A holographic inverse scattering imaging system and method

Abstract

The present invention discloses a holographic inverse scattering imaging system and method. The incident field of the inverse scattering system is generated by a broadband circularly polarized antenna, and the scattering field is received by a broadband dual-polarized antenna. The holographic inverse scattering imaging method of the present invention combines the polarization characteristics of scattering information and the frequency domain characteristics of multiple frequency points. Based on the discrete dipole approximation theory, the received full-polarization scattering information is used as the input condition of imaging at one time, and is equivalent to an inverse source problem. For the results of the inverse source problem at multiple frequency points, they are equivalent to the results of the frequency domain characteristics of the wave source corresponding to each imaging grid element. By performing IFFT on the frequency domain information of the equivalent wave source, the characteristics of the equivalent wave source in the quasi-time domain can be obtained, and the result of the starting position in the quasi-time domain is selected as the inversion result of the target.

Description

Technical Field

[0001] The invention belongs to the technical field of microwave near-field imaging, and in particular relates to a holographic inverse scattering imaging system and method. Background Art

[0002] The electromagnetic field inverse scattering problem is aimed at calculating or estimating the relevant physical information of the scatterer using the information of the incident and scattered electromagnetic fields. It is an important method in the field of microwave near-field imaging. Compared with other commonly used microwave near-field imaging methods, such as millimeter-wave synthetic aperture radar methods, inverse scattering imaging can more directly reflect the physical information of the scatterer target itself. This feature also enables imaging using the inverse scattering method to obtain higher imaging resolution in lower frequency bands such as the S band and C band. Compared with millimeter waves, the use of lower-frequency microwave hardware systems can greatly save hardware costs, while also ensuring the penetration of electromagnetic waves, so that this inverse scattering imaging method can be applied to dielectric targets and targets where dielectrics and metals coexist.

[0003] Whether the obtained scattering field data can reflect the accurate information of the target to be measured is the basic condition that determines whether the target can be imaged through the inverse problem. At present, the electromagnetic field inverse scattering problem is more widely used in the requirements of dielectric target imaging represented by medical judgment. Unlike dielectric targets, metal targets have strong polarity, that is, when different polarization modes are used to incident and receive electromagnetic fields, the imaging results of metal targets using inverse scattering imaging are generally different. At present, the common inverse scattering imaging systems mostly use a single polarization incidence plus a single polarization reception method represented by a conical slot antenna. However, the use of single polarization incidence will make the imaging results of metal targets unable to reflect the complete target information when using lower frequency microwave imaging; and the use of single polarization antennas cannot obtain the required scattering field information in multiple polarization directions with the same phase center. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a holographic inverse scattering imaging system and method, which overcomes the difficulty in the imaging process caused by the strong polarity of metal targets, is suitable for microwave imaging in lower frequency bands such as S band and C band, has high resolution of imaging results, and can greatly save hardware costs compared to high-frequency millimeter wave imaging.

[0005] A holographic inverse scattering imaging system comprises a transmitting antenna, a receiving antenna and an imaging processing unit;

[0006] The transmitting antenna is a broadband circularly polarized antenna, which is used to generate incident electromagnetic waves in the area where the imaging target is located. The transmitting antenna performs full polarization excitation on the imaging target, and the excitation can evenly cover the entire area of ​​the imaging target;

[0007] The receiving antenna includes a plurality of broadband dual-polarization antennas, all of which are arranged around the imaging target, and the broadband dual-polarization antennas can simultaneously obtain scattered field information in two polarization directions at the location;

[0008] The imaging processing unit is used to image the target according to the scattered field information. During the imaging processing, at each frequency point, the scattered field information in two polarization directions received by the receiving antenna is simultaneously processed as input conditions to obtain an inversion result of the imaging target, wherein the inversion result includes full polarization information of the imaging target;

[0009] The inversion results at multiple frequency points are processed to obtain a target image, which simultaneously contains full polarization information and broadband frequency domain information of the imaging target.

[0010] Preferably, the broadband dual-polarized antennas form an antenna array and are parallel to a vertical line passing through the center point of the target. The center point of the antenna array and the aperture surface of the transmitting antenna are both facing the center of the imaging area. The antenna array makes a circular motion around the vertical line, and every other set angle is used as a measurement position. At each measurement position, a broadband dual-polarized antenna in the antenna array represents an observation point, and the observation points all receive electromagnetic waves of different frequencies reflected by the imaging target.

[0011] Preferably, the broadband dual-polarization antennas form a plurality of antenna linear arrays, which are arranged around the imaging target.

[0012] Preferably, the plurality of antenna arrays are arranged on a cylindrical surface around the imaging target.

[0013] Preferably, the broadband dual-polarization antennas are distributed on a circular surface around the imaging target.

[0014] Preferably, the broadband dual-polarization antennas are distributed on a plane.

[0015] Preferably, the transmitting antenna transmits electromagnetic waves in the S band and the C band.

[0016] An imaging method of a holographic inverse scattering imaging system, comprising:

[0017] Delimiting an imaging area including the imaging target on the plane where the imaging target is located, dividing the imaging area into grids, with each grid serving as an imaging element;

[0018] Each imaging element is considered as an equivalent electric dipole source Each electric dipole contains The components p in three directions x ,p y ,p z ;

[0019] The measurement position of the antenna array is changed so that the receiving antenna traverses all observation points. The following equation is listed based on the scattered field information received by all observation points at any frequency:

[0020]

[0021] Wherein, M is the total number of all observation points, N is the number of the imaging elements; the total number of observation points M=D×Q, D is the number of the measurement positions, and Q is the number of the broadband dual-polarization antennas at each measurement position;

[0022]

[0023] is the observation angle of observation points 1, 2, ..., M relative to the imaging target;

[0024]

[0025] Where λ is the wavelength of the incident electromagnetic wave; k = 2π / λ is the wave number of the incident wave; is the position of the mth observation point; is the position of the nth equivalent electric dipole source, m = 1, 2, ..., M; n = 1, 2, ..., N; for point to Unit vector of direction; I is the identity matrix;

[0026] Column vector (p′ 1x ,p′ 1y ,p′ 1z ,…,p′ Nz ) T is the value to be solved of the equation group corresponding to formula (3), which represents the solution result of each equivalent electric dipole source;

[0027] E mz Represents the scattered field information of two polarization directions at the mth measurement point;

[0028] Solve the equation group formed by equation (3) obtained at all frequency points to obtain the solution results of all equivalent electric dipole sources at each frequency point, that is, the inversion result of the imaging target at each frequency point;

[0029] Considering each imaging element as a wave source that can radiate multi-frequency electromagnetic waves, the value of the solution result of the equivalent dipole in any imaging element at multiple frequency points (p′ nx ,p′ ny ,p′ nz )T , which represents the radiation intensity of the wave source; assuming the number of frequency points is P, the solution results of all frequency points on the same component of the same imaging element are subjected to IFFT, that is, the information of the equivalent wave source, that is, the equivalent dipole source in the quasi-time domain t=0, 1, 2, ..., P-1 is obtained; the information of the equivalent dipole source at t=0 in the quasi-time domain is selected as the final solution result of the equivalent dipole source corresponding to the imaging element; the final result of the equivalent dipole source of the three components corresponding to an imaging element is modulo the value, that is, the imaging result of the imaging element; all imaging elements are traversed to obtain the target image.

[0030] The present invention has the following beneficial effects:

[0031] 1. The present invention uses a broadband circularly polarized antenna for incidence and a broadband dual-polarized antenna for reception to obtain the scattered field. This method allows the transmitting antenna to fully polarize the imaging target, and the excitation can evenly cover the entire area of ​​the imaging target; in addition, the broadband dual-polarized antenna can be used to obtain broadband scattered field information in two polarization directions at the antenna required for imaging.

[0032] 2. The imaging system and method of the present invention can use relatively low-frequency microwaves such as S-band and C-band. Using low-frequency microwave devices can save hardware cost and ensure the penetration of electromagnetic waves, so as to image dielectric targets and targets where metal and dielectric coexist.

[0033] 3. In the imaging method of the present invention, at each frequency point, the scattered field information of all polarization directions received by the receiving antenna is simultaneously processed as an input condition to obtain an inversion result of the imaging target, and the inversion result includes the full polarization information of the imaging target; through multi-frequency point measurement, the inversion results at multiple frequency points are processed to obtain a target image, and the target image includes the full polarization information and wide-band frequency domain information of the imaging target. The above imaging method overcomes the drawback of the existing synthetic aperture radar imaging method that only the transmission and reception results of a given polarization mode can be imaged (such as only the HH polarization imaging result map, VV polarization imaging result map, VH polarization imaging result map, and HV polarization result map can be obtained in sequence), so that the imaging result is more accurate and complete. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the imaging system model of the present invention.

[0035] FIG2(a) is a schematic diagram of the target model, which is a metal ring model with an outer radius of 7 cm and an inner radius of 6 cm; FIG2(b) is an imaging result diagram. DETAILED DESCRIPTION

[0036] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0037] The purpose of the present invention is to provide a system and method design for near-field imaging of a target using electromagnetic inverse scattering theory. The method design refers to the design of an inverse scattering problem method, and the system design refers to the sampling array design coordinated therewith. Using the method, the target can be effectively imaged and reproduced, and the scattered field obtained by the system can reflect the full polarization information and wide-band frequency domain information of the scatterer target.

[0038] In terms of imaging methods, it is a holographic imaging method. Holography means "all information". In this method, "holography" is reflected in that, in the imaging process, at each frequency point, the scattered field information of all polarization directions received by the receiving antenna is processed as input conditions at the same time to obtain an inversion result of the imaging target, and the inversion result contains the full polarization information of the imaging target; on the other hand, it is reflected in processing the inversion results at multiple frequency points to obtain a target image, and the target image contains the full polarization and wide-band frequency domain information of the imaging target.

[0039] The method uses a method based on discrete dipole approximation (DDA) theory and transforms the inverse scattering problem into an inverse source problem. Unlike the synthetic aperture radar method, which can only image the transmission and reception results of a given polarization mode (such as only being able to obtain HH polarization imaging result map, VV polarization imaging result map, VH polarization imaging result map, and HV polarization result map respectively), the method uses the scattering field information of all polarization directions received by the receiving antenna as input information of the imaging process at one time to generate a complete target image, which more directly reflects the shape of the target itself. Using multi-frequency, i.e., frequency sweeping mode, the results of the inverse source problem at different frequency points are obtained, i.e., the solution results of the equivalent electric dipole source at each frequency point; each imaging element is considered to contain a wave source that can radiate multi-frequency electromagnetic waves, and the value of the solution result of the equivalent dipole in any imaging element at each frequency point represents the radiation intensity of the wave source; assuming that the number of frequency points is P, the solution results of all frequency points on the same component of the same imaging element are IFFT, and the information of the equivalent wave source, i.e., the equivalent dipole source in the quasi-time domain t=0, 1, 2, ..., P-1 can be obtained, and the distribution of the equivalent dipole source at t=0 in the quasi-time domain is selected as the final solution result of the equivalent source corresponding to the imaging element. The final result of the equivalent source of an imaging element is modulo valued, which is the imaging result of the imaging element; all imaging elements are traversed to obtain the target image. The present invention overcomes the difficulty in imaging caused by the strong polarity of metal targets, and is suitable for microwave imaging in lower frequency bands such as S band and C band. The imaging result has high resolution, and can save hardware costs compared to high-frequency millimeter waves, and ensure the penetration of electromagnetic waves, so that the inverse scattering imaging system can be applied to the study of dielectric targets and the coexistence of dielectric targets and metal targets. The technical solution of the present invention is as follows:

[0040] A holographic inverse scattering imaging system, comprising:

[0041] (1) Establish a spatial rectangular coordinate system O-xyz, take the origin O as the center, define an area in the xOz plane as the imaging area, divide the imaging area into grids, and obtain a set of imaging elements. The imaging target is located in the imaging area.

[0042] (2) Use a broadband circularly polarized antenna A0 as a transmitting antenna, whose aperture surface should face the imaging area, and the incident field propagates along the negative direction of the y-axis. The transmitting antenna is used to generate incident electromagnetic waves in the area where the imaging target is located. The imaging target can be of any shape. The transmitting antenna fully polarizes the imaging target, and the excitation uniformly covers the entire area of ​​the imaging target;

[0043] (3) Using multiple broadband dual-polarized antennas as receiving antennas, wherein the multiple broadband dual-polarized antennas are arranged in a linear array, and the antenna linear array is parallel to the imaging area.

[0044] (4) Using a turntable device, the receiving antenna array is kept facing the target with the aperture of the receiving antenna, and the antenna array rotates around the z-axis, i.e., cylindrical scanning is performed. During the scanning, every fixed angle is taken as a measurement position, and all dual-polarized antennas simultaneously measure the transmission field information at each frequency point at that position to obtain Scattering field information in the polarization direction and z-polarization direction and like Figure 1 shown.

[0045] (5) According to the equivalent principle of electromagnetic field, the inverse scattering problem for metal targets can be equivalent to the inverse source problem for the equivalent current on the surface of the metal target. According to the DDA theory, the equivalent current is described by a number of discrete electric dipole sources with different amplitudes, positions and other parameters, which are used as the forward operator of the inverse problem. The DDA forward operator is modified by using the vector rectangular coordinate and cylindrical coordinate coefficient transformation matrix so that the calculated electric dipole radiation electric field only contains direction and the z-direction polarization component.

[0046] (6) At each frequency point, use the scattered field obtained in (4) and All of them are used as input conditions for the inverse scattering problem constructed in (5). By solving the inverse problem, the solution results of the equivalent dipole source corresponding to the subdivision elements of the imaging area at each frequency point are obtained.

[0047] (7) The solution of the equivalent electric dipole source at each frequency point obtained in (6) is regarded as a wave source that can radiate multi-frequency electromagnetic waves contained in each imaging element, wherein the electromagnetic waves radiated by each wave source at different working frequencies have different amplitude and phase information, which are regarded as its frequency domain characteristics. Assuming the number of frequency points is P, the solution results of all frequency points on the same component of the same imaging element are subjected to IFFT to obtain the quasi-time domain characteristics of the equivalent wave source, i.e., the equivalent dipole source. The quasi-time domain characteristics are specifically manifested as the information of the equivalent dipole source in the quasi-time domain t=0, 1, 2, ..., P-1. The information of the equivalent dipole source at t=0 in the quasi-time domain is selected as the final solution result of the equivalent source corresponding to the imaging element. The modulus of the final solution result of the equivalent dipole source of the three components corresponding to an imaging element is obtained, which is the imaging result of the imaging element.

[0048] (8) (7) is traversed over all imaging elements to obtain the intensities of the equivalent dipole sources of all imaging elements at t=0 in the quasi-time domain, thereby obtaining the target image, that is, the imaging result of the imaging target.

[0049] Example:

[0050] like Figure 1 The figure shows a schematic diagram of the incident antenna, receiving antenna array and imaging target. The incident antenna A0 is a broadband circularly polarized antenna, and the propagation direction of the electromagnetic wave it emits is perpendicular to the plane where the imaging target is located; the receiving antennas A1 to A8 are broadband dual-linear polarized antennas arranged in a linear shape.

[0051] In order to ensure the resolution of imaging while saving the increase in hardware cost caused by high-frequency microwave devices, and to ensure sufficient penetration of electromagnetic waves so that the imaging system of the present invention can be used for medium target imaging, it is not suitable to select too low or too high a frequency as the frequency of the electromagnetic wave in the imaging process. In actual cases, 2.5 to 6 GHz is selected as the frequency used by the imaging system, and its corresponding wavelength is 0.05 to 0.12 m; in principle, the more frequency points used for scanning, the better the target imaging results can be obtained, but too many scanning frequency points will reduce the measurement efficiency and imaging efficiency. Generally, 32 frequency points can be selected.

[0052] For example, the imaging area is a square area with a side length of 0.2m; the imaging area is a square area with the origin as the center and a side length of two wavelengths. Let the plane where the imaging area is located be the xOz plane, and the target is located in the imaging area. The imaging area is divided into 50×50 grids, each with a side length of 2×10 -4 The small grid of m is an imaging element.

[0053] The incident antenna A0 is located at (x, y, z) = (0, 0.5m, 0) in the rectangular coordinate system, and the propagation direction of the incident wave is the negative direction of the y axis, perpendicular to the plane of the imaging area. When a linearly polarized wave is used as the incident field to irradiate a metal target, only the part with the same shape as the polarization direction of the incident field can contribute to the scattered field. Since the imaging target can be of any shape, in order to make the transmitting antenna perform full polarization excitation on the imaging target, that is, the excitation uniformly covers the entire area of ​​the imaging target, the incident antenna A0 should select a broadband circularly polarized antenna.

[0054] The receiving antenna array should be able to simultaneously obtain the scattered field information in the horizontal polarization direction and the vertical polarization direction at each observation point. If the single-line polarization antenna in the prior art is selected as the receiving antenna, it is impossible to obtain the information in the horizontal polarization and vertical polarization directions at the same time. Too many receiving antennas will increase the hardware cost, and too few will make the sampling array sparse and cause the data to fail to reflect the real target. Therefore, the "single-transmit and eight-receive" at each position should be measured simultaneously. Scattering field information in the polarization direction and z-polarization direction and

[0055] Therefore, antennas A1 to A8 should be broadband dual-polarized antennas to form a linear array receiving antenna. The linear array receiving antenna is arranged parallel to the z-axis, and the starting position of the receiving antenna is located at The z-axis coordinates of A1~A8 are -0.40m, -0.32m, -0.24m, -0.16m, 0.16m, 0.24m, 0.32m, and 0.40m respectively.

[0056] The straight line in the cylindrical coordinate system is ρ = 0.5m; As the starting position of the receiving antenna array, the receiving antenna array is rotated every Rotate once to the final position ρ = 0.5m; Finally, 31 observation positions are obtained, so that the observation positions of the scattering field are finally in the form of cylindrical scanning with ρ = 0.5m. Figure 1 shown.

[0057] Each time the receiving antenna array is rotated to an angle, the scattered field information received by the receiving antennas A1 to A8 at each frequency is recorded. At each frequency, a total of 31 groups are obtained (each group is in the form of "single transmission and eight receptions", and the observation point positions of the 8 receiving antennas in each group are respectively and Directional scattered field component and ) scattering field information, that is, each frequency contains 31×8×2=496 scattering field information. The scattering field information is used as input information in the subsequent inverse scattering problem.

[0058] A forward operator based on DDA theory is used to construct the inverse scattering problem, and the inverse scattering problem is converted into an inverse source problem for processing. The equivalent dipole source in each imaging element obtained at different frequency points is regarded as a wave source that can radiate multi-frequency electromagnetic waves contained in each imaging element. The electromagnetic wave radiation of the wave source in each imaging element in the quasi-time domain t=0, 1, 2,..., P-1 is obtained by the IFFT method. The results of all imaging elements at t=0 in the quasi-time domain are obtained and the modulus value is calculated, and the target can be imaged.

[0059] According to the equivalent principle of electromagnetic field, for the scattering problem of metal targets, the calculation of its scattering field can be equivalent to the radiation field of the equivalent current source on the metal surface. Therefore, the electromagnetic inverse scattering problem for metal targets can also be equivalent to the electromagnetic inverse source problem of inverting the equivalent current source on the metal surface. At the same time, the above-mentioned equivalent current source can be described by an array composed of several discrete dipoles. The above is the basic content of DDA theory.

[0060] Each small grid in the image area segmentation result is regarded as an equivalent electric dipole source Each electric dipole consists of The components p in three directions x ,p y ,p z .

[0061] The radiation field calculation formula of the electric dipole is:

[0062]

[0063] Where λ is the wavelength of the incident electromagnetic wave; k = 2π / λ is the wave number of the incident wave; is the observation position of the radiation field, that is, the position of each receiving antenna involved in the invention of the imaging method; is the position of the electric dipole radiation source, that is, the position of each small grid in the segmentation result of the imaging area in this problem; for point to Unit vector of direction; I is the identity matrix.

[0064] At each frequency point, for all electric dipole sources at the grids that divide the imaging area and the scattered fields at all observation positions, the equations for the calculation of the radiation field of the discrete dipole array can be constructed according to the calculation formula of the radiation field of the electric dipole:

[0065]

[0066] Where M is the total number of all observation points, that is, the total number of broadband dual-polarized antennas at all positions, which satisfies M = D × Q, D is the number of measurement positions, Q is the number of broadband dual-polarized antennas at each measurement position; N is the number of imaging elements, that is, the number of grids divided into the imaging area by the total number of observation points, that is, the total number of all electric dipoles. According to the conversion relationship between the vector coefficients in the cylindrical coordinate system and the rectangular coordinate system, the above matrix equations are multiplied on the left by the matrix related to the observation point position, and the following equations can be obtained:

[0067]

[0068] Formula (3) is the equation group form corresponding to the inverse scattering imaging method in the present invention. Among them:

[0069]

[0070] in It is the angle between the straight line perpendicular to the z-axis at the observation position 1, 2, ..., M and the x-axis.

[0071] According to formula (1), we have formula (3):

[0072] is the mth observation position Point to the nth imaging grid position Unit vector of direction;

[0073] The column vector (p′) on the left side of the equal sign in formula (3) 1x ,p′ 1y ,p′ 1z ,…,p′ Nz ) T is the unknown number of the equation group corresponding to equation (3). By solving equation group (3), the solution of the equivalent electric dipole source at each frequency point can be obtained (when a vector network analyzer is used to obtain the S parameters corresponding to the scattered field in actual measurement, the above solution should be compensated for the corresponding phase).

[0074] The column vector on the right side of the equal sign in equation (3) is the input condition of the inverse problem, that is, the scattered field information received at M observation points at each frequency point and

[0075] Generally, in order to ensure the clarity of the imaging result, the imaging area is precisely divided (for example, a 50×50 division is selected in the actual case of the present invention). Therefore, the number of observation points is much smaller than the number of electric dipole sources, that is, M<<N. Furthermore, Equation (3) is an underdetermined linear equation system where the number of equations is less than the number of unknowns. In theory, it has infinitely many solutions, but its optimal solution can be obtained. In the actual case of the present invention, the Tikhonov regularization method is selected to obtain the optimal solution of the inverse problem.

[0076] For all 32 frequency points, 32 sets of equation systems shown in Equation (3) can be correspondingly obtained, and a total of 32 sets of solution results of the electric dipole sources can finally be obtained.

[0077] The following process of the present invention proposes a method to reasonably integrate the 32 sets of solution results of the electric dipole sources to obtain the most suitable imaging result of the inverse problem.

[0078] By solving the equation systems constituted by Equation (3) obtained at all frequency points, the solution results of the equivalent electric dipole sources at each frequency point are obtained.

[0079] Regarding each imaging element as a wave source that can radiate electromagnetic waves of multiple frequencies, the values of the equivalent dipole solution results within any one imaging element at multiple frequency points (p′ nx , p′ ny , p′ nz ) T , that is, represent the radiation intensities of the wave source that can radiate electromagnetic waves of multiple frequencies at each frequency point; perform IFFT on the solution results of all frequency points on the same component of the same imaging element, such as , and the information of this component of the equivalent dipole source in the pseudo-time domain t = 0, 1, 2,..., 31 can be obtained. And select the information of the equivalent dipole source at t = 0 in the pseudo-time domain as the final solution result of the equivalent source corresponding to this imaging element. Take the modulus value of the final solution result, which is the imaging result of this imaging element; traverse all imaging elements to obtain the target image, as shown in Figure 2(b).

[0080] In summary, the above is only the preferred embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A holographic inverse scattering imaging system, characterized in that: It includes a transmitting antenna, a receiving antenna and an imaging processing unit; The transmitting antenna is a broadband circularly polarized antenna, which is used to generate incident electromagnetic waves in the area where the imaging target is located, and the transmitting antenna performs full polarization excitation on the imaging target; The receiving antenna includes a plurality of broadband dual-polarization antennas, all of which are arranged around the imaging target, and the broadband dual-polarization antennas can simultaneously obtain scattered field information in two polarization directions at the location; The imaging processing unit is used to image the target according to the scattered field information. During the imaging processing, at each frequency point, the scattered field information in two polarization directions received by the receiving antenna is simultaneously processed as input conditions to obtain an inversion result of the imaging target; The inversion results at multiple frequency points are processed to obtain a target image.

2. A holographic inverse scattering imaging system as claimed in claim 1, characterized in that: The broadband dual-polarized antennas form an antenna array and are parallel to a vertical line passing through the center point of the target. The center point of the antenna array and the aperture surface of the transmitting antenna are both facing the center of the imaging area. The antenna array performs circular motion around the vertical line, and every other set angle is used as a measurement position. At each measurement position, a broadband dual-polarized antenna in the antenna array represents an observation point, and the observation points all receive electromagnetic waves of different frequencies reflected by the imaging target.

3. A holographic inverse scattering imaging system as claimed in claim 1, characterized in that: The broadband dual-polarization antennas form a plurality of antenna linear arrays and are arranged around the imaging target.

4. A holographic inverse scattering imaging system as claimed in claim 3, characterized in that: The plurality of antenna arrays are arranged on a cylindrical surface around the imaging target.

5. The holographic inverse scattering imaging system according to claim 1, characterized in that: The broadband dual-polarization antennas are distributed on a circumferential surface around the imaging target.

6. The holographic inverse scattering imaging system according to claim 1, characterized in that: The broadband dual-polarization antennas are distributed on a plane.

7. The holographic inverse scattering imaging system according to claim 1, characterized in that: The transmitting antenna transmits electromagnetic waves in the S band and the C band.

8. An imaging method based on the holographic inverse scattering imaging system according to claim 2, characterized in that: include: Delimiting an imaging area including the imaging target on the plane where the imaging target is located, dividing the imaging area into grids, with each grid serving as an imaging element; Each imaging element is considered as an equivalent electric dipole source Each electric dipole contains The components p in three directions x ,p y ,p z ; The measurement position of the antenna array is changed so that the receiving antenna traverses all observation points. The following equation is listed based on the scattered field information received by all observation points at any frequency: Wherein, M is the total number of all observation points, N is the number of the imaging elements; the total number of observation points M=D×Q, D is the number of the measurement positions, and Q is the number of the broadband dual-polarization antennas at each measurement position; is the observation angle of observation points 1, 2, ..., M relative to the imaging target; Where λ is the wavelength of the incident electromagnetic wave; k = 2π / λ is the wave number of the incident wave; is the position of the mth observation point; is the position of the nth equivalent electric dipole source, m = 1, 2, ..., M; n = 1, 2, ..., N; for point to Unit vector of direction; I is the identity matrix; Column vector (p′ 1x ,p′ 1y ,p′ 1z ,…,p′ Nz ) T is the value to be solved of the equation group corresponding to formula (3), which represents the solution result of each equivalent electric dipole source; E mz Represents the scattered field information of two polarization directions at the mth measurement point; Solve the equation group (3) obtained at all frequency points to obtain the solution results of all equivalent electric dipole sources at each frequency point; Considering each imaging element as a wave source that can radiate multi-frequency electromagnetic waves, the value of the solution result of the equivalent dipole in any imaging element at multiple frequency points (p′ nx ,p′ ny ,p′ nz ) T , which represents the radiation intensity of the wave source; assuming the number of frequency points is P, the solution results of all frequency points on the same component of the same imaging element are subjected to IFFT, that is, the information of the equivalent wave source, that is, the equivalent dipole source in the quasi-time domain t=0, 1, 2, ..., P-1 is obtained; the information of the equivalent dipole source at t=0 in the quasi-time domain is selected as the final solution result of the equivalent dipole source corresponding to the imaging element; the final result of the equivalent dipole source of the three components corresponding to an imaging element is modulo the value, that is, the imaging result of the imaging element; all imaging elements are traversed to obtain the target image.

Images