The OLCI product grid has been defined by upstream Level-1B products. Parameters included in the Level-1B products are ortho-geolocated and re-gridded during a specific step including in Level-1 processing (the re-sampling stage).
The grid applied at this stage is evenly spaced at the reference ellipsoid surface in the across track (AC) direction and is quasi-evenly spaced in the along track (AL) direction, since data are evenly sampled in time along the orbit of the satellite.
The AL sampling distance is equal to the Spatial Sampling Distance (SSD) at the Sub-Satellite Point (SSP), i.e. about 300 m in FR mode and 1.2 km in RR mode. The grid used in RR mode is a 4 by 4 sub-grid of the FR product grid.
The nominal observation time window along an orbit is driven by the sun zenith angle at SSP being below 80°. It results in a duration of approximately 44 minutes, yielding approximately 60 000 (15 000) pixels in FR (RR) mode.
Note that acquisition geometry and ECMWF annotations for both resolution products are provided at tie-points on a grid defined as a 64 by 64 subset of the FR product grid. In RR mode, the tie-point grid is identical in terms of ground spatial sampling but shifted with respect to the FR grid (tie-points every 16 pixels).