Relative location system for platforms in a moving swarm of platforms

Abstract

System for relative localization of platforms of a swarm of moving platforms System for relative localization of platforms of 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 - means for engaging (50) cooperation with specifically a second platform equipped with another beacon of the system, - an antenna system (60) for transmitting and receiving radar waves, - means for responding (70) to a wave received by the antenna system by active transmission 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 transmission of a radar wave by the antenna system (60), identified for the purpose of localization of the platforms of the swarm,as the second platform. Abstract figure: 2B,

Description

Title of the invention: System for the relative location of platforms in 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 the cohesion of the swarm. Particular interest is, for an aircraft, in measuring the position of each of the other aircraft in the swarm. This is particularly useful in a situation where GNSS (Geolocation and Navigation by a Satellite System) type radio navigation systems are unavailable, due to jamming of communications or an alteration of any kind to the service in the area where the swarm is moving. In such a situation, an autonomous means of relative location in conditions of jamming of the radio navigation means must be available for the movement of the swarm of aircraft.

[0002] 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 satisfactory spacing between the various pairs of platforms so as to maintain cohesion, i.e., maintain the inter-distances between platforms below a certain limit, for example 1000 m, separation, i.e., maintain the inter-distances between platforms above a certain limit, for example 10 meters, to avoid collisions and satisfy possible operational objectives of the swarm flight, and alignment insofar as it is sought, 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 light and are therefore likely to drift quite quickly. Therefore, their accuracy is not compatible with the minimum distance (up to 10 meters) between two platforms in the state of swarm cohesion. Therefore, an autonomous metric-class location system is required in distance when the platforms are close to each other.

[0004] Furthermore, we are in a swarm organization without master platform and neither slave platform. Each platform must have its own capacity to direct itself without relying on the others, and this is linked in particular to the fact that certain platforms may be forced to leave the swarm, following a breakdown or destruction.

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

[0006] Each aircraft constitutes a reference frame 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 made by the two members of the pair, and merge the measurements obtained on 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 mono-pulse angular measurement for the angular measurements and large instantaneous angular coverage to avoid scanning and / or beamforming solutions, to detect the skin echo of the platforms is discussed in Figure 1, for, as an example, an average transmitted power of 1W, for example using a CW or FMCW continuous wave form, an operation in X-band (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 equivalent surface SER of another platform is at least 0.1 m2, we obtain the signal-to-noise ratio curve SNR after applying a suitable filter (on the ordinate, from 0 to 100) as a function of the distance d between platforms (on the abscissa, from 0 to 1000 m) in the figure: the value decreases from 90 to 10 between 0 and 1000 m. To obtain detection conditions (Pj> Pja) and precision compatible with operational use, in practice we seek a signal-to-noise ratio greater than 20, i.e. greater than +13 dB. This condition is validated, 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 of the order of a few tens of meters, the high signal-to-noise ratio makes it possible to obtain precise angular localization by using point reflectors. This solution fulfills the detection function and the relative localization function.However, it does not fulfill the function of data exchange between two platforms. But in addition, it has various weaknesses, among which the fact that at short distance, the platforms are not punctual 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, we are also interested in platforms with very small radar equivalent surface, and this method is therefore not easily applicable to them. Features of the invention and advantages

[0010] 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.

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

[0012] Each of said beacons comprises, in the case of a beacon carried by a first platform

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

[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 purposes of locating the platforms of the swarm, as being the second platform.

[0017] The proposed solution thus uses communication equipment embedded on the platforms. Each communication equipment can operate in several modes. If a platform A seeks to locate a platform B and exchange data with it in a bidirectional manner, - mode 1 is a mode of transmission and reception, as well as processing of a radar waveform, and is therefore qualified as radar mode, platform A detecting with the help of its communication equipment the echo of the responder beacon carried by platform B and not the skin echo of platform B - this is therefore a special radar mode. But it provides for the use, according to radar methods, of means of spatial characterization, to obtain a speed, a distance, angles of location and their derivatives with respect to time. - mode 2 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 later. - and mode 3 a mode of bidirectional exchange of data by the communication equipment communicating with another platform, for the initiation of cooperation, in particular.

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

[0019] Optionally, and advantageously, the system may further comprise the following characteristics: - The means of engaging in cooperation may include • a means of prior selection of the second platform from a list of platforms in the swarm, said list being predefined and stored in the first platform, • timing means and means for transmitting an identifier of the second platform on a reference frequency of the system, to initiate cooperation,

[0020] and the engagement means only emits said identifier after having noted that a reference frequency Fo of the system is not in use by another beacon. More generally, it remains silent if the reference frequency is in use. This allows the positioning or location measurements to be made without disturbance from one another, since they are never simultaneous. - A platform comprising a direction of movement (for example the platform can be a longitudinal cylinder and move 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 cover only a part of it, in which case each of the beacons also comprises a means of transmitting to the second platform the spatial characterizations of other beacons previously stored by the first platform, for communication from one to another within the swarm of said characterizations. This makes it possible to overcome the presence of blind spots around the platform, if it is decided to only carry a limited number of antennas for radar transmission 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 radar wave received and re-emitted after processing 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 for modifying by multiplication by an integer fraction a frequency of the wave received by the antenna system for the purposes of transposition.

[0021] Frequency transposition makes it possible to easily distinguish useful echoes from the noise of 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 comprise, for the purpose of preparing the signal to be transmitted, an amplification means, the response means re-transmitting the received signal after processing said received wave at least by said amplification means. Here too, such amplification makes it possible to easily distinguish useful echoes from the noise of echoes caused by the environment, which are not amplified. The power used by the radar in initial transmission 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 second platforms, of the relative speed of the second platform relative to the first, and of the circular and elevation angles as well as their derivatives with respect to time, of the second platform relative to the first. - Radar background may include a continuous 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 initiating cooperation may be encrypted or signed by cryptographic means of said means of initiating cooperation. - The antenna system may comprise four antennas to be pressed against the outer surface of the body of the platform, 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 all, 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 an embodiment of the structure of the invention.

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

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

[0027] [Fig.6] illustrates a possibility of implementing an antenna of the invention.

[0028] [Fig.7] illustrates an alternative implementation of the invention on the swarm scale.

[0029] [Fig.8] shows one aspect of a variant of the invention, with respect 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 each other. Each platform is provided with 2 to 4 simple antennas plated on its surface on its periphery. Each antenna allows to transmit and receive in a wide angular field and to measure the direction of arrival of the signals.

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

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

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

[0034] Platform B, initially also in mode 3, receives the communication, and recognizes itself in the code that is contained in the communication, for example by its communication equipment, which has a function for this purpose. It comes out of its rest and then prepares to engage in an exchange specifically with 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 do not pay attention to platform A.

[0035] Optionally, the message from platform A to platform B is digitally signed, which prevents malicious intrusions. For this purpose, a specific 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 have all the public keys of the swarm. A technique for erasing the private key is implemented in the platform to be implemented, platform by platform in the event that a particular platform is no longer able to fulfill its function within the swarm. This requires implementing a means of detecting loss of functionality of the platform, such as for example detecting that it responds abnormally to piloting or navigation instructions.

[0036] During a phase 2, after the end of phase 1, platform B proceeds to an acknowledgment of 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 by its communication equipment, by communicating an acknowledgment message by transmitting at the frequency Fo. The duration of the acknowledgment code is of the order of magnitude of a millisecond, like that of sending the code in phase 1.

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

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

[0039] During a phase 3, called the radar phase, after the end 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 platforms as a convention. The communications equipment on platform B has switched to mode 2 and is acting as a responder beacon. The other platforms in the swarm are silent.

[0040] Platform A then emits a sequence which can be a sustained wave or FMCW continuous wave, or pseudo-random sequences modulated in phase or amplitude, or another code presenting 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 the B band.

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

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

[0044] More precisely, the signal received by the receiving antenna of platform B in band B (that of the radar wave) around Fo is transposed in frequency with a known shift AF then is re-emitted 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 transmission.

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

[0048] But this frequency transposition therefore above all allows platform A to discriminate between useful, cooperative echoes returned by platform B and non-cooperative disturbing environmental echoes, similar to clutter, not transposed in frequency.

[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 accurately measured. The transposition value is known to, and used by, all platforms in the swarm.

[0050] An acknowledgment is made at the end of phase 3 in certain variants, but embodiment variants do not provide for an acknowledgment. The radar phase is finished.

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

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

[0053] The measurements make it possible to establish the state vector (D, Vr, C, E, C, È) of platform B 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 E are respectively the angular derivatives of the previous 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.

[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 returned to A.

[0055] In a variant, 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 separable codes at reception for angular localization in two dimensions. At reception, an antenna and a single-channel receiver, by analyzing the mixture of codes received, locate the initial source of emission, 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 on 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 transmission of information can be encrypted by asymmetric cryptography keys, and can also be signed by such keys.

[0059] [Fig.2B] [Fig.2B] accordingly 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 with specifically a second platform equipped with another similar beacon - this means of engaging cooperation carries out phases 1 (on the side of the beacon initiating the process) and 2 (on the side of the beacon responding) of the process described in [Fig.2A]; It may itself comprise an antenna specific to it, or be connected to an antenna of the beacon 40 described later, for example the antennas of the antenna system which is used for the radar sounding of phase 3, and use it for the purposes of phases 1 and 2, and in any state of cause, it emits and listens at a frequency which is conventionally defined for all the platforms of the swarm, and which allows the platforms to inform themselves 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 starting such a cooperation, so as not to clutter the wavelength band in which the radar waves are sent, and disrupt the cooperation, which must result in an unambiguous location. At the end of the cooperation commitment, one beacon has the quality of interrogator, and another beacon of interrogated.

[0061] - An antenna system 60 for transmitting and receiving radar waves, which comprises one or more antennas 61, 62, 63 (but often 4 in number, without this number being imperative) placed against the external face of the body of the platform, each emitting, during phase 3, a powerful wave (a radar wave) specifically towards the outside of the platform, at an angle which 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 vast 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 the same amplifier to convert the electromagnetic waves received into a single electrical wave (a signal) intended to

[0062] a) if it is an echo from an interrogated beacon, 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, constitute the basis of an active response to this other beacon of the beacon system, according to an advantageous characteristic of the present invention - as presented in the point below;

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

[0065] - And spatial characterization means 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 estimated (being therefore confused) by said beacon 40 for the purpose 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 actively responds using 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 a determination, by known methods, of the position (distances, angles) and the speed of the source of the echo. It is carried out by the beacon which has initiated cooperation as an interrogating beacon.

[0066] The beacon 40 may also comprise, in certain variants, means of communicating to the second platform 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] The 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 on the receiving antenna 100 is amplified by a low noise amplifier LNA (low noise amplifier) ​​110 and is then transposed in frequency by multiplication by a multiplexer 120 by a local oscillator signal whose frequency is of 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 transmitting antenna 160 of the 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 the transposition, namely the local oscillator 130. In fact, the effective Doppler beat which 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 of the order of 30 Hz. Such stability is equivalent to a relative stability of the order of 106 for an offset of 30 MHz.

[0073] In a variant, the signal is retransmitted simultaneously 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 taken which makes it possible to cancel the effect of the frequency drift. of the local oscillator 130. In fact, the frequency bias on - AF cancels out with that on + AF.

[0074] In a variant, these two retransmissions are made not simultaneously, but alternately, one after the other, with the passage from one to the other made quickly to accumulate measurements with one and with the other under the same conditions of transmission, propagation and reception. This method relieves the transmission device of the platform which carries out the transposition and the retransmission (platform B in the case presented above).

[0075] [Fig.4] [Fig.4] presents a PLL (Phase-locked loop) architecture. phase-locked loop (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 (low noise amplifier) ​​210 and is then divided by a factor kl by a divider 220. The divided signal is applied to a PLL 230 locked loop flip-flop 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 (high power amplifier) ​​250 and applied to the transmitting antenna 260 of the platform B which converts it into a radio signal.

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

[0078] fqut L End “L

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

[0080] For example, if Fo = 9000 MHz, k, = 90 and k2 = 91, the retransmission will be done on 9100 MHz. This architecture is that it does not include any asynchronous frequency source, therefore 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 by 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 report is now proposed.

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

[0088] In the case of a transponder whose transmitting and receiving antennas have respectively not a unit gain but the gains GbE and Gb, the capture surface of the receiving antenna of the beacon being = Gt Ga Intercepted power being further amplified by a gain amplifier (the gain of the beacon), and the amplified power is not retransmitted isotropically but concentrated in a sector leading to a gain Gb

[0089] The radar equivalent surface SER is therefore the product of the capture surface by the two factors relating to the re-emission process, namely:

[0090] q _ pqgt~

[0091] At constant angular coverage (i.e. constant gain), the SER increases inversely with the square of the frequency.

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

[0093] Qb - x 4 6 1 Q3

[0094] To obtain an equivalent SER 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 SER of 4.6 m2 is obtained.

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

[0096] [Fig.5] An antenna as mentioned in the introduction and plated on the outside of 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 of circular section, and the four antennas 510, 511, 512 and 513 are placed on the same straight section of the body of the platform 500, at 90° next to each other, creating coverage cones which, depending on their exact opening angle - which is more than 90° - meet at a short distance from the platform to define a complete coverage of the space around it, in the plane perpendicular to the body of the platform, assumed to be cylindrical in revolution. Blind spot zones 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 of [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 the center thereof. 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 the center thereof.

[0100] [Fig.7] In certain variants, each platform does not have permanent visibility, in the sense of a possibility of communicating by sending radar waves, with each of the Nl other platforms. In these variants, the N platforms can be located relatively to each other from close to close.

[0101] In [Fig.7], which is a simplified representation in a plane, a platform 600 has two antennas arranged on its two sides, diametrically opposite each other, and which have a central coverage axis which can be horizontal, for example. These two antennas have a coverage cone of approximately 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 spot remaining downwards. The platform 600 is therefore not able to communicate with these platforms 606.But it was found that platform 605, which like 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 platform 600 via platform 605. Thus, all the platforms can be located relative to each other from near to far.

[0102] We now discuss the duration of the exchanges within the swarm so that all the platform positions are known, on the assumption that all the platforms communicate directly two by two, without needing a transfer of information from one to another.

[0103] With the numerical values ​​cited previously the total duration of sequences 1 to 4 is of 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 two-way exchanges is therefore T - n(N - 1)t-

[0106] [Fig.8] [Fig.8] shows a method, put into an alternative embodiment, within the framework of details of the invention, for the purpose of avoiding temporal collisions of interrogations of one platform by another, which it is desired that they are not 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 the platforms in the swarm, for example by a third-party system. From this moment on, the platforms randomly draw a delay before transmitting the first interrogation. The "random" is intended to prevent all the platforms from transmitting at the same time on the same frequency Fo. The range of drawing these delays is sufficiently long so that the probability of collision of two interrogations is lower than a certain threshold taking into account the number of platforms in the swarm, and sufficiently short to provide acceptable reactivity to the system. At initialization, each platform designates a correspondent, namely another platform, randomly. The first platform to transmit initializes the dialogue sequence. If the attempt is unsuccessful, it is relaunched at the swarm level 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] [Fig.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 requests for measuring the geometry of ordered pairs respect a certain time average time between two successful measurements and time hazards are introduced to avoid systematic collisions of requests.

[0110] It uses long-term tracking of the exchanges. This tracking is implemented to be able to have at any time an estimate of the geometry of the swarm so as to be able to control it. A 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). The tracking is in certain variants distributed within the swarm, on the other hand, but all the platforms of the swarm have the 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 test 703 to find out if 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 draw and wait step 704 including 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 test 705 during which the platform examines whether a platform of the swarm is communicating on the frequency FO or if this frequency is free. If the frequency is not free, a wait step 706 of a random time St much smaller than T is carried out, before returning to test 704 which is repeated. When the response of a 704 test is positive, and therefore the frequency F0 is free, then there is a commitment to an interrogation of a platform of the swarm.

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

[0113] At the end of tests 705 or 708 which proves positive, there is therefore an interrogation of a platform of the swarm during a transmission-reception step 709. At the end of this, the process resumes at step 704. Finally, the position of the platforms in the swarm, relative to each other, is known and can be updated regularly. And on this basis, a control of the platforms of the swarm is put in place so that they move in a coherent manner and the swarm retains its properties. Conclusion

[0114] Thus, complete information on the relative geometry of the swarm is made available to an autonomous (with respect to radio navigation means) and decentralized (in each platform) piloting / guidance means. This is done by a series of measurements by radar systems of the relative geometry of a high number of ordered pairs of members of the swarm (A, B) to exploit this information and determine the geometry of the swarm. By using active beacons on the platforms to respond to interrogations, angular scintillation of non-point targets is avoided, and sorting between "useful" objects and parasitic echoes from the environment is facilitated. This makes it possible to work with modest transmission powers to promote the discretion of the system.

Claims

Claims

1. A system for relative location of platforms of 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 - means for engaging (50) cooperation with specifically a second platform equipped with another beacon of the system, - an antenna system (60) for transmitting and receiving radar waves, - 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 spatial characterization (80), relative to the first platform, of a source of a radar echo received by the antenna system (60) after transmitting a radar wave by the antenna system (60), said source being identified by said beacon (40) for the purpose of locating the platforms of the swarm,as the second platform.,

2. System for relative location of the platforms of a swarm according to claim 1, characterized in that the means for engaging (50) a cooperation comprises a means for prior selection of the second platform from a list of platforms of the swarm, timing means and means for transmitting an identifier of the second platform on a reference frequency of the system, to initiate the cooperation, and only transmits said identifier after having noted that a reference frequency of the system is not in use by another beacon.

3. System for relative location of the platforms of a swarm according to claim 1 or claim 2, characterized in that a platform having 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 only covers a part of it, in which case each of the beacons also comprises a means of transmission (90) to the second platform of the spatial characterizations of other beacons previously stored by the first platform, for close communication within the swarm of said characterizations.

4. A system for relative location of the platforms of a swarm according to one of claims 1 to 3, 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 retransmits 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 whose frequency is the difference in frequencies to be applied for the purposes of the transposition and a multiplexer (120) configured to apply said difference in 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 relative location of the platforms of 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) re-emitting the received wave after processing said received wave at least by said amplification means (160, 260).

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

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

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

10. System for relative location of the platforms of 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 pressed against the external surface of the bodywork of the platform, around a longitudinal axis of the platform (500) which is its average axis of movement.

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