QAA Processor

Overview

This processor is used to derive absorption and backscattering coefficients for open ocean and coastal waters of MERIS images. The Quasi Analytical Algorithm (QAA) has been originally developed by Lee at al. It is based on remote-sensing reflectances models and uses therefor MERIS Level 2 data as input.
An valid pixel expression is used to determine which pixels are processed. This expression is set by default to 'l2_flags.WATER'. This means that only pixels determined as water are processed, other pixels (land, clouds) will be assigned a No-Data value (NaN) to all bands. Also, only bands below 670nm are used in the processing. In the case of MERIS band settings, the first 7 bands are used in the processing.
Besides MERIS Level 2 data it is also possible to process atmospherically corrected MERIS Level 1 data. It is only required that the reflectance bands follow the same name pattern as in MERIS L2. The expression to detect valid pixels can be changed as necessary for the specific product.

Coefficient Bands

The following absorption and backscattering coefficients are generated at different wavelengths:

• a_total - The total absorption coefficient of all water constituents is given for the wavelengths at 412, 443, 490, 510 and 560.
• bb_spm - The backscattering of suspended particulate matter is given for the wavelengths at 412, 443, 490, 510 and 560.
• a_pig - The pigment absorption coefficient is given for the wavelengths at 412, 443 and 490.
• a_ys - The yellow substance absorption coefficient is given for the wavelengths at 412, 443 and 490.

Flag Band

The Quasi-Analytical Processor provides a flag band with the result product. It is named 'analytical_flags' and contains flags specific for the Quasi-Analytical Processor..

• normal - Valid water pixels.
• imaginary_number - Pixels that are classified as water, but an imaginary number would have been produced in step one of the algorithm.
• negative_a_ys - Pixels that are classified as water, but one or more of the bands contain a negative 'a_ys' value.
• non_water - Pixels that are not classified as a water (land/cloud).
• a_total_oob - At least one value of the 'a_total' spectrum is out of bounds.
• bb_spm_oob - At least one value of the 'bb_spm' spectrum is out of bounds.
• a_pig_oob - At least one value of the 'a_pig' spectrum is out of bounds.
• a_ys_oob - At least one value of the 'a_ys' spectrum is out of bounds.

Further Information

QAA Algorithm Specification

QAA Processor Documentation