Calibration Coefficients Computation for Visible-NIR-SWIR Channels
Gain parameters for visible/NIR/SWIR channels are derived from the pixel counts over the visible calibration target (the VISCAL unit) during the period of full illumination by the sun – this occurs once per orbit.
This full illumination period is determined according to the calibration window and the average and standard deviation of all pixels from the VISCAL unit, for all scans during the calibration period. A similar mean and standard deviation are calculated for all valid pixels from a selected Black Body (BB) (the dark signal) over the full orbit, as done previously for the IR channels.
The gain parameter is derived from statistics previously computed, assuming knowledge of the instrument gain settings and the spectral reflectance of the VISCAL unit.