NAVIGATION

FIELD OF THE PRESENTLY DISCLOSED SUBJECT MATTER

The presently disclosed subject matter relates to the field of autonomously controlled aircrafts.

BACKGROUND

During autonomous flight, unmanned aerial vehicles (UAVs, also known as unmanned aerial systems or drones) may navigate in midair according to one or more preconfigured flight paths. The flight route is constructed as a series of waypoints (WP) guiding the UAV from one waypoint to the next along the flight route from its current location to a desired target location.

Published documents considered to be relevant as background to the presently disclosed subject matter are provided below. Acknowledgement of the documents herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

US Patent No. 7,158,877 discloses remote control of an unmanned aerial vehicle, UAV, by a control station by means of a wireless command link. Flight control parameters are monitored, and in case a major alarm condition occurs, the UAV is controlled to follow an emergency route defined by a second set of predefined waypoints.

US Patent No. 6377875 discloses avoidance of an uncontrolled flight of a remotely controlled unmanned air vehicle (UAV), upon loss of radio contact between a control station and the UAV, the UAV flies on a preprogrammed safety route, as required the UAV is guided to a flight path that is remote-controlled from the control station, and, in the event of an interruption of the radio contact, the UAV will fly on a substitute route calculated with on-board equipment, without active intervention from the remote control station.

GENERAL DESCRIPTION

According to an aspect of the presently disclosed subject matter there is provided in an autonomously controlled aircraft, a method of avoiding violation of a flight constraint, the method comprising: with the help of at least one processing unit, performing at least the following operations: responsive to information indicative of a flight constraint, calculating a bank angle limit which allows the aircraft to proceed towards a desired destination along a flight path without violating the flight constraint; and generating instructions to aerial control devices for guiding the aircraft along the flight path.

In addition to the above features, the method according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (i) to (ix) below, in any desired combination or permutation: i. wherein the flight path is characterized by a turning radius which enables the aircraft to fly as fast as possible towards the desired destination without violating the flight constraint. ii. the method further comprising repeatedly calculating the bank angle limit during flight to the desired destination in order to adapt the bank angle limit to real-time changes in the flight constraint data. iii. the method further comprising: obtaining information indicative of a flight route comprising at least two waypoints for directing the aircraft to the desired destination; wherein the flight path directs the aircraft from one waypoint to a proceeding waypoint along the flight route. iv. wherein the aircraft is operating in observation mode or tracking mode the method further comprising: controlling the UAV to fly according to a certain flight pattern, wherein the desired destination is defined based on the certain flight pattern. v. wherein the flight constraint is a no-flight zone constraint flight of the aircraft over a certain ground area. vi. wherein the flight constraint is a BLOS communication link constraint requiring an open communication link between the aircraft and a communication satellite at all times. vii. wherein the flight constraint is a camera LOS constraint requiring an open LOS between an onboard camera and one or more objects of interest at all times. viii. wherein the flight constraint is maximal Radar Cross Section (RCS) constrains specifying a limitation of the aircraft's surface area which is allowed to be exposed in the direction of a radar. ix. wherein the flight constraint is a topographical and/or land cover constraint

According to another aspect of the presently disclosed subject matter there is provided a flight control unit onboard an autonomously controlled aircraft; the flight control unit being operatively connected to at least one processing unit, configured for: responsive to information indicative of a flight constraint, calculating a bank angle limit which allows the aircraft to proceed towards a desired destination along a flight path without violating the flight constraint; and generating instructions to aerial control devices for guiding the aircraft along the flight path.

According to another aspect of the presently disclosed subject matter there is provided an unmanned aerial vehicle configured with the flight control unit.

According to another aspect of the presently disclosed subject matter there is provided a non-transitory program storage device readable by a computer, tangibly embodying a program of instructions executable by the computer to perform a method of avoiding violation of a flight constraint, the method comprising: with the help of at least one processing unit, performing at least the following operations: responsive to information indicative of a flight constraint, calculating a bank angle limit or generation of a certain bank angle which allows the aircraft to proceed towards a desired destination along a flight path without violating the flight constraint; and generating instructions to aerial control devices for guiding the aircraft along the flight path.

The control unit, the storage device and UAV disclosed in accordance with the presently disclosed subject matter can optionally comprise one or more of features (i) to (ix) listed above with respect to the method, mutatis mutandis, in any desired combination or permutation.

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, the subject matter will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

Fig. 1 is a flowchart showing an example of a sequence of operations which are performed in a constraint driven navigation scenario, in accordance with the presently disclosed subject matter;

Fig. 2a is a schematic illustration of a flight route exemplifying some principles of the presently disclosed subject matter;

Fig. 2b is a schematic illustration of a flight constraint according to an example of the presently disclosed subject matter;

Fig. 3a is a schematic illustration exemplifying a flight constraint according to an example of the presently disclosed subject matter;

Fig. 3b is a schematic illustration exemplifying a flight constraint according to an example of the presently disclosed subject matter; Fig. 4 is a functional block diagram schematically illustrating an example of an aircraft comprising a flight control unit, in accordance with the presently disclosed subject matter;

Fig. 5a is a schematic illustration in top view of a UAV tracking an object of interest while flying in spiral flight pattern according to an example of the presently disclosed subject matter;

Fig. 5b is a schematic illustration in top view of a UAV tracking an object of interest while flying in offset-spiral flight pattern according to an example of the presently disclosed subject matter;

Fig. 6 is a flowchart showing another example of a sequence of operations which are performed in a constraint driven navigation scenario, in accordance with the presently disclosed subject matter.

DETAILED DESCRIPTION

In the drawings and descriptions set forth, identical reference numerals indicate those components that are common to different embodiments or configurations. Elements in the drawings are not necessarily drawn to scale.

Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "obtaining ", "generating", "determining", "calculating", 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.

UAV flight control unit 410 comprises or is otherwise operatively connected to at least one computerized device configured to execute operations as disclosed herein. The terms "computerized device", "computer", "processing unit" or variation thereof 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 (e.g. digital signal processor (DSP), a microcontroller, a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), any other electronic computing device, and or any combination thereof.

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" 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" 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 subcombination.

In embodiments of the presently disclosed subject matter, fewer, more and/or different stages than those shown in Figs. 1 and 6 may be executed. In embodiments of the presently disclosed subject matter one or more stages illustrated Figs. 1 and 6 may be executed in a different order and/or one or more groups of stages may be executed simultaneously. Fig. 4 illustrates a general schematic of the system architecture in accordance with an embodiment of the presently disclosed subject matter. Functional elements in Fig. 4 may 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 (e.g. distributed differently) functional elements than those shown in Fig. 4. While the following description refers in some places to a UAV, this is done by way of example only and should not be construed as limiting. The same principles are similarly applicable in a piloted aircraft configured with an auto-pilot flight mode.

A UAV (or a piloted aircraft operating in auto-pilot flight mode) proceeds to a desired destination without the intervention of human operator. During flight, the UAV is controlled by an onboard flight control unit which is a computerized device comprising one or more processing units operatively connected to a computer memory (including non-transitory computer memory) and configured to direct the UAV along a flight route defined by a series of waypoints (WPs). The onboard flight control unit is operatively connected to an aircraft positioning utility (e.g. GPS and/or INS systems) configured to determine the current position of the aircraft. Based on the current aircraft position and the position of the waypoints along the flight route, the onboard flight control unit controls the operations of various aerial control devices, such as stabilizers, ailerons and rudders, for directing the UAV from its current position to the next waypoint along its flight route.

In different flight scenarios the flight route of the UAV is restricted by various constraints. For example, no-flight zones (NFZ) are areas over which aircrafts are not permitted to fly. Similarly, restricted airspace is an area (volume) of airspace in which the air traffic is restricted. Such restrictions are commonly defined by authorities for military, security or safety reasons. For example, for national security reasons aircrafts are often allowed to cross the border between neighboring countries only after being officially authorized to do so. In some cases, where a border separates two belligerent countries, crossing of the border may be altogether prohibited. Likewise, some aircrafts (such as UAVs) may not be allowed to fly over populated areas for human safety reasons.

Other flight constrains are related to the topography and/or the land cover in urban areas. For example, flight routes can be restricted in the vicinity of tall mountains or buildings to avoid the risk of collision. Various additional types of flight constraints are described below in more detail.

Information pertaining to existing flight constraints may not be available at the time the flight route is being planned before takeoff. This can result for example, from lack of intelligence or from changes in real-time conditions which occur during flight and therefore are not foreseeable during flight route planning.

The presently disclosed subject matter includes a computerized method and an autonomous flight control unit which is configured to control an aircraft operating in an autonomous flight mode and generate instructions for directing the aircraft from one waypoint to the next along a flight route while taking into consideration flight constraints and avoiding the violation thereof. Data defining flight constraints is referred herein in general as "flight constrained data".

Fig. 1 is a flowchart illustrating operations carried out in accordance with an example of the presently disclosed subject matter. The operations described with reference to Fig. 1 can be executed, for example, by a flight control unit installed onboard an aircraft. An example of a flight control unit is described below with reference to Fig 4.

According to one example, a mission flight route can be fed to the flight control unit (block 101). As explained above, the flight route comprises a series of waypoints for directing the UAV to a desired destination. The flight route can be provided, for example, by a core UAV control system (CUCS) configured for monitoring and controlling the operation of a UAV or by some other system or device communicating with the UAV. Following takeoff, while operating in autonomous flight mode, the flight control unit navigates the UAV from one waypoint to the next along the mission flight route (block 103). Navigation of the UAV can be accomplished for example with the help of navigation module 421.

During flight, the flight control unit may receive flight constrained data indicative of one or more flight constraints (block 105). The flight control unit is configured, responsive to the received information, to determine whether there is a conflict between the UAV flight path and the one or more flight constraints. Determination as to whether or not there is a conflict between the flight path and flight constraints can be accomplished for example with the help of flight constraints analysis module 423.

If a conflict is indeed identified, flight control unit (e.g. with the help of flight constraint analysis module 423) can be further configured to adapt a flight path extending between at least two waypoints along the mission flight route in order to avoid violating the flight constraint.

According to the presently disclosed subject matter, this can be accomplished by adapting the bank angle of the UAV (increased or decreased) for obtaining a respective flight path directing the UAV from one waypoint to the next under the flight constraint. A fixed wing aircraft turns by banking the wings in the direction of the desired turn at a specific angle known as the bank angle. When the wings of an aircraft are banked, the force acting on the wing is divided into vertical and horizontal components. The horizontal component of the produced lift force turns the aircraft to the desired direction. By changing the bank angle between adjacent waypoints, the UAV can avoid the violation of various flight constraints while maintaining the original flight route (flight along the waypoints) and avoiding the need to calculate and/or use an alternative flight route.

Thus, according to examples of the presently disclosed subject matter, responsive to receiving flight constrained data indicative of a flight constraint, the UAV control unit is configured to limit the bank angle and/or to calculate a bank angle enabling the UAV to proceed from one waypoint to the next without violating the flight constraint (block 107). The flight control unit can then use the calculated bank angle for generating instructions to various aerial control devices in order to control the aircraft to turn in the desired bank angle and reach the next waypoint. Calculation of the bank angle is repeatedly performed (as long as the constraint is valid) in order to repeatedly determine a suitable bank angle according to the real-time conditions, which may be continuously changing. Furthermore, according to one example, the bank angle calculation is directed for maintaining the flight path to the next waypoint as short as possible.

By repeatedly calculating the bank angle it is possible to continuously adapt the bank angle (and respective flight path) to real-time changes in the flight constraint data. For example, assuming the flight constraints require maintaining a clear line of site between the communication antenna and a respective communication satellite, the calculated bank angle may change due to changes in the relative position of the UAV and the satellite. Thus, flight control unit can be configured to repeatedly use updated information regarding the location of the UAV and the location of the object of interest and calculate the bank angle accordingly.

According to some examples, the arc defining the flight path between two waypoints is predetermined, having a constant turning radius or a turning radius within a certain range. In such cases the bank angle is calculated to enable the UAV to fly along the predetermined arc (which can be a perfect arc). Using a predetermined arc enables an operator to determine the exact flight path of the aircraft while avoiding the violation of the constraint.

Fig. 2a is a schematic illustration of a flight route exemplifying some principles of the presently disclosed subject matter. Flight route FR is constructed as a series of waypoints (WP). Consider for example UAV 200 which is deployed on a traffic control or some other urban policing mission. Area 224 is a free flight area and area 222 (striped area) is a no-flight zone, where flight of the UAV is prohibited. Fig. 2a shows waypoint 2 (WP2) and waypoint 3 (WP3) along the flight route. Notably a substantially straight line L2 connecting WP2 and WP3 crosses within the no-flight zone. In order to avoid violating the no-flight zone restriction, UAV control unit is configured to determine a new path (indicated by L3) connecting WP2 and WP3 which circumvents - li

the no-flight zone. As explained above, to this end a desired bank angle is calculated, which enables the UAV to circumvent the no-flight zone.

Similarly, as exemplified in Fig. 2b, UAV 200 can be provided with instructions to reach a given destination (D) along with flight constraint data allowing traveling within a restricted area limited on both sides. Area 224 is a free flight area and area 222 (striped area) is a no-flight zone, where flight of the UAV is prohibited. The flight constraint data prohibits the UAV to fly anywhere but within the allowed area ("sleeve" 224). In order to avoid violating this flight restriction, a UAV control unit is configured to determine a path along the allowed flight area. To do so, a desired bank angle is repeatedly calculated during progress of the UAV along its flight route. Each calculation provides the UAV with a respective change of heading that enables the UAV to advance without deviating from the allowed area. The banking angle is calculated based on parameters including the position of the UAV, its next destination and the location of the boundaries of the allowed flight zone.

A different example of flight constraint is related to beyond line of site (BLOS) communication link with the UAV. A BLOS communication link is a satellite based communication link enabling to communicate with a UAV over greater distance than line of sight communication. In order to maintain BLOS communication continuously available it is imperative to maintain a clear line of site between the communication antenna and a respective communication satellite. However, as illustrated in Fig. 3a while turning, banking of the wings may position the UAV (e.g. the UAV fuselage) between the antenna, located for example on the dorsal side of fuselage, and the direction of the satellite, thus blocking line of sight between the UAV and the satellite.

In order to avoid the violation of a communication loss restriction (i.e. requiring maintaining an open BLOS communication link at all times) UAV control unit is configured to use information indicative of the satellite transmission angle and calculate a maximal bank angle which will not violate the communication loss restriction. To this end UAV control unit is configured to calculate the angles of observation from the UAV to the satellite relative to the aircraft horizontal plane using Euler angle calculations based on the aircraft Yaw, Pitch and Roll angles and based on the UAV satellite locations. Then, the flight control unit uses this information to calculate a maximal bank angle that would result in the fuselage blocking BLOS communication. The path from the current waypoint to the next is adapted according to the limitations of the maximal bank angle which was calculated.

Notably, as the desired bank angle depends also on the current position of the satellite relative to the position of the UAV, it is clear why this information may not be available during flight route planning. Furthermore, during flight from one waypoint to the next, the bank angle can be repeatedly calculated in order to obtain the best results for maneuvering the UAV and taking into considerations any real-time changes in the position of the satellite relative to the UAV while flying along the adapted path.

Likewise, flight constraint can be related to line of sight (LOS) communication link with the UAV (e.g. when using high frequency transmission). In order to maintain LOS communication continuously available it is imperative to maintain a clear line of site between the onboard communication antenna and a respective remote communication antenna. Accordingly, in order to avoid the violation of a communication loss restriction (requiring maintaining an open LOS communication link at all times) UAV control unit is configured to use information indicative of the remote antenna transmission angle and calculate a maximal bank angle which will not violate the communication loss restriction.

Another example of a flight constraint is related to the operation of an onboard sensing unit comprising, for example a sensing device such as a camera. In order to maintain the sensing device continuously operative it is imperative to keep an open line of sight between the sensing device and the object(s) of interest (e.g. in the direction of the ground). However, as illustrated in Fig. 3b, while turning, banking of the wings may position the UAV fuselage between the sensing device (located for example on the ventral side of fuselage) and the direction of the earth, thus blocking _{r L}

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line of sight between the sensing device and the target(s) which is/are sought to be captured. Moreover, steering of payloads LOS may be compromised when positioned along a perpendicular to the gimbal axes. When being in that area, lack of steering ability may cause the operator to lose track of the target. Consequently, it is imperative to avoid approaching the perpendicular to the gimbal axes area. Note that in the illustrated example, banking of the UAV directs the sensing device FOV away from the object of interest 310.

In order to avoid the violation of a sensing device LOS restriction (requiring maintaining an open LOS between the sensing device and the object of interest(s) at all times) UAV control unit is configured to use information indicative of the sensing device FOV, the UAV position and the object of interest position and calculate a maximal bank angle which will not violate the sensing device LOS restriction. Furthermore, in some examples the bank angle calculation is adapted to avoid bringing the LOS close to the perpendicular area (e.g. within a predefined distance from the perpendicular to the gimbal axes). The flight path from the current waypoint to the next is adapted to the limitations of the bank angle which was calculated.

Another example of flight constraint is maximal Radar Cross Section (RCS) restriction. This restriction is related to the maximal UAV surface area which is allowed to be exposed in the direction of a radar. In order to avoid detection, the UAV surface area which is allowed to be exposed in the direction of a radar is limited. While turning, banking of the wings may cause a greater surface area of the UAV to be exposed in the direction of a radar and thus increase the likelihood of detection.

In order to avoid the violation of a maximal RCS restriction, UAV control unit is configured to use information indicative of the position of a radar, the relative position of the UAV and calculate a maximal bank angle (or possibly an optimal bank angle) which will not violate the RCS restriction. This is done using the Euler angle calculations of the observed angles of the UAV by the radar relative to the aircraft horizontal plane and calculating the amount of RCS exposed (the maximal exposure is usually when looking in perpendicular to the aircraft horizontal plane). The flight path from the current waypoint to the next is adapted to the calculated bank angle calculated in order to maintain the UAV surface area which is exposed to the radar within the desired limits.

A further example of a flight constraint which was mentioned above, is topography and/or the land cover restrictions, where the UAV flight path is restricted by the topography or other object (e.g. buildings, trees) covering the land. In order to avoid the violation of this type of flight constraints, UAV control unit is configured to use information regarding the position of the UAV, the position and the topographical (and/or land cover) obstacle (e.g. position of a building) and the position of the current and next waypoints and calculate a bank angle which will not violate the restriction and put the UAV at a collision risk. The flight path from the current waypoint to the next is adapted to the limitations of the bank angle which was calculated.

Notably, the flight control unit can be configured to consider more than one constraint and provide a bank angle which does not violate any one of the constraints. For example, different bank angles can be calculated, each based on a respective constraint, and the one bank angle which does not violate any one of the constraints is the one which is used.

Fig. 4 is a functional block diagram schematically illustrating a UAV control unit, according to an example of the presently disclosed subject matter. While Fig. 4 refers to a UAV, this is done by way of example only, and the same principles and similar functional elements described with reference to Fig. 4 are similarly applicable in a piloted aircraft configured with an auto-pilot flight mode.

Flight control unit 410 is suitably mounted on UAV 400 and is operatively connected to various devices and subsystems onboard the aircraft. Flight control unit 410 is configured in general to autonomously control the UAV during flight and direct it to a desired destination. To this end control unit 410 is configured to receive flight instructions. As explained above, flight instructions can be provided in the form of a flight route consisting of a series of waypoints along with instructions to fly from one waypoint to the next in a certain order. Flight instructions can also be provided in the form of instructions directing the UAV to fly to a specific location or in a specific direction without specifying a detailed flight route.

Flight control unit can comprise communication unit 409 configured to enable communication with the UAV control system via line of site and/or beyond line of site communication links. Flight instructions as well as flight constraints related data can be sent to the UAV via communication module 409. Flight control unit 410 can also be configured to control or assist in other flight related operations which are not described here in detail (e.g. take-off, landing, emergency landing, shutting off, etc.). Flight control unit 410 may be fully automated, but in some implementations it may also react to commands issued by another system or a human operator.

Flight control unit 410 comprises one or more processing units. Each processing unit comprises or is operatively connected to one or more computer processors and computer memory (transitory and/or non-transitory). While Fig. 4 shows only a single processing unit (420) it is noted that this is so for the sake of simplicity and clarity only and should not be construed as limiting.

Processing unit 420 is connected to a number of input interfaces which provide information required for controlling the UAV. Input interface includes for example: airspeed input interface 401a configured to obtain data indicative of airspeed of the UAV from one or more air speed detectors 401 (e.g. implemented as Pitot tubes); Navigation system input interface 403a configured to obtain data indicative of UAV position and heading from a positioning utility such as GPS receiver 403 and INS (not shown); altitude input interface 405a configured to obtain data indicative of current altitude of the UAV from altimeter 405. Altimeter 405 can be implemented, for example, as a pressure altimeter, a sonic altimeter, a radar altimeter, a GPS based altimeter, and so forth. It is noted that control unit 410 can be further configured to obtain additional information (including situation data of all kinds) indicative of flight and state of UAV 400. It is noted that the list above is given by way of non-limiting example only and flight control unit 410 can be operatively connected to additional or different types of input interfaces and/or various devices to those specified above.

Processing unit 420 can comprise for example the following modules:

• Navigation module 421 configured to control the UVA flight in order to direct the UAV to a desired destination and maintain its course along a desired flight route. The flight route can be a predetermined path to a certain destination (e.g. pre- stored in data-repository 422 before takeoff) or can be a flight route provided in realtime after takeoff. Navigation module 421 is configured to determine the desired flight parameters of the UAV for reaching the desired destination. The flight parameters include for example, UAV heading and altitude.

• Flight constraint analysis module 423 configured to analyze information pertaining to a flight constraint (flight constraints related data) and determine whether the current flight path conflicts with relevant flight constrains. Flight constraint analysis module 423 can be configured to calculate the future position and/or banking angle of the aircraft according to its current flight path and determine whether a conflict is predicted to occur with any known flight constrains.

In the event that a conflict is found, flight constrains analysis module is configured to determine a flight path which does not violate such restrictions. Flight constraint analysis module 423 can make use for example of a bank angle calculation module 425 configured for calculating a bank angle which allows proceeding from one waypoint to the next without violating a given flight constraint.

For example, given a no-flight zone flight constraint analysis module 423 is configured to use information pertaining to the position of the UAV, the position of the next destination relative to the no-flight zone and to determine a flight path directed to the next destination (e.g. the UAV's final destination or a subsequent waypoint along a flight route) which circumvents the no-flight zone. A bank angle which allows the UAV to maintain the determined flight path is calculated. The bank angle can be used by navigation module 421 for controlling various aerial control devices and directing the UAV to fly along the flight path.

In a different example related to a communication loss restriction, flight constraint analysis module 423 is configured to calculate a maximal bank angle which would not position the UAV fuselage between the receiving antenna and the direction of the satellite thus avoiding blocking BLOS communication between the UAV and the satellite. Given the calculated bank angle, navigation module is configured to generate instructions to various aerial control devices for directing the UAV to its next destination while avoiding banking at an angle which is greater than the calculated maximal bank angle.

As mentioned above, during flight constraint analysis, module 423 is configured to repeatedly calculate the bank angle limits in order to avoid violating the respective restriction on one hand and minimizing degradation of flight performance on the other hand in view of real-time changes which occur during flight to the next destination. According to some examples, in each calculation the most updated available data with respect to the constraints is used in order to maintain optimal compliance with the flight restrictions. Thus, flight constraint analysis module can be configured to be continuously fed with information pertaining to real-time changes in the flight constrained data (e.g. changes in the location of a satellite relative to the UAV, location of tracked object of interest relative to the UAV and its onboard tracking device, location of a radar relative to the UAV, etc.) allowing bank angle calculation module 425 to use this information in its calculation.

As explained above according to one example, the arc defining the flight path between two waypoints is predetermined, having a constant turning radius or a turning radius within a certain range. In such cases bank angle calculation module 425 is configured to calculate the bank angle along the predefined arc while avoiding any violation of the constraints. Information indicative of the predetermined arc can be preprogrammed (e.g. stored in data repository 422) or alternatively it can be received (e.g. via communication unit 409) from an external source (e.g. from a human operator of the control unit).

As is well known in the art, calculation of the bank angle and respective flight radius is based on various parameters including: lift acting on the aircraft (L); the angle of bank of the aircraft (Θ); the mass of the aircraft (m); and the ground speed of the aircraft which is composed of the aircraft true airspeed and the wind speed (v). In straight level flight, lift (L) is equal to the aircraft weight. In turning flight the lift (L) exceeds the aircraft weight, and is equal to the weight of the aircraft (mg - g is the gravitational field strength) divided by the cosine of the angle of bank. Thus, given a certain bank angle, the respective flight radius can be determined and vice versa.

In order to allow the UAV to turn in an efficient and fully-coordinated manner, it is necessary to have various aerial control devices available. For example, ailerons (rectangular flaps at the back of the wing) are needed to bank and hence initiate the turn, elevator is needed to maintain UAV altitude during the turn, rudder is needed to coordinate the movement of the nose, and the throttle is needed to increase/decrease thrust while turning, thereby affecting the radius of the turn.

• Elevators control module 427 is configured to control the elevators 445 located on the horizontal tail wing. Elevators enable the plane to go up and down through the air. The elevators change the horizontal stabilizer's angle of attack, and the resulting lift either raises the rear of the aircraft (pointing the nose down) or lowers it (pointing the nose skyward).

• Ailerons control module 427 is configured to control the ailerons 441 which are horizontal flaps located near the end of an airplane's wings. Ailerons allow one wing to generate more lift than the other, resulting in a rolling motion that allows the plane to bank left or right. • Rudder control module 429 is configured to control a rudder 443 which is a flap located on the vertical tail wing. The rudder enables the plane to turn left or right.

• Throttle control module 433 is configured to control the throttle 449 which is configured in turn to increase/decrease thrust while turning.

According to an example of the presently disclosed subject matter, based on the bank angle and flight path which are determined by the navigation module 421 instructions are generated by each one of the above modules for controlling a respective control device to obtain the desired UAV maneuver.

Autonomously operating aircrafts and specifically UAVs are sometimes utilized as an airborne system for surveillance for remote observation and tracking of objects. As mentioned above with reference to Fig. 3, to this end UAVs may be equipped with some type of data sensing unit (comprising a sensing device such as a camera, radar, sonar, etc.). The data sensing unit is used for surveying a scene and generating sensing-data, which includes data that was acquired by the sensing device or data generated by the sensing unit in relation to the acquired data (e.g. images of a scene, object-data characterizing identified objects within the images, etc.). The generated data can be transmitted, over a communication link to a control unit where the sensing-data can be displayed on a display device to be viewed by an operator. The sensing unit can be further operable to lock on and track an object located in the surveyed scene. The control unit enables to provide to the sensing unit control-data, including for example, different types of commands, such as lock and track command, zoom-in command, centering command, etc.

In this type of scenario the aircraft does not necessarily fly along a flight route defined by a sequence of predefined waypoints. Rather, an aircraft such as a UAV, flying in observation mode, surveys the area below and provides sensing-data of the surveyed area. Likewise, a UAV in tracking mode does not fly according to a predefined fly route, but rather strives to maintain a certain object of interest (i.e. the object being tracked, also referred to herein as "target") within the FOV of the sensing device, preferably in its center. In these types of flight modes UAV flight control unit 410 can be configured to assume one of a number of possible flight patterns accommodated for observing an area or tracking a target.

In one example of a type of a flight pattern (referred to herein as "spiral flight pattern") UAV flight control unit directs the UAV to advance in spiraling circles. In this type of flight pattern, an area of interest or target can be maintained substantially at the central area of the circles. This flight pattern is schematically illustrated in fig. 5a showing in top view a UAV tracking a moving vehicle along a road in spiral flight maneuvers, where the UAV is located substantially above the moving object and the object is maintained substantially in the center of the spiraling circles. Spiral flight pattern can be likewise executed in observation mode.

In another example of a type of a flight pattern (referred to herein as "circular flight pattern") in observation mode, UAV flight control unit can be configured to direct the UAV to circle a certain area and thus maintain the view on the same surveyed area. This flight pattern is schematically illustrated in fig. 5b showing a UAV in top view tracking a moving vehicle along a road in circular flight maneuvers 503. A variation to circular flight pattern is referred to herein as "circular-offset pattern" where the circle is located in an offset to the area of interest.

When flying according to certain flight pattern, the flight route of the aircraft is defined by the flight pattern which is being used. Thus, given that the UAV is located at a certain location, its next destination depends on the flight pattern in which it flies. In some cases, the flight route of the UAV which is prescribed by the flight pattern cannot be maintained without violating a certain flight constraint. According to the presently disclosed subject matter, while in observation mode or tracking mode, UAV flight control unit 410 can be configured, to consider information related to one or more flight restrictions and adapt the flight path of the UAV in order to avoid violation of the restrictions. Once the flight restriction is no more relevant, flight control unit can direct the UAV to continue and fly according to the flight pattern.

Fig. 6 is a flowchart showing an example of a sequence of operations which are performed in a constraint driven navigation scenario while UAV is flying in observation mode or tracking mode, in accordance with the presently disclosed subject matter. The operations described with reference to Fig. 6 can be executed, for example, by flight control unit installed onboard an aircraft, described above with reference to fig. 4.

At block 601 an aircraft is provided with instructions to fly in a certain flight pattern. In some instances, flight pattern can be automatically selected by UAV flight control unit, e.g. based on the mission, which is being executed. In other instances, flight pattern may be selected by an operator, who can send specific command related data to the UAV instructing to assume a certain flight pattern. While operating in autonomous flight mode, flight control unit maneuvers the UAV according to the selected flight pattern (block 603).

During flight, the flight control unit may receive flight constrained data indicative of one or more flight constraints (block 605). The flight control unit is configured, responsive to the received information, to determine whether there is a conflict between the current flight pattern assumed by the UAV and the received information indicating one or more flight constraints. Determination as to whether or not there is a conflict between the flight pattern and relevant flight constraints can be accomplished for example with the help of flight constraints analysis module 423 as explained above.

If a conflict is indeed identified, flight control unit (e.g. with the help of flight constraint analysis module 423) can be further configured to adapt the flight in order to avoid violating the flight constraint. According to the presently disclosed subject matter, this can be accomplished by adapting the bank angle of the UAV for obtaining a respective flight path which does not violate the flight constraint, while continuing with the desired mission.

Thus, according to examples of the presently disclosed subject matter, responsive to receiving flight constrained data indicative of a flight constraint, the UAV control unit is configured to limit the bank angle and/or to calculate a bank angle enabling the UAV to proceed with the mission (block 607). The flight control unit can then use the calculated bank angle for generating instructions to various aerial control devices in order to control the aircraft to turn in the desired bank angle.

According to some examples, the bank angle is calculated while maintaining the flight path to the desired destination as short as possible without violating the flight constraints.

Calculation of the bank angle is repeatedly performed (as long as the constraint is valid) in order to repeatedly determine a suitable bank angle according to the realtime conditions, which may be continuously changing.

As explained above, by repeatedly calculating the bank angle it is possible to continuously adapt the bank angle (and respective flight path) to real-time changes in the flight constraint data.

For example, in an object tracking mission, assuming the flight constraints require to maintain a clear line of sight between an onboard camera and an object of interest which is being tracked, the calculated bank angle may change (be increased or decreased) due to changes in the relative position of the UAV and the object of interest (e.g. in case the object is moving). Thus, flight control unit can be configured to repeatedly use updated information regarding the location of the UAV and the location of the object of interest and calculate the bank angle accordingly. In the example illustrated in Fig. 5a the turning radius of the spirals can be repeatedly adapted according to the calculated bank angle in order to avoid violation of one or more flight constraints. ln another example, when flying in observation mode in a circular flight pattern, in case the UAV encounters some type of obstacle, flight control unit can be configured to autonomously change the bank angle in order to detour the obstacle. Once the obstacle is passed, the UAV can return to fly according to the flight pattern. This scenario is exemplified in Fig. 5b schematically illustrating UAV 505 flying in a circular flight pattern, which encounters a flight constraint in the form of a structure 501 blocking its flight path. Flight control unit calculates a bank angle which allows the UAV to follow a detour flight path 507, generates flight instructions for circumventing the structure, and then directs the UAV to fly according to its original flight pattern 503. As further mentioned above, according to some examples, the arc defining the flight path between two waypoints is predetermined, having a constant turning radius or a turning radius within a certain range. Likewise, the flight path generated responsive to a flight constraint in one of the flight modes mentioned above (e.g. while surveying an area or tracking a target) in a predefined arc, can be used. For example, flight path 507 may have a predefined turning radius.

In such cases the bank angle is calculated to enable the UAV to fly along the predetermined arc (which can be a perfect arc). Using a predetermined arc enables an operator to determine the exact flight path of the aircraft while avoiding the violation of the constraint.

It is to 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 non-transitory 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 (transitory and non-transitory) tangibly embodying a program of instructions executable by the machine for executing the method of the presently disclosed subject matter. It is also 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.