Passive field reflectance measurements

Abstract

The results of reflectance measurements performed with a three-band passive radiometer with independent channels for solar irradiance reference are presented. Comparative operation between the traditional method that uses downward-looking field and reference white panel measurements and the new approach involving duplicated downward- and upward-looking spectral channels (each latter one with its own diffuser) is analyzed. The results indicate that the latter method performs in very good agreement with the standard method and is more suitable for passive sensors under rapidly changing atmospheric conditions (such as clouds, dust, mist, smog and other scatterers), since a more reliable synchronous recording of reference and incident light is achieved. Besides, having separate channels for the reference and the signal allows a better balancing of gains in the amplifiers for each spectral channel. We show the results obtained in the determination of the normalized difference vegetation index (NDVI) corresponding to the period 2004–2007 field experiments concerning weed detection in soybean stubbles and fertilizer level assessment in wheat. The method may be used to refine sensor-based nitrogen fertilizer rate recommendations and to determine suitable zones for herbicide applications.