Minimize Spatial Re-Sampling

Spatial re-sampling processing defines and fills the output products grids. There are two such grids:

  • Full Resolution (FR) product grid
  • Reduced Resolution (RR) product grid (four times coarser than FR).

Both product grids are built on the basis of equally spaced time samples for the along track dimension and of samples equally spaced in on-ground distance for the across track dimension. The on-ground distance is computed at the surface of the reference ellipsoid. The across track dimension is defined as the intersection between the YZ plane in the satellite actual reference frame and the reference ellipsoid.

Time sampling for the FR product is the OLCI instrument time sampling (44 ms) and the middle sample from four consecutive OLCI time samples for the RR grid. The across track distance sampling step for FR corresponds to the best instrument spatial sampling step (~300 m) and is specified through auxiliary data. The across track distance sampling step for RR is four times that of FR. Product pixels are filled with instrument pixels for FR and averages of surrounding instrument pixels for RR on the basis of the nearest neighbour re-sampling method.

The re-sampling scheme for FR pixels involves parsing the output product grid and, for every product pixel, knowing its across track pointing angle (the angle between the satellite nadir and the projection of the viewing direction onto the YZ plane, assigned with the sign of Y). The array of actual pointing directions of the OLCI pixels (converted into satellite actual reference frame) is searched for the closest one. Once found, the along track depointing of the corresponding OLCI pixel, available from characterisation (expressed in units of along track pixels and referred to as the frame offset) is used to address the appropriate OLCI frame, defining a unique pixel in the instrument 2D spatial grid. All the values affected by the OLCI instrument source pixel are then assigned to the current product pixel. The OLCI pixel is "marked" as used and if selected again, the corresponding product pixel is flagged as "duplicate". This multiple selection occurs often since instrument spatial resolution degrades at FOV edges while the product's spatial resolution remains constant. Processing continues with the next product pixel. As a consequence of the (potential) along track misalignment of OLCI pixels with respect to the theoretical viewing plane, alignment with the YZ plane requires the availability of several frames simultaneously during the re-sampling step.

The resampling scheme for RR follows a very similar scheme but takes advantage of the availability of the highest spatial resolution of the source data to minimise the spatial resolution degradation toward the FOV edges. This can be achieved by averaging a variable number of pixels in the across track direction according to the in-FOV position (or to the across track pointing angle). The number of pixels to be averaged in the along track direction remains constant (four). The number of pixels to be averaged in the across track direction is retrieved as a function of the across track pointing angle from the RR product pixel centre. The RR resampling is otherwise identical to FR resampling.

The regularity of the adopted grid as well as the relatively low accuracy requirements on their associated geo-reference data (accurate geo-reference is provided independently for each OLCI sample) allows the use of a sub-grid called the tie-point grid and bilinear interpolation in between. The tie-point grid has the same ground spacing, in each direction, for the FR and RR product. The corresponding sub- sampling factors in each direction, with respect to the FR product grid, can be defined using configuration parameters, and is currently set to 64 FR pixels (and consequently 16 RR pixels). It should be noted however, that these values may change in the future. To allow interpolation at every pixel, any product shall start with a tie-points frame and stop with a tie-points frame. The first tie-frame is synchronised with the first product frame and an additional tie-frame is added if the last product frame does not match a tie-frame.

It is also only at tie-points that meteorological data are appended. Meteorological data are retrieved from global data grids provided by a weather forecast centre (ECMWF) and interpolated in time and space at tie-point locations. It is assumed that the closest forecasts/analysis files bracketing the OLCI product are available at the time of processing. It is also assumed that the products are provided in one of the ECMWF "reduced Gaussian grids".

 

Logical Flow of Spatial Re-Sampling