Georeferencing of OLCI pixels involves computing for every pixel, the first intersection between the pixel line-of-sight and the Earth's surface. The Earth's surface is assumed to be perfectly represented by the WGS-84 reference ellipsoid completed by a Digital Elevation Model (DEM).
This algorithm includes four main steps:
- Retrieve satellite location and attitude at the time of each OLCI acquisition (mid-exposure). This step is realised through calls to mission CFI functions, on the basis of NAVATT packets information. Direct handling of NAVATT packets by the mission CFI is not available, so the NAVATT data are used to generate orbit and attitude files consistent with their use by the mission CFI routines.
- Retrieve instrument to satellite alignment matrix (accounting for along-orbit thermoelastic effects). This is achieved by a dedicated function providing the alignment matrix as a function of Position On Orbit (POO) (short term effect) and day-of-year (long-term seasonal effect).
- Compute the intersection of each pixel's line-of-sight with the Earth's surface, including a high resolution elevation model. Compute the corresponding sun and view azimuth and zenith angles. This is implemented by the corresponding mission CFI functions, accounting for satellite location and attitude and instrument/satellite alignment derived in previous steps and to Earth/Sun ephemeris embedded in the CFI functionalities.
- On the basis of the GeoReferencing information, pixels are further classified through calls to the Pixels Classification Functions described in next section
In addition to the four algorithm steps described above, implementation requires an additional upstream step for tools initialisation.