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Description  |
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TECHNICAL FIELD
The present invention broadly relates to satellite communication systems
especially of the type employing a satellite placed in geosynchronous
orbit above the earth so as to form a communication link between many
small aperture terminals on the earth. More particularly, the invention
involves a communication satellite having hybrid communication capability
accommodating both two-way and broadcast communication systems. Two-way
communications between small aperture earth terminals is achieved through
multi-fold reuse of a fixed frequency spectrum in contiguous zones of an
area on the earth.
BACKGROUND ART
In domestic communication satellite systems, which interconnect large
numbers of very small aperture earth terminals, the most important
parameters affecting the system capacity are the Effective Isotropic
Radiated Power (EIRP) and the available bandwidth. EIRP refers to a
measure of the satellite's transmitter power which takes into
consideration the gain of the antenna. EIRP is the power of a transmitter
and isotropic antenna that would achieve the same result as the
transmitter and antenna which is actually employed.
In the past, high antenna gain and multiple frequency reuse has been
achieved by employing a plurality of up and down link beams covering the
regions of a country or other areas of the earth to be served. Both
frequency division and time division systems have been used or proposed to
interconnect large numbers of signals from many geographically separated
earth stations. Time division systems permit the satellite transmitters to
operate efficiently. This is because only one time division signal at a
time is amplified in a transmitter, so it may be operated at or close to
signal channel saturation, the most efficient operating point. However,
time division systems require high power ground transmitters and expensive
signal processing and are therefore incompatible with low cost earth
stations. Frequency division systems are better suited to low cost earth
station, but have lower satellite transmitter efficiency because each
transmitter handles multiple carriers. Since multiple carrier amplifiers
generate undesirable intermodulation products that increase in power as
the transmitter efficiency is increased, the optimum compromise between
transmitter efficiency and intermodulation product generation results in a
relatively low transmitter efficiency.
In Ku band, the satellite communication band most suitable for two-way
service between very small terminals, the attenuation of the signals by
rain is an important consideration in the design of the system. In the
previous systems, this attenuation is overcome on the downlink by using
higher satellite transmitter power per channel than would be necessary for
clear weather service, typically four times as much. This accommodation of
rain attenuation therefore results in more expensive satellites having
fewer available channels.
The available bandwidth of a satellite system is determined by the number
of times the allocated frequency spectrum can be reused. Polarization and
spatial isolation of beams have been employed to permit reuse of the
frequency spectrum. As the number of isolated beams is increased, however,
the problem of interconnecting all the users becomes very complicated and
is one of the factors that limit the number of reuses of the frequency
spectrum.
The present invention is directed toward overcoming each of the
deficiencies mentioned above.
SUMMARY OF THE INVENTION
The present invention provides a satellite communication system for
interconnecting large numbers of very small aperture earth terminals and
mobile satellite service users which maximizes satellite EIRP as well as
the available bandwidth. The system employs highly directional, contiguous
beams on the downlink or transmit signal which substantially increases the
EIRP and allows multiple reuse of the assigned frequency spectrum. As a
result, the number of communications that can be provided for
point-to-point service is maximized. High multi-carrier transmitter
efficiency is achieved as a result of the dispersion of intermodulation
products and the deleterious effects of rain on the downlink channels are
easily overcome by the use of pooled transmitter power. The
interconnection of the many users is achieved by a combination of a filter
interconnection matrix and a highly directional addressable downlink beam.
According to the present invention, a system is provided for
interconnecting any of a plurality of earth terminals or mobile terminals
within an area on the earth for two-way communication using a
communications satellite. A plurality of uplink beams are formed which
respectively emanate from contiguous zones covering the area to be
serviced by the satellite. The uplink beams carry a plurality of channels
over a first preset range of uplink frequencies. Each uplink zone uses the
same preset range of frequencies. The uplink frequencies are therefore
reused by each zone, thereby effectively multiplying the number of
communications channels that can be handled by the satellite. A plurality
of downlink beams destined for the downlink zones also carry a plurality
of channels over a second preset range of frequencies. The beams for each
of the downlink zones also use the same second preset range of frequencies
to provide multiple reuse of these frequencies. The satellite employs a
filter interconnection matrix for interconnecting the channels in the
different zones.
The preset range of frequencies are separated into first and second sets.
The preset range of frequencies is spatially distributed over each zone
such that contiguous regions of adjacent zones are not serviced by the
same frequency set. Since contiguous regions of adjacent zones communicate
over different frequency sets, there is sufficient spatial isolation
between contiguous zones to permit frequency reuse. The beams operating
over the first frequency set are simultaneously transmitted to all of the
zones and then the beams operating over the second set are simultaneously
transmitted to all the zones. This is achieved by using a two-position
switch which insures that beams operating over the first and second sets
are alternately transmitted.
A fan beam narrow in one direction, east-west for example, and broad in the
orthogonal direction, is generated by a beam-forming network used in
conjunction with the transmit array antenna. The transmit array may be a
confocal arrangement or a direct radiating array. The east-west direction
of the beam within the covered area is determined by the downlink
frequency, which is related to the uplink frequency by a constant
difference. The uplink frequency therefore determines the downlink
frequency, which by action of the beam-forming network and the transmit
array determine the direction and hence, the destination of the downlink
beam. Such an arrangement is referred to as a frequency addressable beam.
The side lobes of the beam are designed to be low enough to permit reuse
of the frequency spectrum in the adjacent zones.
The transmitters, preferably equal power dual solid state amplifiers, are
embedded in the transmit array antenna, one amplifier being associated
with two staves of the array. All of the amplifiers operate at the same
power level despite the unequal power inputs and outputs. Since all of the
downlink power is provided by this single pool of transmitters, it is easy
to provide relatively high power to those relatively few signals directed
to rain affected areas with only a very small reduction in power available
to the much larger number of unimpaired signals.
Because the transmit beam directions are related to the frequencies of
their signals, and the frequencies of the intermodulation products
generated in the power amplifiers differ from those of the signals which
cause them, the intermodulation products go down in different directions
than the signals. This process results in the spatial dispersion of the
intermodulation products. This dispersion is enhanced by the use of
multiple downlink zones. This results in a lower intermodulation product
density at all ground terminals in the frequency bands to which they are
tuned. This reduced sensitivity to intermodulation products permits the
power amplifiers in the satellite to be operated more efficiently.
It is therefore a primary object of the invention to provide a
communication satellite for interconnecting large numbers of very small
aperture antenna terminals and mobile satellite service users using high
satellite transmit antenna gain and allowing multiple reuse of the
assigned frequency spectrum, to substantially increase the number of
communication channels that can be provided for point-to-point
communication service.
Another object of the invention is to provide the downlink power from a
single pool of transmitters, so that signals being attenuated by rain can
be easily allocated more satellite transmit power.
Another object of the invention is to provide a communication satellite
which disperses intermodulation products in order to increase transmitter
efficiency.
A further object of the invention is to provide a communication satellite
as described above which provides both broadcast, and point-to-point
communications service.
A further object of the invention is to provide a communication satellite
which uses direct radiating array antennas in a system which reuses the
assigned frequency spectrum.
Another object of the invention is to provide point-to-point and broadcast
services on a single polarization.
These, and further objects and advantages of the invention will be made
clear or will become apparent during the course of the following
description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a perspective view of a communications satellite, showing the
antenna subsystems;
FIG. 2 is a top plan view of the antenna subsystems shown in FIG. 1;
FIG. 3 is a sectional view taken along the line 3--3 in FIG. 2;
FIG. 4 is a sectional view taken along the line 4--4 in FIG. 2;
FIG. 5 is a view of the United States and depicts multiple, contiguous
receive zones covered by the satellite of the present invention, the
primary areas of coverage being indicated in cross-hatching and the areas
of contention being indicated by a dimpled pattern;
FIG. 6 is a block diagram of the communication electronics for the
communications satellite;
FIG. 7 is a schematic diagram of a coupling network which interconnects the
point-to-point receive feed horns with the inputs to the communications
electronics shown in FIG. 6;
FIG. 8 is a reference table of the interconnect channels employed to
connect the receive and transmit zones for the point-to-point system;
FIG. 9 is a diagrammatic view of the United States depicting multiple
contiguous transmit zones covered by the satellite and the geographic
distribution of the interconnected channels for each zone, across the
United States;
FIG. 9A is a graph showing the variation in gain of the transmit antenna
beam for each zone in the point-to-point system in relation to the
distance from the center of the beam in the east-west direction;
FIG. 9B is a graph similar to FIG. 9A but showing the variation in gain in
the north-south direction;
FIG. 10 is a detailed schematic diagram of the filter interconnection
matrix employed in the point-to-point system;
FIG. 11 is a detailed, plan view of the beam-forming network employed in
the point-to-point system;
FIG. 12 is an enlarged, fragmentary view of a portion of the beam-forming
network shown in FIG. 11;
FIG. 13 is a front elevational view of the transmit array for the
point-to-point system, the horizontal slots in each transmit element not
being shown for sake of simplicity;
FIG. 14 is a side view of the transmit element of the array shown in FIG.
13 and depicting a corporate feed network for the element;
FIG. 15 is a front, perspective view of one of the transmit elements
employed in the transmit array of FIG. 13;
FIG. 16 is a front view of the receive feed horns for the point-to-point
system;
FIG. 17 is a diagrammatic view showing the relationship between a
transmitted wave and a portion of the transmit feed array for the
point-to-point system;
FIG. 18 is a perspective view of a deployed mobile satellite which forms an
alternate embodiment of the present invention;
FIG. 19 is similar to FIG. 18 but depicts the mobile satellite in its
stowed position;
FIG. 20 is an elevational view of a directly-radiating array antenna
forming part of the satellite of FIG. 18;
FIG. 21 is a view similar to FIG. 5 but depicting zones covered by the
satellite of FIG. 18;
FIG. 22 is an illustration similar to FIG. 9A, but showing the antenna
pattern contours for three zones;
FIG. 23 is a diagrammatic view showing a beam-forming network designed for
a system which reuses the frequency spectrum three times;
FIG. 24 is similar to FIG. 6, but illustrates a block diagram for the
alternate embodiment;
FIG. 25 is a diagrammatic view of an equal power dual amplifier and the
resulting power distribution;
FIG. 26 is a perspective view of another deployed satellite which forms a
further embodiment of the present invention;
FIG. 27 is a view similar to FIG. 5 and FIG. 21, but depicting alternate
receive zones;
FIG. 28 is an alternate illustration, similar to FIG. 9A and FIG. 22, of
the receive beam patterns;
FIG. 29 is similar to FIG. 9 illustrating alternate transmit zones;
FIG. 30 is an illustration of the transmit beam pattern corresponding to
the transmit zones in FIG. 29;
FIG. 31 is a reference table similar to that shown in FIG. 8, but for a
filter interconnection matrix for the mobile satellite system depicted in
FIG. 26; and
FIG. 32 is a block diagram of the repeater employed in the satellite shown
in FIG. 26.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIGS. 1-4, a communications satellite 10 is depicted
which is placed in geosynchronous orbit above the earth's surface. The
satellite's antenna system, which will be described in more detail below,
will typically be mounted on an earth-oriented platform so that the
antenna system maintains a constant orientation with respect to the earth.
The satellite 10 is of a hybrid communications-type satellite which
provides two different types of communication services in a particular
frequency band, for example, the fixed satellite service Ku band. One type
of communication service, referred to hereinafter as point-to-point
service, provides two-way communications between very small aperture
antenna terminals of relatively narrow band voice and data signals.
Through the use of frequency division multiple access (FDMA) and reuse of
the assigned frequency spectrum, tens of thousands of such communication
channels are accommodated simultaneously on a single linear polarization.
The other type of communication service provided by the satellite 10 is a
broadcast service, and it is carried on the other linear polarization. The
broadcast service is primarily used for one-way distribution of video and
data throughout the geographic territory served by the satellite 10. As
such, the transmit antenna beam covers the entire geographic territory.
For illustrative purposes throughout this description, it will be assumed
that the geographic area to be serviced by both the point-to-point and
broadcast services will be the United States. Accordingly, the broadcast
service will be referred to hereinafter as CONUS (Continental United
States).
The antenna system of the satellite 10 includes a conventional omni antenna
13 and two antenna subsystems for respectively servicing the
point-to-point and CONUS systems. The point-to-point antenna subsystem
provides a two-way communication link to interconnect earth stations for
two- way communications. The CONUS antenna system functions as a
transponder to broadcast, over a wide pattern covering the entire United
States, signals received by one or more particular locations on earth. The
point-to-point transmit signal and the CONUS receive signal are vertically
polarized. The CONUS transmit and point-to-point receive signals are
horizontally polarized. The antenna system includes a large reflector
assembly 12 comprising two reflectors 12a, 12b. The two reflectors 12a,
12b are rotated relative to each other about a common axis and intersect
at their midpoints. The reflector 12a is horizontally polarized and
operates with horizontally polarized signals, while the reflector 12b is
vertically polarized and therefore operates with vertically polarized
signals. Consequently, each of the reflectors 12a, 12b reflects signals
which the other reflector 12a, 12b transmits.
A frequency selective screen 18 is provided which includes two halves or
sections 18a, 18b and is mounted on a support 30 such that the screen
halves 18a, 18b are disposed on opposite sides of a centerline passing
diametrically through the satellite 10, as best seen in FIG. 2. The
frequency selective screen 18 functions as a diplexer for separating
different bands of frequencies and may comprise an array of discrete,
electrically conductive elements formed of any suitable material, such as
copper. Any of various types of known frequency selective screens may be
employed in this antenna system. However, one suitable frequency selective
screen, exhibiting sharp transition characteristics and capable of
separating two frequency bands which are relatively close to each other,
is described in U.S. patent application Ser. No. 896,534, filed Aug. 14,
1986, and assigned to Hughes Aircraft Company. The frequency selective
screen 18 effectively separates the transmitted and received signals for
both the CONUS and point-to-point subsystems. It may be appreciated that
the two halves 18a, 18b of the screen 18 are respectively adapted to
separate individual signals which are horizontally and vertically
polarized.
The CONUS subsystem, which serves the entire country with a single beam
has, in this example, eight conventional transponders each having a high
power traveling wave tube amplifier as its transmitter 82 (see FIG. 6).
The CONUS receive antenna uses vertical polarization, sharing the
vertically polarized reflector 12b with the point-to-point transmission
system. CONUS receive signals pass through the frequency selective screen
half 18b and are focused on the receive feed horns 14 located at the focal
plane 28 of reflector 12b. The antenna pattern so formed is shaped to
cover CONUS. The CONUS transmit antenna employs horizontal polarization,
and shares reflector 12a with the point-to-point receive system. Signals
radiating from the transmit feeds 24 are reflected by the horizontally
polarized frequency selective screen 18a to reflector 12a whose secondary
pattern is shaped to cover CONUS.
The point-to-point subsystem broadly includes a transmit array 20, a
subreflector 22, and receive feed horns 16. The transmit array 20, which
will be described later in more detail, is mounted on the support 30,
immediately beneath the screen 18. The subreflector 22 is mounted forward
of the transmit array 20 and slightly below the screen 18. The signal
emanating from the transmit array 20 is reflected by the subreflector 22
onto one half 18b of the screen 18. The subreflector 22 in conjunction
with the main reflector 12 functions to effectively magnify and enlarge
the pattern of the signal emanating from the transmit array 20. The signal
reflected from the subreflector 22 is, in turn, reflected by one half 18b
of the screen 18 onto the large reflector 12b, which in turn reflects the
point-to-point signal to the earth. Through this arrangement, the
performance of a large aperture phase array is achieved. The receive feed
horns 16 are positioned in the focal plane 26 of the reflector 12a. It
consists of four main horns 50, 54, 58, 62 and three auxiliary horns 52,
56, 60 as shown in FIG. 16.
Referring now also to FIGS. 13-15, the transmit array 20 comprises a
plurality, for example forty, transmit waveguide elements 106 disposed in
side-by-side relationship to form an array, as shown in FIG. 13. Each of
the transmit waveguide elements 106 includes a plurality, for example
twenty-six, of horizontal, vertically spaced slots 108 therein which
result in the generation of a vertically polarized signal. As shown in
FIG. 14, the transmit array 20 is fed with a transmit signal by means of a
corporate feed network, generally indicated by the numeral 110 which
excites the array element in four places 114. The purpose of the corporate
feed network 110 is to provide a broadband match to the transmit waveguide
element 106. Signals input to the waveguide opening 112 excite the array
slots 108 so that the slot excitation is designed to give a flat pattern
in the north-south direction.
Attention is now directed to FIG. 5 which depicts a generally rectangular
beam coverage provided by the horizontally polarized point-to-point
receive system. In this particular example, the area serviced by the
point-to-point system is the continental United States. The point-to-point
receive system comprises four beams R1, R2, R3, R4 respectively emanating
from the four uplink zones 32, 34, 36, 38 to the satellite, wherein each
of the beams R1-R4 consists of a plurality of individual uplink beams
originating from individual sites in each zone 32, 34, 36, 38 and carries
an individual signal from that site. The uplink beam signals from the
individual sites are arranged into a plurality of channels for each zone.
For example, zone 32 may include a plurality, e.g. sixteen 27 MHz channels
with each of such channels carrying hundreds of individual beam signals
from corresponding uplink sites in zone 32.
The signal strength for each of the four beam pattern contours,
respectively designated by numerals 32, 34, 36 and 38, are approximately 3
dB down from peaks of their respective beams. The antenna beams have been
designed to achieve sufficient isolation between them to make feasible in
the cross-hatched regions 39, 41, 43, 45 reuse of the frequency spectrum
four times. In the dotted regions 40, 42, and 44, the isolation is
insufficient to distinguish between signals of the same frequency
originating in adjacent zones. Each signal originating in these regions
will generate two downlink signals, one intended and one extraneous. The
generation of extraneous signals in these areas will be discussed later in
more detail.
It may be readily appreciated from FIG. 5 that the four zones covered by
beams 32, 34, 36, 38 are unequal in width. The East Coast zone covered by
beam 32 extends approximately 1.2 degrees; the Central zone covered by
beam 34 extends approximately 1.2 degrees; the Midwest zone covered by
beam pattern 36 extends approximately 20 degrees, and; the West Coast zone
covered by beam pattern 38 extends approximately 2.0 degrees. The width of
each of the four receive zones 32, 34, 36 and 38 is determined by the
number of terminals and thus the population density in the various regions
of the country. Thus, beam pattern 32 is relatively narrow to accommodate
the relatively high population density in the Eastern part of the United
States while beam pattern 36 is relatively wide due to the relatively low
population density in the Mountain states. Since each zone utilizes the
entire frequency spectrum, zone widths are narrower in regions where the
population density is high, to accommodate the greater demand for channel
usage.
As shown in FIG. 9, the point-to-point transmit system comprises four beams
T1, T2, T3, T4 respectively covering the four transmit zones 31, 33, 35,
37, wherein each of the beams T1-T4 consists of a plurality of individual
downlink beams destined for the individual downlink sites in each zone 31,
33, 35, 37 and carries an individual signal to that site. The downlink
beam signals, destined to be received at the individual downlink sites,
are arranged into a plurality of channels for each zone. For example, zone
31 may include a plurality, e.g. sixteen 27 MHz channels with each of such
channels carrying hundreds of individual beam signals to corresponding
downlink sites in zone 32.
The use of multiple downlink zones and downlink zones of unequal widths
assist in causing the intermodulation products, generated by the
later-discussed solid state power amplifiers, to be geographically
dispersed in a manner which prevents most of these products from being
received at the ground terminals. The net effect is that the amplifiers
may be operated more efficiently because the system can tolerate more
intermodulation products. Although the widths of the transmit zones 31,
33, 35, 37 are nearly the same as those of the receive zones R1, R2, R3,
R4, small differences between the two sets have been found to optimize the
capacity of the system.
The half power beam width of the individual transmit beams 29 is
substantially narrower than that of the transmit zones 31, 33, 35, 37.
This results in the desirable high gain, and avoids the zones of
contention 40, 42, 44 characteristic of the receive zone arrangement.
These individual beams 29 must be steered within the zones in order to
maximize the downlink EIRP in the directions of the individual destination
terminals. The transmit point-to-point frequency addressable narrow beams
29 are generated by an array 20 whose apparent size is magnified by two
confocal parabolas comprising a main reflector 12b and a subreflector 22.
The east-west direction of each beam 29 is determined by the phase
progression of its signal along the array 106 of transmit elements 20
(FIGS. 13 and 15). This phase progression is established by a
later-discussed beam-forming network 98 and is a function of the signal
frequency. Each of the transmit array elements 20 is driven by a
later-discussed solid state power amplifier. The power delivered to the
array elements 106 is not uniform but is instead tapered with the edge
elements being more than 10 dB down. Tapering of the beams 29 is achieved
by adjusting the transmit gain according to the position of the transmit
array elements 20. The excitation pattern determines the characteristics
of the transmit secondary pattern, shown in FIG. 9A. Referring to FIG. 9,
the closest spacing between transmit zones 31, 33, 35, 37 occurs between
zones 31 and 33 and is approximately 1.2 degrees. This means that a signal
addressed to zone 33 using a particular frequency would interfere with a
signal using the same frequency in zone 31 with its side lobe 1.2 degrees
from its beam center. However, the individual transmit gains have been
adjusted to provide low side lobe levels, thereby permitting frequency
reuse in adjacent zones. Referring to FIG. 9A, it is seen that the side
lobe level at this angle off beam center is more than 30 dB down, so that
such interference will be negligibly small. The same frequency uses in
zones 35 and 37 are further removed in angle, hence the side lobe
interference in those zones is even smaller.
FIG. 9B is an illustration of the transmit beam pattern in the north-south
direction. The twenty six slots 108 in each of the transmit waveguide
elements 106 are excited in a manner which creates a nearly flat
north-south pattern, extending over the covered range of plus and minus
1.4 degrees from the north-south boresight direction.
Both the point-to-point and CONUS systems may utilize the same uplink and
downlink frequency bands, with the point-to-point system using horizontal
polarization for its uplink polarization, and the CONUS system using
vertical polarization, as previously mentioned. For example, both services
may, simultaneously, utilize the entire 500 MHz uplink frequency band
between 14 and 14.5 GHz, as well as the entire 500 MHz downlink frequency
band between 11.7 and 12.2 GHz. Each of the receive zones 32, 34, 36, 38
and transmit zones 31, 33, 35, 37, employing the point-to-point service
utilizes the entire frequency spectrum (i.e. 500 MHz). Furthermore, this
total frequency spectrum is divided into a plurality of channels, for
example, sixteen channels each having a usable bandwidth of 27 MHz and a
spacing of 30 MHz. In turn, each of the sixteen channels may accommodate
approximately 800 subchannels. Hence, within each zone, approximately
12,500 (16 channels.times.800 subchannels) 32 kilobit per second channels
may be accommodated, at any given moment. As will be discussed below, the
communication architecture of the point-to-point system allows any
terminal to communicate directly with any other terminal. Thus, within a
single polarization, a total of 50,000 subchannels may be accommodated
nationwide.
Referring now particularly to FIGS. 1, 2, 6, 7 and 16, the point-to-point
receive feed array 16 employs seven receive horns 50-62. Horns 50, 54, 58
and 62 respectively receive signals from zones 32, 34, 36 and 38. Horns
52, 56 and 60 receive signals from the zones of contention 40, 42 and 44.
Using a series of hybrid couplers or power dividers C.sub.1 -C.sub.9, the
signals received by horns 50-62 are combined into four outputs 64-70. For
example, a signal originating from the area of contention 44 and received
by horn 60 is divided by coupler C.sub.2 and portions of the divided
signal are respectively delivered to couplers C.sub.1 and coupler C.sub.4
whereby the split signal is combined with the incoming signals received by
horns 58, 62 respectively. Similarly, signals originating from the area of
contention 42 and received by horn 56 are split by coupler C.sub.5. A
portion of the split signal is combined by coupler C.sub.3, with the
signal output of coupler C.sub.4, while the remaining portion of the split
signal is combined, by coupler C.sub. 7, with the signal received by horn
54.
Attention is now particularly directed to FIG. 6 which depicts, in block
diagram form, the electronics for receiving and transmitting signals for
both the CONUS and point-to-point systems. The point-to-point receive
signals 64-70 (see also FIG. 7) are derived from the point-to-point
receive feed network in FIG. 7, whereas the CONUS receive signal 72
derives from the CONUS receive feed horns 14, (FIGS. 1 and 3). Both the
point-to-point and CONUS receive signal are input to a switching network
76 which selectively connects input lines 64-72 with five corresponding
receivers, eight of which receivers are generally indicated at 74. The
receivers 74 are of conventional design, three of which are provided for
redundancy and are not normally used unless a malfunction in one of the
receivers is experienced. In the event of a malfunction, switching network
76 reconnects the appropriate incoming line 64-72 with a back-up receiver
74. Receivers 74 function to drive the filters in a filter interconnection
matrix 90. The outputs of the receivers 74, which are connected with lines
64-70, are coupled by a second switching network 78 through four receive
lines R1-R4 to a filter interconnection matrix 90. As will be discussed
later below, the filter interconnection matrix (FIM) provides
interconnections between the receive zones 32, 34, 36, 38, and the
transmit zones 31, 33, 35, 37. Operating in the above-mentioned 500 MHz
assigned frequency spectrum, separated into sixteen 27 MHz channels, four
sets of sixteen filters are employed. Each set of the sixteen filters
utilizes the entire 500 MHz frequency spectrum and each filter has a 27
MHz bandwidth. As will be discussed later, the filter outputs T1-T4 are
arranged in four groups, each group destined for one of the four transmit
zones 31, 33, 35, 37.
The transmit signals T1-T4 are respectively connected, via switching
network 94, to four of six driving amplifiers 92, two of such amplifiers
92 being provided for back-up in the event of failure. In the event of the
failure of one of the amplifiers 92, one of the back-up amplifiers 92 will
be reconnected to the corresponding transmit signal T1-T4 by the switching
network 94. A similar switching network 96 couples the amplified output of
the amplifiers 92 to a beam-forming network 98. As will be discussed later
in more detail, the beam-forming network 98 consists of a plurality of
transmission delay lines connected at equal intervals along the four delay
lines. These intervals and the width of the delay lines are chosen to
provide the desired centerband beam squint and the beam scan rate with
frequency for the corresponding transmit zones 31, 33, 35, 37 to be
serviced. The transmit signals, coupled from the four delay lines, are
summed in the beam-forming network 98 as shown in FIGS. 11 and 12, to
provide inputs to solid state power amplifiers 100, which may be embedded
in the point-to-point system's transmit array 20. In the illustrated
embodiment discussed below, forty solid state power amplifiers (SSPAs) 100
are provided. Each of the SSPAs 100 amplifies a corresponding one of the
forty signals formed by the beam-forming network 98. The SSPAs 100 posses
different power capacities to provide the tapered array excitation
previously mentioned. The output of the SSPA 100 is connected to the input
112 (FIG. 14) at one of the elements of the transmit array 20.
The receive signal for CONUS on line 72 is connected to an appropriate
receiver 74 by switching networks 76, 78. The output of the receiver
connected with the CONUS signal is delivered to an input multiplexer 80
which provides for eight channels, as mentioned above. The purpose of the
input multiplexers 80 is to divide the one low level CONUS signal into
subsignals so that the subsignals can be amplified on an individual basis.
The CONUS receive signals are highly amplified so that the CONUS transmit
signal may be distributed to very small earth terminals. The outputs of
the input multiplexer 80 are connected through a switching network 84 to
eight of twelve high power traveling wave tube amplifiers (TWTAs) 82, four
of which TWTAs 82 are employed for back-up in the event of failure. The
outputs of the eight TWTAs 82 are connected through another switching
network 86 to an output multiplexer 88 which recombines the eight
amplified signals to form one CONUS transmit signal. The output of the
multiplexer 88 is delivered via waveguide to the transmit horns of the
CONUS transmitter 24 (FIGS. 2 and 3).
Attention is now directed to FIG. 10 which depicts the details of the FIM
90 (FIG. 6). As previously discussed, the FIM 90 effectively interconnects
any terminal in any of the receive zones 32, | | |
