The QAA-RGB: A universal three-band absorption and backscattering retrieval algorithm for high resolution satellite sensors. Development and implementation in ACOLITE

We present the "QAA-RGB", a semianalytical algorithm that retrieves total absorption, particle backscattering, as well as the diffuse attenuation coefficient and the Secchi disk depth, from eighteen metre and decametre satellite sensors, including present and heritage Landsat's data, Sentinel 2 at 10 m and an array of commercial satellites such as PlanetScope, Pleiades and Worldview. The QAA-RGB is a minimal version of the Quasi-Analytical Algorithm (QAA), and therefore keeps its robustness and general validity across different water types. It ingests remote-sensing reflectance at only three bands, centred on red, green and blue wavelengths. Retrievals of non-water absorption at 555 nm (a(nw)(555)) are satisfactory, with a standard deviation of sigma similar to 50% up to a(nw)(555) < 2 m(-1), based on in situ datasets. Careful calibration results in unbiased retrievals across all sensors. Validation to independent in situ data is also provided. The QAA-RGB has been implemented in the ACOLITE generic processor, and is openly accessible to the community. Consistency of near-coincident observations from different satellite sensors is demonstrated.