FLEX: an Imaging Spectrometer for Measurement of Vegetation Fluorescence

Detection of vegetation fluorescence gives information about plant functioning, stress and vitality. During the past decades several ground based laser fluorosensors have been developed to investigate these processes and to demonstrate the value of this technique. FLEX (= FLuorescense EXplorer) is a space mission to measure the fluorescence of vegetation on earth over large areas from space. Such a mission would greatly improve the understanding and enhance the capability to quantify e.g. the role of terrestrial vegetation in global carbon sequestration. Because the fluorescence signal, which is excited by solar irradiation is low with respect to the reflected sunlight the signal from a satellite is proposed to be measured in the solar Fraunhofer lines, where the reflection signal is much reduced. The heart of FLEX is a high resolution imaging spectrometer with 2 channels: channel 1 around the Fraunhofer lines at ‡ = 397 nm, ‡= 423 nm and/or ‡ = 434 nm and channel 2 around the Fraunhofer line at ‡ = 656 nm. The required spectral resolution will depend on the linewidth (0.02-0.3 nm). A first definition of the field of view is 8.4 degrees, leading from an 800 km satellite altitude to a swath of about 120 km. For detection a 1024x1024 pixel frame transfer CCD detector is proposed, with a pixel dimension of 13 x 13 ‡ mm2. The maximum footprint is about 500x500m2. The optical configuration contains a scan mirror for solar calibration, for pointing the FOV in swath direction and for freezing the observed ground scene up to a few seconds to increase the signal to noise performance. At this moment the concept of FLEX is elaborated in a feasibility study. Both the scientific and instrument requirements are updated and the concept is studied in detail. Besides a development plan for FLEX is made. In this paper the idea and the headlines of FLEX are described.