Improved Monitoring of Sun-Induced Fluorescence with the HyPlant Imaging Spectrometer
Guanter, Luis¹; Rascher, Uwe²; Schickling, Anke²; Pinto, Francisco²; Damm, Alexander^3
1Free University of Berlin, GERMANY; ²Forschungszentrum Jülich, GERMANY; ³University of Zürich, SWITZERLAND

Sun induced fluorescence (Fs) is a novel remote sensing signal that is emitted from the core of the photosynthetic machinery in the 650-800 nm window. The fluorescence signal has the potential to quantify the actual rate of photosynthesis and is closely related to vegetation stress and reflects functional limitations of photosynthetic carbon gain. Fs is a rather weak signal, but can be measured in the solar and atmospheric absorption lines using high performance imaging spectrometers.

The first Fs maps from airborne sensors were produced in the last years from measurements by general-purpose imaging spectrometers with spectral resolutions of the order of 2-5 nm. The basic approach for those first retrievals consisted in the evaluation of the Fs in-filling of the O2A-band in 760 nm with an empirical compensation of atmospheric effects with reference non-fluorescent targets. Although very important to demonstrate the feasibility of the remote measurement of Fs, such earlier airborne measurements of Fs were performed under several assumptions driven by instrumental limitations, Fs was normally given in relative units, and only one single spectral point in 760 nm was sampled.

Those technical limitations for high-quality Fs retrieval can be overcome with sub-nanometer spectral resolution measurements in the 600-800 nm window. For this purpose, the HyPlant imaging spectrometer was developed by the Forschungszentrum Jülich and the Finish company Specim. HyPlant is the first imaging spectrometer designed and optimized for Fs retrievals in the entire 650-780nm emission window. This airborne sensor consists of two modules and allows state-of-the-art imaging spectroscopy in combination with high resolution mapping of the red and near-infrared region. After three years of developmental time HyPlant was first employed in the ESA funded HyFLEX campaign late summer 2012 measuring an agricultural area in Germany and natural forest in the Czech Republic. More than 120 flight lines were recorded and radiometric and geometric performance of the HyPlant sensor was characterized in detail. The HyPlant sensor allows mapping of reflectance between 400 and 2500 nm with a high performance spectral window between 670 and 780 nm (0.26 nm FWHM in this spectral range). Spatial resolution was set to 1 and 3 meters depending of the flight height. Airborne data are complemented by a dense network of ground data, including surface reflectance, detailed characterization of the atmosphere using LIDAR and characterization of the functional status of vegetation.

In this contribution we will report on recent work on the retrieval of chlorophyll fluorescence from HyPlant measurements. We will present methods for the retrieval of Fs from HyPlant as well as the analysis of the results and validation of the retrievals making use of the data set collected during HyFLEX. Emphasis will be put on the assessment of the potential of the derived Fs maps for an improved monitoring of photosynthetic processes for a variety of vegetation types. This study is an important component in the preparation of the Earth Explorer 8 candidate satellite mission FLEX, that is currently under evaluation in phase A by the European Space Agency (ESA).