-
Mini-RF采用简极化工作模式,得到4个Stokes参数[20]
$$ {I_{\text{1}}} = \left\langle {{{\left| {{E_{\text {HL}}}} \right|}^2} + {{\left| {{E_{\text {VL}}}} \right|}^2}} \right\rangle $$ (1) $$ {I_{\text{2}}} = \left\langle {{{\left| {{E_{\text {HL}}}} \right|}^2} - {{\left| {{E_{\text {VL}}}} \right|}^2}} \right\rangle $$ (2) $$ {I_{\text{3}}} = 2{\rm Re} \left\langle {{E_{\text {HL}}}E_{\text {VL}}^*} \right\rangle $$ (3) $$ {I_{\text{4}}} = - 2{\rm Im} \left\langle {{E_{\text {HL}}}E_{\text {VL}}^*} \right\rangle $$ (4) 其中:下标L表示发射左旋圆极化波;下标H、V分别表示接收水平极化与垂直极化;I1为总回波强度;E是电场;Re和Im分别表示取实部与取虚部;< >表示多视平均[4]。
圆极化率CPR的定义是与发射电磁波相同圆极化分量(Same sense Circular polarization,SC)和相反圆极化分量(Opposite sense Circular polarization,OC)的比值,Mini-RF数据中CPR定义为[20]
$$ {\mu _{\text{c}}} = \frac{{{S_{{\text{LL}}}} \times S_{{\text{LL}}}^*}}{{{S_{{\text{RL}}}} \times S_{{\text{RL}}}^*}} = \frac{{{I_{\text{1}}} - {I_{\text{4}}}}}{{{I_{\text{1}}} + {I_{\text{4}}}}} $$ (5) 其中:S是散射矩阵参数;下标R表示接收右旋圆极化成分。极化度的定义为[20]
$$ m = {(I_2^2 + I_3^2 + I_4^2)^{1/2}}/{I_1} $$ (6) 去极化度(1–m)可以用于表示雷达回波中的随机成分[20]。
-
月表面雷达回波与表面粗糙度、入射角、介电常数等因素有关,入射角较小时,镜面反射成分主导[21],在大入射角下,小尺度粗糙的贡献主导[22],Mini-RF在月表水平面上的入射角约为49°,该入射角范围内小尺度粗糙的散射成分主导,这里用微扰法来分析这些因素对雷达回波的影响,对于高斯粗糙面,后向散射系数可以写为[22-23]
$$ \sigma _{{\text{pq}}}^{\text{0}} = 4{k^4}{\delta ^2}{L^2}{\cos ^4}{\theta _i}|{a_{{\text{pq}}}}{|^2}\exp ( - {k^2}{L^2}\sin {^2}{\theta _i}) $$ (7) 其中
$$ {a_{{\text{HH}}}} = {R_{\text{H}}} $$ (8) $$ {a_{{\text{VV}}}} = \frac{{({\varepsilon _{\text{r}}} - 1)[{{\rm{sin}}^2}{\theta _{\text{i}}} - {\varepsilon _r}(1 + {{\rm{sin}}^2}{\theta _{\text{i}}})]}}{{{{[{\varepsilon _{\text{r}}}\cos {\theta _{\text{i}}} + {{({\varepsilon _{\text{r}}} - {{\rm{sin}}^2}{\theta _{\text{i}}})}^{1/2}}]}^2}}} $$ (9) $$ {a_{\text {HV}}} = {a_{\text {VH}}} = 0 $$ (10) 其中:k为真空中的波数;
$ \delta $ 为粗糙面的均方根高度;L为相关长度;$ {\theta _{\text{i}}} $ 为入射角;$ {\varepsilon _{\text{r}}} $ 为相对介电常数;RH为水平极化的菲涅尔反射率。发射左旋圆极化波,接收线极化波时的散射系数与线极化散射系数之间满足
$$ {S_{{\text{HL}}}} = ( - \text j{S_{{\text{HH}}}} + {S_{{\text{HV}}}})/\sqrt 2 $$ (11) $$ {S_{{\text{VL}}}} = ( - \text j{S_{{\text{VH}}}} + {S_{{\text{VV}}}})/\sqrt 2 $$ (12) 其中:j为虚部单位。在一阶近似下,小尺度粗糙面的交叉极化项为0,由式(1)、(11)、(12)可得,雷达回波总强度I1正比于
$ (\sigma _{{\text{HH}}}^0 + \sigma _{{\text{VV}}}^{\text{0}}{\text{)}}/2 $ 。图1给出了微扰法计算得到的不同粗糙度和介电常数下的后向散射系数,随着入射角的增大,后向散射系数降低。陨石坑内部斜坡使得局部入射角改变,朝向雷达的坑壁入射角小,回波较强,而背向雷达的坑壁局部入射角大,回波较弱。
媒质介电常数与粗糙度的改变也会使得雷达回波强度改变,介电常数越大,后向散射系数越大,后向散射系数还会随着粗糙面的均方根高度增加而增加,相关长度的增加使得大入射角区域的后向散射系数略有下降。
Analysis of High Resolution SAR Data and Selection of Landing Sites in the Permanently Shadowed Region on the Moon
-
摘要:月球两极永久阴影区(Permanently Shadowed Regions,PSR)全年没有直接光照,搜寻PSR内的月球水冰是“嫦娥七号”的主要任务之一。人类至今没有在PSR着陆,中国的“嫦娥七号”任务计划在PSR附近的太阳直射区域登陆,着陆器上携带的飞跃器将进入PSR进行采样与分析。如何选择登月点和取样点是该项任务的一个关键,结合高精度的数字高程模型,“嫦娥七号”卫星搭载的极化合成孔径雷达(Polarization Synthetic Aperture Radar,Pol-SAR)可以进一步确定月表面的高低起伏状态,以确定平缓表面的登月点与采样点,及小飞跃器的飞行路线。通过将美国“月球轨道勘测器”(Lunar Reconnaissance Orbiter,LRO)上面的微型雷达Mini-RF采集的雷达图像与太阳直射区域光学图像对比,以希吉努斯(Hyginus)陨石坑附近区域和“嫦娥四号”着陆区为例,来分析Pol-SAR在筛选平坦表面作为登月点与采样点时的作用。率先将HRNet网络应用于月球SAR图像分割,并讨论了人工神经网络在探月SAR图像信息获取中的应用。以月球南极的舒梅克(Shoemaker)陨石坑和沙克尔顿(Shackleton)陨石坑为例,找出永久阴影区平坦的区域,作为“嫦娥七号”SAR探测的方法参考。Abstract:There is no direct solar illumination in the permanently shadowed regions (PSR) at the polar region of the Moon. Detecting water-ice in PSR is a significant scientific question. Until now, no spacecraft has landed in PSR. Chang’E-7 mission plans a rover landing at the solar illuminated region near PSR. A mini-flyer carried by the lander will fly to the PSR to collect regolith samples for analysis. Selection of landing site and sampling site is critical for the mission. The Polarization Synthetic Aperture Radar (Pol-SAR) onboard Chang’E-7 satellite can evaluate the roughness of lunar surface, the landing site, the sampling site and the flying trajectory with the assistance of high-resolution digital elevation model. By comparing the SAR data acquired by the Mini-RF onboard Lunar Reconnaissance Orbiter with the optical images at the solar illuminated region, we analyze the role that Pol-SAR play in selecting the flat landing site and sampling site. Regions near Hyginus crater and the landing site of Chang’E-4 mission are taken as examples. HRNet is used in lunar SAR image segmentation. The application of neutral network in lunar SAR image segmentation is discussed. Craters Shoemaker and Shackleton at lunar south pole are analyzed to find flat surface in PSR as potential landing sites. This paper provides a reliable reference for SAR detection in Chang’E-7 mission.
-
Key words:
- Chang’E-7 mission/
- SAR/
- permanently shadowed regions/
- Mini-RF data/
- landing site/
- flat surface
Highlights● Mini-RF data, optical data and DEM data at non-polar region were used to illustrate the effects of SAR images in selecting flat surface for landing. ● The polarized parameters from SAR images can provide more information about the roughness of lunar surface. ● HRNet was used in lunar SAR image segmentation. ● The application of neutral network in lunar SAR image segmentation was discussed. ● Some flat regions in crater Shoemaker and Shackleton were selected for further study. -
-
[1] ZOU Y, LIU Y, JIA Y. Overview of china’s upcoming Chang’E series and the scientifc objectives and payloads for Chang’E-7 mission[C]//The 51st Lunar and Planetary Science Conference. Woodlands, USA: 2020. [2] 吴伟仁,于登云,王赤,等. 月球极区探测的主要科学与技术问题研究[J]. 深空探测学报(中英文),2020,7(3):223-231.WU W R,YU D Y,WANG C,et al. Research on the main scientific and technological issues on lunar polar exploration[J]. Journal of Deep Space Exploration,2020,7(3):223-231. [3] 法文哲, 徐丰, 金亚秋. 基于不规则三角网格剖分的非均匀起伏月球表面SAR成像模拟[J]. 中国科学(F辑: 信息科学), 2009, 39(2): 185-198.FA W Z, XU F, JIN Y Q. SAR imaging simulation for an inhomogeneous undulated lunar surface based on triangulated irregular network[J]. Science In China(Series F: Information Sciences), 2009, 39(2): 185-198. [4] RANEY R K,SPUDIS P D,BUSSEY B,et al. The lunar Mini-RF radars:hybrid polarimetric architecture and initial results[J]. Proceedings of the IEEE,2010,99(5):808-829. [5] LIU N,YE H,JIN Y Q. Dielectric inversion of lunar PSR media with topographic mapping and comment on “quantification of water ice in the Hermite—a crater of the lunar north pole”'[J]. IEEE Geoscience & Remote Sensing Letters,2017,14(9):1444-1448. [6] LIU N,FA W,JIN Y Q. No water-ice invertable in PSR of Hermite—a crater based on Mini-RF data and two-layers model[J]. IEEE Geoscience and Remote Sensing Letters,2018,15(10):1485-1489. [7] LIU N, JIN Y, Q. Selection of a landing site in the permanently shadowed portion of lunar polar regions using DEM and Mini-RF data [J]. IEEE Geoscience and Remote Sensing Letters, 2022, 19: 4503305. [8] LIU N, XU F, JIN Y Q. Anomaly detection in permanently shadowed region at lunar polar using fully polarimetric SAR data of Chandrayanan-2 [J/OL]. IEEE Geoscience and Remote Sensing Letters, 2021, https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=36. [9] LIU N, XU F, JIN Y Q. A Numerical model of CPR of rough surface with discrete scatterers for analysis of Mini-RF data[J]. Radio Science, 2020, 55(5): e2018RS006776. [10] LIU N, JIN Y Q. Simulation of Pol-SAR imaging and data analysis of Mini-RF observation from the lunar surface [J]. IEEE Transactions on Geoscience and Remote Sensing, 2021, 60: 2000411. [11] SPUDIS P D,BUSSEY D B J,BALOGA S M,et al. Evidence for water ice on the Moon:results for anomalous polar craters from the LRO Mini‐RF imaging radar[J]. Journal of Geophysical Research:Planets,2013,118(10):2016-2029. [12] FA W, CAI Y. Circular polarization ratio characteristics of impact craters from Mini-RF observations and implications for ice detection at the polar regions of the Moon[J]. Journal of Geophysical Research: Planets, 2013, 118(8): 1582-1608. [13] CAMPBELL B A. High circular polarization ratios in radar scattering from geologic targets [J]. Journal of Geophysical Research, 2012, 117(E6): E06008. [14] CALLA O P N,MATHUR S,GADRI K L. Quantification of water ice in the Hermite—a crater of the lunar north pole[J]. IEEE Geoscience and Remote Sensing Letters,2016,13:926-930.doi:10.1109/LGRS.2016.2554282 [15] KUMAR A, KOCHAR I M, PANDEY D K, et al. Dielectric constant estimation of lunar surface using Mini-RF and Chandrayaan-2 SAR data [J]. IEEE Transactions on Geoscience and Remote Sensing, 2021, 60: 4600608. [16] FASSETT C I,KING I R,NYPAVER C A,et al. Temporal evolution of S-band circular polarization ratios of kilometer-scale craters on the lunar maria[J]. Journal of Geophysical Research,2018,123:3133-3143. [17] LIU J,REN X,YAN W,et al. Descent trajectory reconstruction and landing site positioning of Chang’E-4 on the lunar farside[J]. Nature Communication,2019,10:4229.doi:10.1038/s41467-019-12278-3 [18] 金亚秋, 法文哲. 行星微波遥感理论方法与应用[M]. 北京: 科学出版社, 2019. [19] SHI X,FU S,CHEN J,et al. Object-level semantic segmentation on the high-resolution Gaofen-3 FUSAR-Map dataset[J]. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing,2021,14(5):3107-3119. [20] RANEY R K, CAHILL J T S, PATTERSON G W, et al. The m-chi decomposition of hybrid dual‐polarimetric radar data with application to lunar craters[J]. Journal of Geophysical Research: Planets, 2012, 117(E12): E00H21. [21] THOMPSON T W, USTINOV E A, HEGGY E. Modeling radar scattering from icy lunar regoliths at 13 cm and 4 cm wavelengths [J]. Journal of Geophysical Research: Planets, 2011, 116(E1): E01006. [22] 金亚秋. 电磁散射和热辐射的遥感理论[M]. 北京: 科学出版社, 1993. [23] ULABY F T, MOORE R K, FUNG A K. Microwave remote sensing fundamentals and radiometry [M]// Microwave Remote Sensing Active & Passive. Boston, MA, USA: Addison-Wesley, 1981. [24] ROBINSON M S,BRYLOW S M,TSCHIMMEL M,et al. Lunar Reconnaissance Orbiter Camera (LROC) instrument overview[J]. Space Science Reviews ,2010,150:81-124.doi:10.1007/s11214-010-9634-2 [25] SMITH D E,ZUBER M T,NEUMANN G A,et al. Summary of the results from the lunar orbiter laser altimeter after seven years in lunar orbit[J]. Icarus,2017,283:70-91.doi:10.1016/j.icarus.2016.06.006 [26] NEISH C D,BLEWEET D T,HARMON J K,et al. A comparison of rayed craters on the Moon and Mercury[J]. Journal of Geophysical Research:Planets,2013,118(10):2247-2261.doi:10.1002/jgre.20166 [27] BHIRAVARASU S S,CHAKRABORTY T,PUTREVU D,et al. Chandrayaan-2 Dual-frequency Synthetic Aperture Radar (DFSAR):performance characterization and initial results[J]. The Planetary Science Journal,2021,2:1-21.doi:10.3847/PSJ/abd022 [28] 徐丰, 王海鹏, 金亚秋. 合成孔径雷达图像智能解译[M]. 北京: 科学出版社, 2020. [29] WANG J,SUN K,CHENG T,et al. Deep high-resolution representation learning for visual recognition[J]. IEEE Transactions on Pattern Analysis and Machine Intelligence,2021,43(10):3349-3364.doi:10.1109/TPAMI.2020.2983686 [30] EKE V R,BARTRAM S A,LANE D A,et al. Lunar polar craters-icy,rough or just sloping?[J]. Icarus,2014,241:66-78.doi:10.1016/j.icarus.2014.06.021