GB2353182A - Satellite data system - Google Patents

Abstract

A satellite communications system where a satellite acts as a relay between a user terminal and an earth station 11 comprises a satellite 10 which is partitioned to provide two or more sets of frequencies, each with its own antenna pattern, provided from a common antenna array. Each partition is independent of any other, can comprise a forward path and a return path, and can be operated from the same, or another, earth station. A partition can, independently, support any type of modulation or coding. In the example given, one partition provides a cellular telephone system using a plurality of spot beams while another partition supports a slow data system using a single, global beam 108.

Description

2353182 Satellite Data System Tbe present invention relates to satellite

communications systems in which an earth station is operative to communicate with a plurality of user terminals, on the surface of the earth, by sending signals to an orbiting satellite, the satellite relaying signals from the earth station to the user terminals and from the user terminals to the earth station.

It is known to use an orbiting satellite to provide bidirectional telephonic communications with user terminals on the surface of the earth. Equally, it is known for such a satellite to cover the visible portion of the surface of the earth with a i o plurality of contiguous spot beams, to create a system, not unlike the contiguous array of cellular telephone cells, whereby the control and progress of a telephone call can be conducted in a manner close to the manner of managing communications in a terrestrial cellular telephone network.

One type of satellite communications network known as the IRIDIUMTM satellite cellular system is described in EP-A-0 365 885 and US Patent No. 5 394 561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.

Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed with an orbital radius in the range of 1020,000 km. Reference is directed to the ICOTm satellite cellular system described for example in GB-A-2 295 296. With this system, the satellite communications link does not permit communication between adjacent satellites. Instead, a signal from a 2 mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to conventional land-based telephone networks. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM. Also simpler satellite communication techniques can be used than with a LEO network.

Reference is directed to "New Satellites for Personal Communications", Scientific American, April 1998, pp. 60 - 67, for an overview of LEO/MEO satellite networks.

Resource allocation in satellite communication systems is by virtue of the power and bandwidth restrictions on individual satellites generally restricted in comparison with terrestrial cellular telephone systems, where bandwidth is re-usable on a small, local basis. One particular bandwidth- hungry application is, potentially, slow data networks. Not only does the slow data terminal have to register on a full voice channel, while requiring only a fraction of the bandwidth available, but the traffic levels are low, giving only fractional use of the available capacity. Equally, while telephones are required to use the cellular (spot beam) environment, tying up a whole spot beam channel for slow and intermittent data transmission is wasteful. Satellite communications systems are ideal for data exchange across large areas of the earth, but the communications structure prohibits most economic provision. The present invention seeks to provide a system and method whereby low rate data can be provided within the coverage area of a communications satellite without uneconomic use of bandwidth, power, or other resources. Such low rate data is applicable, but not limited, to automotive applications such as position reporting, theft alert, roadside assistance provision and engine management. Another area for such a low data rate 3 system includes data and instruction exchange with remote transponders and monitors. There are many other fields of application.

It is known to partition a satellite so that one application will employ one frequency band and another application will employ another frequency band, a typical example being the use of a geostationary satellite to transmit two or more independent television programs. Such partitions are unidirectional and limited in their terrestrial coverage by the considerable weight of antennas, adding to launch and fabrication costs. The present invention seeks to provide a degree of flexibility for satellite partitioning.

According to a first aspect, the present invention comprises a satellite communications system in which a communications satellite is operable to act as a relay between a user terminal and an earth station using an array of antennas to communicate with said user terminal, said system comprising a first partition for a first set of frequencies providing a first antenna pattern for interaction with said user terminal, and a second partition on a second set of frequencies providing a second antenna pattern for interaction with said user terminal, wherein said satellite is operable to simultaneously generate said first antenna pattern and said second antenna pattern.

According to a second aspect, the present invention comprises a method of operating a satellite communications system where a communications satellite is operable to act as a relay between a user terminal and an earth station using an array of antennas to communicate with said user terminal, said method comprising the steps of. creating a first partition in said satellite for a first set of frequencies and providing a first antenna pattern for interaction with said user terminal; creating a second partition in said satellite for a second set of frequencies and providing a 4 second antenna pattern for interaction with said user terminal; and simultaneously generating said first antenna pattern and said second antenna pattern.

The satellite can comprise a plurality of phased array antennas, wherein the phased array antennas are operable to generate the first antenna pattern, and wherein the same plurality of phased array antennas are operable, simultaneously, to generate the second antenna pattern.

The satellite can include a forward path, for sending signals from the earth station to the user terminal, and wherein the second antenna pattern coverage comprises signals from the forward path.

The satellite can further comprise a return path for sending signals from the user terminal to the earth station, and wherein the second antenna pattern comprises signals from the return path.

The satellite can also comprise a forward path, for sending signals from the earth station to the user terminal, and wherein the first antenna pattern comprises signals from the forward path.

In addition, the satellite can comprise a return path for sending signals from the user terminal to the earth station, and wherein the first antenna pattern comprises signals from the return path.

The earth station can comprise a first partition controller for controlling the first partition.

The invention further provides a system and method wherein the earth station comprises a second partition controller for controlling the second partition.

The invention further provides a system and method wherein the first antenna pattern comprises a plurality of spot beams, and wherein the first set of frequencies supports a telephone system.

The invention further fovides a system and method wherein the second antenna pattern comprises a global beam and wherein the second set of frequencies supports a slow data system.

The invention further provides a system and method wherein the slow data system comprises a plurality of slow data channels.

The invention further provides a method and system wherein another earth station can be employed to operate one of the partitions.

According to the invention, there is also provided an earth station in a satellite communications system in which a communications satellite is operable to act as a io relay between a user terminal and the earth station using an array of antennas to communicate with said user terminal, said earth station comprising means for configuring the satellite to provide first and second partitions for respective first and second sets of frequencies, using respective first and second antenna patterns to interact with the user terminal, the satellite being configured to simultaneously generate said first and second antenna patterns.

The invention further provides a user terminal in a satellite communications system in which a communications satellite is operable to act as a relay between the user terminal and an earth station using an array of antennas to communicate with the user terminal, the user terminal being configured to respond to first and second antenna patterns simultaneously generated by said satellite and providing respective first and second partitioned sets of frequencies.

Embodiments of the invention will now be described, by way of example, with reference to the appended drawings, in which:

Figure I shows a constellation of communications satellites in an orbit about the earth; 6 Figure 2 shows a pair of crossed orbits; Figure 3 shows the pattern of spot beams, generated by a satellite, on the surface of the earth; Figure 4 shows a detailed view of a satellite, as seen from the surface of the earth; Figure 5 shows a schematic diagram of the signal processing structure of a satellite; Figure 6 is a detailed view of the user terminal downlink array and the user terminal uplink array, on the satellite; Figure 7 is a cross-sectional view of an antenna element of figure 6; Figure 8 illustrates how amplitude and phase may be adjusted to achieve beam patterns on the antenna arrays of figure 6; Figure 9 shows simultaneous antenna patterns, including spot beams and a global beam; Figure 10 shows the data structure for slow data in the global beam of figure 9 on a forward path and a return path; Figure 11 shows a schematic representation of the elements of an earth station, according to an example of the present invention; Figure 12 shows different ways in which alternate beam pattems can be provided; and Figure 13 shows how multiple partitions can be controlled either from one earth station or more than one earth station.

Attention is firstly drawn to figure 1.

Figure I shows a planar constellation of satellites 10 disposed about the earth 14.

7 The plurality of satellites 10 are evenly disposed around a circular orbit 12 above the surface of the earth 14. Each of the satellites 10 is designed to provide radio communications with apparatus on the surface of the earth 14 when the individual satellite 10 is more than 10 degrees above the horizon. Each satellite 10 therefore provides a cone 16 of radio coverage which intersects with the surface of the earth 14.

The surface of the earth has three types of areas. A first type of area 18 is one which has radio coverage from only one satellite 10. A second type of area 20 is an area where there is radio coverage from more than one satellite 10. Finally, a third lo type of area 22 receives radio coverage from none of the satellites 10 in the orbit 12 shown.

Figure 2 illustrates how the satellites 10 are disposed in orthogonal orbital planes.

The first orbit 12 of figure I is supplemented by a second orbit 12'having satellites 10 disposed there about in a similar manner to that shown in figure 1. The orbits 12, 12' are orthogonal to one another, each being inclined at 45 degrees to the equator 24 and having planes which are orthogonal (at 90 degrees) to each other.

In the example shown, the satellites 10 orbit above the surface of the earth 14 at an altitude in the region of 10 500krn which generally corresponds to the ICOTM system. Those skilled in the art will be aware that other orbital heights and numbers of satellites 10 may be used in each orbit 12, 12'. This configuration is preferred because the example provides global radio coverage of the earth 14, even to the north 26 and south 28 poles, with a minimum number of satellites 10. In particular, the orthogonality of the orbits ensures that the satellites 10 of the second orbit 12' provides radio coverage for the third types of area 22 of no radio coverage for the 8 satellites in the first orbit 12, and the satellites 10 in the first orbit 12 provide radio coverage for those areas 22 of the third type where the satellites 10 of the second orbit 12'provide no radio coverage.

It will become clear that, although the two orbits 12, 12' are here shown to be of the same radius, the system will function with orbits 12, 12'of different radii. Also, there may be more than two orbits 12, 12'.

Each satellite 10 is in bidirectional communication with an earth station I I on the surface of the earth 14 and within the cone of radio coverage 16 of the satellite 10. In turn, the satellite 10 is in potential bidirectional communication with a plurality of i o user terminals 13 (only one shown), also on or near the surface of the earth 14, and anywhere within the cone of radio coverage 16 of the satellite 10. The satellite 10 acts as a simple relay, whereby traffic from the earth station 11, such as telephone calls and, as will later be described, in the preferred embodiment, slow data, is directed to the user terminal (s) 13 and traffic from the user ten-ninal(s) 13 is directed to the earth station 11. Although only one earth station I I is shown, it is to be understood that, to attain global coverage, a sufficient number and distribution of earth stations I I is provided so that all satellites 10 have at least one earth station I I within their respective cones of radio coverage 16.

Figure 3 shows the structure of the cone 16 of radio coverage provided by each satellite 10. For convenience, the radio coverage cone 16 is shown centred, on a map of the earth, at latitude 0 degrees and at longitude 0 degrees. The cone 16 of radio coverage is divided into a plurality of spot beams 30, by means of a phased transmitting antenna array and a phased receiving antenna array on the satellite 10. The satellite 10 is intended for mobile radio telephone communications and each of the spot beams 30 corresponds, roughly, to the equivalent of a cell in a cellular radio 9 telephone network. In figure 3, the cone of radio coverage 16 is distorted due to the geometry of the map of the earth's surface provided. Figure 3 also shows the extent of interaction of the cone 16 of radio coverage down to the edges of the cone 16 being tangential to the earth's surface, that is, to the point where the cone 16 represents a horizontal incidence at its edges, with the surface of the earth. By contrast, figure 1 shows the cone 16 at a minimum of 10 degrees elevation to the surface of the earth, It is to be observed, that because of the curvature of the earth, the spot beams 30 are of near uniform, slightly overlapping circular shape at the centre whereas, at the edges, the oblique incidences of the spot beams 30 onto the surface of the earth 14 j o causes considerable distortion of shape.

Figure 4 is a view, from the surface of the earth 14 showing an orbiting satellite 10 in greater detail.

The satellite 10 comprises a body 32 on which solar panels 34 are mounted on rotating yokes 36. The body 32 of the satellite 10 also supports uplink antennae 3 8 and downlink antennae 40 whereby the satellite 10 can communicate with an earth station 11 for communication and control purposes. The uplink antennae 38, in the example given, provide a reception path for the satellite 10 to receive bulk traffic and command signals, sent from the earth station 11 on a frequency of, for example, 5 GHz. The downlink antenna 40 sends bulk traffic and commands from the satellite 10 to the earth station I I on a frequency of, for example, 7 GHz. The uplink antenna 38 and the downlink antenna 40 are both fairly wide beam so that the earth station 11 can make contact with the satellite 10 over the whole time it is within sight of the earth station 11. Those skilled in the art will appreciate that other frequencies can be employed by the uplink 38 and downlink 40 antennas.

In addition, the satellite 10 comprises a transmission antenna array 42 and a reception antenna array 44 whereby the satellite 10 can maintain contact with user terminal(s) 13, which can, for example, be vehicle mounted or resemble cellular telephone handsets, on the surface of the earth 14. The transmission array 42 operates on a frequency band of, for example, 2170 to 2200 MHz and the reception array 44 operates on a frequency band of, for example, 1980 to 2010 MHz. The bandwidth, each way of 30MHz is, for telephony purposes, split into channels of 25KHz width and spacing, and is used to carry telephone traffic and operational data/commands between the earthbound user terminal(s) 13 and the satellite 10. The satellite 10 relays io the traffic and operational commands/data to the earth station I I via the uplink antenna 38 and the downlink antenna 40.

The solar panels 34 are, for example, automatically steered to face the sun and so power the satellite 10, and the satellite 10 describes 3 60 degree roll, pitch and yaw in each orbit of the earth to ensure that the transmission array 42 and the reception array 44 always face the earth 14 and that the solar panels are always able to face the sun to extract maximum power. The steering of the solar panels 34 and the orbital rotations of the satellite 10 do not form part of the present invention, but are here given by way of example to provide completeness of the system description.

Figure 5 is a schematic block diagram of the internal functions of an exemplary satellite 10.

The satellite 10 comprises a forward path 46 which conducts signals from the earth station I I uplink antenna 38 to the user terminal downlink array 42. A backward path 48 conducts signals from the user terminal uplink array 44 to the earth station I I downlink antenna 40.

Signals from each of the elements in the user terminal uplink array 44 are amplified by a corresponding plurality of low noise amplifiers 50 and then frequency converted to an intermediate frequency by a corresponding plurality of frequency changers 52. The intermediate frequency output from each frequency changer 52 is them converted from an analogue to a digital signal by a corresponding plurality of analogue to digital converters 54. The digital outputs of the analogue to digital converters 54 are provided as input to a multiplexing unit 56 which, in turn, provides input to a backward path digital beam formation network 58 whose f1mction will be described in more detail hereafter, but which, essentially, extracts the 129 signals io received from the individual elements in the receiving array 44 and converts them into the equivalent of 163 spot beams 30.

The 163 signals are then each passed through an equivalent number of respective return path bandpass filters 59, each effectively 150KHz wide (having an edge allowance for doppler shift).

The 163, filtered equivalent spot beam 30 signals are then provided to a multiplexer 60 which provides one signal for each of the elements in the earth station 11 downlink antenna 40, then a corresponding number of digital to analogue converters 62, in turn, drive a corresponding number of intermediate frequency to CBand frequency changers. The output from each intermediate frequency to C Band frequency changer 64 drives a corresponding C Band power amplifier 66, each driving a respective element in the earth station 11 downlink antenna 40. In this manner, the satellite simply relays signals from the user terminals 13s, on the surface of the earth 14, to the earth station 11, elsewhere on the surface of the earth 14.

The forward path 46 has signals, received from the earth station 11, entering at the uplink earth station antenna 38, each antenna element having its received signal 12 amplified by forward link low noise amplifiers 68 and converted to an intermediate frequency by a forward path front end frequency changer 70. Each signal is then provided as input to a respective for-ward path analogue to digital converter 72 where the digital output is provided as input to a forward path demultiplexer 74 providing input to forward path bandpass filters 76 ( each with the same usable 150 KHz width as the return path bandpass filters 59) and a forward path digital beam formation network 78, (corresponding to the return path digital beam formation network 58). Thereafter, signals are each fed to a forward path multiplexer 80 and thence to a respective forward path digital to analogue converter 82 whose analogue outputs are i o provided to a forward path rear end frequency changer 84 which converts the intermediate frequency of the forward path to the frequency used in the user terminal downlink antenna 42. Thereafter, forward path power amplifiers 85 drive each of the individual elements in the user terminal downlink antenna 42. The forward path thus acts as a transparent relay for signals from the earth station I I to the user terminal(s) 13.

A central, controlling processor 88 controls all of elements in the satellite 10, and, in particular, the operation of the forward path digital beam formation network 78 and the return path digital beam formation network 58, the co-operation between which 58, 78, 88 is described, hereafter, in greater detail, to illustrate how the present invention provides, in the case of the preferred embodiment, both spot beams 30 and a global beam.

Figure 6 is a more detailed view of the user terminal downlink array 42 and the user terminal uplink array 44 of the satellite 10.

Each of the user terminal uplink array 44 and the user terminal downlink array 42 comprise a plurality of individual elements 86, arranged in a pattern. In the 13 example shown, there are lj elements 86 arranged as shown. The invention is equally applicable to different numbers of spot beams 30 and differrent numbers and layouts of elements 86. The elements 86 are individually driven to create a pattern of spot beams, on the surface of the earth, rather like the cells of a cellular phone network, whereby terrestrial users with handsets or other equipment may communicate with the satellite 10.

Figure 7 is a cross sectional view of an antenna element 86.

Each antenna element 86 comprises a circular cross section cylinder 88, made of a radio reflective material, and closed at its proximal end to the satellite 10 by an end wall 90. A feed line 92 is connected to a dipole element 94 which is spaced from resonant parasitic elements 96.

Such antenna elements 86 are already known in the art. The antenna element 86 is shown merely by way of example as a type of antenna element 86 which may be individually driven, in a known pattern of spacing, when forming an array of spot beams or any other pattern of radio signals to be projected towards, or received from, the surface of the earth 14.

Figure 8 is a block diagram of the manner in which the individual antenna elements 86 may be electronically phased to produce the pattern shown in figure 3. It is to be appreciated that the pattern shown in figure 3 is merely one of many possible beam patterns for the spot beams 30 for which the present invention is applicable. The spot beams 30 may be fewer or more in number.

The example of Figure 8 is shown in terms of the forward path 46 of figure 5. It reflects the activities of the forward path digital beam formation network 78. It is to be appreciated that exactly the same technique is applied to the return path 48 of figure 5.

14 An input feed 98 comprises signals, which are to be fed to an individual element 86 in the user terminal downlink array 42, and corresponding to one of the plural outputs of the forward path bandpass filter 76, having passed through forward path demultiplexer 74 and the forward path analogue to digital converter 72, which are in the form of a stream of binary words or binary digits representative of the instant amplitude of the demuliplexed analogue input to the forward path analogue to digital converter 72.

The input feed 98 is provided as input to a first fast fourier transformer 100 which converts the stream of binary digits or binary words into a further stream of i o binary digits or binary words representative of the amplitude of the elements of the frequency spectrum of the input feed 98.

The output of the first fast fourier transformer 100 is provided as input to an adjuster 102 which is controlled by the controlling processor 88. Responding to commands from the controlling processor, the adjuster 102 scales the individual binary words to adjust the amplitude of the individual frequency components indicated by the output of the first fast fourier transformer 100 and adjusts the phase thereof by digitally delaying or advancing (relatively) binary words or binary digits.

The output of the adjuster 102 is then fed to a second fast fourier transformer 104 which performs the inverse transformation converting the signal back into a stream of binary digits representative of a signal in the time domain.

The output of the second fast fourier transformer 104 is fed as input to the forward path digital to analogue converter 82 which converts the input stream of binary digits or binary words into a continuous analogue output which is provided as drive ( via the forward path tail end frequency changer 84 and the forward path power amplifier 85 to an individual antenna element 86 in the user terminal downlink array 42.

Precisely the inverse process occurs for antenna elements 86 in the user terminal uplink array 44, the controller 88 providing the same adjustment so that the reception pattern reproduces the spot beam 30 array shown in figure 3, and copies the beam pattern generated by the user terminal downlink array 42.

By the adjuster 102 being able to adjust the amplitude and phase of any particular frequency element at the individual antenna element 86, and by the controlling processor 88 knowing how to adjust the frequency and phase between the i o individual antenna element 86, any beam pattern, within the capability of the numbers and disposition of the elements 86 in an antenna array 42, 44 can be created for any filter block in the bandpass filters 59, 76. Just how any particular antenna radiation pattern can be achieved is known in the art, and described in "Reference Data For Radio Engineers", ISBN 0-672-20678- 1, published in its fifth (1968) revision in its fifth printing (1973) by Howard W. Sams & Co. Inc, and in the extensive bibliography published therein. In brief summary, the control processor 88 is either pre-programmed with amplitude and phase parameters to create the desired beam pattern for a particular filter, or can receive instructions from the earth station 11 as to what the parameters should be.

Figure 9 shows the polar diagram of the user terminal downlink array 42 and the user terminal uplink array 44. These are achieved by the processor 88 instructing the forward path digital beam formation network 78 and the return path digital beam formation network 58 with the correct parameters for the achievement of the desired polar diagram for each frequency group emanating, respectively, from the forward path bandpass filters 76 and return path bandpass filters 59.

16 One of the 150KHz frequency blocks in the forward path bandpass filter 76 is allocated and reserved to form a global beam 108 for transmission from the user terminal do-wrilink array 42. The global beam 108, in this example, fills the entire cone of radio coverage 16 of the satellite 10. Likewise, another allocated and reserved 150KHz frequency block in the return path bandpass filter 59 is elected to form the global beam 108 for reception by the user terminal uplink array 44. It is to be appreciated that the "global beam" 108, in this example, is, in fact, two beams, preferably (but not of necessity) identical to each other. Tbere is one global beam 108 for transmission to user terminals 13 on the earth 14, generated by the user tenninal io downlink array 42, and another globalbeam 108, configured on the user terminal uplink array 44, for reception of signals from user terminals 13 on the earth. The same is true for all of the spot beams 30, only three of which are shown in figure 9 for clarity, it being understood that the other frequency blocks, not reserved or allocated to the global beam 108, from the bandpass filters 59 76, are employed to create the full 163 spot beams, otherwise shown in figure 3 and used for telephony purposes.

Figure 9 shows how a dual service can be obtained. Not only are the normal, plural spot beams 30, used for normal telephonic traffic, present and functional in the normal way, but there is also provided, in addition to the spot beams 30, a global beam 108 having a frequency allocation for messages from the earth station I I to the earth 14 and another frequency allocation for messages from the earth 14 to the earth station 11. Since the satellite 10 is transparent to messages from the earth station I I to the user terminals 13, and vice versa, it follows that the frequency allocations for the global beam 108 are similarly transparent. The earth station I I can use the forward path 46 global beam reserved frequency allocation to pass any kind of message to be transmitted by the global beam 108 on the user terminal downlink array 42. Likewise, 17 The eartb station I I can receive any kind of message within the return path global beam reserved frequency allocation from the user terminal uplink array 44. All this time, the earth station I I can carry on the business of normal telephone traffic, using the spot beams 30 in the normal way and employing all the other, non- reserved and non-allocated frequency blocks present in the forward path bandpass filter 76 and the return path bandpass filter 59. The traffic capacity, created through the global beam 108, is totally independent of all other traffic and can be co-ordinated by the earth station 11, and as will be shown, by another earth station I I B in an independent manner.

Figure 10 shows the manner in which the global beam 108 is used, by way of example, to provide a slow data service.

The forward path 46 has its forward path frequency band 110, corresponding to the reserved, 150KHz wide frequency block in the forward path bandpass filter 76, divided into thirty, contiguous, 5KHz wide forward data bands 112. The return path 4 8 has its return path data band 114, corresponding to the reserved, 15 OKHz wide reserved ftequency block in the return path bandpass filter 59, divided into six, contiguous, 25KHz wide return data bands 116.

In an example of the invention, each forward data band 112 is adapted to carry 1200 bits per second Time Division Multiplex (TDM) carrier data. Each return data band 116 is driven in a CDMA fashion where ten users at a 3 5 0 bits per second at a 11.25 Kcps chip rate are accommodated. It will be appreciated that other exact forms of modulation and bandwidths can be used for data bands 114, 116.

Figure I I shows the earth station 11. A radio frequency transmitter/receiver 118 feeds radio signals to, and receives radio signals ftom, the dish antenna 120, which is pointed at and tracks the satellite 10. An earth station controller 122 passes 18 traffic signals to, and receives traffic signals from, the transmitter/receiver 118. In addition, the earth station controller 122 specifies to the transmitter/receiver 118 on what frequency signals are to be transmitted to the satellite 10 and identifies received radio signals by their frequency.

An interface switch 124 provides an interface between the earth station I I and the global terrestrial telephone network 126, thereby enabling telephone and other calls to be placed through the satellite 10 and the earth station 11.

The earth station 11 further comprises a global beam interface 128 which provides connection between the earth station 11 and a data signal system, which i o could be the global terrestrial telephone network 126, or, equally, could be any other data signal system 130. The global beam interface 128 communicates bidirectionally with the earth station controller 122 so that the data signals are sent through the global beam 108 and received signals, from the global beam 108 are returned to the global beam interface 128. For example, the earth station I I provides an RS232 interface to applications.

Figure 12 shows various exemplary ways in which "global beams" 108, or other forms of beams, can be provided, and disposed within the cone of radio coverage 16 of the satellite 10.

Two or more global beams 108, 108A may be disposed within the cone 16 to provide coverage substantially over the whole of the cone 16. They can provide the same service, or different services, each transparently to the telephone traffic operation of the earth station 11. Equally, a "semi-global beam" 108B can provide partial, shaped cover of the cone 16, together with other shaped global beams 108C, to provide regional rather than global coverage, again, all being transparent in their operation.

19 Lastly, partial global beams 108D, 108E can give spot cover over part of the cone 16.

Each global beam 108, as shown in figure 12, together with its corresponding reserved frequencies in the bandpass filters, provides an independent partition of the satellite 10. Each partition provides its selected beam pattem by the controlling processor 88 providing a selected set of parameters for the beam (or beams) to be formed for that partition by the adjuster 102 in the corresponding digital beam formation network 58 78 shown in figures 5 and 8. Thus, each partition comprises a band of reserved and assigned frequencies in the corresponding bandpass filter 59 76 io together with its selected beam parameters which the controlling processor 88 provides to the adjuster 102.

A single earth station I I can be used for all partitions, in which case the single earth station I I comprises multiple partition controllers 128, 128A, 128B (hereinbefore described as the global beam interface 128) there being a controller 128, 128A, 128B for every partition. Alternatively, another earth station or stations I I B can be used for some or all of the partitions. This permits different operations or operators to share a satellite 10, independently of one another, or through the same earth station, each operation or operator having the beam configuration of their choice. By way of example, three beams 108, 108F, 108G are shown, corresponding to one for each of the three partition controllers 128, 128A, 128B shown in figure 13.

The another earth station 11 B is simply required to transmit to the satellite 10 on the band of frequencies allocated to its partition in the forward path 46, and to receive signals from the satellite 10 on the band of frequencies allocated in the return path 48.

A partition need not be bi-directional. Partitions can exist only in the return path 48 where the independent operation is required only to receive signals from the surface of the earth 14. Partitions can also exist only in the forward path 46, where the purpose of the independent operation is only to transmit signals to the surface of the earth 14.

Any partition can, independently of any other partition, carry signals in any modulation form and corresponding to any protocol or system. For example, two parallel telephone systems can be run independently of each other, each having its own, selected beam 30 array. One telephone system can, for example, be GSM TDMA and the other, CDMA. A partition can carry as much traffic as its allocated bandwidth will allow. A partition can comprise one or more blocks from the bandpass filters 59, 76. Where there is more than one block in a partition, the blocks can be contiguous, or spaced from each other.

Although the invention has been described generally in relation to the ICOTM system, it will be appreciated that it could be equally well applied to any of the satellite mobile telecommunications networks described in Scientific American supra.

Also, whilst the user terminals UT have been described herein as mobile telephone handsets, it will be understood that they may be semi-mobile e. g. mounted on a ship or aircraft. The UT may also be stationary e.g. for use as a payphone in a geographical location where there is no terrestrial telephone network.

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Claims (30)

1. A satellite communications system in which a communications satellite is operable to act as a relay between a user terminal and an earth station using an array of antennas to communicate with said user terminal, said system comprising a first partition for a first set of frequencies providing a first antenna pattern for interaction with said user terminal, and a second partition for a second set of frequencies providing a second antenna pattern for interaction with said user terminal, wherein said satellite is operable to simultaneously generate said first antenna pattern and said i o second antenna pattern.

2. A system according to claim 1, wherein said satellite comprises a plurality of phased array antennas, wherein said phased array antennas are operable to generate said first antenna pattern, and wherein said same plurality of phased array antennas are operable, simultaneously, to generate said second antenna pattern.

3. A system according to claim 1 or 2, wherein said satellite comprises a forward path, for sending signals from said earth station to said user terminal, and wherein said second antenna pattern comprises signals from said forward path.

4. A system according to any one of the preceding claims, wherein said satellite comprises a return path for sending signals from said user terminal to said earth station, and wherein said second antenna pattern comprises signals from said return path.

5. A system according to any one of the preceding claims, wherein said satellite comprises a forward path, for sending signals from said earth station to said user terminal, and wherein said first antenna pattern co. mpnses signals from said forward path.

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6. A system according to any one of the preceding claims, wherein said satellite includes a return path for sending signals from said user terminal to said earth station, and wherein said first antenna pattern comprises signals from said return path.

7. A system according to any of the preceding claims, wherein said earth station comprises a first partition controller for controlling said first partition.

8. A system according to any of the preceding claims, wherein said earth station comprises a second partition controller for controlling said second partition.

9. A system according to any of the preceding claims, wherein said first antenna pattern comprises a plurality of spot beams, and wherein said first set of frequencies io supports a telephone system.

10. A system according to any of the preceding claims wherein said second antenna pattern comprises a global beam and wherein said second set of frequencies supports a slow data system.

11. A system according to claim 10, wherein said slow data system comprises a plurality of slow data channels.

12. A system according to any one of the preceding claims, comprising another earth station to operate one of said partitions.

13. A method of operating a satellite communications system where a communications satellite is operable to act as a relay between a user terminal and an earth station using an array of antennas to communicate with said user terminal, said method comprising the steps of. creating a first partition in said satellite for a first set of frequencies and providing a first antenna pattern for interaction with said user terminal; creating a second partition in said satellite for a second set of frequencies and providing a second antenna pattern for interaction with said user terminal; and simultaneously generating said first antenna pattern and said second antenna pattern.

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14. A method according to claim 13, including the steps of. providing, in said satellite, as said array of antennas, a plurality of phased array antennas, and causing said phased array antennas to generate said first antenna pattern; and causing, simultaneously, said same plurality of phased array antennas to generate said second antenna pattern.

15. A method according to claim 13 or claim 14, wherein said satellite comprises a forward path, for sending signals from said earth station to said user terminal, and wherein said second antenna pattern coverage comprises signals from said forward path.

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16. A method according to claim 13, claim 14 or claim 15, for use where said satellite comprises a return path for sending signals from said user terminal to said earth station, and where said second antenna pattern comprises signals from said return path.

17. A method according to any one of claims 13 to 16, for use where said satellite comprises a forward path, for sending signals from said earth station to said user terminal, and where said first antenna pattern comprises signals from said forward path.

18. A method according to any one of claims 13 to 17, for use where said satellite comprises a return path for sending signals from said user terminal to said earth station, and where said first antenna pattern comprises signals from said return path.

19. A method according to any one of claims 13 to 18, including the provision, at said earth station, of a first partition controller for controlling said first partition.

20. A method according to any one of claims 13 to 19, including the provision, at 24 said earth station, of a second partition controller for controlling said second partition.

21. A method according to any one of claims 13 to 20, including the steps of causing said first antenna pattern to comprise a plurality of spot beams, and providing, on said first set of frequencies signals for a telephone system.

22. A method according to any one of claims 13 to 21, including the steps of causing said second antenna pattern to comprise a global beam and providing, on said second set of frequencies, signals for a slow data system.

23. A method according to claim 22, wherein said slow data system comprises a i o plurality of slow data channels.

24. A method according to any one of claims 13 to 23, including the step of employing another earth station to operate one of said partitions.

25. An earth station for use in a satellite communications system in which a communications satellite is operable to act as a relay between a user terminal and the earth station using an array of antennas to communicate with said user terminal, said earth station comprising means for configuring the satellite to provide first and second partitions for respective first and second sets of frequencies, using respective first and second antenna patterns to interact with the user tenninal, the satellite being configured to simultaneously generate said first and second antenna patterns.

26. A user terminal for use in a satellite communications system in which a communications satellite is operable to act as a relay between the user terminal and an earth station using an array of antennas to communicate with the user terminal, the user terminal being configured to respond to first and second antenna patterns simultaneously generated by said satellite and providing respective first and second partitioned sets of frequencies.

27. A satellite communicatIons system substantially as hereinbefore described with reference to the accompanying drawings.

28. A method of operating a satellite communications system substantially as hereinbefore described with reference to the accompanying drawings.

29. An earth station in a satellite communications system substantially as hereinbefore described with reference to the accompanying drawings.

30. A user terminal for use in a satellite communications system substantially as herein described with reference to the accompanying drawings.