Calibration of the DSCOVR EPIC visible and NIR channels using MODIS Terra and Aqua data and EPIC lunar observations

Abstract

Abstract. The unique position of the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) at the Lagrange 1 point makes an important addition to the data from currently operating low Earth orbit observing instruments. EPIC instrument does not have an onboard calibration facility. One approach to its calibration is to compare EPIC observations to the measurements from polarorbiting radiometers. Moderate Resolution Imaging Spectroradiometer (MODIS) is a natural choice for such comparison due to its well-established calibration record and wide use in remote sensing. We use MODIS Aqua and Terra L1B 1 km reflectances to infer calibration coefficients for four EPIC visible and NIR channels: 443, 551, 680 and 780 nm. MODIS and EPIC measurements made between June 2015 and 2016 are employed for comparison. We first identify favorable MODIS pixels with scattering angle matching temporarily collocated EPIC observations. Each EPIC pixel is then spatially collocated to a subset of the favorable MODIS pixels within 25 km radius. Standard deviation of the selected MODIS pixels as well as of the adjacent EPIC pixels is used to find the most homogeneous scenes. These scenes are then used to determine calibration coefficients using a linear regression between EPIC counts s −1 and reflectances in the close MODIS spectral channels. We present thus inferred EPIC calibration coefficients and discuss sources of uncertainties. The lunar EPIC observations are used to calibrate EPIC O 2 absorbing channels (688 and 764 nm), assuming that there is a small difference between moon reflectances separated by ∼ 10 nm in wavelength and provided the calibration factors of the red (680 nm) and NIR (780 nm) are known from comparison between EPIC and MODIS.

Determining the Shortwave Radiative Flux From Earth Polychromatic Imaging Camera

“…These 10 EPIC channels include blue (443 nm), green (551 nm), and red (680 nm) channels. These channels are calibrated against the MODIS instruments on Terra and Aqua (Geogdzhayev & Marshak, ) and the accuracy of the calibration factors is assessed to be between 1% and 3%. Previous studies using MODIS and MISR instruments (Loeb et al, , ) have demonstrated that these narrowband channels can be used to derive broadband radiances.…”

“…Alternatively, the narrowband measurements from EPIC can also be used to derive daytime SW fluxes by utilizing narrowband‐to‐broadband regressions. As the EPIC visible channels are calibrated against the Moderate Resolution Imaging Spectrometer (MODIS) observations (Geogdzhayev & Marshak, ), and the narrowband‐to‐broadband regressions are developed from MODIS and CERES measurements, this approach lacks a degree of independence from CERES. However, it can address two outstanding issues.…”

EPIC Spectral Observations of Variability in Earth’s Global Reflectance

“…The reflectances are obtained by multiplying the original data values provided in the L1B files in engineering units of count per second by calibration factors for each wavelength (https://eosweb.larc.nasa.gov/project/ dscovr/DSCOVR_EPIC_Calibration_Factors_V02.pdf). These calibration factors were obtained by comparing EPIC observations with measurements taken by low Earth orbit satellite instruments [4,7], and analyzing EPIC moon observations [7]. We use the latitude and longitude of pixels to identify the surface types according to the International Geosphere-Biosphere Programme (IGBP) surface ecosystem classifications.…”

“…At this point, the optical resolution of EPIC images is about 10 km, and the instantaneous field of view of a pixel is about 8 km. To reduce the amount of data transmitted from DSCOVR, four pixels are averaged onboard the spacecraft for all bands except the 443 nm band [4,7]. This yields downloaded images of 1024 × 1024 pixels with a sub-satellite optical resolution of approximately 20 km.…”

A Relationship Between Blue and Near‐IR Global Spectral Reflectance and the Response of Global Average Reflectance to Change in Cloud Cover Observed From EPIC

“…The calibration coefficient was estimated by comparing EPIC observations with LEO measurements for nonoxygen‐absorbing bands (e.g., Aura Ozone Monitoring Instrument and Suomi NPP Ozone Mapping and Profiler Suite for UV channels and MODIS Aqua and Terra for visible and NIR channels) and using lunar observations for oxygen‐absorbing bands (Geogdzhayev & Marshak, ; Herman, Huang, et al, ; Marshak et al, ). Note that reflectance is not normalized by the cosine of the solar zenith angle (SZA) in this study.…”