What was the purpose of FLEXSense 2018?

The 2018 Fluorescence Explorer Sense (FLEXSense) campaign, also known as the FLEX Sentinel Tandem Campaign, combined various field activities in preparation for the FLEX satellite mission.

Two months of campaign activities were undertaken at six sites across Europe, which included the opportunity to measure and interpret the fluorescence signal across various scales (from leaf level to satellite data). The focus of these activities was the recording of complete FLEX-like datasets, which include all relevant elements that are needed for the preparation of the FLEX satellite mission.

The dataset of this campaign has proven to be complete, providing almost all elements that are needed to develop and validate the data products of the FLEX and Sentinel missions, i.e. including ground based characterisation of the structural and functional plant traits, recording of time series of top-of-canopy reflectance, fluorescence and atmospheric properties, extensive airborne mapping with the high-resolution imaging spectrometer HyPlant, the TASI thermal imager and an airborne LiDAR system, as well as satellite based imagery from Sentinel-2, Sentinel-3 and Sentinel-3B in a reprogrammed mode during commissioning phase.

What was the outcome of FLEXSense 2018?

The airborne package consisting of the high-resolution imaging spectrometer HyPlant (hyperspectral surface reflectance and sun-induced fluorescence (SIF)), TASI (surface temperature) and a LiDAR system (surface and canopy structure) were used from an aircraft and built the backbone of these campaign activities. Data from this sensor package were recorded over diverse ecosystems (including agriculture, natural forests, agroforestry, grasslands, coastal regions and inland and offshore waters) and 342 flight lines of combined data were recorded, processed and quality-checked. The airborne data were combined with extensive ground-based measurements of plant function, detailed atmospheric characterisation, and on several ‘golden days’ with synchronous overpasses of the Sentinel-3A and 3B tandem constellation during the commissioning phase, when Sentinel-3B data were provided in a special ‘reprogrammed high-resolution mode’.

A complete diurnal course of maps of SIF, vegetation temperature and reflectance were recorded from a large and diverse agricultural research campus (Campus Klein-Altendorf, Germany). These data were used to develop and test an empirical down-scaling approach, which allows to quantitatively separate structural and functional influences on diurnal canopy SIF measurements of various crop systems.

The relationship between gross primary productivity (GPP) and SIF was re-examined at various ecosystems. Diurnal and seasonal dynamics were described and interpreted showing that a linear relationship between GPP and SIF only exists on aggregated seasonal data, when changes in canopy chlorophyll content determines GPP. Under stressful conditions, when photosynthesis operates below its potential maximum, the linear relationship breaks down and SIF is the only remote sensing parameter that is able to track stress-induced down regulation of photosynthesis.

The relationship between GPP and SIF was analysed in more detail in a nutrient manipulation experiment in a Mediterranean agroforestry, which experienced extreme high temperatures during the campaign activity. We could show that reflectance-based vegetation indices such as the chlorophyll content index (CCI), enhanced vegetation index (EVI), normalised difference vegetation index (NDVI) and near-infrared reflectance of vegetation (NIRv) did not show any significant response to the heatwave. The functionally based parameters, namely photochemical reflectance index (PRI) and SIF were clearly affected by the extreme temperatures that caused a functional downregulation of photosynthesis. The relationship between GPP and SIF was thus reversed demonstrating the potential of SIF to serve as an early stress indicator.

Larger scale HyPlant maps and mobile FloX system measurements were used to investigate the spatio-temporal variations of SIF across different functional plant types. In this context, large agricultural areas in Germany and Italy and diverse Mediterranean forest ecosystems were mapped while on ground the biochemical and photosynthetic characteristics of the diverse vegetation were recorded. These data built the bases to better understand the link between functional diversity of plants and the dynamics in the SIF signal and in reflectance.

Several HyPlant flight lines were acquired over open inland and coastal waters to test the possibility to retrieve SIF from FLEX like satellite data. The data were spatially binned to increase the SNR ratio of level-1c data and then SIF was successfully retrieved from these images. This demonstrates that SIF can also be retrieved from water ecosystem, even though the low radiance signal in combination with the lower fluorescence signal of open waters will require some concepts to improve SNR.

Download the FLEXSense 2018 Final Report.

Digital Object Identifier: European Space Agency, 2023, FLEX Sentinel Tandem Campaign 2018, Version Final, https://doi.org/10.57780/esa-ae7953d.

The campaign data is available online via FTPS upon submission of a data access request. An active EO Sign In account is required to submit the request.

The data can be downloaded via an FTP client (e.g., FileZilla or WinSCP) using the option "Implicit FTP over TLS".

For further information about the EO Sign In Service you can visit TellUs.

Should you need support please contact EOHelp.