The Copernicus Atmosphere Monitoring Service (CAMS)
provides near-real-time forecast and reanalysis of aerosols using the ECMWF
Integrated Forecasting System with atmospheric composition extension,
...constrained by the assimilation of MODIS and the Polar Multi-Sensor Aerosol
Optical Properties (PMAp) aerosol optical depth (AOD). The objective of this
work is to evaluate two new near-real-time AOD products to prepare for their
assimilation into CAMS, namely the Copernicus AOD (collection 1) from the Sea
and Land Surface Temperature Radiometer (SLSTR) on board Sentinel 3-A/B
over ocean and the NOAA EPS AOD (v2.r1) from VIIRS on board S-NPP and
NOAA20 over both land and ocean. The differences between MODIS (C6.1), PMAp
(v2.1), VIIRS (v2.r1), and SLSTR (C1) AOD as well as their departure from the
modeled AOD were assessed at the model grid resolution (i.e., level-3)
using the 3-month AOD average (December 2019–February 2020 and March–May 2020). VIIRS and MODIS show the best consistency across the products, which is
explained by instrument and retrieval algorithm similarities. VIIRS AOD is
frequently lower over the ocean background and higher over biomass burning
and dust source land regions compared to MODIS. VIIRS shows larger spatial
coverage over land and resolves finer spatial structures such as the
transport of Australian biomass burning smoke over the Pacific, which can be
explained by the use of a heavy aerosol detection test in the retrieval
algorithm. Our results confirm the positive offset over ocean (i) between
Terra/MODIS and Aqua/MODIS due to the non-corrected radiometric calibration
degradation of Terra/MODIS in the Dark Target algorithm and (ii) between
SNPP/VIIRS and NOAA20/VIIRS due to the positive bias in the solar reflective
bands of SNPP/VIIRS. SLSTR AOD shows much smaller level-3 values than the
rest of the products, which is mainly related to differences in spatial
representativity at the IFS grid spatial resolution due to the stringent
cloud filtering applied to the SLSTR radiances. Finally, the geometry
characteristics of the instrument, which drive the range of scattering
angles sampled by the instrument, can explain a large part of the
differences between retrievals such as the positive offset between PMAp datasets from MetOp-B and MetOp-A.
The Copernicus Atmosphere Monitoring Service (CAMS) provides near-real-time forecast and reanalysis of aerosols using the ECMWF Integrated Forecasting System with atmospheric composition extension, ...constrained by the assimilation of MODIS and the Polar Multi-Sensor Aerosol Optical Properties (PMAp) aerosol optical depth (AOD). The objective of this work is to evaluate two new near-real-time AOD products to prepare for their assimilation into CAMS, namely the Copernicus AOD (collection 1) from the Sea and Land Surface Temperature Radiometer (SLSTR) on board Sentinel 3-A/B over ocean and the NOAA EPS AOD (v2.r1) from VIIRS on board S-NPP and NOAA20 over both land and ocean. The differences between MODIS (C6.1), PMAp (v2.1), VIIRS (v2.r1), and SLSTR (C1) AOD as well as their departure from the modeled AOD were assessed at the model grid resolution (i.e., level-3) using the 3-month AOD average (December 2019-February 2020 and March-May 2020).
Since the beginning of the 1990s, stable desert sites have been used for the calibration monitoring of many different sensors. Many attempts at sensor intercalibration have been also conducted using ...these stable desert sites. As a result, site characterization techniques and the quality of intercalibration techniques have gradually improved over the years. More recently, the Committee on Earth Observation Satellites has recommended a list of reference pseudo-invariant calibration sites for frequent image acquisition by multiple agencies. In general, intercalibration should use well-known or spectrally flat reference. The reflectance profile of desert sites, however, might not be flat or well characterized (from a fine spectral point of view). The aim of this paper is to assess the expected accuracy that can be reached when using desert sites for intercalibration. In order to have a well-mastered estimation of different errors or error sources, this study is performed with simulated data from a hyperspectral sensor. Earth Observing-1 Hyperion images are chosen to provide the simulation input data. Two different cases of intercalibration are considered, namely, Landsat 7 Enhanced Thematic Mapper Plus with Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Environmental Satellite MEdium Resolution Imaging Spectrometer (MERIS) with Aqua MODIS. The simulation results have confirmed that intercalibration accuracy of 1% to 2% can be achieved between sensors, provided there are a sufficient number of available measurements. The simulated intercalibrations allow explaining results obtained during real intercalibration exercises and to establish some recommendations for the use of desert sites for intercalibration.
Deep convective clouds have been tested to be used as stable reference for calibration purposes: the monitoring of the radiometric changes of space sensors in the spectral range from blue to ...short-wave infrared. After an appropriate selection, the clouds have been characterized for their brightness, spectral aspects, bidirectional signature, stability, and homogeneity. For this, radiative transfer computations using a discrete ordinate code, as well as remote sensing measurements from the PARASOL satellite, were analyzed. The first main result is a confirmation that the monthly mean reflectance over deep convective clouds is quite stable as suggested in other papers. Moreover, the excellent spectral properties of deep convective clouds are really convenient for a temporal monitoring if it can be assumed that a reference band is stable or well characterized with time. If the reference band is perfectly known, the accuracy of the temporal monitoring is about 0.2%. Experimental results are provided with PARASOL data for which the temporal drift is known with an accuracy better than 0.5% for the three years in orbit (accuracy which includes the uncertainty of the reference band).
Abstract
Miniaturized radiance cameras measuring underwater multispectral radiances in all directions at high-radiometric accuracy (CE600) are presented. The camera design is described, as well as ...the main steps of its optical and radiometric characterization and calibration. The results show the excellent optical quality of the specifically designed fish-eye objective. They also show the low noise and excellent linearity of the complementary metal oxide semiconductor (CMOS) detector array that is used. Initial results obtained in various oceanic environments demonstrate the potential of this instrument to provide new measurements of the underwater radiance distribution from the sea surface to dimly lit layers at depth. Excellent agreement is obtained between nadir radiances measured with the camera and commercial radiometers. Comparison of the upwelling radiance distributions measured with the CE600 and those obtained with another radiance camera also shows a very close agreement. The CE600 measurements allow all apparent optical properties (AOPs) to be determined from integration of the radiance distributions and inherent optical properties (IOPs) to be determined from inversion of the AOPs. This possibility represents a significant advance for marine optics by tying all optical properties to the radiometric standard and avoiding the deployment of complex instrument packages to collect AOPs and IOPs simultaneously (except when it comes to partitioning IOPs into their component parts).
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This paper presents the CNES analysis of CCD dark current degradation in orbit. For the first time, data coming from several detectors in orbit have been collected and congregated to extract general ...trends about in-flight detector behaviours. The in-orbit degradation is therefore compared to the on-ground prediction for each detector. This work has been done in collaboration with the CNES detection subsystem office, the CNES image quality office and the CNES components & environment effects office.
Absolute radiometric calibration is one of the main elements that contribute to the quality of measurements obtained with optical remote sensing instruments, but maintaining a good calibration ...accuracy during the whole life of an instrument is a difficult task. Since the sensitivity of an instrument generally changes after launch and degrades with time, many sensors have been equipped with onboard calibration devices. But these devices being not perfectly reliable, independent calibration methods based on natural targets are necessary to validate the results. The Sun glint calibration method is an interband calibration method that uses the specular reflection of the Sun on the ocean surface to transfer the absolute calibration of one reference spectral band to other spectral bands, from visible to short wave infrared wavelengths. Despite the drawback of relying on the absolute calibration of a reference spectral band, this method is one of the rare methods that can provide accurate calibration results for near-infrared spectral bands up to 1650 nm, without requiring costly in situ measurements simultaneously to the satellite overpass. This paper details the Sun glint calibration method and its error budget, and gives the results obtained with the VEGETATION instrument that was recently launched onboard the Systeme Pour l'Observation de la Terre 5 (SPOT-5) satellite. These results compare very well with the results of other calibration methods.
The radiometric sensitivity of the POLarization and Directionality of the Earth's Reflectances (POLDER) instrument in the ocean color spectral bands (443, 490, and 565 nm) was checked vicariously by ...comparing top-of-atmosphere normalized radiances measured by the instrument with those computed for the same geometries using a radiative transfer model. In situ measurements of aerosol optical thickness and marine reflectance at the time of the satellite overpass were used as input to the model. The accuracy of the vicarious calibration coefficients was estimated to better than 3%. A large decrease in the POLDER instrument response was found in the blue, confirming the results previously obtained using alternative techniques.
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