Patent Description:
Air traffic control (ATC) is a service provided by air traffic controllers to the pilots of aircrafts by directing aircrafts and providing support and information. For this purpose, the earth's airspace is divided into flight information regions (FIR) by the International Civil Aviation Organization (ICAO). Each of these flight information regions is serviced by an area control center (ACC), also known as air route traffic control center. A flight information region generally comprises the airspace above a territory of a state in which the area control center is located, or a part thereof. It may also comprise an airspace above international waters. A flight information region may further be divided into sectors.

ATC systems are based on an analog communication between ground stations and aircrafts in the VHF or UHF range. Therein, an analog amplitude modulation of the RF carriers is used. To each sector of a flight information region, a specific frequency is assigned.

For international waters, it may happen that a sector of a flight information region cannot be covered by ground stations located in the state which is responsible for the flight information region or cannot be covered by ground stations at all. In this case all airplanes must be additionally equipped with a special long range communication equipment. This long range communication equipment may use either HF (shortwave) installations or satellites. In both cases these long range communication systems are not using the VHF frequency band and must be available in parallel to the normal VHF equipment.

<CIT> describes a communication and position locating system suitable for air traffic control. Multiple satellites are provided in two geosynchronous constellations. One constellation (e.g., <NUM> satellites) has a first closed ground track. The other constellation (e.g., <NUM> satellites) has a different ground track or is geostationary, located outside of the first ground track. The satellites include communication transceivers so that aircraft can communicate voice and data to other aircrafts and to the ground stations and automatically report their position to air traffic control centers, e.g., via the satellites.

<CIT> describes a satellite-based air traffic control system utilizing a global-coverage satellite communications network to provide voice and data communication between an air traffic control center and one or more aircraft worldwide. Each aircraft is provided with an airborne navigation location reporting system device which has the capability of determining positional information of the aircraft and to transmit and receive voice and data messages to and from the ATC center via the satellite communications network.

<CIT> describes an aircraft position report system. The system comprises a satellite, an aircraft position reporting receiver mounted on the satellite, an antenna element mounted on the satellite, and an antenna interface mounted on the satellite. The aircraft position reporting receiver receives aircraft position reports through the antenna element by associating a spot beam with a narrow coverage area. The aircraft position reports are derived from a signal produced by the antenna element. The antenna interface changes the narrow coverage area associated with the spot beam to receive aircraft position reports from a wide coverage region within a reporting period.

<CIT> describes a system for surveying an aircraft present in an area covered by at least one communication satellite, the aircraft transmitting signals conveying surveillance messages. The coverage area consists of multiple spots, the satellite is configured to periodically apply a switching sequence, the sequence consisting of multiple switching phases, a switching phase corresponding to a duration during which the signals transmitted by the aircraft present in at least one spot are processed by the satellite in such a way as to detect surveillance messages, and the sequence is adapted to allow for the detection, during a predetermined period, of at least one surveillance message by aircraft present in the coverage area.

It is therefore an object of the present invention to provide a method and a system for providing air traffic control from satellites into a geographic sector by using the standard VHF or UHF equipment but no extra expensive long range communication equipment inside the aircrafts.

Further implementation forms are apparent from the dependent claims, the description, and the figures. Therein, the product claims may also be further restricted by the features of the dependent method claims and vice versa.

According to a first aspect, a system for is provided providing air traffic control within a geographic sector (<NUM>, <NUM>) to which a communication frequency in the VHF or UHF range is assigned. The system comprises at least one satellite configured for a communication with an aircraft in the geographic sector using an analog modulated RF signal at the communication frequency. The system is configured in such a way that at each time, only one satellite is transmitting on the communication frequency assigned to the geographic sector.

The geographic sector may for example be a flight information region as defined by the ICAO or one of its sectors. VHF and UHF are frequency ranges internationally standardized by the International Telecommunication Union (ITU). For civilian aeronautical radio navigation service and aeronautical mobile radio communication service a frequency range from <NUM> to <NUM> is reserved so that the system preferably operates within that frequency range. For military users frequencies in the VHF band between <NUM> and <NUM> or in the UHF band between <NUM> and <NUM> are used. In general, the system described herein can provide services from space into a geographical area by using analog modulated RF signals.

Further, the system comprises a plurality of satellites moving in a distance from each other essentially in the same orbit which is a medium earth orbit or low earth orbit, preferably an equatorial orbit.

Further, the system is configured in such a way that one of the plurality of satellites is switched active for the communication with the aircraft when it reaches a predetermined position, and the satellite active up to then is switched off at the same time. The active role of a satellite in this context means the transmission of signals with the analogue RF signal into the geographical area. All other RF links and all other transmit and receive functions may active at all time for all satellites.

In a further implementation form of the first aspect, a number and an altitude of the plurality of satellites is selected in such a way that at each time, at least one of the plurality of satellites has an elevation within a predetermined range, preferably between <NUM>° and <NUM>°.

In a further implementation form of the first aspect, a number and an altitude of the plurality of satellites is selected in such a way that at each time, at least two of the plurality of satellites has an elevation within a predetermined range, preferably between <NUM>° and <NUM>°, so that one of them can be used as the active satellite and the other one can be used as a backup satellite if the active satellite fails.

In a further implementation form of the first aspect, a number and an altitude of the plurality of satellites is selected in such a way that for two geographic sectors at a sufficient distance from the equator resulting in an elevation of the satellites always being smaller than a predetermined upper limit, preferably <NUM>°, so that a first satellite may be used as an active satellite for one of the geographic sectors, a second satellite may be used as an active satellite for the other one of the geo-graphic sectors, and a third satellite may be used as a backup satellite if one of the first and second satellites fail.

In a further implementation form of the first aspect, the system comprises at least two ground antennas following the position of at least two satellites. A supporting link between ground antennas and an actively transmitting satellite may provide relayed upwards and/or downwards ATC voice transmission and preferably embedded telemonitoring and telecommand functionality. A supporting link between a ground antennas and a not actively transmitting satellite may provide maintenance service, preferably orbit tracking, orbit correction, software upload and others.

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

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

It is to be understood that an embodiment of the invention can also be any combination of the dependent claims or above implementation forms with the respective or other independent claims or above aspects.

Further features and useful aspects of the invention can be found in the description of exemplary embodiments with reference to the attached drawings.

In the following, embodiments of an air traffic control system are described with reference to the enclosed drawings. Of these embodiments, the first embodiment forms part of the present invention, while the second embodiment does not form part of the present invention but serves for illustration purposes.

<FIG> shows a schematic view of an air traffic control system illustrating the basic principle of the present invention.

A system <NUM> for providing a traffic control in a flight information region comprises one or multiple ground station(s) <NUM> and one or multiple satellite(s) <NUM>. A supporting link <NUM> (described later) is established between the ground station <NUM> and the satellite <NUM>. A communication link <NUM> is established between the satellite <NUM> and an aircraft <NUM>, for example an airplane. Both the supporting link <NUM> and the communication link <NUM> preferably are bidirectional. Thus, communication between the ground station <NUM> and the aircraft <NUM> is not performed as usual by a direct communication link between the ground station and the aircraft, but indirectly by the supporting link <NUM> between the ground station <NUM> and the satellite <NUM> and the communication link <NUM> between the satellite <NUM> and the aircraft <NUM>.

The communication link <NUM> between the satellite <NUM> and the aircraft <NUM> has to fulfil a number of basic conditions because it is required to work with the existing ATC devices installed in aircrafts. That means that there are some unchangeable parameters such as the communication in the VHF and UHF range, especially in a frequency range of about <NUM> to <NUM> (corresponding to a wavelength between <NUM>,<NUM> and <NUM>) assigned to air traffic control, and the analog amplitude modulation used for air traffic control. For such analog waveforms, there are no automatic channel access mechanisms. Further, the number, the orbits, and the distances of the satellites are required to be selected in such a way that at each time, at least one satellite is visible from the aircraft.

In order to ensure a safe and easy to operate <NUM> hours / <NUM> days communication between the satellite and the aircraft, the system <NUM> according to the present invention is configured in such a way that a single satellite covers a specific geographic sector at least over a reasonable long period of time, and that only one satellite is actively transmitting at each time point. Further, only a single frequency is used for the communication link <NUM> between the satellite and the aircraft within the specific geographic sector so that frequency switching within this sector is not required. Two specific embodiments of a system configured in this way are described below.

In a first embodiment, a plurality of satellites are arranged in a line at a medium earth orbit (MEO) or low earth orbit (LEO), preferably above the equator. MEO includes an altitude range above earth from <NUM>,<NUM> up to the geosynchronous orbit at <NUM>,<NUM> while LEO is located below <NUM>.

<FIG> shows a schematic not-to-scale side view of an air traffic control system <NUM> according to the first embodiment. On the earth's surface <NUM>, a geographic sector <NUM> is defined. This may for example be a flight information region or a sector thereof. A plurality of satellites <NUM> travels one behind the other at the predetermined altitude above the earth's surface <NUM>, preferably above the equator, from west W to east E. A supporting link <NUM> is established between a ground station <NUM> and at least one of the satellites <NUM>. A communication link <NUM> is established between the satellite <NUM> and an aircraft <NUM> in the geographic sector <NUM>.

In order to ensure a safe line-of-sight communication link <NUM> between one of the satellites <NUM> and the aircraft <NUM> in the entire geographic sector <NUM>, an elevation angle β of a position of the satellite <NUM> above the horizon has to be in a specific range, for example between <NUM>° and <NUM>°, for all the locations within the geographic sector <NUM>. The upper limit is due to the fact that a radiation pattern of an aircraft has a minimum at a radiation angle of <NUM>°. The lower limit is due to the fact that a minimum height of the satellite above the horizon is required for a safe communication link. In the figure, an elevation angle β1 for the westernmost point PW of the geographic sector <NUM> and an elevation angle β2 for the easternmost point PE are shown.

From this condition, the time each satellite <NUM> is available for communication can be determined depending on the altitude of the satellite. This time should not be too short in order to enable a reasonable long period of time for the operation of each satellite <NUM> and thus for a communication between an aircraft <NUM> and the corresponding satellite <NUM>.

As a specific (non-limiting) example, a satellite at an altitude of <NUM>,<NUM> which is within the MEO range has an orbit period of <NUM> minutes. If <NUM> satellites are distributed in equal distances at above the equator orbit, every <NUM> minutes a satellite is at a position the preceding satellite had before, so that the satellites may be used for communication one after the other over a time period of <NUM> minutes for each satellite (in this example the rotation of the earth is not taken into consideration).

<FIG> shows a schematic view of an example 100a for the air traffic control system of the first embodiment from above. As a specific (non-limiting) example, the geographic sector <NUM> in this example extends across the equator <NUM>.

A geographic sector <NUM> to be serviced, e.g. a flight information region or a sector thereof as defined by the ICAO, generally has an irregular shape. A service region <NUM> serviced by the satellite <NUM>, for example a region in which the signal emitted by the satellite exceeds a predetermined power density, is selected in such a way that the entire geographic sector <NUM> is included. The fact that also adjacent sectors may thereby be covered by a service region <NUM> is not of relevance since other communication frequencies are assigned to these sectors.

The form of the service region <NUM> depends on the directivity of the satellite's antenna. Typically, helical antennas emitting a circularly polarized signal are used. This makes the communication less dependent on the relative position of satellite and aircraft with regard to each other. Linear polarization is also usable. For the ease of explanation, the service region <NUM> is shown as a rectangle, but the present invention is not limited thereto.

The satellites <NUM> at MEO travel above the equator <NUM> from west W to east E. A satellite directly passing across the geographic sector <NUM> has an elevation angle up to <NUM>° which may be too high for a reliable communication. However, a satellite approaching the geographic sector <NUM> as well as a satellite having already passed the geographic sector <NUM> may be used for a reliable communication. In the following, especially the satellite positions A and B shall be taken into account.

Satellite position A is a position in which the satellite <NUM> has already passed the geographic sector <NUM> and is sufficiently far away from it so that its elevation angle within the entire geographic sector <NUM> is smaller than the maximum permissible elevation angle. On the other hand, satellite position B is a position in which the satellite <NUM> that has already passed position A still is near enough to the geographic sector <NUM> so that its elevation angle within the entire geographic sector <NUM> is greater than the minimum required elevation angle. A satellite in position A as well as a satellite in position B therefore can both be used for a communication with aircrafts in the geographic sector <NUM>.

A distance ds between adjacent satellites <NUM> is selected in such a way that it is possible to find satellite positions A and B having the distance ds which fulfill the above conditions. The following timing scheme can then be used:
When a satellite <NUM> reaches position A, its communication link is switched on. It can then service the entire geographic sector <NUM> until it reaches position B. At the time it reaches position B, the following satellite <NUM> reaches position A. At this time, the communication link of the satellite at position B is switched off, and the communication link of the satellite which now is at position A is switched on. This is repeated every time the active satellite reaches position B.

In this way, it can be made sure that only one communication link at a time is switched on. Thereby, problems occurring in ATC by interference when two or more satellite transmitters are simultaneously active on the same frequency, such as a different Doppler shifts or different signal delays, can be avoided. Even if these problems may also be overcome by using a different VHF or UHF frequency for each satellite, the aircrafts in that case would not know when to switch between those frequencies.

With a system according to the present embodiment, is thus possible to provide space based ATC via satellites <NUM> hours / <NUM> days in the same way as ground based ATC without requiring any additional equipment in the aircrafts and without having to modify the existing equipment and/or the procedures used. Further, it is possible to control the ATC for an entire geographic sector from a single ground station so that the use of ground stations of other states or the building of platforms for ground stations on sea is not required for providing ATC above international waters.

<FIG> shows additional satellite positions that may be used for communication. As indicated above, positions before the satellite has reached the geographic sector <NUM> as well as positions after the satellite has passed the geographic sector <NUM> may be used for communication.

Satellite position C is a position in which the satellite <NUM> still is approaching the geographic sector <NUM> and is sufficiently far away from it that its elevation angle within the entire geographic sector <NUM> is smaller than the maximum permissible elevation angle. On the other hand, satellite position D is a position in which the satellite <NUM> approaching position C still is near enough to the geographic sector <NUM> that its elevation angle within the entire geographic sector <NUM> is greater than the minimum required elevation angle. A satellite in position C as well as a satellite in position D therefore can both be used for a communication with aircrafts in the geographic sector <NUM>.

As an alternative to the above time scheming, the communication link of a satellite may be switched on when the satellite <NUM> reaches position D, and switched off when the satellite <NUM> reaches position C.

This redundancy also makes it possible provide the system with a failover. For the explanation, a case is assumed in which a satellite is active between the positions A and B. When this satellite fails, a satellite which currently is between the positions D and C may take over the operation, thus replacing the defective satellite. When the next satellite reaches position A, it is switched on, and the backup satellite is switched off. For each active satellite, the system thus provides a backup satellite which may take over the operation if the active satellite fails. Thereby, the reliability of the system can be improved.

Also schematically shown in <FIG> is an example for providing supporting links to the individual satellites. At a ground station <NUM>, a plurality of directional antennas 112a, 112b, 112c is provided, i.e. antennas which concentrate the emitted radiation in a specific direction. Each directional antenna 112a, 112b, 112c establishes a supporting link 111a, 111b, 111c to one of the satellites <NUM>. In this specific example, three directional antennas are shown, but the present invention is not limited thereto. There may also be provided more or less than <NUM> directional antennas, and they may be provided at different ground stations instead of the same ground station. It is to be understood that the figure only schematically shows the assignment of the antennas to the satellites and not their real position on earth which generally is at a specific point (or at specific points) of the earth surface, preferably within the state responsible for providing air traffic control to the geographic sector <NUM>.

The directional antennas follow the position of the corresponding satellites during their flight above the equator at least during the entire period of time required for the operation of the satellite, preferably longer, for example during the entire period of their individual visibility. In the present example, the directional antennas follow the satellites at positions between D and B. If a satellite reaches the position B, the corresponding antenna is turned around to establish a link with the satellite which then has arrived at position D.

In this case, the following effects are achieved: The antenna 112a provides a supporting link 111a to a satellite between positions A and B which is the actively transmitting satellite for providing ATC to the geographic sector <NUM>. The supporting link 111a provides relayed upwards/downwards ATC voice transmission and embedded telemonitoring and telecommand functionality (TMTC).

The antenna 112b provides a supporting link 111b to a satellite between positions C and A which is currently not active because its elongation is too large. The supporting link 111b enables the satellite to be monitored for service purposes such as for example orbit tracking, orbit correction, software upload and others. When the satellite reaches position A, it is switched active, and the supporting link 111b then provides the service indicted above for supporting link 111a.

The antenna 112c provides a supporting link 111c to a satellite between positions D and C which generally is not active, but may be switched active as a backup if the satellite between positions A and B fails. If the satellite is active, the supporting link 111c then provides the service indicted above for supporting link 111a. If it is not active, the supporting link 111c provides the service as indicted above for supporting link 111b. Since the link 111c has already been established at the time when the satellite arrived at position D, the satellite between positions D and C can be immediately activated at any time when the satellite between positions A and B fails without causing any interruption at all in the operation of the system.

By the method above in which a supporting link between a ground station and a satellite is provided for a longer period of time than the activity period of the satellite, a sufficiently long time for orbit tracking and other services such as orbit correction and/or software upload is provided.

While the explanation above has been given for a geographic sector crossing the equator, ATC may also be provided to a geographic sector which is located at a distance from the equator.

<FIG> shows a schematic view of another example of the air traffic control system 100b according to the first embodiment. In this specific example, two geographic sectors 102a and 102b are arranged adjacent to each other and at a sufficient distance from the equator <NUM> so that an elevation angle of a satellite <NUM> traveling above the equator <NUM> in the selected MEO orbit never exceeds the maximum permitted value. Therefore, the geographic sectors may also be served by a satellite which momentarily is positioned at the same longitude as the geographic sectors. In the example, the geographic sectors 102a and 102b need not be immediately be adjacent to each other, and the service regions 104a and 104b may or may not overlap each other or be spaced from each other.

Satellite positions F, G, and H shall be explained next. F is a position in which the satellite <NUM> is approaching the geographic sector 102a and already is near enough so that its elevation angle within the entire geographic sector 102a is greater than the minimum required value, G is a position in which the satellite <NUM> is at the same longitude as the geographic region 102a and/or 102b. H is a position in which the satellite <NUM> has already passed the geographic sector 102b and is still near enough so that its elevation angle within the entire geographic sector 102b is greater than the minimum required value.

In the present example, the satellite in position F is active for providing air traffic control to the geographic sector 102a, and the satellite in position H is active for providing air traffic control the geographic sector 102b. Since different frequencies are assigned to the geographic sectors 102a and 102b, the signals sent by the two satellites do not interfere with each other even if the service regions 104a and 104b overlap. The satellite in position G is not active, but may be used as a backup satellite if one of the adjacent satellites fails. The control of the satellites from the ground station may be carried out in a similar way as described above with reference to <FIG>.

In the example explained above, each satellite is active (or ready to become active if another satellite fails) only over a small part of its orbit around earth. It may therefore also be used for servicing geographic sectors in other regions of the earth's surface. It may then be required to swivel the antenna of the satellite in order to be adapted to geographic sectors at different distances from the equator.

In a second embodiment, a single satellite is provided at a geostationary orbit, also called geosynchronous equatorial orbit (GEO) The geostationary orbit has an altitude above earth of about <NUM>,<NUM>.

<FIG> shows a schematic not-to-scale side view of an air traffic control system <NUM> according to the second embodiment. On the earth's surface <NUM>, a geographic sector <NUM> is defined in a similar way as in the first embodiment. As a specific (non-limiting) example, the geographic sector <NUM> in this embodiment extends across the equator <NUM>. A satellite <NUM> is in a geostationary orbit above the equator <NUM> so that it always has the same position (azimuth and elevation) seen from earth.

A service region <NUM> serviced by the satellite <NUM> is selected in such a way that the entire geographic sector <NUM> is included. The service region <NUM> in the present embodiment is shown as an ellipse, but the present embodiment is not limited thereto.

The satellite <NUM> comprises at least one antenna <NUM>, <NUM>, for establishing a communication link 221a, 221b between the satellite <NUM> and an aircraft <NUM> in the geographic sector <NUM>, preferably a directional antenna. In the present embodiment, the directional antenna comprises an active antenna <NUM> for emitting an RF beam 221a in the VHF or UHF frequency range and passive reflector antenna <NUM> which is held at a distance from the satellite <NUM> by an arm <NUM> for reflecting and bundling the emitted RF beam 221a and directing the reflected and bundled RF beam 221b to earth. The active antenna <NUM> may for example be a Yagi antenna comprising an active element and a plurality of passive elements acting as reflectors and directors. The reflector antenna <NUM> may for example be a parabolic offset reflector antenna. Such a parabolic offset reflector antenna is an antenna having a reflector surface in the form of a sector of a paraboloid, wherein the center point of the reflector surface is not the center point of the paraboloid. Since a parabolic reflector for the VHF range is required to have a considerable size (some ten meters), a special light weight construction has to be used. The reflector antenna may for example be formed by a light weight deployable mesh which is for example known as Astro mesh.

Since a relative position of the satellite with regard to the geographic sector <NUM> to be serviced is fixed, and the antenna may exactly be aligned to the target area, a linear polarization of the radiation may be used which results in an advantage to the gain of <NUM> dB compared with circular polarization.

Since the position of the satellite <NUM> is fixed, the supporting link can be performed using a single antenna at a single ground station (not shown in the figure).

The satellite may be configured to emit two or more RF signals having different frequencies for providing air traffic control to two or more geographic sectors within a target area of the directional antenna.

Since only one satellite is active in this embodiment, special control for avoiding that two satellites are active on the same frequency as in the first embodiment is not required.

While the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. From reading the present disclosure, other modifications will be apparent to a person skilled in the art. Such modifications may involve other features, which are already known in the art and may be used instead of or in addition to features already described herein.

Claim 1:
A system (<NUM>, <NUM>, <NUM>) for providing air traffic control within a geographic sector (<NUM>, <NUM>) to which a communication frequency in the VHF or UHF range is assigned,
the system comprising a plurality of satellites (<NUM>, <NUM>, <NUM>) configured for a communication (<NUM>, <NUM>, <NUM>) with an aircraft (<NUM>, <NUM>, <NUM>) in the geographic sector using an analog modulated RF signal at the communication frequency,
wherein the system is configured in such a way that at each time, only one satellite is actively transmitting on the communication frequency assigned to the geographic sector,
the plurality of satellites (<NUM>) move in a distance (ds) from each other essentially in the same orbit which is a medium earth orbit, MEO, or low earth orbit, LEO, preferably an equatorial orbit, and
the system (<NUM>) is configured in such a way that
one of the plurality of satellites (<NUM>) is switched active for the communication (<NUM>) with the aircraft (<NUM>) when it reaches a predetermined position (A), and
the satellite (<NUM>) active up to then is switched off at the same time.