The Lunar Radar Sounder (LRS) experiment on-board the SELENE (SELenological and ENngineering Explorer) spacecraft has been planned for observation of the subsurface structure of the Moon, using HF radar operating in the frequency range around 5 MHz. The fundamental technique of the instrumentation of 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 by using EXOS-B (Jikiken), EXOS-C (Ohzora) and EXOS-D (Akebono) satellites; and the plasma sounder for observations of the Martian ionosphere as well as surface land shape are installed on the Planet-B (Nozomi) spacecraft which will arrive at Mars in 2003. For the exploration of lunar subsurface structures applying the developed sounder technique, discrimination of weak subsurface echo signals from intense surface echoes is important; to solve this problem, a frequency modulation technique applied to the sounder RF pulse has been introduced to improve the resolution of range measurements. By using digital signal processing techniques for the generation of the sounder RF waveform and on-board data analyses, it becomes possible to improve the S/N ratio and resolution for the subsurface sounding of the Moon. The instrumental and theoretical studies for developing the LRS system for subsurface sounding of the Moon have shown that the LRS observations on-board the SELENE spacecraft will give detailed information about the subsurface structures within a depth of 5 km from the lunar surface, with a range resolution of less than 75 m for a region with a horizontal scale of several tens of km. This capability is evaluated to be sufficient for study of the thermal history of the lunar surface region relating to a time scale of several tens of millions of years.
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