Abstract:
An expendable, stand-alone, off-board Electronic Counter-Measure system, airborne RF decoy aimed to provide airborne platforms with protection against multiple radar-based threats including Air-to-Air and Surface-to-Air missiles both active and semi-active ones. The airborne RF decoy has the mechanical outline of standard chaff and flare decoys and is safely ejected from any platform by pyrotechnic elements. The airborne RF decoy deceives enemy radar-based threats as follows: immediately after its ejection from the protected airborne platform, the decoy activates an energy source, stabilizes its path, acquires illuminating signals and analyzes threat parameters. Then the decoy alters the received signals to generate an authentic false target and transmits a deceiving signal towards the radar threat. The radar threats locks on the decoy and follow its path. Thus the threat course is diverted from the protected airborne platform and a large miss distance of the attacking missile (tens to hundreds of meters) is assured.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electronic countermeasures for protecting an aircraft against enemy missile attacks and, more particularly, to an airborne RF decoy that deceives a radar-based missile to track it instead of tracking the aircraft. 
     BACKGROUND OF THE INVENTION 
     Electronic countermeasures (ECM) are a subsection of electronic warfare (EW) which includes any sort of electrical or electronic device designed to deceive radar, sonar, or other detection systems. Electronic countermeasures may be used both offensively and defensively in any method to deny targeting information to an enemy. For example, ECM may cause the detecting radar system to falsely “identify” many separate targets or make the real target appear and disappear or move about randomly. ECM is used effectively to protect aircraft from guided missiles. Most air forces use them to protect their aircraft from attack. 
     Offensive ECM often takes the form of jamming. Defensive ECM includes using chaff and flares against incoming missiles, as well as soids (floating flares that are effective only in the terminal phase of missiles with infrared signature seeker heads), blip enhancement and jamming of missile terminal homers. When employed effectively ECM can keep aircraft from being tracked by search radars, surface-to-air missiles and air-to-air missiles. 
     Electronic counter-countermeasures (ECCM) describe a variety of practices which attempt to reduce or eliminate the effect of ECM on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles. ECCM is also referred to as Electronic Protective Measures (EPM), chiefly in Europe. 
     ECM is practiced by nearly all military units—land, sea or air. Aircraft are the primary weapons in the ECM battle because they can “see” a larger patch of earth than a sea or land-based unit. When employed effectively ECM can keep aircraft from being tracked by search radars, surface-to-air missiles and air-to-air missiles. 
     Modern radar-based threat systems with advanced Electronic Counter-Counter Measures capabilities are immune to existing on-board ECM techniques and pose a real threat to airborne platforms. Several methods for off-board protecting means had been suggested in the past. U.S. Pat. No. 5,333,814 describes a towed body aimed to intercept or collide with incoming threats but without any ECM capability. U.S. Pat. No. 6,492,931 describes an expendable decoy that operates off-board but is dependent on the equipment residing in the protected platform. This decoy is not a stand-alone jammer that can work autonomously against multiple targets and it poses major limitations on the flight envelope of the platform after the launching. Other towed decoy jammers are also known to act in close dependence with the protected platform, both electrically and mechanically. These types of decoy also limit the aircraft maneuvers and lowers the efficiency of other protective measures. 
     U.S. Pat. No. 6,429,800 deals with a true off-board expendable jammer. However, this decoy has no “receive” capability and/or any independent recognition of the enemy threats. It has no Digital Radio Frequency Memory (DRFM)-based equipment that can optimize the deceiving technique, nor any updating mechanism. It has no mechanical and aerodynamical detailed design. The spatial coverage and the frequency coverage are not explicitly described, thus the efficiency against multiple type threats arising from all directions is not proved. 
     SUMMARY OF THE INVENTION 
     The present invention provides an airborne Radio Frequency (RF) decoy that answers to the modern threats which overcomes the above mentioned limitations with full off-board and stand alone capabilities. The goal of the airborne RF decoy of the invention is to “pull/steal” the tracking of the missile and/or radar away from the protected airborne platform and towards the off-board decoy. The decoy thus causes the enemy attacking missile to explode at a sufficiently large distance from the protected airborne platform. 
     The airborne RF decoy of the invention can cope with multiple threats coming from any direction. The decoy does not require intimate knowledge of the technical details of the threats, thus providing a robust ECM solution. 
     The airborne RF decoy of the invention is an expendable, stand-alone, off-board Electronic Counter-Measure (ECM) system aimed to provide airborne platforms with protection against multiple radar-based threats including Air-to-Air (AA) and Surface-to-Air (SAM) missiles both active and semi-active ones. The airborne RF decoy is a stand-alone system that includes a receiver, a transmitter, a digital RF memory (DRFM), a power source and one or more omnidirectional EW antennas, all of which operate dependently of the equipment residing in the protected platform itself. 
     The airborne RF decoy has the mechanical outline of standard chaff and flare decoys and is safely ejected from any platform by pyrotechnic elements. It is compatible with all existing industry dispensers so that no structural or aerodynamical changes are required to the airborne RF decoy, and the operational deployment process is straight forward, that is, identical to the process of ejecting a chaff or a flare. 
     The basic concept of operation of the airborne RF decoy of the invention uses a robust technique to deceive enemy radar-based threats as follows: immediately after its ejection from the protected airborne platform, the airborne RF decoy activates an energy source, stabilizes its path, acquires illuminating signals and analyzes threat parameters. Then the decoy alters the received signals to generate an authentic false target and transmits a deceiving signal towards the radar threat. The radar threats locks on the decoy and follow its path. Thus the threat course is diverted from the protected airborne platform and a large miss distance of the attacking missile (tens to hundreds of meters) is assured. 
     The airborne RF decoy of the invention operates in accordance with a Pre-Flight-Data (PFD) file which defines the most probable threat in the arena. The pre-flight-data file is loaded prior to the mission to each specific decoy by an external data loader via a dedicated connector that is embedded in the decoy. The decoy&#39;s data file can be updated by several methods: before ejection by a wire/proximity link, after ejection via a medium-range wireless link, or via a long-range wireless link with the protected airborne platform. 
     Once a long-range wireless link to the protected airborne platform is established, it can be used for synchronization purposes with the equipment on-board the platform. For example, it can be used for time synchronization with the platform&#39;s radars and self protection suit by blanking the airborne RF decoy at specific time intervals. Alternatively, it can be used for cooperative jamming by blinking between deceiving signals coming from the platform and from the decoy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the protected aircraft ejecting 3 airborne RF decoys, from 3 separate dispensers, towards different directions. 
         FIG. 2  shows the ejected airborne RF decoy attracting an approaching enemy missile towards itself. 
         FIG. 3  illustrates the principle of synchronization/blinking between airborne RF decoy radars of the invention and a protected airborne platform&#39;s radars, via a long range wireless link. 
         FIG. 4  illustrates a physical layout of an airborne RF decoy of the invention. 
         FIG. 5  is an electrical block diagram of an airborne RF decoy of the invention. 
         FIG. 6  depicts a layout of the RF board inside an airborne RF decoy of the invention. 
         FIG. 7  depicts a top view and a bottom view layout of the digital board inside an airborne RF decoy of the invention. 
         FIG. 8  is a schematic diagram of the battery inside an airborne RF decoy of the invention. 
         FIG. 9  is a schematic diagram of EW antennas of the airborne RF decoy of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of various embodiments, reference is made to the accompanying drawings that form a part thereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     The present invention relates to an airborne RF decoy adapted for protecting an airborne platform against multiple enemy radar-based threats, said airborne RF decoy comprising:
         (i) means for receiving a plurality of radar signals from one or more directions;   (ii) means for storing said plurality of radar signals;   (iii) means for analyzing said plurality of radar signals to identify threat parameters;   (iv) means for altering said plurality of radar signals in order to deceive said multiple enemy radar-based threats;   (v) means for transmitting the altered radar signals; and   (vi) an independent power supply source.       

     The airborne RF decoy of the invention is an independent, stand-alone, autonomous flying body. It is not attached to the protected airborne platform by a cable or similar attaching mechanisms, rather the airborne RF decoy flies on its own means, using its own power supply source. 
     The term “airborne platform” as used herein includes fighter aircraft, wide-body transport aircraft, wide-body passenger aircraft, unmanned air vehicles (UAV), unmanned combat aircraft (UCA) and balloons. 
     The installation of the airborne RF decoy of the invention on board of a typical airborne platform is shown in  FIG. 1 . The protected airborne platform  10  may eject at any instant one or several airborne RF decoys  20 . The magazine of RF decoys  30  can be installed at various locations on the airborne platform  10  and the ejection can be directed towards any desired direction.  FIG. 1  shows 3 ejected airborne RF decoys  20 , one in the direction of the flight, a second one ejected sideways and the third airborne RF decoys  20  ejected at the rear of the aircraft, against the flight direction of the airborne platform  10 . In one embodiment of the present invention, the ejection is done using pyrotechnic dispensers. A clear and fluent jettison process ensures the safety of the ejection in any flight positions and speeds of the protected platform. The airborne RF decoy  20  can be ejected by an automatic alert sent either from the on-board Missile Warning System (MWS) or from the Radar Warning Receiver (RWS) or by a manual command of the aircrew. 
     The basic concept of operation uses a generic, robust and coherent technique to deceive the radar-based threats as follows. Once the airborne RF decoy  20  is activated it emits radio frequency (RF) signals that are very similar and coherent to radar signals that are scattered from the protected airborne platform  10  and produce coherent false targets to the enemy radar.  FIG. 2  shows a radar-based threat, which is usually a radar-based missile  40  or a similar flying body, detecting the deceiving signal coming from the airborne RF decoy  20 . The radar-based missile  40  interprets the deceiving signal as a legitimate target and “locks” its attack trajectory  50  towards the airborne RF decoy  20 . Since the airborne RF decoy&#39;s  20  trajectory  60  differs from the trajectory of the protected airborne platform  10 , the radar-based missile  40  hits or flies by (and explodes) the airborne RF decoy  20  at a distance of typically several hundreds of meters from the protected airborne platform  10 . 
     Enemy radar-base threats usually include: air-to-air missiles  40  (both semi active and active), air-to-air fire-control radars (FCR), surface-to-air missiles (SAM)  40 , surface-to-air radars or any combination thereof. 
     The airborne RF decoy  20  emits its deceiving signals within a broad spatial coverage both in azimuth and in elevation. Thus, it can effectively deceive threats coming from all directions. 
     In one embodiment of the present invention, the airborne RF decoy  20  includes means to control the distance between said airborne RF decoy  20  and said airborne platform  10 . For example, the use of rocket propulsion mounted inside the airborne RF decoy  20  can control the relative distance between the protected airborne platform  10  and the airborne RF decoy  20 . In some cases the airborne RF decoy  20  can move in a higher speed than the airborne platform  10  thus operating in front of the airborne platform  10  rather than at its back. The distance between the airborne RF decoy  20  and the airborne platform  10  ranges from tens to hundreds of meters in both range and altitude. 
     The airborne RF decoy  20  opens a large distance of tens to hundreds of meters from the protected airborne platform  10  both in range and in altitude thus any hit of a radar-based threat  40  occurs at a safe range from the airborne platform  10 . 
     The airborne RF decoy  20  can handle multiple radar-based threats  40  simultaneously coming from many directions, thanks to one or more omnidirectional antennas embedded inside the airborne RF decoy  20 . The omnidirectional antenna can receive a plurality of radar signals. The omnidirectional antenna or antennas are implemented without any erection mechanisms or moving parts. In order to improve the probability of deception, the airborne RF decoy  20  operates in accordance with a Pre Flight Data (PFD) file that defines the most probable radar-based threats  40  in the arena. The PFD file is loaded prior to the mission to each individual airborne RF decoy  20  by an external data loader via a dedicated connector. 
     In another embodiment of the present invention, the airborne RF decoy  20  includes means to communicate with the airborne platform  10 . These communication means (links) include: (i) a wire or proximity link; (ii) a short-range wireless link; (iii) a long-range wireless link; or any combination thereof. The proximity link serves a distance of a few centimeters. The short-range link serves typically a distance of a few meters, while the long-range link can operate in a distance of hundreds of meters. 
     The airborne RF decoy  20  PFD can be updated by several methods: the first method is before ejection by a wire or proximity link; the second method is after ejection via a medium-range wireless link; and the third method is via a long-range wireless link with the protected platform.  FIG. 3  illustrates the airborne RF decoy  20  including a wireless radio link  70  which transmits/receives with the protected airborne platform&#39;s  10  wireless radio link  80  via a line of sight communication channel  90 . This long-range communication link provides updating instructions to the airborne RF decoy  20  concerning the actual parameters of the threat such as frequency, bandwidth, transmit power, Pulse Repetition Frequency (PRF), Doppler shift, low frequency modulation (LFM) of the RF signal and others, to ensure the optimal generation of the false target transmission. It should be emphasized that although the airborne RF decoy  20  has the capability to receive and analyze the incoming signal threats, its deception is efficient against all various types of radar-guided threats (active and semi-active) without the need for intimate knowledge of their technical details. This inherent efficiency steams from the physical spatial separation between the airborne RF decoy  20  and the protected airborne platform  10 . 
     In one embodiment of the present invention, the RF decoy  20  includes means for minimizing interferences between the airborne RF decoy  20  and the on-board equipment of the airborne platform  10 . 
     In a further embodiment of the present invention, said means for minimizing interferences include either blanking of said airborne RF decoy  20  so it does not interfere with on-board equipment of the airborne platform  10  when operation of said on-board equipment has higher priority; or blanking on-board systems of the airborne platform  10  that interfere with said airborne RF decoy  20  when operation of said airborne RF decoy  20  has higher priority. 
     Once a long-range wireless link to the protected airborne platform  10  is established it can be used for cooperative jamming with the EW/ECM equipment on-board the airborne platform  10 . For example, it enables generation of combined synchronized blinking between deceiving signals coming from the airborne platform  10  and from the airborne RF decoy  20 . In addition, it can be used for time synchronization by blanking some systems, thus minimizing interferences between the airborne RF decoy  20  and the on-board equipment.  FIG. 3  depicts a possible time sharing between the transmissions from the airborne RF decoy  20  and the transmissions from the radar installed on board of the protected airborne platform  10 . 
     The spatial orientation of the airborne RF decoy  20 , after the ejection from the airborne platform  10 , can be stabilized in the roll plane, in one embodiment, or it is not stabilized in the roll plane, in another embodiment, making use of different embedded antenna polarizations. In most cases the radar-based threats  40  operate in a linear polarization. If the airborne RF decoy  20  is stabilized in the roll plane, its antenna is linear polarized. If the airborne RF decoy  20  is not stabilized in the roll plane, its antenna is circular polarized and has radiation capabilities in all roll angles. 
     In one embodiment of the present invention, said embedded antenna takes the form of a small monopole, an array of two monopoles or an array of three conformal radiating elements when said antenna operates in a linear polarization. 
     In another embodiment of the present invention, said embedded antenna takes the form of helical antennas when said antenna operates in a circular polarization. 
     The aerodynamical stabilization of the airborne RF decoy  20  is achieved by vertical and horizontal wings that are opened automatically after the ejection from the airborne platform  10 . In order to improve the stabilization process, the wings are opened in two steps: a mechanical opening of the wings immediately after ejection followed by a pyrotechnic mechanism, which brings the wings to their final position. In a further embodiment of the present invention, a gas propulsion mechanism can be added to the airborne RF decoy  20 , which enables to accelerate its path and contributes to its flight stability. 
       FIG. 4  shows the physical layout of the airborne RF decoy  20 . An electric battery  110  provides the current and the voltage required for the entire period of operation of the airborne RF decoy  20 . The preferred battery  110  is a thermal battery  110  that can be maintenance-free for a period of at least 10-15 years, being rechargeable or replaceable afterwards. The thermal battery  110  is activated at the instance of the ejection in by an appropriate mechanism  120 . Alternatively, an alkaline battery  110  may be used instead of the thermal battery  110 . 
     The power supply unit  130  is a DC to DC converter which accepts the voltage of the battery (at a nominal value of 12V) and transforms it to several regulated voltages (such as 8V, 5V, 3.3V, 1.8V, 1.2V etc). The RF board  140  includes a microwave low noise receiver operating at a direct conversion technology, a microwave high power transmitter, a frequency synthesizer and a T/R switch (or an isolator). The RF board  140  is connected to an EW antenna  160  mounted on the external envelope of the airborne RF decoy  20  and to a digital board  170  which includes a DRFM with a real time coherent memory and digital control components. 
     The entire body  100  of the airborne RF decoy  20  is stabilized during its flight by horizontal and vertical stabilization wings  180  and possibly by an additional propulsion mechanism. 
     In one embodiment of the present invention, said airborne RF decoy  20  has the external form of a standard chaff decoy or a standard flare decoy. The ejection of the airborne RF decoy  20  can thus be performed by pyrotechnic dispensing mechanisms that are identical to those of standard chaff or flare decoys. The airborne RF decoy  20  can thus be ejected from the airborne platform  10  via a standard housing of chaff or flare dispensers. The RF decoy  20  can thus be implemented in a Mobile Jettison Unit (MJU) such as an MJU-7 envelope (1×2×8 inches) or an MJU-10 envelope (2×2×8 inches). 
     In another embodiment of the present invention, the airborne RF decoy  20  is ejected via a dedicated housing. 
     In addition, the physical layout of the airborne RF decoy  20  may include a standard connector for software loading and tests  200 , a wire/proximity communication module  210 , a medium range communication module  220  and a long-range communication module  70 . 
     The electrical block diagram sketched in  FIG. 5  illustrates the functionality of the airborne RF decoy  20 . The EW antenna  160  receives the RF signals coming from the radar-based threat. The T/R (transmit/receive) switch or the circulator  300  transfer the received signals to a low noise amplifier (receiver)  310  and then the signal is converted into Base band and processed in the Digital RF Memory (DRFM)  320 . The specific EW technique generates a false target, and transfers it to the High Power Transmitter  330 . This false target is then transmitted to the air through the same EW antenna  160 . Additional items of wire/proximity module  210 , medium range module  220 , long-range module  70  and software loading/test connector  200 , all connected to the digital board  170 , are also shown in  FIG. 5 . The airborne RF decoy  20  also includes an independent power supply source  340 . The power supply source  340  can be a standard alkaline battery or a thermal battery that is activated during the ejection of the airborne RF decoy  20  from the airborne platform  10 . 
     In another aspect of the present invention, a method is provided for protecting an airborne platform  10  against multiple enemy radar-based threats by deceiving an enemy to follow a false target, comprising: 
     (i) ejecting an airborne RF decoy  20  from the airborne platform  10 ; 
     (ii) receiving in the airborne RF decoy  20  a plurality of radar signals from one or more directions; 
     (iii) storing said plurality of radar signals on the airborne RF decoy  20 ; 
     (iv) analyzing said plurality of radar signals by the airborne RF decoy  20  to identify threat parameters; 
     (v) altering said plurality of radar signals by said airborne RF decoy  20  in order to deceive said multiple radar-based threats; and 
     (vi) transmitting the altered radar signals by said airborne RF decoy  20 . 
     The airborne RF decoy  20  starts its life cycle by “listening” to said plurality of radar signals in order to identify possible radar-based threats, and acquire the specific active radar-based threats. The plurality of radar signals can be stored in DRFM memory  320  or in any memory with similar functionality. Once the existence of the radar-based threat is confirmed, the airborne RF decoy  20  starts to transmit the deceiving signals. 
     In one embodiment of the present invention, the airborne RF decoy  20  stops transmitting altered signals from time to time and instead analyzes the received radar signals to confirm if each enemy radar-based threat still exists or if the previously identified threat parameters have changed. Threat parameters include frequency, bandwidth, transmission power, pulse repetition frequency (PRF), Doppler shift, low frequency modulation (LMF) of the RF signal, or any combination thereof. The DRFM  320  shown in  FIG. 5  updates the false targets accordingly. 
     The layout of the RF board  140  is further detailed in  FIG. 6 . The Receive channel includes a switch or a circulator  300 , a low noise amplifier  410 , a band pass filter  430  and a balanced mixer  450 . The transmit channel includes a balanced mixer  451 , a phase shifter  440 , a band pass filter  431 , a high power amplifier  420  and the same switch or circulator  300 . The synthesizer unit  460  generates accurate frequency carriers that down convert the signals into low frequency. The RF signals are sampled by I/Q modulator  470  and then transferred unto the DRFM  320 . 
       FIG. 7  shows the layout of the Digital Board  170  including digital processors, analog to digital converters, digital to analog converters, memory units and programmable gate arrays. The real time software that controls the mission of the airborne RF decoy  20  resides in this Digital Board  170 . 
     In yet another embodiment of the present invention, the airborne RF decoy  20  enters automatically into an “end of life” mode with self-destruction capability and complete memory erase for sensitive components that carry data. 
       FIG. 8  shows the layout of a thermal battery  110  including the activation mechanism  120  and the connecting positive port  510 , negative port  520 , and ground port  530 . 
     In yet another embodiment of the present invention, more than one EW antenna  160  is installed in the airborne RF decoy  20 .  FIG. 9  illustrates a schematic diagram with up to three EW antennas  160 . In this case, one EW antenna  160  serves to receive signals while the other one serves to transmit signals. It is even possible to include a third antenna  160  mounted in the front of the airborne RF decoy  20 . All the antennas  160  are fed by the RF Board  140  and receive/transmit to the air in a broad spatial coverage (up to 360 degrees in azimuth and at least 90 degrees in elevation). The antenna  160  can be built to operate in a linear polarization while the airborne RF decoy&#39;s  20  body  100  is stabilized in the roll plane or in a circular polarization while the airborne RF decoy&#39;s  20  body  100  is not stabilized in the roll plane. Thus the antenna  160  enables the airborne RF decoy  20  to operate against multiple enemy radar-based missiles  40  that approach the airborne platform  10  from different directions. The implementation of the antenna can take the form of a small monopole  610 , an array of two monopoles, an conformal array of three radiating elements  620  or a helix structure. The monopole antenna  610  is connected to an antenna feed  600 . An electric layer  630  connects all the antennas  160 . All antenna  160  implementations are mounted on the airborne RF decoy  20  without any erection mechanisms or moving parts. The broad coverage is achieved by a unique combination of scattering by the metallic airborne RF decoy&#39;s  20  body  100  itself, such that the body  100  acts as an antenna. 
     Although the invention has been described in detail, nevertheless changes and modifications, which do not depart from the teachings of the present invention, will be evident to those skilled in the art. Such changes and modifications are deemed to come within the purview of the present invention and the appended claims.