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

Abstract

The invention discloses an RCS measurement method based on target inherent feature extraction and near-field single-station measurement data. According to the method, an equivalent surface is introduced to wrap a target and is dispersed into a plurality of discrete units, a scattered field is expressed as effective current distribution on all the discrete units, effective current is expressed as tangential components of an incident wave electric field and a magnetic field by introducing a pair of union vector kernel functions, and the discrete form of the union vector kernel functions is called as a target inherent characteristic parameter; by obtaining near-field single-station measurement data, a system of linear equations is constructed to solve inherent characteristic parameters of a target, and a scattering field and a radar cross section (RCS) for plane wave incidence are calculated; the theory is strict, the degree of freedom is controlled by adjusting the size of a discrete unit, the higher the degree of freedom is, the higher the solving complexity is, and the higher the precision is; for a target with a simple structure, the coupling effect among most scattering units is ignored, so that the degree of freedom and the solving complexity are greatly reduced; for RCS measurement of an electrically large target, the method has great application value.

Description

TECHNICAL FIELD

[0001] The present application relates to electromagnetic scattering measurement technology, and in particular to a RCS measurement method based on target inherent feature extraction and near-field single-station measurement data. BACKGROUND

[0002] Radar Cross Section (RCS) is a core parameter to measure the scattering characteristics of a target. For the RCS measurement under far-field conditions, the required experimental measurement distance will quickly increase with the electrical size of the target, and the anechoic chamber for measurement often cannot meet this distance requirement. Therefore, it is extremely important to predict the far-field RCS through near-field measurement data. In the past few decades, many algorithms have been developed to predict the far-field RCS through near-field measurement, and many of these methods have been practically applied.

[0003] Near-Field-to-Far-Field Transformation (NFFFT) is an algorithm to process near-field data and predict far-field RCS, so using NFFFT can greatly reduce the required distance of measurement. Some well-known methods include: Melin filtering method, inverse source-based method and image transformation-based method. However, different NFFFT has its own limitations, such as the requirement of the shortest distance or the requirement of the target structure. Melin filtering method proposed in Measuring radar cross section at short distance shortens the required distance of measurement, but sacrifices the angle resolution. After Falconer proposed the near-field-to-far-field transformation method based on physical optics method in Extrapolation of near-field RCS measurements to the far zone, the method of solving the equivalent current and magnetic flow on the target surface using scattering near-field and then extrapolating was developed; however, due to the phase approximation, the accuracy of these methods is poor. Odendaal found in Radar cross section measurements using near-field radar imaging that the near-field radar scattering image is highly similar to the far-field radar scattering image, and applied this image to calculate the far-field RCS; Broquetas introduced the focusing factor to correct the phase inconsistency caused by the near-field spherical wave illumination in Spherical wave near-field imaging and radar cross-section measurement; after these two works, the image-based NFFFT algorithm gradually matured. Watanabe introduced the problem of accuracy decline when the target is offset from the measurement center in Far-field radar cross section determination from near-field 3-D synthetic aperture imaging with arbitrary antenna-scanning surfaces; Watanabe introduced the problem of prediction inaccuracy when there is strong coupling between targets in Image-based radar cross section synthesis for a cluster of multiple static targets.The image-based near-field to far-field transformation method has limitations due to some unrealistic approximations or assumptions, two of which are: first, the scattering centers of the target are independent of each other, and there is no coupling between them; 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 is constant, as the size of the target increases or the structure becomes more complex, the two assumptions are no longer applicable. SUMMARY

[0004] In view of the problems in the prior art, the present application provides a RCS measurement method based on target inherent feature extraction and near-field single station measurement data.

[0005] The RCS measurement method based on target inherent feature extraction and near-field single station measurement data of the present application comprises the following steps: 1) Near-field single station measurement: The antenna measures the target to obtain near-field single station measurement data; 2) Introduction and division of equivalent surface: An equivalent surface is introduced to wrap the target; further, the equivalent surface is discretely divided into a plurality of discrete units, and the size of the discrete unit depends on the distance from the discrete unit to the antenna; 3) Introduction of target inherent feature parameters and scattering field representation: Under the illumination of the incident field generated by the transmitting antenna, the scattering field at the position of the receiving antenna is represented by the effective current distribution on the discrete units, and the effective current is represented by the tangential components of the incident wave electric field and magnetic field through the introduction of two dyadic kernel functions. After discretization processing, the scattering field is represented as the sum of the scattering fields generated by the effective currents on all discrete units, i.e. 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 target inherent feature parameter; 4) Solving target inherent feature parameters using near-field single station measurement data: According to the incident wave electric field and magnetic field at the center position of the discrete unit, a coefficient matrix is formed, and the discrete form of the scattering field is written as a linear equation group. The excitation column vector on the right side of the linear equation group is generated by the near-field single station measurement data, and the linear equation group is solved by the least square method with the ridge parameter optimization to obtain the target inherent feature parameters; 5) Calculation of scattering field and radar cross section under plane wave illumination: After the target inherent feature parameters are solved, the corresponding scattering 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 single station and double station radar cross section (RCS) of the target.

[0006] In step 1), the near-field monostatic scattering measurement system generally comprises a turntable carrying the target, a pair of antennas for transmitting and receiving, a vector network analyzer (VNA) for simultaneously providing signals to the transmitting antenna and processing signals of the receiving antenna, and the transmitting antenna and the receiving antenna are located very close to each other and are approximately regarded as the same position, forming a monostatic scattering measurement system. By rotating the turntable, moving the antenna, adjusting the target placement posture and the like, full-range measurement is realized, and there is no limitation on the distance between the antenna and the target, and the farther the distance is, the better, and the target does not require symmetry or other special properties.

[0007] The near-field monostatic measurement data is composed of the electric field measurement values of the scattering field of the target; the measurement antenna and the transmitting antenna are in the same position, which is called monostatic, and the measurement antenna and the transmitting antenna are not in the same position, which is called bistatic. The near-field measurement method of the present application refers to the distance from the target to the antenna, which can be arbitrarily close. The closer the distance is, the more the number of discrete units is, and the more complex the model solving is, but it will not affect the effectiveness of the method, and it is theoretically strict. The distance of the previous near-field measurement method is still limited, and the theory itself is approximate.

[0008] The theory of the present application has strictness, so the method of the present application has no limitation on the distance between the antenna and the target, and the target does not need to be in the center of the measurement system, and is suitable for complex targets or cases where there is a significant coupling effect between components. Through the method of the present application, the antenna can measure the target at a position one times the target scale or even closer to the center of the target, and the antenna measurement surface is also arbitrary. In general, the turntable carrying the target rotates horizontally around the center of the target, the rotation axis is in the vertical direction, and the antenna can move up and down or along the longitude line, thereby forming a cylindrical or spherical measurement surface. According to the shape of the target, different measurement regions and ranges are selected, for example, for a horizontally placed flat target, the antenna may only need to perform one round of measurement in a ring shape or the antenna does not move and the turntable rotates one round, and the measurement point interval is generally about half a wavelength. The measurement environment is in a microwave darkroom or an outdoor site, and the smaller the background noise is, the better, and there should be no strong scattering interference near the target, otherwise there should be elimination measures.

[0009] In step 2), the closer the equivalent surface to the target surface is, the better, and the shape of the equivalent surface is not limited as long as it can wrap the target so that the target is located in the equivalent surface; but in order to be general, a minimum cuboid surface that can wrap the target is generally selected, and the length, width and height of the cuboid are the maximum length, width and height of the target. The size of the discrete unit depends on its distance to the antenna, and the smaller the distance is, the smaller the size is; or the discrete units are uniformly discretized according to the minimum size. For the equivalent surface of the non-cuboid surface, mature commercial software is used to discretize it into multiple discrete units.

[0010] According to the prior knowledge of the target, a virtual surface wrapping the target is constructed and is called an equivalent surface, and the equivalent surface is denoted asS , the equivalent surface S is divided into N discrete units, the surface of the n discrete unit is denoted by , and the center position is . The denser the division, the higher the accuracy, but the longer the calculation time. According to the required RCS prediction accuracy, the division accuracy is flexibly selected. The size of the discrete unit satisfies the far-field condition: or wherein is the position of the transmitting antenna, is the center position of the n discrete unit, d n is the size of the n discrete unit, n =1,…, N ; N is the number of discrete units, λ is the wavelength of the electromagnetic wave. According to the accuracy requirement: (1).

[0011] In step 3), according to the equivalent principle, the scattering field of the target is represented by the equivalent current and magnetic current on the equivalent surface S ; according to the single-source method, the scattering field is represented by a single effective current as: (2) wherein is the imaginary unit, is the vacuum wave number, , is the angular frequency of the electromagnetic wave, is the vacuum permittivity, is the vacuum permeability, is the free-space wave impedance, , is the free-space dyadic Green's function; r denotes the position of the field, denotes the position of the source, denotes the integration surface element on the equivalent surface, denotes the vector path from the source point to the field point, denotes the effective current on the equivalent surface; in theory, the effective current is determined by the tangential components S and of the incident wave electric field and magnetic field on the equivalent surface , and is represented 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 target intrinsic characteristic parameters (IFPs) 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 center position of the mth discrete element to the receiving antenna position unit directional vector; is the vector from the transmitting antenna position to the center position of the mth discrete element n of the monostatic measurement Substitute equations (3) to (6) into equation (2), and the discrete manner of the scattering field is approximated as: (7) wherein (8a) (8b) wherein, is the scattering field measurement value at the receiving antenna position, and are two weighting factors related to the discrete element, and are the differences of the two directions, respectively; by changing the receiving antenna position , a sufficient number of scattering field measurement values at the receiving antenna position are collected, which is at least more than the number of unknowns to be solved, equation (7) is assembled into a linear algebraic equation set to solve the target intrinsic characteristic parameters IFPs, and the total number of unknowns is wherein represents the number of dyads or matrices, each matrix has 4 elements, and there are 2 types of matrices corresponding to the tangential components of the incident wave electric field and magnetic field, respectively; the linear equation set is assembled into the form of: (9) wherein, is an excitation column vector composed of all scattering field measurement values, i.e., the near-field monostatic measurement data, so that the excitation column vector on the right side of the linear equation set is generated through the near-field monostatic measurement data, is a column vector composed of all IFPs, is a coefficient matrix formed according to the incident wave electric field and magnetic field at the center position of the discrete element; the least square principle with ridge parameter optimization is used to solve the linear equation set, and equation (9) is solved as: (10) wherein, is the conjugate transpose of ​​is a ridge parameter; in order to reduce the unknowns, the following approximations are made, whose applicability mainly depends on the complexity of the target: (1) if the coupling between discrete units can be ignored but the cross-polarization coupling of the same discrete unit cannot be ignored, then If , the unknowns are reduced to 8 N ; (2) if the coupling between discrete units can be ignored and the cross-polarization coupling of the same discrete unit can also be ignored, then becomes a diagonal matrix, and the unknowns are reduced to 4 N ; (3) if the coupling between discrete units cannot be ignored but the cross-polarization coupling can be ignored, the unknowns are reduced to 4 N 2 Whether the coupling between discrete units and the cross-polarization coupling of the same discrete unit can be ignored depends on the complexity of the target and the accuracy requirement, and the usual measurement accuracy requirement is 1-5 dB; the higher the accuracy requirement is, the less it can be ignored, and the lower the accuracy requirement is, the more it can be ignored.

[0013] In step 5), the incident wave is set as a plane wave, the incident wave electric field and the incident wave magnetic field are expressed as: , (11a) , (11b) wherein, and represent the polarization direction and the incident direction, respectively; by substituting the plane wave incident field into formula (7), the scattering field and the bistatic radar cross section (RCS) of any position can be calculated: (12) wherein, is the p polarization component of the scattering field, p represents the receiving field polarization, q represents the incident field polarization, represents the scattering direction, ; the monostatic radar cross section corresponds to .

[0014] Advantages of the present application: 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 Antenna position is located by infrared positioning instrument; two antennas are placed with same polarization (H / V) and connected with vector network analyzer (VNA, N5247A) through coaxial cable, and the vector network analyzer is set to send signal frequency of 10 GHz; when the transmitting antenna transmits electromagnetic wave, the receiving antenna receives the same polarization scattering electromagnetic wave from the target; the scattering signal is read by the vector network analyzer; the measurement point interval is about half a wavelength; the measurement environment is in a microwave darkroom with small background noise, eliminating strong scattering interference near the target; 2) Introduction and division of equivalent surface: The approximate outline of the target is known, and an equivalent surface of a cuboid surface is constructed S The target is wrapped, and the nearest distance of each surface to the target is about 2mm; the equivalent surface S is divided into N square discrete units, and the surface of the first n discrete unit is denoted as , and the center position is , and theoretically, the side length of the discrete unit meets the far-field condition: or wherein, d n is the size of the first n discrete unit, and λ is the wavelength of the electromagnetic wave; according to the accuracy requirement, the following is taken: (1) In this embodiment, when , the prediction error of RCS is less than 1dB; according to the required measurement accuracy, the reference size is , wherein is the distance from the antenna to the nearest discrete unit; 3) Introduction of target intrinsic characteristic parameters and representation of scattering field: According to the equivalent principle, the scattering field of the target is represented by the equivalent current and magnetic current on the equivalent surface S ; according to the single source method, the scattering field is represented by a single effective current as follows: (2) wherein, is the imaginary unit, is the vacuum wave number, , is the angular frequency of the electromagnetic wave, is the vacuum dielectric constant, is the vacuum permeability, is the 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) where, is the tangential component of the incident wave electric field at the center position of the n th discrete element; is the center position of the m th discrete element is the vector distance from the center position of the th discrete element to the receiving antenna position is the unit direction vector from the center position of the m th discrete element to the receiving antenna position is the vector distance from the transmitting antenna position to the center position of the th discrete element n ; substituting (3)~(6) into (2), the discrete approximation of the scattering field is obtained as: (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 and incident wave magnetic field at all discrete element center positions , ; substituting them into equation (7) gives the scattering field at the receiving antenna; where , (8a) , (8b) (8c) where, is the measured value of the scattering field at the receiving antenna position, , is the unit dyadic, is the vector differential operator; Rotate the turntable, which is equivalent to rotating the antenna to change the receiving antenna position , transmit signals and receive scattering near-field data at a certain frequency according to the rotation speed, and each interval is equivalent to the antenna moving about half a wavelength; assuming that L near-field single-station scattering data at different positions are obtained, written as an excitation column vector : (A1) where, T denotes transposition, is the center position of the i ​​The 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 formally the same, just replace with : (A6) where is the matrix associated with the tangential component of the incident wave magnetic field, and are the elements of the matrix associated with the tangential component of the incident wave magnetic field, and ; Collect enough scattering field measurements at the positions of the receiving antennas , more than the number of unknowns to be solved, assemble equation (7) into a linear algebraic equation system, solve the target eigenparameters, the total number of unknowns is , where represents the number of vectors or matrices, each matrix has 4 elements, and there are 2 types of matrices corresponding to the tangential components of the incident wave electric field and magnetic field respectively; the form of the assembled linear equation system is: (9) where is the excitation column vector composed of all scattering field measurements, which is the near-field single-station measurement data, so as to generate the excitation column vector on the right side of the linear equation system through the near-field single-station measurement data, and the scattering field measurements are is a column vector composed of all IFPs, is the coefficient matrix formed, the equation system is solved by using the least square principle and ridge parameter optimization, and equation (9) is solved as: (10) where is the conjugate transpose of , is a ridge parameter; in order to reduce the unknowns, the following approximations are made, and their applicability 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 If , the unknowns are reduced to 8 N ; (2) if the coupling between discrete units can be ignored, and the cross-polarization coupling effect of the same discrete unit can also be ignored, then becomes a diagonal matrix, and the unknowns are reduced to 4 N ; (3) if the coupling between discrete units cannot be ignored, but the cross-polarization coupling effect can be ignored, the unknowns are reduced to 4 N 2Whether the coupling between discrete units and the cross coupling of the same discrete unit can be ignored depends on the complexity of the target and the accuracy requirement, if the general measurement accuracy requirement is 1~5dB; the higher the accuracy requirement is, the less it can be ignored, and the lower the accuracy requirement is, the more it can be ignored; 5) Calculate the scattering field and radar cross section under the plane wave irradiation: The incident wave is set as a plane wave, the incident wave electric field and the incident wave magnetic field are expressed as: , (11a) , (11b) wherein, and represent the polarization direction and the incident direction respectively; by substituting this plane wave incident field into equation (7), the scattering field and bistatic radar cross section (RCS) at any position can be calculated : (12) wherein, is the p polarization component of the scattering field, p represents the receiving field polarization, q represents the incident field polarization, represents the scattering direction, ; the monostatic RCS corresponds to .

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

Claims

1. A method for RCS measurement based on target intrinsic feature extraction and near-field mono-static measurement data, characterized in that, The measurement method comprises the following steps: 1) Near-field single-station measurement: The antenna measures the target to obtain near-field single-station measurement data; 2) Introduction and division of the equivalent surface: An equivalent surface is introduced to wrap the target, and the equivalent surface is discretely divided into a plurality of discrete units; 3) Introduction and scattering field representation of the target intrinsic characteristic parameters: Under the illumination of the incident field generated by the transmitting antenna, the scattering field at the position of the receiving antenna is represented by the effective current distribution on the discrete units; the effective current is represented by the tangential components of the incident wave electric field and magnetic field by introducing two dyadic kernel functions; the scattering field is represented as the sum of the scattering fields generated by the effective currents on all discrete units after discretization processing; and the discrete form of the dyadic kernel function is referred to as the target intrinsic characteristic parameter IFP; 4) Solving the target intrinsic characteristic parameters by using the near-field single-station measurement data: The incident wave electric field and magnetic field at the center position of the discrete unit are used to calculate the coefficient matrix; the discrete form of the scattering field is written as a linear equation group; the excitation column vector of the linear equation group is generated by the near-field single-station measurement data; and the linear equation group is solved by the least square method with the ridge parameter optimization to obtain the target intrinsic characteristic parameters; 5) Calculation of the scattering field and radar cross section under plane wave illumination: After the target intrinsic characteristic parameters are solved, the corresponding scattering 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 single-station and double-station radar cross section of the target.

2. The measurement method of claim 1, wherein, In step 2), a virtual surface called the equivalent surface is constructed to wrap around the target, and the equivalent surface is denoted as S The equivalent surface S is divided into N discrete units, the surface of the n discrete unit is denoted as , the center position is , and the size of the discrete unit is d n satisfies: wherein is the position of the transmit antenna, the n center position of the i-th discrete unit, n = 1, …, N ; N is the number of discrete units, λ is the wavelength of the electromagnetic wave.

3. The method of measuring of claim 2, wherein, In step 3), the scattering field of the target is represented by equivalent currents and magnetic currents on the equivalent surface S According to the single-source method, the scattering field is represented by a single effective current as （2） where is the imaginary unit, is the vacuum wave number, is the free space wave impedance, , is the free space dyadic Green's function; r denotes the position of the field, denotes the position of the source, denotes the integration surface element on the equivalent surface, denotes the dyadic path from the source point to the field point, denotes the effective current on the equivalent surface; the effective current is determined by the tangential components of the electric and magnetic field of the incident wave on the equivalent surface S and and is given by （3） where and are the introduced dyadic kernel functions related to the tangential components of the incident electric and magnetic fields; the tangential components of the incident electric and magnetic fields are calculated according to the employed antenna for the measurements and ; according to the partitioning of step 2), the continuous functions are turned into discrete functions, the dyadic kernel functions related to the tangential components of the incident electric and magnetic fields are 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 ; The discrete element of the dyadic kernel function is related to the tangential component of the incident wave's electric or magnetic field.

4. The measurement method of claim 3, wherein, In step 4), for each discrete unit, the antenna is located in the scattering far field thereof, and the approximation is made: （5） （6） in, For the first n Tangential components of the incident electric field at the center of each 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） wherein （8a） （8b） where is the measured value of the scattered field at the receiving antenna position, and are two weighting factors associated with the discrete elements, and are the differences between the two directions, respectively; by changing the receiving antenna position sufficient measured values of the scattered field at the receiving antenna position are collected Equation (7) is assembled into a system of linear algebraic equations in the form （9） where, is the excitation column vector composed of all the scattered field measurements, is the column vector composed of all the IFPs, is the coefficient matrix formed by the incident electric and magnetic fields at the center of the discrete element; the least square principle with ridge parameter optimization is used to solve the linear equations, and the equation (9) is solved as follows: （10） wherein is the conjugate transpose of is a ridge parameter, resulting in target eigenfeature parameters.

5. The measurement method of claim 4, wherein, Approximation of linear equations: (1) if the coupling between discrete units can be neglected but the cross-coupling of the same discrete unit cannot be neglected, then , the unknowns are reduced to 8 N ; (2) if the coupling between discrete units can be neglected and the cross-polarization coupling of the same discrete unit can also be neglected, then becomes a diagonal matrix, and the unknowns are reduced to 4 N ; (3) if the coupling between discrete units cannot be neglected, but the cross-polarization coupling can be neglected, the unknowns are reduced to 4 N 2 .

6. The measurement method of claim 5, wherein, In step 5), the incident wave is set as a plane wave, and the incident wave electric field and the incident wave magnetic field are expressed as: ， （11a） ， （11b） where and represent the polarization direction and the incident direction, respectively; substituting this plane wave incident field into (7) to calculate the scattering field and bistatic radar cross section at any position : （12） wherein is the scattered field p polarization component, p represents the polarization of the received field, q represents the polarization of the incident field, denotes the scattering direction; monostatic radar cross section corresponds to .

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