Patent Publication Number: US-2019177006-A1

Title: Airborne Wind Turbine Tower

Description:
BACKGROUND 
     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. 
     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. 
     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. 
     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 
     An airborne wind turbine (AWT) system provides a viable way to harness wind energy in applications that were previously unavailable. Various systems and devices for efficiently and safely operating an AWT system are disclosed herein. Example embodiments include a tower and associated structures that place an aerial vehicle of an AWT in a position for more efficient energy generation and simpler storage or parking of the aerial vehicle. Embodiments include aspects that mitigate or prevent damage and increase reliability of components of an AWT system when the aerial vehicle is parked (i.e., not in a flight or power generation mode) or during wind or power failures experienced by the AWT system. Embodiments further include aspects that reduce shear stress experienced by components of the AWT system when the aerial vehicle is in a the power generation flight mode. Moreover, aspects of embodiments described herein also provide for increased safety to the environment surrounding an example AWT system. 
     In a first aspect, a system is provided. The system includes a tower, a gimbal assembly, a ring, and an aerial vehicle. The tower extends upwards from a surface. In some examples the surface may be a ground surface, while in other examples the surface may be a water surface. The gimbal assembly is coupled to the tower above the surface and is configured to move in multiple axes relative to the tower. Moreover, the ring is also coupled to the tower. The ring is coupled between the surface and the gimbal assembly, and further, the ring extends radially away from the tower a first radial distance. The aerial vehicle is configured for at least a power generating flight mode and a parked mode. Additionally, the aerial vehicle is coupled to the gimbal assembly via a tether. When the aerial vehicle is in power generating flight mode the aerial vehicle flies downwind from the tower. Moreover, when the aerial vehicle is in the parked mode, the aerial vehicle hangs from the tether such that the tether supports the weight of the vehicle. Furthermore, when in the parked mode, the ring is configured to contact the tether at a contact point along the tether. 
     In a second aspect, another system is provided. The system includes a tower, a gimbal assembly, a first landing surface, and an aerial vehicle. The tower extends upwards from a base surface. Further, the gimbal assembly is coupled to the tower above the base surface. The gimbal assembly is configured to move in multiple axes relative to the tower. Moreover, the first landing surface is also coupled to the tower between the base surface and the gimbal assembly. The first landing surface extends radially from the tower a first radial distance. Additionally, the aerial vehicle is configured for at least a power generating flight mode and a parked mode. The aerial vehicle is coupled to the gimbal assembly via a tether. Further, a linear distance along a length of the tether from the gimbal assembly to a furthest end of the aerial vehicle from the tether is less than an elevation distance of the gimbal assembly above the base surface such that the aerial vehicle is prevented from contacting the base surface. When the aerial vehicle is in power generating flight mode, the aerial vehicle flies downwind from the tower. When the aerial vehicle is in the parked mode, the aerial vehicle rests on the first landing surface. 
     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 DRAWINGS 
         FIG. 1  depicts an Airborne Wind Turbine (AWT), according to an example embodiment. 
         FIG. 2  is a simplified block diagram illustrating components of an AWT, according to an example embodiment. 
         FIG. 3  depicts an aerial vehicle, according to an example embodiment. 
         FIG. 4  depicts an aerial vehicle coupled to a ground station via a tether, according to an example embodiment. 
         FIG. 5A  depicts an AWT in a first operational mode, according to an example embodiment. 
         FIG. 5B  depicts an AWT in a second operational mode, according to an example embodiment. 
         FIG. 6A  depicts an AWT in a first operational mode, according to an example embodiment. 
         FIG. 6B  depicts an AWT in a second operational mode, according to an example embodiment. 
         FIG. 7A  depicts an AWT in a first operational mode, according to an example embodiment. 
         FIG. 7B  depicts an AWT in a second operational mode, according to an example embodiment. 
         FIG. 8  depicts an AWT, according to an example embodiment. 
         FIG. 9  depicts an AWT, according to an example embodiment. 
         FIG. 10A  depicts an AWT in a first operational mode, according to an example embodiment. 
         FIG. 10B  depicts an AWT in a second operational mode, according to an example embodiment. 
         FIG. 11A  depicts an AWT in a first operational mode, according to an example embodiment. 
         FIG. 11B  depicts an AWT in a second operational mode, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods, systems, and devices are described herein. Any example embodiment or feature described herein is not necessarily to be construed as preferred or advantageous over other embodiments or features. The example embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed systems and methods can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein. 
     Furthermore, the particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other embodiments might 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 example embodiment may include elements that are not illustrated in the Figures. 
     I. OVERVIEW 
     Illustrative embodiments relate to aerial vehicles, which may be used in a wind energy system, such as an Airborne Wind Turbine (AWT). In particular, illustrative embodiments relate to or take the form systems and devices relating to a tower that serves as a support for a gimbal that is connected to a tether that is connected to an aerial vehicle. Illustrative embodiments further include aspects of the tower that may serve to increase the efficiency of the AWT system as well as prevent damage to components of the system. Such aspects may increase the safety of the use and operation of AWT systems. 
     By way of background, an AWT may include an aerial vehicle that flies in a closed path, such as a substantially circular path, to convert kinetic wind energy to electrical energy. In an illustrative implementation, the aerial vehicle may be connected to a ground station via a tether. While tethered, the aerial vehicle can: (i) fly at a range of elevations and substantially along the path, and return to the ground or a landing platform, and (ii) transmit electrical energy to the ground station via the tether. In some implementations, the ground station may transmit electricity to the aerial vehicle for take-off and/or landing. The ground station may be located on land or offshore. In some embodiments, the ground station may be fixedly installed, while in other embodiments the ground station may be easily transportable. The ground station may include a tower structure that supports a ground station gimbal (or “gimbal assembly”) that is in turn, connected to the tether. 
     In an AWT, an aerial vehicle may rest or land in and/or on a ground station, landing platform/surface, or perch when the wind is not conducive to power generation. When the wind is conducive to power generation, such as when a wind speed may be 3.5 meters per second (m/s) at an altitude of 100 meters, the ground station may deploy (or launch) the aerial vehicle. In addition, when the aerial vehicle is deployed and the wind is not conducive to power generation, the aerial vehicle may return to the ground station. When the aerial vehicle as at rest on a landing surface or perched, for example, the aerial vehicle is in a parked mode. 
     Moreover, in an AWT, an aerial vehicle may be configured for hover flight and crosswind flight. Crosswind flight may be used to travel in a motion, such as a substantially circular motion, and thus may be the primary technique that is used to generate electrical energy (i.e., a power generation flight mode). When the aerial vehicle is flying in crosswind flight the aerial vehicle is considered to be in a crosswind flight mode. Hover flight in turn may be used by the aerial vehicle to prepare and position itself for crosswind flight. In particular, the aerial vehicle could ascend to a location for crosswind flight based at least in part on hover flight. Further, the aerial vehicle could take-off and/or land via hover flight. 
     In hover flight, a span of a main wing of the aerial vehicle may be oriented substantially parallel to the ground, and one or more propellers of the aerial vehicle may cause the aerial vehicle to hover over the ground. In some implementations, the aerial vehicle may vertically ascend or descend in hover flight. Moreover, in crosswind flight, the aerial vehicle may be oriented, such that the aerial vehicle may be propelled by the wind substantially along a closed path, which as noted above, may convert kinetic wind energy to electrical energy. In some implementations, one or more rotors of the aerial vehicle may generate electrical energy by slowing down the incident wind. 
     Towers as part of an AWT system described herein may be configured to place the aerial vehicle in a power generating flight mode, such as a crosswind flight mode, that provides efficient energy generation as the aerial vehicle flies along a closed path. As described, when the aerial vehicle is in crosswind flight the airflow acting on the moving aerial vehicle is faster than just the wind experienced by a stationary object. This apparent wind experienced at the aerial vehicle spins rotors of the aerial vehicle, thus generating electricity that is transmitted back to the ground station. Beneficially, a tower, included as part of AWT systems described herein, may position the aerial vehicle such that when the aerial vehicle flies along a closed loop flight path (while in the crosswind flight mode) an inclination angle of a center of the closed loop flight path relative to the wind direction may be reduced. An ideal inclination angle may be zero degrees, e.g. parallel to the wind, in some circumstances. 
     Among various factors, the angle of the center of the closed path flown by the aerial vehicle with respect to the wind, the inclination angle, affects the efficiency of the energy generation. When the inclination angle between the center of the closed path of the aerial vehicle and the wind is lower, the aerial vehicle is more efficient at using the power of the wind for kinetic energy of the aerial vehicle which in turn allows rotors coupled to the wing to more efficiently generate energy. Put another way, a plane of flight formed by the closed path is perpendicular to a ground surface and/or a wind direction when the inclination angle is zero degrees. More particularly, the rotors of the aerial vehicle will spin the most when a crosswind path of the aerial vehicle (e.g., when the aerial vehicle is in the crosswind flight mode) is perpendicular to the wind. Moreover, when the inclination angle is lower, components of the AWT system may experience less shear stress than configurations that create a greater inclination angle. 
     As such, it may be beneficial to place a gimbal assembly, which acts a point of rotation as the aerial vehicle goes around a closed loop flight path, at an elevation so that the center of the crosswind closed loop path maintains a constant and more efficient (i.e., lower) inclination angle to the wind. In scenarios where the gimbal assembly is located at or near a surface, such as the ground or surface of a body of water, the wing of the aerial vehicle may be at larger, less efficient inclination angles to the wind at various points along the closed loop. Therefore, it may be beneficial to locate the gimbal assembly at a higher elevation (e.g., between 50 m and 500 m above the ground) by coupling the gimbal assembly to a tower that extends vertically above the ground surface. Again, locating the gimbal assembly at an elevation may result is more efficient and better power production from the aerial vehicle. Furthermore, the gimbal assembly coupled to a taller tower may allow for operation at an inclination angle that reduces stress and strain on the tether and aerial vehicle, as well as other components. This also may increase reliability and the lifespan of the AWT system. 
     Such a configuration of the AWT system may also have other benefits. For example, for the same flight elevation, a length of the tether connecting the aerial vehicle to the gimbal assembly on a tall tower may be less than in a scenario with the gimbal assembly at or near the ground. Having a shorter tether may allow for small, more efficient loops as well as less overall weight that has to be supported by the aerial vehicle during takeoff, flight, and landing operations. Unlike tethered systems that may require complicated retrieval and/or launching mechanisms, AWT systems utilizing tower configurations disclosed herein may not require complex retrieval or launching systems such as reeling in an paying out a tether, among examples. In other examples, additional features may be designed into the tower that provide even more efficient takeoff, landing, and parking operations for the aerial vehicle. Such features may also mitigate or prevent accidental damage that may otherwise occur. 
     The Figures described in detail below are for illustrative purposes only and may not reflect all components or connections. Further, as illustrations the Figures may not reflect actual operating conditions, but are merely to illustrate embodiments described. For example, while a perfectly straight tether may be used to illustrate the described tether embodiments, during orbiting crosswind flight the tether may in practice exhibit some level of droop between the ground station and the aerial vehicle. Further still, the relative dimensions in the Figures may not be to scale, but are merely to illustrate the embodiments described. 
     II. ILLUSTRATIVE SYSTEMS 
     A. Airborne Wind Turbine (AWT) 
       FIG. 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  FIG. 1 , the tether  120  may be connected to the aerial vehicle on a first end and may be connected to the ground station  110  on a second end. 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 three 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 . 
     The ground station  110  may be used to hold and/or support the aerial vehicle  130  until it is in an operational flight mode. The ground station  110  may also be configured to allow for the repositioning of the aerial vehicle  130  such that deploying of the aerial vehicle  130  is possible. Further, the ground station  110  may be further 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, crosswind flight, and other flight modes, such as forward flight (which may be referred to as airplane-like flight). In some implementations, a ground station  110  may be configured for use on land. However, a ground station  110  may also 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 offshore 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. Although not depicted in  FIG. 1 , the ground station  110  may further include a ground station gimbal or gimbal assembly that is supported by a tower. The tower may include other features such as a ring to support the aerial vehicle  130  and/or landing surfaces for the aerial vehicle  130  when the aerial vehicle  130  is not in flight. The tower may allow for AWT configurations that do not require active storage of the tether  120 . 
     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 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 or at the ground station  110 , the one or more components may be configured to reel in the tether  120 . 
     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  may also 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. 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 at least 70 meters. In another example, the tether  120  may have a length of 140 meters. 
     The aerial vehicle  130  may be configured to fly substantially along a closed 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. 
     The aerial vehicle  130  may include or take the form of various types of devices, such as a kite, a helicopter, a wing and/or an airplane, among other possibilities. The aerial vehicle  130  may be formed of solid 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. 
     The closed path  150  may be various different shapes in various different embodiments. For example, the closed path  150  may be substantially circular. As the aerial vehicle  130  flies along the closed path  150  the aerial vehicle  130  may fly at various elevations. And in at least one example, the closed path  150  may have a radius of up to 265 meters. In other examples, the closed path  150  may have a radius that is 1.5 times a wingspan of the aerial vehicle  130 . 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 closed path  150  may be an oval, such as an ellipse, the shape of a jelly bean, the shape of the number of 8, etc. 
     The aerial vehicle  130  may be operated to travel along one or more revolutions of the closed path  150 . 
     B. Illustrative Components of an AWT 
       FIG. 2  is a simplified block diagram illustrating components of the AWT  200 . The AWT  100  may take the form of or be similar in form to the AWT  200 . In particular, the AWT  200  includes a ground station  210 , a tether  220 , and an aerial vehicle  230 . The ground station  110  may take the form of or be similar in form to the ground station  210 , the tether  120  may take the form of or be similar in form to the tether  220 , and the aerial vehicle  130  may take the form of or be similar in form to the aerial vehicle  230 . 
     As shown in  FIG. 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. 
     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. 
     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. 
     In a further respect, the ground station  210  may include a communication system  218 . The communication 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, Wi-Fi (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 . 
     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 “hotspot”; 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. 
     For example, the ground station  210  may provide a Wi-Fi connection to the remote device, and serve as a proxy or gateway to a cellular service provider&#39;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 center, which the remote device might not be able to otherwise access. 
     Moreover, as shown in  FIG. 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 electrical conductors that are configured to transmit electricity. And in at least one such example, the one or more electrical 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). 
     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 . 
     Further, as shown in  FIG. 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 , program instructions  246 , and a control system  248 . 
     The sensors  232  could include various different sensors in various different embodiments. The sensors  232  may also include one or more probes coupled to strength members of the tether  220 . In another example, the sensors  232  may further 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 (GNNS)), 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. 
     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. 
     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 may also 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. 
     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 to 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. 
     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. In addition, the aerial vehicle  230  may include one or more load cells configured to detect forces distributed between a connection of the tether  220  to the aerial vehicle  230 . 
     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. 
     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 . 
     As noted, the aerial vehicle  230  may include the power generation/conversion components  236 . The power generation/conversion components  236  could take various different forms in various different embodiments. For example, the power generation/conversion components  236  may include one or more generators, such as high-speed, direct-drive generators. With this arrangement, the one or more generators may be driven by one or more rotors. And in at least one such example, the one or more generators may operate at full rated power wind speeds of 11.5 meters per second at a capacity factor which may exceed 60 percent, and the one or more generators may generate electrical power from 40 kilowatts to 600 megawatts. 
     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 . 
     In some implementations, the aerial vehicle  230  may be configured to function as a “hotspot”; 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. 
     For example, the aerial vehicle  230  may provide a Wi-Fi connection to the remote device, and serve as a proxy or gateway to a cellular service provider&#39;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. 
     As noted, the aerial vehicle  230  may include the one or more processors  242 , the program instructions  246 , and the data storage  244 . 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 . 
     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. 
     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  220  and/or the tether  120 . 
     C. Illustrative Aerial Vehicle 
       FIG. 3  depicts an aerial vehicle  330 , according to an example embodiment. The aerial vehicle  130  and/or the aerial vehicle  230  may take the form of or be similar in form to the aerial vehicle  330 . In particular, the aerial vehicle  330  may include a main wing  331 , pylons  332   a ,  332   b , rotors  334   a ,  334   b ,  334   c ,  334   d , a tail boom  335 , and a tail wing assembly  336 . 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  330  forward. 
     The main wing  331  may provide a primary lift force for the aerial vehicle  330 . The main wing  331  may be one or more rigid or flexible airfoils, and may include various control surfaces, such as winglets, flaps (e.g., Fowler flaps, Hoerner flaps, split flaps, and the like), rudders, elevators, spoilers, dive brakes, etc. The control surfaces may be used to stabilize the aerial vehicle  330  and/or reduce drag on the aerial vehicle  330  during hover flight, forward flight, and/or crosswind flight. 
     The main wing  331  and pylons  332   a ,  332   b  may be any suitable material for the aerial vehicle  330  to engage in hover flight, forward flight, and/or crosswind flight. For example, the main wing  331  and pylons  332   a ,  332   b  may include carbon fiber and/or e-glass, and include internal supporting spars or other structures. Moreover, the main wing  331  and pylons  332   a ,  332   b  may have a variety of dimensions. For example, the main wing  331  may have one or more dimensions that correspond with a conventional wind turbine blade. As another example, the main wing  331  may have a span of 8 meters, an area of 4 meters squared, and an aspect ratio of 15. 
     The pylons  332   a ,  332   b  may connect the rotors  334   a ,  334   b ,  334   c , and  334   d  to the main wing  331 . In some examples, the pylons  332   a ,  332   b  may take the form of, or be similar in form to, a lifting body airfoil (e.g., a wing). In some examples, a vertical spacing between corresponding rotors (e.g., rotor  334   a  and rotor  334   b  on pylon  332   a ) may be 0.9 meters. 
     The rotors  334   a ,  334   b ,  334   c , and  334   d  may be configured to drive one or more generators for the purpose of generating electrical energy. In this example, the rotors  334   a ,  334   b ,  334   c , and  334   d  may each include one or more blades, such as three blades or four blades. The rotor blades may rotate via interactions with the wind and be used to drive the one or more generators. In addition, the rotors  334   a ,  334   b ,  334   c , and  334   d  may also be configured to provide thrust to the aerial vehicle  330  during flight. With this arrangement, the rotors  334   a ,  334   b ,  334   c , and  334   d  may function as one or more propulsion units, such as a propeller. Although the rotors  334   a ,  334   b ,  334   c , and  334   d  are depicted as four rotors in this example, in other examples the aerial vehicle  330  may include any number of rotors, such as less than four rotors or more than four rotors (e.g., eight rotors). 
     A tail boom  335  may connect the main wing  331  to the tail wing assembly  336 , which may include a tail wing  336   a  and a vertical stabilizer  336   b . The tail boom  335  may have a variety of dimensions. For example, the tail boom  335  may have a length of 2 meters. Moreover, in some implementations, the tail boom  335  could take the form of a body and/or fuselage of the aerial vehicle  330 . In such implementations, the tail boom  335  may carry a payload. 
     The tail wing  336   a  and/or the vertical stabilizer  336   b  may be used to stabilize the aerial vehicle  330  and/or reduce drag on the aerial vehicle  330  during hover flight, forward flight, and/or crosswind flight. For example, the tail wing  336   a  and/or the vertical stabilizer  336   b  may be used to maintain a pitch of the aerial vehicle  330  during hover flight, forward flight, and/or crosswind flight. The tail wing  336   a  and the vertical stabilizer  336   b  may have a variety of dimensions. For example, the tail wing  336   a  may have a length of 2 meters. Moreover, in some examples, the tail wing  336   a  may have a surface area of 0.45 meters squared. Further, in some examples, the tail wing  336   a  may be located 1 meter above a center of mass of the aerial vehicle  330 . 
     While the aerial vehicle  330  has been described above, it should be understood that the systems described herein could involve any suitable aerial vehicle that is connected to an airborne wind turbine tether, such as the tether  120  and/or the tether  220 . 
     D. Aerial Vehicle Coupled to a Ground Station Via a Tether 
       FIG. 4  depicts the aerial vehicle  330  coupled to a ground station  410  via the tether  120 , according to an example embodiment. Referring to  FIG. 4 , the ground station  410  may include a drum  412  and a platform  414 . The ground station  110  and/or the ground station  210  may take the form of or be similar in form to the ground station  410 .  FIG. 4  is for illustrative purposes only and may not reflect all components or connections. 
     As shown in  FIG. 4 , the tether  120  may be coupled to a gimbal assembly  442  at a proximate tether end  122  and to the aerial vehicle  330  at a distal tether end  124 . Additionally or alternatively, at least a portion of the tether  120  (e.g., at least one electrical conductor) may pass through the gimbal assembly  442 . In some embodiments, the tether  120  may terminate at the gimbal assembly  442 . Moreover, as shown in  FIG. 4 , the gimbal assembly  442  may also be coupled to the drum  412  which in turn may be coupled to the platform  414 . The aerial vehicle  330  may perch or land on the platform  414  when the aerial vehicle  330  is not in flight. In some embodiments, the tether gimbal assembly  442  may be configured to rotate about one or more axes, such as an altitude axis and an azimuth axis, in order to allow the proximate tether end  122  to move in those axes in response to movement of the aerial vehicle  330 . 
     A rotational component  444  located between the tether  120  and the gimbal assembly  442  may allow the tether  120  to rotate about the long axis of the tether  120 . The long axis is defined as extending between the proximate tether end  122  and the distal tether end  124 . In some embodiments, at least a portion of the tether  120  may pass through the rotational component  444 . Moreover, in some embodiments, the tether  120  may pass through the rotational component  444 . Further, in some embodiments, the rotational component  444  may include a fixed portion  444   a  and a rotatable portion  444   b , for example, in the form of one or more bearings and/or slip rings. The fixed portion  444   a  may be coupled to the tether gimbal assembly  442 . The rotatable portion  444   b  may be coupled to the tether  120 . 
     The use of the word fixed in the fixed portion  444   a  of the rotational component  444  is not intended to limit fixed portion  444   a  to a stationary configuration. In this example, the fixed portion  444   a  may move in axes described by the gimbal assembly  442  (e.g., altitude and azimuth), and may rotate about the ground station  410 , but the fixed portion  444   a  will not rotate about the tether  120 , i.e., with respect to the long axis of the tether  120 . Moreover, in this example, the rotatable portion  444   b  of the rotational component  444  may be coupled to the tether  120  and configured to substantially rotate with the rotation of tether  120 . 
     Via the rotational component  444 , the tether  120  may rotate about its centerline along the long axis as the aerial vehicle  330  orbits. The distal tether end  124  may rotate a different amount than the proximate tether end  122 , resulting in an amount of twist along the length of the tether  420 . With this arrangement, the amount of twist in the tether  420  may vary based on a number of parameters during crosswind flight of the aerial vehicle  330 . 
     III. ILLUSTRATIVE AWT TOWERS 
       FIG. 5A  and  FIG. 5B  depict an AWT  500  in a first operational mode and a second operational mode, respectively, according to an example embodiment. Example operational modes include a parked mode (when an aerial vehicle is at rest or parked), a hover flight mode, and a power generating flight mode (or a crosswind flight mode), among other possibilities. As illustrated, the AWT  500  is in a power generating flight mode in  FIG. 5A , while the AWT  500  is in a parked mode in  FIG. 5B . 
     The AWT  500  may take the form of or be similar in form to the AWT  200  of  FIG. 2  and/or the AWT  100  of  FIG. 1 . As depicted in  FIG. 5A  and  FIG. 5B , the AWT  500  includes a gimbal assembly  510 , a tether  520 , an aerial vehicle  530 , and a tower  540 . The tower  540  includes a plurality of guy wires  545  and a ring  550 . The gimbal assembly  510  is capable of moving in multiple axes relative to the tower  540  in order to facilitate movement of the tether  520 . The guy wires  545  are secured to a surface  505  (a ground surface or a floating platform) and stabilize the tower  540  against tension in the tether  510  from the aerial vehicle  530 . 
     As shown in  FIG. 5A , a wind  580  may be blowing at a speed such that the AWT  500  may generate electricity and thus, the aerial vehicle  530  may be in power generating flight mode (e.g., a crosswind flight mode of operation). While in the power generating flight mode, the aerial vehicle  530  may be traveling along a closed path  570  (that may be similar to closed path  150  of  FIG. 1 ). The tether  520  may be coupled to the gimbal assembly  510  at a first end of the tether  520 . Further, the tether  520  may be coupled to the aerial vehicle  530  at a second end of the tether  520 . In some aspects, the tether  520  may include a bridle at the second end and the bridle may be coupled to the aerial vehicle  530 . In some embodiments, a length of the tether  520  between the gimbal assembly  510  and the aerial vehicle  530  is fixed. 
     The gimbal assembly  510  may be coupled to the tower  540 . Preferably, the gimbal assembly  510  is coupled to the tower  540  at an elevation  510 A between about 50 m and about 500 m above the surface  505 . More preferably, the gimbal assembly  510  is coupled to the tower  540  at an elevation  510 A between about 90 m and about 150 m above the surface  505 . In some aspects, the gimbal assembly  510  may be coupled at the top or to a highest point of the tower  540 . In other aspects, the gimbal assembly  510  may be coupled below the top of the tower  540 . The gimbal assembly  510  may act as a point of rotation for the aerial vehicle  530  and tether  520  when the aerial vehicle  530  is in the power generating flight mode. The elevated location of the gimbal assembly  510  may allow for shorter tether lengths (e.g. 70 meters) compared to lower ground station designs, as well as tethers that are heavier per meter than a carbon fiber tether. Moreover, by locating the gimbal assembly  510  at the elevation  510 A such that there is a low inclination angle, shear stresses experienced by components of the AWT  500  may be reduced. 
     Further, as described above, energy may be more efficiently generated when an inclination angle is reduced or closer to zero degrees. The inclination angle is an angle between a line from the gimbal assembly  510  to a center of the closed loop flight path  570  the aerial vehicle  530  travels and a direction of the wind  580  when the aerial vehicle  530  is in the power generating mode or crosswind flight mode. Within examples, a plane of the closed path  570  may be normal to the direction of the wind  580  and normal to the surface  505  when the inclination angle is zero degrees. In some embodiments, the inclination angle is less than five degrees. In other embodiments, the inclination angle is less than thirty degrees. The inclination angle may be at least partly based on the elevation  510 A. 
     Furthermore, in order to operate within the power generating mode, the aerial vehicle  530  must be above a certain elevation from the ground surface  505 . In scenarios where the gimbal assembly  510  is located on or near the ground surface  505 , a similar AWT system would require a longer and heavier tether to get the aerial vehicle  530  up to an elevation at which energy may be generated efficiently (i.e., low inclination angle). Thus, the AWT  500 , with the tower  540  that locates the gimbal assembly  510  at the elevation  510 A, allows the aerial vehicle  530  to be in the power generating mode with a shorter length tether  520  while maintaining or increasing efficiency of power generation. 
     Moreover, if the gimbal assembly  510  was located on the ground surface  505 , when the aerial vehicle  530  was in crosswind flight, the aerial vehicle  530  alone would be supporting the entire length of the tether  520  that would be entirely above the gimbal assembly  510  at all times during crosswind flight. However, when the gimbal assembly  510  is elevated to a range of altitudes that the aerial vehicle  530  flies at during crosswind flight, the gimbal assembly  510  (and the support tower  540 ) take at least a portion of the loading caused by the weight of the tether  520  when the aerial vehicle  530  is at an elevation less than the elevation  510 A. 
     As depicted in  FIG. 5A , the tower  540  may include a lattice structure that is fixed to the surface  505 . The lattice structure of the tower  540  may include a series of metal posts or beams that are configured to support a mass of the gimbal assembly  510 . Tension from the aerial vehicle  530  in power generating flight mode may be transferred from the tether  520  to the plurality of guy wires  545  which may also be fixed to the surface  505 . The tower  540  with a lattice configuration may have low mass and a low cost when compared to existing traditional onshore wind towers. Moreover, because the AWT  500  utilizes the aerial vehicle  530  for electricity generation, the tower  540  may be configured to bend within an allowance without concern for a loss of operational capacity from the aerial vehicle  530  in the wind  580 . Beneficially, a stiff, bulky, upright tower to support a large top head mass (e.g., a conventional turbine, generator, etc.) is not necessary. 
       FIG. 6A  and  FIG. 6B  depict an AWT  600 , according to another example embodiment. The AWT  600  may take the form of, or be similar in form to, the AWT  500 , except as indicated below. Similar to AWT  500 , the AWT  600  includes a gimbal assembly  510 , a tether  520 , an aerial vehicle  530 , and a ring  550 . These components of the AWT  600  may have similar function as in the AWT  500 . In  FIG. 6A , the aerial vehicle  530  is depicted flying in a power generating flight mode similar to  FIG. 5A , and in  FIG. 6B , the aerial vehicle is shown in a parked mode, similar to  FIG. 5B . Power generating flight mode and parked mode would function similarly in both AWT  500  and AWT  600 . 
     Unlike as in the AWT  500 , the tower  640  is a tubular tower that extending upwards from a surface  505 . Despite having a different design, the AWT  600  may still present similar benefits as the AWT  500  of  FIG. 5 . The tower  640  may still need to support tensions during crosswind flight of the aerial vehicle  530  similar to loading experienced by more traditional towers, the tower  640  may still have a relatively lower mass than more traditional wind turbine towers and the tower  640  does not have to support a top head mass of the magnitude of more traditional wind turbine engines and blades. Although more rigid than a lattice tower (e.g., the tower  540  of  FIG. 5 ), because the tower  640  does not need to be as rigid or carry as much of a top mass load as a traditional wind turbine tower, it may also be relatively lower in cost. 
     A ring  550  may be coupled to the tower  540  or tower  640  between the surface  505  and the gimbal assembly  510 . In some embodiments of AWT  500 , the ring  550  may be coupled to and/or supported by the plurality of guy wires  545 . In some embodiments of AWT  500  or AWT  600 , the ring  550  may be supported by other guy wires that may or may not be connected the tower  540  and/or the surface  505 . The ring  550  extends some distance radially from the tower  540  or the tower  640 . In some embodiments, the ring  550  may be centered around the tower  540  or the tower  640 . The ring  550  may be coupled about the tower  540  or the tower  640  in a plane normal to the tower  540  or tower  640 . 
     As depicted in  FIGS. 5A-6B , the ring  550  may be coupled at an elevation above the surface  505  and along the tower  540  or tower  640  such that when the aerial vehicle  530  is in a parked mode, the aerial vehicle  530  may hang from the gimbal assembly  510 , such that the tether  520  supports the weight of the vehicle. The ring  550  may contact the tether  520  at a contact point  520 C and bias the tether  520  and the aerial vehicle  530  away from the tower  540  or the tower  640 . The contact point  520 C may include a special coating or additional or different material configured to withstand and/or facilitate the contact between the ring  550  and the tether  520 . 
     When the aerial vehicle  530  is in parked mode, a first portion  520 A of the tether  520  may hang from the gimbal assembly  510  to the ring  550 . A second portion  520 B of the tether  520  may hang below the ring  550 . In this configuration, the tether  520  is supporting the weight of the aerial vehicle  530 . 
     When the aerial vehicle  530  is in parked mode, the ring  550  may prevent the aerial vehicle  530  from contacting the tower  540 , the tower  640 , or other components of the AWT  500  or AWT  600 . In other words, the ring  550  may provide clearance around the aerial vehicle  530  in a parked condition to prevent or lessen any damage caused by accident or failure of the AWT  500  or AWT  600 . To do so, a linear distance  520 D from the contact point  520 C to a furthest end of the aerial vehicle  530  from the tether  520  is less than the radial distance  550 A of the ring  550  from the tower  540  or tower  640 . 
     The ring  550  may have a curved outer surface area at the contact point  520 C, i.e. in a direction radially outward from the tower  540  or tower  640 , that is large enough to support a bend radius of the tether  520 . Additionally or alternatively, the ring  550  and/or the tether  520  at the contact point  520 C may also include features such as a snap fit or other mechanical coupling mechanism (e.g. clasps, magnets, etc.) that is configured to connect and hold the tether  520  in a fixed position up against the ring  550 . The coupling mechanism may be passive, i.e., the tether  520  and the ring  550  will couple once they come into contact, or the coupling mechanism may be an active system that locates and/or secures the tether  520 . In another embodiment, the ring  550  may include components or mechanisms that are configured to couple to the bridle between the tether  520  and the aerial vehicle  530 . Coupling the ring  550  to the bridle may reduce a roll moment of the aerial vehicle  530  as the aerial vehicle  530  hangs in a parked configuration. 
     As shown  FIGS. 5A and 6A , when the aerial vehicle  530  is in the power generating flight mode, the aerial vehicle  630  may have an maximum flight altitude  530 A above the surface  505  when it is at the top of the closed loop flight path  570 . Similarly, the aerial vehicle  530  may have an minimum flight altitude  530 B above the surface  505  when it is at the bottom of the closed loop flight path  570 . As shown in  FIGS. 5B and 6B , when the aerial vehicle  530  is in a parked configuration where the tether  520  is in contact with the ring  550  at the contact point  520 C, the aerial vehicle  530  may have an elevation  530 C above the surface  505 . 
     As such, the tower  540  or tower  640 , and the ring  550 , may be a fail-safe for the AWT  500  or AWT  600  in case of a failure. For example, if the aerial vehicle  530  were to lose power while in a power generating flight mode, the arrangement of AWT or AWT  600  would mitigate risk of the aerial vehicle  530  contacting or damaging the tower  540 , the tower  640 , or any of the environment surrounding the AWT  500  or AWT  600 , e.g. structures or people on the ground surface  505 . 
     In one embodiment, the elevation  510 A (i.e., the elevation of the gimbal assembly  510 ), may be more than the minimum flight altitude  530 B but less than the maximum flight altitude  530 A. As such, when the aerial vehicle  530  is in the power generating flight mode (e.g., crosswind flight mode), energy generation components (e.g., rotors) of the aerial vehicle  530  may be more efficiently positioned or angled in the wind  580 . Thus, the elevation  510 A of the gimbal assembly  510  may reduce the inclination angle between a direction of the wind  580  and the center of the closed loop flight path  570  relative the gimbal assembly  510  in power generation mode. In another embodiment, the elevation  510 A may be halfway between the minimum flight altitude  530 B and the maximum flight altitude  530 A of the aerial vehicle  530 . In such an example, the elevation  510 A of the gimbal assembly  510  may be at the same elevation as the center of the closed flight path  570  (i.e., the inclination angle is zero and the plane of the closed flight path  570  is perpendicular to the wind  580  and the ground  505 ). 
       FIG. 7A  and  FIG. 7B  depict an AWT  700 , according to another example embodiment. The AWT  700  may take the form of, or be similar in form to, the AWT  500 , except as indicated below. Similar to AWT  500 , the AWT  700  includes a gimbal assembly  510 , a tether  520 , an aerial vehicle  530 , and a ring  550 . These components of the AWT  700  may have similar function as in the AWT  500 . In  FIG. 7A , the aerial vehicle  530  is depicted flying in a power generating flight mode similar to  FIG. 5A , and in  FIG. 7B , the aerial vehicle is shown in a parked configuration, similar to  FIG. 5B . Power generating flight mode and parked mode would function similarly in both AWT  500  and AWT  700 . 
     Unlike as in the AWT  500 , the AWT  700  is an offshore AWT design. As such, the tower  740  may include or be constructed from a buoy. The tower  740  may be configured to float in a body of water with a surface  705 , with the tower  740  extending upward from a surface  705 , as well as extending below the surface  705 . Portions of the tower  740  may tubular, solid, lattice, or other structural designs. The tower  740  may be allowed to tilt in a direction of the wind  580  when the aerial vehicle  530  is in power generating flight mode. The tension from the aerial vehicle  530  in flight may be distributed to an anchor cable  744  that is secured below the surface  705  (e.g., by anchor, tension lines, or guy wires) and also coupled to the tower  740 . Allowing the tower  740  to tilt may assist in locating the aerial vehicle  530  in a more efficient position relative to the wind  580 . Further, the tower  740  of the AWT  700  allows for offshore wind generation without the cost of constructing a rigid tower that has to remain nearly perfectly vertical in order to operate. In some examples, the tower  740  may be configured to tilt up to 50 degrees from vertical). In park mode (or when not in power generating flight mode), the aerial vehicle  530  may exert less than a significant tilting force on the tower  740  and the tower  740  can return to a stable upright orientation. 
       FIG. 8  depicts an AWT  800 , according to another example embodiment. The AWT  800  may take the form of or be similar in form to the AWT  600  of  FIGS. 6A and 6B , the AWT  500  of  FIGS. 5A and 5B , the AWT  200  of  FIG. 2 , and/or the AWT  100  of  FIG. 1 . As depicted in  FIG. 8 , the AWT  800  includes a gimbal assembly  510 , a tether  520 , an aerial vehicle  530 , and a tower  840 . Further, components of the AWT  800  may take similar form and have similar function to components of the AWT  600  of  FIGS. 6A and 6B , and the AWT  500  of  FIGS. 5A and 5B . The tower  840  may be constructed as a lattice tower, similar to the tower  540  of  FIGS. 5A and 5B . In another embodiment, the tower  840  may be a tubular tower or equivalent. 
     In addition to the other components, the AWT  800  further includes a landing platform  860 . The landing platform  860  may be located onshore and include supporting structures to assist the aerial vehicle  530  in parking or perching when the aerial vehicle  530  is not in flight. In this example embodiment, the tether  520  may be long enough such that the aerial vehicle  530  can land at the landing platform  860 . In some examples, the landing platform  860  may be fixed, while in other examples the landing platform  860  may be allowed to move autonomously or under remotely control. When parked on the landing platform  860  the aerial vehicle is in a parked mode, and the aerial vehicle  530  may charge batteries, download or upload information, undergo mechanical maintenance, or otherwise be maintained. The landing platform  860  may further provide means for allowing the aerial vehicle  530  to perch in a vertical orientation. 
       FIG. 9  depicts an AWT  900 , according to another example embodiment. The AWT  900  may take the form of or be similar in form to the AWT  700  of  FIGS. 7A and 7B , the AWT  600  of  FIGS. 6A and 6B , the AWT  500  of  FIGS. 5A and 5B , the AWT  200  of  FIG. 2 , and/or the AWT  100  of  FIG. 1 . As depicted in  FIG. 9 , the AWT  900  includes a gimbal assembly  510 , a tether  520 , an aerial vehicle  530 , a tower  940 , and a landing platform  960 . The landing platform  960  may be located offshore. In some examples the landing platform  960  may be a floating platform. The tower  940  may be constructed as a buoy tower, similar to the tower  740  of  FIGS. 7A and 7B . 
     Further, components of the AWT  900  may take similar form and have similar function to components of the AWT  700  of  FIGS. 7A and 7B , the AWT  600  of  FIGS. 6A and 6B , and the AWT  500  of  FIGS. 5A and 5B . In some embodiments, the tower  940  may be partially constructed as a lattice tower, similar to the tower  540  of  FIGS. 5A and 5B . 
       FIG. 10A  and  FIG. 10B  depict an AWT  1000 , according to another example embodiment. The AWT  1000  may take the form of, or be similar in form to, the AWT  500 , except as indicated below. Similar to AWT  500 , the AWT  1000  includes a gimbal assembly  510 , a tether  520 , and an aerial vehicle  530 . These components of the AWT  1000  may have similar function as in the AWT  500 . In  FIG. 10A , the aerial vehicle  530  is depicted flying in a power generating flight mode similar to  FIG. 5A , and in  FIG. 10B , the aerial vehicle is shown in a parked configuration. Power generating flight mode would function similarly in both AWT  500  and AWT  1000 . 
     Unlike as in the AWT  500 , the AWT  1000  is a landing platform design. As such, a landing surface  1050  is coupled to the tower  1040  between the base surface  505  and the gimbal assembly  510 . Similar to the ring  550 , the landing surface  1050  extends radially from the tower some radial distance. The landing surface  1050  may be considered a landing platform in some embodiments, and moreover, the landing surface  1050  may include components and aspects of the landing platform  860  of  FIG. 8  or the landing platform  960  of  FIG. 9 . The landing surface  1050  may take the form of an annular track that surrounds at least a portion of the tower  1040 . 
     The AWT  500  is configured such that the aerial vehicle  530  is capable of landing on the landing surface  1050  in a parked mode, but not capable of crashing into the surface  505  in the event of a failure. To accomplish this, the linear distance along the length of the tether  520  from the gimbal to a furthest end of the aerial vehicle  530  from the tether  520  is less than the elevation distance of the gimbal  510  above the base surface  505 . In practical terms, the tether  520  is too short to allow the aerial vehicle  530  to touch the surface  505 . In some embodiments, the tether  520  may be significantly longer than the distance from the gimbal  510  to the landing surface  1050 . In such embodiments, the landing surface  1050  may further be arranged to function similarly to the ring  550 , such that landing surface  1050  has an outer contact surface configured to contact the tether so the aerial vehicle  530  can hang from tether  520  and not contact the surface  505  or the tower  1040 , similar to AWT  500 . This would be beneficial in the event of a failure of the AWT  1000 . 
     While a single landing surface  1050  is depicted, more than one landing surface  1050  may be coupled to the tower  1040 . For example, two or more landing surfaces may be coupled to the tower, and those landing surfaces may have other configurations (e.g., half circle annular tracks), may be at multiple elevations along the tower  1040 , and/or may be coupled to the tower  1040  at different radial locations (e.g., one landing surface on a North side of the tower  1040  and one landing surface on a South side of the tower  1040 ). Moreover, the landing surface  1050  may include additional components that may allow for vertical perching or parking modes of the aerial vehicle  530 . For example, the landing surface  1050  may include supports that provide for vertical take-off and landing/perching of the aerial vehicle  530 . In some examples, particularly when the aerial vehicle  530  is configured to perch when in a parked mode, the landing surface  1050  may be in a plane normal to a base surface, such as a ground surface. In other examples, the landing surface  1050  may be in a plane normal to the tower  1040   
       FIG. 11A  and  FIG. 11B  depict an AWT  1100 , according to another example embodiment. The AWT  1100  may take the form of, or be similar in form to, the AWT  500  and/or AWT  1000 , except as indicated below. Similar to AWT  500 , the AWT  1000  includes a gimbal assembly  510 , a tether  520 , and an aerial vehicle  530 . These components of the AWT  1000  may have similar function as in the AWT  500 . In  FIG. 11A , the aerial vehicle  530  is depicted flying in a power generating flight mode similar to  FIG. 5A , and in  FIG. 11B , the aerial vehicle is shown in a parked configuration, similar to  FIG. 10B . Power generating flight mode would function similarly in both AWT  500  and AWT  1000 , and parked mode would function similar to AWT  1000 . 
     As depicted in  FIGS. 11A and 11B , the tower  1140  includes a landing surface  1180 . The landing surface  1180  may be positioned at one of a first set of landing positions  1180 A or a second set of landing positions  1180 B. In other examples, a second landing surface may be positioned at another of the first set of landing positions  1180 A or the second set of landing positions  1180 B. The first set of landing positions  1180 A may be at a first elevation above the surface  505 , while the second set of landing positions  1180 B may be at a second elevation above the surface  505 . 
     In one embodiment, each set of landing positions  1180 A and  1180 B may include four individual landing positions that are evenly spaced out about the tower  1140 . For example, the first set of landing positions  1180 A may each be 90 degrees apart from each other around the circumference of the tower  1140 . Similarly, the second set of landing positions  1180 B may also be 90 degrees apart from each other. Moreover, each landing position of the first set of landing positions  1180 A may be offset 45 degrees around the tower from each landing position of the second set of landing positions  1180 B. In other embodiments, there may be more or less landing positions. 
     In some examples, when the aerial vehicle  530  lands on the landing surface  1180  (one of the landing positions  1180 B as depicted in  FIG. 11B ), the tether  520  may include some slack that is allowed to droop below an elevation of the landing surface  1180  above a base surface. Within examples, at least a portion of the tether  520  may be below the landing surface  1180  when the aerial vehicle  530  is in a parked mode. In other words, at least a portion of the tether  520  may be at an elevation above the base surface  505  that is less than an elevation above the base surface  505  of the landing surface  1180  when the aerial vehicle  530  is in the parked mode. As such, a configuration of the tower  1140  with the landing surface  1180  may prevent the tether  520  from contacting the base surface without need for a complicated system to reel-in or pay-out the tether  520 . 
     Other landing surface configurations are contemplated herein. For example, while the landing surface  1180  among the landing positions  1180 A and  1180 B is depicted as being horizontally planar, the landing surface  1180  may also include components to allow the aerial vehicle  530  to vertically perch on the landing surface  1180 . As such, in some aspects, particularly wherein the aerial vehicle  530  perches when in the parked mode, the landing surface  1180  may be in a plane that is normal (or nearly normal) to the base surface. 
     IV. CONCLUSION 
     It should be understood that arrangements described herein are for purposes of example only. As such, those skilled in the art will appreciate that other arrangements and other elements (e.g. machines, interfaces, operations, orders, and groupings of operations, etc.) can be used instead, and some elements may be omitted altogether according to the desired results. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location, or other structural elements described as independent structures may be combined. 
     While various aspects and implementations have been disclosed herein, other aspects and implementations will be apparent to those skilled in the art. The various aspects and implementations disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims, along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular implementations only, and is not intended to be limiting.