Chandrayaan-2 dual-frequency SAR: Further investigation into lunar water and regolith

• Published in: Advances in space research 2016-01, Vol.57 (2), p.627-646
• Main Authors: Putrevu, Deepak, Das, Anup, Vachhani, J.G., Trivedi, Sanjay, Misra, Tapan
• Format: Article
• Language: English
• Subjects: Ambiguity analysis; Circular Polarization Ratio; Full-polarimetry; Hybrid-polarimetry; Imaging; Indian spacecraft; ISRO; Lunar probes; Lunar surface; Missions; Noise Equivalent Sigma Zero; Regolith; Synthetic Aperture Radar
• ISSN: 0273-1177; 1879-1948
• DOI: 10.1016/j.asr.2015.10.029

The Space Applications Centre (SAC), one of the major centers of the Indian Space Research Organization (ISRO), is developing a high resolution, dual-frequency Synthetic Aperture Radar as a science payload on Chandrayaan-2, ISRO’s second moon mission. With this instrument, ISRO aims to further the ongoing studies of the data from S-band MiniSAR onboard Chandrayaan-1 (India) and the MiniRF of Lunar Reconnaissance Orbiter (USA). The SAR instrument has been configured to operate with both L- and S-bands, sharing a common antenna. The S-band SAR will provide continuity to the MiniSAR data, whereas L-band is expected to provide deeper penetration of the lunar regolith. The system will have a selectable slant-range resolution from 2m to 75m, along with standalone (L or S) and simultaneous (L and S) modes of imaging. Various features of the instrument like hybrid and full-polarimetry, a wide range of imaging incidence angles (∼10° to ∼35°) and the high spatial resolution will greatly enhance our understanding of surface properties especially in the polar regions of the Moon. The system will also help in resolving some of the ambiguities in interpreting high values of Circular Polarization Ratio (CPR) observed in MiniSAR data. The added information from full-polarimetric data will allow greater confidence in the results derived particularly in detecting the presence (and estimating the quantity) of water–ice in the polar craters. Being a planetary mission, the L&S-band SAR for Chandrayaan-2 faced stringent limits on mass, power and data rate (15kg, 100W and 160Mbps respectively), irrespective of any of the planned modes of operation. This necessitated large-scale miniaturization, extensive use of on-board processing, and devices and techniques to conserve power. This paper discusses the scientific objectives which drive the requirement of a lunar SAR mission and presents the configuration of the instrument, along with a description of a number of features of the system, designed to meet the science goals with optimum resources.