Calculation method of RCS of composite electromagnetic scattering of marine ship targets

Abstract

The present invention discloses a method for calculating composite electromagnetic scattering RCS of a marine ship target, which specifically includes: describing the motion characteristics of marine ship targets with wind and waves, constructing and solving a six-degree-of-freedom motion model of marine ship targets with wind and waves; establishing a geometric model of a composite scene of a dynamic sea surface and a moving ship; performing facet subdivision on the geometric information of marine ship targets, dividing the composite electromagnetic scattering RCS of marine ship targets into four parts of RCS, specifically, the single scattering RCS of the ship, the multiple scattering RCS of the ship, the single scattering RCS of the sea surface, and the multiple scattering RCS between the sea surface and the ship, respectively solving the scattering field intensity corresponding to the above four parts of RCS according to the target facet, vector superposition of the obtained scattering field intensity, and obtaining the composite electromagnetic scattering RCS of marine ship targets. The present invention will provide technical support for the research on the electromagnetic characteristics of ship targets in actual complex marine environments, and is of great significance to the research on marine ship target detection and recognition.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic field and microwave technology, and in particular to a method for calculating composite electromagnetic scattering RCS of a marine ship target. Background Art

[0002] At present, most of the estimation and calculation methods of composite electromagnetic scattering RCS of marine ship targets only consider the motion characteristics of the waves, while ignoring the image of the motion characteristics of the ship target with the waves. However, in reality, due to the random motion characteristics of the waves and the hydrodynamic effect of the waves, the posture of the ship target changes with the waves, and the ship target and the coupled scattering between the ship target and the sea surface also change with time. Therefore, the current calculation method of composite electromagnetic scattering RCS of marine ship targets that ignores the motion characteristics of the ship to the waves is quite different from the actual situation, resulting in a large difference between its calculation results and the composite electromagnetic scattering characteristics of the actual marine ship targets.

[0003] In addition, the current calculation of composite electromagnetic scattering of marine targets mainly includes two widely used schemes. One scheme is to use the ray method to consider the coupling relationship between the sea surface and the ship. The disadvantage is that if the contribution of capillary waves to the coupling field is to be considered, the sea surface needs to be more finely divided, which undoubtedly greatly increases the amount of calculation; the other scheme is to use the complex reflection coefficient to estimate the mutual coupling between the sea surface and the ship, such as the four-path method. This type of scheme cannot well consider the shielding effect between the sea surface and the ship, and its hypothetical sea surface is a plane, while the actual sea surface is a two-scale model of large-scale undulating surges modulating small-scale capillary waves. The reflection of the sea surface is not concentrated in the mirror direction, so it is too simple to only use the complex reflection coefficient to estimate the coupling between the sea surface and the ship. In scenarios with high requirements for calculation accuracy, this scheme will no longer be applicable. Summary of the invention

[0004] In view of the problem that the existing marine ship target composite scattering RCS calculation method is difficult to obtain the actual dynamic scene composite scattering, the present invention discloses a marine target composite electromagnetic scattering RCS calculation method, by considering the ship's motion characteristics with the waves, and combining the ray tracing method with the panel method, a multi-path model is used to calculate the composite scattering of the sea surface and the ship target, and a marine ship target composite scattering model is established to achieve high-precision calculation of the marine ship target composite electromagnetic scattering RCS. The RCS refers to the radar cross section, and its full English name is Radar Cross Section.

[0005] The present invention discloses a method for calculating composite electromagnetic scattering RCS of a marine ship target, the steps of which include:

[0006] S1. According to the geometric information and scene parameters of the maritime ship target, a coordinate system is established to describe the motion characteristics of the maritime ship target with wind and waves, and the motion characteristics description results are obtained. According to the motion characteristics description results, a six-degree-of-freedom motion model of the maritime ship target moving with wind and waves is constructed and solved to obtain the six-degree-of-freedom motion response variables of the ship.

[0007] The scenario parameters include sea surface size, sea surface wind speed, sea surface wind direction, ship movement speed and ship movement direction, etc.

[0008] The establishment of the coordinate system includes establishing a global coordinate system, an inertial coordinate system and a hull fixed coordinate system. The hull fixed coordinate system is a three-dimensional rectangular coordinate system with the ship's hull center of gravity as the origin, the sea level as the XOY plane, and the ship's forward direction as the positive direction of the X axis. It moves with the movement of the ship. The X axis, Y axis and Z axis of the hull fixed coordinate system point to the bow, port side and bottom of the ship respectively. The global coordinate system adopts the geodetic coordinate system, the center of the inertial coordinate system remains consistent with the global coordinate system, the center of the inertial coordinate system does not change with the movement of the hull, and the X axis, Y axis and Z axis of the inertial coordinate system point to the bow, starboard side and bottom of the ship respectively.

[0009] The description of the motion characteristics of the marine ship target following the waves to obtain the motion characteristics description result specifically includes: using six-degree-of-freedom motion response variables to describe the motion state of the marine ship target in the waves, and the six-degree-of-freedom motion response variables include the translational motion variables of the ship hull along the coordinate axis of the fixed coordinate system and the rotational motion variables around the coordinate axis of the fixed coordinate system. The translational motion variables include surge motion variables, sway motion variables and heave motion variables, and the rotational motion variables include roll motion variables, pitch motion variables and yaw motion variables.

[0010] The coordinates of the position point of the maritime ship target in the global coordinate system are {x, y, z}, and the coordinates of the position point of the maritime ship target in the inertial coordinate system are {x I ,yI,z I}, the position coordinates of the target ship at sea in the fixed coordinate system of the ship are {x b ,y b ,z b}, the six-degree-of-freedom motion response variable of the ship in the fixed coordinate system is expressed as u1, u2, and u3 are surge, sway, and heave variables included in the translational motion variables of the marine ship target, respectively; u4, u5, and u6 are roll, pitch, and yaw variables included in the rotational motion variables, respectively; the conversion relationship between the inertial coordinate system of the marine ship target and the fixed coordinate system of the hull is expressed as follows:

[0011]

[0012] Among them, A roll , A pitch and A yaw Are all transformation matrices, and their expressions are:

[0013]

[0014] The conversion relationship between the global coordinate system and the inertial coordinate system of the maritime ship target is expressed as:

[0015]

[0016] Among them, v ship is the ship speed, is the angle between the ship's motion direction and the X-axis of the global coordinate system, and t represents time. The motion characteristics description results include the six-degree-of-freedom motion response variables, the conversion relationship between the inertial coordinate system of the marine ship target and the fixed coordinate system of the hull, and the conversion relationship between the global coordinate system of the marine ship target and the inertial coordinate system.

[0017] The six-degree-of-freedom motion model of the marine ship target moving with wind and waves is constructed according to the motion characteristic description result, which specifically includes:

[0018] According to the wave spectrum and the relationship between wave direction and hull position, the encounter frequency ω with the hull as reference is calculated. e (encounter frequency) and encounter wave spectrum S e , and its calculation process is expressed as:

[0019]

[0020]

[0021] in, Indicates the direction of waves and the direction of ship movement The angle of It means that the direction of the waves is consistent with the direction of the ship's movement. Indicates that the direction of the waves is perpendicular to the direction of the ship's motion. It means that the direction of the wave is opposite to the direction of the ship's motion, ω means the wave period, Indicates that the encounter frequency ω e , Angle and The wave spectrum encountered at time , g is the gravity constant, Indicates that in the wave period ω, the angle The wave spectrum function at ;

[0022] According to the force balance of the ship target on the sea surface, the linear equation of the ship's motion in regular waves is constructed, and its expression is:

[0023]

[0024] in, is the mass matrix, and its expression is:

[0025]

[0026] Among them, m is the mass of the sea ship target, [x G ,y G ,z G ] is the coordinate of the center of gravity of the marine ship target in the global coordinate system, θ xx It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the x-axis, θ yy It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the x-axis, θ zz It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the z-axis, θ xy It represents the mass moment of inertia of the marine ship target when it moves along the x-axis translation and the y-axis rotation, θ xz It represents the mass moment of inertia of the marine ship target when it moves along the x-axis translation and the z-axis rotation, θ yz It represents the mass moment of inertia of a maritime ship target when it performs translational motion along the y-axis and rotational motion along the z-axis. It represents the fluid dynamics exerted by waves on the ship target at sea, which includes the fluid statics part and the fluid dynamics part caused by the ship movement, the incident wave of the sea wave hitting the hull and the diffraction after the collision; the linear equation of the ship motion in regular waves is decomposed and represented to obtain the six-degree-of-freedom motion model of the ship target at sea moving with wind and waves, and its expression is:

[0027]

[0028] in, is the restoring force matrix of the marine ship target, which is generated by the hydrostatics part, Represents the radiation force matrix of the motion of the maritime ship target, is the amplitude matrix of the disturbance force generated by the incident wave hitting the hull and its diffraction after the collision.

[0029] The six-degree-of-freedom motion model for solving the motion of a ship target at sea with wind and waves adopts a slicing method to solve the six-degree-of-freedom motion model for solving the motion of a ship target at sea with wind and waves.

[0030] The slicing method is used to solve the six-degree-of-freedom motion model of the marine ship target moving with wind and waves, which specifically includes:

[0031] The ship hull is evenly divided into several transverse sections along the axial direction of the hull. For each transverse section, the fluid dynamics of the ship hull at the transverse section is obtained by solving the linear equation of the motion of the ship in regular waves.

[0032] The fluid dynamics at each transverse section of the ship hull are superimposed along the axial direction of the hull to obtain the three-dimensional fluid dynamics of the ship hull;

[0033] The three-dimensional fluid dynamics of the ship's hull is used to construct a wave spectrum superposition model, and the six-degree-of-freedom motion response variables of the ship are solved according to the wave spectrum superposition model.

[0034] The three-dimensional fluid dynamics of the ship hull is used to construct a wave spectrum superposition model, and the six-degree-of-freedom motion response variables of the ship are obtained by solving the wave spectrum superposition model, which specifically includes:

[0035] The constructed wave spectrum superposition model is expressed as follows:

[0036]

[0037] Among them, u i (x, y, t) is the six-degree-of-freedom response variable of the marine ship target at the two-dimensional wave surface (x, y) at time t, i = 1, 2, ..., 6, k e is the encounter circle frequency ω e The wave number under the wave, S e (,) is the two-dimensional encounter wave spectrum function, R is the amplitude response operator, N k and N φ are the number of sampling points for frequency and azimuth, respectively. e, l is the encounter circle frequency at the lth frequency sampling point, k e,l is the encounter circle frequency ω e,l The corresponding wave number is is the jth spatial direction angle of the wave, represents the sea surface wind direction angle, Δk e is the change increment of the wave number, is the change increment of the spatial direction angle of the wave, εlj It represents the initial phase corresponding to the lth frequency sampling point and the jth spatial direction angle. Using the wave spectrum superposition model, the six-degree-of-freedom motion response variables of the ship are solved.

[0038] S2, using the six-degree-of-freedom motion response variables of the ship, establish a geometric model of the dynamic sea surface and moving ship composite scene;

[0039] The three-dimensional dynamic sea surface geometric model of the ship target at sea is established by using the linear superposition method. The wave height of a fixed point on the sea surface at a single moment is described by superposition of multiple random cosine waves. Then the wave height η of a fixed point (x, y) on the sea surface at time t is L (x,y,t) is expressed as:

[0040]

[0041] Among them, S e (,) is the two-dimensional encounter wave spectrum function, k l ,ω l , Respectively represent the wave number, circular frequency, and direction angle of the random cosine wave, N k and N φ are the number of sampling points for frequency and azimuth, respectively. represents the sea surface wind direction angle, k e,l is the encounter circle frequency ω e,l The corresponding wave number is is the jth spatial direction angle of the wave, represents the sea surface wind direction angle, Δk e is the change increment of the wave number, is the change increment of the spatial direction angle of the wave, ε lj Represents the initial phase corresponding to the lth frequency sampling point and the jth spatial direction angle.

[0042] The three-dimensional dynamic sea surface geometry model includes three-dimensional dynamic sea surface geometry information. The three-dimensional dynamic sea surface geometry information includes the wave height information of each fixed point (x, y) on the sea surface at each moment.

[0043] At each time sampling point, the geometric information of the maritime ship target and the geometric information of the three-dimensional dynamic sea surface are linearly superimposed to obtain a geometric model of the composite scene of the dynamic sea surface and the moving ship.

[0044] S3, calculate the composite electromagnetic scattering RCS of the marine ship target based on the geometric model of the composite scene of the dynamic sea surface and the moving ship;

[0045] The geometric information of the maritime ship target is segmented by surface elements to obtain several target surface elements. The composite electromagnetic scattering RCS of the maritime ship target is divided into four parts of RCS, namely the single scattering RCS of the ship, the multiple scattering RCS of the ship, the single scattering RCS of the sea surface and the multiple scattering RCS between the sea surface and the ship. The scattering field intensities corresponding to the above four parts of RCS are solved respectively according to the target surface elements, and the obtained scattering field intensities are vector-superimposed to obtain the composite electromagnetic scattering RCS of the maritime ship target.

[0046] The calculation of single scattering RCS of marine ship targets is realized by pure physical optics method, which specifically includes:

[0047] The tangent plane approximation condition is used to locally approximate the excitation source of the scattered RCS, and the induced electromagnetic current of the marine ship target is obtained. The far-field Green's function and the equivalent principle are used to perform near-far field extrapolation to obtain the expression of the electromagnetic field of the marine ship target. Then, the tangent plane approximation method is used to obtain the surface current value and magnetic current value of the marine ship target. According to the surface current value and magnetic current value of the marine ship target, the single scattering RCS of the marine ship target is obtained.

[0048] The bouncing ray method is used to calculate the multiple scattering RCS of marine ship targets.

[0049] When the bouncing ray method is used to calculate the multiple scattering RCS of a ship target at sea, the incident wave direction is determined, and the corresponding incident ray direction information is determined according to the incident wave direction. Each incident ray passes through the center of the corresponding target face element. According to the incident ray direction information, the target face element is blocked and the blockage judgment result is obtained. The blockage judgment result includes the illuminated face element information and the blocked face element information. For the target face element corresponding to the illuminated face element information, the incident ray corresponding to the target face element is tracked and its scattering field is calculated. The ray corresponding to the incident wave is called the incident ray, and the direction of the incident ray is called the incident ray direction.

[0050] The incident ray is tracked, according to the geometric model of the composite scene of the dynamic sea surface and the moving ship, for each incident ray, its propagation path is tracked using the geometric optics method until the incident ray does not intersect with any target surface element or the number of reflections in the propagation path of the incident ray reaches a preset value, and the propagation path of the incident ray is obtained and recorded;

[0051] The scattering field calculation is as follows: according to the propagation path of the incident ray, the intersection point of the incident ray and the target surface element is obtained, and the propagation distance between two adjacent intersection points is calculated; according to each propagation distance, the field intensity relationship between the two adjacent intersection points is calculated, and the field intensity relationship is used as the incident field intensity at the corresponding illuminated surface element, and the far-zone scattering field value on the illuminated surface element is calculated by using the physical optics method; the far-zone scattering field values ​​of all illuminated surface elements are calculated, the far-zone scattering field values ​​of all illuminated surface elements are accumulated to obtain an accumulated value, and the multiple scattering RCS of the marine ship target is obtained by using the accumulated value.

[0052] The calculation of the field strength relationship between the two adjacent intersection points according to each propagation distance specifically includes:

[0053] In the propagation path of the incident ray, the intersection point of the incident ray and the target surface element is I is the number of intersection points, and the propagation distance of the ray between two adjacent intersection points The relationship between the field strengths at two adjacent intersection points is:

[0054]

[0055] in, and The intersection points and intersection The field strength at , k is the wave number of the incident ray electromagnetic wave, (DF) i and The intersection points The divergence factor and reflection coefficient matrix of . The expression of the divergence factor is

[0056]

[0057] Among them, ρ1 and ρ2 are the intersection points The principal curvature radius of the incident ray emission surface and the illuminated surface element corresponding to the intersection point, s represents The reflection distance between the incident ray emission surface and the illuminated surface element corresponding to .

[0058] The occlusion judgment of the target surface element is performed to obtain the occlusion judgment result, which specifically includes:

[0059] First, perform other-occlusion judgment on the target surface element to obtain directly illuminated target surface elements and target surface elements that are not directly illuminated. Then perform self-occlusion judgment on the directly illuminated target surface elements to obtain the illuminated surface elements and the target surface elements occluded by themselves that are finally output. The target surface element information occluded by itself and the target surface element information that is not directly illuminated constitute the occluded surface element information in the occlusion judgment result. The illuminated surface element information that is finally output constitutes the illuminated surface element information in the occlusion judgment result, and the occlusion judgment result is obtained.

[0060] The aforementioned other-blocking judgment specifically includes, along the direction of the incident ray, judging whether the target plane element is blocked by other target plane elements, and if the target plane element is not blocked by other target plane elements, judging that the target plane element is directly illuminated by the incident wave. Performing other-blocking judgment on all target plane elements, obtaining directly illuminated target plane elements and target plane elements not directly illuminated.

[0061] The aforementioned other-occlusion judgment specifically includes that the geometric center point of the target surface element for which the other-occlusion judgment needs to be performed is Q. If the incident ray has an intersection with other target surface elements before reaching point Q, then the target surface element where point Q is located is determined to be blocked by other target surface elements, and the target surface element where point Q is located is considered to be a target surface element that is not directly illuminated, and the target surface element that is not directly illuminated belongs to an occluding surface element. If the incident wave has no intersection with other target surface elements before reaching point Q, then the target surface element where point Q is located is determined to be illuminated by the incident wave, and the target surface element where point Q is located is considered to be a directly illuminated target surface element.

[0062] The specific implementation of the above-mentioned occlusion judgment process is as follows: for a target surface element m on a marine ship target, if the three vertex coordinates of the target surface element m on the marine ship target are A, B and C, the starting point of the incident wave ray is at point P, and the geometric center point of the target surface element m is Q. The coordinate S of any point on the target surface element m is expressed by a parametric equation as follows:

[0063] S=αA+βB+(1-α-β)C,

[0064] Among them, α, β and γ are the parameters to be solved, let γ = 1-α-β; assume that the point with coordinate S is also located on the line segment PQ, that is, S is the intersection of the target surface element m and the line segment PQ, and its coordinates are expressed as:

[0065] S=λP+(1-λ)Q,

[0066] According to the above two expressions of coordinate S, construct the equivalent equation:

[0067] α(AC)+β(BC)+λ(QP)=QC,

[0068] Expressing the equivalent equation in the form of matrix multiplication, we get the equivalent equation system:

[0069]

[0070] Wherein, CA represents the vector from C to A, CB represents the vector from C to B, PQ represents the vector from P to Q, and CQ represents the vector from C to Q. For this equivalent equation group, if det[CA CB PQ]=0, the target surface element is determined to be the target surface element directly illuminated by the incident wave. If det[CA CB PQ]≠0, the equivalent equation group is solved to obtain α, β, λ and γ. If the values ​​of α, β, λ and γ are all within the interval [0,1], the target surface element is determined to be the target surface element that is not directly illuminated. If any one of the four variables α, β, λ and γ is not within the interval [0,1], the target surface element is determined to be the target surface element directly illuminated by the incident wave.

[0071] The self-occlusion judgment of the directly illuminated target surface element specifically includes: for the target surface element directly illuminated by the incident wave obtained through the self-occlusion judgment, calculating the normal vector of the target surface element and the direction vector of the incident wave The inner product of If the inner product Then the target pixel is judged to be the illuminated pixel of the final output; if the inner product It is determined that the target surface element is blocked by itself, and the target surface element is considered to be a target surface element blocked by itself, and the target surface element blocked by itself belongs to an blocking surface element.

[0072] For the calculation of the single scattering RCS of the sea surface, the sea surface is discretized into several inclined surface elements, and the expression of the geometric contour of the inclined surface element is constructed. Each inclined surface element is used as the first target surface element, and a self-occlusion judgment is performed to obtain the first sea surface illuminated surface element. The first sea surface illuminated surface element and the target surface element of the sea ship target are subjected to a other-occlusion judgment to obtain the second sea surface illuminated surface element. For all the second sea surface illuminated surface elements, the semi-deterministic surface element method is used to solve and obtain the single scattering RCS of the sea surface.

[0073] The self-occlusion judgment of the first target surface element specifically includes: calculating the normal vector of the first target surface element and the direction vector of the incident wave The inner product of If the inner product Then the target pixel is judged to be the first illuminated pixel on the sea surface; if the inner product It is determined that the first target plane is blocked by itself, and the first target plane is considered to be a target plane blocked by itself.

[0074] The step of performing other-occlusion judgment on the first sea surface illuminated surface element and the target surface element of the ship target at sea to obtain the second sea surface illuminated surface element specifically includes:

[0075] Along the direction of the incident ray, determine whether the first sea surface illuminated surface element is blocked by the target surface element of the sea ship target. If the first sea surface illuminated surface element is not blocked by the target surface element of the sea ship target, then determine that the first sea surface illuminated surface element is the target surface element directly illuminated by the incident wave. Perform other-blocking judgment on all first sea surface illuminated surface elements to obtain second sea surface illuminated surface elements.

[0076] The aforementioned other-occlusion judgment specifically includes that the geometric center point of the first sea surface illuminated surface element that needs to be judged for other-occlusion is Q; if the incident ray has an intersection with the target surface element of the sea ship target before reaching the Q point, then it is determined that the first sea surface illuminated surface element where the Q point is located is blocked by the target surface element of the sea ship target, and the first sea surface illuminated surface element where the Q point is located is considered to be a target surface element that is not directly illuminated; if the incident wave has no intersection with the target surface element of the sea ship target before reaching the Q point, then it is determined that the first sea surface illuminated surface element where the Q point is located is illuminated by the incident wave, and the first sea surface illuminated surface element where the Q point is located is considered to be a directly illuminated target surface element, and the directly illuminated target surface element is used as the second sea surface illuminated surface element.

[0077] The sea surface is discretized into a number of inclined plane elements, and an expression of the geometric profile of the inclined plane element is constructed. The sea surface is discretized into a number of inclined plane elements, and the inclined plane elements include sea surface rough plane elements and component plane elements of cosine capillary waves. The geometric profile of the inclined plane element is represented by superimposing the component plane elements of cosine capillary waves on the sea surface rough plane elements. The expression of the geometric profile of the inclined plane element at time t is:

[0078] ζ(ρ c ,t)=B(k c )cos(k c ·ρ c -ω c t),

[0079] Among them, ρ c represents the position coordinate vector inside the inclined surface element, k c represents the wave number vector of the cosine capillary wave, B(k c ) represents the amplitude of the cosine capillary wave, ω c represents the wave number vector k c The corresponding angular frequency.

[0080] The method of using a semi-deterministic surface element method to solve all second sea surface illuminated surface elements to obtain the single scattering RCS of the sea surface specifically includes:

[0081] The electromagnetic scattering coefficient of each illuminated surface element of the second sea surface is calculated by the perturbation method. The calculation formula is:

[0082]

[0083] Where k is the wave number of the incident electromagnetic wave, ε is the dielectric constant of the sea surface, ψ(q l ) is the spectral function of the cosine small-scale capillary wave component, q l is the projection of the scattering vector corresponding to the incident electromagnetic wave on the inclined surface element, represents the scattering vector corresponding to the incident electromagnetic wave, represents the incident wave vector corresponding to the incident electromagnetic wave, F pq is the polarization factor, and the scattering vector refers to the vector of the scattered component generated by the incident wave on the sea surface.

[0084] The far-field scattering intensity of the second sea surface illuminated surface element is expressed as

[0085]

[0086] Among them, R0 is the distance from the starting point of the incident wave to the geometric center of the sea surface used to calculate the composite electromagnetic scattering RCS of the marine ship target. is the scattering amplitude of the inclined surface element, and its calculation formula is:

[0087]

[0088] The far-zone scattered field intensity values ​​of all second sea surface illuminated surface elements are accumulated to obtain an accumulated value, and the single scattering RCS of the sea surface is obtained by using the accumulated value.

[0089] The calculation of multiple scattering RCS between the sea surface and the ship target at sea is realized by the ray path tracing method. In the ray path tracing method, the field intensity between rays is calculated by the geometric optics method. When the incident ray irradiates the target plane element of the ship target at sea, the far-field scattering field intensity is calculated by the physical optics method. When the incident ray irradiates the inclined plane element on the sea surface, the far-field scattering field intensity is calculated by the semi-determined plane element method. All the calculated far-field scattering field intensities are superimposed to obtain the multiple scattering RCS between the sea surface and the ship target at sea. In the ray path tracing method, the ray paths between the sea surface and the ship target at sea include ship-sea surface rays, sea surface-ship rays, ship-sea surface-ship rays and sea surface-ship-sea surface rays.

[0090] The beneficial effects of the present invention are as follows: an effective method for calculating the composite scattering RCS of marine ship targets is proposed. This method takes into account the hydrodynamic effects of waves on ship targets, constructs a six-degree-of-freedom motion model of marine ship targets following waves, and improves the fidelity of the geometric model of the composite scene of dynamic sea surface and moving ship targets. On this basis, the composite scattering of sea surface and ship targets is solved by using the semi-deterministic panel method and ray tracing hybrid algorithm, which can effectively improve the calculation accuracy and effectiveness of the RCS of marine ship targets. At the same time, this method also greatly reduces the amount of calculation, which is conducive to engineering implementation. The present invention will provide technical support for the study of the electromagnetic characteristics of ship targets in actual complex marine environments, and is of great significance to the research on the detection and identification of marine ship targets. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 1 is a flow chart for implementing the method of the present invention;

[0092] Figure 2 This is a simulation result diagram of composite scattering RCS of a marine ship target according to the present invention;

[0093] Figure 3 This is a schematic diagram of the six-degree-of-freedom motion of a ship of the present invention;

[0094] Figure 4 It is a schematic diagram of ship speed and heading in the global coordinate system of the present invention;

[0095] Figure 5 is a diagram showing the relationship between the wave direction and the hull position of the present invention;

[0096] Figure 6 It is a schematic diagram of the slicing method solution of the present invention;

[0097] Figure 7 A point source division method for symmetrical and asymmetrical slice contours of the present invention;

[0098] Figure 8 It is a diagram showing the change of the sea surface geometric profile at different times of the present invention;

[0099] Fig. 9 This is a schematic diagram of composite scattering of a marine ship target according to the present invention;

[0100] Fig.10 It is a schematic diagram of incident wave decomposition of the present invention;

[0101] Fig.11 This is a schematic diagram of the target surface element self-occlusion judgment of the present invention;

[0102] Fig.12 This is a schematic diagram of target surface element occlusion judgment of the present invention;

[0103] Fig.13 It is a flowchart of the occlusion judgment of the present invention;

[0104] Fig.14 A schematic diagram of the geometric outline of the dual-scale rough sea surface of the present invention;

[0105] Fig.15 It is a schematic diagram of a single facet geometric model of the present invention;

[0106] Fig.16 This is a flow chart of the calculation of composite electromagnetic scattering of dynamic sea surface and moving ship target of the present invention. DETAILED DESCRIPTION

[0107] In order to better understand the content of the present invention, an embodiment is given here.

[0108] Figure 1 The flowchart of the implementation method of the present invention is shown in Figure 1. In the specific implementation, a threshold of the number of operations can be set. After each calculation of the composite electromagnetic scattering RCS of the marine ship target, the number of operations is calculated. When the number of operations exceeds the threshold, the calculation of the composite electromagnetic scattering RCS of the marine ship target is stopped, and the final calculation result of RCS is output. Figure 2 The following is the composite scattering RCS simulation result of a ship target at sea, where the sea surface size is 250m*50m, the incident wave frequency f=33GHz, and the wind speed 10m above the sea surface is 10m / s. The ship speed is 0m / s, the incident azimuth is 60°, the incident azimuth is 0°, and the polarization mode is VV polarization. The ship target geometric model is as follows: Figure 2 As shown in (a), the composite scattering RCS curve of the ship target at sea varies with time. Figure 2 (b) as shown. Figure 3 This is a schematic diagram of the six-degree-of-freedom motion of a ship of the present invention; Figure 4 It is a schematic diagram of ship speed and heading in the global coordinate system of the present invention; Figure 5 is a diagram showing the relationship between the wave direction and the hull position of the present invention; Figure 6 It is a schematic diagram of the slicing method solution of the present invention; Figure 7 A point source division method for symmetrical and asymmetrical slice contours of the present invention; Figure 8 It is a diagram showing the change of the sea surface geometric profile at different times of the present invention; Fig. 9 This is a schematic diagram of composite scattering of a marine ship target according to the present invention; Fig.10 It is a schematic diagram of incident wave decomposition of the present invention; Fig.11 This is a schematic diagram of the target surface element self-occlusion judgment of the present invention; Fig.12 This is a schematic diagram of target surface element occlusion judgment of the present invention; Fig.13 It is a flowchart of the occlusion judgment of the present invention; Fig.14 A schematic diagram of the geometric outline of the dual-scale rough sea surface of the present invention; Fig.15 It is a schematic diagram of a single facet geometric model of the present invention; Fig.16This is a flow chart of the calculation of composite electromagnetic scattering of dynamic sea surface and moving ship target of the present invention.

[0109] The present invention discloses a method for calculating the composite electromagnetic scattering RCS of marine targets. By considering the motion characteristics of ships with waves, the ray tracing method and the panel method are combined, and the composite scattering of sea surface and ship targets is calculated using a multipath model, and a composite scattering model of marine ship targets is established to achieve high-precision calculation of the composite electromagnetic scattering RCS of marine ship targets. The RCS refers to the radar cross section, and its full English name is Radar Cross Section.

[0110] The present invention discloses a method for calculating composite electromagnetic scattering RCS of a marine ship target, the steps of which include:

[0111] S1, according to the geometric information and scene parameters of the maritime ship target, establish a coordinate system, describe the motion characteristics of the maritime ship target with the waves, construct and solve the six-degree-of-freedom motion model of the maritime ship target with the wind and waves, and obtain the six-degree-of-freedom motion response variables of the ship.

[0112] The scenario parameters include sea surface size, sea surface wind speed, sea surface wind direction, ship movement speed and ship movement direction, etc.

[0113] The sea surface size is the sea surface area used to calculate the composite electromagnetic scattering RCS of marine ship targets.

[0114] The establishment of the coordinate system includes establishing a global coordinate system, an inertial coordinate system and a hull fixed coordinate system. The hull fixed coordinate system is a three-dimensional rectangular coordinate system with the ship's hull center of gravity as the origin, the sea level as the XOY plane, and the ship's forward direction as the positive direction of the X axis. It moves with the movement of the ship. The X-axis, Y-axis and Z-axis of the hull fixed coordinate system point to the bow, port side and bottom of the ship respectively. As the hull moves in different postures, the hull fixed coordinate system also changes continuously. The global coordinate system adopts the geodetic coordinate system, the center of the inertial coordinate system remains consistent with the global coordinate system, the center of the inertial coordinate system does not change with the movement of the hull, and the X-axis, Y-axis and Z-axis of the inertial coordinate system point to the bow, starboard side and bottom of the ship respectively.

[0115] The description of the motion characteristics of the marine ship target with the waves specifically includes: using six-degree-of-freedom motion response variables to describe the motion state of the marine ship target in the waves, such as Figure 3As shown, the six-degree-of-freedom motion response variables include the translational motion variables of the ship hull along the coordinate axes of the fixed coordinate system and the rotational motion variables around the coordinate axes of the fixed coordinate system. The translational motion variables include surge motion variables, sway motion variables and heave motion variables, and the rotational motion variables include roll motion variables, pitch motion variables and yaw motion variables.

[0116] The coordinates of the position point of the maritime ship target in the global coordinate system are {x, y, z}, and the coordinates of the position point of the maritime ship target in the inertial coordinate system are {x I ,y I ,z I}, the position coordinates of the target ship at sea in the fixed coordinate system of the ship are {x b ,y b ,z b}, the six-degree-of-freedom motion response variable of the ship in the fixed coordinate system is expressed as u1, u2, and u3 are the surge, sway, and heave motion variables included in the translational motion variables of the marine ship target, respectively; u4, u5, and u6 are the roll, pitch, and yaw motion variables included in the rotational motion variables, respectively; the conversion relationship between the inertial coordinate system and the hull fixed coordinate system is expressed as

[0117]

[0118] Among them, A roll , A pitch and A yaw Are all transformation matrices, and their expressions are:

[0119]

[0120] The conversion relationship between the global coordinate system and the inertial coordinate system is expressed as:

[0121]

[0122] Among them, v ship is the ship speed, is the angle between the ship's motion direction and the X-axis of the global coordinate system, and t represents time. Figure 4 shown.

[0123] The construction of a six-degree-of-freedom motion model of a maritime ship target moving with wind and waves specifically includes:

[0124] According to the wave spectrum and the relationship between wave direction and hull position, the encounter frequency ω with the hull as reference is calculated. e (encounter frequency) and encounter wave spectrum S e , and its calculation process is expressed as:

[0125]

[0126]

[0127] in, Indicates the direction of waves and the direction of ship movement The angle of Figure 5 As shown, It means that the direction of the waves is consistent with the direction of the ship's movement. Indicates that the direction of the waves is perpendicular to the direction of the ship's motion. It means that the direction of the wave is opposite to the direction of the ship's motion, ω means the wave period, Indicates that the encounter frequency ω e , Angle and The wave spectrum encountered at time , g is the gravity constant, Indicates that in the wave period ω, the angle The wave spectrum function at ;

[0128] According to the force balance of the ship target on the sea surface, the linear equation of the ship's motion in regular waves is constructed, and its expression is:

[0129]

[0130] in, is the mass matrix, and its expression is:

[0131]

[0132] Among them, m is the mass of the sea ship target, [x G ,y G ,z G ] is the coordinate of the center of gravity of the marine ship target in the global coordinate system, θ xx It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the x-axis, θ yy It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the x-axis, θ zz It represents the mass moment of inertia of the marine ship target when it performs translational and rotational motion along the z-axis, θ xy It represents the mass moment of inertia of the marine ship target when it moves along the x-axis translation and the y-axis rotation, θ xzIt represents the mass moment of inertia of the marine ship target when it moves along the x-axis translation and the z-axis rotation, θ yz It represents the mass moment of inertia of a maritime ship target when it moves along the y-axis in translation and the z-axis in rotation. The calculation formula of the mass moment of inertia is:

[0133] θ xx =∫(my 2 (y)+mz(z) 2 )dydz,θ xy =∫(my 2 (y)+mx(x) 2 )dxdy,

[0134] θ yy =∫(mx 2 (x)+mz(z) 2 )dxdz,θ yz =∫(my 2 (y)+mz(z) 2 )dzdy,

[0135] θ zz =∫(mx 2 (x)+my 2 (y))dxdy,θ xz =∫mx 2 (x)+mz(z) 2 dxdz, (7)

[0136] Among them, mx(x), my(y) and mz(z) are the mass distribution functions of the marine ship target along the x-axis, y-axis and z-axis respectively. For a symmetric mass distribution hull, θ xy =θ yz =0. It represents the fluid dynamics exerted by waves on the ship target at sea, which includes the fluid statics part and the fluid dynamics part caused by the ship movement, the incident wave of the sea wave hitting the hull and the diffraction after the collision; the linear equation of the ship motion in regular waves is decomposed and represented to obtain the six-degree-of-freedom motion model of the ship target at sea moving with wind and waves, and its expression is:

[0137]

[0138] in, is the restoring force matrix of the marine ship target, which is generated by the hydrostatics part, Represents the radiation force matrix of the motion of the maritime ship target, is the amplitude matrix of the disturbance force generated by the incident wave hitting the hull and its diffraction after the collision.

[0139] The six-degree-of-freedom motion model for solving the motion of a ship target at sea with wind and waves adopts a slicing method to solve the six-degree-of-freedom motion model for solving the motion of a ship target at sea with wind and waves.

[0140] The slicing method is used to solve the six-degree-of-freedom motion model of the marine ship target moving with wind and waves, which specifically includes:

[0141] The ship hull is evenly divided into several transverse sections along the axial direction of the hull. For each transverse section, the fluid dynamics of the ship hull at the transverse section is obtained by solving the linear equation of the ship's motion in regular waves. The fluid dynamics at each transverse section of the ship hull are superimposed along the axial direction of the hull to obtain the three-dimensional fluid dynamics of the ship hull. The three-dimensional fluid dynamics of the ship hull are used to construct a wave spectrum superposition model. The six-degree-of-freedom motion vector of the sea ship target in the fixed coordinate system of the hull is obtained according to the wave spectrum superposition model. According to the six-degree-of-freedom motion vector of the sea ship target in the fixed coordinate system of the hull, the six-degree-of-freedom motion response variable of the ship is obtained by using the formula for solving the six-degree-of-freedom motion amplitude of the ship.

[0142] like Figure 6 As shown, according to the slicing method, the 2D fluid dynamics at each section of the hull are superimposed along the length of the ship to obtain the 3D fluid dynamics of the hull. In order to ensure the calculation accuracy of the slicing method, it is generally required that the number of slices is not less than 20, and the frequency range covers the entire response range, with approximately 20 to 30 frequencies between 0.20 rad / s and 2.40 rad / s being taken for calculation.

[0143] The following introduces the definitions of various mechanical matrices and basic solution methods of the linear equations for the motion of a ship in regular waves.

[0144] For the restoring force matrix, that is, the hydrostatic matrix When the hull is asymmetric, the restoring force matrix in general is It can be expressed as

[0145]

[0146] Where ρ is the density of seawater, the integral is the integral along the length of the hull, A is the area of ​​the slice (under the average water surface), B is the width of the waterline, and y w (x) is the average transverse coordinate of the slice waterline, (x s ,z s ) is the center coordinate of the slice, and the longitudinal coordinate of all cross-section slices is x, A tr is the section area of ​​the dry stern below the waterline. If the hull has no stern or the stern is wet, then A tr =0,y tr ,z tr is the center of gravity coordinate of the stern.

[0147] The calculation of added mass, Froude-Krilov force and diffraction force is introduced below.

[0148] Basic solution for 2D flow potential:

[0149]

[0150] If the ship has a forward velocity v, then the hull slices encounter frequency ω e Oscillating, two-dimensional flow potential satisfies the following conditions:

[0151] Laplace equation (due to the incompressibility of the fluid): φ yy +φ zz =0,z>0;

[0152] Bottom condition: Shallow water: φ z =0, z=H, H represents the water depth. Deep water:

[0153] Free surface condition: φ tt -gφ z =0,z=0;

[0154] Hull boundary conditions: represents the speed of the hull at a certain point on its surface, Represents the outward normal direction of the point.

[0155] Radiation conditions: The negative sign in the exponential term indicates that the wave propagates in the +y direction, and the positive sign indicates that the wave propagates in the opposite direction.

[0156] For the complex amplitude of the potential From formula (10), we can get

[0157]

[0158] Shallow Water: or deep water:

[0159]

[0160]

[0161]

[0162] According to the slicing method, the complex amplitude of the potential can be It is approximately a superposition of point sources, that is,

[0163]

[0164] Among them, qi For the i ,z i ). Except for the point source located within the slice outline or above z = 0 (y i ,z i ), equation (15) satisfies the Laplace equation (11) everywhere.

[0165] The slice contour is defined by given compensation points. For any contour segment between adjacent compensation points, a source can be generated near the midpoint between the two compensation points. Generally speaking, the location of the source is shifted from the midpoint to the inside of the slice by 1 / 20 times the length of the contour segment. Along the mean water surface z=0, the grid points are automatically generated. Close to the hull, the distance between the grid points is equal to 1.5 times the distance between the contour compensation points on the waterline. On the farther sides, the distance between the grid points increases by a factor of 1.5 until the maximum distance reaches 1 / 12 times the wavelength. At this point, the source point is again located at the midpoint of each free surface segment. For a symmetric hull, only half of the free surface grid points need to be discretized (for example, the number is 55). For an asymmetric hull, the free surface on both sides of the slice needs to be discretized, with a number of grid points of 2×55. Figure 7 The point source partitioning method for each hull slice contour is given.

[0166] For the hull boundary condition (13), the integral along each contour segment at the left end, i.e., the flow induced by the i-th source S through the contour segment between points A and B, is equal to the source intensity multiplied by the angle ASB (β i ) divided by 2π. In this way, the total flow can be obtained by superimposing the flows induced by all sources, that is,

[0167]

[0168] The second term of the free surface condition (12) can be processed in the same way as above. The integral of the first term along the contour segment is approximately

[0169]

[0170] Using the mirror source, equation (15) can accurately satisfy the shallow water condition of bottom condition (12). i ,z i ) is located below the bottom (y i ,2H-z i ) For deep water, the potential approximation (15) of the point source method can automatically satisfy the deep water condition (12).

[0171] According to equation (17), the integral of the radiation condition (14) along the segment between points A and B is

[0172]

[0173] Arrangement can be obtained

[0174]

[0175] Solving the system of linear equations consisting of boundary conditions gives the complex amplitudes of all source intensities. Therefore, the flow potential can be determined according to equation (15).

[0176] Added mass and drag:

[0177] According to Bernoulli's equation, the complex amplitude of pressure is

[0178] p=-ρφ t (20)

[0179] Integrating the pressure amplitude along the slice contour gives the complex amplitudes of the horizontal force, vertical force, and moment. For one of the forces, consider the force amplitude and motion amplitude This proportional relationship can be expressed as follows

[0180]

[0181] Since if the sinusoidal motion occurs with a circular frequency ω, is the complex amplitude of acceleration, so can be understood as an element of the complex additional mass matrix. However, considering the force amplitude When it is the sum of resistance and mass terms, we have

[0182]

[0183] in, is the complex amplitude of the motion velocity. It is not difficult to find that equations (21) and (22) are the same. By comparison, we can get the complex additional mass The real added mass m and the damping coefficient d satisfy the following relationship

[0184]

[0185] Froude-Krilov force:

[0186] According to formula (20), the complex amplitude of the wave pressure is

[0187]

[0188] The Froude-Krilov force can be obtained by integrating the pressure along the slice contour, and the integral of the pressure along the slice contour can be obtained by superimposing the integral of the pressure along the contour segment between each two adjacent compensation points, that is,

[0189]

[0190] In the formula, Δy=y2-y1, Δz=z2-z1.

[0191] Diffraction power:

[0192] The calculation of wave diffraction forces and moments is different from the calculation of added mass forces and moments, which only considers the inhomogeneous terms in the hull boundary conditions. In the calculation of diffraction forces, the wave flow must be offset by the source induced flow.

[0193] According to the flow velocity vector, and The wave flow between can be expressed as

[0194]

[0195]

[0196] Using the dispersion relation ω 2 =gktanhkH, the first fraction in the above formula can be expressed as:

[0197]

[0198] In equations (25) and (27), when α is very small, the direct estimation of the expression [exp(α)-1] / α is inaccurate. Therefore, when |α| < 0.01, the Taylor expansion of the exponential function can be used to approximate [exp(α)-1] / α as

[0199]

[0200] (3) Calculation of radiation force

[0201] make represents the complex amplitude of the forces and moments exerted by the water on the slice, It contains three components: force in the y direction, force in the z direction, and moment about the x axis. Proportional to the motion amplitude vector with three components

[0202]

[0203] All elements in the matrix A The first column i represents the force component, and the second column j represents the force caused by the motion of the j component. The elements in the complex additional mass matrix can be understood as the contribution of the real additional mass and the drag force, and have the same relationship as in equation (23), for example, for the first element we have

[0204]

[0205] For easier understanding, formula (30) is rewritten in another form:

[0206]

[0207] This can be thought of as the time derivative of momentum (mass times velocity). If the ship has a forward velocity v ship , the above motion relationship can be expressed as

[0208]

[0209] The six-component ship motion can be used to convert the three-component cross-sectional velocity Expressed as

[0210]

[0211] In the formula

[0212]

[0213] Three-component cross-sectional force and the six-component force per unit length The transformation matrix between

[0214]

[0215] The force per unit length Integrating along the ship gives the six-component radiation force:

[0216]

[0217] therefore,

[0218]

[0219] (4) Calculation of excitation force

[0220] make represents the complex amplitude of the three-component force and is proportional to the complex amplitude of the wave at the point (x=0, y=0) Including Froude-Krilov forces and diffraction forces.

[0221]

[0222] In the formula, 0 and 7 in the following table represent the Froude-Krilov part and the diffraction part, respectively.

[0223] If the ship's forward speed is v ship When Expand to

[0224]

[0225] For a regular wave with direction u, the complex amplitude of its wave height ζ can be expressed as

[0226]

[0227] in, is the complex amplitude at the origin, k = 2π / λ is the wave number, and λ is the wavelength.

[0228]

[0229] right Multiplying the transformation matrix V on the left can transform it into a six-component force vector in the global coordinate system, so the first part of the contribution of the excitation force vector can be obtained:

[0230]

[0231] For a more accurate calculation of the surge motion and the pitch and yaw, it is necessary to consider the additional longitudinal force exerted on the ship by the water. In this case the diffraction part can be neglected, so the longitudinal force on the ship when only the Froude-Krilov contribution is considered is

[0232]

[0233] In the formula, is the pressure amplitude, where the center of the cross section (x, y x ,z x ) instead of the average value along the entire cross section. The pressure at the center of the cross section is

[0234]

[0235] In the formula, α = -ρgexp[-k(z x +T)]exp(iky x sin(u)).

[0236] Longitudinal force per unit length The contribution to the pitch moment can also be induced, so the second part of the contribution to the six-component excitation force vector can be obtained:

[0237]

[0238] The amplitude response operator is defined as the ship's six-degree-of-freedom motion vector Complex amplitude of regular wave with excitation The ratio of

[0239]

[0240] The method of constructing a wave spectrum superposition model by utilizing the three-dimensional fluid dynamics of the ship hull and solving the wave spectrum superposition model to obtain the six-degree-of-freedom motion vector of the sea ship target in the fixed coordinate system of the hull specifically includes:

[0241] According to the idea of ​​linear superposition, the motion of a ship at a certain point on the wave surface can also be considered to be jointly excited by waves of different frequencies and directions.

[0242] The constructed wave spectrum superposition model is expressed as follows:

[0243]

[0244] Among them, u i (x, y, t) is the six-degree-of-freedom motion amplitude of the marine ship target at the two-dimensional wave surface (x, y) at time t, i = 1, 2, ..., 6, and the six-degree-of-freedom motion amplitude constitutes the six-degree-of-freedom motion vector of the marine ship target in the fixed coordinate system of the hull, k e is the encounter circle frequency ω e The wave number under the wave, S e (,) is the two-dimensional encounter wave spectrum function, R is the amplitude response operator, N k and N φ are the number of sampling points for frequency and azimuth, respectively. e,l is the encounter circle frequency at the lth frequency sampling point, k e,l is the encounter circle frequency ω e,l The corresponding wave number is is the jth spatial direction angle of the wave, represents the sea surface wind direction angle, Δk e is the change increment of the wave number, is the change increment of the spatial direction angle of the wave, ε lj It represents the initial phase corresponding to the lth frequency sampling point and the jth spatial direction angle. Using the wave spectrum superposition model, the six-degree-of-freedom motion vector of the sea ship target in the fixed coordinate system of the hull is solved.

[0245] S2, establish a geometric model of the dynamic sea surface and moving ship composite scene;

[0246] The three-dimensional dynamic sea surface geometric model of the ship target at sea is established by using the linear superposition method. The wave height of a fixed point on the sea surface at a single moment is described by superposition of multiple random cosine waves. Then the wave height η of a fixed point (x, y) on the sea surface at time t is L (x,y,t) is expressed as:

[0247]

[0248] Among them, S e(,) is the two-dimensional encounter wave spectrum function, where the Efouhaily spectrum is used, k l ,ω l , Respectively represent the wave number, circular frequency, and direction angle of the random cosine wave, N k and N φ are the number of sampling points for frequency and azimuth, respectively. represents the sea surface wind direction angle, k e,l is the encounter circle frequency ω e,l The corresponding wave number is is the jth spatial direction angle of the wave, represents the sea surface wind direction angle, Δk e is the change increment of the wave number, is the change increment of the spatial direction angle of the wave, ε lj It represents the initial phase corresponding to the lth frequency sampling point and the jth spatial direction angle, which is uniformly distributed between 0 and 2π.

[0249] The three-dimensional dynamic sea surface geometry model includes three-dimensional dynamic sea surface geometry information. The three-dimensional dynamic sea surface geometry information includes the wave height information of each fixed point (x, y) on the sea surface at each moment. Figure 8 This is a diagram showing the changes in sea surface geometric contours at different times.

[0250] At each time sampling point, the geometric information of maritime ship targets and the geometric information of three-dimensional dynamic sea surface are linearly superimposed to obtain a composite scene geometric model of dynamic sea surface and moving ship, which provides effective scene information for subsequent composite electromagnetic RCS calculation of maritime ship targets.

[0251] S3, calculate the composite electromagnetic scattering RCS of the marine ship target based on the geometric model of the composite scene of the dynamic sea surface and the moving ship;

[0252] The geometric information of the maritime ship target is segmented by surface elements to obtain several target surface elements. The composite electromagnetic scattering RCS of the maritime ship target is divided into four parts of RCS, namely the single scattering RCS of the ship, the multiple scattering RCS of the ship, the single scattering RCS of the sea surface and the multiple scattering RCS between the sea surface and the ship. The scattering field intensities corresponding to the above four parts of RCS are solved respectively according to the target surface elements, and the obtained scattering field intensities are vector-superimposed to obtain the composite electromagnetic scattering RCS of the maritime ship target. Fig. 9 A schematic diagram of the scattering of these four parts is given.

[0253] The calculation of single scattering RCS of marine ship targets is realized by pure physical optics method, which specifically includes:

[0254] The tangent plane approximation condition is used to locally approximate the excitation source of the scattered RCS, and the induced electromagnetic current of the marine ship target is obtained. The far-field Green's function and the equivalent principle are used to perform near-far field extrapolation to obtain the expression of the electromagnetic field of the marine ship target. Then, the tangent plane approximation method is used to obtain the surface current value and magnetic current value of the marine ship target. According to the surface current value and magnetic current value of the marine ship target, the single scattering RCS of the marine ship target is obtained.

[0255] For the calculation of single scattering RCS of marine ship targets, the pure physical optics (PO) method is used. However, it should be noted that in addition to considering the self-occlusion and mutual occlusion of marine ship targets themselves, the occlusion effect of the sea surface on marine ship targets should also be considered, such as Fig. 9 The scattering of the submerged sea ship target will not be calculated.

[0256] The PO method uses the tangent plane approximation condition to make a local approximation of the excitation source. After obtaining the induced electromagnetic current, the far-field Green's function and the equivalent principle are used to perform near-far field extrapolation, and the Huygens equivalent principle is obtained, thereby obtaining the expression of the electromagnetic field:

[0257]

[0258]

[0259] Then, the surface electromagnetic current is obtained by using the tangent plane approximation, is the unit vector of the incident direction. The incident wave can be decomposed into parallel polarization (TM wave) using Represents its unit direction vector, and orthogonal polarization (TE wave), with Represents its unit direction vector, and the reflected wave also has a similar decomposition representation, Fig.10 Only a schematic diagram of the electric field vector is given; the magnetic field vector is similar.

[0260] and The relationship between

[0261]

[0262]

[0263] The relationship between the reflected field and the incident field of the TE or orthogonal polarization component can be expressed as

[0264]

[0265]

[0266]

[0267]

[0268] Among them, the relationship between the reflected wave propagation vector and the incident wave and the normal vector is

[0269] Similarly, the relationship between the reflected field and the incident field in the TM or parallel polarization component can be expressed as

[0270]

[0271]

[0272]

[0273]

[0274] Among them, R TE and R TM are the reflection coefficients of TE wave and TM wave respectively.

[0275] The calculation formulas for equivalent electric and magnetic currents are as follows:

[0276]

[0277]

[0278] Substituting into equation (50) we get the formula for the far-field electric field (replace the direction vector Replace with the scattering direction unit vector and assume that the amplitude of the incident wave is unit amplitude)

[0279]

[0280] in

[0281]

[0282] The bouncing ray method is used to calculate the multiple scattering RCS of marine ship targets.

[0283] When the bouncing ray method is used to calculate the multiple scattering RCS of a ship target at sea, the direction of the incident wave is determined, and the corresponding incident ray direction information is determined according to the incident wave direction. The direction of each incident ray is consistent with the direction of the incident wave, and each incident ray passes through the center of the corresponding target facet. According to the incident ray direction information, the target facet is blocked and the blockage judgment result is obtained. The blockage judgment result includes the illuminated facet information and the blocked facet information. For the target facet corresponding to the illuminated facet information, the incident ray corresponding to the target facet is tracked and its scattering field is calculated. The ray corresponding to the incident wave is called the incident ray, and the direction of the incident ray is called the incident ray direction.

[0284] The incident ray is tracked, according to the geometric model of the composite scene of the dynamic sea surface and the moving ship, for each incident ray, its propagation path is tracked using the geometric optics method until the incident ray does not intersect with any target surface element or the number of reflections in the propagation path of the incident ray reaches a preset value, and the propagation path of the incident ray is obtained and recorded;

[0285] The scattering field calculation is as follows: according to the propagation path of the incident ray, the intersection point of the incident ray and the target surface element is obtained, and the propagation distance between two adjacent intersection points is calculated; according to each propagation distance, the field intensity relationship between the two adjacent intersection points is calculated, and the field intensity relationship is used as the incident field intensity at the corresponding illuminated surface element, and the far-zone scattering field value on the illuminated surface element is calculated by using the physical optics method; the far-zone scattering field values ​​of all illuminated surface elements are calculated, the far-zone scattering field values ​​of all illuminated surface elements are accumulated to obtain an accumulated value, and the multiple scattering RCS of the marine ship target is obtained by using the accumulated value.

[0286] The calculation of the field strength relationship between the two adjacent intersection points according to each propagation distance specifically includes:

[0287] In the propagation path of the incident ray, the intersection point of the incident ray and the target surface element is I is the number of intersection points, and the propagation distance of the ray between two adjacent intersection points The relationship between the field strengths at two adjacent intersection points is:

[0288]

[0289] in, and The intersection points and intersection The field strength at , k is the wave number of the incident ray electromagnetic wave, (DF) i and The intersection points The divergence factor and reflection coefficient matrix of . The expression of the divergence factor is

[0290]

[0291] Among them, ρ1 and ρ2 are the intersection points The principal curvature radius of the incident ray emission surface and the illuminated surface element corresponding to the intersection point, s represents The reflection distance between the incident ray emission surface and the illuminated surface element at the corresponding position. The plane divergence factor is 1. When all target surface elements are plane surface elements, the divergence factor is equal to 1.

[0292] The performing of occlusion judgment and obtaining of the occlusion judgment result specifically includes:

[0293] First, perform other-occlusion judgment on the target surface element to obtain directly illuminated target surface elements and target surface elements that are not directly illuminated. Then perform self-occlusion judgment on the directly illuminated target surface elements to obtain the illuminated surface elements and the target surface elements occluded by themselves that are finally output. The target surface element information occluded by itself and the target surface element information that is not directly illuminated constitute the occluded surface element information in the occlusion judgment result. The illuminated surface element information that is finally output constitutes the illuminated surface element information in the occlusion judgment result, and the occlusion judgment result is obtained.

[0294] Since ship targets generally contain multiple dihedral structures, which will form a large number of multiple reflections, the bounce ray (SBR) algorithm is used to calculate the multiple scattering of ships.

[0295] SBR is a high-frequency algorithm that combines geometric optics (GO) and PO. It uses GO to consider the energy propagation of electromagnetic waves between targets, and uses PO to integrate in the far field. When using SBR to calculate multiple scattering of ship targets, the corresponding incident ray can be determined given the direction of the incident wave. The direction of each incident ray is consistent with the direction of the incident wave, and each ray passes through the center of the corresponding target face element. After the incident ray information is determined, the target face element needs to be occluded first, which includes self-occlusion judgment and other-occlusion judgment.

[0296] The aforementioned other occlusion judgment specifically includes judging whether the target surface element is occluded by other target surface elements along the direction of the incident ray, such as Fig.12 As shown, if the target plane element is not blocked by other target plane elements, the target plane element is determined to be the target plane element directly illuminated by the incident wave. All target plane elements are subjected to other blocking judgment to obtain directly illuminated target plane elements and target plane elements not directly illuminated.

[0297] The aforementioned other-occlusion judgment specifically includes that the geometric center point of the target surface element for which the other-occlusion judgment needs to be performed is Q. If the incident ray has an intersection with other target surface elements before reaching point Q, then the target surface element where point Q is located is determined to be blocked by other target surface elements, and the target surface element where point Q is located is considered to be a target surface element that is not directly illuminated, and the target surface element that is not directly illuminated belongs to an occluding surface element. If the incident wave has no intersection with other target surface elements before reaching point Q, then the target surface element where point Q is located is determined to be illuminated by the incident wave, and the target surface element where point Q is located is considered to be a directly illuminated target surface element.

[0298] The specific implementation of the above-mentioned occlusion judgment process is as follows: for a target surface element m on a marine ship target, if the three vertex coordinates of the target surface element m on the marine ship target are A, B and C, the starting point of the incident wave ray is at point P, and the geometric center point of the target surface element m is Q. The coordinate S of any point on the target surface element m is expressed by a parametric equation as follows:

[0299] S=αA+βB+(1-α-β)C,

[0300] Among them, α, β and γ are the parameters to be solved, let γ = 1-α-β; assume that the point with coordinate S is also located on the line segment PQ, that is, S is the intersection of the target surface element m and the line segment PQ, and its coordinates are expressed as:

[0301] S=λP+(1-λ)Q,

[0302] According to the above two expressions of coordinate S, construct the equivalent equation:

[0303] α(AC)+β(BC)+λ(QP)=QC,

[0304] Expressing the equivalent equation in the form of matrix multiplication, we get the equivalent equation system:

[0305]

[0306] Wherein, CA represents the vector from C to A, CB represents the vector from C to B, PQ represents the vector from P to Q, and CQ represents the vector from C to Q. For this equivalent equation group, if det[CA CB PQ]=0, the target surface element is determined to be the target surface element directly illuminated by the incident wave. If det[CA CB PQ]≠0, the equivalent equation group is solved to obtain α, β, λ and γ. If the values ​​of α, β, λ and γ are all within the interval [0,1], the target surface element is determined to be the target surface element that is not directly illuminated. If any one of the four variables α, β, λ and γ is not within the interval [0,1], the target surface element is determined to be the target surface element directly illuminated by the incident wave.

[0307] The self-occlusion judgment of the directly illuminated target surface element specifically includes: for the target surface element directly illuminated by the incident wave obtained through the self-occlusion judgment, calculating the normal vector of the target surface element and the direction vector of the incident wave The inner product of like Fig.11 As shown. If the inner product Then the target pixel is judged to be the illuminated pixel of the final output; if the inner product It is determined that the target surface element is blocked by itself, and the target surface element is considered to be a target surface element blocked by itself, and the target surface element blocked by itself belongs to an blocking surface element.

[0308] Normal vector of the target surface The direction is along the center of gravity of the hull pointing to the outside of the hull and satisfying the right-hand spiral rule.

[0309] Any point on the target surface element m is expressed by a parametric equation:

[0310] S=αA+βB+(1-α-β)C, (66)

[0311] Among them, the value intervals of constants α, β and γ are all [0,1], then point S is inside the target surface element m, otherwise point S is outside the target surface element m, γ = 1-α-β. The three-dimensional coordinates of any point on the line segment PQ can be written as the following expression:

[0312] S=λP+(1-λ)Q, (67)

[0313] Where λ represents the distance between point S and point Q. If λ is between [0,1], then the point is on the line segment PQ, otherwise it is not. When point S is on both the line segment PQ and the face element m, that is, S is the intersection of the target face element m and the line segment PQ, we have:

[0314] α(AC)+β(BC)+λ(QP)=QC, (68)

[0315] Denoted as:

[0316] α(AC)+β(BC)+λ(PQ)=CQ, (69)

[0317] The expression in matrix form is:

[0318]

[0319] Fig.13 The process of occlusion judgment is given. Solve the above equations to obtain α, β and λ, and take γ=1-α-β. α, β and λ are analyzed to judge the occlusion situation. If det[CA CB PQ]=0, then expression (5) has no solution, no intersection, and the face element is not occluded. If det[CA CB PQ]≠0, then expression (5) has a solution and an intersection. If α, β, λ and γ are all in the interval [0,1], the face element is occluded. If any one of α, β, λ and γ is not in the interval [0,1], the face element is not occluded.

[0320] For the calculation of the single scattering RCS of the sea surface, the sea surface is discretized into several inclined surface elements, and the expression of the geometric contour of the inclined surface element is constructed. Each inclined surface element is used as the first target surface element, and a self-occlusion judgment is performed to obtain the first sea surface illuminated surface element. The first sea surface illuminated surface element and the target surface element of the sea ship target are subjected to a other-occlusion judgment to obtain the second sea surface illuminated surface element. For all the second sea surface illuminated surface elements, the semi-deterministic surface element method is used to solve and obtain the single scattering RCS of the sea surface.

[0321] The self-occlusion judgment of the first target surface element specifically includes: calculating the normal vector of the first target surface element and the direction vector of the incident wave The inner product of like Fig.11 As shown. If the inner product Then the target pixel is judged to be the first illuminated pixel on the sea surface; if the inner product It is determined that the first target plane is blocked by itself, and the first target plane is considered to be a target plane blocked by itself.

[0322] The step of performing other-occlusion judgment on the first sea surface illuminated surface element and the target surface element of the ship target at sea to obtain the second sea surface illuminated surface element specifically includes:

[0323] Along the direction of the incident ray, determine whether the first sea surface illuminated surface element is blocked by the target surface element of the sea ship target, such as Fig.12 As shown, if the first sea surface illuminated surface element is not blocked by the target surface element of the ship target at sea, the first sea surface illuminated surface element is determined to be the target surface element directly illuminated by the incident wave. All first sea surface illuminated surface elements are subjected to other blocking judgment to obtain second sea surface illuminated surface elements.

[0324] The aforementioned other-occlusion judgment specifically includes that the geometric center point of the first sea surface illuminated surface element that needs to be judged for other-occlusion is Q; if the incident ray has an intersection with the target surface element of the sea ship target before reaching the Q point, then it is determined that the first sea surface illuminated surface element where the Q point is located is blocked by the target surface element of the sea ship target, and the first sea surface illuminated surface element where the Q point is located is considered to be a target surface element that is not directly illuminated; if the incident wave has no intersection with the target surface element of the sea ship target before reaching the Q point, then it is determined that the first sea surface illuminated surface element where the Q point is located is illuminated by the incident wave, and the first sea surface illuminated surface element where the Q point is located is considered to be a directly illuminated target surface element, and the directly illuminated target surface element is used as the second sea surface illuminated surface element.

[0325] The sea surface is discretized into a number of inclined surface elements, and an expression for the geometric contour of the inclined surface element is constructed. The rough sea surface dual-scale model is used, and it is assumed that the wave numbers in the continuous ocean spectrum are separated into a sea spectrum dominated by large-scale gravity waves and a sea spectrum dominated by small-scale tension waves. These two wave number ranges can be fully covered by the applicable ranges of the Kirchhoff method and the perturbation method.

[0326] The sea surface is discretized into a number of inclined surface elements, and the geometric contour expression of the inclined surface element is constructed, such as Fig.14 As shown in , the sea surface is regarded as composed of small-scale capillary waves superimposed on large-scale gravity waves, and the sea surface is discretized into several inclined surface elements, including sea surface roughness surface elements and cosine capillary wave component surface elements. The geometric contour of the inclined surface element is represented by the cosine capillary wave component surface element that causes Bragg resonance of the incident electromagnetic wave superimposed on the sea surface roughness surface element, as shown in Fig.15 As shown, the geometric profile of the inclined surface element at time t is expressed as:

[0327] ζ(ρ c ,t)=B(k c )cos(k c ·ρ c -ω c t),

[0328] Among them, ρ c represents the position coordinate vector inside the inclined surface element, k c represents the wave number vector of the cosine capillary wave, B(k c ) represents the amplitude of the cosine capillary wave, ω c represents the wave number vector k c The corresponding angular frequency.

[0329] The method of using a semi-deterministic surface element method to solve all second sea surface illuminated surface elements to obtain the single scattering RCS of the sea surface specifically includes:

[0330] The electromagnetic scattering coefficient of each illuminated surface element of the second sea surface is calculated by the perturbation method. The calculation formula is:

[0331]

[0332] Where k is the wave number of the incident electromagnetic wave, ε is the dielectric constant of the sea surface, ψ(q l ) is the spectral function of the cosine small-scale capillary wave component, q l is the projection of the scattering vector corresponding to the incident electromagnetic wave on the inclined surface element, represents the scattering vector corresponding to the incident electromagnetic wave, represents the incident wave vector corresponding to the incident electromagnetic wave, Fpq is the polarization factor, and the scattering vector refers to the vector of the scattered component generated by the incident wave on the sea surface.

[0333] Since the surface element is tilted under the action of gravity waves, there is a problem of conversion between local coordinates and global coordinates. In local coordinates, the local scattering amplitude element is expressed as F pq_loc ,Right now

[0334]

[0335] F vh_loc =[1-R v (θ i_loc )][1+R h (θ s_loc )]cosθ i_loc sinφ s_loc

[0336] F hv_loc =[1+R h (θ i_loc )][1-R v (θ s_loc )]cosθ s_loc sinφ s_loc

[0337] F hh_loc =[1+R h (θ i_loc )][1+R h (θ s_loc )]cosφ s_loc

[0338] Among them, θ i_loc ,θ s_loc ,φ i_loc ,φ s_loc are the local incident angle, scattering angle, incident azimuth, and scattering azimuth of the incident wave for the inclined surface element. Their values ​​are not only related to the direction of the incident wave, but also to the tilt angle of the surface element. h and R v is the reflection coefficient of different polarizations. After obtaining the local scattering amplitude, the local scattering amplitude is converted into the global scattering amplitude through the following formula.

[0339]

[0340] in, are the global horizontal and vertical polarization vectors, are the local horizontal and vertical polarization vectors.

[0341] The far-field scattering intensity of the second sea surface illuminated surface element is expressed as

[0342]

[0343] Among them, R0 is the distance from the starting point of the incident wave to the geometric center of the sea surface used to calculate the composite electromagnetic scattering RCS of the marine ship target. is the scattering amplitude of the inclined surface element, and its calculation formula is:

[0344]

[0345] The far-zone scattered field intensity values ​​of all second sea surface illuminated surface elements are accumulated to obtain an accumulated value, and the single scattering RCS of the sea surface is obtained by using the accumulated value.

[0346] Since the small facets in the semi-deterministic method have taken into account the Bragg scattering, which has the greatest scattering effect, each small facet can fully reflect the contribution of capillary waves to scattering, so the entire sea surface can be divided into larger facets. Therefore, this algorithm can greatly reduce the amount of calculation.

[0347] The calculation of multiple scattering RCS between the sea surface and the ship target at sea is realized by the ray path tracing method. In the ray path tracing method, the field intensity between rays is calculated by the geometric optics method. When the incident ray irradiates the target plane element of the ship target at sea, the far-field scattering field intensity is calculated by the physical optics method. When the incident ray irradiates the inclined plane element on the sea surface, the far-field scattering field intensity is calculated by the semi-determined plane element method. All the calculated far-field scattering field intensities are superimposed to obtain the multiple scattering RCS between the sea surface and the ship target at sea. In order to simplify the processing of the coupling effect between the sea surface and the ship target at sea, the order of ray tracing is set to the third order, and rays greater than the third order will not be considered. In the ray path tracing method, the ray paths between the sea surface and the ship target at sea include ship-sea surface rays, sea surface-ship rays, ship-sea surface-ship rays, and sea surface-ship-sea surface rays.

[0348] Solve the scattering field intensity corresponding to the above four parts, perform vector superposition on the obtained scattering field intensity, and obtain the composite total scattering field of the ship target at sea, that is,

[0349]

[0350] at last, Fig.16The specific process of calculating the composite electromagnetic scattering of dynamic sea surface and moving ship targets is given. As can be seen from the figure, in the simulation calculation process, the ship target geometry model file is firstly input, and the corresponding scene parameters (such as sea surface size, wind speed, wind direction, ship movement speed, direction, etc.) are set. At each time sampling point, the ship motion characteristics with waves and the three-dimensional sea surface geometry modeling are respectively calculated to obtain the six-degree-of-freedom motion model of the ship target with waves at sea, and the scene information is updated. On this basis, combined with the composite electromagnetic scattering model of the ship target at sea, with the help of ray tracing and semi-deterministic surface element hybrid method, the ship target scattering, sea surface scattering and coupled scattering between the sea surface and the ship at the time sampling point are calculated respectively, and the composite scattering RCS data of the ship target at sea is obtained. Then, the time sampling point is updated, and the above steps are repeated until the preset maximum time sampling point is reached.

[0351] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A method for calculating the composite electromagnetic scattering RCS of a marine ship target, characterized in that: include: S1, according to the geometric information and scene parameters of the maritime ship target, a coordinate system is established to describe the motion characteristics of the maritime ship target with wind and waves, and obtain the motion characteristics description results; according to the motion characteristics description results, a six-degree-of-freedom motion model of the maritime ship target with wind and waves is constructed and solved to obtain the six-degree-of-freedom motion response variables of the ship; S2, using the six-degree-of-freedom motion response variables of the ship, establish a geometric model of the dynamic sea surface and moving ship composite scene; S3, calculate the composite electromagnetic scattering RCS of the marine ship target based on the geometric model of the composite scene of the dynamic sea surface and the moving ship; The scenario parameters include sea surface size, sea surface wind speed, sea surface wind direction, ship movement speed and ship movement direction; The establishing of the coordinate system includes establishing a global coordinate system, an inertial coordinate system and a hull fixed coordinate system; The hull fixed coordinate system is a three-dimensional rectangular coordinate system with the ship's hull center of gravity as the origin, the sea level as the XOY plane, and the ship's forward direction as the positive direction of the X axis. It moves with the movement of the ship. The X axis, Y axis, and Z axis of the hull fixed coordinate system point to the bow, port side, and bottom of the ship, respectively. The global coordinate system adopts the geodetic coordinate system; The center of the inertial coordinate system is kept consistent with the global coordinate system, the center of the inertial coordinate system does not change with the movement of the hull, and the X-axis, Y-axis and Z-axis of the inertial coordinate system point to the bow, starboard and bottom of the ship respectively; The step S3 specifically includes: The geometric information of the marine ship target is divided into several target surface elements, and the composite electromagnetic scattering RCS of the marine ship target is divided into four parts of RCS, namely, the single scattering RCS of the ship, the multiple scattering RCS of the ship, the single scattering RCS of the sea surface, and the multiple scattering RCS between the sea surface and the ship. The scattering field strengths corresponding to the above four parts of RCS are solved according to the target surface elements, and the obtained scattering field strengths are vector-superimposed to obtain the composite electromagnetic scattering RCS of the marine ship target; The calculation of the multiple scattering RCS between the sea surface and the ship is implemented by the bouncing ray method; when the bouncing ray method is used to calculate the multiple scattering RCS of the marine ship target, the direction of the incident wave is determined, and the corresponding incident ray direction information is determined according to the incident wave direction, each incident ray passes through the center of the corresponding target face element, and according to the incident ray direction information, the target face element is blocked and the blockage judgment result is obtained, and the blockage judgment result includes the illuminated face element information and the blocked face element information. For the target face element corresponding to the illuminated face element information, the incident ray corresponding to the target face element is tracked and its scattering field is calculated; the ray corresponding to the incident wave is called the incident ray, and the direction of the incident ray is called the incident ray direction; The incident ray is tracked, and according to the geometric model of the composite scene of the dynamic sea surface and the moving ship, for each incident ray, its propagation path is tracked using a geometric optics method until the incident ray does not intersect with any target surface element or the number of reflections in the propagation path of the incident ray reaches a preset value, and the propagation path of the incident ray is obtained and recorded.

2. The method for calculating composite electromagnetic scattering RCS of a marine ship target according to claim 1, characterized in that: The six-degree-of-freedom motion model for solving the motion of a ship target at sea with wind and waves is solved by a slicing method.

3. The method for calculating the composite electromagnetic scattering RCS of a marine ship target according to claim 2, characterized in that: The slicing method is used to solve the six-degree-of-freedom motion model of the marine ship target moving with wind and waves, which specifically includes: The ship hull is evenly divided into several transverse sections along the axial direction of the hull. For each transverse section, the fluid dynamics of the ship hull at the transverse section is obtained by solving the linear equation of the motion of the ship in regular waves. The fluid dynamics at each transverse section of the ship hull are superimposed along the axial direction of the hull to obtain the three-dimensional fluid dynamics of the ship hull; The three-dimensional fluid dynamics of the ship's hull is used to construct a wave spectrum superposition model, and the six-degree-of-freedom motion response variables of the ship are solved according to the wave spectrum superposition model.

4. The method for calculating composite electromagnetic scattering RCS of a marine ship target according to claim 1, characterized in that: The scattering field calculation obtains the intersection of the incident ray and the target surface element according to the propagation path of the incident ray, and calculates the propagation distance between two adjacent intersection points; calculates the field intensity relationship between the two adjacent intersection points according to each propagation distance, uses the field intensity relationship as the incident field intensity at the corresponding illuminated surface element, and calculates the far-field scattering field value on the illuminated surface element using the physical optics method at the illuminated surface element; The far-zone scattering field values ​​of all illuminated surface elements are calculated, and the far-zone scattering field values ​​of all illuminated surface elements are accumulated to obtain an accumulated value, which is used to obtain the multiple scattering RCS of the marine ship target.

5. The method for calculating composite electromagnetic scattering RCS of a marine ship target according to claim 1, characterized in that: The step of performing occlusion judgment on the target surface element to obtain an occlusion judgment result includes: First, perform other-occlusion judgment on the target plane element to obtain directly illuminated target plane elements and target plane elements that are not directly illuminated, and then perform self-occlusion judgment on the directly illuminated target plane element to obtain the illuminated plane element and the target plane element occluded by itself that are finally output, the target plane element information occluded by itself and the target plane element information that is not directly illuminated constitute the occluded plane element information in the occlusion judgment result, and the illuminated plane element information that is finally output constitutes the illuminated plane element information in the occlusion judgment result, to obtain the occlusion judgment result; The other-occlusion judgment specifically includes: judging whether the target surface element is blocked by other target surface elements along the direction of the incident ray; if the target surface element is not blocked by other target surface elements, then judging that the target surface element is the target surface element directly illuminated by the incident wave; performing the other-occlusion judgment on all target surface elements to obtain the directly illuminated target surface elements and the target surface elements not directly illuminated; The self-occlusion judgment of the directly illuminated target surface element specifically includes: for the target surface element directly illuminated by the incident wave obtained through the self-occlusion judgment, calculating the normal vector of the target surface element and the direction vector of the incident wave The inner product of If the inner product Then the target pixel is judged to be the illuminated pixel of the final output; if the inner product It is determined that the target surface element is blocked by itself, and the target surface element is considered to be a target surface element blocked by itself, and the target surface element blocked by itself belongs to an blocking surface element.

6. The method for calculating composite electromagnetic scattering RCS of a marine ship target according to claim 1, characterized in that: For the calculation of the single scattering RCS of the sea surface, the sea surface is discretized into several inclined surface elements, and the expression of the geometric contour of the inclined surface element is constructed. Each inclined surface element is used as the first target surface element, and a self-occlusion judgment is performed to obtain the first sea surface illuminated surface element. The first sea surface illuminated surface element and the target surface element of the sea ship target are subjected to a other-occlusion judgment to obtain the second sea surface illuminated surface element. For all the second sea surface illuminated surface elements, the semi-deterministic surface element method is used to solve and obtain the single scattering RCS of the sea surface.

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