CA2193573A1 - Process and system for transmitting radioelectric signals between a stationary earth station and subscriber mobile terminals via a satellite network - Google Patents

Abstract

The invention relates to a method and a system for transmitting radio signals between a fixed earth station (ST1) and mobile user terminals (T1, T2), via a network of satellites (Sat1), in inclined circular orbits and with a radius smaller than that of geostationary orbits. The generated beams are subdivided into brushes (P1, P20), each one being associated with a frequency sub-band (F1, F20). These are distributed continuously, increasing or decreasing. The instantaneous transmission frequency is progressively shifted, during the duration of the communication, to the frequency sub-band (F2) of the adjacent brush, at a rate proportional to the speed of rotation, so as to allow inter-brush switching or inter-satellite. Application to transmissions by satellites in non-geostationary orbit. Figure 1a

Description

- _ 1 2 tl 9 3 ~ 3 The present invention relates to a signal transmission method 5 radio via a network of satellites between a fixed earth station and mobile user terminals.
It also relates to a transmission system implementing the process.
According to the invention, by "fixed earth station" is meant a station 10 international traffic terminal, or similar installation; this last which can itself be connected to terrestrial communication networks and / or other stations of the same type, by land or by satellite link.
Similarly, the term "mobile tPrmin ~ l" encompasses many types of terminals, in particular portable terminals, for example of the so-called "pager" type, 5 according to Anglo-Saxon terminology.
We know many satellite communication systems, for mobile services, or for fixed services. In general, the routes followed were very divergent depending on whether we consider one type or the other of these services, whether in terms of frequency bands (L or S band in one case, 20 C, Ku or Ka in the other case), capacity (a few Mbits / s, in one case, hundreds of Mbits / s in the other case), of coverage (global or regional, in a case, global by specific illumination, regional or national, in the other case) or in size and capacity of termin ~ llx (from 1 meter to a few centimeters with a flow of data of a few kbits / s, in one case, from 17 meters to 1 meters with a flow of 25 data of the order of a few Mbits / s to a few hundred Mbits / s).
Recently, the trend for satellite networks for mobile services is that they consist of satellites launched into low orbits (say "LEO") or medium (say "M: EO"). These satellites are capable of serving portable terminals using omnidirectional antennas, usually for 30 very strongly compressed voice transmissions. On the other hand, satellite networks intended for fixed services remain based on satellites launched into orbit geostationaries (called "GEO"). They are intended more specifically for traffic long distance or private (leased networks) by broadband networks.
The "low speed" part of the satellite communications market : ~ 5 (satellite dishes less than one meter in diameter) is affected by portable suitcases and a significant effort has been made in favor of terminals

- 2 2 ~ 93J73 inexpensive laptops. However, this market segment leaves a segment blank.
more efficient mobile terminals, i.e. broadband portable terminals. There is an increasing need for this market, located between traditional "fixed" and "mobile" satellite communication services. This new market also appears to be able to meet new requirements for "broadband mobile communications". Now, it is well known that networks cell phones such ~, at least in the short term, will not be able to offer communication higher than 16 kbits / s.
We can also think of telephony applications or 0 faxes using powerful terminals, for example in remote areas lacking sufficient terrestrial infrastructure: communication "mini-node", etc.
It is therefore necessary to develop a communication system by satellite, with global coverage, requiring a reasonable initial investment, 1 ~ but which offers high level performance (typical data rate of 64 kbits / s minimum, low tr ~ n ~ mi ~. ~ Ion delay) for terminals, with antenna foldable, the size of a pocket calculator. In addition, the system should not require, before putting into operation, long coordination procedures.
Finally, the system must not cause interference with other systems 20 of satellites, notably geostationary.
The method and the system according to the invention aim to solve simultaneously these requirements while overcoming the drawbacks of the prior art.
To do this, the "terrestrial" part of the system according to the invention consists in a large number of broadband portable user terminals and 25 satellite control, preferably duplicated.
The "space" part consists of a network of satellites in medium orbit ("MEO") which are controlled by the aforementioned stations, through telemetry, tracking and control relay stations ("TT&C"). The stations control can be linked to traffic end stations

3 () international. The orbits are circular and inclined, with a nodal period of 8 sidereal hours (that is to say triple synchronization with the rotation of the earth).
The satellites of the aforementioned network systematically repeat the same plot on above the earth every 24 hours, as explained in the article by J. MASS:
"Triply Geosynchronous Orbits for Mobile Communications", published in "15th : ~ AI ~ A Conference on Communication Satellite Systems "", San Diego, 1994, pages 1220-1226. The satellites, in minimum number of six, but preferably of 3 21 ~ 3573 seven, must additionally meet a number of additional criteria as will be shown below.
User portable terminals can be in active mode when their antennas are deployed and controlled by a command station 5 appropriate through a "control link" or, conversely, in passive mode.
In this mode, they can only be called (so-called "paged" mode according to the Anglo-Saxon terminology).
According to an important characteristic of the invention, scanning antennas, on satellites and in portable terminals. In addition, the 0 global frequency band is subdivided, in narrow beams, into contiguous bands, in a preferred embodiment. In what follows, these narrow beams will be called "brushes". We can therefore establish, in portable terminals, a very simple automatic switching mode (without blocking) between brushes, although the satellite is in motion. Just perform 15 an antenna scan command, for example a frequency scan, in ~ a preferred embodiment. According to this mode, the frequency used for the communication link.
Switching from one satellite to another takes place using a method similar. Still in a preferred embodiment, two consecutive satellites 20 sweep the same area of coverage using brushes whose sub-bands of frequency are organized in reverse order. This arrangement allows a t ~ rmin ~ l given to stay in the same frequency sub-band, during the transition from one satellite to another.
Finally, measures are taken to avoid, as much as possible, 25 any interference with already existing systems operating in the same bands frequency, including geostationary satellite systems.
The subject of the invention is therefore a method of transmitting signals via a network of satellites between at least one fixed earth station and user terminals, the satellites being rotated 30 around the terrestrial globe in circular orbits and at fixed speed, the orbits being inclined with respect to the equator and of radius smaller than that of geostationary orbits, the satellites being in triple synchronization with rotation of the terrestrial globe, each satellite comprising an arrangement of antennas making it possible to generate at least one beam with a directed radiation diagram ~ 5 towards the surface of the terrestrial globe and associated with a frequency band of width determined, characterized in that it comprises at least the following stages:

'~ 4 21 ~ 3 ~ i13 - partitioning of said beam into a determined number of contiguous brushes, each being associated with a frequency sub-band, subdivision of said bandwidth of determined width;
- establishment of a communication link, according to at least one s direction, between one of said land terminal stations and one of said terminals to use, via one of said network satellites, by using a frequency instant of transmission included in a first frequency sub-band associated with a first brush, the latter illl] min ~ nt said user terminal to the instant of overflight thereof by this satellite;
o - and progressive shift of said tran frequency ~ mi. ~. ~ instantaneous ion, for the duration of the communication link, to the frequency sub-band next associated with the adjacent brush and a rhythm proportional to said speed of determined rotation, so as to compensate for said movement and to allow said satellite to continue.
The invention also relates to a signal transmission system.
via a network of satellites between a fixed earth station providing implement the process.
The method and the system implementing the method therefore allow all at the same time to reduce the number of satellites required, to offer a large link capacity, particularly in terms of speed, and reduce the complexity of on-board equipment and that of portable terminals. There is not, in particular of need for inter-satellite communications and switching procedures portable terminals are reduced to a minimum.
The invention will be better understood and other characteristics and advantages will appear on reading the description which follows with reference to the appended figures, and among which:
- Figures la and lb schematically illustrate a transmission of radio signals via a satellite network enke a fixed earth station and portable terminals, according to the method of the invention;
3 () - Figures 2a and 2b illustrate switching between two satellites consecutive;
- Figure 3 illustrates a detail of the conforming transmission beams to the invention;
- Figures 4a to 4c schematically illustrate an example .5 arrangement of portable terminal antennas, in deployed and folded positions, respectively;

5 ~ l 93 l73 - Figures Sa and 5b illustrate two modes of reception of the terminals user mobiles;
- Figure 6 illustrates two examples of interference diagrams with a tr ~ n.cmi.csion system by geostationary satellites;
- Figure 7 is a diagram illustrating the land cover of a example of a network of satellites of the system according to the invention;
- Figure 8 illustrates the frequency variation as a function of the time of the signals received over a fixed point on the earth;
- Figure 9 illustrates the allocation of frequency bands between users according to an alternative embodiment for which the beams are virtual;
- Figure 10 illustrates an example of architecture of a compliant satellite to the invention.
The process of the invention will now be described in detail.
As noted, the land portion consists of a large number broadband portable user terminals and control stations for satellites Figures la and lb schematically illustrate the tran. ~ Mi ~ sion of radio signals via a satellite network between a f1xe ST1 terminal and land station and termin ~ llx of mobile users, only two of which have been referenced: T1 and T2. Naturally, the system can include several stations terrestrial. It also includes remote control stations, telemetry and ordered. These aspects are well known and there is no need to describe them further.
Although only one Sat1 satellite has been represented, it is clear that, for cover a given region of the earth at all times, since satellites do not are not geostationary, it is necessary to provide several satellites, preferably six to eight satellites. The precise characteristics of the orbits and the land cover provided will be explained later. We recall briefly that the orbits are circular and inclined with a nodal period of 8 sidereal hours (triple synchronization of the satellites on the rotation of the earth). The same tracing on the earth's surface will be repeated every 24 hours. This aspect is explained in the aforementioned article by J. MASS.
We can also show that we can get coverage optimization of emerged land if we adopt appropriate orbit parameters, by example:
.5 - tilt angle: 75 ~;
- ascending node: 30 ~;

6 21 q357 ~
-- Average anomaly: 0 ~;
In this case the minimum elevation angle is between 20 ~ and 30 ~ (for a satellite network of 6 to 8 units).
The mobile link can take place in the Ku or Ka bands. To fix 5 the ideas, and without this in any way limiting the scope of the invention, it will be assumed in what follows that we operate in the Ka band. The links of communication, for example l1, are established on request for the desired bandwidth, for example typically from 16 kbits / s to 2 Mbits / s between any portable user terminal in the active state, for example T1, and a ground station for 0 appropriate command or international traffic terminal, for example ST1. This last station ST1 then routes the call through a network public or private appropriate, so as to reach the desired final destination. The ST1 station then establishes the reverse routing of incoming "calls", through terrestrial networks, to portable user terminals T 1 or T2. These terminals can be connected to each other by double hops and a transit (or more) by earth stations, for example ST1. It should be noted that transit times however, remain below the time allowed for geostationary satellites.
The "call" channels can be conventional connections based on switched physical circuits but also, as long as the satellite links are 20 concerned, these "call" channels can be virtual, for example in a structured asynchronous multiplexed transmission system called "ATM".
To fix the ideas, each satellite can offer an equivalent capacity at around one thousand 64 kbit circuits, in full duplex, with a single repeater transparent translating in direct frequency. Each satellite, by 25 example Satl, communicates with the portable user terminals, for example T1 or T2, using a scanning antenna arrangement for reception and another scanning antenna arrangement for tr ~ n ~ mi. ~ ion. In order to simplification, only the AS1 receiving antenna arrangement has been shown.
According to one aspect of the invention, each of the antennas generates a number ~ o determined by brushes, for example Z0, referenced P1 to P20, the section of which is strongly ellipsoidal. We will assume below, without this being in any way limiting of the scope of the invention, that the main axis of the ellipse is orthogonal to the vector velocity v of Sat1 satellite. We also assume that it is moving from the south north above the surface of the earth Te.

7 21 93 ~ 13 A priori, the imprints of the beams, transmitted and received, are alike. It is assumed that these beams have crossed polarizations (orthogonal) and occupy frequency bands of substantially equal width.
In one important aspect, the overall frequency band (typically 500 MHz) is subdivided between the different brushes (for example 20), Pl to P20, in sub-bands, F1 to F20. We will finally assume that each brush operates in a specific sub-band, contiguous to the adjacent brush sub-band (25 MHz, in the example described).
By implementing this provision and by modifying the frequency according to 0 a law proportional to the angular velocity v of the satellite Sat1, it is possible to perform an automatic switch between brushes even though the Sat1 satellite is in continuous movement, without blocking.
For example, in FIG. 1 a, the portable user terminal T1 operates in the sub-band of frequencies F3 when the satellite Satl is in position 1. The link 11 is established when the terminal T1 is in the footprint of the brush p3, corresponding to this sub-band F3. We then modify gradually the operating frequency towards the frequencies of the frequency sub-band F2. Sat1 satellite is expected to move from south to the North. The frequency bands associated with the brushes P1 to P20, are in order F1 to F20, from south to north (in the example described). It may also be that the sub-band F2 is reached during the duration of the tr ~ nsmi. ~ sion and that one is in the brush P2, corresponding to this sub-band.
It should be noted that these provisions do not presuppose any processing of data on board satellites, for example Sat1 satellite, but a simple variation of the frequency used at the level of the earth station ST1 during the duration of the link 11 between the latter and the terminal T1. Naturally, the T1 terminal must be able to follow these received frequency variations and to transmit on this instantaneous frequency. This type of switching does not require any high level, complex to implement and, on the other hand, shows itself strictly not blocking.
We have just described the switching procedure between brushes, P1 to P20, of the radiation pattern of the same satellite, in this case the satellite Sat1. However, when the terminal T1 is in the last brush, P20, Sat1 satellite, whose associated sub-band is F1, if the communication in progress 3 ~ is not finished, we will have to organize the switching between this brush and the first brush of the next satellite, the Sat2 satellite. If the sub-bands of - ~ 1 93 ~ 73 _ 8 frequencies are distributed identically to those of the Sat1 satellite, it will be necessary to jump abruptly from sub-band F1 to sub-band F20.
In a preferred embodiment of the invention, the distribution of Sat2 satellite subbands are organized so that they appear in reverse order, i.e. from the F20 frequency sub-band to the frequency band F1, from south to north (in the example described).
Figures 2a and 2b schematically illustrate the switching between two consecutive satellites Satl and Sat2. To simplify the drawings, we do not have represented the earth stations. The brushes corresponding to the sub-bands 0 respectively F1 to F20 are marked P'1 to P'20 On f1gure 2a, the satellites, Sat1 and Sat2, are represented by position 3 in their rotational movements around the earth Te ~ from south to north, with speed v. Terminal T1 is located in the footprint of the last brush, P1, from satellite Sat1 and operates in the frequency sub-band F1 (link ll). The first brush, P'20, of the Sat2 satellite can cover all or part of the last brush, P1, from the Satl satellite.
The frequency sub-bands associated with these two brushes, P1 and P'20 respectively, being identical, the switching between the two satellites, Sat1 and Sat2, can be done simply. This switching can be progressive, the terminal T1 capable of receiving signals from the two satellites, Sat1 and Sat2, such as it will be explained below.
In FIG. 2b, the satellites, Sat1 and Sat2, are represented by position 4 in their rotational movements around the earth Te. The link, referenced 12, is now carried out exclusively between the terminal Tl and the Sat2 satellite. The operating frequency is then gradually modified towards, the frequencies of the sub-band of frequencies F2 (and so on, towards F3, etc.).
In order to avoid interference between brushes and to facilitate switching, the access scheme in each sub-band can be performed, by example and without this being in any way limiting of the scope of the invention, according to the synchronous "AMDC" mode (Multiple Access by Code Difference), whether in transmission or reception. At the global level of satellites, for example Sat1, the diagram will be of the "V-MRF-AMDC" type, that is to say Multiplexing by Distribution Variable Frequency ("MRF") between brushes and "AMDC" inside each ~ 5 brush, that is to say in a frequency sub-band, for example F1. The mode AMDC allows efficient processing of virtual channels in transmission mode - 21 ', 3S13 _ g asynchronous multiplexed ("ATM"). In addition, a wide variety of bit rates can be treated, within the same element flow in "MRF", by setting makes gains from different processes. The "MRF" mode can be replaced by a equivalent mode, such as "MRT", or time division multiplexing, spectrum, for the downstream link (Terminal Station STl ~ satellite Satl ~ terminal Tl, see Figures la and lb), if necessary, for example for frequency acquisition and its easier follow-up.
In addition, the coverage area of each brush can be subdivided in sub-brushes, for example three, as illustrated in Figure 3. Each brush, for example Pl, is subdivided into three sub-brushes, P1A to PlC, aligned on the main axis ~ 1 of the ellipse that forms the section of each brush. The axis ~ 1 is orthogonal, as indicated, to an axis ~ (minor axis of the ellipse), parallel at the velocity vector v. Each series of sub-brushes, for example P1A to P20A ~ can be generated by separate scanning antennas, arranged in parallel, all operating identically.
This arrangement is particularly advantageous for the connection satellite-to-ground to increase the antenna gain, decrease the consumption of power of the on-board electronics and decrease the power spectral density interfering with geostationary systems.
Another improvement is to shrink the brushes and sub-brushes to a single active user, i.e. to individualize the frequency sweep to the user. If we refer again to figure la, this amounts to using a slightly different center frequency of the sub-band, ie F'3, for the spectrum at frequency spreading of terminal T2, for example, although the same procedure 2s of frequency sweep is applied to this terminal T2.
This improvement does not complicate or even modify the antenna onboard, but offers a gain of around 3 dB in the link budget and makes absolutely smooth frequency tracking by mobile user terminals.
It is naturally necessary to implement a tuning of -, o finer frequency per user, but the possible complexity that results is rejected at terrestrial terminal stations.
There is also a slight degradation due to recovery imperfect spectra "AMDC" which are no longer completely synchronous.
However, very close users (for example Tl and T2, in FIG. La) are ~ s almost synchronous and distant users can be discriminated by gains lo 21 935, 3 different antennas at separate frequencies, which leads to rejection of own noises between synchronous and asynchronous "AMDC" levels.
During the orbital movement of the satellite, the mobile terminals of users must continue the brush, whether in reception or in transmission. Figures 4a to 4c schematically illustrates an example of creation of a terminal with its antenna.
The terminal includes two planar antennas, A1 l and A12. These antennas, A1 1 and A12 are scanning antennas. They are arranged on supports, Sup1 and Sup2, respectively. These supports can be constituted by boxes o comprising the usual electronic circuits for receiving signals, transmitting and processing them.
In FIG. 4a, the terminal T1 has been represented in active mode, that is to say say with its antennas, Al1 and A12, deployed.
The pursuit of the satellite, taking into account the provisions which come to be called back, can be done in a simple plan, with a very small error, and not in a volume. This plan is determined simply, if we know the characteristics of satellite orbits (which are repetitive), time and position of the mobile user terminal on the surface of the terrestrial globe Te. Circuits of well known calculations can be used for this purpose.
Each antenna, A11 and A12, is aligned with an orthogonal direction in the plane of the antenna. The surface of the two supports, Sup1 and Sup2, forms a plane single and the antennas, A11 and A12, are inclined at the same angle o ~ relative to this unique plan.
According to an advantageous aspect of the invention, each antenna, A11 and A12, is connected to separate transmit and receive circuits.
If we first consider the "reception" mode, two cases arise present:
- When the satellite, for example Sat1 (see figures la or lb), found substantially at the zenith, the two antennas, A11 and A12, receive the same signal, from the same Sat1 satellite. Figure 5a schematically illustrates this case. The signals received are transmitted to the inputs of two input circuits usual, represented by simple amplifiers, A1 and A2, respectively.
Each channel can be used separately, but signals can be combined advantageously by a Mel mixer circuit, which makes it possible to obtain a signal of ~ 5 Vs output of better quality. This combination is performed at the demodulation. The signal Vs is then processed in a known manner.

_ 11 2t '~ / 3 - When switching between contiguous satellites (see Figures 2a and 2b), each antenna, All and A12, receives the same signal, as for its content, but from different satellites. In the example illustrated in Figure 5b, the All antenna receives the signal from the Satl satellite and the A12 antenna from the 5 Sat2 satellite. The two transmission links ll (see f1gure lb, for example) and 12 (see Figure 2b, for example) remain under the control of the same earth station terminal, for example STl (Figures la and lb). The received power is added by the Mel circuit, as before. This technique is known as of "diversity techniques" and is applicable to the "AMDC" mode. In this way, the o switching between satellites is carried out without discontinuity.
For the transmission, a similar procedure can be implemented.
It is then necessary to have two power amplifiers in the terminal (antenna amplifier).
Since there is no need to track the satellite (s) 5 that in a single plane and not in three dimensions, it is possible to optimize the configuration of the antenna, or more precisely of the antennas of mobile terminals of users. It is in particular possible to minimize the erasing losses and to obtain a radiation diagram, while allowing easy, inter-brush switching of the same satellite (for example Sat1) or inter-satellites (by 20 example figures 2b or 4a: Sat1 to Sat2).
The antenna can be folded back when the terminal T1 switches from active mode in passive mode.
Figures 4b and 4c illustrate two stages of antenna folding A1l and A12. First, the antennas are folded down on the single plane formed by 25 the upper faces of the supports, Sup1 and Sup2. These can be in turn folded around a hinge axis ~ r (f1gure 4b), folded, head to tail, one on the other, and locked in this position (Figure 4c). To achieve these results, organs well known mechanics can be used: hinges, etc.
In passive mode, the mobile user terminals, for example Tl, do not 30 can only be called (so-called "paging" mode according to Anglo-Saxon terminology). he it is possible to implement the same "AMDC" multiplexing procedure, ie by a partial use of the folded flat antennas, A11 and A12, either using an auxiliary antenna (not shown).
Proper deployment, as noted, can be accomplished . ~ simple, since we know the orbital characteristics of the satellites in the network. These data can be saved in tables (programmable type memories 12 2 ~ 9 3 "3 or PROM for example). It is also necessary to know, at least approximately, the time and location of terminal T1.
An initial procedure (using the previous information:
times, location, and / or broadcast data signals from satellites) allows fine adjustment of orientation, frequency of use and that the creation of a signaling channel inside the "AMDC" band (associated with a low flow of information per user).
At the satellite level, there are many solutions for continuation of the terrestrial terminal station: mechanically adjustable reflector, 0 phase control array antenna, multiple feed arrangement based on a Butler matrix or other type of frequency sweep antenna. We can implement an autonomous tracking and switching procedure, similar to that of terminals.
In a preferred embodiment, each terrestrial terminal station (for example ST1, FIGS. 1a and 1b) comprises at least one double antenna (for example of satellite dishes) by satellite orbit in sight. This provision allows a more gradual switching. Each station is responsible for the correct allocation communication channels in the sub-bands (i.e. brushes and frequencies, including frequency sweep to compensate for orbital motion satellites), and in the sub-bands, appropriate "AMDC" codes for initialization procedures and sign ~ ation channels for mobile terminals of users. They are also responsible for connecting to public or private networks, for determining the location of terminals in active mode and for traffic characteristics, together with other terminal stations, as well as all other functions necessary for correct operations.
It is well known, moreover, that one of the major problems encountered with satellite systems on lower orbits than satellites its geostationary systems, are interference and coordination with other systems, both geostationary and non-geostationary.
With regard to similar non-satellite systems geostationary, just use phases of different satellite orbits and / or different ground traces.
With regard to geostationary satellites, working in same frequency bands, it is necessary to control interference .5 mutuals as explained below.

~ _ 13 21 9:) 573 One of the aims of the system according to the invention, as has been indicated, is to solve this problem.
The most critical case, compared to geostationary systems is naturally the interference of very powerful downlink signals (satellite-to-earth), 5 reaching terminals operating in cooperation with satellites geostationaries. The "Equivalent Isotropic Radiated Power" or "EIRP" is very important since it involves communicating with portable terminals of very small sizes, including the antennas, even when deployed, have a very small surface.
In addition, due to the lower latitude than that of geostationary satellites, the lo signals undergo a weaker attenuation.
It can, however, be observed that the satellites of the system according to the invention interferes during a very limited part of their orbit (near the equator, typically between -9 ~ and + 9 ~ latitude) and according to a figure of interference substantially presenting a "banana diagram", as illustrated 15 by FIG. 6. This last figure illustrates two diagrams Dial and Dia2, for two distinct positions of a satellite, for example the Satl satellite. The thickness of this "banana diagram" depends on various parameters: in particular the reception performance outside of the reception axis of greater sensitivity of the terminals (belonging to the geostationary system) and the level of 20 signals (power spectral density) of the system according to the invention compared to nominal signal from geostationary satellites.
If we consider that these "banana diagram" interferences have a form very close to the brush marks and they move all the more quickly that they have a larger surface (i.e. when we 25 approaches latitudes + 9 ~ above), a very simple countermeasure, and yet effective, to avoid unwanted interference can be adopted. The simplest is to remove the brush (es) corresponding to the interference in "diagram banana ". The positions of these brushes can be known a priori since they depend on the instantaneous position of the satellites, positions which are naturally .o permanently known. The brush removal operation in question is simple, due to the frequency scanning characteristics of the antenna satellite. It is enough to interrupt, at the land terminal station (by example STl) controlling the satellite (s) the frequency sub-bands corresponding to the brushes to be deleted.
~ 5 International standardization works are exposed in following publications:

- l ~ 21 '~ 3S73 - "ITU-RRecommandation 580-2";
- "ITU-R Special Committee Results on Coordination between Geostationary and Non-Geostationary Satellite System ".
Given the standardization work mentioned above, a 5 part, and characteristics of resistance to interference of the "AMDC" mode, on the other hand, the duration of such interruptions would be approximately 2 to 3 minutes every 2 hours (it is moreover likely that the sign ~ tion route will still remain active), in the worst case, i.e. for a continuous ring of satellites the along a geostationary arc, all operating in the same frequency band as the satellites of the system according to the invention. The resulting unavailability (approximately 2% of the foreseeable time intervals) is then comparable to the non-availability due to the attenuation in the event of rain (approximately 1% of the foreseeable time intervals) is much better than the non availability due to the occultations tolerated for land mobile systems.
These hypotheses have been retained for a system of 6 to 8 satellites. The Interference could be entirely avoided if a greater number of satellites are used: 10 to 15 for example.
Another way to solve the interference problem is based on the fact that, in each zone, only a subset of frequency sub-bands 20 may interfere with geostationary satellite systems (in this case only the sub-bands corresponding to a scanning angle at which the "banana diagrams" are generated, that is, when the satellite is located between latitudes -9 ~ and + 9 ~). Frequency coordination could be limited to this single set of sub-bands, which can be chosen in the least portion 25 spectrum criticism (for example in the margin of the band allocated to transponders geostationary or taking into account the current actual position of the coordinates satellites and frequencies instead of the aforementioned pessimistic hypothesis, that is to say say that of a continuous ring of satellites).
Other methods and / or combinations of methods may be 30 also implemented.
We will now explain in more detail the different parameters associated with the satellites of the network composing the space part of the system.
As indicated, the orbits are circular and inclined with a nodal period of 8 sidereal hours (28721l ~) (triple synchronization on ~ rotating ~ 5 of the earth). The same pattern on the earth's surface is repeated every 24 hours.

15,219 ~ $ ~ 7 ~

However, to compensate for the longitudinal drift of the ascending node, it is necessary to adopt a slightly shorter orbital period.
Optimized land cover is obtained if we adopt appropriate orbit parameters, for example:
- tilt angle: 75 ~;
- ascending node: 30 ~;
- Average anomaly: 0 ~;
- Orbital period: around 28714 "(instead of 28721"theoretical);
- altitude: 13,901 km.
0 In this case the minimum elevation angle is between 20 ~ and 30 ~ (for a satellite network of 6 to 8 units).
Naturally the satellites of the network, for example 6 minimum, are regularly spaced. However, since the satellites, taken as a whole, have an apparent movement comparable to the crawling of a snake, for a terrestrial observer, it happens that the "head" and the "tail" do not overlap properly for half an hour a day at temperate latitudes. Also for avoid this drawback, it is necessary to provide at least one satellite additional, ie 7 satellites for the network.
To fix ideas, by taking the hypothesis of 7 satellites, the Orbit parameters are as follows:

Satellite N ~: Ascending nodes Right anomalies:
120 ~ 90 ~
2 90 ~ 180 ~
3 60 ~ _90O

4 30 ~ 0 ~
0 ~ 90O
6 -30 ~ 180 ~
7 -60 ~ -90 ~

Figure 7 illustrates the hourly positions of the satellites on a projection terrestrial (trace).
We will now detail the characteristics of the communication, in a preferred embodiment.
Four links are necessary to guarantee communications in full duplex: "terrestrial terminal station ~ satellite" and "satellite ~ terminal ~ ~ 16 21 9 ~, 3 mobile user ", that is to say" earth ~ space "transmissions.
Full duplex communications can be achieved using different frequency bands with different polarizations. As indicated, we can choose, by example, the Ku band for the links "satellite ~ mobile user terminal" and the Ka band for the links "terrestrial terminal station ~ satellite". Other frequencies can be used: millimeter strip, for example.
The access technique will preferably be of the "SD-AMDC" type ("Direct Sequence - Multiple Access by Difference Code"), although the system can operate according to many other techniques. Using a simple paintbrush lo per user is particularly interesting, at least if the system is not too charge.
When a frequency scanning antenna is used in the terminal user portable, for example T1 (see FIGS. 1a and 1b), this terminal receives from high gain antenna, on board the satellite (for example Sat1), a signal whose 15 frequency varies according to the diagram of FIG. 8, according to its relative position by relative to the antenna on board the satellite (see FIGS. 1a and 1b, for example).
This diagram represents the variation of the frequency of the received signal according to the time. The time axis is arbitrary, in the sense that the start (time 0) represents the moment for which the earth station enters the visibility zone 20 of a given satellite (for example Satl). The other frequencies are attenuated in function of the antenna pattern specific to the terminal considered (e.g. T1). In this way, it is possible to divide the coverage area of the satellite (for example Satl) in virtual brushes, each characterized by a nominal frequency band, narrow compared to bandwidth 25 of the system, band for which the antenna has a significant gain.
The virtual brush is fixed on the surface of the earth, but the frequency cenkale evolves as illustrated by figures la, lb and 8. In this way, if the terrestrial elevation (for example ST1) is able to follow the variations of frequency, it is always in the center of the brush, so it allows a greater gain 30 of the satellite antenna (for example Satl): there are no losses due to the drops gains at the edge of the cover.
Since the apparent ground speed of the satellites is important (approximately 5000 km / h), a ground station can "see" pass several brushes during a Call in progress. The systems of the known art require an inter-beams while the earth station in the system according to the invention continues to "see" the same virtual brush, the frequency of which changes as indicated 17 21q ~) ~ 73 above. The frequency variation is known. It's a deterministic function, both at the transmitter and at the receiver. It can be deducted, in particular, the coordinates of the station and the time. It is therefore sufficient that the transmitter varies its frequency in accordance with the aforementioned law and that the receiver changes the frequency of the local oscillator of change of frequency, in the same way, to be able to retrieve band information based.
The mechanism to be adopted requires being able to follow variations in relatively wide frequencies, typically of the order of 50 to 100 kHz / s, for o obtain the correct baseband signal at all times. This requirement, by making call on current technologies, can however be satisfied, even for inexpensive terminals. It should be noted that this significant variation in frequency concerns only the center frequency, which is easier to process than a conventional Doppler device which also requires flow changes from data, with all the consequences that this implies for synchronization, filtering, etc.
Modulation and filtering techniques adopted within the framework of the invention may be the same as those implemented in the systems "AMDC" terrestrial or by satellite of the known art.
Another critical problem is posed by non-satellite geostationary is call forwarding, i.e. switching between land terminal stations. In known prior art satellite systems, of the type "LEO", the apparent speed of the satellites on the Earth track is very important and the elevation angle is very small (about 10 ~), so that the coverage area of the 2 ~ satellite is the same for the land terminal station and for the land station mobile. Switching between terrestrial terminal stations can take place easy, in particular when a mobile earth station initiates a call on the edge of blanket. The satellite links the mobile earth station to the earth terminal station and it connects with a fixed terrestrial communication network. during the call, the satellite continues its movement in orbit, leaves the visibility area of the terrestrial terminal station to enter the visibility zone of the next.
In this case, it is necessary to redirect the communication channels between terminal stations, by operating a dedicated network, via cable or link satellite, so as to reach the original terrestrial fixed communication network.
3 ~ On the contrary, the system according to the invention makes it possible to avoid, in the majority of cases, switching between terrestrial terminal stations.

18 21 93 ~ 73 According to this system, the minimum elevation angle for mobile stations of users (e.g. 25 ~) is more important than the minimum elevation angle for land terminal stations (e.g. 10 ~), so that the visibility area for terminal stations from the satellite is comparatively more important.
s By exploiting this property, the special characteristics of the orbits of the satellites and an appropriate location of earth terminal stations, it is not normally no need to switch between these stations since it is very likely that communication will stop or, at the very least that an inter-satellite switching takes place, before the latter leaves the area 0 of visibility of the terrestrial terminal station.
So the problem is essentially reduced to a sharing of resources between terrestrial terminal stations and a simple configuration of satellites without Inter-terminal terrestrial switching can be considered.
We will now detail the link budget on an example of preferred achievement.
The following assumptions have been made:
- Access is carried out according to the "SD-AMDC" mode already described, which means that users are divided according to different codes for get bandwidth expansion.
- The bit rate of elements is equal to 24.576 MHz;
- The processing gain is 64 for a flow at source of 384 kbits / s and 384 for a source bit rate of 64 kbit / s;
- The receiver of the user's mobile terminal continues, by scanning frequencies, the main lobe of the antenna.
As a consequence of this last characteristic, the different user signals can partially overlap in the same band of frequencies. This arrangement is illustrated in Figure 9. There are shown three frequency bands, allocated to users Ul to U3 and overlapping partially.
Consequently, in the system according to the invention, there is no rigid partition of the coverage area in brushes, since it is not possible to identify a central frequency. To make calculations easier, a brush virtual is defined as the surface of the earth that "sees" the bandwidth of signal transmitted with a center at the nominal frequency. The mobile user terminal .5 is always in the center of the virtual brush coverage area (and the gain satellite antenna is always maximum) but it is not possible to always 19 21 q3 ~ 3 obtain completely orthogonal spread sequences, even if the users remain synchronized in time. However, the situation is more favorable than in classical asynchronous "AMDC" type systems since the own noise close users (received by the same on-board power) is rejected by their quasi-orthogonality, while the own noise of distant users (therefore largely non orthogonal) is reduced by antenna discrimination.
Accurate calculation of self-interference noise is very difficult to carry out. It was admitted an extreme adverse scenario based on an "AMDC"
fully asynchronous, which includes flexibility.
0 To fix the ideas, tables I to VI placed at the end of this description illustrates examples of link budget for, respectively:
- the "outbound" link to the earth terminal station ~ satellite;
- the "go" link satellite station ~ mobile user terminal;
- the overall results of these two routes;
IS - the "return" link mobile user terminal ~ satellite;
- the satellite "return" link ~ t ~ .rmin ~ terrestrial station;
- the overall results of these two routes.
FIG. 10 schematically illustrates an exemplary embodiment possible of a communication satellite according to the invention, in the form of block diagrams.
The satellite, for example Sat1, comprises scanning antennas in frequencies: 1, 2, 3 and 4. The maximum scanning angles are of the order of 35 ~, either from or to mobile user terminals, or from or to stations terrestrial terminals. The go link has two sets of antennas: the antenna transmission 1, in Ku band, and the receiving antenna, in Ka band. Each game cor ~ carries three parallel antennas, 11 to 13 and 21 to 23, respectively; each typically covering 35 ~ parallel to the direction of travel of the satellite Sat1 and 12.5 ~, in an orthogonal direction. For example, the antennas 11 to 13 produce the sub-brushes P1A to PlC shown in FIG. 3.
, ~ The antennas of the "return" links, 3 and 4, respectively, are unique.
Satellite electronics Sat1, includes local oscillator 7 distributing frequency signals on its outputs to the usual circuits of control and processing of signals, in particular to a set 5 of processors of intermediate frequencies, 51 to 53, controlling a set of wave tubes ~ s progressive 60. These feed the antennas ll to 13. A fourth tube to progressive waves, 61, feeds the transmitting antenna 4 to the terminal stations 20 21 9:; 'J ~ 73 terrestrial. The on-board electronics also include amplifiers with low noise, 80 and 81, arranged at the output of the receiving antennas. She understands also a beacon 9, the output signals of which serve as a reference for the local oscillator 7. Finally, it includes various filters and mixers, usual for this type of application, which have not been expressly referenced as well as all organs necessary for the proper functioning of the assembly: power supplies electric, etc.
Each Ku-band antenna scans individual channels "AMDC" communication channel (about 25 MHz) in a direction parallel to the 0 displacement of the Satl satellite, on its global band of 500 ME ~ z. The same band bandwidth can be reused by all three sub-antennas 11 to 13, if the "AMDC" protocol and lateral antenna discrimination can guarantee sufficient protection against mutual interference.
Each 500 MHz band (in antennas 11 to 13) is obtained at from the signals received by the corresponding antennas (21 to 23) in Ka band (by frequency change, from 2.5 MHz to 500 MHz). Signals are transmitted to intermediate frequency processors 51 to 53.
A similar method is implemented for the "back" link. However the receiving 3 and transmitting 4 antennas are unique.
It should be clear that the invention is not limited to only the examples of embodiments precisely described, in particular in relation to FIGS. 1 to 10. In in particular, the scanning of antennas, whether on-board antennas or antennas of mobile user terminals, is not limited to only scanning of frequencies.
Numerical values (frequency bands, bandwidths, etc.) were only provided to better illustrate the invention and essentially depend of the application specifically envisaged. Finally, the transmission protocols usable in the context of the invention ("AMDC", etc.) are multiple.

2 1 9 S, 7 3 TABLE I

Assessment of the "outbound" link "Terrestrial terminal station ~ Satellite"

Frequency GHz 30 Minimum site angle Degrees 10 Distance Km 18 173 Tractive Power dBW 22.7 DBi 66 station peak gain WORST dBW 88.7 WORST per user dBW 58.7 Propagation loss dB 207.1 Atmospheric losses dB 10.5 Satellite antenna gain (3-sector antenna), dBi 24.1 after Butler matrix ("BFN") Satellite noise temperature ("BFN" included) K 900 Boltzmann constant dBW / K / Hz -228.6 Noise spectral density dBW / Hz -199.1 Bit rate kHz 64 Bit rate dBHz 48.1 Eb / No link "amount" dB 16.1 Minimum angle outside the satellite axis ~ 1 25 relative to the aiming station antenna degrees terrestrial terminal for interference acceptable Time interval during which the satellite is ~ 1 aligned with the arc of geostationary minute satellites 2 1 ~ 3 ~ 7 3 TABLE II

Assessment of the "outbound" link "Satellite ~ User mobile station"

Frequency GHz 12 Desiteminimum angle Degrees 25 Distance Km 16741 Trnn ~ power ~ mi ~ sion to the antenna dBW 19.8 Overall satellite antenna gain (3 dBi 32.1 antenna sectors) after Butler matrix ("BFN") EIRP total dBW 51.9 WORST per user dBW 21.9 Propagation losses dB 198.5 Average atmospheric losses dB 1.5 Antenna gain of the mobile user tPrmin ~ l dBi 30 Noise temperature of the K 290 mobile terminal Boltzmann constant dBW / K / Hz -228.6 Noise spectral density dBW / Hz -204 Bit rate kHz 64 Bit rate dBHz 48.1 Eb / No "downlink" dB 16.1 Minimum angle outside the satellite axis ~ 2 compared to an antenna of the degree sight geostationary for acceptable interference Time interval during which the satellite is 3 aligned with the arc of geostationary satellites in minutes below the "off-axis" angle Number of users per satellite number 1000 System margin dB

23 21 9357 ~

TABLE III

Overall assessment of the "outbound" link

5 "Land terminal station ~ Mobile user terminal"

Eb / No (global, end-to-end) dB 6.2 Eb / (No + Io) (global) dB 4.5 Bandwidth of the MHz 520 system o Processing gain - at 64 kbits / s 384 - at 384 kbits / s 64 Element throughput Mbits / s 24,576 Bit error rate (end-to-end) lO-10 21 ~ $ 513 TABLE IV

Assessment of the link "Mobile user terminal ~ Satellite"

Frequency GHz 14 Desiteminimum angle Degrees 25 Distance Km 16741 Total transmission power at the antenna dBW 3 Antenna gain of the mobile user terminal dBi 31 EIRP of the mobile user terminal dBW 34 Propagation losses dB 199.8 Maximum of co-located users (for numbers 24 interference inside the system and with geostationary satellites) Atmospheric losses dB 3 Satellite antenna gain dBi 28.6 Noise temperature of satellite K 600 Boltzmann constant dBW / K / Hz -228.6 Noise spectral density dBW / Hz -200.8 Bit rate kHz 64 Bit rate dBHz 48.1 Eb / No link "amount" dB 11.5 2s Minimum angle outside the axis of a satellite ~ 0.7 geostationary with respect to an antenna of degrees sight of the mobile user terminal for a acceptable interference Time interval during which the satellite is 1 30 aligned with the arc of geostationary satellites in minutes below the "off-axis" angle Eb / Io dB 9.2 System margin dB

~ 5 2 1.3513 TABLE V

Assessment of the "Satellite ~ Terrestrial Terminal Station" link s Frequency GHz 20 Minimum site angle Degrees 10 Distance Km 18 173 Transmission power to the antenna dBW 10.5 lo Satellite antenna gain (after "BFN") dBi 20.1 EIRP from dBW 30.6 satellite WORST per user dBW 0.6 Propagation losses dB 203.7 Atmospheric losses dB 6 Antenna gain of the earth terminal station dBi 61 Noise temperature of the terminal station K 290 earthly Boltzmann constant dBW / K / Hz -228.6 Noise spectral density dBW / Hz -204 Bit rate kHz 64 Bit rate dBHz 48.1 Eb / No "downlink" dB 7,8 Minimum angle "outside" of the satellite axis relative to the antenna of the earth station f1xe degrees 1 <
for targeting a geostationary satellite for acceptable interference Time interval during which the satellite is aligned with the arc of geostationary satellites in minutes 1 <
below the minimum angle "outside" of the axis Number of users per satellite number 1000 2t 93, 73 -TABLE VI

Overall assessment of the "return" link 5 "Mobile user terminal ~ Land terminal station"

Eb / No (global, end-to-end) dB 6.2 Eb / (No + Io) (global) dB 4.5 Bandwidth of the MHz 520 system o Processing gain - at 64 kbits / s 384 - at 384 kbits / s 64 Element throughput Mbits / s 24,576 Lo-lo bit error rate (end-to-end)

Claims (16)

1. Method of transmitting radio signals via a network of satellites (Sat1, Sat2) between at least one fixed earth station (ST1) and terminals of users (T1), the satellites (Sat1, Sat2) being rotated around the terrestrial globe (Te) in circular orbits and at fixed speed, the orbits being inclined with respect to the equator and of radius smaller than that of geostationary orbits, the satellites (Sat1, Sat2) being triple synchronized with the rotation of the terrestrial globe (Te), each satellite (Sat1, Sat2) comprising an arrangement of antennas (1-4) making it possible to generate at least one beam at radiation pattern directed towards the surface of the Earth (Te) and associated with a frequency band of determined width, characterized in that it comprises at minus the following steps:
- partition of said beam into a determined number of contiguous brushes (P1-P20), each associated with a frequency sub-band (F1-F20), subdivision said bandwidth of determined width;
- establishment of a communication link (1 1), according to at least one direction, between one of said land terminal stations (ST1) and one of said user terminals (T1), via one of said network satellites (Sat1), by use of an instantaneous transmission frequency included in a first sub-band of frequencies (F3) associated with a first brush (p3), the latter illuminating said user terminal (T1) at the instant of overflight thereof by this satellite (Sat1), - and progressive shift of said instantaneous transmission frequency, for the duration of the communication link (1 1), to the frequency sub-band next (F2) associated with the adjacent brush (P2) and at a rate proportional to said determined speed of rotation, so as to compensate for said movement and allow the tracking of said satellite (Sat1). 2. Method according to claim 1, characterized in that said strip of frequencies is subdivided into frequency sub-bands (F1-F20) of equal width, in that each of said brushes (P1-P20) is associated with a sub-band of specific frequencies (F1-F20), and in that the arrangement of these frequency sub-bands (F1-F20) forms a continuous spectrum, so that brushes contiguous are associated with equally contiguous frequency sub-bands, evolving in a determined direction. 3. Method according to claim 2, characterized in that, during said frequency shift step, the amplitude of the shift is such that said instantaneous transmission frequency jumps from an initial frequency included in said first frequency sub-band (F3) at a final frequency included in the next frequency sub-band (F2), associated with said adjacent brush (p2), of so as to ensure inter-brush switching (p3, p2) without blocking. 4. Method according to claim 2 or 3, characterized in that, for two consecutive satellites (Sat1, Sat2) of said network, said sub-bands of frequencies (F1-F20) associated with said brushes (p1-p20, p'1-p'20) are arranged in opposite direction, so that when the last brush (p20) of a first satellite (Sat1), associated with a determined frequency sub-band (F1) ceases to illuminate one of said user terminals (T1), it is illuminated by the first brush (p'1) of the next satellite (Sat2), this brush (p'1) being associated with the same frequency sub-band (F1), and so as to achieve switching inter-satellites (Sat1, Sat2) without blocking. 5. Method according to claim 4, characterized in that the imprints of said last (p20) and first (p'1) brushes of two consecutive satellites (Sat1, Sat2) overlap in whole or in part at the surface of the earth (Te), so that said inter-satellite switching (Sat1, Sat2) is progressive. 6. Method according to any one of claims 1 to 5, characterized in that said brushes (p1-p20) have an elliptical section, with high ellipsivity and whose main axis (.DELTA.1) is orthogonal to the direction of movement of the satellite (Sat1) of said network illuminating one of said user terminals (T1) during said communication. 7. Method according to claim 6, characterized in that each brush (p1) is subdivided into a determined number of sub-brushes (p1A, p1B, p1C). 8. Method according to any one of claims 1 to 7, characterized in that said communication link (l1) consists of a full duplex and in that that the "go" and "return" transmissions have crossed, orthogonal polarizations between them. 9. Method according to any one of claims 1 to 8, characterized in that for transmissions there is used an access protocol of the type to Variable Frequency Multiplexing between said brushes (p1-p20) and with access multiple by direct sequence synchronous code difference within these brushes (P1-P20) 10. Method according to any one of claims 1 to 9, characterized in that to avoid interference with satellite systems geostationaries working in transmission frequency bands comprising said instantaneous transmission frequencies, the momentary cut off of the brushes of said satellites of the network illuminating, during the movement of rotation, an area of the terrestrial globe (Te) between two latitudes of determined values, on either side of the equator of the terrestrial globe. 11. Method according to claim 10, characterized in that said zone extends between + 9 ° and -9 ° latitude. 12. System for the transmission of radio signals via a network of satellites (Sat1, Sat2) between at least one fixed earth station (ST1) and user terminals (T1) by implementing the method according to one of claims 1 to 11, this system comprising a network of satellites (Sat1, Sat2), at minus a fixed earth station (ST1) and user terminals (T1), satellites (Sat1, Sat2) being animated by a rotational movement around the terrestrial globe (Te) on circular orbits and at a fixed speed, the orbits being inclined by compared to the equator and of radius smaller than that of the geostationary orbits, the satellites (Sat1, Sat2) being triple synchronized with the rotation of the terrestrial globe (Te), each satellite (Sat1, Sat2) comprising an arrangement of antennas (1-4) making it possible to generate at least one beam with a directed radiation diagram towards the surface of the terrestrial globe (Te) and associated with a frequency band of determined width, characterized in that it comprises:
- means for dividing said beam into a determined number of contiguous brushes (P1-P20) each associated with a respective sub-band of frequencies (F1-F20), subdivision of said bandwidth of determined width; - means for establishing a communication link (l1), according to at least a direction, between one of said earth terminal stations (ST1) and one of said user terminals (T1), via one of said network satellites (Sat1), by use of an instantaneous transmission frequency included in a first sub-band of frequencies (F3) associated with a first brush (p3), the latter illuminating said user terminal (T1) at the instant of overflight thereof by this satellite (Sat1);
- And means for gradually shifting said frequency of instant transmission, for the duration of the communication link (l1), to the next frequency sub-band (F2) associated with the adjacent brush (P2) and a rhythm proportional to said determined speed of rotation, so as to compensate for said movement and allow tracking of said satellite (Sat1). 13. System according to claim 12, characterized in that said satellites (Sat1, Sat2) and user terminals (T1, T2) include antennas scanning (1-4; A11, A12) to form said transmission links (11). 14. System according to claim 13, characterized in that the antennas (1-4; A11, A12) are of the frequency sweep type. 15. System according to claim 14, characterized in that, each said brushes (P1) being subdivided into a number determines N of sub-brushes (P1A, P1B, P1C), said satellites (Sat1, Sat2) each include a first set of N transmit antennas (11-13) and a second set of N receive antennas (21-23), so as to establish "outgoing" and "return" communications, between satellites (Sat1, Sat2) and said user terminals (T1, T2), each antenna being associated with one of said sub-brushes (P1A, P1B, P1C), and in that the satellites (Sat1, Sat2) each further comprise an additional transmitting antenna (4) and an additional receiving antenna (3), so as to establish "outward" and "return" communications, between the satellites (Sat1, Sat2) and said terrestrial terminal stations (ST1), said antennas being scanning antennas frequency. 16. System according to claim 14 or 15, characterized in that said user terminals (T1, T2) are provided with deployable antennas (A11, A12), in that these user terminals (T1, T2) take two distinct states, a first so-called "active" state, when said antennas are unfolded and during which said antennas connections (11) can be established, and a second state, called "passive", when said antennas are folded and during which the user terminals (T1, T2) can only to be called, and in that said antennas (A11, A12) are antennas to frequency sweep.