In the frame of pre-launch activities for Sentinel-3's OLCI-A&B instruments, a first version of the instrument Spectral Response Functions (SRFs) based on on-ground characterisation measurements was derived. The spectral characterisation information is provided in the following files for both A&B units with three different levels of detail:
- The full dataset - OLCI-A / OLCI-B
Includes for each of the 21 OLCI-A or B spectral bands, the centre wavelength, the bandwidth and the in-band solar irradiance at individual OLCI CCD columns (corresponding to the across track dimension of the instrument swath)
- A subset of the dataset - OLCI-A / OLCI-B
The same as the full file, but reduced to the spectral response functions at only three CCD columns (located at Eastern, centre, and Western part) for each module and each one of the 21 spectral bands
- The mean dataset - OLCI-A / OLCI-B
The same as the full file, but reduced to a single representative (mean) spectral response function, in-band solar irradiance, bandwidth and centre wavelength per band.
The S3 Mission Performance Center have assessed the evolution of the spectral characteristics for both OLCI-A & B and have generated two Look-Up-Tables (LUTs) for enabling the calculation of the best estimate of centre wavelength, bandwidth and in-band solar irradiance for all bands and all pixels at a given absolute orbit number. Users making use of the OLCI bands 13, 14 and 15 around 760 nm, surrounding the O2-A absorption are strongly advised to apply these. It may also be important for the water vapour absorption bands 19 and 20, for the very blue Rayleigh bands 1 and 2 at 400 & 412 nm, for the chlorophyll fluorescence bands and the red-edge bands around 750 nm. The LUT's will be updated in line with the evolution and currently it is not foreseen to apply these in the operational Level-1 processing. As a consequence, only the in-band solar irradiance provided in the Level 1b data shall be used to convert L1b radiance into TOA reflectance. Please be aware that the provided LUTs are not of the same format or content as the SRFs and only replace the corresponding parameters central wavelength, FWHM and in-band irradiance. The Spectral Characterisation Technical Note gives guidelines on how to use the LUTs. Moreover there are two Jupyter Notebook scripts SRF alignment and calculation of centre wavelengths and FWHM, respectively.
Note: The spectral characteristics of both OLCI-A and OLCI-B will be monitored throughout the entire mission duration and this might again lead to updates of the instrument SRFs at a later stage in the mission lifetime.