GB2277408A - Radar - Google Patents

Abstract

A radar tracking and guidance system comprising a Cassegrain reflector system including a main reflector dish and a sub-reflector for directing feed radiation from a feed system to the main reflector dish, and including a first polarisation filter positioned over the sub-reflector and selectively rotatable between a first position in which the filter is wholly or mainly transparent to said feed radiation to permit the main reflector dish to be illuminated to produce a tracking beam, and a second position in which the filter is substantially less transparent to said feed radiation so that the filter reflects the whole or a portion of said feed radiation onto only a portion of the main reflector dish whereby a relatively wide beam (RIF) is generated. <IMAGE>

Description

RADAR The present invention relates to a radar system, in particular to the generation of a radar beam which can be used in guiding a missile or other object to its target or destination.

In particular the present invention has application to the simultaneous generation of a radar tracking beam and a radar information field beam, the information field beam being as disclosed and claimed in our copending application (F20698).

However the present invention is not limited to the system disclosed in that application and is of use in any radar system where there is a requirement to provide a narrow tracking beam for example to track a target and a wider beam, for example a so-called "radar information field" (RIF) which is used by a guided missile or other object to enable it to reach the target.

The present invention is concerned with the Cassegrain focussing system in which a radar signal feed assembly is positioned at the centre of a main radar dish in order to direct radiation at a sub-reflector which may have a hyperbolic reflecting surface in order to reflect the radiation onto the whole extent of the main radar dish which is usually in the form of a parabola so as to generate the required beam.

Previously it has been necessary to employ separate feed and reflector systems for generating the tracking beam and the RIF.

In accordance with the present invention, it has been realised that a single feed and reflector system may generate the tracking beam and the RIF, wherein a filter is positioned over the sub reflector and selectively rotated between first and second positions so that in the first position the filter is wholly or mainly transparent to the beam from the feed system and permits the whole of the main reflector dish to be illuminated to produce a narrow (e.g. tracking) beam, and in the second position the filter is substantially less transparent to the beam from the feed system so that the filter reflects the whole or a portion of the feed system beam onto only a portion of the main reflector dish whereby a wide radar beam (e.g.

radar information field) is generated.

In a preferred arrangement, a plurality of polarisation filters are provided which are independently rotatable into operative positions whereby different areas of the main reflector dish may be illuminated whereby to generate beams of a selected width; hence the width of the RIF beam may be controlled.

In a preferred form of filter, the filter consists of metallic wires in a parallel array extending across the sub-reflectors, the wires being spaced apart by the wavelength of the radar beam under consideration (-1 millimetre for 100 gigahertz radiation) and the width of the metallic wires is very fine in comparison to the wavelength. The metallic wires may either be formed as discrete members or alternatively they may be formed as metallic tracks printed on a thin substrate of low dielectric constant, for example plastics, perspex or glass.

The transfer characteristic of the filter is such that it provides a very narrow band of high attenuation with the wires at 90" to the incident electric field where the energy is mostly reflected to the main reflector. As significant angles away from 900 the attenuation is zero and hence most of the radiation passes straight through the filter and is reflected from the sub-reflector. However it is possible in accordance with the invention to position the filter so that an intermediate position of attenuation is achieved on one of the side edges or slopes of the transfer function. In this arrangement, the filter partially reflects the beam to provide a wide field and is partially transparent to the beam, which beam part is reflected from the sub-reflector to provide the tracking beam.

It has been found necessary in many practical situations to maintain the main target tracking beam at all times for efficient tracking whereas it is possible to provide the radar information field at intervals which are sufficiently short that the missile guidance system will not react to the collapsing of the RIF. Thus the filter is switched between a position in which it is wholly transparent to the feed beam and a position in which it is partially transparent to the feed beam.

In other applications, it may not be necessary to rotate the filter, the filter remaining in an accurately maintained intermediate position in which partial reflection takes place.

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 radar track and guidance system comprising a tracker beam for tracking a target and a radar information field for guiding a missile to the target; Figure 2 is a schematic view of a radar reflector dish with a Cassegrain focussing system; and, Figure 3 is a schematic view of polarisation filter in accordance with the invention.

Referring now to the drawings, figure 1 shows a radar track and guidance system comprising a single reflector dish 2 providing a narrow width tracker beam 4 arranged to track a target 6, commonly an oncoming missile. Reflector dish 2 also provides a wider beam radar information field 8 which is operative in accordance with known techniques to guide a further missile 10 towards the target missile 6.

The reflector system in accordance with the invention is shown in figure 2. A main reflector 20 in the form of a parabolic dish has a feed horn 22 mounted at its centre which transmits a plurality of channels of radar wave energy to a sub-reflector system 24. The feed assembly is known as a monopulse feed assembly. The subreflector system comprises a reflector surface of hyperbolic section 26. First and second radar information field filters 28, 30 are provided positioned over reflector surface 26. These filters are selectively rotatable relative to the reflector surface by means of a motor and gearbox system 32.

Referring to figure 3 this shows a front view of a polarisation filter 28, as it would appear to radar energy directed from the feed assembly 22. The filter comprises a circular member 30 comprising a substrate of low dielectric constant, for example perspex, having printed thereon an array of metallic conductors 32 which are spaced apart approximately by the wavelength of the oncoming radiation (which for 100 gigahertz radiation is approximately 3 mm) and the width of the conductors is very thin in relation to the radiation. It will be understood that the perspex substrate 30 is actually hyperbolic in shape to conform to the shape of the sub-reflector and this means that the lines 32 will in fact be projected onto a hyperbolic surface in such a way that when viewed in the direction of figure 3 they appear as straight lines. In the position shown in figure 3, for vertically polarised incident radiation with the electric vector in a vertical direction, the incident radiation is wholly reflected from the wires 32 onto the main reflector dish. By selection of an appropriate sub-reflector geometry, a selected amount of the main reflector dish is illuminated, the narrower the amount of reflector dish illuminated, the wider being the beam generated. This is indicated schematically by the lines 40 in figure 2. Thus it may be arranged that the different filters 28, 30 have different hyperbolic curvatures so as to illuminate different sections of the main reflector dish in order to generate different width RIF beams.

The attenuation characteristics of the filter are indicated in figure 4 for the amount of attenuation of the beam in relation to the orientation of the wires of the filter in relation to an incident vertically polarised beam. Thus for horizontally oriented wires there is a large amount of attenuation, indicating that most of the radiation is reflected. However for relatively small angles of rotation from the horizontal direction, the attenuation decreases rapidly, the 3db point 44 may be only between 5 and 10 degrees away from the horizontal position. For an angle of rotation of about 20 , the radiation beam is not significantly attenuated by the filter and the radiation passes through the filter to be reflected by the main sub-reflector 26.

Thus it may be seen that only a small amount of rotation of the filter is necessary to move it from a wholly transparent orientation to a wholly reflective orientation. In a practical situation it has been found necessary to maintain the tracker beam at all times in order accurately to track the target and it is therefore undesirable that the filter wholly reflect the radiation since in such a situation the tracker beam will collapse. Accordingly it is convenient to rotate the filter from a position in which it is wholly transparent the incoming beam to positions such as that indicated at 46, i.e. a 6db point where half the signal amplitude is reflected from the filter and half the signal amplitude is reflected from the main sub-reflector. Known techniques may be employed to reduce the slope of the attenuation characteristic so that point 46 may be more accurately located.

Claims (7)

1. A radar tracking and guidance system comprising a Cassegrain reflector system including a main reflector dish and a sub-reflector for directing feed radiation from a feed system to the main reflector dish, and including a first polarisation filter positioned over the subreflector and selectively rotatable between a first position in which the filter is wholly or mainly transparent to said feed radiation to permit the main reflector dish to be illuminated to produce a narrow beam, and a second position in which the filter is substantially less transparent to said feed radiation so that the filter reflects the whole or a portion of said feed radiation onto only a portion of the main reflector dish whereby a relatively wide beam is generated.

2. A system according to claim 1 wherein in said first position the filter is wholly transparent to the feed radiation and in said second position the filter is partially transparent to the feed radiation, whereby to maintain the narrow beam at all times and to provide the wide beam at intervals which are relatively short in relation to time constants of the guidance system of an object guided by the wide beam.

3. A system according to claim 1 wherein the polarisation filter comprises an array of wires which appear parallel to one another in the direction of the feed radiation, the wires being spaced by about the wavelength of the radiation and being thus in relation to the wavelength of the radiation.

4. A system according to claim 1 wherein a plurality of filters are provided, each filter being arranged to illuminate a different area of the main reflector dish whereby to provide an RIF of selectable width. Each filter being formed on an appropriate hyperbolic surface.

5. A system as claimed in claim 1, wherein the narrow beam is arranged as a tracking beam to track a target, and the relatively wide beam is arranged as a radar information field to guide an object to the target.

6. A system as claimed in claim 2, modified in that the filter is maintained in said second position and is not rotated in use to said first position.

7. A radar tracking and guidance system substantially as described with reference to the accompanying drawings.

7. A radar tracking and guidance system substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radar tracking and guidance system comprising a Cassegrain reflector system including a main reflector dish and a sub-reflector for directing feed radiation from a feed system to the main reflector dish, and including a first polarisation filter positioned over the sub-reflector and selectively rotatable between a first position in which the filter is wholly or mainly transparent to said feed radiation to permit the main reflector dish to be illuminated to produce a narrow beam, and a second position in which the filter is substantially less transparent to said feed radiation so that the filter reflects the whole or a portion of said feed radiation onto only a portion of the main reflector dish whereby a relatively wide beam is generated.

2. A system according to claim 1 wherein in said first position the filter is wholly transparent to the feed radiation and in said second position the filter is partially transparent to the feed radiation, whereby to maintain the narrow beam at all times and to provide the wide beam at intervals relatively short in dependence on the time period that the filter occupies said second position.

3. A system according to claim 1 wherein the polarisation filter comprises an array of wires which appear parallel to one another in the direction of the feed radiation, the wires being spaced by about the wavelength of the radiation.

4. A system according to claim 1 wherein a plurality of filters are provided, each filter being arranged to illuminate a different area of the main reflector dish whereby to provide an RIF of selectable width each filter being formed on an appropriate hyperbolic surface.

5. A system as claimed in claim 1, wherein the narrow beam is arranged as a tracking beam to track a target, and the relatively wide beam is arranged as a radar information field to guide an object to the target.

6. A system as claimed in claim 2, modified in that the filter is maintained in said second position and is not rotated in use to said first position.