GB2266191A - Radar reflector devices - Google Patents

Abstract

A radar reflector device (50) comprises a plurality n of (preferably three) stacked reflector elements (10, 20, 30) each comprising two pairs (10A, 10B) of corner reflectors arranged back-to-back, elements (10) in adjacent layers of the stack being relatively rotated by 360 DEG /2n around the axis of the stack. The top and bottom of each element (10) have location holes for the insertion of corresponding pegs (41) to interengage adjacent elements. Reflecting surfaces may be formed by coating the appropriate surfaces within a cut solid body with reflective foil or paint. <IMAGE>

Description

Radar Reflector Devices The present invention relates to radar reflector devices.

With reference to octahedral reflectors, it is known that adjacent orthogonal corners, supplemented by an intervening dihedral 'flash', produce a lobe response having a shape similar to a butterfly. U.K. Patent 681-666 discloses a reflector comprised solely of a plurality of pairs of corner reflectors in a helical disposition. European Patents A-000-447 & A-0206-054 employ similar arrangements of trihedral (three reflections) and dihedral (two reflections) elements but are designed to avoid mutual phase related cancellations by ensuring adjacent corners do not overlap. 360 degrees azimuthal cover is maintained without gaps by the number of lobes deployed.

The present invention seeks to provide a radar reflector device with an improved response.

According to the present invention there is provided a radar reflector device comprising a plurality, 2n, of pairs of corner reflectors, each pair comprising two radar reflecting surfaces arranged substantially perpendicularly to each other and having a common edge and a divider reflector member arranged generally centrally of and substantially perpendicular to both of the reflecting surfaces, wherein the pairs of corner reflectors are arranged at n levels one above the other to define a longitudinal axis, two pairs of corner reflectors being arranged back-to-back in each layer with their common edges substantially parallel and/or substantially coincident, said corner edges being substantially perpendicular to the longitudinal axis and each common edge being rotated relative to the common edges of the next layer by an angle substantially equal to 360 /2n.

In preferred arrangements n is an integer equal to 3 or more.

The present invention seeks, by exploiting 'glint', to provide a radar reflector device with a significant improvement in radar response. Adjacent corners overlap and their optimum axes of radar response are all directed horizontally. They are so displaced vertically that, within an angle of 6 tilt of the major axes of the device relative to the scanning radar, a h shift between the corners will occur.

Various embodiments of the basic unit have been examined and their radar response measured in a radar anechoic chamber. Noting the requirements of standard ISO-8279 an improvement approaching 100% over reflectors of a similar size and construction has been achieved. Recordings taken of one reflector's radar response vertically (along the major axis) show that the required response is maintained to +/- 30 of tilt.

Thus the present invention is based on the realisation that, provided constructive interference occurs at least some of the time between overlapping lobes, interference is not necessarily detrimental.

The invention is also based on the realisation that radar reflector devices mounted on small boats or other crafts will inevitably be subjected to at least a few degrees of rocking motion. Thus in a reflector device with relatively-vertically disposed reflector elements, the responses from the elements at different vertical heights can be arranged to change their degree of mutual interference as the boat rocks; this produces what is known as "glint".

The present invention also seeks to provide a radar reflector in which the parts forming the reflective surfaces are securely held in position and are protected from harsh environments while maintaining as light a weight as possible.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 is a side elevation of an element of a radar reflector device in accordance with the present invention; Fig. 2 is a face elevation from direction 2 of Fig. 1; Fig. 3 is a vertical cross-sectional view taken along line 3-3 of Fig. 1; Fig. 4 shows the lobe distribution of a threeelement radar reflector device according to the present invention; Fig. 5 shows the lobe distribution in the vertical array; Fig. 6 shows the lobe overlap in a single sector; Fig. 7 shows a reflector plate of a radar reflector element; Fig. 8 illustrates one method of assembling a reflector element; and Fig 9 is a side perspective view, at a smaller scale, of a radar reflector device in accordance with the present invention.

Referring to the drawings, Fig. 1 shows a side view of a radar reflector element 10 comprising two corner reflector pairs 10A and lOB arranged back to back. As can be seen from Figs. 2 and 3, which show front and cross-sectional plan views respectively, each reflector comprises a pair of reflective surfaces 11, 12, arranged perpendicularly to each other, and a reflective divider plate 13 in a central plane of the reflector element and perpendicular to both surfaces 11 and 12. Reflector 10A thus defines two box-corner reflectors to give the conventional butterfly response mentioned above. Reflector lOB gives a similar response at 180 to the response of reflector 10A, i.e.

in the opposite direction.

As can be seen from Fig. 3, the reflector element of the present embodiment is of circular cross-section and portions 15 comprise lips of non-reflective material.

The fact that plates 11 and 12 do not extend to the circumference of the device does not adversely affect performance and has the advantage of reducing the total height of the device. Regions 16 may be conveniently filled with packing material to provide support for the means defining the reflective surfaces.

To form a radar reflector device 50 in accordance with the present invention, element 10 is located above two substantially identical elements 20 and 30, see Fig. 9.

Each element 20, 30 is rotated by 60 from element 10 and from each other so that the entire device gives six butterfly responses equally spaced at 600 intervals viewed azimuthally, see Fig. 4. To connect the elements together they are provided with three equallyspaced location holes 40, the positions of which are also indicated in Fig. 4. Pegs 41 are inserted in the holes 40 to secure the elements together in their desired relative positions. It will be seen from Figs.

3 and 9 that the reflector device 50 is of cylindrical shape. The device 50 may be supported by a rod 51 which may be arranged to extend through a hole along the central axis of the device.

Fig. 4 represents the response of the reflector device 50 to incoming radar waves. The dihedral reflections (i.e. two reflections off surfaces 11 and 12 only) are indicated at 10A, lOB, 20A, 20B, 30A and 30B.

The centres of the two main symmetrical lobes of the butterfly response of reflector 10A are indicated lOAL and LOAN, and similar labelling is used for the five other trihedral responses. These represent reflections off surfaces 11, 12 and the divider plate 13, i.e.

three reflections. It will be noted that the lobes overlap. The overlap between lobes lOAR and 20AL is indicated in Fig. 6.

With reference to the relevant standard for radar reflector devices ISO 8279, and in particular paragraphs 5 thereof, the size of the radar response at the optimum position of the lobes and substantially perpendicular to the vertical axis is completely satisfactory for a cylindrical device of 230 mm diameter and 488 mm height. For locations between the peak regions of the lobes it should be noted that a half wavelength shift at radar frequencies is obtained with less than a six degree tilt of the major longitudinal axis of the reflector device. Thus at at least one position in this angular range, "glint" will occur, i.6. there will be constructive interference between overlapping lobes from reflector elements 10, 20, 30 at different levels.

A convenient way of manufacturing the device 50 is to prepare three cylinders of foamed plastics material each corresponding to the shape of a reflector element 10, 20, 30. Such a material is transparent to radar waves. The material is then cut along the surfaces defining the positions of the reflector surfaces 11, 12 and the divider plates 13. One of the thus-exposed surfaces in each case is then coated with a radar reflective coating, e.g in the form of foil or a paint.

The cut away material is then adhered back into its original place. Three elements 10, 20, 30 prepared in this way are then connected to each other by means of holes 40 and pegs 41, rod 51 may be provided, and the entire assembly can then conveniently be shrinkwrapped in a radar transparent plastics film.

A device produced in this way has the advantages that the reflective surfaces are protected from the weather, the assembly is extremely light, and the reflector surfaces are held firmly in their correct location.

Furthermore the assembly has a neat and smooth appearance.

Numerous modifications may be made to the abovedescribed radar reflector device. For example, the entire device 50 may be made from a single block of foamed plastics material, but cutting away the various portions is not as simple a procedure as described above. Alternatively after providing the regions of material 16 with reflective coatings, the cut off portions of material do not need to be restored; in this case however separate divider plates 13 of relatively rigid material then need to be inserted.

Another method of making the reflective surfaces 11, 12 is to take two slotted plates 61 as shown in Fig. 7, engage the closed ends 63 of their slots 62 and then provide means for securing them perpendicular to each other, e.g. packing material.

Yet another method of manufacturing a radar reflector element 70 is illustrated in Fig. 8. In this case element 70 is collapsible and comprises reflector plates 71 - 74 hinged together centrally along axis 75.

Divider plates 77, 78 are hinged along their edges to the adjacent reflector plate, and are arranged to have central fold lines 79 to allow the entire assembly to fold flat. The assembly may be spring-loaded into its erected disposition. Alternatively, suitable parts of the assembly may be attached to the interior of an inflatable cover or envelope of radar transparent material; inflation of the envelope is arranged to pull the reflecting plates into their correct position.

The rod 51 may be replaced or supplemented by a straining wire along the central vertical axis. If desired, adjacent elements in the stack may be separated by a circular sheet of radar transparent material; this slightly increases the height of the device.

The device may comprise four reflector elements arranged in a vertical stack with adjacent elements being relatively rotated by substantially 45" instead of 60". Even two reflector elements rotated by substantially 900 instead of 600 provide a significant improvement in response over existing reflector devices.

All the angles mentioned above are preferable values; substantial variation may occur without an excessive fall in performance. For example it is possible for the elements to be arranged at up to 15 to the horizontal instead of precisely horizontal.

Furthermore since there is considerable overlap between the lobes (see Figs. 4, 5 and 6), the relative angle of rotation of the elements about the device axis may also vary to increase the overlap in one or more sectors while reducing the overlap elsewhere, possibly down to zero overlap. Thus with a three element reflector device, an individual element may be rotated by up to 26 relative to the described arrangement.

With a four element reflector device, an individual element may be rotated up to 11

Claims (16)

1. Claims 1. A radar reflector device comprising a plurality, 2n, of pairs of corner reflectors, each pair comprising two radar reflecting surfaces arranged substantially perpendicularly to each other and having a common edge and a divider reflector member arranged generally centrally of and substantially perpendicular to both of the reflecting surfaces, wherein the pairs of corner reflectors are arranged at n levels one above the other to define a lontitudinal axis, two pairs of corner reflectors being arranged back-to-back in each layer with their common edges substantially parallel and/or substantially coincident, said corner edges being substantially perpendicular to the longitudinal axis and each common edge being rotated relative to the common edges of the next layer by an angle substantially equal to 3600/211.
2. 2. A device according to claim 1 where n is an integer equal to 3 or more.
3. 3. A device according to claim 1 or 2, wherein the two pairs of corner reflectors in each level constitute a reflector element, said reflector element being of substantially circular shape in plan view.
4. 4. A device according to claim 3, wherein nonreflective regions of the reflector element are located between the edges of said radar reflecting surfaces remote from said common edge, and the circumference of said circular shape.
5. 5. A device according to claim 3 or 4, wherein each reflector element is provided at its top and/or bottom with location means for interengaging with an adjacent reflector element.
6. 6. A device according to any preceding claim wherein the radar reflecting surfaces are constituted by a radar reflective coating on an interface within a solid body.
7. 7. A device according to claim 6 wherein the solid body is a cut and reassembled block of foamed plastics material.
8. 8. A device according to claim 6 or 7 wherein the reflective coating is foil.
9. 9. A device according to claim 6 or 7 wherein the reflective coating is paint.
10. 10. A radar reflector device substantially as hereinbefore described with reference to each of the accompanying drawings.
11. 11. A method of manufacturing a radar reflector device according to any of claims 6 to 9 comprising cutting a substantially cylindrical solid body to define the position of the reflecting surfaces and the divider reflector members, applying the radar reflective coating to one of the thus exposed surfaces in each case, and adhering the cut-away material back into its original place.
12. 12. A method of manufacturing a radar reflector device substantially as hereinbefore described with reference to each of the accompanying drawings.
13. 13. A radar reflector element for a radar reflector device according to any of claims 1 to 10 comprising two pairs of corner reflectors each pair comprising two radar reflecting surfaces arranged substantially perpendicularly to each other and having a common edge and a divider reflector member arranged generally centrally of and substantially perpendicular to both of the reflecting surfaces.
14. 14. A reflector element according to claim 13 provided at its top and/or bottom with location means for interengaging with an adjacent similar reflector element.
15. 15. A reflector element according to claim 14, wherein the location means are constituted by equally-spaced holes.
16. 16. A radar reflector element substantially as hereinbefore described with reference to each of the accompanying drawings.