Two grids can be defined for SLSTR outputs: the measurement grids, also called instrument reference frame, and the image grids. In addition, each grid contains a number of variations according to the different instrument view and resolution. Note however that, to be compliant with the continuity requirement, all SLSTR L1b and L2 files are defined on image grid. The L0 product is still associated with the instrument grid.
SLSTR products are generated separately in two instrument views:
- nadir view is almost vertical at the sub-satellite point
- oblique view has a local zenith angle close to 55°.
Two resolutions are then defined:
- a 500 m resolution for Visible/NIR/SWIR channels (S1 to S6)
- a 1 km resolution for thermal and fire channels (S7 to S9, F1 to F2).
The measurement grids are the native grids on which the SLSTR collects data. Each datum is indexed by scan, pixel and detector numbers. The scan number identifies the number of the scan line, starting at the ANX. The pixel number identifies, in a scan line, the pixel count, starting at one end. The detector number maps, for each pixel, the rows and the columns (aligned with the along track and across track directions) onto a two-dimensional matrix of detectors.
The image grids are locally cartesian and are regular grids aligned with the sub-satellite track. All measurement data are provided on this grid to hold an "image-like" data and to be compliant with the continuity requirement (i.e. SLSTR granules should be continuous).
In addition to these two grids, a lower resolution grid is defined and called the tie-point grid. Some annotations such as meteorological or viewing geometry data are produced on this grid. It is aligned with the 1 km and 0.5 km image grids and is regular in distance across track (16 km) and regular in time along track (1 km to fulfill the continuity requirement).