DEVELOPMENT AND MODELLING OF A POINT SOURCE INTEGRATING CAVITY ABSORPTION METER (PSICAM)

The absorption coefficient is a fundamental parameter in understanding the underwater light field, for solving the Radiative Tranfer Equation and understanding/interpreting remotely sensed data from the ocean. Measuring the absorption coefficient is particularly complicated in coastal areas where the optical properties of the water body are the result of a complex mixture of dissolved and particulate components, but mainly because of the interfering effect that scattering has upon the measurements. A great variety of in situ instruments and laboratory techniques have been developed to measure total absorption or the absorption by the various fractions that constitute the total absorption. They are, however, all affected by scattering and empirical corrections need to be applied. Among the instruments to measure absorption, a promising one appeared to be one based on an integrating cavity. Kirk (1995, 1997) outlined the principle and theory of an absorption meter based on an integrating sphere: a Point Source Integrating Cavity Absorption Meter (PSICAM). He argued that owing to its design, a PSICAM would be insensitive to scattering. A novel Monte Carlo code was written to simulate the behaviour of a PSICAM of various cavity radiuses. The results of the simulations carried out with this code showed that such an absorption meter should indeed be unaffected by scattering even with high levels of scatterers. One important disadvantage deduced from numerical modelling for a PSICAM is its sensitivity to the reflectivity of the integrating cavity. Several prototype PSICAMs of increasing quality were built and tested with scattering-free standard solutions. A major difficulty in the development of the prototype was found to be the calibration of the integrating sphere reflectivity. A final laboratory instrument made of a Spectralon sphere was built and tested with artificial and natural water samples containing different levels of scattering particles and compared with existing in situ and laboratory techniques: the ac-9 transmissometer and the filter paper technique for particulate absorption as well as measurement of Coloured Dissolved Organic Matter. Compared with the ac-9 transmissometer, the PSICAM showed remarkable agreement even for water with very high content of Suspended Particulate Matter. Very good correlations were obtained when compared with traditional CDOM measurement. In some cases, significant discrepancies occurred with filter paper measurements of particulate absorption. From laboratory to in situ experiments the PSICAM proved to be a reliable instrument assuming that the instrument was regularly and carefully calibrated. Finally, the PSICAM was deployed during a cruise around the Antarctic Peninsula where total and dissolved absorption measurements were carried out together with chlorophyll absorption measurements after extraction in acetone.Qinetiq Malvern (Formerly DERA Malvern

A Holistic Perspective on the Calibration and Validation of Sentinel-2 L2A products: Contribution From the CCVS Project

In this presentation, we report on the preliminary findings of the H2020 project “Copernicus Cal/Val Solution” (CCVS), whose objective is to define a holistic solution to the cal/val of the Copernicus Sentinel missions. We focus more specifically on synergies of the Sentinel-2 mission with other Sentinel or third-party missions, in terms of cal/val requirements as well as reference data sources. Regarding the first aspect, CCVS will consolidate cal/val requirements for all missions with a unified approach. For instance, we compare validation requirements for Sentinel-2 L2A AOD and Water Vapour products to other optical missions like Sentinel-3 OLCI and SLSTR, as well as atmospheric composition missions. In addition, user-driven inter-operability requirements could lead to specific calibration or validation needs. A first example concerns the radiometric inter-calibration between Sentinel-2A and B, which could be ensured with better accuracy than the absolute calibration of either satellites. Geometric co-registration with other optical missions like Landsat could be also monitored. In terms of data sources, CCVS will first establish a survey of existing sources, including natural targets and in-situ data acquired in the frame of systematic measurement programs or ad-hoc campaigns. In a second step, we investigate potential data sources needed for calibration and validation, with a specific focus on directional surface reflectance and cloud mask

Aerosol Climatology over Pseudo-Invariant Calibration Sites: Application for African and Arabian Desert Sites

Calibration over natural targets is a very powerful way to derive the in-flight calibration of sensors (e.g., PARASOL, Vegetation, Pleiades) or if not, to validate the calibration derived using on-board devices (e.g., MODIS, MERIS). Various type of targets can be considered among oceanic sites (Rayleigh scattering, sunglint), atmospheric targets (deep convective clouds), terrestrial sites (desert, snow), or extra-terrestrial targets (moon, stars). Pseudo-invariant calibration sites (so-called PICS) are used for a very long while for many sensors because of their suitable properties in term of stability, moderated bidirectional variations, accessibility in term of cloudiness. Calibration approaches over PICS directly compare top-of-atmosphere (TOA) radiances measured by two sensors, or try to resolve spectral differences between both sensor responses by considering atmospheric and surface properties through a more or less complex algorithm (e.g. Lachérade et al., 2013). PICS are known to provide very stable surface properties, long-term or seasonal. It is also often assumed the same property for atmosphere as if historical studies have shown limitations. We derived a climatology of aerosol content over the 20 desert sites located in Africa and Arabia. These time series were derived using data from 3 sensors designed for aerosol retrieval: MODIS-Aqua and –Terra aerosol monthly products over land (from GIOVANNI) for the 2000-2014 period, and PARASOL aerosol daily products from for the 2005-2013 period. The aerosol load seasonal cycle is described for all desert sites and in general, a similar behaviour is observed year after year. According MODIS, the aerosol optical thickness at 550nm may vary from 0.2 to 0.6 depending of site and season. A sensible difference is observed with PARASOL, first for the aerosol optical thickness (in general smaller), but also for the seasonal variation that may be opposite to the one derived by MODIS. We interpret this signature as a very strong variation of the aerosol type during the season because PARASOL, which uses the directional and polarized signature of aerosol, is only sensitive to the aerosol fine mode while MODIS, which uses a spectral information up to the SWIR domain, is sensitive to both coarse and fine aerosol modes. A summary of the different behaviours observed for various sites will be presented. Results are confronted to observation from Aeronet for (not exact but) consistent locations. A generally good consistency is observed between datasets. This is an important validation of the conclusion derived from spaceborne sensors. As a conclusion, a single aerosol content cannot be a sufficient hypothesis when cross-calibration sensors, especially when allowing matching from different dates. In order to improve the accuracy of the calibration over desert sites (cross-calibration as well as temporal monitoring) a seasonal cycle of aerosol has to be considered on the data processing

A comparison between semi-analytical and empirical reflectance model in the case of a high oceanic phytoplankton bloom in the South Western Atlantic ocean

Semi-analytical (SA) and empirical ocean color algorithms are routinely used to monitor global chlorophyll biomass in the oceans. SA models are particularly attracting since they relate inherent optical properties of the water to measured variables like remote sensing reflectance. In addition to this, SA models have the capacity to derive various parameters from a single remote sensing reflectance. Empirical algorithms make no use of optical theory and are derived from statistical relationship linking remote sensing reflectances with chlorophyll a concentration. Maritorena et. al (2002) has produced a new data set for the chlorophyll specific absorption based on a procedure to optimize SA ocean color models. The resulting SA model: GSM01 is now routinely used by NASA/SeaDAS program. Empirical algorithms are regularly updated as the SeaBASS global database of ocean color data increases but rarely high concentration of chlorophyll have been reported in case 1 waters. During 2004 Brazilian OPERANTAR 23 spring cruise along the Argentinean shelf break a high oceanic phytoplankton bloom of diatoms and dinoflagellates was observed. The dataset consists of 18 stations of radiometric measurements together with chlorophyll concentration measurements along the high chlorophyll band. The radiometric measurements include surface downwelling irradiances and subsurface upwelling radiances measurements from which remote sensing reflectances at five wavelengths (412, 443, 490, 510 and 555 nm) were calculated. The measured surface chlorophyll a concentrations ranged from 1.8 to 19.9 mg.m-3. A SA reflectance model was used to retrieve chlorophyll concentration as well as particulate backscattering and CDOM absorption. The global ocean color empirical algorithms OC2v4 and OC4v4 were used to retrieve chlorophyll concentration. With this dataset, the OC4 performed better than the OC2 (r2 = 0.896, 0.852 for the OC4, OC2 respectively). The SA model showed variable performances depending on the dataset used for the chlorophyll specific absorption.Pages: 1305-130

System Vicarious Calibration of Sentinel-3 OLCI

Sentinel-3A (S3A), carrying the Ocean and Land Colour Instrument (OLCI), was successfully launched on February 16th 2016. It was the first of the series planned by the European Commission (EC) in the frame of COPERNICUS Sentinel program. Sentinel-3B is planned for launch in late 2017, bearing identical instruments, thus improving the global Earth coverage. The OLCI series providing global coverage at 300m resolution will therefore represent a major breakthrough in the family of ocean colour sensors. For being an operational mission feeding in downstream Copernicus services like CMEMS, it is essential to ensure product quality prior public release. As for most ocean color missions, this supposes the implementation of a system vicarious calibration (SVC). Based on the methodologies applied to MERIS (and historically to older sensors), SVC of OLCI is performed separately for near-infrared (NIR) and visible (VIS) bands. NIR bands SVC is performed over dedicated oligotrophic targets. Over such waters the marine signal is negligible in this spectral region compared to the contribution of the atmosphere. This avoids the use of in situ colocated data and provides enough sensor observations for the analysis and the computation of vicarious calibration gains. Different methodologies are tested and their intercomparison provides the conclusion that a simple unweighted regression on the NIR aerosol reflectances provides robust consistency in the obtained gains. On the other hand, VIS bands SVC relies on OLCI observations matching very high quality in situ measurements after calibration of the NIR. For the time being, only two operational stations provide sufficiently high quality data for this purpose: BOUSSOLE in the Mediterranean Sea and MOBY in the Pacific Ocean. The number of accurate vicarious calibration matchups to ensure statistically reliable gains is discussed. To increase the statistics and improve reliability, an alternative procedure based on the use of global daily climatologies is shown to provide consistent additional measurements for the computation of robust SVC gains in the VIS. In this presentation, the implemented SVC procedures for OLCI are described along with the analysis ensuring their reliability. OLCI product quality improvement brought by SVC is shown through the analysis of individual user products as well as by comparison with in situ data and other sensors

ProVal: A New Autonomous Profiling Float for High Quality Radiometric Measurements

An efficient system to produce in situ high quality radiometric measurements is compulsory to rigorously perform the vicarious calibration of satellite sensors dedicated to Ocean Color Radiometry (OCR) and to validate their derived products. This requirement is especially needed during the early stages of an OCR satellite activity or for remote areas poorly covered by oceanographic cruises with possible bio-optical anomalies. Taking advantage of Argo's profiling float technology, we present a new autonomous profiling float dedicated to in situ radiometric measurements. The float is based on the Provor CTS5 (manufacturer NKE) with an added novel two protruding arm design allowing for sensor redundancies, shading mitigation and near-surface data. Equipped with two identical radiometers on each arm that measure downward irradiance and upwelling radiance at seven wavelengths, the ProVal float generates both redundant radiometric profiles as well as an estimate of Remote Sensing Reflectance. Results from 449 profiles obtained in the NW Mediterranean Sea and in the Indian sector of the Southern Ocean are presented to illustrate the ProVal float technical maturity. Analysis of the behavior of the profiling float, including tilting and ascent speeds is presented. The vertical stability of the ProVal exhibits 85% of surface data of the Mediterranean Sea with a tilt smaller than 10 degrees. This percentage is 40% in the Southern Ocean due to rougher seas. Redundant sensors provide a characterization of the relative drift between sensors over the deployment which is found to be <0.15% per month over a year. Post-cruise calibration of a recovered float revealed no significant drift. As an example of the utility of ProVal floats, a match-up of Remote Sensing Reflectance measured with the European Space Agency Ocean and Land Color Imager (OLCI onboard Sentinel-3A) is shown. It follows that profiling floats, such as ProVal, could provide a significant contribution to an upcoming global System Vicarious Calibration of space-based radiometers

Evaluation of global leaf area index and fraction of absorbed photosynthetically active radiation products over North America using Copernicus Ground Based Observations for Validation data

With a growing number of Earth observation (EO) products available through operational programmes such as the European Union’s Copernicus, there is increasing emphasis on product accuracy and uncertainty, necessitating evaluation against in situ reference measurements. Whilst existing reference datasets have proven a valuable resource, they incorporate little data with which products from recent EO instruments can be assessed. A reliance on individual field campaigns has also led to several inconsistencies, whilst limiting the extent to which temporal variations in EO product performance can be captured. Recently established environmental monitoring networks such as the National Ecological Observatory Network (NEON), which collect routine in situ measurements using standardised instruments and protocols, provide a promising opportunity in this respect. The Copernicus Ground Based Observations for Validation (GBOV) service was initiated in recognition of this fact. In the first component of the project, raw observations from existing networks have been collected and processed to provide reference data for a range of EO land products. In this study, we focus on leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR). Raw digital hemispherical photography (DHP) from twenty NEON sites was processed to derive in situ reference measurements, which were then upscaled to provide high spatial resolution reference maps. Using these data, we assess the recently released Copernicus Global Land Service (CGLS) 300 m Version 1 (V1) products derived from PROBA-V, in addition to existing products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Radiometer Suite (VIIRS). When evaluated against reference data, the CGLS 300 m V1 product demonstrated the best agreement (RMSD = 0.57 for LAI and 0.08 for FAPAR), followed by the Collection 6 VNP15A2H and MOD15A2H products (RMSD = 0.81 to 0.89 for LAI and 0.12 for FAPAR). Differing assumptions of the products and in situ reference measurements, which cause them to be sensitive to slightly different quantities, are thought to explain apparent biases over sparse vegetation and forest environments. To ensure their continued utility, future work should focus on updating the GBOV in situ reference measurements, implementing additional corrections, and improving their geographical representativeness

Evaluation of global leaf area index and fraction of absorbed photosynthetically active radiation products over North America using Copernicus Ground Based Observations for Validation data

With a growing number of earth observation (EO) products available through operational programmes such as the European Union's Copernicus, there is increasing emphasis on product accuracy and uncertainty, necessitating evaluation against in situ reference measurements. Whilst existing reference datasets have proven a valuable resource, they incorporate little data with which products from recent EO instruments can be assessed. A reliance on individual field campaigns has also led to several inconsistencies, whilst limiting the extent to which temporal variations in EO product performance can be captured. Recently established environmental monitoring networks such as the National Ecological Observatory Network (NEON), which collect routine in situ measurements using standardised instruments and protocols, provide a promising opportunity in this respect. The Copernicus Ground Based Observations for Validation (GBOV) service was initiated in recognition of this fact. In the first component of the project, raw observations from existing networks have been collected and processed to provide reference data for a range of EO land products. In this study, we focus on leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR). Raw digital hemispherical photography (DHP) from twenty NEON sites was processed to derive in situ reference measurements, which were then upscaled to provide high spatial resolution reference maps. Using these data, we assess the recently released Copernicus Global Land Service (CGLS) 300 m Version 1 (V1) products derived from PROBA-V, in addition to existing products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Radiometer Suite (VIIRS). When evaluated against reference data, the CGLS 300 m V1 product demonstrated the best agreement (RMSD = 0.60 for LAI and 0.08 for FAPAR), followed by the Collection 6 VNP15A2H and MOD15A2H products (RMSD = 0.78 to 0.87 for LAI and 0.11 for FAPAR). Differences in the definition of the products and in situ reference measurements are thought to explain apparent biases over sparse vegetation and forest environments. To ensure their continued utility, future work should focus on updating the GBOV in situ reference measurements, implementing additional corrections, and improving their geographical representativeness.JRC.D.6-Knowledge for Sustainable Development and Food Securit