Communications: directive radio wave systems and devices (e.g. – Radio wave absorber – For camouflage
Reexamination Certificate
2002-05-14
2004-05-18
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radio wave absorber
For camouflage
C342S005000
Reexamination Certificate
active
06738007
ABSTRACT:
FIELD OF INVENTION
The present invention is directed to improvements in or relating to vehicles and structures, and is more particularly directed to methods for reducing the radar signature of structures by directing the radar reflections from structures away from specified directions.
BACKGROUND TO THE INVENTION
Air-borne and water-borne defence vessels are under constant threat of detection and possible attack from non-friendly artillery such as ground-based, sea-going or air-borne weapons systems. Vessel detection is usually by means of radar, since conventional vessels, to a greater or lesser extent, all possess a radar signature.
Radar operates by transmitting a pulse of electromagnetic energy and measuring the time between the transmitted pulse and the receipt of the reflection of the pulse from a target. The range at which a target can be detected is a function of, amongst other things, the intensity of the pulse energy and the size of the target (also known as radar cross-section) with respect to the frequency of operation of the radar. In its simplest form, the best return of the electromagnetic pulse to the radar is generated when the surface reflecting the pulse is normal to the direction from which the pulse is transmitted. In this example, the strength of the return signal is proportional to the square of the area of the surface and the square of the radar frequency.
A number of proposals have been put forward with a view to reducing the radar signature of such vessels. In general these proposals involve purpose-building the vessel with radar cross-section reduction capability. This involves, in the case of a water-borne craft shaping the hull and other components of the craft. One such design is the so-called “stealthy” vehicle. These vehicles are designed so that the shape reflects radar energy away from the threat sectors into more benign areas.
For example, an F117 bomber's threat region is forward and below the aircaft. Hence design is such that energy from a radar signal is reflected upwards and sideways from its angled surface facets. Stealth ships require protection from sea-skimming missiles which normally approach parallel with the sea surface. Such a ship therefore has sloping sides which reflect the radar energy upwards and away from the threat direction. A sharply defined or specular reflected beam emerges, as opposed to a diffuse reflection.
It will be appreciated that vessels incorporating such designs involve significant expense over and above the basic vessel. With tightening of government spending generally and defence budgets specifically, the prospect of replacing an ageing fleet with a new fleet which incorporates an expensive design, even if the des may reduce the likelihood of loss of the vessel when on a war footing, is not an attractive proposition.
For vehicles that are not designed as “stealthy”, the usual method of treatment to reduce radar cross-section is to cover the vehicle with a radar absorbent material. These materials are usually electrical attenuators such as carbon granules embedded in a membrane that is then fastened to the vehicle. This material works by attenuating the signal as the pulse energy passes through it towards the reflecting surface, and then again after reflection so that the resulting return signal is reduced. These methods rely on attenuating the radar beam. Radar absorbent materials of this type are expensive to purchase and install, and create major maintenance problems by trapping moisture and dirt and promoting corrosion of the substrate such as the ship super structure. Other proposals for radar absorbency have included the use of absorbent rubber sheets and paints.
SUMMARY OF INVENTION
The present invention accordingly provides in one embodiment a method for retrofitting a vessel to reduce its radar signature, the method including the step of attaching to surfaces of the vessel structure an array comprising a plurality of elements, the elements having reflective surfaces with substantially planar faces, the arrangement being such that when attached to surfaces of the vessel structure the faces are oriented so as to reflect an incident radar signal in a direction away from its direction of incidence for a given range of incident directions.
The present invention provides in another separate embodiment an array when used for retrofitting to a vessel to reduce its radar signature, the array capable of being fastened to surfaces of the vessel structure and comprising a plurality of elements, the elements having reflective surfaces with substantially planar aces, the arrangement being such that when the array is fastened to the vessel structure the faces are oriented so as to reflect an incident radar signal in a direction away from its direction of incidence for a given range of incident directions.
The present invention provides in another separate embodiment a vessel having surfaces of its structure retrofitted with an array according to the invention.
The orientation of the faces is preferably such that the faces are oblique to the direction of incident radar for a given range of incident directions. Preferably any edges defining the boundary of the reflective surfaces are also oriented so as to be oblique to the direction of incident radar. In this embodiment of the invention, with a surface orientation that is oblique to the direction of incident radar, the strength of the return signal is then only proportional to the square of the length of the edge of the surface normal to the incident radar. If the face is oblique to the direction of incident radar, and the edges of the surface are oblique to the direction of incident radar, then only the corners of the surface will reflect as point sources. As points have no spatial dimension, the strength of the return signal from each point will vary inversely with the square of the radar frequency. In this way the radar cross-section of the vessel, and hence its radar signature, can be reduced for a given range of incident radar directions.
The present invention is capable of providing a degree of control over the direction in which electromagnetic waves are redirected, permitting other directions as well as the incident direction to be avoided where required.
A vessel according to the invention is any ship or vehicle requiring defence against radar threat. A vessel structure according to the invention includes any surface on the vessel capable of reflecting a radar signal. The structure will therefore, in the case of a water-borne craft, include the hull, bulkhead, decking, the bridge, any weapons or weapon turrets, and rigging.
In the case of a water-borne craft, incident radar is generally parallel to the sea surface, although naturally in some cases incident radar will emanate from aircraft. In accordance with the invention the reflective surfaces are preferably oriented so as to reflect an incident radar signal by up to about 30 degrees. More preferably the reflective surfaces are oriented so as to reflect an incident radar signal by up to about 15 degrees and more typically by up to about 8 to 10 degrees away from its direction of incidence for a given range of incident directions.
A surface to which an array according to the invention is attached will include substantially vertical surfaces, substantially horizontal surfaces, surfaces disposed at an angle to the vertical, and curved surfaces.
An element according to the invention may take any suitable form. The element will typically be triangular, polyhedral, pyramidal or prismatic in shape or in cross-section. The element may be an elongated triangle, polyhedron or pyramid. The element may be open-sided. It may be a solid figure. Where an element according to the invention defines an apex, the apex will typically be disposed in a region which is forward (colinear with respect to the incident radar signal) relative to the planar faces of the reflective surfaces.
An array according to the invention may take any suitable form. It may be uniform or non-uniform. If uniform, the array may comprise a gri
Andrea Brian
Clark and Brody
Tenix Defence Systems PTY LTD
LandOfFree
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