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 orbit Earth 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 polar orbiting 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 1km reflectances to infer calibration coefficients for four EPIC visible and NIR channels: 443 nm, 551 nm, 680 nm and 780 nm. MODIS and EPIC measurements made between June 2015 and June 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/sec 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 O2 absorbing channels (688 nm and 764 nm) assuming that there is a small difference between moon reflectances separated by ~10 nm in wavelength provided the calibration factors of the red (680 nm) and near-IR (780 nm) are known from comparison between EPIC and MODIS.
Highlights
The Deep Space Climate Observatory (DSCOVR) occupies a unique location among Earth-observing instruments in the Lagrange point L1 between the Sun and the Earth at about 1.5 million km from Earth
Since the launch in June 2015 Earth Polychromatic Imaging Camera (EPIC) provided regular Earth images in 10 narrow spectral channels ranging from UV to near infrared (NIR)
The large scattering angle of EPIC observations is a significant difference compared to the observations from low Earth orbit (LEO) instruments
Summary
The Deep Space Climate Observatory (DSCOVR) occupies a unique location among Earth-observing instruments in the Lagrange point L1 between the Sun and the Earth at about 1.5 million km from Earth. The large scattering angle of EPIC observations is a significant difference compared to the observations from low Earth orbit (LEO) instruments. One approach to its calibration is to compare EPIC observations to the measurements from polar-orbiting radiometers Another is to use the images of the moon regularly observed by the instrument. In this study we use Moderate Resolution Imaging Spectroradiometer (MODIS; King et al, 2003) Level 1b reflectances and collocated EPIC measurements to derive the calibration coefficients in four EPIC visible and NIR channels. Version 2 data include refined geolocation, flat-fielding and stray light correction algorithm (Marshak et al, 2017) These differences require the derivation of separate sets of calibration coefficients for each of the two versions.
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