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.