Developed by ADS under CNES responsibility in partnership with Eumetsat, IASI-NG payload on board of METOP-SG satellite will deliver data for operational meteorology, climate monitoring, and atmospheric chemistry in the next decades. In order to improve by a factor two compared to IASI the spectral resolution and radiometric error, the instrument is designed around a Mertz compensated interferometer, whose good spectral performances have been checked at subassembly level in 2020. The PFM is now fully integrated and will go through functional tests, performance tests at ambient and in TVAC through 2022. The aim of this paper is to give an overview on these activities.
MicroCarb is a space program which uses a passive Short Wave InfraRed (SWIR) spectrometer instrument. MicroCarb will provide measurements of the atmospheric concentration of the carbon dioxide at global level with a precision sufficient in order to permit to the scientific community to improve the model of the carbon cycle by understanding the mechanisms governing the exchanges of CO2 between the sources and the sinks.
The MicroCarb Instrument is currently under development at Airbus Defense & Space (ADS). Among the spectral requirements that it must meet, some concern the Instrumental Spectral Response Function (ISRF), which is a key quantity of spectrometers, needed for spectral instrument calibration and atmospheric inversion calculations.
During the development of the Instrument, ADS proposed a method based on several Fourier Transforms to compute the ISRF, based on the one developed by R. Berlich for the FLORIS Instrument [1], in order to have an accurate model of its spectral performances.
In this paper, we will explain the simulation method, which allows to take into account diffraction at the system entrance pupil, the spectrometer slit and the dispersion grating, and also the theoretical optical quality of the Instrument. We will describe the preliminary experiments ADS performed to validate this approach. We will then present the work done at ADS and CNES for validating and cross-checking the results of our computations, that required the use of advanced light propagation modules of CODE V.
Once validated, this tool allows to accurately compute the theoretical performances of a given working point, such as the Full-Width at Half Maximum (FWHM) of the ISRF, the resolution of the spectrometer and the likeness of the ISRF to a gaussian function. It is also a powerful tool to quickly compute many ISRFs and thus carry out trade-offs on several parameters of the instrument.
Finally, by including this static computation into a loop and by varying the radiance inside the slit, it is possible to obtain dynamic ISRFs of complex scenes on the ground, and thus have a complete and validated model of the spectral performances of the MicroCarb instrument.
KEYWORDS: Telescopes, Space telescopes, Modulation transfer functions, Mirrors, Synthetic apertures, Satellites, Space operations, Wavefront sensors, Tolerancing, Imaging systems
For very large telescope diameters, typically above 4 meters, monolithic telescopes can hardly be envisaged for space applications. Optical aperture synthesis can be envisaged in the future for improving the image resolution from high altitude orbits by co-phasing several individual telescopes of smaller size and reconstituting an aperture of large surface. The telescopes can be deployed on a single spacecraft or distributed on several spacecrafts in free flying formation. Several future projects are based on optical aperture synthesis for science or earth observation. This paper specifically discusses the limitations and interest of aperture synthesis technique for Earth observation from high altitude orbits, in particular geostationary orbit. Classical Fizeau and Michelson configurations are recalled, and system design aspects are investigated: synthesis of the Modulation Transfer Function (MTF), integration time and imaging procedure are first discussed then co-phasing strategies and instrument metrology are developed. The discussion is supported by specific designs made at EADS Astrium. As example, a telescope design is presented with a surface of only 6.6 m2 for the primary mirror for an external diameter of 10.6 m allowing a theoretical resolution of 1.2 m from geostationary orbit with a surface lower than 10% of the overall surface. The impact is that the integration time is increasing leading to stringent satellite attitude requirements. Image simulation results are presented. The practical implementation of the concept is evaluated in terms of system impacts in particular spacecraft attitude control, spacecraft operations and imaging capability limitations.
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