FLEX Bridge Study – Task Descriptions
The FLEX Bridge Study Project kicked off on October 1, 2014 ("KO")
and concluded on September 30, 2015 ("KO+12").
The Final Review was held in December 2015, and the Final Project Report was published in January 2016.
Tasks, Milestones, and Deliverables
Nine organizations from eight countries were involved in the FLEX Bridge Study, necessitating
an effective strategy for overall management and coordination of activities. The study was been divided
into five tasks and twenty-eight workpackages, summarized here and detailed in the
sections below. Thirteen deliverables were scheduled. The findings from the
FLEX Bridge Study formed the basis of advice and recommendations developed throughout the
project, eventually culminating in the Final Report and Technical Data Package.
TASK 1. Management and coordination of activities
Key aspects included in Task 1 were: establishment of a study website — essentially,
continuation of the private website developed for the
2012 FLEX/Sentinel-3 Tandem Mission Photosynthesis Study — to provide
project updates and document sharing; overall coordination and management of the study to meet
the specified deliverables; and provision of an effective interface between the FLEX Bridge Study
and any other FLEX studies and campaign activities.
TASK 2. SIF Retrieval Optimization & Analysis
This task built upon the activities of the previous Performance Analysis and Requirement Consolidation Study (PARCS)
which was completed in 2014 and which focused on development and evaluation of approaches for retrieval of the fluorescence
signal. Task 2 targeted further optimization in retrieval of the SIF spectrum to extract key features, with the main
goal being to produce an accurate fluorescence retrieval. (This will have practical benefits for
implementing stress indicators and providing inputs to photosynthetic modelling.)
The main objectives of Task 2 were:
(i) to consolidate algorithms for atmospheric correction in FLEX configuration;
(ii) to optimise and improve a set of algorithms for retrieval of sun-induced chlorophyll fluorescence;
(iii) to formalise fluorescence derived indices; and
(iv) to advance the development of biophysical products.
This task will also utilize the FLEX End-to-End Mission Performance Simulator (FLEX-E) developed under PARCS
in generating images for datasets. The task culminated in the definition of higher Level-2 products that will lead to
development of retrieval algorithms for biophysical or canopy state variables for an advanced exploitation of FLEX data products.
TASK 3. Development of vegetation stress indicators & applications
This task built upon an activity of the 2012 FLEX/Sentinel-3 Tandem Mission Photosynthesis Study which identified
stress indices and associated research needs from literature reviews. Task 3 extended the range
of potential applications of SIF in stress detection for the stewardship & management of resources such as food, feed,
fibre, and fuel, and it helped identify potential sources of error. We linked to airborne campaigns from other
ESA activities, which have generated a substantive database available for the first time for in-depth analysis with respect to
future applications. From this database, stress-related experiments were extracted studying the link between
sun-induced fluorescence and stress. Additionally, we evaluated non-photochemical protection
mechanisms and their link to the Photochemical Reflectance Index (PRI) and sun-induced fluorescence
to better understand the added value of combined PRI and fluorescence measurements.
Stress-related datasets were extracted from airborne and ground-based campaign data to perform a quantitative analysis
of the relation between sun-induced fluorescence and selected vegetation stresses. Some open questions and limitations
regarding the link between NPQ and PRI were identified. Strategies to measure NPQ beyond the
commonly used NPQ-PRI linkage were conceptually developed. Campaign data from the HyPlant airborne sensor
was used to extract and test quantitative relationships between stress-induced changes in non-photochemical protection
and remote sensing parameters. Finally, we further extended the database generated in the PS Study with
more publications and datasets from recently published studies and also unpublished experiments investigating water,
temperature, and nitrogen deficiency stress effects on SIF.
TASK 4. Photosynthesis model optimization, updates, applications
Task 4 advanced the photosynthesis model (SCOPE v1.53) developed in the Photosynthesis Study with updated
functionality and content, and addressed its applications. Four aspects of model optimization were targeted:
(i) inclusion of new elements such as PSI/PSII relative contributions and xanthophyll effects;
(ii) definition of model parameters for specific plant functional types usable in operational global
dynamic vegetation models;
(iii) further quantification of vegetation structural effects on the measured fluorescence signal; and
(iv) investigation of data assimilation techniques to support applications in global ecology and
climate & carbon modelling.
The work has advanced the utility of the model for stress applications and illustrated useful relationships between
fluorescence, photosynthesis, stress factors and canopy structure. It also built upon the initial
validation work conducted in the Photosynthesis Study to evaluate the model's representation of the fluorescence emission
signal and quantification of photosynthesis.
TASK 5. Calibration & validation ("cal/val") strategy
Task 5 developed a strategy for the calibration and validation of the FLORIS sensor and selected FLEX products.
(A thorough calibration/validation plan is essential to ensure a timely development and deployment of in-situ observations
representing the relevant space and time scales. Product validation is an essential part of future
algorithm development, fine-tuning, and eventually mission performance evaluation.) The strategy complies
with cal/val standards at a global scale and metrics established in the FLEX Mission Requirements Document,
covering from the network level to the instrumentation, measuring and sampling protocols, with the horizon for implementation
at the timeframe of 2022+.
We evaluated multiple distances/metrics, focusing on particular characteristics of the parameter being evaluated and identifying possible
error propagation in the processing chain. Steps included: determination of the biomes and
climatic regions that need to be represented in the cal/val strategy; setting of the requirements on the
cal/val sites and possible synergies with existing networks; definition of measuring protocols, including the
determination of optimal sampling strategies according to the limitations inherent to
and identification of possible new instrumentation and methodologies for the cal/val activity.
FINAL. Conclusions and recommendations
This last piece – a component of Task 1 – was to prepare and submit the final deliverables to ESA.
(The deliverables from all of the previous tasks were used as inputs to create the Final Report,
its Abstract, and the Executive Summary.)
The Final Presentation, which presented an overview of all the work done in the study and its findings,
provided an opportunity to receive feedback from ESA. Submission of the
Technical Data Package – containing all science and software deliverables – in January 2016 completed the
contractual requirements of the FLEX Bridge Study.
About the FLEX Bridge Study...
The goal of the FLEX Bridge Study was to optimize approaches for SIF retrievals and applications in assessment of photosynthesis
and stress status in terrestrial vegetation, including development of a calibration/validation strategy for FLEX products.
The study has advanced the work of the PS Study and
PARCS projects with new and additional science to address priorities
identified by these previous studies and in ongoing FLEX preparatory activities. FLEX Bridge will form the foundation of subsequent
science & applications development during mission development and deployment. The study consolidated follow-on activities
leading up to the User Consultation Meeting (UCM) held in Krakow, Poland, on September 15-16, 2015.
FLEX has now been selected as ESA's new
Earth Explorer 8 mission.
For further information about FLEX Bridge, click here.
To read or download the Final Report, click here.
To see a list of publications from the Study, click here.