Heureusement que les biologistes animaliers ne sont pas censés passer sur ce blog, il pourriaent être choqués par ce qui suit.
Bon, expliquons nous !
En rentrant de nos vacances dans les Alpes, nous avons passé une journée près des gorges du Verdon, le grand Canyon européen (je sais, il y a une différence d'échelle avec l'Américain). Ceci dit, le paysage était superbe, comme vous pouvez le voir dans le panorama ci-dessous.
Mais ne vous inquiétez pas, je ne vais pas vous parler ici de mes aventures estivales, je réserve cela pour les collègues à l'heure du repas, et je pense que les deux premiers de la semaine leur ont suffi.
Allons au fait, j'ai par la même occasion remarqué un grand nombre de pins desséchés, voire morts, ce qui m'a étonné parce que la région a été relativement humide ce printemps et début d'été. J'ai demandé au gens du coin, qui m'ont dit que c'est la sécheresse de l'été et l'automne 2017 en Provence qui en est la cause, particulièrement dans les zones où la roche est juste sous la surface et la profondeur du sol est très faible..
Coming back from our stay in the Alps, we stayed one day near the Verdon Gorges, the European Grand Canyon (I know there is a difference in scale with the American one). Anyway, the landscape was gorgeous as you can seen on the panorama below.
But don't worry, I will not tell you about all my holidays adventures, I keep that for my close colleagues at lunch time, and it seems they have had enough of it with the two past days.
Let's go to the point, I also noticed quite a number of brown pine trees, which surprised me as the spring and summer had been rather wet for the region so far. I asked some locals who told me it was due to the severed drought that happened last year in Provence, which happened during summer and fall 2017 and damaged mainly the pine forest, particularly where the soil root zone is thin due to the presence of rock.
After a very difficult period, and thanks to the installation of new improvements MUSCATE ground segment is back in shape, and margins have been found to add new sites.
We have added two new zones where Sentinel-2 data are processed to Level 2A, which provide surface reflectance after atmospheric correction and a good quality cloud mask, tanks to MAJA processor. These two sites are Lebanon and Telangana region in India. The data are processed in near real time, since May 2018, and we will later on add the acquired before that date.
As usual, the data are available for download from https://theia.cnes.fr
You might have noticed the apparition of the first Venµs L2A products on Theia web site within the real time production, since last Friday. A first global processing will start this summer, to provide you with the data acquired from November until now. There will be probably further reprocessings to benefit from the fine tuning of all the parameters and to propagate the further evolution of Level 1 improvement.
Even if it took us a few months to check the software and set the parameters up, what took us very long... was waiting for the level 1 validation and calibration phase. As you know, our colleagues from CNES did a great work to rescue the Venµs raw data which were full of surprises. They started to provide us with calibrated products in April only, and that's when we started the validation.
We were quite happy with the first results, as our processor MAJA did not show any bug, and the first images looked good. But the first validation results were quite poor, with undetected thin clouds, with biases in the estimates of atmospheric properties (Aerosol, water vapour), as well as biases in reflectances (with a lot of negative values). We then started iterating tests on the parameters, and after several iterations we corrected several errors in the parameters (Venµs band numbers are different from those of Sentinel-2, and in a couple of cases, I forgot to change them:( ), and we tuned better all the thresholds. Among those, we had to change the calibration of band 910 band by 6% (this band is hard to calibrate in flight due to the presence of water vapour and is also affected by some newly discovered stray light).
The following table compares the results we had initially, on the left, and the results obtained after tuning the parameters, on the right. Of course, what we distribute is on the right ! We will of course need to increase the number of validation points, and we expect that the low level stray light in band 910 that was discovered during the commissioning phase and is not yet corrected will introduce some site related bias in the water vapour estimates. We will therefore need a reprocessing after this defect has been fixed, if the Level 1 team finds a way to fix it. And finally, we have still some issues to solve with the shadows mask which can often be quite poor.
|Before tuning||After Tuning|
RGB Quicklook with cloud mask contour
RGB Quicklook with cloud mask contour
Water vapour in g/cm2 compared with Aeronet
Water vapour in g/cm2 compared with Aeronet
Aerosol Optical Thickness compared with Aeronet (sorry for the scale different from that on the right)
Aerosol Optical Thickness compared with Aeronet (sorry for the scale different from that on the left)
The Theia workshop for Sentinel-2 L2A MAJA products was held in Toulouse on the 13th and 14th of June 2018.
About 80 people participated either on the 13th or 14th, and nearly 70 participants attended each day of this workshop, whose object was to collect feedback and share experiences on the quality, use and applications of the L2A surface reflectance products delivered by Theia from Sentinel-2 data.
Until V0.9, only a small proportion of Venµs L1C data were available at L1C level on Theia's website. Now, with the version V1.0, a greater proportion of products will be distributed to users, but we advise users to check their geometric quality in the metadata, as explained below.
Distribution of V1.0 products should start very soon !
The MUSCATE ground segment that delivers Sentinel-2A products has had several issues these past two weeks. We have had some performance issues with the data base which tells MAJA processor which product must be processed next. These lower performances caused traffic jams and disruptions in the data production, and the system crashed quite frequently. Because of these crashes, our real time production of L2A and snow products has currently a delay above one week for most sites.
The teams are working hard to find the roots of these issues and correct them. We are very sorry for the inconvenience this may cause to your applications with good Level2A data or snow products.
The Sentinel-2 Level 2A data which are being produced now have a new version number: version 1.8. The novelty with these products is the cirrus clouds correction which was implemented within MAJA thanks to our collaboration with DLR (the German aerospace agency) and thanks to ESA funding.
The quality of the correction is generally good, but depends on the cirrus cloud thickness, and does not correct for the underlying shadow. As a result, we only apply the correction where a cirrus cloud has been detected, and we keep these pixels flagged as cloudy, even if the correction is good. For now, we advise you to use these pixels only after having checked visually the correction. If you are processing data automatically without any visual control, our advise is to go on discarding these pixels, except if you do not need a high accuracy.
We have just updated the MAJA/THEIA workshop website to add a draft program. The workshop will be held in Toulouse, from 13th to 14th of June, and will be hosted by the ENSEEIHT engineering school in Toulouse historical center. Registrations are still open, until the third of June.
The aim of this workshop is to collect feedback and share experiences on the quality, use and applications of the L2A surface reflectance products delivered by Theia from Sentinel-2 data.
The meeting objectives are as follows :
We look forward to meeting you soon !
The organising Comittee (Arnaud Sellé, Olivier Hagolle, Céline Arnal)