Remote sensing of chlorophyll fluorescence using Unmanned Aerial Vehicles (UAVs)

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This project aims to improve our understanding of plant photosynthesis with the help of innovative remote sensing technology. A small drone, Unmanned Aerial Vehicle (UAV), will measure the sun-induced chlorophyll fluorescence (SIF) of plants. SIF enables measuring the photosynthetic activity of plants which decreases under stress e.g. through disease. By measuring SIF, plant stress can be detected before it becomes visible in the crop. This method creates new opportunities for the research of ecosystems and agriculture with low environmental impact and high yield.In this research project, a field spectrometer is mounted on a UAV to measure SIF. A field spectrometer measures the intensity of light reflected from a surface. The field spectrometer enables differentiating between closely located wavelengths, which is essential when measuring SIF. One problem is that the SIF signal is only 2% of the total reflectance signal and interference effects can obfuscate this signal. The first objective is to calibrate and validate the fluorescence signal with light-emitting diodes (LEDs) in the fluorescence wavelength range.In addition, factors affecting the signal will be examined: the solar irradiation, the texture of the foliage, the vertical structure of the plants, and flying height. To assess the spatial distribution of such effects, many point measurements will be acquired by the UAV-mounted spectrometer in the area of interest. A spatially-explicit fluorescence surface may be interpolated from this dataset. Based on this surface, the spatial variation of the signal may be investigated, for example in relation to plant structure.The signal varies depending on observing an entire landscape or a single field. As a result, the sensor and the carrier platform are adapted according to the observation scale e.g. hand or tractor-mounted systems, aircraft and satellites. Small drones with a total weight up to 5 kg fill the gap between observations on the ground and in high altitude by flying up to a few 100 m above the surface and being versatile and inexpensive systems with regard to the flexibility and efficiency of spatial data collection.The comparison of the field spectrometer with an established ground-based measurement system (LIFT sensor) is an additional objective of this research project, which will improve interpreting the SIF signal. The findings from this comparison combined with the analysis of the signal in different altitudes can serve as a basis for preparing satellite missions e.g. the European FLEX mission. Ultimately, the data captured in the proposed DFG project will contribute to a better understanding of the SIF signal in different observation scales.