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).
The European Space Agency (ESA), in collaboration with the European Commission (EC) and
EUMETSAT, is developing as part of the EC’s Copernicus programme, a space-borne observing system for quantification of anthropogenic carbon dioxide (CO2) emissions. The anthropogenic CO2 monitoring (CO2M) mission will be implemented as a constellation of identical LEO satellites, to be operated over a period > 7 years and measuring CO2 concentration in terms of column-averaged dry air mole fraction (denoted as XCO2). Industrial activities for the phase B2CD have been kicked-off Mid 2020.
The demanding requirements necessitate a payload composed of a suite of instruments,
which simultaneously perform co-located measurements. A push-broom imaging spectrometer will perform co-located measurements of top-of-atmosphere radiances in the Near Infrared (NIR) and Short-Wave Infrared (SWIR) at high to moderate spectral resolution (NIR: 747- 773nm @0.1nm, SWIR-1: 1595-1675nm @0.3nm, SWIR-2: 1990-2095nm @0.35nm) for retrieving XCO2. These observations are complemented in the same spectrometer by measurements in the visible spectral range (405-490 nm @0.6nm), providing vertical column measurements of nitrogen dioxide (NO2) that serve as a tracer to high temperature combustion of fossil-fuel and related emission plumes (e.g. from coal-fired power plants and cities). High quality retrievals of XCO2 will be ensured even in situations of large aerosol loading, thanks to co-located measurements of aerosol resulting from a Multiple- Angle Polarimeter (MAP). Polarimetric measurements are performed over 40 angular views and in six spectral channels between 410 and 865 nm. Finally, due to the strong sensitivity of the XCO2 retrieval to cloud contamination, a three-band Cloud Imager (CLIM) will provide the required capacity to detect small tropospheric clouds and cirrus cover with an accuracy of 1% to 5% and a sampling better than 400 m.
CarbonHIGS is a 2D imager designed to measure the enhancement of CO2 concentration in emission plumes from power plants. The origin of HIGS, and the physics of the working principle will be explained. Two possible implementations of HIGS will be presented, i.e. one that uses power splitting while the other one uses polarization splitting. The characteristics of these implementations will be discussed. In the final part of this paper the preliminary design of the CarbonHIGS system will be shown.
The European Space Agency (ESA), in collaboration with the European Commission (EC) and EUMETSAT, is developing a space-borne observing system for quantification of anthropogenic carbon dioxide (CO2) emissions. Forming part of the EC's Copernicus programme, the CO2 monitoring (CO2M) mission will be implemented as a constellation of identical satellites, to be operated over a period > 7 years and measuring CO2 concentration in terms of column-averaged mole fraction (denoted as XCO2). Each satellite will continuously image XCO2 along the satellite track on the sun-illuminated part of the orbit, with a swath width of >250 km. Observations will be provided at a spatial resolution < 2 x 2 km2 near the swath center, with high precision (<0.7 ppm) and accuracy (bias <0.5 ppm). To this end, the payload comprises a suite of instruments addressing the various aspects of the challenging observation requirements: A push-broom imaging spectrometer will perform co-located measurements of top-of-atmosphere radiances in the Near Infrared (NIR) and Short-Wave Infrared (SWIR) at high to moderate spectral resolution (NIR: 747-773nm@0.1nm, SWIR-1: 1595-1675nm@0.3nm, SWIR-2: 1990-2095nm@0.35nm). These observations are complemented by measurements in the visible spectral range (405-490 nm@0.6nm), providing vertical column measurements of nitrogen dioxide (NO2) that serve as a tracer to assist the detection of fossil-fuel emission plumes (e.g. from coal-fired power plants and cities). High quality retrievals of XCO2 will be ensured even over polluted industrial regions, thanks to co-located measurements of aerosols performed by a Multiple-Angle Polarimeter (MAP). Finally, measurements of a three-band Cloud Imager, co-registered with the CO2 observations, will provide the required cloud-flagging capacity at sub-sample level (<200m resolution).
The presentation will review the results of the Phase A/B1 instrument studies carried out in 2018-2019, including technology pre-development activities, and highlight the identified engineering challenges. The preliminary design of the CO2M mission’s instruments at the beginning of the implementation phase will be presented, along with an outlook on the development activities under the Phase B2CD programme.
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.
As part of the European Copernicus Programme, the European Commission (EC) and the European Space Agency (ESA) together with the support of Eumetsat and the European Centre for Medium-Range Weather Forecasts (ECMWF) are initiating the development of operational satellites for measurements of anthropogenic carbon dioxide (CO2) emissions. The CO2 Monitoring (CO2M) mission shall provide atmospheric CO2 measurements at 4 km2 spatial resolution and a precision and systematic error better than 0.7 ppm and 0.5 ppm respectively in column-average dry-air mole fractions of CO2 (XCO2). The demanding requirements necessitate a payload composed of several instruments, which simultaneously perform co-located measurements. The main CO2 instrument is a 250 km swath pushbroom imaging spectrometer allowing to retrieve XCO2 from reflectance measurements in the Near-Infrared (747-773 nm) and Short-Wave Infrared spectral regions (1590-1675 nm and 1990-2095 nm). The observations for CO2 concentration will be complemented by measurements of nitrogen dioxide (NO2) columns over the same area. The NO2 measurements from the visible region (405-490 nm) will serve as a tracer for plumes of CO2 emission resulting from high temperature combustion, which will facilitate plume identification and mapping from (fossil fuel) power plants and large cities. The third component of the payload is a multiple-angle polarimeter, performing high-precision measurements of aerosol (and cloud) properties. Its measurements of polarized radiance under various observation angles will allow a precise light path correction. The resulting improved knowledge of the effective optical path due to scattering will reduce XCO2 bias error. Retrievals will be successful not only under clear sky conditions, but also under moderate aerosols loading and hence significantly increase the yield of useful XCO2 retrievals. The strong sensitivity of the XCO2 retrieval to cloud contamination calls also for a cloud-imager capable of detecting small tropospheric clouds and cirrus cover with an accuracy of 1% to 5% and with a sampling better than 400 m.
In this contribution, an approach for the characterization of various fiber-based slit homogenizer devices in the NIR and SWIR is shown. The devices are to be tested for use in a satellite-based spectrometer for spatial monitoring of anthropogenic greenhouse gases. This leads to the characterization requirement for temporal coherence and spatial incoherence. Speckle noise has to be reduced to a very low level, which is achieved using a fixed diffusor in combination with a rotating diffusor and a tunable (wavelength) laser as well as temporal averaging. Remaining variations due to unwanted interferences at the imager are removed by controlled movement of the sensor with an automated micro positioning stage in combination with image processing. The design, realization and characterization of the measurement breadboard as well as near field homogenization results for different input scenes and polarizations are shown. Additionally, the geometric characteristics and the depolarization effect of the fibers are investigated for a homogeneous input scene. Furthermore, a setup and measurement results concerning the focal ratio degradation of the fibers are presented.
Responding to plans of the European Commission for extending the observation capabilities of the Copernicus programme, the European Space Agency (ESA) has initiated Phase A industrial (technical feasibility) studies for several new space-borne Earth Observation missions. High priority is given to a constellation of LEO satellites in Sunsynchronous orbit with the purpose of observing anthropogenic carbon dioxide (CO2) emissions [European Commission, 2017]. The observing system shall acquire images of CO2 concentration in terms of dry air column-averaged mole fractions (XCO2), providing complete global land coverage at high spatial resolution (4 km2) within five days. The demanding requirements call for a payload comprising a combination of multiple instruments, which perform simultaneous measurements. The XCO2 is inferred from reflectance measurements in the Near-Infrared (NIR) and Short-Wave Infrared spectral regions (SWIR). This requires at least three spatially co-registered push-broom imaging spectrometers, measuring spectral radiance and solar irradiance in the NIR (747-773 nm), SWIR-1 (1595-1675 nm) and SWIR-2 (1990-2095 nm) at moderate spectral resolving power (R~5000-7000). In addition, the observations for CO2 concentration will be complemented by Differential Optical Absorption Spectroscopy (DOAS) measurements of nitrogen dioxide (NO2) over the same area. The NO2 measurements in the visible region (400-500 nm) are expected to serve as a tracer for plumes of high CO2 concentration resulting from high temperature combustion, which will facilitate plume identification and mapping. The third component of the payload is a multiple-angle polarimeter (MAP), performing high-precision measurements of aerosol (and cloud) properties. Its measurements of polarized radiance under various observation angles are expected to reduce XCO2 bias error and significantly increase the yield of useful retrievals from the NIR and SWIR spectra. The complex observation architecture, involving multiple instruments and platforms, call for optimized observational requirements, driven by the primary goal of detecting and quantifying point-sources of greenhouse gas emissions. In particular, high single-sounding precision is essential for identifying plumes of elevated CO2 concentration from instantaneous image acquisitions without regional and temporal averaging. This translates into stringent requirements for Signal-to-noise ratio (SNR), as well as spatial co-registration and spectral stability, which drive the instrument design. The presentation will introduce the different elements of the candidate Copernicus mission, in view of the ambitious mission goals. The payload components and observation requirements are addressed with special emphasis on the derivation of the SNR and spectral resolution requirements, which determine the instrument sizing.
Sentinel-5 is an atmospheric monitoring mission planned in the frame of the joint EU/ESA initiative Global Monitoring for Environment and Security (GMES). The objective of the mission, planned to be launched in 2020, is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate applications.
Sensitivity to polarization is a major design driver for Earth observing dispersive spectrometers. While the measured Earth radiance observed from space in the UV, visible and near IR bands has a strong and highly variable linearly polarized component, most essential components in spectrometers are inherently sensitive to polarization: scan mirrors, gratings, dichroics. Minimisation of the resulting radiometric errors is a challenge and cannot be only achieved with careful optical designs. Depolarization by passive optical components such as birefringent polarization scramblers has been demonstrated with the last generation of atmosphere monitoring instruments (MERIS, OMI). In order to achieve the demanding performances targeted by future instruments (Sentinel-4, Sentinel-5, CarbonSat) the available degrees of freedom left for optimisation shall be explored, and new polarization scrambler designs must be found.
This paper summarizes design rules and performance aspects identified by ESA during phases A/B1 of the Sentinel-4 and Sentinel-5 missions. The following aspects have been investigated and will be discussed: minimization of polarization dependent spectral oscillations, use of a polarization scrambler in converging beam or parallel beam at large angles of incidence, polarization dependent pointing error.
CarbonSat is one of the two candidate missions for the 8th cycle of European Space Agency (ESA) Earth Explorers, currently undergoing feasibility studies with two industrial consortia. The mission aims at quantifying the spatial distribution of carbon dioxide (CO2) and methane (CH4) with high precision (3.0 ppm for CO2 and 12.0 ppb for CH4) and accuracy (0.5 ppm for CO2 and 5 ppb for CH4) at a high spatial resolution (2km x 3km) and with global coverage above 40° latitude every 12 days. It consists of three pushbroom spectrometers measuring the Earth reflectance in each of the following bands: NIR (747nm- 773nm @0.1nm resolution), SWIR-1 (1590-1675nm @0.3nm) and SWIR-2 (1925-2095nm @0.55nm).
Although most requirements for the CarbonSat phase A are defined over spatially homogeneous scenes, it is known from previous missions and studies that the observation of real, spatially heterogeneous scenes create specific measurement errors. One obvious mechanism is a distortion of the instrument spectral response function (ISRF) induced by a non-uniform slit illumination in the along-track (ALT) direction. This error has been analysed for several missions (OMI, Sentinel-4, Sentinel-5). The combination of spectrometer smile with across-track (ACT) scene non-uniformities induces similar errors. In this paper, we report about the analysis efforts carried out during CarbonSat preliminary phases to evaluate and mitigate these effects. In a first section, we introduce common concepts and notations for heterogeneous scenes analysis. An exhaustive list of known error mechanisms is presented. In section 2 we discuss the effect of inhomogeneous slit illumination, and describe hardware mitigation with a slit homogeniser. The combination of spectrometer smile and ACT heterogeneities is studied in section 3.
In the frame of recent studies and missions, ESA has been performing various pre-developments of optical gratings for instruments operating at wavelengths from the UV up to the SWIR. The instrument requirements of Sentinel-4, Sentinel-5, CarbonSat and FLEX are driving the need for advanced designs and technologies leading to gratings with high efficiency, high spectral resolution, low stray light and low polarization sensitivities. Typical ESA instruments (e.g. Sciamachy, GOME, MERIS, OLCI, NIRSpec) were and are based on ruled gratings or gratings manufactured with one holographic photoresist mask layer which is transferred to an optical substrate (e.g. glass, glass ceramic) with dry etching methods and subsequently either coated with a reflective coating or used as a mold for replication. These manufacturing methods lead to blazed grating profiles with a metallic reflective surface. The vast majority of spectrometers on ground are still based on such gratings. In general, gratings based on grooved metallic surfaces tend for instance to polarize the incoming light significantly and are therefore not always suitable for ESA’s needs of today. Gratings made for space therefore evolved to many other designs and concepts which will be reported in this paper.
KEYWORDS: Reflectivity, Polarization, Carbon monoxide, Absorption, Gases, Near infrared, Spectrometers, Spatial resolution, Signal to noise ratio, Short wave infrared radiation
CarbonSat is a candidate mission for ESA's Earth Explorer program, currently undergoing industrial feasibility studies. The primary mission objective is the identification and quantification of regional and local sources and sinks of carbon dioxide (CO2) and methane (CH4). The mission also aims at discriminating natural and anthropogenic fluxes. The space-borne instrument will quantify the spatial distribution of CO2 and CH4 by measuring dry air column-averaged mixing ratios with high precision and accuracy (0.5 ppm for CO2 and 5 ppb for CH4). These products are inferred from spectrally resolved measurements of Earth reflectance in three spectral bands in the Near Infrared (747-773 nm) and Short Wave Infrared (1590-1675 nm and 1925-2095 nm), at high and medium spectral resolution (0.1nm, 0.3 nm, and 0.55 nm). Three spatially co-aligned push-broom imaging spectrometers with a swath width <180 km will acquire observations at a spatial resolution of 2 x 3 km2 , reaching global coverage every 12 days above 40 degrees latitude (30 days at the equator). The targeted product accuracy translates into stringent radiometric, spectral and geometric requirements for the instrument. Because of the high sensitivity of the product retrieval to spurious spectral features of the instrument, special emphasis is placed on constraining relative spectral radiometric errors from polarisation sensitivity, diffuser speckles and stray light. A new requirement formulation targets to simultaneously constrain both the amplitude and the correlation of spectral features with the absorption structures of the targeted gases. The requirement performance analysis of the so-called effective spectral radiometric accuracy (ESRA) establishes a traceable link between instrumental artifacts and the impact on the level-2 products (column-averaged mixing ratios). This paper presents the derivation of system requirements from the demanding mission objectives and report preliminary results of the feasibility studies.
CarbonSat was a candidate satellite mission in the frame of ESA’s Living Planet Programme, which targeted high-precision measurements of carbon dioxide (CO2) and methane (CH4) concentrations from space.
Sentinel-5 is an atmospheric monitoring mission planned in the frame of the joint EC/ESA Copernicus initiative, previously known as Global Monitoring for Environment and Security (GMES). The objective of the mission, due for launch in 2021, is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate applications. It will provide accurate measurements of key atmospheric constituents such as ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide, methane, formaldehyde, and aerosol properties. The space segment will be implemented as an imaging spectrometer to be flown on EUMETSAT's Metop Second Generation satellites. From a sunsynchronous LEO orbit Sentinel-5 measurements will provide a daily global coverage at an unprecedented spatial resolution of 7x7 km at nadir and will complement the Sentinel-4 GEO data over Europe. The pushbroom imaging grating spectrometer will acquire continuous spectra of Earthshine radiance covering the UV (270-370 nm), VIS (370- 500 nm), NIR (685-773 nm) and SWIR (1590-1675 nm; 2305-2385 nm) spectral regions, with spectral resolution ranging from 0.25 nm to 1 nm.
CarbonSat is a proposed Earth observation mission, which was selected in 2010 as one of two candidates for becoming the European Space Agency’s (ESA) eighth Earth Explorer (EE8). It is currently undergoing parallel feasibility studies (phase A) performed by two industrial consortia. CarbonSat aims at a better understanding of the natural and anthropogenic sources and sinks of the two most important anthropogenic greenhouse gases CO2 and CH4, which will contribute to a better understanding of climate feedback and forcing mechanisms. To achieve these objectives the instrument will quantify and monitor the spatial distribution of carbon dioxide (CO2) and methane (CH4). It will deliver global data sets of dry air column-averaged mixing ratios of these gases with high precision (1 - 3 ppm for CO2 and 6 - 12 ppb for CH4) and accuracy (0.5 ppm for CO2 and 5 ppb for CH4). The measurements will provide global coverage every 12 days above 40 degrees latitude at a spatial resolution of 2 x 3 km2. The retrieval products are inferred from observations of Earth radiance and solar irradiance at high to medium spectral resolution (0.1-0.55 nm) in the Near Infrared (747-773 nm) and Short Wave Infrared (1590- 1675 nm and 1925-2095 nm) spectral regions. The combination of high spatial resolution and global coverage requires a swath width larger than 180 km for three spatially co-aligned push-broom imaging spectrometers. The targeted product accuracy translates into stringent radiometric, spectral and geometric requirements for the instrument. This paper presents the system requirements derived from the demanding mission objectives and reports preliminary results of the feasibility studies. It highlights the key components of the instrument, focusing on the optical conceptual design, and addresses the identified critical performance aspects.
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