Patent Publication Number: US-6336030-B2

Title: Method and system for providing satellite coverage using fixed spot beams and scanned spot beams

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 09/159,332 filed Sep. 23, 1998, which is a continuation-in-part of Ser. No. 08/867,672 filed Jun. 2, 1997, now issued as U.S. Pat. No. 6,125,261, issue date Sep. 26, 2000. 
     This invention is a continuation-in-part of co-pending application Ser. No. 08/867,672 FILED Jun. 2, 1997 entitled “Method And System For Communicating High Data Rate In a Satellite-Based Communications Network”, having the same assignee as the present invention, and which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to methods and systems for communicating high rate data to customers in satellite-based communications networks. 
     BACKGROUND OF THE INVENTION 
     A number of applications continue to drive the need for high-speed data transport. Industry specific examples include remote film editing, medical image transport, and financial service data consolidation and backup. Business communications and training needs further accelerate information transfer needs across all sectors. As business, government and educational institutions disseminate more information, greater importance is attached to data transfer. In this environment, reliable, high-speed video and data transport becomes even more critical. 
     Furthermore, a tremendous growth in Internet traffic has caused a strain on the capacity of telephony networks. Network shortcomings include network outages, insufficient access bandwidth, and insufficient internode bandwidth. Currently, providers need to make significant investments, as well as experience installation delays, to upgrade network infrastructure, yet they cannot pass the costs on to the end users. 
     Corporate LANs/WANs also generate an insatiable demand for higher bandwidth. The demand for bandwidth goes up as more and more users are connected. The users, in turn, demand more services and improved network speed. Personal computers are being used to process not only text, but graphics and video as well, all on networks that are increasingly global. Widespread implementation of corporate intranets and extranets further drive the move to increased bandwidth applications. High-speed networking is also driven by the growth of video distribution, client/server technology, decentralized systems, increased processing power and developments in storage capacity. 
     Thus, it is important to relieve congestion among the heavily used communications links in high-density areas and to bring such service to isolated rural areas which have not been able to participate fully in the communications world. While existing satellite systems offer ubiquitous service, they do not offer direct connection to the end user at moderate to high data rates. Existing Fixed Satellite Service (FSS) systems employ wide channel bandwidths and relatively large beamwidths making them more suited to point-to-point trunking service rather than to end user connectivity. The wide area coverage, limited Equivalent Isotropically Radiated Power (EIRP), and constrained flexibility of these systems makes any attempt to serve many small users both inefficient and costly. 
     The emerging cellular type satellite services serve a very large number of potential subscribers but only at very low data rates. The on-board processing and packet-switched nature of their signal structure severely limits the practical user data rates that can be accommodated within the technology limitations of the processor. Thus, there exists a need for a satellite communications system that serves the demand for high data rate business users including the high-end individual as well as small business users that demand direct and affordable connection. 
     It would therefore be desirable to provide a satellite-based communications network providing reliable high data rate communications service to customers throughout the world while maintaining flexibility to reconfigure the beam patterns often to tailor the beam pattern according to user needs. 
     SUMMARY OF THE INVENTION 
     In carrying out the above features, and advantages of the present invention, a satellite-based communications network provides a plurality of communications satellites each having uplink and downlink antennas capable of receiving and transmitting a plurality of signals utilizing a plurality of beams having fixed spot beams and scanned spot beams to a plurality of spot coverage areas and a plurality of scanned spot areas respectively at a predetermined range of frequencies. The plurality of satellites each have receiving and transmitting beam forming networks coupled to the uplink and downlink antennas respectively. The antennas have adjacent reconfigurable receiving and transmitting antenna elements. A controller located on the satellite is coupled to said beam forming network, the controller selecting either a fixed spot beam or a scanned spot beam. 
     One advantage of the invention is that the satellite system allows the use of both fixed and scanned spot beams from the same satellite and same antenna. This is important in high frequency applications such as those in V-band because the beams in high frequency bands generate narrow beams. Thus, blanket coverage for large land areas such as the continental United States (CONUS) is difficult without an inordinate number of satellite beams. The present invention allows fixed coverage over high traffic area while allowing scanned beams to be quickly moved between areas not requiring a dedicated fixed beam. 
     The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation illustrating a satellite communication system of the present invention. 
     FIG. 2 is a schematic block diagram illustrating a communications subsystem within the satellites of the present invention. 
     FIG. 3 is a schematic illustration of the constellation of communications satellites utilized in the present invention. 
     FIG. 4 is a schematic illustration of a portion of a constellation of communications satellites utilized in the present invention. 
     FIG. 5 is a schematic illustration of a satellite operating both scanned beams and fixed beams. 
     FIG. 6 is a block diagram of a transmitter according to the present invention. 
     FIG. 7 is a block diagram of a receiver according to the present invention. 
     FIG. 8 is a schematic view of a first example of a suitable antenna. 
     FIG. 9 is a schematic view of a second example of a suitable antenna. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a satellite-based communications network  10  with a typical geometry for practicing the present invention is diagrammatically illustrated. In general, the network  10  includes a plurality of communications satellites  12  in geosynchronous orbit, a ground station  14  for controlling and maintaining operation of each of the plurality of satellites  12 , and a plurality of user terminals  16 . The user terminals  16  may interconnect with a single computer  18 , a group of networked PC/Workstation users  20 , a group of linked mini/main frame users  22 , a mega computer  24 , or a service provider  26  that provides service to any number of independent systems  28 . 
     The geosynchronous satellites  12  are positioned in orbit locations supporting Fixed Satellite Service (FSS) coverage for domestic service and accommodating a primary range of frequencies and a secondary range of frequencies, such as 50/40 GHz V-band as well as 13/11 GHz Ku-band operation. The locations of satellites  12  must accommodate emissions along with other co-orbiting satellites, and must support service to and from high population metropolitan and business areas throughout the world. The ground terminal elevation angles to satellites  12  must be 30 degrees or greater to minimize adverse propagation effects especially in the presence of rain and other disturbances. The preferred orbit locations include four satellites over the U.S., two each at 99EW and 103EW. To accommodate global growth and provide coverage to western Europe, central Europe, Middle East, and Africa, the preferred orbit locations further include eight other satellites, two each at 10EE and one at 63EW, 53EW, 48EE, 63.5EE, 115.4EE, and 120.6EE. 
     Each of the satellites  12  are high power satellites having 15-20 KW payload capability, such as an HS 702L High Power Spacecraft manufactured by Hughes Electronics Corporation, the assignee of the present invention. The HS 702L is a three-axis body-stabilized spacecraft that uses a five panel solar array system, along with outboard radiator panels attached to the main body to dissipate heat generated from the high powered Traveling Wave Tubes (TWTs). A schematic block diagram illustrating a communications subsystem, or payload,  30  within satellites  12  is shown in FIG.  2 . 
     The payload  30  includes a primary communication payload  32 , a secondary communication payload  34 , an inter-hemisphere link  35 , and an intersatellite link  36 . Primary communication payload  32  supports the majority of the communications signals. Secondary communication payload  34  is utilized for thin route satellite traffic and as back up for weather outages of primary communication payload  32 . Primary communication payload  32  operates preferably in the 50/40 GHz FSS region, or any other similar high frequency range, to provide high capacity service and utilizes 3 GHz of spectrum (47.2 to 50.2 GHz) for uplinks and another 3 GHz of spectrum (38.6 to 41.6 GHz) for downlinks. Data rates from 1.544 Mbps (equivalent to T1) to 155 Mbps (OC3 equivalent) can, thus, be supported. Users operating at data rates below the Ti level can be accommodated by submultiplexing the signals at the user terminal  16 . Secondary communication payload  34  preferably operates in the 13/11 Ku-band FSS region with 500 MHz of bandwidth to provide ubiquitous thin route and high link availability service and connection between the northern and southern hemispheres. 
     Primary communication payload  32  includes an uplink antenna  38  and a downlink antenna  44  for receiving and transmitting spot beams carrying signals at the primary range of frequencies. Narrow spot beams allow a greater power to be received and transmitted in the area it covers, thereby supporting higher data rates than those of wider beams. A single antenna can produce many spot beams. Many small feed horns are positioned so that their signals are reflected in narrow beams by a parabolic dish of the antenna. Different antenna feeds are switched on and off, via uplink antenna beam switch  39  and downlink antenna beam switch  41 , thereby selecting the spot beam to be used in each case. Not only do satellites with multiple narrow beam antennas give a higher EIRP per beam, but the same frequency and bandwidth can also be reused several times for different portions of the earth. Even further, if the spot beams also have dual polarization capability, the number of beams is doubled, thereby increasing spectral reuse also by a factor of two. For example, for twenty spot beams each with dual polarization, the spectral reuse if forty times. 
     In the present invention, a surface, or area, such as CONUS, to receive communications services of the present invention, is divided into a plurality of coverage areas  43 , as shown in FIG.  3 . Uplink and downlink antennas  38 , 44 , respectively, can support a predetermined number of coverage areas  43 , e.g.,  200 . However, a subset of the plurality of coverage areas  43  is chosen to be used by uplink and downlink antennas  38 , 44 , respectively, to support communications services in predetermined metropolitan areas having heavy traffic. This configuration is controlled by a routing table  45  stored on the payload  30 . Thus, the spot beams  43  are semi-fixed in position, until reconfigured at a later time. Reconfiguration of uplink and downlink antenna beam switches  39 , 41 , respectively, is possible by updating routing table  45  as needed. This updated information is transmitted by ground station  14 . Thus, usage of available satellite resources, such as weight and power, are utilized for only those beams that are selected and active. 
     Preferably, uplink antenna  38  and downlink antenna  44  each consists of an east-mounted and a west-mounted multifeed antenna assembly having a multibeam array  40 , 44  and a reflector  42 , 48  to provide CONUS and coastal coverage. The offset parabolic reflectors  42 , 48  are deployed from the east and west side of the satellite  12 , yet the feed arrays  40 , 44  are fixed to the nadir face and do not deploy. Each reflector  42 , 48  is populated by a fifty-one horn dual circularly polarized feed array  40 , 44 . Each horn of the feed array  40 , 44  is diplexed for both transmit and receive frequencies. In addition, each horn provides either a single sense of circular polarization or dual circular polarization. Consequently, there are 400 total input ports to the 200 horns that comprise the antenna assemblies  38 , 44 . Alternatively, uplink and downlink antennas  38 , 44  may comprise a phased array antenna. 
     As discussed above, antenna beam switches  39 ,  41  select twenty spot beams from the 200 horn array, each with dual circular polarization to provide forty beams per satellite. Each beam and each polarization makes full use of the 3 GHz of spectrum with a total of forty times spectrum reuse (120 GHz) in all. The selected forty spot beams  43  are directed towards major metropolitan population centers and business areas included within the ubiquitous area, as shown in FIG.  3 . In addition, any twenty beams included in the uplink array of receive beam locations and any twenty of the array of downlink beam locations can be selected independently of each other on orbit to accommodate variations in traffic or satellite relocation at a later date. 
     Each beam is divided into ten Frequency Division Multiple Access (FDMA) channels, with each channel nominally 300 MHz wide, including guard bands. Each FDMA channel is divided into 100 Time Division Multiple Access (TDMA) channels, with each TDMA channel having a nominal burst rate of 150 Mbps. Thus, a total of 100 users may use the same frequency channel in the same beam. Upon subscribing to the service provided by the network  10  of the present invention, a dedicated communications link is assigned to a user at a source location in one of the coverage areas  43  and a user at a destination location in another one of the coverage areas  43 . This dedicated link is assigned an exclusive time channel in one of the frequency channels for transmitting and receiving communications signals. 
     Satellite payload  30  includes a Time Division Multiple Access (TDMA) circuit switch  62  operating at a suitable intermediate frequency (IF). Circuit switch  62 , driven by routing table  45 , provides interlinking of all beams, services and users and dedicated point-to-point and point-to-multipoint services. Circuit switch  62  circuit switches signals to be transmitted either to the same uplink beam as the source signal was transmitted from or by another downlink beam based on the time interval assigned the source signal according to routing table  45 . Circuit switch  62  is gated within the time domain to provide precise windows of time to different desired outputs. Loopback information is included within the transmission to provide necessary synchronization of user terminals  16  with the satellites  12 . Circuit switch  62  also routes crosslink traffic as well as traffic to and from the hemispheric coverage beam, discussed below. 
     As with primary communication payload  32 , secondary communication payload  34  includes an uplink antenna  50  having a multibeam array  52  and a reflector  54 , and a downlink antenna  56  having a corresponding multibeam array  58  and reflector  60 . Secondary communication coverage is preferably provided by two nadir-mounted dual-gridded reflector antennas, each illuminated by eight diplexed feeds for transmit and receive frequencies. Secondary communication antennas  50 , 56  provide a total of eight dual polarized, elliptical area (3E×1E) coverage beams  57 , as shown in FIG. 3, for uplink and downlink services. Thus, secondary communication payload  34  provides an eight-fold reuse of the spectrum for a total useable bandwidth of 4 GHz. 
     To provide for inter-hemisphere interconnectivity, inter-hemisphere link  35  includes a single steerable horn  61 , diplexed for transmit and receive frequencies providing one dual linearly polarized spot beam for uplink and downlink services. Horn  61  transmits a 6E×6E, 13/11 GHz area beam  63  towards the southern hemisphere, allowing thin route coverage of southern regions such as South America, as shown in FIG.  3 . This beam may also provide north-south interconnection coverage to areas such as Europe and Africa. 
     Intersatellite link  36  is included so that traffic from one satellite covering a particular region or selected metropolitan areas can be linked to a second satellite covering the same or other areas and regions. Intersatellite link  36  may be an optical (or laser) link operating in the 1.6 micron region via two 9 inch laser-telescope assemblies  71 , 73 . Alternatively, intersatellite link  36  may be a radio frequency (RF) link operating in the 60 GHz region. Data is frequency converted and multiplexed to provide a maximum 3 Gbps data rate for inter-satellite connectivity. 
     Returning to FIG. 1, user terminals  16  include a primary antenna  64  for communicating with each of the satellites  12  in the primary range of frequencies, such as V-band frequencies. Thus, user terminals support data rates between 1.544 Mbps (equivalent to T1) and 155 Mbps (OC3 equivalent) via V-band antenna  64 . Data rates below T1 are accommodated at user terminals  16  by submultiplexing the data to T1 (or higher) rates before transmission. Each of the user terminals  16  time-share the FDMA channels, with 100 TDMA channels in each 300 MHz FDMA channel. Since each TDMA channel supports a data rate of 1.544 Mbps, the network  10  provides a data throughput rate of 1.544 Gbps (100×1.544 Mbps×10) for each of the forty effective beams per satellite  12 . For each FDMA channel, the channel data rate is 274.8 Mbps, which includes overhead for coding, transport protocol, network signaling, and access management. Uplink operation at each of the user terminals  16  operates in a burst mode at a data rate determined by the full FDMA channel plan. 
     Thirty watt high power amplifiers (HPA&#39;s) operate at saturation in the user terminals  16 , with the user terminals  16  in each beam operating time shared on one of ten unique carrier frequencies. Out of band emissions are minimized in each user station  16 . Each of the forty 3.0 GHz bandwidth beams is received and down converted, routed through circuit switch  62 , upconverted, and amplified by a TWTA associated with a particular downlink beam. The downlink beams each have ten carriers, one for each FDMA channel. Each TWTA uses linearizers and operates with sufficient output backoff to ensure minimum out of band emissions and intermodulation products. 
     User terminals  16   a  that cannot tolerate the expected loss of transmission due to weather outages further include a secondary communication antenna  65  for transmitting and receiving signals at the secondary range of frequencies. Secondary communication antenna  65  may or may not be the same as the primary communication antenna  64 . User terminals  16   a  subscribing to this type of service include a link quality monitoring center  69  to monitor the quality of service of primary communication payload  32  and routes it to a higher quality link, i.e., secondary communication payload  34 , in the presence of adverse link propagation disturbance. The rerouting of traffic to a higher availability link is accomplished by communicating such conditions to ground station  14 . 
     As discussed above, each of the satellites  12  are also in communication with a ground station  14 . Ground station  14  has two functions. Satellite control center  68  manages the health and status of all the satellites  12  and maintains their orbits. If rain attenuation exceeds the link budget allocation at the primary range of frequencies, e.g., 50/40 GHz, the satellite  12  shall be commanded by satellite control center  68  to provide service via the secondary communication payload  34  until the weather front passes, at which time the satellite  12  is restored to primary services. Secondary communication payload  34  resource is then free to backup another metropolitan area, if needed, or to provide ubiquitous thin route services over CONUS. To be an effective backup, secondary communication payload  34  must have a sufficient capacity, on the order of 10% of the primary communication payload  32 , in order to backup the entire service. 
     Network operations center  70  of ground station  14  provides resource management, fault management, accounting, billing, customer interfacing, and service. Ground station  14  is preferably placed in low-rain sections of friendly countries so as to provide line-of-sight communications with each of the satellites  12 . 
     The network of the present invention provides communications capabilities that will significantly contribute to the National and Global Information Infrastructures. It provides high data rate communications to customers throughout the United States and most of the rest of the world as well. The system provides true broadband capability, including high speed access to the Internet in particular and high-technology telecommunications in general. The innovative design of the system insures that this capability can be provided at a much lower cost than installing fiber, thereby taking advantage of the distance insensitivity of satellite-based service. It is also particularly attractive at making first and last mile connections, which is a problem with the present copper and optical fiber cable systems. 
     Referring now to FIG. 4, the present invention is suitable for use in a satellite system  110  generally depicted by a first satellite  112  and a second satellite  112 . Satellites  112  and  114  are generally interconnected by an optical link generally represented by arrows  116 . As shown on map  118 , the footprint  120  of various regions of desired coverage are illustrated. A second beam  121  is generated by satellite  112 . Second beam  121  may have a different diameter than beams  120 . By using different satellites the size of the beam may be different between the satellites. Satellites  112 ,  114  may be MEOs, LEOs or GEOs. As mentioned above, satellites  114  and  112  may be reconfigured to provide coverage for each of footprints  120 . One problem, however, with satellites  112  and  114  is that if a high frequency is used, the beams generated by satellites  112  and  114  cover only a narrow area. Therefore, coverage may not extend entirely over CONUS. 
     Referring now to FIG. 5, satellite  114  generates a plurality of fixed spot beams  122 . Fixed spot beams may be repositioned as described above. The spot beams are used to both transmit and receive information. To further expand the coverage provided by satellite  114 , a plurality of scanned spot beams  124  may also be used. As illustrated, one scanned spot beam  124  with footprints  126  is illustrated. Scanned spot beams  124  may be used to provide coverage to lower traffic areas which would otherwise not fully utilize a fixed spot beam  122 . The combination of the scanned spot areas more fully utilize the beam capacity. Also, the use of scanned beams may be used in conjunction with other beams in the same area. That is, beams of narrow diameter may be interlaced with wider beams. This would promote frequency reuse in highly populated regions. 
     Both scanned spot beams  124  and fixed spot beams  122  are generated from the same antennas. As will be described below, each of the transmitting and receiving elements of the antenna may be reconfigured to be fixed spot beams  122  or scanned spot beams  124 . 
     Referring now to FIG. 6, signals to be transmitted to earth are routed through switch  126 . Switch  126  may, for example, be a TDMA switch shown at  62  in FIG.  2 . Switch  126  directs signals to be transmitted through power dividers  128  to beam forming networks  130 . Usually, a plurality of beams are formed by the satellite. Each beam has a beam forming network  130  and a power divider  128 . 
     Beam forming networks  130  are coupled to a transmitting antenna  132 . Preferably, beam forming networks are coupled to transmitting antenna  132  through a summer  134  and a solid state amplifier  136  in a conventional manner. Transmitting antenna  132  may, for example, be a multiple beam array antenna or a phased array antenna. Of course, other types of antennas may be used according to the teachings of the present invention. Transmitting antenna  132  has a plurality of transmitting elements  138 . Transmissions from transmitting elements  138  form a beam that is transmitted to the earth. As described above, the beams generated by transmitting elements  138  may be fixed or scanned. Each beam forming network  130  has a plurality of beam forming elements  140 . As illustrated, beam forming elements  140  have a phase coefficient  142  and an amplitude coefficient  144 . Each of phase coefficients  142  and amplitude coefficients  144  are independently tunable or adjustable. 
     Generally, to form a scanned beam, only phase coefficients  142  within the associated beam forming network needs to be adjusted. The phase elements provide a time delay which, when summed with the other elements of the associated beam forming network, effectively steers the beam away from the perpendicular to the antenna array plane. It is believed that up to 1,000 changes of phase coefficient  142  may be performed per second. Preferably, each transmitting element  138  has an associated phase coefficient  142 . This allows the satellite to have maximum versatility. To have a fixed beam, the phase coefficient  142  may remain stable or unchanged. 
     In certain situations, it may also be helpful to adjust amplitude coefficient  144 . This may be done to assist in beam steering. By changing amplitude coefficients  144  with the associated beam forming network, better control of side lobe levels of the transmitted beam may be achieved. That is, at the edge of the beam, the amplitude levels of the transmitted signal is preferably reduced. This prevents or reduces interference with adjacent beams. By reducing interference between adjacent beams, frequency reuse is promoted. 
     A controller  146  controls the operation of beam forming network and controls the calculation of the phase coefficients  142  and amplitude coefficients  144 . Although controller  146  may have other functions, the controller of the present invention at least has a scanning beam logic algorithm  148 , a beam-forming processor  150 , and an amplitude phase weighting coefficient generator  152 . Controller  146  has a plurality of beam characteristic inputs  154 . Although a variety inputs may be used for beam forming, a current beam position input  156 , an antenna beam width input  158 , an antenna side lobe level input  160 , and a beam sharing request input  162  are illustrated. Various events take place in controller  146  to insure that the generated beam has the proper direction and does not interfere with other beams that are being generated or that have been requested to be generated by the satellite. Scanning beam logic algorithm  146  uses current beam position input  156  and beam sharing request  162  to determine whether the beam that is to be generated will interfere with any of the beams that are or will be generated by antenna elements  138 . Antenna beam width input  158  may also be used. Antenna beam width input  158  is generated by a calibration that is typically performed before the satellite is launched. By knowing the geometry of the transmitting elements  138 , an actual beam can be transmitted and measured for each of the elements. Due to the great distances that signals are transmitted, a slight change in the geometry of the transmitting element  138  may cause the beam width to vary. Beam forming processor  150  is coupled to scanning beam logic algorithm  148 . Once scanning beam logic algorithm  148  checks to determine whether the beam that is requested is acceptable, beam forming processor  150  chooses the proper beam forming network for the generation of the requested beam. 
     Amplitude phase and weighting coefficients portion of controller  146  generates phase coefficients  142  and amplitude coefficients  144 . As discussed above, phase coefficients  142  are used to direct the beam to the proper angle with respect to the array plane. Also as described above, it may be important to control the antenna side lobe levels of the generated beam. By controlling amplitude coefficients  144  and by monitoring antenna side lobe level input  160 , amplitude and phase weighting coefficient generator  152  may properly control the side lobe levels to prevent interference with adjacent beams. 
     Referring now to FIG. 7, a receiving network  164  is illustrated. Receiving network  164  operates in a similar manner to that described above with respect to the transmitting network illustrated in FIG. 6 except that the signals originate with an uplink antenna  166 , which transmits the signals ultimately to a switch  168 . Switch  168  may be the same switch as switch  126  of FIG.  6 . Uplink antenna  166  has a plurality of receiving elements  170 . Each receiving element  170  is coupled to a low noise amplifier  172  and a power divider  174 . Each receiving element  170  is coupled through low noise amplifiers (LNA)  172  and power dividers  174  to receiving beam forming networks  176 . Preferably, the signal is amplified in low noise amplifier prior to power division. 
     Receiving beam forming networks  176  have beam forming elements  178  similar to that of beam forming elements  140  of FIG.  6 . Beam forming elements  178  have phase coefficients  180  and amplitude coefficients  182 . Phase coefficients  180  essentially steer the direction of the beam of the receiving elements  170  in a similar manner as that described in conjunction with transmitting by using different phase coefficients  142 . The sum of each beam forming element  178  is added together in summer  184 . Summer  184  forms the signal which is transmitted to switch  168  within the satellite. The received beam may be retransmitted through the same satellite or may be transmitted to another satellite by switch  168  through an optical interface as described above. 
     The controller  146 ′, scanning beam logic algorithm  148 ′, beam forming processor  150 ′, and amplitude and phase weighting coefficient generator  152 ′ all operate in a similar manner and based on beam characteristics  154 ′. The operation of processor is essentially the same as that described in FIG.  6 . 
     Referring now to FIG. 8, a near-field Cassegrainian antenna  186  is illustrated as one of many suitable antennas. Antenna  186  has a reflector  188  and a subreflector  189 . In this example, a phased-array feed  190  has a plurality of transmitting elements  191 . The transmitting elements  191  are coupled to a beam forming network as described above. 
     Referring now to FIG. 9, an offset-fed Gregorian antenna is illustrated having a phase-array feed  194  having transmitting elements  195 . Transmitting elements  199  transmit or receive signals from an offset subreflector  196  that directs the signals to or receives signals from a reflector  197 . 
     The antenna designs illustrated in FIGS. 8 and 9 are representative of two suitable antenna designs. However, other antenna designs may also be suitable for use in transmitting and receiving various signals using scanned beams. 
     In operation, the beams for transmitting and receiving signals to and from a ground station are similar. The satellite  114  has several fixed spot beams and scanned spot beams that are allocated according to the usage of the satellite. As described above, cities or regions of the country having sufficient usage to support a dedicated fixed spot beam have phase parameters that remain fixed unless reconfigured. Scanned spot beams  126  may scan several regions of land mass so that the full capacity of the scanned spot beam is extensively utilized. To obtain the proper direction, the phase coefficients of the beam forming network are adjusted. The processor calculates the proper phase coefficient by reviewing several inputs such as the current beam position, the antenna beam width, the antenna side lobe levels, and the beam sharing requests of the other beams. 
     If side lobe characteristics are important, the side lobe input  160  may be used to monitor the side lobe characteristics. Amplitude and phase weighting coefficient generator  152  may be used to generate various amplitudes to be directed to beam forming network. By controlling the side lobe of the formed beam, interference with other beams may be minimized or eliminated. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.