Abstract:
The present invention provides an improved compact antenna system with three degrees of freedom positioned on a moving platform to maintain orientation of the antenna for continuous tracking of a satellite. The system includes a cross-elevation sub-frame having two pivotal joints at each end to support an antenna reflector. The system also includes an azimuth sub-frame connected to the cross-elevation sub-frame. The system further includes a dome enclosing the reflector, cross-elevation sub-frame and the azimuth sub-frame The cross-elevation sub-frame is oriented at an angle substantially about a midpoint between the elevation angle ranges of axis of rotation for the reflector such that the reflector rotates at a point substantially to a center of the dome.

Description:
CROSS REFERENCES 
       [0001]    This patent application claims the benefit of U.S. Provisional Application Ser. No. 61/244,630 filed Sep. 22, 2009, the contents of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally related to the field of satellite communications and antenna systems, and is more specifically directed to a compact antenna system with three independent displacements or aspects of motion (a.k.a. three degrees of freedom). 
       BACKGROUND OF THE INVENTION 
       [0003]    Many antenna systems mainly include two axes pointing systems but are subject to keyhole limitations when the satellite is right above the antenna such that the antenna seeks to track a satellite moving perpendicular to both axes. In other words, the antenna tries to track a satellite when the satellite tracking planes are even slightly off co-planar. In such situations, the antenna would require infinite velocity to rotate the antenna to maintain a lock on the satellite. This rotational motion of antenna causes substantial problems in the acceleration of the antenna and could result in technical failure. 
         [0004]    In the current art, there exist three axes, a.k.a. three degrees of freedom (3DOF) pointing antenna systems that can solve the keyhole problem but sacrifice additional size/volume/footprint over the two axes system with the same antenna reflector diameter. Until now three degrees of freedom (3DOF) antennas have been relegated to larger designs. Several existing antenna manufacturers utilize an azimuth, elevation, cross-elevation mechanism for the three axis of freedom, however these antennas position the cross elevation axis at a much lower angle and mount the cross-elevation sub-frame to the front of the azimuth sub-frame. As a result, these antennas are much larger in size. 
         [0005]    Additionally, many antenna systems have three axes of motion with the third axis substantially orthogonal to the other two axes, so they are perpendicular to each other. In the design, one needs to adjust each of the individual axes in order to rotate the antenna in all various directions. Therefore, the current systems have three independent orthogonal axes, which take up more physical space. 
         [0006]    U.S. Pat. No. 6,911,949 discloses an antenna stabilization system for two antennas mounted on a single pedestal on a moving platform. The pedestal includes an upper alignment system, a lower alignment system and an intermediate element between the two systems. The upper alignment system has three rotational degrees of freedom for pointing the antennas relative to the intermediate element in order to provide an angular displacement between the antennas and their respective satellites. The lower alignment system has three rotational degrees of freedom to maintain the orientation of the intermediate element in order to compensate for rotation of the mobile platform such that antennas are maintained and pointed towards their respective satellites. This antenna stabilization system includes many components and requires at least two antennas. 
         [0007]    Thus, there is a need in the art to minimize the size of the antenna system having 3DOF that can track orbiting satellites without being subject to keyhole limitations. 
       SUMMARY OF THE INVENTION 
       [0008]    One of the objectives of the present invention is to provide a compact antenna system having three degrees of freedom to be accommodated in a small size dome. 
         [0009]    Another objective is utilizing the three degrees of freedom, i.e. azimuth, elevation, and cross-elevation axes as the three axes of motion of the antenna system to allow the antenna to track orbiting satellites without being subject to keyhole limitations at high elevations as will be describe in greater detail below. 
         [0010]    The objectives are accomplished by designing an antenna system having a dome enclosing a reflector, a cross-elevation sub-frame and an azimuth sub-frame. The reflector is mounted directly to the cross-elevation sub-frame via first and second pivoting joints. The cross-elevation sub-frame is divided between first and second frames to form space there-between to allow a portion of an azimuth sub-frame to be securely connected between the first and the second frames. The cross-elevation sub-frame is positioned to be oriented about midway between the elevation ranges of the reflector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawings, of which: 
           [0012]      FIG. 1A  depicts a schematic drawing of one embodiment of the antenna stabilization system of the present invention. 
           [0013]      FIG. 1B  depicts a schematic drawing of a partial back view of the antenna stabilization system of  FIG. 1A . 
           [0014]      FIG. 1C  depicts a schematic drawing of side view of the antenna stabilization system of  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1A  illustrates a schematic view of an antenna stabilization system  100  installed on a roof of a moving platform (not shown) in accordance with an embodiment of the present invention. The system  100  includes three rotational degrees of freedom enclosed in a dome  101 . The three rotational degrees of freedom include an azimuth sub-frame  102  creating the azimuth axis, a cross-elevation sub-frame  106  creating the cross-elevation axis and two pivoting joints  106   a  and  106   b  creating the elevation axis. These elements together function to adjust the orientation of a reflector  108  in order to allow the antenna to track on a separate plane than that of a satellite (not shown) to continuously track the satellite. As illustrated in  FIG. 1A , the system  100  includes a reflector dish  108  mounted directly to the cross-elevation sub-frame  106  via the first and second pivoting joints  106   a  and  106   b.  The cross elevation sub-frame  106  is substantially rectangular in shape and is supported by the azimuth sub-frame  102  via joint  102   a  of the sub-frame  102 . As shown in  FIG. 1A , the azimuth sub-frame  102  is substantially elongate with a the joint  102   a  of a substantially circular shape at one end connected to the cross-elevation sub-frame  106  and another joint  102   b  connected to a base  109 . Although not shown, each of the axes includes a drive motor and a bearing to provide movement to the reflector  108 . 
         [0016]    The reflector  108  of the present invention has diameter in the range of about 18 inches to about 50 inches. In a preferred embodiment the reflector  108  of the system  100  has a diameter of 24 inches and the dome  101  has a diameter of about 26 inches and height of about 31 inches, thus resulting in a very compact system in accordance with the present invention. These dimensions are about 13 to 25 percent smaller compared to currently available antenna systems having a reflector of same diameter, i.e. about 24 inches with a dome of about 34 inches in diameter and having height of about 36 inches. 
         [0017]    Referring to  FIG. 1B , there is shown a partial back view of the antenna stabilization system  100  of  FIG. 1A . As illustrated in  FIG. 1B , the cross-elevation sub-frame  106  is divided into a primary frame  106   c  and a secondary frame  106   d  providing for an opening  107  between the frames  106   c  and  106   d.  This division of the sub-frame  106  allows for the circular portion  102   a  of the azimuth sub-frame  102  to be securely placed at this opening  107  between the two frames  106   c  and  106   d  as illustrated in  FIG. 1B . This division of the frame  106  and the placement of portion of the azimuth sub-frame  102  between the sub-frames  106  causes the two frames  102  and  106  to be further distanced from the center of the reflector  108 , which in turn leaves more space available in the back of the reflector  108  for the feed components as shown in  FIGS. 1A and 1B . Preferably, the distance between the back of the reflector  108  and the sub-frame  106  is about 6 inches. Furthermore, by placing the azimuth sub-frame  102  in between the cross elevation sub-frames  106   c  and  106   d;  the frame components of the system  100  may preferably be joined together in a compact form. 
         [0018]    The present invention further reduces the size of the system by orienting the cross-elevation sub-frame  106  midway between the travel limits of the elevation angle range of the reflector  108 . In order to determine the orientation of the cross-elevation sub-frame  106 , an optimal sub-frame angle is first calculated. This optimal sub-frame angle is the angle between the axis of rotation of the antenna reflector  108  and the axis of rotation of the sub-frame  106 . So, if a is the high angle value (preferably in degrees) of the elevation angle range of the axis of rotation for the reflector  108  and b is the low angle value (preferably in degrees) of the elevation angle range of the axis of rotation for the reflector  108 , then optimal sub-frame angle, θ is calculated using the computation provided below: 
         [0000]      θ= a+ ( b−a )/2
 
         [0019]    For example, if the elevation angle range of the axis of rotation for the antenna is designed to be between 25 degrees below the horizon (i.e. a=−25°) and 115 degrees above the horizon (i.e. b=115°), then the optimal sub-frame angle, θ is 45° (using the computation formula above) as illustrated below: 
         [0000]    
       
         
           
             θ 
             = 
             
               
                 
                   
                     - 
                     25 
                   
                    
                   ° 
                 
                 + 
                 
                   
                     
                       115 
                        
                       ° 
                     
                     - 
                     
                       ( 
                       
                         
                           - 
                           25 
                         
                          
                         ° 
                       
                       ) 
                     
                   
                   2 
                 
               
               = 
               
                 45 
                  
                 ° 
               
             
           
         
       
     
         [0020]    In the above example, the cross-elevation sub-frame  106  is oriented at 45° with respect to the reflector  108  in order to make certain that the reflector  108  is not in a co-planar position with the satellite. As a result, the reflector  108  may preferably be maintained to track the satellite in orbit regardless of the movement of the antenna and/or the moving platform. It is noted that the actual angle maybe adjusted from the ideal angle θ if needed. 
         [0021]      FIG. 1C  illustrates a side view of the system  100  of  FIG. 1A  in which the reflector  108  is positioned pointing straight up towards the azimuth axis and rotates about this axis. In this position, the cross-elevation sub-frame  106  is rotating at about 45 degrees with respect to the base  109 . Further, in this position, it is assumed that the satellite (not shown) is directly above the antenna dish  108 . However, if a satellite (not shown) moves away from the azimuth-axis towards the cross-elevation axis, the reflector  108  need to be rotated towards the cross-elevation axis in order to track the satellite. The reflector  108  is rotated by the movement of the cross-elevation sub-frame  106 , which swings around to point the reflector  108  toward the satellite. The antenna  108  rotates about the cross-elevation axis relative to the cross-elevation sub-frame  106  and may also move in either the clockwise or the counter-clockwise orientation depending on the position of the satellite. 
         [0022]    As described above, rotation of the reflector  108  in the cross-elevation axis as described above results in a change in the orientation of the cross-elevation axis. Since the cross-elevation is not orthogonal with respect to the azimuth axis and elevation axis, this change in angle in the cross-elevation axis will require the adjustment in the angles of the other two axes, i.e. the azimuth and the elevation axis in order for the system  100  to continuously track the orbiting satellites. This adjustment can be preferably be made by any known software designed to automatically readjust angles of the two axes upon change in angle of the third axis. 
         [0023]    Thus, in the present invention, the third axis, cross-elevation axis allows the antenna to move in an axial direction that can be imagined as concentric with the elevation axis. It is this movement that results in elimination of the keyhole. Also, the 45° cross elevation approach described above solves the size/volume/footprint problem by allowing the three DOF systems to be similar in size to the two DOF systems with the same antenna reflector diameter. Furthermore, the configuration of cross elevation at 45° to the azimuth puts the reflector  108  at a center of rotation closest to the center of the radome  101  and hence allows a smaller antenna than the prior art three DOF systems that offset the center of rotation for the reflector. 
         [0024]    While the present invention has been described with respect to what are some embodiments of the invention, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.