Sentinel-2 Level-3A time series

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April, despite a certain amount of cloudiness, once again offered us a beautiful, cloudless synthesis. In fact, Theia's Level 3A products use 46 days periods centered on the 15th of the month, and the good weather of the last days of March or the first days of May compensates the cloudy April. As every month, Peter Kettig from CNES produced the Level 3A syntheses from the previous month's Sentinel-2 data. As you can see, a column of tiles is degraded in western France. This is due to a problem during the download of Sentinel data by PEPS. We're going to reprocess it.

Translated with www.DeepL.com/Translator

 
The full resolution data, and the corresponding data quality masks, can be downloaded from Theia's distribution server at CNES.
 
If you are not afraid to spend too much time while you have urgent things to do, you may have a look to the mosaic of Sentinel-2 monthly syntheses for each month since July over France. Each monthly synthesis is accessible using the following links :

Or you may also use the nice viewer below (merci Michel Lepage !) to compare with the previous months.

See it full screen
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Spot the odd one out

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There is an odd image in this time series of L2A products of the 31TCJ Sentinel-2 tile (Toulouse region). Can you guess which one ?

 

Yes it is the last one, acquired on the 26th of February. But what's odd with it ?

  • The black South East corner ? No, just a different orbit
  • The date ? Close enough...
  • Two images separated by one day (25 th, 26 th of January) ? You're boiling !

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WASP source and executable are now openly available

WASP (Weighted Average Synthesis Processor) is the tool we use to compute the nice (mostly) cloud free syntheses of Sentinel-2 surface reflectances, as shown in the images below. A full resolution viewer is also available in this post, or you may also download the products you can download from Theia. As promised (but it took a while to get the allowance), we have just released it as an open source software and we are also providing a compiled version for linux platforms.

The WASP method was developed at CESBIO by O.Hagolle, in 2007, during the preparation of the Venµs mission. It then evolved and improved with the help of several persons at CESBIO (V.Debaecker, M.Huc, D.Morin, M.Kadiri). Then an operational version was developed by CS Romania within the Sen2Agri consortium funded by ESA, which is distributed as open source. WASP was finally adapted to work in Theia context, and improved by P.Kettig. Peter also set up distribution of the software.

So finally, here is how you can download the software :

  • the source code is available within CNES github repository. You will also find there a forum (issues) and a readme file to compile and use the software.
  • but P.Kettig also compiled an executable version (which is tested on Redhat and Ubuntu), which is available from CNES software distribution server. Until now, only a couple of users have used it, so in case of problems, please open an issue on the github platform.

 

Satellite detection by satellite

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Airplanes largely disrupt our remote sensing images, because of the ice contrails they leave behind them, which often turn into cloud cover. We had to set up a method for detecting and correcting aircraft contrails.

LANDSAT 8 image acquired over Paris on 14/04/2013. On the left, RGB color composition, on the right, image of the 1.38μm band. Given the number of traces of planes,  we might have to choose between flying or observing the earth.


But a new nuisance is appearing: the satellites themselves. More than 4000 satellites orbit around the earth, and with the nanosatellites mode, launches of space objects have multiplied. 450 new objects appeared last year, more than 500 are expected in 2019. As most of these satellites are launched in low orbit, between 400 and 600 km altitude, they orbit between our favorite observation satellites and the Earth.

 

And the future is quite worrying (generally speaking, the future is more worrying than the past): according to my colleague from CNES, Christophe Bonnal: "The US company One Web has the ambition to deploy 600 satellites within three years three to offer broadband internet access from space. Several companies have similar projects in drawers : Boeing has announced the sending of 2400 satellites, Samsung sits at 4000, while Elon Musk speaks bluntly of 12,000 spacecrafts ".

 

The 12,000 satellites in the Starlink constellation would be located at 3 different altitudes (340 km, 550 km and 1,200 km). Two of these altitudes will therefore be visible from the Sentinel-2 orbit. And already, the company Planet has about 200 satellites at an altitude of 400 km.

 

Given the large number of satellites, I wondered if it was possible that the images of Sentinel-2 were disturbed by the presence of satellites located a little lower. The possibility is quite high, because finally, most optical observation satellites seek to make their observations around 10:30 in the morning. With a good orbit propagator, and thanks to Norad's data, it's pretty easy to find the moments when one of the Sentinel-2 passes over one of the Planet satellites just below. And with that information, accurate to a few tenths of meters, we can search for the satellite in the image

 

Here are three examples of results obtained, two on very recent images, and one older. Although the planet satellites are quite small, their metal surface reflects the sun well and therefore leaves a visible mark on Sentinel-2 images.

 

Sentinel-2 image from March 28th, over China. The satellite can be seen within the red circle. See the zoom on the image on the right. The bright point, just right of the image center is satellite Planet Flock 1C-11,
 

Sentinel-2 image from March 27th, over Spain. The satellite can be seen within the red circle. See the zoom on the image on the right.

The bright point, just right of the image center is satellite Planet Flock 3R-8. On both images, my computation predicted it would be in the image center. There must be a bias.

This interesting case was observed just after Flock 3P launch with PSLV on the 12 the of January 2018. Three satellites, whose obits a re still quite close, can be seen just left of the image center.
 

(Sentinel-2 image observed over New Caledonia, on 13th Janury 2018.)

 

Artist view of Flock 3P launch from PSLV on January 12th.

 

Well, the phenomenon is still modest, thanks to the small size and low orbit of Planet satellites, but if large constellations are launched at an altitude closer to that of Sentinel-2, they can cover many pixels.Will we have to resort to the technique recently developed by the indian government to  avoid multiplying white spots on our images ?

 

And of course, this text was published on the first of April, and the "satellites" shown in the images are just white spots, probably not satellites, but who knows :)

 

[MUSCATE news] back to nominal production

Good news, MUSCATE is back to nominal production !

The source of the issue (archiving the produced data) is not solved yet, but it has been mitigated to allow to come back to real time production. A big thanks to the exploitation team and to the CNES computing center who gave us extra disk space to store the files we do not send immediately to the archiving facility.

 

 

[MUSCATE news] Slow production / Production ralentie

Following the installation of MUSCATE version 2.5, the production has resumed, but very slowly. It seems to be hampered by the archiving of data, which takes too much time. This causes traffic jams in the scheduler, an the team has no other solution right now than reducing the throughput of MUSCATE.We hope to solve this issue very soon.

We apologize for the delays due to this issue.

 

Après l'installation de la version 2.5 de MUSCATE, la production a repris, mais très lentement. La production semble être limitée par l'opération d'archivage des produits, ce qui cause des embouteillages dans notre orchestrateur. L'équipe d'exploitation a donc du réduire la taille des zones à traiter pour éviter ces plantages.Nous espérons résoudre ces problèmes au plus vite.

Nous vous prions de nous excuser pour les retards actuels de notre production.