The satellite
FLEX is a single, three‑axis stabilised satellite with a launch mass of approximately 400 kg.
The satellite is based on a recurrent low‑Earth‑orbit platform and has been designed to provide high pointing accuracy, thermal stability and low disturbance levels to support demanding optical measurements.
The satellite structure is built around an aluminium panel architecture, providing mechanical stiffness and stable thermal behaviour. Power is provided by body‑mounted and deployable solar arrays, with rechargeable batteries supporting operations during eclipse periods.
The onboard avionics system manages attitude and orbit control, power distribution, thermal control, payload operations, data handling and communications with ground stations.
Orbit
FLEX operates in a Sun‑synchronous low Earth orbit at an altitude of about 815 km, with an inclination of approximately 98.6°. The orbit has a local time of descending node close to 10:00, ensuring stable and repeatable illumination conditions for land surface observations.
A key feature of the mission is the convoy flight configuration: FLEX flies in close temporal proximity – typically 6 to 15 seconds – ahead of a Copernicus Sentinel‑3 satellite in the same orbital plane. This configuration allows near‑simultaneous observations of Earth by the two satellites.
The orbital repeat cycle is 27 days, providing near‑global coverage between approximately 56° South and 75° North.
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FLORIS
The satellite carries a single measuring instrument: FLORIS (Fluorescence Imaging Spectrometer).
FLORIS is an imaging spectrometer specifically developed to measure solar‑induced chlorophyll fluorescence from vegetation. The instrument comprises:
- Two high‑resolution spectral channels centred on oxygen absorption bands (O₂‑A and O₂‑B),
- One low‑resolution spectral channel covering the visible to near‑infrared range.
The instrument observes Earth with a swath width of 150 km and a ground sampling distance of approximately 300 m × 300 m.
To achieve the required radiometric accuracy and stability, FLORIS is mounted on a thermally controlled optical bench. The instrument detectors and key optical components are maintained within tightly regulated temperature ranges using a combination of passive insulation and active thermal control.
Attitude and orbit control
The satellite’s attitude and orbit control system provides high pointing accuracy and stability during science acquisition phases. This is essential for isolating the very weak fluorescence signal from reflected sunlight.
The observation geometry, timing and co‑registration with Sentinel‑3 are managed through coordinated mission planning and orbit control.
Data handling and communications
Science data acquired by FLORIS are stored onboard and downlinked to ground stations via high‑rate communication links. The data are processed through the FLEX ground segment and combined with auxiliary information – primarily from Sentinel‑3 – to generate calibrated geophysical products.
Mission lifetime and operations
FLEX is designed for a nominal mission lifetime of 3.5 years, with consumables to allow possible extension up to five years.
Operations are conducted via ESA’s flight operations infrastructure, with routine monitoring, commanding and anomaly handling performed from ground control centres. The satellite is designed for largely autonomous operation, supported by periodic ground intervention.
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