Abstract:
A synthetic aperture radar (SAR) image is produced by using all phase histories of a set of phase histories to produce a first pixel array having a first azimuth resolution, and using less than all phase histories of the set to produce a second pixel array having a second azimuth resolution that is coarser than the first azimuth resolution. The first and second pixel arrays are combined to produce a third pixel array defining a desired SAR image that shows distinct shadows of moving objects while preserving detail in stationary background clutter.

Description:
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD 
     The present work relates generally to processing SAR images and, more particularly, to processing SAR images for use in SAR video products. 
     BACKGROUND 
     U.S. Pat. No. 7,498,968 (incorporated by reference herein) describes a synthetic aperture radar (SAR) system that is capable of forming high-resolution SAR images at near video rates, e.g., many times a second. The rate of image update allows users to determine when moving targets start moving, when and where they stop, the direction of motion, and how fast they are moving. Further, the radar shadows of moving targets can be used to determine the position and identity of the targets (including even slow moving targets), and display the target motion from frame to frame. SAR systems and techniques such as described in U.S. Pat. No. 7,498,968 are also referred to herein generally as “VideoSAR”. 
     A VideoSAR movie is a sequence of individual SAR images. The video product produced by VideoSAR is typically either a clip or a stream of images. A VideoSAR clip product is a file containing a closed set of SAR images, for example, thousands of SAR images captured over a few minutes. A VideoSAR stream product may be a true real-time video constructed as a sequence of SAR images 
     One application of Video SAR is the observation of shadows from moving objects. Fine azimuth resolution SAR images provide high quality detail of stationary objects, but shadows of moving objects often disappear. Coarse azimuth resolution SAR images provide dark shadows of moving objects, but with less detail of stationary objects. The darkness of the shadow from a moving object in a SAR image is a function of the percentage of the aperture time that the stationary clutter behind the object was obscured by the object. Coarse azimuth resolutions have shorter aperture times, which give darker shadows of moving objects. However, coarse azimuth resolution SAR images do not provide much detail of the stationary background clutter. On the other hand, fine azimuth resolution SAR images provide detail of the background clutter, but the shadows of moving objects are often very faint. 
     It is desirable in view of the foregoing to provide for producing a SAR image that shows distinct shadows of moving objects while preserving detail in stationary background clutter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  diagrammatically illustrates apparatus and associated processes capable of producing SAR images and corresponding VideoSAR products according to example embodiments of the present work. 
         FIG. 2  diagrammatically illustrates a portion of  FIG. 1  in more detail according to example embodiments of the present work. 
         FIG. 3  diagrammatically illustrates a portion of  FIG. 1  in more detail according to further example embodiments of the present work. 
         FIG. 4  diagrammatically illustrates operation of the subset selector of  FIG. 2  according to example embodiments of the present work. 
         FIG. 5  diagrammatically illustrates sizes of pixel arrays of  FIGS. 1-3  according to example embodiments of the present work. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  diagrammatically illustrates apparatus and associated processes capable of producing SAR images and corresponding VideoSAR products according to example embodiments of the present work. The apparatus of  FIG. 1  operates on a set of SAR phase histories, shown diagrammatically at  11 . In some embodiments, the SAR phase histories are captured using conventional techniques and functionalities (not explicitly shown). U.S. Pat. No. 6,608,586, which is incorporated herein by reference, describes conventional examples of these techniques and functionalities in detail. Briefly, an antenna arrangement receives a SAR return signal, which is processed by a receiver to produce a raw SAR video signal that includes real, or in-phase (I), and imaginary, or quadrature (Q) components. The raw SAR video signal is digitized to produce the set of phase histories. 
     The set of phase histories  11  is input to a fine resolution image former  12  and a coarse resolution image former  13 . In some embodiments, the image former  12  is responsive to the set of phase histories  11  to produce, in conventional fashion, a pixel array  16  having range and azimuth dimensions. (Note that  FIG. 5  illustrates range and azimuth dimensions of various pixel arrays shown and described herein.) The image former  13  is responsive to the set of phase histories  11  to produce a pixel array  17  having the same range and azimuth dimensions as the pixel array  16 , but having an azimuth resolution (i.e., a resolution in the azimuth dimension) that is lower (i.e., coarser) than the azimuth resolution of the pixel array  16 . Thus, the pixel array  16  corresponds to a SAR image having a relatively finer azimuth resolution (fine resolution image), and the pixel array  17  corresponds to a SAR image having a relatively coarser azimuth resolution (coarse resolution image). As is conventional, the phase history capture process is configured to provide in the set  11  sufficient phase histories to support a fine resolution SAR image (corresponding to pixel array  16 ) that is acceptably focused and provides acceptable background clutter detail. It should also be noted that this fine resolution SAR image may also provide fine details of relatively slower movers. 
     In some embodiments, the fine resolution image former  12  uses all of the phase histories contained in the set  11  to produce the pixel array  16 . On the other hand, the coarse resolution image former  13  selects a subset of the set of phase histories  11  (i.e., less than all phase histories contained in the set  11 ), and uses the selected subset to produce the pixel array  17 . An example of this is shown in  FIG. 4 . The set of captured phase histories  11  is arranged in a phase history array having fast-time and slow-time to dimensions, as is conventional. The phase histories in each column of the phase history array correspond to respective range direction distances in the associated pixel array, and the phase histories of each row of the phase history array correspond to respective azimuth direction distances in the associated pixel array. As previously mentioned, the coarse resolution image former  13  selects only a subset of the phase histories (shown at  25  between broken lines in  FIG. 4 ) to use for SAR image formation. 
     The subset  25  is selected such that it is centered on the same phase history (phase history  31  in  FIG. 4 ) as is the set  11 . The subset  25  has the same fast-time dimension as the set  11 , but has a smaller slow-time dimension than does the set  11 . More specifically, whereas the set  11  is arranged as an array containing N columns, the subset  25  is a sub-array that contains less than N columns. In some embodiments, the sub-array contains N×(F/C) columns as shown in  FIG. 4 , where the ratio F/C establishes a relationship between the azimuth resolution F of the fine resolution image, and the azimuth resolution C of the coarse resolution image. In some embodiments, the parameters F and C are quantified in terms of azimuth direction pixel spacing, and F&lt;C. The fine resolution parameter F is defined by the phase history capture process which, as indicated above, provides a phase history array (see  FIG. 4 ) that results in a SAR image (associated with the pixel array  16 ) that is acceptably focused and has acceptable background clutter detail. The coarse resolution parameter C may be chosen such that the corresponding SAR image (associated with pixel array  17 ) presents sufficiently dark shadows of moving objects. 
     The coarse resolution image former  13  (see  FIG. 1 ) includes a subset selector that appropriately selects the subset from the set  11 .  FIG. 2  shows such a subset selector at  21 . The  FIG. 2  example shows that the subset selector  21  selects the subset  25  from the set  11  in accordance with the ratio F/C (see also  FIG. 4 ).  FIG. 2  also shows that, in some embodiments, the image formers  12  and  13  of  FIG. 1  share a single SAR image formation unit  22  that receives both the full set of phase histories  11  and the subset of phase histories  25 . In some embodiments, the SAR image formation unit  22  implements conventional techniques to produce the pixel array  16  (see also  FIG. 1 ) from the full set  11 , and also to produce an intermediate pixel array  24  from the subset  25 . 
     The intermediate pixel array  24  has the same range dimension as the pixel arrays  16  and  17 , but its azimuth dimension is less than that of the pixel arrays  16  and  17 . Referring again to the example of  FIG. 4 , if the full array of phase histories at  11  contains N columns, and the sub-array at  25  contains N×(F/C) columns, then the intermediate pixel array  24  as produced from the sub-array will be smaller in azimuth dimension than the pixel array  16  as produced from the full array. An interpolator shown at  23  in  FIG. 2  interpolates between adjacent values of the intermediate pixel array  24  in the azimuth dimension sufficiently to provide the pixel array  17  (see also  FIG. 1 ) with the same azimuth dimension as the pixel array  16 , but with lower (coarser) azimuth resolution than the pixel array  16  (because fewer columns of the phase history array are used to produce pixel array  24 ). In some embodiments, the interpolation is carried out in conventional fashion. Referring also to  FIG. 5 , it can be seen that the interpolator  23  transforms the pixel array  24 , having a smaller azimuth dimension, into the pixel array  17  having a larger azimuth dimension (equal to that of the pixel array  16 ). 
       FIG. 3  illustrates an example arrangement similar to that of  FIG. 2 , but with the single SAR image formation unit  22  of  FIG. 2  replaced by separate SAR image formation units  41  and  42  that respectively produce the pixel arrays  16  and  24 . 
     Referring again to  FIG. 1 , the pixel arrays  16  and  17  are input to a combiner  14  that is configured to combine the pixel arrays  16  and  17  to produce a resultant pixel array  18  having the same range and azimuth dimensions as the pixel arrays  16  and  17  (see also  FIG. 5 ). In some embodiments, the combiner  14  combines the values of respectively corresponding pixels of the arrays  16  and  17  to determine a value for each corresponding pixel of the array  18 . In some embodiments, the pixel value combining performed by combiner  14  includes comparing pixel values. In some embodiments, the combiner  14  selects, for each value of the resultant pixel array  18 , the lower of the corresponding values of the pixel arrays  16  and  17 . 
     By the operation of the combiner  14 , the resultant pixel array  18  has a “composite” azimuth resolution achieved by the combination of the fine resolution “component” associated with the array  16  and the coarse resolution “component” associated with the array  17 . In general, the aforementioned ratio F/C may be selected to achieve a desired balance between background clutter detail and shadow darkness of moving objects. Various embodiments determine F/C in various ways. For example, a preferred F/C ratio may be determined based on empirical observations of results obtained with: (1) a selected value of F and various values of C; or (2) various values of F and C. An F/C ratio that results in a SAR image (associated with the resultant pixel array  18 ) exhibiting acceptably dark shadows of moving objects and acceptable detail in the background clutter may be selected. 
     A given set of phase histories  11  is part of a series of sets of phase histories captured sequentially, for example, during a mission flight. This series of phase history sets results in a corresponding series of resultant pixel arrays  18  suitable for presentation as a movie (or similar video product) by, for example, a conventional VideoSAR unit, as indicated by broken line in  FIG. 1 . 
     Various embodiments provide one or more further coarse resolution image formers, i.e., in addition to the one shown at  13  in  FIG. 1 . Each additional coarse resolution image former provides an associated SAR image with an associated azimuth resolution that is coarser than the azimuth resolution of the SAR image  16 . These additional coarser azimuth resolution SAR images, and the SAR image at  16 , are then suitably combined to produce a resultant SAR image. In some embodiments, the combining compares the corresponding pixel values among each of the various SAR images, and selects the lowest of those pixel values to be the corresponding pixel value in the resultant image. 
     Although example embodiments of the present work are described above in detail, this does not limit the scope of the present work, which can be practiced in a variety of embodiments.