US3039093A

US3039093A - Reflective radar target

Info

Publication number: US3039093A
Application number: US588347A
Authority: US
Inventors: Charles H Rockwood
Original assignee: Cook Electric Co
Current assignee: Cook Electric Co
Priority date: 1956-05-31
Filing date: 1956-05-31
Publication date: 1962-06-12
Anticipated expiration: 1979-06-12

Description

June 12, 1962 c. ROCKWOOD REFLECTIVE RADAR TARGET Filed May 31, 1956 Miq 3,039,093 REFLECTIVE RADAR TARGET Charles H. Rockwood, Evanston, 11]., assignor, by mesne assignments, to Cook Electric Company, Chicago, 11]., a corporation of Delaware Filed May 31, 1956, Ser. No. 588,347 3 Claims. (Cl. 343-18) This invention relates to an improved electromagnetic reflector and more particularly to an improved radar reflector having a large effective cross section and substantially omnidirectional characteristics.

A theoretically perfect omnidirectional radar reflector, that is a reflector which will reflect incident waves uniformly in all directions is a sphere. However, the convex reflective surface which a sphere presents to incident Waves substantially reduces the percentage of transmit-ted energy which is reflected back to a receiver. The use of various shaped directional reflectors has been investigated extensively and the theoretical optimum unidirectional response of a reflector is produced by a perfectly conductive planar member in which the incident radar energy is transmitted toward the plane normal thereto. However, such a reflector is highly directional and even a slight departure from the normal axis produces a substantial reduction in the usuable reflected energy.

It is therefore one particular object of this invention to provide an improved reflector for electromagnetic waves having a large effective electromagnetic cross section.

-It is another object of this invention to provide an improved electromagnetic reflector having a large effective cross section over a substantial range of directions.

It is a further object of this invention to provide an omnidirectional reflector for electromagnetic waves which has a large substantially uniform electromagnetic cross section for incident energy from any arbitrary direction.

It is a still further object of this invention to provide an improved omnidirectional reflector for electromagnetic waves which may be constructed of relatively simple and inexpensive parts with a minimum cost of fabrication.

Further and additional objects of this invention will become manifest from a consideration of this description, the accompanying drawings and the appended claims.

In one form of this invention a solid reflector is provided comprising a plurality of triangular corner reflectors disposed in edge to edge relationship whereby the outer edges thereof form a portion of an icosahedron. For the purposes of this description, the triangular corner reflectors may be termed trihedral angles or trihedrons. More particularly, fifteen right trihedrons are assembled in edge to edge relationship with their apexes directed inwardly toward a common center and the planes of their outer edges defining an icosahedron with five adjacent surfaces removed.

For a more complete understanding of this invention reference will now be made to the accompanying drawing wherein:

FIG. 1 is a perspective view of one embodiment of this invention;

FIG. 2 is a reduced bottom plan view of the embodiment of FIG. 1;

FIG. 3 is a top plan view of the embodiment of FIG. 1, this figure also representing the bottom view of a modified embodiment;

FIG. 4 is a bottom plan view of the embodiment of FIG. 1 with the bottom plate removed to indicate the internal construction of the reflector; and

FIG. 5 is a perspective view of one trihedron forming a part of the embodiment of FIG. 1.

Referring now to the drawings and more particularly 3,39,ii93 Patented June 12, 1962 to FIG. 1, a radar reflector is illustrated mounted on a vertical post .12. The radar reflector comprises a total of forty-five triangular surfaces 14- disposed in a unique manner to produce optimum reflection of incident radar waves from all directions above the ground.

The faces 14 are assembled together in sets of three,

as clearly illustrated in FIG. 5, to form trihedral corner reflectors comprising three

faces

14a, 14b, and 14c, secured together along adjacent edges and having a common apex 16. In the preferred embodiment of this invention the corner reflector 18 is formed very precisely as the corner of a cube. That is, the angle formed by any two faces '14 of the trihedron 18 will be 90 when measured normal to the common edge thereof. Thus, the angle formed by faces 14a and 14b when measured normal to the common edge 26a is 90". Similarly, the

' angle formed between faces 14b and 140 measured norsides. In a regular icosahedron, the sides are triangular and are joined in edge to edge relationship insuch a manner that at every apex of the figure five sides meet. In the instant embodiment each of the planar portions forming a side of the icosahedron are defined by the edges 24 and the area defined by these edges is open with the three triangular faces 14 defining a hollow trihedral angle therein.

In the particular embodiment described, the bottom fifteen triangular faces which would define the bottom five icosahedron sides have been removed and a flat plate 22 substituted therefor whereby the reflector is especially adapted for ground mounting. In the particular embodiment described the vertical post 12 is secured to the bottom plate 22 and the post will normally maintain the reflector above the ground at a predetermined height for most efficient signal reflection. As all incident signals will be arriving at the reflector parallel to or above the ground plane, the removal of the lower sides of the icosahedron will in no way impair the efliciency of the reflector.

It is contemplated that the reflector of this invention may be utilized in many ways. For example, a reflector may be used as a drop from flying aircraft, missiles and the like for testing, military countermeasures, as decoys and the like. In such operations it will be desirable to utilize the entire symmetrical figure comprising a total of sixty triangular faces formed into twenty corner reflectors or trihedrons as a complete balanced solid in the nature of an icosahedron.

Referring now to FIG. 4, a bottom view with the bottom plate removed illustrates the internal construction of the reflector 10. Therein it can be seen that the lower five

trihedrons

18a, 18b, 18c, 18d and lfle extend into the reflector with the apexes thereof directed to the center of the body but disposed from the center a substantial distance. Intermediate the lower trihedrons 18a-e are trihedrons 18g-k which can be clearly seen in FIGS. 1, 3 and 4. The combination of the lower trihedrons 18a-e and the intermediate trihedrons 18g-k form a closed hollow body open at the top and bottom. A top assembly ting edges described and illustrated may be secured together in any manner depending upon the nature of the material employed. The only requirement in choosing materials for use in this invention is that the outer surfaces of the faces have good reflective properties for electromagnetic waves.

In the event that a complete solid is desired, a second star-like assembly identical to the upper assembly comprising stnlctures 18m-q may be constructed and secured to the lower edges 24d-h of the illustrated embodiment. The bottom view of the reflector will then be as shown in FIG. 3, rather than as shown in FIG. 2. As is clear from the showing of FIG. 4 the apexes of all the trihedrons 18a-q are directed toward the center of the solid but are disposed a substantial distance therefrom. The precise spacing of the apexes from the center will, of course, be determined by the nature of each trihedron, the shape of the faces thereof, and the angle therebetween. It has been found that optimum response is accomplished through the utilization of the particular type trihedron described above in which the various faces are disposed in normal relationship. In such a construction the assembly illustrated in FIG. 4 necessarily results.

The advantages of a reflector constructed in accordance with this invention may be roughly determined for certain theoretical calculations. While these calculations are theoretically accurate in determining the approximate response of various reflectors, it has been found in practice that the actual radiations or reflections vary substantially from the calculated values. However, the reflector described hereinabove has proven more uniform throughout the entire range of azimuth and elevation than any reflector heretofore known of comparable size and complexity.

In a typical assumed situation, it has been theoretically determined that the same power ratio (that is the same ratio between incident and reflected power) will be obtained from targets having the following configurations and dimensions: A sphere having a 270 ft. diameter for incident energy having a 1 cm. wavelength, a sphere of 90 ft. for 3 cm. wavelength energy, a sphere of 27 ft. for cm. wavelength energy, and a reflector of the type described having a corner length of 2 ft. for all wavelength small with respect to the corner length.

This was determined by estimating the effective radar cross section of a sphere from the equation:

The effective radar cross section of a single corner reflector for incident energy along the corner axis is estimated from the equation:

where a is the corner length and A is the wavelength.

These equations may be substituted in the general radar equation for calculating power ratio in a closed reflection system:

P, (44r)3R wherein P is the power received at the receiver; P, is the transmitted power; G is the antenna gain; R is the radar range; and F is the pattern propagation factor. For the purposes of this study, it is assumed that the pattern propagation factor F is a constant, namely, unity, and that the radar range, and antenna gain are fixed. Thus the power ratio is a function of effective radar cross section and wavelength only.

As will be clear, substituting Equation 2 in Equation 3 will eliminate the factor and thus power ratio from a corner reflector is independent of wavelength.

Theoretical calculations and tests indicate that an assembly of triangular corner reflectors as shown and described above produce average minimum reflection characteristics of the order of 70% of the reflection along the axis of one corner reflector in all directions and that the reflection is sufliciently uniform to maintain reliable radar indications for all angles of incidence. Thus by multiplying the combined equation of Equations 2 and 3 by the factor .7, the minimum power ratio for a reflector as taught by this invention is determined.

Without further elaboration, the foregoing will so fully explain the character of my invention that others may by applying current knowledge, readily adapt the same for use under varying conditions of service, while retaining certain features which may properly be said to constitute the essential items of novelty involved, which items are intended to be defined and secured to me by the following claims.

I claim:

1. A reflective radar target comprising a closed polyhedron having sixty plane surfaces in the shape of right triangles, said surfaces being made of a material which is reflective to electromagnetic radar radiation, said surfaces being joined edge-to-edge and being arranged in twenty open trihedrons having open sides facing outwardly and apexes pointing inwardly toward the center of said polyhedron, each of said trihedrons being composed of three of said surfaces joined at right angles to one another in a configuration corresponding to an internal corner segment of a cube.

2. A reflective radar target comprising at least a seg-- ment of a polyhedron having sixty plane surfaces in the shape of right triangles, said surfaces being made of a material which is reflective to electromagnetic radar radiation, said surfaces being joined edge-to-edge and being arranged in open trihedrons having open sides facing outwardly and apexes pointing inwardly toward the center of said polyhedron, each of said trihedrons being composed of three of said surfaces joined at right angles to one another in a configuration corresponding to an internal corner segment of a cube.

3. A reflective radar target, comprising a flat base plate in the shape of a pentagon and surmounted by a forty-five sided segment of a sixty-sided polyhedron having plane surfaces in the form of right triangles, said surfaces being made of a material which is reflective to electromagnetic radar radiation, said surfaces being joined edge-to-edge and being arranged in fifteen open trihedrons having open sides facing outwardly and apexes pointing toward the center of said polyhedron, five of said surfaces having edges joined to said pentagonal base plate, each of said trihedrons being composed of three of said surfaces joined at right angles to one another in a configuration corresponding to an internal corner segment of a cube.

References Cited in the file of this patent UNITED STATES PATENTS 2,576,255 Hudspeth et a1 Nov. 27, 1951 2,746,035 Norwood May 15, 1956 2,763,000 Graham Sept. 11, 1956 FOREIGN PATENTS 696,834 Great Britain Sept. 9, 1953 OTHER REFERENCES Cundy et al.: Mathematical Models by Cundy and Rollett, QA11C8, published at Oxford at the Clarendon Press, Oxford University Press, Amen House, London, first edition 1951, reprinted lithographically in Great Britaim at the University Press, Oxford, from corrected sheets of the first edition 1954, 1956, 1957, see pages 13, 82, 86 and Plate 1.