Abstract:
A radar system for detecting the presence of threat providing electronic manmade devices placed in a given area of operation comprising: a radar transmitter for transmitting a radar signal designated as a probe signal to the given area of operation to cause said threat providing devices to produce anomalous energy signals, a radar receiver for receiving a reflected radar signal from said area of operation including said anomalous energy signals, designated as Target Anomalous Response (TAR) signals, a circuit responsive to the received signal for processing the signal to detect the (TAR) anomalous energy signals indicative of a threat producing device.

Description:
FIELD OF INVENTION 
     This invention relates to radar systems and more particularly to a parametric radar employed for manmade device detection. 
     BACKGROUND 
     There have been many threats against various facilities and human rights promulgated by terrorists or other groups intent on destroying property and killing people in order to impart panic and terror in today&#39;s society. Many of these threats and attempts at disruption employ man made devices, which devices can be placed in a small vehicle or hidden in conventional places. Such threat enabling devices may be activated by means of remote control or various other mechanisms, and explode at will. It is often difficult to detect or otherwise discriminate such man made devices from their environment by conventional means. The detection and location of these man made devices is of great importance in mitigating many kinds of threats. 
     Small threats are typically very difficult to detect since they are of low radar cross-section, may be slow moving, or perhaps static, and may be situated in an environment of clutter interference. For example, of importance is the detection of electronic hardware or circuits associated with small classes of weapons and unmanned vehicles. This is especially true in the detection of small UAV systems. In such circumstances, conventional radars that depend upon primary (or direct mirror-like) radio reflections of their transmitted wave forms are relatively non-effective detectors of man made devices and of such threats. In a conventional radar system, increasing the illuminating signal power and correspondingly the energy returned from such objects within a radar beam to achieve improved transmission/range or improved detection sensitivity does not fundamentally improve an object&#39;s signal to clutter signal ratio and thus it&#39;s detectability under clutter dominated conditions (note that in this instance both the signal power of the object and it&#39;s background clutter power will be raised by the same degree and that this does not increase the object&#39;s signal to background clutter power ratio that is fundamental for increased detectability). Increased radar receiver gain yields the same, no net detection value result under clutter dominated conditions. Thus, by providing greater transmitted radar illumination power and/or greater receiver gain, one does not change the object&#39;s signal to clutter signal ratio. Moreover, Doppler discrimination is of limited value in the discrimination of slow moving or static objects of interest within a background of clutter having similar behavior. It is understood that if one has a non-moving object then Doppler radar is ineffective in detecting the object. 
     In combination with the above-noted problems the additional challenge of detecting weak signals from relatively small objects within a large background of interfering signal energy is extremely difficult utilizing conventional radar systems. Secondary radars or transponders are typically employed to detect and track the above-noted types in like environments. However, these objects must be technically cooperative or friendly in the sense of operating active transponding electronics designed for such applications. Thus, as one can ascertain, one can track a small vehicle by conventional radars if the vehicle contains a suitable transponder, and therefore is capable of returning a signal which can be detected as distinct from background signals. However, non-cooperative targets would not use such transponders. For example, a terrorist utilizing a small vehicle or having a man made destructive device on the vehicle would not incorporate or use such transponders. 
     In order to solve the above-noted problems a parametric radar system utilizes radio energy analogous to that associated with conventional radars. However, the system is designed to ignore or otherwise not receive or process primary or directly reflected energy from objects. 
     SUMMARY 
     A parametric radar (PR) system transmits radar signals, which signals are designed to promote the production of anomalous RF energy within electronic devices contained in a threat vehicle or other device/object. Such radar transmitted signals may be referred to as probe signals. Probe signals can be directed to impinge upon the threat object and due to electronic non-linearities therein causing such electronics to produce target anomalous response signals, which response signals are returned to the radar and which are processed by the processing circuitry in the radar system to provide output signals indicative of the nature and location of the threat target. The PR system anticipates the effects that the probe signals will produce due to known classes of nonlinear effects and thereby optimizes the receiver and detection of such objects while rejecting signals of non-interest from other passive reflections at primary radar signal frequencies. The signals from the PR radar pulses which generate the TAR (Target Anomalous Response) signals or return energy that is manufactured within a targeted electronic object by its associated device non-linearities and by related RF convolution mixing and harmonic distortion. These signals can be at a PR radar signal harmonic and/or at radar signal cross-product frequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an application of a high power, parametric radar system according to an embodiment of the invention. 
         FIG. 2  depicts typical transmitted and received signal spectra according to an embodiment of the invention. 
         FIG. 3A  is a system block diagram of processing apparatus for producing and receiving signals according an embodiment of the invention. 
         FIG. 3B  is a block diagram of an exemplary receiver suitable for use within the system of  FIG. 3A . 
         FIG. 3C  is a block diagram of another exemplary receiver suitable for use within the system of  FIG. 3A . 
         FIG. 4  is a block diagram of processing apparatus for producing and receiving signals according an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  there is shown a parametric radar system according to an embodiment of the present invention, which system can be implemented for example, as an airborne radar, surface radar, interceptor or can employ all of the above.  FIG. 1  depicts a schematic view of an aircraft  20  having a high power radar located thereon. The high power radar is directed to an area of operation  25 . The area of operation is an area which for example is known to include a small unmanned vehicle threat or object  21 . The object  21  may be a slow moving automobile or truck or a slow moving device or a static device. The area of operation  25  may contain a great deal of ground clutter which would normally interfere with the reflective signals from conventional radar systems. The threat area can be covered by a surface radar  23  which also directs a high power beam within the area  25  which beam will impinge upon the vehicle threat  21 . Also shown is an interceptor  22 , which interceptor device is located within the desired area, and which interceptor device  22  will receive the signals reflected by the transmission from the airborne radar  20  or by the ground surface radar  23 . It is of course understood that all such devices can be employed and therefore one can utilize an airborne radar system  20  in conjunction with a surface radar system  23  and interceptor  22 . 
     Referring to  FIG. 2 , there is shown one class of receive spectra which would be processed by the parametric radar system according to an exemplary embodiment of this invention. As seen in  FIG. 2  the center frequencies designated as  30  and  31  are typical transmitted frequency spectra such as would be transmitted by the airborne radar or by a surface ground radar. The transmit spectra  30  and  31  are shown and they are transmitted at high power. Shown adjacent thereto are target anomalous responses (TAR) as for example  34  and  35  to the left of signals  30  and  31 , and  36  and  37  to the right thereof. These TAR signals are those responses which would be received and processed by the parametric radar and would be indicative of vehicle or other threats located within the area. 
     Referring to  FIG. 1  in conjunction with  FIG. 2 , the parametric radar (PR) system utilizes radio energy somewhat analogous to that associated with conventional radars. However, the system is adapted to ignore or otherwise not receive or process primary or direct reflections from remotely located objects such as the threat object  21 . Conventional radars utilize primary returns from objects and depend upon various means to reject clutter interference that competes directly with a targeted object&#39;s primary return. The objective of these processes is to yield a sufficiently positive signal to clutter power ratio for reliable detection and subsequent location of the targeted object. Radars utilize spatial directivity, temporal sampling, frequency filtering, signal correlation, normalization and Doppler discrimination processes as fundamental tools to separate primary returns from a target object of interest and from competing reflections that originate from nearby objects of minor interest. None of these techniques by themselves adequately allow for the discrimination of man made electronic objects from passive objects which is the objective of the PR system. The PR system generates signals designed to promote the production of anomalous RF energy within electronic devices to be detected. Such PR generated signals are referred to as probe signals. The PR system not only transmits probe signals appropriate to elicit non-linear responses from electronic devices, it also anticipates the effects of those probe signals as potentially manifested within the energy returned from electronic objects/threats/targets of interest. The system does this in order to optimize the detection of such objects and otherwise reject signals of non-interest. 
     For example, anomalous energy at harmonic and/or cross-product frequencies can be manufactured within an electronic object by device/component nonlinearities and related RF convolution (mixing). Signal energy derived from such anomalous responses can be engineered to occur at other than normal primary radar frequencies or be otherwise separable from energy directly reflected from objects within the radar beam. The type and degree of separation anticipated as a response from the probe signal drives detection parameters within the PR radar that in turn uniquely tag objects of interest while rejecting energies from most passive or non-electronic objects. This energy rejection mechanism thereby eliminates the majority of clutter interference associated with non-electronic objects. In a conventional radar system, increasing the signal power illuminating and therefore energy returned from objects within a radar beam, either through improved transmission or improved receive sensitivity (greater radar illumination power, gain) does nothing to effect a fundamental change in returned object signal to clutter signal ratio. Both signal and clutter scale together in a linear fashion result in no net first order improvement in the radar&#39;s ability to discriminate the desired object from clutter. Increased illumination in a PR system on the other hand, promotes significant signal nonlinearities within electronic objects being illuminated and attendant potentials for higher returned power at harmonic and cross-product frequencies. The class of anomalous response is referred to as the Target Anomalous Response (TAR). This of course has been discussed in conjunction with  FIGS. 1 and 2 . While both electronic and non-electronic objects may be simultaneously illuminated within a PR transmission there is essentially no power returned at TAR frequencies of interest from normal (non-electronic) objects. Direct reflections from non-electronic objects are at primary or probe signal frequencies and not at receiver/detector anticipated TAR frequencies. 
     Thus, the signal to clutter (or “signal to noise”) power ratio at TAR associated cross-product frequencies increases as the illumination power increases. The effect is non-linear and can be dramatic at high illumination power such as to clearly delineate slow moving or static electronic objects of interest from surrounding reflections originating from inanimate objects such as trees, animals or other non-electronic objects. Given that the transmitted direction of probe signals is known and/or received TAR direction is determinable using conventional radar techniques, one can define the bearing angle to the target. Given that the probe signal is modulated analogously to a conventional radar (e.g. pulsed radar) one can utilize the round trip time from probe signal transmission to reception of the TAR as a ranging measurement as in normal radar processing. Thus PR can determine a target&#39;s location. 
     One possible class of probe signal useful within a PR system is one containing two (or more) carriers, separated by a known frequency (or frequencies), both generated simultaneous within the envelope of the transmitted signal. Referring to  FIG. 2  these two carriers are represented by numerals  30  and  31 . Possible cross-product TAR signals generated from such an illumination signal are represented by a family of signals with each signal component separated from its neighbor by the carrier frequency separation within the probe signal. These signals are represented by numerals  34 ,  35 ,  36  and  37  of  FIG. 2 . Typically the first set of newly generated TAR carriers, those closest in frequency to the Probe transmitted carrier, is the strongest and is referred to as third order cross-product terms. Given such a dual carrier probe signal spectrum as depicted in  FIG. 2 , processing within the PR receiver can include a bank of filters with frequency range, spacing and resolution that match that of possible TAR. The PR system can appropriately select, combine and otherwise optimize detection of TAR from that of the primary returns (and other noise) that can potentially compete with or mask the TAR signal. Thus signal detection is enhanced by avoiding clutter dominated probe signal filter bank outputs. In an exemplary configuration, the threat providing electronic device includes a radio receiver having electronic circuitry containing one or more non-linear devices (e.g. a diode) responsive to the probe signal to cause the threat providing electronic device to produce TAR signals. The electronic circuitry may include a circuit board such as a printed circuit board or PCB containing one or more non-linear devices. In another exemplary configuration, the threat providing electronic device includes a radio receiver having an external antenna, where the external antenna receives the probe signal to cause the threat providing electronic device to produce TAR signals. 
     Referring now to  FIG. 3A , there is shown a general block diagram of a radar system and associated processing components according to an embodiment of the present invention. As shown in  FIG. 3A , antenna  40   a  is coupled to transmit/receiver (Tx/Rx) switch  140 . The switch  140  couples the antenna  40   a  to the radar transmitter  100  in a transmit mode for transmitting RF waveform  111  (RF Waveform # 1 ) toward an object  21  in order to attempt to elicit anomalous RF energy from electronic components/circuitry associated with the object. In similar fashion, antenna  40   b  is coupled to Tx/Rx switch  141 . The switch  141  couples radar transmitter  101  which transmits RF waveform  112  (RF Waveform # 2 ) during a transmit mode toward object  21 . The Tx/Rx switches  140 ,  141  are also coupled to respective radar receivers  102  and  103 . Both receivers  102  and  103  are coupled to the Threat Target detection and tracking processor  49 . Processor  49  operates in conventional fashion to measure various signal parameters associated with the output from receivers  102 ,  103  such as signal output power, signal direction and history (include range and/or angle history) for radar operator or other such use. 
     In operation, probe signals having waveforms  111 ,  112  as shown in  FIG. 3A  are transmitted from the radar assembly  300  via antennas  40   a ,  40   b  and impinge on object or threat target  21 . In receive mode, radar return signals are received by the antennas  40   a ,  40   b  and include received primary signals  310  (reflected from the target and associated background or clutter  210 ), in addition to TAR signals  320  transmitted from the threat target  21  in response to stimulation via the probe signals. The received signals are passed to the radar receivers  102 ,  103  via the respective Tx/Rx switches (operating in receive mode). The radar receivers may be adapted so as to anticipate the effects of the probe signals as potentially manifested by the energy returned from the targets and optimized for detecting the resulting TAR frequencies while otherwise rejecting signals of non-interest. Detection and tracking processor  49  coupled to the radar receivers  102 ,  103  operates to process the received signal data for determining TAR energy signals indicative of passive threat target device  21 . 
       FIG. 3B  shows a radar receiver configuration which can be employed as receiver  102  and  103  of  FIG. 3A . An alternative receiver configuration is shown in  FIG. 3C  which also can be employed as receiver  102  and/or receiver  103 . 
     Referring now to  FIG. 3A  and  FIG. 3B , the received or return signal which enters antennas  40 A and  40 B are conventionally amplified or otherwise processed in preprocessing module  41  and applied to a bank of filters  42 . Module  41  includes RF amplifier  70  and RF downconverter  71  having its output applied to filter bank  42 . Each filter in the filter bank is set at a particular frequency and is separated from one another in frequency based on the transmitted probe signal frequencies so as to allow one to discriminate between desired and undesired signal frequencies. For example, for a static target or a Doppler compensated radar receiver, the filter bank is adapted so that the frequency separation of the two primary transmitted signals is used to anticipate projected TAR signal frequency locations (above and below the two primary signals). In one configuration, the frequency difference between the two primary signals is used (e.g. doubly added) to obtain two TAR signal locations above the higher primary signal, and likewise by subtracting (doubly) the two primary signals using the lower primary signal, such that two additional TAR signal locations (below the lower of the primary signals) are obtained. Thus, in the illustrated embodiment a total of four TAR signal components can be individually integrated (via signal integration module  47 ) and passed to the power detection circuitry  49 . 
     As one can see, the outputs TAR 1  to TAR N  of filter bank  42  are directed to the input of processing circuit  45 . The processor circuit  45  enables determination and selection of potential TAR component frequencies according to the received and filtered RF signal data and has an output which is directly input to signal integration circuit or module  47  which provides as an output the received signals to module  49  for subsequent detection and tracking ( FIG. 3A ). 
     As one can ascertain, electronic objects come in a variety of forms, with differing attributes and resulting susceptibilities to PR probe signals and detection. Most electronic devices however, can be generally described as having the ability to couple RF energy to and from free space, whether directly in the form of a discrete antenna or indirectly as a consequence of interconnections and enclosures. One particularly important class of electronic object that is germane to detection by the present PR system and which would be included in unmanned vehicles and other threats is the radio receiver. Radio receivers are necessary command and control links critical to threat function. As such, radio receivers are attached to an external antenna optimized for the receipt of weak radio signals over operational frequency ranges. Antennas provide an ideal means by which PR probe signals can enter and create suitable TAR within radio receiver electronics at designed receive frequencies and/or otherwise susceptible ones (harmonically related, image related . . . etc.). Additionally, if the probe signal spectrum is dynamically changed or swept across the frequency band associated with known threats susceptibilities, the system may be able to optimize the probability of detecting threat receivers and locating them in range and bearing. Once detected, the specific probe signal frequency versus TAR frequency and amplitude responses can be captured as a record. This record of TAR signature susceptibilities may in addition be used to effectively fingerprint and/or identify and track particular threats. 
     Another important class of electronic object for PR detection and localization is the radio transmitter. Note that transmitter bearing is often the only localization parameter determinable by conventional radio location devices; thus the addition of a PR ranging function is significant. This and the ability to dual utilize radar assets for both passive &amp; active localization provide significant advantages to the use of PR. Radio transmitters are associated with key threat communications equipment required to relay sensor data, provide status and acknowledge commands. They are often associated with corresponding receivers and their transceiver architecture, such as a cell phone or other transceiver arrangements. Transmitters alone, whether they are actively transmitting through an antenna or simply connected to an antenna can generate TAR effects like those previously discussed for receivers. However, when one examines an actively transmitting system, there is an additional attribute of interest. The relatively high levels of RF power associated with final stages of the transmitter can act in the role of a local oscillator, mixer and modulator. Mixing between the signal to be transmitted (acting as a local oscillator) with an incoming probe signal provides an opportunity for the generation of significant amounts of TAR. The TAR generated will be the convolution of the spectrum of the “native” threat transmitter spectrum and the spectrum of the incoming probe signal. As a result, newly generated TAR will have spectral components that exceed the product of both spectrums. Given that high levels of TAR are generated that are outside the native threat transmission spectrum, broad amplitude/energy detection techniques in the TAR band may be sufficient to ensure detection and localization. Localization can be performed by measuring round-trip delay to derive range and PR beam steering to derive bearing. 
     It is possible to provide a means to achieve additional sensitivity in the detection of threat transmitter generated TAR signals. This can be performed while also providing rejection of the generally strong transmitter signal itself, by processing PR receive signals within a TAR signal matched correlator. The objective is to apply correlation gain to TAR components while suppressing noise, components of the transmitter signal and those components associated with primary reflections of the PR probe signal itself. 
     It is important to derive a suitable TAR correlation reference signal in the face of a spectrally dynamic, data modulation driven threat object transmitter spectrum. In order to accomplish this, the PR system provides a receiver processor function that generates a baseline TAR or TAR reference signal (synthesized TAR Reference signal) by first filtering the un-TARed threat object transmitter signal components from the received signals that include both PR Probe and TAR signals. This is illustrated with reference to the receiver of  FIG. 3C  adapted for active threat transmissions, wherein the received RF signals from the Tx/Rx switches are preprocessed via module  41  and provided to threat signal filter module  47  which operates to filter the un-TARed threat object transmitter signal components and provides this to the input of mixer  54  whereupon continuously available probe signal replicas  57  are modulated with the unTARed threat object transmit signal components. The filtered threat object transmitter signal  47   b  is independently convoluted with a locally derived PR probe signal replica  57  within the PR processor itself as shown at module  54 . (The probe signal copy can be a suitably range matched replica of the PR signal that would have impinged upon the threat transmitter for the currently being received TAR signal or simply be a continuously applied, CW spectrum). This reference signal  56  thus represents potential TAR components as a real time emulation of the convolution that potentially occurred within the threat object  21  transmitter, at the time the probe signal arrived there, for the time length of the probe pulse. In other words, the convolution circuit operates to process the filtered threat transmitter signal with the probe signal copy. This results in a real time emulation of the potential convolution occurring within the threat transmitter, at the time the probe signal would have arrived there. This emulation can also be accommodated in the correlator circuit  55  shown in  FIG. 3C . 
     In any event, the probe signal replica  57  is obtained from mixer  44   b  of  FIG. 3A . By applying the synthesized TAR signal  56  to one input (A) and the potential threat TAR signals  47   a  output from filter module  51  (which performs filtering for the TAR signal components of the received signal) to the other input (B), correlations between the internally derived PR processor reference and incoming signals provide a signal/correlation time record upon which to base threat detections. This is performed by correlation module  55  of  FIG. 3C . As previously described, mixer  54  provides a synthesized TAR reference signal which is applied as shown at input (A) and in the exemplary illustration stored in memory as indicated by stored TAR reference record module  55   a . This enables a sampled history of the data that may be processed in non-real time as is understood by those skilled in the art. In similar fashion, the anticipated TAR signal spectrum  47   a  from filter  51  is applied as shown at input (B) and in the exemplary illustration stored in memory as indicated by stored received TAR signal record module  58 . The potential threat TAR signals from memory  58  and TAR reference signals from memory  55   a  are combined in multiplier  60 . The output of multiplier  60  is then integrated by baseband integrator  61  and again provides as an output signal vector information corresponding to the received signals for power detection and tracking via processing circuitry  49  ( FIG. 3A ). 
     Typical of signal correlators, sets of overlapping time sampled data records are independently generated for both TAR received signals and for the new reference signal. A subsequent multiplication, signal time sample by respective reference time sample, followed by integration over a given record length of the resulting products produces a single correlation time sample for each correlated record. Correlation in this manner can result in a TAR signal matched filter gain proportional to the record length processed. Thus the correlation approach as previously discussed is able to use knowledge of the threat transmission itself, which in the case of a modulated threat transmission may be spectrally dynamic, in conjunction with stored knowledge of the probe signal and its anticipated effects upon the threat transmission, to synthesize a correlation reference signal and perform related correlations in search of localization information. This correlation process provides ranging, important in the localization of the source of a given threat transmission. The above techniques are applicable in the localization of both ends of a communications link e.g. both the unmanned threat itself and of manned threats employed in the control and data processing ends of its communications link. 
     One may gain high strategic value in implementing the above described technique. Command and control assets include ground control stations, airborne control stations and possibly intermediate relay stations which can be manned or unmanned stations. 
     In another area of PR application one can employ a bi-static radar technique to advantage. An example of this would be the use of an airborne PR as an illuminator for both air and surface searches, while employing a remotely located TAR receiver, that may be in local proximity to potential threats. This, for example, can be employed in  FIG. 1  as using an airborne system  20  and the interceptor  22 . Local proximity gives a sensitivity advantage to the TAR receiver, being local to the threat, as well as a way to avoid integration issues on the airborne PR platform and a need for high power PR generation at the TAR receiver. The receiver can perform ellipse based ranging with knowledge of the probe signal transmission time and locations of both PR platform and TAR receiver. Antenna directivity at the TAR receiver provides further refinement. Given mobility, this bi-static receiver can be employed to physically home in on detected threats. Thus, as one can ascertain, the system proposed and indicated above has many novel attributes which include the high signal to clutter detection aspects of only processing TAR signals. The system has the ability to work with both active and passive target objects and has the flexibility to match a wide range of threats. The system can work with existing high power radars and can exploit current Doppler processing at higher TAR frequencies. 
     A PR Probe spectrum can be generated as a composite of two separate radars and transmission signals working cooperatively, with individual spectral components assigned to each of the radars. In one configuration, two relatively simple radars, one airborne and one terrestrial, for example, can cooperatively illuminate common search volumes to detect &amp; localize potential PR responses. 
       FIG. 4  shows a simplified embodiment wherein a first radar system  300 ′ (e.g. a fixed terrestrial platform) and a second radar system  300 ″ (e.g. an airborne platform) cooperate to illuminate a same area containing a target of interest. In an exemplary embodiment, and referring to  FIG. 4  (in conjunction with the elements illustrated in  FIG. 3A ), the airborne radar platform  300 ″ includes the same functional elements as shown in  FIG. 3A  except that only element  40   a  operates as a transmitter in transmit mode to transmit an appropriately formed waveform to illuminate the target area while both  40   a  and  40   b  of radar  300 ″ operate to receive and process the received or returned signals. In this manner, the configuration of radar system  300 ″ differs from that of system  300  of  FIG. 3A  by the omission of various modules. Fixed platform  300 ′ comprises a transmitter  40   b ′ that operates to transmit an appropriately formed waveform to illuminate the target area. Additional receiver/processor functionality is not needed in this configuration. For purposes of clarity, only the transmit waveforms are illustrated onto the target volume. 
     A communication backchannel link  330  may be established between radars  300 ′ and  300 ′ to coordinate certain information such as GPS data, scheduling of the fixed terrestrial platform transmission times, and global position of the target volume, for example. Through this synchronization (shown schematically as module  340 ) the transmit pulses from each of the waveforms  111 ,  112  of  FIG. 4  arrive at the same time both spatially and temporally. Information as to the location to illuminate, angle, range, transmit coordinates and the like may thus be coordinated. Based on the velocity of the moving platform the TAR components may be determined to arrive at appropriate frequency differentials (i.e. using Doppler frequency analysis). Thus, processing at the receiver associated with moving platform  300 ″ operates in similar fashion to that depicted in  FIG. 3A  except for taking into account the appropriate Doppler waveform shifts associated with the moving platform and the resultant change in TAR component spacing as received by the processor. While the above example describes a first moving platform and a second fixed platform, it is also contemplated by the present invention to utilize two moving platforms with appropriate Doppler frequency discrimination processing to provide appropriate TAR components associated with a targeted device. Furthermore, it is also contemplated that the receiver functionality may reside only on either one or the other of the two radar platforms or subsystems, with a single transmitter configured on the other one of the platforms. 
     To further enhance effectiveness the spectrum of the PR formed convolution (TAR emulation) can be further tailored to exclude all but optimal spectral components of the anticipated TAR. Thus, the output from the convolution circuit and an output from the processing circuit is directed to the correlator. This basically provides correlation gain to the TAR components while further suppresses the main components of the transmitter signal and those components associated with primary reflections of the PR probe signal itself. Repeated correlations are performed between the PR derived reference and incoming signals to provide a signal/correlation time record upon which to base threshold detections. The threshold detection occurs in module  49  which after the above-noted steps provides a signal indicative of the particular type of threat as described above, which threat is located within the threat area. 
     It would be apparent to those skilled in the art that there are many modifications which will be deemed to be obvious and are encompassed within the spirit and scope of the claims appended hereto.