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

Abstract

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.

De-shadowing of airborne imagery using at-sensor downwelling irradiance data

“…The roll, pitch, and yaw movements of the aircraft significantly affect the ILS measurements as several researchers stress (Choi and Milton 2001;Homolova, Alanko-Hotari, and Scheapman 2009;Milton 2000;Richter and Schläpfer 2013). Therefore, the correction of the irradiance data for the effects induced by the aircraft movement is mandatory before their exploitation.…”

“…This is because the aircraft usually flies over the lower troposphere, where the main bulk of aerosols and water vapour is located (Kneizys et al 1996;Wallace and Hobbs 2006, 375). Unfortunately, the existing scientific references about the use of FODIS/ILS measurements are very few, since they have limited uses, and mostly focus on the correction of such data due to the movement of the aircraft (Choi and Milton 2001;Homolova, Alanko-Hotari, and Scheapman 2009;Milton 2000;Richter and Schläpfer 2013).…”