Method and device for extracting backscattering energy of an observed object based on SAR images

Abstract

The present invention relates to a method and device for extracting the backscattering energy of an observed object based on SAR images. By extracting the spatial incident angle of a radar beam relative to a known observed object and the corresponding scattering energy from a spaceborne SAR image, and constructing a backscattering mapping database of the observed object in the spaceborne SAR image according to the spatial incident angle, the scattering energy, and the acquisition parameters of the spaceborne SAR image; then, based on this mapping database, the scattering energy of the observed object to be measured is obtained according to the acquisition parameters of the spaceborne SAR image and the spatial incident angle of the radar beam relative to the observed object to be measured; the present invention can make the backscattering energy of the observed object extracted from the SAR image have practical application significance by obtaining the electromagnetic wave incident angle relative to the observed object in an accurate spatial resolution manner.

Description

Technical Field

[0001] The present invention relates to the technical field of spaceborne synthetic aperture radar applications, and more specifically, to a method and device for extracting the backscattering energy of an observed object based on SAR images. Background Art

[0002] Backscattering describes the ability of an object to scatter incident electromagnetic waves in the incident direction and is an important electromagnetic scattering characteristic of the object. The backscattering energy is characterized by the pixel value in a Synthetic Aperture Radar (SAR) image. The stronger the backscattering energy, the greater the detected scattering intensity and the larger the corresponding pixel value. Backscattering is usually related to the structure and physical size of the observed object. Therefore, in the field of microwave applications, such as SAR image processing and applications, it is a common technical approach to analyze the scattering ability, shape, structure, and other scattering characteristics of the observed object based on the detected scattering intensity.

[0003] Due to the extremely complex interaction mechanism between electromagnetic waves and the observed object and the scattering process, it is difficult to describe the backscattering expression of an object with a complex structure through an analytical formula. Currently, the techniques for calculating or extracting the backscattering of common objects mainly fall into three categories.

[0004] The first category is to model the scattering process based on electromagnetic scattering theory and deduce the analytical expression of backscattering; the second category is to use electromagnetic scattering simulation means, combined with object modeling, to simulate and calculate the electromagnetic scattering process to obtain the backscattering energy values of the object at different incident angles; the third category is to conduct simulation experiments on full-scale or scaled physical objects or object models in a laboratory or microwave anechoic chamber to obtain the backscattering energy at different incident angles.

[0005] Among them, the first category of methods is limited by the complex scattering process and can only calculate the backscattering energy of some objects with simple structures at limited angles. The second category of methods only has high-precision simulation capabilities for electrically small-sized objects, while for electrically large-sized objects, the simulation accuracy needs to be sacrificed significantly due to computational efficiency limitations. The third category of methods is also limited by laboratory conditions, with restrictions on the object sizes that can be experimented on.

[0006] At the same time, the above methods are usually only applicable to near-field simulations, and the accuracy for far fields is limited. In addition, there are significant differences between the above three technical means and the conditions for obtaining scattering information of observed objects in actual applications. Therefore, the numerical values obtained by existing methods are difficult to directly adopt in actual applications. Summary of the Invention

[0007] In view of this, the present invention provides a method and device for extracting the backscattering energy of an observed object from a SAR image, aiming to obtain the incident angle of electromagnetic waves relative to the observed object in an accurate spatial resolution manner, so that the backscattering energy of the observed object extracted from the SAR image has practical application significance.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] On the one hand, the present application provides a method for extracting the backscattering energy of an observed object from a SAR image, including the following steps:

[0010] S1. Determine the spatial incident angle and corresponding scattering energy of the radar beam relative to a known observed object based on the spaceborne SAR image, and construct a spaceborne SAR image observed object backscattering mapping database according to the spatial incident angle, the scattering energy, and the acquisition parameters of the spaceborne SAR image;

[0011] Among them, the step of determining the incident angle of the radar beam relative to the known observed object based on the spaceborne SAR image includes:

[0012] S11. Determine the spatial coordinate of the intersection point of the satellite radar beam center and the ground;

[0013] S12. Obtain the spatial coordinates of the known observed object according to the intersection point spatial coordinates;

[0014] S13. Calculate the incident angle of the radar beam relative to the spatial coordinates of the known observed object;

[0015] S14. Obtain the spatial incident angle of the radar beam relative to the known observed object according to the spatial attitude of the known observed object and the incident angle of the radar beam relative to the spatial coordinates of the known observed object;

[0016] S2. Based on the spaceborne SAR image observed object backscattering mapping database, obtain the scattering energy of the observed object to be measured according to the acquisition parameters of the spaceborne SAR image and the spatial incident angle of the radar beam relative to the observed object to be measured.

[0017] Preferably, step S11 includes the following steps:

[0018] S111. Establish a spatial coordinate system with the satellite sub-satellite point as the origin;

[0019] S112. Obtain the abscissa of the intersection point spatial coordinates according to the satellite altitude and the beam center downward viewing angle;

[0020] S113. Obtain the ordinate of the intersection point spatial coordinates according to the satellite altitude, the beam center downward viewing angle, and the beam center slant viewing angle.

[0021] Preferably, the intersection point spatial coordinates are expressed as follows:

[0022]

[0023] In the formula, h is the satellite altitude among the parameters of the spaceborne SAR image system, and θ l is the nadir angle of the beam center, and θ sq is the squint angle of the beam center.

[0024] Preferably, in S12, the spatial coordinates of the known object are (x, y, 0), where x is obtained by the following steps, and y is the same as the ordinate in the intersection spatial coordinates;

[0025] S121. Extract the minimum circumscribed rectangle of the known object in the spaceborne SAR image, and use the center pixel coordinates of the minimum circumscribed rectangle as the pixel coordinates of the known object in the SAR image;

[0026] S122. Obtain the range pixel difference ΔN between the pixel coordinates of the known object in the SAR image and the pixel point coordinates corresponding to the radar beam center in the SAR image;

[0027] S123. According to the range pixel difference, use the following formula to obtain the abscissa in the spatial coordinates of the known object;

[0028] x = x 0 + δ ra × (N T - N 0 )

[0029] In the formula, N 0 is the range pixel coordinate of the radar beam center in the SAR image, NT is the range pixel coordinate of the known object in the SAR image, δ ra is the range resolution, and x 0 is the abscissa in the intersection spatial coordinates.

[0030] Preferably, in S13, the incident angle includes the incident azimuth angle and the incident elevation angle θ T , and the calculation methods are respectively:

[0031]

[0032]

[0033] In the formula, h represents the altitude of the satellite in the space coordinate system.

[0034] In this embodiment, the spatial distance between the satellite and the object is also calculated, and the calculation method is:

[0035]

[0036] Preferably, S14 includes the following steps:

[0037] S141. Establish an observation object center coordinate system with a known observation object as the origin;

[0038] S142. When the spatial attitude of the known observation object is known and measurable, use the angle between the normal direction of the measured observation object and the satellite when the beam passes as the spatial incident pitch angle of the radar beam relative to the observation object, and use the sum of the incident azimuth angle in the incident angle of the radar beam relative to the spatial coordinates of the known observation object and the orientation azimuth angle of the known observation object as the spatial incident azimuth angle of the radar beam relative to the observation object; that is

[0039]

[0040] In the formula, is the orientation azimuth angle of the known observation object, is the incident azimuth angle of the radar beam relative to the spatial coordinates of the known observation object.

[0041] S143. When the spatial attitude of the known observation object is unknown and unmeasurable, establish an actual spatial attitude coordinate system according to the spatial attitude of the known observation object, and obtain the spatial incident azimuth angle and pitch angle of the radar beam relative to the observation object according to the coordinates of the satellite in the actual spatial attitude coordinate system of the known observation object.

[0042] The specific steps are as follows:

[0043] 1) According to the incident azimuth angle and pitch angle θ s of the satellite in the known observation object center coordinate system, convert the satellite coordinates in the star point coordinate system to the coordinates in the known observation object center coordinate system;

[0044] where the incident azimuth angle and pitch angle θ s are obtained through the following formula:

[0045]

[0046] In the formula, is the orientation azimuth angle of the known observation object, is the incident azimuth angle of the radar beam relative to the spatial coordinates of the known observation object, and θ T is the incident pitch angle of the radar beam relative to the spatial coordinates of the known observation object.

[0047] The satellite coordinate conversion method is:

[0048]

[0049] In the formula, (x s ), ys , z s ) is the satellite coordinate in the known observation object center coordinate system; and z s = h;

[0050] 2) By successively rotating the known observation object center coordinate system around the x-axis and y-axis, establish the actual space attitude coordinate system of the known observation object; and determine the coordinates of the satellite in the actual space attitude coordinate system of the known observation object;

[0051] Among them, the rotation angle around the x-axis is α, then the rotation matrix R x (α) is:

[0052]

[0053] The rotation angle around the y-axis is β, then the rotation matrix R y (β) is:

[0054]

[0055] ″′

[0056] And the coordinates of the satellite in the actual space attitude coordinate system of the known observation object (x S , y S , z S ) can be obtained by the following formula:

[0057]

[0058] 3) According to the coordinates of the satellite in the actual space attitude coordinate system of the known observation object, obtain the spatial incident azimuth angle and elevation angle of the radar beam relative to the observation object.

[0059] The calculation method is as follows:

[0060]

[0061] In the formula, is the spatial incident azimuth angle of the radar beam relative to the observation object, and θ is the elevation angle.

[0062] Preferably, the scattering energy of the known observation object includes backscattering energy and maximum scattering energy; the acquisition formulas are respectively:

[0063]

[0064] In the formula, E is the backscattering energy of the known observation object, P is the maximum scattering energy of the known observation object, I represents the image amplitude value within the minimum circumscribed rectangle of the known observation object, and H and W are the height and width of the minimum circumscribed rectangle.

[0065] Preferably, in S1, the information generated by each known object sample constitutes an object backscattering mapping table, and the spaceborne SAR image object backscattering mapping database is formed by connecting multiple encoded object backscattering mapping tables end to end.

[0066] On the other hand, the present invention also provides a device for extracting the backscattering energy of an object based on an SAR image, including:

[0067] A backscattering mapping database construction module, configured to determine the incident angle of the radar beam relative to a known object and the corresponding scattering energy based on the spaceborne SAR image, and construct a spaceborne SAR image object backscattering mapping database according to the incident angle, the scattering energy, and the acquisition parameters of the spaceborne SAR image;

[0068] Among them, the step of determining the incident angle of the radar beam relative to a known object based on the spaceborne SAR image includes:

[0069] S11. Determine the spatial coordinates of the intersection point between the center of the satellite radar beam and the ground;

[0070] S12. Obtain the spatial coordinates of the known object according to the intersection point spatial coordinates;

[0071] S13. Calculate the incident angle of the radar beam relative to the spatial coordinates of the known object;

[0072] S14. Obtain the incident angle of the radar beam relative to the known object according to the spatial attitude of the known object and the incident angle of the radar beam relative to the spatial coordinates of the known object;

[0073] A scattering energy extraction module for the object to be measured, configured to obtain the scattering energy of the object to be measured based on the spaceborne SAR image object backscattering mapping database, according to the acquisition parameters of the spaceborne SAR image and the incident angle of the radar beam relative to the object to be measured.

[0074] It can be seen from the above technical solutions that the present invention discloses a method and device for extracting the backscattering energy of an object based on an SAR image. Compared with the prior art, the present invention has the following advantages:

[0075] 1. It can directly utilize the existing massive spaceborne SAR image data to obtain the backscattering energy of the observed object at a specific incident angle. The method is much more efficient and economical than laboratory simulation measurement, and the acquisition conditions of the backscattering energy are closer to the actual application environment and can directly assist in actual applications. At the same time, compared with the electromagnetic simulation method, the numerical reliability and accuracy of the backscattering energy extracted by the present invention are higher;

[0076] 2. It has high-precision geometric calculation ability, can accurately calculate the relative geometric relationship of the attitude of the observed object with respect to the satellite platform, and improve the reliability and accuracy of the extraction and application of the backscattering energy.

[0077] 3. At the same time, the on-board SAR image observation object backscattering mapping database designed in this application covers parameters such as synthetic aperture length and polarization mode that may affect the scattering energy of the observed object while recording the results of the backscattering energy extraction. It also records the theoretical scattering information of the observed object obtained from existing theoretical research or simulation, providing a reference for method comparison, error analysis, and correction of measured energy values, and providing effective and powerful data support for the research and application of the backscattering of the observed object. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0079] Figure 1 It is a flowchart of the method for extracting the backscattering energy of the observed object based on the SAR image of the present invention.

[0080] Figure 2 It is a geometric schematic diagram of satellite motion and synthetic aperture radar imaging in the sub-satellite point space coordinate system.

[0081] Figure 3 It is a schematic diagram of the observed object center space coordinate system. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0082] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0083] Spaceborne SAR is an important means of microwave remote sensing in China at present. SAR images are also the main carriers of the electromagnetic scattering information of the observed objects in microwave remote sensing. The image values are directly related to the backscattering ability of the observed objects. Compared with electromagnetic simulation and laboratory measurement, SAR images can provide more general, more real and more practical backscattering data closer to the actual environment. At the same time, with the development and deployment of China's SAR system, it can provide a large amount of data support, and the data acquisition and experimental costs are lower than those in the laboratory. Therefore, the backscattering energy of the observed objects extracted from SAR images has practical application significance.

[0084] However, different from laboratory conditions, the objects observed by spaceborne SAR cannot be placed in a pre-specified posture. At the same time, the imaging geometry of spaceborne SAR is complex, and the posture calculation of the observed objects is complex. Therefore, even though the backscattering intensity of the observed objects can be directly read from SAR images, the incident angle of the electromagnetic wave emitted from the satellite relative to the observed objects seriously affects the reference and application value of the obtained backscattering energy of the observed objects. Therefore, accurately obtaining the incident angle of the radar beam is crucial for the extraction of the backscattering energy of the observed objects.

[0085] In response to this, the embodiments of the present invention disclose a method and device for extracting the backscattering energy of observed objects based on SAR images. By obtaining the incident angle of the electromagnetic wave relative to the observed objects in an accurate spatial analysis manner, and then constructing a database by combining the scattering energy corresponding to the incident angle, the present invention can quickly obtain the backscattering energy of the observed objects at a specific incident angle based on the database and a large number of spaceborne SAR images.

[0086] Embodiment 1

[0087] The method for extracting the backscattering energy of observed objects based on SAR images disclosed in this embodiment, as Figure 1 shown, includes:

[0088] S1. Determine the spatial incident angle and scattering energy of the radar beam relative to the known observed objects based on the spaceborne SAR images. According to the spatial incident angle and scattering energy, and the acquisition parameters of the spaceborne SAR images, construct a backscattering mapping database of the observed objects in the spaceborne SAR images;

[0089] S2. Based on the backscattering mapping database of the observed objects in the spaceborne SAR images, obtain the scattering energy of the to-be-detected observed objects according to the acquisition parameters of the spaceborne SAR images and the spatial incident angle of the radar beam relative to the to-be-detected observed objects.

[0090] In this embodiment, before constructing the backscattering mapping database of the observed objects in the spaceborne SAR images, it is preferably to obtain the system parameters of the spaceborne SAR image system, the spaceborne SAR image data, and the information of the known observed objects in the image, so as to facilitate the subsequent extraction and calculation of the spatial incident angle and scattering energy.

[0091] Among them, the SAR image system parameters at least include: satellite altitude h, radar frequency f, polarization mode, beam center squint angle θ sq , beam center depression angle θ l , range resolution δ ra , synthetic aperture length L, and the imaging algorithm adopted.

[0092] The SAR image data at least includes: an image in single-look complex data format or intensity format that has been geometrically corrected but not geocoded, and the range coordinate (column coordinate of the image) N of the SAR image pixel corresponding to the radar beam center 0 ,

[0093] The known observation object information in the image at least includes: observation object category, observation object name, and the corresponding pixel set and pixel coordinates of the observation object in the SAR image.

[0094] The implementation process of step S1 is mainly described below:

[0095] In step S1, the steps of determining the incident angle of the radar beam relative to the known observation object based on the spaceborne SAR image include:

[0096] S11. According to the spaceborne SAR image system parameters, use the synthetic aperture radar imaging geometry and satellite motion geometry relationship to determine the spatial geometric coordinates T of the intersection point of the satellite radar beam center and the ground 0 ; The calculation method includes:

[0097] S111. Establish a space coordinate system with the satellite sub-satellite point as the origin;

[0098] According to the spaceborne SAR image system parameters, establish a space coordinate system with the sub-satellite point in the system parameters as the origin, where the sub-satellite point O is the origin of the space coordinate system, the satellite motion azimuth direction v in the system parameters is used as the y-axis, the range direction in the system parameters is used as the x-axis, and the normal direction of the ground plane is used as the z-axis. The space coordinate system is called the sub-satellite point space coordinate system or sub-satellite point coordinate system. The x, y, and z axes of the sub-satellite point coordinate system satisfy the right-hand rule. The relationship between the sub-satellite point space coordinate system and satellite motion and synthetic aperture radar imaging geometry is as Figure 2 shown;

[0099] In Figure 2 , O represents the origin of the space coordinate system (sub-satellite point); S represents the satellite position, coordinates: (0, 0, h); v represents the unit direction vector of satellite motion, (0, 1, 0); A - B represents the intersection line of the beam and the ground, that is, the beam range; T represents the observed object, coordinates: (x, y, 0); ST 0 represents the beam center; θ sq represents ∠X0 -S-T 0 , is the oblique viewing angle of the beam center; θ 1 represents ∠O-S-X 0 , is the downward viewing angle of the beam center; T 0 represents the intersection point of the beam center and the ground; X 0 represents T 0 represents the range projection point, with coordinates (xo, 0, 0); Y represents T 0 The azimuth projection point, with coordinates (0, y, 0); X represents the range projection point of T, with coordinates (x, 0, 0);

[0100] S112. According to the satellite altitude and the downward viewing angle θ of the beam center l , obtain the abscissa of the intersection point's space coordinates; specifically, according to the satellite altitude h and the downward viewing angle θ in the parameters of the spaceborne SAR image system l , calculate the range space coordinate x of the intersection point of the beam center and the ground according to the following formula 0 of T 0 ,

[0101] R 1 = OX 0 = x 0 = h × tan(θ l )

[0102] where the symbol OX 0 represents the length of the line segment between points O and X 0 , and the value is represented by R 1 . According to the geometric relationship shown in Figure 2 , it can be known that the value of R 1 is the range coordinate (abscissa) X of point T 0 ; 0 ;

[0103] S113. According to the satellite altitude h, the downward viewing angle θ of the beam center, l , and the oblique viewing angle θ of the beam center sq , calculate the azimuth space coordinate of the intersection point of the beam center and the ground.

[0104] That is, first calculate the intermediate variables R 2 and R 3 :

[0105]

[0106] Subsequently, according to the intermediate variables R 2 and R 3 , calculate the space coordinates (x 0 , y) of the intersection point of the beam center and the ground;

[0107]

[0108] S12. Obtain the spatial coordinates of the known object according to the intersection spatial coordinates; in this embodiment, the spatial coordinates of the known object are (x, y, 0), where x is obtained by the following steps, and y is the same as the ordinate in the intersection spatial coordinates;

[0109] S121. Extract the minimum circumscribed rectangle of the known object in the spaceborne SAR image. The four corner points of the rectangle are represented by the row and column numbers of the corresponding image pixel positions. The coordinates represented by the pixel row and column numbers are called pixel coordinates, where the row coordinate corresponds to the azimuth direction and the value range is from 1 to the image height H, and the column coordinate N corresponds to the range direction and the value range is from 1 to the image width W. Take the central pixel coordinate of the minimum circumscribed rectangle as the pixel coordinate of the known object in the SAR image, and use N T to represent;

[0110] S122. Obtain the coordinates of the SAR image pixel point corresponding to the beam center according to the system parameters of the spaceborne SAR image, and calculate the range-direction pixel difference ΔN between the pixel coordinates of the known object and the pixel coordinates of the beam center. If the corresponding pixel point coordinates are not clearly given in the system parameters, take the midpoint of the image range direction as the coordinates of the SAR image pixel point corresponding to the beam center:

[0111] ΔN = N T - N 0

[0112] In the formula, N 0 is the range-direction pixel coordinate of the SAR image corresponding to the radar beam center, and N T is the range-direction pixel coordinate of the known object in the SAR image.

[0113] S123. Calculate the range-direction spatial distance of the spatial position of the known object relative to the intersection of the beam center and the ground according to the range-direction resolution δ ra in the system parameters of the spaceborne SAR image, and obtain the spatial coordinates of the known object in space. The calculation method is as follows:

[0114] x = x 0 + δ ra × ΔN

[0115] The finally obtained spatial coordinates of the object are (x, y, 0).

[0116] S13. Calculate the spatial incident angle and slant range R T (the spatial distance between the satellite and the object) of the radar beam relative to the spatial point where the known object is located, where the incident angle includes the incident azimuth angle with the incident pitch angle θ T ;

[0117] In this embodiment, referring to Figure 2 , the incident azimuth angle refers to ∠Y-T-O. Since is parallel to the line segment on the X-axis, this angle is numerically equal to ∠T-O-X. That is, ∠Y-T-O can be obtained by calculating ∠T-O-X. In the triangle TOX in the XOY plane of Figure 2 , according to the trigonometric relationship and the spatial coordinates of the observed object point, the tangent value of ∠T-O-X is the ratio of the length x of the opposite side to the length y of the adjacent side . Therefore, the incident azimuth angle

[0118]

[0119] Furthermore, according to the Pythagorean theorem, in the triangle TOX in the XOY plane of Figure 2 , calculate its hypotenuse This hypotenuse is also the base of the inclined triangle TSX, and its length is represented by R 4 :

[0120]

[0121] The incident pitch angle θ T refers to the angle between and the normal of the observed object point. According to the parallel relationship between the normal of the observed object point and the line segment on the z-axis, it can be known that this angle is numerically equal to ∠T-S-O. And the tangent of ∠T-S-O can be calculated in the triangle TSX, and its tangent is equal to the ratio of the length R of the opposite side 4 to the length h of the adjacent side . Therefore, θ T can be calculated by the arctangent function:

[0122]

[0123] Furthermore, in the triangle TSX, calculate the length of the hypotenuse . This length is the distance from the satellite to the observation point, that is, the slant range R T :

[0124]

[0125] where R4 is an intermediate variable.

[0126] S14. Obtain the spatial incident angle of the radar beam relative to the known object based on the spatial attitude of the known object and the incident angle of the radar beam relative to the spatial coordinates of the known object.

[0127] The specific steps are as follows:

[0128] S141. Establish an object-centered coordinate system with the known object as the origin. In this embodiment, taking the known object in the image as the origin, a right-handed spatial coordinate system is established with the orientation of the object obtained by on-site measurement as the positive direction of the x-axis and the ground normal as the positive direction of the z-axis. This coordinate system is called the object-centered spatial coordinate system or the object-centered coordinate system. The object-centered coordinate system is as shown in Figure 3 shown;

[0129] Figure 3 where x represents the x-axis coordinate, and its direction is the same as the orientation of the object; n represents the normal of the object. Among them, n yz , n xz , n xy are the projection points of the normal n on the planes yTZ, xTz, and xTy respectively; b represents the beam direction vector, which is parallel to the ST connection line; is ∠O-T-Xs, representing the beam azimuth angle in the spherical coordinate system; θ s is Lz-T-S, representing the beam elevation angle in the spherical coordinate system; Xs represents the projection of the sub-satellite point on the x-axis, with coordinates (xs, 0, 0); Ys represents the projection of the sub-satellite point on the y-axis, with coordinates (0, ys, 0);

[0130] Then, measure the object attitude and calculate the spatial incident angle of the radar beam relative to the object in the object-centered coordinate system.

[0131] In this embodiment, according to whether the angle of the satellite relative to the object normal can be directly measured by the instrument during the measurement of the object spatial attitude and when the beam passes, two calculation strategies are adopted, which are as follows:

[0132] S142. When the relative position relationship between the object and the satellite is known and the angle of the satellite relative to the object normal can be directly measured, directly use the angle measuring instrument to measure the angle between the object normal and the satellite when the beam passes to obtain the spatial incident elevation angle θ of the radar beam relative to the object. When using the instrument for measurement, taking the protractor as an example, place the protractor at the position of the object, adjust its attitude to be the same as that of the object, and align the scale side with the position of the satellite when the beam passes, and record the elevation angle θ by reading the scale;

[0133] According to the object orientation φ D and the incident azimuth angle φ T in the sub-satellite point coordinate system, calculate the spatial incident azimuth angle φ of the radar beam relative to the object. The calculation method is as follows:

[0134]

[0135] Among them, the azimuth angle of the observed object represents the included angle between the orientation of the observed object and the satellite's running direction. When the orientation of the observed object is perpendicular to the satellite's running direction and points to one side of the satellite, it is 0. If the observed object rotates clockwise it is positive, and if it rotates counterclockwise it is negative, and its value range is from -π to +π.

[0136] S143. When the relative position relationship between the observed object and the satellite is unknown and the normal angle of the satellite relative to the observed object cannot be directly measured, a known actual space attitude coordinate system of the observed object is established, and based on the coordinates of the satellite in the known actual space attitude coordinate system of the observed object, the spatial incident azimuth angle and pitch angle of the radar beam relative to the observed object are obtained.

[0137] The specific steps are as follows:

[0138] 1. According to the incident azimuth angle and pitch angle θ s of the satellite in the central coordinate system of the known observed object, convert the satellite coordinates in the star point coordinate system into the coordinates in the central coordinate system of the known observed object;

[0139] 1.1 Among them, according to the orientation φ D of the observed object and the incident azimuth angle φ T in the sub-satellite point coordinate system, calculate the azimuth angle φ s and pitch angle θ s of the satellite in the central coordinate system of the observed object. The calculation method is as follows:

[0140]

[0141] Among them, the azimuth angle φ D of the observed object represents the included angle between the orientation of the observed object and the satellite's running direction. When the orientation of the observed object is perpendicular to the satellite's running direction and points to one side of the satellite, φ D is 0. If the observed object rotates clockwise, φ D is positive, and if it rotates counterclockwise, φ D is negative. The value range of φ D is from -π to +π. The pitch angle θ s is complementary to the pitch angle of the satellite relative to the observed object in the sub-satellite point coordinate system;

[0142] 1.2 Convert the satellite coordinates (0, 0, h) in the sub-satellite point coordinate system into the coordinates (x s , y s , zs ) The calculation method is as follows:

[0143]

[0144] In the formula, (x s , y s , z s ) are the satellite coordinates in the known observation object's central coordinate system; and z s = h;

[0145] 2. By successively rotating the known observation object's central coordinate system around the x-axis and y-axis, establish the actual spatial attitude coordinate system of the known observation object; and determine the coordinates of the satellite in the actual spatial attitude coordinate system of the known observation object;

[0146] 2.1 According to the known information of the observation object or on-site measurement, obtain the current attitude of the observation object, and assume that the current actual attitude of the observation object is obtained by successively rotating around the x-axis and y-axis in the origin coordinate system of the observation object with the normal of the observation object perpendicular to the ground plane and the observation object pointing to the positive direction of the y-axis of the observation object's central coordinate system. Among them, the target initial attitude is towards the positive direction of the x-axis and the normal is the positive direction of the z-axis. First, rotate by an angle α with the x-axis as the rotation axis, as Figure 3 , at this time the target normal also rotates in the yTz plane, changing from parallel to the z-axis to pointing from the origin to n yz , that is, Figure 3 The ∠z-T-n in yz is the rotation angle α, and the corresponding rotation matrix R x (α) is:

[0147]

[0148] Subsequently, the target further rotates around the y-axis, and the rotation angle is β, as Figure 3 , the target normal also rotates out of the yTz plane around the y-axis, that is, from rotating around the y-axis to n, Figure 3 The corresponding ∠z-T-n in xz is the rotation angle β, and the rotation matrix R y (β) is:

[0149]

[0150] 2.2 According to the actual attitude of the observation object, establish a new right-handed coordinate system with the direction of the observation object as the y-axis and the normal of the observation object as the z-axis. And the coordinates (x S ′, y S ′, z S ′) of the satellite in the new coordinate system can be calculated by rotation through the coordinates in the original observation object's central coordinate system:

[0151]

[0152] This formula is equivalent to first rotating the newly established coordinate system by -β around the y-axis and then by -α around the x-axis, so that the new coordinate system overlaps with the central coordinate system of the observed object. The coordinates (x s , y s , h) of the satellite in the central coordinate system of the observed object also undergo the same rotation, and the coordinates (x S ′, y S ′, z S ′) equivalent to those in the new coordinate system are calculated.

[0153] 3. Converting these coordinates to spherical coordinates can obtain the spatial incident elevation angle and azimuth angle of the satellite relative to the observed object. The calculation method is as follows:

[0154]

[0155] In the formula, is the spatial incident azimuth angle of the radar beam relative to the observed object, and θ is the elevation angle.

[0156] Furthermore, in step S1, the scattered energy of the known observed object obtained includes the backscattered energy E and the maximum scattered energy P. Among them, E is the total energy of the image within the minimum circumscribed rectangle, and P is the maximum energy of the image within the minimum circumscribed rectangle. The acquisition formulas are respectively:

[0157]

[0158] In the formula, I represents the image amplitude value within the minimum circumscribed rectangle of the known observed object, H and W are the height and width of the minimum circumscribed rectangle, and E and P characterize the energy and are related to the backscattering ability of the observed object.

[0159] In an exemplary embodiment, the present application further constructs a backscattering mapping database of the observed object in the spaceborne SAR image according to the spatial incident angle and scattered energy, as well as the acquisition parameters of the spaceborne SAR image; that is, according to the spaceborne SAR image system parameters, known observed object information, the imaging geometric parameters, and the SAR image data, establish the relationship between the azimuth angle θ of the satellite relative to the observed object and the total energy E of the minimum circumscribed rectangle of the observed object image, and constitute the backscattering mapping database of the observed object in the spaceborne SAR image.

[0160] In this embodiment, the on-orbit SAR image observed object backscattering mapping database at least includes: synthetic aperture radar satellite name, imaging mode, radar frequency, polarization mode, synthetic aperture length, imaging algorithm, observed object category, observed object name, observed object image slice, spatial distance between the satellite and the observed object, beam incident azimuth angle, beam incident elevation angle, maximum scattering energy of the observed object, backscattering energy of the observed object, and theoretical backscattering energy of the observed object. Specifically, refer to the following table;

[0161]

[0162] In the present invention, the information generated by each observed object sample constitutes an "observed object backscattering mapping table", and the on-orbit SAR image observed object backscattering mapping database is formed by connecting multiple encoded "observed object backscattering mapping tables" end to end.

[0163] The present invention establishes a formatted database, which can provide a technical and data basis for the electromagnetic scattering analysis, research, and application of observed objects.

[0164] Embodiment 2

[0165] This embodiment also provides a device for extracting the backscattering energy of an observed object based on an SAR image, including:

[0166] A backscattering mapping database construction module, configured to determine the incident angle and scattering energy of the radar beam relative to a known observed object based on the on-orbit SAR image, and construct an on-orbit SAR image observed object backscattering mapping database according to the incident angle and scattering energy, and the acquisition parameters of the on-orbit SAR image;

[0167] Among them, the step of determining the incident angle of the radar beam relative to a known observed object based on the on-orbit SAR image includes:

[0168] S11. Determine the spatial coordinates of the intersection point between the satellite radar beam center and the ground;

[0169] S12. Obtain the spatial coordinates of the known observed object according to the intersection point spatial coordinates;

[0170] S13. Calculate the incident angle of the radar beam relative to the spatial coordinates of the known observed object;

[0171] S14. Obtain the incident angle of the radar beam relative to the known observed object according to the spatial attitude of the known observed object and the incident angle of the radar beam relative to the spatial coordinates of the known observed object;

[0172] The scattered energy extraction module for the object to be measured is used to obtain the scattered energy of the object to be measured based on the backscattering mapping database of the object in the spaceborne SAR image, according to the acquisition parameters of the spaceborne SAR image and the incident angle of the radar beam relative to the object to be measured.

[0173] Since the implementation principle of this device is the same as that of the above method, it will not be repeated here.

[0174] In this specification, the various embodiments are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the various embodiments, reference can be made to each other. For the device disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method section.

[0175] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for extracting backscattered energy of an observed object based on SAR images, characterized in that: include: S1. Extracting the spatial incident angle and corresponding scattering energy of the radar beam relative to a known observation object based on the spaceborne SAR image, and constructing a backscattering mapping database of the spaceborne SAR image observation object according to the spatial incident angle and the scattering energy, and acquisition parameters of the spaceborne SAR image; The step of determining the spatial incident angle of the radar beam relative to the known observation object based on the spaceborne SAR image includes: S11, determining the spatial coordinates of the intersection of the satellite radar beam center and the ground; S12, obtaining the spatial coordinates of the known observed object according to the spatial coordinates of the intersection point; S13, calculating the incident angle of the radar beam relative to the known spatial coordinates of the observed object; S14, obtaining a spatial incident angle of the radar beam relative to the known observed object according to the spatial posture of the known observed object and the incident angle of the radar beam relative to the spatial coordinates of the known observed object; S2. Based on the spaceborne SAR image observation object backscatter mapping database, the scattering energy of the object to be measured is obtained according to the acquisition parameters of the spaceborne SAR image and the spatial incident angle of the radar beam relative to the object to be measured.

2. The method for extracting backscattered energy of an observed object based on a SAR image according to claim 1, characterized in that: In S11, the step of determining the spatial coordinates of the intersection of the radar beam center and the ground includes: S111, establishing a space coordinate system with the satellite sub-satellite point as the origin; S112, obtaining the horizontal coordinate of the spatial coordinate of the intersection point according to the satellite altitude and the viewing angle below the beam center; S113. Obtain the ordinate of the spatial coordinates of the intersection point according to the satellite altitude, the downward viewing angle of the beam center, and the oblique viewing angle of the beam center.

3. The method for extracting backscattered energy of an observed object based on SAR images according to claim 1, characterized in that: In S12, the spatial coordinates of the known observed object are (x, y, 0), where x is obtained by the following steps, and y is the same as the ordinate in the spatial coordinates of the intersection point; S121, extracting the minimum bounding rectangle of a known observed object in the spaceborne SAR image, and using the central pixel coordinates of the minimum bounding rectangle as the pixel coordinates of the known observed object in the SAR image; S122, obtaining the distance pixel difference between the pixel coordinates of the known observed object in the SAR image and the pixel point coordinates corresponding to the center of the radar beam in the SAR image; S123, obtaining the horizontal coordinate in the spatial coordinate of the known observed object according to the distance pixel difference according to the following formula; x=x0+δ ra ×(N T -N0) Where N0 is the pixel coordinate of the radar beam center in the SAR image, N T is the distance pixel coordinate of the known observed object in the SAR image, δ ra is the distance resolution, and x0 is the horizontal coordinate in the intersection space coordinates.

4. The method for extracting backscattered energy of an observed object based on SAR images according to claim 3, characterized in that: In S13, the incident angle includes the incident azimuth angle With the incident pitch angle θ T , the calculation methods are: Where h represents the altitude of the satellite in the space coordinate system.

5. The method for extracting backscattered energy of an observed object based on SAR images according to claim 4, characterized in that: In S13, the spatial distance between the satellite and the observed object is also calculated as follows:

6. The method for extracting backscattered energy of an observed object based on SAR images according to claim 1, characterized in that: S14 includes the following steps: S141. Establish the central coordinate system of the observed object with the known observed object as the origin; S142. When the spatial attitude of the known observed object is known and measurable, the angle between the measured normal direction of the observed object and the satellite when the beam passes through is used as the spatial incidence pitch angle of the radar beam relative to the observed object, and the sum of the incident azimuth angle of the radar beam relative to the known observed object spatial coordinates and the known observed object orientation azimuth angle is used as the spatial incidence azimuth angle of the radar beam relative to the observed object; S143. When the spatial attitude of the known observed object is unknown and cannot be measured, an actual spatial attitude coordinate system is established according to the spatial attitude of the known observed object, and the spatial incident azimuth and pitch angle of the radar beam relative to the known observed object are obtained according to the coordinates of the satellite in the actual spatial attitude coordinate system.

7. The method for extracting backscattered energy of an observed object based on SAR images according to claim 6, characterized in that: S143 includes: According to the incident azimuth and elevation angle of the satellite in the known observation center coordinate system, the satellite coordinates in the star point coordinate system are converted into coordinates in the observation center coordinate system; By rotating the center coordinate system of the observed object around the x-axis and the y-axis in sequence, an actual space attitude coordinate system based on the known observed object is established; and the coordinates of the satellite in the actual space attitude coordinate system are determined; According to the coordinates of the satellite in the actual space attitude coordinate system, the spatial incident azimuth and pitch angle of the radar beam relative to the observed object are obtained.

8. The method for extracting backscattered energy of an observed object based on SAR images according to claim 1, characterized in that: It is known that the scattering energy of the observed object includes backscattering energy and maximum scattering energy; the acquisition formulas are: Where E is the backscattering energy of the known observation object, P is the maximum scattering energy of the known observation object, I represents the image amplitude value within the minimum bounding rectangle of the known observation object, and H and W are the height and width of the minimum bounding rectangle.

9. The method for extracting backscattered energy of an observed object based on SAR images according to claim 1, characterized in that: In S1, the information generated by each known observation object sample constitutes an observation object backscatter mapping table, and the spaceborne SAR image observation object backscatter mapping database is composed of multiple encoded observation object backscatter mapping tables connected end to end.

10. A device for extracting backscattered energy of an observed object based on SAR images, characterized in that: A method for extracting backscatter energy of an observed object based on a SAR image according to any one of claims 1 to 9, comprising: A backscatter mapping database construction module is used to determine the spatial incident angle and corresponding scattering energy of the radar beam relative to the known observation object based on the spaceborne SAR image, and to construct a backscatter mapping database of the spaceborne SAR image observation object according to the spatial incident angle and the scattering energy, and acquisition parameters of the spaceborne SAR image; The step of determining the incident angle of the radar beam relative to the known observation object based on the spaceborne SAR image includes: S11, determining the spatial coordinates of the intersection of the satellite radar beam center and the ground; S12, obtaining the spatial coordinates of the known observed object according to the spatial coordinates of the intersection point; S13, calculating the incident angle of the radar beam relative to the known spatial coordinates of the observed object; S14, obtaining a spatial incident angle of the radar beam relative to the known observed object according to the spatial posture of the known observed object and the incident angle of the radar beam relative to the spatial coordinates of the known observed object; The module for extracting scattered energy of the object to be measured is used to obtain the scattered energy of the object to be measured based on the backscatter mapping database of the object to be measured in the spaceborne SAR image, according to the acquisition parameters of the spaceborne SAR image and the spatial incident angle of the radar beam relative to the object to be measured.

Images