Abstract:
A method and apparatus for reconfigurably transmitting shaped beam satellite signals via reflector array antennas are disclosed. The apparatus comprises a reflector for reflecting RF signals having a reflector focal plane and a feed array comprising a plurality of feed elements wherein said feed array is defocused from said reflector focal plane, yet produces a wavefront substantially similar to a wavefront that would be produced by a feed array located at the reflector focal plane. The method of transmitting a signal in accordance with the present invention comprises forming a wavefront with a feed array, wherein said feed array is defocused from a reflector focal plane, yet produces a wavefront substantially similar to a wavefront that would be produced by a feed array located at the reflector focal plane and reflecting said wavefront to a coverage area.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods for transmitting satellite signals, and in particular to a system and method for reconfigurably transmitting shaped beam satellite signals via reflector array antennas. 
     2. Description of the Related Art 
     Communications satellites are in widespread use. The communications satellites are used to deliver television and communications signals around the Earth for public, private, and military uses. 
     The primary design constraints for communications satellites are antenna beam coverage and radiated Radio Frequency (RF) power. These two design constraints are typically thought of to be paramount in the satellite design because they determine which customers on the Earth will be able to receive satellite communications service. Further, the satellite weight becomes a factor, because launch vehicles are limited as to how much weight can be placed into orbit. 
     Many satellites operate over fixed coverage regions, such as the Continental United States (CONUS), and employ polarization techniques, e.g. horizontal and vertical polarized signals, to increase the number of signals that the satellite can transmit and receive. These polarization techniques use overlapping reflectors where the reflector surfaces are independently shaped to produce substantially congruent coverage regions for the polarized signals. This approach is limited because the coverage regions are fixed and cannot be changed on-orbit, and the cross-polarization isolation for wider coverage regions is limited to the point that many satellite signal transmission requirements cannot increase their coverage regions. 
     Many satellite systems would be more efficient if they contained antennas with an on-orbit reconfigurable beam, capable of modifying the shape and translation (or scan) of the beam on the Earth. These objectives are typically met using a multi-feed reflector antenna system that reconfigures the beam coverage by individually varying signal amplitude with variable attenuators or amplifiers and varying the signal phase with variable phase shifters at the feed elements located along the reflector focal plane. 
     However, the antenna feed system and beamforming network (BFN) of such prior art multi-feed reflector antennas is complex, lossy, heavy, difficult to integrate, test, and repair or replace, requiring excessive time and labor costs. Furthermore, the complexity of the antenna feed system of such prior art multi-feed reflector antenna systems makes them more difficult to operate. Particularly, the amplifiers of prior art multi-feed reflector antenna systems do not operate at a fixed power level when reconfiguring the beam coverage. In addition, reconfiguring the beam coverage of prior art multi-feed reflector antenna systems requires switching power among a plurality of feeds. 
     Another approach to meet the previous beam reconfigurability objectives is to use a Direct Radiating Array (DRA). In the DRA solution, no reflector is used. The feed elements are arranged in a grid pattern and pointed directly at the coverage area. The antenna beam phase can be reconfigured by varying the excitation phase at the feed elements with variable phase shifters. The disadvantage of this solution is that to obtain the same directivity as a reflector antenna, a very large number of feed elements and phase shifter are needed, making such an antenna system very heavy and complex. 
     There is therefore a need in the art for a reconfigurable multi-feed reflector antenna system without the attendant complexity of prior art systems. There is also a need in the art for a reconfigurable multi-feed reflector antenna system that is easier to integrate and test. There is a further need in the art for a reconfigurable multi-feed reflector antenna system using amplifiers operating at a fixed gain. There is yet another need in the art for a reconfigurable multi-feed reflector antenna system that is reconfigured without switching power among a plurality of feeds. 
     The present invention satisfies these needs. 
     SUMMARY OF THE INVENTION 
     To address the requirements described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an apparatus and method for transmitting signals with a phase-only reconfigurable multi-feed reflector antenna. The present invention further discloses a feed network. 
     In general, the reconfigurable multi-feed reflector antenna system of the present invention is achieved by employing a less complicated approach to reshape and scan the beam of a multi-feed reflector by varying only the relative excitation phase at each feed element, while maintaining a fixed signal gain at the feed elements. The phase of each element can be controlled using ordinary variable phase shifters. In addition, the system and method of the present invention may be implemented with a reflector gimbal mechanism to further extend beam coverage translations. 
     A reconfigurable multi-feed antenna system in accordance with the present invention comprises a reflector for reflecting RF signals having a reflector plane and a feed array comprising a plurality of feed elements wherein said feed array is defocused from said reflector focal plane, yet produces an RF wavefront substantially similar to an RF wavefront that would be produced by a feed array located at the reflector focal plane. 
     A feed network in accordance with the present invention comprises a BFN comprising a signal divider for dividing an input signal into a plurality of divided signals, a plurality of variable phase adjusters, each receiving one of the plurality of divided signals and outputting a phase adjusted signal, and at least one fixed gain amplifier for amplifying each phase adjusted signal and outputting an amplified signal for each phase adjusted signal. The feed network further comprises a feed array, defocused from a reflector, comprising a plurality of feed elements, each receiving an amplified signal and radiating a radiated signal, wherein the combination of the radiated signals forms a wavefront. 
     A method of transmitting a signal in accordance with the present invention comprises forming an RF wavefront with a feed array, wherein said feed array is defocused from a reflector focal plane, yet produces an RF wavefront substantially similar to an RF wavefront that would be produced by a feed array located at the reflector focal plane and reflecting said wavefront to a coverage area. 
     The foregoing allows the use of a constant value for the gain at each feed element which in turn enables three fundamental advantages of the present invention. First, the present invention provides the advantage that the amplifiers feeding the elements have a fixed operating power level, regardless of the coverage shape. Second, reconfiguring the beam coverage does not require switching power among feeds. Third, the overall antenna feed system is less complex and simpler to control than prior art systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 illustrates a reconfigurable multi-feed reflector antenna system of the prior art; 
     FIGS. 2A-2B are example mappings of the feed elements to the coverage of Japan of a prior art reconfigurable multi-feed antenna at a spacecraft yaw of 0° and −90°, respectively; 
     FIGS. 3A-3B illustrate the principle of the reconfigurable phase-only multi-feed reflector antenna system of the present invention; 
     FIG. 4 illustrates the reconfigurable phase-only multi-feed reflector antenna system of the present invention; 
     FIG. 5 is a block diagram of the feed network of the present invention; 
     FIGS. 6A-6B are example mappings of the coverage of Japan at a spacecraft yaw of 0° and −90°, respectively, of a reconfigurable phase-only multi-feed reflector antenna system of the present invention showing antenna directivity contours; 
     FIG. 7 is an example mapping of the coverage of Japan of a reconfigurable phase-only multi-feed reflector antenna system of the present invention with the reflector gimbaled by 2° showing antenna directivity contours; and 
     FIG. 8 is an example mapping of the CONUS coverage at a spacecraft yaw of 0° of a reconfigurable phase-only multi-feed reflector antenna system of the present invention with the reflector gimbaled by 3° showing antenna directivity contours. 
    
    
     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 principle of the present invention is best illustrated through a comparison between a prior art multi-feed reflector antenna and the phase-only reconfigurable multi-feed antenna of the present invention. 
     FIG. 1 illustrates a reconfigurable multi-feed reflector antenna system  100  of the prior art. The feed array  108  is comprised of a plurality of feed horns or radiating elements (hereinafter, feed elements  110 ) arranged in a grid pattern, preferably a hexagonal pattern. The feed array  108  is located at the reflector focal plane  104  at an offset  106  distance. Signals are delivered to the individual feed elements  110  through a beam forming network  112  (BFN). The feed array  108  produces a wavefront  114  which is reflected off the reflector  102  to a coverage area. Importantly, there is direct correspondence between each feed element  110  positioned at the reflector focal plane  104  and the beam location produced by that feed element  110  on the coverage area. 
     FIGS. 2A-2B are example mappings of the feed element coverage of Japan of a prior art reconfigurable multi-feed antenna at a spacecraft yaw of 0° and −90°, respectively. Such spacecraft reorientation may occur when a spacecraft is in a highly elliptical orbit and the spacecraft yaw orientation must vary continuously by up to 360° to align the solar panels with the sun. Of the thirty-seven (37) feed elements  110  blanketing the overall coverage area  200 , only those feed elements  110  which project beams  204  to the receiving coverage area  202  are activated to optimize the performance of the system. 
     For example, in FIG. 2A the active feed elements  110  correspond to beams  204  numbered  1 - 7 ,  14  and  15 . To produce the receiving coverage area  202  an RF signal is distributed with the proper gain and phase among the active feed elements  110  through a BFN. The remaining feed elements  110 , corresponding to beams  204  numbered  8 - 13  and  16 - 37 , are not used. 
     In FIG. 2B, a reorientation of the spacecraft to a yaw of −90° necessitates a redistribution of the active feed elements  110 . The active feed elements  110  now correspond to beams  204  numbered  1 - 7 ,  11  and  12 , with the remaining feed elements inactive. In general, each time the coverage is rotated or translated with respect to the antenna boresight, the power must be redistributed among the feed elements  110  to maintain proper coverage. A BFN which supports the task of redistributing the power can be very complex. The present invention eliminates the need for such a BFN. 
     Principle of the Present Invention 
     FIGS. 3A and 3B illustrate the principle of the reconfigurable phase-only multi-feed reflector antenna system. FIG. 3A depicts a multi-feed reflector antenna system  300 . Feed elements  110  of a prior art feed array  108  are located at the reflector focal plane  104 . A repeater device  308  located at a defocused plane  302  intercepts a cone angle between the feed array  108  and the outside rim of the reflector  102 . The repeater device  308  receives an incoming wavefront  310  from the feed array  108  at a receiver array  304  and repeats it at discrete points from a transmit array towards the reflector  102 . The illumination of each feed element  110  on the repeater device  308  is closely Gaussian with a maximum around the center of the repeater device  308 . 
     FIG. 3B depicts the feed array  314  and reflector  102  configuration of the present invention. The original feed array  108  and the repeater device  308  are replaced by a single feed array  314  located at the same defocused plane  302 . The new feed array  314  is designed to substantially reproduce the wavefront  312  as would have been produced by the original feed array  108  (as shown in FIG. 1, wavefront  114 ). The defocused plane  302  must be positioned to allow enough sampling points on the wavefront while maintaining a feed element size larger than at least one wavelength to reduce mutual coupling effects. 
     Configuration of the Present Invention 
     FIG. 4 illustrates the reconfigurable phase-only multi-feed reflector antenna system  400 . The feed array  402  is positioned in a defocused plane  302  from the focal plane  104  of the reflector  406 . The defocusing of each feed element  414  broadens the beam that it produces, allowing coverage of substantially the entire potential coverage area  416 . The combination of the contributions of each feed element  414  after proper phasing between them, produces a wavefront  312  of a shaped beam concentrated only on the desired geographical area within the coverage area  408 . The BFN  404  delivers signals to each feed element  414  at a fixed gain but with a variable phase to produce a wavefront  312 . The wavefront  312  is reconfigured by the BFN  404  through reconfiguration of the variable phase adjusters of the BFN  404 . When the shape of the desired coverage area changes, due to a satellite maneuver or for any other reason, the phase of the BFN  404  can be reconfigured to concentrate the beam on the new coverage area. The wavefront  312  reflects off the reflector  406  to a coverage area  408 , producing antenna directivity contours  410  representing varying signal strength across the coverage area  408 . 
     In one embodiment, the power at each feed element  414  is fixed with an imposed circularly symmetric taper at the feed array  402  with a maximum at the center of the feed array  402 . For example, in a thirty-seven element hexagonal array, a taper of −8 dB may be used. The seven center feed elements  414  operate at 0 dB, the surrounding twelve feed elements  414  operate at −4 dB and the outermost eighteen feed elements  414  operate at −8 dB. The phase of each feed element  414  is selected to optimally blanket the coverage area  408 . 
     Reconfiguration of the variable phase adjusters of the BFN  404  can alter both the shape and the scan of the coverage area  408 . In addition, the scan of the coverage area may be further extended through the use of a gimbal mechanism  412 . 
     Importantly, the reflector geometry must accommodate a sufficiently large offset  414  with respect to the focal axis of the reflector  406 , yet allow enough room for feed defocusing without obstructing the reflector  406 . 
     FIG. 5 is a block diagram of the feed network of the present invention. The feed network  500  of the present invention comprises a BFN  512  and a feed array  514 . A signal is applied at the input  510  to a signal divider  508  of the BFN  512 . The signal divider  508 , which may be a passive signal divider, appropriately divides the signal among the feed elements  502  of the feed array  514 . Each divided signal is directed to a variable phase adjuster  506  and a fixed gain amplifier  504  before arriving at the appropriate feed element  502 . In a preferred embodiment, the variable phase adjusters  506  will perform five-bit shifting quantization. 
     Although FIG. 5 depicts an individual fixed gain amplifier  504  for each feed element  502 , such an arrangement is not required. Equivalent systems may incorporate a fixed gain amplifier system wherein more than one of the signals are amplified by a single amplifier. 
     Example Coverage Mappings of the Present Invention 
     FIGS. 6A-6B are example mappings of the coverage of Japan at varying spacecraft yaw by a reconfigurable phase-only multi-feed reflector antenna system of the present invention showing antenna directivity contours  600 . FIG. 6A depicts the coverage area shape  602  using a phase-only multi-feed reflector antenna of the present invention at a spacecraft yaw of 0°. FIG. 6B depicts the coverage area shape  604  reconfigured to accommodate a spacecraft yaw of −90°. 
     FIG. 7 is an example mapping of the coverage of Japan at a high scan by a reconfigurable phase-only multi-feed reflector antenna system of the present invention showing antenna directivity contours  702 . To achieve the coverage area shape  700  with the antenna boresight shifted by 4° east of the original position, the reflector was first gimbaled by 2° to repoint the reflector boresight before reconfiguring the variable phase adjusters of the BFN to further alter the coverage shape and scan. 
     FIG. 8 is an example mapping of the CONUS coverage shape  800  at a spacecraft yaw of 0° of a reconfigurable phase-only multi-feed reflector antenna system of the present invention. The reflector is first gimbaled by 3° before reconfiguring the variable phase adjusters of the BFN to further alter the coverage shape and scan. 
     Those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. For example, those skilled in the art will recognize that any combination of the above components, or any number of different components, and other devices, may be used with the present invention. 
     Conclusion 
     This concludes the description of the preferred embodiments of the present invention. In summary, the present invention describes an apparatus and method for a phase-only reconfigurable multi-feed reflector antenna system. The present invention provides the advantage that feed element amplifiers have a fixed operating power level, regardless of the coverage shape. The present invention also provides the advantage that reconfiguring the beam coverage does not require switching power among feed elements. In addition, the present invention provides the advantage that the overall antenna feed system is less complex and simpler to control than prior art systems. The present invention combines the reconfiguration flexibility of a phased array antenna with the concentrating efficiency of a large reflector antenna, but with much fewer elements than would normally be required by an ordinary phased array antenna. 
     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.