A Field Platform for Continuous Measurement of Canopy Fluorescence

This paper presents a field platform for continuous measurement of fluorescence at the canopy level. It consists of a 21-m-high crane equipped for fluorescence measurements. The crane is installed in the middle of the fields dedicated to agricultural research. Thanks to a jib of 24 m and a railway of 100 m distance, fluorescence measurements can be performed at nadir viewing over various field crops. The platform is dedicated to the development and test of future passive or active airborne and space-borne vegetation sensors. A new fully automatic instrument, called TriFLEX, has been installed at the end of the jib. TriFLEX is designed for passive measurement of fluorescence in the oxygen A and B absorption bands. It is based on three spectrometers and allows for continuous measurements with a repetition rate of about 1 Hz. The data products are the radiances of the target, the fluorescence flux at 687 and 760 nm, and several vegetation indexes, including the photochemical reflectance index and the normalized difference vegetation index. A new algorithm for fluorescence retrieval from spectral bands measurement is described. It improves upon the well-known Fraunhofer line discriminator method applied to passive fluorescence measurement by taking into account the spectral shape of fluorescence and the reflectance of vegetation. A measurement campaign of 38 days has been carried out in summer 2008 over a sorghum field. The evolution of the signals showed that the crop was suffering from stress due to lack of water. After several rainy days, a reversion of the water stress was observed.