A three-dimensional lunar imaging method based on ground-based radar with optimal image quality

Abstract

The present invention relates to a method for three-dimensional lunar imaging based on a ground-based radar with optimal image quality, and belongs to the technical field of synthetic aperture radar. The method comprises the following steps: Step 1: selecting two tracks of radar echo data that meet interferometric processing requirements for interferometric imaging, obtaining an elevation information estimate of the observation scene, and obtaining a DEM model; Step 2: obtaining SAR three-dimensional imaging results of lunar surface targets under multiple radar motion tracks using a three-dimensional BP imaging algorithm with inter-track non-coherent superposition; Step 3: interpolating the DEM model obtained in Step 1 to construct an altitude window function that matches the BP imaging grid in Step 2; and Step 4: performing altitude windowing on the SAR three-dimensional imaging result in Step 2 according to the DEM-based altitude window function obtained in Step 3, to obtain a final imaging result.

Description

Technical Field

[0001] The present invention relates to a three-dimensional moon imaging method based on a ground-based radar with optimal image quality, and belongs to the technical field of synthetic aperture radar. Background Art

[0002] Ground-based synthetic aperture radar (GB SAR) is a radar system built on Earth for high-resolution lunar imaging. It achieves lunar observation by transmitting and receiving electromagnetic waves from Earth. Compared to other lunar observation methods such as optical remote sensing, ground-based radar observations offer advantages such as all-day, all-weather, low-cost, and high reusability. Currently, GB SAR demonstrates tremendous potential. For example, it can achieve high-resolution lunar imaging, thus supporting lunar geological science research and the formulation and implementation of lunar exploration projects. However, as the requirements of corresponding scientific research tasks continue to increase, these applications require GB SAR to obtain more comprehensive and accurate lunar surface information, placing more stringent requirements on GB SAR lunar imaging systems.

[0003] Currently, GBSAR lunar imaging is limited to two-dimensional (2D) and interferometric imaging, with no published results for three-dimensional (3D) lunar imaging. 2D imaging cannot obtain lunar surface elevation information, while interferometric imaging cannot obtain scattering information from the observed scene and may result in altitude overlap. Both have limitations. In GBSAR lunar imaging, the relative motion trajectory of the radar is generated by the relative motion between the Earth and the Moon. By selecting multiple radar trajectories, both azimuth and elevation synthetic apertures can be simultaneously acquired, enabling ground-based radars to achieve 3D lunar imaging. However, due to the limitations of the Earth-Moon observation geometry, the spatial distribution of the radar trajectories results in significant sparseness and non-uniformity in elevation sampling, significantly degrading the elevation quality of 3D imaging. Currently, no suitable Earth-Moon 3D imaging algorithm exists to support GBSAR 3D lunar imaging. Summary of the Invention

[0004] To address this problem, the present invention proposes a new method for three-dimensional lunar imaging based on ground-based radar with optimal image quality, which can obtain the three-dimensional morphological features of the lunar surface.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for three-dimensional lunar imaging based on ground-based radar with optimal image quality, comprising:

[0007] Step 1: Select two radar echo data tracks that meet the interferometric processing requirements for interferometric imaging, obtain the elevation information estimate of the observation scene, and obtain the DEM model;

[0008] Step 2: Obtain SAR 3D imaging results of lunar surface targets under multiple radar motion tracks through the 3D BP imaging algorithm of inter-track non-coherent superposition;

[0009] Step 3: interpolate the DEM model obtained in step 1 and construct a height window function that matches the BP imaging grid in step 2;

[0010] Step 4: According to the DEM-based altitude window function obtained in step 3, the SAR three-dimensional imaging result in step 2 is windowed in the altitude direction to obtain the final imaging result.

[0011] Furthermore, the present invention selects two tracks of radar echo data that meet the interferometric processing requirements by screening vertical baselines within a critical baseline to obtain two tracks of radar echo data that meet the interferometric processing requirements.

[0012] Furthermore, the calculation formula of the critical baseline of the present invention is:

[0013]

[0014] Where λ is the system wavelength, R is the distance between the radar and the observation center in the main trajectory, and θ max is the maximum pitch angle of the radar relative to the observation center in the main track, ρ r is the range resolution, and η is the terrain slope angle.

[0015] Furthermore, the height window function of the present invention is:

[0016]

[0017] Among them, w B,L (·) is the elevation window function corresponding to latitude B and longitude L, DEM B,L is the known digital elevation information at latitude B and longitude L, ρ h is the height resolution, and H is the high-level information.

[0018] Furthermore, the present invention also adjusts the height position of the window function based on the criterion of optimal image quality to suppress irregular high side lobes in the height direction.

[0019] Beneficial effects:

[0020] First, the present invention uses the lunar DEM information obtained by interferometry to perform altitude windowing on the 3D BP imaging results, and adjusts the position of the altitude window function based on the criterion of optimal image quality, effectively overcoming the difficulties brought by the non-uniform and sparse altitude sampling points of GB SAR to 3D Earth-Moon imaging.

[0021] Second, the computer simulation results of the present invention show that the method is feasible and has excellent performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 Schematic diagram of DEM windowing based on BP grid of the present invention;

[0024] Figure 2 This is a schematic diagram of one-dimensional stretch target setting in an embodiment of the present invention;

[0025] Figure 3(a) to Figure 3(b) 3(a) is the imaging result before and after DEM windowing of the one-dimensional extended target in the embodiment of the present invention, and FIG3(b) is the imaging result after DEM windowing.

[0026] Figure 4(a) to Figure 4(b) 4(a) is a schematic diagram of a three-dimensional scene, and FIG4(b) is a top view of the scene;

[0027] Figure 5(a) to Figure 5(d) 5(a) is a three-dimensional imaging result diagram before and after DEM windowing in an embodiment of the present invention, wherein FIG5(b) is a top view of FIG5(a), FIG5(c) is a three-dimensional imaging result diagram after DEM windowing, and FIG5(d) is a top view of FIG5(c). DETAILED DESCRIPTION

[0028] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0029] It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments may be combined with each other; and, based on the embodiments in this disclosure, all other embodiments obtained by persons of ordinary skill in the art without creative work are within the scope of protection of this disclosure.

[0030] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on this disclosure, it should be understood by those skilled in the art that an aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects described herein can be used to implement an apparatus and / or practice a method. In addition, other structures and / or functionalities other than one or more of the aspects described herein can be used to implement this apparatus and / or practice this method.

[0031] First, two radar echo data tracks that meet the interferometric processing requirements are selected for interferometric SAR imaging. According to the requirements of spatial coherence, the vertical baselines within the critical baseline are screened to obtain a track combination that meets the interferometric processing requirements. The calculation formula for the critical baseline is:

[0032]

[0033] Where λ is the system wavelength, R is the distance between the radar and the observation center in the main trajectory, and θ max is the maximum pitch angle of the radar relative to the observation center in the main track, ρ r is the range resolution, and η is the terrain slope angle. Multiple interferometric baselines can then be formed through azimuth aperture segmentation and pairing of the master and slave tracks, and lunar surface DEM information can be obtained through the interferometric imaging processing process.

[0034] Based on the radar echo data of each track, 3D BP imaging is performed to obtain a set of 3D SAR images. Afterwards, in order to suppress the irregular high sidelobe problem caused by the non-uniform and sparsely distributed sampling points in the altitude, it is necessary to perform altitude windowing on the 3D imaging results on the 3D BP imaging grid based on the DEM information obtained by interference. The schematic diagram of altitude windowing on the BP imaging grid is shown in the figure below. Figure 1 As shown in Figure 2, the elevation information corresponding to the latitude and longitude grids in the BP imaging grid can be obtained through two-dimensional scattered point interpolation. On this basis, a Gaussian window function can be constructed and the imaging latitude and longitude grids can be windowed in the height direction one by one to improve the imaging quality. The window function model can be expressed as:

[0035]

[0036] Among them, w B,L (·) represents the elevation window function corresponding to latitude B and longitude L, DEM B,L is the known digital elevation information at latitude B and longitude L, ρ h Represents the height resolution.

[0037] In combination with the above analysis, the ground-based radar three-dimensional lunar imaging method proposed in the present invention will be introduced in detail below. The specific process is as follows:

[0038] Step 1: Select two radar echo data tracks that meet the interferometric processing requirements for interferometric imaging, obtain the elevation information estimate of the observation scene, and obtain the DEM model;

[0039] To obtain a lunar surface DEM model through interferometric imaging, two radar echoes with good coherence are selected for interferometric imaging. The DEM model is obtained through master-slave image registration, interferogram generation, flat ground phase removal, phase filtering, phase unwrapping, and elevation inversion.

[0040] Step 2: Obtain SAR 3D imaging results of lunar surface targets under multiple radar motion tracks through the 3D BP imaging algorithm of inter-track non-coherent superposition;

[0041] First, a 3D imaging grid is created based on the imaging scene. For each radar track used for imaging, a corresponding 3D SAR image is obtained using a 3D BP imaging algorithm. The original 3D SAR image is then obtained by non-coherently superimposing images from different tracks.

[0042] Step 3: interpolate the DEM model obtained in step 1 and construct a height window function that matches the BP imaging grid in step 2;

[0043] The original 3D SAR image has poor altitude quality, so altitude windowing is required. Because the known DEM information does not necessarily match the grid division, a 2D scattered point interpolation is performed on the DEM information to obtain an altitude window function that matches the BP imaging grid in step 2.

[0044] Step 4: According to the DEM-based height window function obtained in step 3, the SAR 3D imaging result in step 2 is height-windowed to obtain the final imaging result;

[0045] For the altitude imaging results corresponding to each latitude and longitude grid in the three-dimensional BP imaging grid, altitude windowing can be performed based on the altitude window function constructed in step 2 (as shown in formula (2)), and the altitude position of the window function can be adjusted based on the criterion of optimal image quality to suppress irregular high side lobes in the altitude direction and obtain the final imaging result.

[0046] Example

[0047] Computer simulation was used to verify the effectiveness of the ground-based radar lunar interferometry-three-dimensional cascade imaging method proposed in the present invention.

[0048] In the simulation of one-dimensional stretched targets, the target scenario is set as follows Figure 2As shown in Figure 3, the main research object is a one-dimensional extended target on the azimuth altitude plane, which is used to visually verify the effect of DEM windowing on altitude imaging. The imaging results of the one-dimensional extended target before and after DEM windowing are shown in Figure 3, and the peak sidelobe ratio comparison results are shown in Table 1.

[0049] Table 1 Comparison of peak sidelobe ratio of one-dimensional extended target before and after windowing

[0050] Evaluation Metrics DEM before windowing DEM windowed Peak-to-sidelobe ratio 13.33dB -8.12dB

[0051] In the simulation of the 2D extended target, the target scene is set up as shown in Figure 4. The main research object is the 2D simulation scene generated based on the actual lunar topography. The imaging results of the 2D extended target before and after DEM windowing are shown in Figure 5, and the average elevation error is shown in Table 2.

[0052] Table 2 Comparison of average elevation errors of two-dimensional extended targets before and after windowing

[0053] Evaluation Metrics DEM before windowing DEM windowed Average elevation error 840m 156m

[0054] In summary, the effectiveness of the method proposed in the present invention has been demonstrated through computer simulation.

[0055] The above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for three-dimensional lunar imaging based on ground-based radar with optimal image quality, characterized in that: include: Step 1: Select two radar echo data tracks that meet the interferometric processing requirements for interferometric imaging, obtain the elevation information estimate of the observation scene, and obtain the DEM model; Step 2: Obtain SAR 3D imaging results of lunar surface targets under multiple radar motion tracks through the 3D BP imaging algorithm of inter-track non-coherent superposition; Step 3: interpolate the DEM model obtained in step 1 and construct a height window function that matches the BP imaging grid in step 2; Step 4: According to the DEM-based height window function obtained in step 3, the SAR 3D imaging result in step 2 is height-windowed to obtain the final imaging result; The selecting of two-track radar echo data that meets the interferometric processing requirement comprises: screening vertical baselines within the critical baseline to obtain two-track radar echo data that meets the interferometric processing requirement; The calculation formula of the critical baseline is: in, is the system wavelength, is the distance between the radar and the observation center in the main track, is the maximum pitch angle of the radar relative to the observation center in the main track, is the range resolution, is the terrain slope angle; The height window function is: in, For latitude , longitude is The corresponding elevation window function is For latitude , longitude is The known digital elevation information, is the height resolution, For high-level information.

2. The method for three-dimensional lunar imaging based on ground-based radar with optimal image quality according to claim 1, characterized in that: The height position of the window function is also adjusted based on the criterion of optimal image quality to suppress irregular high side lobes in the height direction.

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