FR2818812A1 - METHOD FOR MEASURING AND COMPENSATING FOR THE DEFORMATION OF THE RADIANT SURFACE OF AN ANTENNA AND ANTENNA COMPRISING MEANS EMPLOYING SUCH A METHOD - Google Patents

Abstract

The present invention relates to a method for determining the error of the radiation pattern of an electronically scanned antenna, the error being due to the mechanical deformation of the radiating surface of the antenna, and the determination, with a view to the correction of this error, being carried out from measurements of the mechanical deformation of the radiating surface. The method compensates for the deformation measured by calculating and applying a phase difference correction to the phase shifters. The present invention also relates to an antenna comprising means implementing such a method. The mechanical deformation is estimated by optical measurements. of measurement points is less than the number of radiating elements of the antenna The error correction is carried out during operation of the antenna The invention applies in particular to cost and mass electronic scanning antennas reduced.

Description

The present invention relates to the field of scanning array antennas

  electronic; and more particularly a method of determining the error of the radiation diagram due to the deformation of the radiating face of a network antenna and of compensating for this error. We will first briefly describe the operation of such an antenna. It consists of a plurality of radiating elements, of the dipole type for example, generally arranged at the nodes of a regular plane mesh (rectangular, triangular, or more generally bi-periodic). Each of these radiating elements is associated with an electronically controlled phase shifting device. To orient the radio beam in a given pointing direction, the signals transmitted or received by the different radiating elements are out of phase with each other so that the contributions to the radiation of the antenna of the various radiating elements are added in phase in the direction chosen. The command setpoint value for each phase shifter is produced by a computer called a pointer. Electronic antenna pointing control allows multiple radar functions to be implemented on the same antenna, by

  example watch, pursuit and fire control.

  The array antenna is in the form of a slab having a radiating surface and a thickness corresponding to the stack of a radiating element, a phase shifter, a distribution

  microwave and in some cases a microwave generator.

  The thickness is for example a few tens of centimeters.

  In a network antenna, the quality of the radiation pattern and the pointing accuracy of the radio beam

  largely depend on the control of the electric wave plane.

  To meet the requirements of pointing accuracy and quality of the diagram, the method currently used consists in defining a mechanical structure, supporting the radiating elements, whose flatness state makes it possible to obtain the quality of the electric wave plane. For this, we define very tight manufacturing tolerances for the mechanical structure and the radiating elements and we design a mechanical structure whose rigidity is very high. This rigidity makes it possible to prevent the mechanical deformations induced by the operating and environmental conditions, in particular the rotational movement of the antenna and the

  wind, do not degrade the flatness of the emission surface.

  The main drawbacks of the above method for ensuring the quality of the transmission surface of the array antenna are that it leads to equipment whose mass and cost are high. A network antenna is generally 5 to 10 times heavier than a passive radar antenna. Satisfying the rigidity requirements leads to the design of heavy structures, which penalize the mobility and installation of the equipment. Moving such equipment may be impossible by air transport and requires a high capacity road vehicle. The installation means are important, for example with a crane, and the reception structure is sophisticated. Installation on a ship may in particular be impossible. Reduced mobility, the importance of time and means of installation of a mobile system comprising an antenna with current network limit

notably the tactics.

  An object of the invention is in particular to allow the production of an antenna with an electronic scanning array, the mobility, implantation and tactics of which are improved. To this end, the invention relates to a method for determining the error of the radiation diagram of an electronic scanning antenna comprising an array of N radiating elements, the error being due to the mechanical deformation of the radiating surface. of the antenna, such as the determination is carried out from measurements of the mechanical deformation of n points of said surface in view

correcting this error.

  The invention particularly has the advantage that it allows an embodiment of electronic scanning antenna whose structure is lightened and simplified without affecting the quality of its radiation pattern. It makes it possible to produce an antenna at low cost and whose

moving is facilitated.

  Preferably, the method uses optical measurements of the

deformations of the antenna plane.

  Preferably, the method according to the invention is implemented in

  using light sources and optical sensors fixed on the antenna itself

  even. It does not use any device or system for measuring error outside the antenna and allows the total autonomy of the antenna to be preserved,

especially for a mobile antenna.

  The invention makes it possible to compensate for mechanical deformations of the mechanical plane of the antenna induced by the operating and environmental conditions. The invention makes it possible to maintain the same quality of radiation pattern with an antenna whose deformations are ten to one hundred times greater than the tolerated deformations without implementing the invention. The invention makes it possible to reduce

  notably the rigidity constraints.

  The invention makes it possible to compensate for a permanent defect in the radiating surface of the antenna, it has the advantage of increasing the

  mechanical manufacturing tolerances of the antenna surface.

  The invention also relates to a scanning antenna.

  electronics comprising means implementing the method.

  Other characteristics and advantages of the invention will appear

  with the aid of the description which follows made with reference to the appended drawings which

  represent: - Figure 1, an exemplary embodiment of an antenna for implementing the method; - Figure 2, an embodiment of an optical fiber strip according to the invention; - Figure 3, an alternative embodiment of displacement measurement according to the invention; - Figure 4, a sensor for the alternative embodiment of

measured.

  FIG. 1 schematically illustrates an exemplary embodiment of an array electronic scanning antenna with a device for measuring the deformation of the radiating surface. In the embodiment of FIG. 1, the deformation measurement is an optical type measurement. A light source includes for example a laser or a diode

  electroluminescent placed behind the radiating surface 2 of the antenna.

  In this example, the radiating surface of the antenna is substantially planar, it materializes, apart from defects, a reference plane for the radar transmission. A volume hologram 1 is placed at a point on the radiating surface 2 considered as a reference point on the surface

radiating 2 from the antenna.

  The light source associated with a hologram allows the emission, from the reference point of the surface 2, of a light beam 3 in a plane substantially parallel to the reference plane of the radiating surface. Bars of optical fibers 4, 5 are fixed on the radiating surface. The antenna has n fiber optic bars. The receiving plane of a strip 4 is substantially perpendicular to the radiating surface on the one hand and substantially perpendicular to the beam

  luminous 3 which is received by the bar on the other hand.

  The hologram is constructed for example by ultraviolet or short wavelength irradiation. The hologram allows the light from the light source to be focused on each of the n bars, for example by forming n beams. The light beam 3 is anamorphic. The

  beam emitted by the source becomes elliptical when passing through the hologram.

  The volume hologram has the advantages of a small footprint

and an easy realization.

  FIG. 2 illustrates the reception plan of an example of an optical fiber array according to the invention. Each fiber strip is a bundle of optical fibers having a flat and rigid end. The rest of the bundle is preferably flexible. The reception plan comprises sections 6 of optical fibers of small diameter, for example around 250 micrometers. Each silica fiber core 7 preferably comprises a

  8 polymer coating transparent to electronic radar radiation.

  Optical fiber has the advantage of being immune to radar radiation.

  The bundle of optical fibers 6 extends behind the plane of FIG. 2. In the example illustrated by FIG. 2, the optical fibers are distributed in three superposed layers in the reception plane. A lower layer 9 has sections of juxtaposed fibers whose centers of hearts are aligned along a line segment 12. The intermediate layer 10 has juxtaposed sections whose centers of hearts are aligned along a second straight line segment 13 at first. An upper layer 11 is parallel to the two layers already described 9, 10. The centers 14, 15, 16 of the first fiber of each of the three layers are offset in the direction common to the first and second straight segments 12, 13 corresponding to that an axis 17 of the abscissa of the fiber centers. The intercentre, or fiber diameter, is the distance between the centers of two fibers juxtaposed with the same layer. The spacing between the centers 14, 15 of two fibers juxtaposed with two offset layers is less than the intercentre. In the example of FIG. 2, the abscissa of the center 15, 16 of the first fiber of a layer 10, 11 is one third of the intercenter greater than the abscissa of the center of the first fiber 14, 15 of the diaper immediately

lower 9, 10.

  Each optical fiber in a strip is connected to a CCD receiver.

  CCD receivers are installed directly on the associated fiber bar or are carried inside the antenna. The embodiment with transfer inside the antenna, behind the radiating elements, by an adapted length of the optical fiber bundle of the strip has the advantage of avoiding disturbance of the microwave field emitted by

  the antenna by CCD semiconductors.

  A CCD group grouping the CCD receivers associated with a fiber strip makes it possible to identify the optical fiber or fibers illuminated by the light beam 3 on the receiving plane of the fiber strip. Each CCD group provides a measurement of the displacement of the light beam on the associated strip of fibers, the displacement is measured along the abscissa axis. The resolution of the measurement depends on the discrete spacing of the fibers along this axis, the diameter of the core of the fibers and the width of the illumination beam along this axis. The measurement of a strip comprising a single layer of juxtaposed fibers has better resolution when the coating of the fibers is removed. In the example in FIG. 2, the different layers of superimposed and offset fibers allow good resolution to be obtained while retaining the coating of each.

  fibers. The bar according to Figure 2 is robust and inexpensive.

  Each CCD group measures a displacement in one direction

  substantially perpendicular to the emission plane of the light beam 3.

  Each CCD group provides a measurement of the displacement of the fixing point

  to the radiating surface of the optical fiber strip associated therewith.

  Each movement is measured relative to the reference point of the radiating surface. The device described above makes it possible to measure the relative displacement of n points of the radiating surface of the antenna with respect to the reference point and in the direction substantially

  perpendicular to the reference plane of the radiating surface.

  All of the n CCD groups are connected by an interface to a calculation card. From the number n of points on the radiating surface whose displacement is measured, the calculation card makes an estimate of the actual displacement of each of the N numerous radiating elements of the radiating surface of the antenna. For example, for an antenna having a few hundred to a few thousand radiating elements, the number n of displacement measurement points is advantageously between

one and a few dozen.

  In an example of a planar antenna mounted outside, the calculation card develops an approximate form of the real deformation of the radiating surface by a linear combination of six elementary forms of deformation corresponding in this example to the first three eigen modes of deformation of the planar antenna and the three extreme forms of wind deformation. For a combination of six elementary shapes, the number of points on the surface whose displacement is measured is between 10 and 20 and preferably between 12 and 18, and these points are placed in zones characteristic of each shape. Certain points can be associated with several elementary forms when the identification method makes it possible to discriminate their participation in each of them. In the method of the invention, the position of each measurement point and their number is calculated beforehand. Each pair of a measurement point and an elementary shape is associated with a coefficient which characterizes the part of the displacement of the measurement point relative to the maximum displacement seen by the surface in this elementary shape. The measurement points used allow the identification of the contribution of each elementary form when the defined coefficients are comparable to the coefficients

  of an independent system of linear equations.

  The position of each attachment point of the fiber strips makes it possible to separate the contribution of each elementary shape to the shape

  approximate to the real deformation.

  The elementary forms of deformation depend on the geometry of the radiating surface, for example square, rectangular, round, and on the

  fixing of the antenna plate on its support.

  The calculation card then elaborates and for each radiating element of the antenna, an estimate of the distance between its theoretical position in the absence of deformation of the surface and its estimated real position. This estimated distance represents a measure of the error of the radiation pattern of the deformed antenna. From the estimated distance of a radiating element, the calculation card generates a phase difference correction value to be applied to the phase shifter device associated with the radiating element. A digital link transmits the phase difference correction information to the radar pointing computer which controls the phases of the radiating elements. In the invention, the number of deformation forms to be used for the estimation of the real deformation of the antenna depends on the precision of the phase shifters of the antenna. It is at least three. For inaccurate phase shifters, the number of deformation forms can be limited to 3. For high quality phase shifters with high precision, a greater number of deformation forms allows a better estimate. All of the correction calculations and their transmission to the pointing calculator is carried out in less than ten milliseconds, it allows a fairly rapid correction taking into account the mechanical strain rates whose natural frequencies are in the range of one tenth to ten from Hertz. The optical measurements of displacement of the n points of the radiating surface are renewed at a high frequency in a range of one hundred to a thousand Hertz allowing correction of the pointing of the radar at a rate faster than that of the annoying degradation of the diagram of

  radiation due to mechanical deformations of the radiating surface.

  The correction can be made outside of a burst of pulses.

  The application of the correction calculated according to the invention allows the emission of a radar wave whose radiation diagram is corrected for disturbances due to the deformation of the radiating surface of the antenna measured during the operation of the antenna. The invention makes it possible in particular to reduce the increase in the width of the main lobe and the rise in the level of the secondary lobes of the diagram which are observed when a radar antenna deforms. The invention allows

  improve the performance of a deforming antenna.

  Radar emission is corrected in real time. The invention allows the emission of a plane electromagnetic wave from a mechanical plane

  distorted from the antenna surface.

  For an antenna with a surface area between a few square meters and a few tens of square meters, the number n of strain measurement sensors is from one to a few tens. For example, with a deformation measurement accuracy of one tenth of a millimeter, the invention makes it possible to correct deformation amplitudes up to ten millimeters relative to the reference plane of the antenna. The invention is particularly suitable for a multifunction radar which requires a large

  precision, it eases the rigidity constraints of its antenna.

  The invention also makes it possible to improve the performance of an antenna having a permanent geometry defect, for example of manufacturing or accidental. The invention makes it possible in particular to soften the flatness tolerances in manufacturing. The invention makes it possible to correct amplitudes of deformation increased by a factor of ten to one hundred compared to the amplitudes of deformations tolerated to guarantee control of the electric wave plane.

in the prior art.

  In an alternative embodiment of the invention illustrated by FIGS. 3 and 4, the displacement measurements of the n points of the antenna are

estimated as follows.

  The antenna shown in FIG. 3 comprises n light sources 19, 20 placed at n points on the radiating surface and sequentially illuminating a sensor 21 fixed at a reference point on the radiating surface, which is represented by its reference plane 22. A light source preferably comprises a laser behind the plane of the radiating surface and a mirror at the point of the surface whose displacement is to be measured. The non-metallic mirror is produced for example by stacking optical dielectric layers, it returns the light wave from the laser to the sensor 21. The light source has the advantage of not disturbing the electromagnetic radiation from the antenna. FIG. 4 illustrates the sensor 21, it comprises a matrix 24 of micro lenses whose plane, represented in FIG. 4 by the straight line 25, is substantially perpendicular to the plane of the radiating surface. The sensor comprises a CCD strip 23 perpendicular to the surface of the antenna and it is connected to calculation means. The sensor 21 is preferably placed on the edge of the antenna in order to avoid disturbances of the electromagnetic field emitted by the latter. The radius of the light wave 26 emitted by the source placed at a first point 27 of the surface of the antenna results in a spacing between two successive focusing spots of the micro lenses. The measurement of the spacing on the CCD strip 23 makes it possible to estimate the distance d between the

  source at the first point 27 and the matrix 24 with very high precision.

  When the antenna is deformed, the source moves in particular in the direction perpendicular to the initial plane of the antenna, that is to say parallel to the CCD strip and it is represented in FIG. 4 by the second point 28, the focusing spots of the micro lenses move and the measurement of the general displacement D allows the calculation means to evaluate the distance x of the displacement of the source between the first and second points 27 and 28. The sequential illumination of the sensor by the n sources allows the successive evaluation of the relative displacement of each of the n points of the surface of the antenna where the light sources are placed, the displacement being evaluated relative to the reference point for fixing the sensor 21 and in the direction substantially perpendicular to the reference plane 22 of the radiating surface. The n light sources illuminate

  preferably directionally to the sensor 21.

  This variant has the advantage of using a sensor

  and a commercially available electronic processing assembly.

  The variant can be modified by adding an additional light source, for example in the center of the radiating surface, and by making the difference between the displacement of the additional source and the displacements of the other sources. This modification makes it possible to provide displacement measurements with respect to a reference placed at a point

  any surface of the antenna, for example the center.

Claims (20)

  1. Method for determining the error of the radiation pattern of an electronic scanning antenna comprising an array of N radiating elements, characterized in that the error being due to the mechanical deformation of the radiating surface of the antenna, determination is made from measurements of mechanical deformation   of n points of said surface with a view to correcting this error.   2. Method according to claim 1, characterized in that the   mechanical strain measurements are optical measurements.   3. Method according to claim 2, characterized in that the measurements of the mechanical deformation are carried out by means of a   light source and n optical sensors.   4. Method according to claim 2, characterized in that the measurements of the mechanical deformation are carried out by means of n   light sources and a single optical sensor.   5. Method according to claim 4, characterized in that the n   light sources sequentially illuminate said sensor.   6. Method according to one of the preceding claims,   characterized in that the radiating surface of the antenna having elementary forms of deformation, the position of the n measurement points is determined by calculation as a function of an integer of these forms elementary deformation.   7. Method according to one of the preceding claims,   characterized in that the radiating surface of the antenna having elementary forms of deformation, the whole number of forms   elementary is at least 3.   8. Method according to one of the preceding claims,   characterized in that the radiating surface of the antenna having elementary forms of deformation, the whole number of elementary forms is equal to 6 and in that the number n of measurement points is between 12 and 18.   9. Method according to one of the preceding claims,   characterized in that the radiating surface of the antenna is substantially planar.   10. Method according to one of the preceding claims,   characterized in that it compensates for said error by calculating and applying a phase difference correction on the phase shifters associated with the N radiating elements. 1The. Electronic scanning antenna comprising an array of N radiating elements, characterized in that it comprises means for measuring the deformation of its radiating surface, said means estimating the   deformation from n points of said surface.   12. Antenna with electronic scanning according to claim 11, characterized in that the radiating surface of the antenna having elementary forms of deformation, the position of the n measurement points is determined by calculation as a function of an integer of these forms elementary deformation.   13. Antenna with electronic scanning according to claim 12, characterized in that the radiating surface of the antenna having elementary forms of deformation, the whole number of forms   elementary is at least 3.   14. Antenna with electronic scanning according to claim 12, characterized in that the radiating surface of the antenna having elementary forms of deformation, the whole number of elementary forms is equal to 6 and in that the number n of measurement points East between 12 and 18.   15. Electronic scanning antenna according to one of the   claims 11 to 14, characterized in that the radiating surface of the antenna is substantially flat.   16. Electronic scanning antenna according to one of the   Claims 11 to 15, characterized in that it includes means for   calculation of phase differences, said phase differences being applied to the phase shifters of the radiating elements compensate for the error of the radiation diagram due to the mechanical deformation of the radiating surface 17. Antenna with electronic scanning according to one of the   claims 11 to 16, characterized in that said means comprise   optical strips illuminated by a laser source.   18. Antenna with electronic scanning according to claim 17, characterized in that said means comprise a hologram   volume located between the laser source and the light bars.   19. Electronic scanning antenna according to one of the   claims 17 and 18, characterized in that said light bars   have at least two layers of optical fibers superimposed and offset.   20. Electronic scanning antenna according to one of the   claims 17 and 18, characterized in that said light bars   have three layers of optical fibers superimposed and offset.   21. Electronic scanning antenna according to one of the   claims 11 to 16, characterized in that said means comprise   light sources sequentially illuminating an optical sensor located on the radiant surface.   22. An electronic scanning antenna according to claim 21, characterized in that said light sources comprise means for returning light to said sensor, said means comprising a   non-metallic mirror located on the radiating surface.