雷达探测仪对月球次表层结构探测的理论建模与反演方法
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摘要
月球是地球的唯一一颗天然卫星,它的内部结构是一直人类不断探索的谜题之一。在漫长的月球历史中,由于受到陨石小天体的撞击以及太阳-宇宙射线的照射,月球表层储存了月球地质和太阳辐射活动的线索。研究月球的结构有助于提高对月球资源的科学认识,对于未来的探月、将月球作为人类探测更遥远天体的理想平台等有着十分重要的意义。
在月球探测中,星载高频雷达探测仪(Radar Sounder)是一种用于探测月球次表层结构的有效工具。与其它微波频段相比较,高频波段(3-30MHz)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。雷达探测仪接收到月球表层的回波主要有：表面天底点回波,表面非天底点回波和次表面天底点回波。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和强度来判断月球次表层的深度以及物质成分。
本文首先讨论了高频雷达探测仪频率与带宽的选取准则,介绍了基于粗糙面电磁散射的Kirchhoff近似与几何光学射线追踪方法对月表雷达回波的快速模拟方法。在雷达探测仪对月球次表层结构探测的回波模拟中,由于选取的月球表面及次表面都是有限的,会使得月球表面及次表面场景边缘所对应的射程距离处产生虚假的峰值回波。因此在模拟中,需要在边缘对应的射程距离处对接收到的回波进行截断。作为本研究的第一个问题,本文讨论了回波截断处所对应的射程距离与月表层参数之间的定量关系。
电磁波在月球次表层内部传播过程中受衰减、透射、散射等影响,次表面天底点回波往往会很微弱。受月球表面粗糙度、环形山等月表地形的影响,来自表面非天底点的强杂波往往会淹没微弱的次表面回波,成为对月球次表层结构探测的最大障碍。对于如何从具有强烈背景杂波的雷达探测仪回波中提取微弱次表面回波的研究,到目前为止还比较少。为有效探测月球次表层结构,本文基于月球次表面天底点回波和表面非天底点回波(杂波)的相干与非相干特性,提出在月球次表面地形变化不大的情况下,由累积取平均的方法来抑制表面非天底点回波从而识别次表面回波。以数值模拟的月海与月陆表面雷达探测仪回波为例,验证了该方法的正确性与可行性,并讨论了累积平均数目对次表面回波提取结果的影响。
本文所述方法也可以应用到火星等其他外星球次表层结构、以及液态水的探测中。
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 can 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.
Spaceborne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration. Compared with radio waves at microwave frequencies, high frequency (HF) radar wave can penetrate much deeper into the lunar subsurface layer due to its lower frequency. Center frequency selection for the high-frequency radar sounder is discussed firstly as a compromise of penetration depth and range resolution. In the simulation for high frequency radar sounder detection of the lunar subsurface structure, the surface and subsurface areas in simulation are finite. Because of this limitation, it makes a false peak echo in the simulation result at the edge of the surface and subsurface scene. Therefore, the echoes whose range is greater than that from the edge of the scene should be cut. This simulation work on the lunar surface and subsurface area is also discussed.
The primary strategy for radar sounder detection of subsurface structure is through the time delay and intensity difference of the nadir echoes from the surface and subsurface. Due to the inhomogeneous undulation of lunar surface, the weak subsurface echoes can be easily masked by the strong off-nadir surface echoes (clutter), which is the main obstacle in lunar subsurface exploration. In this study, an effective approach for subsurface echo extraction is developed on the basis of the coherent and incoherent properties of the surface nadir echoes and subsurface off-nadir echoes (clutter). By stacking and averaging the received radar sounder echo at different time series, subsurface echo can be identified under the condition that the subsurface topography variation is not too large. Taken the simulated radar sounder echoes from lunar maria and highlands area as an example, the feasibility of this approach is verified numerically. Finally, the influences on subsurface echo identification by the stacking number, surface roughness are also discussed. The approach in this study can be applied to Martian and other planetary exploration.
在月球探测中,星载高频雷达探测仪(Radar Sounder)是一种用于探测月球次表层结构的有效工具。与其它微波频段相比较,高频波段(3-30MHz)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。雷达探测仪接收到月球表层的回波主要有：表面天底点回波,表面非天底点回波和次表面天底点回波。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和强度来判断月球次表层的深度以及物质成分。
本文首先讨论了高频雷达探测仪频率与带宽的选取准则,介绍了基于粗糙面电磁散射的Kirchhoff近似与几何光学射线追踪方法对月表雷达回波的快速模拟方法。在雷达探测仪对月球次表层结构探测的回波模拟中,由于选取的月球表面及次表面都是有限的,会使得月球表面及次表面场景边缘所对应的射程距离处产生虚假的峰值回波。因此在模拟中,需要在边缘对应的射程距离处对接收到的回波进行截断。作为本研究的第一个问题,本文讨论了回波截断处所对应的射程距离与月表层参数之间的定量关系。
电磁波在月球次表层内部传播过程中受衰减、透射、散射等影响,次表面天底点回波往往会很微弱。受月球表面粗糙度、环形山等月表地形的影响,来自表面非天底点的强杂波往往会淹没微弱的次表面回波,成为对月球次表层结构探测的最大障碍。对于如何从具有强烈背景杂波的雷达探测仪回波中提取微弱次表面回波的研究,到目前为止还比较少。为有效探测月球次表层结构,本文基于月球次表面天底点回波和表面非天底点回波(杂波)的相干与非相干特性,提出在月球次表面地形变化不大的情况下,由累积取平均的方法来抑制表面非天底点回波从而识别次表面回波。以数值模拟的月海与月陆表面雷达探测仪回波为例,验证了该方法的正确性与可行性,并讨论了累积平均数目对次表面回波提取结果的影响。
本文所述方法也可以应用到火星等其他外星球次表层结构、以及液态水的探测中。
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 can 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.
Spaceborne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration. Compared with radio waves at microwave frequencies, high frequency (HF) radar wave can penetrate much deeper into the lunar subsurface layer due to its lower frequency. Center frequency selection for the high-frequency radar sounder is discussed firstly as a compromise of penetration depth and range resolution. In the simulation for high frequency radar sounder detection of the lunar subsurface structure, the surface and subsurface areas in simulation are finite. Because of this limitation, it makes a false peak echo in the simulation result at the edge of the surface and subsurface scene. Therefore, the echoes whose range is greater than that from the edge of the scene should be cut. This simulation work on the lunar surface and subsurface area is also discussed.
The primary strategy for radar sounder detection of subsurface structure is through the time delay and intensity difference of the nadir echoes from the surface and subsurface. Due to the inhomogeneous undulation of lunar surface, the weak subsurface echoes can be easily masked by the strong off-nadir surface echoes (clutter), which is the main obstacle in lunar subsurface exploration. In this study, an effective approach for subsurface echo extraction is developed on the basis of the coherent and incoherent properties of the surface nadir echoes and subsurface off-nadir echoes (clutter). By stacking and averaging the received radar sounder echo at different time series, subsurface echo can be identified under the condition that the subsurface topography variation is not too large. Taken the simulated radar sounder echoes from lunar maria and highlands area as an example, the feasibility of this approach is verified numerically. Finally, the influences on subsurface echo identification by the stacking number, surface roughness are also discussed. The approach in this study can be applied to Martian and other planetary exploration.
引文
[1]Y.-Q. Jin. Electromagnetic Scattering Modelling for Quantitative Remote Sensing [M]. Singapore:World Scientific,1994.
[2]Y.-Q. Jin. Theory and Approach of Information Retrieval from Electromagnetic Scattering and Remote Sensing [M]. Netherlands:Springer,2005.
[3]金亚秋.空间微波遥感数据验证理论与方法[M].北京：科学出版社,2005.
[4]Phillips, R. J., G. F. Adams, Jr. W. E. Brown, R. E. Eggleton, P. L. Jackson, R. L. Jordan, W. I. Linlor, W. J. Peeples, L. J. Porcello, J. Ryu, G. Schaber, W. R. Sill, T. W. Thompson, S. H. Ward, and J. S. Zelenka. Apollo lunar sounder experiment. NASA Technical report,1973.
[5]Porcello, L. J., R. L. Jordan, J. S. Zelenka, G. F. Adams, R. J. Phillips, Jr. E. W. Brown, S. H. Ward, and P. L. Jackson. The Apollo lunar sounder radar system. Proceeding of the IEEE,1974,62 (6):769-788.
[6]Ono, T. and H. Oya. Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft. Earth Planets Space,2000,52:629-637.
[7]Picardi, G, D. Biccari, R. Seu, L. Marinangeli, W. T. K. Johnson, R. L. Jordan, J. Plaut, A. Safaenili, D. A. Gurnett, G. G. Ori, R. Orosei, D. Calabrese, and E. Zampolini. Performance and surface scattering models for the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). Planetary and Space Science,2004,52:149-156.
[8]Seu, R., D. Biccari, L. V. Lorenzoni, R. J. Phillips, L. Marinangeli, G. Picardi, A. Masdea, and E. Zampolini. SHARAD:the MRO 2005 shallow radar, Planetary and Space Science,2004,52:157-166.
[9]Kobayashi, T., H. Oya, and T. Ono. A-scope analysis subsurface radar sounding of lunar mare region. Earth Planets Space,2002,54:973-982.
[10]Kobayashi, T., H. Oya, and T. Ono. B-scan analysis of subsurface radar sounding of lunar highland region. Earth Planets Space,2002,54:983-991.
[11]Nouvel, J. F., A. Herique, W. Kofman, and A. Safaeinili. Radar signal simulation: Surface modeling with the Facet Method. Radio Science,2004,39:RS1013.
[12]法文哲,金亚秋.雷达探测仪对月球次表层结构的探测模拟方法.中国科学D,2010,40(4)：473-485,
[13]Heiken, G., D. Vaniman, and B. French. Lunar Source-Book: A User's Guide to the Moon. New York: Cambridge University Press,1991.
[14]金亚秋,刘鹏,叶红霞.随机粗糙面与目标复合散射数值模拟理论与方法.北京：科学出版社,2008.
[15]Fa, W., F. Xu, and Y.-Q. Jin. SAR imaging simulation for an inhomogeneous undulated lunar surface based on triangulated irregular network. Science in China F,2009,52(4):559-574.
[16]Philips, R.J., G. F. Adams, Jr. W. E. Brown, R.E. Eggleton, P. L. Jackson, R.L Jordan, W. I. Linlor, S. H. Ward, and J. S. Zelenka. Apollo lunar sounder experiment [R]. NASA Technical report,1973.
[17]Porcello, L. J., R.L. Jordan, Jr. W. E. Brown, S. H. Ward, and P. L. Jackson. The Apollo lunar sounder radar system [J]. Proceeding of the IEEE,1974,62(6); 769-788.
[18]Kobayashi, T. and T. Ono. SAR/InSAR observation by an HF Sounder [J]. Journal of Geophysical Research,2007,112:E03s90.
[19]Heiken, G., D. Vaniman, and B. French. Lunar Source-Book:A User's Guide to the Moon [M]. New York:Cambridge University Press,1991.
[20]Kong, J. A. Electromagnetic-Wave Theory [M]. Massachusetts:EMW Publishing, 2005.
[21]Spudis P D, Bussey B, Lichtenberg C, et al. Mini-SAR:an imaging radar for the Chandrayaan 1 mission to the Moon [C]. In:Proceedings of the 36th Lunar Science Conference. New York:Pergamon Press,2005.1153
[22]Sen, P. N., C. Scala, and M. H. Cohen. A self-similar model for sedmentary rocks with applications to the dielectric constants for fused glass beads [J]. Geophysics, 1981,46:781-795
[23]Greely, R. Planetary Landscapes[M], Boston:Allen and Unwin,1987.
[2]Y.-Q. Jin. Theory and Approach of Information Retrieval from Electromagnetic Scattering and Remote Sensing [M]. Netherlands:Springer,2005.
[3]金亚秋.空间微波遥感数据验证理论与方法[M].北京：科学出版社,2005.
[4]Phillips, R. J., G. F. Adams, Jr. W. E. Brown, R. E. Eggleton, P. L. Jackson, R. L. Jordan, W. I. Linlor, W. J. Peeples, L. J. Porcello, J. Ryu, G. Schaber, W. R. Sill, T. W. Thompson, S. H. Ward, and J. S. Zelenka. Apollo lunar sounder experiment. NASA Technical report,1973.
[5]Porcello, L. J., R. L. Jordan, J. S. Zelenka, G. F. Adams, R. J. Phillips, Jr. E. W. Brown, S. H. Ward, and P. L. Jackson. The Apollo lunar sounder radar system. Proceeding of the IEEE,1974,62 (6):769-788.
[6]Ono, T. and H. Oya. Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft. Earth Planets Space,2000,52:629-637.
[7]Picardi, G, D. Biccari, R. Seu, L. Marinangeli, W. T. K. Johnson, R. L. Jordan, J. Plaut, A. Safaenili, D. A. Gurnett, G. G. Ori, R. Orosei, D. Calabrese, and E. Zampolini. Performance and surface scattering models for the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). Planetary and Space Science,2004,52:149-156.
[8]Seu, R., D. Biccari, L. V. Lorenzoni, R. J. Phillips, L. Marinangeli, G. Picardi, A. Masdea, and E. Zampolini. SHARAD:the MRO 2005 shallow radar, Planetary and Space Science,2004,52:157-166.
[9]Kobayashi, T., H. Oya, and T. Ono. A-scope analysis subsurface radar sounding of lunar mare region. Earth Planets Space,2002,54:973-982.
[10]Kobayashi, T., H. Oya, and T. Ono. B-scan analysis of subsurface radar sounding of lunar highland region. Earth Planets Space,2002,54:983-991.
[11]Nouvel, J. F., A. Herique, W. Kofman, and A. Safaeinili. Radar signal simulation: Surface modeling with the Facet Method. Radio Science,2004,39:RS1013.
[12]法文哲,金亚秋.雷达探测仪对月球次表层结构的探测模拟方法.中国科学D,2010,40(4)：473-485,
[13]Heiken, G., D. Vaniman, and B. French. Lunar Source-Book: A User's Guide to the Moon. New York: Cambridge University Press,1991.
[14]金亚秋,刘鹏,叶红霞.随机粗糙面与目标复合散射数值模拟理论与方法.北京：科学出版社,2008.
[15]Fa, W., F. Xu, and Y.-Q. Jin. SAR imaging simulation for an inhomogeneous undulated lunar surface based on triangulated irregular network. Science in China F,2009,52(4):559-574.
[16]Philips, R.J., G. F. Adams, Jr. W. E. Brown, R.E. Eggleton, P. L. Jackson, R.L Jordan, W. I. Linlor, S. H. Ward, and J. S. Zelenka. Apollo lunar sounder experiment [R]. NASA Technical report,1973.
[17]Porcello, L. J., R.L. Jordan, Jr. W. E. Brown, S. H. Ward, and P. L. Jackson. The Apollo lunar sounder radar system [J]. Proceeding of the IEEE,1974,62(6); 769-788.
[18]Kobayashi, T. and T. Ono. SAR/InSAR observation by an HF Sounder [J]. Journal of Geophysical Research,2007,112:E03s90.
[19]Heiken, G., D. Vaniman, and B. French. Lunar Source-Book:A User's Guide to the Moon [M]. New York:Cambridge University Press,1991.
[20]Kong, J. A. Electromagnetic-Wave Theory [M]. Massachusetts:EMW Publishing, 2005.
[21]Spudis P D, Bussey B, Lichtenberg C, et al. Mini-SAR:an imaging radar for the Chandrayaan 1 mission to the Moon [C]. In:Proceedings of the 36th Lunar Science Conference. New York:Pergamon Press,2005.1153
[22]Sen, P. N., C. Scala, and M. H. Cohen. A self-similar model for sedmentary rocks with applications to the dielectric constants for fused glass beads [J]. Geophysics, 1981,46:781-795
[23]Greely, R. Planetary Landscapes[M], Boston:Allen and Unwin,1987.