Recovery System for UAV

Abstract

An unmanned airborne vehicle (UAV) recovery system for in-flight recovery of a fixed wing UAV includes a catching device and a plurality of hover-capable drones, such as multi-rotors. The catching device is for catching the fixed wing UAV via a hook system during flight of the fixed wing UAV and takes the form of a line between the recovery drones. The recovery drones are arranged to support the catching device as it spans a gap in a horizontal orientation with the catching device suspended between at least two recovery drones that are in flight and spaced apart horizontally from each other either side of the gap. During a recovery operation the recovery drones co-ordinate their movement to adopt a flight path relative to a flight path of the fixed wing UAV to define a virtual runway for interception of the fixed wing UAV by the recovery system.

Claims

1. An unmanned airborne vehicle (UAV) recovery system for in-flight recovery of a fixed wing UAV, the recovery system comprising: a catching device comprising a line for catching the fixed wing UAV during flight of the fixed wing UAV via a hook system; and a plurality of hover-capable recovery drones; wherein the recovery drones are arranged to support the line as it spans a gap in a horizontal orientation with the line suspended between at least two recovery drones that are in flight and spaced apart horizontally from each other either side of the gap; and wherein the recovery drones are arranged to co-ordinate their movement to adopt a flight path relative to a flight path of the fixed wing UAV to define a virtual runway for interception of the fixed wing UAV by the recovery system.

2. A UAV recovery system as claimed in claim 1, wherein the line of the catching device is a flexible line arranged to hang suspended between the at least two recovery drones.

3. A UAV recovery system as claimed in claim 1, wherein the line has a length of 5-100 m and is suspended across a gap of 10-40 m.

4. A UAV recovery system as claimed in claim 1, including the hook system for attachment to the fixed wing UAV, wherein the hook system includes a hook and a hook line holding the hook.

5. A UAV recovery system as claimed in claim 4, wherein the hook system is provided with a housing for holding the hook line in a stowed arrangement during normal flight.

6. A UAV recovery system as claimed in claim 4, wherein the hook system is arranged to deploy the hook when the fixed wing UAV is within a certain distance of the recovery drones and/or when the fixed wing UAV is within a certain flight time from interception with the catching device at the virtual runway.

7. A UAV recovery system as claimed in claim 4, wherein the hook and line are deployed using gravity with the weight of the hook pulling the hook line from its stowed arrangement to a deployed arrangement where the hook and the hook line hang beneath the fixed wing UAV.

8. A UAV recovery system as claimed in claim 4, wherein the hook or the hook line is provided with aerodynamic features for using the airspeed of the UAV to generate a downward force to aid deployment of the hook.

9. A UAV recovery system as claimed in claim 4, wherein the recovery system or the hook system is arranged to provide a signal to the fixed wing UAV to trigger a shut down of the fixed wing UAV's propulsion system when the hook engages with the catching device or when the UAV or the hook is within a certain distance of the catching device.

10. A UAV recovery system as claimed in claim 1, wherein the recovery drones are arranged to co-ordinate their movement to fly in the same direction as the fixed wing UAV so that the relative velocity of the recovery system and the fixed wing UAV is reduced.

11. A UAV recovery system as claimed in claim 1, wherein the recovery drones provide a virtual runway that travels in the same direction as the fixed wing UAV at a lower speed than the fixed wing UAV.

12. A UAV recovery system as claimed in claim 1, wherein the recovery system is arranged to use control of the recovery drones to damp and/or absorb impact forces and the recovery drones are arranged to react to impact of the fixed wing UAV at the catching device by applying lift to counteract the impact forces by control of each recovery drone such that in reaction to motion and/or forces arising due to impact of the fixed wing UAV then the recovery drones apply additional lift to maintain, restore or modify the flight path of the recovery drones.

13. A UAV recovery system as claimed in claim 1, wherein the recovery system is arranged to use control of the recovery drones to damp and/or absorb impact forces and the recovery drones are arranged to detect motion or forces induced by impact of the fixed wing UAV via sensors on the recovery drone, and to use input from the sensors as well as control inputs from feedback systems on the recovery drone, such as gyroscopic systems, to react to impact of the fixed wing UAV at the catching device by applying lift to counteract the impact forces in order to maintain stable flight of the recovery drones as well as to maintain, restore or modify the flight path.

14. A UAV recovery system as claimed in claim 1, wherein the recovery system is arranged to use control of the recovery drones to damp and/or absorb impact forces, the recovery system includes one or more sensors for sensing the magnitude and/or direction of tension forces applied to the drone by the catching device to allow for detection of the impact forces, and the recovery drones are arranged to react to impact of the fixed wing UAV at the catching device by applying lift to counteract the impact forces in order to maintain stable flight of the recovery drones as well as to maintain, restore or modify the flight path taking account of the direction and/or magnitude of the tension applied to the recovery drone by the catching device.

15. A UAV recovery system as claimed in claim 12, wherein the recovery drone flight path is modified in order to absorb impact forces by permitting the recovery drones to move into the horizontal gap that was spanned by the catching device prior to impact of the fixed wing UAV.

16. A UAV recovery system as claimed in claim 12, wherein the recovery drone flight path is modified in order to absorb impact forces by acceleration or deceleration of the flight speed along the flight path of the virtual runway.

17. A UAV recovery system as claimed in claim 1, wherein the recovery system includes shock absorbing features at the catching device comprising: a shock absorbing line, such as a line that extends under load and absorbs or dissipates forces during extension; and/or shock absorbers attached to the catching device within the load path and using damping or plastic deformation to absorb or dissipate forces.

18. A method for recovery of a fixed wing unmanned airborne vehicle (UAV) during flight, the method comprising: using a plurality of hover-capable recovery drones, supporting a catching device as it spans a gap in a horizontal orientation with the catching device suspended between at least two recovery drones that are in flight and spaced apart horizontally from each other either side of the gap, wherein the catching device is a line for catching the fixed wing UAV during flight of the fixed wing UAV using a hook system attached to the UAV; co-ordinating movement of the recovery drones in order to that they adopt a flight path relative to a flight path of the fixed wing UAV; and thereby defining a virtual runway for interception of the fixed wing UAV by the recovery system.

19. A method as claimed in claim 18, comprising using a recovery system as claimed in claim 1.

20. A method as claimed in claim 18, wherein the hook system includes a hook with a hook line attached, wherein the hook line has a length allowing for the fixed wing UAV to fly above the recovery drones whilst the hook hangs below the level of the line supported between the drones and the method includes catching the fixed wing UAV by hooking the hook onto the catching device line so that the fixed wing UAV is then attached to the catching device, and hence to the recovery drones, by the hook line.

21. A method as claimed in claim 20, comprising using the hook system to deploy the hook when the fixed wing UAV or the hook is within a certain distance of the recovery drones and/or is within a certain flight time from interception with the catching device at the virtual runway.

22. A method as claimed in claim 18, wherein each of the recovery drones is flown in co-ordination both with the other recovery drone(s) and with the flight path of the fixed wing UAV to be recovered and the method includes controlling the fixed wing UAV in a way that is blind to the presence of the recovery system, with the recovery system matching its movements with the fixed wing UAV flight path.

23. A method as claimed in claim 18, wherein the movement of the recovery drones is in the same direction as the fixed wing UAV so that the relative velocity of the recovery system and the fixed wing UAV is reduced.

24. A method as claimed in claim 18, wherein flight path of the UAV and hence the flight path of the virtual runway is aligned with weather conditions so that the fixed wing UAV is flying into the wind when it lands.

25. A method as claimed in claim 18, comprising absorbing impact forces from the catching of the fixed wing UAV via shock absorbing features of the catching device and/or via control of the recovery drones to damp and/or absorb forces from the impact.

26. A method as claimed in claim 25, comprising using control of the recovery drones to damp and/or absorb forces from the impact by controlling the recovery drones to react to impact of the fixed wing UAV at the catching device by applying lift to counteract the impact forces such that in reaction to motion and/or forces arising due to impact of the fixed wing UAV then the recovery drone applies additional lift to maintain, restore or modify the flight path of the recovery drone.

27. A method as claimed in claim 26 including detecting motion or forces induced by impact of the fixed wing UAV by using sensors on the recovery drone, and then using input from the sensors as well as control inputs from feedback systems on the recovery drone, such as gyroscopic systems, in order to maintain stable flight of the recovery drone as well as to maintain, restore or modify the flight path.

28. A method as claimed in claim 26, wherein the system includes one or more sensors for sensing the magnitude and/or direction of tension forces applied to the drone by the catching device and the method includes detecting a change in magnitude and/or direction in order to detect the impact forces, and then controlling the recovery drone to react to impact of the fixed wing UAV at the catching device by applying lift to counteract the impact forces taking account of the direction and/or magnitude of the tension applied to the recovery drone by the catching device.

29. A method as claimed in claim 26, wherein the recovery drone flight path is modified in order to absorb impact forces by permitting the recovery drone to move into the horizontal gap that was spanned by the catching device prior to impact of the fixed wing UAV and/or by accelerating or decelerating the co-ordinated movement of the recovery drones along the flight path of the virtual runway.

30. A method as claimed in claim 18, comprising catching the fixed wing UAV, absorbing impact forces from impact of the fixed wing UAV, and then carrying the fixed wing UAV as a suspended load.

31. An unmanned airborne vehicle (UAV) recovery system for in-flight recovery of a fixed wing UAV, the recovery system comprising: a catching device for catching the fixed wing UAV during flight of the fixed wing UAV; and a plurality of hover-capable recovery drones; wherein the recovery drones are arranged to support the catching device as it spans a gap in a horizontal orientation with the catching device suspended between at least two recovery drones that are in flight and spaced apart horizontally from each other either side of the gap; and wherein the recovery drones are arranged to co-ordinate their movement to adopt a flight path relative to a flight path of the fixed wing UAV to define a virtual runway for interception of the fixed wing UAV by the recovery system.

32. A UAV recovery system as claimed in claim 31, wherein the catching device is a line supported across a horizontal span by drones at either end of the line, or a net that is hung from the recovery drones and/or suspended between the recovery drones.

33. A UAV recovery system as claimed in claim 31, wherein the catching device is a flexible line arranged to hang suspended between the recovery drones.

34. A method for recovery of a fixed wing unmanned airborne vehicle (UAV) during flight, the method comprising: using a plurality of hover-capable recovery drones, supporting a catching device as it spans a gap in a horizontal orientation with the catching device suspended between at least two recovery drones that are in flight and spaced apart horizontally from each other either side of the gap, wherein the catching device is for catching the fixed wing UAV during flight of the fixed wing UAV; co-ordinating movement of the recovery drones in order to that they adopt a flight path relative to a flight path of the fixed wing UAV; and thereby defining a virtual runway for interception of the fixed wing UAV by the recovery system.

35. A method as claimed in claim 34, comprising use of a UAV recovery system as defined in claim 1 and/or comprising method steps as defined in claim 18.

36. A computer programme product comprising instructions that, when executed on a control network for controlling hover-capable recovery drones, will configure them to operate in accordance with the method of claim 18, including supporting a catching device and co-ordinating the drones' flight to define a virtual runway.

Description

[0045] Certain preferred embodiments of the current invention will now be described by way of example only and with reference to the accompanying drawings in which:

[0046] FIGS. 1a and 1b show flight paths for recovery drones defining a virtual runway for recovery of a fixed wing UAV;

[0047] FIG. 2 illustrates multi-rotors holding a catching device in the form of a line for catching a fixed wing UAV; and

[0048] FIG. 3 illustrates multi-rotors holding a catching device in the form of a net.

[0049] As discussed above, it is beneficial to be able to recover a fixed wing UAV during flight without the need for the recovery system to be linked to any particular location such as a ship or a ground anchor. In UAV recovery systems 10 proposed herein, as shown in the Figures, a horizontally arranged catching device 12 is suspended between two multi-rotor drones 14 and this is used to intercept a fixed wing UAV 16 during flight. The catching device takes the form of a line 12. In order to use the catching device 12 to intercept the fixed wing UAV 16 the recovery drones 14 are flown in a co-ordinated manner to define a virtual runway 20.

[0050] Key benefits of the proposed approach include: [0051] Operational flexibility: When recovering a fixed wing UAV 16, it is crucial to travel against the wind to minimize the ground speed, and thus the catching device 12 needs to be aligned with this path. Even in marine vessels equipped with Dynamic Positioning (DP) systems, turning the ship can be undesired as it may interfere with operations. The multirotors 14 can however quickly react to changing wind conditions, and align the catching device 12 against the wind without interfering with other ship operations. [0052] Not affected by waves and turbulence in marine operations: Since the catching device 12 is suspended free from the ship, heave motion induced by waves on the ship will not affect the landing. Also, there is no impact from turbulence caused by the ship super-structure. [0053] Not affected by ground conditions during inland or littoral operations: Since the catching device 12 is airborne and the recovery drones can take off and land without a runway then the recovery of the fixed wing UAV 16 can be performed in any location irrespective of ground conditions. [0054] Safety: By having the catching device 12 suspended by two multirotor UAVs 14, the recovery operation can be used away from the operators. Thus, no operators or staff risk coming in contact with the incoming fixed wing UAV 16. [0055] Smaller impact force: By having the two multirotors 14 move against the wind with the fixed wing UAV 16, the relative speed difference between it and the catching device 12 can be made smaller, thus decreasing the structural load on the fixed wing body during impact. [0056] Smaller footprint: By moving the landing operation off ship, operations with UAVs can be conducted from smaller ships, not needing a large open deck with a catching device 12 to support the mission. Land-based operations may only require small vehicles without any specialised adaptations and in some cases recovery via a land-bound operator could be done with manual handling only, i.e. with the multirotors 14 and catching device 12 carried by hand and the recovery system as well as the fixed wing UAV 16 being retrieved by hand as well.

[0057] Autonomous recovery of a fixed wing UAV 16 in a suspended catching device 12 is a complex task, so the functionality is split into several key components. The overall mission is executed in the following fashion: [0058] The fixed wing UAV 16 is instructed to follow a path against the wind, with the minimal airspeed required for safe flying. This path can be used to define a virtual runway 20, where the multirotor 14 recovery drones are arranged to fly in such a way as to set a virtual runway 20 that will intercept the flight path of the fixed wing UAV 16. [0059] Both multirotors 14 are equipped with coordinated controllers that keep the inter-formation of the two intact, while lifting the suspended catching device 12. [0060] The current position and the velocity of the fixed wing UAV 16 is transmitted to a coordination controller in one of the multirotors 14, which sends desired setpoints to the formation controllers according to the phases of the mission, in order to catch the fixed wing UAV 16.

[0061] FIGS. 1a and 1b show two possible flight paths for the recovery drones 14 to define a virtual runway 20, where the catching device 12 can be used to intercept the fixed wing UAV 16. Precise navigation is crucial for precision landing of UAVs. Navigation for both the recovery drones and the fixed wing UAV 16 can utilize Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS). This is a navigation technique using the carrier wave of the incoming signals from the satellites, and comparing the signals to that received by a base station. By computing the phase shift between the signals at the fixed wing UAV 16 or the multirotor 14 and the base, the location can be locked in at centimetre-level accuracy. Note that the fixed wing UAV 16 acts as a reference generator (master) in the proposed control scheme, as it is not affected by the current position of the recovery drone multirotors 14. Depending on the type of fixed wing UAV 16 used, it is preferred to keep a steady flight envelope, rather than correcting for minor deviations from the catching device 12 position. This is much better handled by the agility of the multirotors 14. Thus, in effect the virtual runway 20 can adjust to adapt to deviations in the fixed wing UAV 16 flight path, rather than needing to continually adjust the fixed wing UAV 16 flight path for accurate alignment with a traditional runway or with a catching device 12 fixed to the ground or to a ship, as in the prior art.

[0062] The fixed wing UAV 16 moves with a constant course and altitude and its position and velocity is communicated to the other vehicles. This is controlled by an on-board autopilot.

[0063] In the next sections, let p.sub.i.sup.n , i{1,2} be the position of multirotor 14 i in the inertial frame n. Further, we define the position p.sup.n as the centroid of the two multirotors 14 plus a height offset to compensate for the position of the net. Further, the states of the fixed wing UAV 16 is denoted with subscript f.

[0064] The virtual runway 20 defines a path frame {p} at constant altitude, which is defined by an origin p.sup.n and a rotation around the {n} z-axis such that R.sup.n=R.sub.z (). Then a position p.sup.n can be decomposed in {p} by the transformation p.sup.p=(R.sup.n).sup.T(p.sup.np.sup.n). By dividing the path frame into a cross-track plane and an along-track distance, we can design controllers for each part separately.

[0065] A supervisor of the control system for the recovery drones 14 monitors the position and velocity of the fixed wing UAV 16 relative to the virtual runway 20 in order to switch between the different modes in the manoeuvre. Each mode enables a certain controller and reference which gives a desired velocity setpoint. In addition, the supervisor monitors the manoeuvre as it is progressing. If, because of wind or other factors, the fixed wing UAV 16 misses the catching device 12 then it instructs the vehicles to try the manoeuvre again. Further, if the projected proximity of the fixed wing UAV 16 and multirotors 14 are too small, the supervisor can abort the operation. Depending on the situation, an abort can involve the multirotors 14 climbing and repositioning for a retry, or releasing the net and abort the mission entirely.

[0066] For control purposes, the virtual runway 20 is divided into a cross-track plane and an along-track distance. The position of the catching device 12 is controlled according to the fixed wing UAV 16 position in the cross-track plane along the virtual runway 20. A cross-track frame {p*} is defined as the yz-plane in the path frame {p}, such that the position in the crosstrack plane can be extracted from the position in the path frame.

[0067] To control the velocity setpoint in the crosstrack plane for the multirotors, a modified pure-pursuit scheme is introduced. Given a desired position p.sub.d and the position error p*:=p.sub.dp* the following controller is used

v.sup.p*=K.sub.p,p{tilde over (p)}.sup.p*+K.sub.d,p{tilde over (p)}.sup.p*+K.sub.i,p{dot over ({tilde over (p)})}.sup.p*

[0068] where K.sub.j,pR.sup.22 for j{p, i, d}. The desired position p.sub.d=p.sub.f,2:3 is defined as the current position of the fixed wing UAV 16 projected along the virtual runway to the cross-track plane.

[0069] It should be noted that the position of the catching device 12 is not measured explicitly, and furthermore it is not a desirable control target as the catching device 12 may swing during the transit. Therefore we seek to control a position p.sup. defined in the cross-track plane. Hence, p.sup.p*={tilde over (p)}2:3.

[0070] The relative velocity between the net and the fixed-wing UAV is reduced by accelerating the net to a desired velocity. In order to control the point of impact an, open loop scheme is proposed.

[0071] For the final recovery phase, the along-track velocity of the fixed-wing UAV 16 is assumed constant. The virtual runway 20 defines a point r.sub.c along the runway as the designated recovery point. While waiting at the start of the virtual runway 20, the multirotor UAVs 14 should monitor the location of the fixed-wing UAV 16. Based on a operator-defined relative speed to be achieved by the multirotors 14 at the point of recovery r.sub.c, the multirotor UAVs 14 will start a pre-defined velocity profile along the virtual runway, to intercept the incoming fixed-wing UAV 16 at r.sub.c. By knowing the type of velocity profile used, the distance to the fixed-wing UAV 16, r.sub.0, can be calculated based on the desired relative speed and along-track velocity of the fixed-wing UAV 16.

[0072] A dynamical model of a multirotor 14 can be derived by Newtonian or Lagrangian methods as is known in this field. By further assuming the presence of an internal attitude controller, the relevant dynamics for the control design is extracted.

[0073] Let the dynamics of multirotor i be modelled by

{dot over (p)}.sub.i=v.sub.i

m.sub.i{dot over (v)}.sub.i=m.sub.ig+R.sub.if.sub.i

{dot over (R)}.sub.i=R.sub.iS(.sub.i)

I.sub.i{dot over ()}.sub.i=S(Iw.sub.i).sub.i+M.sub.i

[0074] where p.sub.i is the UAV position in the inertial frame {n}, v.sub.i the translational velocity in n, R.sub.i a rotation matrix from the body-fixed frame b.sub.i to the inertial frame n, the angular velocity of the UAV, represented in b.sub.i. Further, the operator S( ) is the skew-symmetric transformation, such that p q=S(p)q. m.sub.i is the mass of the multirotor 14, and I.sub.i the body-fixed inertia matrix. f.sub.i is upwards thrust directed along the negative body-aligned z-axis, M.sub.i are applied moment about the multirotor 14 centre of gravity, and g=[0 0 g].sup.T where g is the gravitational constant. Consider now the net being suspended in the centre of gravity of the UAV. This will affect the translational motion by a force.sub.L,i, given by the load dynamics, but the rotational motion is unaffected. As control of the attitude of the multirotor 14 is not considered, the model considering the translational motion is now

m.sub.i{dot over (v)}.sub.i=m.sub.ig+R.sub.if.sub.i+.sub.L,i

[0075] Further, assume now that a sufficiently fast attitude controller is present. The direction of the applied force for translational motion (5) is given by R.sub.i, and by manipulating the roll and pitch of the UAV we can apply force in a desired direction. Thus, the term R.sub.if.sub.i can be replaced by an inertial control force F.sub.i, resulting in the linear dynamics

m.sub.i{dot over (v)}.sub.i=m.sub.ig+F.sub.i+.sub.L,i

[0076] The formation controller can be designed in two steps using a passivity-based approach, where an inner loop controller takes a velocity setpoint from an outer controller, and the stability of the cascaded structure is proved by passivity theory. While the inner controller uses only its own measurements, the outer uses available information from the other multirotor 14 to reach the desired formation.

[0077] First, let

F.sub.i=.sub.L,i+m.sub.igK.sub.i(v.sub.iv.sub.d)+m.sub.i{dot over (v)}.sub.d+u.sub.i

[0078] where v.sub.d is the desired common velocity from the coordination controller, known to both vehicles. u.sub.i R.sup.3 is the input from the outer loop formation controller, which acts as an injection to achieve a desired formation, to be specified later. Note that we assume we can measure the disturbance force .sub.L,i of the suspended catching device 12, so it can be compensated using feed-forward by the controller.

[0079] Next, let z=p.sub.1p.sub.2 be the vector between the two multirotors 14 in {n}.

[0080] A standard linear consensus protocol can now be applied as

u.sub.i=d.sub.iK.sub.i(zz.sub.d)

[0081] where d.sub.1=1, d.sub.2=1, where z.sub.d is the desired link vector.

[0082] To provide a thorough understanding of the dynamical motion of the of the combined multirotor 14-catching device 12 system during the recovery manoeuvre a simulator can be used that includes the full 6-DOF dynamics of the multirotors 14, fixed wing UAV 16, and the catching device 12 suspended under the multirotors 14. This can be used to model the impact forces during collision, with consequential adaptations being made in the control of the multirotors 14.

[0083] By having the catching device 12 attached to the multirotors 14, we have in effect a system of constrained motion where each wire connecting the catching device 12 removes one degree of freedom. For simplicity, the catching device 12 is considered as a rigid body. To model this behaviour, one can reduce the state-space and use only generalized coordinates that cover the configuration space. This, however, will hide the forces acting on the wires during impact. Instead, one chose to model the interconnected system with constrained coordinates. The Udwadia-Kalaba equation, can be used to explicitly calculate the forces of constraints acting on each body.

[0084] To consider the dynamics during the impact, when the fixed wing UAV 16 gets arrested by the suspended catching device 12, the collision can be assumed to be perfectly inelastic such that the bodies will stick together after the collision. In order to calculate the forces and moments on the suspended catching device 12, conservation of momentum can be applied.

[0085] A numerical simulation was carried out using the controllers and models presented in the previous sections. We consider two multirotors 14, with a mass of m1,2=6 kg, recovering an incoming fixed wing UAV 16 at mf=3 kg. The fixed wing UAV 16 is approaching at a constant speed of 15 m/s, and the multirotors 14 are set to reach an approach-speed of 7 m/s. Further, the multirotors 14 are equipped with a basic autopilot that handles attitude setpoints, as discussed above, which is implemented as a PD control structure. Next, we consider the catching device 12 as a net with a width and height of 5 and 3 m, respectively. FIG. 3 shows a recovery system using a net as the catching device 12. The numerical simulation is conducted in MATLAB, using Runge Kutta 4 as integrator at 50 Hz. The total thrust of each multirotor 14 is configured such that it uses half of the available power at hover. Due to the construction of the multirotor 14, the available torque is likewise limited so that each motor does not exceed its maximum. The multirotor 14 has a motor-to-motor diameter of 1 m. Further, discrete time sampling is implemented with a zero-order-hold. The net is attached 10 cm below the Centre of Gravity of each multirotor 14.

[0086] In the simulation the multirotors 14 successfully intercept the incoming fixed wing UAV 16 and are able to handle the load during impact. The tension force on each of the multirotors 14 oscillates around a steady value in the z axis that is equivalent to half of the weight of combined net and fixed wing UAV 16. Due to a slight twist in the net when it swings, a slight transient can be seen on the y-component of the tension force. After impact, some residual oscillations remain due to the swinging payload.

[0087] An example operation procedure for recovery of a fixed wing UAV 16 with multirotor recovery drones 14 is described below.

[0088] The fixed wing UAV 16 takes off based on its standard operation procedure. For ship-based operations, this will typically include a catapult or similar. Before takeoff, the recovery pod bay, consisting of a releasable wire with a hook 18 and communication system is mounted under the fixed wing UAV 16 at a hard point.

[0089] To initiate the recovery, two or more multirotors 14 are prepared. As they are able to take off and land vertically, minimal space allocation is required. For the sake of this example, consider two multirotors 14.

[0090] The two multirotors 14 are set up 5 m from each other on the ship deck. The catching device 12 is attached to the bottom of the multirotors 14, and connected between the two multirotors 14.

[0091] Before launch of the multirotors 14, the location of the virtual runway is specified. This is either specified by an operator, based on surroundings, local regulations, weather, or done automatically. The operator can in the latter case adjust and accept the suggested virtual runway. The virtual runway can be oriented such that the recovery is performed against the wind, thus minimizing ground speed.

[0092] Parameters for the recovery operation, such as rendezvous speed, fixed wing cruise speed, and safety-constraints are set based on the physical parameters of the multirotors 14 and fixed wing UAV 16.

[0093] The location and orientation of the virtual runway is transmitted to the fixed wing UAV 16, which is instructed to follow this runway with the prescribed speed. Alternatively the virtual runway can be defined based on a previously set flight path of the fixed wing UAV 16.

[0094] To minimize the battery-drain on the multirotors 14, the time of which the multirotors 14 takes off from the ship is calculated based on location of the virtual runway from the ship, total flight time, and location of the incoming fixed wing UAV 16. Based on the speed of the fixed wing UAV 16, the multirotor take-off is initiated so they reach the start of the virtual runway in time to start the recovery manoeuvre.

[0095] Navigation solution of the multirotors 14 is checked, based on high precision satellite system or other local radio-based localization systems.

[0096] The multirotors 14 are set in a special control mode, coordinated mode, in which they act as a single unit. Take-off is initiated either automatically, or operated by a single pilot. In the automatic mode, the two multirotors 14 simultaneously and coordinatedly fly straight up and away from the ship deck. In the operator assisted mode, the operator uses a single control stick to fly both multirotors 14. Special control algorithms make sure the position between the multirotors 14 stays the same.

[0097] When the multirotors 14 have reached cruising altitude (either automatic or pilot assisted), the multirotors 14 go to a formation (relative position) optimal for cruise travel towards the start of the virtual runway. The optimal cruise relative position may depend on the recovery mechanism used (net or rope), the number of multirotors 14, wind direction, etc. They then move to the start of the virtual runway, and align themselves with the recovery mechanism (net or line) perpendicular to the virtual runway.

[0098] Position and velocity data from the fixed wing UAV 16 is transmitted to the multirotors 14 from the communications device in the pod. Based on this information, the multirotors 14 automatically starts to move along the virtual runway with a prescribed velocity as to recover the fixed wing UAV 16 at a designated location. Alternatively they may take a fixed position for interception of the fixed wing UAV 16 with zero groundspeed for the multirotors 14.

[0099] A graphical user interface (GUI) on the Command Control Centre (CCC) computer on the ship deck lets the operator continuously monitor the progress of the recovery manoeuvre, including vital parameters such as battery capacity, wind conditions, communication link status, etc. At any point, the operator can issue commands to the system to stop, retry or abort the recovery.

[0100] The system automatically detects a successful recovery based on on-board sensors. In the case of a miss or aborted recovery (due to communication loss, localization misalignment or changing weather conditions), the system can issue a retry. The fixed wing UAV 16 is instructed to fly around, and the multirotors 14 move towards the start of the runway again for a second attempt.

[0101] For a successful recovery, the multirotors 14 automatically moves back to the ship, with the fixed wing UAV 16 still attached to the catching device 12. When close to the ship, case must be taken to safely land and touch down the fixed wing UAV 16 on the ship deck. For automatic operations, the multirotor UAVs measure the position of the ship deck from satellite or other local measurement systems (camera, radio), and position themselves directly over the ship deck. They lower the fixed wing UAV 16 until touch-down is confirmed either by tensile sensors on the attachment device on each multirotor or other mechanism. When the fixed wing UAV 16 is safely on the ground then the multirotors 14 release the catching device 12 and drop it over the fixed wing. For pilot assisted touch-down, special control software lets a single pilot operate the location, altitude and orientation of the multirotors 14 with the suspended fixed wing UAV 16. The pilot guides multirotors 14 towards a safe landing area on the ship deck, and lowers the multirotors 14 until the fixed wing UAV 16 successfully touches down on the ship deck. Various measurement systems, such as radio, satellite or camera can assist the pilot to compensate for inherent motion of the ship deck do to waves and wind.

[0102] When the multirotors 14 have detached the suspended fixed wing UAV 16, either through pilot assisted or automatic operations, the multirotors 14 are landing on the ship deck one by one. Based on the number of multirotors 14, a queuing system might be used. The multirotors 14 either land automatically, based on positioning data from satellites or local measurements systems as described above, or manually from a single pilot one by one.

[0103] The complete system is built to be modularized, such that the recovery mechanism, multirotors 14 and housing for the hook system 18 can be interchanged to match each recovery mission criteria.