Baseline Atmospheric Correction
Baseline Atmospheric Correction
The BAC algorithm removes all contributions to TOA reflectance that are not due to ocean colour. This means the correction of glint and white caps effect is carried out, followed by estimation of the aerosol type and load to allow atmospheric correction. This algorithm is divided into three major steps.
- Glint and white caps correction. Glint reflectance is compared to a low glint threshold. If the glint reflectance is below this low glint threshold then no glint correction for this pixel is applied. If the pixel is bright, it is flagged as ice or high aerosol load and no further processing is carried out. If the glint reflectance is above the low glint threshold then the glint reflectance is compared to a medium glint threshold. If the glint reflectance is above the medium glint threshold then no correction is applied and the pixel is flagged as uncorrected sun glint. If the glint reflectance is below the medium glint threshold then a medium glint flag is raised and the pixel is corrected for glint reflectance.
- Case 2 NIR reflectance estimation. This step is necessary for sediment loaded waters, to comply with the aerosol estimation scheme initially designed for clear waters that are black in the near infra-red. Firstly, a detection of Case 2 turbid waters is based on radiometry (reflectance corrected for stratospheric aerosol, gaseous absorption, sun glint, Rayleigh), The algorithm is based on optical properties of the water and performs an iterative procedure with a combination of (a) single scattering aerosol reflectance and (b) water-leaving reflectance including the contribution of sediments. It iterates until the model best approximates the measured Rayleigh corrected reflectances at b709, b779, b865 and b885.An estimate of the total suspended matter used internally allows identification of sediment-dominated Case 2 waters through a dedicated flag.
- Aerosol and Rayleigh correction. The aerosol type and load are estimated from the NIR bands at 779 and 865 nm once contributions from the water at those channels are known. The Rayleigh (molecular scattering) contribution is determined from tabulated results of radiative transfer computations, corrected for local variations in atmospheric pressure (impacting the amount of scatter). The aerosol type and load are determined together using the level of signal (marine contribution deduced) at 865 nm and the spectral slope between 865 nm and 779 nm by identifying the set of predefined aerosol models that best fit the measured data. A mixture of the closest models is then determined to exactly match the measurements and the correction can take place using tabulated values for the same two models and mixtures at all other channels.
Note: steps 2 and 3 above account for the spectral smile i.e. the in-FOV variation of the central wavelengths of OLCI channels, within the so-called "pressure adjustment" that adjusts the Rayleigh optical thickness according to actual channels central wavelength (smile), local atmospheric pressure conditions, and latitudinal variation of the Rayleigh optical thickness at standard pressure.