Polarimetric characterization of Chandrayaan-3 landing site near lunar south pole using high resolution Chandrayaan-2 DFSAR data

Abstract

The Chandrayaan-3 (CH3) Vikram lander presents an unique opportunity to study the radar scattering behavior of the landing site as well as human-made dihedral structure on the lunar surface. This opportunity is made possible by the Dual-Frequency Synthetic Aperture Radar (DFSAR) sensor onboard the Chandrayaan-2 orbiter, which has the highest resolution and polarimetric capabilities compared to any planetary SAR sensor. To explore this, we utilized DFSAR to capture high-resolution images of the CH3 landing site during pre-landing and post-landing condition, with a pixel spacing as fine as 1 m, in a hybrid-pol mode. The landing site exhibits dominant volume and even-bounce radar scattering behavior similar to an ideal dihedral geometry. Furthermore, we observed an exceptionally high Circular Polarization Ratio value at the landing site (1.99 ± 0.30), a rarity among natural features on the lunar surface. Besides, the landing site is characterized by enhanced average dielectric constant value (5.76 ± 3.11). The post-landing DFSAR image reveals a 177 m2 area, surrounding the CH3 landing location, characterized by high CPR and elevated even bounce and volume scattering. The drastic enhancement of the average CPR value (7-times), dielectric value (2-times), even bounce and volume scattering in the landing site, in comparison with the pre-landing DFSAR observation, is due to presence of lander module and disturbance in the regolith structure in the landing area. The polarimetric characteristics of the landing site distinguish it from the major natural features on the lunar surface, such as regolith, debris flow, and impact ejecta. This investigation is of utmost importance as it emphasizes the effectiveness of high-resolution DFSAR acquisitions for evaluating the polarimetric behavior of small-scale features, which can be invaluable for characterizing landing sites in upcoming missions.