Method for establishing bidirectional optical communication between a satellite and a ground station

Abstract

Method for establishing a bidirectional optical communication between a ground station (10) and a particularly geostationary satellite (14), to which a lead angle (PAA) is assigned, by which an uplink light beam (20) to be sent from the ground station (10) to the satellite (14) is offset from the direction of incidence of a downlink light beam (16) previously sent from the satellite (14) to the ground station (10), wherein in the method - a satellite (14) transmits a downlink light beam (16) through the atmosphere to a ground station (10) in a direction of incidence and - an orbital relay device (22) is provided, flying in the direction of movement (12) of the satellite (14) at a distance (24) behind the latter, which - receives an uplink light beam (20) sent by the ground station (10) and forwards it to the satellite (14) or - receives an uplink light beam (20) transmitted by the ground station (10) and transmits signals representing data of the uplink light beam (20) to the satellite (14) or - receives an uplink light beam (20) transmitted by the ground station (10) and transmits signals processed by the relay device (22) on the basis of data transmitted by the uplink light beam (20) to the satellite (14), - wherein the distance (24) between the satellite (14) and the relay device (22) is determined based on the difference in movement speed between the satellite (14) and the ground station (10) and on the distance of the satellite (14) from the ground station (10) and thus on the lead angle (PAA) assigned to the satellite (14), and - the uplink light beam (20) is transmitted from the ground station (10) to and received by the relay device (22) at a lead angle (PAA) of zero degrees and thus in a direction opposite to the direction of incidence of the downlink light beam (16).

Description

[0001] The invention relates to a method for establishing bidirectional optical communication between a satellite and a ground station, as is particularly suitable for optical feeder links of geostationary communication satellites, optical satellite downlinks (LEO or GEO) to small ground terminals and / or for optical transmission of frequency standards (“synchronization of optical clocks”).

[0002] Geostationary (GEO) communications and television satellites require high uplink data rates to transmit the data from the ground gateway to the satellite (from where it is transmitted via radio transponders as a television signal to households or as a communications signal to users on the ground). These radio links between the ground station and the GEO (so-called GEO feeder link, GFL) must become increasingly high-speed to meet the system requirements, while at the same time, the available frequency spectrum is becoming increasingly scarce. One solution to this problem is to switch from microwave (radio) link technology to optical point-to-point radio. There are no spectrum restrictions in the optical range. In addition, optical data links – as is known from terrestrial fiber optic technology – enable significantly higher data rates, namely – from today's perspective – up to 100 Gbps per channel, which can be increased approximately a hundredfold using wavelength division multiplexing technology.

[0003] However, optical ground loops (OGFLs) are subject to atmospheric interference. For example, cloud cover above the optical ground station (OGS) can block the connection to the satellite. Studies have shown that OGS diversity can adequately counteract this.

[0004] Another atmospheric influence is refractive index turbulence (BIT), which disrupts the optical wavefront and thus leads to intensity fluctuations (scintillations) during further propagation. Depending on the location of the OGS and the time of day, the wavelength used, and the elevation of the link (angle between the satellite, ground station, and horizon), BIT can cause significant field disturbances, causing the signal to fluctuate extremely sharply at the GEO satellite. Depending on the transmission method and BIT situation, this can severely disrupt or even prevent signal reception.

[0005] The BIT disturbances are noticeable to the receiver through two effects: - intensity disturbances (or field amplitude fluctuations) and - Wavefront disturbances; these lead to small but rapid fluctuations in the angle of incidence of the received signal across the receiving aperture.

[0006] With regard to BIT, there is a "cone of view," i.e., a solid angle range (starting from the OGS) within which two laser beams experience the same BIT perturbations (i.e., the same directional deflections). This angle is called the isoplanatic angle (IPA), which depends, among other things, on the satellite's elevation angle. Depending on the BIT strength, wavelength, and length of the atmospheric path (given by the link elevation), the IPA ranges from a few µrad to less than 100 µrad.

[0007] Another effect to consider in optical transmission between ground and satellite is the point-ahead angle (PAA). This angle is caused by the absolute difference in speed between the satellite and the ground station and is, for example, 18 µrad for geostationary orbit. The transmitted beam from the ground station must "lead" the direction of incidence of the received beam by this angle (known as pointing) in order to reach the satellite. This applies to both the alignment at the ground station and the satellite, and not just to GEO satellites but to any satellite in orbit.

[0008] If, in a specific scenario, IPA < PAA, the received and transmitted beams no longer pass through the equally turbulent atmospheric volume, and the angle of incidence fluctuations caused by atmospheric BIT (measured at the receiver) can no longer be used to correctly point the transmitted beam. Likewise, with adaptive optics, the measured received wavefront distortion can no longer be used to pre-distort the wavefront of the transmitted beam (pre-distortion in this context means that the transmitted wave is pre-distorted – usually by appropriate adjustable mirrors – precisely so that the BIT disturbance of the atmosphere leads to the equalization of the wavefront and thus to a flat / ideal wave above the atmosphere relevant for the BIT disturbance).

[0009] The Fig. 1 to 3 are intended to cover the cases IPA>PAA and IPA <PAA illustrieren. Der Satellit ist dabei doppelt (einmal mit gestrichelten und einmal mit durchgezogenen Linien) dargestellt, um seine Bewegung zwischen dem Absenden des Downlink-Lichtstrahls und dem Empfangen des Uplink-Lichtstrahls zu verdeutlichen.

[0010] In Fig. Figure 1 shows a high elevation scenario (little atmospheric refractive index turbulence in the link path) where the IPA is larger than the PAA. Fig. 2, however, shows the more general situation in which, for example, due to a satellite elevation that is too small (“low-lying satellite”), IPA <PAA ist; hier gehen Empfangsstrahl und Sendestrahl durch Volumina der Atmosphäre mit unterschiedlicher Brechungsindexturbulenz-Struktur. Fig. Figure 3 shows the beam path in the optical GEO downlink and uplink. The two bent laser beams illustrate the PAA effect; in fact, this is a dynamic effect due to the signal propagation time limited by the speed of light.

[0011] The problem that the IPA is smaller than the PAA (IPA <PAA) ist unvermeidbar bei optischen Punkt-zu-Punkt Verbindungen zwischen Erdoberfläche und einem Satelliten, es sei denn, der Satellit bewegt sich sehr langsam (dazu müsste er aber weiter entfernt sein als der geostationäre Orbit) oder die Atmosphäre ist ungewöhnlich stabil (was aber in den allermeisten Anwendungsszenarien nicht gegeben ist). Damit kann weder der Sendestrahl anhand der verfolgten -atmosphärisch verursachten - Einfallswinkelschwankungen unter dem zum Erreichen des Satelliten erforderlichen „richtigen Winkel“ ausgerichtet werden, noch kann eine „adaptive-Optik Vorverzerrung“ erfolgreich eingesetzt werden.

[0012] DE 10 2011 113 148 A1 describes a system and method for bidirectional communication between two communication platforms, namely between a low-Earth communication platform, such as an aircraft, and a far-Earth communication platform, such as a satellite. For example, an uplink beam is transmitted from the aircraft to the satellite, whereupon the satellite transmits a downlink beam to a ground station or transmits the downlink beam to another satellite, which in turn establishes the connection to Earth.

[0013] A similar method of bidirectional communication is also known from US 2002 / 72853 A1.

[0014] The problem that a downlink beam and an uplink beam returned upon reception by, for example, a ground station can pass through different volumes of the atmosphere, resulting in different channel conditions, is known from M. Reyes et al.: Propagation statistics of ground-satellite optical links with different turbulence conditions. In: Proc. SPIE 5572, Optics in Atmospheric Propagation and Adaptive Systems VII, 11 Nov. 2004, pp. 211-222.

[0015] The object of the invention is to provide a method for establishing a bidirectional optical communication between a satellite and a ground station, in which the problem is addressed by simple means in the case that the isoplanasia angle (IPA) is smaller than the point-ahead angle (or lead angle, PAA).

[0016] To achieve this object, the invention proposes a method for establishing a bidirectional optical communication between a satellite and a ground station, wherein in the method - a satellite sends a downlink light beam through the atmosphere to a ground station in a direction of incidence and - an orbital relay device is provided, flying in the direction of movement of the satellite at a distance behind the latter, which relays an uplink light beam transmitted by the ground station to the satellite or receives an uplink light beam transmitted by the ground station and transmits signals representing data of the uplink light beam or signals processed on the basis of data transmitted by the uplink light beam to the satellite, wherein the distance between the satellite and the relay device is determined on the basis of the difference in movement speed between the satellite and the ground station and on the basis of the distance of the satellite from the ground station, and - wherein the uplink light beam is transmitted from the ground station in the opposite direction to the direction of incidence of the downlink light beam and thus to the relay device.

[0017] The method according to the invention establishes bidirectional optical communication between a transmitter and a receiver. For this purpose, electromagnetic waves are used in the downlink and uplink. The electromagnetic waves are, in particular, coherent light waves, i.e., laser light, although the term "light" does not imply that the wavelengths must be in the visible range.

[0018] In the method according to the invention, bidirectional optical communication takes place without taking the lead angle (PAA) into account. This allows the downlink light beam and the uplink light beam to traverse the same atmospheric volume. The omission of the PAA is compensated for by an orbital relay device that flies at a suitable distance behind the satellite. The distance depends on the satellite speed (absolute and / or relative to the ground station), the signal transmission speed (speed of light), and the distance between the satellite and the ground station.

[0019] The PAA is calculated precisely, taking into account the OGS proper motion (like the GEO satellite, the Earth's surface also rotates eastward), to 17.6 µrad for an OGS at the equator. For an OGS closer to the pole, the proper motion is correspondingly somewhat smaller. Exactly at the pole, the PAA is 20.44 µrad; a typical value of 18 µrad is commonly used.

[0020] The distance covered by the GEO satellite within the PAA is accordingly a minimum of 621 m to a maximum of 858 m (the latter at the pole, where the distance between the OGS and the GEO satellite is also greater than at the equator), typically 700 m for an OGS at a typical latitude.

[0021] It's not so crucial to determine the exact distance between the GEO satellites and the relay, as this is relative to the uplink beam diameter. This is typically 10 µrad*H_orbit (the altitude of the GEO orbit is approximately 36,000 km), or approximately d / 2. The beam should therefore illuminate the relay with an accuracy of about 50 m.

[0022] The PAA is relatively independent of the GEO satellite's line of sight (the link's elevation angle above the horizon), as one always looks fairly perpendicular to the satellite's direction of travel (this can also be calculated precisely, as it is a deterministic problem). Therefore, the PAA is not critical for GEO satellites as far as the GEO satellite's position is concerned when it is in line of sight with the ground station. However, this is different for Low Earth Orbit (LEO) satellites, for which the PAA strongly depends on the satellite's elevation. However, even for a LEO satellite, the required distance to the relay device can be calculated, as this is a deterministic problem.

[0023] The application of the invention is interesting, for example, for the International Space Station (ISS) (orbital altitude approximately 380 km) as an example of a low-earth-orbit (LEO) satellite, since the ISS has such a large extension that it is in the range of the distance between the satellite and the relay device, meaning that both can be arranged on the ISS. Such a mechanically "fixed" connection between the satellite and the relay device is therefore also covered by the invention.

[0024] The orbital relay device can, for example, be a subsidiary satellite or similar additional satellite; however, a reflector that flies through the orbit independently or mechanically connected to the satellite is also conceivable and feasible. The relay device can also be designed as a receiving and transmitting unit that receives the uplink light beam from the ground station and, if necessary, forwards it to the satellite after processing. The communication connection between the satellites and the relay device can be wired or wireless (for example, as a free-space transmission channel). As already mentioned above, a mechanical connection between the satellite and the relay device is also conceivable but not absolutely necessary, provided the relay device has its own drive systems for correcting the trajectory.

[0025] In a further advantageous embodiment of the invention, it can therefore be provided 1. that the relay device is another satellite that communicates with the satellite transmitting the download light beam, 2. that the relay device has a receiver and that the receiver communicates with the satellite by wiring (e.g. cable, with simultaneous mechanical coupling) or wirelessly, in particular e.g. by a free-space transmission channel, 3. that the receiver is mechanically connected / coupled to the satellite (e.g. feasible with a LEO satellite of corresponding size such as ISS - see above) and / or 4. that the relay device is a reflector that reflects the uplink light beam towards the satellite.

[0026] In a further advantageous embodiment of the invention, it can further be provided that angle of incidence fluctuations caused by refractive index turbulences in the atmosphere, with which the downlink light beam reaches the ground station, are determined and that the uplink light beam is transmitted from the ground station in a pre-distorted manner by modulating the light beam in accordance with the angle of incidence fluctuation or deviation determined for the downlink light beam (from the expected direction of incidence which can be determined on the basis of the relative positions of the satellite and the ground station).

[0027] The invention can also be applied in a corresponding "reverse" manner, in that a satellite receives an uplink light beam and transmits a downlink light beam in the direction opposite to the direction of reception of the uplink light beam, where it then hits the ground behind the OGS from which the uplink light beam came and is received by a relay device and forwarded to the OGS.

[0028] The invention is explained in more detail below using an exemplary embodiment and with reference to the drawings. In detail: Fig. 1 to 3 illustrations of scenarios in which PAA<IPA und PAA> IPA applies, and Fig. 4 a schematic representation of an embodiment according to the invention for the method according to the invention.

[0029] In Fig.4, reference numeral 10 indicates a ground station for receiving and transmitting an optical (usually modulated) communications light beam. In the scenario described here, ground station 10 receives a downlink light beam 16 from a (particularly GEO) satellite 14 moving in the direction of arrow 12, which traverses the atmosphere 18. Due to the known relative positions of ground station 10 and satellite 14, the direction of incidence and the angle at which the downlink light beam 16 strikes ground station 10 are known. Ground station 10 transmits an uplink light beam 20, which also traverses the atmosphere 18.

[0030] The invention solves the potential problem that the isoplanarity angle (IPA) could be smaller than the lead angle (PAA) by having the ground station 10 transmit the uplink light beam in the opposite direction to the direction of incidence of the downlink light beam, whereby both light beams traverse the same atmospheric volume and are therefore exposed to refractive index turbulence in the same way. The inventive non-consideration of the PAA is compensated for by having a relay device 22 flying behind the satellite 14, which receives the uplink light beam 20 transmitted by the ground station 10. The relay device 22 relays the uplink light beam 20 to the satellite 14 (if necessary after processing the information or data of the modulated uplink light beam).The distance 24 results from the distances between satellite 14 and ground station 10 as well as from the satellite speed (and direction of movement relative to the ground station) and the speed of light.

[0031] According to the invention, the receive and transmit beams (downlink and uplink light beams) pass through exactly the same atmospheric volume, thus solving the problem of “IPA <PAA“ vollkommen gelöst ist. Die Berücksichtigung des Vorhaltewinkels ist nicht notwendig, da der Empfang des Uplink-Lichtstrahls beim Satelliten 14 durch beispielsweise einen „Tochtersatelliten“ o.dgl. Weiterleitvorrichtung 22 (beispielsweise ein Reflektor) geschieht, der exakt um den PAA-Winkel hinter dem Satelliten 14 hinterher fliegt, wobei beide beispielsweise mit einem Kabel zur Stromversorgung und zum Datenaustausch verbunden sein können; alternativ kann auch eine Freistrahl-Übertragungsstrecke für die Datenkommunikation vorgesehen sein. Für geostationäre Satelliten beträgt der Abstand zwischen dem Satelliten 14 und der Weiterleitvorrichtung 22 etwa 720 m (bei 40000 km Orbitalhöhe und einem dabei gegebenen PAA-Winkel von 18 µrad).

[0032] The invention solves the potential problem that the isoplanasia angle (IPA) could be smaller than the lead angle (PAA), which now allows optimal and without loss of efficiency - Pointing by tracking, - Predistortion Adaptive Optics and / or - Channel Reciprocity ([1] and [2]) apply.

[0033] The transmission quality in an optical feeder link to GEO and LEO satellites can be significantly improved by applying one or more of the three measures mentioned above. LIST OF ABBREVIATIONS GEO geostationary GFL GEO feeder link OGFL optical GFL OGS optical ground station BIT refractive index turbulence (also IRT index of refraction turbulence) IPA isoplanatic angle PAA lead angle (point-ahead angle) LEO Low Earth Orbit BIBLIOGRAPHY [1] N. Perlot, D. Giggenbach, Scintillation correlation between forward and return spherical waves, Applied Optics, Vol. 51, No. 15, Optical Society of America, May 20, 2012 [2] D. Giggenbach, W. Coley, K. Grant, N. Perlot, Experimental verification of the limits of optical channel intensity reciprocity, Applied Optics, Vol. 51, No. 16, Optical Society of America, June 1, 2012 LIST OF REFERENCE SYMBOLS 10 Ground station 12 Direction of movement of the satellite 14 (especially GEO) satellite 16 Downlink light beam 18 Atmosphere 20 uplink light beam 22 Forwarding device 24 distance

Claims

[1] Method for establishing bidirectional optical communication between a ground station (10) and a satellite (14), in particular a geostationary satellite, to which a lead angle (PAA) is assigned by which an uplink light beam (20) to be sent from the ground station (10) to the satellite (14) is offset relative to the direction of incidence of a downlink light beam (16) previously sent from the satellite (14) to the ground station (10), wherein in the method - a satellite (14) sends a downlink light beam (16) through the atmosphere to a ground station (10) in an incoming direction and - an orbital relay device (22) flying in the direction of movement (12) of the satellite (14) at a distance (24) behind it is provided, which - receives an uplink light beam (20) sent from the ground station (10) and forwards it to the satellite (14) or - receives an uplink light beam (20) sent from the ground station (10) and transmits signals representing data of the uplink light beam (20) to the satellite (14) or - receives an uplink light beam (20) sent from the ground station (10) and transmits signals, which are processed by the relay device (22) on the basis of data transmitted by the uplink light beam (20), to the satellite (14), - wherein the distance (24) between the satellite (14) and the relay device (22) is determined based on the difference in speed of movement between the satellite (14) and the ground station (10) and based on the distance of the satellite (14) to the ground station (10) and thus based on the lead angle (PAA) assigned to the satellite (14), and - the uplink light beam (20) is sent from the ground station (10) at a leading angle (PAA) of zero degrees and thus in the opposite direction to the direction of incidence of the downlink light beam (16) to the relaying device (22) and is received by it. [2] Method according to claim 1, wherein an angle of incidence variation caused by refractive index turbulence of the atmosphere (18) is determined, with which the downlink light beam (16) reaches the ground station (10), and wherein the uplink light beam (20) is pre-distorted by modulation from the ground station (10) according to the angle of incidence variation determined for the downlink light beam (16). [3] Method according to claim 1 or 2, wherein the relay device (22) is a further satellite which communicates with the satellite (14) emitting the downlink light beam (16). [4] Method according to any one of claims 1 to 3, wherein the forwarding device (22) has a receiver and wherein the receiver communicates with the satellite (14) via a wired connection. [5] Method according to claim 4, wherein the receiver is mechanically connected to the satellite (14). [6] Method according to any one of claims 1 to 3, wherein the forwarding device (22) has a receiver and wherein the receiver communicates wirelessly with the satellite (14). [7] Method according to claim 6, wherein the receiver communicates with the satellite (14) via a free-ray transmission channel. [8] Method according to claim 1, wherein the forwarding device (22) is a reflector that reflects the uplink light beam (20) towards the satellite (14).

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