GB2531939A

GB2531939A - Method for obtaining a reference target with low monostatic RCS in a determined direction

Info

Publication number: GB2531939A
Application number: GB1518726.3A
Authority: GB
Inventors: Vacus Olivier; Saint-Flour Carol
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2014-10-22
Filing date: 2015-10-22
Publication date: 2016-05-04
Anticipated expiration: 2035-10-22
Also published as: GB201518726D0; FR3027684A1; FR3027684B1; GB2531939B

Abstract

The invention relates to a reference target, and a method of obtaining such a target, having a small monostatic radar cross section (RCS) at a frequency of interest and in a determined direction. The reference target, which may be a sphere (100), has rotational invariance of 2π/N, wherein N is an integer such that N≥3, relatively to a rotational symmetry axis (Oz). The target has over at least part of its surface a one-dimensional grating of grooves (110) for which the periodicity is substantially less than the wavelength. The depth of the grooves is selected in order to cancel out the backscatter coefficient of an incident wave along the axis (Oz). The grooves may be parallel to one another and form a predetermined angle, such as a right angle, with the rotational symmetry axis (fig. 1), or they may be arranged in a radial direction (fig. 3). The target may have symmetry of revolution around the rotational symmetry axis. The target may be made in a PEC (perfect electrical conductor), typically a metal material, and the grooves may be filled with a dielectric material.

Description

METHOD FOR OBTAINING A REFERENCE TARGET WITH LOW MONOSTATIC RCS IN A DETERMINED DIRECTION

DESCRIPTION

TECHNICAL FIELD

The present invention relates to the field of electromagnetic characterization of an object and more particularly of the measurement of the equivalent surface area (RCS for radar cross-section) of such an object.

Conventionally, a target is characterized by its radar equivalent surface area or RCS.

The RCS of a radar target is defined from the power balance of the wave emitted towards the target and of the power of the wave received by the radar. In a far field and by approximating the waves to plane waves, the radar equation radar is actually written as: 22 (1) = PG a 1 G e 42rd-42rd' 47z-wherein P, and Pr respectively are the powers of the waves emitted and received by the radar, G, and Gr the antenna gains upon emission and reception, d the distance between the radar and the target, A, the wavelength used by the radar. The coefficient a is homogenous to a surface and only depends on the relevant target, this is the RCS of the target.

In the expression (1) it is assumed that the radar used for illuminating the target was the same as the one used for receiving the diffracted wave, this is then referred to as monostatic RCS. As a general rule, the monostatic RCS depends on the direction of the incident wave, on the frequency f of the radar and of the respective polarizations Ire and with which the incident wave is emitted and the received wave is analyzed. It is noted as SER(f,yo,19,n-"zr,,), wherein 0) respectively are the roll and relative bearing angles of the radar in a reference system bound to the target. Each of the polarizations re and;re may either be horizontal or vertical, i.e. H or V; irr, = PI or V. It is also useful to characterize a target by its bistatic RCS. Unlike the monostatic RCS, the latter is obtained by illuminating the target with a given emission power along a first direction and by measuring the diffracted power along a second direction distinct from the first. The bistatic RCS therefore depends on the illumination angle of the target and on the angle under which the diffracted wave is analyzed but also, like the monostatic RCS, on the frequency of the radar, on the polarization of the emitted wave as well as on the polarization according to which the received wave is analyzed.

The measurement of an RCS is conducted in an anechoic room, i.e. in a room for which the walls are coated with absorbents, so as to avoid parasitic echoes. The efficiency of the absorbents is proportional to their size expressed in wavelength. Therefore it is conceivable that it is delicate, for congestion reasons, to produce an anechoic chamber having good absorption performances at low frequencies (LF).

Further, the target is positioned by means of a slightly echogenic positioner, generally of a polystyrene vertical column which may be oriented around its own axis. In the case of positioning by means of a vertical column, only the RCS in the equatorial plane may be acquired (in other words, for all the relative bearing angles B but for a constant roll angle 90).

In any event, in spite of the presence of absorbents, the RCS measurements are perturbed by parasitic reflections (further called couplings) whether these occur on the walls or on other elements present in the anechoic chamber, such as the positioner mentioned above.

The assembly formed by the illumination antenna, the target and its environment, and the receiving antenna may be considered as a microwave quadripole. The measurement of the RCS is then inferred from the parameter S1, of the equivalent quadripole by means of: a = 4 71. I g I (2) wherein 4a-is the solid angle corresponding to the totality of the sphere. The parameter S12 is further designated as a backscatter coefficient.

In order to reduce the influence of the couplings on the measurement of RCS, it is known how to conduct a first measurement of the backscatter coefficient in the absence of the target. This first measurement, noted as.812, expresses the contribution of the environment. Further, a second measurement of the backscatter coefficient is conducted by means of a reference target for which one knows how to calculate the theoretical RCS. The reference target may for example be a metal sphere. This second measurement is noted as Si, and the RCS of the reference target as 6th, as obtained by the calculation.

The RCS of the target may then be obtained by means of the expression: s,, Se -S°Sl°2 with o-th =47-0,12 wherein Sf2 is the theoretical backscatter coefficient of the reference target.

From the expression (3) it is understood that by subtracting both at the numerator and at the denominator the term 5102, an operation commonly designated as « empty chamber subtraction », it is possible to partly do without the couplings due to the environment upon measuring respective backscatter coefficients of the target and of the reference target.

However, by subtracting the empty chamber it is not possible to entirely suppress the influence of the couplings on the measurement of RCS. Indeed, certain couplings are due to multiple reflections between the target and the environment, and therefore are not naturally taken into account in the empty chamber measurement.

In order to allow analysis and emission of these couplings, it is desirable to be able to have a collection of reference targets (i.e. targets for which one knows to how to (3) calculate the monostatic and bistatic RCSes in an analytical way or by numerical simulation) having zero monostatic RCS in certain predetermined directions, for a given frequency. Indeed, by placing the reference target in the location and place of the object in the anechoic chamber, it is possible to estimate the coupling with the chamber by measuring the monostatic RCS, at the frequency of interest, in one of the predetermined directions. If the measured monostatic RCS is negligible, the conclusion may be drawn that there is no coupling (or parasitic signal) in the relevant direction. By default, the coupling level may be estimated in this direction in order to either validate or not the RCS measurements in this direction.

A first approach for producing reference targets with low monostatic RCS would be to use metal reference targets covered with absorbent coatings. However, on the one hand, these coatings generally considerably modify the bistatic RCS both in level and in angular dependency. On the other hand they raise delicate problems for controlling manufacturing. Now, if their electromagnetic properties are not sufficiently well known, the use of these targets as standards is excluded.

The object of the present invention is therefore to propose a method for obtaining reference targets having a zero or quasi-zero RCS at a given frequency and in certain determined directions.

DISCUSSION OF THE INVENTION

The present invention is defined, according to a first embodiment, by a reference target for testing a system for measuring the radar cross section at a given frequency f, the reference target having a rotation invariance of -2ff, wherein N is an integer such that N 3, around a rotational symmetry axis, the reference target further having on all or part of its surface grooves parallel with each other, arranged according to a one dimensional network along said rotational symmetry axis, with a periodicity d substantially less than the wavelength 2=c1 f, at least some of the grooves of said grating having a depth h= , (2k +1) wherein k is an integer and cg, respectively are the relative 41 Vitgeg permittivity and the relative permeability in the groove so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said rotational symmetry axis.

Generally, the depth axis of each groove forms a predetermined angle with said rotational symmetry axis.

The predetermined angle may be a right angle, each groove being delimited by two planes orthogonal to the rotational symmetry axis, spaced apart by the width of the groove.

According to a second embodiment, the invention relates to a reference target for testing a system for measuring the radar cross section at a given frequency f, the reference target having rotation invariance of 27.1-, wherein N is an integer such that N 3 around a rotational symmetry axis (Oz), the reference target further having on all or part of its surface grooves, the depth axis of each groove coinciding with the normal to the surface at the point where it opens onto this surface, the grooves being arranged at the surface with a transverse periodicity d, measured along an arc orthogonal to the grooves, said transverse periodicity being substantially less than the wavelength =cif, the grooves all having a depth h= C (2/c +1) wherein k is an integer and cg,pg 4f JAcgeg respectively are the relative permittivity and the relative permeability in the groove so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said rotational symmetry axis.

Advantageously, the reference target has symmetry of revolution around said rotational symmetry axis.

Advantageously, the reference target is made in a metal material.

According to an alternative, the grooves are filled with a dielectric material.

The invention further relates, according to a first embodiment, to a method for obtaining a reference target having, at a given frequency f, a small monostatic radar cross section along a given axis, wherein: - said reference target is selected as having a rotation invariance of 3!--"c, wherein N N is an integer such that N 3, around said axis; - the geometrical characteristics of a grating of parallel grooves are determined, the pitch of the grating along said axis being selected so as to be substantially less than the wavelength A=c1f, the depth of the grooves being calculated by means of h- (2k +1) wherein k is an integer and eg "rig respectively are the relative 4fjpgeg permittivity and the relative permeability in the groove; - grooving of the surface of the reference target is carried out according to the grating of parallel grooves, for which the geometrical characteristics have been thus determined so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said axis.

The invention finally relates to a method for obtaining a reference target having, at a given frequency f, a small monostatic radar cross section along a given axis, wherein: - said reference target is selected as having rotation invariance of -2x, wherein N is an integer such that 3, around said axis; - the geometrical characteristics of a grating of grooves of identical depth are determined, the depth axis of each groove coinciding with the normal to the surface in each point where the groove opens onto this surface, the pitch of the grating along this surface being selected so as to be substantially less than the wavelength A=c1f, the depth of the grooves being calculated by means of h= (2k+1) wherein k is an integer and 41 jpgeg g, pg respectively are the relative permittivity and the relative permeability in the groove; - grooving (630) of the surface of the reference target is carried out according to the grating of grooves for which the geometrical characteristics have thus been determined so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said axis.

Advantageously, the reference target is selected so as to have symmetry of revolution around said axis.

The reference target is preferably in a metal material.

According to an alternative, after grooving, the grooves are filled with a dielectric material.

SHORT DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent upon reading a preferential embodiment of the invention with reference to the appended figures wherein: Fig. 1 schematically illustrates a reference target example according to a first embodiment of the invention; Fig. 2 illustrates the monostatic RCS versus the frequency of a reference target according to Fig. 1; Fig. 3 schematically illustrates a reference target example according to a second embodiment of the invention; Fig. 4 illustrates the monostatic RCS versus frequency of a reference target according to Fig. 3; Fig. 5 schematically illustrates a first method for obtaining a reference target having a small monostatic RCS in a rotational symmetry axis; Fig. 6 schematically illustrates a second method for obtaining a reference target having a small monostatic RCS in a rotational symmetry axis.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

Subsequently, a reference target will be considered for which one knows, as an assumption, how to determine the monostatic and/or bistatic RCS. Such a reference target is traditionally a perfectly conducting metal object with a known shape, having symmetry of revolution around an axis, for example a metal sphere.

The idea at the basis of the invention is to simulate a reference target coated with an absorbent by grooving all or part of its surface. For the reasons explained later on, the reference target has rotation invariance of 27EIN"V around an axis, designated hereafter as a rotational symmetry axis, the grooving being achieved by means of periodically or quasi-periodically distributed grooves along this axis, at the surface of said reference target, the periodicity or quasi-periodicity of the grooves being selected so as to be substantially less than the wavelength of interest.

In a first embodiment, the grooves are parallel with each other. Subsequently it will be assumed, with no loss of generality, that the depth axis of each groove is perpendicular to the axis (Oz).

Fig. 1 schematically illustrates an exemplary embodiment of a reference target according to the first embodiment of the invention.

The illustrated example is a sphere, but it will be understood that this embodiment may be applied to any shape having a rotational symmetry axis, Oz, in the sense defined above. Subsequently a system of cylindrical coordinates (0,r,q3,z) is adopted. In other words, a point M of the sphere is described by its coordinate z along the axis (02) and its polar coordinates (,.,y0) in the plane orthogonal to this axis and containing the point M. The radial unit vector is noted as u" the tangent unit vector as u0, and the unit vector on the axis (0z) as uz.

The sphere, 100, is grooved with grooves of circular shape, 110, the grooves being parallel to the plane (0, ur, uW).

It is assumed that the grooves are periodically distributed along the axis (Oz), with a predetermined period d and that they have a rectangular shape of width a. In other words, each groove is delimited by two planes perpendicular to the axis (0z) and spaced apart by a.

Moreover, the depth of a groove is noted as h. According to a first alternative, not shown, the depth h may be selected to be constant, regardless of the position of the groove along the axis (0z). In other words, in this case all the grooves then have the same depth. According to another alternative, the depth h depends on the position of the groove along the axis (Oz). In the case illustrated in Fig. 1, the grooves are made in a surface film of the sphere with a thickness e. Thus, the depth of the grooves varies from a minimum value hm,, = e for z = 0 to a maximum value h",, when lz1= R wherein R is the radius of the sphere.

It is assumed that the sphere is made in a perfectly conducting material or PEC (perfect electrical conductor), typically a metal material.

As shown in the article of F.J. Garcia et al., entitled "Surfaces with holes in them: new plasmonic metamaterials" published in Journal of Optics A; Pure Appl. Opt, Vol. 7, pp. 597-5101 (2005), a planar surface of a PEC material, provided with a grating of linear grooves of rectangular section, of width a and of depth h, repeated with a periodicity d, may be considered as electromagnetically equivalent to a fictitious surface layer, 120, homogeneous but anisotropic of thickness h on a PEC material. More specifically, if it is assumed that the grooves are aligned along an axis (Oy) and are repeated along an axis (Ox) orthogonal to (Oy) in the plane of the surface and if the axis (Oz) is orthogonal to this surface so that the reference system (0, x, y, z) is direct, the permittivity tensor in the surface layer may be written, in this reference system, as: din 0 0" c= 0 0 (4) 0 0 -Do In other words, the permittivity value et along the repetition axis of the grooves is finite and equal to the ratio dla of the repetition period to the width of the grooves while the values of permittivity along the axes (Oy) and (Oz) are infinitely large (in absolute value).

Given that the waves may propagate within grooves at the speed of light both in the direction (Oy) of alignment of the grooves and in the depth direction (Oz) of the grooves, one has the relationship: VEvi.1), = lex/1_=1 (5) wherein izy and /2, are respectively the permeability values along the axes (Oy) and (Oz).

It is inferred therefrom that the permeability tensor in the surface layer may be expressed in the reference system (0, x, y, z) in the form: (1 0 0 PL= 0 ald 0 (6) 0 ald1 In the same way, a PEC sphere with radius R grooved as illustrated in Fig. 1, is electromagnetically equivalent to a PEC sphere of radius R-e covered with a homogenous and anisotropic surface layer of thickness e. The anisotropy of the surface layer is due to the presence of the grooves which induce behavioral anisotropy between the directions ur and tc, on the one hand (isotropy in a plane perpendicular to 04 and the direction z, on the other hand.

Given the symmetry of revolution of the sphere around of the axis (04, the permittivity tensor in the surface layer is expressed in cylindrical coordinates in the reference system (0,r,c9,z) as: z-03 0 0 £ = 0 -00 (7) \0 0 dla1 Similarly, the permeability tensor in the surface layer is expressed in this same reference system as: (aid 0 IP p= 0 ct I d 0 (8) 0 0 I If an incident plane wave is now considered, propagating in the direction Oz, the tangential component of the total electric field (incident field and reflected field) verifies in the plane tangent to the surface of the sphere E -(n.E)n = Z, (n x II) (9) wherein Z, = 4 (Z: 0 ", 0 Z, is the impedance 7f =zsa /z wherein = - vacuum and itto u and Zsc° = Z;VZ, , wherein Z,°.Z:' are the surface impedances in the directions u, and II, , respectively wherein n is a unit vector normal to the sphere and u, is the vector orthogonal to; , the vectors (us) defining the tangential plane.

E0=Z11 and Ev=Z:He (10) wherein (E,,E,) are components of the electric field along the vectors u, and tico, respectively. Also, (H,,H,) are the components of the magnetic field along the vectors u, and;0, respectively.

In the reference system (0,r,co) only the impedance along z is non zero and equal to: 2n:J17\

C

By performing a change of reference system from (u,uz,uc,, ) to (n,u,,uy), wherein uz is the unit vector along the axis Oz, it may be shown that: 41= 2: sin sin 0 27r fh c (12) wherein cz and gp respectively are the relative permittivity and the relative permeability of the surface layer in the directions ur and u,/,.

Taking into account (7) and (8), the expression (12) may then be rewritten as: Zr = -j sin G.tan 2g fh) (12') ) According to the Weston theorem extended to the anisotropic case, as shown by K.S. Yee et al. in the article entitled "Scattering theorems with anisotropic surface boundary conditions for bodies of revolution", published in IEEE Trans. on Antennas and Propagation, Vol. 39, No. 7, pp. 1041-1043 (1998), if the condition: (13-1) or, equivalently, if the condition: zs(.1.r; = (z0)2 (13-2) is met, then the reflection coefficient on the surface layer is zero. The result of this is then that the monostatic RCS along the axis Oz is also zero.

This conclusion is valid, according to the aforementioned article, when the object has symmetry of revolution along the Oz axis. However, it may be extended to a solid only having a rotational invariance of order -271-N 3, as described in the article of C. Monzon entitled "Effect of rotational invariance on the monostatic characteristics of matched bodies", published in J. Opt. Soc. Am. A, Vol. 22, No. 6, June 2005, pp. 1035-1041.

Given that z?' -= 0, the condition (13-1) can only be met if 219. =c,-.), i.e. if 271-fr -(2k + I)-, wherein k is an integer, in other words for an incident wave frequency 2 equal to: = 4h(2k + I) (14) In the preceding embodiment, it was assumed that the grooves in the surface layer were empty. Alternatively, they may be filled with a dielectric material of relative permittivity, sg, or more generally with a material of refractive index ng = s gpg wherein sg and mg are the relative permittivity and the relative permeability of the material respectively.

Fig. 2 represents the monostatic RCS of a reference target according to the exemplary embodiment of Fig. 1, versus the frequency of the incident wave. The monostatic RCS is relative to an incident wave propagating along the axis Oz.

The curve 210 is relative to a PEC sphere with a smooth surface and a radius equal to 99.5 mm.

The curve 220 is relative to a PEC sphere of radius R =99.5 mm having a surface layer of thickness e =3 mm in which grooves with a width a=1 mm, with a periodicity d =5 mm along the axis Oz have been made.

It is seen that the monostatic RCS under zero incidence has a first minimum at 5.8 4i GHz corresponding to the resonance frequency f = 4h (i.e., h = -) in the groove with the greatest depth (h= 12.8 mm). Moreover it is noted that when the grooves are filled with a dielectric material of relative permittivity eg, this minimum varies according to f = 4hje, which strengthens the preceding interpretation. It should be noted that at this frequency, the assumption c/ << A. is well verified (d /10).

In the second embodiment, the grooves in the surface of the reference target are no longer necessarily parallel with each other: the depth axis of each groove coincides with the normal to the surface at the point where it opens onto the surface.

Fig. 3 schematically illustrates an exemplary embodiment of a reference target according to the second embodiment of the invention.

The example illustrated here is a sphere, but it will be understood subsequently that this embodiment may apply to any shape having a rotational symmetry of order N L-3 around an axis, in the sense defined above.

More specifically, the sphere, 300, is grooved with grooves, 310, here made in the radial direction, in other words the depth axis of each groove coincides with the normal to the sphere at the relevant point. Like in the example of Fig. 1, each groove has an axisymmetrical shape around the axis Oz, the section of the groove having a rectangular shape of width a and of depth h in the radial direction of the sphere. The radius of the sphere is subsequently noted as R. Unlike the preceding exemplary embodiment, the grooves all have the same depth and are distributed according to an angular 0 periodicity (and not according to a periodicity along the axis (Oz)). Equivalently, two successive grooves are separated by an arc of length d in the transverse direction 0.

It is still assumed that the sphere is made in a perfectly conducting material or PEC (perfect electrical conductor), typically a metal material.

Like in the first embodiment, the electromagnetic behavior of the thereby grooved sphere is equivalent to that of a sphere of radius R-e covered with a fictitious surface layer of thickness e=h. This surface layer is anisotropic in the sense that the grooves introduce a behavioral anisotropy between the longitudinal direction of the grooves, W, and the transverse direction, 0.

The permittivity tensor in the surface layer is expressed in spherical coordinates in the reference system ( 0, r, 97, ) as: -co 0 0 = 0 -ro 0 (15) 0 0 dlai Similarly, the permeability tensor in the surface layer is expressed in this same reference system as: aid 0 0 p= 0 ald 0 (16) 0 0 1 If the plane tangent to the sphere (u,,uco) in a point is considered and if an incident plane wave propagating in the direction Oz is considered, the tangential component of the total electric field (incident field and reflected field) verifies in the plane ( no): E -OLE* = Z, (n x H) (17) wherein Z, = Zo zs wherein Zo = 6=I1 is the vacuum impedance and 7: = 0 r: Po and 2: = Zr/Z0, wherein 48. are the surface impedances in the directions 0 and p, respectively and wherein n is a unit vector normal to the sphere, such that (II, , n) is a direct reference system.

The expression (15) may further be written as: Ea =Z [Iv and E,,,=Z He (18) wherein (E0,E,) are the components of the electric field along the vectors u9 and uc, respectively. Also, (Hio,H,) are the components of the magnetic field along the vectors tic, and u,p, respectively.

It may be shown that =0 (current parallel to the groove) and that: 2n-fh\ (19) tan (\ls",pc, . se c wherein e0 and pc, are the relative permittivity and the relative permeability of the surface layer in the directions u, and u,, respectively.

Taking into account (15) and (16), the expression (19) may be rewritten as: 79 --tan tan ( 271fh'.

c (20) It will be noted that, in this second embodiment, the surface impedance is uniform on the object.

According to the aforementioned Weston theorem, if the relationship zsa.z7 = (zo)' is met, the reflection coefficient on the surface layer is then zero. As seen earlier, this conclusion is valid not only for an object having symmetry of revolution but also a rotational symmetry of order N 3 around a symmetry axis.

Given that Zm =0, the condition z,tl.Z: = (zo)2 can only be verified if 2: =oo, i.e., g i if 2n-fh -(2k+ 0-, in other words for a frequency of the incident wave equal to: f = (2k +1)-c-4h (21) In other words, there again the (first) minimum is attained, like in the first embodiment, for a stationary mode in the groove corresponding to h=-2. In the case of a groove filled with a dielectric material of index this minimum varies as f -4hng wherein ng=.18gpa wherein aree the relative permittivity and the relative permeability of the relevant dielectric, respectively.

Fig. 4 illustrates the monostatic RCS of a reference target according to the exemplary embodiment of Fig. 3, versus the frequency of the incident wave. The monostatic RCS is relative to an incident wave propagating along the axis Oz.

The curve 410 is relative to a PEC sphere with a smooth surface and a radius equal to 99.5 mm.

The curve 420 is relative to a PEC sphere of a radius R =99.5 mm having a surface layer of thickness a=3 mm in which grooves with the thickness of a=1 mm with a curvilinear periodicity of ci =5 mm have been radially made.

The relative permittivity e0 was taken at 80=5 and the relative permeability pg, was taken at pc,, =0.2.

It is seen that the monostatic RCS under zero incidence has a minimum at a frequency close to 25 GHz, corresponding, as provided, to the aforementioned resonance condition in the groove.

Fig. 5 illustrates a first method for making a reference target having a small monostatic RCS along a rotational symmetry axis, at a frequency of interest, f.

In step 510, a reference target is selected having a rotation symmetry of order N > 3, in other words, the shape of this target is rotation invariant by -2g, around a rotational symmetry axis. A fortiori, the reference target may have symmetry of revolution (which also comes under the preceding case with N arbitrarily large), around a rotational symmetry axis (Oz).

This object is advantageously in PEC and its monostatic RCS (and optionally also bistatic RCS) is determined either by simulation or by an analytical calculation.

In step 520, the geometrical characteristics of grooves periodically or quasi-periodically distributed at the surface of the object are determined along the axis (Oz). In other words, the grooves are parallel with each other and the depth axis of the grooves forms a constant angle with the axis (Oz), for example a right angle. The width a of the grooves and the repetition period d of the grooves are selected so that a < d «A and the depth h of the groove is calculated such that: h =-c(2k +1) (22) 4f wherein k E N (set of all natural numbers).

According to an alternative, if it is provided that the grooves will be filled with a dielectric material of index ng, the depth of the groove is then calculated by means of: 4frg (2k +1) (23) wherein k e N. Generally, the formula (23) will be applied in every case, with ng =1 in the absence of any dielectric.

In step 530, grooving of the object is carried out by producing the grating of grooves at the surface of the object according to the geometrical characteristics determined in step 520.

Fig. 6 represents a second embodiment of a reference target having a small monostatic RCS along a rotational symmetry axis, at a frequency of interest, f As earlier, in step 610, a reference target is selected having a rotational symmetry of order N>_3, in other words the shape of this target is invariant by a rotation of 221-, N 3, around a rotational symmetry axis (Oz). Its monostatic (and optionally bistatic) RCS is determined by simulation or analytically.

In step 620, the geometrical characteristics of grooves periodically or quasi-periodically distributed at the surface of the object are determined. However, unlike the preceding embodiment, the depth axes of the different grooves are not parallel with each other but orthogonal to the surface of the object at the points where they open onto the latter.

The width a of the grooves and the repetition period d of the grooves (measured along an arc orthogonal to the groove) are selected such that a < ci « . The depth h of the grooves, as for it, is calculated such that: h = ±-(2k +1) 4f (24) wherein k e N. According to an alternative, if it is provided that the grooves will be filled with a dielectric material of index ng, the depth of the groove is then calculated by means of: h= (2k +1) king wherein k EN N. As earlier, formula (24) will be applied in every case, with ng =1 in the absence of any dielectric.

In step 630, grooving of the object is carried out by producing a grating of grooves at the surface of the object according to the geometrical characteristics determined in step 620.

The method of Fig. 5 or of Fig. 6 gives the possibility of obtaining zero monostatic RCS of a reference target in the direction of its rotational symmetry axis without any substantial modification of its bistatic RCS.

This reference target may then be used for testing the system for measuring RCS and notably for controlling the coupling level in the direction corresponding to the rotational symmetry axis of this target.

The different embodiments described above are relative to a grating of grooves of depth h depending on the frequency of interest f. However, one skilled in the art will understand that a reference target may be provided with several gratings of grooves of different depths for having a zero monostatic RCS or a very small RCS at a plurality of frequencies. (25)

Claims

CLAIMS1. A reference target for testing a system for measuring a radar cross section at a given frequency f, the reference target having rotation invariance of -2g, wherein N is an integer such that N around a rotational symmetry axis, the reference target being characterized in that it has over all or part of its surface grooves parallel with each other arranged according to a one-dimensional grating along said rotational symmetry axis, with a periodicity d substantially less than the wavelength.1,=c1, at least some of the grooves of said grating having a depth h- c (2k+1) wherein lc is an integer and 4.f fige, g, ug are respectively the relative permittivity and the relative permeability in the groove so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said rotational symmetry axis..
2. The reference target according to claim 1, characterized in that the depth axis of each groove forms a predetermined angle with said rotational symmetry axis.
3. The reference target according to claim 2, characterized in that the predetermined angle is a right angle, each groove being delimited by two planes orthogonal to the rotational symmetry axis, spaced apart by the width of the groove.
4. The reference target for testing a system for measuring a radar cross section at a given frequency f, the reference target having rotation invariance of k-r, wherein N is an integer such that N>_3, around a rotational symmetry axis (Oz), the reference target being characterized in that it has over all or part of its surface grooves, the depth axis of each groove coinciding with the normal to the surface at the point wherein it opens onto this surface, the grooves being arranged at the surface with transverse periodicity d, measured along an arc orthogonal to the grooves, said transverse periodicity being substantially less than the wavelength 2. =c/ f, the grooves all having a depth h- c (2k+1) wherein k is an integer and eg,pg are respectively the relative 4/ V,LIgEg permittivity and the relative permeability in the groove so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said rotational symmetry axis.
5. The reference target according to one of the preceding claims, characterized in that it has symmetry of revolution around said rotational symmetry axis.
6. The reference target according to one of the preceding claims, characterized in that it is made in a metal material.
7. The reference target according to one of the preceding claims, characterized in that the grooves are filled with a dielectric material.
8. A method for obtaining a reference target having, at a given frequency small monostatic radar cross section along a given axis, characterized in that: -said reference target is selected (510) as having rotational invariance of -2n-, wherein N is an integer such that N 3, around said axis; -the geometrical characteristics of a grating of parallel grooves, are determined (520), the pitch of the grating along said axis being selected so as to be substantially less than the wavelength A =c I f, the depth of the grooves being calculated by means of h- c (2k +1) wherein k is an integer and sg,pg are the relative permittivity and 4,f.the relative permeability in the groove, respectively; -grooving of the surface of the reference target is carried out (530) according to the grating of parallel grooves for which the geometrical characteristics have thus been determined so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said axis.
9. A method for obtaining a reference target having, at a given frequency f, a small monostatic radar cross section along a given axis, characterized in that: - said reference target is selected (610) as having rotation invariance of 271-, wherein N is an integer such that N z 3, around said axis; - the geometrical characteristics of a grating of grooves of identical depth are determined (620), the depth axis of each groove coinciding with the normal to the surface in each point where the groove opens onto this surface, the pitch of the grating along this surface being selected so as to be substantially less than the wavelength 2 =elf, the depth of the grooves being calculated by means of h = (2k +1) wherein k is an 4.f figEg integer and 5g' /1g are the relative permittivity and the relative permeability in the groove, respectively; - grooving (630) of the surface of the reference target is carried out according to the grating of grooves for which the geometrical characteristics have thus been determined so that the reference target exhibits a null or quasi-null radar cross section at said given frequency in the direction of said axis.
10. The method for obtaining a reference target according to claim 8 or 9, characterized in that the reference target is selected so as to have symmetry of revolution around said axis.
11. The method for obtaining a reference target according to one of claims 8 to 10, characterized in that the reference target is in metal material.
12. The method for obtaining a reference target according to one of claims 8 to 11, characterized in that after grooving, the grooves are filled with a dielectric material.
13. A reference target for testing a system for measuring a radar cross section generally as herein described with reference to and/or as illustrated in the accompanying drawings.
14. A method for obtaining a reference target generally as herein described with reference to and/or as illustrated in the accompanying drawings.