DEVICE FOR REDUCING THE SER OF A POSITIONER IN AN ANECHOIC CHAMBER

Abstract

The present invention relates to a device for reducing the radar signature of a positioner intended for positioning a target in an anechoic chamber, the positioner comprising at least one attachment element to which the target is fixed. This device is in the form of a shell made of a material that is substantially electromagnetically transparent in a measurement frequency range of the target's radar cross-section (RCS), said shell comprising a first and a second half-shell intended to be fixed to one another, said shell being filled with a material that is electromagnetically absorbent in said measurement frequency range, and encompassing said attachment element. Figure for abbreviation: Fig. 1

Description

Title of the invention: DEVICE FOR REDUCING THE SER OF A POSITIONER IN AN ANECHOIC CHAMBER Technical field

[0001] The present invention relates generally to the field of measuring the radar equivalent surface (SER) of a target in an anechoic chamber. STATE OF THE PRIOR ART

[0002] The radar equivalent surface or SER is a fundamental quantity characteristic of a target. It is used both in the military field and in the civil field (for example for air traffic control) for the purpose of discriminating an object such as an aircraft or part of an aircraft.

[0003] The measurement of SER of a target is conventionally carried out in an electromagnetic anechoic chamber, by means of a single measurement antenna (operating in transmission and reception) when it is a question of measuring a monostatic SER or of a pair of antennas (one operating in transmission and the other in reception) when it is a question of measuring a bistatic SER. The measurement of SER is generally carried out at a plurality of frequencies in a measurement frequency band.

[0004] To perform the SER measurement, the target is positioned facing the measuring antenna on a support which must guarantee the precision and stability of the distance between the target and the antenna, the height of the target as well as the angular incidence of the beam on the latter. The support is generally rotatable to allow the illumination of the target from different angles of incidence. The reproducibility and precision of the positioning are essential to reduce the margin of error on the SER measurement, particularly at high frequencies. This constraint leads to choosing supports that are all the more robust as the target is massive.

[0005] It is also necessary for the support, also called positioner, to have the lowest possible specific SER so as not to disturb the SER measurement of the target. More precisely, the measurement of the SER in the anechoic chamber involves carrying out a first measurement of the SER of the target in position on the positioner and a second measurement of the SER in the absence of the target. The difference between these two measurements makes it possible to eliminate common parasitic echoes and it is therefore necessary to reduce all deviations other than those due to the presence and absence of the target. A slight modification of the support, due for example to a sagging or, more generally, to an alteration of its shape, under the weight of a massive target can lead to incorrect elimination of common parasitic echoes and consequently a measurement error.

[0006] In practice, the supports most commonly used for SER measurements in anechoic chambers are polystyrene columns, a relatively transparent material in the measurement frequency range (from a few GHz to a few tens of GHz) but insufficiently robust to support targets of high weight and / or large size (for example an aircraft). In this case, it is necessary to position the target in the anechoic chamber using a hanging system composed of slings and shackles, capable of supporting significant loads.

[0007] However, such a hanging system also has drawbacks for measuring the SER of the target:

[0008] First of all, the SER of the attachment system, that is to say its electromagnetic signature, is often very high, and sometimes even higher than that of the target itself.

[0009] Then, the SER of the attachment system can vary greatly depending on the angle of presentation of the target relative to the measuring antenna, which makes its elimination complex. In addition, couplings can occur between the attachment system and the target itself.

[0010] Finally, the elimination by subtraction of the parasitic contributions of the attachment system cannot be applied. Indeed, once the target is unhooked, the slings and shackles of the attachment system are in a geometric configuration different from that under load and consequently their contribution to the SER is different.

[0011] To eliminate the parasitic contributions of the attachment system, a radar image filtering technique is then used. However, this method requires a large number of measurements to be carried out to obtain a radar image without aliasing, in other words, using both a small frequency step and a small angular step. The location of the parasitic echoes is then carried out manually by the operator, this being made difficult by the presence of artifacts due to the couplings between shackles and slings, shackles and target, etc.

[0012] The object of the present invention is therefore to propose a positioner which makes it possible to position massive and / or large targets in an anechoic chamber without requiring complex post-processing to eliminate the parasitic contribution of the positioner. Statement of the invention

[0013] The present invention is defined by a device for reducing the radar equivalent surface of a positioner intended to position a target in an anechoic chamber, said positioner comprising at least one attachment element to which the target is fixed, said device being original in that it comprises a first and a second half-shells made of a material that is electromagnetically substantially transparent in a measurement frequency range of the radar equivalent surface (SER) of the target, the first and second half-shells being intended to be fixed to each other so as to form a shell encompassing said attachment element, said shell being filled with an electromagnetically absorbing material in said measurement frequency range.

[0014] The shell advantageously has a substantially spherical shape, the half-shells being able to have a mesh structure.

[0015] The half-shells can be obtained by additive manufacturing.

[0016] The electromagnetically substantially transparent material in said measurement frequency range may be a dielectric material, homogeneous or composite, preferably having an equivalent permittivity less than or equal to 3.

[0017] The electromagnetically absorbing material in said measurement frequency range may be a carbon-loaded foam. Brief description of the drawings

[0018] Other characteristics and advantages of the invention will appear on reading a preferred embodiment of the invention, made with reference to the attached figures among which:

[0019] [Fig.l] schematically represents a partial view of the SER reduction device of a positioner according to an embodiment of the invention;

[0020] [Fig.2] shows the impulse responses of a target attached to a positioner equipped or not with a SER reduction device according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] We consider in the following a SER measurement system comprising an electromagnetic anechoic chamber, a weakly echogenic positioner placed within this chamber, a measurement antenna making it possible to illuminate the target and receive the backscattered signal, and a network analyzer, connected to the measurement antenna and making it possible to measure the backscattering coefficients S3 j( f], from the emitted signal and the backscattered signal.

[0022] The positioner comprises slings from which the target is suspended by means of metal attachment elements such as shackles. The length of each sling can be adjusted, for example, by means of a motorized pulley and reel system. By adjusting the length of the different slings, the target can be moved and oriented in the anechoic chamber.

[0023] The attachment elements are generally of complex shapes and their electromagnetic signatures are difficult to model. The idea behind the invention is to significantly reduce these electromagnetic signatures by including these hanging elements by a shell made of a suitable shape, advantageously spherical, filled with an electromagnetic absorbing material in the measurement frequency range of the SER.

[0024] The material of the shell is non-metallic: it is advantageously chosen to be electromagnetically substantially transparent in the aforementioned measurement range.

[0025] The spherical shape is preferred for several reasons: it is simple to implement since it does not require any fine positioning or alignment, it disperses the unabsorbed residual energy, and its effect on the SER of the target is independent of its orientation. However, other shapes may be considered by those skilled in the art provided that they only return a few echoes to the measuring antenna (low specular reflection).

[0026] The shell advantageously comprises two half-shells made of non-metallic material, which can be clipped or screwed onto each other.

[0027] Advantageously, the structure of the shell is not solid but meshed.

[0028] Such a mesh shell can advantageously be produced by additive manufacturing. Once the shell is arranged around the attachment element, a filling material, an electromagnetic absorber in the measurement frequency range, or even a broadband absorbent material, can be injected or inserted into the shell so as to reduce / mask the reflection of the shackle.

[0029] Such a mesh shell allows the electromagnetic wave emitted by the measuring antenna to enter: it is absorbed a first time by the filling element on the outward path and a second time, on the return path after reflection on the attachment element.

[0030] Alternatively, the shell may be solid, the interior of each half-shell being coated with an element made from said filling material and substantially matching the shape of the shackle.

[0031] The solid shell being made of a transparent material in the measurement frequency range, it reflects little of the wave emitted by the measurement antenna, the transmitted fraction being absorbed as previously by the filling material.

[0032] In all cases, the filling material may be a carbon-loaded foam.

[0033] The diameter of the sphere is chosen to be large enough to encompass the attachment element without significantly obscuring the target, at least its main SER contributing points.

[0034] [Fig.l] represents a partial view of a SER reduction device of a positioner according to an embodiment of the invention, and more precisely a spherical meshed half-shell, 110, produced by additive manufacturing. A first orifice, 121, is provided in the sphere so as to allow the sling to pass through at the end of which the attachment element is fixed. A second orifice, 122, symmetrical to the first with respect to the center of the sphere may also be provided.

[0035] The material of the half-shell may be chosen from those transparent to radar waves in the aforementioned frequency range, for example a dielectric material, homogeneous or composite, preferably having an equivalent permittivity less than or equal to 3.

[0036] [Fig.2] shows the impulse responses of a target attached to a positioner equipped or not with a SER reduction device according to the present invention.

[0037] The impulse responses were obtained from measurements of the backscattering coefficient 5] j( / ) in the frequency band of interest, then inverse Fourier transform.

[0038] The target used for this demonstration of effectiveness is here a metal sphere of 400 mm diameter.

[0039] Curve 210 represents the theoretical impulse response of a metal sphere 400 mm in diameter. Curve 220 represents that obtained by measuring the SER of a metal sphere 400 mm in diameter, equipped with a shackle intended to position it in the anechoic chamber. Finally, curve 230 represents that of this same sphere equipped with the same shackle when the latter is included in a shackle cover of the type shown in [Fig.l], namely two meshed half-shells, made of plastic, filled with an electromagnetically absorbing material in the frequency range considered, and fixed to each other.

[0040] The first peak of the theoretical curve 210 (located at - 0.2 m) corresponds to the specular reflection on the metal sphere (namely the target). This peak is naturally found in the curves 220 and 230 obtained by measurement; the curve 220 also has several nearby and high-level parasitic peaks (221, 222 in particular) due to the reflection on the shackle and to the multiple interactions between it and the sphere. It can be seen on the curve 230 that, as soon as the shackle is equipped with the shackle cover, these peaks are attenuated by more than 25 dB, which shows the effectiveness of the SER reduction device according to the present invention.

Claims

Claims

1. Device for reducing the radar equivalent surface of a positioner intended to position a target in an anechoic chamber, said positioner comprising at least one attachment element to which the target is fixed, characterized in that it comprises a first and a second half-shell made of a material that is electromagnetically substantially transparent in a measurement frequency range of the radar equivalent surface (SER) of the target, the first and second half-shells being intended to be fixed to each other so as to form a shell encompassing said attachment element, said shell being filled with an electromagnetically absorbing material in said measurement frequency range.

2. Device for reducing the radar equivalent surface of a positioner according to claim 1, characterized in that the shell has a substantially spherical shape.

3. Device for reducing the radar equivalent surface of a positioner according to claim 1 or 2, characterized in that the half-shells have a mesh structure.

4. Device for reducing the radar equivalent surface of a positioner according to claim 3, characterized in that the half-shells are obtained by additive manufacturing.

5. Device for reducing the radar equivalent surface of a positioner according to one of the preceding claims, characterized in that the material is electromagnetically substantially transparent in said measurement frequency range.

6. Device for reducing the radar equivalent surface of a positioner according to claim 5, characterized in that the electromagnetically substantially transparent material in said measurement frequency range is a dielectric material, homogeneous or composite, having an equivalent permittivity less than or equal to 3.

7. Device for reducing the radar equivalent surface of a positioner according to one of the preceding claims, characterized in that the electromagnetically absorbing material in said measurement frequency range is a carbon-loaded foam.

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