Continental ecosystems monitoring

Removing the effects of atmospheric components on the response of land and water surfaces to solar illumination is the primary mandatory step to the retrieval of the biophysical properties of vegetation, soils and inland waters. Through its tools, SMAC_CL and Polymer, HYGEOS contributes to major European initiatives focusing on land ecosystems monitoring.

A continuous involvement in the Copernicus services

HYGEOS takes part in the global component of the Copernicus Land Monitoring Service from the beginning of the service operations in 2012. HYGEOS is involved in the consortium in charge of the production of the “Vegetation and Energy” variables, led by VITO, and in the consortium in charge of the production of the “Cryosphere and Water” variables led by CLS Group. In both consortia, HYGEOS is the expert team for the definition of atmospheric corrections methods, applied in near real time, and validation of their performance. In one hand, SMAC_CL is used to retrieve Top-Of-Canopy (TOC) reflectances which are the common input to assess a set of basic vegetation parameters. On the other hand, Polymer is used to retrieve the lake surface reflectances which are the input to assess some water quality indices. Furthermore, HYGEOS coordinates the scientific activities of the first consortium, in particular concerning the evolution of portfolio, the validation of products and the communication towards users and other Copernicus actors.

Similarly, HYGEOS is involved in the Copernicus Climate Change Service from the beginning of the service operations in 2018. HYGEOS participates in the consortium in charge of the generation of long-term data records describing the evolution of some biosphere Essential Climate Variables (ECV) (surface albedo, Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), Leaf Area Index (LAI), land cover, burnt areas and fire radiative power). SMAC is used to perform the atmospheric correction with a focus on the propagation of uncertainties from Top-of-Atmosphere reflectance to the biophysical variables and taking into account multiple aerosol models and BRDF corrections.

The atmospheric corrections benefit from the lastest upgrades of the SMAC_CL code. In particular, the Jacobians allow calculating and propagating the part of the TOC reflectance uncertainty due to each key model parameter like the aerosol optical depth or the column of water vapor (example on the figure at right). The uncertainty due to the aerosol model is also taken into account.

Input TOA reflectances and interpolated auxiliary data (Ozone content, Water Vapour content, Pressure of Vurface and Aerosol Optical Thickness at 550nm) from top to bottom (left column) and absolute values of their respective output Jacobians (right column), all in SMAC_CL units. The last row represents the resulting TOC reflectance (left) and its uncertainty (right).

Example for an area of about 45km x 45km centered on the AERONET station of Carpentras, France, extracted from Sentinel-3 OLCI spectral band at 865nm on 3rd of July 2018. White pixels are clouds.

maps of SMAC inputs (TOA reflectance, ozone, water vapour pressure of surface and aerosol optical thickness) and of their corresponding jacobians, map of the resulting TOC reflectance and its uncertainty. Example for OLCI band at 865nm

Evolutions through research project

The evolutions of the Copernicus Climate Change Service are prepared through research projects like the ESA-funded Climate Change Initiative Vegetation Parameters project. This latter performs the necessary research and development needed for the generation of high quality, global, multi-mission time series of two vegetation ECVs (LAI and FAPAR), maximising the consistency between resolutions (1km and 300m) of different satellite missions and between different ECVs. Once again, SMAC_CL is used to perform the atmospheric correction on data of all satellite radiometers available during the period from 2000 to the present. HYGEOS assesses the performance of the atmospheric correction using the aerosol properties available in the AERONET dataset and the 6S accurate atmospheric correction method acting on Top-Of-Atmosphere reflectance extracted around AERONET sites that cover different biomes, locations and meteorological conditions. In this project, a particular attention is given to overlapping time period between several sensors in order to improve the multi-sensor consistency and to consolidate the error budget.

Funder : European Commission (Copernicus Services) & ESA (CCI)

Duration : From 2012 onwards

Contact at HYGEOS : Roselyne Lacaze