面向嫦娥五号探月雷达的月壤结构逆时成像和反演研究

Lunar penetrating radar is an important tool for understanding the moon and exploiting its resources. China is going to launch Chang’e-5 lunar exploration probe in 2019, which is equipped with a lunar penetrating radar system. This radar system consists of twelve Vivaldi antennas, of which the center frequency is 2 GHz, and can detect the near-surface structure of the lunar regolith. Different from the Chang’e-3 lunar penetrating radar, which is equipped on the lunar rover “Yutu”, the Chang’e-5 penetrating radar is equipped under the lander and cannot move. Due to the air-coupling characteristic of the Vivaldi antenna, the target reflection will suffer from the antenna coupling, as well as the strong reflection from the bottom plate and other metal structures of the Chang’e-5 lander. The bouncing signal between the bottom plate and the lunar surface would also interfere with the reflection signal from the object embedded in the lunar regolith. Therefore, this project proposes to suppress these clutter and eliminate their impacts on the high-resolution imaging of the lunar regolith, which is of significance for the regolith-drilling task of the Chang’e-5 exploration mission. .The research work, which is going to be carried out in this project, consists of the following four parts. Firstly, a data processing flowchart is going to be proposed to suppress the interfering clutter caused by the Chang’e-5 lander, based on the experimental data acquired over a volcanic ash pit in laboratory in the past three years. Secondly, a fast numerical method using the domain decomposition method is going to be used to simulate the whole system, which consists of the lander, the radar system and the lunar regolith. The electromagnetic response of the lunar regolith and rocks are analyzed and their time-frequency response is used to guide the interpretation of the probing data to be measured on the moon. Thirdly, a reverse time migration (RTM) algorithm is going to be developed based on the Dyadic Green’s function in layered media. From the lunar-probing data, the fast RTM algorithm is going to produce an image of the lunar regolith and possible rocks beneath the Chang’e-5 lander with a resolution of several centimeters in less than three hours. The imaging results can provide key information for the lunar sample drilling task. Fourthly, a full waveform inversion (FWI) algorithm is going to be developed based on the afore-mentioned fast simulation method. Distorted Born iteration method is employed to reconstruct the distribution of the dielectric permittivity and conductivity of the lunar regolith and rocks from the lunar-probing data. The dependence of the dielectric properties of the lunar regolith on the depth and the temperature upon the lunar surface is also going to be investigated. The results of this project can aid the accomplishment of the scientific aims of Chang’e-5 exploration mission, and are also useful for understanding the generation and evolution of the lunar regolith.

探月雷达对于认识月球和开发月球资源具有重要意义。我国将于2019年发射嫦娥五号探测器，其搭载一套高频探月雷达系统。与嫦娥三号雷达不同，嫦娥五号探月雷达不能移动，其采集的数据受到着陆器金属构件的电磁干扰非常严重，影响月壤结构的反演成像。本课题将在嫦娥五号探月雷达地面试验的基础上，研究复杂电磁环境下嫦娥五号探月数据的杂波去除方法；采用区域分解法对包含着陆器、月壤结构和雷达探测仪的全模型进行快速仿真实验，分析月壤结构和月岩的电磁响应特征，以指导月表实测数据的解译；开发基于格林函数的月壤结构频率域逆时偏移算法，实现三小时内完成月壤结构和月岩分布的高分辨率成像，为后续月壤采样任务提供信息支持；研究基于变形玻恩迭代法的全模型全波形反演算法，获取着陆器下方月壤和月岩的介电参数分布，并分析其随温度和深度的变化规律。本课题研究成果将有助于嫦娥五号采样返回科学任务的顺利实施，深化月壤起源和演化机理的认识。

本项目提出了一种二维时域有限元 (FETD)算法，可模拟在色散和有损耗介质中的电磁场，与传统的时域有限差分法相比，具有更好的几何灵活性和相当的模拟精度，更适用于模拟包含曲面的复杂几何模型。其次，研究了基于TV正则化的多尺度全波形反演算法，利用嫦娥五号月壤结构探测仪的仿真数据重建了月壤风化层的高分辨率结构，可估算浅风化层的相对介电常数和电导率。再次，提出了一种考虑天线方向图修正的背向投影成像算法，实验结果表明可抑制地下目标的绕射伪影，提升成像质量。然后，研究了适合嫦娥五号月壤结构雷达探测仪的数据处理流程，在复杂干扰环境中提取有效的弱目标信号，在此基础上，提出了一种基于分层介质的并矢格林函数频域逆时偏移成像算法，可获得与时域逆时偏移成像算法几乎一样的高精度地下目标成像结果，相比时域逆时偏移成像算法，计算效率可提高2-3个数量级；编制了嫦娥五号月壤结构探测仪雷达数据处理与成像软件，在北京航天指挥中心对嫦娥五号月壤结构探测仪着陆实测数据进行了准实时处理，获取了钻头下方2 m深度范围内的月壤结构高精度成像结果，为月壤钻取任务的顺利实施提供了关键的信息支持。最后，利用嫦娥五号月壤结构探测仪的雷达实测数据，通过速度谱分析计算嫦娥五号着陆器足盘在月壤中的侵彻深度，由此估算了嫦娥五号着陆点月壤摩擦角的原位值，为月壤的产生和演化提供了新的认识，并为未来月球基地的建设提出参考意见