Observed radar echoes taken with the LRS near the Poisson crater (30.4S, 10.6E) on November 20, 2007, in a 20 second period from 18:22:50 to 18:23:10 (JST). 

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... Lunar Radar Sounder (LRS) on-board the KAGUYA (Selene) lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (~5MHz) frequency range (Ono and Oya, 2000; Ono et al. 2007). Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the moon, but also of the moon’s regional and global thermal history of the moon and of the origin of the Earth-Moon system (Yamaji et al., 1998). In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz (Kumamoto et al., 2007; Kasahara et al., 2007). After completing all the pre-flight model integration tests, the KAGUYA (Selene) spacecraft was launched on September 14, 2007. The technique of the instrumentation of the LRS is based on the plasma waves and sounder experiments which have been established through the observations of the earth’s magnetosphere, plasmasphere and ionosphere on-board Jikiken, Ohzora and Akebono satellites, and extended to observations of the Martian ionosphere as well as surface land shape on-board the Nozomi spacecraft. To provide necessary efficiency for transmission of high-power (800Watts) pulses and detection of reflected echoes with 5MHz frequency, Bi- Stem antennas with length of 15m are equipped. By using digital signal processing techniques for the RF waveform generation and on-board data analyses, it becomes possible to improve the S/N ratio and resolution, as well as capability of data handling for the subsurface sounding of the Moon. The instrumental and theoretical studies had already shown that the LRS observation on-board the SELENE spacecraft is able to provide detailed information about the subsurface structures within a depth of 5 km from the moon’s surface, with a range resolution of less than 75 m for a region with a horizontal scale of several tens of km (Kobayashi et al., 2002, 2006). This paper provides initial results from the LRS function test on-board the spacecraft in the lunar orbit. Initial results on the lunar subsurface structure were obtained using the LRS sounder mode observation data collected on November 20 and 21, 2007. The received radar echo (Figure 1) was detected as it has been expected through a computer simulation (Kobayashi et al., 2002). The extraction of radar echo signals reflected by subsurface structures was demonstrated to be satisfactory. There is well agreeing feature with the simulated echoes of the simulation study in the following aspects: - Trace and amplitude variation of the surface crater echoes in the 2-D diagram. - Behavior of the surface clatters (unwanted radar echoes caused by adjacent craters that interfere with the detection of subsurface echoes, particularly in highland regions). These observed characteristics of echoes ensure validity of the new data analysis method established with the computer simulation results. Here both crater echoes and surface clatters are classified as interference due to surface reflection, and we refer to the coherent echoes as "crater echoes" and the incoherent ones as "surface clatters". To verify the newly developed data analysis method, we tried to adopt a method that uses not only the amplitudes of the echoes but also their phases was proved feasible. This method utilizes the synthetic aperture radar (SAR) technique with foci of variable depths and ensures robust detection of radar echoes from subsurface structures. Shown in the upper left panel of Figure 2 are results of the SAR detection of reflective subsurface structures down to 500 m by focusing on relatively shallow regions. The depth refers to the 1737.4 km sphere centered at the center of the mass of the Moon. Red lines in the lower left panel show evident reflecting interfaces in the section, which is 180 km long. True dips of the interfaces are very small, as those sections are vertically exaggerated by a factor of ~30. This means that the mare is underlain by horizontally lying strata as thick as 500 m, probably composed of lava, volcanic ashe and ejecta blankets. This experiment verified the performance of the LRS sounder mode observation, and at the same time, provided direct evidence for the existence of depositional units underneath the Mare Imbrium. 1. Ono, T., A. Kumamoto, Y. Yamaguchi, A. Yamaji, T. Kobayashi, Y. Kasahara, and H. Oya, Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft, Earth Planets Space, 59 (in press), 2007 2. Kumamoto, A., T. Ono, Y. Kasahara, Y. Goto, Y. Iijima, and S. Nakazawa, Electromagnetic compatibility (EMC) evaluation of the SELENE spacecraft for the lunar radar sounder (LRS) observations, Earth Planets Space, 59 (in press), 2007. 3. Kasahara, Y., Y. Goto, K. Hashimoto, T. Imachi, A. Kumamoto, T. Ono and H. Matsumoto, Plasma Wave Observation using Waveform Capture (WFC) on LRS/SELENE, Earth Planets Space, 59 (in press), 2007. 4. Kobayashi, T., and T. Ono, SAR/InSAR observation by an HF sounder, 112, E03S90, doi:10.1029/2005JE002576, 2007. 5. Kobayashi T., T. Ono, Estimation of planetary surface roughness using radio sounder A-scope data, J. Geophys. Res., 111, E06S10, doi:10.1029/2005JE002575, 2006. 6. Kobayashi, T.,H. Oya, and T. Ono, A-scope analysis of subsurface radar sounding of lunar mare region, Earth Planets Space, 54, 973-982, 2002. 7. Kobayashi, T., H. Oya, and T. Ono, B-scan analysis of subsurface radar sounding of lunar highland region, Earth Planets Space, 54, 983-991, 2002. 8. Ono, T., and H. Oya, Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft, Earth Planets Space, 52, 629-637, 2000. 9. Yamaji, A., S. Sasaki, Y. Yamaguchi, T. Ono, J. Haruyama, and T. Ogada, Lunar tectonics and its implications for the origin and evolution of the moon, Mem. Geolo. Soc. Japan, No. 50, 213-226, July, ...