ESA initiated in 2018 an architectural design study to prepare the development of the next generation of the optical component of the current Sentinel-2 and Sentinel-3 observation functions i.e. Sentinel-3 without the SAR altimeter and related payloads. The aim of this activity was to analyse and trade-off different architectural options for the next-generation of the Copernicus Space Component optical imaging missions, with an implementation in the 2032-time horizon. A holistic approach was taken to assess architectural option to secure the respective observation capabilities in future, not only extrapolating the current scenario, but to openly assess e.g. the merging of observation functions of different instruments as well as to consider potential data flows from third part missions and service providers, institutional as well as commercial. The outcome of the analysis, taking technological as well as programmatic aspects into account, resulted in the conclusion that Sentinel-2 MSI NG as well as Sentinel-3 OLCI NG together with SLSTR NG on the same platform, are most efficiently implemented as free flyers, with dedicated instruments. The data flow of these two missions will be ideally supplemented by the Land Surface Temperature Mission (LSTM) and the Hyperspectral Mission (CHIME), which are going to be implemented in the frame of the High Priority Candidate Missions (HPCMs) of Copernicus. Those operational data flows, building the backbone of the operational services, can benefit significantly by including the data flows of reliable sources e.g. as from the Landsat series of satellites. The needs for very high resolution (VHR) imaging data, can best be served by data from third party missions, as currently implemented in the Copernicus framework; a Sentinel VHR mission is currently not baselined.
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.
Human activities have been identified as critical contributors to climate changes. Modern industrial development and increasing urbanisation have been affecting the environment at an unprecedented scale since the 19th century. In the two last decades the process has become even faster, also due to the impressive development of largely populated countries like India and China. Historical data records testify a direct correlation between increase in atmospheric CO2 levels and Earth's temperatures but processes underlying climate regulation and changes are only partially known. The improvement of knowledge of atmosphere and climate processes needs the availability of complete and reliable data about atmospheric composition and properties and space-based observations play a primary role. Implementation of a demonstration mission based on an occultation technique at optical wavelengths is proposed. Observations in the infrared spectral range vest a particular importance because this band exhibits many absorptive spectral lines due to greenhouse gases, identified as the main responsible of global warming, thus H2O, CO2, CH4, N2O, O3, and others can be observed with high accuracy. It is expected that the mission will demonstrate technical feasibility of an optical payload for limb sounding observations and provide useful inputs to climatic benchmarking (greenhouse gases and wind profile, as well as atmospheric thermodynamic properties). The identification and the preliminary definition of the instrument architecture and the identification of the critical technologies have been among the main tasks. Possible design options for the laser transmitter and the receiver are discussed, considering available technological solutions and technical constraints. Potential technological criticalities are illustrated too. The creation of performance models, analytical and numerical, facilitates and addresses the payload design activity, both at instrument level and at general system level.
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.
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.
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 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.