Mean Solar Exo-atmospheric Irradiances (ESUN) is an important parameter to calculate the apparent
reflectance based on the satellite sensor measured DN values. GF-4 was launched in 2015, the ESUN
of this satellite has not been officially reported, however. To determine which solar spectrum curve is
best fitted to GF-4, this study calculated the ESUN of GF-1 at first, by using six distinct solar spectrum
curves and spectral response curves of GF-1. Next, the results were validated by comparing with the
operational released values. It indicates that the World Radiation Center (WRC) solar spectrum is the
most accurate and reliable solar spectrum curve for GF-1, with a total error less than 0.1% for 4 bands.
Finally, the ESUN of GF-4 was calculated by making use of the WRC solar spectrum curve.
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