System for the relative locating of platforms in a swarm of platforms in motion

Abstract

System for the relative locating of platforms in a swarm of platforms in motion, comprising individual beacons for platforms in motion, each of said beacons comprising, concerning a beacon (40) carried by a first platforma means for engaging (50) cooperation with specifically a second platform equipped with another beacon of the system,an antenna system (60) for emitting and receiving radar waves,a means for responding (70) to a wave received by the antenna system by active emission by the antenna system of a wave based on the received wave,and means for spatial characterization (80), relative to the first platform, of a source of a radar echo received by the antenna system (60) after emission of a radar wave by the antenna system (60), identified for the purposes of locating the platforms of the swarm, as being the second platform.

Description

System for the relative locating of platforms in a swarm of platforms in motion The invention relates to a device for measuring the relative geometry of a swarm of autonomously guided aircraft for the purposes of maintaining the cohesion of the swarm. Interest is in particular focused, for an aircraft, on measuring the position of each of the other aircraft of the swarm. This is notably useful in a situation where radionavigation systems of the GNSS type (Geolocation and Navigation by a Satellite System) are unavailable, due to a jamming of communications or an alteration of any nature of the service in the zone where the swarm is moving. In such a situation, an autonomous means of relative locating under conditions of jamming of the radionavigation means must be available for the movement of the swarm of aircraft.Starting from a state where the platforms do not form a coordinated swarm, the determination of the relative positions of the platforms provides to an automaton the elements necessary for rallying and then for maintaining a state of cohesion in which the swarm is coordinated with a satisfactory spacing between the various pairs of platforms so as to maintain, the cohesion, that is to say maintain the inter-distances between platforms below a certain limit, for example 1000 m, the separation, that is to say maintain the inter-distances between platforms above a certain limit, for example 10 meters, to avoid collisions and satisfy possible operational objectives of swarm flight, and the alignment insofar as it is sought, in the state of cohesion of the swarm, that the latter behave with strong collaboration between platform, all the platforms following approximately parallel trajectories.The platforms are equipped with inertial navigation systems but, for these are light and are therefore likely to drift fairly rapidly. Consequently, their accuracy is not compatible with the minimum distance (down to 10 meters) between two platforms in the state of cohesion of the swarm. A metric-class autonomous locating system in distance is therefore needed when the platforms are close to one another.Moreover, one is placed in an organization of the swarm without master platform and nor slave platforms. Each platform must have its own ability to steer itself without relying on the others, and this is related notably to the fact that certain platforms may be led to leave the swarm, following a failure or a destruction.Guidance or piloting and cohesion-maintaining algorithms are known, and the invention provides a manner of determining the relative positions of the aircraft, using sensors and their use.Each aircraft constitutes a frame of reference in which it seeks to know the position of the other aircraft, by angle and distance values. The aircraft, in pairs, exchange the reciprocal measurements performed by the two members of the pair, and fuse the measurements obtained on either side.Each platform knows its own attitude (including pitch, roll and yaw) by virtue of a simple inertial system.It is therefore desired to locate a reference point of another platform and to exchange bidirectional data without confusing the other platforms of the swarm.The use of small active radars with mono-pulse angular measurement for the angular measurements and with large instantaneous angular coverage to avoid scanning and / or beam-forming solutions, to detect the skin echo of the platforms is discussed in figure 1, for, by way of example, an average emitted power of 1W, for example using a sustained waveform or FMCW continuous wave, operation in X band (around 10 GHz) with a wavelength , a transmit-receive antenna made up of four patches arranged in a square, spaced by approximately a half-wavelength, of gain close to 8 (9 dB), an aggregated noise and losses factor of 6 dB, and an integration time of 40 ms. Assuming that the radar cross section RCS of another platform is at least 0.1 m², the signal-to-noise ratio curve SNR is obtained after application of a matched filter (on the ordinates, from 0 to 100) as a function of the distance d between platforms (on the abscissas, from 0 to 1000 m) of the figure: the value decreases from 90 to 10 between 0 and 1000 m. To obtain detection conditions and accuracy compatible with operational use, in practice a signal-to-noise ratio greater than 20, that is greater than +13 dB, is sought. This condition is validated, in the preceding example, up to a distance of 750 m. When the swarm has reached its cohesion geometry, that is distances between platforms all on the order of a few tens of meters, the high signal-to-noise ratio makes it possible to obtain precise angular locating by the use of point reflectors. This solution fulfills the detection function and the relative locating function. However, it does not fulfill the function of exchanging data between two platforms. But moreover it has various weaknesses, among which the fact that at short distance, the platforms are not point-like and cause angular scintillation, generating significant noise on the measured position, and the fact that the skin echoes of the platforms are sometimes confused with ground echoes. Finally, interest is moreover focused on platforms of very small radar cross section, and this method is consequently not readily applicable to them.It is desired to maintain the cohesion of a swarm of aircraft without using GNSS measurements, and without using optical systems, nor sophisticated inertial navigation means.Faced with these limitations, a system for the relative locating of platforms in a swarm of platforms in motion is proposed, comprising individual beacons for platforms in motion.Each of said beacons comprises, concerning a beacon carried by a first platforma means for engaging cooperation with specifically a second platform equipped with another beacon of the system,an antenna system for emitting and receiving radar waves,a means for responding to a wave received by the antenna system by preparation of a wave based on the received wave and emission of the prepared wave by the antenna system,and means for spatial characterization, relative to the first platform, of a source of a radar echo received by the antenna system after emission of a radar wave by the antenna system, said source being identified by said beacon for the purposes of locating the platforms of the swarm, as being the second platform.The proposed solution thus uses communication equipment carried on board the platforms. Each communication equipment can operate according to several modes. If a platform A seeks to locate a platform B and to exchange data with it bidirectionally,mode 1 is a mode of emitting and receiving, as well as processing a radar waveform, and consequently qualified as radar mode, the platform A detecting with the aid of its communication equipment the responder beacon echo carried by the platform B and not the skin echo of the platform B – this is therefore a special radar mode. But it provides for the use, according to radar methods, of spatial characterization means, to obtain a speed, a distance, locating angles and their derivatives with respect to time.mode 2 an operation as a responder beacon of the communication equipment - this mode is called “transponder” mode or “beacon” mode, the platform operates as a responder beacon and possibly performs a frequency translation, as will be developed further below.and mode 3 a mode of bidirectional data exchange by the communication equipment communicating with another platform, for engaging cooperation, notably.The use of a responder beacon avoids emitting too much power in emission, which would be necessary in the absence of cooperation of the target, and therefore promotes the discretion and the simplicity of the device, facilitates sorting between cooperative and non-cooperative echoes, and limits the target to a geometry and size assimilable to a disk of very small diameter – the target is said to be point-like.Optionally, and advantageously, the system may further comprise the following features:The means for engaging cooperation may comprisea means for prior selection of the second platform in a list of the platforms of the swarm, said list being predefined and stored in the first platform,timing means and means for emitting an identifier of the second platform on a reference frequency of the system, to engage cooperation,and the engagement means emits said identifier only after having established that a reference frequency F0 of the system is not being used by another beacon. More generally, it remains silent if the reference frequency is used. This allows the positioning or locating measurements to be performed without disturbance from one another, since they are never simultaneous.A platform comprising a direction of movement (for example the platform may be a longitudinal cylinder and move along its axis of revolution), the antenna system may cover all the directions of the circumference of a section of the platform transversely to the direction of movement, or conversely cover only a part thereof, in which case each of the beacons also comprises a means for transmitting to the second platform the spatial characterizations of other beacons previously stored by the first platform, for a step-by-step communication within the swarm of said characterizations. This makes it possible to overcome the presence of blind angles around the platform, if it is decided to carry on board only a limited number of antennas for radar emission and reception.The response means may comprise, for the purposes of preparing the wave to be emitted in response, a frequency transposition means applicable to the received radar wave and reemits the received wave after processing of said received wave at least by said frequency transposition means, said frequency transposition means comprisinga local oscillator for generating a wave of which a frequency is the difference of frequencies to be applied for the purposes of the transposition and a multiplexer configured to apply said difference of frequencies,or a fractional phase-locked loop system for modifying by multiplication by an integer fraction a frequency of the wave received by the antenna system for the purposes of the transposition.The frequency transposition makes it possible to easily distinguish the useful echoes from the noise of echoes caused by the environment, which are not frequency-transposed. The power used by the radar in initial emission may then be lower.The response means may comprise, for the purposes of preparing the wave to be emitted, an amplification means, the response means reemitting the received wave after processing of said received wave at least by said amplification means. Here too, such amplification makes it possible to easily distinguish the useful echoes from the noise of echoes caused by the environment, which are not amplified. The power used by the radar in initial emission may then be lower.The spatial characterization, relative to the first platform, of said source, identified as being the second platform, may comprise the determination of a distance value between the first and the second platform, of the relative speed of the second platform with respect to the first, and of the circular and elevation angles as well as their derivatives with respect to time, of the second platform with respect to the first.the radar wave may comprise a sustained wave or a noise code.The antenna system may be made up of antennas for emission and reception in complementary angular fields, said antennas being monopulse or / and MIMO.information exchanged for the purposes of engaging cooperation may be encrypted or signed by cryptographic means of said means for engaging cooperation.The antenna system may comprise four antennas to be placed against the outer surface of the bodywork of the platform, around a longitudinal axis of the platform which is its average axis of movementThe spatial characterization determined by the platform may be broadcast to the entire swarm in a broadcast communication mode (to all, without discrimination).Figure 1 shows the signal-to-noise ratio as a function of the distance for an active radar solution.Figure 2A shows an embodiment of the phases of implementing the invention.Figure 2B shows an embodiment of the structure of the invention.Figures 3 and 4 each show an alternative embodiment of certain aspects of the invention, in this case the means for responding to radar probing, in a beacon in transponder mode.Figure 5 shows an embodiment of the invention, in cross section.Figure 6 illustrates a possibility of implementing an antenna of the invention.Figure 7 illustrates an alternative implementation of the invention at the scale of the swarm.Figure 8 shows an aspect of an alternative of the invention, with regard to engaging cooperation between two beacons.The system is decentralized, without master nor slaves, the platforms being the pairs of one another. Each platform is provided with 2 to 4 simple antennas placed on its surface on its periphery. Each antenna makes it possible to emit and receive in a wide angular field and to measure the direction of arrival of the signals.The sequence of operations is the following, and is represented in figure 2A. On the upper part of figure 2A, the signals emitted and received by the platform A have been represented, and on the lower part of figure 2A, the signals emitted and received by the platform B have been represented. The axis of the abscissas represents time, and the two axes of the ordinates present the emission or reception frequencies used, with a reference indicated for the frequency F0 (for example 9000 MHz).During a phase 1, the communication equipment of the platform A is in mode 3 (see above), and the platform A decides to engage a method for locating the platform B. The communication equipment of the platform A then emits at the frequency , reference frequency of the system, a digital code, identifying the platform A in a list of platforms known to all the platforms of the swarm, and containing all the platforms of the swarm. The code moreover designates the target platform B with the aid of its identifier in the list shared by all the platforms. The duration of the code is on the order of magnitude of the millisecond.Apart from the platform A, all the platforms are at rest, including at this moment the platform B. The platforms at rest are in mode 3 and listen at the frequency with the aid of a receiver connected to the “Sum” channel of the receiving antenna. This receiver is configured in a mode compatible with receiving a data transmission.The platform B, initially in mode 3 as well, receives the communication, and recognizes itself in the code which is contained in the communication, for example by its communication equipment, which has a function for this purpose. It exits its rest and then prepares to engage an exchange specifically with the platform A. Conversely, the other platforms of the swarm, which have also received the communication, have not recognized themselves in the code, and consequently ignore the communication and pay no attention to the platform A.Optionally, the message from the platform A to the platform B is digitally signed, which makes it possible to avoid malicious intrusions. For this purpose a private key of its own is carried on board each platform and the message from the platform A to the platform B is signed using known asymmetric techniques of digital signature and authentication. All the platforms have the set of public keys of the swarm. A technique for erasing the private key is implemented in each platform to be implemented, platform by platform in the case where a particular platform would no longer be able to fulfill its function within the swarm. This requires implementing a means for detecting loss of functionality of the platform, such as for example detecting that it responds abnormally to the piloting or navigation setpoints.During a phase 2, after the end of phase 1, the platform B proceeds with an acknowledgment of the message. The communication equipment of the platform A is still in mode 3. The platform B, still in mode 3 as well, having recognized the code addressed to it, acknowledges the request by its communication equipment, by communicating an acknowledgment message by emitting at the frequency F0. The duration of the acknowledgment code is on the order of magnitude of the millisecond, like that of the sending of the code in phase 1.Optionally, this acknowledgment message may be signed as in phase 1. Optionally also, this acknowledgment phase may be omitted – the acknowledgment is considered implicit, for example after a predefined delay defined by convention.The other platforms of the swarm, which were in mode 3 as well, observe, on the frequency F0, phases 1 and 2 identifying the platforms A and B as initiating in exchange (and therefore not identifying any of the other platforms with regard to this exchange). The other platforms therefore understand that a communication is in the process of being established by two platforms, and in reaction to this, these other platforms switch into a silence mode, to allow the exchange to take place in the best conditions.During a phase 3, called radar phase, after the end of phases 1 and 2, the communication equipment of the platform A has now switched to mode 1, following reception of the acknowledgment and / or after a predefined delay known to all the platforms as convention. The communication equipment of the platform B has for its part switched to mode 2 and behaves as a responder beacon. The other platforms of the swarm are in silence.The platform A then emits a sequence which may be a sustained wave or FMCW continuous wave, or pseudo-random sequences modulated in phase or in amplitude, or another code having a well-marked autocorrelation peak, on the frequencies of a useful band on the order of about thirty MHz around the frequency .The platform B, having captured and recognized this sequence, emits in response in the band .It does so with a delay. The delay introduced by the transponder due to its latency is small and well calibrated.The platform B emits around a frequency offset with respect to , (the offset may be positive or negative and its order of magnitude is in one embodiment a few tens of MHz).More precisely, the signal received by the receiving antenna of the platform B in the band (that of the radar wave) around is frequency-transposed with a known offset then is reemitted by the emitting antenna after amplification. The platform B therefore acts as a transponder.The platform A captures and recognizes the waves emitted by the platform B. It then measures distance, Doppler speed and angle parameters.So as to separate the signals returned by the platform B from echoes returned passively by the environment, the platform is interested only in echoes on the transposed frequency, and not in echoes on the frequency that it initially used for emission.This transposition also makes it possible, in addition to promoting the distinction between natural echoes and active echoes from the transponder, to overcome coupling problems in the transponder beacon B between reception and emission, which could lead to its self-oscillation.But this frequency transposition therefore above all allows the platform A to discriminate the useful, cooperative echoes, returned by the platform B, from the disturbing non-cooperative environmental echoes, assimilable to clutter, not frequency-transposed.The offset must be generated in a precise and stable manner over time so that the Doppler effect on the returned and transposed echo can be precisely measured. The transposition value is known to all the platforms of the swarm, and used by them.An acknowledgment is performed at the end of phase 3 in certain alternatives, but embodiments do not provide for an acknowledgment. The radar phase is ended.The platform A performs calculations taking into account the delay in the response of the platforms.The band must be sufficiently wide to obtain adequate distance resolution, that is to say with metric-class accuracy. Typically, the band is on the order of 30 MHz, which offers a resolution of 5 m.The measurements make it possible to establish the state vector of the platform B seen from the platform A. In this notation, and are respectively the circular and elevation angles of the platform B in the reference frame of the platform A and and are respectively the angular derivatives of the preceding angles. D is the distance between A and B and Vr is the relative speed of B with respect to A, or of A with respect to B.In a first alternative, there is measurement of the direction of the emitter of the platform A by the receiver of B with the aid of a monopulse antenna of the platform B and return of this direction measured on board B to A.In an alternative, the colored emission technique (or “MIMO”, multiple input multiple output, or multiple inputs, multiple outputs in French) is used with the emission of at least three codes separable at reception for angular locating in two dimensions. At reception, an antenna and a single-channel receiver, by analyzing the mixture of received codes, locate the initial source of emission, namely the platform A, with respect to the platform B.In a third alternative, the two preceding alternatives are combined to increase the accuracy: a measurement is implemented with the colored emission signals plus a monopulse measurement at reception.During a phase 4, after the end of phase 3, the platform A broadcasts to the entire swarm the position of the platform B that it has measured.This transmission of information may be encrypted by asymmetric cryptography keys, and may also be signed by such keys.Figure 2B consequently shows the structures used in the invention. A beacon 40 installed on a platform is composed of the following elements:A means 50 for engaging cooperation with specifically a second platform equipped with another similar beacon – this means for engaging cooperation performs phases 1 (on the side of the beacon engaging the process) and 2 (on the side of the responding beacon) of the process described in figure 2A; It may itself comprise an antenna that is specific to it, or be connected to an antenna of the beacon 40 described further below, for example the antennas of the antenna system that is used for the radar probing of phase 3, and use it for the purposes of phases 1 and 2, and in any event, it emits and listens at a frequency that is defined conventionally for all the platforms of the swarm, and that allows the platforms to be informed of the fact that a pair of platforms is in the process, or not, of carrying out an exchange (a cooperation) in which case, the other platforms must wait before initiating such cooperation, so as not to congest the wavelength band in which the radar waves are sent, and disturb the cooperation, which must result in an ambiguity-free locating. At the end of the cooperation engagement, one beacon has the quality of interrogator, and another beacon of interrogated.An antenna system 60 for emitting and receiving radar waves, which comprises one or more antennas 61, 62, 63 (but often numbering 4, without this number being imperative) placed against the outer face of the bodywork of the platform, each emitting, during phase 3, a powerful wave (a radar wave) toward specifically the exterior of the platform, in an angle that may be, in the plane transverse to the direction of movement of the platform, 120°; the fact of having several antennas in the antenna system 60 on the different faces of the platform makes it possible to probe a wide angular space, or even the angular space of the entire circumference of the platform; in this case, the antennas are for example connected to the same source of electrical waves to form the same electromagnetic wave to be emitted in different directions and to a same amplifier to convert the received electromagnetic waves into a single electrical wave (a signal) intended toa) if it is an echo coming from an interrogated beacon, be analyzed, so that the beacon determines the position and the speed of the echo according to known radar methods orb) if it is a probing coming from an interrogating beacon, constitute the basis of an active response to this other beacon of the system of beacons, according to an advantageous feature of the present invention – as presented in the point below;The beacon therefore also comprises a means 70 for responding to a wave received by the antenna system 60 by preparation of a wave based on the received signal and emission by the antenna system 60 of this prepared wave– this prepared wave is advantageously frequency-transposed, and it is emitted by the antenna system 60 without delay after reception of the received wave; This preparation of a wave for responding is performed if the beacon had engaged cooperation with another beacon in the position of interrogated beacon. Two main architectures are mentioned further below for the response means 70, in connection with figures 3 and 4.And means 80 for spatial characterization, relative to the first platform, of a source of a radar echo received by the antenna system 60 after emission of a radar wave by the antenna system 60, said source being estimated (therefore being confused) by said beacon 40 for the purposes of locating the platforms of the swarm, as being the second platform – this assimilation of the source to the second platform is justified insofar as the second platform responds actively with the aid of an amplified wave, such a reaction of the environment not being possible without the presence of a beacon of the system. The spatial characterization is for its part a determination, by known methods, of the position (distances, angles) and of the speed of the source of the echo. It is conducted by the beacon that engaged the cooperation in the quality of interrogating beacon.The beacon 40 may also comprise, in certain alternatives, means for communicating to the second platform the last known spatial characterizations of one or more platforms of the swarm, in particular the spatial characterization of the second platform which was determined during the cooperation.The cooperation ends after this communication of one or more spatial characterizations.A first functional architecture for the response means 70 is represented in figure 3: it is a transposition architecture.The radioelectric signal received on the receiving antenna 100 is amplified by a low-noise amplifier LNA (low noise amplifier) 110 then is frequency-transposed by multiplication by a multiplexer 120 by a local oscillator signal the frequency of which is on the order of 50 MHz, produced by the local oscillator 130. Then the multiplexed signal is filtered by a band selection filter 140, which eliminates a lower band or an upper band. The filtered signal is then amplified by a high-power amplifier HPA (high power amplifier) 150 and applied, once amplified, to the emitting antenna 160 of the platform B which converts it into a radioelectric signal.The accuracy of the Doppler measurement is linked to the accuracy of the source of frequency difference serving for the transposition, namely the local oscillator 130. Indeed, the effective Doppler beat that is measured on board the platform A is: A Doppler speed error less than 0.5 m / s is sought, that is a frequency stability on the order of 30 Hz. Such stability is equivalent to a relative stability on the order of 10-6 for an offset of 30 MHz.In an alternative, the signal is reemitted simultaneously with two transpositions, one at and the other at and the Doppler effect is measured on the two transmitted signal. An average is then taken which makes it possible to cancel the effect of the frequency drift of the local oscillator 130. Indeed, the frequency bias on cancels with that on .In an alternative these two reemissions are made not simultaneously, but alternately, one after the other, with the passage from one to the other being made rapidly to accumulate measurements with one and with the other under the same emission, propagation and reception conditions. This method relieves the emission device of the platform which performs the transposition and the reemission (the platform B in the case presented above).Figure 4 presents a PLL (Phase-locked loop or phase-locked loop or phase-locked loop) architecture of the fractional PLL type.The radioelectric signal received on the receiving antenna 200 is amplified by a low-noise amplifier LNA (low noise amplifier) 210 then is divided by a factor k1 by a divider 220. The divided signal is applied to a PLL loop latch 230 centered on the frequency FIN / k1, and the signal thus processed by the loop, is multiplied by a factor k2 by a multiplier 240. Then me multiplied signal is amplified by a high-power amplifier HPA (high power amplifier) 250 and applied to the emitting antenna 260 of the platform B which converts it a radioelectric signal.This architecture consists in reemitting a signal of which the output frequency FOUT is linked to the input frequency FIN by a proportionality relationship expressed in the form  With and two coprime and neighboring integers.For example, if , et , the reemission will be performed on 9100 MHz. This architecture is that it comprises no asynchronous frequency source, therefore no frequency drift on the Doppler measurement w linked to a local oscillator.The Doppler effect on the path from the platform A to the platform B is FAB = FAB = . This effect is multiplied by in the transposition.The Doppler effect on the path from the platform B to the platform A is furthermore FBA = The total Doppler effect is therefore the sum of the two components thus evoked: In an alternative, a four-antenna solution is used.To facilitate the appreciation of the interest of the presented embodiments, a radar balance is now proposed.An object of radar cross section SER is defined by the fact that the flux of the incident wave, assumed plane, is intercepted by a surface , and the intercepted power corresponding to the product of the equivalent surface and of the flux is reemitted isotropically throughout space, that is over steradians for an antenna of unit gain.In the case of a transponder of which the emitting and receiving antennas respectively have not a unit gain but the gains and , the capture surface of the receiving antenna of the beacon being , the intercepted power being moreover amplified by an amplifier of gain (the gain of the beacon), and the amplified power is not reemitted isotropically but concentrated in a sector leading to a gain .The radar cross section SER is therefore the product of the capture surface by the two factors relating to the reemission process, that is: At constant covered angular domain (that is to say at constant gain), the SER increases in inverse proportion to the square of the frequency.With the value mentioned in the introduction by way of example for the antenna gains, namely a gain of dB (, and furthermore, a transmission in band, X one obtains: To obtain an equivalent RCS of 1 m², the amplification must therefore have a gain for the beacon of . In the case of an amplification gain an equivalent SER of 4.6 m² is obtained.The geometry and the positioning of the antennas are now discussed.An antenna such as mentioned in the introduction and placed on the exterior of the structure of a platform has at most a hemispherical coverage, and rather in practice a conical coverage of a vertex angle close to 120°.One embodiment consequently uses two antennas and other embodiments use a higher number of antennas.An interesting solution uses four antennas arranged as represented in figure 5: the platform 500 has a cylindrical body of circular section, and the four antennas 510, 511, 512 and 513 are placed on a same straight section of the body of the platform 500, at 90° next to one another, creating coverage cones which, depending on their exact aperture angle – which is more than 90° - meet at short distance from the platform to define complete coverage of the space around the latter, in the plane perpendicular to the body of the platform, assumed cylindrical of revolution. Blind angle zones exist at very close proximity to the platform and are visible in the figure but have no effect on the operation of the invention.Each of the four antennas of figure 5 has a radiation pattern as represented in figure 6 in a plane perpendicular to the normal to the plane of the antenna, taken at the center thereof. The radiation intensities are homogeneous in the entire 360° sector of the antenna, and are higher near the normal to the plane of the antenna, taken at the center thereof.In certain alternatives, each platform does not permanently have visibility, in the sense of a possibility of communicating by the sending of radar waves, with each of the N-1 other platforms. In these alternatives, the N platforms can be located relative to one another step by step.In figure 7, which is a simplified representation in a plane, a platform 600 has two antennas arranged on its two flanks, diametrically opposite one another, and which have a central coverage axis that may be horizontal, for example. These two antennas have a coverage cone of about 90°. The left antenna, in the figure, sees two platforms 601 and 602 of the swarm in its cone, and the right antenna, in the figure, sees three platforms 603, 604 and 605 in its cone, but on the other hand, there are five platforms 606 outside each of these two coverage cones, in this case at a lower altitude than the platform 600 and more precisely in a blind angle subsisting downward. The platform 600 is therefore not able to communicate with these platforms 606. But it noted that the platform 605, which like the platform 600 has two antennas oriented with a horizontal central coverage axis manages to communicate with each of the platforms 606, the platforms 606 being in the cone of one of the antennas. The coordinates of the platforms 606 are then communicated to the platform 600 through the intermediary of the platform 605. Thus, all the platforms can be located relative to one another step by step.The duration of the exchanges within the swarm is now discussed so that all the positions of platforms are known, in the hypothesis where all the platforms communicate directly two by two, without needing a step-by-step transfer of information.With the numerical values cited previously the total duration of sequences 1 to 4 is on the order of τ = 50 ms.Each platform has a label. At the instant a platform is a platform of label “A” and designates a platform of label “B”, at , the platform “B” becomes platform “ A’ ” and the platform “A” becomes platform “ B’ ”, at , , the platform “A’ ” becomes platform “ A’’ ” and designates a platform “ B’’ ”, and so on according to a logic aimed at measuring the distances between pairs of platforms as regularly as possible.The total duration of the bidirectional exchanges is therefore .Figure 8 shows a method, implemented in an embodiment alternative, within the framework of details of the invention, for the purposes of avoiding temporal collisions of interrogations of a platform by another, which it is desired that they not be simultaneous, while ensuring an exchange between a pair of platforms, on average, at a time interval T.A one-off initialization signal is sent to all the platforms of the swarm, for example by a third-party system. Starting from this instant the platforms randomly draw a delay before emitting the first interrogation. The “randomness” is intended to avoid all the platforms emitting at the same time on the same frequency. The range of drawing of these delays is sufficiently long for the probability of collision of two interrogations to be less than a certain threshold taking into account the number of platforms of the swarm, and sufficiently short to provide acceptable reactivity to the system. At initialization, each platform designates a correspondent, namely another platform, in a random manner. The first platform to emit initializes the dialogue sequence. In the case where the attempt is unsuccessful, it is relaunched at the scale of the swarm until a platform in reception acknowledges an interrogation. Each platform must listen to the frequency to verify that no emission is in progress before attempting an emission.Figure 8 shows a process implemented in an embodiment alternative, to avoid temporal collisions of platform interrogations while ensuring an exchange between a pair of platforms, on average at a time interval T.The requests for measuring the geometry of the ordered pairs respect a certain average time between two successful measurements and temporal randomnesses are introduced to avoid systematic collisions of requests.It uses tracking over a long period of the exchanges. This tracking is implemented to be able to have at any instant an estimation of the geometry of the swarm so as to be able to pilot it. A possible solution is to track the state vector of a platform B in each of the ordered pairs of platforms (A, B). The tracking is in certain alternatives distributed within the swarm, in contrast, but all the platforms of the swarm have the tracked state vectors.The process begins with an initialization 701, followed by a random drawing and waiting step 702 comprising a random drawing of a delay between 0 and T and a wait of this random value . Then there is a test 703 to know whether there has already been an exchange between two platforms. This is often the case, but at the beginning of the process, there has not yet been an exchange and the answer is no. If as is often the case, an exchange has already taken place, then there is a random drawing and waiting step 704 comprising a random drawing of a delay between 0 and 2T this time and a wait of this random value . Then there is a test 705 during which the platform examines whether a platform of the swarm is in the process of communicating on the frequency F0 or whether this frequency is free. If the frequency is not free, a step of waiting 706 for a random time much smaller than T is performed, before returning to the test 704 which is repeated. When the answer of a test 704 is positive, and therefore the frequency F0 is free, then, there is engagement of an interrogation of a platform of the swarm.If during the test 703, it is established that no exchange has been engaged previously, a random drawing 707 of a platform of the swarm is carried out, then during a test 708 it is verified that the frequency is free. If it is not free, then one proceeds to step 704 and to those that follow it. If it is free, one proceeds to the engagement of an interrogation of a platform of the swarm.At the end of the tests 705 or 708 which proves positive, there is therefore engagement of an interrogation of a platform of the swarm during an emission-reception step 709. At the end of the latter, the process resumes at step 704. Finally, the position of the platforms in the swarm, relative to one another, is known and can be updated regularly. And on this basis, a piloting of the platforms of the swarm is put in place so that they move coherently and so that the swarm preserves its properties.Thus, complete information on the relative geometry of the swarm is made available to an autonomous (with respect to radionavigation means) and decentralized (in each platform) piloting / guidance means. This is done by a series of measurements by the radar systems of the relative geometry of a high number of ordered pairs of members of the swarm (A, B) for by exploiting this information, determining the geometry of the swarm. By using active beacons on the platforms to respond to the interrogations, angular scintillation of non-point targets is avoided, and sorting between “useful” objects and parasitic echoes of the environment is facilitated. This makes it possible to work with modest emission powers to promote the discretion of the system.

Claims

1. System for the relative locating of platforms in a swarm of platforms in motion, comprising individual beacons for platforms in motion, each of said beacons comprising, concerning a beacon (40) carried by a first platform- a means for engaging (50) cooperation with specifically a second platform equipped with another beacon of the system,- an antenna system (60) for emitting and receiving radar waves,- a means for responding (70) to a wave received by the antenna system by preparation of a wave based on the received wave and emission of the prepared wave by the antenna system (60),- and means for spatial characterization (80), relative to the first platform, of a source of a radar echo received by the antenna system (60) after emission of a radar wave by the antenna system (60), said source being identified by said beacon (40) for the purposes of locating the platforms of the swarm, as being the second platform.

2. System for the relative locating of platforms in a swarm according to claim 1, characterized in that the means for engaging (50) cooperation comprises a means for prior selection of the second platform in a list of the platforms of the swarm, timing means and means for emitting an identifier of the second platform on a reference frequency of the system, to engage cooperation, and emits said identifier only after having established that a reference frequency of the system is not being used by another beacon.

3. System for the relative locating of platforms in a swarm according to claim 1 or claim 2, characterized in that a platform comprising a direction of movement, the antenna system (60) covers all the directions of the circumference of a section of the platform transversely to the direction of movement, or conversely covers only a part thereof, in which case each of the beacons also comprises a means for transmitting (90) to the second platform the spatial characterizations of other beacons previously stored by the first platform, for a step-by-step communication within the swarm of said characterizations. 4. System for the relative locating of platforms in a swarm according to one of claims 1 to 3, characterized in that the response means (70) comprises, for the purposes of preparing the wave to be emitted, a frequency transposition means applicable to the received radar wave and reemits the received wave after processing of said received wave at least by said frequency transposition means, said frequency transposition means comprising a local oscillator (130) for generating a wave of which a frequency is the difference of frequencies to be applied for the purposes of the transposition and a multiplexer (120) configured apply said difference of frequencies, or a fractional phase-locked loop system (220, 230, 240) for modifying by multiplication by an integer fraction a frequency of the wave received by the antenna system for the purposes of the transposition.

5. System for the relative locating of platforms in a swarm according to one of claims 1 to 4, characterized in that the response means (70) comprises, for the purposes of preparing the wave to be emitted, an amplification means (160, 260), the response means (70) reemitting the received wave after processing of said received wave at least by said amplification means (160, 260).

6. System for the relative locating of platforms in a swarm according to one of claims 1 to 5, characterized in that the spatial characterization (80), relative to the first platform, of said source, identified as being the second platform, comprises the determination of a distance value between the first and the second platforms, of the relative speed of the second platform with respect to the first, and of the circular and elevation angles as well as their derivatives with respect to time, of the second platform with respect to the first.

7. System for the relative locating of platforms in a swarm according to one of claims 1 to 6, characterized in that the radar wave comprises a sustained wave or a noise code.

8. System for the relative locating of platforms in a swarm according to one of claims 1 to 7, characterized in that the antenna system (60) is made up of antennas (61, 62, 63) for emission and reception in complementary angular fields, said antennas being monopulse or MIMO.

9. System for the relative locating of platforms in a swarm according to one of claims 1 to 8, characterized in that information exchanged for the purposes of engaging cooperation is encrypted or signed by cryptographic means of said means for engaging (50) the cooperation.

10. System for the relative locating of platforms in a swarm according to one of claims 1 to 8, characterized in that the antenna system (60) comprises four antennas (510, 511, 512, 513) to be placed against the outer surface of the bodywork of the platform, around a longitudinal axis of the platform (500) which is its average axis of movement.

Images