METHOD FOR DETECTING II / SI IDENTIFICATION CODE CONFLICTS OF RADAR MODE S AND SURROUNDING RADAR DEVICES, AND SECONDARY RADAR FOR IMPLEMENTING THIS METHOD

Description

[0001] The invention is in the field of air traffic control (ATC) for which radar performance is fundamental, particularly in the detection of aircraft in mode S, with a success rate of around 99% expected.

[0002] Air traffic control is primarily based on Mode S radar, whose detection and decoding reliability is widely recognized. The performance of a Mode S radar is notably linked to the fact that radars are now identified by aircraft using their II / SI (Interrogator Identifier) ​​code. To limit electromagnetic interference and thus increase the reliability of radar transactions, the Mode S radar locks the targets it manages, within its coverage area, to their II / SI identity so that these targets no longer respond to non-selective Mode S interrogations.

[0003] One technical problem to be solved is the following: In the event of overlapping coverage between nearby radars, the radars must have different II / SI identifiers. Otherwise, if two nearby radars share the same II / SI code, neither radar will see the targets that the other radar has locked onto first. This results in a major security vulnerability.

[0004] Prior art has proposed various solutions for detecting aircraft within a radar's coverage area that are not responding to general radar calls because they are locked onto by a nearby radar using the same II / SI code. One such solution is disclosed in document GB201000946. By design, this solution only works with aircraft equipped with ADS-B_out. It requires that the radar be equipped with, or associated with, a local ADS-B_in receiver with which it communicates.

[0005] Another solution is proposed in document GB201316553. It consists of detecting an IL / SI code conflict between nearby sensors by comparing the incoming and outgoing detection ranges of the radar. Indeed, an aircraft exiting the coverage of radar A is managed in selective mode, therefore up to the operational coverage limit of radar A, whereas an aircraft entering the coverage of radar A (coming from the coverage of radar B) is only seen by radar A's station when radar B stops locking onto it. If there is an overlap in coverage between radars A and B, this therefore occurs within radar A's coverage, and thus at a shorter distance than its coverage limit.

[0006] The document "Principles of Mode S Operation and Interrogator Codes" (Eurocontrol) presents the operating principles of Mode S and interrogator codes. The documents "Results of Validation of SSR Mode S Interrogator Identifier Code Coordination" (Koga et al.) and "Blended Secondary Surveillance Radar Solutions to Improve Air Traffic Surveillance" (Kim Euhio et al.) address Code II conflict issues. Document EP3502735, which describes a method for measuring the antenna pattern of a secondary radar, does not disclose a method for detecting Code II / SI conflicts.

[0007] Prior art is therefore limited in its ability to detect areas with code II / SI conflicts: Either by the equipment (ADS-B_out) necessary for aircraft to be seen; Or by the a posteriori observation of the gap between incoming and outgoing coverage; Finally, not proposing a means of circumventing the Il / SI code conflict to detect aircraft. Another standardized approach to using Mode S stations involves coordinating radars using the same IL / SI identifier in a Surveillance Coordination Network (SCN) cluster. This requires a cross-border ground infrastructure for them to voluntarily exchange target positions in their shared areas via a highly reliable ground network. In addition to the drawback of a complex structure, this approach does not eliminate the problem of human error.

[0008] One object of the invention is, in particular, to overcome the problems associated with code sharing between nearby radars (II / SI) without the drawbacks of the prior art. To this end, the invention relates to a method for detecting code conflicts between II / SI radars surrounding a secondary Mode S radar according to claim 1.

[0009] The invention also relates to a radar according to claim 4.

[0010] Other features and advantages of the invention will become apparent from the following description, made with reference to the accompanying drawings which depict: [ Fig.1 ] There figure 1 , an example of a synoptic diagram of a mode S radar according to the prior art; [ Fig. 2 ] There figure 2 , an example of a synoptic diagram of a mode S radar capable of implementing the invention; [ Fig.3 ] There figure 3 , an illustration of the steps for implementing the process according to the invention; [ Fig. 4 ] There figure 4 , an illustration of the operational range and extended range of a secondary radar at a given altitude; Fig. 5 ] There figure 5 , an illustration of the overlapping ranges between two secondary radars; [ Fig. 6a ] There figure 6a an illustration of electromagnetic pollution from a surrounding radar on a reference radar; [ Fig. 6b ] There figure 6bConversely, an illustration of the electromagnetic pollution of the reference radar on the surrounding radar; [ Fig. 7 ] There figure 7 , an illustration of how an overlapping area is divided into several sub-areas based on the responses generated by the targets with the two radars; Fig. 8 ] There figure 8 , a presentation of the responses exchanged by the targets with the R1 and R2 radars in the overlap zone by sub-zones; Fig. 9 ] There figure 9 , a presentation of the R1 radar's code II / SI conflict azimuth zone based on the responses exchanged in the overlap zone by sub-zone; Fig. 10a ] There figure 10a , an illustration of the detection of DF11 plots outside the operational coverage of the reference radar by increasing the duration of the AC periods; Fig. 10b ] There figure 10b, an illustration of the detection of DF11 plots outside the operational coverage of the reference radar in parallel with the following period without increasing the duration of the AC periods; Fig. 11 ] There figure 11 , an illustration of the asynchronicity of fruits due to surrounding radar; Fig. 12 ] There figure 12 , an illustration of target search and pre-localization in an IL / SI code conflict zone by temporal and azimuth analysis of the fruit generated by the surrounding radar; Fig. 13 ] There figure 13 , an illustration of the detection of locked targets without operational disruption to the reference radar; Fig. 14 ] There figure 14 , another illustration of the process according to the invention.

[0011] In relation to the figure 1This diagram represents an example of a Mode S radar schematic, and the principles of such a radar are reviewed. The principle of the secondary Mode S radar (whose interfaces with the transponder are defined in detail by ICAO Annex 10, Vol. 4) consists of: Sending selective queries: either indicating the recipient: a single target designated by its Mode S address; or indicating the sender's identifier; receiving selective replies: • either indicating the sender's identifier: the same Mode S address of the target; • or indicating the recipient: the interrogator's identifier; whose main content depends on the message: ∘ in the acquisition phase (transient at the beginning of the track) ▪ DF11 : Target Mode S address; ∘ in ELS (Elementary Surveillance) ▪ DF4 : altitude; ▪ DF5 : identity (code A); ∘ in EHS (Enhanced Surveillance) ▪ DF20:altitude + BDS register whose number is known, among other things, from the query that requested it; ▪ DF21 : identity (code A) + BDS register whose number is known in part by the query that requested it.

[0012] In its usual use, the secondary radar operates in synchronous mode, that is to say it sends an interrogation and waits for a response consistent with it, which allows it to locate by measurement (in azimuth and range) and to identify (by the Mode S address) the target.

[0013] To perform this task efficiently, the radar is equipped with an antenna 1 ( figure 1 ) having several diagrams 11, 12, 14, 15 whose roles are classically: a sum diagram 11, hereafter noted SUM, to interrogate and detect the synchronous response of the target; a difference diagram 12, noted DIFF, to finely locate the target in the SUM beam; a first control diagram 15, noted CONT_Front, to block and reject responses from targets facing the antenna not present in the main SUM beam; a second control diagram 14, noted CONT_back, to block and reject responses from targets behind the antenna (therefore necessarily not present in the main SUM beam).

[0014] Depending on the missions and therefore the expected performance of the radar, the antenna can be: of several diagrams: 4 diagrams: SUM, DIFF, CONT_Front & CONT_Back; 3 diagrams: SUM, DIFF, CONT (CONT_Front and CONT_Back are grouped at the antenna level); 2 diagrams: SUM, DIFF / CONT (DIFF, CONT_Front & CONT_Back are grouped at the antenna level); of adapted dimensions: in width: ∘ having a large width to have a thin main beam providing high gain as well as to be selective and precise in azimuth; in height: ∘ having a large height, of the Large Vertical Aperture (LVA) type providing gain and protection against ground reflections (mainly in ATC); ∘ having a small height, of the "beam" type providing mobility (mainly used in IFF). While the SUM and DIFF diagrams are classically thin with 3dB lobes between 2.4° and 10°, the CONT_Front and CONT_Back diagrams seek to cover almost 180° each. The antenna can also be: of fixed diagram, called "mechanical" and rotating; of evolving diagram, with electronic scanning, called "AESA" fixed or rotating.

[0015] The following description describes the most complete antenna configuration, namely 4 diagrams for a rotating antenna. Other configurations are handled similarly regardless of the number of antenna diagrams used, whether the antenna is rotating or fixed. To simplify the description, however, the 3-diagram configuration can be used by employing CONT as a grouping of CONT_Front and CONT_Back.

[0016] Antenna 1 ensures the transmission of interrogations at 1030 MHz and the reception of return replies at 1090 MHz, according to the four diagrams: SUM, DIFF, CONT_Front and CONT_Back, or three diagrams (SUM, DIFF, CONT) or according to two diagrams (SUM, DIFF / CONT).

[0017] A rotating joint 2 and antenna down cables, for a rotating antenna, ensure: RF coupling of signals transmitted at 1030 MHz and received at 1090 MHz independently for the four diagrams between the rotating and fixed part of the radar; broadcasting of the azimuth position 201 of the main lobe axis of the antenna.

[0018] An RF treatment involves: a duplexer or circulator 3 ensuring RF coupling between the signals transmitted at 1030 MHz and received at 1090 MHz independently for the four diagrams; a transmitter 4 ensuring: the transmission of interrogations at 1030 MHz on the SUM diagram; the blocking of transponders outside the SUM lobe at 1030 MHz by the CONT_Front and CONT_Back diagrams; this for the different secondary protocols: IFF, SSR and Mode S; a receiver 5 ensuring the reception of replies at 1090 MHz on the four diagrams SUM, DIFF, CONT_Front and CONT_Back, and the calculation of the deviation for the different secondary protocols: IFF, SSR and Mode S.

[0019] Real-time processing includes: a spatio-temporal management 6 ensuring real-time management of interrogation and listening periods associated with the different secondary protocols: IFF, SSR and Mode S; a signal processing 7 ensuring: the processing of responses in the listening periods associated with interrogations for the different secondary protocols: IFF, SSR and Mode S; the detection and decoding of synchronous responses in the main lobe of the antenna by exploiting the four diagrams: ∘ SUM: to detect the responses received in the main lobe; ∘ DIFF: to precisely locate in azimuth the responses received in the main SUM lobe and possibly for the discrimination of tangled responses; ∘ CONT_Front and CONT_Back: to reject the responses received on the secondary lobes of SUM and DIFF in the case of detection in the main lobe of SUM.

[0020] A treatment in the main lobe of the antenna includes: a management 8 of the targets present in the lobe, ensuring: the preparation of the transactions (interrogations and responses) to be carried out in the next lobe for the different secondary protocols IFF, SSR and Mode S; the management of the periods IFF, SSR, Mode S "All Call" and Mode S "Roll Call" in the lobe; the dynamic placement of the Mode S selective interrogations and responses in the future "Roll call" period according to the state of the transactions just carried out and the new aircraft entering the lobe; extractors 9 ensuring the constitution of plots for each of the different secondary protocols IFF, SSR and Mode S, from the synchronous responses received in the lobe according to the protocol used during the interrogations.

[0021] A multi-turn 10 treatment includes: a 101 management of Mode S tasks to be performed with targets in the coverage, ensuring the prediction of target positions (antenna rendezvous) and the preparation of tasks to be performed associated with these positions according to internal and external requests and the state of transactions from previous rounds; an association of plots and a 102 tracking of targets in the coverage ensuring the tracking of targets to improve performance (elimination of false plots, control of decoded data in particular) and to predict the future position of these mainly, but not only, in Mode S.

[0022] A user interface allows the radar to take into account different requests and to visualize plots and target tracking.

[0023] Having reviewed the operation of a Mode S secondary radar, and before describing the invention in more detail, its implementation principle is presented below. The solution according to the invention analyzes the environment of a radar A by exploiting the "fruits" that this radar A receives, or rather, that it no longer receives in the event of an IL / SI code conflict. "Fruits" (an acronym for "False Reply Unsynchronized in Time") are asynchronous responses not solicited by the radar. These fruits are: either requested by another sensor (another Radar, WAM, TCAS, ...) sharing the same space; or automatically generated by the target itself (ADS-B, ..). The fruits are characterized by: the target's mode S address; the received power according to the antenna diagrams; the antenna azimuth; the radar time of their reception.

[0024] These Mode S Fruits are messages (Mode S replies of various types or Downlink Format_DF) that aircraft exchange with other radars (R2, R3, R4, etc.). Depending on the message type, it provides the aircraft's identity and sometimes, depending on the nature of the reply, the identity of another radar (R2, R3, R4, etc.) with which the aircraft is communicating. If the aircraft is referenced by radar R1 using its Mode S address, also present in the Fruits, this allows for the localization, through geographic analysis, of an area of ​​space exhibiting an IL / SI code conflict between radar R1 and radar R2, almost in real time.

[0025] Indeed, when the same IL / SI code is used by two nearby radars, R1 and R2, sharing a common coverage area, differences are observed in the distribution of fruit in the environment compared to operation with different IL / SI codes. These differences include the following: No target in the common RF zone (the one beyond the operational zone) generates DF11 fruits because both radars R1 and R2 lock onto aircraft on the same Il / SI code; targets in each non-common coverage zone of each radar R1 or R2 generate DF11 fruits of the same Il / SI code; because each radar R1, R2, locks onto aircraft in its coverage on its Il / SI code which is the same as that of the other radar while aircraft outside the two coverage zones of the two radars R1, R2 generate DF11 fruits with the Il / SI code of both radars R1, R2; because they are no longer locked onto by either of the two radars, neither R1, nor R2.

[0026] This analysis allows us to identify and locate, in the azimuth of R1 (and R2 respectively), an area exhibiting a code II / SI conflict between radars R1 and R2. Furthermore, in the shared area, aircraft generate fruit. DF4 giving the altitude, DF5 giving the identity -code A-, DF20 giving the altitude and the required BDS register, DF21 giving the identity and the required BDS register of operational Mode S Elementary Surveillance (ELS) or Enhanced Surveillance (EHS) transactions of the R2 radar that the R1 radar can detect: allowing the detection of a mode S target not yet known by the R1 radar.

[0027] The near-synchronous generation (time analysis) of DF4 / 5 / 20 / 21 fruits from such a target with DF11 fruits from other targets with the same R2 radar allows, in a few turns: to isolate the DF4 / 5 / 20 / 21 fruits of each target with the R2 radar; to pre-locate in azimuth of the A radar a new target not yet detected in the II / SI conflict zone: either by temporal analysis based on the time difference between the DF4 / 5 / 20 / 21 fruits of this target with the DF11 fruits generated by the other targets whose position is known by R1; or by the signature that the R1 radar assigns to each DFxx fruit according to the energy received from the Fruit on each antenna diagram (SUM, DIFF, CONT) - see French patent No. 1800657 for the details of this principle applied to ADS-B squitters (DF17).

[0028] The code conflict between II and SI in the shared area with R2 renders the DF11 response to a general UF11 call from radar R1 ineffective; however, it remains useful for acquiring new, closer targets. Therefore, to precisely locate this target in range (and more accurately in azimuth), radar R1 will supplement its All Call periods, solely within the target's pre-located azimuth, with a third selective interrogation using the target's Mode S address over a fairly long listening range (since the asynchronous responses to this target do not provide distance information). It is possible to add a third Mode S interrogation (UF4 or UF5) within the same All Call period that already includes a non-selective Mode S interrogation (UF11) and an SSR interrogation (MA / MC / M1 / M2), as these three interrogations are intended for different targets, and furthermore, the associated synchronous responses are in different formats, thus preventing any misinterpretation.

[0029] To reduce the size of the selective listening window, the target's distance can be estimated from the power received in the target's fruit. To do this, for each fruit received from the target, once its azimuth has been determined, the target's distance is calculated, taking into account the antenna gain at the received fruit's bearing. This allows, in particular, for the near real-time acquisition and precise localization of targets, even those locked to R1's general UF11 calls by radar R2, and then for their subsequent management, like all other targets, in selective transactions during Roll Call (RC) periods.

[0030] As will be shown in the rest of the description, the invention has at least the following advantages: The invention is based on the very concept of the use of the Il / SI code, namely the locking of Mode S targets: it is therefore applicable as soon as a target is involved, i.e., is Mode S, without requiring other equipment on board (such as ADS-B_out); the analysis of the fruits of all the targets in the area makes it possible to define the Il / SI code conflict zone with greater geographical precision (thanks to the position of all these targets) and this very quickly even if the number of targets is high; the analysis of the fruits of a target with the R2 radar makes it possible to pre-locate it in azimuth; Selective interrogation in the "All Call" with a large tolerance in distance of this target at the pre-located azimuth makes it possible to detect it precisely without affecting the operational performance of the R1 radar.

[0031] We return to the synoptic diagram of the mode S radar presented at the figure 1with the developments for the implementation of the invention. The new diagram, suitable for implementing the invention, is presented in figure 2 The main components and added treatments specific to the invention, applied to a mode S radar having a 4-diagram antenna, are shown in bold on the figure 2 .

[0032] While the operation of a Mode S radar is synchronous, the processing added for the invention is not related to the transmission and only exploits the azimuth position of the main lobe axis of the antenna. Most of the elements remain unchanged, thus verifying both: The non-intrusion of the invention into the operational functioning of the mode S radar; The use of the same elements as those used by the radar: In aerials in the broadest sense: antenna, rotating joint, antenna down cables, duplexers; In processing: the receiver. This allows, in particular, a correlation of synchronous and asynchronous responses from the same aircraft. The main added features are described below.

[0033] In real-time processing, and more specifically in signal processing: addition of continuous processing of asynchronous Mode S 21 responses (independent of listening periods associated with interrogations) ensuring the detection and decoding of asynchronous responses by exploiting separately but also the 4 diagrams SUM, DIFF, CONT_Front and CONT_Back: to detect all received responses: Asynchronous and Synchronous; to decode responses of all types (DF4 / 5 / 11 / 20 / 21), the message data and especially extract the Mode S address; to enrich each decoded response with its characteristics: detection time, azimuth of the main lobe of the antenna at the time of detection, powers received on SUM, DIFF, CONT_Front and CONT_Back; enrichment of synchronous responses by the power measured on SUM, DIFF, CONT_Front and the antenna azimuth; detection of synchronous responses outside the operational coverage of the radar.

[0034] In the main lobe processing, the 22 Mode S extractor of DF11 plots is added outside the operational range, the DF11 plot extraction being carried out outside this range to locate and identify targets by their Mode S address.

[0035] In multi-turn processing: addition of processing 23 of Code Conflict Detection II / SI; association of fruits (Asynchronous response) with synchronous plots of the coverage (operational range and beyond); geographical analysis of the sources of fruits DF11_R2 (DF11_R2 meaning that the fruit DF11 is an asynchronous response whose source is an R2 radar, i.e. an asynchronous response not solicited by R1_ following a general call issued by this R2 radar); Addition of Precise Detection and Localization 23 of target locked by R2 in code II / SI conflict zone: isolation of the presence of a target not detected by radar R1 in the conflict zone by analysis of the fruits DF4 / 5 / 20 / 21_R2 (DF4 / 5 / 20 / 21_R2 meaning that the fruit is of type DF4 or DF5 or DF20 or DF21 generated by a target during its operational management by radar R2); evaluation of its pre-localization in azimuth with respect to R1 in the conflict coverage zone;Precise detection and localization of this target in terms of distance and azimuth, in order to then continue its surveillance like all other targets.

[0036] There figure 3 This illustrates the various steps for implementing the invention, carried out by an operational radar 30. This radar performs its usual surveillance mission within its area of ​​responsibility in operational coverage (ELS or EHS), that is, detecting and locating all possible Mode S targets via their enhanced synchronous responses. The synchronous responses, requested by the radar, are received using the SUM and DIFF azimuth selective diagrams. The three steps implementing the invention are described below.

[0037] The first step 31 comprises three substeps SE1, SE2, and SE3. In the first substep SE1, the radar 30 detects the fruit. These asynchronous responses, not solicited by the radar, are received via the four diagrams SUM, DIFF, CONT_front, and CONT_back. According to the invention, the radar processing notably exploits the fruit, whose format is identical to that of the synchronous responses, both in RF signal and response structure.

[0038] To exploit the fruit, still in substep SE1, a detection and decoding chain for these asynchronous responses, which a conventional radar typically rejects, is added to the processing. These asynchronous responses are qualified with the usual attributes of a response, including the following: Detection time; Antenna azimuth at the time of fruiting; Mode S address of the transmitting transponder; Message content; Fruiting power according to each antenna diagram.

[0039] Depending on the target's distance from the radar, the fruit may be detected on several diagrams simultaneously. Under these conditions, during this first step, the multiple detections (at the same time) are concatenated to ensure only one asynchronous response message per fruit. At this stage, the source of the fruit, which could be: Either solicited by another sensor (another radar, WAM, TCAS ...) sharing the same space; Or automatically generated by the target itself (ADS-B, ...), this possibility not being exploited by the invention.

[0040] In substep SE2, the radar's detection coverage is extended (knowing that the operational coverage is often set by the user to be less than its guaranteed maximum range) to its significantly greater, or even maximum, synchronous detection range, in order to ensure a measurement area that allows the IL / SI code conflict zone to be defined. The additional synchronous responses thus obtained (with the same general UF11 radar call interrogations, therefore without influencing the radar's operational functioning) are processed like the other synchronous responses from the radar's All Call (AC) coverage to create plots that therefore have the usual elementary attributes of a basic Mode S plot, such as: Center Plot detection time; Mode S address of the transmitting transponder; Azimuth of the center plot; Distance from the plot; for each reply that constituted the plot; Detection time (classically on the order of 50ns); Antenna azimuth; Success or failure of the interrogation (reply received or not); Pointing in the lobe; Message content; Reply power according to each antenna diagram (SUM, DIFF and CONT_Front).

[0041] In substep SE3, the fruits are associated with the Mode S plots of the extended radar coverage. For each target located within the operational or extended radar coverage area by the radar via selective interrogations that generated synchronous responses (DF4 / 5 / 11 / 20 / 21), the invention associates with that target the fruits it generated (based on the transponder's unique Mode S address as the target identifier): either between two successive synchronous detections in Mode S (close to the tower); or on a tower basis, for example. Since the fruit is asynchronous by nature, the position of the target at the time of fruit detection is determined by interpolating the target's position from its trajectory established by the radar in its operational functions at the time of fruit reception.

[0042] In the second step 32, the radar performs the detection and characterization of a potential code II / SI conflict zone by analyzing different geographical areas between the two radars. This operation corresponds to substep SE4 in which the radar performs this detection and characterization by analysis for different geographical areas between the two radars: The presence of DF11 target fruits in two zones D1 and D2 framing the code conflict zone II / SI (zones D1 and D2 of the figure 7) ; From the absence of DF11 fruit of the targets in the non-overlapping operational coverage area A of the R2 radar (areas A of the figure 7 ); The II / SI code associated with the same fruits as that of the R1 radar in zones D1 and D2; The absence of a synchronous DF11 plot beyond the operational coverage of the radar in a C zone (C zones of the figure 7 ). If a conflict between the IL and SI codes is confirmed, the affected area is transmitted by the radar to ATM supervision to correct the problem.

[0043] Step 33 comprises at least three substeps, SE5, SE6, and SE7. In this step, to ensure the security of radar monitoring (essential in ATC), in the event of a code II / SI conflict detection, the invention allows: to isolate the presence of a target not detected by radar in the conflict zone; to assess the pre-location of this target in azimuth in the conflict zone; to detect and precisely locate this target in distance and azimuth in order to then continue its surveillance like all other targets.

[0044] In sub-step SE5, the radar isolates the presence of an undetected target within the conflict zone (in the event of a Code II / SI conflict detection). To this end, it detects the presence of a target locked by another R2 radar potentially within the Code II / SI conflict zone (zone B or C of the figure 7The radar first isolates, from all the captured fruits, the DF4 / 5 / 20 / 21 fruits due to the other radar R2 (triggered by it during its operational use with the targets) whose Mode S address is not yet known to R1. Consequently, a target, referenced by its Mode S address, has some of its DF4 / 5 / 20 / 21 fruits: temporally between DF11_R2 fruits of known targets of R1 in zones D1 and D2, and temporally synchronous between DF11_R2 fruits of zones D1 and D2 over P turns of R1 (around ten turns for example), is a potentially locked target in zones B or C, some of whose fruits are caused by this other R2 radar (these zones will be defined later). The synchronism criterion between the fruits of the target and the fruits of each target in D1 and D2 takes into account a tolerance of the time gap between these fruits considering the known evolution in azimuth of the targets in D1 and D2 on the P turns and the unknown position of the interrogation of these in the lobe of R2. A target generates fruits with different radars; this temporal selection allows us to select only those due to R2 from a target in zone B or C.

[0045] In substep SE6, the radar performs pre-localization of the target's azimuth within the code II / SI conflict zone. More precisely, the radar evaluates the pre-localization of an isolated target within the conflict zone by exploiting the absolute value of the time difference between the fruits of the target under analysis and the fruits of known targets in zones D1 and D2. Assuming that the rotation speed of R2 is stable, and that the targets in D1 and D2 are localized in azimuth and range by R1 at each revolution, this allows for the estimation of the azimuth of the target under analysis at each revolution by simple interpolation using the time of fruit detection by R1. This is done over the P revolutions of the previous temporal analysis (or more, depending on the desired accuracy). At each revolution, each pair of a fruit from the target with a fruit from a known target in D1 or D2 provides an azimuth estimate. The average of these estimates constitutes a pre-localization of the target's azimuth.

[0046] In substep SE7, the invention uses R1 to detect the azimuth position and calculates the distance to the target locked by R2, allowing subsequent monitoring as with all other targets. To achieve this, the additional selective interrogation for this target, whose Mode S address is known by its results, is scheduled during the AC period (usually intended for general non-selective call interrogations), thus without altering the operational function of selectively monitoring known targets during Roll Calls (RC). Indeed, since the distance to this target is not yet precisely known, the associated listening window is large, and its use during the RC period would occupy approximately half of the sequence time, thus depriving the other targets to be managed (A and B zones).

[0047] In the process illustrated by the figure 3The SE4 substep for detecting an Il / SI code conflict, followed by an external alert 39 indicating the code conflict zone, constitutes a first processing chain 31 for improving the overall security of ATC surveillance. The SE5, SE6, and SE7 substeps constitute a second processing chain 32 for ensuring radar surveillance 30 in the event of an Il / SI code conflict, this surveillance being guaranteed by the detection and localization of aircraft within the Il / SI code conflict zone.

[0048] We will now describe in more detail the context and phases of the invention presented earlier. We begin by recalling the context of the fruit received by a secondary radar. The fruit always originates from real targets, primarily those within the radio coverage of the secondary radar.

[0049] There figure 4This illustrates the different radar ranges in an XY plane. To ensure its surveillance role in a given area, a secondary radar, hereafter designated R1, typically has a margin in transmission to guarantee a detection probability greater than 99% within its coverage area 41. Within this area 41, even a target equipped with a transponder at the lower limits of sensitivity at 1030 MHz can correctly interpret the Mode S interrogation and a transponder at the lower limits of power at 1090 MHz can be correctly detected by the radar. Consequently, targets whose transponder is more centered within the standard sensitivity range of 1030 MHz and power range of 1090 MHz, or even at the maximum values ​​of the standard, can still correctly interpret (and therefore respond) over a maximum range well beyond this area 41. This results in a wider area 42 where most targets are still likely to respond to interrogations.Furthermore, a radar must detect the synchronous 1090 MHz responses it has requested through its 1030 MHz interrogations. Consequently, its reception range is often, in practice, much greater than its maximum transmission range, resulting in a reception zone 43 that encompasses the preceding zones 41 and 42. The transmission range corresponding to the boundary of zone 42 will be denoted Range_TX, and the reception range corresponding to the boundary of zone 43 will be denoted Range_RX. The secondary radar involved is designated R1; it is located in the center of zone 41, and the transmission and reception ranges will be denoted Range_TX1 and Range_RX1, respectively, in reference to this radar R1. Moreover, radar operators often use their radars below their guaranteed radio ranges, thus having an operational target lock-on coverage of less than 41, primarily due to limitations imposed by the radar site's visibility of the targets.

[0050] There figure 5 illustrates a dual-radar configuration, with a second radar R2 located in the vicinity of radar R1. More precisely, the figure 5 This illustrates the overlap in coverage areas between the two radars. The analysis in the following description is performed at the level of radar R1; a reciprocal analysis can be performed at the level of radar R2. Radar R2 interrogates targets within the coverage area of ​​R1 up to its transmit range, Range_TX2.

[0051] THE figures 6a and 6b show the pollution zones between the two radars R1, R2. figure 6a presents the pollution volume of R2 on R1 corresponding to zone 61. figure 6b presents the pollution from R1 on R2 corresponding to zone 62.

[0052] As shown by figure 7The overlap zone between two radars (here, the case of interference from R2 on R1) is divided into different sub-zones A, B, C, D1 / D2, E, and F1 / F2, depending on the type of message each radar exchanges with the targets in the sub-zone. The principle of the invention consists of analyzing in detail the presence or absence of this type of message to define these azimuth sub-zones, as with the device described in French patent application FR 1800914. In the present invention, the aim is to detect code conflicts between II and SI. The formats of the responses exchanged between a radar and an aircraft transponder are known to those skilled in the art. The type of response depends both on the radar and on the task assigned to it within the radar's area where the target is located.

[0053] The area concerned in case of code conflict Il / SI is zone B in which if a target arrives towards R1 from the coverage of R2 (zones E, C, B) is not detected by R1 before zone A because in zone B, being locked by R2, it does not respond to the general All Call (DF11) calls of R1 which therefore does not see it.

[0054] Table 1 below summarizes, for an example of typical operation of a mode S radar, with different II / SI codes (II1 for R1 and II2 for R2), the respective tasks of R1 and R2 according to the zone belonging. [Table 1] Sub-Zone Radar R1 (II1) Radar R2 (112) Answer Synchronous for R1 Answer Asynchronous for R1 (due to R2) Zone A Target Under Surveillance Locked on II1 Target Out of Surveillance Not Locked on 112 DF4, DF5, DF20, DF21 DF11 Zone B Target Under Surveillance Locked on II1 Target under surveillance, locked on 112 DF4, DF5, DF20, DF21 DF4, DF5, DF20, DF21 Zone C Target Out of Surveillance Not Locked on II1 Target under surveillance, locked on 112 DF11 DF4, DF5, DF20, DF21 Zone_ D1 / D2 Target Out of Surveillance Not Locked on II1 Target Out of Surveillance Not Locked on 112 DF11 DF11 Zone E No Synchronous Response Target under surveillance, locked on 112 - DF4, DF5, DF20, DF21 Zone_ F1 / F2 No Synchronous Response Target Out of Surveillance Not Locked on 112 - DF11

[0055] There figure 8This table presents the types of responses by sub-zone for different R1 / R2 codes. More specifically, it shows the messages exchanged in the overlap zone according to the sub-zone in relation to Table 1 above. Synchronous responses for R1 are indicated in bold, and asynchronous responses for R1 are indicated in regular font; the latter are the result of R2. The extensions S for Synchronous and F for Result are also used.

[0056] Table 2 below summarizes the respective tasks of R1 and R2 according to the belonging of a target to a sub-zone in the case of the use of the same code Il / SI by the two radars, this code being noted IIc. [Table 2] Sub-Zone Radar R1 ( IIc ) Radar R2 ( IIc ) Answer Synchronous for R1 Answer Asynchronous for R1 due to R2 Zone_A Target under surveillance, locked on IIc Target Out of Watch Not Locked On IIc DF4, DF5, DF20, DF21 The R1 radar locks onto the aircraft. IIc Zone_B Target under surveillance, locked on IIc Target under surveillance, locked on IIc DF4, DF5, DF20, DF21 Conflict Zone Code II / SI DF4, DF5, DF20, DF21 Conflict Zone Code II / SI Zone C Target Out of Watch Not Locked On IIc Target under surveillance, locked on IIc ► The R2 radar locks onto the aircraft IIc DF4, DF5, DF20, DF21 Zone_D1 / D2 Target Out of Watch Not Locked On IIc Target Out of Watch Not Locked On IIc DF11S ► plot R1 tj present because R2 does not lock (on IIc ) DF11F ► fruit of R2 tj present because R1 does not lock (on IIc ) Zone_E No Synchronous Response Target under surveillance, locked on IIc - DF4, DF5, DF20, DF21 Zone_F1 / F2 No Synchronous Response Target Out of Watch Not Locked On IIc - DF11

[0057] There figure 9This table presents, for a given II / SI code, the types of responses by sub-zone. More specifically, it shows the messages exchanged in the overlap zone according to the sub-zone in relation to Table 2. Characters in bold indicate DF11 responses that disappeared when the same II / SI (IIc) code was used. Characters in regular font indicate synchronous and asynchronous responses that were always present. The distribution of DF11 responses to the general All Calls of each radar has evolved compared to the figure 8 , following the use of the same II / SI (IIc) code by the R1 and R2 radars: in sub-zones D1 and D2, synchronous responses for R1 and asynchronous responses for R2 are still present; moreover they use the same II C code; in sub-zone A, targets in the coverage of R1 do not generate a DF11 fruit with R2; in sub-zone C, targets leaving the coverage (sub-zone B) of R1 no longer respond to R1's general All Call calls while R1 no longer locks them.

[0058] Table 3 below summarizes for R1 the differences according to sub-zones in case of code II / SI conflict or in the absence of conflict. [Table 3] AREA Different II / SI Codes Code II / SI identical Comments Plot in blanket Plot hors blanket Fruits Plot in cover Plot out of cover Fruits A Yes DF11_R2 Yes 0 Planes controlled by R1 no longer produce fruit with R2 B Yes, outgoing range = incoming range DF4_R2, DF5_R2, DF20_R2, DF21_R2 Yes Outgoing range > Incoming range DF4_R2, DF5_R2, DF20_R2, DF21_R2 The maximum range of targets exiting cover (known in A) is greater than that of targets entering cover because they are not seen in B. C Yes Outgoing range = Incoming range DF4_R2, DF5_R2, DF20_R2, DF21_R2 0 DF4_R2, DF5_R2, DF20_R2, DF21_R2 The plots exiting operational coverage B from R1 towards C no longer produce synchronous plots DF11_R1 D Yes. Outgoing range = incoming range. DF11_R2 Yes. Outgoing range = incoming range. DF11_R2 The fruits of R2 have the same II / SI code as R1 ( IIc )

[0059] Depending on the distribution of aircraft in space among the four sub-zones, 1 to 4 criteria can be met: zone D1 and D2: presence of plots of R1 making fruits of the same code IIc with R2; zone C: the plots of R1 leaving zone B do not respond to the general calls (UF11) of R1; zone A; the plots of R1 do not make fruit of code II C which is the identifier of R2; the incoming plots are not seen by R1 in zone B they appear in AC only in zone A: outgoing range > incoming range. Following this analysis, radar R1 considers it to have a presumption of code conflict II / SI in an azimuth zone between the two zones D1 and D2, synoptically delimited by the two lines 91, 92 on the figure 9 In practice, the position of these lines is obtained by the line passing through radar R1 and: in D1: the target of R1 generating fruits of the same Ilc code having the highest azimuth seen from R1; in D2: the target of R1 generating fruits of the same Ilc code having the lowest azimuth seen from R1.

[0060] In relation to the figure 10a Or figure 10b Having illustrated the detection of DF11 plots outside operational radar coverage, we now describe the specific processing of fruit to confirm the code conflict azimuth II / SI. In order to detect and define the code conflict azimuth II / SI, it is necessary at the R1 radar level to process its synchronous responses in both zones D1 and D2 without interfering with the operational functioning of the radar.

[0061] In its operational function, the radar manages two types of periods for aircraft within its coverage (zones A and B): All Call (AC) to detect incoming Mode S aircraft: Mode S Radar Beam Management positions a general call interrogation UF11 in each AC; Mode S Reply Processing detects the DF11 responses within the radar's operational range; Mode S Extractor constructs a Mode S DF11 plot, also within the radar's operational range; Roll Call (RC) to monitor Mode S aircraft (at ELS or EHS level): Mode S Radar Beam Management selectively interrogates aircraft previously detected in AC via UF4 / 5 / 20 / 21; Mode S Reply Processing detects the DF4 / 5 / 20 / 21 responses within the radar's operational range; Mode S Extractor constructs an enriched Mode S plot, also within the radar's operational range; then the radar locks onto the target thus acquired in RC so that it no longer responds to general calls from this radar, therefore not to its identifier, which is its II / SI code.

[0062] In the present invention, the mode S function in the asynchronous response processing 21 references all the fruits DF11 of the R1 II1 code received relative to the last UF11 interrogation of the AC periods of the R1 radar, and this beyond the radar's operational range. Two approaches are possible, as shown: either the figure 10a by increasing the duration of the AC periods; that is, the figure 10b where the listening of the synchronous DF11s is located beyond the AC period or during the RC period. Both solutions allow the DF11 extraction function 22 to construct synchronous DF11 plots outside operational coverage (beyond the usual listening period AC), therefore for zones D1, C and D2. figures 10a And 10b This demonstrates an additional listening 110, beyond the operational listening AC period (which is not an operational requirement for radars according to prior art), thus without disrupting the operational functioning of the radar in the case of the figure 10b This additional listening is carried out in area 42, in reference to the figure 4 It should therefore be noted that the synchronous DF11 responses in the operational coverage of the radar are not processed by this new function 22 since they are already used for operational AC operation by the radar.

[0063] The duration of this additional listening to the DF11 is in fact limited only by the duration of the RC period, since it takes place in parallel with this period before the next AC period in the case of the figure 10bIn practice, since an RC period is approximately 1.5 to 2 times the duration of an AC period, it allows for listening well over twice the operational range. Consequently, the maximum listening range is limited in practice only by the radio range.

[0064] In relation to the figure 11 We describe the search for targets in zone B using a time-domain approach. For this time-domain approach, we consider the predominant assumption of rotating mechanical antenna radars, which correspond to the vast majority of ATC radars. figure 11 presents as an example, in the form of a simplified diagram, the case of the previous R1 and R2 radars for which all targets in the common RF coverage area are Northeast for radar R1; Southwest for radar R2. having in addition as a complementary assumption (in this example) a rotation period of the antenna of R2 of the order of ¾ of that of R1.

[0065] There figure 11 Figure 111 shows the synchronous responses for R1 and the synchronous responses for R2, as well as the asynchronous responses for R1 (fruits) of targets in this common area. Due to the natural slippage between the two antenna rotations, it is confirmed that the fruits due to R2 are also asynchronous in azimuth with respect to R1, which can nevertheless accurately time-date the fruits to subsequently calculate their azimuth.

[0066] The principle of the invention, for detecting the presence of a target locked by R2 potentially in zone B, first requires isolating, among all the signals captured by R1, the DF4 / 5 / 20 / 21 signals generated by the R2 radar during its operational use with targets, including those in zone B or C. It should be noted that the DF4 / 5 / 20 / 21 messages do not contain the R2 identifier but only the target's Mode S address; therefore, they are not self-carrying from the radar that generated them: they only allow the target to be identified.

[0067] To that end, as the figure 12 The invention exploits the fact that the fruits of R2 are almost synchronous with each other in turn from the antenna of R2. The previous analysis allowed R1 to locate the targets generating DF11 fruits due to R2 (according to its identifier) ​​in the D1 area of ​​Southeast R1 (Southwest of R2) and the D2 area of ​​Northeast R1 (West of R2) according to the example of the figure 9 The next step in the analysis is to search for DF4 / 5 / 20 / 21 fruits that are synchronous with these DF11 fruits from R2. This is done by analyzing, over several turns, those fruits whose time difference is almost constant with the fruits of the targets in zones D1 and D2, taking into account the movement of these targets (which R1 can calculate since these targets produce synchronous plots for R1 outside of operational coverage). Consequently, these DF4 / 5 / 20 / 21 fruits: temporally between fruits DF11_R2 of known targets of R1 in zones D1 and D2 in turn, and temporally synchronous between fruits DF11_R2 of zones D1 and D2 over P turns of R1 (around ten turns - parameter of the invention), are from targets potentially in zones B or C and caused by R2.

[0068] There figure 12This illustrates the successive azimuth scans of R1 by the R2 antenna, starting from a scan N (labeled scan_N) along the time axis. It represents successive temporal zooms as the R2 antenna moves from the "D1_Southwest of R2" zone to the "D2_West of R2" zone, passing through zones B and C. The dashed lines represent the remainder of the R2 antenna's rotation at other azimuths of R2, intentionally reduced in time scale to highlight the zoomed-in sections. In the "D1_Southwest of R2" and "D2_West of R2" zones, only targets emitting DF11_R2 fruits originating from R2, represented by solid circles, are considered. In the central time period between zones D1 and D2, the DF4 / 5 / 20 / 21 fruits that R1 receives are of two types.The radar according to the invention analyzes these fruits target by target (S mode address available in DF4 / 5 / 20 / 21) in time difference at each turn with respect to the fruits DF11_R2 of the two zones D1 and D2: . Those from R2 for targets in zone B or C are then almost stable with respect to the DF11 fruits of R2 since they come from the same radar (represented by bold circles); those from other radars for targets in all azimuths, which R1 will nevertheless detect by its omnidirectional antenna which is the CONT diagram, are not temporally stable with respect to the fruits of R2 considering the rotation differences between the antennas of these radars with R2 (represented by the other non-bold circles). According to the figure 12 , the target represented by the bold circles with mode S address MS1 (@MS1) being stable (within a configurable temporal tolerance ΔT) with respect to the fruits DF11_R2 (@MS HAS from D1 and @MS B of D2) on an analysis of P turns depth (parameter) is considered to be in zone B or C. The analysis of the figure 12 This allows you to position the fruits in relation to each other.

[0069] To pre-locate in azimuth a target potentially locked as being in zone B without responding to the general UF11 calls of R1, at least two methods can be used: a pre-location in azimuth by time gap according to the invention or a pre-location in azimuth by use of the unclaimed antenna pattern.

[0070] For the azimuth pre-localization, we note that during the previous step, we used the stability of the time gap between the fruits of the target at address MS1 (@MS1) and the DF11_R2 fruits of zones D1 and D2 to isolate its belonging to zone B or C. However, the R1 radar knows the azimuth position in its reference frame of the targets in zones D1 and D2 that generated DF11_R2 fruits.

[0071] The absolute value of the time difference between the target under analysis (@MS1) and each target (@MS A and @MS B) in zones D allows us to estimate the azimuth of target @MS1 at each turn over the P turns of the previous analysis (or more, depending on the desired accuracy). The average of these estimates constitutes a preliminary azimuth localization of target @MS1 over one turn. A linear regression over P turns allows us to both refine the azimuth and also evaluate the angular velocity of the target relative to R1.

[0072] The other principle of pre-localization in azimuth, by use of the antenna pattern, is described in particular in patent application FR1800657.

[0073] A potentially locked target in zone B has been located by azimuth; it now needs to be located by range. In the continuation of the previous example, we consider the target @MS1. This target @MS1 needs to be located by range relative to R1: is not in zone A because it would then have answered R1's UF11 query; is potentially in zone B, therefore in R1's area of ​​responsibility, in which case it must be detected; is potentially in zone C, therefore outside R1's area of ​​responsibility, in which case it is not necessary to detect it. It is necessary to search for target @MS1 beyond the maximum distance of zone A and below the minimum distance of zone B. At least two methods allow for the detection and precise positioning of target @MS1: either by exploiting a feature of the mode S protocol; or by exploiting a selective interrogation of the target zone B.

[0074] The Mode S protocol proposes, in case of an Il / SI code conflict, to force the unlocking of targets receiving the UF11 general call interrogation, forcing a transponder locked to the Il / SI code of UF11 to respond. This approach is both: very polluting since all targets in zone A as well as zone B that the R1 radar already knows will respond, not only to the transmission azimuth of this UF11 call but also to the azimuth width of the transmitting antenna lobe (EBW_TX close to 6°); not always effective because all these Mode S responses will also become entangled: if the number of targets is high in this azimuth direction; or even simply if a target is close in distance to @MS1, Detection @MS1 will have a high probability of not occurring on the 1st er trial and error will require several trials over several rounds, each time provoking a number of useless responses from targets already known.

[0075] In the other method, illustrated by the figure 13 To avoid triggering responses from targets already known in zone A or zone B, a selective interrogation of target @MS1 is sent at its pre-located azimuth. As shown in the figure 13The selective polling for the target with mode S address @MS1 is positioned during the AC call period (usually intended for general non-selective call polling), thus without modifying the operational behavior of selective monitoring of known targets during RC calls. In the example of the figure 13 , during the AC period at the target azimuth, a selective interrogation UF4 for the address @MS1 is added in the AC period m+2 in addition to the general interrogation UF11.

[0076] In light of the description of the implementation of the invention just given, we illustrate with the help of the figure 14 the principle of the invention in a three-radar case, more particularly the second step 32 and the third step 33 (see figure 3The secondary radar considered is always radar R1, with two surrounding radars here: radar R2 and radar R3. The principle stated for R2 is applicable to R3. Radar R1 therefore detects targets not responding to general calls that may be present in the coverage area; these targets are locked by radar R2 due to a code conflict (II / SI). In the first step (31), radar R1, having previously extended its range to an extended area (42) and having detected fruit in this area (42), performs the following detection conditions: radar R1: detects 141 DF11_R2 fruits, having as their source the R2 radar, in an extended area 42 outside the coverage area 41, of the same code Il / SI (detection in sub-areas D) as R1; notes the absence of 142 DF11_R2 fruits in the coverage area of ​​R1 not overlapping the coverage area of ​​R2 (sub-area A); notes the absence of a synchronous DF11_R1 response from aircraft leaving its operational coverage (sub-area C). Under these conditions, radar R1 deduces a code conflict II / SI between the two radars R1 and R2 (R2 locking the responses), corresponding to sub-step SE4. The presence of DF4, DF5, DF20 or DF21 fruits, originating from R2, in the B or C overlap zone between the coverages of the two radars R1 and R2, radar R1 deduces 143 the presence of an aircraft in these zones B or C. The azimuth and distance of the unseen target are then determined as described previously.

Claims

1. Method for detecting conflicts in the ll / SI identification code of radars near a secondary mode-S radar (R1), characterized by comprising at least: - a first step (31) in which: - the coverage of said radar (R1) is extended (SE2) to obtain synchronized replies in an extended radar coverage; - said radar (R1) detects (SE1) unsolicited unsynchronized replies, named fruits, in a region (42) of extended radar coverage; - the fruits are associated with the mode-S targets of said extended radar coverage (SE3), the position of said fruits being determined by interpolation with the synchronized detections of said targets, on the basis of their mode-S address; - a second step (32) in which said radar (R1) detects a conflict in II / SI code by analyzing geographic regions of radar coverage (A, B, C, D1 and D2) common to said radar (R1) and to at least one nearby radar (R2), a conflict being detected if said radar (R1): - detects (141) the presence of DF11 fruits of the same ll / Sl code as said radar (R1), having as source said nearby radar (R2), in two sub-regions of said extended radar coverage (region_D1, region_D2),; - observes (142) the absence of DF11 fruits caused by said nearby radar (R2) in that region of radar coverage of said radar (R1) which does not overlap with the region of radar coverage of said nearby radar (region A); - observes the absence of a synchronous DF11 reply to a UF11 all-call interrogation from the radar (R1) by aircraft leaving the region of radar coverage of said radar; the region of overlap (region_B) in the radar coverage of said radar (R1) and the radar coverage of said nearby radar (R2) forming a region of conflict in ll / Sl code; and - a third step (33) in which said radar (R1) detects (SE5) targets locked by said nearby radar (R2) in said region of conflict (region_B, region_C) on the basis of detection (143) of DF4, DF5, DF20 or DF21 fruits caused by said nearby radar (R2) in said region of conflict (region_B, region_C), indicating the presence of a target in said region of conflict; said detection being performed by analyzing, over several passes, the DF4, DF5, DF20, or DF21 fruits, for which the time difference is nearly constant with respect to the DF11 fruits of the targets in the two sub-regions of said extended radar coverage (region_D1, region_D2), taking into account the movement of these targets; and said targets being pre-located (SE6) in an azimuthal region inside said region of conflict (region_B, region_C) by exploiting the absolute value of the time difference between each of its DF4, DF5, DF20 or DF21 fruits of said targets due to said nearby radar (R2) and each of the DF11 fruits of the other targets of the two sub-regions of said region of extended radar coverage (region_D1, region_D2) that are caused by said nearby radar (R2), the azimuthal position of each of said other targets being known.

2. Method according to claim 1, characterized in that a precise distancewise and azimuthal location of a target is obtained by positioning selective UF4 or UF5 interrogations (131, SE7) in said pre-location azimuthal region of said target, associated with a listening window, in which the reply is listened for, taking into account the uncertainty in the distance of said target, which is estimated on the basis of the power of the fruits of this target and of the characteristics of the radar, said selective interrogations being transmitted to said target during the roll-call period, or during the all-call period, in addition to the UF11 all-call mode-S interrogation.

3. Method according to any one of the preceding claims, characterized in that, said radar transmitting so-called UF11 all-call interrogations, in said first step (31, SE2), in response to said UF11 all-call interrogations of said radar, listening for synchronized DF11 replies after the all-call period and during the roll-call period provides additional synchronized replies (110) in said region of extended radar coverage (42), said additional replies thus obtained being processed as the other synchronized replies in the all-call period to construct DF11 hits having the attributes of a conventional mode-S hit.

4. Secondary radar, characterized in that it is configured to implement the method according to any one of the preceding claims.

5. Radar according to claim 4, characterized in that, comprising listening periods associated with interrogations transmitted by said radar, it comprises means (21) for continuously processing the unsynchronized mode-S replies, independently of said listening periods.

6. Radar according to claim 5, characterized in that, said radar comprising an antenna (1) having antenna radiation patterns (11, 12, 14, 15), said processing means detect and decode said unsynchronized replies by exploiting said radiation patterns of the antenna of said radar separately: - to detect all the, unsynchronized and synchronized, replies received via said antenna; - to decode replies of any type, the data of the messages and to extract the mode-S address therefrom; - to enrich each decoded reply with its characteristics, said characteristics being at least the detection time, the azimuth of the main lobe of the antenna on detection and the power received through the antenna radiation patterns.

7. Radar according to any one of claims 4 to 6, characterized by comprising an extractor (22) of DF11 hits in said region of extended coverage (42), DF11 hits being extracted beyond the operational range of said radar solely with a view to location and identification of the targets via their mode-S address.

8. Radar according to any one of claims 4 to 7, characterized in that it comprises processing means (23) for detecting conflicts in II / SI codes and for detecting and locating targets locked by said nearby radar in any regions of conflict in II / SI code, said means (23): - associating fruits with synchronized hits; - geographically analyzing the sources of DF11 fruits that are unsynchronized replies the cause of which is a nearby radar (R2); - isolating the presence of targets not detected by said radar in said region of conflict; - evaluating the azimuthal pre-location of said targets with respect to said radar in said region of conflict; - detecting and locating said targets distancewise and azimuthally in order to allow said radar to continue its surveillance function as for all the other targets.