Radar detection imaging method

Abstract

The invention discloses a radar detection imaging method, which comprises the following steps: firstly, calculating the electromagnetic field intensity of a radar signal of a radar to a target point after being transmitted and propagated through an evaporation waveguide medium by using an electromagnetic propagation model; utilizing an electromagnetic scattering model to calculate a radar signal backward far region scattering field of the target and the random sea surface composite scene, namely electromagnetic scattering characteristics; and finally, electromagnetic imaging is carried out on the radar signal backward far region scattered field by using an electromagnetic imaging algorithm. The evaporation waveguide serves as a transmission medium of radar electromagnetic waves, the detection capacity of the radar can be remarkably improved, beyond-visual-range detection can be achieved, meanwhile, compared with free space transmission, the enhancement effect of the evaporation waveguide layer on target electromagnetic signal echoes can improve the detection signal-to-noise ratio of signals, and the detection precision is improved. The method can be applied to a high frequency band to provide a high radar resolution, radar electromagnetic waves are propagated in the evaporation waveguide at a low glancing angle, sea clutter interference in the target echo detection process is reduced, and the precision and reliability of target detection are improved.

Description

Technical Field

[0001] The invention belongs to the field of radar imaging, and in particular relates to a radar detection imaging method. Background Art

[0002] At present, the detection of small targets at sea mainly adopts air-based detection and shore-based blind spot detection. Air-based radar systems, especially airborne radars equipped on drones, have been widely used in marine environment detection due to their unique advantages of flexibility and efficiency. However, when air-based radar detects the sea surface, due to the large incident angle and the corresponding Bragg scattering effect, the waves within the radar wave irradiation area will cause a large amount of sea wave clutter. When the radar echo intensity caused by the radar cross-section (RCS) of the small target on the sea surface is similar to the echo intensity caused by the sea surface waves and the distribution is uneven, this similarity and category imbalance often cause a large number of false detections, which will pose a huge challenge to the detection and identification of sea surface targets. The commonly used shore-based blind spot radar solutions currently use high-frequency ground wave radar and microwave ocean radar. Among them, high-frequency ground wave radar uses the characteristics of low-frequency electromagnetic waves diffracting along the sea surface to achieve over-the-horizon detection, and has been proven to provide effective monitoring capabilities for low-flying targets that are difficult to detect. However, due to the frequency limitation of high-frequency ground wave radar, its radar has low spatial resolution and poor detection effect on small targets. At the same time, microwave ocean radar is also widely used in ocean radar monitoring due to its high spatial resolution and strong anti-interference ability to rain and fog. When the microwave radar antenna is installed at a low height, the accuracy and reliability of target detection can be improved by taking advantage of its low grazing emission angle and the characteristic that it is not easily interfered by sea clutter. However, this is to obtain more accurate radar echo information of sea surface targets at the expense of detection distance. In summary, in the current field of marine small target detection technology, it is difficult to simultaneously achieve the detection capability of beyond-horizon targets and the high-resolution recognition capability of small targets. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a radar detection imaging method that can cope with small target detection at sea and improve the accuracy and reliability of target detection.

[0004] The present invention is implemented by the following technical solutions:

[0005] A radar detection imaging method comprises the following steps:

[0006] Step (1): using an electromagnetic propagation model, calculate the electromagnetic field strength of the radar signal after it is transmitted through the evaporative waveguide medium to the target point;

[0007] Step (2): using the electromagnetic scattering model, calculate the far-field backscattering field of the radar signal of the target and random sea surface composite scene, i.e., the electromagnetic scattering characteristics;

[0008] Step (3): Use an electromagnetic imaging algorithm to perform electromagnetic imaging on the far-field backscattering field of the radar signal.

[0009] Furthermore, the electromagnetic propagation model is solved using a parabolic equation model, and the parabolic equation model uses a step-by-step Fourier algorithm to solve the parabolic equation of electromagnetic wave propagation.

[0010] Furthermore, a bouncing ray method is used to analyze the electromagnetic scattering characteristics of the electromagnetic scattering model, and the bouncing ray method includes a geometric optics method and a physical optics method.

[0011] Furthermore, the electromagnetic imaging algorithm is an inverse synthetic aperture radar imaging technology, which includes performing a two-dimensional inverse Fourier transform on the scattering field of the target, and the radar related parameters include: center frequency f, radar peak power P t , antenna gain G, radar receiving threshold T h , beam width BW and antenna height h.

[0012] The beneficial effects of the present invention are as follows:

[0013] The present invention uses the evaporation waveguide as the transmission medium of the radar electromagnetic wave, which can significantly expand the detection capability of the radar and realize over-the-horizon detection. Compared with free space transmission, the enhancement effect of the evaporation waveguide layer on the target electromagnetic signal echo can improve the detection signal-to-noise ratio of the signal and can be applied to higher frequency bands to provide higher radar resolution. The radar electromagnetic wave propagates at a low grazing angle in the evaporation waveguide, which reduces the sea clutter interference in the target echo detection process and improves the accuracy and reliability of target detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of aviation airborne radar application scenarios, where (a) is the low-altitude mode: the airborne radar incident angle is 60°, and (b) is the high-altitude mode: the airborne radar incident angle is 20°;

[0015] Figure 2 Schematic diagram of the application scenario of shipborne radar deployed at low antenna height at sea, where (a) is the short-range mode: the target distance is 18km, and (b) is the long-range mode: the target distance is 30km;

[0016] Figure 3 is the RCS of the icosahedral corner reflector;

[0017] Figure 4 Corrected refractive index profile for evaporative waveguide atmosphere;

[0018] Figure 5Transmission loss of 10 GHz electromagnetic waves in different airborne and shipborne scenarios in the offshore evaporation duct environment, where (a) airborne, h = 9 km, (b) airborne, h = 17 km, (c) shipborne, h = 5 m;

[0019] Figure 6 These are the ISAR simulation images of sea surface targets under the conditions of incident angles of 60° and 20° for airborne radar, where (a) airborne radar, incident angle of 60°, with target, (b) airborne radar, incident angle of 20°, with target;

[0020] Figure 7 These are ISAR simulation images of targets at different distances in the evaporation duct environment, where (a) evaporation duct environment, 18 km, with a target, (b) evaporation duct environment, 30 km, with a target;

[0021] Figure 8 These are ISAR images of sea clutter under different radar configuration conditions at 5 km, including (a) shipborne radar, evaporative duct environment, 5 km, no target (b) airborne radar, incident angle 60°, 5 km, no target. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0023] Step (1): using an electromagnetic propagation model, calculate the electromagnetic field strength of the radar signal after it is transmitted through the evaporative waveguide medium to the target point;

[0024] First, obtain the radar equation in the atmospheric propagation environment:

[0025]

[0026] Among them, P r and P t are radar receiving power and transmitting power respectively, G is antenna gain, c is electromagnetic wave propagation speed, F is propagation factor, σ is radar cross-sectional area of ​​target, f is radar signal frequency, K is Boltzmann constant, T0 is standard room temperature, B is radar receiver receiving bandwidth, (S / N) min is the minimum detectable signal-to-noise ratio, and R represents the transmission distance of the electromagnetic wave.

[0027] The propagation factor F of the radar signal in the evaporative waveguide environment is derived through the electromagnetic propagation model, and the electromagnetic propagation model adopts the parabolic equation model (PE). The parabolic equation model is an approximate algorithm for calculating the propagation of electromagnetic waves. It is the result of a parabolic approximation of the Helmholtz wave equation. It can simulate the forward propagation of electromagnetic waves, calculate the electromagnetic wave transmission loss at long distances, and effectively solve the electromagnetic wave propagation problem in an environment with uneven refractive index under the premise of only obtaining partial propagation boundary conditions. For different propagation areas and frequency domains, the parabolic equation does not need to use different approximations and algorithms, nor does it need to express the solution in the form of multiple simple normal waves. Therefore, the parabolic equation is often used in theoretical research, link design evaluation, offshore experimental analysis, etc.

[0028] The parabolic equation model uses the split-step Fourier method (SSFT) to solve the parabolic equation of electromagnetic wave propagation. First, the attenuation function along the x-axis is (where u(x,z) represents the field intensity in space, ψ(x,z) is the two-dimensional space wave, is a plane wave solution), the approximate formula of the Helmholtz wave equation is introduced to obtain the expression of electromagnetic wave propagation in space:

[0029]

[0030] Among them, x represents the horizontal propagation distance, z represents the vertical ground height, and k0 represents the free space wave number.

[0031] Then, the step-by-step Fourier algorithm is used to traverse and obtain the field strength at each point in space:

[0032]

[0033] Where M(x,z) represents the modified refractive index of the atmosphere that varies with altitude and horizontal distance, and p = k0sinθ p , where θ p represents the angle with the horizontal ground, and are Fourier transform and inverse Fourier transform respectively, Δx is the horizontal step size, u(x0,z) is the initial field distribution, and the propagation factor is obtained:

[0034]

[0035] The PE model is used to calculate the electromagnetic wave propagation intensity at the small target on the sea surface in the propagation path, and it is used as the incident field of the subsequent electromagnetic scattering model.

[0036] Step (2): using the electromagnetic scattering model, calculate the far-field backscattering field, i.e., the electromagnetic scattering characteristics, of the target in a composite scene of a random sea surface at different frequencies and different rotation angles;

[0037] The bouncing ray method is used to quickly analyze the electromagnetic scattering characteristics of the electromagnetic scattering model. The bouncing ray method includes a geometric optics method and a physical optics method.

[0038] First, an electromagnetic scattering model is established to accurately simulate the interaction between electromagnetic waves and targets. This requires not only accurate modeling of the geometric shape of the target, but also in-depth research on the propagation and scattering effects of electromagnetic waves. The bouncing ray method provides an effective means for analyzing the echo of the target by simulating the reflection and scattering of electromagnetic rays on the target surface. The bouncing ray method (SBR algorithm) uses the principle of geometrical optics (GO) to determine the reflection path and illuminated area of ​​the multiple reflection process of electromagnetic waves (rays), and tracks the ray intensity of the reflected rays. Then, the far-field scattering field integral formula of the physical optics method (PO) is used to solve the far-field scattering field of the bright area. Finally, the far-field scattering fields of all bright areas are vector superimposed to obtain the total scattering field of the target.

[0039] The geometric optics method treats electromagnetic waves as electromagnetic rays and can be used to approximately deal with the direct, reflected and refraction problems in the process of electromagnetic wave propagation. According to the GO principle, light follows the Fresnel law during the reflection process. The mathematical expression of the Fresnel law is as follows:

[0040]

[0041] in, represents the incident direction vector, represents the reflection direction vector, Represents the normal vector of the surface element at the incident point.

[0042] The ray propagation path is tracked by the ray tube. The geometric optics method is used to accurately track the ray path, and the center line of the ray tube is used to track the ray intensity. When the ray tube and the target surface are reflected multiple times, the center line of the ray tube and the target surface will form a series of intersections (r1, r2, ..., r n ), the field strength relationship between adjacent points is:

[0043]

[0044] Among them, (DF) i and Represents point r i The divergence factor and reflection coefficient matrix at . The divergence factor describes the change of the electric field intensity in the ray tube with the cross-sectional area of ​​the ray tube.

[0045] Reflection coefficient matrix and reflected electric field E r and the incident electric field E inThe relationship is:

[0046]

[0047] In formula (7), Γ V and Γ H represents the vertical and parallel polarization reflection coefficients, and the corresponding calculation expressions are:

[0048]

[0049] in, and is the intrinsic impedance of the medium, θ in and θ2 represent the angle of incidence and the angle of refraction, respectively.

[0050] In the process of ray tracing and field intensity tracing using geometric optics, when the reflected ray no longer intersects with other facets of the target, the far-field integration method of physical optics (PO) is used to calculate the scattering field of the target in the far zone based on the field intensity data of the last reflection. The physical optics method (PO) is an approximate method based on the three assumptions of the locality principle, the tangent plane approximation, and the dark zone scattering field being zero. The corresponding far-zone scattering field integral formula is:

[0051]

[0052] In formula (11), E s represents the far-field scattered field; E i and H i represents the incident electric field and the incident magnetic field; Z0 is the wave impedance of free space; is the unit normal vector of the surface element; is the unit vector of the scattering direction; r′ represents the coordinate of the surface element; ds is the area of ​​the integrated surface element.

[0053] The far-zone scattered field of the bright area is solved, and finally the far-zone scattered fields of all bright areas are vector superimposed.

[0054] Finally, the far-field scattering field of the target in the ocean environment is calculated by the SBR method as follows:

[0055]

[0056] Where i represents the number of ray bounces.

[0057] Step (3): Use an electromagnetic imaging algorithm to perform electromagnetic imaging on the far-field backscattering field of the radar signal.

[0058] In the field of electromagnetic imaging, Synthetic Aperture Radar (SAR) and Inverse Synthetic Aperture Radar (ISAR) technologies can generate high-resolution radar target images using scattered electromagnetic waves. However, for maritime detection, it is difficult to move shore-based early warning radars and synthesize radar images in engineering. At the same time, ISAR imaging technology is widely used for imaging of moving targets at sea and in the air because it can use the relative motion between the radar and the target to capture the subtle changes and dynamic characteristics of the target from the echo data. ISAR technology is used to image maritime detection targets, and the evaporation waveguide is combined to enhance the echo signal strength and reduce sea clutter interference. At the same time, the SBR algorithm is used to significantly improve the efficiency of electromagnetic simulation calculations in composite scenarios, effectively improving the ISAR imaging quality and radar detection performance.

[0059] The mathematical formula for the target scattering field under small broadband and small rotation angle can be expressed as:

[0060]

[0061] Where k is the spatial frequency variable, is the rotation angle variable, A0 is the backscattering field amplitude, k c is the wave number relative to the center frequency.

[0062] The ISAR image can be calculated by performing a two-dimensional inverse Fourier transform on the target's scattering field. The specific expression is:

[0063]

[0064] Therefore, the final ISAR image expression can be obtained:

[0065]

[0066] Among them, A0 represents the amplitude of electromagnetic reflection, and δ(x-x0,y-y0) represents the two-dimensional pulse function at the point (x0,y0).

[0067] The scattering field of the target can be approximated as the sum of the scattering of a finite number of scattering centers on the target. Therefore, the approximate expression of ISAR imaging of the target scattering field is:

[0068]

[0069] Where N is the total number of target scattering centers, A m represents the backscattering field amplitude of the mth scattering center; from formula (16), we can get the ISAR image composed of the sum of N scattering centers and their electromagnetic scattering coefficients.

[0070] In order to verify the feasibility of using the evaporation duct environment to detect and identify small targets at sea, a corresponding simulation study was carried out based on the theory and the above algorithm model. In the simulation scenario, a control experiment based on aviation airborne radar and low antenna height radar was designed to detect small targets on the sea surface.

[0071] The radar parameters used in the simulation are as follows: center frequency is 10 GHz, radar peak power is 1000 kW, antenna gain is 45 dB, radar receiving threshold is -140 dBm, beam width is 0.1°, and antenna height is 5 m.

[0072] Aviation airborne radar settings: distance R from the airborne radar t For a target at 18km, the airborne radar detects it. Assume that the detection is carried out at low altitude, that is, in low altitude mode, the configuration height H1 of the airborne radar is 9km, and the incident angle is 60°. For detection at high altitude, that is, in high altitude mode, the configuration height H2 of the airborne radar is 17km, and the incident angle of the radar is 20°. Figure 1 shown.

[0073] Shipborne radar settings with low-altitude antenna arrangement: The shipborne radar antenna arrangement height h is 5m, the radar beam incident angle with the sea surface is 89°, and the target distance R from the shipborne radar is set h The horizontal distances are 18km for short-distance mode and 30km for long-distance mode. Figure 2 shown.

[0074] Target and sea surface wave simulation settings: During the simulation, an icosahedral corner reflector with a side length of 1m is used as a small sea surface target model. The electromagnetic scattering characteristics of the target are calculated by applying the SBR algorithm, and its RCS changes with the incident angle as follows: Figure 3 As shown in the figure, the average RCS value is 0.5492 dBsm, which can approximately replace the target scattering characteristics of the small unmanned boat on the sea surface. In the simulation process, the wind speed at 19.5 m above the sea surface is set to 5 m / s and the wind direction angle is 0°. The Pierce-Moskowitz sea spectrum model is used to simulate the sea surface, as shown in Figure 3 shown.

[0075] Evaporation duct environment settings: The evaporation duct height is set to the commonly used 23m. Therefore, the propagation of shipborne radar signals at low antenna heights (5m antenna height) at sea is affected by the evaporation duct. The vertical refractive index profile of the evaporation duct atmosphere is crucial to the propagation of electromagnetic waves. The variation of the modified refractive index M with height can be defined by the following formula:

[0076]

[0077] Among them, zM represents the atmospheric height, n represents the refractive index, and a represents the radius of the earth. The atmospheric correction refractive index profile of the evaporation waveguide is as follows Figure 4 shown.

[0078] In the simulation, the PE model is first used to simulate the electromagnetic propagation loss of the radar signal to determine the input electromagnetic field of electromagnetic scattering in different scenarios and at different distances. Figure 5 (a) is the transmission loss when the transmitting antenna height is 9 km in the airborne radar simulation scenario 1. Figure 5 (b) is the transmission loss when the transmitting antenna height is 17 km in the airborne radar simulation scenario 2. Figure 5 (c) is the transmission loss of the shipborne radar simulation scenario 1 when the antenna height is 5 meters. As can be seen from the figure, Figure 5 In (c), the evaporative waveguide environment traps the radar electromagnetic waves in a waveguide layer of a certain thickness on the sea surface, and is mainly concentrated in the height range near the transmitting antenna, which greatly limits the diffusion of electromagnetic waves in vertical space. Compared with the propagation of electromagnetic waves in free space, the propagation of electromagnetic waves in the waveguide has a smaller path loss, which allows the evaporative waveguide environment to break through the limitation of the earth's curvature and achieve over-the-horizon propagation. At close range, the propagation distance is about 5km. Figure 5 The propagation characteristics of (a), (b), and (c) are basically consistent with those of free space. It can be observed that part of the electromagnetic wave penetrates the waveguide layer and escapes into the free space, while the other part of the electromagnetic wave is captured by the evaporated waveguide. Figure 5 (c) It can be seen that during long-distance propagation, due to the influence of the roughness of the sea surface waves, a small amount of electromagnetic waves leaks from the evaporation waveguide, but the energy density of the electromagnetic waves is very low.

[0079] The above simulation shows that the low-altitude antenna arrangement can effectively utilize the offshore evaporation duct environment to reduce the propagation loss of radar electromagnetic waves, thereby enhancing the radar signal echo intensity of the target. In addition, the low-altitude antenna arrangement makes the incident angle of the radar beam close to 90°, which can effectively reduce the intensity of the backscattered echo of sea clutter.

[0080] The following simulation studies the ISAR image characteristics under different airborne radar and shipborne radar settings in the process of detecting small targets on the sea surface. In the simulation process, the evaporation duct height is set to 23m, the radar related parameters are the parameters set above, and the incident angles are 20°, 60°, and 89° respectively. In the following figures, the X-axis and Y-axis represent the distance dimension and azimuth dimension of the ISAR image, respectively, and the color bar on the right represents the received radar signal strength, which ranges from [-140, -90] dB.

[0081] Figure 6 (a) and Figure 6(b) shows the scattering ISAR image of electromagnetic waves from sea surface targets under free space propagation conditions, corresponding to the scenes when the airborne radar is incident at 60° and 20°, and the sea surface target is about 18 km away from the radar. Figure 6 (b) It can be seen that when the radar incident angle is small, the sea clutter interference caused by the sea surface is significantly enhanced, making it difficult to distinguish the target signal from the sea clutter, thereby reducing the radar receiver's ability to identify the target. As the radar incident angle increases, the sea clutter interference gradually weakens, but the effect is still poor. Figure 6 (a) is shown, that is, Figure 6 (a) is an airborne radar with an incident angle of 60° and a target. Figure 6 (b) The condition of the airborne radar incident angle of 20° and the presence of a target.

[0082] Figure 7 (a) and Figure 7 (b) is the ISAR image of a small target on the sea surface in an evaporative duct environment using a shipborne low-altitude antenna setting, corresponding to a target distance of 18 km and 30 km from the radar, respectively. Figure 7 Combination of (a) and (b) Figure 5 (c) The simulation results clearly show that the shipborne low antenna height radar combined with the evaporation waveguide can effectively detect small targets on the sea surface. The evaporation waveguide can effectively reduce the propagation loss of radar signals when they propagate over long distances at sea, and effectively extend the detection range of the radar, completing line-of-sight detection and beyond-line-of-sight detection. In addition, the larger beam incidence angle can greatly reduce the impact of sea clutter on target detection, and clearly identify the target from the ISAR image. Figure 7 The difference between (a) and (b) is that the image amplitude intensity at 30 km is lower than that at 18 km. However, the corresponding target electromagnetic scattering characteristics are still retained.

[0083] Finally, the characteristics of sea surface clutter were studied using shipborne radar and airborne radar settings in a target-free setting at a close distance of 5km. Figure 8 (a) and Figure 8 (b) The ISAR images of sea clutter detected by shipborne radar and airborne radar respectively when there is no target on the sea surface 5 km away from the transmitting antenna. Figure 6 (a) and Figure 8 As shown in (b), at this time, in a free space environment, the echo intensity of a target at a longer distance is very close to the echo intensity of sea clutter at a closer distance, which poses a huge challenge to the setting of the radar receiving threshold and the decision threshold. In addition, since the number of samples of sea clutter echoes and targets is extremely unbalanced, this may lead to the so-called "class imbalance problem", which increases the complexity of target detection. In addition, compared with Figure 7 (a) and Figure 8 (a) It can be concluded that in the evaporative waveguide environment, the distinction between the echo intensity of the target at a longer distance and the echo intensity of the sea surface clutter at a closer distance is still high, and judgment and classification can be made relatively clearly, which helps to improve the recognition performance of small targets on the sea surface.

[0084] In summary, evaporation ducts, as a special atmospheric duct phenomenon that frequently appears above the sea surface, have a significant impact on electromagnetic wave transmission. The present invention utilizes the evaporation duct environment to enhance the detection performance of low antenna height microwave radar for small targets on the sea surface, and models and simulates the electromagnetic scattering characteristics and ISAR images of small targets on the sea surface in the evaporation duct environment. The transmission characteristics of electromagnetic waves in the evaporation duct are analyzed by the PE model, that is, the electromagnetic propagation model, and the SBR algorithm is used to calculate the backscattering field and electromagnetic scattering characteristics of small targets on the sea surface and their rough surfaces. Finally, the electromagnetic scattering characteristics of the target in space are displayed using ISAR imaging technology, that is, the electromagnetic imaging algorithm, which verifies the effectiveness of the evaporation duct in suppressing sea surface clutter interference, reducing electromagnetic signal transmission loss, and supporting over-the-horizon propagation. Therefore, by using the evaporation waveguide as the transmission medium of the radar electromagnetic wave and performing calculations through the steps of the present invention, the detection capability of the radar can be significantly expanded, and over-the-horizon detection can be achieved. At the same time, compared with free space transmission, the enhancement effect of the evaporation waveguide layer on the target electromagnetic signal echo can improve the detection signal-to-noise ratio of the signal, and can be applied to higher frequency bands to provide higher radar resolution. The radar electromagnetic wave propagates at a low grazing angle in the evaporation waveguide, which reduces the sea clutter interference in the target echo detection process and improves the accuracy and reliability of target detection.

Claims

1. A radar detection imaging method, characterized in that: The following steps are involved: Step (1): using an electromagnetic propagation model, calculate the electromagnetic field strength of the radar signal after it is transmitted through the evaporative waveguide medium to the target point; Step (2): using the electromagnetic scattering model, calculate the far-field backscattering field of the radar signal of the target and random sea surface composite scene, i.e., the electromagnetic scattering characteristics; Step (3): Use an electromagnetic imaging algorithm to perform electromagnetic imaging on the far-field backscattering field of the radar signal.

2. A radar detection imaging method according to claim 1, characterized in that: The electromagnetic propagation model is solved by using a parabolic equation model, and the parabolic equation model uses a step-by-step Fourier algorithm to solve the parabolic equation of electromagnetic wave propagation.

3. The radar detection imaging method according to claim 1, characterized in that: The bouncing ray method is used to analyze the electromagnetic scattering characteristics of the electromagnetic scattering model, and the bouncing ray method includes a geometric optics method and a physical optics method.

4. The radar detection imaging method according to claim 1, characterized in that: The electromagnetic imaging algorithm is an inverse synthetic aperture radar imaging technology, which includes performing a two-dimensional inverse Fourier transform on the scattering field of the target.

5. The radar detection imaging method according to claim 2, characterized in that: Step (1) is specifically implemented as follows: First, obtain the radar equation in the atmospheric propagation environment: Among them, P r and P t are radar receiving power and transmitting power respectively, G is antenna gain, c is electromagnetic wave propagation speed, F is propagation factor, σ is radar cross-sectional area of ​​target, f is radar signal frequency, K is Boltzmann constant, T0 is standard room temperature, B is radar receiver receiving bandwidth, (S / N) min is the minimum detectable signal-to-noise ratio, and R represents the transmission distance of the electromagnetic wave; The propagation factor F of the radar signal in the evaporative waveguide environment is derived through an electromagnetic propagation model, wherein the electromagnetic propagation model adopts a parabolic equation model PE; The parabolic equation model uses the split-step Fourier algorithm SSFT to solve the parabolic equation of electromagnetic wave propagation; first, the attenuation function along the x-axis is By introducing the approximate formula of the Helmholtz wave equation, we can get the expression of electromagnetic wave propagation in space; where u(x,z) represents the field intensity in space, ψ(x,z) is the two-dimensional space wave, is a plane wave solution: Where x represents the horizontal propagation distance, z represents the vertical ground height, and k0 represents the free space wave number; Then, the step-by-step Fourier algorithm is used to traverse and obtain the field strength at each point in space: Where M(x,z) represents the modified refractive index of the atmosphere that varies with altitude and horizontal distance, and p = k0sinθ p , where θ p represents the angle with the horizontal ground, and are Fourier transform and inverse Fourier transform respectively, Δx is the horizontal step size, u(x0,z) is the initial field distribution, and the propagation factor is obtained: The PE model is used to calculate the electromagnetic wave propagation intensity at the small target on the sea surface in the propagation path, and it is used as the incident field of the subsequent electromagnetic scattering model.

6. The radar detection imaging method according to claim 3, characterized in that: The specific method of step (2) is as follows: First, an electromagnetic scattering model is established to accurately simulate the interaction between electromagnetic waves and targets. The bouncing ray method uses the geometric optics GO principle to determine the reflection path and illuminated area of ​​the electromagnetic wave multiple reflection process, and tracks the ray intensity of the reflected rays. Then, the far-field scattering field integral formula of the physical optics method PO is used to solve the far-field scattering field of the bright area. Finally, the far-field scattering fields of all bright areas are vector-superimposed to obtain the total scattering field of the target. The geometric optics method treats electromagnetic waves as electromagnetic rays. According to the GO principle, light follows the Fresnel law during reflection. The mathematical expression of the Fresnel law is as follows: in, represents the incident direction vector, represents the reflection direction vector, represents the normal vector of the surface element at the incident point; The ray propagation path is tracked by the ray tube. The geometric optics method is used to accurately track the ray path, and the center line of the ray tube is used to track the ray intensity. When the ray tube and the target surface are reflected multiple times, the center line of the ray tube and the target surface will form a series of intersections (r1, r2, ..., r n ), the field strength relationship between adjacent points is: Among them, (DF) i and Represents point r i The divergence factor and reflection coefficient matrix at ; the divergence factor describes the change of the electric field intensity in the ray tube with the cross-sectional area of ​​the ray tube; Reflection coefficient matrix and reflected electric field E r and the incident electric field E in The relationship is: In formula (7), Γ V and Γ H represents the vertical and parallel polarization reflection coefficients, and the corresponding calculation expressions are: in, and is the intrinsic impedance of the medium, θ in and θ2 represent the angle of incidence and the angle of refraction, respectively; In the process of ray tracing and field intensity tracing using the geometric optics method, when the reflected ray no longer intersects with other facets of the target, the far-field integration method of physical optics is used to calculate the scattering field of the target in the far zone based on the field intensity data of the last reflection; the physical optics method is an approximate method based on the three assumptions of the locality principle, the tangent plane approximation, and the dark zone scattering field being zero. The corresponding far-zone scattering field integral formula is: In formula (11), E s represents the far-field scattered field; E i and H i represents the incident electric field and the incident magnetic field; Z0 is the wave impedance of free space; is the unit normal vector of the surface element; is the unit vector of the scattering direction; r′ represents the coordinate of the surface element; ds is the area of ​​the integrated surface element; Solve the far-zone scattered field of the bright area, and finally perform vector superposition on the far-zone scattered fields of all bright areas; Finally, the far-field scattering field of the target in the ocean environment is calculated by the SBR method as follows: Where i represents the number of ray bounces.

7. The radar detection imaging method according to claim 4, characterized in that: The specific method of step (3) is as follows: The mathematical formula for the target scattering field under small broadband and small rotation angle can be expressed as: Where k is the spatial frequency variable, is the rotation angle variable, A0 is the backscattering field amplitude, k c is the wave number relative to the center frequency; The ISAR image can be calculated by performing a two-dimensional inverse Fourier transform on the target's scattering field. The specific expression is: Therefore, the final ISAR image expression can be obtained: Where A0 represents the amplitude of electromagnetic reflection, δ(x-x0,y-y0) represents the two-dimensional pulse function at the point (x0,y0); The scattering field of the target can be approximated as the sum of the scattering of a finite number of scattering centers on the target. Therefore, the approximate expression of ISAR imaging of the target scattering field is: Where N is the total number of target scattering centers, A m represents the backscattering field amplitude of the mth scattering center; from formula (16), we can get the ISAR image composed of the sum of N scattering centers and their electromagnetic scattering coefficients.

Images