Method for measuring antenna pattern of secondary radar and secondary radar

Abstract

Each pattern is associated with a reception channel and, during a given time period: - unsolicited asynchronous replies of the long ADS-B intermittent transmission type, transmitted by targets present in the air environment of the radar, are detected; - for each detection, the long ADS-B intermittent transmission is decoded; - for each detection that is retained, the time of the detection, the value of the azimuth of the main beam of the antenna and the value of the received power level on each of the reception channels are associated with the detection, the position information contained in the intermittent transmission giving the elevation bin in which the detection is located; the values ​​obtained during the time period are stored and the measured pattern is sampled from the stored values ​​according to the elevation bins.

Description

Technical Field

[0001] The present invention relates to a method for measuring the azimuth accuracy of an antenna pattern and an angle error measurement function of a secondary radar and also to a secondary radar implementing such a method.

[0002] A preferred field of the invention is air traffic control (ATC), for which the performance of the radar is crucial both for the detection of aircraft and for very accurate positioning of targets up to hundreds of kilometres in azimuth. Background Art

[0003] The accuracy performance level of a secondary radar employed in ATC or IFF is linked, inter alia, to the quality of the pattern of the antenna with which the secondary radar is equipped.

[0004] In the following description, for simplicity, antenna will be used to refer extensively to a component consisting of:

[0005] - mainly antennas for radars of any type (large vertical aperture LVA or beam);

[0006] - A rotating joint connecting the rotating and fixed parts of the radar;

[0007] -Downlead cable.

[0008] Currently, measurement of the azimuth accuracy of a secondary radar requires an external beacon, the reply of which the radar uses to locate it and therefore control the correct operation of the angle error measurement function of the radar.

[0009] A more independent radar measurement of the quality of the pattern in the main lobe (essentially between -10° and +10° in ATC) of the antenna mounted at the radar site requires two things:

[0010] - Switching the station to maintenance mode, which reduces radar coverage to the system level;

[0011] - Use external tools to measure the antenna's pattern (SUM, CONT and DIFF) when receiving (at 1090 MHz);

[0012] This is usually performed only for elevation values ​​as the tool is usually very close to the radar and therefore has very low elevation values, usually close to zero (same altitude as the radar), while aircraft are mainly located between 0.5° and 20° depending on the type of radar use ("airport" or "en route" configuration).

[0013] In the absence of regular preventive measurements of the radar's antenna pattern and azimuth accuracy, or even between preventive measurement sessions, the degradation of these patterns becomes perceptible to the user only when the radar's performance level degrades, sometimes to the point where it no longer fulfils its mission. In this case, interruption of service is mandatory and repairs to the antenna must be performed urgently, bearing in mind that this involves the most important and complex radar maintenance tasks.

[0014] Recall that angular error measurement is a method for measuring the misalignment of targets appearing in the main beam relative to the axis of the antenna, and since a secondary radar must, among its primary mission, ensure very accurate localization of all detected targets both in range and azimuth relative to the radar, its angular error measurement functionality is crucial to it since it contributes directly to its azimuth accuracy. Summary of the invention

[0015] One object of the invention is especially to make it possible to measure the azimuth accuracy of the antenna pattern of a secondary radar and thereby assess its degradation.

[0016] To this end, the subject of the invention is a method for measuring antenna patterns and angle error measurements according to elevation segments of a secondary radar, each pattern being associated with a reception channel, characterized in that, during a given period of time:

[0017] - detecting unsolicited asynchronous replies of the long ADS-B squitter type, transmitted by targets present in the air environment of said radar, each of said squitters containing 3D position information about the target that sent it;

[0018] - For each detection, the long ADS-B intermittent transmission signal is decoded to check that the detected object is located according to the position information contained in the intermittent transmission signal, and non-conforming detections are rejected;

[0019] - for each detection that is retained, the time of the detection, the value of the azimuth angle of the axis of the main beam of the antenna, the angle error measurement voltage and the value of the received power level on each of the SUM, DIFF, CONT_Front, CONT_Back reception channels are associated with the detection, and the position information contained in the intermittent transmission signal is calculated to give the elevation segment in which the detection is located;

[0020] The values ​​obtained during the time period are stored, and the measured patterns and angle error measurements are sampled per elevation segment based on the stored values.

[0021] The antenna, for example, includes one of the following pattern sets:

[0022] - a sum pattern (SUM), a difference pattern (DIFF), a control pattern for rejecting replies from targets facing the antenna (CONT_Front) and a control pattern for rejecting targets behind the antenna (CONT_Back);

[0023] -Sum pattern (SUM), difference pattern (DIFF), control pattern (CONT);

[0024] -Sum pattern (SUM) and difference and control pattern (DIFF / CONT).

[0025] In a particular implementation, a reply having a long ADS-B intermittent transmission signal type is characterized by a characteristic representing acquisition of the reply, the characteristic being, for each reply, at least one of the following characteristics:

[0026] - received power according to each pattern of the antenna;

[0027] - angular error measurements in the main lobe of the antenna;

[0028] - the time of detection of said response;

[0029] - the azimuth of the axis of the main beam of said antenna at the time of said detection.

[0030] For example, for each intermittent transmission signal received in the main beam, the method performs the following operations:

[0031] - Utilize the information contained in the intermittently transmitted signal;

[0032] - Calculate the relative power and then calculate the error in the gain of the antenna pattern based on the relative orientation:

[0033] In the same beam, the radar measures the power of the synchronous trace, which is the maximum of the sum pattern;

[0034] The long ADS-B intermittent transmission signal reply is characterized by measuring the power of the sum pattern, difference pattern and control pattern. The following items are derived for the relative position of the intermittent transmission signal according to the method of the present invention:

[0035] o SUM_Relative (dB) = SUM (in dBm) - SUM_max (in

[0036] dBm);

[0037] o DIFF_Relative (dB) = DIFF (in dBm) - SUM_max (in

[0038] dBm);

[0039] o CONT_Relative (dB) = CONT (in dBm) - SUM_max

[0040] (in dBm);

[0041] The gain error for each pattern is established by comparing these values ​​with the values ​​of the reference of the present invention (the pattern measured when the radar is received in the factory or when the radar site is accepted):

[0042] o SUM_Error(dB) = SUM_Relative(dB) - SUM_Reference(dB);

[0043] o DIFF_error (dB) = DIFF_relative (dB) - DIFF_reference (dB);

[0044] o CONT_error (dB) = CONT_relative (dB) - CONT_reference (dB);

[0045] -Calculate the angle error measurement error by taking into account the deformation of the beam in elevation relative to the antenna.

[0046] Over the duration of the analysis, the values ​​are accumulated over time according to relative orientation, for example according to:

[0047] - the azimuth of the antenna;

[0048] - The elevation angle of the target.

[0049] During the given time period, the angle error measurement error and the error of each pattern are accumulated, for example, in a table with three inputs, one table is associated with the angle error measurement error and the error of each antenna pattern, one input is the relative azimuth according to the axis of the main lobe of the antenna, the second input is the elevation angle of the target according to the horizontal alignment of the antenna, and the third input is the antenna azimuth. Each unit (relative azimuth, elevation angle) of the angle error measurement error and the error of each pattern is established, for example, by averaging, histogram or other types of methods.

[0050] The given time period is, for example, defined to reveal a current degradation of the pattern.

[0051] Under operator control or automatically, the radar locates in azimuth a synchronization target in the main beam using a table of angle error measurements, the table being corrected, for example, based on measured angle error measurement errors, to ensure good azimuth accuracy of the radar in the event of degradation of a collection of components known as an aerial.

[0052] The antenna equipped with the secondary radar of ATC or IFF type includes at least two patterns, for example. The antenna has fixed or rotating electronic scanning.

[0053] The measurements of the angular error measurement error and the antenna pattern are used, for example, to assess the degradation level for each of the following elements of the antenna unit: antenna, downconductor cable, swivel joint.

[0054] The measurements of the pattern are used, for example, to assess the level of degradation of a collection of components known as antenna elements.

[0055] Another subject of the invention is a secondary radar for implementing such a method. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Other features and advantages of the present invention will become apparent from the following description, which is given with reference to the accompanying drawings, which show:

[0057] - Figure 1 : Example of a block diagram of a Mode S secondary radar;

[0058] - Figure 2 : Illustration of the hardware implementation of the present invention;

[0059] - Figure 3 : Illustration of an example of an ATC antenna;

[0060] - Figure 4 : Illustration of typical synchronization operation of secondary radar by example;

[0061] - Figure 5 : Illustration of received power as a function of distance to the target;

[0062] - Figure 6 : Illustration of the principle of the invention by the following example: Sampling the antenna pattern by asynchronous squitters sent by different targets;

[0063] - Figure 7 : An example of a pattern measured by the method according to the present invention;

[0064] - Figure 8 : Plot of the angle error measurement voltage as a function of the misalignment of the target in the main lobe;

[0065] - Fig. 9 : The radar azimuth accuracy error as a function of the misalignment of the target in the main lobe;

[0066] - Fig.10 : A new curve of the radar's azimuth accuracy error as a function of misalignment after a new table of angle error measurements has been calculated according to the invention;

[0067] - Fig.11 : Amplification of the useful part of the main lobe comparing the azimuth accuracy error of the radar as a function of misalignment between the new angle error measurement table according to the invention and the old angle error measurement table. DETAILED DESCRIPTION

[0068] Regarding an example of a block diagram representing a Mode S radar Figure 1 , the principles of this radar are reviewed.

[0069] The principles of Mode S secondary radar (defined in detail by ICAO Annex 10 Volume 4) include:

[0070] -Send optional inquiry:

[0071] Indicates to the receiver: a single target specified by its Mode S address;

[0072] or an identifier indicating the transmitter;

[0073] -Receive Selective Acknowledgement:

[0074] Indicator of transmitter: the same Mode S address of the target;

[0075] Or to indicate to the recipient: the identifier of the interrogator.

[0076] In its normal use, the secondary radar operates in synchronous mode, that is to say it sends interrogations and waits for replies consistent therewith, which allow it to locate by measurements (in azimuth and range) and to identify the target (by means of the Mode S address).

[0077] To perform this task efficiently, the radar is equipped with an antenna 1 ( Figure 1 ), the antenna has a plurality of patterns 11, 12, 14, 15, and their functions are generally:

[0078] - a sum pattern 11, denoted SUM in the following, which is used to interrogate and detect a synchronous reply from a target;

[0079] - a difference pattern 12, denoted DIFF, which is used to finely locate the target in the SUM beam;

[0080] - a first control pattern 15, denoted CONT_Front, which is used to block and reject replies from targets facing the antenna that are not present in the main SUM beam;

[0081] - A second control pattern 14, denoted CONT_back, which is used to block and reject replies from targets behind the antenna (and therefore not necessarily present in the main SUM beam).

[0082] Depending on the mission and therefore the expected performance level of the radar, the antenna can have:

[0083] -Several patterns:

[0084] 4 patterns: SUM, DIFF, CONT_Front and CONT_Back;

[0085] 3 patterns: SUM, DIFF, CONT (CONT_Front and CONT_Back are grouped together at antenna level);

[0086] • 2 patterns: SUM, DIFF / CONT (DIFF, CONT_Front and CONT_Back are grouped together at antenna level).

[0087] -Different sizes:

[0088] In terms of width:

[0089] o have a large width to have a thin main beam providing high gain and being selective in azimuth and accurate;

[0090] In terms of height:

[0091] o Large height of the Large Vertical Aperture (LVA) type, which increases gain and protection from ground reflections (mainly in ATC);

[0092] o Has a small height of the "beam" type, which increases mobility (mainly in IFF

[0093] ).

[0094] While the SUM and DIFF patterns are traditionally good for 3dB lobes between 2.4° and 10°, the CONT_Front and CONT_Back patterns both attempt to cover almost 180°.

[0095] Antennas can also:

[0096] - has a fixed pattern called "mechanical" and rotates;

[0097] -Has a changing pattern with an electronic scan called a fixed or rotating "AESA".

[0098] In the following description, the most comprehensive antenna configuration is described, in other words, 4 patterns in the rotating antenna mode, it should be remembered that other configurations are handled similarly regardless of the number of antenna patterns used, regardless of whether the antenna is rotating or fixed. However, in order to simplify the description, it will be possible to use a 3-pattern configuration by using CONT as a combination of CONT_Front and CONT_Back.

[0099] In operational use of a radar, it receives an unsolicited reply (with no associated interrogation on the radar's side) which is called a "False Reply Unsynchronized in Time" or Fruit. It is so named because:

[0100] - they are not what the radar that rejected them expected ("error");

[0101] - they are replies very similar to replies (“replies”) that are synchronized and obtained from the same target within the coverage of the same radar, with the same frequency and the same message format;

[0102] - They are not associated with an interrogation from this radar, but with an interrogation from another radar or even sent regularly by the target (eg ADS-B intermittent transmission signals) ("time asynchronous").

[0103] Due to its asynchronous nature, the radar receives Fruit on all of its antenna patterns.

[0104] Finally, S-Mode Fruit can be identified by the single S-Mode address associated with each target.

[0105] While Fruit generated by secondary radars are currently processed as faults that must be filtered before processing, the present invention advantageously utilizes Fruit as long ADS-B intermittent transmission signals during processing to measure the radar's angular error measurement accuracy and antenna pattern.

[0106] In summary, the measurement of the pattern of the main lobe of the antenna is performed continuously without influence on the operational operation of the radar and is applicable regardless of the type of antenna described.

[0107] In Mode S Fruit received by radar, the ADS-B bursts are not solicited by another interrogator (radar, WAM, etc.), but are generated unsolicited by the aircraft themselves to signal their presence and, in the case of long ADS-B bursts (DF17), also give their position, which is accurately calculated by utilizing GPS signals.

[0108] The basic principles of a receiver using the same Mode S protocol, long ADS-B intermittent transmission signal (the message is defined in detail in ICAO Annex 10 Volume 4) are:

[0109] - Receiving an unsolicited (and therefore asynchronous) selective reply:

[0110] Indicates the transmitter's identifier: the target's same Mode S address as sent to the radar in the selective interrogation and reply described above (24-bit field);

[0111] The nature of the content of the message (DF=17) is variable according to the TC field of the message:

[0112] o 1 to 4 "Aircraft identification"

[0113] o 5 to 8 "surface positions"

[0114] o 9 to 18 "Air Position (Baro Alt (Pressure Altitude))"

[0115] o 19 is "air speed"

[0116] o 20 to 22 “Aerial position (GNSS altitude)”

[0117] o 23 "Test Message"

[0118] o 24 "Surface system status"

[0119] o 25 to 27 "Reserved"

[0120] o 28 "Extended intermittent transmission signal AC state"

[0121] o 29 “Target status and conditions (V.2)”

[0122] o 30 "Reserved"

[0123] o 31 “Aircraft operating status”.

[0124] The above list is given by way of example, is indicative and is subject to variation.

[0125] Specifically, the invention exploits the DF17 intermittent transmissions of the corresponding TC fields from 9 to 18 or from 20 to 22, which give the position in 3 dimensions of a target equipped with a transponder broadcasting an intermittent transmission associated with altitude (either barometric or GNSS).

[0126] Therefore, in its normal use, the ADS-B_in receiver operates in asynchronous mode, that is, it listens over 360° for Mode S messages, which are very similar to the radar's messages for the position (azimuth and range) and identification (Mode S address) of targets.

[0127] To perform this task efficiently, the ADS-B_in receiver is equipped with:

[0128] - Omnidirectional antenna covering 360°, which is the normal configuration;

[0129] - or several wide pattern antennas covering a total of 360°:

[0130] Two antennas back-to-back with coverage greater than 180°, the most widely used configuration;

[0131] Less frequently, three antennas covering more than 120°, or even four antennas covering more than 90°;

[0132] According to the format reviewed above, its role is simply to detect an asynchronous reply from the target by means of a single pattern (with and type) and to decode its content.

[0133] Given that secondary radars and ADS-B receivers utilize nearly identical messages (same 1090 MHz frequency, same waveform, same data structure of reply messages), it is easy to incorporate in radar the functionality of listening for asynchronous ADS-B intermittent transmissions by listening for them via different patterns of the radar's antenna (and for this purpose, primarily but not exclusively via omnidirectional patterns):

[0134] - by several receivers each associated with its antenna pattern:

[0135] For antennas with 4 patterns: SUM, DIFF, CONT_Front and CONT_Back;

[0136] For antennas with 3 patterns: SUM, DIFF, CONT;

[0137] For antennas with 2 patterns: SUM, DIFF / CONT.

[0138] In the context of the present invention, the radar is equipped according to this configuration, ie one receiver is associated with each pattern of its antenna, whether the antenna comprises four, three or two patterns.

[0139] Before describing the present invention in more detail, Figure 1 The components of the S-mode radar. This block diagram reveals the synchronized operation of the S-mode radar:

[0140] - in the left part 100, by generating a query;

[0141] - in the right part 200, by synchronous processing of the associated reply,

[0142] and the synchronization between the latter is revealed by the arrows spanning between the left and right.

[0143] The following reviews the functions of the main components:

[0144] Antenna 1 ensures that interrogations are radiated at 1030 MHz and replies are returned at 1090 MHz according to four patterns (SUM, DIFF, CONT_Front and CONT_Back) or three patterns (SUM, DIFF, CONT) or two patterns (SUM, DIFF / CONT).

[0145] The swivel joint 2 and downconductor cable for the swivel antenna ensure:

[0146] - RF coupling for signals transmitted at 1030 MHz and received at 1090 MHz, for four patterns between the rotating and fixed parts of the radar separately;

[0147] - The position is broadcast in azimuth 201 of the axis of the antenna's main lobe.

[0148] RF processing includes:

[0149] a duplexer or circulator 3 ensuring RF coupling between the signal transmitted at 1030 MHz and the signal received at 1090 MHz for the four patterns individually;

[0150] - Transmitter 4, ensure that:

[0151] Send an inquiry at 1030 MHz on a SUM pattern;

[0152] Blocking transponders outside the SUM lobe at 1030 MHz through CONT_Front and CONT_Back patterns;

[0153] Same for different auxiliary protocols: IFF, SSR and S-Mode;

[0154] - Receiver 5 ensures that the four SUM, DIFF, CONT_Front and CONT_Back

[0155] The reply is received at 1090 MHz on the pattern and the angle error measurements are calculated for the different auxiliary protocols (IFF, SSR and S-mode).

[0156] Real-time processing includes:

[0157] - Space-time management 6 ensures real-time management of the associated interrogation and listening periods for the different auxiliary protocols (IFF, SSR and S-mode);

[0158] - Signal processing 7 ensures:

[0159] Handling of replies in listening periods associated with interrogations for different auxiliary protocols (IFF, SSR and S-Mode);

[0160] Detect and decode synchronization replies in the main lobe of the antenna by utilizing four patterns:

[0161] o SUM: used to detect replies received in the main lobe;

[0162] o DIFF: used to finely locate in azimuth the replies received in the SUM main lobe and possibly to distinguish entangled replies;

[0163] o CONT_Front and CONT_Back: used to reject the acknowledgements received in the auxiliary SUM and DIFF lobes if detected in the SUM main lobe. The processing in the main lobe of the antenna includes:

[0164] - Management 8 of the objects present in this lobe, which serves to ensure:

[0165] Prepare transactions (queries and replies) to be performed in the next lobe for the different auxiliary protocols (IFF, SSR, and S-Mode);

[0166] · According to the status of the transaction that has just been executed, the "roll call" in the future

[0167] During this period, Mode S interrogation and reply are placed;

[0168] The extractor 9 ensures the formation of a trace for each of the different auxiliary protocols IFF, SSR and S-mode (according to the synchronization replies received in this lobe based on the protocol adopted in the interrogation).

[0169] The multiple rounds of processing 10 include:

[0170] - management 101 of the Mode S missions to be performed with the targets in coverage, which ensures the prediction of the targets' positions (rendez-vous) and the preparation of the missions to be performed using these positions, according to internal and external requests and the status of transactions of previous rounds;

[0171] - Association of a plot with the tracking 102 of the target in the overlay, which ensures the tracking of the target to enhance the performance level (elimination of erroneous plots, in particular checking of decoded data) and to predict its future position.

[0172] The interface with the user makes it possible for the radar to take into account different requests and to display point tracks and target tracks.

[0173] Therefore, the selective interrogation principle of Mode S is conceptually highly sensitive to the accuracy of these measurements: it is based on interrogating in the next pass by using the position prediction performed from earlier measurements. The measured position error causes the predicted position error of the Mode S radar in the next pass, and when this happens it is possible that:

[0174] - lead to meaningless selective interrogations because they are poorly positioned in azimuth relative to the target's true position;

[0175] - or in the worst case, causing the round to not detect the target.

[0176] It is therefore very important to "monitor" the accuracy of the radar's azimuth measurements based on the use of angular error measurements.

[0177] Figure 2 The hardware implementation of the present invention applied to an S-mode radar having an antenna with four modes is shown. Figure 1 The block diagram of a radar of the type shown in FIG. 1 is shown in FIG. 1 with the addition of elements specific to the present invention. The main elements of the present invention applied to a Mode S radar are shown in FIG. Figure 2 The center is indicated by a thick dashed line.

[0178] Although the operation of the Mode S radar is synchronous, it can be seen that the added processing 21, 22 is not transmission dependent and only utilizes the azimuth position of the axis of the antenna's main lobe.

[0179] Most elements remain unchanged, confirming two things:

[0180] - The present invention does not disrupt the operation of Mode S radar;

[0181] -Measurements are made via the same receiver, using the same elements as those used by radar:

[0182] ·antenna;

[0183] Rotary joint;

[0184] Down conductor cable.

[0185] A first process 21 is added to handle the reception of ADS-B intermittent transmissions. It ensures continuous processing of asynchronous replies in Mode S (independent of the listening period associated with the interrogation), which ensures the detection and decoding of asynchronous replies by utilizing the four antenna patterns (SUM, DIFF, CONT_Front and CONT_Back) separately but equally to:

[0186] - Check all received replies, whether asynchronous or synchronous;

[0187] -decode the reply to extract the Mode S address from it;

[0188] - enrich each decoded response with its following properties, specifically:

[0189] · Detection time;

[0190] The azimuth of the antenna’s main lobe during detection;

[0191] Received power in SUM, DIFF, CONT_Front and CONT_Back patterns;

[0192] • Angular error measurements of the ADS-B reply, that is, measurements of the misalignment of the reply relative to the axis of the antenna.

[0193] To this end, the space-time management 6 performs an operation 23 of sending the azimuth position of the main lobe of the antenna to the continuous processing 21 of the S-mode asynchronous reply.

[0194] In parallel and advantageously, a characterization of the synchronization acknowledgement can be obtained through the powers measured with respect to the SUM, DIFF, CONT_Front and CONT_Back patterns, supplemented by an angle error measurement.

[0195] At the extractor 9, a representation of the S-mode trace of the power measured with respect to SUM is also obtained.

[0196] The added second process 22 stores and correlates the data contained in the long ADS-B intermittent signal replies (DF17) and the radar measurements of the characteristics of these intermittent signal replies. Specifically, the calculation of the 3 SUM, DIFF, CONT_Front and CONT_Back antenna patterns in elevation and the calculation of the angular error measurement error in elevation are performed by using the position given in the long ADS-B intermittent signal. This second process is added in multiple rounds of processing, where the position of the target is sent in the intermittent signal with the S-mode address, and the time and position are supplemented by the values ​​of the power measured for SUM, DIFF, CONT_Front and CONT_Back, the angular error measurement and the antenna azimuth measured using this same intermittent signal reply.

[0197] The added second process 22 realizes the principle of the present invention, which is to make use of the following facts:

[0198] - Long ADS-B intermittent transmissions are inherently asynchronous between the period of the ADS-B intermittent transmission and the rotation of the radar;

[0199] - Long ADS-B intermittent transmissions are received at any instant, so they sample the pattern of the antenna's main beam at different relative bearings;

[0200] - Long ADS-B intermittent transmissions are sent by different targets that sample the pattern of the antenna's main beam at different elevation angles, the elevation angles of the targets causing these intermittent transmissions to be generated;

[0201] - Long ADS-B TX signals send accurate position measurements in addition to the radar's position measurements (GPS position of the long ADS-B TX signal);

[0202] - The long ADS-B intermittent transmission makes it possible to perform radar type measurements on the same intermittent transmission.

[0203] Before describing the present invention in more detail, Figure 3 The principle of the invention is recalled by the representation of the ATC antenna of the secondary radar shown. In this example, the antenna is of LVA type. It comprises N rods, each containing P dipoles (e.g. 35 rods, each with 11 elements). The invention exploits the content of the DF17 message for specifying the position of the transponder (LAT-LONG) and the detection and measurement of its position by the radar. The distribution of the energy received between the N rods makes it possible to measure the position in azimuth, and the distribution of the energy received between the dipoles of each rod defines the elevation pattern.

[0204] In order to isolate local degradations at the distributor level between the bars, or at the level of the bars or at the level of the dipoles of the bars, the invention accumulates the measurements in elevation segments. It is the asynchronicity of the long ADS-B intermittent transmission signal (DF17) and the differences in the height and range (and therefore in elevation) of the targets that make it possible to cover almost all antenna lobes in a given period of time (e.g., a day).

[0205] Thus, as will be described below, the present invention consists essentially in: measuring misalignment relative to the antenna's axis in terms of elevation segments of ADS-B intermittent transmission signals received by the radar in its main beam; measuring errors in the angle error measurement curve used by the radar for its angle error measurement function based on the difference between the position given by the ADS-B intermittent transmission signal and the position measured by the radar's simultaneous angle error measurement of the same ADS-B intermittent transmission signal; and then integrating these errors in terms of elevation segments over a long period of time (e.g., daily).

[0206] Similarly, measurement of the power of each pattern associated with the ADS-B intermittent transmission signal makes it possible to calculate the relative power of SUM, DIFF, and CONT_Front compared to SUM_max for misalignment of the ADS-B intermittent transmission signal in the lobe, and then integrate these measurements by elevation segment over a long period of time (e.g., daily).

[0207] In this way, without interfering with the operational operation of the radar and in the absence of beacons, using only aircraft equipped with feasible ADS-B, it is possible to automatically detect current degradations of the radar's, in particular its azimuth accuracy (or more globally, its main beam), per elevation segment alone.

[0208] The principle consists in using Fruit (asynchronous replies), which are long ADS-B intermittent transmissions received by the radar in its main beam, corresponding to the situation in which an aircraft intermittent transmission appears in the main lobe of the radar when it is sent. In fact, by its main task of locating the target, the long ADS-B intermittent transmission DF17 is essentially related to the latitude-longitude position (LAT-LONG) of the target.

[0209] Like any other Fruit, when an ADS-B intermittent transmission is received, it is characterized by the power received on different patterns of the radar antenna and (for any response received in the radar's lobe) an angular error measurement measured (the angular error measurement is only meaningful in the main beam, more specifically, near the axis of the antenna, i.e., in the region of 2.5° to +2.5° for an ATC antenna with a 2.4° 3dB lobe).

[0210] The present invention utilizes only radar-verified ADS-B intermittent transmission signals, that is, it uses selective interrogations that have been positively responded to by the same targets in the radar operation mission covered by the airborne responsibility of the surveillance radar, thereby avoiding taking into account ADS-B intermittent transmission signals from the following: spoofers whose declared positions may be false, or targets whose flights have shown various errors in the radar's previous detection.

[0211] Therefore, the present invention advantageously utilizes long ADS-B intermittent transmission signals, which are located in relative azimuth relative to the axis of the antenna so that they are located in the main beam (the parameters depend on the antenna type, ATC or IFF). Therefore, for each long ADS-B intermittent transmission signal, the present invention performs the following operations:

[0212] - accurately calculate its relative position by using the LAT-LONG position of the intermittently transmitted signal and the LAT-LONG position of the radar, giving its misalignment in the beam of the radar's antenna;

[0213] - Calculate the relative power measured by the same receive chain of the radar from the SUM, DIFF and CONT_Front patterns compared to the SUM_max of the quasi-simultaneous synchronous traces of the intermittently transmitted signal (necessary for the detection function of the radar);

[0214] - Calculation of its angular error measurement voltage measured by the same receiving chain of the radar (necessary for the azimuth position function of the radar);

[0215] -Error about the theoretical curve of the antenna:

[0216] Based on the target's elevation angle relative to the radar's position;

[0217] Based on the azimuth of the intermittently transmitted signal;

[0218] o In terms of relative magnitude, for SUM, DIFF, and CONT compared to

[0219] SUM_max pattern;

[0220] o In the case of angle error measurement, reference is made to an angle error measurement table which gives, for the same reply, the angle error measurement voltage of the reply as a function of the misalignment of the target being measured without interfering with the operational operation of the radar.

[0221] It should be noted that in order to extend the range of measurements and therefore improve the accuracy of this monitoring of the pattern of the main beam, the invention performs calculations for each elevation segment in order to take into account the influence of the expansion of the antenna's pattern (and therefore, in particular, the angle error measurement function) in terms of the cosine of the target's elevation angle relative to the radar.

[0222] In order to ensure good accuracy of the individual measurements, the following are used, for example:

[0223] - Intermittent transmission of strong level (short distance) in order to limit the effect of receiver noise while ensuring sufficient dynamic range for measuring SUM, DIFF and CONT_Front beams;

[0224] - intermittent transmissions from targets at short and medium ranges (depending on the elevation angle) in order to minimize the impact on the accuracy of the target's position transmitted by the target when the ADS-B intermittent transmission is detected (variable delay between the measured GPS position of the intermittent transmission and the time of reception of the intermittent transmission by the radar),

[0225] Thereby optimizing the measurement result of the angle error measurement error.

[0226] The accumulation of these measurements over a considerable period of time makes it possible to cover all the values ​​of elevation, azimuth and relative orientation used by the radar and thus improve the accuracy of the calculation of the errors according to the three axes retained. In addition, the accumulation of these errors (measured in terms of magnitude and angle errors of relative orientation) according to azimuth segments (e.g., with a pitch of one degree) makes it possible to reveal singular points associated with a given azimuth.

[0227] These errors are analyzed (e.g., by automated tools). If the analysis results in an estimated excessive amount of error (e.g., in magnitude, in the case of angular error measurements), a hypothesis can be made about the source of the degradation:

[0228] - If it does not depend on the azimuth or relative azimuth or elevation of the intermittently transmitted signal, then:

[0229] It may be related to the deterioration of the downconductor cable;

[0230] - If it depends on the azimuth of the intermittently transmitted signal, but not on the relative azimuth or elevation, then:

[0231] It may be related to the deterioration of the rotating joint that connects the fixed and rotating parts of the radar’s antenna;

[0232] - If it depends on the relative position, but not on the azimuth or elevation of the intermittently transmitted signal, then:

[0233] It may be related to the degradation of the vertical dipole of the antenna or the antenna's divider;

[0234] - If it depends on the relative azimuth and elevation, but not on the azimuth of the intermittently transmitted signal, then:

[0235] • It may be related to the degradation of the radiating element of the vertical dipole of the antenna.

[0236] Other assumptions can be made about the source of the degradation.

[0237] Therefore, in a HUMS (“Health and Usage Monitoring System”) type approach, continuous analysis of these measurements allows automatic alerts to provide repair actions when necessary, which can be scheduled before the performance level of the radar degrades.

[0238] Regarding decoding of "Long ADS-B Intermittent Transmission" replies, Chapter 3 of Appendix 10 of Aeronauticaltelecommunications gives an overview of the Mode S reply (downlink) format.

[0239] The long ADS-B intermittent transmissions (DF17 and DF18) do have the same message format as the synchronous replies processed by the radar. Therefore, using similar processing for the asynchronous replies as for the synchronous replies makes it possible to obtain the same pattern measurements from both messages and therefore be able to infer a common conclusion from them, namely that the degradation of the antenna measured by the asynchronous replies is fully applicable to the synchronous replies.

[0240] The detection of replies by the radar according to the invention differs according to whether these replies are synchronous or asynchronous, as summarized according to the antenna pattern in the following Table 1, in which the numerical data are given by way of example:

[0241] Table 1

[0242]

[0243] According to the present invention, all asynchronous replies are characterized by the power measured according to each pattern.

[0244] The different S-Mode responses (synchronous and asynchronous) are very similar with respect to the S-Mode address in the following items:

[0245] - in the parity field;

[0246] - or in the data field.

[0247] The Mode S address is a unique identifier for the transponder and, therefore, makes it possible to identify the target in an asynchronous process (as traditionally in a synchronous process) and to ensure, through selective interrogation of the radar, that the ADS-B intermittent transmission is indeed a real target within the radar's coverage.

[0248] The present invention does not take into account ADS-B intermittent transmissions that are considered suspicious (ie, not verified by the selective interrogation of the radar).

[0249] Figure 4 A typical synchronous operation of a secondary radar is shown by way of example, utilizing the main beam of a conventional ATC antenna with three patterns (SUM, DIFF, CONT_Front) in both transmission and reception, within an azimuth zone bounded by two limits 40 (to ≈ -2°), 40' (to ≈ +2°):

[0250] -SUM is used to:

[0251] Using the query 41 ′, a query is generated for the transponder (curve 41 );

[0252] · Detecting the response of the transponder (curve 42) using the response 42';

[0253] -DIFF is used for:

[0254] The position of the reply relative to the axis of the antenna (curve 43);

[0255] For some types of radar, distinguishing between entangled replies;

[0256] - CONT_Front, which is used to delimit the replies to be processed in the beam of the antenna (curve 44):

[0257] ISLS: Blocks transponders outside the core of the main beam in terms of transmission (1030 MHz) (see Figure 4 , which corresponds to queries rejected by the transponder);

[0258] RSLS: Rejects replies outside the core of the main beam on receive (1090 MHz).

[0259] Figure 5 The received power is shown as a function of the distance to the target. Figure 4The curves representing the patterns of the antennas show that the power dynamic range of the signal to be processed in the main beam has a relative magnitude of 25 dB to 35 dB between these patterns. The curve 51 indicates the average power received per target (that is, per trace) as a function of the distance of the target. This curve 51 (a typical downlink radar budget) makes it possible to evaluate the power of the signal received from the target. In order to ensure a good dynamic range and accuracy, the method according to the invention uses only nearby targets whose power level is at least 35 dB higher than the threshold 52 for the detection and decoding of the ADS-B reply by the radar. Given that the long ADS-B intermittent transmission signals considered by the invention are long ADS-B intermittent transmission signals received in the main beam of the radar, their levels are close to their levels when the same target replies to the radar in a synchronized mode. In practice, this results, for example, in that for the measurement of the pattern, only long ADS-B intermittent transmission signals for which the target is located within 50 Nm of the radar (this value can be adjusted by the operator through parameters) are used.

[0260] Figure 6 The principle of the invention is illustrated by the following example: the antenna pattern is sampled by about ten intermittent transmissions sent by different targets when receiving at 1090 MHz. These patterns are similar to those described in the case of synchronous use. Figure 4 The patterns are the same, only the interrogation curve 41 is eliminated. The present invention advantageously utilizes the long ADS-B intermittent transmission signal to persistently measure with high accuracy the pattern of the main beam of the secondary radar antenna, where opportunistic targets at different elevation angles are selected at a certain distance depending on the type of measurement performed.

[0261] When the radar's main beam illuminates the target, the radar selectively interrogates the target, e.g. Figure 4 As shown in , and these targets can automatically send intermittent transmission signals (including long ADS-B intermittent transmission signals) at their own rates, such as Figure 6 as shown in .

[0262] Figure 6 A typical ATC or IFF antenna is shown having 3 patterns: SUM, DIFF and CONT_Front, which are represented by curves 42, 43, 44 respectively.

[0263] At a given moment, different targets present at azimuths between -5° and +5° to the antenna's axis may send long intermittent transmissions. Each intermittent transmission detected in a lobe is characterized by:

[0264] - its received power levels on the three curves 42, 43, 44 - thus, the intermittent transmission signal is received at a power level value 61 on the SUM curve 42, at a power level value 611 on the DIFF curve 43, and at a power level value 612 on the CONT_Front curve 44;

[0265] - the angular error measurement voltage calculated by the radar from the reply signals received on DIFF and SUM;

[0266] - Its relative orientation compared to the axis of the antenna (abscissa X axis).

[0267] Depending on the distance to the target, the dynamic range of the signal (ordinate Y axis) given by the reception level compared to the measurement threshold of the radar makes it possible to have the required dynamic range (approximately 35 dB).

[0268] Each intermittent transmission signal is represented by the target's S-mode address:

[0269] -Some intermittent transmissions give the LAT-LONG position of the target;

[0270] - Other intermittent transmissions give the target's altitude (barometric or GNSS).

[0271] Asynchronicity between:

[0272] - The position of the target in azimuth and elevation;

[0273] - Rotation of the radar beam;

[0274] - Spontaneous transmission of long ADS-B intermittent transmission signals by the target;

[0275] This makes it possible to sample all possible positions of the radar antenna during its rotation over a long period of time:

[0276] - in relative orientation;

[0277] - in elevation (which is derived from the LAT-LONG position given the range and altitude of the target);

[0278] - In azimuth of the axis of the antenna beam.

[0279] The accumulation of measurements from ADS-B intermittent transmission signals is also performed. To this end, the present invention tracks all long ADS-B intermittent transmission signals (asynchronous) in a manner similar to the synchronous track of radar to construct ADS-B tracks. These tracks are mainly represented by the following items:

[0280] -S mode address;

[0281] - The current position of the target in 3D (azimuth, range, altitude) and its velocity vector;

[0282] - Current power of the target;

[0283] - Possible use of this track according to the method of the invention.

[0284] Even if intermittent transmissions from a target are not received in the antenna's main beam, they are tracked in order to characterize the track for the above data.

[0285] In fact, the long ADS-B intermittent transmission signal DF17 includes references, in particular the LAT-LONG position and altitude, depending on the value of TC.

[0286] For each available intermittent transmission signal (intermittent transmission signal verified by the radar) received in the main beam (e.g., misaligned with the antenna axis in relative azimuth by up to -10° to 10° when detected, keeping in mind that this angular range is not limiting and the invention can be applied from -180° to 180°), the method according to the invention performs the following operations:

[0287] - Utilize the information contained in the intermittently transmitted signal;

[0288] -Complete the missing information based on data from its track;

[0289] - Calculate the relative power and then calculate the error in the gain of the antenna pattern based on the relative orientation:

[0290] The radar measures the power of synchronized traces in the same beam (see Figure 4 ): This is the SUM_max value (in dBm) that can be obtained with this radar configuration at this distance from the target;

[0291] The reply of the long ADS-B intermittent transmission signal is characterized by the power measurement results (in dBm) of the SUM, DIFF and CONT patterns. The following items are derived for the relative position of the intermittent transmission signal according to the method of the present invention:

[0292] o SUM_Relative (dB) = SUM (in dBm) - SUM_max (in

[0293] dBm);

[0294] o DIFF_Relative (dB) = DIFF (in dBm) - SUM_max (in

[0295] dBm);

[0296] o CONT_Relative (dB) = CONT (in dBm) - SUM_max

[0297] (in dBm);

[0298] SUM_Relative(dB), SUM(in dBm), and SUM_max(in dBm) are defined as follows: Figure 4 Medium (or Figure 6 The curves in Figure 1 (a) and (b) represent the relative gains (in dB) of the SUM, DIFF, CONT antenna patterns according to the misalignment in the relative orientation of the antenna's axis (relative to the maximum power SUM_max in the antenna's axis). When an ADS-B reply is received with a certain misalignment, a measurement of the power (in dBm) is performed according to these three patterns.

[0299] For the DIFF and CONT patterns, DIFF_Relative (dB) and DIFF (in dBm), CONT_Relative (dB) and CONT (in dBm) are similarly defined.

[0300] By comparing the power of the synchronization point trace closest in time to the ADS-B fruit (here referred to as SUM_max, in dBm) with the power of the ADS-B on the three patterns, according to Figure 7 to obtain the relative gain points of measured pattern 42" (SUM), measured pattern 43" (DIFF) and measured pattern 44" (CONT), based on the misalignment presented in Figure 7 This will be described below.

[0301] The gain error for each pattern is established by comparing these values ​​with the values ​​of the reference of the present invention (the pattern measured when the radar is received in the factory, or when the radar site is accepted):

[0302] o SUM_Error(dB) = SUM_Relative(dB) - SUM_Reference(dB);

[0303] o DIFF_error (dB) = DIFF_relative (dB) - DIFF_reference (dB);

[0304] o CONT_error (dB) = CONT_relative (dB) - CONT_reference (dB);

[0305] For the main lobe, the angular error measurement error is established by comparing the position measured by the radar with the position sent in the ADS-B intermittent transmission signal for its misalignment in the beam, taking into account the deformation of the beam in elevation relative to the antenna.

[0306] During the analyzed time period, these different values ​​are accumulated according to relative position based on:

[0307] - the azimuth of the antenna (e.g., up to a certain number of degrees of spacing);

[0308] - and regardless of the azimuth;

[0309] -Elevation angle of the target.

[0310] Using the positionally independent ADS-B intermittent transmission signals detected in the main lobe provides two main advantages:

[0311] - For the measurement of the pattern in the main beam (-10° to +10° from the axis), after quasi-simultaneous reception of the ADS-B STOP signal and the synchronous dot trace, the power transmitted by the target is constant for both the synchronous reply of the dot trace and the asynchronous reply of the STOP signal, which makes it possible to have very accurate relative measurements of the 3 SUM, DIFF, CONT patterns;

[0312] - For "monitoring" of the azimuth accuracy of the radar, the radar method of measuring the azimuth position of a target that has sent an ADS-B intermittent transmission signal directly from the ADS-B reply makes it possible to have a very accurate measurement of the angular error measurement error, because the target has not moved between the transmitted position and the measured position.

[0313] Figure 7 An accumulation of these values ​​is shown, which makes it possible to reconstruct the SUM pattern (measured pattern 42"), the DIFF pattern (measured pattern 43") and the CONT pattern (measured pattern 44") between -5° and 5°. In this example, the bolded portion of the SUM, DIFF, CONT pattern is obtained using approximately 18,000 responses.

[0314] Factory acceptance point tracking is traditionally done with a relative azimuth spacing equal to approximately 0.05°.

[0315] In order to have an accuracy of this order, the analysis time must be large enough (for example, of the order of a day) to have enough samples in each cell (azimuth, elevation, relative bearing). In fact, it is the target considered according to its relative position to the radar in the site that samples the antenna pattern, so only the duration of the analysis makes it possible to collect enough measurements.

[0316] It should be noted that the numbers given here are only an indication of order of magnitude and that in practice they can be parameterized by the operator (especially according to his or her needs) in terms of accuracy, in terms of refresh rate and in terms of the rate of fruit that can be obtained from aircraft approaching the radar.

[0317] Figure 8 An enlargement of the useful part of the curve of the angle error measurement voltage as a function of misalignment in the main lobe is presented. This curve is inverted and stored in an angle error measurement table used by the radar to estimate its misalignment relative to the antenna axis based on the measurement of the DIFF / SUM ratio of synchronous or asynchronous replies received in the main lobe.

[0318] The vertical dashed lines 81 , 82 delimit the useful region of the curve which is conventionally exploited by radars associated with LVA antennas having an angle of 2.4° at -3 dB.

[0319] Fig. 9 The plot of the azimuth error as a function of the misalignment of the target in the main lobe for a large number of replies (more than 8000) is presented. It will be noted that it starts to show a slight slope compared to the azimuth accuracy expected for the radar in terms of a standard deviation of 0.08°, with errors approaching + / -0.1° at the lobe edge. For example, as defined in this radar, the angular error measurement table is limited to the range between the dashed lines 91, 92, and consistently, errors are normal (for example) in the outer regions where the radar is not utilized, crossing to the far end.

[0320] The present invention establishes templates of different SUM, DIFF, CONT and angle error measurement error antenna patterns based on the factory traces (at the time of factory acceptance) of the antennas equipped at the radar sites.

[0321] After calculating the antenna and angular error measurement error patterns, typically on a daily basis, the method according to the invention compares the measurements performed on the template and produces a summary of the points outside the template, weighting these points by taking into account the quantity and quality of the results used for the measurement pattern and the angular error measurements.

[0322] This HUMS summary (usually daily) makes it possible to assess the potential current degradation of the antenna (which includes the cables and swivel joints).

[0323] Furthermore, by establishing characteristic deformations of the antenna pattern and deformations of the angle error measurement curve specifically according to different types of degradation of the antenna element (aerial), the present invention proposes degradation elements:

[0324] - Uniform level of loss of SUM, DIFF and / or CONT patterns regardless of azimuth, relative bearing and elevation;

[0325] RF cable losses increase by xdB;

[0326] - Loss of azimuth accuracy regardless of azimuth, relative bearing and elevation;

[0327] RF cable, SUM or DIFF losses increase by xdB;

[0328] - loss of azimuth accuracy at certain azimuths, regardless of relative azimuth and elevation;

[0329] Degradation of the rotary joint in amplitude of XX or in phase of YY at one or more given azimuth angles;

[0330] - deformation of the pattern at certain relative orientations, regardless of azimuth and elevation;

[0331] • Degradation of the radiating column or at the level of the divider of the antenna.

[0332] Other indicia of downgrade may also be obtained.

[0333] This continuous analysis of the three antenna patterns and the angle error measurements is performed with respect to several templates. This makes it possible to quantify in a functional manner the degradation of the performance level, which can ultimately declare a radar or ADS-B receiver using the same antenna element as degraded or faulty.

[0334] In addition to the common templates for guaranteeing performance, dedicated templates may be provided in order to detect normal degradations of the antenna element, where deformations of the antenna pattern after these degradations would be relevant.

[0335] Finally, tracking degradation trends on a day-by-day basis makes it possible to schedule a limit date for intervention at a radar site before degradation affects the radar's performance level in such a way as to render it non-operational.

[0336] Furthermore, a further advantage of the invention is as follows: the error of the angle error measurement function is established for example daily, making it possible to correct the angle error measurement table (which the radar uses to locate the synchronization target in the beam in azimuth) while the angle error measurement function remains within a tolerance template still acceptable to the operator, and thus still ensure good azimuth accuracy of the radar when degradation of the antenna unit or aging of the antenna unit sets in. The antenna unit is an assembly consisting of at least an antenna 1, a downconductor cable and a swivel joint.

[0337] Fig.10 The plot of the new azimuth error curve after calculating a new table of angle error measurements based on the misalignment of the target in the main lobe according to the present invention over the same large number of replies (more than 8000) is presented. Note that the curve is almost flat, showing peak-to-peak errors that are significantly less than + / -0.05° throughout.

[0338] Fig.11 By using the old angular error measurement table or the new angular error measurement table according to the invention on the same large number of replies (more than 8000), the amplification of the useful part of the lobe is presented by comparing the points of the azimuth error curve as a function of the misalignment of the target.

[0339] Note the gain in the radar's azimuth accuracy that follows from the misalignment of the synchronous reply: with the new angle error measurement curve, the azimuth error is very low and almost constant regardless of the position of the target in the lobe.

Claims

1. A method for measuring antenna patterns and angle error measurements according to elevation segments of a secondary radar, each antenna pattern (11, 12, 14, 15) being associated with a receiving channel, characterized in that In a given time period: - detecting unsolicited asynchronous replies of the type of long ADS-B bursts, each of which contains 3D position information about the target that sent it, sent by targets present in the air environment of said radar; - for each detection, the long ADS-B intermittent transmission signal is decoded to check that the detected target is located according to the position information contained in the intermittent transmission signal, and non-conforming detections are rejected; - for each detection that is retained, the time of the detection, the value of the azimuth angle of the axis of the main beam of the antenna, the angle error measurement voltage and the value (61, 611, 612) of the received power level on each of the SUM, DIFF, CONT_Front, CONT_Back reception channels are associated with the detection, the position information contained in the intermittent transmission signal giving the elevation segment in which the detection is located by calculation; The values ​​obtained during the time period are stored, and the measured patterns (42", 43", 44") and the angle error measurement results are sampled from the stored values ​​according to the elevation segment, wherein the values ​​obtained during the time period include the detection time, the azimuth angle value of the axis of the main beam of the antenna, the angle error measurement voltage and the reception power level value on each of the SUM, DIFF, CONT_Front, CONT_Back reception channels.

2. The method according to claim 1, characterized in that The antenna includes one of the following pattern sets: - a sum pattern SUM, a difference pattern DIFF, a control pattern CONT_Front for rejecting replies from targets facing the antenna and a control pattern CONT_Back for rejecting targets behind the antenna; -Sum pattern SUM, difference pattern DIFF, control pattern CONT; -Sum pattern SUM and difference and control pattern DIFF / CONT.

3. The method according to any one of claims 1 and 2, characterized in that The reply having a long ADS-B intermittent transmission signal type is characterized by a characteristic representing acquisition of the reply, the characteristic being, for each reply, at least one of the following characteristics: - received power according to each pattern of the antenna; - said angle error measurement of said response in the main lobe of said antenna; - said time of said detection of said response; - said azimuth of said axis of said main beam of said antenna at said detecting.

4. The method according to claim 3, characterized in that For each intermittent transmission signal received in the main beam, the method performs the following operations: - utilizing information contained in said intermittently transmitted signal; - Calculate the relative power and then calculate the error of the gain of each antenna pattern of the antenna according to the relative orientation: In the same beam, the radar measures the power of the synchronization trace, which is the maximum of the sum pattern; The long ADS-B intermittent transmission signal reply is characterized using measurements of power related to sum pattern, difference pattern and control pattern, and the method derives the following for the relative position of the intermittent transmission signal: o SUM_relative = SUM - SUM_max; o DIFF_Relative = DIFF - SUM_max; o CONT_relative = CONT - SUM_max; where SUM_relative, DIFF_relative and CONT_relative are in dB, and SUM, DIFF, CONT and SUM_max are in dBm; The gain error of each antenna pattern is established by comparing the values ​​of SUM_Relative, DIFF_Relative and CONT_Relative to a reference consisting of the pattern measured when the radar is received in the factory or when the radar site is accepted: o SUM_error = SUM_relative - SUM_reference; o DIFF_error = DIFF_relative - DIFF_reference; o CONT_error = CONT_relative - CONT_reference; where SUM_error, DIFF_error, CONT_error, SUM_reference, DIFF_reference and CONT_reference are in dB; - Calculating the angular error measurement error by taking into account the deformation of the main beam in elevation relative to the antenna.

5. The method according to claim 4, characterized in that Over the duration of the analysis, the gain error and the angle error measurement errors are accumulated over time according to the relative orientation according to: - the azimuth of the antenna; - The elevation angle of the target.

6. The method according to claim 5, characterized in that During the given time period, the angle error measurement error and the gain error of each antenna pattern are accumulated in a table having three inputs, which are associated with the angle error measurement error and the gain error of each antenna pattern, one input is the relative orientation of the axis of the main lobe of the antenna, the second input is the elevation angle of the target based on the horizontal alignment of the antenna, and the third input is the antenna azimuth angle.

7. The method according to claim 6, characterized in that For each cell of the table, each cell comprising a relative azimuth and elevation angle: the angular error measurement error and the gain error for each antenna pattern are established by averaging, histogram or other type of method.

8. The method according to claim 1, characterized in that The given time period is defined to reveal the current degradation of the antenna pattern.

9. The method according to claim 1, characterized in that: Under operator control or automatically, the radar locates in azimuth a synchronization target in the main beam using a table of angle error measurements, the table being corrected based on measured angle error measurement errors to ensure good azimuth accuracy of the radar in the event of degradation of a collection of components called an antenna unit.

10. The method according to claim 1, characterized in that The antenna equipped with the ATC or IFF type secondary radar includes at least two patterns.

11. The method according to claim 1, characterized in that: The antenna (1) has fixed or rotating electronic scanning.

12. The method according to claim 1, characterized in that The measurements of the angular error measurement error and the antenna pattern are used to assess the degradation level for each of the following elements of the antenna unit: antenna, downconductor cable, swivel joint.

13. The method according to claim 1, characterized in that The measurements of the antenna pattern are used to assess the degradation level of a collection of components called an antenna element.

14. A secondary radar, characterized in that: It enables the method according to any one of claims 1 to 13 to be implemented.

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