Yutu-2 Rover of the Chang’e-4 (CE-4) is equipped with the Visible and Near-Infrared Imaging Spectrometer (VNIS) to investigate the mineralogy and rock lithologies on the Von Kármán crater floor in South Pole–Aitken (SPA) basin of the lunar farside. Ninety-six hyperspectral measurements were performed (for 18 lunar days until 29 May 2020) over relatively uniform terrain and the rover is still active as of 23 September. However, these in-situ spectral datasets were acquired with different illumination conditions. It is widely known that in addition to mineralogy, reflected light entering the spectrometer is also influenced by illumination conditions, causing albedo differences due to observation geometries. Thus, photometric normalization should be conducted to normalize these spectra to a standard illumination condition (i = 30°, e = 0°, g = 30°) before deriving the quantitative mineralogy for accurate geologic interpretations.
The planetary science group of Shandong University, together with other research groups, produced accurate empirical wavelength-dependent phase functions of the Lommel-Seeliger model (450-2395 nm) using in-situ imaging spectra and laboratory photometric datasets. Values of phase functions exhibit declining trends with the increase in phase angles and an opposition effect and phase reddening effect were observed. The shadows caused by surface roughness have significant impacts on the reflectance spectra acquired during lunar in-situ VNIS observations and proper corrections for shadows were introduced. Photometric corrections of CE-4 and laboratory VNIS spectra were conducted using the Lommel-Seeliger model. Generally, photometrically normalized spectra exhibit smaller spectral slopes, lower FeO contents and thus a larger OMAT than that of spectra without correction. The band centers of the 1 and 2 μm absorption of spectra after photometric normalization exhibit a more concentrated distribution, indicating the compositional homogeneity of soils at the CE-4 landing site after Yutu-2 traveling ~448 m.
The paper was published in the journal "Remote Sensing".
Original link: https://www.mdpi.com/2072-4292/12/19/3211
Qi, X., Ling, Z., Zhang, J., Chen, J., Cao, H., Liu, C., ... & Liu, J. (2020). Photometric Normalization of Chang'e-4 Visible and Near-Infrared Imaging Spectrometer Datasets: A Combined Study of In-Situ and Laboratory Spectral Measurements. Remote Sensing, 12(19), 3211.
This research was funded by Strategic Priority Research Program of Chinese Academy of Sciences; the National Natural Science Foundation of China; the pre-research project on Civil Aerospace Technologies supported by China National Space Administration (CNSA); the National Key Research and Development Program of China,; National Science and Technology Infrastructure Work Projects; the Natural Science Foundation of Shandong Province; the Beijing Municipal Science and Technology Commission; Physical-Chemical Materials Analytical and Testing Center of Shandong University at Weihai.