Abstract:
A communication spacecraft generates a plurality of spot beams having a given spatial relationship. Four beams of the plurality are used to maintain the antenna aligned with a ground station. The four beams are sequenced about the ground location, and the signal characteristics of each beam are used to maintain pointing. The sequencing requires continual switch operation, and failure to maintain antenna position, as might be occasioned by the failure of an element such as a switch or oscillator, is ameliorated by a redundant source of beacon signals, together with an arrangement which duplicates the beacon signal, and uses redundant switch cascades and combiners to perform the switching required for sequencing the beams.

Description:
FIELD OF THE INVENTION 
     This invention relates to a high-reliability spacecraft arrangement in which overlapping antenna beams are sequentially generated, as for control of the pointing of an antenna. 
     BACKGROUND OF THE INVENTION 
     This invention relates to spacecraft for cellular communications systems, and more particularly to such systems which provide coverage between terrestrial terminals in a region by way of a spacecraft, where some of the terrestrial terminals may be mobile terminals, and some may be gateways which link the cellular system with a terrestrial network such as a public switched telephone network (PSTN). 
     A salient feature of a spacecraft communication satellite is that all of the electromagnetic transmissions to the user terminals originate from one, or possibly a few, spacecraft. Consequently, the spacecraft communication antenna must form a plurality of beams, each of which is directed toward a different portion of the underlying target region, so as to divide the target area into cells. The cells defined by the beams will generally overlap, so that a user communication terminal may be located in one of the beams, or in the overlap region between two beams, in which case communication between the user communication terminal and the spacecraft is accomplished over one of the beams, generally that one of the beams which provides the greatest gain or signal power to the user terminal. Operation of spacecraft communication systems may be accomplished in many ways, among which is Time-Division Multiple Access, (TDMA), among which are those systems described, for example, in conjunction with U.S. Pat. Nos. 4,641,304, issued Feb. 3, 1987, and 4,688,213, issued Aug. 18, 1987, both in the name of Raychaudhuri. Spacecraft time-division multiple access (TDMA) communication systems are controlled by a controller which synchronizes the transmissions to account for propagation delay between the terrestrial terminals and the spacecraft, as is well known to those skilled in the art of time division multiple access systems. The TDMA control information, whether generated on the ground or at the spacecraft, is ultimately transmitted from the spacecraft to each of the user terminals. Consequently, some types of control signals must be transmitted continuously over each of the beams in order to reach all of the potential users of the system. 
     More specifically, since a terrestrial terminal may begin operation at any random moment, the control signals must be present at all times in order to allow the terrestrial terminal to begin its transmissions or reception (come into time and control synchronism with the communication system) with the least delay. 
     When the spacecraft is providing cellular service over a large land mass, many cellular beams may be required. In one embodiment, the number of separate spot beams is one hundred and forty. As mentioned above, each beam carries control signals. These signals include frequency and time information, broadcast messages, paging messages, and the like. Some of these control signals, such as synchronization signals, are a prerequisite for any other reception, and so may be considered to be most important. When the user communication terminal is synchronized, it is capable of receiving other signals, such as paging signals. 
     Communication spacecraft are ordinarily powered by electricity derived from solar panels. Because the spacecraft may occasionally go into eclipse, the spacecraft commonly includes rechargeable batteries and control arrangements for recharging the batteries when the power available from the solar panels exceeds the power consumed by the spacecraft payload. When a large number of cellular beams are produced by the antenna, a correspondingly large number of control signals must be transmitted from the spacecraft. When one hundred and forty beams are transmitted, one hundred and forty control signals must be transmitted. When the power available from the solar panels is divided between the information and data transmission channels of the spacecraft, the power available to the synchronization and paging signals may be at a level such that a user communication terminal in an open-air location may respond, but a similar terminal located in a building may not respond, due to attenuation of electromagnetic signals by the building. 
     FIG. 1 is a simplified block diagram of a spacecraft or satellite cellular communications system  10 , as described in U.S. patent application Ser. No. 08/986,611, filed Dec. 8, 1997 in the name of Kent et al. In system  10 , a spacecraft  12  includes a transmitter (TX) arrangement  12   t , a receiver (RX) arrangement  12   r , and a frequency-dependent channelizer  12   c , which routes bands of frequencies from the receiver  12   r  to the transmitter  12   t . Spacecraft  12  also includes an array of frequency converters  12   cv , which convert each uplink frequency to an appropriate downlink frequency. Spacecraft  12  includes a power source which includes a solar panel (SP) illustrated as  12   s , and a power converter (PC) or conditioner  12   p  for converting the solar array power into power suitable for powering the transmitter, receiver, and converters, and other devices on the spacecraft, such as, for example, attitude control systems. A transmitting antenna  12   at  mounted to the spacecraft body by a two-axis gimbal  12   gt  generates a plurality  20  of spot beams, one or more spot beams for each frequency band. Some of the spot beams  20   a ,  20   b , and  20   c  of set  20  are illustrated by their outlines. Each antenna beam  20   x  (where x represents any subscript) defines a footprint on the surface  1  of the Earth below. The footprint associated with spot beam  20   a  is at the nadir  3  directly under the spacecraft, and is designated  20   af . The footprint associated with spot beam  20   c  is designated  20   cf , and is directed toward the horizon  5 , while the footprint  20   bf  associated with spot beam  20   b  is on a location on surface  1  which lies between nadir  3  and horizon  5 . It will be understood that those antenna beams which are illustrated in “lightning bolt” form also produce footprints. As is known to those skilled in the art, the footprints of antenna beams from a spacecraft may overlap (overlap not illustrated in FIG.  1 ), to provide continuous coverage of the terrestrial region served by the antennas. Spacecraft body  12   b  also carries, by way of a two-axis gimbal  12   gr , a receiving antenna  12   ar , which produces spot beams which are intended to be identical to those of transmitting antenna  12   at.    
     Spacecraft  12  also includes a further transmit-receive antenna  72   a , which produces a single, or possibly two or three, broad transmit beam(s) and corresponding receive beam(s), such as those designated as  20   d  and  20   e , which are illustrated by “lightning bolt” symbols in order to simplify the drawing. 
     For completeness, it should be noted that each separate antenna beam forms an infinite number of more-or-less concentric “footprints” centered about the maximum-beam-intensity point on the ground, with each being a fraction of a decibel (dB) greater than the next inner footprint. When “a” footprint is discussed, a selected energy distribution across the “footprint” is assumed. Thus, a common assumption is that the beam intensity will not vary more than 3 dB across the footprint, which defines the extent of the footprint by the 3 dB contour of the antenna beam. Similarly, overlap of the beams is taken to mean overlap at the designated beam intensity. It should further be noted that a receiving antenna also preferentially receives signals within a receiving “beam,” and for a given antenna, the receiving “beam” is “dimensionally” identical to the transmitting beam, in that it has the same beamwidth and gain. 
     As illustrated in FIG. 1, a group  16  of mobile terrestrial user terminals or stations includes three user terminals, denominated  16   a ,  16   b , and  16   c , each of which is illustrated as having an upstanding whip antenna  17   a ,  17   b , and  17   c , respectively. User terminal  16   a  lies on or within the footprint  20   af , user terminal  16   b  lies within footprint  20   bf , and user terminal  16   c  lies within footprint  20   cf . User terminals  16   a ,  16   b , and  16   c  provide communications service to users, as described below. Those skilled in the art will recognize that the illustration of a single user terminal in each footprint is only for ease of understanding, and that many such user terminals may be found in each footprint. More particularly, each illustrated user terminal  16   a  represents one of a plurality of user terminals which may be found within footprint  20   af , and likewise illustrated user terminals  16   b  and  16   c  each represent one of a plurality of user terminals which may be found in footprints  20   bf  and  20   cf , respectively. 
     FIG. 1 also illustrates a terrestrial gateway terminal (a fixed site, tower, or station)  14 , which lies in a footprint (not designated) of the (or an) antenna beam  20   e . While not illustrated, it should be understood that the footprint associated with beam  20   e  may also contain user terminals such as  16   x . Gateway terminal  14  communicates with antenna  72   a  of spacecraft  12  by way of C-band electromagnetic signals transmitted from an antenna  14   al , and receives C-band signals from the spacecraft by way of the same antenna. Gateway terminal  14  is coupled by a data path  9  with a land-line network or public switched telephone system (PSTN) illustrated as a block  8 , and provides communication between spacecraft cellular communications system  10  and the PSTN  8 . While a single gateway  14  is illustrated, the system  10  may contain many gateways at spaced-apart locations, to allow the spacecraft communication system to access different PSTNs. The signals traversing antenna beam  20   e  represent information signals from the user terminals  16  to the gateway terminal  14 , and information signals from the gateway to various ones of the user terminals. The information signals are designated generally as COMM. 
     A network control center (NCC)  18  is illustrated in FIG. 1 as a terrestrial terminal lying in a footprint (not designated) of antenna beam  20   d , originating from antenna  72   a . The footprint may also contain user terminals (not illustrated). Network control center  18  includes an antenna  18   a  for communication with the spacecraft, and for communication by way of the spacecraft to the user terminals  16  and the gateway(s)  14 . Network control center  18  also includes a GPS receiving antenna  18   g  for receiving global positioning time signals, to provide position information and an accurate time clock. Network control center  18  performs the synchronization and TDMA slot control which the spacecraft cellular communications network requires. The functions of network control center  18  may be distributed throughout the communication system  10 , but unlike the arrangement of the land-based GPS cellular communication system, in which control of the slot timing is independently set at each cell center or tower, there is only one network control center associated with the spacecraft communication system  10 , for the required control of the time-division multiple access slots cannot be applied simply to one cell or antenna beam, but rather must be applied across the entire system. While network control center  18  is illustrated in FIG. 1 as being separate from gateway  14 , those skilled in the art will recognize that the network control center  18  includes functions, such as the antenna  18   a , which are duplicated in the gateway  14 , and that it may make economic sense to place the network control center  18 , or the portions which together make up the network control center, at the site(s) of the gateway(s) such as gateway  14 , so as to reduce the overall system cost by taking advantage of the redundancies to eliminate expensive subsystems. 
     The signals traversing antenna beam  20   d  between NCC  18  and spacecraft  12  of FIG. 1 represent control signals. “Forward” control signals proceed from the NCC  18  to the remainder of the communication system  10  by way of spacecraft  12 , and “reverse” or “return” control signals are those which originate at terrestrial terminals other than the NCC, and which are sent to the NCC by way of the spacecraft. Forward control signals include, for example, commands from the NCC  18  to the various user terminals  16   x , indicating which slot set is to be used by each user terminal for communication, while an example of a return control signal may be, for example, requests by various user terminals  16   x  for access to the communication system  10 . Other control signals are required, some of which are described in more detail below. As mentioned, those control signals flowing from NCC  18  to other portions of the communication system  18  are termed “forward” control signals, while those flowing in a retrograde direction, from the communication system  10  toward the NCC, are denominated “return” control signals. 
     The spacecraft  12  of FIG. 1 may need to produce many transmitted spot beams  20  from its antennas  12   at  and  12   ar , and the transmissions over the spot beams may require substantial electrical power, at least in part because of the relatively low gain of the simple antennas  17  of the user terminals  16 . In order to reduce the power required by the transmitters in the spacecraft, the largest number of downlink frequencies, namely those used for transmissions from the spacecraft to terrestrial user terminals, are desirably within a relatively low frequency band, to take advantage of increased component efficiencies (lower component losses) at the lower frequencies. The user terminals transmit to the spacecraft at the lower frequencies, for like reasons. The transmissions to and from the spacecraft from the NCC  18  and the gateway(s)  14  may be within a higher frequency band, in part because of FCC frequency allocation considerations, and in part to take advantage of high antenna gain available at the higher frequencies from large antennas at fixed installations, such as antennas  14   al  and  18   a . In a specific embodiment, the uplinks and downlinks of the NCC and the gateways by way of antenna  72   a  may be at C-band (frequencies at about 3400 to 6700 MHz.), while the uplinks and downlinks of the user terminals by way of antennas  12   at  and  12   ar  are at L-band (frequencies at about 1500-1700 MHz). Thus, the downlink signals from transmit antenna  12   at , by way of antenna beams  20   a ,  20   b , and  20   c  of FIG. 1, are at frequencies within the relatively low L-band, while the uplink and downlink signals in antenna beams  20   d  and  20   e  of antenna  72   a  are at the higher C-band. The uplink signals from the terrestrial user terminals at L-band travel on receive spot beams (not illustrated in FIG. 1) of spacecraft receive antenna  12   ar , which, at least in principle, correspond exactly with transmit beams  20   a ,  20   b , and  20   c . At the distances from the Earth&#39;s surface  1  at which geosynchronous spacecraft orbit, the distance between the transmit and receive antennas  12   at  and  12   ar  does not materially affect the beam correspondence, and even at low Earth orbit, is of almost no consequence. 
     FIG. 2 is similar to FIG. 1, except that, instead of illustrating the antenna beams  20   x  (where the subscript x represents any one of the C- or L-band antenna beams) as a whole, some of the carriers contained in the beams are illustrated separately. For example, some of the forward control signals flowing from network control center  18  to C-band spacecraft antenna  72   a  over C-band antenna beam  20   d  are designated  105 ,  109 , and  113 , while some of the C-band return control signals flowing from antenna  72   a  of spacecraft  12  to the NCC  18  by way of antenna beam  20   d  are designated  106 ,  110 , and  114 . Each of these control signals is transmitted on a carrier of a different frequency, for reasons described below. Thus, the designations  105 ,  106 ,  109 ,  110 ,  113 , and  114  in FIG. 2 may each be imagined to represent a different carrier frequency within C band. In practice in one embodiment, each of the forward control signals has a bandwidth of 200 KHz. As described below, each of the different uplinked control signal carriers will ultimately be routed to a different one of the L-band antenna downlink spot beams and its associated footprint; three footprints attributable to L-band downlinks are illustrated in FIGS. 1 and 2, so three uplinked forward control C-band signal carriers are illustrated, namely carriers  105 ,  109 , and  113 . Similarly, each of the different return control C-band signal carriers  106 ,  110 ,  114  downlinked from spacecraft  12  is generated by, or originates from, an L-band uplink from a user terminal  16  in a different one of the footprints illustrated in FIGS. 1 and 2; three footprints are illustrated, so the downlink portion of antenna beam  20   e  as illustrated includes the three carriers  106 ,  110 , and  114 . 
     As mentioned above in relation to the discussion of FIG. 1, the spacecraft  12  includes frequency-dependent channelizers  12   c  and frequency converters  12   cv . The three forward control signals  105 ,  109 , and  113  uplinked from NCC  18  of FIG. 2 to the spacecraft are received at antenna  72   a  of the spacecraft, and routed by way of receiver (RX)  12   r  to the channelizers  12   c  of the spacecraft, thence to an appropriate one of the frequency converters  12   cv , where they are frequency converted to L-band. For example, uplinked forward control signal  105  of FIG. 2 arriving at antenna  12   ar  of the spacecraft over antenna beam  20   d  at C-band is converted from C-band to a frequency within L-band. In order to make it easy to track the flow of signals in FIG. 2, the L-band frequency corresponding to C-band frequency  105  is also designated  105 . It is easy to keep the meaning of these identical designations in mind, by viewing them as identifying the control signals being transmitted; the forward control information on C-band uplink “frequency”  105  is retransmitted from the spacecraft, after frequency conversion to L-band, within antenna beam  20   a  produced by transmit antenna  12   at , as downlink  105 . Thus, the forward control signal information for all user terminals  16   a  lying within footprint  20   af  is uplinked from NCC  18  in C-band to the spacecraft over antenna beam  20   d , and converted to L-band downlink frequency  105  at the spacecraft, and transmitted in the L-band form over antenna beam  20   a  for use by all user terminals  16   a  within footprint  20   af . Similarly, uplinked control signal  109  arriving at the spacecraft over antenna beam  20   d  at C-band is converted from C-band to a frequency within L-band. In order to make it easy to track the flow of signals, the L-band frequency corresponding to C-band frequency  109  is also designated  109 . The control information on C-band uplink “frequency”  109  is retransmitted from the spacecraft on L-band, within antenna beam  20   b , as downlink  109 . Thus, the forward control signal information for all user terminals  16   b  lying within footprint  20   bf  is uplinked from NCC  18  in C-band to the spacecraft over antenna beam  20   d , and converted to an L-band downlink frequency  109  at the spacecraft, and transmitted in the L-band form over antenna beam  20   b  for use by all user terminals  16   b  within footprint  20   bf . For completeness, control signals generated at NCC  18  for ultimate transmission to user terminals  16   c  in footprint  20   cf  is generated at C-band at a frequency  113  different from frequencies  105  and  109 , and is uplinked from NCC  18  to spacecraft  12 . The C-band control signal  113  received at spacecraft  12  is frequency-converted to a frequency, designated as  113 , in L-band, and transmitted over antenna beam  20   c  for use by all user terminals  16   c  lying in footprint  20   cf.    
     It should be noted that the fact that forward control signals are transmitted on the same carriers to a group of user terminals  16  of FIG. 2 lying in a particular footprint does not necessarily mean that all the user terminals lying within that footprint must operate simultaneously or in the same manner; instead, within each control signal carrier, a plurality of TDMA slots are available, and each set of slots is capable of being directed or assigned to a different one of the user terminals within the footprint being controlled, so that the user terminals are individually controllable. Of course, simultaneous reception of broadcast forward control signals by all user terminals within a footprint is possible, and all user terminals receive information signals “simultaneously,” in that they may all be receiving transmissions at the same “time” as measured on a gross scale, although each individual message is received in a different time slot allocation. It should also be noted that, while control signals have not been described as being transmitted over antenna beam  20   e  between spacecraft  12  and gateway  14 , the gateway (and any other gateways throughout the system) also require such control signal transmissions. In the event that the NCC and a gateway are co-located, the control signals flowing therebetween may be connected directly, rather than by being routed through the spacecraft. 
     When a user terminal  16   x  (where the subscript x represents any one of the user terminals) of FIG. 2 is initially turned on by a user, the user terminal will not initially have an assigned slot. In order to advise the NCC  18  that the user terminal is active and wishes to be assigned a slot by which it may communicate, the user terminal must first synchronize to the forward control signals, and then transmit a reverse control signal to the NCC  18  by way of spacecraft  12 , requesting access in the form of assignment of an information carrier time slot. Thus, in addition to the forward control signals flowing from NCC  18  to the user terminals  16   x , additional return control signals also flow from the user terminals to the NCC  18 . These control signals originating from the user terminals lying within a particular footprint are modulated onto uplink carriers at L-band and transmitted to the spacecraft, where they are converted to frequencies lying in C-band for transmission to the NCC  18 . More particularly, return control signals originating at user terminals  16   a  lying within footprint  20   af  are modulated onto an L-band uplink carrier frequency designated as  106  in FIG.  2 . The return control signals are received by spacecraft antenna  12   ar  and receiver  12   r  by way of spot beam  20   a , and routed by channelizer  12   c  to the appropriate frequency converter of converter array  12   cv  for conversion to C-band frequency  106 . C-band frequency  106  is routed by way of a C-band transmitter (not illustrated) to C-band transmit-receive antenna  72   a , for transmission over antenna beam  20   d  to NCC  18 . Similarly, return control signals originating at user terminals  16   b  lying within footprint  20   bf  are modulated onto an L-band uplink carrier frequency designated as  110  in FIG.  2 . The return control signals are received by spacecraft antenna  12   ar  in beam  20   b , and routed by channelizer  12   c  to the appropriate frequency converter  12   cv  for conversion to C-band frequency  110 . C-band frequency  110  is routed by way of antenna  72   a , for transmission over antenna beam  20   d  to NCC  18 . For completeness, return control signals from user terminals  16   c  in footprint  20   cf  are modulated onto an L-band uplink carrier frequency designated as  114 , and are received by spacecraft antenna  12   ar  in beam  20   c , routed to the appropriate frequency converter  12   cv , converted to C-band frequency  114 , and transmitted over antenna beam  20   d  to NCC  18 . Thus, NCC  18  transmits a single forward control signal carrier to each downlink spot beam  20   a ,  20   b ,  20   c , . . . on an L-band carrier at a frequency which identifies the downlink spot beam to which the forward control signal is directed. NCC  18  receives return control signals from the various user terminals in footprints associated with the spot beams, and one return carrier is associated with each spot beam. In each spot beam, user terminals receive forward control signals over a carrier in an L-band downlink, and transmit return control signals over an L-band uplink. Spot beam  20   a  is associated with forward and return control signal carriers  105  and  106 , respectively, spot beam  20   b  is associated with forward and return control signal carriers  109  and  110 , respectively, and beam  20   c  is associated with forward and return control signal carriers  113  and  114 , respectively. 
     Only the control signal carriers have been so far described in the arrangement of FIG.  2 . The whole point of the communication system  10  is to communicate information signals among the users, so each antenna beam also carries signal carriers on which information signals are modulated or multiplexed by FDMA/TDMA, under control of the NCC  18 . It should first be noted that NCC  18  of FIG. 2 does not need any information signal carriers (unless, of course, it is associated with a gateway terminal as described above). In general, information signals flow between gateways and user terminals. More particularly, signals from public switched telephone system  8  of FIG. 2 which arrive over data path  9  at gateway terminal  14  must be transmitted to the designated user terminal  16   x  or other gateway  14   x , which is likely to be served by an antenna beam other than beam  20   d  which serves gateway  14 . Gateway  14  must communicate the identity of the desired recipient by way of a return control signal to NCC  18 , and receive instructions as to which uplink carrier is to be modulated with the data from PSTN  8 , so that the data carrier, when frequency-converted by the frequency converters  12   cv  in spacecraft  12 , is routed to that one of the antenna beams which serves the desired recipient of the information. Thus, when information is to be communicated from gateway  14  to the remainder of communication system  10 , it is transmitted on a selected one of a plurality of C-band uplink carriers, where the plurality is equal to the number of spot beams to be served. In the simplified representation of FIG. 2, three spot beams  20   a ,  20   b , and  20   c  are served in the system, so gateway  14  must produce information signal carriers at three separate C-band uplink frequencies. These three carrier frequencies are illustrated as  107 ,  111 , and  115 . The information signal is modulated onto the appropriate one of the carriers, for example onto carrier  107 , and transmitted to the spacecraft  12 . At the spacecraft, the C-band carrier  107  is converted to an L-band frequency carrier, also designated  107 , which is downlinked over spot beam  20   a  to those user terminals (and gateways, if any) lying in footprint  20   af . Within footprint  20   af , that particular one of the user terminals  16  to which the information signal is destined, and which has been assigned a TDMA slot set, recovers that portion of the frequency carrier  107  associated with the TDMA slot set, and therefore recovers the information signal. Similarly, information modulated at gateway  14  onto C-band uplink carrier  111 , and transmitted to the spacecraft, is converted to L-band carrier  111 , and downlinked over spot beam  20   b  to user terminals lying in footprint  20   bf . For completeness, information modulated at gateway  14  onto C-band uplink carrier  115 , and transmitted to the spacecraft, is converted to L-band carrier  115 , and downlinked over spot beam  20   c  to user terminals lying in footprint  20   cf . Within each footprint, the various user terminals select the information signals directed or addressed to them by selecting the particular time slot set assigned by NCC  18  for that particular communication. 
     Each user terminal lying in a footprint (and gateway, if any) of system  10  of FIG. 2 must be able to transmit information to the spacecraft for reradiation to a desired recipient. In general, all user terminals communicate only with gateways. If a user terminal of the system wishes to communicate with another user terminal of the system, the information may be routed first to one of the gateways, and then from the gateway back to the intended recipient user terminal. In one mode of operation, however, the user terminals may communicate directly with other user terminals in other spot beams. Thus, any user terminal  16   a  lying in footprint  20   af  of FIG. 2 communicates its information signals by modulating them onto (a selected slot set of) an L-band carrier  108 . The transmission is received by antenna  12   ar  of spacecraft  12 , and the signal is routed by way of channelizers  12   c  to the appropriate frequency converter of converter array  12   cv , where conversion to a C-band frequency takes place. For example, L-band uplink information signal carrier  108  received by the spacecraft in spot beam  20   a  is converted to a C-band carrier frequency also designated  108 , which is downlinked over antenna beam  20   e  to gateway  14 . Similarly, L-band uplink information signal carrier  112  received by the spacecraft in spot beam  20   b  is converted to a C-band carrier frequency also designated  112 , which is downlinked over antenna beam  20   e  to gateway  14 , and uplink signal carrier  116  of antenna beam  20   c  is converted to downlink carrier  116  of antenna beam  20   e  to gateway  14 . The user terminals (and gateways) in each spot beam thus transmit their information signals on uplink carriers having frequencies selected so that, after frequency conversion and channelization at the spacecraft, the resulting downlink carriers travel the particular antenna beam which is directed toward the recipient gateway. Similarly, signals originating at a gateway are modulated onto carriers which, after frequency conversion and channelizing at the spacecraft, traverse that one of the spot beams associated with the footprint in which the designated recipient is located. It should be noted that part of the system control performed by the NCC  18  is to determine the spot-beam in which a designated mobile recipient is located by keeping a record of the last location of each identifiable user, so that each spot beam does not have to be individually polled each time a connection to a mobile user is requested, to “find” the desired mobile user. 
     FIG. 3 illustrates details of one embodiment of spacecraft  12 . As illustrated in FIG. 1, the spacecraft  12  includes a body  12   b , which supports two deployed solar panel arrays  12   s1  and  12   s2 . The spacecraft body  12   b  also supports deployed transmit antenna  12   at  and receive antenna  12   ar . As mentioned, antennas  12   at  and  12   ar  preferably each produce a plurality of relatively narrow spot radiation beams directed towards the surface of the Earth. In one embodiment, the spot beams  20   a ,  20   b , and  20   c  are less than two degrees wide (as conventionally measured at their 3 dB points). 
     FIG. 3 also illustrates a C-band antenna  72   a , adapted for transmitting and receiving signals at C-band. As described below, these signals are communicated between a gateway terminal, or other fixed terrestrial terminal, and antenna  72   a , for the described purposes. The pattern of spot receiving beams produced by antenna  12   ar  is ideally identical to the spot transmitting beams produced by antenna  12   at , so that the radiation beams are congruent. Those skilled in the art of antennas know that, even if antennas  12   at  and  12   ar  produce identical beam patterns, misalignment between the transmit and receive antennas may result in misalignment of at least some of the spot beams, as a result of which some terrestrial terminals  16   x  lying within one spot transmit beam will lie within a receive beam which does not correspond to the transmit beam. Such misalignments may be due to (a) long-term or seasonal errors including thermal distortion, orbit and ephemeris uncertainty, (b) diurnal errors attributable to attitude control errors including gyro drift, (c) short-term errors due to reflector resonances and attitude control system error, and (d) residual errors. Errors may also be caused by antenna integration misalignments andor incorrect deployment. 
     As illustrated in FIG. 3, transmit antenna  12   at  takes the form, when deployed, of a parabolic reflector  12   atr  and a feed array  12   atf . Feed array  12   atf  is mounted on the spacecraft body at a location near the focus of the parabolic reflector. Similarly, receive antenna  12   ar  includes a deployed reflector  12   arr  in conjunction with a feed array  12   arf . The feed arrays include an array of feed horns. 
     The gimbals  12   gt  and  12   gr  are mounted at the junctures of spacecraft body with reflector supports  12   gts  and  12   gtr.    
     FIG. 4 illustrates the layout of the horn apertures of feed horn arrangement  12   atf  of FIG.  3 . In FIG. 4, a map of a portion of Asia is superposed on some of the circles representing apertures, distorted to appear as it would from a spacecraft to the East of the Asian coast. More particularly, Asia, together with its principal islands is designated generally as  410 ,  412  represents India,  414  represents the combination of Vietnam, Cambodia, and Thailand, and  416  represents the island and mainland portions of Malaysia. Some of the islands of Indonesia are represented as  418 . New Guinea is illustrated as  420 , and Taiwan (Formosa) by  422 . The Korean peninsula is  424 , and the Japanese islands are represented as  426 . The circles, some of which are designated  430 , represent the apertures of the various feed horns of the feed array  12   atf  of transmit antenna  12   at  of FIG.  3 . Not all of the feed horn apertures are illustrated, because there are eighty-eight feed horn apertures, and illustrating them all would make the illustration difficult to interpret. For the most part, the peripheral horns of the array have been illustrated, together with a line, which is illustrated by the arrows  432 , of horns across the region being served. However, it will be understood that the entire continent of Asia, and its offshore islands out as far as the Philippines, are served by spot beams originating from the eighty-eight feed horn apertures which are illustrated, in part, in FIG.  4 . More particularly, the feed horn array  12   atf  of FIGS. 1,  2 , and  3  may be represented by the outline of FIG. 4, completely filled in by circles. The exact arrangement of the horn apertures is not particularly material, and the appropriate arrangement for use with a parabolic reflector will be readily understood to those skilled in the antenna arts. It should be noted that the circles of FIG. 4 do not represent the spot beam footprints themselves, but may roughly be conceived of as being a version of the footprints which each horn itself would form if it were energized independently, without a beamformer. 
     FIG. 5 illustrates a beamformer arrangement  500  which is used in conjunction with transmitting antenna  12   at , to allow A feed horns (where A is eighty-eight in the example) to produce a total of N spot antenna beams, where N is  140  in the example. In short, the beamformer combines the signals associated with, or “from” each feed horn with the signals from adjacent ones of the feed horns, in such a manner as to produce the desired beams. The beamformer  500  of FIG. 5 includes an “input” port (considering the antenna in its transmitting mode) for each of the N beams to be generated from the antenna; the input ports are then  510   B1 ,  510   B2 , . . . ,  510   BN , where N represents the total number of beams to be generated, namely  140  beams in the example. The ports  510   B1 ,  510   B2 , . . . ,  510   BN  are coupled to an RF power divider network designated generally as  520 . Network  520  includes a plurality N of individual beam power divider networks  520   D1 ,  520   D2 , . . . ,  520   DN , each of which transforms a single one of the signals applied to a port  510   B1 ,  510   B2 , . . . , or  510   BN  into J output signals, each having a specified amplitude weighting and phase weighting, all as known in the art, which J output signals, when applied to a like number J of antenna radiating elements, produce a beam in space. The power division for weighting is typically accomplished by junctions of several transmission-line sections having various selected relative impedances, or by couplers formed by transmission lines spaced from each other by specific distances over specific lengths. Phase weighting is ordinarily accomplished by selecting among various physical lengths of transmission line, which inherently have different electrical lengths. The outputs of each of the power dividers  520   D1 ,  520   D2 , . . . ,  520   DN  of set  520  are coupled to input ports of a plurality of power combiners  532   C1 ,  530   C2 , . . . ,  530   CA , where A is the number of separate radiating elements of the array which are to be involved in the generation of each beam. Each power combiner  530   C1 ,  530   C2 . . . ,  530   CA  of set  530  combines the signals from K of the power dividers, and couples the powers so combined to the associated one of the A antenna elements. 
     Those skilled in the art know that the term “RF” when used in this context means “radio-frequency,” and that the term originally had a meaning which limited the range of frequencies to the range of 550 to 1600 KHz. The term is now very broadly used to refer to any frequency range extending from audio frequencies (up to around 20 KHz) to frequencies just below infrared frequencies. 
     The transmit antenna  12   at  of FIG. 1 is aligned by use of a fixed ground station located in a region lying between adjacent ones of the spot beams. The spacecraft transmits a beacon signal over the beams, and the beacon signals on the beams are received by the fixed site. One or more characteristics of the multiple beacon signals received over the various beams are processed to determine the deviation of the spacecraft antenna from its proper position. A control signal is generated from the deviation information, which ultimately restores the transmit antenna to its proper position. More particularly, if the fixed ground station is located at a position which is ideally half-way between two adjacent spot beams, one of which is east, and one west of the location, a signal transmitted by way of both beams with equal amplitude should be received at the fixed site with equal amplitudes if the antenna in question is properly aligned. A deviation in amplitude is indicative of an error in positioning. 
     FIG. 6 illustrates a portion of the region of Asia previously illustrated in FIG. 4, showing four spot beam footprints designated N, S, E, and W, which are made larger than their actual dimensions for ease of representation. The four footprints overlap at a dot which represents Batam, at which a fixed terrestrial Beacon Reference Terminal (BRT) site will be used to receive the beacon signals, for adjusting the position of the transmit antenna  12   at  in a manner which aligns it with the underlying surface. The receive antenna  12   ar  is aligned in a similar manner, so that both the transmit antenna  12   at  and the receive antenna  12   ar  are aligned with a feature of the target surface, and therefore with each other. 
     FIG. 7 a  illustrates two adjacent antenna beams V N  and V S , which overlap symmetrically about a solid vertical line at an angle designated  2   0 , which represents a symmetrical a overlap of the beams at  2 = 0 E. If a misalignment occurs, so that the fixed terrestrial terminal is located at an error angle  2   K , a difference results between the signals received on or from the two antenna beams V N  and V S , in an amount V N −V S . FIG. 7 b  is a plot of an error voltage which results from processing the difference signal V N −V S  by normalization, (V N −V S )/(V N +V S ), showing how a misalignment results in a linear change of the error signal relative to angular displacement. 
     FIG. 8 is a simplified block diagram of an antenna alignment control system as described in the abovementioned Kent et al. application. In FIG. 8, a Frequency Generation unit (FGU)  810  generates a baseband beacon signal, which is applied to an upconverter  812 . A portion  818  of beamforming network  500  is associated with the beacon signal, while the remaining portion of beamformer  500 , and its input ports  510 , are associated with the communications beams of the communications system. The upconverted beacon signal is applied to a clocked one-of-four switch  816 , which sequentially applies the upconverted beacon signal to input ports  818 N,  818 S,  818 E, and  818 W of the beamformer portion  818 , corresponding to the N, S, E, and W beams illustrated in FIG.  7 a. The beacon signal is transmitted in time sequence from the ports  819 N,  819 S,  819 E, and  819 W (although not necessarily in the stated order) to the antenna elements  898 N,  898 S,  898 E, and  898 W of feed array  12   atf  of transmit antenna  12   t , for generation of (or transmission over) the N, S, E, and W beams. The beacon signals are received in time sequence at L-band antenna  14   a   2  at the fixed terrestrial Beacon Reference Terminal (BRT)  14   BRT . The received beacon signals are applied from BRT  14   BRT  to a clocked error signal generator  820 , which removes or demodulates the time sequence using a switch arrangement T 1 , T 2 , T 3 , T 4  synchronized to the corresponding switches of beam selection switch  816 , and temporarily stores the resulting four signals in stores (not illustrated). The four stored signals represent the beacon signals V N , V S , V W , and V E  received by BRT receiver  14   BRT  from each of the N, S, E, and W beams. The V N  and V S  signals are applied to a subtractor  821  to produce signals representative of V N −V S , and the V N  and V S  signals are applied to an adder  822  to produce a signal representative of V N +V S . Similarly, the V W  and V E  signals are applied to a subtractor  823  to produce signals representative of V E −V W , and the V W  and V E  signals are applied to an adder  824  to produce a signal representative of V E +V W . The V N −V S , and V N +V S  signals are applied to a divider  826  to produce a normalized error signal V error NS   
     
       
           V   error NS =( V   N   −V   S )/( V   N   +V   S ) 
       
     
     to determine the north-south error. The V W −V E , and V W +V E  signals are applied to a divider  828 , to produce a signal representing an EW error signal V error EW   
     
       
           V   error EW =( V   E   −V   W )/( V   E   +V   W ) 
       
     
     The NS and EW error signals are applied by way of first and second signal paths to a computer  840 . The first signal path includes a buffer  831 , an analog-to-digital converter (ADC)  832 , and an error processor (E-P)  833 , and the second signal path includes a buffer  834 , ADC  835 , and E-P  836 . Computer  840  converts the error value to signals which can be transmitted by antenna  14   al  over a C-band uplink  850  to C-band antenna  72   a  of the spacecraft  12 , for two-axis control of the gimbal  12   gt  by which the transmit antenna  12   at  is supported. The arrangement of FIG. 8 controls the gimbal  12   gt  in the NS and EW planes under control of the two error signals in a manner which maintains the four N, S, E, and W antenna beams centered about the Batam BRT. It should be noted that, in the arrangement of FIG. 8, the processing by taking differences and dividing which is performed in blocks  821 ,  822 ,  823 ,  824 ,  826 , and  828  is illustrated as being performed by analog processing blocks, which might be at RF frequencies, or it might be at intermediate frequencies, if an appropriate frequency converter were used in BRT receiver  14   BRT . As an alternative, the BRT receiver  14   BRT  may perform analog-to-digital conversion, so that all the processing performed in block  820  is performed by a digital processor (or a portion of a digital processor), thereby eliminating the need for analog-to-digital converters  832  and  834 . 
     FIG. 9 is a simplified block diagram which illustrates a corresponding control arrangement for receive antenna  12   ar  of FIGS. 1,  2 , and  3 . The system of FIG. 9 operates in the same fundamental manner as the arrangement of FIG. 8, but is arranged so that the receiving antenna  12   ar  is required only to receive, rather than transmit. In FIG. 9, elements corresponding to those of FIG. 8 are designated by like reference numerals. In FIG. 9, a baseband beacon signal is generated by a frequency generator unit (FGU)  910 , and is upconverted to L-band by a frequency converter  912 . The up-converted signal is transmitted from antenna  14   a   2  toward spacecraft  12 . In addition to the other functions performed by the receiving antenna  12   ar  of spacecraft  12 , it produces four beams which are centered about the BRT  14  location. These beams are available at ports  918 N,  918 S,  918 E, and  918 W of the beamforming network (BFN)  918 . The beacon signal appears simultaneously at beamformer ports  918 N,  918 S,  918 E, and  918 W, and the signals are applied from the beamformer ports to switches T 1 , T 2 , T 3 , and T 4  of commutating switch  916 . The four switches T 1 , T 2 , T 3 , and T 4  are closed (contact is made) in time sequence under the control of clock  917 , so that the four received beacon signals appear in time sequence at output port  916   o  of commutating switch  816 . The commutated beacon signal is coupled from output port  916   o  to a downconverter  914 , where the beacon signal is converted to baseband, but has the modulation of the antenna beam switching. 
     The downconverted, commutated beacon signal needs to be relayed to the ground, where the error is processed. In order to accomplish the relaying of the downconverted, commutated beacon signal to the ground without using additional dedicated beacon tracking equipment, such as an on-board receiver and processor, the downconverted, commutated beacon signal is converted by downconverter  914  of FIG. 9 to C-band (in one embodiment), and applied from downconverter  914  to a C-band transponder  924 , otherwise necessary in the spacecraft for transmission of the C-band information signals., which converts the commutated baseband beacon signal to C-band, for transmission by way of a transmit-receive device  950  to antenna  72   a  for transmission to gateway ground station  14 . The C-band signal from antenna  72   a  traverses the downlink to antenna  960  and is coupled by a transmit-receive device  961  to a C-band receiver  962 . Receiver  962  demodulates the commutated beacon signal, and makes it available to a further synchronized commutator and processor  820 , which is identical in principle to element  820  of FIG.  8 . Synchronized commutator and processor  820  of FIG. 9 generates error signals in the same manner as that described in conjunction with FIG. 8, and couples the error signals by way of buffers  831  and  834 , ADCs  832  and  835 , and error processors  833  and  836  to computer  840 . It will be recognized that the structure extending from block  820  to block  840  of FIG. 9 is identical to the similar structure of FIG. 8, and operates in the same manner to produce correction signals at the output of computer  840 . The correction signals are coupled from computer  840  of FIG. 9 to transmit-receive device  961 , and are coupled by way of antenna  960  and an uplink path to antenna  72   a . From antenna  72   a , transmit-receive device  950  directs the correction signals to gimbal  12   gr , for correcting the position of receive antenna  12   ar . Thus, the commutated beacon signal is transmitted from the spacecraft to the ground station at C-band, and the beacon signal is extracted at the ground station and processed as in FIG.  8 . This technique avoids burdening the spacecraft with dedicated equipment used only by the beacon. 
     SUMMARY OF THE INVENTION 
     An electromagnetic beam selection arrangement according to an aspect of the invention includes a first plurality of beamforming antenna elements, each having a port. Each of these beamforming elements may be a single radiating element of an array, or each may be a subarray of radiating elements. The arrangement includes a first set of hybrids including a plurality, equal to the first plurality, of three-dB hybrids. Each of the three-dB hybrids includes at least first, second, third, and fourth ports, and each of the three-dB hybrids provides transmission between the first port and the second and third ports with a relative amplitude of −3 dB and with quadrature phase between the signal appearing at the second and third ports (so long as the fourth port is terminated in the appropriate impedance). The first ports of each of the three-dB hybrids of the first set of three-dB hybrids is coupled by way of amplifiers to the ports of the beamforming elements, and the fourth ports of the hybrids of the first set are terminated in the appropriate impedance. The arrangement also includes a second set of three-dB hybrids. The second set includes a plurality, equal to the first plurality, of three-dB hybrids. Each of the three-dB hybrids of the second plurality is electrically identical to a three-dB hybrid of the first set. The fourth ports of each of the three-dB hybrids of the second set are terminated in the appropriate impedance. A redundant one of a source and sink of signal is also provided in the arrangement. The redundant source or redundant sink includes a plurality of ports, where the plurality is equal to the first plurality. Each of the ports of the redundant source of signal or redundant sink of signal is coupled to a first port of one of the hybrids of the second set of hybrids, and not to the ports of others of the hybrids of the second set of hybrids. A set of solid-state RF switch cascades is also provided. Each of the switch cascades includes a cascade of at least two solid-state switches, and each of the cascades of switches is electrically coupled between one of (a) a second port of a hybrid of the first set of hybrids and a second port of a corresponding hybrid of the second set of hybrids and (b) a third port of a hybrid of the first set of hybrids and a third port of a corresponding hybrid of the second set of hybrids. As a result of these connections, or whereby, a pair of the cascades connects each three-dB hybrid of the first set of hybrids to a corresponding one of the three-dB hybrids of the second set of hybrids. The arrangement also includes a controller coupled to the switches of the set of solid-state switch cascades. A controller is coupled to the switches of the set of solid-state switch cascades, for controlling the switches to accomplish two functions, (a) and (b). The first or (a) function is to operate at least some of the switches of each cascade always to the same state, as for example by operating all serially coupled switches of a cascade ON or OFF together. If there are interleaved series and parallel switches, for any particular coupling condition of the cascade, each of the series switches would be set to conduct, and the parallel switches to not conduct, or vice versa. The second or (b) function is to operate the switches of the pair-sets of cascades so as to cycle coupling among sets of cascades, so that the beacon signal is sequenced to the beamforming antenna elements. More particularly, the (b) function is to operate the switches of at least one cascade, of that particular pair of cascades which couple together a selected hybrid of the first set of hybrids with a corresponding hybrid of the second set of hybrids, to a state which provides coupling between the selected hybrid and the corresponding hybrid by way of the at least one cascade of the selected pair of cascades. At the same time, or simultaneously, at least some of the switches of those pairs of cascades which couple together non-selected ones of the first set of hybrids with corresponding ones of the hybrids of the second set of hybrids are controlled or operated to states which provide reduced coupling. The second or (b) function further includes the sequential selection, in turn, of each of the hybrids of the first set of hybrids. As a result of this control function applied to the structure, each of the beamforming antenna elements is sequentially selected and is coupled by at least one switch path of redundant switch paths to the redundant one of the source and sink. 
     In another embodiment of the invention, the controller controls the switches for (a) operating the switches of each cascade always to the same state, and (b) closing the switches of that pair of cascades which couple together a selected hybrid of the first set of hybrids with a corresponding hybrid of the second set of hybrids while, or simultaneously with, opening the switches of those pairs of cascades which couple together non-selected ones of the first set of hybrids with corresponding ones of the hybrids of the second set of hybrids. The controller also controls the switches for sequentially selecting, in turn, each of the hybrids of the first set of hybrids, whereby each of the beamforming antenna elements is sequentially selected and is coupled by redundant switch paths to the redundant one of the source and sink. In a particularly advantageous embodiment of the invention, the plurality is four. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified diagram of a spacecraft cellular communications system as described in a copending patent application, illustrating some antenna beams which define system cells; 
     FIG. 2 is a simplified diagram similar to FIG. 1, illustrating some of the signals which flow over the various antenna beams; 
     FIG. 3 is a simplified perspective or isometric view of the spacecraft of FIGS. 1 and 2 with its solar panels and antennas deployed; 
     FIG. 4 is a simplified representation of the feed-horn array of one of the L-band transmit or receive antennas of the arrangement of FIG. 3, with a distorted representation of a portion of the continent Asia superposed thereon; 
     FIG. 5 is a simplified representation of a beamformer which may be used in conjunction with a feed-horn array to generate a plurality of spot beams; 
     FIG. 6 illustrates a portion of the region of Asia shown in FIG. 4, showing four spot beam footprints; 
     FIG. 7 a  illustrates two adjacent antenna beams V N  and V S , which overlap symmetrically about a solid vertical line at a designated angle, and FIG. 7 b  is a plot of an error voltage which results from processing the difference signal V N −V S  by normalization, showing how a misalignment results in a linear change of the error signal relative to angular displacement; 
     FIGS. 8 and 9 are simplified block diagrams of antenna alignment control systems associated with the system of FIGS. 1 through 7; 
     FIG. 10 is a simplified block diagram of a portion of an apparatus according to an aspect of the invention, for sequentially generating a plurality of partially-overlapping beams from a transmit antenna; 
     FIGS. 11,  12 ,  13 , and  14  are simplified block diagrams of various portions of the arrangement of FIG. 10; 
     FIG. 15 is a simplified block diagram of a portion of an apparatus according to an aspect of the invention, for sequentially generating a plurality of partially-overlapping beams from a receive antenna; and 
     FIG. 16 is a simplified block diagram of a portion of the arrangement of FIG.  15 . 
    
    
     DESCRIPTION OF THE INVENTION 
     The very large capital cost of a communications spacecraft, and the large income which it is capable of producing, make it imperative that the spacecraft communication system be as reliable as possible. The beacon signal arrangement, as for example the beacon signal described in conjunction with FIG. 8, constitutes a system portion which must operate correctly in order for the communication system as a whole to remain functional, as deviation of the antenna pointing will disrupt system operation. In order to maximize the reliability of the beacon signal sequencing, the arrangement of FIG. 10 is used. In FIG. 10, an arrangement  1008  according to an aspect of the invention includes a redundant source designated as  1010 , which includes four output ports designated  1010   o1 ,  1010   o2 ,  1010   o3 , and  1010   o4 . An upconverted beacon signal, equivalent to that produced by FGU  810  and upconverter  812  of FIG. 8, is generated at the four output ports  1010   o1 ,  1010   o2 ,  1010   o3 , and  1010   o4 . The redundant upconverted beacon signals at the output ports of redundant source  1010  are coupled to the input ports, designated together as  1030   i , of a block  1030  representing a set of three-dB hybrids denominated as a “second” set. The second set of hybrids coacts with a first set of hybrids and a set of switches, as described below, to provide redundant switching of the redundant upconverted beacon signal. The output of block  1030  is applied by way of a set of paths  1032  to a block  1040 , which represents a set of cascades of solid-state RF switches. Solid-state switches are used because, in general, they require less power to operate than do mechanical switches which perform the same function, and also because solid-state devices tend to be more reliable in operation than devices with moving parts. The output of the switch array  1040  is applied over a set of paths  1042  to a further set  1020  of hybrids, denominated a “first” set of hybrids. As illustrated in FIG. 10, block  1020  has four output paths, which lead to a set  1050  of amplifiers, which include amplifiers  1051 ,  1052 ,  1053 , and  1054 . The amplifiers amplify the signals produced at the output of block  1020 , and apply the amplified signals to the input ports  1098 N,  1098 S,  1098 E, and  1098 W, respectively, of a set of four beamforming antenna elements  898 N,  898 S,  898 E,  898 W. In operation of the arrangement of FIG. 10, the switches of switch array  1040  are controlled in a manner such that only one of the beamforming antenna elements  898 N,  898 S,  898 E,  898 W receives the current one of the redundant upconverted beacon signals. 
     FIG. 11 illustrates details of redundant source  1010  and “second” set  1030  of hybrids. In FIG. 11, block  1010  includes a first frequency generation unit (FGU)  810   a  and a second FGU  810   b . FGU  810   a  drives an upconverter  812   a  to produce an RF beacon signal, and FGU  810   b  drives a second upconverter  812   b  to similarly generate a second or redundant RF beacon signal. One or the other of the redundant RF beacon signal generators is energized at any one time, so that only one RF beacon signal is generated, while the redundant generator is held off-line for use if the first one should fail. The first RF beacon signal, if generated, is applied on a signal path which is coupled to the second port (port  2 ) of a three-dB hybrid (H)  1110 , and the second RF beacon signal, if generated, is applied on a signal path which is coupled to the third port (port  3 ) of the three dB hybrid  1110 . 
     As is well known to those skilled in the art; a three-dB hybrid including four ports can produce at ports  1  and  4  a signal which is the combination or sum of the signals applied to ports  2  and  3 . Since only one upconverted beacon signal at a time is applied to hybrid  1110 , the upconverted beacon signal appears at ports  1  and  4  of hybrid  1110  regardless of which source of the two sources is operated. Since there is but one signal involved, no phase differences need to be taken into account. Thus, redundant RF beacon signal appears at both output ports of hybrid  1110 . However, operation of the illustrated system requires that the redundant beacon signal be available at four ports, and so a further set of hybrids  1112  and  1114  is used to split the signals from ports  1  and  4  of hybrid  1110 . Thus, equal-amplitude versions of the redundant RF beacon signal are produced at output ports  1  and  4  of hybrids  1112  and  1114 , corresponding to output ports  1010   a ,  1010   b ,  1010   c , and  1010   d  of redundant RF beacon source  1010 . 
     The redundant RF beacons signals generated at output ports  1010   a ,  1010   b ,  1010   c , and  1010   d  of redundant RF beacon source  1010  are applied by way of a set, illustrated as being four in number, of input ports designated generally as  1030   i , to the first ports (the ports designated as  1 ) of three-dB hybrids (H)  1030   a ,  1030   b ,  1030   c , and  1030   d  of a set  1030  of hybrids. The fourth ports (the ports designated  4 ) of hybrids  1030   a ,  1030   b ,  1030   c , and  1030   d  are terminated in an appropriate impedance, such as the characteristic impedance of the transmission lines of the system or of the hybrids. Each of hybrids  1030   a ,  1030   b ,  1030   c , and  1030   d  acts as a power splitter or power divider, so as to produce at its output ports  2  and  3  equal-amplitude, mutually quadrature-phase versions of the signal applied to its input port  1 . The two equal-amplitude versions of the currently-selected one of the redundant RF beacon signal which are generated at the output ports  2  and  3  of three-dB hybrid  1030   a  are applied to transmission lines or conductors  1032   a  and  1032   b , respectively, of a signal path or bus  1032 . The two equal-amplitude versions of the currently-selected one of the redundant RF beacon signal which are generated at the output ports  2  and  3  of three-dB hybrid  1030   b  are applied to transmission lines or conductors  1032   c  and  1032   d , respectively, of signal path  1032 . Similarly, the two equal-amplitude versions of the currently-selected one of the redundant RF beacon signal which are generated at output ports  2  and  3  of three-dB hybrid  1030   c  are applied to transmission lines or conductors  1032   e  and  1032   f , respectively, of signal path  1032 . For completeness, the two equal-amplitude versions of the currently-selected one of the redundant RF beacon signal which are generated at output ports  2  and  3  of three-dB hybrid  1030   d  are applied to transmission lines or conductors  1032   g  and  1032   h , respectively, of signal path  1032 . The arrangement of FIG. 11, then, is one which takes the current one of the redundant RF beacon signals from source  1010 , and splits it into a plurality, which in the illustrated case is eight, of mutually identical RF beacon signals. 
     FIG. 12 illustrates details of switch arrangement  1040  of FIG.  10 . In FIG. 12, the eight mutually identical versions of the RF beacon signal arrive on the various “input” signal paths of path or bus  1032 , and each one is applied to a switch cascade of a set  1040  of switch cascades. More particularly, the RF beacon signals arriving by way of signal paths  1032   a  and  1032   b  are applied to switch cascades  1040   a  and  1040   b , respectively, the RF beacon signals arriving by way of signal paths  1032   c  and  1032   d  are applied to switch cascades  1040   c  and  1040   d , respectively, the RF beacon signals arriving by way of signal paths  1032   e  and  1032   f  are applied to switch cascades  1040   e  and  1040   f , respectively, and the RF beacon signals arriving by way of signal paths  1032   g  and  1032   h  are applied to switch cascades  1040   g  and  1040   h , respectively. Each switch cascade  1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g , and  1040   h  of set  1040  of switches of FIG. 12 is capable of assuming one of two states, under the control of a controller illustrated as  1210 . These states are an ON or conductive state, in which signal applied from input bus  1032  is coupled to a path of an output bus  1042 , and an OFF or nonconductive state, in which signal applied from bus  1032  is blocked from proceeding, and is not coupled to a path of output bus  1042 . More particularly, when cascade  1040   a  is ON, signal arriving by way of path  1032   a  is coupled to a corresponding path  1042   a  of output bus  1042 , and when cascade  1040   a  is OFF, signal arriving by way of input path  1032   a  is blocked from proceeding to output path  1042   a  of output bus  1042 . When cascade  1040   b  is ON, signal arriving by way of path  1032   b  is coupled to a corresponding path  1042   b  of output bus  1042 , and when cascade  1040   b  is OFF, signal arriving by way of input path  1032   b  is blocked from proceeding to output path  1042   b  of output bus  1042 . For brevity, it is noted that all of the cascades operate in the same fashion as that described for cascades  1040   a  and  1040   b , blocking progress of the RF beacon signal when in the OFF state, and allowing the signal to pass in the ON state. 
     Each switch cascade  1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g , and  1040   h  of set  1040  of switches of FIG. 12 is a cascade of controllable solid-state switches, many types of which are known in the art, including series-connected andor parallel-connected diodes, and various transistor-based arrangements. The reason for cascading such switches is for reliability. FIG. 13 is a simplified schematic diagram illustrating details of two of the cascades, namely cascades  1040   a  and  1040   b . In FIG. 13, cascade  1040   a  can be seen to include a plurality of series-connected switches, including switches  1212   a , . . .  1212 N, each of which is represented by a mechanical switch symbol. Those skilled in the art know that such a mechanical representation is solely for the purpose of explanation, and that in actuality each individual switch, such as switch  1212   a , includes one or more semiconductor devices. Similarly, cascade  1214  includes a cascade of a plurality of series-connected semiconductor or solid-state switches  1214   a , . . . ,  1214 N. As illustrated in FIG. 13 solely for the purpose of explanation, the switches  1212   a , . . . ,  1212 N of cascade  1040   a  are in the OFF state, so that one of the redundant RF beacon signals applied by way of path  1032   a  cannot reach output path  1042   a . Also as illustrated in FIG. 13, the switches  1214   a , . . . ,  1214 N of cascade  1040   b  are in the ON state, so that the same one of the redundant RF beacon signals, applied by way of path  1032   b  reaches output path  1042   a . It will be appreciated that controller  1210  can be programmed to control the switches in any desired manner. For example, controller  1210  could be controlled so as to never operate pairs of cascades in a manner such that one cascade is ON during those intervals in which the other cascade of the pair is OFF; it would instead, control both to the ON state simultaneously, and to the OFF state simultaneously. Each cascade pair can be identified by the fact that it receives its signals from the outputs of one hybrid of set  1030  of hybrids of FIG.  11 . More particularly, the outputs of hybrid  1030   a  are applied to cascades  1040   a  and  1040   b , and these two cascades therefore constitute a pair. The outputs of hybrid  1030   b  are applied to cascades  1040   c  and  1040   d , and these two cascades therefore constitute a pair. Similarly, the outputs of hybrid  1030   c  are applied to cascades  1040   e  and  1040   f , and these two cascades constitute a pair. For completeness, the outputs of hybrid  1030   d  are applied to cascades  1040   g  and  1040   h , and these two cascades constitute a pair. If there were more hybrids, the cascades coupled to the outputs of each of those additional hybrids would be paired similarly. In a preferred control arrangement, each pair of cascades has one cascade designated as primary, and the other as secondary, and the controller  1210  is programmed to operate or switch only the primary cascade, until such time as the primary cascade fails, in which case the secondary cascade of the pair is brought on-line and operated. With the described simultaneous control of the pairs of cascades, the output of each hybrid  1030   a ,  1030   b ,  1030   c , and  1030   d  of set  1030  of hybrids will ordinarily reach both of the output paths of set  1042  of paths when the associated cascades are in the ON condition Controller  1210  of FIGS. 12 and 13 controls each pair of cascades so that only one pair of cascades is ON at any one time, and the others pairs are OFF. Controller  1210  also controls so that the pair of cascades which is selected to be ON cycles among the available pairs, so that the RF beacon signal is applied in sequence to beamforming elements  898 N,  898 S,  898 E, and  898 W (although the ordering of the sequence is irrelevant). This allows the beacon signal to perform the location function as described in conjunction with FIG.  8 . 
     The paired cascade arrangement of solid-state switches illustrated and described in conjunction with FIGS. 12 and 13 has the advantage of high reliability. If a single switch in either cascade fails in the ON or conductive state (that is, cannot be rendered nonconductive), the remaining switches, in their OFF state, still control the passage of the signal, and the operation of the cascade is unaltered (although the isolation of the cascade may be degraded). If one of the switches of a cascade fails in the OFF (nonconducting) state, that particular cascade is disabled, for it cannot thereafter be operated to the ON (conducting) state. However, since the cascades are paired, and they operate on two different versions of the same signal, there remains another cascade which can be operated to both the ON and OFF states to switch the signal, and the signal routed through the remaining operable cascade is divided into two portions by the following hybrid, so that the sequencing is unaltered. 
     FIG. 14 illustrates details of block  1020  of FIG. 10, and its relationship to the signal routing. Under ordinary conditions, when all cascades are operating normally in FIG. 14, the selected one of the redundant RF beacon signals arrives at a particular hybrid of set  1020  by way of either (or both) paths associated with a particular pair of cascades. More particularly, when one of cascades  1040   a  or  1040   b  is ON and the other cascades are OFF, the selected RF beacon signal arrives at one of ports  2  or  3  of hybrid  1220   1 , and becomes available at output port  1  of the hybrid for application to amplifier  1051  and the North beam forming element, and when the cascades  1040   a  and  1040   b  are ON and the other cascades are OFF, the selected RF beacon signal arrives at both ports  2  and  3  of hybrid  1220   1 , and the two versions are summed (taking phase shifts into account, if necessary) to produce the signal at port  1  for application to amplifier  1051  and the North beam forming element. At the time at which the RF beacon signal passes through one or both of ON-state cascades  1040   a  and  1040   b , the other cascades are OFF, and the RF beacon signal does not reach any other hybrid, so no other beam forming element receives beacon signal. Consequently, only the North beam is generated. Similarly, with all cascades working properly (or with at least one of the cascades of each pair operating properly), and with controller  1210  controlling at least one of the cascades  1040   c  and  1040   d  of the second pair to the ON state, and the remaining cascades to the OFF state, RF beacon signal is applied only to generate the S beam. With all cascades working properly, and with controller  1210  controlling at least one of the cascades  1040   e  and  1040   f  of the third pair to the ON state, and the remaining cascades to the OFF state, RF beacon signal is applied only to generate the E beam. Finally, with all cascades working properly, and with controller  1210  controlling the at least one of the cascades  1040   g  and  1040   h  of the fourth pair of cascades to the ON state, and the remaining cascades to the OFF state, RF beacon signal is applied only to generate the W beam. 
     If a switch of one cascade of FIG. 12 fails in the ON state, that cascade can still be turned OFF, as mentioned above. Consequently, the failure to the ON state of less than all of the solid-state switches of a cascade has no effect on the operation described above, and the sequencing of the beacon beams continues. If a switch of a cascade fails in the OFF state, that cascade of switches cannot thereafter be operated to the ON state, and so is disabled. However, beam sequencing can continue, since the other switch cascade of the pair continues to function, and to switch its version of the RF beacon signal to the output hybrid. For example, if cascade  1040   a  of FIG. 12 were to fail in the OFF state, no RF beacon signal could pass therethrough to port  2  of hybrid  1220   1  of FIG.  14 . However, its paired cascade would be switched into service and continue to be operated to the ON and OFF states by controller  1210 , with the result that the RF beacon signal would still be sequenced to port  3  of hybrid  1220   1  of FIG.  14 . If only one cascade of a pair were ordinarily in service, the switchover would have no effect on the amplitude of the beacon signal. If both switches of each cascade had been in operation, the switchover to use of only one of the switch pairs would have the effect of reducing the RF beacon signal amplitude applied to by way of amplifier  1051  to antenna  898 N, which would reduce the transmitted signal power, but would also allow operation to continue. Ideally, for such a situation amplifier  1051  would be provided with a gain control, so that the amplification could be increased by 3 dB in order to overcome even this minor effect. 
     As a result of this arrangement, the RF beacon signal leaving port  1  of a hybrid of set  1020  of hybrids is applied by way of a power amplifier to a port of one beamforming antenna element. More particularly, the signal at port  1  of hybrid  1220   1 , of FIG. 14 is applied to an amplifier  1051 , and the amplified signal is made available for application to port  1098 N of antenna  898 N, the signal at port  1  of hybrid  1220   2  of FIG. 14 is applied to an amplifier  1052 , and the amplified signal is made available for application to port  1098 S of antenna  898 S, the signal at port  1  of hybrid  1220   3  of FIG. 14 is applied to an amplifier  1053 , and the amplified signal is made available for application to port  1098 E of antenna  898 S, and the signal at port  1  of hybrid  1220   4  of FIG. 14 is applied to an amplifier  1054 , and the amplified signal is made available for application to port  1098 W of antenna  898 W. 
     Thus, the arrangement of the invention described in conjunction with FIG. 10 has a redundant or selectable source of RF beacon signal, and the beacon signals are applied to a “second” set of hybrids, in which the selected one of the RF beacon signals is replicated, to produce, or make available, two beacon signals for each antenna element to be driven. As described in conjunction with FIGS. 10,  11 ,  12 , and  13 , hybrid set  1030  produces eight replicas (four pairs) of the selected one of the RF beacon signals for driving four beamforming elements. Each pair of the replicated RF beacon signals is applied to a pair of cascades of switches in a switch array  1040 , and the switches of at least one of the pairs of cascades are operated while the other cascade of the pair is held in reserve, or the switches of each pair of cascades are operated to the same state simultaneously (either both ON or both OFF). The pairs of cascades are turned ON and OFF in timed sequence by a controller, so that the RF beacon signal is cyclically applied, in turn, to each of the beamforming elements, so as to sequentially form the four beams necessary, in one embodiment of the invention, to maintain the antenna pointed at a particular location on the Earth&#39;s surface. 
     FIG. 15 illustrates a receiving arrangement according to an aspect of the invention. In FIG. 15, a set  898  of receiving antennas includes  898 N,  898 S,  898 E, and  898 W antenna elements. Elements  898 N,  898 S,  898 E, and  898 W are coupled to the input ports of low-noise amplifiers  1551 ,  1552 ,  1553 , and  1554 , respectively, of a set  1550  of amplifiers. The amplified signals from amplifier set  1550  are applied to a block  1520 , representing a first set of hybrids, which form the received and amplified signals into redundant pairs. The redundant pairs of signals are applied to a switch array illustrated as a block  1540 . The switch array of block  1540  includes a plurality of cascades, which are controlled to sequence the beams produced by the antenna elements of antenna  898 . The sequenced received signals are applied over a set  1542  of signal paths to a second set of hybrids, illustrated as a block  1530 . Block  1530  receives the four redundant received beacon signals, and converts them into signals on four separate paths, for application to input ports  1510   i1 ,  1510   i2 ,  1510   i3 , and  1510   i4  of a redundant signal sink illustrated as a block  1510 . The sink  1510  may include redundant receivers andor downconverters. Those skilled in the art will recognize the arrangement of FIG. 15 as being the receive version of the arrangement of FIG.  10 . 
     For definiteness, FIG. 16 illustrates details of the arrangement of FIG.  15 . In FIG. 16, low-noise amplifiers  1551 ,  1552 ,  1553 , and  1554  apply their amplified signals to input ports  1  of hybrids (H)  1520   1 ,  1520   2 ,  1520   3 , and  1520   4 , respectively, of first hybrid set  1520 . Each hybrid  1520   1 ,  1520   2 ,  1520   3 , and  1520   4  makes its own received signal redundant, by hybrid division into two portions, each on one signal path. The redundant received beacon signals from hybrid  1520   1  (originating from antenna element  898 N) are applied to switch cascades  1540   a  and  1540   b  of switch array  1540 , the redundant received beacon signals from hybrid  1520   2  (originating from antenna element  898 S) are applied to switch cascades  1540   c  and  1540   d  of switch array  1540 , the redundant received beacon signals from hybrid  1520   3  (originating from antenna element  898 E) are applied to switch cascades  1540   e  and  1540   f  of switch array  1540 , and the redundant received beacon signals from hybrid  1520   4  (originating from antenna element  898 E) are applied to switch cascades  1540   g  and  1540   h  of switch array  1540 . As mentioned, the switch cascades of array  1540  are controlled, with the control being provided by a controller designated  1610 , which may be identical to controller  1210  of FIG. 12, and may be operated in any of the modes described therewith. 
     In the arrangement of FIG. 16, the redundant beacon signals originating from antenna  898 N are sequenced by cascades  1540   a  and  1540   b , and applied over signal path(s)  1542   a  (and  1542   b , if appropriate), respectively, to input ports  2  and  3  of a hybrid  1530   a  of set  1530 . Similarly, the redundant beacon signals originating from antenna  898 S are sequenced by cascades  1540   c  and  1540   d , and applied over signal paths  1542   d  and  1542   d , respectively, to input ports  2  and  3  of a hybrid  1530   b  of set  1530 , the redundant beacon signals originating from antenna  898 E are sequenced by cascades  1540   e  and  1540   f , and applied over signal paths  1542   e  and  1542   f , respectively, to input ports  2  and  3  of a hybrid  1530   c  of set  1530 , and the redundant beacon signals originating from antenna  898 W are sequenced by cascades  1540   g  and  1540   h , and applied over signal paths  1542   g  and  1542   h , respectively, to input ports  2  and  3  of a hybrid  1530   d  of set  1530 . The hybrids of second set  1530  combine the two redundant beacon signals applied to them, and produce at their output ports  1  a signal associated with one of the antenna elements. More particularly, the signal originating from antenna element  898 N and amplified by amplifier  1551 , as switched by the redundant switch array, is applied from port  1  of hybrid  1530   a  to input port  1510   i1 , of redundant sink  1510 , the signal originating from antenna element  898 S and amplified by amplifier  1552 , as switched by the redundant switch array, is applied from port  1  of hybrid  1530   b  to input port  1510   i2  of redundant sink  1510 , the signal originating from antenna element  898 E and amplified by amplifier  1553 , as switched by the redundant switch array, is applied from port  1  of hybrid  1530   c  to input port  1510   i3  of redundant sink  1510 , and the signal originating from antenna element  898 W and amplified by amplifier  1554 , as switched by the redundant switch array, is applied from port  1  of hybrid  1530   d  to input port  1510   i4  of redundant sink  1510 . 
     As illustrated in FIG. 16, redundant sink block  1510  includes three hybrids, namely hybrids  1110 ,  1112 , and  1114 , which are identical to those of FIG. 11, which coact to combine the sequenced beacon signals applied to four input ports  1510   i1 ,  1510   i2 ,  1510   i3 , and  1510   i4  into two paths, connected to ports  2  and  3  of hybrid  1110 . The signals on these two paths constitute redundant versions of the sequenced beacon signals arriving at the antenna elements of antenna  898 . The two signals from hybrid  1110  are applied to redundant receivers. In the specific arrangement of FIG. 16, a first of the receivers includes a downconverter  1612   a  coupled to a receiver (RX)  1610   a , and the second of the receivers includes a downconverter  1612   b  coupled to a receiver (RX)  1610   b . In operation of the arrangement of FIGS. 15 and 16, the beacon transmitter on the spacecraft transmits continuously. Each of the four elements of the receiving antenna  898  has its beam directed somewhat away from the line-of-sight between the ground station and the spacecraft. The sequencing of the signals results in a change in amplitude, phase, or both between N/S and E/W antenna element pairs if the pairs are not symmetrically disposed about the line-of-sight. This asymmetry may be used to correct the alignment. 
     Other embodiments of the invention will be apparent to those skilled in the art. For example, while serially-connected switches have been illustrated in each cascade of switches, parallel-connected switches may be used, or an intermixture of serial- and parallel-connected switches. 
     Thus, an electromagnetic beam selection arrangement ( 1008 ,  1508 ) according to an aspect of the invention includes a first plurality (four) of beamforming antenna elements ( 898 N,  898 S,  898 E, and  898 W), each having a port ( 1098 N,  1098 S,  1098 E, and  1098 W). Each of these beamforming elements ( 898 N,  898 S,  898 E, and  898 W) may be a single radiating element of an array, or each one may be a subarray of radiating elements, interconnected by an appropriate beamformer or beamformer portion. The arrangement ( 1008 ,  1508 ) includes a first set ( 1020 ,  1520 ) of hybrids, including a plurality, equal to the first plurality (four), of electrically identical three-dB hybrids ( 1220   1 ,  1220   2 ,  1220   3 ,  1220   4 ;  1520   1 ,  1520   2 ,  1520   3 ,  1520   4 ). Each of the three-dB hybrids ( 1220   1 ,  1220   2 ,  1220   3 ,  1220   4 ;  1520   1 ,  1520   2 ,  1520   3 ,  1520   4 ) of the first set ( 1020 ,  1520 ) of three-dB hybrids includes at least first (1), second (2), third (3), and fourth (4) ports, and each of the three-dB hybrids provides transmission between the first port (1) and the second (2) and third (3) ports with a relative amplitude of −3 dB, and possibly with quadrature phase between the signal appearing at the second (2) and third (3) ports (so long as the fourth port (4) is terminated in the appropriate impedance). The first ports (ports  1 ) of each of the three-dB hybrids ( 1220   1 ,  1220   2 ,  1220   3 ,  1220   4 ;  1520   1 ,  1520   2 ,  1520   3 ,  1520   4 ) of the first set ( 1020 ,  1520 ) of three-dB hybrids is coupled by way of paths ( 1050 ;  1550 ) including amplifiers ( 1051 ,  1052 ,  1053 ,  1054 ;  1551 ,  1552 ,  1553 ,  1554 ) to the ports ( 1098 N,  1098 S,  1098 E, and  1098 W) of the beamforming elements ( 898 N,  898 S,  898 E, and  898 W), and the fourth ports (ports  4 ) of the hybrids of the first set are terminated in the appropriate impedance. The arrangement ( 1008 ,  1508 ) also includes a second set ( 1030 ,  1530 ) of three-dB hybrids. The second set ( 1030 ,  1530 ) includes a plurality, equal to the first plurality (four), of three-dB hybrids ( 1030   a ,  1030   b ,  1030   c ,  1030   d ;  1530   a ,  1530   b ,  1530   c ,  1530   d ). Each of the three dB hybrids ( 1030   a ,  1030   b ,  1030   c ,  1030   d ,  1530   a ,  1530   b ,  1530   c ,  1530   d ) of the second plurality ( 1030 ,  1530 ) of three-dB hybrids is electrically identical to a three-dB hybrid of the first set. The fourth ports (ports  4 ) of each of the three-dB hybrids ( 1030   a ,  1030   b ,  1030   c ,  1030   d ,  1530   a ,  1530   b ,  1530   c ,  1530   d ) of the second set ( 1030 ,  1530 ) are terminated in the appropriate impedance. A redundant one of a source ( 1010 ) and sink ( 1510 ) of signal is also provided in the arrangement ( 1008 ,  1508 ). The redundant source ( 1010 ) or redundant sink ( 1510 ) includes a plurality of ports ( 1010   o1 ,  1010   o2 ,  1010   o3 ,  1010   o4 ;  1510   i1 ,  1510   i2 ,  1510   i3 ,  1510   i4 ), where the plurality is equal to the first plurality (four). Each of the ports ( 1010   o1 ,  1010   o2 ,  1010   o3 ,  1010   o4 ;  1510   i1 ,  1510   i2 ,  1510   i3 ,  1510   i4 ) of the redundant source ( 1010 ) of signal or redundant sink ( 1510 ) of signal is coupled to a first port of one of the hybrids ( 1030   a ,  1030   b ,  1030   c ,  1030   d ,  1530   a ,  1530   b ,  1530   c ,  1530   d ) of the second ( 1030 ,  1530 ) set of hybrids, and not to other ports of the hybrids of the second set of hybrids. A set ( 1040 ;  1540 ) of solid-state RF switch cascades ( 1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g ,  1040   h ;  1540   a ,  1540   b ,  1540   c ,  1540   d ,  1540   e ,  1540   f ,  1540   g ,  1540   h ) is also provided. Each of the switch cascades ( 1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g ,  1040   h ;  1540   a ,  1540   b ,  1540   c ,  1540   d ,  1540   e ,  1540   f ,  1540   g ,  1540   h ) includes a cascade of at least two solid-state switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ), and each of the cascades ( 1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g ,  1040   h ;  1540   a ,  1540   b ,  1540   c ,  1540   d ,  1540   e ,  1540   f ,  1540   g ,  1540   h ) of switches is electrically coupled between one of (a) a second port (port  2 ) of a hybrid ( 1220   1 ,  1220   2 ,  1220   3 ,  1220   4 ;  1520   1 ,  1520   2 ,  1520   3 ,  1520   4 ) of the first set ( 1020 ,  1520 ) of hybrids and a second port (port  2 ) of a corresponding hybrid of the second set ( 1030 ,  1530 ) of hybrids and (b) a third port (port  3 ) of a hybrid of the first set ( 1020 ,  1520 ) of hybrids and a third port (port  3 ) of a corresponding hybrid of the second set ( 1030 ,  1530 ) of hybrids. As a result of these connections, or whereby, a pair of the cascades of the set ( 1040 ,  1540 ) of cascades connects each three-dB hybrid of the first set ( 1020 ,  1520 ) of hybrids to a corresponding one of the three-dB hybrids of the second set ( 1030 ,  1530 ) of hybrids. The arrangement ( 1008 ,  1508 ) also includes a controller ( 1210 ) coupled to the switches (such as switches  1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of the set ( 1240 ) of solid-state switch cascades. According to a preferred control, the controller ( 1210 ) is coupled to the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of the set ( 1040 ) of solid-state switch cascades, for controlling the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) to accomplish two functions, (a) and (b). The first or (a) function is to operate at least some of the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of each cascade always to the same state, as for example by operating all serially coupled switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of a cascade ON or OFF together. If there are interleaved series and parallel switches, for any particular coupling condition of the cascade, each of the series switches would be set to conduct, and the parallel switches to not conduct, or vice versa. The second or (b) function is to operate the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of the pair-sets of solid-state switch cascades ( 1040 ) so as to cycle coupling among sets of cascades ( 1040 ), so that the beacon signal is sequenced to the beamforming antenna elements. More particularly, the (b) function is to operate the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of at least one cascade, of that particular pair of cascades ( 1040   a ,  1040   b , for example) which couple together a selected hybrid of the first set of hybrids with a corresponding hybrid of the second set of hybrids, to a state which provides coupling between the selected hybrid and the corresponding hybrid by way of the at least one cascade of the selected pair of cascades. At the same time, or simultaneously, at least some of the switches ( 1212   a , . . . ,  1212   N ;  1214   a , . . . ,  1214   N ) of those pairs of cascades ( 1040   a ,  1040   b ,  1040   c ,  1040   d ,  1040   e ,  1040   f ,  1040   g ,  1040   h ;  1540   a ,  1540   b ,  1540   c ,  1540   d ,  1540   e ,  1540   f ,  1540   g ,  1540   h ) which couple together non-selected ones of the first set of hybrids with corresponding ones of the hybrids of the second set of hybrids are controlled or operated to states which provide reduced coupling. In this context, “coupling” and “reduced coupling” correspond to the ON and OFF states, respectively, of a cascade, in which the ON state may undesirably exhibit some loss, and the OFF state may undesirably exhibit some leakage. The second or (b) function further includes the sequential selection, in turn, of each of the hybrids of the first set of hybrids. As a result of this control function applied to the structure, each of the beamforming antenna elements is sequentially selected and is coupled by at least one switch path of redundant switch paths to the redundant one of the source and sink. The controller ( 1210 ) controls the switches for (a) operating the switches of each cascade always to the same state (all ON or all OFF), and (b) closing the switches of that pair of cascades which couple together a selected hybrid of the first set ( 1020 ,  1520 ) of hybrids with a corresponding hybrid of the second set ( 1030 ,  1530 ) of hybrids while, or simultaneously with, opening the switches of those pairs of cascades which couple together non-selected ones of the first set ( 1020 ,  1520 ) of hybrids with corresponding ones of the hybrids of the second set ( 1030 ,  1530 ) of hybrids. The controller also controls the switches for sequentially selecting, in turn, each of the hybrids of the first set ( 1020 ,  1520 ) of hybrids, whereby each of the beamforming antenna elements ( 898 N,  898 S,  898 E, and  898 W) is sequentially selected and is coupled by redundant switch paths to the redundant one of the source ( 1010 ) and sink ( 1510 ). In a particularly advantageous embodiment of the invention, the plurality is four.