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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. |
1.INTRODUCTIONThe fluorescence of vegetation is an indicator of stress and vitality. Since several decades both in laboratories and in the field studies has been performed to investigate at one hand the fundamental mechanisms of the fluorescence in relation to the photosynthesis and the functioning of the vegetation and on the other hand to explore the possibilities of this type of measurement for remote sensing purposes. Concerning the science related aspects many studies have been published and the relation between fluorescence intensity and vitality of the vegetation is clearly demonstrated. Since more than 20 years Laser Induced Fluorosensors have been developed for studies in laboratory and in the field. These sensors are using pulse lasers for excitation and detect the fluorescense signal in a short time window to obtain a sufficient S/N performance. For example about 10 years ago in the Netherlands a sensor called LEAF (= Laser Environmental Active Fluorosensor) was developed both for field measurements (distance < 100 m) and conditioned laboratory investigations3, 4. Herewith the first steps for remote sensing application have been made, albeit for application at a moderate distance. For application from space detection of solar induced fluorescence (so called “natural” fluorescence) is the only realistic possibility. Contrary to Laser Induced Fluorescence with monochromatic excitation natural fluorescence results from broadband (solar) excitation. However the basic fluorescence phenomena are the same, only the relative distribution of the emission may be different. Practically observation and measurement of the solar induced vegetation fluorescence is difficult because of the weakness of the signal relative to the reflected signal. The only feasible method is sensing the fluorescence using the Fraunhofer lines in the solar spectrum, where the irradiance is low enough so that the fluorescence signal is not too small in comparison to the reflected signal. This technique has already been explored more than 25 years ago by Plascyk e.a., who developed a Fraunhofer Line Discriminator (FLD), based on a solid etalon Fabry-Perot filter5, 6. In later years investigations with the FLD have been continued especially in the US on a low level scale 7, 8, 9, 10. The FLEX instrument is an European initiative, that is based on high resolution spectroscopy, making use of advanced 2-dim. detector arrays11, 12. FLEX is proposed in 1998 for a space mission and is being studied at this moment w.r.t. a more detailed definition of the science – and instrument requirements, demonstration of feasibility by a more elaborated instrumental design and performance estimate and additional laboratory experiments. The present study is being performed by TNO Institute of Applied Physics (instrumental aspects) and the University of Strasbourg and of Karlsruhe (scientific aspects). 2.SCIENTIFIC REQUIREMENTSThe primary goal of the FLEX mission is the quantative measurement from space of plant fluorescence by measurements at defined (Fraunhofer) wavelengths. These measurements of course must provide information about the state of the observed vegetation w.r.t. factors such as stress and vitality, which affect plant function. For FLEX this leads to:
3.INSTRUMENTAL REQUIREMENTS.For the FLEX instrument design the main starting points are:
Table 1:Fraunhofer lines.
Besides these starting points the following factors determine to a great extent the optical instrumental design of FLEX.
4.CONCEPT DESIGNBased on the starting points as mentioned in chapter 3 a concept design of FLEX is made. The schematic configuration is given in figure 1. FLEX is an imaging spectrometer, that consists essentially of a telescope and a spectrometer. Via a scan mirror in front of the telescope the observation or the calibration mode can be selected. The various elements in the configuration are: The telescopeThe telescope images the earth (or solar radiation) on the entrance slit of the spectrometer. It consists of a single mirror with a focal length of 320 mm. A telescope exit slit (= spectrometer entrance slit) of 0.16 × 46.8 mm2 leads to a FOV of 0.03° (flight direction) × 8.4° (swath direction). The spectrometerThe (imaging) spectrometer consists of 2 spectral channels. In each channel after dispersion by a grating a (limited) spectral range is imaged in one direction on the detector. In perpendicular direction the FOV of 8.4° is imaged on the detector. The components of the spectrometer are:
The scanmirrorWith the scanmirror in front of the telescope FLEX can be set in 2 modes:
In table 2 an overview is given of the main data of the FLEX concept. Table 2.FLEX data
5.CONCLUDING REMARKSIn this paper the concept design of an instrument for measurements of fluorescence from space is described. After study of the potential and possibilities of the fluorescence technique as tool for diagnostics of the vitality of vegetation during the last decades both in Europe and US the possibilities for application from space are being explored now, where in the US the interferometric techniques are studied and in Europe high resolution spectroscopy is being explored. At this moment a feasibility study of FLEX is being performed, which will bring us a step further to the realisation of a spaceborne mission. This will be a first step for future development of a new generation of sensors for earth observation. 6.6.REFERENCESE.W. Chapelle, F.M. Wood, J.E. McMurtrey and W.W. Newcomb:,
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