CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Patent Application No. 16/233,767, filed December 27, 2018, which is hereby incorporated by reference in its entirety.

BACKGROUND

[0002] Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

[0003] Power generation systems may convert chemical and/or mechanical energy (e.g., kinetic energy) to electrical energy for various applications, such as utility systems. As one example, a wind energy system may convert kinetic wind energy to electrical energy.

[0004] The use of wind turbines as a means for harnessing energy has been used for a number of years. Conventional wind turbines typically include large turbine blades positioned atop a tower. The cost of manufacturing, erecting, maintaining, and servicing such wind turbine towers, and wind turbines is significant.

[0005] An alternative to the costly wind turbine towers that may be used to harness wind energy is to use an aerial vehicle attached to a ground station with an electrically conductive tether. Such an alternative may be referred to as an Airborne Wind Turbine or AWT.

SUMMARY

[0006] In one aspect, a method includes: (i) determining a period of natural oscillation of a floating ground station in an airborne wind turbine, wherein an aerial vehicle is coupled to the floating ground station via a tether, and wherein each natural-oscillation period comprises a forward displacement and a backward displacement of the floating ground station with respect to the aerial vehicle; (ii) determining a phase of oscillation of the floating ground station; and (iii) operating the aerial vehicle to fly in a closed path with: (a) a looping period that matches the natural-oscillation period of the floating ground station, and (b) a looping phase that aligns with the oscillation phase of the floating ground station such that movement of the aerial vehicle on a downstroke portion the closed path corresponds to forward displacement of the floating ground station, and movement of the aerial vehicle on an upstroke portion the closed path corresponds to reverse displacement of the floating ground station. [0007] In another aspect, airborne wind turbine (AWT) system includes an aerial vehicle, a floating ground station, and a control system. The control system is configured to: (a) determine a period of natural oscillation of a floating ground station in an airborne wind turbine, wherein an aerial vehicle is coupled to the floating ground station via a tether, and wherein each natural-oscillation period comprises a forward displacement and a backward displacement of the floating ground station with respect to the aerial vehicle; (b) determine a phase of oscillation of the floating ground station; and (c) operate the aerial vehicle to fly in a closed path with: (i) a looping period that matches the natural-oscillation period of the floating ground station, and (ii) a looping phase that aligns with the oscillation phase of the floating ground station such that movement of the aerial vehicle on a downstroke portion the closed path corresponds to forward displacement of the floating ground station, and movement of the aerial vehicle on an upstroke portion the closed path corresponds to reverse displacement of the floating ground station.

[0008] These as well as other aspects, advantages, and alternatives, will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

[0009] Figure 1 depicts an Airborne Wind Turbine (AWT), according to an example embodiment.

[0010] Figure 2 is a simplified block diagram illustrating components of an AWT, according to an example embodiment.

[0011] Figures 3a and 3b depict an example of an aerial vehicle transitioning from hover flight to crosswind flight, according to an example embodiment.

[0012] Figures 4a-c are graphical representations involving an angle of ascent, according to an example embodiment.

[0013] Figures 5a and 5b depict a tether sphere, according to an example embodiment.

[0014] Figures 6a-c depict an example of an aerial vehicle transitioning from crosswind flight to hover flight, according to an example embodiment.

[0015] Figure 7 depicts a side view of an airborne wind turbine system with a moored floating ground station, according to an example embodiment.

[0016] Figure 8A depicts a side view of an airborne wind turbine system, according to an example embodiment. [0017] Figure 8B depicts a side view of another airborne wind turbine system, according to an example embodiment.

[0018] Figure 9 is a flow chart illustrating a method 900, according to an example embodiment.

[0019] Figure 10 depicts a timeline showing movement an aerial vehicle and a ground station, according to an example method.

DETAILED DESCRIPTION

[0020] Exemplary methods and systems are described herein. It should be understood that the word“exemplary” is used herein to mean“serving as an example, instance, or illustration.” Any embodiment or feature described herein as“exemplary” or“illustrative” is not necessarily to be construed as preferred or advantageous over other embodiments or features. More generally, the embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed methods systems and can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein.

I. Overview

[0021] Airborne wind turbines may provide a significant advantage over conventional wind turbines when it comes to offshore power generation. In particular, strong, consistent winds may be found in deep offshore locations (e.g., in water that is 30 meters deep or deeper). Accordingly, floating airborne wind turbines may be implemented at such offshore locations, in order to take advantage of these favorable wind conditions.

[0022] Such floating airborne wind turbines may include a floating platform or ground station, which can be used to anchor the tether of an aerial vehicle operable for power generation (e.g., an energy kite), and possibly provide a platform for takeoff and landing of the aerial vehicle. The ground station may utilize a floating spar buoy platform, which is coupled to a mooring on the sea bed by a mooring system that includes one or more mooring lines or“tendons.”

[0023] In a spar buoy configuration, a floating ground station will move in response to unsteady loads from waves and/or currents, and will also move in response to unsteady loads from the tether (resulting from closed-loop flight of the tethered aerial vehicle). This motion of the floating ground station is characterized by certain natural frequencies. Further, this natural motion of the platform can pull the tether anchor point (on the ground station) back and forth. As such, there may be relationship between the motion of the floating ground station and the flight dynamics of the aerial vehicle, and each may be able to affect the other. [0024] The oscillating motion of the floating ground station is characterized by a certain natural frequency or frequencies (e.g., a resonant frequency of the airborne wind turbine system). The natural oscillation frequency may be defined by the characteristics of the airborne wind turbine system, and possibly by other factors such as the current and/or wind at its location. However, when waves are present, the waves may affect the oscillation of the floating ground station, such that it differs from the natural oscillation. In other words, the presence of waves can change the frequency (and period), amplitude, phase, and/or direction at which the floating ground station operates.

[0025] Additionally or alternatively, during flight by the aerial vehicle, the tension on the tether can also alter the frequency and/or phase of the floating ground station’s oscillation. And, in a scenario where waves are not present, the phase of the aerial vehicle’s periodic flight on a closed path (e.g., looping flight) can drive the phase of the ground station’s oscillation. Alternatively, in a scenario where waves are present, the phase of the ground station’s oscillation may in large part be driven by the combination of the aerial vehicle’s closed loop flight and the waves.

[0026] Further, in an illustrative airborne wind turbine, the aerial vehicle may generate power by flying in a substantially circular closed loop (or more generally, in a closed path), while tethered to the ground station. The downward force of gravity will tend to cause the aerial vehicle to speed up on the downstroke of each loop, and to slow down on the upstroke of each loop. However, this variation in speed may be undesirable because it can reduce the total energy generation of the airborne wind turbine, result in inefficient energy generation, and/or result in undesirable periodic variations in energy generation. Therefore, it is desirable to reduce and/or minimize the variations in speed as the aerial vehicle traverses each loop.

[0027] According to example embodiments, a control system matches the looping period of the aerial vehicle to natural period of oscillation for the floating platform (e.g., using active control of the kite, and possibly certain ground- station features). Further, in some cases, the control system can coordinate the phase of the aerial vehicle’s flight on the closed path (e.g., the looping phase) with the phase of the floating ground station’s oscillation. Configured as such, the airborne wind turbine may be able to extract energy from the aerial vehicle during its downstroke movement, and store that energy in the motion of the floating ground station (effectively pulling the ground station towards the aerial vehicle). In turn, on the upstroke, the energy stored in the floating grand station can be used to pull the aerial vehicle up the upstroke (e.g., with the platform moving away from the kite during the upstroke). This alignment of the aerial vehicle’s closed-path flight with the ground station’s oscillation results in constructive interference that reduces variations in aerial-vehicle speed and thus helps to improve energy capture by the system.

[0028] Further, in practice, when attempting to match the period (or frequency) and/or phase of the aerial vehicle’s closed-path flight with the period and/or phase of the ground station’s oscillation, there will typically be some phase lag due to, e.g., damping that occurs when moving the floating ground station through water. Accordingly, references to“matching” the looping period, frequency, and/or phase of the aerial vehicle’s closed-path flight to the period, frequency, and/or phase of the ground station’s oscillation, do not require an exact alignment of phase or exactly the same period or frequency. The function of matching the looping period, frequency, and/or phase should thus be understood to include any process undertaken in an effort to match the looping period frequency, and/or phase, even if such process does not result in an exact match (e.g., even if the result is a looping phase that lags slightly behind the phase of the ground station’s oscillation and/or some phase drift occurs).

[0029] Systems and information described with respect to Figures 1 through 6c are generally illustrative of airborne wind turbines. Systems, methods, an d charts described with respect to Figures 7 and beyond are illustrative of airborne wind turbines with floating ground stations.

II. Illustrative Systems

A. Airborne Wind Turbine (AWT)

[0030] Figure 1 depicts an AWT 100, according to an example embodiment. In particular, the AWT 100 includes a ground station 110, a tether 120, and an aerial vehicle 130. As shown in Figure 1, the aerial vehicle 130 may be connected to the tether 120, and the tether 120 may be connected to the ground station 110. In this example, the tether 120 may be attached to the ground station 110 at one location on the ground station 110, and attached to the aerial vehicle 130 at two locations on the aerial vehicle 130. However, in other examples, the tether 120 may be attached at multiple locations to any part of the ground station 110 and/or the aerial vehicle 130.

[0031] The ground station 110 may be configured to hold and/or support the aerial vehicle 130 until it is in an operational mode. The ground station 1 10 also may be configured to allow for the repositioning of the aerial vehicle 130 such that deployment of aerial vehicle 130 is possible. Further, the ground station 110 may be configured to receive the aerial vehicle 130 during a landing. The ground station 110 may be formed of any material that can suitably keep the aerial vehicle 130 attached and/or anchored to the ground while in hover flight, forward flight, and/or crosswind flight. In some implementations, a ground station 110 may be configured for use on land. However, a ground station 110 also may be implemented on a body of water, such as a lake, river, sea, or ocean. For example, a ground station could include or be arranged on a floating off-shore platform or a boat, among other possibilities. Further, a ground station 110 may be configured to remain stationary or to move relative to the ground or the surface of a body of water.

[0032] In addition, the ground station 110 may include one or more components (not shown), such as a winch, that may vary a length of the deployed tether 120. For example, when the aerial vehicle 130 is deployed, the one or more components may be configured to pay out and/or reel out the tether 120. In some implementations, the one or more components may be configured to pay out and/or reel out the tether 120 to a predetermined length. As examples, the predetermined length could be equal to or less than a maximum length of the tether 120. Further, when the aerial vehicle 130 lands in the ground station 110, the one or more components may be configured to reel in the tether 120.

[0033] The tether 120 may transmit electrical energy generated by the aerial vehicle 130 to the ground station 110. In addition, the tether 120 may transmit electricity to the aerial vehicle 130 in order to power the aerial vehicle 130 for takeoff, landing, hover flight, and/or forward flight. The tether 120 may be constructed in any form and using any material which may allow for the transmission, delivery, and/or harnessing of electrical energy generated by the aerial vehicle 130 and/or transmission of electricity to the aerial vehicle 130. The tether 120 also may be configured to withstand one or more forces of the aerial vehicle 130 when the aerial vehicle 130 is in an operational mode. For example, the tether 120 may include a core configured to withstand one or more forces of the aerial vehicle 130 when the aerial vehicle 130 is in hover flight, forward flight, and/or crosswind flight. The core may be constructed of any high strength fibers. In some examples, the tether 120 may have a fixed length and/or a variable length. For instance, in at least one such example, the tether 120 may have a length of 140 meters.

[0034] The aerial vehicle 130 may include or take the form of various types of devices, such as a kite (as illustrated in Figure 1), a wing, and/or an airplane, among other possibilities. The aerial vehicle 130 may be formed of structures of metal, plastic and/or other polymers. The aerial vehicle 130 may be formed of any material which allows for a high thrust-to-weight ratio and generation of electrical energy which may be used in utility applications. Additionally, the materials may be chosen to allow for a lightning hardened, redundant and/or fault tolerant design which may be capable of handling large and/or sudden shifts in wind speed and wind direction. Other materials may be possible as well.

[0035] The aerial vehicle 130 may be configured to fly substantially along a path 150 to generate electrical energy. The term“substantially along,” as used in this disclosure, refers to exactly along and/or one or more deviations from exactly along that do not significantly impact generation of electrical energy as described herein and/or transitioning an aerial vehicle between certain flight modes as described herein.

[0036] The path 150 may be various different shapes in various different embodiments. For example, the path 150 may be substantially circular. And in at least one such example, the path 150 may have a radius of up to 265 meters. The term“substantially circular,” as used in this disclosure, refers to exactly circular and/or one or more deviations from exactly circular that do not significantly impact generation of electrical energy as described herein. Other shapes for the path 150 may be an oval, such as an ellipse, the shape of a jelly bean, the shape of the number of 8, etc.

[0037] As shown in Figure 1, the aerial vehicle 130 may include a main wing 131, a front section 132, rotor connectors 133A-B, rotors 134A-D, a tail boom 135, a tail wing 136, and a vertical stabilizer 137. Any of these components may be shaped in any form which allows for the use of components of lift to resist gravity and/or move the aerial vehicle 130 forward.

[0038] The main wing 131 may provide a primary lift for the aerial vehicle 130. The main wing 131 may be one or more rigid or flexible airfoils, and may include various control surfaces, such as winglets, flaps, rudders, elevators, etc. The control surfaces may be used to stabilize the aerial vehicle 130 and/or reduce drag on the aerial vehicle 130 during hover flight, forward flight, and/or crosswind flight.

[0039] The main wing 131 may be any suitable material for the aerial vehicle 130 to engage in hover flight, forward flight, and/or crosswind flight. For example, the main wing 131 may include carbon fiber and/or e-glass. Moreover, the main wing 131 may have a variety dimensions. For example, the main wing 131 may have a span of 8 meters, an area of 4 meters squared, and an aspect ratio of 15. The front section 132 may include one or more components, such as a nose, to reduce drag on the aerial vehicle 130 during flight.

[0040] The rotor connectors 133 A-B may connect the rotors 134A-D to the main wing 131. In some examples, the rotor connectors 133 A-B may take the form of or be similar in form to one or more pylons. In this example, the rotor connectors 133 A-B are arranged such that the rotors 134A-D are spaced above and below the main wing 131. In some examples, a vertical spacing between corresponding rotors (e.g., rotor 134A and rotor 134B or rotor 134C and rotor 134D) may be 0.5 to 3 meters.

[0041] The rotors 134A-D may be configured to drive one or more motor-generators for the purpose of generating electrical energy when the vehicle is operated in an electrical power generation mode. The rotors 134A-D may each include one or more blades, such as two, three, four, five, or more blades. The one or more rotor blades may rotate via interactions with the wind (or apparent wind) and be used to drive the one or more motor-generators. In addition, the rotors 134A-D also may be configured to provide a thrust to the aerial vehicle 130 during flight. With this arrangement, each of the rotors 134A-D may function as a propulsion units, such as a propeller, driven by a motor-generators when the vehicle is operated in a thrust flight mode. Although the rotors 134A-D are depicted as four rotors in this example, in other examples the aerial vehicle 130 may include any number of rotors, such as less than four rotors or more than four rotors.

[0042] The tail boom 135 may connect the main wing 131 to the tail wing 136. The tail boom 135 may have a variety of dimensions. For example, the tail boom 135 may have a length of 2 to 6 meters. Moreover, in some implementations, the tail boom 135 could take the form of a body and/or fuselage of the aerial vehicle 130. And in such implementations, the tail boom 135 may carry a payload.

[0043] The tail wing 136 and/or the vertical stabilizer 137 may be used to stabilize the aerial vehicle and/or reduce drag on the aerial vehicle 130 during hover flight, forward flight, and/or crosswind flight. For example, the tail wing 136 and/or the vertical stabilizer 137 may be used to maintain a pitch of the aerial vehicle 130 during hover flight, forward flight, and/or crosswind flight. In this example, the vertical stabilizer 137 is attached to the tail boom 135, and the tail wing 136 is located on top of the vertical stabilizer 137. The tail wing 136 may have a variety of dimensions. For example, the tail wing 136 may have a length of 1 to 5 meters. Further, in some examples, the tail wing 136 may be located above a center of mass of the aerial vehicle 130.

[0044] While the aerial vehicle 130 has been described above, it should be understood that the methods and systems described herein could involve any suitable aerial vehicle that is connected to a tether, such as the tether 120.

B. Illustrative Components of an AWT

[0045] Figure 2 is a simplified block diagram illustrating components of the AWT 200. The AWT 200 may take the form of or be similar in form to the AWT 100. In particular, the AWT 200 includes a ground station 210, a tether 220, and an aerial vehicle 230. The ground station 210 may take the form of or be similar in form to the ground station 110, the tether 220 may take the form of or be similar in form to the tether 120, and the aerial vehicle 230 may take the form of or be similar in form to the aerial vehicle 130.

[0046] As shown in Figure 2, the ground station 210 may include one or more processors 212, data storage 214, and program instructions 216. A processor 212 may be a general-purpose processor or a special purpose processor (e.g., digital signal processors, application specific integrated circuits, etc.). The one or more processors 212 can be configured to execute computer-readable program instructions 216 that are stored in a data storage 214 and are executable to provide at least part of the functionality described herein.

[0047] The data storage 214 may include or take the form of one or more computer-readable storage media that may be read or accessed by at least one processor 212. The one or more computer-readable storage media can include volatile and/or non-volatile storage components, such as optical, magnetic, organic or other memory or disc storage, which may be integrated in whole or in part with at least one of the one or more processors 212. In some embodiments, the data storage 214 may be implemented using a single physical device (e.g., one optical, magnetic, organic or other memory or disc storage unit), while in other embodiments, the data storage 214 can be implemented using two or more physical devices.

[0048] As noted, the data storage 214 may include computer-readable program instructions 216 and perhaps additional data, such as diagnostic data of the ground station 210. As such, the data storage 214 may include program instructions to perform or facilitate some or all of the functionality described herein.

[0049] In a further respect, the ground station 210 may include a communication system 218. The communications system 218 may include one or more wireless interfaces and/or one or more wireline interfaces, which allow the ground station 210 to communicate via one or more networks. Such wireless interfaces may provide for communication under one or more wireless communication protocols, such as Bluetooth, WiFi (e.g., an IEEE 802.11 protocol), Long-Term Evolution (LTE), WiMAX (e.g., an IEEE 802.16 standard), a radio-frequency ID (RFID) protocol, near-field communication (NFC), and/or other wireless communication protocols. Such wireline interfaces may include an Ethernet interface, a Universal Serial Bus (USB) interface, or similar interface to communicate via a wire, a twisted pair of wires, a coaxial cable, an optical link, a fiber-optic link, or other physical connection to a wireline network. The ground station 210 may communicate with the aerial vehicle 230, other ground stations, and/or other entities (e.g., a command center) via the communication system 218.

[0050] In an example embodiment, the ground station 210 may include communication systems 218 that allows for both short-range communication and long- range communication. For example, the ground station 210 may be configured for short-range communications using Bluetooth and for long-range communications under a CDMA protocol. In such an embodiment, the ground station 210 may be configured to function as a “hot spot”; or in other words, as a gateway or proxy between a remote support device (e.g., the tether 220, the aerial vehicle 230, and other ground stations) and one or more data networks, such as cellular network and/or the Internet. Configured as such, the ground station 210 may facilitate data communications that the remote support device would otherwise be unable to perform by itself.

[0051] For example, the ground station 210 may provide a WiFi connection to the remote device, and serve as a proxy or gateway to a cellular service provider’s data network, which the ground station 210 might connect to under an LTE or a 3G protocol, for instance. The ground station 210 could also serve as a proxy or gateway to other ground stations or a command station, which the remote device might not be able to otherwise access.

[0052] Moreover, as shown in Figure 2, the tether 220 may include transmission components 222 and a communication link 224. The transmission components 222 may be configured to transmit electrical energy from the aerial vehicle 230 to the ground station 210 and/or transmit electrical energy from the ground station 210 to the aerial vehicle 230. The transmission components 222 may take various different forms in various different embodiments. For example, the transmission components 222 may include one or more conductors that are configured to transmit electricity. And in at least one such example, the one or more conductors may include aluminum and/or any other material which allows for the conduction of electric current. Moreover, in some implementations, the transmission components 222 may surround a core of the tether 220 (not shown).

[0053] The ground station 210 could communicate with the aerial vehicle 230 via the communication link 224. The communication link 224 may be bidirectional and may include one or more wired and/or wireless interfaces. Also, there could be one or more routers, switches, and/or other devices or networks making up at least a part of the communication link 224.

[0054] Further, as shown in Figure 2, the aerial vehicle 230 may include one or more sensors 232, a power system 234, power generation / conversion components 236, a communication system 238, one or more processors 242, data storage 244, and program instructions 246, and a control system 248.

[0055] The sensors 232 could include various different sensors in various different embodiments. For example, the sensors 232 may include a global positioning system (GPS) receiver. The GPS receiver may be configured to provide data that is typical of well-known GPS systems (which may be referred to as a global navigation satellite system (GNSS)), such as the GPS coordinates of the aerial vehicle 230. Such GPS data may be utilized by the AWT 200 to provide various functions described herein.

[0056] As another example, the sensors 232 may include one or more wind sensors, such as one or more pitot tubes. The one or more wind sensors may be configured to detect apparent and/or relative wind. Such wind data may be utilized by the AWT 200 to provide various functions described herein.

[0057] Still as another example, the sensors 232 may include an inertial measurement unit (IMU). The IMU may include both an accelerometer and a gyroscope, which may be used together to determine the orientation of the aerial vehicle 230. In particular, the accelerometer can measure the orientation of the aerial vehicle 230 with respect to earth, while the gyroscope measures the rate of rotation around an axis, such as a centerline of the aerial vehicle 230. IMUs are commercially available in low-cost, low-power packages. For instance, the IMU may take the form of or include a miniaturized MicroElectroMechanical System (MEMS) or a NanoElectroMechanical System (NEMS). Other types of IMUs also may be utilized. The IMU may include other sensors, in addition to accelerometers and gyroscopes, which may help to better determine position. Two examples of such sensors are magnetometers and pressure sensors. Other examples are also possible.

[0058] While an accelerometer and gyroscope may be effective at determining the orientation of the aerial vehicle 230, slight errors in measurement may compound over time and result in a more significant error. However, an example aerial vehicle 230 may be able mitigate or reduce such errors by using a magnetometer to measure direction. One example of a magnetometer is a low-power, digital 3-axis magnetometer, which may be used to realize an orientation independent electronic compass for accurate heading information. However, other types of magnetometers may be utilized as well.

[0059] The aerial vehicle 230 may also include a pressure sensor or barometer, which can be used to determine the altitude of the aerial vehicle 230. Alternatively, other sensors, such as sonic altimeters or radar altimeters, can be used to provide an indication of altitude, which may help to improve the accuracy of and/or prevent drift of the IMU.

[0060] As noted, the aerial vehicle 230 may include the power system 234. The power system 234 could take various different forms in various different embodiments. For example, the power system 234 may include one or more batteries for providing power to the aerial vehicle 230. In some implementations, the one or more batteries may be rechargeable and each battery may be recharged via a wired connection between the battery and a power supply and/or via a wireless charging system, such as an inductive charging system that applies an external time- varying magnetic field to an internal battery and/or charging system that uses energy collected from one or more solar panels.

[0061] As another example, the power system 234 may include one or more motors or engines for providing power to the aerial vehicle 230. In some implementations, the one or more motors or engines may be powered by a fuel, such as a hydrocarbon-based fuel. And in such implementations, the fuel could be stored on the aerial vehicle 230 and delivered to the one or more motors or engines via one or more fluid conduits, such as piping. In some implementations, the power system 234 may be implemented in whole or in part on the ground station 210.

[0062] As noted, the aerial vehicle 230 may include the power generation / conversion components 236. The power generation / conversion components 326 could take various different forms in various different embodiments. For example, the power generation / conversion components 236 may include one or more motor-generators, such as high-speed, direct-drive motor-generators. With this arrangement, the one or more motor-generators may drive and be driven by one or more rotors, such as the rotors 134A-D. And in at least one such example, the one or more motor-generators may operate at full rated power at wind speeds of 11.5 meters per second at a capacity factor which may exceed 60 percent, and the one or more motor-generators may generate electrical power from 40 kilowatts to 600 megawatts.

[0063] Moreover, as noted, the aerial vehicle 230 may include a communication system 238. The communication system 238 may take the form of or be similar in form to the communication system 218. The aerial vehicle 230 may communicate with the ground station 210, other aerial vehicles, and/or other entities (e.g., a command center) via the communication system 238.

[0064] In some implementations, the aerial vehicle 230 may be configured to function as a“hot spot”; or in other words, as a gateway or proxy between a remote support device (e.g., the ground station 210, the tether 220, other aerial vehicles) and one or more data networks, such as cellular network and/or the Internet. Configured as such, the aerial vehicle 230 may facilitate data communications that the remote support device would otherwise be unable to perform by itself.

[0065] For example, the aerial vehicle 230 may provide a WiFi connection to the remote device, and serve as a proxy or gateway to a cellular service provider’s data network, which the aerial vehicle 230 might connect to under an LTE or a 3G protocol, for instance. The aerial vehicle 230 could also serve as a proxy or gateway to other aerial vehicles or a command station, which the remote device might not be able to otherwise access.

[0066] As noted, the aerial vehicle 230 may include the one or more processors 242, the program instructions 244, and the data storage 246. The one or more processors 242 can be configured to execute computer-readable program instructions 246 that are stored in the data storage 244 and are executable to provide at least part of the functionality described herein. The one or more processors 242 may take the form of or be similar in form to the one or more processors 212, the data storage 244 may take the form of or be similar in form to the data storage 214, and the program instructions 246 may take the form of or be similar in form to the program instructions 216.

[0067] Moreover, as noted, the aerial vehicle 230 may include the control system 248. In some implementations, the control system 248 may be configured to perform one or more functions described herein. The control system 248 may be implemented with mechanical systems and/or with hardware, firmware, and/or software. As one example, the control system 248 may take the form of program instructions stored on a non-transitory computer readable medium and a processor that executes the instructions. The control system 248 may be implemented in whole or in part on the aerial vehicle 230 and/or at least one entity remotely located from the aerial vehicle 230, such as the ground station 210. Generally, the manner in which the control system 248 is implemented may vary, depending upon the particular application.

[0068] While the aerial vehicle 230 has been described above, it should be understood that the methods and systems described herein could involve any suitable aerial vehicle that is connected to a tether, such as the tether 230 and/or the tether 110.

C. Transitioning an Aerial Vehicle from Hover Flight to Crosswind Flight

[0069] Figures 3a and 3b depict an example 300 of transitioning an aerial vehicle from hover flight to crosswind flight, according to an example embodiment. Hover flight may be an example of the AWT operating in thrust flight mode, with the motor-generators consuming power and driving the rotor rotation to provide thrust to the AWT. Crosswind flight may be an example of drag flight mode, with the wind or apparent wind rotating the rotors and the rotors driving the motor-generators to generate electrical power. Example 300 is generally described by way of example as being carried out by the aerial vehicle 130 described above in connection with Figure 1. For illustrative purposes, example 300 is described in a series of actions as shown in Figures 3a and 3b, though example 300 could be carried out in any number of actions and/or combination of actions.

[0070] As shown in Figure 3a, the aerial vehicle 130 is connected to the tether 120, and the tether 120 is connected to the ground station 110. The ground station 110 is located on ground 302. Moreover, as shown in Figure 3, the tether 120 defines a tether sphere 304 having a radius based on a length of the tether 120, such as a length of the tether 120 when it is extended. Example 300 may be carried out in and/or substantially on a portion 304A of the tether sphere 304. The term“substantially on,” as used in this disclosure, refers to exactly on and/or one or more deviations from exactly on that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

[0071] Example 300 begins at a point 306 with deploying the aerial vehicle 130 from the ground station 110 in a hover-flight orientation. With this arrangement, the tether 120 may be paid out and/or reeled out. In some implementations, the aerial vehicle 130 may be deployed when wind speeds increase above a threshold speed (e.g., 3.5 m/s) at a threshold altitude (e.g., over 200 meters above the ground 302).

[0072] Further, at point 306 the aerial vehicle 130 may be operated in the hover-flight orientation. When the aerial vehicle 130 is in the hover-flight orientation, the aerial vehicle 130 may engage in hover flight. For instance, when the aerial vehicle engages in hover flight, the aerial vehicle 130 may ascend, descend, and/or hover over the ground 302. When the aerial vehicle 130 is in the hover-flight orientation, a span of the main wing 131 of the aerial vehicle 130 may be oriented substantially perpendicular to the ground 302. The term“substantially perpendicular,” as used in this disclosure, refers to exactly perpendicular and/or one or more deviations from exactly perpendicular that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

[0073] Example 300 continues at a point 308 while the aerial vehicle 130 is in the hover-flight orientation positioning the aerial vehicle 130 at a first location 310 that is substantially on the tether sphere 304. As shown in Figure 3a, the first location 310 may be in the air and substantially downwind of the ground station 110.

[0074] The term “substantially downwind,” as used in this disclosure, refers to exactly downwind and/or one or more deviations from exactly downwind that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

[0075] For example, the first location 310 may be at a first angle from an axis extending from the ground station 110 that is substantially parallel to the ground 302. In some implementations, the first angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the first angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

[0076] As another example, the first location 310 may be at a second angle from the axis. In some implementations, the second angle may be 10 degrees from the axis. In some situations, the second angle may be referred to as elevation, and the second angle may be between 10 degrees in a direction above the axis and 10 degrees in a direction below the axis. The term “substantially parallel,” as used in this disclosure refers to exactly parallel and/or one or more deviations from exactly parallel that do not significantly impact transitioning an aerial vehicle between certain flight modes described herein.

[0077] At point 308, the aerial vehicle 130 may accelerate in the hover-flight orientation. For example, at point 308, the aerial vehicle 130 may accelerate up to a few meters per second. In addition, at point 308, the tether 120 may take various different forms in various different embodiments. For example, as shown in Figure 3a, at point 308 the tether 120 may be extended. With this arrangement, the tether 120 may be in a catenary configuration. Moreover, at point 306 and point 308, a bottom of the tether 120 may be a predetermined altitude 312 above the ground 302. With this arrangement, at point 306 and point 308 the tether 120 may not contact the ground 302.

[0078] Example 300 continues at point 314 with transitioning the aerial vehicle 130 from the hover-flight orientation to a forward-flight orientation, such that the aerial vehicle 130 moves from the tether sphere 304. As shown in Figure 3b, the aerial vehicle 130 may move from the tether sphere 304 to a location toward the ground station 110 (which may be referred to as being inside the tether sphere 304).

[0079] When the aerial vehicle 130 is in the forward-flight orientation, the aerial vehicle 130 may engage in forward flight (which may be referred to as airplane-like flight). Forward flight may be an example of the AWT operating in thrust flight mode, with the motor-generators consuming power and driving the rotor rotation to provide thrust to the AWT. For instance, when the aerial vehicle 130 engages in forward flight, the aerial vehicle 130 may ascend. The forward-flight orientation of the aerial vehicle 130 could take the form of an orientation of a fixed-wing aircraft (e.g., an airplane) in horizontal flight. In some examples, transitioning the aerial vehicle 130 from the hover-flight orientation to the forward-flight orientation may involve a flight maneuver, such as pitching forward. And in such an example, the flight maneuver may be executed within a time period, such as less than one second.

[0080] At point 314, the aerial vehicle 130 may achieve attached flow. Further, at point 314, a tension of the tether 120 may be reduced. With this arrangement, a curvature of the tether 120 at point 314 may be greater than a curvature of the tether 120 at point 308. As one example, at point 314, the tension of the tether 120 may be less than 1 KN, such as 500 newtons (N).

[0081] Example 300 continues at one or more points 318 with operating the aerial vehicle 130 in the forward-flight orientation to ascend at an angle of ascent AA1 to a second location 320 that is substantially on the tether sphere 304. As shown in Figure 3b, the aerial vehicle 130 may fly substantially along a path 316 during the ascent at one or more points 318. In this example, one or more points 318 is shown as three points, a point 318A, a point 318B, and a point 318C. However, in other examples, one or more points 318 may include less than three or more than three points.

[0082] In some examples, the angle of ascent AA1 may be an angle between the path 316 and the ground 302. Further, the path 316 may take various different forms in various different embodiments. For instance, the path 316 may be a line segment, such as a chord of the tether sphere 304.

[0083] In some implementations, the aerial vehicle 130 may have attached flow during the ascent. Moreover, in such an implementation, effectiveness of one or more control surfaces of the aerial vehicle 130 may be maintained. Further, in such an implementation, example 300 may involve selecting a maximum angle of ascent, such that the aerial vehicle 130 has attached flow during the ascent. Moreover, in such an implementation, example 300 may involve adjusting a pitch angle of the aerial vehicle 130 based on the maximum angle of ascent and/or adjusting thrust of the aerial vehicle 130 based on the maximum angle of ascent. In some examples, adjusting thrust of the aerial vehicle 130 may involve using differential thrusting of one or more of the rotors 134A-D of the aerial vehicle 130. The pitch angle may be an angle between the aerial vehicle 130 and a vertical axis that is substantially perpendicular to the ground 302.

[0084] As shown in Figure 3b, at point 314 the aerial vehicle 130 may have a speed V31 and a pitch angle PA31; at point 318A the aerial vehicle 130 may have a speed V32 and a pitch angle PA32; at point 318B the aerial vehicle 130 may have a speed V33 and a pitch angle PA33; and at point 318C the aerial vehicle 130 may have a speed V34 and a pitch angle PA34.

[0085] In some implementations, the angle of ascent AA1 may be selected before point 318 A. With this arrangement, the pitch angle PA31 and/or the pitch angle PA32 may be selected based on the angle of ascent AA1. Further, in some examples, the pitch angle PA32, the pitch angle PA33, and/or the pitch angle PA34 may be equal to the pitch angle PA31. However, in other examples, the pitch angles PA31, PA32, PA33, and/or PA34 may be different than each other. For instance, the pitch angle PA31 may be greater or less than pitch angles PA32, PA33, and/or PA34; the pitch angle PA32 may be greater or less than pitch angles PA33, PA34, and/or PA31 ; the pitch angle PA33 may be greater or less than pitch angles PA34, PA31, and/or PA32; and the pitch angle PA34 may be greater or less than pitch angles PA31, PA32, and/or PA33. Further, the pitch angle PA33 and/or PA34 may be selected and/or adjusted during the ascent. Further still, the pitch angle PA31 and/or PA32 may be adjusted during the ascent.

[0086] Moreover, in some implementations, the speed V31 and/or the speed V32 may be selected based on the angle of ascent AA1. Further, in some examples, the speed V32, the speed V33, and the speed V34 may be equal to the speed V31. However, in other examples, speeds V31, V32, V33, and V34 may be different than each other. For example, the speed V34 may be greater than the speed V33, the speed V33 may be greater than the speed V32, and the speed V32 may be greater than the speed V31. Further, speeds V31, V32, V33, and/or V34 may be selected and/or adjusted during the ascent.

[0087] In some implementations, any or all of the speeds V31, V32, V33, and/or V34 may be a speed that corresponds with a maximum (or full) throttle of the aerial vehicle 130. Further, in some implementations, at the speed V32, the aerial vehicle 130 may ascend in a forward- flight orientation. Moreover, at the speed V32, the angle of ascent AA1 may be converged.

[0088] As shown in Figure 3b, the second location 320 may be in the air and substantially downwind of the ground station 110. The second location 320 may be oriented with respect to the ground station 110 in a similar way as the first location 310 may be oriented with respect to the ground station 110.

[0089] For example, the second location 320 may be at a first angle from an axis extending from the ground station 110 that is substantially parallel to the ground 302. In some implementations, the first angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

[0090] In addition, as shown in Figure 3b, the second location 320 may be substantially upwind of the first location 310. The term“substantially upwind,” as used in this disclosure, refers to exactly upwind and/or one or more deviations from exactly upwind that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

[0091] At one or more points 318, a tension of the tether 120 may increase during the ascent. For example, a tension of the tether 120 at point 318C may be greater than a tension of the tether 120 at point 318B, a tension of the tether 120 at point 318B may be greater than a tension of the tether 120 at point 318A. Further, a tension of the tether 120 at point 318A may be greater than a tension of the tether at point 314.

[0092] With this arrangement, a curvature of the tether 120 may decrease during the ascent. For example, a curvature the tether 120 at point 318C may be less than a curvature the tether at point 318B, and a curvature of the tether 120 at point 318B may be less than a curvature of the tether at point 318 A. Further, in some examples, a curvature of the tether 120 at point 318 A may be less than a curvature of the tether 120 at point 314.

[0093] Moreover, in some examples, when the aerial vehicle 130 includes a GPS receiver, operating the aerial vehicle 130 in the forward-flight orientation to ascend at an angle of ascent may involve monitoring the ascent of the aerial vehicle 130 with the GPS receiver. With such an arrangement, control of a trajectory of the aerial vehicle 130 during the ascent may be improved. As a result, the aerial vehicle 130’s ability to follow one or more portions and/or points of the path 316 may be improved.

[0094] Further, in some examples, when the aerial vehicle 130 includes at least one pitot tube, operating the aerial vehicle 130 in a forward-flight orientation to ascend at an angle of ascent may involve monitoring an angle of attack of the aerial vehicle 130 or a side slip of the aerial vehicle 130 during the ascent with the at least one pitot tube. With such an arrangement, control of the trajectory of the aerial vehicle during the ascent may be improved. As a result, the aerial vehicle 130’s ability to follow one or more portions and/or points of the path 316 may be improved. The angle of attack may be an angle between a body axis of the aerial vehicle 130 and an apparent wind vector. Further, the side slip may be an angle between a direction substantially perpendicular to a heading of the aerial vehicle 130 and the apparent wind vector.

[0095] Example 300 continues at a point 322 with transitioning the aerial vehicle 130 from the forward-flight orientation to a crosswind-flight orientation. In some examples, transitioning the aerial vehicle 130 from the forward-flight orientation to the crosswind-flight orientation may involve a flight maneuver.

[0096] When the aerial vehicle 130 is in the crosswind-flight orientation, the aerial vehicle 130 may engage in crosswind flight. For instance, when the aerial vehicle 130 engages in crosswind flight, the aerial vehicle 130 may fly substantially along a path, such as path 150, to generate electrical energy. In some implementations, a natural roll and/or yaw of the aerial vehicle 130 may occur during crosswind flight.

[0097] As shown in Figure 3b, at points 314-322 a bottom of the tether 120 may be a predetermined altitude 324 above the ground 302. With this arrangement, at points 314-322 the tether 120 may not touch the ground 302. In some examples, the predetermined altitude 324 may be less than the predetermined altitude 312. In some implementations, the predetermined altitude 324 may be greater than one half of the height of the ground station 110. And in at least one such implementation, the predetermined altitude 324 may be 6 meters.

[0098] Thus, example 300 may be carried out so that the tether 120 may not contact the ground 302. With such an arrangement, the mechanical integrity of the tether 120 may be improved. For example, the tether 120 might not catch on (or tangle around) objects located on the ground 302. As another example, when the tether sphere 304 is located above a body of water (e.g., an ocean, a sea, a lake, a river, and the like), the tether 120 might not be submersed in the water. In addition, with such an arrangement, safety of one or more people located near the ground station 110 (e.g., within the portion 304A of the tether sphere 304) may be improved.

[0099] In addition, example 300 may be carried out so that a bottom of the tether 120 remains above the predetermined altitude 324. With such an arrangement, the mechanical integrity of the tether 120 may be improved as described herein and/or safety of one or more people located near the ground station 110 (e.g., within the portion 304A of the tether sphere 304) may be improved.

[00100] Moreover, one or more actions that correspond with points 306-322 may be performed at various different time periods in various different embodiments. For instance, the one or more actions that correspond with point 306 may be performed at a first time period, the one or more actions that correspond with point 308 may be performed at a second time period, the one or more actions that correspond with point 314 may be performed at a third time period, the one or more actions that correspond with point 318A may be performed at a fourth time period, the one or more actions that correspond with point 318B may be performed at a fifth time period, the one or more actions that correspond with point 318C may be performed at a sixth time period, and the one or more actions that correspond with point 322 may be performed at a seventh time period. However, in other examples, at least some of the actions of the one or more actions that correspond with points 306-322 may be performed concurrently.

[00101] Figures 4a-c are graphical representations involving an angle of ascent, according to an example embodiment. In particular, Figure 4a is a graphical representation 402, Figure 4b is a graphical representation 404, and Figure 4c is a graphical representation 406. Each of graphical representations 402, 404, and 406 may be based on example 300.

[00102] More specifically, in Figures 4a-c, an aerial vehicle in an example of transitioning the aerial vehicle from hover flight to crosswind flight may have a thrust-to- weight ratio (T/W) of 1.3 and a coefficient of drag (CD) equal to the equation 3 + (CL ² / eARn), where CL is coefficient of lift, e is span efficiency of the aerial vehicle, and AR is aspect ratio of the aerial vehicle. However, in other examples, aerial vehicles described herein may have various other thrust-to-weight ratios, such as a thrust-to-weight ratio greater than 1.2. Further, in other examples, aerial vehicles described herein may have various other values of CD, such as a value of CD between 0.1 and 0.2.

[00103] As noted, Figure 4a is the graphical representation 402. In particular, the graphical representation 402 depicts an angle of ascent of an aerial vehicle in relation to air speed. In graphical representation 402, the angle of ascent may be measured in degrees, and the airspeed may be measured in m/s. As shown in Figure 4a, a point 402A on the graphical representation 402 may represent a maximum angle of ascent of an aerial vehicle for attached flow during an ascent, such as at one or more points 318 in example 300. In graphical representation 402, the maximum angle of ascent may be about 65 degrees, and an airspeed that corresponds with the maximum angle of ascent may be about 11 m/s.

[00104] Moreover, as noted, Figure 4b is the graphical representation 404. In particular, the graphical representation 404 depicts an angle of ascent of an aerial vehicle in relation to CL of the aerial vehicle. In graphical representation 404, the angle of ascent may be measured in degrees, and CL may be a value without a unit of measurement. As shown in Figure 4b, a point 404A on the graphical representation 404 may represent a maximum angle of ascent of an aerial vehicle for attached flow during an ascent, such as at one or more points 318 in example 300. In graphical representation 404, the maximum angle of ascent may be about 65 degrees, and the CL that corresponds with the maximum angle of ascent may be about 0.7.

[00105] Further, as noted, Figure 4c is the graphical representation 406. In particular, the graphical representation 406 depicts a first component of a speed of an aerial vehicle in relation to a second component of the speed of the aerial vehicle. In graphical representation 406, the first and second components of speed of the aerial vehicle may be measured in m/s. In some examples, the first component of the speed of the aerial vehicle may be in a direction that is substantially parallel with the ground. Further, in some examples, the second component of the speed of the aerial vehicle may be in a direction that is substantially perpendicular with the ground.

[0106] As shown in Figure 4c, a point 406A on the graphical representation 406 may represent a first and second component of a speed of the aerial vehicle when the aerial vehicle is at a maximum angle of ascent for attached flow during an ascent, such as at one or more points 318 in example 300. In graphical representation 406, the first component of the speed of the aerial vehicle that corresponds with the maximum angle of ascent may about 5 m/s, and the second component of the speed of the aerial vehicle that corresponds with the maximum angle of ascent may be about 10.25 m/s.

[0107] Figures 5a and 5b depict a tether sphere 504, according to an example embodiment. In particular, the tether sphere 504 has a radius based on a length of a tether 520, such as a length of the tether 520 when it is extended. As shown in Figures 5a and 5b, the tether 520 is connected to a ground station 510, and the ground station 510 is located on ground 502. Further, as shown in Figures 5a and 5b, relative wind 503 contacts the tether sphere 504. In Figures 5a and 5b, only a portion of the tether sphere 504 that is above the ground 502 is depicted. The portion may be described as one half of the tether sphere 504.

[0108] The ground 502 may take the form of or be similar in form to the ground 302, the tether sphere 504 may take the form of or be similar in form to the tether sphere 304, the ground station 510 may take the form of or be similar in form to the ground station 110 and/or the ground station 210, and the tether 520 may take the form of or be similar in form to the tether 120 and/or the tether 220.

[0109] Examples of transitioning an aerial vehicle between hover flight and crosswind flight described herein may be carried out in and/or substantially on a first portion 504A of the tether sphere 504. As shown in Figures 5a and 5b, the first portion 504A of the tether sphere 504 is substantially downwind of the ground station 510. The first portion 504A may be described as one quarter of the tether sphere 504. The first portion 504A of the tether sphere 504 may take the form of or be similar in form to the portion 304A of the tether sphere 304.

[0110] Moreover, examples of transitioning an aerial vehicle between hover flight and crosswind flight described herein may be carried out at a variety of locations in and/or on the first portion 504A of the tether sphere 504. For instance, as shown in Figure 5a, while the aerial vehicle is in a hover-flight orientation, the aerial vehicle may be positioned at a point 508 that is substantially on the first portion 504A of the tether sphere 504.

[0111] Further, as shown in Figure 5b, when the aerial vehicle transitions from the hover-flight orientation to a forward-flight orientation, the aerial vehicle may be positioned at a point 514 that is inside the first portion 504A of the tether sphere 504. Further still, as shown in Figure 5b, when the aerial vehicle ascends in the forward-flight orientation to a point 518 that is substantially on the first portion 504A of the tether sphere 504, the aerial vehicle may follow a path 516. The path 516 may take the form of a variety of shapes. For instance, the path 516 may be a line segment, such as a chord of the tether sphere 504. Other shapes and/or types of shapes are possible as well.

[0112] The point 508 may correspond to point 308 in example 300, the point 514 may correspond to point 314 in example 300, the point 518 may correspond to point 318C in example 300, and the path 516 may take the form of or be similar in form to the path 316.

[0113] Further, in accordance with this disclosure, the point 508 and the point 518 may be located at various locations that are substantially on the first portion 504A of the tether sphere 504, and the point 514 may be located at various locations that are inside the first portion 504A of the tether sphere 504.

D. Transitioning an Aerial Vehicle from Crosswind Flight to Hover Flight

[0114] Figures 6a-c depict an example 600 of transitioning an aerial vehicle from crosswind flight to hover flight, according to an example embodiment. Example 600 is generally described by way of example as being carried out by the aerial vehicle 130 described above in connection with Figure 1. For illustrative purposes, example 600 is described in a series of actions of the aerial vehicle 130 as shown in Figures 6a-c, though example 600 could be carried out in any number of actions and/or combination of actions.

[0115] As shown in Figure 6a, the aerial vehicle 130 is connected to the tether 120, and the tether 120 is connected to the ground station 110. The ground station 110 is located on the ground 302. Moreover, as shown in Figure 6a, the tether 120 defines the tether sphere 304. Example 600 may be carried out in and/or substantially on the portion 304A of the tether sphere 304.

[0116] Example 600 begins at a point 606 with operating the aerial vehicle 130 in a crosswind- flight orientation. When the aerial vehicle is in the crosswind-flight orientation, the aerial vehicle 130 may engage in crosswind flight. Moreover, at point 606 the tether 120 may be extended.

[0117] Example 600 continues at a point 608 with while the aerial vehicle 130 is in the crosswind-flight orientation, positioning the aerial vehicle 130 at a first location 610 that is substantially on the tether sphere 304. (In some examples, the first location 610 may be referred to as a third location). As shown in Figure 6a, the first location 610 may in the air and substantially downwind of the ground station 110. The first location 610 may take the form of or be similar in form to the first location 310. However, in some examples, the first location 610 may have an altitude that is greater than an altitude of the first location 310.

[0118] For example, the first location 610 may be at a first angle from an axis that is substantially parallel to the ground 302. In some implementations, the angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the first angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

[0119] Moreover, at point 606 and point 608, a bottom of the tether 120 may be a predetermined altitude 612 above the ground 302. With this arrangement, at point 606 and point 608 the tether 120 may not contact the ground 302. The predetermined altitude 612 may be greater than, less than, and/or equal to the predetermined altitude 312.

[0120] Example 600 continues at a point 614 with transitioning the aerial vehicle from the crosswind-flight orientation to a forward-flight orientation, such that the aerial vehicle 130 moves from the tether sphere 120. As shown in Figure 6b, the aerial vehicle 130 may move from the tether sphere 304 to a location toward the ground station 110.

[0121] When the aerial vehicle 130 is in the forward-flight orientation, the aerial vehicle may engage in forward flight. In some examples, transitioning the aerial vehicle 130 from the crosswind-flight orientation to the forward-flight orientation may involve a flight maneuver, such as pitching forward. Further, in such an example, the flight maneuver may be executed within a time period, such as less than one second.

[0122] At point 614, the aerial vehicle 130 may achieve attached flow. Further, at point 314, a tension of the tether 120 may be reduced. With this arrangement, a curvature of the tether 120 at point 614 may be greater than a curvature of the tether 120 at point 608.

[0123] Example 600 continues at one or more points 618 with operating the aerial vehicle 130 in the forward-flight orientation to ascend at an angle of ascent AA2 to a second location 620. (In some examples, the second location 620 may be referred to as a fourth location). As shown in Figure 6b, the aerial vehicle 130 may fly substantially along a path 616 during the ascent at one or more points 618. In this example, one or more points 618 includes two points, a point 618 A and point 618B. However, in other examples, one or more points 618 may include less than two or more than two points.

[0124] In some examples, the angle of ascent AA2 may be an angle between the path 618 and the ground 302. Further, the path 616 may take various different forms in various different embodiments. For instance, the path 616 may a line segment, such as a chord of the tether sphere 304. Other shapes and/or types of shapes are possible as well. The angle of ascent AA2 may take the form of or be similar in form to the angle of ascent AA1, and the path 616 may take the form of or be similar in form to the path 316.

[0125] In some implementations, at one or more points 618, the aerial vehicle 130 may ascend with substantially no thrust provided by the rotors 134A-D of the aerial vehicle 130. With this arrangement, the aerial vehicle 130 may decelerate during the ascent. For instance, at one or more points 618, the rotors 134A-D of the aerial vehicle 130 may be shutoff. The term “substantially no,” as used in this disclosure, refers to exactly no and/or one or more deviations from exactly no that do not significantly impact transitioning between certain flight modes as described herein.

[0126] Moreover, in some implementations, the aerial vehicle 130 may have attached flow during the ascent. And in such an implementation, effectiveness of one or more control surfaces of the aerial vehicle 130 may be maintained. Further, in such an implementation, example 600 may involve selecting a maximum angle of ascent, such that the aerial vehicle 130 has attached flow during the ascent. Moreover, in such an implementation, example 600 may involve adjusting a pitch angle of the aerial vehicle based on the maximum angle of ascent and/or adjusting thrust of the aerial vehicle 130 based on the maximum angle of ascent. In some examples, the adjusting thrust of the aerial vehicle 130 may involve using differential thrusting of one or more of the rotors 134A-D of the aerial vehicle 130.

[0127] As shown in Figure 6b, at point 614 the aerial vehicle 130 may have a speed V61 and a pitch angle PA61; at point 618A the aerial vehicle 130 may have a speed V62 and a pitch angle PA62; and at point 618B the aerial vehicle 130 may have a speed V63 and a pitch angle PA63.

[0128] In some implementations, the angle of ascent AA2 may be selected before point 618 A. With this arrangement, the pitch angle PA61 and/or the pitch angle PA62 may be selected based on the angle of ascent AA2. Further, in some examples, the pitch angle PA62 and the pitch angle PA63 may be equal to the pitch angle PA61. However, in other examples, the pitch angles PA61, PA62, and PA63 may be different than each other. For instance, PA61 may be greater or less than PA62 and/or PA63; PA62 may be greater or less than PA63 and/or PA61; and PA63 may be greater or less than PA61 and/or PA62. Further, PA63 may be selected and/or adjusted during the ascent. Further still, PA61 and/or PA62 may be adjusted during the ascent.

[0129] Moreover, in some implementations, the speed V61 and/or the speed V62 may be selected based on the angle of ascent AA2. Further, in some examples, the speed V62, and the speed V63 may be equal to the speed V61. However, in other examples, the speeds V61, V62, V63 may be different than each other. For example, the speed V63 may be less than the speed V62, and the speed V62 may be less than the speed V61. Further, speeds V61, V62, and V63 may be selected and/or adjusted during the ascent.

[0130] In some implementations, any of speeds V61, V62, and/or V64 may be a speed that corresponds with a minimum (or no) throttle of the aerial vehicle 130. Further, in some implementations, at the speed V62, the aerial vehicle 130 may ascend in a forward-flight orientation. Moreover, at the speed V62, the angle of ascent AA2 may be converged. As shown in Figure 6, the second location 620 may be in the air and substantially downwind of the ground station 110. The second location 620 may be oriented with respect to the ground station 110 a similar way as the first location 610 may be oriented with respect to the ground station 110.

[0131] For example, the first location 610 may be at a first angle from an axis that is substantially parallel to the ground 302. In some implementations, the angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the first angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

[0132] As another example, the first location 610 may be at a second angle from the axis. In some implementations, the second angle may be 10 degrees from the axis. In some situations, the second angle may be referred to as elevation, and the second angle may be between 10 degrees in a direction above the axis and 10 degrees in a direction below the axis.

[0133] At one or more points 618, a tension of the tether 120 may increase during the ascent. For example, a tension of the tether 120 at point 618B may be greater than a tension of the tether at point 618 A, and a tension of the tether at point 618 A may be greater than a tension of the tether at point 614.

[0134] With this arrangement, a curvature of the tether 120 may decrease during the ascent. For example, a curvature the tether 120 at point 618B may be less than a curvature of the tether 120 at point 618 A. Further, in some examples, a curvature of the tether 120 at point 618 A may be less than a curvature of the tether 120 at point 614.

[0135] Moreover, in some examples, when the aerial vehicle 130 includes a GPS receiver, operating the aerial vehicle 130 in the forward-flight orientation to ascend at an angle of ascent may involve monitoring the ascent of the aerial vehicle with the GPS receiver. With such an arrangement, control of a trajectory of the aerial vehicle 130 during the ascent may be improved. As a result, the aerial vehicle 130’s ability to follow one or more portions and/or portions of the path 616 may be improved.

[0136] Further, in some examples, when the aerial vehicle 130 includes at least one pitot tube, operating the aerial vehicle 130 in the forward-flight orientation to ascend at an angle of ascent may involve monitoring an angle of attack of the aerial vehicle 130 or a side slip of the aerial vehicle 130 during the ascent with the at least one pitot tube. With such an arrangement, control of the trajectory of the aerial vehicle 130 during the ascent may be improved. As a result, the aerial vehicle’s ability to follow one or more portions and/or points of the path 616 may be improved.

[0137] Moreover, as shown in Figure 6b, at point 614 and point 618 a bottom of the tether 120 may be a predetermined altitude 624 above the ground 302. With this arrangement, at point 614 and point 618 the tether 120 may not touch the ground 302. In some examples, the predetermined altitude 624 may be less than the predetermined altitude 612. And the predetermined altitude 624 may be greater than, less than, and/or equal to the predetermined the predetermined altitude 324. In some implementations, the predetermined altitude 624 may be greater than one half of the height of the ground station 110. And in at least one such implementation, the predetermined altitude 624 may be 6 meters.

[0138] Example 600 continues at a point 622 with transitioning the aerial vehicle 130 from the forward-flight orientation to a hover-flight orientation. In some examples, transitioning the aerial vehicle 130 from the forward-flight orientation to the hover-flight orientation may involve a flight maneuver. Further, transitioning the aerial vehicle 130 from the forward-flight orientation to the hover-flight orientation may occur when the aerial vehicle 130 has a threshold speed, such as 15 m/s. In some implementations, transitioning the aerial vehicle 130 from the forward-flight orientation to the hover-flight orientation may occur when the speed V63 is 15 m/s. Further, at point 622, a tension of the tether 120 may be greater than a tension of the tether at point 618B. [0139] During the transition from the forward-flight orientation to the hover-flight orientation, the aerial vehicle 130 may be positioned at third location 624 (In some examples, the third location 624 may be referred to as a fifth location). As shown in Figure 6c, the third location 624 may be in the air and substantially downwind of the ground station 110. In some implementations, the third location 624 could be the same as or similar to the second location 620. When the third location 624 is not substantially on the tether sphere 304, after point 622 the aerial vehicle 130 may be blown by the wind to a fourth location (not shown) that is substantially on the tether sphere 304.

[0140] Moreover, as shown in Figure 6c, at point 622 a bottom of the tether 120 may be a predetermined altitude 626 above the ground 302. With this arrangement, at point 626 the tether 120 may not touch the ground 302. In some examples, the predetermined altitude 626 may be greater than the predetermined altitude 612 and/or the predetermined altitude 624.

[0141] Thus, example 600 may be carried out so that the tether 120 may not contact the ground 602. With such an arrangement, the mechanical integrity of the tether 120 may be improved. For example, the tether 120 might not catch on (or tangle around) objects located on the ground 302. As another example, when the tether sphere 304 is located above a body of water described herein, the tether 120 might not be submersed in the water. In addition, with such an arrangement, safety of one or more people located near the ground station 110 (e.g., within the portion 304 A of the tether sphere 304) may be improved.

[0142] In addition, example 600 may be carried out so that a bottom of the tether 120 remains above the predetermined altitude 624. With such an arrangement, the mechanical integrity of the tether 120 may be improved as described herein and/or safety of one or more people located near the ground station may be improved.

[0143] Moreover, one or more actions that correspond with points 606-622 may be performed at various different time periods in various different embodiments. For instance, the one or more actions that correspond with point 606 may be performed at a first time period, the one or more actions that correspond with point 608 may be performed at a second time period, the one or more actions that correspond with point 614 may be performed at a third time period, the one or more actions that correspond with point 618 A may be performed at a fourth time period, the one or more actions that correspond with point 618B may be performed at a fifth time period, and the one or more actions that correspond with point 622 may be performed at a seventh time period. However, in other examples, at least some of the actions of the one or more actions that correspond with points 606-622 may be performed concurrently. [0144] Although example 600 has been described above with reference to Figures 6a-c, in accordance with this disclosure, point 608 and point 622 may occur at various locations that are substantially on the portion 304A of the tether sphere 304, and point 614 and one or more points 618 may occur at various locations that are inside the portion 304 A of the tether sphere.

III. Floating Ground Stations

[0145] Figure 7 depicts a side view of an airborne wind turbine system 700 with a moored floating ground station 704 partially submerged beneath a water surface 701, according to an example embodiment. As illustrated, the ground station 704 is in a quiescent state with no substantial oscillation as a result of wave-induced motion. Ground station 704 includes a top platform 712 A and a spar buoy 712B that may rotate independently of each other. The ground station 704 may be similar to, perform similarly to, and/or include components described with respect to, other ground stations described herein, for example, ground station 100. An aerial vehicle 702, shown here in crosswind flight about path 703, is coupled to a winch drum 710 via tether 706. The winch drum 710 is mounted to the top platform 712A. Tether 706 may be similar to, perform similarly to, and/or include components described with respect to, other tethers described herein, for example, tether 120. Tether 706 preferably contains one or more electrical conductors and data pathways linking the aerial vehicle 702 to the ground station 704. The winch drum 710 may be used to take up or payout the tether 706, particularly during landing or takeoff. Alternatively or additionally, other tether take up or payout apparatuses may be present, including but not limited to, internal reels or submersion systems. As illustrated, the ground station 704 is coupled to a single aerial vehicle 702; however, in another embodiment, more than one aerial vehicle may be coupled to a single ground station 704.

[0146] A perch 708 is coupled to the top platform 712A and may be used to directly couple the aerial vehicle 702 to the ground station 704 when the aerial vehicle 702 is landed. The top platform 712A may rotate to help align the aerial vehicle 702 during takeoff, landing, and/or crosswind flight. For example, it may be desirable to align the perch 708 and/or aerial vehicle 702 in a downwind position, or in a position relative to wave direction, or in a position based on a combination of wind and wave direction.

[0147] The ground station 704 is moored to a mooring 718 via a mooring line 716. The spar buoy 712B may have a rotational couple with the mooring line 716 in one or more axis at interface 720. As illustrated, the ground station 704 is anchored in a tension leg configuration via the single mooring line 716; however, other configurations in other embodiments are also possible. For example, one or more mooring lines may anchor the ground station 704 at interface 720 or at another rotational couple location along the body of the spar buoy 712B. Alternatively, the ground station 704 may utilize a floating platform configuration other than a spar buoy 712B.

[0148] Submerged thrusters 714B and 714A (714A not visible in this view) are mounted below the water surface 701 on opposing sides of the spar buoy 712B. Submerged thrusters 714A-B may independently employ forward or reverse thrust. Note that the submerged thrusters 714A- B are optional. As such, ground station 704 may not include any means for actively controlling its own movement, or may include other mechanisms and systems for controlling its movement, in addition or in the alternative to submerged thrusters 714A-B (e.g., rudders or other submerged features that can take advantage of energy from water flow around the ground station to achieve desired movements).

[0149] Figure 8A depicts a side view of the airborne wind turbine system 700 where the ground station 704 is in an oscillatory state as a result of wave-induced motion. As illustrated for clarity, the wave-induced oscillation may be primarily about the rotational couple at the interface 720. However, depending on the configuration of the system 700, the wave-induced oscillation may alternatively or additionally be about mooring 718. The oscillation is depicted with a total amplitude of 2*Ao degrees. Amplitude may be also be considered in terms of distance displacement instead of degrees of displacement. For simplicity of illustration, the oscillation of ground station 704 is depicted as symmetrical about a vertical axis; however, the oscillation may be about a tilted axis, for example where a tension force from the tether 706 is acting on the ground station 704. Further, when waves are moving in a certain direction, the amplitude (and thus the angular displacement) of the ground station may be greater moving in the direction of the waves, than it is in the direction opposite the motion of the waves.

[0150] Ground station 704 may include motion sensors, environmental sensors, positional sensors, and/or other types of sensors that can provide quantitative and/or qualitative data about waves and/or wind being experienced at the floating ground station 704. For example, such sensor may be operable to determine wave direction, wave height, wave speed, wind direction and/or speed, orientation of the ground station 704 and/or platform 712A with respect to the wind and/or wave direction, ground station 704 tilt, amplitude of oscillation of the entire ground station 704 and/or one or more positions on the ground station 704, frequency of oscillation (e.g., oscillations per unit time), direction of oscillation in an absolute direction (e.g., compass direction) or relative direction to wave, wind, or aerial vehicle direction, and/or wave periodicity, including swell period, wind-wave period, and/or dominant period where the kinetic energy is at a maximum (i.e., peak period), or some combination thereof. As non limiting examples, the ground station 704 may utilize anemometers, wind vanes, mechanical wind sensors, ultrasonic wind sensors, radar, acoustic wave sensors, pressure-based wave sensors, gyroscopic sensors, and/or inertial measurement units (IMUs).

[0151] Figure 8B depicts a side view of an airborne wind turbine system 750. Some or all components of airborne wind turbine system 750 may be similar to or the same as components of airborne wind turbine system 700. However, Figure 8B shows the ground station 754 in an oscillatory state resulting from unsteady loads from water movement and/or unsteady loads applied by the tether 756 (resulting from circular flight of aerial vehicle 752). Such unsteady loads may occur due to the combination of water current and/or wind, force exerted on the spar buoy 762 by the mooring line 766, and/or varying force applied to the ground station 754 via the tether 756 during closed-loop flight. The oscillation of the ground station 754 illustrated in Figure 8B may be referred to as the“natural” movement or oscillation of the ground station. Such natural oscillations can exist independent from any wave force acting on the ground station 754 (as illustrated by the flat state of water surface 751). Further, while Figure 8B shows a scenario where there are no waves, it should be understood that the oscillation of a floating ground station due to currents and/or wind, may also exist in scenarios with waves. In such scenarios, the oscillation frequency of the floating ground station may driven by the wave force (and possibly by tether tension as well).

[0152] In the illustrated example, a natural oscillation may be primarily about the rotational couple at the interface 770. In other implementations, a natural oscillation may additionally or alternatively be about the mooring 768. Or, a secondary natural oscillation of the interface 770 may be determined, such that a natural oscillation of the spar buoy may be determined about the oscillating coupling point at interface 770 (e.g., measured at the top of the ground station or the point where the tether decouples from the winch drum).

[0153] The oscillation 782 is depicted with a total amplitude 0t equal to the sum of q ₃ plus 0b, where 0t, q _3. and 0b are measured in degrees. Amplitude may also be measured in terms of displacement distance instead of degrees. For simplicity of illustration, the oscillation of ground station 754 is depicted as symmetrical about a vertical axis; however, the oscillation may be about a tilted axis, for example where a tension force from the tether 756 is acting on the ground station 754. Additionally or alternatively, while q ₃ and 0b appear to be symmetrical, the oscillation of ground station 754 may be asymmetrical, such that movement in one direction is greater than the movement in another direction during each period. In any event, there is a natural frequency (and period) that characterizes the motion for a given configuration of an airborne wind turbine (in the absence of waves and tether tension).

[0154] Using techniques such as those described herein, an aerial vehicle 752 may generate power during flight on the closed path 780. As shown in Figure 8B, the closed path 780 may include a down stroke 780a and an upstroke 780b. Generally, gravity tends to increase the speed of the aerial vehicle 752 on the down stroke 780a, and decrease the speed of the aerial vehicle 752 on the upstroke 780b. Such variation in speed may be undesirable since it can reduce the total energy capture by the aerial vehicle 752.

[0155] In a further aspect, airborne wind turbine system 750 may include a control system, and/or be in communication with a remotely-located control system (e.g., a computing system or systems such as those described herein integrated in and/or in communication with the floating ground station) which implements methods described herein to coordinate closed-path flight of the aerial vehicle with the natural oscillation period (or frequency) of the floating ground station (e.g., the oscillation period and frequency in the absence of waves). The control system may include and/or be communicatively coupled to sensor systems and/or communications systems in order to determine the natural-oscillation period (or frequency) and/or oscillation phase of the floating ground station, and/or receive data that can be utilized to determine the natural-oscillation period (or frequency) and/or oscillation phase of the floating ground station.

[0156] When waves are not present, the tension of tether 706 during flight of the aerial vehicle may drive the oscillation of the ground station 704, such that it does not oscillate at its natural frequency. Since sensor systems on the floating ground station will generally provide data indicative of the current movement of the floating ground station, the control system may utilize other information to determine the natural oscillation period. For example, the period (or frequency) of natural oscillation could be pre-determined as a characteristic of the system, based on structural and/or mechanical parameters of the ground station, mooring line, tether, and/or other components of the airborne wind turbine, to determine the current and/or future period and/or phase of the oscillation of the floating ground station. The natural oscillation phase may then be determined using sensor data (since the phase may vary depending upon current, wind, and/or other factors). Other data may also be utilized to pre-determine the natural oscillation frequency of the floating ground station 704.

[0157] In a further aspect, even when the natural oscillation frequency (and period) of the floating ground station is predetermined, sensor data may be utilized to determine the phase of oscillation. Accordingly, airborne wind turbine system 750 may include some or all of the sensor systems described in reference to airborne wind turbine system 700, and possibly other sensor systems as well or in the alternative.

[0158] Configured as described herein, airborne wind turbine system 750 can determine the natural oscillation period, and/or the oscillation phase, of the floating ground station. The control system can then operate the aerial vehicle to fly in a closed path with: (a) a looping period that matches the natural-oscillation period of the floating ground station, and (b) a looping phase that aligns with the oscillation phase of the floating ground station such that movement of the aerial vehicle on a downstroke portion the circular path corresponds to forward displacement of the floating ground station, and movement of the aerial vehicle on an upstroke portion the closed path corresponds to reverse displacement of the floating ground station.

IV. Controlling Constructive Interference with Natural Ground-Station Oscillation

[0159] Methods and systems described herein may help to reduce variations in the speed of aerial vehicle 752 as it travels around a closed path (such as path 780), which in turn can help a floating airborne wind turbine more efficiently capture energy generated by the aerial vehicle. To do so, a control system for a floating airborne wind turbine may control the flight of aerial vehicle 752 to: (a) match the period with which the aerial vehicle 752 repeats the closed path 780 with the period of natural oscillation of floating ground station 754, and (b) coordinate the phase of the aerial vehicles flight around closed path 780 with phase of the floating ground station’s natural oscillation. Herein, the period with which an aerial vehicle repeats a closed path (e.g., a substantially circular path) may be referred to as the“looping period,” and the phase of the aerial vehicle’s movement on the closed path may be referred to as the“looping phase” (e.g., the timing during each looping period at which the upstroke to downstroke transition, or downstroke to upstroke transition, occurs).

[0160] Figure 9 is a flow chart illustrating a method 900, according to an example embodiment. Method 900 is described by way of example as being implemented by a control system (e.g., for an aerial vehicle or for the floating airborne wind turbine). However, it should be understood that example methods such as method 900 could also be implemented by combinations of control systems, and/or by other systems described herein alone or in combination, including but not limited to, a ground station system and/or an aerial vehicle system. [0161] As shown by block 902, method 900 involves a control system in an airborne wind turbine determining a period of natural oscillation of a floating ground station in an airborne wind turbine, where each natural-oscillation period comprises a forward displacement and a backward displacement of the floating ground station with respect to the aerial vehicle. The control system may also determine a phase of the oscillation of the floating ground station, as shown by block 904. The control system then operates the aerial vehicle to fly in a substantially circular path with: (a) a looping period that matches the natural-oscillation period of the floating ground station, and (b) a looping phase that aligns with the oscillation phase of the floating ground station that movement of the aerial vehicle on a downstroke portion the circular path corresponds to forward displacement of the floating ground station, and movement of the aerial vehicle on an upstroke portion the circular path corresponds to reverse displacement of the floating ground station, as shown by block 906.

[0162] At block 902, various techniques may be utilized to determine the period (or frequency) of natural oscillation of the floating ground station. In embodiments, the period (or frequency) of natural oscillation could be pre-determined based at least in part on characteristics of the airborne wind turbine, such as structural and/or mechanical parameters of the ground station, mooring line, tether, and/or other components of the airborne wind turbine, among other possibilities. Other data may also be utilized to pre-determine the natural oscillation frequency of the floating ground station 704.

[0163] At block 904, various techniques may be utilized to determine the floating ground station’s oscillation phase. In particular, a control system may use sensor data generated by sensors systems at or near the floating ground station and/or pre-determined data to determine the oscillation phase of the floating ground station. For example, the ground station’s oscillating phase at a given time may be determined based on motion sensor data generated at or near the given time, environmental sensor data generated at or near the given time, weather forecast data indicating characteristics of currents and/or winds at or near the floating ground station at or near the given time, and/or other data sources. In some cases, the ground station may also utilize structural and/or mechanical parameters of the ground station, mooring line, tether, and/or other components of the airborne wind turbine, to determine the oscillation phase corresponding to the natural frequency. For instance, the phase may be determined based on the natural oscillation frequency and motion sensor data indicating the timing with which the floating ground station changes its direction of movement. [0164] At block 906, various techniques may be utilized to match the looping period of the aerial vehicle to the natural-oscillation period of the floating ground station. For instance, the control system may actively control the flight of the aerial vehicle to match the looping period to the natural-oscillation phase. For example, the control system could send flight control instructions indicating when and/or how the aerial vehicle should change its airspeed and/or its direction of flight (e.g., to fly on a closed path with a smaller or larger radius). Other examples are also possible.

[0165] In some cases, at block 906, various techniques may be utilized to coordinate the looping phase of the aerial vehicle with the natural-oscillation phase of the floating ground station. For instance, the control system may actively control the flight of the aerial vehicle to coordinate the looping phase with the natural-oscillation phase such that the downstroke aligns with natural forward movement of the floating ground station and the upstroke aligns with natural backward movement of the floating ground station. (Note that for ease of description, “forward” movement, displacement, or oscillation is considered as movement of the top of the ground station in a direction towards the aerial vehicle and “backward” movement, displacement, or oscillation is considered as movement of the top of the ground station in a direction away from the aerial vehicle.) To do so, the control system may utilize motion sensor data indicating the movement of the floating ground station, and generate flight-control instructions for the aerial vehicle such that transitions between the upstroke and downstroke substantially match or follow when the ground station changes its direction of movement relative to the aerial vehicle. Such flight-control instructions may indicate when and/or how the aerial vehicle should change its airspeed and/or its direction of flight (e.g., to fly on a closed path with a smaller or larger radius).

[0166] By matching the looping period of the aerial vehicle to the natural-oscillation period of the floating ground station at block 906, and aligning the aerial-vehicle looping phase and the ground station oscillation phase, the control system may increase and/or improve the efficiency of energy capture by the floating airborne wind turbine. In particular, the control system may align the looping phase of the aerial vehicle with the natural-oscillation phase such that during each looping period, downstroke movement of the aerial vehicle substantially corresponds to forward displacement of the floating ground station (e.g., movement of the top of the ground station towards the aerial vehicle), while upstroke movement of the aerial vehicle substantially corresponds to backward displacement of the floating ground station (e.g., movement of the top of the ground station away the aerial vehicle). [0167] In some cases, such as when waves are not present, the phase of the ground station’s oscillation may be partially or completely driven by variations in tether tension, which in turn is driven by the closed-loop flight of the aerial vehicle. In such cases, there may be no need to actively match the phase of the closed-path flight with the floating ground station’s oscillation phase. In other words, when the aerial vehicle flies on a closed path with a certain looping phase, the oscillation phase of the floating ground station will follow (e.g., match) the aerial vehicle’s looping phase, such that further steps to actively determine and match the ground station’s oscillation phase to the looping phase may be unnecessary. As such, variations on exemplary methods may involve matching the aerial vehicle’s looping period with the oscillation period for the ground station, without performing block 904 to determine the oscillation phase.

[0168] In a further aspect, since forward displacement of the floating ground station tends to decrease the ground-station tensioning force applied to the tether, aligning the downstroke of the looping period with forward displacement of the ground station can result in energy that otherwise might have been captured by the aerial vehicle during the downstroke to instead be stored as kinetic energy in the motion of the floating ground station. Similarly, because backward displacement of the floating ground station tends to increase the ground-station tensioning force applied to the tether, aligning the upstroke of the looping period with backward displacement of the ground station results in energy from the oscillation of the ground station being transferred to the aerial vehicle via the tether and pulling the aerial vehicle on the upstroke. Thus, the above-described alignment of period (or frequency) and phase results in a constructive interference that can increase the aerial vehicle’s speed on the upstroke and/or decrease the aerial vehicle’s speed on the downstroke. And, by reducing variation of the aerial vehicle’s speed over a looping phase, this alignment helps to improve energy capture by the floating airborne wind turbine.

[0169] In a further aspect, methods described herein could include the additional initial step of determining whether or not waves are present, or have more than some threshold amplitude or wave force, at the floating ground station. In such implementations, the use of exemplary processes to match the aerial vehicle’s looping period (or frequency) with the floating ground station’s natural oscillation period (or frequency) may be conditioned upon an initial determination that waves are not present or are below a

[0170] Figure 10 depicts a simplified timeline 1000 showing movement of an aerial vehicle 752 and movement of a ground station 754 between time 1002 and time 1018, according to an example implementation of method 900. As shown, aerial vehicle 752 flies through a circular path twice between time 1002 and time 1018. In other words, the time period between time 1002 and time 1018 spans two looping periods of the aerial vehicle. The first looping period of aerial vehicle 752 starts at time 1002, with aerial vehicle 752 at the top of the circular path. The aerial vehicle 752 then undergoes downstroke movement between time 1002 and time 1006, and upstroke movement between time 1006 and time 1010, at which point the first looping period ends. The aerial vehicle then repeats this movement during the second looping period, which occurs between time 1010 and time 1018.

[0171] Note that in Figure 10, rotation of the ground station 754 counterclockwise represents backward displacement of the ground station with respect to aerial vehicle 752, while rotation of the ground station 754 clockwise represents forward displacement of the ground station with respect to aerial vehicle 752. Further, the position of floating ground station 754 at times 1002, 1010, and 1018 represents the maximum backward displacement of the ground station with respect to aerial vehicle 752, and the position of floating ground station 754 at times 1006 and 1014 represent the maximum forward displacement of the ground station with respect to aerial vehicle 752. As such, Figure 10 shows two back-and-forth oscillations of the floating ground station 754. More specifically, a first natural-oscillation period occurs between time 1002 and time 1010, when the floating ground station 754 moves from a maximum backward displacement to a maximum forward displacement, and then back to the maximum backward displacement. The ground station then repeats this back-and-forth motion during a second oscillation period, which occurs between time 1010 and time 1018.

[0172] In accordance with method 900, the looping period of the aerial vehicle 752 is matched to the natural-oscillation period of the floating ground station 754, which results in the time period between time 1010 and time 1018 spanning exactly two looping periods and exactly two natural-oscillation periods. Also in accordance with method 900, the looping phase of the aerial vehicle 752 is aligned with the oscillation phase of the floating ground station 754 between time 1002 and time 1018 such that movement of the aerial vehicle 752 on a downstroke portion the circular path corresponds to forward displacement of the floating ground station 754, and movement of the aerial vehicle 752 on an upstroke portion the circular path corresponds to reverse displacement of the floating ground station 754. In particular, the downstroke movement of the aerial vehicle 752 between time 1002 and time 1006 aligns with forward displacement that occurs as the ground station 754 moves from its maximum backward displacement at time 1002 to its maximum forward displacement at time 1006. And, the upstroke movement of the aerial vehicle 752 between time 1006 and time 1010 aligns with backward displacement that occurs as the ground station 754 moves from its maximum forward displacement at time 1006 to its maximum backward displacement at time 1010. The movements of the aerial vehicle 752 and the ground station 754 are then repeated with the same phase alignment during the second looping period, between time 1010 and 1018.

V. Conclusion

[0173] The particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other embodiments may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an exemplary embodiment may include elements that are not illustrated in the Figures.

[0174] Additionally, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated herein.