Remote sensing of chlorophyll fluorescence using Unmanned Aerial Vehicles (UAVs)
Final Report Abstract
This project aimed at developing an Unmanned Aerial Vehicle (UAV) payload capable of measuring Solar Induced chlorophyll Fluorescence (SIF) from the air “AirSIF”. SIF is a small amount of energy emitted by plants during photosynthesis. Measuring absorbed photosynthetically active radiation, heat dissipation, and emitted fluorescence allows estimating a plant’s photosynthetic activity and contributes to our understanding of the global carbon cycle. Chlorophyll fluorescence is well studied in leaves, but poorly understood in canopies. A UAV-based system helps to improve the understanding of these canopy observations. Within the project, the system was built and a field protocol for UAV-based SIF observations was developed. Depending on application, two operational modes are available. For accurate measurements of a specific object, stop&go mode is used, where the UAV hovers over an object for a given time. For mapping applications where covering a large area is important, the continuous or mapping mode is applied, with continuous observations along a flight line. Data processing involves applying a series of calibrations, corrections, and finally the SIF retrieval. Selecting a suitable retrieval technique depends on the instrument specifications and how well is characterised in the lab. Retrieval algorithms are under constant development. Since development of the UAV payload took longer than expected for technical reasons, field data was also collected later than expected. Field experiments aimed at exploring the influence of flight protocol, environmental conditions, and vegetation structure on the SIF signal. At the end of this project, sufficient data has been collected and data analysis is ongoing. Preliminary results show that it is possible to measure SIF from a UAV. The sensor used for this measurement is a point spectroradiometer, i.e. a non-imaging device. A challenge was to accurately obtain the position of the UAV for each measurement and account for flying height, UAV movement, topography, and time taken per measurement (separate work of a PhD student in the same project). The extent of the measured area is finally overlayed on an orthophoto. This way, a map of SIF could be generated, showing the spatial distribution of the signal in a field. Measured SIF values are within the expected range but a missing thorough atmospheric correction is limiting the currently obtainable data quality. Recent advances in radiative transfer modelling enable using the acquired data to improve our understanding of SIF on the canopy scale. Ultimately, this work has produced a functioning UAV-system for SIF measurements and enabled collecting important data for calibration and validation of current airborne (Hyplant) and future satellite missions (Fluorescence Explorer, FLEX).
