Satellite with lockable optical beam steering

Abstract

Disclosed methods and apparatuses provide for optical beam steering in a laser communications terminal (LCT) onboard a satellite, in which an electromagnetic braking system advantageously locks the steering mechanism when not in use. Locking the steering mechanism in this manner offers multiple advantages, such as isolating or otherwise resisting disturbances that might be imparted by the steering system to the optical assembly even when steering is not active, which in turn offers the opportunity for using lower-cost steering motors, and the opportunity for reducing average power consumption by removing power from the steering motors when not reorienting the optical assembly. Other example advantages of integrating an electromagnetic braking system into the steering system include inherent protection for the optical assembly during launch.

Description

SATELLITE WITH LOCKABLE OPTICAL BEAM STEERINGTECHNICAL FIELD

[0001] Disclosed methods and apparatuses relate to satellite communications systems and, particularly, relate to the use of satellite-based laser communications terminals (LCTs) with steerable optics.BACKGROUND

[0002] Satellites play an indispensable role in supporting global communication networks, enabling data transmission for internet access, broadcasting, and various other applications. Exponential growth in data demand, particularly for high-definition video streaming, cloud computing, and low-latency applications, place significant bandwidth demands on the “feeder links” used to couple satellites with their supporting ground network. Optical feeder links (OFLs) have emerged as a high-bandwidth alternative to older radiofrequency (RF) feeder links, with OFLs leveraging the advantages of optical wavelengths to deliver high bandwidth, reduced latency, and secure data transmission.

[0003] Complementary laser communications terminals (LCTs) in a supporting ground stations and in a satellite anchor respective ends of the OFL, and provide for bidirectional optical communications based on the free space propagation of laser beams going between the ground station and the satellite. LCT implementation onboard the satellite raises key challenges going well beyond the conventional challenges of size, weight, and power (SWAP) restrictions.

[0004] For example, the onboard LCT must establish and maintain precise optical alignment with the supporting ground station and may need to steer its optical assembly, such as an optical telescope, for aligning with different ones among multiple ground stations. While beam steering mechanisms provide the ability for ground station reselection over some defined range of angular articulation, achieving the necessary steering precision entails introducing additional sources of disturbance with respect to maintaining optical alignment on the order of microradians.SUMMARY

[0005] Disclosed methods and apparatuses provide for optical beam steering in a laser communications terminal (LCT) onboard a satellite, in which an electromagnetic braking system advantageously locks the steering mechanism when not in use. Locking the steering mechanism in this manner offers multiple advantages, such as isolating or otherwise resisting disturbances that might be imparted by the steering system to the optical assembly even when steering is notactive, which in turn offers the opportunity for using lower-cost steering motors, and the opportunity for reducing average power consumption by removing power from the steering motors when not reorienting the optical assembly. Other example advantages of integrating an electromagnetic braking system into the steering system include inherent protection for the optical assembly during launch.

[0006] An example embodiment comprises a satellite configured for use in a satellite communications system (SCS). The satellite includes a communications payload and a LCT that is configured to couple the communications payload to an optical ground station (OGS) of the SCS. The LCT includes an optical head comprising an optical telescope configured for free space transmission or reception of an optical beam and a beam steering assembly (BSA) configured for pointing the optical telescope. The BSA includes axial mounts for carrying the optical telescope, a motor drive system integrated with the axial mounts and configured to provide controlled rotation of the optical telescope over a range of azimuth angles and a range of elevation angles, and an electromagnetic braking system. The electromagnetic braking system is integrated with the axial mounts and has a locked state in which the optical telescope is locked against rotation, and further has an unlocked state in which the optical telescope is not locked against rotation.

[0007] Another embodiment comprises a method of operating a satellite configured for use in a SCS. The method includes pointing an optical telescope of the satellite at a selected OGS of the SCS, based on unlocking an electromagnetic braking system used to lock a BSA used to steer the optical telescope, and applying motor control signals to a motor drive system associated with the BSA. The method further includes removing power from the motor drive system and relocking the electromagnetic braking system, subsequent to pointing the optical telescope at the selected OGS, and, subsequent to relocking the electromagnetic braking system and removing power from the motor drive system, controlling a fast steering mechanism (FSM) that is located behind the optical telescope, to maintain optical alignment between the LCT and the selected OGS.

[0008] 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

[0009] Figure 1 is a block diagram of a satellite communications system (SCS), according to an example embodiment.

[0010] Figure 2 is a perspective view of a laser communications terminal (LCT), according to an example embodiment.

[0011] Figure 3 is a cut-away perspective view of the example LCT shown in Figure 2.

[0012] Figure 4 is a simplified diagram of example optical paths and optical components associated with an optical telescope and corresponding optical bench of the example LCT.

[0013] Figure 5 is a front view of the example LCT.

[0014] Figure 6 is another cut-away perspective view of the example LCT, and illustrates the optical beam and disturbances affecting line-of-sight point, along with example details for a fast steering mechanism (FSM).

[0015] Figure 7 is a cut-away diagram of an electromagnetic brake according to one embodiment, for use in friction-locking the BSA of a LCT.

[0016] Figures 8 and 9 are block diagrams of example circuit arrangements for selectively applying supply power to or removing supply power from an electromagnetic brake and a steering motor, respectively.

[0017] Figure 10 is a logic flow diagram of a method of operation by a satellite, according to an example embodiment.DETAILED DESCRIPTION

[0018] Figure 1 illustrates an example embodiment of a satellite communications system (SCS) 10, including a ground segment 12 and a space segment 14. A ground network 16 comprised in or constituting the ground segment 12 includes a communications processing system (CPS) 18 and a plurality of optical ground stations (OGSs) 20. Space segment entities include one or more satellites 22, with one satellite 22 shown for ease of illustration. In one or more embodiments, the satellite(s) 22 are geosynchronous and, in at least one embodiment, the satellite(s) 22 are geostationary.

[0019] Example arrangements of the CPS 18 include one or more computer servers or other computer nodes that are specially adapted to interface with one or more external networks 24, such as the Internet or other data and telecommunications networks. User traffic 26 flows into the ground network 16 from the external network(s) 24, for relaying by the SCS 10 as forward link user traffic 28 to targeted user terminals (UTs) 30 among an overall population of UTs 30 served by the SCS 10.

[0020] In at least one embodiment, the satellite 22 communicatively couples to the ground network 16 via one or more optical feeder links (OFLs) 32. For example, a selected one among a plurality of available, geographically distributed OGSs 20 serves the satellite 22 at any given time, with one or more optical feeder uplink signals 34 in the OFL 32 carrying forward user traffic for targeted ones among the population of UTs 30 served by the satellite 22. In this regard, each OGS 20 includes one or more laser communications terminals (LCTs), which are not shown in the diagram, and the satellite 22 carries one or more LCTs 36, with one example LCT 36 shown in the diagram.

[0021] Each LCT 36 onboard the satellite 22 supports an OFL 32 and, in one or more embodiments herein, the LCT 36 is steerable, meaning that it can be pointed at any given OGS 20 among a plurality of OGSs 20 that are available for serving the satellite 22, as a selected OGS 20. Whether for maintenance reasons or reasons of optical link quality or network loading, the satellite 22 may be commanded to point its LCT 36 towards different OGSs 20 at different times. Regardless, with the LCT 36 supporting a respective OFL 32 with the selected OGS 20, the satellite 22 receives the optical feeder uplink signal(s) 34 that convey the forward link user traffic 28 targeting respective ones among the population of UTs 30 served by the satellite 22.

[0022] The satellite 22 relays this forward link user traffic 28 to the targeted UTs 30 based on transmitting user downlink signals 38. In at least one embodiment, the user downlink signals 38 are radiofrequency (RF) downlink signals 38 that are transmitted via a user link antenna subsystem 40 as forward user beams 42. The forward user beams 42 may be formed by the user link antenna subsystem 40 as spot beams via the use of respective feedhorns and reflectors, or may be formed electronically in embodiments where the user link antenna subsystem comprises a phased array of antenna elements.

[0023] Regardless, each forward user beam 42 may be oriented towards a respective forward user beam coverage area and, although the diagram suggests a single UT 30 per forward user beam 42, there may be many UTs 30 located within each user beam coverage area. Correspondingly, the user downlink signals 38 may be multiplexed signals conveying forward user traffic 28 for many UTs 30.

[0024] In one or more embodiments, the satellite 22 functions as a bent-pipe satellite, meaning that the user downlink signals 38 are recovered or otherwise generated from the received optical feeder uplink signal(s) 34 without digital processing and remodulation. For example, a forward link transmitter 44 comprised in a communications payload 46 onboard the satellite 22 in one or more embodiments comprises a plurality of electrical signal paths, such aslow-noise amplifiers (LNAs), frequency converters, and power amplifiers (PAs), for relaying intermediate frequency or RF signals recovered from the optical feeder uplink signal(s) 34 as the aforementioned user downlink signals 38. In one or more other embodiments, the satellite 22 performs digital domain processing, i.e., it recovers digital-domain signals from the optical feeder uplink signal(s) 34 and performs remodulation of those signals, for generation of the user downlink signals 38.

[0025] In the return link direction, respective UTs 30 transmit RF user uplink signals 50, which are received by the satellite 22 via the user link antenna subsystem 40. While not shown explicitly in the diagram, return link beams or beamforming may be used, such that the RF user uplink signals 50 are received or otherwise processed on a return user beam basis. In any case, a return link transmitter 52 included in the communications payload 46 generates one or more optical domain signals from the received user uplink signals 50, which are transmitted on the OFL 32 by the LCT 36 as one or more optical feeder downlink signals 54. As with the forward link, the return link transmitter 52 may be configured for bent pipe relaying in the analog domain, or may be configured for digital domain processing and remodulation.

[0026] In either case, the optical feeder downlink signal(s) 54 convey return link user traffic 56 from respective ones of the UTs 30 served by the satellite 22, and the selected OGS 20 supporting the ground end of the OFL 32 cooperates with the CPS 18 to recover the return link user traffic 56 for forwarding to the one or more external network(s) 24 as outgoing user traffic 58.

[0027] While the foregoing details are useful for understanding generalized operations of the SCS 10 in one or more example embodiments, a particular focus herein regards steering of the LCT 36, such as is done when selecting or reselecting a particular one of the available OGSs 20 for use as the serving OGS 20. In one or more embodiments where the satellite 22 includes more than one LCT 36 for supporting more than one OFL 32 at the same time, or for providing redundancy, each such LCT 36 may be steerable independently and may be configured and operated in like manner.

[0028] For the illustrated LCT 36, control circuitry 60 onboard the satellite 22 is configured to perform pointing control of the LCT 36. Although shown in a centralized form, the control circuitry 60 may be implemented in distributed fashion, such as where LCT-specific pointing control circuitry 62 is integrated into or directly coupled with the LCT 36 for pointing control. Further portions of the overall control circuitry 60 include, for example, spacecraft control circuitry for station keeping, attitude control, etc.

[0029] Similarly, the depicted power supply 64 onboard the satellite 22 may comprise a multiplicity of power supplies or regulated power supply subsystems providing different voltage levels, amperage limits, and power qualities. These various subsystems provide supply power for the various electrical / electronic components onboard the satellite 22, including power for the control circuitry 60 / 62, the communications payload 46, etc.

[0030] As a basis for discussing LCT component and configuration details, Figure 2 illustrates an example embodiment of the LCT 36, which is carried onboard a satellite 22 and is configured to couple the communications payload 46 to an OGS 20 of a SCS 10. The LCT 36 in this example depiction includes an optical head comprising an optical telescope 100 configured for free space transmission or reception of an optical beam, e.g., a laser beam. Figure 2 depicts two laser beams, including an incoming laser beam 102 that conveys the aforementioned one or more optical feeder uplink signals 34, and an outgoing laser beam 104 that conveys the aforementioned one or more optical feeder downlink signals 54. The laser beams 102 and 104 comprise the OFL 32 and provide for free space transmission of user traffic and control signaling between the satellite 22 and a selected OGS 20, which has its own LCT 106.

[0031] Because the particular OGS 20 that is selected at any given time may change, the LCT 36 is steerable, meaning that it can be pointed in different directions. For example, the azimuth pointing angle and the elevation pointing angle may be adjusted or swept through corresponding angular ranges, allowing the LCT 36 to be pointed at any one among a plurality of geographically separated OGSs 20. Here, “pointed at” refers to alignment of the optical axis of the LCT 36 with the optical axis of the remote LCT 106 included in the selected OGS 20.

[0032] A beam steering assembly (BSA) 110 is configured for pointing the optical telescope 100 and it includes axial mounts 112 for carrying the optical telescope 100. The BSA 110 comprises a gimbal arrangement, for example, and includes a first pair of axial mounts 112-1 and 112-2 allowing for setting the azimuth angle of the optical telescope 100. A second pair of axial mounts 112-3 and 112-4 allow for setting the elevation angle of the optical telescope 100. A motor drive system 114 is integrated with the axial mounts 112, for pointing the optical telescope 100.

[0033] In at least one embodiment, the motor drive system 114 comprises a first motor 116-1 that is integrated into the first axial mount 112-1, for driving (rotating) the optical telescope 100 in the azimuthal plane. The pair of first and second axial mounts 112-1 and 112-2 are axially aligned for rotation of the optical telescope 100 in the azimuthal plane, and the first motor 116-1comprises a first harmonic drive, for example, that is responsive to first motor control signals 118-1.

[0034] Continuing with this example embodiment, the motor drive system 114 further comprises a second motor 1 16-2 that is integrated with the third axial mount 112-3. The pair of third and fourth axial mounts 112-3 and 112-4 are axially aligned for rotation of the optical telescope 100 in the elevational plane, and the second motor 116-2 comprises a second harmonic drive, for example, that is responsive to second motor control signals 118-2. The pointing control circuitry 62 introduced in Figure 1 is configured to generate the first and second motor control signals 118-1 and 118-2. For example, the pointing control circuitry 62 comprises digital control circuitry and associated analog motor interfaces, for generating the first and second motor control signals 118-1 and 118-2. The first and second motors 116-1 and 116-2 provide rotational or positional feedback, for example, such that the pointing control circuitry 62 senses the angular position of the optical telescope 100 in the azimuthal and elevational planes.

[0035] Further, an electromagnetic braking system 120 is integrated with the axial mounts 112 and it has a locked state in which the optical telescope 100 is locked against rotation, and further has an unlocked state in which the optical telescope 100 is not locked against rotation. For example, the electromagnetic braking system 120 comprises a first electromagnetic brake 122-1 that locks the axial mount 112-2 against rotation, thereby locking the BSA 110 against rotation in the azimuthal plane, and further comprises a second electromagnetic brake 122-2 that locks the axial mount 112-4 against rotation, thereby locking the BSA 110 against rotation in the elevational plane. The first and second electromagnetic brakes 122-1 and 122-2 are friction brakes in one or more embodiments, that friction-lock the respective axes of rotation. While the electromagnetic braking system 120 acts on respective axial mounts 112 of the BSA 110, one may equivalently say that the electromagnetic braking system 120 selectively locks or unlocks the optical telescope 100 with respect to its axes of rotation (e.g., azimuth and elevation).

[0036] The BSA 110 in one or more embodiments provides at least one additional axis of rotation and with the motor drive system 114 and the electromagnetic braking system 120 correspondingly expanded to operate on the additional axis / axes. In one or more embodiments, for each axis of rotation, there is at least one corresponding electromagnetic brake 122 that acts on a respective axial pin and includes a friction lock that is engaged via the magnetic field of an included permanent magnet during times when an included electromagnetic coil is unenergized and disengaged during times when the included electromagnetic coil is energized. As such, each electromagnetic brake 122 has the locked state as its default state — i.e., when no input power isapplied to the electromagnetic brake 122, its electromagnetic coil is not energized and the permanent magnet acts to friction-lock the brake. In at least one embodiment, the friction locking is sufficient to prevent axial rotation even when the corresponding motor 116 is active.

[0037] As noted, the axial mounts 1 12 in one or more embodiments are comprised in a multi-axis gimbal arrangement and the electromagnetic braking system 120 comprises a respective electromagnetic brake 122 per axis of the gimbal. Note that here and throughout this disclosure, suffixing is used when needed for clarity or to otherwise denote a particular one among several like entities or components. When suffixing is not used, the corresponding reference number may be used to refer to any given one or ones of the involved entities or components. For example, “112” without suffixing refers to any given axial mount or mounts, “116” without suffixing refers to any given motor or motors, and “122” without suffixing refers to any given electromagnetic brake or brakes.

[0038] Power savings and reliability or simplicity of control flow from the electromagnetic braking system 120 being configured such that the locked state attains in the absence of electrical power being applied to the electromagnetic braking system 120. This arrangement means that the locked state is a default state of the electromagnetic braking system 120, whereas the unlocked state attains only in the presence of electrical power being applied to the electromagnetic braking system 120.

[0039] In at least one embodiment, an electrical path used to provide power to the electromagnetic braking system 120 is open by default, and the pointing control circuitry 62 actively controls whether the electrical path is open or closed. Such circuitry may also be referred to as “BSA control circuitry” and there may be redundant electrical paths and associated switching circuitry, to prevent single-point failures from prevent energization of the electromagnetic braking system 120 for pointing of the optical telescope 100.

[0040] Broadly, in one or more example implementations, the electromagnetic braking system 120 is configured to be in the locked state absent operation of BSA control circuitry and the BSA control circuitry is configured to temporarily transition the electromagnetic braking system 120 from the locked state to the unlocked state, for adjusting the pointing of the optical telescope via actuation of the motor drive system. Here, “adjusting the pointing” shall be understood as meaning that the pointing direction of the optical telescope 100 is changed. That is, when the motor drive system 114 is not active, the optical telescope 100 is fixed in a particular pointing direction, or is at least nominally static in the absence of micro- vibrations or other perturbations.

[0041] The BSA control circuitry in one or more embodiments is configured to cause power to be applied to the motor drive system 114 for adjusting the pointing of the optical telescope 100 and to otherwise cause power to be removed from the motor drive system 114. In complementary fashion, the electromagnetic braking system 120 is in its locked state when the motor drive system 114 is not powered, and is in its unlocked state when the motor drive system 114 is powered. At least two meaningful advantages flow from such an arrangement, including the following: (1) power consumption is reduced by removing power from the motor drive system 114 during times when the optical telescope 100 is not being steered; (2) motor-induced disturbances of the optical alignment of the optical telescope 100 are eliminated by removing power from the motor drive system 114 during times when the optical telescope 100 is not being steered; (3) power consumption is reduced by requiring power only for unlocking the electromagnetic braking system; and BSA / telescope robustness is improved by having the locked state attain in the absence of power to the electromagnetic braking system 120.

[0042] One point worth elaborating is that item (2) in the foregoing list of example advantages also means that cheaper motors may be used in the motor drive system 114. That is, if maintaining a given pointing direction of the optical telescope 100 required ongoing dynamic control — powered “holding” — of the motors 1 16, any positional vibration or oscillation of the motors 116 would have to be below exacting thresholds. Further, if the motors 116 were used to maintain a given pointing direction, they would have to exhibit a high degree of insensitivity to electrical noise from the power supply 64.

[0043] By powering the motors 116 only when actively adjusting the pointing of the optical telescope 100, such problems are avoided, as the motors 116 need only be precise enough to move the optical telescope 100 into alignment with the selected OGS 20, after which they are shut down and the optical telescope is locked against rotation via the electromagnetic braking system 120. For example, in one or more embodiments, the pointing control circuitry 62 is configured to: (a) establish a coarse optical alignment of the LCT 36 with the OGS 20 by using the BSA 110 to adjust the pointing of the optical telescope 100, based on unlocking the electromagnetic braking system 120 and applying motor control signals 118 to the motor drive system 114; (b) lock the coarse optical alignment by returning the electromagnetic braking system 120 to the locked state and powering down the motor drive system 114; and (c) control fine optical alignment of the LCT 36 with the OGS 20, based on dynamic control of a fast steering mechanism (FSM) that is not shown but is “behind” the optical telescope 100 in theoptical sense. In the illustrated example, the LCT 36 includes an optical bench 124 that is carried on or in association with the BSA 110, where the optical bench 124 includes the FSM.

[0044] As a further example, the pointing control circuitry 62 (or the overall control circuitry 60) of the satellite 22 in one or more embodiments is configured to perform an OGS selection procedure based on the control circuitry being configured to: (a) determine which OGS 20 from among a plurality of candidate OGSs 20 is the selected OGS; (b) point the optical telescope 100 at the selected OGS 20 using the BSA 110, based on unlocking the electromagnetic braking system 120, applying power to the motor drive system 1 14, and applying motor control signals 118 to the motor drive system 114; (c) remove power from the motor drive system 114 and relock the electromagnetic braking system 120, subsequent to pointing the optical telescope 100 at the selected OGS 20; and (d) subsequent to relocking the electromagnetic braking system 120 and removing power from the motor drive system 114, control the FSM, to maintain optical alignment between the LCT 36 and the selected OGS 20.

[0045] The pointing control circuitry 62 in one or more embodiments is configured to use open loop control of the BSA 110, for pointing the optical telescope at the selected OGS 20. For example, the open loop control is based on ephemeris and knowledge of the geographic coordinates of the selected OGS 20. In one or more of the same embodiments, or in one or more other embodiments, the pointing control circuitry 62 is configured to use closed loop control of the FSM, to maintain the optical alignment between the LCT 36 and the selected OGS 20. For example, the optical bench 124 includes an optical sensor that indicates alignment error of the incoming laser beam 102, with closed loop control of the FSM being driven by a corresponding alignment error signal. In an example case here, the precision or control quality of the motor(s) used to orient the FSM need not have the micro-radian precision associated with fine adjustments of the optical alignment of the LCT 36 with the selected OGS 20, because the rotational precision of such motors is magnified by the optical gain of the optical telescope 100. That gain may be significant, e.g., a gain of 20 or more.

[0046] Figure 3 depicts the example LCT 36 in a cut-away or cross-section view, which exposes example details for the optical bench 124. Here, the term “optical bench” denotes a stable structure and precision components that include, for example, beam steering mirrors, optical isolators (for wavelength filtering), beam splitters, focal plane arrays, fiber optic interfaces to / from the communications payload, and the aforementioned FSM. In particular, Figure 3 depicts an optical path 130 within the optical bench 124, with a FSM 132 disposed within the optical path 130.

[0047] The FSM 132 provides fast, fine control of the optical alignment of the LCT 36, but over a very limited angular range, e.g., micro-radians, while the BSA 110 provides coarse or gross pointing control of the optical telescope 100 over a much larger range, e.g., a few degrees. When the motor drive system 114 is powered down or otherwise idled, the FSM 132 may be controlled dynamically, to compensate for BSA jitter and any attitude disturbances affecting the satellite 22.

[0048] Figure 4 adds further example details by depicting the optical telescope 100 and optical bench 124 in a simplified form that eases discussion of the germane elements and features. The optical telescope 100 provides a received (RX) optical beam to the optical bench 124, being all or portion of the laser beam 102 impinging on the optical telescope 100. Conversely, the optical bench 124 provides a transmit (TX) optical beam to the optical telescope 100, for transmission as the outgoing laser beam 104, to be received at the selected OGS 20. The optical path 130 may comprise two or more paths or path segments associated with optical transmission and reception, respectively, and there may be respective FSMs 132-1 and 132-2. In one example, each FSM 132 is a mounted mirror that is rotatable in two axes corresponding to azimuth angle and elevation angle.

[0049] In at least one embodiment, a motor drive system similar to the one used for rotation of the BSA 110 may be used for rotating the FSM mirror. Again, with the FSMs 132 being located “behind” the optical telescope 100, the precision of such rotational control benefits from the optical gain. There may be one or more FSMs 132; for example, two or three FSMs 132 work in conjunction to achieve precise beam stabilization along both the transmission and reception paths. One FSM 132 in a multi-FSM embodiment may be used to set an intentional angular offset between the transmission and reception paths.

[0050] Figure 4 also highlights optical fibers (OFs) 140-1 and 140-2, which are used to convey optical signals to / from the communications payload 46 of the satellite 22. In the optical RX direction (forward link direction), received optical signals propagate via OF into optoelectronic circuitry included in the forward link transmitter 44, for recovery of electrical-domain communication signals to be transmitted by the satellite 22 in the forward link. In the optical TX direction (return link direction), the return link transmitter 52 includes opto-electronic circuitry configured to convert electrical-domain communication signals into the optical domain, for fiber propagation into the optical bench 124.

[0051] Finally, Figure 4 illustrates a pointing-acquisition-tracking (PAT) sensor 142, which comprises, for example, a multi-quadrant optical sensor that outputs an alignment error signalindicative of angular direction and extent by which the LCT 36 is offset from a nominal centering of the optical alignment with the OGS 20. This alignment error signal is used by the pointing control circuitry 62, for example, for closed loop control of the FSM(s) 132, for zeroing out the alignment error or otherwise keeping the alignment error below some defined threshold.

[0052] Figure 5 illustrates another view of the LCT 36, looking into the optical telescope 100. This front or facing view provides a better illustration of the respective first and second motors 116-1 and 116-2, along with the respective first and second electromagnetic brakes 122-1 and 122-2. The first motor 116-1 causes rotation of the optical telescope 100 about a first axis, with the first electromagnetic brake 122-1 locking the optical telescope 100 against rotation about that first axis. Similarly, the second motor 116-2 causes rotation of the optical telescope 100 about a second axis, with the second electromagnetic brake 122-2 locking the optical telescope 100 against rotation about that second axis.

[0053] Figure 6 provides another perspective view of the example LCT 36, illustrating FSM- based pointing — fine optical alignment adjustment — and further illustrating that FSM control may be used to compensate for micro-vibrations of the satellite 22, once gross pointing is achieved via the BSA 110 and the electromagnetic braking system 120 is locked. Note that the indicated BSA jitter is associated with operation of the BSA during the unlocked state of the electromagnetic braking system 120. The electromagnetic braking system 120 in its locked state prevents such jitter and, in any case, in at least one embodiment, the motor drive system 114 is powered down when BSA adjustments are not active.

[0054] FFSM control may provide for fine adjustments over the angular ranges needed for ground station tracking. Successful optical ground station tracking entails receiving one or more coaxial optical beams from the optical ground station and correcting the line of sight to accurately direct the corresponding outgoing optical beam(s) to the ground station.

[0055] Figure 7 illustrates a cross-sectional view of an example embodiment of an electromagnetic brake 122 with friction locking. The electromagnetic brake 122 as an overall assembly includes a base plate 150 for mounting on an axial pin or shaft of the BSA 110, as suggested in Figure 1. Further included is a permanent magnet 152, an electromagnetic coil 154, an armature / frictional disc 156, and a spring 158.

[0056] When the electromagnetic coil 154 is energized, its magnetic field disengages the armature / frictional disc 156, thereby unlocking the electromagnetic brake 122. Conversely, when the electromagnetic coil 154 is de-energized — not powered — the armature / frictional disc 156 is urged into frictional engagement via the magnetic field of the permanent magnet 152 and thespring 158. In one or more other embodiments, the electromagnetic brake 122 relies on spring force alone for friction braking, such that brake 122 automatically locks under spring force, in the absence of electricity.

[0057] Put simply, when unpowered, the electromagnetic brake 122 is in the locked state and when powered, the electromagnetic brake 122 is in the unlocked state.

[0058] Figure 8 illustrates an arrangement for selective application of supply power (Vdd) to an electromagnetic brake 122. The electrical path 160 for coupling supply power into the electromagnetic brake 122 includes a switch 162, such as an electro- mechanical relay or transistor-based switch. The pointing control circuitry 62 is configured to output a control signal (CNTL) that causes the switch 162 to open or close. In at least one embodiment, the switch 162 is a normally open switch, such that active control by the pointing control circuitry 62 is required to close it.

[0059] Figure 9 illustrates a similar arrangement for selective application of supply power (Vdd) to a motor 116. Here, the supply power may be the same as or different from the supply power used to power the electromagnetic brake 122. The electrical path 170 for coupling supply power into the motor 116 includes a switch 172, such as an electro-mechanical relay or transistor-based switch. The pointing control circuitry 62 is configured to output a control signal (CNTL) that causes the switch 172 to open or close. In at least one embodiment, the switch 172 is a normally open switch, such that active control by the pointing control circuitry 62 is required to close it.

[0060] With respect to both Figures 9 and 10, there may be redundant electrical paths and switches, and there may be correspondingly redundant circuits within the pointing control circuitry 62, such that no single point of failure causes a failure of the motor drive system 114 that includes the motor(s) 116 or a failure of the electromagnetic braking system 120, with its electromagnetic brakes 122.

[0061] Figure 10 illustrates a method 1000 of operating a satellite configured for use in a SCS. For instance, the example satellite 22 discussed and illustrated herein performs the method 1000.

[0062] The method 1000 includes the satellite 22: pointing (Block 1002) an optical telescope 100 of the satellite 22 at a selected OGS 20 of the SCS 10, based on unlocking an electromagnetic braking system 120 used to lock a BSA 110 that is used to steer the optical telescope 100, and applying motor control signals 118 to a motor drive system 114 associated with the BSA 110; removing (Block 1004) power from the motor drive system 114 andrelocking the electromagnetic braking system 120, subsequent to pointing the optical telescope 100 at the selected OGS 20; and subsequent to relocking the electromagnetic braking system 120 and removing power from the motor drive system 114, controlling (Block 1006) a FSM 132 that is located behind the optical telescope 100, to maintain optical alignment between the LCT 36 and the selected OGS 20.

[0063] In at least one embodiment of the method 1000, pointing the optical telescope 100 comprises performing open loop pointing control, based on ephemeris and knowledge of the geographic coordinates of the selected OGS 20. In the same embodiment, or in one or more other embodiments, controlling the FSM 132 to maintain the optical alignment comprises performing closed loop control of the FSM 132. As noted, closed loop control may be driven by an alignment error signal.

[0064] In one or more embodiments, the method 1000 further comprises the control circuitry 60 onboard the satellite 22 determining which OGS 20 from among a plurality of OGSs 20 included in the SCS 10 is the selected OGS 20. For example, the satellite 22 receives control signaling via the OFL 32, which identifies the selected OGS 20 from among a plurality of OGS 20 that are candidates for selection. In this regard, the control circuitry 60, including the pointing control circuitry 62, comprises, in one or more embodiments, digital circuitry in the form of one or more of microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, field programmable gate arrays, complex programmable devices, etc., along with analog interface circuitry, such as for driving the motors 116, sensing the angular position of the optical telescope 100, sensing the alignment error signal(s) from a PAT sensor 142, closing power switches 162 and 172, etc.

[0065] In at least one embodiment, the control circuitry 60 comprises supporting interface circuitry and one or more microprocessors that are specially adapted to carry out the pointing control described herein, based on executing stored computer program instructions held in computer readable media included with the control circuitry 60. In one or more other embodiments, the control circuitry 60 comprises configured logic blocks contained in one or more field programmable gate arrays or other programmable logic devices, along with supporting interface circuitry.

[0066] However the control circuitry 60 is implemented, in one or more embodiments, the motor drive system 114 provides coarse pointing control of the optical telescope 100 for selecting any particular one among two or more geographically separated OGSs 20, and the method 1000 further comprises, when not repointing the optical telescope 100 towards a newlyselected OGS 20, the control circuitry 60 / 62 leaving the electromagnetic braking system 120 in a locked state and removing power from the motor drive system 114, such that the satellite 22 relies on control of the FSM(s) 132 to mitigate perturbations affecting the optical alignment.

[0067] With the above examples in mind, the various embodiments described herein address problems arising from the use of “conventional” BSAs for pointing control in satellite-based LCTs. Here, a “conventional” BSA lacks the motor and braking control mechanisms and logic described herein, and they correspondingly exhibit BSA-induced pointing jitter in the range of a few micro-radians or more. Such jitter is significant in the context of fine optical alignment of the LCT with a ground station. Such jitter may be difficult to compensate with conventional FSM control, and power consumption and control complex may correspondingly increase in the satellite.

[0068] An advantageous recognition herein is that the BSA in a geo-satellite is idle — not intentionally rotating — for the majority of the time, because it is used for pointing the included optical telescope at the same ground station for potentially long periods of time. The mechanisms and control logic disclosed herein for de-powering BSA drive motors and friction locking the rotational mounts / optical telescope of the BSA therefore can be understood as simultaneously reducing average power consumption and removing at least one significant source of perturbations affecting the fine alignment that must be maintained via ongoing FSM control.

[0069] In a typical operational scenario, the advantageous arrangements illustrated herein for the satellite 22 allow the motor drive system 114 to be powered only during brief periods, such as when the satellite 22 is commanded to or otherwise decides to change from one OGS 20 to another. As a further benefit, because the electromagnetic braking system 120 in one or more embodiments is configured to be in the locked state in the absence of electrical power, it safeguards the BSA / optical telescope assembly of the satellite 22 during satellite launch. That is, no additional “launch lock” mechanism need be included onboard the satellite 22, which yields further size and weight savings.

[0070] 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 satellite configured for use in a satellite communications system (SCS), wherein the satellite comprises: a communications payload; and a laser communications terminal (LCT) configured to couple the communications payload to an optical ground station (OGS) of the SCS, the LCT including an optical head comprising: an optical telescope configured for free space transmission or reception of an optical beam; and a beam steering assembly (BSA) configured for pointing the optical telescope, the BSA comprising: axial mounts for carrying the optical telescope; a motor drive system integrated with the axial mounts and configured to provide controlled rotation of the optical telescope over a range of azimuth angles and a range of elevation angles; and an electromagnetic braking system integrated with the axial mounts and having a locked state in which the optical telescope is locked against rotation, and further having an unlocked state in which the optical telescope is not locked against rotation.

2. The satellite according to claim 1, wherein the electromagnetic braking system comprises one or more electromagnetic brakes, each electromagnetic brake selectively locking the optical telescope against rotation for a respective one of azimuth and elevation axes.

3. The satellite according to claim 2, wherein each electromagnetic brake acts on a respective axial pin and includes a friction lock that is engaged via the magnetic field of an included permanent magnet during times when an included electromagnetic coil is unenergized and disengaged during times when the included electromagnetic coil is energized, such that the locked state is a default state requiring no input power to electromagnetic brake.

4. The satellite according to any one of claims 1-3, wherein the axial mounts are comprised in a multi-axis gimbal arrangement and wherein the electromagnetic braking system comprises a respective electromagnetic brake per axis of the gimbal.

5. The satellite according to any one of claims 1-4, wherein the electromagnetic braking system is configured such that the locked state attains in the absence of electrical power being applied to the electromagnetic braking system, such that the locked state is a default state of the electromagnetic braking system, and wherein the unlocked state attains in the presence of electrical power being applied to the electromagnetic braking system.

6. The satellite according to any one of claims 1-5, wherein an electrical path used to provide power to the electromagnetic braking system is open by default, and wherein active control by BSA control circuitry included in the satellite is required to close the electrical path.

7. The satellite according to any one of claims 1-6, wherein the electromagnetic braking system is configured to be in the locked state absent operation of BSA control circuitry included in the satellite, and wherein the BSA control circuitry is configured to temporarily transition the electromagnetic braking system from the locked state to the unlocked state, for adjusting the pointing of the optical telescope via actuation of the motor drive system.

8. The satellite according to any one of claims 1-7, wherein BSA control circuitry included in the satellite is configured to cause power to be applied to the motor drive system for adjusting the pointing of the optical telescope and to otherwise cause power to be removed from the motor drive system, such that the motor drive system is inactive when not adjusting the pointing of the optical telescope.

9. The satellite according to any one of claims 1-8, wherein the satellite includes control circuitry configured to: establish a coarse optical alignment of the LCT with the OGS by using the BSA to adjust the pointing of the optical telescope, based on unlocking the electromagnetic braking system and applying motor control signals to the motor drive system; lock the coarse optical alignment by returning the electromagnetic braking system to the locked state and powering down the motor drive system; andcontrol fine optical alignment of the LCT with the OGS, based on dynamic control of a fast steering mechanism that is behind the optical telescope.

10. The satellite according to any one of claims 1-9, wherein the OGS is a selected OGS among a plurality of OGSs of the SCS and the satellite further comprises control circuitry operative to perform an OGS selection procedure based on the control circuitry being configured to: determine which OGS is the selected OGS; point the optical telescope at the selected OGS using the BSA, based on unlocking the electromagnetic braking system, applying power to the motor drive system and applying motor control signals to the motor drive system; remove power from the motor drive system and relock the electromagnetic braking system, subsequent to pointing the optical telescope at the selected OGS; and subsequent to relocking the electromagnetic braking system and removing power from the motor drive system, control a fast steering mechanism (FSM) that is located behind the optical telescope, to maintain optical alignment between the LCT and the selected OGS.

11. The satellite according to claim 10, wherein the control circuitry is configured to use open loop control of the BSA, for pointing the optical telescope at the selected OGS, the open loop control based on ephemeris and knowledge of the geographic coordinates of the selected OGS.

12. The satellite according to claim 10 or 11, wherein the control circuitry is configured to use closed loop control of the FSM, to maintain the optical alignment between the LCT and the selected OGS.

13. A method of operating a satellite configured for use in a satellite communications system (SCS), the method comprising: pointing an optical telescope of the satellite at a selected optical ground station (OGS) of the SCS, based on unlocking an electromagnetic braking system used to lock a beam steering assembly (BSA) used to steer the optical telescope, and applying motor control signals to a motor drive system associated with the BSA;removing power from the motor drive system and relocking the electromagnetic braking system, subsequent to pointing the optical telescope at the selected OGS; and subsequent to relocking the electromagnetic braking system and removing power from the motor drive system, controlling a fast steering mechanism (FSM) that is located behind the optical telescope, to maintain optical alignment between the LCT and the selected OGS.

14. The method according to claim 13, wherein pointing the optical telescope comprises performing open loop pointing control, based on ephemeris and knowledge of the geographic coordinates of the selected OGS.

15. The method according to claim 13 or 14, wherein controlling the FSM to maintain the optical alignment comprises performing closed loop control of the FSM.

16. The method according to any one of claims 13-15, wherein the method further comprises determining which OGS from among a plurality of OGSs included in the SCS is the selected OGS, based on uplink control signaling received at the satellite from a ground network of the SCS.

17. The method according to any one of claims 13-16, wherein the motor drive system provides coarse pointing control of the optical telescope for selecting any particular one among two or more geographically separated OGSs, and wherein the method further comprises, when not repointing the optical telescope towards a newly selected OGS, leaving the electromagnetic braking system in a locked state and removing power from the motor drive system, such that the satellite relies on control of the FSM to mitigate perturbations affecting the optical alignment.

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