Abstract:
A set of co-registered coherent change detection (CCD) products is produced from a set of temporally separated synthetic aperture radar (SAR) images of a target scene. A plurality of transformations are determined, which transformations are respectively for transforming a plurality of the SAR images to a predetermined image coordinate system. The transformations are used to create, from a set of CCD products produced from the set of SAR images, a corresponding set of co-registered CCD products.

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 imaging with synthetic aperture radar (SAR) and, more particularly, to co-registration of coherent change detection (CCD) products. 
     BACKGROUND 
     Coherent change detection (CCD) offers the capability for detecting changes that occur in a target scene between SAR imaging passes. A sequence of SAR imaging passes produces a sequence of complex SAR images that respectively correspond to the imaging passes. The CCD process produces a set of CCD products, each of which represents a comparison of a pair of SAR images of the target scene that have been captured during time-wise adjacent airborne imaging passes. The creation of each CCD product involves a sub-pixel co-registration of the associated pair of SAR images. The set of CCD products is then registered to a common coordinate system, resulting in a set of co-registered CCD products for use in producing statistical models about the target scene, on a pixel-by-pixel basis. Traditionally, however, for a large set of SAR images taken across a long span of time, it has been difficult to produce a co-registered set of CCD products with sufficient accuracy to permit collection of useful statistics. 
     In particular, extending the aforementioned pair-wise, sub-pixel co-registration approach across many images presents some problems. First, every time an image is transformed a small amount of error is introduced. Second, creating a good CCD product requires very accurate sub-pixel registration. Accurate sub-pixel registration cannot be achieved with complex SAR images that are significantly separated in time, which leads to poor quality CCD products. These problems make it difficult to use simple solutions such as registering all of the CCD products to a common coordinate system. 
     It is desirable in view of the foregoing to provide for producing a set of co-registered CCD products that is suitable for statistical analysis and avoids problems such as those associated with longer image sequences and increased time between image passes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  diagrammatically illustrates conventional use of sub-pixel registration techniques to produce a set of CCD products from a set of SAR images. 
         FIGS. 2-4  illustrate operations used to produce a set of co-registered CCD products according to example embodiments of the present work. 
         FIG. 5  diagrammatically illustrates an apparatus configured to produce a set of to co-registered CCD products according to example embodiments of the present work. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments of the present work provide for co-registering a large set of SAR CCD products that span a significant amount of time. The present work exploits the fact that conventional techniques for producing a CCD product from a pair of complex SAR images use extremely accurate sub-pixel registration between the pair of complex SAR images. According to example embodiments of the present work, transformations produced by co-registration of the complex SAR images are used to co-register the corresponding CCD products. Some embodiments maintain the best image registration possible for CCD products while reducing the number of image transformations, producing high quality CCD products with minimal error introduction. 
     Example embodiments of the present work create a co-registered set of CCD products corresponding to a time interval of SAR image capture by first producing a set is of time-ordered CCD products according to a conventional technique. In particular, the raw complex SAR images are ordered with respect to the time they were taken, and indexed as SAR i, for i=1, 2, . . . , as shown at  11  in  FIG. 1 . The SAR images shown at  11  are captured sequentially during an image capture interval. Starting with the second image in the sequence, SAR  2 , each image is sub-pixel registered to the immediately preceding image, as shown at  13  in  FIG. 1 , to create a corresponding CCD product designated by CCD(i, i+1), as shown at  15  in  FIG. 1 . The aforementioned sub-pixel registration is also designated herein as fine registration. Techniques for capturing a sequence of SAR images are known in the art, as are techniques for performing sub-pixel registration between sequentially adjacent pairs of the captured SAR images to produce corresponding CCD products. 
     According to example embodiments of the present work, the set of complex SAR images  11  is processed to co-register the images. The process of co-registering the set of complex SAR images is also referred to herein as coarse registration. The coarse registration process produces transformations for transforming the SAR images to a predetermined image coordinate system. These transformations produced during the coarse registration are used to co-register the set of CCD products  15 . In some embodiments of co-registering the SAR images, a base image is initially selected from the set of SAR images  11 . Some embodiments select as the base image the image closest in time to the middle of the SAR image capture interval encompassed by the set of SAR images  11 . By using this base image, error propagation may be minimized, because all images may be transformed directly into the coordinate system of the base image. 
     Each SAR image is registered to the base image in conventional fashion, using a corresponding transformation into the coordinate system of the base image. For a given image SAR i at  11 , the corresponding transformation T i  that transforms that image to the coordinate system of the base image is applied to the CCD product that corresponds to image SAR i, namely, CCD(i, i+1). The result is a set of co-registered CCD products. 
     The longer the SAR image capture interval, the more likely it is that attempts to register some of the captured SAR images to the base image will fail, due to changes in the scene being imaged. Over a relatively long image capture interval, these changes can be drastic and encompass the entire scene. For example, manmade structures may be added or removed, trees and other plants may grow, die, or be cut down, or fields can be tilled, planted, or harvested. Some embodiments use an intermediate base image when a registration failure occurs. A registration failure is declared for a SAR image if, during the conventional process of registering that image to the base image, the number of tie points detected is below a predetermined threshold. If a registration failure occurs, the last SAR image that was successfully registered to the current base image is selected as a new, intermediate base image. The image that failed to register, and subsequently processed images, are then registered to the intermediate base image. The corresponding CCD product will therefore not be transformed directly to the coordinate system of the original base image, but rather will be transformed according to a compound transformation wherein the image-to-intermediate base transformation is applied to the CCD product, followed by the intermediate base-to-original base transformation. Some embodiments determine a suitable tie point threshold for failure declarations based on empirical observations under anticipated operating conditions. 
     The use of an intermediate base image is readily extended to one or more additional intermediate base images. That is, if a registration failure occurs relative to the aforementioned intermediate base, some embodiments select a further intermediate base in the same manner as the previous intermediate base was selected. This further to intermediate base is registered to the previous intermediate base, which is in turn registered to the original base. One or more still further intermediate base images may be selected as necessary. The use of one or more intermediate base images to register a given image SAR i to the coordinate system of the original base image means that the corresponding CCD product, CCD(i, i+1), will have associated therewith a set of transformations, also referred to herein as a compound transformation. This compound transformation is applied to the CCD product, thereby co-registering that CCD product to the original base image. For example, if two intermediate bases are required to register an image SAR i to the original base image, this results in a compound transformation that includes the following three transformations: (1) a transformation of SAR i to the coordinate system of the second intermediate base image; (2) a transformation of the second intermediate base image to the coordinate system of the first intermediate base image; and (3) a transformation of the first intermediate base image to the coordinate system of the original base image. 
     Some embodiments are capable of producing a set of co-registered CCD products corresponding to a sequence of 350 SAR images captured over a six-month image capture interval. A limiting factor is the amount of major permanent changes (e.g., demolition of a building) that occur in the scene. As described above, the use of intermediate bases helps ameliorate these types of limitations. 
       FIG. 2  illustrates operations performed according to example embodiments of the present work. The SAR images (see also  11  in  FIG. 1 ) are obtained at  21 . At  22 , CCD products are produced from the SAR images (see also  13  in  FIG. 1 ). Also at  22 , transformations for transforming the SAR images to the coordinate system of an original base image are produced. At  23 , each CCD product, CCD(i, i+1), has applied thereto the transformation (direct or compound) that transforms the associated image, SAR i, to the coordinate system of the original base image, thereby producing the desired set of co-registered CCD products. 
       FIG. 3  illustrates operations used to produce transformations (see also  22  in  FIG. 2 ) according to example embodiments of the present work. At  31 , an original base SAR image is selected. At  32 , the identity of the selected base SAR image is recorded. At  33 , the next SAR image is registered to the base SAR image, and information indicative of the transformation used to accomplish the registration is saved. If it is determined at  34  that the registration at  33  was successful, then registration of the next SAR image commences at  33 , unless it is determined at  36  that all SAR images are registered, in which case operation is completed. If it is determined at  34  that the registration at  33  has failed (for example, because the number of tie points detected is below a threshold), then the last successfully-registered SAR image is selected at  35  as an intermediate base image. The identity of this intermediate base image is then recorded at  32 , and the registration process continues at  33 . 
       FIG. 4  illustrates operations for transforming a set of CCD products to produce a set of co-registered CCD products (see also  23  in  FIG. 2 ) according to example embodiments of the present work. A CCD product (see also  15  in  FIG. 1 ) is obtained at  41 . It is then determined at  42  whether any intermediate base is applicable to the CCD product. If not, then a direct transformation to the original base is applied to the CCD product at  43 . The next CCD product is then obtained at  41 , unless it is determined at  45  that all CCD products are co-registered, in which case operation is completed. If applicability of any intermediate base(s) is determined at  42 , then a compound transformation is applicable. The transformation(s) to the intermediate base(s) is/are applied to the CCD product at  44 , after which operation proceeds to  43 , where transformation to the original base is applied, completing the compound transformation. 
       FIG. 5  diagrammatically illustrates an apparatus for producing a set of co-registered CCD products according to example embodiments of the present work. A coarse registration unit  52  is configured to produce, from the set of raw SAR images  11 , information  54  indicative of the transformations required to register all of the SAR images to a base SAR image selected from the set  11 . In some embodiments, the information  54  includes only direct transformations to an original base image. In some embodiments, the information  54  includes both direct transformations and compound transformations. A fine registration unit  13  is configured to produce a set of CCD products  15  from the set of SAR images  11 . As mentioned above relative to  FIG. 1 , such production of CCD products is known in the art. A transformation unit  56  is configured to produce a set of co-registered CCD products by suitably applying to the CCD products  15  the transformations indicated by information  54 . 
     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.