A-scope analysis of subsurface radar sounding of lunar mare region

Abstract

Lunar Radar Sounder (LRS) is a spaceborne HF radar system and is a science mission of Japanese lunar exploration project, SELENE, which is scheduled to be launched in 2005. The primary objective of LRS is to investigate the geologic structure of lunar subsurface from orbit. Computer simulations of LRS observation of lunar mare region have been carried out by utilizing a newly developed simulation code, the Kirchhoff-approximation Sounding Simulation (KiSS) code. The purpose of the simulations is to understand the nature of reflection/refraction of HF wave at the lunar surface as well as at the lunar subsurface boundary, and to confirm that the lunar subsurface structure can be investigated from orbit by means of an HF radar. Gaussian random rough surfaces are employed to represent the surface feature of a lunar mare region. From simulation results, we have found that the power flux of both surface nadir echo and subsurface nadir echo vary little if roughness of either/both surface or/and subsurface boundary interface changes. However, their intensity of surface off-nadir backscattering echo varies following a power law of (kσ0)2, where k is the wave number of LRS transmission pulse, and σ0 is the RMS height of the surface. Thus slight roughness of the surface causes significant increase of the power flux of surface offnadir backscattering echo, which easily masks weak subsurface echoes. These observations have been understood qualitatively by geometrical optics approximation and quantitatively by examining the Stratton’s integral formula in an analytic way. Computer simulations have revealed that subsurface echoes are received even if they are completely masked by surface off-nadir backscattering echo. To distinguish those subsurface echoes from strong surface backscattering echo, the data stacking technique has been proved to be effective on reducing surface backscattering echoes due to their random nature.