MAJA/THEIA workshop Program

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 :

  • to provide information about L2A product status and validation
  • to gather feedback from users about L2A product quality
  • to show applications and results of L2A Sentinel-2 time series
  • to share experiences on how to use the products
  • to collect suggestions for improvements.

We look forward to meeting you soon !

The organising Comittee (Arnaud Sellé, Olivier Hagolle, Céline Arnal)

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.

 

Theia started the distribution of Venµs products in Near Real Time


Great news for our little Venµs satellite project: Theia has started distributing Venµs L1C data in near real time. The data acquired yesterday are already available on https://theia.cnes.fr

 

 

These L1C products are tagged with 0.9 version, which means that their quality is not as good as what we think we will be able to deliver in a couple of months. The multi-spectral and multi-temporal registration can still be improved, even if it is not too far from our expectations yet in most cases. The image quality teams are finishing an error budget of the current situation, and also preparing the improvement with our Israeli partner who manage the satellite.  We will tell you about that.

 

The L2A delivery will also start in a few weeks, but the L1C teams, who had much more work than expected with this satellite, handled us correct products very late, and we still need to tune a few parameters to provide good quality products.

 

On the Theia website, you will find several ways to download the tiles. My little download tool has also been updated. Once you have registered, and updated the config.cfg file, you will be able to download at once all the products in Australia with the following command line :

python theia_download.py -l 'Australia' -c VENUS -a config.cfg -d 2018-01-01

The data format is explained here. Its packaging with useless zips and tars is still provisional and will be simplified very soon.

 

 

Let's ask ESA to improve Sentinel-2 multi-temporal registration

Update from May 4th 2018 : ESA has started planning for a global reprocessing in 2019

 

The information gathered by Sentinel-2 system on Sentinel-2 orbit, attitude, date accuracy, and viewing directions of all detectors allows an excellent accuracy for the geolocation of all Sentinel-2 pixels. The overall geo-location accuracy is better than 11 or 12 meter, for about 97 % of the cases, which is about the size of one Sentinel-2 pixel. Such a performance is more or less equivalent to that obtained for Pleiades, but Pleiades has a resolution of 0.7m. It is therefore really an achievement, which is  to be credited to ESA, to the satellite and instrument manufacturer, and to the image quality teams (including my CNES colleagues).

 

But even if it is excellent, it is not enough. The standard need for multi-temporal registration errors is 0.3 pixels, and the current performances show that for more than 50% of the cases, the performance does not meet that requirement.

From Sentinel-2 data quality report

 

 

For many  users, I guess, these figures do not mean much, and it is not easy to figure out their impact on real life applications. The animation below (made by the twitter star Simon Gascoin) makes that much more concrete :

Time series of Sentinel-2 images of the construction of Nour solar power station near Ouarzazate in Morocco. (made by Simon Gascoin using Sentinelhub)

 

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MAJA now corrects thin cirrus clouds on Landsat-8 and Sentinel-2 images

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Thanks to our collaboration with  DLR,, who developed the method [1], and with an ESA funding, we added within MAJA a correction for thin cirrus clouds that works for LANDSAT 8 and Sentinel-2. This correction uses the cirrus band  at 1.38 µm, which allows to estimate cirrus reflectance, which is then subtracted from the other bands with an empiric factor derived from the images.

 

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