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

Abstract

The present application relates to a secondary radar for improving air safety through ultra-long-range ADS-B detection. The secondary radar comprises an antenna having a radiation pattern forming a sum channel denoted as SUM, a radiation pattern forming a difference channel denoted as DIFF, and a pattern forming a control channel denoted as CONT, and locates a target by performing the following steps: detecting ADS-B messages received via the CONT channel (102), via the SUM channel (103), and via the DIFF channel (104); measuring at least the power of the message and its azimuth relative to the radar (110); calculating the position of the target sending the ADS-B message by detecting at least one ADS-B message based on the latitude and longitude position of the radar and the azimuth measurement relative to the radar, and selecting a location cell denoted as a CPR cell encoded in the message via the azimuth measurement.

Description

Technical Field

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

[0002] Currently, air traffic control is primarily based on secondary radar, whose detection reliability is widely recognized. Secondary radar ensures synchronized surveillance of aircraft via the SSR and Mode S protocols. Furthermore, the asynchronous reception of extended ADS-B messages (ADS-B stands for Automatic Dependent Surveillance-Broadcast) (messages are position information transmitted by aircraft; they are not triggered in response to radar interrogation signals but are transmitted automatically from the aircraft's transmitters) designed to prevent collisions when implemented on aircraft (TCAS: Traffic Alert and Collision Avoidance System) is used by ground ATC, providing part of an interactive surveillance system.

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

[0004] The operational use of these different activities in a single structure that combines these sensors requires simultaneously:

[0005] - Simultaneous monitoring of the secondary SSR / Mode S radar and the IFF interrogator (if the latter is used). These simultaneous mode monitoring operations utilize beamformed SUM and DIFF.

[0006] - Listening to civilian ADS-B messages or military Mode 5 messages in asynchronous mode, these asynchronous mode listening operations utilize the omnidirectional coverage of a control pattern (denoted as CONT) consisting of a forward control pattern (denoted as CONT_Front) and a rearward control pattern (denoted as CONT_Back).

[0007] Traditionally, ADS-B messages are sent without a request from a secondary radar at a rate specific to the transmitter, which depends, among other things, on the transmitter's state and the nature of the information being sent. Thus:

[0008] -ADS-B receivers must be able to listen in all directions; however:

[0009] Antenna gain is typically low, e.g., from 11 to 15 DBi, depending on the number of antennas used to achieve 360° coverage in the case of dedicated standalone ADS-B receivers;

[0010] The specified guaranteed range is approximately 150 Nmi, or even more, depending on the quality of the transponder transmitting the message and the number of antennas used by the ADS-B receiver;

[0011] - In addition, in principle, two almost consecutive position messages are required to locate a target in ADS-B mode.

[0012] Receiving and processing ADS-B information has been identified as an essential function for the new generation of secondary radars. When ADS-B functionality is integrated into the structure of a secondary radar, reception primarily utilizes CONT-Front and CONT-Back patterns to achieve as close to 360° coverage as possible. The gain of these patterns is typically over 17dB, lower than the maximum gain of the sum pattern (denoted as SUM). However, the latter, used for simultaneous detection in civilian or military applications, has a gain of approximately 27dBi, allowing for a detection range of only approximately 250nm.

[0013] Compared to synchronous mode, in civilian or military applications, radar range is typically limited primarily by the range of the 1030 MHz uplink (i.e., limited by the power of the transmitted interrogation and the receiver sensitivity of the transponder), and ADS-B range is limited only by the 1090 MHz downlink, i.e., by the power of the transponder and the sensitivity of the ADS-B receiver. Indeed, by the very nature of the secondary radar architecture, the downlink (receive) is more sensitive than the uplink (transmit) to ensure that any reply generated by the transponder is receivable and usable.

[0014] As a result, the difference in operating gain between a synchronized radar and one listening for asynchronous ADS-B transmissions (otherwise, all other aspects being identical with respect to the architecture of the two sensors) is approximately 8 to 10 dB. This results in a guaranteed ADS-B range of typically around 100 Nmi for an ADS-B receiver architecture integrated into a secondary radar, although it should be noted that asynchronous replies received via the control pattern (CONT) are significantly more susceptible to garbled signals than synchronized replies received via the SUM pattern, as they are inherently much narrower (approximately 3°). Furthermore, overlapping replies (known as clutter), i.e., replies that coincide with each other, make detection and decoding more difficult. Consequently, in a crowded aircraft environment, the guaranteed ADS-B detection rate at the receiver is two to three times lower than that of a Mode S secondary radar.

[0015] Prior art solutions that attempt to overcome these performance deficiencies are known. Specifically, in dedicated ADS-B receivers, prior art involves increasing the number of independent antennas to cover 360°, with the goal of providing protection through azimuth selectivity, while also achieving improvements by associating a beam with each receiver. The receivers are coupled to a common processing stage that combines detection operations, thus avoiding duplicate detections, especially during the passage of targets from one receiver to the next (which allows the construction of a single ADS-B track).

[0016] One disadvantage of this type of solution is in particular the cost of the ADS-B system infrastructure, which increases with the number of antennas, which is typically 4 to 8 (multiple antennas, multiple downlinks, multilateral towers, multiple receivers, global ADS-B processing, etc.).

[0017] When an ADS-B receiver is integrated into a radar architecture, in addition to detection via the CONT pattern, detection via the sum pattern (SUM) and difference pattern (DIFF) also allows for higher gain and better azimuth selectivity, but only for a small fraction of the time (approximately 1.4%, corresponding to 5° / 360°). Therefore, this only ensures better listening time coverage on the antenna axis, exceeding 360°, in addition to the ADS-B listening operation performed via the gain of the CONT pattern. Specifically, as is known, the global detection of an ADS-B receiver requires two consecutive position messages to generate a track. Therefore, even if the first reply is received via the SUM pattern, the second reply will inevitably be received via the CONT pattern, and thus, ultimately, the ADS-B reception chain integrated into the architecture of the secondary radar is limited to the range of the CONT pattern. Summary of the Invention

[0018] One object of the present invention is to alleviate the drawbacks of the prior art by, in particular, allowing ultra-long-range ADS-B detection of targets. To this end, a subject of the invention is a method for locating a target transmitting an ADS-B message, the target being located by implementing the following steps:

[0019] -Detect ADS-B messages received via CONT channel, SUM channel and DIFF channel;

[0020] - measuring at least the power of the message and its bearing relative to the radar;

[0021] The position of the target sending the ADS-B message is calculated by at least using the detection of a single ADS-B message based on the latitude and longitude position of the radar and a bearing measurement relative to the radar, and a position cell denoted as a CPR cell encoded in the message is selected via the bearing measurement.

[0022] In the event that two coded CPR positions encoded in the message may be located in the same azimuth sector, the two positions are distinguished using, for example, the measured power and altitude encoded in the ADS-B message, the position retained is the one with the highest probability based on an estimator based on the visibility of the target and based on the consistency of the power received by the radar and the range of the target from the radar.

[0023] In a specific implementation mode, the method comprises the following steps, wherein once a target enters the interrogation coverage of the radar, roll-call capture of the target is ensured by:

[0024] - once a target enters the reception coverage of the radar, using the position encoded in the ADS-B message transmitted by the target;

[0025] - using the target's paging identity, contained in its Mode S address encoded in the message,

[0026] The antenna transmits a round-robin interrogation every N revolutions in order to lock the target to the radar station's code, thereby avoiding contamination by the M DF11 replies typically generated by the target per revolution once the potential target enters the interrogation coverage area and reaches the area of ​​the radar's operating range.

[0027] The method for example comprises the steps wherein the targets are detected and located via their ADS-B messages prior to the interrogation coverage of the radar, the targets are associated with ADS-B tracks once 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 coverage of the operational radar to allow detection of their potential lock status, i.e. whether they are locked to another radar, i.e. there is no synchronized DF11 reply, thereby potentially enabling provision of II / SI code conflict warnings to other Mode S targets that do not have ADS-B capability, while ensuring that targets are placed on Mode S call surveillance once they enter the operational coverage of the radar.

[0028] The method comprises, for example, a step in which, within a moving time window, for each 3D geographical cell, the average value of the difference between the distance of a target measured by the radar and the distance of the target encoded in the ADS-B message sent by the target is calculated, this being performed for each target passing through the cell, and then each distance of any target measured by the radar in any mode is corrected according to the average difference calculated for the geographical cell through which the target flies.

[0029] The method comprises, for example, the steps of measuring the sensitivity of a transponder, then, whether in round-robin mode or not, by interrogating a target transmitting an ADS-B message located via the method upon entering the interrogation coverage of the radar, and detecting a transponder having undesirable sensitivity to the interrogation transmitted by the radar at 1030 MHz by calculating the sensitivity of the transponder based on the power received by the transponder on its first reply to the interrogation at 1030 MHz, and declaring the transponder to have undesirable sensitivity if the power is above a given threshold.

[0030] The method, for example, includes the following steps, wherein an unsatisfactory altimeter associated with a transponder of an ADS-B target is detected by utilizing a deviation of the difference between the barometric altitude and geometric altitude, encoded in an ADS-B message transmitted by the target, in a series of geographic cells flown by the target relative to an average value of the difference between the barometric altitude and geometric altitude of other targets per cell calculated for each of the geographic cells flown by the target in a moving time window; if the deviation exceeds a given threshold, the altimeter is determined to be unsatisfactory.

[0031] Another subject matter of the invention is a secondary radar capable of implementing such a method. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other features and advantages of the present invention will become apparent from the following description given with reference to the accompanying drawings.

[0033] Figure 1 Possible steps for implementing the present invention are shown;

[0034] Figure 2 A diagram showing the directional pattern of a conventional ATC antenna;

[0035] Figure 3 A diagram showing the azimuth sensitivity of a conventional ATC antenna in integrated ADS-B mode;

[0036] Figure 4 A diagram showing the architecture of a secondary radar without ADS-B reception according to the prior art;

[0037] Figure 5An example of a change in the architecture of a secondary radar implementing the method according to the present invention is shown;

[0038] Figure 6 shows a range of various interrogation and detection terminals utilized by radars according to the present invention;

[0039] Figure 7 A diagram showing the principle of global ADS-B detection with two consecutive messages;

[0040] Figure 8 A diagram showing the ADS-B detection principle with a single message according to the present invention;

[0041] Figure 9 A diagram showing achievable ADS-B disambiguation;

[0042] Figure 10 A diagram showing the various ranges involved in tracking a target that enters radar coverage;

[0043] Figure 11 An example of an area of ​​II / SI code conflict between radars with overlapping coverage is shown;

[0044] Figure 12 Example of a map showing the average value of the difference between pressure altitude and geometric altitude;

[0045] Figure 13 shows the shape of an exemplary distribution of height deviations in a 3D geographic cell;

[0046] Figure 14 Amplification of target height deviations consistent with targets in the same geographic cell is shown;

[0047] Figure 15 A zoom-in on a target that exhibits an altitude deviation that is inconsistent with targets in the same geographic cell is shown. DETAILED DESCRIPTION

[0048] Figure 1 The steps for implementing the present invention are shown. This implementation requires at least two steps. For this implementation, in view of the presence of ADS-B detection functionality integrated into the secondary radar structure 101, the present invention provides the following steps:

[0049] - continuous detection 102 must be performed with very high sensitivity via the CONT_Front and CONT_Back patterns;

[0050] -But in addition, optionally, successive complementary detections are performed via the directional patterns of the main lobe (detection 103 via the SUM directional pattern and detection 104 via the DIFF directional pattern) with a lower sensitivity of about 15 dB, in order to link the antenna gains of the CONT directional patterns (CONT_Front and CONT_Back) and thereby insert a gain gap occupying about 5° in the CONT pattern on the antenna axis.

[0051] Figure 2 and Figure 3 The antenna patterns involved are shown, at the receiving end at 1090 MHz. Figure 2 The SUM, DIFF, and CONT antenna patterns of an antenna commonly used for the ATC function of a secondary radar are shown. The gain of the SUM and DIFF patterns is much higher than the gain of the CONT pattern around the antenna axis. Figure 3 Shown is the azimuth-integrated ADS-B sensitivity of a conventional ATC antenna, with the SUM and DIFF channels aligned at the same receiver sensitivity as the CONT channel.

[0052] return Figure 1 In a first step 110, ultra-long range detection of the ADS-B message is performed, thereby:

[0053] - Detect ADS-B messages with very high sensitivity via SUM and DIFF patterns;

[0054] - Computing the position of the target using a single message using conventional error signal methods based on detection via SUM and DIFF, as will be described in detail in the remainder of this specification.

[0055] The terms ADS-B message and ADS-B reply will be used interchangeably hereinafter.

[0056] In a second step 120, the first step 110 is operatively exploited. Passive detection of aircraft beyond the synchronous operating range of the radar via ADS-B is performed up to twice as fast, allowing the aircraft to be exploited before it even enters the radar coverage area, improving the operation of the radar and the safety of the ATC surveillance system, as described below.

[0057] In the field of radar, and more generally in the field of ATC air safety:

[0058] - Reduce RF pollution by locking onto the target at a low frequency once it enters the radar's transmission coverage, thus limiting pollution by replacing all all-call (DF11) replies with a much smaller number of round-call (DF04) replies before entering the radar's operational coverage;

[0059] - Improved detection of II / SI code conflicts within the radar range limits and ensured detection of ADS-B targets within the radar coverage area that were falsely locked by another radar (i.e., more specifically, ADS-B targets that are beyond the ADS-B intercept range achievable via the CONT pattern);

[0060] -Improve radar synchronous ranging accuracy by compensating for ionospheric propagation distortion.

[0061] In the field of ATC safety, for aircraft equipped with ADS-B transmitters, perform the following actions before entering the operational coverage area of ​​the radar:

[0062] - pollution-free detection of undesirable sensitivity of transponders;

[0063] - Detection of an unsatisfactory altimeter associated with a transponder.

[0064] The principles of the present invention and the use of various detection zones will be described in more detail below.First, possible modifications of the secondary radar architecture for implementing the present invention will be described.

[0065] therefore, Figure 4 and Figure 5 shows the changes in the architecture of the Mode S radar of the present invention, Figure 4 shows the structure of the traditional prior art, Figure 5 The structure for implementing the present invention is shown.

[0066] Reference Figure 4 , which shows an example of a diagram of a conventional Mode S radar, the constituent elements of such a radar will be reviewed. Figure 4 The minimal architecture of a Mode S secondary radar is shown without integrated ADS-B reception via the CONT_Front and CONT_Back patterns, which is not a standard reception mode in traditional secondary radar configurations. The following description proposes a solution based on four channels: SUM, DIFF, CONT_Front, and CONT_Back, although the present invention is of course applicable to solutions based on three channels: SUM, DIFF, and CONT.

[0067] When used in a conventional manner, secondary radars operate in synchronous mode, ie sending interrogations and waiting for replies consistent therewith, which allows locating (via Mode S address) and identifying (via measurement) of targets. Figure 4 The diagram below shows this synchronized operation of the Mode S radar:

[0068] - the left hand part shows the generation of the query;

[0069] - the right-hand part shows the synchronous processing of the associated reply;

[0070] and the synchronization between them, represented by the horizontal arrows between left and right.

[0071] To perform this task, the radar is equipped with an antenna 1 which transmits signals in four directions. Figure 11 、 12 , 14, 15 send inquiries at 1030 MHz and receive replies at 1090 MHz. The effect of the directional pattern is usually:

[0072] -SUM pattern, simultaneous response of inquiry and detection targets;

[0073] -DIFF pattern, fine positioning of the target in the SUM beam;

[0074] -CONT_Front pattern, blocks and rejects replies from targets facing the antenna but not present in the main SUM beam;

[0075] - CONT_Back pattern, blocks and rejects replies from targets behind the antenna (and therefore necessarily not present in the main SUM beam).

[0076] While the SUM and DIFF patterns are generally narrow, with 3dB lobes between 2.4° and 10°, the CONT_Front and CONT_Back patterns attempt to cover effectively 180° each.

[0077] Antennas can also have:

[0078] - Setting the directivity pattern, the so-called "mechanical" antenna, and rotating it;

[0079] - Active electronically scanned pattern, so-called AESA, and remains fixed or rotates.

[0080] For rotating antennas, the rotating joint 2 and the antenna downlink cable ensure that:

[0081] - RF coupling of signals transmitted at 1030 MHz and received at 1090 MHz, independent for the four patterns between the rotating and fixed parts of the radar;

[0082] - A shift in the azimuthal position 201 of the axis of the main lobe of the antenna.

[0083] The RF processing stage includes:

[0084] - a duplexer or circulator 3 ensuring RF coupling between the signal transmitted at 1030 MHz and the signal received at 1090 MHz, independently for the four patterns;

[0085] - a transmitter 4 which:

[0086] Sending inquiries via the SUM pattern at 1030 MHz;

[0087] Transmit pulses at 1030 MHz via the CONT_Front and CONT_Back patterns to block transponders outside the SUM lobe.

[0088] The transmitter does this for various secondary protocols (IFF, SSR, and Mode S).

[0089] - The receiver 5 receives the replies at 1090 MHz via the four patterns SUM, DIFF, CONT_Front and CONT_Back and calculates error signals for the various secondary protocols IFF, SSR and S-Mode.

[0090] The real-time processing phase includes:

[0091] - a space-time manager 6 which manages in real time the interrogation periods and the associated listening periods for the various secondary protocols IFF, SSR and Mode S;

[0092] - Signal Processor 7:

[0093] Processing replies during the listening period associated with interrogations for various secondary protocols: IFF, SSR, and Mode S;

[0094] Detecting and decoding synchronization replies in the antenna main lobe by utilizing four radiation patterns:

[0095] οSUM: detects the replies received in the main lobe;

[0096] oDIFF: fine-tune the replies received in the main SUM lobe in azimuth and potentially remove scrambled replies;

[0097] o CONT_Front and CONT_Back: Reject replies received via the SUM and DIFF sidelobes.

[0098] The stages of processing the antenna main lobe include:

[0099] - A target manager 8 exists in a lobe, which:

[0100] For the various secondary protocols IFF, SSR, and S-Mode, prepare (inquiry-reply) traffic to be executed in the next lobe;

[0101] Management of IFF, SSR, all-call S-mode, and round-robin S-mode periods in the lobes;

[0102] Dynamically conducts selective Mode S interrogations and replies during the next round-robin period based on the state of the traffic just executed and any new aircraft entering the lobe;

[0103] An extractor 9 which generates a plot for each of the various secondary protocols (IFF, SSR and S-mode) based on the synchronization replies received in the lobes and according to the protocol used for the interrogation.

[0104] The multi-rotation processing stage 10 includes:

[0105] - a manager 101 of Mode S missions to be performed on targets in coverage, which predicts target positions (antenna intersection points) and prepares the missions to be performed associated with these positions based on internal requests, external requests and traffic status of previous rotations;

[0106] - Association and tracking 102 of the profiles of targets within coverage, ensuring that the targets are tracked with a view to improving performance (particularly by removing erroneous profiles and checking decoded data) and predicting their future positions, primarily but not exclusively in Mode S.

[0107] The user interface allows the radar to take into account various requests, as well as the plots and trajectories of the targets to be viewed.

[0108] Figure 5 Shows relative to Figure 4 The conventional architecture of the present invention is modified, as indicated by the bold dashed lines. Although the Mode S radar operates synchronously, it can be seen that the processing operations 31 and 32 added for the present invention are independent of transmission, and that only the azimuth position of the antenna main lobe axis is utilized for the first step 110 of the present invention. Most elements remain unchanged, thus satisfying the present invention's criterion of not interfering with the functional operation of the Mode S radar.

[0109] The main components added have the following functions:

[0110] In SpaceTimeManager 6:

[0111] - sending 33 the azimuth position of the antenna main lobe to operation 31 for processing the asynchronous ADS-B reply (see below);

[0112] In signal processor 7:

[0113] - Adding a processing operation 31 which is continuous (i.e., independent of the period in which the interrogation is sent) and detects and decodes the non-synchronized ADS-B replies by utilizing the four patterns SUM, DIFF, CONT_Front and CONT_Back separately but equally at their respective maximum sensitivities:

[0114] To detect all received ADS-B messages: DF17;

[0115] Extract the S-mode address from it;

[0116] Enrich each decoded reply with its characteristics: time of detection; orientation of the antenna main lobe at the time of detection; received power via SUM, DIFF, CONT_Front, and CONT_Back; and the off-axis angle of the ADS-B message in the main SUM and DIFF beams, calculated using the error signal method;

[0117] In the non-real-time processing stage (multi-rotation processing stage)

[0118] - Added remote position 32 of ADS-B target;

[0119] - Decision 34 to perform a selective interrogation when the simultaneous interrogation range is reached.

[0120] Reference Figure 6 The principles of the present invention will now be explained. The radar according to the present invention still utilizes the CONT pattern in the conventional manner to detect ADS-B messages. The radar range for this conventional ADS-B processing is illustrated by the first circle 41. In addition, as will be described below, the present invention advantageously utilizes the range limitation of the signals transmitted by the transponders of surrounding aircraft (SUM and DIFF) via channels SUM and DIFF, which is illustrated by the larger diameter circle 42. The reception range itself is greater than the maximum range of the interrogations transmitted by the radar (TX) and interpreted by the transponder, which is illustrated by the third circle 43 corresponding to the high-sensitivity transponder.

[0121] In other words, the principle of long-range ADS-B detection according to the present invention is based on exploiting the range difference between:

[0122] - The radar's operational coverage, which is shown by the final circle 44 and is based on the guaranteed values:

[0123] A transponder with minimum sensitivity for 1030 MHz transmission;

[0124] A transponder with minimum power for synchronous reception at 1090 MHz;

[0125] The gain of the radar antenna's SUM and DIFF patterns in synchronized mode;

[0126] The coverage area of ​​radar use is necessarily narrower than the three criteria above;

[0127] a maximum range 43 of the interrogation at 1030 MHz, taking into account the transponder with maximum sensitivity, which is utilized in the second step 120 ;

[0128] - Considering a transponder transmitting at maximum power, the maximum receiving range at 1090 MHz is 42;

[0129] - For ADS-B functionality integrated into radar, ADS-B operational range 41, based on guaranteed values:

[0130] The gain of the antenna’s CONT pattern used in legacy ADS-B mode to ensure continuous detection of ADS-B messages (see below);

[0131] • For asynchronous reception at 1090 MHz, the transponder has the minimum power.

[0132] Figure 7 The principle of ADS-B global detection with two messages (replies below) is shown, which is applied in the prior art reviewed in the background. It is well known in the literature that the position information of the ADS-B responder is encoded in a format represented by the CPR format (CPR is the acronym for Compact Position Reporting) in two ADS-B replies, one reply is sent in a first georeferenced frame called an even frame, and the other reply is sent in a second georeferenced frame called an odd frame. For example, Figure 7 Nine reply cells 61 are shown, with even-numbered replies 62 drawn in bold and odd-numbered replies 63 drawn in finer text. Each CPR cell 62, 63 contains latitude and longitude information associated with the cell, with the cells being represented by an axis system, where the y-axis represents latitude and the x-axis represents longitude. As is well known, an ADS-B receiver applying this global detection must perform the following operations to locate the source of the ADS-B transmission:

[0133] - receiving two consecutive ADS-B messages of two different types: an even-numbered ADS-B message frame 62 and an odd-numbered ADS-B message frame 63;

[0134] - Determine the source (destination) location by calculating the unique CPR cells 64 corresponding to even and odd locations.

[0135] Thus, the absolute latitude and longitude position in the Earth's reference frame (rather than relative to the cell) can be reconstructed.

[0136] Figure 8 The principle of ADS-B detection according to the present invention is shown. Here, a single frame reply is shown, for example an uneven frame. The principle of the present invention is to locate the position of a target based on a single (even or odd) ADS-B reply by simultaneously utilizing the following information:

[0137] - the latitude and longitude position of the radar 70;

[0138] - A bearing measurement 71 made by the radar 70 when receiving the ADS-B message, the measurement being made using the SUM and DIFF pattern.

[0139] Specifically, since the latitude and longitude position of the radar and the azimuth measurement of the message are known (with a given tolerance), it is possible to calculate the CPR cell 72 that satisfies the following two conditions:

[0140] - Concise ADS-B position sent in received messages;

[0141] - bearing measurement of the message as seen from the radar's latitude and longitude position,

[0142] Therefore, an ADS-B target can be located through a single ADS-B message. Figure 8 , the position measurement 71 allows the selection of a position 72 (CPR cell) from the 9 possible positions (cells) shown in the figure.

[0143] To summarize the detection process according to the present invention, it can be recalled that when an asynchronous message is captured via SUM or DIFF, the radar measures the same characteristics as the synchronous reply, obtaining:

[0144] - azimuth position of the antenna;

[0145] - Off-axis angle of the target in the SUM beam using the error signal method;

[0146] -Power of the message in SUM and DIFF.

[0147] Since distance is meaningless in the case of asynchronous replies, the decoded message provides information about the target:

[0148] -Relative latitude position (even or odd CPR);

[0149] - Relative longitude position (even or odd CPR);

[0150] -high.

[0151] In the long-range ADS-B position of a target, geographical confusion may be encountered when two possible positions 81, 82 of the target (in two adjacent CPR cells) are aligned with the position of the radar, e.g. Figure 9 In this case, the bearing 71 measured alone is no longer a discriminant that allows to define the correct cell. Then, two other characteristics of the received ADS-B message are used:

[0152] - the power of the message in SUM and / or DIFF;

[0153] - The declared altitude of the target.

[0154] Since the characteristics of the radar are known: the geographical location (including its altitude) and the RF loss of the facility, the highest probability location 82 between two possible locations can be defined by building an estimator based on the radar's visibility of the target (which depends on the altitude of the radar and the target) and the consistency of the received message power with the two locations.

[0155] This is achieved by simultaneously utilizing the following information:

[0156] - Asynchronous reception range of the radar via SUM and DIFF patterns;

[0157] - Detection and localization of targets using a single message.

[0158] Radar advantageously detects ADS-B targets at very long ranges, typically more than double the radar's operating range44 (see Figure 6 ).

[0159] Figure 10 The various detection zones are shown for a path 99 of an aircraft, for example, at FL500 (approximately 15,000 meters altitude), and for the visibility of the aircraft, represented by curve 90, depending on the altitude of both the radar and the target. In this example, as a function of the target distance on the x-axis and the target altitude on the y-axis, Figure 10 Allowed to see:

[0160] - a maximum long-range ADS-B detection area, defined by an almost vertical line 92 , depending on the power of the transponder and the sensitivity of the ADS-B receiver in the radar's SUM pattern;

[0161] - a minimum guaranteed long-range ADS-B detection area, which is defined by an almost vertical line 95;

[0162] - the maximum contamination area of ​​the all-call DF11 reply (transmit contamination range), which is bounded by the almost vertical line 93 and depends on the sensitivity of the Mode S transponder to radar interrogations via the SUM pattern;

[0163] - the guaranteed operating range of the radar, which is defined by the almost vertical line 94;

[0164] The conventional detection area of ​​the integrated ADS-B receiver via the CONT pattern, which is bounded by the almost vertical line 91 .

[0165] It is important to note that, generally, since transponders broadcasting ADS-B messages are close in nature, their RF characteristics are usually good and therefore higher than the minimums for any given Mode S transponder. This generally limits the operating range of the radar, which must process all Mode S targets, even those not equipped with ADS-B capabilities.

[0166] Looking to the left of the aircraft's visibility limit 90, which depends on the radar's altitude due to the Earth's curvature, one can see the region between the most favorable long-range detection of ADS-B messages according to the present invention (line 92) and the detection guarantee limit of ADS-B messages (line 95), in which radially flying aircraft can be detected before entering the contaminated region 93, then the radar's operating region 94, and finally the integrated ADS-B detection region 91 (guaranteed conventional ADS-B detection).

[0167] If an aircraft entering the radar coverage at, for example, FL500 is considered, the invention allows three areas to be considered:

[0168] - Area A, where ADS-B messages are detected remotely before DF11 contamination can begin;

[0169] - Region B, corresponding to the beginning of DF11 contamination before radar coverage;

[0170] - Area C, corresponding to radar coverage including legacy integrated ADS-B reception.

[0171] As shown in the above description, the present invention is based on the fact that the operations required to integrate ADS-B functionality into the radar architecture are almost identical to the Mode S signal processing operations implemented by the radar to process synchronous (DF11, DF04 / 05, DF20 / 21) replies, since the ADS-B message is actually just an extended Mode S reply. The present invention also allows, in the main SUM or DIFF lobe of the radar beam:

[0172] - For asynchronous replies instead of synchronous replies as in the traditional case, the received message is located with respect to bearing and power by performing traditional very precise radar measurements:

[0173] - Achieving high antenna gain of approximately 15 dB above the gain of the CONT pattern typically used for ADS-B detection, i.e., approximately four times the range of a typical (conventional) integrated ADS-B (for ease of illustration, the various separations are not shown to scale);

[0174] -Ultra-long range target location via a single message (rather than the two normally required when attempting ultra-long range detection) by utilizing the radar position, precise bearing measurements made by that radar, and knowledge of the relative latitude and longitude CPR position encoded in the detected ADS-B message.

[0175] Allowing the time coverage to remain low, according to the width of the SUM pattern less than or equal to 1.4%, the present invention allows the secondary radar to be ultra-long range (such as Figure 10Long range targets are processed similarly to those processed conventionally for closer targets by conventional integrated ADS-B receivers utilizing an omnidirectional CONT pattern (shown between 200 Nmi and 500 Nmi).

[0176] The following will now describe the second step 120 ( Figure 1 ), which advantages may be achieved by utilizing the present invention, particularly relate to:

[0177] -About radar:

[0178] Reduced pollution at 1090MHz;

[0179] Detection of II / SI code conflicts and detection of false locks on targets in the overlap area between conflicting radars;

[0180] Correction of radar-measured distances (depending on propagation);

[0181] -About ATC safety:

[0182] Detect transponders with substandard sensitivity at 1030 MHz;

[0183] Generate atmospheric pressure maps and detect unsuitable altimeters.

[0184] These five operational improvements advantageously achieved by the present invention are described below.

[0185] Reduces 1090MHz pollution and the number of replies from targets outside the radar's operating range.

[0186] Currently, the occupation of the 1090MHz spectrum is becoming one of the weak points of ATC monitoring. The pollution encountered is attributed to its success, and the responses used are of different types, such as:

[0187] -DF11: used to acquire new targets using Mode S radar;

[0188] -DF04 / 05: For Mode S ATC surveillance: Basic Surveillance (ELS);

[0189] -DF20 / 21: For Mode S ATC surveillance: Enhanced High Speed ​​Surveillance (EHS);

[0190] -DF0: for active TCAS collision avoidance;

[0191] -DF17: ADS-B message for passive ACAS collision avoidance (ACAS is the acronym for Airborne Collision Avoidance System).

[0192] Given the purpose of the functions they perform in Mode S radars when in operational use, D04 / 05 / 17 / 20 / 21 replies will continue to exist. DF0 replies will eventually be replaced by DF17 replies. DF11 replies are generated by targets unknown to the radar and can only be partially replaced by another means, such as:

[0193] - Mode S radar cluster;

[0194] However, this only reduces the contamination inside its coverage area, and DF11 contamination continues to exist outside the common coverage area of ​​the cluster;

[0195] -Trajectory initialization based on ADS-B message;

[0196] However, the RF coverage of an omnidirectional ADS-B receiver is approximately two times smaller than that of a radar, so it can only be utilized within the short- to medium-range radar range to initialize radar tracks based on the ADS-B message; therefore, it does not prevent long-range contamination and limits the radar's range.

[0197] To describe the contribution of the present invention to reducing 1090 MHz spectrum pollution, refer to Figure 10 : If we consider a target flying at FL500 that enters radar coverage:

[0198] -Zone A: Almost as long as the target is visible, it can be detected by long-range ADS-B detection;

[0199] - Region B: if the radar is enabled in this bearing, the radar can lock onto the target via round-robin UF4 interrogations as soon as the target enters the radar's transmission range, and therefore even before it starts responding to DF11 all-call interrogations; the radar then continues to interrogate at a low rate (about 15 to 18 seconds), which is enough to prevent the target from being unlocked: the DF11 contamination (3 times per 5-second rotation) is thus reduced to a lower DF04 contamination (1 time per three 5-second rotations), so the ratio is close to 10;

[0200] - Zone C: The target enters the operational coverage area of ​​the radar; then advantageously a track has already been established.

[0201] The invention thus allows reducing the contamination caused by friendly radar and reducing the number of replies generated unnecessarily by the transponder, while speeding up radar tracking when the target eventually enters the operational coverage of the Mode S radar.

[0202] Long-range detection of conflict zones in II / SI codes and detection of falsely locked ADS-B targets.

[0203] The following will refer to Figure 11Let's recall the principle of II / SI code conflicts within the limits of radar range. Several solutions (see, in particular, patent application FR1913154) propose detecting areas of radar coverage where there is an II / SI code conflict with another radar R2 close to the radar R1 in question, whose operating coverage areas 111, 112 overlap. The overlapping area 113 is the area of ​​II / SI code conflict (if II1=II2) between the two operating coverage areas, and therefore the area where both radars R1, R2 lock onto targets with the same II / SI code that enter their coverage areas, thereby shielding them from the other radar.

[0204] For example, considering various existing methods, based on the position of ultra-long-range radar R1 in the common bearing of R1 and R2, the maximum range difference between R1's outgoing target (which is far away because it is already locked by R1) and the incoming target (which can only be seen when it is closer because it is already locked by R2) is typically 250 nm (the inverse of R2). This method is only indicative (based on the probability of behavior) and does not allow for the observation that the incoming target has not yet been seen in the conflict area, thus failing to mitigate the problem and therefore failing to ensure ATC safety.

[0205] In this common bearing of R1 and R2, Mode S radar tracking (via UF11 / DF11 all-call or by Mode S cluster) of a target not yet detected within the entire ADS-B coverage area of ​​radar R1, using the Mode S address and the position given by the ADS-B message broadcast by the target, allows:

[0206] - Resolve II / SI code conflicts in the common area 114 between R2’s operational coverage and R1’s ADS-B coverage;

[0207] - Ensure high reactivity, so an ADS-B target locked by R2 is detected as soon as it reaches the ADS-B coverage area of ​​R1;

[0208] However, this cannot cover the entire deconfliction zone of the long-range radar, as the conventional range of the R1’s integrated ADS-B receiver is limited by the lower gain (approximately 15dB) of the R1’s CONT channel (compared to the SUM channel).

[0209] For the same reasons, the analysis of the presence and distribution of FRUITS, such as that described in patent application FR1913154, is hereafter applicable to all Mode S aircraft, whether or not they are equipped with ADS-B OUT capability, but with respect to the ADS-B range of R1, which is limited by its lower CONT and SUM gain; therefore, this analysis is applicable to medium-range radars.

[0210] According to the present invention, before the radar's operating coverage, that is, the area receiving the UF11 and DF11 all-calls, this is also the area of ​​the UF04 / 05 and DF04 / 05 round-robin inquiries, that is, at this distance away from the radar, such as Figure 10 As shown, ADS-B targets are detected and located via their messages and associated with ADS-B tracks.

[0211] If the above-mentioned decontamination principle is not applied (for example, in the event that the user refuses to allow the lock to occur outside the coverage area and therefore in zone B), radar R1 can still interrogate the target as soon as it enters its operational coverage area (zone C), thus ensuring perfect Mode S surveillance within the entire deconfliction zone of the II / SI code, but only if the target is equipped with ADS-B capability.

[0212] A conflict in the II / SI code is then declared only within the radar's operational coverage area 94: if an ADS-B target is locked in area B to reduce the contamination of DF11 replies, the invention proposes to cancel the lock when entering area C and, if such operation is authorized, to verify the absence of DF11 replies from the target in area C, with the intention of declaring a II / SI code conflict if the radar receives a DF04 / 05 reply to a call interrogation for the same target; thus, with this method, the target is first detected and only then is the presence of a II / SI code conflict verified, in particular to provide a warning of potential non-detection of aircraft equipped with Mode S but not equipped with ADS-B functionality.

[0213] If the ADS-B target is not locked in zone C, then there is no DF11 reply to the radar before it enters zone B, thus becoming an indicator of a II / SI code conflict before it is locked within the radar's operational coverage.

[0214] Corrects the radar's measured distance (depends on propagation).

[0215] It is well known that secondary radar waves at 1030 MHz and 1090 MHz do not propagate in a straight line due to refractive index distortion in the layers of the ionosphere; therefore, targets appear to be farther away than they actually are.

[0216] According to the present invention, to correct the distance measured by the radar, the same ADS-B message as used for long-range ADS-B detection can be used:

[0217] - ADS-B "Air Position" messages sent approximately every 0.5 seconds deliver latitude / longitude position and barometric altitude.

[0218] Since step 110 allows the determination of the latitude and longitude position and altitude of the ADS-B target using a single message, associated with the known latitude and longitude position of the radar, it is possible to recalculate the exact distance to the target, i.e. the exact distance from the geographical location and altitude of the target to the radar, which is mainly used for high-altitude radars (altitudes above 1000 m) with visibility of medium-altitude targets at very long ranges from the radar.

[0219] Therefore, the present invention proposes to collect, for each 3D geographic cell and each altitude slice, statistics on the difference between the range measured by the radar and the range calculated for the ADS-B targets present in the cell, in a moving time base that can be adjusted to reflect the stability of the ambient atmosphere. Thus, for each cell using ADS-B targets, the average value of the difference between the radar-measured range and the geometric range is associated. This analysis can be performed in Area C or even Area B, if the radar is allowed to interrogate targets outside its operational coverage area.

[0220] If the statistics of each cell are representative (i.e., based on a sufficient number of samples that exhibit stable range differences and consistency with neighboring cells), then, for each simultaneous SSR, Mode S or IFF target detection by a radar, whether or not the target is ADS-B capable, the measured range (thus including the effects due to propagation through the ionosphere) can be completed by an evaluation of the geometric range in order to correct the measured range by the average difference of that cell at that altitude, and therefore the contribution of ionospheric reflections. Thus, the range measured by the radar is corrected according to the average difference calculated in the geographical cells that the target flies through successively. For multi-radar management, this range correction also allows to improve the correlation of tracks generated by different radars for the same target, thereby compensating for the unequal stratum propagation thus observed.

[0221] Detecting low-sensitivity transponders

[0222] One purpose of this optional step of the invention is to detect the presence of an aircraft including a low-sensitivity transponder, i.e., a transponder with a sensitivity below a standard threshold, as soon as possible after entering the distant coverage area of ​​the radar. Thus, a warning can be provided about the potential vulnerability of these aircraft to simultaneous detection by the radar, i.e., an aircraft that reduces the radiated field received from the radar during maneuvers may no longer be detected. The International Civil Aviation Organization (ICAO) defines the expected RF characteristics of transponder antennas installed on aircraft in Annex 10, Volume IV:

[0223] - Regarding the ability to monitor interrogation (synchronous mode only), regarding sensitivity: -71 to -77 dBm;

[0224] - Regarding the sending of synchronous and asynchronous replies, regarding power: 51 to 57 dBm (125 to 500 W).

[0225] Patent application FR1800479 proposes to evaluate the sensitivity of a transponder in an active configuration by sending various interrogations as a supplement to target surveillance, usually during takeoff from an airport, and therefore a priori short-range. The power of these interrogations is attenuated (both by a reduction in the radar's transmitter and by using a reduction in antenna gain when the target is very off-axis in the beam) to determine the power at which the target no longer responds. In particular, since the power of the transponder's reply remains at a maximum even at close range from the interrogator, it is known that the radar's failure to detect a reply can only be due to the transponder's failure to interpret the interrogation.

[0226] The present invention here has the same purpose and proposes to evaluate the sensitivity of a transponder in a quasi-passive configuration, i.e. without generating any additional interrogations and therefore no additional contamination, which would be contrary to the previous method, with the same purpose of providing a warning of the presence of an insufficiently sensitive transponder.

[0227] To this end, once an ADS-B message is received in the antenna main lobe (SUM and / or DIFF), the target is identified by its Mode S address and located relative to bearing and range. In order to limit the contamination caused by the DF11 reply sent by the target immediately after interpreting the interrogation, the target is selectively locked as quickly as possible by using the full power of the radar's transmitter in its central lobe (therefore one of maximum gain) at a very low repetition rate (typically 1 every 15 seconds).

[0228] Since the radar locates the target via its ADS-B message, it can be assumed that when the target starts responding to the UF11 general call or the UF04 / 05 round-robin call interrogation, the power transmitted by the transponder will also be within the specifications; the latter is just above the transponder's receiver sensitivity threshold. The radar can then calculate the power received by the transponder antenna by using the characteristics described in the aforementioned patent application FR1800479, which are:

[0229] About Radar:

[0230] - Antenna gain (maximum gain minus the antenna gain loss due to the off-axis angle of the target during interrogation);

[0231] - Losses caused by the cable between the transmitter and the antenna;

[0232] - the peak power of its transmitter; and

[0233] About the goal:

[0234] - antenna gain loss as a function of target altitude as seen from the radar antenna;

[0235] - Due to the close proximity of the frequencies (1030 MHz for interrogation and 1090 MHz for reply), the propagation loss is estimated assuming that the propagation effects remain similar between interrogation and reply, taking into account other known characteristics associated with the two frequencies.

[0236] In both cases, collectively, once the target appears in the airspace, and although there are differences between the two approaches, as will be shown below, they complement each other and both work towards the same goal.

[0237] With regard to patent application FR1800479, an attempt is made to carry out sensitivity measurements, usually during take-off from an airport and therefore at a small distance, by reducing the power transmitted in successive interrogations until a sensitivity threshold is crossed at the receiver, resulting in a non-response of the transponder: the search is therefore active and contaminated, but still acceptable since it is carried out only at the time of path initialization.

[0238] According to the present invention, an attempt is made to measure sensitivity remotely, typically even before an aircraft enters the radar's airspace. Assuming the downlink (aircraft-to-radar) budget is good because ADS-B replies are being detected, if an aircraft does not respond to an all-call or turn-call interrogation, this means its transponder is still below the receiver sensitivity threshold for the 1030 MHz interrogations sent by the secondary radar. Additional turn-call interrogations intended for the transponder will not cause contamination because they will not be sensed until the receiver sensitivity threshold is crossed.

[0239] The solution implemented by the present invention is beneficial for ATC safety, since the latter requires that any failure of the monitoring system be detected as quickly as possible during the flight of an aircraft.

[0240] Detection of substandard altimeters.

[0241] Aircraft altitude separation is a fundamental element of air safety, both for air traffic control and for automated systems such as onboard collision avoidance systems. Barometric pressure measurements define the local altitude of an aircraft, thus ensuring pressure altitude separation between aircraft in the same flight path. It is well known that altimeter failure can lead to mid-air disasters. Therefore, it is important to detect a barometric altimeter failure as quickly as possible, even in real time, before or after an aircraft enters the airborne coverage area of ​​a secondary radar, as the present invention allows.

[0242] To detect an unsatisfactory barometric altimeter, the first step involves establishing the average atmospheric pressure for each 3D geographic cell, allowing for the generation of a map of the difference between barometric altitude and geometric altitude. The long-range ADS-B coverage area of ​​the present invention is pre-divided into geographic cells. The cell size can typically be 1° latitude and 1° longitude.

[0243] Information obtained from the airspeed message, a second type of ADS-B message, sent every 0.5 seconds, provides the altitude difference between the barometric pressure measurement and the geometric measurement (specifically obtained via GNSS satellite positioning). According to the present invention, statistics on this difference are collected for each geographic cell in a mobile time base that can be adjusted to take into account the atmospheric stability of the environment. Figure 12 An example of a map showing the average values ​​of the differences between pressure altitude and geometric altitude obtained in this way is shown. The levels of the average values ​​correspond to the levels of grayscale on the map.

[0244] Associated with these averages for each cell, the following factors are also considered:

[0245] - the number of measurements in the cell that allows the establishment of an average value;

[0246] -The standard deviation of the measurements in each cell reflects the measurement stability in the cell, and thus reflects the stability of the atmospheric pressure in the cell.

[0247] According to the present invention, at a rate of a mobile time base (where the average of the differences for each 3D geographic cell is taken):

[0248] - For each geographic cell, analyze the distribution of the difference between pressure altitude and geometric altitude within the range of possible values ​​delivered in the ADS-B message, typically with a 25-foot interval, e.g. Figure 13 , which shows an example of a distribution 131 of these differences for all targets present in a geographical cell;

[0249] -Then if:

[0250] On the one hand, the standard deviation of the differences is lower than the parameter of the invention indicating the stability of the height differences in the cell; and

[0251] On the other hand, some differences in a cell are higher than another parameter, indicating the representativeness of the average value of the cell;

[0252] Thus, any target of the radar present in the cell has its (barometric / geometric) altitude difference evaluated relative to the mean value of the altitude differences.

[0253] Two parameters of the invention allow operators to adjust the invention to the characteristics of their radar sites:

[0254] -Atmospheric stability via standard deviation

[0255] - By measuring the number of aircraft congestion.

[0256] If the height difference is close to the distribution of other differences of the cell, then according to the given proximity criteria 142, the target is considered to have a consistent pressure / geometric height difference and the value of its ADS-B track score is increased. Figure 14 The figure shows the target differences via curve 141 , which approximates the distribution of the total differences 131 .

[0257] If the difference 151 exceeds the distribution of other differences of the cell by too much 152, such as Figure 15 As shown, the target is considered to have an inconsistent pressure / geometric altitude difference and is identified by assigning it a value of -1 instead of +1 as in the previous case ( Figure 14 ), the value of its ADS-B tracking score decreases.

[0258] The consistency score of the pressure / geometric altitude difference for each ADS-B track is analyzed based on a given number of comparisons, performed with the intention of notifying the radar user if the score is below the assumed value of an unsatisfactory altimeter. This value is defined taking into account the accuracy of the ADS-B position measurements sent in the message, which are used to establish the pressure / geometric altitude difference for both the cell reference and the target in question. Therefore, the more precise the altimetry error detection, the fewer targets are used, and therefore the less interest is tested, and the invention must integrate this smaller data per cell in a longer moving window. This parameter allows adjusting the general trade-off between the measured position and the stationarity of the measurement context.

[0259] The ultra-long-range detection of ADS-B messages allows this statistical analysis to be performed before the target enters the coverage area of ​​the Mode S radar and therefore allows the air traffic controller to manage the separation between the target and its surrounding targets with a larger margin to ensure ATC safety even in the event of an altimeter failure.

Claims

1. A method for locating a target transmitting an ADS-B message using a secondary radar, the secondary radar comprising an antenna (1), the antenna (1) having a radiation pattern forming a sum channel denoted as SUM, a radiation pattern forming a difference channel denoted as DIFF, and a pattern forming a control channel denoted as CONT, the secondary radar having a predefined interrogation coverage, the method being characterized in that the target is located by implementing the following steps: -Detect ADS-B messages received via CONT channel, SUM channel and DIFF channel; - measuring at least the power of the message and its bearing relative to the radar; The position of the target sending the ADS-B message is calculated by at least detecting a single ADS-B message based on the latitude and longitude position of the radar and a bearing measurement (71) relative to the radar, the position cell denoted as CPR cell (72) encoded in the message being selected via the bearing measurement.

2. The method according to claim 1, characterized in that In the event that two CPR positions (81, 82) encoded in the message may be located in the same azimuth sector, the two CPR positions (81, 82) are distinguished using the measured power and the altitude encoded in the ADS-B message, the CPR position (82) retained being the position with the highest probability based on an estimator based on the visibility of the target and based on the consistency between the power received by the radar and the range of the target from the radar.

3. The method according to claim 1 or 2, characterized in that The method comprises step (120), wherein once a target enters the interrogation coverage of the radar, the target is captured by: - once a target enters the reception coverage of the radar, using the position encoded in the ADS-B message transmitted by the target; - using the target's call identification, which is contained in its Mode S address encoded in the message, A round-robin interrogation is sent every N revolutions of the antenna in order to lock the target to the radar station code, thus avoiding contamination by DF11 replies generated by the target at each revolution once the potential target enters the interrogation coverage area and reaches the area of ​​the radar's operating range.

4. The method according to claim 1 or 2, characterized in that The invention comprises the step (120) wherein said targets are detected and located via their ADS-B messages prior to interrogation coverage of said radar, said targets being associated with ADS-B tracks once they enter said interrogation coverage, allowing them to be acquired by said secondary radar, said tracks then being unlocked upon their entry into operational coverage of said radar to allow detection of their potential lock status, i.e. whether they are locked to another radar, i.e. there is no simultaneous DF11 reply, thereby potentially enabling provision of II / SI code conflict warnings to other Mode S targets that do not have ADS-B capability, while ensuring that Mode S call surveillance is placed on targets once they enter operational coverage of said radar.

5. The method according to claim 1 or 2, characterized in that The invention comprises a step (120) in which, in a moving time window, for each geographical cell over which the target flies, the average value of the difference between the distance of the target measured by the radar and the distance of the target encoded in the ADS-B message transmitted by the target is calculated, this being performed for each target passing through the cell, and then each distance measured by the radar of any target in any mode is corrected according to the average difference calculated for the geographical cell over which the target flies.

6. The method according to claim 1 or 2, characterized in that The method comprises the following steps (120): measuring the sensitivity of a transponder, then detecting a transponder having undesirable sensitivity to an interrogation sent by said radar at 1030 MHz by interrogating a target transmitting an ADS-B message located by said method upon entering the interrogation coverage of said radar, whether in round-robin mode or not, and calculating the sensitivity of the transponder based on the power received by the transponder on its first reply to an interrogation at 1030 MHz, and declaring the transponder to have undesirable sensitivity if said power is above a given threshold.

7. The method according to claim 1 or 2, characterized in that The method comprises the step (120) of detecting an unsatisfactory altimeter associated with a transponder of an ADS-B target by utilizing a deviation of the difference between the pressure altitude and geometric altitude in a series of geographic cells flown by the target and encoded in an ADS-B message transmitted by the target relative to an average of the difference between the pressure altitude and geometric altitude of other targets in each cell calculated for each of the geographic cells flown by the target in a moving time window, wherein the altimeter is determined to be unsatisfactory if the deviation exceeds a given threshold.

8. Secondary radar, characterized in that Arranged to implement the method according to any one of the preceding claims.

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