The European Space Agency (ESA) is pleased to announce the opening of the calibration and validation (cal/val) announcement of opportunity (AO) for the Fluorescence Explorer (FLEX) mission. With this AO, ESA invites research groups and individuals worldwide to contribute to cal/val efforts supporting the FLEX mission.

FLEX is ESA’s eighth Earth Explorer mission, developed under the FutureEO programme.

The FLEX launch is planned for September 2026 together with the Sentinel-3C satellite, on a Vega-C launcher. FLEX is the first mission designed to measure vegetation fluorescence from space on a global scale. By detecting the faint glow emitted by plants during photosynthesis - known as solar-induced chlorophyll fluorescence (SIF) - and other quantitative physiological indicators, FLEX will provide a unique perspective on plant health, stress, and productivity. These quasi-direct measurement of plants’ photosynthetic activity are critical to better understanding vegetation functioning and the global carbon and water cycles.

FLEX will carry the FLORIS (FLuORescence Imaging Spectrometer) instrument, which operates in the 500–780 nm spectral range with high spectral resolution. These observations will occur over a swath width of 150 km with a spatial resolution of 300 m. FLEX will fly in a sun-synchronous orbit with an altitude of 814.5 km, an inclination of 98.65°, and a repeat cycle of 27 days. It is expected that it will orbit ahead of a Sentinel-3 satellite in order to collect complementary measurements over the same location within 6 - 15 seconds of each other.

This AO is an open call to contribute to cal/val activities of FLEX products, starting from the commissioning phase (Phase E1) and throughout the operational phase (Phase E2). ESA welcomes participation from all interested institutions and individuals involved in vegetation monitoring, plant physiology, remote sensing, and related fields. Cal/val activities are expected to cover a range of ground-based, Unmanned Aerial Vehicle (UAV), airborne, and modelling efforts that will support the validation of FLEX products and contribute to improved understanding and interpretation of SIF and other advanced FLEX products.

Funding for activities conducted under this AO shall be secured through national or institutional sources. ESA will not provide financial support through this call. However, successful applicants will gain early access to FLEX products and to the FLEX collaborative platform, and will be integrated into the FLEX cal/val science team. Successful applicants will also have the opportunity to collaborate with other FLEX cal/val principal investigators (PIs) during dedicated events. In fact, successful participants to this AO will be invited to join ad-hoc FLEX cal/val workshop and present their work. Further details, including date and access information, will be communicated at a later stage.

All proposals submitted in response to this AO will be reviewed by the ESA FLEX cal/val Scientific Committee. All participants will be informed via email following the conclusion of the evaluation process.

The following criteria will be used in the evaluation of all proposals:

• Relevance of the proposed project to FLEX cal/val activities
• Quality of the proposed project
• Collaborative benefit derived from participating as a FLEX cal/val team member
• Feasibility and probability of success

The deadline for submission is 31 July 2026.
Proposals submitted by this date will be considered for inclusion in the current evaluation round. Early evaluation may be possible for proposals submitted significantly ahead of the deadline (TBC depending on the reviewers’ availability). 

How to apply

Please apply at the following link. The closing deadline is the 31 July 2026 

Documents

Please Note: The documentation provided below will evolve over time, e.g. to include the Algorithm Theoretical Baseline Document (ATBD) for the FLEX Level 1 and Level 2 products, as well as future evolutions of the cal/val plans and product format specifications.

Download documentation related to the FLEX calibration and validation call:

Mission definition:

• Mission Requirements Document

Product format definition:

• FLEX Instrument Performance Simulator and Ground Processor Prototype Product Definition Document
• FLEX Level 1C and Level 2 IPF product format definition

Calibration/validations plans:

• FLEX calibration / validation plan overview
• FLEX Level 1C vicarious calibration / validation plan
• FLEX Level 2A calibration / validation plan
• FLEX Level 2B calibration / validation plan
• FLEX Level 2C calibration / validation plan