CN115184689B - Method for calculating electromagnetic scattering of target in evaporation waveguide environment - Google Patents
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Abstract
The invention relates to the technical field of radars, and discloses a method for calculating electromagnetic scattering of a complex target in an evaporation waveguide environment, which comprises the steps of firstly establishing a three-dimensional electromagnetic wave propagation model in the evaporation waveguide by adopting a ray tracing method according to an atmospheric refractive index parameterized model; pre-modulating an incident wave of the SBR method according to a three-dimensional electromagnetic wave propagation model in the evaporation waveguide, establishing an incident wave virtual emitting surface in an evaporation waveguide environment, and correcting general forms such as plane waves, conical waves and the like of the incident wave in the SBR method into a special form considering the influence of the evaporation waveguide on the electromagnetic wave propagation; finally, according to a special incident wave form in the evaporation waveguide environment, calculating the total scattering field of the incident wave on the surface of the target; the invention can simplify the calculation of the scattered field of the complex target in the evaporation waveguide environment, and greatly reduce the calculation cost under good precision.
Description
Technical Field
The invention relates to the technical field of radars, in particular to a method for calculating target electromagnetic scattering in an evaporation waveguide environment.
Background
The evaporation waveguide has extremely high occurrence probability at sea, is a special surface waveguide frequently occurring in the marine atmosphere environment, and is formed by the fact that the atmospheric humidity in a very small height range on the sea surface is reduced sharply along with the height due to the evaporation of water vapor on the sea surface. It is manifested in the field of radiosurgery as an anomaly in the refractive index of the atmosphere, trapping electromagnetic waves therein, causing anomalous propagation. Therefore, the electromagnetic scattering of the target in the evaporation waveguide environment is an important issue. Overcoming this problem has the following challenges: firstly, due to the influence of the environment of the evaporation waveguide, the traditional method that the incident wave is set to be a plane wave or a conical wave is not applicable any more when electromagnetic scattering is calculated, and the conditions of the incident wave are required to be preprocessed by acquiring the emergent field of the evaporation waveguide; second, an electrically large target requires a large number of grids to describe its geometric contour, which lead to a surge in computation in electromagnetic simulation. All these problems have made great attention to how to effectively model electromagnetic scattering in an evaporative waveguide environment.
In the field of electromagnetic scattering, all simulation methods can be divided into two categories: numerical methods (e.g., moment method (MoM), finite Element Method (FEM), etc.) and high frequency methods (e.g., bouncing ray method (SBR), iterative Physical Optics (IPO), etc.). The high frequency method is more practical for the electrically large-sized target in complex environments. In order to calculate the electromagnetic scattering characteristics of a target in an evaporation waveguide, firstly, the propagation characteristics of electromagnetic waves in the environment need to be obtained, and a parabolic equation method is widely used for solving the propagation problem of the electromagnetic waves in the evaporation waveguide; however, since the parabolic equation method does not study the ray trajectories, it is difficult to determine the power loss during propagation. This method requires neither a lot of computer resources nor the subsequent acquisition of the radiation information required for the calculation of the scatter by means of the bouncing radiation method (SBR).
Disclosure of Invention
In order to overcome the disadvantages of the prior art, the present invention aims to provide a method for calculating the electromagnetic scattering of a target in an evaporation waveguide environment, which can preprocess an incident waveform in the evaporation waveguide environment according to the electromagnetic wave propagation characteristics, considering the influence of the evaporation waveguide environment, and calculate the spatial scattering power of the target in the evaporation waveguide environment.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a method of calculating electromagnetic scattering of a target in an evaporative waveguide environment, comprising the steps of:
s1, establishing a three-dimensional electromagnetic wave propagation model in an evaporation waveguide by adopting a ray tracing method according to an atmospheric refractive index parameterized model;
s2, pre-modulating an incident wave of the SBR method according to a three-dimensional electromagnetic wave propagation model in the evaporation waveguide, establishing a virtual emitting surface of the incident wave in the evaporation waveguide environment, and correcting the general forms of plane waves and conical waves of the incident wave in the SBR method into special forms considering the influence of the evaporation waveguide on the electromagnetic wave propagation;
s3, according to a special incident wave form in the evaporation waveguide environment, calculating induction current generated by the incident wave on the surface of the target, and calculating to obtain a total scattered field.
The method for establishing the three-dimensional electromagnetic wave propagation model in the evaporation waveguide in the step S1 comprises the following steps:
the atmosphere correction refractive index M and the atmosphere correction refractive index M using curvature correction, their relationship with the refractive index N and the refractive index N are expressed as:
in the above, r e =6370 Km is the average earth radius, z is the altitude from the sea surface;
the evaporation waveguide is described using the following logarithmic refractive index model:
wherein M (0) is the bottom layer corrected refractive index, 330, c 0 And z 0 Taking 0.125 and 1.5X10 as constants respectively -4 When the evaporation waveguide height d=0, the refractive index profile in the case of standard atmosphere corresponds;
a Ray-Tracing method (RT) is adopted to establish a three-dimensional electromagnetic wave propagation model in the evaporation waveguide; uniformly layering the evaporation waveguide to obtain the radiation propagation direction and track in each layer, wherein the radiation propagation direction and track are as follows:
wherein z is the height from the sea surface, X is the distance between the transmitting antenna and the receiving antenna along the X-axis direction, θ is the elevation angle of radiation propagation, Y is the distance between the transmitting antenna and the receiving antenna along the Y-axis direction,g is the corrected refractive index gradient for azimuth; in the above, subscripts 1 and 2 represent the parameters of the previous and present layers, x, respectively, of the refractive index layer 1 Initial value of 0, θ 1 Taking the initial value of-0.3 degrees and z 1 The initial value of (1) is the antenna height; inside the refractive index stratification, the trajectory of the rays can be three: monotonically up, monotonically down, or reaching the highest (lowest) point within a layer and changing propagation direction; for the third case, the division into two segments at the highest (lowest) point is calculated separately, thus becoming monotonically upward and monotonically downwardIs a combination of two cases.
The method for pre-modulating the incident field in the step S2 comprises the following steps:
according to the established three-dimensional electromagnetic wave propagation model, establishing a virtual emission surface at a given distance to acquire the direction and field intensity of each ray at the position after the rays are propagated through the evaporation waveguide environment, wherein the rays emitted from the virtual emission surface are regarded as the incident field of a bouncing ray method (Shooting and Bouncing Rays, SBR) in the evaporation waveguide environment, and the distance between the virtual emission surface and the center of a target is 1.5-2 times of the size of the target;
after the distance between the antenna and the virtual emitting surface is given, the coordinates of the incident point and the emergent point of the ray in each refractive index layering near the virtual emitting surface are obtained through a three-dimensional electromagnetic wave propagation model; obtaining intersection points of rays and the virtual emitting surface through a geometric relationship, namely, source point coordinates of each ray in an incident field in the SBR method;
ε′ r ＝ε r +i60σλ
wherein ii-1 and ii respectively represent subscript numbers of ray track points adjacent to the left side and the right side of the virtual emitting surface, x, Y and z are coordinates of the ray track points, yy and zz are Y axes of intersection points of rays and the virtual emitting surfaceCoordinates and Z-axis coordinates, tt is a geometric scale parameter, thus (x [ ii ]]-x[ii-1],y[ii]-y[ii-1],z[ii]-z[ii-1]) Is the direction of propagation of the ray, (distance, yy [ ii ]],zz[ii]) Is the intersection point of the ray and the virtual emitting surface,is an angle->The gain coefficient of the antenna pattern corresponding to the rays is 1 when the antenna is omni-directional; Γ is the Fresnel reflection coefficient, is a function of the incidence complementary angle, and subscripts H and V are the Fresnel reflection coefficients for horizontal polarization and vertical polarization, respectively, where θ i For glancing incidence angle ε' r For complex relative permittivity of sea surface epsilon r Is relative dielectric constant, sigma is conductivity, lambda is wavelength, E is field intensity after L times of reflection of ray passes through path S, E 0 For the initial field strength, k is the wave number, from which the field strength on the virtual emission surface can be obtained.
The step S3 is a method for calculating the scattering field of the incident electromagnetic wave on the target surface in the evaporation waveguide environment, which comprises the following steps:
the reflection direction of the Nth reflection of the incident electromagnetic wave is as follows:
wherein the method comprises the steps ofFor the direction of the incident wave before the nth reflection, +.>For a unit normal vector at the intersection position of the ray and the target, the ray generates induced electromagnetic current on the surface of the target, and the obtained scattered field is as follows:
where j is an imaginary unit, k is the wave number of the electromagnetic wave, k=2pi/λ, λ is the wavelength of the electromagnetic wave, r is the distance from the intersection point of the ray and the target to the receiving radar,for the scattering direction unit vector, η=120pi is the wave impedance in vacuum, and J and M are the induced electromagnetic currents generated by rays on the target surface respectively>For the incident electric field before the nth reflection, +.>Is the incident magnetic field before the Nth reflection, deltas is the area where the tube intersects the target surface;
the total scattered field of the target is
The target space scattering power is:
compared with the prior art, the invention has the beneficial effects that:
according to the atmospheric refractive index parameterized model, a ray tracing method is adopted to analyze electromagnetic wave propagation characteristics in an atmospheric waveguide environment, a three-dimensional electromagnetic wave propagation model in an evaporation waveguide is established, then an incident field form in the evaporation waveguide environment is obtained by using the model, and the model is applied to an SBR method in the evaporation waveguide environment to calculate the space scattering power of a target. Experiments show that the method has higher simulation precision, and can calculate the space scattering power of the target in the evaporation waveguide environment by using less computer resources.
Drawings
Fig. 1 is a refractive index profile of an evaporation waveguide.
Fig. 2 is an angular schematic diagram of an electromagnetic wave propagation model in an evaporation waveguide.
Fig. 3 is a graph of ray propagation trajectories in an evaporated waveguide refractive index stratification.
FIG. 4 is a schematic diagram of a positional relationship and a geometric relationship; wherein the graph (a) is the position relation among the antenna, the virtual emitting surface and the target, and the graph (b) is the geometrical relation among the incidence point and the emergence point of the ray and the intersection point of the ray and the virtual emitting surface in the refractive index layering.
FIG. 5 is a schematic diagram of ray tracing.
FIG. 6 is a comparison of RT method with AREPS3.6 software simulation results; wherein the graph (a) is an electromagnetic wave propagation track in an evaporation waveguide environment, the graph (b) is an AREPS3.6 software simulation result graph, and the graph (c) is a three-dimensional electromagnetic wave propagation model in the evaporation waveguide environment.
FIG. 7 is a graph of field strength distribution over virtual emission surfaces at different distances, respectively; wherein figure (a) shows the field strength distribution on the virtual transmitting surface at a distance of 5km from the antenna to the virtual transmitting surface; the graph (b) shows the field intensity distribution on the virtual emitting surface when the distance between the antenna and the virtual emitting surface is 10km, the graph (c) shows the field intensity distribution on the virtual emitting surface when the distance between the antenna and the virtual emitting surface is 15km, the graph (d) shows the field intensity distribution on the virtual emitting surface when the distance between the antenna and the virtual emitting surface is 30km, the graph (e) shows the field intensity distribution on the virtual emitting surface when the distance between the antenna and the virtual emitting surface is 40km, and the graph (f) shows the field intensity distribution on the virtual emitting surface when the distance between the antenna and the virtual emitting surface is 50 km.
FIG. 8 is a comparison of SBR process with MoM in Feko; wherein, the graph (a) is a simple boat model, and the graph (b) is a graph of the comparison result of the SBR method and the double-station RCS of the MoM method in Feko.
Fig. 9 is a coordinate relationship of an antenna and a target.
FIG. 10 is a distribution of scattered power over an xoy plane at a radius of 50m for different bow rotation angles; wherein figure (a) is the bow rotation angleThe scattering power distribution on the xoy plane at a radius of 50m, graph (b) is bow rotation angle +.>Scattering power distribution over the xoy plane at a radius of 50 m.
FIG. 11 is a schematic diagram showing the positions of two different spaces and targets.
FIG. 12 is a graph of spatial fringe field distributions at different bow rotation angles; wherein figure (a) is the bow rotation angleThe scattering power distribution of the space A and the space B; the diagram (b) is the bow rotation angle +.>The scattering power distribution of the space A and the space B; FIG. (c) shows the bow rotation angle +.>The scattering power distribution of the space A and the space B; FIG. d shows the bow rotation angleAnd scattering power distribution of the space A and the space B.
Detailed Description
The present invention will be described in detail with reference to the drawings and examples, but it should be understood that the scope of the present invention is not limited to the examples.
A method of calculating electromagnetic scattering of a target in an evaporative waveguide environment, comprising the steps of:
s1, establishing a three-dimensional electromagnetic wave propagation model in an evaporation waveguide by adopting a ray tracing method according to an atmospheric refractive index parameterized model;
as shown in fig. 1, in order to consider the influence of the earth curvature, the refractive index profile of the evaporation waveguide uses an atmosphere correction refractive index M and an atmosphere correction refractive index M with curvature correction, and the relationship between them and the refractive index N is expressed as:
in the above, r e =6370 Km is the average earth radius, z is the altitude from the sea surface;
typically, the evaporation waveguide is described using a logarithmic refractive index model as follows:
wherein M (0) is the bottom layer corrected refractive index, 330, c 0 And z 0 Taking 0.125 and 1.5X10 as constants respectively -4 When the evaporation waveguide height d=0, the refractive index profile in the case of standard atmosphere corresponds;
according to a parameterized model of the evaporation waveguide, a Ray-Tracing method (RT) is adopted to establish a three-dimensional electromagnetic wave propagation model in the evaporation waveguide; since the modified index of refraction varies with height, the direction of propagation of the rays at different heights is different; uniformly layering the evaporation waveguide to obtain the radiation propagation direction and track in each layer, wherein the radiation propagation direction and track are as follows:
wherein z is the height from the sea surface, X is the distance between the transmitting antenna and the receiving antenna along the X-axis direction, and θ is the radiationThe elevation angle of line propagation, Y is the distance between the transmitting antenna and the receiving antenna along the Y-axis direction,azimuth, as shown in fig. 2; g is the modified refractive index gradient; in the above, subscripts 1 and 2 represent the parameters of the previous and present layers, x, respectively, of the refractive index layer 1 Initial value of 0, θ 1 Taking the initial value of-0.3 degrees and z 1 The initial value of (1) is the antenna height; inside the refractive index stratification, the trajectory of the rays can be three: monotonically up, monotonically down, or reaching the highest (lowest) point within a layer and changing propagation direction; in the third case, the method is split into two sections at the highest (lowest) point for calculation, so that the method becomes a monotone upward and monotone downward case, and the method for calculating the segmentation RT under the atmosphere layering condition simplifies the operation and is convenient for calculating the propagation path length. As shown in fig. 3. Thus, a three-dimensional electromagnetic wave propagation model in the evaporation waveguide can be obtained;
s2, pre-modulating an incident wave of the SBR method according to a three-dimensional electromagnetic wave propagation model in the evaporation waveguide, establishing a virtual emitting surface of the incident wave in the evaporation waveguide environment, and correcting the general forms of plane waves and conical waves of the incident wave in the SBR method into special forms considering the influence of the evaporation waveguide on the electromagnetic wave propagation;
according to the established three-dimensional electromagnetic wave propagation model, establishing a virtual emitting surface at a given distance to acquire the direction and field intensity of each ray at the place after the rays are propagated through the evaporation waveguide environment, wherein the rays emitted from the virtual emitting surface are regarded as incident fields of a bouncing ray method (Shooting and Bouncing Rays, SBR) in the evaporation waveguide environment, and according to experience, the distance between the virtual emitting surface and the center of a target is 1.5-2 times of the size of the target;
after the distance between the antenna and the virtual emitting surface is given, the coordinates of the incident point and the emergent point of the ray in each refractive index layering near the virtual emitting surface can be obtained through a three-dimensional electromagnetic wave propagation model; the intersection point of the rays and the virtual emitting surface can be obtained through the geometric relationship, namely the source point coordinate of each ray in the incident field in the SBR method.
ε′ r ＝ε r +i60σλ
Wherein ii-1 and ii respectively represent subscript numbers of ray trace points adjacent to left and right sides of the virtual emission surface, x, Y, Z are ray trace point coordinates, yy, zz are Y-axis coordinates and Z-axis coordinates of an intersection point of the ray and the virtual emission surface, tt is a geometric proportion parameter, and thus (x [ ii ]]-x[ii-1],y[ii]-y[ii-1],z[ii]-z[ii-1]) Is the direction of propagation of the ray, (distance, yy [ ii ]],zz[ii]) Is the intersection point of the ray and the virtual emitting surface,is an angle->The gain coefficient of the antenna pattern corresponding to the rays is 1 when the antenna is omni-directional; Γ is the Fresnel reflection coefficient, is a function of the incidence complementary angle, and subscripts H and V are the Fresnel reflection coefficients for horizontal polarization and vertical polarization, respectively, where θ i For glancing incidence angle ε' r For complex relative permittivity of sea surface epsilon r Is relative dielectric constant, sigma is conductivity, lambda is wavelength, E is field intensity after L times of reflection of ray passes through path S, E 0 K is the wave number, which is the initial field intensity, and the field intensity on the virtual emitting surface can be obtained by the formula; fig. 4 (a) is a schematic diagram of the positional relationship among the antenna, the virtual emitting surface and the target, and (b) is a schematic diagram of the geometrical relationship between the incident point and the exit point of the ray and the intersection point of the ray and the virtual emitting surface in the refractive index hierarchy. The method comprises the steps of carrying out a first treatment on the surface of the
S3, according to a special incident wave form in the evaporation waveguide environment, calculating induction current generated by the incident wave on the surface of the target, and calculating to obtain a total scattered field;
the ray exit coordinates, direction and field intensity are all obtained in step S2, as shown in fig. 5, the reflection direction of the nth reflection is:
wherein the method comprises the steps ofFor the direction of the incident electromagnetic wave before the nth reflection, < >>Is the unit normal vector at the intersection of the ray with the target. The ray scattering field is:
where j is an imaginary unit, k is the wave number of the electromagnetic wave, k=2pi/λ, λ is the wavelength of the electromagnetic wave, r is the distance from the intersection point of the ray and the target to the receiving radar,for the scattering direction unit vector, η=120pi is the wave impedance in vacuum, and J and M are the induced electromagnetic currents generated by rays on the target surface respectively>For the incident electric field before the nth reflection, +.>Is the incident magnetic field before the Nth reflection, deltas is the area where the tube intersects the target surface;
the total scattered field of the target is
The target space scattering power is:
simulation results:
first, an electromagnetic wave propagation model in an evaporation waveguide environment obtained by a ray tracing algorithm was verified with ares 3.6 software. In this simulation, the evaporation waveguide height was d=20m, the antenna height was 5m, the ray elevation angle θ was-0.5 ° to 0.5 °, and the interval was 0.01 °. Azimuth angleIs-20 DEG to 20 DEG, and the interval is 5 deg. Fig. 6 (a) is a graph of electromagnetic wave propagation tracks in an evaporation waveguide environment, fig. b is a graph of AREPS3.6 software simulation results, and fig. c is a three-dimensional electromagnetic wave propagation model in the evaporation waveguide environment. FIG. 6 shows that the RT method of the invention has better consistency with the simulation result of AREPS3.6 software, and proves the correctness of the RT method of the invention.
And then, obtaining the field intensity distribution on the virtual emitting surface by a three-dimensional electromagnetic wave propagation model in the evaporation waveguide environment. In this simulation, the antenna is an omni-directional antenna, the evaporation waveguide height is d=20m, the initial field strength E 0 =50, frequency f=10 GHz. Fig. 7 shows the field intensity distribution on the virtual emission surface at different distances, respectively, from which it can be seen that the field intensity varies with the distance, the farther the distance, the weaker the field intensity. At distances of 5km and 10km, there is still a field distribution above 20m, compared to other distances, because of some in the evaporating waveguide environmentRays have not been trapped at short distances. And it can also be seen that at the same distance the field strength is in a layered distribution, since the atmospheric correction refractive index varies with height. And the field intensity value in the middle of the emitting surface is slightly larger than that of the edge in the Y-axis direction due to propagation loss.
Next, the SBR method of the present invention was verified by MoM in Feko, in which the incident wave was in the form of a plane wave, the frequency of the incident wave was f=5 GHz, and the incident angle was θ i ＝45°,The scattering angle is theta s ＝45°，/>From 0 to 360 degrees. FIG. 8 is a diagram (a) showing a simple boat model, and FIG. (b) showing the comparison result of the SBR method of the present invention with the two-station RCS of the MoM algorithm in Feko. The good agreement of the two curves indicates the accuracy of the SBR method in this case.
Then, field information on the virtual emitting surface is used as an incident field to be applied to an SBR method, and scattering power distribution of an actual ship target on an xoy surface at a radius of 50m under different rotation angles is calculated. In this simulation, the antenna was an omni-directional antenna, a distance from the origin of coordinates of 50km, an evaporation waveguide height of d=20m, an initial field strength E 0 =50, frequency f=10 GHz. The ship dimensions are as follows: 31.5m long, 5m wide and 4.3m high, with its center coordinates at the origin of coordinates. Fig. 9 is a coordinate relationship of an antenna and a target. FIG. 10 (a) shows the bow rotation angleThe scattering power distribution on the xoy plane at a radius of 50m, graph (b) is bow rotation angle +.>Scattering power distribution over the xoy plane at a radius of 50 m. It can be seen from fig. 10 that the scattering power distribution of the ship varies with the bow rotation angle at the scattering angle +.>At the time of bow rotation angle +.>Is smaller than the bow rotation angle +.>Because the boat side is more reflective of radiation than the boat head.
Next, the scattering power distribution of the target in different spaces is calculated, fig. 11 is a schematic diagram of positions of two different spaces and the target, the x coordinate of the space a is-100 m to-10 m, the x coordinate of the space B is 50m to 100m, the y coordinate of the two spaces is-100 m to 100m, and the z coordinate is 0m to 30m. Fig. 12 shows the spatial fringe field distribution at different bow angles of rotation. From the image it is possible that the scattered power exhibits a lamellar distribution, since its incident field strength is also lamellar. When (when)When the ship is symmetric, the space scattering power can be seen to be symmetrically distributed due to the symmetry of the target ship, and the scattering power is changed along with the change of the angle of the ship bow.
The invention discloses a method for calculating target electromagnetic scattering in an evaporation waveguide environment, which combines a three-dimensional electromagnetic wave propagation model in the evaporation waveguide environment with a high-frequency method to provide a new target electromagnetic scattering model in the evaporation waveguide environment. Based on a ray tracing algorithm, establishing a three-dimensional electromagnetic wave propagation model in an evaporation waveguide environment; then, based on a three-dimensional electromagnetic wave propagation model, a virtual emitting surface is established to acquire information such as the direction, the field intensity and the like of rays propagated through the evaporation waveguide, and the information is used as an incident field of an SBR method for calculating scattering subsequently. Finally, the special incident field form is applied to the SBR method to obtain the electromagnetic scattering property of the target in the evaporation waveguide environment. The accuracy of the method of the invention is demonstrated by comparison with the numerical method.
The above disclosure is merely an embodiment of the present invention, but the embodiment of the present invention is not limited thereto, and any changes that can be thought by those skilled in the art should fall within the protection scope of the present invention.
Claims (4)
1. A method of calculating electromagnetic scattering of a target in an evaporative waveguide environment, comprising the steps of:
s1, establishing a three-dimensional electromagnetic wave propagation model in an evaporation waveguide by adopting a ray tracing method according to an atmospheric refractive index parameterized model;
s2, pre-modulating an incident wave of the SBR method according to a three-dimensional electromagnetic wave propagation model in the evaporation waveguide, establishing a virtual emitting surface of the incident wave in the evaporation waveguide environment, and correcting a general form of a plane wave or a conical wave of the incident wave in the SBR method into a special form considering the influence of the evaporation waveguide on the electromagnetic wave propagation;
s3, according to a special incident wave form in the evaporation waveguide environment, calculating induction current generated by the incident wave on the surface of the target, and calculating to obtain a total scattered field.
2. The method according to claim 1, wherein the method for creating the three-dimensional electromagnetic wave propagation model in the evaporation waveguide in step S1 is:
the atmosphere correction refractive index M and the atmosphere correction refractive index M using curvature correction, their relationship with the refractive index N and the refractive index N are expressed as:
in the above, r e =6370 Km is the average earth radius, z is the altitude from the sea surface;
the evaporation waveguide is described using the following logarithmic refractive index model:
wherein M (0) is the bottom layer corrected refractive index, 330, c 0 And z 0 Taking 0.125 and 1.5X10 as constants respectively -4 When the evaporation waveguide height d=0, the refractive index profile in the case of standard atmosphere corresponds;
a Ray-Tracing method (RT) is adopted to establish a three-dimensional electromagnetic wave propagation model in the evaporation waveguide; uniformly layering the evaporation waveguide to obtain the radiation propagation direction and track in each layer, wherein the radiation propagation direction and track are as follows:
wherein X is the distance between the transmitting antenna and the receiving antenna along the X-axis direction, θ is the elevation angle of radiation propagation, Y is the distance between the transmitting antenna and the receiving antenna along the Y-axis direction,g is the corrected refractive index gradient for azimuth; in the above, subscripts 1 and 2 represent the parameters of the previous and present layers, x, respectively, of the refractive index layer 1 Initial value of 0, θ 1 Taking the initial value of-0.3 degrees and z 1 The initial value of (1) is the antenna height; inside the refractive index stratification, the trajectory of the rays can be three: monotonously up, monotonously down, or up to the highest and lowest point within the layer and change the propagation direction; for the third case, split at the highest and lowest pointsThe two sections are calculated separately, thus changing into two situations of monotonous upward and monotonous downward.
3. The method according to claim 1, wherein the method of pre-modulating the incident field in step S2 is:
according to the established three-dimensional electromagnetic wave propagation model, establishing a virtual emission surface at a given distance to acquire the direction and field intensity of each ray at the position after the rays are propagated through the evaporation waveguide environment, wherein the rays emitted from the virtual emission surface are regarded as the incident field of a bouncing ray method (Shooting and Bouncing Rays, SBR) in the evaporation waveguide environment, and the distance between the virtual emission surface and the center of a target is 1.5-2 times of the size of the target;
after the distance between the antenna and the virtual emitting surface is given, the coordinates of the incident point and the emergent point of the ray in each refractive index layering near the virtual emitting surface are obtained through a three-dimensional electromagnetic wave propagation model; obtaining intersection points of rays and the virtual emitting surface through a geometric relationship, namely, source point coordinates of each ray in an incident field in the SBR method;
ε′ r ＝ε r +i60σλ
wherein ii-1 and ii respectively represent subscript numbers of ray trace points adjacent to the left and right sides of the virtual emitting surface, x, Y and Z are coordinates of the ray trace points, yy and zz are coordinates of a Y-axis and a Z-axis of an intersection point of the ray and the virtual emitting surface, and tt is a geometric proportion parameter, so x [ ii ]]-x[ii-1]、y[ii]-y[ii-1]、z[ii]-z[ii-1]Is the direction of ray propagation, distance, yy [ ii ]]、zz[ii]Is the intersection point of the ray and the virtual emitting surface,is an angle->The gain coefficient of the antenna pattern corresponding to the rays is 1 when the antenna is omni-directional; Γ is the Fresnel reflection coefficient, is a function of the incidence complementary angle, and subscripts H and V are the Fresnel reflection coefficients for horizontal polarization and vertical polarization, respectively, where θ i For glancing incidence angle ε' r For complex relative permittivity of sea surface epsilon r Is relative dielectric constant, sigma is conductivity, lambda is wavelength, E is field intensity after L times of reflection of ray passes through path S, E 0 For the initial field strength, k is the wave number, from which the field strength on the virtual emission surface can be obtained.
4. The method according to claim 1, wherein the step S3 of calculating the scattering field of the incident electromagnetic wave on the target surface in the evaporation waveguide environment is:
the reflection direction of the Nth reflection of the incident electromagnetic wave is as follows:
wherein the method comprises the steps ofFor the direction of the incident wave before the nth reflection, +.>For a unit normal vector at the intersection position of the incident wave and the target, the incident wave generates an induced electromagnetic current on the surface of the target, and a scattered field is obtained as follows:
where j is an imaginary unit, k is the wave number of the electromagnetic wave, k=2pi/λ, λ is the wavelength of the electromagnetic wave, and r is
The distance from the point of intersection of the incident wave with the target to the receiving radar,for the unit vector of the scattering direction, η=120pi is the wave impedance in vacuum, and J and M are the induced current and the induced magnetic current generated by the incident wave on the target surface respectively, +.>For the incident electric field before the nth reflection, +.>For the incident magnetic field before the nth reflection, Δs is the area where each row of incident waves intersects the target surface;
the total scattered field of the target is
The target space scattering power is:
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