RCS measurement method based on target inherent feature extraction and near-field single station measurement data

Abstract

This invention discloses an RCS measurement method based on target intrinsic feature extraction and near-field monostation measurement data. The invention introduces an equivalent surface to enclose the target, discretizing it into multiple discrete elements. The scattered field is represented as the effective current distribution on all discrete elements. By introducing a pair of dyadic kernel functions, the effective current is represented as the tangential components of the incident wave's electric and magnetic fields. The discrete form of the dyadic kernel function is called the target's intrinsic feature parameters. By obtaining near-field monostation measurement data, a system of linear equations is constructed to solve for the target's intrinsic feature parameters, calculating the scattered field and radar cross section (RCS) for plane wave incidence. The theory of this invention is rigorous; the degree of freedom is controlled by adjusting the size of the discrete elements. Higher degrees of freedom result in higher solution complexity but also higher accuracy. For targets with simple structures, the coupling effect between most scattering elements is ignored, thus significantly reducing the degree of freedom and solution complexity. This invention has significant application value for RCS measurement of electrically large targets.

Description

Technical Field

[0001] This invention relates to electromagnetic scattering measurement technology, specifically to an RCS measurement method based on target intrinsic feature extraction and near-field monostation measurement data. Background Technology

[0002] Radar cross section (RCS) is a core parameter for measuring the scattering characteristics of a target. For far-field RCS measurements, the required experimental measurement distance increases rapidly with the electrical size of the target, and the anechoic chamber used for measurement is often insufficient to meet this distance requirement. Therefore, the need to predict far-field RCS using near-field measurement data has become extremely important. Over the past few decades, many algorithms have been developed to predict far-field RCS from near-field measurements, and many of these methods have been put into practical use.

[0003] Near-Field to Far-Field Transformation (NFFFT) is an algorithm for processing near-field data to predict the far-field RCS, thus significantly reducing the distance required for measurement. Some well-known methods include Merlin filtering, inverse source-based methods, and image transformation-based methods. However, different NFFFT methods have their own limitations, such as requirements for the shortest distance or target structure. Merlin's filtering method in "Measuring radar cross section at short distance" shortens the measurement distance but sacrifices angular resolution. Following Falconer's proposal of a physical optics-based near-field to far-field transform method in "Extrapolation of near-field RCS measurements to the far zone," methods using scattered near-field data to solve for the equivalent current and magnetic flux on the target surface and then extrapolating have been developed; however, due to phase approximation, these methods have relatively poor accuracy. In "Radar cross-section measurements using near-field radar imaging," Odendaal discovered the high similarity between near-field and far-field radar scattering images and applied this image to calculate the far-field RCS. Broquetas, in "Spherical wave near-field imaging and radar cross-section measurement," introduced a focusing factor to correct for phase inconsistencies caused by near-field spherical wave illumination. These two works contributed to the maturation of image-based NFFFT algorithms. Watanabe, in "Far-field radar cross-section determination from near-field 3-D synthetic aperture imaging with arbitrary antenna-scanning surfaces," introduced the problem of decreased accuracy when the target deviates from the measurement center. Watanabe, in "Image-based radar cross-section synthesis for acluster of multiple static targets," introduced the problem of inaccurate predictions when there is strong coupling between targets.Image-based near-field to far-field transformation methods have limitations due to some unrealistic approximations or assumptions. Two fundamental assumptions are: first, the scattering centers of the target are independent and not coupled to each other; second, the scattering centers are isotropic, and the RCS of each scattering center is independent of the incident angle. If the distance from the antenna to the target remains constant, these assumptions no longer apply as the target size increases or its structure becomes more complex. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention proposes an RCS measurement method based on target intrinsic feature extraction and near-field single-station measurement data.

[0005] The RCS measurement method of the present invention based on target intrinsic feature extraction and near-field single-station measurement data includes the following steps: 1) Near-field single-station measurement: The antenna measures the target to obtain near-field monostation measurement data; 2) Introduction and division of equivalent surfaces: An equivalent surface is introduced to enclose the target; further, the equivalent surface is discretized into multiple discrete elements, the size of which depends on the distance from the discrete element to the antenna. 3) Introduction of inherent characteristic parameters of the target and representation of the scattering field: Under the illumination of the incident field generated by the transmitting antenna, the scattered field at the receiving antenna position is represented by the effective current distribution on the discrete element. The effective current is represented by the tangential components of the electric and magnetic fields of the incident wave by introducing two dyadic kernel functions. After discretization, the scattered field is represented as the sum of the scattered fields generated by the effective currents on all discrete elements, that is, a weighted linear combination with discrete dyadic kernel functions as coefficients. The dyadic kernel function is only related to the target itself and is independent of the incident wave. The discrete form of the dyadic kernel function is called the inherent characteristic parameter of the target. 4) Solve for the inherent characteristic parameters of the target using near-field single-station measurement data: The coefficient matrix is ​​calculated based on the incident electric and magnetic fields at the center of the discrete unit. The discrete form of the scattered field is written as a system of linear equations. The excitation column vector on the right side of the linear equations is generated by near-field single-station measurement data. The linear equations are then optimized and solved using the least squares method of the Galing parameters to obtain the inherent characteristic parameters of the target. 5) Calculate the scattered field and radar cross-section under plane wave illumination: After solving for the inherent characteristic parameters of the target, the corresponding scattered field is obtained by changing the incident wave; the incident wave is set as a plane wave and the receiving point is set at infinity to obtain the target's monostatic and bistatic radar cross sections (RCS).

[0006] In step 1), the near-field monostatic scattering measurement system generally includes a turntable carrying the target, a pair of antennas for transmitting and receiving, and a vector network analyzer (VNA) that simultaneously provides signals to the transmitting antenna and processes the signals from the receiving antenna. The transmitting and receiving antennas are located very close to each other and can be approximated as the same location, forming a monostatic scattering measurement system. By rotating the turntable, moving the antennas, and adjusting the target's orientation, omnidirectional measurement can be achieved. There is no limit to the distance between the antenna and the target; the farther the better, provided that conditions permit. The target does not need to have special characteristics such as symmetry.

[0007] Near-field monostation measurement data consists of electric field measurements of the target's scattered field. A monostation is defined as the measurement antenna and transmitting antenna being located at the same position, while a bistation is defined as the measurement antenna and transmitting antenna being located at different positions. The near-field measurement method of this invention allows for arbitrarily close distances from the target to the antenna. The closer the distance, the more discrete elements are required, and the more complex the model solution becomes, but this does not affect the effectiveness of the method; theoretically, it is rigorous. In contrast, previous near-field measurement methods still had limitations on the distance between the two elements, and their theories were approximate.

[0008] The theory of this invention is rigorous; therefore, the method of this invention has no limitation on the distance between the antenna and the target, and the target does not need to be at the center of the measurement system. It is applicable to complex targets or situations where there are significant coupling effects between components. Using the method of this invention, the antenna can measure the target from a distance of one time the target's size or even closer to the target's center, and the antenna measurement surface is arbitrary. Generally, a turntable carrying the target rotates horizontally around the target's center, with the rotation axis in the vertical direction. The antenna can move up and down or along a longitude line, thereby forming a cylindrical or spherical measurement surface. Different measurement areas and ranges are selected according to the target's shape. For example, for a horizontally placed flat target, the antenna may only need to perform one circular measurement or the antenna may remain stationary while the turntable rotates one revolution. The measurement point interval is generally about half a wavelength. The measurement environment is in a microwave anechoic chamber or outdoors, with the lowest possible background noise. There should be no strong scattering interference near the target; otherwise, cancellation measures should be implemented.

[0009] In step 2), the closer the equivalent surface is to the target surface, the better. The shape of the equivalent surface is not limited, as long as it can encompass the target and place the target within the equivalent surface. However, for versatility, a minimum rectangular surface that can encompass the target is generally selected, with the length, width, and height of the rectangular surface equal to the maximum length, width, and height of the target. The size of the discrete element depends on its distance from the antenna; the closer the distance, the smaller the size. Alternatively, the discrete elements can be uniformly discretized according to the minimum size. For equivalent surfaces that are not rectangular, mature commercial software is used to discretize them into multiple discrete elements.

[0010] Based on prior knowledge of the target, a virtual surface called the equivalent surface is constructed to enclose the target. This equivalent surface is denoted as...S , will the equivalent surface S Divided into N The nth discrete unit, the nth n The surface of a discrete unit is denoted as The central position is A denser grid will result in higher accuracy, but also longer computation time. The grid density should be flexibly chosen based on the required RCS prediction accuracy. The scale of the discrete elements satisfies the far-field condition: or in, The location of the transmitting antenna. No. n The center position of each discrete unit d n For the first n The scale of a discrete unit n =1,…, N ; N The number of discrete units. λ The wavelength of the electromagnetic wave. Based on the required precision, it is taken as: (1).

[0011] In step 3), according to the equivalence principle, the target's scattered field is represented by an equivalent surface. S The equivalent current and magnetic current on the surface are used to represent the scattered field; according to the single-source method, the scattered field is... Using a single effective current Represented as: (2) in, The imaginary unit, Vacuum wavenumber, , The angular frequency of the electromagnetic wave. The vacuum permittivity, The permeability of free space, For free space wave impedance, , For free space dyadic Green's functions; r Indicates the location of the field. Indicates the location of the source. This represents the integral element on the equivalent surface. This represents the radius vector from the source point to the field point. This represents the effective current on the equivalent surface; theoretically, the effective current... It is composed of equivalent surfaces S Tangential components of the incident electric and magnetic fields and The decision is expressed as: (3) in, and These are the dyadic kernel functions introduced and related to the tangential components of the incident wave's electric and magnetic fields, respectively. They reflect the structural properties of the target under test and are independent of the incident field. The tangential components of the incident wave's electric and magnetic fields are calculated based on the antenna used in the measurement. and Based on the division in step 2), the continuous function is transformed into a discrete function, and the dyadic kernel function related to the tangential components of the incident wave's electric and magnetic fields is approximated in the following way. and : , (4) Among them, the superscript "( e / h )"express"( e ) "or"( h ")" corresponds to electric field or magnetic field, respectively. Indicates from the first n The center position of each discrete unit To the m The center position of each discrete unit The unit direction vector, S m For the first m The surface of a discrete unit m =1,…, N ; Discrete elements of the dyadic kernel function, which are related to the tangential components of the incident wave's electric or magnetic field, are called intrinsic characteristic parameters (IFPs) of the target, because they are unique characteristics of the target under test and are independent of the transmitting and receiving antennas.

[0012] In step 4), the target's inherent characteristic parameters (IFPs) are solved using the measured near-field monostation measurement data. Near-field monostation measurement data is used because monostation data is much easier to obtain than bistation / multistation data. For each discrete element, with the antenna located in its far-field scattering region, an approximation is made: (5) (6) in, For the first n Tangential components of the incident wave electric field at the center of a discrete unit; It is the first m The center position of each discrete unit To the receiving antenna position The radius vector; From the firstm The center position of each discrete unit To the receiving antenna position The unit direction vector; From the position of the transmitting antenna To the n The center position of each discrete unit The radius vector, for single-station measurements Substituting equations (3) to (6) into equation (2), the discretization method of the scattered field is approximately as follows: (7) in (8a) (8b) in, The scattered field measurement at the location of the receiving antenna. and These are two weighting factors related to the discrete unit. and These are the differences between the two directions; by changing the position of the receiving antenna. Collect sufficient scattered field measurements at the receiving antenna locations. The number of unknowns is at least greater than the number of unknowns to be solved. Equation (7) is assembled into a system of linear algebraic equations to solve for the target intrinsic characteristic parameter IFPs. The total number of unknowns is , among them This represents the number of dyadic vectors or matrices. Each matrix has four elements, and there are two types of matrices corresponding to the tangential components of the incident wave's electric and magnetic fields, respectively. The assembled linear equation system takes the form: (9) in, It is an excitation column vector composed of all scattered field measurements, which are the near-field single-station measurement data. Thus, the excitation column vector on the right side of the linear equation system is generated from the near-field single-station measurement data. It is a column vector consisting of all IFPs. The coefficient matrix is ​​formed based on the incident electric and magnetic fields at the center of the discrete unit; the linear equations are solved using the least squares principle with ridge parameter optimization, and equation (9) is solved as follows: (10) in, for The conjugate transpose of . It is a ridge parameter; in order to reduce the number of unknowns to be solved, the following approximations are made, the applicability of which mainly depends on the complexity of the objective: (1) If the coupling between discrete units can be ignored but the cross-coupling of the same discrete unit cannot be ignored, then like The unknown quantity is reduced to 8 N (2) If the coupling energy between discrete units is negligible, and the cross-polarization coupling of the same discrete unit is also negligible, then By making it a diagonal matrix, the number of unknowns is reduced to 4. N (3) If the coupling between discrete units cannot be ignored, but the cross-polarization coupling can be ignored, the number of unknowns is reduced to 4. N 2 Whether the coupling between discrete units and the cross-coupling within the same discrete unit can be ignored depends on the complexity of the target and the accuracy requirements. The typical measurement accuracy requirement is 1~5dB; the higher the accuracy requirement, the more it cannot be ignored, while the lower the accuracy requirement, the more it can be ignored.

[0013] In step 5), the incident wave is set as a plane wave, and the incident wave electric field... and incident wave magnetic field Represented as: , (11a) , (11b) in, and represent the polarization direction and the incident direction, respectively; by substituting this plane wave incident field into equation (7), the scattered field and bistatic radar cross section (RCS) at any location can be calculated. : (12) in, For the scattered field p polarization components, p Represents the polarization of the receiving field. q Represents the polarization of the incident field. Indicates the scattering direction. The single-station radar cross-section corresponds to .

[0014] Advantages of this invention: This invention proposes a new near-field to far-field transformation algorithm. By analyzing the equivalent current and magnetic current on the equivalent surface, it is found that there are inherent characteristic parameters of the target that are independent of the incident field and the scattered field. By measuring the scattered near field under different incident fields, these inherent characteristic parameters of the target can be solved. After obtaining them, the scattered field for any incident field can be calculated. Applying this method to the scattering measurement in the traditional microwave anechoic chamber or outdoor open field does not require modification of the measurement system and relaxes the distance limit between the antenna and the target. Its advantages include: (1) This method is theoretically rigorous and can complete high-precision measurement at any distance; (2) The complexity and diversity of the target will not affect the measurement accuracy of this method; (3) The measurement system requirements are simple, single-station measurement can be used, and it can be flexibly adapted to the existing measurement environment, with wide applicability. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a measurement system according to an embodiment of the RCS measurement method based on target intrinsic feature extraction and near-field single-station measurement data of the present invention; Figure 2 This is the main flowchart of the RCS measurement method based on target inherent feature extraction and near-field single-station measurement data of the present invention. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0017] The RCS measurement method based on target intrinsic feature extraction and near-field single-station measurement data in this embodiment is as follows: Figure 2 As shown, it includes the following steps: 1) Measure the target to obtain near-field single-station measurement data: like Figure 1 As shown, a styrene foam frustum is placed on a turntable, with the target, a scaled-down airplane model with a maximum size of 30cm, positioned at the center of the frustum. Two identical standard horn antennas (HD-100SGAH20 N) are fixed on the small turntable, 30cm from the center of the frustum. The turntable rotates horizontally around the center of the target, with the rotation axis vertical. The antennas can move up and down or along a longitude line, thus forming a cylindrical or spherical measurement scanning surface. The transmitting antenna is positioned as follows: The receiving antenna is located at The two antennas are placed next to each other, assuming The antenna position was determined using an infrared locator; the two antennas were placed with the same polarization (H / V) and connected to a vector network analyzer (VNA, N5247A) via a coaxial cable. The vector network analyzer was set to emit a signal at a frequency of 10 GHz; when the transmitting antenna emitted electromagnetic waves, the receiving antenna received the same polarized scattered electromagnetic waves from the target; the scattered signal was read by the vector network analyzer; the measurement point interval was approximately half a wavelength; the measurement environment was in a microwave anechoic chamber with low background noise, eliminating strong scattering interference near the target; 2) Introduction and division of equivalent surfaces: Given the approximate outline of the target, construct an equivalent surface of a cuboid. S Wrap the target with each facet approximately 2mm from the target at its closest point; then wrap the equivalent facet. S Dissected N The nth square discrete unit, the nth n The surface of a discrete unit is denoted as The central position is Theoretically, the side lengths of the discrete elements must satisfy the far-field condition: or in, d n For the first n The scale of a discrete unit λ The wavelength of the electromagnetic wave; selected according to accuracy requirements: (1) In this embodiment, let At that time, the prediction error of RCS is less than 1dB; the reference size is determined according to the required measurement accuracy. ,in This is the distance from the antenna to the nearest discrete element; 3) Introduction of inherent characteristic parameters of the target and representation of the scattering field: According to the equivalence principle, the target's scattered field is represented by an equivalent surface. S The equivalent current and magnetic current on the surface are used to represent the scattered field; according to the single-source method, the scattered field is... Using a single effective current Represented as: (2) in, The imaginary unit, Vacuum wavenumber, , The angular frequency of the electromagnetic wave. The vacuum permittivity, The permeability of free space, For free space wave impedance, , For free space dyadic Green's functions; r Indicates the location of the field. Indicates the location of the source. This represents the integral element on the equivalent surface. This represents the radius vector from the source point to the field point. This represents the effective current on the equivalent surface; theoretically, the effective current... It is composed of equivalent surfaces S Tangential components of the incident electric and magnetic fields and The decision is expressed as: (3) in, and These are the dyadic kernel functions introduced and related to the tangential components of the incident wave's electric and magnetic fields, respectively. They reflect the structural properties of the target under test and are independent of the incident field. The tangential components of the incident wave's electric and magnetic fields are calculated based on the antenna used in the measurement. and Based on the division in step 2), the continuous function is transformed into a discrete function, and the dyadic kernel function related to the tangential components of the incident wave's electric and magnetic fields is approximated in the following way. and : , (4) Among them, the superscript "( e / h )"express"( e ) "or"( h ")" corresponds to electric field or magnetic field, respectively. Indicates from the first n The center position of each discrete unit To the m The center position of each discrete unit The unit direction vector, S m For the first m The surface of a discrete unit m =1,…, N ; Discrete elements of the dyadic kernel function, which are related to the tangential components of the incident electric or magnetic field, are called intrinsic characteristic parameters (IFPs) of the target, because they are unique characteristics of the target under test and are independent of the transmitting and receiving antennas. 4) Solve for the inherent characteristic parameters of the target using near-field single-station measurement data: For each discrete element, the antenna is located in its far-field scattering region, and an approximation is made: (5) (6) in, For the first n Tangential components of the incident wave electric field at the center of a discrete unit; It is the first m The center position of each discrete unit To the receiving antenna position The radius vector; From the first m The center position of each discrete unit To the receiving antenna position The unit direction vector; From the position of the transmitting antenna To the n The center position of each discrete unit The radius vector, for single-station measurements Substituting equations (3) to (6) into equation (2), the discretization method of the scattered field is approximately as follows: (7) Using a standard horn antenna, the field it produces is known and can be found in textbooks or calculated using simulation software to obtain the incident wave electric field at the center of all discrete units. and incident wave magnetic field , Substitute them into formula (7) to obtain the scattered field at the receiving antenna; where , (8a) , (8b) (8c) in, The scattered field measurement at the location of the receiving antenna. , For unit vector, It is a vector partial differential operator; Rotating the turntable is equivalent to rotating the antenna to change the position of the receiving antenna. Based on the rotation speed, signals are transmitted and scattered near-field data are received at a certain frequency, with each interval equivalent to the antenna moving approximately half a wavelength; assuming the acquisition... L Near-field monostatic scattering data at different locations are represented as an excitation column vector. : (A1) in, T Indicates transpose. For the first iThe scattered field measurements at each receiving antenna location, i =1,…, L , subscript " "Mark the dimensions; the column vector of the IFPs to be solved." : , (A2) in, and These are elements in the IFPs corresponding to the tangential components of the incident wave's electric and magnetic fields, respectively. They are sub-column vectors with four parameters, such as... ,in, For the elements in IFPs corresponding to the tangential component of the incident wave electric field, s =1 or 2, t= 1 or 2; the matrix is ​​calculated based on the incident wave electric and magnetic fields at the center of the discrete element. for: (A3) Each element is actually a 1×4 sub-row vector. Let be an arbitrary sub-row vector related to the tangential direction of the incident wave's magnetic field, and an arbitrary sub-row vector related to the tangential direction of the incident wave's electric field. ( ): (A4) in, Indicates the first i The secondary transmitting and receiving antennas are located at... , It is the polarization direction of the received electric field. row vectors From the first m The center position of each discrete unit To the i Each receiving antenna position The radius vector; and It is the first m Two tangential direction vectors for each discrete element, and a matrix related to the tangential direction of the incident wave electric field. : (A5) in, and The incident wave at the th n Two tangential direction components on a discrete unit and It is the first n Two tangential direction vectors of a discrete unit; and They are the same in form, just need to... Change to : (A6) in, The matrix is ​​related to the tangential direction of the incident wave's magnetic field. and These are elements in the matrix that are tangentially related to the magnetic field direction of the incident wave. and ; Collect sufficient scattered field measurements at the receiving antenna locations. The number of unknowns is at least greater than the number of unknowns to be solved. Equation (7) is assembled into a system of linear algebraic equations to solve for the inherent characteristic parameters of the target. The total number of unknowns is , among them This represents the number of dyadic vectors or matrices. Each matrix has four elements, and there are two types of matrices corresponding to the tangential components of the incident wave's electric and magnetic fields, respectively. The assembled linear equation system takes the form: (9) in, It is an excitation column vector composed of all scattered field measurements, which are the near-field single-station measurement data. Therefore, the excitation column vector on the right-hand side of the linear equation system is generated from the near-field single-station measurement data, and the scattered field measurements are... It is a column vector consisting of all IFPs. The coefficient matrix is ​​formed. The equation system is solved using the least squares principle plus ridge parameter optimization. Solving equation (9) yields: (10) in, for The conjugate transpose of . It is a ridge parameter; in order to reduce the number of unknowns to be solved, the following approximations are made, the applicability of which mainly depends on the complexity of the target: (1) If the coupling between discrete units can be ignored but the cross coupling of the same discrete unit cannot be ignored, then like The unknown quantity is reduced to 8 N (2) If the coupling energy between discrete units is negligible, and the cross-polarization coupling effect of the same discrete unit is also negligible, then By making it a diagonal matrix, the number of unknowns is reduced to 4. N (3) If the coupling between discrete units cannot be ignored, but the cross-polarization coupling effect can be ignored, the number of unknowns is reduced to 4. N 2Whether the coupling between discrete units and the cross-coupling within the same discrete unit can be ignored depends on the complexity of the target and the accuracy requirements. If the typical measurement accuracy requirement is 1~5dB, then it cannot be ignored if the accuracy requirement is higher, and it can be ignored if the accuracy requirement is lower. 5) Calculate the scattered field and radar cross-section under plane wave illumination: If the incident wave is defined as a plane wave, the electric field of the incident wave... and incident wave magnetic field Represented as: , (11a) , (11b) in, and represent the polarization direction and the incident direction, respectively; by substituting this plane wave incident field into equation (7), the scattered field and bistatic radar cross section (RCS) at any location can be calculated. : (12) in, For the scattered field p polarization components, p Represents the polarization of the receiving field. q Represents the polarization of the incident field. Indicates the scattering direction. A single-station RCS corresponds to .

[0018] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A method for RCS measurement based on target intrinsic feature extraction and near-field single-station measurement data, characterized in that, The measurement method includes the following steps: 1) Near-field single-station measurement: The antenna measures the target to obtain near-field monostation measurement data; 2) Introduction and division of equivalent surfaces: An equivalent surface is introduced to enclose the target, and the equivalent surface is discretized into multiple discrete units; 3) Introduction of inherent characteristic parameters of the target and representation of the scattering field: Under the illumination of the incident field generated by the transmitting antenna, the scattered field at the receiving antenna position is represented by the effective current distribution on the discrete element. By introducing two dyadic kernel functions, the effective current is represented as the tangential components of the incident wave electric field and magnetic field. After discretization, the scattered field is represented as the sum of the scattered fields generated by the effective current on all discrete elements. The discrete form of the dyadic kernel function is called the target intrinsic characteristic parameter IFPs. 4) Solve for the inherent characteristic parameters of the target using near-field single-station measurement data: The coefficient matrix is ​​calculated based on the incident electric and magnetic fields at the center of the discrete unit. The discrete form of the scattered field is written as a system of linear equations. The excitation column vector of the system of linear equations is generated by near-field single-station measurement data. The system of linear equations is then optimized and solved by the least squares method of the ridge parameters to obtain the inherent characteristic parameters of the target. 5) Calculate the scattered field and radar cross-section under plane wave illumination: After solving for the inherent characteristic parameters of the target, the corresponding scattered field is obtained by changing the incident wave; by setting the incident wave as a plane wave and the receiving point at infinity, the monostatic and bistatic radar cross sections of the target are obtained.

2. The measurement method as described in claim 1, characterized in that, In step 2), a virtual surface called the equivalent surface is constructed to enclose the target. This equivalent surface is denoted as S, and divided into N discrete units. The surface of the nth discrete unit is denoted as... The central position is The scale d of the discrete unit n satisfy: in, The location of the transmitting antenna. The center position of the nth discrete unit, n=1,…,N; N is the number of discrete units, and λ is the wavelength of the electromagnetic wave.

3. The measurement method as described in claim 2, characterized in that, In step 3), according to the equivalence principle, the target's scattered field is represented by the equivalent current and magnetic current on the equivalent surface S; according to the single-source method, the scattered field is... Using a single effective current Represented as: （2） in, The imaginary unit, Vacuum wavenumber, For free space wave impedance, , Let r be the free space dyadic Green's function; r represents the position of the field. Indicates the location of the source. This represents the integral element on the equivalent surface. This represents the radius vector from the source point to the field point. Represents the effective current on the equivalent surface; effective current It consists of the tangential components of the incident electric and magnetic fields on the equivalent surface S. and The decision is expressed as: （3） in, and These are the dyadic kernel functions introduced, which are related to the tangential components of the incident wave's electric and magnetic fields; the tangential components of the incident wave's electric and magnetic fields are calculated based on the antenna used for measurement. and Based on the division in step 2), the continuous function is transformed into a discrete function, and the dyadic kernel function related to the tangential components of the incident wave's electric and magnetic fields is approximated in the following way. and : ， （4） The superscript "(e / h)" indicates that "(e)" or "(h)" corresponds to the electric field or magnetic field, respectively. Represents the center position from the nth discrete unit To the center position of the m-th discrete unit The unit direction vector, S m Let m be the surface of the m-th discrete unit, where m = 1, ..., N; The discrete elements of the dyadic kernel function are related to the tangential components of the incident wave's electric or magnetic field.

4. The measurement method as described in claim 3, characterized in that, In step 4), for each discrete element, the antenna is located in its far-field scattering region, and an approximation is made: （5） （6） in, Let be the tangential component of the incident wave electric field at the center of the nth discrete unit. It is the center position of the m-th discrete unit. To the receiving antenna position The radius vector; From the center position of the m-th discrete unit To the receiving antenna position The unit direction vector; From the position of the transmitting antenna To the center position of the nth discrete unit The radius vector, for single-station measurements Substituting equations (3) to (6) into equation (2), the discretization method of the scattered field is approximately as follows: （7） in （8a） （8b） in, The scattered field measurement at the location of the receiving antenna. and These are two weighting factors related to the discrete unit. and These are the differences between the two directions; by changing the position of the receiving antenna. Collect sufficient scattered field measurements at the receiving antenna locations. Equation (7) can be assembled into a system of linear algebraic equations, which takes the form of: （9） in, It is an excitation column vector composed of all scattered field measurements. It is a column vector consisting of all IFPs. The coefficient matrix is ​​formed based on the incident electric and magnetic fields at the center of the discrete unit; the linear equations are solved using the least squares principle with ridge parameter optimization, and equation (9) is solved as follows: （10） in, for The conjugate transpose of . It is a ridge parameter, which yields the inherent characteristic parameters of the target.

5. The measurement method as described in claim 4, characterized in that, Approximating the linear equation system: (1) If the coupling between discrete units can be ignored but the cross-coupling of the same discrete unit cannot be ignored, then =0 The unknown quantity is reduced to 8N; (2) If the coupling energy between discrete units is negligible, and the cross-polarization coupling of the same discrete unit is also negligible, then Becoming a diagonal matrix, m=n, the number of unknowns is reduced to 4N; (3) If the coupling between discrete units cannot be ignored, but the cross-polarization coupling can be ignored, the number of unknowns is reduced to 4N. 2 .

6. The measurement method as described in claim 5, characterized in that, In step 5), the incident wave is set as a plane wave, and the incident wave electric field... and incident wave magnetic field Represented as: ， （11a） ， （11b） in, and Let represent the polarization direction and the incident direction, respectively; substituting this plane wave incident field into equation (7), the scattered field and bistatic radar cross section at any location can be calculated. : （12） in, Let q be the p-polarization component of the scattered field, where p represents the polarization of the receiving field and q represents the polarization of the incident field. Indicates the scattering direction; the monostatic radar cross section corresponds to .

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