Patent Publication Number: US-6992616-B2

Title: Millimeter-wave active imaging system

Description:
BACKGROUND 
   Millimeter wave signals are used for radar and telecommunications. They are also capable of being used for producing an image of a subject by directing millimeter-wave signals at the subject and detecting the reflected signal. Examples of such imaging systems have been described in U.S. Pat. Nos. 5,455,590; 5,557,283; 5,859,609; and 6,507,309; and U.S. patent application Ser. No. 10/607,552 filed Jun. 26, 2003 and a continuation-in-part of that application filed Oct. 30, 2003 entitled “Detecting Concealed Objects at a Checkpoint”; and U.S. patent application Ser. No. 10/301,522 filed Nov. 21, 2002 and a continuation-in-part of that application filed Oct. 30, 2003 entitled “Detection of a Concealed Object”, which patent references are incorporated herein by reference. 
   When imaging systems are used for surveillance of persons, it may be desirable for the system to quickly, conveniently and safely perform the surveillance. This is particularly true in situations where the surveillance delays the intended progress of the person being surveilled, such as prior to boarding a public transportation vehicle, or prior to entering a public or protected facility. Accordingly, different surveillance situations may be benefited by using differently configured surveillance or interrogation stations in which a person is positioned during imaging. 
   BRIEF SUMMARY OF THE DISCLOSURE 
   Active imaging systems can include an antenna apparatus configured to transmit toward and receive from a subject in a subject position, millimeter-wave electromagnetic radiation. The antenna apparatus transmits and receives radiation from positions spaced from the subject position and distributed along a locus of points or aperture facing the subject. A controller can include a transceiver configured to operate the antenna apparatus and produce an output representative of the received radiation, and a processor adapted to convert the transceiver output into image data representative of an image of the subject. 
   Various configurations of an antenna apparatus are possible. A particular configuration can be selected as appropriate for a particular application. For example, the antenna apparatus may include one or a plurality of antenna units, such as a linear or two-dimensional array of antenna units. The antenna unit or units may move along a curved path or be in an array that is curved or straight, and may be fixed or move in a curved or straight path. Antenna units or one or more arrays of antenna units may be fixed in position and pivot to scan a subject from one or more positions distributed about the subject. An assembly, in which an antenna array is adapted to move along a defined path, may move in various ways. For example, the assembly may move along a path extending at least partially around the subject, toward or away from the subject, or in an opposite direction to an associated assembly. Such antenna units may also be oriented at different angular positions along an array. Antenna arrays may also be formed of a plurality of array segments, and a group of arrays may be combined to form an antenna apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general diagram showing an active imaging system. 
       FIG. 2  is a diagram depicting general examples of interrogation stations that may be used in imaging systems. 
       FIGS. 3–9  are simplified top views of different interrogation stations. 
       FIGS. 10A–10C  are top views of another interrogation station illustrating a sequence of operation. 
       FIGS. 11A–11D  are top views of yet another interrogation station illustrating a sequence of operation. 
       FIG. 12  is a side view of an interrogation station illustrating an exemplary antenna array configuration. 
       FIG. 13  is a front view of an optional embodiment of the antenna array of  FIG. 12 . 
       FIG. 14  is a top view of an interrogation station illustrating another exemplary antenna array configuration that can be the same interrogation station shown in  FIG. 12 . 
       FIG. 15  is a top view of an interrogation station having laterally offset antenna array assemblies. 
       FIG. 16  is a general diagram depicting an imaging system having an interrogation station with a plurality of antenna apparatus segments. 
       FIG. 17  is a general diagram depicting one example of an antenna apparatus segment usable in the interrogation station of  FIG. 16 . 
       FIG. 18  is a plan view of a plurality of antenna array segments that may be used to form an antenna array. 
       FIG. 19  is an isometric view of one example of an antenna apparatus segment. 
       FIG. 20  is a top view of an interrogation station having a plurality of antenna apparatus segments. 
   

   DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS 
   Shown generally at  20  in  FIG. 1  is an active imaging system. System  20  includes an antenna apparatus  22  and a controller  24 . The system is active in the sense that the antenna apparatus transmits electromagnetic radiation  26  toward a subject  28 , and in response, the subject emits or reflects electromagnetic radiation  30  that is detected by the antenna apparatus. A subject includes all that is presented in an interrogation station of an imaging system for imaging, whether human, animal, or inanimate object. For example, if a person is in an interrogation station for imaging, the subject includes the person as well as any objects supported on the person, such as watches, keys, jewelry, pocket or other knives, coins, clothing accessories, guns, or any other objects that can be imaged. A subject may include one or more persons, animals, objects, or combination of these. 
   Electromagnetic radiation may be selected from an appropriate frequency range, such as in the range of about 200 megahertz (MHz) to about one terahertz (THz), generally referred to herein as millimeter-wave radiation. Satisfactory imaging may be realized using electromagnetic radiation in the reduced frequency range of one gigahertz (GHz) to about 300 GHz. Radiation in the range of about 5 GHz to about 110 GHz may also be used for producing acceptable images. Such radiation may be either at a fixed frequency or over a range or set of frequencies using several modulation types, e.g. chirp, pseudorandom frequency hop, pulsed, frequency modulated continuous wave (FMCW), or continuous wave (CW). 
   Many variations of an antenna apparatus are possible. The antenna apparatus may include one or more antenna units, and each antenna unit may include one or more transmitting antennae and one or more receiving antennae. An antenna unit may include a plurality of antennae that may receive radiation in response to transmission by a single antenna. The antennae may be any appropriate type configured to transmit or receive electromagnetic radiation, such as a slot line, patch, endfire, waveguide, dipole, semiconductor, or laser. Antennae may both transmit and receive. The antennae units may have one or more individual antennae that transmit or receive like polarization or unlike polarized waveforms such as plane, elliptical, or circular polarization, and may have narrow or broad angular radiation beam patterns, depending on the application. Beam width may be relatively broad, i.e. 30–120 degrees for imaging applications that use holographic techniques, while narrow beam widths in the range of 0 to −30 degrees may be used for applications having a narrow field of view requirement. Further, a single antenna may scan a subject by mechanically moving about the subject in a one- or two-dimensional path. A one- or two-dimensional array of antenna units may electronically and mechanically scan a subject. An imaging system may include one or a plurality of antenna apparatus, such as a second antenna apparatus  22 ′. The antennae apparatus may be protected from the environment by suitable radome material which may be part of the apparatus, or separate, depending on the mechanical motion that is required of the antennae apparatus or array. 
   An imaging system may include an antenna-apparatus moving mechanism  32 , represented by a motor, which moves antenna apparatus  22  relative to a subject  28 . Moving mechanism  32  may be mounted relative to a frame  34  for moving the antenna along a path defined by a movement control mechanism, such as a guide  36 , including associated motor indexers, encoders or other controls, as appropriate. The moving mechanism may be any appropriate mechanism that moves the antenna apparatus, and may include a stepper motor, servo motor, or other suitable device. 
   Controller  24  may control operation of motor  32 , and coordinate the operation of antenna apparatus  22  with movement of the antenna apparatus. Controller  24  may include hardware, software, firmware, or a combination of these, and may be included in a computer, computer server, or other microprocessor-based system capable of performing a sequence of logic operations. In addition, processing can be distributed with individual portions being implemented in separate system components. In one example, controller  24  may include a transceiver  38 , a processor  40 , and a memory  42  coupled to the processor for storing data and operating instructions. Such instructions may be embodied as hardware, firmware, or software. 
   The transceiver, as contemplated herein, includes all structure and functions appropriate for generating, routing, processing, transmitting and receiving millimeter-wave signals between the antenna apparatus and the processor. The transceiver, then in this comprehensive sense, may include multiplexed switching among the antenna units, transmit and receive electronics, and electronic and logic units. The transceiver may be wholly or partly included with a central controller or be wholly or partly resident in an interrogation station  44  housing the antenna apparatus. In certain cases, more than one transceiver is desirable, such as for multiple antenna apparatus or for two dimensional array imaging systems. The transceiver thus sends to and receives from the antenna apparatus scanning signals  46 , and outputs received signals  48  to processor  40 . 
   The processor may be any analog or digital computational device, or combination of devices, such as a computer(s), microprocessor(s), or other logic unit(s) adapted to control scanning of a subject and receiving received signals  48  and produce image data  50  representative of an image of at least a portion of the subject. Image data may include any data, whether processed, partially processed or unprocessed, or sub-sets of the data, such as data for a portion of a subject, data that is manipulated in order to separate, for viewing by an operator or by another processor, objects that may represent a desired class of objects, such as man-made objects, non-physiological or non-living objects, or the like, data identifying or facilitating identification of an object or subject, or measurements or other information relating to a subject that is derived from received signals. The image data may be output to an output device  52 , such as a storage device, communication link, such as a network hub, another computer or server, or directly to a display device, such as a video monitor. Memory  42  may be a single device or a combination of devices, and may be local to the processor or remote from it and accessible on a communication link or network. 
     FIG. 2  illustrates another form of interrogation station  60  for use in some imaging systems, such as system  20  discussed above. Interrogation station  60  includes an antenna apparatus assembly  62  and an antenna path-moving assembly  64 . The antenna apparatus assembly may include an antenna apparatus  66 , a motor  68 , a guide  70 , and an intermediate frame  72 , similar to interrogation station  44  of imaging system  20 . Components  66 ,  68  and  70  may be mounted relative to intermediate frame  72  for moving the antenna apparatus along an antenna path. Path-moving assembly  64  correspondingly may include a motor  74 , a guide  76  and a base frame  78 . Motor  74  can act on antenna apparatus assembly  62  to move the position of the antenna path. As is discussed further below, this movement of the antenna apparatus assembly may be used to control access by a person (subject) to the subject position in the interrogation station, or to provide extended scanning of the subject with the antenna apparatus. 
   Following are various embodiments and configurations of interrogation stations or portions of interrogation stations that may be used in an imaging system, such as in one or more of the imaging systems illustrated in  FIGS. 1 and 2 .  FIG. 3  illustrates a top view of an interrogation station  80  having an antenna apparatus  82  spaced from a subject position  84  having a subject center  86 . Antenna apparatus  82  provides for transmitting and receiving electromagnetic radiation along a locus  88  of points, represented by an arc  90 . In this example, arc  90  has a center of curvature  92  that is on the opposite side of subject center  86  from the antenna apparatus. 
   The shape of the locus of points may vary along its length in one or more directions. The locus of points may thus appear to be concave or convex to a subject being imaged, or may have any of various curvilinear configurations, such as an S-shaped curve, or a curve with an incrementally or continuously varying curvature, a configuration with one or more rectilinear segments, or any combination of such configurations. As used herein, the center of curvature of a portion of a locus of points containing three adjacent points is a point equally distant from the three points. The distance from the center of curvature to the three points then corresponds to the radius of an arc passing through the three points. 
   The antenna apparatus may include at least one antenna unit  94  that moves along a path  96  conforming to arc  90 . Various positions of antenna unit  94  along path  96  are illustrated. In an embodiment having only a single antenna unit, the subject position is scanned by mechanically moving the antenna unit along an aperture of interest, such as along arc  90 . 
   Antenna unit  94  also may be part of a vertical antenna array  98  that extends vertically along the height or a portion of the height of a subject  100 , such as a person, generally occupying subject position  84 . In such a case, the vertical array travels along path  96  in an imaging system as illustrated in  FIG. 1 . The various positions of antenna unit  94  along the path correspond to positions of antenna array  98  along the path. 
   Optionally, antenna unit  94  may be part of a horizontal array  102 , with the horizontal array also having additional antenna units, such as antenna units  104 ,  106 ,  108  and  110 . Horizontal array  102  extends along arc  90 . A subject in the subject position may be scanned both electronically along the array and mechanically by movement of the array vertically. 
   In some embodiments, a two-dimensional array  112  may be used. Array  112  thus extends vertically and horizontally, and includes antenna units  94 ,  104 ,  106 ,  108 ,  110 , and others extending vertically from the antenna units along arc  90 . The subject position may then be electronically scanned from the locus of points covering an area or aperture  114  including arc  90 . 
   Extended imaging may be provided by one or more of extending arc  90 , providing one or more additional antenna apparatus, rotating a subject  100  about center  86 , as indicated by circle  116 , such as on a platform, and moving arc  90  around the subject position. Arc  90  may be any length considered appropriate for a particular application. 
   The distance D 1  from the center of the subject position to arc  90  may vary along the arc. An imaging system based on a cylindrical aperture for imaging, as disclosed in U.S. Pat. No. 5,859,609, can be modified in computing image data corresponding to a cylindrical system, by compensating for the difference in distance along the arc. Such a difference can be computed during data processing, or difference values can be stored in memory, such as in a look-up table. 
   It will be appreciated that by positioning the center of curvature  92  of arc  90  on the opposite side of the subject center  86  from the antenna apparatus  82 , the curve of the arc is more elongate along the length of the arc relative to the subject position. The longer the radius of curvature, the more gradual or straight the arc becomes. Various configurations are accordingly possible. For instance, when the interrogation station is used as a portal for an imaging system at an entrance to a facility, such as an airport, it may be desirable to have people walk along a continuous path. Having the antenna apparatus extending along gentle arcs along the sides of the path reduces the width of the imaging system, allowing it to be used in a smaller space. 
   If the subject, and therefore the subject position, has a shape that is cylindrical, then a cylindrical arc centered on the subject center would provide a uniform distance between the arc and the subject. This, however, also may not be the case. The shape of arc  90  may generally conform to a side  118  of a subject  100  facing arc  90  and that extends more along one axis, such as a long axis  120 , than along a transverse, short axis  122 , as may be the case with people, particularly with the backs of people. As a result, an arc  90  may be selected that provides a distance D 2  between the locus  88  of points along arc  90  that generally conforms to an expected general shape of at least a portion of a subject. 
     FIG. 4  illustrates a top view of a further variation of an interrogation station  130  usable in an imaging system, such as imaging system  20 . Interrogation station  130  has an antenna apparatus  132  spaced from a subject position  134  having a subject center  136 . Antenna apparatus  132  may provide for transmitting and receiving electromagnetic radiation along a locus  138  of points, represented by an arc  140 . In this example, arc  140  has a center of curvature  142  that is on the same side of subject center  136  from the antenna apparatus. Otherwise, interrogation station  130  is similar to interrogation station  80  described above. 
   The antenna apparatus may include at least one antenna unit  144  that moves along a path  146  conforming to arc  140 . Various positions of antenna unit  144  along path  146  are illustrated. In an embodiment having only a single antenna unit, the subject position is scanned by mechanically moving the antenna unit along an aperture of interest, such as along arc  140 . 
   Antenna unit  144  also may be part of a vertical antenna array  148  that extends vertically along the height or portion of the height of a subject  150 , such as a person, generally occupying subject position  134 . In such a case, the vertical array travels along path  146  in an imaging system as illustrated in  FIG. 1 . The various positions of antenna unit  144  along the path correspond to positions of antenna array  148  along the path. 
   Optionally, antenna unit  144  may be part of a horizontal array  152 , with the horizontal array also having additional antenna units, such as antenna units  154  and  156 . There also may be additional units between those shown. Horizontal array  152  extends along arc  140 . A subject in the subject position may be scanned both electronically along the array and mechanically by movement of the array vertically. 
   In some embodiments, a two-dimensional array  158  may be used. Array  158  thus may extend vertically as well as horizontally along arc  140 , and may include antenna units  144 ,  154  and  156 , and others extending vertically from the antenna units along arc  140 . The subject position may then be electronically scanned from the locus of points covering an area or aperture  160  including arc  140 . 
   Extended imaging may be provided by one or more of the following: extending arc  140 ; providing one or more additional antenna apparatus; rotating a subject  150  about center  136 , as indicated by circle  162 , such as on a platform; and moving arc  140  around the subject position. Arc  140  may be any length and shape considered appropriate for a particular application. 
   The distance D 3  from the center of the subject position to arc  140  may vary along the arc. As discussed above, an imaging system based on a cylindrical aperture for imaging, as disclosed in U.S. Pat. No. 5,859,609, can be modified in computing image data corresponding to a cylindrical system, by compensating for the difference in distance along the arc. Such a difference can be computed during data processing, or difference values can be stored in memory, such as in a look-up table. 
   It will be appreciated that by positioning the center of curvature  142  of arc  140  on the same side of the subject center  136  as the antenna apparatus  132 , the curve of the arc is tighter along the length of the arc relative to the subject position. The shorter the radius of curvature, the sharper the curve of the arc becomes. Various configurations are accordingly possible. For instance, when the interrogation station is used as a portal for an imaging system at an entrance to a facility, such as an airport, it may be desirable to have people walk along a continuous path. Having the antenna apparatus positioned on the sides of the path and shaped to conform generally with the sides of a person positioned on the path may be desirable. 
   Following this line of thinking, the shape of arc  140  may generally conform to a side  164  of a subject  150  exposed to arc  140  and that extends less along one axis, such as a short axis  166 , than along an orthogonal, long axis  168 , as may be the case with people, particularly with the sides of people. As a result, an arc  140  may be selected that provides a distance D 4  between the locus  138  of points along arc  140  that generally conforms to an expected general shape of a portion of a subject. 
     FIG. 5  illustrates a top view of an interrogation station  170  having a different design than that of interrogation station  130 , but for which an antenna unit  172  of an antenna apparatus  174  moves along an arc  176  having a center of curvature  178  that is on the same side of a subject center  180  of a subject position  182  as the antenna apparatus. Antenna unit  172  may be part of an antenna array  184  that may extend along arc  176  and/or vertically. 
   The antenna unit may have a beam  186  that may be narrow or broad, depending upon the application. By pivoting the antenna unit about a pivot axis  188  passing through center of curvature  178 , the antenna unit moves along arc  176 . During such movement, the beam may scan across subject position  182 , as represented by double-arrow  190 . In some examples, a plurality of such pivoting antenna units, such as antenna units  192  and  194 , may be distributed along a further arc, such as arc  196 . Arc  196  may be an arc similar to arcs  90  and  140  mentioned above. Optionally, antenna unit  172  may also move along arc  196 . 
   Also, in an optional embodiment, an array  184  may include additional antenna units, such as antenna units  198  and  200  positioned along arc  176 . Scanning of the subject position then may be accomplished electronically by activating each of the antenna units along the arc. 
     FIG. 6  illustrates a top view of an interrogation station  210  that is similar to interrogation station  170 , but differs in that a pivot axis  212  for pivoting an antenna unit  214  of an antenna apparatus  216  is positioned opposite from a subject position  218  having a subject center  220 . Pivot axis  212  may thus be coincident with a center of curvature  222  of an arc  224  along Which the antenna unit moves. By pivoting about pivot axis  212 , antenna unit  214  may scan a beam  226  across the subject position  218 , as indicated by arrow  228 . 
   Pivoting of the antenna unit may be provided in various ways by a moving mechanism  230 . One way is to support the antenna unit on an arm  232  that is adapted to pivot relative to a frame  234 . Arm  232  may pivot by reciprocatingly moving an arm end  232   a  by a drive element  236 . Drive element  236  may be driven in a reciprocating fashion by a drive mechanism  238 , such as a solenoid or stepper motor. The drive mechanism may be controlled by a controller via a control line  240 . A similar moving mechanism also may be used for pivoting antenna units of interrogation station  170 . 
   Optionally, a plurality of antenna units may be positioned along arc  224 , such as antenna units  242  and  244 , of an antenna array  246 . Additionally, the antenna units may move along a larger arc  248 , or pivoting antenna units may be distributed along arc  248 , such as antenna units  250  and  252  of an antenna array  254 . 
     FIG. 7  illustrates a top view of an interrogation station  260  that may be a further variation of interrogation stations  170  and  210 . Interrogation station  260  may include an antenna apparatus  262  that is fixed in position on a frame  264  that may include an enclosure or housing for the antenna apparatus. The antenna apparatus includes an antenna unit  266  that may be part of an antenna array  268 . Each antenna unit may have a beam, represented by line  270 . A moving mechanism, not shown, may pivot the antenna unit(s) of antenna apparatus  262  about a pivot axis  272  shown aligned with the antenna units. As illustrated in  FIGS. 5 and 6 , the pivot axis may also be spaced from the antenna units. During pivoting, beam  270  may scan across a subject position  274  having a subject center  276 . When the antenna apparatus includes an array of antenna units, the antenna units may be individually pivoted or may be pivoted collectively. 
     FIG. 8  illustrates an interrogation station  280  formed of a plurality of antenna apparatus, such as antenna apparatus  282 ,  284 ,  286  and  288 . These antenna apparatus may each be configured like the antenna apparatus  262  of interrogation station  260 , illustrated in  FIG. 7 . They may be distributed around a subject position  290  having a subject center  292 . Antenna apparatus  282 ,  284 ,  286  and  288  have respective antenna units, represented by antenna units  294 ,  296 ,  298  and  300 . The respective antenna units may pivot about respective pivot axes  302 ,  304 ,  306  and  308  for scanning respective beams  310 ,  312 ,  314  and  316  across the subject position. The various antenna apparatus may be fixedly mounted to a frame  318 . As illustrated in  FIGS. 5 and 6 , the pivot axis may also be spaced from the antenna unit. 
   More or fewer antenna apparatus may be used. The antenna apparatus are positioned to provide a desired coverage of the surface of a subject positioned in the subject position. In the configuration shown, the antenna apparatus are positioned to allow a subject to enter the interrogation station along a path  320  at an entrance  322 , stand at the subject position during imaging, and leave through an exit  324  opposite from the entrance. 
     FIG. 9  illustrates an interrogation station  330  having first and second antenna apparatus  332  and  334  positioned on opposite sides of a subject position  336  having a subject center  338 . In the embodiment shown, each antenna apparatus has an antenna unit  340  that may be part of an antenna array  342  that transmits and receives electromagnetic radiation along an arc  344 . 
   Antenna apparatus  332  and  334  may be formed as combinations of antenna apparatus  82  and  132  described previously. Each arc  344  may have an intermediate portion  344   a , with a respective center of curvature  346  and  348  positioned between the associated arc portion and the subject center. Each arc  344  also may have end portions  344   b  and  344   c  that have respective centers of curvature  350  and  352  positioned on the opposite side of the subject center from the associated arc portion. Optionally, terminal ends  344   d  and  344   e  may be formed in a straight line so that an antenna unit on that portion of the arc more directly faces a subject in the subject position. 
   Antenna units may be distributed along the respective arcs, may move along the arc, or both. Optionally, the arcs of the antenna apparatus, such as arcs  353  and  354 , may include other arc portions. These arcs thus may include intermediate portions having respective centers  350  and  352 , and end portions having respective centers  346  and  348 . 
   In the example shown, first antenna apparatus  332  is fixed in position relative to the subject position, and second antenna apparatus  334  is adapted to move relative to the subject position. Second antenna apparatus  334  may be mounted relative to an apparatus frame  355  with a moving mechanism that is adapted to move antenna apparatus  334  relative to a base frame  356 . The second antenna apparatus may thus be shifted between a first position  358 , close to or proximal the subject position for imaging, and a distal, second position  360  spaced further away from the subject position. The antenna apparatus, thus, moves in a direction  362  that is transverse to associated arc  344 . 
   Interrogation station  330  may be useful for providing a close-fitting enclosure, defined by opposing arcs  344 , around a subject during imaging that has reduced-width passageways  364  and  366 . A subject may then move into and out of the interrogation station through the passageways when the distance between the antenna arrays is increased. As an optional design, both of the antenna arrays can move toward and away from the subject position, and thereby toward and away from each other. In this case, first antenna apparatus  332  may be mounted relative to an apparatus frame  355 ′ with associated moving mechanism that is adapted to move antenna apparatus  332  relative to a base frame  356 ′. If more antenna arrays surround the subject position, any combination of them can be made to move toward and away from the subject position. 
     FIGS. 10 and 11  illustrate top views interrogation stations that include barriers around arcs associated with antennae apparatus in which the barriers move in the direction of the arcs. These arcs may be concentric or eccentric relative to the center of the subject position. 
     FIGS. 10A–10C  depict an interrogation station  370  having first and second antenna apparatus assemblies  372  and  374 , each having a respective antenna apparatus  376  and  378 . Each antenna apparatus correspondingly includes one or more antenna units  380  and, if appropriate, an antenna array  382 , such as has been described with reference to the previous figures. In particular, apparatus assemblies  372  and  374  include respective barriers  384  and  386  associated respectively with antenna apparatus  372  and  374 . These barriers may cover or enclose associated arcs  387  and  388 , along which electromagnetic radiation is transmitted and received, as has been described. The barriers may conform to the arcs, as shown, although other shapes may also be used. Apparatus assemblies  372  and  374  may be moved along an apparatus path, such as defined by a track  389 . 
   As viewed in the figures, a subject  390  may enter interrogation station  370  along a subject path  392  through an entrance  394 . Initially, barriers  384  and  386  may be abutting in a position downstream relative to a subject position  396  along path  392  and blocking an exit  398  from the interrogation station. This barrier supports the objective of having the subject stop on the subject position and remain there during imaging. 
   Initially, the antenna apparatus  376  and  378  may perform imaging along arcs  387  and  388  with the barriers in a starting or blocking position as shown in  FIG. 10A . The barriers shown each span an arc of about 90 degrees, so the two antenna apparatus are able to scan half of a cylindrical aperture surrounding the subject position. Other lengths and shapes of arcs and other numbers of antenna apparatus assemblies may be used. 
   After scanning the first side of the subject, the antenna apparatus assemblies may move in opposite directions along track  389  to an upstream position on path  392  at the entrance  394  to the interrogation station. The other side of the subject is then scanned, and with the exit open, the subject may be allowed to leave the interrogation station, as shown in  FIG. 10B . The antenna apparatus assemblies are then again moved along track  389  in opposite directions to the initial position blocking path  392  at exit  398 , and opening entrance  394 , permitting a second subject  390 ′ to enter the interrogation station. 
   An interrogation station  400 , illustrated in  FIGS. 11A–11D , provides for continuous rotation of an antenna apparatus assembly  402 . Assembly  402  is shown as having an antenna apparatus  404  extending along an arc  406  of about 120 degrees, although other arc lengths longer or shorter than arc  406  may be used. Antenna apparatus assembly  402  includes an antenna unit  408 , which may be included in an antenna array  410 . A barrier  412  may extend along the arc. The antenna apparatus assembly may be adapted to move along a path defined by a guide, as represented by a track  414 . Track  414  extends at least partially around a subject position  416 . 
   Initially, a subject  418  may enter the interrogation station through an entrance  420  and along a path  422 , stopping at the subject position. The antenna apparatus  404  then performs imaging from positions along arc  406  with the barrier in a starting or blocking position shown in  FIG. 11A . The barrier may block an exit  424  from the interrogation station, downstream along path  422  from the subject position. After scanning a first side of the subject, the antenna apparatus assembly moves along track  414  to a second position, which may be complementary to the initial position, as shown in  FIG. 11B . The next 120 degrees of image of the subject may then be scanned. 
   Assembly  402  then may move to a third position at which the final 120 degrees of scanning of the subject is performed, as shown in  FIG. 1C . The exit from the interrogation station is open when assembly  402  in this third position, allowing the subject to leave the interrogation station. The antenna apparatus assembly  402  may then move along track  414  to the initial position, blocking path  422  at exit  424 . With, the assembly in this position, entrance  420  is again unobstructed, permitting a second subject  418 ′ to enter the interrogation station. 
     FIG. 12  illustrates a feature of an interrogation station  430  that may be used in various of the interrogation stations illustrated in the other figures.  FIG. 12  illustrates simplistically an antenna apparatus  432  including a vertical array  434  of antenna units, including antenna units  436 ,  437 ,  438  and  439 , represented by cone shapes for simplicity. Array  434  is mounted along a vertical frame  442 . The array is generally directed toward a subject position  444  extending generally parallel to the array. 
   In a lower portion  434   a  of the array, the antenna units, including antenna unit  439 , are mounted relative to the array to extend generally perpendicular to a line  446  corresponding to frame  442 . In an upper portion  434   b  of the array, the antenna units are mounted at an acute angle to the line of the array. For instance, antenna unit  436  is shown to be mounted at an angle A 1  of about 30 degrees from the perpendicular to the line of the array, or about 60 degrees relative to the line of the array. Any suitable angle may be used in a partucular application, as is appropriate to obtain the desired coverage of the subject Antenna units  437 ,  438  and others are at progressively increased angles relative to the line of the array until they align with the perpendicular to the array, as in lower array portion  434   a . In this example, upper array portion  434   b  generally extends above the level of the subject position. The subject position can correspond to the expected general position of a subject located in the subject position. Accordingly, all of the antenna units are directed toward the subject position. The antenna units in upper array portion  434   b  then are able to provide an imaging perspective from above the subject position. 
     FIG. 13  illustrates a simplified front view of a variation of antenna apparatus  432  illustrated in  FIG. 12 . In this example, the antenna units in the lower portion  434   a  of array  434  are directed in different circumferential orientations or directions about the line  446  of the array. For example, antenna unit  439  is directed forward. Antenna units  447  and  448 , respectively above and below antenna unit  439 , may be directed left and right, respectively, as viewed in the figure. This pattern may be repeated along the array. In upper array portion  434   b , the antenna units are variously directed left, forward, and right as in the lower array portion. Additionally, the antenna units may be directed downwardly at an acute angle, as was described with reference to  FIG. 12 . For example, antenna unit  436  may point downwardly and forward, antenna unit may point downwardly and to the right (from the perspective of a viewer of the figure), and antenna unit  438  may point downwardly and to the left. In this example, the amount that the antenna units are directed downwardly at an acute angle to the line of the array decreases with increasing distance down the array. Many other variations in antenna unit orientation may be used, as appropriate to provide imaging characteristics desired for a given application. 
   Although this discussion has been directed to a vertical antenna array, it also applies to horizontal antenna array. For example,  FIG. 14  illustrates a simplified top view of another interrogation station  450  having an antenna apparatus  452  positioned adjacent to a subject position  454 . In this example, the antenna apparatus extends along a line in the form of an arc  456 . An antenna array  458  includes a plurality of antenna units, including antenna units  460 ,  461 ,  462 ,  463  and  464 , are distributed along the arc. Arc  456  has a center of curvature  466  spaced from a center  468  of the subject position, as shown. Other shapes and curvatures of the arc may be used. For instance, the array could extend along a rectilinear line  470  and have antennae positioned at different angles versus position as shown in  FIG. 12 , but in a horizontal position. 
   Array  458  may include an intermediate portion  458   a  along which the antenna units, such as antenna unit  462 , extend normal to the line of the array, which in this example is arc  456 . The array also includes end portions  458   b  and  458   c . The antenna units, such as antenna units  460 ,  461 ,  463  and  464 , in these end portions are mounted at an acute angle relative to the arc. For example, antenna unit  461  is mounted at an angle A 2  of about 10 degrees, corresponding to a complementary angle of about 80 degrees relative to the arc. The antenna units, then, in these end portions may be oriented more directly toward subject position  454  than would be the case if they were mounted orthogonally to the arc of the array. 
   Antenna array  458  may extend only horizontally along arc  456 , in which case the side of a subject in a subject position facing the array may be fully scanned by mechanically moving the arc vertically. Optionally, an array  458  may be part of a two-dimensional array  472  of an interrogation station  474 , which array also includes array  434  depicted in  FIG. 12 . For instance, the view of the array in  FIG. 14  may be the view taken along line  13 — 13  in  FIG. 12 . If array  472  is large enough it may be possible to obtain an image of the side of a subject facing the array by electronically scanning the subject. In some examples, an image of only a portion of a subject may be desired; in which case, the array may correspond to only a portion of the subject. 
   A top view of yet another form of interrogation station is illustrated in  FIG. 15 . An interrogation station  480  includes first and second antenna apparatus  482  and  484  positioned or adapted to be positioned relative to a subject position  486 . Antenna apparatus  482  may include an antenna unit or array of antenna units adapted to transmit and receive electromagnetic radiation along an arc  488 . Arc  488  in this example is semi-circular, extending from a first end  488   a  to a second end  488   b  around a center of curvature  490 . Similarly, antenna apparatus  484  extends in a semi-circular arc  492  having first and second ends  492   a  and  492   b , about a center of curvature  494 . Barriers may be associated with these arcs, as has been described, and the arcs may correspond to movement or location of one or more antenna units during imaging of a subject. It is seen that center of curvature  490  is at the end  492   a  of arc  492 . Similarly, center of curvature  494  is at the end  488   a  of arc  488 . In this particular construction, then, the four arc ends, the two centers of curvature and the subject position are aligned along a common straight line  496 . 
   In this configuration, first ends  488   a  and  492   a  of the arcs are positioned closer to the subject position than the other ends. The two arcs may be considered to be offset relative to each other, in that the arcs form an asymmetrical configuration about line  496 . The arcs, thereby, form a partial definition of a subject path  498  having an entrance  500  and an exit  502 . The subject position is located where the antenna arcs are located the closest to the subject path. Further, when the subject is in the subject position, the arcs surround the subject, allowing for imaging of all sides of the subject without moving an antenna apparatus or moving the subject. Further, the arcs define the boundaries of the path between the entrance and exit. Other configurations may also be used. 
   Imaging systems may be used in a wide variety of applications. They may be used in conventional fixed locations where security checks are currently made, such as in restricted facilities, or at entrances to public facilities that may be considered likely to be of interest to persons having destructive or adverse inclinations. In such installations, it is important to keep the imaging systems functional in order to interrogate a potentially continuous flow of personnel through the associated interrogation station or stations. 
   Imaging systems are also useful for the temporary interrogation of subjects in ad hoc applications that do not warrant a fixed or permanent installation. Examples include special or infrequent events, such as sports or political activities. Another example is a military operation in which troops are moving across a large territory, and check-points are set up at points of access for non-military personnel. For these applications, it may be desirable to have an imaging system that can be readily assembled for use, and then disassembled for reuse at another location. 
     FIGS. 15–19  illustrate various aspects of imaging systems that can facilitate assembly, disassembly, upgrading the imaging system, and maintenance.  FIG. 16  is a block diagram of an imaging system  510  having an antenna apparatus  512  and a controller  514 . The function and structure of imaging system  510  corresponds to imaging systems described above, and have the particular features described now. 
   Antenna apparatus  512  can include a plurality of segments or modules, such as array modules  516 ,  517  and  518 . These modules in combination can form the antenna apparatus. A frame or mounting assembly  520  can join the various modules together in a given configuration, such as in an arc  522  of an array  524 . 
   Optionally, the modules  516 ,  517  and  518  can form one of a plurality of interrogation segments, such as interrogation segment  526 . Additional interrogation segments, such as interrogation segment  528 , may be formed of associated modules, such as array modules  530 ,  531  and  532 . Modules  530 ,  531  and  532  may be joined by a frame or mounting assembly  534 . In turn, interrogation segments  526  and  528 , and others, may be joined by a base mounting assembly  538 , to form antenna apparatus  512 . 
   A communication link, such as link  540 , connects each module to a module input/output device  542  of controller  514 . 
     FIG. 17  is a general schematic of one configuration of a first array module, such as module  516 , connected to a second array module, such as module  517  of an antenna apparatus, such as antenna apparatus  512 . Module  516  may include a frame  544  that is attached to a corresponding frame  546  of module  517 . Frames  544  and  546  can be attached together in various configurations, such as by connection to a common frame or mounting assembly, or by joining them as segment frames, as shown, with an attachment assembly  552 , such as brackets  554  and  556 . Other structures may be used to attach the array modules together. 
   Each array module can have a plurality of antenna units, such as antenna units  558 ,  559  and  560 . When the plurality of array modules is mounted together, the respective antenna units may form collectively an antenna array  562 . Each antenna unit is in communication with a transceiver  564 , which transceiver may be resident as part of the array module; mounted on a common base mounting assembly; or at a location remote from the antenna apparatus. The transceiver may then be in communication with a processor or other signal controller device, such as via an input/output device  566 . 
     FIG. 18  depicts a plurality of array modules, such as modules  516 ,  517  and  518 , positioned along an arc  568  and directed toward a subject position  570  to form an antenna apparatus  571 . In this example, each array module has a plurality of antenna units  572  disposed along a rectilinear line, such as lines  574  and  576  associated with modules  516  and  517 . The array modules may be mounted at respective angles, such as angle A 3 , relative to each other. The result is that arc  568  is comprised of a plurality of chords. The same array modules can accordingly be configured to form any appropriate arc, including a rectilinear line for which the radius of curvature may be considered to be at an infinite distance. 
   An example of an interrogation station  580  using a plurality of array modules  516 ,  517  and  518  to form an antenna apparatus  571  is shown in  FIG. 19 . The array modules form an array  582  extending horizontally around a subject position  584 . The array may be moved up and down to scan mechanically a subject in the subject position by a moving mechanism  586 . A frame  588  forms a barrier that also supports the antenna apparatus. A transceiver  590  may control operation of the antenna array and antenna units during scanning of a subject. 
   Optionally, interrogation station  580  may be configured as an interrogation segment  592  that forms part of an interrogation station  594  shown in  FIG. 20 . As illustrated, interrogation station  594  may include a plurality of interrogation segments, such as segments  592 ,  595  and  596 . The interrogation segments may be mounted collectively on a segment frame  598  to form a combined antenna array  600  formed by the arrays of each of the interrogation segments and extending along a combined arc  602 . The transceivers  590  of the interrogation segments may be in communication with a shared controller  604 . Optionally, the interrogation segments may share a single transceiver mounted separately, or a transceiver included in controller  604 , as was described with reference to controller  24  of imaging system  20 . 
   It will be appreciated that the foregoing imaging systems, interrogation stations and antenna apparatus have varying characteristics and features. Various of these features may be used in various combinations. For example, the antenna arrays described with reference to any of the interrogation stations may be constructed with array segments each having a plurality of arrays, transceivers, and/or may be constructed as interrogation segments. These array segments and interrogation segments facilitate maintenance, since faulty parts can be readily replaced, or the entire system may be readily assembled or disassembled, as appropriate. Further, an interrogation station may have antenna units mounted at differing angles along an arc or with different polarizations and beam angular width, either vertically or horizontally for the entire antennae apparatus, transmitting or receiving arrays, or individual antenna units within an arrayarray arc may be formed of antenna units formed in groups or segments extending along a line that varies from the arc, but collectively form the arc. Array segments may be oriented along the line of an array that varies from the perpendicular line of the array. Various combinations and configurations are therefore possible. 
   Some embodiments of imaging systems having any of the various described interrogation stations may use a transmitting signal that incorporates frequencies in the range of 24 to 30 GHz, FMCW modulation, and having signal content that meets FCC unlicensed operation requirements and is outside of any restricted US Government frequency bands. Pulse lengths may range from 2–10 microseconds. Antennae beam widths may range from 20–120 degrees for broad beam implementation, or from 1 to 30 degrees for narrow beam width applications, depending on the image formation signal processor requirements. Various system polarizations may be used. Examples include same polarization, cross polarization, elliptical polarization, right circular polarization, and/or left circular polarization. 
   Accordingly, while the inventions defined in the following claims have been particularly shown and described with reference to the foregoing preferred embodiments, those skilled in the art will understand that many variations may be made therein without departing from the spirit and scope of the inventions. Other combinations and sub-combinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the present disclosure. The foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or later applications. Where the claims recite “a” or “a first” element or the equivalent thereof, such claims should be understood to include one or more such elements, neither requiring nor excluding two or more such elements. Further, cardinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, nor does it generally indicate a particular position or order of such elements. 
   INDUSTRIAL APPLICABILITY 
   The described imaging systems and components of imaging systems, as well as the methods relating thereto, are applicable to surveillance, metric, and other industries in which subject images are utilized.