FR2735303A1 - Telecommunication system - Google Patents

Abstract

The system includes a group of moving satellites which establish communication links between ground stations (11,15) and terminals (12,13,14,16). The ground stations and terminals lie within defined geographical regions (9,10). A communication link between a particular station and the terminals with which it is associated is established by one particular satellite (1) in the satellite group. This satellite is controlled to provide both an orientatable transmission beam (7,8) and an orientatable reception beam (5,6), pointing at the defined regions (9,10) which include the particular ground stations. Together these provide the communication channel. Control of the satellite is performed from one of the ground stations transmitting control signals to the satellite. The directed beams are established w.r.t. aiming data stored within the satellite and used to measure its progress.

Description

SCROLLING SATELLITE COMMUNICATION SYSTEM,
SATELLITE, STATION AND TERMINAL INCLUDED
The present invention relates to a communication system comprising a set of scrolling satellites establishing communication links between ground stations and fixed ground terminals, a communication link originating from a terminating station & associated terminals located in an area. limited geographic including this station.

 A system of this type is described in the document "GLOBALSTAR: a transparent system", by D. Rouf fet, published in the Revue des Télécommunications, 1st quarter 1993 and in the documents cited in its bibliography.

 In the system described in this document, the terminals are not fixed, but mobile. They have an omnidirectional antenna and communicate with the satellite or satellites providing them with the strongest signal. The satellites, for their part, cover a plurality of elementary surfaces on the ground, by a plurality of beams of different carrier frequencies, which move at the same time as the satellite. Seen from the ground, the scrolling of the elementary surfaces requires for the stations and the terminals of numerous frequency swings, which harms the overall transmission efficiency. In addition, for this system, the bandwidth that can be transmitted between a station and an associated terminal is relatively limited.

 The present invention, in contrast, relates to a system in which the number of frequency switches is considerably reduced, and which allows broadband links between stations and terminals.

 This objective, and others still, are achieved by the system of the invention which is characterized in that the connection between such a particular station and the terminals associated with it is established by a satellite belonging to said set of satellites, controlled to establish for this purpose a directional beam of emission and a directional beam of reception, pointing a said limited area including said particular station, and supporting said communication link.

 The use of orientable beams kept fixed relative to the ground allows the same satellite to establish and maintain a communication link between a station and its terminals, as it moves from one horizon to the other of these , that is to say as long as possible, taking into account the movement of the satellites, which contributes to reducing the number of link switches and therefore to improving the overall efficiency of the transmission.

 In one embodiment of the invention, said beams are established on command from said particular station, transmitted to said satellite. Each station thus requests visible beams from the visible satellites, in terms of frequency, bandwidth, shape and orientation.

 According to an alternative, said beams are established from data previously recorded in said satellite and used as the satellite progresses.

 According to another alternative, said beams are established from data previously recorded in the station and supplied to the satellite as it progresses.

 In the case of the first form of implementation mentioned, a satellite being in view of said particular station, the latter is arranged to order a connection, which consists in transmitting to it data which identify it itself. even and of the beam establishment control data, the satellite being arranged to verify the membership of the station in the system and to establish accordingly the beams which it requests.

 Likewise, a satellite being in view of said particular station, the latter is arranged to command it to delete a link, which consists in transmitting to it data which identify it itself and beam deletion control data. , the satellite being arranged to verify the membership of the station in the system and to consequently delete the beams established for its benefit.

 Finally, said satellite is controlled to establish said beams only outside a celestial area where it would be likely to disturb other space communications. This allows said satellite to transmit on a frequency allocated to said other space communications, or even only on such frequencies.

 Furthermore, preferably, the satellites are in orbits such that they travel a closed path in a defined number of orbits, and then repeat it identically.

 This simplifies the ephemeris in stations and terminals.

The various objects and characteristics of the invention will now be detailed in the following description of an example of implementation of the invention, provided without limitation, with reference to the attached figures which represent
FIG. 1, a schematic view of a communication system according to the invention,
FIG. 2, the trajectory, projected onto the surface of the Earth, of satellites suitable for implementing the invention,
FIG. 3, a plan showing the reuse of the frequencies in the system of FIG. 1,
FIG. 4, a schematic plan of an embodiment of the equipment of a satellite of the system of FIG. 1,
- Figure 5, a schematic plan of the equipment of a station or terminal of the system of Figure 1.

 The satellite communication system of the invention is illustrated very schematically in Figure 1. I1 comprises a set of scrolling satellites, only one of which is shown at 1 in the figure.

 I1 comprises a receiving antenna 2 and a transmitting antenna 3, between which a repeater 4 is placed. The signals actually received from the Earth by the receiving antenna 2 are amplified in the repeater 4 and retransmitted towards the Earth by l transmission antenna 3. Thus, from the point of view of communication management, satellite 1 is transparent. It constitutes only a relay of transmission between sources and recipients of signals both on the ground. This reduces the cost of the space segment.

More specifically, the antennas 2 and 3 each generate a multiplicity of beams. The antenna 2 generates for example upward beams including in particular the beams 5 and 6. Similarly, the antenna 3 generates downward beams, including the beams 7 and 8. These beams can have different characteristics (frequency, polarization, width band, etc.) As is well known, network antennas can be used for this. A description can be found, for example, in the article "A Ku Band Antenna Program" by D. Michel et al, published in the minutes of the "15th International
Communications Satellite Systems Conference ", organized by
The AAIA, from February 27 to March 3, 1994. An ascending beam, 5 for example, and a descending beam, 7 for example, cover the same geographically limited area, 9, on the surface of the Earth. Likewise, the beams 6 and 8 cover another zone 10.

 In an area such as 9, there is a station Il and terminals 12, 13, 14. The principle is that station 11 communicates with terminals 12, 13, 14 via a satellite, 1, including transmission and reception beams are oriented on a zone, 9, which contains the station and the associated terminals. The signals sent by the station are relayed to the terminals by satellite. Similarly, the signals sent by the terminals are relayed by the satellite to the station. This establishes a communication link between them. It is the same with regard to zone 10 in which there is a station 15 and terminals 16 which communicate with each other as has just been indicated.

 By way of example, the operation of the communication link between such a station and the associated terminals can be of the type applied in terrestrial radiotelephony systems; the communication link then offers several frequency stepped channels, some of these channels at least supporting several channels by time multiplexing. The terminals call the station on a common access channel and the station answers on a control channel. A conventional procedure, of the "ALOHA" or "slotted-ALOHA" type, enables conflicts of access to the access channel to be resolved. The control channel is used to assign a time channel between a terminal and the station, in each direction of transmission. From this moment the communication can take place, for example between the terminal 14 and the station 11. The latter can itself connect this terminal 14 to another terminal, 12 for example, by similar means. It can also, via links 17, 18, 19, extend the communication to other stations, in particular station 15, or to other networks, depending on the connection location of another terminal to which the terminal requests access. 14.

 The links between stations can be terrestrial links. These links can also be satellite links. The same satellites can be used for this purpose. They then contain an additional communication system, similar to that of the invention, reserved for communications between stations and dimensioned accordingly.

The ground areas, in particular, will be large enough to include multiple stations.

 The communication link between the station 11 and the associated terminals 12, 13, 14, the satellite being transparent is therefore a multiplexed link from the station to all the terminals. It can include one or more frequency-separated channels and on each of them one or more time-division multiplex channels, for example asynchronous time-division multiplexing. In the context of the present invention, it suffices to indicate that it employs well-known and proven techniques in the field of terrestrial communications.

 In the system of the invention, the satellite 1 is a scrolling satellite which moves along the arrow 20.

This means that the beams 5 to 8, to constantly target zones 9 and 10 must be pointed at these zones and follow them. The satellite 1 includes for this purpose a beam pointing device 21, comprising a table of ephemeris read periodically and which provides the beam pointing coordinates as the satellite moves, so that the beams remain pointed at the zones 9 and 10, as long as satellite 1 is within sight of these. Array antennas of the type mentioned above make it possible to meet this need.

 Furthermore, the satellite 1 moving will ultimately cease to be in visibility of the area 9 for example.

Before that, beams 5 and 7 must be switched off. Another satellite, similar to satellite 1, will be used, instead of satellite 1, to, using beams similar to beams 5 and 6, maintain the communication link between station 11 and terminals 12, 13, 14 This will happen just as when satellite 1 previously arrived for areas 10 and 9.

 To do this, in accordance with the invention, the satellite comprises means for establishing beams 22, which cause the establishment of beams suitable for establishing the necessary communication links for the areas which the satellite encounters on its route.

 The satellite route being known to the stations, each of them, according to a first embodiment of the invention, requests the satellite 1, when it rises at its horizon, to establish the beams which it needs . Thus the station 11, seeing satellite 1 rise on the horizon, sends it a message, on a control channel not shown in FIG. 1, addressed to the beam-setting means 22, so that the satellite establishes the beams 5 and 6. Of course, the stations such as 11, in relation to one another by the links such as 17, 18, 19, coordinate in advance their requests for connection taking into account the communication capacity of the satellite, so that the demands of the different stations can always be met.

 Additionally, the station 11 is arranged to command a connection to the satellite 1 by transmitting to it data which, on the one hand, identify it itself and, on the other hand, specify the beams to be established; for its part, the satellite is arranged to verify the membership of the station in the communication system and to consequently establish the beams it requests.

 This is to protect access to the satellite against any fraudulent intrusion.

 According to a variant, the beams are established from data previously recorded in the satellite and used as the satellite progresses around the Earth.

 According to another variant, the data is stored in the stations and supplied to the satellite over as it progresses.

 It remains to be considered that the communication system of the invention comprises a set of satellites capable of permanently supplying the communication links required by each of the stations to communicate with the associated terminals of its area and that, correspondingly, each of the stations, to be able to provide a permanent service must always see at least one satellite capable of providing it with a communication link with the terminals associated with it, but also see a second satellite rising above the horizon then that whoever connects it with its terminals is not yet too far away.

According to one embodiment, these satellites circulate in circular orbits at an altitude of 1626.5 km, inclined at 55. Figure 2 shows the ground trace of such an orbit. Given the rotation of the Earth and the natural precession due to the flattening of the
Earth, this trace closes on itself after 12 revolutions. This simplifies the pointing of the antennas, which is particularly important with regard to the antennas of the terminals. In FIG. 2, the routes 23, 24, 25 ..., 26 have been identified, after which a satellite finds itself at its starting point. In an exemplary embodiment of the satellite system, it is possible to provide several orbits similar to that of FIG. 2, regularly spaced apart. The phase of the satellites on each orbit being calibrated relative to those which surround it, so that the distribution of the satellites is regular and that, seen from a ground station, the satellites always follow the same trajectory in the firmanent.

 Each satellite establishes a maximum number n of uplink beams and the same maximum number n of downlink beams. For example, n = 61. Each beam covers an area of approximately 200 km in diameter on the ground. With 32 satellites, coverage is complete at temperate latitudes. The transition to 40 satellites makes it possible to cover the entire planet between 800S and 800N. In regions where the population is sparse, beams with larger apertures may be adopted, covering for example areas of 500 km in diameter.

 By considering only one direction of transmission, from the satellite to the ground for example, each zone is assigned its own reception frequency. The reception frequencies of the neighboring zones are different.

However, the total number of frequencies used by the system may be moderate. By way of example, FIG. 3 represents, according to a well known model, zones 30 forming a regular paving covering an entire region. The frequencies specific to each of these zones are indicated.

They are 7, fl to f7. The zones 9 and 10 of FIG. 1 are shown in FIG. 3. It can be seen that they have, in this example, the same frequency F1. The same arrangement is used in the upward direction.

 Seen from a ground station, the satellites parade in the sky along repeating trajectories. We can therefore use simplified ephemeris tables to describe these trajectories, which facilitates calculations in each station and in each terminal.

 Indeed, as will be seen below, the communication link established for a station by a satellite comprises not only a beam pointed by the satellite at the area containing the station, as illustrated in FIG. 1, but also a pointed beam from the station to the satellite. The station will preferably use high gain antennas. And each station will include at least two antennas, so that one is used for the active communication link and the other for preparing the establishment of the communication link by another satellite. As for the terminals, they may have antennas of small dimensions, therefore of low directivity, which will facilitate their pointing towards the satellites.

 We will now turn to FIG. 4 which schematically represents an embodiment of the equipment of a satellite of the communication system of the invention, for example of the satellite 1 of FIG. 1.

Essentially, for communication between stations and terminals, the satellite includes a repeater system including:
- a reception antenna 40,
a plurality of reception beam forming networks 41.1, 41.2, up to 41, n,
a plurality of reception mixers 42 42, n,
for each channel thus unlaced in frequency, a variable gain amplifier 43, followed by an adjustable bandwidth filter 44,
a plurality of emission mixers 45.1 to 45, n,
a plurality of transmission beam forming networks 46.1, 46.2, up to 46, n,
- a transmitting antenna 47,
a local oscillator supplying the transmission mixers on the one hand and the reception mixers on the other hand local frequency change waves enabling the communication channels of the uplink frequency band to be transferred to a lower intermediate frequency, in order to allow, under good conditions, amplification and filtering and to replace the communication links in the downlink frequency band.

 A bus 49 connects the beam forming networks, the mixers, the amplifiers and the filters to a processor 50 which provides all these elements with the adjustment quantities which they need.

 The antenna 40 is a network antenna of the type described in the document cited above; together with the beam forming networks 41.1 to 41, n, it produces n beams (up to 61 in the example indicated above).

Its bandwidth includes the 7 frequency bands used to cover the different areas.

 The signal existing in the beam formed by the network 41.1, coming from the station and the terminals of a given area towards which this beam is pointed, is transferred in the intermediate frequency band by the mixer 42.1, the local oscillator 48 providing the appropriate local frequency. After amplification, in the amplifier 43, according to the gain prescribed to the amplifier by the processor 50, the filter 44 selects the useful frequency band for this zone. The resulting signal is moved in frequency, in the mixer 45.1, according to the local wave supplied by the local oscillator 48, to take its place in the downward frequency band. The signal thus prepared is supplied to the beamforming network 46.1 and the corresponding energy is directed by the antenna 47 to the same area considered. These means define a communication link relating to this area.

 It is thus clear that the two beam forming networks 41.1 and 46.1, of the same communication link, produce beams having the same orientation. They receive from the processor 50 the same pointing information. The processor 50 extracts this information from a database 51 having a location by area on the ground and containing data which make it possible to locate this area relative to the satellite, over time, and therefore to define the orientation to be given to the satellite antenna beams, so that they reach it.

This information is transmitted to the networks 41.1 and 46.1 which use it to orient the beams. The processor is arranged to periodically address the database; each time, new pointing information is sent to the networks 41.1 and 46.1 which consequently modify the pointing of the beams. The frequency of this addressing need not be very high.

 It is recalled that, as a variant, this information can be directly supplied to the satellite by the ground station when the satellite needs it.

 Of course, what has just been explained about a communication link is also valid for each of the n communication links that the satellite can establish simultaneously. The processor provides the corresponding elements of the repeater with the pointing information and other attributes under the same conditions, by time sharing.

 Of course again, the satellite only establishes the necessary communication links and each with the required power level and bandwidth. This means inter alia that it removes any communication link which is no longer necessary, either that the area for which this connection was established is no longer in view, or that it is served by another satellite.

 According to the invention, the satellite is therefore controlled to establish communication links and this command consists in transmitting to it data used to establish and update the database 51.

To this end, the satellite comprises, in cascade
- a control antenna 60,
- a coupler 61,
a set of amplification and filtering on reception 62,
a demodulator 63, and,
also cascading
- a modulator 64 and
a set of amplification and filtering in emission 65 attacking the coupler 61.

 A source of control data for the satellite can thus transmit control data to it on a control frequency which the satellite receives, amplifies and demodulates, the output of the demodulator 63 supplying the data in question to the processor 50 which introduces them into the database 51.

 Of course, beforehand, the control data source must interact with the satellite and, in particular, identify itself. As several stations are likely to want to communicate with the satellite at the same time, an access procedure allowing conflict to be resolved must be applied. An ALOHA type procedure similar to that which was mentioned previously can also be applied here.

 In a first form of the communication system of the invention, this data stored in the database 51 comes from a single pilot station; they are updated regularly. The stations note the establishment, then the suppression of the communication links by the satellite which flies over them. They use the communication links established by the various satellites, switching from one to the other at the appropriate time. In each station, the data which make it possible to follow the satellite and to switch the communication link from one satellite to another is also supplied by a pilot station of the communication system.

 According to another form of the communication system of the invention, the data relating to each communication link is transmitted to the satellite by the station requesting such a link when this station sees the satellite approaching. In response, the satellite establishes the requested communication link. The same means will be used to allow a station to control the removal of a communication link. The role of the satellite vis-à-vis the data of the database 51 is then much more passive than in the first case, since it simply has to execute orders to establish and delete communication links.

 According to yet another form, the data relating to each communication link is transmitted to the satellite by the station having requested such a link each time the satellite needs it. Practically then, the database 51 no longer plays any role, its content being found in the databases of the stations, as will be seen.

 We will now turn to FIG. 5 which very schematically represents an embodiment of the transmission equipment of a ground station. In this case it is a station with two antennas.

 The station in FIG. 5 therefore comprises two antennas 70 and 71, each coupled to a transmission / reception chain 73, 74 coupled, one or the other, by an inverter switch 75, to a modulation / demodulation assembly. 76 leading to a user interface 77. The two antennas are oriented by a control processor 78 having for this a database 79, like what has been described for the satellite with reference to FIG. 4 .

 Additionally, a data transmission channel 80 allows the processor 78 to communicate in both directions with a pilot station, for example, capable of supplying it with data to be recorded in the database 79. As indicated above, this communication channel Data transmission can be established via the satellites of the communication system, provided with additional means for this purpose.

 The processor 78 and the database 79, as we have seen for the processor 50 and the database 51 in FIG. 4, orient the beams of the antennas 70 and 71 towards a first and a second satellite. One of the two satellites establishes a communication link on behalf of the station considered, that aimed by the antenna 71 for example, and the switch 75 is oriented accordingly, as indicated in FIG. 5, under the control of the processor 78, by a control channel 81.

 In this way, the station 11, for example, through this user interface 77 communicates with the satellite and, through the latter, since it is transparent from the communication point of view, with the terminals in its area.

 When the first satellite moves away, the reversing switch 75 is switched to the antenna 70 which points to a second satellite which will be used to establish the communication link in place of the first.

 Additionally, the station has control channel equipment 72, coupled to the antenna 70 or 71 oriented by the processor 78, for the transmission of data to the satellite. This control channel allows the station to transmit control data to the satellite, as described above.

 Control data, arriving at the station by the link 80 is first recorded by the processor 78 in the database 79 which therefore contains, not only the control data which it needs to acquire and track the satellites of the system communication, but also those that must be transmitted to the satellites.

 The station of FIG. 5 can thus play the role of pilot station and transmit to each of the satellites of the system, a complete set of control data.

 Alternatively, each of the stations similar to that of FIG. 5 can limit itself to transmitting to each satellite with which it comes into contact the data which will enable it to establish a communication link for its own benefit.

 In this case, the station must also transmit a link delete command later.

 Alternatively again, the control link makes it possible to supply the satellite with the control data it needs, from the database 79, when it needs it, during its progression.

 No figure has been provided to illustrate the terminals. Figure 5, in fact also applies to the case of a terminal.

 This has one or two orientable antennas and one or two reception chains, as well as a transmission chain. The user interface delivers the entire spectrum of the communication link. The communication channel actually used by the terminal in its relationship with the station is determined by the communication procedure which was mentioned above and which, in any case, is outside the scope of the present invention.

 Likewise, the terminal has a processor and a database for pointing its antenna (s) under the conditions which have been specified previously. The data entered in its database are transmitted to it by the station with which it is associated, by the control link 82, or by any other equivalent thereof.

 It remains to be considered finally a peculiarity of the communication system of the invention which is that, seen from the ground the satellites appear as bright sources which are switched on and off according to the communication needs of each station. The system can easily be extended so that a station uses several satellites at the same time to communicate with the terminals associated with it. But also, it is easy to plan to extinguish the beams of a satellite whose position in the sky would be likely to cause the disturbance of other space communications. Then, it becomes possible to use for the communication system of the invention a frequency allocated to said other space communications, or even only such frequencies. The system of the invention therefore does not require any specific frequency allocation.

Claims (15)

 1. Communication system comprising a set of scrolling satellites establishing communication links between ground stations and terminals, a link coming from a station leading to associated terminals located in a limited geographical area including this station, characterized in that a communication link between such a particular station and the terminals associated with it is established by a satellite belonging to said set of satellites, controlled to establish for this purpose a directional beam of emission and a directional beam of reception, pointing to said limited area including said particular station, and supporting said communication link.  2. Communication system according to claim 1, characterized in that said beams are established on command of said particular station, transmitted to said satellite.  3. Communication system according to claim 1, characterized in that said beams are established from data previously recorded in said satellite and used as the satellite progresses.  4. Communication system according to claim 1, characterized in that said beams are established from data previously recorded in said station and supplied to the satellite as it progresses.  5. Communication system according to claim 1 or 2, characterized in that, a satellite being in view of said particular station, the latter is arranged to command a connection, which consists in transmitting to it data which identifies it itself and the beam establishment control data, the satellite being arranged to verify the station's membership in the system and to consequently establish the beams it requests.  6. Communication system according to claim 1 or 2, characterized in that, a satellite being in view of said particular station, the latter is arranged to command it to delete a link, which consists in transmitting to it data which identifies it itself and beam deletion control data, the satellite being arranged to verify the station's membership in the system and to consequently delete the beams established for its benefit.  7. Communication system according to claim 5 and 6, characterized in that said particular station is arranged to detect the entry into visibility of the satellites of the system and to predict their exit from visibility, to decide at any time by which satellite in visibility it must be linked and to transmit to visibility satellites, accordingly, the link setting and link removal commands, in order to remain in communication with the terminals in its area.  8. Communication system according to any one of claims 1 to 7, characterized in that said satellite is controlled to establish said beams only outside a celestial area where it would be likely to disturb other space communications.  9. Communication system according to claim 8, characterized in that said satellite transmits on a frequency at least allocated to said other space communications.  10. Communication system according to any one of the preceding claims, characterized in that said station comprises at least two orientable antennas and two transmission / reception chains for tracking at least two satellites at the same time.  11. Communication system according to any one of the preceding claims, characterized in that said terminals each comprise at least one orientable antenna and a transmission / reception chain for tracking at least one satellite.  12. Communication system according to any one of the preceding claims, characterized in that satellites are in an orbit such that their track on the ground closes after a defined number of revolutions.  13. Satellite arranged to conform to the communication system defined by any one of claims 1 to 12.  14. Station arranged to conform to the communication system defined by any one of claims 1 to 12.  15. Terminal arranged to conform to the communication system defined by any one of claims 1 to 12.