Abstract:
A method and apparatus are disclosed for forming an image from millimeter waves. A field of view scanned using two geometrically orthogonal, intersecting copolarized fan beams ( 110, 120 ) to receive millimeter wave radiation. The received millimeter wave radiation from said fan beams are then cross-correlated ( 250, 650 ). Also, a method and antenna ( 400, 610 ) for receiving millimeter wave radiation are disclosed. The antenna includes first and second fan beam antennas ( 410, 420 ) for receiving millimeter wave radiation and a filter ( 430, 440 ) for rotating polarization of incident millimeter wave radiation through 90 degrees received by the second fan beam antenna ( 410 ). The respective first and second beams ( 110, 120 ) intersect and are co-polarized and geometrically orthogonal to each other. Still further, a millimeter wave imaging system ( 600 ) and method are also disclosed, which utilise an antenna ( 610 ) for receiving millimeter wave radiation, process the received millimeter wave radiation from the antenna ( 610 ), and build up the image ( 682 ) using a filtered, cross-correlated signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to millimeter imaging systems and in particular to a realtime millimeter imaging system for detecting millimeter wave radiation and generating a corresponding image. 
   BACKGROUND 
   Millimeter-wave imaging systems produce a picture of a scene by detecting thermally generated radiation in the 30-300 GHz range, which is emitted or reflected by objects in the field of view of the instrument. Such systems offer advantages over equivalent instruments detecting infrared and visible light, because the millimeter-wave radiation can penetrate low visibility and obscuring conditions (e.g., caused by clothing, walls, clouds, fog, haze, rain, dust, smoke, sandstorms) without the high level of attenuation that occurs at the other noted wavelengths. This is particularly the case in specific “windows” for atmospheric transmission of radio waves that occur between 90 and 110 GHz and between 210 and 250 GHz. 
   Millimeter-wave imaging systems may be used in a range of important applications such as: aids to aircraft landing; collision warning in air, land and sea transport; detection and tracking of ground based vehicular traffic; covert surveillance for intruders, contraband and weapons. In such applications, the availability of real-time, “movie-camera” like imaging is highly desirable. However, for such systems to find wide acceptance in the commercial market-place, the sensing instrumentation must be light in weight, small in size, and affordable in cost. 
   A range of millimeter-wave imaging systems have been reported, but fail to meet the size, weight, and cost requirements for wide commercial acceptance of the technology, while at the same time offering real-time moving images. Such systems use two distinct technologies: mechanical scanning of the beam of a single antenna, and two-dimensional arrays. 
   Mechanical scanning of the beam of a single antenna connected to a single receiving system is performed in a raster pattern over a scene to detect the emitted radiation and produce a map or image of the brightness. The angular resolution of the resultant image is determined by the width of the antenna beam, whereas the scan angle determines the field of view. Rapid real-time imaging is difficult or inadequate, because physically large and cumbersome antenna elements (required to achieve high angular resolution) must be moved quickly at high rates. 
   Two-dimensional arrays of electrically-small antennas and integrated receivers sample the magnitude of the received millimeter-wave signal at the focal plane of an antenna system. This information is then used to produce a snap-shot of the brightness in the field of view of the instrument. In any given plane, the angular resolution of the resultant image is determined by the number of elements across the array and the outer dimensions of the array. In contrast, the field of view is determined by the beam-width of the individual antenna-array elements. Rapid real-time imaging can be achieved with these systems. However, this occurs at the expense of large numbers (1000&#39;s) of millimeter-wave receiving sub-systems and complex electronic phase shifting and amplitude weighting networks. Because of the large number of receivers required, heterodyne systems are avoided (in view of the local oscillator distribution problems) in favour of direct detection systems, with the attendant problems of gain stability and poorer sensitivity. Coherent local oscillator distribution to such a large number of millimeter-wave heterodyne receivers presents significant difficulties. 
   Thus, a need clearly exists for an improved real-time millimeter-wave imaging system capable of producing real-time, movie-like imaging, in which the system is more compact, less complex, and less expensive to produce. 
   SUMMARY 
   In accordance with a first aspect of the invention, an image is formed from millimeter waves. To do so, a field of view is scanned using two geometrically orthogonal, intersecting co-polarized fan beams to receive millimeter-wave radiation. The components of received millimeter-wave radiation from the two fan beams are cross-correlated. The polarizations of the electric fields of the two fan beams are arranged to be substantially parallel in alignment. This may be achieved by polarization rotation filtering of the millimeter-wave radiation received in one of the fan beams. The two fan beams may be scanned in azimuth and elevation defining a scan range. The intersection region of the two fan beams is able to cover any point in the scan range. The scan range determines the field of view and a beam width of each fan beam in the narrow direction determines an angular resolution of the image. The cross-correlated output is measured at each point in the field of view to produce a map of the brightness. The position of the two geometrically orthogonal, intersecting fan beams may be controlled to generate the cross-correlated output at each fan beam intersection point in the field of view. Preferably, the scanning is implemented using a dual fan-beam antenna The dual fan-beam antenna may have two modified pill-box antennas and a polarization rotator to change the direction of the incident polarization for one of the modified pill-box antennas. An image may be formed from millimeter waves of a different polarization by having a polarization rotator to change the direction of the incident polarization for a different modified pill-box antenna, only one polarization rotator being used at any time. 
   In accordance with a second aspect of the invention, millimeter-wave radiation is received. A field of view is scanned using a fan beam to receive millimeter-wave radiation. Polarization of incident millimeter-wave radiation is rotated through 90 degrees, and the field of view is scanned using another fan beam to receive the polarization-rotated millimeter-wave radiation. The fan beams intersect and are geometrically orthogonal to each other, yet the radiation is co-polarized. The fan beams are provided by respective fan-beam antennas. Each such antenna may include a modified pill-box antenna. Preferably, the modified pill-box antenna includes: a metal housing with an elongated aperture in at least one side of the housing, a curved primary reflector surface located within the housing and opposite the aperture, a feed horn within the housing, and one or more sub-reflectors for coupling the feed horn to the primary reflector surface. At least one of the sub-reflectors is designed to rotate, providing one-dimensional beam scanning in the narrow direction of the fan beam. The polarization rotation for a fan beam may be implemented using a polarization rotating transreflector. 
   Preferably, the transreflector includes: a planar metallic reflector, and a grid of closely spaced wires. The wires are preferably spaced n×λ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimeter-wave radiation. The polarization rotating transreflector may be positioned at a 45 degree angle relative to the aperture of the second fan-beam antenna and at a substantially 45 degree angle relative to the direction of incident millimeter-wave radiation. The polarization rotation for a fan beam may be switched by exchanging a polarization rotating transreflector and a planar metallic reflector, both aligned in the same way. An exchange may be effected by turning a polarization rotating transreflector by 180 degrees to use its back surface as a planar metallic reflector. An exchange may be effected by making the wires of a polarization rotating transreflector out of a material that has a switchable conductivity. 
   In accordance with a third aspect of the invention, millimeter wave radiation is received for generating an image. To do so, millimeter wave radiation is received in accordance with first and second fan beams. The first and second fan beams are geometrically orthogonal to each other and intersecting. The millimeter wave radiation received in accordance with the second fan beam is co-polarized with the millimeter wave radiation received in accordance with the first fan beam. Components of the millimeter wave radiation received in accordance with the first and second beams are downconverted to generate respective intermediate frequency (IF) signals. The IF signals are cross-correlated. The resulting cross-correlated signal is filtered to provide a value proportional to brightness at each point in the scene. The received millimeter wave radiation may be amplified in accordance with the first and second beams prior to the step of downconverting. 
   In accordance with a fourth aspect of the invention, millimeter-wave imaging is disclosed. To do so, millimeter-wave radiation is received. The receiving includes: receiving millimeter-wave radiation by scanning a field of view using a fan beam, rotating the polarization of incident millimeter-wave radiation through 90 degrees, and receiving the polarization-rotated millimeter-wave radiation by scanning a field of view using another fan beam. The fan beams intersect and are geometrically orthogonal to each other. The received millimeter-wave radiation is processed. The processing step includes: receiving components of millimeter-wave radiation from the antenna received in accordance with the fan beams, downconverting respective components of the received millimeter wave radiation received to generate respective intermediate frequency (IF) signals, cross-correlating the IF signals; and filtering the resulting cross-correlated signal. The filtered, cross-correlated signal is proportional to the brightness at each point in the field of view as the antenna beams are scanned. In this way, an image of the scene may be built up. The scanning of each fan beam may be independently controlled as required so that the image can be generated from the filtered, cross-correlated output signal which provides a value proportional to the brightness of the scene at each point in said field of view. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A small number of embodiments are described hereinafter with reference to the drawings, in which: 
       FIG. 1  is a radiation pattern of two crossed fan beam antennas in accordance with the embodiments of the invention; 
       FIG. 2  is a simplified block diagram of a real-time millimeter-wave imaging system in accordance with an embodiment of the invention; 
       FIG. 3  is a perspective view of an example of a pill-box antenna for implementing a scanned-beam imaging system in accordance with another embodiment of the invention; 
       FIG. 4  is a perspective view of a combination of two pill-box antennas and a metallic reflector for producing a dual-scanning beam antenna with co-polarized far-field response in accordance with a further embodiment of the invention; 
       FIG. 5  is a perspective view of a combination of two pill-box antennas and two polarization rotating transreflectors that may be exchanged for planar metallic reflectors, for producing a dual-scanning beam antenna with co-polarized far-field response of either of two polarizations, in accordance with a further embodiment of the invention; and 
       FIG. 6  is a block diagram illustrating a real-time cross-correlating millimeter-wave imaging system in accordance with a further embodiment of the invention, incorporating the dual fan-beam antenna of  FIG. 4  or  FIG. 5  in a modified millimeter-wave imaging system of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   A method and an apparatus for forming an image from millimeter waves, a method and an antenna for receiving millimeter wave radiation, a method and an apparatus for receiving millimeter wave radiation for generating an image, and a method and a system for millimeter wave imaging are disclosed. In the following description, numerous specific details are set forth. In the other instances, details well known to those skilled in the art may not be set out so as not to obscure the invention. It will be apparent to those skilled in the art in the view of this disclosure that modifications, substitutions and/or changes may be made without departing from the scope and spirit of the invention. 
   The embodiments of the invention involve improved imaging methods, antennas, and systems that enable the realization of a simple, low-cost instrument, capable of realtime imaging of moving targets. In broad terms, the embodiments produce a map or image of the millimeter-wave brightness in the field of view of the instrument by cross-correlating the signal received from two orthogonal, intersecting fan-beams. 
   Fan-Beam Antennas Generally 
   An antenna with a fan-beam radiation pattern detects radiation from a region in the field of view that is of narrow angular extent in one direction only, while possessing a broad pattern in the orthogonal plane. Typically, a fan-beam can be generated by an antenna, or array of antennas, which is essentially one-dimensional (e.g., a long narrow slot, a linear array of slots, or a linear array of patch antennas). The width of the beam in the narrow direction is inversely proportional to the electrical length of the aperture or array. In contrast, the beam-width in the broad direction is inversely proportional to the width of the aperture or an individual element of the array. The angular position of the fan-beam in the narrow direction may be scanned across the field of view by producing a varying linear gradient in the phase of the electrical excitation across the aperture or across the elements of the array. 
   In accordance with embodiments of the invention, two such fan beams are arranged so that the beams intersect at right angles in the field of view of the instrument.  FIG. 1  is a plot illustrating the radiation pattern  100  of two crossed fan beam antennas. The pattern  100  includes an E-plane, fan-beam antenna pattern  110  and an H-plane, fan-beam antenna pattern  120 , and a pencil beam pattern  130 . The polarization of the electric field in each beam is arranged to be parallel in alignment. As the fan-beams  110 ,  120  are scanned in azimuth and elevation, the intersection region  130  can be made to cover any point in the scan range. Thus, the scan range determines the field of view of the instrument and the beam-width of the fan-beam in the narrow direction determines the angular resolution of the image. The millimeter-wave brightness at any point in the image is proportional to the cross-correlation between the signals received by the two antenna systems. 
   Imaging Receiver System 
   A significant component of the imaging system is the receiver, which takes the output from the antennas, amplifies the signals, and then down-converts the amplified signals to a convenient intermediate frequency at which the cross-correlation can take place. There are a number of possible implementations for such receiving systems, depending upon the design of the fan-beam antenna. 
   An imaging receiver system  200  in accordance with an embodiment of the invention shown in  FIG. 2  uses only two receivers, one connected to an antenna  202  scanning in the vertical direction and the other to an antenna  204  scanning in the horizontal plane, to sample the whole image. The antenna  202  is an E-plane antenna, and the antenna  204  is an H-plane antenna. The E-plane antenna is coupled to one or more radio frequency (RF) low noise amplifiers (LNAs)  212   a ,  212   b . The output of the one or more low noise amplifiers  212   b  is coupled to a respective block down converter  232 . Similarly, the H-plane antenna  204  is coupled to one or more LNAs  214   a ,  214   b . The output of the LNA  214   b  is coupled to a further block down converter  234 . A local oscillator  220  provides an input to both block down converters  232 ,  234 . 
   The respective block down converters  232 ,  234  produce respective intermediate frequency (IF) signals that are both provided to a correlator  240 . The output of the correlator  240  is provided to a low pass filter  250 , which produces the output signal  260 . A map of the millimeter-wave brightness at each point in the field of view is produced by scanning the antenna beams over the field and at each field point measuring the cross correlation between the receiver outputs using a broadband analogue multiplier  240 . 
   A polarization rotating filter (not shown) may be placed in front of one of the antenna apertures so that both fan beams operate in the same polarization. 
   Antenna for Imaging System 
   In accordance with an embodiment of the invention, a simple, inexpensive implementation uses a multiple reflector “pill-box” style antenna  300  shown in  FIG. 3 . In this simplified example, a shaped primary reflector  334  is coupled to a single feed-horn  330 ,  332  via a rotating sub-reflector  320 , which provides beam scanning as the sub-reflector  320  spins. More than one sub-reflector may be practiced, with at least one sub-reflector rotating to provide beam scanning. With careful mechanical and electrical design, in which the rotating sub-reflector  320  rotates about its center of mass, high speed scanning can be achieved. Preferably, the sub-reflector  320  is disc-like in form. A significant advantage of this system is that only a single heterodyne receiver per beam is needed. This is advantageous from the point of view of system simplicity and cost and also because a simple local oscillator distribution system is possible without the need for complex array phasing. 
   In a conventional “pill-box” antenna, a parabolic cylinder is used as the reflector. The “pill-box” is formed by two parallel planes which cut through the parabolic cylinder perpendicular to the cylinder elements. Typically, the focal line of the cylinder is positioned in the center of the aperture formed by the open ends of the parallel plates. When a feed horn is placed at the focal line, the feed horn blocks a significant portion of the aperture, resulting in large sidelobes in the far-field pattern of the antenna as well as standing waves within the “pill-box” itself. 
   Much improved performance can be obtained when an offset feeding arrangement is used, so that only one side of the “pill-box” is illuminated. The arc of the parabola does not include its vertex, and the feed horn points to illuminate this arc. Even though the illumination is asymmetric, good sidelobe performance is obtained. Alternatively, the “pill-box” antenna may be symmetrical about the axis of the parabola, but arranged as a folded lens to avoid blockage. Such an antenna, however, is more difficult to manufacture than an unfolded design. 
   The millimeter-wave fan-beam antenna  300  shown in  FIG. 3  includes a metal housing  310  with a radiating aperture  312  formed in one side of the metal housing. The length of the radiating aperture  312  is approximately 200 wavelengths (λ) and the width of the aperture  312  is approximately one wave length (1λ). These measurements are preferred and other dimensions may be practiced without departing from the scope and spirit of the invention. The direction of the electric field at the aperture is indicated by an arrow  314 . Located within the metal housing  310  is the primary reflector surface  334  coupled to the tapered wave guide feed-horn  330  with a wave guide input/output  332  oppositely positioned relative to the radiating aperture  312  within the housing  310 . At the bottom of the tapered wave guide feed-horn  330  within the metal housing  310  is the rotating sub-reflector  320  for one dimensional beam scanning. 
   The antenna  300  uses one or more sub-reflectors  320  to couple the feed horn  330 ,  332  in an offset “pill-box” structure. The primary reflector  334  is shaped away from the traditional parabola to provide enhanced off-axis scanning angle with good sidelobe performance over the widest possible range of scan. The primary reflector  334  is coupled to the single feed-horn  330  via one or more sub-reflectors  320 , which are also designed to have a profile that enhances the scan performance of the complete antenna assembly  300 . One of these secondary mirrors  320  is arranged so that this sub-reflector  320  rotates, providing main beam scanning as the sub-reflector  320  spins. With careful mechanical and electrical design, in which the rotating sub-reflector  320  rotates about its center of mass, high speed scanning can be achieved. 
   For the imaging system, a pair of independently-scanned, orthogonally-oriented fan beams are required, with the sense of electric polarization aligned in each beam. Two “pill-box” antennas  410 ,  420  of the type shown in  FIG. 3  are used, configured  400  as shown in  FIG. 4 . The antenna  410  has an aperture  414  oriented lengthwise in a horizontal sense, while the other antenna  420  has an aperture  424  lengthwise in a vertical sense, as depicted in  FIG. 4 . The direction  412 ,  422  of the electric field in the respective apertures  414 ,  424  are shown. Thus, the aperture  424  couples directly to the observed scene, while the other aperture  414  is arranged at a right angle so that the aperture  414  is coupled via a passive reflecting screen  430 ,  440  and is oriented so that the narrow dimension of the far-field pattern of the aperture  414  is at right angles to the pattern of the other antenna  420 . 
   The passive reflecting screen  430 ,  440  is generally configured at an angle of 45° relative the surface of the fan-beam antenna  410  having the aperture  414 . The passive reflecting screen preferably has a planar metallic reflector  430  spaced apart by a multiple of a quarter wavelength (nλ/4) from a closely spaced, fine wire grid  440 . The grid  440  is located between the reflector  430  and the antenna  410 . The wires of the grid  440  are aligned at 45° to the direction of incident field polarization. This arrangement  400  results in orthogonal polarization in the far-field, if a standard plane reflector  430  is used. 
   Another way to achieve a co-polarized far-field response may be to modify the feed for the “pill-box” antenna  410 ,  420 , so that the E-field vector is rotated through 90 degrees and aligned parallel to the long direction of the aperture. For this configuration, small variations in the surface quality and spacing of the metallic walls may cause significant degradation in antenna performance. However, for this arrangement, the polarization rotating filter  430 ,  440  is no longer required to be included. 
   The preferred way to achieve co-polarization is by the use of a “transreflector”  430 ,  440 . The transreflector  430 ,  440  consists of the wire grid  440 , with wires aligned at 45 degrees to the incident electric field vector, backed by the planar metallic mirror  430  spaced away by an odd-multiple of a quarter wavelength at the operating frequency. The wire spacing and wire diameter must both be small compared to the operating wavelength. Over a limited bandwidth determined by the spacing between the grid  440  and the reflector  430  (the higher the number of quarter wavelengths, the narrower the bandwidth), this arrangement results in a rotation of the polarization of the incident wave through 90 degrees, without significantly altering the far-field radiation pattern of the antenna system. 
   Two “pill-box” antennas  510 ,  520  of the type shown in  FIG. 3  configured  500  in an alternative manner are shown in  FIG. 5 . Generally,  FIG. 5  shows how two pill-box antennas can be placed with their flat sides parallel and the apertures oriented 90 degrees apart. In front of both apertures is a polarization rotating transreflector that can be exchanged with a planar metallic reflector, such that only one aperture receives polarization-rotated radiation at any time. This leads to a more compact structure than  FIG. 4  that is capable of forming an image of either of two polarizations. The axes of the rotating sub-reflectors are parallel, so a simple gearing mechanism can be used to give the relative rotation rates needed for the intersection of the fan beams to perform a raster scan. 
   The antenna  510  has an aperture  530  oriented lengthwise in a horizontal sense, while the other antenna  520  has an aperture  540  oriented lengthwise in a vertical sense, as depicted in  FIG. 5 . In front of both apertures is a polarization rotating transreflector that can be exchanged with a planar metallic reflector, such that only one aperture receives polarization-rotated radiation at any time. In  FIG. 5  the horizontal aperture  530  is coupled to the observed scene via a transreflector  550 , while the vertical aperture  540  is coupled to the observed scene via a planar metallic reflector  560 . The transreflector  550  may be exchanged with a planar metallic reflector, and the planar metallic reflector  560  may be exchanged with a transreflector, as indicated by the dotted lines on the reflector  560 . An exchange may be effected by turning a polarization rotating transreflector by 180 degrees to use its back surface as a planar metallic reflector. An exchange may be effected by making the wires of a polarization rotating transreflector out of a material that has a switchable conductivity. The advantages of this configuration  500  over the configuration  400  in  FIG. 4  are that the configuration  500  occupies a smaller overall volume and is capable of forming an image from either of two polarizations. The axes of the rotating sub-reflectors  320  are parallel in this configuration  500 , so a simple gearing mechanism (not shown) can be used to achieve relative rotation rates that cause the intersection  130  of the fan beams  110 ,  120  to perform a raster scan of the field of view. 
     FIG. 6  is a block diagram illustrating an implementation of a real-time, cross-correlating, millimeter-wave imaging system  600  in accordance with a further embodiment of the invention. For purposes of illustration only, the system is shown in  FIG. 6  with a tree  602  as the object of imaging in the field of view. A dual, fan-beam antenna  610  is used to scan the object  602  and respective horizontal and vertical scans  604 ,  606  generated by the antenna  610  are shown. The dual fan-beam antenna  610  is of the type  400  shown in  FIG. 4 . Alternatively, the dual fan-beam antenna  610  may be of the type  500  shown in  FIG. 5 . The dual fan-beam antenna  610  provides respective E-plane and H-plane outputs to an imaging receiver system, similar to that shown in  FIG. 2 . 
   The E-plane output is provided to a low noise amplifier  612  and the H-plane output is provided to a different low noise amplifier  614 . In turn, the low noise amplifiers  612 ,  614 , acting as RF amplifiers, are coupled to respective mixers  620 ,  622 . Further, a local oscillator  630  is coupled to both of mixers  620  and  622 . The respective outputs of mixers  620  and  622  are provided as inputs to IF amplifiers  640 ,  642 . The output of the IF amplifiers  640 ,  642  are provided to a cross-correlator  652 . 
   The output of the cross-correlator  652  is provided to a base band filter  660 . The base band filter  660  provides the output signal for the system. The output of the base band filter  660  is provided to an analogue to digital (A/D or ADC) converter  670 . The ADC  670  produces digital data from the output signal that is provided as input to a computer  680 . The computer  680  using hardware and/or software can produce a computer image  682  using the digital data from the ADC  670 . In turn, using the digital data, the computer  680  can provide scan control signals  690  (indicated by dashed lines) to the dual fan-beam antenna  610 . As shown in  FIG. 6 , the scan control signals  690  are preferably provided to each of the pill-box antennas. 
   The embodiments of the invention have various advantages including one or more of: 
   Use of a “pill-box” antenna to implement a scanned-beam imaging system; 
   A “pill-box” antenna in which the beam is scanned in one dimension using a rotating sub-reflector; 
   Use of a wire-grid transreflector to achieve a dual-scanning-beam system with co-polarized far-field response; 
   Use of two wire-grid transreflectors, exchangeable for planar metallic reflectors, to achieve switchable polarisation of the far-field response. 
   Use of a mechanically scanned beam so that only a single heterodyne receiver per beam is needed. 
   Use of two intersecting fan beams so that each antenna is required to scan only in one direction. 
   Thus, a method and an apparatus for forming an image from millimeter waves, a method and an antenna for receiving millimeter wave radiation, a method and an apparatus for receiving millimeter wave radiation for generating an image, and a method and system for millimeter wave imaging have been disclosed. In the light of this disclosure, it will be apparent to those skilled in the art that modifications, substitutions and/or changes may be made without departing from the scope and spirit of the invention.