Satellite detection by satellite



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 :)


Posted under: In English, Sentinel-2

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  • Denis FEURER on 01/04/2019 at 13:25 said:

    Many thanks for your article, which reveals an - unfortunately - future common problem in remote sensing.

    I also had read (but I cannot find the source back) that low-orbit satellites with ultra-precise orbit positioning and specific external coating with controlled spectral reflectance are already used to hinder sensitive sites from being mapped by commercial EO satellites, in a somehow comparable manner to the camouflage used for planes factories during the 2nd World War

      • Denis FEURER on 02/04/2019 at 12:26 said:

        Totally ! Would my supposed source had ever been published, it would certainly have been published on April 1st or on the 29th of Ferbuary of a non-leap year :-B

        Sigh.. These special coatings with controlled reflectance would have been so useful for cal/val !!! :-)


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