A spectral calibration algorithm for the hyperspectral geostationary environmental monitoring spectrometer (GEMS) onboard
GEO-KOMPSAT-2B (GK-2B) planned to launch in 2019 has been developed. Although spectral registration for
the CCD detector is done by the optical parameters prepared during the ground test of the instrument, the algorithm is
applied for the improved spectral accuracy. The prototype algorithm is based on the best fitting of the measured
spectrum to the known high resolution reference spectrum such as the solar irradiance. To characterize the prototype
algorithm, a series of sensitivity tests for various spectral parameters, such as squeeze, shift, spectral response function,
and reference solar spectrum, has been performed. The prototype algorithm shows a minimal sensitive to the
uncertainties associated with several parameters such as squeeze, shift, or spectral band. However, the algorithm
performance degrades by an order if the spectral response function including its shape has uncertainty. Thus, it is
recommended to measure the spectral response function at the ground test as accurately as possible. Furthermore, the
prototype algorithm is also highly sensitive to the used reference solar spectrum, which needs further investigation.
The five channel meteorological imager (MI) on-board the geostationary Communication, Ocean, and Meteorological
Satellite (COMS) of Korea has been operationally used since April 2011. For a better utilization of the MI data, a rigorous
characterization of the four infrared channel data has been conducted using the GSICS (Global Space-based
Inter-Calibration System) approach with the IASI (Infrared Atmospheric Sounding Interferometer) on-board the European
Metop satellite as the reference instrument. Although all four channels show the uncertainty characteristics that are in line
with the results from both the ground tests and the in-orbit-test, there shows an unexpected systematic bias in the water
vapor channel of MI, showing a cold bias at the warm target temperature and a warm bias with the cold target temperature.
It has been shown that this kind of systematic bias could be introduced by the uncertainties in the spectral response function
(SRF) of the specific channel which is similar to the heritage instruments on-board GOES series satellite. An extensive
radiative transfer simulation using a radiative transfer model has confirmed that the SRF uncertainty could indeed
introduce such a systematic bias. By using the collocated data set consisting of the MI data and the hyperspectral IASI data,
the first order correction value for the SRF uncertainty is estimated to be about 2.79 cm-1 shift of the central position of the
current SRF.
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