FR3144870A1 - STATE SWITCHING STANDARD FOR CALIBRATION OF A RADAR EQUIVALENT SURFACE MEASUREMENT SYSTEM - Google Patents

Abstract

The present invention relates to a standard for radar equivalent area (SER) measurements in an electromagnetic anechoic chamber. The standard can switch from an absorbing state in which its monostatic/bistatic SER is negligible to an active state in which it exhibits a calibrated monostatic SER. The standard is in the form of a metal sphere covered with an absorber, on the surface of which a horn antenna with a highly directivity radiation pattern is integrated. A battery-powered processing circuit is housed inside the sphere: it includes a duplexer, a low-noise amplifier and a programmable phase shifter and attenuator. The attenuator can be programmed to a high value to place the standard in an absorbing state. The present invention also relates to a method for measuring SER using this standard as a reference. Fig. 1

Description

STATE-SWITCHING STANDARD FOR CALIBRATION OF A RADAR CROSS-SECTION AREA MEASURING SYSTEM

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

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

The measurement of the SER of an object is classically carried out in an electromagnetic anechoic chamber, using a single antenna (operating in transmission and reception) when it is a question of measuring a monostatic SER or a pair of antennas (one operating in transmission and the other in reception) when it is a question of measuring a bistatic SER.

The SER measurement is generally carried out in a frequency band, which allows, from Fourier transform-based processing, to determine the impulse response of the object and/or to improve the quality of the measurement.

More precisely, the (complex) backscattering coefficient of the target placed within the anechoic chamber is determined from the signal transmitted and the signal received by the measuring antenna, for a plurality of frequencies equidistributed in the frequency band of interest, if necessary for two distinct polarizations. The same measurements are also carried out in the empty anechoic chamber, i.e. in the absence of the target. The backscattering coefficient of the empty chamber is then subtracted to the backscattering coefficient of the target in the chamber to obtain a backscattering coefficient of the target alone for a plurality of frequencies .

In the same way, in a calibration phase, we determine the complex backscattering coefficient of a standard object at these same frequencies, , . To take into account a possible variation in the chamber environment between the calibration phase and the target measurement phase, due for example to the fact that the standard support differs from that of the target, the chamber coefficient is measured again, i.e. here in the absence of the standard, i.e. .

It should be noted that the calibration phase can occur before or after the target measurement phase.

Knowing the theoretical response of the standard, obtained by simulation, by analytical calculation or even by a previous and validated calibrated measurement, we estimate the target's own backscattering coefficient, , by means of:

Ultimately the estimation of the SER of the target at the frequency is none other than .

Subtracting the response of the empty anechoic chamber allows us to overcome the intrinsic signature of the anechoic chamber. The ratio between the difference in measurements in the numerator and the difference in measurements in the denominator allows us to overcome the transfer function of the measurement system.

The standards commonly used for measurements in anechoic chambers are well-defined, perfectly conducting (PEC) geometry objects whose SER can be determined by calculation (e.g. Mie series), generally metal spheres or plates. Metal spheres have the advantage of an isotropic SER while that of a metal plate depends on its orientation in space. An orientation error of a metal plate in the chamber can lead to a significant measurement bias.

Since SER measurement campaigns can be particularly long for large targets, it is necessary to compensate for drifts in the acquisition chain as well as changes in the chamber characteristics by regularly carrying out a calibration phase as described above. In such a case, the target should be removed from the chamber and replaced with a standard to carry out the calibration. This handling operation is particularly complex to implement for heavy and bulky targets: it can lead to a degradation of the anechoic chamber configuration or the positioner.

In addition, as previously mentioned, it is necessary to frequently perform a SER measurement of the empty chamber (chamber qualification procedure). This involves opening the chamber each time and removing the target or the standard sphere (or a metal sphere that has already been calibrated with respect to the standard sphere) using a nacelle. Here again, the handling operations are long and complex, particularly for large chambers, and can damage its absorbent coating.

The object of the present invention is therefore to propose a procedure for calibrating a SER measurement system, in particular for large targets, which does not have the aforementioned drawbacks, in particular which is rapid and requires little or no handling operations. Another object of the present invention is to propose a standard which makes it possible to implement such a calibration procedure.

According to a first embodiment, the present invention is defined by a standard for calibrating a radar equivalent surface measurement system in an anechoic chamber, said standard being original in that it is in the form of a hollow metal sphere coated with a layer of electromagnetic absorber in the measurement frequency band, the wall of the sphere comprising a first recess in which a horn antenna is housed and a second recess in which an access hatch is provided for accessing the interior of the sphere, said access hatch being made of or covered by said electromagnetic absorber, the hollow metal sphere carrying a processing circuit comprising a duplexer connected by its common port to the horn antenna, a low-noise amplifier for amplifying the signal received from the reception port of the duplexer, a phase shifter for phase-shifting the signal thus amplified and an attenuator for attenuating the signal thus phase-shifted, the signal thus attenuated being supplied to the transmission port of the duplexer, the attenuator and the phase shifter being programmable by the measurement system, the attenuation being able to be programmed to a maximum value to place the standard in an absorbing state, the standard being in an active state otherwise.

According to a second embodiment, the invention is defined by a standard for calibrating a radar equivalent surface measurement system in an anechoic chamber, said standard being original in that it is in the form of a hollow metal sphere coated with a layer of electromagnetic absorber in the measurement frequency band, the wall of the sphere comprising a first recess in which a dual-polarization horn antenna is housed and a second recess in which an access hatch is provided for accessing the interior of the sphere, said access hatch being made of or covered by said electromagnetic absorber, the hollow metal sphere carrying a processing circuit comprising: a first duplexer whose common port is connected to a first port of the antenna, associated with a vertical polarization; a second duplexer whose common port is connected to a second port of the antenna, associated with a horizontal polarization; a first quadrature coupler receiving on its non-phase-shifted input the signal from the reception port of the first duplexer and on its quadrature input the signal from the reception port of the second duplexer; a low-noise amplifier for amplifying the signal received from the reception port of the duplexer, a phase shifter for shifting the signal thus amplified and an attenuator for attenuating the signal thus shifted, the signal thus attenuated being supplied to the input port of a second quadrature coupler whose non-phase-shifted output port is connected to the transmission port of the second duplexer and the quadrature output port is connected to the transmission port of the first duplexer, the attenuator and the phase shifter being programmable by the measurement system, the attenuation being able to be programmed to a maximum value to place the standard in an absorbing state, the standard being in an active state otherwise.

Whatever the embodiment, the hollow sphere can carry an inertial unit.

The present invention also relates to a method for measuring the radar equivalent surface of a target in an anechoic chamber, by means of a measurement system comprising a measurement antenna coupled to a network analyzer, said measurement antenna being intended to emit a wave in the direction of the target and to receive a backscattered wave, the measurement being carried out at a plurality of frequencies. , of a frequency band of interest, the network analyzer being adapted to measure the backscattering coefficients for each frequency of said plurality from the complex amplitude of the emitted wave and the complex amplitude of the received wave, said method being original in that the backscattering coefficient is measured of the anechoic chamber at said plurality of frequencies, the target and the standard as defined above are then introduced into the anechoic chamber, and that for each frequency of said plurality the standard is placed in the absorbing state and the backscattering coefficient of the target is measured in the chamber in the presence of the standard in the absorbing state, , then the standard is switched to its active state and the backscattering coefficient of the standard in its active state is measured in the presence of the target in the chamber, , the measurement system deducing the monostatic SER of the target from the backscattering coefficients , , .

Advantageously, the monostatic SER, , the target is then obtained by means of , Or is the monostatic SER associated with the active backscattering component of the state-switching standard, obtained by calibration with a passive standard.

The invention also relates to a method for measuring the radar equivalent surface of a target in an anechoic chamber, by means of a measurement system comprising a measurement antenna coupled to a network analyzer, said measurement antenna being intended to emit a wave in the direction of the target and to receive a backscattered wave, the measurement being carried out at a plurality of frequencies. , of a frequency band of interest, the network analyzer being adapted to measure the backscattering coefficients for each frequency of said plurality from the complex amplitude of the emitted wave and the complex amplitude of the received wave, said method being original in that the backscattering coefficient is measured of the anechoic chamber including the standard in its absorbing state, at said plurality of frequencies, then that the target is introduced into the anechoic chamber, and that for each frequency of said plurality, the standard is placed in the absorbing state and the backscattering coefficient of the target is measured in the chamber in the presence of the standard in the absorbing state, , then the standard is switched to its active state and the backscattering coefficient of the standard in its active state is measured in the presence of the target in the chamber, , the measurement system deducing the monostatic SER of the target from the backscattering coefficients , , .

Advantageously, the monostatic SER, , the target is then obtained by means of , Or is the monostatic SER associated with the active backscattering component of the state-switching standard, obtained by calibration with a passive standard.

Preferably, the backscattering coefficient of the target is measured twice, for a plurality of orientations of said target, a first time with the standard in its active state and another time with the standard in its absorbing state.

Advantageously, when measuring the backscattering coefficient of the standard in its active state in the presence of the target in the chamber, , the orientation of said standard is verified by means of an inertial unit embedded in the hollow sphere.

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

schematically represent different views of a state switching standard according to an embodiment of the present invention;

schematically represents a first example of a microwave circuit embedded in the state-switching standard of Figs. 1A, 1B and 1C;

schematically represents a first example of a microwave circuit embedded in the state-switching standard of Figs. 1A, 1B and 1C;

schematically represents a method of measuring SER using calibration using a state-switching standard according to the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

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 for illuminating the target and receiving the backscattered signal, and a network analyzer, connected to the measurement antenna and for measuring the backscattering coefficients. , from the emitted signal and the backscattered signal.

The idea behind the present invention is to replace passive standards, in particular the metal spheres mentioned in the introductory part, with a standard that can switch in a controlled manner between an active state, in which it has a setpoint SER in a specific direction, and an absorbing state, in which the monostatic SER and the bistatic SER of the standard are negligible, i.e. substantially lower than the setpoint monostatic SER, and this throughout the measurement frequency band. Such a standard will be referred to hereinafter as a state-switching standard.

More specifically, as shown schematically in Figs. 1A, 1B and 1C, such a standard, 100, is in the form of a hollow metal sphere, 110, in other words a spherical metal wall, coated with an absorbent dielectric layer in the measurement frequency band. A first recess is provided in the spherical wall to house a horn antenna, 120. Advantageously, a second recess (not shown) is provided in the spherical wall, preferably on the side opposite the first recess relative to the center of the sphere. A metal hatch (not shown) coated with a layer of absorbent dielectric material, or even a hatch made of this material, closes this second recess hermetically. The access hatch can for example be fixed by screws on a shoulder provided along the contour of this second recess, said screws having a diameter substantially smaller than the wavelength in the measurement frequency band. This hatch provides access to the inside of the sphere, in particular to house a processing circuit detailed later as well as an autonomous power source (battery). This processing circuit and this battery can be arranged on insulating plates, 130, for example on discs made of insulating material fixed within the sphere, as shown in the exploded view in .

There schematically represents a first example of a processing circuit embedded in the hollow metal sphere of Figs. 1A, 1B and 1C.

This circuit, 200, comprises a duplexer 210 whose common channel is connected to the aforementioned horn antenna, 220. The signal received on the reception port of the duplexer, is amplified by a low noise amplifier (LNA), 230, before being phase shifted in a phase shifter (for example a delay line), 240, and, if necessary, attenuated in an attenuator, 250, then finally injected into the transmission port of the duplexer.

The phase shifter 240 and the attenuator 250 are programmable and can be controlled remotely by a control module of the measurement system, either by wire, or (preferably) by an auxiliary radio signal (in a frequency band not absorbed by the dielectric forming the cover of the second compartment), or by means of a low-frequency modulation of the measurement signal.

When the attenuator is programmed to its maximum attenuation value, the circuit is made inactive and the standard is in its absorbing state.

Alternatively, a switch (not shown) may be provided between the common port of the duplexer 210 and the antenna 220, so as to be able to make the circuit inactive. Those skilled in the art will understand that this switch may be placed if necessary in the processing loop between the reception port and the transmission port of the duplexer.

The separation between the received wave and the transmitted wave can be ensured by the delay line if the measurement signal is sufficiently brief.

Alternatively, this separation may be achieved spatially by means of separate horn antennas for transmitting and receiving, in which case the duplexer is no longer necessary, the receiving antenna being connected to the input of the amplifier and the transmitting antenna to the output of the attenuator. A switch on each antenna may be provided to place the standard in an active state or in an absorbing state.

Alternatively, the separation between the received wave and the emitted wave can be ensured by crossed polarizations or reverse polarization directions in the case of circularly polarized waves.

There schematically represents a second example of an on-board processing circuit in the case where the separation between the received wave and the emitted wave are circularly polarized waves (RHCP or LHCP).

The antenna used may be a dual-polarization horn antenna, 220, with a first port (VV) associated with the vertical polarization component and a second port (HH) associated with the horizontal polarization component. The first port of the antenna 220 is connected to the common port of a first duplexer 211 and the second port of this antenna is connected to the common port of a second duplexer 212.

A first quadrature coupler, 215, receives on its non-phase-shifted input the signal from the reception port of the first duplexer, 211, and on its quadrature input, 211, the signal from the reception port of the second duplexer, 212. The output of the first coupler is connected to the input of the low-noise amplifier, 230.

Symmetrically, a second quadrature coupler, 216, receives on its input the output signal of the attenuator, 250, and provides a non-phase-shifted signal to the transmission port of the second duplexer, 212, as well as a quadrature signal to the transmission port of the first duplexer, 211.

The processing loop consisting of the low noise amplifier, 230, the phase shifter, 240, and the attenuator, 250, is identical to that already described in relation to the example of the . In particular, the phase shifter and the attenuator are programmable and can be controlled remotely by a control module of the measuring system. The variants for placing the standard in an absorbing state or in an active state also apply here.

Regardless of the embodiment considered, the processing circuit allows the standard to be placed in an absorbing state or in an active state. Furthermore, in the active state, the programming of the attenuator and the phase shifter allows the effect of a desired SER contributor to be reproduced with one or more setpoint SER values. This SER value can be obtained in a prior calibration phase relative to a conventional passive standard, for example a metal sphere. Furthermore, the state-switching standard can also include a positioning device and/or an attitude determination device, which can be activated on demand, so as to verify that its position and orientation relative to the measurement antenna are unchanged. Finally, the state-switching standard can be placed in different different orientations, with different attenuations and/or phase shifts so as to allow a wide variety of calibrations. This versatility allows the anechoic chamber to be qualified in a single pass before introducing the target into the anechoic chamber.

The presence of an amplifier, 230, within the processing circuit makes it possible to obtain SER values much higher than those of a passive standard, for example a metal sphere, of the same dimensions.

The high directivity of the radiation pattern of the antenna integrated in the state-switching standard makes it possible to avoid parasitic echoes on the walls of the chamber and, more generally, on the environment of the measurement system.

Finally, and most importantly, the fact that the state-switching standard can be easily placed in an absorbing state allows it to be left in the anechoic chamber during target SER measurements, without the risk of coupling with the target. This considerably reduces the target handling operations, particularly when the target is large.

There schematically represents a method of measuring SER using a calibration procedure using a state-switching standard, as described above.

It is assumed that the state-switching standard has been previously calibrated with respect to a passive standard, such as a metal sphere for example. More precisely, the monostatic SER due to the active component of the state-switching standard is determined with reference to this passive standard, in the measurement frequency band, . To do this, the backscattering coefficient of the state-switching standard is measured in the absorbing state and then subtracted from that measured in the active state for each measurement frequency, and the monostatic SER corresponding to this difference is determined by reference to that of the passive standard. If necessary, the backscattering coefficient of the standard in its absorbing state compared to that in its active state, in the measurement frequency band, can be neglected and therefore the first measurement omitted.

In a first step, 310, the anechoic chamber is empty and the backscattering coefficient is measured , in a plurality of frequencies in the measurement frequency band. This backscattering coefficient is measured at frequencies .

In a second step, 320, the target whose SER is to be measured is introduced into the chamber as well as the state switching standard.

A measurement campaign is then carried out by selecting the frequencies in turn , the measurements being repeatable, if necessary, for a plurality of orientation directions of the target. This selection step is shown at 330.

During this campaign, the measurement antenna remains fixed relative to the chamber and is sufficiently non-directive to illuminate both the target and the state-switching standard.

For each frequency, the following procedure is followed:

We first place the state-switching standard in its absorbing state at 340 and we measure at 350 the backscattering coefficient relative to the anechoic chamber, target and standard assembly in absorbing state, noted .

The state-switching standard is then switched to its active state at 360 and a measurement of the backscattering coefficient is carried out again at 370, all other things being equal. The backscattering coefficient thus measured is relative to the assembly of anechoic chamber, target and standard in its active state, noted .

The difference represents the contribution to the SER due to the active backscattering component of the standard, with the contributions from the chamber and target eliminated.

Finally, we calculate in 380, the monostatic SER of the target, by :

When the SER measurement campaign is completed, 390, the SER measurement method stops at 395. Otherwise, the measurement process continues at 330 by selecting a new measurement frequency. This campaign can then be repeated by selecting a new target orientation.

In the description of Fig. 3, it was assumed that the standard was first placed in its absorbing state to perform a first backscattering measurement, , then in its active state, to carry out a first backscattering measurement, at this same frequency. Of course, the person skilled in the art will understand that the order of this sequence is indifferent and that one can start by placing the standard first in its active state before switching it to its absorbing state.

Alternatively, the measurement campaign can be carried out by placing the standard first in its absorbing state, and selecting the different frequencies in turn. to measure backscattering coefficients at these frequencies, then switching the standard to its active state, and again selecting the frequencies in turn to measure backscattering coefficients . Of course, here again, the order of selection of the absorbing state and the active state of the standard matters little.

According to yet another variant, the state-switching standard is left permanently in the anechoic chamber, step 310 then consists of measuring the backscattering coefficient of the chamber comprising the standard in its absorbent state, namely , at frequencies . In step 320, only the target is introduced into the chamber and the measurement campaign takes place as described previously in relation to steps 330-370. However, in step 380, the calculation of the monostatic SER of the target is this time obtained by ignoring the anechoic chamber response including the standard (in its absorbing state), namely:

Finally, it should be noted that, whatever the variant considered, when measuring the backscattering coefficient of the standard in its active state in the presence of the target in the chamber, , in step 370, the orientation and position of the standard can be verified by means of the on-board positioning and/or attitude determination device, this verification being able to take place immediately before or after the measurement to validate it.

Claims (9)

Standard for calibrating a radar equivalent surface measurement system in an anechoic chamber, characterized in that it is in the form of a hollow metal sphere coated with a layer of electromagnetic absorber in the measurement frequency band, the wall of the sphere comprising a first recess in which a horn antenna is housed and a second recess in which an access hatch is provided for accessing the interior of the sphere, said access hatch being made of or covered by said electromagnetic absorber, the hollow metal sphere carrying a processing circuit comprising a duplexer (210) connected by its common port to the horn antenna (220), a low-noise amplifier (230) for amplifying the signal received from the reception port of the duplexer, a phase shifter (240) for phase-shifting the signal thus amplified and an attenuator (250) for attenuating the signal thus phase-shifted, the signal thus attenuated being supplied to the transmission port of the duplexer. duplexer, the attenuator (250) and the phase shifter (240) being programmable by the measurement system, the attenuation being able to be programmed to a maximum value to place the standard in an absorbing state, the standard being in an active state otherwise. Standard for calibrating a radar equivalent surface measurement system in an anechoic chamber, characterized in that it is in the form of a hollow metal sphere coated with a layer of electromagnetic absorber in the measurement frequency band, the wall of the sphere comprising a first recess in which a dual-polarization horn antenna is housed and a second recess in which an access hatch is provided for accessing the interior of the sphere, said access hatch being made of or covered by said electromagnetic absorber, the hollow metal sphere carrying a processing circuit comprising: a first duplexer (211) whose common port is connected to a first port of the antenna, associated with a vertical polarization; a second duplexer (212) whose common port is connected to a second port of the antenna, associated with a horizontal polarization; a first quadrature coupler (215) receiving on its non-phase-shifted input the signal from the reception port of the first duplexer and on its quadrature input the signal from the reception port of the second duplexer; a low noise amplifier (230) for amplifying the signal received from the receiving port of the duplexer, a phase shifter (240) for phase shifting the signal thus amplified and an attenuator (250) for attenuating the signal thus phase shifted, the signal thus attenuated being supplied to the input port of a second quadrature coupler (216) whose non-phase shifted output port is connected to the transmit port of the second duplexer (212) and whose quadrature output port is connected to the transmit port of the first duplexer (211), the attenuator (250) and the phase shifter (260) being programmable by the measurement system, the attenuation being programmable to a maximum value to place the standard in an absorbing state, the standard being in an active state otherwise. Standard for calibrating a radar equivalent surface measurement system in an anechoic chamber according to claim 1 or 2, characterized in that the hollow sphere carries an inertial unit. Method for measuring the radar equivalent surface of a target in an anechoic chamber, by means of a measurement system comprising a measurement antenna coupled to a network analyzer, said measurement antenna being intended to emit a wave in the direction of the target and to receive a backscattered wave, the measurement being carried out at a plurality of frequencies , of a frequency band of interest, the network analyzer being adapted to measure the backscattering coefficients for each frequency of said plurality from the complex amplitude of the emitted wave and the complex amplitude of the received wave, characterized in that the backscattering coefficient is measured of the anechoic chamber at said plurality of frequencies (310), the target and the standard according to claim 1 or 2 are then introduced into the anechoic chamber (320), and that for each frequency of said plurality (330) the standard is placed in the absorbing state (340) and the backscattering coefficient of the target in the chamber is measured (350) in the presence of the standard in the absorbing state, , then the standard is switched to its active state (360) and the backscattering coefficient of the standard in its active state is measured (370) in the presence of the target in the chamber, , the measurement system deducing (380) the monostatic SER of the target from the backscattering coefficients , , . Method for measuring the radar equivalent surface of a target in an anechoic chamber according to claim 4, characterized in that the monostatic SER, , of the target is obtained by means of , Or is the monostatic SER associated with the active backscattering component of the state-switching standard, obtained by calibration with a passive standard. Method for measuring the radar equivalent surface of a target in an anechoic chamber, by means of a measurement system comprising a measurement antenna coupled to a network analyzer, said measurement antenna being intended to emit a wave in the direction of the target and to receive a backscattered wave, the measurement being carried out at a plurality of frequencies , of a frequency band of interest, the network analyzer being adapted to measure the backscattering coefficients for each frequency of said plurality from the complex amplitude of the emitted wave and the complex amplitude of the received wave, characterized in that the backscattering coefficient is measured of the anechoic chamber including the standard in its absorbing state, at said plurality of frequencies, then that the target is introduced into the anechoic chamber, and that for each frequency of said plurality (330) the standard is placed in the absorbing state (340) and the backscattering coefficient of the target in the chamber is measured (350) in the presence of the standard in the absorbing state, , then the standard is switched to its active state (360) and the backscattering coefficient of the standard in its active state is measured (370) in the presence of the target in the chamber, , the measurement system deducing (380) the monostatic SER of the target from the backscattering coefficients , , . Method for measuring the radar equivalent surface of a target in an anechoic chamber according to claim 6, characterized in that the monostatic SER, , of the target is obtained by means of , Or is the monostatic SER associated with the active backscattering component of the state-switching standard, obtained by calibration with a passive standard. Method for measuring the radar equivalent surface of a target in an anechoic chamber according to one of claims 4 to 7, characterized in that the backscattering coefficient of the target is measured twice, for a plurality of orientations of said target, a first time with the standard in its active state and another time with the standard in its absorbent state. Method for measuring the radar equivalent surface of a target in an anechoic chamber according to one of claims 4 to 8, characterized in that, during a measurement (370) of the backscattering coefficient of the standard in its active state in the presence of the target in the chamber, , the orientation of said standard is verified by means of an inertial unit embedded in the hollow sphere.