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The Fluorescence Explorer (FLEX) mission is currently subject to feasibility (Phase A) study as one of the two
candidates of ESA’s 8th Earth Explorer opportunity mission. The FLuORescence Imaging Spectrometer (FLORIS) will
be an imaging grating spectrometer onboard of a medium sized satellite flying in tandem with Sentinel-3 in a Sun
synchronous orbit at a height of about 815 km. FLORIS will observe vegetation fluorescence and reflectance within a
spectral range between 500 nm and 780 nm. It will thereby cover the photochemical reflection features between 500 nm
and 600 nm, the Chlorophyll absorption band between 600 and 677 nm, and the red-edge in the region from 697 nm to
755 nm being located between the Oxygen A and B absorption bands.
By this measurement approach, it is expected that the full spectrum and amount of the vegetation fluorescence radiance
can be retrieved, and that atmospheric corrections can efficiently be applied. FLORIS will measure Earth reflected
spectral radiance at a relatively high spectral resolution of ~0.3 nm around the Oxygen absorption bands. Other spectral
band areas with less pronounced absorption features will be measured at medium spectral resolution between 0.5 and 2
nm. FLORIS will provide imagery at 300 m resolution on ground with a swath width of 150 km. This will allow
achieving global revisit times of less than one month so as to monitor seasonal variations of the vegetation cycles. The
mission life time is expected to be at least 4 years. The fluorescence retrieval will make use of information coming from
OLCI and SLSTR, which are onboard of Sentinel-3, to monitor temperature, to detect thin clouds and to derive
vegetation reflectance and information on the aerosol content also outside the FLORIS spectral range.
In order to mitigate the technological and programmatic risk of this Explorer mission candidate, ESA has initiated two
comprehensive bread-boarding activities, in which the most critical technologies and instrument performance shall be
investigated and demonstrated. The breadboards will include representative optics and dispersive elements in a
configuration, which is expected to be very close to the instrument flight configuration. This approach follows the
guideline to reach, before it goes into the implementation phase, a technology readiness level of at least 5. It thereby
requires a demonstration of predicted performance in a configuration, where the basic technological components are
integrated with reasonably realistic supporting elements such that it can be tested in a simulated environment.
We will report, within the limits of the competitive nature of the industrial studies, on the currently running or planned
preparatory activities. We will present the mission configuration, the imposed instrument requirements and the identified
instrument concepts as derived by the Phase A studies.
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