Patent Publication Number: US-2022219832-A1

Title: Devices, systems and methods for refueling air vehicles

Description:
TECHNOLOGICAL FIELD 
     The presently disclosed subject matter relates to systems and methods for refueling air vehicles, especially aircraft, in particular for refueling aircraft during flight. 
     BACKGROUND 
     Airborne refueling (also referred to interchangeably herein as air refueling, in-flight refueling, air to air refueling (AAR), aerial refueling, tanking, and the like) refers to the process of transferring fuel from a tanker aircraft to a receiver aircraft during flight. 
     Two types of airborne refueling systems are currently in use for refueling airborne military aircraft:
         the so-called “hose and drogue” system, used by the US Navy and many non-US air forces;   the so-called “boom and receptacle” or “flying boom” system, used by the US Air Force, and also used by Israel, Turkey and the Netherlands.       

     In the hose and drogue system, the refueling aircraft is provided with one or more non-rigid refueling lines, at the end of each of which there is a drogue which functions as a stabilizer and includes a receptacle, while the receiver aircraft is fitted with a probe that is configured for engaging with the receptacle. In use, the drogue is not actively controlled, but rather aligns itself freely in the airflow aft of the tanker. The pilot of the receiver aircraft controls the flight path thereof to ensure engaging contact between the probe and the receptacle. Thereafter, the receiver aircraft is refueled via the refueling line and probe. 
     In the boom and receptacle system, the tanker includes a so-called “flying boom”, which is a rigid tube that telescopes outwardly and is gimbaled to the rear of the tanker aircraft, and is otherwise retracted into the tanker fuselage when not in use. The boom carries a fuel line and comprises a fuel transfer nozzle at the end thereof, and the boom is equipped with adjustable flight control surfaces. Once the tanker and receiver aircraft are in close proximity and flying in formation, with the receiver aircraft at a position behind the tanker within an air refueling envelope (i.e., safe limits of travel for the boom with respect to the receiver aircraft and within which contact between the receiving aircraft and the boom is safe), a dedicated operator in the tanker controls the position of the boom via the control surfaces, and inserts the end of the boom including the nozzle into a receptacle provided on an upper part of the receiving aircraft, ensuring proper mating between the nozzle and receptacle, after which fuel transfer can begin. During refueling, and while the boom is engaged with the receptacle, the pilot of the receiver aircraft must continue to fly within the air refueling envelope, and if the receiver aircraft approaches these limits the operator in the tanker requires the receiver aircraft pilot to correct the position thereof, and if necessary the boom is disconnected to prevent accidents. All current tankers of this type carry a single boom and can refuel a single receiver aircraft of this type at a time. 
     In addition, there are some tankers that comprise a flying boom system and at least one hose and drogue system as well, and are commonly known as Multi-Point Refueling Systems (MPRS). In some cases a hose and drogue system is provided at the aircraft tail, and thus only this system or the flying boom system may be used at any one time. In other cases, two under-wing hose and drogue pods, known as Wing Air Refueling Pods (WARPs), can be provided, one under each wing, in addition to the flying boom system. 
     U.S. Pat. No. 7,562,847 discloses an autonomous in-flight refueling hose end unit including a first end configured to be coupled to a fuel hose of a tanker aircraft. and a second end configured to be coupled to receiver aircraft and adjustable control surfaces, and a flight control computer autonomously controls the control surfaces to fly the refueling hose end into contact with the receiver aircraft. 
     In GB 2,237,251 an in flight refueling apparatus mountable on a tanker aircraft has a probe receptor coupled with a fuel line and is arranged to be deployed outboard of the aircraft, and can be provided on a drogue or a boom. In one mode, the apparatus is arranged to provide a parameter which is representative of the deviation of the path of the receptor from a predetermined initial path for actuating control means for changing automatically the position of the receptor relative to the initial path. In another mode, a parameter which is representative of the relative angular position of the receptor with respect to the probe of an approaching refueling aircraft for actuating control means for changing automatically the relative angular position to achieve alignment of receptor and probe. 
     Additional references considered to be relevant as background to the presently disclosed subject matter are listed below: US 2007/108339, US 2007/084968, US 2006/065785, US 2006/043241, US 2006/060710, US 2006/060709, US 2005/224657, US 2004/102876, U.S. Pat. Nos. 7,097,139, 6,966,525, 6,994,294, 6,644,594, 5,906,336, 5,785,276, 5,499,784, 5,326,052, 4,282,909, 4,126,162, 4,072,283, 3,948,626, 3,091,419, 3,059,895, 2,954,190, 2,582,609, U.S. Des. 439,876, DE 100 13 751. 
     Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter. 
     GENERAL DESCRIPTION 
     In accordance with an aspect of the presently disclosed subject matter, there is provided a variety of refueling devices, systems and methods for use in in-flight refueling. In at least one example one such device is towed by a tanker aircraft via a fuel hose at least during in-flight refueling, and has a boom member with a boom axis. The boom member enables fuel to be transferred from the fuel hose to a receiver aircraft along the boom axis during in-flight refueling. The device maintains a desired non-zero angular disposition between the boom axis and a forward direction at least when the refueling device is towed by the tanker aircraft in the forward direction via the fuel hose. 
     In accordance with an aspect of the presently disclosed subject matter, there is provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising automatically steering a refueling device to an engagement enabling position, including:
         (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the steering commands to the spatial control system;   whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The method can optionally further comprise one or more of the features (c1) to (c15), in any desired combination or permutation: 
     (c1) providing an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory for automatically engaging with the fuel receptacle. 
     (c2) wherein the boom member has a boom axis and wherein at least a final part of the predetermined trajectory is parallel to the boom axis. 
     (c3) determining an engagement area specification condition; repeatedly calculating maneuvering instructions for the receiver aircraft based on the spatial dispositions and an engagement area specification; and invoking the automatic steering in response to meeting the engagement area specification condition. 
     (c4) wherein the refueling device is connected to a tanker aircraft by a fuel hose, and further comprising providing the maneuvering instructions to at least one of a pilot of the receiver aircraft pilot or a pilot of the tanker aircraft. 
     (c5) wherein providing the maneuvering instructions comprises activating a signaling system, optionally mounted on the refueling device or the tanker aircraft. 
     (c6) activating a force generating arrangement in the refueling device for generating force in the direction of the fuel receptacle of the receiver aircraft in response to receiving an engagement command for enabling refueling. 
     (c7) wherein the determining a spatial disposition comprises acquiring an image of said receiver aircraft, comparing the image with a reference image depicting a desired spatial disposition of the refueling device with respect to a receiver aircraft, and determining, based on the comparing, the spatial disposition of the refueling device with respect to the receiver aircraft. 
     (c8) wherein the spatial control system characteristics are related to operation parameters of aero-dynamic control surfaces of the refueling device. 
     (c9) wherein the aero-dynamic control surfaces are one or more vanes. 
     (c10) wherein the spatial control system characteristics are related to operation parameters of reaction control thrusters associated with the refueling device and capable of steering the refueling device. 
     (c11) wherein the engagement area specification condition is a spatial disposition within a pre-determined volume with respect to the refueling device and wherein the pre-determined volume is optionally substantially in the shape of a cube or substantially in the shape of a sphere. 
     (c12) wherein the calculating steering commands comprises obtaining data of an initial trail position of the refueling device and wherein the steering commands are based also on the data of an initial trail position. 
     (c13) wherein the data of an initial trail position includes at least one of a pitch angle of the refueling device, a yaw angle of the refueling device, and a deployment length of a fuel hose connecting the refueling device to the tanker aircraft. 
     (c14) wherein the automatic steering and the automatic engaging are performed autonomously by the refueling device. 
     (c15) wherein said refueling device is non-aircraft-fixed. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) automatically steering a refueling device to an engagement enabling position, including:
           (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the steering commands to the spatial control system;   
           (b) providing an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c4) to (c15) detailed hereinabove and/or one or more of the features (d1) to (d2), in any desired combination or permutation: 
     (d1) invoking the automatic steering in response to a spatial disposition between the refueling device and the receiver aircraft meeting an engagement area specification condition. 
     (d2) repeatedly calculating maneuvering instructions for the receiver aircraft based on spatial dispositions and an engagement area specification, for establishing the spatial disposition between the refueling device and the receiver aircraft that meets the engagement area specification condition. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) repeatedly calculating maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met;   (b) in response to meeting the engagement area specification condition, automatically steering a refueling device to an engagement enabling position, including:
           (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the refueling device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the steering commands to the spatial control system;   
           (c) providing an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c4) to (c15) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a steering control module configured to automatically steer a refueling device to an engagement enabling position, including:
         (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the steering commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The system can optionally further comprise one or more of the features (c2) and/or (c8) to (c11) and/or (c13) and/or (c15) detailed hereinabove and/or one or more of the features (e1) to (e10), in any desired combination or permutation: 
     (e1) an engagement/disengagement module configured to provide an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory to automatically engage with the fuel receptacle. 
     (e2) a maneuvering instructions module configured to determine an engagement area specification condition, to repeatedly calculate maneuvering instructions for the receiver aircraft based on the spatial dispositions and an engagement area specification, and to invoke the steering control module to automatically steer the refueling device to the engagement enabling position in response to meeting the engagement area specification condition. 
     (e3) wherein the refueling device is connected to a tanker aircraft by a fuel hose, and wherein the maneuvering instructions module is further configured to provide the maneuvering instructions to at least one of a pilot of the receiver aircraft pilot or a pilot of the tanker aircraft. 
     (e4) wherein the maneuvering instructions module is configured to activate a signaling system in order to provide the maneuvering instructions, the signaling system is optionally mounted on the refueling device or the tanker aircraft. 
     (e5) wherein the engagement/disengagement module is further configured to activate a force generating arrangement in the refueling device for generating force in the direction of the fuel receptacle of the receiver aircraft in response to receiving an engagement command for enabling refueling. 
     (e6) wherein the steering control module is configured to perform the following steps in order to determine a spatial disposition: acquire an image of the receiver aircraft; compare the image with a reference image depicting a desired spatial disposition of the refueling device with respect to a receiver aircraft; determine, based on the comparing, the spatial disposition of the refueling device with respect to the receiver aircraft. 
     (e7) wherein the steering control module is further configured to obtain data of an initial trail position of the refueling device and wherein the calculate steering commands is based also on the obtained data of an initial trail position. 
     (e8) wherein at least the steering control module and the engagement/disengagement module are fitted within the refueling device for enabling autonomously controlling in-flight refueling of the receiver aircraft by the refueling device. 
     (e9) wherein at least the steering control module and the engagement/disengagement module are fitted within the receiver aircraft. 
     (e10) wherein at least the steering control module and the engagement/disengagement module are fitted within the tanker aircraft. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a steering control module configured to automatically steer a refueling device to an engagement enabling position, including:
         (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the steering commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c7) to (c11) and/or (c13) and/or (c15) and/or (e3) to (e5) and/or (e7) to (e10) detailed hereinabove and/or one or more of the features (f1) to (f2), in any desired combination or permutation: 
     (f1) a maneuvering instructions module configured to invoke the steering control module to automatically steer the refueling device to the engagement enabling position in response to meeting an engagement area specification condition. 
     (f2) wherein the maneuvering instructions module is further configured to repeatedly calculate maneuvering instructions for the receiver aircraft based on spatial dispositions and an engagement area specification, for establishing the spatial disposition between the refueling device and the receiver aircraft that meets the engagement area specification condition. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a maneuvering instructions module configured to repeatedly calculate maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met, and in response to meeting the engagement area specification condition, activate a steering control module; the steering control module is configured to automatically steer a refueling device to an engagement enabling position, including:
         (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate steering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the steering commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c7) to (c11) and/or (c13) and/or (c15) and/or (e3) to (e5) and/or (e7) to (e10) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising: 
     automatically maneuvering a refueling device to an engagement enabling position, including:
         (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the maneuvering commands to the spatial control system;   whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The method can optionally further comprise one or more of the features (c1) and/or (c2) and/or (c5) and/or (c7) and/or (c9) and/or (c11) detailed hereinabove and/or one or more of the features (g1) to (g12), in any desired combination or permutation: 
     (g1) wherein the refueling device is non-aircraft-fixed and wherein the maneuvering commands are steering commands for steering the refueling device in six degrees of freedom. 
     (g2) wherein the refueling device is aircraft fixed and wherein the maneuvering commands are alignment commands for aligning the refueling device in three degrees of freedom. 
     (g3) determining an engagement area specification condition; repeatedly calculating maneuvering instructions for the receiver aircraft based on the spatial dispositions and an engagement area specification; and invoking the automatically maneuvering in response to meeting the engagement area specification condition. 
     (g4) providing the maneuvering instructions to at least one of a pilot of the receiver aircraft or a pilot of a tanker aircraft. 
     (g5) wherein the refueling device is non-aircraft-fixed and wherein the method further comprising activating a force generating arrangement in the refueling device for generating force in the direction of the fuel receptacle of the receiver aircraft in response to receiving an engagement command for enabling refueling. 
     (g6) wherein the determining a spatial disposition comprises: acquiring an image of the receiver aircraft, the image comprising depth data and electromagnetic data; comparing the depth data and the electromagnetic data with look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft; determining, based on the comparing, the spatial disposition of the refueling device with respect to the receiver aircraft. 
     (g7) wherein the image is acquired by a Light Detection And Ranging (LIDAR) unit. 
     (g8) wherein the refueling device is non-aircraft-fixed and wherein the spatial control system characteristics are related to operation parameters of aero-dynamic control surfaces of the refueling device. 
     (g9) wherein the refueling device is non-aircraft-fixed and wherein the spatial control system characteristics are related to operation parameters of reaction control thrusters associated with the refueling device and capable of maneuvering the refueling device. 
     (g10) wherein the calculating maneuvering commands comprises obtaining data of an initial trail position of the refueling device and wherein the maneuvering commands are based also on the data of the initial trail position. 
     (g11) wherein the refueling device is non-aircraft-fixed and wherein the data of the initial trail position includes at least one of a pitch angle of the refueling device, a yaw angle of the refueling device, and a deployment length of a fuel hose. 
     (g12) wherein the automatically maneuvering and the automatically engaging are performed autonomously by the refueling device. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) automatically maneuvering a refueling device to an engagement enabling position, including:
           (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the maneuvering commands to the spatial control system;   
           (b) providing an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c5) and/or (c7) and/or (c9) and/or (c11) and/or (g1) to (g12) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) repeatedly calculating maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met;   (b) in response to meeting the engagement area specification condition, automatically maneuvering a refueling device to an engagement enabling position, including:
           (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the refueling device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the maneuvering commands to the spatial control system;   
           (c) providing an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c5) and/or (c7) and/or (c9) and/or (c11) and/or (g1) and/or (g2) and/or (g4) to (g12) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         a steering control module configured to automatically maneuver a refueling device to an engagement enabling position, including:
           (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the maneuvering commands to the spatial control system for automatically maneuvering the refueling device to the engagement enabling position;   
           whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The system can optionally further comprise one or more of the features (c2) and/or (c9) and/or (c11) and/or (e4) and/or (e6) and/or (e8) to (e10) and/or (g1) to (g2) and/or (g7) and/or (g9) and/or (g11) detailed hereinabove and/or one or more of the features (h1) to (h7), in any desired combination or permutation: 
     (h1) an engagement/disengagement module configured to provide an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory to automatically engage with the fuel receptacle. 
     (h2) wherein the boom member has a boom axis and wherein at least a final part of the predetermined trajectory is parallel to the boom axis. 
     (h3) a maneuvering instructions module configured to determine an engagement area specification condition, to repeatedly calculate maneuvering instructions for the receiver aircraft based on the spatial dispositions and an engagement area specification, and to invoke the steering control module to automatically maneuver the refueling device to the engagement enabling position in response to meeting the engagement area specification condition. 
     (h4) wherein the maneuvering instructions module is further configured to provide the maneuvering instructions to at least one of a pilot of the receiver aircraft or a pilot of a tanker aircraft. 
     (h5) wherein the refueling device is non-aircraft-fixed and wherein the engagement/disengagement module is further configured to activate a force generating arrangement in the refueling device for generating force in the direction of the fuel receptacle of the receiver aircraft in response to receiving an engagement command for enabling refueling. 
     (h6) wherein the steering control module is configured to perform the following steps in order to determine a spatial disposition: acquiring an image of the receiver aircraft, the image comprising depth data and electromagnetic data; comparing the depth data and the electromagnetic data with look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft; determining, based on the comparing, the spatial disposition of the refueling device with respect to the receiver aircraft. 
     (h7) wherein the steering control module is further configured to obtain data of an initial trail position of the refueling device and wherein the calculate maneuvering commands is based also on the obtained data of the initial trail position. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         a steering control module configured to automatically maneuver a refueling device to an engagement enabling position, including:
           (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the maneuvering commands to the spatial control system for automatically maneuvering the refueling device to the engagement enabling position;   
           the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c9) and/or (c11) and/or (e4) and/or (e6) and/or (e8) to (e10) and/or (g1) to (g2) and/or (g7) and/or (g9) and/or (g11) and/or (h2) to (h7) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         a maneuvering instructions module configured to repeatedly calculate maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met, and in response to meeting the engagement area specification condition, activate a steering control module;   the steering control module is configured to automatically maneuver a refueling device to an engagement enabling position, including:
           (i) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculate maneuvering commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) send the maneuvering commands to the spatial control system for automatically maneuvering the refueling device to the engagement enabling position;   
           the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c9) and/or (c11) and/or (e4) and/or (e6) and/or (e8) to (e10) and/or (g1) to (g2) and/or (g7) and/or (g9) and/or (g11) and/or (h2) and/or (h4) to (h7) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a non-aircraft-fixed refueling device for use in in-flight refueling operation between a tanker aircraft and a receiver aircraft, comprising:
         a selectively steerable body configured for being towed by the tanker aircraft via a fuel hose at least during in-flight refueling, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from the fuel hose to the receiver aircraft along the boom axis during the in-flight refueling operation;   a controller configured for selectively maneuvering the body to an engagement enabling position spaced with respect to the receiver aircraft and for aligning the boom axis in an engagement enabling orientation at the spaced position, and for subsequently moving the boom member along the boom axis towards the receiver aircraft for enabling fuel communication therebetween.       

     In accordance with an aspect of the presently disclosed subject matter, there is provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising automatically aligning a refueling device at an engagement enabling position, including:
         (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the alignment commands to the spatial control system;   whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The method can optionally further comprise one or more of the features (c1) to (c3) and/or (c5) and/or (c7) and/or (c11) detailed hereinabove and/or one or more of the features (i1) to (i6), in any desired combination or permutation:
         (i1) wherein the maneuvering commands are alignment commands for aligning said refueling device in three degrees of freedom.   (i2) providing the maneuvering instructions to at least one of a pilot of the receiver aircraft pilot or a pilot of the tanker aircraft.   (i3) determining an engagement area specification condition; repeatedly calculating maneuvering instructions for said receiver aircraft based on said spatial dispositions and an engagement area specification; and invoking said automatically aligning in response to meeting said engagement area specification condition.   (i4) wherein the calculating alignment commands comprises obtaining data of an initial trail position of the refueling device and wherein the alignment commands are based also on the data of an initial trail position.   (i5) wherein the automatic aligning and the automatic engaging are performed autonomously by the refueling device.   (i6) wherein said refueling device is aircraft fixed.       

     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) automatically aligning a refueling device at an engagement enabling position, including:   (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the alignment commands to the spatial control system;   (b) providing an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c5) and/or (c7) and/or (c11) and/or (i1) to (i6) detailed hereinabove and/or feature (l1), in any desired combination or permutation: 
     (l1) invoking the automatic aligning in response to a spatial disposition between the refueling device and the receiver aircraft meeting an engagement area specification condition. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a method for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising:
         (a) repeatedly calculating maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met;   (b) in response to meeting the engagement area specification condition, automatically aligning a refueling device at an engagement enabling position, including:   (i) repeatedly determining a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the refueling device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (ii) repeatedly calculating alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (iii) sending the alignment commands to the spatial control system;   (c) providing an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory for automatically engaging with the fuel receptacle.       

     The method can optionally further comprise one or more of the features (c2) and/or (c5) and/or (c7) and/or (c11) and/or (i1) to (i2) and/or (i4) to (i6) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a steering control module configured to automatically align a refueling device at an engagement enabling position, including:
         (iv) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (v) repeatedly calculate alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (vi) send the alignment commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   whereby at the engagement enabling position, the boom member of the refueling device is capable of engaging with the fuel receptacle to enable refueling of the receiver aircraft.       

     The system can optionally further comprise one or more of the features (c2) and/or (c11) and/or (i1) to (i2) and/or (i4) to (i6) detailed hereinabove and/or feature (o1), in any desired combination or permutation: 
     (o1) a maneuvering instructions module configured to determine an engagement area specification condition, to repeatedly calculate maneuvering instructions for the receiver aircraft based on the spatial dispositions and an engagement area specification, and to invoke the steering control module to automatically align the refueling device at the engagement enabling position in response to meeting the engagement area specification condition. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a steering control module configured to automatically align a refueling device at an engagement enabling position, including:
         (iv) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (v) repeatedly calculate alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (vi) send the alignment commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, when it arrives at the engagement enabling position, for causing the refueling device to move the boom member along a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c7) and/or (c11) and/or (e4) to (e5) and/or (e8) to (e10) and/or (f2) and/or (i1) to (i2) and/or (i4) to (i6) detailed hereinabove and/or one or more of the features (p1) to (p2), in any desired combination or permutation: 
     (p1) wherein the steering control module is further configured to obtain data of an initial trail position of the refueling device and wherein the calculate alignment commands is based also on the obtained data of an initial trail position. 
     (p2) a maneuvering instructions module configured to invoke the steering control module to automatically align the refueling device to the engagement enabling position in response to meeting an engagement area specification condition. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a system for controlling in-flight refueling of a receiver aircraft having a fuel receptacle, comprising a maneuvering instructions module configured to repeatedly calculate maneuvering instructions for the receiver aircraft based on spatial dispositions of the receiver aircraft and an engagement area specification until an engagement area specification condition is met, and in response to meeting the engagement area specification condition, activate a steering control module; the steering control module is configured to automatically align a refueling device at an engagement enabling position, including:
         (iv) repeatedly determine a spatial disposition of the refueling device with respect to the receiver aircraft, the refueling device being capable of engaging and refueling the receiver aircraft via a boom member, when the device arrives to the engagement enabling position at which the boom member is in a predetermined spaced and spatial relationship with respect to the fuel receptacle of the receiver aircraft;   (v) repeatedly calculate alignment commands based at least on the repeatedly determined spatial dispositions and characteristics of a spatial control system of the refueling device;   (vi) send the alignment commands to the spatial control system for automatically steering the refueling device to the engagement enabling position;   the system further comprises an engagement/disengagement module configured to provide an instruction to the refueling device, in response to its arriving at the engagement enabling position, causing the refueling device to move the boom member in a predetermined trajectory to automatically engage with the fuel receptacle.       

     The system can optionally further comprise one or more of the features (c2) and/or (c7) and/or (c11) and/or (e4) to (e5) and/or (e8) to (e10) and/or (i1) to (i2) and/or (i4) to (i6) and/or (p1) detailed hereinabove, in any desired combination or permutation. 
     In accordance with an aspect of the presently disclosed subject matter, there is yet further provided a refueling device for use in in-flight refueling operation between a tanker aircraft and a receiver aircraft, comprising a selectively steerable body configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from the fuel hose to a receiver aircraft along the boom axis during the in-flight refueling operation; a controller configured for selectively steering the body to an engagement enabling position spaced with respect to the receiver aircraft and for aligning the boom axis in an engagement enabling orientation at the spaced position, and for subsequently moving the boom member along the boom axis towards the receiver aircraft for enabling fuel communication therebetween. 
     According to at least one aspect of the presently disclosed subject matter, there is provided a refueling device for use in in-flight refueling operation between a tanker aircraft and a receiver aircraft, comprising: 
     a selectively steerable body configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from said fuel hose to a receiver aircraft along said boom axis during said in-flight refueling operation; 
     a controller configured for selectively steering the body to an engagement enabling position spaced with respect to the receiver aircraft and for aligning said boom axis in an engagement enabling orientation at said spaced position, and for subsequently moving the boom member along said boom axis towards the receiver aircraft for enabling fuel communication therebetween. 
     For example, moving the boom member along said boom axis towards the receiver aircraft for enabling fuel communication therebetween can be achieved by any one of the following, for example:
         by moving the body towards the fuel receptacle of the receiver aircraft along the direction of the boom axis,   by telescopically extending the boom member towards the towards the fuel receptacle of the receiver aircraft along said boom axis while the body is maintained at the engagement enabling position,   partially by moving the body towards the fuel receptacle of the receiver aircraft along the direction of the boom axis, and partially by telescopically extending the boom member towards the towards the fuel receptacle of the receiver aircraft along said boom axis while the body is maintained at the engagement enabling position.       

     The refueling device according to this aspect of the presently disclosed subject matter can optionally comprise a spatial control system configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and said forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose. 
     Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (A) to (S) below, additionally or alternatively including one or more of features (j1) to (j6) below, additionally or alternatively including one or more of features (k1) to (k15) below, additionally or alternatively including one or more of features M1 and or M2 and/or (m1) to (m4) below, additionally or alternatively including one or more of features (n1) to (n4) below, additionally or alternatively including one or more of features (q1) to (q6) below, mutatis mutandis, in any desired combination or permutation. 
     Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise a force generating arrangement configured for selectively generating a force along said boom axis in a direction generally away from said fuel hose, i.e., towards the fuel delivery end of the boom member. Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (AA) to (LL) below, mutatis mutandis, in any desired combination or permutation. 
     Additionally or alternatively to the above features, the body according to this aspect of the presently disclosed subject matter can optionally comprise a fuel delivery lumen configured for fluid communication with said fuel hose at least during the in-flight refueling operation, said lumen being configured to enable fuel to be transferred from the fuel hose to a receiver aircraft during said in-flight refueling operation, and the fuel delivery device comprises a coupling having a hose interface configured for connecting said lumen to the fuel hose, said coupling configured for allowing relative rotation between the hose and said body in at least one degree of freedom while maintaining said fuel communication. Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (AAA) to (LLL) below, mutatis mutandis, in any desired combination or permutation. 
     According to at least one aspect of the presently disclosed subject matter, there is provided a refueling device for use in in-flight refueling operation, comprising:
         a body having a longitudinal axis and configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling operation, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from said fuel hose to a receiver aircraft along said axis during said in-flight refueling operation;   said boom member being pivotable with respect to said body, between a retracted position and a deployed position, wherein in said retracted position said boom axis is at a smaller angular disposition with respect to said longitudinal axis than in said deployed position   spatial control system including two sets of longitudinally spaced control surfaces configured for enabling selectively steering said refueling device while concurrently selectively maintaining a desired non-zero angular disposition between said boom axis and said longitudinal axis at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose.       

     The refueling device according to this aspect of the presently disclosed subject matter can optionally comprise a spatial control system configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and said forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose. 
     Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (A) to (S) below, additionally or alternatively including one or more of features (j1) to (j6) below, additionally or alternatively including one or more of features (k1) to (k15) below, additionally or alternatively including one or more of features M1 and or M2 and/or (m1) to (m4) below, additionally or alternatively including one or more of features (n1) to (n4) below, additionally or alternatively including one or more of features (q1) to (q6) below, mutatis mutandis, in any desired combination or permutation. 
     Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise a force generating arrangement configured for selectively generating a force along said boom axis in a direction generally away from said fuel hose, i.e., towards the fuel delivery end of the boom member. Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (AA) to (LL) below, mutatis mutandis, in any desired combination or permutation. 
     Additionally or alternatively to the above features, the body according to this aspect of the presently disclosed subject matter can optionally comprise a fuel delivery lumen configured for fluid communication with said fuel hose at least during the in-flight refueling operation, said lumen being configured to enable fuel to be transferred from the fuel hose to a receiver aircraft during said in-flight refueling operation, and the fuel delivery device comprises a coupling having a hose interface configured for connecting said lumen to the fuel hose, said coupling configured for allowing relative rotation between the hose and said body in at least one degree of freedom while maintaining said fuel communication. Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (AAA) to (LLL) below, mutatis mutandis, in any desired combination or permutation. 
     According to at least one aspect of the presently disclosed subject matter, the refueling device comprises:
         a) a body configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling operation, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from said fuel hose to a receiver aircraft along said axis during said in-flight refueling operation;   (b) spatial control system configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and a forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose.       

     The above refueling device can optionally comprise one or more of the following features, in any desired combination or permutation:
         A. A controller configured for selectively steering the body to an engagement enabling position spaced with respect to the receiver aircraft and for aligning said boom axis in an engagement enabling orientation at said spaced position, and for subsequently moving the boom member along said boom axis towards the receiver aircraft for enabling fuel communication therebetween.   B. The boom member comprises a nozzle at a terminus thereof in fluid communication with a fuel delivery lumen comprised in said body, said nozzle being configured for reversible engagement with a fuel receptacle of a receiver aircraft.   C. The fuel hose is substantially non-rigid and/or said body is selectively steerable.   D. The desired non-zero angular disposition is selectively controllable and/or said angular disposition is defined on a vertical plane.   E. The spatial control system configured for at least maintaining a selectively controllable non-zero angular disposition between said boom axis and a datum direction (different from said boom axis); the datum direction can be a forward direction of the body, i.e., direction of motion of the body when towed via the hose; the said angular disposition can be or comprise an angle of attack of said boom axis with respect to said forward direction.   F. The said angular disposition is such as to ensure that the boom axis is at a predetermined design angle with respect to the receiver aircraft, in particular with respect to a longitudinal axis of the receiver aircraft; the design angle is such as to ensure proper alignment and engagement between the nozzle and the fuel receptacle; for example, the design angle may be between about 25° and about 35°, for example about 30°.   G. The said angular disposition is defined about a pitch axis of said refueling device. Additionally or alternatively, said angular disposition is defined about at least one of a yaw axis and a roll axis of said refueling device. Additionally or alternatively, the said angular disposition is in a range between about 5° and about 85°; preferably between about 10° and about 80°; more preferably between about 15° and about 70°; more preferably between about 20° and about 60°; more preferably between about 25° and about 50°; more preferably between about 20° and about 40°; more preferably between about 25° and about 40°; more preferably between about 28° and about 32°; or said angular disposition is about 30°.   H. Wherein said refueling device is configured for maintaining said desired non-zero angular disposition between said boom axis and said forward direction at least prior to engagement of said nozzle with a fuel receptacle of a receiver aircraft that flying in formation aft of the tanker aircraft.   I. Wherein said spatial control system is further configured for selectively providing control moments in at least one of pitch, yaw and roll wherein to enable the refueling device to be flown while towed by the tanker aircraft in said forward direction via said fuel hose.   J. Wherein said device can optionally comprise one or more of the following features, in any desired combination or permutation:
           (j1) wherein said body is elongate having a longitudinal axis generally aligned with said boom axis.   (j2) wherein said body comprises a longitudinal axis, and said boom member is pivotably mounted with respect to said body, and pivotable between a retracted position and a deployed position, wherein in said retracted position said boom axis is at a smaller angular disposition with respect to said longitudinal axis than in said deployed position.   (j3) wherein in said retracted position said boom axis is at an angular disposition with respect to said longitudinal axis of between 0° and 15°, and wherein in said deployed position, said angular disposition is greater than 15°.   
           (j4) wherein in said deployed position said boom axis is at an angular disposition with respect to said longitudinal axis between 20° to 40°.
           (j5) wherein said boom member is in said deployed position during said in-flight refueling operation.   (j6) wherein said body comprises a longitudinal axis, and said boom member is mounted with respect to said body (for example fixedly mounted, or non-pivotably mounted, or mounted for relative translation therebetween), such as to maintain a generally parallel spatial disposition between said boom axis and said longitudinal axis at least during said in-flight refueling operation.   
           K. Wherein said spatial control system comprises selectively controllable aerodynamic control system. The selectively controllable aerodynamic control system can optionally comprise one or more of the following features, in any desired combination or permutation:
           (k1) wherein said selectively controllable aerodynamic control system comprises a forward set of aerodynamic control surfaces mounted to said body, and an aft set of aerodynamic control surfaces mounted to said body in longitudinally aft spaced relationship with respect to said forward set of aerodynamic control surfaces.   (k2) a center of gravity of said body is disposed in longitudinally intermediate said forward set of aerodynamic control surfaces and said aft set of aerodynamic control surfaces.   (k3) wherein said aft set of aerodynamic control surfaces comprises at least two said control surfaces mounted to said body in Vee configuration; or wherein said aft set of aerodynamic control surfaces comprises a high H-tail configuration, comprising two vertical stabilizers, one each on either side of a horizontal stabilizer—the H-tail configuration can be mounted to the upper side of the body, and optionally: each vertical stabilizer comprises a controllably pivotable rudder, and/or the horizontal stabilizer comprises one, two or more pivotable elevators, which optionally are controllably actuated by an actuator system for example controlled by a controller.   (k4) wherein said aft set of aerodynamic control surfaces further comprises at least one said control surfaces mounted to said body in vertical configuration.   (k5) wherein said forward set of aerodynamic control surfaces comprises at least two said control surfaces mounted to said body in Vee configuration.   (k6) wherein said forward set of aerodynamic control surfaces comprises at least four said control surfaces mounted to said body in cruciform configuration, for example cruciform “X” configuration or cruciform “+” configuration.   (k7) wherein at least one said control surface is pivotably mounted to said body via a respective boss laterally projecting from an outer surface of said body.   (k8) wherein each said boss houses an actuator configured for actuating the respective control surface.   (k9) wherein each said boss comprises an aerofoil shaped cross-sectional shape having a respective chord.   (k10) wherein each said chord is angularly displaced from said boom axis such as to become generally aligned with said forward direction when said boom axis is at said non-zero angular disposition with respect to said forward direction.   (k11) wherein said forward set of aerodynamic control surfaces comprises a canard configuration, and said aft set of aerodynamic control surfaces comprises one or more wing elements.   (k12) wherein said aft set of aerodynamic control surfaces comprises an H-tail configuration, comprising two vertical stabilizers, one each on either side of a horizontal stabilizer; and/or wherein said forward set of aerodynamic control surfaces comprises at least four said control surfaces mounted to said body in cruciform configuration.   (k13) wherein said spatial control system is configured for enabling the refueling device to be steered in one, or two, or three degrees of freedom in translation, and in one, or two, or three degrees of freedom in rotation, independently of the tanker aircraft or of the refueling aircraft.   (k14) wherein said spatial control system is configured for providing at least one of:
               one or more of: sideslip, up/down translation, forward-aft translation, relative to the tanker aircraft and/or to the refueling aircraft, independently of rotational moments in roll pitch and/or yaw;   rotational moments in one or more of roll pitch and/or yaw, relative to the tanker aircraft and/or to the refueling aircraft, independently of sideslip, up/down translation, forward-aft translation.   
               (k15) wherein said spatial control system is configured for providing an angle of attack for the body with respect to the forward direction, between −10° and +10°.   L. Wherein said spatial control system comprises a thrust vector system.   
           M. A force generating arrangement including one or more of:
           M1—a force generating arrangement configured for selectively generating a force along said boom axis in an aft direction, i.e., a direction towards said nozzle. The force generating arrangement can optionally comprise one or more of the following features, in any desired combination or permutation:
               (m1) wherein said force generating arrangement comprises a selectively deployable and/or actuable drag inducing arrangement.   (m2) wherein said force generating arrangement comprises a selectively deployable and/or actuable air brake arrangement.   (m3) wherein said air brake arrangement comprises a plurality of airbrakes laterally mounted to at least one of said body and said boom member.   (m4) wherein said force generating arrangement is configured for selectively generating said force along said boom axis in a direction towards said nozzle responsive to said nozzle being in predetermined proximity to the fuel receptacle of the receiver aircraft whereby to force said nozzle into engagement with the fuel receptacle.   
               M2—an aerodynamic stabilizer arrangement, different from the spatial control system, for example wherein said aerodynamic stabilizer arrangement is in the form of a drogue structure having a stowed configuration, in which drogue structure generates a minimum drag, and a deployed configuration in drogue structure generates greater drag than in the inactive configuration.   
           N. Said body comprises a fuel delivery lumen configured for fluid communication with said fuel hose and said boom member at least during the in-flight refueling operation, and said body comprises a coupling having a hose interface configured for connecting said lumen to the fuel hose, said coupling configured for allowing relative rotation between the hose and said body in at least one degree of freedom while maintaining said fuel communication. The coupling can optionally comprise one or more of the following features, in any desired combination or permutation:
           (n1) wherein said coupling configured for allowing relative rotation between the hose and said body in at least two degrees of freedom.   (n2) wherein said coupling configured for allowing relative rotation between the hose and said body in three degrees of freedom.   (n3) wherein at least one said rotational degree of freedom has the respective axis of rotation generally orthogonal to a plane defining said non-zero angular disposition between said boom axis and said forward direction.   (n4) wherein said coupling comprises a universal coupling.   
           O. Wherein said boom member is selectively reversibly telescopically deployable along said boom axis with respect to said body or wherein said boom member is not reversibly telescopically deployable along said boom axis with respect to said body.   P. Wherein said boom member is pivotably mounted to said body.   Q. A data acquisition system configured for providing spatial data relating to a relative spatial disposition between a fuel delivery nozzle of the refueling device and a fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic and/or autonomous and/or manual engagement of the fuel delivery nozzle to the fuel receptacle of the receiver aircraft. In at least one example, the system comprises an imaging system for providing said data including at least image data corresponding to a field of regard with respect to the refueling device. The data acquisition system can be in the form of an imaging system, and can optionally comprise one or more of the following features, in any desired combination or permutation:   (q1) wherein said imaging system is configured for providing at least one of 2D images, stereoscopic images, and 3D images of a volume defined by said field of regard.   (q2) wherein said imaging system is configured for providing said image data in real time.   (q3) wherein said imaging system comprises or is operatively connected to a computing system configured for identifying a fuel receptacle of a receiver aircraft within said field of regard from said image data, and for determining a spatial disposition of said nozzle with respect to the fuel receptacle.   (q4) wherein said imaging system comprises a first set of electromagnetic energy modules configured for illuminating said field of regard with electromagnetic energy (for example laser energy), and a second set of electromagnetic energy modules configured for receiving electromagnetic energy from said illuminated field of regard.   (q5) wherein said imaging system comprises a first set of electromagnetic energy modules configured for transmitting electromagnetic energy in a direction generally along said boom axis and generally opposed to said forward direction, and a second set of electromagnetic energy modules configured for receiving electromagnetic energy from a direction generally along said boom axis and generally along said forward direction.   (q6) wherein said imaging system comprises at least one flash ladar unit and/or at least one LIDAR unit.   R. The controller can comprise, for example, a computer system, operatively connected to said spatial data acquisition system and/or to said spatial control system, and/or optionally configured as an automatic or autonomous system for enabling refueling device to be steered to an engagement enabling position to provide engagement of the nozzle with the fuel receptacle of the receiver aircraft, and thereafter enable refueling of the receiver aircraft.   S. A suitable communication system to transmit image data and to receive control commands/signals. For example, the communications system can be operatively connected to the controller for controlling operation of the refueling device.       

     According to at least one aspect of the presently disclosed subject matter, the refueling device comprises: 
     a body configured for connection to a tanker aircraft via a fuel hose at least during in-flight refueling operation thereof while said body is in towed configuration with respect to the tanker aircraft via said fuel hose, and further comprising a substantially rigid boom member having a boom axis and configured to enable fuel to be transferred from the tanker aircraft to a receiver aircraft during said in-flight refueling operation; 
     spatial control system configured for selectively maintaining a desired non-zero angular disposition between said boom axis and a datum direction. 
     The refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of the following features, in any desired combination or permutation:
         Wherein said datum direction is parallel to a longitudinal axis of the receiver aircraft.   The desired non-zero angular disposition is selectively controllable.   The datum direction is different, i.e. non-parallel, from said boom axis.   The datum direction can be parallel to a forward direction of the body, i.e., direction of motion of the body when towed via the hose.   The spatial control system is also configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and said forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose.   The boom member comprises a nozzle at a terminus thereof in fluid communication with a fuel delivery lumen comprised in said body, said nozzle being configured for reversible engagement with a fuel receptacle of a receiver aircraft.   Additionally or alternatively to the above features, the refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (A) to (S), additionally or alternatively including one or more of features (j1) to (j6), additionally or alternatively including one or more of features (k1) to (k15), additionally or alternatively including one or more of features M1 and/or m2 and/or (m1) to (m4), additionally or alternatively including one or more of features (n1) to (n4), additionally or alternatively including one or more of features (q1) to (q6), mutatis mutandis, in any desired combination or permutation.       

     According to at least one other aspect of the presently disclosed subject matter, the refueling device comprises:
         (aa) a body configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling operation, and comprising a boom member having a boom axis and configured to enable fuel to be transferred from said fuel hose to a receiver aircraft along said axis during said in-flight refueling operation;   (bb) a force generating arrangement configured for selectively generating a force along said boom axis in a direction generally away from said fuel hose.       

     A fuel delivery nozzle is comprised at a terminus of the boom member and is in fluid communication with a fuel delivery lumen comprised in said body, the lumen configured for fluid communication with said fuel hose and said fuel member at least during in flight refueling operation, said nozzle being configured for reversible engagement with a fuel receptacle of a receiver aircraft. 
     The refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of the following features, in any desired combination or permutation:
         (AA) Wherein said force generating arrangement comprises a selectively deployable drag inducing arrangement.   (BB) Wherein said force generating arrangement comprises a selectively deployable air brake arrangement.   (CC) Wherein said air brake arrangement comprises a plurality of airbrakes laterally mounted to at least one of said body and said boom member.   (DD) Wherein said force generating arrangement is configured for selectively generating a force along said boom axis in a direction towards said nozzle responsive to said nozzle being in predetermined proximity to the fuel receptacle of the receiver aircraft wherein to force said nozzle into engagement with the fuel receptacle.   (EE) Wherein the fuel hose is substantially non-rigid and/or wherein said body is selectively steerable.   (FF) Wherein said boom member is selectively reversibly telescopically deployable along said boom axis with respect to said body.   (GG) Wherein said boom member is pivotably mounted to said body.   (HH) a controller configured for selectively steering the body to an engagement enabling position spaced with respect to the receiver aircraft and for aligning said boom axis in an engagement enabling orientation at said spaced position, and for subsequently moving the boom member along said boom axis towards the receiver aircraft for enabling fuel communication therebetween.   (II). A suitable communication system to transmit image data and to receive control commands/signals. For example, the communications system can be operatively connected to the controller for controlling operation of the refueling device.   (JJ) A spatial control system configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and said forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose, and/or configured for at least providing directional stability at least during deployment of drag generating system, the spatial control system being different from said force generating arrangement. The spatial control system according to at least this aspect of the presently disclosed subject matter can optionally comprise one or more of features (B) to (L), optionally including one or more of features (kl) to k( 15 ), mutatis mutandis, in any desired combination or permutation.   (KK) A coupling having a hose interface configured for connecting said lumen to the fuel hose, said coupling configured for allowing relative rotation between the hose and said body in at least one degree of freedom while maintaining said fuel communication. The coupling according to at least this aspect of the presently disclosed subject matter can optionally comprise one or more of features (n1) to (n4), mutatis mutandis, in any desired combination or permutation.   (LL) A data acquisition system configured for providing spatial data relating to a relative spatial disposition between said fuel delivery nozzle and a fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic or autonomous or manual engagement of the fuel delivery end to the fuel receptacle, said system optionally comprising an imaging system configured for providing said data including image data corresponding to a field of regard aft of the refueling device, and wherein optionally said imaging system comprises or is operatively connected to a computing system configured for identifying a fuel receptacle of a receiver aircraft within said field of regard from said image data, and for determining a spatial disposition of said fuel delivery nozzle with respect to the fuel receptacle. The data acquisition system can be in the form of an imaging system, and can optionally comprise one or more of features (q1) to (q6), mutatis mutandis, in any desired combination or permutation.       

     According to at least one other aspect of the presently disclosed subject matter, the refueling device comprises:
         (aaa) a body configured for being towed by a tanker aircraft via a fuel hose at least during in-flight refueling operation, and comprising a fuel delivery lumen configured for fluid communication with said fuel hose at least during the in-flight refueling operation, said lumen being configured to enable fuel to be transferred from the fuel hose to a receiver aircraft during said in-flight refueling operation;   (bbb) a coupling having a hose interface configured for connecting said lumen to the fuel hose, said coupling configured for allowing relative rotation between the hose and said body in at least one degree of freedom while maintaining said fuel communication.       

     The refueling device according to this aspect of the presently disclosed subject matter can optionally comprise one or more of the following features, in any desired combination or permutation:
         (AAA) Wherein said coupling is configured for allowing relative rotation between the hose and said body in at least two degrees of freedom.   (BBB) Wherein said coupling is configured for allowing relative rotation between the hose and said body in three degrees of freedom.   (CCC) Wherein said body comprises a boom member having a boom axis, and wherein said lumen is configured to enable fuel to be transferred from the fuel hose to a receiver aircraft via said boom member during said in-flight refueling operation, and/or at least one said degree of freedom has the respective axis of rotation generally orthogonal to a plane defining said non-zero angular disposition between said boom axis and said forward direction.   (DDD) Wherein said coupling comprises a universal coupling.   (EEE) Wherein the fuel hose is substantially non-rigid.   (FFF) Wherein said boom member is selectively reversibly telescopically deployable along said boom axis with respect to said body.   (GGG) Wherein said boom member is pivotably mounted to said body.   (HHH) A data acquisition system configured for providing spatial data relating to a relative spatial disposition between a fuel delivery end of said boom member and a fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic or autonomous or manual engagement of the fuel delivery end to the fuel receptacle, said system optionally comprising an imaging system configured for providing said data including image data corresponding to a field of regard aft of the refueling device, and wherein optionally said imaging system comprises or is operatively connected to a computing system configured for identifying a fuel receptacle of a receiver aircraft within said field of regard from said image data, and for determining a spatial disposition of said fuel delivery end with respect to the fuel receptacle.   (III) A suitable communication system to transmit image data and to receive control commands/signals. For example, the communications system can be operatively connected to the controller for controlling operation of the refueling device.   (JJJ) A spatial control system configured for selectively ensuring maintaining a desired non-zero angular disposition between said boom axis and said forward direction at least when said refueling device is towed by the tanker aircraft in said forward direction via said fuel hose, and/or, configured for at least providing directional stability. The spatial control system according to at least this aspect of the presently disclosed subject matter can optionally comprise one or more of features (B) to (L), optionally including one or more of features (k1) to k(15), mutatis mutandis, in any desired combination or permutation.   (KKK) A force generating arrangement configured for selectively generating a force along said boom axis in a direction towards said nozzle. The force generating arrangement according to at least this aspect of the presently disclosed subject matter can optionally comprise one or more of features M1 and/or M2 and/or (m1) to (m4), mutatis mutandis, in any desired combination or permutation.   (LLL) A data acquisition system configured for providing spatial data relating to a relative spatial disposition between a fuel delivery nozzle of the refueling device and a fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic and/or autonomous and/or manual engagement of the fuel delivery nozzle to the fuel receptacle of the receiver aircraft. In at least one example, the system comprises an imaging system for providing said data including at least image data corresponding to a field of regard with respect to the refueling device. The data acquisition system can be in the form of an imaging system, and can optionally comprise one or more of features (q1) to (q6), mutatis mutandis, in any desired combination or permutation.       

     According to at least one other aspect of the presently disclosed subject matter, there is provided a refueling system comprising a refueling fuel reservoir connected to a refueling device via a hose, the refueling device being as defined in the examples herein, in particular as defined above and optionally including one or more of the features listed above in A to S, AA to LL, and AAA to LLL, in any desired combination or permutation. Optionally, the refueling system can be housed in a suitable pod configured to be fixedly attached to a tanker aircraft. 
     According to at least one other aspect of the presently disclosed subject matter, there is provided a tanker aircraft comprising at least one refueling system as defined herein, for example comprising one refueling system as defined herein, or comprising two refueling systems as defined herein, or comprising three refueling systems as defined herein, or comprising more four or more refueling systems as defined herein. 
     According to the tanker aircraft may be a manned tanker aircraft or a UAV, and/or at least one receiver aircraft may be a manned aircraft or a UAV. 
     Optionally, a tanker aircraft according to the presently disclosed subject matter can comprise one or two such refueling systems mounted to the wings (e.g. via pods) and additionally comprise one conventional “flying boom” system in the aft fuselage. Thus, it is readily apparent that existing tanker aircraft already fitted with conventional “flying boom” systems can be retrofitted with refueling systems according to the first aspect of the presently disclosed subject matter, for example one such refueling system fitted onto each wing, thereby effectively tripling the refueling efficiency/capability of such a tanker aircraft, enabling up to three receiver aircraft having fuel receptacles to be refueled concurrently. 
     Optionally, a tanker aircraft according to the presently disclosed subject matter can comprise one or more such refueling systems, as well as at least one conventional “hose and drogue” system, enabling receiver aircraft of both types to be refueled concurrently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the presently disclosed subject matter and to see how it may be carried out in practice, examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a top view of an example of a tanker system according to the presently disclosed subject matter. 
         FIG. 2  is a side view of the tanker system of  FIG. 1 . 
         FIG. 3  is an isometric view of an example of a refueling device according to the presently disclosed subject matter. 
         FIG. 4  is a side view of the refueling device of  FIG. 3 . 
         FIG. 5  is a top view of the refueling device of  FIG. 3 . 
         FIG. 6( a )  is a front view of the refueling device of  FIG. 3 ;  FIG. 6( b )  is an aft view of the refueling device of  FIG. 3 . 
         FIG. 7  is a cross-sectional side view of the refueling device of  FIG. 5  taken along B′-B′. 
         FIG. 8  is a cross-sectional side view of the refueling device of  FIG. 4  taken along A′-A′. 
         FIG. 9( a )  is an isometric view of the refueling device of  FIG. 3 , with the airbrakes and boom member in the deployed positions;  FIG. 9( b )  is a top view of the refueling device of  FIG. 9( a ) . 
         FIG. 10( a )  is a partial side view of the example of the boom member of the refueling device of  FIG. 3  in proximity to a fuel receptacle of a receiver aircraft;  FIG. 10( b )  is an alternative variation of the example of  FIG. 10( a ) . 
         FIG. 11  is an isometric view of the refueling device of  FIG. 3 , further schematically illustrating a volume aft thereof. 
         FIGS. 12( a ) to 12( d )  are respective isometric, side, top and front views of an alternative variation of the example of refueling device of  FIG. 3 . 
         FIGS. 13( a ) to 13( e )  are respective isometric views of other alternative variations of the example of refueling device of  FIG. 3 . 
         FIGS. 14( a ) and 14( b )  are respective isometric views of other alternative variations of the example of refueling device of  FIG. 3 . 
         FIGS. 15( a ) to 15( d )  illustrate another alternative variation of the example of refueling device of  FIG. 3 , in isometric view, top view, side view and aft view, respectively. 
         FIGS. 16( a ) to 16( d )  illustrate another example of a refueling device according to the presently disclosed subject matter, in isometric view, side view, top view and front view, respectively. 
         FIGS. 17( a ) to 17( e )  illustrate an alternative variation of the example of the refueling device  FIGS. 16( a ) to 16( d ) , in isometric view (stowed configuration), isometric view (deployed configuration), side view (deployed configuration), front view (deployed configuration), and top view (deployed configuration), respectively;  FIGS. 17( f ) to 17( g )  illustrate an alternative variation of the example of the refueling device  FIGS. 16( a ) to 16( d ) , in isometric view, in stowed configuration and in deployed configuration, respectively;  FIG. 17( h )  illustrates another alternative variation of the example of the refueling device  FIGS. 16( a ) to 16( d ) , in isometric view, in deployed configuration. 
         FIG. 18  is a block diagram schematically illustrating a system for controlling in-flight refueling, according to certain examples of the presently disclosed subject matter; 
         FIG. 19  is a flowchart illustrating a sequence of operations carried out for performing in-flight refueling, according to certain examples of the presently disclosed subject matter; 
         FIG. 20  is a flowchart illustrating a sequence of operations carried out for providing maneuvering commands for positioning a receiver aircraft within an engagement area related thereto, according to certain examples of the presently disclosed subject matter; 
         FIG. 21  is a flowchart illustrating a sequence of operations carried out for providing steering commands to a refueling device for maneuvering to an engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 22  is a flowchart illustrating a sequence of operations carried out for determining the receiver aircraft spatial disposition with respect to the engagement area related thereto, according to certain examples of the presently disclosed subject matter; 
         FIG. 23  is a flowchart illustrating a sequence of operations carried out for determining the refueling device spatial disposition with respect to the engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 24  is an illustration of an example of a receiver aircraft positioned outside a virtual engagement area, according to certain examples of the presently disclosed subject matter; 
         FIG. 25  is an illustration of an example of a receiver aircraft positioned inside a virtual engagement area, according to certain examples of the presently disclosed subject matter; 
         FIG. 26  is an illustration of an example of a refueling device not in an engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 27  is an illustration of an example of a refueling device positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 28  is an illustration of an example of a sensed image indicating that the refueling device is not positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 29  is an illustration of an example of a sensed image indicating that the refueling device is positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter; 
         FIG. 30  is a partial side view of another example of a tanker system according to certain examples of the presently disclosed subject matter; 
         FIG. 31  is a schematic illustration of an image acquisition system according to certain examples of the presently disclosed subject matter; 
         FIG. 32  is a schematic illustration of a scene sensed the LIDAR unit according to certain examples of the presently disclosed subject matter; 
         FIG. 33  is a schematic representation of the depth and electromagnetic data relating to the boom refueling device and to the fuel receptacle of the receiver aircraft as acquired by the LIDAR unit according to certain examples of the presently disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In the drawings and descriptions set forth, identical reference numerals indicate those components that are common to different embodiments or configurations. 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “steering”, “determining”, “calculating”, “providing”, “causing”, “activating”, “receiving”, “acquiring”, “comparing”, “obtaining”, or the like, include action and/or processes of a computer that manipulate and/or transform data into other data, said data represented as physical quantities, e.g. such as electronic quantities, and/or said data representing the physical objects. The term “computer” should be expansively construed to cover any kind of electronic device with data processing capabilities, including, by way of non-limiting example, a personal computer, a server, a computing system, a communication device, a processor/processing unit (e.g. digital signal processor (DSP), a microcontroller, a microprocessor, a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), any other electronic computing device, and or any combination thereof. 
     The operations in accordance with the teachings herein may be performed by a computer specially constructed for the desired purposes or by a general purpose computer specially configured for the desired purpose by a computer program stored in a computer readable storage medium. 
     As used herein, the phrase “for example,” “such as”, “for instance” and variants thereof describe non-limiting embodiments of the presently disclosed subject matter. Reference in the specification to “one case”, “some cases”, “other cases”, “one example”, “some examples” or variants thereof means that a particular feature, structure or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the presently disclosed subject matter. Thus the appearance of the phrase “one case”, “some cases”, “other cases”, “one example”, “some examples” or variants thereof does not necessarily refer to the same embodiment(s). 
     It is appreciated that certain features of the presently disclosed subject matter, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the presently disclosed subject matter, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
     In embodiments of the presently disclosed subject matter, fewer, more and/or different stages than those shown in  FIGS. 19 to 23  can be executed. In embodiments of the presently disclosed subject matter one or more stages illustrated in  FIGS. 19 to 23  can be executed in a different order and/or one or more groups of stages may be executed simultaneously.  FIGS. 7 and 18  illustrate a general schematic of the system architecture in accordance with an embodiment of the presently disclosed subject matter. Each module in  FIGS. 7 and 18  can be made up of any combination of software, hardware and/or firmware that performs the functions as defined and explained herein. The modules in  FIGS. 7 and 18  can be centralized in one location or dispersed over more than one location. In other embodiments of the presently disclosed subject matter, the system may comprise fewer, more, and/or different modules than those shown in  FIGS. 7 and 18 . 
     According to a first aspect of the presently disclosed subject matter, there are provided systems and devices for in-flight refueling of aircraft. 
     Referring to  FIGS. 1 and 2 , a tanker system according to one example of the presently disclosed subject matter, generally designated  10 , comprises a tanker aircraft  12  comprising one or more in-flight refueling systems  50 . In this example, the tanker aircraft  12  has three such in-flight refueling systems  50 , one comprised on each one of the port wing  14  and starboard wing  16 , and a third one comprised on the rear portion of the fuselage  15 , and the tanker aircraft  12  is configured for in-flight concurrent refueling of up to three receiver aircraft  20 . In alternative variations of this example the tanker aircraft  12  can have at least one, or two, or more than three in-flight refueling systems  50 , arranged in any suitable configuration with respect to the tanker aircraft  12 . 
     By way of non-limiting example, such a tanker aircraft  12  can be a suitably equipped Boeing 767 and each receiver aircraft  20  can include any one of suitably equipped F-15, or F-16, or B2 stealth bomber, or other suitably equipped fighter, bomber or other aircraft. Alternatively, and also by way of non-limiting example, the tanker aircraft may be a UAV, and/or at least one of the receiver aircraft may be a UAV. 
     Also by way of non-limiting example, a refueling flight envelope for use with such a tanker system can include a forward speed of between about 220 knots and about 320 knots (typically about 280 knots), and an altitude of between 500 ft and between about 30,000 ft and about 40,000 ft, and in general not below about 10,000 ft, in which refueling can take place between the tanker aircraft  12  and each receiver aircraft  20 , flying in formation, depending on the operating limits of the tanker aircraft and of the receiver aircraft, as well as other factors. 
     Each in-flight refueling system  50  comprises an elongate, non-rigid, fuel delivery hose  52 , reversibly extendible from the tanker aircraft  12 . A first end (not shown) of the hose  52  is connected to a refueling fuel tank (not shown) carried by the tanker aircraft  12 . For example, such a refueling fuel tank can be an internal fuel tank of the tanker aircraft  12 , for example the tanker aircraft&#39;s own fuel tanks, or a special fuel reservoir mounted internally in the tanker aircraft  12 , for example in the fuselage, or externally and carried in fuel pods, for example. 
     The hose  52  is flexible and can be retracted into a roll up drum (not shown), suitably provided in the tanker aircraft  12 , and selectively deployed therefrom when required. 
     The second (aft) end  54  of hose  52  is operatively connected to a respective refueling device that is towed in a forward direction A by the tanker aircraft  12  via hose  52  when the hose  52  is extended and the tanker aircraft  12  is in flight. 
     In this example, one in-flight refueling system  50  is centrally-located and mounted with respect to the rear fuselage of the tanker aircraft  12 , and each of the other two in-flight refueling systems  50  is comprised in a respective pod  51  that is attached to the underside of the respective wing. 
       FIGS. 3 to 11  illustrate a refueling device according to a first example of the presently disclosed subject matter, generally designated  100 , for use with an in-flight refueling system, for example at least one of the in-flight refueling systems  50  illustrated  FIGS. 1 and 2 . 
     For convenience, and referring to  FIG. 3  for example, a roll axis R, a pitch axis P and a yaw axis Y can be conventionally defined with respect to the refueling device  100 . The roll axis R is parallel to or co-axial with the longitudinal axis  111  of the device  100 ; the pitch axis P is generally in lateral and orthogonal relationship to the roll axis R (i.e., parallel to the horizontal when the body is at a zero roll angle); and yaw axis Y is in orthogonal relationship to the roll axis R the pitch axis P (i.e., parallel to the vertical when the body is at a zero pitch angle). 
     Refueling device  100  is affixed to the end  54  of hose  52  and comprises an elongate body  110  comprising a longitudinal axis  111  and a general oval cross section (as best seen in  FIGS. 6( a ) and 6( b ) ), although in alternative variations of this example the body  110  can have any suitable cross-sectional shape, for example circular cross-section, polygonal cross-section, and so on. Referring in particular to  FIGS. 7 and 8 , the body  110  comprises a fuel delivery lumen  120  and a boom member  130  (which at least in the disclosed examples is a substantially rigid boom member) in fluid communication therewith. The boom member  130  defines a boom axis  131  and comprises a fuel delivery nozzle  135  at a terminus  136  of the boom member  130 . The nozzle  135  is configured for reversibly engaging with the fuel receptacle  22  of a receiver aircraft  20  (see also  FIGS. 1, 2 and 11 ), and thus can comprise any conventional design of such nozzles, which are well known, or indeed can comprise any other current or future design of such an in-flight refueling nozzle. 
     The boom member  130  is telescopically mounted to body  110 , and is reversibly extendable from a stowed position illustrated in  FIGS. 3 to 8  in which most of the boom member  130  is accommodated in a sleeve within the body  110 , to the fully extended position illustrated in  FIGS. 9( a ) and 9( b ) , by means of a controllable actuation mechanism (not shown). Optimally, the boom member  130  is telescopically extendable to a controllably variable extended position in a general aft direction from the aft end  112  of body  110 , up to the aforesaid the fully extended position. While in this example the boom axis  131  is parallel and co-axial with longitudinal axis  111 , in at least some alternative variations of this example the boom axis can be parallel but not co-axial with the body longitudinal axis, or the boom axis can be non-parallel with respect to the body longitudinal axis. 
     Thus, the boom member  130  is mounted with respect to the body  110  such as to maintain a generally parallel spatial disposition between the boom axis  131  and the longitudinal axis  111 , at least during in-flight refueling operation of device  100 . 
     In an alternative variation of this embodiment, the boom member  130  is permanently extended with respect to the body  110 , and is not telescopically or reversibly extendible therefrom. In another alternative variation of this example, the boom member  130  is permanently retracted with respect to the body  110 , and is not telescopically or reversibly extendible therefrom, and thus may only comprise a relatively short section extending aft from the body  110  to connect to the nozzle  135 . 
     In alternative variations of this example, or in other examples, the boom member may have any other suitable structure configured for coupling with the receiver aircraft, in particular the fuel receptacle thereof. 
     The body  110  comprises a coupling  140  at forward end  114  thereof. The coupling  140  comprises a hose interface  142  configured for connecting the lumen  120  to the hose  52 , and thereby to the tanker aircraft  12 . The coupling  140  is configured for allowing relative rotation between the body  110  and the hose  52  while maintaining fluid communication between the lumen  120  and the hose  52  and thus the refueling tank. In this example, the coupling  140  is in the form of a universal joint or the like (also referred to as a universal coupling, a Cardan joint, a Hardy-Spicer joint or a Hooke&#39;s joint, and so on), and is thus configured for allowing relative rotation between the body  110  and the hose  52  in three degrees of freedom. In alternative variations of this example and in other examples, the coupling can instead be configured for allowing relative rotation between the body  110  and the hose  52  in one degree of freedom, or in two degrees of freedom. In particular, the coupling allows the body  110 , and in particular the boom member  130  and the boom axis  131  to freely pivot with respect to the hose  52 , in particular the second end  54 , about at least one axis B (see  FIGS. 3 and 5 , for example), so that the spatial orientation of the refueling device  100  can be controllably changed without significant mechanical resistance thereto being generated by the hose  52  about axis B, which is typically parallel the pitch axis P of the refueling device  100 , but may be alternatively inclined to the pitch axis P and/or to the roll axis R and/or to the yaw axis Y. 
     The body  110  can optionally be formed as an integral and/or unitary structure incorporating the boom member  130  and the coupling  140 . 
     In alternative variations of this example the coupling  140  can be omitted and replaced with a fixed coupling that is configured to maintain a fixed relative spatial disposition between the body  110  and the hose  52  while maintaining fluid communication between the lumen  120  and the hose  52 . For example such a spatial disposition may be an angle (see  FIG. 1 ) of about 0°; or about 30°; or in a range between about 5° and about 85°; or in a range between about 10° and about 80°; or in a range between about 15° and about 70°; or in a range between about 20° and about 60°; or in a range between about 25° and about 50°; or in a range between about 20° and about 40°; or in a range between about 25° and about 40°; or in a range between about 28° and about 32°. 
     The refueling device  100  further comprises a spatial control system  160 , configured for controlling a spatial disposition of the refueling device  100  when towed aft of the tanker aircraft  12  via the hose  52 , and enables the refueling device  100  to be steered and/or to adopt any desired stable spatial disposition while being towed at the end  54  of hose  52 . 
     In particular, spatial control system  160  is configured for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  131  and the forward direction A, and enables this angle θ to be selectively maintained between the boom axis  131  and the forward direction A at least for a part of the time when the refueling device  100  is being towed by the tanker aircraft  12  via hose  52 , in particular during the engagement operation of the fuel device  100  to the receiver aircraft  20  and during refueling thereof. In particular, angle θ is in pitch, i.e., about a pitch axis P of the refueling device  100  and is defined on a plane including the roll axis R and the yaw axis Y of the refueling device  100 . Angle θ is thus representative of an angle of attack of the refueling device  100  in the airflow, or at least of the boom axis  131  with respect to forward direction A (which is typically, but not exclusively, parallel to the horizontal direction). Nevertheless, and depending on specific conditions during any particular refueling operation, angle θ can include an angular displacement component between the boom axis  131  and the forward direction A in yaw (i.e., about yaw axis Y), for example due to sideslip angle, and/or in roll (i.e. about roll axis R), instead of or in addition to an angular displacement component in pitch (i.e., about pitch axis P). 
     The refueling device  100 , in particular the boom member  130 , nozzle  135  and lumen  120  can be sized to allow suitable fuel flow rates for refueling a wide range of receiver aircraft. By way of non-limiting example, relative high fuel flow rates (for example up to 1000 US gallons/6,500 lb per minute) can be provided for refueling operations of large aircraft (for example transport aircraft, bombers, etc), while for fighter aircraft that cannot accept fuel at the maximum flow rate of the refueling device  100 , the refueling pressure can be correspondingly reduced. Alternatively the refueling device  100 , in particular the boom member  130 , nozzle  135  and lumen  120  can be sized to allow suitable fuel flow rates for refueling a narrow range of receiver aircraft., for example only fighter aircraft or only larger aircraft (for example about 400 US gallons/2,600 lb per minute). 
     Thus, the spatial control system  160  is configured for providing stability to the refueling device  100 , while tethered to and towed by the tanker aircraft  12  via the hose  52 , and while the boom axis  131  is at any desired pitch and/or yaw and/or roll angle corresponding to the aforesaid angle θ. 
     In particular, and referring to  FIG. 10( a )  and  FIG. 10( b ) , angle θ is such as to provide a design angle (angle θ des ) that is within a particular angular range which corresponds to the design relative angular position of the boom member  130  (and boom axis  131 ) with respect to the receiver aircraft  20 . 
     In particular, design angle θ des  is the design relative angular position of the boom axis  131  with respect to the longitudinal axis of the receiver aircraft  20  (the receiver aircraft  20  being at a predetermined spatial orientation relative to the forward direction, typically in horizontal forward flight), to enable the boom member  130  to align and engage the nozzle  135  with respect to the fuel receptacle  22 . Thus, angle θ (which can have an angular component in yaw and/or in pitch and/or in roll) compensates for any off-nominal pitch of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero angle of attack) and/or for any off-nominal roll of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero roll angle) and/or for any off-nominal yaw of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero sideslip angle) to ensure that the actual angular disposition between the boom axis  131  and the receiver aircraft longitudinal axis is maintained at design angle θ des  even as the relative spatial orientation between the receiver aircraft  20  and the forward direction changes. 
     Thus, design angle θ des  the boom axis  131  is in an engagement enabling orientation with respect to the receiver aircraft  20 , and in particular with respect to the fuel receptacle  22 . 
     In non-limiting examples, angle θ (and in particular angle θ des ) can be in a range between about 5° and about 85°; or in a range between about 10° and about 80°; or in a range between about 15° and about 70°; or in a range between about 20° and about 60°; or in a range between about 25° and about 50°; or in a range between about 20° and about 40°; or in a range between about 25° and about 40°; or in a range between about 28° and about 32°. 
     In one non-limiting example, angle θ des  can be about 30°, and operation of the refueling device  100  to adopt this angle automatically renders it compatible for use with existing receiver aircraft  20 , in which the fuel receptacles  22  are configured for receiving and engaging with a nozzle at the end of a boom where the boom is at about 30° to the longitudinal axis of the receiver aircraft, without the need for modifying the configuration of the fuel receptacle thereof. 
     Thus, when angle θ is equal to design angle θ des , the receiver aircraft travelling along direction A with zero angle of attack and zero sideslip and zero roll, and boom axis  131  is at the required spatial orientation to the forward direction A of the tanker aircraft and the receiver aircraft such as to ensure engagement between the nozzle  135  in the fuel receptacle  22 , without the need for modifying the configuration of the fuel receptacle thereof. 
     In this example, the spatial control system  160  comprises a selectively controllable aerodynamic control system  170 , comprising a forward set  172  of aerodynamic control surfaces  173  mounted to body  110  at a forward portion thereof, and an aft set  174  of aerodynamic control surfaces  175  mounted to the body  110  at an aft portion thereof. Referring in particular to  FIG. 4 , the aft set  174  is thus in aft spaced relationship with respect to the forward set  172 , and the center of gravity CG of the body  110  is disposed longitudinally intermediate the aft set  174  and the forward set  172 , noting that the actual longitudinal position of the center of gravity CG can shift between two extreme longitudinal positions according to, inter alia, whether the boom member  135  is extended or retracted, and whether fuel is present in the refueling device  100  or absent therefrom. In alternative variations of this example and in other examples, the center of gravity can be forward of both the forward set and the aft set of aerodynamic surfaces, which are configured to provide the required stability to the refueling device  100  with the boom axis  131  at any desired pitch and/or yaw and/or roll angle. 
     In this example, the forward set  172  comprises four aerodynamic control surfaces  173  in cruciform “X” configuration (see in particular  FIGS. 6( a ) and 6( b ) ). Each aerodynamic control surface  173  is in the form of a vane, pivotably mounted to the body  110  via a respective boss  183  laterally projecting from the surface of body  110 . Each boss  183  houses an actuator (not shown) for controlling the angular position of the respective vane about a respective pivot axis, and is controlled by controller  180 . The pivot axes of the vanes are, in at least this example, orthogonal to at least one of longitudinal axis  111  and boom axis  135 , and can also intersect the respective at least one of longitudinal axis  111  and boom axis  135 . 
     In this example, controller  180  comprises any suitable computer control system, and is housed in the refueling device  100  (see  FIG. 7 ). In alternative variations of this example and in other examples, the controller  180  or portions thereof can instead comprise any suitable electronic control unit, or any other suitable control unit. Additionally or alternatively, the controller or portions thereof can be comprised elsewhere in the in-flight refueling system  50  or in the tanker aircraft  12 . 
     In this example, the aft set  174  comprises two aerodynamic control surfaces  175  in “Vee” configuration (see in particular  FIGS. 6( a ) and 6( b ) ). Each aerodynamic control surface  175  is in the form of a vane, pivotably mounted to the body  110  via a respective boss  186  laterally projecting from the surface of body  110 . Each boss  186  houses an actuator (not shown) for controlling the angular position of the respective vane, and is also controlled by controller  180 . 
     In this example, and referring in particular to  FIG. 4 , each boss  183  has an aerofoil-shaped cross section defining a chord  185 , and each boss  186  has an aerofoil-shaped cross section defining a chord  185 . Furthermore the chord  185  is oriented with respect to at least one of the longitudinal axis  111  and the boom axis  131  such as to align the chord  185  with the forward direction A, which is nominally the airflow direction with respect to the refueling device  100 , when the refueling device  100  is at spatial orientation in which the boom axis  131  is at angle θ des  with respect to the forward direction A Similarly, each chord  185  is oriented with respect to at least one of the longitudinal axis  111  and the boom axis  131  such as to align the chord  185  with the forward direction A when the refueling device  100  is at spatial orientation in which the boom axis  131  is at angle θ des  with respect to the forward direction A. 
     In alternative variations of this example or in other examples, the forward set aerodynamic control surfaces can comprise two or three or four or more than four vanes (or any other type of aerodynamic control surfaces), in any suitable configuration, including for example four vanes in cruciform “+” configuration, and/or, each vane (or other type of aerodynamic control surfaces) can be pivotable about a respective axis having any suitable spatial relationship with respect to the longitudinal axis of the refueling device and/or the axis of the boom member. Additionally or alternatively, the aft set aerodynamic control surfaces can comprise two more than two vanes (or any other type of aerodynamic control surfaces), in any suitable configuration. Additionally or alternatively, the respective aerodynamic control surfaces of the spatial control system, in the form of pivotable vanes or any other suitable configuration, are mounted to respective bosses, which can be aerodynamically shaped but at a different orientation with respect to the longitudinal axis  111  and/or the boom axis  131 , or wherein the respective bosses can have a different shape, for example in the form of cylinders or any other prismatic shape or other shape, or wherein the respective aerodynamic control surfaces are directly mounted to the body  110  without bosses (in which case the respective actuators can be provided in the body  110 ). 
     For example, one such alternative variation of the refueling device example of  FIGS. 3 to 11  is illustrated in  FIGS. 12( a ) to 12( d ) , in which the respective example of the refueling device, designated  1000 , comprises all the elements and features of the refueling device  100 , mutatis mutandis, with the main difference that the aerodynamic control system  170  of the example of the refueling device  100  is replaced with an alternative configuration for the spatial control system  160 , comprising aerodynamic system  1170 . Thus, the refueling device  1000  comprises a body  1110 , forward end  1114 , aft end  1112 , longitudinal axis  1111 , fuel delivery lumen (not shown), boom member  1130 , boom axis  1131 , fuel delivery nozzle  1135 , terminus  1136 , coupling  1140 , hose interface  1142 , substantially similar to the corresponding components as described herein for the example of the refueling device  100  or alternative variations thereof, mutatis mutandis, i.e., respectively: body  110 , forward end  114 , aft end  112 , longitudinal axis  111 , fuel delivery lumen  120 , boom member  130 , boom axis  131 , fuel delivery nozzle  135 , terminus  136 , coupling  140 , hose interface  142 . In addition, the refueling device  1000  optionally comprises a force generating arrangement (not shown) for example substantially similar to force generating arrangement  190  as described hereinbelow, mutatis mutandis, and/or a suitable data acquisition system (not shown) for example substantially similar to imaging system  150 , as described hereinbelow, mutatis mutandis, and/or a controller  1180 , for example similar to controller  180  as described herein, mutatis mutandis, and/or a control computer system (not shown), for example similar to control computer system  155  as described herein, mutatis mutandis. 
     The aerodynamic system  1170  comprises a forward set  1172  of aerodynamic control surfaces  1173  mounted to body  110  at a forward end  1114  thereof, and an aft set  1174  of aerodynamic control surfaces  1175  mounted to the body  1110  at an aft portion  1112  thereof. The aft set  1174  is thus in aft spaced relationship with respect to the forward set  1172 , and the center of gravity of the body  1110  is disposed longitudinally therebetween, though in alternative variations of this example and in other examples, the center of gravity can be forward or aft of both the forward set and the aft set of aerodynamic surfaces, which are configured to provide the required stability to the refueling device  1000  with the boom axis  1131  at any desired pitch and/or yaw and/or roll angle. 
     In this example, the forward set  1172  comprises four aerodynamic control surfaces  1173  in cruciform “+” configuration, and each aerodynamic control surface  1173  is in the form of a vane, pivotably mounted to the body  1110  and operatively connected to an actuator system (not shown) for controlling the angular position of the respective vane about a respective pivot axis, and is controlled by controller  1180 . The pivot axes of the vanes are, in at least this example, orthogonal to at least one of longitudinal axis  1111  and boom axis  1135 , and can also intersect the respective at least one of longitudinal axis  1111  and boom axis  1135 . In alternative variations of this example, the forward set  1172  may comprise any suitable configuration or vanes, wings, RCS, etc. 
     In this example, the aft set  1174  comprises a high H-tail configuration, comprising two vertical stabilizers  1175 , one on either side of a horizontal stabilizer  1171 , which in turn is mounted to the upper side of the aft end  1112 . Each vertical stabilizer  1175  comprises a controllably pivotable rudder  1178 , and the horizontal stabilizer  1171  comprises a pair of pivotable elevators  1179 , which are controllably actuated by an actuator system (not shown) also controlled by controller  1180 . 
     For example, four other such alternative example variations are illustrated in  FIG. 13( a )  to  FIG. 13( d ) , respectively, in which for the respective refueling devices  100 ″ a,    100 ″ b,    100 ″ c  and  100 ″ d,  respectively, the respective forward set  172 ″ comprises two aerodynamic control surfaces  173 ″ in “Vee” configuration, and the respective aft set  174 ″ comprises two aerodynamic control surfaces  175 ″ in “Vee” configuration as in the first example, mutatis mutandis. In the example of  FIG. 13( a )  the aerodynamic control surfaces  173 ″ are smaller than the aerodynamic control surfaces  175 ″, while in the examples of  FIG. 13( b )  to  FIG. 13( d )  the aerodynamic control surfaces  173 ″ are the same size nominally as the aerodynamic control surfaces  175 ″. In yet other alternative variations of the example of  FIGS. 13( a ) to 13( d ) , the aerodynamic control surfaces  173 ″ are larger than the aerodynamic control surfaces  175 ″. 
     For example, another such alternative example variation is illustrated in  FIG. 13( e ) , in which the respective aft set  174 ′ for the refueling device  100 ′ comprises three aerodynamic control surfaces, two aerodynamic control surfaces  175 ′ in “Vee” configuration as in the first example, mutatis mutandis, and a third vane  175 ″ in vertical and downwardly depending relationship with respect to the respective body  110 ′. 
     For example, two such alternative example variations are illustrated in  FIGS. 14( a ) and 14( b )  in which the respective forward set  172 ′″ for each respective refueling device  100 ′″ a,    100 ′″ b  comprises two aerodynamic control surfaces  173 ″ with zero dihedral, and the respective aft set  174 ′″ also comprises two aerodynamic control surfaces  175 ′″ with zero dihedral. In the example of  FIG. 14( b ) , each aerodynamic control surfaces  175 ′″ further comprises a vertical vane  176 ′″ in upwardly depending relationship with respect to the aerodynamic control surfaces  175 ′″ at the respective wing tips. 
     For example, in the alternative example variations illustrated in  FIGS. 13( a ), 13( c ) and 13( d ) , the respective forward aerodynamic control surfaces  173 ″ are pivotably mounted to cylindrically shaped bosses  183 ″, and the respective aft aerodynamic control surfaces  175 ″ are pivotably mounted to cylindrically shaped bosses  185 ″. On the other hand, in the examples illustrated in  FIGS. 13( b ), 14( a ) and 14( b ) , the respective forward and aft aerodynamic control surfaces are pivotably mounted directly to the body of the respective refueling device. 
     Referring again to the example of  FIGS. 3 to 10 , the aerodynamic control system  170  is configured for allowing the refueling device  100  to adopt any desired angle θ while maintaining a zero pitching moment (and/or zero yawing moment and/or zero rolling moment), as the forward set  172  of aerodynamic control surfaces  173  is configured for trimming any pitching moment (and/or yawing moment and/or rolling moment, respectively) generated by aft set  174  of aerodynamic control surfaces  175  at a given pitch angle (and/or yaw angle and/or roll angle, respectively) of the body  110 , or vice versa. In this example, where the center of gravity CG is longitudinally intermediate the forward set  172  and the aft set  174 , the trimming pitching moment generated by the forward set  172 , for example is in a counter-rotational direction with respect to the pitching moment generated by aft set  174  to maintain a particular pitch angle for angle θ, while the pitch forces generated by forward set  172  and the aft set  174  are in the same direction. In alternative variations of this example or in other examples in which the center of gravity of the refueling device is forward of both the forward set and the aft set of aerodynamic control surfaces, the trimming pitching moment generated by the forward set of aerodynamic control surfaces, for example, is also in a counter-rotational direction with respect to the pitching moment generated by aft set of aerodynamic control surfaces to maintain a particular pitch angle for angle θ, but the pitch forces generated by forward set of aerodynamic control surfaces and the aft set of aerodynamic control surfaces are in opposite directions. In yet other examples, the refueling device comprises the spatial control system in the form of a single set of aerodynamic control surfaces which are configured for generating zero pitch moment for a desired range of pitch angles corresponding to angle θ, and the spatial control system is self-trimming to provide stable pitch angle corresponding to angle θ. 
     In the first example, the aft aerodynamic control surfaces  175  are larger than the forward aerodynamic control surfaces  173 , though in alternative variations of this example and in other examples, the aft aerodynamic control surfaces  175  can be the same size or smaller than the forward aerodynamic control surfaces  173 . 
     In other variations of this example and in other examples, the spatial control system  160  comprises a non-adjustable aerodynamic control system that is configured for allowing the refueling device  100  to adopt a particular, pre-set, desired angle θ while maintaining a zero pitching moment (and/or zero yawing moment and/or zero rolling moment), this being the design angle θ des , at least at one set of conditions associated with the refueling—for example at a particular forward speed and altitude. Thus, once the refueling device is towed behind the tanker aircraft  12  via the hose  52 , the boom axis automatically adopts the particular design angle θ des , and stably maintains this relative spatial disposition at the aforesaid set of conditions until the refueling device is retracted back into the tanker aircraft  12 . 
     In other variations of this example and in other examples, the spatial control system  160  comprises a selectively controllable control system that is not based on aerodynamic control surfaces. For example, a plurality of suitable thrust nozzles or other suitable reaction control thruster system (RCS) can be mounted to the body to provide thrust vector control and maintain the boom axis  131  at any desired angle θ. Such thrusters or RCS can be operatively connected to a suitable compressed air supply or compressed gas supply, for example carried by the refueling device itself, or carried by the tanker aircraft and supplied to the refueling device via pneumatic or gas lines, or generated by the tanker aircraft and/or the refueling device using a suitable compressor taking air from the atmosphere. 
     Referring in particular to  FIGS. 3, 4, 5, 9 ( a ),  9 ( b ) and  10 , the refueling device  100  further comprises a force generating arrangement  190 . The force generating arrangement  190  is configured for selectively generating a force F ( FIGS. 9( a )  to  11 ) along the boom axis  131  in a direction towards nozzle  135 . In this example, the force generating arrangement  190  selectively generates force F as a drag, and is in the form of a selectively and reversibly deployable drag inducing arrangement  192 , comprising a selectively and reversibly deployable air brake system  194 . The air brake system  194  comprises a port air brake  195  and a starboard air brake  196 , each comprising a curved plate  197  pivotably hinged laterally to the body  110  via hinges  198  between a closed position, in which the plate is received in a recess  199  (best seen in  FIGS. 9( a ) and 9( b ) ) and the outer surface of the plate  197  is flush with the outer surface of body  110 , and an open position in which the plate offers a maximum frontal surface area to the airflow and thereby generates drag. The hinges  199  are forwardly disposed so that the convex outer surface of each one of the port air brake  195  and of the starboard air brake  196  faces the airflow. Suitable actuators (not shown) are operatively connected to and operate the air brakes  195 ,  196 , controlled by controller  180 . Alternatively, and as illustrated for the example of  FIG. 13( a ) , the hinges  199  can be disposed aft of the respective plates  197  so that the convex outer surface of each one of the port air brake  195  and the starboard air brake  196  faces away from the airflow. In the example of  FIG. 13( d )  the force inducing arrangement  190  is an airbrake in the form of plate  920  that is selectively laterally deployable and retractable with respect to airbrake housing  910 . 
     The force generating arrangement  190  is in particular configured for selectively generating a force F having a magnitude sufficient for forcing the nozzle  135  into engagement with the fuel receptacle  22  of the receiver aircraft ( FIG. 11 ) when the nozzle  135  (and the boom member  130 ) and the fuel receptacle  22  are in a predetermined relative spatial disposition, i.e., when the refueling device  100  reaches an engagement enabling position and the boom axis is in the engagement enabling orientation with respect to the receiver aircraft  20 , and in particular with respect to the fuel receptacle  22 . 
     The force generating arrangement  190  is further configured for selectively operating in this manner responsive to the nozzle  135  being in a predetermined proximity to the fuel receptacle  22 , i.e. responsive to the nozzle  135  being in a predetermined spacing with respect to the fuel receptacle  22 , typically the engagement enabling spatial position, and can be operated manually or automatically to provide such a force F, as will become clearer herein. 
     Thus, at the engagement enabling position, when the boom member  130 , or the boom axis  131 , is in a predetermined spatial disposition with respect to the fuel receptacle  22  and the nozzle  135  being in a predetermined spacing with respect to the fuel receptacle  22  (i.e., at the engagement enabling position the boom axis is at the engagement enabling orientation—corresponding to the design angle θ des ), the force generating arrangement  190  can be selectively actuated to compel the boom member  130  to follow a predetermined trajectory, for example aligned with the boom axis  131  in the direction of the receiver aircraft  20 , to ensure alignment and engagement between the nozzle  135  and the fuel receptacle  22 . In this example, the boom  130  is telescopically extended to the extended position in a direction along the boom axis  131 , which is maintained at the engagement enabling orientation—corresponding to the design angle θ des , while the body  110  remains at the same spatial disposition with respect to the receiver aircraft  20 . In alternative variations of this example, the boom  130  is partially or fully telescopically extended towards the receiver aircraft  20  while the device  100  can be moved towards or away from the receiver aircraft  20  to effect engagement between the nozzle  135  and the fuel receptacle  22 . In other alternative variations of this example, the boom member  130  remains retracted, and the body  110  itself is moved towards the receiver aircraft  20  along a the direction of the boom axis, maintaining the boom axis  131  at the engagement enabling orientation—corresponding to the design angle θ des , to effect engagement between the nozzle  135  and the fuel receptacle  22 . 
     Once the nozzle  135  is forced into engagement with the fuel receptacle  22  of the receiver aircraft  20 , the tanker aircraft  12  can begin refueling the receiver aircraft  20 . 
     In alternative variations of this example and in other examples, the force generating arrangement  190  can comprise any other suitable drag inducing arrangement, for example spoilers on the vanes  175 . 
     In yet other variations of this example and in other examples, the force generating arrangement  190  can be configured for generating a thrust force in the required direction. For example, one or a plurality of suitable thrust nozzles can be mounted to the body to provide the required thrust vector parallel to the boom axis  131  towards nozzle  135 . Such thrust nozzle(s) can be operatively connected to a suitable compressed air or compressed gas supply, for example carried by the refueling device itself, or carried by the tanker aircraft and supplied to the refueling device via pneumatic or gas lines, or generated by the tanker aircraft and/or the refueling device. 
     In yet other alternative variations of this example and in other examples, the force generating arrangement can be omitted, and for example the receiver aircraft and/or the boom member can comprise suitable means for mechanically engaging the nozzle to the fuel receptacle that does not require such a force F to be generated by the device  100 . For example, the fuel receptacle can comprise a suitable mechanical clamp that engages the terminus  136  of the boom member  130 , and pulls in the nozzle  135  into engagement with the fuel receptacle  22 . 
     Referring in particular to  FIGS. 4, 5, 10 and 11  the refueling device  100  further comprises a suitable data acquisition system for providing or enabling the calculation of spatial data relating to the relative spatial dispositions between the refueling device  100  and the receiver aircraft  20 , in particular the relative spatial dispositions between the fuel delivery nozzle of the refueling device  100  and the fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic (optionally including autonomous) and/or manual steering of the refueling device  100  to the engagement enabling position and subsequent selective engagement of the fuel delivery nozzle to the fuel receptacle of the receiver aircraft. At least in the example of  FIGS. 4, 5, 10 and 11 , the data acquisition system is in the form of imaging system  150 , in particular configured for providing imaging data of any object coming within a field of regard (FOR) aft of the refueling device  100 . Such a field of regard has a predetermined depth aft of the imaging system and in this example comprises sensing volume  159  aft of the imaging system  150 , which while in this example comprises a prismoidal volume in alternative variations of this example the FOR can have any suitable shape, for example conical, frustoconical, cylindrical, spherical, part-spherical (e.g. hemispherical), parallelepiped (for example cubic) or any other regular or irregular shape. The sensing volume  159 , i.e., the predetermined depth of the FOR, extends aft further than is required corresponding to the engagement enabling position, i.e., further than the maximum extension of the boom member  130  when this is in its fully deployed position. The imaging system  150  is operatively connected to a control computer system  155 , which can be integral with, connected to, or independent from controller  180  (see  FIG. 8 ). In particular, and referring particularly to  FIGS. 10 and 11 , such an object is the receiver aircraft  20  and more particularly a part AP thereof including the fuel receptacle  22 , and the sensing volume  159  defines an outer envelope limit  158  in which image data of part AP can be processed, inter alia, by control computer system  155  to provide control signals, for example steering commands, to the spatial control system  160  and/or the force generating arrangement  190 , for example via controller  180  to control operation of the refueling device  100 , in particular the relative spatial position and orientation of the refueling device  100  with respect to the receiver aircraft  20 , in particular the position and orientation of the boom member  130  and nozzle  135  with respect to the fuel receptacle  22 , so that the nozzle  135  can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner. The manner of operation of the imaging system  150  and control computer system  155  will be described in greater detail further herein. 
     In this example, the imaging system  150  comprises two pairs of flash ladar units  151 , also referred to interchangeably herein as FLADAR units, one pair on the trailing edge of each boss  186 . Suitable FLADAR units can include, for example, a PMD [vision]® CamCube 3.0, provided by PMD Technologies, Germany, and adapted for operating within the refueling unit and at the flight conditions thereof. 
     In operation the FLADAR units  151  illuminate the sensing volume  159  and any object therein, in particular part AP of the receiver aircraft  20  and thereafter acquire suitable image data corresponding thereto which is sent to control computer system  155  for processing to provide the aforesaid control signals for controlling the refueling device  100 . In particular, by means of the FLADAR units  151 , a 3D image of the areas AP is reconstructed, and manipulated via a computer system to determine the relative position and orientation of the nozzle  135  with respect to the fuel receptacle  22 . 
     The sensing volume  159  thus includes the engagement enabling position. 
     In alternative variations of this example and in other examples, the imaging system  150  can comprise any other suitable imaging system (for example, but not limited to, systems providing 2D images and/or stereoscopic images and/or 3D images of (including reconstruction of 3D data corresponding to) the sensing volume  159 , in particular but not limited to images that are updated in real time, for example in the form of a video stream) that operate to provide suitable data to the control computer system  155  to, in turn, enable selectively controlling the refueling device  100  to provide autonomous and/or manual engagement of the nozzle  135  to the fuel receptacle  22  of the receiver aircraft  20 . 
     In alternative variations of this example, the imaging system  150  can be replaced with any other suitable data acquisition system for providing the aforesaid spatial data. 
     In yet other alternative variations of this example and in other examples, the refueling device  100  can omit the imaging system  150  and can be actively controlled by an operator, for example, to control the relative spatial position and orientation of the refueling device  100  with respect to the receiver aircraft  20 , in particular the spatial position and orientation of the boom member  130  and/or nozzle  135  with respect to the fuel receptacle  22 , so that the nozzle  135  can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner, for example via direct visual tracking of the device by the operator. Alternatively, the refueling device can be operated as a free flying refueling device towed at the end of hose  52 , and the relative spatial position and orientation of the refueling device  100  with respect to the receiver aircraft  20  (in particular the position and orientation of the boom member  130  and nozzle  135  with respect to the fuel receptacle  22 , so that the nozzle can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner) is achieved by maneuvering the receiver aircraft only. In such a case, the spatial control system  160  can optionally comprise a non-adjustable aerodynamic stability system that is configured for allowing the refueling device  100  to adopt a particular, pre-set, desired angle θ while maintaining a zero pitching moment (and/or zero yawing moment and/or zero rolling moment), this being the design angle θ des  as discussed above for example 
     Optionally, a suitable air-driven generator can be provided in the refueling device  100  to provide electrical power thereto. Additionally or alternatively, electrical power can be provided to the refueling device  100  by the tanker aircraft  12 . Additionally or alternatively, electrical power can be provided to the refueling device  100  by one or more batteries in the refueling device  100 . Additionally or alternatively, electrical power can be provided to the refueling device  100  by one or more ram air turbines (RAT), affixed internally or externally with respect to the refueling device  100 . 
     In at least some alternative variations of the first example the refueling device can comprise an aerodynamic stabilizer arrangement, different from the spatial control system  160  or from the force generating arrangement  190 . For example, each one of the alternative example variations illustrated in  FIGS. 13( c ) and 13( d )  comprises such an aerodynamic stabilizer arrangement in the form of a respective drogue structure  180 ″ fixed to the aft portion of the body. Such a drogue structure  180 ″ can be utilized for generating a drag which in turn induces a tension to the hose  52 , thereby aiding reduction or dampening of vibrations or oscillations in the hoe  52  that can otherwise occur. Such a drogue structure can also be provided for other examples, for example the first example illustrated in  FIG. 3  or alternative variations thereof. 
     In the example of  FIGS. 15( a ) to 15( d ) , the respective refueling device  100 E comprises an aerodynamic stabilizer arrangement in the form of a drogue structure  180 E forwardly spaced from a forward end  114 E of the frustoconical body  110 E of refueling device  100 E by a length pipe  52 E, which is flexible but can be articulated instead, and the body  110 E comprises a spatial control system  160 E comprising a selectively controllable aerodynamic control system  170 E, comprising a forward set  172 E of two swept back aerodynamic control surfaces  173 E mounted directly to body  110 E at a forward portion thereof, and an aft set  174 E of two aerodynamic control surfaces  175 E directly mounted to the body  110 E at an aft portion thereof in “Vee” configuration, a deployable airbrake system  190 E provided on the aft end of multi-segmented telescopic boom  130 E, which comprises a nozzle  135 E at the terminus  136 E thereof. 
     The in-flight refueling systems  50  including the first example of the refueling device  100  or at least some alternative variations thereof, can be operated in a number of different ways to provide selective engagement of the nozzle  135  with the fuel receptacle  22  of a receiver aircraft  20 , and enable subsequent refueling of the receiver aircraft  20  from the  30  tanker aircraft  12  in flight, for example as disclosed herein. 
     Referring to  FIGS. 16( a ) to 16( d ) , a second example of the refueling device, designated herein with reference numeral  200 , comprises the elements and features of the first example and/or of at least some alternative variations thereof, mutatis mutandis, with some differences, as will become clearer herein, and the refueling device  200  is configured for use with an in-flight refueling system, for example at least one of the in-flight refueling systems  50  illustrated  FIGS. 1 and 2 . 
     For convenience, and referring to  FIG. 16( a )  for example, a roll axis R, a pitch axis P and a yaw axis Y can be conventionally defined with respect to the refueling device  200 . The roll axis R is parallel to or co-axial with the longitudinal axis  211  of the device  200 ; the pitch axis P is generally in lateral and orthogonal relationship to the roll axis R (i.e., parallel to the horizontal when the body is at a zero roll angle); and yaw axis Y is in orthogonal relationship to the roll axis R the pitch axis P (i.e., parallel to the vertical when the body is at a zero pitch angle). 
     Refueling device  200  is affixed to the end  54  of hose  52  and comprises body  210  comprising a longitudinal axis  211 , a fuel delivery lumen  220 , and a substantially rigid boom member  230  in fluid communication therewith. The boom member  230  comprises a plurality of telescopic elements  232 , defines a boom axis  231 , and comprises a fuel delivery nozzle  235  at a terminus  236  of the boom member  230 . The nozzle  235  is configured for reversibly engaging with the fuel receptacle  22  of a receiver aircraft  20 , and thus can be similar to the nozzle  135  of the first example and as disclosed above, mutatis mutandis. 
     The boom member  230  is telescopically and pivotably mounted to body  210  about axis C (generally parallel to the pitch axis P of the body  210 ), and is reversibly movable from a stowed position in which the telescopic elements  232  are retracted and nested in one another and the boom member  230  is pivoted about axis C into a position accommodated in body  210  (wherein optionally the boom axis  231  can be generally parallel to longitudinal axis  211 ), to a deployed position illustrated in  FIGS. 16( a ) and 16( b ) . In the deployed position, the boom member  230  can be, by means of a controllable actuation mechanism (not shown), controllably variably extended in an aft direction from the aft end  212  of body  210 , up to the fully extended position illustrated in  FIGS. 16( a ) and 16( b ) , and/or variably pivoted about pivot axis C in a downward direction to provide a non-zero angular displacement, angle θ′, between boom axis  231  and longitudinal axis  211 . In this example, angle θ′ is in pitch, though in alternative variations of this example angle θ′ may also include angular components in yaw and/or roll. Such angular components in yaw and/or roll may be additionally provided by suitably orienting the device  200  with respect to the yaw axis Y and/or roll axis R, respectively. In variations of this example where the boom axis  231  can only be pivoted with respect to the body  210  about an axis parallel to the pitch axis P, such angular components in yaw and/or roll may be alternatively and exclusively provided by suitably orienting the device  200  with respect to the yaw axis Y and/or roll axis R, respectively. 
     The body  210  optionally comprises a coupling  240  at forward end  214  thereof, similar to the coupling  140  of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     The refueling device  200  further comprises a spatial control system  260 , configured for controlling a spatial disposition of the refueling device  200  when towed aft of the tanker aircraft  12  via the hose  52 . In particular, spatial control system  260  is configured for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  231  and the forward direction A, and enables this angle θ to be selectively maintained between the boom axis  231  and the forward direction A when the refueling device  200  is being towed by the tanker aircraft  12  via hose  52 , similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. Thus, in particular, angle θ is in pitch, i.e., about a pitch axis P of the refueling device  200  and is defined on a plane including the roll axis R and the yaw axis Y of the refueling device  200 . Nevertheless, and depending on specific conditions during any particular refueling operation, angle θ can instead include an angular displacement component between the boom axis  231  and the forward direction A in yaw (i.e., about yaw axis Y) for example due to sideslip angle, and/or in roll (i.e. about roll axis R), in addition to an angular displacement component in pitch (i.e., about pitch axis P). 
     Thus, the spatial control system  260  is configured for controllably flying the refueling device  200 , and for providing stability to the refueling device  200 , while tethered and towed via the hose  52 , and while the boom axis  231  is at any desired pitch and/or yaw and/or roll angle corresponding to the aforesaid angle θ, and in particular, angle θ is a design angle (angle θ des ) is within a particular angular range which corresponds to the design relative angular position of the boom member  230  (and boom axis  231 ) with respect to the receiver aircraft  20  similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     In the second example, though, at least a part of angle θ, in particular a part of the design angle θ des  is provided by angle θ′, i.e., by pivoting the boom member  230  about axis C, depending on the magnitude of angle ϕ, which is the relative angular disposition between the longitudinal axis  211  and the forward direction A. The angle ϕ can be positive (as illustrated in  FIG. 16( b ) ), representing a positive angle of attack of body  210  with respect to forward direction A. Alternatively, angle ϕ can be negative, or zero. 
     In this example, the spatial control system  260  is configured for providing a zero or near zero angle ϕ when the boom member  230  is in its deployed position pivoted at angle θ′, and comprises a selectively controllable aerodynamic control system  270 . The aerodynamic control system  270  comprises a forward set  272  of aerodynamic control surfaces  273  in the form of low aspect ratio wing members fixedly mounted to body  210  at a forward portion thereof and having controllably movable ailerons  271 . The aerodynamic control system  270  further comprises an aft set  274  of aerodynamic control surfaces  275  mounted to the body  210  at an aft portion thereof in “Vee” configuration. The spatial control system  260  is also configured for providing the required pivoting angle θ′ so that angle θ′ together with angle ϕ provide the desired angle θ between the boom axis  231  and the forward direction A in order to maintain the required design angle θ des  between the boom axis  231  and the longitudinal axis of the receiver aircraft  20 . Thus, angle θ (which can have an angular component in yaw and/or in pitch and/or in roll) compensates for any off-nominal pitch of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero angle of attack) and/or for any off-nominal roll of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero roll angle) and/or for any off-nominal yaw of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero sideslip angle) to ensure that the actual angular disposition between the boom axis  231  and the receiver aircraft longitudinal axis is maintained at design angle θ des  even as the relative spatial orientation between the receiver aircraft  20  and the forward direction A changes. 
     In other variations of the example and in other examples, the spatial control system  260  can be similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     The refueling device  200  can optionally further comprise a force generating arrangement (not shown), similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     The refueling device  200  can optionally further comprise a suitable spatial data acquisition system including an imaging system (not shown), similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis, or can omit such an imaging system and can be actively controlled by an operator, for example, similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     The in-flight refueling systems  50  including the second example of the refueling device  200 , and at least some alternative variations thereof, can also be operated in a number of different ways to provide selective engagement of the nozzle  235  with the fuel receptacle  22  of a receiver aircraft  20 , and enable subsequent refueling of the receiver aircraft  20  from the tanker aircraft  12  in flight. 
     Referring to  FIGS. 17( a ) to 17( e ) , a variation of the second example of the refueling device, designated herein with reference numeral  200 B, comprises the elements and features of the second example of the refueling device and/or of at least some alternative variations thereof, and/or of the first example of the refueling device and/or of at least some alternative variations thereof, mutatis mutandis, with some differences, as will become clearer herein. In a similar manner thereto, the refueling device  200 B is also configured for use with an in-flight refueling system, for example at least one of the in-flight refueling systems  50  illustrated  FIGS. 1 and 2 . 
     For convenience, and referring to  FIG. 17( a )  for example, a roll axis R, a pitch axis P and a yaw axis Y can be conventionally defined with respect to the refueling device  200 B in a similar manner to that of the second example of  FIGS. 16( a ) to 16( d ) , mutatis mutandis. Thus, for example, the roll axis R is parallel to or co-axial with the longitudinal axis  211 B of the device  200 B, while the pitch axis P and the roll axis R each are in orthogonal relationship to the roll axis R. 
     Refueling device  200 B is affixed to the end  54  of hose  52  and comprises body  210 B in the form of an elongate fuselage and comprising a longitudinal axis  211 B. The refueling device  200 B also comprises a substantially rigid boom member  230 B, which defines a boom axis  231 B, and comprises a fuel delivery nozzle  235 B at a terminus  236 B of the boom member  230 B. The nozzle  235 B is configured for reversibly engaging with the fuel receptacle  22  of a receiver aircraft  20 , and thus can be similar to the nozzle  235  of the second example of nozzle  135  of the first example, or of alternative variations thereof, and as disclosed above, mutatis mutandis. 
     In this variation of the second example, the boom member  230 B has a fixed axial length and is thus not extensible, providing for a relatively simple construction. However, optionally, the boom member  230 B can instead comprise a plurality of telescopic elements, for example similar to the plurality of telescopic elements  232  of the second example of refueling device  200  illustrated in  FIGS. 16( a ) to 16( d ) , mutatis mutandis. 
     The boom member  230 B is pivotably mounted to body  210 B about axis C (generally parallel to the pitch axis P of the body  210 B) at pivot joint  219 B, and is reversibly pivotable between a stowed or retracted position and a deployed position. 
     In the stowed or retracted position, illustrated in  FIG. 17( a ) , boom member  230 B is pivoted about axis C into a position where the terminus  236 B is closest to the underside of body  210 . In this position, the boom axis  231  is generally parallel to and displaced away from longitudinal axis  211 B in a downward direction with respect to body  210 B. In the deployed position illustrated in  FIG. 17( b ) , boom member  230 B is variably pivoted about pivot axis C in a downward direction to provide a non-zero angular displacement, angle θ′, between boom axis  231 B and longitudinal axis  211 B (best seen in  FIG. 17( c ) . In this example, angle θ′ is in pitch with respect to the refueling device  200 B. 
     In any case, in the retacted position, the boom axis  230 B is at a smaller angular disposition with respect to said longuitudinal axis  211 B than in the deployed position. For example, in the retracted position the boom axis  230 B is at an angular disposition with respect to said longuitudinal axis  211 B of 0°, or 15°, or between 0° and 15°, for example any one of 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°. For example, in the deployed position the boom axis  230 B is at an angular disposition with respect to said longuitudinal axis  211 B of greater than 15°, for example 20°, or 45°, or between 20° and 40°, or between 20° and 45°, for example any one of 21°, 22°, 23°, 24°, 25°, 26°, 27°, 28°, 29°, 30°, 31°, 32°, 33°, 34°, 35°, 36°, 37°, 38°, 39°, 40°, 41°, 42°, 43°, 44°. 
     In general the boom member  230 B is in the deployed position at least during in-flight refueling operation of the device  200 B. 
     An actuation mechanism  290 B is provided for controllably pivoting the boom member  230 B between the stowed or retracted position and the deployed position. Actuation mechanism  290 B includes an articulated strut  291 B comprising upper strut  292 B connected to lower strut  293 B at pivoting joint  294 B. The upper strut  292 B is pivotably connected to an underside of body  210 B, while the lower strut  293 B is pivotably connected to an upper side of boom member  230 B. An actuator (not shown) operates to selectively and effectively bring close or distance away the pivoting joint  294 B with respect to the body  210 B. At the maximum distancing away of the pivoting joint  294 B, illustrated in  FIGS. 17( b ), 17( c ) and 17( d ) , the upper strut  292 B is aligned with (i.e., at about 180° with respect to) the lower strut  293 B, and as the pivoting joint  294  is brought closer to the body  210 B, the articulated strut  291 B adopts a V-configuration, where the pivot angle between upper strut  292 B is and the lower strut  293 B at the pivoting joint  294 B progressively reduces from about 180° (at the fully deployed position) to about 0° (at the stowed or retracted position). The actuator or actuation mechanism  290 B can be configured to selectively lock the articulated strut  291 B only at each one of the stowed/retracted position and the deployed position, to provide a fixed angle θ′; alternatively, the actuator or actuation mechanism  290 B can be configured to selectively lock the articulated strut  291 B at each one of the stowed/retracted position and  20  the deployed position, and at any angular disposition therebetween, to provide a variable angle θ′. 
     Other alternative configurations for actuation mechanism  290 B are of course possible. 
     Angular components in yaw and/or roll can be provided to the boom axis  231 B with respect to the forward direction A by suitably orienting the device  200 B with respect to the yaw axis Y and/or roll axis R, respectively. 
     The boom member  230 B comprises a coupling  240 B at forward end  214 B thereof, fixed to the underside of body  210 B. The coupling  240 B is similar to the coupling  240  of the second example or alternative variations thereof, or of coupling  140  of the first example or alternative variations thereof, and as disclosed above, mutatis mutandis. Furthermore, the pivot joint  219 B can be part of coupling  240 B, or integrated therewith, or can be affixed thereto and/or to the body  210 . The coupling  240 B is connected to the body  210 B for example at the underside of the body  210 B. 
     The refueling device  200 B further comprises a spatial control system  260 B and an aerodynamic stabilizer arrangement, different from the spatial control system  260 B. For example, the aerodynamic stabilizer arrangement is in the form of a respective drogue structure  280 B fixed to the aft portion of the body  210 . Referring to  FIGS. 17( a ) and 17( b )  respectively, the drogue structure  280 B has an inactive (or stowed) configuration, in which drogue structure  280 B generates minimum drag, and an active (or deployed) configuration in drogue structure  280 B generates more drag than in the inactive configuration, up to a maximum drag. Such a drogue structure  280 B can be utilized for generating a drag (when in the active configuration of  FIG. 17( b ) ) which in turn induces a tension to the hose  52 , thereby aiding reduction or dampening of vibrations or oscillations in the hose  52  that can otherwise occur. 
     The spatial control system  260 B is configured for controlling a spatial disposition of the refueling device  200 B when towed aft of the tanker aircraft  12  via the hose  52 . In particular, and in a similar manner to the second example illustrated in  FIGS. 16( a ) to 16( d ) , mutatis mutandis, spatial control system  260 B is also configured for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  231 B and the forward direction A, and enables this angle θ to be selectively maintained between the boom axis  231 B and the forward direction A when the refueling device  200 B is being towed by the tanker aircraft  12  via hose  52 , also similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     Thus, in particular, angle θ is in pitch, i.e., about a pitch axis P of the refueling device  200 B and is defined on a plane including the roll axis R and the yaw axis Y of the refueling device  200 B. Nevertheless, and depending on specific conditions during any particular refueling operation, angle θ can instead include an angular displacement component between the boom axis  231 B and the forward direction A in yaw (i.e., about yaw axis Y) for example due to sideslip angle, and/or in roll (i.e. about roll axis R), in addition to an angular displacement component in pitch (i.e., about pitch axis P). 
     Thus, the spatial control system  260 B is configured for controllably flying the refueling device  200 B, and for providing stability to the refueling device  200 B, while tethered and towed via the hose  52 , and while the boom axis  231 B is at any desired pitch and/or yaw and/or roll angle corresponding to the aforesaid angle θ, and in particular, angle θ is a design angle (angle θ des ) is within a particular angular range which corresponds to the design relative angular position of the boom member  230 B (and boom axis  231 B) with respect to the receiver aircraft  20  similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis. 
     As with the first example of the refueling device or alternative variations thereof, or of the second example of the refueling device or other alternative variations thereof, the spatial control system  260 B of refueling device  200 B, in particular the selectively controllable aerodynamic control system  270 B, is configured for enabling the device  200 B to be steered in one, two, or three degrees of freedom in translation and in one, two, or three degrees of freedom in rotation, independently of the tanker aircraft  12  or of the refueling aircraft  20 . Thus, the spatial control system  260 B, in particular the selectively controllable aerodynamic control system  270 B, is configured providing:
         one or more of: sideslip, up/down translation, forward-aft translation, relative to the tanker aircraft  12  and/or to the refueling aircraft  20 , independently of rotational moments in roll pitch and/or yaw;   and/or   rotational moments in one or more of roll pitch and/or yaw, relative to the tanker aircraft  12  and/or to the refueling aircraft  20 , independently of sideslip, up/down translation, forward-aft translation.       

     The spatial control system  260 B is also configured for providing an angle of attack for the body  210 B with respect to the forward direction, for example up ±10°. 
     In this variation of the second example of refueling device  200 B, though, at least a part of angle θ, in particular a part of the design angle θ des  is provided by angle θ′, i.e., by pivoting the boom member  230 B about axis C, depending on the magnitude of the relative angular disposition ϕ between the longitudinal axis  211 B and the forward direction A, referring to  FIG. 17( c ) . This angular disposition can be positive, representing a positive angle of attack of body  210 B with respect to forward direction A, or can be negative, or can be zero. In this variation of the second example, the spatial control system  260 B is configured for providing a zero or near zero said angular disposition when the boom member  230 B is in its deployed position pivoted at angle θ′. 
     The spatial control system  260 B comprises a selectively controllable aerodynamic control system  270 B. In this variation of the second example of the refueling device  200 B, the aerodynamic control system  270 B comprises a forward set  272 B of aerodynamic control surfaces  273 B and an aft set  274 B of aerodynamic control surfaces  275 B. 
     Furthermore, in the example of  FIGS. 17( a ) to 17( e ) , the a forward set  272 B of aerodynamic control surfaces  273 B is in the form of four canards  273 K in cruciform “X” arrangement around a forward part of the body  210 , while the aft set  274 B of control surfaces  275 B is in the form of a H-tail, in particular a high, cropped delta wing  276 B, mounted to the upper aft part of body  210 , and comprising vertical fins  277 B above and below port and starboard wing tip elements  278 B of the cropped delta wing  276 B. The forward canards  273 K can be fixed, or instead can be pivotable, or instead can comprise pivotable surfaces to provide control moments to the device  200 B. The delta wing  276 B, and/or vertical fins  277 B, are each pivotable or can instead comprise pivotable surfaces to provide control moments to the refueling device  200 B. 
     However, other arrangements are possible for selectively controllable aerodynamic control system  270 B and/or for the body  210 B. 
     For example the forward set  272 B of aerodynamic control surfaces  273 B and/or the aft set  274 B of aerodynamic control surfaces  275 B can each be configured to have any one or more of the following features, in any combination:
         monoplane configuration, including any one of: high wing configuration (or shoulder wing configuration), low wing configuration or mid wing configuration   mounted on or near an upper part, a lower (bottom) part of inbetween the upper and lower part, respectively, of the body  210 ; parasol wing configuration—mounted to the body  210 B via cabane struts of the like; shoulder wing configuration;   biplane, triplane, quadruplane, multiplane configurations, having two, three four, or more than four wing plane elements, respectively, of similar size or dissimilar size with respect to one another, stacked one above the other in unstaggered, or forward staggered, or backwards staggered arrangement;   combined or closed wing configurations, in which two or more wing elements are joined structurally at or near the respective wing tips in some way; for example a box wing configuration, in which at least one set of overlying (staggered or unstaggered) wing elements is joined together between their tips by vertical fins; tandem box wings; rhomboidal wings in which at least one set of overlying wing elements has a forward swept wing plane and a swept back wing plane, joined between the tips directly or via a vertical fins; annular or ring wing, which can be flat (in the form of the rim of a flat disc) or cylindrical (the wing is shaped as a cylinder), for example;   at least one wing element is cantilevered (self-supported) and/or externally supported to the body  210  via struts and/or braces;   wings elements, wherein each can comprise low aspect ratio, moderate aspect ratio or high aspect ratio;   wings elements, wherein each wing element can be swept forward or swept back or have zero sweep, and/or the sweep angle can be fixed or varied along the span, and/or each wing element can have fixed wing geometry or variable wing geometry, for example variable sweep or oblique wing configurations;   wings elements, wherein each wing element can have a respective wing chord that can be fixed or varied along the span of the wing element, for example including at least one of the following plan shapes: elliptical plan; constant chord plan, tapered plan; trapezoidal plan; reverse tapered plan; compound tapered plan;   wings elements, wherein each wing element can be based on a delta design, including at least one of the following: regular delta; cropped delta (wing tip is cropped) compound delta; double delta; ogival delta;   wings elements, wherein each wing element can have dihedral or anhedral angle;   wings elements, wherein the wing elements can be formed as fins, for example in cruciform “X” or cruciform “+” configuration, or having one, two, three, four, or more than four fins arranged on the body  210 B in any geometrical arrangement;   wings elements, wherein the wing elements can comprise vertical fins or the like, attached on the upper part and/or the lower part at any spanwise position including the tip; and/or the fins can be swept forward or swept back or have zero sweep, and/or sweep angle can be fixed or varied along the span, and/or each vertical fin can have fixed wing geometry or variable wing geometry, for example variable sweep or oblique wing configurations;   each wing element can be fixed, or can be movably mounted to the body  210  and fully pivotable to operate independently as an integral control surface, or can be fixedly mounted to the body  210  and comprises a pivotal control surface;   each wing element can be movably mounted to the body  210  to allow for selective relative translational movement therebetween.       

     For example the forward set  272 B of aerodynamic control surfaces  273 B can have any suitable configuration regarding its geometrical and spatial relationship with respect to the aft set  274 B of aerodynamic control surfaces  275 B, for example as follows:
         conventional configuration, in which the forward set  272 B of aerodynamic control surfaces  273 B forms the main lift-generating wing arrangement of the device  200 B, while the aft set  274 B of aerodynamic control surfaces  275 B, forms part of the stabilizer or tail;   canard configuration, in which the aft set  274 B of aerodynamic control surfaces  275 B forms the main lift-generating wing arrangement of the device  200 B, while the forward set  272 B of aerodynamic control surfaces  273 B can be in the form of canards or fins as the stabilizer;   tandem configuration, in which both the aft set  274 B of aerodynamic control surfaces  275 B and the forward set  272 B of aerodynamic control surfaces  273 B are configured to provide lift and to provide stability;   tailess configuration, in which the forward set  272 B of aerodynamic control surfaces  273 B is omitted, and the aft set  274 B of aerodynamic control surfaces  275 B is configured to provide lift and to provide stability;   three-surface or triplet configuration, in which in which the aft set  274 B of aerodynamic control surfaces  275 B forms the main lift-generating wing arrangement of the device  200 B, while the forward set  272 B of aerodynamic control surfaces  273 B can be in the form of canards or fins forming part of the stabilizer, and further comprising a third set of aerodynamic control surfaces aft of the aft set  274 B of aerodynamic control surfaces  275 B, forms part of the stabilizer.       

     For example the forward set  272 B of aerodynamic control surfaces  273 B and/or the aft set  274 B of aerodynamic control surfaces  275 B can be blended with the body  210 B to provide a blended body configuration. 
     For example one of the forward set  272 B of aerodynamic control surfaces  273 B and/or the aft set  274 B of aerodynamic control surfaces  275 B can be omitted, and the other one of forward set  272 B of aerodynamic control surfaces  273 B and/or the aft set  274 B of aerodynamic control surfaces  275 B can be formed as a flying wing configuration, incorporating therein the functions of body  210 B, which can then be omitted. 
     For example the forward set  272 B of aerodynamic control surfaces  273 B and/or the aft set  274 B of aerodynamic control surfaces  275 B can both be omitted, and the body  210  can be formed as a lifting body, integrally providing the functions of the aerodynamic control system  270 B. 
     For example, the aerodynamic control system  270 B can be replaced with or supplemented by reaction control thrusters. 
     Thus, for example, and referring to  FIGS. 17( f ) and 17( g ) , the aerodynamic control system  270 B comprises a canard configuration, in which: the forward set  272 B of aerodynamic control surfaces comprises canards, for example comprising two horizontal, swept, zero taper canards  273 C in mid-wing configuration, one on each side of the body  210 B, and optionally including vertical swept-back zero taper fins  273 D above and/or below the canards  273 C at the respective tips thereof; the aft set  274 B of aerodynamic control surfaces can be in the form of a H-tail, in particular comprising two horizontal, swept, zero-taper wing elements  273 E in high-wing configuration, on the upper part of the body  210 B, and optionally including fins, for example vertical swept-back zero taper fins  273 F above and below the wing elements  273 E at the respective tips thereof. In this example, one or more of the canards  273 C, fins  273 D, wing elements  273 E and fins  273 F, is fully pivotable to operate independently as an integral control surface or is fixedly mounted to the body  210  and comprises a pivotal control surface. Optionally, one or more of the canards  273 C, fins  273 D, wing elements  273 E and fins  273 F, can be movably mounted to the body  210  to allow for selective relative translational movement therebetween. 
     In another example, and referring to  FIG. 17( h ) , the aerodynamic control system  270 B comprises a canard configuration, in which: the forward set  272 B of aerodynamic control surfaces comprises a canard configuration, for example comprising two horizontal, swept, zero taper canards  273 C in mid-wing configuration, one on each side of the body  210 B, and optionally including vertical swept-back zero taper fins  273 D above and below the canards  273 C at the respective tips thereof; the aft set  274 B of aerodynamic control surfaces can be in the form of a H-tail, in particular comprising two horizontal, swept, zero-taper wing elements  273 G in mid-wing configuration, one on each side of the body  210 B, and optionally including fins, for example vertical swept-back zero taper fins  273 H above and below the wing elements  273 G at the respective tips thereof. In this example, one or more of the canards  273 C, fins  273 D, wing elements  273 G and fins  273 H, is fully pivotable to operate independently as an integral control surface or is fixedly mounted to the body  210  and comprises a pivotal control surface. Optionally, one or more of the canards  273 C, fins  273 D, wing elements  273 G and fins  273 H, can be movably mounted to the body  210  to allow for selective relative translational movement therebetween. 
     It is to be noted that in other variations of the second example, and in other examples, of the refueling device  200 , the respective spatial control system  260  can be similar to the spatial control system  260 B the variation of the example of the refueling device  200 B or alternative variations thereof and as disclosed above, mutatis mutandis. 
     The refueling device  200 B can optionally further comprise a force generating arrangement (not shown), similar to the corresponding feature of the first example or alternative variations thereof and as disclosed above, mutatis mutandis, and/or can be operated in a corresponding manner. 
     In the variations of the second example illustrated in  FIGS. 17( a ) to 17( h ) , a respective force generating arrangement  295 B is configured for selectively generating a force FB (see  FIG. 17( c ) ) along the boom axis  231 B in a direction towards nozzle  235 B. 
     In these variations of the second example, the force generating arrangement  290 B comprises (a) at least some elements of the spatial control system  260 B, in particular the selectively controllable aerodynamic control system  270 B; and optionally (b) at least some elements of the aerodynamic stabilizer arrangement, in particular in the form of a respective drogue structure  280 B. 
     The force generating arrangement  290 B is configured for selectively generating force FB in a direction aligned with the boom axis  231 B by generating a negative lift force LF (or reducing the lift force by force LF) and a drag force LD, which together provide force FB of the required magnitude and vector. The negative lift force LF can be generated by suitably controlling the spatial control system  260 B, in particular the selectively controllable aerodynamic control system  270 B. For example, appropriately changing an angle of attack, and/or providing a flap angle to the respective control surfaces of the control system  270 B can reduce the lift generated by the control system  270 B, and thus result in a net downwards force corresponding to negative lift force LF. Concurrently, in at least some cases, the drag force LD can also be generated by suitably controlling the spatial control system  260 B, in particular the selectively controllable aerodynamic control system  270 B. For example, appropriately changing an angle of attack, and/or providing a flap angle to the respective control surfaces of the control system  270 B can also change the drag generated by the control system  270 B, and thus result in an increase in drag corresponding to drag force LD. Additional drag force can be generated, where necessary to complement or replace the drag generated by the control system  270 B to provide the appropriate drag force LD, by controlling the drag generated by the aerodynamic stabilizer arrangement, in particular in by the drogue structure  280 B. 
     The force generating arrangement  295 B is in particular configured for selectively generating a force FB having a magnitude sufficient for forcing the nozzle  235 B into engagement with the fuel receptacle  22  of the receiver aircraft when the nozzle  295 B (and  30  the boom member  230 B) and the fuel receptacle  22  are in a predetermined relative spatial disposition, i.e., when the refueling device  200 B reaches an engagement enabling position and the boom axis  231 B is in the engagement enabling orientation with respect to the receiver aircraft  20 , and in particular with respect to the fuel receptacle  22 . 
     The force generating arrangement  295 B is further configured for selectively operating in this manner responsive to the nozzle  235 B being in a predetermined proximity to the fuel receptacle  22 , i.e. responsive to the nozzle  235 B being in a predetermined spacing with respect to the fuel receptacle  22 , typically the engagement enabling spatial position, and can be operated manually or automatically to provide such a force FB, as will become clearer herein. 
     Thus, at the engagement enabling position, when the boom member  230 B, or the boom axis  231 B, is in a predetermined spatial disposition with respect to the fuel receptacle  22  and the nozzle  235 B being in a predetermined spacing with respect to the fuel receptacle  22  (i.e., at the engagement enabling position the boom axis is at the engagement enabling orientation—corresponding to the design angle θ des ), the force generating arrangement  295  can be selectively actuated to compel the boom member  230 B to follow a predetermined trajectory (together with the device  200 B), for example aligned with the boom axis  231 B in the direction of the receiver aircraft  20 , to ensure alignment and engagement between the nozzle  234 B and the fuel receptacle  22 . In this example, the boom member  230 B (and thus the boom axis  231 B) is maintained at the engagement enabling orientation—corresponding to the design angle θ des , while the body  210 B remains at the same spatial disposition with respect to the receiver aircraft  20 . The body  210 B is moved towards the receiver aircraft  20  along a the direction of the boom axis  231 B, maintaining the boom axis  231 B at the engagement enabling orientation—corresponding to the design angle θ des , to effect engagement between the nozzle  235 B and the fuel receptacle  22 . In alternative variations of this example, the boom  230 B is telescopic, and is partially or fully telescopically extended towards the receiver aircraft  20  while the device  200 B can be moved towards or away from the receiver aircraft  20  to effect engagement between the nozzle  235 B and the fuel receptacle  22 . 
     Once the nozzle  235 B is forced into engagement with the fuel receptacle  22  of the receiver aircraft  20 , the tanker aircraft  12  can begin refueling the receiver aircraft  20 . 
     In alternative variations of this example and in other examples, the force generating arrangement  295 B can comprise any other suitable drag inducing arrangement, for example spoilers on the boom member  230  and/or on other parts of the device  200 B. 
     In yet other variations of this example and in other examples, the force generating arrangement  295 B can be configured for generating a thrust force in the required direction. For example, one or a plurality of suitable thrust nozzles can be mounted to the body  210 B and/or to the boom member  230 B to provide the required thrust vector parallel to the boom axis  231 B towards nozzle  235 B. Such thrust nozzle(s) can be operatively connected to a suitable compressed air or compressed gas supply, for example carried by the refueling device itself, or carried by the tanker aircraft and supplied to the refueling device via pneumatic or gas lines, or generated by the tanker aircraft and/or the refueling device. 
     In yet other alternative variations of this example and in other examples, the force generating arrangement can be omitted, and for example the receiver aircraft and/or the device  200 B can comprise suitable means for mechanically engaging the nozzle  235 B to the fuel receptacle that does not require such a force FB to be generated by the device  200 B. For example, the fuel receptacle and/or the boom member  230 B can comprise a suitable mechanical clamp that engages the terminus  236 B of the boom member  230 B to the fuel receptacle  22 , and pulls in the nozzle  235 B into engagement with the fuel receptacle  22 . 
     The device  200 B comprises controller  285 B for controlling operation of one or more of the force generating arrangement  290 B, the spatial control system  260 B (in particular the selectively controllable aerodynamic control system  270 B), the aerodynamic stabilizer arrangement (in particular in the form of a respective drogue structure  280 B), for example similar to controller  180  as described herein, mutatis mutandis, and thus for example comprises any suitable computer control system, and can be internally or externally mounted in the refueling device  200 B. In alternative variations of this example and in other examples, the controller  285 B or portions thereof can instead comprise any suitable electronic control unit, or any other suitable control unit, and/or the controller  285 B or portions thereof can be comprised elsewhere in the in-flight refueling system  50  or in the tanker aircraft  12 . 
     The refueling device  200 B further comprises a suitable spatial data acquisition system, also referred to herein as a data acquisition system  299 B for providing or enabling the calculation of spatial data relating to the relative spatial dispositions between the refueling device  200 B and the receiver aircraft  20 , in particular the relative spatial dispositions between the fuel delivery nozzle  235 B of the refueling device  200 B and the fuel receptacle of the receiver aircraft, to enable selectively controlling the refueling device to provide automatic (optionally including autonomous) and/or manual steering of the refueling device  200 B to the engagement enabling position and subsequent selective engagement of the fuel delivery nozzle to the fuel receptacle of the receiver aircraft. 
     In this variation of the second example of the device  200 B, the data acquisition system is in the form of imaging system  289 B, in particular configured for providing imaging data of any object coming within a field of regard (FOR) aft of the refueling device  200 B. 
     The imaging system  289 B is operatively connected to a control computer system  255 B, for example similar to control computer system  155  as described herein, mutatis mutandis, and which can be integral with, connected to, or independent from controller  285 B. In particular, and referring particularly to  FIG. 17( b ) , such an object is the receiver aircraft  20  and more particularly a part AP thereof including the fuel receptacle  22 , and the sensing volume  259 B defines an outer envelope limit  258 B in which image data of part AP can be processed, inter alia, by control computer system  255 B to provide control signals, for example steering commands, to the spatial control system  260 B and/or the force generating arrangement  290 B, for example via controller  285 B to control operation of the refueling device  200 B, in particular the relative spatial position and orientation of the refueling device  200 B with respect to the receiver aircraft  20 , in particular the position and orientation of the boom member  230 B and nozzle  235 B with respect to the fuel receptacle  22 , so that the nozzle  235 B can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner. The manner of operation of the imaging system  289 B and control computer system  255 B will be described in greater detail further herein. 
     In alternative variations of this example, the imaging system  289 B can be replaced with any other suitable data acquisition system for providing the aforesaid spatial data. 
     In yet other alternative variations of this example and in other examples, the refueling device  200 B can omit the imaging system  289 B and can be actively controlled by an operator, for example, to control the relative spatial position and orientation of the refueling device  200 B with respect to the receiver aircraft  20 , in particular the spatial position and orientation of the boom member  230 B and/or nozzle  235 B with respect to the fuel receptacle  22 , so that the nozzle  235 B can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner, for example via direct visual tracking of the device by one or more operators (for example, an operator can be in the tanker aircraft  12  and/or an operator can be in the refueling aircraft  20 ). Alternatively, the refueling device can be operated as a free flying refueling device towed at the end of hose  52 , and the relative spatial position and orientation of the refueling device  200 B with respect to the receiver aircraft  20  (in particular the position and orientation of the boom member  230 B and nozzle  235 B with respect to the fuel receptacle  22 , so that the nozzle can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner) is achieved by maneuvering the receiver aircraft  20  only. In such a case, the spatial control system  260 B can optionally comprise a non-adjustable aerodynamic stability system that is configured for allowing the refueling device  200 B to adopt a particular, pre-set, desired angle θ while maintaining a zero pitching moment (and/or zero yawing moment and/or zero rolling moment), this being the design angle θ des  as discussed above for example. 
     Optionally, a suitable air-driven generator can be provided in the refueling device  200 B to provide electrical power thereto. Additionally or alternatively, electrical power can be provided to the refueling device  200 B by the tanker aircraft  12 . Additionally or alternatively, electrical power can be provided to the refueling device  200 B by one or more batteries in the refueling device  200 B. Additionally or alternatively, electrical power can be provided to the refueling device  200 B by one or more ram air turbines (RAT), affixed internally or externally with respect to the refueling device  200 B. 
     In this variation of the second example of device  200 B, the imaging system  289 B is for example similar to imaging system  350  as described herein, mutatis mutandis, and comprises one or more Light Detection And Ranging (LIDAR) units  351 , for example similar as described herein, mutatis mutandis, which can utilize eye-safe laser. The imaging system  350  in this example is located on the underside of the body  210 B near the nose of body  210 B, but in alternative variations of this example the imaging system  289 B can be located elsewhere on the device  200 B, for example the at  289 B′, near the tail of the body  201 B, or on the spatial control system  260 B, so long as the respective sensing volume  359  extends beyond the position of the nozzle  235 B to include the nozzle  235 B, and part AP of the receiver aircraft  20  when the part AP is in close proximity to the nozzle  235 B. 
     In alternative variations of this example, the LIDAR unit  351  can be replaced with any other suitable imaging system  350  that provides depth data and electromagnetic intensity data of objects within the sensing volume  359  (including, but not limited to, Flash LADAR, 3D Flash LIDAR Camera, etc.). In still further alternative variations of this example, the LIDAR unit  351  can be replaced with other suitable imaging systems such as stereoscopic cameras, conventional cameras, various radar systems, etc. 
     In any case, the control computer system  355 , included in controller  285 B and/or controller  255 B for example, comprises a memory including geometrical data of the shape of at least part of the receiver aircraft  20 , in particular part AP and or the fuel receptacle  22 . The control computer system  355  is further configured for operating on the depth data and the geometrical data to enable identification of a first part of the depth data that corresponds to the part AP and or the fuel receptacle  22 . 
     According to certain examples of the presently disclosed subject matter the data acquisition system  299 B further includes fuel receptacle marker  342 , comprised on the receiver aircraft  20  in a pre-determined location with respect to the fuel receptacle  22  thereof. The fuel receptacle marker  342  is at a fixed and known geometrical relationship with respect to the fuel receptacle  22 , and is electromagnetically visible to the imaging system  350 , at least during operation thereof. 
     In this example, the fuel receptacle marker  342  comprises a retro-reflective surface that reflects incident beams along the same path, and thus provides a strong intensity reflection of the respective reflected beam when illuminated by a beam, as compared with the reflection intensity obtained from other surfaces of the receiver aircraft  20 , for example. Such a retro-reflective surface may be provided via a retro-reflective material affixed to a certain known location visible to the imaging system  350  on the receiver aircraft  20 . Such retro-reflective materials are well known in the art, and can include for example retro-reflective tape or retro-reflective paint. 
     It is to be noted that the data acquisition system  299 B can include any one of the boom tip marker  340  and the fuel receptacle marker  342 , or a combination of both. Alternatively, in some cases, the data acquisition system  299 B can include none of the markers (in this case the natural reflective properties of the surfaces can be used). 
     The control computer system  355  is further configured for operating on the depth data and the electromagnetic intensity data, as disclosed herein, mutatis mutandis, to enable identification of a second part of the depth data that corresponds to the high intensity reflection originating from the boom tip marker  340 , which in turn enables identification of the part of a third depth data that corresponds to nozzle  316  since the relative spatial relationship between the boom tip marker  340  and nozzle  316  is known. 
     Accordingly, when the aforesaid first part and second part (first part and third part) of the depth data is known, the control computer system  355  can determine the relative disposition between the boom system  300 , and in particular the nozzle  316 , and the receiver aircraft  20 , in particular the fuel receptacle  22  thereof. 
     The in-flight refueling systems  50  including this alternative variation of the second example of the refueling device  200 B, and at least some alternative variations thereof, can also be operated in a number of different ways to provide selective engagement of the nozzle  235 B with the fuel receptacle  22  of a receiver aircraft  20 , and enable subsequent refueling of the receiver aircraft  20  from the tanker aircraft  12  in flight, for example as disclosed herein for the first example or alternative variations thereof, or the second example, mutatis mutandis. 
     Referring to  FIG. 30 , a tanker system according to another example of the presently disclosed subject matter, generally designated  10 ′, comprises a tanker aircraft  12 ′ comprising an aircraft-fixed flying boom system, designated by the reference numeral  300 , and may optionally further comprise one or more non-aircraft-fixed in-flight refueling systems. For example, each non-aircraft-fixed in-flight refueling system can comprise an in-flight refueling system  50  as disclosed above with reference to  FIGS. 1 to 29 , mutatis mutandis. For example, the tanker aircraft  12  can have two such in-flight refueling systems  50 , one comprised on each one of the port wing and starboard wing, and, together with the fixed flying boom system  300 , the tanker aircraft  12 ′ is configured for in-flight concurrent refueling of up to three receiver aircraft  20 . In yet other alternative variations of this example the tanker aircraft  12 ′ can have no such in-flight refueling systems  50 , or at least one, or two, or more than three in-flight refueling systems  50 , arranged in any suitable configuration with respect to the tanker aircraft  12 ′. 
     The fixed flying boom system  300  comprises a refueling device in the form of telescoping boom fuelling unit  310 , which comprises a spatial control system including at least one of a mechanical connection  320  and a motion control system  330 . 
     The boom fuelling unit  310  is movably affixed at its forward end to an underside of the aft end of the fuselage of the tanker aircraft  12 ′ via the mechanical connection  320 , such as for example an articulation joint, gimbals, and so on. The mechanical connection  320  is configured for providing the boom fuelling unit  310  with two rotational degrees of freedom about the yaw and pitch axes at mechanical connection  320 , relative to the tanker aircraft  12 ′. 
     The boom fuelling unit  310  comprises an elongate boom member  312 , and a telescoping aft section  314  configured for being selectively telescopically deployed and selectively telescopically retracted into boom member  312 , along boom axis  311 , under the control of controller  390  (that in some cases can be controller  180  or part thereof), providing the boom system  300  with a translational degree of freedom with respect to the tanker aircraft  12 ′. At the boom tip or terminus of aft section  314 , the boom fuelling unit  310  comprises a fuel delivery nozzle module  317  including fuel delivery nozzle  316 , which is in selective fuel communication, via a hose, pipe and so on (not shown), with a refueling fuel tank (not shown) carried by the tanker aircraft  12 ′. The nozzle  316  is configured for reversibly engaging with the fuel receptacle  22  of a receiver aircraft  20  (in a similar manner to the examples illustrated in  FIGS. 1 to 29  disclosed herein, mutatis mutandis), and thus can comprise any conventional design of such nozzles, which are well known, or indeed can comprise any other current or future design of such an in-flight refueling nozzle. 
     The boom fuelling unit  310  further comprises motion control system  330  configured for controlling the position of the boom fuelling unit  310  in elevation and azimuth, i.e., about the pitch and yaw axes at mechanical connection  320 . In this example, the motion control system  330  comprises aerodynamic lift/control surfaces, also known as ruddevators  325 , operatively connected to controller  390 . While in this and other examples ruddevators  325  can be in a V-tail type configuration, in yet other alternative variations of this example the aerodynamic lift/control surfaces can have any other suitable configuration. Selectively and controllably changing the incidence angles of the ruddevators  325  via controller  390  generates aerodynamic forces that enable the boom fuelling unit  310  to be aligned or aimed in any particular direction within a predefined envelope. Additionally or alternatively, the motion control system  330  comprises reaction control thrusters, which can also be operatively connected to controller  390 . 
     In particular, motion control system  330  is configured for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  311  and the longitudinal axis of the receiver aircraft  20 . 
     The boom system  300 , in particular the nozzle  316  can be sized to allow suitable fuel flow rates for refueling a wide range of receiver aircraft. By way of non-limiting example, relative high fuel flow rates (for example up to 1000 US gallons/6,500 lb per minute) can be provided for refueling operations of large aircraft (for example transport aircraft, bombers, etc), while for fighter aircraft that cannot accept fuel at the maximum flow rate of the boom system  300 , the refueling pressure can be correspondingly reduced. Alternatively the boom system  300  can be sized to allow suitable fuel flow rates for refueling a narrow range of receiver aircraft., for example only fighter aircraft or only larger aircraft (for example about 400 US gallons/2,600 lb per minute). 
     In operation of boom system  300 , angle θ can be chosen such as to be a nominal design angle (angle θ des ) that is within a particular allowable angular range which corresponds to the range of relative angular positions of the boom system  300  (and boom axis  311 ) with respect to the receiver aircraft  20 , which allow for engagement between the nozzle  316  and the fuel receptacle  22 . 
     In practice, the actual pitch, roll and yaw angles of the boom axis (and the extension of the telescoping aft section  314  with respect to boom member  312 ) determine the spatial position of the boom tip including the nozzle  316  with respect to the tanker aircraft  12 ′. In order to allow for engagement, the boom tip including the nozzle  316  needs to be within the respective refueling geometrical envelope and this position needs to be matched with the position of receptacle  22  of the receiver aircraft  20 . The refueling geometrical envelope represents the safe limits of movement for the boom fuelling unit  310  with respect to the receiver aircraft  20  and within which contact between the receiving aircraft  20  and the boom fuelling unit  310  is permitted, and can correspond to at least part of the sensing volume  359 . This position for the boom tip including the nozzle  316  can be achieved by adjusting angular alignment of the boom axis in elevation and azimuth, i.e., about the pitch and yaw axes at mechanical connection  320 , together with axial extension along the boom axis via extension/retraction of the telescoping aft section  314  with respect to boom member  312 . Thus, at a particular engagement enabling position, the boom tip and nozzle  316  can be positioned close to the receptacle  22 , with the boom axis being aligned at an angle θ which can deviate from the nominal design angle (angle θ des ) by a allowable angular clearance (+Δθ, −Δθ), but nevertheless still within the aforesaid allowable angular range (angle θ max  to angle θ min ). The structure of the nozzle  316  and the receptacle  22  can be such as to allow for such deviations of angle θ from angle θ des  within the aforesaid allowable angular range. For example, the nozzle  316  can incorporate a ball joint and the receptacle  22  can include a funnel guide for the nozzle  316  to provide the allowable angular clearance (+Δθ, −Δθ), for example in a similar manner to that schematically illustrated in  FIG. 10( b )  regarding other examples, mutatis mutandis. By way of non-limiting example, where the nominal design angle θ des  is +30 degrees, engagement between the nozzle  316  and receptacle  22  can occur even when the boom axis has an angle of +40 degrees in elevation and +8 degrees in azimuth, assuming that the nozzle  316  can engage with the receptacle  22  with a relative spatial disposition therebetween of −10 degrees in elevation and −8 degrees in azimuth. 
     In at least some cases, angle θ (which can have an angular component in yaw and/or in pitch and/or in roll) can be further compensated for any off-nominal pitch of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero angle of attack) and/or for any off-nominal roll of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero roll angle) and/or for any off-nominal yaw of the receiver aircraft  20  (for example if the receiver aircraft  20  is traveling along direction A at a non-zero sideslip angle) to ensure that the actual angular disposition between the boom axis  311  and the receiver aircraft longitudinal axis is maintained at design angle θ des  even as the relative spatial orientation between the receiver aircraft  20  and the forward direction changes. Such compensation can be achieved, for example, by steering the tanker aircraft  12 ′ in a corresponding manner. 
     Thus, at the design angle θ des  (and for other angles θ within the aforesaid allowable angular range (angle θ max  to angle θ min )) the boom axis  311  is in an engagement enabling orientation with respect to the receiver aircraft  20 , and in particular with respect to the fuel receptacle  22 . 
     In non-limiting examples, angle θ (and in particular angle θ des ) can be any suitable angle in a range between about 5° and about 85°; or in a range between about 10° and about 80°; or in a range between about 15° and about 70°; or in a range between about 20° and about 60°; or in a range between about 25° and about 50°; or in a range between about 20° and about 40°; or in a range between about 25° and about 40°; or in a range between about 28° and about 32°. 
     In one non-limiting example, angle θ des  can be about 30°, and operation of the boom system  300  to adopt this angle automatically renders it compatible for use with existing receiver aircraft  20 , in which the fuel receptacles  22  are configured for receiving and engaging with a nozzle at the end of a boom where the boom is at about 30° to the longitudinal axis of the receiver aircraft, without the need for modifying the configuration of the fuel receptacle thereof. 
     Thus, when angle θ is equal to design angle θ des , or within the aforesaid allowable angular range (angle θ max  to angle θ min ), the receiver aircraft travelling along direction A with zero angle of attack and zero sideslip and zero roll, and boom axis  311  is at the required spatial orientation to the forward direction A of the tanker aircraft and the receiver aircraft such as to ensure engagement between the nozzle  316  in the fuel receptacle  22 , without the need for modifying the configuration of the fuel receptacle thereof. 
     The boom system  300  further comprises a suitable data acquisition system  360  for providing or enabling the calculation of spatial data relating to the relative spatial dispositions between the boom system  300  and the receiver aircraft  20 , in particular the relative spatial dispositions between the fuel delivery nozzle module  317  including fuel delivery nozzle  316  of the boom system  300 , and the fuel receptacle  22  of the receiver aircraft, to enable selectively controlling the boom system  300  to provide automatic (optionally including autonomous) and/or manual alignment of the boom system  300  in the engagement enabling position and subsequent selective engagement of the fuel delivery nozzle to the fuel receptacle of the receiver aircraft. 
     In this example the data acquisition system  360  includes imaging system  350 , in particular configured for providing imaging data of any object coming within a field of regard (FOR), in particular the fuel delivery nozzle module  317  including fuel delivery nozzle  316 , and the fuel receptacle  22 . Such a field of regard has a predetermined depth aft of the imaging system and in this example comprises sensing volume  359  generally aft of the imaging system  350 , which for example comprises a prismoidal volume or any other suitable shape, for example conical, frustoconical, cylindrical, spherical, part-spherical (e.g. hemispherical), parallelepiped (for example cubic) or any other regular or irregular shape. The sensing volume  359 , i.e., the predetermined depth of the FOR, extends beyond the position of the fuel delivery nozzle module  317  further than is required corresponding to the engagement enabling position, i.e., further than the maximum extension of the aft section  314  when this is in its fully deployed position. The imaging system  350  is operatively connected to a control computer system  355 , which can be integral with, connected to, or independent from controller  390 . In particular, such an object is the receiver aircraft  20  and more particularly a part AP thereof including the fuel receptacle  22  and/or another part of the receiver aircraft  20  (including a part that does not include the fuel receptacle  22 , but, when recognized, can enable estimation of the fuel receptacle  22  position and orientation), and the sensing volume  359  defines an outer envelope limit  358  in which image data of part AP can be processed, inter alia, by control computer system  355 . The control computer system  355  can then provide control signals, for example alignment commands, to the motion control system  330 , for example via controller  390 , to control operation of the boom system  300 , in particular the relative orientation of the boom fuelling unit  310  and the relative position of the nozzle  316  with respect to the receiver aircraft  20 , in particular with respect to the fuel receptacle  22 , so that the nozzle  316  can be controllably brought into selective engagement with the fuel receptacle  22  in a safe and effective manner. 
     In this example, the imaging system  350  comprises one or more Light Detection And Ranging (LIDAR) units  351 , which can utilize eye-safe laser. The imaging system  350  in this example is located on the underside of the fuselage of the tanker aircraft  12 ′, but in alternative variations of this example the imaging system  350  can be located elsewhere on the tanker aircraft  12 ′, for example the wings, elevators, etc., so long as the sensing volume  359  extends beyond the position of the nozzle  316  to include the nozzle module  317  and nozzle  316 , and part AP when the part AP is in close proximity to the nozzle  316 . One exemplary non-limiting alternative location is shown in the figure under reference numeral  350 ′. 
     In particular, and referring also to  FIG. 31 , the LIDAR unit  351  is configured for providing depth data and electromagnetic data (for example electromagnetic intensity data) relating to any object within the sensing volume  359 , in particular relating to one or more of the nozzle module  317 , nozzle  316  and part AP. 
     In this example, the LIDAR unit  351  comprises a fiber laser unit  352  configured for generating and radiating a laser beam B 1  towards horizontally rotated polygon  353 , which in turn deflects the beam B 1  to a mirror  354 , which can be controllably moved in elevation. The controlled combined motion of the mirror  354  and polygon  353  scans the outgoing beam B 1  in a two-dimensional pattern SC in sensing volume  359 , along azimuth and elevation within sensing volume  359 , for example with respect to one or more planes BB orthogonal to a depth direction radiating from the LIDAR unit  351 . Each time the outgoing beam B 1  impinges on an object within the sensing volume  359 , a reflected beam B 2  returns along a path similar to that of the outgoing beam Bl, deflected by the mirror  354  and polygon  353 , and detected by detector  356 . 
     It is to be noted that in some cases, a single mirror which can be rotated in two dimensions can be used instead of the mirror  354  and polygon  353 . 
     Control module  369  includes one or more of a fast Analog to Digital Converter (ADC) card, Field Programmable Gate Array (FPGA), Digital Signal Processor (DSP), memory and power supply for operation of the LIDAR unit  351 . 
     Fast Analog to Digital Converter (ADC) card can be one having a typical conversion speed of about 1.5 GHZ or more. The fiber laser unit  352  can generate a laser pulse having a very fast rising time therefore the returning (i.e. the reflected) beam requires a very fast detector such as an Avalanche Photo Diode. An Avalanche Photo Diode is a very sensitive Photo Diode which enables measuring the time of light with an accuracy of typically about 10 centimeters. Such accuracy can be further increased for example using the known functional time evolution of the laser pulse. Using such algorithm, accuracies of few millimeters can be achieved. 
     The Field Programmable Gate Array (FPGA) can be used to enable the required processing power for processing the data acquired by the LIDAR unit  351 . 
     The Digital Signal Processor (DSP) can be a real time processor that can be configured to analyze the data acquired by the LIDAR unit  351  and provide the electromagnetic data and the depth data. Such data can be stored in the memory and can be transferred (e.g. transmitted) for further computations in one or more additional/alternative computers. 
     The fiber laser unit  352  is a Fiber laser which can comprise an optical fiber to which diode lasers (e.g. having a larger frequency than the required laser radiation) are attached for providing the necessary excited medium (e.g. the light amplifying medium). In some cases, the initiating laser diode at the required frequency is attached to the fiber and starts the laser operation. The laser beam is amplified in the fiber and an amplified beam goes out from the fiber to a collimator device to start the laser operation. A laser beam at 1.5 microns can be used, which is an eye safe light, since it cannot penetrate the cornea of the pilot and hence cannot focus on the retina and cause any damage. The laser beam can be typically composed of about one hundred thousand pulses per second. It is to be noted that the higher the number of pulses is, the higher resolution can be achieved. In some cases, the pulse duration of the laser beam can be about 2 to 10 nanosecond width. The energy of the pulsed laser beam can be set according to the required sampling distance. In some cases, no more than one hundred meters are needed, and in such cases the required energy is typically about five to twenty micro joules per pulse. The rise time of the laser beam can be of about few tenths of a nanosecond. This allows for accurate measurement of the time of light of the laser to the target. It is to be noted that in some cases gas or solid state laser can be alternatively used. 
     At any particular position of the mirror  354  and polygon  353  (corresponding to a two-dimensional position on plane BB, for example), the time interval between the outgoing beam B 1  and the return beam B 2  being detected by the detector  356  provides a measure of the depth of the part of the object which reflected the beam, thereby providing the depth data. In addition, the detector  356  also detects the intensity of the part of the object which reflected the beam to provide the intensity data. 
     In alternative variations of this example, the LIDAR unit  351  can be replaced with any other suitable imaging system  350  that provides depth data and electromagnetic intensity data of objects within the sensing volume  359  (including, but not limited to, Flash LADAR, 3D Flash LIDAR Camera, etc.). In still further alternative variations of this example, the LIDAR unit  351  can be replaced with other suitable imaging systems such as stereoscopic cameras, conventional cameras, various radar systems, etc. 
     In any case, the control computer system  355  comprises a memory including geometrical data of the shape of at least part of the receiver aircraft  20 , in particular part AP and or the fuel receptacle  22 . The control computer system  355  is further configured for operating on the depth data and the geometrical data to enable identification of a first part of the depth data that corresponds to the part AP and or the fuel receptacle  22 . 
     according to certain examples of examples of the presently disclosed subject matter the data acquisition system  360  further includes boom tip marker  340 , comprised in the fuel delivery nozzle module  317  proximate to the nozzle  316 . The boom tip marker  340  is at a fixed and known geometrical relationship with respect to the nozzle  316 , independently of the relative motion between the boom system  300  and the tanker aircraft  12 ′, and is electromagnetically visible to the imaging system  350 , at least during operation thereof. 
     In this example, the boom tip marker  340  comprises a retro-reflective surface that reflects incident beams along the same path, and thus provides a strong intensity reflection of the respective beam B 2  when illuminated by beam B  1 , as compared with the reflection intensity obtained from other surfaces of the boom fuelling unit  310 , for example. Such a retro-reflective surface may be provided via a retro-reflective material affixed to the fuel delivery nozzle module  317 . Such retro-reflective materials are well known in the art, and can include for example retro-reflective tape or retro-reflective paint. 
     According to certain examples of examples of the presently disclosed subject matter the data acquisition system  360  further includes fuel receptacle marker  342 , comprised on the receiver aircraft  20  in a pre-determined location with respect to the fuel receptacle  22  thereof. The fuel receptacle marker  342  is at a fixed and known geometrical relationship with respect to the fuel receptacle  22 , and is electromagnetically visible to the imaging system  350 , at least during operation thereof. 
     In this example, the fuel receptacle marker  342  comprises a retro-reflective surface that reflects incident beams along the same path, and thus provides a strong intensity reflection of the respective beam B 2  when illuminated by beam B 1 , as compared with the reflection intensity obtained from other surfaces of the receiver aircraft  20 , for example. Such a retro-reflective surface may be provided via a retro-reflective material affixed to a certain known location visible to the imaging system  350  on the receiver aircraft  20 . Such retro-reflective materials are well known in the art, and can include for example retro-reflective tape or retro-reflective paint. 
     It is to be noted that the data acquisition system  360  can include any one of the boom tip marker  340  and the fuel receptacle marker  342 , or a combination of both. Alternatively, in some cases, the data acquisition system  360  can include none of the markers (in this case the natural reflective properties of the surfaces can be used). 
     The control computer system  355  is further configured for operating on the depth data and the electromagnetic intensity data to enable identification of a second part of the depth data that corresponds to the high intensity reflection originating from the boom tip marker  340 , which in turn enables identification of the part of a third depth data that corresponds to nozzle  316  since the relative spatial relationship between the boom tip marker  340  and nozzle  316  is known. 
     Accordingly, when the aforesaid first part and second part (first part and third part) of the depth data is known, the control computer system  355  can determine the relative disposition between the boom system  300 , and in particular the nozzle  316 , and the receiver aircraft  20 , in particular the fuel receptacle  22  thereof. 
     The tanker  12 ′ can be configured to provide the maneuvering instructions for enabling positioning of the receiver aircraft  20  within an engagement area, for example in a similar manner to that disclosed herein with reference to the examples of  FIGS. 1 to 29 , mutatis mutandis, for example by utilizing a signaling system. Thus, such signaling system can be mounted, for example, on the tanker aircraft  12 ′, at any location visible to the receiver aircraft  20  pilot. In some cases, the signaling system can provide the receiver aircraft  20  pilot with maneuvering instructions on three axes: forward-backward, left-right and up-down, thus enabling it to maneuver the receiver aircraft  20  to the corresponding engagement area that allows for engagement between the fuel nozzle  316  and the fuel receptacle  22 . In some cases the signaling system can be a light directing system. Alternatively or additionally, the maneuvering instructions can be provided to by using voice commands (e.g. by utilizing speakers, pilot headset, etc.) or by any other means known per se. In some cases, a maneuvering instructions module can be configured to communicate the maneuvering instructions to an auto pilot system of the receiver aircraft  20 , if such system exists, for causing the auto pilot system to maneuver the receiver aircraft  20  accordingly. 
     The manner of operation of the imaging system  350  and control computer system  355  will be described in greater detail further herein. 
     In operation, the LIDAR unit  351  illuminates the sensing volume  359  and any object therein, in particular part AP of the receiver aircraft  20  and thereafter acquire suitable image data corresponding thereto which is sent to control computer system  355  for processing to provide the aforesaid control signals for controlling operation of the boom system  300 , in particular, to align the boom fuelling unit  310  to provide a desired relative position and orientation of the nozzle  316  with respect to the fuel receptacle  22 , in particular the engagement enabling position. 
     In alternative variations of this example and in other examples, the imaging system  350  can comprise any other suitable imaging system (for example, but not limited to, systems providing 2D images and/or stereoscopic images and/or 3D images of (including reconstruction of 3D data corresponding to) the sensing volume  359 , in particular but not limited to images that are updated in real time, for example in the form of a video stream) that operate to provide suitable data to the control computer system  355  to, in turn, enable selectively controlling the boom system  300  to provide autonomous and/or manual engagement of the nozzle  316  to the fuel receptacle  22  of the receiver aircraft  20 . For example the imaging system  350  can be similar to the imaging system disclosed herein with respect to the examples of  FIGS. 1 to 29 . 
     In alternative variations of this example, the fuelling unit  310  can be replaced with any other conventional or non-conventional so-called “flying boom” systems. 
     In alternative variations of this example, the part AP can also comprise a retro-reflective surface to help identify the part of the depth data corresponding to the part AP. 
     In alternative variations of this example, the boom tip marker  340  and/or the fuel receptacle marker  342  can comprise an electromagnetic source, for example a light source of different wavelength from that of the illumination beam Bl, and the illumination data received from the respective boom tip marker  340  and/or the respective fuel receptacle marker  342  (via the imaging system  350  and/or a second imaging system). 
     In alternative variations of this example, the imaging system  350  can be located in alternative locations on the refueling aircraft  12 ′ and/or on the boom member  312 , or on any other location. 
     There is now provided a description of certain examples of systems of controlling in-flight refueling. 
     Reference is now made to  FIG. 18 , which is a block diagram schematically illustrating a system for controlling in-flight refueling, according to certain examples of the presently disclosed subject matter. The system  1805  comprises at least one processing unit  1801 . The processing unit  1801  can be a microprocessor, a microcontroller or any other computing device or module, including distributed and/or multiple processing units, which are adapted to independently or cooperatively process data for controlling relevant system  1805  components and for enabling operations related to system  1805  components. 
     In some cases, the processing unit  1801  can be the control computer system  155 , or part thereof. In some cases, the processing unit can be the controller  180 . Alternatively, the processing unit  1801  can be a separate component. 
     In some cases, the processing unit  1801  can be located on-board the refueling device  100 , or on-board the receiver aircraft  20 , or on-board the tanker aircraft  12 . In some cases, more than one processing unit can be used and the plurality of processors can be cooperatively operated. 
     In some cases, the system  1805  can be distributed between the refueling device  100  and/or the receiver aircraft  20 , and/or the tanker aircraft  12  and/or any other location, including remote locations. The communication between the various components of the system  1805  can be realized by any communication components, protocols and modules, and can be wired or wireless. 
     The system  1805  further comprises a sensor control module  1810 . According to some examples of the presently disclosed subject matter, the sensor control module  1810  can be configured to utilize at least one sensor  1890  (possibly according to instructions from the processing unit  1801 ) as part of the operation of and control over the refueling process. 
     The sensor control module  1810  can be operatively connected to at least one sensor  1890  and can be configured to control the operation of the sensor  1890 . The sensor  1890  can be a suitable data acquisition system, for example, any image acquisition means such as a camera (e.g. a digital still camera, a digital video camera, a Flash LADAR camera, etc.). Alternatively, the sensor  1890  can be a Light Detection And Ranging (LIDAR) unit (including one using an eye-safe laser), a radar (including a radar utilizing an eye-safe laser), a laser array (including a laser array that utilizes eye-safe lasers), electro-acoustic sensors, etc. An exemplary non-limiting LIDAR unit that can be used is described herein, inter alia with reference to  FIGS. 31-33 . In some cases, sensor  1890  can be configured inter alia for providing data of any object coming within a field of regard (FOR) aft of the refueling device  100  (and/or the tanker aircraft  12 ). In some cases, the sensor  1890  can be the imaging system  150  or imaging system  350  detailed herein. In some cases, multiple sensors can be utilized, including, for example, a combination of the imaging system  150  and the imaging system  350 , or multiple redundant sensors of same type. In some cases, such combination of sensors can enable determination of spatial data also in cases when one of the imaging system  150  and the imaging system  350  does not provide the required data from some reason (e.g. due to a malfunction or due to environmental conditions such as clouds, dazzling light, etc. or due to partial concealment of the FOR by other system components, such as concealment of receptacle by telescope, that can affect the sensors and/or the data received therefrom, or from any other reason). 
     In some cases, imaging system  150  can be configured inter alia for providing imaging data (including spatial disposition determination enabling data such as data received from a LIDAR unit, etc.) of any object coming within a field of regard (FOR) aft of the refueling device  100  (and/or the tanker aircraft  12 ). The sensor control module  1810  is configured to operate sensor  1890  (or multiple sensors, mutatis mutandis) in order to acquire data that enables, inter alia, repeated determination of spatial data such as the spatial disposition of the receiver aircraft  20  with respect to an engagement area related thereto and/or determination of the spatial dispositions of the refueling device  100  with respect to an engagement enabling position, etc. as further detailed herein, inter alia with respect to  FIGS. 22 and 23 . 
     The system  1805  can further comprise a maneuvering instructions module  1820 , a steering control module  1830 , a safety module  1840 , and an engagement/disengagement control module  1850 . 
     Maneuvering instructions module  1820  can be configured to calculate maneuvering instructions for enabling positioning of the receiver aircraft  20  within an engagement area related thereto, and for providing the calculated maneuvering instructions to a pilot of the receiver aircraft  20 , as further detailed, inter alia with respect to  FIG. 20 . 
     Steering control module  1830  can be configured, when a non-aircraft-fixed in-flight refueling system is used, to calculate and provide steering commands (e.g. in six degrees of freedom) to the refueling device  100  for steering the refueling device  100  to an engagement enabling position or, when utilizing an aircraft-fixed flying boom system, to calculate and provide alignment commands (e.g. in three degrees of freedom) to the boom fueling unit  310  for aligning the refueling device  100  in an engagement enabling position, as further detailed herein, inter alia with respect to  FIG. 21 . It is to be noted that the steering commands (e.g. in six degrees of freedom) and the alignment commands (e.g. in three degrees of freedom) are also referred to as maneuvering commands interchangeably. 
     Safety module  1840  can be configured to monitor hazardous situations in the refueling process, as further detailed herein, inter alia with respect to  FIG. 19 . The hazardous situations can be defined by a set of thresholds and/or parameters and respective safety conditions. For example, safety module  1840  can be configured to monitor that the refueling device  100  does not approach the receiver aircraft  20  (or vice versa) in an unsafe manner, and/or that the refueling device  100  does not approach the tanker aircraft  12  (or vice versa) in an unsafe manner, etc. 
     Engagement/disengagement control module  1850  can be configured to provide an engagement command to the refueling device  100  for causing the refueling device  100  to engage with the fuel receptacle  22  of the receiver aircraft  20  for performing refueling, and to provide a command to the refueling device  100  to disengage from the fuel receptacle  22  of receiver aircraft  20 , as further detailed herein. According to examples of the presently disclosed subject matter, engagement/disengagement control module  1850  can be responsive to an indication that the receiver aircraft  20  is positioned in an engagement enabling position. The engagement enabling position, in some cases, can depend on a spatial disposition of the refueling device  100  with respect to the receiver aircraft  20 , as further detailed herein. 
     The system  1805  can further comprise a configuration data repository  1860  (hereinafter: “Configuration DR”) and a reference data repository  1870  (hereinafter: “Reference DR”). Configuration DR  1860  comprises data indicative of various predefined configurations that are used in the refueling process. According to examples of the presently disclosed subject matter, the configuration DR  1860  can include configuration data related to an engagement area and an engagement enabling position. Further, by way of example, the configuration data related to the engagement area and the engagement enabling position can be used to determine the engagement area and/or the engagement enabling position in a given scenario (and for a given set of parameters). Additional data with respect to the configuration data will be provided herein, inter alia with respect to  FIGS. 22 and 23 . Reference DR  1870  comprises reference data to be used, inter alia, for determining (it is to be noted that in some cases such determination is made, for example, repeatedly) the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area related thereto and the refueling device&#39;s  100  spatial disposition with respect to the engagement enabling position, etc. According to some examples, the reference data can be used in combination with dynamic data acquired by the sensor  1890  for enabling evaluation of the sensor&#39;s  1890  data. Further explanations regarding the reference data are provided herein, inter alia with respect to  FIGS. 22 and 23 . It is to be noted that in some cases, Reference DR  1870  can also be used by the safety module  1840  for determining hazardous situations. 
     It is to be noted that according to some examples of the presently disclosed subject matter, some or all of the Configuration DR  1860 , the Reference DR  1870 , the sensor control module  1810 , the maneuvering instructions module  1820 , the steering control module  1830 , the safety module  1840 , and the engagement/disengagement control module  1850  can be combined and provided as a single system/module, or, by way of example, at least one of them can be realized in a form of two or more systems/modules, each of which can in some cases be distributed over more than one location. 
     The system can still further comprise an interface  1880  for enabling one or more components of the system  1805  to operate in cooperation with auxiliary units, devices, systems or modules. For example, the interface  1880  can implement various protocols, software languages, drive signals, etc. Further, by way of example, the interface  1880  can be used to operate certain auxiliary units, devices, systems or modules on board one or more of the refueling device  100 , the receiver aircraft  20  or the tanker aircraft  12 . 
     According to another aspect of the presently disclosed subject matter, there are provided methods for in-flight refueling of aircraft. While such methods can be applied to the systems and devices for in-flight refueling of aircraft according to another aspect of the presently disclosed subject matter, and as disclosed herein, for example, the methods can also be applied to other suitable systems and devices for in-flight refueling of aircraft, mutatis mutandis. 
     According to the second aspect of the presently disclosed subject matter, there are at least three alternative operation modes for in-flight refueling of aircraft: 
     Operation Mode I—in which a refueling device is automatically (and in some cases autonomously) flown to engagement with the receiver aircraft fuel receptacle. 
     Operation Mode II—in which an operator in the tanker aircraft or elsewhere (via suitable communications link—for example satellite link, another aircraft including the receiver aircraft, ground control, and so on) controls flying of a refueling device while towed behind the tanker aircraft to engagement with the receiver aircraft fuel receptacle. 
     Operation Mode III—in which the refueling device is not flown or controlled per se, but instead attains a stable configuration with the boom member at the required inclination angle with respect to the forward direction, and the receiver aircraft maneuvers to a position where it can engage the nozzle to the receiver aircraft fuel receptacle. 
     Examples of these operation modes will now be described in greater detail. 
     Operation Mode I 
     In this operation mode, once the tanker aircraft  12  and receiver aircraft  20  are in close proximity and flying in formation, with the receiver aircraft  20  at a position behind the tanker aircraft  12 , the refueling device  100  automatically (and in some cases, autonomously) flies into engagement with the fuel receptacle  22  of the receiver aircraft  20 . 
     Turning to  FIG. 19  there is provided a flowchart illustrating a sequence of operations carried out for enabling performance of in-flight refueling, according to certain examples of the presently disclosed subject matter, in particular relating to the example of a system for controlling in-flight refueling, as illustrated in  FIG. 18 . The sequence of operations begins with performance of an engagement sequence  1905 , comprising  3  blocks:  1910 ,  1920  and  1930 . 
     Turning at first to block  1910 , in some cases, maneuvering instructions module  1820  is configured to calculate and provide maneuvering instructions for enabling positioning the receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, within an engagement area related to the receiver aircraft  20  (as in cases where more than one receiver aircraft  20  exists, each receiver aircraft  20  can be associated with a different engagement area) (block  1910 ), as further detailed herein with respect to  FIGS. 20 and 22 . Inter alia, some examples of methods that can be used for providing the maneuvering instructions to the pilot of the receiver aircraft  20  are also provided herein. 
     It is to be noted that such maneuvering instructions can be required in some cases, where the pilot of the receiver aircraft  20  has no line of sight to the refueling device  100  or boom fuelling unit  310  during all or part of the refueling process, inter alia in light of the receiver aircraft  20  fuel receptacle  22  position. Thus, there can be a need to provide the pilot of the receiver aircraft  20  with maneuvering instructions, as further detailed herein. 
     As mentioned above, according to examples of the presently disclosed subject matter, the refueling process can include providing maneuvering instructions for positioning the receiver aircraft  20  within an engagement area related thereto. The engagement area is a virtual volume in which the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) can be maneuvered in order to engage with the fuel receptacle  22  of the receiver aircraft  20 . According to some examples of the presently disclosed subject matter, the engagement area can be defined by various specifications that depend on several parameters. According to one example, the parameters are associated with maneuvering capabilities of the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system). Such maneuvering capabilities can be defined, for example, by the range and types of motion that can be achieved by utilizing the spatial control system  160  and/or the force generating arrangement  190  of the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or the mechanical connection  320  and/or the motion control system  330  of the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system), etc. 
     According to certain examples of the presently disclosed subject matter, the parameters defining the engagement area can further include, inter alia, the length of the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the length of the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system), the flight speed, the flight altitude, weather conditions, the fuel pressure within the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the location of the fuel receptacle  22  of the receiver aircraft  20 , etc. In some cases, the engagement area can be substantially in the shape of a cube, a sphere, or any other shape, including a non-regular shape, with a certain volume. The various engagement area specifications can be stored, for example, on configuration DR  1860 . For example, the engagement area specifications can include a set of spatial dispositions between the refueling device  100  and the receiver aircraft  20  or any other volumetric specification. According to further examples of the presently disclosed subject matter, the engagement area specifications can be used in combination with reference data for enabling the refueling device  100 , based on dynamic data acquired by the sensor  1890 , to identify when the receiver aircraft  20  is within a position that meets the engagement area specification. In this case, correlations can be computed between the data acquired by the sensor  1890  and the reference data, in order to determine if and when the receiver aircraft  20  is within the engagement area, as further detailed herein, inter alia with reference to  FIG. 30-32 . 
     Before moving on to describe  FIG. 19 , and for the purpose of providing a visual illustration of an exemplary engagement area, attention is drawn to  FIG. 24 , showing an illustration of one example of a receiver aircraft positioned outside a virtual engagement area, according to certain examples of the presently disclosed subject matter. The engagement area  2410  in the illustrated example is a virtual pre-determined volume, shaped substantially like a cube, in which the refueling device  100  (or, when utilizing an aircraft-fixed flying boom system—the boom fuelling unit  310 ) can navigate until engaging with the fuel receptacle  22  of the receiver aircraft  20 , as detailed herein. Although in this example the virtual engagement area is shaped substantially like a cube, the virtual engagement area can have any other shape with a certain volume. The virtual engagement area can be defined by a set of parameters that correspond to a volumetric shape. It can be further appreciated that in the illustrated example, the receiver aircraft  20  is not positioned within the engagement area  2410 . The illustration of  FIG. 24  is provided for clarity of explanation only and is by no means binding. 
     Returning to  FIG. 19 , in some cases, the engagement area specifications can be defined using, inter alia, a parameter denoting a position of the center of such an engagement area, or any other point within the engagement area, and a set of offset vectors, collectively representing a volume. In some cases, the center (or any other point of reference) of the engagement area can be determined in accordance with one or more of the following parameters: the length of the hose  52  in a deployed position (when a non-aircraft-fixed in-flight refueling system is used), the length of the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system), a given pitch angle between the boom axis  131  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) and the forward direction A, a given yaw angle between the boom axis  131  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) and the forward direction A and a given fuel pressure within the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), etc. One or more of the parameters which are used to determine the center (or any other point of reference) of the engagement area can vary during flight and/or during the engagement sequence  1905  and the system  1805  can measure the relevant parameters dynamically for determining the center (or any other point of reference) of the engagement area. 
     In some cases (e.g. when a non-aircraft-fixed in-flight refueling system is used), the point of reference for the engagement area can be positioned in a position from which utilization of the force generating arrangement  190  enables the nozzle  135  to engage with the fuel receptacle  22  of the receiver aircraft  20  and the engagement area is defined with reference to this point. In other cases (e.g. when utilizing an aircraft-fixed flying boom system), the point of reference for the engagement area can be positioned in a position which enables the nozzle  316  to engage with the fuel receptacle  22  of the receiver aircraft  20  (e.g. by extending the telescoping aft section  314  thus applying force on the fuel receptacle  22  due to reaction force on the other side of the boom fuelling unit  310 , at the mechanical connection  320 ) and the engagement area is defined with reference to this point. 
     In some examples, this point of reference is defined in the system as an engagement enabling position, and can also be used by an engagement/disengagement control module  1850 , for controlling engagement and of the nozzle  135  (or nozzle  316 ) with the fuel receptacle  22  of the receiver aircraft  20 . It will be appreciated that this point can also depend, inter alia, on the parameters described herein, and in some cases, is dynamically calculated according to the relevant parameters during the engagement sequence  1905 . 
     According to some examples of the presently disclosed subject matter, e.g. when a non-aircraft-fixed in-flight refueling system is used, the engagement enabling position can be characterized by the boom member  130  being in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . According to some examples of the presently disclosed subject matter, e.g. when utilizing an aircraft-fixed flying boom system, the engagement enabling position can be characterized by the boom fuelling unit  310  (or nozzle  316  thereof) being in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . 
     Throughout the description and the claims, reference is made interchangeably to the terms spatial relationship and spatial disposition. The terms spatial relationship and spatial disposition or the like can relate to spatial distances, spatial angles (including orientations), or any other spatial reference that is used for characterizing a spatial relationship between two objects, e.g. between any two of the following: the tanker aircraft  12 , the receiver aircraft  20  (and a fuel receptacle  22  thereof), the refueling device  100  and the boom fuelling unit  310 . In some cases the spatial relationship can include aligning the boom axis  131  of the boom member  130  (e.g. when a non-aircraft-fixed in-flight refueling system is used) or the boom axis  311  of the boom fuelling unit  310  (e.g. when utilizing an aircraft-fixed flying boom system), in an engagement enabling orientation. 
     In some cases, when a non-aircraft-fixed in-flight refueling system is used, the maximal spaced relationship between the boom member  130  of the refueling device  100  and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position can depend on various parameters, such as: the hose  52  length and flexibility, the flight speed, the flight altitude, the characteristics of the force generating arrangement  190 , etc., and in such cases, the maximal space can be calculated as necessary, inter alia dynamically during the refueling process, based on current values of the respective parameters. For example, it can be appreciated that the less flexible the hose  52 , the maximal space between the boom member  130  of the refueling device  100  and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position is reduced. In some cases, the maximal space between the boom member  130  of the refueling device  100  and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position can be defined by the movement range of the boom member  130  in the direction of the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, when utilizing an aircraft-fixed flying boom system, the maximal spaced relationship between the boom member  312  of the boom fuelling unit  310  and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position can also depend on various parameters, such as: the flight speed, the flight altitude, etc., and in such cases, the maximal space can be calculated as necessary, inter alia dynamically during the refueling process, based on current values of the respective parameters. In some cases, the maximal space between the boom member  312  of the boom fuelling unit  310  and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position can be defined by the extension range of the telescoping aft section  314  in the direction of the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, the spatial relationship between the boom member  130  of the refueling device  100  (e.g. when a non-aircraft-fixed in-flight refueling system is used), or the boom fuelling unit  310  (e.g. when utilizing an aircraft-fixed flying boom system), and the fuel receptacle  22  of the receiver aircraft  20  at the engagement enabling position can also depend on various parameters, such as the characteristics of the fuel receptacle  22  of the receiver aircraft  20 , etc. According to examples of the presently disclosed subject matter the spatial relationship with which the engagement enabling position is associated can also include an angle parameter. In this regard, it can be appreciated that in case the fuel receptacle  22  of the receiver aircraft  20  has an entrance angle, the boom axis  131  of the boom member  130  (e.g. when a non-aircraft-fixed in-flight refueling system is used), or the boom axis  311  of the boom fuelling unit  310  (e.g. when utilizing an aircraft-fixed flying boom system), should be in a spatial disposition that enables the nozzle (nozzle  135  or nozzle  316 ) to engage therewith (e.g. the boom axis  131  or boom axis  311  needs to be aligned with a fuel receptacle  22  of the receiver aircraft  20 ). 
     It has been explained herein that when a non-aircraft-fixed in-flight refueling system is used spatial control system  160  can, in accordance with certain examples, be configured for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  131  and the forward direction A, that enables this angle θ to be selectively maintained between the boom axis  131  and the forward direction A at least for a part of the time when the refueling device  100  is being towed by the tanker aircraft  12  via hose  52 , and in particular during the engagement operation of the fuel device  100  to the receiver aircraft  20  and during refueling thereof. It is to be noted that such angle θ can be predetermined. In some cases the angle θ can be stored in the configuration DR  1860 . 
     It is to be noted that when utilizing an aircraft-fixed flying boom system, a mechanical connection  320  and/or a motion control system  330  can be utilized for selectively and controllably providing a non-zero angular disposition, angle θ, between the boom axis  311  and the receiver aircraft (e.g., between the boom axis  311  and. forward direction, A when the receiver aircraft is also aligned with forward direction A) and selectively maintain this angle θ at least for a part of the time during the engagement operation of the boom fuelling unit  310  to the receiver aircraft  20  and during refueling thereof. It is to be noted that such angle θ can be predetermined, and can lie within a range of angles in which such engagement between the boom fuelling unit  310  to the receiver aircraft  20 . In some cases the angle θ can be stored in the configuration DR  1860 . 
     In some examples, it is to be noted that the engagement enabling position is not necessarily a specific x, y, z coordinate, as, under certain conditions, the exact coordinates of the point of reference for the engagement area can vary, or some tolerance can be accepted (for example using tolerance ranges). In addition, there can be more than one engagement enabling position that meets the conditions detailed herein, each of which is an engagement enabling position. 
     Before moving on to describe  FIG. 19 , and for the purpose of providing a visual illustration of an exemplary engagement enabling position, attention is drawn to  FIG. 26  and  FIG. 27 . Reverting to  FIG. 26 , there is shown an illustration of an example of a refueling device not in an engagement enabling position, according to certain examples of the presently disclosed subject matter. It can be appreciated that the receiver aircraft  20 , and more particularly a fuel receptacle  22  thereof, is positioned within the engagement area  2410 , however, the refueling device  100  is not positioned in an engagement enabling position. In this example, as illustrated in  FIG. 26 , it can be appreciated that the refueling device  100  is not aligned with the fuel receptacle  22  of the receiver aircraft  100 . The illustration of  FIG. 26  is provided for clarity of explanation only and is by no means binding. Reverting to  FIG. 27 , there is shown an illustration of an example of a refueling device positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter. It can be appreciated that the refueling device  100  is positioned in an engagement enabling position that can enable engagement with the fuel receptacle  22  of the receiver aircraft  100 . The illustration of  FIG. 27  is provided for clarity of explanation only and is by no means binding. 
     It is to be noted that a refueling device  100  is shown in  FIGS. 24-27  for illustration purposes only and other refueling systems can also be used, including, but not limited to, a boom fuelling unit  310  as further detailed herein. 
     Returning to  FIG. 19 , in some cases, maneuvering instructions module  1820  can be configured to provide the maneuvering instructions for enabling positioning of the receiver aircraft  20  within an engagement area, for example by utilizing a signaling system. Such signaling system can be mounted, for example, on the tanker aircraft  12 , at any location visible to the receiver aircraft  20  pilot. In some cases, the signaling system can provide the receiver aircraft  20  pilot with maneuvering instructions on three axes: forward-backward, left-right and up-down, thus enabling it to maneuver the receiver aircraft  20  to the engagement area  2410 . In some cases the signaling system can be a light directing system. Alternatively or additionally, the maneuvering instructions can be provided to by using voice commands (e.g. by utilizing speakers, pilot headset, etc.) or by any other means known per se. In some cases, maneuvering instructions module  1820  can be configured to communicate the maneuvering instructions to an auto pilot system of the receiver aircraft  20 , if such system exists, for causing the auto pilot system to maneuver the receiver aircraft  20  accordingly. 
     Before moving on to describe  FIG. 19 , and for the purpose of providing a visual illustration of an exemplary engagement area, attention is drawn to  FIG. 25 , showing an illustration of an example of a receiver aircraft positioned inside a virtual engagement area, according to certain examples of the presently disclosed subject matter. It can be noted that the receiver aircraft  20 , and more particularly the fuel receptacle  22  thereof, are positioned within the engagement area  2410 . Also in this illustration the engagement area  2410  is a virtual pre-determined volume, shaped substantially like a cube, in which, when a non-aircraft-fixed in-flight refueling system is used, the refueling device  100  can navigate until arriving at an engagement enabling position (in which the boom member  130  when a non-aircraft-fixed in-flight refueling system is used or the boom fuelling unit  310  (or nozzle  316  thereof) when utilizing an aircraft-fixed flying boom system, is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ) and engaging with the fuel receptacle  22  of the receiver aircraft  20 , as detailed herein. 
     Although also in this example the virtual engagement area is shaped substantially like a cube, the virtual engagement area can have any other shape with a certain volume. It can be noted that the illustration further illustrates an example of a signaling system  2420  mounted on tanker aircraft  12 . It can be appreciated that such a signaling system can be used by maneuvering instructions module  1820  for providing maneuvering instructions to the pilot of the receiver aircraft  20 . It is to be noted that alternative and/or additional signaling systems can be used. The illustration of  FIG. 25  is provided for clarity of explanation only and is by no means binding. 
     Bearing all this in mind, and returning to  FIG. 19 , it is to be noted that block  1910  can be performed repeatedly (e.g. every pre-determined period) or continuously, at least until the nozzle  135  or nozzle  316  is engaged with the fuel receptacle  22  of the receiver aircraft  20 , as further detailed herein. Thus, while the receiver aircraft  20  is not positioned within the engagement area, maneuvering instructions module  1820  provides the pilot of the receiver aircraft  20  (and, in some cases, an auto pilot system that controls the maneuvering of the receiver aircraft  20 ) with maneuvering instructions for positioning the receiver aircraft  20  within an engagement area. 
     Although the process above (referring to block  1910 ) was described with respect to maneuvering of the receiver aircraft  20 , it is to be noted that in some cases, in addition or as an alternative to maneuvering of the receiver aircraft  20  for approaching the refueling device  100  or the boom fuelling unit  310 , the tanker aircraft  12  can approach the receiver aircraft  20 , thus bringing the refueling device  100  or the boom fuelling unit  310  in the direction of the receiver aircraft  20 . In such cases, the maneuvering instructions can be additionally or alternatively provided to the pilot of the tanker aircraft  12  mutatis mutandis. 
     It is to be further noted that the process relating to block  1910  can in some cases be an independent process, and in other cases it can be performed as part of a sequence of processes, such as engagement sequence  1905 . 
     Turning now to block  1920  in  FIG. 19 , in some cases, when a non-aircraft-fixed in-flight refueling system is used, once an engagement area specification condition is met (e.g. it is determined that the receiver aircraft  20  is positioned within the engagement area), steering control module  1830  can be configured to provide commands for causing the steering of the refueling device  100  to an engagement enabling position, in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20  (block  1920 ), as further detailed herein, inter alia with respect to  FIGS. 21 and 23 . 
     In some cases, the steering commands module  1830  can be operatively connected to the spatial control system  160  and/or to the force generating arrangement  190  of the refueling device  100 . In such cases, the steering commands module  1830  can provide steering commands for controlling the spatial control system  160  and/or to the force generating arrangement  190  and thus enabling steering the refueling device  100  to an engagement enabling position, in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, the steering commands can be based, inter alia, on characteristics of the spatial control system  160 . Such characteristics can be, for example, operation parameters of reaction control thrusters associated with the refueling device and capable of steering the refueling device  100  and/or operation parameters of aero-dynamic control surfaces of the refueling device  100 . 
     In some cases, based on the steering commands from the steering commands module  1830 , the refueling device  100  can be adapted to steer automatically. In particular, in some cases, using the commands, autonomous steering of the refueling device  100  can be achieved (e.g. when all the necessary components are fitted within the refueling device  100 ). 
     Thus, in accordance with some examples of the presently disclosed subject matter, there can be provided a refueling device  100  which can bring itself automatically, and in some cases (when all the necessary components are fitted within the refueling device  100 ) autonomously, into fluid communication with the fuel receptacle  22  of the receiver aircraft  20 . In further examples, there can be provided a refueling device which can automatically, and in some cases (when all the necessary components are fitted within the refueling device  100 ) autonomously, align its boom axis  131  with a fuel receptacle  22  of the receiver aircraft  20 , and move the boom member  130  in a predetermined trajectory towards the receiver aircraft  20  and thus enable the refueling device  100  to automatically bring itself into engagement with the fuel receptacle  22  of the receiver aircraft  20 . 
     It is to be noted that block  1920  can be performed repeatedly (e.g. every pre-determined period) or continuously. For example, block  1920  can be performed until engagement of the nozzle  135  to the fuel receptacle  22  of the receiver aircraft  20 , as further detailed herein, and in some cases even after such engagement. While the refueling device  100  is not positioned within an engagement enabling position, steering control module  1830  provides the refueling device  100  with steering commands for maneuvering the refueling device  100  to the engagement enabling position, in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . 
     According to some examples of the examples of the presently disclosed subject matter, when utilizing an aircraft-fixed flying boom system, once an engagement area specification condition is met (e.g. it is determined that the receiver aircraft  20  is positioned within the engagement area), steering control module  1830  can be configured to provide commands for aligning the boom fuelling unit  310  in an engagement enabling position, in which the boom member  311  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, the steering commands module  1830  can be operatively connected to the mechanical connection  320  and/or a motion control system  330  and/or to the telescoping aft section  314  of the boom fuelling unit  310 . In such cases, the steering commands module  1830  can provide alignment commands for controlling a mechanical connection  320  and/or a motion control system  330  and/or to a telescoping aft section  314  of the boom fuelling unit  310  and thus enabling aligning the refueling device  100  at an engagement enabling position, in which the boom fuelling unit  310  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . It is to be noted that the alignment commands can result in motion of the aircraft-fixed flying boom system in three degrees of freedom (e.g. pitch, yaw and translation). 
     In some cases, the alignment commands can be based, inter alia, on characteristics of the mechanical connection  320  and/or the motion control system  330  and/or the telescoping aft section  314  of the boom fuelling unit  310 . 
     In some cases, based on the alignment commands from the steering commands module  1830 , the boom fuelling unit  310  can be adapted to align itself automatically. In particular, in some cases, using the commands, autonomous aligning of the boom fuelling unit  310  can be achieved (e.g. when all the necessary components are fitted within the boom fuelling unit  310 ). 
     Thus, in accordance with some examples of the presently disclosed subject matter, there can be provided a boom fuelling unit  310  which can bring itself automatically, and in some cases (when all the necessary components are fitted within the boom fuelling unit  310 ) autonomously, into fluid communication with the fuel receptacle  22  of the receiver aircraft  20 . In further examples, there can be provided a boom fuelling unit  310  which can automatically, and in some cases (when all the necessary components are fitted within the boom fuelling unit  310 ) autonomously, align itself with a fuel receptacle  22  of the receiver aircraft  20 , and move itself in a predetermined trajectory towards the receiver aircraft  20  and thus enable the boom fuelling unit  310  to automatically bring itself into engagement with the fuel receptacle  22  of the receiver aircraft  20 . 
     It is to be noted that block  1920  can be performed repeatedly (e.g. every pre-determined period) or continuously. For example, block  1920  can be performed until engagement of the boom fuelling unit  310  to the fuel receptacle  22  of the receiver aircraft  20 , as further detailed herein, and in some cases even after such engagement. While the boom fuelling unit  310  is not positioned within an engagement enabling position, steering control module  1830  provides the boom fuelling unit  310  with alignment commands for maneuvering the boom fuelling unit  310  to the engagement enabling position, in which it is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . 
     It is to be further noted that the process relating to block  1920  can in some cases be an independent process, and in other cases it can be performed as part of a sequence of processes, such as engagement sequence  1905 . 
     Attention is now drawn to block  1930  in  FIG. 19 . In some cases, when a non-aircraft-fixed in-flight refueling system is used, once the receiver aircraft  20  is positioned within the engagement area and the refueling device  100  is positioned in an engagement enabling position (in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), the engagement/disengagement control module  1850  can be configured to provide the refueling device  100  with an engagement command for causing the refueling device  100  to engage to the fuel receptacle  22  of the receiver aircraft  20  for enabling refueling of the receiver aircraft  20  (block  1930 ). It is to be noted that at the engagement enabling position the nozzle  135  is properly aligned with the fuel receptacle  22  (the boom member  130  and boom axis  131  being at the design angle θ des  to the forward direction A) and sufficiently close thereto, i.e., at a predetermined spacing from the receiver aircraft, said boom axis being aligned in an engagement enabling orientation at said spaced position. 
     In some cases, the engagement command can cause activation of the force generating arrangement  190 , e.g., by deploying the air brakes  195 ,  196 , thus generating a force along boom axis  131  that effectively pushes the nozzle  135  into engagement with the fuel receptacle  22  of the receiver aircraft  20 . Such force can cause the boom member  130  to move in a predetermined trajectory towards the receiver aircraft  20  for enabling engagement between the nozzle  135  and the fuel receptacle  22  of the receiver aircraft  20  (e.g. for enabling fuel communication therebetween). In some cases, prior to deploying the air brakes  195 ,  196 , the boom member  130  can be extended, for example until reaching a pre-determined space from the fuel receptacle  22  of the receiver aircraft  20 . In some cases, the engagement command can cause extension of the boom member  130  until connection with the fuel receptacle  22  of the receiver aircraft  20 , with no use of any air brakes mechanism. In other words, once the refueling device  100  is at the aforesaid engagement enabling orientation and spaced position, the boom member is subsequently moved along said boom axis towards the receiver aircraft for enabling fuel communication therebetween. Movement of the boom member can be effected in one of two ways, or combination thereof: the refueling device  100  remains in the spaced position, and the boom member  130  is extended telescopically; the boom member  130  can be in the retracted or extended position, and the refueling device  100  is bodily moved towards the receiver aircraft for enabling fuel communication therebetween. 
     In accordance with some examples of the presently disclosed subject matter, there can be provided a refueling device  100  which can bring itself automatically, and in some cases (when all the necessary components are fitted within the refueling device  100 ) autonomously, into fluid communication with the fuel receptacle  22  of the receiver aircraft  20 . In further examples, there can be provided a refueling device  100  which can automatically, and in some cases (when all the necessary components are fitted within the refueling device  100 ) autonomously, engage the nozzle  135  with the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, prior to providing the refueling device  100  with an engagement command, maneuvering instructions module  1820  can be configured to cause a signaling system to provide the pilot of the receiver aircraft  20  with a notification indicating that the refueling device  100  is about to engage to the fuel receptacle  22 , thus requiring the pilot of the receiver aircraft  20  to stabilize it. 
     According to some examples of the examples of the presently disclosed subject matter, when utilizing an aircraft-fixed flying boom system, once the receiver aircraft  20  is positioned within the engagement area and the boom fuelling unit  310  is positioned in an engagement enabling position (in which it is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), the engagement/disengagement control module  1850  can be configured to provide the boom fuelling unit  310  with an engagement command for causing the boom fuelling unit  310  to engage to the fuel receptacle  22  of the receiver aircraft  20  for enabling refueling of the receiver aircraft  20 . It is to be noted that at the engagement enabling position the boom member  312  is aligned such that the nozzle tip is sufficiently close to the fuel receptacle  22  of the receiver aircraft  20 , i.e., at a predetermined spacing from the fuel receptacle  22  of the receiver aircraft  20 , said boom axis  311  being aligned in an engagement enabling orientation at said spaced position. 
     In some cases, the engagement command can cause extension of the telescoping aft section  314  thus applying force on the fuel receptacle  22  due to reaction force on the other side of the boom fuelling unit  310 , at the mechanical connection  320 . 
     In accordance with some examples of the presently disclosed subject matter, there can be provided a boom fuelling unit  310  which can bring itself automatically, and in some cases (when all the necessary components are fitted within the boom fuelling unit  310 ) autonomously, into fluid communication with the fuel receptacle  22  of the receiver aircraft  20 . In further examples, there can be provided a boom fuelling unit  310  which can automatically, and in some cases (when all the necessary components are fitted within the boom fuelling unit  310 ) autonomously, engage the nozzle  316  with the fuel receptacle  22  of the receiver aircraft  20 . 
     In some cases, prior to providing the boom fuelling unit  310  with an engagement command, maneuvering instructions module  1820  can be configured to cause a signaling system to provide the pilot of the receiver aircraft  20  with a notification indicating that the boom fuelling unit  310  is about to engage to the fuel receptacle  22 , thus requiring the pilot of the receiver aircraft  20  to stabilize it. 
     It is to be further noted that the process relating to block  1930  can in some cases be an independent process, and in other cases it can be performed as part of a sequence of processes, such as engagement sequence  1905 . 
     Attention is now drawn to Block  1940  in  FIG. 19 . In some cases, following engagement sequence  1905 , the system  1805  can be configured to provide a command to the refueling device  100  to perform refueling of the receiver aircraft  20  by pumping fuel to the receiver aircraft  20  from the tanker aircraft  12  (block  1940 ). In some cases, when a non-aircraft-fixed in-flight refueling system is used, at any time following engagement sequence  1905 , the system  1805  can be further configured to deactivate the force generating arrangement  190 , e.g. by retracting the air brakes  195 ,  196 .In one example, the command to deactivate the force generating arrangement  190  and the refueling command can be issued by the engagement/disengagement module  1850 . Further by way of example, engagement/disengagement module  1850  can be configured to provide a deactivation command when an indication is received that the refueling device  100  is engaged with the receiver aircraft  20 . Further by way of example, the refueling command can be issued for example when an engagement indication is issued. 
     Attention is now drawn to Block  1950  in  FIG. 19 . the engagement/disengagement control module  1850  can be further configured to provide the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) with a disengagement command for causing it to disengage from the fuel receptacle  22  of the receiver aircraft  20  in response to receiving an indication that the fuel level in the fuel tank of the receiver aircraft  20  reached a desired level or when an indication of a hazard is issued (block  1950 ). 
     In some cases, prior to providing the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) with a disengagement command, maneuvering instructions module  1820  can be configured to cause the signaling system to provide the pilot of the receiver aircraft  20  with a notification indicating that the refueling is done, and in some cases instruct the pilot of the receiver aircraft  20  to perform a maneuver in order to fly away from the refueling device  100 . 
     It is to be noted that throughout the refueling process, safety module  1840  can monitor certain parameters, for example parameters that relate to the spatial dispositions between any two or more of the following: the receiver aircraft  20 , the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) and the tanker aircraft  12 , as well as other parameters including the angles, etc., possibly in comparison to predefined reference thresholds or parameters or ranges, to identify possible hazardous situations. Such hazardous situations can include, inter alia, a dangerous approach of the receiver aircraft  20  to the refueling device  100  or to the tanker aircraft  12 , a dangerous movement of the receiver aircraft  20 , and/or the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) and/or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) and/or the tanker aircraft  12 , including such movement when the refueling device  100  or the boom fuelling unit  310  is engaged to the receiver aircraft  20 , etc. It is to be noted that such hazardous situations can be caused for example due to a human error, environmental conditions (weather, wind, etc.), as well as other causes. It is to be further noted that for monitoring such hazardous situations, safety module  1840  can be configured to utilize, inter alia, sensor control module  1810  for sensing the spatial dispositions of the receiver aircraft  20 , and/or the refueling device  100  and/or the boom fuelling unit  310  and/or the tanker aircraft  12 . 
     In some cases, when safety module  1840  identifies a hazardous situation (e.g. a safety condition is met or, in some cases, is not met), it can be configured, inter alia, to instruct steering control module  1830  to provide steering instructions for causing the refueling device  100  to steer away from the receiver aircraft  20  (when a non-aircraft-fixed in-flight refueling system is used) or to provide commands for maneuvering the boom fuelling unit  310  away from the receiver aircraft  20 , and/or to provide an indication to the pilot of the receiver aircraft  20  that a hazardous situation has been identified, thus enabling the pilot to maneuver the receiver aircraft  20  away from danger, etc. It is to be noted that in case the hazardous situation is identified after the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) engaged with the receiver aircraft  20 , safety module  1840  can be further configured to instruct engagement/disengagement module  1850  to cause the refueling device  100  or the boom fuelling unit  310  to disengage from the receiver aircraft  20  prior to performing the maneuvering as detailed herein. 
     It is to be noted that in some cases, when the receiver aircraft  20  is positioned within the engagement area, or at an earlier stage, the refueling device  100  (when a non-aircraft-fixed in-flight refueling system is used) and/or the boom fuelling unit  310  (when utilizing an aircraft-fixed flying boom system) can be deployed to an initial trail position. Such initial trail position can be defined in terms of a spatial disposition with respect to the tanker aircraft  12  and can be characterized, inter alia, by one or more of the following:
         When a non-aircraft-fixed in-flight refueling system is used—a required deployment length of the hose  52  (that in some cases can depend, inter alia, on the flight speed, the flight altitude, the receiver aircraft  20  type, the engagement area specification, etc., whereas in other cases it can be for example pre-determined) or, when utilizing an aircraft-fixed flying boom system, the extension length of the telescoping aft section  314 ;   A required pitch angle between the boom axis  131  or boom axis  311  and the forward direction A is maintained;   A required yaw angle between the boom axis  131  or boom axis  311  and the forward direction A is maintained.       

     In some cases, the steering control module  1830  can be configured to monitor the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  with respect to the tanker aircraft  12  and validate that the refueling device  100  and/or the boom fuelling unit  310  is positioned in the initial trail position with respect to the tanker aircraft  12 . For that purpose, steering control module  1830  can utilize, for example, sensor control module  1810  for repeatedly, and in some cases in real time (for example in the form of a video stream) acquiring an image of the area in which the refueling device  100  and/or the boom fuelling unit  310  is expected to be positioned when in the initial trail position. The image can be acquired by a sensor  1890  that can be mounted, for example, on the tanker aircraft  12  in a position that enables it to acquire images of the area in which the refueling device  100  and/or the boom fuelling unit  310  is expected to be positioned when in the initial trail position. Such sensor position can be, for example, on the tanker aircraft  12  wing, elevators, tail, on the underside of the fuselage, etc. Utilization of such an image can enable determination of the refueling device&#39;s  100  and/or the boom fuelling unit  310  spatial disposition with respect to the tanker aircraft  12  (it is to be noted that in some cases such determination is made, for example, repeatedly). For example, the acquired image can be compared with a pre-stored image (e.g. stored on reference data repository  1870 ) of the refueling device  100  and/or the boom fuelling unit  310 , illustrating a desired spatial disposition with respect to the tanker aircraft  12 , thus enabling determination of the relative spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  with respect to the tanker aircraft  12  (it is to be noted that in some cases such determination is made, for example, repeatedly). Such desired spatial disposition can, in some cases, depend on various factors, such as, inter alia, the flight speed, the flight altitude, the refueling device  100  weight, etc. It can be appreciated that pre-stored images of different spatial dispositions of the refueling device  100  and/or the boom fuelling unit  310  with respect to the tanker aircraft  12  can be stored in reference DR  1870  and a set of parameters, inter alia, flight speed, flight altitude, refueling device  100  weight, etc., can be specified in association with one or more of the pre-stored images. 
     According to examples of the presently disclosed subject matter, for a given set of parameters, steering control module  1830  can determine which image is to be used as a reference image. For example, the steering control module  1830  can receive a set of measurements which are correlated with the parameters associated with the images and can compare the current measurements to the various sets of parameters and identify which set is, for example, most closely correlated with the measurements and the steering control module  1830  can select the image with which the parameters are associated as the reference image. Following the selection of the reference image, the steering control module  1830  can repeatedly (e.g. every pre-determined period), or continuously, compare images obtained by the sensor  1890  during the refueling process to the selected reference image, and calculate the spatial disposition of the refueling device  100  with respect to the initial trail position. 
     In some cases, 3-D models can be used instead of images. According to further examples, the reference DR  1870  can store one or more generic 3-D models (e.g. one for each type of aircraft), and as part of determining the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  with respect to the tanker aircraft  12 , an appropriate 3-D model can be selected (for example according to the type of the receiver aircraft  20 ) and the 3-D model can be adapted using current measurements (e.g. obtained by the sensor  1890 ) and respective parameters of the 3-D model. 
     According to other examples of the presently disclosed subject matter, steering control module  1830  can search among the different pre stored reference images for an image which most closely correlates with a current image and can determine the spatial disposition using the pre-stored parameters associated with the selected image. 
     In some cases, the sensor  1890  can be a LIDAR unit  351 . In such cases, the sensor can acquire the images as further detailed herein, inter alia with reference to  FIGS. 30-32 . The images obtained by the LIDAR unit  351  can comprise both depth data and electromagnetic data within the sensing volume  359 . In such cases, the depth and electromagnetic data can be compared with look up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft  20  and/or the boom fuelling unit  310 , optionally based on the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.) and/or the type of the boom fuelling unit  310 . Based on the comparison, the spatial disposition of the refueling device  100  or the boom fuelling unit  310  with respect to the tanker aircraft  12  and/or to the fuel receptacle  22  of the receiver aircraft  20  can be calculated. In some cases, a full or partial 3-D model of the refueling device  100  and/or the boom fuelling unit  310  can be calculated based on the depth and electromagnetic data received from the LIDAR unit  351  and such full or partial 3-D model can be compared with one or more pre-stored generic full or partial 3-D models (e.g. one for each type of aircraft and/or one for each type of boom fuelling unit). Based on the comparison, the spatial disposition of the refueling device  100  or the boom fuelling unit  310  with respect to the tanker aircraft  12  and/or to the fuel receptacle  22  of the receiver aircraft  20  can be calculated. The look up tables and/or the 3-D models can be stored for example on the reference DR  1870 . 
     It is to be noted that various other methods and techniques can be used in order to determine the refueling device&#39;s  100  and/or the boom fuelling unit  310  spatial disposition with respect to the tanker aircraft  12  and/or to the fuel receptacle  22  of the receiver aircraft  20  (it is to be noted that in some cases such determination is made, for example, repeatedly). 
     It is to be noted that, with reference to  FIG. 19 , some of the blocks can be integrated into a consolidated block or can be broken down to a few blocks and/or other blocks may be added. Furthermore, in some cases, the blocks can be performed in a different order than described herein. It should be also noted that whilst the flow diagrams are described also with reference to the system elements that realizes them, this is by no means binding, and the blocks can be performed by elements other than those described herein. 
     Turning to  FIG. 20 , there is shown a flowchart illustrating a sequence of operations carried out for providing maneuvering commands for positioning a receiver aircraft within an engagement area related thereto, according to certain examples of the presently disclosed subject matter. Maneuvering instructions module  1820  can be configured to determine (it is to be noted that in some cases, as indicated herein, such determination is made, for example, repeatedly) the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area related thereto (block  2005 ), as further detailed with respect to  FIG. 22 . 
     Following determination of the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area related thereto, maneuvering instructions module  1820  can be configured to check if the receiver aircraft  20  is positioned within the engagement area (block  2010 ), based for example on current measurements from sensor/s  1890  as described herein. In some examples, in case the receiver aircraft  20  is positioned within the engagement area, and until the refueling device  100  is engaged with the fuel receptacle  22  of the receiver aircraft  20  (in some cases this process can continue until the refueling process ends), the maneuvering instructions module  1820  can be configured to return to block  2005  and re-determine the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area related thereto. 
     In some examples, in case the receiver aircraft  20  is not positioned within the engagement area, maneuvering instructions module  1820  can be configured to calculate maneuvering instructions for positioning the receiver aircraft  20  within the engagement area (block  2020 ). It can be appreciated that once the maneuvering instructions module  1820  determines the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area related thereto, it can also calculate maneuvering instructions for positioning the receiver aircraft  20  within the engagement area related thereto. Maneuvering instructions module  1820  can also provide the calculated maneuvering instructions for positioning the receiver aircraft  20  (and, in some cases, an auto pilot system that controls the maneuvering of the receiver aircraft  20 ) within the engagement area related thereto to the pilot of the receiver aircraft  20  (block  2030 ) and return to block  2005 . 
     It is to be noted that, in some cases, as indicated herein, maneuvering instructions module  1820  can be configured to provide the maneuvering instructions by using a light directing system. Such light directing system can be mounted, for example, on the tanker aircraft  12 , at any location visible to the receiver aircraft  20  pilot. In some cases, the light directing system can provide the pilot of the receiver aircraft  20  with maneuvering instructions on three axes: forward-backward, left-right and up-down, thus enabling it to maneuver the receiver aircraft  20  to the engagement area  2110 . Alternatively or additionally, the maneuvering instructions can be provided by using voice commands (e.g. by utilizing speakers, pilot headset, etc.) or by any other means known per se. In some cases, maneuvering instructions module  1820  can be configured to communicate the maneuvering instructions to an auto pilot system of the receiver aircraft  20 , if such system exists. 
     It is to be noted that, with reference to  FIG. 20 , some of the blocks can be integrated into a consolidated block or can be broken down to a few blocks and/or other blocks may be added. Furthermore, in some cases, the blocks can be performed in a different order than described herein. It should be also be noted that whilst the flow diagrams are described also with reference to the system elements that realizes them, this is by no means binding, and the blocks can be performed by elements other than those described herein. 
     It is to be further noted that in some cases instead of monitoring that the receiver aircraft  20  is positioned within the engagement area, an alternative light directing system can be used. Such an alternative lighting system can be designed to display alternative light indications depending on the angle from which it is viewed. Thus, in some cases, viewing the light directing system from its bottom side can result in display of light in a certain color, looking at the same light directing system from its upper side can result in display of light in a second color, looking at the same light directing system from its right side can result in display of light in a third color, and looking at the same light directing system from its left side can result in display of light in a fourth color. Additional angles can result in display of additional colors. When using such a light directing system, upon arrival of the receiver aircraft  20  to the engagement area, the light directing system can be automatically directed to the receiver aircraft  20  (based on its current determined spatial disposition) and provide a color indication indicating that it is positioned within the engagement area. If the receiver aircraft  20  does not maintain its position, the pilot will receive appropriate color indications from the light directing system so that he will be able to make the required corrections to maintain the receiver aircraft&#39;s  20  spatial disposition. 
     Turning to  FIG. 21 , there is shown a flowchart illustrating a sequence of operations carried out for providing steering commands to a refueling device for maneuvering to an engagement enabling position, according to certain examples of the presently disclosed subject matter. 
     When a non-aircraft-fixed in-flight refueling system is used steering control module  1830  can be configured to determine (it is to be noted that in some cases, as indicated herein, such determination is made, for example, repeatedly) the refueling device&#39;s  100  spatial disposition with respect to the engagement enabling position (in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ) related thereto (block  2105 ), as further detailed with respect to  FIG. 23 . 
     Following determination of the refueling device&#39;s  100  spatial disposition with respect to the engagement enabling position related thereto, steering control module  1830  can be configured to check if the refueling device  100  is positioned within the engagement enabling position (block  2110 ). In case the refueling device  100  is positioned within the engagement enabling position (in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), and at least until the refueling device  100  is engaged with the fuel receptacle  22  of the receiver aircraft  20  (in some cases this process can continue until the refueling process ends), the steering control module  1830  can be configured to return to block  2105  and re-determine the refueling device&#39;s  100  spatial disposition with respect to the engagement enabling position related thereto. 
     In case the refueling device  100  is not positioned within the engagement enabling position (in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), steering control module  1830  can be configured to calculate steering commands for maneuvering the refueling device  100  to an engagement enabling position, in which the boom member  130  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20  (block  2120 ). It can be appreciated that once the steering control module  1830  determines the refueling device&#39;s  100  spatial disposition with respect to an engagement enabling position related thereto, it can also calculate steering commands for maneuvering the refueling device  100  to an engagement enabling position related thereto. Steering control module  1830  can also provide the refueling device  100  with calculated steering commands for maneuvering the refueling device  100  to an engagement enabling position related thereto (block  2130 ) and return to block  2105 . The steering commands can control the operation of components of the spatial control system  160  and/or the force generating arrangement  190 . 
     When utilizing an aircraft-fixed flying boom system, steering control module  1830  can be configured to determine (it is to be noted that in some cases, as indicated herein, such determination is made, for example, repeatedly) the boom fuelling unit  310  spatial disposition with respect to the engagement enabling position (in which the boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ) related thereto (block  2105 ), as further detailed with respect to  FIG. 23 . 
     Following determination of the boom fuelling unit  310  spatial disposition with respect to the engagement enabling position related thereto, steering control module  1830  can be configured to check if the boom fuelling unit  310  is positioned within the engagement enabling position (block  2110 ). In case the boom fuelling unit  310  is positioned within the engagement enabling position (in which the boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), and at least until the boom fuelling unit  310  is engaged with the fuel receptacle  22  of the receiver aircraft  20  (in some cases this process can continue until the refueling process ends), the steering control module  1830  can be configured to return to block  2105  and re-determine the boom fuelling unit  310  spatial disposition with respect to the engagement enabling position related thereto. 
     In case the boom fuelling unit  310  is not positioned within the engagement enabling position (in which the boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ), steering control module  1830  can be configured to calculate alignment commands for maneuvering the boom fuelling unit  310  to an engagement enabling position, in which the boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20  (block  2120 ). It can be appreciated that once the steering control module  1830  determines the boom fuelling unit  310  spatial disposition with respect to an engagement enabling position related thereto, it can also calculate alignment commands for maneuvering the boom fuelling unit  310  to an engagement enabling position related thereto. Steering control module  1830  can also provide the boom fuelling unit  310  with calculated alignment commands for maneuvering the boom fuelling unit  310  to an engagement enabling position related thereto (block  2130 ) and return to block  2105 . The alignment commands can control the operation of components of the motion control system  330  and/or the mechanical connection  320  and/or the telescoping aft section  314 . 
     It is to be noted that, with reference to  FIG. 21 , some of the blocks can be integrated into a consolidated block or can be broken down to a few blocks and/or other blocks may be added. Furthermore, in some cases, the blocks can be performed in a different order than described herein. It should also be noted that whilst the flow diagrams are described also with reference to the system elements that realizes them, this is by no means binding, and the blocks can be performed by elements other than those described herein. 
     Turning to  FIG. 22 , there is provided a flowchart illustrating a sequence of operations carried out for determining the receiver aircraft&#39;s spatial disposition with respect to the engagement area related thereto, according to certain examples of the presently disclosed subject matter. In some cases, maneuvering instructions module  1820  can be configured to acquire an image of the receiver aircraft  20  (block  2210 ). 
     For that purpose, in some cases, maneuvering instructions module  1820  can be configured to utilize sensor control module  1810  for repeatedly (e.g. every pre-determined period) or continuously (e.g. in the form of a video stream) acquiring an image of the area aft the refueling device  100  and/or aft the tanker aircraft  12 , at a predetermined Field of View. Such Field of View can depend, inter alia, on the distance from which a receiver aircraft  20  is to be identified. In some cases, the farther it is required to identify the receiver aircraft, the larger the Field of View is. 
     It is to be noted that although reference in the description is sometimes made to an image, any other data that can be indicative of presence of a receiver aircraft can be utilized mutatis mutandis (e.g. radar data, acoustic signature data, etc.). It is to be further noted that in some cases, when reference is made to an image (throughout the description) it can also include an image based on data acquired by the LIDAR unit  351 . 
     For the purpose of acquiring an image, sensor control module  1810  can be configured to utilize sensor  1890 . In some cases, one or more sensor/s  1890  can be mounted on the refueling device  100  and/or on the tanker aircraft  12 . As indicated herein, in some cases the imaging system  150  can be used as one or more of the sensor/s  1890 . It is to be noted that, as indicated herein, the imaging system  150  can, in some cases, comprise one or more FLADAR units and/or one or more LIDAR units  351 . 
     It can be appreciated that for acquiring an image of the receiver aircraft  100 , the receiver aircraft  100  should be present in the sensed area (e.g. the sensing volume  159  or the sensing volume  359 ). In some cases the receiver aircraft  20  is expected to approach the refueling device  100  and/or the boom fuelling unit  310  and/or the fuel tanker  12  for the refueling process to begin. In some cases, the approach of receiver aircraft  20  to the refueling device  100  and/or the boom fuelling unit  310  is made from a certain direction or through a virtual funnel such as, for example, from the rear and bottom side of the refueling device  100  and/or the boom fuelling unit  310  and/or the fuel tanker  12  while the pilot of the receiver aircraft  20  has a line of sight to the refueling device  100  and/or the boom fuelling unit  310  and/or the fuel tanker  12 . However, in other cases, other directions of approach are also possible (e.g. approach from the front and bottom side of the refueling device  100  and/or the boom fuelling unit  310  and/or the fuel tanker  12 , etc.), depending, inter alia, on the characteristics of the receiver aircraft  20  (e.g. the location of the fuel receptacle  22  of the receiver aircraft  20 , etc.). 
     In some cases, maneuvering instructions module  1820  can be configured to analyze sensed images in order to determine if a receiver aircraft  20  can be identified within the sensed image. It is to be noted that such analysis can be performed using various known methods and techniques, such as, in the case of digital images−image correlation, in the case of LIDAR based images—comparison with look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft, optionally based on the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.), etc. 
     In some cases, maneuvering instructions module  1820  can be configured to cause the signaling system to provide the pilot of the receiver aircraft  20  with a notification indicating that its location with respect to the engagement area has been acquired. Such indication can be provided, for example, by a signaling system (e.g. a lighting system mounted on the refueling device  100  and/or the boom fuelling unit  310  and/or on the tanker aircraft  12 , etc.). Additionally or alternatively, the indication can be a voice indication provided to the pilot of the receiver aircraft  20  (e.g. by utilizing speakers, pilot headset, etc.). Additionally or alternatively, the indication can be any other indication (including visual or voice indication) provided to the pilot of the receiver aircraft  20  (e.g. by utilizing speakers, pilot headset, a display, a light, etc.). 
     In some cases, maneuvering instructions module  1820  can be further configured to fetch configuration data (block  2220 ), including, inter alia, data indicative of the engagement area specification. As detailed herein, the engagement area can be defined by various specifications that depend on several parameters, such as, for example, the length of the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the extension length of the telescoping aft section  314  (when utilizing an aircraft-fixed flying boom system), the flight speed, the flight altitude, weather conditions, the fuel pressure within the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the location of the fuel receptacle  22  of the receiver aircraft  20 , etc. 
     In some cases, maneuvering instructions module  1820  can be further configured to fetch a reference image of a reference receiver aircraft (block  2230 ). In some cases the reference image is fetched inter alia according to the fetched configuration data and/or the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.). Such a reference image can be an image of an aircraft similar to the actual receiver aircraft  20 , and in some cases of identical type as the actual receiver aircraft  20 . If necessary, the maneuvering instructions module  1820  can obtain current measurements for certain parameters in the configuration DR  1860 , to compute appropriate engagement area specifications. 
     It is to be noted that such a reference image should depict a scene in which the reference aircraft is positioned within the engagement area having the fetched specification (fetched in block  2220 ). In some cases, the reference image can depict a scene in which the reference aircraft is not positioned within the engagement area having the fetched specification, however the offset of the reference receiver aircraft from the engagement area can be calculated or alternatively is a-priori known. 
     Maneuvering instructions module  1820  can be further configured to utilize the reference image of a reference receiver aircraft within the engagement area having the fetched specification (or not within such engagement area but with data of its offset from the engagement area) for calculating the relative spatial disposition of the receiver aircraft with respect to the engagement area (block  2240 ), for example using methods and techniques known per se (such as, in the case of digital images, image correlation, etc.). 
     In some cases, 3-D models can be used instead of images. According to further examples, the reference DR  1870  can store one or more generic 3-D models (e.g. one for each type of aircraft), and as part of determining the spatial disposition of the receiver aircraft  20  with respect to the engagement area, an appropriate 3-D model can be selected (for example according to the type of the receiver aircraft  20 ) and the 3-D model can be adapted using current measurements (e.g. obtained by the sensor  1890 ) and respective parameters of the 3-D model. 
     It is to be noted that various other methods and techniques can be used in order to determine the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement area. 
     One example of such alternative technique is using LIDAR. In such cases, the sensor  1890  can be a LIDAR unit  351  that can acquire the images as further detailed herein, inter alia with reference to  FIGS. 30-32 . As indicated herein, the images obtained by the LIDAR unit  351  can comprise both depth data and electromagnetic data within the sensing volume  359 . In such cases, the depth and electromagnetic data can be compared with one or more look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft, optionally based on the type of the receiver aircraft  20  (e.g. F-15, F- 16 , etc.). Based on the comparison, the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  and/or the receiver aircraft  20  with respect to the tanker aircraft  12  can be calculated. In some cases, a full or partial 3-D model of the receiver aircraft  20  and/or the refueling device  100  and/or the boom fuelling unit  310  can be calculated based on the depth and electromagnetic data received from the LIDAR unit  351  and such full or partial 3-D model can be compared with one or more pre-stored generic full or partial 3-D models (e.g. one for each type of aircraft). Based on the comparison, the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  and/or the receiver aircraft  20  with respect to the tanker aircraft  12  can be calculated. The look up tables and/or the 3-D models can be stored for example on the reference DR  1870 . 
     It is to be noted that, with reference to  FIG. 22 , some of the blocks can be integrated into a consolidated block or can be broken down to a few blocks and/or other blocks may be added. Furthermore, in some cases, the blocks can be performed in a different order than described herein. It should be also be noted that whilst the flow diagrams are described also with reference to the system elements that realizes them, this is by no means binding, and the blocks can be performed by elements other than those described herein. 
     Turning to  FIG. 23  there is provided a flowchart illustrating a sequence of operations carried out for determining the refueling device&#39;s  100  (or boom fuelling unit  310 ) spatial disposition with respect to the engagement enabling position, according to certain examples of the presently disclosed subject matter. In some cases, steering control module  1830  can be configured to acquire an image of the receiver aircraft  20  (block  2310 ), and, in some cases, more specifically, of the area of the fuel receptacle  22  of the receiver aircraft  20 . 
     For that purpose, in some cases, steering control module  1830  can be configured to utilize sensor control module  1810  for repeatedly (e.g. every pre-determined period) or continuously acquiring an image (it is to be noted again that although reference in the description is made to an image, any other data that can be indicative of presence of a receiver aircraft can be utilized mutatis mutandis. It is to be further noted that in some cases, when reference is made to an image it can also include an image based on data acquired by the LIDAR unit  351 ) of the area aft the refueling device  100  or boom fuelling unit  310 , at a predetermined Field of View. Such Field of View can depend, inter alia, on the distance from which a receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, is to be identified. In some cases, the farther it is required to identify the receiver aircraft, and more specifically a fuel receptacle  22  thereof, the larger the Field of View is. 
     For the purpose of acquiring an image, sensor control module  1810  can be configured to utilize sensor  1890 . In some cases, one or more sensor/s  1890  can be mounted on the refueling device  100  and/or the boom refueling unit  310  and/or on the tanker aircraft  12 . As indicated herein, in some cases the imaging system  150  can be used as one or more of the sensor/s  1890 . It is to be noted that, as indicated herein, the imaging system  150  can, in some cases, comprise one or more FLADAR units and/or one or more LIDAR units  351 . 
     It can be appreciated that for acquiring an image of the receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, the receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, must be present in the sensed area (e.g. the sensing volume  159 ). In some cases, the receiver aircraft  20  is expected to be positioned within the engagement area related thereto. 
     In some cases steering control module  1830  can be configured to analyze a sensed image in order to determine if a receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, can be identified within the sensed image. A series of images can be analyzed, each substantially immediately after it has been captured (for example as described herein), in order for the steering control module  1830  to be able to provide steering or alignment commands that are based on the actual (dynamic) relative position of the refueling device  100  and/or the boom fuelling unit  310  and the engagement enabling position. 
     In some cases, 3-D models can be used instead of images. According to further examples, the reference DR  1870  can store one or more generic 3-D models (e.g. one for each type of aircraft), and as part of determining the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  with respect to the engagement enabling position, an appropriate 3-D model can be selected (for example according to the type of the receiver aircraft  20 ) and the 3-D model can be adapted using current measurements (e.g. obtained by the sensor/s  1890 ) and respective parameters of the 3-D model. 
     It is to be noted that such analysis can be performed using various known methods and techniques, such as, in the case of digital images—image correlation, in the case of LIDAR based images—comparison with look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft, optionally based on the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.), etc. 
     In some cases, steering control module  1830  can be further configured to fetch configuration data (block  2320 ), including, inter alia, data indicative of the engagement enabling position specification. As detailed herein, the engagement enabling position can be defined by a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . As further indicated herein, such configuration data can depend on several parameters, such as, for example, the length of the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the extension length of the telescoping aft section  314  (when utilizing an aircraft-fixed flying boom system), the flight speed, the flight altitude, weather conditions, the fuel pressure within the hose  52  (when a non-aircraft-fixed in-flight refueling system is used), the location of the fuel receptacle  22  of the receiver aircraft  20 , etc. The configuration data is fetched according to current measurements and respective parameters stored in association with each set of configuration data. 
     In some cases, steering control module  1830  can be further configured to fetch a reference image of a reference receiver aircraft, and more specifically a fuel receptacle thereof (block  2330 ). In some cases the reference image is fetched inter alia according to the fetched configuration data (which, in turn, was fetched according to current measurements and respective parameters stored in association with each set of configuration data). Such a reference image can be an image of an aircraft, and more specifically a fuel receptacle thereof, similar, and in some cases identical to, the actual receiver aircraft  20 , and more specifically, an actual fuel receptacle  22  of the actual receiver aircraft  20 . 
     It is to be noted that such a reference image should depict a scene in which the reference aircraft, and more specifically a fuel receptacle thereof, is positioned within an engagement enabling position having the fetched specification (fetched in block  2320 ). In some cases, the reference image can depict a scene in which the reference aircraft, and more specifically a fuel receptacle thereof, is not positioned within the engagement enabling position having the fetched specification, however the offset of the reference receiver aircraft from the engagement enabling position can be calculated or alternatively is a-priori known. 
     In some cases, steering control module  1830  can be further configured to utilize the reference image of a reference receiver aircraft, and more specifically a fuel receptacle thereof, within the engagement area having the fetched specification (or not within such engagement area but with data of its offset from the engagement area) for calculating the relative spatial disposition of the receiver aircraft  20 , and more specifically the fuel receptacle  22  thereof, with respect to an engagement enabling position (block  2340 ), for example using methods and techniques known per se (such as, in the case of digital images, image correlation, etc.). 
     In some cases, 3-D models can be used instead of images. According to further examples, the reference DR  1870  can store one or more generic 3-D models (e.g. one for each type of aircraft), and as part of determining the spatial disposition of the refueling device  100  with respect to engagement enabling position, an appropriate 3-D model can be selected (for example according to the type of the receiver aircraft  20 ) and the 3-D model can be adapted using current measurements (e.g. obtained by the sensor  1890 ) and respective parameters of the 3-D model. 
     It is to be noted that various other methods and techniques can be used in order to determine the refueling device  100  and/or the boom fuelling unit  310  spatial disposition with respect to the engagement area. 
     One example of such alternative technique is using LIDAR. In such cases, the sensor  1890  can be a LIDAR unit  351  that can acquire the images as further detailed herein, inter alia with reference to  FIG. 30-32 . As indicated herein, the images obtained by the LIDAR unit  351  can comprise both depth data and electromagnetic data within the sensing volume  359 . In such cases, the depth and electromagnetic data can be compared with one or more look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft, optionally based on the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.). Based on the comparison, the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  and/or the receiver aircraft  20  with respect to the tanker aircraft  12  can be calculated. In some cases, a full or partial 3-D model of the receiver aircraft  20  and/or the refueling device  100  and/or the boom fuelling unit  310  can be calculated based on the depth and electromagnetic data received from the LIDAR unit  351  and such full or partial 3-D model can be compared with one or more pre-stored generic full or partial 3-D models (e.g. one for each type of aircraft). Based on the comparison, the spatial disposition of the refueling device  100  and/or the boom fuelling unit  310  and/or the receiver aircraft  20  with respect to the tanker aircraft  12  can be calculated. The look up tables and/or the 3-D models can be stored for example on the reference DR  1870 . 
     It is to be noted that, with reference to  FIG. 23 , some of the blocks can be integrated into a consolidated block or can be broken down to a few blocks and/or other blocks may be added. Furthermore, in some cases, the blocks can be performed in a different order than described herein. It should be also be noted that whilst the flow diagrams are described also with reference to the system elements that realizes them, this is by no means binding, and the blocks can be performed by elements other than those described herein. 
     Looking, by way of example, at  FIG. 28 , there is shown an illustration of an example of a sensed image indicating that the refueling device is not positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter. In some cases, the sensed image of the receiver aircraft  20 , and more specifically a fuel receptacle  22  thereof, and the reference image of the reference receiver aircraft, and more specifically a fuel receptacle thereof, can contain some elements (e.g.  2810 ,  2820 ,  2830 ,  2840 , etc.) that enable determination of the sensed image offset from the reference image (thereby enabling determination of the offset of the refueling device  100  or the boom fuelling unit  310  from an engagement enabling position, in which the boom member  130  or boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 ). It is to be noted that virtual cross  2850  indicates the center of the image. Such elements (e.g.  2810 ,  2820 ,  2830 ,  2840 ) can be used for example by various known per se image correlation algorithms in order to determine the spatial disposition of the receiver aircraft  20 , and more specifically the fuel receptacle  22  thereof, with respect to an engagement enabling position. The illustration of  FIG. 28  is provided for clarity of explanation only and is by no means binding. 
     Attention is now drawn to  FIG. 29 , showing an illustration of an example of a reference image indicating that the refueling device is positioned in an engagement enabling position, according to certain examples of the presently disclosed subject matter. Looking at the illustration, it can be appreciated that the fuel receptacle of a receiver aircraft should be aligned with the cross  2850  (that indicates the center of the image) and that elements  2810 ,  2820 ,  2830 , and  2840  should also be aligned with the cross  2850  vertical and horizontal axis, thus indicating that a refueling device is positioned in an engagement enabling position, in which the boom member  130  or boom member  312  is in a predetermined maximal spaced and spatial relationship with respect to the fuel receptacle  22  of the receiver aircraft  20 . It can be appreciated that there is an offset between the sensed image shown in  FIG. 28  and the reference image shown in  FIG. 29 , thus indicating that at the time the sensed image shown in  FIG. 28  was sensed, the refueling device  100  or the boom fuelling unit  310  was not in an engagement enabling position. The illustration of  FIG. 29  is provided for clarity of explanation only and is by no means binding. 
     It has been indicated herein that when a non-aircraft-fixed in-flight refueling system is used the engagement enabling position is a position from which utilization of the force generating arrangement  190  enables the nozzle  135  to engage with the fuel receptacle  22  of the receiver aircraft  20 . When utilizing an aircraft-fixed flying boom system, the engagement enabling position is a position from which the nozzle  316  is able to engage with the fuel receptacle  22  of the receiver aircraft  20 . Therefore, in some cases, there can be more than one engagement enabling position that meets such criteria and thus, in some cases, a certain offset between the sensed image and the reference image can be allowed, as long as the refueling device is in a position from which, when a non-aircraft-fixed in-flight refueling system is used, utilization of the force generating arrangement  190  enables the nozzle  135  to engage with the fuel receptacle  22  of the receiver aircraft  20  or from which, when utilizing an aircraft-fixed flying boom system, the nozzle  316  is able to engage with the fuel receptacle  22  of the receiver aircraft  20 . 
     It is to be noted that various other methods and techniques can be used in order to determine the receiver aircraft&#39;s  20  spatial disposition with respect to the engagement enabling position, including the known per se 3-D modeling adaptation and/or the selection of the reference image which provides the highest correlation to the sensed data. 
     Operation Mode II 
     In this operation mode, once the tanker aircraft  12  and receiver aircraft  20  are in close proximity and flying in formation, with the receiver aircraft  20  at a position behind the tanker aircraft  12 , the refueling device is flown into engagement with the fuel receptacle  22  of the receiver aircraft  20  by an operator. 
     In a first example of Operation Mode II, the operator is stationed in the tanker aircraft  12 , which comprises a suitable control station operatively connected to the refueling device, which can be refueling device  100  according to the first example or alternative variations thereof, or refueling device  200  according to the second example or alternative variations thereof, or a refueling device according to other suitable examples thereof according to the first aspect of the presently disclosed subject matter. The control station comprises a display device for suitably displaying data relating to the spatial disposition of the refueling device at least with respect to the receiver aircraft  20  and the fuel receptacle  22  thereof, and an output device for providing control signals to the refueling device for controlling the flight thereof. 
     For example, and referring to refueling device  100  according to the first example or alternative variations thereof, the display device can comprise a screen display that displays real time images (2D, and/or stereoscopic images, and/or 3D images), for example in video streams, provided by the imaging system  150 . Additionally or alternatively, such imaging can be provided or augmented via suitable cameras or other imaging units, provided on the tanker aircraft  12  and/or the receiver aircraft  20  and/or any other suitable air vehicle in the vicinity of the refueling device  100 , and thus in at least some such examples the refueling device  100  can omit the imaging system  150 . 
     The output device can comprise, for example, a joystick that is hand-manipulated by the operator to provide the required control signals to the spatial control system  160 , in particular the controllable aerodynamic surfaces thereof, to provide the required design angle θ des  between the boom axis  131  and the forward direction A, while flying the refueling device  100  into proximity with the receiver aircraft  20  and the fuel receptacle  22  thereof. 
     The operator first ensures that the refueling device  100  is being towed behind the tanker aircraft  12  at a suitable distance therefrom, and can control this spacing by extending or retracting the hose  52  with respect to the tanker aircraft  12 . 
     When the operator determines that the nozzle  135  is properly aligned with the fuel receptacle  22  (the boom member  130  and boom axis  131  being at the design angle θ des  to the forward direction A) and sufficiently close thereto, i.e., at the engagement enabling position at a predetermined spacing from the receiver aircraft, said boom axis being aligned in an engagement enabling orientation at said spaced position, the operator provides a suitable control signal to the refueling device  100  to activate the force generating arrangement  190 , i.e., by deploying the air brakes  195 ,  196 , generating a force along boom axis  131  that effectively pushes the nozzle into engagement with the receptacle  22 . In other words, once the refueling device  100  is at the aforesaid engagement enabling orientation and spaced position, the boom member is subsequently moved along said boom axis towards the receiver aircraft for enabling fuel communication therebetween. Movement of the boom member can be effected in one of two ways, or combination thereof: the refueling device  100  remains in the spaced position, and the boom member  130  is extended telescopically; the boom member  130  can be in the retracted or extended position, and the refueling device  100  is bodily moved towards the receiver aircraft for enabling fuel communication therebetween. 
     Thereafter, the air brakes  195 ,  196  are retracted, and fuel is pumped to the receiver aircraft  20  from the tanker aircraft  12 . The refueling device  100  can be automatically or manually controlled to maintain the required design angle θ des  between then boom axis  131  and the forward direction A during refueling. 
     Once refueling is completed, the operator disengages the nozzle  135  from the fuel receptacle  22  and flies the refueling device  100  at least to a safe position away from the receiver aircraft  20 , and/or the receiver aircraft  20  maneuvers to such a position, and the refueling device  100  can be retracted back into the tanker aircraft  12 , or reused with another receiver aircraft  20 . 
     It is to be noted that the same operator can carry out Operation Mode II with each of the plurality of refueling systems  50  of the tanker aircraft  12 . Alternatively, the tanker aircraft  12  can comprise a dedicated control station operatively connected to each refueling device  100 , and operated by a respective dedicated operator; thus different operators control each of the refueling devices  100 . 
     In an alternative variation of this example of Operation Mode II, the operator is stationed in another aircraft different from the receiver aircraft  20  or tanker aircraft  12 , or is located in a ground station, and Operation Mode II can be carried out in a similar; manner to that described above for the first example, mutatis mutandis, with the main difference that the operator receives the data relating to the spatial disposition of the refueling device at least with respect to the receiver aircraft  20  and the fuel receptacle  22  thereof, and provides control signals to the refueling device for controlling the flight thereof, via a suitable communications link respect to the refueling device  100 , which is correspondingly equipped with suitable communication system. 
     In another alternative variation of this example of Operation Mode II, the operator is stationed in the receiver aircraft  20  rather than in tanker aircraft  12 , and Operation Mode II can be carried out in a similar; manner to that described above for the first example, mutatis mutandis, with the main difference that the operator receives the data relating to the spatial disposition of the refueling device at least with respect to the receiver aircraft  20  and the fuel receptacle  22  thereof, and provides control signals to the refueling device for controlling the flight thereof, via a suitable communications link with respect to the refueling device  100 , and the receiver aircraft  20  and the refueling device  100  are each correspondingly equipped with a suitable communication system. Alternatively, in at least some circumstances, the operator can have the refueling device  100  in particular the boom member  130  and nozzle  135 , and the fuel receptacle  22  in the operator&#39;s visual field of view, and does not require the aforesaid spatial disposition data in order to control the refueling device  100 , and thus in such cases the refueling device  100  can omit the imaging system  150 . 
     Clearly, Operation Mode II can be applied to other variations of the first example of refueling device  100 , or to the second example of refueling device  200  or alternative variations thereof, in a similar manner to that described above for the first example of refueling device  100 , mutatis mutandis. 
     It is to be noted that according to Operation Mode I and/or Operation Mode II, the refueling unit can be selectively controlled to adopt an aligned configuration with the hose  52 . By way of non-limiting example, such a situation is illustrated in  FIG. 2  for the fuselage-mounted system  50 . Such a configuration can include controlling the spatial control system  160  to align the longitudinal axis  111  with the forward direction A in the case of the first example of refueling device  100  or alternative variations thereof, or maintaining the boom  230  in a retracted configuration accommodated in body  210  in the case of the second example of refueling device  200  or alternative variations thereof, for example 
     Operation Mode III 
     In this operation mode, the tanker aircraft  12  and receiver aircraft  20  are maneuvered to be in close proximity and flying in formation, with the receiver aircraft  20  at a position behind the tanker aircraft  12 . It is first ensured that the refueling device  100  is being towed behind the tanker aircraft  12  at a suitable distance therefrom, and an operator (typically in the tanker aircraft) can control this spacing by extending or retracting the hose  52  with respect to the tanker aircraft  12 . 
     In a first example of Operation Mode III, the refueling device  100  is not flown or controlled per se, but attains a stable configuration with the boom member  135  at the required angle with respect to the forward direction A, design angle θ des . Accordingly the refueling device  100  can optionally omit the controllable spatial control system  160 , and instead comprises a suitable configuration that provides stability to the boom member  135  at this spatial disposition. 
     The receiver aircraft  20  maneuvers to a position where it can engage the nozzle to the receiver aircraft fuel receptacle, and when an operator in the receiver aircraft (for example the pilot) determines that the nozzle  135  is properly aligned with the fuel receptacle  22  and sufficiently close thereto, the operator provides a suitable control signal to the refueling device  100  to activate the force generating arrangement  190 , i.e., by deploying the air brakes  195 ,  196 , generating a force along boom axis  131  that effectively pushes the nozzle into engagement with the receptacle  22 . Thereafter, the air brakes  195 ,  196  are retracted, fuel is pumped to the receiver aircraft  20  from the tanker aircraft  12 , and the refueling device  100  maintains the required design angle θ des  between then boom axis  131  and the forward direction A during refueling. 
     The receiver aircraft can comprise a display device for suitably displaying data relating to the spatial disposition of the refueling device at least with respect to the receiver aircraft  20  and the fuel receptacle  22  thereof. 
     For example, and referring to refueling device  100  according to the first example or alternative variations thereof, the display device can comprise a screen display that displays to the operator (for example the pilot or navigator of the receiver aircraft) real time images (2D, and/or stereoscopic images, and/or 3D images), for example in video streams, provided by the imaging system  150 . Additionally or alternatively, such imaging can be provided or augmented via suitable cameras or other imaging units, provided on the tanker aircraft  12  and/or the receiver aircraft  20  and/or any other suitable air vehicle in the vicinity of the refueling device  100 , and thus in at least some such examples the refueling device  100  can omit the imaging system  150 . 
     Alternatively, in at least some circumstances, the operator can have the refueling device  100  in particular the boom member  130  and nozzle  135 , and the fuel receptacle  22  in the operator&#39;s visual field of view, and does not require the aforesaid spatial disposition data, and thus in such examples the refueling device  100  can omit the imaging system  150 . 
     In at least some circumstances, the operator/pilot can maneuver the receiver aircraft  12  such as to provide a suitable force via the fuel receptacle  22  to engage the nozzle  135  thereto, or alternatively the receiver aircraft can comprise a suitable arrangement configured for engage the nozzle  135  to the fuel receptacle  22  without the need to generate such a force, and thus in such examples the refueling device  100  can omit the force generating arrangement  190 . 
     Once refueling is completed, the operator disengages the nozzle  135  from the fuel receptacle  22  and the receiver aircraft  20  maneuvers to at least a safe position spaced from the refueling devoice  100 , and the refueling device  100  can be retracted back into the tanker aircraft  12 , or reused with another receiver aircraft  20 . 
     It is to be noted that Operation Mode III can be carried out with each of the plurality of refueling systems  50  of the tanker aircraft  12 . 
     Clearly, Operation Mode III can be applied to other variations of the first example of refueling device  100 , or to the second example of refueling device  200  or alternative variations thereof in a similar manner to that described above for the first example of refueling device  100 , mutatis mutandis. 
     Reference is made to  FIG. 32 , showing a schematic illustration of a scene sensed by the LIDAR unit according to certain examples of the presently disclosed subject matter. 
     In this example, the LIDAR unit  351  scans the sensing volume  359 , while at least one scan line intersects with the fuel receptacle  22  (or another recognizable part from which the position and orientation of the fuel receptacle  22  can be estimated) of the receiver aircraft  20  and at least one scan line intersects with a certain location on the boom member  312  (e.g. a boom tip marker  340  located thereon) or refueling device  100 . 
     It is to be noted that although in this example the scan lines are horizontal alternative scan pattern can be used as well. 
     Turning to  FIG. 33 , there is shown a representation of the depth and electromagnetic data relating to the boom refueling device and to the fuel receptacle of the receiver aircraft as acquired by the LIDAR unit according to certain examples of the presently disclosed subject matter. 
     The boom fueling device  310  electromagnetic data, shown as element  405  in the figure, shows that at a certain area, having a certain width, within a certain scan line of the LIDAR unit  351 , the electromagnetic value was higher than the electromagnetic value in its surrounding area, thus indicating that the boom member  312  (or, sometimes, more specifically, boom tip marker  340 ) is located in this area. As indicated above, in some cases the intensity can be affected, for example, by a fuel receptacle marker  342  located at a pre-determined location on the boom member  312  and causing strong intensity reflection of the respective beam B 2  when illuminated by beam B 1 , as compared with the reflection intensity obtained from other surfaces of the boom fuelling unit  310 , for example. 
     The boom fueling device  310  depth data, shown as element  410  in the figure, shows that at a certain area, having a certain width, within a certain scan line of the LIDAR unit  351 , the depth value was lower than the depth value in its surrounding area, thus also indicating that the boom member  312  is located in this area. The area in which the boom member  312  is located is closer to the LIDAR unit  351  in comparison to its surrounding area, and thus the time interval between the outgoing beams and the return beams associated with the area comprising the boom member  312  is lower than the time interval between the outgoing beams and the return beams associated with areas surrounding it. 
     The fuel receptacle  22  electromagnetic data, shown as element  415  in the figure, shows that at a certain area, having a certain width, within a certain scan line of the LIDAR unit  351 , the electromagnetic value was lower than the electromagnetic value in its surrounding area, thus indicating that the fuel receptacle is located in this area. The electromagnetic value is lower at that area, for example since the fuel receptacle  22  located within the corresponding area is deeper than its surrounding area. 
     As indicated above, in some cases the intensity can be affected by a fuel receptacle marker  342  comprised on the receiver aircraft  20  in a pre-determined location with respect to the fuel receptacle  22  thereof and causing strong intensity reflection of the respective beam B 2  when illuminated by beam B 1 , as compared with the reflection intensity obtained from other surfaces of the receiver aircraft  20 , for example. 
     The fuel receptacle  22  depth data, shown as element  420  in the figure, shows that at a certain area, having a certain width, within a certain scan line of the LIDAR unit  351 , the depth value was higher than the depth value in its surrounding area, thus also indicating that the fuel receptacle  22  is located in this area. The area in which the fuel receptacle  22  is located is farther from the LIDAR unit  351  in comparison to its surrounding area, and thus the time interval between the outgoing beams and the return beams associated with the area comprising the boom member  312  is lower than the time interval between the outgoing beams and the return beams associated with areas surrounding it. 
     It is to be noted that, as any person of ordinary skill in the art can appreciate, the depths and width of the fuel receptacle depth data and/or the boom fueling device depth data can enable calculation of the spatial dispositions of the fuel receptacle  22  and/or the boom fueling device  310  (and its tip) with respect to the fuel tanker  12  and with respect to each other. 
     It is to be further noted that the boom fueling device  310  electromagnetic data shown as element  405 , the boom fueling device  310  depth data shown as element  410 , the fuel receptacle  22  electromagnetic data shown as element  415  and the fuel receptacle  22  depth data shown as element  420  can be compared with pre-stored look-up tables comprising reference depth data and reference electromagnetic data relating to reference spatial dispositions with respect to the receiver aircraft, optionally based on the type of the receiver aircraft  20  (e.g. F-15, F-16, etc.), thus enabling calculation of various spatial relationships, e.g. between any two of the following: the boom fueling device  310 , the fuel receptacle  22 , the refueling device  100 , the receiver aircraft  20 , the engagement enabling position, the engagement area. 
     It is to be further noted that although the description of  FIG. 33  refers to a boom fueling device  310  the same also applies to a non-aircraft-fixed in-flight refueling system (e.g. refueling device  100 , etc.). 
     In the method claims that follow, alphanumeric characters and Roman numerals used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps. 
     It should be noted that the word “comprising” as used throughout the appended claims is to be interpreted to mean “including but not limited to”. 
     While there has been shown and disclosed examples in accordance with the presently disclosed subject matter, it will be appreciated that many changes may be made therein without departing from the spirit of the presently disclosed subject matter. 
     It is to be understood that the presently disclosed subject matter is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The presently disclosed subject matter is capable of other embodiments and of being practiced and carried out in various ways. Hence, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present presently disclosed subject matter. 
     It will also be understood that the system according to the presently disclosed subject matter may be a suitably programmed computer. Likewise, the presently disclosed subject matter contemplates a computer program being readable by a computer for executing the method of the presently disclosed subject matter. The presently disclosed subject matter further contemplates a machine-readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the presently disclosed subject matter.