Abstract:
A method, apparatus, article of manufacture, and a memory structure for generating reconfigurable beams is disclosed herein. The apparatus comprises a stationary feed array having a plurality of selectably activatable feed array elements, the feed array having a feed array sensitive axis; a reflector, illuminated by the selectably activatable feed array elements; a first mechanism, coupled to the reflector, for varying a position of the reflector along the feed array axis; wherein a desired beam size of the antenna system is selected by varying the reflector position along the feed array sensitive axis and by selectably activating the feed array elements.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to systems and methods for transmitting/receiving data, and in particular to a system and method for on-orbit reconfiguration of beams transmitted/received by satellite antennas. 
   2. Description of the Related Art 
   Commercial and military satellites often require the flexibility in terms of changing the coverage size and the beam location over the global field-of-view. It is also important to keep the feed(s) stationary for most applications either due to the high power required to carry multiple frequency channels on-board the satellite or to avoid long cables required to move the feed(s). 
   Many existing satellite designs have fixed beam coverages and therefore can not provide any flexibility in terms of coverage patterns on ground and also can not be adapted to changing service requirements once the satellite has been launched. 
   Future applications for both commercial and military satellites may require the beam shape as well, as the beam location to be reconfigured over the global coverage based on changes in traffic demand, changes in the coverage scenario and/or the need for a service back-up for an on-orbit or launch failure. This flexibility is critical to many satellite operators in order for them to provide uninterrupted service to their customers. 
   Existing methods of beam reconfiguration involve either moving the feed of a reflector antennas or use of phase array antennas. These are risky due to the high power going through the feed, long and glossy cabling requirement, or very expensive hardware with increased power consumption on satellite. 
   In the paper “Variable Beamwidth Dual-Reflector Antenna’, IEEE Conference on Antennas &amp; Propagation (ICAP)”, Publication # 407, pp.92-96, April 1998, which is hereby incorporated by reference herein, authors J. U. I. Syed and A.D. Olver describe a method of changing the beam size by moving the feed of a reflector antenna. They employ a symmetrical Cassegrain reflector antenna with main and sub-reflectors which inherently has high sidelobes and low beam efficiency due to blockage effects caused by the feed and the sub-reflector. This method has limited beam shape reconfiguration due to the fact that the main beam splits or bifurcates for beam aspect ratios greater than 1:2.5 and therefore resulting in poor gain performance. 
   In another paper, “A Novel Semi-Active Multibeam Antenna Concept”, IEEE Antennas &amp; Propagation Symposium Digest, pp. 1884-1887,July 1990, authors A Roederer and M. Sabbadini describe a semi-active multibeam antenna concept for mobile satellites. The beams are reconfigured using a Butler matrix and a semi-active beamformer whereby a limited number of feed elements (typically three or seven) are used for each beam and the beam reconfiguration is achieved by varying the phases through the active BFN. This scheme provides limited reconfigurability over a narrow bandwidth and employs complicated and expensive hardware. 
   U.S. Pat. No. 6,198,455, entitled “Variable Beamwidth Antenna Systems” and issued to Luh on Mar. 6, 2001, which is hereby incorporated by reference herein, describes an offset dual-reflector antenna in the Gregorian configuration. This requires feed movement and also reflector movement (main or sub-reflector) and also has limited range of beam size reconfiguration (beam size aspect ratio of less than 1:2) due to the use of single feed and has disadvantages associated with feed movement. 
   U.S. Pat. No.5,859,619, entitled “Small Volume Dual Offset Reflector Antenna”, and issued to T. Wu, B. Yee and G. H. Sinkins on Jan. 12, 1999, which is hereby incorporated by reference herein, describes a compact dual-offset Cassegrain antenna system that requires the position of the feed, position of the sub-reflector and the feed axial direction that need to be changed in order to arrive at a compact antenna configuration. This is mainly intended for fixed beam applications and does not provide the beam size flexibility. 
   What is needed is an antenna system that provides for control of the beam size as well as the beam direction, and is compatible with a high-power and stationary feed array requirements. What is also needed is a system that extends the range that the beam size can be reconfigured and provides high beam efficiency values over the beam zooming range while minimising scan loss. The present invention satisfies that need. 
   SUMMARY OF THE INVENTION 
   To address the requirements described above, the present invention discloses a method and apparatus for generating reconfigurable beams. 
   The apparatus comprises a stationary feed array having a plurality of selectably activatable feed array elements, the feed array having a feed array sensitive axis; a reflector, illuminated by the selectably activatable feed array elements; a first mechanism, coupled to the reflector, for varying a position of the reflector along the feed array axis; wherein a desired beam size of the antenna system is selected by varying the reflector position along the feed array sensitive axis and by selectably activating the feed array elements. 
   The method comprises the steps of illuminating a reflector from a stationary feed array having plurality of feed array elements; and changing a width of the beam by varying a distance of the reflector from the feed array along a feed array sensitive axis and selectably activating the feed array elements. 
   The foregoing provides the desired flexibility in high power applications, by keeping the feed array stationary, and extends the range of beam size reconfiguration by using a variable size feed array and reflector movement. It also provides high beam efficiency values over the zooming range of the beams, while achieving mini scan loss by using reflector gimbaling to scan the beams. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
       FIG. 1A  is a diagram of one embodiment of the reconfigurable antenna system; 
       FIG. 1B  is a diagram depicting one embodiment of the feed array, 
       FIG. 2  is a schematic diagram of a driver network that drives the feed array, 
       FIG. 3  is a plot of the typical beam coverage from a Medium Earth Orbit (MEO) satellite located at 110 degree west orbital location and typical beamsets covering the Earth; 
       FIGS. 4 and 5  are diagrams showing the operation of the antenna system in the deployed state; 
       FIG. 6A  is a diagram showing the performance of the antenna system for smaller beam foot-print of 600 km wherein only the primary central element of the feed array is used at a first frequency L1=1.585 GHz; 
       FIG. 6B  is a diagram showing the performance of the antenna system for smaller beam foot-print of 600 km wherein only the primary central element of the feed array is used at a second frequency L2=1.226 GHz; 
       FIG. 7  is a diagram showing a typical beam pattern azimuth cuts for the three east-west beams shown in  FIG. 6A ; 
       FIGS. 8A and 8B  are diagrams illustrating computed beam directivity contours using all the eight feed elements for a 2000 km foot-print for frequencies L1 and L2, respectively; 
       FIG. 9  is a plot showing the azimuth pattern cuts for three beams shown in  FIG. 8A ; 
       FIG. 10  is a plot of computed directivity contours for 1500 km beam footprints using a conventional seven element feed array, 
       FIG. 11  is a diagram plotting the two variable beam sizes of the antenna system, in which the narrow beams use the primary element to generate beam sizes in the range 500 km to 1200 km and the broader beam using all of the secondary elements to generate beam sizes in the range 1200 km to 2500 k; and 
       FIGS. 12A and 12B  are diagrams depicting another embodiment of the present invention, using a feed array with more elements. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
   Overview 
   The reconfigurable beam antenna employs an offset reflector illuminated with a feed array. The feed array is stationary, and the reflector can either be stationary or movable axially towards the feed array. The desired beam reconfigurability is achieved through the use of one or more of the following techniques: (1)varying the number of feed elements through high power switch and a beamforming network (BFN), (2) moving the reflector mechanically towards the feed array along the axial direction, and (3) using gimbal mechanisms behind the reflector to steer the beam(s) over the earth coverage. The first two techniques provide beam size reconfiguration while the third technique provides beam location reconfiguration. The use of a fixed feed array with high power switches and a BFN allows the number of feed array elements to vary depending on the size of the coverage beam. 
   The reconfigurable antenna system disclosed herein employs an offset reflector being illuminated with a feed array. In one embodiment, the antenna system includes an offset single reflector (solid or mesh type) whereby the reflector surface can either be parabolic or arbitrarily shaped. The reflector may be illuminated with a feed array where the number of elements are varied on-orbit depending on the beam size. The feed array is stationary and the reflector can be mechanically moved over a limited distance along the feed axial direction using articulated mechanisms. The feed array can be located in the focal plane of the reflector or can be defocused. The reflector can be gimbaled along the east-west and north-south directions by using azimuth and elevation gimbal mechanisms. The feed array uses high power switches and beamforming networks (BFN) in order to vary the number of feed elements. The antenna system also consists of a reflector support structure, including a boom for deploying the reflector on-orbit. 
   By proper combination of the number of elements in the feed array, excitation coefficients of the BFN and the reflector movement, the beam size on ground can be reconfigured over a 1:5 aspect ratio. The antenna system also improves the beam efficiency for larger beams by eliminating the flower-shaped beams associated with conventional designs. This is done by reducing the size of outer elements and adding an additional element, to form an eight element array instead of the conventional seven element array. 
   DETAILED DESCRIPTION 
     FIG. 1A  is a diagram of one embodiment of the reconfigurable antenna system  100 . It uses a large deployable offset reflector  102  being fed with an 8 element feed array  104 . The reflector has a 252 inch diameter projected aperture, a focal length of 160 inches, and an offset clearance of 50 inches in order to avoid the feed array  104  blockage. In the illustrated embodiment, the reflector  102  shape is parabolic but can be other shapes as well, to suit the particular application. 
     FIG. 1B  is a diagram depicting one embodiment of the feed array  104 . The feed array  104  includes a primary element  120  and a plurality of secondary elements  122 A- 122 G disposed about the periphery of and surrounding the primary element  120 . In one embodiment, the primary element  120  includes a first cup-dipole and the secondary elements include seven or more second cup-dipoles smaller in diameter than the first cup dipole (e.g. 13.0 and 10.0 inches in diameter, respectively. 
     FIG. 2  is a schematic diagram of a driver network  200  that drives the feed array  104 . The feed array  104  employs an 8-element cup  202  and crossed dipole  204  array fed by a switching network  206  comprising a first high power switch  208  and a second high power switch  210 , and a coupler  212 . The feed array also employs a 1:7 power dividing network  214 , and a diplexer  216  to separate the L1 and L2 frequency bands. 
   For smaller beam sizes, only the primary element  120  of the feed array  104  is used. This is accomplished by selecting the state of switches  208  and  210  to pass signals as shown in the arrows labeled “1” in FIG.  2 . For larger beams, the primary element and one or more of the seven secondary elements are utilized. This is accomplished by selecting the state of switches  208  and  210  to pass signals as shown in the arrows labeled “2” in FIG.  2 . 
   The efficiency and performance of larger beams is significantly improved by using eight elements ( 102  and  122 A- 122 G). This eliminates the flower-shaped beam contour patterns associated with the conventional 7-element array design. The amplitude and phase excitations of the seven element power divider  214  and the coupling value of the coupler  212  are optimized based on all the beams covering the Earth. 
   The driver network  200  uses hybrid couplers  218  and  220 A- 220 G behind each cup-dipole element in order to generate circular polarization over wide bandwidth and a high-level BFN (1:7 power divider  214 ) implemented using a low-loss squarex (TEM-line) medium. Two high power switches  208  and  210  and a coupler  212  allow the flexibility to select either 1 or 8 elements of the feed array  104 . The high power diplexer  216  separates the L1 and L2 frequencies with sufficient isolation in order to separate the two frequency bands and minimize their intermodulation products generated by different carrier frequencies. 
     FIG. 3  is a plot of the typical beam coverage from a MEO satellite located at 110 degrees West orbital location. This plot shows a 600 km (1.7 degree diameter) and a 1500 km (4.23 degree diameter) beam pair over 9 different locations over the Earth (one central beam and 8 peripheral beams located 14.3 degree radially from the central beam). These nine beams are used to optimize the beam performance over the Earth coverage. 
     FIGS. 4 and 5  are diagrams showing the operation of the antenna system  400  in the deployed state. The center-mounted reflector  102  is attached to a two-axis gimbal mechanism  408 , which provides the capability to steer the spot beams in azimuth and elevation over a 14.3 degree half cone angle of the Earth for a MEO orbit. The reflector assembly  102 ,  408  is mated to the spacecraft bus structure  416  by a two segment  404 ,  402  boom structure that uses two deployment actuators (only one is shown  406 ) to achieve its final on-orbit configuration. The physical movement of the reflector  102 , required for larger beams, is achieved through a rotary positioning mechanism (RPM)  406  located between the boom joints  402  and  404  and the gimbal mechanism  408  at the center of the reflector  102 . The two-gimbal mechanism  408  allows the beams to steer over the Earth&#39;s coverage in both North-South and East-West directions. 
   Turning to  FIG. 5 , a 5 degree rotation of the RPM  406  accomplishes a 14 inch reflector  102  movement towards the feed array  104  and along the feed axis  410  (moving boom segment  402  to position  402 A). The change in the antenna boresight direction (from 412 to 412B) caused by the RPM  406  rotation is corrected by the gimbal mechanism  408 , which rotates the reflector  102  by 5 degrees in the opposite direction of the RPM  406  to position  412 A to realign the antenna boresight. 
     FIGS. 6A and 6B  are plots showing the performance of the antenna system  400  for smaller beam foot-print of 600 km (1.7 degrees diameter) wherein only the primary element  120  of the feed array  104  is used.  FIG. 6A  depicts the performance at L1=1.585 Hz, and  FIG. 6B  depicts the performance at L2=1.226 GHz. The beam size has been expanded to account for radial pointing error of +/−0.15 degrees, caused by the spacecraft and antenna pointing uncertainties, and the radio frequency (RF) performance has been evaluated over an expanded beam diameter of 2.0 degrees. the reflector remains at its normal position for the smaller beans and does not require physical movement. The reflector is gimbaled to reconfigure its beam location. Worst case directivity values evaluated over the 9 beams (this represents the worst case performance over the earth&#39;s field-of-view) are 33.5 dBi and 32.9 dBi at L1 and L2 frequencies. 
     FIG. 7  is a diagram showing a typical beam pattern azimuth cuts for the three east-west beams shown in FIG.  6 . It shows that efficient beams are formed over the global coverage, achieving low side lobe levels. 
   Larger beam performance has been optimized by using all eight elements of the feed array and by moving the reflector  102  towards the feed array  104  and along the feed axis  410 . The extent of the reflector  102  movement depends on the desired beam size ( 14  in. for 2500 km beam). All of the secondary elements  122  of the feed array  104  are excited with uniform amplitude and phase in order to simplify the BFN  214  and achieve the desired broad bandwidth of 26%. The coupler  212  value is determined based on the optimum excitation value of the outer array (the array of secondary elements  122 ) relative to the primary element  120 . This coupler  222  value is optimized over the desired range of beam foot-prints on ground (1200 km to 2500 km for this example), and for the parameters described above, is about 5.5 dB. 
   The illustrated beam patterns were computed using these optimized feed array excitations and by moving the reflector  102  towards the feed array  104  (0 to 14 in. physical movement of the reflector  102 ). 
     FIG. 8  is a diagram illustrating computed beam directivity contours for a 2000 km foot-print (5.67 deg+2=0.1 5 deg=5.97 deg)for L1 and L2 frequencies. Minimum directivity value for 5.97 degree beam is 26.41 dBi for both L1 and L2 frequencies over the globe (minimum value based on 9 beams). 
     FIG. 9  is a plot showing the azimuth pattern cuts for three beams (the beam numbers  8 ,  1  and  4  shown in FIG.  8 ). The contour plot of  FIG. 8  shows that the circularity of the 5.97 degree beam is well maintained with the 8-element antenna system  400 , while the conventional design with 7-element array shows flower-shaped contours, as plotted in  FIG. 10 , even for smaller beam size of 4.53 degrees (1500 km foot-print) diameter. 
     FIG. 11  is a diagram plotting the two variable beam sizes of the antenna system  400 , in which the narrow beams use the primary element  120  to generate beam sizes in the range 500 km to 1200 km and the broader beam using all the 8 elements  120  and  122 A- 122 G to generate beam sizes in the range 1200 km to 2500 km. 
   Table I shows a summary of the directivity performance reconfigurable antenna system at the L1 frequency (1.585 GHz). Table II shows a summary of the directivity performance reconfigurable antenna system at the L2 frequency (1.226 GHz). Worst case directivity over the Earth&#39;s coverage is shown as the bottom line of each table. 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE I 
             
           
           
             
                 
                 
             
             
                 
               1 Feed 
               8 Feeds 
             
           
        
         
             
               1.585 GHz 
               600Km 
               1000Km 
               1500Km 
               2000Km 
             
             
                 
             
           
        
         
             
               1 
               35.51 
               30.27 
               26.41 
               26.41 
             
             
               2 
               34.64 
               29.59 
               26.5 
               26.5 
             
             
               3 
               34.01 
               29.58 
               26.59 
               26.57 
             
             
               4 
               33.52 
               29.56 
               26.62 
               26.51 
             
             
               5 
               34.13 
               30.11 
               26.48 
               26.48 
             
             
               6 
               34.85 
               30.95 
               26.62 
               26.62 
             
             
               7 
               34.82 
               30.22 
               26.93 
               26.85 
             
             
               8 
               34.26 
               29.36 
               27.04 
               26.5 
             
             
               9 
               34.66 
               29.91 
               26.76 
               26.74 
             
             
               W.C. 
               33.5 
               29.4 
               26.4 
               26.4 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE II 
             
           
           
             
                 
                 
             
             
                 
               1 Feed 
               8 Feeds 
             
           
        
         
             
               1.226 GHz 
               600Km 
               1000Km 
               1500Km 
               2000Km 
             
             
                 
             
           
        
         
             
               1 
               34.59 
               31.38 
               28.31 
               26.41 
             
             
               2 
               33.92 
               31.17 
               28.18 
               26.79 
             
             
               3 
               33.35 
               30.77 
               28.13 
               26.6 
             
             
               4 
               32.92 
               30.36 
               27.99 
               26.48 
             
             
               5 
               33.28 
               30.67 
               27.95 
               26.43 
             
             
               6 
               33.84 
               31.07 
               28.16 
               26.7 
             
             
               7 
               33.82 
               31.06 
               28.11 
               26.71 
             
             
               8 
               33.69 
               30.79 
               27.93 
               26.41 
             
             
               9 
               33.71 
               30.83 
               28.13 
               26.77 
             
             
               W.C. 
               32.9 
               30.4 
               27.9 
               26.4 
             
             
                 
             
           
        
       
     
   
   The present invention can be extended to larger beam aspect ratios (beam size reconfigurability beyond the 1:5 ratio) by using a larger feed army  104  with increased number of elements  120 ,  122 A- 122 G, and  120 F. 
     FIGS. 12A and 12B  are diagrams depicting another embodiment of the present invention. 
     FIG. 12A  is a diagram depicting another embodiment of the feed array  104 . In this embodiment, the secondary elements  1222 ,  1224  are disposed around the periphery of the primary element  1220  in a plurality of rings including an inner ring R 2 , indicated it by the solid line in  FIG. 12A , and an outer ring R 3 , indicated by the dashed line in FIG.  12 A. Inner ring R 2  includes a plurality of secondary elements  1222  disposed about the primary element  1220 , and outer ring R 3  includes a plurality secondary elements  1224  disposed about the periphery of inner ring R 2 . In a more general case, the number of rings can be extended beyond three. 
     FIG. 12B  Is a diagram of a driver network  1200  that can be used with the feed array depicted in  FIG. 12A  Primary element  1220 , switches  1208  and  1210 A, coupler  1212 A, BFN  1214 A, and secondary elements  1222  are coupled and operate analogously to the corresponding features depicted in FIG.  2 . In this embodiment, however, these elements a operate with a secondary network  1230 . 
   Secondary network  1230  includes a first switch  1210 C coupled to high-power diplexer  1216 . The first switch  1210 C directs energy to the secondary elements in ring R 3 , or to switch  1210 B (and thereby to switch  1210 A) and the elements  1222  in ring R 2 , thus providing for the selective activation of secondary elements  1222  in ring R 2 . Elements  1224 , BFN  1214 B, and coupler  1212 B operate analogously to the elements  1222  of ring ( 2 , BFN  1214 A, and coupler  1212 A. 
   Hence, the primary element  1220  alone can be activated (by selection of switches  1208 ,  1210 A,  1210 B, and  1210 C to route signals as shown in the arrows labeled “1” in FIG.  12 B), the primary element  1220  and secondary elements  1222  in the second ring (by selection of switches  1208 ,  1210 A,  1210 B, and  1210 C to route signals as shown in the arrows labeled “2” in FIG.  12 B), or the primary element  1220 , and the secondary elements  1222 ,  1224  in both rings R 2  and R 3  (by selection of switches  1208 ,  1210 A,  1210 B, and  1210 C to route signals as shown in the arrows labeled “3” in FIG.  12 B). 
   When compared to the embodiment shown in  FIG. 2 , this feed array network can achieve more flexibility in terms of beam size reconfiguration, but this improvement comes at the expense of increased complexity and cost. 
   The embodiment shown in  FIGS. 12A and 12B  can be expanded to accommodate further rings RN of feed elements. It is also noted that the elements disposed in ring R 3  can differ in design from those of ring R 2 . For example, feed elements  1224  can be lower power elements than feed elements  1222 , if desired. Also, each of the elements in rings R 2  or R 3  need not be identical in design. For example, elements  1222  of ring R 2  may each be designed to output different power levels, or to be controllable in different ways, as required to achieve beam control and reconfiguration requirements. The Applicants&#39; invention is also applicable to other frequency bands such as C, Ku &amp; Ka used for communication satellites to provide fixed-satellite (FSS) and broadcast-satellite (BSS) services. 
   CONCLUSION 
   This concludes the description of the preferred embodiments of the present invention. The reconfigurable beam antenna system described above provides a simple and an efficient way to reconfigure the beams of communications satellites on orbit without the need for active components such as variable phase shifters and variable attenuators. It is also inexpensive, yet provides high degree of beam reconfiguration. 
   The antenna system employs an offset single reflector illuminated with a feed array. The beam size is controlled by keeping the feed array stationary while varying in the number of elements in feed array according to the desired beam size. This is accomplished through the use of high power switch(es) and passive beam-forming network(s) realized at high-level by using low-loss transmission media. Additional control over the beam size is achieved by moving the reflector along the axial direction towards the feed array by one or more articulating mechanisms behind the reflector. This defocusing technique extends the range of beam size reconfiguration beyond that which is achievable by other techniques. The beam can also be relocated in direction as well as size, by use of a gimbal mechanism behind the center of the reflector. The gimbal mechanism steers the reflector and hence the beams along the east-west and north-south directions over the earth&#39;s field-of-view. 
   The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.