Complex terrain SAR echo simulation method based on polygon shadow judgment

Abstract

The invention discloses a complex terrain SAR (Synthetic Aperture Radar) echo simulation method based on polygon shadow judgment, which comprises the following steps: acquiring radar parameters and complex terrain data, and generating a beam irradiation area polygon, a height vector and a contour line in a slow time aiming at a current azimuth; for each terrain point at the current height, obtaining a shadow judgment result by judging the position relationship between the projection of the point on the zero-height surface and the polygon of the beam irradiation area and the polygon of the projection generated on the zero-height surface by the complex terrain with the height higher than that of the terrain point; the interior and exterior of the complex terrain are distinguished by dyeing the contour line of the current height, a projection polygon of the interior area of the complex terrain of the current height on the zero-height plane is generated, and the projection polygon of shadow judgment is updated after processing; calculating the slow time SAR echo of the current azimuth based on the shadow judgment results of all height terrain points; and obtaining SAR echoes of the complex terrain according to the SAR echoes of the slow time in all the directions. According to the method, an accurate shadow judgment result can be quickly generated.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of radar, and particularly relates to a complex terrain SAR echo simulation method based on polygon shadow judgment. BACKGROUND

[0002] Synthetic aperture radar (SAR) echo simulation technology can quickly obtain SAR echoes under any region and flight trajectory, and has been widely used in SAR system development and optimization. Compared with obtaining SAR echoes by actual flight, simulation of SAR echoes has the characteristics of low cost, fast speed, no trajectory limitation, and no environmental risk. However, when SAR echoes of complex terrain are simulated, the shadow effect in the SAR image is more obvious due to the influence of terrain undulation.

[0003] The key to complex terrain SAR echo simulation is to make shadow judgment on complex terrain, and however, the traditional complex terrain shadow judgment method cannot balance accuracy and calculation time. The light tracing method will cause the calculation time to increase sharply as the scene width becomes larger, so that the shadow judgment result cannot be obtained due to too long time when the method is used to make shadow judgment on a large scene. The shadow judgment method based on the comparison of the downward angle is a high-efficiency shadow judgment method. For example, the patent document “A complex terrain SAR echo simulation method based on dynamic occlusion judgment” (application number CN202110633947.6, application publication number CN113447896A) applied by Chongqing University discloses a complex terrain SAR echo simulation method, and the core of the method is a complex terrain shadow judgment method. The main idea of the method is as follows: basic parameters for SAR simulation and a digital elevation model (DEM) of a target scene are obtained; the depression angle of each scattering point target in the target scene is calculated; the depression angle of all points on the line connecting each judgment point and the radar is found along the distance direction, and if there is no corresponding point on the current distance direction, the point is supplemented by linear interpolation; the depression angles of the points on the same line are compared, and the depression angle is continuously updated, so that only the depression angles of the current point and the previous point need to be compared, and the shadow judgment result is obtained by judging all points; and SAR echoes are simulated according to the shadow judgment result. The method is simple in operation, and can quickly obtain complex terrain SAR echoes considering shadow. However, the method is based on interpolation operation, which can make the edge of the complex terrain smooth and cause incorrect shadow judgment result, and the required time sharply increases as the scene becomes larger. For large width and severe undulation terrain, the method has problems of insufficient calculation accuracy and low efficiency, and the method can only make shadow judgment on complex terrain stored in the form of DEM, so the applicability is low. SUMMARY

[0004] To address the aforementioned problems in existing technologies, this invention provides a SAR echo simulation method for complex terrain based on polygon shadow determination. The technical problem to be solved by this invention is achieved through the following technical solution: A SAR echo simulation method for complex terrain based on polygon shadow judgment includes: S1, acquire radar parameters and complex terrain data corresponding to the target area; S2, for the current azimuth slow time, based on the radar parameters and complex terrain data, generate the corresponding beam illumination area polygon on the zero height surface; by layering the complex terrain data, generate multiple height vectors from top to bottom, and generate closed contour lines based on the height vectors; S3, for the current height traversed from top to bottom, for each terrain point at the current height, by judging the positional relationship between the projection point of the terrain point on the zero height plane, the polygon of the beam illumination area of ​​the current azimuth slow time, and the projection polygon of the complex terrain at a height higher than the terrain point on the zero height plane, the shadow judgment result of the terrain point is obtained. S4. By coloring the contour lines at the current height, the interior and exterior of the complex terrain are distinguished, thereby generating the projection polygon of the complex terrain interior area at the current height onto the zero height plane. Using this polygon and the projection polygon of the complex terrain interior area at the previous height onto the zero height plane, the projection polygon used for shadow judgment is updated to support the next shadow judgment. S5, based on the shadow judgment results of terrain points at all altitudes, calculates the SAR echo of the current azimuth at a slow time. S6, based on the SAR echoes of all directions at slow time, obtain the SAR echoes of the complex terrain of the target area.

[0005] In one embodiment of the present invention, the radar parameters include: carrier frequency, platform speed, platform height, platform closest slant range, bandwidth, pulse repetition frequency, pulse width, modulation frequency, azimuth beamwidth, and elevation beamwidth. The complex terrain data includes, but is not limited to, DEM data.

[0006] In one embodiment of the present invention, in S2, based on the radar parameters and complex terrain data, a corresponding beam illumination area polygon is generated on the zero-height surface for the current azimuth time, including: Based on the radar parameters and complex terrain data, the depression angle and azimuth angle of any point illuminated on the complex terrain relative to the SAR platform are calculated under the current azimuth slow time. Based on the depression and azimuth angles of each illuminated point on the complex terrain relative to the SAR platform, a corresponding beam illumination area polygon is generated on the zero-height surface, where the zero-height surface is the xoy surface.

[0007] In one embodiment of the present invention, the formulas for calculating the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform are expressed as follows: ; ; in, and These represent the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform, respectively. Indicates azimuth time; the SAR platform position under azimuth time is represented as... The coordinates of any point illuminated on complex terrain are represented as follows: ; Indicates a modulo operation; The polygon of the beam illumination region generated at the zero-height surface can be expressed by the formula: ; in, and These represent the depression angle and azimuth angle of the beam center, respectively. and These represent the beamwidth in the elevation and azimuth directions, respectively.

[0008] In one embodiment of the present invention, the height vector is represented as: ; in, Indicates the number generated from top to bottom One height; , and These represent the minimum altitude, maximum altitude, and altitude resolution of the complex terrain in the target area, respectively.

[0009] In one embodiment of the present invention, S3 includes: Regarding the current altitude For each terrain point, obtain the coordinates of that terrain point. The terrain point is then projected onto the zero-height plane to obtain its coordinates. ; Get height as The projected polygons generated by projecting complex terrain onto the zero-height surface ; Based on the coordinates of the projected image of this terrain point and the polygon of the beam illumination area in the current azimuth over slow time. and height is The projected polygons generated by projecting complex terrain onto the zero-height surface The shadow determination result of the terrain point is obtained according to the preset shadow determination formula; wherein, the preset shadow determination formula is expressed as: ; in, Indicates coordinates as The terrain points are in the direction of slow time The shadow judgment result is 1, which means that the shadow is not occluded and 0 means that the shadow is occluded.

[0010] In one embodiment of the present invention, S4 includes: At current altitude The contour lines are colored to distinguish the interior and exterior of complex terrain, and the colored polygons are projected onto the zero-height surface to generate the current height. The projected polygon of the complex terrain interior region on the zero-height surface; By looking at the previous altitude The complex terrain interior region's projected polygon on the zero-height surface and its current height The complex terrain interior region is XORed with the projected polygon on the zero-height plane to generate... The projection polygon of the interior region of complex terrain onto the zero-height plane; By The projected polygon and height of the complex terrain are The projected polygons generated by projecting complex terrain onto the zero-height surface Perform a union operation to update the projected polygon. This can be used for the next shadow detection.

[0011] In one embodiment of the present invention, contour lines at the same height have two positional relationships: outside and inside. At current altitude The contour lines are colored to distinguish the interior and exterior of complex terrain, and the colored polygons are projected onto the zero-height surface to generate the current height. The projected polygons of the complex terrain interior regions onto the zero-height plane include: For polygons corresponding to externally separated contour lines, directly color them to represent the interior of complex terrain; For the polygons corresponding to the contained contour lines, the outermost polygon is divided into multiple regions by the inner polygons, numbered from the outside in, with odd numbers indicating the interior of complex terrain, and then colored accordingly. Project the points that form the contour lines onto the zero-height surface, keeping the colored areas unchanged, to obtain the current height. The projection polygon of the complex terrain interior region onto the zero-height surface.

[0012] In one embodiment of the present invention, by adjusting the previous height The projected polygon and the current height The complex terrain interior region is XORed with the projected polygon on the zero-height plane to generate... Projected polygons of complex terrain include: From height Choose any target polygon from the projected polygons; At an altitude of Find the intersecting polygons in the projected polygons that intersect with the target polygon; If no intersecting polygons exist, directly add the target polygon. In the projected polygons of complex terrain; If intersecting polygons exist, perform an XOR operation on the target polygon and the intersecting polygons, and then add the result. In the projected polygons of complex terrain; Delete the arbitrarily selected target polygon and intersecting polygon from their respective projected polygons, and then reselect the target polygon and repeat the process until the height is [missing information]. The projection polygon is generated until there are no polygons left in the projection polygon. Projected polygons of complex terrain.

[0013] In one embodiment of the present invention, in step S5, based on the shadow determination results of all terrain points at the current altitude, the SAR echo at the current azimuth slow time is calculated using the following formula: ; in, It's about the distance and time. It is an amplitude constant. This is the reflection coefficient corresponding to the azimuth time slow. In slow time, the coordinates are: The Euclidean distance between the terrain points and the SAR platform. At the speed of light, For rectangular window functions, The pulse width. For carrier frequency, To adjust the frequency, It is the imaginary unit.

[0014] The beneficial effects of this invention are: This invention proposes a SAR echo simulation method for complex terrain based on polygon shadow judgment. The core of this method is a polygon-based shadow judgment approach for complex terrain. Addressing the problems caused by block division or interpolation operations, this invention transforms shadow judgment into the positional relationship between polygons and points, performing shadow judgment on a two-dimensional plane. This eliminates the need to find the same line-of-sight point, thus avoiding block division or interpolation operations and mitigating issues such as unstable shadow judgment results or smooth edges of complex terrain caused by these operations. Furthermore, this method does not require generating any points outside the original complex terrain data, better preserving the characteristics of the complex terrain and saving time. To address the issue of poor applicability, this invention first converts the complex terrain into contour lines and uses these contour lines as input for shadow judgment. This allows for shadow judgment on various complex terrain data, enhancing its applicability. This invention avoids block division or interpolation operations, improving the accuracy of shadow judgment results and ensuring that the simulated complex terrain SAR echo is closer to the measured SAR echo. It can also perform SAR echo simulation on various complex terrain data types, demonstrating strong applicability, especially suitable for SAR echo simulation of complex terrain. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating a complex terrain SAR echo simulation method based on polygon shadow judgment provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the processing procedure of the complex terrain SAR echo simulation method based on polygon shadow judgment provided in the embodiment of the present invention; Figure 3 This is a schematic diagram of the beam illumination area in an embodiment of the present invention; Figure 4 This is a schematic diagram illustrating the method flow proposed in the embodiments of the present invention; Figure 5 This is a schematic diagram illustrating the understanding of contour lines in an embodiment of the present invention; Figure 6 This is a schematic diagram illustrating the understanding of the polygon-based shadow judgment criterion in an embodiment of the present invention; Figure 7 This is a schematic diagram illustrating the two positional relationships of contour lines at the same altitude in an embodiment of the present invention; Figure 8 This is a complex topographic map of the target area measured in this embodiment of the invention; Figure 9a and Figure 9b The results are obtained using the Z-buffer algorithm and the method of this invention, respectively. Figure 10a and Figure 10bThe images show simulated echo imaging results obtained using the Z-buffer algorithm and the method of this invention, respectively. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0017] When using synthetic aperture radar (SAR) to image complex terrain, the undulations in the terrain can cause some areas to be obscured by electromagnetic waves, resulting in shadows in the imaging results. This shadowing effect is particularly pronounced in areas with significant terrain undulations, such as mountains or tall buildings. Therefore, when simulating and generating SAR images of complex terrain, it is essential to first consider shadow detection for the complex terrain.

[0018] In existing shadow identification methods, traditional depression angle comparison algorithms divide the imaging region into blocks to find points on the same line of sight, and then compare the depression angles of these points to obtain shadow identification results. This often results in different shadow identifications for the same point on different lines of sight, and the unstable shadow identification results produce inaccurate SAR echoes. Ultimately, this causes SAR images generated from simulated echoes to fail to accurately reflect the actual shadow distribution, rendering many shadow-based SAR image application algorithms unusable. To obtain stable shadow identification results, researchers have replaced the imaging region block operation with interpolation to find more accurate and unique points on the same line of sight. However, interpolation can smooth the edges of complex terrain, leading to errors in shadow identification results and producing inaccurate SAR echoes for complex terrain. Furthermore, depression angle-based methods can only be used with digital elevation models, exhibiting poor applicability. Therefore, there is an urgent need to develop a computationally efficient shadow identification method that avoids interpolation and is applicable to various complex terrain data types to quickly generate accurate shadow identification results for complex terrain SAR echo simulations.

[0019] To address the problem of existing methods struggling to generate accurate shadow determination results in SAR echo simulation of complex terrain, this invention provides a method for SAR echo simulation of complex terrain based on polygon shadow determination, such as... Figure 1 and Figure 2 As shown, the method may include the following steps: S1, acquire radar parameters and complex terrain data corresponding to the target area; It is understood that the embodiments of the present invention are intended to simulate SAR echoes in complex terrain. Therefore, the simulation scene parameters must first be set, including radar parameters and complex terrain data corresponding to the illuminated target area.

[0020] The radar parameters include: carrier frequency, platform speed, platform height, platform closest slant range, bandwidth, pulse repetition frequency, pulse width, modulation frequency, azimuth beamwidth, and elevation beamwidth. In this embodiment of the invention, the complex terrain data includes, but is not limited to, DEM data, and can be any type of complex terrain data.

[0021] S2, for the current azimuth slow time, based on the radar parameters and complex terrain data, generate the corresponding beam illumination area polygon on the zero height surface; by layering the complex terrain data, generate multiple height vectors from top to bottom, and generate closed contour lines based on the height vectors; Please see Figure 3 The diagram illustrating the beam illumination area shows that the flight direction corresponds to the radar's movement direction. Due to the limitation of radar beam width, the beam cannot completely illuminate the imaging area in a certain azimuth over a short period of time. Therefore, it is first necessary to determine which parts of the complex terrain are located within the beam illumination area. Complex terrain outside the beam illumination area is naturally treated as shadows, eliminating the need for subsequent shadow identification, thus improving the computational efficiency of the method.

[0022] Therefore, in S2, for the current azimuth slow time, based on the radar parameters and complex terrain data, generating a corresponding beam illumination area polygon on the zero-height surface can include: Step a1: Based on the radar parameters and complex terrain data, calculate the depression angle and azimuth angle of any point illuminated on the complex terrain relative to the SAR platform at the current azimuth slow time. Assumption This indicates the azimuth time delay, representing the SAR platform position at azimuth time delay. and the coordinates of any point illuminated on complex terrain It can be represented as: ; ; Therefore, the formulas for calculating the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform are expressed as follows: ; ; in, and These represent the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform, respectively. This indicates a modulo operation. Step a2: Based on the depression angle and azimuth angle of each illuminated point on the complex terrain relative to the SAR platform, generate the corresponding beam illumination area polygon on the zero-height surface, where the zero-height surface is the xoy surface.

[0023] The shadow determination method provided by this invention is based on the positional relationship between polygons and points. The beam-illuminated area is used for subsequent shadow determination, and the beam-illuminated area is represented as a polygon generated on a zero-height surface.

[0024] Specifically, the polygon of the beam illumination area generated on the zero-height surface can be expressed by the formula: ; in, and These represent the depression angle and azimuth angle of the beam center, respectively; and These represent the beamwidth in the elevation and azimuth directions, respectively. It can be understood that each azimuth time interval corresponds to a polygon of the beam's illumination area.

[0025] Shadows are created when higher, more complex terrain blocks electromagnetic waves, preventing lower, more complex terrain from being illuminated. Therefore, to simulate the shadow generation process, this embodiment of the invention calculates shadows layer by layer from top to bottom along the height of the complex terrain. Specifically, multiple heights are generated from top to bottom based on the complex terrain data, forming a height vector. This height vector defines the direction and step size for shadow calculation.

[0026] For details, please see Figure 4 Understood, the height vector is represented as: ; in, Indicates the number generated from top to bottom The height, which is the height in the height vector. One element; , and These represent the minimum altitude, maximum altitude, and altitude resolution of the complex terrain in the target area, respectively, which can be obtained from the complex terrain data.

[0027] at the same time, Figure 4 The process of the method proposed in this invention is also described, namely, first projecting complex terrain, and then updating the projected polygon.

[0028] This invention's embodiment determines shadows based on the positional relationship between polygons and points. This requires the polygons to be closed, as only closed polygons can clearly define their positional relationship with points. Therefore, the complex terrain data is first expanded, and closed contour lines are generated based on the height vector. It can be understood that the height vector contains multiple heights divided into layers, each corresponding to a contour line. This invention's embodiment can input the height vector into a preset function in MATLAB to generate closed contour lines. For details, please refer to relevant technical explanations, which will not be elaborated here. Of course, the implementation process of closed contour lines is not limited to this and is not restricted here. Please refer to... Figure 5 To understand the contour lines of a real undulating terrain, all contour lines are closed.

[0029] S1~S2 of this embodiment of the invention embodies the initialization process. It should be noted that only the polygon of the beam illumination area needs to be recalculated in different directions with slow time, while the height vector and the closed contour lines only need to be generated once and can be reused in other directions with slow time.

[0030] S3, for the current height traversed from top to bottom, for each terrain point at the current height, by judging the positional relationship between the projection point of the terrain point on the zero height plane, the polygon of the beam illumination area of ​​the current azimuth slow time, and the projection polygon of the complex terrain at a height higher than the terrain point on the zero height plane, the shadow judgment result of the terrain point is obtained. The embodiments of the present invention are from arrive We perform a traversal, and for ease of understanding, we'll use the current height reached during the traversal. For example, S3 refers to the current height. Shadow detection is performed on each terrain point.

[0031] The shadow cast on a terrain point is caused by the electromagnetic waves being blocked by complex terrain higher than that point, preventing it from being illuminated. Treating the complex terrain as a three-dimensional geometric shape and projecting it onto a zero-height plane with the radar as the projection center results in a two-dimensional closed polygon. These polygons are called projection polygons and can be used... express.

[0032] The inventors discovered that if the projection of a terrain point onto a zero-height plane falls within a projection polygon generated by complex terrain that is higher than the terrain point, the terrain point will be occluded and cast a shadow; if it is not within such a polygon, the terrain point can be illuminated by electromagnetic waves.

[0033] For details, please see Figure 6 Understanding, assuming a polygon The outline of a terrain with varying elevations. and These are the terrain points to be determined. The point is the projection center, and the terrain outline is projected. , terrain point and Projecting onto the zero-height plane generates a projected polygon. Projection point and We discovered that, Terrain outline Obstruction, and It is not obstructed. Meanwhile, on the zero-height surface, In the terrain outline Projected polygons In the middle, and No. Therefore, we obtain a polygon-based shadow judgment criterion, which can be expressed as follows: if the projection of a terrain point onto the zero-height plane lies within a projection polygon generated by a more complex terrain at a higher elevation, the terrain point will be occluded by the complex terrain; if the projection of the terrain point onto the zero-height plane does not lie within a projection polygon generated by a more complex terrain at a higher elevation, the terrain point will not be occluded by the complex terrain. Therefore, in this embodiment of the invention, shadow judgment is performed on terrain points at each elevation using a polygon-based shadow judgment criterion based on the elevation vector.

[0034] S3 may include the following steps: S31, for the current altitude For each terrain point, obtain the coordinates of that terrain point. The terrain point is then projected onto the zero-height plane to obtain its coordinates. ; As mentioned earlier, the coordinates of this terrain point ,at this time Coordinates are Projecting this terrain point onto the zero-height plane yields the projected point. coordinate Represented as: ; in, ; ; S32, obtain the height as The projected polygons generated by projecting complex terrain onto the zero-height surface ; At a height of When determining shadows for terrain points, a height of [height missing] is required. The projected polygon generated by projecting complex terrain onto the zero-height plane is denoted as... , This was obtained after the last execution of S4, which will be explained in detail later.

[0035] It should be noted that for heights of When determining shadows for terrain points, the projected polygons generated by projecting complex terrain onto the zero-height plane are used. It is an empty set.

[0036] S33, based on the coordinates of the projection point of the terrain point and the polygon of the beam illumination area in the current azimuth slow time. and height is The projected polygons generated by projecting complex terrain onto the zero-height surface Based on the preset shadow judgment formula, the shadow judgment result of the terrain point is obtained; The preset shadow determination formula is expressed as follows: ; in, Indicates coordinates as The terrain points are in the direction of slow time The shadow judgment result is 1, which means that the shadow is not occluded and 0 means that the shadow is occluded.

[0037] Based on the above processing procedure, the current altitude Each terrain point on the map can obtain a corresponding shadow determination result.

[0038] S4. By coloring the contour lines at the current height, the interior and exterior of the complex terrain are distinguished, thereby generating the projection polygon of the complex terrain interior area at the current height onto the zero height plane. Using this polygon and the projection polygon of the complex terrain interior area at the previous height onto the zero height plane, the projection polygon used for shadow judgment is updated to support the next shadow judgment. S4 is when the height is After all terrain points have completed shadow detection, a projected polygon is generated for the next shadow detection. .

[0039] Specifically, S4 may include the following steps: S41, regarding the current altitude The contour lines are colored to distinguish the interior and exterior of complex terrain, and the colored polygons are projected onto the zero-height surface to generate the current height. The projected polygon of the complex terrain interior region on the zero-height surface; Contour lines can only represent the outline of complex terrain; only the interior of complex terrain can block electromagnetic waves. Correspondingly, only the interior of complex terrain can generate a projected polygon on a zero-height surface. Therefore, it is first necessary to define the contour lines at height... The contour lines are colored to distinguish the interior and exterior of complex terrain.

[0040] SeeFigure 7 It is understandable that contour lines at the same altitude can have two positional relationships: externally separated and internally contained. These are respectively as follows: Figure 7 The two lines above and below are shown.

[0041] Based on this, S41 may include the following steps: Step b1: For the polygons corresponding to the outer contour lines, directly color them to represent the interior of the complex terrain; Step b2: For the polygons corresponding to the contained contour lines, the outermost polygon is divided into multiple regions by the inner polygons, numbered from the outside to the inside, with odd numbers indicating the interior of complex terrain, and then colored. The contained polygons do not intersect; they form a ring within a ring. The ring structure divided by the outermost polygons is numbered, starting from the first layer, which is numbered 1, and so on inwards. Odd numbers indicate the interior of complex terrain, and these layers are then colored.

[0042] Step b3: Project the points that form the contour lines onto the zero-height surface, keeping the colored area unchanged, to obtain the current height. The projection polygon of the complex terrain interior region onto the zero-height surface.

[0043] S42, via the previous altitude The complex terrain interior region's projected polygon on the zero-height surface and its current height The complex terrain interior region is XORed with the projected polygon on the zero-height plane to generate... The projection polygon of the interior region of complex terrain onto the zero-height plane; It should be noted that in each loop, the projected polygons of the complex terrain interior region at the current altitude onto the zero-altitude plane are generated, and the previous altitude is also considered. The projected polygons of the complex terrain interior region onto the zero-height plane are generated in the previous loop. During the initial calculation, the projected polygons of the complex terrain interior region at the current height onto the zero-height plane are an empty set.

[0044] right What is needed to update is Projected polygons of complex terrain. The projection polygon of the interior region of complex terrain onto the zero-height plane is a polygon relative to the height. The projection polygon and height of the complex terrain interior region on the zero-height surface The polygons generated are obtained by XORing the projected polygons on the zero-height surface within the complex terrain interior region. When performing the XOR operation, it is necessary to match the projected polygons at different heights to avoid the XOR operation result being an empty set when two polygons have no intersection, causing both polygons to be ignored.

[0045] Specifically, S42 may include the following steps: Step c1, from a height of Choose any target polygon from the projected polygons; Height is The projected polygon is the current height. The projection polygon of the complex terrain interior region onto the zero-height plane, with a height of The projected polygon is the previous height. The projection polygons of the complex terrain interior regions onto the zero-height plane are all simplified here.

[0046] Step c2, at a height of Find the intersecting polygons in the projected polygons that intersect with the target polygon; Step c3: If no intersecting polygons exist, directly add the target polygon. In the projected polygons of complex terrain; If no intersecting polygons exist, it means the target polygon is appearing for the first time, generated at the top of some terrain feature. This is equivalent to an empty set of intersecting polygons, so no XOR operation is needed; it can be added directly.

[0047] Step c4: If intersecting polygons exist, perform an XOR operation on the target polygon and the intersecting polygons, and then add the result. In the projected polygons of complex terrain; If intersecting polygons exist, it means that the target polygon and the intersecting polygons were generated from the same terrain structure. In the current loop, what we need is... The projected polygon of the complex terrain needs to be selected from the target polygon. The part that is, the intersecting polygons, is removed, which corresponds to the XOR operation.

[0048] Step c5: Delete the arbitrarily selected target polygon and intersecting polygon from their respective projected polygons, and then reselect the target polygon and repeat the process until the height is [missing information]. The projection polygon is generated until there are no polygons left in the projection polygon. Projected polygons of complex terrain.

[0049] Since the arbitrarily selected target polygon and intersecting polygons have already been used to generate the projected polygon, they need to be deleted to prevent them from being selected again for duplicate calculations. When the height is... If there are no polygons in the projected polygons, it means that all polygons have completed the matching operation and have been used for generation. Projected polygons of complex terrain.

[0050] S43, by The projected polygon and height of the complex terrain are The projected polygons generated by projecting complex terrain onto the zero-height surface Perform a union operation to update the projected polygon. This can be used for the next shadow detection.

[0051] Understandably, during the initial calculation, the projected polygon used for shadow determination... The set is empty. After each loop, the projected polygon used for shadow detection is... The updated projection polygon is used for the next shadow determination.

[0052] This invention presents a novel shadow detection method that transforms the three-dimensional shadow detection problem into a two-dimensional problem using the positional relationships between polygons and points. This avoids the use of block division or interpolation operations, thus preventing inaccurate shadow detection results caused by these methods. Furthermore, this invention utilizes contour lines to calculate the projection of complex terrain. By treating contour lines as polygons, the contour lines are first colored to distinguish the interior and exterior of the complex terrain. Then, through XOR and union operations, the complex terrain is projected onto a zero-height surface, generating polygons for shadow detection. This method allows complex terrain data to be converted into contour lines before shadow detection, making it applicable to different types of complex terrain data.

[0053] S5, based on the shadow judgment results of terrain points at all altitudes, calculates the SAR echo of the current azimuth at a slow time. Specifically, S5 simulates SAR echoes based on shadow judgment results. For the current azimuth slow time, shadow judgments are performed on terrain points at all altitudes to obtain the shadow judgment results for the entire complex terrain at the current azimuth slow time. The downsampled echo at the current azimuth slow time is calculated using the following formula.

[0054] ; in, It's about the distance and time. It is an amplitude constant. This is the reflection coefficient corresponding to the azimuth time slow. In slow time, the coordinates are: The Euclidean distance between the terrain points and the SAR platform. At the speed of light, For rectangular window functions, The pulse width. For carrier frequency, To adjust the frequency, It is the imaginary unit.

[0055] S6, based on the SAR echoes of all directions at slow time, obtain the SAR echoes of the complex terrain of the target area.

[0056] It is understandable that if S2~S5 obtains the SAR echo of the current azimuth at a slow time, then repeating the above process for all azimuths at a slow time can obtain the SAR echo of the complex terrain of the target area.

[0057] This invention proposes a SAR echo simulation method for complex terrain based on polygon shadow judgment. The core of this method is a polygon-based shadow judgment approach for complex terrain. Addressing the problems caused by block division or interpolation operations, this invention transforms shadow judgment into the positional relationship between polygons and points, performing shadow judgment on a two-dimensional plane. This eliminates the need to find the same line-of-sight point, thus avoiding block division or interpolation operations and mitigating issues such as unstable shadow judgment results or smooth edges of complex terrain caused by these operations. Furthermore, this method does not require generating any points outside the original complex terrain data, better preserving the characteristics of the complex terrain and saving time. To address the issue of poor applicability, this invention first converts the complex terrain into contour lines and uses these contour lines as input for shadow judgment. This allows for shadow judgment on various complex terrain data, enhancing its applicability. This invention avoids block division or interpolation operations, improving the accuracy of shadow judgment results and ensuring that the simulated complex terrain SAR echo is closer to the measured SAR echo. It can also perform SAR echo simulation on various complex terrain data types, demonstrating strong applicability, especially suitable for SAR echo simulation of complex terrain.

[0058] To verify the effectiveness of the method in the embodiments of the present invention, the simulation results are given below.

[0059] Figure 8 This is complex terrain data of the target area measured in an embodiment of the present invention, with a resolution of 1m. 1m 1m.

[0060] Table 1 shows the SAR echo simulation parameters for complex terrain considering shadows, and the corresponding radar parameters.

[0061] Table 1

[0062] Figure 9a and Figure 9b These are the shadow judgment results obtained using the Z-buffer algorithm and the method of this invention, respectively. Figure 10a and Figure 10b The images show simulated echo imaging results obtained using the Z-buffer algorithm and the method of this invention, respectively. (Comparison)Figure 9a and Figure 9b It can be seen that the method of the present invention takes into account the influence of beamwidth, and at the same time avoids the generation of snowflake-like shadow judgment results by not dividing the imaging scene into grids. Meanwhile, in comparison... Figure 10a and Figure 10b It can be seen that the shadow area generated by the method of the present invention is cleaner and more consistent with the characteristics of the shadow area in the actual SAR image.

[0063] Shadowing is more pronounced in SAR images of complex terrain. To generate accurate SAR echoes of complex terrain, shadow identification is essential. This invention transforms complex terrain into contour lines and uses polygons and point positional relationships as a basis to determine shadows. The generated simulated SAR echoes of complex terrain, considering shadows, provide significant data support for SAR system development and the design of various algorithms utilizing SAR image shadows, as detailed below: (1) Assisting in the development of SAR system: During the development of SAR system, it is difficult to obtain SAR echoes in complex terrain. SAR echoes under different flight trajectories and radar parameters can be obtained through simulation and input into the SAR system to find possible problems in the system and complete the development of SAR system.

[0064] (2) Design of early warning algorithms for landslides and glaciers: It is difficult to acquire a large number of SAR images of actual landslides and glaciers. At the same time, the acquired samples also have problems such as narrow coverage of displacement changes, small scene width, and low resolution. This invention can simulate and generate high-resolution SAR images of mountains, glaciers and other complex terrains with different displacement degrees, which can meet the data requirements during algorithm design.

[0065] (3) Assisting in the design of height inversion algorithms using shadows: Using shadows for 3D reconstruction of scene information improves the accuracy of scene height reconstruction, providing more accurate scene information for autonomous obstacle avoidance, geographic mapping, and target contour reconstruction. This invention can generate SAR images that consider shadows for specific scenes. The simulated shadows are free from noise interference, providing high-quality simulated SAR images in the early stages of algorithm development. Furthermore, in the later stages of algorithm development, low-quality SAR images can be generated by artificially injecting noise to test and modify the algorithm, thereby improving its applicability.

[0066] (4) Assisting in the design of target tracking algorithms using shadows: In SAR imaging results of moving targets, the target will be out of focus, making it impossible to track the target based on the imaging results. Currently, some research teams have discovered that the shadow imaging results generated by moving targets are not out of focus, and because the shadows have a large contrast with the surrounding environment, moving targets can be tracked using shadows. Although this invention involves shadow judgment and echo generation for complex terrain, it can also convert the scene composed of the target and the ground into contour lines first, then perform shadow judgment, and finally generate SAR echoes. Through simulation, SAR images of specific moving targets under specific trajectories can be generated, providing a large amount of data support for algorithm design in the early stages of developing target tracking algorithms using shadows.

[0067] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A SAR echo simulation method for complex terrain based on polygon shadow judgment, characterized in that, include: S1, acquire radar parameters and complex terrain data corresponding to the target area; S2, for the current azimuth slow time, based on the radar parameters and complex terrain data, generate the corresponding beam illumination area polygon on the zero height surface; by layering the complex terrain data, generate multiple height vectors from top to bottom, and generate closed contour lines based on the height vectors; S3, for the current height traversed from top to bottom, for each terrain point at the current height, by judging the positional relationship between the projection point of the terrain point on the zero height plane, the polygon of the beam illumination area of ​​the current azimuth slow time, and the projection polygon of the complex terrain at a height higher than the terrain point on the zero height plane, the shadow judgment result of the terrain point is obtained. S4. By coloring the contour lines at the current height, the interior and exterior of the complex terrain are distinguished, thereby generating the projection polygon of the complex terrain interior area at the current height onto the zero height plane. Using this polygon and the projection polygon of the complex terrain interior area at the previous height onto the zero height plane, the projection polygon used for shadow judgment is updated to support the next shadow judgment. S5, based on the shadow judgment results of terrain points at all altitudes, calculates the SAR echo of the current azimuth at a slow time. S6, based on the SAR echoes of all directions at slow time, obtain the SAR echoes of the complex terrain of the target area.

2. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 1, characterized in that, The radar parameters include: carrier frequency, platform speed, platform height, platform closest slant range, bandwidth, pulse repetition frequency, pulse width, modulation frequency, azimuth beamwidth, and elevation beamwidth. The complex terrain data includes, but is not limited to, DEM data.

3. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 1 or 2, characterized in that, In S2, for the current azimuth slow time, based on the radar parameters and complex terrain data, a corresponding beam illumination area polygon is generated on the zero-height surface, including: Based on the radar parameters and complex terrain data, the depression angle and azimuth angle of any point illuminated on the complex terrain relative to the SAR platform are calculated under the current azimuth slow time. Based on the depression and azimuth angles of each illuminated point on the complex terrain relative to the SAR platform, a corresponding beam illumination area polygon is generated on the zero-height surface, where the zero-height surface is the xoy surface.

4. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 3, characterized in that, The formulas for calculating the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform are as follows: ； ； in, and These represent the depression angle and azimuth angle of any illuminated point on complex terrain relative to the SAR platform, respectively. Indicates azimuth time; the SAR platform position under azimuth time is represented as... The coordinates of any point illuminated on complex terrain are represented as follows: ; Indicates a modulo operation; The polygon of the beam illumination region generated at the zero-height surface can be expressed by the formula: ； in, and These represent the depression angle and azimuth angle of the beam center, respectively. and These represent the beamwidth in the elevation and azimuth directions, respectively.

5. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 4, characterized in that, The height vector is represented as: ； in, Indicates the number generated from top to bottom One height; , and These represent the minimum altitude, maximum altitude, and altitude resolution of the complex terrain in the target area, respectively.

6. The complex terrain SAR echo simulation method based on polygon shadow judgment according to claim 5, characterized in that, S3 include: Regarding the current altitude For each terrain point, obtain the coordinates of that terrain point. The terrain point is then projected onto the zero-height plane to obtain its coordinates. ; Get height as The projected polygons generated by projecting complex terrain onto the zero-height surface ; Based on the coordinates of the projected image of this terrain point and the polygon of the beam illumination area in the current azimuth over slow time. and height is The projected polygons generated by projecting complex terrain onto the zero-height surface The shadow determination result of the terrain point is obtained according to the preset shadow determination formula; wherein, the preset shadow determination formula is expressed as: ； in, Indicates coordinates as The terrain points are in the direction of slow time The shadow judgment result is 1, which means that the shadow is not occluded and 0 means that the shadow is occluded.

7. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 6, characterized in that, S4 includes: At current altitude The contour lines are colored to distinguish the interior and exterior of complex terrain, and the colored polygons are projected onto the zero-height surface to generate the current height. The projected polygon of the complex terrain interior region on the zero-height surface; By looking at the previous altitude The complex terrain interior region's projected polygon on the zero-height surface and its current height The complex terrain interior region is XORed with the projected polygon on the zero-height plane to generate... The projection polygon of the interior region of complex terrain onto the zero-height plane; By The projected polygon and height of the complex terrain are The projected polygons generated by projecting complex terrain onto the zero-height surface Perform a union operation to update the projected polygon. This can be used for the next shadow detection.

8. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 7, characterized in that, Contour lines at the same altitude can have two positional relationships: outside and inside. At current altitude The contour lines are colored to distinguish the interior and exterior of complex terrain, and the colored polygons are projected onto the zero-height surface to generate the current height. The projected polygons of the complex terrain interior regions onto the zero-height plane include: For polygons corresponding to externally separated contour lines, directly color them to represent the interior of complex terrain; For the polygons corresponding to the contained contour lines, the outermost polygon is divided into multiple regions by the inner polygons, numbered from the outside to the inside. Odd numbers indicate the interior of complex terrain, and these regions are then colored. Project the points that form the contour lines onto the zero-height surface, keeping the colored areas unchanged, to obtain the current height. The projection polygon of the complex terrain interior region onto the zero-height surface.

9. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 8, characterized in that, By looking at the previous altitude The projected polygon and the current height The complex terrain interior region is XORed with the projected polygon on the zero-height plane to generate... Projected polygons of complex terrain include: From height Choose any target polygon from the projected polygons; At an altitude of Find the intersecting polygons in the projected polygons that intersect with the target polygon; If no intersecting polygons exist, directly add the target polygon. In the projected polygons of complex terrain; If intersecting polygons exist, perform an XOR operation on the target polygon and the intersecting polygons, and then add the result. In the projected polygons of complex terrain; Delete the arbitrarily selected target polygon and intersecting polygon from their respective projected polygons, and then reselect the target polygon and repeat the process until the height is [missing information]. The projection polygon is generated until there are no polygons left in the projection polygon. Projected polygons of complex terrain.

10. The SAR echo simulation method for complex terrain based on polygon shadow judgment according to claim 9, characterized in that, In S5, based on the shadow determination results of all terrain points at the current altitude, the slow-time SAR echo at the current azimuth is calculated using the following formula: ； in, It's about the distance and time. It is an amplitude constant. This is the reflection coefficient corresponding to the azimuth time slow. In slow time, the coordinates are: The Euclidean distance between the terrain points and the SAR platform. At the speed of light, For rectangular window functions, The pulse width. For carrier frequency, To adjust the frequency, It is the imaginary unit.

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