Patent Publication Number: US-2021181768-A1

Title: Controllers for Lighter-Than-Air (LTA) Vehicles Using Deep Reinforcement Learning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 16/667,424 filed Oct. 29, 2019, and U.S. patent application Ser. No. 16/667,441, filed Oct. 29, 2019, all of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF INVENTION 
     Computing devices such as personal computers, laptop computers, tablet computers, cellular phones, and countless types of Internet-capable devices are increasingly prevalent in numerous aspects of modern life. This prevalence of Internet-capable devices can enable us to connect people all over the world, and as such, the demand for data connectivity via the Internet, cellular data networks, and other such networks, is growing rapidly. In areas where conventional Internet connectivity-enabling infrastructure exists (e.g., urban or other relatively densely populated areas), people can connect to make phone calls in an emergency, get access to the weather forecasts (e.g., to plan for crop planting, to plan logistics for major events, etc.) and to education. However, there are many areas of the world where data connectivity is unavailable, unreliable and/or extremely costly due to difficulties in building and maintaining conventional infrastructure in these areas. In these areas, the growing demand for data connectivity is not being met, thus they can benefit greatly from data connectivity delivered by fleets of aerial vehicles. 
     There are various types of aerial vehicles that may provide these types of connectivity services, floating or flying at different altitudes and regions of the atmosphere. Particularly, in the stratosphere where winds are stratified, wind currents are strong and quickly changing, and each layer of wind may vary in speed and direction, unmanned high altitude aerial vehicles may navigate from location to location using these stratospheric wind currents. However, many factors, in addition to predicted weather forecasts, are important in generating navigation policies for planning and/or controlling movement of such aerial vehicles. Conventional methods for generating flight controllers are unable to use large numbers of variable factors and objectives under uncertainty, such as imperfect weather forecasts, to optimize flight objectives. Typical flight controllers also have difficulty adapting to localized differences, such that a controller would not be tuned to optimize a vehicle&#39;s flight over two or more regions (e.g., over a country, island, ships, or bodies of water, near the equator versus another farther North, South, or closer to a pole) with different flight regulations, no-fly restrictions, and other environmental or regulatory differences. 
     Conventional flight navigation techniques have largely been unique to a particular type of vehicle and determined based on a combination of heuristics (i.e., rules) and conventional models. The particular set of heuristics and conventional models for determining flight paths and actions often will be tailored to a specific vehicle system, with a particular set of characteristics such as size, weight, materials, whether the vehicle is floating or self-propelled, or other characteristics. Thus, conventional techniques are unable to generate controllers for many different types of vehicles without a lot of custom human design, and generally require a significant amount of human engineering and tailoring to achieve each defined objective, much less to achieve multiple objectives at the same time, such as reaching a target destination while optimizing for power consumption, avoiding unauthorized airspace, and reducing time and/or distance to the target destination. 
     Thus, improved systems and methods for generating controllers for lighter-than-air vehicles is desirable. 
     BRIEF SUMMARY 
     The present disclosure provides for a system and method for generating controllers for lighter-than-air vehicles, particularly controllers directed to achieving particular objectives. A computer-implemented method for generating an objective-directed controller for an aerial vehicle may include defining an action space for an objective-directed controller; providing a set of feature vectors and the action space as input to a simulation module, the set of feature vectors associated with a desired objective; training, by a learning module, the objective-directed controller according to a reward function correlated with the desired objective; evaluating the trained objective-directed controller; and storing the trained objective-directed controller. In some examples, the action space is defined with continuous directions. In some examples, the action space is defined with discretized directions. In some examples, the action space is defined with a set of actions comprising up, down and stay. In some examples, the action space is defined with a set of actions comprising lateral propulsion in a given direction at a given speed. In some examples, the action space is defined with power levels on and off. In some examples, the action space is defined with a plurality of power levels. 
     In some examples, the reward function is configured to generate a cumulative incursion score. In some examples, the reward function is configured to assess a large first-entry penalty. In some examples, the reward function is configured to reward progress made on an objective. In some examples, the reward function is configured to generate a fixed penalty incursion score. In some examples, the desired objective comprises navigation toward a target heading using an altitude control system. In some examples, the desired objective comprises navigation toward a target heading using an altitude control system and lateral propulsion system. In some examples, the desired objective comprises station seeking. In some examples, the desired objective comprises map following. In some examples, the desired objective comprises restricted zone avoidance. In some examples, the desired objective comprises storm avoidance. In some examples, the set of feature vectors includes a standard feature vector used for a plurality of objectives. In some examples, the set of feature vectors includes an objective-directed feature vector. 
     A distributed computing system may include a storage system configured to store objectives, feature vectors, and trained objective-directed controllers; and one or more processors configured to: define an action space for an objective-directed controller, provide a set of feature vectors and the action space as input to a simulation module, the set of feature vectors associated with a desired objective, train, by a learning module, the objective-directed controller according to a reward function correlated with the desired objective, evaluate the trained objective-directed controller, and store the trained objective-directed controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  are diagrams of exemplary operational systems in which learned flight policies may be implemented for navigating an aerial vehicle, in accordance with one or more embodiments; 
         FIG. 2  is a diagram of an exemplary aerial vehicle network, in accordance with one or more embodiments; 
         FIG. 3  is a simplified block diagram of an exemplary computing system forming part of the systems of  FIGS. 1A-2 , in accordance with one or more embodiments; 
         FIG. 4  is a simplified block diagram of an exemplary distributed computing system, in accordance with one or more embodiments; 
         FIG. 5A  is a simplified block diagram of an exemplary flight policy training system, in accordance with one or more embodiments; 
         FIG. 5B  is a simplified block diagram of another exemplary flight policy training system, in accordance with one or more embodiments; 
         FIG. 6  is a simplified block diagram of an exemplary meta-learning system, in accordance with one or more embodiments; 
         FIG. 7A  is a flow diagram of an exemplary method for generating learned flight policies, in accordance with one or more embodiments; 
         FIG. 7B  is a flow diagram of an exemplary method for evaluating learned flight policies and learning systems, in accordance with one or more embodiments; 
         FIG. 8  is a flow diagram of an exemplary method for deploying a learned flight policy in an operational aerial vehicle system, in accordance with one or more embodiments; 
         FIG. 9  is a map showing a flight path resulting from controlling an aerial vehicle using a learned flight policy, in accordance with one or more embodiments; 
         FIG. 10  is a flow diagram of an exemplary method for generating a controller for an LTA vehicle using a deep reinforcement learning architecture, in accordance with one or more embodiments; and 
         FIG. 11  is a map showing an exemplary flight path of an aerial vehicle being controlled by an objective-directed controller, in accordance with one or more embodiments. 
     
    
    
     The figures depict various example embodiments of the present disclosure for purposes of illustration only. One of ordinary skill in the art will readily recognize from the following discussion that other example embodiments based on alternative structures and methods may be implemented without departing from the principles of this disclosure, and which are encompassed within the scope of this disclosure. 
     DETAILED DESCRIPTION 
     The Figures and the following description describe certain embodiments by way of illustration only. One of ordinary skill in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. 
     The above and other needs are met by the disclosed methods, a non-transitory computer-readable storage medium storing executable code, and systems for navigating aerial vehicles in operation, as well as for generating flight policies for such aerial vehicle navigation using deep reinforcement learning. 
     Aspects of the present technology are advantageous for high altitude systems (i.e., systems that are operation capable in the stratosphere, approximately at or above 7 kilometers above the earth&#39;s surface in some regions, and at or above 20 kilometers above the earth&#39;s surface in other regions, or beyond in the exosphere or cosmic space), such as High Altitude Platforms (HAPs), High Altitude Long Endurance (HALE) aircraft, unmanned aerial vehicles (UAVs), including lighter than air vehicles (e.g., floating stratospheric balloons), propelled lighter than air vehicles (e.g., propelled floating stratospheric balloons), fixed-wing vehicles (e.g., drones, rigid kites), various types of satellites, and other high altitude aerial vehicles. In some examples, high altitude systems are configured to fly above an altitude reserved for commercial airline flights. One way to provide enhanced network access is through a network of aerial vehicles carrying Internet, cellular data, or other network capabilities. To maintain a network, each aerial vehicle in a fleet or network of aerial vehicles may travel to a particular location. In some embodiments, lighter than air aerial vehicles (i.e., propelled or not) may rely on rapidly changing and extreme (i.e., strong, high speed, and volatile) wind conditions to assist in navigation efforts to different locations. Other environmental and non-environmental factors may impact an aerial vehicle&#39;s flight plan or policy. In view of this, large scale simulations may be performed to evaluate operational characteristics or capabilities (e.g., power system availability, ambient temperatures, software and hardware versions implemented or accessible onboard, integrity of various components) and life cycle of individual aerial vehicles or fleets. Such simulations may be used to manage the life cycle of an aerial vehicle to manage risk of failures and to optimize availability for service delivery. 
     This disclosure is directed to a deep reinforcement learning system (hereinafter “learning system”) for generating optimal flight policies to control aerial vehicles according to a desired goal (i.e., objective), along with methods for performing training learned flight policies that are improved and optimized for one or more objectives, and deploying said learned flight policies in an aerial vehicle navigation system. As described in more detail below, the learning system comprises a simulation module (including one or more “Workers” or simulators), one or more replay buffers, a learning module comprising a deep reinforcement learning architecture designed to train flight policies, and one or more servers or repositories that store learned policies from the learning module. 
     In a training loop, the simulation module simulates an aerial vehicle&#39;s flight through a region of the atmosphere (e.g., stratosphere) according to a given policy (e.g., encoded into a neural network, for determining an action by the aerial vehicle in a given environment and aerial vehicle state). The simulation module generates a frame, represented by one or more feature vectors, for each time step, and feeds the frames of each simulation to one or more replay buffers. The replay buffers store the frames (e.g., in sequential order in a circular buffer or at random), and the learning module requests a set of frames from one or more replay buffers as inputs. An input may comprise a random sample of frames from a circular buffer, a prioritized sample of frames according to optimization criteria, or a set of frames with other characteristics. 
     The training loop continues with the learning module processing the frames according to a deep reinforcement learning architecture to determine, in a given situation (i.e., a given vehicle state in a given environment), which action provides a larger or largest estimated reward. Actions to be taken by an aerial vehicle may include ascending, descending, maintaining altitude, and propelling itself in a direction, among others, and may be manifested as discrete actions such as up, down, or stay (i.e., maintain altitude). In some examples, the learning module may run one or more neural networks that output a value or other representation of an action, or a command associated with an action, and a magnitude associated with said action or command. The deep reinforcement learning architecture may be configured to run one or more variations of reinforcement learning, including value-based methods, distributional methods, and policy-based methods. Some examples of reinforcement learning techniques include, without limitation, Q-learning, double Q-learning, distributional Q-learning, categorical Q-learning, quantile regression Q-learning; policy gradient, actor-critic, soft actor-critic, and trust region policy optimization, among others. The reinforcement learning algorithm in the learning module is characterized by a reward function corresponding to the objective of the flight policy training. The learning module generates learned flight policies (e.g., encoded in neural networks), and scores them according to the reward function. The learned policies may be stored in a policy server, from which the simulation module can pull learned policies to run further simulations. 
     A reward function is defined according to a desired objective. Example objectives include: flying within a predetermined radius of a target location; following a mapped trajectory; flying in a given direction; arriving at a location at a desired date and time; maximizing (or otherwise optimizing) the amount of time an aerial vehicle provides connection services to a given area; conserving energy or minimizing energy consumption during a time period of flight or in achieving any of the aforementioned objectives; and achieving any of the aforementioned objectives in coordination with other aerial vehicles (i.e., in the context of a fleet of aerial vehicles). In some examples, the systems described herein may be tuned with a reward function that optimizes for multiple objectives (e.g., any combination of the example objectives above), and may further account for other factors, such as minimizing wear on a vehicle, avoiding inclement weather, etc. A learned flight policy may be deemed high performing if it scores well according to the reward function, and threshold scores may be defined to determine whether a learned flight policy is high performing, low performing, operation-ready, or otherwise should be kept (i.e., stored for further use in simulations or operation) or discarded. 
     Multiple learning systems may be run in parallel as a meta-learning system, with varied parameters in each learning system. Parameters that may be varied may include: the reward function (e.g., objective being optimized, slope, additional factors, etc.), reward tuning or modification (e.g., period of time or times of day during which full reward is valid or awarded, penalties for various characteristics of simulation or resulting reward), the number of frames that a learning module requests per input, characteristics of said frames for input (e.g., random, prioritized, or other), the depth of the reinforcement learning architecture (e.g., number of layers, hidden or otherwise) in the learning module, number of objectives, types of objectives, number of available actions, types of available actions, length of time into the future that is being predicted, and more. 
     A learning system also may store high performing learned flight policies (i.e., store neural networks encoded with high performing flight policies) in a policy repository for use by an operational navigation system to control movement of an aerial vehicle according to one or more desired objectives. These high performing learned flight policies may be used to determine actions for an aerial vehicle in a given situation. In an embodiment described herein, an aerial vehicle system may generate an input vector characterizing a state of the aerial vehicle, which may be provided to a learned flight policy that processes the input vector to output an action optimized for an objective of the learned flight policy. Such input vectors may be generated onboard an aerial vehicle in some examples, and in other examples, may be generated offboard (e.g., in a datacenter, which may or may not be integrated with a ground station or other cloud infrastructure, or the like). The action may be converted to a set of commands configured to cause the aerial vehicle to perform the action. In some examples, the input vector may include more than an aerial vehicle&#39;s physical and operational state (e.g., battery levels, location, pose, speed, weight, dimensions, software version, hardware version, among others), but also may include one or more of the following environmental inputs: sensor inputs (e.g., measuring temperature, pressure, humidity, precipitation, etc.), weather forecasts, map information, air traffic information, date and time. 
     Using the systems and methods described herein, one can customize flight policies across vehicle types and also to particular environments (Peru vs. Ecuador vs. Kenya, if it is useful, for the same vehicle) to improve performance with minimal human interference. For example, a system can customize flight policies for particular countries, each with its own set of regulations and no-fly restrictions, as well as weather and wind forecasts specific to its region. Thus, these systems and methods for aerial vehicle navigation can generate controllers for many types of aerial vehicles with minimal custom human design. Also, for any given vehicle and environment, one can process more data, and more types of data, to generate improved controllers. 
     Using the learning systems described herein, an objective-directed controller may be trained. In some examples, one or more objective-directed controllers may be used in combination (e.g., turned on and off) to improve navigation of an LTA vehicle. An action space may be defined for each objective-directed controller with continuous (or nearly continuous) or discretized (i.e., a predetermined number of) directions in two or three (or more) dimensions. In some examples, directions for an action space may be based on an absolute directional framework (e.g., North, South, East, West and combinations thereof), relative to a target heading or location (e.g., a discretized set of directions relative to a target heading or a continuous set of directions relative to a target heading), or dynamically determined based on other factors. For example, in a dynamic directional action space, an objective-directed controller may cause a vehicle with lateral propulsion to turn and/or propel towards, away, or tangential to a target location (i.e., station), depending on an objective (e.g., station seeking versus map following) and relevant current and expected wind patterns. The action space also may be defined with respect to actions a given vehicle can take (e.g., up, down, stay, apply lateral propulsion in a given direction, for example, at a given speed and/or power level). The action space may further be defined with respect to power levels (e.g., on, off, continuum of power levels) being applied to an altitude control system and/or a propulsion system. 
     In an example, actions defined in the action space may be on a spectrum or in intervals between two values (e.g., −1 to +1, wherein −1 may be down and +1 may be up with stay being 0, or wherein the two extremes represent other opposing actions), power levels may be on a spectrum or in intervals between two values (e.g., 0 to 1, wherein 0 is off and 1 is full power on), and directions may be on a spectrum or in intervals between two values (e.g., −π to π). A reward score may be generated when an objective-directed controller is trained on a set of values representing each of these aspects of the action space given a set of feature vectors, a state of the vehicle, and a reward function corresponding to the objective (e.g., argmax or other optimization function). In some examples, a reward or cost function may include a cumulative incursion score, wherein a penalty is applied for each incursion (i.e., instance or amount of time or distance spent in an undesirable zone (e.g., a no-fly zone, storm, or the like) or outside of a desirable zone (e.g., a proximity of a station or mapped path)), a reward otherwise (i.e., time or distance spent outside of an undesirable zone or within a desirable zone), and a straightforward computation of a value based on said penalty and reward. In other examples, the reward or cost function may include a large first-entry penalty as well as cumulative incursion score, wherein a larger penalty may be assessed for entering an undesirable zone (e.g., if the vehicle was not in a restricted zone or storm in an immediately preceding in time frame) or exiting a desirable zone (e.g., if the vehicle was within a threshold proximity of a station or mapped path in an immediately preceding in time frame), and a smaller penalty for remaining in the undesirable zone (e.g., the vehicle was already in a restricted zone or storm in the immediate preceding frame) or outside of the desirable zone (e.g., the vehicle was already outside of a threshold proximity of a station or mapped path). In other examples, a reward or cost function may give rewards for making progress on an objective (e.g., following a mapped path). In still other examples, a reward or cost function may return a fixed penalty incursion score, wherein there is a fixed penalty for intersecting an undesirable zone in a current frame (i.e., without considering earlier or later in time frames) and a fixed reward for intersecting a desirable zone. 
     Objective-directed controllers may each be directed to a desired objective (e.g., navigation using an altitude control system (e.g., toward a target location or heading), navigation using an altitude control system and lateral propulsion system (e.g., toward a target location or heading), station seeking (i.e., remaining within a proximity of a target location (aka station)) for a threshold amount of time—a minimum desired amount or for as long as possible), map following (i.e., progressing along, or closely following, a mapped trajectory or flight plan), restricted zone avoidance (e.g., no-fly areas), storm avoidance, and the like). A set of feature vectors associated with a desired objective may be provided as input to a learning system to train an objective-directed controller. In some examples, a set of standard or generic feature vectors may be provided for all controllers, including without limitation, position-related, pressure-related (e.g., minimum, maximum, current), power-related (e.g., altitude control system power, available solar power), solar-related (e.g., solar elevation, solar angle, next sunrise), and the like. A set of objective-directed feature vectors may be provided in addition to the standard or generic set. For example, storm avoidance feature vectors may include one or more of storm-related predictions, a storm proximity threshold (e.g., dynamic depending on nature and severity of storm, or a predetermined radius), a time until incurring a risk of breaching a storm proximity threshold or a risk in a given look-ahead time period for each of a range of pressures (i.e., altitudes) (e.g., expressed as an average trajectory risk, first time to non-zero risk, or other risk factor). In another example, map following feature vectors may include one or more of a speed (e.g., ground speed), direction (e.g., vehicle heading), and progress on a mapped path (e.g., map lookups compared with vehicle trajectory). In an example where map following is aided by lateral propulsion, additional feature vectors may include an effectiveness factor for application of lateral propulsion (i.e., at different levels of power in different directions). 
     In an example, a combination of objective-directed controllers may be concurrently trained and used for more comprehensive control of a vehicle. Such a combination may include one of a directional system controller (e.g., an altitude control system, an altitude control system plus lateral propulsion system), one of an objective-specific controller (e.g., station seeking, map following), and all overriding controllers (e.g., restricted zone avoidance, storm avoidance). Other combinations may include fewer or more types of controllers. Such combinations may be trained together to generate combination-specific objective-directed controllers. 
     A learning system, as described herein, may be configured to output an objective-directed controller trained according to a reward function corresponding to a desired objective, the objective-directed controller configured to select an action that maximizes a probability of achieving the desired objective (i.e., highest reward score). 
     Example Systems 
       FIGS. 1A-1B  are diagrams of exemplary operational systems in which flight policies may be implemented for navigating an aerial vehicle, in accordance with one or more embodiments. In  FIG. 1A , there is shown a diagram of system  100  for navigation of aerial vehicle  120   a . In some examples, aerial vehicle  120   a  may be a passive vehicle, such as a balloon or satellite, wherein most of its directional movement is a result of environmental forces, such as wind and gravity. In other examples, aerial vehicles  120   a  may be actively propelled. In an embodiment, system  100  may include aerial vehicle  120   a  and ground station  114 . In this embodiment, aerial vehicle  120   a  may include balloon  101   a , plate  102 , altitude control system (ACS)  103   a , connection  104   a , joint  105   a , actuation module  106   a , and payload  108   a . In some examples, plate  102  may provide structural and electrical connections and infrastructure. Plate  102  may be positioned at the apex of balloon  101   a , and may serve to couple together various parts of balloon  101   a . In other examples, plate  102  also may include a flight termination unit, such as one or more blades and an actuator to selectively cut a portion and/or a layer of balloon  101   a . ACS  103   a  may include structural and electrical connections and infrastructure, including components (e.g., fans, valves, actuators, etc.) used to, for example, add and remove air from balloon  101   a  (i.e., in some examples, balloon  101   a  may include an interior ballonet within its outer, more rigid shell that is inflated and deflated), causing balloon  101   a  to ascend or descend, for example, to catch stratospheric winds to move in a desired direction. Balloon  101   a  may comprise a balloon envelope comprised of lightweight and/or flexible latex or rubber materials (e.g., polyethylene, polyethylene terephthalate, chloroprene), tendons (e.g., attached at one end to plate  102  and at another end to ACS  103   a ) to provide strength to the balloon structure, a ballonet, along with other structural components. 
     Connection  104   a  may structurally, electrically, and communicatively, connect balloon  101   a  and/or ACS  103   a  to various components comprising payload  108   a . In some examples, connection  104   a  may provide two-way communication and electrical connections, and even two-way power connections. Connection  104   a  may include a joint  105   a , configured to allow the portion above joint  105   a  to pivot about one or more axes (e.g., allowing either balloon  101   a  or payload  108   a  to tilt and turn). Actuation module  106   a  may provide a means to actively turn payload  108   a  for various purposes, such as improved aerodynamics, facing or tilting solar panel(s)  109   a  advantageously, directing payload  108   a  and propulsion units (e.g., propellers  107  in  FIG. 1B ) for propelled flight, or directing components of payload  108   a  advantageously. 
     Payload  108   a  may include solar panel(s)  109   a , avionics chassis  110   a , broadband communications unit(s)  111   a , and terminal(s)  112   a . Solar panel(s)  109   a  may be configured to capture solar energy to be provided to a battery or other energy storage unit, for example, housed within avionics chassis  110   a . Avionics chassis  110   a  also may house a flight computer (e.g., computing device  301 , as described herein), a transponder, along with other control and communications infrastructure (e.g., a controller comprising another computing device and/or logic circuit configured to control aerial vehicle  120   a ). Communications unit(s)  111   a  may include hardware to provide wireless network access (e.g., LTE, fixed wireless broadband via 5G, Internet of Things (IoT) network, free space optical network, or other broadband networks). Terminal(s)  112   a  may comprise one or more parabolic reflectors (e.g., dishes) coupled to an antenna and a gimbal or pivot mechanism (e.g., including an actuator comprising a motor). Terminal(s)  112 ( a ) may be configured to receive or transmit radio waves to beam data long distances (e.g., using the millimeter wave spectrum or higher frequency radio signals). In some examples, terminal(s)  112   a  may have very high bandwidth capabilities. Terminal(s)  112   a  also may be configured to have a large range of pivot motion for precise pointing performance. Terminal(s)  112   a  also may be made of lightweight materials. 
     In other examples, payload  108   a  may include fewer or more components, including propellers  107  as shown in  FIG. 1B , which may be configured to propel aerial vehicles  120   a - b  in a given direction. In still other examples, payload  108   a  may include still other components well known in the art to be beneficial to flight capabilities of an aerial vehicle. For example, payload  108   a  also may include energy capturing units apart from solar panel(s)  109   a  (e.g., rotors or other blades (not shown) configured to be spun by wind to generate energy). In another example, payload  108   a  may further include or be coupled to an imaging device, such as a downward-facing camera and/or a star tracker. In yet another example, payload  108   a  also may include various sensors (not shown), for example, housed within avionics chassis  110   a  or otherwise coupled to connection  104   a  or balloon  101   a . Such sensors may include Global Positioning System (GPS) sensors, wind speed and direction sensors such as wind vanes and anemometers, temperature sensors such as thermometers and resistance temperature detectors, speed of sound sensors, acoustic sensors, pressure sensors such as barometers and differential pressure sensors, accelerometers, gyroscopes, combination sensor devices such as inertial measurement units (IMUs), light detectors, light detection and ranging (LIDAR) units, radar units, cameras, and more. These examples of sensors are not intended to be limiting, and those skilled in the art will appreciate that other sensors or combinations of sensors in addition to these described may be included without departing from the scope of the present disclosure. 
     Ground station  114  may include one or more server computing devices  115   a - n , which in turn may comprise one or more computing devices (e.g., computing device  301  in  FIG. 3 ). In some examples, ground station  114  also may include one or more storage systems, either housed within server computing devices  115   a - n , or separately (see, e.g., computing device  301  and repositories  320 ). Ground station  114  may be a datacenter servicing various nodes of one or more networks (e.g., aerial vehicle network  200  in  FIG. 2 ). 
       FIG. 1B  shows a diagram of system  150  for navigation of aerial vehicle  120   b . All like-numbered elements in  FIG. 1B  are the same or similar to their corresponding elements in  FIG. 1A , as described above (e.g., balloon  101   a  and balloon  101   b  may serve the same function, and may operate the same as, or similar to, each other). In this embodiment, aerial vehicle  120   b  further includes, as part of payload  108   b , propellers  107 , which may be configured to actively propel aerial vehicle  120   b  in a desired direction, either with or against a wind force to speed up, slow down, or re-direct, aerial vehicle  120   b . In this embodiment, balloon  101   b  also may be shaped differently from balloon  101   a , to provide different aerodynamic properties. 
     As shown in  FIGS. 1A-1B , aerial vehicles  120   a - b  may be largely wind-influenced aerial vehicle, for example, balloons carrying a payload (with or without propulsion capabilities) as shown, or fixed wing high altitude drones (e.g., aerial vehicle  211   c  in  FIG. 2 ). However, those skilled in the art will recognize that the systems and methods disclosed herein may similarly apply and be usable by various other types of aerial vehicles. 
       FIG. 2  is a diagram of an exemplary aerial vehicle network, in accordance with one or more embodiments. Aerial vehicle network  200  may include aerial vehicles  201   a - b , user devices  202 , and ground infrastructure  203 , in Country A. Aerial vehicle network  200  also may include aerial vehicles  211   a - c , user devices  212 , and ground infrastructure  213  in Country B. Aerial vehicles  201   a - b  and  211   a - c  may comprise balloon, other floating (i.e., lighter than air), propelled or partially propelled (i.e., propelled for a limited amount of time or under certain circumstances, and not propelled at other times or under other circumstances), fixed-wing, or other types of high altitude aerial vehicles, as described herein. User devices  202  and  212  may include a cellular phone, tablet computer, smart phone, desktop computer, laptop computer, and/or any other computing device known to those skilled in the art. Ground infrastructure  203  and  213  may include always-on or fixed location computing device (i.e., capable of receiving fixed broadband transmissions), ground terminal (e.g., ground station  114 ), tower (e.g., a cellular tower), and/or any other fixed or portable ground infrastructure for receiving and transmitting various modes of connectivity described herein known to those skilled in the art. User devices  202  and  212 , and ground infrastructure  203  and  213 , all may be capable of receiving and transmitting signals to and from aerial vehicles  201   a - b  and  211   a - c , as well as to and from each other. Aerial network  200  may further include satellite  204  and Internet  210 . Aerial vehicles  201   a - b  and  211   a - b  may be the same or similar to aerial vehicles  120   a - b  described above. Aerial vehicle network  200  may support ground-to-vehicle communication and connectivity, as shown between ground infrastructure  203  and aerial vehicle  201   b , as well as aerial vehicles  211   b - c  and ground infrastructure  213 . In these examples, aerial vehicles  201   b  and  211   b - c  each may exchange data with either or both a ground station (e.g., ground station  114 ) and a tower. Aerial vehicle network  200  also may support vehicle-to-vehicle (e.g., between aerial vehicles  201   a  and  201   b , between aerial vehicles  211   a - c ), satellite-to-vehicle (e.g., between satellite  204  and aerial vehicles  201   a - b ), and vehicle-to-user device (e.g., between aerial vehicle  201   a  and user devices  202 , between aerial vehicle  211   a  and user devices  212 ), communication and connectivity. In some examples, ground stations within ground infrastructures  203  and  213  may provide core network functions, such as connecting to the Internet and core cellular data network (e.g., connecting to LTE EPC or other communications platforms, and a software defined network system) and performing mission control functions. In some examples, the ground-to-vehicle, vehicle-to-vehicle, and satellite-to-vehicle communication and connectivity enables distribution of connectivity with minimal ground infrastructure. For example, aerial vehicles  201   a - b  and  211   a - c  may serve as base stations (e.g., LTE eNodeB base stations), capable of both connecting to the core network (e.g., Internet and core cellular data network), as well as delivering connectivity to each other and to user devices  202  and  212 . As such, aerial vehicles  201   a - b  and  211   a - c  represent a distribution layer of aerial vehicle network  200 . User devices  202  and  212  each may access cellular data and Internet connections directly from aerial vehicles  201   a - b  and  211   a - c.    
       FIG. 3  is a simplified block diagram of an exemplary computing system forming part of the systems of  FIGS. 1A-2 , in accordance with one or more embodiments. In one embodiment, computing system  300  may include computing device  301  and storage system  320 . Storage system  320  may comprise a plurality of repositories and/or other forms of data storage, and it also may be in communication with computing device  301 . In another embodiment, storage system  320 , which may comprise a plurality of repositories, may be housed in one or more of computing device  301 . In some examples, storage system  320  may store learned flight policies, as described herein, and other various types of information as described herein. This information may be retrieved or otherwise accessed by one or more computing devices, such as computing device  301  or computing devices  401   a - n  in  FIG. 4 , in order to perform some or all of the features described herein. Storage system  320  may comprise any type of computer storage, such as a hard-drive, memory card, ROM, RAM, DVD, CD-ROM, write-capable, and read-only memories. In addition, storage system  320  may include a distributed storage system where data is stored on a plurality of different storage devices, which may be physically located at the same or different geographic locations (e.g., in a distributed computing system such as system  400  in  FIG. 4 ). Storage system  320  may be networked to computing device  301  directly using wired connections and/or wireless connections. Such network may include various configurations and protocols, including short range communication protocols such as Bluetooth™, Bluetooth™ LE, the Internet, World Wide Web, intranets, virtual private networks, wide area networks, local networks, private networks using communication protocols proprietary to one or more companies, Ethernet, WiFi and HTTP, and various combinations of the foregoing. Such communication may be facilitated by any device capable of transmitting data to and from other computing devices, such as modems and wireless interfaces. 
     Computing device  301  also may include a memory  302 . Memory  302  may comprise a storage system configured to store a database  314  and an application  316 . Application  316  may include instructions which, when executed by a processor  304 , cause computing device  301  to perform various steps and/or functions, as described herein. Application  316  further includes instructions for generating a user interface  318  (e.g., graphical user interface (GUI)). Database  314  may store various algorithms and/or data, including neural networks (e.g., encoding flight policies and controllers, as described herein) and data regarding wind patterns, weather forecasts, past and present locations of aerial vehicles (e.g., aerial vehicles  120   a - b ,  201   a - b ,  211   a - c ), sensor data, map information, air traffic information, feature vectors, vehicle states, telemetry, among other types of data. Memory  302  may include any non-transitory computer-readable storage medium for storing data and/or software that is executable by processor  304 , and/or any other medium which may be used to store information that may be accessed by processor  304  to control the operation of computing device  301 . 
     Computing device  301  may further include a display  306 , a network interface  308 , an input device  310 , and/or an output module  312 . Display  306  may be any display device by means of which computing device  301  may output and/or display data. Network interface  308  may be configured to connect to a network using any of the wired and wireless short range communication protocols described above, as well as a cellular data network, a satellite network, free space optical network and/or the Internet. Input device  310  may be a mouse, keyboard, touch screen, voice interface, and/or any or other hand-held controller or device or interface by means of which a user may interact with computing device  301 . Output module  312  may be a bus, port, and/or other interface by means of which computing device  301  may connect to and/or output data to other devices and/or peripherals. 
     In some examples computing device  301  may be located remote from an aerial vehicle (e.g., aerial vehicles  120   a - b ,  201   a - b ,  211   a - c ) and may communicate with and/or control the operations of an aerial vehicle, or its control infrastructure as may be housed in avionics chassis  110   a - b , via a network. In one embodiment, computing device  301  is a data center or other control facility (e.g., configured to run a distributed computing system as described herein), and may communicate with a controller and/or flight computer housed in avionics chassis  110   a - b  via a network. As described herein, system  300 , and particularly computing device  301 , may be used for planning a flight path or course for an aerial vehicle based on wind and weather forecasts to move said aerial vehicle along a desired heading or within a desired radius of a target location. Various configurations of system  300  are envisioned, and various steps and/or functions of the processes described below may be shared among the various devices of system  300 , or may be assigned to specific devices. 
       FIG. 4  is a simplified block diagram of an exemplary distributed computing system, in accordance with one or more embodiments, comprising two or more computing devices  301   a - n . In some examples, each of  301   a - n  may comprise one or more of processors  404   a - n , respectively, and one or more of memory  402   a - n , respectively. Processors  404   a - n  may function similarly to processor  304  in  FIG. 3 , as described above. Memory  402   a - n  may function similarly to memory  302  in  FIG. 3 , as described above. 
       FIGS. 5A-5B  are simplified block diagrams of exemplary flight policy training systems, in accordance with one or more embodiments. Flight policy training system  500  may include simulation module  502 , one or more replay buffers  504 , learning module  506 , and policy server  508 . Simulation module  502  may include one or more simulators configured to run flight simulations over and over. Each simulator is configured to generate feature vectors, and each simulation is characterized by a set of feature vectors, wherein actions taken in a time step of a simulation corresponds to a feature vector of a frame, and the resulting time step captured in the frame is associated with a reward for the behavior (i.e., the actions taken). When starting with an empty policy server  508 , simulation module  502  may begin a simulation by randomly moving an aerial vehicle at random (e.g., up, down, maintaining altitude, etc.). In other examples, simulation module  502  may obtain an existing flight policy or learned flight policy (“existing policy”) from policy server  508 , and run simulations according to said existing policy (i.e., using the existing policy to determine actions for an aerial vehicle in the simulation). Simulation module  502  may provide various inputs to said existing policy, including an input vector characterizing a state of the vehicle, as well as relevant weather data, onboard and offboard sensor data, and one or more desired objectives, in order to obtain actions to be taken by the aerial vehicle in the simulation. Each simulation produced by simulation module  502  may include a frame for each time step of the simulation. A frame comprises a feature vector (e.g., representing a given situation by characterizing one or more operational or environmental features of a simulation). Example operational and environmental features include an aerial vehicle&#39;s altitude (e.g., current pressure altitude), atmosphere contents (e.g., gases, liquids or solids that are present, ratio or percentages of said gases, liquids or solids, any other particles or contents), angle or heading to a target location, distance to a target location, angle of a wind current, wind speed at current altitude, view of a map of a geographic area surrounding the aerial vehicle, aerial vehicle speed and acceleration, ambient temperature, power stored in the aerial vehicle&#39;s battery, time of day (e.g., solar elevation), current or most recent action, mode of operation (e.g., low power mode, fallback mode, normal mode), among others. Feature vectors may be informed by various sources of data, including onboard and offboard sensors, maps, weather forecasts, and the like, as described herein. 
     Simulation module  502  may be configured to feed frames of simulations to replay buffers  504 , which serve to randomize and store said frames independent of the particular simulations from which they came. A plurality of simulators in simulation module  502  (e.g., comprising several or ten or more simulators) may work to feed simulation frames to a plurality of replay buffers  504  (comprising one or more replay buffers). 
     Learning module  506  may be configured to pull sets of frames (e.g., comprising 32 or 64 frames, or any number of frames ranging from a dozen to multiples of fives, tens or dozens of frames) from replay buffers  504  to train learned flight policies. Learning module  506  may comprise a deep reinforcement learning architecture configured to run one or more variations of reinforcement learning, such as Q-learning, double Q-learning, distributional Q-learning, or other policy learning methods. The reinforcement learning algorithm in learning module  506  may be configured to maximize a sum of rewards generated by a reward function corresponding to the objective of the flight policy training. The reward function may be correlated with an objective (i.e., a control objective related to navigation or operation of an aerial vehicle, such as a high altitude aerial vehicle). Examples of objectives may include: following a map (e.g., following the gradient or map of a heuristic function built to indicate how to efficiently cross an ocean); spending the most (or otherwise optimal) amount of time within a radius of a target location (e.g., a latitude-longitude, a city, an island, a group or chain of islands, a group of buoyed structures such as offshore wind farms); following a mapped trajectory; flying in a given direction; arriving at a location at a desired date and time; maximizing (or otherwise optimizing) the amount of time an aerial vehicle provides connection services to a given area; following, or remaining within a given radius of, a terrestrial or nautical vehicle (e.g., a cruise ship, an all-terrain vehicle, etc.); and any of the above in coordination with other aerial vehicles (i.e., in the context of a fleet of aerial vehicles). Thresholds may be predetermined to categorize learned flight policies into various levels of performance (e.g., high, medium, or low performing) based on how a learned flight policy scores according to the reward function (i.e., how high of a reward produced by said learned flight policy). In some examples, the reward score may comprise a value. In other examples, where certain types of reinforcement learning is implemented (e.g., distributional Q-learning), a reward score may comprise a probability distribution or a distribution of rewards. In an example, learning module  506  may be configured to provide medium and high performing learned flight policies to policy server  508  for storage and further use by simulation module  502  to run simulations, and also to provide high performing learned flight policies to operation-ready policies server  510  for use in operational navigation systems, according to methods described below. In another example, learning module  506  may be configured to provide medium and high performing learned flight policies to policy server  508 , and to provide a separate category of highest performing learned flight policies to operations policies server  510  for use in operational navigation systems. Learning module  506  may discard (i.e., delete) low-performing learned flight policies. 
     Turning to  FIG. 5B , in an alternative embodiment, learning module  506  may store all learned flight policies in policy server  508 , either ad hoc (i.e., as the learned flight policies are generated) or periodically (i.e., in batches), without regard to a resulting reward score. An evaluation server  512  may then retrieve the learned flight policies and evaluate the learned flight policies to determine whether they are operations ready (e.g., should be stored in operation-ready policies server  510 ). In some examples, evaluation server  512  may retrieve and evaluate every learned flight policy in policy server  508 . In other examples, evaluation server  512  may retrieve and evaluate samples of the learned flight policies in policy server  508 . In some examples, evaluation server  512  may run a set of simulations on the learned flight policies and use the resulting rewards to score the progress of learning by learning module  506 . The set of simulations run by evaluation server  512  may be unchanging or consistent so as to evaluate each learned flight policy according to the same criteria. 
     In some examples, system  500  also may include an operation-ready policies server  510  separate from policy server  508 , in which learned flight policies that meet or exceed a certain threshold score defined for an appropriate category of flight policies (e.g., high performing, highest performing, operation-ready, and other appropriate categories) may be stored. The operation-ready flight policies in operation-ready policies server  510  may be provided to aerial vehicles (e.g., aerial vehicles  120   a - b ,  201   a - b ,  211   a - c ) for navigation of said aerial vehicles (i.e., to determine actions to be performed by said aerial vehicles to achieve an objective), according to methods described in more detail below. 
       FIG. 6  is a simplified block diagram of an exemplary meta-learning system, in accordance with one or more embodiments. Meta-learning system  600  may comprise a plurality of learning systems  602   a - n , coordinator  604 , evaluation server  512 , and operation-ready policies server  510 . Each of learning systems  602   a - n  may include some or all of the same or similar components of flight policy training system  500 . For example, learning systems  602   a - n  may include simulation modules  502   a - n , replay buffers  504   a - n , learning modules  506   a - n , and policy servers  508   a - n , respectively. In meta-learning system  600 , a plurality (e.g., from a few to tens to 20-60, or more) of learning systems  602   a - n  may be run in parallel. In some examples, parameters may be varied among the plurality of learning systems  602   a - n . Examples of parameters that may be varied among the plurality of learning systems  602   a - n  may include any of the parameters mentioned herein, for example in the context of learning systems  602   a - n , without limitation: depth of (e.g., how many layers, hidden or otherwise) the reinforcement learning system within learning modules  506   a - n ; characteristics of a reward function (e.g., how steep is the slope of the function, how the function is defined according to an objective) by which learning modules  506   a - n  processes inputs; objective of the reward function (e.g., the objectives outlined herein, as well as variations on each objective); how many frames to include in each input to learning modules  506   a - n ; characteristics of the frames in each input to learning modules  506   a - n ; the feature vectors to be included in each input (i.e., input vector) to learning modules  506   a - n ; the number of replay buffers in each of buffers  504   a - n ; the number of simulators in each of simulation modules  502   a - n ; the reward score thresholds for discarding or storing a learned flight policy, for example in policy servers  508   a - n  and operation-ready policies server  510 ; the type of reinforcement learning being implemented in learning modules  506   a - n ; length of time into the future that is being predicted in each simulation; step size; and other parameters. Coordinator  604  may provide instructions to the various stacks ( 602   a - c ) on parameters to use and when to start and finish. In some examples, a learning system  602   n  may in itself comprise a stack of learning systems running on the same set of parameters. 
     In an example, a subset of 30-60 of learning systems  602   a - n  may be run in parallel with the objective of training optimal flight policies to navigate an aerial vehicle (e.g., aerial vehicles  120   a - b ,  201   a - b ,  211   a - c ) from a starting location to a target location. In this example, the objective amongst this subset of learning systems  602   a - n  may be the same, but each of learning systems  602   a - n  may be run with one or more of the following parameter variations: distances between said starting and target locations, the geographical locations of the starting and target locations (i.e., differing hemispheres or regions of the world, thereby invoking very different environmental conditions), starting times, number of frames per input (e.g., 16, 32, 64 or more), the reward function slope (i.e., how steep is the slope, which translates into how strictly the reward may be scored), number of simulators. In this example, said subset of 30-60 of learning systems  602   a - n  may further be grouped into subset groups, each subset group running variations on a single parameter or a few related parameters (e.g., a subset group running variations on the reward function, another subset group running variations on geographical locations, another subset group running variations on input vectors, etc.). In another example, another subset of learning systems  602   a - n  may be training on the same objective with the same parameter variations using a different type of reinforcement learning (e.g., Q-learning in one subset providing value scores, distributional Q-learning in another subset providing probability distribution scores). Evaluation server  512  may evaluate the performance of the learned flight policies being generated by learning systems  602   a - n  (e.g., retrieving from policy servers  508   a - n  or directly from learning modules  506   a - n ). Evaluation server  512  may determine which learned flight policies should be stored in operation-ready policies server  510 , and provide feedback on which stacks are doing well or doing poorly. For example, where certain ones of said subset of 30-60 of learning systems  602   a - n , characterized by certain sets of parameters, perform poorly (i.e., produce poor performing learned flight policies), the poor performing learning systems (i.e., sets of parameters) may be discontinued. As certain of learning systems  602   a - n  are discontinued, new ones with new sets of parameters may be added to particular subsets or subset groups. 
     In some examples, meta-learning system  600  may be implemented in a distributed computing environment wherein a plurality of copies of a stack of learning systems may be maintained along with a plurality of policy servers, each policy server being dedicated to a learning stack. 
     Example Methods 
       FIG. 7A  is a flow diagram of an exemplary method for generating learned flight policies, in accordance with one or more embodiments. Method  700  may start with simulating an aerial vehicle&#39;s flight through a region of the atmosphere according to a flight policy at step  702 . The flight policy may be an existing flight policy stored in a policy server and may be a learned flight policy previously output and provided by a learning module, as described herein. The simulation may be performed by one or more simulators in a simulation module, as described herein. In some examples, a simulation may begin with causing the aerial vehicle to perform random actions to generate feature vectors characterizing the simulation. 
     The simulation module may generate a plurality of frames, each frame representing a time step of a simulation at step  704 , each frame comprising a feature vector representing a set of features of the aerial vehicle state and environment in said time step, wherein actions taken in each simulation in said time step correspond to a feature vector of a frame, and the resulting time step captured in the frame is rewarded for the behavior (i.e., the actions taken). At step  706 , the plurality of frames may be stored in a replay buffer, the replay buffer configured to provide a random or scrambled set of frames (e.g., to disrupt the plurality of frames from their original time sequence order) for input into a learning module. As described herein, the replay buffer may comprise one or more buffers. 
     The method may continue, at step  708 , with requesting, by a learning module, a set of frames from the replay buffer. The learning module may comprise a deep reinforcement architecture, as described above. The learning module may process the set of frames using a reinforcement learning algorithm at step  710 , in order to then generate a learned flight policy that is scored according to a reward function defined for that learning module at step  712 . The learned flight policy, encoded in a neural network, may be stored in a policy server at step  714 . 
     Turning to  FIG. 7B , method  750  is an exemplary method for evaluating learned flight policies and learning systems, in accordance with one or more embodiments. A learned flight policy may be obtained from the policy server at step  716 , and a reward for the learned flight policy may be evaluated at step  718 . As discussed above, an evaluation server (e.g., evaluation server  512 ) may run a set of simulations on a learned flight policy to evaluate its performance. At  720 , a determination may be made as to whether the reward for the learned flight policy meets or exceeds a performance threshold (e.g., a threshold for operation-ready flight policies). If yes, the learned flight policy that was evaluated may be stored in an operation-ready policies server at step  722 . If no, the learned flight policy may be discarded at step  724 . At step  726 , an evaluation server (e.g., evaluation server  512 ) may further determine whether a learning system or set of learning systems (i.e., learning stack) that produced the discarded learned flight policy is performing poorly. For example, an evaluation server may determine that a number of previously-evaluated learned flight policies resulting from the same learning system or stack also have been discarded. In another example, an evaluation server may determine that a number of previously-evaluated learned flight policies resulting from the same learning system or stack have met a poor-performance threshold. If yes, the learning system or stack from which the discarded learned flight policy resulted is performing poorly, the evaluation server may send feedback through the system to discard said poor performing learning system or stack at step  728 . Whether or not the learning stack is performing poorly, the evaluation server may continue to obtain and evaluate learned flight policies. 
     A threshold may be predetermined to define whether a learned flight policy is high performing or low performing, and said threshold may comprise one or more thresholds used to define more granular performance categories (e.g., high performing threshold, very high performing threshold, low performing threshold, medium performing threshold, simulation-ready threshold, operation-ready threshold, discard threshold, anomalous threshold, and others, any of which may be a value-based threshold wherein the score comprises a value or a distribution-based threshold wherein the score comprises a probability distribution). 
     In an alternative embodiment, in some examples, the learning module itself may determine whether a threshold is met (i.e., the score for the learned flight policy meets or exceeds the threshold). The learned flight policy, which may be encoded in a neural network, that meets or exceeds said threshold may be stored in the policy server configured to store neural networks, at step  714 . Once the learned flight policy is stored in the policy server, it may be provided to a simulation module, or pulled by a simulation module from the policy server, to run further simulations. If the threshold is not met (i.e., the score for the learned flight policy falls below the threshold), the learned flight policy may be discarded. Where the score comprises a value, the value score will be evaluated against a predetermined threshold value. Where the score comprises a probability distribution or distribution of rewards, the score may be evaluated against a predetermined distribution threshold. 
     In some examples, the operation-ready threshold for storing in the operation-ready policies server may be different from other thresholds, e.g., for storing in a normal policies server (e.g., if the learned flight policy score meets or exceeds a different and higher threshold). In still other examples, methods  700  and  750  may include evaluating a learned flight policy score against still other thresholds to gate the grouping and treating of learned flight policies in other ways. 
     Method  700  may be performed by flight policy training system  500  and method  750  may be performed by a flight policy training system  550 , either or both of which may be implemented in a distributed computing system such as distributed computing system  400 . In some examples, method  700  may be performed as a training loop in each of a plurality of training systems, such as in a meta-learning system  600 . Method  750  also may be performed as part of a training loop in a meta-learning system  600 . 
     In still other embodiments, methods  700  and  750  may be implemented using real world flight data or other sources of flight data, rather than simulations. Therefore, the training methods described herein may be performed using simulated data, historical data, fresh data collected during operation, or a mixture of these, to generate input for a learning module. 
       FIG. 8  is a flow diagram of an exemplary method for deploying a learned flight policy in an operational aerial vehicle system. Method  800  may start with selecting a trained neural network encoding a learned flight policy from a policy server at step  802 . An input vector comprising a set of characteristics representing a state of an aerial vehicle may be generated at step  804 . In some examples, an input vector may comprise a feature vector, as described above. In some examples, an input vector may comprise more than a characterization of an aerial vehicle&#39;s physical and operational state (e.g., battery levels, location, pose, speed, weight, dimensions, software version, hardware version, among others), but also may include one or more of the following environmental inputs: sensor inputs (e.g., measuring temperature, pressure, humidity, precipitation, etc.), weather forecasts, map information, air traffic information, date and time. 
     An action may be selected by the trained neural network based on the input vector at step  806 . The action may be converted into a set of commands, at step  808 , the set of commands configured to cause the aerial vehicle to perform the action. For example, an action “descend” or “down” may be converted to a set of commands that includes spinning an ACS fan motor of the aerial vehicle at a predetermined number of watts. A control system, as described herein, may then cause the aerial vehicle to perform the action using the set of commands at step  810 . In some examples, a determination may be made whether the aerial vehicle operation should continue at step  812 . If yes, method  800  may return to step  804  to generate further input vectors to select further actions. If no, method  800  may end. 
     It would be recognized by a person of ordinary skill in the art that some or all of the steps of methods  700  and  800 , as described above, may be performed in a different order or sequence, repeated, and/or omitted without departing from the scope of the present disclosure. 
       FIG. 9  is a map showing a flight path resulting from controlling an aerial vehicle using a learned flight policy, according to one or more embodiments described herein. Map  900  shows an aerial vehicle  902  having navigated a path  904  from a starting location  910  to within a target area  905  (e.g., a radius or area surrounding a target location) using a learned flight policy as described herein. Map  900  further shows a path  906  that may otherwise have been taken by aerial vehicle  902  or another aerial vehicle using a conventional, or less optimal flight policy. As shown, path  904  may have resulted from a high performing learned flight policy, and may be more efficient than path  906  in a variety of ways, including less distance traveled to target area  905 , less power consumption in traveling to target area  905 , better avoidance of unauthorized areas or regions of map  900 , better avoidance of risky weather conditions (i.e., planning and flying a path through lower risk weather conditions), and in achieving other objectives described herein. In other examples, although not shown, aerial vehicle  902  also may be able to remain within target area  905  for a longer period of time using said learned flight policy as described herein. 
       FIG. 10  is a flow diagram of an exemplary method for generating a controller for an LTA vehicle using a deep reinforcement learning architecture, according to one or more embodiments. Learning system  600  in  FIG. 6  may be used to train a plurality of objective-directed controllers. Method  1000  begins with defining an action space for an objective-directed controller at step  1002 . As described herein, the action space may be defined by values indicating directions, types of actions, power levels, and other parameters. A continuous set of directions may include an infinite or very high number of directions in a two- or three-dimensional space. A discretized set of directions may include two or more discrete directions (e.g., absolute directions according to a standard framework, such as North, South, East and West, or relative directions, such as with respect to a target heading). A set of feature vectors and the action space may be provided as input to a simulation module at step  1004 , the set of feature vectors associated with a desired objective. In some examples, the set of feature vectors also may include a generic or standard set of feature vectors, as well as a specialized set of feature vectors for the desired objective. The objective-directed controller may be trained by a learning module according to a reward function correlated with the desired objective at step  1006 . Reward functions may be configured to determine a cumulative incursion score, a large first-entry penalty plus cumulative incursion score, a progress score, a fixed penalty incursion score, or other reward and/or cost score. Desired objectives may include, without limitation, navigation using an altitude control system, navigation using an altitude control system and lateral propulsion system, station seeking, map following, restricted zone avoidance, storm avoidance, and combinations thereof, by way of example. The trained objective-directed controller may be evaluated at step  1008 . In some examples, the trained objective-directed controller may be evaluated based on reward scores being generated by the trained objective-directed controller. If a reward score for the trained objective-directed controller meets a reward score threshold, it may be stored at step  1010 , for example, to use in controlling vehicles in a fleet. In other examples, the trained objective-directed controller may be evaluated based on visual review of its performance in a plurality of simulations (e.g., mapped results), and selection of high performing objective-directed controllers for storage and use may be determined based thereon. Aerial vehicles may then be controlled (i.e., caused to take or not take an action) using a selected, trained objective-directed controller, particularly ones that are high performing (i.e., meet or exceed a reward score threshold, or otherwise satisfy performance criteria). 
       FIG. 11  is a map showing an exemplary flight path of an aerial vehicle being controlled by an objective-directed controller, in accordance with one or more embodiments. Map  1100  shows a target heading (i.e., station)  1102 , radius  1104  around station  1102 , lateral propulsion direction and magnitude arrows  1106   a - c  (along with other lateral propulsion direction and magnitude arrows), passive wind-driven path portions  1108   a - b  (along with other wind-driven paths), starting location  1110  and ending location  1112 . In this example, the aerial vehicle may begin by being controlled by a trained objective-directed controller for navigating toward target heading  1102  using ACS and lateral propulsion. In some examples, the same or another controller may be used to control the aerial vehicle to follow a map to target heading  1102 . The aerial vehicle may begin by using lateral propulsion in a given direction with a given speed and/or level of power, as indicated by arrows  1106   a . As the aerial vehicle nears or enters radius  1104 , the same or different controller may be used to control the aerial vehicle to station seek around station  1102 . For example, the aerial vehicle may be caused to turn off its lateral propulsion and change to a wind-driven state, as shown by dots on the map (e.g., path portions  1108   a - b ). Radius  1104  may comprise a desirable proximity to station  1102  wherein the aerial vehicle may provide a service to a target area. In this example, once near or within radius  1104 , a controller for station seeking using ACS and lateral propulsion may determine when to turn on lateral propulsion, in which direction and at what magnitude (i.e., speed or power level), in order to optimize the aerial vehicle&#39;s time within radius  1104 . 
     While specific examples have been provided above, it is understood that the present invention can be applied with a wide variety of inputs, thresholds, ranges, and other factors, depending on the application. For example, the time frames and ranges provided above are illustrative, but one of ordinary skill in the art would understand that these time frames and ranges may be varied or even be dynamic and variable, depending on the implementation. 
     As those skilled in the art will understand, a number of variations may be made in the disclosed embodiments, all without departing from the scope of the invention, which is defined solely by the appended claims. It should be noted that although the features and elements are described in particular combinations, each feature or element can be used alone without other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general-purpose computer or processor. 
     Examples of computer-readable storage mediums include a read only memory (ROM), random-access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks. 
     Suitable processors include, by way of example, a general-purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), a state machine, or any combination of thereof.