Abstract:
Methods and apparatus for enhancing the resolution of a radar image in the cross-range direction. An example method includes receiving a plurality of received power samples in the cross-range dimension as the radar antenna scans and calculating a window function from the antenna beam response pattern. Then for each of a plurality of positions of the window function along the azimuth axis, multiplying the received response pattern by the window function at that position, yielding a product function for each position. Finally, the method includes calculating an estimated azimuth bin offset, resulting estimated target location, and a reflected power value corresponding to the integral of the product function from the product function of each position. A reconstructed azimuth bin array developed from the estimated target locations and reflected power values is substituted for the original received cross-range received power values, yielding a resolution-enhanced map image.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Airborne radar is used in aircraft navigation to generate a radar map of the ground in the vicinity of the aircraft. In some cases, the antenna aperture area available to an airborne radar device is limited, leading to a relatively large angular width of the anterna&#39;s main beam. The width of the beam can result in substantial “smearing” of the map imagery in the cross-range direction, especially in the case of highly reflective targets that are narrow relative to the beam width along the length of the azimuth. In some instances, image smearing can make it difficult for a pilot to identify features on the ground important for the effective use of the imagery. 
         [0002]    Signal-processing techniques exist that improve the cross-range resolution of radar ground maps. One commonly used technique uses the gradient in Doppler frequency across the antenna&#39;s main beam to sharpen the image. But a disadvantage of this technique is that the gradient in Doppler frequency approaches zero as the pointing direction of the antenna approaches the direction of the velocity vector. Therefore, this technique carries the disadvantage of not being effective in the direction in which the aircraft is travelling. 
         [0003]    Another technique for resolution enhancement uses monopulse radar. Processing of monopulse radar signals provides cross-range enhancement independent of direction, but unlike Doppler methods does not provide true resolution improvement. The mono-pulse technique also comes at the cost of significant additional expense and complexity. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention includes a method and apparatus for enhancing the resolution of a radar image in the cross-range direction. In accordance with the invention, the method includes pointing a radar antenna in the direction of one of a plurality of azimuth directions, transmitting a radar signal, receiving a reflected radar signal, processing the reflected radar signal, repeating the steps of pointing, transmitting, receiving and processing for a plurality of azimuth directions as the antenna is scanned in azimuth, composing an array of values corresponding to the plurality of reflected radar signal power samples received and processed, and converting the array into a map image. 
         [0005]    In accordance with another aspect of the invention, the method further includes creating a received power azimuth bin array, calculating a window function of a form similar to the antenna beam discrete target response pattern, calculating at least one product function by multiplying the function consisting of the array of received signal power samples by the window function, calculating an estimated azimuth bin offset from the mean of the product function, calculating the azimuth direction of an estimated target location from the azimuth bin offset, and adding a reflected power value corresponding to the integral of the product function to an element of a new azimuth bin array corresponding to the estimated target location. 
         [0006]    In accordance with yet another aspect of the invention, the method further includes calculating a plurality of product functions for a plurality of positions of the window function along the azimuth, the individual product functions corresponding to individual window function positions, adding at least one reflected power value to at least one value stored in an element of the azimuth bin array, whereby the element of the new azimuth bin array corresponds to the azimuth direction of the estimated target location, and substituting a pattern of values stored in the new azimuth bin array for the received signal power samples, whereby the resolution of the map image becomes enhanced. 
         [0007]    In accordance with the invention, the apparatus includes an antenna, a transmit circuit, a receive circuit, a map display, a control device and a processor. The processor constructs a function in the cross-range, or azimuth, dimension for a number of range bins from received radar signal samples and converts the azimuth functions for each of the ranges into a map image. In a further aspect of the invention, the processor enhances the map image resolution using the method described above. 
         [0008]    One benefit of the method is that the method is not dependent on the direction of the velocity vector, as with Doppler resolution-enhancing methods. Another advantage is that the disclosed method can result in substantial cost savings over monopulse methods for resolution enhancement due to the invention&#39;s lower level of complexity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0010]      FIG. 1  shows a schematic diagram of an example airborne radar device formed in accordance with the present invention; 
           [0011]      FIG. 2  shows a flow diagram of a method for creating and displaying a radar ground map as performed by the airborne radar device of  FIG. 1 ; 
           [0012]      FIG. 3  shows a flow diagram of a method for resolution enhancing a radar ground map performed by the airborne radar device of  FIG. 1 ; 
           [0013]      FIG. 4  shows a geometric drawing of a radar beam transmitted through an example azimuth containing a target as performed by the method of  FIG. 2 ; 
           [0014]      FIGS. 5-1  through  5 - 6  show example radar beam directions subsequent to the beam direction of  FIG. 4  during a radar beam sweep along the example azimuth; 
           [0015]      FIG. 6  shows a plot of an example measured power azimuth bin array corresponding to reflected radar signal power samples received and processed by the method of  FIG. 2 ; 
           [0016]      FIG. 7  shows a plot of an example window function for the measured power azimuth bin array of  FIG. 6 ; 
           [0017]      FIG. 8  shows on a first plot the measured power azimuth bin array of  FIG. 6  and the window function of  FIG. 7 , with the window function mean positioned at azimuth direction  4 , and on a second plot the product of the two functions calculated according to the method of  FIG. 3 ; 
           [0018]      FIG. 9  shows a plot of a reconstructed power azimuth bin array of the method of  FIG. 3  for the product function of  FIG. 8 ; 
           [0019]      FIG. 10  shows on a first and second plot the responses of  FIG. 8 , except with the window function mean positioned at azimuth direction  5 ; 
           [0020]      FIG. 11  shows on a first and second plot the responses of  FIG. 8 , except with the window function mean positioned at azimuth direction  6 ; 
           [0021]      FIG. 12  shows on a first and second plot the responses of  FIG. 8 , except with the window function mean positioned at azimuth direction  7 ; and 
           [0022]      FIG. 13  shows a plot of the reconstructed power azimuth bin array summed for the product functions of FIGS.  8  and  10 - 12 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIG. 1  is an example airborne radar device  10  used in aircraft navigation to generate a radar map of the ground in the vicinity of an aircraft. The airborne radar device  10  includes an antenna  11 , a transmit circuit  12 , a receive circuit  13 , a processor  14 , a memory  15 , a map display  16 , and a control device  17 . The antenna  11  is in signal communication with the transmit circuit  12  and the receive circuit  13 . The transmit circuit  12  and the receive circuit  13  are each in signal communication with the processor  14 . The processor  14  is in signal communication with the memory  15 , the map display  16 , and the control device  17 . 
         [0024]    The airborne radar device  10  transmits and receives radar signals, converts the received radar signal into processor-interpretable data, and displays the processed image data in map form. Outgoing radar power signals generated by the transmit circuit  12  and incoming radar power signals reflected from a target are transmitted and received by the antenna  11 . The received radar signal is decoded into a processor-interpretable signal by the receive circuit  13 . The processor-interpretable signal is interpreted by the processor  14  in a signal processing operation that includes extraction of image data from the processor-interpretable signal. Command of the transmit circuit  12 , delivery of image data to the map display  16 , receipt of control parameters from the control device  17 , and measures to enhance the resolution of the map image are other functions performed by the processor  14 . Data and control parameters received from the processor  14  are either temporarily or permanently stored by the memory  15 . Image data corresponding to the received power signal is displayed by the map display  16 . Input control parameters selected by a radar operator are received by the control device  17 . 
         [0025]    As shown in  FIG. 2 , the airborne radar device  10  displays a radar map image according to a radar mapping method  20 . At a first block  21 , the radar antenna  11  initiates a scan in a cross-range, or azimuth, dimension. Next at a block  22 , the antenna  11  transmits a radar power signal in the direction of the scanned azimuth of block  21 . Next at a block  23 , while still scanning the azimuth dimension of block  21 , the antenna  11  receives a reflected radar response signal. Next at a block  24 , the processor  14  processes the received radar response signals of block  23  and records values in bins of a measured power azimuth bin array corresponding to the intensity of the reflected signal. Individual bins of the measured power azimuth bin array correspond to individual directions along the scanned azimuth dimension. Next at a block  26 , the processor  14  converts the values of the measured power azimuth bin array into a reconstructed azimuth bin array according to an image enhancement method  28  of  FIG. 3 . The airborne radar device  10  displays a radar map image corresponding to the reconstructed azimuth bin array. 
         [0026]    As shown in  FIG. 3 , the processor  14  enhances the resolution of the radar map displayed in  FIG. 2  by the image enhancement method  28 . At a first block  30 , the processor  14  calculates a product function by multiplying the measured power azimuth bin array of block  24  in  FIG. 2  by a pre-defined window function for one position of the window function along the azimuth. The pre-defined window function is derived from a two-way antenna radiation pattern in azimuth. In one embodiment, the two-way antenna radiation pattern is modeled by a Gaussian function therefore, a Gaussian form for the window function is appropriate. In other embodiments another model could be used, as long as the selected function is similar to the two-way antenna main beam pattern. 
         [0027]    Next at blocks  32  and  34 , the processor  14  calculates the mean and a reflected power value of the product function. The reflected power value corresponds to the integral of the product function. 
         [0028]    Next at a block  36 , the processor  14  calculates an estimated azimuth bin offset by subtracting window function mean from the product function mean, multiplying by two, and rounding to the nearest integer. The estimated azimuth bin offset corresponds to an estimate of the distance along the azimuth between the mean of the product function and an estimate of the target location. Next at a block  38 , the processor  14  calculates the estimated target location by adding the estimated azimuth bin offset to the window function mean. Next at a block  39 , the processor  14  adds the reflected power value of block  34  to the bin of the reconstructed azimuth bin array corresponding to estimated target location of block  39 . 
         [0029]    Next at a decision block  40 , the processor  14  either calculates another product function at the block  30  for a new position of the window function along the azimuth, or alternately at a block  41  stops processing. The degree to which image resolution improves is approximately proportional to the number of unique positions of the window function along the azimuth for which blocks  30  through  39  are executed. Therefore blocks  30  through  39  may be re-executed for subsequent positions of the window function along the azimuth in order to further improve map image resolution. 
         [0030]    As shown in  FIG. 4 , a radar beam  44  is transmitted so that the beam  44  intersects one of a plurality of azimuth directions  45  along an azimuth  43 . Although the radar beam  44  is pointed toward only one azimuth direction  45 , the beam&#39;s width may intersect a plurality of neighboring azimuth directions  45  at one time. At each azimuth direction  45 , a radar power signal is transmitted and a reflected radar response signal is received, according to blocks  22  and  23  of  FIG. 2 . The beam  44  then advances to a subsequent direction  45  along the azimuth  43 . 
         [0031]      FIGS. 5-1  through  5 - 6  exhibit six individual radar beam directions  50 ,  51 ,  52 ,  53 ,  54 ,  55  of the radar beam  44  of  FIG. 4 . Taken in succession, radar beam directions  50 ,  51 ,  52 ,  53 ,  54 ,  55  in  FIGS. 5-1  through  5 - 6  can be viewed as a scan of the azimuth  43  by the radar beam  44 . In  FIGS. 5-1  though  5 - 3  the beam  44  approaches a target  46 . In  FIG. 5-4  the beam  44  intersects the target  46 . In  FIGS. 5-5  and  5 - 6  the beam  44  moves away from the target  46 . Because beam  44  has width that may include several azimuth directions  45  at one time, even at azimuth directions that do not directly intersect the target  46 , the reflected response signal may still include power reflected from the target  46 . 
         [0032]    As shown in  FIG. 6 , the reflected radar response signals of a scan of azimuth  43  can be used to identify the approximate location of a target  46  along the azimuth. The plot of  FIG. 6  shows a measured power azimuth bin array  60  corresponding to the six radar beam directions of the scan in  FIG. 5 . The intensity of the received (reflected) power increases the closer the radar beam is pointed to the target  46 . The location of the target  46  can be identified by the highest intensity bin of the measured power azimuth bin array  60 , but because neighboring bins also have received power, it&#39;s not possible to precisely identify the width and location of the target  46 . 
         [0033]      FIG. 7  shows a window function  70  selected to correspond with the measured power azimuth bin array  60  of  FIG. 6 . The window function  70  is a function that is non-negative within some chosen interval, and zero or approaching zero outside the chosen interval. By multiplying the window function  70  through the measured power azimuth bin array  60 , the width and location of the target  46  can be more precisely identified, as will be shown. 
         [0034]    As shown in  FIG. 8 , the multiplication of the measured power azimuth bin array  60  and the window function  70  produces a product function  74  that is shaped similarly to its two factors  60 ,  70  and has as its mean  76  an azimuth direction midway between the peaks of the two factor functions  60 ,  70 .  FIG. 8  shows this multiplication for the case where the window function  70  is positioned with its mean  72  at azimuth direction  4 . The scaled power at each azimuth direction  45  of the product function  74  is the product of the received power from the azimuth bin array  60  and the magnitude of the window function  70  at each azimuth direction  45 . For example, at azimuth direction  5  the received power equals four and the magnitude of the window function  70  equals eight, therefore the scaled power at azimuth direction  5  equals  32 . Calculation of the product function  74  is the first operation, the block  30 , of the image enhancement method  28  of  FIG. 3 . 
         [0035]      FIG. 8  also shows the result of a calculation of the product function mean  76  and a reflected power value  80 , as described in blocks  32  and  34  of the image enhancement method  28  of  FIG. 3 . The product function mean  76  is calculated by taking the mean of the product function  74 . The reflected power value  80  is calculated by taking the integral of the product function  74 . 
         [0036]    From the product function mean  76  and the window function mean  72 , an estimated azimuth bin offset is calculated. As shown in  FIG. 8 , first an intermediate offset value  78  is calculated by subtracting the window function mean  72  from the product function mean  76 . The estimated azimuth bin offset  84  is then calculated from the intermediate value  78  by multiplying the intermediate offset value  78  by two, and rounding to the nearest integer. This operation coincides with block  36  of the image enhancement method  28 . 
         [0037]    From the estimated azimuth bin offset  84 , an estimated target location  88  is calculated. As shown in  FIG. 9 , by adding the estimated azimuth bin offset  84  to the window function mean  72  of  FIG. 8 , the estimated target location  88  along the azimuth  43  is identified. This operation coincides with the block  38  of the image enhancement method  28 . 
         [0038]    The reflected power value  80  calculated at the block  34  is then added to a bin of a reconstructed azimuth bin array  90  coinciding with the estimated target location  88 . As shown in  FIG. 9 , the reflected power value  80  from  FIG. 8  equal to 25 is plotted at azimuth direction  7 , which is the estimated target location  88 . This operation coincides with the block  39  of the image enhancement method  28 . 
         [0039]    As shown in  FIGS. 10-12 , the operations within the image enhancement method  28  are repeated for a series of positions of the window function  70  on the azimuth  43 . There is a product function  74 , and a new estimated target location  88  and reflected power value  80  for each position of the window function  70 . 
         [0040]    As shown in  FIG. 13 , repeated execution of the image enhancement method  28  leads to an accumulation of added reflected power values  80  in the same or nearly the same bin as the estimated target location  88 .  FIG. 13  shows a plot of the reconstructed azimuth bin array  90  for an incomplete series of window function positions (only directions  4 - 7 ), but demonstrates that over repeated iterations of the image enhancement method  28 , the reconstructed azimuth bin array  90  increases the reflected power value stored in bins close to the target location, and reduces the reflected power value in bins of the array away from the target. 
         [0041]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, the invention can be practiced with equal success using various techniques for selecting a window function. One skilled in the art would also acknowledge that the multiplication factor of two used in the calculation of the azimuth bin offset could be varied. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.