Abstract:
Generating an image matrix includes accessing a round-trip time matrix for a space having points. The round-trip time matrix describes an estimated round-trip time for a signal to travel from a transmit antenna, to a point, and to a receive antenna. Signals reflected from an object of the space are received at the receive antennas. The following are repeated for at least a subset of the points to generate an image matrix: select a point of the subset of points; for each receive antenna, establish a waveform of a signal received by a receive antenna and identify a waveform value of the established waveform that corresponds to the selected point according to the round-trip time matrix; and combine the waveform values for the selected point to yield an image value for the selected point. The image matrix is generated from the image values.

Description:
TECHNICAL FIELD 
   This invention relates generally to the field of imaging systems and more specifically to generating three-dimensional images using impulsive radio frequency signals. 
   BACKGROUND OF THE INVENTION 
   Radar imaging devices may be used to detect an object behind an obstruction such as a wall and to generate an image of the object. Some known radar imaging devices, however, are not able to display certain types of images such as three-dimensional images. Moreover, other known radar imaging may not be able to detect certain targets such as stationary targets. It is generally desirable to display certain images and to detect certain targets. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the present invention, disadvantages and problems associated with previous techniques for generating images may be reduced or eliminated. 
   According to one embodiment of the present invention, generating an image matrix includes accessing a round-trip time matrix for a space having discrete points. The round-trip time matrix describes an estimated round-trip time for a signal to travel from a transmit antenna, to a point, and to a receive antenna. Signals reflected from an object of the space are received, where each signal is received at a corresponding receive antenna. The following are repeated for at least a subset of the points to generate an image matrix: select a point of the subset of points; for each receive antenna, establish a waveform of a signal received by a receive antenna and identify a waveform value of the established waveform that corresponds to the selected point according to the round-trip time matrix; and combine the waveform values for the selected point to yield an image value for the selected point. The image matrix is generated from the image values. 
   Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that round-trip times may be used to generate a three-dimensional image. Another technical advantage of one embodiment may be that stationary targets may be detected. 
   Certain embodiments of the invention may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagram illustrating an environment that includes one embodiment of an imaging system that generates an image of an object that may be located behind an obstruction; 
       FIGS. 2A and 2B  illustrate an example imaging system; 
       FIG. 3  is a flowchart illustrating one embodiment of a method for generating an image that may be used with the imaging system of  FIG. 1 ; and 
       FIG. 4  illustrates example waveforms of received signals. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention and its advantages are best understood by referring to  FIGS. 1 through 4  of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1  is a diagram illustrating an environment  10  that includes an imaging system  20  that generates a three-dimensional image  22  of an object  24  that may be located behind an obstruction  28 . In general, imaging system  20  transmits signals such as radio frequency signals through an obstruction  28  towards object  24 . Imaging system  20  detects signals reflected from object  24  and generates image  22  of object  24  in accordance to the round-trip times of the signals. 
   Object  24  may comprise any suitable object of any suitable shape or size that can reflect signals such as radio frequency (RF) signals. Object  24  may comprise any suitable material such as a metallic, non-metallic, or a composition of both metallic and non-metallic material. Examples of object  24  include a living organism such as a human, a machine such as a weapon, other suitable object, or any combination of the preceding. 
   According to the illustrated embodiment, object  24  is located within an actual space  30 . Actual space  30  refers to a plurality of points p i  mapped to a physical region in order to represent the physical region. In the illustrated example, actual space  30  has three dimensions to represent a three-dimensional physical region, but actual space  30  may have two dimensions to represent a two-dimensional physical region. In the illustrated embodiment, a point p i  of actual space  30  is expressed as p i =(x i , y i , z i ), and is used to represent a generally cubic region about point p i . 
   Obstruction  28  may comprise any suitable material that passes through at least some signals that impinge on its surface. Examples of obstruction  28  may include a wall, ground matter, clothing, or any combination of the preceding. 
   Imaging system  20  generates image  22  of object  24 . According to the illustrated embodiment, imaging system  20  includes an antenna system  36 , a computing system  38 , and a display  40 . In general, antenna system  36  transmits signals such as radio frequency signals through obstruction  28  towards object  24 . Antenna system  36  detects signals reflected from object  24  and sends the reflected signals to computing system  38 . Computing system generates an image matrix, which is used to form image  22  of object  24  on display  40 . 
   According to one embodiment, antenna system  36  has one or more transmit antennas T j , j=1, . . . ,J, for transmitting signals and one or more receive antennas R k , k=1, . . . ,K, for receiving signals. Antenna system  36  may have, for example, more receive antennas R k  than transmit antennas T j  such as multiple receive antennas R k  and one transmit antenna T j . According to the illustrated embodiment, antenna system  36  has one transmit antenna T 1  and three receive antennas R 1 , R 2 , and R 3 . 
   An antenna of antenna system  36  may comprise, for example, a coaxial antenna such as an embodiment of a coaxial cavity antenna disclosed in U.S. Pat. No. 6,356,241, which is herein incorporated by reference. Coaxial cavity antennas may reduce coupling between the receive antennas R k , which may provide higher-fidelity image reconstruction. The antennas of antenna system  36  may be arranged in any suitable configuration such as a planar configuration that may allow for placement of the antennas proximate to a flat obstruction  28 . 
   Antenna system  36  may operate in an active mode or in a passive mode. In an active mode, antenna system  36  emits a signal that is reflected from object  24  back to antenna system  36 . In a passive mode, antenna system  36  does not emit signals but only receives signals reflected from object  24 . The passive mode may be used for direction finding purposes. 
   The signals may comprise ultra-wideband radio frequency signals that have impulse-like waveforms of extremely short duration relative to typical continuous wave radar waveforms. The signals may have pulses of one to three nanoseconds. Ultra-wide band is defined to have a relative bandwidth of at least twenty-five percent. For example, if a waveform has a center frequency of one gigaHertz, the bandwidth is at least two hundred fifty megahertz. The signals may be emitted at high power with a low pulse repetition rate or at a low power with high pulse repetition rate. 
   The antennas may polarize the signals at diverse orientations. The signals may be vertically or horizontally polarized to detect vertical or horizontal objects, respectively. The signals may be multiply polarized to detect objects that reflect signals at diverse orientations or to reduce multi-path effects. 
   Computing system  38  processes waveforms of signals received by antenna system  36  to generate image  22 , and may operate according to the method described with reference to  FIG. 3 . According to the illustrated embodiment, computing system  38  includes an interface (IF)  50 , a processor  52 , a memory  54 , and an image generator  56  coupled as shown in  FIG. 1 . Interface  50  receives and sends data. As used in this document, the term “interface” refers to any suitable structure of a device operable to receive input for the device, send output from the device, or both, and may comprise one or more ports. 
   Processor  52  manages the operation of computing system  38 , and may comprise any suitable hardware, software, other logic, or any combination of the preceding. As used in this document, the term “processor” refers to any suitable device operable to execute instructions and manipulate data to perform operations. Examples of processors include a digital signal processor and a field programmable gate array. 
   Memory  54  stores and facilitates retrieval of information used by processor  52 . As used in this document, the term “memory” refers to any structure operable to store and facilitate retrieval of information used by a processor, and may comprise Random Access Memory (RAM), Read Only Memory (ROM), magnetic drives, disk drives, Compact Disk (CD) Drives, Digital Video Disk (DVD) drives, removable media storage, any other suitable data storage device, or a combination of any of the preceding. 
   Image generator  56  generates an image matrix for image  22  of object  24  in accordance with the round-trip times of the signals. A round-trip time rtt jk (p i ) refers to the time it takes for a signal to travel from a transmit antenna T j  to a point p i  of actual space  30  and back to a receive antenna R k . The transmit time a j (p i ) refers to the time it takes for a signal to travel from transmit antenna T j  to point p i , and the receive time r k (p i ) refers to the time it takes for a signal to travel from point p i  back to receiver antenna R k . Accordingly, the round-trip time rtt jk (p i ) for a signal to travel from transmit antenna T j  to point p i  and return to receive antenna R k  is equal to a j (p i )+r k (p i ). If there is only one transmit antenna T j , the transmit time may be written as a(p i ), and the round-trip time rtt jk (p i ) is a(p i )+r k (p i ). 
   A round-trip time matrix refers to a matrix that records round-trip times rtt jk (p i ). An entry RTT(p i ) of a round-trip time matrix may record the round-trip times for each transmit antenna T j  and each receive antenna R k  of antenna system  36  for a point p i . If there is only one transmit antenna T j , an entry RTT(p i ) may record the round-trip times for each receive antenna R k , k=1, . . . ,K, of antenna system  36  for a point p i . For example, an entry RTT(p i ) may be written as a K-tuple &lt;rtt 1 (p i ), . . . ,rtt K (p i )&gt;. 
   The round-trip times may be used to generate an image matrix for image  22 . An image matrix refers to a matrix that includes an image value for at least some points p i  of virtual space  60 . An image value refers to one more values for one or more parameters used to generate image  22 . The parameters may include, for example, intensity, instantaneous frequency, polarization, other parameter, or any combination of the preceding. An image matrix may be generated for a particular time period such as a time period of from a few microseconds to several seconds. Image matrices for successive time periods may be used to display successive images  22  of object  24 . 
   The image values may determined from waveform values of the waveforms. A waveform value may refer to an amplitude or other suitable value of a waveform. The image value for a point p i  may determined from waveform values corresponding to point p i  according to the round-trip time rtt(p i ) of point p i . For example, if a waveform is transmitted at time t=to, then the waveform value at time t=t 0 +rtt(p i ) corresponds to point p i . If there is more than one waveform for a point p i , the waveform values of the waveforms may be combined to determine an image value for point p i . The waveform values may be combined by, for example, multiplying or adding them together to yield an image value for point p i . 
   Computing system  38  may perform other operations that may, for example, improve signal-to-noise (SNR) ratio. As a first example, computing system  38  may scale the waveforms to compensate for differences in waveform amplitude due to the different round-trip times of the signals. According to one embodiment, computing system  38  may scale the waveforms according to a range-amplitude correction matrix. A range-amplitude correction matrix includes range-amplitude correction values for the waveform points of a waveform. A range-amplitude correction value refers to a value that is used to correct a waveform point to compensate for the differences in waveform amplitude. For example, the amplitude value of a waveform point may be multiplied by a range-amplitude correction value to correct the amplitude. A range-amplitude correction value rac jk (p i ) may be used to correct a waveform of a signal transmitted from transmit antenna T j  to point p i  and received by receive antenna R k . If there is one transmit antenna, the range-amplitude correction value may be written as rac k (p i ). 
   As another example, image generator  56  may reduce or subtract a background from the waveforms to reduce or remove transmitter-receiver coupling. Background may represent an empty actual space  30  such as a space that does not include objects or that does not include targeted objects. Background measurements may be subtracted from received waveforms to reduce or subtract the background. Background measurements refer to measurements made of only the background, which may be made during an initial calibration of imaging system  20  and may be updated by periodic calibration of imaging system  20 . 
   As yet another example, image generator  56  may suppress interference such as narrow band interference. Interference may be suppressed by detecting interfering signals, filtering out the interfering signals, and amplifying the pulses of the signals. According to one embodiment, narrow band interference may be suppressed by converting the waveforms to the frequency domain using a windowed fast Fourier transform. Narrow band peaks may be zeroed out, or removed. The waveforms may then be converted back to the time domain using an inverse fast Fourier transform. As yet another example, image generator  56  may average the waveforms, which may improve the final image. Image generator  56  may average any suitable number of waveforms, such as from 10 to 100 waveforms. 
   As yet another example, image generator  56  may generate image matrices that display both stationary and moving targets, only stationary targets, or only moving targets. Stationary and moving targets may be displayed by generating successive images  22  from successive image matrices. Stationary targets may be displayed by averaging together any suitable number of image matrices, for example, from 5 to 100 images, or by utilizing an alpha filter such as a low pass filter with an alpha value greater than 0.9. The resulting image matrix may then be used to generate image  22 . 
   Moving targets may be identified by calculating the differences between image matrices of successive time periods. Images  22  that have different positions in successive image matrices may be identified as moving. The difference in position may be required to satisfy a threshold value to be considered moving. The images  22  of the moving targets may be displayed. 
   Interface  50 , processor  52 , memory  54 , and image generator  56  may be integrated or separated according to particular needs. For example, the present invention contemplates the functions of both processor  52  and memory  54  being provided using a single device, for example, a computer. If any of interface  50 , processor  52 , memory  54 , or image generator  56  are separated, separate elements may be coupled using a bus or other suitable link. 
   Display  40  displays image  22  of object  24 . Display  40  may comprise, for example, a computer screen, a goggle display, or other suitable display. In the illustrated embodiment, display  40  comprises a two-dimensional screen that is operable to display a three-dimensional image  22 . According to the illustrated embodiment, image  22  is presented in a virtual space  60  that corresponds to actual space  30  in which object  24  is located. The points p i  of virtual space  60  correspond to points p i  of actual space  30 . Image  22  may be presented in any suitable manner. As an example, image  22  may be rotated in three-dimensional space in order to display a different view of image  22 . As another example, image  22  generated from waves of a specific polarization may be displayed. 
   Imaging system  20  may be deployed in any suitable embodiment. For example, imaging system  20  may be deployed in a smaller format to be carried by a person, or may be deployed in a larger format to map a building compound. An example embodiment is described with reference to  FIGS. 2A and 2B . 
   Alterations or permutations such as modifications, additions, or omissions may be made to imaging system  20  without departing from the scope of the invention. Imaging system  20  may have more, fewer, or other modules. For example, the operations of image generator  56  may be performed by more than one module. Additionally, operations of imaging system  20  may be performed using any suitable logic comprising software, hardware, other logic, or any suitable combination of the preceding. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     FIGS. 2A and 2B  illustrate an example imaging system  70 .  FIG. 2A  illustrates a side of device  70  through which signals are emitted and received. System  70  includes a transmit antenna  72 , receive antennas  74 , a wave source  78 , a computing system  80 , and a housing  82  coupled as shown. 
   Wave source  78  may generate signals such as ultra-wideband radio frequency signals. Wave source  78  may include, for example, a seventy picosecond rise-time, nine to thirty volt ultra-wideband source. Transmit antenna  72  transmits signals, which are reflected from object  22 , and received by receive antennas  74 . Transmit antenna  72  and receive antenna  74  may comprise, for example, a coaxial antenna. 
   Computing system  80  operates to generate image  22  from the reflected signals, and may operate according to the method described with reference to  FIG. 3 . According to the illustrated embodiment, computing system  80  includes a processor  90 , a digitizer  92 , radio frequency components  94 , and a power distributor  96  coupled as shown. Digitizer  92  may comprise a multi-channel digitizer to capture the waveforms from each antenna. Radio frequency components  96  may comprise multi-stage low-noise ultra-wideband radio frequency amplifiers. Housing  82  serves to hold the components of system  70 . Housing is described in more detail with reference to  FIG. 2B . 
     FIG. 2B  illustrates a perspective view of system  70 . Housing  82  may comprise any suitable material that can hold the components of system  70 . For example, housing  82  may comprise fiberglass. Portions of housing  82  through which signals are transmitted and received may expose the transmit and receive antennas to allow the antennas to transmit and receive signals, respectively. Alternatively or additionally, housing  82  may cover antennas with a material through which the signals may pass. 
   Alterations or permutations such as modifications, additions, or omissions may be made to imaging system  70  without departing from the scope of the invention. Imaging system  70  may have more, fewer, or other modules. Additionally, operations of imaging system  70  may be performed using any suitable logic comprising software, hardware, other logic, or any suitable combination of the preceding. 
     FIG. 3  is a flowchart illustrating one embodiment of a method for generating an image that may be used with system  10  of  FIG. 1 . The method begins at step  100 , where a round-trip time matrix and a range-amplitude correction matrix are accessed. A round-trip time matrix refers to a matrix that records the round-trip times rtt jk (p i ) takes for a signal to travel from a transmit antenna T j  to a point p i  of actual space  30  and back to a receive antenna R k . A range-amplitude correction matrix includes range-amplitude correction values for each waveform point of a waveform. 
   Signals are transmitted at step  104  by transmit antenna T j . The signals pass through obstruction  28  to object  24 , and are reflected back towards antenna system  36 . Receive antennas R k  receive the reflected signals at step  108 . Waveforms representing the signals are sent to computing system  38 . Example waveforms are illustrated in  FIG. 4 . 
     FIG. 4  illustrates example waveforms  90  received by receive antennas. According to the illustrated embodiment, receive antenna R 1  receives waveform  90   a , receive antenna R 2  receives waveform  90   b , and receive antenna R 3  receives waveform  90   c . Waveforms  90  are presented as examples only, and are not meant to narrow the scope of the invention. 
   Referring back to  FIG. 3 , image generator  56  of computing system  38  may subtract the background from the waveforms at step  112 . Background measurements may be subtracted from the waveforms to subtract the background. Interference may be suppressed at step  116 . Narrow band interference may be suppressed by converting the waveforms to the frequency domain using a windowed fast Fourier transform, removing narrow band peaks, and then converting the waveforms back to the time domain using an inverse fast Fourier transform. The waveforms for each point p i  may be averaged to remove further interference. The waveforms may be scaled according to the range-amplitude correction matrix at step  124 . A range-amplitude correction matrix includes range-amplitude correction values used to correct the amplitude of a waveform point to compensate for the differences in range. 
   A point p i  of space  30  is selected at step  128 . The waveform values corresponding to the selected point p i  are identified at step  132  according to the round-trip time matrix. For example, if a waveform is transmitted at time t=t 0 , then the waveform value at time t=t 0 +rtt(p i ) corresponds to point p i . The waveform values are combined at step  134  to yield the image value for the selected point p i . The waveform values may be combined by multiplying the values together. 
   The image values are stored in an image matrix at step  140 . The image matrix may include image values for each point p i  used to generate image  22 . If there is a next point p i  of image space  30  at step  144 , the method proceeds to step  128  to select the next point p i . If there is no next point p i  at step  144 , the method proceeds to step  150 . 
   A display option is selected at step  150 . Image  22  may be displayed in any suitable manner. For example, both stationary and moving targets, only stationary targets, or only moving targets may be displayed. If both stationary and moving targets are to be displayed, the method proceeds to step  152  to perform a stationary plus moving targets procedure. Image  22  of stationary and moving targets is generated from successive image matrices. 
   If only stationary targets are to be displayed, the method proceeds to step  154  to perform a stationary targets procedure. Stationary targets may be displayed by averaging together a suitable number of image matrices and generating image  22  from the averaged image matrix. If only moving targets are to be displayed, the method proceeds to step  154  to perform a moving targets procedure. Moving targets are identified by determining the images  22  that have different positions in successive image matrices. The moving targets may then be displayed. Image  22  is displayed using display  40  at step  160 . After displaying image  22 , the method terminates. 
   Alterations or permutations such as modifications, additions, or omissions may be made to the method without departing from the scope of the invention. The method may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order without departing from the scope of the invention. 
   Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that round-trip times may be used to generate a three-dimensional image. Another technical advantage of one embodiment may be that stationary targets may be detected. 
   While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims. 
   To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims to invoke paragraph 6 of 35 U.S.C. § 112 as it exists on the date of filing hereof unless the words “means for” or “step for” are used in the particular claim.