Calibration of EPIC UV channels

Since EPIC was launched without an accurate laboratory calibration, in-flight calibration transfer using other well-calibrated satellites was necessary. There were two suitable LEO satellites satellite instruments Aura/OMI (Ozone Monitoring Instrument) and Suomi-NPP/OMPS (National Polar-orbiting Partnership / Ozone Mapping and Profiler Suite) that contain similar wavelength channels and are able to observe scenes that closely match in location and angles with those observed by EPIC. Of these, the best calibrated was OMPS, which has an albedo accuracy of 2% and a wavelength accuracy of better than 1%.

Reflectance calibration was chosen, since the ratio of reflected radiance to the incoming solar flux mostly cancels the strong Fraunhofer line structure. The lack of line structure permits accurate interpolation needed to match the wavelength bands of EPIC. EPIC measures raw counts per second (C/S) based on permanently fixed exposure times designed to fill the CCD wells to approximately 80% for the brightest scenes in each of all 10 channels. Comparison with identical OMPS scenes produces EPIC multiplicative albedo calibration coefficients Kλ (Table 1) to convert C/S into top of the atmosphere reflectance πI/Sλ. Here, I is the radiance measured by OMPS at the top of the atmosphere and Sλ is the solar flux corrected for the sun-earth distance. For the UV channels, there is a small secular change of a few percent per year. A more complete discussion is given in Herman et al. (2018).

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.

λ (nm) Center FWHM (nm) Kλ Sλ(mW/m2/nm)
317.5±0.1 1.0 1.216E-04 819.0
325.0±0.1 1.0 1.111E-04 807.7
340.0±0.3 2.7 1.975E-05 995.8
388.0±0.3 2.6 2.685E-05 1003.

Table 1. Calibration factors (Kλ) and irradiance at 1 AU (Sλ) for four UV channels.

Ref.

Herman, J.R., L. Huang, R.D. McPeters, J. Ziemke, A. Cede, and K. Blank, 2018: Synoptic ozone, cloud reflectivity, and erythemal irradiance from sunrise to sunset for the whole Earth as viewed by DSCOVR spacecraft from the earth-sun Lagrange-1, Atmos. Meas. Tech., 11, 1-18, https://avdc.gsfc.nasa.gov/pub/DSCOVR/JayHerman/.