OLCI Level-2 processing is divided into two main processes:
- Ocean processing, providing OL_2_WFR and OL_2_WRR products
- Land processing, providing OL_2_LFR and OL_2_LRR products.
In addition a common pre-processing and product formatting process aims to read and check the input, and to define and write the outputs.
It is important to note that, for each geophysical parameter included in the OLCI Level-2 product, a switch has been defined in the OLCI Level-2 configuration file. Each parameter and its associated flags are produced only if the associated switch is set to '1'. As a consequence, the module generating each parameter is triggered only if the appropriate switch is set to '1'.
The pre-processing module, starting from the Level-1B TOA radiances, derives reflectances corrected for gaseous absorption. The consolidation of pixel classifications from Level-1B and the definition of water vapour retrieval are included in this module.
The algorithm is divided into five successive steps:
- The conversion from radiances to reflectances step-checks the Level-1B products and converts radiances into reflectances (also known as first instrumental correction).
- To be correctly taken into account or to be rejected from the algorithm, pixels have to be differentiated according to four criteria: cloud, land, water and invalid pixels. The first pixel classification focuses on identification of cloudy pixels. This Cloud masking will be improved in the frame of coming evolutions, including the improvement tested and validated during the MERIS 4th reprocessing.
- Gaseous correction: correcting reflectances for gaseous absorption (i.e. O2, H2O and O3). Five OLCI bands are dedicated to this correction and are not used after this step: Oa13 to Oa15, Oa19 and Oa20.
- The second pixel classification estimates glint reflectance and completes pixel classification starting at the second step by consolidating the classification land and water pixels.
- The water vapour process retrieves atmospheric water vapour content from clear sky pixels.
The ocean processing module provides water-leaving reflectances, computing and subtracting from the total signal, the atmosphere contribution and deriving the necessary geophysical products.
The first step is performed using two algorithms, selectable by a dedicated switch (included in the OLCI Level-2 configuration file):
- Baseline Atmospheric Correction removes all the contributions to TOA reflectances, beginning with glint correction and white cap effects, followed by estimation of the near-infra-red water-leaving reflectance to perform atmospheric correction. This step accounts for the spectral smile i.e. the in-FOV variation of the central wavelengths of OLCI channels.
- Alternate Atmospheric Correction uses a neural network approach to provide water-leaving reflectances. This step accounts for the spectral smile i.e. the in-FOV variation of the central wavelengths of OLCI channels.
- Several steps (OC4Me Chlorophyll, IMT Neural Net, Transparency Product, PAR Product) compute all the needed products from the water-leaving reflectances.
The land processing module consists of two independent sections (one for each product):
- The Green Instantaneous Fraction of Absorbed Photosynthetically Active Radiation (GI-FAPAR) section combines the information contained in the blue band with that contained in the bands at 681 and 865 nm to generate "rectified channels" at these latter two wavelengths.
- The OLCI Terrestrial Chlorophyll Index (OTCI) section uses Rayleigh correction to produce the necessary index. This step accounts for the spectral smile i.e. the in-FOV variation of the central wavelengths of OLCI channels.