World Intellectual Property Organization, WIPO

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TIME REVERSAL ANTENNA NETWORK DIRECTED ENERGY SYSTEMS FBLD OF THE INVENTION

[0001] The invention relates to high power microwave (HPM) directed energy systems and methods.

BACKGROUND

[0002] High power microwave (HPM) directed energy weapon (DEW) systems generally can be classified into one of two categories, narrow band (NB) or ultra wide band (UWB). NB systems transmit a continuous wave (CW) frequency and are most effective when it is known that the target is vulnerable to a particular frequency. UWB systems transmit a very short high-intensity pulse and are generally useful when specific frequency vulnerabilities of the target are not known.

[0003] The advantages of HPM DEW systems are accompanied by some practical limitations. A major practical limitation is that for the power output level required in most DEW applications the prime power source is often excessively large, confining the use of HPM DEW systems to either ground- based stations or to aircraft capable of carrying a generator which can produce the appropriate amount of electrical power. A related limitation is that at output power levels high enough to achieve significant electromagnetic field strength at a sufficient stand-off distance from the target, the extremely intense fields at the radiating aperture may result in field breakover, undermining the aperture’s effectiveness. Though these power-related limitations can generally be overcome, this tradeoff results in a large, heavy radiating aperture that places limits on the applicability of such systems.

[0004] Phased array antennas have been recognized as being able to overcome many of the power-weight limitations discussed above. When the total radiated power comes from an array of N apertures, the amount of power required from a single array element is reduced by a factor of N. Since each array element can have its own source of prime power, the energy burden per element is greatly reduced. This also mitigates the breakover issues, as well as the size and weight problems associated with using a single aperture. There are other advantages in favor of an array of apertures. It is well known that a phased array antenna may be steered electronically providing for rapid, dynamic beamforming, including the ability to shape and thus direct the beam, all without physically moving the aperture. Since arrays can occupy a greater spatial area than does a single antenna, the larger effective aperture size provides a narrower, more focused beam, providing a capability for surgical strike using a HPM DEW.

[0005] Regarding tracking and dispersion limitations, phased array systems present unique difficulties that have impeded their wider use. Traditional delay-and-sum or Fourier beamforming methods require a priori knowledge of a target’s precise location. However, in a likely scenario, both the target and the array platform airborne are in motion, with the target generally moving at high speed. For the RF beam to track the moving target, a sophisticated feedback control system is typically needed to dynamically generate the time delays needed to correctly steer the beam produced by array element motion. Additionally, it is generally necessary that the location of the radiating elements be precisely known, but this is usually very difficult to accomplish in the context of airborne platforms, particularly if each element is aboard a separate vehicle. Moreover, and perhaps most significantly, the medium through which the HPM beam propagates is generally random and inhαmogeneous, owing to atmospheric effects, such as clouds, rain, or the stratosphere, and/or owing to man-made obscurants, such as smoke, rocket plume, and debris. These conditions can make determination of the correct Fourier beamforming weights effectively impossible.

16

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] There are shown in the drawings, embodiments which are presently preferred. It is expressly noted, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0007] FIG. 1 is a schematic view of an unmanned combat air vehicles (UCAV) formation that collectively forms a phased array high power microwave (HPM) weapon system, according to an embodiment of the invention.

[0008] FIG. 2 is schematic view of an exemplary UAV platform for implementing Time Reversal Antenna Network for Directed Energy Weapons (TRANDEW), according to another embodiment of the invention.

[0009] FIGS. 3(a) and (b) are schematic views of two desired energy distributions (beam patterns) on a target plane, according to the invention, one illustrating the energy distribution generated by a planar array flight formation, and the other illustrating the energy distribution generated by a linear array flight formation.

[0010] FIG. 4 is a schematic view of a plurality of individual HPM radiators collectively forming a phased array HPM weapon system to protect airport assets, according to another embodiment of the invention.

[0011] FIG. 5 is a schematic view of a ship-based HPM array for defending against an incoming threat, according to yet another embodiment of the invention. [0012] FIG.6 is a schematic view of an exemplary aircraft-based HPM defense system, according to still another embodiment of the invention.

[0013] FIG. 7 is a schematic view of Time Reversal Antenna Network for Directed Energy Weapons (TRANDE W)-based RF target-painting and/or tracking, according to yet another embodiment of the invention.

DETAILED DESCRIPTION

[0014] According to one aspect of the invention, a directed energy system comprises an antenna array and a microwave power source coupled to each antenna element in the array. The power level of the source, as well as its bandwidth, is specified once a particular application is established. At least one computer or processor is communicably connected, such as over the air, to the array of antenna elements and the sources of radiation. Although generally described as being a directed energy weapon (DEW) system, the inventive system can be used for other applications, including homeland security, airport protection, and other types of security applications.

[0015] Operatively, at least one antenna in the array of antennas transmits broadband microwave radiation toward a suspected target. Backscattered radiation emanating from the target is received by the array of antennas and is time gated by the computer or processor to form time-gated signals. The computer or processor calculates weights for each antenna in the array using an adaptive beamforming algorithm to provide a weighted, time-reversed (TR) signals for transmission to the target. The computer or processor then triggers the source of radiation for each antenna in the array to transmit a high power time reversed signal toward the target. The inventive approach implements a physical time reversal, wherein energy is directed to a single target location and adaptive weights are utililzed to ensure that all other targets receive little or no radiation. The system, according to one embodiment, comprises a plurality of movable unmanned combat air vehicles (UCAVs), wherein each of the UCAVs includes at least one of the antennas comprising the array, a radiation source and a computer or processor.

[0016] The TR process can implemented according to the known principle of optimal matched filtering in order to maximize the signal-to-noise ratio. Although TR is by its very nature non-causal, time reversal of a finite time duration signal can be made causal by introducing a sufficient, though otherwise arbitrary, amount of excess time delay. The TR process receives the backscattered signal emanated by the irradiated target, time reverses that signal, and then retransmits the TR signal.

[0017] Conventional systems, in contrast to the invention described herein, have typically only utilized a TR process for applications where the radiation source and the target are fixed in location. As noted above, however, in the much more likely scenario in which a DEW system would be utilized, both the target and the array platform are airborne and in motion, with the target typically moving at high speed. Motion induces system complexity as compared to a fixed system, including:

1. The target motion is generally unknown and generally subject to abrupt change. For conventional TR applications, the radiators and target are stationary.

2. For the DE application the target may try to evade being radiated, unlike conventional TR applications. Moreover, unlike conventional TR applications, the DE application requires that target tracking be utilized. The inventive TR approach as described herein readily performs this task.

3. In conventional TR applications the nature of the target and resulting frequency sensitivities are generally known. For DE applications the nature of the target may not be known.

4. For the DE application the intervening medium is often changing (smoke, rain, pocket plume, etc.) and unpredictable, unlike in conventional TR applications where the intervening medium is fixed.

5. For conventional TR applications the target location is known to within a rather limited field of view. For DE applications, a full 360-degree is generally required.

[0018] For DEW applications, amplification of a broadband signal is generally required to obtain a power level that, when generally combined with the TR signals from other array radiators, can inflict significant stress on a target. In certain applications, the target is ten kilometers or more distant from the array. The realization of a broadband amplifier capable of producing such high power output levels is presently challenging. However, if a plurality of UCAVs are used, the individual UCAVs can each be outfitted with at least one antenna or an array which would once again bring the power/bandwidth requirements for each array element to be within reasonable levels based on currently available devices.

[0019] Thus, employing an array antenna as the effective radiating aperture mitigates the problem of requiring one source to provide all the energy needed to produce the desired fluence on the target. This in turn allows for greater flexibility and relaxed requirements on each array source selection. The time reversal antenna network-DEW (TRANDEW) system and associated process, according to one embodiment of the invention, mitigates the problems associated with determining the correct timing signals for array control and automatically accounts for the fact that the HPM beam will propagate in an inhomogeneous, random media for which an analytic determination of proper beam weights is not otherwise possible. The TRANDEW process also provides compensation when the target and/or the array platform are in motion through updated calculations.

[0020] In a preferred embodiment, the backscattered signals from an initial interrogatory (typically low energy) microwave radiation of the target are generally digitized (ADC) and preferably pre-conditioned using on-board digital signal processing (DSP). The preconditioning of the backscattered signals generally includes removal of antenna inter-element coupling effects (if any), removal of reflections from unwanted targets, and a time gate to retain substantially only the predominantly direct paths of the signals from the target location. These time-gated signals are time-reversed and weighted using an adaptive beamforming algorithm, such as the preferred RCB algorithm described below, and are then used to trigger the HPM generators which form the TRANDEW. Using all the elements in the array to transmit the TR processed versions of their received signals automatically results in a highly accurate confluence or “refocusing” of energies on the scattering target. The theoretical basis underlying this refocusing is the reciprocity of the wave equation.

[0021] The reception of the backscattered signals will be generally corrupted by noise and interference. Since the received signal are captured by an array of antennas, signal extraction is preferably accomplished using array processing methods, such as adaptive beamforming algorithms. A preferred adaptive beamforming algorithm is the data-adaptive Robust Capon Beamforming (RCB) algorithm, as described in U.S. Pat. No. 6,798,380 (the ‘380 patent) to Li, one of the present inventors, and titled “Robust capon beamforming,” or as described in related U.S. Pat. No. 6,894,642 (the ‘642 patent), also to Li, et al. and titled “Doubly constrained robust capon beamformer.” The ‘380 application discloses a method for enhanced Capon beamforming, referred to therein as an advanced robust Capon beamformer, which includes the steps of providing a sensor array including a plurality of sensor elements, wherein an array steering vector corresponding to a signal of interest (SOI) is unknown. The array steering vector is represented by an ellipsoidal uncertainty set. A covariance fitting relation for the array steering vector is bounded with the uncertainty ellipsoid. The matrix fitting relation is solved to provide an estimate of the array steering vector. Both the ‘380 application and the ‘642 application are hereby incorporated by reference into the present application in their entireties. The following publications also provide details regarding the RCB and related algorithms: J. Li and P. Stoica, Ed., Robust Adaptive Beamforming. New York, NY: John Wiley & Sons, 2005, and J. Li, P. Stoica, and Z. Wang, “On robust capon beamforming and diagonal loading,” IEEE Transactions on Signal Processing, vol. 51, pp. 1702-1715, July 2003.

[0022] The RCB algorithm is a data-adaptive beamforming method which has higher resolution and much better interference suppression capability than its data-independent counterpart, such as the delay-and-sum (DAS) and space-time (Fourier) beamforming methods. The RCB algorithm can also be used to place nulls in the beam pattern at locations where the HPM DEW effects are not desired, such as at locations of unmanned combat air vehicles (UCAVs) when a formation of CAVs forms a phase array PPM weapon system.

[0023] When compared with other methods, a significant advantage of the proposed TRANDEW approach is that it compensates for the inhomogeneous media in which the HPM beam propagates, thus providing improved fluence on the target. Since TR can be performed and updated at a rapid speed using currently available high speed signal processors, both target and weapon platform motion can be accounted and compensated for. Finally, knowledge of the precise location of each antenna element is not required, since the TR process automatically includes the necessary information about the antenna location and the environment in which energy radiates. The combination of the TR process, time gating and RCB weighting ensures that the energy contained in the UWB signal will be directed substantially only to the target of interest which scattered the interrogation signal and not to the numerous other scatterers that are likely to exist.

[0024] An exemplary application involves a UCAV-based weapon system. Although generally described relative to weapon-based systems, the invention can more broadly be used for radiation applications where the source and target are both generally moving, such as air-to- air engagement or even in an air-to-ground engagement where the ground target is a vehicle (e.g., a tank or other military vehicle) which is in motion. Moreover, although the goal is generally to irradiate a single target, the invention can be used to radiate more than one target. When more than one target is engaged, the respective targets are generally engaged one at a time. [0025] The invention can also be applied beyond weapons systems, such as for various homeland security application, some of which are described below. For example, the invention can be embodied as an airport defense system as described below.

[0026] It is widely recognized that area dominance is an essential requirement in modern day military conflicts, and emphasis is being directed toward the use of Unmanned Combat Air Vehicles (UCAVs) to achieve this dominance. The use of UCAVs now allows the deployment of an array-based HPM weapon system without significant additional overhead. For a swarm of N UCAVs, each individual aircraft needs only a fraction (N) of the prime power needed for the collective DEW as well as greatly relaxing the individual UWB source and radiating aperture requirements.

[0027] A high-level depiction of an operational scenario illustrating one possible application of a system, according to the invention, is schematically shown in FIG 1. The HPM phased array comprises single radiating elements on each aircraft in a swarm of UCAVs 101 – 109. Each UCAV carries a DEW prime power source (not shown) that needs only to provide the energy required for its RF array element. Each UCAV is generally able to communicate with every other UCAV such that the swarm forms a fully connected network, such as based on an RF link. Such communication is generally required if RCB array processing is used. As typical of a weapon system, a priori knowledge of the existence of a target and a reasonably accurate initial estimate of its location is generally required. This information can be obtained from an on-board or remote radar system or from an intelligence gathering agent. The information obtained can then be communicated to the UCAV swarm. Once the existence and approximate location of the target has been provided, the system performing the TRANDEW process can proceed as described in the paragraph below.

[0028] A low-power pulse is transmitted from an antenna on any one of the UCAVs 101-109. All UCAV antennas are now set to operate in the receive mode, such as by using a transmit/receive (T/R) switch. The backscattered signal received from the target is captured by each UCAV antenna. A time gate is employed to exclude any unwanted reflections such as signals reflected from other UCAV swarm members or other targets whose range is different (and its associated time of arrival longer) from that of the desired signal backscattered by the target. The backscattered signal from the target automatically contains the delay information about the inhomogeneous, random nature of the medium in which the signal propagated. This information is utilized in the time reversal from the backscattered signal using an adaptive beamforming algorithm, such as the RCB algorithm.

[0029] The weight factors provided by the RCB algorithm on the time gated signals are used to ensure that the TR process does not direct the energy to unwanted directions such as friendly targets or toward other array elements. The RCB algorithm also minimizes the influence of noise as would be encountered in a jammed environment. When these gated signals are used to trigger the HPM UWB sources, the propagation path from the array element to the target will be such that the energy from each source arrives at the target essentially simultaneously and hence maximizes the fluence on-target. The process can be repeated at a rate determined by the HPM UWB source. If the target and/or sources are in motion, their new location will automatically be accounted for in the TRANDEW process.

[0030] FIG. 2 schematically illustrates an exemplary UAV platform 200 according to the invention. Each UAV (node in the network) includes the subsystems illustratively shown in the figure. The HPM antenna 205, illustratively being a monopole in FIG. 2, is a radiator with a broad beamwidth so to enable versatile electronic steering of the array with almost full spherical (isotropic) coverage. The transmit/receive (T/R) module 210 allows the HPM antenna 205 also to be used in a receive mode as well for target identification and tracking. An Analog-to-Digital Converter (ADC) 215 digitizes the backscattered signal for extraction of the target information via DSP by a dedicated on-board computer 220. The on-board computer 220 communicates with other node members via an RF link 225 to accommodate the RCB array processing as described above. Once the required weight information has been obtained, the on-board computer 220 triggers the HPM source 235 that emits the UWB pulse. The weighting that has been coordinated with all other UCAV radiators results in maximum fluence on the target. Prime power 245 is a power source that provides power to the respective subsystems.

[0031] It is noted that the formation pattern in which the UCAV members fly can be used to determine the HPM radiation pattern. The effect of the formation patter is schematically illustrated in FIG. 3 (a), which shows that when the UCAVs fly in a manner to form a planar array pattern, a generally focused beam results. Alternately, when the UAVs fly to form a linear array as schematically illustrated shown in FIG. 3(b), a fan-shaped beam results. Other patterns are possible. The ability to adapt the pattern in this way allows for versatility in how a given target is attacked.

[0032] The invention can support a variety of other systems. Some exemplary systems are described below. Airport Anti-Aircraft Missile Defense

[0033] It is widely recognized that airport security is essential for homeland defense and HPM DEW systems have been identified as being a viable way of securing this defense. For an assembly of N HPM Array Elements according to the invention, each individual element needs only a fraction (TV1) of the prime power needed for the collective DEW as well as greatly relaxing the individual UWB source and radiating aperture requirements. A high-level operational scenario is an airport anti-aircraft missile defense system is schematically illustrated in FIG. 4, which shows a number of individual HPM radiators that collectively form a phased array HPM weapon system to protect airport assets.

[0034] The HPM phased array is comprised of single radiating elements, each conveniently located at various locations around the airport, including in one embodiment on mobile platforms. Each HPM array element requires a DEW prime power source that needs to provide only the energy required for its own RF array element. It is assumed that each array element is able to communicate with every other array element such that the array forms a fully connected network. As with any weapon system, a priori knowledge of the existence of a target and a reasonably accurate initial estimate of its location are generally required. This information can be obtained from an on-board or remote radar system or from some other intelligence gathering agent such as an infrared detection system, and then communicated to the array elements.

[0035] Once the existence and approximate location of the target has been ascertained the TRANDEW process according to the invention preferably proceeds as follows:

• A low-power pulse is transmitted from any one of the HPM Array Elements on the antenna. The HPM Array Elements may be conveniently located on the roofs of buildings, in remote areas of the airport, or on mobile platforms.

• All HPM antennas are first set to operate in the receive mode.

• The backscattered signal from the target is captured by each HPM antenna.

• A time gate is employed to exclude any unwanted reflections such as signals reflected from other antenna elements or other targets whose range is different from that of the desired target.

• The TRANDEW process ensures that energy is not placed in unwanted directions, such as the assets being protected, by placing spatial nulls in those directions.

• When these gated signals are used to trigger the HPM UWB sources, the propagation path from the array element to the target will be such that the energy from each source arrives at the target simultaneously and hence maximizes the fluence on-target.

• The process is repeated at a rate determined by the HPM UWB source. If the target and/or sources are in motion, their new location will automatically be accounted for in the TRANDEW process.

[0036] Each HPM Array Element (node in the network) contains the subsystems previously illustrated schematically in FIG. 2. The HPM antenna, shown as a monopole in FIG. 2, is a radiator with a broad beamwidth to allow for versatile electronic steering of the array with almost full spherical coverage. The optimal radiating element used will be determined by the requirements of the specific application. The use of a transmit/receive (T/R) module allows for the HPM antenna to be used in the receive mode as well for target identification and tracking. An Analog-to-Digital Converter (ADC) digitizes the backscattered signal for the extraction of the target information via DSP by a dedicated on-board computer. The on-board computer communicates with other node members via the RF link to accommodate the TRANDEW processing as discussed. Once the required weight information has been obtained, the on-board computer triggers the HPM source that emits the UWB pulse, the weighting which has been coordinated with all other HPM radiators to result in maximum fiuence on the target.

Shipboard Anti-Aircraft Missile Defense and Other Application Scenarios

[0037] The versatility of the TRANDEW approach allows it to adapt readily to a variety of other applications. For example, the invention can be used to protect ships, as shown in FIG. 5. For the case of ship defense a major problem encountered is the ability to find and track an incoming missile that is flying close to the surface of the water. The excellent time resolution provided by the TRANDEW process can isolate the incoming threat and make ineffective its ability to seek its target.

[0038] Analogously, a single aircraft can be retrofitted to include HPM radiating elements to be used as a defensive or offensive array. This is illustrated in FIG. 6. Though using distributing array elements over the body of an aircraft, referred to as a conformal array, is generally known, array processing has used either Fourier or delay-and-sum beamforming, which has been problematic in this particular application. The TRANDEW approach overcomes many of the problems associated with conformal arrays and provides for almost any aircraft to incorporate an HPM defense system.

[0039] Another application of the TRANDEW system is known as “RF target painting.” As shown in FIG. 7, the TRANDEW system maintains an HPM signal on a target so that a traditional RF-guided kinetic energy weapon may be guided to the target. The TRANDEW approach once again allows for the HPM generating sources to be spatially dispersed, for example, by using the UCAV swarm illustrated in FIG. 7. This makes it more difficult to destroy the illuminating source thus allowing for a continued HPM signal, though reduced in strength, even is some subset of the UCAV swarm is destroyed.

[0040] The foregoing description of preferred embodiments of the invention have been presented for the purposes of illustration. The description is not intended to limit the invention to the precise forms disclosed. Indeed, modifications and variations will be readily apparent from the foregoing description. Accordingly, it is intended that the scope of the invention not be limited by the detailed description provided herein.

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~ by blombladivinden on May 6, 2011.

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