Method and system for providing air traffic control within a geographic sector

Abstract

The invention relates to a method and system for providing air traffic control within a geographical sector. A system (1, 100, 200) for providing air traffic control within a geographical sector (102, 202) which is allocated a communication frequency in the VHF or UHF range, the system comprising at least one satellite (20, 120, 220) for communicating (21, 121, 221) with aircraft (30, 130, 230) in the geographical sector using analog modulated RFF signals on the communication frequency. The system is configured in such a way that only one satellite at a time actively transmits on the communication frequency allocated to the geographical sector.

Description

Technical Field

[0001] The present invention relates to methods and systems for providing air traffic control within a geographic sector, and more particularly to methods and systems for providing space-based air traffic control using satellites. Background Art

[0002] Air traffic control (ATC) is the service provided by air traffic controllers to aircraft pilots by guiding aircraft and providing support and intelligence. For this purpose, the International Civil Aviation Organization (ICAO) divides the earth's airspace into flight information regions (FIRs). Each of these FIRs is served by an Area Control Center (ACC), also known as an Air Route Traffic Control Center. A FIR usually includes the airspace over the territory or part of the country where the ACC is located. It may also include the airspace over international waters. FIRs may be further divided into sectors.

[0003] The ATC system is based on analog communication between ground stations and aircraft in the VHF or UHF range. Here, analog amplitude modulation of an RF carrier is used. A specific frequency is assigned to each sector of the flight information region.

[0004] For international waters, situations may occur where sectors of the flight information region cannot be covered by ground stations located in the country responsible for the flight information region, or cannot be covered by ground stations at all. In such cases, all aircraft must be additionally equipped with special telecommunication equipment. Such telecommunication equipment can use HF (shortwave) facilities or satellites. In both cases, these telecommunication systems do not use the VHF band and must be available in parallel with normal VHF equipment. Summary of the invention

[0005] It is therefore an object of the present invention to provide a method and system for providing air traffic control from a satellite to a geographic sector using standard VHF or UHF equipment without the need for additional expensive telecommunications equipment inside the aircraft.

[0006] The above and other objects are achieved by the features of the independent claims. Further implementation forms are obvious from the dependent claims, the description and the drawings. Among them, the product claim may also be limited by the features of the dependent method claim, and vice versa.

[0007] According to a first aspect, a system for providing air traffic control within a geographical sector (102, 202) is provided, to which a communication frequency in the VHF or UHF range is allocated. The system comprises at least one satellite configured to communicate with aircraft in the geographical sector using analog modulated RF signals on the communication frequency. The system is configured in such a way that only one satellite transmits at a time on the communication frequency allocated to the geographical sector.

[0008] The geographic sector may be, for example, a flight information region defined by ICAO or one of its sectors. VHF and UHF are frequency ranges internationally standardized by the International Telecommunication Union (ITU). For civil aviation radionavigation services and aeronautical mobile radiocommunication services, the frequency range of 108 to 137 MHz is reserved, so the system preferably operates in this frequency range. For military users, frequencies in the VHF band of 137 to 144 MHz or the UHF band of 225 to 400 MHz are used. In general, the system described herein can provide services from space to a geographic sector by using analog modulated RF signals.

[0009] In an implementation form of the first aspect, the system comprises a plurality of satellites moving at a distance from each other in substantially the same orbit, which is a medium Earth orbit or a low Earth orbit, preferably an equatorial orbit.

[0010] In another implementation form of the first aspect, the system is configured in such a way that one of the plurality of satellites is switched to be active for communication with the aircraft upon reaching a predetermined position, and the satellite active until then is simultaneously switched off. In this context, the active role of a satellite means that a signal with an analog RF signal is sent to a geographic sector. All other RF links and all other transmit and receive functions may be active for all satellites at all times.

[0011] In another implementation form of the first aspect, the number and altitude of the plurality of satellites are selected in such a way that each time, at least one of the plurality of satellites has an elevation within a predetermined range, preferably between 10° and 80°.

[0012] In another implementation form of the first aspect, the number and altitude of the multiple satellites are selected as follows: at each time, at least two of the multiple satellites have an elevation angle within a predetermined range, preferably between 10° and 80°, so that one of them can be used as an active satellite, and if the active satellite fails, the other can be used as a backup satellite.

[0013] In another implementation form of the first aspect, the number and altitudes of the multiple satellites are selected as follows: for two geographic sectors that are sufficiently far from the equator so that the elevation angle of the satellite is always less than a predetermined upper limit (preferably 80°), the first satellite can be used as an active satellite for one of the geographic sectors, the second satellite can be used as an active satellite for the other geographic sector, and if one of the first and second satellites fails, the third satellite can be used as a backup satellite.

[0014] In another implementation form of the first aspect, the system includes at least two ground antennas that follow the positions of at least two satellites. The support links between the ground antennas and the actively transmitting satellites can provide ATC voice transmissions that are relayed upward and / or downward, and preferably provide embedded remote monitoring and remote command functions. The support links between the ground antennas and the satellites that are not actively transmitting can provide maintenance services, preferably providing orbit tracking, orbit correction, software upload, etc.

[0015] In another implementation form of the first aspect, the system is configured in such a way that each satellite can be switched to be active more than once during one orbit around the earth in order to provide air traffic control to multiple geographical sectors. Preferably, each satellite is configured to rotate its antenna in order to reach geographical sectors at different distances from the equator.

[0016] In another implementation form of the first aspect, a system includes a satellite disposed in a geostationary orbit.

[0017] In another implementation form of the first aspect, the satellite comprises a directional antenna for the VHF or UHF range.

[0018] In another implementation form of the first aspect, the directional antenna comprises an active antenna and a passive reflector antenna. The active antenna is preferably a Yagi antenna. The reflector antenna is preferably a parabolic offset reflector antenna, and more preferably a deployable mesh antenna.

[0019] In another implementation form of the first aspect, the satellite is configured to transmit a linearly polarized VHF or UHF signal.

[0020] In another implementation form of the first aspect, at least one satellite, preferably all satellites, are configured to be able to operate on at least two different communication frequencies assigned to one or different geographical sectors or at least on a communication frequency assigned to a geographical sector and on an emergency frequency of the aircraft.

[0021] According to a second aspect, there is provided a communication method for providing air traffic control within a geographical sector, a communication frequency in the VHF or UHF range being allocated to the geographical sector. The method comprises establishing a communication link between at least one satellite and an aircraft in the geographical sector using an analog modulated RF signal on the communication frequency, wherein only one satellite is actively transmitting at a time on the communication frequency allocated to the geographical sector.

[0022] According to a third aspect, a satellite is provided. The satellite is configured to communicate using analog modulated RF signals. The satellite is also configured to be used in a system according to the first aspect and / or its implementation forms or in a method according to the second aspect.

[0023] It should be understood that an embodiment of the present invention may also be any combination of a dependent claim or the above implementation form and a corresponding or other independent claim or the above aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further features and useful aspects of the present invention can be found in the description of exemplary embodiments with reference to the accompanying drawings.

[0025] Figure 1 A schematic diagram of an air traffic control system is shown, illustrating the basic principles of the invention.

[0026] Figure 2 A schematic diagram of an air traffic control system according to a first embodiment of the present invention is shown.

[0027] Figure 3 A schematic diagram of an air traffic control system according to a first embodiment is shown from above.

[0028] Figure 4 Additional satellite positions that may be used for communications are shown.

[0029] Figure 5 A schematic diagram of an air traffic control system according to a variation of the first embodiment is shown.

[0030] Figure 6 A schematic diagram of an air traffic control system according to a second embodiment of the present invention is shown. DETAILED DESCRIPTION

[0031] Hereinafter, embodiments of the present invention are described with reference to the accompanying drawings.

[0032] Figure 1 Shown is a schematic diagram of an air traffic control system illustrating the basic principles of the invention.

[0033] The system 1 for providing traffic control in a flight information region comprises one or more ground stations 10 and one or more satellites 20. A support link 11 (described later) is established between the ground station 10 and the satellite 20. A communication link 21 is established between the satellite 20 and the aircraft 30. Both the support link 11 and the communication link 21 are preferably bidirectional. Therefore, the communication between the ground station 10 and the aircraft 30 is not performed as usual through a direct communication link between the ground station and the aircraft, but indirectly through the support link 11 between the ground station 10 and the satellite 20 and the communication link 21 between the satellite 20 and the aircraft 30.

[0034] The communication link 21 between the satellite 20 and the aircraft 30 must meet several basic conditions, since it needs to work with the existing ATC equipment installed in the aircraft. This means that there are some parameters that cannot be changed, such as communication in the VHF and UHF range, in particular in the frequency range of about 100 to 400 MHz (corresponding to wavelengths between 0.75m and 3 m) allocated to air traffic control, and analog amplitude modulation used for air traffic control. For such analog waveforms, there is no automatic channel access mechanism. In addition, the number of satellites, their orbits and their distances need to be chosen in such a way that at least one satellite is visible from the aircraft at any one time.

[0035] In order to ensure that the communication between satellite and aircraft is secure and easy to operate 24 / 7, the system 1 according to the invention is configured in such a way that a single satellite covers a specific geographical sector at least for a reasonably long period of time, and at each point in time only one satellite is transmitting. In addition, only a single frequency is used for the communication link 21 between aircraft and satellite within a specific geographical sector, so that frequency switching within the sector is not required. Two specific embodiments of a system configured in this way are described below.

[0036] In a first embodiment, a plurality of satellites are arranged in a row in a medium earth orbit (MEO) or a low earth orbit (LEO), preferably above the equator. MEO includes an altitude range from 2000 km above the earth up to geosynchronous orbit at 35786 km, while LEO is located below 2000 km.

[0037] Figure 2 A schematic, not to scale, side view of an air traffic control system 100 according to a first embodiment is shown. On the surface of the Earth 101, a geographical sector 102 is defined. This may be, for example, a flight information region or a sector thereof. A plurality of satellites 120 travel one after another at a predetermined height above the surface of the Earth 101, preferably above the equator, from West W to East E. A support link 111 is established between a ground station 110 and at least one satellite 120. A communication link 121 is established between the satellite 120 and an aircraft 130 in the geographical sector 102.

[0038] In order to ensure a secure line-of-sight communication link 121 between one of the satellites 120 and the aircraft 130 in the entire geographic sector 102, the elevation angle β of the position of the satellite 120 above the horizon must be within a certain range, for example between 10° and 80°, for all positions within the geographic sector 102. The upper limit is due to the fact that the radiation pattern of the aircraft has a minimum at a radiation angle of 90°. The lower limit is due to the fact that a secure communication link requires a minimum altitude of the satellite above the horizon. In the figure, the elevation angle β1 of the westernmost point PW and the elevation angle β2 of the easternmost point PE of the geographic sector 102 are shown.

[0039] According to this condition, the time that each satellite 120 can be used for communication can be determined according to the altitude of the satellite. This time should not be too short so as to achieve a reasonably long period of time for the operation of each satellite 120 and therefore for the communication between the aircraft 130 and the corresponding satellite 120.

[0040] As a specific (non-limiting) example, a satellite at an altitude of 4000 km in the MEO range has an orbital period of 175 minutes. If 16 satellites are distributed above the equatorial orbit at equal distances, then every 10.9 minutes, a satellite is located at the position previously occupied by the previous satellite, so that for each satellite, the satellites can be used to communicate successively within a period of 10.9 minutes (the rotation of the Earth is not taken into account in this example).

[0041] Figure 3 A schematic diagram of an example 100a of an air traffic control system for a first embodiment is shown from above. As a specific (non-limiting) example, the geographical sector 102 in this example straddles the equator 103 .

[0042] The geographical sector 102 to be served, such as a flight information region or a sector thereof as defined by ICAO, generally has an irregular shape. The service area 104 served by the satellite 120, such as an area in which the signal transmitted by the satellite exceeds a predetermined power density, is selected in such a way that the entire geographical sector 102 is included. The fact that adjacent sectors can also be covered by the service area 104 does not matter, because other communication frequencies are allocated to these sectors.

[0043] The form of the service area 104 depends on the directivity of the satellite's antenna. Typically, a helical antenna that transmits circularly polarized signals is used. This makes communication less dependent on the relative positions of the satellite and the aircraft relative to each other. Linear polarization is also available. For ease of illustration, the service area 104 is shown as a rectangle, but the present invention is not limited to this.

[0044] Satellites 120 at MEO move from west W to east E over the equator 103. Satellites that pass directly over geographic sector 102 have elevation angles up to 90°, which may be too high for reliable communication. However, satellites that are close to geographic sector 102 and satellites that have passed through geographic sector 102 can be used for reliable communication. In the following, satellite positions A and B are particularly considered.

[0045] Satellite position A is a position at which satellite 120 has passed through geographic sector 102 and is far enough away from it so that its elevation angle in the entire geographic sector 102 is less than the maximum allowed elevation angle. On the other hand, satellite position B is a position at which satellite 120, which has passed through position A, is still close enough to geographic sector 102 so that its elevation angle in the entire geographic sector 102 is greater than the minimum required elevation angle. Therefore, both the satellite at position A and the satellite at position B can be used for communication with aircraft in geographic sector 102.

[0046] The distance ds between adjacent satellites 120 is chosen in such a way that it is possible to find satellite positions A and B having a distance ds that satisfies the above condition. The following timing scheme may then be used.

[0047] When satellite 120 arrives at position A, its communication link is opened. It can then provide service to the entire geographic sector 102 until it reaches position B. When it reaches position B, the next satellite 120 arrives at position A. At this point, the communication link of the satellite at position B is closed, and the communication link of the satellite now at position A is opened. This operation is repeated each time an active satellite reaches position B.

[0048] In this way, it is ensured that only one communication link is open at a time. Thus, problems that occur in ATC due to interference when two or more satellite transmitters are active on the same frequency at the same time, such as different Doppler shifts or different signal delays, can be avoided. Even these problems can be overcome by using a different VHF or UHF frequency for each satellite, in which case the aircraft does not know when to switch between these frequencies.

[0049] Thus, using the system according to the present embodiment, space-based ATC can be provided via satellite 24 hours / 7 days in the same manner as ground-based ATC, without requiring any additional equipment in the aircraft, and without modifying existing equipment and / or procedures in use. In addition, ATC for an entire geographic sector can be controlled from a single ground station, making it unnecessary to use ground stations in other countries or to build platforms for ground stations at sea to provide ATC over international waters.

[0050] Figure 4Additional satellite positions that may be used for communication are shown.As described above, positions before the satellite has arrived at geographic sector 102 and positions after the satellite has passed geographic sector 102 may be used for communication.

[0051] Satellite position C is a position at which satellite 120 is still approaching geographic sector 102 and is far enough away that its elevation angle within the entire geographic sector 102 is less than the maximum allowed elevation angle. On the other hand, satellite position D is a position at which satellite 120, which is approaching position C, is still close enough to geographic sector 102 that its elevation angle within the entire geographic sector 102 is greater than the minimum required elevation angle. Therefore, both the satellite at position C and the satellite at position D can be used to communicate with aircraft in geographic sector 102.

[0052] As an alternative to the above timing scheme, the satellite's communication link may be opened when satellite 120 reaches position D and closed when satellite 120 reaches position C.

[0053] This redundancy can also provide failover for the system. For illustration, assume a situation where a satellite is active between location A and location B. When this satellite fails, the satellite currently between location D and location C can take over, replacing the failed satellite. When the next satellite arrives at location A, it is turned on, and the backup satellite is turned off. For each active satellite, the system thus provides a backup satellite that can take over when the active satellite fails. Thus, the reliability of the system can be improved.

[0054] exist Figure 4 Also schematically shown in the figure are examples for providing support links to various satellites. A plurality of directional antennas 112a, 112b, 112c, i.e., antennas that concentrate the emitted radiation in a specific direction, are provided at the ground station 110. Each directional antenna 112a, 112b, 112c establishes a support link 111a, 111b, 111c to one of the satellites 120. In this particular example, three directional antennas are shown, but the present invention is not limited to this. More or less than three directional antennas may also be provided, and they may be provided at different ground stations rather than the same ground station. It should be understood that the figure only schematically illustrates the allocation of antennas to satellites, and not their actual locations on the earth, which are typically at a specific point (or multiple specific points) on the earth's surface, preferably within the state responsible for providing air traffic control to the geographic sector 102.

[0055] The directional antennas follow the position of their respective satellites during their flight over the equator, at least during the entire period of time required for the satellites to operate, for example during the entire period of their respective visibility. In this example, the directional antennas follow the positions of the satellites between D and B. If the satellite reaches position B, the corresponding antenna is turned around to establish a link with the satellite which then reaches position D.

[0056] In this case, the following is achieved: Antenna 112a provides a supporting link 111a between locations A and B to a satellite that is an active transmitting satellite for providing ATC to geographic sector 102. Support link 111a provides up / down relayed ATC voice transmissions and embedded remote monitoring and remote command functions (TMTC).

[0057] Antenna 112b provides a support link 111b to a satellite between positions C and A, which is currently inactive due to its excessive elongation. Support link 111b enables the satellite to be monitored for service purposes such as orbit tracking, orbit corrections, software uploads, etc. When the satellite reaches position A, it is switched to an active state, and support link 111b then provides the services indicated above for support link 111a.

[0058] Antenna 112c provides a support link 111c to a satellite between locations D and C, which is normally inactive but can be switched to standby if a failure occurs in the satellite between locations A and B. If the satellite is active, support link 111c then provides the services indicated above for support link 111a. If not active, support link 111c provides the services indicated above for support link 111b. Since link 111c is already established when the satellite arrives at location D, the satellite between locations D and C can be immediately activated at any time when a failure occurs in the satellite between locations A and B, without causing any interruption in the operation of the system at all.

[0059] By the above method, the period of time for providing a supporting link between the ground station and the satellite is longer than the active period of the satellite, thus providing enough time for orbit tracking and other services (such as orbit correction and / or software upload).

[0060] Although the above description is given for a geographic sector that passes through the equator, ATC can also be provided to a geographic sector located at a distance from the equator.

[0061] Figure 5A schematic diagram of another example of an air traffic control system 100b according to the first embodiment is shown. In this particular example, two geographic sectors 102a and 102b are adjacent to each other and are at a sufficient distance from the equator 103 so that the elevation angle of a satellite 120 traveling above the equator 103 on a selected MEO orbit never exceeds a maximum allowed value. Therefore, a geographic sector may also be served by a satellite temporarily located at the same longitude as the geographic sector. In this example, the geographic sectors 102a and 102b do not need to be directly adjacent to each other, and the service areas 104a and 104b may overlap or not overlap or be spaced apart from each other.

[0062] Next, satellite positions F, G, and H will be described. F is a position where satellite 120 is approaching geographic sector 102a and is close enough so that its elevation angle within the entire geographic sector 102a is greater than the minimum required value. G is a position where satellite 120 is located at the same longitude as geographic sectors 102a and / or 102b. H is a position where satellite 120 has passed geographic sector 102b and is still close enough so that its elevation angle within the entire geographic sector 102b is greater than the minimum required value.

[0063] In this example, the satellite at position F is active to provide air traffic control to geographic sector 102a, and the satellite at position H is active to provide air traffic control to geographic sector 102b. Because different frequencies are assigned to geographic sectors 102a and 102b, the signals transmitted by the two satellites do not interfere with each other even though service areas 104a and 104b overlap. The satellite at position G is inactive but can be used as a backup satellite in the event of a failure of one of the neighboring satellites. Control of the satellites from the ground station can be performed in the same manner as described above with reference to FIG. Figure 4 Proceed in a similar manner as described.

[0064] In the example above, each satellite is active only over a small portion of its orbit around the Earth (or ready to become active if another satellite fails). It can therefore also be used to serve geographic sectors in other areas of the Earth's surface. The satellite's antenna may then need to be rotated to accommodate geographic sectors at different distances from the equator.

[0065] In a second embodiment, a single satellite is provided in a geostationary orbit, also known as a geosynchronous equatorial orbit (GEO). The geostationary orbit has an altitude above ground of approximately 35786 km.

[0066] Figure 6A schematic, not-to-scale side view of an air traffic control system 200 according to a second embodiment is shown. On the Earth's surface 201, geographical sectors 202 are defined in a similar manner as in the first embodiment. As a specific (non-limiting) example, the geographical sectors 202 in this embodiment extend across the equator 203. A satellite 220 is in a geostationary orbit above the equator 203, so that it always has the same position (azimuth and elevation) as seen from the Earth.

[0067] The service area 204 served by the satellite 220 is selected in such a way that the entire geographical sector 102 is included. The service area 204 in the present embodiment is shown as an ellipse, but the present invention is not limited thereto.

[0068] Satellite 220 includes at least one antenna 222, 223, for establishing communication links 221a, 221b between satellite 220 and aircraft 230 in geographic sector 202, preferably directional antennas. In the present embodiment, the directional antenna includes: an active antenna 222 for transmitting an RF beam 221a in the VHF or UHF frequency range; and a passive reflector antenna 223, which is kept at a distance from satellite 220 by an arm 224, for reflecting and beaming the transmitted RF beam 221a, and pointing the reflected and beamed RF beam 221b to the earth. The active antenna 222 can be, for example, a Yagi antenna, which includes an active element and a plurality of passive elements used as a reflector and a director. The reflector antenna 223 can be, for example, a parabolic offset reflector antenna. This parabolic offset reflector antenna is an antenna having a reflector surface in the form of a sector of a parabola, wherein the center point of the reflector surface is not the center point of the parabola. Since the parabolic reflectors for the VHF range are required to be of considerable size (tens of metres), special lightweight constructions have to be used. The reflector antenna can for example be formed by a lightweight deployable net, such as the so-called Astro-net.

[0069] Since the relative position of the satellite with respect to the geographic sector 202 to be served is fixed and the antenna can be precisely aimed at the target area, linear polarization of the radiation can be used, which brings a 3 dB gain advantage compared to circular polarization.

[0070] Since the location of satellite 220 is fixed, the support link may be performed using a single antenna at a single ground station (not shown).

[0071] The satellite may be configured to transmit two or more RF signals having different frequencies to provide air traffic control to two or more geographic sectors within the target area of ​​the directional antenna.

[0072] Since only one satellite is active in this embodiment, no special control is required to avoid two satellites being active on the same frequency as in the first embodiment.

[0073] Although the present invention has been illustrated and described in detail in the accompanying drawings and the foregoing description, such illustration and description will be considered illustrative or exemplary rather than restrictive. The present invention is not limited to the disclosed embodiments. By reading this disclosure, other modifications will be apparent to those skilled in the art. Such modifications may involve other features known in the art and that may be used as an alternative or supplement to the features already described herein.

[0074] For example, in addition to the frequencies assigned to the geographical sector to be served, each satellite may also be active on aircraft emergency frequencies, such as International Air Distress (IAD) on 121.5 MHz or 243 MHz.

[0075] Although the invention has been described with reference to specific features and embodiments thereof, it is apparent that various modifications and combinations may be made thereto without departing from the spirit and scope of the invention. Therefore, the specification and drawings are to be regarded as illustrative only of the invention.

Claims

1. A system (100) for providing air traffic control to a plurality of aircraft (130) within a predetermined geographical sector (102), a predetermined communication frequency in the VHF or UHF range being allocated to the geographical sector, each of the plurality of aircraft (130) using the same predetermined communication frequency allocated to the geographical sector (102), The system comprises at least one satellite (120) configured for communication (121) with an aircraft (130) in the geographic sector using analog modulated RF signals on the same predetermined communication frequency assigned to the geographic sector (102), in, A plurality of satellites (120) move at a distance (ds) from each other in the same orbit, which is a medium earth orbit (MEO) or a low earth orbit (LEO), The number and altitude of the plurality of satellites (120) are selected in such a way that at least one of the plurality of satellites (120) has an elevation angle (β) within a predetermined range for all locations within the predetermined geographic sector (102), and The system (100) is configured as follows: When a first satellite of the plurality of satellites (120) reaches a predetermined first position (A) relative to the predetermined geographic sector (102), the first satellite of the plurality of satellites (120) is switched to be active for communication (121) with the plurality of aircraft (130), and The first satellite of the plurality of satellites (120) is switched off when a second satellite of the plurality of satellites (120) reaches a predetermined second position (B) relative to the predetermined geographical sector (102), the second satellite having previously passed the first position (A) and having been switched active for communication (121) with the plurality of aircraft (130), wherein the first position (A) and the second position (B) are determined based on the number and altitude of the plurality of satellites (120) in the following manner: while the first satellite in the plurality of satellites (120) is switched to active, the second satellite in the plurality of satellites (120) is switched off, Thereby only one satellite (120) is actively transmitting at a time on the predetermined communication frequency assigned to the geographic sector (102).

2. The system (100) according to claim 1, wherein: The orbit is an equatorial orbit.

3. The system (100) according to claim 1, wherein: The number and altitude of the plurality of satellites (120) are selected in such a way that at least two of the plurality of satellites (120) have an elevation angle (β) within a predetermined range in the geographical sector at each time, so that one of them can be used as an active satellite and the other can be used as a backup satellite if the active satellite fails.

4. The system (100) according to claim 1 or 3, wherein: The predetermined range is a range between 10° and 80°.

5. The system (100) according to claim 1, wherein: The number and altitude of the plurality of satellites (120) are selected in such a way that, for two geographical sectors (102a, 102b) which are sufficiently far from the equator so that the elevation angle (β) of the satellite is always less than a predetermined upper limit, a first satellite can be used as an active satellite for one of the geographical sectors (102a), a second satellite can be used as an active satellite for the other geographical sector (102b), and if one of the first satellite and the second satellite fails, the third satellite can be used as a backup satellite.

6. The system (100) according to claim 5, wherein: The upper limit is 80°.

7. The system (100) according to claim 1, comprising at least two ground antennas (112a, 112b) following the positions of at least two satellites, in, A supporting link (111a) between a ground antenna (112a) and an actively transmitting satellite provides for uplink and / or downlink relay of ATC voice transmissions, and / or A support link (111b) between a ground antenna (112b) and a satellite that is not actively transmitting provides maintenance services.

8. The system (100) according to claim 7, wherein A supporting link (111a) between a ground antenna (112a) and an actively transmitting satellite provides embedded remote monitoring and remote command capabilities, and / or A support link (111b) between a ground antenna (112b) and a satellite that is not actively transmitting provides orbit tracking, orbit corrections, and software uploads.

9. The system (100) of claim 1, configured in such a way that each satellite (120) can be switched to active more than once during one orbit around the earth to provide air traffic control for more than one geographic sector (102).

10. The system (100) of claim 9, wherein each satellite (120) is configured to rotate its antenna in order to reach geographic sectors (102) at different distances from the equator.

11. The system according to claim 1, wherein: At least one satellite (120, 220) is configured to be able to operate on at least two different communication frequencies assigned to the same or different geographical sectors (102a, 102b) or at least on a communication frequency assigned to a geographical sector (102, 202) and on an aircraft emergency frequency.

12. The system (100) according to claim 11, wherein: All satellites (120) are configured to be able to operate on at least two different communication frequencies assigned to the same or different geographic sectors (102a, 102b) or at least on a communication frequency assigned to a geographic sector (102, 202) and on an aircraft emergency frequency.

13. A communication method for providing air traffic control to a plurality of aircraft within a predetermined geographical sector, a predetermined communication frequency in the VHF or UHF range being allocated to the geographical sector, each of the plurality of aircraft using the same predetermined communication frequency allocated to the geographical sector, in, The method includes establishing a communication link between at least one satellite and the plurality of aircraft in the geographic sector using analog modulated RF signals on the same predetermined communication frequency assigned to the geographic sector, wherein a plurality of satellites move at a distance from each other in the same orbit, which is a medium earth orbit or a low earth orbit, each time at least one of the plurality of satellites has an elevation angle within a predetermined range for all locations within the predetermined geographic sector, When a first satellite of the plurality of satellites reaches a predetermined first position relative to the predetermined geographic sector, the first satellite of the plurality of satellites is switched to be active for communication with the plurality of aircraft, and the first one of the plurality of satellites being turned off when a second one of the plurality of satellites reaches a predetermined second position relative to the predetermined geographic sector, the second one of the plurality of satellites having previously passed the first position and having been switched active for communication with the plurality of aircraft, wherein the first position and the second position are determined based on the number and altitude of the plurality of satellites in the following manner: while the first satellite among the plurality of satellites is switched to be active, the second satellite among the plurality of satellites is switched off, Thereby only one satellite at a time is actively transmitting on the predetermined communication frequency assigned to the geographic sector.

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