Fourier Transform Spectrometer (FTS) observes sunlight reflected from the earth's surface and light emitted from the atmosphere and the surface. The former is observed in the spectral bands 1 through 3 of FTS in the daytime, and the latter is captured in band 4 during both the day and the night. Prior to reaching the detectors of the instrument, the light in the bands 1 through 3 is split into two orthogonally-polarized components (P and S components) and measured independently. The light in the band4, however, is not split. The instrument thereby observes the incoming light in seven different channels. The reflection character of sunlight over land and that of over water differ significantly. Observation of reflected light over the surfaces of oceans and lakes is difficult since the water absorbs sunlight. The water, however, reflects sunlight specularly in certain directions. The instrument thus targets such points when observing over large water surfaces.
Cloud and Aerosol Imager (CAI) visualizes the state of the atmosphere and the ground surface during the daytime. The image data from CAI are used to determine the cloud existence over an extended area that includes the FTS's field of view (FOV). When clouds and aerosols are detected in FOV, cloud characteristics and aerosol amounts are calculated. The information is used to correct the effects of the clouds and the aerosols on the spectra obtained with FTS.