Abstract:
A linear FM pulse radar with Doppler processing of co-polarized and cross-polarized radar return signals isolates the target echo signal content associated with a moving pedestrian to provide high quality target echo data for standoff HCE detection based on polarimetric signature analysis. Baseband co-polarized and cross-polarized radar return signals are repeatedly and coherently integrated across numerous successive radar return pulses to create co-polarized and cross-polarized range vs. velocity (Doppler) data maps. The co-polarized data map is used to identify a moving pedestrian, and co-polarized and cross-polarized data subsets corresponding to the identified pedestrian are extracted and subjected to polarization signature analysis to determine if the pedestrian is bearing explosive devices. Low pass filtering of the of the baseband co-polarized and cross-polarized radar return signals prior to integration provides range aliasing to reject signal content associated with objects beyond the unambiguous range of the radar apparatus.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to the detection of human carried explosive (HCE) devices by processing polarized radar reflections, and more particularly to a radar apparatus and processing method that provide enhanced HCE detection.  
       BACKGROUND OF THE INVENTION  
       [0002]     Various counter-terrorism systems have been devised for detecting the presence of HCE devices that are concealed from view, by the clothing of a suicide bomber, for example. A particularly effective system, described in the U.S. Pat. No. 6,967,612 to Gorman et al., develops radar range profiles of potential targets within a field of regard and analyzes diversely polarized radar signal reflections for each target to detect polarization signatures that are characteristic of a person that is carrying explosive devices. Radar or video camera data is processed to identify or track one or more targets within the field of regard, and a servo mechanism aims the radar at specified targets for data acquisition and polarimetric signature analysis.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention is directed to a radar apparatus and processing method for use in HCE detection based on polarimetric signature analysis. Doppler processing of orthogonally polarized radar return signals isolates the radar signal content associated with a moving pedestrian in the radar field of view to provide high quality target echo data for polarimetric signature analysis. The Doppler processing produces co-polarized and cross-polarized range vs. velocity (Doppler) data maps, and target echo data corresponding to the moving pedestrian is identified. Co-polarized and cross-polarized data subsets associated with the range and velocity coordinates of the identified moving pedestrian are extracted and subjected to polarization signature analysis to determine if the pedestrian is bearing explosive devices. Low pass filtering of the of the co-polarized and cross-polarized radar return signals prior to Doppler processing provides range aliasing to reject signal content associated with objects beyond the unambiguous range of the radar apparatus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a diagram of a real-world scenario in which the radar apparatus of the present invention is utilized to detect HCE devices;  
         [0005]      FIG. 2  is a block diagram of a linear FM pulse radar architecture, including a digital signal processor (DSP) for processing co-polarized and cross-polarized target echo signals according to the present invention;  
         [0006]      FIG. 3  graphically depicts a FM pattern for the radar transmit signal of  FIG. 2 ; and  
         [0007]      FIG. 4  is a block diagram describing the functionality of the DSP of  FIG. 2 .  
         [0008]      FIG. 5  is a representation of range vs. velocity (Doppler) data collected by the DSP of  FIG. 2  in the case of a pedestrian moving toward the radar apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0009]     As indicated above, the present invention is directed to a radar apparatus and processing method for use in a standoff HCE detection system of the type disclosed in the aforementioned U.S. Pat. No. 6,967,612 to Gorman et al., incorporated herein by reference. As described by Gorman et al., the target echo response to a polarized radar transmission is received in the form of a co-polarized response and a cross-polarized response, and the presence of HCE devices in the radar field of view is detected based on the relative magnitudes of the co-polarized and cross-polarized responses. In a preferred implementation, the polarimetric signature analysis involves forming a ratio of the polarized responses and comparing the ratio with reference values characteristic of HCE devices. The radar transmits a narrow beam, and an aiming apparatus points the beam at a given object-of-interest such as a pedestrian (i.e., a potential suicide bomber) moving toward a protected region. However, even an accurately aimed narrow radar beam is likely to illuminate closely spaced objects in addition to the object-of-interest when the system is operated in a real-world environment. Such “not-of-interest” objects may include, for example, other moving or stationary pedestrians, parked or moving cars, trees, buildings, etc.  
         [0010]      FIG. 1  illustrates a representative real-world scenario in which a HCE detection system is used. The reference numeral  10  generally designates a HCE detection system  10  including a radar sensor  12 , a digital signal processor (DSP)  14  and a user interface device  16 . The HCE detection system  10  is positioned at or near the perimeter  18  of a protected region  20 , and an aiming apparatus (not shown) points the main beam  22  of radar sensor  12  at a selected pedestrian  24 . The field of regard, which may include the entire scene forward of perimeter  18 , also includes various not-of-interest objects such as other pedestrians  26   a,    26   b,    26   c,    26   d,    26   e,    26   f,    26   g  (moving or stationary), a post or tree  28 , and a parked car  30 . Certain of these not-of-interest objects are at least partially illuminated by the main beam  22 ; in  FIG. 1 , the main beam illuminates the pedestrian  26   e,  the post/tree  28  and the parked car  30  in addition to the object-of-interest (i.e., pedestrian  24 ). Moreover, the transmit signal of radar sensor  12  will typically include side-lobes  32   a,    32   b  in addition to the main beam  22 , and these side-lobes can illuminate various not-of-interest objects such as the pedestrians  26   f,    26   g.  Due to the radar illumination of not-of-interest objects, the co-polarized and cross-polarized responses processed by DSP  14  will include not-of-interest signal content (i.e., clutter) in addition to the object-of-interest signal content. Since such clutter can distort or disguise the polarimetric signature of the object-of-interest (pedestrian  24 ), it is important to maximize the signal-to-clutter power ratio (SCR) of the HCE system  10 . For similar reasons, the radar sensor  12  must have a reasonably high signal-to-noise ratio (SNR) to produce high quality polarimetric signatures, while rejecting signal content associated with distant not-of-interest objects having large radar cross-section, such as the car  30  in  FIG. 1  or a building.  
         [0011]     The present invention addresses the performance issues outlined above, and produces high quality polarimetric signatures for accurate and reliable standoff HCE detection even in cluttered and highly variable environments. The invention utilizes a pulse-Doppler radar apparatus to acquire target echo data uniquely associated with moving objects, since movement (particularly movement toward a protected region  20 ) is strongly characteristic of a suicide bomber. With appropriate signal processing, this automatically rejects clutter associated with stationary objects within the radar field of view, as well as objects moving at different velocities than the object-of-interest. The SNR is optimized for a given transmitter output power level by utilizing a linear frequency-modulated transmit waveform with relatively long pulse-widths, and coherently integrating range bin data in the target echo signal across numerous pulses. Additionally, hardware and DSP filtering are utilized to squelch signals associated with not-of-interest objects beyond the radar&#39;s unambiguous range and velocity limits.  
         [0012]     Referring to  FIG. 2 , the radar apparatus  12  according to the present invention may be generally described as a pulse or chirp Doppler radar with a polarized transmit antenna  40 , a co-polarized receive antenna  42  and a cross-polarized (i.e., orthogonally polarized) receive antenna  44 . The co-polarized and cross-polarized return signals are supplied to DSP  14  for target identification and tracking, and polarimetric analysis, and the HCE detection results are outputted to interface device  16 .  
         [0013]     The transmit waveform is produced by voltage-controlled oscillator (VCO)  46  and supplied to transmit antenna  40  via amplifier  48 . Signal generator  50  produces the control voltage for VCO  46 , a linear ramp that is periodically reset by DSP  14  via line  52 . This produces a linear frequency modulation in VCO  46  as graphically depicted in  FIG. 3 . By way of example, the modulation waveform of  FIG. 3  may have a pulse-width of 10-2000 μsec, and the radiated transmit waveform may have a frequency bandwidth of 2-10,000 MHz. Although not illustrated in  FIG. 3 , there is a very brief blanking interval (such as 1-5 μsec) between pulses to allow substantial decay of reset-related transients. The long duration transmit pulses and near 100% duty cycle provide excellent range resolution even at short ranges and maximize sensitivity for a given transmitter output power level.  
         [0014]     The co-polarized and cross-polarized target echo signals respectively received by antennas  42  and  44  are passed through amplifiers  54  and  56 , and then mixed with replicas of the transmit waveform in mixers  58  and  60  to produce co-polarized and cross-polarized baseband signals on lines  62  and  64 . In the diagram of  FIG. 2 , the transmit waveform replica is obtained by a pickup device  66  and duplicated by splitter  68 . Since the radar return signals are mixed with replicas of the transmit signal, the baseband signals on lines  62  and  64  exhibit a frequency that is proportional to the delay of the respective target echo signals. And as the transmitted signal is sweeping in frequency the baseband signals are conditioned by high-pass filters (HPF)  70  and  72  and low-pass filters  71  and  73 . The filtered baseband signals are then digitized by A/D converters  74  and  76 , and supplied to DSP  14  via lines  78  and  80 . If desired, splitter  68  can be configured to delay the replica waveforms relative to the transmit waveform to provide extended range coverage. The high-pass filters  70  and  72  primarily operate to reject low frequency (below 50 kHz, for example) signal content due to receiver noise and mixer bias, and may alternatively be implemented as band-pass filters. Additionally, the high-pass filtering tends to attenuate short-range target echo signals, thereby normalizing the signal response to target range. The purpose of low-pass filters  71  and  73  is discussed below.  
         [0015]     The block diagram of  FIG. 4  represents the functionality of DSP  14  with respect to the present invention. The digital co-polarized and cross-polarized baseband radar return signal inputs on lines  78  and  80  are processed by the blocks  82 - 108  to form an HCE detection decision, and the decision is outputted to the interface device  16  per  FIGS. 1-2 . Initially, the co-polarized and cross-polarized baseband signal inputs are filtered by low-pass filters  82  and  84  for rejection of signal content associated with distant not-of-interest objects. This is important, especially in the case of large distant objects such as buildings and vehicles, because such objects typically have a much higher radar cross-section (RCS) than the usual object-of-interest, a pedestrian. If not rejected, the target echo signals from such distant objects (i.e., objects beyond the unambiguous range of the radar sensor  12 ) will “fold into” the unambiguous range and distort target echo signals associated with objects-of-interest. According to the present invention, the distant target echo returns are initially attenuated to some degree by analog low-pass filters  71  and  73 . The distant target echo signals are further attenuated by over-sampling the baseband signals at A/D blocks  74  and  76  in conjunction with low-pass filtering of the digitized baseband signals at blocks  82  and  84 . The high frequency roll-off of the low-pass filters  71  and  73  complements the low-pass function of blocks  82  and  84 . The A/D over-sampling enhances the performance of digital filters  82  and  84 , and also extends the radar&#39;s unambiguous range to enhance the rejection capabilities of the analog filter blocks  71  and  73 .  
         [0016]     The filtered co-polarized and cross-polarized baseband signals are then subjected to Fast-Fourier Transform (FFT) analysis as indicated by blocks  86  and  88 . For each target echo pulse in the co-polarized and cross-polarized baseband signals, the FFT blocks  86  and  88  generate range profiles, each profile comprising a sequence of data samples corresponding to signal strength in successive and contiguous range bins projected onto the illuminated field of view. The range profile data generated by FFT block  86  is stored in data table or map  90 , and the range profile data generated by FFT  88  is similarly stored in data table  92 . In the illustration, each data table row stores range profile data for a given target echo pulse, and the tables  90  and  92  are filled with range profile data as the FFT blocks  86  and  88  process a succession of target echo pulses.  
         [0017]     When the data tables  90  and  92  are filled, the co-polarized target echo response for any given range can be obtained by reading the range profile data in the corresponding column of table  90 , and the cross-polarized response can be similarly obtained from table  92 . The FFT  94  integrates the co-polarized target echo response for each range bin of table  90  to measure pulse-to-pulse phase (Doppler) shifts of targets therein. The FFT  96  similarly analyzes the cross-polarized target echo responses stored in table  92 . Since target movement toward the radar sensor  12  (i.e., targets having a radial velocity) produces a nearly constant change in phase shift between successive pulses, the FFT integration of blocks  94  and  96  acts as a matched filter for moving targets with constant radial velocity. The integration interval of FFT blocks  94  and  96  is relatively long (on the order of 50 milliseconds) to maximize the radar&#39;s SNR for a given transmitter output power level, but is nevertheless short in relation to a pedestrian&#39;s capacity to accelerate. Accordingly, the velocity of the object-of-interest (i.e., pedestrian  24  of  FIG. 1 ) will be constant over the integration interval, and the integration of FFT blocks  94  and  96  effectively constitutes a matched filter for moving pedestrians in the radar field of view. Consequently, signal clutter associated with stationary objects and objects moving at different velocities than the object-of-interest within the radar field of view are automatically rejected, substantially enhancing the radar&#39;s SCR. This is demonstrated in the real-world example of  FIG. 5 , as described below.  
         [0018]     The velocity (Doppler) signal strength (amplitude) data generated by FFT  94  for each range bin of table  90  is stored in table  98 , and the data generated by FFT  96  is similarly stored in data table  100 . After each of the range bins of tables  90  and  92  have been integrated, and the data stored in tables  98  and  100 , respectively, the range vs. velocity data is ready for analysis.  
         [0019]      FIG. 5  is a graphical representation of the co-polarized range vs. velocity data stored in table  98  for a real-world scenario such as depicted in  FIG. 1  where a pedestrian  24  is walking toward the radar sensor  12  at a range of approximately 100 m. In this case, the not-of-interest objects are stationary or have no substantial velocity toward the radar sensor  12 , and produce a series of echoes in various range bins, but at substantially zero velocity. Such echoes are disregarded as clutter. In contrast, the object-of-interest  24  produces a significant non-zero velocity return in the appropriate range bin, allowing clear separation of the target echo signal content associated with the target of interest.  
         [0020]     Returning to  FIG. 4 , the blocks  102 - 106  process the co-polarized range vs. velocity data stored in table  98  over an extended period of time such as 0.5-10 seconds, forming a data sequence for the object-of-interest. Although the cross-polarized data stored in table  100  could be similarly processed, the co-polarization data is preferred for detection and tracking since it is typically significantly higher in power than the corresponding cross-polarization data. Since there may be several objects-of-interest in a real-world field-of-regard, the blocks  102 - 106  are also designed to separately group data sequences for each such target-of-interest. Each time the table  98  is filled with range vs. velocity data, block  102  (detector) scans table  98  and locates the co-polarized response for a target-of-interest and produces (range, velocity) coordinates where each target response is located. The output of detector block  102  is thus a list of (range, velocity) coordinates, one set of coordinates for each detected target. The block  104  (tracker) tracks the range and velocity coordinates of each target-of-interest over time by assigning the vector outputs of block  102  to a target-specific historical data tracks. Each time the block  104  assigns a vector output to a historical data track, the block  106  (data extraction) fetches response data subsets from both the co-polarized data table  98  and the cross-polarized data table  100  for polarimetric signature analysis. The block  106  stores a matrix of extracted co-polarized and cross-polarized data for each target-of-interest, and supplies the data to block  108  (PSA) for polarimetric signature analysis as described in the aforementioned U.S. Pat. No. 6,967,612 to Gorman et al. As described in that patent, the block  108  maintains a historical record of polarimetric ratios for each target-of-interest and decides if a HCE device is present. The outcome of the HCE decision is then supplied to the interface device  16  as mentioned above.  
         [0021]     In summary, the present invention enhances the utility of HCE device standoff detection using polarimetric signature analysis, particularly as applied to moving pedestrians. While the invention has been described with respect to the a illustrated embodiment, it will be recognized that numerous modifications and variations in addition to those mentioned above will occur to those skilled in the art. In general, the radar sensor  12  must be configured to transmit polarized radar energy and receive reflected radar energy in the form of co-polarized and cross-polarized target echo signals that can be isolated according to the range of the illuminated objects. As will be appreciated by those skilled in the art, this objective can be achieved with various radar techniques including, for example, narrow pulse transmissions, non-linear frequency modulation, phase or amplitude modulation, or modulation by noise or pseudo-random noise sequences. Similarly, VCO  46  may be replaced with a fully synthesized signal source, and the Doppler processing can be mechanized with a bank of analog filters in lieu of the described digital mechanization. Likewise, various transmit and receive antenna configurations and polarization combinations may be utilized, as described for example in the aforementioned U.S. Pat. No. 6,967,612 to Gorman et al. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.