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 :)

 

Sentinel-2 Level-3A time series

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This year, good weather came back earlier, and the synthesis of February can therefore be shown, event if the low sun elevation does not help. As every month, Peter Kettig from CNES processed the Sentinel-2 L3A composites of France from the Month before. And I am very pleased to announce that thanks to his brilliant work, Peter has now been hired by CNES on a permanent position.
 
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.

 
In February 2019 the weather was nice, hot and dry. Sentinel-2 was able to acquire several cloud free images in nearly all regions In February, snow is abundant, winter crops are green, and deciduous forests are brown. That's a normal february image, except for the clouds. Speaking of clouds, a low cloud escaped the detection within MAJA just North of Bourges. A good reminder to go on improving our methods. MAJA 3.3 should improve the results, and it's nearly ready. If you zoom on the snow covered regions, you will see atefacts, which come from WASP. We know how to improve them, we just need to find some time. And finally, because of the low sun elevation, directional effects are increased and our correction model is not perfect. Some swath edges are visible in the West of France. But all in all, it is a good February synthesis.

 
In November, in France, we had a... French November weather, and several zones stayed overcast for all Sentinel-2 overpasses during the synthesis period of 45 days. In that case, we try to provide a value, which is the minimum reflectance in the blue band. Of course this value is flagged as invalid. So the November synthesis is not as nice as the previous ones, due to the presence of remaining clouds. As in October (see below), we now also see artefacts at the edges of the swath.

Anyway, in many regions, the results are rather correct and they allow us to see the changes. Forests are now brown, soils are wetter and darker, winter crops have started, and the highest mountains are turning white.

 
In October, we had the first the opportunity to observe a neat swath edge effect in four months, near Cambrai, North of France. The Western part of the artefact is browner than the Eastern part. Because of the cloud cover, the average date used in the eastern part is several days before the average date of the western part. , due to the observation at very different dates on each side of the swath. So the only way to improve that with the current method would be to add a third or even a fourth Sentinel satellite.
 

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[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.

 

 

Sentinel-2 Level 3A products : syntheses or composites ?

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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.

Comparison of a synthesis obtained with WASP + MAJA, with a composite product from S2GM + Sen2cor, on the Toulouse region, in October 2018. (Click on image to enlarge)

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.