Relative location system for the platforms of a moving platform swarm

Abstract

Relative positioning system for the platforms of a moving platform swarm. Relative positioning system for the platforms of a moving platform swarm, comprising individual beacons for moving platforms, each of said beacons comprising, in the case of a beacon (40) carried by a first platform: - a means for engaging (50) in cooperation specifically with a second platform equipped with another beacon of the system, - an antenna system (60) for transmitting and receiving radar waves, - a means for responding (70) to a wave received by the antenna system by actively transmitting by the antenna system a wave based on the received wave, - and means for spatially characterizing (80), relative to the first platform, a source of a radar echo received by the antenna system (60) after transmission of a radar wave by the antenna system (60), identified for the purpose of positioning the platforms of the swarm.as being the second platform. Abbreviation figure: 2B,

Description

Title of the invention: Relative localization system for the platforms of a swarm of moving platforms. Technical context

[0001] The invention relates to a device for measuring the relative geometry of a swarm of autonomously guided aircraft for the purpose of maintaining swarm cohesion. In particular, it is of interest to measure the position of each of the other aircraft in the swarm for a given aircraft. This is especially useful in situations where GNSS (Geolocation and Navigation Satellite System) radio navigation systems are unavailable due to communication jamming or any other type of service disruption in the area where the swarm is moving. In such a situation, an autonomous means of relative positioning under conditions of radio navigation jamming must be available for the movement of the aircraft swarm.

[0002] Starting from a state where the platforms do not form a coordinated swarm, the determination of the relative positions of the platforms provides an automaton with the elements necessary for rallying and then 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 cohesion, i.e. maintain the interdistances between platforms below a certain limit, for example 1000 m, separation, i.e. maintain the interdistances between platforms above a certain limit, for example 10 meters, to avoid collisions and satisfy possible operational objectives of swarm flight, and alignment insofar as we seek, in the state of cohesion of the swarm, that it behaves with strong collaboration between platforms, all the platforms following approximately parallel trajectories.

[0003] The platforms are equipped with inertial navigation systems, but these are lightweight and therefore prone to drift quite rapidly. Consequently, their accuracy is not compatible with the minimum distance (up to 10 meters) between two platforms in the swarm's cohesive state. Therefore, an autonomous localization system with a metric range is required when the platforms are close to each other.

[0004] Furthermore, we are considering a swarm organization without master or slave platforms. Each platform must have its own ability to orient itself without relying on the others, and this is linked in particular to the fact that some platforms may have to leave the swarm following a failure or destruction.

[0005] Algorithms for guidance or piloting and maintaining cohesion are known, and the invention provides a way to determine the relative positions of aircraft, using sensors and their use.

[0006] Each aircraft constitutes a reference frame in which it seeks to determine the position of other aircraft, using angle and distance values. The aircraft, in pairs, exchange the reciprocal measurements taken by the two members of the pair, and merge the measurements obtained from both sides.

[0007] Each platform knows its own attitude (including trim, heel and yaw) thanks to a simple inertial system.

[0008] We therefore wish to locate a reference point of another platform and exchange bidirectional data without confusing the other platforms of the swarm.

[0009] [Fig-1] The use of small active radars with single-pulse angular measurement for the angular and large instantaneous angular coverage measurements to avoid scanning and / or beamforming solutions, to detect the skin echo of platforms is discussed in Figure 1, for, by way of example, an average emitted power of 1W, for example using a continuous or continuous FMCW waveform, X-band operation (around 10 GHz) with a wavelength a = 3 cm, a transmit-receive antenna consisting of four patches arranged in a square, spaced about half a wavelength apart, with a gain close to 8 (9 dB), an aggregate noise and loss 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 (SNR) curve after applying a suitable filter (ordinate, from 0 to 100) is obtained as a function of the distance d between platforms (abscissa, from 0 to 1000 m) as shown in the figure: the value decreases from 90 to 10 between 0 and 1000 m. To obtain detection conditions (Pj > Pja) and accuracy compatible with operational use, a signal-to-noise ratio greater than 20, or greater than +13 dB, is sought in practice. This condition is met, in the previous example, up to a distance of 750 m. When the swarm has reached its cohesion geometry, i.e., distances between platforms all on the order of a few tens of meters, the high signal-to-noise ratio allows for precise angular localization using point reflectors. This solution fulfills both the detection function and the relative localization function.However, it does not fulfill the function of exchanging data between two platforms. Furthermore, it has several weaknesses, including the fact that at short distances, the platforms are not point-like and cause angular scintillation, generating significant noise in the measured position, and the fact that skin echoes from the platforms are sometimes confused with ground echoes. Finally, the focus is on platforms with very small radar cross-sections, and this method is therefore not easily applicable to them. Features of the invention and advantages

[0010] We wish to maintain the cohesion of a swarm of aircraft without using GNSS measurements, and without using optical systems, or sophisticated inertial navigation means.

[0011] In response to these limitations, a system for the relative localization of the platforms of a swarm of moving platforms is proposed, comprising individual beacons for moving platforms.

[0012] Each of said tags includes, in the case of a tag carried by a first platform

[0013] - a means of engaging in cooperation specifically with a second platform equipped with another beacon of the system,

[0014] - an antenna system for transmitting and receiving radar waves,

[0015] - a means of responding to a wave received by the antenna system by preparation of a wave based on the received wave and emission of the wave prepared by the antenna system,

[0016] - and means of spatial characterization, relative to the first platform, from 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 purpose of locating the swarm platforms, as being the second platform.

[0017] The proposed solution thus uses communication equipment embedded on the platforms. Each communication device can operate in several modes. If a platform A seeks to locate a platform B and exchange data with it bidirectionally, Mode 1 is a mode for transmitting, receiving, and processing a radar waveform, and is therefore called a radar mode. Platform A detects, using its communication equipment, the echo of the responding beacon carried by platform B, and not the skin echo of platform B. This is thus a special radar mode. However, it involves the use, according to radar methods, of spatial characterization tools to obtain velocity, distance, location angles, and their derivatives with respect to time. - mode 2 an operation in responding beacon of the communication equipment - this mode is called "transponder" mode or "beacon" mode, the platform operates in responding beacon and possibly performs a frequency translation, as will be developed later. - and mode 3 a mode of bidirectional data exchange by the communication equipment communicating with another platform, for the engagement of cooperation, in particular.

[0018] The use of a responding beacon avoids emitting too much power in transmission, which would be necessary in the absence of cooperation from the target, thus promoting the discretion and simplicity of the device, facilitates the sorting between cooperative and non-cooperative echoes, and limits the target to a geometry and size comparable to a disk of very small diameter - the target is said to be a point.

[0019] Optionally, and advantageously, the system may further include the following features: - The means of engaging in cooperation may include • a means of pre-selecting the second platform from a list of platforms in the swarm, said list being predefined and stored in the first platform, • time delay mechanisms and means for transmitting an identifier from the second platform on a system reference frequency, to initiate cooperation,

[0020] and the engagement means transmits said identifier only after determining that a reference frequency Fo of the system is not being used by another beacon. More generally, it remains silent if the reference frequency is in use. This allows positioning or location measurements to be carried out without interfering with each other, since they are never simultaneous. - For a platform with a direction of movement (for example, the platform could be a longitudinal cylinder moving along its axis of revolution), the antenna system can cover all directions of the circumference of a section of the platform transverse to the direction of movement, or conversely, only a portion of it. In the latter case, each beacon also includes a means of transmitting to the second platform the spatial characteristics of other beacons previously stored by the first platform, for step-by-step communication within the swarm of said characteristics. This makes it possible to overcome the presence of blind spots around the platform if it is decided to carry only a limited number of antennas for radar transmission and reception. - The response means may include, for the purpose of preparing the response wave, a frequency transposition means applicable to the received radar wave and re-emits the received wave after processing of said received wave at least by said frequency transposition means, said frequency transposition means comprising • a local oscillator to generate a wave whose frequency is the difference in frequencies to be applied for the purposes of transposition and a multiplexer configured to apply said frequency difference, • or a fractional phase-locked loop system to modify, by multiplication by an integer fraction, a frequency of the wave received by the antenna system for the purpose of transposition.

[0021] Frequency transposition makes it easy to distinguish useful echoes from noise echoes caused by the environment, which are not frequency transposed. The power used by the radar in initial transmission can then be lower. The response means may include, for the purpose of preparing the Fonde signal for transmission, an amplification means, the response means re-emitting the received Fonde signal after processing the received wave at least by said amplification means. Here too, such amplification makes it easy to distinguish useful echoes from echo noise caused by the environment, which is not amplified. The power used by the radar in initial transmission can then be lower. - The spatial characterization, relative to the first platform, of said source, identified as the second platform, may include the determination of a distance value between the first and second platforms, the relative velocity of the second platform with respect to the first, and the circular and elevation angles and their derivatives with respect to time, of the second platform with respect to the first. - Radar background may include a sustained wave or a noise code. - The antenna system may consist of antennas for transmission and reception in complementary angular fields, said antennas being monopulse and / or MIMO. - information exchanged for the purpose of engaging in cooperation may be encrypted or signed by cryptographic means of said means of engaging in cooperation. - The antenna system may include four antennas to be attached to the outer surface of the platform body, around a longitudinal axis of the platform which is its average axis of movement - The spatial characterization determined by the platform can be broadcast to the entire swarm in a broadcast communication mode (to everyone, without discrimination). List of figures

[0022] Fig. 1 shows the signal-to-noise ratio as a function of distance for an active radar solution.

[0023] Fig. 2A shows an embodiment of the implementation phases of the invention.

[0024] Fig. 2B shows one embodiment of the structure of the invention.

[0025] Figures 3 and 4 each show a variant embodiment of certain aspects of the invention, in this case the means of responding to a radar sounding, in a transponder mode beacon.

[0026] Figure 5 shows an embodiment of the invention in cross-section.

[0027] Figure 6 illustrates one possible implementation of an antenna of the invention.

[0028] Figure 7 illustrates a variant implementation of the invention on the swarm scale.

[0029] Fig. 8 shows an aspect of a variant of the invention, with regard to the engagement of cooperation between two beacons. Detailed description

[0030] [Fig. 2A] The system is decentralized, without master or slaves, the platforms being pairs of one another. Each platform is equipped with 2 to 4 simple antennas attached to its surface around its perimeter. Each antenna allows transmission and reception within a wide angular field and measurement of the direction of arrival of the signals.

[0031] The sequence of operations is as follows, and is represented in [Fig.2A]. In the upper part of [Fig.2A], the signals emitted and received by platform A are shown, and in the lower part of [Fig.2A], the signals emitted and received by platform B are shown. The x-axis represents time, and the two y-axes show the transmission or reception frequencies used, with a reference indicated for the frequency Fo (for example 9000 MHz).

[0032] During phase 1, the communication equipment of platform A is in mode 3 (see above), and platform A decides to initiate a process for locating platform B. The communication equipment of platform A then transmits, at frequency F{), the system reference frequency, a numerical code identifying platform A in a list of platforms known to all platforms in the swarm, and containing all the platforms in the swarm. The code also designates the target platform B using its identifier in the list shared by all platforms. The duration of the code is on the order of milliseconds.

[0033] Apart from platform A, all platforms are at rest, including platform B at this time. The platforms at rest are in mode 3 and are listening at frequency Fo using a receiver connected to the "Sum" channel of the receiving antenna. This receiver is configured in a mode compatible with receiving a data transmission.

[0034] Platform B, initially also in mode 3, receives the communication and recognizes itself in the code contained therein, for example, through its communication equipment, which has a function for this purpose. It wakes up from its resting state and then prepares to initiate an exchange specifically with platform A. Conversely, the other platforms in the swarm, which have also received the communication, have not recognized themselves in the code and therefore ignore the communication and pay no attention to platform A.

[0035] Optionally, the message from platform A to platform B is digitally signed, which helps prevent malicious intrusions. For this purpose, a unique private key is embedded in each platform, and the message from platform A to platform B is signed using known asymmetric digital signature and authentication techniques. All platforms possess all the public keys of the swarm. A private key erasure technique is implemented in each platform to be activated platform by platform in the event that a particular platform is no longer able to perform its function within the swarm. This requires implementing a means of detecting platform malfunctions, such as detecting abnormal responses to piloting or navigation commands.

[0036] During phase 2, after the end of phase 1, platform B acknowledges the message. The communication equipment of platform A is still in mode 3. Platform B, also still in mode 3, having recognized the code addressed to it, acknowledges the request via its communication equipment by transmitting an acknowledgment message at frequency Fo. The duration of the acknowledgment code is on the order of milliseconds, like that of the code transmission in phase 1.

[0037] Optionally, this acknowledgment message can be signed as in phase 1. Optionally also, this acknowledgment phase can be omitted - acknowledgment is considered implicit, for example after a predefined delay defined by convention.

[0038] The other platforms in the swarm, which were also in mode 3, observe, on the Fo frequency, phases 1 and 2, identifying platforms A and B as initiating an exchange (and therefore not identifying any of the other platforms regarding this exchange). The other platforms thus understand that communication is being established between two platforms, and in response to this, these other platforms switch to a silent mode, to allow the exchange to proceed under the best conditions.

[0039] During a phase 3, referred to as the radar phase, after the completion of phases 1 and 2, the communication equipment of platform A has now switched to mode 1, following receipt of the acknowledgment and / or after a predefined delay known to all the platforms as a convention. The communication equipment of platform B has switched to mode 2 and is behaving as a responding beacon. The other platforms in the swarm are silent.

[0040] Platform A then emits a sequence which can be a sustained wave or continuous FMCW wave, or pseudo-random sequences modulated in phase or amplitude, or another code exhibiting a well-marked autocorrelation peak, on the frequencies of a useful band B of the order of thirty MHz around the frequency F{).

[0041] Platform B, having captured and recognized this sequence, transmits in response in band B.

[0042] It does so with a delay. The delay introduced by the transponder due to its latency is small and well calibrated.

[0043] Platform B emits around a frequency offset from Fo, Fo-AF (the AF offset can be positive or negative and its order of magnitude is in one embodiment of a few tens of MHz).

[0044] More specifically, the signal received by the receiving antenna of platform B in the B band (that of the radar wave) around Fo is frequency transposed with a known AF shift and then retransmitted by the transmitting antenna after amplification. Platform B therefore acts as a transponder.

[0045] Platform A captures and recognizes the waves emitted by platform B. It then measures parameters of distance, Doppler speed and angles.

[0046] In order to separate the signals returned by platform B from the echoes returned passively by the environment, the platform is only interested in the echoes on the transposed frequency, and not in the echoes on the frequency that it initially used for the transmission.

[0047] This transposition also makes it possible, in addition to facilitating the distinction between natural echoes and active echoes of the transponder, to overcome the coupling problems in the transponder beacon B between reception and transmission, which could lead to its self-oscillation.

[0048] But this frequency transposition allows platform A to discriminate useful, cooperative echoes returned by platform B from disruptive, non-cooperative environmental echoes, which are like clutter and not frequency transposed.

[0049] The AF shift must be generated accurately and stably over time so that the Doppler effect on the returned and transposed echo can be precisely measured. The transposition value is known to all platforms in the swarm and used by them.

[0050] An acknowledgement is made at the end of phase 3 in some variants, but some embodiments do not provide for an acknowledgement. The radar phase is complete.

[0051] Platform A performs calculations taking into account the delay in the response of the platforms.

[0052] The B band must be sufficiently wide to obtain adequate distance resolution, i.e., with metric-class accuracy. Typically, the B band is on the order of 30 MHz, which provides a resolution of 5 m.

[0053] The measurements allow us to establish the state vector (D, Vr, C, E, C, È) of platform B as seen from platform A. In this notation, C and E are respectively the circular and elevation angles of platform B in the frame of reference of platform A, and C and È are respectively the angular derivatives of the preceding angles. D is the distance between A and B, and Vr is the relative velocity of B with respect to A, or of A with respect to B.

[0054] In a first variant, the direction of the transmitter of platform A is measured by the receiver of B using a monopulse antenna of platform B and this direction measured on board B is sent back to A.

[0055] In one embodiment, the colored transmission technique (or "MIMO," multiple input multiple output) is used, with the transmission of at least three separable codes at the receiver for two-dimensional angular localization. At the receiver, an antenna and a single-channel receiver, by analyzing the mixture of received codes, locate the initial transmission source, namely platform A, relative to platform B.

[0056] In a third variant, the two previous variants are combined to increase precision: a measurement is implemented with the colored emission signals plus a monopulse measurement at reception.

[0057] During a phase 4, after the end of phase 3, platform A broadcasts to the entire swarm the position of platform B that it has measured.

[0058] This information transmission can be encrypted by asymmetric cryptography keys, and can also be signed by such keys.

[0059] [Fig. 2B] Figure 2B shows the structures used in the invention. A beacon 40 installed on a platform is composed of the following elements:

[0060] - A means of engaging 50 in cooperation specifically with a second platform equipped with another similar beacon - this means of engaging in cooperation carries out phases 1 (on the side of the beacon initiating the process) and 2 (on the side of the responding beacon) of the process described in [Fig. 2A]; It may itself comprise a specific antenna, or be connected to an antenna of the beacon 40 described later, for example the antennas of the antenna system that is used for the radar sounding of phase 3, and use it for the purposes of phases 1 and 2, and in any case Because of this, it transmits and listens at a frequency conventionally defined for all platforms in the swarm, allowing the platforms to determine whether a pair of platforms is currently exchanging signals (cooperating). If not, the other platforms must wait before initiating such cooperation, so as not to clutter the wavelength band in which the radar waves are transmitted and disrupt the cooperation, which must result in an unambiguous location. At the end of the cooperative engagement, one beacon becomes the interrogator, and another the interrogated beacon.

[0061] - An antenna system 60 for transmitting and receiving radar waves, which includes One or more antennas 61, 62, 63 (but often four, although this number is not mandatory) are attached to the outer face of the platform's body, each emitting, during phase 3, a powerful wave (a radar wave) specifically outwards from the platform, at an angle that can be 120° in the plane transverse to the direction of movement of the platform. Having several antennas in the antenna system 60 on the different faces of the platform makes it possible to probe a large angular space, 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 the same amplifier to convert the received electromagnetic waves into a single electrical wave (a signal) intended for

[0062] a) if it is an echo from an interrogated beacon, to be analyzed so that the beacon determines the position and speed of the echo according to known radar methods or

[0063] b) if it is a sounding from an interrogating beacon, to constitute the basis of an active response to that other beacon of the beacon system, according to an advantageous feature of the present invention - as presented in the point below;

[0064] - The beacon therefore also includes a means of responding 70 to a wave received by The antenna system 60 prepares a wave based on the received signal and transmits this prepared wave. This prepared wave is advantageously frequency-transposed and transmitted by the antenna system 60 without delay after receiving the received signal. This wave preparation for replying is performed if the beacon had engaged in cooperation with another beacon in the position of the interrogated beacon. Two main architectures are mentioned later for the reply means 70, in connection with Figures 3 and 4.

[0065] - And means of spatial characterization 80, relative to the first platform, from a source of a radar echo received by antenna system 60 after emission of a radar wave by antenna system 60, said source being estimated (being therefore (confused) by said beacon 40 for the purpose of locating the swarm platforms, as being the second platform – this assimilation of the source to the second platform is justified insofar as the second platform actively responds with an amplified wave, such a reaction from the environment not being possible without the presence of a beacon of the system. Spatial characterization, for its part, is a determination, using known methods, of the position (distances, angles) and velocity of the echo source. It is carried out by the beacon that initiated the cooperation as the interrogating beacon.

[0066] The beacon 40 may also include, in certain variants, means of communication to the second platform of the latest 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.

[0067] Cooperation ends after this communication of one or more spatial characterizations.

[0068] [Fig.3] A first functional architecture for the response means 70 is represented in [Fig.3]: this is a transposition architecture.

[0069] The radio signal received at the receiving antenna 100 is amplified by a low-noise amplifier (LNA) 110 and then frequency-shifted by multiplication by a multiplexer 120 with a local oscillator signal of approximately 50 MHz, produced by the local oscillator 130. The multiplexed signal is then filtered by a band-selector filter 140, which eliminates either a lower or an upper band. The filtered signal is then amplified by a high-power amplifier (HPA) 150 and, once amplified, applied to the transmitting antenna 160 of platform B, which converts it into a radio signal.

[0070] The accuracy of the Doppler measurement is linked to the accuracy of the frequency difference source used for transposition, namely the local oscillator 130. Indeed, the effective Doppler beat that is measured on board platform A is:

[0071] F _ w zAF AF x Doppler path Doppler path Frequency error transposition A to BB to .4

[0072] We are looking for a Doppler velocity error of less than 0.5 m / s, i.e., a frequency stability on the order of 30 Hz. Such stability is equivalent to a relative stability on the order of 10⁶ for a shift of 30 MHz.

[0073] In one embodiment, the signal is simultaneously retransmitted with two transpositions, one at Fo- AF and the other at Fo+ AF, and the Doppler effect is measured on the two transmitted signals. An average is then calculated, which cancels out the effect of frequency drift. of the local oscillator 130. Indeed, the frequency bias on - AF cancels out with that on + AF.

[0074] In one variant, these two retransmissions are performed not simultaneously, but alternately, one after the other, with the switchover from one to the other being done rapidly to accumulate measurements with both under the same transmission, propagation, and reception conditions. This method relieves the transmission device of the platform that performs the transposition and retransmission (platform B in the case presented above).

[0075] [Fig.4] [Fig.4] presents a PLL (Phase-locked loop) architecture. phase-locked or phase-locked loop) of fractional PLL type.

[0076] The radio signal received on the receiving antenna 200 is amplified by a low noise amplifier (LNA) 210 and then divided by a factor kl by a divider 220. The divided signal is applied to a PLL latching loop flip-flop 230 centered on the frequency FIN / kl, and the signal thus processed by the loop is multiplied by a factor k2 by a multiplier 240. Then the multiplied signal is amplified by a high power amplifier (HPA) 250 and applied to the transmitting antenna 260 of platform B which converts it into a radio signal.

[0077] This architecture consists of re-emitting a signal whose output frequency F0Ut is related to the input frequency F1N by a proportionality relationship expressed in the form

[0078] fqut L End “L

[0079] With k, and k2 two relatively prime and neighboring integers.

[0080] For example, if Fo = 9000 MHz, k1 = 90 and k2 = 91, the retransmission will take place on 9100 MHz. This architecture has the advantage that it does not contain any asynchronous frequency source, therefore there is no frequency drift on the Doppler measurement w linked to a local oscillator.

[0081] The Doppler effect on the path from platform A to platform B is FAB = vrrfq . c This effect is multiplied in the transposition.

[0082] The Doppler effect on the path from platform B to platform A is also Fba = c L-

[0083] The total Doppler effect is therefore the sum of the two components mentioned above:

[0084] F _ 9½}½ ^“^AB* k| 2k, c

[0085] In one variant, a four-antenna solution is used.

[0086] To facilitate the appreciation of the interest of the embodiments presented, a radar summary is now proposed.

[0087] A radar equivalent area object SER ° is defined by the fact that the flux ¢ of the incident wave, assumed to be planar, is intercepted by a surface °, and the intercepted power corresponding to the product of the equivalent area and the flux ¢(7 is re-emitted isotropically throughout space, i.e. on steradians for an antenna of unit gain.

[0088] In the case of a transponder whose transmitting and receiving antennas have not a unit gain but rather the gains GbE and Gb respectively, the receiving area of ​​the beacon's antenna being = Gt Ga, the intercepted power is further amplified by a gain amplifier (the beacon's gain), and the amplified power is not re-emitted isotropically but concentrated in a sector leading to a gain of Gb

[0089] The radar cross-section (RCS) is therefore the product of the capture area and the two factors relating to the re-emission process, namely:

[0090] q _ pqgt~

[0091] At constant angular coverage (i.e. at constant gain), the SER grows in inverse proportion to the square of the frequency.

[0092] With the value mentioned in the introduction as an example for antenna gains, namely a gain of 9 dB (q — 0 — 9 ct). Furthermore, a band transmission, X we obtain:

[0093] Qb - x 4 6 1 Q3

[0094] To obtain an equivalent RCS of 1 m², the amplification must therefore have a gain for the beacon of gb = 23.4 dB. In the case of an amplification gain gb = 30 dB, an equivalent RCS of 4.6 m² is obtained.

[0095] We are now discussing the geometry and positioning of the antennas.

[0096] [Fig.5] An antenna as mentioned in the introduction and mounted on the outside the structure of a platform has at most a hemispherical cover, and rather in practice a conical cover with an apex angle close to 120°.

[0097] One embodiment therefore uses two antennas and other embodiments use a higher number of antennas.

[0098] An interesting solution uses four antennas arranged as shown in [Fig. 5]: the platform 500 has a cylindrical body with a circular cross-section, and the four antennas 510, 511, 512 and 513 are placed on the same cross-section of the body of the platform 500, at 90° to each other, creating coverage cones which, depending on their exact opening angle - which is greater than 90° - meet at a short distance from the platform to define complete coverage of the space around it, in the plane perpendicular to the body of the platform, assumed to be cylindrical in revolution. Blind spots exist very close to the platform and are visible in the figure but have no impact on the operation of the invention.

[0099] [Fig. 6] Each of the four antennas in [Fig. 5] has a radiation pattern as shown in [Fig. 6] in a plane perpendicular to the normal to the plane of the antenna, taken at its center. The radiation intensities are homogeneous throughout the 360° sector of the antenna, and are higher near the normal to the plane of the antenna, taken at its center.

[0100] [Fig. 7] In certain variants, each platform does not have permanent visibility, in the sense of being able to communicate by sending radar waves, with each of the other Nl platforms. In these variants, the N platforms can be located relative to each other step by step.

[0101] In [Fig. 7], which is a simplified planar representation, a platform 600 has two antennas arranged on its two sides, diametrically opposed to each other, and which have a central coverage axis that can be horizontal, for example. These two antennas have a coverage cone of approximately 90°. The antenna on the left in the figure sees two platforms 601 and 602 of the swarm within its cone, and the antenna on the right in the figure sees three platforms 603, 604, and 605 within its cone. However, there are five platforms 606 outside each of these two coverage cones, specifically at a lower altitude than platform 600 and more precisely in a remaining downward blind spot. Platform 600 is therefore unable to communicate with these platforms 606.However, he observed that platform 605, which like platform 600 has two antennas oriented with a horizontal central axis of coverage, manages to communicate with each of the 606 platforms, the 606 platforms being within the cone of one of the antennas. The coordinates of the 606 platforms are then communicated to platform 600 via platform 605. Thus, all the platforms can be located relative to each other step by step.

[0102] We now discuss the duration of exchanges within the swarm so that all platform positions are known, assuming that all platforms communicate directly two by two, without needing a step-by-step transfer of information.

[0103] With the numerical values ​​cited above the total duration of sequences 1 to 4 is on the order of r = 50 ms.

[0104] Each platform has a label. At time 1, a platform is a platform with label "A" and designates a platform with label "B". At t + T, platform "B" becomes platform "A'" and platform "A" becomes platform "B'". At t + 2t, platform “A’” becomes platform “A” and designates a platform “B””, and so on following a logic aimed at measuring the distances between pairs of platforms as regularly as possible.

[0105] The total duration of the bidirectional exchanges is therefore T - n(N - 1)t-

[0106] [Fig.8] Fig.8 shows a method, shown in a variant embodiment, within the framework of details of the invention, for the purpose of avoiding temporal collisions of queries of one platform by another, which are not desired to be simultaneous, while ensuring an exchange between a pair of platforms, on average, at a time interval T.

[0107] A one-time initialization signal is sent to all platforms in the swarm, for example, by a third-party system. From that moment, the platforms randomly draw a delay before transmitting the first interrogation. This randomness is intended to prevent all platforms from transmitting simultaneously on the same frequency Fo. The range of these delays is long enough that the probability of two interrogations colliding is below a certain threshold, taking into account the number of platforms in the swarm, and short enough to provide acceptable responsiveness to the system. At initialization, each platform randomly designates a correspondent, namely another platform. The first platform to transmit initiates the dialogue sequence. If the attempt is unsuccessful, it is repeated throughout the swarm until a receiving platform acknowledges an interrogation.Each platform must listen to the Fo frequency to verify that no transmission is in progress before attempting a transmission.

[0108] Figure 8 shows a process implemented in an alternative embodiment, for avoid temporal collisions of platform queries while ensuring an exchange between a pair of platforms, on average at a time interval T.

[0109] The queries for measuring the geometry of ordered pairs respect a certain average time between two successful measurements and temporal randomness is introduced to avoid systematic collisions of queries.

[0110] It uses long-term tracking of exchanges. This tracking is implemented to provide an estimate of the swarm's geometry at any given time, enabling its control. One possible solution is to track the state vector (D, Vr, GE, C, É) of a platform B in each of the ordered pairs of platforms (A, B). In some variants, the tracking is distributed within the swarm, but all platforms in the swarm have tracked state vectors.

[0111] The process begins with an initialization 701, followed by a random draw and wait step 702 comprising a random draw of a delay AT between 0 and T and A wait for this random value AT. Then there is a 703 test to determine if there has already been an exchange between two platforms. This is often the case, but at the start 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 occurred, then there is a 704 random draw and wait step involving a random draw of a delay AT between 0 and 2T this time, and a wait for this random value AT. Then there is a 705 test during which the platform examines whether a platform in the swarm is communicating on the FO frequency or if this frequency is free. If the frequency is not free, a 706 wait step of a random time St much smaller than T is performed, before returning to the 704 test, which is repeated. When the response of a 704 test is positive, and therefore the F0 frequency is free, then there is an engagement of an interrogation of a platform of the swarm.

[0112] If, during test 703, it is found that no exchange has been initiated previously, a platform from the swarm is randomly selected 707, and then, during test 708, it is verified that the frequency F0 is free. If it is not free, step 704 and subsequent steps are then carried out. If it is free, an interrogation of a platform from the swarm is initiated.

[0113] Following a successful test in step 705 or 708, an interrogation of a swarm platform is initiated during a transmission-reception step 709. After this step, the process resumes at step 704. Ultimately, the relative positions of the platforms within the swarm are known and can be updated regularly. Based on this information, a control system is implemented to ensure that the swarm platforms move coherently and that the swarm maintains its properties. Conclusion

[0114] Thus, complete information on the relative geometry of the swarm is made available to an autonomous (independent of radio navigation systems) and decentralized (on each platform) piloting / guidance system. This is achieved through a series of measurements by radar systems of the relative geometry of a large number of ordered pairs of swarm members (A, B). By exploiting this information, the geometry of the swarm is determined. By using active beacons on the platforms to respond to interrogations, angular scintillation from non-point targets is avoided, and the distinction between "useful" objects and spurious echoes from the environment is facilitated. This allows operation with modest transmission powers to promote the system's stealth.

Claims

Demands

1. A relative platform localization system for a swarm of moving platforms, comprising individual beacons for moving platforms, each of said beacons comprising, in the case of a beacon (40) carried by a first platform: - a means for engaging (50) in cooperation specifically with a second platform equipped with another beacon of the system, - an antenna system (60) for transmitting and receiving radar waves, - a means for responding (70) to a wave received by the antenna system by preparing a wave based on the received wave and transmitting the prepared wave by the antenna system (60), - and means for spatially characterizing (80), relative to the first platform, a source of a radar echo received by the antenna system (60) after transmission of a radar wave by the antenna system (60), said source being identified by said beacon (40) for the purpose of localizing the platforms of the swarm,as being the second platform, the means of engaging in cooperation (50) comprising a means of prior selection of the second platform from a list of platforms in the swarm, time-delay means and means of transmitting an identifier of the second platform on a reference frequency of the system, to engage in cooperation, and transmits said identifier only after having determined that a reference frequency of the system is not in use by another beacon.

2. A relative localization system for the platforms of a swarm according to claim 1, characterized in that, for a platform having a direction of movement, the antenna system (60) covers all 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 includes a means for transmitting (90) to the second platform the spatial characterizations of other previously stored beacons. through the first platform, for a step-by-step communication within the swarm of said characterizations.

3. Relative localization system of the platforms of a swarm according to any one of claims 1 or 2, characterized in that the response means (70) comprises, for the purpose of preparing the wave to be transmitted, a frequency transposition means applicable to the received radar wave and re-transmits 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 transposition and a multiplexer (120) configured to 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 transposition.

4. Relative localization system of the platforms of a swarm according to any one of claims 1 to 3, characterized in that the response means (70) comprises, for the purpose of preparing the wave to be emitted, an amplification means (160, 260), the response means (70) re-emitting Fonde received after processing of said received wave at least by said amplification means (160, 260).

5. Relative localization system of the platforms of a swarm according to any one of claims 1 to 4, characterized in that the spatial characterization (80), relative to the first platform, of said source, identified as the second platform, includes the determination of a distance value between the first and second platforms, the relative velocity of the second platform with respect to the first, and the circular and elevation angles and their time derivatives of the second platform with respect to the first.

6. Relative localization system of the platforms of a swarm according to any one of claims 1 to 5, characterized in that the radar wave comprises a sustained wave or a noise code.

7. A relative localization system for the platforms of a swarm according to any one of claims 1 to 6, characterized in that the antenna system (60) consists of antennas (61, 62, 63) for the emission and reception in complementary angular fields, said antennas being monopulse or MIMO.

8. Relative localization system of the platforms of a swarm according to any one of claims 1 to 7, characterized in that information exchanged for the purpose of engaging in cooperation is encrypted or signed by cryptographic means of said means of engaging (50) in cooperation.

9. Relative localization system of the platforms of a swarm according to any one of claims 1 to 7, characterized in that the antenna system (60) comprises four antennas (510, 511, 512, 513) to be pressed against the outer surface of the body of the platform, around a longitudinal axis of the platform (500) which is its mean axis of movement.

Images