Significant progress has been made over the past years in the understanding of Earth’s climate; however, there are still uncertainties in the modelling of atmospheric radiative processes. The main radiative processes relevant for climatological models are related to atmospheric water vapour, clouds, and surface emissivity across the full thermal infrared spectrum. Whilst the mid-infrared outgoing radiation is currently measured from space (e.g. IASI), there is a lack of spectrally resolved measurements over a significant portion of Earth’s thermal emission in the Far-InfraRed (FIR) i.e. between 100 and 667 cm-1 (15-100 micron). FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected as the ESA’s ninth Earth Explorer mission in September 2019. FORUM aims at measuring the Earth’s Top-Of-Atmosphere emission spectrum from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). Two independent instrument concepts, based on a Fourier Transform Spectrometer, have been thoroughly studied in Phase A preparatory activities, including the development of a breadboard of the interferometer. In addition, critical technologies have been identified and subjected to comprehensive breadboard studies in order to mitigate development risks and to raise the Technology Readiness Level (TRL). This has led to the development and characterisation of several beamsplitter candidates, the measurement of the emissivity of the black coating for the on-board blackbody, the verification of noise and responsivity performance of pyroelectric detectors and the characterisation of the interferometer mechanism.
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.
Arnaud Lecuyot, Charlotte Pachot, Pascal Hallibert, H. Oetjen, Francois Laurent, Bruno Napierala, Louis Moreau, Gaetan Perron, M. Lekouara, K. Holmlund
This paper presents the Arctic and Nordic Imager as a concept investigated by the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). The concept addresses a recognized gap in the near-real-time coverage at high latitudes (above 55 degrees) from geostationary satellites, due to viewing geometry over polar areas. The paper recalls the context, summarises the requirements customized for it, describes the main trades, features and sizing parameters of the mission and sensor, and illustrates potential implementations. The requirements are similar to those of geostationary systems. They call for near-real-time multispectral optical observations in the Visible, Near Infrared, Short Wave Infrared, Water Vapour, and Thermal Infrared bands, at moderate spatial resolution. The mission profile proposed uses Highly Elliptical Orbits (HEO). The sensor concept is largely derived from GEO imagers in operation or in development, due to very similar requirements. In the current instrument concepts, the area of interest is covered using a scanning mirror in two directions. A telescope and complementing optics then redirect the optical beam to focal plane assemblies and optical detectors. The instrument also includes calibration subsystems and all necessary equipment i.e. cryocoolers for infrared detectors and electronics for data handling. The major trades are to do with meeting the very demanding optical performance, and to a lesser degree with the radiometric one. These include aperture sizing, definition of scanning law and principle, telescope concept, cryostat design, and image registration. The resulting concepts yield state-of-the-art large space optical instruments.
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