Sensitivity of the ground-based downwelling irradiance recorded by the FODIS sensor in respect of different angular positions

Some airborne hyperspectral sensors (e.g. AISA) can measure spectral downwelling irradiance using an additional cosine sensor mounted on a roof of an aircraft. The downwelling irradiance data, however, are rarely used for any atmospheric correction or compensation of different sun-sensor geometry, partly because they are sensitive towards continuous motion of the airborne platform. The airborne hyperspectral system AISA Eagle (Specim, Ltd., Finland), combined with the Fiber Optic Downwelling Irradiance Sensor (FODIS), were used for ground-based outdoor static measurements. The FODIS sensor was tilted into various zenith and azimuth angles. The data analysis revealed high sensitivity of the raw recorded FODIS signal towards different angular position. Simple cosine corrections reduced variation in the recorded FODIS signal. The variability (standard deviation of all measurements) decreased by 88% after the cosine correction was applied.