Patent Publication Number: US-7724176-B1

Title: Antenna array for an inverse synthetic aperture radar

Description:
TECHNICAL FIELD OF THE DISCLOSURE 
   This disclosure generally relates to radars, and more particularly, to an antenna array for an inverse synthetic aperture radar and a method of using the same. 
   BACKGROUND OF THE DISCLOSURE 
   Synthetic aperture radars generate imagery by processing multiple received signals that have been reflected from a moving target. Inverse synthetic aperture radars include a particular class of synthetic aperture radars that generate imagery using movement of its antenna relative to the target. Synthetic aperture radars and inverse synthetic aperture radars may serve many useful purposes including generation of imagery that may be difficult to obtain using visual image generation mechanisms, such as video cameras, that generate imagery using the visible light spectrum. For example, synthetic aperture radars may generate imagery through generally opaque walls or during periods of inclement whether when fog or other type of precipitation may cause relatively poor visibility. 
   SUMMARY OF THE DISCLOSURE 
   According to one embodiment, an antenna array includes a plurality of racks that are each configured with a plurality of antenna elements. Each rack may be rotated relative to the other racks through an axis that is generally parallel to the axis of other racks. Each antenna element within each rack has an axial orientation that is generally similar to and has an elevational orientation that is individually adjustable relative to one another. 
   Some embodiments of the disclosure may provide numerous technical advantages. For example, one embodiment of the antenna array may be less complicated and thus cheaper and easier to operate than other known antenna arrays used by inverse synthetic aperture radars. In many cases, antenna signals are acquired using a relatively fixed orientation the various transmit and receive beams generated by individual elements of the antenna array. The antenna array of the present disclosure utilizes an articulated configuration in which the azimuthal and elevational orientation of each antenna element may be adjusted by manual intervention to provide a structure that may be easy to use and maintain relative to other more complicated antenna arrays for inverse synthetic aperture radars. 
   Some embodiments may benefit from some, none, or all of these advantages. Other technical advantages may be readily ascertained by one of ordinary skill in the art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of embodiments of the disclosure will be apparent from the detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a perspective view of one embodiment of an antenna array according to the teachings of the present disclosure; 
       FIG. 2  is a plan view of the antenna array of  FIG. 1  showing how the racks may be oriented to generated imagery of one or more targets; 
       FIG. 3  is an enlarged, perspective view of one embodiment of an upper coupling mechanism that may be used to couple each rack to the upper rail of the antenna array of  FIG. 1 ; 
       FIG. 4  is an enlarged, elevational view of one embodiment of a lower coupling mechanism that may be used to couple each rack to the lower rail of the antenna array of  FIG. 1 ; 
       FIGS. 5A and 5B  are enlarged, perspective views of one embodiment of a collar that may be implemented to couple each antenna element to its respective rack; and 
       FIG. 6  is a flowchart showing one embodiment of a series of actions that may be performed to operate the antenna array of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
   As previously described, inverse synthetic aperture radars may be useful for generating imagery in conditions that may be relatively difficult to obtain using visible image generating devices, such as video cameras. An inverse synthetic aperture radar typically uses an antenna array that transmits microwave radiation and receives radiation that is reflected by one or more targets of interest. Due to the relative complexity and size of known antenna arrays configured for use with inverse synthetic aperture radars, however, their applications may be limited. For example, inverse synthetic aperture radars are typically implemented with active electronically scanned arrays that may be relatively complicated to use and operate. 
     FIG. 1  shows one embodiment of an antenna array  10  that may provide a solution to these problems and other problems. Antenna array  10  includes a plurality of racks  12  that are each configured with a plurality of antenna elements  14 . Racks  12  are spatially separated from one another for transmitting and receiving microwave radiation at various angles relative to one or more targets of interest. According to the teachings of the present disclosure, the azimuthal orientation of the antenna elements  14  configured on a particular rack  12  is adjustable relative to the azimuthal orientation of the antenna elements  14  of another rack  12 . Antenna elements configured on each rack  12  have an elevational orientation that is also adjustable relative to other antenna elements  14  in its respective rack  12 . Thus, the scan pattern developed by antenna array  10  may be tailored according to the nature and type of imagery to be generated and/or the characteristics of the terrain or other background objects in the vicinity of various targets of interest. 
   Each rack  12  has an upper coupling mechanism  16  and a lower coupling mechanism  18  that are each coupled to an elongated upper rail  20  and an elongated lower rail  22 , respectively. Upper coupling mechanism  16  and lower coupling mechanism  18  forms an axis for rotation of its respective rack  12  relative to the other racks  12 . In one embodiment, upper rail  20  is disposed above lower rail  22  for maintaining racks  12  in a generally vertical orientation. In this manner, antenna elements  14  may transmit or receive microwave radiation from a generally lateral direction. In other embodiments, upper rail  20  and lower rail  22  may support racks  12  at any suitable orientation for transmission or receipt of microwave radiation at virtually any orientation. Each rack  12  supports a plurality of antenna elements  14  at a desired azimuthal orientation relative to upper rail  20  and thus to each other. Each antenna element  14  is coupled to its respective rack  12  through a collar  24  that extends around the periphery of its respective antenna element. Details of upper coupling mechanism  16 , lower coupling mechanism  18 , and collar  24  will be described in detail below. 
   Antenna elements  14  may be include any type of device that transmits and/or receives microwave radiation for generation of inverse synthetic aperture radar imagery. In the particular embodiment shown, antenna elements  14  are generally horn-shaped and operate in the L-band of the microwave spectrum, which includes frequencies in the range of 40 to 60 Giga-Hertz (GHz). Given this range of frequencies, each antenna element  14  has a length of approximately 1.5 feet and a front aperture of approximately 1.0 foot by 1.0 foot. 
   Inverse synthetic aperture radars typically operate by moving a transmit and receive beam of microwave radiation across a target of interest in a controlled manner. In some cases, the transmit and receive beam may be rotated across the target of interest while multiple signals from the received beam are processed. Techniques used for this mode of movement may include a motorized mechanism that spins its antenna array across a target or an active electronically scanned array (AESA) that scans its transmit and receive beams across the target using the combined radiation pattern of multiple antenna elements. In the present embodiment, antenna elements  14  may have an orientation that remains relatively fixed during acquisition of microwave radiation reflected from the target. The generally static nature of antenna elements  14  may, therefore, be relatively less complex and smaller in size than other antenna elements configured for use with inverse synthetic aperture radars in some embodiments. 
     FIG. 2  is a diagram showing how antenna array  10  may be used to acquire multiple reflected signals from one or more targets  28  for generation of inverse synthetic aperture radar imagery. Antenna array  10  is configured on a movable platform  30  that moves in a direction  32  laterally with respect to one or more targets  28 . Movable platform  30  may include any movable structure, such as, for example, an automobile, a train, a bus, a watercraft, or an aircraft. For example, movable platform  30  may be an automobile that moves antenna array  10  over a roadway for generating inverse synthetic aperture radar imagery of targets  28  that may include buildings or other structures in close proximity to the roadway. 
   In the particular embodiment shown, antenna elements in rack  12   a  are configured to transmit microwave radiation, while antenna elements  14  configured in rack  12   b  and  12   c  are configured to receive microwave radiation such that a total of three racks are implemented. In other embodiments, any plurality of racks  12  may by used in which any subset of racks  12  may be delegated for transmission of microwave radiation while the other racks  12  are delegated for receipt of microwave radiation. In another embodiment, certain antenna elements  14  within each rack  12  may be alternatively delegated for transmission or receipt of microwave radiation. In yet another embodiment, each antenna element  14  in each rack  12  may be configured to transmit and receive microwave radiation. 
   Movement of movable platform  30  relative to targets provide spatial separation along its direction of movement while the azimuthal orientation and physical separation of each rack  12  from one another provide spatial separation normal to the movable platform&#39;s direction  32 . Spatial separation along these axes provide for the generation of inverse synthetic aperture radar imagery. As shown, rack  12   a  transmits microwave radiation at a direction θ t  relative to movable platform  30  while racks  12   b  and  12   c  receives reflected microwave radiation from targets  28  at directions θ r1  and θ r2 , respectively. Directions θ t , θ r1 , and θ r2  of racks  12   a ,  12   b , and  12   c , respectively, may be selected according to various factors, such as the anticipated velocity of movable platform  30 , the size and complexity of targets  28 , and/or the type of background terrain features around targets  28 . 
     FIG. 3  is an enlarged, cross-sectional, elevational view of one embodiment an upper coupling mechanism  16  that couples one rack  12  to upper rail  20 . Upper coupling mechanism  16  includes a rail mount  36 , a universal joint  38 , a radial locking mechanism  40 , and a shock absorber  42  that are coupled to one another between upper rail  20  and rack  12  as shown. Rail mount  36  couples upper rail  20  to universal joint  38  and is slidingly engaged in a channel  44  formed in upper rail  20 . Thus, rail mount  36  provides for lateral movement of rack  12  along upper rail  20 , while universal joint allows bending of rack  12  relative to upper rail  20 . Shock absorber  42  is made of any suitable material, such as rubber, and is disposed between radial locking mechanism and rack  12  for absorbing vibrational energy from upper rail  20 . In one embodiment, shock absorber  42  has an elastic coefficient such that rack  12  and shock absorber  42  have a natural resonant frequency of approximately 18 Kilo-Hertz (KHz). A natural resonant frequency of 18 Kilo-Hertz may be higher than most anticipated perturbations during movement on movable platform  30  that may reduce potential unwanted oscillation of racks  12  during movement. 
   Radial locking mechanism  40  includes a plate  46  and an arm  48  that is rigidly coupled to universal joint  38  for remaining at a fixed angular orientation relative to upper rail  20 . A pin  50  is provided that may be selectively inserted through one of a plurality of holes configured in plate  46  and a hole configured in arm  48  for maintaining rack  12  at a desired angular orientation relative to upper rail  20 . 
     FIG. 4  is an enlarged, cross-sectional view of one embodiment of a lower coupling mechanism  18  that may be used to couple rack  12  to lower rail  22 . Lower coupling mechanism  18  includes a shock absorber  52 , a spherical bearing  54 , and a rail mount  56  that are coupled together as shown. Shock absorber  52  is formed of a suitable material, such as rubber, for absorbing vibrational energy from lower rail  22 . Similar to shock absorber  42 , shock absorber  42  has an elastic coefficient such that rack  12  and shock absorber  42  have a natural resonant frequency of approximately 18 Kilo-Hertz. Spherical bearing  54  is provided to allow bending of rack  12  relative to lower rail  22 . In a manner similar to rail mount  36  of  FIG. 3 , rail mount  56  is slidingly coupled to lower rail  22  for providing lateral movement of rack  12  relative to lower rail  22 . 
     FIG. 5A  is an enlarged, partial, perspective view of one embodiment of a rack  12  in which a collar  24  is implemented for securing one antenna element  14  to rack  12 . Collar  24  is hingedly coupled to rack  12  by a rod  56  that extends through both beams of rack  12 . Collar  24  includes a plate  58  that is configured with a plurality of holes. In its operational position, plate  58  lies proximate one beam of rack  12  such that a pin  60  may be selectively inserted through rack  12  and one hole configured in plate  58  for maintaining antenna element  14  at a desired elevational orientation relative to its respective rack  12 . 
     FIG. 5B  shows a perspective view of collar  24  that has been removed from rack  12  and its associated antenna element  14 . As shown, collar  24  may be cut from a single piece of sheet metal and bent several times to produce a shape suitable for securing antenna element  14  to rack  12 . 
     FIG. 6  is a flowchart showing one embodiment of a series of actions that may be performed to use the antenna array  10 . In act  100 , the process is initiated. 
   In act  102 , antenna array  10  is configured on a suitable movable platform  30  that may be moved in close proximity to one or more targets  28  of interest. In one embodiment, targets  28  are located at a position that is in close proximity to a road such that antenna array  10  may be configured on a vehicle for movement over the road during acquisition of imagery of targets  28 . In this particular case, the axes of racks  12  are mounted vertically such that the orientation of antenna elements  14  are directed laterally from the vehicle. 
   In act  104 , the azimuthal orientation of each rack  12  is adjusted relative to one another. In one embodiment, antenna elements  14  of one rack  12  are configured to transmit microwave radiation while the other two racks  12   b  and  12   c  are configured to receive microwave radiation reflected from targets  28 . Given this configuration, the scan pattern of the transmit beam generated by antenna elements  14  in rack  12   a  or the scan pattern of the receive beams from antenna elements  14  in racks  12   b  and  12   c  may be controlled in a relatively consistent and easy manner. 
   In act  106 , the elevational orientation of each antenna element  14  configured in each rack  12  is independently adjusted relative to other antenna elements  14 . Individual adjustment of the elevational orientation of each antenna element  14  may provide control over the scan pattern of the transmit or receive beam that is normal to the direction of movable platform  30 . For example, antenna elements  14  may be adjusted to have a relatively wide variation in elevational orientation for acquisition of imagery from targets  28 , such as tall buildings, while antenna elements  14  may be adjusted to have a relatively narrow variation in elevational orientation for shorter buildings or other targets  28  that may be further away. 
   In act  108 , the spacing between each rack  12  is adjusted. Spacing between each rack  12  affects spatial separation between the transmit beam and receive beam. For example, spacing between racks  12  may be increased due to an anticipated speed of a particular movable platform  30  that may be relatively slower than normal. For the embodiment described above in which antenna elements  14  of two racks  12   b  and  12   c  form the receive beam, spacing between these two racks  12   b  and  12   c  may also be tailored to obtain a desired spatial separation or the received beams. 
   In act  110 , the movable platform  30  is moved within the vicinity of the one or more targets  28  of interest. During this time, synthetic aperture radar imagery is generated by the transmit and receive beams generated by antenna elements  14  as they cross through the target&#39;s location in act  112 . 
   The previously described process continues throughout acquisition of imagery to gather information about targets  28 . When operation of antenna array  10  is no longer needed or desired, the process ends in act  114 . 
   Modifications, additions, or omissions may be made to the method without departing from the scope of the disclosure. The method may include more, fewer, or other acts. For example, azimuthal rotation of racks  12  and/or elevational rotation of individual antenna elements  14  may be provided by servo motors that provide adjustments during acquisition of inverse synthetic aperture radar imagery in accordance with one embodiment. Thus, the azimuthal and elevational orientations of antenna elements  14  may be adjusted while imagery is being acquired. 
   Although the present disclosure has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompass such changes, variations, alterations, transformation, and modifications as they fall within the scope of the appended claims.