Patent Publication Number: US-9834297-B2

Title: Uni-penetration tendon retention and fill port system for a balloon envelope

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of U.S. Non-Provisional patent application Ser. No. 14/153,020, filed Jan. 11, 2014, which is hereby incorporated by reference in its entirety. 
     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. 
     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. As such, the demand for data connectivity via the Internet, cellular data networks, and other such networks, is growing. However, there are many areas of the world where data connectivity is still unavailable, or if available, is unreliable and/or costly. Accordingly, additional network infrastructure is desirable. 
     SUMMARY 
     An apex fitting described herein advantageously provides a base plate configured to be securable to the exterior of a balloon envelope via a single perforation through the balloon envelope. This arrangement may minimize air leaks from the balloon envelope. In addition, the base plate may be beneficially configured to be coupled to a retention ring without perforating the balloon envelope. The retention ring may further hold tendons in place at the apex of the balloon envelope. In addition, a fill-port body may be advantageously coupled to the base plate, rather than being coupled to the soft balloon envelope, to remove stress from the balloon envelope. 
     In one aspect, an example apparatus involves: (a) a base plate having a top surface and a bottom surface, wherein the base plate defines an opening, and wherein the base plate is configured to be securable to an exterior of a balloon envelope, (b) at least one stud coupled to the base plate, wherein the at least one stud is configured to be securable to a tendon, (c) a retention ring defining at least one opening configured to receive the at least one stud, (d) a fill-port body defining a cavity, wherein a flange is coupled to the fill-port body, wherein the fill-port body is arranged coaxially with and extends through the opening of the base plate such that the flange lies adjacent to the bottom surface of the base plate, and (e) a locking body coupled to the fill-port body, wherein the locking body defines an opening arranged coaxially with the fill-port body, wherein the fill-port body extends through the opening of the locking body such that a portion of the locking body lies adjacent to the top surface of the base plate. 
     In another aspect, an example method involves: (a) affixing a base plate to a balloon envelope, wherein the balloon envelope defines an opening at an apex of the balloon envelope, wherein the base plate defines an opening, wherein the opening of the base plate is aligned with the opening of the balloon envelope, and wherein a plurality of studs are coupled to the base plate, (b) placing a fill-port body through the opening of the balloon envelope and through the opening of the base plate such that a flange of the fill-port body lies adjacent to the balloon envelope, (c) securing a locking body to the fill-port body and/or the base plate such that the locking body lies adjacent to the base plate, (d) securing a plurality of tendons to the plurality of studs, and (e) securing a retention ring to the base plate. 
     In a further aspect, a balloon is provided having a balloon envelope and means for retaining tendons at the apex of a balloon envelope and means for filling the balloon envelope with air. 
     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  is a simplified diagram illustrating a balloon network, according to an example embodiment. 
         FIG. 2  is a diagram illustrating a balloon-network control system, according to an example embodiment. 
         FIG. 3  is a simplified diagram illustrating a high-altitude balloon, according to an example embodiment. 
         FIG. 4  is a simplified diagram illustrating a balloon network that includes super-nodes and sub-nodes, according to an example embodiment. 
         FIG. 5A  is a perspective view of an example apparatus, according to an example embodiment. 
         FIG. 5B  is a detail view of the example apparatus shown in  FIG. 5A . 
         FIG. 6A  is a cross-sectional side view of the example apparatus shown in  FIG. 5A . 
         FIG. 6B  is a detail cross-sectional side view a fill-port body of the example apparatus shown in  FIG. 6A . 
         FIG. 6C  is a detail cross-sectional side view of an example locking pin of the example apparatus shown in  FIG. 6A . 
         FIG. 6D  is a detail cross-sectional side view of an example stud of the example apparatus shown in  FIG. 6A . 
         FIG. 7  shows a high-altitude balloon, according to an example embodiment. 
         FIG. 8  is a flow chart of a method according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods and systems 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 may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an example embodiment may include elements that are not illustrated in the Figures. 
     1. Overview 
     Example embodiments disclosed herein may generally relate to a data network formed by balloons, and in particular, to a mesh network formed by high-altitude balloons deployed in the stratosphere. In some applications, it may be desirable for high altitude balloons to retain lift gas for one hundred days or more. Example balloons may be configured to have a top chamber to receive and retain lift gas and a bottom chamber to receive and retain air. In one embodiment, neither chamber may initially contain perforations, but a hole may be defined in the balloon envelope to receive a fill port to administer lift gas and fill the balloon envelope. The present invention may also be used with balloons that are made of a number of gores seamed together resulting in holes at the apex and base of the balloon, for example, among other possibilities. 
     Example embodiments may advantageously provide a single perforation in the top chamber of the balloon envelope. This advantage may be accomplished by relocating the rigid fill port from the soft film of the balloon to a rigid base plate at the apex of the balloon envelope such that the fill port is the only perforation in the balloon envelope in communication with lift gas. Placing the fill port in the base plate beneficially removes stress from the balloon envelope. 
     In addition, example embodiments may advantageously provide a base plate that may be securable to the exterior of a balloon envelope via the single perforation. For example, the base plate may be initially secured to the balloon envelope via tape or adhesive. The base plate may define a plurality of studs configured to be coupled to the ends of a plurality of tendons. The opposite ends of the plurality of tendons may be coupled to a fitting on the underside of the balloon for film-load assistance when the balloon is pressurized. Once the balloon is pressurized, friction between the tendons and the outward pressing envelope may be great enough to hold the apex fitting in place. Known apex fittings typically have over forty perforations in the balloon envelope and the base plate. Each perforation is a source for potential air leaks from the balloon envelope. Thus, the single perforation of the present invention has the benefit of minimizing air leaks. 
     To ensure the tendons stay in place, a retention ring may be coupled to the base plate, such that the tendon loops are sandwiched in between the base plate and the retention ring. The retention ring defines a plurality of openings configured to receive the plurality of studs. Each stud may have a body and a flange coupled to the free end of the body. The plurality of openings in the retention ring may each have a first portion sized to receive the flange of a corresponding stud and a second portion defining a channel having a width narrower than a diameter of the flange. This arrangement allows the first portion of the openings to slide over the flange of the studs and down to the body of the studs. The retention ring may then be rotated such that the body of the studs is received in the second portion of the openings, locking the retention ring in place. One or more locking pins may then be disposed through the retention ring to prevent the retention ring from rotating to an unlocked position. 
     In some embodiments, the base plate may define one or more apertures or windows therein, which advantageously reduces the weight of the apex fitting. In addition, the base plate may beneficially include electrical passages for coupling to one or more sensors. 
     2. Example Balloon Networks 
     In an example balloon network, the balloons may communicate with one another using free-space optical communications. For instance, the balloons may be configured for optical communications using ultra-bright LEDs (which are also referred to as “high-power” or “high-output” LEDs). In some instances, lasers could be used instead of or in addition to LEDs, although regulations for laser communications may restrict laser usage. In addition, the balloons may communicate with ground-based station(s) using radio-frequency (RF) communications. 
     In some embodiments, a high-altitude-balloon network may be homogenous. That is, the balloons in a high-altitude-balloon network could be substantially similar to each other in one or more ways. More specifically, in a homogenous high-altitude-balloon network, each balloon is configured to communicate with nearby balloons via free-space optical links. Further, some or all of the balloons in such a network, may also be configured to communicate with ground-based station(s) using RF communications. (Note that in some embodiments, the balloons may be homogenous in so far as each balloon is configured for free-space optical communication with other balloons, but heterogeneous with regard to RF communications with ground-based stations.) 
     In other embodiments, a high-altitude-balloon network may be heterogeneous, and thus may include two or more different types of balloons. For example, some balloons may be configured as super-nodes, while other balloons may be configured as sub-nodes. Some balloons may be configured to function as both a super-node and a sub-node. Such balloons may function as either a super-node or a sub-node at a particular time, or, alternatively, act as both simultaneously depending on the context. For instance, an example balloon could aggregate search requests of a first type to transmit to a ground-based station. The example balloon could also send search requests of a second type to another balloon, which could act as a super-node in that context. 
     In such a configuration, the super-node balloons may be configured to communicate with nearby super-node balloons via free-space optical links. However, the sub-node balloons may not be configured for free-space optical communication, and may instead be configured for some other type of communication, such as RF communications. In that case, a super-node may be further configured to communicate with sub-nodes using RF communications. Thus, the sub-nodes may relay communications between the super-nodes and one or more ground-based stations using RF communications. In this way, the super-nodes may collectively function as backhaul for the balloon network, while the sub-nodes function to relay communications from the super-nodes to ground-based stations. Other differences could be present between balloons in a heterogeneous balloon network. 
       FIG. 1  is a simplified diagram illustrating a balloon network  100 , according to an example embodiment. As shown, balloon network  100  includes balloons  102 A to  102 F, which are configured to communicate with one another via free-space optical links  104 . Balloons  102 A to  102 F could additionally or alternatively be configured to communicate with one another via RF links  114 . Balloons  102 A to  102 F may collectively function as a mesh network for packet-data communications. Further, balloons  102 A to  102 F may be configured for RF communications with ground-based stations  106  and  112  via RF links  108 . In another example embodiment, balloons  102 A to  102 F could be configured to communicate via optical link  110  with ground-based station  112 . 
     In an example embodiment, balloons  102 A to  102 F are high-altitude balloons, which are deployed in the stratosphere. At moderate latitudes, the stratosphere includes altitudes between approximately 10 kilometers (km) and 50 km altitude above the surface. At the poles, the stratosphere starts at an altitude of approximately 8 km. In an example embodiment, high-altitude balloons may be generally configured to operate in an altitude range within the stratosphere that has lower winds (e.g., between 5 and 20 miles per hour (mph)). 
     More specifically, in a high-altitude-balloon network, balloons  102 A to  102 F may generally be configured to operate at altitudes between 17 km and 25 km (although other altitudes are possible). This altitude range may be advantageous for several reasons. In particular, this layer of the stratosphere generally has mild wind and turbulence (e.g., winds between 5 and 20 miles per hour (mph)). Further, while the winds between 17 km and 25 km may vary with latitude and by season, the variations can be modelled in a reasonably accurate manner. Additionally, altitudes above 17 km are typically above the maximum flight level designated for commercial air traffic. Therefore, interference with commercial flights is not a concern when balloons are deployed between 17 km and 25 km. 
     To transmit data to another balloon, a given balloon  102 A to  102 F may be configured to transmit an optical signal via an optical link  104 . In an example embodiment, a given balloon  102 A to  102 F may use one or more high-power light-emitting diodes (LEDs) to transmit an optical signal. Alternatively, some or all of balloons  102 A to  102 F may include laser systems for free-space optical communications over optical links  104 . Other types of free-space optical communication are possible. Further, in order to receive an optical signal from another balloon via an optical link  104 , a given balloon  102 A to  102 F may include one or more optical receivers. Additional details of example balloons are discussed in greater detail below, with reference to  FIG. 3 . 
     In a further aspect, balloons  102 A to  102 F may utilize one or more of various different RF air-interface protocols for communication ground-based stations  106  and  112  via RF links  108 . For instance, some or all of balloons  102 A to  102 F may be configured to communicate with ground-based stations  106  and  112  using protocols described in IEEE 802.11 (including any of the IEEE 802.11 revisions), various cellular protocols such as GSM, CDMA, UMTS, EV-DO, WiMAX, and/or LTE, and/or one or more propriety protocols developed for balloon-to-ground RF communication, among other possibilities. 
     In a further aspect, there may scenarios where RF links  108  do not provide a desired link capacity for balloon-to-ground communications. For instance, increased capacity may be desirable to provide backhaul links from a ground-based gateway, and in other scenarios as well. Accordingly, an example network may also include downlink balloons, which could provide a high-capacity air-ground link. 
     For example, in balloon network  100 , balloon  102 F could be configured as a downlink balloon. Like other balloons in an example network, a downlink balloon  102 F may be operable for optical communication with other balloons via optical links  104 . However, downlink balloon  102 F may also be configured for free-space optical communication with a ground-based station  112  via an optical link  110 . Optical link  110  may therefore serve as a high-capacity link (as compared to an RF link  108 ) between the balloon network  100  and a ground-based station  112 . 
     Note that in some implementations, a downlink balloon  102 F may additionally be operable for RF communication with ground-based stations  106 . In other cases, a downlink balloon  102 F may only use an optical link for balloon-to-ground communications. Further, while the arrangement shown in  FIG. 1  includes just one downlink balloon  102 F, an example balloon network can also include multiple downlink balloons. On the other hand, a balloon network can also be implemented without any downlink balloons. 
     In other implementations, a downlink balloon may be equipped with a specialized, high-bandwidth RF communication system for balloon-to-ground communications, instead of, or in addition to, a free-space optical communication system. The high-bandwidth RF communication system may take the form of an ultra-wideband system, which provides an RF link with substantially the same capacity as the optical links  104 . Other forms are also possible. 
     Balloons could be configured to establish a communication link with space-based satellites in addition to, or as an alternative to, a ground-based communication link. 
     Ground-based stations, such as ground-based stations  106  and/or  112 , may take various forms. Generally, a ground-based station may include components such as transceivers, transmitters, and/or receivers for communication via RF links and/or optical links with a balloon network. Further, a ground-based station may use various air-interface protocols in order communicate with a balloon  102 A to  102 F over an RF link  108 . As such, ground-based stations  106  and  112  may be configured as an access point with which various devices can connect to balloon network  100 . Ground-based stations  106  and  112  may have other configurations and/or serve other purposes without departing from the scope of the invention. 
     Further, some ground-based stations, such as ground-based stations  106  and  112 , may be configured as gateways between balloon network  100  and one or more other networks. Such ground-based stations  106  and  112  may thus serve as an interface between the balloon network and the Internet, a cellular service provider&#39;s network, and/or other types of networks. Variations on this configuration and other configurations of ground-based stations  106  and  112  are also possible. 
     2a) Mesh Network Functionality 
     As noted, balloons  102 A to  102 F may collectively function as a mesh network. More specifically, since balloons  102 A to  102 F may communicate with one another using free-space optical links, the balloons may collectively function as a free-space optical mesh network. 
     In a mesh-network configuration, each balloon  102 A to  102 F may function as a node of the mesh network, which is operable to receive data directed to it and to route data to other balloons. As such, data may be routed from a source balloon to a destination balloon by determining an appropriate sequence of optical links between the source balloon and the destination balloon. These optical links may be collectively referred to as a “lightpath” for the connection between the source and destination balloons. Further, each of the optical links may be referred to as a “hop” on the lightpath. 
     To operate as a mesh network, balloons  102 A to  102 F may employ various routing techniques and self-healing algorithms. In some embodiments, a balloon network  100  may employ adaptive or dynamic routing, where a lightpath between a source and destination balloon is determined and set-up when the connection is needed, and released at a later time. Further, when adaptive routing is used, the lightpath may be determined dynamically depending upon the current state, past state, and/or predicted state of the balloon network. 
     In addition, the network topology may change as the balloons  102 A to  102 F move relative to one another and/or relative to the ground. Accordingly, an example balloon network  100  may apply a mesh protocol to update the state of the network as the topology of the network changes. For example, to address the mobility of the balloons  102 A to  102 F, balloon network  100  may employ and/or adapt various techniques that are employed in mobile ad hoc networks (MANETs). Other examples are possible as well. 
     In some implementations, a balloon network  100  may be configured as a transparent mesh network. More specifically, in a transparent balloon network, the balloons may include components for physical switching that is entirely optical, without any electrical involved in physical routing of optical signals. Thus, in a transparent configuration with optical switching, signals travel through a multi-hop lightpath that is entirely optical. 
     In other implementations, the balloon network  100  may implement a free-space optical mesh network that is opaque. In an opaque configuration, some or all balloons  102 A to  102 F may implement optical-electrical-optical (OEO) switching. For example, some or all balloons may include optical cross-connects (OXCs) for OEO conversion of optical signals. Other opaque configurations are also possible. Additionally, network configurations are possible that include routing paths with both transparent and opaque sections. 
     In a further aspect, balloons in an example balloon network  100  may implement wavelength division multiplexing (WDM), which may help to increase link capacity. When WDM is implemented with transparent switching, physical lightpaths through the balloon network may be subject to the “wavelength continuity constraint.” More specifically, because the switching in a transparent network is entirely optical, it may be necessary to assign the same wavelength for all optical links on a given lightpath. 
     An opaque configuration, on the other hand, may avoid the wavelength continuity constraint. In particular, balloons in an opaque balloon network may include the OEO switching systems operable for wavelength conversion. As a result, balloons can convert the wavelength of an optical signal at each hop along a lightpath. Alternatively, optical wavelength conversion could take place at only selected hops along the lightpath. 
     Further, various routing algorithms may be employed in an opaque configuration. For example, to determine a primary lightpath and/or one or more diverse backup lightpaths for a given connection, example balloons may apply or consider shortest-path routing techniques such as Dijkstra&#39;s algorithm and k-shortest path, and/or edge and node-diverse or disjoint routing such as Suurballe&#39;s algorithm, among others. Additionally or alternatively, techniques for maintaining a particular Quality of Service (QoS) may be employed when determining a lightpath. Other techniques are also possible. 
     2b) Station-Keeping Functionality 
     In an example embodiment, a balloon network  100  may implement station-keeping functions to help provide a desired network topology. For example, station-keeping may involve each balloon  102 A to  102 F maintaining and/or moving into a certain position relative to one or more other balloons in the network (and possibly in a certain position relative to the ground). As part of this process, each balloon  102 A to  102 F may implement station-keeping functions to determine its desired positioning within the desired topology, and if necessary, to determine how to move to the desired position. 
     The desired topology may vary depending upon the particular implementation. In some cases, balloons may implement station-keeping to provide a substantially uniform topology. In such cases, a given balloon  102 A to  102 F may implement station-keeping functions to position itself at substantially the same distance (or within a certain range of distances) from adjacent balloons in the balloon network  100 . 
     In other cases, a balloon network  100  may have a non-uniform topology. For instance, example embodiments may involve topologies where balloons are distributed more or less densely in certain areas, for various reasons. As an example, to help meet the higher bandwidth demands that are typical in urban areas, balloons may be clustered more densely over urban areas. For similar reasons, the distribution of balloons may be denser over land than over large bodies of water. Many other examples of non-uniform topologies are possible. 
     In a further aspect, the topology of an example balloon network may be adaptable. In particular, station-keeping functionality of example balloons may allow the balloons to adjust their respective positioning in accordance with a change in the desired topology of the network. For example, one or more balloons could move to new positions to increase or decrease the density of balloons in a given area. Other examples are possible. 
     In some embodiments, a balloon network  100  may employ an energy function to determine if and/or how balloons should move to provide a desired topology. In particular, the state of a given balloon and the states of some or all nearby balloons may be input to an energy function. The energy function may apply the current states of the given balloon and the nearby balloons to a desired network state (e.g., a state corresponding to the desired topology). A vector indicating a desired movement of the given balloon may then be determined by determining the gradient of the energy function. The given balloon may then determine appropriate actions to take in order to effectuate the desired movement. For example, a balloon may determine an altitude adjustment or adjustments such that winds will move the balloon in the desired manner. 
     2c) Control of Balloons in a Balloon Network 
     In some embodiments, mesh networking and/or station-keeping functions may be centralized. For example,  FIG. 2  is a diagram illustrating a balloon-network control system, according to an example embodiment. In particular,  FIG. 2  shows a distributed control system, which includes a central control system  200  and a number of regional control-systems  202 A to  202 B. Such a control system may be configured to coordinate certain functionality for balloon network  204 , and as such, may be configured to control and/or coordinate certain functions for balloons  206 A to  206 I. 
     In the illustrated embodiment, central control system  200  may be configured to communicate with balloons  206 A to  206 I via number of regional control systems  202 A to  202 C. These regional control systems  202 A to  202 C may be configured to receive communications and/or aggregate data from balloons in the respective geographic areas that they cover, and to relay the communications and/or data to central control system  200 . Further, regional control systems  202 A to  202 C may be configured to route communications from central control system  200  to the balloons in their respective geographic areas. For instance, as shown in  FIG. 2 , regional control system  202 A may relay communications and/or data between balloons  206 A to  206 C and central control system  200 , regional control system  202 B may relay communications and/or data between balloons  206 D to  206 F and central control system  200 , and regional control system  202 C may relay communications and/or data between balloons  206 G to  206 I and central control system  200 . 
     In order to facilitate communications between the central control system  200  and balloons  206 A to  206 I, certain balloons may be configured as downlink balloons, which are operable to communicate with regional control systems  202 A to  202 C. Accordingly, each regional control system  202 A to  202 C may be configured to communicate with the downlink balloon or balloons in the respective geographic area it covers. For example, in the illustrated embodiment, balloons  206 A,  206 F, and  206 I are configured as downlink balloons. As such, regional control systems  202 A to  202 C may respectively communicate with balloons  206 A,  206 F, and  206 I via optical links  206 ,  208 , and  210 , respectively. 
     In the illustrated configuration, where only some of balloons  206 A to  206 I are configured as downlink balloons, the balloons  206 A,  206 F, and  206 I that are configured as downlink balloons may function to relay communications from central control system  200  to other balloons in the balloon network, such as balloons  206 B to  206 E,  206 G, and  206 H. However, it should be understood that it in some implementations, it is possible that all balloons may function as downlink balloons. Further, while  FIG. 2  shows multiple balloons configured as downlink balloons, it is also possible for a balloon network to include only one downlink balloon. 
     Note that a regional control system  202 A to  202 C may in fact just be a particular type of ground-based station that is configured to communicate with downlink balloons (e.g. the ground-based station  112  of  FIG. 1 ). Thus, while not shown in  FIG. 2 , a control system may be implemented in conjunction with other types of ground-based stations (e.g., access points, gateways, etc.). 
     In a centralized control arrangement, such as that shown in  FIG. 2 , the central control system  200  (and possibly regional control systems  202 A to  202 C as well) may coordinate certain mesh-networking functions for balloon network  204 . For example, balloons  206 A to  206 I may send the central control system  200  certain state information, which the central control system  200  may utilize to determine the state of balloon network  204 . The state information from a given balloon may include location data, optical-link information (e.g., the identity of other balloons with which the balloon has established an optical link, the bandwidth of the link, wavelength usage and/or availability on a link, etc.), wind data collected by the balloon, and/or other types of information. Accordingly, the central control system  200  may aggregate state information from some or all the balloons  206 A to  206 I in order to determine an overall state of the network. 
     The overall state of the network may then be used to coordinate and/or facilitate certain mesh-networking functions such as determining lightpaths for connections. For example, the central control system  200  may determine a current topology based on the aggregate state information from some or all the balloons  206 A to  206 I. The topology may provide a picture of the current optical links that are available in the balloon network and/or the wavelength availability on the links. This topology may then be sent to some or all of the balloons so that a routing technique may be employed to select appropriate lightpaths (and possibly backup lightpaths) for communications through the balloon network  204 . 
     In a further aspect, the central control system  200  (and possibly regional control systems  202 A to  202 C as well) may also coordinate certain station-keeping functions for balloon network  204 . For example, the central control system  200  may input state information that is received from balloons  206 A to  206 I to an energy function, which may effectively compare the current topology of the network to a desired topology, and provide a vector indicating a direction of movement (if any) for each balloon, such that the balloons can move towards the desired topology. Further, the central control system  200  may use altitudinal wind data to determine respective altitude adjustments that may be initiated to achieve the movement towards the desired topology. The central control system  200  may provide and/or support other station-keeping functions as well. 
       FIG. 2  shows a distributed arrangement that provides centralized control, with regional control systems  202 A to  202 C coordinating communications between a central control system  200  and a balloon network  204 . Such an arrangement may be useful to provide centralized control for a balloon network that covers a large geographic area. In some embodiments, a distributed arrangement may even support a global balloon network that provides coverage everywhere on earth. A distributed-control arrangement may be useful in other scenarios as well. 
     Further, it should be understood that other control-system arrangements are possible. For instance, some implementations may involve a centralized control system with additional layers (e.g., sub-region systems within the regional control systems, and so on). Alternatively, control functions may be provided by a single, centralized, control system, which communicates directly with one or more downlink balloons. 
     In some embodiments, control and coordination of a balloon network may be shared between a ground-based control system and a balloon network to varying degrees, depending upon the implementation. In fact, in some embodiments, there may be no ground-based control systems. In such an embodiment, all network control and coordination functions may be implemented by the balloon network itself. For example, certain balloons may be configured to provide the same or similar functions as central control system  200  and/or regional control systems  202 A to  202 C. Other examples are also possible. 
     Furthermore, control and/or coordination of a balloon network may be de-centralized. For example, each balloon may relay state information to, and receive state information from, some or all nearby balloons. Further, each balloon may relay state information that it receives from a nearby balloon to some or all nearby balloons. When all balloons do so, each balloon may be able to individually determine the state of the network. Alternatively, certain balloons may be designated to aggregate state information for a given portion of the network. These balloons may then coordinate with one another to determine the overall state of the network. 
     Further, in some aspects, control of a balloon network may be partially or entirely localized, such that it is not dependent on the overall state of the network. For example, individual balloons may implement station-keeping functions that only consider nearby balloons. In particular, each balloon may implement an energy function that takes into account its own state and the states of nearby balloons. The energy function may be used to maintain and/or move to a desired position with respect to the nearby balloons, without necessarily considering the desired topology of the network as a whole. However, when each balloon implements such an energy function for station-keeping, the balloon network as a whole may maintain and/or move towards the desired topology. 
     As an example, each balloon A may receive distance information d 1  to d k  with respect to each of its k closest neighbors. Each balloon A may treat the distance to each of the k balloons as a virtual spring with vector representing a force direction from the first nearest neighbor balloon i toward balloon A and with force magnitude proportional to d i . The balloon A may sum each of the k vectors and the summed vector is the vector of desired movement for balloon A. Balloon A may attempt to achieve the desired movement by controlling its altitude. 
     Alternatively, this process could assign the force magnitude of each of these virtual forces equal to d i ×d I , wherein d I  is proportional to the distance to the second nearest neighbor balloon, for instance. 
     In another embodiment, a similar process could be carried out for each of the k balloons and each balloon could transmit its planned movement vector to its local neighbors. Further rounds of refinement to each balloon&#39;s planned movement vector can be made based on the corresponding planned movement vectors of its neighbors. It will be evident to those skilled in the art that other algorithms could be implemented in a balloon network in an effort to maintain a set of balloon spacings and/or a specific network capacity level over a given geographic location. 
     2d) Example Balloon Configuration 
     Various types of balloon systems may be incorporated in an example balloon network. As noted above, an example embodiment may utilize high-altitude balloons, which could typically operate in an altitude range between 17 km and 25 km.  FIG. 3  shows a high-altitude balloon  300 , according to an example embodiment. As shown, the balloon  300  includes an envelope  302 , a skirt  304 , a payload  306 , and a cut-down system  308 , which is attached between the balloon  302  and payload  304 . 
     The envelope  302  and skirt  304  may take various forms, which may be currently well-known or yet to be developed. For instance, the envelope  302  and/or skirt  304  may be made of a highly-flexible latex material or may be made of a rubber material such as chloroprene. In one example embodiment, the envelope and/or skirt could be made of metalized Mylar or BoPet. Other materials are also possible. Further, the shape and size of the envelope  302  and skirt  304  may vary depending upon the particular implementation. Additionally, the envelope  302  may be filled with various different types of gases, such as helium and/or hydrogen. Other types of gases are possible as well. 
     The payload  306  of balloon  300  may include a processor  312  and on-board data storage, such as memory  314 . The memory  314  may take the form of or include a non-transitory computer-readable medium. The non-transitory computer-readable medium may have instructions stored thereon, which can be accessed and executed by the processor  312  in order to carry out the balloon functions described herein. 
     The payload  306  of balloon  300  may also include various other types of equipment and systems to provide a number of different functions. For example, payload  306  may include optical communication system  316 , which may transmit optical signals via an ultra-bright LED system  320 , and which may receive optical signals via an optical-communication receiver  322  (e.g., a photodiode receiver system). Further, payload  306  may include an RF communication system  318 , which may transmit and/or receive RF communications via an antenna system  340 . 
     The payload  306  may also include a power supply  326  to supply power to the various components of balloon  300 . The power supply  326  could include a rechargeable battery. In other embodiments, the power supply  326  may additionally or alternatively represent other means known in the art for producing power. In addition, the balloon  300  may include a solar power generation system  327 . The solar power generation system  327  may include solar panels and could be used to generate power that charges and/or is distributed by power supply  326 . 
     Further, payload  306  may include various types of other systems and sensors  328 . For example, payload  306  may include one or more video and/or still cameras, a GPS system, various motion sensors (e.g., accelerometers, magnetometers, gyroscopes, and/or compasses), and/or various sensors for capturing environmental data. Further, some or all of the components within payload  306  may be implemented in a radiosonde or other probe, which may be operable to measure, e.g., pressure, altitude, geographical position (latitude and longitude), temperature, relative humidity, and/or wind speed and/or wind direction, among other information. 
     As noted, balloon  300  includes an ultra-bright LED system  320  for free-space optical communication with other balloons. As such, optical communication system  316  may be configured to transmit a free-space optical signal by modulating the ultra-bright LED system  320 . The optical communication system  316  may be implemented with mechanical systems and/or with hardware, firmware, and/or software. Generally, the manner in which an optical communication system is implemented may vary, depending upon the particular application. The optical communication system  316  and other associated components are described in further detail below. 
     In a further aspect, balloon  300  may be configured for altitude control. For instance, balloon  300  may include a variable buoyancy system, which is configured to change the altitude of the balloon  300  by adjusting the volume and/or density of the gas in the balloon  300 . A variable buoyancy system may take various forms, and may generally be any system that can change the volume and/or density of gas in the envelope  302 . 
     In an example embodiment, a variable buoyancy system may include a bladder  310  that is located inside of envelope  302 . The bladder  310  could be an elastic chamber configured to hold liquid and/or gas. Alternatively, the bladder  310  need not be inside the envelope  302 . For instance, the bladder  310  could be a ridged bladder that could be pressurized well beyond neutral pressure. The buoyancy of the balloon  300  may therefore be adjusted by changing the density and/or volume of the gas in bladder  310 . To change the density in bladder  310 , balloon  300  may be configured with systems and/or mechanisms for heating and/or cooling the gas in bladder  310 . Further, to change the volume, balloon  300  may include pumps or other features for adding gas to and/or removing gas from bladder  310 . Additionally or alternatively, to change the volume of bladder  310 , balloon  300  may include release valves or other features that are controllable to allow gas to escape from bladder  310 . Multiple bladders  310  could be implemented within the scope of this disclosure. For instance, multiple bladders could be used to improve balloon stability. 
     In an example embodiment, the envelope  302  could be filled with helium, hydrogen or other lighter-than-air material. The envelope  302  could thus have an associated upward buoyancy force. In such an embodiment, air in the bladder  310  could be considered a ballast tank that may have an associated downward ballast force. In another example embodiment, the amount of air in the bladder  310  could be changed by pumping air (e.g., with an air compressor) into and out of the bladder  310 . By adjusting the amount of air in the bladder  310 , the ballast force may be controlled. In some embodiments, the ballast force may be used, in part, to counteract the buoyancy force and/or to provide altitude stability. 
     In another embodiment, a portion of the envelope  302  could be a first color (e.g., black) and/or a first material from the rest of envelope  302 , which may have a second color (e.g., white) and/or a second material. For instance, the first color and/or first material could be configured to absorb a relatively larger amount of solar energy than the second color and/or second material. Thus, rotating the balloon such that the first material is facing the sun may act to heat the envelope  302  as well as the gas inside the envelope  302 . In this way, the buoyancy force of the envelope  302  may increase. By rotating the balloon such that the second material is facing the sun, the temperature of gas inside the envelope  302  may decrease. Accordingly, the buoyancy force may decrease. In this manner, the buoyancy force of the balloon could be adjusted by changing the temperature/volume of gas inside the envelope  302  using solar energy. In such embodiments, it is possible that a bladder  310  may not be a necessary element of balloon  300 . Thus, various contemplated embodiments, altitude control of balloon  300  could be achieved, at least in part, by adjusting the rotation of the balloon with respect to the sun. 
     Further, a balloon  306  may include a navigation system (not shown). The navigation system may implement station-keeping functions to maintain position within and/or move to a position in accordance with a desired topology. In particular, the navigation system may use altitudinal wind data to determine altitudinal adjustments that result in the wind carrying the balloon in a desired direction and/or to a desired location. The altitude-control system may then make adjustments to the density of the balloon chamber in order to effectuate the determined altitudinal adjustments and cause the balloon to move laterally to the desired direction and/or to the desired location. Alternatively, the altitudinal adjustments may be computed by a ground-based or satellite-based control system and communicated to the high-altitude balloon. In other embodiments, specific balloons in a heterogeneous balloon network may be configured to compute altitudinal adjustments for other balloons and transmit the adjustment commands to those other balloons. 
     As shown, the balloon  300  also includes a cut-down system  308 . The cut-down system  308  may be activated to separate the payload  306  from the rest of balloon  300 . The cut-down system  308  could include at least a connector, such as a balloon cord, connecting the payload  306  to the envelope  302  and a means for severing the connector (e.g., a shearing mechanism or an explosive bolt). In an example embodiment, the balloon cord, which may be nylon, is wrapped with a nichrome wire. A current could be passed through the nichrome wire to heat it and melt the cord, cutting the payload  306  away from the envelope  302 . 
     The cut-down functionality may be utilized anytime the payload needs to be accessed on the ground, such as when it is time to remove balloon  300  from a balloon network, when maintenance is due on systems within payload  306 , and/or when power supply  326  needs to be recharged or replaced. 
     In an alternative arrangement, a balloon may not include a cut-down system. In such an arrangement, the navigation system may be operable to navigate the balloon to a landing location, in the event the balloon needs to be removed from the network and/or accessed on the ground. Further, it is possible that a balloon may be self-sustaining, such that it does not need to be accessed on the ground. In other embodiments, in-flight balloons may be serviced by specific service balloons or another type of aerostat or aircraft. 
     In a further aspect, balloon  300  includes a gas-flow system, which may be used for altitude control. In the illustrated example, the gas-flow system includes a high-pressure storage chamber  342 , a gas-flow tube  350 , and a pump  348 , which may be used to pump gas out of the envelope  302 , through the gas-flow tube  350 , and into the high-pressure storage chamber  342 . As such, balloon  300  may be configured to decrease its altitude by pumping gas out of envelope  302  and into high-pressure storage chamber  342 . Further, balloon  300  may be configured to move gas into the envelope and increase its altitude by opening a valve  352  at the end of gas-flow tube  350 , and allowing lighter-than-air gas from high-pressure storage chamber  342  to flow into envelope  302 . 
     Note that the high-pressure storage chamber  342 , in an example balloon, may be constructed such that its volume does not change due to, e.g., the high forces and/or torques resulting from gas that is compressed within the chamber. In an example embodiment, the high-pressure storage chamber  342  may be made of a material with a high tensile-strength to weight ratio, such as titanium or a composite made of spun carbon fiber and epoxy. However, high-pressure storage chamber  342  may be made of other materials or combinations of materials, without departing from the scope of the invention. 
     In a further aspect, balloon  300  may be configured to generate power from gas flow out of high-pressure storage chamber  342  and into envelope  302 . For example, a turbine (not shown) may be fitted in the path of the gas flow (e.g., at the end of gas-flow tube  350 ). The turbine may be a gas turbine generator, or may take other forms. Such a turbine may generate power when gas flows from high-pressure storage chamber  342  to envelope  302 . The generated power may be immediately used to operate the balloon and/or may be used to recharge the balloon&#39;s battery. 
     In a further aspect, a turbine, such as a gas turbine generator, may also be configured to operate “in reverse” in order to pump gas into and pressurize the high-pressure storage chamber  342 . In such an embodiment, pump  348  may be unnecessary. However, an embodiment with a turbine could also include a pump. 
     In some embodiments, pump  348  may be a positive displacement pump, which is operable to pump gas out of the envelope  302  and into high-pressure storage chamber  342 . Further, a positive-displacement pump may be operable in reverse to function as a generator. 
     Further, in the illustrated example, the gas-flow system includes a valve  346 , which is configured to adjust the gas-flow path between envelope  302 , high-pressure storage chamber  342 , and fuel cell  344 . In particular, valve  346  may adjust the gas-flow path such that gas can flow between high-pressure storage chamber  342  and envelope  302 , and shut off the path to fuel cell  344 . Alternatively, valve  346  may shut off the path high-pressure storage chamber  342 , and create a gas-flow path such that gas can flow between fuel cell  344  and envelope  302 . 
     Balloon  300  may be configured to operate fuel cell  344  in order to produce power via the chemical reaction of hydrogen and oxygen to produce water, and to operate fuel cell  344  in reverse so as to create hydrogen and oxygen from water. Accordingly, to increase its altitude, balloon  300  may run fuel cell  344  in reverse so as to generate gas (e.g., hydrogen gas), which can then be moved into the envelope to increase buoyancy. Specifically, balloon may increase its altitude by running fuel cell  344  in reverse, adjusting valve  346  and valve  352  such that hydrogen gas produced by fuel cell  344  can flow from fuel cell  344 , through gas-flow tube  350 , and into envelope  302 . 
     To run fuel cell  344  “in reverse,” balloon  300  may utilize an electrolysis mechanism in order to separate water molecules. For example, a balloon may be configured to use a photocatalytic water splitting technique to produce hydrogen and oxygen from water. Other techniques for electrolysis are also possible. 
     Further, balloon  300  may be configured to separate the oxygen and hydrogen produced via electrolysis. To do so, the fuel cell  344  and/or another balloon component may include an anode and cathode that attract the positively and negatively charged O− and H− ions, and separate the two gases. Once the gases are separated, the hydrogen may be directed into the envelope. Additionally or alternatively, the hydrogen and/or oxygen may be moved into the high-pressure storage chamber. 
     Further, to decrease its altitude, balloon  300  may use pump  348  to pump gas from envelope  302  to the fuel cell  344 , so that the hydrogen gas can be consumed in the fuel cell&#39;s chemical reaction to produce power (e.g., the chemical reaction of hydrogen and oxygen to create water). By consuming the hydrogen gas the buoyancy of the balloon may be reduced, which in turn may decrease the altitude of the balloon. 
     It should be understood that variations on the illustrated high-pressure storage chamber are possible. For example, the high-pressure storage chamber may take on various sizes and/or shapes, and be constructed from various materials, depending upon the implementation. Further, while high-pressure storage chamber  342  is shown as part of payload  306 , high-pressure storage chamber could also be located inside of envelope  302 . Yet further, a balloon could implement multiple high-pressure storage chambers. Other variations on the illustrated high-pressure storage chamber  342  are also possible. 
     It should also be understood that variations on the illustrated air-flow tube  350  are possible. Specifically, any configuration that facilitates movement of gas between the high-pressure storage chamber and the envelope is possible. 
     Yet further, it should be understood that a balloon and/or components thereof may vary from the illustrated balloon  300 . For example, some or all of the components of balloon  300  may be omitted. Components of balloon  300  could also be combined. Further, a balloon may include additional components in addition or in the alternative to the illustrated components of balloon  300 . Other variations are also possible. 
     3. Balloon Network With Optical And Rf Links Between Balloons 
     In some embodiments, a high-altitude-balloon network may include super-node balloons, which communicate with one another via optical links, as well as sub-node balloons, which communicate with super-node balloons via RF links. Generally, the optical links between super-node balloons may be configured to have more bandwidth than the RF links between super-node and sub-node balloons. As such, the super-node balloons may function as the backbone of the balloon network, while the sub-nodes may provide sub-networks providing access to the balloon network and/or connecting the balloon network to other networks. 
       FIG. 4  is a simplified diagram illustrating a balloon network that includes super-nodes and sub-nodes, according to an example embodiment. More specifically,  FIG. 4  illustrates a portion of a balloon network  400  that includes super-node balloons  410 A to  410 C (which may also be referred to as “super-nodes”) and sub-node balloons  420  (which may also be referred to as “sub-nodes”). 
     Each super-node balloon  410 A to  410 C may include a free-space optical communication system that is operable for packet-data communication with other super-node balloons. As such, super-nodes may communicate with one another over optical links. For example, in the illustrated embodiment, super-node  410 A and super-node  401 B may communicate with one another over optical link  402 , and super-node  410 A and super-node  401 C may communicate with one another over optical link  404 . 
     Each of the sub-node balloons  420  may include a radio-frequency (RF) communication system that is operable for packet-data communication over one or more RF air interfaces. Accordingly, each super-node balloon  410 A to  410 C may include an RF communication system that is operable to route packet data to one or more nearby sub-node balloons  420 . When a sub-node  420  receives packet data from a super-node  410 , the sub-node  420  may use its RF communication system to route the packet data to a ground-based station  430  via an RF air interface. 
     As noted above, the super-nodes  410 A to  410 C may be configured for both longer-range optical communication with other super-nodes and shorter-range RF communications with nearby sub-nodes  420 . For example, super-nodes  410 A to  410 C may use using high-power or ultra-bright LEDs to transmit optical signals over optical links  402 ,  404 , which may extend for as much as 100 miles, or possibly more. Configured as such, the super-nodes  410 A to  410 C may be capable of optical communications at speeds of 10 to 50 GB/sec or more. 
     A larger number of balloons may be configured as sub-nodes, which may communicate with ground-based Internet nodes at speeds on the order of approximately 10 MB/sec. Configured as such, the sub-nodes  420  may be configured to connect the super-nodes  410  to other networks and/or to client devices. 
     Note that the data speeds and link distances described in the above example and elsewhere herein are provided for illustrative purposes and should not be considered limiting; other data speeds and link distances are possible. 
     In some embodiments, the super-nodes  410 A to  410 C may function as a core network, while the sub-nodes  420  function as one or more access networks to the core network. In such an embodiment, some or all of the sub-nodes  420  may also function as gateways to the balloon network  400 . Additionally or alternatively, some or all of ground-based stations  430  may function as gateways to the balloon network  400 . 
     4. Example Apex Fitting 
     The present embodiments advantageously provide an apex fitting  500  that may reduce air leaks from and stress on a balloon envelope.  FIGS. 5A to 7  show an apex fitting  500  that includes a base plate  510  having a top surface  511  and a bottom surface  512 . The base plate  510  may define an opening  515 . In a preferred embodiment, the opening  515  of the base plate  510  may be centered in the base plate  510 . The opening may alternatively be located off-center within the base plate  510 . The base plate  510  may further be configured to be securable to an exterior of a balloon envelope  520  via a single opening  521  in the balloon envelope  520 , as described in more detail below. In addition, the base plate  510  may be made from a strong lightweight material, such as aluminum or steel, engineered plastics or composite materials, among other possibilities. In various embodiments, the base plate  510  has a diameter ranging from about 15 inches to about 30 inches. As used herein, “about” means ±5%. 
     At least one stud  525  may be coupled to the base plate  510 . In a preferred embodiment, a plurality of studs  525  may be coupled to the base plate  510 . Each stud  525  may further be configured to be securable to a tendon  530 . For example, a tendon  530  may be looped about a corresponding stud  525 . In another preferred embodiment, the plurality of studs  525  may have a spaced-apart arrangement about a periphery of the base plate  510  to substantially evenly distribute the tendons  530  about the periphery of the balloon envelope  520 . Further, in one embodiment, each stud  525  may include a body  527  and a flange  526  coupled to the body at a free end. In one embodiment, the flange  526  may comprise a nut with threads defined on an interior surface that may be joined to a stud  525  via mating threads defined on the exterior of the stud  525  at the free end. 
     The apex fitting  500  may further include a retention ring  535 . The retention ring  535  may be used to hold the tendons  530  looped in place over the studs  525 . For example, the retention ring  535  may define at least one opening  540  that may be configured to receive a corresponding stud  525 . In a preferred embodiment, a plurality of openings  540  are defined in the retention ring  535 . In one embodiment, the openings  540  of the retention ring  535  may have a first portion  541  sized to receive the flange  526  of a corresponding stud  525  and a second portion  542  defining a channel or slot having a width that is less than a diameter of the flange  526  of the corresponding stud  525 . Further, the second portion  542  of the opening  540  may be configured to receive the body  527  of a corresponding stud. This arrangement allows the retention ring to be rotated relative to the base plate  510  and locked into place. In this locked position, the flange  526  of each stud  525  is aligned over the channel or slot of the second portion  542  of opening  540 , preventing the removal of the retention ring  535  from the base plate  510 . In a further embodiment, the second portion  542  of the openings  540  may each be configured as a detent that is capable of retaining the body  527  of a corresponding stud  525  once the retention ring  535  has been rotated into the locked position. In various embodiments, the retention ring  535  may be made from a strong lightweight material, such as aluminum or steel, among other possibilities. 
     In one embodiment, the apparatus further includes at least one locking pin  550  disposed through the retention ring  535  to hold the retention ring  535  in the locked position. In a preferred embodiment, the at least one locking pin  550  includes a plurality of locking pins  550 . In various embodiments, the locking pins  550  may include retention screws, rivets or bolts, among others possibilities, disposed through the retention ring  535  and coupled to a fitting  551  in the base plate  510 . In various other embodiments, the locking pins  550  may comprise pins or plugs, for example, disposed through the first portion  541  of the plurality of openings  540  of the retention ring  535 . 
     The apex fitting  500  may also include a fill-port body  545  defining a cavity. A flange  546  may be coupled to the fill-port body  545 . The flange  546  is preferably located at or near a base of the fill-port body  545 . The fill-port body  545  may be arranged coaxially with and extend through the opening  515  of the base plate  510  such that the flange  546  lies adjacent to the bottom surface  512  of the base plate  510 . In addition, a locking body  547  may be coupled to the fill-port body  545  to hold the flange  546  against the base plate  510  and/or the balloon envelope  520  (as described below). The locking body  547  may further define an opening arranged coaxially with the fill-port body  545 . The fill-port body  545  may extend through the opening of the locking body  547  such that a portion  548  of the locking body  547  lies adjacent to the top surface  511  of the base plate  510 . In one embodiment, the locking body  547  may be press-fit onto the fill-port body  545  such that the locking body  547  is received in a detent  549  defined by the fill-port body  545 . In another embodiment, the locking body  547  may be press-fit onto the fill-port body  545  such that at least one detent (not shown) defined on the locking body  547  receives at least one protuberance (not shown) coupled to the fill-port body  545 . In a further embodiment, the locking body  545  may be coupled to the base plate  510  or the fill-port body  545  via one or more connectors, such as screws  544  or rivets. In yet another embodiment, the locking body  547  may be coupled to the base plate  510  or the fill-port body  545  via adhesive. In a further embodiment, the locking body  547  may include threads (not shown) defined along the opening of the locking body  547 , and the fill-port body  545  may include mating threads (not shown) defined along an exterior of the fill-port body  545 , such that the locking body  547  may be screwed onto the fill-port body  545 . Still other possibilities exist to couple to the locking body  547  to the fill-port body  545  or the base plate without creating additional perforations in the balloon envelope  520 . 
     In one embodiment, the apex fitting  500  further includes a balloon envelope  520 . The balloon envelope  520  may define an opening  521  at an apex  522  of the balloon envelope  520 . The fill-port body  545  may extend through the opening  521  of the balloon envelope  520  such that a portion  523  of the balloon envelope  520  may be disposed between the flange  546  of the fill-port body  545  and the bottom surface  512  of the base plate  510 . In a further embodiment, a gasket (not shown) may be disposed between the flange  546  of the fill-port body  545  and the balloon envelope  520 . The purpose of the gasket is to minimize stress on the opening  521  of the balloon envelope  520  and to create a more effective seal between the base plate  510  and the balloon envelope  520 . 
     In one embodiment, the base plate  510  may define one or more apertures  513 . In this embodiment, the aperture(s)  513  may take many forms, including a polygonal, a circular or a half moon shape, among other possibilities. In one embodiment, the base plate  510  may include a single aperture (not shown). In another embodiment, the base plate  510  may include a plurality of apertures  513 . In further embodiment, shown in  FIG. 5A , a plurality of apertures  513  are arranged such that the base plate  510  has a spoke-like configuration. 
     In another embodiment, the apex fitting  500  may further include a flight termination system (not shown) that is configured to puncture the balloon envelope  520  through at least one aperture  513  of the base plate  510 . The flight termination system is of a type known in the art and is configured to vent lift gas by puncturing the balloon envelope  520 . 
     In one embodiment, one or more electrical passages are disposed through a cap  555 , where the cap  555  may be removably coupled to the fill-port body  545 . These electrical passages may be configured to be coupled to one or more sensors, for example. In one embodiment, these one or more sensors (not shown) may be coupled to the balloon envelope  520  and aligned with at least one of the apertures  513  of the base plate  510 . In another embodiment, inductive power and data transfer may be utilized with the one or more sensors via one or more of the apertures  513 . The sensor(s) may be configured to measure any number of parameters, for example, temperature, air pressure, lift gas purity or moisture, among other possibilities. 
     5. Illustrative Methods 
       FIG. 8  is a flow chart of a method, according to an example embodiment. Example methods, such as method  800  of  FIG. 8 , may be carried out by a human operator or a control system for automated manufacturing. A control system may take the form of program instructions stored on a non-transitory computer readable medium and a processor that executes the instructions. However, a control system may take other forms including software, hardware, and/or firmware. Example methods may be implemented as part of the manufacturing or maintenance process for a balloon. 
     As shown by block  810 , method  800  involves providing affixing a base plate to a balloon envelope, where the balloon envelope defines an opening at an apex of the balloon envelope, where the base plate defines an opening, where the opening of the base plate is aligned with the opening of the balloon envelope, and where a plurality of studs are coupled to the base plate. Then at block  820 , a fill-port body may be placed through the opening of the balloon envelope and through the opening of the base plate such that a flange of the fill-port body lies adjacent to the balloon envelope. A locking body may then be secured to the fill-port body and/or the base plate, at block  830 , such that the locking body lies adjacent to the base plate. At block  840 , a plurality of tendons may be secured to the plurality of studs. Then, at block  850 , a retention ring may be secured to the base plate. 
     In one embodiment, securing the retention ring to the base plate may include (i) aligning the first portion of the plurality of openings in the retention ring with the plurality of studs on the base plate, (ii) sliding the retention ring over the flanges of the plurality of studs such that the flanges of the plurality of studs extend through the first portion of the plurality of openings of the retention ring and (iii) rotating the retention ring relative to the base plate such that a body of each of the plurality of studs are received within the second portion of the plurality of openings of the retention ring. In a further embodiment, securing the retention ring to the base plate also includes coupling at least one locking pin to the retention ring. 
     In another embodiment, securing the lock body to the fill-port body and/or the base plate may include (i) press-fitting the locking body onto the fill-port body such that the locking body is received in a detent defined by the fill-port body, (ii) press-fitting the locking body onto the fill-port body such that at least one detent defined on the locking body receives at least one protuberance defined on the fill-port body, (iii) coupling the locking body to the base plate or the fill-port body via one or more connectors, (iv) coupling the locking body to the base plate or the fill-port body via adhesive, and/or (v) screwing the lock nut onto the fill-port body via threads coupled to the opening of the locking body and mating threads coupled to an exterior of the fill-port body, among other possibilities. 
     6. Conclusion 
     The above detailed description describes various features and functions of the disclosed systems, devices, and methods with reference to the accompanying figures. While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.