Satellite communication system with a leo satellite relaying communications between a geo satellite and earth stations, the uplink and downlink using the same frequency band and time multiplexing.

[0001] The invention relates to the field of long distance data transmission systems. It applies in particular to systems and methods of data transfer (data transfer, remote control, monitoring of terminals ...) between users with small mobile terminals.

Context of the invention and the problem posed [0002] The problem of long distance data transmission from or to a mobile terminal occurs especially in the case of connections between computers ("machine to machine" or M2M). This field of transmission is characterized by the required data flow substantially lower than in the case of image or internet connections, and / or the possibility of using intermittent rather than continuous connections.

[0003] The first approach to this problem is known to be used in existing data transmission systems such as Orbcomm and Argos, which systems use the constellations of Low Earth Orbit satellites (LEO). In this approach, the normal operation mode of each satellite in a low LEO orbit requires that it be seen both by the ground control and connection station and the user terminal.

[0004] The satellite then serves as a communication link between the two parts, and the delay time of receipt and message acknowledgment is a function of the distance between the satellite and the ground station (GES).

[0005] However, the coverage area provided by the network of ground stations of systems using low-orbit satellites such as Orbcomm and Argos is limited by the quantitative expansion of ground stations (GES), and existing systems provide only limited coverage of the Earth in this mode. Each ground station actually allows coverage in an area of ​​about 3,000 km, and each of these systems has about twenty ground stations.

[0006] It can therefore easily be seen that the coverage zones represent large "white" zones for which the system is unusable. These zones cover in particular a large part of the ocean zones and even a significant part of the continents such as Africa or Australia.

[0007] In cases where the LEO satellite does not have simultaneous visibility of the user terminal and ground control station (GES), it is necessary to use a storage and transmission type communication method (known to those skilled in the art under the name "store & forward" - remember and send). In this method, the message is stored on board the satellite, which moves in its orbit until it flies over the GES ground station to which it transmits the stored message.

[0008] With this mode of operation, communication delays are large and hinder two-way communication under acceptable conditions, given that delays are usually between a few minutes and 100 to 150 minutes of the full orbit period of the LEO satellite.

[0009] There are also known examples of hybrid telecommunications systems for data transmission between users. These hybrid systems consist of geostationary satellites and low-orbit satellite constellations.

[0010] In particular, the first US 6208625 can be cited.

[0011] This document describes a network formed of LEO and geostationary (GEO) satellites that can communicate with each other. On the ground, user terminals have the ability to receive / transmit (Rx / Tx) with LEO and GEO satellites. The LEO component filters the data traffic received from terminals, and depending on the urgency of the received data traffic, it routes this data either in the internal system to LEO satellites or to GEO satellites.

[0012] In the second patent document EP 0883252, a satellite communication system is proposed enabling global coverage, reducing transmission delay (Tx) and maximizing the use of system efficiency (broadband satellite communication by interconnecting many constellations in mid-orbit - MEO - and geostationary - GEO -).

[0013] See also document US-A-2002/0132578 (Wiedeman).

[0014] MEO and GEO satellites communicate with each other directly over inter-satellite links, which allows traffic routing (in terms of voice and data) on board satellites based on certain rules.

[0015] In addition, this document proposes to divide and reuse the spectrum between the GEO and MEO satellites in the very high frequency range (for example, in the range from 40 to 60 GHz) to enable the use of the function known as "seamless handover" for terminals portable (moving from a mobile network to a fixed network without breaking the ongoing connection).

[0016] It is clear that current hybrid systems have great complexity, meaning high implementation and use costs.

Objects of the invention [0017] The object of the invention is to provide a solution based on the simplest possible relay satellites, ensuring good parameters (connection balance, availability) within the coverage area of ​​each satellite.

Presentation of the invention [0018] According to a first embodiment, the present invention relates to a payload of a satellite of a telecommunications relay system, said relay of satellite being intended to be placed in orbit above the surface of the celestial body, and the payload is configured to relay data received from the stationary satellite over the surface of the celestial body to the terminal essentially on the surface of the celestial body and to transmit the received data from the terminal to the stationary satellite. Said payload is also configured to:

- using the same frequency band to transmit data to a stationary satellite, called 'up-link broadcasting', and to transmit data to a terminal, called 'down-link broadcasting', and

- up-link and down-link time division multiplexing.

[0019] Such solutions allow a better distribution of power on board a relay satellite. In addition, such solutions allow the optimization of the payload structure in terms of reducing manufacturing costs, space and / or take-off mass.

[0020] It should be noted that by the term "substantially on the surface of the celestial body" is meant especially the terminals of ground, sea or air users. Similarly, said terminals may be placed, for example, in ground, sea or air.

[0021] The following are specific embodiments for the payload of the relay satellite. The invention also relates to all technically possible combinations of these particular embodiments.

[0022] According to a particular embodiment, the payload comprises at least one power amplifier, and said payload is configured to use said power amplifier for both up-link transmission and down-link transmission.

[0023] According to a particular embodiment, the payload includes:

- at least one up-link transmitting antenna,

- at least one down-link transmitting antenna,

- means for routing the signal at the output of the power amplifier, representative of the data to be transmitted, to said up-link transmit antenna, or to said down-link transmit antenna.

[0024] According to a particular embodiment, the payload comprises means for multiplexing up-link transmission signals and down-link transmission signals at the input of the power amplifier.

[0025] According to a particular embodiment, the payload includes:

- many down-link transmitting antennas,

- means for routing the signal at the output of the power amplifier, representative of the data to be transmitted, to any of these down-link transmission antennas.

[0026] According to a specific embodiment, the payload is configured to use the same frequency band used for uplink and down-link transmission to receive data to be forwarded to the terminal, called 'up-link pickup' and for receiving data to be transmitted to a stationary satellite called 'down-link reception'.

[0027] According to a particular embodiment, the payload comprises a plurality of up-link receive antennas, and said payload is configured to receive simultaneously on each of these up-link receive antennas.

[0028] According to a particular embodiment, the payload is configured for multiplexing during up-link transmission and up-link reception and / or for multiplexing during down-link transmission and down-link reception.

[0029] According to a particular embodiment, the payload is configured to use the first frequency subband from the frequency band to receive up-link and transmit up-link, and to use the same frequency subband to receive down-link and transmit down-link .

[0030] According to a particular embodiment, the payload is configured to conduct up-link reception simultaneously with down-link transmission, and to conduct down-link reception simultaneously with up-link transmission.

[0031] According to a second embodiment, the present invention relates to a telecommunications system for transmitting data between at least one terminal located substantially on the surface of a celestial body and at least one stationary satellite above the surface of the celestial body. The system has one or more satellite signal repeaters, said repeaters moving above the surface of the celestial body and carrying the payload according to the invention.

[0032] The following are specific embodiments of the telecommunications system according to the invention. The invention also relates to all technically possible combinations of these particular embodiments.

[0033] According to a particular embodiment, the same frequency band is used for connections between terminals and the constellation of the satellite satellite as for the connections between the stationary satellite and the relay satellite. Preferably, when the terminal is in the coverage area of ​​the stationary satellite, the same frequency band is also used for direct connections between the terminals and the stationary satellite.

[0034] This is a significant advantage over known solutions. The frequency band used by the system can be in any of the bands allocated to satellite telecommunications services (in the lower bands, for example, UHF or VHF, up to high frequencies such as Ka or Q bands). Preferably, the frequency band used is the L band (from 0.9 to 2.0 GHz), which is particularly suitable for mobile connections via satellites.

[0035] According to a particular embodiment, the system has at least one ground station for communication with stationary satellites. Connections between the earth and the relay satellite are provided via stationary satellites and stations connecting these stationary satellites. These connections include both data exchange between users and potentially connections related to remote control and remote measurements of space repetition means. This embodiment therefore does not require the use of a ground connection station assigned to the relay satellite.

[0036] According to a particular embodiment, the at least one relay satellite is placed in a polar or quasi-polar orbit (inclination of the orbit greater than 70 °) around the celestial body.

[0037] According to a particular embodiment:

- at least one relay satellite contains means for performing amplification without frequency offset of the signal received from the stationary satellite,

- the radio interface used is a CDMA (Code Division Multiple Access) interface,

- at least one terminal has means for managing the appearance of two signals containing delay and Doppler differences.

[0038] Preferably in this case, the means for managing the appearance of two signals containing delay and Doppler differences belonging to the terminal are "Rake" type receivers well known to those skilled in the art.

[0039] According to a particular embodiment:

- the radio interface is a TDMA interface,

- the system uses two separate signals: one for stationary satellites and one for relay satellite,

- time multiplexing is used to distribute performance between stationary satellites and relay satellites with protection intervals and initial Doppler compensation at the level of the relay satellite.

[0040] According to a particular embodiment, the at least one terminal has means to use spatial diversity or MIMO (Multiple Input Multiple Output) techniques to recombine signals coming from both a stationary satellite and a relay satellite.

[0041] According to a particular embodiment:

- at least one relay satellite transmits the signal, either transparently or regeneratively, without shifting the frequency of the received signal, and

- the radio interface has means for limiting interference at the terminal level between signals originating from the stationary satellite and signals transmitted via the relay satellite.

[0042] According to a particular embodiment:

- at least one relay satellite transmits the signal, either transparently or regeneratively, on an adjacent channel before it is sent again, and

- the telecommunications system has a coordinating unit for coordinating the frequency levels between stationary satellites and the relay satellite.

[0043] According to a third embodiment, the present invention relates to a telecommunications method for transmitting data between a terminal located substantially on a surface of the celestial body and a stationary satellite above the surface of the celestial body, wherein the data transfer between said terminal and said stationary satellite is via a relay satellite configured to forward data received from said stationary satellite to said terminal, and to forward data received from that terminal to said stationary satellite. Also:

- the relay satellite uses the same frequency band for transmitting data to the stationary satellite, called 'up-link broadcasting', and for transmitting data to the terminal, called 'down-link broadcasting',

- the relay satellite multiplexes up-link and downlink broadcasting in time.

[0044] The following are specific embodiments of the telecommunications method. The invention also relates to all technically possible combinations of these particular modes of implementation.

[0045] According to a particular embodiment, the relay satellite uses the same power amplifier for both up-link and down-link connections.

[0046] According to a specific embodiment, the relay satellite uses the same frequency band, used for up-link broadcasting and down-link broadcasting, to receive data to be forwarded from a terminal, called "up-link reception", and to receive data for transmission from a stationary satellite called 'down-link reception'.

[0047] According to a particular embodiment, the relay satellite multiplexes up-link transmission and up-link reception and / or said satellite repeater multiplexes down-link transmission and down-link reception in time.

[0048] According to a particular embodiment, the relay satellite simultaneously performs down-link transmission and up-link reception, and simultaneously performs up-link transmission and down-link reception.

[0049] According to a particular embodiment, the relay satellite uses the same first frequency subband for up-link reception and uplink transmission, and uses the same second frequency subband for frequency down-link reception and down-link transmission.

Brief description of the drawings [0050] The objects and advantages of the invention will become more apparent in the light of the following description and figures of specific embodiments, given as non-limiting examples, and in which the drawings show:

- Fig. 1: general system architecture,

- Fig. 2: shows the positions of flying LEO satellites and stationary GEO on the planner, at a given moment,

- Fig. 3: table of orders of magnitude of delays between stationary satellite signals

GEO and LEO satellites for different heights of LEO orbits,

- Fig. 4: coverage zones of LEO satellites and GEO stationary constellations of this constellation,

- Fig. 5: an example of the use of the frequency band in a system according to the invention,

- Fig. 6: schematic representation of the payload of a passing satellite

LEO according to the preferred form of implementation,

- Figures 7a and 7b: examples of multiplexing during connections between a user terminal, a LEO flying satellite and a GEO stationary satellite,

- Fig. 8: a schematic representation of the payload according to a variant embodiment of Fig. 6.

[0051] In these figures, identical reference numerals designate identical or similar elements.

Detailed Description of Embodiments of the Invention [0052] The structure of the telecommunications system according to the invention is shown in Figs. 1 and 2. As can be seen in these figures, the proposed system uses two satellite constellations. [0053] The first constellation consists of one or more stationary satellites (hereinafter also referred to as GEO).

[0054] In the present case, the system described here as a non-limiting example is based on the constellation of three GEO1 stationary satellites,

GEO2, GEO3 located in geostationary orbits above the three main continental zones (for example, at longitudes of 265 ° E, 25 ° E, 145 ° E, respectively, as shown in Fig. 2). Stationary satellites GEO1, GEO2, GEO3 operate in the band called MSS L (1.5 / 1.7 GHz).

[0055] The constellation of stationary satellites GEO1, GEO2, GEO3 is controlled by one or more ground control stations located within the visibility range of the stationary satellites GEO1, GEO2, GEO3 they control, which stations perform control and remote control functions. The constellation of stationary satellites GEO1, GEO2, GEO3 is connected to terrestrial telecommunications networks through one or more GES ground connection stations located within the visibility range of the stationary satellites GEO1, GEO2, GEO3 via the FL (Feeder Link) link in a known manner as such.

[0056] The system is complemented by a second constellation of three satellites flying in low or medium orbits (also called LEO satellites or MEO satellites), with orbital heights generally between 400 and 20,000 km, operating as a relay satellite. It is clear that the system can use more or fewer satellites in each constellation of LEO satellites and stationary GEO satellites, with the difference being more or less covering the Earth's surface.

[0057] In the example described herein, it is assumed that the flying satellites are of the low orbit type (called LEO), and are located in heliosynchronous orbits at an altitude of 567 km with a slope of 97.7 ° in three different orbital planes (with right ascension) ascending nodes at 0 °, 60 ° and 120 °). It should be recalled that a heliosynchronous orbit is defined by the fact that each passing satellite flies, in many orbits, over the same point of the Earth at the same time of local solar time. This system uses three satellites in low orbit: LEO1, LEO2, LEO3, whose orbit traces are shown in Fig. 2 as a non-limiting example.

[0058] In this example, these three low-orbiting satellites LEO1, LEO2, LEO3 may be payloads taken as "passengers" on satellites whose main payload is intended for another mission, such as Earth observation.

[0059] It is clear that the constellation of satellites passing through LEO1, LEO2, LEO3 can include satellites moving in orbits of a different height or slope.

[0060] These LEO1, LEO2, LEO3 satellites operate in the same frequency band as the stationary satellites GEO1, GEO2, GEO3, and in the present case in the MSS L band (1.5 / 1.7 GHz).

[0061] The telecommunications system is directed at any user, especially mobile on the Earth's surface, equipped with a sending / receiving terminal (also called the REC terminal).

[0062] In the example shown in Fig. 1, the system is particularly directed towards a user equipped with a REC1 sending / receiving terminal, sending or receiving data to / from another user, possibly also equipped with a REC2 sending / receiving terminal, possibly also mobile on the surface earth. The REC3 user can also be connected to the terrestrial network (IP, PSTN ...) and can be connected to the REC1 user through a GES station.

[0063] Each user terminal REC1 and REC2 is a portable terminal, especially having a user interface, for example, a type of keyboard, touch screen or data link to the electronic device, battery and / or power means, processor and / or electronic control system, means for storing programs or data, and means for transmitting and receiving signals operating in the MSS L frequency band, in this example described herein without limitation.

[0064] In this example, each user terminal REC1 and REC2 is equipped with an omnidirectional antenna, adapted to receive signals without difference from any of the satellites passing LEO1, LEO2, LEO3 in low orbit, or from any of the stationary satellites GEO1, GEO2, GEO3 in geostationary orbit.

[0065] In the embodiment described herein, each user terminal REC1 and REC2 has a "Rake" type receiver, well known to those skilled in the art, for receiving purposes. It should be recalled that the "Rake" type receiver is a radio receiver, originally designed for terrestrial systems to compensate for attenuation caused by multiple radio waves. It is based on the concept that reflected signals can be distinguished (usually when using the CDMA multiplexing technique) and in this way can be combined in an appropriate manner, using multiple propagations. As regards broadcasting, GEO stationary satellites are considered "transparent" and the "Rake" receiver is set at the GES (GEO satellites communication station) level.

[0066] It should be noted, however, that in the event that the GEO stationary satellites were of the regenerative type, it would be necessary to take on board the "Rake" receiver (instead of the receiver located at the GES level).

[0067] Communication between the two user terminals REC1, REC2, assuming that they can see both satellites passing through, LEO1, LEO2 and the same stationary satellite GEO1, respectively, involves a series of steps, as shown schematically in Fig. 1:

- the first REC1 user terminal transmits the first S1 to the first low-orbit LEO1 satellite,

- the LEO1 passing satellite receives and amplifies the S1 signal transmitted by the REC1 user terminal on the ground and sends it as an S2 signal to the stationary satellite GEO1 in geostationary orbit,

- the GEO1 stationary satellite receives the S2 signal and, if the conditions allow it, the S1 signal, and transmits it in the form of an S3 signal to the second LEO2 satellite in low orbit, either directly (with routing on board the satellite) or via a GES communication station . The S1 and S2 signals are processed using the "Rake" receiver either on board (assuming routing on board) or at the GES station level (this solution is advantageous due to simplified implementation),

- the LEO2 passing satellite receives and amplifies the S3 signal transmitted by the stationary GEO1 satellite and sends it as a S4 signal to the REC2 user terminal on the ground.

- the REC2 user terminal receives the S4 signal and potentially the S3 signal if the conditions allow it. The "Rake" receiver enables the combination of these two signals at the user terminal level.

[0068] In the case of user terminals within two different GEO1, GEO2 stationary satellites, the connection between the two user terminals further includes a segment of connection between these satellites, for example, but without limitation, via GES communications stations and ground links, or via a direct inter-satellite GEO link, if any.

[0069] It is naturally understood that the connection of a REC1 user having a transmit / receive terminal to another REC3 user connected through a "classic" terrestrial telecommunications network (PSTN, IP, ...) via a GES communication station may also be considered.

[0070] In this case:

- the first REC1 user terminal transmits the first S1 signal to the first low-orbit LEO1 satellite,

- the LEO1 passing satellite receives and amplifies the S1 signal transmitted by the REC1 user terminal on the ground and sends it as an S2 signal to the stationary satellite GEO1 in geostationary orbit,

- the GEO1 stationary satellite receives the S2 signal and potentially the S1 signal and sends it as the S5 signal to the GES communication station,

- the GES communication station receives the S5 signal (by connecting the S1 and S2 signals contained in S5 with the help of the "Rake" receiver if necessary) and sends it as the S6 signal to the REC3 user terminal on the ground via a classic terrestrial network.

[0071] It should be noted that for the purpose of simplifying the drawing, Fig. 1 does not indicate direct links between user terminals REC1, REC2 and REC3 and the stationary satellite GEO1.

[0072] Different approaches may be considered with respect to the space repeater installed on board LEO1, LEO2, LEO3.

[0073] Or preferably, simple amplification without frequency offset of the signal received from the GEO stationary satellite. However, this involves using a radio interface capable of handling the arrival of two signals differing in terms of delay and Doppler effect. This is, for example, the case of the CDMA (Code Division Multiple Access) radio interface associated with the "Rake" receiver.

[0074] Or, alternatively, the use of two separate signals (one for the GEO stationary satellite and one for the LEO passing satellite). For example, a TDMA (Time Division Multiple Access) radio interface, known per se, can be used to either divide the bandwidth between LEO satellites and stationary GEO satellites (with protection breaks and Doppler pre-compensation on level of a LEO satellite). Alternatively or additionally, two different frequency band channels can also be used (one for the GEO stationary satellite and one for the LEO passing satellite).

[0075] In the embodiment described here, the first approach has been chosen as an example, because it offers a simple and effective solution.

[0076] In fact, it uses a variety of satellites, because signals from both LEO satellites and stationary GEO satellites can be combined in a "Rake" receiver to get a better signal-to-noise ratio. This technique to improve the signal-to-noise ratio allows for a lower transmission error rate ("bit error rate"), lower transmitted power EIRP (Effective

Isotropically Radiated Power - or a larger margin in the connection balance.

[0077] Anyway, in the case of the REC user terminal, which is simultaneously within the visibility range of the LEO flying satellite and the GEO stationary satellite, if the propagation conditions cause the loss of one link towards one of the satellites with which it is connected (due to the change in the geometry of the link with the LEO flying satellite) changing as a function of time, or due to obstacles on the target line of one of the two satellites, a passing LEO and a stationary GEO), a second link may be used to maintain the connection.

[0078] This concept of simple amplification without offsetting the frequency of the signal received from a GEO stationary satellite can be implemented thanks to the option offered by the "Rake" receiver, built into the REC user terminal, combining various incoming signals by different routes, originating from a LEO satellite and a GEO stationary satellite.

[0079] In the satellite data transmission scenario that is the subject of this implementation, the multi-path component is usually negligible. In this case, the "Rake" receiver is simply used to combine multiple direct signals from multiple LEO satellites and GEO land satellites, because the various signals can be considered as fictitious "multi-path" components.

[0080] The received signals can then be combined at the REC user terminal using three main algorithms known to those skilled in the art and thus not described in more detail in this document:

- by choosing the best signal (known as 'selection combining' in English),

- by means of a simple, uniform combination of signals (also known as 'equal gain combining' in English), or

- by balanced signal recombination to maximize the total signal-to-noise ratio (known as "Maximal Ratio Combining" or MRC). This algorithm is a favorable solution because it is the most effective in terms of the signal-to-noise ratio.

[0081] One of the main problems associated with signal merging is that each path used is possibly very different in length due to the relative position of the elements: user - LEO satellite - GEO stationary satellite. To compensate for the propagation time difference, which also changes over time, appropriate data buffers should be provided at the "Rake" receiver level. The dimensioning of these buffers depends on the difference in delay in the worst case between different routes and on the maximum data transfer rate used. [0082] In the proposed system, the time difference remains less than 5 ms for the considered constellation of LEO1, LEO2, LEO3 satellites. The table in Fig. 3 gives several orders of magnitude of delay for different heights of low orbits relative to the stationary GEO satellite on geostationary reflection.

[0083] It should also be noted that the telecommunications services considered with respect to the system according to the invention are preferably low data rate transmissions. Therefore, the necessary data buffer size remains within reasonable limits.

[0084] Due to this "Rake" receiver approach, in the case of the CDMA multiplexing technique, the LEO satellites and the stationary GEO satellites separate the same frequency band (MSS L band in this example) respectively without causing harmful interference.

[0085] Frequency planning and coverage problems should also be taken into account because the LEO and GEO coverage zones need to be coordinated to ensure proper system operation. In the proposed approach, the coverage of GEO stationary satellites consists of a global beam covering all visible on the Earth's surface. This approach allows avoiding or limiting the transmission procedures for LEO satellites (known to those skilled in the art under the name "hand over") between individual beams from one (or more) GEO stationary satellites. The LEO coverage is therefore included in the GEO coverage, as shown in Fig. 4. LEO satellites therefore simply transmit the signals of the stationary GEO satellites under which they are located.

[0086] In the following example:

- the LEO1 passing satellite transmits signals from and to the GEO1 stationary satellite,

- LEO2 and LEO3 satellites transmit signals to and from the stationary satellite

GEO2.

[0087] There is no LEO flying satellite in the coverage area of ​​the GEO3 stationary satellite at the moment illustrated in Fig. 4. In fact, the LEO3 flying satellite is currently connected to the GEO2 stationary satellite.

[0088] As any passing LEO satellite moves in the coverage area of ​​the GEO stationary satellites, it may be within the visibility range of various GEO stationary satellites. However, it is assumed that it is currently connected to one single geostationary satellite. When many GEO satellites are in the visibility zone of LEO satellites, different strategies can be used to select a GEO satellite that the LEO satellites should connect to (for example, based on the best signal received at the LEO satellite or geometric criterion of minimizing the distance between the LEO satellite and the GEO stationary satellite, which can be predicted in advance based on the ephemeris satellites). In the above example, the LEO satellite is connected to a GEO stationary satellite providing the best received signal.

[0089] Based on these hypotheses, there is no need to develop complex frequency planning strategies and all satellites (three GEO stationary satellites and three LEO satellites flying through) can operate, for example, on the only frequency band channel used, with CDMA multiplexing.

[0090] In contrast to known Orbcomm or Argos approaches, the proposed system is able to provide bi-directional data transfer due to the fact that the GEO stationary satellite forwards LEO satellites connections.

[0091] According to this approach, as soon as the user terminal REC1, REC2 is in the coverage zone of the flying satellite LEO1, LEO2, LEO3, it can be communicated with in a bi-directional way in real time. There is no longer a requirement for simultaneous visibility through the satellite of a passing LEO1, LEO2, LEO3, user terminal REC1, REC2 and ground communication station, which allows taking into account the total coverage of the Earth.

[0092] The delay in communicating with the REC1 user terminal on earth is then only a function of the frequency of passage of the satellites passing through LEO1, LEO2, LEO3, which depends directly on the orbits selected for these satellites and on the number of these satellites (which may reach continuous coverage of the entire Earth) .

[0093] Fig. 5 schematically illustrates the use by the telecommunications system of the invention of the same frequency band for connections between REC user terminals and LEO satellites, and for connections between said LEO satellites and GEO stationary satellites.

[0094] Fig. 5 shows in particular an example of using the L band for MSS applications.

[0095] As shown in Fig. 5, the L band is organized in the form of subbands used either for up-link connections (from the REC user terminal to the LEO satellite and / or the GEO stationary satellite and from the LEO satellite to the GEO stationary satellite), or for down-link connections (from a GEO stationary satellite to a LEO flying satellite and / or REC user terminal, and from a LEO satellite to REC user terminal). In this example , the L band generally corresponds to frequencies in the range between 1.518 GHz and 1.675 GHz, and:

- the subband used for downlink links corresponds to frequencies between 1.518 and 1.559 GHz,

- the subband used for up-link connections corresponds to frequencies between 1.6265 and 1.6605 GHz and to frequencies between 1.668 and 1.675 GHz.

[0096] The use of separate L-band frequency subbands for up-link and down-link connections corresponds to the frequency multiplexing of said up-link and down-link connections, known in the Anglo-Saxon literature under the name "Frequency Division Duplex" (FDD). It should also be understood that other types of up-link and down-link multiplexing are possible, such as, for example, time division multiplexing or "Time Division Duplex" (TDD), multiplexing by CDMA type code separation, etc. However, the use of FDD frequency multiplexing corresponds to a favorable implementation method, because interference between up-link and down-link connections are reduced, and because TDD time-division multiplexing can be complicated, given the propagation delays inherent in satellite-based telecommunications systems .

[0097] In the example shown in Fig. 5 and in the case of an up-link connection, the REC user terminal sends data on a specific channel (L band frequency subband used for up-link connections, preferably with a series of such channels) towards a LEO satellite. Preferably, the LEO satellite transmits this data, either transparently or regeneratively, to the GEO stationary satellite using the same channel that the REC user terminal uses. As noted above, a LEO passing satellite can also alternatively relay this data on an adjacent frequency subband channel used for up-link connections.

[0098] In an analogous manner, in the case of a down-link connection, a GEO stationary satellite sends data on a given channel (the L-band frequency subband used for down-link connections, preferably containing a number of such channels) to a LEO flying satellite. Preferably the LEO satellite passes this data, either transparently or regeneratively, to the REC user terminal using the same channel that the GEO land satellite uses. As said above, a LEO passing satellite can also alternatively transmit this data on an adjacent frequency subband channel used for up-link connections.

[0099] According to a particularly preferred embodiment of the invention, the up-link broadcast by a LEO flying satellite intended for a stationary GEO satellite and the down-link broadcast by said LEO flying satellite intended for a REC user terminal are multiplexed in time. In other words, up-link broadcasting and down-link broadcasting by the same LEO flying satellite takes place during different time periods.

[0100] In fact, the severe limitation of the concept of payload of a telecommunications satellite and its manufacturing cost is due to the maximum instantaneous transmit power being delivered. Due to multiplexing during up-link and down-link broadcasting, the said maximum instantaneous transmission power is reduced in the case of simultaneous up-link and down-link broadcasting. This is even more advantageous because for small satellites, the maximum instantaneous transmit power available is usually limited (for example, in the order of 60 to 150 W for a satellite with a mass of 100 to 200 kg).

[0101] Fig. 6 schematically illustrates a preferred embodiment of the payload of a LEO relay relay satellite configured for multiplexing during up-link transmission to a GEO stationary satellite and for down-link transmission to a REC user terminal. It should be noted that the payload of the LEO satellite may also contain other elements not shown in Fig. 6.

[0102] As shown in Fig. 6, the payload 10 includes a radio step for connections to a satellite or GEO stationary satellites, called a "LEO / GEO step" 100, as well as a radio step for connections to a terminal or terminals of the REC users, called "step LEO / REC ”110.

[0103] In the non-limiting example shown in Fig. 6, the LEO / GEO 100 stage includes four antennas 101 that serve both for up-link transmission and down-link reception. This use of antennas 101 simultaneously for up-link transmission and down-link reception is possible because they occur in the same frequency band, for example, in the L-band frequency subbands described with reference to Fig. 5. Antennas 101 may be any suitable type, for example, horn antennas, microstrip antennas, etc.

[0104] Each antenna 101 is coupled to two channels, a transmitting channel and a receiving channel, for example, by means of a circulator 102 of a known type. This circulator 102 is adapted to route the signals of the transmit channel to the antenna 101 and to route the signals received by the antenna to the receive channel.

[0105] Each receiving channel includes a low-noise amplifier or LNA 103 (English acronym "Low-Noise Amplifier"), which can be of any known type.

[0106] Similarly, the LEO / REC 110 stage includes four antennas 111, which serve both for down-link transmission and up-link reception, and can be of any suitable type, for example, horn antennas, microstrip antennas, etc.

[0107] Each of the antennas 111 is coupled to two channels, a transmission channel and a receiving channel, for example, via a circulator 112 of a known type. Each receiving channel contains a low-noise amplifier or LNA 113, which can be of any known type.

[0108] In addition, payload 10 includes a power amplifier or PA 120 (English acronym for "Power Amplifier"), which can be of any known type, for example, SSPA ("Solid State Power Amplifier"), tube amplifier, etc. .

[0109] In the preferred embodiment shown in Fig. 6, the PA 120 amplifier is separated by the LEO / GEO 100 stage and the LEO / REC 110 stage. In other words, the PA 120 amplifier is used for both up-link broadcasting and broadcasting down-link. This is possible, on the one hand, because the up-link transmission and down-link transmission takes place in the same frequency band, for example, in the L-band frequency subbands described with reference to Fig. 5. On the other hand, this is possible because said up-link broadcast and said downlink broadcast are multiplexed in time.

[0110] To this end, payload 10 includes means for routing the signal at the output of the PA 120 amplifier, presenting the data to be transmitted, either to LEO / GEO 100 antennas 101 or to LEO / REC 110 antennas 111. Said routing means occur, in the non-limiting example shown in Fig. 6, in the form of a switching system 121.

[0111] Also for this purpose, the payload 10 comprises means for multiplexing, at the input of the PA amplifier 120, up-link transmission signals and downlink transmission signals. In the non-limiting example shown in Fig. 6, said multiplexing means are also in the form of a switching system 122.

[0112] The payload 10 also includes demodulating circuits 130, of a known type, coupled to each of the receiving channels, at the output of which there are two 90 ° phase shift channels, known under the names I and Q. The signals received in each of these channels are processed then to digital form using analog / digital converters 131, of known type, for further processing by a processing module 150, such as, for example, a microcontroller and / or FPGA.

[0113] The signals processed by the processing module 150, corresponding to the transmission data, in the form of two channels I and Q for transmission with a phase shift of 90 °, are converted into analog signals using digital / analog converters 141. The signals at the output of these digital / converters analog 141 is then fed to the input of modulation circuits 140, of the known type, which are coupled to switching circuit 122 at the input of the PA 120 amplifier.

[0114] Furthermore, although not shown in Fig. 6, the processing module 150 is coupled to the switching circuit 122 at the input of the PA amplifier 120 and the switching circuit 121 at the output of the PA amplifier 120. The processing module 150 is further configured to control these switching circuits 121 , 122 in a substantially synchronous manner to use said PA 120 amplifier once for up-link transmission and once for down-link transmission.

[0115] It should be understood that the advantage of the preferred embodiment shown in Fig. 6 is in particular that the same PA power amplifier 120 is used once for up-link transmission and once for down-link transmission. In essence, this reduces the number of power amplifiers to be installed in the LEO satellite, which has at least the following advantages:

- power amplifiers are devices that generally consume a lot of energy, reducing the number of needed power amplifiers allows to reduce the energy demand of the LEO satellite,

- power amplifiers are devices generally large, reducing the number of power amplifiers needed allows the production of more compact LEO satellites,

- this leads to LEO satellites flying cheaper to manufacture and place in orbit.

[0116] According to an embodiment of the payload 10, also shown in Fig. 6, the LEO / REC 110 stage further comprises means for routing the signal at the output of the PA amplifier, especially at the output of the switching circuit 121, to any of said antennas 111 for down-emitting link.

[0117] In the non-limiting example shown in Fig. 6, these routing means are in the form of a switching system called "scanning system" 114, for example controlled by processing module 150 (control not shown in Fig. 6). [0118] Thanks to the scanning system 114, the payload 10 is adapted, in the case of down-link transmission, to switch antennas 111 in turn. If, for example, antennas 111 are directional antennas with different aiming directions, this enables different zones on the Earth's surface to be scanned to transmit data to different REC user terminals, transmitting in each of these zones with maximum instantaneous transmit power.

[0119] Preferably, said payload 10 is configured to be switched on simultaneously in the receiving mode of each of the LEO / REC 110 degree 111 antennas. In other words, during the time intervals during which the payload 10 will be receiving up-link data from the terminals REC users, all antennas 111 will be connected by a receiving channel to a processing module 150 that will process the signals received simultaneously by each of these antennas 111. In essence, this makes it possible to increase the time during which signals from REC user terminals can be received.

[0120] According to specific embodiments, there is no obstacle to using a more complex scanning system 114, enabling the simultaneous activation of multiple antennas 111 (two antennas on four, three antennas on four, etc.) and the simultaneous activation of multiple antenna groups 111.

[0121] Furthermore, according to specific embodiments, there is no obstacle to also provide a scanning system for LEO / GEO 100 antennas 101. It may be advantageous, for example, to communicate with various GEO stationary satellites, whereby the antennas 101 have different aiming directions.

[0122] It should be noted that in the payload embodiment 10 of Fig. 6, the same switching system having one input (coupled to the PA 120 amplifier) ​​and five outputs (four outputs coupled to each of the four LEO / REC 111 antennas respectively 110 and one output coupled with all LEO / GEO 100 antennas 101) can be used instead of switching 121 (at the output of PA 120) and scanning 114.

[0123] According to a preferred embodiment of the invention, at the level of the LEO in-flight satellite, the LEO in-flight satellite multiplexes in time up-link transmission and up-link reception. In other words, a LEO flying satellite does not perform an up-link reception at the time intervals in which it carries out or can carry out up-link broadcasting.

[0124] Preferably, the LEO in-flight satellite also multiplexes down-link transmission and down-link reception in time. In other words, a LEO passing satellite does not receive down-link reception at the time intervals in which it carries out or can carry out down-link broadcasting.

[0125] According to the invention, connections between REC user terminals and LEO satellites, and connections between said LEO satellites and GEO stationary satellites, use the same frequency band.

[0126] More specifically, if we put ourselves in the non-limiting example shown in Fig. 5, up-link connections use the first L-band frequency subband, while down-link connections use the second L-band frequency subband.

[0127] It is therefore understandable that from the point of view of a LEO satellite, up-link broadcasting to a GEO stationary satellite can interfere with the simultaneous up-link reception carried out by this LEO satellite as far as these connections are made in the same frequency subband L band. From the point of view of a LEO passing satellite, this is also the case in the case of simultaneous down-link transmission and down-link reception carried out by this LEO satellite.

[0128] Such interference can be reduced by isolating the LEO / GEO 100 stage from the LEO / REC 110 stage, but such isolation would be accompanied by the addition of additional isolation devices and / or increased distance of the 101 stage LEO / GEO 100 antennas and the 111 stage LEO / REC 110 antennas. as a result of multiplexing in time, at the level of the LEO satellite, on the one hand, up-link transmission and up-link reception, and on the other hand, down-link transmission and down-link receiving, such interference is eliminated. This removal action is achieved without the need for additional isolation devices. In addition, 101 degree LEO / GEO 100 antennas and 111 degree LEO / REC 110 antennas can be approached, for example, by deploying them on adjacent surfaces of a LEO passing satellite. [0129] It should be noted that time multiplexing, at the level of a LEO satellite, on the one hand, up-link broadcasting and up-link reception, on the other hand, down-link broadcasting and down-link reception, allows for a significant simplification of the design solution degrees LEO / GEO 100 and LEO / REC 110 in terms of electromagnetic isolation. Therefore, such solutions can be taken into account regardless of multiplexing during up-link and down-link broadcasts, which allow better distribution of the power occurring on board the LEO satellite.

[0130] It is understood, however, that the combination of multiplexing during up-link transmission and down-link broadcasting and multiplexing during up-link transmission and up-link reception on one side, and down-link transmission and down-link reception on the other site, enables the design of LEO satellites with a particularly compact design, with better cost control.

[0131] Fig. 7a and Fig. 7b show two embodiments.

[0132] In the examples shown, reference numeral F1 denotes the frequency subband used for up-link connections, and reference F2 denotes the frequency subband used for down-link connections.

[0133] It should be noted that the examples shown in Fig. 7a and Fig. 7b are shown from a LEO satellite viewpoint. In particular, the following points should be noted:

- up-link F1 arrow between the REC user terminal and the flying satellite

LEO indicates the time interval during which the flying LEO satellite receives reception on the F1 subband. No such up-link F1 arrow means that said LEO satellite does not receive reception on the F1 subband, but does not imply that the REC user terminal does not send a signal on said F1 subband; in particular, if conditions allow, the REC user terminal can send a signal directly for the GEO stationary satellite (the coexistence of signals sent in the F1 subband by the REC user terminal and the passing LEO satellite can take place, for example, using the CDMA radio interface and using the "Rake" receiver at the GES ground communication station level);

- the F2 down-link arrow between the stationary GEO satellite and the LEO flying satellite indicates the time interval during which the LEO flying satellite receives reception in the F2 subband. The absence of such a down-link F arrow means that said LEO satellite does not receive reception on the F2 subband, but does not imply that the GEO stationary satellite does not send a signal on said F2 subband; in particular, if conditions allow, the GEO landline satellite may send a signal directly for the user terminal (the coexistence of signals sent in the F2 subband by the stationary GEO satellite and the passing LEO satellite may take place, for example, using the CDMA radio interface and "Rake" receiver at the REC user terminal level).

[0134] In the example shown in Fig. 7a, time multiplexing at the level of a LEO flying satellite occurs mainly within three time intervals

I1, I2 and I3.

[0135] During the first time interval I1, a LEO passing satellite performs downlink transmission on the F2 subband towards one or more REC user terminals.

[0136] During the second time interval I2, a LEO flying satellite conducts up-link broadcasting on the F1 subband towards one or more GEO stationary satellites.

[0137] During the third time interval I3, the LEO flying satellite simultaneously performs up-link reception on subband F1 and down-link reception on subband F2.

[0138] This association of time intervals I1, I2 and I3 is preferably performed recursively. For example, the duration of each of these time intervals may be a fixed constant or a variable to be determined according to established criteria. For example, the duration of each of these time intervals may be between a few tenths of a second and tens of seconds.

[0139] In a preferred embodiment, such as shown in Fig. 7b, the payload 10 is configured to conduct up-link reception simultaneously with downlink transmission, and to conduct down-link reception simultaneously with up-link transmission.

[0140] As shown in Fig. 7b, time multiplexing then takes place within two time intervals I1 and I2.

[0141] During the first time interval I1, a LEO flying satellite transmits down-link broadcast on the F2 subband towards one or more REC user terminals as well as up-link reception on the F1 subband signals that are optionally sent by the REC user terminal or terminals.

[0142] During the second time interval I2, a LEO flying satellite performs up-link broadcasting on the F1 subband towards one or more GEO stationary satellites, as well as down-link reception on the F2 subband signals possibly sent by a satellite or GEO stationary satellites.

[0143] This association of time intervals I1 and I2 is preferably performed recursively. For example, the duration of each of these time intervals may be a fixed constant or a variable to be determined according to established criteria. For example, the duration of each of these time intervals may be between a few tenths of a second and tens of seconds.

[0144] Fig. 8 is a variant of the payload implementation of Fig. 6 adapted to operate the payload of a LEO transceiver relay satellite as shown in Fig. 7b.

[0145] In relation to the payload 10 shown in Fig. 6, a greater number of components were used both for connections to one or more GEO satellites and for connections to one or more REC user terminals.

[0146] This is possible because these connections, from the point of view of the LEO flying satellite, are multiplexed in time, as shown in Fig. 7b.

[0147] Thus, in relation to the payload 10 shown in Fig. 6, the following components are separated:

- modulation systems 140: the separation of these components makes it possible to reduce their number from two to one,

- D / A converters 141: separating these components allows you to reduce their number from four to two,

- demodulating systems 130: the separation of these components allows their number to be reduced from eight to four,

- analog / digital converters 131: Separation of these components allows reducing their number from sixteen to eight.

[0148] Relative to payload 10 of Fig. 6, switching circuit 122 at the input of PA amplifier 120 has been removed, and multiplexing is performed directly by processing module 150.

[0149] In contrast, switching circuits 160 controlled by the processing module 150 (control not shown in the drawing) have been added at the input of the demodulating circuits 130 to connect them simultaneously with the LEO / GEO 100 stage and the LEO / REC 110 stage.

[0150] It is therefore understood that by using the same frequency band and by multiplexing in time with connections to stationary satellites and connections with user terminals, particularly optimized payloads 10 can be obtained.

[0151] It should be noted that it is also possible to separate other components as, for example, LNA amplifiers 103, 113.

Advantages of the invention [0152] By combining the constellation of LEO satellites (which enables the delivery of a more efficient service in polar areas) and the constellation of GEO satellites (which provides high quality services in equatorial areas and at low latitudes), the average duration of the lack of visibility of the satellite by the user's terminal is eliminated or significantly reduced compared to known systems, especially when when large elevation angles are sought (for mobile connections via satellite, the signal blocking ratio decreases at high elevation, which leads to better service availability).

[0153] It is therefore understood that a system such as described allows offering substantially greater accessibility (due to quasi-continuous coverage) for users who need to move in remote areas and poorly covered by traditional communication systems. This occurs, for example, in polar zones, using constellations in polar or quasi-polar orbit.

[0154] In addition, multiplexing adapted to various tasks of transmitting / receiving payload of LEO satellites allows to significantly simplify their design solutions and reduce the cost of their production, while maintaining a high level of operating parameters.

[0155] Other advantages of the invention are particularly apparent when compared to existing solutions using either low-orbit satellites or geostationary satellites or hybrid constellations.

[0156] Compared to satellite communications solutions using low-orbit satellite constellations (such as, for example, Orbcomm or Argos), the benefits of the invention are numerous:

- no need to develop communication stations for connections between LEO satellites and terrestrial network infrastructure; in fact, the station of communication of the satellite or stationary GEO satellites provides permanent access to satellites passing LEO;

- there is no need to develop a remote control / measurement and control (TT&C) constellation of passing satellites, because TT&C signals are also transmitted by satellite or GEO stationary satellites; it should be understood that the control of LEO satellites is carried out from the ground by means of the station of control of stationary GEO satellites, through these stationary satellites of GEO;

- a substantially simplified design solution for LEO satellites due to the fact that the payload performance of these LEO satellites allows the installed power to be split between up-link and down-link broadcasting, and reduces the need for electromagnetic isolation between the LEO / GEO stage and the degree LEO / REC.

[0157] Compared with a satellite communication solution using geostationary orbit, the benefits of the invention are:

- extension of the coverage of the GEO stationary satellite to cover, for example, polar zones,

- reduced time delay when accessing the network and when receiving confirmation of receipt.

[0158] Compared to the system (contained in known solutions) of satellite telecommunications using hybrid satellites, comprising one or more satellites in geostationary orbits and a constellation of satellites in low orbits, the advantages of the invention are:

- using a common protocol between the two systems, more effective than each protocol separately;

- sharing of the frequency band used, with mechanisms ensuring reduction of intra-system interference.

[0159] Due to its concept, the design solution is particularly suited to the terminals of users having poor directionality and not requiring maintenance of targeting satellites in low orbits. In fact, it is sufficient for the user terminal to be pointing towards a stationary GEO satellite or a flying LEO to provide connectivity.

[0160] The considered applications relate to the improvement of future mobile systems implemented by satellites in geostationary orbit (MSS or "Mobile Satellite Services"), including mobile air services by satellites such as AMSS ("Aeronautical Mobile Satellite Service") and AMSRS ("Aeronautical Mobile Satellite Route Services "), in the UHF, L, S band, C or X based on the development of a constellation in a low orbit much less complex than existing MSS constellations (such as Globalstar or Iridium) for voice, data or machine-to-machine (M2M) message connections.

[0161] This concept may also apply to systems of mobile satellite broadcasting of data, television or radio programs (e.g., broadcasting radio programs in the S-DAB standard using BSS allocation in the L band or mobile broadcasting television programs in the DVB-SH standard using MSS allocation in the S band).

[0162] Another use of this concept relates to data exchange for navigation applications (especially sea or air navigation). In these applications, known to those skilled in the art as SBAS ("Satellite Based Augmentation System"), there is currently no means to cover mobile terminals located in the vicinity of poles. The invention solves this problem by means of a satellite in geostationary orbit and a satellite in low orbit flying in the range of visibility of the mobile device.

[0163] Similarly, within the geostationary satellite working zone there are shadow zones in which the geostationary satellite is not directly visible to the surface terminals due to obstructions by buildings or natural elements (trees, mountains, ...). Covering the shadow zone of these traditional communications devices is possibly possible as soon as the zone can be seen by satellites flying in low orbit.

[0164] The use of the telecommunications system or method according to the invention thus allows the extension of the coverage zone of the communication means comprising the shadow zones, and in particular the extension of the coverage zone of the SBAS system using a geostationary satellite.

[0165] In the case where the space relay transmits regeneratively in a channel adjacent to the GEO landline satellite channel, the advantage of the invention relates to the possibility of possible simplification of exchange protocols between user terminals and LEO satellites.

[0166] LEO satellites may in particular perform conversion to a special exchange protocol for GEO (to take into account, for example, restrictions on the propagation delay specific to GEO), or message aggregation and optimization of bandwidth usage.

[0167] Another important, attractive aspect of this concept is the ability to achieve continuous connection and almost real-time inspection and mission time, and the constellation of LEO satellites passing through a communications station and a GEO relay.

[0168] It is, moreover, understandable that the system does not necessarily require the development of a special constellation of LEO satellites or stationary GEO satellites. Indeed, the available transmission possibilities in existing constellations of GEO stationary satellites can be used.

[0169] In this case, of course, the frequency band of the constellations of the GEO stationary satellites to be used as the operating frequency band of the LEO satellites is selected. This makes it possible to solve the problem of the low number of frequency bands available for mobile satellite services using non-geostationary satellites, and therefore presents a regulatory advantage when developing a constellation of LEO satellites operating as an auxiliary in the same frequency band as the satellite or stationary GEO satellites.

[0170] Similarly, the functions considered for LEO satellites can actually be implemented by means of payload placed as "passengers" on LEO satellites intended primarily for other functions. In this case, the decisive criterion is the orbit taken into account for the LEO satellite. The preferred choice is the choice of Earth observation satellites, which often use a heliosynchronous orbit with a large slope, and thus covering large latitudes.

[0171] This introduction of payload useful as "passengers" is naturally very cost-effective to improve the system.

[0172] The system described herein is therefore a simple and economical solution compared to other possible alternatives, such as:

- developing a large number of ground stations to offer a continuous connection between LEO satellites and earth, which is a costly and complex solution to implement, especially to cover the oceans (the constellation Globalstar is a good example of this difficulty);

- the use of inter-satellite connections to obtain a permanent connection between LEO satellites and a limited number of ground stations; this solution has the disadvantage of being complicated and at an additional cost associated with the space issue (the Iridium constellation is a suitable illustration of such a solution).

[0173] Finally, a favorable and cheaper payload structure such as that described above allows the cost of the system to be lowered without sacrificing its parameters. In this case, the significant reduction in the complexity of the relay platforms used for flying satellites allows their price to be reduced and / or the number of satellites used in the telecommunications system to be increased.

Variants of the Invention [0174] The use of spatial diversity (or MIMO techniques) at the user terminal level to combine signals coming simultaneously from the stationary GEO satellite and the LEO flying satellite can be considered to further improve connection balance.

[0175] The satellite relay may be a simple "transparent" analog relay, which is the simplest solution, but imposes design restrictions on the radio interface to reduce terminal-level interference between signals originating from a GEO stationary satellite and signals transmitted via a LEO satellite.

[0176] An alternative solution is to transmit a signal (transparently or regeneratively) on a channel of the same frequency band aboard a relay satellite. This solution requires a coordinating unit to coordinate frequency plans between GEO stationary satellites and LEO flying overhead.

[0177] The constellation of relay satellites can also perform additional functions ("store & forward" - remember and send; signal aggregation).

[0178] A relay satellite constellation can provide global or partial coverage of the Earth, depending on the intended purposes.

[0179] A relay satellite constellation can provide continuous coverage over time (for real-time services available at any time) or only access with some delay (for real-time services) using constellations with fewer satellites.

[0180] It is also clear that the described concept, using the same frequency band for communication between surface terminals and a relay satellite, and for communication between a relay satellite and stationary satellites, can apply to communications in only one direction or in both directions .