FR2707386A1 - Captive drone aerial observation system - Google Patents

Abstract

The invention relates to a system of aerial observation by a captive drone (12) which carries at least one electromagnetic sensor, radar or camera. The system comprises a fixed central point (1) which holds, by means of a cable (3), onto a drone (2) in captive circular flight around the central point (1). The drone (2) comprises at least one wing-shaped aerofoil (4) and an engine (6, 10) and an elevon (5) or stabiliser (9). It carries an active (7) or passive (12) radar antenna or holds taught at least one wire antenna (31, 32). It carries at least one optronic sensor (8) complementary to the antenna. Application to battlefield surveillance, site protection.

Description

AERIAL WATCHING SYSTEM WITH CAPTIVE DRONE
The present invention relates to an aerial observation system, in captive flight animated by a movement around a point of attachment.

The aerial observation means are of the electromagnetic type, such as radars or optical cameras, in the visible or in the invisible.

Terrestrial surveillance and observation systems, such as radars, can be improved in detection capacity, i.e. in efficiency and range, by:
- the antenna elevation,
- the diversity of wavelengths used,
- the relative movement of the antennas,
- optical complementarity.

 Indeed, to detect objects on the ground (vehicles) or flying at very low altitude (planes), it is interesting that the radar antennas are well cleared of the surrounding vegetation, constructions and reliefs, in order to avoid the " soil clutter "which is a consequence of the soil effect.

 Furthermore, the objects to be detected by surveillance radars are increasingly stealthy, and to detect them it is sometimes necessary to use low frequencies for which the radar equivalent surfaces are closely linked to the geometric dimensions of the object and no longer its surface state and surface materials. But sometimes it is more interesting to use high frequencies when the objects of small sizes present peculiarities of surface or form. It is therefore important to be able to change the electromagnetic frequency of observation in significant proportions.

 In addition, to detect and locate certain targets with precision, to examine details, it is advantageous to place oneself at appropriate wavelengths and to carry out appropriate processing on the data. For this, it is necessary to use antennas adapted to these treatments, in particular provided with a relative movement relative to the illuminated object.

 Finally, it is good to be able to cross-check the information provided by two different means of observation in their principles. Thus, a radar antenna makes it possible to detect an object, in motion or stationary, and an optical sensor, such as an infrared camera for example, associated with the radar, makes it possible to determine whether the said object is hot, if its engine is running, which provides good optical complementarity to the radar data.

 Finally, with regard to the vulnerability of radars to aerial attacks, the transmitting and receiving antennas of a radar are usually placed on stationary infrastructures or on sea, land or air carriers. The position of the antenna reveals the position of the infrastructure or of the carrier and - ultimately the destruction of the antenna results in the radar operator being unable to continue operating.

Reducing the vulnerability to observing a radar system requires:
- the dispersion of the radiating elements, and their movement,
- the mobility of the entire radar, and its means
associated optronics,
- operating in passive mode, and reducing the vulnerability to destruction requires:
- the permanent movement
- the use of long wavelengths
The aerial observation system according to the invention provides a solution to the requirements which have been mentioned, and it makes it possible to increase the detection capacity and to reduce the vulnerability for a given radar.

 This system comprises as aerial observation means at least one radar antenna1 flying around a central point to which it is retained by a cable. To fly, the antenna is provided with at least one lift plane and at least one motor, and it is controlled from the central point, which supplies, via the cable, the energy necessary for the flight as well as transmitting signals to the antenna and receiving signals back.

 More precisely, the invention consists of an aerial observation system characterized in that it comprises at least one means of observation by electromagnetic waves, this means being carried by a bearing surface called a drone, in captive flight animated by a circular movement around a central point, on the ground, to which it is retained by a cable.

The invention will be better understood by the more detailed description which follows of an exemplary embodiment and of some of its possible variants, this presentation being supported by the attached figures, which represent:
FIG. 1: overall diagram, seen in space, of an aerial observation system according to the invention, in a first embodiment,
FIG. 2: view in a longitudinal and vertical section of the drone of the previous figure,
FIG. 3: overall diagram of a first variant of the observation system according to the invention,
- Figures 4 and 5: overall diagram of a second variant of the invention, and organization of the corresponding drone,
- Figures 6 and 7: representation of the orientation mode of the drone,
- Figure 8: use of the movement of the antenna to create a Doppler effect.

In order to make the presentation clearer and more precise, the invention will be explained by relying on the example of an air system which carries a radar and an optronic sensor, but this conciseness in no way limits the scope of the invention, which applies to a system in which the drone carries at least one observation means by emission and / or reception of electromagnetic waves, in the radar, visible camera fields,
Infrared or ultraviolet, or even particle detection, for example. The means of observation is active or passive.

 A small aerial vehicle, unmanned and remotely controlled, is called a drone.

 FIG. 1 gives a view in space of an aerial observation system according to the invention. As with all the other figures, the relative scales are not preserved at all, but on the contrary adapted so as to allow a better representation of the drone.

 To solve all of the problems which have been analyzed previously, the system according to the invention comprises a central point 1 to which the drone 2 is retained by a cable 3. The central point 1 is for example a land mobile machine, such as a truck or a tank, and it is advantageously provided with a mast, intended for the launching and the recovery of the drone. To this mast is fixed the retaining cable 3.

 The drone itself comprises the means of observation, using electromagnetic waves in a wide range of wavelengths including visible and infrared light and radar waves, these means being active or passive and driven by a movement at a distance. and around the central point 1, of which the drone is captive.

In a first example shown in Figure 1, this drone includes:
a bearing surface 4, in the shape of a wing section, provided with at least one elevon 5,
a propulsion means 6, which is, for example, an electric blower or a small turbo propeller, depending on the weight and the size of the drone,
- at least a first observation means, for example a radar antenna 7, fixed in a position such that it makes it possible to observe a predetermined area, to which we will return later,
- This first observation means can be supplemented by another observation mode, such as for example at least one optronic sensor 8, television camera or infrared sensor for example.

The radar antenna 7 is preferably oriented towards the outside of the circular path followed by the drone 2, in its captive flight around the central point 1, and it is oriented, as shown in FIG. 2, relative to the bearing surface 4 , so as to monitor a predetermined horizon. Depending on the position, or the distance from the area to be monitored, the field of observation can be defined:
either by varying the altitude of the drone 2, therefore its flight angle relative to the horizontal of the place, the radar antenna 7 being fixed relative to the drone,
- Or by modifying the angle of attachment of the radar antenna 7 relative to the drone, the flight of which is maintained at constant altitude.

 or by means of a vertical scanning antenna, the horizontal scanning being provided by the circular flight movement.

 Cable 3 is a composite element, due to the different functions it provides. It is first of all the wire which connects the drone to the central point and ensures the retaining function. Its strength is not the same in normal flight, at the time of recovery and at launch; which leads to consider a section of wire in material supporting the traction to cover all cases. This cable has a length of the order of 50 to 100 meters, or even up to 300 meters, for a flight altitude of the drone of the order of 50 meters, if the mast of the central point is 15 meters in height.

 However, this cable 3 also has functions of electrical supply and, where appropriate, of antenna and transmission.

 It therefore comprises the electrical conductors necessary to transmit the primary energy for the propulsion to the drone, the source of which in all cases is on the ground, as well as other electrical conductors, at least for the piloting of the drone by its elevon 5 and for that of the cable attachment point 3. With regard to the energy for the radar, the power source for the radar transmission may be on board the drone, and in this case the signals received by the antenna 7 are transmitted to the ground either by coaxial conductor or by optical fiber, which are therefore incorporated in the cable 3. But the power source for the emission can also remain on the ground, and be transmitted to the antenna in flight by various means, including electromagnetic radiation.

 Finally, this cable may include an optical fiber for the routing of information in the uplink and downlink directions.

 This cable can also include, in certain variants, an antenna part as well as a protective coating. The antenna is isolated at its end near the drone and an adaptation device can be added to it.

The cable is protected against icing by its own operating temperature.

 As in any detection system, it is necessary to know well the position of the means which carries out said detection. The position, attitude and movement of the mobile antennas are measured with electromagnetic means, part of which is on the ground and another part on board each of the antennas, if the system includes several antennas.

The vulnerability of these systems is relatively low because:
- the system is mobile.

- the antennas are scattered over a large area,
each has a very small apparent surface and each
is moving,
- the missile seeker in the metric bands and
decametrics are quite imprecise,
The launch and recovery of the drone does not require the support of a means external to the system itself. For launching the drone is either fixed at the end of an arm a few meters, or hung from a mast. It is rotated until it reaches its minimum lift speed. While being retained at the central point, it is then released and departs slowly, gaining altitude and speed.

 For recovery in both cases, the drone is slowly drawn towards the center where a recovery arm previously rotated recovers it.

 The recovery operation can be carried out even in the event of loss of propulsion on the drone at any point of its trajectory because the total energy possessed by the drone is at any time higher than that which is necessary for it to reach the position of recovery.

 FIG. 3 gives an overview of a system according to the invention, presenting several variants. The drone has a bearing surface 4, in wing profile, which is provided with a fin 9 with horizontal stabilizing plane and elevator, and it is propelled by a propeller engine 10. But above all the radar system includes means 11 transmission-reception from the ground, for example on board a cabin or a vehicle preferably located near the central point 1 of the system. The antenna 12 carried by the drone 2 is then nothing more than a deflection lens, to return to the observed area the signals transmitted from the central point to the ground. The connection between the source of radar signals, at 11, and the deflector 12 is by radio, and it is symbolized, in FIG. 3, by arrows.

 Yet another variant of the invention is shown in Figures 4 and 5. It essentially relates to the shape of the antenna, and the shape of the carrying surface is adapted to house, in its thickness, a Yagi antenna. The drone therefore comprises a bearing surface element 4, comparable to those represented in FIGS. 1 and 3, but it is extended by a wider element 13 which carries a stabilizer 9 and a motor 10, as well as an elevon 5. The Yagi antenna is housed inside a third element 14 which extends the first two elements 4 and 13 from the bearing surface.

In FIG. 5, the directors of the Yagi antenna are sketched at 15, which are in the third element 14, and at 16 the dipoles, as well as at 17 the antenna reflector. Electromagnetic absorbents are housed in a compartment 18, adjacent to the reflector 17, and the antenna power is placed at 19, in the external part of the drone, relative to the motor axis 10 - stabilizer 9. It is necessary to effect that this variant of drone is stable during flight, but its elongated wing profile requires stabilization aid, which is provided in part by the stabilizer 9 and in part by weight balancing, the most high stabilize by centrifugal force if they are in the second or third element of the bearing surface.

 A Yagi antenna mounted on board a drone such as that of FIGS. 4 and 5 can also be oriented, in the vertical plane, like the antennas of FIGS. 1 and 3. For this purpose, the cable 3 is fixed under the bearing surface 4 at a variable point, along a radius of the rotating system.

Thus, in FIG. 6, the cable 3 is fixed at a point on the bearing surface 4 relatively close to the center of gravity of the drone, and the latter is balanced in its flight: it flies horizontally. On the other hand, in FIG. 7, the cable 3 is fixed on the "inside" edge 41 of the bearing surface 4 - considered with respect to the circular path - and the drone, subjected to centrifugal force, is in the extension of the cable 3 : The Yagi antenna is inclined relative to the ground.

The angle of inclination a can be modified either by controlling the cable hooking position, or by controlling the flight height of the drone, since it is captive of the central point 1.

 The use of two superimposed Yagi antennas results in the production of two bearing surfaces also superimposed and integral.

 The antennas mounted on drones, as described in the previous figures, are not of the Doppler effect type, nor are the corresponding radars. However, the rotational movement of the drone and the antenna creates a relative movement of the antenna which gives a Doppler effect, as explained in FIG. 8, on which 0 represents the orbit of the drone 2, of which A, B and C are three successive positions of which B is the closest position to the object observed. The antenna 7 or 12 emits at points A and C two lobes which make it possible to define the position P of an object observed.

 When the antenna rotates and observes a stationary object at P, the distance from this point to the antenna varies.

 In FIG. 8, the distance dA observed from position A is greater than the distance dB observed by the same drone from position B, it becomes dC again, equal to what it was in A from position C.

 Thus for a given distance an immobile object at a characteristic Doppler slope.

 This feature can be used to artificially refine the antenna pattern, as is done in synthetic antennas; it can also be used to discriminate moving objects which do not have the same Doppler as fixed objects.

 These treatments have particular characteristics for these radar systems because the flight is circular.

 The invention has been exposed so far based on examples of antennas or reflectors attached to the drone. FIGS. 9 and 11 illustrate two cases of aerial observation systems whose antenna is a radiating strand incorporated in the cable 3.

 In FIG. 9, a single drone 2 drives and tends by centrifugal force a single cable 3 comprising, in addition to the energy-piloting links already mentioned, a metal casing constituting the wire antenna. The antenna is fixed to the end of a wing 4 of the drone which, no longer having to incorporate an antenna with a radiating surface, can take the form of a small plane. The double radiation lobe of this linear antenna is represented in FIG. 10, in which the information processing center in which the analyzes of the received signals are carried out is also shown at 20. This data center 20 is of the same type as those which assist the radars of the preceding variants of the invention, but which have not been shown.

The use of two drones 21 and 22, carrying two strands of wire antennas 31 and 32 captive of a single central point 1, as shown in FIG. 11, is very advantageous because the equivalent antenna is an antenna in V. The characteristics of the V antennas are thus defined in the book "The antennas" by Brault and Piat, ETSF edition:
"The antenna takes its name from its shape. It consists of two antennas having a length equal to one or more wavelengths and forming between them an acute and obtuse angle. The power is supplied at the top of the angle or at the end of one of the sides.

 This angle is not chosen at random but in such a way that the combination of the radiation patterns of the two antennas gives the maximum gain in the bisector plane of the V.

 As the half-wave, whole-wave, double-wave, etc. antennas do not have the same radiation patterns, the opening angle of the V will vary with the length of its sides.

 If we choose the angle of V such that it is twice that of the main lobes of each antenna, and if we feed the two antennas in phase opposition, the combination of radiation in the direction perpendicular to the bisector of the angle produces a reinforcement of the radiation in the direction of the bisector. This therefore gives a gain since there is an increase in radiation in a preferred direction.

 We know that the longer an antenna, the more its directivity increases in the direction of the wire and the smaller the angle formed by the main lobes ".

 This variant of the invention with V-shaped antenna therefore requires either the adaptation of the lengths of antenna strands, or the control of the angle between the two strands, in order to keep good flexibility of use of the system. observation, and being able, as needed, to change the radar frequency or increase the directivity of the antenna.

 In fact, changing the length of an antenna is not the quickest solution. It is preferable to operate with a fixed antenna length, compatible with the wavelength used and with the flight requirements of the two drones, and to adjust the wavelength as well as the opening angle of the V.

 We can easily and quickly change the frequency of an oscillator, therefore the operational wavelength. But if the length of the antenna strands is fixed, it does not necessarily correspond to two different wavelengths, or to their multiples or submultiples: by way of nonlimiting example, an antenna of 103.4 meters corresponds at 10 X (h = wavelength) at 29 MHz and at 11 X at 31.91 MHz. If the wavelength used is located between two integer multiples n1 k and n2k, the V-shaped antenna is optimized by adjusting the opening angle between the two antenna strands. By way of example, in FIG. 11, the two strands form an angle of 31 "between them, which corresponds to a frequency 29 MHz.

 In practice, we observe from the ground what the actual opening of the V is, and we emit in the appropriate frequency by possibly bringing into play an adjustment of adaptation which can be located in the drone. For example, the position of the second drone can be slaved to that of the first drone by an infrared distance measurement automation, which controls the engine of the second drone.

 FIG. 12 shows the advantage of this variant of the invention: the two antenna strands 31 and 32 define a directional lobe, having its origin at the fixed point 1 of the observation system, and having a gain which, by way of example, varies from 3 to 17 dB when the angle between the two strands varies from 90 "to 23".

 The aerial observation system according to the invention is more particularly intended for monitoring battlefields. It is specified by the following claims.

Claims (12)

 1 - Aerial observation system characterized in that it comprises at least one means of observation by electromagnetic waves (7), this means being carried by a bearing surface (4) called a drone (2), in captive flight animated by 'a circular movement around a central point (1), on the ground, to which it is retained by a cable (3).  2 - System according to claim 1, characterized in that the observation means on board the drone (2) is a radar antenna (7).  3 - System according to claim 1, characterized in that the observation means on board the drone (2) is an optronic sensor (8) of the television camera or infrared sensor type.  4 - System according to claim 1, characterized in that the drone (2) is powered by an engine (6,10), propeller or air turbine, supplied with energy and controlled from the central point (1) of the system .  5 - System according to claim 1, characterized in that the drone (2) is provided with at least one control device such as an elevon (5) or a stabilizer (9).  6 - System according to claim 1, characterized in that the drone (2) is attached to the central point (1) by a cable (3) which provides at least the energy supply to the drone (2).  7 - System according to claim 2, characterized in that the radar antenna (7) is active and is connected to the central point (1) of the system by the cable (3) which also transports the radar signals on transmission and at the reception.  8 - System according to claim 2, characterized in that the radar antenna (12) is passive, formed by a radar deflector, joined by radio means to the means (11) of radar transmission-reception located on the ground.  9 - System according to claim 2, characterized in that the antenna is of the wired type and is incorporated in the cable (3) retaining the drone (2), which tends the antenna by centrifugal force.  10 - System according to claim 9, characterized in that it comprises two wire antennas (31. 32). tensioned by two drones (21,22), these two antennas forming a V-shaped antenna.  11 - System according to claim 1, characterized in that the bearing surface comprises at least one portion of wing (4)  12 - System according to claim 11, characterized in that the orientation of the drone (2) relative to the ground plane is obtained by modification of the cable attachment point (3) under the bearing surface (4) of the drone (2 )