Although the validation of level 3 data cannot replace the validation of level 2 data, it may contribute to the estimation of the measurements quality by providing information where in-situ data does not exist, at a global scale almost every day, at daily, monthly, seasonal and annual temporal scales almost everywhere, on the long-term instrument stability and on instrument ageing.

The statistical analysis of the spatial variations of the level 3 data combined with oceanographic knowledge can lead us to a certain level of validation or invalidation of the products. Of course, the huge number of available points in the level 3 products increase the quality of the statistics.

Comparisons of level 3 products between sensors can be also of great interest to estimate the coherency between the couples instrument + processing as well as to characterise some possible inter-calibration between the products, for example before any merging attempt.

The analysis has been applied to the MERIS monthly level 3 products available on the ESA web site (1/12° sinusoidal grid) and the MODIS and SeaWiFS level 3 products available on the NASA Ocean Colour web site (both at 9 km on an Plate-Carré grid). The MERIS products have been reprojected on the same 9km Plate-Carré grid before any computation (using the Sutherland-Hodgeman area clipping and flux-conserving algorithms).

The following marine parameters are covered by the quality control tasks:

• chlorophyll-a, case 1 water
• normalised water leaving radiances (nLw) at 412, 443, 490, 510 and 560 nm
• aerosol optical thickness over water at 865 nm
• Angstrom coefficient over water at 865 nm

The statistical analysis has been performed at global level and on a set of dedicated zones.