18.104.22.168.6.2 Turbid water screening and corrections (steps 2.6.8, 2.6.10)
This section describes the algorithms used
1. in step 2.6.8, to detect Case 2 turbid waters based on radiometry (reflectance corrected for stratospheric aerosol, gaseous absorption, Sun glint); in the process, Rayleigh reflectance above water is computed;
2. in step 2.6.10, to compute the water-leaving reflectance for Case 2 turbid waters at 510, 705, 775 and 865 nm, needed before entering the atmospheric corrections processing over Case 1 waters (step 2.6.9) and provide an estimate of the total suspended matter used in turn to identified sediment dominated case 2 waters through a dedicated flag.
22.214.171.124.6.2.1 Rayleigh correction 1 (step 126.96.36.199)
This correction requires to estimate the Rayleigh reflectance in all useful bands, which will be re-used for further ocean pixels processing. The Rayleigh reflectance is interpolated in a LUT (RD 7, 3.6.1) as a function of pixel geometry, wind speed and wavelength.
The Rayleigh correction consists in subtracting the Rayleigh reflectance, corrected for atmosphere pressure in this step, from the total reflectance (already corrected for stratospheric aerosol, gaseous absorption and sun glint) in each band (RD 8, 2.7, 188.8.131.52.2).
184.108.40.206.6.2.2 Turbid water and White Scatterers identification (step 220.127.116.11)
Turbid water identification
The TOA marine reflectance is computed at 705 nm (channel 9) from Rayleigh corrected reflectances at 775 nm (channel 12) and 865 (channel 13) using the Angström exponent method. Then this TOA marine reflectance at 705 nm is compared to a threshold (interpolated in a LUT as a function of geometry). If it exceeds the threshold then the turbid water flag (Case2_S) is raised.
White scatterer identification
An estimate of the spectral slope of marine basckscatter is computed using Rayleigh corrected reflectance and pure water specific absorption. This estimated spectral slope is compared to a threshold below which the White Scatterer Flag is raised.
18.104.22.168.6.2.3 Turbid water correction (step 2.6.10)
When a water pixel has been detected as contaminated by a water signal in the infra-red by test 2.6.8, the algorithm called bright pixel procedure performs an estimate of the water-leaving reflectance at four bands used later by the atmosphere corrections above water (see 8.3 below). The algorithm is based on optical properties of the water and performs an iterative procedure with a combination of:
- single scattering aerosol reflectance;
- water-leaving reflectance;
- Suspended Particular Matter concentration.
The atmospheric attenuation of water-leaving reflectance is taken into account.