Device and method for testing an array antenna during operation.

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METHOD AND DEVICE FOR TESTING A MULTISOURCE ANTENNA IN OPERATION

The present invention relates to a method and a device for on-line testing, that is to say in operation of an antenna composed of a plurality of radiating sources.

A particular field of application of the invention relates to secondary radar antennas of the monopulse type, making it possible to radiate in space the pulses produced by an IFF interrogator (Identification friend or foe, that is to say identification friend. enemy) and capture any responses from the questioned planes. Such antennas consist of a plurality of radiating elements, called columns. On site, it is very difficult to identify whether one or more of these columns are faulty, and, in the event of a drop in antenna performance, we do not know how to precisely locate the column (s) that are there. the cause.

A known practical method of locating broken down columns consists in placing a detector successively in front of each column to check whether their radiation is indeed that expected. The drawback of such a method lies in the interaction of the radiation diagrams of the columns neighboring the tested column, which makes the measurements carried out imprecise. In addition, this method requires the temporary shutdown of the antenna in its functions of interrogation and reception of responses from aircraft.

There are also different methods that work directly on the antenna radiation pattern and are based on the following principles:

- the average level of the secondary lobes can make it possible to detect a failure. In fact, this method is discarded because it does not make it possible to extract reliable information concerning the end columns for which the variations in the mean level on the secondary lobes are too low;

- the characteristic function of the alignment of the dipoles is canceled as many times as there are radiating elements in the alignment. Observation of the number of zeros should therefore allow fault detection. However, this is an unreliable method in that in practice reflections on obstacles can cause zeros to disappear;

- the radiation diagram of an antenna is deduced from the Fourier transform of the illumination of this antenna. By calculating the inverse Fourier transform, we should therefore be able to reconstruct the appearance of the illumination of the antenna, and thus detect holes at the location of the broken down columns. In fact, this method cannot be used because it is not possible to recover the essential information of the signal, namely its phase.

The object of the invention is to alleviate the above drawbacks by proposing a method and a device for on-site testing of the operation of a multi-source antenna which make it possible to precisely locate the radiating source (s) which have failed, without having to the primary function of this antenna is hampered.

Although the description of the method and of the device according to the invention is given in the context of the application to a secondary radar antenna, it is already important to note that the invention is not limited to this application. particular, as we will see later.

The subject of the invention is more precisely a method for on-site testing of an antenna comprising a plurality of radiating sources, characterized in that it consists in:

- constitute a library of antenna radiation patterns on the one hand, when all the radiating sources are operating normally and on the other hand, when at least one of said sources has failed;

- note the radiation pattern of the antenna during its operation on site;

- calculating the correlation coefficient of said radiation pattern with each of the radiation patterns contained in said library;

- calculate the maximum value of the correlation coefficient, so as to detect a possible failure of one or more of said radiating sources.

The invention also relates to a device for on-site testing of an antenna comprising a plurality of radiating sources, characterized in that it comprises:

- first means for measuring and storing in a library of the antenna radiation patterns on the one hand, when all the radiating sources are operating, and on the other hand, when at least one of said sources has failed ;

- second means for measuring the radiation pattern of the antenna during its operation on site;

means for calculating the correlation coefficient of the radiation pattern of the antenna and of each pattern contained in said library;

means for calculating the maximum value of the correlation coefficient in order to detect a possible failure.

The test method and device will be better understood from the following description relating to the following figures:

- Figure 1 is a representation of the front face of a secondary radar antenna;

FIG. 2 is a representation of the rear face of this same antenna;

- Figure 3 shows the three radiation patterns of the previous antenna;

FIG. 4 is a block structure representative of the steps to be carried out for the test according to the invention.

FIGS. 1 and 2 respectively represent the front face and the rear face of a monopulse type secondary radar antenna. This antenna comprises a beam 1 on which are fixed radiating columns (2, 3), for example thirty six in number, the thirty five columns 2 being placed on the front of the antenna, and column 3 being placed at the back. The radiation can be obtained for example by a plurality of radiating dipoles placed on each column. The antenna is intended, on the one hand, to radiate in space the signals produced by an interrogator (not shown) and, on the other hand, to pick up the responses emanating from the transponders mounted on the airplanes. For this purpose, it has three radiation diagrams shown schematically in FIG. 3, obtained by different distributions, in amplitude and in phase, of the energy between the thirty six radiating columns. The antenna first radiates a diagram (Σ) 3 having a directional main lobe 4 surrounded by secondary lobes 5, thanks to which it emits a pair of interrogation pulses. To prevent a transponder on board a nearby aircraft from responding to the pulses emitted by the sidelobes 5, a third impulse interspersed with the previous ones is emitted according to the quasi-omnidirectional ray diagram (Ω) 6, which completely covers the sidelobes 5 . Thus, only the aircraft located in the coverage area of ​​the main lobe 4 are required to respond to the interrogations of the antenna. The last diagram 7, or diagram (Δ), makes it possible to refine the position of the airplane which emits a response.

FIG. 4 explains the different steps of the method according to the invention.

A first step referenced 11 consists in storing in a library the three diagrams Σ ^, Ω ^ and measured for example on the site of the antenna (i representing the column index) in the particular configuration where the column i is in breakdown. The index 0 corresponds to the case where no column has failed.

The need to use the triplet (Σ, Δ, Ω) of the radiation diagrams is justified by the fact that a single diagram is not sufficient for the precise location of a faulty column. Indeed, the energy applied to the input of the channel (Σ) of the antenna is distributed between the thirty five front columns with the same phase and decreasing amplitudes from the center of the antenna towards the ends. Thus, the diagram (Σ) only makes it possible to detect the failures of the columns close to the center of the antenna, with moreover an ambiguity on the relative position of these columns with respect to this center. For the radiation diagram (Δ), the seventeen columns located to the left of the central column are supplied in phase opposition with respect to the seventeen columns located to the right. This makes it possible to dispel the preceding ambiguity. In addition, the amplitude distribution of the Δ diagram makes it possible to deal more specifically with the columns placed on either side of the center of each half of the antenna. Finally, the diagram (Ω), for which only the rear central column is supplied in phase opposition with respect to the thirty-five other columns, makes it possible to detect the failure of this rear column.

The second step of the method according to the invention, referenced 22, consists in measuring on site at predetermined times the three radiation patterns Σ, Ω, Δ of the antenna during its operation in IFF interrogation.

The third step denoted 44 is the calculation of the correlation coefficient for each pair of signals (Σ ^, Σ), (Δ ^, Δ), (Ωρ Ω), knowing that this correlation coefficient for a pair of signals (x ^ (t), (x (t)) is given by the relation:

x ^ Ct) x (t) p „ ⁼ .....;..............................—............

^Xi / - - I yx | (t). x²(t)

This coefficient p makes it possible to evaluate the resemblance of the i two signals x (t) and x ^ (t), knowing that the maximum resemblance is reached theoretically for p - 1, and the i total decorrelation for p = 0.

i

Steps 44 and 55 thus make it possible to detect the faulty column i. More precisely, step 44 makes it possible to calculate the maximum values ​​of the coefficients and p ^; these iii values ​​are not necessarily 1 in practice because the resemblance is never perfect. Column i identified as probably broken by step 55 corresponds to the highest correlation coefficient. In the event that several columns are down, the operation is repeated from step 33 in order to examine all of the columns. Instead of proceeding by iteration, one could just as easily design a library larger than the previous one, containing fault configurations on more than one column.

The invention also proposes a device for implementing the method as described above.

Steps 11 and 22 of the method are readings of radiation patterns on site. A beacon on the ground placed in the optical range of the radar (typically between 1 km and 50 km) is used to make it, this beacon functioning as an aircraft transponder, except that it responds to all the interrogations emitted by the aircraft. 'antenna. The three diagrams (Σ), (Δ) and (Ω) are recorded by measuring the peak amplitudes of the three pulses constituting the response of the beacon.

This survey is carried out over several antenna towers, which makes it possible to obtain very precise radiation patterns. The idea is based on the fact that the position of the antenna is generally known thanks to an optical encoder for example, comprising n bits, 360 ° and whose resolution is -. We can get

2^not very precise radiation diagrams by recording the peak amplitudes of the beacon responses for an antenna position varying from 0 ° to 360 ° by a step which is as close as possible to the resolution of the encoder. Obtaining all the points requires taking the measurements over several antenna towers. However, this is not a constraint because the antenna continues to perform its function of IFF interrogator.

This device thus makes it possible to measure very precisely on the one hand, the triplets (Σ ^, Δ ^, Ω ^) that must therefore be stored in a library, and on the other hand, the triplet (Σ,

Δ, Ω) in order to detect one or more possible faults. The rest of the test device can be produced by any means making it possible to calculate the three correlation coefficients, p & and p &, to find their maximum, and thus to determine the column that is probably faulty.

As we have already noted previously, the invention is not in any way limited to antennas of secondary radars of the monopulse type and can extend to all cases of antennas having at least two radiating sources.

In addition, the constitution of the library can also be done by simple mathematical calculation.