Method for calibrating an antenna, pointer and antenna

Abstract

A calibration method for an electronically scanned array antenna (A) comprising channels (Vi), each comprising a radiating element (Ei) and capable of operating in a set of contiguous sub-bands forming an overall frequency band, each sub-band comprising a center frequency, the calibration method comprising the following step: - Generating phase-shift commands intended to be applied to the respective channels, in operational phase in a transmit or receive mode, so that the antenna points in a predetermined pointing direction to a first frequency belonging to a first sub-band of the set of sub-bands, the phase-shift commands being defined, for each channel (Vi), by the difference between a theoretical insertion phase of the channel, in the operating mode,and an overall phase shift being the sum of a channel insertion phase in the operating mode at the center frequency of the first frequency sub-band, and a transit phase shift introduced by a transit time, between the center frequency of the first frequency sub-band and a reference center frequency, in the operating mode, when the reference center frequency does not belong to the first frequency sub-band.

Description

[0001] The invention relates to the field of multi-channel array antennas, each channel comprising a radiating element. It concerns, in particular, electromagnetic antennas of the active electronically scanned type, used notably for equipping radars or seekers.The field of application is that of antennas capable of operating in a narrowband mode on different sub-bands of limited frequency extent, commonly called "instantaneous frequency bands", contiguous having the same reduced width in frequency and allowing a classic operating mode of the moving target visualization or MTI (from the Anglo-Saxon "Moving target indicator") type and a so-called wideband operating mode, i.e. frequency scanning over a wide frequency band including several of the frequency sub-bands, for example, for a synthetic aperture radar SAR (from the Anglo-Saxon "Synthetic aperture radar") or for a pulse compression radar.

[0002] In narrowband mode, only measurements obtained in one of the frequency sub-bands are used to form an image, without using measurements obtained in the other frequency sub-bands.

[0003] In broadband mode, a frequency scan is performed across a wide range of frequencies. Measurements taken at different frequencies within different sub-bands are used to calculate the distance to a target and ultimately to create a single radar image. Using measurements obtained by scanning a wide frequency range increases the range resolution.

[0004] The invention relates to the calibration of such antennas. The article "Internal calibration and range replica extraction scheme for ultra-high resolution spaceborne SAR", Feng Fan et al., 2019, 6th Asia-Pacific conference on synthetic aperture radar (APSAR), IEEE, November 26, 2019, pages 1 to 4; discloses a method for calibrating a radar antenna.

[0005] In narrowband operation, such as in MTI radar, a broadband antenna with an overall frequency of, say, around 500 MHz operates in instantaneous frequency bands (or frequency sub-bands), each with a span of, say, a few tens of MHz, for example, 50 MHz. A phase shift command is applied to each antenna channel to point the antenna in a specific direction, forming a specific angle with a reference direction linked to the network, at a specific frequency within one of the frequency sub-bands. The phase shift command applied to each channel is the same regardless of the operating frequency within the considered frequency sub-band.

[0006] Thus, for a linear array with N channels of order i, each comprising a radiating element of order i, and in which the N radiating elements of order i are regularly distributed along a straight line and spaced two by two by a distance d, it is known, for example, that for an antenna pointed along a pointing direction defined by an angle θ₀, formed with the normal to the line on which the radiating elements are arranged, at the center frequency Fc j of a frequency sub-band ΔF j, phase shifts φ1 ij, defined as follows, must be introduced by the different channels of order i, on an input signal Si injected at the input of the channel so that the antenna points in the predetermined pointing direction, the signals Si injected at the input of the respective channels having the same phase, in transmission or reception: φ 1 ij = 2 ∗ π ∗ x i ∗ sin θ 0 / λ c j Or x i = (i - 1)*d is the position of the radiating element along the linear lattice, and where the wavelength λ c j of the beam at the center frequency Fc j of the sub-band ΔF j is given by: λc j = c / Fc j where it is the speed of light.

[0007] A calibration operation is performed to balance the N channels of the antenna network in phase in order to correct the phase differences introduced between the different channels due to differences between the N transmitting or receiving chains.

[0008] For zero angular pointing (θ 0 = 0°) at the center frequency Fc j Within the sub-band ΔF j, the N channels must be in phase. The phase shifts introduced by the different channels i are measured and stored in the computer, called the pointer computer. These phase shifts, or insertion phases, constitute a set CALj of calibration coefficients, called principal CALj i, with i = 1 to N. The principal calibration coefficient CALj i is the phase shift measured, for channel i, between the output signal of channel i and the signal injected at the input of said channel, in transmit or receive mode, at the center frequency. Fc j of the sub-band ΔF j for zero angular pointing.

[0009] A set CALj of principal calibration coefficients CALj i, with i = 1 to N, is determined for each sub-band ΔF j, that is, for j = 1 to M, where M is the number of contiguous frequency sub-bands ΔF j forming the overall frequency band. This yields a calibration table comprising the M sets CALj.

[0010] The pointer computer generates, for pointing the beam in a direction defined by the angle θ 0, for any frequency in the sub-band ΔF j, the following phase shift command φ ij addressed to channel i: φ ij = φ 1 ij − CALj i CALj i is called the main calibration command.

[0011] φ1 ij is the theoretical phase shift control applied to channel i in the sub-band ΔF j.

[0012] These phase shift commands φ ij are the phase shifts applied by respective phase shifters of the respective antenna channels V i for any operating frequency of the sub-band ΔF j.

[0013] However, we want to be able to use this antenna in broadband mode.

[0014] A known solution exists for pointing in a predetermined direction defined by a pointing angle θ₀. This solution consists of applying to the different antenna channels, for any frequency within the broadband to be covered, the phase shift commands determined, for this angle θ₀, at the center frequency Fcc of the broadband to be covered. In other words, the phase shift commands are adjusted using the calibration set defined for the angle θ₀, at the center frequency Fcc of the center frequency band ΔFc of the broadband to be covered.

[0015] However, this solution leads to a rapid degradation of the antenna's radiation pattern when the frequency moves outside the central sub-band ΔFc, and a variation in the beam pointing angle as the frequency deviates from the central frequency Fcc. At the edge of the broad frequency band to be covered, the quality of the radiation pattern is insufficient to ensure good radar image quality if measurements taken at this frequency are to be used to create the image.

[0016] A second solution involves replacing the phase shifters used for angular scanning of the radiated beam with programmable length lines (PLLs), which introduce time delays (phase shifts) between the different channels. The radiated beam then remains angularly fixed with the frequency. The main drawback of this solution is the size of the LLPs, which depends on the antenna dimensions and the maximum beam pointing angle. Another limitation is the ohmic losses inherent in this type of component. Finally, this second solution does not allow for increasing the capabilities of an existing antenna usable in MTI mode to enable its use in broadband mode without significant modifications to its architecture.

[0017] One aim of the present invention is to limit at least one of the aforementioned disadvantages.

[0018] In connection with the present invention, the applicant has made the findings set forth below.

[0019] In the following text, the insertion phase of a channel refers to the phase shift introduced by a channel between an input signal and an output signal. This is the phase shift between the channel's excitation signal and the signal radiated by the channel in transmit mode, and, in receive mode, between the signal transmitted to the channel and the signal measured at the channel's output (i.e., a signal generated at the channel's output). The signal radiated by the antenna is measured at any predetermined point in the environment, and the signal transmitted to the channel is transmitted from any predetermined point in the environment.To compare the insertion phases of the same channel at different frequencies, in transmit or receive mode, during a calibration measurement phase, the signal radiated by the antenna at the same point in the environment is measured at different frequencies from the same input signal, or conversely, the same signal is transmitted to the channel from the same point in the environment at different frequencies. To compare the insertion phases at the same frequency between different channels, during a calibration measurement phase, the signal radiated by the respective channels is measured at identical relative positions with respect to their respective antenna channels, with the antenna input signal being the same, or conversely, the same signal is transmitted to the respective channels from identical relative positions with respect to their respective antenna channels, and the output signal of the respective channels is measured.

[0020] One solution, in order to limit the problems related to the use, for all frequencies of the broadband frequency, of the main calibration coefficients defined at the center frequency Fcc of the center frequency band ΔFc, could be to use the different calibration coefficients CALj i defined previously.

[0021] In this case, the pointer computer would use, for pointing the beam in a direction defined by an angle θ 0, for any frequency of the sub-band ΔF j, the following phase shift command φ ij addressed to the radiating element i: φ ij = φ 1 ij − CALj i

[0022] In the adjacent upper sub-band ΔF j+1, pointing in the same direction would be obtained with different initial pointing commands corresponding to the frequency Fc j+1 = Fc j + ΔF: φ ij + 1 = φ 1 ij + 1 − CAL j + 1 i

[0023] It would not be possible to form high-resolution radar images from signals measured in separate sub-bands because there would be no coherence of the signals and therefore no phase continuity between the sub-bands.

[0024] Indeed, the operation of phasing the different channels, that is to say the operation of adjusting the phase shifts to avoid the introduction, between the different channels, of phase shifts other than those which must be introduced to make the antenna point in the desired direction consists, to the first order, of controlling the phase shift of each channel i by a main calibration command (- CALj i ) equal to the opposite of the main calibration coefficient CALj i (or phase shift) measured for channel i.

[0025] By performing such a calibration, it is assumed that the phase shift introduced by channel i does not vary with frequency in each of the sub-bands, and the opposite of the phase shift CALj i measured at the center frequency Fcj is applied in order to maintain this phase shift constant. This amounts to imposing a zero insertion phase at each center frequency Fcj.

[0026] We can see in figure 1 , on which we have represented insertion phase measurements of a channel i in the same operating mode (transmit or receive), at different frequencies fk regularly spaced over a first frequency band made up of three contiguous sub-bands, when the phase shift control is the sum of the theoretical phase shift control and the main calibration control - CALj i for j = 1 to 3. The frequencies fk are included in the three contiguous sub-bands ΔF x+1 , ΔF x+2 and ΔF x+3 . The center frequencies Fc x+1, Fcx+2, Fc x+3 of the respective sub-bands ΔF x+1, ΔF x+2, and ΔF x+3 have respective order numbers 80, 96, and 112. There are 16 frequencies per sub-band in this non-limiting example. figure 1 .

[0027] Since each CALj term was previously determined at the center frequency Fcj of each sub-band ΔFj, it can be observed that at each center frequency Fcx+1, Fcx+2, Fcx+3, the insertion phase is approximately zero. For example, in the sub-band ΔFx+1, the center frequency or calibration frequency is the frequency f80.

[0028] However, this calibration fails to account for the fact that the insertion phase measured by a measuring instrument is the sum of a phase shift independent of frequency and a phase shift related to the transit time the signal takes to propagate through the corresponding channel, i.e., related to the electrical length of the channel. If we assume that these media are not dispersive, which is the case in practice, this transit time is constant in the sense that it does not depend on the frequency. On the other hand, the transit time causes a phase shift that depends on the frequency and, more precisely, varies linearly with the frequency.

[0029] This is also confirmed by the figure 1 Indeed, we observe, on the figure 1 that in each of the sub-bands, the insertion phase varies linearly with frequency with a slope denoted δphi where δphi = Phase(fk+1) - Phase(fk), where Phase(fk) is the insertion phase at frequency fk. This phase difference is caused by the transit time.

[0030] Thus at frequency f81, the insertion phase is equal to δphi, at frequency f82, the insertion phase is equal to 2*δphi, at frequency f79, the phase is equal to -δphi.

[0031] In contrast, the insertion phase varies in a sawtooth pattern across the first frequency band, which consists of three contiguous sub-bands Fc x+1, Fcx+2, and Fc x+3. There is a phase jump at each change of frequency sub-band, that is, between two consecutive frequencies belonging to two contiguous sub-bands. The absolute value of the phase difference DF between these two consecutive frequencies is significantly greater than that of δphi. Therefore, there is no continuity of insertion phases between the different sub-bands.

[0032] This discontinuity is incompatible with a broadband application, as the measurements used to create a radar image must be frequency coherent.

[0033] However, the phase difference between two consecutive frequencies is caused solely by the transit between these two frequencies.

[0034] Therefore, there should not be a phase jump between two consecutive frequencies belonging to two contiguous sub-bands, but rather a phase difference equal to δphi. The insertion phase should have the shape shown in figure 2 on which the insertion phases are located on a straight line with a slope equal to δphi / δf where δf is the difference between two consecutive frequencies, this difference being equal to unity on the figure 1 .

[0035] Furthermore, in radar, the distance Dist is measured using propagation phase measurements phi at different frequencies f: Phi = 4 * π * f * Dist / c

[0036] Adding a phase shift due to antenna transit time only introduces a small bias into the distance measurement. However, if different transit time phase shifts are introduced at different frequencies due to a calibration error, noise is added to the phase measurements at those frequencies, making it impossible to deduce the distance with good resolution.

[0037] The present invention therefore proposes to use, in the transmission and / or reception phase, an estimation of the phase shift due to the transit time between two consecutive frequencies to adjust the phase shift commands so as to avoid the introduction of an insertion phase discontinuity when passing from one sub-band to the contiguous sub-band.

[0038] This calibration process, by restoring phase continuity between the different sub-bands, allows the antenna to be used in wideband mode.

[0039] To this end, the invention relates to a method for calibrating an electronically scanned array antenna according to claim 1.

[0040] Advantageously, the first insertion phase is measured at a center frequency of one of the sub-bands.

[0041] Advantageously, the process includes a step of correcting the first and second insertion phases by eliminating, before the step of estimating global phase shifts, insertion phases introduced by a device measuring the first insertion phase and the second insertion phase.

[0042] The invention also relates to a method for estimating a distance comprising the following steps: to transmit, using the antenna, during a transmission phase, microwave signals at several transmission frequencies of several frequency sub-bands of a first set of sub-bands; to measure, using the antenna, during a reception phase, signals received at several reception frequencies of several frequency sub-bands of a second set of sub-bands following the transmission of microwave signals; to calculate a distance from the signals received by the antenna. during the transmission and reception stages, the phase shift commands for the phase shifters of the different channels are generated by the calibration process according to the invention.

[0043] The invention also relates to a pointer for generating phase shift commands for the channels of an electronically scanned array antenna according to claim 4.

[0044] Advantageously, the pointer includes a memory storing a calibration table defined for the operating mode, the calibration table comprising, for each channel of the antenna, a set of global phase shifts defined for the center frequencies of the respective sub-bands, each global phase shift being the sum of an insertion phase of channel i, in the operating mode, at one of the center frequencies of the sub-bands and a transit phase shift introduced by a transit time between the center frequency and a reference center frequency.

[0045] The invention also relates to a pointer in which the command generation means are configured to implement the following steps during a distance estimation step: generate initial phase-shift commands for the antenna channels so that the antenna, during a transmission phase, emits microwave signals at several transmission frequencies from several sub-bands of a first set of sub-bands; generate second phase-shift commands for the antenna channels so that the antenna, during a reception phase, measures received signals at several reception frequencies from several sub-bands of a second set of frequency sub-bands. the pointer including distance estimation means configured to estimate a distance from the measured signals.

[0046] The invention also relates to an electronically scanned array antenna comprising a set of antenna channels, each comprising a radiating element, the array antenna being capable of operating in a set of contiguous frequency sub-bands of width forming an overall frequency band, the electronically scanned array antenna comprising the pointer according to the invention, the commands generated by the pointer being intended to control phase shifters of the antenna channels.

[0047] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying drawings given by way of example, which represent, respectively: There figure 1 The previously described method represents insertion phase measurements of an antenna channel, in the same operating mode, at different frequencies regularly spaced within a first frequency band consisting of three contiguous sub-bands, with the phase shift controls being set by main calibration controls; figure 2 The already described represents the shape that the channel insertion phase should have as a function of frequency; figure 3 , represents an example of the architecture of an antenna according to the invention and a measuring device; The figure 4 , schematically represents the steps of a calibration process according to the invention; The figure 5 , schematically represents, for a given channel, the phase difference measured in the near field between consecutive frequency points spaced at a constant interval, when the phases are adjusted solely by means of the principal phase shifts; The figure 6 , schematically represents, for the route of the figure 5 , the phase difference measured in the near field between consecutive frequency points spaced at the same interval as on the figure 5 , when the phases are adjusted using global phase shifts; The figure 7 , schematically represents the phase deviations of the main lobe between consecutive frequencies spaced at a constant interval, measured in an antenna measurement baseline for a beam pointed in the 0° direction, the antenna being one whose radiating element array is two-dimensional and comprises several hundred channels; The figure 8 , schematically represents the phase deviations of the main lobe between consecutive frequencies spaced at the same interval as on the figure 7 for the same antenna as for the figure 7 , for a beam pointed in direction 60.

[0048] From one figure to another, the same elements are identified by the same references.

[0049] The invention relates to a calibration method, that is to say, the calibration of a network antenna.

[0050] A network antenna A typically includes, as shown in figure 3 , a plurality of antenna channels V i each comprising a radiating element E i with i = 1 to N where N is the number of radiating elements i.e. channels of antenna A, and a phase shifter D i capable of applying a phase shift to an input signal of channel V i in reception and / or transmission from a phase shift command corresponding to the phase shift applied by the phase shifter D i.

[0051] The radiating elements E i form a network R of radiating elements.

[0052] The R network can be linear, as on the figure 3 , the radiating elements E i with i = 1 to N being distributed along a straight line D. Alternatively, the network of radiating elements is two-dimensional or three-dimensional.

[0053] In the non-limiting example of the figure 3 , the radiating elements E i are regularly distributed along the line D. They are separated in pairs by a distance d.

[0054] The antenna of the figure 3 is capable of operating alternately in transmit and receive. The alternation of transmit / receive functions is ensured by switches C1 i , C2 i , C3 i controlled by a synchronization clock H.

[0055] Network antenna A is an electronically scanned antenna.

[0056] In the non-limiting example of the figure 3 The array antenna comprises a set of MC1 control modules with i = 1 to N. A single MC1 control module configured to control the radiating element E1 is shown in figure 3 but in reality, each channel includes a control module of the same architecture as the MC 1 control module.

[0057] In the non-limiting example of the figure 3 , the MC i control module includes a variable attenuator A i and an electronic phase shifter D i allowing respectively to apply attenuation and to apply a phase shift to an input signal the channel V i both in transmission and reception, from an attenuation command and respectively, from a phase shift command.

[0058] The phase shift commands and the attenuation commands are generated by a pointer P. In particular, the phase shift commands addressed to the respective phase shifters D i are generated by a pointer CP computer from theoretical phase shift commands (or theoretical insertion phases) and global phase shift commands stored in a MEM memory of the pointer P.

[0059] The theoretical phase shifts depend on the pointing angle.

[0060] In transmit mode, an RC splitter / combiner with an ERC input / output feeds the MC i control modules. Phase shifting and attenuation are applied to the signal received at the input of the MC i control module by the phase shifter D i and, respectively, the attenuator A i, from the phase shifting and attenuation commands generated by the pointer P. The switches C1 i, C2 i and C3 i are controlled by the clock H, and the signal output from the phase shifter D i is amplified by a power amplifier AP i, before exciting the radiating element E i.

[0061] In receive mode, the RC combiner receives signals routed by the MC i control modules from the radiating element E i. In the control modules, the signals from the radiating elements E i are switched by switches C1 i, C2 i, and C3 i onto the receive channel and pass through a low-noise amplifier AF i. Then, phase shifting and attenuation are applied by the phase shifter D i and the attenuator A i, controlled by the pointer P.

[0062] This network antenna architecture is well known to those skilled in the art, but other electronically scanned antenna architectures are of course conceivable.

[0063] In one variant, the antenna is capable of operating only in transmission or reception.

[0064] The phase shifters D i are controlled by the pointer P configured to generate phase shift commands from a pointing setpoint corresponding to a pointing direction.

[0065] The phase shifters Di are controlled by the respective phase shift commands so that the phase shifters apply the respective phase shifts, corresponding to the respective phase shift commands, to the different channels Vi, so that the antenna points in the pointing direction.

[0066] Antenna A is capable of operating alternately in different contiguous frequency sub-bands ΔFj, of width ΔF, forming an overall frequency band of width ΔFG.

[0067] The present invention relates to a method for calibrating an antenna, or a method for generating phase shift commands corresponding to phase shifts intended to be applied, in operational phase in a transmit or receive mode of operation, by the respective channels on the respective input signals of said channels, so that the antenna points in a predetermined pointing direction at a first frequency belonging to a first sub-band of the set of sub-bands in which, for each channel i, a phase shift command is generated, defined by the difference between a theoretical insertion phase of channel i, in the operating mode, and an overall phase shift being the sum of a main insertion phase of channel i, in the operating mode, at the center frequency of the first frequency sub-band,and a transit phase shift introduced by the antenna due to a transit time between the center frequency of the first frequency sub-band and a reference center frequency, in the reception mode, when the reference center frequency does not belong to the first frequency sub-band. The reference center frequency is the same for all frequencies and all channels.

[0068] This is therefore the case for any first frequency belonging to a first sub-band that does not contain the reference center frequency. When the first frequency belongs to the same sub-band as the reference center frequency, then, for each channel, the overall phase shift is the main insertion phase of the channel, at the center frequency of the first frequency sub-band being the reference center frequency, in the operating mode.

[0069] The center frequency of a frequency sub-band is the frequency located at the center of the frequency sub-band.

[0070] The phase shift introduced by the transit time, or transit-time induced insertion phase, is defined from an estimate of the phase shift caused by the transit time between two frequencies separated by a frequency difference equal to ΔF. This phase shift is called the inter-sub-band transit phase shift dphi in the rest of the text.

[0071] Thus, the calibration process according to the invention makes it possible to eliminate phase jumps between adjacent frequency sub-bands and thus to obtain phase continuity between the sub-bands, which allows the use of the antenna for broadband applications.

[0072] Advantageously, a global calibration table is determined including the global phase shifts defined, in an operating mode, for each channel of the antenna, for each frequency sub-band.

[0073] The calibration table includes, for each channel of the antenna, a set of global phase shifts defined for the center frequencies of the respective sub-bands, each global phase shift being the sum of an insertion phase introduced by channel i, in the operating mode, at one of the center frequencies and a transit phase shift induced by a transit time between the center frequency and a reference center frequency.

[0074] Each global phase shift used to generate the phase shift commands for the respective channels is taken from the global calibration table defined for the desired operating mode (transmit or receive).

[0075] Advantageously, for an antenna capable of operating in both transmit and receive modes, a calibration table for transmit and a calibration table for receive are determined. Alternatively, a calibration table for transmit or a calibration table for receive is determined.

[0076] To determine a calibration table for transmission, all the measurements described below are performed in transmission mode, and the calculations described below are derived from measurements obtained in transmission mode. To determine a calibration table for reception, all the measurements described below are performed in reception mode, and the calculations described below are derived from measurements obtained in reception mode. This distinction is not repeated in the rest of the text to avoid making the description cumbersome. In the following text, it is assumed that the measurements and associated calculations are performed in one of the two modes: transmission or reception. The phase shift commands are then generated, in either transmission or reception mode, from the calibration table determined in that mode.

[0077] Advantageously, the calibration process includes, as shown in figure 4 , a step 100 of estimating the inter-sub-band transit phase shift.

[0078] To estimate the transit phase shift, it is necessary to measure a phase shift between two frequencies sufficiently close such that the phase shift introduced by the transit time between these two frequencies is less than 2π. Indeed, the phase is modulo 2π.

[0079] The calibration process according to the invention therefore includes, for at least one of the radiating elements R i, a step 10 of measuring phase shifts dΦik (or insertion phases) respectively introduced by the channel i considered (when this radiating element of the channel is the only radiating element of the set R of radiating elements to operate, or alternatively, the only radiating element of the set R in electromagnetic visibility of the measuring probe S) at different measurement frequencies fk spaced two by two by a step such that the phase shift introduced between these two frequencies is less than 2π.

[0080] The measurement frequencies belong, for example, to the same sub-band, but this is not mandatory.

[0081] The measurement frequencies are advantageously spaced by a frequency difference less than ΔF.

[0082] The phase shift measurement dΦik (or insertion phase) is, for example, carried out in the near field, that is, from a measurement made at a distance on the order of the wavelength of the radiating element E i at the frequency fk.

[0083] The measurement step 10 includes, for example, for a channel i, a measurement step of the insertion phase dϕik of the signal emitted at different measurement frequencies fk regularly spaced by a step δf in frequency.

[0084] For example, we can measure the insertion phases dϕik introduced by channel i of the antenna at K = 512 measurement frequencies fk (with k = 1 to K) regularly spaced across the overall frequency bandwidth ΔFG. The measurement frequencies are spaced two by two at a unit step δf = 1, corresponding to a predetermined frequency difference. The frequency bandwidth ΔFG is then 512 units. If the antenna, comprising 512 measurement frequencies, includes M = 32 sub-bands, then each sub-band ΔFj comprises K / M = 16 measurement frequencies and has a bandwidth ΔF = δf * K = 16 units.

[0085] Each measurement 10 is carried out by the same measuring device DM.

[0086] The DM measuring device includes, for example, as shown in figure 3 A measurement probe S is connected by a first CA cable 1 to a measurement bay BAI, itself connected by a second CA cable 2, to the ERC input / output of the RC combiner splitter. The measurement bay BAI includes measurement means, for example a network analyzer, allowing the transmission phase of the insertion of channel V i to be measured from the phase of the signal received by the probe S and that of the excitation signal injected into the ERC input of the RC combiner splitter and generated, for example, by a microwave signal generator in the measurement bay BAI.

[0087] The BAI measurement bay also includes a microwave signal generator to excite the antenna at its ERC input so that it emits a signal towards the probe.

[0088] In reception, the BAI measurement bay measures the insertion phase of the V i channel from a signal emitted by a microwave signal generator in the BAI bay and radiated by the S probe and a signal delivered at the ERC output of the combiner splitter and measured by a network analyzer in the BAI bay.

[0089] The BAI measurement bay advantageously includes a positioner allowing the probe to be moved relative to the antenna and to be positioned relative to the array of radiating elements in predetermined relative positions relative to the array R of radiating elements so that the insertion phase measurements carried out for each channel in the operating mode are carried out in a predetermined relative position between the radiating element of the channel and the probe, this relative position being the same for all channels.

[0090] For example, in the non-limiting case of a linear R network as represented in figure 3 , the positioner is configured to allow the probe to be moved by a step equal to d along the line D and to be held in position relative to the network R in each of the positions spaced by the step d.

[0091] Advantageously, the probe is positioned in the near field of the radiating element of the track for which the measurement is being carried out.

[0092] The process advantageously includes a step 20 of correction of each phase shift dϕik, or insertion phase, by subtracting, from the phase shift dϕik, a phase shift dϕck introduced by the measuring device DM at the measurement frequency at which the phase shift dϕik is measured.

[0093] Indeed, the DM measurement device introduces phase shifts related to the transit time of electrical signals, particularly in the CA 1 and CA 2 cables and in the BAI measurement bay. The phase shifts introduced by the DM measurement device, due to the electrical length of the measuring device, vary according to the signal frequency and therefore disrupt the continuity between the frequency sub-bands.

[0094] The corrected phase shift dcϕik obtained by the correction step is given by: dcϕik = dϕik − dϕck .

[0095] In order to calculate the corrected phase shift, the process advantageously includes a step 15 of prior measurement of the phase shifts dϕck introduced by the DM measuring device at frequencies fk.

[0096] These measurements are carried out, by the DM measuring device, by replacing the complete antenna with a single radiating element E0 and by connecting the second cable CA 2 to the radiating element E0. Alternatively, the radiating element E0 is within a passive network of charged radiating elements, representing the surrounded radiating element.

[0097] The process then includes a step of estimating the average unit transit phase shift δphi introduced by the antenna, due to the transit time in channel i between two measurement frequencies separated by the step δf.

[0098] For this purpose, we calculate, for example, during a step 30, from the corrected phase shifts dcϕik obtained, the average unit transit phase shift δphi between two transit phase shifts obtained for respective measurement frequencies spaced by δf.

[0099] This average unit transit phase shift δphi is an estimate of a phase difference introduced, under the effect of transit time, by channel i, between two frequencies separated by δf.

[0100] In step 40, the inter-sub-band transit phase shift dphi between the center frequencies of two contiguous sub-bands is then estimated from the average unit transit phase shift: dphi = ΔF / δf × δphi = ΔF × δphi lorsque δf = 1 .

[0101] Alternatively, the inter-sub-band transit phase shift dphi is estimated from average unit transit phase shifts estimated for different channels i. The process then includes steps of estimating the average unit transit phase shifts for these different channels and a step of calculating an average unit transit phase shift from the average unit transit phase shifts determined for several channels i.

[0102] Alternatively, the average unit phase shift is estimated from phase shifts measured for a smaller number of measurement frequencies spaced δf apart for one or more channels.

[0103] Alternatively, the average derivative of the unit phase shift is estimated from the estimated phase shifts, and the inter-sub-band transit phase shift is estimated from this derivative.

[0104] The process then includes a step 50 of storing the estimated transit phase shift dphi in a memory, for example, a memory of the measurement bay BAI.

[0105] The calibration process according to the invention also includes a step 200 of determining a so-called main calibration table comprising M sets of main phase shifts CCALj, where M is the number of center frequencies at which the main phase shifts are determined over the overall frequency band.

[0106] These center frequencies are separated in pairs by the frequency gap ΔF which is fixed over the entire overall frequency band.

[0107] Each CCALj set comprises N principal phase shifts CCALj i with i = 1 to N.

[0108] The main phase shift CCALj i is the phase shift introduced by channel i at the center frequency Fcj of the sub-band ΔF j on an input signal of channel i in operating mode. This is the insertion phase of channel i.

[0109] Step 200 is carried out as described previously with reference to steps 10 to 20, the main phase shift CCALj i being the phase shift introduced by a channel i at a particular frequency fk corresponding to the center frequency Fcj of the ΔF j band.

[0110] In other words, step 200 includes, for each channel i, a step 210 of measurement of phase shifts dϕij respectively introduced by the channel i considered (when this radiating element of the channel is the only radiating element of the set R of radiating elements to operate, or alternatively, the only radiating element of the set R in electromagnetic visibility of the measuring probe S), at the different center frequencies Fcj belonging to the frequency sub-bands ΔF j.

[0111] Step 210 is, for example, carried out by the DM measuring device.

[0112] The phase shifts introduced by the different channels are measured relative to the same reference signal for all channels in the operating mode considered.

[0113] The process advantageously includes a step 220 of correction of each phase shift dϕij by subtracting, from the phase shift dϕij, the phase shift dϕcj introduced by the measuring device DM at the frequency Fcj.

[0114] The corrected phase shift dcϕij obtained by correction step 220 is given by: dcϕij = dϕij − dϕcj .

[0115] In order to calculate the corrected phase shift, the process advantageously includes a step 215 of prior measurement of the phase shifts dϕcj introduced by the measuring device DM at the frequency Fcj as described previously with reference to step 15.

[0116] The process then includes a 300 storage step of the main calibration table, i.e. the main phase shifts CCALji determined for j= 1 to M and i = 1 to N in a memory, for example a memory of the measurement bay.

[0117] The method includes, for each channel i, the determination 60 of an overall phase shift NCALj i for each center frequency Fcj. This overall phase shift is determined from the principal phase shift CCALji determined for the channel considered at the center frequency Fcj and from the transit phase shift dphi.

[0118] The overall phase shift is the sum of the main phase shift (main insertion phase) and the transit phase shift induced by the transit time between the center frequency Fcj and a reference center frequency Fcr, that is, from the reference center frequency Fcr to the center frequency Fcj.

[0119] The transit phase shift dϕjr induced by the transit time between the center frequency Fcj and a reference frequency Fcr is given by: dϕjr = j − r * dphi

[0120] Thus, taking the first-order center frequency Fc1 as the reference, the overall phase shifts NCALj i are defined as follows, for channel i, for each center frequency j: NCAL1 i = CCAL1 i NCAL 2 i = CCAL 2 i + dphi NCAL 3 i = CCAL 3 i + 2 * dphi ... NCALj i = CCALj i + j − 1 * dphi ... NCALM i = CCALM i + M − 1 * dphi

[0121] The process advantageously includes a step 70 of storing, in a MEM memory, global phase shifts NCALj i for i = 1 to N and j = 1 to M.

[0122] In the method according to the invention, the global phase shifts NCALj i are then used to adjust the phase commands applied to the channels i so that the antenna points in a predefined pointing direction at a frequency fk belonging to one of the sub-bands ΔFcj so as to substantially compensate for the phase shifts introduced between the different channels i by the elements composing the different channels, at the center frequency Fcj of the sub-band ΔFcj and so that a phase shift introduced by a transit time between the center frequency Fcj of the sub-band ΔFcj and a reference center frequency varies linearly with the center frequency Fcj.

[0123] The overall phase shifts NCALj i are used to correct the theoretical phase commands, which are the theoretical insertion phases that must actually be introduced by the respective channels V i to point the antenna in a predetermined pointing direction at the frequency fk of the sub-band ΔFcj. In other words, the theoretical phase commands are the phase commands that must be applied to the respective channels to point the antenna in a pointing direction at the frequency fk in the absence of phase shifts introduced, between the different channels, by the components of the different channels, at the center frequency Fcj of the sub-band ΔFcj, and phase shifts related to the transit time between the center frequency Fcj and a reference center frequency Fcr.

[0124] In other words, the calibration method according to the invention makes it possible to fix the phase difference between two consecutive frequencies fk to fk+1 at δphi within the same sub-band but also at the transition from one sub-band ΔFj to the next sub-band ΔFj+1.

[0125] By taking the same reference frequency Fcr for each of the sub-bands, we allow the use of measurements made in a plurality of arbitrary frequency sub-bands of the overall frequency band.

[0126] To this end, the process includes a step 80 of determining phase shift commands, in order to point the antenna in a given direction at the frequency fk belonging to a sub-band ΔFj, forming a predetermined pointing angle with respect to a direction related to the array of radiating elements.

[0127] This step 80 includes, for each channel i, a step of calculating a difference between the theoretical control and the associated global phase shift.

[0128] Theoretical phase commands can be stored beforehand in a memory or calculated in operation by the pointer computer by means of an equation stored in a memory and then possibly stored in a pointer memory.

[0129] For a given angular pointing, the phase shift command Φij addressed to each channel i, i.e., to each phase shifter, for each frequency within the frequency sub-band ΔF is given by: Φ ij = ϕt ij − NCALj i .

[0130] Where Φt ij is the theoretical phase shift command to be applied to the center frequency Fcj at channel i for angular pointing.

[0131] The process then includes a step of applying the phase shifts to the different channels comprising a step 90 of transmitting the phase shift commands Φ ij to the phase shifters and a step 91 of applying the corresponding phase shifts by the phase shifters.

[0132] In figure 5 For a given channel i, the phase difference measured in the near field between two frequency points separated by δf is shown, when the phase shift controls are adjusted solely by means of the principal phase shifts, applying the -CCALj i corrections to the theoretical phase controls. The abscissa represents the frequency k in the overall band. These frequencies are separated by δf.

[0133] This figure shows an average phase difference of approximately 14.5° between consecutive frequencies within the same sub-band, and several points exhibiting a phase difference of approximately 145° between two consecutive frequency points. These points correspond to the transition from a sub-band ΔFj to the adjacent sub-band ΔFj+1 (in this case, the theoretical value is: dphi0 = -ΔF / δf x δphi). Within the same sub-band ΔFj, the transit time thus induces, here, between two frequency points separated by δf, a phase shift of approximately 14.5°.

[0134] In figure 6 For a given channel i, the phase difference measured in the near field between two frequency points separated by δf is shown, when the phases are adjusted using the global phase shifts NCALj i. As in the previous figure, the abscissa represents the frequency's order number in the overall band. Note that the phase difference scale is expanded compared to that of the figure 5 .

[0135] We observe, in this figure, the same phase differences δphi between the frequencies spaced by δf over the entire global band, up to quantization and calibration errors.

[0136] For example, the figures 7 And 8illustrate measurements made in the far field on the radiation pattern of an array antenna whose radiating element network is two-dimensional, comprising several hundred channels. These figures represent the phase deviations of the main lobe between frequencies fk spaced by δf, measured in an antenna measurement baseline, on the one hand for a beam pointed in the 0° direction ( figure 7 ), on the other hand, for a beam angled at 60° in the circular plane ( figure 8 ). Each main lobe is obtained by the radiation of all the radiating elements excited by excitation signals whose phase shifts are adjusted by means of the global phase shifts.

[0137] The network exhibits perfect behavior over a wide frequency band. The average phase shift between consecutive frequencies is virtually constant.

[0138] Advantageously, the global phase shifts NCALj i are generated during a so-called preliminary calibration step, prior to using the antenna in operational mode.

[0139] Measurements of the insertion phases can be performed in the near field as explained previously. Alternatively, these measurements are performed in the far field.

[0140] The invention also relates to a broadband method for estimating the distance between an antenna and a point originating an echo, comprising a step of generating phase-shift commands during transmission so that the antenna emits microwave signals at several transmission frequencies within several sub-bands of transmission frequencies, a step of generating phase-shift commands during reception so that the antenna delivers measurements of received signals at several reception frequencies within several sub-bands of reception frequencies under the effect of the emission of microwave signals, and a processing step consisting of estimating a distance, separating the antenna from a point originating an echo (for example, a target or part of a target), from measurements of the signals received by the antenna during a reception step and, advantageously,To construct an image representing echo intensities as a function of the distance separating the antenna from the points originating the echoes.

[0141] The process further advantageously includes the steps of transmission and reception via antenna channels.

[0142] Advantageously, the phase shift commands for the phase shifters of the different channels in the different sub-bands, during the transmission and reception phases, are generated by the calibration method according to the invention.

[0143] The invention also relates to the pointer P comprising a pointer calculator CP comprising a control generator capable of generating phase shift commands from the calibration table and from theoretical insertion phases.

[0144] The invention also relates to a pointer P whose control generator is configured to implement the phase shift control generation steps, during a broadband distance estimation process, such that the antenna comprising the pointer implements the transmission and reception steps of this process, and comprising an estimator EST configured to estimate the distance from the measurements delivered during the reception phase.

[0145] The invention relates to an antenna configured to implement the distance estimation method according to the invention.

[0146] Distance estimation can be based on linear frequency modulation or on a multicarrier code.

[0147] One advantage of the invention lies in the fact that global phase shifts can also be used in a narrowband operating mode to control phase shifters so as to transmit or receive electromagnetic waves from one or more frequencies belonging to the same sub-band and to use measurements of signals received only in the sub-band to construct an image. Indeed, the transit phase shifts added to the respective principal phase shifts to generate the global control are the same for all channels in a given sub-band. They therefore do not affect the antenna's radiation pattern.

[0148] This avoids having to memorize several sets of calibration tables; only the calibration tables defined in transmission and / or reception, including the global phase shifts NCALj i, need to be stored in an antenna memory for use in operational phase in wideband and narrowband modes.

[0149] In the described embodiment, a calibration table is stored for each operating mode. The calibration table includes the overall phase shifts. Alternatively, the calibration table includes the principal phase shifts, and the inter-sub-band transit phase shift or its derivative is stored, and the overall phase shifts are calculated from the principal phase shifts and the transit phase shift or its derivative.

[0150] Each computer may include one or more dedicated electronic circuits or a general-purpose circuit. Each electronic circuit may include a reprogrammable computing machine (a processor or a microcontroller, for example) and / or a computer executing a program comprising a sequence of instructions and / or a dedicated computing machine (for example, a set of logic gates such as an FPGA, a DSP or an ASIC, or any other hardware module).

Claims

1. Method for calibrating an electronically scanned array antenna (A) comprising a set of channels (Vi) each comprising a radiating element (Ei), the array antenna (A) being suitable for operating in a set of contiguous frequency sub-bands (ΔFj) forming an overall frequency band, each frequency sub-band comprising a central frequency, the calibration method comprising the following steps: - generating phase shift commands intended to be applied to the respective channels, in operational phase in a transmission or reception operating mode, so that the antenna points in a predetermined pointing direction at a first frequency belonging to a first sub-band of the set of sub-bands, the phase shift commands being defined, for each channel (Vi), by the difference between a theoretical insertion phase of the channel, in the operating mode, and an overall phase shift which is the sum of an insertion phase of the channel in the operating mode at the centre frequency of the first frequency sub-band, and a transit phase shift which is an estimate of the phase shift caused by the transit time between the centre frequency of the first frequency sub-band and a reference centre frequency, in the operating mode, when the reference centre frequency does not belong to the first frequency sub-band, - for each channel, measuring a first insertion phase of the channel in the operating mode, - measuring, for at least one of the channels, second insertion phases of said channel, in the operating mode, at different measurement frequencies separated two by two by a frequency difference less than the width ΔF of the sub-bands, - estimating, from the second insertion phases; an inter-sub-band transit phase shift introduced, by a transit time, between two frequencies separated by the width ΔF, in the operating mode, - estimating, for each channel, in the operating mode, an overall phase shift, from the first measured insertion phase, and from the inter-sub-band transit phase shift estimated in the operating mode.

2. Calibration method according to the preceding claim, comprising a step of correcting the first and second insertion phases by eliminating, before the step of estimating the overall phase shifts, insertion phases introduced by a device for measuring the first insertion phase and the second insertion phase.

3. Method for estimating a distance, comprising the following steps: - transmitting, by means of the array antenna, during a transmission phase, microwave signals at a plurality of transmission frequencies of a plurality of frequency sub-bands of a first set of sub-bands, - measuring, by means of the array antenna, during a reception phase, signals received at a plurality of reception frequencies of a plurality of frequency sub-bands of a second set of sub-bands following the transmission of microwave signals, - calculating a distance from the signals received by the antenna, during the transmission and reception steps, the phase shift commands transmitted to the different channels during the transmission and reception phases being generated by the calibration method according to any one of the preceding claims.

4. Pointer for generating phase shift commands for the channels of an electronically scanned array antenna (A) comprising a set of channels (Vi) each comprising a radiating element (Ei), the array antenna (A) being suitable for operating in a set of contiguous frequency sub-bands (ΔFj) forming an overall frequency band, the pointer comprising command generation means configured to generate phase shift commands intended to be applied to the respective channels, in operational phase in a transmission or reception operating mode, so that the antenna points in a predetermined pointing direction at a first frequency belonging to a first sub-band of the set of sub-bands, the phase shift commands being defined, for each channel (Vi), by the difference between a theoretical insertion phase of the channel, in the operating mode, and an overall phase shift which is the sum of an insertion phase of the channel in the operating mode at the centre frequency of the first frequency sub-band, and a transit phase shift introduced by a transit time between the centre frequency of the first frequency sub-band and a reference centre frequency, in the operating mode, when the reference centre frequency does not belong to the first frequency sub-band.

5. Pointer according to any one of the preceding claims, comprising a memory storing a calibration table defined for the operating mode, the calibration table comprising, for each channel of the antenna, a set of overall phase shifts defined for the centre frequencies of the respective sub-bands, each overall phase shift being the sum of an insertion phase of the channel i, in the operating mode, at one of the centre frequencies and a transit phase shift induced by a transit time between the centre frequency and a reference centre frequency, the overall phase shift being taken from the calibration table.

6. Pointer according to any one of claims 4 to 5, wherein the command generation means are configured to implement the following steps during a distance estimation step: - generating first phase shift commands intended for the antenna channels so that, during a transmission phase, the antenna transmits microwave signals at a plurality of transmission frequencies of a plurality of frequency sub-bands of a first set of sub-bands, - generating second phase shift commands intended for the antenna channels so that the antenna measures, during a reception phase, signals received at a plurality of reception frequencies of a plurality of sub-bands of a second set of frequency sub-bands, the pointer comprising distance estimation means configured to estimate a distance from the measured signals.

7. Electronically scanned array antenna comprising a set of antenna channels each comprising a radiating element, the array antenna being suitable for operating in a set of contiguous frequency sub-bands forming an overall frequency band, the electronically scanned array antenna comprising the pointer according to any one of claims 4 to 6, the commands generated by the pointer being intended to control phase shifters of the antenna channels via a control module (MCi) associated with each antenna channel respectively.

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