Improved method for far-field calibration of a network antenna and calibrated network antenna

Abstract

Improved method for far-field calibration of a network antenna and calibrated network antenna. The calibration method comprises the steps of: - simulating (210) an surrounded radiation pattern for each channel of the network antenna; then, an angular range of network antenna pointing being subdivided into one or more intervals, for each interval, - balancing (240) the surrounded radiation patterns of each channel so as to determine a correction factor so that a dispersion on the surrounded radiation patterns is minimized; - measuring (110) for each channel an overall error affecting an illumination law in transmission and / or reception; - correcting (120) the overall error measured with the correction factor of the channel considered, so as to obtain a corrected error; and, - storing (130) the corrected error for each channel in the calibration table of the network antenna pointing calculator.Figure for the abbreviation: Figure 2.

Description

Title of the invention: Improved method for far-field calibration of an array antenna and calibrated array antenna

[0001] The invention relates to the technical field of active electronically scanned array antennas - AESA (Active Electronically Scanned Array), and more particularly to the calibration methods of such AESA antennas.

[0002] In order for an AESA antenna to meet the desired radioelectric performance, it is necessary to identify and compensate for errors in the complex illumination law (i.e. in amplitude and phase) of the radiating array.

[0003] These errors can be classified into different categories:

[0004] - an error on the "conducted signal" resulting from a manufacturing dispersion of the different paths (dispersions in mechanical realization, dispersions in the electrical characteristics of the components used, effects of so-called internal electromagnetic couplings between components, cascading of components creating a combination of mismatch, etc.)

[0005] - an error in the "radiated signal" resulting from coupling effects external electromagnetic fields between the channels.

[0006] - an error in "temperature" resulting from a change in the behavior of the components and pathways with temperature.

[0007] - an error in "frequency" resulting from a change in the behavior of elements and pathways with frequency; and,

[0008] - an "aging" error resulting from an evolution of the characteristics of components over time.

[0009] With regard more specifically to the error on the "radiated signal," the design of an AESA antenna involves an initial phase in which the radiating array is considered infinite, that is, comprising an infinite number of radiating elements. This allows for the optimization of the matching of the radiating elements over a given frequency band and angular range. The elementary radiation pattern of each channel is therefore identical from one channel to the other of the antenna.

[0010] However, the AESA antenna actually manufactured is a finite radiating array, that is to say comprising a finite number of radiating elements.

[0011] Due to electromagnetic coupling, each channel of the finite radiating array then exhibits a specific radiation pattern, which depends on its environment, in the presence of all other antenna channels. This phenomenon is known as "active" or "surrounded" diagrams.

[0012] In particular, edge effects are observed (i.e., when the radiating element considered is located near the edge of the network) which cause distortions of the elementary radiation pattern of a channel, both in amplitude and in phase, compared to the ideal behavior in an infinite radiating network.

[0013] In a finite radiating network, each elementary diagram undergoes different deformations, possibly up to symmetries.

[0014] The phenomenon of "surrounded" radiation patterns is all the more critical when the array antenna is small (measured in wavelength). This phenomenon is also present between sub-array radiation patterns when several radiating elements are associated with an active module.

[0015] These distortions of the elementary radiation patterns degrade the illumination law of the array, particularly in phase in the accuracy of pointing the antenna and in amplitude in the relatively high level of "diffuse" (i.e. the gain of the antenna beyond the main and secondary lobes).

[0016] The “calibration” (or more precisely the “calibration”) of an AESA antenna consists of measuring the complex electrical path errors (i.e. in amplitude and phase) between each channel of the AESA antenna.

[0017] In the remainder of this document, the case of calibration in reception will be presented in detail (the calibration probe emits and the antenna receives the wave emitted by the probe), but the case of calibration in transmission is quite similar (the antenna emits and the calibration probe receives the wave emitted by the antenna).

[0018] The commands applied to the controllable components of each channel in order to modify, both in transmission and reception, the amplitude and phase of the signal, transmitted or received, by the associated radiating elements, must take these deviations into account, in particular in order to compensate for amplitude dispersion to avoid excessive scatter feedback and / or to compensate for phase dispersion to avoid excessive native axis deviation

[0019] According to the prior art, the calibration method for an AESA antenna consists of:

[0020] - place the antenna in a mechanical reference position with controls of amplitude and reference phase on each active channel; then,

[0021] - to perform measurements using a probe disposed along the axis normal to the radiating surface of the antenna and passing through its center, in order to determine, for each channel, amplitude and phase biases, respectively Cal_RX_A_i and Cal_RX_P_i, that is to say the amplitude and phase deviations on each channel relative to the theoretical expected value; and finally,

[0022] - store the measured errors for the considered frequency in a table of calibration.

[0023] These errors are then compensated by the pointing calculator when using the ASEA antenna at the working frequency, by applying an appropriate phase shift and gain between network channels, in transmission or reception, in order to depoint the radiated beam in the desired direction, with the desired beam shape and secondary lobe topology.

[0024] The pointing calculator calculates a target setting to compensate for these errors.

[0025] Thus, the measurement during calibration of the transfer function of each channel i, Cal_RX_A_i and Cal_RX_P_i, provides input to the pointing computer, which deduces adjustment targets for each channel in amplitude and phase, Obj_RX_A_i and Obj_RX_P_i:

[0026] Obj_RX_A_i = E_RX_A_i - Cal_RX_A_i + Corr_T_RX_A_i * (T-T20)

[0027] Obj_RX_P_i = E_RX_P_i - Cal_RX_P_i + Corr_T_RX_P_i * (T-T20) + Phi_i

[0028] where:

[0029] - E_RX_A_i and E_RX_P_i correspond to the illumination law in reception in amplitude and phase (understood as the theoretical adjustment objective allowing to obtain a radiation pattern with the desired characteristics);

[0030] - Corr_T_RX_A_i and Corr_T_P_i correspond to a temperature correction at to provide for the current temperature T, linked to the deviation of the known value at temperature T20; and,

[0031] - Phi_i is the phase shift to be introduced in order to point the antenna beam into a desired direction.

[0032] The values ​​of these different terms may depend on the operating frequency and / or aging.

[0033] In a manner known per se, the calibration of an AESA antenna can be carried out either "in the near field" or in the "far field":

[0034] In the "near-field" approach, the probe, which is close to the antenna, is moved in front of each channel of the antenna in order to perform measurements to characterize each channel. This is the method generally used for large antennas with a large number of channels, such as, for example, a forward-facing radar antenna on a fighter jet.

[0035] In the "far-field" approach, the probe is positioned far from the antenna. This approach is the method generally used for small antennas, expressed in wavelengths.

[0036] The "far-field" approach can be carried out either outdoors or inside an anechoic chamber (while respecting the far-field distance defined by the Fraunhofer criterion), or even in a small room (or compact room) by using a parabolic reflector to simulate an electromagnetic environment corresponding to a very distant area.

[0037] In the "near-field" approach, since the probe is close to the active radiating element, the antenna's radiation pattern is not formed, and only the conducted signal is considered to be measured. This measurement is therefore unaffected by the error in the radiated signal caused by electromagnetic coupling effects between channels.

[0038] For AESA antennas whose behavior is close to that of an infinite array, i.e., AESA antennas with a very large number of active channels (several hundred) and an amplitude-weighted irradiance law to obtain low-level sidelobes, the channels can be considered to behave identically (i.e., like a "central" source). Consequently, the impact of the difference in behavior of the channels located at the periphery of the array is considered small. This assumption is justified, firstly, because these peripheral channels are few in number relative to all the antenna channels, and secondly, because they are strongly amplitude-weighted.

[0039] In the far-field approach, the probe is far from the AESA antenna to be characterized, i.e., in a region where the antenna's radiation pattern is formed. The measurement performed by the probe is therefore affected by coupling effects between channels. Thus, in the far-field approach, the measurement is affected by the conducted signal error and the radiated signal error for each channel, but without being able to distinguish between these two contributions.

[0040] If a conducted signal error is substantially constant over all AESA antenna pointing directions, a radiated signal error varies according to the pointing direction.

[0041] The prior art calibration process therefore leads to a non-optimal adjustment of the AESA antenna, resulting in a high level of diffuse radiation and / or an error in the pointing direction.

[0042] The problem to be solved is therefore to propose a far-field calibration method that allows us to distinguish between the conducted signal error and the radiated signal error in order to take them into account appropriately, in particular by compensating for their effects according to the pointing direction.

[0043] It should be noted that in the "far-field" calibration in a "box," with the probe located in an intermediate zone, a detailed electromagnetic simulation is used to extract the conducted signal error and the radiated signal error. The results are then extrapolated to determine the far-field radiation behavior. with new complex electromagnetic simulations. However, some simplifications are possible for large-channel array antennas.

[0044] The aim of this invention is to solve this problem.

[0045] To this end, the invention relates to a far-field calibration method for an array antenna of the type active electronically scanned array antenna, the array antenna comprising a plurality of channels, each channel combining a transmit-receive module and at least one radiating element, the antenna also comprising a pointing computer adapted to individually control each transmit-receive module, the calibration method comprising the steps of:

[0046] - to simulate, by means of an electromagnetic simulation of the antenna, a diagram of surrounding radiation for each channel of the array antenna;

[0047] then, an angular range for the pointing of the network antenna being subdivided into one or more intervals, for each interval,

[0048] - to balance, over the interval considered, the radiation diagrams surrounded by each channel of the array antenna in such a way as to determine a correction factor for each channel so that dispersion on the surrounding radiation patterns is minimized over the interval considered;

[0049] - measure, over the interval considered, for each channel of the network antenna, an error global affecting an illumination law in transmission and / or reception of the channel in question;

[0050] - correct the overall measured error with the correction factor of the channel considered, so as to obtain, over the interval considered, a corrected error for each channel of the network antenna; and,

[0051] - to store, for the interval considered, the corrected error for each channel of the network antenna in the calibration table of the network antenna pointing calculator.

[0052] According to particular embodiments, the calibration process comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0053] - the step of balancing the surrounding radiation patterns consists of to transform each surrounded radiation pattern in order to minimize a section of an envelope containing all the transformed surrounded radiation patterns;

[0054] - the transformation is the application of a gain to the radiation pattern in the environment considered, the correction factor corresponds to the gain value for the minimum dispersion;

[0055] - a channel comprising a single radiating element, each diagram of surrounded radiation is a diagram of surrounded elementary radiation;

[0056] - a path comprising a plurality of radiating elements so as to define a sub-array in the array antenna, each surrounded radiation pattern is a surrounded sub-array radiation pattern.

[0057] The invention also relates to an array antenna of the type active array antenna with electronic beam scanning, calibrated by implementing a calibration method as defined above.

[0058] The invention and its advantages will be better understood upon reading the following detailed description of a particular embodiment, given solely by way of non-limiting example, this description being made with reference to the accompanying drawings in which:

[0059] [Fig-1] The [Fig.1] is a schematic representation of an AESA antenna;

[0060] [Fig.2] Fig.2 is a block representation of the process of calibration of the antenna of [Fig. 1]; and,

[0061] [Fig.3] Fig.3 represents three graphs respectively of the superposition of the elementary radiation patterns of the receiving antenna channels with the implementation of a calibration method according to the prior art, of the superposition of the elementary radiation patterns of the receiving antenna channels with the implementation of the calibration method according to the invention, and the amplitude of the antenna radiation pattern on the one hand by implementing a calibration method according to the prior art and on the other hand by implementing the calibration method according to the invention.

[0062] The present invention relates to a method for calibrating an AESA antenna.

[0063] Fig. 1 schematically represents an AESA antenna 1.

[0064] It comprises a plurality of N paths. For example, in [Fig.1], three paths are shown. They are referenced 10, 20 and 30 in [Fig.1].

[0065] The different paths are similar to each other.

[0066] Taking the example of channel 10, a channel includes a transmission-reception module, or TR module, 11 associated with a radiating element 12. The latter protrudes beyond the front face of the antenna 1, which is made up of a metallic plane 40.

[0067] Module TR 11 includes, for example:

[0068] - a switch 13 for selecting the transmission channel or the channel of reception ;

[0069] - a circulator 16 connected on the one hand to the transmission and reception chains and on the other hand to the radiating element;

[0070] - along the emission chain, from the switch 13 to the circulator 16, a phase shifter 14 followed by an amplifier 15;

[0071] - along the receiving chain, from the circulator 16 to the switch 13, a the amplifier 17 followed by a phase shifter 18;

[0072] - finally, a controllable device 19 for modifying the phase and / or the gain which are introduced along the transmission chain, respectively the reception chain, by the phase shifter 14, respectively the phase shifter 18.

[0073] A distributor circuit 50 ensures, in transmission, the distribution of the signal to each of the TR modules and, in reception, the summation of the signals coming from the TR modules.

[0074] The antenna also includes a pointing calculator 60. The latter stores a calibration table 65. When using the AESA antenna, the pointing calculator is designed to calculate a target per channel and to control the device 19 of this channel to introduce, in reception and / or transmission, a phase shift and / or an appropriate gain.

[0075] Axis A is orthogonal to plane 40. By adjusting the phase and gain of the different channels, both in transmission and reception, the antenna beam can point in a pointing direction D making a pointing angle 0 with respect to axis A.

[0076] With reference to [Fig.2], a preferred embodiment of the "far field" calibration method for tuning the AESA antenna 1 will now be presented.

[0077] The process 100 comprises a calibration phase 101 and a correction determination phase 201.

[0078] Phase 201 begins with an electromagnetic simulation step 210.

[0079] This step involves the implementation of electromagnetic simulation software. This software is used to simulate the elementary radiation pattern of each channel of the AESA antenna 1.

[0080] For this simulation, the AESA antenna is considered in its real geometry, i.e. finite.

[0081] For each channel of the antenna, the theoretical elementary radiation pattern of that channel (only active channel) is simulated in reception, by loading all the other channels of the antenna (non-active channels) with a characteristic impedance.

[0082] This characteristic impedance is preferably that of the circulator 16 of the TR 11 module of the AESA antenna channels.

[0083] Alternatively, the characteristic impedance is a predefined value, for example 50 Ohms.

[0084] Step 210 allows obtaining a plurality of N simulated elementary radiation diagrams.

[0085] The electromagnetic simulation is only interested in Fonde received by the radiating element of the active channel, taking into account its electromagnetic environment.

[0086] It therefore simulates the radiated contribution.

[0087] On the other hand, it is not interested in the contribution conducted.

[0088] The simulated elementary radiation diagrams therefore exhibit a dispersion that is solely due to a radiated contribution.

[0089] Step 210 therefore makes it possible to obtain a simulated radiated contribution.

[0090] Once the various theoretical elementary radiation diagrams have been obtained by simulation, the process 100 advantageously includes a step 230.

[0091] For step 230, the angular range of beam pointing of antenna 1 is subdivided into a plurality of intervals, preferably maintaining symmetry with respect to 0=0.

[0092] For example, the range -0max+0max is divided into three intervals -0max-0max / 3, -0max / 3 +0max / 3, and +0 max / 3 +0 max.

[0093] Step 230 then consists of selecting an interval.

[0094] For the selected interval, in step 240, an amplitude balancing of the theoretical elementary radiation diagrams is carried out.

[0095] The relative level of the diagrams is balanced in order to reduce the dispersion between channels.

[0096] For example, this balancing is achieved by applying a transformation to each elementary radiation diagram in order to minimize the cross-section of an envelope containing, over the interval considered, all the surrounded elementary radiation diagrams thus transformed.

[0097] The transformation of a diagram is, for example, the application of a gain on that diagram.

[0098] This optimization can be done at one point of the interval (for example the midpoint), at a plurality of points of the interval, or over the entire length of this interval.

[0099] Each diagram is thus corrected by an amplitude correction factor. This factor is denoted Corr_RX_A_i_0.

[0100] Finally, in step 250, the amplitude correction factors determined in step 240 for each channel i of the antenna over the selected interval at 0 are stored.

[0101] Steps 230, 240 and 250 are iterated over the next interval of the depointing range.

[0102] By symmetry, a similar correction factor can be used on two intervals symmetrical to each other with respect to the zero angle, corresponding to a direction of pointing along the axis A.

[0103] Phase 201 is completed when correction factors have been determined on each subdivision interval of the angular range.

[0104] The calibration phase 101 begins with a step 110 of far-field measurement of the calibration coefficient in reception Cal_RX_A_i_0 of each of the antenna channels.

[0105] This measure therefore includes both the conducted contribution and the radiated contribution.

[0106] Preferably, each subdivision interval of the pointing range is considered successively.

[0107] The probe is placed at an angle 0 with respect to the axis A of the antenna, the angle 0 belonging to the interval considered.

[0108] One channel of the antenna is activated, while the other channels remain inactive.

[0109] A measurement is performed by activating the probe in transmit mode and measuring the signal received via the activated channel.

[0110] This measurement is denoted Cal_RX_A_i_0 where i is an integer indexing the channels (i between 1 and N).

[0111] This measurement is iterated by successively activating the different channels so as to obtain at least one measurement for each of the antenna channels for the interval considered.

[0112] In a step 120, the emission calibration coefficient Cal_RX_A_i_0 of channel i is corrected with the result of the balancing step 240 for the same pointing interval.

[0113] For example, for the different intervals of the antenna's pointing range and for the different channels, we calculate:

[0114] CalCor_RX_A_i_0 = Cal_RX_A_i_0 - Corr_RX_A_i_0

[0115] In a step 130, the corrected calibration coefficients CalCor_RX_A_i_0 are stored in the calibration table 65.

[0116] Finally, following calibration procedure 100, during phase 300 of antenna A operation, when it is desired to shift the antenna's transmission direction by an angle 0, for each channel i of the antenna, an amplitude setpoint Obj_Amp_i_0 is calculated from the corrected calibration coefficients CalCorr_RX_Amp_i_0 present in the calibration table. This setpoint is applied to the adjustable element 19 of module TR 11 of channel i of the AESA antenna.

[0117] Obj_RX_A_i_0 = E_RX_A_i_0 - CalCorr_RX_A_i_ 0 + Corr_T_RX_A_i * (T-T20)

[0118] By implementing a calibration process similar to that just described, a target amplitude in transmission is determined for the use of the antenna.

[0119] Figure 3A represents the superposition of the elementary radiation patterns of an AESA antenna (or more precisely of a sampling of the elementary radiation patterns taking into account the existence of top / bottom and left / right symmetries on the surface of the antenna considered) after the implementation of a calibration method according to the state of the art.

[0120] The dispersion between the levels of the elementary radiation diagrams can reach a high amplitude out of the normal direction, whereas it has been greatly reduced, for example canceled (all the curves intersecting at the same point), by calibration in a single direction, for example the normal direction.

[0121] Consequently, there remains a significant degradation of the illumination law, particularly at high pointing distances, a degradation induced by the significant imbalance of the different diagrams of the active channels.

[0122] As shown in curve Cl of Figure 3C, for a given non-zero angular offset, this leads to a high level of scatter (the lobes of the radiation pattern far from the beam pointing axis are significant). Such a level of scatter is detrimental to the proper functioning of the antenna.

[0123] Figure 3B represents the superposition of the elementary radiation patterns of an AESA antenna (or more precisely of a sampling of the elementary radiation patterns taking into account the existence of top / bottom and left / right symmetries on the surface of the antenna considered) after the implementation of the calibration method according to the invention.

[0124] It is observed that the dispersion between the levels of the elementary radiation diagrams is balanced over the entire pointing range.

[0125] As shown on curve C2 in Figure 3C, the improvement in diffuse is substantial, better than 5 dB on the maximum diffuse level.

[0126] The level of the secondary lobes is also reduced, although not as significantly.

[0127] We have previously focused on taking into account the amplitude dispersion of the elementary radiation diagrams with the objective of reducing the level of diffuse radiation.

[0128] Alternatively or in combination, a similar calibration process can be applied for the phase, in order to restrict the error on the pointing direction.

[0129] The simulation and / or measurement of the transfer function of each channel as a function of the pointing direction provides input for the pointing algorithm, which deduces Obj_RX_P_i_0 adjustment objectives:

[0130] Obj_RX_P_i_0 = E_RX_P_i_0 - CalCorr_RX_P_i_0 + Corr_T_RX_P_i * (T-T20) + Phi_i

[0131] Here again, instead of a receiving target, a transmitting target can, following a similar calibration procedure, be calculated during the use of the antenna.

[0132] Correcting the phase, in particular on the edges of the pointing domain, makes it possible to reduce the pointing error to less than 3 mrad.

[0133] Systematic correction is only of interest if the error that one seeks to compensate is significantly greater than the quantization error of the phase shifters of the TR modules of the channels.

[0134] Unlike near-field calibration, far-field calibration de facto includes radiated signal errors, therefore the effects of dispersion of the elementary diagrams, and conducted signal errors, therefore the manufacturing dispersion effects between the different channels.

[0135] With the calibration method according to the invention, the radiated signal error is simulated, which then allows the overall measured error to be corrected. It is this corrected overall error that is then used for defining the objectives when using the antenna.

[0136] Alternatively, a TR module can be associated with more than one radiating element, thus defining a sub-array in the AESA antenna. Each sector is equipped with an additional distribution circuit between the TR module and the various radiating elements. In other words, a channel comprises a plurality of radiating elements. In this case, it is the diagram of the plurality of radiating elements associated with a channel, or sub-array diagram, that is measured during far-field calibration and adjusted during calibration, and no longer the elementary diagram of each radiating element, since when a channel is activated, all the radiating elements of the sub-array are operational and the elementary diagrams are not individually accessible for measurement. The method according to the invention then applies, but at the level of a sub-array.Optionally, the simulation step may involve the simultaneous simulation of all radiating elements of the active path to determine the sub-network diagram, or the simulation of each radiating element of the active path, before combining these simulated elementary diagrams to obtain the sub-network diagram.

[0137] The different sub-arrays of the same AESA antenna may have a geometry and group a number of channels which are not necessarily identical, but generally exhibit at least a top / bottom and left / right symmetry in front view of the radiating plane of the AESA antenna.

[0138] The present method is of particular interest for antennas of small dimensions (measured in wavelength) and with a reduced number of channels, i.e. for AESA antennas whose behavior is highly sensitive to the phenomenon of "active" diagrams, insofar as a small AESA antenna has a proportionally larger number of peripheral radiating elements and that these peripheral radiating elements are subject to edge effects and different mutual couplings.

[0139] This is further reinforced in the case of AESA antennas made up of sub-arrays each grouping several radiating elements, with a unique tuning component per sub-array.

[0140] The present invention finds application in the field of radars (combat or surveillance), in the field of missile seekers, in the field of telecommunications (in receiving configuration), or even in the field of electronic warfare.

Claims

Demands

1. Method (100) of far-field calibration of an array antenna of the type active array antenna with electronic beam scanning, the array antenna comprising a plurality of channels, a channel associating a transmit-receive module and at least one radiating element, the antenna also comprising a pointing computer adapted to individually control each transmit-receive module, the calibration method being characterized in that it comprises the steps of: - simulating (210), in the far field, by means of an electromagnetic simulation of the antenna, an surrounded radiation pattern for each channel of the array antenna;then, an angular range of array antenna pointing being subdivided into one or more intervals, for each interval, - balance (240), over the interval considered, the surrounding radiation patterns of each channel of the array antenna so as to determine a correction factor for each channel so that a dispersion on the surrounding radiation patterns is minimized over the interval considered, the correction factor corresponding to a radiated signal error; - measure (110), in the far field, over the interval considered, for each channel of the array antenna, a global error affecting an illumination law in transmission and / or reception of the channel considered; - correct (120) the global error measured with the correction factor of the channel considered, so as to obtain, over the interval considered, a corrected error for each channel of the array antenna, the corrected error corresponding to a conducted signal error;and, - memorize (130), for the interval considered, the corrected error for each channel of the network antenna in the calibration table of the network antenna pointing calculator.;

2. A method according to claim 1, wherein the step of balancing the surrounded radiation patterns consists of transforming each surrounded radiation pattern to minimize a section of an envelope containing all the transformed surrounded radiation patterns.

3. A method according to claim 2, wherein the transformation is the application of a gain to the considered surrounded radiation pattern, the correction factor corresponding to the value of the gain for the minimum dispersion.

4. A method according to any one of claims 1 to 3, wherein, a channel comprising a single radiating element, each surrounded radiation pattern is a surrounded elementary radiation pattern.

5. A method according to any one of claims 1 to 3, wherein, a channel comprising a plurality of radiating elements so as to define a sub-array in the antenna array, each surrounded radiation pattern is a surrounded sub-array radiation pattern.

6. Array antenna (1) of the type active array antenna with electronic beam scanning, characterized in that it is calibrated by implementing a calibration method in accordance with any one of the preceding claims.

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