In 2019, the FORUM mission (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected by ESA as Earth Explorer 9 mission. FORUM aims to record the emission spectrum of the Earth’s stratosphere and troposphere in the spectral range of 100 to 1600 cm-1 (i.e. 6.25μm to 100 μm). It will measure the spectral features of the far-infrared contribution to the Earth radiation budget with the focus on the water vapour contribution, the cirrus cloud properties, and the ice/snow surface emissivity. FORUM’s primary optical instrument is a non-imaging Fourier Transform Spectrometer (FTS) scanning the Earth atmosphere in a step-and-stare acquisition mode sounding a 15 km ground sampling with a ground sampling distance of 100 km. A consortium led by TAS-UK as mission and platform prime and OHB as instrument prime are currently developing the satellite and payload together with Leonardo Italy and MICOS during an ESA phase A / B1 study. This consortium is one of the 2 independent consortia selected for FORUM’s phase A/B1. The paper will describe the design concept of the FORUM FTS instrument with main focus on the optical design. The key optical subsystems are the pointing unit and a double pendulum interferometer with an entrance three-mirror-anastigmatic telescope. The prediction of the performance with respect to threshold and goal requirements will be presented. Furthermore, the status of the beam splitter pre-development will be reported, in particular the development of an optical element based on a CVD-diamond substrate and a technology to manufacture broad band anti-reflective microstructures on a diamond surface. The microstructures are designed to suppress parasitic reflections in the spectral range between 6.25μm and 100μm. The highest efficiency occurs between 13 and 30μm, where the Fresnel reflection is reduced from 17% down to below 3.5%.
The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected in September 2019 as the 9th Earth Explorer mission of the European Space Agency. The mission aims to measure the Earth’s Top-Of- Atmosphere (TOA) emission spectrum in the spectral region from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). This will fill the current observational gap from space in the far-infrared region (FIR) from 100 to 667 cm-¹ (i.e. from 15 to 100 μm). FORUM measurements will improve the understanding of the climate system by providing, for the first time with high resolution, the spectral features of the far-infrared emission of the Earth with a focus on the contribution to the radiation budget of the continuum absorption of the water vapour rotational bands, on cirrus cloud properties, and on ice/snow surface emissivity. The FORUM mission requires a payload able to spectrally-resolve the Earth's outgoing longwave radiation across the Far-InfraRed (FIR) spectral range with high absolute radiometric accuracy. Characterisation of the surface, atmospheric and cloud/surface heterogeneity in the observed field-of-view is also required to help interpret the measured spectral radiance. These needs dictate the use of two instruments: a spectrometer and an infrared imager. The concepts of both instruments, thoroughly studied in phase A preparatory activities by two independent industrial consortia, are presented in this paper.
We present the first steps executed to space qualify an assembly technique for miniaturized optical components that already demonstrated its maturity for the ground segment. Two different types of demonstrators have been manufactured and submitted to various tests: endurance demonstrators placed in simulated environment reproducing strong space environmental constraints that may potentially destroy the devices under test, and a functional demonstrator put in operational conditions as typically found in a satellite environment. The technology, the realized demonstrators and the results of the tests are reported.
A development program was conducted to further improve the technology readiness level of the Generic Flight
Interferometer (GFI), a candidate technology for the future hyperspectral sounder on MTG. Interferometer-based
sounders have already demonstrated their performance and reliability in conducting advanced sounding tasks in recent
missions (METOP-A, IBUKI, SCISAT). The transition from previous single-pixel (or few) to large-format array
detectors offering strong hyperspectral capabilities adds technical challenges to the interferometer design. Some of the
improvements required to address those challenges have already been implemented in recent deployment of
hyperspectral commercial products but must be adapted to the space environment and constraints. Other improvements
are dictated by mission specifics but still tend to be recurrent in recent opportunities. The GFI design intent is to regroup
these innovations in a generic modular interferometer platform in order to address a variety of missions with minor
modifications and hence lower development costs and risks.
KEYWORDS: Sensors, Signal to noise ratio, Interferometers, Simulation of CCA and DLA aggregates, Mirrors, Electronics, Spectral resolution, Fourier transforms, Telescopes, Temperature metrology
The SciSat/ACE mission provided, and still provides, high quality and high spectral resolution measurements of the
atmosphere with a FTS sounder in sun-occultation configuration. Based on the comprehensive results and models of
SciSat/ACE it is foreseen that most of the desired information can also be retrieved from lower spectral resolution
measurements with higher signal-to-noise ratio (SNR) and appropriate data treatment. With the Canadian Space Agency
under the Space Technologies Development Program, ABB Analytical developed a small size sun-occultation sounder
compatible with a micro-satellite platform that has identical throughput, spectral bandwidth and vertical resolution as
ACE. The spectral resolution is decreased by a factor 25 (0.6 cm-1 instead of 0.024 cm-1 for ACE) whereas the SNR
performance is highly increased with an equal factor (target of 2500 instead of 100 for ACE over most of the spectral
bandwidth between 750 and 4000 cm-1).A prototype of the sun-occultation sounder was built, tested under various
thermal conditions and subjected to vibrations similar to those expected at launch. An outdoor experiment was also
conducted to test the instrument in sun-occultation conditions. The good behavior of the instrument indicates interesting
opportunities for such small footprint sounder on a low-cost micro-satellite mission and potentially good earth coverage
if several of such instruments are used in coordination. Depending on the scientific needs, it is possible to adapt the
proposed instrument to increase the vertical resolution and/or to extend the measurements on lower altitudes due to the
higher SNR performances.
Experimental, analytical and finite-element-simulation approaches are presented for the characterization of fibre Bragg grating sensors written in conventional monomode and polarization-maintaining fibers subjected to transverse loading. Firstly, a diametrical-load configuration is considered. Numerical simulations show the behavior to be nonlinear as a function of the applied load when an appropriate analytical model for the opto-mechanical response is employed. Secondly, experiments are carried out with the sensors embedded in epoxy specimens, when the latter are subjected to transversal biaxial loading. The response is monitored as a function of the vertical/horizontal load ratio. A finite-element model of the specimen with the embedded fibre and the previous analytical procedure are used to calculate the strain distributions in the fibre core resulting from loading, and predict the corresponding Bragg wavelength shifts. Experimental results are then compared to numerical predictions.
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