Level‐2 processing of Chandrayaan‐2 Imaging Infrared Spectrometer (<scp>IIRS</scp>) data for generation of surface reflectance

Abstract Spectroscopy‐based approach for remote exploration of any planetary body is significant in providing detailed understanding of surface composition, vital to any scientific exploration. Imaging Infrared Spectrometer (IIRS) is a hyperspectral imaging sensor flown over ISRO's Chandrayaan‐2 (Ch‐2) orbiter for mapping mineral composition and complete characterization of hydration feature on the lunar surface. With the extended spectral range (0.8–5 μm), high‐spatial resolution (80 m) and high signal‐to‐noise ratio, IIRS data are capable of measuring surface composition based on diagnostic spectral absorption features of known/unknown characteristic minerals present on the lunar surface. The present paper discusses for the first time the methodology to process Ch‐2 IIRS data to generate photometrically corrected reflectance images after thermal correction. Spectrally and radiometrically calibrated Level‐1 IIRS spectral radiance data were subjected to various data processing techniques including thermal emission correction beyond 2.5 μm, conversion to apparent reflectance, and empirical line correction for smoothing the observed reflectance spectra. The thermally corrected IIRS reflectance data in the 0.8–3.3 μm range after correction for standard geometry were calibrated with ground‐based observations of the lunar surface from the Apollo 16 site to generate Level‐2 product. The results generated for the selected study regions representing the dominant landforms of the Moon (Mare, Highland and Polar region) were analyzed based on overall spectral reflectance variation and prominent absorption features at particular wavelengths corresponding to their surface properties. Finally, the results were compared with observations from Chandrayaan‐1 Moon Mineralogy Mapper (M 3 ) data within the overlapping spectral range from the same region to validate the absolute reflectance of the IIRS. Overall, slight differences in reflectance have been observed in the spectral profile from both the sensors in the lower wavelength range attributed mainly due to differences in resolution and observation geometry. However, beyond 2 μm, the spectral slope variation could be clearly visible, possibly because of thermal contributions that have been removed efficiently in the case of Ch‐2 IIRS spectra.