New datasets have been released for three campaigns, covering activities supporting the upcoming FLEX mission.

ESA conducts airborne and ground-based campaigns to support the development of new instruments, and calibration and validation of existing instruments. These campaigns simulate satellite-based instruments and are conducted all over the world in support of a wide range of applications.

FLEXSense, also known as the Technical assistance for airborne measurements during the FLEX Sentinel Tandem Experiment, was conducted in preparation for ESA’s FLuorescence EXplorer (FLEX) mission, an upcoming Earth Explorer satellite. FLEX will provide global maps of vegetation fluorescence that can reflect photosynthetic activity, and plant health and stress, furthering our understanding of the global carbon cycle and aiding agricultural management and food security. The satellite will fly in tandem with Copernicus Sentinel-3.

PhotoProxy, also known as the Technical assistance for the Photosynthetic-Proxy Experiment, focused on subjects needed to develop proposed Level-2D data products for FLEX.

FLEXSense 2018

During the first FLEXSense campaign in 2018, the OLCI (Ocean and Land Colour Instrument) onboard the Sentinel-3B satellite was reprogrammed to operate in a mode simulating the future tandem operations with FLEX, supporting validation of the concept as closely as possible. This FLEX mode on Sentinel-3B took place over an eight-week period between June and August.

The campaign was conducted at sites in Germany, Italy, France, Spain, and Switzerland, with the goal to acquire FLEX-like datasets. Airborne and ground-based activities were conducted as part of the campaign, in parallel with the space segment, requiring coordination between a large number of institutions.

The airborne aspect of FLEXSense 2018 involved the use of the HyPlant instrument, TASI thermal imager, and a LIDAR, which were operated on a series of flights from each of the test sites, including underflights of Sentinel-3A and 3B. HyPlant was the core instrument used in the airborne activities, and as the first airborne instrument optimised to retrieve full-spectrum fluorescence, it served as a demonstrator for the FLORIS (Fluorescence Imaging Spectrometer) instrument that will be used on FLEX.

The ground-based aspect of FLEXSense used the FloX field spectroscopy system, which consists of two spectrometers that measure infrared bands and fluorescence emissions. FloX systems were installed at all of the test sites, which gathered measurements of plant function.

During the campaign, daily measurements of Sun-Induced Fluorescence (SIF), vegetation temperature, and surface reflectance were recorded. These data were analysed and indicated that SIF has the potential to support a number of applications, including providing early indicators of plant stress. The HyPlant instrument also successfully retrieved SIF over inland and coastal waters, providing potential for FLEX to monitor water ecosystems.

Another study during the campaign examined the relationship between gross primary productivity (GPP) and SIF in different ecosystems. The data acquired indicated that a linear relationship between GPP and SIF only exists in aggregated seasonal data, when changes in canopy chlorophyll content determines GPP. When seasonal changes cause photosynthesis to operate below potential maximum, the linear relationship breaks down and SIF is a very important remote sensing parameter that is still able to track stress-induced down regulation of photosynthesis. This highlights that FLEX will be able to acquire measurements during conditions current techniques cannot support.

FLEXSense 2018 acquired extensive measurements over diverse ecosystems, including agriculture, natural forests, agroforestry, grasslands, coastal regions, and inland and offshore waters. This resulted in several complete datasets combining satellite, airborne, and ground-based sources, which demonstrated the potential applications for the FLEX mission concept. The datasets provide valuable references for use in developing FLEX data products and calibration and validation for the mission.

FLEXSense 2019

In 2019, the second FLEXSense campaign took place during three weeks of summer in Germany and Italy. The campaign continued the goals of its predecessor the year before, with a focus on compiling a complete set of high-resolution experimental data to support FLEX preparations. The 2019 campaign also addressed outstanding issues raised during previous FLEX-related campaigns, such as introducing new quality measurements for the HyPlant instrument, to better integrate its data with what are expected for calibration and validation of FLEX.

The operations of FLEXSense 2019 closely followed the 2018 campaign, making use of the same airborne and ground-based instruments and performing test flights during overflights of Sentinel-3A and 3B.

During the campaign, additional experiments were successfully conducted to test whether FLEX could support early drought stress detection, and fire detection products.

The outcome of the campaign was an expanded dataset that supports activities for the upcoming FLEX mission. The secondary experiments also validated the concept for drought and fire detection products, demonstrating that FLEX could offer the potential to support these applications.

PhotoProxy

The PhotoProxy campaign was conducted in parallel with FLEXSense 2018, and utilised the combined dataset FLEXSense produced. PhotoProxy combined the FLEXSense dataset with local flux measurements to derive optical remote sensing parameters and GPP.

In addition to FLEX, there are other existing and upcoming missions dedicated to studying Earth’s carbon cycle, such as ESA’s Biomass, and CMOS, GEDI, NiSAR, and TanDEM-X/L. Data from FLEX are expected to offer synergy with these missions creating an unprecedented set of complementary data.

ESA’s Carbon Science Constellation Initiative created clusters of studies, research activities, campaigns, and development of tools that will help the scientific community to explore potential synergies between different missions in the field of carbon cycle research.

PhotoProxy was created within this initiative and addressed quantitative assessment of vegetation photosynthesis and vegetation stress from space. The campaign explored a combination of fluorescence and reflectance-based measurements of vegetation to determine if this offers a better approach to assessing photosynthesis and GPP than solely fluorescence-based measurements.

Activities for PhotoProxy took place in Germany, Italy, Spain, and the United States. The HyPlant spectrometer was used at the sites in Germany, Italy, and Spain, while in the United States a combination of the Airborne Imaging Spectrometer for Applications (AISA) and IBIS fluorescence imager was used, which is similar to HyPlant. At all sites, the FloX system gathered ground-based measurements, as with FLEXSense.

The results from all four study sites verified the use of SIF, but the activities in Spain provided a notable demonstration of the benefits. During this part of the campaign, a series of flights took place during a heatwave at Majada in the summer of 2018.

The influence of stress events such as extreme temperature or drought on the relationship between GPP and fluorescence signal F760 is not currently understood with sufficient detail. Data acquired during the PhotoProxy flights in Spain revealed that signals F760 and F680 were strongly affected by the heatwave and that GPP and SIF respond differently to extreme temperature.

During the heatwave, the usually positive relationship between GPP and F760 became negative, highlighting the non-linear nature of the relationship between GPP and F760. The two peaks of SIF (F760 and F680) reacted differently to the heatwave, indicating that the two peaks of SIF carry out different information and can be used together to understand vegetation stress. Assessment indicated that Photochemical Reflectance Index (PRI) was the best predictor of GPP during the heatwave, which indicates that photosynthetic activity was highly controlled by heat dissipation processes. These results suggest that including PRI in an empirical model together with F760 might improve GPP prediction, especially during stress conditions.

Results of the campaigns

All three campaigns were successful and verified the FLEX mission concept, demonstrating the feasibility and benefits of using Sun-Induced Fluorescence to measure photosynthetic activity.

The two FLEXSense campaigns generated complete datasets, which provide almost all of the elements that are needed to develop and validate data products for FLEX and Sentinel missions.

The PhotoProxy campaign complements the FLEXSense datasets with data that combined the future FLEX-like measurements with surface reflectance measurements, offering new insights into the capabilities of these techniques.

How to access the data

Datasets from all three of these campaigns are freely available for download upon submission of a data access request.

Learn more about the campaigns and how to request the data:

• FLEXSense 2018
• FLEXSense 2019
• PhotoProxy