Method for detecting water ice distribution and content in the lunar south pole assisted by LOLA and M 3 Method for detecting water ice distribution and content in the lunar south pole assisted by Mini-RF

Abstract

The application relates to a LOLA and M 3 The application relates to an auxiliary Mini-RF lunar south pole water ice distribution and content detection method, which comprises the following steps: based on Mini-RF radar image analysis, multiple water ice polarization properties are analyzed, and water ice detection and identification are carried out in combination with terrain factors; in combination with M 3 A mineral mapping instrument is used for supplementing and cross-verification of water ice detection results of data blank parts of radar images; an m-chi mixed polarization decomposition model is established, radar scattering characteristics of water ice point full internal reflection are analyzed; a dielectric constant inversion model is established based on backscattering coefficients, dielectric constant inversion results are obtained; on the basis of dielectric constant measurement values of simulated lunar soil at different water ice content ratios, a fitting model of water ice content and dielectric constant is established; water ice content of each water ice point in the lunar south pole is obtained based on the fitting model, and average water ice content of water ice points in a meteorite crater permanent shadow area is analyzed. Compared with the prior art, the application has the advantages of clearly detecting detailed points of lunar south pole water ice, specific water ice content and the like.

Description

Technical Field

[0001] This invention relates to the field of lunar exploration, and in particular to a method based on LOLA and M 3 A supplementary Mini-RF method for detecting the distribution and content of water ice in the lunar south pole. Background Technology

[0002] Most of the Moon is directly exposed to sunlight, and heat conduction causes the surface temperature to rise to tens or even hundreds of degrees Celsius, causing water to sublimate into water vapor and escape from the lunar surface. Compared to the mid- and low-latitude regions, most of the lunar poles are highlands. Furthermore, because the angle between the Moon's orbital plane and the ecliptic plane is less than 1.54°, the solar altitude near the lunar poles is very low, resulting in complex lighting conditions. The bottoms of impact craters in the lunar poles permanently block the sun, receiving only heat from space and the Moon's interior; these are called permanently shadowed regions. Water ice in the PSR (Polar Stress Retention System) can remain stable over long geological periods in the extremely low-temperature environment of the Moon. High-resolution full-moon maps reveal numerous impact craters in the lunar poles, with dozens densely distributed in the south polar region. Due to lighting conditions and topographical factors, many areas of these impact craters are permanently shadowed regions, potential storage areas for water ice. Therefore, water ice detection is a key focus of many lunar exploration missions, and accurately understanding the distribution and mass of lunar water ice is of great significance to lunar science and human exploration.

[0003] Due to the complex forms and significant uncertainties inherent in water ice resources, precise development and utilization of these resources are challenging, necessitating improved accuracy in water ice detection. Currently, various remote sensing technologies are applied to lunar exploration, such as visible light imaging, infrared imaging, laser altimeters, and radar detection. Water ice detection is a process that progresses from global to local detection. First, orbiters conduct surveys of key areas, such as the lunar polar regions, to identify large-scale water ice areas. Then, rovers explore these areas to achieve more refined in-situ detection. Radar signals possess a certain penetrating power and can identify water ice based on the polarization and reflection characteristics of the echo. Therefore, given the current inability to reach permanently shadowed regions for in-situ detection, using orbiter radar remote sensing for water ice detection at the lunar south pole is both necessary and feasible.

[0004] However, due to the highly complex interaction between radar waves and the lunar surface, existing technologies have not considered the problem of high circular polarization caused by a rough lunar surface, resulting in the inability to detect or low detection accuracy. Summary of the Invention

[0005] The purpose of this invention is to provide a method based on LOLA and M 3A Mini-RF method for detecting the distribution and content of water ice at the lunar south pole was developed. This method detects the distribution of water ice at the lunar south pole (87°S-90°S) and overcomes the limitation that visible light cannot detect permanently shadowed areas of the moon. Using radar signals, the polarization properties of the lunar south pole were analyzed based on Stokes parameters. The location of water ice points was detected based on radar scattering characteristics, and the spatial distribution of water ice was obtained. A dielectric constant inversion model was established, and the dielectric constant of the lunar south pole (87°S-90°S) was inverted. Fitting models of simulated lunar soil with different water ice content ratios and dielectric constants were obtained. Combining the water ice detection results and the dielectric constant values, the water ice content was calculated, providing a reference for subsequent lunar exploration projects.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method based on LOLA and M 3 The auxiliary Mini-RF method for detecting the distribution and content of water ice in the lunar south pole includes the following steps:

[0008] Step 1) Acquire Mini-RF radar images and perform data preprocessing to obtain a large-scale image of the lunar south pole, and obtain data characterizing the polarization properties of the lunar south pole based on Stokes parameters;

[0009] Step 2) Analyze the polarization properties of the lunar south pole, including the circular polarization ratio, backscattering coefficient, and polarizability;

[0010] Step 3) Based on the multiple water ice polarization properties and combined with topographic factors, water ice detection and identification are carried out to obtain water ice detection results in the lunar south polar region;

[0011] Step 4) Combine with M 3 The mineral mapper supplements the data gaps in the Mini-RF radar images with water ice detection results and cross-validates the detection results;

[0012] Step 5) Establish an m–χ hybrid polarization decomposition model, obtain the polarization decomposition results of the entire lunar south pole region, and analyze the radar scattering characteristics of total internal reflection at the water ice point.

[0013] Step 6) Establish a dielectric constant inversion model based on the backscattering coefficient to obtain the dielectric constant inversion results for the lunar south pole region;

[0014] Step 7) Based on the measured dielectric constant values ​​of lunar soil with different water ice content ratios, establish a fitting model between water ice content and dielectric constant;

[0015] Step 8) Obtain the water ice content of each water ice point at the lunar south pole based on the fitted model, and analyze the average water ice content of water ice points in the permanently shadowed areas of the crater.

[0016] The latitude range of the lunar south pole is 87°S-90°S.

[0017] Step 1) includes the following steps:

[0018] Step 11) Obtain the S-band Mini-RF secondary data after radiometric and polarization correction, perform projection conversion and mosaicking to obtain Mini-RF data of the lunar antarctic region;

[0019] Step 12) Considering the Mini-RF as a coherent dual-polarization SAR, transmitting with left-hand circular polarization and receiving with horizontal and vertical linear polarization at the receiver, four Stokes parameters are obtained, expressed as:

[0020]

[0021] In the formula, E is the complex voltage of subscript polarization, * is the complex conjugate voltage, <...> indicates average, Re and Img are the real and imaginary parts of the complex cross product amplitude, respectively, L represents the left-hand circular polarization of the transmitter, H represents the horizontal linear polarization of the receiver, V represents the vertical linear polarization of the receiver, R represents the right-hand circular polarization of the receiver, S1 is the total intensity of the backscattered field, S2 is the difference between the horizontal and vertical components of the electromagnetic field polarization, and S3 and S4 are the real and imaginary parts, representing the cosine and sine of the average phase between the horizontal and vertical polarization components of the field, describing the polarization characteristics of the backscattered wave.

[0022] Step 2) includes the following steps:

[0023] Step 21) Determine the circular polarization ratio:

[0024] Numerous sub-parameters characterizing the properties of electromagnetic waves are derived from the Stokes parameters. The circular polarization ratio is defined as the ratio of the total received energy of unidirectional polarization to the total energy of anti-polarization.

[0025]

[0026] On flat, smooth surfaces, unidirectional scattering mainly occurs, with a relatively large total reverse polarization energy and a low circular polarization ratio; on rough surfaces, multiple scattering occurs, with the total energy of the returned unidirectional polarization and the total energy of the reverse polarization being approximately equal, and the circular polarization ratio approaching 1.

[0027] Step 22) Determine the polarizability:

[0028] Polarizability is defined as the ratio of the polarization power to the total power of an electromagnetic wave.

[0029]

[0030] The polarization properties include cases of complete polarization, partial polarization, or complete depolarization. The polarizability of random or depolarized backscattering is close to 0, while the polarizability of polarized backscattering is close to 1. For surface scattering and double reflection scattering that belong to the polarized part, the value of m is closer to or equal to 1, while for volume scattering that belongs to the random or completely depolarized part, the value of m is closer to or equal to 0.

[0031] Step 23) Determine the Poincaré ellipticity:

[0032] The Poincaré ellipticity is a shape parameter that describes the degree of polarization ellipticity. The shape of the ellipse is determined by the magnitude and relative phase of the horizontal and vertical components of the electric field vector.

[0033]

[0034] The Poincaré ellipticity χ ranges between +45° and -45°. An ellipticity χ = +45° corresponds to left-hand circular polarization, and χ = -45° corresponds to right-hand circular polarization. The χ parameter is more sensitive to even-order and odd-order bounce scattering.

[0035] Step 24) Determine the relative phase:

[0036] The relative phase δ describes the measure of electromagnetic wave change. It is the angular difference in phase between the two components of the electric field vector and indicates the phase reversal indicated in the scattering type.

[0037]

[0038] Double bounce scattering involves two phase inversions, which results in the received and transmitted symbols having the same phase. In other words, the relative phase is quite sensitive to double bounce scattering.

[0039] Step 25) Determine the backscattering coefficient:

[0040] The backscattering coefficient is the radar reflectivity per unit cross-sectional area of ​​a target in the incident direction, representing the radar intensity in the incident direction or the average backscattering cross-section per unit area of ​​the target.

[0041] LH intensityσ LH =(S1+S4) / 2

[0042] The backscattering coefficient is a combination of three factors: unit cross section, reflectivity, and directionality. In addition to being related to radar system parameters, it is also related to the dielectric constant, surface roughness, and geometric properties of the object. The horizontal polarization backscattering coefficient and the vertical polarization backscattering coefficient are calculated based on the Stokes parameters.

[0043] Step 26): Due to the low-loss property of water ice, when the increment of the scattering intensity is twice or higher than the average value, the water ice in the near-surface of the moon can be separated from the rock according to the enhanced scattering intensity, and the position of water ice enrichment in the crater is determined:

[0044]

[0045] In the formula, α is the increment of the SC echo, γ is the increment of the OC echo, and the weighted sum is the increment of the total power; the conditions for water ice enrichment are as follows:

[0046]

[0047] The specific content of step 3) is as follows: High-precision water ice detection is carried out according to the form of electromagnetic wave transmission and reception by Mini-RF combined with radar reflection characteristics, and the influence of roughness on the circular polarization ratio is eliminated by combining the lunar surface roughness data to realize the division of water ice and the rough lunar surface. The method for water ice detection and identification based on the polarization properties of water ice and combined with terrain factors is as follows:

[0048] Due to the transmission of radar waves in the water ice medium, the co-polarization energy and cross-polarization energy of the received radar polarization echo are more, and the CPR of water ice with volume scattering characteristics is greater than 1; for volume scattering, the polarization degree m value is closer to or equal to 0, and the area where m is less than 0.2 is regarded as a possible point for water ice detection; water ice has volume scattering characteristics, and substances with a relative phase in the range of 0° < δ < 80° and 100° < δ < 180° are regarded as possible water ice; the high value of the scattering coefficient represents the scattering of the rock surface and sediment spallum, and the area where the backscattering coefficient is less than -15 dB is regarded as a possible location for water ice detection; according to the co-polarization reflection energy and cross-polarization reflection energy, three different types of craters, namely rough, frozen, and abnormal / double scattering, are divided, and the incremental weight of the frozen crater is set to 0.5 < w < 1; in water ice detection, the rough area of the lunar surface with a certain roughness is removed to eliminate the higher CPR value caused by surface roughness to a certain extent.

[0049] Step 4) includes the following steps:

[0050] Step 41): Based on the radar water ice detection results obtained in step 3), combined with the M3 mineral mapper, water ice detection is carried out based on the spectral analysis method to complete the radar water ice detection results, specifically as follows:

[0051] Water ice substances have three significant absorption peaks, and the absorption characteristics of the mineral mapper spectrum near 1.3, 1.5, and 2.0 μm are consistent with the absorption characteristics of water ice. It is considered that the pixels in the area that meet this absorption characteristic contain water ice;

[0052] Step 42) Verify the water ice detection results by combining data from different types of detectors. Within a certain range of the water ice point, the water ice detection results obtained by the two methods are considered to be consistent. Outside a certain range, they are inconsistent. In order to obtain more accurate cross-validation results, a buffer analysis is performed on the water ice detection results obtained from the two types of data, and the overlap area is calculated to quantitatively analyze the water ice detection results.

[0053] Step 5) specifically involves:

[0054] Based on the physical properties of lunar surface materials represented by the sub-parameters derived from the Stokes parameters, and using the Poincaré ellipticity χ as the second decomposition variable in polarization decomposition, an m–χ mixed polarization decomposition model is established to perform polarization decomposition to distinguish between three types of radar backscattering: surface scattering, double scattering, and volume scattering.

[0055]

[0056] In the formula, the relative contribution of surface scattering is represented by BS; the relative contribution of double scattering is represented by RD; and the relative contribution of volume scattering is represented by GV.

[0057] Step 6) includes the following steps:

[0058] Step 61) As a dual-polarization hybrid radar system, Mini-RF receives orthogonal horizontal and vertical backscattered information. The intensity of the backscattered radar signal is mainly a function of surface roughness and dielectric properties.

[0059] Step 62) Due to the orthogonal polarization Fresnel transmission coefficient T H and T V The differences between LH and LV backscattering coefficients are well represented by their ability to characterize the polarization properties of the subsurface. When the radar signal returns, the radar echo includes surface and subsurface echoes. The subsurface echo is modified through two channels passing through the upper interface and attenuated according to loss characteristics and depth. The observed LH / LV ratio is given by the following formula:

[0060]

[0061] In the formula, A is the attenuation factor. According to the above expression, Setting it to 0 yields the minimum dielectric constant of the surface:

[0062]

[0063] In the formula, θ is the incident angle of the radar signal. It is the backscattering coefficient of horizontal polarization. ε is the backscattering coefficient of vertical polarization, and ε is the obtained dielectric constant.

[0064] Step 7) specifically involves: based on the measured dielectric constant of lunar soil with different water ice content ratios, establishing an empirical fitting model of the relationship between water ice-lunar soil mixtures with different water ice content ratios and dielectric constants, and actually quantifying the water ice in the permanently shadowed region of the lunar south pole.

[0065] Step 8) includes the following steps:

[0066] Step 81) Based on the empirical fitting model of the relationship between lunar soil mixtures with different water ice content ratios and dielectric constants, determine the water ice content ratio at each water ice point according to the dielectric constant obtained in Step 6).

[0067] Step 82) Based on the water ice content inversion results, the water ice content of the permanent shadowed areas of each impact crater is statistically analyzed, and the average water ice content of the permanent shadowed areas is calculated. For connected impact craters, they are counted as a single permanent shadowed area.

[0068] Compared with the prior art, the present invention has the following beneficial effects:

[0069] (1) Given the current inability to reach permanently shadowed areas for in-situ exploration, this invention utilizes Mini-RF dual-polarization synthetic aperture radar data to detect lunar south polar water ice. It detects lunar south polar water ice using radar data, combined with M... 3 The mineral mapping instrument enabled the precise location of detailed water ice points in the lunar south polar region.

[0070] (2) This invention studies the spatial distribution characteristics and scattering polarization properties of water ice in the lunar south pole region, analyzes the influencing factors of water ice detection, and combines the establishment of a dielectric constant inversion model to perform water ice inversion, thereby realizing the accurate analysis of water ice content in the permanent shadow region of the south pole.

[0071] (3) This invention takes into account the problem that a rough lunar surface may lead to high circular polarization of radar waves. Based on the form of transmitting and receiving electromagnetic waves by Mini-RF, combined with the radar reflection characteristics such as the echo intensity and backscattering coefficient of water ice, high-precision water ice detection is carried out. In addition, the influence of roughness on the circular polarization ratio is eliminated by combining lunar surface roughness data, so as to realize the separation between water ice and rough lunar surface and improve detection accuracy. Attached Figure Description

[0072] Figure 1 This is a flowchart of the method of the present invention;

[0073] Figure 2 This is a diagram of the case study area for the embodiment;

[0074] Figure 3 Analysis results of the circular polarization ratio in the lunar range of 87°S-90°S;

[0075] Figure 4Results of water ice detection at the lunar south pole (87°S-90°S);

[0076] Figure 5 Mini-RF water freezing point and reference water freezing point in the range of 87°S-90°S from the lunar south pole;

[0077] Figure 6 The water ice content at the water freezing point of the lunar south pole (87°S-90°S). Detailed Implementation

[0078] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0079] Limited by lighting conditions and its own imaging characteristics, optics lack the ability to image and detect permanently shadowed areas, while radar detection has a certain penetrating power and can be used to detect permanently shadowed areas at the lunar poles. When radar electromagnetic waves penetrate the lunar surface, water ice, as a volatile substance with total internal reflection properties, can cause multiple reflections of the radar waves, i.e., changes in polarization direction. This makes the radar echo have the same polarization direction as the incident radar electromagnetic wave, producing a coherent backscattering counter-effect under certain conditions, thereby enhancing the echo in the same direction. Furthermore, water ice is a low-loss medium; the transmission loss of frozen volatiles is lower than that of silicate rocks, resulting in weaker absorption of electromagnetic waves and more reflected electromagnetic waves, thus producing a higher average radar reflectivity. The total internal reflection property of water ice preserves the circular polarization propagation in the scattered signal, which can be enhanced through constructive interference. Therefore, the circular polarization ratio of the radar signal scattered by water ice is key to using radar data for water ice detection.

[0080] The interaction between radar waves and the lunar surface is highly complex. It's necessary to consider that a rough lunar surface can also lead to high circular polarization. High-precision water ice detection is achieved by combining the form of electromagnetic waves transmitted and received by Mini-RF with radar reflection characteristics such as water ice echo intensity and backscattering coefficient. Furthermore, lunar surface roughness data is used to eliminate the influence of roughness on the circular polarization ratio, thus distinguishing between water ice and the rough lunar surface. Polarization decomposition analysis provides unique interpretation information about the vertical distribution of materials and the presence of coherent water ice sediments. This method, based on Stokes sub-parameters characterizing the physical properties of lunar surface materials such as polarizability, ellipticity, and circular polarization ratio, uses two or more sub-parameters in combination for polarization decomposition to differentiate between surface, two-way, and volume backscattering radar backscattering categories. This allows for qualitative derivation of the spatial distribution of lunar surface components, revealing the interaction between electromagnetic waves and the lunar regolith, and better reflecting the reliability of water ice detection results, providing corroboration for water ice detection results at the lunar south pole.

[0081] As a dual-polarization hybrid radar system, the Mini-RF transmits a left-handed circularly polarized signal while simultaneously receiving orthogonal horizontal and vertical backscattered information. For a pair of polarized images of the same area, the intensity of the backscattered radar signal is primarily a function of surface roughness and dielectric properties. Due to the Fresnel transmission coefficient T of the two orthogonal polarizations... H and T V The differences in horizontal and vertical backscattering coefficients are quite helpful in characterizing the polarization properties of the subsurface. The main factors affecting the backscattering properties of the lunar surface and subsurface are the roughness of the upper interface and the actual dielectric constant, as well as the electromagnetic loss characteristics of each layer. The dielectric constant of the subsurface material can be inverted by the relationship between the S-band horizontal and vertical polarization backscattering coefficients and the dielectric constant obtained by Mini-RF.

[0082] Radar signals have a certain penetrating ability and can identify water ice based on the polarization and radar reflection characteristics of the echo. Given the current inability to reach the permanently shadowed region for in-situ detection, it is necessary and feasible to conduct lunar south pole water ice detection based on Mini-RF dual-polarization synthetic aperture radar data. By detecting lunar south pole water ice through radar data, we can achieve water ice detection in the lunar south pole region, study the spatial distribution characteristics and scattering polarization properties of water ice, analyze the influencing factors of water ice detection, and combine the establishment of a dielectric constant inversion model to invert water ice content and analyze the water ice content in the permanently shadowed region of the south pole.

[0083] Based on the above analysis, this embodiment provides a method based on LOLA and M 3 Auxiliary Mini-RF methods for detecting the distribution and content of water ice in the lunar south pole, such as Figure 1 As shown, it includes the following steps:

[0084] Step 1) Acquire Mini-RF radar images and perform data preprocessing to obtain a large-scale image of the lunar south pole (87°S-90°S). Obtain data characterizing the polarization properties of the lunar south pole based on Stokes parameters.

[0085] This embodiment specifically examines Mini-RF dual-polarization radar data used for water ice detection and LRO LOLA and Chandrayaan-1M. 3 Auxiliary data is used for data preprocessing.

[0086] Step 11) Obtain the S-band Mini-RF secondary data after radiometric and polarization correction, perform projection conversion and mosaicking to obtain Mini-RF data of the lunar antarctic region.

[0087] Step 12) Considering the Mini-RF as a coherent dual-polarization SAR, transmitting with left-hand circular polarization and receiving with horizontal and vertical linear polarization at the receiver, four Stokes parameters are obtained, expressed as:

[0088]

[0089] In the formula, E is the complex voltage of subscript polarization, * is the complex conjugate voltage, <...> indicates average, Re and Img are the real and imaginary parts of the complex cross product amplitude, respectively, the first subscript indicates the transmitting polarization, the second subscript indicates the receiving polarization; L indicates the transmitting left-hand circular polarization, H indicates the receiving horizontal linear polarization, V indicates the receiving vertical linear polarization; R indicates the receiving right-hand circular polarization; S1 is the total intensity of the backscattered field; S2 is the difference between the horizontal and vertical components of the electromagnetic field polarization; S3 and S4 are the real and imaginary parts, representing the cosine and sine of the average phase between the horizontal and vertical polarization components of the field, describing the polarization characteristics of the backscattered wave.

[0090] The research area targeted in this embodiment is as follows: Figure 2 As shown.

[0091] Step 2) A detailed analysis of the polarization properties of water ice, such as the circular polarization ratio, polarizability, and scattering intensity, is conducted from the overall to the local level. The CPR of the inner region of the crater is generally higher than that of the outer region. At the same time, the backscattering intensity of the water ice-rich region is relatively lower. The backscattering signal polarizability at the bottom of the crater is low, exhibiting volume scattering characteristics, which is consistent with the radar scattering properties of water ice.

[0092] Step 21) Determine the circular polarization ratio:

[0093] Numerous sub-parameters characterizing the properties of electromagnetic waves are derived from the Stokes parameters. The circular polarization ratio is defined as the ratio of the total received energy of unidirectional polarization to the total energy of anti-polarization.

[0094]

[0095] On flat, smooth surfaces, unidirectional scattering mainly occurs, with a relatively large total reverse polarization energy and a low circular polarization ratio. On rough surfaces, multiple scattering occurs, with the returned total unidirectional and reverse polarization energies being approximately equal, and the circular polarization ratio approaching 1. Figure 3 As shown;

[0096] Step 22) Determine the polarizability:

[0097] Polarizability is defined as the ratio of the polarization power to the total power of an electromagnetic wave.

[0098]

[0099] The polarization properties include cases of complete polarization, partial polarization, or complete depolarization. The polarizability of random or depolarized backscattering is close to 0, while the polarizability of polarized backscattering is close to 1. For surface scattering and double reflection scattering that belong to the polarized part, the value of m is closer to or equal to 1, while for volume scattering that belongs to the random or completely depolarized part, the value of m is closer to or equal to 0.

[0100] Step 23) Determine the Poincaré ellipticity:

[0101] The Poincaré ellipticity is a shape parameter that describes the degree of polarization ellipticity. The shape of the ellipse is determined by the magnitude and relative phase of the horizontal and vertical components of the electric field vector.

[0102]

[0103] The Poincaré ellipticity χ ranges between +45° and -45°. An ellipticity χ = +45° corresponds to left-hand circular polarization, and χ = -45° corresponds to right-hand circular polarization. The χ parameter is more sensitive to even-order and odd-order bounce scattering.

[0104] Step 24) Determine the relative phase:

[0105] The relative phase δ describes the measure of electromagnetic wave change. It is the angular difference in phase between the two components of the electric field vector and indicates the phase reversal indicated in the scattering type.

[0106]

[0107] Double bounce scattering involves two phase inversions, which results in the received and transmitted symbols having the same phase. In other words, the relative phase is quite sensitive to double bounce scattering.

[0108] Step 25) Determine the backscattering coefficient:

[0109] The backscattering coefficient is the radar reflectivity per unit cross-sectional area of ​​a target in the incident direction, representing the radar intensity in the incident direction or the average backscattering cross-section per unit area of ​​the target.

[0110] LH intensityσ LH =(S1+S4) / 2

[0111] The backscattering coefficient is a combination of three factors: unit cross section, reflectivity, and directionality. In addition to being related to radar system parameters, it is also related to the dielectric constant, surface roughness, and geometric properties of the object. The horizontal polarization backscattering coefficient and the vertical polarization backscattering coefficient are calculated based on the Stokes parameters.

[0112] Step 26): Due to the low-loss property of water ice, when the increment of the scattering intensity is twice the average value or higher, the water ice near the lunar surface can be separated from the rock according to the enhanced scattering intensity, and the location of water ice enrichment in the crater is determined:

[0113]

[0114] In the formula, α is the increment of the SC echo, γ is the increment of the OC echo, and the weighted sum is the increment of the total power; the conditions for water ice enrichment are as follows:

[0115]

[0116] Step 3): Based on the multiple water ice polarization properties and combined with topographic factors, water ice detection and identification are carried out to obtain the water ice detection results in the lunar south pole region.

[0117] Specifically, high-precision water ice detection is carried out according to the form of electromagnetic wave emission and reception by Mini-RF and combined with radar reflection characteristics, and the influence of roughness on the circular polarization ratio is eliminated by combining the lunar surface roughness data, realizing the division of water ice and the rough lunar surface. The method for water ice detection and identification based on water ice polarization properties and combined with topographic factors is as follows:

[0118] Due to the transmission of radar waves in the water ice medium, more co-polarized energy and cross-polarized energy of the received radar polarization echo are obtained, and the CPR of water ice with volume scattering characteristics is greater than 1; for volume scattering, the polarization degree m value is closer to or equal to 0, and the region where m is less than 0.2 is taken as a possible point for water ice detection; water ice has volume scattering characteristics, and substances with relative phase in the range of 0° < δ < 80° and 100° < δ < 180° are regarded as possible water ice; the high value of the scattering coefficient represents the scattering of the rock surface and sediment spallation, and the region where the backscattering coefficient is less than -15 dB is taken as a possible location for water ice detection; according to the co-polarized reflection energy and cross-polarized reflection energy, three different types of craters, rough, frozen, and abnormal / double scattering, are divided, and the increment weight of the frozen crater is set to 0.5 < w < 1; in water ice detection, the rough lunar surface area with a certain roughness is removed, and the higher CPR value caused by surface roughness is eliminated to a certain extent.

[0119] In this embodiment, water ice detection results are obtained based on Mini-RF radar data, and a total of 1570 water ice points are obtained, as Figure 4 shown, and relevant spatial distribution analysis is carried out. In the overall range of the lunar south pole (87°S - 90°S), water ice shows an aggregated distribution; within a single crater, water ice shows a random distribution.

[0120] Step 4): Combine M 3The mineral mapper supplements the data gaps in the Mini-RF radar imagery with water ice detection results and cross-validates the detection results, such as... Figure 5 As shown.

[0121] Step 41) Based on the radar water ice detection results obtained in Step 3), and combined with the M3 mineral mapper, water ice detection is performed using spectral analysis methods to complete the radar water ice detection results, as detailed below:

[0122] Water ice material has three significant absorption peaks. The absorption characteristics of mineral plotter spectra near 1.3, 1.5 and 2.0 μm are consistent with the absorption characteristics of water ice. It is believed that the pixels in the region that meet the absorption characteristics contain water ice.

[0123] Step 42) Verify the water ice detection results by combining data from different types of detectors. Within a certain range of the water ice point, the water ice detection results obtained by the two methods are considered to be consistent. Outside a certain range, they are inconsistent. In order to obtain more accurate cross-validation results, a buffer analysis is performed on the water ice detection results obtained from the two types of data, and the overlap area is calculated to quantitatively analyze the water ice detection results.

[0124] Step 5) Establish an m–χ hybrid polarization decomposition model to obtain the polarization decomposition results of the entire lunar south pole region and analyze the radar scattering characteristics of total internal reflection at the water ice point.

[0125] Specifically, based on the physical properties of lunar surface materials represented by the sub-parameters derived from the Stokes parameters, and using the Poincaré ellipticity χ as the second decomposition variable in polarization decomposition, an m–χ hybrid polarization decomposition model is established to perform polarization decomposition to distinguish between three types of radar backscattering: surface scattering, double scattering, and volume scattering.

[0126]

[0127] In the formula, the relative contribution of surface scattering is represented by BS; the relative contribution of double scattering is represented by RD; and the relative contribution of volume scattering is represented by GV.

[0128] The rationale for the presence of water ice in the 87°S-90°S region of the lunar south pole was analyzed based on the hybrid polarization decomposition model. The analysis results for the overall 87°S-90°S region and the local Shackleton crater indicate that the intensity of both volume scattering and double scattering is higher than that of surface scattering, which is consistent with the radar scattering characteristics of total internal reflection at the water ice point.

[0129] Step 6) Establish a dielectric constant inversion model based on the backscattering coefficient to obtain the dielectric constant inversion results for the lunar south pole region.

[0130] Step 61) As a dual-polarization hybrid radar system, Mini-RF receives orthogonal horizontal and vertical backscattered information. The intensity of the backscattered radar signal is mainly a function of surface roughness and dielectric properties.

[0131] Step 62) Due to the orthogonal polarization Fresnel transmission coefficient T H and T V The differences between LH and LV backscattering coefficients are well represented by their ability to characterize the polarization properties of the subsurface. When the radar signal returns, the radar echo includes surface and subsurface echoes. The subsurface echo is modified through two channels passing through the upper interface and attenuated according to loss characteristics and depth. The observed LH / LV ratio is given by the following formula:

[0132]

[0133] In the formula, A is the attenuation factor. According to the above expression, Setting it to 0 yields the minimum dielectric constant of the surface:

[0134]

[0135] In the formula, θ is the incident angle of the radar signal. It is the backscattering coefficient of horizontal polarization. ε is the backscattering coefficient of vertical polarization, and ε is the obtained dielectric constant.

[0136] Step 7) Based on the measured dielectric constant values ​​of lunar soil with different water ice content ratios, establish a fitting model between water ice content and dielectric constant.

[0137] Specifically, based on the measured dielectric constant of lunar soil with different water ice content ratios, an empirical fitting model was established to establish the relationship between the dielectric constant and the water ice-lunar soil mixture with different water ice content ratios. The water ice in the permanently shadowed region of the lunar south pole was actually quantified, and the results are shown in Table 1.

[0138] Table 1. Fitting relationship between water ice content and dielectric constant

[0139]

[0140] Step 8) Obtain the water ice content of each water ice point at the lunar south pole based on the fitted model, and analyze the average water ice content of water ice points in the permanently shadowed areas of the crater.

[0141] Step 81) Based on the empirical fitting model of the relationship between lunar soil mixtures with different water-ice content ratios and dielectric constants, and according to the dielectric constant obtained in Step 6), determine the water-ice content ratio at each water-ice point, such as... Figure 6 As shown.

[0142] Step 82) Based on the water ice content inversion results, the water ice content of the permanent shadowed areas of each impact crater is statistically analyzed, and the average water ice content of the permanent shadowed areas is calculated. For connected impact craters, they are counted as a single permanent shadowed area.

[0143] The water ice detection results and average water ice content obtained in this embodiment are shown in Tables 2 and 3.

[0144] Table 2. Water content and freezing point at lunar south poles (87°S-90°S).

[0145] water ice content ratio Number of water freezing points 0％ 540 0％-1％ 139 1％-2％ 92 2％-3％ 128 3％-4％ 84 4％-5％ 122 5％-6％ 109 6％-7％ 110 7％-8％ 85 8％-9％ 66 9％-10％ 41 10％-11％ 28 11％-12％ 11 >＝12％ 15

[0146] Table 3. Average water ice content in the permanently shadowed areas of each meteorite crater.

[0147] Meteorite Crater Number of water freezing points Water ice content (wt%) Haworth 200 2.1 Shoemaker 95 2.6 De gerlache 25 3.8 Henson+Sverdrup 139 3.5 Shackleton 46 3.9 Slater 44 1.6 Faustin+Malinkin 183 5.9

[0148] Based on the above results, it is shown that the method of the present invention can detect the water ice distribution at the lunar south pole (87°S-90°S) and overcome the defect that visible light cannot detect the permanently shadowed areas of the moon. Using radar signals, the polarization properties of the lunar south pole were analyzed based on Stokes parameters, and the location of water ice points was detected based on radar scattering characteristics to obtain the spatial distribution of water ice. A dielectric constant inversion model was established, and the dielectric constant of the lunar south pole (87°S-90°S) was inverted. Fitting models of simulated lunar soil with different water ice content ratios and dielectric constants were obtained. Combining the water ice detection results and the dielectric constant values, the water ice content was calculated with high precision.

[0149] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method based on LOLA and M 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Includes the following steps: Step 1) Acquire Mini-RF radar images and perform data preprocessing to obtain a large-scale image of the lunar south pole, and obtain data characterizing the polarization properties of the lunar south pole based on Stokes parameters; Step 2) Analyze the polarization properties of the lunar south pole, including the circular polarization ratio, backscattering coefficient, and polarizability; Step 3) Based on the multiple water ice polarization properties and combined with topographic factors, water ice detection and identification are carried out to obtain water ice detection results in the lunar south polar region; Step 4) Combine M 3 The mineral mapper supplements the data gaps in the Mini-RF radar images with water ice detection results and cross-validates the detection results; Step 5) Establish m – A hybrid polarization decomposition model was used to obtain polarization decomposition results for the entire lunar south pole region, and the radar scattering characteristics of total internal reflection at the water ice point were analyzed. Step 6) Establish a dielectric constant inversion model based on the backscattering coefficient to obtain the dielectric constant inversion results for the lunar south pole region; Step 7) Based on the measured dielectric constant values ​​of lunar soil with different water ice content ratios, establish a fitting model between water ice content and dielectric constant; Step 8) Obtain the water ice content of each water ice point at the lunar south pole based on the fitted model, and analyze the average water ice content of water ice points in the permanently shadowed areas of the crater. Step 1) includes the following steps: Step 11) Obtain the S-band Mini-RF secondary data after radiometric and polarization correction, perform projection conversion and mosaicking to obtain Mini-RF data of the lunar antarctic region; Step 12) Considering the Mini-RF as a coherent dual-polarization SAR, transmitting with left-hand circular polarization and receiving with horizontal and vertical linear polarization at the receiver, four Stokes parameters are obtained, expressed as: In the formula, E is the complex voltage of subscript polarization, ∗ is the complex conjugate voltage, <...> represents the average, Re and Img are the real and imaginary parts of the complex cross product amplitude, respectively, L represents the transmit left-hand circular polarization, H represents the receive horizontal polarization, V represents the receive vertical polarization, and R represents the receive right-hand circular polarization. This represents the total intensity of the backscattered field. This is the difference between the horizontal and vertical components of the electromagnetic field polarization. and For the real and imaginary parts, denoted as the cosine and sine of the average phase between the horizontal and vertical polarization components of the field, respectively, describing the polarization characteristics of the backscattered wave.

2. A method based on LOLA and M according to claim 1 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, The latitude range of the lunar south pole is 87°S-90°S.

3. A method based on LOLA and M according to claim 1 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 2) includes the following steps: Step 21) Determine the circular polarization ratio: Numerous sub-parameters characterizing the properties of electromagnetic waves are derived from the Stokes parameters. The circular polarization ratio is defined as the ratio of the total received energy of unidirectional polarization to the total energy of anti-polarization. On flat, smooth surfaces, unidirectional scattering mainly occurs, with a relatively large total reverse polarization energy and a low circular polarization ratio; on rough surfaces, multiple scattering occurs, with the total energy of the returned unidirectional polarization and the total energy of the reverse polarization being approximately equal, and the circular polarization ratio approaching 1. Step 22) Determine the polarizability: Polarizability is defined as the ratio of the polarization power to the total power of an electromagnetic wave. Polarization properties include cases of complete polarization, partial polarization, or complete depolarization. Random or depolarized backscattering has a polarizability close to 0, while polarized backscattering has a polarizability close to 1. For surface scattering and double reflection scattering that belong to the polarized portion... m A value closer to or equal to 1 is more favorable for volume scattering that belongs to the random or fully depolarized portion. m The value is closer to or equal to 0; Step 23) Determine the Poincaré ellipticity: The Poincaré ellipticity is a shape parameter that describes the degree of polarization ellipticity. The shape of the ellipse is determined by the magnitude and relative phase of the horizontal and vertical components of the electric field vector. Poincaré ellipticity The value of χ ranges between +45° and -45°. The ellipticity χ = +45° corresponds to left-hand circular polarization, and χ = -45° corresponds to right-hand circular polarization. The χ parameter is more sensitive to even-order and odd-order bounce scattering. Step 24) Determine the relative phase: The relative phase δ describes the measure of electromagnetic wave change. It is the angular difference in phase between the two components of the electric field vector and indicates the phase reversal indicated in the scattering type. Double bounce scattering involves two phase inversions, resulting in the received and transmitted symbols having the same phase. In other words, the relative phase is quite sensitive to double bounce scattering. Step 25) Determine the backscattering coefficient: The backscattering coefficient is the radar reflectivity per unit cross-sectional area of ​​a target in the incident direction, representing the radar intensity in the incident direction or the average backscattering cross-section per unit area of ​​the target. The backscattering coefficient is a combination of three factors: unit cross section, reflectivity, and directionality. In addition to being related to radar system parameters, it is also related to the dielectric constant, surface roughness, and geometric properties of the object. The horizontal polarization backscattering coefficient and the vertical polarization backscattering coefficient are calculated based on the Stokes parameters. Step 26) Due to the low-loss nature of water ice, when the increase in scattering intensity is twice or higher than the average value, the location of water ice enrichment in the crater can be determined based on the ability of enhanced scattering intensity to separate water ice from rocks near the lunar surface. In the formula, α is the increment of the SC echo, γ is the increment of the OC echo, and the weighted sum is... It is the increment of total power; the conditions for water ice enrichment are as follows: 。 4. A method based on LOLA and M according to claim 3 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 3) specifically involves: using the form of Mini-RF transmitted and received electromagnetic waves combined with radar reflection characteristics to perform high-precision water ice detection, and combining lunar surface roughness data to eliminate the influence of roughness on the circular polarization ratio, thereby achieving the distinction between water ice and the rough lunar surface. The method for water ice detection and identification based on water ice polarization properties and combined with topographic factors is as follows: Due to the transmission of radar waves in the water-ice medium, the received radar polarization echo has more in-phase and out-of-phase energy, and the CPR of water-ice, which has volume scattering characteristics, is greater than 1; for volume scattering, the polarizability... m The value is closer to or equal to 0, m Regions with a backscattering coefficient less than 0.2 are considered potential locations for water ice detection. Water ice exhibits volume scattering characteristics; materials with a relative phase between 0° < δ < 80° and 100° < δ < 180° are considered potential water ice. High scattering coefficients represent scattering from rock surfaces and sedimentary ejecta; regions with a backscattering coefficient less than -15 dB are considered potential locations for water ice detection. Based on co-directional and aniso-directional reflection energy, impact craters are classified into three different categories: rough, frozen, and anomalous / double-scattering. The incremental weight for frozen impact craters is set to 0.5 < δ ... w <1; In water ice detection, a certain degree of roughness in the lunar surface rough area is eliminated, which to some extent eliminates the high CPR value caused by surface roughness.

5. A method based on LOLA and M according to claim 4 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 4) includes the following steps: Step 41) Based on the radar water ice detection results obtained in Step 3), and combined with the M3 mineral mapper, water ice detection is performed using spectral analysis methods to complete the radar water ice detection results, as detailed below: Water ice material has three significant absorption peaks. The absorption characteristics of mineral plotter spectra near 1.3, 1.5 and 2.0 μm are consistent with the absorption characteristics of water ice. It is believed that the pixels in the region that meet the absorption characteristics contain water ice. Step 42) Verify the water ice detection results by combining data from different types of detectors. Within a certain range of the water ice point, the water ice detection results obtained by the two methods are considered to be consistent. Outside a certain range, they are inconsistent. In order to obtain more accurate cross-validation results, a buffer analysis is performed on the water ice detection results obtained from the two types of data, and the overlap area is calculated to quantitatively analyze the water ice detection results.

6. A method based on LOLA and M according to claim 5 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 5) specifically involves: Based on the physical properties of lunar surface materials represented by the sub-parameters derived from the Stokes parameters, and using the Poincaré ellipticity χ as the second decomposition variable in polarization decomposition, a system is established. m – A hybrid polarization decomposition model is used to perform polarization decomposition to distinguish between three types of radar backscattering: surface scattering, double scattering, and volume scattering. In the formula, the relative contribution of surface scattering is represented by BS; the relative contribution of double scattering is represented by RD; and the relative contribution of volume scattering is represented by GV.

7. A method based on LOLA and M according to claim 6 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 6) includes the following steps: Step 61) As a dual-polarization hybrid radar system, Mini-RF receives orthogonal horizontal and vertical backscattered information. The intensity of the backscattered radar signal is mainly a function of surface roughness and dielectric properties. Step 62) Due to the Fresnel transmission coefficient of orthogonal polarization and The differences between LH and LV backscattering coefficients are well represented by their ability to characterize the polarization properties of the subsurface. When the radar signal returns, the radar echo includes surface and subsurface echoes. The subsurface echo is modified through two channels passing through the upper interface and attenuated according to loss characteristics and depth. The observed LH / LV ratio is given by the following formula: In the formula, A is the attenuation factor. According to the above expression, Setting it to 0 yields the minimum dielectric constant of the surface: In the formula, It is the radar signal incident angle. It is the backscattering coefficient of horizontal polarization. It is the backscattering coefficient of vertical polarization. The dielectric constant is obtained.

8. A method based on LOLA and M according to claim 7 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 7) specifically involves: based on the measured dielectric constant values ​​of lunar soil with different water ice content ratios, establishing an empirical fitting model of the relationship between water ice-lunar soil mixtures with different water ice content ratios and dielectric constants, and actually quantifying the water ice in the permanently shadowed region of the lunar south pole.

9. A method based on LOLA and M according to claim 8 3 A Mini-RF method for detecting the distribution and content of water ice in the lunar south pole, characterized in that, Step 8) includes the following steps: Step 81) Based on the empirical fitting model of the relationship between lunar soil mixtures with different water ice content ratios and dielectric constants, determine the water ice content ratio at each water ice point according to the dielectric constant obtained in Step 6. Step 82) Based on the water ice content inversion results, the water ice content of the permanent shadowed areas of each impact crater is statistically analyzed, and the average water ice content of the permanent shadowed areas is calculated. For connected impact craters, they are counted as a single permanent shadowed area.

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