Abstract:
A passive radar decoy comprises a substantially hemispherical upper body portion having its inner surface coated with an RF reflective material, a weighted nose piece, a tapered lower body portion interconnecting the upper body to the nose piece, an RF reflectively-coated corner reflector mounted coaxially within the upper body portion, and means to ram-air inflate the decoy when it is released from an aircraft in flight such that RF energy from a ground source of such energy impinges on the RF reflective surfaces to retro-reflectively return the RF energy back to the source and provide a scintillating and doppler frequency return and enhanced radar target cross section.

Description:
This invention is a continuation-in-part of prior application Ser. No. 06/378,313 filed May 14, 1982 now allowed entitled, “Ram-Air Inflated Passive Radar Decoy and Chaff Package Therefor.” 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to passive radar decoys and more particularly to a self-inflated aero-dynamic body incorporating a unique RF retro-reflective configuration for radar target enhancement and improved radar cross section at millimeter frequencies. 
     The use of chaff to defeat the radar function by denying it range and direction (azimuth and elevation) information is a well known and practiced technique in the art of radar jamming and/or countermeasures. Passive chaff elements in the form of discrete dipoles are dispensed by an aircraft to form a distince cloud which creates a credible false target to the ground-based radar. The dipoles are generally low mass slivers of metalized milar, glass, or other suitable dialectric material and these are very compactly and densely packaged into canisters and loaded into ejection equipment aboard the aircraft. The ejection equipment fire the chaff out of the canisters and into the aircraft windstream where vehicle-induced turbulance or wind shear effects are available for cloud dispersion. The low mass chaff slivers, upon being ejected, rapidly slow down and fall at an almost constant rate. For example, a widely used one mil metalized glass chaff has a settling rate of about 50 feet per minute. 
     One of the problems with present chaff systems is that the low mass slivers are easily damaged by the high compressive force necessary to eject them from the canister and into the aircraft windstream. Being compressed, the chaff dipoles may not uniformly disperse in the windstream and will therefore not provide the desired radar countermeasures performance. 
     Another problem with present chaff systems is the fact that the low mass slivers rapidly slow down upon ejection and therefore the useful life is shortened for lack of a doppler frequency return to the radar. Thus, the radar can update its return information and easily determine the location of the aircraft because of its doppler frequency. 
     It is in accordance with one aspect of the present invention, an object to provide a highly effective false target for enhancement of radar target cross section. 
     It is in accordance with another aspect of the invention, an object to provide a self-inflated, passive radar decoy that achieves coverage at millimeter wave frequencies and upon ejection from an aircraft provides a scintillating and doppler frequency return to a ground-based radar. 
     In accordance with still another aspect of the invention, it is an object to provide a ram-air inflated passive decoy that has a unique RF retro-reflective configuration for millimeter wave radar target performance. 
     The various aspects and advantages of the invention are met in a passive radar decoy comprising a substantially hemispherical upper body portion having an inner surface coated with an RF reflective material, a weighted nose piece, a tapered lower body portion interconnecting the upper body to the nose piece, an RF reflectively-coated corner reflector mounted coaxially within the hemispherically-shaped upper body portion; and means to ram-air inflate the decoy when it is released from the aircraft such that RF energy from a ground source of such energy impinges on the RF reflective surfaces to retro-reflectively return said RF energy back to the source of said energy and provide a scintillating and doppler frequency return and enhanced radar target cross section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 pictorially illustrates the application of the passive radar decoy concept comprising the instant invention; 
     FIG. 2 is an elevational view of a prior art inflatable decelerator, the basics of which are applied to this invention; 
     FIG. 3 is an elevational view of a preferred configuration for the passive decoy that meets the needs of this invention; 
     FIG. 4 illustrates the orientation of the decoy upon ejection from an aircraft in flight; 
     FIG. 5 is a sectional elevational view of the ram-air inflated decoy of this invention when in its operational configuration shortly after ejection from the aircraft; and 
     FIG. 6 is a cross-sectional elevational view of the decoy as it may be packaged with chaff dipoles in an ejection canister. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, FIG. 1 illustrates a radar countermeasures technique as may be employed by an aircraft  10  flying over a hostile territory to create false targets  12  to an enemy radar  14  by dispensing decoys  20  which effectively reflect radar energy to produce a target signal such as may be displayed at  12 . In the usual practice, a plurality of chaff clouds  16  may also be ejected from the aircraft  10  so as to provide a number of false targets  12  and thus saturate defense radar discrimination and computation capacity. 
     In accordance with this invention, the signal frequency performance of the chaff elements  18  may be dramatically increased by dispensing at least one passive decoy  20 , which by reason of its high reflectivity to millimeter wave RF energy, enhances the radar target  12  as seen by the enemy radar  14 . More specifically, the decoy  20  is an aerodynamic body of the BALLUTE™ type capable of ram-air inflation. A typical configuration of the BALLUTE is illustrated in FIG. 2 which shows the external appearance of these type devices. Most notable of the features are a substantially hemispherical main body  22 , a burble fence  24  about the hemispherical portion and slightly above the equator  26  of the body, ram-air inlets  28 , and a ram-air exit port  30 . Similar type BALLUTES are being used as retarding devices for bombs and as safety lowering devices for equipment and the like and are usually attached at the tapered forward end at  22   a.    
     A preferred configuration for the decoy  20  is illustrated in FIGS. 3 and 5. The decoy  20  essentially comprises a substantially hemispherical main body portion  32  that tapers to a forward end  32   a  having a weighted nose piece  34 . The weighted nose piece  34  may include a ram-air inlet  34   a  and whether or not it includes such inlet will depend upon what provisions are made for ram-air inflation of the body. For example, the decoy  20  of the preferred embodiment is made in two parts, a substantially hemispherical main body portion  36  and a tapered lower body portion  38 , the upper and lower portions being connected about their peripheral edges  36   a ,  38   a  by a plurality of cords generally indicated at  40 . In this configuration the separation of the body portions  36 ,  38  by the cords  40  allows for rapid ram-air entry into the body and inflation thereof immediately upon being ejected from the aircraft. While various other configurations of the BALLUTE body may be applied to this invention, the open body concept illustrated in FIG. 3 may be packaged into a smaller storage space and further is found to ram-air inflate in the shortest time over other configurations using ram-air inlets  28  as shown in FIG.  2 . In any case, the decoy  20  is provided with a plurality of ram-air exit holes  42  about the main body portion  36  that function to bleed off excessive ram air and thus stabilize the aerodynamic operation of the body. An additional ram-air exit port  44  may be provided at the top of the hemisphere  36  in line with the body axis however test results indicate that such additional exit port may only be necessary in a fully enclosed BALLUTE body of the type illustrated in FIG.  2 . 
     ™ Goodyear Aerospace Corporation, Akron,. Ohio  
     The decoy body  20 , comprised of portions  36 ,  38 , may be made from any of a number of well-known materials including fabrics and films which are able to be packaged in a minimum of space while also being capable of rapid inflation and strong enough to accept the force of ram-air upon ejection from the aircraft. 
     The decoy  20  has for its primary purpose the ability to reflect millimeter wave RF energy and in this respect the interior surface  40  of the hemispherical body portion  36  is coated with an appropriate RF reflective material. It will be appreciated by those knowledgeable in the art that the hemispherical body provides an ideal geometrical configuration for the return of the “glory rays”  52  of the incoming radar as illustrated in FIG.  5 . In this respect, the diameter of the hemisphere  36  is chosen so as to cover the millimeter frequency range of the incoming radar to be reflected. 
     For example, a 12-inch diameter hemisphere will reflect RF energy in the frequency range of 36 GHZ and again at about 95 GHZ. 
     Mounted within the hemispherical body portion  36  is a second RF reflective member  60  comprised of two orthogonally intersecting triangular planes  62 ,  64  which are mounted on a substantially square shaped base  66 . The combination of the elements  62 ,  64 ,  66  comprise a three dimensional reflector  60  that represents a very strong radar target due to the inherent backscattering gain accomplished by this type configuration. As illustrated in FIG. 5, the return  54  provided by the corner reflector  60  is additive and combines with the “gloryray” return of the hemispherical portion and a phase difference is exhibited between them. The addition of the two radar returns will produce the requisite scintillating signal needed to successfully simulate the radar echo of the aircraft  10 . Of course, the corner reflector  60  may be made from numerous and various materials that may be coated with a radar reflective film. For example, extremely light weight but strong fabrics, films, and crushable foams are available and any of these materials may be applied to this application. The corner reflector  60  may also be mounted within the hemispherical portion  36  by various means such as for example cords  68 . The exact position and location of the cords  68  may vary and the inventive concept is not considered limited by the manner of such mounting. 
     FIG. 6 illustrates the packaged configuration of the decoy  20  and chaff  18  as these may be mounted in an ejection canister or tube  70  aboard the aircraft  10 . The canister  70  includes a piston  72  at one end thereof for forceful ejection of the contents of the tube out of the opposite end  70   a . The decoy  20  is mounted within an inner tube  74  with the weighted end  34  positioned at either end of the tube. As shown in the embodiment illustrated in FIG. 6 the weight  34  may be positioned adjacent the piston  72  and the inner tube  74  abuts the weight  34 . In this circumstance, a force F on the piston  72  is transmitted to the weight  34  and tube  74  and not to the crushable contents of the canister  70  which includes chaff elements  18 . The chaff elements  18  are cut to various lengths to cover a specific range of RF frequencies and are densely packaged between the inner tube  74  and the walls of the canister  70 . The alternative packaging arrangement, i.e., the weight  34  at the opposite end of the canister  70 , is described in the prior application referenced earlier in this specification. 
     Irrespective of the mounting arrangement within the ejection tube  70 , the decoy  20  will reach its most advantageous orientation with respect to the search radar  14  almost immediately upon being ejected. FIG. 4 illustrates ejection of the decoy-chaff package from an aircraft  10 . At the time of ejection, the weight  34  is moving at substantially the same velocity “v” and in the same direction as the aircraft  10 , and ram-air inflation of the hemispherical portion  36  of the decoy body will provide a doppler and scintillating frequency return signal to the radar  14  that is a credible false target. Because the trajectory of the decoy  20  is in the same direction as the aircraft  10 , it provides a doppler frequency return for at least 0.5 seconds after ejection. Thereafter trajectory falloff is quite sharp and its useful life from a practical countermeasures standpoint is terminated. However, the useful life is sufficiently long to fool the radar and additional decoy/chaff packages may be ejected to, continue to decoy the radar. 
     While certain representative embodiments and details are shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made thereto without departing from the spirit or scope of the invention.