The Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) mission was launched in December 2004 and was a member of the A-train up to ...the end of 2009. The mission was definitively stopped in December 2013 after nine years of operation. A temporal decrease of the PARASOL radiometric sensitivity occurred with a complex signature within the field of view. Using several calibration techniques, all based on the use of natural targets, it was possible to derive an efficient set of new calibration parameters which includes the absolute calibration, as well as the temporal evolution characterized by a mean decrease plus a variation within the field of view. The physical model used to fit parameters is a classical exponential decrease with a time constant of 0.018 month -1 and an amplitude from 16% in 490 nm down to 1%-2% in the near infrared. The mean drift can be considered as estimated with an accuracy better than 0.5% per decade. The variation within the field of view, which is estimated with accuracy between 0.2% and 0.5% rms, follows a similar spectral shape as the mean instrumental drift. Some typical features were found which illustrate the ageing of the optical part of the instrument (lenses). The updated calibration, based on a synergy between various calibration methods, has significantly improved the radiometric consistency for all spectral bands of the PARASOL data archive, which represents nearly nine years of acquisitions. Finally, the PARASOL absolute calibration was confirmed with accuracy close to 2%. The last reprocessing of the full PARASOL level-1 archive has implemented these new radiometric corrections.
The Polar Multi-Sensor Aerosol product (PMAp) is based on the synergistic use of three instruments from the Metop platform, GOME-2, AVHRR, and IASI. The retrieval algorithm includes three major ...steps: a pre-identification of the aerosol class, a selection of the aerosol model, and a calculation of the Aerosol Optical Depth (AOD). This paper provides a detailed description of the PMAp retrieval, which combines information provided by the three instruments. The retrieved AOD is qualitatively evaluated, and a good temporal as well as spatial performance is observed, including for the transition between ocean and land. More quantitatively, the performance is evaluated by comparison to AERONET in situ measurements. Very good consistency is also observed when compared to other space-based data such as MODIS or VIIRS. The paper demonstrates the ability of this first generation of synergistic products to derive reliable AOD, opening the door for the development of synergistic products from the instruments to be embarked on the coming Metop Second Generation platform. PMAp has been operationally distributed in near-real-time since 2014 over ocean, and 2016 over land.
As part of the Copernicus program, Sentinel-2 is the optical imaging mission designed for the operational monitoring of land and coastal areas. It offers a unique combination of global coverage with ...a wide field of view, a high revisit capability (5 days with two satellites), a high resolution and multi-spectral imagery.
CNES, the French Space Agency, was involved in the commissioning of both Sentinel-2 satellites and is currently working in collaboration with ESA on their long-term monitoring. This paper reviews all the techniques used to ensure an absolute calibration of the 13 spectral bands to better than 5% (the target is 3%) at TOA level. After a brief description of the mission and its related radiometric calibration scheme, we show how standard vicarious calibration methods based on acquisitions over natural targets (oceans, deserts, and Antarctica during winter) are used to check and improve the accuracy of the absolute calibration coefficients. Finally, the verification scheme, exploiting photometer in-situ measurements over the La Crau plain in France and Gobabeb in Namibia, is described. The paper concludes with a summary that includes spectral coherence, agreement between the results obtained with various calibration methods and temporal evolution.
•Multi-angle, multi-spectral and polarised radiances from the Metop SG satellite.•An easy to use and well characterized polarized radiance product by EUMETSAT.•Radiometric error characterization by ...measuring sub-pixel spatial frequencies.
The Multi-viewing, Multi-channel and Multi-polarisation Imager (3MI) on board the Metop-SG satellites will observe polarised multi-spectral radiances of a single target within a very short time period from the visible to the shortwave infrared region with daily global coverage. In order to provide the users of 3MI data with an easy to use and well characterised radiance product EUMETSAT will make a geoprojected and regridded 3MI level-1C product available to users within 70 min of sensing. The paper describes the methodologies of geoprojection and regridding used for the processing of such a product. In addition, the colocation of ancillary information, in particular from the METimage 20-channel imager providing subpixel information of the radiance field and of clouds is described in detail. The latter information is provided as colocated geometric average values in the product and is also used to provide a realistic scene-dependent error introduced by the radiance regridding. Initial estimates, using a synthetic test dataset of top-of-atmosphere radiances of 3MI and METimage at native instrument resolution, provide an upper limit for the additional radiance error contribution depending on the scene homogeneity. Colocated METimage cloud-top height information is also used for parallax correction of the coregistered radiance data either to the cloud height or to the surface elevation, depending on the origin of the dominant radiance signal within the line-of-sight.
Radiometric cross calibration of Earth observation sensors is a crucial need to guarantee or quantify the consistency of measurements from different sensors. Twenty desert sites, historically ...selected, are revisited, and their radiometric profiles are described for the visible to the near-infrared spectral domain. Therefore, acquisitions by various sensors over these desert sites are collected into a dedicated database, Structure d'Accueil des Données d'Etalonnage, defined to manage operational calibrations and the required SI traceability. The cross-calibration method over desert sites is detailed. Surface reflectances are derived from measurements by a reference sensor and spectrally interpolated to derive the surface and then top-of-atmosphere reflectances for spectral bands of the sensor to calibrate. The comparison with reflectances really measured provides an estimation of the cross calibration between the two sensors. Results illustrate the efficiency of the method for various pairs of sensors among AQUA-Moderate Resolution Imaging Spectroradiometer (MODIS), Environmental Satellite-Medium Resolution Imaging Spectrometer (MERIS), Polarization and Anisotropy of Reflectance for Atmospheric Sciences Couples With Observations From a Lidar (PARASOL)-Polarization and Directionality of the Earth Reflectances (POLDER), and Satellite pour l'Observation de la Terre 5 (SPOT5)-VEGETATION. MERIS and MODIS calibrations are found to be very consistent, with a discrepancy of 1%, which is close to the accuracy of the method. A larger bias of 3% was identified between VEGETATION-PARASOL on one hand and MERIS-MODIS on the other hand. A good consistency was found between sites, with a standard deviation of 2% for red to near-infrared bands, increasing to 4% and 6% for green and blue bands, respectively. The accuracy of the method, which is close to 1%, may also depend on the spectral bands of both sensor to calibrate and reference sensor (up to 5% in the worst case) and their corresponding geometrical matching.
In this study, we present a radiative transfer model, so-called OSOAA, that is able to predict the radiance and degree of polarization within the coupled atmosphere-ocean system in the presence of a ...rough sea surface. The OSOAA model solves the radiative transfer equation using the successive orders of scattering method. Comparisons with another operational radiative transfer model showed a satisfactory agreement within 0.8%. The OSOAA model has been designed with a graphical user interface to make it user friendly for the community. The radiance and degree of polarization are provided at any level, from the top of atmosphere to the ocean bottom. An application of the OSOAA model is carried out to quantify the directional variations of the water leaving reflectance and degree of polarization for phytoplankton and mineral-like dominated waters. The difference between the water leaving reflectance at a given geometry and that obtained for the nadir direction could reach 40%, thus questioning the Lambertian assumption of the sea surface that is used by inverse satellite algorithms dedicated to multi-angular sensors. It is shown as well that the directional features of the water leaving reflectance are weakly dependent on wind speed. The quantification of the directional variations of the water leaving reflectance obtained in this study should help to correctly exploit the satellite data that will be acquired by the current or forthcoming multi-angular satellite sensors.
•Operational spaced-based system for aerosol and cloud determination.•Definition of the Multi-view, Multi-spectral and Multi-polarization concept.•Characterization of the system, on-ground as well as ...in-orbit.•Ground processing of the Level-1 products.
The 3MI instrument is one of the missions of the EUMETSAT Polar System Second Generation (EPS-SG) program to be launched in 2021. This polarimetric mission has a direct heritage from the POLDER mission, with improved capabilities and is implemented within a fully operational long-term framework. The spectral range was extended from the visible-near-infrared (410–910 nm) to the shortwave-infrared domain (up to 2200 nm). The spatial resolution (4 km at nadir) and the instantaneous swath (2200 × 2200 km²) were also improved compared to previous POLDER instruments. The 3MI concept of the multi-viewing, multi-spectral and multi-polarized Imaging is described, in particular how these 3 pieces of information are acquired together within a single simple instrument concept. Achieving the performance necessary to meet the mission requirements will rely on a comprehensive initial pre-launch characterization and subsequently extensive in-orbit monitoring based on a vicarious calibration strategy. The level 1 products available to the users will be geo-located Stokes vectors on the native viewing geometry (Level 1B) and geo-projected multi-directional and spectral Stokes vectors (Level 1C). Level-2 products will provide geophysical and microphysical parameters for aerosol and clouds.
•How aerosol retrieval is influenced by the geometry.•Definition of the scattering angle range distribution.•Typical information content for some earth observing sensors.•Recommendation for the ...development of aerosol retrieval and products.
For decades now, the retrieval of aerosol property has been successfully achieved from space-borne sensors from which measurements it is possible to derive specific parameters such as optical thickness, absorption, type, refractive index, or size distribution. In the reflective spectral domain, remote sensing of aerosol properties relies on the top-of-atmosphere measurement of the sun irradiance scattered by aerosols in different directions. This measure is intrinsically linked to the aerosol phase function. Ground-based measurements are made for many viewing directions providing a good description of this phase function. For satellite remote sensing, the phase function cannot be measured in so much detail. Only a single scattering angle for mono-viewing sensors, or a limited range for multi-view sensors, is accessible. The associated geometry varies very significantly along the swath, from East to West, and along the orbit, from North, to Tropics, and South. Whatever the considered aerosol retrieval approach, the performance cannot be the same from these very different geometrical configurations, and may significantly differ. This aspect is in general not well documented. In this paper, the scattering angle range distribution (ScARD) is described in the case of the EPS-SG/3MI multi-view polarimeter. Based on reference aerosol phase functions, it is anticipated how the retrieval performance could be impacted. Other cases are simulated trying to extrapolate the conclusion to other types of sensors having more limited swath or number of views. The ScARD is described for these different situations, including the variation along the orbit and along the swath. Important recommendations are drawn including the need to document the geometrical part of the information content provided by the sensor (not only spectral), to better consider the associated classes of viewing geometry for the development of retrieval algorithms (which could limit the ability to retrieve some parameters), but also for the validation of products.
The Ocean and Land Color Instrument (OLCI) onboard the Copernicus Sentinel-3A satellite is a medium-resolution and multi-spectral push-broom imager acquiring radiance in 21 spectral bands covering ...from the visible to the far near-infrared. These measurements are primary dedicated to land & ocean color applications, but actually include also reliable information for atmospheric aerosol and surface brightness characterization. In the framework of the EUMETSAT funded study to support the Copernicus Program, we describe the retrieval of aerosol and surface properties from OLCI single-viewing multi-spectral Top-Of-Atmosphere (TOA) radiances based on the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm. The high potential of the OLCI/GRASP configuration stems from the attempt to retrieve both aerosol load and surface reflectance simultaneously using a globally consistent high-level approach. For example, both over land and ocean surfaces OLCI/GRASP uses 9 spectral channels (albeit with different weights), strictly the same prescribed aerosol models and globally the same a priori constraints (though with some differences for observations over land and ocean). Due to the lack of angular multi-viewing information, the directional properties of underlying surface are strongly constrained in the retrieval: over ocean the Fresnel reflection together with foam/whitecap albedo are exclusively computed using a priori wind speed; over land, the Bidirectional Reflectance Distribution Function (BRDF) is slightly adjusted from a priori values of climatological Ross-Li volumetric and geometric terms. Meanwhile, the isotropic reflectance is retrieved globally under mild spectral smoothness constraints. It should be noticed that OLCI/GRASP configuration employs innovative multi-pixel concept (Dubovik et al., 2011) that enhance retrieval by simultaneously inverting large group of pixels. The concept allows for benefiting from knowledge about natural variability of the retrieved parameters.
The obtained OLCI/GRASP products were validated with the Aerosol Robotic Network (AERONET) and Maritime Aerosol Network (MAN) and intercompared with the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and surface products. The overall performance is quite comparable to the community-referenced MODIS. Over ocean the OLCI/GRASP results are encouraging with 67% of the AOD (550 nm) satisfying the Global Climate Observing System (GCOS) requirement using AERONET coastal sites and 74% using MAN deep ocean measurements, and an AOD (550 nm) bias 0.01 with AERONET and nearly zero bias with MAN. Over land, 48% of OLCI/GRASP AOD (550 nm) satisfy the GCOS requirement and a bias within ±0.01 for total and AOD < 0.2. Key challenges are identified and discussed: adequate screening of cloud contaminations, retrieval of aerosol over bright surfaces and in the regions containing complex mixtures of aerosol.
•We retrieve aerosol and surface from OLCI/Sentinel-3A based on GRASP algorithm.•The directional BRDF are constrained using wind speed and climatology.•OLCI/GRASP products are validated with AERONET, MAN and intercompared with MODIS.
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