SYN Level-1 processing aims to retrieve OLCI and SLSTR radiances and brightness temperature in their acquisition geometry. The same computation is done for their associated annotations. This processing also aims to compute the correspondence grids between the OLCI reference channel and all other OLCI and SLSTR channels.
SYN Level-1 processing is divided into the following parts:
- A pre-processing step which reads and adapts the Level-1B data for subsequent processing.
The main processing steps:
- Inter-instrument mis-registration estimation aims to correspond, for each OLCI camera module, the reference SLSTR visible reference channel with the reference OLCI channel.
- Computation of correspondence between the OLCI reference channel and all other OLCI and SLSTR channels, based on the mis-registration estimated at the previous step and on intra-instrument mis-registration estimation stored in a characterisation ADF.
- Projection of all OLCI/SLSTR and subsampled parameters on the same SYN reference grid (i.e. the OLCI acquisition grid)
- Post-processing to define and write the Level-1 product, SY_1_MISR
The pre-processing focuses on reading and adaptation of OLCI and SLSTR Level-1B product and is divided into two sub-steps.
- An extended land/sea mask is applied to focus only on land surface a) (Note that this distinction ocean/land can be taken into account to now include ocean pixels in global aerosol processing)
- The OLCI retrieval section retrieves the full images of the five OLCI camera modules in their acquisition geometry. This processing involves radiometric measurement and associated annotations. The TOA radiance from OLCI reference channels is converted to TOA reflectance. A first selection Ground Control Point (GCP) is also included. This Ground Control Point grid can be computed from an external database or regularly created.
- A similar process, except GCP selection, is done for each channel of the SLSTR nadir and oblique views during SLSTR retrieval. The conversion is done for TOA radiance from SLSTR reference channels.
The main processing section is divided into four sections, gathering inter and intra-measurement spatial mis-registration.
- The extraction of OLCI/SLSTR imagette pairs extracts imagettes around each GCP, one in the OLCI reference channel (called the context imagette) and one larger imagette in the SLSTR reference channel (called the search imagette). The search imagette is projected onto the OLCI geometry. This section also includes rejection tests on selected Ground Control Points.
- The sub-pixel shift estimation at GCP sub-step computes a correlation surface between the two imagettes that is shifted around the GCP according to shift vectors and finds its sub-pixel maximum. This sub-step also includes final rejection tests on selected GCP. These two last sub-steps are performed for each selected GCP.
- The deformation model estimation sub-step computes the parameters of a piece-wise deformation model giving the mis-registration of the reference SLSTR channel at each pixel of the OLCI selected channel.
- The correspondence between reference OLCI and other channels sub-step uses all previously computed parameters and mis-registration ADFs to compute the correspondence between the OLCI reference channel image and all other OLCI channels, and SLSTR nadir channels. The correspondence between the OLCI reference channel and the SLSTR oblique channel is computed by estimating the common swath between these two images and establishing a superimposition in the gridded Level-1B product. These results are used to project all OLCI/SLSTR parameters on the same grid, the SYN reference grid, i.e. the OLCI acquisition one.
The post-processing step gathers all de-registration information, inter and intra-instrument and writes this information to the Level-1 product.