HF (5 MHz) Imaging of the Moon by Kaguya Lunar Radar Sounder Off Nadir Echo Data

Abstract

HF (5MHz) imaging of lunar surface was attempted using off nadir echo data of Kaguya Lunar Radar Sounder (LRS). LRS observation data of multiple orbits were processed and mapped onto a surface which was defined based on the Kaguya Digital Elevation Model. The transmitting/receiving antenna of LRS is a dipole antenna which illuminates lunar surface on both sides of orbit, consequently, in a single-orbit observation, a detected target location has ambiguity in terms of the side of the orbit. However, use of multiple orbit data enables to resolve this ambiguity problem, i.e., radar illumination is controlled. We demonstrated this HF imaging technique by reconstructing surface images of Rupes Recta region using LRS observation data of 61 orbits. Control of radar illumination was confirmed by the presence/absence of Rupes Recta image in the reconstructed surface images. As it was anticipated, the reconstructed surface images presented false images which were identified as mirror images of major surface features. We also carried out simulation of these LRS observations of 61 orbits over the Rupes Recta site using Kirchhoff-approximation Surface Scattering (KiSS) simulation code. Comparison of the images of LRS observation and those of KiSS simulation exposed some discrepancies. Our interpretation is that the discrepancies are attributed to shallow subsurface scatterings which the KiSS simulation does not take into account. This implies the possibility of imaging shallow lava tubes by LRS, although we did not find one in this particular site of Rupes Recta region.