Patent Publication Number: US-10308375-B2

Title: Capturing hook for aerial system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of and claims priority to U.S. application Ser. No. 14/213,450, filed on Mar. 14, 2014, entitled “Aerial System and Vehicle for Continuous Operation,” which claims priority to U.S. Provisional Patent Application No. 61/851,866, filed on Mar. 14, 2013, entitled “Aerial System and Vehicle for Continuous Operation,” both of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to systems and methods for use with Unmanned Aerial Vehicles (“UAVs”) and Unmanned Aerial Systems (“UASs”). More specifically, the present invention relates to systems and methods for enabling the operation of an autonomous, self-charging aerial vehicle surveillance system. 
     BACKGROUND INFORMATION 
     The use of Unmanned Aerial vehicles (“UAVs”) and Small Unmanned Aerial Systems (“SUASs”) has grown in recent years and such UAVs and SUASs are employed in a wide variety of applications, including both military and civilian uses. In some applications, a UAV or SUAS may be required to maneuver quickly, or in tight spaces, over a wide range of speeds. Accordingly, several efforts have been made to improve UAV and SUAS performance to yield a fully autonomous UAV system. 
     For example, U.S. Pat. No. 6,960,750, to Doane, discusses an optical system and method for positioning a first object with respect to a second object, such as a refueling aircraft and an unmanned air vehicle, including a pattern of reflectors, an optical receiver, an optical transmitter, and a processor. U.S. Pat. No. 7,318,564, to Marshall, discusses a surveillance aircraft recharging system based on energy collection by magnetic induction from the current flowing in a randomly selected alternating current transmission line conductor. U.S. Pat. No. 7,714,536, to Silberg, discusses a method and apparatus for charging energy supplies in a UAV. U.S. Pat. No. 8,167,234, to Moore, discusses a micro air vehicle (MAV) that comprises features that emulate insect-like topology and flight, including a dangling three-part body ( 100   a ,  100   b ,  100   c ); wing-like, dual side rotors ( 107 ,  107   a ) positioned to either side on rotor arms ( 103 ) providing tilt and teeter motions to vector thrust and allowing crawling along improved surfaces; and elevators ( 101 ) that approximate the center of gravity and center of pressure control employed by insects via the inertial reaction and aerodynamic influence of a repositionable abdomen. U.S. Pat. No. 8,172,177, to Lovell, discusses a stabilized UAV recovery system. United States Patent Publication No. 2003/0222173, to McGeer, discusses a method and an apparatus for capturing a flying object. 
     While a number of UAVs and UAV systems are disclosed through the above references, existing UAVs and UAV systems are deficient in at least two respects. First, existing UAVs are not entirely self-sufficient and require routine upkeep, such as charging and/or refueling. Second, existing UAVs are generally concerned only with the cable capture (e.g., landing), but do not consider both the autonomous capture and release of the vehicle. Accordingly, the present application provides systems and methods for providing a self-charging UAV and UAV system capable of autonomous capture and release. 
     SUMMARY 
     The present disclosure endeavors to provide systems and methods for enabling the operation of an autonomous self-charging aerial vehicle surveillance system. 
     According to a first aspect of the present invention, an aerial vehicle system for gathering data comprises: a waypoint location, wherein the Waypoint Location comprises an arresting cable; a ground control station, wherein the ground control station comprises a charging cable; an aerial vehicle, wherein the aerial vehicle comprises an onboard battery, a capturing hook and a sensor payload for generating surveillance data; wherein the aerial vehicle is configured to autonomously travel between the Waypoint Location and the ground control station; wherein the aerial vehicle is configured to couple and decouple with the arresting cable via the capturing hook; wherein the aerial vehicle is configured to perch from the charging cable via the capturing hook; wherein the aerial vehicle is configured to electronically couple and decouple with the charging cable via the capturing hook to facilitate charging the aerial vehicle&#39;s onboard battery. 
     According to a second aspect of the present invention, a capturing hook for engaging a cable during capture and release of an aerial vehicle comprises: a first gate pivotally supported at a first end by a base portion and movable between (i) a closed position and (ii) an open position; a first return spring biasing the first gate to the closed position; a second gate pivotally supported at a first end by the base portion and movable between (i) a closed position and (ii) an open position; a second return spring biasing the second gate to the closed position; and a latch device comprising a movable locking part biased by a return spring to a locked position to lock the second gate in the closed position. 
     According to a third aspect of the present invention, a vision-based aerial vehicle navigation system for capturing an arresting cable comprises: a camera; an infrared illuminator positioned on an aerial vehicle; two or more infrared reflectors positioned on an arresting cable; and an onboard vision processor configured to calculate the centers of each of said two or more infrared reflectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other advantages of the present invention will be readily understood with reference to the following specifications and attached drawings, wherein: 
         FIG. 1  illustrates an example Continuous Operation System; 
         FIG. 2  illustrates a block diagram of a UAV communicatively coupled with a Ground Control Station; 
         FIGS. 3 a  and 3 b    illustrate an example UAV for use with a System; 
         FIGS. 4 a  through 4 d    illustrate example cable capture and release maneuvers; 
         FIGS. 5 a  and 5 b    provide an enlarged view of an example arresting device; 
         FIGS. 6 a  through 6 e    provide enlarged views of the arresting device&#39;s capturing hook during the cable capture and release maneuver of  FIGS. 4 a    through  4   d;    
         FIG. 7  illustrates a Ground Control Station  102  positioned on a rooftop; 
         FIG. 8  illustrates a schematic diagram of a Ground Control Station  102 ; 
         FIG. 9 a    illustrates a block diagram of an example vision-based navigation system; 
         FIG. 9 b    illustrates an example process of analyzing an image using a vision-based navigation system; 
         FIG. 10 a    illustrates a cross-sectional view of a charging cable electrically coupling with a conductive contacts; 
         FIG. 10 b    illustrates an example configuration for positioning conductive contacts on an arresting device capturing hook; and 
         FIG. 11  illustrates the geometry of the guidance elements in the camera frame. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail because they would obscure the invention in unnecessary detail. For this application, the following terms and definitions shall apply: 
     The terms “communicate” and “communicating,” as used herein, refer to both transmitting, or otherwise conveying, data from a source to a destination and delivering data to a communications medium, system, channel, network, device, wire, cable, fiber, circuit, and/or link to be conveyed to a destination. 
     The term “computer,” as used herein, refers to a programmable device designed to sequentially and automatically carry out a sequence of arithmetic or logical operations, including without limitation, personal computers (e.g., those available from Gateway, Hewlett-Packard, IBM, Sony, Toshiba, Dell, Apple, Cisco, Sun, etc.), handheld, processor-based devices, and any other electronic device equipped with a processor or microprocessor. 
     The term “processor,” as used herein, refers to processing devices, apparatus, programs, circuits, components, systems and subsystems, whether implemented in hardware, tangibly embodied software or both, and whether or not programmable. The term “processor,” as used herein includes, but is not limited to, one or more computers, hardwired circuits, signal modifying devices and systems, devices and machines for controlling systems, central processing units, programmable devices and systems, field-programmable gate arrays, application-specific integrated circuits, systems on a chip, systems comprising discrete elements and/or circuits, state machines, virtual machines and data processors. 
     The present disclosure endeavors to provide systems and methods for enabling the operation of an autonomous self-charging aerial vehicle surveillance system. More specifically, the present disclosure endeavors to provide systems and methods for providing a self-charging UAV and UAV system capable of autonomous capture and release for use in a Continuous Operation System. While utilizing utility power lines for both perching and inductance charging is possible, a UAV system of the present invention preferably utilizes off-site charging through direct current lines (e.g., via a charging cable  804 ). An advantage of this approach is that the charging mechanisms on the UAV may be simplified, thus reducing the cost and weight of the UAV. Moreover, by using off-site charging stations equipped with direct current lines, UAVs are not forced to rely on ideal conditions of the power line to facilitate charging (e.g., current, voltage, diameter, etc.), thus expanding the scope of suitable Waypoint Locations. 
     For example, as illustrated in  FIG. 1 , a Continuous Operation System  100  may permit continuous, fully autonomous operation of one or more UAVs  106  for surveillance purposes. Each UAV  106  may employ one or more sensors to facilitate autonomous flight, including, but not limited to, ultrasonic sensors, infrared sensors, radar and the like. To collect data and monitor an area, the UAV  106  may be equipped with a traditional intelligence, surveillance, and reconnaissance (ISR) payload. For example, the UAV  106  may be equipped with a payload pod comprising one or more cameras, audio devices and other sensors. Any video, image, audio, telemetry and/or other sensor data (“Surveillance Data”), collected by the UAV  106  may be locally stored or wirelessly communicated from the UAV  106  (e.g., at the Waypoint Location  104  or during flight) to a Ground Control Station  102  in real time using an antenna coupled with an onboard wireless communication device, such as a transmitter/receiver. Alternatively, Surveillance Data may be communicated, or otherwise transferred, to the Ground Control Station  102  or another party via a wired connection. 
     In operation, a UAV  106  may alternate between a Waypoint Location  104  and a charging location, such as a Ground Control Station  102 . The UAV  106  should be capable of autonomous landing and takeoff using, for example, an optical sensing system with an onboard precision vision-processing computer. At each of the Waypoint Location  104  and Ground Control Station  102 , the UAV  106  may capture an arresting cable  310  to arrest itself and perch. As used herein, the two general types of arresting cables  310  include perching cables and charging cables  804 . Each of the perching cable and the charging cable  804  are capable of capturing and supporting a UAV  106  while it perches, however, as will be discussed in greater detail below, a charging cable  804  provides the additional function of charging the UAV  106 &#39;s batteries. 
     For example, at the Waypoint Location  104 , a utility power transmission line may function as a perching cable. To facilitate targeting when perching, a predetermined perching point on the perching cable may be marked using markers, such as IR reflectors. A UAV  106  may be further configured to autonomously charge itself upon return to the Ground Control Station  102 , or other charging stations, through an electrified charging cable  804  on which the UAV  106  may perch and recharge. Accordingly, at the Ground Control Station  102 , a charging cable  804  may comprise two direct current wires carrying power and ground transmission. As with the Waypoint Location  104 , a predetermined perching point may be marked on the charging cable  804  using markers. 
     Depending on the operation, the Ground Control Station  102  may be permanently installed or portable to facilitate on-the-move operations. By employing a plurality of UAVs  106  in a Continuous Operation System  100 , continuous fully autonomous surveillance is enabled, thus enabling continuous surveillance by providing a real time Surveillance Data feed to the Ground Control Station  102  and/or another monitoring facility. 
       FIG. 2  provides a block diagram for a UAV  106  communicatively coupled with a Ground Control Station  102  via a wireless data link. Each UAV  106  typically includes an onboard processor  108  that controls the various UAV components and functions. The processor  108  may be communicatively coupled with an Inertial Navigation System (“INS”)  114  (e.g., Vector Nav VN-100) that is communicatively coupled with an inertial measurement unit  116  and GPS receiver, an onboard data storage device  112  (e.g., hard drive, flash memory, or the like), a surveillance payload  118 , one or more batteries  142 , a battery system  130 , a wireless communication device  120 , or virtually any other desired services  110 . 
     To collect data and monitor an area, the UAV  106  may be equipped with a traditional ISR surveillance payload  118 . For example, the UAV  106  may be equipped with one or more cameras  118   a , audio devices, and/or other sensors  118   b . Any Surveillance Data collected by the UAV  106  may be wirelessly communicated to the Ground Control Station  700  in real time via the wireless communication device  120 . The UAV  106  may be further equipped to store said Surveillance Data to an onboard data storage device  112 . However, if the UAV  106  is operated in an unfriendly zone, it may be advantageous to encrypt all stored data, including Surveillance Data, or to implement a data self-destruction protocol. The UAV  106  may be programmed to erase, or otherwise destroy, the onboard data storage device  112  if the UAV  106  determines that it may have fallen into an enemy&#39;s possession. For example, the UAV  106  onboard data storage device may be erased automatically when communication between the Ground Control Station  102  and UAV  106  is lost or upon touching down in a location outside of a predefined radius from the Ground Control Station  102  and/or Waypoint Location  104 , based on GPS calculations, or, if a crash is detected, e.g., based on detection of a sudden impact. 
     Data may be communicated between the UAV  106  and Ground Control Station  102  via the wireless communication device  120 , which is operatively coupled to the processor  108 . For example, the wireless communication device  120  may be configured to communicate data (e.g., Surveillance Data and/or flight control data) with the Ground Control Station  102 . To facilitate optional wireless communication, the UAV  106  may further comprise an air communication link  120  enabled to transmit (“TX”) and receive (“RX”) data using one or more antennas (e.g., top and bottom) via a circulator  126 , LNE  122  and RFE  124 . The antenna may be controlled via the processor  108  that is operatively coupled to an RF switch  128 . 
     In urban environments, multipath interference can become a problem. Thus, standard analog video transmitters and many digital transmission methods may not be able to cope with this type of interference. Therefore, to mitigate this problem, the UAV may be equipped with coded orthogonal frequency division multiplexing (“CoFDM”) radios. CoFDM is a modulation format that is highly resistant to multipath interference. Since an operation may call for the UAV  106  to communicate Surveillance Data from a stationary Waypoint Location  104  below a roofline, a multipath resistant radio may be useful by eliminating the need to launch and re-land to improve radio reception from a perching point. 
     The Ground Control Station  102  typically includes a processor  132  that controls the various Ground Control Station  102  components and functions. The processor  132  may be communicatively coupled with a communication transceiver  136 , an I/O device  140 , a power supply  138  and a charging cable  804 . When the UAV  106  is perched on the Ground Control Station  102 &#39;s charging cable  804 , the UAV  106 &#39;s battery system  130  is electrically coupled with the Ground Control Station  102 &#39;s Power Supply  138 , thereby charging the UAV  106 &#39;s one or more onboard batteries  142 . As noted above, to reduce weight and cost, it is preferable to directly couple the battery system  130  with the Ground Control Station  102 &#39;s Power Supply  138  via the charging cable  804 . However, other methods are possible, such as inductance charging. 
     The Ground Control Station  102 &#39;s communication transceiver  136  may be used to wirelessly communicate data signals with the UAV  106  and/or an end user. Specifically, Surveillance Data collected by the UAV  106  may be transmitted in real time to the end user for live viewing, or to an apparatus (e.g., a computer) where it may be stored and/or displayed. Similarly, flight control data (i.e., flight commands from the end user or a flight computer) may be communicated between the Ground Control Station  102  and UAV  106  using the communication transceiver  136 . Alternatively, the Ground Control Station  102  may employ separate communication transceivers for communicating with the UAV  106  and with an end user. For example, the Ground Control Station  102  may communicate with an end user through a pre-configured high bandwidth directional data link and/or a satellite-based tactical data link. As illustrated, the Ground Control Station  102  may be electronically coupled to a power supply  138 . The power supply  138  may be, for example, a battery, a generator, line current (e.g., from a power grid), a solar cell, etc. The I/O Device  140  may be coupled with one or more sensors, such as a wind gauge  706 . 
     To further enhance data communication, the Ground Control Station  102  may be equipped with an enhanced data receiving system. For example, the Ground Control Station  102  may be provided with a mechanically steered antenna system, or a multi-antenna diversity system that can allow much higher gain antennas to be used, thereby greatly extending the range of the UAV&#39;s data link without increasing the power consumption of the UAV  106 &#39;s transmitting radios. With each additional antenna added to the system, a higher gain antenna can be utilized. For example, the Ground Control Station  102  may employ a smart antenna (e.g., an adaptive array antenna, multiple-antenna and multiple-input and multiple-output) combined with smart signal processing algorithms for (i) identifying spatial signal signatures such as the direction of arrival (DOA) of the signal, and (ii) calculating beam-forming vectors to track and locate the antenna beam on the mobile/target. 
     While multiple identical UAVs  106  are illustrated in  FIG. 1 , a Continuous Operation System  100  may employ a plurality of UAVs  106  of different types and sizes. Indeed, specially equipped UAVs  106  may be deployed to a particular Waypoint Location  104  to meet a specific need. However, compact lightweight UAVs are generally advantageous as they yield minimal detection and reduce weight imposed on the arresting cable  310 . Indeed, as will be shown below, a suitable UAV  106  that may be modified to facilitate Continuous Operation System operations includes the back-packable Skate™ UAS, available from Aurora Flight Sciences. 
     The Skate™ system is able to fly with its ISR payload, The Skate™ system can carry out autonomous missions from takeoff to landing without pilot intervention, but it is not able to land and take off again without assistance. The Skate™ system uses independently articulating thrust vectoring motor pods to allow rapid transition between vertical and horizontal flight. Transitioning from vertical take-off and landing (VTOL) to wing-borne flight increases the endurance and range of the system to levels characteristic of a fixed-wing platform and far beyond those of a traditional VTOL asset. The thrust vectoring provided by the motor pods allows the Skate™ UAV to fly both vertically and horizontally indoors and out, enabling rapid navigation of cluttered environments such as city streets or building interiors. For additional information regarding the Skate™ system, see Aurora Flight Science&#39;s website and commonly owned U.S. Pat. No. 8,721,383 (filed Sep. 9, 2009) and U.S. Pat. No. 8,500,067 (filed Aug. 4, 2012), which are each entitled “Modular Miniature Unmanned Aircraft With Vectored-Thrust Control.” 
     While the present invention illustrates a modified Skate™ UAV in the Continuous Operation System  100 , one of skill in the art would appreciate that the present invention should not be limited to use with the Skate™-type UAVs. On the contrary, virtually any small UAV or SUAS may be modified to meet the objectives of a Continuous Operation System  100 . Such features including, for example, unattended recharging, autonomous cable capture and launch, and video-based flight controls that permit accurate perching point targeting, as well as the extended endurance to a one hour mission with a half-pound payload. However, a modified Skate™ UAV is illustrated in the following examples because of its advantageous airframe characteristics. Moreover, the Skate™ UAV may be equipped with an autopilot capable of flying to waypoints (e.g., a Waypoint Location  104 ) and performing many fully autonomous missions through a full suite of sensors including, for example, GPS, barometric pressure for altitude, differential pressure (e.g., a Pitot tube) for airspeed, and a full 9-DOF inertial measurement unit. 
     The existing Skate™ UAS configuration represents a balance of the need for vertical launch/recovery, with the desire for persistent presence, and the physical constraints imposed by back packability (e.g., the ability to carry the UAV in a backpack). Therefore, the resulting Skate™ planform compromises by providing an aspect ratio selection driven by maneuvering and payload constraints, as opposed to an optimized cruise case, and wingspan limited by packaging requirements. This low aspect ratio planform provides a wide angle of attack envelope, facilitating inbound perch transition and controlled steep glide slopes for landing/maneuvering in confined spaces, but increasing induced drag at cruise. However, relaxing the aspect ratio constraint, by incrementally increasing wingspan, can improve cruise performance with minimal impact on maneuvering and storage capabilities. Therefore, because back packability is not necessarily required in a Continuous Operation System  100 , lengthening the wingspan of the UAV  106  can both increase the aspect ratio and reduce the wing loading, translating directly to lower cruise power requirements. 
     Indeed, as illustrated in  FIG. 3 a   , a Skate™ UAV may be modified using, for example, 3-inch wingtip extension  302  on each wing. By implementing wingtip extensions  302 , changes to the Skate™ UAV&#39;s mechanical/propulsion systems are not required. To account for the increase in wingspan, the vertical stabilizers  304  may also be enlarged, or raked, to meet new tail volume requirements. To enable perching on an arresting cable  310 , any sub fins may be eliminated from the underside of the UAV  106  thereby avoiding interference with any landing/capture devices, including arresting devices  306 . 
     The UAV  106  should also be configured to capture onto, or otherwise engage, an arresting cable  310  using one or more arresting devices  306  located on the underside of the UAV  106 . During and after the arresting operation, the UAV  106  may be configured to swing down and hang inverted from the arresting cable  310  to execute its surveillance objectives as illustrated in  FIG. 3 b   . Providing the arresting devices  306  on the underside of the UAV  106  allows for increased clearance between the oncoming arresting cable  310  and the propellers  312 , but also allows for excellent ground visibility from the surveillance payload pod  308 . Accordingly, a UAV  106 &#39;s ISR payload pod  308  may be preferably located on the top of the UAV  106  (e.g., opposite the arresting device  306 ). Therefore, when the UAV  106  is hanging inverted, the payload pod  308  faces downward, and provides the opportunity to mount a gimbaled camera, or other sensors, with a full 360-degree view of the ground. This orientation also maximizes exposure of the payload pod  308  with the ground where, from this position, the UAV  106  will be able to observe any ground location below the arresting cable  310 . 
     To facilitate surveillance functionality, the UAV may wirelessly transmit Surveillance Data back to the Ground Control Station  102 . From there, the data may be relayed to end users (e.g., remote operators) either through a pre-configured high bandwidth directional data link or through a satellite-based tactical data link. Alternatively, the UAV may wirelessly transmit any data directly to the remote operators. 
     To facilitate capture (landing) and release (takeoff), the cable-arresting mechanical system, which may comprise one or more arresting devices  306 , may be designed to release the UAV  106  either on command (i.e., actively) or when the UAV  106  generates enough thrust to lift off the arresting cable  310  (i.e., passively). For example, the UAV  106  may power its motors to reaches a stable condition wherein the vehicle is pointed nearly vertically (e.g., perpendicular to the ground) and is pulling against the arresting cable  310 . At this point, the autopilot simultaneously applies increased power to the motors and actuates the servo releasing the one or more arresting devices  306 &#39;s hook from the arresting cable  310 . Thus, the UAV  106  releases from the arresting cable  310  and launches vertically to a predetermined altitude before resuming level flight and navigation to a waypoint location (e.g., Ground Control Station  102  and/or Waypoint Location  104 ). Before the takeoff sequence, the UAV  106  hangs inverted, but since the propellers  312  are offset from the arresting devices, rotating back to vertical is not a difficult operation. 
     To facilitate capture and release of the arresting cable  310 , the UAV&#39;s flight control system may be provided with camera-derived estimates of the target arresting cable  310 &#39;s relative azimuth, elevation, and range. Together with the state estimate of the vehicle itself, this information is sufficient to determine line-of-sight rate and range rate to the target, which in turn can be used to implement a homing algorithm such as pure pursuit, proportional navigation (PN), or variations on PN that reduce the reliance on range rate information (which may be of lower accuracy in windy situations). For example, in operation, the UAV may guide itself to the desired landing site using GPS, along the approach heading designated by the installation crew. Once the UAV comes within a predetermined distance of the landing zone (e.g., about 20-30 feet from the perch point) the UAV can activate the sensing system. The UAV may detect the cable markers (e.g., IR illuminators and/or IR reflectors) and may utilize a terminal guidance algorithm to impact the arresting cable  310  at a slower cruise. In the event that the sensing system does not detect the markers, or the UAV encounters a wind gust and misses the arresting cable  310 , the UAV can perform an abort operation. An abort operation may comprise, for example, climbing rapidly above obstructions. Since the UAV may be capable of a vertical takeoff, the UAV can rapidly ascend to a safe altitude and fly around for another attempt. 
     The UAV  106  may employ an onboard vision processing system capable of performing real time centroiding on the incoming video and calculating relative altitude estimates. This may be done at a conservative minimum of 30 frames per second (fps), although 60 fps may be preferable. For example, an OMAP™ 3-based cellular phone processor may be used to provide a vision processing system because it is highly miniaturized and designed for low power operation. The perch point cable markers may be placed on an arresting cable  310  and may be detected using, for example, IR beacons, coupled with the onboard vision processing system. This method is advantageous because IR light can be effectively utilized in both day and night with proper selection of IR frequency (e.g., 940 nm). Since centroiding generates sub-pixel resolution accuracy, high pixel camera resolution may not be necessary to achieve high accuracy results. Consequently, the primary metrics for selection of the camera may be size, weight and ease of integration. For example, a suitable camera may be a miniature camera based on an Aptina monochrome image sensor. Testing of the marker performance is analyzed through an RGB intensity graph. 
     Accordingly, as illustrated in  FIG. 8 , arresting cables  310 , whether charging or perching cables, may be marked with a marker  812 , such as active IR illuminator (e.g., Phoenix Jr. Infrared Beacons) and/or IR reflectors (e.g., retro-reflective tape), which functions in IR (e.g., “Glint Tape” that is available from U.S. Tactical Supply; Emdom retro-reflective ID Marker; 3M 3000X or 3M 7610 Reflective Tape; or another all-purpose adhesive light strips). The IR illuminator has the advantage of not requiring an IR light source on the UAV, but would require power of some kind (e.g., onboard batteries.) Conversely, the IR reflectors, which are passive, would require an illuminator on the UAV. 
     For example, two markers may be attached to the arresting cable  310  to enable the vehicle to easily detect relative bank angle compared to the cable, relative pitch and heading as well as to estimate a rough distance to target. The onboard vision processing can centroid the incoming images, and determines the centers of the IR beacons in the field of view. To identify a perching point, the vision processing system can input these centroid coordinates and calculate the relative altitude estimates to feed into the landing control system. 
       FIGS. 4 a  through 4 d    illustrate an example cable capture and release maneuver. In  FIG. 4 a   , once the perching point has been located, the UAV  106  approaches the arresting cable  310 , whether a perching cable or charging cable  804 , at cruise speed with the arresting devices  306  lowered. The UAV  106  may be equipped with one or more retractable arresting devices  306  on the underside of the UAV  106 . The distal end of each arresting device  306  may be provided with a capturing hook  314  (e.g., a capture and release mechanism), such as the passively locking jaw illustrated in  FIGS. 5 a  and 5 b   . Moreover, to increase lateral stability, it is preferable to capture the arresting cable  310  at two or more points, which may be accomplished by employing two arresting devices  306  or a single apparatus having two or more capturing hooks  314 . 
     With the arresting device  306  hanging below the UAV  106 , aircraft components (e.g., the propeller, ISR pod and tails) do not pose a snagging risk with the arresting cable  310 , and an abort maneuver can be performed simply by pitching up and away from the arresting cable  310 . As illustrated in  FIG. 4 b   , the arresting device  306  may strike the arresting cable  310  and capture it via the one or more capturing hooks  314 . The arresting device  306  may be configured such that an arresting cable  310  may strike at any point along the shank  510  and result in a capture. Once locked, the UAV  106  swings forward and/or down to hang below the arresting cable  310 . To increase strength, the arresting device  306  may be secured to the UAV&#39;s main structure (e.g., the payload pod  308 &#39;s attachment brackets) thus transferring arresting loads (e.g., energy) into the main structure of the airframe. Once the UAV  106  is hanging below the arresting cable  310 , the payload  308  has an unobstructed view of the surroundings. 
     After charging, or surveying an area of interest, the UAV  106  is ready to re-launch. By throttling up the motors to a thrust-weight setting, as illustrated in  FIG. 4 c   , the UAV  106  will rotate around the arresting cable  310  and be pulled into a vertical launch position. At this point, the arresting device  306  will release. By omitting sub-fins, the portion of the airframe behind the arresting device  306  is obstruction free. As illustrated in  FIG. 4 d   , the UAV  106  climbs up and away from the arresting cable  310 , transitioning back to a cruise altitude. The arresting device  306  may then retract into the fuselage of the UAV  106  to reduce drag. 
       FIGS. 5 a  and 5 b    provide an enlarged view of an example arresting device  306  while  FIGS. 6 a  through 6 d    provide enlarged views of the arresting device  306 &#39;s capturing hook  314  during the cable capture and release maneuver of  FIGS. 4 a  through 4 d   . As illustrated, the capturing hook  314  for engaging an arresting cable  310  during capture and release of an aerial vehicle generally comprises a first gate  502  pivotally supported at a first end by a base portion of the shank  510  and movable between (i) a closed position (e.g.,  FIGS. 6 a  and 6 c -6 e   ) and (ii) an open position (e.g.,  FIG. 6 b   ) and a second gate  504  pivotally supported at a first end by the base portion of the shank  510  and movable between (i) a closed position and (e.g.,  FIGS. 6 a -6 d   ) (ii) an open position (e.g.,  FIG. 6 e   ). To prevent the first gate  502  from inadvertently opening, a first return spring  516  biases the first gate  502  in the closed position. Similarly, to prevent the second gate  504  from inadvertently opening, a latch device  514  comprising a movable locking part  506  biased by a return spring  518  to a locked position to lock the second gate  504  in the closed position. When the first gate  502  and the second gate  504  are both in the closed position, the first gate  502  may be configured such that the first gate  502 &#39;s second end  520  slips within a recess  522  at the second end of the second gate  504 , thereby preventing the arresting cable  310  from inadvertently slipping out of the hook recess  512 . 
     More specifically,  FIG. 6 a    illustrates the arresting device  306 &#39;s capturing hook  314 , which is a passively locking jaw resembling a clevis type hook, in a lowered position as the UAV  106  approaches an arresting cable  310  (e.g., a charging cable  804  comprising a wire pair) at cruise speed.  FIG. 6 b    illustrates the capturing hook  314  in the course of catching the arresting cable  310 . In operation, the arresting cable  310  may either (i) make direct contact with the capturing hook  314 &#39;s first gate  502 , or (ii) pass along the arresting device  306 &#39;s shank  510  until it reaches the capturing hook  314 &#39;s first gate  502 . Regardless of the initial contact, once the arresting cable  310  makes contact with the first gate  502 , the force of the arresting cable  310  can cause the first gate  502  to push backward in direction C about pivot point Y, thus providing access to the capturing hook  314 &#39;s hook recess  512 . Once the arresting cable  310  is in the hook recess  512  of the capturing hook  314 , a force (e.g., an extension spring) may cause the first gate  502  to snap back in direction C, as illustrated in  FIG. 6 c   , thereby securing the arresting cable  310  in the hook recess  512 . 
       FIG. 6 d    illustrates the capturing hook  314  prior to releasing the arresting cable  310 . While perched and during the takeoff maneuver, the arresting cable  310  generates a force against the second gate  504  in direction A. To release the arresting cable  310  from the hook recess  512 , a servo-controlled release wire may be configured related to the latch device  514  by pulling the a movable locking part  506  in direction B about pivot point Z, thus enabling the second gate  504  to open by pivoting about pivot point X as illustrated in  FIG. 6 d   . As illustrated in  FIG. 6 e   , the lift and thrust of the UAV  106  applies a tension to the arresting cable  310  during takeoff that pulls the second gate  504  open thereby releasing the UAV  106  to facilitate free flight, as shown in  FIGS. 4 c  and 4 d   . Once released, the force from the arresting cable  310  pulling on the second gate  504  is gone and an extension spring pulls the second gate  504  back into its closed position and allows the latch device  514  to lock it in place, ready for the next capture. The arresting device  306 &#39;s design eliminates the need for a controls/sensing intensive dynamic perch maneuver. By approaching the arresting cable  310  at or near cruise speed, the UAV  310  can be much less susceptible to gusts, and the controls approach can be greatly simplified (i.e. not in the post-stall regime). The arresting device  306 &#39;s design enables the Continuous Operation System  100 . 
     As illustrated in  FIG. 7 , a Ground Control Station  102  may be positioned on a rooftop, or other substantially clear area (e.g., clear from landing/takeoff obstruction) near the area of surveillance. Once a Ground Control Station  102  has been configured, a Waypoint Location  104  having an arresting cable  310  (e.g., power, telephone or specially-installed cable) for the UAV  106  may be identified or created. Since the arresting cable  310  may be pre-surveyed, an operator can designate an approach direction; the GPS coordinate location and/or altitude of the perching zone (e.g., the area immediately surrounding the arresting cable  310 ). The arresting cable  310  may also be marked or equipped with markers, such as IR beacons, that allow the UAV  106  to locate the arresting cable  310  using its onboard sensory system. After surveying and marking an arresting cable  310  within a perching zone, the UAV may approach and land on the arresting cable  310 . 
     Since the UAV  106 &#39;s operation is autonomous, the UAV  106  may be configured to launch from a Ground Control Station  102 , fly a mission (e.g., perch at a Waypoint Location  104 ), then return to the Ground Control Station  102  to charge itself in preparation for the next autonomous launch. To facilitate this functionality, a Ground Control Station  102  may provide a charging cable  804  for electrically connecting to the UAV  106  to facilitate charging after the UAV  106  has perched on the charging cable  804 . A diagram of an example Ground Control Station  102  is illustrated in  FIG. 7 . 
     To facilitate autonomous takeoff and capture (landing), a Ground Control Station  102  may comprise a charging cable  804 , a communications transceiver  136  and wind gauge  704 . The charging cable  804  may be supported by one or more dedicated posts  708 . Accordingly, the Ground Control Station  102  may be configured to be self-supporting so that it may stand on its own in an open field. Alternatively, the charging cable  804  may be coupled with independent structures, such as buildings, telephone poles, etc. In fact, the entire Ground Control Station  102  may be coupled with, or integrated with, a vehicle to provide a mobile Ground Control Station  102  where the UAV  106  can locate the Ground Control Station  102  via the wireless antenna and/or GPS tracking. Similarly, the wind gauge  704  may be integrated with the Ground Control Station  102  via an I/O device  140  to provide wind data (e.g., speed and direction) or remotely located wherein the wind data is communicated to the Ground Control Station  102  via communications transceiver  136 . Regardless of the configuration, to account for landing and takeoff maneuvers, the charging cable  804  should be designed and positioned sufficiently off the ground to provide an adequate amount of landing length for the UAV. For example, the charging cable  804  length may be 15 feet and positioned substantially parallel to the ground at a height of 15 feet. 
     A notable feature of the charging cable  804  is that it may directly electrically interface with the capturing hook  314  to facilitate charging the UAV  106 &#39;s battery while perched on (e.g., hanging from) the charging cable  804 . Specifically, the charging cable  804  preferably comprises conductive wires (e.g., ground and power) that interface with the UAV  106 &#39;s battery system via the capturing hook  314 . In doing so, the conductive wires should be configured to minimize the risk of shorting (e.g., making contact with each other) directly, or through some part of the UAV  106 &#39;s hook, once the UAV has perched and is hanging in steady state. 
     A cable management system for use in Ground Control Station  102  according to the present invention may be illustrated by the following example. This example is provided to aid in the understanding of the invention and is not to be construed as a limitation thereof. As illustrated in  FIG. 8 , two extendable (e.g., telescoping) poles  802  may be spaced apart and extended into the air. For a small UAV  106 , the poles  802  may be spaced 15 feet apart and extend 15 feet into the air. To tether the poles  802  to the ground, a first end of two or more guy-lines  804  may be coupled to the top of each pole  802  via one or more mounts while a second end may be coupled to one or more ground attachment points  806  (e.g., ground stakes). While the guy-lines  804  may be constructed from rope, cable may be employed to reduce any displacement resulting from guy-line stretching. The mount for the guy-lines may be configured such that each guy-line may attach to its own mount with some distance between them, thus adding moment resistance to the setup, thereby reducing twisting of the poles  802 . Moreover, ratcheting tensioning devices may be provided in line with the guy-line to facilitate larger adjustments while turnbuckles may be provided to facilitate fine adjustments. 
     A charging cable  804  may be stretched between the poles  808  to capture and charge a UAV  106 . The charging cable  804  may be further threaded through one or more pulleys  808  and coupled to one or more cable management devices  810 . Each cable management device  810  may be configured to provide a constant charging cable  804  tension augmented by a shock absorber to absorb energy during UAV capture. Indeed, the charging cable  804  should be configured as to provide a soft catch to the UAV  106 , thereby minimizing the risk of damage after repeated use. Example cable management devices  810  may include, for example, a winch coupled with one or more shock absorbers, springs (linear or torsional), elastic cables, or hydraulics. To increase reliability, the cables may be kept on the pulleys with cable guards or routed through cable housings. 
     While the following example is applied to a charging cable  804 , the same functionality may be used in conjunction with any perch point, such as an arresting cable  310  positioned at a Waypoint Location  104 . For example, as discussed above, the charging cable  804  may be marked with two or more markers  812 , such as active IR beacon and/or retro-reflective tape. The markers  812  may be attached to the charging cable  804  to enable the UAV  106  to detect relative bank angle compared to the charging cable  804 , relative pitch and heading as well as to estimate a rough distance to target. The UAV  106 &#39;s onboard vision based navigation system can centroid the incoming images to determine the centers of the IR beacons in the field of view, thereby identifying the charging cable  804 . The vision navigation algorithm is continually trying to identify a target. When a possible target is recognized, an internal counter verifies that it has been continually identified for several frames. Upon recognition of the beacons, a signal is sent to the outer-loop controller and the vehicle guidance is switched into a vision-based tracking routine. Specifically, heading and altitude commands may track the location of the target to the center of the field of view. The target was set as the central markers, detected by the vision based navigation system algorithms where offsets in the lateral direction commands heading, and vertical offset commands altitude. Accordingly, a low level altitude and heading tracker was implemented in the autopilot. 
     Accordingly, the general objective of a vision based navigation system is to determine the estimation and control approach for a UAV  106  flying towards an identified visual source and using the information of the observed location of two or more known markers  812  in the camera frame to reach a specific location (e.g., a Ground Control Station  102  or Waypoint Location  104 ). Measurements available to the vision based navigation system may include, for example, position data from a GPS device, attitude with respect to a global reference frame (e.g., using VectorNav) and the location of predetermined points in the camera frame (e.g., using Sanford image processing). Markers  812  or other beacon points may be located in the environment and the MAV may be provided with data regarding the markers  812 &#39;s location with respect to the charging cable  804 . For example, the specific location should be visually accessible to the camera at least up to a point where the UAV  300  may achieve a final approach with increased certainly. That is, as the UAV  300  gets closer to the specific location, the markers  812  should fall within the field of view of the camera up to a very short distance to the target location. The markers  812  are known, or assumed, to be located at some predetermined points on the cable. 
       FIG. 9 a    illustrates a block diagram of an example vision based navigation system  900 . As illustrated, the vision based navigation system  900  may comprise one or more image capture devices  902 , a point correlation device  904 , a thresholding device  906 , a feature tracking device  908 , a likelihood filter  910 , and a Kalman estimate device  912 . 
     The one or more image capture devices  902  may be configured to receive, or generate, an image of an area. The one or more image capture devices  902  may include, for example, an onboard camera. The image of the area may be a still photo or a video, which generally comprises a series of still photos known as frames. The thresholding device  906  may be configured to determine the location of features within the image. For example, markers  812  may be used to provide image points with high intensity levels that can be extracted from the image by thresholding. Accordingly, the information from the image capture devices  902  (e.g., camera) can provide the coordinates (u,v) for points within a corresponding threshold. For example, a detected light that exceeds a predetermined threshold intensity value may be represented as a coordinate within the image. The feature tracking device  908  may employ a feature tracking algorithm, such as Lukas-Kanade, to calculate the motion of the image locally using the coordinates by tracking the motion of a feature (e.g., a coordinate) from frame to frame. This process allows for filtering out coordinates that do not correspond to the markers  812  as they are tracked. However, additional calculations may further be employed to track of the markers, or other features. Specifically, an algorithm may track features over different frames but may not identify which ones are the markers. 
     The point correlation device  904  and likelihood filter  908  may be used to reduce the coordinates by eliminating outlier coordinates based on, for example, a linear correlation. Since the locations of the markers is typically known (e.g., on a power line), the markers should meet a known geometric constraint. Accordingly, a first approach may be to assume that a valid set of points should lie on a line (or close to it) as shown in  FIG. 9 b   . Therefore for vision based navigation system  900  selected n-sets of points, the likelihood filter  908  can determine whether the points are within a threshold of being in a line by checking the error from a linear fit. From here, there are two possible approaches to beacon detection: one based on closed-loop, time-varying thresholding and a Kalman Filter-based approach. The Kalman estimate device  912  comprises a Kalman filter, which uses input from GPS, the IMU and the Camera to produce the best estimate of the position and/or attitude with respect to the markers. Generally speaking, a Kalman filter is an algorithm that uses a series of measurements observed over time, containing noise (random variations) and other inaccuracies, and produces estimates of unknown variables. Moreover, the routine may identify features that are consistent over time in spite of the time-varying brightness threshold to eliminate noise and reflections that are less consistent in brightness over time. A standard Extended Kalman Filter approach is used to predict the state and update the covariance:
 
 {circumflex over (x)}   k|k−1   =f ( {circumflex over (x)}   +   k-1|k-1   ,u   k-1 )
 
 P   k|k-1   =F   k-1   P   k-1|k-1   F   k-1   T   +Q   k  
 
     To filter out sources of noise, reflections, and non-beacon lights, a closed-loop thresholding algorithm may be used to alters the brightness threshold in real-time to select a subset of points (e.g., 5 or so). The closed-loop thresholding algorithm may work in conjunction with the likelihood filter that may have more time-varying brightness levels than the beacons themselves. 
       FIG. 9 b    illustrates an example process of analyzing an image using a vision based navigation system  900 . At step  1 , the vision based navigation system  900  generates, or otherwise receives, an image of a given area. The area may be the view of the UAV  300  during flight (e.g., akin to the view from a manned aircraft&#39;s cockpit). The camera parameters may be assumed, such as focal length and camera configuration. 
     At step  2 , a series of coordinates are identified based on the image using thresholding techniques. Specifically, the various light sources detected in the image are represented using one or more coordinates on a coordinate plane. The light sources are represented on the plane when they exceed a predetermined threshold intensity value. As illustrated, in addition to the two markers, the various street lamps also exceed the threshold intensity and thus are similarly represented on the coordinate plane. Using intense beacons will provide image points with high intensity levels that can be extracted from the image by thresholding. The information from the camera will provide the (u,v) coordinates for points within a corresponding threshold. The measurement model used is the standard pinhole model: 
     
       
         
           
             u 
             = 
             
               
                 X 
                 f 
               
               Z 
             
           
         
       
       
         
           
             v 
             = 
             
               
                 Y 
                 f 
               
               Z 
             
           
         
       
     
     a convenient way used to express this model is: 
     
       
         
           
             p 
             = 
             
               
                 λ 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         u 
                       
                     
                     
                       
                         v 
                       
                     
                     
                       
                         1 
                       
                     
                   
                   ) 
                 
               
               = 
               
                 
                   
                     ( 
                     
                       
                         
                           f 
                         
                         
                           τ 
                         
                         
                           
                             o 
                             x 
                           
                         
                       
                       
                         
                           0 
                         
                         
                           
                             η 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             f 
                           
                         
                         
                           
                             o 
                             y 
                           
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                       
                     
                     ) 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           
                             X 
                             cam 
                           
                         
                       
                       
                         
                           
                             Y 
                             cam 
                           
                         
                       
                       
                         
                           
                             Z 
                             cam 
                           
                         
                       
                     
                     ) 
                   
                 
                 = 
                 
                   KX 
                   cam 
                 
               
             
           
         
       
     
     Where τ, η, o x , o y  are parameters of the camera describing distortion and offset. As mentioned in the assumptions, τ, η, will be considered 0, 1 (no shear or compression distortion). The location of a point p A  in the camera frame can then be calculated as:
 
λ p   A   =KR (− X+x   A )  (1)
 
with:
 
λ= R   3 (− X+x   A )  (2)
 
     Where 
             R   =     [           R   1               R   2               R   3           ]           
is the rotation matrix from the absolute frame to the camera frame:
 
 R=R   cam/body   R   body/X  
 
     Where, X is the coordinates of the camera with respect to the target location, and x A  is the location of the beacon with respect to the target location. The gradient of the measurement equation H can be calculated by implicit differentiation of equations (1) and (2) to be: 
     
       
         
           
             
               
                 
                   
                     H 
                     k_p 
                   
                   = 
                   
                     
                       
                         
                           ∂ 
                           
                             p 
                             A 
                           
                         
                         
                           ∂ 
                           X 
                         
                       
                       ⁢ 
                       
                         | 
                         
                           
                             x 
                             ^ 
                           
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                     = 
                     
                       
                         
                           - 
                           
                             1 
                             λ 
                           
                         
                         ⁢ 
                         
                           ( 
                           
                             KR 
                             + 
                             
                               
                                 p 
                                 A 
                               
                               ⁢ 
                               
                                 R 
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                                 T 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                         | 
                         
                           
                             x 
                             ^ 
                           
                           k 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     At step  3 , a RANdom SAmple Consensus (RANSAC) algorithm may sample different combinations of n-points. At step  4 , the coordinates are reduced by eliminating outlier coordinates based on a known geometric constraint, for example, a linear correlation. That is, outliers that correspond to 3D points reflecting or emitting IR that may not be discerned from the correct beacons to be tracked. Thus, a preliminary filter that can reduce the data points by rejecting outliers is based on a linear correlation. Since the beacons are known to be located on a power line, they should meet a specific geometric constraint. As a first approach we will assume that a valid set of points should lie on a line (or close to it). Therefore for every selected pair of points we can check if they are within a threshold of being in a line by checking the error from a linear fit. After the image processing, we have a set of points:
         P k :{p k1 , p k2 , p k3 , p k4 , p k5  . . . }       

     Which we can group into sets that represents mutually exclusive events:
         Z k :{z k     1   , z k     2   , z k     3   , z k     4   , z k     5   , . . . }       

     That is: 
     
       
         
           
             
               z 
               k 
             
             = 
             
               
                 
                   [ 
                   
                     
                       
                         
                           
                             p 
                             ^ 
                           
                           A 
                         
                       
                     
                     
                       
                         
                           
                             p 
                             ^ 
                           
                           B 
                         
                       
                     
                     
                       
                         
                           
                             p 
                             ^ 
                           
                           C 
                         
                       
                     
                   
                   ] 
                 
                 k 
               
               = 
               
                 
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                             ⁢ 
                             
                                 
                             
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                 ⁢ 
                 
                     
                 
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                   or 
                   ⁢ 
                   
                       
                   
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                             ⁢ 
                             
                                 
                             
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                 ⁢ 
                 
                     
                 
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                   or 
                   ⁢ 
                   
                       
                   
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                           p 
                           
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                             ⁢ 
                             
                                 
                             
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                             ⁢ 
                             
                                 
                             
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                             ^ 
                           
                           
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                             ⁢ 
                             
                                 
                             
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                             4 
                           
                         
                       
                     
                   
                   ] 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   or 
                   ⁢ 
                   
                       
                   
                   [ 
                   
                     
                       
                         
                           p 
                           
                             k 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                             p 
                             ^ 
                           
                           
                             k 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                       
                     
                     
                       
                         
                           
                             p 
                             ^ 
                           
                           
                             k 
                             ⁢ 
                             
                                 
                             
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                             2 
                           
                         
                       
                     
                   
                   ] 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 … 
               
             
           
         
       
     
     Finally, at step  5 , the markers, indicated in the figure using a dotted circle, are identified using likelihood filtering and/or Kalman filter. Indeed, a standard Extended Kalman Filter approach may be used to predict the state and update the covariance:
 
 {circumflex over (x)}   k|k-1   =f ( {circumflex over (x)}   +   k-1|k-1   ,u   k-1 )
 
 P   k|k-1   =F   k-1   P   k-1|k-1   F   k-1   T   +Q   k  
 
     Each point z k  indicates a combinatorial of feature points, which indicate mutually exclusive events. Therefore, we are interested in selecting one of the possible events as the correct one. This may be accomplished by selecting the one with the largest likelihood as measured by the innovation vector and its covariance. After performing the state prediction, we can look for the event z k     n    that maximizes the likelihood by projecting the innovation of each event (variation from predicted state) in its probability space, and finding distance to the origin (maximum likelihood). This is equivalent to finding the event n that minimizes:
 
 e   k     n     ={tilde over (y)}   k     n     T   S   k   −1   {tilde over (y)}   k     n    
 
With:
 
 {tilde over (y)}   k     n   =( z   k     n     −h ( {circumflex over (x)}   k|k-1 ))
 
 S   k   =H   k   P   k|k-1   H   k   T   +R   k  
 
     After selecting the measurement with highest likelihood, the filtered data point is used in the update of the Kalman State estimate:
 
 K   k   =P   k|k-1   H   k   T   S   k   −1  
 
 {circumflex over (x)}   k|k   ={circumflex over (x)}   k|k   +K   k   {tilde over (y)}   k     n    
 
 P   k|k =( I−K   k   H   k ) P   k|k-1  
 
     One the markers are identified, the UAV  300  may navigate to the arresting cable  310 &#39;s perch point using an onboard autopilot. For example, a terminal guidance control approach may be employed. Using this approach, assumptions may be used to reduce the complexity of an approach. Specifically, one assumption may be that the UAV  300  can initially identify the beacons on the camera plane, that is, the points in the image that correspond to the beacons are known. Additionally, given the ambiguity of the measurements we have to make some assumptions of states that will be controlled through internal loops. 
     The beacons may also be tracked from image to image by finding the set that maximizes the probability of being the same beacon from the previous frame by calculating the set that minimizes:
 
 e=dp   T   R   −1   dp  
 
     Where dp=(du,dv) is an array of the difference in the measured positions between the identified beacons in one frame, and the detected points in the next one, R is the covariance matrix of the measurements. Using that information, a line in the image may be defined by two points. In general, the information from the camera in  FIG. 11  may be used, which illustrates the geometry of the guidance elements in the camera frame. The guidance elements are: 
     
       
         
           
             α 
             = 
             
               atan 
               ⁡ 
               
                 ( 
                 
                   
                     
                       v 
                       2 
                     
                     - 
                     
                       v 
                       1 
                     
                   
                   
                     
                       u 
                       2 
                     
                     - 
                     
                       u 
                       1 
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             β 
             = 
             
               atan 
               ⁡ 
               
                 ( 
                 
                   
                     
                       v 
                       2 
                     
                     + 
                     
                       v 
                       1 
                     
                   
                   
                     
                       u 
                       2 
                     
                     + 
                     
                       u 
                       1 
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             γ 
             = 
             
               β 
               - 
               α 
             
           
         
       
       
         
           
             
               e 
               θ 
             
             = 
             
               
                 
                    
                   e 
                    
                 
                 ⁢ 
                 cos 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 γ 
               
               - 
               
                 
                    
                   e 
                    
                 
                 ⁢ 
                 sin 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 γ 
               
             
           
         
       
       
         
           
             
               e 
               ψ 
             
             = 
             
               
                 
                    
                   e 
                    
                 
                 ⁢ 
                 sin 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 γ 
               
               + 
               
                 
                    
                   e 
                    
                 
                 ⁢ 
                 cos 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 γ 
               
             
           
         
       
       
         
           
             
                
               e 
                
             
             = 
             
               
                 1 
                 2 
               
               ⁢ 
               
                 
                   
                     
                       ( 
                       
                         
                           u 
                           2 
                         
                         + 
                         
                           u 
                           1 
                         
                       
                       ) 
                     
                     2 
                   
                   + 
                   
                     
                       ( 
                       
                         
                           v 
                           1 
                         
                         + 
                         
                           v 
                           2 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
           
         
       
     
     Where p i =[u i , v i ], i∈(1,2), is the location of the line extreme points on the camera frame. The control transfer functions K(s) can be a set of static gains or a dynamic transfer functions set to compensate the dynamics of the non-linear input to output system. 
     A first approach may be a constant pitch angle approach. To perform this approach it may be assumed that an internal loop tries to maintain a constant pitch angle, the altitude variations are small, and the altitude is regulated using thrust. Thus, in general, Pitch angle is constant. Small variations with respect to the level direction. (Defining level direction as vector from camera to target point is aligned). Under the constant pitch angle assumption we can define an input-output system: 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       e 
                       θ 
                     
                   
                 
                 
                   
                     
                       e 
                       ψ 
                     
                   
                 
                 
                   
                     α 
                   
                 
               
               ] 
             
             = 
             
               G 
               ⁡ 
               
                 ( 
                 
                   
                     
                       h 
                     
                   
                   
                     
                       ψ 
                     
                   
                   
                     
                       ϕ 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     Control approach will then be based on trying to regulate the altitude, heading and roll based on the observed line (defined by the extreme points) in the image. 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       h 
                       . 
                     
                   
                 
                 
                   
                     
                       ϕ 
                       . 
                     
                   
                 
               
               ] 
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         
                           K 
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       
                         
                           K 
                           ψα 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       
                         
                           K 
                           ϕα 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                   
                 
                 ] 
               
               ⁡ 
               
                 [ 
                 
                   
                     
                       
                         e 
                         θ 
                       
                     
                   
                   
                     
                       
                         e 
                         ψ 
                       
                     
                   
                   
                     
                       α 
                     
                   
                 
                 ] 
               
             
           
         
       
     
     Where θ, ψ, ρ are pitch yaw and roll angles respectively. 
     A second approach may be control of velocity vector. An objective of the control law is to maintain the camera vector aligned with the final target point, given the assumption that the camera vector is aligned with the velocity direction, the trajectory converges to the target. 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       e 
                       θ 
                     
                   
                 
                 
                   
                     
                       e 
                       ψ 
                     
                   
                 
                 
                   
                     α 
                   
                 
               
               ] 
             
             = 
             
               G 
               ⁡ 
               
                 ( 
                 
                   
                     
                       h 
                     
                   
                   
                     
                       ψ 
                     
                   
                   
                     
                       ϕ 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The velocity of the vehicle is aligned with the camera vector. This can be performed by providing inner control loops that regulate the thrust to achieve such behavior. 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       θ 
                       . 
                     
                   
                 
                 
                   
                     
                       ψ 
                       . 
                     
                   
                 
                 
                   
                     
                       ϕ 
                       . 
                     
                   
                 
               
               ] 
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         
                           K 
                           θ1 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       
                         
                           K 
                           θ2 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       0 
                     
                   
                   
                     
                       
                         
                           K 
                           ψ1 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       
                         
                           K 
                           ψ1 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       0 
                     
                   
                   
                     
                       
                         
                           K 
                           ϕ1 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       0 
                     
                     
                       
                         
                           K 
                           ϕ2 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                   
                 
                 ] 
               
               ⁡ 
               
                 [ 
                 
                   
                     
                       
                         e 
                         θ 
                       
                     
                   
                   
                     
                       
                         e 
                         ψ 
                       
                     
                   
                   
                     
                       α 
                     
                   
                 
                 ] 
               
             
           
         
       
     
     Where θ, ψ, ρ are pitch yaw and roll angles respectively. 
     A third approach may be glideslope. This approach considers a glideslope defined by an estimated distance to the target from the size of the line in the camera plane and assumes that the pitch angle is held constant. 
     
       
         
           
             h 
             = 
             
               1 
               - 
               
                 
                   ( 
                   
                      
                     
                       
                         p 
                         2 
                       
                       - 
                       
                         p 
                         1 
                       
                     
                      
                   
                   ) 
                 
                 
                   Y 
                   max 
                 
               
             
           
         
       
     
     Inner loops may be employed to achieve the glideslope. 
       FIG. 10 a    illustrates a cross sectional diagram of an example charging cable  804  comprising two conductive wires  1002  separated by an non-conductive insulator  1004 , which resembles a twin lead RF cable. To reduce the risk of shorting the circuit, the arresting device  306  and capturing hook  314  may be fabricated from a non-conductive material equipped with conductive contacts  1006  positioned in the hook recess  510  of the capturing hook  314  to facilitate an effective electrical contact between the UAV  106 &#39;s battery charging system and the wire conductors  1002 . An example non-conductive material includes synthetic polymers, such as plastic. To increase conductivity and prevent corrosion, the conductive contacts  1006  may be fabricated from a non-corrosive conductor, such as gold. Providing wider conductive contacts  1006  allows for a large amount of angular movement of the UAV  106  on the cable while maintaining electrical contact. 
       FIG. 10 b    illustrates an example configuration for positioning conductive contacts  1006  on the arresting device  306 &#39;s capturing hook  314  of  FIGS. 6 a  through 6 e   . For example, a first conductive contact  1006  may be placed on the second gate  504 , and a second conductive contact  1006  may be placed on the base portion of the shank  510 . Specifically, the first and second conductive contacts  1006  should be placed at or near the point where the second gate  504  meets the base portion of the shank  510 , such that each of the arresting cable  310 &#39;s conductive wires  1002  may be electronically coupled with its respective conductive contact  1006 . To account for conductive contact  1006  placement, the non-conductive insulator  1004 &#39;s width may be increased or decreased to ensure sufficient contact between each conductive wire  1002  and an associated conductive contact  1006 . 
     Although the present invention has been described with respect to what are currently considered to be the preferred embodiments, the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     All U.S. and foreign patent documents, articles, brochures and other published documents discussed above are hereby incorporated by reference into the Detailed Description of the Preferred Embodiment.