OLCI Level-1 processing is divided into three independent sections:
1. Earth Observation (EO) processing
Earth Observation (EO) processing inputs are Level-0 products, the orbit scenario file and several auxiliary data files providing calibration coefficient, surface classification or threshold for bright and glint classification. EO processing output is Level-1B data, i.e. radiometrically calibrated, geo-referenced and annotated radiances.
EO processing involves the calibrating of the the numerical counts contained in ISPs into radiances, geo-locating the acquired pixels on the Earth's surface and re-sampling the image onto an orthogonal product grid, representing the instrument's ideal swath. The final steps involve quality flags, meteorological annotations and pixel classification flags, appended with computed variables to the outputted Level-1B products.
EO processing is divided into seven steps:
- Data extraction and quality checks from ISP products.
- Radiometric scaling: derivation of calibrated TOA radiance values from the numerical counts previously extracted. This section is itself divided into six sub-sections: initialisation, non-linearity correction, dark signal correction, smear correction, absolute gain calibration and cosmetic pixel filling.
- Stray light correction: a two-step process that estimates and corrects stray light contamination.
- Geo-referencing: Computation, for every pixel, of the first intersection between the pixel line-of-sight and the Earth's surface (assumed to be perfectly represented by the WGS84 Reference Ellipsoid completed by a Digital Elevation Model).
- Pixel classification: characterisation of the pixels according to classes of underlying surface, whatever the atmospheric conditions, to provide preliminary detection of cloudy pixels and to detect pixels showing a risk of contamination by sun glint.
- Spatial re-sampling: definition and filling of the output products grids, taking into account the full Resolution and the Reduced Resolution grids.
- Product formatting: production of the OL_1_EFR and OL_1_ERR products.
Note that data extraction, quality checks, instrument count corrections (included in the radiometric scaling step) and product formatting sub-sections are common, or share commonalities with the three processing levels.
2.OLCI Radiometric Calibration (RC)
OLCI Radiometric Calibration (RC) processing is based on the same inputs as EO processing and outputs a set of calibration Look-Up-Tables (LUTs). The associated steps are:
- Acquisition geometry.
- Diffuser radiance computation, determining the radiance at instrument entrance, from knowledge of the acquisition geometry, sun flux and the diffuser bi-directional reflectance.
- Stray light computation, similar to stray light correction in EO processing.
- Radiometric LUT computation.
3.OLCI Spectral Calibration (SC)
OLCI Spectral Calibration (SC) processing accurately determines the central wavelengths of specific rows of the detector arrays and contributes to the overall accuracy and reliability of the instrument spectral model used during RC and EO processing.
SC processing uses two Level-0 products, pertaining to two successive orbits. The associated steps are:
- Relative spectral calibration containing a data extraction and quality check sub-process (once for each input Level-0 product) and an instrument count correction sub-process (once for each input Level-0 product). A final sub-process derives the spectral diffuser's relative spectral BRDF from corrected counts of both diffusers.
- Wavelength calibration analysing the above and deriving absolute wavelength characterisation of the instrument, over the whole field of view, by comparison of measured spectral BRDF to a reference.