Secondary radar improves aviation safety through ultra-long range ADS-B detection

Abstract

To provide a secondary radar that improves aviation safety through ultra-long range ADS-B detection.SOLUTION: A secondary radar comprises an antenna having a radiation pattern that forms a sum channel, labeled SUM, a radiation pattern that forms a difference channel, labeled DIFF, and a pattern that forms a control channel, labeled CONT, where localization is performed by detecting an ADS-B squitter received via a CONT channel 102, a SUM channel 103 and a DIFF channel 104, and measuring at least the power of the squitter and an azimuth with respect to a radar 110; the position of a target transmitting the ADS-B squitter is calculated using the detection of at least one ADS-B squitter in view of the radar latitude and longitude and the azimuth with respect to the radar; and positional cells, labeled CPR cells coded in the squitter, are selected via azimuth measurement.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the field of civil and military air traffic control (ATC). [Background technology]

[0002] Currently, air traffic control is mainly based on secondary radars, whose detection reliability is widely recognized, which ensure synchronous monitoring of aircraft through SSR and Mode S protocols. In addition, asynchronous reception of enhanced ADS-B squitters (ADS-B stands for Automatic Dependent Surveillance-Broadcast) (squitters are aircraft-supplied position information that is not triggered in response to a radar interrogation signal, but is automatically transmitted from the aircraft transmission means) (TCAS: Airborne Collision Avoidance System) for collision avoidance purposes when implemented on board aircraft is used by ground ATC, thus providing part of an interactive surveillance system.

[0003] This surveillance may be combined with IFF (Identification Friend or Foe) type surveillance, which ensures the identification of aircraft according to various military protocols.

[0004] Within a single structure that groups these sensors together, the operational use of these various activities is simultaneous: - if an IFF interrogator is used, the secondary SSR / Mode S radar and the IFF interrogator are listened to simultaneously, and their synchronous mode listen operation utilizes the SUM and DIFF of the formed beams; Civil ADS-B squitters and military Mode 5 squitters are listened to in asynchronous mode, and these asynchronous mode listen operations utilize the omnidirectional coverage of a control pattern (denoted CONT) consisting of a front control pattern (denoted CONT_Front) and a back control pattern (denoted CONT_Back). is required.

[0005] Traditionally, ADS-B squitter is transmitted without solicitation from a secondary radar at a rate specific to the transmitter, which rate depends, among other things, on the state of the transmitter and the characteristics of the information being transmitted. - ADS-B receivers must be able to listen in all directions, Antenna gain is often small, e.g. 11-15 DBi for dedicated standalone ADS-B receivers depending on the number of antennas utilized to achieve 360° coverage. The specified guaranteed range is around 150 Nmi or possibly slightly more depending on the quality of the transponder transmitting the squitter and the number of antennas utilized by the ADS-B receiver. In principle, in addition, two almost consecutive position squitters are required to locate a target in ADS-B mode.

[0006] Receiving and processing ADS-B information has been identified as an essential function for the new generation of secondary radars. When the ADS-B function is integrated into the structure of a secondary radar, the receiving function mainly utilizes the CONT-Front and CONT-Back patterns to achieve as close to 360° listening coverage as possible. These patterns generally have a gain of just over 17 dB, which is less than the maximum gain of the sum pattern (denoted SUM). However, the sum pattern used for synchronous detection in civil or military applications allows a gain of just 27 dBi to ensure a detection range of just 250 Nmi.

[0007] In contrast to the synchronous mode, in civil or military applications, radar range is traditionally limited primarily by the range of the uplink at 1030 MHz (i.e., by the power of the transmitted interrogation and the receiving end sensitivity of the transponder), and ADS-B is limited only by the downlink at 1090 MHz, i.e., by the power of the transponder and the sensitivity of the ADS-B receiver. Indeed, in the construction of a secondary radar, the downlink (receive) essentially has a higher sensitivity than the uplink (transmit) to ensure that any reply generated by the transponder is receivable and usable.

[0008] As a result, the difference between the operating gain of a synchronous radar and one listening to asynchronous ADS-B transmissions is, all else being equal, on the order of 8-10 dB for a two-sensor architecture. This leads to a guaranteed ADS-B range, typically on the order of 100 Nmi, for an ADS-B receiver structure integrated into a secondary radar, but in addition, it will be noted that asynchronous replies received via the control pattern (CONT) are much more likely to be distorted than synchronous replies received via the SUM pattern, since this is much narrower in characteristics (having a 3° degree or so). Furthermore, the overlap of replies, which is said to result in distortion, i.e. replies occurring at the same time as each other, makes detection and decoding more difficult. Thus, the guaranteed ADS-B detection rate in a congested environment by the receiving aircraft is 1 / 2 to 1 / 3 that of a Mode S secondary radar.

[0009] Prior art solutions are known that attempt to overcome these performance shortcomings. Specifically, in dedicated ADS-B receivers, the prior art involves increasing the number of independent antennas to cover 360°, with the aim of providing protection through azimuth selectivity and achieving improvements by associating one beam with each receiver at a time. The receivers are coupled to a common processing stage that groups together detection operations, thus avoiding duplicate detections and in particular the passing of targets from one receiver to the next (thus allowing the construction of a single ADS-B track).

[0010] One drawback of this type of solution is the cost of infrastructure, especially for the ADS-B system, which grows with the number of antennas, which are typically between four and eight (multiple antennas, multiple downleads, multi-side pylons, multiple receivers, global ADS-B processing, etc.).

[0011] In the case of an ADS-B receiver integrated in a radar architecture, detection via sum and difference patterns (SUM and DIFF) allows a larger gain and better azimuth selectivity to be realized in addition to detection via the CONT pattern, but only for a very small part of the time (of the order of 1.4% corresponding to 5° / 360°). Therefore, this only allows the guarantee of a better listening time coverage over 360° on the axis of the antenna in addition to the ADS-B listening operation performed via the gain of the CONT pattern. In particular, it is known that global detection by an ADS-B receiver requires two successive position squitters to generate a track. Thus, even if the first reply is received via the SUM pattern, the second one will necessarily be received via the CONT pattern and therefore will ultimately be limited to the range of only the CONT pattern for the ADS-B receiving chain integrated in the secondary radar architecture. Summary of the Invention [Problem to be solved by the invention]

[0012] One object of the present invention is in particular to alleviate the drawbacks of the prior art by making it possible to obtain a very long range ADS-B detection of a target. To this end, one subject of the present invention is a method for locating a target transmitting ADS-B squitter by means of a secondary radar comprising an antenna having a radiation pattern forming a sum channel denoted SUM, a radiation pattern forming a difference channel denoted DIFF and a pattern forming a control channel denoted CONT, said target being: - detecting ADS-B squitter received via the CONT channel, via the SUM channel and via the DIFF channel; - measuring at least the power of said squitter and its azimuth angle relative to said radar. wherein the position of a target transmitting an ADS-B squitter is calculated by utilizing detection of at least a single ADS-B squitter in light of the latitude and longitude position of the radar and an azimuth measurement relative to the radar, and a location cell, denoted a CPR cell, encoded within the squitter is selected via the azimuth measurement. [Means for solving the problem]

[0013] If two coded CPR positions coded in the squitter are possibly located within the same azimuth sector, the two positions are discriminated between using, for example, the measured power and altitude coded in the ADS-B squitter, and the position retained is the most likely position based on an estimator based on the visibility of the target and the consistency of the power received by the radar with the distance from the target to the radar.

[0014] In one particular mode of implementation, the method further comprises the steps of: obtaining a roll-call of targets; - a location coded in an ADS-B squitter transmitted by the target as soon as the target enters the receiving coverage of the radar; - a roll-call identification of the target contained in the Mode S address of the target encoded within the squitter; By utilizing a roll-call interrogation scheme, it is ensured that as soon as a target enters the interrogation coverage of the radar, a single roll-call interrogation is sent every N revolutions to lock the target onto the radar station's code, potentially as soon as the target enters the interrogation coverage and into the area of ​​operational range of the radar, and thus avoid contamination by the M DF11 replies that a target would normally generate on each revolution.

[0015] The method may include, for example, steps in which the targets are detected and located via their ADS-B squitter prior to interrogation coverage of the radar, the targets are associated with ADS-B tracks as soon as they enter the interrogation coverage, allowing them to be acquired by the secondary radar, and then the tracks are temporarily unlocked upon their entry into the operational radar coverage to allow their potentially locked state, i.e. whether they are locked onto another radar, i.e. the absence of a synchronous DF11 reply, to be detected, and thus potentially providing collision warnings in II / SI code for other Mode S targets without ADS-B capability, while ensuring Mode S roll-call monitoring of the target as soon as it enters the operational coverage of the radar.

[0016] The method may, for example, comprise a step in which within one moving time window, for each 3D geographical cell, an average of the difference between the range of a target measured by the radar and the range of the target coded in the ADS-B squitter transmitted by the target is calculated, this is performed for all targets passing through the cell, and then each range of any target measured by the radar in any mode is then corrected in view of the average difference calculated for the geographical cell over which the target is flown.

[0017] The method includes the steps of interrogating a target transmitting ADS-B squitter located via the method upon entry into the interrogation coverage of the radar, whether in roll-call mode or not, and measuring the sensitivity of the transponder by calculating the sensitivity of the transponder based on the power received by the transponder in its first reply to the interrogation at 1030 MHz, and then detecting a transponder with unsatisfactory sensitivity to the interrogation at 1030 MHz transmitted by the radar, a transponder being declared to be of unsatisfactory sensitivity if the power is above a given threshold.

[0018] The method may, for example, include a step in which an unsatisfactory altimeter associated with a transponder of an ADS-B target is detected by utilizing a deviation of the difference between the pressure and geometric altitudes coded in the ADS-B squitter transmitted by the target from an average, within a moving time window, of the differences between the pressure and geometric altitudes calculated for other targets per cell, for each of the geographic cells flown over by the target, within a series of geographic cells flown over by the target, and the altimeter is determined to be unsatisfactory if the deviation exceeds a given threshold.

[0019] Another subject of the invention is a radar capable of implementing such a method.

[0020] Other characteristics and advantages of the invention will become apparent from the following description given with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0021] [Figure 1] 1 shows possible steps for implementing the present invention. [Diagram 2] FIG. 1 is a diagram of a conventional ATC antenna pattern. [Diagram 3] FIG. 1 is a diagram of the azimuth sensitivity of a conventional ATC antenna in integrated ADS-B mode. [Figure 4] FIG. 1 is a diagram of a prior art secondary radar architecture without ADS-B reception. [Diagram 5] 4 is an example of a modification to the architecture of a secondary radar implementing the method according to the invention; [Figure 6] 1 shows various interrogation and detection ranges utilized by a radar according to the present invention. [Figure 7] FIG. 1 is a diagram of the global ADS-B detection principle with two consecutive squitters. [Figure 8] 1 is a diagram of the ADS-B detection principle with a single squitter according to the present invention. [Figure 9] FIG. 1 is a diagram of possible ADS-B disambiguation. [Figure 10] FIG. 1 is a diagram of the various ranges involved in tracking a target entering radar coverage. [Figure 11] This is an example of an area of ​​conflict in II / SI codes between radars whose coverage overlaps. [Figure 12] 1 is an example of a map of the average difference between pressure altitude and geometric altitude. [Figure 13] 1 is a shape of an exemplary distribution of altitude deviations in 3D geographic cells. [Figure 14] FIG. 13 is a zoomed-in view of target altitude deviation consistent with targets within the same geographic cell. [Figure 15] FIG. 13 is a zoomed-in view of a target showing altitude deviations that are inconsistent with targets within the same geographic cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] FIG. 1 shows the steps used for the implementation of the present invention. For this implementation, at least two steps are required. For this implementation, the present invention: - a continuous detection 102 via the CONT_Front and CONT_Back patterns, which necessarily has a very high sensitivity, but further optionally, successive complementary detections via the patterns of the main lobe with lower sensitivity of around 15 dB (detection via the SUM pattern 103 and detection via the DIFF pattern 104) with the aim of linking the antenna gains of the CONT patterns (CONT_Front and CONT_Back) and thus filling in a gain gap occupying around 5° in the CONT diagram on the axis of the antenna. The present invention provides for the presence of ADS-B detection capability integrated into the secondary radar structure 101, which includes:

[0023] Figures 2 and 3 show the antenna patterns associated with the receiving end at 1090 MHz. Figure 2 shows the SUM, DIFF and CONT antenna patterns of an antenna traditionally used for the ATC function of secondary radars. The gain of the SUM and DIFF patterns is much greater than the gain of the CONT pattern about the axis of the antenna. Figure 3 shows the azimuth and integrated ADS-B sensitivity of a traditional ATC antenna, where the SUM and DIFF channels are aligned with the same receiving end sensitivity as the CONT channel.

[0024] Referring again to FIG. 1, in a first step 110, ultra-long range detection of ADS-B squitters is performed, which results in: - ADS-B squitter is detected via SUM and DIFF patterns with extremely high sensitivity. The target position is calculated by a single squitter using conventional error-signal methods based on detection via SUM and DIFF, as detailed in the remainder of this description.

[0025] The terms ADS-B squitter and ADS-B reply are used interchangeably hereinafter.

[0026] In a second step 120, the first step 110 is operationally exploited. Passive detection of aircraft via ADS-B beyond, and possibly up to twice, the radar's synchronous operating range allows this aircraft to be exploited before it enters the radar's coverage, thereby improving both the operation of the radar and the safety of the ATC surveillance system, as will be shown below.

[0027] In the field of radar and more generally in the field of ATC flight safety, - RF pollution is reduced by replacing the all-call (DF11) replies with several roll-call (DF04) replies, which are much smaller in number, and by locking out low frequency targets as soon as the targets enter the radar's transmit coverage and therefore well before entering the radar's operational coverage, to limit pollution; - detection of collisions in II / SI codes at the limits of the radar range has been improved and detection of ADS-B targets within the radar coverage and locked incorrectly by another radar (i.e. more specifically ADS-B targets beyond the ADS-B listen range that is achievable via the CONT pattern) has been ensured; - The accuracy of radar synchronous ranging is improved by compensating for ionospheric propagation distortions. In the area of ​​ATC safety, for aircraft equipped with ADS-B transmitters, the following items are performed prior to entry into the radar's operational coverage: - non-polluting detection of unsatisfactory sensitivity of transponders, - Detection of an unsatisfactory altimeter associated with a transponder.

[0028] In the following the principles of the invention and its use in various areas of detection will be explained in more detail, firstly a possible adaptation of the secondary radar architecture to implement the invention will be described.

[0029] Thus, Figures 4 and 5 show modifications to the architecture of a Mode S radar according to the present invention, with Figure 4 showing a conventional prior art structure and Figure 5 showing a structure embodying the present invention.

[0030] The components of such a radar will be recalled with reference to Fig. 4, which shows an example of a diagram of a conventional Mode S radar. Fig. 4 shows a minimal architecture of a Mode S secondary radar without integrated ADS-B reception via CONT_Front and CONT_Back patterns, which is not a standard reception mode in conventional secondary radar configurations. The following description presents a solution based on four channels: SUM, DIFF, CONT_Front and CONT_Back, but it will be appreciated that the invention also applies to a solution based on three channels: SUM, DIFF and CONT.

[0031] When used in its conventional manner, the secondary radar operates in a synchronous mode, i.e., it transmits an interrogation and waits for a coherent reply, thereby enabling target location via measurements (azimuth and range) and identification (via Mode S address). The diagram in Figure 4 illustrates this synchronous operation of a Mode S radar, - The left hand part indicates the generation of the interrogation, - The right hand part shows the synchronous processing of the associated replies, and the synchronization therebetween is indicated by the cross arrows between the left and right.

[0032] To accomplish this task, radar must be able to: - SUM patterns to interrogate and detect target synchronous replies; - DIFF pattern for fine target localization within the SUM beam, - CONT_Front pattern to block and reject replies from targets facing the antenna but not in the main SUM beam, - A CONT_Back pattern to block and reject replies from targets behind the antenna (and therefore not necessarily in the main SUM beam), The system is equipped with an antenna 1 which transmits interrogations at 1030 MHz via four patterns 11, 12, 14, 15 and receives replies at 1090 MHz in response.

[0033] The SUM and DIFF patterns have 3dB lobes from 2.4° to 10°, conventionally narrow, while the CONT_Front and CONT_Back patterns each seek to cover practically 180°. - may have a set pattern, i.e. what are called "mechanical" antennas, and may rotate; - It may have an active electronically scanned pattern, called AESA, and may remain stationary or rotate.

[0034] The rotary joint 2 for the rotating antenna and the antenna down cable are - RF coupling of signals transmitted at 1030 MHz and received at 1090 MHz independently in four patterns between the rotating and fixed parts of the radar; - Transfer of the azimuth position 201 of the axis of the antenna's main lobe guarantee.

[0035] The RF processing stage consists of a duplexer or circulator 3 ensuring RF coupling between the signals transmitted independently at 1030 MHz and received at 1090 MHz for the four patterns; a transmitter 4, Transmits interrogation at 1030MHz via SUM pattern, Transmits pulses to block transponders outside the SUM lobe at 1030MHz via the CONT_Front and CONT_Back patterns; Transmitter 4, which implements it for various secondary protocols: IFF, SSR and Mode S; - Receiver 5, which receives replies at 1090MHz via four patterns: SUM, DIFF, CONT_Front and CONT_Back, and calculates various secondary protocol error signals: IFF, SSR and Mode S. Includes.

[0036] The real-time processing stage is - A Space-Time Manager 6 that manages the interrogation periods and associated listen periods for the various secondary protocols IFF, SSR and Mode S in real time; a signal processor 7, - Processing replies with interrogations and associated listen periods for various secondary protocols, IFF, SSR and Mode S; -Synchronous reply within the main lobe of the antenna, SUM to detect replies received within the main lobe, DIFF to finely locate in azimuth replies received within the main SUM lobe and de-distort potentially distorted replies; CONT_Front and CONT_Back to reject replies received via the side SUM and DIFF lobes A signal processor 7 detects and decodes the four radiation patterns by using the Includes.

[0037] The stage that processes the antenna's main lobe is a manager 8 of a target present in a lobe, Prepares the transactions to be executed on the next lobe for the various secondary protocols of IFF, SSR and Mode S (Interrogation-Reply); Manages IFF, SSR, All-Call Mode S and Roll-Call Mode S durations within a lobe; Manager 8, which dynamically arranges selective Mode S interrogations and replies at the next roll-call period depending on the status of the transaction just executed and any new aircraft entering the lobe; - Extractor 9, which generates plots for each of the various secondary protocols (IFF, SSR and Mode S) based on the sync replies received within the lobe and depending on the protocol being used for interrogation. Includes.

[0038] The multi-rotation processing stage 10 includes: - a manager 101 of Mode S tasks executed for targets in coverage, which predicts target positions (antenna crossings) and prepares the tasks that need to be executed that are associated with these positions, according to internal requests, external requests and the state of the transactions of the previous rotations; - correlation of plots and tracks 102 of targets in coverage, improving performance (in particular by eliminating false plots and checking the decoded data) and ensuring that targets are tracked with the aim of predicting their future positions, mainly but not only in Mode S; The user interface allows the radar to take into account various requirements and plots and tracks of observed targets.

[0039] Figure 5 shows modifications to the conventional architecture of Figure 4, these modifications are indicated by the use of thick dashed lines. It can be seen that while the operation of the Mode S radar is synchronous, the processing operations 31, 32 added for the present invention are not linked to the transmission and only utilize the azimuth position of the axis of the antenna's main lobe in the context of the first step 110 of the present invention. The majority of the elements remain unchanged, thereby fulfilling the criterion that the present invention does not go as far as functional operation of the Mode S radar.

[0040] The main elements added have the following functions: In Space-Time Manager 6, - transmitting 33 the azimuth position of the antenna main lobe for operation 31 for processing asynchronous ADS-B replies (see below); In the signal processor 7, a processing operation 31 for detecting and decoding asynchronous ADS-B replies continuously (i.e. independent of the duration of the interrogation transmission) and separately, but also with its maximum sensitivity, by utilizing four patterns SUM, DIFF, CONT_Front and CONT_Back, For detecting all received ADS-B squitters: DF17, to extract the Mode S address from it, Addition to enrich each decoded reply by its characteristics of the time of detection, the azimuth angle of the antenna main lobe at the time of detection, the power received via SUB, DIFF, CONT_Front and CONT_Back, and the off-boresight angle of the ADS-B squitter in the main SUM and DIFF beams, calculated using the error-signal method; In the non-real-time processing stage (multi-rotation processing stage), - Added 32 long range locations for ADS-B targets; - Decision 34 to conduct selective interrogation upon reaching synchronous interrogation range.

[0041] In the following, the principle of the invention will be explained with reference to Fig. 6. The radar according to the invention still uses the CONT pattern to detect ADS-B squitters in a conventional manner. The range of the radar for this conventional ADS-B processing is indicated by the first circle 41. In addition, as will be explained later, the invention advantageously uses the range limit of reception (RX) via channels SUM and DIFF of signals transmitted by the transponders of the surrounding aircraft, which range limit is indicated by a circle 42 of a larger diameter. The reception range is itself greater than the maximum range of interrogation transmitted (TX) by the radar and interpreted by the transponder, which range is indicated by a third circle 43 corresponding to a highly sensitive transponder.

[0042] In other words, the principle of long-range ADS-B detection according to the present invention is as follows: - the operational coverage of the radar, as indicated by the last circle 44 and based on the following guaranteed values: A transponder with the lowest sensitivity for transmission at 1030 MHz, Minimum power transponder for synchronous reception at 1090 MHz, Radar antenna SUM and DIFF pattern gain in synchronous mode, - necessarily a cumulatively narrower radar coverage area than the three preceding standards; - the maximum range of interrogation at 1030 MHz 43, taking into account the transponder with the greatest sensitivity, whose maximum range is used in the second step 120; - maximum receiving range at 1090 MHz, taking into account the transponder transmitting with maximum power 42; - ADS-B operating range 41 for the ADS-B functionality integrated into the radar, based on the following guaranteed values: The gain of the CONT pattern of the antenna used in conventional ADS-B mode to ensure continuous detection of ADS-B squitters (see below); Minimum power transponder for asynchronous reception at 1090MHz This is based on taking advantage of the difference in range between the

[0043] Fig. 7 shows the principle of ADS-B global detection with two squitters (hereafter replies) applied in the prior art recalled in the introduction. It is known in the literature that the position information of the ADS-B responders is coded in two ADS-B replies in a format denoted CPR (CPR is an abbreviation for Compact Position Report) format, one reply transmitted in a first geographical reference frame called the even frame and the other reply transmitted in a second geographical reference frame called the odd frame. Fig. 7 shows as an example nine reply cells 61, the even reply 62 is depicted in bold and the odd reply 63 by finer characters. Each CPR cell 62, 63 contains latitude and longitude information related to the cell, the cells being represented in a system of axes where the y-axis represents latitude and the x-axis represents longitude. As is known, an ADS-B receiver employing such global detection may use the following techniques to locate the source of an ADS-B transmission: - two consecutive ADS-B squitters of two different types must be received: an even ADS-B squitter frame 62 and an odd ADS-B squitter frame 63; The location of the source (target) must be determined by computing the unique CPR cell 64 that the even and odd locations correspond to. It is therefore possible to reconstruct absolute latitude and longitude positions in the Earth's reference frame (rather than relative to a cell).

[0044] FIG. 8 illustrates the principle of ADS-B detection according to the present invention. Here, a reply of a single frame, e.g. a non-even frame, is shown. The principle of the present invention is: - the latitude and longitude position of the radar 70; - the azimuth measurement 71 obtained by the radar 70 upon reception of this ADS-B squitter, obtained using a SUM or DIFF pattern; The method includes locating a target based on a single (even or odd) ADS-B reply by simultaneously utilizing

[0045] Specifically, the latitude and longitude position of the radar and the azimuth measurement of the squitter are known (within a given tolerance), so the following two conditions are met: - compact ADS-B position transmitted within received squitter, - Azimuth measurements of the squitter observed from the radar's latitude and longitude position It is then possible to compute a CPR cell 72 that satisfies (i.e., 0.1 V, 1.2 V, 1.3 V, 1.4 V, 1.5 V, 1.6 V, 1.7 V, 1.8 V, 1.9 V, 2.0 V, 2.1 V, 2.2 V, 2.3 V, 2.4 V, 2.5 V, 2.6 V, 2.7 V, 2.8 V, 2.9 V, 3.0 V, 3.1 V, 3.2 V, 3.3 V, 3.4 V, 3.5 V, 3.6 V, 3.7 V, 3.8 V, 3.9 V, 3.1 V, 3.2 V, 3.4 V, 3.5 V, 3.6 V, 3.7 V, 3.8 V, 3.9 V, 3.8 V, 3.

[0046] To summarize the detection process according to the present invention, when acquiring an asynchronous squitter via SUM or DIFF, the radar measures its characteristics in the same way as a synchronous reply, thereby: - azimuth position of the antenna, - Off-boresight angle of the target in the SUM beam using the error-signal method, - SUM and DIFF power of SKITTERS Since distance has no meaning in the case of asynchronous replies, the step of decoding the squitter involves obtaining, for the target, - relative latitude position (even or odd CPR), - Relative longitude position (even or odd CPR) - Altitude supplies.

[0047] In the long-range ADS-B location of a target, a geographic ambiguity may be encountered when two possible positions 81, 82 of the target (within two consecutive CPR cells) are aligned with the radar position, as shown in Figure 9. The measured azimuth angle 71 alone is no longer a discriminant that allows the correct cell to be defined. Then, - The power of the squitter in SUM and / or DIFF, - Declared altitude of the target Two other characteristics of the received ADS-B squitter are exploited.

[0048] Since the radar characteristics of the geographic location (including its altitude) and the RF losses of the installation are known, a maximum likelihood location 82 can be defined between two possible locations by establishing an estimator based on the radar visibility of the target (as a function of the radar and target altitude) and the consistency of the received power of the squitter with the two locations.

[0049] this is, - Asynchronous reception range of the radar via SUM and DIFF patterns, - Detection and location of targets using a single squitter This is done by simultaneously utilizing the radar and ADS-B. The radar advantageously detects ADS-B targets at very long ranges, more than twice the operating range of conventional radars.

[0050] Figure 10 shows, for example, various regions of detection for FL500 (approximately 15000m altitude) and the visibility of the aircraft and its path 99, this visibility being represented by a curve 90 depending on the altitude of both the radar and the target. Figure 10 makes it possible to observe, in this example, the following as a function of the target distance on the X-axis and the target altitude on the Y-axis: - the maximum long-range ADS-B detection area bounded by the almost vertical line 92, depending on the power of the transponder and the sensitivity of the ADS-B receiver in the radar SUM pattern; - the minimum guaranteed long-range ADS-B detection area bounded by the nearly vertical line 95; - the maximum area of ​​contamination by the All-Call DF11 reply (transmit contamination range), bounded by the nearly vertical line 93, depending on the sensitivity of the Mode S transponder to radar interrogation via the SUM pattern; - the guaranteed operating range of the radar, bounded by an almost vertical line 94; - The conventional detection area of ​​an integrated ADS-B receiver via the CONT pattern, bounded by the almost vertical lines 91. It should be noted that transponders that broadcast ADS-B squitter are typically modern in nature and therefore their RF characteristics are typically good and exceed the minimum for any given Mode S transponder, which must therefore handle all Mode S targets, even those not equipped with ADS-B capabilities, that typically constrain the operational range of a radar.

[0051] Looking to the left of the radar altitude dependent aircraft visibility limit 90 as a result of the curvature of the earth, one can observe the area between the most preferred long range detection of ADS-B squitter according to the present invention (line 92) and the guaranteed limit of ADS-B squitter (line 95), where an aircraft in radial flight conditions can be detected before entering the contamination region 93, then the radar 94 operating region and finally the integrated ADS-B detection region 91 (guaranteed conventional ADS-B detection).

[0052] For example, considering an aircraft approaching radar coverage at altitude FL500, the present invention: - Area A, where ADS-B squitters are detected at long range before the possible onset of DF11 contamination; - area B, which corresponds to the beginning of DF11 contamination before radar coverage; - Area C corresponds to radar coverage including conventional and integrated ADS-B reception This allows three areas to be taken into account:

[0053] As the above description shows, the present invention is based on the fact that the operations required to integrate ADS-B functionality into a radar architecture are almost identical to the Mode S signal processing operations implemented by a radar to process synchronous (DF11, DF04 / 05, DF20 / 21) replies, since ADS-B squitters are in fact just extended Mode S replies. The present invention is based on the fact that the operations required to integrate ADS-B functionality into a radar architecture are almost identical to the Mode S signal processing operations implemented by a radar to process synchronous (DF11, DF04 / 05, DF20 / 21) replies, since ADS-B squitters are in fact just extended Mode S replies. - the received squitter is located in azimuth and power by performing conventional very high precision radar measurements on the asynchronous replies, instead of the synchronous replies as in the conventional case; - to obtain a high antenna gain, on the order of 15 dB higher than the gain of the CONT pattern normally used for ADS-B detection, i.e. a level of range only four times greater than the normal (conventional) integrated ADS-B range (for reasons of ease of illustration, the various separations are not drawn to scale); - Locating targets at very long ranges with a single squitter (and not the two conventionally required when very long range detection is not required) by utilizing knowledge of the radar's position, the highly accurate azimuth measurements obtained by this radar, and the relative latitude and longitude CPR positions encoded in the detected ADS-B squitters. This makes it even more possible.

[0054] However, if the temporal coverage remains low, at or below 1.4% depending on the width of the SUM pattern, the present invention enables the secondary radar to treat distant targets at very long ranges (200 Nmi to 500 Nmi as shown in FIG. 10) in the same manner as is conventionally performed with closer targets by a conventional integrated ADS-B receiver utilizing an omnidirectional CONT pattern.

[0055] In the following, the advantages made possible by the present invention, which are utilized in the second step 120 (FIG. 1), are described, which are: - Regarding radar, Reduced pollution at 1090MHz, Detection of collisions in II / SI codes and detection of falsely locked targets in areas of overlapping coverage between two radars in a collision; Correction for distances measured by radar (propagation dependence), - Regarding ATC safety, Detection of transponders with poor sensitivity at 1030MHz, -Generating atmospheric pressure maps and detecting insufficient altimeters In the following, five of these operational improvements advantageously enabled by the present invention are described.

[0056] Reducing the number of replies from targets outside the operational coverage of contamination and radar at 1090MHz. Currently, the occupation of the 1090 MHz spectrum is becoming one of the weak spots in ATC surveillance. The pollution encountered is due to its success, and the reply utilized is - DF11 for acquiring new targets by Mode S radar, - DF04 / 05 for Mode S-ATC surveillance, Basic Surveillance (ELS), - DF20 / 21 for Mode S-ATC surveillance, Improved Surveillance (EHS), - DF0 for active TCAS collision avoidance, - DF17: ADS-B Squitter for Passive ACAS Collision Avoidance (ACAS stands for Airborne Collision Avoidance System) and so on.

[0057] The D04 / 05 / 17 / 20 / 21 replies will continue to exist, given the purpose of the functions they perform on a Mode S radar when operationally used. The DF0 reply will eventually be replaced by the DF17 reply. The DF11 reply is still unknown to the radar, and - Mode S radar cluster This reduces contamination only inside its coverage, and DF11 contamination is generated by targets that are only partially replaceable by another means such as: - Track initialization based on ADS-B squitter Although ADS-B continues to exist outside of the radar, the RF coverage of omnidirectional ADS-B receivers is approximately half that of radar, and therefore can only be used at short and medium radar ranges to initiate radar tracks based on ADS-B squitters; this therefore does not prevent long-range contamination and does not limit the range of radar.

[0058] To explain the contribution of the present invention to reducing pollution of the 1090 MHz spectrum, refer to Figure 10. Consider a target flying at FL500 entering the radar coverage: - Region A: Almost as soon as a target becomes visible, it can be detected by long-range ADS-B detection; - Region B: If the radar is allowed in this azimuth, it can lock onto the target via roll-call UF4 interrogation as soon as the target enters the radar's transmit coverage and thus before it starts responding to the DF11 all-call interrogation, after which the radar keeps performing roll-call interrogations at a lower rate (about 15-18 seconds), which is just enough to prevent the target from going into an unlocked state, and thus DF11 contamination (3 in each 5 second rotation) is reduced by a smaller amount than DF04 contamination (1 per 3 5 second rotations) and thus by a factor closer to 10; - Region C: The target enters the operational coverage of the radar, so that advantageously a track has already been established. Thus, the present invention allows both reduced contamination caused by our radar and reduced number of replies unnecessarily generated by the transponder, while accelerating radar tracking when the target eventually enters the Mode S radar's operational coverage.

[0059] Detection of areas of collision in II / SI codes at long range and detection of mislocked ADS-B targets In the following, the principle of collision in II / SI code at the limit of the radar range is recalled with reference to Fig. 11. Several solutions (see in particular FR 1 913 154 A1) propose to detect areas of radar coverage in which there is a collision in II / SI code with another radar R2 in the vicinity of the radar of interest R1, whose operational coverages 111, 112 overlap. The area of ​​overlap 113 is the area of ​​collision in II / SI code between the two operational coverages (if II1 = II2) and thus the area in which the two radars R1, R2 lock onto identical II / SI code targets entering their respective coverages, thus concealing them from the other radar.

[0060] As an example, if various existing methods are considered, depending on the location of a very long range radar R1, which is usually 250 Nmi (the one of R2 is bidirectional), the difference between the maximum distances between the existing target (which is therefore located far away because it is already locked by R1) and the incoming target of R1 (which is therefore only observed when approaching closer because it is already locked by R2) at the common azimuth angle of R1 and R2 indicates the probability of collision. This method only indicates (probability based on behavior), it does not allow to recognize that the incoming target was not observed in the area of ​​collision and therefore does not alleviate this problem and therefore cannot guarantee ATC safety.

[0061] At this common azimuth of R1 and R2, Mode S radar tracking of the still undetected target (via UF11 / DF11 All-Call or by Mode S cluster) using the Mode S address and location given by the ADS-B squitter broadcast by the target throughout the ADS-B coverage of radar R1: - conflicts in II / SI codes within the common area 114 of R2 operational coverage and R1 ADS-B coverage are addressed; - High responsiveness is guaranteed as soon as the target reaches the ADS-B coverage of R1, and therefore the locked ADS-B target is detected by R2. However, the conventional range of the R1's integrated ADS-B receiver is limited by the lower gain of the CONT channel compared to the R1's SUM channel (approximately 15 dB), which does not cover the entire area of ​​radar impact at long ranges.

[0062] For the same reasons, the analysis of the presence and dispersion of FRUITS, such as that described in FR 1 913 154, is henceforth applicable to all Mode S aircraft, whether or not equipped with ADS-B OUT capability, but with the ADS-B range of R1 limited by the lower gain of CONT vs SUM of R1, and therefore this analysis applies to radars of intermediate range.

[0063] According to the present invention, the ADS-B target is thereafter detected and located via its squitter and associated with the ADS-B track well before the operational coverage of the radar, i.e., the area of ​​reception of UF11 and DF11 All-Calls, which is also the area of ​​UF04 / 05 and DF04 / 05 Roll-Call interrogation, i.e., at this distance far away from the radar as shown in FIG. 10.

[0064] However, even if the above-mentioned contamination elimination principle does not apply (e.g., if the user refuses to allow a lockout to occur outside of coverage and therefore in area B), radar R1 can still interrogate a target as soon as it enters its area of ​​operational coverage (area C), provided that the target is equipped with ADS-B capability, thereby ensuring complete Mode S surveillance in the full area of ​​conflict in the II / SI code.

[0065] Thereafter, a collision in II / SI code is declared only within the operational coverage of the radar 94, and if an ADS-B target has been locked out in region B to reduce contamination by DF11 replies, the present invention proposes to cancel the lockout upon entry into region C, if permission to do so is granted, in order to verify the absence of a DF11 reply from this target in region C, with the purpose of declaring a collision in II / SI code if the radar receives a DF04 / 05 reply to a roll-call interrogation of this same target, thus in this manner the target is first detected, and only then is it verified whether a collision in II / SI code exists, in order to provide a warning of potential non-detection, particularly for aircraft equipped with Mode S, but not ADS-B capability. If an ADS-B target was not locked in region C, the absence of a DF11 reply from the target to the radar prior to its entry into region B, and thus prior to the target being locked within the operational coverage of the radar, is an indicator of a collision in II / SI code.

[0066] Correction for distances measured by radar (propagation dependence) It is known that secondary radar waves at 1030 MHz and 1090 MHz do not propagate in a straight line as a result of refractive index distortions in the ionosphere, with the result that targets appear to be farther away than they actually are.

[0067] In accordance with the present invention, the same ADS-B squitter used for long range ADS-B detection can be used to correct the distance measured by the radar. - The ADS-B "Airborne Position" message, transmitted approximately every half second, provides both latitude / longitude position and pressure altitude.

[0068] Since step 110 allows the latitude and longitude position and altitude of an ADS-B target to be determined by a single squitter, it is possible to recalculate the exact distance to the radar from the target, i.e. the geographical site and altitude at which the target is located, in relation to the known latitude and longitude position of the radar, which is mainly useful in the case of high altitude (above 1000m altitude) radars with visibility of medium altitude targets at very long distances from the radar.

[0069] Therefore, the present invention proposes to collect, on a travel time basis that may be adjustable to reflect the atmospheric stability of the environment, statistics on the difference between the distances measured by the radar and the distances calculated by the ADS-B targets present in the cell for each 3D geographical cell and altitude slice. The use of ADS-B targets per cell is therefore associated with the average difference between the distances measured by the radar and the geometric distance. This analysis can be performed in region C or possibly in region B, if the radar is allowed to interrogate targets outside its operational coverage.

[0070] Thus, if the statistics per cell are representative (i.e. based on a sufficient number of samples with stable range differences and consistency with neighboring cells), each synchronous SSR, Mode S or IFF target detection by the radar, regardless of whether said target has ADS-B capabilities or not, allows the evaluation of the measured distance (thus including the effects due to propagation through the ionosphere) to be completed by an evaluation of the geometric distance, so as to correct the measured distance according to the average difference of this cell at this altitude and therefore for the contribution of ionospheric reflections. The distance measured by the radar is thus corrected according to the average difference calculated within the geographical cells over which the target is successively flown. With regard to multi-radar management, this correction of the distance also allows an improved correlation of the tracks generated by various radars of the same target, and thus also to correct for the unequal layer propagation observed thereby.

[0071] Low sensitivity transponder detection One purpose of this optional step of the present invention is to detect the presence of aircraft containing transponders with low sensitivity, i.e., sensitivity below the standard threshold, immediately after entering the radar's far-flung coverage, thus providing a warning of the potential vulnerability of the radar's synchronous detection of these aircraft, i.e., aircraft that may no longer be detected as a result of maneuvers that result in a reduction in the radiation field received from the radar. ICAO defines in Annex 10, Volume IV, the RF characteristics expected from transponder antennas installed on aircraft. - Sensitivity: -71 to -77 dBm (synchronous mode only) for the ability to listen for interrogation; - Power: 51-57 dBm (125-500 W) for transmitting synchronous and asynchronous replies.

[0072] FR 1 800 479 proposes to evaluate the sensitivity of the transponder in an active configuration a priori, usually at the take-off of the aircraft from the airport and therefore at close range, by transmitting various interrogations that complement the monitoring of the target. The power of these interrogations is attenuated (both through reduction at the radar transmitter and also through the use of attenuation of the antenna gain when the target is lobed and very off-boresight) in order to determine the power at which the target no longer responds. In particular, it turns out that the power of the transponder replies remains at a maximum even at short distances from the interrogator, so that non-detection of the replies by the radar can only be attributed to non-interpretation of the interrogations by the transponder.

[0073] The invention now proposes to evaluate the sensitivity of a transponder in a quasi-passive configuration, i.e. without generating any further interrogation and therefore without further contamination, which would work contrary to the previous schemes, with the same objective of providing a warning of the presence of a responder that does not have a sufficiently high sensitivity.

[0074] For this purpose, as soon as an ADS-B squitter is received in the main antenna lobe (SUM and / or DIFF), the target is identified via its Mode S address and located in terms of azimuth and distance. To limit the pollution generated by the DF11 reply that the target transmits as soon as it interprets the interrogation, the target is selectively locked out as soon as possible via a roll-call interrogation performed using the full power of the radar's transmitter in its central lobe (and thus of maximum gain) at a very low repetition rate (typically once every 15 seconds).

[0075] Also, when the radar locates a target via its ADS-B squitter and the target starts responding to a UF11 All-Call or UF04 / 05 Roll-Call interrogation, it may be taken for granted that the power transmitted by the transponder will be within the specifications, which will therefore just exceed the receiving end sensitivity threshold of the transponder, and the radar will then be able to calculate the power received by the transponder's antenna, as described in the abovementioned patent application FR 1 800 479, by taking advantage of the characteristics, which are: Regarding radar, - Antenna gain (maximum gain - loss of antenna gain due to off-boresight angles of the target azimuth during interrogation), - losses due to the cable between the transmitter and the antenna, - the peak power of its transmitter, and Regarding the target, - loss of antenna gain as a function of the elevation angle of the target as seen by the radar antenna; - Propagation loss estimated by assuming that propagation effects remain similar between interrogation and reply since the frequencies are very close together (1030 MHz for interrogation and 1090 MHz for reply), while taking into account other known characteristics related to the two frequencies.

[0076] What both cases have in common is that as soon as the target appears in space, and although there are differences between the two methods, which are presented below, they complement each other and both work towards the same goal.

[0077] With regard to FR 1 800 479, it is usually sought to obtain sensitivity measurements at the time of take-off of the aircraft from the airport and therefore at small distances, by reducing the transmitted power with successive interrogations until the point at which the receiving end sensitivity threshold is crossed, thus preventing the transponder from responding; thus the search is active and causes pollution, but remains acceptable, since it is only performed at the initialization of the route.

[0078] According to the present invention, one usually seeks to obtain sensitivity measurements at long ranges even before the aircraft enters the radar's space. If an aircraft does not respond to an all-call or roll-call interrogation when the downlink (aircraft to radar) budget is good because an ADS-B reply is detected, this means that the transponder is still below the receiver sensitivity threshold for interrogations at 1030 MHz transmitted by the secondary radar. Further roll-call interrogations aimed at the transponder will not cause pollution because they will not detect them until they cross the receiver sensitivity threshold.

[0079] The solution made possible by the present invention is advantageous for the integrity of the ATC, since this requires that any failure of the surveillance system be detected as soon as possible when the aircraft is in flight.

[0080] Unsatisfactory altimeter detection The separation in terms of altitude of aircraft is one of the fundamental elements of aviation safety, both for air traffic control and for automatic systems such as on-board collision avoidance systems. Measurement of atmospheric pressure makes it possible to define the local altitude of aircraft and thus to guarantee the separation in terms of pressure altitude between aircraft on the same flight route. It is known that failures of an altimeter can result in aviation disasters. It is therefore important to detect failures of a barometric altimeter as soon as possible, or possibly in real time, with the invention enabling this before or as soon as an aircraft enters the air coverage of a secondary radar.

[0081] With the aim of detecting unsatisfactory barometric altimeters, the first step involves establishing the mean atmospheric pressure for each 3D geographical cell, which allows a map of the difference between barometric altitude and geometric altitude to be generated, the long-range ADS-B coverage of the present invention is pre-divided into geographical cells, the size of which may typically be 1° of latitude and 1° of longitude.

[0082] Information obtained from the airborne speed message, the second type of ADS-B message and transmitted every 0.5 seconds, supplies the altitude difference between the pressure measurement and the geometric measurement (obtained in particular via GNSS satellite positioning). According to the invention, statistics are collected on this difference for each geographical cell on a travel time basis, which may be adjustable, to take into account the atmospheric stability of the environment. Figure 12 shows an example of a map of the average of the difference between the pressure altitude and the geometric altitude thus obtained. The gradation of the average corresponds to the gradation of the greyscale levels across the map.

[0083] Also considered in relation to these averages per cell are: - the number of measurements in a cell that allows the establishment of an average, - The standard deviation of the measurements in each cell, which reflects the stability of the measurements within the cell and therefore the stability of the atmospheric pressure within this cell.

[0084] According to the invention, on a travel-time basis (where the average of the differences per 3D geographical cell is taken), - for each geographical cell, an analysis of the distribution of the differences between the pressure altitude and the geometric altitude over the range of possible values ​​provided in the ADS-B squitter is carried out, usually with a pitch of 25 feet, as shown in Figure 13, which shows an example of the distribution 131 of all these differences for targets present in the geographical cell; - after that, on the other hand, if the standard deviation of the differences is less than a parameter of the invention that indicates the stability of the differences at altitude within this cell, and On the other hand, if the number of differences in a cell exceeds another parameter that indicates the representativeness of the cell's average, Any radar target present within the cell will have its (barometric / geometric) altitude difference evaluated relative to the average altitude difference. Two parameters of the present invention allow the operator to tune the invention to the characteristics of his radar site. - Atmospheric stability via standard deviation, - Aircraft congestion through the number of measurements.

[0085] If this altitude difference is close to the distribution of other differences in the cell, according to a given proximity criterion 142, the target is considered to have a consistent barometric / geometric altitude difference and its ADS-B track score is increased in value, which is shown in Figure 14. This figure shows the target's difference via a curve 141 that is close to all the distributions of the differences 131.

[0086] If this difference 151 lies beyond the distribution of other differences in the cell by an excessive amount 152, as shown in FIG. 15, the target is considered to have an inconsistent pressure / geometric altitude difference, and its ADS-B track score is decremented by assigning it a value of −1 instead of a value of +1 as in the previous case (FIG. 14).

[0087] The consistency score of the pressure / geometric height difference of each ADS-B track is analyzed based on a given number of comparisons performed with the aim of declaring to the radar user if this score is less than the estimated unsatisfactory altimeter value. This value is defined taking into account the accuracy of the ADS-B position measurements transmitted in the squitter used to establish the pressure / geometric height difference, both for the cell reference and for the target of interest. Thus, the more accurate the altimeter error detection, which reduces the contamination of the targets used and therefore concerned by the test, the more this smaller amount of data per cell must be integrated in a longer moving window. This parameter allows a normal compromise between the stationarity of the measurement position and the context of the measurement to be adjusted.

[0088] The very long range detection of ADS-B squitters allows this statistical analysis to be performed before the target enters the Mode S radar's responsible coverage, and therefore allows air traffic controllers to manage the separation between such a target and its surroundings with a large margin to ensure ATC safety even in the event of an altimeter malfunction. [Explanation of symbols]

[0089] 1 Antenna 2 Rotary joint 3 Circulator 4 Transmitter 5 Receiver 6. Time Manager 7 Signal Processor 8. Manager 9 Extractor 10 Multi-rotation Processing Stages 11 Patterns 12 Patterns 14 Patterns 15 Patterns 31 Processing operations 32 Processing operations 33 Send 34 Decision 41 ADS-B operating range 42 Maximum range 43 Maximum range 44 Operating range 61 Reply Cell 62 cells 63 Cells 64 CPR Cells 70 Radar 71 Azimuth 72 CPR Cell 81 position 82 positions 90 Visibility Limit 91 ADS-B Detection Area 92 Line 93 Contaminated Areas 94 Radar 95 Line 99 Routes 101 Manager 102 Detection 103 Detection 104 Detection 110 First Step 111 Operational Coverage 112 Operational Coverage 113 areas 114 Common Area 120 Second Step 131 Distribution 141 Curve 152 quantity 201 Azimuth position

Claims

1. 1. A method of locating a target transmitting ADS-B squitter by means of a secondary radar including an antenna (1) having a radiation pattern forming a sum channel denoted SUM, a radiation pattern forming a difference channel denoted DIFF, and a pattern forming a control channel denoted CONT, the secondary radar having a predefined interrogation coverage (43), the method comprising: - detecting ADS-B squitters received via said CONT channel (102), via said SUM channel (103) and via said DIFF channel (104); - measuring at least the power of said squitter and its azimuth angle relative to said radar (110); and a position of a target transmitting the ADS-B squitter is calculated by utilizing the detection of at least one ADS-B squitter in view of the latitude and longitude position of the radar and the azimuth measurement (71) relative to the radar, and a location cell, designated a CPR cell (72), encoded within the squitter is selected via the azimuth measurement.

2. 2. The method of claim 1, characterized in that, when two CPR positions (81, 82) coded in the squitter are possibly located in the same azimuth sector, the two positions (81, 82) are discriminated between using the measured power and the altitude coded in the ADS-B squitter, and the retained position (82) is the maximum likelihood position according to an estimator based on the visibility of the target and the consistency of the power received by the radar with the distance from the target to the radar.

3. The target roll-call is obtained. - a position coded in the ADS-B squitter transmitted by the target as soon as it enters the radar's receiving coverage (42); the roll-call identification of said target contained in said target's Mode S address, encoded in said squitter; 3. The method according to claim 1 or 2, further comprising a step (120) of ensuring that the target enters the interrogation coverage (43) of the radar by utilizing a roll-call interrogation signal, characterized in that a single roll-call interrogation is transmitted every N revolutions of the antenna (1) in order to potentially lock the target onto a radar station's code as soon as the target enters the interrogation coverage (43) and into the area of ​​operational range (44) of the radar, and thus to avoid contamination by DF11 replies that a target would normally generate on each revolution.

4. 4. The method according to claim 1, further comprising a step (120) in which the targets are detected and located via their ADS-B squitters prior to the interrogation coverage (43) of the radar, the targets being associated with ADS-B tracks as soon as they enter the interrogation coverage, allowing them to be acquired by the secondary radar, and the tracks then being temporarily unlocked upon their entry into the operational radar coverage, to allow their potentially locked state, i.e. whether they are locked onto another radar, i.e. the absence of a synchronous DF11 reply, to be detected, and therefore to provide collision warnings in II / SI code for potentially other Mode S targets, without ADS-B capability, while ensuring Mode S roll-call monitoring of the targets as soon as they enter the operational coverage of the radar.

5. 5. The method according to claim 1, further comprising a step (120) of calculating, for each geographical cell overflown by a target within a moving time window, an average of the difference between the range of the target measured by the radar and the range of the target coded in the ADS-B squitter transmitted by the target, this being performed for all targets passing through the cell, after which the respective range of any target measured by the radar in any mode is corrected in view of the average difference calculated for the geographical cell overflown by the target.

6. 6. The method according to claim 1, further comprising the step of: interrogating the target transmitting ADS-B squitter located via the method upon entry into the interrogation coverage of the radar, whether in roll-call mode or not, and measuring the sensitivity of the transponder by calculating the sensitivity of the transponder based on the power received in its first reply to the interrogation at 1030 MHz, and then detecting (120) a transponder with poor sensitivity to the interrogation at 1030 MHz transmitted by the radar, characterized in that a transponder is declared to be of poor sensitivity if the power is above a given threshold.

7. 7. The method according to claim 1, further comprising a step (120) in which an unsatisfactory altimeter associated with a transponder of an ADS-B target is detected by utilizing a deviation of the difference between the pressure and geometric altitudes coded in the ADS-B squitter transmitted by the target from the average, within a moving time window, of the differences between the pressure and geometric altitudes calculated for other targets per cell, for each of the geographical cells flown over by the target, within a series of geographical cells flown over by the target, and an altimeter is determined to be unsatisfactory if the deviation (152) exceeds a given threshold.

8. A secondary radar, characterized in that it is adapted to carry out the method according to any one of the preceding claims.

Images