Method for verifying Mars sky radiance simulation result based on Mars real-shot photos

Abstract

The invention relates to a method for verifying a Mars sky radiance simulation result based on a Mars real-shot photo, and belongs to the technical field of Mars sky background radiance. Comprises: image acquisition; performing radiation calibration; correcting dark current; performing flat field correction; converting a radiation value; and geometric correction. The Mars sky radiance simulation method breaks through the dependence of an existing model on idealized hypothesis, performs dynamic verification by using actual shot image data, remarkably improves the authenticity and reliability of a Mars sky radiance simulation result, and effectively solves the problem of systematic deviation under a specific wave band; by constructing a complete process from image acquisition, radiation calibration, geometric modeling to brightness inversion and comparison verification, dynamic feedback and closed-loop correction of a theoretical radiation model are realized, model adaptability is improved, and fine modeling under multi-temporal and multi-meteorological conditions is supported; the method can be applied to an existing Mars rover navigation camera and an image processing link of the Mars rover navigation camera, does not need additional modification, has good engineering practicability and task compatibility, and is convenient to popularize and apply.

Description

TECHNICAL FIELD

[0001] The present application relates to a method based on sky radiance simulation results, and belongs to the technical field of Mars sky background radiance. BACKGROUND

[0002] In some spectral bands, due to the high spectral radiance of the sky background or the close spectral radiance of the strong scattering radiation or infrared radiation of the target, the target may be submerged by the background radiation and difficult to identify, and the image recognition is not accurate. Accurate modeling of Mars sky background radiance is the core technical basis for understanding the characteristics of atmospheric aerosols, optimizing the design of the optical system of the probe, and ensuring the success rate of deep space exploration missions.

[0003] The existing modeling method of sky radiance in the Mars dust environment mainly simulates through a theoretical radiation transfer model combined with orbital remote sensing data or limited ground scientific instruments. However, in practical applications, the modeling has significant limitations due to excessive reliance on idealized assumptions. For example, the Simulation of Polarized Light Transmission Characteristics in Martian Dust Environment simulates the transmission characteristics of polarized light in the Mars dust environment, which assumes the content of atmospheric dust. This method is used to study the transmission characteristics of polarized light in the Mars dust environment. However, this assumption ignores the vertical distribution characteristics of the optical thickness of the dust with height, and similar idealized assumptions and single assumptions in the existing technology result in systematic deviations between the simulation results of specific spectral bands and the actual sky radiance distribution.

[0004] Therefore, it is urgent to propose a method for verifying the simulation results of Mars sky radiance based on real photos of Mars to solve the above technical problems. SUMMARY

[0005] To solve the above problems, a method for verifying the simulation results of Mars sky radiance based on real photos of Mars is provided. In the following, a brief overview of the present application is given to provide a basic understanding of some aspects of the present application. It should be understood that this overview is not an exhaustive overview of the present application. It is not intended to determine the key or important parts of the present application, nor to limit the scope of the present application.

[0006] Technical scheme of the present application: The method for verifying the simulation results of Mars sky radiance based on real photos of Mars comprises the following steps: Step one, image acquisition; Step two, radiation calibration, including: Dark current correction: average the dark field image to generate a dark field noise image, establish a temperature-dependent exponential model, divide the dark current correction into effective area and mask area, the dark current of effective area is used to correspond to the part of the field of view exposed to the detector, and the dark current of the mask area is used to correspond to the accumulated charge value during readout; Flat field correction: acquire a standard flat field image by shooting a uniform illumination target to generate pre-flight flat field reference data; whether to complete the correction before flight can be determined according to the flag of PDS header file FLAT_FIELD_CORRECTION_FLAG, and the pixel response difference can be compensated in real time according to the polynomial function model in the navigation camera; Radiometric conversion: convert the DN value of the acquired PDS image into absolute radiance; Step three, geometric correction: use the CAHVOR model to realize the projection of a three-dimensional world point to a two-dimensional image point, accurately describe the camera imaging geometry through six groups of parameters, and support coordinate conversion from three-dimensional space to two-dimensional image and lens distortion correction.

[0007] Preferably, in step one, the original image (EDR file) containing the sun is shot by the navigation camera, and the shooting time, solar azimuth angle, elevation angle and navigation camera parameters are recorded.

[0008] Preferably, in the current correction of step two, the expression of the dark current signal is: (1) Wherein, DF 掩蔽区域 , DF 有效区域 respectively, the average and normalized value of the zero exposure time and non-zero exposure time dark image, T CCD refers to the temperature of the CCD, t exp refers to the exposure time, i , j respectively, the row and column pixel positions in the image, d 0、 d 1 refers to the average rate of the effective area, which is a constant value, D 遮掩 , D 有效区域 respectively, the dark current correction values of the mask area and the effective area.

[0009] Preferably, in the flat field correction of step two, when the PDS header file FLAT_FIELD_CORRECTION_FLAG flag is TRUE, it represents that this correction is completed; otherwise, the parameter correction stored in the PDS header file can be used; the expression of the polynomial function model for real-time compensation of pixel response difference is: (2) Wherein, the parameters a, b are optical center offset, c, d, e are distortion coefficients.

[0010] Preferred: the expression of absolute radiance in the conversion of radiation values is: (3) Wherein, K 0 is the offset coefficient ; K 1 is the slope coefficient .

[0011] Preferred: in step three, each letter of the six groups of parameters is a vector coordinate, and is saved in the same label record as the original image; C is the vector from the origin (X-Y-Z) of the ground coordinate system to the center of the camera center of the camera view angle; A is the unit vector of the camera axis; H and V are the horizontal and vertical information vectors; O represents the optical center unit vector, which is mainly used for radial distortion correction; R mainly represents the nonlinear polynomial coefficient of lens radial distortion; The projection of the target point to its corresponding image point (x, y) is represented as: i, j (4) Wherein, i , j respectively indicate the row and column pixel positions in the image; P is the ground coordinate system vector of the target point; According to the P-C vector, the elevation angle and azimuth angle of each pixel in the image are obtained in combination with the ground coordinate system, and in addition, the scattering angle is obtained by using the camera light principle, at which time the geometric network image can be constructed; in the three-dimensional ground coordinate system, the azimuth angle of the object point P relative to the observation point C can be expressed as: after projecting the vector φ onto the XY plane, the inverse tangent value of the vertical component and the horizontal projection length, that is, P-C = arctan( φ ); the calculation of the elevation angle y / x is based on the two-dimensional projection of the vector θ on the XY plane, which is characterized as the plane angle between the projection vector and the positive direction of the Z axis: P-C (5) Wherein, n is the normal vector in the vertical direction.

[0012] Preferred: the method further comprises step four, comparison verification: the sky radiance at each pixel position is obtained by using the actually taken image.​​

[0013] Preferably: in step four, since the radiometric calibration camera directly calibrates the radiance value, a synchronous sky image taken by the radiometric calibration camera at the same Mars local time and the same solar geometry is selected; a region is demarcated in the image, the corresponding ground coordinates are inversely deduced through the CAHVOR model, the azimuth and scattering angle network is obtained, and the radiance value measured by the method is compared with the calibration value provided by the radiometric calibration camera angle by angle.

[0014] The present application has the following beneficial effects: 1. The present application breaks through the dependence of the existing model on idealized assumptions, uses actual image data for dynamic verification, significantly improves the authenticity and reliability of the simulation results of the Mars sky radiance, and effectively solves the problem of systematic deviation under specific wavebands; 2. The present application constructs a complete process from image acquisition, radiometric calibration, geometric modeling to brightness inversion and comparison verification, realizes dynamic feedback and closed-loop correction of the theoretical radiometric model, improves the adaptability of the model, and supports fine modeling under multiple time phases and multiple weather conditions; 3. The method proposed by the present application can be directly applied to the existing Mars rover navigation camera and its image processing link without additional hardware modification, has good engineering practicability and task compatibility, and is convenient for popularization and application on different task platforms. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a method flowchart for verifying the simulation results of the Mars sky radiance based on the actual photos of Mars; Figure 2 It is an image geometric model diagram; Figure 3 It is a camera light transmission geometric principle diagram; Figure 4 It is the sky radiance obtained according to the image.

[0016] Figure 3 In the figure, 1 is a camera, 2 is an optical center, 3 is a focal length, and 4 is a pixel plane. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the present application more clear and obvious, the present application will be described below by specific embodiments shown in the drawings. However, it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present application. In addition, in the following description, the description of known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present application.

[0018] Specific implementation method one: combined with Figures 1-4The method for verifying the simulation result of the sky radiance of Mars based on the actual photograph of Mars in the embodiment, and the core of the application is to dynamically verify the accuracy of the simulation result of the sky radiance of Mars by combining the actual image of the Mars engineering camera with the radiation transmission model, including the following steps: Step one, image acquisition: the original engineering data record is acquired through the image shot by the navigation camera of the Mars vehicle, the original image (EDR file) shot by the navigation camera containing the sun, the shooting time, the solar azimuth angle, the solar elevation angle and the camera parameters (such as exposure time, CCD temperature) are recorded, and the solar azimuth angle, the solar elevation angle, the exposure time and the CAHVOR parameter information at the moment are extracted from the data record label file; the dark field image can be obtained by shooting the image under the condition of complete light shielding; Step two, radiation calibration, including dark current correction, flat field correction and radiation value conversion; Dark current correction: the dark field image is processed by averaging to generate a dark field noise image, a temperature-dependent exponential model is established, for example: the method proposed in the journal of laser and infrared, “Large-range radiation temperature inversion based on image gray level” can be used to establish the actual image, the dark field noise image is subtracted to eliminate the dark current noise, and the correction effect is further optimized by the temperature-dependent exponential model, the exponential dependence is the T CCD in the following formula related to temperature CCD , exp(T 掩蔽区域 ), which relies on the exponential model to divide the dark current correction into an effective area and a masked area, the dark current in the effective area is used to correspond to the exposure of the detector to the field of view part, and the dark current in the masked area is used to correspond to the cumulative charge value during reading; the specific calibration file and parameters are determined according to the type of the camera, wherein the dark current correction is performed according to formula (1), and the normalized data is the dark field image in the cruise stage; (1) wherein, DF 掩蔽区域 , DF 有效区域 , and T CCD is the temperature of the CCD, t exp is the exposure time, i , j respectively, d 0, d 1 is the average rate of the effective area, which is a constant value, D 遮掩 , D 有效区域 respectively, are the dark current correction values of the masked area and the effective area; Flat field correction: In the radiation calibration process, flat field correction is used to eliminate the difference in quantum efficiency between CCD pixels and the illumination response deviation caused by the inhomogeneity of the optical system; it is mainly obtained by taking pictures of uniformly illuminated targets (such as the diffuse scattering surface of an integrating sphere) to obtain standard flat field images, thereby generating pre-flight flat field reference data. The calibration can be determined by the flag FLAT_FIELD_CORRECTION_FLAG in the PDS header file. The pixel response difference can be compensated in real time according to the polynomial function model in the navigation camera; in addition, the navigation camera firmware uses a polynomial function model to compensate for the pixel response difference in real time. After completing dark current correction and normal correction in sequence, the radiation value conversion is performed, referring to formula (2): (2) Where parameters a and b are optical center offsets, c, d, and e are distortion coefficients, and x and y are independent variables; when FLAT_FIELD_CORRECTION_FLAG in the PDS header file is marked TRUE, it indicates that this correction is completed; otherwise, the correction can be performed using the parameters stored in the PDS header file. Radiometric Conversion: Radiometric conversion converts the DN values ​​of the acquired PDS image into absolute radiance (W / m²). 2 ·nm·sr): (3) in, K 0 represents the offset coefficient W*m -2 *nm -1 sr -1 *(DN / s) -1 ; K 1 represents the slope coefficient W*m -2 *nm -1 sr -1 (DN / s) -1 °C -1 ; Step 3, Geometric Correction: The CAHVOR model is used to project points from a 3D world to 2D image points. The CAHVOR model is specifically designed for the geometric calibration and projection model of planetary exploration cameras. It accurately describes the camera imaging geometry through six sets of parameters and supports coordinate transformation from 3D space to 2D image and lens distortion correction. Combination Figure 2 , Figure 3 The geometric network diagram, based on the CAHVOR parameters (C - camera center, A - optical axis, H / V - horizontal / vertical vectors, O - distortion center, R - radial distortion coefficient) in the image data label, converts the pixel coordinates (i,j) into azimuth angles in the ground coordinate system. φThe parameters (θ) and elevation angle are used to calculate the geometric relationship between the sun, target, and observer for each pixel, and to determine the scattering angle of each image pixel. That is, each letter of the six sets of parameters is a vector coordinate, and they are stored in the same tag record as the original image (EDR file). C It is the vector from the origin (XYZ) of the ground coordinate system to the center of the camera's viewpoint; A The unit vector is the camera axis. H and V These are horizontal and vertical information vectors; O Represents the optical center unit vector, primarily used for radial distortion correction; R It mainly represents the nonlinear polynomial coefficients of the lens radial distortion; Map the target point to its corresponding image point ( i, j The projection of ) is represented as: (4) in, i , j These refer to the row and column pixel positions in the image, respectively. P The ground coordinate vector of the target point; Based on the PC vector and the ground coordinate system, the elevation and azimuth angles of each pixel in the image are obtained. Furthermore, the scattering angle is obtained using camera ray principles, allowing the construction of a geometric network image. In the three-dimensional ground coordinate system, object points... P Relative to the observation point C azimuth φ This can be expressed as: [taking the vector] P-C After being projected onto the XY plane, its vertical component is the arctangent of the horizontal projection length, i.e. φ = arctan( y / x ); Elevation angle θ The calculation is based on vectors. P-C The two-dimensional projection onto the XY plane is represented by the complementary angle between the projection vector and the positive Z-axis: (5) Where n is the normal vector in the vertical direction; This invention breaks through the dependence of existing models on idealized assumptions, uses actual captured image data for dynamic verification, comprehensively considers the actual situation, significantly improves the authenticity and reliability of the simulation results of Martian sky radiance, and effectively solves the problem of systematic deviation under specific wavebands; Step 4: Comparison and verification. The sky radiance at each pixel location is obtained using real-world images. Since the radiometric calibration camera directly calibrates radiance values, synchronized sky images taken at the same Martian local time and solar geometry angle as the radiometric calibration camera are selected. The region is demarcated in the image, the corresponding ground coordinates are obtained by backstepping the CAHVOR model, the azimuth and scattering angle network are obtained, and the radiance value measured by the method is compared with the calibration value provided by the radiation calibration camera at each angle. The present application realizes dynamic feedback and closed-loop correction of the theoretical radiation model by constructing a complete process from image acquisition, radiation calibration, geometric modeling to brightness inversion and comparison verification, improves the model adaptability, and supports fine modeling under multi-temporal and multi-weather conditions. The method proposed in the present application can be directly applied to the existing Mars rover navigation camera and its image processing link without additional hardware modification, has good engineering practicability and task compatibility, and is convenient for popularization and application on different task platforms.

[0019] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be arranged and combined. Those skilled in the art can exhaust all possibilities according to the mathematical knowledge of arrangement and combination, so the present application will not explain the technical solutions after arrangement and combination one by one, but it should be understood that the technical solutions after arrangement and combination have been disclosed by the present application.

[0020] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various changes and modifications to the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for validating the simulation results of the sky radiance on Mars based on the real photos of Mars, characterized in that: The method comprises the following steps: Step one, image acquisition; Step two, radiation calibration, comprising: Dark current correction: average processing of dark field image to generate dark field noise image, establishment of temperature-dependent exponential model, dark current correction is divided into effective area and masked area, effective area dark current is used for corresponding detector exposed to field of view part, and masked area dark current is used for corresponding readout time cumulative charge value; Flat field correction: standard flat field image is obtained by shooting uniform illumination target, so as to generate pre-flight flat field reference data; the navigation camera adopts a polynomial function model to compensate the pixel response difference in real time; Radiation value conversion: convert the DN value of the obtained PDS image into absolute radiance; Step three, geometric correction: realize the projection of three-dimensional world point to two-dimensional image point by using CAHVOR model, accurately describe the camera imaging geometric relationship through six groups of parameters, support three-dimensional space to two-dimensional image coordinate conversion and lens distortion correction.

2. The method of claim 1, wherein the method is based on a real photograph of Mars. In step one, the original image containing the sun is shot by the navigation camera, and the shooting time, solar azimuth angle, elevation angle and navigation camera parameters are recorded.

3. The method of claim 2, wherein the method is based on a real photograph of Mars. In the current correction of step two, the dark current correction expression is: （1） where, DF 掩蔽区域 , DF 有效区域 denote the average and normalized value of the dark image for zero and non-zero exposure time, respectively, T CCD denotes the temperature of the CCD, t exp denotes the exposure time, i , j denote the row and column pixel positions in the image, respectively, d 0, d 1denotes the average rate of the active area, which is a constant value, D 遮掩 , 有效区域 denote the dark current correction values for the masked and active areas, respectively.

4. The method of claim 3, wherein the method is based on a real photograph of Mars. In the flat field correction of step two, the expression of the polynomial function model for real-time compensation of pixel response difference is: （2） Wherein, the parameters a and b are optical center offset, and c, d and e are distortion coefficients.

5. The method of claim 4, wherein the method is based on a real photograph of Mars. The expression of absolute radiance in radiation value conversion is: （3） wherein, K 0 is an offset coefficient ; K 1 is a slope coefficient .

6. The method of claim 5, wherein the method is based on a real photograph of Mars. In step three, each letter of the six groups of parameters is a vector coordinate, which is saved in the same label record as the original image; C is the vector from the origin of the ground coordinate system (X-Y-Z) to the center of the camera; A is the unit vector of the camera axis; H and V are the horizontal and vertical information vectors; O represents the optical center unit vector, mainly used for radial distortion correction; R mainly represents the nonlinear polynomial coefficients of the lens radial distortion; Map the target point to its corresponding image point ( i, j The projection of ) is represented as: （4） wherein, i , j denote the row and column pixel positions in the image, respectively; P is the ground coordinate system vector for the target point. Based on the PC vector and the ground coordinate system, the elevation and azimuth angles of each pixel in the image are obtained. Furthermore, the scattering angle is obtained using camera ray principles, allowing the construction of a geometric network image. In the three-dimensional ground coordinate system, object points... P Relative to the observation point C azimuth φ This can be expressed as: [taking the vector] P-C After being projected onto the XY plane, its vertical component is the arctangent of the horizontal projection length, i.e. φ = arctan( y / x ); Elevation angle θ The calculation is based on vectors. P-C The two-dimensional projection onto the XY plane is represented by the complementary angle between the projection vector and the positive Z-axis: （5） Wherein, n is the normal vector in the vertical direction.

7. The method of claim 6, wherein the method is based on a real photograph of Mars. The method further comprises step four, contrast verification: the sky radiance at each pixel position is obtained by using the actually shot image.

8. The method of claim 7, wherein the method is based on a real photograph of Mars. In step four, since the radiation calibration camera directly calibrates the radiance value, the synchronous sky image is selected, which is shot by the radiation calibration camera at the same Mars local time and the same solar geometry angle; the region is demarcated in the image, the corresponding ground coordinates are obtained by reverse calculation through the CAHVOR model, the azimuth angle and scattering angle network are obtained, and the radiance value measured by the method is compared with the calibration value provided by the radiation calibration camera at each angle.

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