Calibration ball device for hyperspectral characteristic extraction of high-resolution SAR (Synthetic Aperture Radar) image
By designing a calibration ball device including metal spheres, base brackets and wave absorbing materials, the problem that existing SAR calibration equipment cannot meet the RCS flatness requirements required for high-resolution SAR overspectral characteristics extraction is solved, and RCS response flatness and band sharing within the distance bandwidth is realized, reducing design difficulty and cost.
Patent Information
- Application Number
- CN202510160575.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-13
- Publication Date
- 2025-06-27
AI Technical Summary
Existing SAR calibration equipment cannot meet the RCS flatness requirements within the distance bandwidth required for high-resolution SAR overspectral characteristics extraction, and there are problems such as high design difficulty, high cost, and difficulty in sharing bands.
A calibration ball device including a metal sphere, a base bracket and a wave absorbing material was designed. The metal sphere was welded by watermelon flap steel plate. The base bracket adopts a four-point support structure, and the wave absorbing material was used to reduce the additional scattering of the base bracket and the clutter of the ground background.
The RCS response is flat within the distance bandwidth, reducing design difficulty and cost, suitable for SAR overspectral characteristics extraction of multiple bands, and band sharing is realized.
Smart Images
Figure CN120214709A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of radar detection, and particularly relates to a calibration sphere device for extracting the hyperspectral characteristics of high-resolution SAR images. Background Technique
[0002] Synthetic Aperture Radar (SAR) is a microwave remote sensing imaging radar, which has wide applications in both civilian and military fields. SAR uses the principles of range broadband signal pulse compression and azimuth synthetic aperture to achieve two-dimensional focusing and obtain a two-dimensional grayscale image of the ground object. The grayscale of this image corresponds to the center frequency point, and the scattering intensity of the ground object under the forward-looking observation geometry, that is, the Radar Cross Section (RCS) or the backscattering coefficient, which approximately holds at narrowband and low resolution. For the application of ground object classification and recognition based on the grayscale of SAR images, first, through the radiometric calibration of the SAR system, an approximate quantitative relationship between the image grayscale and the RCS or backscattering of the ground object at the center frequency point of the SAR system is established, so as to complete the ground object classification and recognition based on the RCS or backscattering coefficient extracted from the SAR image.
[0003] The high-resolution SAR system has a relatively wide range bandwidth, resulting in the above approximate quantitative relationship at the center frequency point no longer being strictly valid. The difference in the center frequency points of the range sub-bands is relatively large, resulting in relatively large differences in the scattering characteristics of the ground object, which also brings potential applications for classification and recognition using the spectral characteristics of the ground object. The extraction of the hyperspectral characteristics of high-resolution SAR images refers to using the spectral characteristics (so-called hyperspectrum) of the ground object at the center frequency points of each range sub-band rather than the amplitude at the center frequency point of the entire bandwidth for classification and recognition, which has important potential applications. However, the prerequisite for the application of hyperspectral characteristics is to have a SAR hyperspectral radiometric calibration device with a flat RCS frequency response within the range bandwidth, so as to calibrate and correct the spectral response error of the high-resolution SAR system, and then quantitatively extract the hyperspectral characteristics of the ground object.
[0004] Existing SAR calibration devices are generally passive corner reflectors or active calibrators. A typical passive calibration device such as a trihedral corner reflector has an RCS that increases with the square of the frequency. For example, within the bandwidth of 9 GHz - 10 GHz in the X-band, the fluctuation exceeds 0.9 dB; the active calibrator is also affected by the in-band amplitude-frequency error of active microwave devices, with an in-band fluctuation of about 0.5 dB. Even if complex in-calibration techniques for active calibrators are used, there will still be a random error of 0.3 dB. The design is difficult and the cost is high; theoretically, the RCS of an ideal metal sphere is independent of the wavelength. However, currently, the largest diameter of the metal spheres used for calibration or microwave metrology is 70 cm, and the RCS is too small, only -4 dBm. 2, it cannot meet the calibration requirements of spaceborne SAR, and it is difficult to design and process large-sized spheres. In addition, the metal sphere is an omnidirectional corner reflector and is easily affected by the sphere support and ground background clutter, directly affecting the in-band RCS flatness characteristic.
[0005] In summary, there are some deficiencies in existing calibration devices: (1) The RCS of the passive corner reflector varies greatly with the square of the frequency and cannot meet the requirements of hyperspectral calibration; (2) The active calibrator is affected by the in-band amplitude-frequency error of active microwave devices, with high design difficulty and cost. Moreover, for each SAR band, an active calibrator needs to be designed separately and cannot be shared among bands; (3) Although the existing metal spheres theoretically meet the hyperspectral calibration requirements, their sizes are limited, it is difficult to design and process large-sized ones, and they are easily affected by the support and ground background clutter, directly affecting the in-band RCS flatness characteristic. Summary of the Invention
[0006] The purpose of the present invention is to propose a calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images, thereby solving the problem of a calibration device with flat RCS within the range bandwidth required for extracting hyperspectral characteristics of high-resolution SAR.
[0007] A calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images; comprising: a metal sphere, a base support, and an absorbing material; wherein: the metal sphere is composed of a northern hemisphere and a southern hemisphere, and the entire metal sphere is formed by pressing and welding multiple watermelon-petal-shaped steel plates; the northern hemisphere is larger than the southern hemisphere, and the two are connected by countersunk head screws, and the screws are flush with the spherical surface after connection; a lifting ring and a plug are provided below the southern hemisphere for hoisting the metal sphere; the base support adopts a four-point support structure, and insulating rubber plates are laid at the four support points; the absorbing material is a wedge-shaped absorbing material, which is used to reduce the additional scattering of the base support, ground clutter, and RCS error of the calibration sphere, and at the same time improve the signal-to-clutter ratio of the calibration sphere relative to the background ground objects.
[0008] The metal sphere serves as an omnidirectional reflector of radar signals, reflecting the transmitted signals from the SAR radar to provide a flat RCS response within the range bandwidth.
[0009] The base support is used to stably carry the metal sphere, so that the position of the calibration sphere is fixed during field work.
[0010] The absorbing material is used to reduce the additional scattering of the base support and ground clutter, so that the RCS of the calibration sphere meets the accuracy requirements and the signal-to-clutter ratio required for hyperspectral characteristic extraction.
[0011] 5. A calibration sphere device for extracting the hyperspectral characteristics of high-resolution SAR images according to claim 4, characterized in that the base bracket is fixed on a relatively flat outdoor field on the ground, the entire metal sphere is lifted by a lifting ring and placed on the base bracket, then the lifting ring is removed and a plug is installed, and finally, an absorbing material is laid on the base bracket and around the calibration sphere.
[0012] The metal sphere adopts a hollow sphere structure, and there are reinforcing ribs inside the metal sphere to maintain the strength and rigidity of the overall structure and prevent deformation during subsequent processing and clamping.
[0013] The SAR transmitted signal comes from the top and the upper side, and the reflecting surface of the metal sphere is located in the Northern Hemisphere, and the Northern Hemisphere is larger than the Southern Hemisphere to avoid the influence of the equatorial seam on the RCS accuracy.
[0014] The base bracket is made of Q235 steel plate and coated with an absorbing coating to keep the whole metal sphere off the ground and at a certain height from the ground. The height is greater than the SAR image range resolution cell to avoid the dihedral angle scattering formed by the ground background and the metal sphere and the scattering center of the metal sphere falling into the same range cell. The base bracket adopts a four-point support structure, and insulating rubber sheets are laid at the four support points to protect the metal sphere.
[0015] The absorbing material adopts wedge-shaped flexible polyurethane foam material, and the wedge height is selected according to the SAR working band.
[0016] The metal sphere is fixed on the base bracket, and the insulating rubber sheets at the four support points of the base bracket are used to protect the metal sphere to prevent damage to the surface of the metal sphere during the installation and connection process; part of the absorbing material is wrapped on the base bracket, and part of it is distributed on the ground area not less than 10 resolution cells in front, behind, left and right of the ground projection of the sphere scattering center.
[0017] The beneficial technical effects brought by the technical solution proposed by the present invention include: (1) By utilizing the characteristic that the RCS of the metal sphere does not change with frequency, the device overcomes the fluctuation of the RCS of the existing passive corner reflector with frequency, and reduces the processing difficulty of the whole sphere through the watermelon slice type assembly and welding method, which is suitable for the processing of large spheres with high RCS required for outdoor SAR calibration tests; (2) The device adopts a simple passive mechanical structure form, avoiding the inherent defect of large in-band amplitude-frequency error of active microwave devices of active calibrators, can meet the extraction of hyperspectral characteristics of multiple-band SARs, realize band sharing, and reduce the design difficulty and production cost; (3) The device uses absorbing materials to reduce the influence of additional scattering of the base bracket, avoids the problem that the RCS of the traditional sphere is too small and the signal-to-clutter ratio relative to the ground object background is too low, reduces the influence of outdoor ground object background clutter, and is suitable for high-resolution SAR outdoor tests. Description of the Drawings
[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.
[0019] Figure 1 Structural diagram of a hyperspectral calibration sphere device according to an embodiment of the present invention.
[0020] Figure 2 Schematic diagram of the metal sphere of the hyperspectral calibration sphere device according to an embodiment of the present invention.
[0021] Figure 3 Schematic diagram of the connection of the northern and southern hemispheres and the lifting ring plug of the hyperspectral calibration sphere device according to an embodiment of the present invention.
[0022] Figure 4 Schematic diagram of the base bracket of the hyperspectral calibration sphere device according to an embodiment of the present invention. Specific embodiments
[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.
[0024] First embodiment
[0025] A calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR includes three parts: a metal sphere, a base bracket, and absorbing material. As Figure 1 shown.
[0026] The metal sphere is composed of a northern hemisphere and a southern hemisphere. The entire metal sphere is divided into multiple watermelon-petal-shaped steel plates, which are pressed and then welded together. As Figure 2 shown. The northern hemisphere is slightly larger than the southern hemisphere and is connected by countersunk head screws. After connection, the screws are flush with the spherical surface. A lifting ring and a plug are provided below the southern hemisphere to facilitate the lifting of the metal sphere. As Figure 3 shown.
[0027] The base bracket of the base bracket adopts a four-point support structure, and insulating rubber sheets are laid at the four support points. On the one hand, it can protect the metal sphere and prevent damage to the metal sphere. As Figure 4 shown.
[0028] The microwave absorbing material used is a pyramidal microwave absorbing material, which is used to reduce the additional scattering of the base bracket and reduce the clutter of the ground object background, thereby reducing the RCS error of the calibration sphere introduced by the base bracket and improving the signal-to-clutter ratio of the calibration sphere relative to the background ground object.
[0029] The metal sphere serves as an omnidirectional reflector of radar signals, reflecting the transmitted signal of the SAR radar from the SAR, and providing a flat RCS response within the range bandwidth for high-resolution SAR; the base bracket is used to stably and reliably carry the metal sphere to ensure that the calibration sphere is fixed in position during field work; the microwave absorbing material is used to reduce the additional scattering of the base bracket and the clutter of the ground object background, thereby ensuring the RCS accuracy of the calibration sphere and the signal-to-clutter ratio required for the extraction of hyperspectral characteristics.
[0030] In actual use, first fix the base bracket on a relatively flat field on the ground, then lift the entire metal ball through the lifting ring and place it on the base bracket, remove the lifting ring, install the plug, and finally lay the microwave absorbing material around the base bracket and its periphery of the calibration sphere.
[0031] Second Embodiment
[0032] A calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images includes three components: a metal sphere (1), a base bracket (2), and a microwave absorbing material (3).
[0033] Among them, the metal sphere (1) is used to reflect the transmitted signal from the SAR. The diameter of the whole sphere is 2000 mm. According to the geometric optics RCS prediction method, the designed RCS value is 4.97 dBm. 2 The metal sphere adopts a hollow sphere structure to effectively reduce weight; its southern and northern hemispheres are made of Q235 steel plate material with a thickness of 30 mm to ensure the structural strength; due to the large size of the whole sphere, the metal sphere is divided into multiple watermelon-petal-shaped steel plates, which are pressed and then welded together. Reinforcing ribs are designed inside the metal sphere to ensure the strength and rigidity of the overall structure and prevent deformation during subsequent processing and clamping, as Figure 2 shown. Since the SAR transmitted signal comes from the top and the upper side, the main reflection surface of the metal sphere is located in the northern hemisphere. Therefore, in the design, the northern hemisphere is slightly larger than the southern hemisphere to avoid the influence of the seam near the equator on the RCS accuracy. Four lifting rings are evenly arranged under the southern hemisphere to facilitate the lifting of the metal ball, as Figure 3 shown. The lifting rings are only used during the field layout and installation of the calibration sphere and are removed after installation. The lifting ring holes are filled with metal plugs, and after filling, the lifting ring holes can be flush with the spherical surface.
[0034] The base bracket (2) is used to carry the entire metal sphere. It is made of Q235 steel plate and coated with an absorbing coating on the surface. The metal sphere can be lifted off the ground as a whole and maintained at a certain height from the ground. The height is designed to be greater than a SAR image distance resolution unit. For example, if the SAR distance resolution is 0.3m, the height of the sphere should be greater than 0.3m, thereby avoiding the influence of the dihedral scattering formed by the ground background and the metal sphere and the scattering center of the metal sphere falling into the same distance unit on the RCS accuracy of the calibration sphere. The base bracket adopts a four-point support structure and is welded from Q235 steel plates as a whole. Insulating rubber plates are laid at the four fulcrums to protect the metal sphere and prevent damage to the surface of the metal sphere.
[0035] The absorbing material (3) is used to reduce the additional scattering of the base support, reduce the background clutter of the ground, and improve the signal-to-clutter ratio required for the SAR hyperspectral characteristic extraction. The absorbing material is made of a wedge-shaped flexible polyurethane foam material, which is light in weight and convenient for field use. The area of a single piece is 600mm×600mm. The height of the wedge can be selected according to the SAR working band. For example, for an X-band SAR, a height of 300mm can be selected with a reflection loss greater than 30dB, thereby effectively reducing the influence of the base support scattering and the background clutter of the ground.
[0036] The connection relationship between the above-mentioned components is as follows: the metal sphere (1) is fixed on the base bracket (2), and the metal sphere is protected by insulating rubber plates at the four supporting points of the base bracket to prevent damage to the surface of the metal sphere during the installation and connection process; a part of the absorbing material is wrapped on the base bracket, and a part of the absorbing material is distributed on a ground area of not less than 10 resolution units in front, behind, left and right of the ground projection of the scattering center of the sphere. If the SAR resolution unit is 0.3m×0.3m, the total ground layout area of the absorbing material is not less than 10×2×10×2×0.3m*0.3m=36m 2 .
[0037] The specific working process of this device is as follows: In actual use, first fix the base bracket on a relatively flat outdoor site, install four lifting rings on the southern hemisphere of the metal sphere, use a crane to lift the entire metal sphere through the lifting rings, and place it on the base bracket, remove the lifting rings, install the plugs, and finally lay absorbing materials on the base bracket and surrounding areas of the calibration ball to complete the field installation and deployment of this device.
[0038] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification. The above embodiments only express several implementation manners of the present application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.
Claims
1. A calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images, characterized in that: The device comprises: a metal sphere, a base support, and a wave absorbing material; wherein: The metal sphere is composed of a northern hemisphere and a southern hemisphere. The entire metal sphere is formed by welding a plurality of watermelon-shaped steel plates after being pressed. The northern hemisphere is larger than the southern hemisphere. The two are connected by countersunk screws. After connection, the screws are flush with the spherical surface. A lifting ring and a plug are provided below the southern hemisphere for lifting the metal sphere. The base bracket adopts a four-point support structure, and insulating rubber plates are laid at four supporting points; The absorbing material is a wedge-shaped absorbing material, which is used to reduce the additional scattering of the base bracket, the background clutter of the ground object, and the RCS error of the calibration sphere, and at the same time improve the signal-to-clutter ratio of the calibration sphere relative to the background ground object.
2. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 1, characterized in that: The metal sphere acts as an omnidirectional reflector of the radar signal, reflecting the transmission signal from the SAR radar to provide a flat RCS response within the range bandwidth.
3. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 2, characterized in that: The base bracket is used for stably carrying the metal sphere, so that the calibration ball is fixed in position when working in the field.
4. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 3, characterized in that: The absorbing material is used to reduce the additional scattering of the base bracket and the background clutter of the ground object, so that the RCS of the calibration sphere meets the accuracy requirements and the signal-to-clutter ratio required for the extraction of hyperspectral characteristics.
5. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 4, characterized in that: Fix the base bracket on a relatively flat outdoor site, lift the entire metal ball through the lifting ring and place it on the base bracket, then remove the lifting ring and install the plug, and finally lay the absorbing material on the base bracket and surrounding area of the calibration ball.
6. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 5, characterized in that: The metal sphere adopts a hollow sphere structure, and has reinforcing ribs inside the metal sphere to maintain the strength and rigidity of the overall structure and prevent deformation during subsequent processing and clamping.
7. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 6, characterized in that: The SAR transmission signal comes from the top and the upper side. The reflecting surface of the metal sphere is located in the northern hemisphere, and the northern hemisphere is larger than the southern hemisphere to avoid the influence of the equatorial seam on the RCS accuracy.
8. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 7, characterized in that: The base bracket is made of Q235 steel plate and coated with an absorbing coating on the surface, so that the metal ball is completely separated from the ground and maintains a certain height from the ground. The height is greater than the SAR image distance resolution unit to prevent the dihedral scattering formed by the ground background and the metal sphere and the scattering center of the metal sphere from falling into the same distance unit. The base bracket adopts a four-point support structure, and insulating rubber plates are laid at the four fulcrums to protect the metal sphere.
9. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 8, characterized in that: The wave absorbing material is a wedge-shaped flexible polyurethane foam material, and the height of the wedge is selected according to the SAR working band.
10. The calibration sphere device for extracting hyperspectral characteristics of high-resolution SAR images according to claim 9, characterized in that: The metal sphere is fixed on the base bracket, and is protected by insulating rubber plates at the four fulcrums of the base bracket to prevent damage to the surface of the metal sphere during the installation and connection process; part of the absorbing material is wrapped on the base bracket, and part of it is distributed on a ground area of no less than 10 resolution units in front, behind, left and right of the ground projection of the scattering center of the sphere.