Method for obtaining scattering field of rough target and ISAR imaging based on vortex electromagnetic wave decomposition

Abstract

The application provides a rough target scattering field acquisition and ISAR imaging method based on vortex electromagnetic wave decomposition, and is used for solving the problem that there is no rough target scattering field acquisition method and imaging method under vortex electromagnetic wave incidence in the current industry. The rough target scattering field acquisition method provided by the application is as follows: a target is meshed, a random rough surface is generated, and a rough target geometric model is established; the vortex electromagnetic wave is expanded into a plane electromagnetic wave superposition form with different elevation angles and different phase distributions; a vortex electromagnetic wave incidence coordinate system and a rough target geometric model scattering coordinate system are converted, far zone scattering fields of each plane electromagnetic wave acting on a single facet element of the target are solved, and then far zone scattering fields of all single facet elements of the rough target under vortex electromagnetic wave incidence are obtained; and a total far zone scattering field of the rough target under vortex electromagnetic wave incidence is obtained through a vector superposition principle.

Description

TECHNICAL FIELD

[0001] The present application relates to vortex electromagnetic wave, and particularly relates to a rough target scattering field acquisition and ISAR imaging method based on vortex electromagnetic wave decomposition. BACKGROUND

[0002] The research on the orbital angular momentum characteristics is initially started from the optical field, and many specific applications such as optical wrench, optical tweezers and optical communication are generated. In recent years, the vortex electromagnetic wave is generated by using an antenna array, so that the research on the vortex electromagnetic wave characteristics and applications becomes a hotspot. The vortex electromagnetic wave has great potential in improving the performance of the radio communication and radar system due to the spiral wave front phase distribution and the orbital angular momentum mode.

[0003] The previous research on the vortex electromagnetic wave is mainly concentrated in the generation and communication application. Among them, the target electromagnetic scattering characteristics and radar imaging research is an important part in the field of radar system design. In the field of vortex electromagnetic wave scattering, for the optical frequency band, some scholars theoretically study the scattering characteristics of high-order Bessel vortex beams on spherical particles, and for the electromagnetic frequency band, some scholars measure the backscattering characteristics of corner reflector and flat plate target under the vortex electromagnetic wave. However, these researches are concentrated on regular objects such as spherical objects and flat plate objects, and the theoretical research on the vortex electromagnetic wave scattering of complex targets is very little. At present, there is no scattering field acquisition method and imaging method of rough target under the vortex electromagnetic wave in the industry. SUMMARY

[0004] The present application aims to solve the problem that there is no scattering field acquisition method and imaging method of rough target under the vortex electromagnetic wave in the industry, and provides a rough target scattering field acquisition and ISAR imaging method based on vortex electromagnetic wave decomposition.

[0005] In order to achieve the above purpose, the technical scheme is adopted to realize the present application:

[0006] A rough target scattering field acquisition method based on vortex electromagnetic wave decomposition, which is characterized by comprising the following steps:

[0007] 1. The target is meshed to obtain a plurality of target facet elements; at the same time, a random rough surface is generated, and a rough target geometric model capable of determining the roughness and correlation length distribution under the vortex electromagnetic wave incidence is established by combining the obtained plurality of target facet elements;

[0008] 2. The vortex electromagnetic wave is expanded into a superposition form of a plurality of plane electromagnetic waves with different elevation angles and different phase distributions by Bessel function integral transformation;

[0009] 3】Convert the incident coordinate system of the vortex electromagnetic wave to the scattering coordinate system of the rough target geometric model, solve the far-zone scattering field of each facet element of the rough target geometric model obtained in step 1】 under the action of each plane electromagnetic wave expanded in step 2】 by plane wave scattering theory, and then coherently superimpose the far-zone scattering fields of each facet element of the rough target under the action of each plane electromagnetic wave to obtain the far-zone scattering field of each facet element of the rough target under the incidence of the vortex electromagnetic wave;

[0010] 4】Repeat step 3】 to obtain the far-zone scattering field of all single facet elements of the rough target under the incidence of the vortex electromagnetic wave;

[0011] 5】Coherently superimpose the far-zone scattering fields of all single facet elements of the rough target under the incidence of the vortex electromagnetic wave obtained in step 4】 by the vector superposition principle to obtain the total far-zone scattering field of the rough target under the incidence of the vortex electromagnetic wave.

[0012] Further, step 1】 is specifically:

[0013] 1.1】According to the incident frequency of the vortex electromagnetic wave, the target is meshed to obtain a plurality of target facet elements;

[0014] 1.2】A Gaussian random rough surface is generated by using a linear filtering method;

[0015] 1.3】The generated Gaussian random rough surface fluctuation is mapped to the plurality of target facet elements to establish a rough target geometric model capable of determining the roughness and correlation length distribution under the irradiation of the vortex electromagnetic wave.

[0016] Further, in step 2】, the vortex electromagnetic wave is expanded into a superposition form of multiple plane electromagnetic waves with different elevation angles and different phase distributions, wherein the expressions of the expanded vortex electric field E and vortex magnetic field H are respectively:

[0017]

[0018] wherein k x =k ρ cosυα, k y =k ρ sinυ, k z =k cosα, k ρ =k sinα, α represents the elevation angle of the plane electromagnetic wave, υ represents the phase angle of the plane electromagnetic wave, k represents the incident wave number in free space, j represents the imaginary unit; A(k x ,k y ) / k ρ represents the amplitude of the expanded plane electromagnetic wave; represents the x-direction component in the incident coordinate system of the vortex electromagnetic wave, represents the y-direction component in the incident coordinate system of the vortex electromagnetic wave, represents the z-direction component in the coordinate system of the incident vortex electromagnetic wave; l represents the orbital angular momentum mode of the vortex electromagnetic wave; a represents the amplitude of the x-direction component of the electric field of the vortex electromagnetic wave; b represents the amplitude of the y-direction component of the electric field of the vortex electromagnetic wave; ω represents the angular frequency of the incident wave in free space; and μ represents the magnetic permeability in free space.

[0019] Further, the step 3 is specifically:

[0020] 3.1】Converting the incident coordinate system of the vortex electromagnetic wave and the scattering coordinate system of the rough target geometric model to obtain the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model;

[0021] 3.2】According to the obtained incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model, combining the tangent plane approximation and the integral equation method, the far-zone scattering fields of each plane electromagnetic wave acting on the single facet element of the rough target in the rough target geometric model obtained in step 1 are respectively solved;

[0022] 3.3】Coherently superimposing the far-zone scattering fields of each plane electromagnetic wave acting on the single facet element of the rough target to obtain the far-zone scattering field of the single facet element of the rough target under the incidence of the vortex electromagnetic wave The specific expression form is:

[0023]

[0024] Wherein, represents the normal vector of the tangent plane, represents the scattering wave vector, η1 represents the characteristic impedance in free space, R = |r-r'| represents the distance from the observation point to the source point, ds' represents the area element of the single facet element of the rough target, ε1 represents the dielectric constant in free space, J(r') represents the vortex current of the single facet element of the rough target, and M(r') represents the vortex magnetic current of the single facet element of the rough target.

[0025] The application also provides a rough target ISAR imaging method based on vortex electromagnetic wave decomposition, which is characterized by comprising the following steps:

[0026] 1】Grid partitioning is performed on the target to obtain a plurality of target facet elements; at the same time, a random rough surface is generated, and a rough target geometric model capable of determining the roughness and correlation length distribution under the incidence of the vortex electromagnetic wave is established in combination with the obtained plurality of target facet elements;

[0027] 2】The vortex electromagnetic wave is expanded into a plurality of plane electromagnetic wave superposition forms with different elevation angles and different phase distributions through Bessel function integral transformation;

[0028] 3】Convert the incident coordinate system of the vortex electromagnetic wave and the scattering coordinate system of the rough target geometric model, solve the far-zone scattering field of each facet element of the rough target geometric model obtained in step 1】 under the action of each plane electromagnetic wave expanded in step 2】 by plane wave scattering theory, and then coherently superimpose the far-zone scattering fields of each facet element of the rough target under the action of each plane electromagnetic wave to obtain the far-zone scattering field of each facet element of the rough target under the incidence of the vortex electromagnetic wave;

[0029] 4】Repeat step 3】 to obtain the far-zone scattering field of each facet element of the rough target under the incidence of the vortex electromagnetic wave;

[0030] 5】Use the turntable imaging algorithm to combine the far-zone scattering field distribution of each facet element of the rough target under the incidence of the vortex electromagnetic wave obtained in step 4】 to perform ISAR imaging of the rough target under the incidence of the vortex electromagnetic wave.

[0031] Further, step 1】 is specifically:

[0032] 1.1】According to the incident frequency of the vortex electromagnetic wave, the target is meshed to obtain a plurality of target facet elements;

[0033] 1.2】A linear filtering method is used to generate a Gaussian random rough surface;

[0034] 1.3】Map the generated Gaussian random rough surface fluctuation to the plurality of target facet elements to establish a rough target geometric model capable of determining the roughness and correlation length distribution under the irradiation of the vortex electromagnetic wave.

[0035] Further, in step 2】, the vortex electromagnetic wave is expanded into a superposition form of multiple plane electromagnetic waves with different elevation angles and different phase distributions, wherein the expressions of the expanded vortex electric field E and vortex magnetic field H are respectively:

[0036]

[0037]

[0038] wherein, k x =k ρ cosυα, k y =k ρ sinυ, k z =k cosα, k ρ =k sinα, α represents the elevation angle of the plane electromagnetic wave, υ represents the phase angle of the plane electromagnetic wave, k represents the incident wave number in free space, j represents the imaginary unit; A(k x ,k y ) / k ρ represents the amplitude of the expanded plane electromagnetic wave; E x (r) = a l e -j l · r E y (r) = b l e -j l · r E z (r) = 0

[0039] Further, step 3】 is specifically:

[0040] 3.1】Convert the incident coordinate system of the vortex electromagnetic wave and the scattering coordinate system of the rough target geometric model to obtain the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model;

[0041] 3.2】According to the obtained incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model, combine the tangent plane approximation and the integral equation method to solve the far-zone scattering field of each plane electromagnetic wave acting on the single facet element of the rough target in the rough target geometric model obtained in step 1】;

[0042] 3.3】Coherently superimpose the far-zone scattering fields of each plane electromagnetic wave acting on the single facet element of the rough target to obtain the far-zone scattering field of the single facet element of the rough target under the incidence of the vortex electromagnetic wave The specific expression is:

[0043]

[0044] wherein, is the normal vector of the tangent plane, is the scattering wave vector, η1 is the characteristic impedance in free space, R = |r-r'| is the distance from the observation point to the source point, ds' is the area element of the single facet element of the rough target, ε1 is the dielectric constant in free space, J(r') is the vortex current of the single facet element of the rough target, and M(r') is the vortex magnetic current of the single facet element of the rough target.

[0045] Further, step 5】 is specifically:

[0046] Perform two-dimensional inverse Fourier transform on the far-zone scattering field distribution of all single facet elements of the rough target under the incidence of the vortex electromagnetic wave obtained in step 4】 by the turntable imaging algorithm to complete the ISAR imaging of the rough target under the incidence of the vortex electromagnetic wave, and the expression of the imaging intensity f(x,y) is:

[0047]

[0048] wherein, θ mWhere, represents the total rotation angle of the target, represents the observation angle, and x and y represent the position coordinates of the target in the x direction and y direction of the scattering coordinate system of the rough target geometric model, respectively.

[0049] The beneficial effects of the present application are as follows:

[0050] The present application innovatively proposes a rough target scattering field acquisition method based on vortex electromagnetic wave decomposition and an IASR imaging method, which can accurately and effectively obtain the vortex electromagnetic wave scattering field and ISAR imaging of the rough target, and fills the blank of the scattering theory research of the rough target under the vortex electromagnetic wave incidence. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 The flowchart of the rough target scattering field acquisition method based on vortex electromagnetic wave decomposition according to the present application is shown in the figure.

[0052] Figure 2 The conversion diagram of the incidence coordinate system of the vortex electromagnetic wave and the scattering coordinate system of the rough target geometric model in the method step 3 of the present application is shown in the figure. DETAILED DESCRIPTION

[0053] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, but not all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0054] A rough target scattering field acquisition method based on vortex electromagnetic wave decomposition is shown in the figure, which specifically includes the following steps. Figure 1 The figure shows the rough target scattering field acquisition method based on vortex electromagnetic wave decomposition.

[0055] 1. Establishing a rough target geometric model

[0056] 1.1. Grid partitioning the target according to the incidence frequency of the vortex electromagnetic wave to obtain a plurality of target facet elements. In this embodiment, the target is grid partitioned by using the simulation software FEKO, and a grid data file is exported. The exported grid data file contains the number of the target facet element, the coordinates of three vertices, and the normal vector of the target facet element. The position vector of the three vertices of the i-th target facet element is denoted as The normal vector is denoted as

[0057] 1.2. Generating a Gaussian random rough surface by using a linear filtering method. The root mean square height δ of the generated Gaussian random rough surface and the correlation length l of the Gaussian random rough surface satisfy the Gaussian power spectrum function, that is:

[0058]

[0059] where S(k) denotes the Gaussian power spectrum function, and k is the spatial wave number.

[0060] Assuming the number of target facets is N, the linear filtering method can be used to obtain the roughness function f of N points.

[0061] 1.2】Since the center position vector of the i-th target facet is The generated Gaussian random roughness is mapped to the smooth target facets, i.e., the vector of the center position of the i-th target facet is replaced by Thus, the rough target geometry model that satisfies the determined roughness and correlation length distribution under vortex wave incidence is established.

[0062] 2】The vortex electromagnetic wave is expanded into a superposition form of multiple plane electromagnetic waves with different elevation angles and different phase distributions; different elevation angles can be represented by α1, α2, ….

[0063] Assuming the electric field E of the vortex electromagnetic wave is:

[0064]

[0065] where a represents the amplitude of the x-direction component of the vortex electromagnetic wave electric field, b represents the amplitude of the y-direction component of the vortex electromagnetic wave electric field, e -jlφ is the vortex phase, represents the x-direction component in the incident coordinate system of the vortex electromagnetic wave, represents the y-direction component in the incident coordinate system of the vortex electromagnetic wave, represents the z-direction component in the incident coordinate system of the vortex electromagnetic wave, E z represents the longitudinal component of the vortex electromagnetic wave, E(z, ρ) e -jlφ represents the transverse component of the vortex electromagnetic wave.

[0066] Using the Bessel function integral transform, the transverse component of the vortex electromagnetic wave can be expressed as:

[0067]

[0068] i.e., the vortex phase e -jlφ is converted to e -jlυ , and the following relationship holds:

[0069] k x = k ρ cosυα, k y = k ρ sinυ, k z = k cosα, α represents an elevation angle of the plane electromagnetic wave, υ represents a phase angle of the plane electromagnetic wave, k represents an incident wave number in free space, and j represents an imaginary unit.

[0070] According to the transverse-longitudinal relationship of the electromagnetic field, expressions of vortex electric field E and vortex magnetic field H are obtained as follows:

[0071]

[0072] wherein A(k x ,k y ) / k ρ represents an amplitude of the expanded plane electromagnetic wave, l represents an orbital angular momentum mode of the vortex electromagnetic wave; ω represents an angular frequency of the incident wave in free space, and μ represents a magnetic permeability in free space.

[0073] 3】The scattering coordinate system of the rough target geometric model is converted from the vortex electromagnetic wave incident coordinate system, and the far-field scattering field of the rough target under the vortex electromagnetic wave incidence is solved by the plane wave scattering theory. The far-field scattering field of the rough target under the vortex electromagnetic wave incidence is solved by taking the vortex electromagnetic wave decomposition combined with the physical optics method as an example.

[0074] 3.1】As shown in Figure 2 , the conversion relationship between the scattering coordinate system of the rough target geometric model and the vortex electromagnetic wave incident coordinate system is as follows:

[0075]

[0076] By using the conversion relationship between the scattering coordinate system of the rough target geometric model and the vortex electromagnetic wave incident coordinate system, the incident vector k i of the expanded plane wave in the scattering coordinate system of the rough target geometric model and the incident electric field E i (r) in the scattering coordinate system of the rough target geometric model can be obtained, and the specific expressions are as follows:

[0077]

[0078] wherein

[0079]

[0080] is a polarization unit vector of the incident wave.

[0081] 3.2】According to the obtained incident electric field E i (r) in the scattering coordinate system of the rough target geometric model, the far-field scattering field of each plane electromagnetic wave acting on the single facet element of the rough target in the rough target geometric model obtained in step 1】 is solved by combining the tangent plane approximation and the integral equation method.

[0082] 3.3】Coherent superposition of the far-zone scattering field of each plane electromagnetic wave acting on the single facet element of the rough target, to obtain the far-zone scattering field of the single facet element of the rough target under the incidence of the vortex electromagnetic wave The specific expression is:

[0083]

[0084] Wherein, represents the normal vector of the tangent plane, represents the scattering wave vector, η1 represents the characteristic impedance in free space, R = |r-r'| represents the distance from the observation point to the source point, ds' represents the area element of the single facet element of the rough target, ε1 represents the dielectric constant in free space, J(r') represents the vortex current of the single facet element of the rough target, and M(r') represents the vortex magnetic current of the single facet element of the rough target.

[0085] Specifically, the expressions of the vortex current J(r') and the vortex magnetic current M(r') of the single facet element of the rough target are as follows:

[0086]

[0087] Wherein, R hh and R vv are Fresnel reflection coefficients, represents the horizontal polarization component,

[0088] represents the vertical polarization component, represents the incident wave vector.

[0089] 4】Repeat step 3】to obtain the far-zone scattering field of all single facet elements of the rough target under the incidence of the vortex electromagnetic wave.

[0090] 5】Through the vector superposition principle, the far-zone scattering field of all single facet elements of the rough target under the incidence of the vortex electromagnetic wave obtained in step 4】is coherently superposed, to obtain the total far-zone scattering field of the rough target under the incidence of the vortex electromagnetic wave.

[0091] The application also provides a rough target ISAR imaging method based on vortex electromagnetic wave decomposition, specifically comprising steps 1】-5】.

[0092] 5】The far zone scattering field distribution of all single facet elements of the rough target under the vortex electromagnetic wave incidence obtained in step 4】 is two-dimensionally inverse Fourier transformed by a turntable imaging algorithm, and the ISAR imaging of the rough target under the vortex electromagnetic wave incidence is completed, and the expression of the imaging intensity f(x, y) is as follows:

[0093]

[0094] Wherein, θ m represents the total rotation angle of the target, θ represents the observation angle, and x and y respectively represent the position coordinates of the x direction and the y direction in the scattering coordinate system of the rough target geometric model.

[0095] The rough target scattering field and the ISAR imaging under the vortex electromagnetic wave can be accurately and effectively obtained by using the rough target scattering field acquisition and ISAR imaging method based on vortex electromagnetic wave decomposition provided by the application.

[0096] The above is only the preferred embodiment of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for obtaining the scattered field of a rough target based on vortex electromagnetic wave decomposition, characterized in that, Includes the following steps:

1. The target is meshed to obtain multiple target facets; at the same time, random rough surfaces are generated, and a rough target geometric model is established by combining the obtained multiple target facets to determine the roughness and related length distribution under the incident of vortex electromagnetic waves. 2】Through the Bessel function integral transform, the vortex electromagnetic wave is expanded into a superposition of multiple plane electromagnetic waves with different elevation angles and different phase distributions; 3】 Transform the vortex electromagnetic wave incident coordinate system and the rough target geometric model scattering coordinate system. Solve the far-field scattering field of each plane electromagnetic wave expanded in step 2】acting on the rough target geometric model obtained in step 1】 using plane wave scattering theory. Then coherently superimpose the far-field scattering fields of each plane electromagnetic wave acting on the rough target individual small surface element to obtain the far-field scattering field of the rough target individual small surface element under vortex electromagnetic wave incident. 4】Repeat step 3】 to obtain the far-field scattering field of all individual small surface elements of the rough target under the incident vortex electromagnetic wave; 5】The far-field scattering fields of all individual small surface elements of the rough target under the incident vortex electromagnetic wave are coherently superimposed by the principle of vector superposition to obtain the total far-field scattering field of the rough target under the incident vortex electromagnetic wave.

2. The method for obtaining the rough target scattering field based on vortex electromagnetic wave decomposition according to claim 1, characterized in that, Step 1】Specifically: 1.1】The target is meshed according to the incident frequency of the vortex electromagnetic wave to obtain multiple target small surface elements; 1.2】A Gaussian random rough surface is generated using a linear filtering method; 1.3】The generated Gaussian random rough surface undulations are mapped onto multiple target small surface elements to establish a rough target geometric model that can determine the roughness and related length distribution under vortex electromagnetic wave illumination.

3. The method for obtaining the rough target scattering field based on vortex electromagnetic wave decomposition according to claim 2, characterized in that: In step 2, the vortex electromagnetic wave is expanded into a superposition of multiple plane electromagnetic waves with different elevation angles and phase distributions. The expanded expressions for the vortex electric field E and the vortex magnetic field H are as follows: Where, k x =k ρ cosυα,k y =k ρ sinυ,k z =kcosα,k ρ =ksinα, where α represents the elevation angle of the plane electromagnetic wave, υ represents the phase angle of the plane electromagnetic wave, k represents the incident wave number in free space, and j represents the imaginary unit; A(k x ,k y ) / k ρ This represents the amplitude of an expanded plane electromagnetic wave; This represents the x-axis component of the vortex electromagnetic wave incident in the coordinate system. This represents the y-axis component in the incident coordinate system of the vortex electromagnetic wave. ω represents the z-direction component of the incident coordinate system of the vortex electromagnetic wave; l represents the orbital angular momentum mode of the vortex electromagnetic wave; a represents the amplitude of the x-direction component of the electric field of the vortex electromagnetic wave; b represents the amplitude of the y-direction component of the electric field of the vortex electromagnetic wave; ω represents the angular frequency of the incident wave in free space; and μ represents the permeability in free space.

4. The method for obtaining the rough target scattering field based on vortex electromagnetic wave decomposition according to claim 3, characterized in that, Step 3: Specifically, it is as follows: 3.1】Transform the incident coordinate system of the vortex electromagnetic wave with the scattering coordinate system of the rough target geometric model to obtain the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model. 3.2】Based on the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the obtained rough target geometric model, and combining the tangent plane approximation and the integral equation method, the far-field scattering field of each plane electromagnetic wave expanded in step 2】acting on a single small surface element of the rough target in the rough target geometric model obtained in step 1】 is solved respectively. 3.3】The far-field scattered fields of individual small surface elements of a rough target are coherently superimposed by the plane electromagnetic waves acting on them to obtain the far-field scattered field of an individual small surface element of a rough target under the incident vortex electromagnetic wave. Its specific form of expression is: in, The vector representing the normal to the tangent plane. Let denot η1 represent the characteristic impedance in free space, R = |r - r'| represent the distance from the observation point to the source point, ds' represent the area element of a single small surface element of the rough target, ε1 represent the free space dielectric constant, J(r') represent the eddy current of a single small surface element of the rough target, M(r') represent the eddy magnetic current of a single small surface element of the rough target, and j represent the imaginary unit.

5. A coarse target ISAR imaging method based on vortex electromagnetic wave decomposition, characterized in that, Includes the following steps:

1. The target is meshed to obtain multiple target facets; at the same time, random rough surfaces are generated, and a rough target geometric model is established by combining the obtained multiple target facets to determine the roughness and related length distribution under the incident of vortex electromagnetic waves. 2】Through the Bessel function integral transform, the vortex electromagnetic wave is expanded into a superposition of multiple plane electromagnetic waves with different elevation angles and different phase distributions; 3】 Transform the vortex electromagnetic wave incident coordinate system and the rough target geometric model scattering coordinate system. Solve the far-field scattering field of each plane electromagnetic wave expanded in step 2】acting on the rough target geometric model obtained in step 1】 using plane wave scattering theory. Then coherently superimpose the far-field scattering fields of each plane electromagnetic wave acting on the rough target individual small surface element to obtain the far-field scattering field of the rough target individual small surface element under vortex electromagnetic wave incident. 4】Repeat step 3】 to obtain the far-field scattering field of all individual small surface elements of the rough target under the incident vortex electromagnetic wave; 5】Using the turntable imaging algorithm combined with the far-field scattering distribution of all individual small elements of the rough target under vortex electromagnetic wave incident obtained in step 4】ISAR imaging is performed on the rough target under vortex electromagnetic wave incident.

6. The coarse target ISAR imaging method based on vortex electromagnetic wave decomposition according to claim 5, characterized in that, Step 1】Specifically: 1.1】The target is meshed according to the incident frequency of the vortex electromagnetic wave to obtain multiple target small surface elements; 1.2】A Gaussian random rough surface is generated using a linear filtering method; 1.3】The generated Gaussian random rough surface undulations are mapped onto multiple target small surface elements to establish a rough target geometric model that can determine the roughness and related length distribution under vortex electromagnetic wave illumination.

7. The coarse target ISAR imaging method based on vortex electromagnetic wave decomposition according to claim 6, characterized in that: In step 2, the vortex electromagnetic wave is expanded into a superposition of multiple plane electromagnetic waves with different elevation angles and phase distributions. The expanded expressions for the vortex electric field E and the vortex magnetic field H are as follows: Where, k x =k ρ cosυα,k y =k ρ sinυ,k z =kcosα,k ρ =ksinα, where α represents the elevation angle of the plane electromagnetic wave, υ represents the phase angle of the plane electromagnetic wave, k represents the incident wave number in free space, and j represents the imaginary unit; A(k x ,k y ) / k ρ This represents the amplitude of an expanded plane electromagnetic wave; This represents the x-axis component of the vortex electromagnetic wave incident in the coordinate system. This represents the y-axis component in the incident coordinate system of the vortex electromagnetic wave. ω represents the z-direction component of the incident coordinate system of the vortex electromagnetic wave; l represents the orbital angular momentum mode of the vortex electromagnetic wave; a represents the amplitude of the x-direction component of the electric field of the vortex electromagnetic wave; b represents the amplitude of the y-direction component of the electric field of the vortex electromagnetic wave; ω represents the angular frequency of the incident wave in free space; and μ represents the permeability in free space.

8. The coarse target ISAR imaging method based on vortex electromagnetic wave decomposition according to claim 7, characterized in that, Step 3: Specifically, it is as follows: 3.1】Transform the incident coordinate system of the vortex electromagnetic wave with the scattering coordinate system of the rough target geometric model to obtain the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the rough target geometric model. 3.2】Based on the incident electric field of the vortex electromagnetic wave in the scattering coordinate system of the obtained rough target geometric model, the far-field scattering field of each plane electromagnetic wave expanded in step 2】acting on a single small surface element of the rough target in the rough target geometric model obtained in step 1】is solved by combining the tangent plane approximation and the integral equation method. 3.3】The far-field scattered fields of individual small surface elements of a rough target are coherently superimposed by the plane electromagnetic waves acting on them to obtain the far-field scattered field of an individual small surface element of a rough target under the incident vortex electromagnetic wave. Its specific form of expression is: in, The vector representing the normal to the tangent plane. Let denot η1 represent the characteristic impedance in free space, R = |r - r'| represent the distance from the observation point to the source point, ds' represent the area element of a single small surface element of the rough target, ε1 represent the free space dielectric constant, J(r') represent the eddy current of a single small surface element of the rough target, M(r') represent the eddy magnetic current of a single small surface element of the rough target, and j represent the imaginary unit.

9. A coarse target ISAR imaging method based on vortex electromagnetic wave decomposition according to claim 8, characterized in that, Step 5 is as follows: The far-field scattering distribution of all individual small elements of the rough target under vortex electromagnetic wave incident is obtained in step 4] by performing a two-dimensional inverse Fourier transform on the turntable imaging algorithm, thus completing the ISAR imaging of the rough target under vortex electromagnetic wave incident. The expression for its imaging intensity f(x,y) is as follows: Where, θ m θ represents the total rotation angle of the target, θ represents the observation angle, and x and y represent the position coordinates in the x and y directions of the scattering coordinate system of the rough target geometric model, respectively.

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