Sensing of Plant Fluorescence From the Fill-in of Solar Fraunhofer lines

Solar-induced chlorophyll fluorescence (ChlF) by terrestrial vegetation is linked closely to photosynthetic efficiency that can be exploited to monitor the plant health status and to assess the terrestrial carbon budget from space. Current optical remote sensing approaches have mainly relied on reflectance data offered by several Earth observing systems to provide estimates of potential photosynthesis rates of plant ecosystem. Another alternative approach that has been investigated for many years uses solar-induced chlorophyll fluorescence emitted by the photosynthetic process itself. This approach has the advantages over the solar reflectance one since it provides an early and more direct monitoring of the vegetation health status before any significant reduction in chlorophyll content. The solar-induced ChlF from vegetation exhibits two broad peaks in the red and the far-red at ~690 nm and ~740 nm. It adds a weak, broad continuum signal to the reflected solar signals that can be detected from the filling of strong O2 absorption lines or solar Fraunhofer lines. In this paper, we present the design of a tunable triple etalon Fabry-Perot imaging instrument that has the capability to provide vegetation fluorescence measurements from a geostationary platform over all daylight hours from high resolution reflected spectra of several key solar Fraunhofer lines. A prototype instrument designed and constructed specifically for airborne and groundbased ChlF measurements has been used to demonstrate this solar Fraunhofer line fill-in ChlF measurement technique. Specifically, we will focus on solar Fraunhofer line features near the ChlF red peak (~ 690 nm), a spectral region that is dominated by O2 and H2O absorption lines and has large reflectance spectral variability. The ChlF retrieval statistical and systematic uncertainties associated with this prototype sensor will be discussed in details based on demonstrated measurements conducted on the ground and in the laboratory.