The main processing routine estimates inter-instrument de-registration of the two reference channels (one in OLCI and one in SLSTR) at each OLCI pixel. Several tests are dedicated to the selection of Ground Control Points (GCP).
Deregistration is estimated at each GCP in the following way:
- SLSTR (sub) pixels (around a GCP) are referenced by OLCI pixels (through their ortho-geolocation information) and then interpolated on the OLCI grid.
- A correlation surface is calculated between the two reference channels and its maximum is searched for, yielding the pixel de-registration. Sub-pixel deregistration is obtained by interpolating the correlation surface around the found maximum on a finer grid. This process is repeated a fixed number of times (or until convergence has been reached) and gives the desired sub-pixel deregistration for a grid point.
- The parameters of a deformation model for the whole image are estimated using a Thin Plate Spline (TPS) model.
- A local deformation model is estimated as a piece-wise linear model.