Minimize Shift Estimation at Ground Control Points

De-registration at GCPs between the reference channels of OLCI (Oq) and SLSTR (Su) is first computed at pixel scale and subsequently at sub-pixel scale.

Pixel scale de-registration is obtained by calculating cross-correlation coefficients between the context and search imagettes (obtained from the previous module). The radiometric content of the context window (context imagette) is shifted around the GCP according to the shift vector. At each shift the cross-correlation is computed, yielding a cross-correlation surface for the whole search window. The value of the shift vector where the cross-correlation surface reaches its maximum denotes the pixel scale de-registration of both channels.

A sub-pixel de-registration is obtained by zooming and interpolating the cross-correlation coefficient on a finer grid. This is done in several steps until the desired resolution and de-registration at sub-pixel level is found (dichotomic search).

A GCP rejection routine is also implemented. Several criteria are defined, allowing rejection of GCP (e.g. no distinct correlation maximum found, flat correlation surface around maximum).

Logical flow of sub-pixel shifts estimation module