S3 L1C SYN Tool - Mis-registration Algorithm

Mis-registration Algorithm

The S3 L1C tool can co-register SLSTR and OLCI data by using a so called mis-registration file. This file provides mapping information for the correspondence grids between the OLCI reference channel and other OLCI and SLSTR near-nadir channels.
Here the implemented algorithm for applying the provided mis-registration information shall be briefly explained.

Input Data

The mis-registration algorithm uses SENTINEL-3 (A/B) OLCI EFR and SLSTR L1B RBT data as inputs. Besides the two satellite data products an ancillary product is used. This mis-registration file, provides information on how the OLCI and the SLSTR product must be mapped onto each other. In the following the expected versions are defined of the input files:

OLCI Data
S3IPF PDS 004.1 - Product Data Format Specification - OLCI Level 1 version=2.4
SLSTR Data
S3IPF.PDS.005.1 - Product Data Format Specification - SLSTR Level 1 version=2.9
MISR Data
S3IPF PDS 006 - i1r12 - Product Data Format Specification - SYNERGY
The tool accepts mis-registration files as 3-minute granules and also as half-orbit files. The memory requirement is higher when using the half-orbit files.

Algorithm

The algorithm constructs a lookup-table that maps the SLSTR image grid (x/y) to the corresponding OLCI image grid. To do this it uses the so called instrument grid (scan/pixel/detector). The lookup-table is created by the 5 following steps, the necessary data is read from the input files:

Step 1 - Mapping from SLSTR image grid to SLSTR instrument grid
SLSTR(x, y) -> SLSTR(idx_scan, idx_pixel, idx_detector)

Step 2 - Mapping between SLSTR instrument grid and MISR grid
SLSTR(idx_scan, idx_pixel, idx_detector) -> MISR(misr_row, misr_col)

Step 3 - Mapping between SLSTR (row/col) and OLCI instrument grid
mapping SLSTR(misr_row, misr_col) -> OLCI(camera, line, detector)

Step 4 - Mapping between OLCI instrument grid and OLCI image grid
mapping OLCI(camera,line,detector) -> OLCI(row,col)

Step 5 - Mapping between SLSTR image grid and OLCI image grid
mapping SLSTR(x,y) -> OLCI(row,col) -> OLCI(row,col)

When the lookup-table is created, the mapping is applied.

  1. Create target raster (OLCI dimension) and initialize with FillValue
  2. For each Pixel:
    1. Get SLSTR (x/y) from OLCI (x/y)
    2. If mapped (x/y) inside SLSTR grid
      1. Read SLSTR variable at (x/y)
      2. If value read > 0 set value to target raster at OLCI(x/y)
  3. If after the previous steps any of pixels on the final raster has no value, but its position (x/y) has a value in the original OLCI product in the reference band “Oa17_radiance”, then the closest (euclidian distance) value in a 9x9 window around the pixel is searched. This value replaced the no-data. If no such pixel exists, then the value remains no-data.