Abstract:
A synthetic aperture radar (SAR) image of a wide coverage area is acquired during a frame containing a first plurality of ambiguities induced in the SAR image from radar scatterers within the area. The area is illuminated with radar pulses and a segmented receive antenna oriented towards the area. The segmented receive antenna has a second plurality of sub-apertures, where the second plurality of sub-apertures is larger than the first plurality of ambiguities. Each sub-aperture has its own receiver. The digital stream from each receiver is stored in a computer for the duration of the frame to obtain frame data. A SAR image is extracted from the frame data. The first plurality of ambiguities are identified from analysis of the frame data, and a correction is computed to account for the first plurality of ambiguities contained within the synthetic aperture image. The correction is applied to reduce distortions caused by the ambiguities in the SAR image.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention is in the field of Synthetic Aperture Radar (SAR) imaging using a plurality of receive channels of a segmented antenna to identify and compensate for ambiguities in wide coverage SAR images. 
   2. Description of the Related Art 
   Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform. 
   The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up a frame length. During the frame length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the frame length. 
   The plurality of pulses transmitted during an SAR frame length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the frame length. 
   Range target resolution in SAR images is determined by the radar bandwidth. Range resolution is inversely proportional to radar bandwidth. That is, the higher the bandwidth, the smaller the details of a radar scatterer can be discerned in a range bin. Therefore, ideally, radar imaging is best enhanced by short pulses of high peak power. 
   It is desirable to produce a SAR image covering as much of an area as possible in one frame length. Unfortunately, both for spotlight and side looking, swath SAR modes, there is an inherent constraint in the size of the area that can be acquired in one frame length. If the frame is too long in the range direction, or too wide in the cross range direction, or both, ambiguities will arise in the imaging process. That is, because of the processing involved, certain scatterers will appear in more than one location in the final SAR image. This ghost like superposition on the real image tends to degrade the actual image, affecting its clarity thereby limiting its utility. 
   SUMMARY OF THE INVENTION 
   Above limitation is avoided by a method for acquiring a synthetic aperture image of an area during a frame, said synthetic aperture radar (SAR) image containing a first plurality of ambiguities. Said ambiguities are induced in the SAR image from radar scatterers within said area. Said SAR image has range/azimuth pixels, and is acquired with a radar system on a moving platform for the duration of said frame. 
   The method comprises the steps of: 
   
       
       
         
           transmitting radar pulses for illuminating said area for the duration of said frame, said area reflecting a radar return; 
           receiving said radar return using a segmented receive antenna oriented towards said area, said segmented receive antenna having a second plurality of sub-apertures said second plurality of sub-apertures larger than said first plurality of ambiguities; 
           connecting each of said sub-apertures to a radar receiver, said second plurality of sub-apertures receiving radar returns from said area, each of said radar receivers generating a digital stream, said digital stream representative of said radar return received at one of said sub-apertures during said frame; 
           storing said digital stream from each receiver in a computer for the duration of said frame to obtain frame data; 
           computing said synthetic aperture image from said frame data; 
           identifying said first plurality of ambiguities from analysis of said frame data; 
           computing a correction for said first plurality of ambiguities contained within said synthetic aperture image; 
           applying said correction to reduce distortions caused by said first plurality of ambiguities on said synthetic aperture image of said area. 
         
       
     
  
   Specifically, in mathematical notation,
         {overscore (Y)} is said range/azimuth SAR pixel computed from said frame data generated by one or more of said second plurality of antenna sub-apertures;   {right arrow over (G)} 1 , {right arrow over (G)} 2 , . . . {right arrow over (G)} N  are complex vectors of spatial channel gain associated with said second plurality sub apertures, said receivers and a path from said radar scatterers to each of said sub-apertures;   α 1 , α 2 , . . . α N &#39;s are said radar scatterer cross sections ambiguously contributing to one of said range/azimuth pixel of said SAR image;   {right arrow over (n)} are a receiver/antenna/target noise in each of said sub-apertures;   said computing step comprising a least square fit to said frame data by solving
 
[ {right arrow over (Y)}−{circumflex over (α)}   1   {right arrow over (G)}   1 −α 2   {right arrow over (G)}   2 − . . . −{right arrow over (α)} N   {right arrow over (G)}   N   ]·[{right arrow over (Y)}−{circumflex over (α)}   1   {right arrow over (G)}   1 −α 2   {right arrow over (G)}   2 − . . . −{right arrow over (α)} N   {right arrow over (G)}   N ]*={right arrow over (n)}·{right arrow over (n)}*  (2) 
 
to identify said α 1 , α 2 , . . . α n  from 
         (           ⁢             α   ^     1                 α   ^     2                       ⁢   ⋮                 α   ^     N           ⁢           )     =         (           ⁢         1               G   -&gt;     1   *     ·       G   -&gt;     2             G   -&gt;     1     ·       G   -&gt;     1   *                     G   -&gt;     1   *     ·       G   -&gt;     3             G   -&gt;     1     ·       G   -&gt;     1   *             ⋯               G   -&gt;     1   *     ·       G   -&gt;     N             G   -&gt;     1     ·       G   -&gt;     1   *                         G   -&gt;     2   *     ·       G   -&gt;     1             G   -&gt;     2     ·       G   -&gt;     2   *             1               G   -&gt;     2   *     ·       G   -&gt;     3             G   -&gt;     2     ·       G   -&gt;     2   *             ⋯               G   -&gt;     2   *     ·       G   -&gt;     N             G   -&gt;     2     ·       G   -&gt;     2   *                         G   -&gt;     3   *     ·       G   -&gt;     1             G   -&gt;     3     ·       G   -&gt;     3   *                     G   -&gt;     3   *     ·       G   -&gt;     2             G   -&gt;     3     ·       G   -&gt;     3   *             1       ⋰       ⋮           ⋮       ⋮       ⋰       ⋰               G   -&gt;       N   -   1     *     ·       G   -&gt;     N             G   -&gt;       N   -   1       ·       G   -&gt;       N   -   1     *                         G   -&gt;     N   *     ·       G   -&gt;     1             G   -&gt;     N     ·       G   -&gt;     N   *                     G   -&gt;     N   *     ·       G   -&gt;     2             G   -&gt;     N     ·       G   -&gt;     N   *             ⋯               G   -&gt;     N   *     ·       G   -&gt;       N   -   1               G   -&gt;     N     ·       G   -&gt;     N   *             1         ⁢           )       -   1       ·     (           ⁢               Y   -&gt;     ·       G   -&gt;     1   *             G   -&gt;     1     ·       G   -&gt;     1   *                       Y   -&gt;     ·       G   -&gt;     2   *             G   -&gt;     2     ·       G   -&gt;     2   *                 ⋮                 Y   -&gt;     ·       G   -&gt;     N   *             G   -&gt;     N     ·       G   -&gt;     N   *               ⁢           )           
       

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In the Drawing: 
       FIG. 1  is a SAR spotlight configuration of the prior art; and 
       FIG. 2  is a SAR spotlight configuration using multiple receive channels in a segmented antenna using the method of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention describes an apparatus and method for improving a SAR image due to ambiguities acquired because of wide area coverage. 
     FIG. 1  shows the typical prior art geometric relationship between a moving platform carrying a radar transmitter/receiver using Synthetic Aperture (SAR) spotlight methods and target area  101  to be imaged by said radar transmitter/receiver. The moving platform is initially at position  103 , travels with velocity V in the direction shown to position  105 . In SAR spotlight mode, the SAR antenna is actively oriented towards scatterer  101  as the platform moves with respect to scatterer  101  with velocity V. The moving platform moves from position  103  to position  105 , while adjusting the side looking angle from φ to φ+Δφ for spotlight mode so that the antenna keeps illuminating target area  101 . Antenna illumination with radar energy covers area  107  during a frame length, and includes target area  101 . Similarly, the antenna receive pattern covers area  107 , and includes target area  101 . Radar pulses are transmitted and corresponding returns received at many points during the frame length between position  103  and position  105 . SAR radar is well known in the art and described by W. Carrara, et al, in  Spotlight Synthetic Aperture Radar , Artech house, 1995, incorporated herein be reference in its entirety. 
   In the prior art, as detailed in High Resolution Radar, by Donald R. Wehner, Artech House, ISBN 0-089006-194-7, in spotlight mode, slant range resolution is 
       c     2   ⁢   n   ⁢           ⁢   Δ   ⁢           ⁢   f         
 
   The size of area  107  is limited by slant range ambiguity. This ambiguity is defined by 
             c     2   ⁢           ⁢   Δ   ⁢           ⁢   f             (   PA1   )             
 
   Similarly, cross range ambiguity length is 
               R   ⁢           ⁢   λ       2   ⁢     nT   2     ⁢     v   p     ⁢   sin   ⁢           ⁢   ϕ             (   PA2   )             
         where   c is the speed of light;   n number of pulses in a burst;   Δf is frequency step size;   R is range to scatterer in vicinity of target  101 ;   λ is wavelength of the radar operating frequency;   T 2  is pulse repetition interval (PRI);   v p  is phase delay for frequency error;   φ is azimuth angle at start of frame, φ+Δφ is angle at end of frame, assuming SAR spotlight operation.       

   Above equations compute the area covered by the radar antenna transmit/receive pattern, such as area  107 . If greater coverage is attempted, by beam spoiling or the like, azimuth/range ambiguities are introduced in the imaging of scatterers within area  107 . 
   The result of increasing area covered beyond that computed in equations PA1 and PA2 above are blurred SAR images. This is a marked problem at close range where a typical antenna pattern covers a relatively small region resulting in low SAR area coverage. One option, using beam spoiling to make SAR antenna coverage wider, results in lowered transmit and receive antenna gain, thus lower clutter to noise ratio (CNR), reducing SAR map quality. The other option, reducing PRF, affects negatively the Clutter to noise ratio as less average power is available for SAR imaging. 
   The present invention avoids above ambiguity problem and those of related spoiled beam/reduced PRF approaches and facilitates wide area coverage for SAR images. By forming multiple receive beams, in the limit, one receive beam oriented at each SAR image pixel, the CNR loss due to receive beam spoiling can be reduced and higher CNR can be achieved. 
   Shown in  FIG. 2  is a radar system of the present invention for acquiring a synthetic aperture image of an area  202 . Area  202  is a wide coverage area, larger than typical area coverage  107 . The synthetic aperture images derived form area  202  contain a first plurality of ambiguities from scatterers (targets) such as  208 ,  210  and  212 . The radar system is on a moving platform. Radar transmitter and transmit antenna  200  illuminates area  202 . Area  202  reflects a radar return into a segmented receive antenna  204  oriented towards area  202 . The segmented receive antenna  204  has a second plurality of sub-apertures  1 ,  2 ,  3  . . . N. This second plurality of sub-apertures  1 ,  2 ,  3  . . . N is larger than the first plurality of ambiguities present in the radar image. Each receive sub-aperture has larger coverage area than the original receive antenna because each sub-aperture has a smaller physical extent than the original receive antenna. 
   A second plurality of radar receivers AMP 1 , AMP 2 , AMP 3  . . . AMP N is connected to each of said second plurality of sub-apertures for receiving radar returns from area  202 . Each of the radar receivers AMP 1 , AMP 2 , AMP 3 , . . . AMP N generates a digital stream. Each of the digital streams from each radar receiver AMP 1 , AMP 2 , AMP 3  . . . AMP N is representative of the radar return received at one (of the second plurality) of the sub-apertures. 
   The digital streams are transmitted to computer  206  for storing and processing. Computer  206  generates a correction from the second plurality of digital streams sent by radar receivers AMPS, AMP 2 , AMP 3  . . . AMP N. This correction corrects the blurring of the synthetic aperture image of area  202  caused by the first plurality of ambiguities from targets  208 ,  210  and  212 . 
   The output from computer  206  is a SAR display. The equation for the radar return in an image pixel from all receivers connected to different receive sub-apertures is given by equation (1)
 
{right arrow over (Y)}=α 1 {right arrow over (G)} 1 +α 2 {right arrow over (G)} 2 + . . . +α N {right arrow over (G)} N +{right arrow over (n)}  (1) 
         where   {right arrow over (Y)} is a specific range/azimuth SAR pixel composed of information derived from different spatial channels (antenna sub-apertures);   {right arrow over (G)} 1 , {right arrow over (G)} 2 , . . . {right arrow over (G)} N  are complex vectors of spatial channel gain associated with AMP 1 , AMP 2 , AMP 3  . . . AMP N.   α&#39;s are scatterer/ambiguity cross sections related to scatterer  208 ,  212 ,  210  ambiguously contributing to a particular range/azimuth pixel of the SAR image.   1 . . . . N is the second plurality of sub apertures of the receive antenna gain;   {right arrow over (n)} is the receiver/antenna/target noise in each spatial channel.   {right arrow over (Y)} is obtained using typical SAR image processing, as detailed in  FIG. 2 , for example performing the steps of:   1) Transmit a series of bursts of narrow band pulses to illuminate area  202  for the duration of a frame, where each pulse is generally frequency coded for subsequent pulse compression.   2) Collect I and Q samples of the echo responses received from the radar illuminated area over the frame from a plurality of receive antenna sub-apertures, each aperture connected to its own amplification/digitization channel, each of the sub apertures oriented towards area  202 .   3) Store I and Q samples for the area for pulses within a frame yielding a complex quantity, I+jQ.   4) Apply weighting, motion compensation, corrections for phase and amplitude ripple, sampling bias and imbalance errors to each I+jQ sample from each channel corresponding to each sub aperture.   5) Compute an inverse discrete Fourier Transform (DFT −1 ) of I+jQ samples covering the area.   6) Extract SAR image pixels descriptive of the area from DFT −1  results, the pixels descriptive of radar scatterers within the area.       

   One condition of this invention is that the number of spatial channels is larger than the number of ambiguities present in the area covered by the SAR image. 
   A least square fit of the data to the antenna gain pattern is performed with the estimates, {circumflex over (α)} of ground reflectivity in the ambiguous direction by solving:
 
[ {right arrow over (Y)}−{circumflex over (α)}   1   {right arrow over (G)}   1 −α 2   {right arrow over (G)}   2 − . . . −{right arrow over (α)} N   {right arrow over (G)}   N   ]·[{right arrow over (Y)}−{circumflex over (α)}   1   {right arrow over (G)}   1 −α 2   {right arrow over (G)}   2 − . . . −{right arrow over (α)} N   {right arrow over (G)}   N ]*={right arrow over (n)}·{right arrow over (n)}*  (2) 
         using the same definitions for variables as in equation 1 above, and   {right arrow over (n)} is receiver/path/target noise, assumed independent from receive channel to receive channel.   {circumflex over (α)} 1  . . . {circumflex over (α)} N &#39;s are the ground reflectivity, or target cross section(s), contributing to a pixel in the SAR image in the ambiguous direction(s).       

   The solution to equation 2, is expressed by equation 3, using the same variables as in equation 1 and 2. 
               (           ⁢             α   ^     1                 α   ^     2                       ⁢   ⋮                 α   ^     N           ⁢           )     =         (           ⁢         1               G   -&gt;     1   *     ·       G   -&gt;     2             G   -&gt;     1     ·       G   -&gt;     1   *                     G   -&gt;     1   *     ·       G   -&gt;     3             G   -&gt;     1     ·       G   -&gt;     1   *             ⋯               G   -&gt;     1   *     ·       G   -&gt;     N             G   -&gt;     1     ·       G   -&gt;     1   *                         G   -&gt;     2   *     ·       G   -&gt;     1             G   -&gt;     2     ·       G   -&gt;     2   *             1               G   -&gt;     2   *     ·       G   -&gt;     3             G   -&gt;     2     ·       G   -&gt;     2   *             ⋯               G   -&gt;     2   *     ·       G   -&gt;     N             G   -&gt;     2     ·       G   -&gt;     2   *                         G   -&gt;     3   *     ·       G   -&gt;     1             G   -&gt;     3     ·       G   -&gt;     3   *                     G   -&gt;     3   *     ·       G   -&gt;     2             G   -&gt;     3     ·       G   -&gt;     3   *             1       ⋰       ⋮           ⋮       ⋮       ⋰       ⋰               G   -&gt;       N   -   1     *     ·       G   -&gt;     N             G   -&gt;       N   -   1       ·       G   -&gt;       N   -   1     *                         G   -&gt;     N   *     ·       G   -&gt;     1             G   -&gt;     N     ·       G   -&gt;     N   *                     G   -&gt;     N   *     ·       G   -&gt;     2             G   -&gt;     N     ·       G   -&gt;     N   *             ⋯               G   -&gt;     N   *     ·       G   -&gt;       N   -   1               G   -&gt;     N     ·       G   -&gt;     N   *             1         ⁢           )       -   1       ·     (           ⁢               Y   -&gt;     ·       G   -&gt;     1   *             G   -&gt;     1     ·       G   -&gt;     1   *                       Y   -&gt;     ·       G   -&gt;     2   *             G   -&gt;     2     ·       G   -&gt;     2   *                 ⋮                 Y   -&gt;     ·       G   -&gt;     N   *             G   -&gt;     N     ·       G   -&gt;     N   *               ⁢           )               (   3   )             
 
   Once the {circumflex over (α)} 1 , {circumflex over (α)} 2  . . . {circumflex over (α)} N  are extracted as shown by above equations, the related SAR image is corrected for the effect of the ambiguities. 
   All references cited in this document are incorporated herein in their entirety by reference. 
   Although presented in exemplary fashion employing specific embodiments, the disclosed structures are not intended to be so limited. 
   Those skilled in the art will also appreciate that numerous changes and modifications could be made to the embodiment described herein without departing in any way from the invention.