Method and apparatus for performing optical beam correction in a satellite communication system

Abstract

The technology disclosed in this specification relates to the correction of an optical beam (laser beam) in a satellite communication system based on the use of a beacon satellite that redirects an optical downlink reference beam from a communication satellite so that it can be received by a ground terminal. The beacon satellite flies on the same orbit in front of the communication satellite by a distance corresponding to the pointing angle of arrival (PAA) that a ground terminal is expected to use for transmitting an optical uplink communication beam. Thus, the optical downlink reference beam serves as a direct reference for the ground terminal to detect wavefront distortion associated with the atmospheric path through which the optical uplink communication beam passes.

Description

Technical Field

[0001] The technology disclosed in this specification relates to the correction of the influence of the atmosphere on the optical uplink beam of a satellite communication system.

Background Art

[0002] FIG. 1 shows a typical satellite communication system 10, which includes a ground terminal 12 that receives an optical downlink communication beam 14 from a communication satellite 16 moving along a defined orbital path 18. Since the satellite 16 moves relative to the ground terminal 12, the ground terminal 12 transmits an optical uplink communication beam 20 towards the communication satellite 16 using a pointing angle (PAA).

[0003] The need for PAA is due to the crossing speed between the satellite 16 and the Earth. When a signal is transmitted from the ground terminal 12 to the satellite 16, the satellite moves along its orbital path 18. In order for the optical uplink communication beam 20 to reach the satellite 16 reliably, the ground terminal 12 must predict where the satellite will be when the optical uplink communication beam arrives and aim slightly ahead of the current position of the satellite 16. PAA is the angle between the line of sight and the future position. Using the annotations in the figure, PAA explains the movement of the satellite 16 along the orbital path 18 from time t0 to time t1.

[0004] For example, the pointing angle for a GEO satellite is approximately 18.5 microradians. A GEO satellite moves approximately 800 meters during the uplink and downlink flights. One effect of PAA is that the atmospheric path through which the optical downlink communication beam 14 passes cannot accurately represent the atmospheric path through which the optical uplink communication beam 20 passes. This fact limits the ability of the ground terminal 12 to use the optical downlink communication beam 14 as a reference for determining the correction to be applied to the optical uplink communication beam 20. Here, the desired correction for the optical uplink communication beam mitigates the wavefront distortion imparted to the optical uplink communication beam by the atmospheric turbulence in the near field of the ground terminal 12.

[0005] In an ideal case, the ground terminal 12 can accurately estimate the wavefront distortion imparted by the atmosphere to its optical uplink communication beam 20. Correspondingly, by applying inverse wavefront distortion (predistortion) when transmitting the optical uplink communication beam 20, the ground terminal 12 mitigates the atmospheric distortion.

[0006] One approach for more accurately estimating the uplink atmospheric path relies on the transmission by the ground terminal 12 of an excitation beam in the direction of aberration to create an "artificial star". This star results from the excitation by a laser beam focused on sodium atoms in the mesosphere, and the ground terminal evaluates the faint light returning from the artificial star for the estimation of the uplink path. However, the artificial star approach has several drawbacks in a free space optical communication environment, such as the need to transmit a relatively high-power laser from the ground terminal dedicated to both day and night links, and during the day being dominated by background light and the returning sodium fluorescence signal level being significantly reduced. SUMMARY OF THE INVENTION

[0007] The technology disclosed herein relates to the correction of an uplink optical communication beam (laser beam) in a satellite communication system based on the use of a beacon satellite to redirect an optical downlink reference beam from a communication satellite for reception by a ground terminal. The beacon satellite flies ahead of the communication satellite on the same orbit by a distance corresponding to the predicted angle of arrival (PAA) that the ground terminal is expected to use for transmitting the optical uplink communication beam, and the optical downlink reference beam provides a direct basis for the ground terminal to detect the wavefront distortion associated with the atmospheric path through which the optical uplink communication beam passes.

[0008] Exemplary embodiments include a method of operation by a ground terminal of a satellite communication system. The method includes receiving an optical downlink communication beam transmitted by a communication satellite towards the ground terminal and, simultaneously, receiving an optical downlink reference beam transmitted by the communication satellite towards a beacon satellite. The beacon satellite precedes the communication satellite by a defined distance along the same orbit, and the beacon satellite redirects the optical downlink reference beam towards the ground terminal. Further, the method includes the ground terminal controlling the wavefront pre-distortion of an optical uplink communication beam transmitted by the ground terminal towards the communication satellite as a function of the wavefront distortion detected in the optical downlink reference beam.

[0009] Related embodiments include a ground terminal of a satellite communication system. The ground terminal includes one or more optical receivers and optical transmitters. The one or more optical receivers are configured to receive an optical downlink communication beam transmitted by a communication satellite towards the ground terminal and, simultaneously, receive an optical downlink reference beam. The communication satellite transmits the optical downlink reference beam towards a beacon satellite that precedes the communication satellite by a defined distance along the same orbit, and the beacon satellite redirects the optical downlink reference beam towards the ground terminal. The optical transmitter of the ground terminal is configured to control the wavefront pre-distortion of an optical uplink communication beam transmitted towards the communication satellite as a function of the wavefront distortion detected in the optical downlink reference beam.

[0010] Another embodiment includes an operating method by a beacon satellite and a communication satellite of a satellite communication system. This method includes a communication satellite transmitting an optical downlink communication beam toward a ground terminal and simultaneously transmitting an optical downlink reference beam toward a beacon satellite, and the beacon satellite flying in formation with the communication satellite along the same orbital path by a defined distance depending on the orbit of the communication satellite as a mother ship satellite. This method further includes the beacon satellite operating as a much smaller passive daughter satellite and redirecting the optical downlink reference beam transmitted from the communication satellite toward the ground terminal. Then, the ground terminal uses this redirected and downlinked signal when determining the wavefront pre-distortion applied to the optical uplink communication beam transmitted from the ground terminal to the communication satellite.

[0011] In at least one embodiment, this method further includes, at the ground terminal, receiving the optical downlink communication beam and the optical downlink reference beam, and controlling the wavefront pre-distortion of the optical uplink communication beam transmitted by the ground terminal for the communication satellite as a function of the wavefront distortion detected in the optical downlink reference beam. For example, the ground terminal includes a deformable mirror or other adaptive optics for controlling the wavefront pre-distortion.

[0012] In another exemplary embodiment, a satellite communication system includes a communication satellite and a beacon satellite. The communication satellite is configured to transmit an optical downlink communication beam toward a ground terminal and simultaneously transmit an optical downlink reference beam toward the beacon satellite when the beacon satellite is flying ahead of the communication satellite along the same orbit by a defined distance. The beacon satellite is configured to redirect the optical downlink reference beam from the beacon satellite toward the ground terminal, which is used when the ground terminal determines the wavefront prediction distortion applied to the optical uplink communication beam transmitted from the ground terminal to the communication satellite.

[0013] The satellite communication system can further include a ground terminal. In one or more embodiments, the ground terminal is configured to (a) receive an optical downlink communication beam and an optical downlink reference beam at the ground terminal, and (b) control the wavefront pre-distortion of the optical uplink communication beam transmitted by the ground terminal to the communication satellite as a function of the wavefront distortion detected in the optical downlink reference beam.

[0014] Of course, the present invention is not limited to the above features and advantages. In fact, those skilled in the art will recognize additional features and advantages upon reading the following detailed description and viewing the accompanying drawings.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0016] Figure 2 shows an exemplary satellite communication system 30 according to one embodiment. The satellite communication system 30 includes a communication satellite 32 and a beacon satellite 34. During operation, the beacon satellite 34 flies a defined distance "d" in front of the communication satellite 32 along the same orbit as the orbit 36 followed by the communication satellite 32. The defined distance d takes into account the speed and altitude of the communication satellite 32 and corresponds to the pointing angle of arrival (PAA) used for the ground terminal 40 to transmit the optical uplink communication beam 42. Here, the term "optical beam" as used herein and in other parts of the present disclosure means a laser beam, and "optical communication beam" means a laser beam that carries communication data (e.g., user traffic).

[0017] The communication satellite 32 is configured to transmit an optical downlink communication beam 44 so that the ground terminal 40 can receive it directly, and the ground terminal 40 is configured to transmit an optical uplink communication beam 42 so that the communication satellite 32 can receive it directly. Figure 2 represents a single point in time, and it will be understood that the communication satellite 32 has moved to the position occupied by the beacon satellite 34 during the time it takes for the optical uplink communication beam 42 to reach the communication satellite 32.

[0018] According to the above arrangement, the communication satellite 32 and the beacon satellite 34 cooperate to provide the ground terminal 40 with an optical downlink reference beam 46 that moves along the downlink path corresponding to the uplink path of the optical uplink communication beam 42. In particular, the optical downlink reference beam 46 takes into account the PAA used by the ground terminal 40.

[0019] To realize this relationship, the communication satellite 32 is configured to transmit the optical downlink reference beam 46 towards the ground terminal 40 and, at the same time, transmit the optical downlink communication beam 44 towards the beacon satellite 34. The beacon satellite 34 is configured to redirect the optical downlink reference beam 46 towards the ground terminal 40. In one or more embodiments, the redirection of the optical downlink communication beam 44 depends on the passive redirection element 38 mounted on the beacon satellite 34. Exemplary passive redirection elements include reflective cones, combinations of mirrors, prism-shaped passive optical systems, or other passive optical redirection means. In one or more embodiments, the passive redirection element 38 is steerable for pointing adjustment.

[0020] In the illustrated embodiment, the ground terminal 40 includes an interface circuit 50 configured to receive traffic and control signals from one or more terrestrial network nodes 52 of the satellite communication system 30 for transmission to the communication satellite 32 via the optical uplink communication beam 42. Further, the interface circuit 50 is configured to transmit the traffic and control signals received from the communication satellite 32 via the optical downlink communication beam 44. Such traffic may be related to user traffic input and output from one or more external networks 54 such as the Internet.

[0021] The interface circuit 50 comprises a physical layer circuit for performing wired or wireless transmission and reception on a medium used to interconnect the terrestrial terminal 40 with one or more terrestrial network nodes 52. In at least one embodiment, the interface circuit 50 comprises a data network interface circuit and an associated protocol processor. Further elements of the terrestrial terminal 40 include an optical transceiver 56 that is locally communicatively coupled to the interface circuit 50 for receiving traffic and control signaling transmitted via the optical uplink communication beam 42, recovering traffic and control signaling arriving on the optical downlink communication beam 44, and transferring such information to the terrestrial network node(s) 52.

[0022] The optical transceiver 56 includes one or more optical receivers and optical transmitters. FIG. 3 shows an exemplary apparatus including an optical receiver 60 that outputs a received signal 62, for example, in the electrical domain, corresponding to information transmitted via the optical downlink communication beam 44. The communication circuit 64 may be included in the interface circuit 50 shown in FIG. 2 and processes the received signal 62 to couple back to the terrestrial network node(s) 52 via the interface circuit 50.

[0023] The communication circuit 64 also outputs a transmission signal 66, for example, in the electrical domain, and the transmission signal carries data for transmission by the communication satellite 32. The optical transmitter 68 forms the optical uplink communication beam 42 in response to the transmission signal 66, such as by modulating a source laser beam according to the transmission signal 66.

[0024] In one or more embodiments, the optical downlink communication beam 44 and the optical downlink reference beam 46 are of different wavelengths, and the optical receiver 60 includes an optical filter 70 or other optical discriminator for separating the two beams into respective optical paths 72 and 74. The optical path 72 is associated with the reception of the optical downlink communication beam 44, and the optical path 74 is associated with the use of the optical downlink reference beam 46 for the determination of the predistortion applied in the transmission of the optical uplink communication beam 42. Correspondingly, the reference numeral "76" in FIG. 3 indicates the internal routing or directioning of the received optical downlink reference beam 46 for use in determining the predistortion for the optical uplink communication beam 42.

[0025] FIG. 4 shows selected details focusing on the use of the optical downlink reference beam 46 for determining the predistortion applied in the transmission of the optical uplink communication beam 42 to mitigate the effects of the atmosphere.

[0026] The optical telescope 82 associated with the optical head assembly 80 receives and transmits beams. The telescope 82 includes a lens 84, and further optical elements include a deformable mirror 86 and a dichroic beam splitter 88 as an example of the optical filter 70 shown in FIG. 3. The optical elements further include an aberration correction mirror 90 and a lens 92.

[0027] In operation, the dichroic beam splitter 88 directs the received optical downlink reference beam 46 onto the lens 92, illuminating the wavefront sensor 94. The wavefront sensor 94 outputs a signal 96 indicative of the detected wavefront distortion of the received optical downlink reference beam 46 to the real-time processor 98. These distortions result from the turbulence and other atmospheric effects that affect the optical downlink reference beam 46 along the atmospheric path through which the optical downlink reference beam 46 passes.

[0028] The real-time processor 98 generates a control signal 100 for controlling the deformable mirror 86 so as to impart distortion to the outgoing optical uplink communication beam 102. The outgoing optical uplink communication beam 102 is a distorted version of the optical uplink communication beam 42, and the applied distortion is inverse with respect to the distortion detected by the wavefront sensor 94. Thus, the deformable mirror 86 can be understood as an example of an adaptive optical system used by the ground terminal 40 to transmit the optical uplink communication beam 42 using wavefront pre-distortion that mitigates the atmospheric effects of the atmospheric path through which the optical uplink communication beam 42 passes.

[0029] The real-time processor 98 comprises fixed circuitry or programmatically configured circuitry or a hybrid of both. In one or more embodiments, the real-time processor 98 comprises digital processing circuitry including any one or more of one or more microprocessors, one or more digital signal processors, one or more field programmable gate arrays, one or more complex programmable logic devices, or one or more application specific integrated circuits. In at least one embodiment, all or part of the real-time processor 98 comprises digital processing circuitry specially adapted to control the deformable mirror 86 in response to wavefront distortion detected via the wavefront sensor 94 based on the execution of stored computer program instructions.

[0030] FIG. 5 shows a method 500 of operation by the ground terminal 40 of the satellite communication system 30. The method 500 includes the ground terminal 40 receiving an optical downlink communication beam 44 transmitted by the communication satellite 32 towards the ground terminal 40 (block 502), and simultaneously receiving an optical downlink reference beam 46 transmitted by the communication satellite 32 towards a beacon satellite 34 that precedes the communication satellite 32 by a defined distance along the same orbital path 36. The beacon satellite 34 redirects the optical downlink reference beam 46 towards the ground terminal 40.

[0031] Furthermore, method 500 includes the ground terminal 40 controlling the wavefront pre-distortion of the optical uplink communication beam 42 transmitted by the ground terminal 40 to the communication satellite 32 as a function of the wavefront distortion detected in the optical downlink reference beam 46 (block 504). In at least one embodiment, controlling the wavefront pre-distortion of the optical uplink communication beam 42 includes continuously detecting the wavefront distortion of the optical downlink reference beam 46 via the wavefront distortion detector 84, determining a complementary wavefront pre-distortion for application to the optical uplink communication beam 42, and applying the complementary wavefront pre-distortion to the optical uplink communication beam 42 via the adaptive optics of the optical transmitter 68 of the ground terminal 40. The adaptive optics includes, for example, a deformable mirror 86.

[0032] In one or more embodiments, method 500 includes the ground terminal 40 receiving the optical downlink communication beam 44 and the optical downlink reference beam 46 via the same optical receiver 60 of the ground terminal 40. The optical receiver 60 in at least one such embodiment includes a first optical path 72 for processing the optical downlink communication beam 44 and a second optical path 74 for directing the optical downlink reference beam 46 for wavefront distortion detection.

[0033] The optical downlink communication beam 44 and the optical downlink reference beam 46 are of different wavelengths in one or more embodiments, and method 500 includes the ground terminal 40 performing wavelength-based filtering to direct the optical downlink communication beam 44 to the first optical path 72 and the optical downlink reference beam 46 to the second optical path 74. In one or more other embodiments, method 500 includes the ground terminal 40 distinguishing the optical downlink reference beam 46 from the optical downlink communication beam 44 based on the optical downlink reference beam 46 being modulated at a particular rate.

[0034] One advantage of the various apparatuses shown as examples in this specification is that the defined distance by which the beacon satellite 34 precedes the communication satellite 32 corresponds to the PAA used by the ground terminal 40 for the transmission of the optical uplink communication beam 42. Thus, the wavefront distortion detected in the optical downlink reference beam 46 corresponds to the atmospheric path defined by the PAA. That is, the optical downlink reference beam 46 and the optical uplink reference beam 42 are transmitted along substantially the same atmospheric path. As a result, the ground terminal 40 can continuously detect the wavefront distortion of the optical downlink reference beam 46 during reception of that beam and apply the corresponding inverse distortion to the outgoing optical uplink communication beam 42.

[0035] Another advantage is that the beacon satellite 34 does not need to be complex. Except for basic telemetry and guidance functions, it only needs to be equipped with a passive redirection element 38 for redirecting the optical downlink reference beam 46 from the communication satellite 32 towards the ground terminal 40. As described above, the passive redirection element 38 may be a mirror for reflective redirection. Other options include prism elements or lenses for transmissive redirection.

[0036] In one or more embodiments, the ground terminal 40 is a satellite access node (SAN) of the satellite communication system 30. Such an embodiment is shown in FIG. 2. Correspondingly, in such an embodiment, the optical downlink communication beam 44 is an optical feeder downlink signal, and the optical uplink communication beam 42 is an optical feeder uplink signal. The terms "optical earth station" and "OGS" can be used interchangeably with a satellite access node or SAN.

[0037] FIG. 6 shows an operating method 600 by a satellite communication system 30 including a beacon satellite 34 and a communication satellite 32. The method 600 includes transmitting an optical downlink communication beam 44 from the communication satellite 32 towards the ground terminal 40 (block 602), and simultaneously transmitting an optical downlink reference beam 46 towards the beacon satellite 34. During operation, the beacon satellite 34 flies ahead of the communication satellite 32 by a defined distance along the same orbital path 36. The defined distance corresponds to the PAA used by the ground terminal 40 for transmitting an optical uplink communication beam 42 to be received by the communication satellite 32.

[0038] The method 600 further includes redirecting the optical downlink reference beam 46 from the beacon satellite 34 towards the ground terminal 40 (block 604). The redirection is based on, for example, the use of a simple optical redirection element 38 mounted on the beacon satellite 34. The redirection of the optical downlink reference beam 46 is for use by the ground terminal 40 in determining the wavefront pre-distortion applied by the ground terminal 40 to the optical uplink communication beam 42 transmitted by the ground terminal 40 for the communication satellite 32.

[0039] The method 600 in one or more embodiments includes controlling the communication satellite 32 and / or the beacon satellite 34 to maintain the beacon satellite 34 at a defined distance from the communication satellite 32. The control may be based on transmitting a control signal from the ground to the beacon satellite 34. Alternatively, the ground terminal 40 and / or other ground nodes of the satellite communication system 30 are configured to transmit a control signal to the communication satellite 32, and the communication satellite 32 transmits it to the beacon satellite 34 via an inter-satellite link.

[0040] In one or more embodiments, method 600 is considered to include the operations depicted for method 500. That is, method 600 can include operations at ground terminal 40 that include (a) receiving an optical downlink communication beam 44 from communication satellite 32 and an optical downlink reference beam 46 redirected by beacon satellite 34, and (b) controlling wavefront pre-distortion of optical uplink communication beam 42 transmitted by ground terminal 40 to communication satellite 32 as a function of wavefront distortion detected in optical downlink reference beam 46.

[0041] Controlling the wavefront pre-distortion includes ground terminal 40 continuously detecting, e.g., via wavefront distortion detector 84, the wavefront distortion of optical downlink reference beam 46 and determining complementary wavefront pre-distortion for application to optical uplink communication beam 42. These complementary inverse wavefront pre-distortions are applied, e.g., via an adaptive optics system.

[0042] Generally, according to the disclosed technology, a beacon satellite flies in formation with a primary communication satellite and, in one or more embodiments, is passive and includes only an optical system for redirecting a beacon beam from the communication satellite. This arrangement has several advantages, such as reduced power requirements, since the beacon satellite (also referred to as a "dormant" or "companion" satellite) does not need to carry an active beacon laser.

[0043] In particular, modifications and other embodiments of the disclosed invention(s) will be apparent to those skilled in the art who benefit from the teachings presented in the foregoing description and the related drawings. Accordingly, it is to be understood that the invention(s) is / are not limited to the specific embodiments disclosed and that alterations and other embodiments are intended to be included within the scope of the present disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. An operating method by a ground terminal of a satellite communication system, the method comprising: Receiving an optical downlink communication beam transmitted by a communication satellite towards the ground terminal, and simultaneously receiving an optical downlink reference beam transmitted by the communication satellite towards a beacon satellite that precedes the communication satellite by a predetermined distance along the same orbit, wherein the beacon satellite redirects the optical downlink reference beam towards the ground terminal; Controlling wavefront pre-distortion of an optical uplink communication beam transmitted by the ground terminal towards the communication satellite as a function of wavefront distortion detected by the optical downlink reference beam. A method comprising the above.

2. Controlling the wavefront pre-distortion of the optical uplink communication beam comprises continuously detecting the wavefront pre-distortion of the optical downlink reference beam via a wavefront sensor, determining a complementary wavefront pre-distortion for application to the optical uplink communication beam, and applying the complementary wavefront pre-distortion to the optical uplink communication beam via an adaptive optical system in an optical transmitter of the ground terminal. The method according to claim 1.

3. The method according to claim 2, wherein the adaptive optical system comprises a deformable mirror.

4. The method according to any one of claims 1 to 3, comprising receiving the optical downlink communication beam and the optical downlink reference beam via the same optical receiver of the ground terminal, the optical receiver comprising a first optical path for processing the optical downlink communication beam and a second optical path for processing the optical downlink reference beam.

5. The optical downlink communication beam and the optical downlink reference beam have different wavelengths, and the method comprises performing wavelength-based filtering to direct the optical downlink communication beam to the first optical path and the optical downlink reference beam to the second optical path. The method according to claim 4.

6. The method according to claim 4, comprising the ground terminal distinguishing the optical downlink reference beam from the optical downlink communication beam based on the optical downlink reference beam being modulated at a specific speed.

7. The method according to any one of claims 1 to 3, including receiving the optical downlink communication beam and the optical downlink reference beam via respective optical receivers of the terrestrial terminal.

8. The method according to any one of claims 1 to 7, wherein the specified distance by which the beacon satellite precedes the communication satellite corresponds to the angle of expectation used by the terrestrial terminal for transmitting the optical uplink communication beam.

9. The method according to any one of claims 1 to 7, wherein the wavefront distortion detected by the optical downlink reference beam corresponds to the optical uplink path defined by the angle of expectation.

10. The method according to any one of claims 1 to 9, wherein the terrestrial terminal is a satellite access node (SAN) of the satellite communication system, the optical downlink communication beam is an optical feeder downlink signal, and the optical uplink communication beam is an optical feeder uplink signal.

11. A method of operating a satellite communication system including a beacon satellite and a communication satellite, transmitting an optical downlink communication beam from the communication satellite towards the terrestrial terminal, and simultaneously transmitting an optical downlink reference beam towards the beacon satellite, wherein the beacon satellite precedes the communication satellite along the same orbit by a specified distance; redirecting the optical downlink reference beam from the beacon satellite towards the terrestrial terminal, for use by the terrestrial terminal in determining the wavefront pre-distortion to be applied to the optical uplink communication beam transmitted by the terrestrial terminal to the communication satellite; including.

12. The method according to claim 11, wherein the specified distance by which the beacon satellite precedes the communication satellite corresponds to the angle of expectation used by the terrestrial terminal for transmitting the optical uplink communication beam.

13. The method according to claim 12 or 13, wherein the specified distance depends on the altitude of the communication satellite.

14. The method according to any one of claims 11 to 13, wherein the optical downlink communication beam and the optical downlink reference beam have different optical wavelengths and can be separated by the terrestrial terminal.

15. The method according to any one of claims 10 to 13, wherein the optical downlink reference beam is transmitted at a specific modulation rate and the ground terminal can separate two beams.

16. The method according to any one of claims 10 to 15, further comprising controlling the communication satellite and the beacon satellite to maintain the beacon satellite at the specified distance from the communication satellite.

17. The method according to any one of claims 10 to 16, wherein the redirection of the optical downlink reference beam includes reflecting the optical downlink reference beam through a mirror mounted on the beacon satellite.

18. The method according to any one of claims 10 to 17, wherein a reflection element mounted on the beacon satellite enables the redirection of the optical downlink reference beam, and the method further comprises actively adjusting the pointing angle of the reflection element.

19. The method according to any one of claims 10 to 18, wherein the ground terminal is a satellite access node (SAN) of the satellite communication system, the optical downlink communication beam is an optical feeder downlink signal, and the optical uplink communication beam is an optical feeder uplink signal.

20. The satellite communication system further comprises the ground terminal, and the method further comprises the ground terminal receiving the optical downlink communication beam from the communication satellite and the optical downlink reference beam redirected by the beacon satellite, controlling the wavefront pre-distortion of the optical uplink communication beam transmitted by the ground terminal to the communication satellite as a function of the wavefront distortion detected by the optical downlink reference beam, The method according to any one of claims 10 to 19, including.

21. Controlling the wavefront pre-distortion continuously includes detecting the wavefront distortion of the optical downlink reference beam via a wavefront sensor, determining a complementary wavefront pre-distortion for application to the optical uplink communication beam, and applying the complementary wavefront pre-distortion to the optical uplink communication beam via an adaptive optics subsystem within the ground terminal. The method according to claim 20.

22. The method according to claim 21, wherein the adaptive optics system comprises a deformable mirror.

23. The method includes receiving the optical downlink communication beam and the optical downlink reference beam via the same optical receiver of the ground terminal, the optical receiver including a first optical path for processing the optical downlink communication beam and a second optical path for processing the optical downlink reference beam, the method according to any one of claims 20 to 22.

24. The optical downlink communication beam and the optical downlink reference beam occupy different optical wavelengths, and the method includes performing wavelength-based filtering at the ground terminal to direct the optical downlink communication beam to the first optical path and the optical downlink reference beam to the second optical path, the method according to claim 23.

25. The method includes receiving the optical downlink communication beam and the optical downlink reference beam via respective optical receivers of the ground terminal, the method according to claims 20 to 22.

26. The wavefront pre-distortion matches the characteristics estimated for the optical path between the ground terminal and the communication satellite, the method according to any one of claims 20 to 25.

27. The specified distance by which the beacon satellite precedes the communication satellite corresponds to the angle of view used by the ground terminal for transmitting the optical uplink communication beam, the method according to any one of claims 20 to 26.

28. The ground terminal is a satellite access node (SAN) of the satellite communication system, the optical downlink communication beam is an optical feeder downlink signal, and the optical uplink communication beam is an optical feeder uplink signal, the method according to any one of claims 20 to 27.

29. The specified distance depends on the altitude of the communication satellite, the method according to any one of claims 20 to 28.

30. The method further includes controlling the communication satellite and the beacon satellite to maintain the beacon satellite at the predetermined distance from the communication satellite, the method according to any one of claims 20 to 29.

31. A ground terminal of a satellite communication system, the ground terminal One or more optical receivers configured to receive an optical downlink communication beam transmitted from the communication satellite toward the ground terminal and, at the same time, receive an optical downlink reference beam transmitted from the communication satellite along the same orbit toward a beacon satellite that precedes the communication satellite by a predetermined distance, wherein the beacon satellite redirects the optical downlink reference beam to the ground terminal, and the optical receiver; An optical transmitter configured to control wavefront pre-distortion of an optical uplink communication beam transmitted toward the communication satellite as a function of wavefront distortion detected by the optical downlink reference beam; A ground terminal comprising:

32. The ground terminal according to claim 31, wherein the optical transmitter is configured to continuously detect the wavefront distortion of the optical downlink reference beam via a built-in wavefront sensor, determine a complementary wavefront distortion for application to the optical uplink communication beam, and apply the complementary wavefront distortion to the optical uplink communication beam via an adaptive optical system in the optical transmitter, thereby controlling the wavefront pre-distortion of the optical uplink communication beam.

33. The ground terminal according to claim 32, wherein the adaptive optical system includes a deformable mirror.

34. The ground terminal according to any one of claims 31 to 33, wherein the one or more optical receivers include an optical receiver having a first optical path for processing the optical downlink communication beam and a second optical path for processing the optical downlink reference beam.

35. The ground terminal according to claim 34, wherein the optical downlink communication beam and the optical downlink reference beam have different wavelengths, and the optical receiver includes a filter that guides the optical downlink communication beam to the first optical path and the optical downlink reference beam to the second optical path.

36. The ground terminal according to claim 34, wherein the one or more optical receivers are configured to distinguish the optical downlink reference beam from the optical downlink communication beam based on the optical downlink reference beam being modulated at a specific speed.

37. The ground terminal according to any one of claims 31 to 33, wherein the one or more optical receivers include respective optical receivers for receiving the optical downlink communication beam and the optical downlink reference beam.

38. The predetermined distance by which the beacon satellite precedes the communication satellite corresponds to the angle of expectation that the ground terminal is expected to use for transmitting the optical uplink communication beam, for the ground terminal according to any one of claims 31 to 37.

39. The wavefront distortion detected by the optical downlink reference beam corresponds to the optical uplink path defined by the angle of expectation, for the ground terminal according to any one of claims 31 to 38.

40. The ground terminal is a satellite access node (SAN) of the satellite communication system, the optical downlink communication beam is an optical feeder downlink signal, and the optical uplink communication beam is an optical feeder uplink signal, for the ground terminal according to any one of claims 31 to 39.

41. A communication satellite, and a beacon satellite A satellite communication system comprising: The communication satellite is configured to transmit an optical downlink communication beam from the communication satellite towards the ground terminal, and at the same time, transmit an optical downlink reference beam towards the beacon satellite that flies a predetermined distance forward along the same orbit as the communication satellite. The beacon satellite is configured to redirect the optical downlink reference beam from the beacon satellite towards the ground terminal, and the ground terminal is used when determining the wavefront pre-distortion applied by the ground terminal to the optical uplink communication beam transmitted to the communication satellite by the ground terminal. Communication satellite system.

42. When the beacon satellite and the communication satellite are in orbit, the predetermined distance by which the beacon satellite precedes the communication satellite corresponds to the angle of expectation used by the ground terminal for transmitting the optical uplink communication beam, for the satellite communication system according to claim 41.

43. Further comprising the ground terminal, the ground terminal receives the optical downlink communication beam and the optical downlink reference beam at the ground terminal, is configured to control the wavefront pre-distortion of the optical uplink communication beam transmitted by the ground terminal to the communication satellite as a function of the wavefront distortion detected by the optical downlink reference beam. The satellite communication system according to claim 41 or 42.

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