CA1326307C

CA1326307C - Satellite communications system having frequency addressable high gain downlink beams

Info

Publication number: CA1326307C
Application number: CA000543182A
Authority: CA
Inventors: Harold A. Rosen
Original assignee: Hughes Aircraft Co
Current assignee: DirecTV Group Inc
Priority date: 1986-08-14
Filing date: 1987-07-28
Publication date: 1994-01-18
Anticipated expiration: 2011-01-18
Also published as: DE3781670T2; EP0277173A1; WO1988001457A1; CN87105572A; EP0277173B1; JPH01500710A; JPH0552098B2; US4823341A; DE3781670D1; CN1007862B

Abstract

SATELLITE COMMUNICATIONS SYSTEM
HAVING FREQUENCY ADDRESSABLE
HIGH GAIN DOWNLINK BEAMS

ABSTRACT OF THE DISCLOSURE

A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Description

- 132~3~7 SAT~LLIT~ COMMUNICATIONS SYSTEM
HAVING FREQUENCY ADDRESSABLE
HIGH GAIN DOWNLINX BEAMS

TECHNICAL ~IELD

, The present invention broadly relatex to satellite communication systems, e~pecially oî the type employing a satellite placed in geo~ynchronous orbit 90 as to provide two-way comr~nications between S the satellite and n~ny small aperture ten~nals on the earth. More particularly, the invention involves a communications satellite having the abillty to simultaneously provide thousands OI narrow, high~ain, *equency addressable downlinlc antenna b~sam without reducing the satellite's oversll channel capacity.
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BACKGROUND ART
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~-~ Cwmunication satellites have, in the past, employed multlple antenna be~ns to perndt both high antenna gain to directed zones and rrequency reu~e in the zones. Coverage OI contiguous zones was achieved by alternating the polarization b~stween adjacent zones so li that a gap wide enough to pe~t spatlal isolation OI the beam was a-,railable in each polarization. Thus, less trlsquency reuse could be achieved than in a system providing contiguous coverage ot the zones wlth trequency reuse in each zone but using only one polarization. In su¢h a sy~tem, the ~econd polaPization could then be used similarly, thus doubling the *equency reuse or ~or another useiul purpose such as providing broqdcast service. A system which permits frequency reuse in ,. . .' , .. .

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-2- 132~3~7 contiguous zones and a single polarization and which covers the zones with narrow high gain frequency addressable bean~ is the subject of the present invention.

SUMMARY OF THE INVENTION

The satellite comnunications syste~n of ~e present invention includes sn earth-orbiting satellite comT~Dicatively interconnecting a plurality of earth termdnal sites distributed over an srea of the earth. In accordance with one aspect of the invention, the system includes first means for fornnng a radio frequency uplink beam between the sstellite snd a plurality of uplink terrrunal sites in the area, and second means for forn~ng a plurality of radio frequency downlink besms, each covering only a portion of the area. The downlink be~ are relatively narrow in width compared to the srea serviced by the satellite, and therefore hsve exceptionslly high gain. Esch OI the downlink beams is frequency addressable by the frequency of the signals carried by the uplink beam. Consequently, the downlink site destination of an uplink signal is determhned by the frequency Or the uplink signsl. The area serviced by the satellite may be arranged in zones in which the identicsl range o~ assigned frequencies is used, thereby permitting nPultfple reuse of the assigned frequency spectrlun and a corresponding increase in the channel capacity of the system.

According to another aspect of the invention, a method is provided for comnunicatively interconnecting a plurality of te~ninal sites in an area on the earth u~ing an earth-orbiting comr~nications satellite which includes the steps of: forming an uplink radio frequency besm between the sate~lite snd each of a plurality oî upllnk terminal ~ites in the area, rorming a plurality Or individual do~mlink beam between the satellite and a plurality of respectively corre~ponding portions Or the area whereln each Or the downlink beams carrles a frequency addressable signal representing a comnunication from an uplink tern~nal site in the areat selecting a particul~r downlink terminal site to receive a connunicl~tlon trom r p~rticul~r upiink terrln~l slte by selecting 9 ' .:.

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3 1~2~3~7 frequency for the uplink beam signal which is related to the particular downlink terminal site which is to receive the communication, and addressing the frequency addressable signals of the downlink beams using the frequency selected for the uplink signal.
It is therefore an object of one aspect of the present invention to provide a satellite communications system for simultaneously providing thousands of narrow, high-gain, ~ frequency addressable downlink beams without reducing ~he - 10 satellite's overall channel capacity.
An object of another aspect of the present invention is to provide a system as described above which may be employed to service multiple zones on the earth in which the assigned frequency spectrum is reused.
An object of yet another aspect of the present invention is to provide a system as described above employing a novel network for forming the narrow, high-gain : antenna beams.
; These, and further objects of the invention, will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.
Various aspects of this invention are as follows:
A satellite communications system for communicatively interconnecting a plurality of terminal sites distributed over an area of the earth, comprising:
an earth orbiting satellite; and frequency responsive means carried by said satellite i for forming a plurality of beams of electromagnetic radiation between said area on the ealth and said satellite with each beam of radiation having a carrier fre~uency, said beam forming means including an array of radiating elements and an antenna for producing all of said beams, said beams being arranged in at least two groups thereof respectively covering two essentially contiguous zones within said area, the beams in said groups thereof intersscting said area on the earth in seriatim along an , ."
: ., j ~ ' 3a 1321~307 - arc with the position of each of said beams along said arc being determined by the carrier frequency of the beam, said groups of beams being arranged essentially end-to-end along said arc, the beams in each group thereof being carried bv the same preselected set of frequencies such said set of frequencies is reused by said beams.
A method of communicatively interconnecting a plurality of terminal sites in an area on the earth using an earth orbiting communications satellite, comprising the steps of:
(A) forming a first plurality of beams of electromagnetic radiation in an arc between said satellite and the earth and covering a first zone of said area and arranged in seriatim, said beams in said first plurality thereof respectively having carrier frequencies defining a preselected set: and (B) forming a second plurality of beams of electromagnetic radiation in said arc between said ~ satellite and the earth and covering a second zone of said : 20 area essentially contiguous to said first zone and arranged in seriatim, said first and second pluralities of beams being arranged essentially end-to-end along said arc, said beams in said second plurality thereof respectively having the same carrier frequencies as said first plurality of said beams, such that said preselected set of frequencies is reused, each of said beams in said first and second ; pluralities thereof being formed such that the position of j the beam along said arc is determined by the carrier ~ frequency of the beam.
BRIEF DESC~IPTION OF THE DRAWINGS
. In the accompanying drawings:
: Figure 1 is a perspective view of a communications satellite, showing the antenna subsystems;
Figure 2 i5 a top plan view of the antenna subsystems ~` 35 shown in Figure l;
Figure 3 is a sectional view taken along the line 3-3 in Figure 2;

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-4- ~3263~7 Figure 4 is a sectional view taken along the line 4-4 in Figure 2;

Figure 5 is a view of the United States snd depicts ~ltiple, contiguous receive zones covered by the satellite of the present : 5 invention, the prin~ry areas of coverage being indicated in cross-hatching and the areas of contention being indicated by a din~led pattern;

Figure 6 is a block diagram of the comr1unication electronics ~or the comnunications satellite;

Pigure 7 i~ a schem~tic diagram of a coupling network which interconnects the point-to-point receive feed horns with the inputs to the communications electronics shown in Pigure 6;

Figure 8 is a reference table of the interconnect channels employed to connect the rece}ve snd transmit zones for the point-to-point system;

~ igure 9 is a diagramnatic view of the United States depicting multiple contiguous transmit zones covered by the satellite and the geographic distr}bution of the interconnected channels for each zone, scross the Un}ted States;

~ igure 9A is a graph showing the var}ation in gain of the transmit sntenna beam l~or each zone in the point-to-point system in relst}on to the di~tance from the center o~ the be~n in the east-west d}rection;

~igure 9B is a graph similar to ~igure 9A but showing the variation in gain in the north-south direct}on;

~igure 10 is a detailed schematic diagrsm of the filter intercoMection matr~c employed in the point-to-point sy~telr4 .

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~ igure ll is a detailed, plan view of the beam-forming network en~loyed in the point-to-point system;

Figure 12 is an enlarged, fr~grr#ntary view of a porffon of the beam-forndng network shown in Figure ll;

Figure 13 is a front elevationsl view of the transmit - srray for the point-to-point system, the horizontal slots in each trsnsmit element not being shown for sake of simplicity;

~igure 14 is a side view of the trans;mit element of the array shown in Flgure 13 and depicting a corporate feed network for the element;
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; ~igure 15 is a front, perspective view of one oî the tran~nit elen~nts employed in the transnnt array of Figure 13;

Figure 16 is a front view of the receive feed horn~ for the point-to-point system; and Pigure 17 is a diagr~m2atic view ~howing the relationship between a transrnitted wave and a portion of the transTIit ~eed array ~or the point-to-point ~ystem.

DESCRlPTION OP THE PREPERRED EMBODIMENTS
;"'' Reierring first to Figures 1-4, a commulications s&tellite 10 is depicted which ic placed in geosynchronous orbit above the ;, earth's surface. The satellite's antenna system, which will be described In m~re detQil below, will typically be mounked on an earth-oriented plQtrorm 90 that the sntenna system maint~ins a constant orientation with ~ respect to the earth.
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-6- ~ 3 ~ 7 The satellite 10 is of a hybrid communications-type satellite which provides two different types of comnunication services in a particular frequency band, for example, the fixed satellite service Ku - band. One type of com~nication service, referred to hereinafter as point-to-point service, provides two-way comnunications between very E~nall aperture antenna tern~nals of relatively narrow band voice and data signals. Through the use OI frequency division multiple access (~DMA) and reuse of the assigned frequency spectr~n, tens of thousands of such comnunication channels are accomnodated simultaneously on a single linear polarization. The other type of cornrD~nication service provided by the satellite lQ is a broadcsst service, and it is carried on the other linear polarization. The bro~dcast service is prin~rily used for one-way distribution of video and data throughout the geographic territory served by the sQtellite 10. As such, the transmit antenna beam covers the entire geogrsphic territory. For illustrative purposes throughout this ''J description, it will be assumed that the geographic area to be serviced by both the point-to-point and broadcast services will be the United States.
Accordingly, the broadcast service will be referred to hereinaiter as CONUS (Continental United Ststes).

The antenna system of the sstellite 10 includes a conventional omni antenna 13 and two antenna subsystems for respectively servicing the point-to-point and CONUS ~y~tem3. The point-to-point antenna subsystem provides a two-way communication link to interconnect earth stations for tWo-wQy communications. -The CONUS antenna system ~unctions as a transponder to broadcast, over a wide pattern covering the entire United States, signal~ received by one or more parffcular locations on earth. The point-to-point transmit signal and the CONUS receive slgnal are vertically polarized. The CONUS tr~m~nit and point-to-point receive signQls are horizontally polarized. The antenna system includes a large renector assembly 12 comprising two renectors 12a, 12b. The two reflectors 12a, 12b are rotated relative to each other about a common axis and intersect at their midpoints. The renector 12a is horizontally polarized and operates with horizontally polarized signals, while the . .

-~L32~07 reflector 12b iq vertically polarized flnd therefore operate~ with vertically poL~qrized signals. Consequently, e~ch o~ the ref~ctors 12a, 12b renect~ signals which the other reflector 12a, 12b transmts.

A ~requency sel0ctive screen 18 ~ provided which S includes two halves or ~ections 18~, 18b and i9 mountæd on ~ support 30 such that the screen h~lves 1&, 18b are di~ ed on opposite side~ oi~ a centerline passing dianRtrically through the s~tellite 10, as best seen in Pigure 2. The frequency ~elcctiv~ ~reen 18 ~unction~ as a diplexer ~or - separating ditrerent bands o~ ~requencie~ and m~y co~ri~e ~n arrsy of discrete, electrically conductive element~ forll#d o~ any suitable n~terial, such a~ copper. Any o~ variou~ types o~ known frequency selective ~creen~ r~y be elr~loyed in thiq antenna ~ystenL However, one ~uitable rrequen¢y selqctive screen, exhibiting ~arp trandtion ch~r~cteristi¢s and capable ot separating two ~equeney bands which ~ re~ffvelY close to : 15 esch other, is described ~n Canadian patent application 543,179, filed July 28, 1987, and assign~d ~ H~ Aircra~ O~q~ny.
The ~requency selective ~creen 18 e~e¢tively ~eparate!l tha transnitted and recelved signaLs ~r both the CONUS and point-to-point subsy~tem.
:; It may be awraciated that the two halve~ 1&, 18b Or Ule ~creen 18 are resp~ctively adapted to separate individu~l signal~ whlch are ho~izontally and vertlcally polarizod.
, Th~ CONUS alb~y~ten~ which ~erve~ the entire cowltry with a single be~n, Iw, in thi~ example, eight convenUonal tran~lponder~
each having a high powor traveling w~ve tube ampli ier as it~ tr~nitter 82 (seo Figuro . 6). n~O CON~ receivo antennA u~es vertical pol~rization, sharing the vertically pol~rized retl~ctor 12b wlth the point-to-point tranom~ion ~y~tem. CONU9 reeeiv~ sign~ p~s Ulrough the ~equen¢y sol~¢tivo wreen halt 18b and are ~wused on the receive ~eed horns 14 located at the ~ocal plane 2~ o~ retloctor 12b. The sntenn~ pattern 80 rorn~d iJ l~d to cover CONU9. The CONU5 tran~mit antenna employ~ horizontsl polarization, and shsre~ renector 12a ¢ with the point-to-point receive ~ystem. Signals radiating rrom the tru~t reed- ~ re renec~ed by the horizont~lly pol-rized rrequcncy : ' . ~

', ~ ' -8- 13263~7 1 selective screen 18a to reflector 12a whose secondary pattern is shaped to cover CONUS~

The point-to-point subsystem broadly includes a - tran~nit array 20, a subreflector 22, and receive feed horns 16. The S trsn~nit array ao, which will be described later in more detail, is molmtedon the support 30, imnediately beneath the screen 18. The subreflector 22 i~ mounted forward of the transmit array 20 snd slightly below the screen 18. The signal emanating from the transmit array 20 is renected by the subreflector 22 onto one half 18b of the screen 18. The subrenector 22 in conjunction with the main reflector 12 functions to ' ~ effectively m~gnify and enlArge the pattern of the signal emanating from the tru~rnt array 20. The signal re~ected ~rom the ~ubreflector 22 is, in turn, reilected by one halt 18b of the 3creen 18 onto the large reflector 12b, which in turn renects the point-to-point signal to the earth. Through this arrangement, the perfolmance of a l~rge aperture phase array is achieved. The receive ~eed horns 16 are positioned in the ~ocal plane 26 of the reflector 12a. It consists of four main horns 50, 54, 58, 62 and three auxiL;ary horns 52, 56, 60 as sho~m in Figure 16.

Re~erring now also to ~igures 13-15, the transndt array 20 comprises a plurality, for example forty, transmit waveguide elements 106 disposed in side-by-side relationship to ~orm sn array, as shown in ~igure 13. Each of the transmit wavegulde elements 106 includes a plurality, for exuDple twenty-six, o~ horizontal, vertically spaced slots 108 therein which Fesult in the generation of a vertically polarized dgnal. As shown in Figure 14, the transmit array 20 i9 fed with a tranE~ it signal by means o~ a corporate ~eed network, generally indicated - by the numeral 110 which excltes the array element in ~our places 114.
The purpose Or the corporate ~eed network 110 is to provide a broadband j mateh to the transmit waveguide element 10~. Signsls input to the waveguide opening 112 excite the array slots 108 80 that the slot txcltatlon i~ deslgned to give a l~t pattern in the north-south dlreetion.

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3 ~ 7 _9_ Attention is now directed to Figure 5 which depicts a generslly rect~ngular beam coverage provided by the horizonta11y polarized point-to-point receive system. In this particular ex~mple, the srea serviced by the point-to-point system is the continental United States. The point-to-point receive system con~rises four bearns Rl, R2, R3, R4 respectively emanating from the four uplink zones 32, 34, 36, 38 to the satellite, wherein each of the beams Rl-R4 consists of a plurality s of individual uplink be~rns originating from individual sites in each zone 32, 34, 36, 38 and carries an individual signal from that site. The uplink bearn signals from the individual sites are arranged into a plurality of channels for each zone. For example, zone 32 may include a plurality, e.g. sixteen 27 M~lz channels with each of such channels carrying hundreds of individual beam ~ignals from corre~ponding uplink sites in zone 32.

The signal ~trength for each oî the four beam psttern contours, respectively de~ignated by numerals 32, 34, 36 and 38, are approximately 3 dB down from peaks of their respective beans. The antenns bean~ have been designed to achieve su~ficient isolation between them to m~ke ~easible in the cross-hatched regions 39, 41, 43, 45 reuse Or the frequency spectrum four times. In the dotted regions 40, 42, and 44, the isolation is insuificient to distinguish between dgnals of the sarne frequency originating in ad~acent zones. Each signal originating in these re~ions will generate two downlink signals, one intended snd one extraneous. The generation oi extraneous signals in these areas will be discussed later in more detail.
', It msy be readily appreciated from ~igure 5 that the four zones covered by bea~ 32, 34, 36, 38 are unequal in undth. llle East Coast zone covered by beam 32 extends approximstely 1.2 degrees;
the Central zone covered by besm 34 extends approximEItely 1.2 degrees;
the Midwest zone covered by be~m pattern 36 extends Approximately 2.0 degrees, and; the West Coa~t zone covered by beam pattern 38 extends approximately 2.0 degrees. The width oi each of the i~our receive zones 32, 34, 3ff and 38 is determined by the number of terminal~ and thus the ,::

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population density in the various regions of the country. Thlls, beam pattern 32 is relatively narrow to accomnodate the relatively high population density in the Eastern part of the United States while beam pattern 36 is relatively wide due to the relatively low population density in the Mountain states. Since each zone utilizes the enffre frequency spectrum, zone widths are narrower in regions where the population density is high, to accomnodate the greater demand for channel usage.
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As shown in ~igure 9, the point-to-point tren~t system comprises four beams T1, T2, T3, T4 respectively covering the four transmit zones 31, 33, 35, 37, wherein each of the beams Tl~T4 consists of Q plurslity of individual downlink beans destined for the individual downlink sites in each zone 31, 33, 35, 37 and carries an indlvidual signal to that site. The downlink beam signals, destined to be received at the individual downlink sites, are arranged into a plurality of channel~ ror each zone. For example, zone 31 may include a plurality, e.g. sixteen 27 MHz channels with esch of such channels carrying hundreds of individual beam signals to corresponding downlink sitas in zone 32.

The use of multiple downlink zones and downlink zones of unequal widths assist in causing the intermodulation products, generated by the later-discussed solid state power ampllfiers, to be geographically dispersed in a manner which prevents most of these produots from being received at the ground terminals. The net effect is that the amplifiers m~y be operated more efflciently because the system can tolerate mDre intermodulation products. Although the widths o$ the transmit zones 31, 33, 35, 37 are nearly the sarsle as those of ~e receive zones R1, R2, R3, R4, small diIferences between the two sets have been ~ound to optimize l~e capaclty Or the system.

The hfllf power bearn width of the individual transmit beam~ 29 is substantially narrower than that Or the tran~mit zones 31, 33, 35, 37. This results in the desirable high gain, and avoids the zones oi~
contention 40, 42, 44 characteristic Or the receive zone arrangement.

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These individual bearns 29 must be steered within the zones in order to maximi~e the downlink EIRP in the directions of the individual destination ter~nals. The transmit point-to-point frequency addressable narrow beam; 29 are generated by an array 20 whose apparent size is n~gnified by two confocal parabolas con~rising a main reflector 12b and a subreflector 22. The east-west direction of each beam 29 is determined by the phase progression of its signal along the array 106 oi transmit elements 20 (Figures 13 and 15). This phase progression is established by a later-discussed beam-forming network 98 and is a function of the signal frequency. Each of the tran mit array slements 20 is driven by a later-discussed solid state power amp]ifier. me power delivered to the array elements 106 is not uniform but is instead tapered with the edge elements being more than 10 dB down. Tapering of the beaz~E 29 is achieved by ad~usting the trar~t gain according to the position OI the transmit array elements 20. The excitation pattern deter~unes t~e characteristics of the transrmt secondary pattern, shown in Figure 9A.
Referring to Figure 9, the closest ~pacing between transmit zones 31, 33, 35, 37 occurs between zones 31 and 33 and i~ approximately l.2 degrees.
This means that a 3ignal addressed to zone 33 using a particular frequency would interfere with a signal using the s~me ~requency in zone 31 with its side lobe 1.2 degrees from its beam center. However, the individual transmit gains h~ve been adjusted to provide low side lobe levels, thereby permitting i~requency rewe in ad~acent zones. Referring to Figure 9A, it is seen that the side lobe bvel at this angle oif beam center is more than 30 dB down, 80 that such interferenoe will be negligibly small. The same irequency u~es in zones 35 and 37 s$e ~urther removed in angle, hence the side lobe interference in those zones is even ~naller.

Figure 9B is an illustration of the transmit beam 3U pattern in the north-south direction. The twentg ~ix slot3 108 in each oi ~e trQnsmit wavegulde elements 106 ~re excited in a mqnner which creates a nearly nat north-south pattern, extending over the covered range of plu~ and minus 1.4 degrees irom the north-south boresight direction.

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., . ' ~ ' '' 1~263~7 Both the point-to-point and CONUS systems may utilize the same uplink and downlink frequency bands, with the point-to-point system using horizontal polarization for its uplink polarization, and the CONUS system using vertical polarization, as previously mentioned. For example, both services mqy, simultaneously, utilize the entire 500 MHz uplink frequency band between 14 and 14.5 ~Hz, as well a~ the entire 500 MHz downlink frequency band between 11.7 and 12.2 GHz. Each ot the receive zones 32, 34, 36, 38 and transmit zones 31, 33, 35, 37, employing the point-to-point service utilizes the entire frequency spectrwn (i.e. 500 MHz). FurthermDre, this total frequency spectrum is divided into a plurality of channels, for example, sixteen channels each having a usable bandwidth of 27 MHz and a spacing of 30 MHz. In turn, each of the sixteen channels may accomnodate ~pproximately 800 subchannels. Hence, within each zone, approxim~tely 12,500 (16 channels x 800 subchannels) 32 kilobit per second channels rn~y be accornn~dated, at any given moment. As will be discussed below, the conmunication architecture of the point-to-point system allows any terminal to comn~nicate directly with any other te~ninal. Thus, within a ~ingle polarization, a total of 50,000 subchannels mE~y be accommodated nationwide.

Referring now particularly to Figures 1, 2, 6, 7 and 16, the point-to-point receive teed array 16 enploys seven receive horns 50-62. Horns 5D, 54, S8 and 62 respectively receive signals from zones 32, 34, 36 and 38. Horns 52, 56 and 60 receive signals trom the zones of ., .
contention 40, 42 and 44. Using a series of hybrid couplers or power dividers C1-Cg, the signals received by horns 50-62 are combined into ~our outputs 64-70. Por example, a signal originating from the area oi contention 44 and received by horn B0 is divided by coupler C2 and portions ot the divided signal are respectively delivered to couplers C1 and coupler C4 whereby the split dgllal is combined with the incoming dBnals recelved by horns 58, 62 respectively. Sin~larly, signals originating from the area ot contention 42 and received by horn 56 are split by coupler Cs. A portion of the split signal is combined, by coupler ' ' .

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Attention is now particularly directed to Figure 6 which depicts, in block disgram form, the electronics for receiving and trsnsmitting ~ignals for both the CONUS and point-to-point systens. The point-to-point receive signals 64-70 (see also Figure 7) are derived from the point-to-point receive feed network in Figure 7, whereas the CONUS
receive signal 72 derives from the CONUS receive feed horns 14, (Figures 1 and 3). Both the point-to-point and CONUS receive signal are input to a switching network 76 which selectively coMects input lines 64-72 with five corresponding receivers, eight of which receivers are generally indicated at 74. The receivers 74 are o~ conventional design, three o~
which are provided for redundancy and are not nornYIlly u~ed unless a mal~un¢tion in one of the receivers is experienced. In the event of a - n~lfunction, switching network 76 reconnects the appropriate incoming line B4-72 wlth a back-up receiver 74. Recei-vers 74 function to drive the 2ilters in Q ~ilter interconnection matrix 90. The outputs of the receivers 74, which are connected with lines 64-70, are coupled by a second switching network 78 through ~our receive lines R1-R4 to a filter interconnection matrix 90. As will be discussed later below~ the ~ilter interconnection mstrix (FIM~ provides interconnections between the receive zone~ 32, 34, 36, 38, and the transmit zone~ 31, 33, 35, 37.
Operating in the above-mentioned 500 MHz assigned ~requency spectrwTI, ~eparated into sixteen 27 MHz chHnnels, four sets of sixteen ~ilters are employed. ~ach set o~ the sixteen ~ilters utilizes the entire 500 MHz frequency spectrum snd each ~ilter has a 27 M~z bandwidth. As will be discussed lster, the ~ilter outputs T1-T4 are arranged in ~our groups, each group destlned ~or one of the ~our transmit zones 31, 33, 35, 37.
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- 30 The transmit signQls Tl-T4 are respectively connected, vla switching network 94, to four o~ six drlving Amplifiers 92, two o~ such ampli~iers 92 being provided ~or back-up in the event o~ ~ailure. In the event o~ the ~a~lure o~ one o~ the smpli~iers 92, one OI the back-up . ~

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-14- 132~7 amplifiers 92 will be reconnected to the corresponding transmit signal Tl-T4 by the switching network 94. A similar switching network 96 couples the ~nplified output of the amplifiers 92 to a bearn-fom~ng network 98.
As will be discussed later in more detail, the beam-forming network 98 consists of a plurality of transmission delay lines connected at equal intervals along the four delay lines. These intervals and the width of the delay lines are chosen to provide the desired centerband beam squint and the bearn scan rate with frequency for the eorresponding trar~mit zones 319 33, 35, 37 to be serviced. The transmit signals, coupled from the four delay lines, are sumned in the beam forn~ing network 98 a shown in Figures 11 and 12, to provide inputs to solid state power amplifiers 100, which may be ernbedded in the point-to-point system's transn~it array ao. In the illustrated embodiment discussed below, forty solid state power amplifiers (SSPAs) 100 are provided. Each of the SSPAs 100 13 ~mplifies a corresponding one oi the forty signals formed by the beam-forming network 98. The SSPAs 100 possess different power capacities to provide the tapered array excitation previously mentioned. The output o~ the SSPA 100 is connected to the input 112 (~igure 14) at one of the elements OI the transmit array 20.
The receive signal ior CONUS on line 72 is connected to an appropriate receiver 74 by switching networks 76S 78. me output of the receiver connected with the CONUS signal is delivered to an input multiplexer 80 which provides ~or eight channels, as mentioned above.
The purpose oi the input multiplexers 80 is to divide the one low level CONUS signal lnto sub6ignals so that the subsignals can be amplified on an individual basis. The CONUS receive signals are highly ~pliffed so that the CONUS transmit signal may be distributed to very smEIll earth terminals. The outputs Or the input multiplexer 80 are connected through a switching network 84 to eight oi twelve high power traveling wave tube ampll2~ers (TWTAs) 82, ~our o~ which TWTAs 82 are employed ror back-up In the event o~ ~ailure. The outputs Or the eight TWTAs 82 are connected through another switching network 86 to an output mutliplexer 88 which recombines the eight ampliffed signals to ~orm one CONUS
transmit signal. The output of the nalltiplexer 88 is delivered via 13263~7 waveguide to the transmit horns of the CONUS tran~mitter 24 (Figures 2 and 3).

Attention is now directed to Figure 10 which depicts the details of the FIM 90 (Figure 6). As previously discussed, the FIM
90 effectively interconnects any te~inal in any of the receive zones 32, 34, 36, 38 ~Figures 5) with any terminal in any of the transmit 7ones 31, 33, 35, 37. The FIM 90 includes ~our waveguide inputs 120, 122, 124 and 126 for respectively receiving the receive sign~ls Rl, R21 R3 and R4.
As previously mentioned, receive signals R1-R4, which originate from a corresponding receive zone 32, 34, 36, 38 (Figure 5), esch contain the entire assigned frequency spectrun-, (e.g. 500 MHz), and are separated into a plurality of channels, (e.g. sixteen 27 MHz channels). The channels are ~urther separated into a plurality of subchannels, where each of the subch~nnels carries a signal from Q corresponding uplink site.
The FIM 90 includes 64 filters, one of which is indicated by the nurneral 102. Each of the filter~ 102 has a passband corresponding to one of the - channel~ (e.g. 1403-1430 MHz). The filters 102 are arr~nged in four groups, one for each receive zone 32, 34, 36, 38, with each group ulcluding two banks or subgroups of eight filters per subgroup. Qne subgroup of f11ters 102 contains those filters for the even-numbered channels and the other subgroup in eàch group contains eight filters for the odd- nu~Tbered channels. Thus, ~or example, the filter group for receive signal Rl comprises subgroup 104 of filters 102 for odd channels, and subgroup 106 of filters 102 for even ch~nnels. me following table 2~ relates the receive signals and zones to their filter subgroups:
~ ' ; Filter Subg~oups Receive Zone Receive Si~nal Odd Chamlels Even Channels ' :
:

-16- ~2~3~
The filters are grouped in a unique manner such that when the receive signals R1-R4 are filtered, the filtered outputs are conbined to forrn the transrnit signals. The translmt signals T1-T4 also utilize the entire a~signed frequency spectrun~ (e.g 500 MHz). In the illustrated embodiment, each of the transmit signals T1-T4 possesses sixteen 2q MHz wide channels, and comprises four channels from each of the four receive zones 32-38 (Figure 5).

The incoming receive signals R1-R4 are divided into the corresponding subgroups by respectively associated hybrid couplers 128-134 which effectively divert 50% of the signal power to each subgroup. Hence~ for example, one-half of the Rl signal input at waveguide 120 is diverted to tran~ission line 136 which services the subgroup 104 of filters 102, and the remaining half of the R1 signal is diverted to transmission llne 138 which service3 subgroup lD6 of filters 102. In a dmilar nsnner, each of the subgroups 104-118 of filters 102 is served by a corresponding distribution line, similar to lines 136 and 138.

me construction of subgroup 104 wi~l now be described in more detail, it being understood that the relT~ining subgroups 106-118 are identical in architecture te subgroup 104. At intervals along the transmission line 136, there are eight ferrite circulators 140, one associated with each of the odd- numbered channel filters 102. The function of the circulators 140 is to coMect the transmission line 136 to each of the odd channel filters 102 in a lossless manner. Thus, for example, the R1 signal enters the ~irst circulator 140a and circulates it counterclockwise whereby the 27 MElz band of signals corresponding to ci~annel 1 passes through it to circulator 142. A~l other frequencies are renected. These renected signals propagate ~ria the circulator toward the next filter where the process is repeated. Through this process, the R1 receive dgnal is filtered into sixteen channels by the sixteen filters 104-108 corresponding to the R1 signals. Hence, the R1 signal with frequencies in the range of channel 1 will pass through the flrst ferrite clrculator 140a and it will be filtered by filter 1 oY group 104.

-17- 13~3~7 me outputs from a filter subgroup 104-118 are selectively coupled by a second set of ferrite circulators 142 which SWT~, in a criss-cross pattern, the outpu$s from an adjacent group of filters 102. For exarnple, the outputs of channel filters 19 5, 9, and 13 of group 104 are swnned with the outputs of channel filters 3, 7, 11 and 15 of tilter group 112. This s~un appears at the output terminal for Tl 144.
Referring to Figure 8, these signals correspond to the connections between receive zones Rl and R3 and to transmit zone T1.

Attention is now directed to Figures 8 and 9 which depict how the trans~t and receive signals are interconnected by the ~IM 90 to allow two-way comnunication between any tenninals.
Specifically, ~igure 8 provides a table showing how the receive and transmit zones are connected together by the interconnect channels while Fi~ure 9 depicts how these interconnect channels are distributed geographically across the transn~t zone~ 31, 33, 35, 37. In Figure 8, the receive signals R1-R4 are read across by rows of interconnect channels and the transmit signals T1-T4 are read by colun~s of interconnect channels. It can be readily appreciated from Figure 8 that each of the transmit signals T1-T4 i5 made up ot sixteen chaMels arranged in four ,~ 20 groups respectively, where each group is as~ociated with one of the receive ~ignals R1-R4. The satellite communications system of the present invention iB intended to be used in conjunction with a ground station referred to as a satellite network control center which coordlnates comnunlcations between the ground terminals via packet switched signals. The network control center assigns an uplink user with sn uplink frequency based on the locatlon ot the desired downlink, assigning the available frequency whose downlink longitude is closest to that o~ the destination. The frequency addres~able downlink transmit beams 29 are thus addressed by the trequencies of the uplink signals.
ThiB strategy rr~cimizes the gain ot the downlinlc signal.

. , .

13~3~7 - 1 . .
As shown in Figure 9, the continental United States lS
divided into four primary zones 31, 33, 35, 37. Zone 31 may be rePerred to as the East Coast zone, zone 33 is the Central zone, zone 35 is the Mountain zone, and zone 37 is the West Coast zone. As previously mentioned, each of the zones 31, 33, 35, 37 utilizes the entire assigned frequency spectrum (e.g. 500 MHz). Thus, in the case of a 500 MHz assigned frequency band, there exists sixteen 27 MHz channe~s plus guard bands in each of the zones 31, 33, 35, 37.

The nwnbers 1-16 repeated four times above the beams 29 in Figure 9 indicate the longitude of the beams corresponding to the - center frequencies of the channels so nurrbered. Because of the frequency sensitivity of the besm, the longitude spsn between the lowest snd highe~t frequency narrow band signal in a channel is approxim~tely one channel width. Each beam i9 0.6 degrees wide between its half power point, about half the zone width in the East Coast and Central zones and nearly one-third the zone width in the Mountain and West Coast zones.
The antenna beans 29 overlap each other to ensure a high signal density, the more that the bearns overlap, the greater channel capacity in a given area. Hence, in the East Coast zone 31, there is ~ greater overlap than in the Mountain zone 35 because the signal traffic in the East Coast zone 31 is considerably greater than that in the Mountain zone 35.

The interconnect scheme described above will now be explained by wsy of a typical caTmunication between terminals in different zones. In this exsmple, it will be sssumed that a caller in Detroit, Michigan wishes to place a csll to a tenmnal in Los Angeles, California. Thus, Detroit, Michigan, which is lossted in the Central zone 34, is the uplink site, and Los Angeles, California, which is located in the West Co~st zone 37, is the downlinlc destination. As shown in Figure 9, esch geographic location in the continents9 United Ststes can be sssociated with a speci~ic channel in a specific zone. Thus, Los Angeles bl podtioned botween channelo 14 ~md lS In tran~mit zone 37.

;

. ~ ' .
.~

.

-19- 132~3~7 Referring now concurrently to ~igures 5, 8 and 9 particularly, receive and tran~nit zones R1 and T1 lie within the East Coast zone 32 and 31, R2 and T2 lie within the Central zone 34 and 33, R3 and T3 lie within the Mountain zone 36 and 35, and R4 and T4 lie within the West Coast zone 38 and 37. Since Detroit lies in the Central or R2 zone 34~ it can be seen that the only channels over ~ich ~ignals can be transmitted to the West Coast or T4 zone 37 are channels 1, 5, 9 and 13. This is determined in the table OI Figure 8 by the intersection of row R2 and column T4. Therefore, from Detroit, the uplink user would uplink on either channel 1, 5, 9 or 13, whichever of these channels is closest to the downlink destination. Since Los Angeles is located between channels 14 and 15, the network control center would uplink the signal on channel 13 because channel 13 is the closest to channel 14.
The downlink beam width is broad enough to provide high gain at Los Angeles.
r:
Conversely, if the uplink site is in Los Angeles and the downlink destination is in Detroit, the intersection of row R4 and col~nn T2 in ~igure 8 must be consulted. This intersection reveals that the signal can be tran~mitted on channels 1, 5, 9 or 13 depending upon which channel is closest to the downlink destination. The network control center would uplink the signal rrom Los Angeles on channel 9 since channel 9 is closest to channel 11 which, in turn, is closest to Detroit.

Returning now to Figure 10, the conversion Or a receive signal to a transmit signal will be described in connection with the example mentioned sbove in ~ich the uplink site is in Detroit and the downlink site is in Los Angeles. The uplink signal transmitted from Detroit would be transmitted on channel 13 carriied by receive signal R2.
Thus, the R2 receive slgnal is input to transmission line 122 and a portion o~ such input signal Is diverted by the hybrid coupler 130 to the input line o~ ~ubgroup 108 o~ fllters 102. Subgroup 108 includes a bank of eight filters ror the odd channela, including channel 13. Thus, the incom~ng signal is riltered through by ~ilter 13 and is output on a line 164 along with other signals ~rom subgroups 108 and 116. The channel 13 .

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-20- 132~7 signal present on line 164, is con~ined by the hybrid coupler 158, with signals emanating from subgroup 106 and 114, and forms the T4 signal on output line 150. The transn~it signal T4 is then downlinked to L.os Angeles.

It is to be understood that the above exanple is somewhat sD~lified inasmuch as the network control cen~er would assign a more specific channel than a 27 MHz wide band channel, since the 27 MHz wide channel n~y actually comprise a ~ltiplicity of sma~ler channels, for exarr~>le, 800 subchannels of 3a KHz bandwidth.

Referring now again to Figures 5, 8 and 9, in the event that an uplink signal originates from one of the areas of contenffon, 40, 42, 44 (Pigure 5), such signal will not only be trsnsmitted to its desired downlink destination, but a non-neglible signal will be transmitted to another geographic sres. For ex~nple, a~sume that the uplink signal originates from Chicago, Illinois which is in the sres of contention 42 and that the signal is destined for Los Angeles, CQlifornia. The area of contention 42 is produced by the overlap OI the besm orming zones 34 and 36. Hence, the uplink signal CQn be transmitted as receive signals R2 or R3. The network control center determines whether the uplink communication is carried by receive ~ign~ls R2 or R3. In the present example, since Chicago is closer to zone 36, the uplink comTIunicstion i8 carried on recelve slgnal R3.

As previously dis¢ussed, the downlink destination, Los Angeles, i8 located in zone 37 and lies between channels 14 snd 15. As shown in Figure 8, the intersection of R3 with colwm T4 yields the possible channels over which the comnunication c~n be routed. Thus, the Chicago uplink ~ignal will be transmitted over one of channels 2, 6,10 or 14. 8ince Lo~ Angeles is closest to channel 14, channel 14 is selected by the network control center as the uplink channel. Note, however, that an undesired signal is also transmitted from zone 34 on channel 14.
To determlne where the undesired signal will be downllnked, the table of Pigure 8 is consulted. The table o~ Pigure 8 reveals that uplink signals : ~ ' ' ' ' ',.:
. . . .

' 21- 132~
carried on channel 14 in the R2 zone 34 are do~linked to the T1 transmit zone 31. The desired signal is transmitted to Los Angeles and the undesired signal (i.e. an extraneous signal~ is transrnitted to the East Coast (i.e. zone 31). The network control center keeps track of these S extraneous signals when mEIking frequency assigr~nts. The effect of these extraneous signaLs is to reduce slightly the capacity of the system.

Referring now again to Figure 6, the beam-fornnng network 98 receives the transmit signaLs T1-T4 and functions to couple all of the individual commJnication signaLs in these trsnsmit signaLs together so that a transmit antenna beam for each signal is formed. In the example di~cussed above in which the assigned freguency spectrum is 500 MHz, a total of approximately 50,000 overlapping antenna beami are formed by the beam-fom~ing network 98 when the system is îully loaded with narrow band signaLs. Each anteMa beam is iormed in a manner so that it can be pointed in a direction which optimizes the perfom~nce of the system. The incremental phase 3hift between adjacent elements determines the dlrection o$ the antenna beam. Since this phase shift is determined by the signal ~requency, the system is referred to cs $requency addressed.
ao Attention is now directed to Figures 11 and 12 which depict the details of the beam-forming network 98. The beam-$orming network, generally indicated by the numeral 98 in Figure 11, is arranged in the general $orm o~ an arc and may be conveniently mounted on the communication shel2 (not shown) of the satellite. The arc shape o$ the 2S beam~$orming network 98 facilitates an arrangement which assures that the paths of the signals passing therethrough are of correct length.
, .
The beam-~ormi-~ network ~ includes a ~irst set o~
circum$erentially extending transmission delay lines 168, 170, a second set o$ transmission delay lines 172, 174 which are radially spaced from delay lines 168 and 170, and a plurality Or radially extending waveguide Assemblies 176. In the illustrated embodiment, forty waveguide asse~lies 176 are provided, one $or each of the elements 106 of the transmit array ~' .
:, ..
., .
.;,...
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. . .

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-22- 132~3a7 20 (Figure 13). The waveguide assemblies 176 intersect each of the delay lines 168-174 and are equally spaced in angle.

- Esch of the waveguide assemblies 17S defines a radial line sumner and intersects each of the delay lines 168-174. As shown in Figure 12, at the points of intersection, between the radial line sumners 176 ~nd the tran~ni~sion delay lines 168-174, a cro~sguide coupler 180 is -: provided. The crossguide coupler 180 connects the delay lines 168-174 ~ with the rndial line sumners 176. The function of the crossguide couplers 180 will be discussed later in more detail.

Four delay li~es 168-174 are provided respectively for the four transmit zones T1-T4 (Figure 9). Hence, transmit signal T1 is provided to the input of delay line 170, T2 i~ provided to input of delay line 168, T3 i8 provided to the input of delay line 174, and T4 is provided to the input of delay line 172. As shown in Figure 12, the distance between the radial line sumners is indicated by the letter "1" and the width of each of the radial delay lines is design~ted by the letter "wn.
Although the radial line sumners 176 are spaced at equal angular intervals along the delay lines 168-174, the distance between them varies from delay line to delay line due to the fact that the delay lines 168-174 are radially spaced from each other. Thus, the hlrther from the center of the arc, which i8 formed by the radial line sumners 176, the greater the distance between the radial line sumners 176, at the point where they intersect wlth the delay lines 168-174. In other words, the sp~cing "1--between radial line summers 176 for delay line 168 is less than the as spaclng between ad~acent radial line summers 176 than for delay line 174.
Typical values for the dimensions "1" and "w" are as follows:

Delay Line Si~nal 1, inches w, inches 168 T2 1.66 0.64 170 T1 1.72 0.66 172 T4 2.45 0.74 174 T3 2.55 0.76 1 3 2 ~ 7 The width of the delay lines 168-174, "w", and the distance "1" between adjacent radial line surnners are chosen to provide the desired center beam squint and beam scan rate so that the beRm pointing is correct for each channel. This results in the desired start and stop points for each of the transnit zones T1-T4.

Referring particularly to Figure 12~ the transnit signal T2 propagates down the delay line 168 for a precise distance, at which point it reaches the first radial line sumner 176. A portion of the T2 signal pssses through the crossguide coupler 180, which may, for exa~ple, be a 20 dB coupler, such that one percent of the tran~nitted power of transmit signal T2 i~ diverted down the radial line summer 176. This diverted energy then propagates down the waveguide 176 towards a corresponding solid state power amp]ifier 100 (~igures 6 and 11). This process is repeated for signal Tl which propagates down delay line 170.
The portions o~ signals T1 and T2 which are diverted by the crossguide couplers 180 (i.e. 0.01 T1 Hnd 0.01 T2) are summed together in the radial line surnmer 176 and the combined signal 0.01 (T1 I T2) propagates radlally outwardly toward the next set of delay lines 172, 174. This same coupling process is repeated for signals T3 and T4 in delay lines 174 and 172 respectively. That is, 0.01 of signals T3 and T4 are coupled via crossguide couplers 180 to the radial line sumner 176. The resulting conbined signal 0.01 (T1 + T2 + T3 + T4) propagates radially outwardly to an a3sociated solid state power amplifier 100 where it is ampli~ied in preparation for transmhssion.
After encountering the first radial line summer 176, the remalning 0.99 Or signals T1-T4 propagate to the second redial line summer where an additional one percent of the SigrlQ]13 is diverted to lthe summer 176. This process Or divertlng one percent of the signals Tl-T4 is repeated ror each Or the radial line summers 176.

The signals, propagating through the radial line summers 176 towards the SSPA~ 100, are a mixture Or all four point-to-point .,~
,,:
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-;

,: . . ~ ., . .. - . . ~ . .. ~.. . .. . . .... ., , ., - - .

i326~

transmit signals T1-T4. However, each of the transmit signals T1-T4 may ~omprise 12,500 subsignals. Consequently, the forty signals propagating through the r~dial line surnners 176 m~y be a mixture of all 50,000 signalC
in the case of the errbodiment mentioned above where the assigned frequency spectrurn is 500 MHz wide. Therefore, each of the SSPAs 100 amplifies all 50,000 signals which emanate from each of the plurality of wave guide assemblies 176.

Since each of the S~PAs 100 amplifies all 50,00û signals which are destined for all regions of the country, it can be appreciated that a~l of the narrow, high gain downlink beans are formed from a commo~ pool of transmitters, i.e. all of the SSPAs 100. This ~rrangement may be thought of as effectively providing a nationwide pool of power since each o~ the downlink beams covering the entire country is produced using all of the SSPAs 100. Consequently, it is possible to divert a portion OI this nationwide pool of power to accomnDdate speciffc, disadvantaged downlink users on an individual basis without n~terially reducing the signal power of the other beam. For example, a downlink user may be "disadvantaged" by rain in the downlink destination which attenuates the signal strength o~ the beam. Such a rain disadvantaged user may be individually accommodated by increasing the signal strength of the corresponding uplink beam~ This is accomplished by diverffng to the disadvantaged downlink beam, a small portion of the power from the pool oi nationwide transmitter power (i.e. a fraction of the power supplied by all o~ the SSPAs lO0). The power of an individual up}ink beam iB proportional to that o~ the corresponding downlink beam.
Consequently, in order to increase the power o~ the downlink beam it is merely necessary to increase the power OI the uplink beanL

In praotice, the previously ml~ntioned network control center keeps track OI all o~ those regions OI the country in which it is raining and determines which o~ the uplink users are placing communications to downlink destinations in rain affected areas. The network control center then instructs each of these uplink users, using packet switched signals, to increase its uplink power ~or those signals .

. .
:.
:

. . .
.
: ' .

1326~7 destined for a rain affected area. me increase in power of the uplink user's signals results in greater co~1ective dr~lificstion of these signals by the SSPAs 100, to produce corresponding downlink beems to ~he rain affected areas, which have power levels increased sufficiently to compensate for rain attenuation. Typically, the number of signals destined for rain aifected areas is small relative to the total n~rber of signals being handled by the total pool of SSPAs 100. ~ccordingly, other downlink users not in the rain affected zones do not suffer substantial signal loss since the smQIl loss that may occur in their signals is ~pread out over the many thousand users.

The SSPAs 100 (Figures 8 and 11) may be mounted, for example, on the rim of the com~n~nication shelf (not shoY.n) of the satellite. The signals amplified by the SSPAs 100 are ~ed into the corresponding elements 106 oi the tran~mit array 20 (Figure 13 and 14).

As previously discussed, an incremental phase shift is achieved between the signals that are coupled in the ~orty rsdial line summers 17B. Hence, the beam-fo~7mng network 98 permits the antenna beams ernanating ~rom the transmit array 20 ~Pigures 1, a, and 13) to be steered by frequency assignment. The incremental phase shlft i~ related ; 20 to the time delay between the waveguides 17B as well ss ~reguency.
AthnUon is now directed to Pigure 17 which is a diagrammatic view oi four o~ the torty trnnsmit array elements lOB (Pigure 13), showing the wave~ront e~anating there~rom~ wherein "d" is equal to the spacing between transmlt array elements 106. The resulting antenna ~am has an angular tilt Or 0 , where 0 is de~ined as the beam scan engle. This means that ~ is the angle ~rom normal oi the transmit beam center.
lhe incremental phase shi~ produced by the del~ line ~rrangement is The relationship between ~ and 0 i8 given by ;
., ~ ~ = ~ un 0 ~1 .

,.;

.~, , ., .
, -26- 13263~7 where:
= signal waYelength = beam scan angle d = spacing between array elements S Hence, the east-west direction of the antenna beam is dete~nned by the incremental phase shift which is different for the four delay lines 168-174 oi the beam-forming network 98, resulting in the four transn~e zones Tl-T4 previously noted.

Having thus described the invention, it is recognized that those skilled in the art may make various n~diffcations or additions to the preielTed embodiment chosen ts illustr~te the invention without departing rrom the ~pirit and ~cope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the sub~ect matter ~ 15 claimed and all equivalents thereof tairly within the scope of the ; invention.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A satellite communications system for communicatively interconnecting a plurality of terminal sites distributed over an area of the earth, comprising:
an earth orbiting satellite; and frequency responsive means carried by said satellite for forming a plurality of beams of electromagnetic radiation between said area on the earth and said satellite with each beam of radiation having a carrier frequency, said beam forming means including an array of radiating elements and an antenna for producing all of said beams, said beams being arranged in at least two groups thereof respectively covering two essentially contiguous zones within said area, the beams in said groups thereof intersecting said area on the earth in seriatim along an arc with the position of each of said beams along said arc being determined by the carrier frequency of the beam, said groups of beams being arranged essentially end-to-end along said arc, the beams in each group thereof being carried by the same preselected set of frequencies such said set of frequencies is reused by said beams.

2. The satellite communications system of claim 1, wherein the carrier frequencies of immediately adjacent portions of two contiguous zones serviced by two respectively associated beams are sufficiently different in frequency to prevent communication interference therebetween.

3. The satellite communications system of claim 1, wherein said preselected set of frequencies includes a first range of up-link carrier frequencies and a second range of down-link carrier frequencies.

4. The satellite communications system of claim 3, including means for selecting the up-link carrier frequency for an uplink terminal site as a function of the location of the downlink terminal site along said arc which is intended to receive communications signals from said up-link terminal site.

5. The satellite communications system of claim 1, wherein the number of beams formed using downlink carrier frequencies is equal to the number of beams formed using the uplink carrier frequencies.

6. The satellite communications system of claim 3, wherein said uplink carrier frequencies are arranged in a plurality of frequency bands defining a set of uplink channels and said downlink carrier frequencies are arranged in a plurality of frequency bands defining downlink channels respectively associated with said uplink channels, said system including means carried by said satellite for interconnected said uplink channels with said downlink channels in a preselected pattern such that uplink beams signals on each of said uplink channels is delivered to a downlink terminal site on a preselected downlink channel.

7. The satellite communications system of claim 6, wherein said interconnecting means includes:
at least one input for receiving signals over said uplink channels, a plurality of outputs, and means connected between said inputs and said outputs for separating said uplink channels and for routing each of said separated uplink channels with a preselected one of said outputs.

8. The satellite communications system of claim 7, wherein said separating and routing means includes a plurality of frequency filters for respectively filtering said uplink channels.

9. The satellite communications system of claim 1, wherein said beam forming means includes:
a first plurality of lines for respectively carrying transmit signals to be transmitted to earth by said beams;
a second plurality of spaced apart lines intersecting said first plurality of lines at crossover points, each of said second plurality of lines being electromagnetically coupled with each of said first plurality of lines at said crossover points such that a portion of the energy of each of the transmit signals carried by each of the first plurality of lines is transferred to each of said second plurality of lines, each of said second plurality of lines having an output for outputting all of said transmit signals.

10. The satellite communications system of claim 9, wherein the distance between adjacent ones of said crossover points and the width of each of said plurality of lines is preselected to produce a desired shift in the phase of said transmit signals.

11. The satellite communications system of claim 9, wherein said second plurality of lines extend essentially radially from a reference point such that said second plurality of lines diverge from each other.

12. The satellite communications system of claim 11, wherein said first plurality of lines extend circumferentially about said reference point and are radially spaced relative to said reference point such that the distance between adjacent crossover points increases with increasing radial distance of the crossover points from said reference point.

13. The satellite communications system of claim 9, wherein said first plurality of lines includes a plurality of transmission lines for conveying electromagnetic energy and said second plurality of lines includes a plurality of electromagnetic energy waveguides.

14. The satellite communications system of claim 13, wherein each of said transmission lines is coupled with each of said waveguides by an electromagnetic wave crossguide coupler.

15. The satellite communications systems of claim 9, wherein at least two of said first plurality of lines are substantially contiguous to each other.

16. The satellite communications system of claim 9, wherein at least two of said first plurality of lines are spaced from each other.

17. A method of communicatively interconnecting a plurality of terminal sites in an area on the earth using an earth orbiting communications satellite, comprising the steps of:
(A) forming a first plurality of beams of electromagnetic radiation in an arc between said satellite and the earth and covering a first zone of said area and arranged in seriatim, said beams in said first plurality thereof respectively having carrier frequencies defining a preselected set; and (B) forming a second plurality of beams of electromagnetic radiation in said arc between said satellite and the earth and covering a second zone of said area essentially contiguous to said first zone and arranged in seriatim, said first and second pluralities of beams being arranged essentially end-to-end along said arc, said beams in said second plurality thereof respectively having the same carrier frequencies as said first plurality of said beams, such that said preselected set of frequencies is reused, each of said beams in said first and second pluralities thereof being formed such that the position of the beam along said arc is determined by the carrier frequency of the beam.

18. The method of claim 17, wherein:
step (A) and (B) each include the steps of forming a plurality of uplink beams of electromagnetic radiation respectively having a set of uplink carrier frequencies, and forming a plurality of downlink beams of electromagnetic radiation respectively having a set of downlink carrier freqencies, and said method further includes the step of selecting the uplink carrier frequency for an uplink terminal site as a function of the location of the downlink terminal site in either of said zones which is intended to receive communication signals from an uplink terminal site.

19. The method of claim 18, including the steps of:
assigning uplink and downlink channels to groups of carrier frequencies respectively in said uplink and downlink carrier frequencies, and interconnecting said uplink channels with said downlink channels in a preselected pattern such that uplink signals on each of said uplink channels is delivered to a downlink terminal site on a preselected downlink channel.

20. The method of claim 17, including the step of selecting carrier frequencies respectively in immediately adjacent portions of contiguous zones which are substantially different in frequency whereby to prevent communication interference therebetween.