【作者】 法文哲；

【导师】 金亚秋；

【作者基本信息】 复旦大学 ， 电路与系统， 2008， 博士

【摘要】 在漫长的月球地质历史时期中,月球表面受陨石小天体的撞击,受太阳—宇宙线的照射,储存了月球地质与太阳辐射活动的历史。对月球表层物质结构、状态及其分布的了解,可以为月球地质提供重要的信息,同时也对月球资源的科学认识、未来的探月、登月以及月球开发等都具有十分重要的意义。月表物质为低损耗介质,适当频率的电磁波可以穿透到月表以下一定深度,从而可以揭示月球表层的地质结构特征。近来,在地球遥感中广泛应用的星载微波遥感技术已经开始应用到对月球表层结构的探测中。本文以电磁波与月球表层物质的相互作用为物理基础,结合月球表面特有的地形地貌与分层结构特征,研究和发展了一套用于探测月球表面月壤特性、次表层结构特征的主被动微波遥感理论模型和参数反演方法,旨在初步解决目前微波遥感在月球探测中的一些基本问题,也希望能对我国现行的和将来的探月工程提供理论依据和指导。本文分为被动微波遥感对月壤特性的研究、主动雷达遥感对月球表面和次表层的研究以及作为扩展的火星探测中的微波遥感技术三部分。在被动微波遥感对月壤特性的研究中,给出了月球表面微波热辐射的理论建模,完成了月表辐射亮度温度的正向模拟,提出了一种月表辐射亮度温度定标方法,进而给出了月壤厚度的反演方法,并对反演的误差进行了分析,最后定量估算了整个月壤层中³He（氦-3）资源的含量。在主动雷达遥感对月表的探测中,结合月表特定的地形地貌给出了月球表面合成孔径雷达（SAR）成像模拟,提出了全极化脉冲波对月壤层结构的探测方案,最后模拟了高频雷达探测仪对月球次表层结构探测的回波,分别实现了雷达对月球表面、浅层月壤结构和次表层结构的探测。作为全文的扩展,结合我国深空探测的进一步需要,回顾了国际上对火星的探测历史及其研究现状,提出了主被动微波遥感对火星表层土壤层、水（或冰）探测的几种可行方案。在被动微波遥感对月球表面的观测中,月表的辐射亮度温度主要与月壤层厚度、月壤介电常数和物理温度等参数有关。首先根据月壤厚度与月球表面高度的一种对应关系,由Apollo（阿波罗）着陆点月壤厚度的实测值和月球表面数字高程分布,试验性地构造整个月表的月壤厚度分布。由美国Clementine（克莱门汀）探月卫星给出的月球表面紫外-可见光光学数据,计算了整个月球表面月壤中FeO+TiO₂含量分布,再根据月壤介电常数与FeO+TiO₂含量之间的关系,给出了整个月球表面月壤的介电常数分布。根据月壤的热导率等特性,研究了月壤物理温度随深度的变化。由月球表面物理温度的实际测量值和理论计算值,给出了月球表面和月壤深层温度随纬度单调变化的经验性关系。在这些条件的基础之上,将月球表层分为三层,即月尘层、月壤层、月岩层,由三平行分层介质热辐射的起伏逸散定理模拟了整个月球表面多通道辐射亮度温度分布,并讨论了辐射亮度温度随有关参数的变化情况。在作为逆问题的月壤厚度反演中,将以上辐射亮度温度的模拟值加随机噪声作为理论观测,提出一种月壤厚度的反演方法。由于FeO+TiO₂含量较高时高频通道的穿透深度小,选择月球表面同一纬度不同经度处月壤中FeO+TiO₂含量较高的地方作为参考点,由两层月壤热辐射模型反演月尘层和月壤层物理温度随纬度的变化。然后,以上述参考点反演的物理温度为已知量,由穿透深度大的低频通道的辐射亮度温度反演整个月球表面的月壤厚度分布。由辐射亮度温度、月壤厚度与月壤层各参数之间的关系,对月壤厚度的反演误差进行了分析与讨论。根据Apollo着陆点月壤厚度和FeO+TiO₂含量的实测值,提出了一种辐射亮度温度验证与定标的方法,最后给出了月壤厚度的实际反演方案。在获得了月球表面辐射亮度温度以及准确反演出月壤厚度之后,可以对月球表面月壤层中³He等资源的含量与总量进行估算。太阳风注入月球表面月壤层的³He是一种可供人类长期使用的潜在月球资源。月壤中³He含量主要与太阳风通量、月壤成熟度以及月壤中TiO₂含量等因素有关。根据受地球磁尾影响的太阳风通量模型,给出了月球表面归一化太阳风通量随经纬度的分布。根据Clementine紫外—可见光光学数据,计算了整个月球表面月壤光学成熟度和月壤中TiO₂含量的分布。在此基础之上,根据Apollo月壤样品的测量结果,给出月壤表层³He含量与月表归一化太阳风通量、月壤光学成熟度以及TiO₂含量之间的关系,并由此计算了整个月球表面月壤表层³He含量的分布。以月球表面数字高程经验性构造的月壤厚度为例,给出了月球表面整个月壤层单位面积内³He含量分布,并估算了整个月壤层内所蕴含的³He总量。在主动雷达遥感对月球表层结构的探测中,不同频率的电磁波可以获取月球表层不同深度处的信息,本文研究了月球表面合成孔径雷达成像模拟,提出了低空飞行全极化L波段雷达窄脉冲对月壤层结构的探测方案,模拟了高频雷达探测仪对月球次表层地质结构探测的回波,分别实现了雷达对月球表面、浅层月壤结构和次表层结构特征的探测。为模拟月球表面的合成孔径雷达成像,按照月球表面环形山数目和形态的统计特征,数值构造了不同特征环形山分布的非均匀起伏月球表面地形。根据月球表面地形起伏坡度的非均匀特征,提出了一种由不规则三角形网格剖分来构造月球表面数字地形的方法。对于地形起伏变化小的地方,剖分面元尺寸大,而对于起伏变化大的地方,剖分面元尺寸小,能用尽量少的面元表征非均匀月球表面的随机高度起伏。在该数字模型的基础上,由Kirchhoff近似给出了月球表面雷达回波的计算,数值模拟了两种不同形状环形山以及密集环形山分布的非均匀起伏月球表面的合成孔径雷达成像。以Apollo 15着陆点地区月球表面真实的数字高程和Clementine紫外-可见光光学数据为基础,给出了Apollo 15地区月球表面SAR成像模拟结果。最后,对模拟的SAR图像特征做了定量分析,并与月球表面真实SAR图像做了定性比较。为探测浅表层月壤的特性,进一步提出了低空飞行全极化L波段雷达窄脉冲探测月壤层厚度与结构的建议。月壤层为一层具有上下随机粗糙界面的有耗介质层,在下垫月岩粗糙界面上有一层随机分布的碎石散射体。推导出了包含面散射、体散射,以及面-体相互作用七种散射机制的全极化脉冲波Mueller矩阵解。以月壤特征参数如月壤层厚、FeO+TiO₂含量、介电常数、月壤界面粗糙度、碎石分布等为函数,用时域Mueller矩阵解数值模拟验证了该方法的可行性,结果表明L波段窄脉冲极化回波波形能用于反演或估算月壤厚度与分层结构。与微波频段相比较,高频波段的电磁波可以穿透到月表以下几百米至几千米的月表深层,从而可以揭示出月球次表层结构特征。在月球次表层探测中,高频雷达探测仪主要通过表面天底点和次表面天底点回波的时延差与强度来给出月球次表层的结构与物质状态。由于电磁波在次表层传播过程中受衰减、透射、散射等影响,次表面的回波往往很微弱。受月球表面非均匀起伏地形的影响,来自月球表面非天底点的较强回波往往会淹没次表面天底点回波,成为次表层结构探测中的最大障碍。因此,对电磁波在月球表层内传播、散射、衰减的定量模拟,对于如何从雷达探测仪回波中提取微弱的次表面天底点回波,进而获取月球次表层结构特征具有非常重要的意义。基于Kirchhoff近似、几何光学和射线追踪方法,提出一种快速模拟月球表面雷达探测仪回波的方法。首先给出雷达探测仪对月球次表层结构探测的原理与雷达探测仪回波的模拟流程。由偶极子天线局部坐标系与月球主坐标系之间的关系,给出了雷达探测仪在月球表面的辐射场分布。根据月球表面地形特征,以模拟构造的月球表面数字高程为例,由规则三角形网格对月表地形进行了剖分。根据Kirchhoff近似,给出了月球表面回波的模拟原理,由射线追踪、几何光学和Kirchhoff近似等方法给出了次表面回波的模拟原理。最后,以数值构造的月海表面地形为例,数值模拟了雷达探测仪对次表层结构探测的回波,并分析了有关参数对回波的影响,得出了有效探测次表层结构的几个准则。作为全文的扩展部分,从微波遥感的角度出发,回顾了国际上对火星的探测历史,归纳了目前国际上对火星的探测现状,列出了对微波遥感探测有影响的火星表层土壤和岩层的结构、分布及其介电特性等参数的已有探测结果,以及对火星地壳表层水（或冰）存在的可能性及其分布状态的研究动向。结合地球表面微波遥感技术的最新进展,提出用主动与被动微波遥感探测火星表面浅层土壤物质状态和分层结构的可行性分析,初步研讨了火星表层是否有水（或冰）存在的探测方案。最后,对这些方案的可行性进行了分析,并对其中一些关键技术进行了探讨。由被动微波遥感对月壤特性的研究、主动雷达遥感对月球表层与次表层结构的探测以及作为扩展部分的火星探测中的微波遥感技术构成的月球表面主被动微波遥感的理论建模与参数反演方法是作者五年多博士研究工作的全面总结,期望能初步解决目前微波遥感在月球探测中的一些基本问题,也希望本文工作能够对我国的探月工程以及未来的深空探测等提供理论依据和指导。当然,在本文工作中还有许多有待完善和深入研究的地方,加上对月球、火星等深空探测的进一步需求和微波遥感技术本身突飞猛进的发展,这些都需要进一步更加深入与广泛地研究。更多还原

【Abstract】 THE lunar regolith layer, the uppermost layer of the lunar surface, preserves the geological history of the Moon, and knowledge of its structure, composition and distribution might provide important information concerning lunar geology and resources for future lunar exploration. Owing to the low dielectric property of the lunar regolith, microwaves at appropriate frequencies should penetrate through the lunar surface to great depth and hence provide a complementary view about subsurface geologic structure to the observations that have been obtained in the visible, infrared, and thermal infrared regimes. In China’s current Chang-E 1 lunar program, a multi-channel microwave radiometer is being employed to measure the brightness temperature of the lunar surface for the retrieval of regolith layer properties, and it is planned that a radar sounding technique will be utilized for characterizing the lunar surface and subsurface structures in the future Chang-E 2 lunar program. The present thesis focuses on theoretical modeling of microwave radiometer and radar observations of the lunar surface and the potential information retrieval from those observations from the viewpoint of electromagnetic wave interactions with lunar surface layers, with particular application to China’s current and future Chang-E lunar program.In passive microwave remote sensing, the brightness temperature of the lunar surface mainly depends on the thermal and dielectric properties of the regolith and the possible variation of these properties with depth. Using a global distribution of FeO and TiO₂ content as determined from the Clementine UVVIS multispectral data, the distribution of regolith dielectric permittivity over the lunar surface is obtained on the basis of a relationship between dielectric permittivity, bulk density and FeO+TiO₂ content. The physical temperature distribution over the lunar surface and its dependence on regolith depth is investigated based on the previous study results of regolith thermal properties from both direct measurements and theoretical calculation, and an empirical formula of physical temperature distribution over the lunar surface is presented. A correlation of the lunar regolith layer thickness with lunar topography is proposed in order to tentatively construct a global map of lunar regolith layer thickness. On the basis of the aforementioned work, the brightness temperature of the lunar regolith layer in passive microwave remote sensing is numerically simulated using a three-layer model （the layering dust, regolith and underlying rock media） using fluctuation dissipation theorem. Taking the simulated brightness temperature with random noise as observations, the potential inversion of regolith layer properties is studied. Because the penetration depth is small for those areas with high FeO+TiO₂ content and at high frequencies, e.g. 19.35GHz and 37.0GHz in the Chang-E project, the physical temperature of the top dust layer and the regolith layer can be inverted by use of the brightness temperature at these high frequency channels using a two layer model. Utilizing the statistics of those points with high FeO+TiO₂ content along each latitude band as a reference, temperature variations as a function of latitude can be retrieved. Finally, the regolith layer thickness can be inverted from the brightness temperature at lower frequency channels, such as 1.4GHz or 3.0GHz.On the basis of the regolith layer thickness that will be inverted from the Chang-E radiometer data, the global distribution of lunar resources, in particular ³He （Helium-3）, is studied using a combination of both optical and microwave remote sensing data. ³He in the lunar regolith implanted by the solar wind is one of the most valuable resources because of its potential as a fusion fuel. The abundance of ³He in the lunar regolith is related to many factors, such as the solar wind flux, lunar surface maturity and TiO₂ content. A model of the solar wind flux, which takes into account variations due to shielding of the lunar nearside when the Moon is in the Earth’s magnetotail, is used to determine the global distribution of relative solar wind flux over the Moon’s surface. The global distribution of lunar surface optical maturity （OMAT） and TiO₂ content in the lunar regolith are calculated from Clementine UVVIS multispectral data. On the basis of the Apollo regolith samples, a linear relation between the ³He abundance and normalized solar wind flux, optical maturity and TiO₂ content is presented in order to produce a global map of regolith ³He abundances. The total amount of ³He per unit area in the lunar regolith layer and the global inventory of ³He will be able to be calculated after an inversion for regolith layer thickness from the actual Chang-E radiometer data.To explore the potential utility of lower frequency （L band） radar pulse penetration for future lunar exploration, a theoretical model of a stratified lunar regolith and a numerical simulation of polarized radar pulse echoes are developed. The lunar regolith layer consists of the low lossy layer with randomly rough top and bottom interfaces, and a layer of random stone-scatterers overlaying the underlying rock media. The time-domain Mueller matrix solution derived from vector radiative transfer formulations contains several scattering mechanisms of the stratified media, including surface scattering from the rough top and bottom interfaces, volumetric scattering from random stone scatterers, and multiple scattering interactions. Temporal characteristics and structure of the polarimetric echo profile as a function of model parameters such as the regolith layer thickness and FeO and TiO₂ content are numerically simulated, and well display an image of the regolith’s structure. Simulated results show that polarimetric pulse echoes could reveal rich information about the lunar regolith layer depth and other structural properties, and it demonstrates a potential new way to explore the lunar surface in China’s future Chang-E program.Compared with radio waves at microwave frequencies, high frequency （HF） radar sounding penetrates much deeper into the lunar regolith due to its lower frequency. The main strategy for HF radar sounding in lunar exploration is to detect weak nadir subsurface echoes, which in general suffers attenuation during subsurface propagation as well as interference by strong off-nadir surface echoes （i.e., clutter）. Thus, it is important to study the surface echo properties in order to detect weak subsurface echoes. High resolution lunar surface topography for both the relatively smooth mare surface and the cratered highland terrain are numerically generated on the basis of statistical study results of the lunar surface terrain. A triangulated irregular network （TIN） is utilized to divide the undulating surface into discrete triangular meshes as a digital elevation model. The mesh dimensions are variable, with large dimension for areas with small slope variations and small dimensions for areas with large slope variations. This model takes into account the inhomogeneous lateral variations in the surface slopes, and makes the number of meshes as small as possible. Finally, Kirchhoff approximation （KA） for rough surface scattering is applied to compute radar echoes from the surface meshes. Radar echoes from both mare and cratered highland surface areas are simulated and their variation with various surface parameters such as surface roughness, crater population and size are discussed. Furthermore, using the simulated off-nadir lunar surface echoes on the basis of a digital elevation model and the Clementine UVVIS optical data for the Apollo 15 landing site, an synthetic aperture radar （SAR） image is simulated using Back Projection （BP） algorithm, which would also provide a tool for data and image evaluation, feature identification and information extraction in future lunar exploration such as the Mini-SAR on Chandrayaan-1 and Lunar Reconnaissance Orbiter （LRO）.In order to meet the development of the Chinese Mars exploration program, a brief review of Mars exploration from the viewpoint of microwave remote sensing （e.g., available research results such as the distribution, structure and dielectric properties of Martian subsurface layers and the possibility of water existence） is presented. Some potential approaches, such as the detection and mapping of fluvial features beneath the Martian dust layer using synthetic aperture radar and the possible detection of water in the Martian subsurface layer using orbital radar sounder and ground penetrating radar are discussed. Finally, some examples and technical key points are presented and discussed.Some primary issues for investigating the lunar surface and subsurface properties from microwave remote sensing and HF radar sounding are studied using both forward simulation and inverse studies. However, many aspects and issues still remain for further study, not mentioning the rapidly developing microwave and radar remote sensing techniques as well as the increasing needs of lunar and other planetary exploration.更多还原