GPR observation of the Moon from orbit: Kaguya Lunar Radar Sounder

Subsurface exploration of the Moon by an orbiting HF radar is reported. Lunar Radar Sounder (LRS) is an FMCW radar which was installed on a lunar exploration platform of Japan, Kaguya, in 2007. LRS operated in a frequency range of 4-6 MHz. A 30 m long dipole antenna was used as the transmitting/receiving antenna. A weighting function was applied to the transmitting pulse waveform so as to suppress range side lobes, which resulted in the range resolution of 150 m. During the operation, LRS covered the whole surface of the Moon with its footprint (~5km in diameter) and carried out lunar subsurface exploration. Uncompressed observation data were returned to the ground station. Synthetic Aperture Radar process was applied to the data. Layered structures were found in many sites in “mare” regions where the surface may be regarded flat and smooth, which indicate past lava flow activities in the regions. Some craters exhibit subsurface echoes which indicate inhomogeneous subsurface structure below the bottom of the craters.