Antenna system for hybrid communication satellite
TECHNICAL FIELD 
     The present invention broadly relates to satellite communication systems especially of the type employing a spin-stabilized satellite placed in geosynchronous orbit above the earth so as to form a communication link between many small aperture terminals on the earth. More particularly, the invention involves an antenna system for a communication satellite having hybrid communication capability accommodating both two-way and broadcast communication systems. 
     BACKGROUND ART 
     Communications satellites have in the past typically employed several antenna subsystems for receiving and transmitting signals from and to the earth respectively. These antenna subsystems are often mounted on a &#34;despun&#34; platform of the satellite so as to maintain a constant antenna orientation relative to the earth. The antenna subsystems may be either fixed or steerable and may operate on different polarizations. For example, one known type of antenna subsystem includes a pair of primary reflectors mounted in aligned relationship to each other, one behind the other. One of the reflectors is vertically polarized and is operative to reflect one of the transmit and receive signals. The other reflector is horizontally polarized and is operative to reflect the other of the transmit and receive signals. 
     Because of space constraints in communication satellites, the antenna systems for such satellites must be as compact and utilizes a few components as possible. To partially satisfy this objective, imaging reflector arrangements have been deviced to form a scanning beam using a small transmit array. These arrangements achieve the performance of a large aperture phased array by combining a small phased array with a large main reflector and an imaging arrangement of smaller reflectors to form a large image of a small array over the main reflector. An electronically scannable antenna with a large aperture is thus formed, using a small array. One important feature of this imaging arrangement is that the main reflector need not be fabricated accurately, since small imperfections can be corrected efficiently by the array. 
     In order to provide a compact antenna system, so called quasi-optical diplexers have been employed in the past to separate coincident radio signals of different frequency bands, e.g. a transmit signal and a receive signal. A compact imaging arrangement employing a quasi-optical diplexer of the type discussed above is disclosed in &#34;Imaging Reflector Arrangements to Form a Scanning Beam Using a Small Array&#34;, C. Dragone and M. J. Gans, The Bell System Technical Journal, Volume 5, No. 2, Feb. 9, 1979. This publication discloses a frequency diplexer positioned between a transmit array and an imaging reflector. The receive array is positioned on one side of the diplexer, opposite that of the transmit array. Signals in the transmit band pass from the transmit array through the diplexer to the imaging reflector. The diplexer is reflective of signals in the receive band, consequently, a signal in the receive band which is incident on the diplexer is reflected onto the receive array. 
     With the increasing cost of placing a communications satellite in geosynchronous orbit, it has become increasingly important for the satellite to handle a maximum number of channels, and if possible, different types of communications services. The present invention is directed toward achieving these objectives. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an antenna system is provided for a communications satellite which comprises a first subsystem suitable for providing two-way, point-to-point communications service, and a second subsystem for providing broadcast service. Each of the subsystems include a transmitter and a receiver. Both subsystems employ a main reflector assembly comprising a pair of parabolic reflectors which intersect each other along a common axis and are respectively vertically and horizontally polarized. 
     The point-to-point transmitter and broadcast receiver of the subsystems each use a vertically polarized signal and cooperate with the vertically polarized main reflector. The broadcast transmitter and point-to-point receiver of the subsystems each operate with a horizontally polarized signal and cooperate with the horizontally polarized reflector. The transmitter of the point-to-point subsystem includes an imaging reflector arrangement utilizing a small subreflector to form a large image of the small transmitter array over the main reflector, thereby obtaining the performance of a large aperture phased array. 
     A pair of quasi-optical diplexers defined by frequency selective screens are employed to separate the transmit and receive signals of each of the subsystems. 
     It is therefore a primary object of the present invention to provide an antenna system for a communications satellite which includes subsystems forming independent communication links with the areas serviced by the satellite. 
     A further object of the present invention is to provide an antenna system as described above which is particularly compact and simple in construction. 
     A further object of the invention is to provide an antenna system as described above which includes a first receiver and transmitter allowing two-way communication between any of a plurality of ground stations and a second receiver and transmitter providing a broadcast service for the area serviced by the satellite. 
     Another object of the present invention is to provide an antenna system as described above which utilizes a pair of frequency selective screens for respectively separating the transmit and receive signals of each of the subsystems. 
     A further object of the invention is to provide an antenna system as described above which includes an electronically scannable antenna with a large aperture using a small phased array. 
     Another object of the present invention is to provide an antenna reflector assembly comprising a pair of reflectors of respectively different polarizations which intersect each other along a common axis and form a compact assembly. 
     These, and further objects and advantages of the invention will be made clear or will become apparent during the course of the following description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a perspective view of a communications satellite, showing the antenna subsystems; 
     FIG. 2 is a top plan view of the antenna subsystems shown in FIG. 1; 
     FIG. 3 is a sectional view taken along the line 3--3 in FIG. 2; 
     FIG. 4 is a sectional view taken along the line 4--4 in FIG. 2; 
     FIG. 5 is a view of the United States and depicts multiple, contiguous receive zones covered by the satellite of the present invention, the primary areas of coverage being indicated in crosshatching and the areas of contention being indicated by a dimpled pattern; 
     FIG. 6 is a block diagram of the communications electronics for the communication satellite; 
     FIG. 7 is a schematic diagram of a coupling network which interconnects the point-to-point receive feed horns with the inputs to the communications electronics shown in FIG. 6. 
     FIG. 8 is a reference table of the interconnect channels employed to connect the receive and transmit zones for the point-to-point system; 
     FIG. 9 is a diagrammatic view of the United States depicting multiple contiguous transmit zones covered by the satellite and the geographic distribution of the interconnected channels for each zone, across the United States; 
     FIG. 9A is a graph showing the variation in gain of the transmit antenna beam for each zone in the point-to-point system in relation to the distance from the center of the beam in the east-west direction; 
     FIG. 9B is a graph similar to FIG. 9A but showing the variation in gain in the north-south direction; 
     FIG. 10 is a detailed schematic diagram of the filter interconnection matrix employed in the point-to-point system; 
     FIG. 11 is a detailed, plan view of the beam-forming network employed in the point-to-point system; 
     FIG. 12 is an enlarged, fragmentary view of a portion of the beam-forming network shown in FIG. 11; 
     FIG. 13 is a front elevational view of the transmit array for the point-to-point system, the horizontal slots in each transmit element not being shown for sake of simplicity; 
     FIG. 14 is a side view of the transmit element of the array shown in FIG. 13 and depicting a corporate feed network for the element; 
     FIG. 15 is a front, perspective view of one of the transmit elements employed in the transmit array of FIG. 13; 
     FIG. 16 is a front view of the receive feed horns for the point-to-point system; and 
     FIG. 17 is a diagrammatic view showing the relationship between a transmitted wave and a portion of the transmit feed array for the point-to-point system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1-4, a communications satellite 10 is depicted which is placed in geosynchronous orbit above the earth&#39;s surface. The satellite&#39;s antenna system, which will be described in more detail below, will typically be mounted on an earth-oriented platform so that the antenna system maintains a constant orientation with respect to the earth. 
     The satellite 10 is of a hybrid communications-type satellite which provides two different types of communication services in a particular frequency band, for example, the fixed satellite service Ku band. One type of communication service, referred to hereinafter as point-to-point service, provides two-way communications between very small aperture antenna terminals of relatively narrow band voice and data signals. Through the use of frequency division multiple access (FDMA) and reuse of the assigned frequency spectrum, tens of thousands of such communication channels are accommodated simultaneously on a single linear polarization. The other type of communication serivce provided by the satellite 10 is a broadcast service, and it is carried on the other linear polarization. The broadcast service is primarily used for one-way distribution of video and data throughout the geographic territory served by the satellite 10. As such, the transmit antenna beam covers the entire geographic territory. For illustrative purposes throughout this description, it will be assumed that the geographic area to be serviced by both the point-to-point and broadcast services will be the United States. Accordingly, the broadcast service will be referred to hereinafter as CONUS (Continental United States). 
     The antenna system of the satellite 10 includes a conventional omni antenna 13 and two antenna subsystems for respectively servicing the point-to-point and CONUS systems. The point-to-point antenna subsystem provides a two-way communication link to interconnect earth stations for two-way communications. The CONUS antenna system functions as a transponder to broadcast, over a wide pattern covering the entire United States, signals received by one or more particular locations on earth. The point-to-point transmit signal and the CONUS receive signal are vertically polarized. The CONUS transmit and point-to-point receive signals are horizontally polarized. The antenna system includes a large reflector assembly 12 comprising two reflectors 12a, 12b. The two reflectors 12a, 12b are rotated relative to each other about a common axis and intersect at their midpoints. The reflector 12a is horizontally polarized and operates with horizontally polarized signals, while the reflector 12b is vertically polarized and therefore operates with vertically polarized signals. Consequently, each of the reflectors 12a, 12b reflects signals which the other reflector 12a, 12b transmits. 
     A frequency selective screen 18 is provided which includes two halves or sections 18a, 18b and is mounted on a support 30 such that the screen halves 18a, 18b are disposed on opposite sides of a centerline passing diametrically through the satellite 10, as best seen in FIG. 2. The frequency selective screen 18 functions as a diplexer for separating different bands of frequencies and may comprise an array of discrete, electrically conductive elements formed of any suitable material, such as copper. Any of various types of known frequency selective screens may be employed in this antenna system. However, one suitable frequency selective screen, exhibiting sharp transition characteristics and capable of separating two frequency band which are relatively close to each other, is described in U.S. patent application Ser. No. 896,534, filed Aug. 14, 1986, and assigned to Hughes Aircraft Company. The frequency selective screen 18 effectively separates the transmitted and received signals for both the CONUS and point-to-point subsystems. It may be appreciated that the two halves 18a, 18b of the screen 18 are respectively adapted to separate individual signals which are horizontally and vertically polarized. 
     The CONUS subsystem, which serves the entire country with a single beam, has, in this example, eight conventional transponders each having a high power traveling wave tube amplifier as its transmitter 82 (see FIG. 6). The CONUS receive antenna uses vertical polarization, sharing the vertically polarized reflector 12b with the point-to-point transmission system. CONUS receive signals pass through the frequency selective screen half 18b and are focused on the receive feed horns 14 located at the focal plane 28 of reflector 12b. The antenna pattern so formed is shaped to cover CONUS. The CONUS transmit antenna employs horizontal polarization, and shares reflector 12a with the point-to-point receive system. Signals radiating from the transmit feeds 24 are reflected by the horizontally polarized frequency selective screen 18a to reflector 12a whose secondary pattern is shaped to cover CONUS. 
     The point-to-point subsystem broadly includes a transmit array 20, a subreflector 22, and receive feed horns 16. The transmit array 20, which will be described later in more detail, is mounted on the support 30, immediately beneath the screen 18. The subreflector 22 is mounted forward of the transmit array 20 and slightly below the screen 18. The signal emanating from the transmit array 20 is reflected by the subreflector 22 onto one half 18b of the screen 18. The subreflector 22 in conjunction with the main reflector 12 functions to effectively magnify and enlarge the pattern of the signal emanating from the transmit array 20. The signal reflected from the subreflector 22 is, in turn, reflected by one half 18b of the screen 18 onto the large reflector 12b, which in turn reflects the point-to-point signal to the earth. Through this arrangement, the performance of a large aperture phase array is achieved. The receive feed horns 16 are positioned in the focal plane 26 of the reflector 12a. It consists of four main horns 50, 54, 58, 62 and three auxiliary horns 52, 56, 60 as shown in FIG. 16. 
     Referring now also to FIGS. 13-15, the transmit array 20 comprises a plurality, for example forty, transmit waveguide elements 106 disposed in side-by-side relationship to form an array, as shown in FIG. 13. Each of the transmit waveguide elements 106 includes a plurality, for example twenty-six, of horizontal, vertically spaced slots 108 therein which result in the generation of a vertically polarized signal. As shown in FIG. 14, the transmit array 20 is fed with a transmit signal by means of a corporate feed network, generally indicated by the numeral 110 which excites the array element in four places 114. The purpose of the corporate feed network 110 is to provide a broadband match to the transmit waveguide element 106. Signals input to the waveguide opening 112 excite the array slots 108 so that the slot excitation is designed to give a flat pattern in the north-south direction. 
     Attention is now directed to FIG. 5 which depicts a generally rectangular beam coverage provided by the horizontally polarized point-to-point receive system. In this particuular example, the area serviced by the point-to-point system is the continental United States. The point-to-point receive system comprises four beams R1, R2, R3, R4 respectively emanating from the four uplink zones 32, 34, 36, 38 to the satellite, wherein each of the beams R1-R4 consists of a plurality of individual uplink beams originating from individual sites in each zone 32, 34, 36, 38 and carries an individual signal from that site. The uplink beam signals from the individual sites are arranged into a plurality of channels for each zone. For example, zone 32 may include a plurality, e.g. sixteen 27 MHz channels with each of such channels carrying hundreds of individual beam signals from corresponding uplink sites in zone 32. 
     The signal strength for each of the four beam pattern contours, respectively designated by numerals 32, 34, 36 and 38, are approximately 3 dB down from peaks of their respective beams. The antenna beams have been designed to achieve sufficient isolation between them to make feasible in the cross-hatched regions 39, 41, 43, 45 reuse of the frequency spectrum four times. In the dotted regions 40, 42, and 44, the isolation is insufficient to distinguish between signals of the same frequency originating in adjacent zones. Each signal originating in these regions will generate two downlink signals, one intended and one extraneous. The generation of extraneous signals in these areas will be discussed later in more detail. 
     It may be readily appreciated from FIG. 5 that the four zones covered by beams 32, 34, 36, 38 are unequal in width. The East Coast zone covered by beam 32 extends approximately 1.2 degrees; the Central zone covered by beam 34 extends approximately 1.2 degrees; the Midwest zone covered by beam pattern 36 extends approximately 2.0 degrees, and; the West Coast zone covered by beam pattern 38 extends approximately 2.0 degrees. The width of each of the four receive zones 32, 34, 36 and 38 is determined by the number of terminals and thus the population density in the various regions of the country. Thus, beam pattern 32 is relatively narrow to accommodate the relatively high population density in the Eastern part of the United States while beam pattern 36 is relatively wide due to the relatively low population density in the Mountain states. Since each zone utilizes the entire frequency spectrum, zone widths are narrower in regions where the population density is high, to accommodate the greater demand for channel usage. 
     As shown in FIG. 9, the point-to-point transmit system comprises four beams T1, T2, T3, T4 respectively covering the four transmit zones 31, 33, 35, 37, wherein each of the beams T1-T4 consists of a plurality of individual downlink beams destined for the individual downlink sites in each zone 31, 33, 35, 37 and carries an individual signal to that site. The downlink beam signals, destined to be received at the individual downlink sites, are arranged into a plurality of channels for each zone. For example, zone 31 may include a plurality, e.g. sixteen 27 MHz channels with each of such channels carrying hundreds of individual beam signals to corresponding downlink sites in zone 32. 
     The use of multiple downlink zones and downlink zones of unequal widths assist in causing the intermodulation products, generated by the later-discussed solid state power amplifiers, to be geographically disperseed in a manner which prevents most of these products from being received at the ground terminals. The net effect is that the amplifiers may be operated more efficiently because the system can tolerate more intermodulation products. Although the widths of the transmit zones 31, 33, 35, 37 are nearly the same as those of the receive zones R1, R2, R3, R4, small differences between the two sets have been found to optimize the capacity of the system. 
     The half power beam width of the individual transmit beams 29 is substantially narrower than that of the transmit zones 31, 33, 35, 37. This results in the desirable high gain, and avoids the zones of contention 40, 42, 44 characteristic of the receive zone arrangement. These individual beams 29 must be steered within the zones in order to maximize the downlink EIRP in the directions of the individual destination terminals. The transmit point-to-point frequency addressable narrow beams 29 are generated by an array 20 whose apparent size is magnified by two confocal parabolas comprising a main reflector 12b and a subreflector 22. The east-west direction of each beam 29 is determined by the phase progression of its signal along the array 106 of transmit elements 20 (FIGS. 13 and 15). This phase progression is established by a later-discussed beam-forming network 98 and is a function of the signal frequency. Each of the transmit array elements 20 is driven by a later-discussed solid state power amplifier. The power delivered to the array elements 106 is not uniform but is instead tapered with the edge elements being more than 10 dB down. Tapering of the beams 29 is achieved by adjusting the transmit gain according to the position of the transmit array elements 20. The excitation pattern determines the characteristics of the transmit secondary pattern, shown in FIG. 9A. Referring to FIG. 9, the closest spacing between transmit zones 31, 33, 35, 37 occurs between zones 31 and 33 and is approximately 1.2 degrees. This means that a signal addressed to zone 33 using a particular frequency would interfere with a signal using the same frequency in zone 31 with its side lobe 1.2 degrees from its beam center. However, the individual transmit gains have been adjusted to provide low side lobe levels, thereby permitting frequency reuse in adjacent zones. Referring to FIG. 9A, it is seen that the side lobe level at this angle off beam center is more than 30 dB down, so that such interference will be negligibly small. The same frequency used in zones 35 and 37 are further removed in angle, hence the side lobe interference in those zones is even smaller. 
     FIG. 9B is an illustration of the transmit beam pattern in the north-south direction. The twenty six slots 108 in each of the transmit waveguide elements 106 are excited in a manner which creates a nearly flat north-south pattern, extending over the covered range of plus and minus 1.4 degrees from the north-south boresight direction. 
     Both the point-to-point and CONUS systems may utilize the same uplink and downlink frequency bands, with the point-to-point system using horizontal polarization for its uplink polarization, and the CONUS system using vertical polarization, as previously mentioned. For example, both services may, simultaneously, utilize the entire 500 MHz uplink frequency band between 14 and 14.5 GHz, as well as the entire 500 MHz downlink frequency band between 11.7 and 12.2 GHz. Each of the receive zones 32, 34, 36, 38 and transmit zones 31, 33, 35, 37, employing the point-to-point service utilizes the entire frequency spectrum (i.e. 500 MHz). Furthermore, this total frequency spectrum is divided into a plurality of channels, for example, sixteen channel each having a usable bandwidth of 27 MHz and a spacing of 30 MHz. In turn, each of the sixteen channels may accommodate approximately 800 subchannels. Hence, within each zone, approximately 12,500 (16 channels×800 subchannels) 32 kilobit per second channels may be accommodated, at any given moment. As will be discussed below, the communication architecture of the point-to-point system allows any terminal to communicate directly with any other terminal. Thus, within a single polarization, a total of 50,000 subchannels may be accommodated nationwide. 
     Referring now particular to FIGS. 1, 2, 6, 7 and 16, the point-to-point receive feed array 16 employs seven receive horns 50-62. Horns 50, 54, 58 and 62 respectively receive signals from zones 32, 34, 36 and 38. Horns 52, 56 and 60 receive signals from the zones of contention 40, 42 and 44. Using a series of hybrid couplers or power dividers C 1  -C 9 , the signals received by horns 50-62 are combined into four outputs 64-70. For example, a signal originating from the area of contention 44 and received by horn 60 is divided by coupler C 2  and portions of the divided signal are respectively delivered to couplers C 1  and coupler C 4  whereby the split signal is combined with the incoming signals received by horns 58, 62 respectively. Similarly, signals originating from the area of contention 42 and received by horn 56 are split by coupler C 5 . A portion of the split signal is combined, by coupler C 3 , with the signal output of coupler C 4 , while the remaining portion of the split signal is combined, by coupler C.sub. 7, with the signal received by horn 54. 
     Attention is now particularly directed to FIG. 6 which depicts, in block diagram form, the electronics for receiving and transmitting signals for both the CONUS and point-to-point systems. The point-to-point receive signals 64-70 (see also FIG. 7) are derived from the point-to-point receive feed network in FIG. 7, whereas the CONUS receive signal 72 derives from the CONUS receive feed horns 14, (FIGS. 1 and 3). Both the point-to-point and CONUS receive signal are input to a switching network 76 which selectively connects input lines 64∝72 with five corresponding receivers, eight of which receivers are generally indicated at 74. The receivers 74 are of conventional design, three of which are provided for redundancy and are not normally used unless a malfunction in one of receivers is experienced. In the event of a malfunction, switching network 76 reconnects the appropriate incoming line 64-72 with a back-up receiver 74. Receivers 74 function to drive the filters in a filter interconnection matrix 90. The outputs of the receivers 74, which are connected with lines 64-70, are coupled by a second switching network 78 through four receive lines R1-R4 to a filter interconnection matrix 90. As will be discussed later below, the filter interconnection matrix (FIM) provides interconnections between the receives zones 32, 34, 36, 38, and the transmit zones 31, 33, 35, 37. Operating in the above-mentioned 500 MHz assigned frequency spectrum, separated into sixteen 27 MHz channels, four sets of sixteens filters are employed. Each set of the sixteen filters utilizes the entire 500 MHz frequency spectrum and each filter has a 27 MHz bandwidth. As will be discussed later, the filter outputs T1-T4 are arranged in four groups, each group destined for one of the four transmit zones 31, 33, 35, 37. 
     The transmit signals T1-T4 are respectively connected, via switching network 94, to four of six driving amplifiers 92, two of such amplifiers 92 being provided for back-up in the event of failure. In the event of the failure of one of the amplifiers 92, one of the back-up amplifiers 92 will be reconnected to the corresponding transmit signal T1-T4 by the switching network 94. A similar switching network 96 couples the ampliified output of the amplifiers 92 to a beam-forming network 98. As will be discussed later in more detail, the beam-forming network 98 consists of a plurality of transmission delay lines connected at equal intervals along the four delay lines. These intervals and the width of the delay lines are chosen to provide the desired centerband beam squint and the beam scan rate with frequency for the corresponding transmit zones 31, 33, 35, 37 to be serviced. The transmit signals, coupled from the four delay lines, are summed in the beam-forming network 98 as shown in FIGS. 11 and 12, to provide inputs to solid state power amplifiers 100, which may be embedded in the point-to-point system&#39;s transmit array 20. In the illustrated embodiment discussed below, forty solid state power amplifiers (SSPAs) 100 are provided. Each of the SSPAs 100 amplifies a corresponding one of the forty signals formed by the beam-forming network 98. The SSPAs 100 possess different power capacities to provide the tapered array excitation previously mentioned. The output of the SSPA 100 is connected to the input 112 (FIG. 14) at one of the elements of the transmit array 20. 
     The receive signal for CONUS on line 72 is connected to an appropriate receiver 74 by switching networks 76, 78. The output of the receiver connected with the CONUS signal is delivered to an input multiplexer 80 which provides for eight channels, as mentioned above. The purpose of the input multiplexers 80 is to divide the one low level CONUS signal into subsignals so that the subsignals can be amplified on an individual basis. The CONUS receive signals are highly amplified so that the CONUS transmit signal may be distributed to very small earth terminals. The outputs of the input multiplexer 80 are connected through a switching network 84 to eight of twelve high power traveling wave tube amplifiers (TWTAs) 82, four of which TWTAs 82 are employed for back-up in the event of failure. The outputs of the eight TWTAs 82 are connected through another switching network 86 to an output multiplexer 88 which recombines the eight amplified signals to form one CONUS transmit signal. The output of the multiplexer 88 is delivered via waveguide to the transmit horns of the CONUS transmitter 24 (FIGS. 2 and 3). 
     Attention is now directed to FIG. 10 which depicts the details of the FIM 90 (FIG. 6). As previously discussed, the FIM 90 effectively interconnects any terminal in any of the receive zones 32, 34, 36, 38 (FIG. 5) with any terminal in any of the transmit zones 31, 33, 35, 37. The FIM 90 includes four waveguide inputs 120, 122, 124 and 126 for respectively receiving the receive signals R1, R2, R3 and R4. As previously mentioned, receive signals R1-R4, which originate from a corresponding receive zones 32, 34, 36, 38 (FIG. 5), each contain the entire assigned frequency spectrum, (e.g. 500 MHz), and are separated into a plurality of channels (e.g. sixteen 27 MHz channels). The channels are further separated into a plurality of subchannels, where each of the subchannels carries a signal from a corresponding uplink site. The FIM 90 includes 64 filters, one of which is indicated by the numeral 102. Each of the filters 102 has a passband corresponding to one of the channels (e.g. 1403-1430 MHz). The filters 102 are arranged in four groups, one for each receive zone 32, 34, 36, 38, with each group including two banks or subgroups of eight filters per subgroup. One subgroup of filters 102 contains those filters for the even-numbered channels and the other subgroup in each group contains eight filters for the odd-numbered channels. Thus, for example, the filter group for receive signal R1 comprises subgroup 104 of filters 102 for odd channels, and subgroup 106 of filters 102 for even channels. The following table relates the receive signals and zones to their filter subgroups: 
     
         ______________________________________                                    
               Filter Subgroups                                           
Receive Zone                                                              
         Receive Signal                                                   
                     Odd Channels                                         
                                 Even Channels                            
______________________________________                                    
32       R1          104         106                                      
34       R2          108         110                                      
36       R3          112         114                                      
38       R4          116         118                                      
______________________________________                                    
 
    
     The filters are grouped in a unique manner such that when the receive signals R1-R4 are filtered, the filtered outputs are combined to form the transmit signals. The transmit signals T1-T4 also utilize the entire assigned frequency spectrum, (e.g. 500 MHz). In the illustrated embodiment, each of the transmit signals T1-T4 possesses sixteen 27 MHz wide channels, and comprises four channels from each of the four receive zones 32-38 (FIG. 5). 
     The incoming receive signals R1-R4 are divided into the corresponding subgroups by respectively associated hybrid couplers 128-134 which effectively divert 50% of the signal power to each subgroup. Hence, for example, one-half of the R1 signal input at waveguide 120 is diverted to transmission line 136 which services the subgroup 104 of filters 102, and the remaining half of the R1 signal is diverted to transmission line 138 which services subgroup 106 of filters 102. In a similar manner, each of the subgroups 104-118 of filters 102 is served by a corresponding distribution line, similar to lines 136 and 138. 
     The construction of subgroup 104 will now be described in more detail, it being understood that the remaining subgroups 106-118 are identical in architecture to subgroup 104. At intervals along the transmission line 136, there are eight ferrite circulators 140, one associated with each of the odd-numbered channel filters 102. The function of the circulators 140 is to connect the transmission line 136 to each of the odd channel filters 102 in a lossless manner. Thus, for example, the R1 signal enters the first circular 140a and circulates it counterclockwise whereby the 27 MHz band of signals corresponding to channel 1 passes through it to circulator 142. All other frequencies are reflected. These reflected signals propagate via the circulator toward the next filter where the process is repeated. Through this process, the R1 receive signal is filtered into sixteen channels by the sixteen filters 104-108 corresponding to the R1 signals. Hence, the R1 signal with frequencies in the range of channel 1 will pass through the first ferrite circulator 140a and it will be filtered by filter 1 of group 104. 
     The outputs from a filter subgroup 104-108 are selectively coupled by a second set of ferrite circulators 142 which sums, in a criss-cross pattern, the outputs from an adjacent group of filters 102. For example, the outputs of channel filters 1, 5, 9, and 13 of group 104 are summed with the outputs of channel filters 3, 7, 11 and 15 of filter group 112. This sum appears at the output terminal for T1 144. Referring to FIG. 8, these signals correspond to the connections between receive zones R1 and R3 and to transmit zone T1. 
     Attention is now directed to FIGS. 8 and 9 which depict how the transmit and receive signals are interconnected by the FIM 90 to allow two-way communication between any terminals. Specifically, FIG. 8 provides a table showing how the receive and transmit zones are connected together by the interconnect channels while FIG. 9 depicts how these interconnect channels are distributed geographically across the transmit zones 31, 33, 35, 37. In FIG. 8, the receive signals R1-R4 are read across by rows of interconnect channels and the transmit signals T1-T4 are read by columns of interconnect channels. It can be readily appreciated from FIG. 8 that each of the transmit signals T1-T4 is made up of sixteen channels arranged in four groups respectively, where each group is associated with one of the receive signals R1-R4. The satellite communications system of the present invention is intended to be used in conjunction with a ground station referred to as a satellite network control center which coordinates communications between the ground terminals via packet switched signals. The network control center assigns an uplink user with an uplink frequency based on the location of the desired downlink, assigning the available frequency whose downlink longitude is closest to that of the destination. The frequency addressable downlink transmit beams 29 are thus addressed by the frequencies of the uplink signals. This strategy maximizes the gain of the downlink signal. 
     As shown in FIG. 9, the continental United States is divided into four primary zones 31, 33, 35, 37. Zone 31 may be referred to as the East Coast zone, zone 33 is the Central zone, zone 35 is the Mountain zone, and zone 37 is the West Coast zone. As previously mentioned, each of the zones 31, 33, 35, 37 utilizes the entire assigned frequency spectrum (e.g. 500 MHz). Thus, in the case of 500 MHz assigned frequency band, there exists sixteen 27 MHz channels plus guard bands in each of the zones 31, 33, 35, 37. 
     The numbers 1-16 repeated four times above the beams 29 in FIG. 9 indicate the longitude of the beams corresponding to the center frequencies of the channels so numbered. Because of the frequency sensitivity of the beams, the longitude span between the lowest and highest frequency narrow band signal in a channel is approximately one channel width. Each beam is 0.6 degrees wide between its half power point, about half the zone width in the East Coast and Central zones and nearly one-third the zone width in the Mountain and West Coast zones. The antenna beams 29 overlap each other to ensure a high signal density; the more that the beams overlap, the greater channel capacity in a given area. Hence, in the East Coast zone 31, there is a greater overlap than in the Mountain zone 35 because the signal traffic in the East Coast zone 31 is considerably greater than that in the Mountain zone 35. 
     The interconnect scheme described above will now be explained by way of a typical communication between terminals in different zones. In this example, it will be assumed that a caller in Detroit, Mich. wishes to place a call to a terminal in Los Angeles, Calif. Thus, Detroit, Mich., which is located in the Central zone 34, is the uplink site, and Los Angeles, Calif., which is located in the West Coast zone 37, is the downlink destination. As shown in FIG. 9, each geographic location in the continental United States can be associated with a specific channel in a specific zone. Thus, Los Angeles is positioned between channels 14 and 15 in transmit zone 37. 
     Referring now concurrently to FIGS. 5, 8, and 9 particularly, receive and transmit zones R1 and T1 lie within the East Coast zone 32 and 32, R2 and T2 lie within the Central zone 34 and 33, R3 and T3 lie within the Mountain zone 36 and 35, and R4 and T4 lie within the West Coast zone 38 and 37. Since Detroit lies in the Central or R2 zone 34, it can be seen that the only channels over which signals can be transmitted to the West Coast or T4 zone 37 are channels 1, 5, 9 and 13. This is determined in the table of FIG. 8 by the intersection of row R2 and column T4. Therefore, from Detroit, the uplink user would uplink on either channel 1, 5, 9 or 13, whichever of these channels is closest to the downlink destination. Since Los Angeles is located between channels 14 and 15, the network control center would uplink the signal on channel 13 because channel 13 is the closest to channel 14. The downlink beam width is broad enough to provide high gain at Los Angeles. 
     Conversely, if the uplink site is in Los Angeles and the downlink destination is in Detroit, the intersection of row R4 and column T2 in FIG. 8 must be consulted. This intersection reveals that the signal can be transmitted on channels 1, 5, 9 or 13 depending upon which channel is closest to the downlink destination. The network control center would uplink the signal from Los Angeles on channel 9 since channel 9 is closest to channel 11 which, in turn, is closest to Detroit. 
     Returning now to FIG. 10, the conversion of a receive signal to a transmit signal will be described in connection with the example mentioned above in which the uplink site is in Detroit and the downlink site is in Los Angeles. The uplink signal transmitted from Detroit would be transmitted on channel 13 carried by receive signal R2. Thus, the R2 receive signal is input to transmission line 122 and a portion of such input signal is diverted by the hybrid coupler 130 to the input line of subgroup 108 of filters 102. Subgroup 108 includes a bank of eight filters for the odd channels, including channel 13. Thus, the incoming signal is filtered through by filter 13 and is output on a line 164 along with other signals from subgroups 108 and 116. The channel 13 signal present on line 164, is combined by the hybrid coupler 158, with signals emanating from subgroup 106 and 114, and forms the T4 signal on output line 150. The transmit signal T4 is then downlinked to Los Angeles. 
     It is to be understood that the above example is somewhat simplified inasmuch as the network control center would assign a more specific channel than a 27 MHz wide band channel, since the 27 MHz wide channel may actually comprise a multiplicity of smaller channels, for example, 800 subchannels of 32 KHz bandwidth. 
     Referring now again to FIGS. 5, 8 and 9, in the event that an uplink signal originates from one of the areas of contention, 40, 42, 44 (FIG. 5), such signal will not only be transmitted to its desired downlink destination, but a non-neglible signal will be transmitted to another geographic area. For example, assume that the uplink signal originates from Chicago, Ill. which is in the area of contention 42 and that the signal is destined for Los Angeles, Calif. The area of contention 42 is produced by the overlap of the beams forming zones 34 and 36. Hence, the uplink signal can be transmitted as receive signals R2 or R3. The network control center determines whether the uplink communication is carried by receive signals R2 or R3. In the present example, since Chicago is closer to zone 36, the uplink communication is carried on receive signal R3. 
     As previously discussed, the downlink destination, Los Angeles, is located in zone 37 and lies between channels 14 and 15. As shown in FIG. 8, the intersection of R3 with column T4 yields the possible channels over which the communication can be routed. Thus, the Chicago uplink signal will be transmitted over one of channels 2, 6, 10 or 14. Since Los Angeles is closest to channel 14, channel 14 is selected by the network control center as the uplink channel. Note, however, that an undesired signals is also transmitted from zone 34 on channel 14. To determine where the undesired signal will be downlinked, the table of FIG. 8 is consulted. The table of FIG. 8 reveals that uplink signals carried on channel 14 in the R2 zone 34 are downlinked to the T1 transmit zone 31. The desired signal is transmitted to Los Angeles and the undesired signal (i.e. an extraneous signal) is transmitted to the East Coast (i.e. zone 31). The network control center keeps track of these extraneous signals when making frequency assignments. The effect of these extraneous signals is to reduce slightly the capacity of the system. 
     Referring now again to FIG. 6, the beam-forming network 98 receives the transmit signals T1-T4 and functions to couple all of the individual communication signals in these transmit signals together so that a transmit antenna beam for each signal is formed. In the example discussed above in which the assigned frequency spectrum is 500 MHz, a total of approximately 50,000 overlapping antenna beams are formed by the beam-forming network 98 when the system is fully loaded with narrow band signals. Each antenna beam is formed in a manner so that it can be pointed in a direction which optimizes the performance of the system. The incremental phase shift between adjacent elements determines the direction of the antenna beam. Since the phase shift is determined by the signal frequency, the system is referred to as frequency addressed. 
     Attention is now directed to FIGS. 11 and 12 which depict the details of the beam-forming network 98. The beam-forming network, generally indicated by the numeral 98 in FIG. 11, is arranged in the general form of an arc and may be conveniently mounted on the communication shelf (not shown) of the satellite. The arc shape of the beam-forming network 98 facilitates an arrangement which assures that the paths of the signals passing therethrough are of correct length. 
     The beam-forming network 98 includes a first set of circumferentially extending transmission delay lines 168, 170, a second set of transmission delay lines 172, 174 which are radially spaced from delay lines 168 and 170, and a plurality of radially extending waveguide assemblies 176. In the illustrated embodiment, forty waveguide assemblies 176 are provided, one for each of the elements 106 of the transmit array 20 (FIG. 13). The waveguide assemblies 176 intersect each of the delay lines 168-174 and are equally spaced in angle. 
     Each of the waveguide assemblies 176 defines a radial line summer and intersects each of the delay lines 168-174. As shown in FIG. 12, at the points of intersection, between the radial line summers 176 and the transmission delay lines 168-174, a crossguide coupler 180 is provided. The crossguide coupler 180 connects the delay lines 168-174 with the radial line summers 176. The function of the crossguide couplers 180 will be discussed later in more detail. 
     Four delay lines 168-174 are provided respectively for the four transmit zones T1-T4 (FIG. 9). Hence, transmit signal T1 is provided to the input of delay line 170, T2 is provided to input of delay line 168, T3 is provided to the input of delay line 174, and T4 is provided to the input of delay line 172. As shown in FIG. 12, the distance between the radial line summers is indicated by the letter &#34;l&#34; and the width of each of the radial delay lines is designated by the letter &#34;w&#34;. Although the radial line summers 176 are spaced at equal angular intervals along the delay lines 168-174, the distance between them varies from delay line to delay line due to the fact that the delay lines 168-174 are radially spaced from each other. Thus, the further from the center of the arc, which is formed by the radial line summers 176, the greater the distance between the radial line summers 176, at the point where they intersect with the delay lines 168-174. In other words, the spacing &#34;l&#34; between radial line summers 176 for delay line 168 is less than the spacing between adjacent radial line summers 176 than for delay line 174. Typical values for the dimensions &#34;l&#34; and &#34;w&#34; are as follows: 
     
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Delay Line  Signal    l, inches  w, inches                                
______________________________________                                    
168         T2        1.66       0.64                                     
170         T1        1.72       0.66                                     
172         T4        2.45       0.74                                     
174         T3        2.55       0.76                                     
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     The width of the delay lines 168-174, &#34;w&#34;, and the distance &#34;l&#34; between adjacent radial line summers are chosen to provide the desired center beam squint and beam scan rate so that the beam pointing is correct for each channel. This results in the desired start and stop point for each of the transmit zones T1-T4. 
     Referring particularly to FIG. 12, the transmit signal T2 propagates down the delay line 168 for a precise distance, at which point it reaches the first radial line summer 176. A portion of the T2 signal passes through the crossguide coupler 180, which may, for example, be a 20 dB coupler, such that one percent of the transmitted power of transmit signal T2 is diverted down the radial line summer 176. This diverted energy then propagates down the waveguide 176 toward a corresponding solid state power amplifier 100 (FIGS. 6 and 11). This process is repeated for signal T1 which propagates down delay line 170. The portions of signals T1 and T2 which are diverted by the crossguide couplers 180 (i.e. 0.01 T1 and 0.01 T2) are summed together in the radial line summer 176 and the combined signal 0.01 (T1+T2) propagates radially outwardy toward the next set of delay lines 172, 174. This same coupling process is repeated for signals T3 and T4 in delay lines 174 and 172 respectively. That is, 0.01 of signals T3 and T4 are coupled via crossguide couplers 180 to the radial line summer 176. The resulting combined signal 0.01 (T1+T2+T3+T4) propagates radially outwardly to an associated solid state power amplifier 100 where it is amplified in preparation for transmission. 
     After encountering the first radial line summer 176, the remaining 0.99 of signals T1-T4 propagate to the second radial line summer where an additional one percent of the signals is diverted to the summer 176. This process of diverting one percent of the signals T1-T4 is repeated for each of the radial line summers 176. 
     The signals, propagating through the radial line summers 176 towards the SSPAs 100, are a mixture of all four point-to-point transmit signals T1-T4. However, each of the transmit signals T1-T4 may comprise 12,500 subsignals. Consequently, the forty signals propagating through the radial line summers 176 may be a mixture of all 50,000 signals in the case of the embodiment mentioned above where the assigned frequency spectrum is 500 MHz wide. Therefore, each of the SSPAs 100 amplifies all 50,000 signals which emanate from each of the plurality of wave guide assemblies 176. 
     Since each of the SSPAs 100 amplifies all 50,000 signals which are destined for all regions of the country, it can be appreciated that all of the narrow, high gain downlink beams are formed from a common pool of transmitters, i.e. all of the SSPAs 100. This arrangement may be thought of as effectively providing a nationwide pool of power since each of the downlink beams covering the entire country is produced using all of the SSPAs 100. Consequently, it is possible to divert a portion of this nationwide pool of power to accommodate specific, disadvantaged downlink users on an individual basis without materially reducing the signal power of the other beams. For example, a downlink user may be &#34;disadvantaged&#34; by rain in the donwlink destination which attenuates the signal strength of the beam. Such a rain disadvantaged user may be individually accommodated by increasing the signal strength of the corresponding uplink beam. This is accomplished by diverting to the disadvantaged downlink beam, a small portion of the power from the pool of nationwide transmitter power (i.e. a fraction of the power supplied by all of the SSPAs 100). The power of an individual uplink beam is proportional to that of the corresponding downlink beam. Consequently, in order to increase the power of the downlink beam it is merely necessary to increase the power of the uplink beam. 
     In practice, the previously mentioned network control center keeps track of all of those regions of the country in which it is raining and determines which of the uplink users are placing communications to downlink destinations in rain affected areas. The network control center then instructs each of these uplink users, using packet switched signals, to increase its uplink power for those signals destined for a rain affected area. The increase in power of the uplink user&#39;s signals results in greater collective amplification of these signals by the SSPAs 100, to produce corresponding downlink beams to the rain affected areas, which have power levels increased sufficiently to compensate for rain attenuation. Typically, the number of signals destined for rain affected areas is small relative to the total number of signals being handled by the total pool of SSPAs 100. Accordingly, other downlink users not in the rain affected zones do not suffer substantial signal loss since the small loss that may occur in their signals is spread out over the many thousand users. 
     The SSPAs 100 (FIGS. 8 and 11) may be mounted for example, on the rim of the communication shelf (not shown) of the satellite. The signals amplified by the SSPAs 100 are fed into the corresponding elements 106 of the transmit array 20 (FIGS. 13 and 14). 
     As previously discussed, an incremental phase shift is achieved between the signals that are coupled in the forty radial line summers 176. Hence, the beam-forming network 98 permits the antenna beams emanating from the transmit array 20 (FIGS. 1, 2, and 13) to be steered by frequency assignment. The incremental phase shift is related to the time delay between the waveguides 176 as well as frequency. Attention is now directed to FIG. 17 which is a diagrammatic view of four of the forty transmit array elements 106 (FIG. 13), showing the wavefront emanating therefrom, wherein &#34;d&#34; is equal to the spacing between transmit array elements 106. The resulting antenna beam has an angular tilt of θ, where θ is defined as the beam scan angle. This means that θ is the angle from normal of the transmit beam center. The incremental phase shift produced by the delay line arrangement is ΔΦ. The relationship between ΔΦ and θ is given by ##EQU1## where: λ=signal wavelength 
     θ=beam scan angle 
     d=spacing between array elements. 
     Hence, the east-west direction of the antenna beam is determined by the incremental phase shift which is different for the four delay lines 168-174 of the beam-forming network 98, resulting in the four transmit zones T1-T4 previously noted. 
     Having thus described the invention, it is recognized that those skilled in the art may make various modifications or additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.