[MUSCATE news] Two new sites added to Sentinel-2 L2A production : Lebanon and Telangana (India)

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.

Number of L2A products produced every day.

 

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

[Venµs news] Distribution of Level2A has started

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)

Slides and conclusions of the MAJA/Theia worskhop

The Theia workshop for Sentinel-2 L2A MAJA products was held in Toulouse on the 13th and 14th of June 2018.

 

Attendance on the 13th of June

 

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.

Slides

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[Venµs News] L1C processor upgraded to V1.0

Summary

 

Until V0.9, only a small proportion of Venµs L1C data were available at L1C level on Theia's website: 888 products only in 2 months.

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 !

 

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Are Sentinel-2 water vapour products useful ?

Atmospheric absorption: in blue, the surface reflectance of a vegetation pixel, as a function of wavelength. In red, the reflectance of the same pixel at the top of atmosphere. For a wavelength of 1.38 µm, water vapour totally absorbs the light that comes from the earth surface at sea level. At 0.94 µm (940nm), a weaker water vapour absorption band only partly absorbs the photons.

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Sentinel-2B has two channels centered on water vapour absorption bands: channel 9 (940 nm) and channel 10 (1380 nm). Band 10 corresponds to a very strong absorption, strong enough to prevent any photon to reach ground from the Sun without being absorbed in the atmosphere. This band is intensively used to detect and correct high clouds.

 

In this blog, we discussed much less band 9 (940 nm) yet. Here, water vapour absorption is not strong enough to catch all the photons which reach the surface. The proportion of absorbed photons depends on the water vapour atmospheric content, and also on the viewing and solar zenith angles. We use band 9 for atmospheric correction, but it could be useful to study convection phenomena within the atmosphere too.

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Des applications pour la vapeur d'eau observée par Sentinel2 ?

Absorption atmosphérique. En bleu, la réflectance de surface pour un pixel couvert de végétation, en fonction de la longueur d'onde, en rouge la réflectance au sommet de l'atmosphère pour ce même pixel. A 1.38 µm, la vapeur d'eau absorbe totalement la lumière provenant de la surface au niveau de la mer. Autour de 0.94 µm, une autre bade d'absorption de la vapeur d'eau est visible, mais l'absorption y est moins intense

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Sentinel-2B dispose de deux canaux centrés sur des bandes d'absorption de la vapeur d'eau: la bande 9 (940nm) et la bande 10 (1380 nm). La bande 10 correspond à une très forte absorption, telle qu'en général, dans cette bande, les photons n'ont aucune chance d'atteindre le sol, et encore moins le satellite après réflexion sur le sol. Cette bande est utilisée pour détecter les nuages hauts.

 

Dans ce blog, nous avons jusqu'ici moins parlé de la bande d'absorption à 940 nm. Ici l'absorption des photons passant par la surface terrestre n'est pas totale, seule une forte proportion d'entre eux est absorbée, et cette proportion dépend bien sûr de la quantité de vapeur d'eau, mais aussi de l'inclinaison des directions solaires et d'observation. Cette bande nous sert à la correction atmosphérique, mais pourrait aussi, je pense, intéresser les spécialistes de l'atmosphère.

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MAJA V3.1 will be distributed this May

Example of cirrus cloud correction


We will start distributing MAJA V3.1 this May to replace MAJA V1 on CNES free software platform.

 

It is also in the pipeline of enhancements of Theia processing platform (MUSCATE), but this pipeline is quite full, so we will need to be patient (which requires a big effort for me, patience not being my best quality...)

 

MAJA V3 comes with a lot of enhancements compared to V1 :

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ESA is making plans for a global Sentinel2 reprocessing in 2019 to enhance multi-temporal registration

A good piece of news, directly from ESA's S2 project manager: ESA is now making plans for a global reprocessing of Sentinel-2 archive in 2019. As explained here, Sentinel-2 data multi-temporal registration isn't perfect yet. It should be improved in the first quarter of 2019, thanks to the use of ground control points obtained via automatic matching, but the issue with the reprocessing of Sentinel-2 archive had not been addressed. It is now, but of course not before 2019.

 

I let you imagine the amount of processing required to do so for the complete 3.5 years archive of Sentinel-2 at that time, so it will be costly and require hard work, but yet it is indispensable. Let's thank ESA and Copernicus for considering this and letting us know !

 

[MUSCATE news] Early Sentinel-2B L2A images over France are available

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MUSCATE sentinel-2 L2A counter just reached 80000 products, but a lot more products were processed these past weeks. Indeed, the MUSCATE team started processing the early Sentinel-2B images acquired from July to October 2017, which were missing on our catalog. Indeed. We only started Level 2A treatments in October 2017, in real time (a bit late, but before the ESA;)). We take advantage of this processing to reprocess Sentinel-2A data, since the quality of the products from MAJA improves with the repetitiveness of the observations. And since we replace old versions with new ones (v 1.7), the product counter is not affected by the reprocessing of S2A data.

 

The reprocessing of data on France and its overseas territories is finished, and the treatment of the other European zones is in progress. We will continue soon with the African zones then those of the rest of the world. If you are using the Sentinel-2 data acquired in the second half of 2017, we encourage you to download them again. We took advantage of this reprocessing to also produce snow products in the France area.

 

 

 

 

 

[Nouvelles de MUSCATE] Le traitement au niveau 2A des anciennes données Sentinel-2B sur la France est terminé

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Le compteur de produits de Niveau 2A de Sentinel-2 de MUSCATE vient de franchir les 80000 produits, mais en fait, MUSCATE en a produit beaucoup plus. En effet, l'équipe d'exploitation (Joel, Raquel, Sylvie) a commencé à traiter les données de Sentinel-2B acquises de juillet à octobre 2017, qui manquaient dans notre catalogue. Nous n'avions démarré les traitements de niveau 2A qu'en octobre 2017, en temps réel (avec un peu de retard, mais avant l'ESA ;) ). Nous profitons de ce traitement pour retraiter les données Sentinel-2A, puisque la qualité des produits issus de MAJA s'améliore avec la répétitivité des observations. Et comme nous remplaçons les anciennes versions par les nouvelles (v 1.7), le compteur de produits n'est pas affecté par le retraitement des données S2A.

 

 

Le retraitement des données sur la France et ses territoires d'outre-mer est terminé, et le traitement des autres zones Européennes est en cours. Nous poursuivrons prochainement avec les zones Africaines puis celles du reste du monde. Si vous utilisez les données Sentinel-2 acquises au deuxième semestre 2017, nous vous encourageons donc a les télécharger à nouveau. Nous avons profité de ce retraitement pour produire également les produits neige sur la zone France.