indicated in Figures 2 and 3. The main tracking beam T is arranged to track target 26 whereas side beams (A. B, C, D) are arranged to guide missile 28 in a manner described more particularly below.

Referring now to Figure 4 a source of microwave energy 40.

which may be for example a gun effect oscillator is connected to an amplifying traveling wave tube 42 to provide a high amplitude microwave output on waveguide 44. A microwave switch arrangement 46 is arranged selectively to switch the microwave energy into azimuthal channels 48 and/or the elevation channels 50.

Referring to figure 3, the azimuthal channels are defined by the pairs of feed channels (a, c) and (b, d) appropriate energisation of these pairs of channels giving the side beams A, C in accordance with the table of figure 3. Thus in order to achieve appropriate switching of these channels, a further switch 52 is provided for coupling to sum and difference inputs 54, 56 of a "magic T" waveguide switching device 58 which is a known device to give at its outputs appropriate antiphase signals depending on whether the sum or difference inputof the magic-T device is energised.

The elevational channel system 50 is similar to that of the azimuthal channel and similar parts are denoted by the same reference numerals. The channel system 50 is operative to energise pairs of feed channels (a, b) and (c, d) in antiphase with one another when switch 52 is connected to input 56 whereby to generate beams B and D.

As an alternative to a "magic T" device, an electronic phase shifter may be employed as shown in figure 5 to generate appropriate phase shift signals for the pairs of feed channels.

The missile to be guided by the radar information field incorporates an antenna and receiving system as shown in figure 5.

Referring to figure 5, this shows a missile 60 having near its rear end an antenna 62 in the form of a yagi array comprising a row of metallic conductors on the missile outer surface. This provides an appropriate narrow beam width receiving antenna for receiving radar pulses from the base station, which signals are fed into a receiver which includes a mixer 64 for mixing the input signal of the local oscillator signal, an IF amplifier 66 and a simple diode detector 68. The detected signal is sampled in an A/D converter 70 and the digitised signals are processed in a microprocessor 72 in order to derive appropriate output signals at 74 for controlling the missile guidance system 76.

As an alternative to the arrangement shown in figure 5, a simple cnrstal detector may be employed as shown in figure 6 comprising a crystal filter 80 tuned to the radar frequency followed by a diode detector 82 and integrating circuit 84.

Thus in operation of the present invention, the base station is operative to generate a set of radar beams. RF energy is supplied to the four horn feed system at an RF frequency in pulses. The pulses are treated in a time division multiplex manner so that the first pulse is arranged to energise all four channels of the four horn system simultaneously whereby to generate a main tracker beam. Referring to figure 4, switches 52 of the azimuth and elevation channels 48, 50 are switched to the summing inputs 54 whereby to generate in phase signals on the left and right channels to create the central tracker beam according to the table shown in figure 3.

On the next pulse, it will be necessary to create side beams A and C and in order to do this feed horns a and c should be fed with pulses in antiphase to those in horns b, d. Accordingly, switch 59 of the azimuth channel is switched to the differencing input 56 whereby to generate antiphase signals on the left and right output lines. In this way the required antiphase energisation is achieved For the next pulse, it is necessary to generate side beams B and D and to this end elevational channel pair (a, b) is energised in antiphase with elevational pair (c, d); this is done by appropriate switching within channel 50. On the following radar pulse, all four feed horn channels are energised in phase simultaneously to generate the central tracker beam.

In an alternative system, streams of pulses may be provided, the first stream of radar pulses generating the central tracker beam, and the second and sequential pulses generating the side beams.

With such an arrangement, the central beam can track the target in accordance with known procedures and the radar dish 2 of figure 2 is moved appropriately in order to track the target. At the same time the side beams are sequentially energised in order to create a deep null which is co-axial with the tracker beam. The relative signal strengths within the null relative to the side beams over the greater part of the null is about 30db; this is obviously a very large signal difference which can simply be detected. Toward the end of the side beams as shown in figure 2, the relative signal strength within the null and within the beam diminishes to about 3db; but this is ample to enable a simple receiver system within the missile 8 to detect the relative signal strengths as it reaches the target.

The receiving system for the missile for guiding the missile within the null is shown in figure 5 as comprising a conventional superhetrodyne receiver coupled to a diode detector. The output from the diode detector is digitised and the digitised signals are processed in order to provide appropriate signals to the missile guidance system. Since the missile guidance system has a very long time constant in relation to the radar pulses, the fact that the side beams are generated in a time sequential manner is not apparent to the missile guidance system.

In alternative arrangements, the missile guidance system may be programmed to be positioned radially of the central null.

Whilst in accordance with the invention the missile is enabled to reach the target, if desired, in an optional arrangement, when the missile is approaching the target 6 at the end of the side beams, then the missile would be close enough to the target to detect reflections from the target and will thus be enabled to approach the target by means of a suitable antenna and receiving system.

Claims

1. A method of guiding an object by means of a radar information field, wherein a field is created comprising a beam which has a generally annular cross sectional region defining a central null in the field, controlling the beam so that the null is directed towards a destination or target, and positioning said object in or at a defined position relative to the null, said object including an antenna and receiver means responsive to the signal strength difference between the annular region of the information field and the null in order to guide the object to the destination or target.

2. A method according to claim 1, wherein said annular region is defined by a plurality of beam lobes closely positioned about the central null.

3. A method according to claim 2, wherein the radar information field is generated by a monopulse generating system wherein a plurality of feed channels are provided to direct radiation at a radar reflector dish, and the feed channels are energised in a time division multiplexed manner in order to generate in sequential pulses or sequences of pulses side beams which together define a central null, and energising all the feed channels in phase together in order to generate a central tracker beam.

4. A method according to claim 3 wherein four feed channels are provided in a rectangular array, and adjacent pairs of channels are energised in antiphase an order to generate in a time division multiplexed scheme a pair of side beams in azimuth, and a pair of side beams in elevation.

5. A method as claimed in any preceding claim wherein said signal strength difference is sufficient to enable the object to reach the target or destination.

6. A method as claimed in any of claims 1 to 4 wherein when the object approaches the target, the object detects radar signals reflected from the target to enable the object to reach the target.

7. A method of guiding an object as claimed in claim 1 and substantially as described with reference to the accompanying drawings.

8. Apparatus for guiding an object by means of a radar information field, including transmitter means for generating a radar beam having a region which is generally annular in cross-section defining a central null, means for controlling the transmitter means for directing the radar beam so that the null is directed towards a target or destination, said object including means for detecting the signal strength of the beam and for guiding the object in dependence upon the detected signal strength at a position within the central null at a defined position relative to the central null to the target or destination.

9. Apparatus as claimed in claim 6 and substantially as described with reference to the accompanying drawings.

9. Apparatus as claimed in claim 8, wherein said transmitter means is arranged to generate a plurality of side beams closely positioned about said central null.

10. Apparatus as claimed in claim 9, wherein said transmitter means comprises a monopulse generating system comprising a plurality of feed channels arranged to direct radiation at a radar reflector dish, and circuit switching means for energising the channels in a time multiplexed manner in order to generate in sequential pulses or sequences of pulses side beams, by energising selected channels in antiphase, for defining the central null, and, by energising all channels in phase, generating a central tracker beam.

11. Apparatus as claimed in claim 10, including four feed channels arranged in a rectangular array, and means for energising adjacent pairs of channels in antiphase in order to generate pairs of side beams disposed in azimuth and elevation.

12. Apparatus as claimed in claim 10, wherein said circuit switching means comprises an azimuthal channel and an elevational channel. each such channel including a switching device for energising pairs of the feed channels in antiphase or in phase depending on the input of the switching device selected.

1 3. Apparatus as claimed in claim 8 and substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of guiding an object by means of a radar information field, wherein the field is generated by means of a monopulse system comprising a multi-horn feed system wherein pairs of horns are energised so as to create a plurality of side beams defining a central null in the field, controlling the beam so that the null is directed towards a destination or target, and positioning said object in or at a defined position relative to the null, said object including an antenna and receiver means responsive to the signal strength difference between the side beams of the information field and the null in order to guide the object to the destination or target, and wherein a plurality of feed channels are provided for respective horns of the multi-horn feed system, and the feed channels are energised in a time division multiplexed manner in order to generate in sequential pulses or sequences of pulses the side beams which together define the central null, and all the feed channels are energised in phase together in order to generate a central tracker beam.

2. A method according to claim 1, wherein four feed channels are provided in a rectangular array, and adjacent pairs of channels are energised in antiphase in order to generate in a time division multiplexed scheme a pair of side beams in azimuth, and a pair of side beams in elevation.

3. A method as claimed in claim 1 or 2, wherein said signal strength difference is sufficient to enable the object to reach the target or destination.

4. A method as claimed in claim 1 or 2, wherein when the object approaches the target, the object detects radar signals reflected from the target to enable the object to reach the target.

5. A method of guiding an object as claimed in claim 1 and substantially as described with reference to the accompanying drawings.

6. Apparatus for guiding an object by means of a radar information field, including monopulse transmitter means comprising a multi-horn feed system for generating a radar field having a plurality of side beams defining a central null, means for controlling the transmitter means for directing the radar beam so that the null is directed towards a target or destination, said object including means for detecting the signal strength of the beam and for guiding the object in dependence upon the detected signal strength at a position within the central null or a defined position relative to the central null to the target or destination, and wherein said transmitter means comprises a plurality of feed channels coupled to respective horns of the multi-horn feed system, and circuit switching means for energising the channels in a time multiplexed manner in order to generate in sequential pulses or sequences of pulses the side beams, by energising selected channels in antiphase, for defining the central null, and for energising all channels in phase to generate a central tracker beam.

7. Apparatus as claimed in claim 6, including four feed channels arranged in a rectangular array, and means for energising adjacent pairs of channels in antiphase in order to generate pairs of side beams disposed in azimuth and elevation.

8. Apparatus as claimed in claim 7, wherein said circuit switching means comprises an azimuthal channel and an elevational channel, each such channel including a switching device for energising pairs of the feed channels in antiphase or in phase depending on the input of the switching device selected.