Evaluation of EPIC oxygen bands stability with radiative transfer simulations over the South Pole





Citation of Original Publication

Zhou, Yaping, Peng-Wang Zhai, and Yuekui Yang. “Evaluation of EPIC Oxygen Bands Stability with Radiative Transfer Simulations over the South Pole.” Journal of Quantitative Spectroscopy and Radiative Transfer 310 (December 1, 2023): 108737. https://doi.org/10.1016/j.jqsrt.2023.108737.


This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
Public Domain


The Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) satellite orbiting the Sun at the Lagrange-1 point was launched without onboard calibration systems. Vicarious calibration is conducted for 8 of the 10 UV/VIS/NIR channels using other low earth orbiting satellite instruments, while its two O₂ bands (688 nm and 764 nm) rely on indirect moon-view calibrations because the same narrow-band O₂ bands are not readily available from other in-flight instruments. This study compares EPIC measurements from the four O₂ bands aiming at examining sensor stability over a uniquely suited location, i.e., the permanently snow-covered South Pole. The study utilizes radiative transfer model simulations with in-situ atmospheric soundings taken at South Pole during months of December and January from 2015 to 2022. The absolute discrepancy between the model simulations and observations is less than 1.0% for the two reference bands, but 5.75% and 15.63% for the 688 nm, and 764 nm absorption bands, respectively. The simulated A-band and B-band ratios are 16.09% and 4.74% higher than that from the observations. Various sensitivities are conducted to estimate possible contributions to the discrepancies from input atmospheric profiles, spectral surface albedos and surface BRDF. While none of the input uncertainties is likely to account for the large discrepancies in the oxygen absorption bands, a small shift in the instrument response function could be the main reason for these biases. On the other hand, the model simulations are able to capture systematic variations with observed angular measurements and explain the multi-year trends found in observed O₂ band ratios due to satellite orbit shifting. When model simulated contributions from the angle variations are deducted from the observed O₂ band ratios, the residual O₂ band ratios are found to be stable since 2015.