Calibration of EPIC visible and near-IR channels
We used MODIS Aqua and Terra L1B 1-km bands 3 (central wavelength is 469 nm), 4 (555 nm), 1 (645 nm) and 2 (858.5 nm) reflectance values to infer calibration factors for four EPIC visible and near-IR channels: 443, 551, 680 and 780 nm, respectively. For each EPIC pixel we identified favorable MODIS pixels as follows: (i) spatially collocated within 25 km; (ii) temporally collocated within 10 min; and finally, (iii) having the same scattering angles within 0.5o.
We selected EPIC pixels that had at least 40 MODIS pixels within 25 km radius. Relative standard deviation was then calculated for the matching MODIS and EPIC pixels. In the latter case, a 5x5 pixel neighborhood was used to calculate the standard deviation. The value of the relative standard deviation was used to select the most homogeneous scenes. Two methods were used to determine the calibration coefficients from the most homogeneous scenes: first, linear regression between EPIC counts and MODIS reflectance values and second, mean MODIS/EPIC ratio for high MODIS reflectance (> 0.6) and small relative standard deviation (< 0.1).
The differences in the position and spectral width of the corresponding EPIC and MODIS channels may result in discrepancies when scenes with different spectral signatures are observed by the two instruments. To compensate, we employ spectral band adjustment factors (SBAFs) which convert MODIS reflectance values to equivalent EPIC reflectances for various surface types. These factors, in the form of linear regression coefficients, were obtained from http://www-angler.larc.nasa.gov/SBAF, and are based on the analysis of SCHIAMACHY hyperspectral data. The land cover type was identified based on a 0.5x0.5 degree re-projected version of the Global Mosaics of the standard MODIS land cover type data product (MCD12Q1) in the IGBP Land Cover Type Classification. Separate adjustment factors were used for MODIS Aqua and MODIS Terra data (Geogdzhayev and Marshak, 2018).
At the time of this writing, no degradation in the EPIC visible and near-IR bands has been detected, while the UV channels have a very small secular change (link to calibration in UV channels). The calibration factors Kλ for these channels are given in Table 2.
The calibration factors for all 10 EPIC channels are also publicly available at https://eosweb.larc.nasa.gov/project/dscovr/DSCOVR_EPIC_Calibration_Factors_V02.pdf.
Geogdzhayev, I. and A. Marshak, 2018. Calibration of the DSCOVR EPIC visible and NIR channels using MODIS Terra and Aqua data and EPIC lunar observations. Atmos. Meas. Tech. 11, 359-368, https://doi.org/10.5194/amt-11-359-2018.
|λ (nm) Center||FWHM (nm)||Kλ|
Table 1. Calibration factors for three visible and one near-IR channel.