For the past few months, Copernicus has been distributing Level 3A products for the Sentinel-2 mission as part of the S2GM (Sentinel-2 Global Mosaics) .
This ambitious project aims to provide monthly, quarterly or annual Level 3A products worldwide. The user defines his zone and period of interest and orders the product. The web site seems to be hosted by SInergise, and as everything done by this company, is easy and very straightforward to use.
The call for tender, with two or three million euros, was launched by the Copernicus program of the European Union two years ago. It required the use of ESA Level 2A official products obtained with Sen2Cor. For this reason, we decided not to participate because our Level 3A method, which calculates a weighted average of non-cloudy observations, requires very good cloud masks, which is not quite the case of Sen2cor products.
The tender was won by a consortium of Brockman Consult, Geoville and SInergise companies. To compensate for the poor quality of cloud detection, the authors of the S2GM product had to use a BAP method: "Best Available Pixel". This method chooses for each pixel the best date according to certain criteria (no cloud or shadow detected, minimum reflectance in the blue, maximum NDVI ...). This method minimizes cloud disturbances when clouds are not detected correctly, but also has the disadvantage of suddenly changing the date from one pixel to another, which causes artifacts and noise. Outputs are therefore composite products , which assemble pieces of images acquired for the different dates available over the period.
Theia Level 3 products are not composites, but syntheses, which use all cloudless observations of a single pixel over the entire monthly observation period to find the value that best represents the surface reflectance we would have had at the central date of the product. Theia's syntheses use the WASP (Weighted Average Synthesis Processor) chain, which calculates a weighted average of surface reflectances over a month, after atmospheric correction and detection of clouds obtained from Level 2A products generated by our MAJA channel , of course. If the clouds are badly detected, they enter into the synthesis and disturb it.
The animation provided above shows a full resolution comparison over Toulouse region, of a synthesis of WASP and of the corresponding composite of S2GM obtained on the same date in October 2018. We see very quickly that the composite of S2GM is very noisy, much more than the synthesis from WASP. It is quite often possible to locate the areas where the synthesis tool has chosen to change the date in its composite. You will also notice the appearance of many white dots, which are in fact pixels without clouds, but quite bright that Sen2Cor systematically classifies as clouds.
In short, provided you have a good level 2A product, syntheses can provide much better results than composites.