FLEX (Fluorescence Explorer) Feasibility study

摘要

The fluorescence signal emitted from vegetation is directly linked to the photosynthesis and as such may be usedrnas an indicator for plant functioning, stress and vitality. Observation of solar induced fluorescence from space isrnproposed by measuring the weak signal contribution in the Fraunhofer line wavelengths.rnThe scientific requirements have been studied in detail, looking to aspects such as the selection of Fraunhoferrnlines, the solar induced fluorescence radiance, measurement accuracy, spatial resolution, atmosphere influencernetc. This has resulted in instrument requirements, that are the basis for a trade off study of optical observationrntechniques. The main choice was between applying a grating spectrometer or a filter spectrometer, each of whichrnhave advantages and disadvantages for Fraunhofer line detection (FLD). From both spectrometer types arnpreliminary optical design has been made. A model has been developed to evaluate the different configurationsrnfor S/N, integration time, radiance level etc.rnIn the study the feasibility of Fraunhofer line detection from space is demonstrated, albeit, that the observationrnstrategy will depend on the real amount of the solar excited fluorescence intensity.rnThe main conclusions of the study are:rn- The atmospheric effects on Fraunhofer Line Detection (FLD) are mainly the Rotational RamanrnScattering (RRS). The hereby induced "filling in" of the Fraunhofer lines can be predicted with goodrnaccuracy and can be removed with appropriate models.rn- The signal modelling shows, that depending of the intensity of solar induced fluorescence of vegetationrnmeasuring of fluorescence from space is feasible with reasonable integration times, albeit that thernstrategy of "scene freezing" might be necessary.rn- It is of great importance to acquire on short term reliable data on the fluorescence intensity ofrnvegetation, caused by solar excitation.rn- Two possible spectrometer configurations can be used, an imaging spectrometer or a filter spectrometerrndepending on different spectral and spatial measurement strategies.rnThe results of the study are a good starting point for further development of a passive spaceborne fluorosensor.