Abstract:
A method of generating an image comprises accessing a dataset gathered from a sensor system and generating, from the dataset, a first image including a plurality of image elements. The method further includes generating a training image by combining the dataset with known sensor responses and generating a weighting factor by correcting the training image in view of the known sensor responses. The method further includes applying the weighting factor to one of the plurality of image elements to form a weighted image element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to co-pending U.S. Provisional Application No. 61/601,208 filed Feb. 21, 2012 and titled “SYSTEMS AND METHODS FOR IMAGE SHARPENING,” which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Radars detect targets by transmitting a beam of electro-magnetic energy toward the target and measuring the electro-magnetic energy reflected from the target. Synthetic aperture radar (SAR) is a signal processing technique that may be implemented for use with radars to form imagery by processing reflected electro-magnetic energy acquired at differing orientations and positions relative to the target. Imagery generated by synthetic aperture radars are often used for detection of objects that may be otherwise difficult to obtain using other imagery generating devices such as video cameras that generate imagery using visible light. Conventional radar imaging systems and image processing techniques often generate distortions and artifacts such as sidelobes and aliasing, and therefore, are not entirely satisfactory in all respects. 
       SUMMARY OF THE INVENTION 
       [0003]    Aspects and embodiments are directed to methods and apparatus for automatically suppressing elevation sidelobes in three dimensional (3D) synthetic aperture radar (SAR) imagery using sparse (efficient) collections. Elevation impulse response sidelobes can blur an image to the point that features are not readily extracted using automated algorithms, particularly when the elevation aperture is sparse and undersampled. Aspects and embodiments provide an elevation sharpening method that uses independent voxel-by-voxel weighting based on statistics of a back-projected signal from antenna element to antenna element in a SAR array. As discussed further below, according to certain embodiments, phase and amplitude variations, or noise estimates (per voxel) are used to deweight returns from sidelobes and/or aliasing by forming 3D volumes from every elevation sample and then combining the returns coherently with the weighting. This method allows for remote radar-based 3D imaging of building interiors, and the ability to extract useful features from the imagery. 
         [0004]    According to one embodiment, a method of generating an image comprises accessing a dataset gathered from a sensor system, generating, from the dataset, a first image including a plurality of image elements, generating a training image by combining the dataset with known sensor responses, generating a weighting factor by correcting the training image in view of the known sensor responses, and applying the weighting factor to one of the plurality of image elements to form a weighted image element. 
         [0005]    In one example of the method the sensor system is a radar system. The plurality of image elements may include a plurality of voxels. In one example, the known sensor responses include known three-dimensional responses. In another example the known sensor responses include known point target responses. The step of generating a weighting factor may include generating a plurality of weighting coefficients for weighting an amplitude parameter of at least one of the plurality of image elements. The step of generating a weighting factor may include generating a plurality of weighting coefficients for weighting a phase parameter of at least one of the plurality of the image elements. The method may further comprise accessing a second dataset gathered from the sensor system, generating, from the second dataset, a second image including a second plurality of image elements, applying the weighting factor to one of the second plurality of image elements to form a second weighted image element, and combining the weighted image elements into a combined image. 
         [0006]    According to another embodiment an image generation system comprises a sensor processor operable to access a dataset gathered from a sensor system, generate, from the dataset, a first image including a plurality of image elements, generate a training image by combining the dataset with known sensor responses, generate a weighting factor by correcting the training image in view of the known sensor responses, and apply the weighting factor to one of the plurality of image elements to form a weighted image element. 
         [0007]    In one example the sensor system includes a radar system operable to gather a plurality of radar datasets. In one example the known sensor responses include known three-dimensional responses. In another example the known sensor responses include known point target responses. The sensor processor may be further operable to generate the weighting factor by generating a plurality of weighting coefficients for weighting an amplitude parameter of at least one of the plurality of image elements. The sensor processor may be further operable to generate the weighting factor by generating a plurality of weighting coefficients for weighting a phase parameter of at least one of the plurality of the image elements. In one example the sensor processor is further operable to access a second dataset gathered from the sensor system, generate, from the second dataset, a second image including a second plurality of image elements, apply the weighting factor to one of the second plurality of image elements to form a second weighted image element, and combine the weighted image elements into a combined image. 
         [0008]    According to another embodiment, a synthetic aperture radar (SAR) image generation method comprises accessing a dataset gathered from a SAR system, generating, from the dataset, a first 3D image including a plurality of image voxels, generating a training image by combining the dataset with training SAR responses, deriving one or more weighting coefficients by correcting the training image in view of the training SAR responses, determining a weighting factor based on at least one property of the dataset and the one or more weighting coefficients, and applying the weighting factor to one of the plurality of image elements to form a weighted image element. 
         [0009]    In one example of the method the training SAR responses are point responses. In another example the training SAR responses are 3D responses. Deriving one or more weighting coefficients by correcting the training image in view of the training SAR responses may include minimizing at least one sidelobe in the training image. In one example the at least one property of the dataset is a phase variance. In another example the at least one property of the dataset is an amplitude variance. 
         [0010]    Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
           [0012]      FIG. 1  is an illustration showing an example of a radar imaging system according to aspects of the present invention; 
           [0013]      FIG. 2  is a schematic diagram of one example of a radar imaging system according to aspects of the present invention; and 
           [0014]      FIG. 3  is a flow chart showing an example of a method of generating an image according to aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Aspects and embodiments are directed to a method of elevation image sharpening that allows for the exploitation of 3D images from efficiently-sampled datasets. In particular, aspects and embodiments provide a method by which 3D imagery is formed from multiple sensors or antennas, using a weighting scheme in which weights are applied voxel by voxel, to coherently combine multiple 3D images into one final image. As discussed further below, phase and amplitude variations or noise estimates (per voxel) are used to deweight returns from sidelobes and/or aliasing, thereby suppressing sidelobes in the final image. The weights used to combine the individual 3D images into the one final image are determined from training using known results. This approach allows for training of coefficients to match the overall system response, and does not suppress weak features while de-weighting the sidelobes. The methods disclosed herein evolved from a monopulse elevation beam sharpening technique which is generalized for the application of N receiver channels, where N&gt;2. The concept of using phase in monopulse to sharpen the resulting beam (elevation response) is extended as disclosed herein to using amplitude and phase statistics to accomplish similar objectives with more than two channels. 
         [0016]    It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. 
         [0017]    Referring to  FIG. 1 , there is illustrated an example of an environment in which a radar imaging system according to various embodiments may be used. The radar imaging system may include an image processing system  102  communicating with a radar system  104 . In one embodiment, the radar system  104  is mounted on a movable platform such as a truck  106  or other vehicle, for example. In alternative embodiments, the radar system may be mounted or an air, space or water based movable platform, or may be located on a stationary platform. The radar system  104  generates electromagnetic energy  110  that may be reflected from a target of interest  108  and the surrounding terrain. The radar system  104  is also configured to receive reflected electromagnetic energy  112  from the target of interest  108  and the surrounding terrain. Single or multi-channel receivers may be used. The reflected energy or pulses  112  are converted into data that is sent to the image processing system  102  to be processed into images of the target of interest  108  and the surrounding terrain. In this embodiment, the target of interest may be a building. In alternative embodiments, the target of interest may be any type of point or moving target in an environment of interest. 
         [0018]    Some radar systems may be operated in a synthetic aperture radar (SAR) mode. The synthetic aperture radar (SAR) mode combines reflected pulses at differing locations to form an image of a region or one or more targets of interest. The pulses may be combined by separating information in each image according to its frequency and range components. Some radars may also be operated in an inverse synthetic aperture radar (ISAR) mode in which the radar receives multiple pulses from one or more moving targets. 
         [0019]    Imagery generated by radars using SAR and ISAR processing techniques may provide certain benefits over other imagery generating devices. For example, radars typically acquire information using electro-magnetic energy reflected from targets of interest. Upon reflection, electro-magnetic energy penetrates into objects to a certain degree according to the principle of penetration depth. The penetration depth of electro-magnetic energy is based upon several factors including the frequency of electro-magnetic energy, conductivity of the object, and the density of the target. Knowledge of these factors when processing reflected electro-magnetic energy may be used to derive useful information about internal features of targets. Thus, reflected electro-magnetic energy may include information about features internal to targets that is not typically obtained using conventional imagery generating devices, such as video cameras that generate imagery using visible light. 
         [0020]    In some cases, it may be useful to determine internal features of static structures such as buildings. In some intelligence, surveillance, and reconnaissance activities for example, it would be useful to generate three-dimensional imagery of buildings or other structures that may be situated in metropolitan or other civilian regions. Although SAR and ISAR devices have been proposed for this purpose, the quality and resolution of imagery has been limited by the ability suppress sidelobes, aliasing, and other distortions. When the elevation aperture is sparse and undersampled as may be the case with efficient data collection techniques, traditional automated algorithms that rely exclusively on non-adaptive channel weighting may not sufficiently extract imaged features and suppress sidelobes, aliasing or other distortions. 
         [0021]      FIG. 2  is a schematic diagram of a radar imaging system  114  according to one embodiment of the present disclosure. As previously described, the radar system  104  communicates with the image processing system  102 . The image processing system  102  is a computing system comprising a processor  116 , a communication link  118 , and a memory  120 . The processor  116  includes an image generator  122  and the memory  120  one or more databases  124  for storing, for example, known radar responses, images and other datasets compiled from the radar system  104 . The radar imaging system  114  also may include a display system  126 . The image generator  122  includes instructions stored in the memory  120  and executable by the processor  116  of image processing system  102 . The image processing system  102  may be any suitable computing system to provide imagery on the timeline required, such as a network coupled computing system or a stand-alone computing system. Examples of stand-alone computing systems may include a personal computer, a personal digital assistant, a smartphone, a laptop computer, or a mainframe computer. A network computing system may be a number of computers coupled together via a network, such as a local area network (LAN), a metropolitan area network (MAN), or a wide area network (WAN), such as the internet, that collectively execute the instructions of the image generator  122 . The communication link  118  may support wired or wireless communication. 
         [0022]      FIG. 3  is flow chart showing a method  130  of generating an image according to one embodiment of the present disclosure. The method  130  may be implemented, for example, on the image processing system  102 . At step  132 , a dataset that was gathered from a radar system, such as radar system  104 , may be accessed, for example from a database  124 . In addition to echo data received from radar system  104 , the accessed dataset may include identifying information about the dataset such as the target area geolocation, the radar kinematics, path of travel, and other relevant information about the sensors and/or information about the antennas required to form imagery. At step  134 , the accessed dataset may be processed to produce a matrix of image elements. Any known image reconstruction methods may be used to produce the matrix of image elements, including for example, backprojection. The image elements may be either pixels for two-dimensional images or voxels for three-dimensional images. 
         [0023]    A training subprocess  135  may be performed to identify training weights that may be applied to each image element to reduce sidelobes, aliasing, and other distortions in the image matrix. The training subprocess  135  may access known training responses associated with a particular radar  104 . A database  136  may include “truth information” or common known two- or three-dimensional responses. For example, predetermined three-dimensional responses corresponding to known features of structural components, weapons, vehicles, aircraft, or other types of structures or technology may be stored in the database  136 . Known three-dimensional responses may be constructed from numerical simulation of various features of interest or from precision measured data. 
         [0024]    The training subprocess  135  may additionally or alternatively include accessing a database  140  that includes parametrics associated with sampled elevation datasets. Dataset parametrics may include the spacing of the receivers in units of wavelengths in order to assess the expected amount of aliasing. 
         [0025]    At step  138 , the dataset accessed at step  132  may be imaged with one or more known three dimensional responses from the database  136 . Alternatively or additionally, at step  142 , the dataset may be imaged with known point target responses, disposed in a similar manner as how scattering features are observed by the radar, based on simulations from the database  140 . In one example, point targets are simulated as perfect omni-directional reflectors at various x, y, z locations with respect to the radar. Derived point target responses may then only contain impurities associated with the radar and the desired collection geometry. 
         [0026]    At step  144 , training weights or coefficients are determined for sharpening the known point target or three dimensional structure responses. Sharpening the known responses includes reducing sidelobes, aliasing, and other distortions associated with the known responses. Because the known responses correspond to know structural and target characteristics, distortions may be more easily identified and corrected. For example, training weights or coefficients may be determined to restore an image point response (IPR) for the known point responses. The training weights may be selected to match and effectively deconvolve the overall system response. In one example, only one set of coefficients is derived. A set of coefficients may be derived from a blending of the known system responses and the point target responses. 
         [0027]    At step  146 , one or more beam sharpening or weighting factors may be calculated for each image element to deweight those image elements associated with sidelobes, aliasing, or other distortions. In one embodiment, the empirically derived training weights from step  144  for a particular three dimensional voxel element may be, for example, 
         [0028]    P1=0.50 
         [0029]    P2=1.25 
         [0030]    P3=1.75 
         [0031]    P4=0.75 
         [0000]    The voxel element weighting factor or beam sharpening factor may be expressed as follows: 
         [0000]      Weighting Factor=[(1−(12*σ p   2 /4/π 2 ) P1 ) P2 ]/[(1+(σ A   2 /μ A   2 ) P3 ) P4 ], where
 
         [0000]    σ p   2 =Phase variance across receivers
 
σ A   2 =Amplitude variance across receivers
 
μ A =Amplitude mean across receivers
 
A Weighting Factor of 1 may indicate that the voxel should be identified as corresponding to a real target rather than corresponding to a distortion. This is because a Weighting Factor of 1 may indicate a low phase and amplitude variance associated with a real target. Distortions may have a higher phase and/or amplitude variance.
 
         [0032]    At step  148 , the weighting factor developed in step  146  is applied to each of the image elements in the matrix of image elements to create a sharpened image with deweighted sidelobes, aliasing, and other distortions. The process steps  132 - 148  may be repeated for each of the remaining datasets corresponding to different elevations, different radar paths and directions, different sensors, and/or different antennas. At step  150 , the sharpened images associated with each dataset may be processed into a single image. The training subprocess  135  may be executed any time the system is modified or the environment, including heat and/or humidity, changes. Also, at system start-up, an automatic calibration program may be initiated. 
         [0033]    Although the described distortion reduction system and technique may be suitable for radar imaging as described, other types of imaging may also benefit from the use of real and simulated data to improve the quality of the imaging. Other types of three dimensional imaging techniques that use multiple sensors, such as medical, commercial, subsurface, sonar, satellite, and other imaging technologies may benefit from the use of the described image sharpening methods. 
         [0034]    Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.