Minimize Level-1

OLCI Level-1 processing is divided into three independent sections, each computed only if the dedicated switch (configured by the users) is set to '1'.

Earth Observation (EO) processing inputs are Level-0 products, the orbits scenario file and several auxiliary data files providing calibration coefficient, surface classification or threshold for bright and glint classification. Its output is Level-1B data, i.e. radiometrically calibrated, geo-referenced and annotated radiances.

EO processing mainly consists of calibrating the numerical counts contained in ISPs into radiances, geo-locating 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:

  1. Data extraction and quality checks from ISP products.
  2. Radiometric scaling: aiming to derive 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.
  3. Stray light correction: a two-step process aiming to estimate and correct stray light contamination.
  4. Geo-referencing: aiming to compute, for every pixel, 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).
  5. Pixel classification: aiming to characterise 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.
  6. Spatial re-sampling: aiming to define and fill the output products grids, taking into account the full resolution and the reduced resolution grids.
  7. Product formatting: aiming to produce 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.

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:

  1. Acquisition geometry.
  2. Diffuser radiance computation, determining the radiance at instrument entrance, from knowledge of the acquisition geometry, sun flux and the diffuser bi-directional reflectance.
  3. Stray light computation, similar to stray light correction in EO processing.
  4. Radiometric LUT computation.

OLCI spectral calibration processing aims to accurately determine 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 radiometric calibration and Earth Observation data processing. Its input is two Level-0 products, pertaining to two successive orbits. The associated steps are:

  1. 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.
  2. 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.