Method for in-situ spectral on-orbit calibration and reflectance determination on Mars

Abstract

The application provides a method for in-orbit calibration and reflectivity determination of a Mars in-situ exploration spectrum. The method comprises the following steps: under the same detection condition, a first calibration plate and a second calibration plate carried by a short-wave infrared spectrometer in a Mars surface component detector are used to obtain a first remote sensing image pixel brightness value corresponding to the first calibration plate and a second remote sensing image pixel brightness value corresponding to the second calibration plate; a regression analysis method is used to determine the in-orbit radiation calibration coefficient ratio of each wave band according to the first remote sensing image pixel brightness value and the second remote sensing image pixel brightness value; for a target detection object, the short-wave infrared spectrometer in the Mars surface component detector is used to obtain a third remote sensing image pixel brightness value corresponding to the target detection object; and the target reflectivity is determined according to the in-orbit radiation calibration coefficient ratio and the third remote sensing image pixel brightness value.

Description

Technical Field

[0001] This invention relates to the field of detection technology, and more specifically to a method for on-orbit calibration of spectra and determination of reflectance for in-situ Mars probes. Background Technology

[0002] The Short-Wave Infrared Spectrometer (SWIR) is a component of the Mars Surface Composition Detector (MarSCoDe) payload aboard the Zhurong Mars rover. It is used to acquire in-situ short-wave infrared spectra of the Martian surface, qualitatively identify the mineral species of the detected objects, and quantitatively retrieve mineral abundance information. SWIR is a single-point spectrometer with a wavelength range of 850-2400 nm, a spectral resolution of approximately 5 nm, 311 bands, a field of view of 36.5 mrad, and a field diameter of 7.3 cm at a distance of 2 m.

[0003] The raw data acquired by the Shortwave Infrared Spectrometer (SWIR) consists of DN values ​​without physical meaning. These values ​​require preprocessing using calibration parameters obtained from ground-based laboratories, including dark current subtraction, instrument temperature correction, and radiometric calibration (both relative and absolute). This process transforms the data into physically meaningful radiance values. However, ground-based laboratories struggle to accurately simulate the Martian environment and its variations. Furthermore, the Martian atmospheric environment and illumination conditions change with the location of the Zhurong rover and the duration of its exploration. Consequently, laboratory calibration parameters are no longer entirely applicable, introducing errors into the processing results and affecting the determination and quantitative inversion of mineral species in the probe. Summary of the Invention

[0004] In view of the above problems, the present invention provides an on-orbit calibration method for spectra and reflectance determination of in-situ Mars probes.

[0005] According to a first aspect of the present invention, a method for on-orbit calibration of spectra and determination of reflectance suitable for in-situ Mars probes is provided, comprising: under the same probe conditions, using a first calibration plate and a second calibration plate carried by a shortwave infrared spectrometer in a Mars surface composition probe to obtain a first remote sensing image pixel brightness value corresponding to the first calibration plate and a second remote sensing image pixel brightness value corresponding to the second calibration plate; determining the ratio of on-orbit radiometric calibration coefficients corresponding to each band using regression analysis based on the first remote sensing image pixel brightness value and the second remote sensing image pixel brightness value; for a target probe object, using the shortwave infrared spectrometer in the Mars surface composition probe to obtain a third remote sensing image pixel brightness value corresponding to the target probe object; and determining the target reflectance based on the ratio of on-orbit radiometric calibration coefficients and the third remote sensing image pixel brightness value.

[0006] According to an embodiment of the present invention, the step of determining the ratio of on-orbit radiometric calibration coefficients corresponding to each band using regression analysis based on the pixel brightness values ​​of the first remote sensing image and the second remote sensing image includes using δ(λ) satisfying the following formula as the ratio of on-orbit radiometric calibration coefficients:

[0007] DN G (λ)=m(λ)×DN W (λ)+(m(λ)-1)δ(λ)

[0008] Where λ is the wavelength value, DN G (λ) represents the pixel brightness value of the first remote sensing image, DN W (λ) represents the pixel brightness value of the second remote sensing image, and m(λ) represents the ratio of the reflectance of the second calibration plate to that of the first calibration plate as measured in the laboratory.

[0009] According to an embodiment of the present invention, determining the target reflectivity based on the ratio of the on-orbit radiometric calibration coefficients and the pixel brightness value of the third remote sensing image includes using R(λ) satisfying the following formula as the target reflectivity:

[0010]

[0011] Where λ is the wavelength value, DN(λ) is the pixel brightness value of the third remote sensing image, δ(λ) is the ratio of on-orbit radiometric calibration coefficients, and DN... standard (λ) represents the pixel brightness value of the first remote sensing image or the pixel brightness value of the second remote sensing image, R standard (λ) represents the reflectance value of the first calibration plate calibrated by the ground laboratory or the reflectance value of the second calibration plate calibrated by the ground laboratory.

[0012] According to an embodiment of the present invention, the method further includes: determining the mineral type matching the target detection object based on the target reflectance, and quantitatively retrieving the mineral content information.

[0013] According to an embodiment of the present invention, the same detection conditions include atmospheric conditions and illumination geometry conditions. Attached Figure Description

[0014] The above-described features, other objects, and advantages of the present invention will become clearer from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0015] Figure 1 A flowchart illustrating a method for on-orbit calibration and reflectance determination of spectra applicable to in-situ Mars probes according to an embodiment of the present invention is shown. Detailed Implementation

[0016] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the invention for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0017] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0018] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0019] When using expressions such as "at least one of A, B, and C", they should generally be interpreted in accordance with the meaning that is commonly understood by a person skilled in the art (e.g., "a system having at least one of A, B, and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B, and C, etc.).

[0020] pass Figure 1 The disclosed embodiments provide a detailed description of the on-orbit calibration and reflectance determination method for in-situ Mars probe spectra.

[0021] Figure 1 A flowchart illustrating a method for on-orbit calibration and reflectance determination of spectra applicable to in-situ Mars probes according to an embodiment of the present invention is shown. Figure 1 As shown, this embodiment includes operations S101 to S104.

[0022] In operation S101, under the same detection conditions, the first calibration plate and the second calibration plate carried by the shortwave infrared spectrometer in the Mars surface composition detector are used to obtain the brightness value of the first remote sensing image pixel corresponding to the first calibration plate and the brightness value of the second remote sensing image pixel corresponding to the second calibration plate.

[0023] In operation S102, based on the pixel brightness values ​​of the first and second remote sensing images, regression analysis is used to determine the ratio of on-orbit radiometric calibration coefficients for each band.

[0024] In operation S103, for the target object, the short-wave infrared spectrometer in the Mars Surface Composition Detector is used to obtain the brightness value of the third remote sensing image corresponding to the target object.

[0025] In operation S104, the target reflectivity is determined based on the ratio of on-orbit radiometric calibration coefficients and the pixel brightness values ​​of the third remote sensing image.

[0026] It is understandable that for in-situ detection data at a specified wavelength λ, the target reflectivity can be calculated according to Equation 1, such as using R(λ) as the target reflectivity.

[0027]

[0028] In the formula, λ is the wavelength value, I(λ), I standard R(λ) represents the reflected radiance values ​​of the detection target and the calibration plate, respectively; R(λ) is the reflectance factor (REFF) of the detection target. standard (λ) represents the reflectance value of the calibration plate calibrated in the ground laboratory. For white and gray boards, it can be represented by R0. W (λ) and R G (λ) represents this.

[0029] During its on-orbit exploration, the Zhurong Mars rover's SWIR spectral detection procedure acquired data sequentially using a white background plate, a gray background plate, and then the target surface object. Theoretically, the reflectance of the target surface object can be calculated using Equation 1 based on the reflectance of either the white or gray background plate, and the results obtained from both calibration plates (white and gray) should be consistent.

[0030] However, in practice, the reflectance curves calculated using Equation 1 for the two calibration boards (white and gray) show significant deviations, with an average relative deviation of 6.8% in amplitude and 89.6% in slope. This significant deviation is mainly due to the fact that the laboratory calibration parameters are no longer fully applicable to on-orbit data processing.

[0031] Because calibration parameters from ground-based laboratories can introduce errors, the Zhurong Mars rover carried two types of diffuse reflection calibration plates—a white plate and a gray plate—for on-orbit calibration of SWIR data to mitigate these errors. The reflectance of these two calibration plates has been precisely measured in ground-based laboratories; for example, the reflectances of the white plate and the gray plate at 1000 nm are 0.989 and 0.482, respectively. In this embodiment, the white plate is used as the first calibration plate, and the gray plate as the second calibration plate. It is understood that those skilled in the art can also use the gray plate as the first calibration plate and the white plate as the second calibration plate.

[0032] The embodiments of the present invention can utilize the two calibration plates described above to perform on-orbit calibration of the shortwave infrared spectrometer's in-situ detection spectrum, thereby achieving accurate calculation of the shortwave infrared spectral reflectance of the Martian surface.

[0033] Specifically, embodiments of the present invention can determine the reflectance of the target object by utilizing the pixel brightness values ​​of remote sensing images from the two calibration plates (e.g., the first and second remote sensing image pixel brightness values), the pixel brightness value of the target object from remote sensing images (e.g., the third remote sensing image pixel brightness value), the ratio of on-orbit radiometric calibration coefficients, and the reflectance of the calibration plates calibrated in the laboratory (first and second calibration plates). This invention can directly calculate the target reflectance from the DN values ​​(pixel brightness values) of the original detection data, thus simplifying the determination process. Furthermore, it is applicable to the reflectance calculation of the spectra of all in-situ planetary probes carrying at least two calibration plates.

[0034] For example, firstly, the raw DN values ​​of the in-situ detection spectra of two calibration plates under the same detection conditions are obtained, and preprocessing such as dark leveling subtraction, relative calibration, instrument temperature correction, and wavelength correction is performed to obtain meaningful radiance values, namely the pixel brightness values ​​of the first remote sensing image corresponding to the first calibration plate and the pixel brightness values ​​of the second remote sensing image corresponding to the second calibration plate. Then, based on the pixel brightness values ​​of the first and second remote sensing images, regression analysis is used to calculate the ratio of on-orbit radiometric calibration coefficients. Finally, the target reflectance is calculated based on the obtained pixel brightness value of the third remote sensing image corresponding to the target object and the ratio of the on-orbit radiometric calibration coefficients.

[0035] Furthermore, for example, δ(λ) satisfying the following formula can be used as the ratio of on-orbit radiometric calibration coefficients:

[0036] DN G (λ)=m(λ)×DN W (λ)+(m(λ)-1)δ(λ)

[0037] Where λ is the wavelength value, DN G (λ) represents the pixel brightness value of the first remote sensing image, DN W (λ) represents the pixel brightness value of the second remote sensing image, and m(λ) represents the ratio of the reflectance of the second calibration plate to that of the first calibration plate as measured in the laboratory.

[0038] Furthermore, for example, R(λ) satisfying the following formula can be used as the target reflectivity:

[0039]

[0040] Where λ is the wavelength value, DN(λ) is the pixel brightness value of the third remote sensing image, δ(λ) is the ratio of on-orbit radiometric calibration coefficients, and DN... standard (λ) represents the pixel brightness value of the first remote sensing image or the pixel brightness value of the second remote sensing image, R standard (λ) represents the reflectance value of the first calibration plate calibrated by the ground laboratory or the reflectance value of the second calibration plate calibrated by the ground laboratory.

[0041] To better understand this invention, the following description, in conjunction with the technical principles, further elaborates on the content of this invention, but this invention is not limited to the technical principles described below.

[0042] Absolute radiometric calibration is the process of establishing a quantitative relationship between the payload output value and the absolute physical quantity (radiance) of the observed target. Calibration is performed in a ground-based laboratory using a standard light source, satisfying the following formula:

[0043] I(λ)=k(λ)×DN(λ)+b(λ) Formula 2

[0044] Where k(λ) and b(λ) are the absolute radiometric calibration coefficients, referred to as gain and bias, respectively, and λ is the wavelength value. It is understandable that the absolute calibration coefficients k(λ) and b(λ) will change due to factors such as probe platform vibration during launch and flight, the on-orbit working environment (mainly including illumination conditions, temperature, air pressure, etc.), and component aging. Therefore, continuing to use the ground-calibrated coefficients (i.e., the calibration parameters from the ground laboratory) for data processing will inevitably result in errors. A solution could be to carry an on-orbit calibration device and use the payload to recalibrate the absolute calibration coefficients based on the detection data from the calibration device.

[0045] According to Equation 1, the distribution of detection data from the white plate (first calibration plate) and gray plate (second calibration plate) carried by the SWIR shortwave infrared spectrometer satisfies:

[0046] IW (λ)=k(λ)×DN W Equation 3 (λ)+b(λ)

[0047] I G (λ)=k(λ)×DN G (λ)+b(λ) Equation 4

[0048] Among them, I W (λ), I G (λ) represents the radiance values ​​of the white board (first calibration board) and the gray board (second calibration board), respectively, DN W (λ), DN G (λ) represents the original DN values ​​(remote sensing image pixel brightness values) of the white board (first calibration board) and the gray board (second calibration board), respectively. When the calibration board material remains unchanged, the reflectance measured on the ground will not change with variations in the working environment (mainly including lighting conditions, temperature, air pressure, etc.). Under the same observation conditions, with the constraint that the reflectance of the same target calculated using the two calibration boards is equal, the following relationship can be derived according to Equation 1:

[0049]

[0050] Where m(λ) is the ratio of the reflectance of the second calibration plate to that of the first calibration plate as measured in the laboratory, and m(λ) < 1. Dividing both sides of Equations 3 and 4 simultaneously still forms an equation. Then, substituting Equation 5 into the new equation, we define the ratio of on-orbit radiation calibration coefficients δ(λ) = b(λ) / k(λ), and obtain the following relationship:

[0051] DN G (λ)=m λ ×DN W Equation 6: (λ)+(m(λ)-1)δ(λ)

[0052] Using the DN value data (i.e., the pixel brightness value of the first remote sensing image corresponding to the first calibration board and the pixel brightness value of the second remote sensing image corresponding to the second calibration board) of the white board (first calibration board) and gray board (second calibration board) acquired by multiple sets of detections by the shortwave infrared spectrometer (SWIR), each set of detections includes three types of detection data: white board, gray board, and target detection object. The ratio of on-orbit radiometric calibration coefficients δ(λ) is calculated by regression analysis. Combining Equations 1, 3, and 4, the following relationship can be obtained:

[0053]

[0054] In the formula, λ is the wavelength value; R(λ) is the target reflectivity of the target object; DN(λ) is the DN value of the target object, such as the pixel brightness value of the third remote sensing image; δ(λ) is the ratio of on-orbit radiometric calibration coefficients obtained by regression calculation using Equation 6; DNstandard (λ) represents the DN value of the calibration plate obtained during detection. For the white plate (first calibration plate) and the gray plate (second calibration plate), DN can be used respectively. W (λ), DN G (λ) represents the pixel brightness value of the first remote sensing image or the pixel brightness value of the second remote sensing image; R standard (λ) represents the reflectance value of the calibration plate calibrated in the ground laboratory. For the white plate (first calibration plate) and the gray plate (second calibration plate), it can be represented by R. W (λ) and R G (λ) represents either the reflectance value of the first calibration plate calibrated by the ground laboratory or the reflectance value of the second calibration plate calibrated by the ground laboratory.

[0055] As shown in Equation 7, based on each set of SWIR detection data, including three types of detection data—white board (first calibration board), gray board (second calibration board), and the detection target (target object)—the reflectivity of the detection target can be calculated using the original DN value of any calibration board obtained by SWIR, the original DN value of the detection target, the ratio of on-orbit radiometric calibration coefficients, and the reflectivity of the calibration board calibrated on the ground. Considering that the ratio of on-orbit radiometric calibration coefficients is obtained through regression calculation based on the on-orbit detection data of the calibration board, the calculation result in Equation 7 has undergone on-orbit calibration.

[0056] Furthermore, it can be understood that, based on the determined target reflectivity, the mineral species matching the target object can be identified, and the mineral content information can be quantitatively retrieved. This enables qualitative identification of the mineral species of the detected object and quantitative retrieval of its content information.

[0057] Furthermore, it can be understood that the same detection conditions may include atmospheric conditions and illumination geometry, etc.

[0058] The embodiments of the present invention can efficiently realize the accurate calculation of the short-wave infrared spectral reflectance of the Martian surface, thereby improving the errors that exist in the calculation results using only laboratory parameters, which is beneficial for qualitative identification of the mineral types of the detected object and quantitative inversion.

[0059] To evaluate the effectiveness of the embodiments of the present invention, a set of SWIR data acquired by the Zhurong Mars rover on Martian day Sol 43 was selected, including white and gray backgrounds and Martian surface targets. The target reflectance curves calculated using the on-orbit calibration and reflectance determination method for in-situ Mars probe spectra provided in the embodiments of the present invention showed average relative deviations and slope deviations of 2.1% and 8.4%, respectively, which were reduced by approximately 3 times and 11 times compared to the results processed using laboratory parameters. It is evident that the on-orbit calibration and reflectance determination method for in-situ Mars probe spectra provided in the embodiments of the present invention, through on-orbit calibration calculations, significantly improves upon the errors present in calculations using only laboratory parameters. Furthermore, compared to traditional reflectance calculation methods, the on-orbit calibration and reflectance determination method for in-situ Mars probe spectra provided in the embodiments of the present invention does not undergo absolute radiometric calibration processes, but directly calculates the target reflectance from the DN values ​​of the original probe data, making the process much simpler.

[0060] Those skilled in the art will understand that the features described in the various embodiments and / or claims of the present invention can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in the present invention. In particular, the features described in the various embodiments and / or claims of the present invention can be combined and / or combined in various ways without departing from the spirit and teachings of the present invention. All such combinations and / or combinations fall within the scope of the present invention.

[0061] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. The scope of the invention is defined by the appended claims and their equivalents. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.

Claims

1. A method for on-orbit calibration of spectra and determination of reflectance suitable for in-situ Mars probes, comprising: Under the same detection conditions, using the first calibration plate and the second calibration plate carried by the short-wave infrared spectrometer in the Mars surface composition detector, the brightness values ​​of the first remote sensing image pixels corresponding to the first calibration plate and the brightness values ​​of the second remote sensing image pixels corresponding to the second calibration plate are obtained. Based on the pixel brightness values ​​of the first remote sensing image and the second remote sensing image, regression analysis was used to determine the ratio of on-orbit radiometric calibration coefficients for each band. For the target object, the short-wave infrared spectrometer in the Mars surface composition detector is used to obtain the brightness value of the third remote sensing image corresponding to the target object; as well as The target reflectivity is determined based on the ratio of the on-orbit radiometric calibration coefficients and the pixel brightness values ​​of the third remote sensing image. The step of determining the ratio of on-orbit radiometric calibration coefficients for each band using regression analysis based on the pixel brightness values ​​of the first and second remote sensing images includes applying the following formula: As a ratio of on-orbit radiometric calibration factors: in, This is the wavelength value. The pixel brightness value of the first remote sensing image. The pixel brightness value of the second remote sensing image. The ratio of the reflectance of the second calibration plate to that of the first calibration plate, as measured in the laboratory; The determination of target reflectivity based on the ratio of on-orbit radiometric calibration coefficients and the pixel brightness values ​​of the third remote sensing image includes applying the following formula: As a target reflectivity: in, This is the wavelength value. The pixel brightness value of the third remote sensing image. This represents the ratio of on-orbit radiation calibration factors. The pixel brightness value of the first remote sensing image or the pixel brightness value of the second remote sensing image. The reflectance value of the first calibration plate calibrated by the ground laboratory or the reflectance value of the second calibration plate calibrated by the ground laboratory.

2. The method according to claim 1, further comprising: Based on the target reflectivity, determine the mineral type that matches the target detection object, and quantitatively retrieve the mineral content information.

3. The method according to claim 1, wherein, The same detection conditions include atmospheric conditions and illumination geometry conditions.

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