GB2476149A - Formation flight control - Google Patents

Abstract

Control of a flight formation 1 formed by at least a first aerial vehicle 2a and a second aerial vehicle 2b, is based on emitting, by the first aerial vehicle 2a an electromagnetic signal SNET based on IEEE 802.11 standards, detecting, by the second aerial vehicle 2b, the electromagnetic signal SNET, determining by the second aerial vehicle 2b, a respective value of a quantity, for example strength of the signal, associated to the electromagnetic signal, and determining by the second aerial vehicle 2b, information associated to a relative position of the second aerial vehicle 2b with respect to the first aerial vehicle 2a on the basis of the value of the quantity associated to the electromagnetic signal.

Description

INTELLECTUAL

. ... PROPERTY OFFICE Application No. GB 1020303.2 RTM Date:5 April 2011 The following term is a registered trademark and should be read as such wherever it occurs in this document: Wi-Fi Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk

METHOD AND SYSTEM FOR AUTOMATIC CONTROL OF THE FLIGHT

FORMATION OF UNMANNED AERIAL VEHTCLES

The present invention relates to a method and a system for automatic control of the flight formation of unmanned aerial vehicles. Unmanned aerial vehicles (UAVs) are currently used in numerous contexts, for example for acquiring territorial information of the space flown over, to bring aid, such as for example medical aid, in remote areas at competitive costs, for the control of criminal activities, etc. 1 0 The control of a flight formation of unmanned aerial vehicles, which fly in spatial proximity to one another (typically at a distance of between 20 m and 100 m apart), presents a potential problem of collision in flight between aerial vehicles that are close to one another. Tn the event of collision, it is highly probable that the aerial vehicles involved will be damaged to the point where they are not able to proceed the flight and crash. The T 15 loss of one or more aerial vehicles has above all a high economic impact, but also causes the loss of the information collected by the aircraft during flight. (\J

To overcome said problems, various solutions have been proposed. For example, technical solutions are known that enable close flight, in line, of at least two aerial vehicles 2 0 communicating with one another by means of laser or infrared signals. According to said technical solutions, each aerial vehicle comprises a laser or infrared transmitter and receiver, configured for transmitting and receiving a signal to and from the other aerial vehicle and, on the basis of the signal received, for modifying its own flight position approaching or moving away from the other aerial vehicle. It is evident how said solution may easily be liable to problems of interruption of the path of the signal between the two aerial vehicles, for example on account of atmospheric perturbations and/or of a sudden change of altitude of one of the aerial vehicles, thus causing a divergence in the course of the aerial vehicles or a collision between them.

Other solutions envisage the use of a GPS receiver on board each aerial vehicle. Each aerial vehicle knows its own position via the data received via GPS and, by means of techniques of radiofrequency communication between the aerial vehicles of one and the same formation, each of them can send to the other aerial vehicles of its own fonTnation its own flight co-ordinates. In this way, when the flight co-ordinates of one aerial vehicle are close to those of another aerial vehicle, it is possible to undertake automatic variations of the course to prevent any collision in flight. Said solution, however, requires the use of high-precision GPS receivers, in so far as errors of detection of position or altitude greater than a minimum threshold (for example, equal to the distance that separates two aerial vehicles flying alongside one another) cannot be tolerated. Said problem can be solved by increasing the distance between aerial vehicles during flight in such a way the distance is greater than the error of precision provided by the GPS system used. This solution would, however, have as consequence an excessively extensive formation, with aerial vehicles too far apart from one another. Even though it is possible to overcome said drawback by using high-precision GPS (for example, DGPS) receivers, said receivers require precise geographical references on the ground for continuous radio communication, from these sites, of a signal of correction of the GPS positions, typically in UHF band. Said infrastructures on the ground are burdensome and not easy to install, and are mostly aimed T 15 at maritime use for favouring approach and docking. Consequently, the DGPS service that they make available exclusively covers distances of some tens of miles from ports. To C\J offer wider coverage, suited to avionic use, the corrective signal calculated at the stations on the ground is sent to satellites designed for the purpose and re-transmitted by these to the mobile DGPS receivers. Said method is known as DGPS-WAAS in North America and its counterpart will be soon available in Europe under the acronym EGNOS.

Finally, systems have been proposed for controlling a flight formation of unmanned aerial vehicles in which the distance between aerial vehicles is maintained stable thanks to a continuous data exchange that, in addition to the GPS, position envisages also other flight information between the aerial vehicles belonging to the same formation, for example information on course, speed, etc. All these systems, however complex, have in common the need for continuous exchange of information between aerial vehicles. However, both data transmission between aerial vehicles and the GPS signal can be subject to disturbance due to atmospheric events, temporary interruptions of service, or intentional radio disturbance (i.e., jamming). Even though the sensitivity of a GPS device to interruptions of service can be compensated by inertial miniaturized measuring devices (inertial measurement units or IMUs) that continue to track the movement of the aircraft in space even in the absence of any signal, the impossibility of communicating its own position exposes each aerial vehicle to the risks inherent in a flight that is "blind" in regard to the aerial vehicles that are flying nearby. Tn this latter case, the aerial vehicles could be dispersed or collide with one another.

The aim of the present invention is to provide a method and a system for automatic control of the flight formation of unmanned aerial vehicles that will enable said drawbacks to be overcome.

1 0 Consequently, according to the present invention a method and a system for automatic control of the flight formation of unmanned aerial vehicles are provided as defined in Claims 1 and 17.

For a better understanding of the present invention, preferred embodiments thereof are T 15 now described purely by way of non-limiting example and with reference to the attached drawings, wherein: (\J -Figure 1 shows a plurality of unmanned aircraft in triangular flight formation in communication with one another by means of a local wireless network of a partial-mesh 0) te yp, -Figure 2 shows the unmanned aircraft of Figure 1 communicating with one another through a fully connected local wireless network (full mesh); -Figure 3 shows, in top view in a horizontal plane xz, an aircraft belonging to the formation of Figure 1 or Figure 2 during use, representing schematically directive wireless signals emitted by the aircraft with the aim of controlling the flight formation, according to one embodiment of the present invention; -Figure 4 shows, in side view in a horizontal plane xy, the aircraft of Figure 3, representing schematically further directive wireless signals emitted by the aircraft with the aim of controlling the flight formation, according to one embodiment of the present invention; -Figure 5 shows, in perspective view, the aircraft of Figure 3 and Figure 4; -Figure 6 shows the flight formation of Figure 1 or Figure 2 representing schematically the directive wireless signals emitted by the aircraft of Figure 3 with the aim of controlling the flight formation; -Figures 7-9 show successive steps of a procedure of creation of the flight formation of Figure 6 by using the directive wireless signals of Figures 3 and 4; -Figure 10 shows, by means of a flowchart, steps of a method of verification and maintenance of a correct direction of flight (or heading) by the aircraft of the flight formation of Figure 1; -Figure 11 shows in top view in a horizontal plane xz, an aircraft belonging to the formation of Figure 1 or Figure 2 during use, representing schematically directive wireless signals emitted by the aircraft with the aim of controlling the flight formation, according to another embodiment of the present invention; -Figure 12 shows, in side view in a horizontal plane xy, the aircraft of Figure 11; -Figure 13 shows a plurality of aircraft according to the embodiment of Figures 11 and 12 during flight in formation, representing schematically the directive wireless signals emitted with the aim of controlling the flight formation; -Figure 14 shows, in perspective view, the aircraft of Figures 11 and 12, by showing T 15 further directive wireless signals emitted by the aircraft; and -Figure 15 shows, in side view, the aircraft of Figure 11, representing schematically C\J further wireless signals emitted by the aircraft with the aim of controlling the flight formation, according to a further embodiment of the present invention. 0)

2 0 The ensuing description is presented to enable a person skilled in the art to implement and use the invention. Various modifications to the embodiments will be evident to persons skilled in the art, without thereby departing from the scope of the present invention, as claimed. Consequently, the present invention is not understood as being limited to the embodiments illustrated, but must be granted the widest sphere of protection consistent with the principles and characteristics illustrated herein and defined in the annexed claims.

Provided according to the present invention are a system and a method for controlling the flight formation of unmanned aerial vehicles, in which the distances between the aircraft, their disposition in flight formation and the manoeuvres for tracking of the course are managed automatically by the aircraft themselves without the aid of signals or warnings generated outside the formation. In detail, each aircraft is provided with an omnidirectional antenna of its own configured for emitting an omnidirectional signal at a frequency of its own and with a plurality of directive antennas configured so as to generate respective radiation cones (or lobes) of directive wireless signals. According to one embodiment of the present invention, the maintenance of a certain flight formation (preferably triangular) is guaranteed by the fact that each aircraft that follows an aircraft that precedes it (with the exception, of course, of the leading aircraft of the formation) flies keeping itself within a specific radiation cone generated by the aircraft that precedes it.

This is made possible through a continuous measurement (or a measurement at intervals predefined) of the level of power of the signal received. For example, a power received higher than a predefined threshold value is indicative of a correct position of an aircraft within the radiation cone generated by the aircraft that precedes it, whereas a power 1 0 received lower than the threshold value is indicative of a position of the aircraft outside said radiation cone. By measuring moreover the value of received power of the omnidirectional signal is possible to determine the distance between two aircraft. In fact, a received power of the omnidirectional signal that is excessively high is indicative of an excessive closeness between two aircraft, and, on the other hand, a received power that is T 15 excessively low is indicative of an excessive distance between them.

C\J According to a further embodiment of the present invention, the aircraft that follows does not fly within a radiation cone but in a low-signal zone comprised between two or more 0) radiation cones. In this case, the aircraft that follows uses the directive signals as signal 2 0 "walls" for monitoring possible lateral variations and of altitude with respect to the aircraft that precedes it. A possible entry of the aircraft that follows within a radiation cone generated by the aircraft that precedes it is indicated by a detected power higher than a predefined threshold. The control of distance between the two aircraft is ensured also in this case through the measurement of the detected power of the omnidirectional signal.

It should be noted that the radiation cones can be modelled (or equalized via software in time real) in order to render them as much as possible similar to geometrical cones (at least in the range of distances that are of interest, for example 15-100 m). The modelling of the radiation cones can, in the absence of sufficiently directive antennas, avail itself of radio-absorbent polyurethane foams commercially available as sheets or fabrics that can be folded and cut to be applied directly in the proximity of the antennas for masking the directions of propagation of the signal deemed not of interest for the purposes of the present invention.

Conditions of divergence of one of the aircraft on account of an erroneous interpretation of its own position on the basis of the signals received can be managed by implementing a control of lateral distance between the aircraft (in the case of triangular formation comprising at least three aircraft). For this purpose, an omnidirectional signal is generated by each of the aircraft belonging to one and the same flight formation. Each omnidirectional signal supplies to each aircraft of the formation an indication of the distance from all the other aircraft of the formation, and in particular from those laterally close thereto, at the same time preventing the aircraft from coming excessively close to or 1 0 moving excessively away from one another. Each aircraft is uniquely identified within the formation through "beacon" packets transmitted via the omnidirectional signal and/or the directive signals. Said packets are managed at level one and two of the SO/OSI protocol standard, and consequently the setting-up of a data-exchange network (for example, based upon the IP protocol) is not necessary for mere maintenance of the flight formation.

T 15 However, the setting-up of a Wi-Fi network, for example based upon the 802.1 lb/g/a protocol, can be envisaged during configuration on the ground of the flight formation in C\J order to handle a dynamic initialization of the formation by setting up the affiliations between the aircraft. Furthermore, during use, each aircraft can be entrusted with a specific task, for example acquisition of photos or films or other data of the territory flown over.

2 0 To maximize the redundancy of storage of said data, the latter can be shared and stored by more than one aircraft belonging to one and the same formation. In this case, the Wi-Fi network can be used also during flight with the aim of sharing information and data, which, however, as has been said, are not essential for maintaining the formation. The course of the entire flight formation can be stored in an appropriate memory of the leading aircraft of the formation or communicated, in flight, by a base station on the ground to the leading aircraft alone.

Figure 1 shows, in top plan view, a formation 1 of aircraft 2a-2f that form a triangle, during flight. According to one embodiment of the present invention, each aircraft 2a-2f is an unmanned aerial vehicle (UAV), or drone. Each aircraft 2a-2f of the formation 1 comprises a network device 4, provided with an appropriate transmitter/receiver module 5 and configured for setting up, by emitting an omnidirectional signal SNET of its own, a local wireless network with network devices 4 arranged on other aircraft 2a-2f of the formation 1.

The leading aircraft 2a of the formation 1 is moreover configured for communicating with a base station 10 via a radio-base communication device 8 of its own (for example, for receiving and sending mission indications, such as the course to be followed or other commands). The base station 10 can be located in any point on the Earth's surface, for example on terra firma, or else on a marine platform, or again on a ship. Alternatively, the base station 10 can be located in the air, for example on an aerial vehicle (helicopter, aeroplane, etc.) that flies at a distance from the formation 1, but in radio communication 1 0 therewith. Furthermore, it is evident that a number of base stations 10 may be present, and the aircraft 2a can communicate with a plurality of base stations 10 simultaneously or with just one of them, for example by choosing the one from which it receives a stronger signal.

In detail, the base station 10 comprises: a communication device 12 of its own, designed to T 15 set up a radio-base connection 15 with the radio-base communication device 8 arranged on the aircraft 2a; a processor 16, connected to the communication device 12; and a storage C\J device 18, for storing the data received from the aircraft 2a.

0) Advantageously, a plurality (or all) of the aircraft 2a-2f comprises a radio-base 2 0 communication device 8 of its own. In this way, during the mission, each aircraft 2a-2f can assume the role of leading aircraft 2a of the formation 1. This may become necessary, for example, in the case where the leading aircraft 2a is shot down; in this situation, another aircraft from among the aircrafl 2b-2f of the formation 1 can assume the role of leader of the formation 1 and set up a communication with the base station 10.

In use, each aircraft 2a-2f is specialized for acquiring a certain type of data (for example, images in the visible, images in the infrared, films, data on the conditions of flight, etc., but also for detecting signals and communications coming from the ground or from other aircraft, or other information still) and, after having acquired and stored them in a memory of its own, can send them to the other aircraft 2a-2f, which store them also. Furthermore, the leading aircraft 2a of the formation 1, in addition to storing said data, or as an alternative to storage, can send them to the base station 10 via the radio-base connection 15, to obtain a high redundancy of storage. Said data acquired are exchanged between the aircraft 2a-2f preferably by means of the local wireless network set up through the network devices 4. Tn particular, each aircraft 2a-2f, when it activates its own network device 4, acts as an access point of a wireless network. Designated by 17 and illustrated graphically via dashed arrows in Figure 1 is a wireless network, for example Wi-Fi based upon the 802.1 lb standard, set up between the network devices 4 of the aircraft 2a-2f and configured to enable a data exchange between the aircraft 2a-2f.

Said local wireless network 17 can be used for distribution and the transfer between the aircraft 2a-2f of data and information acquired by means of appropriate sensors or devices.

Said information, as has been said, can be transferred also to the base station 10 for being stored. In this way, even if the aircraft 2a-2f were lost, for example because they had been shot down, the data detected would be in any case recoverable and analysable. This solution requires a constant link free from disturbance between the aircraft 2a and the base station 10, and moreover involves a high energy consumption for remote transmission. On T 15 the other hand, local storage of the information and data acquired on each individual aircraft 2a-2f is risky in so far as the loss of a single aircraft 2a-2f would cause the loss of (\J all the information and data stored therein. By transferring and storing the information and the data acquired between all the aircraft 2a-2f forming part of the local wireless network 17, there is generated a redundancy of data storage such that the loss of one or more of the 2 0 aircraft 2a-2f does not imply a consequent loss of the information and data acquired.

Since the information and data that are transferred can be intercepted, the aircraft 2a-2f exchange information via the local wireless network 17 preferably using high-security encryption algorithms (for example, codes of a WPA-PSK, WPA2 type, or digital certificates with EAP-TLS).

By using the 802.11 b (or 802.11 g) standard for the Wi-Fi (centre band at 2.44 GHz), it is possible to create channels from 11 to 54 Mbps, which in terms of payload (i.e., of payload but for the network-management packets) drops by approximately 50% in good conditions of signal-to-noise ratio. In practice, this means a channel capacity such as to support, on average, from 8 to 20 data flows of 800 KB/s each. A more stable encoding is obtained by adopting, for example, the 802.llb standard with DQPSK encoding.

Figure 2 shows the aircraft 2a-2f connected to one another by means of a fully connected local wireless network 17. n this case, each aircraft 2a-2f sets up a wireless link for data exchange with all the other aircraft 2a-2f of the formation 1. Tf each aircraft 2a-2f comprises a device or sensor configured for capturing specific environmental data and information, and complete redundancy on all the aircraft 2a-2f of the formation 1 is desired, then the traffic on the local wireless network 17 rises quickly, all the more if the data are video data. Each aircraft 2a-2f transmits data and information, via the connections that constitute the local wireless network 17, to the other aircraft 2a-2f and receives from the latter respective data and information; it follows that in a full-redundancy system of N aircraft (i.e., N nodes from the standpoint of the local wireless network 17) 2(N-1) links are set up for each aircraft 2a-2f. Furthermore, in this case each aircraft 2a-2f receives and stores the data received on the N-i links set up with the other aircraft 2a-2f. In this case, the use of a system of direct memory access, for example DMA, is preferable.

T 15 Figures 3 and 4 show, according to an embodiment of the present invention, in a top plan view and in side view respectively, an aircraft forming part of the formation 1 of Figure 1, (\J for example the aircraft 2a. However, all the aircraft 2a-2fofthe formation 1 can be of the same type as those illustrated in Figures 3 and 4. a)

With joint reference to Figures 1-4, the aircraft 2a comprises: the network device 4, which in turn includes a transmitter/receiver module 5 and is configured for communicating in transmission and reception with network devices 4 of other aircraft 2b-2f of the formation 1 using, for example, the Wi-Fi 802.iib standard; an orientation device 6, which includes, for example, a GPS receiver and one or more gyroscopic compasses, and is configured for indicating the orientation (also referred to as heading) of the aircraft 2a with respect to the magnetic North; a radio-base communication device 8, for example one that uses a communication system based upon the TETRA standard, designed to communicate in transmission and reception with the base station 10 located at a distance from the aircraft 2a; at least one data-acquisition device 7, for example a photographic camera or a video camera, configured for acquiring environmental information, for example images of the territory flown over and/or other sensor devices, for example for the acquisition of meteorological data or avionic data (for instance, speed with respect to the ground and/or with respect to the air); a flight memory 9, configured for storing a plurality of flight and -10 -course data and/or the environmental information acquired; an automatic pilot device 11, configured for piloting the aircraft 2a automatically; a first directional antenna 19, arranged at the tail 18 of the aircraft 2a and configured for emitting a first directive wireless signal 29, for example of a Wi-Fi type based upon the 802.1 lb standard, in a direction substantially parallel to the direction of flight of the aircraft 2a but with opposite sense; a second directional antenna 20, arranged at the tail 18 of the aircraft 2a and configured for emitting a second directive wireless signal 30, for example of a Wi-Fi type based upon the 802.1 lb standard, to one side of the first directive wireless signal 29 (as viewed in Figure 3, to the left of the first directive wireless signal 29) and overlapping it; a 1 0 third directional antenna 21, arranged at the tail 18 of the aircraft 2a and configured for emitting a third directive wireless signal 31, for example of a Wi-Fi type based upon the 802.llb standard, to one side of the first directive wireless signal 29 (as viewed in Figure 3, to the right of the first directive wireless signal 29) and overlapping it; a fourth directional antenna 22 arranged at the tail 18 of the aircraft 2a and configured for emitting T 15 a fourth directive wireless signal 32, for example of a Wi-Fi type based upon the 802.1 lb standard, underlying the first directive wireless signal 29 (see Figure 4) and overlapping it; (J a fifth directional antenna 23 arranged at the tail 18 of the aircraft 2a and configured for emitting a fifth directive wireless signal 33, for example of a Wi-Fi type based upon the 802.1 lb standard, overlying the first directive wireless signal 29 (see Figure 4) and 2 0 overlapping it; and a microcontroller 14, connected to the network device 4, to the orientation device 6, to the radio-base communication device 8, to the flight memory 9, to the data-acquisition device 7, and to the automatic pilot device 11. The first, second, third, fourth, and fifth directional antennas 19, 20, 21, 22, 23 (connected to and managed by respective transmission modules or, alternatively, by a common transmission module, not illustrated) can constitute a standalone transmission system, not necessarily connected to the microcontroller 14; the microcontroller 14 can optionally be connected to the antennas 19-23 with the aim of monitoring their operation and/or centralizing the electrical supply.

The directive wireless signals 29-3 3 are represented schematically in Figure 3 in top view (in the horizontal plane xz) and in Figure 4 in side view (in the vertical plane xy). For clarity of representation, Figure 4 shows only the first directive wireless signal 29, the fourth directive wireless signal 32, emitted underneath the first directive wireless signal 29, and then the directive wireless signal 33, emitted above the first directive wireless -11 -signal 29, but does not show the second directive wireless signal 30 and the third directive wireless signal 31. The latter are, instead, illustrated in Figure 3. However, in use, the aircraft 2a can emit either all or just some of the directive wireless signals 29-33.

The directive wireless signals 29-33 are represented ideally according to the views of Figure 3 and 4 as having a substantially triangular shape, with vertex on the tail 18 of the aircraft 2a (i.e., where the directional antennas 19-23 are set).

With reference to Figures 3 and 4, the first directive wireless signal 29 has an angle of aperture ctC in the horizontal plane xz (Figure 3) and an angle of aperture yC in the vertical plane xy (Figure 4), which have values comprised between 70° and 150°, and preferably the same as one another and equal to 100°. Said angles refer to an attenuation of at least -dB with respect to the line of sight of the antennas.

T 15 With reference to Figure 3, the second directive wireless signal 30 and the third directive wireless signal 31 have, if considered in the horizontal plane xz, a respective angle of (\J aperture ctL and ctR, each of them having a value comprised between 40° and 90°, preferably the same as one another and equal to 60°. In addition, the second and third directive wireless signals 30, 31 overlap the first directive wireless signal 29 on side 2 0 portions of the first directive wireless signal 29 opposite to one another (see Figure 3) SO as to generate respective portions of space 35', 35" of overlapping signals.

With reference to Figure 4, the fourth directive wireless signal 32 and the fifth directive wireless signal 33 have, if considered in the vertical plane xy, respective angles of aperture yD and yU having a value comprised between 25° and 90°, preferably the same as one another and equal to 60°. In addition, also the fourth and fifth directive wireless signals 32, 33 overlap the first directive wireless signal 29 on portions of the latter opposite to one another so as to generate respective portions of space 36', 36" of overlapping signals.

Furthermore, also the second and third directive wireless signals 30, 31 have, if considered in the vertical plane xy, respective angles of aperture (not illustrated), for example having the same value as the angles of aperture aL and ctR. The fourth and fifth directive wireless signals 32, 33 preferably have respective angles of aperture in the horizontal plane xz of approximately 180°. With the current technology, said angle of aperture is provided by -12 -omnidirectional antennas with high gain in the horizontal plane and markedly directive in the vertical plane.

In three dimensions, the directive wireless signals 29-33 are radiation cones (or lobes), ideally represented schematically as illustrated in Figure 5. To be precise, given the technical considerations set forth previously, the fourth and fifth directive wireless signals 32 and 33 look more like "thick disks".

The spatial extension of each directive wireless signal 29-33 (extension of the radiation 1 0 cone along its own axis) depends upon the transmitting power. For each directive wireless signal 29-33, extensions of the radiation cone along its own axis that are considered acceptable for the application envisaged are comprised between 100 m and 500 m.

Figure 6 shows the formation 1 of Figure 1, by schematically representing the first, T 15 second, and third directive wireless signals 29-31, emitted by the aircraft 2a-2f with the aim of controlling the flight formation. For clarity of representation, the fourth, and fifth C\J directive wireless signals 32 and 33, which are, however, present and emitted by the aircraft 2a-2c are not shown in Figure 6. In what follows, the first, second, third, fourth, and fifth directive wireless signals 29-3 3 will be referred to as a whole as directive wireless signals 29-33.

During flight, each aircraft 2a-2c emits respective directive wireless signals 29-33 in a direction substantially opposite to the direction of flight, as illustrated in Figures 3-6. The aircraft 2d-2f, instead, do not emit any directive wireless signal 29-33 in so far as they are aircraft that close the triangular shape of the formation 1 (i.e., they are not followed by any further aircraft and the emission of directive wireless signals would be of no use for controlling the flight formation). Each aircraft 2a-2f moreover emits an omnidirectional signal SNET of its own to enable maintenance of a correct distance between aircraft. The aircraft 2a-2f are moreover configured for receiving a plurality of wireless signals, in particular the directive wireless signals 29-3 3 and the omnidirectional signal SNET. The reception occurs according to known techniques through the transmitter/receiver module 5 or by means of dedicated receiver modules, of a known type.

-13 -Each directive wireless signal 29-33 transmitted by each aircraft 2a-2c is uniquely identified by a pair formed by an aircraft identifier IDA (which identifies uniquely each aircraft 2a-2c of the formation 1 that emits that particular directive wireless signal) and by a signal identifier IDS (which identifies uniquely the directive wireless signal emitted).

Since each directive wireless signal 29-3 3 carries an identifier that includes both the aircraft identifier IDA and the signal identifier IDS, each directive wireless signal 29-33 is uniquely identified within the formation 1.

Since, for one and the same aircraft 2a-2c, the frequencies of transmission of the directive 1 0 wireless signals 29-33 are different from one another, directive wireless signals 29-33 transmitted by one and the same aircraft 2a-2c and overlapping do not interfere with one another to the point ofjeopardizing proper evaluation of the value of power received by an aircraft 2b-2f. Tn this sense, the distinction of the parameters of signal strength and link quality (RSSI) assumes particular importance, only the latter parameter being an indirect T 15 measurement of the phenomenon of interference. Both are measured by the receiver modules, but only the parameter of signal strength assumes a significant value for (\J maintenance of the flight formation.

A different solution envisages that, for reasons of confidentiality, the directive wireless 2 0 signals 29-3 3 hide their own identifiers IDS ("SSID hiding" procedure) so that their recognition is made on the basis of the analysis of the frequencies of transmission alone (the frequency-allocation plan is obviously known to the control logic of each aircraft 2a-2f).

In addition to the directive wireless signals 29-3 3, each aircraft 2a-2f, in setting up the local wireless network 17, emits an omnidirectional signal SNET of its own, at a frequency of its own in order to prevent any interference with other omnidirectional signals SNET and with the directive wireless signals 29-33. Said omnidirectional signals SNET, irrespective of whether they carry information data or not, carry the aircraft identifier IDA (for example via "beacon" signals, which enable unique identification of the aircraft 2a-2f that has generated them). Each aircraft 2a-2f of the formation 1, by detecting the level of power of the omnidirectional signal SNET received from each of the other aircraft 2a-2f of the formation 1, is able to determine its own distance therefrom, and in -14 -particular from the aircraft 2a-2f flying alongside.

For instance, at an altitude of flight of approximately 100 m, the directive wireless signals 29-3 3 and the omnidirectional signals SNET emitted with a power equal to approximately 28 dBm (EIRP value, i.e., including the antenna gain) undergo an attenuation of a decreasing-exponential type, as one moves away along the line of sight, of approximately 6 dBm at each doubling of distance, in optimal meteorological conditions. At altitudes of flight lower than 100 m, the reflections of the signals created by the ground (in particular by bodies of water) and/or buildings, may superimpose on the lines of propagation of the 1 0 original signals, thus creating new areas of radiation and altering the geometries desired for the radiation cones of the directive wireless signals 29-33. The table below shows an example of a possible progression of the attenuation (in dBm) along the line of sight (measured in metres moving away from the source of emission of the signal).

metres 2 4 8 16 32 64 128 256 512 dBm -35 -41 -47 -53 -59 -65 -71 -77 -83 If a ("daughter") aircraft detects a value of power of the omnidirectional signal SNET (or of the directive wireless signal 29-33) emitted by the ("mother") aircraft that precedes it equal to -50 dBm, then, with reference to the table appearing above, it can infer that it is at a distance of approximately 12 m from the mother aircraft. These values are, however, markedly dependent upon the meteorological conditions and upon whether the daughter aircraft is more or less on the line of sight of the directional antenna 19-23 that is transmitting and upon the gain of the receiving antenna. In the best possible conditions of reception, the values in metres appearing in the table above could even be doubled. It follows that the process of estimation of the distance as a function of the power of the signal received provides a range of distances rather than a precise value of distance.

However, since the disclosure according to the present invention is aimed at maintaining a cohesive formation of aerial vehicles, and hence basically at preventing the aerial vehicles from getting dispersed or colliding with one another, it is sufficient to introduce a quantization of the signal/distance relation such as to overcome the randomness represented by the meteorological conditions and by the angles that the directional antennas 19-23 and the receiver modules come to assume with respect to one another -15 -during flight.

Consequently, by measuring the power of the signals received, each aircraft 2b-2f is able to derive its own position within the formation 1 in a relative way with respect to the other aircraft 2a-2f, and consequently adapt its own course in order to prevent any collision (above all lateral collisions).

Furthermore, each aircraft 2b-2f uses the information on the variation of power of the signals received to execute turns. In this way, if the leading aircraft 2a of the formation 1 0 turns, the aircraft 2b and 2c affiliated thereto follow it by turning themselves (seeking to maintain the power of the signals received at an optimal value). In turn, the aircraft 2d-2f affiliated to the aircraft 2b and 2c execute a similar manoeuvre, keeping within the radiation cone of the aircraft 2b and 2c.

T 15 A procedure of creation of a local wireless network 17 between the aircraft 2a-2f of Figure 1 and a procedure of take-off of the aircraft 2a-2f themselves are now described. (\J

The creation of the network starts when the aircraft 2a-2f are on the ground, before take-off. The network devices 4 of the aircraft 2a-2f are turned on and set up, in a known way, a 2 0 local wireless network 17, of a tree type. In particular, the first network device 4 that starts the transmission names itself "root node" of the local wireless network 17. In flight, the aircraft corresponding to the root node has the function of leading aircraft 2a of the formation 1.

Any possible conflict between network devices 4 that transmit simultaneously is managed according to the normal rules of creation of a WLAN based upon 802.1 lb. The root node then manages affiliation in regard to itself of possible intermediate nodes and/or leaf nodes (i.e., peripheral nodes that do not affiliate further nodes) of the local wireless network 17.

In turn, each intermediate node affiliates possible further intermediate nodes and/or leaf nodes. At the end of creation of the local wireless network 17, each node affiliates at the most two intermediate/leaf nodes and is in turn affiliated (except for the root node) to just one node. With reference to the formation 1 of Figure 1, the leading aircraft 2a has the role of root node of the local wireless network 17, the aircraft 2b and 2c, affiliated to the -16 -aircraft 2a, have the role of intermediate nodes and in turn affiliate the aircraft 2d, 2e and 2f, which have the role of leaf nodes. A further level of communication is established between nodes of one and the same level (intermediate nodes with intermediate nodes and leaf nodes with leaf nodes).

In this way a local wireless network 17 of a partial-mesh type, hence not fully connected, is configured. From a strictly topological standpoint, instead, the formation of aircraft is of a tree type or else a delta type (or, in the case of absence of one or more leaf nodes, a degenerate-tree type).

The local wireless network 17 is used for fast evaluation of the distances between aircraft that are useful for preventing collision and for exchange of information coming from the sensors mounted on board the aircraft 2a-2f through analysis of the values of power of the omnidirectional signals SNET that provide the links of the wireless network 17 itself. Via T 15 appropriate encryption algorithms, the wireless network 17 offers characteristics of authentication and confidentiality. Albeit enabling an evaluation of the relative distances C\J between the aircraft 2a-2f the detection of the received power of each omnidirectional signal SNET of the wireless network 17 does not enable evaluation of the relative spatial 0) orientation between the aircraft 2a-2f that transmits and the aircraft 2a-2f that receives.

The relative spatial orientation between two aircraft is rendered possible by the emission of the directive wireless signals 29-3 3 and by the fact that each affiliated aircraft 2b-2f knows the frequencies of transmission of the directive wireless signals 29-3 3 used by the aircraft 2a-2c that affiliates it (and the signal identifier IDS, if it is transmitted) and the spatial orientation of emission of said signals. The affiliated aircraft 2b-2f is, that is, aware of the fact that, by setting itself behind an aircraft 2a-2c, the first directive wireless signal 29 is emitted in a direction opposite to the direction of flight of the aircraft 2a-2c that emits it, the second directive wireless signal 30 is emitted to the left of the directive wireless signal 29, the third directive wireless signal 31 is emitted to the right of the directive wireless signal 29, and the fourth and fifth directive wireless signals 32, 33 are, respectively, oriented at an altitude lower and higher than the first directive wireless signal 29. The data regarding the orientation of emission of the directive wireless signals 29-3 3 can be stored in the memory 9 of each aircraft 2a-2f, in a database. Furthermore, during -17 -creation of the local wireless network 17, each affiliated aircraft 2b-2f (internal node or leaf node of the local wireless network 17) is uniquely assigned the aircraft identifier TDA (and possibly the signal identifier IDS) of the aircraft 2a-2c that it must follow during flight. This means that each aircraft 2b-2f knows not only which other aircraft 2a-2f it must follow (identified by the aircraft identifier IDA), but also within which radiation cone defined by each of the directive wireless signals 29-3 3 it must fly (identified by the signal identifier IDS or by the frequency, which is known, of transmission of the directive wireless signal).

The information on the affiliations of each aircraft 2b-2f can be propagated through the local wireless network 17 SO that each aircraft 2a-2f knows the affiliations of the other aircraft 2a-2f.

Alternatively, the affiliations between the various aircraft 2a-2f can be fixed and T 15 established beforehand. In this case, the affiliations commonly in use (which may obviously be as few as possible) are the object of continuous monitoring via cyclic C\J exploration of the Wi-Fi channels (for monitoring the directive wireless signals 29-3 3 received). This mechanism also underlies the capacity of change of roles between the aircraft 2a-2f in the case where there is a breakdown or one or more of them is shot down.

At the end of the step of creation of the local wireless network 17 (with the aircraft 2a-2f still on the ground), it is possible to proceed with the take-off phase, which preferably occurs with the aircraft 2a-2f arranged in a row, with the directional antennas 19-23 active in transmission of the respective directive wireless signals 29-3 3 and with the network device 4 active in transmission of the omnidirectional signal SNET.

In the first place (Figure 7), the aircraft 2a takes off and, after a few seconds (for example 5-10 seconds), the power of its omnidirectional signal SNET, as a result of the progressive recession of the aircraft 2a from the other aircraft that are still on the ground, will be received by the latter as progressively weaker. When the power of the omnidirectional signal SNET emitted by the aircraft 2a drops below a certain threshold that can be freely set, the aircraft 2b (and only this in so far as all the aircraft 2b-2f are aware of their own identity, of the state of the respective signals emitted, and hence of the respective -18 - affiliations and take-off priorities) activates the maximum propulsion to proceed to take-off. During take-off of the aircraft 2b, which occurs just after the aircraft 2a has taken off, the aircraft 2b detects one or more directive wireless signals 29-33 emitted from the tail by the aircraft 2a and, as a consequence thereof, executes one or more turns in order to position itself within the radiation cone of the directive wireless signal 29-3 3 previously assigned thereto during creation of the wireless network 17. In particular, in the example illustrated in Figure 7, the aircraft 2b positions itself within the radiation cone corresponding to the directive signal 30 emitted by the aircraft 2a. Said positioning occurs by monitoring the power of all the directive wireless signals 29-3 3 received to identifiy in 1 0 what position the aircraft 2b is located with respect to the aircraft 2a, and executes possible appropriate turns or changes of altitude. It should be considered that, by way of example, in good meteorological conditions or even only fair meteorological conditions (slight rain), given a second directive wireless signal 30 transmitted by the aircraft 2a at a power of 30 dBm (1 Watt, the maximum allowed according to the IEEE 802.11 standard), the reception T 15 by the aircraft 2b of a signal with a power of -45±3 dBm indicates a distance between the two aircraft 2a, 2b certainly not greater than 30 m, whilst the reception of a signal with a C\J power of -65±3 dBm indicates a distance between the two aircraft 2a, 2b of at least 60 m. 0. .

Said values are to be considered purely indicative; effective values are to be venfied case by case on the basis of the power transmitted, the antenna gain (also that of the receiving 2 0 one), the type of antennas used, etc. Advantageously, the aircraft 2b can be positioned in an area of boundary between the first and second directive wireless signals 29, 30. In this way, a line of flight of the aircraft 2b that is excessively lateral (to the left) with respect to the line of flight of the aircraft 2a is identified by the aircraft 2b, which detects the power of the directive wireless signal 30 as predominant with respect to that of the directive wireless signal 29, consequently inducing a corrective turn to the right. Likewise, a line of flight of the aircraft 2b that is too far inside the radiation cone of the directive wireless signal 29 is identified by the aircraft 2b, which detects a power of the first directive wireless signal 29 as predominant with respect to the power of the second directive wireless signal 30, consequently inducing a corrective turn to the left.

The use of an orientation device 6 based upon one or more gyroscopes (for example, of an -19 -integrated type, in MEMS technology) enables the microcontroller 14 to govern the automatic pilot device 11 so as to mitigate the effect of the turns and re-enter within a correct line of flight. n other words, the use of one or more gyroscopes introduces a dampening factor that prevents the automatic pilot device 11 from setting up a positive feedback and causing one or more aircraft of the formation 1 to go astray. The longitudinal distance between the aircraft 2a and the aircraft 2b is instead regulated via actions on the engine of the aircraft 2b. In fact, during approach of the aircraft 2b following the aircraft 2a, there is an instant in which the aircraft 2b, accelerating behind the aircraft 2a, will be excessively close to the aircraft 2a (the received power of the omnidirectional signal SNET 1 0 generated by the aircraft 2a is detected by the aircraft 2b as being above a certain threshold, for example higher than -40 dBm). The aircraft 2b, controlled by its own automatic pilot device 11, then undertakes an action of deceleration, to assume a regime of stable propulsion such as to allow it to receive from the aircraft 2a directive wireless signals 29-3 3 and the omnidirectional signal SNET that have a power of a substantially T 15 stable value. It is evident that, in the case where the power of the signallsignals received were to decrease excessively (for example, the power of the omnidirectional signal SNET C\J drops below -70 dBm), the aircraft 2b would undertake a new action of acceleration, and so forth. 0)

2 0 An external observer, observing the formation 1 (also during its constitution), would see the aircraft 2b-2f execute periodic longitudinal and lateral adjustments and adjustments of altitude. Simulations conducted by the applicant show how said adjustments would occur at a rate not higher than once every 5 seconds.

In the situation of Figure 7, with just two aircraft 2a, 2b, it cannot be ruled out that the aircraft 2b diverges with respect to the direction of flight of the aircraft 2a and that the consequent reductions in the second directive wireless signal 30 and in the omnidirectional signal SNET received by the aircraft 2b induce the latter to accelerate rather than turn; if the angle of divergence were considerable, execution of the subsequent turn could prove in any case insufficient, and the final effect of the manoeuvres of the aircraft 2b would be to have further increased its distance from the aircraft 2a instead of reducing it; the further reduction of signal would cause a new acceleration and so forth, i.e., would generate a positive feedback that could lead to loss of the aircraft 2b. The possibility of the aircraft 2b -20 -diverging considerably from the direction of flight behind the aircraft 2a is prevented thanks to the presence of the orientation device 6, which, by including, as has been said, one or more gyroscopes, enables the automatic pilot device 11 of the aircraft 2b to maintain with good approximation the same angular position of the direction of flight as that of the aircraft 2a. The error due to the inevitable angular drift in the long term can be appropriately compensated, for example as illustrated by steps 35-4 1 of the flowchart of Figure 10 (and described hereinafter with reference to said figure). The possible exchange of information of angular position between the aircraft 2a and 2b, albeit not strictly necessary, can occur through the local wireless network 17.

With reference to Figure 8, proceeding in the description of the take-off of the remaining aircraft 2c-2f following upon take-off of the aircraft 2b, also the aircraft 2c, perceiving a reduction of the power of the omnidirectional signal of the aircraft 2b and hence a recession of the aircraft 2b, activates its maximum propulsion and takes off -15 The aircraft 2c detects, via its own network device 4, the presence of the aircraft 2a and of C\J the aircraft 2b, detecting their respective omnidirectional signals SNET. When the aircraft 2c arrives in the vicinity of the tail of the aircraft 2b and of the aircraft 2a, it detects one or more of the directive wireless signals 29-3 3 generated by the aircraft 2b and one or more 2 0 of the directive wireless signals 29-33 generated by the aircraft 2a. Since the aircraft 2c has been assigned the role of daughter of the aircraft 2a (during creation of the wireless network 17), the aircraft 2c makes lateral turns so as to position itself alongside the aircraft 2b, at a distance deemed safe (depending upon various parameters, for example the wing span of the aircraft 2b, 2c). Then (Figure 9), the aircraft 2c accelerates to draw up alongside the aircraft 2b and position itself within the radiation cone of the third directive wireless signal 31 of the aircraft 2a. A further control on the received power of the omnidirectional signal SNET enables the aircraft 2c to keep itself at a lateral safety distance from the aircraft 2b. In this way, a triangular flight formation is formed comprising the aircraft 2a, 2b, 2c.

In the case where, during take-off, the aircraft 2c is unable to detect the directive wireless signals 29-3 3 emitted by the aircraft 2a, the aircraft 2c can use the directive wireless signals 29-33 emitted by the aircraft 2b to draw up alongside it, approaching the aircraft 2a -21 -and hence setting itself in a position for reception of the directive wireless signals 29-3 3 generated thereby.

The triangular formation thus formed by the aircraft 2a, 2b, 2c increases the degree of confidence as regards one of the aircraft which form part thereof moving away excessively: the aircraft 2b and 2c in fact can monitor the relative lateral distances by analysing the value of power of the omnidirectional signal SNET received from one another. In this case, the triangular flight formation can be maintained by using only the omnidirectional signals SNET emitted by the three aircraft 2a-2c, without using the directive signals 29-33. Each aircraft 2a-2c can in fact monitor its distance from the other aircraft by analysing exclusively the received power of the omnidirectional signals SNET.

Also in this case, it is possible, however, to consider an exception. For example, the aircraft 2b and 2c, which have the same mother aircraft 2a, could be subject to a lateral drift in the same direction (for example, both to the right or both to the lefi). In this T 15 situation, the estimation of the lateral distance between 2b and 2c (provided by the evaluation of the received power of the omnidirectional signal SNET) would be adequate, C\J whilst the distance from the aircraft 2a of both of the aircraft 2b and 2c would increase (and the power of the signal received by the aircraft 2a would decrease) to such a point that a command for acceleration forwards could be activated, which could lead the aircraft 2 0 2b and 2c to overtake the aircraft 2a and even go astray in the most serious cases. Such an event is prevented, according to the present invention, by the analysis of the directive wireless signals 29-33. In fact, an excessive divergence of flight course of an aircraft 2b-2f is detected by the aircraft 2b-2f itself simply by evaluating the variation of the intensity of the first (central) directive wireless signal 29 with respect to that of the signal belonging to said aircraft 2b-2f (second or third, lateral, directive wireless signal 30 or 31). Said mechanism intervenes autonomously, irrespective of the presence of an orientation device 6 and of any communication with other aircraft 2a-2f (which may, however, be present and have a value that provides a confirmation, but is not necessary). Evaluation of the omnidirectional signals SNET enables prevention of excessive mutual recessions or approaches of the aircraft even when the latter maintain the correct angular positions with respect to one another.

Next, one after another, the aircraft 2d-2f take off, acting as described with reference to the -22 -aircraft 2b, 2c, to position themselves correctly behind the aircraft 2b, 2c to form the formation 1 of Figure 6.

What has been set forth so far regarding the autonomy of the aircraft in the control of its spatial orientation applies in three dimensions by virtue of the presence of the directive wireless signals 32 and 33.

Figure 10 shows, by means of a flowchart, a method of compensation of the error due to the angular drift. The steps of the method described are executed by each aircraft 2b-2c belonging to the formation 1. For simplicity of description reference will be made in what 1 0 follows to just one aircraft 2b.

During a step of start of the method of Figure 10, a variable Var_Head containing a value of relative heading of the aircraft 2b with respect to the aircraft 2a is allocated in the memory 9 of the aircraft 2b. The variable Var_Head is initialized at a reference value, for

T 15 example zero.

C%J Then (step 35), a check is made to is verify whether the aircraft 2b is flying within the radiation cone to which it belongs (for example, consistently with the examples illustrated previously, within the cone of the second directive wireless signal 30). This check is made 2 0 by monitoring the value of the received power of the second directive wireless signal 30 and by comparing said value of received power with a reference value (or range of values).

In the case (step 36) where the aircraft 2b is not flying within the radiation cone of the second directive wireless signal 30 and the aircraft 2b does not enter the radiation cone of the second directive wireless signal 30 within a certain time interval TOl (for example, 10 seconds), then (step 37), an emergency procedure ("fail-safe procedure") is started, and control returns to step 35. The emergency procedure can, for example, consist in a turn such as to cause the aircraft 2b to re-enter rapidly within the radiation cone to which it belongs. Said re-entry could occur with a certain angle due to the turn, for example an angle of 100, with respect to the orientation of flight of the aircraft 2a that precedes it. Said variation of heading, due to the turn, is detected by the gyroscope/s of the orientation device 6 of the aircraft 2b, which thus governs/governs a counter-turn tending to restore the uniformity of heading between the aircraft 2b and the aircraft 2a. It may here be expedient to recall that only the leading aircraft 2a maintains an absolute heading, on the -23 -basis of position data acquired, for example, via GPS. The aircraft 2b that follows the aircraft 2a maintains a relative heading with respect to the aircraft 2a and not an absolute heading.

If, during step 35, the direction of flight of the aircraft 2b is assumed as being correct (the aircraft 2b flies within the radiation cone defined by the second directive wireless signal 30), then (output YES from the step 35), the variation of the value of heading of the aircraft 2b with respect to the aircraft 2a is defined as being zero, and the variable Var_Head is reset to the reference value (step 38). The verification of the heading is made, 1 0 for reasons of safety, with a fixed timing, for example every 30 seconds. In fact, on account of the normal angular drifts, it is estimated that, on average, the value of heading of the aircraft 2b (obtained, for example, by means of an orientation device 6 of a gyroscopic type) can vary by approximately 10 every 30 seconds of flight.

T 15 If during the 30 seconds subsequent to verification of the value of heading, the latter varies by more than 3° with respect to the current value (step 40), the aircraft 2b is governed C\J (step 41) in such a way as to make a turn such as to bring the aircraft 2b back into the correct position (seeking, that is, to restore the optimal heading value). Otherwise, control 0) returns to step 35.

If the term of timing envisaged (in this example, 30 seconds) elapses, and the heading value has varied by an amount of less than 3°, then control returns to step 35.

The continuous turns aimed at restoring a correct heading value can, however, be a cause of an increasingly imperfect re-alignment. The aircraft could, in this situation, start to turn alternately between the extremes of the radiation cone to which it belongs with increasingly wide oscillations up to the point where there is a risk of loss of control due to excessive lateral recession. In this case, the problem is posed of containing the discrepancy between an effective direction of flight and the indications provided by the orientation device 6, for example within 3° in absolute value. The state of the art enables the orientation devices 6 of a gyroscopic type to contain said discrepancy for a duration of between 3 and 5 minutes (depending also upon the dynamic stresses). It is hence convenient to correct the heading value supplied by an orientation device 6 of a gyroscopic -24 -type with the absolute value with respect to the magnetic North obtained via GPS, for example once every 3-5 minutes. The value of absolute heading is in this case measured individually by each aircraft 2b-2f. Each aircraft 2b-2f is consequently equipped for this purpose with a GPS device of its own, which may be associated to an orientation device 6 of a gyroscopic type and is used, only when necessary, for restoring a correct absolute heading.

The applicant has found that, on account of the continuous adjustments of direction of the aircraft 2b-2f, there is a non-zero probability of lateral collisions, wing against wing, 1 0 between aircraft 2b-2f flying alongside one another. In fact, the lateral motion due to the turns presents a dynamics of higher intensity as compared to longitudinal variations on account of the mechanical inertia and aerodynamic resistance, which manifest their effect prevalently in the direction of flight. Tn the short period of time in which the aircraft 2b-2c flying alongside one another manifest a lateral motion of approach with respect to one T 15 another, albeit mitigated by the action of the gyroscopes, the constant monitoring of the power of the directive wireless signals 29-3 3 received could fail to reveal in due time (in (\J general 3 to 4 seconds are necessary) exit from the radiation cone to which they belong.

For this reason, it is advantageous to set up, by means of the omnidirectional signal SNET, a direct connection on a specific channel between aircraft 2b-2f located alongside one 2 0 another during flight. Said arrangement enables in fact times for evaluation of the power of the signal received that are 5 to 10 times shorter than what can be achieved by exploring all the directive wireless signals 29-33. According to the scenario just described, the result of the continuous monitoring of the omnidirectional signal SNET has priority over the result of the monitoring of the directive wireless signals 29-3 3 in activating or otherwise anti-collision manoeuvres.

Once a pre-set altitude of flight has been reached (which can, for example, by with barometric flight instrumentation or via GPS, if present) the leading aircraft 2a sets itself in a configuration of plane and levelled flight, and the aircraft 2b-2f that follow it emulate its characteristics of flight thereof for tracking the first, second, and third directive wireless signals 29-31 and maintain the same altitude for tracking the fourth and fifth directive wireless signals 32, 33.

-25 -Finally, the landing phase envisages a gradual abandonment of the formation 1 starting from the aircraft 2d-2f that close the formation 1, one at a time.

Since the directive wireless signals and the omnidirectional signals SNET are, by their very nature, unstable and subject to a plurality of environmental conditions, such as for example atmospheric events, the power of the signals that are emitted by the directional antennas 19-23 and by the network device 4 (in the case of the omnidirectional signal SNET) of any one of the aircraft 2a-2f, which is detected by one or more aircraft 2b-2f that fly in the proximity (either behind or alongside) said aircraft 2a-2f can vary irrespective of 1 0 an effective variation of position or course of the aircraft 2a-2f that emits the signal. It follows that the evaluation of the power of the signals received does not enable the aircraft 2a-2f to estimate the relative position with a precision of metres, but more likely with a higher order of magnitude. In fact, random variations of the mutual position of aircraft 2a- 2f of the formation 1 are due principally to three factors: aerodynamic effects, difficulty of T 15 maintaining rigorously one and the same speed, difficulty of maintaining rigorously one and the same attitude (angles of heading, pitch, and roll), etc. The formation-control (\J system described is designed to recognize spatial variations of flight in the region of approximately 5-10 metres in so far as only at such a distance can the power of the signals received be considered having effectively undergone variation on account of a drift of the 2 0 direction of flight and not simply on account of the intrinsic instability and variability of the wireless signals. The aircraft 2b-2f that follow the leading aircraft 2a hence show an apparently random undulating motion that respects the positions of the desired topology only as average in time.

The continuous modification of the relative spatial positions between the aircraft 2a-2f, in addition to lying at the basis of the principle of dynamic equilibrium that enables the formation I to fly in a cohesive way, also reduces the vulnerability of the aircraft 2a-2f to an attack (which typically occurs from the ground). In fact, from the ground it is more complex to make an attack to aircraft 2a-2f that change their position suddenly and continuously, thus rendering the formation 1 difficult to shoot down (at least by visual pointing of the individual aircraft 2a-2f). The difficulty in shooting down the aircraft 2a-2f is further increased by the possibility of rendering the signals not detectable outside the formation 1 (there may be estimated a spatial extension of the signals emitted of -26 -approximately 500 m, in any case depending upon the power of emission of the signals and upon the direction of listening). For this purpose, also the choice of the IEEE 802.1 lb standard is advantageous as compared to TEEE 802.llg/a. In fact, the 802.llb encoding technique is of a DSSS (direct-sequence spread spectrum) type, particularly resistant to attempts at jamming (radio disturbance generated with the intention of hindering radio-communication).

Figures 10 and 11 show, respectively in top plan view (in the plane xz) and in side view (in the plane xy), an aircraft forming part of the formation 1 of Figure 1 or Figure 2, for 1 0 example the aircraft 2a, according to a further embodiment of the present invention.

However, all the aircraft 2a-2f of the formation 1 can be of the same type as the one illustrated in Figures 10 and 11.

According to the embodiment illustrated in Figures 10 and 11, the aircraft 2a does not T 15 comprise a plurality of antennas on the tail 18, but comprises a first directive antenna 50 and a second directive antenna 51, which are arranged at a first wing 55 (for example the (\J left wing), preferably in an outer lateral portion of the first wing 55, and a third directive antenna 58 and a fourth directive antenna 59, which are arranged at a second wing 65 (the right wing), preferably in an outer lateral portion of the second wing 65.

The first 50, second 51, third 58, and fourth 59 directive antennas are configured for emitting, during use, a respective directive wireless signal at a frequency of its own. In particular, the first directive antenna 50 emits a first directive wireless signal 69, for example at a power of 30 dBm and at a frequency of 2.412 GHz; the second directive antenna 51 emits a second directive wireless signal 70, for example at a power of 30 dBm and at a frequency of 2.437 GHz; the third directive antenna 58 emits a third directive wireless signal 71, for example at a power of 30 dBm and at a frequency of 2.437 GHz (the same as that of the directive wireless signal 70, in so far as they are spatially separated and hence do not interfere); and the fourth directive antenna 59 emits a fourth directive wireless signal 72, for example at a power of 30 dBm and at a frequency of 2.4 12 GHz (the same as that of the directive wireless signal 69, in so far as they are spatially separated and hence do not interfere).

-27 -The first and second directive antennas 50, 51 emit the respective directive wireless signal 69, 70 at an angular distance pL comprised between 1200 and 180°, preferably 1500.

Likewise, also the third and fourth directive antennas 58, 59 emit their own respective directive wireless signal 71, 72 at an angular distance pR comprised between 120° and 180°, preferably 150°.

By appropriately choosing the first 50, second 51, third 58, and fourth 59 directive antennas so that the angle of aperture of the respective radiation cone is comprised between 60° and 90°, preferably 75°, it is possible to define a first low-signal zone 74 1 0 comprised between the first and the second directive wireless signals 69, 70, and a second low-signal zone 76 comprised between the third and the fourth directive wireless signals 71, 72. The low-signal zones 74 and 76 are approximately 30° wide and must be oriented so that their bisectrices form an angle comprised between 30° and 45° with respect to the longitudinal axis of the aircraft, towards the outside of the fuselage. This is obtained by T 15 orienting by one and the same angle the two sets of antennas 50, 51 and 59, 58. The figures might not respect faithfully the angles described, but the working principle remains C\J unaltered.

0) During flight in formation, as illustrated in Figure 13 for just three aircraft 2a-2c of the formation 1, the aircraft 2b and 2c fly keeping themselves within the first and second low-signal zones 74, 76, and not within the radiation cones as described with reference to Figures 5-9.

The concept of low signal is to be understood according to a comparison value (for example, -65 dBm) that can be adapted as a function of the estimated distance between two aircraft. Alternatively, it can be equal to the minimum threshold of sensitivity that can be achieved by current receivers, usually to -83 dBm.

This embodiment of the present invention proves particularly advantageous in the case where sufficiently directive antennas such as to define precisely paths (radiation cones) set alongside one another and sufficiently delimited laterally are not available.

According to what is illustrated in Figure 13, the aircraft 2b sets itself behind the aircraft -28 - 2a in such a way as to use the lateral fronts 69', 70' of the radiation cones of the directive wireless signals 69, 70 as "walls" delimiting the path to be followed (first low-signal zone 74). The fact that they are "thick" walls (radiation cones), with non-parallel walls with an angular divergence qL of up to of 150°, is advantageous during possible periods of latency (even 3, 4 seconds according to the network device or the receiver modules used) of the function of monitoring of the signals received, in so far as the aircraft 2b can penetrate in depth in said radiation cones and in any case recover a useful signal at the end of the period of latency. What has been said likewise applies for the aircraft 2c, which sets itself behind the aircraft 2a so as to use the lateral fronts 71', 72' of the radiation cones of the 1 0 directive wireless signals 71, 72 as "walls" delimiting the path to be followed (second low-signal zone 76).

A lateral safety distance for the aircraft 2b and 2c is maintained on the basis of the power of the omnidirectional signal SNET monitored by each of the aircraft 2b, 2c with respect T 15 to any other aircraft 2c, 2b (as already described previously, also in this case an excessively high power of the omnidirectional signal SNET causes a turn of the aircraft C\J 2b, 2c in mutually opposite directions).

The control of the altitude can be made in a way similar to what has been described with reference to Figures 3 and 4, i.e., by setting directive antennas 22 and 23 on the tail 18 of the aircraft 2a (and likewise on the other aircraft 2b, 2c, but also on the aircraft 2d-2f, which, at least in theory, could have further aircraft behind them) or, alternatively, as illustrated in perspective view in Figure 14, by setting further directive antennas on the wing 55 so as to emit a fifth directive wireless signal 80 and a sixth directive wireless signal 81 with a cone that starts from the wing 55 so as to delimit the first low-signal zone 74 vertically in the plane xy, and by setting directive antennas on the wing 65 so as to emit a seventh directive wireless signal 82 and an eighth directive wireless signal 83 with a cone that starts from the wing 65 so as to delimit the second low-signal zone 76 vertically in the plane xy.

Figure 15 shows a further embodiment of the present invention, which is less burdensome from an economic standpoint and also in computational terms, as an alternative to the embodiment illustrated in Figure 14. Tn this case (Figure 15), each aircraft 2a-2c (only the -29 -aircraft 2a is illustrated in the figure) comprises, as an alternative to the fourth and fifth antennas 22 and 23, a first whip antenna 101 and a second whip antenna 102, each of which is omnidirectional with high gain (for example, higher than or equal to 10 dBi), which are arranged at the tail. Figure 15 shows an advantageous mode of installation of the whip antennas 101, 102, arranged at the empennage (in particular on vertical surfaces of the empennage) in aircraft provided with said vertical structures that extend in opposite directions of the tail plane parallel to the plane xy. Other arrangements of the whip antennas 101, 102 can in any case be envisaged for generic aircraft, having different structures of the empennage. In particular, the whip antennas 101, 102 are arranged at the aircraft 2a and configured so as to emit a respective omnidirectional signal 110, 112 only in the horizontal plane xz, and are instead highly directive in the vertical plane xy (for example, with an aperture of just 30° or less). Said configuration is particularly suited to the control of flight altitude. In this case, the radiation geometries of the whip antennas 101, 102 can be represented schematically as disks (in the figure the signals 110, 112 are T 15 represented sideways on, hence only the thickness of the "disks" in the plane xy can be appreciated). The whip antennas 101, 102 are preferably set pointing in opposite directions so that the first whip antenna 101 points upwards, whereas the second whip antenna 102 points downwards so that they form an angle (evaluated from the rear, i.e., from the standpoint of the aircraft that follows) comprised between 210° and 240°, preferably 220°.

2 0 In use, between the two radiation disks generated by the whip antennas 101, 102 a low-signal angular sector 104 is created, which has a width of approximately 180° in a horizontal direction (and the plane of symmetry of which is parallel, ideally coinciding, with the plane xz in which the aircraft proceeds) and a width, in a vertical direction, of a value selectable between a few degrees (ideally 0°) and 30°, preferably 10° (depending upon distance that is to be kept on average between the aircraft and upon the tolerance on the observance of the altitude that it is desired to achieve).

Practical reasons suggest not reducing the aperture of the angular sector 104 to a value lower than 5° to prevent continuous adjustments on the horizontal rudders from occurring on account of inevitable aerodynamic fluctuations of the angle of pitch of the aircraft (pitching phenomenon). In theory, the altitude would be controllable within just ±3 m at a distance of 30 m and within not more than ±9 m at a distance of 100 m. In practice, the performance depends to a large extend upon the quality of the aerodynamic design of the -30 -aircraft 2a and upon the accuracy in the control of the servo-actuators.

Said first and second whip antennas 101, 102 are unlikely to be able to replace the first, second, third, and fourth lateral-control directive antennas 50, 51, 58, 59 of Figure 11; in fact, the restricted aperture would create excessively thin signal "walls" that could be easily passed right through during the time of latency of monitoring of the wireless signals.

With reference to the embodiment of Figures 10 and 11 and Figure 13 or Figure 14, the 1 0 procedures of take-off and constitution of the formation 1, as likewise the operations of turn due to a change of direction of flight of the leading aircraft 2a, are similar to the ones described with reference to Figures 7-9, with the difference that in this case each aircraft 2b-2f sets itself within the low-signal zone 74, 76 assigned to it during creation of the tree network and not within a radiation cone. -15

In order to enable sudden variations of course and position, such as to follow immediately (\J the variations of the wireless signal and prevent collisions due to delayed or excessively slow changes of course, it is preferable for the aircraft 2a-2f to be aircraft of small size (for example, with a wing span of between 4 and 5 metres), agile, and able to change their 2 0 course rapidly. In this way, the aircraft of the formation 1 can fly at a small distance from one another, even just 20 metres and with mean values of around 3 0-40 metres. Aircraft of larger size and less agile can be used. However, it is advisable for said aircraft to be kept at greater average distances apart, preferably greater than 30-50 m. On the other hand, small aircraft, with a wing span not larger than 3 metres, albeit able to transport the weight of the devices, would probably encounter difficulties of an aerodynamic nature on account of the encumbrance of the directive antennas 29-33; said aerodynamic difficulties would become negligible in the case where helicopters, or deltaplanes with pilot, or large drones are used.

Small aircraft 2a-2f are, however, preferable to reduce the running costs (consumption) and the costs of loss in the case where they are shot down, in addition to reducing the risks for civilian population in the case where one of said aircraft 2a-2f crashes in inhabited areas.

-31 -The overall weight of the electronics mounted on board each aircraft 2a-2f, in a minimal configuration that excludes devices for acquisition of video data or other sensors, may be estimated at approximately 3 kg, of which 1 kg is the overall weight estimated for the antennas and 0.5 kg is the weight of a battery pack, for example, a lithium-ion battery pack, capable of supplying the system described for at least 30 minutes. Said minimal equipment could be installed on amateur model aeroplanes capable of supporting a payload of even 4 kg.

Passing now to a professional implementation, hence abandoning the technical and 1 0 legislative restrictions of the amateur field on dimensions, power of the engines, range of action, and purposes of use, it is obviously possible to obtain payloads of even several tens of kilograms, and the weight of the electronics and of the sensor system would not constitute a problem. The payload of a mission would obviously benefit from the cumulative effect of all the individual aircraft making up of the formation. -15

From an examination of the characteristics of the invention described and illustrated (\J herein, the advantages that it affords are evident.

In particular, the present invention enables control of a flight formation of unmanned 2 0 aircraft without the need to transfer information and data of flight or course between all the aircraft belonging to one and the same formation. This characteristic renders the logic for controlling the course particularly simple and reliable.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the sphere of protection of the present invention, as defined in the annexed claims.

For example, the local wireless network 17 can be of a type different from what has been described, for example, based upon the IEEE 802.11 g standard or IEEE 802.11 a standard, or upon a mixed use of IEEE 802.1 lb/g/a or yet another standard for wireless networks.

Each aircraft can emit further directive wireless signals in directions different from the ones illustrated, for example laterally.

-32 -Furthermore, it is possible to provide on each aircraft 2a-2f (in a way not illustrated) a plurality of wireless-signal receiver modules (similar to the transmitter/receiver module 5), each of which is tuned on a specific frequency and configured for receiving only one signal among the directive wireless signals 29-33 and the omnidirectional signal SNET. In this way, the latency in acquiring the power of the directive wireless signals 29-3 3 would drop from 3-4 seconds (as previously indicated) to just 0.5 seconds. The advantages of this embodiment are: the possibility of a further reduction of the average lateral distance between the aircraft 2b-2f (less than 30 m, for example, 20 m); faster turns; and movements due to adjustments of course about a mean position of equilibrium limited in amplitude and more frequent so as to render each aircraft 2a-2f less vulnerable from the ground.

Finally, each aircraft 2a-2f can have a different payload according to the missions, but can T 15 also be a standalone radio relay system, independent, that is, of the configuration of the devices or sensors described previously. (\J a)

Claims (32)

1. -33 -CLAIMS1. A method for controlling a flight formation formed by at least a first aerial vehicle and a second aerial vehicle, comprising the steps of: -emitting, by the first aerial vehicle, at least a first electromagnetic signal of a Wi-Fi type; -detecting, by the second aerial vehicle, the first electromagnetic signal; -determining, by the second aerial vehicle, a respective value of a quantity associated to the first electromagnetic signal; and -determining, by the second aerial vehicle, information associated to a relative position of the second aerial vehicle with respect to the first aerial vehicle on the basis of the value of said quantity associated to the first electromagnetic signal.
2. 2. The method according to Claim 1, wherein said quantity associated to the first electromagnetic signal is a power. -1 5
3. 3. The method according to Claim 1 or Claim 2, further comprising the step of controlling a C\J variation of flight co-ordinates of the second aerial vehicle on the basis of said information associated to the relative position of the second aerial vehicle with respect to the first aerial vehicle.
4. 4. The method according to any one of the preceding claims, wherein the step of emitting a first electromagnetic signal comprises emitting a signal of a directive type defining a first spatial region in which said quantity is higher than a first threshold value, and a second spatial region, external to the first spatial region, in which said quantity is lower than the first threshold value, and the step of determining information associated to a relative position of the second aerial vehicle with respect to the first aerial vehicle (2a) comprises detecting whether the second aerial vehicle is within the first spatial region or the second spatial region.
5. 5. The method according to Claim 4, wherein the step of determining information associated to a relative position of the second aerial vehicle with respect to the first aerial vehicle comprises evaluating a lateral deviation of the second aerial vehicle with respect to the first aerial vehicle.

   -34 -
6. 6. The method according to any one of the preceding claims, wherein the step of detecting, by the second aerial vehicle, the first electromagnetic signal comprises detecting a frequency of transmission of the first electromagnetic signal and/or an identification code carried by the first electromagnetic signal.
7. 7. The method according to any one of Claims 1-3, wherein the first electromagnetic signal is an electromagnetic signal of an omnidirectional type.
8. 8. The method according to any one of Claims 1-5, further comprising the steps of: -emitting, by the first aerial vehicle, a second electromagnetic signal of an omnidirectional Wi-Fi type; -detecting, by the second aerial vehicle, the second electromagnetic signal; -determining, by the second aerial vehicle, a respective value of said quantity associated to the second electromagnetic signal; and T1 5 -determining, by the second aerial vehicle, a relative distance of the second aerial vehicle with respect to the first aerial vehicle on the basis of the value of said quantity associated to C\J the second electromagnetic signal.
9. 9. The method according to Claims 6 and 8, wherein the step of detecting, by the second aerial vehicle, the second electromagnetic signal comprises detecting a frequency of transmission of the second electromagnetic signal, different from the frequency of transmission of the first electromagnetic signal, and/or an identification code carried by the second electromagnetic signal, different from the identification code carried by the first electromagnetic signal.
10. 10. The method according to Claim 8 or Claim 9, further comprising the steps of: -emitting, by the first aerial vehicle, a third electromagnetic signal of a directive Wi-Fi type defining a third spatial region in which said quantity is higher than a second threshold value, and defining a fourth spatial region, external to the third spatial region, in which said quantity is lower than the second threshold value; and -emitting, by the first aerial vehicle, a fourth electromagnetic signal of a directive type defining a fifth spatial region in which said quantity is higher than a third threshold value, and defining a sixth spatial region, external to the fifth spatial region, in which said quantity -35 -is lower than the third threshold value, said first, third, and fifth spatial regions having respective axes of symmetry belonging to a first plane.
11. 11. The method according to Claim 10, further comprising the steps of: -emitting, by the first aerial vehicle, a fifth electromagnetic signal of a directive type defining a seventh spatial region in which said quantity is higher than a fourth threshold value, and moreover defining an eighth spatial region, external to the seventh spatial region, in which said quantity is lower than the fourth threshold value; and -emitting, by the first aerial vehicle, a sixth electromagnetic signal of a directive type defining a ninth spatial region in which said quantity is higher than a fifth threshold value, and moreover defining a tenth spatial region, external to the ninth spatial region, in which said quantity is lower than the fifth threshold value, said first, seventh, and ninth spatial regions having respective axes of symmetry belonging to r"i 5 a second plane transverse to the first plane. r

    C\J
12. 12. The method according to Claim 11, wherein the step of determining information associated to a relative position of the second aerial vehicle with respect to the first aerial vehicle ftirther comprises the steps of: -detecting, by the second aerial vehicle, the third, fourth, fifth, and sixth electromagnetic signals; -calculating, by the second aerial vehicle, respective values of said quantity associated to the third, fifth, seventh, and ninth spatial regions; and -associating the values just calculated to a position of said second aerial vehicle with respect to the third, fourth, fifth, and sixth spatial regions.
13. 13. The method according to Claim 11 or Claim 12, wherein the step of controlling a variation of flight co-ordinates of the second aerial vehicle comprises controlling a flight disposition of the second aerial vehicle in a corridor area delimited between at least two spatial regions chosen from among the first, third, fifth, seventh, and ninth spatial regions.
14. 14. The method according to Claim 12, wherein the step of controlling a variation of flight co-ordinates of the second aerial vehicle comprises controlling a flight disposition of the -36 -second aerial vehicle within a spatial region chosen from among the first, third, fifth, seventh, and ninth spatial regions.
15. 15. The method according to any one of the preceding claims, wherein said flight formation is moreover formed by a third aerial vehicle, further comprising the steps of: -emitting, by the second aerial vehicle, a seventh electromagnetic signal of an omnidirectional Wi-Fi type; -detecting, by the third aerial vehicle, the seventh electromagnetic signal; -emitting, by the third aerial vehicle, an eighth electromagnetic signal of an omnidirectional 1 0 Wi-Fi type; -detecting, by the second aerial vehicle, the eighth electromagnetic signal; -determining respective values of said quantity associated to the seventh, and eighth electromagnetic signals; and -determining information associated to a relative position of the second aerial vehicle with r1 5 respect to the third aerial vehicle and of the third aerial vehicle with respect to the second aerial vehicle. (\J
16. 16. The method according to Claim 15, wherein the step of detecting, by the third aerial vehicle, the seventh electromagnetic signal comprises detecting a frequency of transmission of the seventh electromagnetic signal and/or an identification code carried by the seventh electromagnetic signal, and the step of detecting, by the second aerial vehicle, the eighth electromagnetic signal comprises detecting a frequency of transmission of the eighth electromagnetic signal and/or an identification code carried by the eighth electromagnetic signal.
17. 17. A system for controlling a flight formation of aerial vehicles formed by a first aerial vehicle and a second aerial vehicle, said system being configured for being arranged on the second aerial vehicle and comprising: -a Wi-Fi receiver device, configured for detecting a first electromagnetic signal generated by a first signal-emitting device of a Wi-Fi type arranged on the first aerial vehicle; -means for determining a respective value of a quantity associated to the first electromagnetic signal; -processing means, configured for determining information associated to a relative position -37 -of the second aerial vehicle with respect to the first aerial vehicle on the basis of the value of said quantity associated to the first electromagnetic signal; and -an automatic pilot device, coupled to said processing means and configured for varying flight co-ordinates of the second aerial vehicle on the basis of the information associated to the relative position of the second aerial vehicle with respect to the first aerial vehicle.
18. 18. The system according to Claim 17, wherein said quantity associated to the first electromagnetic signal is a power.
19. 19. The system according to Claim 17 or Claim 18, wherein the first signal-emitting device arranged on the first aerial vehicle is configured for emitting a first electromagnetic signal of an omnidirectional Wi-Fi type.
20. 20. The system according to Claim 17 or Claim 18, wherein the first signal-emitting device T1 5 arranged on the first aerial vehicle is configured for emitting a first electromagnetic signal of a directive Wi-Fi type. (\J

    0. . . .
21. 21. The system according to Claim 20, wherein said processing means comprise a microcontroller configured for calculating a respective value of said quantity associated to the first spatial region and associating the value calculated to a position of said second aerial vehicle with respect to the first spatial region.
22. 22. The system according to any one of Claims 17-21, wherein the receiver device is configured for detecting a frequency of transmission of the first electromagnetic signal and/or an identification code carried by the first electromagnetic signal.
23. 23. The system according to any one of Claims 17-22, wherein the receiver device is moreover configured for detecting a second electromagnetic signal generated by a second signal-emitting device arranged on the first aerial vehicle; said means for determining are moreover configured for determining a respective value of said quantity associated to the second electromagnetic signal; and said processing means are moreover configured for determining information associated to a relative position of the second aerial vehicle with respect to the first aerial vehicle on the basis of the value of said quantity associated to the -38 -second electromagnetic signal.
24. 24. The system according to any one of Claims 17-23, further comprising: -a third signal-emitting device arranged on the first aerial vehicle, configured for emitting a third electromagnetic signal of a Wi-Fi and directive type; and -a fourth signal-emitting device arranged on the first aerial vehicle, configured for emitting a fourth electromagnetic signal of a Wi-Fi and directive type.
25. 25. The system according to Claim 24, further comprising: -a fifth signal-emitting device arranged on the first aerial vehicle, configured for emitting a fifth electromagnetic signal of a Wi-Fi and directive type; and -a sixth signal-emitting device arranged on the first aerial vehicle, configured for emitting a sixth electromagnetic signal of a Wi-Fi and directive type.

    T1 5
26. 26. The system according to Claim 25, wherein the receiver device is moreover configured for detecting the third, fourth, fifth, and sixth electromagnetic signals, and the processing (J means are moreover configured for calculating respective values of said quantity associated to the third, fourth, fifth, and sixth electromagnetic signals and associating the values just calculated to a position of said second aerial vehicle with respect to the third, fourth, fifth, and sixth electromagnetic signals.
27. 27. The system according to Claim 25, further comprising a plurality of receiver devices, each receiver device being configured for detecting just one from among said first, second, third, fourth, fifth, and sixth electromagnetic signals.
28. 28. The system according to Claim 19, wherein the first emitter device is moreover configured for setting up a wireless data network with other emitter devices that is based upon the protocol IEEE 802.11.
29. 29. The system according to, further comprising a data-acquisition sensor, configured for acquiring images and/or films and/or meteorological data and/or avionic data and/or data on the position of targets flown over.

    -39 -
30. 30. The system according to Claim 28, further comprising a memory connected to the data-acquisition sensor.
31. 31. The system according to any one of Claims 17-30, further comprising a communication device, configured for communicating with a base station located at a distance from the flight formation.
32. 32. The system according to any one of Claims 17-3 0, further comprising an orientation device configured for indicating the magnetic North and/or geographical North. (\J a)