The detection and quantification of greenhouse gas (GHG) emissions, in particular carbon dioxide (CO2) and methane (CH4), is presently one of the main goals of remote sensing of atmospheric gasses on a global scale, for the strong impact these molecules have on climate change. Of particular urgency is the quantification of emissions from anthropogenic sources, a high-priority task addressed by the ESA Copernicus mission CO2M, which will provide global coverage detection of CO2 and CH4. The observation of CO2M, capable of quantifying emissions from the major sources, can be complemented by other observation systems addressing the smaller, and more numerous, sources. In this domain, static interferometers can offer several advantages. This paper reports on the main results of two activities completed within the ESA Future Missions activities in the Earth Observation Program, for the development of small instruments based on static interferometer designs, for the detection of CO2. The two studies, named Carbon-HIGS and Carbon-CGI, investigated two instruments operating in the SWIR and NIR bands, with a targeted precision of 2 ppm and an accuracy of 1 ppm for CO2 atmospheric concentration, covering a relatively small swath of 50 km at a spatial sampling better than 300 m. We summarize the general detection principles, the result of the design activities, and the estimated instrument performances. Both concepts are suitable candidates to work in conjunction with the Copernicus mission offering a zoom-mode observation, for quantification of medium-sized GHG sources and improved localization and understanding of anthropogenic emissions. Additional presentation content can be accessed on the supplemental content page.
The paper presents the results of the 2021 CarbonCGI project, specified by ESA Future Earth Observation department, dedicated to high-resolution observations of GHG (Greenhouse Gas) with CGI (Compact Gas Imager). CarbonCGI aims at detecting and characterizing faint anthropogenic emissions of Carbon dioxide and Methane gas, from low orbit satellite to complement and extend CO2M mission [1]. CGI are developed in an integrated team of scientists and engineers involved in the framework of CarbonCGI project, the IRT (Research and Technological Institute) NS3 (New Space Small Sensor) project and the scientific activities of the industrial chair TRACE [2]. Compact Gas Imagers developments cover the atmospheric transport inverse modelling (level 4), the radiative transfer modelling (level 2), the simulation of acquisition chain, data correction, registration and calibration, as well as detailed design of sensor and critical components (level 0-1).
This paper describes the outcomes of a study funded by the European Space Agency aimed at identifying the technical challenges and trade-offs at the system level, performing preliminary designs of an active correction loop for large deployable telescopes, and defining technological roadmaps for the development of the active correction loop for the selected designs. This study has targeted two very different application cases, one for High Resolution Earth Observation from Geostationary orbit (called GeoHR, with a 4m diameter entrance pupil) and one for Science missions requiring very large telescopes (with a up to 18 m diameter entrance pupil) with high-contrast imaging capabilities for exo-Earth observations and characterization. For both application cases, this paper first summarizes the mission objectives and constraints that have influence on the telescope designs. It then presents the high-level trade-offs that have been led and the optical and mechanical design that have been developed, including the deployable aspects. Finally, the performance assessment is presented, and is the basis for the justification of an active optics correction chain, with a preliminary set of requirements for typical components of the system. The presentation is concluded with proposed technological roadmaps that aim to allow the development of the building blocks on which the next generation instruments will be able to rely on.
MOEMS-based instruments could be the next stepping stone in ground-based and space telescopes for Universe and Earth Observation. Targeted instruments are next generation multi-object spectrographs, as MOEMS can be used as reconfigurable slit masks at the MOS entrance, allowing increased instrument compacity and new observational modes such as SNR optimization. For Earth Observation, the use of MOEMS acting like programmable slit masks also allows a dynamic removal of the bright sources in the field of view, therefore enhancing the global SNR of the instrument. We propose a MOEMS-based spectro-imager named BATMAN, using a Digital-Micromirror-Device (DMD) to split the light between its imaging and spectrograph arms. This instrument is intended to be installed at the TNG telescope in the Canary Islands in 2022. This concept has been extended for space Universe and Earth Observation. A new MOEMS-based spectro-imager for Earth Observation has been designed, with constraints of wide 2D field-ofview (3°x1°), image quality (< 2 pixels = 11 μm) and compacity (Fig. 1). The instrument is panchromatic with a medium spectral resolution between 1000 and 2000, fitting in a 80 cm x 80 cm x 40 cm box. A 3-mirror solution for both imaging and spectrograph arms has been designed, using only aspheric surfaces, allowing for easier alignment and tolerancing. The compact design contains a convex grating to disperse light. In order to optimize the spectrograph efficiency, this convex grating must be blazed at the right angle for maximizing the light in the first order of diffraction. For Universe Observation, a new design for a DMD-based high-resolution spectro-imager has been done. For the spectrograph, the large 370 – 950 nm wavelength range is divided into 4 channels for a spectral resolution of 15 000, all with an image quality of under two pixels. Moreover, the 3-mirror design of the instrument allows for a high throughput in comparison to classical systems which use catadioptric optics. These results show the great interest of MOEMS-based spectro-imagers for space Earth and Universe Observation.
MOEMS-based instruments could be the next stepping stone for ground-based and space telescopes, allowing for multi-object spectroscopy in a 2D field of view. These programmable slit masks will optimize the SNR and generate compact and efficient spectro-imagers for Universe and Earth observation. BATMAN, a MOEMS-based spectro-imager for Universe observation to be installed at the TNG at the Canaria Islands, uses a Digital-Micromirror-Device to split the light between its imaging and spectrograph arms. ROBIN, the BATMAN demonstrator, confirmed its feasibility and image quality over the FOV.
A new design for a DMD-based high-resolution spectro-imager for Universe observation is presented: a solution using only one detector for all the wavelength range is obtained. We are dividing the 370 – 950 nm wavelength range into four channels and a spectral resolution of 15 000 is achieved for every one of them, all within an image quality below two detector pixels. Moreover, the 3-mirror design of the instrument implies a high throughput in comparison to catadioptric systems which are more commonly used for this science case.
A new MOEMS-based spectro-imager for Earth observation has been designed, with constraints of wide 2D field of view (3°×1°), image quality (< 2 pixels = 11 μm) and compacity. The instrument is panchromatic with a medium spectral resolution between 1000 and 2000, fitting in a 40 cm × 50 cm × 90 cm box. A 3-mirror solution for both imaging and spectrograph arms has been designed, using only aspheric surfaces thus allowing for easier alignment and tolerancing.
As of today, Earth and planetary high-resolution observations rely on linear focal plane arrays used in TDI mode to acquire images delivered by the telescope. To both reach high angular resolution and keep a large field-of-view, homothetic imaging systems as used in Pleiades satellites or Mars Reconnaissance Orbiter [1] would lead to prohibitive linear focal plane dimensions. Reducing this parameter would therefore allow for a significant decrease in the size of space optical systems. The solution described in this paper intends to optically reduce this dimension by segmenting the linear FOV into smaller sub-fields that are stacked on a CMOS TDI sensor. It is therefore possible to drastically reduce the linear size of the array, at the expense however of a small increase in terms of complexity of the optical system.
We report on the design and fabrication of a reflection grating for hyperspectral applications operating in the range from 340 nm to 1040 nm wavelength. The blazed grating is based on an effective medium approach, where the desired functionality is realized using a binary surface relief structure. For each period, a gradient in size of the local grating features mimics an interface which adds a linear phase profile to the illuminating beam – thus introducing diffraction. The surface relief structure is composed of 2D structures - pillars with diameters from 200 nm to 350 nm to voids with diameters from 300nm to 120 nm. Overall, an entire number of ~50 such features are arranged to establish an overall unit cell of the grating over a length of 30 μm. By purposeful design of size, shape and arrangement of the sub-wavelength features such gratings offer novel opportunities in tailoring the spectral response, i.e. particular broadband efficiency or the enhancement of the efficiency in specific sub-domains of the spectrum. We will present measured performance results of a grating covering a circular area of 80mm in diameter manufactured on a 4inch-wafer. Finally, we will give an outlook on how such structures can be applied to curved surfaces and even ultra-broadband operation.
Today, optical instruments with FOV of ±30 degrees or more can be achieved by combining several optical modules thus increasing the complexity at instrument level in the design, integration, validation and calibration phases. For instance the MERIS instrument on the ENVIronment SATellite (ENVISAT) has a 68.5 degree FOV shared between 5 identical modules. Each one of those modules needs to be calibrated independently and the modules require to be co-registrated, which leads to stringent constraints at system level in terms of stability. Reducing the number of modules to achieve large FOVs will have potentially beneficial impact on the mass and volume of the instrument and will ease the calibration and co-registration process.
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