Dual optical heads for optical feeder link in a satellite communications system

Abstract

A laser communications terminal (LCT) includes separate optical head assemblies (OHAs) for optical signal transmission and reception, with respective pointing systems allowing separate pointing control of the respective OHAs. In a satellite communications system (SCS) having an optical ground station (OGS) configured for communicatively coupling with a satellite of the SCS via an optical feeder link (OFL), the satellite and / or the OGS implements a respective dual OHA (DOHA). In one or more embodiments, the DOHA is configured for the use of different optical wavelengths with respect to optical communications beams on the uplink and downlink. Among several advantages flowing from use of the DOHA is isolation of the optical paths carrying the outgoing optical beam(s) and the incoming optical beam(s), along with the ability to optimize and simplify the corresponding optics. Further, providing separate pointing control relieves the need for manufacturing the DOHA with precise inter-head alignment.

Description

DUAL OPTICAL HEADS FOR OPTICAL FEEDER LINK IN A SATELLITE COMMUNICATIONS SYSTEMTECHNICAL FIELD

[0001] Techniques disclosed herein relate to a laser communication terminal (LCT) with dual optica] heads and the use thereof for an optical feeder link (OFL) of a satellite communications system (SCS).BACKGROUND

[0002] Optical feeder links (OFLs) represent a key innovation in the deployment of satellite communications systems (SCSs) that use high-throughput satellites of the sort needed to provide evolving communication services, such as broadband Internet access. The term “feeder” refers to communications going between a ground station and a satellite, including forward link traffic transmitted from the ground station to the satellite, for transmission to respective terminals within a population of terminals served by the satellite. Feeder communications further include reverse link traffic transmitted from respective terminals to the satellite, for relay or retransmission from the satellite to the ground station. Here, a ground station may be understood as one endpoint of the satellite link, with each terminal being a respective other endpoint of the satellite link. Terminals are, for example, user terminals belonging, or assigned, to subscribers of the SCS.

[0003] In a typical implementation, an OFL comprises one or more uplink optical beams going from the ground station to the satellite, and one or more downlink optical beams going from the satellite to the ground station. The term “optical beam” refers to the directed or guided propagation of light waves. For example, a laser- based emission source outputs light that is then focused and guided in a particular direction, where example guiding operations include any one or more of collimating, focusing, filtering, splitting, deflecting, and redirecting. The light used to form an optical beam may be unmodulated or it may be modulated according to data or control signaling, such that the optical beam is information bearing. In this sense, an optical beam may or may not convey an optical signal.

[0004] An optical beam may contain light at a single optical wavelength or it may contain light at more than one optical wavelength. For example, an optical communications beam transmitted on the OFL between a ground station and a satellite as part of the OFL may contain light at multiple optical wavelengths, with each wavelength being a different optical channel signal and with each optical channel signal being modulated light corresponding to a respective information stream that itself may comprise multiplexed forward traffic for multiple terminals.

[0005] Further, more than one optical beam may be transmitted along the same optical path, but possibly with different divergence. For example, an OGS may transmit an optical uplink beacon beam and an optical uplink communications beam along the same optical path, such that the satellite receives a combined or composite optical beam containing light at the wavelength(s) used to form the optical uplink beacon beam and at the wavelength(s) used to form the optical uplink communications beam. The optical paths onboard the satellite used for the reception of such composite optical beams may be configured for beam separation, allowing for separate treatment or use of the constituent optical beams incoming to the satellite as a composite optical beam. Of course, similar arrangements may be used in the OGS, with respect to composite optical beams incoming to the OGS from the satellite.

[0006] As compared to a radiofrequency (RF) feeder link, an OFL offers much higher capacity but the use of OFLs brings with them a host of design and operational challenges. Establishing and maintaining optical alignment between the satellite and the OGS stands as key among the challenge. Here, “optical alignment” refers to the correct orientation of the involved optics at the satellite and ground station for optical beam transmission and reception. Known approaches to meeting the alignment challenges include the use of a single optical head assembly (OHA) in which the optical transmit (TX) function and the optical receive (RX) function are implemented via the same optical telescope. Here, the term “optical telescope” broadly refers to the optical system(s) used to focus, collect, or transmit light to and from a distant location. Using a single OHA offers several advantages, not least being that only one alignment system is needed for both the optical transmitter and the optical receiver. Of course, the alignment system itself may be complicated, such as having two or more alignment sensors of differing resolutions to support coarse and fine alignment sensing.SUMMARY

[0007] A laser communications terminal (LCT) includes separate optical head assemblies (OHAs) for optical signal transmission and reception, with respective pointing systems allowing separate pointing control of the respective OHAs. In a satellite communications system (SCS) having an optical ground station (OGS) configured for communicatively coupling with a satellite of the SCS via an optical feeder link (OFL), the satellite and / or the OGS implements a respective dual OHA (DOHA). In one or more embodiments, the DOHA is configured for the use of different optical wavelengths with respect to optical communications beams on the uplink and downlink. Among several advantages flowing from use of the DOHA is isolation of the optical paths carrying the outgoing optical beam(s) and the incoming optical beam(s), along with theability to optimize and simplify the corresponding optics. Further, providing separate pointing control relieves the need for manufacturing the DOHA with precise inter-head alignment.

[0008] One embodiment comprises a LCT configured for use at one end of an OFL between a satellite and an OGS of a SCS. The example LCT includes a receive optical head assembly (ROHA) configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA, and a transmit optical head assembly (TOHA) configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA. The LCT also includes pointing control circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

[0009] In at least one embodiment, a satellite configured for use in a SCS includes one or more radiofrequency (RF) antenna systems, and a RF communications payload configured to transmit forward traffic to respective terminals via the one or more RF antenna systems, and to receive return traffic from the respective terminals via the one or more RF antenna systems. Further, the satellite includes a LCT that is configured to receive the forward traffic via an optical uplink communications beam transmitted by an OGS of the SCS and transmit the return traffic to the OGS via an optical downlink communications beam. The LCT onboard the satellite includes a receive optical head assembly (ROHA) configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA, and a transmit optical head assembly (TOHA) configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA. The LCT also includes pointing control circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

[0010] An example OGS configured for use in a SCS includes interface circuitry configured to receive forward traffic from a communications processing system (CPS) of the SCS, for forwarding to respective ones among terminals served by a satellite of the SCS and configured to transmit return traffic from respective ones of the terminals towards the CPS. The OGS further includes communication circuitry configured to generate forward communication signals conveying the forward traffic and configured to recover return communication signals conveyingthe return traffic. The OGS further includes a LCT that is configured to transmit the forward communication signals to the satellite via an optical uplink communications beam, and configured to receive the return communication signals via an optical downlink communications beam transmitted by the satellite. The LCT of the OGS includes a receive optical head assembly (ROHA) configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA, and a transmit optical head assembly (TOHA) configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA. The LCT also includes pointing control circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

[0011] In another embodiment, a method of communicating over an OFL between a satellite and an OGS of a SCS is performed by a LCT at one end of the OFL. The method includes the LCT receiving an optical communications beam incoming to the LCT via free space propagation from a remote LCT at the other end of the OFL, the incoming optical communications beam received via a ROHA of the LCT, the ROHA having a first pointing system for pointing control of the ROHA. Further, the method includes the LCT transmitting an optical communications beam outgoing from the LCT via free space propagation towards the remote LCT, the outgoing optical communications beam transmitted via a TOHA of the LCT, the TOHA having a second pointing system for pointing control of the TOHA. Still further, the method includes performing pointing control separately for the ROHA and the TOHA, based on respective evaluations of a first alignment signals output by a first alignment camera in the ROHA and second alignment signals by a second alignment camera in the TOHA.

[0012] Another embodiment comprises a SCS that includes an OGS configured for operation at one end of an OFL, and a satellite configured for operation at the other end of the OFL. The satellite includes one or more RF antenna systems and a RF communications payload configured to transmit forward link traffic to respective terminals via the one or more RF antenna systems, and to receive return link traffic from the respective terminals via the one or more RF antenna systems. Further, the satellite includes a LCT having a ROHA configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA, and further having a TOHA configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA. Still further, the LCT includes pointingcontrol circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

[0013] Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a block diagram of an example optical ground station and a satellite with included laser communications terminals (LCTs), and with one or both LCTs using a dual optical head assembly (DOHA).

[0015] Figure 2 is a block diagram of a DOHA arrangement for the satellite LCT, according to one embodiment.

[0016] Figure 3 is a diagram of an example optical field of view (FoV) for the receive optical head assembly (ROHA) of the satellite LCT, with Figure 4 illustrating a corresponding example FoV for the transmit optical head assembly (TOHA) of the satellite LCT.

[0017] Figure 5 is a block diagram of a DOHA arrangement for the OGS LCT, according to one embodiment.

[0018] Figure 6 is a diagram of an example optical field of view (FoV) for the receive optical head assembly (ROHA) of the OGS LCT, with Figure 7 illustrating a corresponding example FoV for the transmit optical head assembly (TOHA) of the OGS LCT.

[0019] Figure 8 is a block diagram of example details for a DOHA-based LCT, according to an example embodiment.

[0020] Figure 9 is a block diagram of an example pointing system for orienting the FoV of the ROHA of the LCT shown in Figure 8.

[0021] Figure 10 is a block diagram of an example optical arrangement for the ROHA of the LCT shown in Figure 8.

[0022] Figure 11 is a block diagram of example supporting electronics for the ROHA of theLCT shown in Figure 8.

[0023] Figure 12 is a block diagram of an example pointing system for orienting the FoV of the TOHA of the LCT shown in Figure 8.

[0024] Figure 13 is a block diagram of an example optical arrangement for the TOHA of the LCT shown in Figure 8.

[0025] Figure 14 is a block diagram of example supporting electronics for the TOHA of the LCT shown in Figure 8.

[0026] Figure 15 is a block diagram of a satellite communications system (SCS), according to an example embodiment, where an OGS and a satellite include respective implementations of the LCT shown in Figure 8.

[0027] Figure 16 is a diagram of an optical feeder link (OFL) arrangement used for coupling an OGS to a satellite, according to an example embodiment.

[0028] Figure 17 is a logic flow diagram illustrating a method of pointing control for a DOHA, according to an example embodiment.

[0029] Figure 18 is a diagram illustrating an example search pattern that can be used for beacon-beam transmission by a LCT during acquisition of optical alignment with a remote LCT.DETAILED DESCRIPTION

[0030] Use of an optical feeder link (OFL) between a ground station and a satellite requires precise optical alignment between the optical transceiver of the ground station and the optical transceiver onboard the satellite. Here, “optical alignment” means that the optical field of view (FoV) is pointed in the correct direction for optical beam transmission / reception.Correspondingly, the terms “pointing” and “steering” may be used interchangeably in reference to orienting the FoV, e.g., orienting it in terms of elevational and azimuthal angles by means of mechanical adjustments within the involved optical path(s). Example approaches to orienting the FoV include controlling movable elements such as mirrors, lenses, or prisms within the involved optical path(s).

[0031] Although known approaches to OFL implementation exploit the simplified alignment gained with the use of single optical head assemblies (OHAs) that integrate RX and TX functions into the same, shared optical telescope, it is recognized herein that increasing OFL capacity depends on the use of increasingly complex modulation and coding schemes at the optical transmitter. The ability to detect the corresponding optical signals requires higher signal- to-noise ratios (SNRs) at the optical receiver and that requirement favors higher optical transmission (TX) power.

[0032] As TX power becomes higher and within increasing receiver sensitivity, optical isolation between the TX and RX functions within a single OHA becomes increasingly challenging. An advantageous approach disclosed herein is an optical transceiver that includes a dual OHA (DOHA) comprising a separate RX OHA (ROHA) and a TX OHA (TOHA). Use of the separate OHAs optically isolates the RX and TX functions and allows respective optimizations of the RX and TX optical systems. Further, the disclosed DOHA arrangementsprovide separate pointing control of the ROHA and the TOHA, which simplifies the design, assembly, and operation of a DOHA-based optical transceiver. A particular advantage is that separate pointing control relieves the need for precise inter-head alignment between the ROHA and the TOHA because each one is configured to he pointed independent of the other one.

[0033] Figure 1 illustrates an example arrangement where an optical ground station (OGS) 10 of a satellite communications system (SCS) operates at one end of an OFL 12, with a satellite 14 of the SCS operating at the other end of the OFL 12. The term “OFL” here can be understood as a logical reference to the optical beam-based communicative coupling between the OGS 10 and the satellite 14. Optical beams transmitted from the OGS 10 propagate in free space along an optical path defined by the TX pointing direction of the OGS 10, with the OGS 10 being the beam source and the satellite 14 being the beam destination. Similarly, optical beams transmitted from the satellite 14 propagate in free space along an optical path defined by the TX pointing direction of the satellite 14, with the satellite 14 being the beam source and the OGS 10 being the beam destination.

[0034] The OGS 10 includes an optical transceiver 20, which comprises one or more OHAs 22 that are pointed via one or more pointing systems 24, which are controlled by pointing control circuitry 26. Similarly, the satellite 14 includes an optical transceiver 30, which includes one or more OHAs 32 that are pointed via one or more pointing systems 34, which are controlled by pointing control circuitry 36. The term “laser communications terminal” (LCT) is interchangeable with “optical transceiver” unless otherwise noted, and hereafter the optical transceiver 20 is referred to as the LCT 20 and the optical transceiver 30 is referred to as the LCT 30.

[0035] In the example embodiment illustrated, the LCT 30 transmits an optical communications beam 40 for reception at the LCT 20, and, on the same optical axis, transmits an optical beacon beam 42 for reception at the LCT 20. In complementary fashion, the LCT 20 transmits an optical communications beam 44 for reception at the LCT 30, and, on the same optical axis, transmits an optical beacon beam 46 for reception at the LCT 30.

[0036] In one or more embodiments, a first one of the LCTs 20 and 30 uses a single OHA that integrates RX and TX functions via a shared optics and has a single pointing system, while a second one of the LCTs 20 and 30 uses a DOHA having respective pointing systems for the included ROHA and TOHA. In an example scenario, the OGS 10 uses a single OHA and the satellite 14 uses a DOHA. In another example scenario, the OGS 10 uses a DOHA and the satellite 14 uses a single OHA, although DOHA usage onboard the satellite 14 may offerparticular advantages. In at least one embodiment, the OGS 10, and the satellite 14 use respective DOHAs.

[0037] Figure 2 illustrates an example DOHA implementation of the LCT 30 onboard the satellite 14, including a ROHA 50. According to the illustrated embodiment, the ROHA 50 includes an alignment sensing camera 52 and a first pointing system 34-1. The first pointing system 34-1 includes hierarchical pointing subsystems comprising a beam steering assembly (BSA) 54 as a first pointing subsystem and a fast pointing assembly (FPA) 56 as a second pointing subsystem. The pointing control circuitry 36 in one or more embodiments implements cooperative, orchestrated control of the BSA 54 and the FPA 56 for alignment operations, whereby it uses the FPA 56 for fast, fine alignment adjustments over a first angular range to deal with minor perturbations and precise alignment control, and uses the BSA 54 for slower, coarser adjustments over a larger second angular range that encompasses the first angular range. For example, the pointing control circuitry 36 uses the BSA 54 for gross pointing control, such as for initial pointing towards a selected OGS 10 and making larger, slower adjustments, and uses FPA 56 for faster, finer pointing control, such as for dynamic fine centering control of the ROHA FoV on the selected OGS 10.

[0038] The alignment sensing camera 52 in one or more embodiments comprises a geometric array or grid of pixels, with the particular pixel(s) illuminated by the incoming optical beam(s) indicating alignment of the ROHA FoV. Alignment sensing signals 60 output by the alignment sensing camera 52 indicate the per-pixel illuminations. In a particular embodiment, the alignment sensing camera 52 is a four-quadrant sensor and the alignment sensing signals 60 comprise per-quadrant signals indicating the absolute or relative amounts of light impinging on each quadrant. With this arrangement, centering of the ROHA FoV on the beam source results in a characteristic illumination pattern or relationship for the four quadrants. Thus, the centered condition or any deviation from that condition is detected from the respective signal levels of the alignment sensing signals 60. The pointing control circuitry 36 in one or more embodiments incorporates analog-to-digital converters (ADCs) for digitization of the alignment sensing signals and corresponding comparison of the digital samples in the digital processing domain.

[0039] In one or more embodiments, the BSA 54 comprises one or more optical elements driven by one or more servo actuators for coarse shifting the ROHA FoV in terms of elevational and azimuthal angles. However implemented, the BSA 54 provides one or more position feedback signals 62 to the pointing control circuitry 36 for tracking angular position(s), and the processing circuitry 36 provides one or more BSA drive signals 64 to adjust the angular position(s). Similarly, the FPA 56 in one or more embodiments comprises one or more opticalelements driven by one or more servo actuators for fine shifting the ROHA FoV in terms of elevational and azimuthal angles. However implemented, the FPA 56 provides one or more position feedback signals 66 to the processing circuitry 36 for tracking angular position(s), and the processing circuitry 36 provides one or more FPA drive signals 68 to adjust the angular position(s).

[0040] The LCT 30 further includes a TOHA 70 with an alignment sensing camera 72 and a pointing system 34-2. The pointing system 34-1 is a first pointing system for orienting the ROHA FoV, with the pointing system 34-2 being a second pointing system for orienting the TOHA FoV. Having separating pointing systems allows for separate control of the ROHA and TOHA pointing directions, which means that the LCT 30 need not rely on having precise mechanical alignment between the ROHA 50 and the TOHA 70 or otherwise slaving the pointing control of one head to the other head.

[0041] Like the pointing system 34-1, the pointing system 34-2 includes a BSA 74 and a FPA 76. Further like the ROHA 50, the alignment sensing camera 72 outputs alignment sensing signals 80 — e.g., per-quadrant signal — and the BSA 74 and the FPA 76 output position feedback signals 82 and 86, respectively. The pointing control circuitry 36 controls the BSA 74 via BSA drive signals 84 and controls the FPA 76 via FPA drive signals 88.

[0042] Figures 3 and 4 correspond with the DOHA example of Figure 2, with Figure 3 depicting the FoV of the ROHA 50 of the LCT 30 onboard the satellite 14, and Figure 4 depicting the FoV of the TOHA 70 of the LCT 30. Each FoV encompasses a corresponding spatial region and may be logically subdivided into respective sectors, with Figures 3 and 4 using an example arrangement of sixteen sectors.

[0043] Assuming that the ROHA 50 and the TOHA 70 are each at least coarsely aligned with a selected OGS 10, the OGS 10 “appears” within the respective fields of view of the ROHA 50 and the TOHA 70. However, as suggested in the figures, the separate alignment control of the ROHA 50 and the TOHA 70 means that the same OGS 10 may occupy different positions within the respective fields of view. Ideally, with fine positioning control performed separately for the ROHA 50 and the TOHA 70, the selected OGS 10 will appear at the respective centers of the ROHA FoV and the TOHA FoV. In some embodiments, or under some operating circumstances, the ROHA 50, and the TOHA 70 of the same LCT 30 onboard a satellite 14 are always oriented towards the same selected OGS 10, while in other embodiments or under other operating circumstances, the ROHA 50, and the TOHA 70 of the same LCT 30 are each oriented towards a respective one of two OGSs 10.

[0044] Figure 5 is similar to Figure 2 but illustrates an example DOHA implementation of the LCT 20 at an OGS 10. Although the powers, sizes, optics, and beam characteristics may differ as between a DOHA implementation at the LCT 30 versus a DOHA implementation at the LCT 20, the underlying principles and advantages may be the same as regards optical isolation, separate pointing control of the respective OHAs, etc.

[0045] In the example of Figure 5, the LCT 20 includes a ROHA 90 with an alignment sensing camera 92 and a pointing system 24-1 that includes a BSA 94 and a FPA 96. The alignment sensing camera 92 outputs alignment sensing signals 100 — e.g., per-quadrant signal — and the BSA 94 and the FPA 96 output position feedback signals 102 and 106, respectively. The pointing control circuitry 26 controls the BSA 94 via BSA drive signals 104 and controls the FPA 96 via FPA drive signals 108. The LCT 20 further includes a TOHA 110 with an alignment sensing camera 112 and a pointing system 24-2. Like the pointing system 24-1, the pointing system 24-2 includes a BSA 114 and a FPA 116. The alignment sensing camera 112 outputs alignment sensing signals 120 — e.g., per-quadrant signals — and the BSA 114 and the FPA 116 output position feedback signals 122 and 126, respectively. The pointing control circuitry 26 receives position feedback signals 122 from the BSA 114 and controls the BSA 114 via BSA drive signals 124. Likewise, the pointing control circuitry 26 receives position feedback signals 126 from the FPA 116 and controls the FPA 116 via FPA drive signals 128.

[0046] Figures 6 and 7 illustrate example ROHA and TOHA fields of view for the ROHA 90 and the TOHA 110 of the DOHA arrangement depicted in Figure 5 for the LCT 20 of an example OGS 10. The spatial region or volume encompassed by each such FoV depends, of course, upon the pointing direction of the FoV.

[0047] As with the satellite related fields of view depicted in Figures 3 and 4, the fields of view of the ROHA 90 and the TOHA 110 may be logically divided into sectors. Assuming at least coarse initial optical alignment of the ROHA 90 and the TOHA 110 with respect to a selected satellite 14, the selected satellite 14 appears within each respective ROHA and TOHA fields of view, although not necessarily at the same relative position, given the separate pointing control used for orienting the ROHA 90 and the TOHA 110. Also, in at least one embodiment or under at least some operating circumstances, the OGS 10 with a DOHA-based LCT 20 may orient the ROHA 90 of the LCT 20 towards a first selected satellite 14, while orienting the TOHA 110 of the LCT 20 towards a second selected satellite 14. Similarly, a DOHA-based LCT 30 onboard a satellite 14 may, in some embodiments or under certain operating scenarios, orient its ROHA for reception from a first remote LCT and orient its TOHA for transmission towards asecond remote LCT. For example, the satellite 14 may act as a relay between neighboring satellites.

[0048] Figure 8 depicts a DOHA-based LCT 140 with further example details. The LCT 140 represents a further example implementation of the LCT 20 in an OGS 10 and / or the LCT 30 in a satellite 14. In other words, the LCT 140 serves as a more general example of a DOHA LCT, which may be used in either the OGS context or the satellite context. As such, the LCT 140 may be configured according to any of the context-specific details shown in Figure 2 or 5.

[0049] The illustrated LCT 140 includes a ROHA 142 and a TOHA 144. The ROHA 142 includes an optical head 150 that includes objective optics 152, along with an RX optical telescope 154 and supporting RX optics 156. The supporting RX optics 156 output an optical RX communications signal 158 corresponding to an optical communications beam incoming to the ROHA 142 via the objective optics 152.

[0050] Supporting electronics 160 of the LCT 140 output an electrical-domain RX communications signal 162 corresponding to the optical RX communications signal 158, for input to downstream RX circuitry, which is not shown in the diagram. The supporting electronics 160 further include pointing control circuitry 164 that receives alignment sensing signals 166 output from an alignment sensing camera 168 of the ROHA 142. The pointing control circuitry 164 outputs one or more pointing control signals 170 to adjust the FoV of the ROHA 142. For example, although not shown in the diagram, the ROHA 142 includes a BSA for gross pointing control of the FoV of the ROHA 142 and includes a FPA for fine pointing control of the ROHA 142.

[0051] The TOHA 144 of the LCT 140 includes an optical head 180 that includes objective optics 182, a TX optical telescope 184, and supporting TX optics 186. The supporting TX optics 186 are configured to receive an optical TX communications signal 188 that the TOHA 144 outputs as an outgoing optical communications beam. The supporting electronics 160 generate the optical TX communications signal 188 responsive to an electrical-domain TX communications signal 190 received from TX communications circuitry that is not shown in the diagram.

[0052] The pointing control circuitry 164 is configured to implement both open- loop and closed-loop control of the ROHA 142 and the TOHA 144, with pointing control of the ROHA 142 separate from pointing control of the TOHA 144. For example, the pointing control circuitry 164 comprises one or more microprocessors, digital signal processors, or other programmatically configured circuitry that performs separate pointing control of the ROHA 142 and the TOHA 144. The pointing control circuitry 164 implements open-loop pointing control when initiallypointing the ROHA 142 or TOHA 144, and then implements closed-loop pointing control once incoming optical beams are sensed.

[0053] For closed-loop control of the ROHA 142, the pointing control circuitry 164 uses the alignment sensing signals 166 to determine the optical alignment of the ROHA 142 with respect to an optical beam that is incoming to the ROHA 142 via the objective optics 152, and it outputs one or more pointing control signals 170 to the ROHA 142. Like the earlier ROHA / TOHA examples, the ROHA 142 in one or more embodiments includes a BSA for gross, slow pointing control and a FPA for fine, fast pointing control. Thus, the pointing control signals 170 generated by the pointing control circuitry 164 in an example embodiment include a first control signal for gross control of elevational angle, a second control signal for gross control of azimuthal angle, a third control signal for fine control of elevational angle, and a fourth control signal for fine control of azimuthal angle.

[0054] For closed-loop control of the TOHA 144, the pointing control circuitry 164 uses the alignment sensing signals 192 to determine the optical alignment of the TOHA 144 with respect to an optical beam incoming to the TOHA 144 via the objective optics 182. On this point, it should be noted that although the primary purpose of the TOHA 144 is optical beam transmission, it also functions as an optical receiver inasmuch as it senses an incoming optical beam or beams, for the limited purpose of detecting and adjusting its optical alignment with respect to the source of the incoming optical beam(s). As an example, if the LCT 140 is onboard a satellite 14, both the ROHA 142 and the TOHA 144 may be aligned using the same optical beam transmitted by the selected OGS 10.

[0055] Based on its evaluation of the alignment sensing signals 192 output by an alignment sensing camera 194 of the TOHA 144, the pointing control circuitry 164 outputs one or more pointing control signals 196 to the ROHA 142. The TOHA 144 in one or more embodiments includes a BSA for gross, slow pointing control and a FPA for fine, fast pointing control. Thus, the pointing control signals 196 in an example embodiment include a first control signal for gross control of elevational angle, a second control signal for gross control of azimuthal angle, a third control signal for fine control of elevational angle, and a fourth control signal for fine control of azimuthal angle.

[0056] Consistent with the details immediately above, Figure 9 illustrates an example pointing system 200 for the ROHA 142, comprising a BSA 202 that is responsive to first and second control signals 204 and 206, for gross pointing adjustment of the FoV of the ROHA 142. The pointing system 200 for the ROHA 142 further includes a FPA 208 that is responsive tothird and fourth pointing control signals 210 and 212, for fine pointing adjustment of the FoV of the ROHA 142.

[0057] Figure 10 illustrates example optical details for the ROHA 142. An incoming optical beam 214 strikes a mirror as an example implementation of the objective optics 152 introduced in Figure 8, with the BSA 202 incorporating or otherwise articulating the objective optics 152 for elevational tilt and azimuthal rotation. The BSA 202 in this example depiction includes a first servo actuator 216 that provides elevational tilt control of the objective optics 152 in response to the first position control signal 204 and outputs a position feedback signal 218. The BSA 202 further includes a second servo actuator 220 that provides azimuthal rotation control of the objective optics 152 in response to the second position control signal 206 and outputs a position feedback signal 222.

[0058] The incoming optical beam 214 contains light at one or more optical wavelengths and it may comprise a single beam or it may comprise a composite beam — i.e., two axially coincident beams incoming to the ROHA 142. For example, a remote LCT initially may transmit an optical beacon beam for detection at the ROHA 142 and then may transition to transmission of an optical communications beam instead of or in addition to the beacon beam. In any case, the objective optics 152 deflect the incoming optical beam 214 into the RX optical telescope 154, with the telescope directing it onto a further optical element 224 that is included in or otherwise acted upon by the FPA 208. The optical element 224 is a mirror, and the FPA 208 provides elevational tilt control of the optical element 224 via a third servo actuator 226 and provides azimuthal rotation control of the optical element 224 via a fourth servo actuator 228. Pointing control signals 210 and 212 incoming from the pointing control circuitry 164 control the respective servo actuators 226 and 228, with the respective servo actuators 226 and 228 providing corresponding position feedback signals 230 and 232.

[0059] The optical element 224 deflects the incoming optical beam 214 into a first collimator 234, with the resulting collimated light impinging on a beam splitter 236. The beam splitter 236 is configured to deflect at least a portion of the incoming optical beam 214 as output from the collimator 234 onto the alignment sensing camera 168. In the depicted example, the alignment sensing camera 168 is a four-quadrant sensor, having quadrants 238, 240, 242, and 244, with each quadrant comprising light-sensing pixels.

[0060] The beam splitter 236 passes the remaining beam portion along to a RX collimator 246, which provides corresponding collimated light into a fiber collimator 250. The fiber collimator 250 couples that light into an optical fiber 252 as the optical RX communications signal 158. More particularly, the optical RX communications signal 158 exists or otherwiseconveys information only when the incoming optical beam 214 comprises or includes an optical communications beam — i.e., a light beam containing modulated light.

[0061] In an example scenario, the ROHA 142 is open loop pointed towards a remote LCT via control of the BS A 202 by the pointing control circuitry 164. The initial pointing is based on the pointing control circuitry 164 having knowledge of the position of the remote LCT. The initial pointing results in the ROHA 142 receiving an optical beacon beam that is transmitted by the remote LCT to aid fine optical alignment. That is, initial pointing positions the remote LCT somewhere within the FoV of the ROHA 142, such that the alignment sensing camera 168 is illuminated by the optical beacon beam as projected onto it by the optical element 236. Where the projected beam strikes the surface of the alignment sensing camera 168 indicates the alignment / misalignment, with the alignment sensing signals 166 output by the alignment sensing camera 168 indicating the position of the remote LCT within the FoV of the ROHA 142. Correspondingly, the pointing control circuitry 164 controls the FPA 208 to “center” the remote LCT in the FoV. In this example, centering the remote LCT in the FoV of the ROHA 142 means shifting the FoV in elevation and azimuth, as needed, to produce equal illumination on the four quadrants of the alignment sensing camera 168.

[0062] After it gains mutual optical alignment with the LCT 140, the remote LCT may begin transmitting an optical communications beam that is coincident with the optical beacon beam it transmits, or it may stop transmitting the optical beacon beam once it begins transmitting the optical communications beam. In at least one embodiment, the optical beacon beam, and the optical communications beam from the remote LCT are at different optical wavelengths, with the beam splitter 236 deflecting at least some of the incoming optical beacon beam onto the alignment sensing camera 168 and deflecting some but not all of the incoming optical communications beam onto the alignment sensing camera 168. One advantage to illuminating the alignment sensing camera 168 with a portion of the incoming optical communications beam is that it increases the available light power for alignment detection — i.e., it improves the signal- to-noise ratio (SNR) for alignment detection, without unduly reducing the corresponding optical RX communications signal 158.

[0063] Figure 11 illustrates an example implementation of the supporting electronics 160 with respect to the ROHA 142. The optical RX communications signal 158 is received by an optical amplifier 300, which provides its output optical signal to an optical demultiplexer 302. Here, it is assumed that the optical communications beam incoming to the ROHA 142 contains light at multiple optical wavelengths, with each such wavelength corresponding to a respective optical channel for conveying a respective information stream, which itself may comprise amultiplexed arrangement of information sub-streams. Thus, the optical demultiplexer 302 is based on wavelength division multiplexing (WDM) and outputs a plurality of RX optical channel signals 304, with five such signals 304-1, 304-2, 304-3, 304-4, and 304-5 shown merely for example.

[0064] Corresponding optical phase detectors (PDs) 306 convert the respective optical channel signals 304 into electrical-domain RX communications signals 308, shown as electrical RX communications signal 308-1 corresponding to the optical RX channel signal 304-1, electrical RX communications signal 308-2 corresponding to the optical RX channel signal 304- 2, and so on. The optical PDs 306 operate with respective local oscillator (LO) signals 310 output by respective RX oscillators collectively identified by reference number 312. An optical phase locked loop (OPLL) 314 may be used to provide reference frequencies to the RX oscillators 312.

[0065] A TX communications payload 316, which is not considered part of the LCT 140 in the illustrated embodiment receives the electrical RX communications signals 308 for processed or non-processed retransmission of them. In an example where the LCT 140 is onboard a satellite 14, the TX communications payload 316 may comprise a plurality of bent- pipe signal transmission paths for transmitting the electrical RX communications signals 308 as radiofrequency (RF) downlink signals targeting respective terminals among a population of terminals served by the satellite 14.

[0066] Here, “bent pipe” means that the satellite 14 does not recover the information being carried by the electrical RX communications signals 308, and instead merely passes those signals, possibly with amplification, filtering, and frequency translation. For example, the respective optical channel signals 304 comprise light that is radiofrequency (RF) modulated, with the modulations comprising the information content. In such embodiments, the electrical RX communications signals 308 are RF signals. In a related embodiment, the respective RX optical channel signals 304 comprise light that is digitally modulated, with the TX communications payload 316 modulating respective RF carriers according to such modulations. Alternatively, the TX communications payload 316 may comprise a processing payload that demodulates and decodes the electrical RX communications signals 308, and then generates corresponding new outgoing RF signals for transmission.

[0067] A RX BSA elevation driver 320 provides the position control signal 204, with RX BSA elevation sensor electronics 322 receiving the corresponding position feedback signal 218. A RX BSA azimuth driver 324 provides the position control signal 206, with RX BSA azimuth sensor electronics 326 receiving the corresponding position feedback signal 222. RX FPAelevation / azimuth drivers 328 provide the pointing control signals 210 and 212, with RX FPA elevation sensor electronics 330 receiving the corresponding position feedback signals 230 and 232.

[0068] RX alignment sensor electronics 332 receive the alignment sensing signals 166 from the alignment sensing camera 168 and comprise, for example one or more ADCs that provide digitized samples to a LCT controller 340 that includes, for example, one or more CPUs 342, along with supporting memory 344 for storing one or more computer programs 346 and configuration and working data 348. In at least one embodiment, the LCT controller 340 comprises one or more microprocessors or other digital processing circuitry that is specially adapted based on the execution of computer program instructions stored in the memory 344. The LCT controller 340 and its supporting interface electronics shall be understood as an example of the supporting electronics 160 and the pointing control circuitry 164 comprised therein. Further elements of such circuitry include, for example, an optical-amplifier (OA) driver / controller 350, for controlling the OA 300.

[0069] Consistent with the details discussed for the LCT 140 shown in Figure 8, Figure 12 illustrates an example pointing system 400 for the TOHA 144 of the LCT 140. The pointing system 400 comprises a BSA 402 that is responsive to first and second control signals 404 and 406, for gross pointing adjustment of the FoV of the TOHA 144 in terms of elevation and azimuth. The pointing system 400 for the TOHA 144 further includes a FPA 408 that is responsive to third and fourth pointing control signals 410 and 412, for fine pointing adjustment of the FoV of the TOHA 144 in terms of elevation and azimuth.

[0070] Figure 13 illustrates example optical details for the TOHA 144 of the example LCT 140. The supporting TX optics package 186 of the TOHA 144 includes a fiber-coupled collimator 420 for receiving the optical TX communications signal 188 via an input optical fiber 422. The collimator 420 collimates the optical TX communications signal 188 as an outgoing optical communications beam 424 directed towards a further collimator 428, which directs it towards a beam splitter 430. The beam splitter 430 deflects a portion of the beam light towards an optical dump 432, while passing the remaining portion along to a second beam splitter 434. For simplicity of discussion, this remaining portion of light is still referred to as the outgoing optical communications beam 424.

[0071] The supporting TX optics package 186 further includes a beacon laser unit 436 that outputs a laser beam responsive to a beacon control signal 438, with the laser beam serving as an outgoing optical beacon beam 440 to be transmitted by the TOHA 144 for use by a remote LCT in optically aligning itself with the LCT 140. A collimator 442 directs the outgoing opticalbeacon beam 440 towards an optical element 260, such as a deflection mirror, which directs the outgoing optical beacon beam 440 to the beam splitter 430, which directs it towards a further beam splitter 434. This further beam splitter 434 directs a portion of the light comprised in the outgoing beacon beam towards an optical dump 444, while passing a remaining portion of it towards an optical element 446 comprised in the FPA 408 of the TOHA 144.

[0072] With this arrangement, when the TOHA 144 is transmitting both the outgoing optical communications beam 424 and the outgoing optical beacon beam 440, the beam splitter 430 deflects some of the communications-beam light towards the dump 432, while passing the remaining communications-beam light towards the beam splitter 434. Correspondingly, the beam splitter 434 passes the outgoing communications-beam light impinging on it from the beam splitter 430, while deflecting a portion of the outgoing beacon-beam light to the dump 444 and passing the remaining portion towards the optical element 446. Thus, at least a portion of the supporting optics package 186 includes a shared optical path for the outgoing beacon and communications beams.

[0073] The optical element 446 directs the outgoing optical communications beam 424 and the outgoing optical beacon beam 440 into the optical telescope 184, which directs the beams onto an optical element as an example of the objective optics 182 shown in Figure 8. The optical element 182 comprises a portion of the BSA 402 of the TOHA 144 and the diagram illustrates the outgoing optical communications beam 424 and the outgoing optical beacon beam 440 exiting the TOHA 144 for free space propagation towards the remote LCT.

[0074] Although the primary function of the TOHA 144 is optical beam transmission, separate alignment control of the TOHA 144 requires sensing its alignment relative to the remote LCT. Thus, the TOHA 144 is configured to receive an incoming optical beam 450 on at least a portion of the same optical path used for beam transmission, with the diagram depicting these incoming and outgoing beams via a single bidirectional arrow for simplicity. The incoming optical beam is, for example, the same optical beacon and / or communications beam incoming to the ROHA 142, and it is deflected by the optical element 182 into the main optical path of the supporting TX optics package 186 via the TX optical telescope 184. The depicted optical path directs the incoming optical beam 450 into a collimator 452, which directs it onto the alignment sensing camera 194, which outputs the aforementioned alignment sensing signals 192, e.g., per- quadrant signals from a four-quadrant configuration of the alignment sensing camera 194.

[0075] The pointing control circuitry 164 uses the alignment sensing signals 192 to sense the optical alignment of the FoV of the TOHA 144 with respect to the incoming optical beam 450 or, phrased differently, with respect to the remote LCT that transmits the optical beam 450. Thepointing control circuitry 164 uses the BSA 402 for gross pointing control of the TOHA 144 and uses the FPA 408 for fine pointing control of the TOHA 144. In one or more embodiments, the pointing control circuitry 164 implements open loop pointing control via the BSA 402, e.g., with the FPA 408 positioned at its default or nominal center position, for initial alignment of the TOHA 144 with the remote LCT. This same initial alignment approach may be implemented for the ROHA 142.

[0076] That initial alignment positions the optical beam 450 somewhere within the FoV of the TOHA 144, with the alignment sensing signals 192 then indicating to the pointing control circuitry 164 the fine FPA adjustments that are needed to center the incoming optical beam 450 within the FoV. From there, the pointing control circuitry 164 may cause the TOHA 144 to “track” the incoming optical beam 450 by using the BSA 402 to compensate for slow misalignment trends while using the FPA 408 to compensate for smaller, faster perturbations. Tracking in this context means controlling the BSA 402 / FPA 408 to keep the incoming optical beacon beam centered within the TOHA FoV. Separately, the pointing control circuitry 164 performs the same tracking control of the ROHA FoV.

[0077] The pointing control circuitry 164 knows the current positions of the BSA 402 and the FPA 408 in terms of their elevational and azimuthal angles, based on position feedback signals. A first servo actuator 460 of the BSA 402 controls the elevational angle of the optical element 182 responsive to the first position control signal 404 and provides a corresponding position feedback signal 462. A second servo actuator 464 of the BSA 402 controls the azimuthal angle of the optical element 182 responsive to the second position control signal 406 and provides a corresponding position feedback signal 466. A first servo actuator 470 of the FPA 408 controls the elevational angle of the optical element 446 responsive to the third pointing control signal 410 and provides a corresponding position feedback signal 472. A second servo actuator 474 of the FPA 408 controls the azimuthal angle of the optical element 182 responsive to the fourth pointing control signal 412 and provides a corresponding position feedback signal 476.

[0078] Further points of interest in the diagram include articulation of the optical element 260 for “scanning” of the outgoing optical beacon beam 440. For example, with the LCT 140 implemented in an OGS 10, the OGS 10 may scan the outgoing optical beacon beam 440 within its optical FoV, as part of an advantageous pointing, acquisition, and tracking (PAT) algorithm.

[0079] Multiple approaches to articulation are possible, with Figure 13 merely illustrating an example approach based on a servo actuator 480 for azimuthal rotation of the optical element 260, and a servo actuator 482 for elevational rotation of the optical element 260. An Az controlsignal 484 controls azimuthal rotation, with corresponding positional feedback provided via a position feedback signal 486. Likewise, an El control signal 488 controls elevational rotation, with corresponding positional feedback provided via a position feedback signal 490.

[0080] Figure 14 illustrates example details for the supporting electronics 160 of the LCT 140 in one embodiment, with respect to controlling the TOHA 144. The LCT controller 340 introduced in Figure 11 is configured to exercise or otherwise coordinate such control, e.g., based on its execution of stored computer program instructions.

[0081] Further circuitry includes a TX BSA elevation driver 500 that operates under control of the LCT controller 340 and outputs the first position control signal 204. Corresponding TX BSA elevation sensor electronics 502 receive the corresponding position feedback signal 462. A TX BSA azimuth driver 504 operates under control of the LCT controller 340 and outputs the second position control signal 206. Corresponding TX BSA azimuth sensor electronics 506 receive the corresponding position feedback signal 466. TX FPA elevation and azimuth drivers 508 operate under control of the LCT controller 340 and output the third and fourth position control signals 210 and 212. Corresponding TX FPA elevation / azimuth sensor electronics 510 receive the corresponding position feedback signals 472 and 476.

[0082] TX alignment-camera sensor electronics 512 interface the alignment sensing camera 194 of the TOHA 144 with the LCT controller 340, e.g., by digitizing the alignment sensing signals 192 output by the alignment sensing camera 194. Further interface details include a beacon laser control circuit 514 configured to provide the LCT controller 340 with on / off and, in some embodiments, modulation control, of the beacon laser unit 436 of the TOHA 144.

[0083] Still further, the LCT controller 340 controls an optical power amplifier (OPA) 516 via an OPA driver 518, with the OPA 516 configured to output the optical TX communications signal 188 that is received into the TOHA 144 for transmission as the outgoing optical communications beam 424 via the input optical fiber 422. Assuming that the optical TX communications signal 188 is formed as a combined optical signal containing a plurality of optical channel signals 520, e.g., optical channel signals 520-1 through 520-5, the supporting electronics 160 include an optical multiplexer 522 that is configured to combine the optical channel signals 520 via WDM, to produce the optical TX communications signal 188 as a combined or composite optical signal having a plurality of optical channel signals 520 that are multiplexed in the optical frequency domain.

[0084] Each optical channel signal 520 corresponds to a different electrical-domain RX communications signal 524 output by a RX communications payload 526, which is not considered to comprise part of the LCT supporting electronics 160. Although the RXcommunications payload 526 outputs the electrical-domain RX communications signals 524 for optical transmission by the LCT 140, the “RX” designation refers to the fact that these RX communications signals 524 are received for optical transmission by the LCT 140.

[0085] Taking the example case of five optical channel signals 520-1 , 520-2, 520-3, 520-4, and 520-5, each optical channel signal 520 is at a respective optical wavelength and is modulated according to a corresponding one of the electrical-domain RX communications signals 524 via respective optical modulator 528 that is provided a corresponding LO signal 530 at the correct frequency via a respective one among a plurality of TX oscillators 532. One or more OPLLs 534 operating under control of the LCT controller 340 may be configured to provide reference frequency signals into the TX oscillators 532.

[0086] A scanning control circuit 536 interfaces the LCT controller 340 with the servo actuators 480 and 482, used for scanning control of the outgoing optical beacon beam 440 in one or more embodiments. For example, the scanning control circuit 536 includes respective driver circuits for driving the servo actuators 480 and 482 responsive to digital and / or analog signals output from the LCT controller 340. The scanning control circuit 536 in one or more embodiments further includes level- shifting or buffering circuits configured to interface the position feedback signals 486 and 490 with the LCT controller 340. Thus, when performing beacon beam scanning, the LCT controller 340 “moves” the outgoing optical beacon beam 440 within the optical FoV via the signals 484 and 488, and reads the current “position” of the outgoing optical beacon beam 440 via the signals 486 and 490.

[0087] Figure 15 illustrates an example satellite communications system (SCS) 600, including an OGS 602 communicatively coupled to a satellite 604 via an OFL 606. The OGS 602 includes a first LCT 140-1 as discussed above, with the LCT 140-1 transmitting one or more optical uplink beams 608 as part of the OFL 606, such as an optical uplink communications beam and an optical uplink beacon beam. Correspondingly, the satellite 604 includes a LCT 140- 2 as described above, with the LCT 140-2 transmitting one or more optical downlink beams 610, such as an optical downlink communications beam and an optical downlink beacon beam.

[0088] Figure 16 illustrates an example arrangement for the OFL 606, where the LCT 140-1 in the OGS 602 transmits an optical uplink (UL) beacon beam 612 and, at the same or different times, transmits an optical UL communications beam 614, e.g., conveying a plurality of optical channel signals at respective optical wavelengths. The LCT 140-2 onboard the satellite 604 transmits an optical downlink (DL) beacon beam 616 and, at the same or different times, transmits an optical DL communications beam 618. In at least one embodiment, the optical UL communications beam is at a first optical wavelength, the DL2.

[0089] In one or more embodiments, the optical UL communications beam 614 is at a first optical wavelength, the optical DL communications beam 618 is at a second optical wavelength, the optical UL beacon beam 612 is at a third optical wavelength, and the optical DL beacon beam 616 is a fourth optical wavelength. In at least one such embodiment, the first and third optical wavelengths are within a first range of optical wavelengths and the second and fourth optical wavelengths are within a second range of optical wavelengths, and where the first and second ranges of optical wavelengths are disjoint.

[0090] For one or more embodiments, saying that the optical UL communications beam 614 is at a first optical wavelength can be understood as meaning that the optical UL communications beam 614 is based on an optical carrier having a carrier center frequency corresponding to the first optical wavelength. The optical UL communications beam 614 may, in fact, contain light at more than one optical wavelength, e.g., centered around the carrier center frequency, and the same holds for the optical DL communications beam 618. This arrangement allows an optical communications beam to convey multiple information streams, e.g., multiple RF communications signals.

[0091] In one or more embodiments, the optical UL beacon beam 612 and the optical UL communications beam 614 are close in optical wavelength. For example, the optical wavelength of the optical UL communications beam 614 is 1550 nm and the optical wavelength of the optical UL beacon beam 612 is 1590 nm. Similarly, in the same or other one or more embodiments, the optical DL beacon beam 616 and the optical DL communications beam 618 are close in optical wavelength. For example, the optical wavelength of the optical DL communications beam 618 is 1064 nm and the optical wavelength of the optical DL beacon beam 616 is 974 nm.

[0092] Turning back to example details in Figure 15, in addition to the OGS 602, a ground segment 620 of the SCS 10 includes a core network 622 that includes one or more computer servers or other computer systems configured to operate as a communications processing system (CPS) 624 operative to interface the SCS 600 to one or more external networks 626, such as the Internet. In one or more embodiments, the CPS 624 is configured to receive incoming packet streams targeting respective ones among a population of terminals 628 served by the satellite 604, and provide them to the OGS 602 as forward traffic for DL transmission by the satellite 604, for reception by the targeted terminals 628, which may comprise user terminals or other subscriber endpoint devices.

[0093] The OGS 602 includes interface circuitry 630 configured to interface the OGS 602 to the CPS 624, e.g., data network interface circuitry. Communications circuitry 632 generates oneor more TX communications signals corresponding to the forward traffic incoming to the OGS 602 from the CPS 624 via the interface circuitry 630. In one embodiment, the terminals 628 are distributed over a geographic area referred to as a satellite service area covered by the satellite 604. The satellite service area is divided logically into a plurality of beam coverage areas, with the satellite 604 transmitting RF DL beams covering the respective beam coverage areas.

[0094] Correspondingly, in an example embodiment, the communications circuitry 632 in the OGS 602 generates a RF forward beam signal for each beam coverage area, with each such signal comprising multiplexed forward traffic for the terminals 628 located within the corresponding beam coverage area. The LCT 140-1 forms its optical UL communications beam 614 as a multiplexed optical signal comprising a plurality of optical channel signals, each optical channel signal conveying a respective one of the forward beam signals. Continuing with the forward direction, the LCT 140-2 in the satellite 604 receives the optical UL communications beam 614 as an incoming optical RX communications beam, and it outputs corresponding RF signals that may be understood as recovered versions of the RF forward beam signals. See, for example, the electrical-domain RX communications signals 308 depicted in Figure 11.

[0095] The satellite 604 includes a TX communications payload 316 as described earlier, which uses one or more RF antenna systems 642 to transmit RF DL communications signals 644 corresponding to electrical-domain forward traffic signals provided to it by the LCT 140-2. In one or more embodiments, the RF DL communications signals 644 are transmitted via RF beamforming corresponding to the beam coverage areas, as part of a RF user link 646. The TX communications payload 316 may be a bent-pipe payload with no signal decoding, or it may be a processed payload that performs signal decoding and remodulation for transmission.

[0096] Respective ones among the population of terminals 628 transmit return traffic via RF UL signals 648, with these signals received via the one or more RF antenna systems 642 onboard the satellite 604. A RX communications payload 526 as described earlier provides these signals, or signals derived therefrom, to the LCT 140-2 as electrical-domain return-traffic signals, for transmission to the OGS 602 via the optical DL communications beam 618. The LCT 140-1 provides the correspondingly recovered versions of these return-traffic signals to the CPS 624.

[0097] While Figure 15 is simplified by illustrating one OGS 602 and one satellite 604, the SCS 600 in one or more embodiments includes a plurality of OGS 602 and a plurality of satellites 604. For example, a given satellite 604 may be supported by different OGSs 602 at different times. Moreover, one or more of the OGSs 602 may include more than one LCT 140-1, with each LCT 140-1 usable for supporting a respective OFL 606, meaning that one such OGS 602 may support two or more OFLs 606 simultaneously, with the same satellite 604 or withmultiple satellites 604. Similarly, in one or more embodiments, at least one satellite 604 of the SCS 600 includes more than one LCT 140-2, which increases the capacity of the satellite 604 because it can exchange forward / reverse traffic simultaneously over more than one OFL 606.

[0098] Figure 17 illustrates a method 1700 of operation at the LCT 140-2 onboard the satellite 604. The. LCT controller 340 initially performs (Block 1702) open loop pointing separately of the ROHA 142 and the TOHA 144, based on knowledge of its current location and orientation and the location of a selected OGS 602, where the SCS 600 in one or more embodiments comprises a plurality of geographically distributed OGSs 602 and where the satellite 604 is switchable from one OGS 602 to another, and with the LCT controller 340 operative to reorient the ROHA 142 and the TOHA 144 as needed.

[0099] Assuming that the selected OGS 602 performs complementary open loop pointing of the ROHA 142 and TOHA 144 in its LCT 140-1, the LCT 140-2 will begin receiving the optical UL beacon beam 612 from the OGS 602. Correspondingly, the LCT 140-1 will begin receiving the optical DL beacon beam 616 from the LCT 140-2.

[0100] The method 1700 continues with acquisition processing (Block 1704), wherein the LCT controller 340 in the LCT 140-2 uses the optical UL beacon beam 612 as detected by the alignment sensing camera 168 of the ROHA 142 and the alignment sensing camera 194 of the TOHA 144 to acquire optical alignment with respect to the LCT 140-1. Likewise, the LCT controller 340 in the LCT 140-1 uses its alignment sensing cameras 168 and 194 to perform optical alignment of its own ROHA 142 and TOHA 144 with respect to the LCT 140-2. Upon acquisition, the LCT 140-1 and LCT 140-2 begin transmitting their respective optical communications beams 614 and 618, and the method 1700 continues with tracking (Block 1706) during ongoing communications. Tracking comprises the LCT controller 340 in the LCT 140-2 controlling the BSA / FPA of its ROHA 142 and the BSA / FPA of its TOHA 144, to maintain optical alignment of the ROHA 142 and the TOHA 144 with the LCT 140-1. The same or similar tracking operations are carried out in the LCT 140-1, in the OGS 602.

[0101] In more detail, to establish the communication link between a given OGS 602 and a given satellite 604 of the SCS 600, the respective LCTs 140-1 (OGS) and 140-2 (satellite) first perform a spatial Pointing, Acquisition and Tracking (PAT) sequence to optically co-align with each other. To speed up the acquisition time, the respective beacon beams in one or more embodiments carry modulated service channels with encoded beacon pointing data. Consider the following example (PAT) sequence.

[0102] Pointing operations in the PAT sequence include the LCTs 140-1 and 140-2 using the BSAs in their respective ROHAs and TOHAs to point their beacons towards each other coarsely, in open-loop fashion, based on known GPS and orbital data.

[0103] After pointing, acquisition operations begin. The satellite beacon in one or more embodiments has a divergence angle large enough such that initial pointing results in the beacon beam of the satellite immediately illuminating the OGS 602, with the OGS 602 thus acquiring the satellite 604 relatively quickly. However, to improve usable signal at the satellite 604, the communications beam transmitted by the OGS 602 may have a narrower divergence angle, with the beacon beam transmitted by the OGS 602 having the same characteristically narrow divergence angle.

[0104] The OGS 602 thus “scans” its outgoing beacon beam within its optical FoV, e.g., via El / Az control of movable mirror, such as the optical element 260 shown in Figure 13, that affects the beacon beam without affecting the optical FoV. See Figure 18, illustrating the FoV of the OGS TOHA, which transmits the outgoing optical beacon beam to be sensed by the satellite 604. Although Figure 18 illustrates a spiral scan or search pattern over the FoV, other patterns may be used, such as horizontal or diagonal raster scanning. In any case, the LCT controller 340 in the LCT 140-1 of the OGS 602 modulates the outgoing beacon beam at least during the acquisition phase of the PAT algorithm, such that the outgoing beacon beam dynamically encodes the scan path or the respective FoV sectors that are traversed by the scan path. In other words, different modulation codes applied to the beacon beam correspond to different scan positions.

[0105] With the satellite 604 being located within the optical FoV of the OGS 602 according to the example depiction of Figure 18, the LCT 140-2 of the satellite 604 generally will not detect the beacon beam from the OGS 602 unless / until the beacon beam is scanned through the sector numbered as “12” in the example. Or, while the satellite 604 may detect the UL beacon beam from the OGS 602 for more than one scanned sector of the FoV of the OGS 602, the detected beam will be strongest for the sector in which the satellite 604 is located.

[0106] The LCT 140-2 demodulates the detected optical UL beacon beam from the OGS 602, to recover the encoded sector information, and it modulates its own optical DL beacon beam with the sector information, to feed back the sector information to the OGS 602. In the illustrated example, as the OGS 602 scans its optical uplink beacon beam through sector 12 of its optical FoV, the satellite 604 detects the optical uplink beacon beam (or most strongly detects it), recovers the corresponding sector code that was modulated onto the optical UL beacon beam by the OGS 602, and returns the recovered code to the OGS 602 by modulating its optical DL beacon beam with the recovered sector code.

[0107] Upon reception of the recovered sector code, the OGS 602 knows that the satellite is positioned in or near sector 12 of its optical FoV and it initiates a new scan over a reduced area one or around sector 12, possibly with increased sector resolution. In particular, in one or more embodiments, the scan is based on a higher resolution sectorization around the position determined from the prior scan. Thus, the modulation codes in one scan may not correspond to the same absolute FoV locations represented by them in the prior scan. In any case, the OGS 602 may perform one or more additional scans for refinement of its optical alignment and, for each additional scan, may perform FPA adjustment, e.g., for fine recentering of the FoV on the “position” of the remote LCT determined in the prior scan. Upon conclusion of the scanningbased alignment, the satellite 604 will be positioned at the center of the optical FoV of the OGS 602, within tolerance and error limits.

[0108] With the OGS 602 and the satellite 604 then optically coaligned, the optical UL / DL communications beams can be established, with the FPAs in the OGS 602 and the satellite 604 now being used for actively maintaining optical coalignments and compensating for drifts and vibrations. In addition, both LCTs 140-1 and 140-2 start compensating for point ahead angle.

[0109] Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is / are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this 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

CLAIMSWhat is claimed is:

1. A laser communications terminal (LCT) configured for use at one end of an optical feeder link (OFL) between a satellite and an optical ground station (OGS) of a satellite communications system, the LCT comprising: a receive optical head assembly (ROHA) configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA; a transmit optical head assembly (TOHA) configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA; and pointing control circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

2. The LCT according to claim 1 , wherein the ROHA is configured to receive an incoming optical communications beam at a first optical wavelength, and the TOHA is configured to transmit an outgoing optical communications beam at a second optical wavelength.

3. The LCT according to claim 2, wherein the ROHA is configured to receive an incoming optical beacon beam at a third optical wavelength, the first and third optical wavelengths being within a first range of optical wavelengths and the second optical wavelength being outside the first range of optical wavelengths, with the ROHA including a shared optical path configured to direct the incoming optical beacon beam towards the ROHA alignment sensing camera and direct at least a portion of the incoming optical communications beam towards a receive (RX) fiber collimator.

4. The LCT according to claim 3, wherein the shared optical path of the ROHA is further configured to direct a portion of the incoming optical communications beam towards the ROHA alignment sensing camera, such that either or both the incoming optical beacon beam and the incoming optical communications beam are useable by the pointing control circuitry for controlling the pointing the of the ROHA.

5. The LCT according to claim 3 or 4, wherein the TOHA is configured to receive the incoming optical beacon beam, with the TOHA including a shared optical path of the TOHA that is configured to direct the incoming optical beacon beam towards the TOHA alignment sensing camera and direct the outgoing optical communications beam for transmission from the TOHA.

6. The LCT according to claim 5, wherein the TOHA is further configured to transmit an outgoing optical beacon beam, for pointing control use by a remote LCT at the other end of the OFL, the shared optical path of the TOHA configured to direct the outgoing optical beacon beam for transmission from the TOHA.

7. The LCT according to claim 6, wherein the outgoing optical beacon beam is at a fourth optical wavelength, the second and fourth optical wavelengths being within a second range of optical wavelengths, the second range of optical wavelengths not overlapping with the first range of optical wavelengths.

8. The LCT according to any one of claims 1-7, wherein the ROHA includes a first optical telescope dedicated to the ROHA, and the TOHA includes a second optical telescope dedicated to the TOHA.

9. The LCT according to any one of claims 1-8, wherein the pointing control circuitry is configured to control the pointing direction of the ROHA responsive to first alignment signals output by the ROHA alignment sensing camera, and control the pointing direction of the TOHA responsive to second alignment signals output by the TOHA alignment sensing camera.

10. The LCT according to claim 9, wherein each of the ROHA alignment sensing camera and the TOHA alignment sensing camera comprises a first four-quadrant sensor, with the first alignment signals comprising a detection signal for each quadrant, indicating an amount of light impinging on the quadrant.

11. The LCT according to any one of claims 1-10, wherein the first and second pointing systems each comprises a first pointing control element that is configured to provide gross pointing adjustment and a second pointing control element that is configured to provide fine pointing adjustment.

12. The LCT according to claim 11, and wherein the pointing control circuitry is configured to implement a pointing, acquisition, and tracking (PAT) algorithm for each of the ROHA and the TOHA as an OHA, wherein the PAT algorithm includes the pointing control circuitry: initially pointing the OHA on an open-loop basis, for initiating beacon-beam reception; and performing closed-loop pointing control of the OHA responsive to beacon-beam reception.

13. The LCT according to any one of claims 1-12, wherein the pointing control system is configured to align a first one of the ROHA and the TOHA on at least an initial basis, determining alignment assistance information based on aligning the first one, and use the alignment assistance information for faster alignment of the second one of the ROHA and the TOHA.

14. A satellite configured for use in a satellite communications system (SCS), the satellite comprising: one or more radiofrequency (RF) antenna systems; a RF communications payload configured to transmit forward traffic to respective terminals via the one or more RF antenna systems, and to receive return traffic from the respective terminals via the one or more RF antenna systems; and the LCT of any one of claims 1-13, wherein the LCT is configured to receive the forward traffic via an optical uplink communications beam transmitted by an optical ground station (OGS), the OGS comprised in the SCS, and is further configured to transmit the return traffic to the OGS via an optical downlink communications beam.

15. An optical ground station (OGS) configured for use in a satellite communications system (SCS), the OGS comprising: interface circuitry configured to receive forward traffic from a communications processing system (CPS) of the SCS, for forwarding to respective ones among terminals served by a satellite of the SCS, and configured to transmit return traffic from respective ones of the terminals towards the CPS;communication circuitry configured to generate forward communication signals conveying the forward traffic and configured to recover return communication signals conveying the return traffic; and the LCT of any one of claims 1-13, wherein the LCT is configured to transmit the forward communication signals to the satellite via an optical uplink communications beam, and configured to receive the return communication signals via an optical downlink communications beam transmitted by the satellite.

16. A method of communicating over an optical feeder link (OFL) between a satellite and an optical ground station (OGS) of a satellite communications system, the method performed via a laser communications terminal (LCT) at one end of the OFL and comprising: receiving an optical communications beam incoming to the LCT via free space propagation from a remote LCT at the other end of the OFL, the incoming optical communications beam received via a receive optical head assembly (ROHA) of the LCT, the ROHA having a first pointing system for pointing control of the ROHA; transmitting an optical communications beam outgoing from the LCT via free space propagation towards the remote LCT, the outgoing optical communications beam transmitted via a transmit optical head assembly (TOHA) of the LCT, the TOHA having a second pointing system for pointing control of the TOHA; and performing pointing control separately for the ROHA and the TOHA, based on respective evaluations of a first alignment signals output by a first alignment camera in the ROHA and second alignment signals by a second alignment camera in the TOHA.

17. The method according to claim 16, wherein performing pointing control separately for the ROHA and the TOHA comprises separately detecting optical alignment of the ROHA and the TOHA with respect to an optical beacon beam originating from the remote LCT and incoming to both the ROHA and the TOHA after an initial pointing of the ROHA and the TOHA, and wherein separately detecting the optical alignment of the ROHA and the TOHA comprises detecting the optical alignment of the ROHA via a first alignment sensing camera in the ROHA and detecting the optical alignment of the TOHA via a second alignment sensing camera in the TOHA.

18. The method according to claim 16, wherein performing pointing control separately for the ROHA and the TOHA comprises initially pointing at least one of the ROHA and the TOHA on an open-loop basis, as an initially-pointed OHA, and performing further pointing adjustments of the initially-pointed OHA responsive to detecting a beam position of an optical beacon beam transmitted by the remote LCT and impinging on an alignment-detection camera of the initially- pointed-OHA as a consequence of the initial pointing.

19. The method according to any one of claims 16-18, wherein the incoming optical communications beam is at a first optical wavelength and the outgoing optical communications beam is at a second optical wavelength that is spaced from the first optical wavelength.

20. A satellite communications system (SCS) comprising: an optical ground station (OGS) configured for operation at one end of an optical feeder link (OFL); a satellite configured for operation at the other end of the OFL, the satellite comprising: one or more radiofrequency (RF) antenna systems; a RF communications payload configured to transmit forward link traffic to respective terminals via the one or more RF antenna systems, and to receive return link traffic from the respective terminals via the one or more RF antenna systems; and a laser communications terminal (LCT) comprising: a receive optical head assembly (ROHA) configured for optical beam reception, the ROHA including a first pointing system configured for controlling a pointing direction of the ROHA; a transmit optical head assembly (TOHA) configured for optical beam transmission, the TOHA including a second pointing system for controlling a pointing direction of the TOHA independent of the pointing direction of the ROHA; and pointing control circuitry configured to control the first pointing system responsive to first alignment signals output from a ROHA alignment sensing camera, and further configured to control the second pointing system responsive to second alignment signals output from a TOHA alignment sensing camera.

21. The SCS according to claim 20, wherein the ROHA is configured to receive an incoming optical communications beam at a first optical wavelength, and the TOHA is configured to transmit an outgoing optical communications beam at a second optical wavelength.

22. The SCS according to claim 21, wherein the ROHA is configured to receive an incoming optical beacon beam at a third optical wavelength, the first and third optical wavelengths being within a first range of optical wavelengths and the second optical wavelength being outside the first range of optical wavelengths, with the ROHA including a shared optical path configured to direct the incoming optical beacon beam towards the ROHA alignment sensing camera and direct at least a portion of the incoming optical communications beam towards a receive (RX) fiber collimator.

23. The SCS according to claim 22, wherein the shared optical path of the ROHA is further configured to direct a portion of the incoming optical communications beam towards the ROHA alignment sensing camera, such that either or both the incoming optical beacon beam and the incoming optical communications beam are useable by the pointing control circuitry for controlling the pointing the of the ROHA.

24. The SCS according to claim 22 or 23, wherein the TOHA is configured to receive the incoming optical beacon beam, with the TOHA including a shared optical path of the TOHA that is configured to direct the incoming optical beacon beam towards the TOHA alignment sensing camera and direct the outgoing optical communications beam for transmission from the TOHA.

25. The SCS according to claim 24, wherein the TOHA is further configured to transmit an outgoing optical beacon beam, for pointing control use by a remote LCT at the other end of the OFL, the shared optical path of the TOHA configured to direct the outgoing optical beacon beam for transmission from the TOHA.

26. The SCS according to claim 25, wherein the outgoing optical beacon beam is at a fourth optical wavelength, the second and fourth optical wavelengths being within a second range of optical wavelengths, the second range of optical wavelengths not overlapping with the first range of optical wavelengths.

27. The SCS according to any one of claims 20-26, wherein the ROHA includes a first optical telescope dedicated to the ROHA, and the TOHA includes a second optical telescope dedicated to the TOHA.

28. The SCS according to any one of claims 20-27, wherein the pointing control circuitry is configured to control the pointing direction of the ROHA responsive to first alignment signals output by the ROHA alignment sensing camera, and control the pointing direction of the TOHA responsive to second alignment signals output by the TOHA alignment sensing camera.

29. The SCS according to claim 28, wherein each of the ROHA- alignment sensing camera and the TOHA alignment sensing camera comprises a first four-quadrant sensor, with the first alignment signals comprising a detection signal for each quadrant, indicating an amount of light impinging on the quadrant.

30. The SCS according to any one of claims 20-29, wherein the first and second pointing systems each comprises a first pointing control element that is configured to provide gross pointing adjustment and a second pointing control element that is configured to provide fine pointing adjustment.

31. The SCS according to claim 30, and wherein the pointing control circuitry is configured to implement a pointing, acquisition, and tracking (PAT) algorithm for each of the ROHA and the TOHA as an OHA, wherein the PAT algorithm includes the pointing control circuitry: initially pointing the OHA on an open-loop basis, for initiating beacon-beam reception; and performing closed-loop pointing control of the OHA responsive to beacon-beam reception.

32. The SCS according to any one of claims 20-31, wherein the pointing control system is configured to align a first one of the ROHA and the TOHA on at least an initial basis, determining alignment assistance information based on aligning the first one, and use the alignment assistance information for faster alignment of the second one of the ROHA and the TOHA.

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