The ESA – SEOM funded CIRCAS (ConsIstent Retrieval of Cloud Aerosol Surface) project aim at providing a set of atmospheric (cloud and aerosol) and surface (albedo) products derived from S3A/SLSTR observations retrieved using the same radiative transfer physics and assumptions. The retrieval is based on the CISAR (Combined Inversion of Surface and Atmosphere pRoperties) algorithm. CISAR is an advanced mathematical method developed by Rayference for the joint retrieval of surface reflectance and atmospheric (cloud and aerosols) properties from observations acquired by space-based imagers.

The CISAR algorithm relies on the FASTRE radiative transfer model that describes surface reflectance and atmospheric absorption/scattering processes. The lowest level represents the surface. The lower layer hosts the aerosols. Molecular scattering and absorption are also taking place in that layer which is radiatively coupled with the surface for both the single and the multiple scattering. The upper layer is only subject to molecular absorption.

The inversion of the FASTRE model within the CISAR algorithm against satellite observations provides accurate estimates of the surface reflectance field, aerosol or cloud optical thickness and single scattering properties in each processed spectral band. An estimate of the retrieval uncertainty is also provided.

As the proposed method retrieved both cloud and aerosol properties with the same retrieval algorithm, no cloud mask is needed to perform the retrieval. Additionally, the same algorithm can be applied over any type of surfaces, including dark or bright surfaces or water bodies.


11/12/2019 Correction of the SLSTR solar band calibration

Rayference has been working on the verification and correction of the S3A/SLSTR solar bands. This activity has been reported in web story of the Eradiate project entitled "Estimating the accuracy of 1D radiative transfer models over the Libya-4 site". Following this study, the S3A/SLSTR Level 1b TOA BRF corrections recommended by Rayference, displayed in the following table,

Bands S1 S2 S3 S5 S6
Correction 0.95 1.10 1.10
Correction 0.92 0.93 0.93

will be used by the CIRCAS project. These calibration correction factors should multiply the delivered top-of-atmosphere bidirectional reflectance factor (TOA BRF) values. The complete analysis is presented in a dedicated report.

06/12/2018 Mid-Term Review

The CIRCAS mid-term review took place in Hamburg. The objective of the meeting was to review the CIRCAS progress during the first year of the project.

11/01/2018 Science Review

The CIRCAS science review took place at Rayference premises located in Brussels. The objective of the meeting was to review the Requirement Baseline Document and the draft version of the ATBD.

29/09/2017 Kick-off meeting

The CIRCAS kick-off meeting took place at ESA-ECSAT (UK).


FASTRE model
Example of radiative coupling between surface reflectance and atmospheric scattering for a sun zenith angle set to 35°. The green line shows the surface BRF in the principal plane in the absence of any atmospheric effects. The other colours show the bottom of atmosphere surface reflectance for various values of scattering Optical Thickness (OT).


The CIRCAS project team includes Rayference as prime contractor and Brockmann Consult as partner.

Rayference Rayference

Rayference is an Earth Observation research and development company providing technical and scientific expertise in 1D and 3D radiative transfer, both in the atmosphere and at the surface, and inverse modelling. This expertise is applied in areas such as retrieval of aerosol and surface properties from space observations, vicarious calibration, fundamental and thematic climate data record generation or sensitivity analyses for the preparation of new space missions and associated services.

Rayference project activities include:
  • Project management
  • ATBD preparation
  • Adaptation of the CISAR algorithm for the processing of SLSTR dual view observation
  • Improvement of the CISAR algorithm for the consitent retrieval of surface, cloud and aerosol properties

BC Brockmann Consult

Brockmann Consult is a private commercial company (SME) founded in 1999 in Hamburg as private business and turned into GmbH in 2002. BC offers services for the management of environmental data in particular from Earth Observation. The company comprises two complementary branches: The Environmental Informatics Department and the Geo-Information Services Department.

The Environmental Informatics Department develops custom-made software solutions for environmental data. The cornerstone of the department’s activities is system development for Earth Observation data processing systems and data analysis software. Important EO software products include the Hadoop-based Calvalus System for massive parallel processing of big data, the ESA Climate Change Initiative Toolbox, and SNAP, the Sentinel Application Platform. The staff members of the department are internationally recognized experts combining professional software engineering with natural sciences expertise. They advise customers on the design and implementation of processing systems and provide turn-key system solutions.

Brockmann Consult project activities include:
  • Adaptation of CISAR processing environment to Calvalus
  • Processing of SLSTR data with the CISAR algorithm
  • Cloud, aerosol, surface albedo product validation


The CIRCAS project has been presented in the following conferences and workshops: