Abstract
Growing interest in the proximal sensing of sun‐induced chlorophyll fluorescence (SIF) has been boosted by space-based retrievals and up-coming missions such as the FLuorescence EXplorer (FLEX). The European COST Action ES1309 “Innovative optical tools for proximal sensing of ecophysiological processes” (OPTIMISE, ES1309; https://optimise.dcs.aber.ac.uk/) has produced three manuscripts addressing the main current challenges in this field. This article provides a framework to model the impact of different instrument noise and bias on the retrieval of SIF; and to assess uncertainty requirements for the calibration and characterization of state-of-the-art SIF-oriented spectroradiometers. We developed a sensor simulator capable of reproducing biases and noises usually found in field spectroradiometers. First the sensor simulator was calibrated and characterized using synthetic datasets of known uncertainties defined from laboratory measurements and literature. Secondly, we used the sensor simulator and the characterized sensor models to simulate the acquisition of atmospheric and vegetation radiances from a synthetic dataset. Each of the sensor models predicted biases with propagated uncertainties that modified the simulated measurements as a function of different factors. Finally, the impact of each sensor model on SIF retrieval was analyzed. Results show that SIF retrieval can be significantly affected in situations where reflectance factors are barely modified. SIF errors were found to correlate with drivers of instrumental-induced biases which are as also drivers of plant physiology. This jeopardizes not only the retrieval of SIF, but also the understanding of its relationship with vegetation function, the study of diel and seasonal cycles and the validation of remote sensing SIF products. Further work is needed to determine the optimal requirements in terms of sensor design, characterization and signal correction for SIF retrieval by proximal sensing. In addition, evaluation/validation methods to characterize and correct instrumental responses should be developed and used to test sensors performance in operational conditions.
Highlights
The estimation of passive sun-induced chlorophyll fluorescence (SIF) has interested the remote sensing community for some decades due to its potential to monitor photosynthesis [1]
Uncertainties in the location of λc and size of FWHM were 0.59·10−3 nm and 1.34·10−3 nm, respectively. These values describe the incertitude on the determination of these parameters; they do not represent a difference between the nominal and the characterized features or any variation induced by external factors
This work analyzed the effect of different instrumental responses on the retrieval of sun-induced chlorophyll fluorescence
Summary
The estimation of passive sun-induced chlorophyll fluorescence (SIF) has interested the remote sensing community for some decades due to its potential to monitor photosynthesis [1]. A lot of effort has been invested in the preparation of the FLEXmission [6,7]; which has been selected as the European Space Agency’s (ESA) 8th Earth Explorer mission and is planned to be launched in 2022 These developments have boosted the interest of the near-ground remote sensing (proximal sensing) community in the retrieval of SIF. New portable field spectroradiometers have been developed well as their use has spread in the scientific community These spectroradiometers feature higher spectral resolutions (e.g., FWHM ≤ 0.39 nm), higher signal-to-noise ratios (e.g., SNR ~ 1000:1), and extended dynamic ranges of 200,000 counts, which enable SIF to be retrieved in different atmospheric dark lines from the same measurement. The intention is to provide an update to the previous review performed a decade ago by Meroni et al [1]
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