What do satellites look like
How satellites see
Since the 1970s satellites have been used which were developed to observe the earth's surface and which are equipped with special sensors for this task.
Today several hundred satellites orbit our planet, many of them for earth observation - equipped with increasingly precise technology. Geoscientists receive important data from them with which they can register environmental changes over long periods of time, monitor large-scale events on the earth's surface and derive models for the future from this.
It is astonishing that satellites can still see anything at all from a distance of several hundred kilometers on the earth's surface. And otherwise her view of the earth is very special and a fascinating matter.
Beautiful colorful world
The world presents itself to us humans in the most beautiful colors. The amazing thing is: the brain mixes this colorful world from just three color impressions. The retina in the human eye has receptors for the colors blue, green and red. These are just small "windows" within the electromagnetic spectrum of sunlight.
Measured against the bandwidth of this spectrum, which ranges from extremely high-energy gamma rays to very long-wave radio waves, the part of this spectrum that is visible to us is only a tiny fraction. We can no longer perceive even the short-wave ultraviolet and long-wave infrared range, which are only slightly shorter-wave than blue and slightly longer-wave than red.
I can see what you can not see
Earth observation satellites have sensors that can record far more than this small section of the electromagnetic spectrum that is visible to us. Modern earth observation satellites with so-called hyperspectral sensors can register around 200 different wavelengths. Each of these "windows" reveals different information.
For example, the "near infrared" is very important for geoscientists. This is the portion of solar radiation that is not absorbed by the chlorophyll in the plants, but is reflected back very intensively. Green is also reflected by the plants (which is why plants give us this color impression), but we can no longer perceive the infrared - the satellite sensors, on the other hand, can.
With this "near infrared" data, geoscientists can assess the health of forests, for example. This is helpful in assessing the extent of forest damage, such as forest fires or massive clearing. These infrared measurements are also very important for agriculture.
Passive and active satellites
Satellites with sensors that only receive the solar radiation reflected from the earth's surface are called "passive satellites". These include, for example, the satellites of the LANDSAT program of NASA (National Aeronautics and Space Administration), which have been providing images of the earth since the 1970s, or the RapidEYE satellite of the DLR (German Aerospace Center) for several years.
"Active satellites", on the other hand, not only have sensors that can detect radiation - they also actively send signals to earth. For example radar radiation - these are microwaves with a wavelength in the centimeter range.
The radar beams are reflected on the earth's surface and then recorded again by sensors on the satellite. The distance to the targeted object can be calculated extremely precisely from the time difference between "send signal" and "receive signal" and thus create exact height profiles of the earth's surface.
Such radar satellites can also be used to analyze the nature of the ground or to determine the thickness of ice layers. With their "active radar eyes", the satellites can even "see" through clouds and in pitch darkness.
Two radar eyes see more - the earth in 3D
The DLR is carrying out a globally unique radar satellite mission with the pair of satellites TerraSAR-X and TanDEM-X. The two satellites circle the earth in tandem flight and with a stereoscopic radar view from a height of 514 kilometers and only about 200 meters apart.
They have been providing geoscientists with data since 2010, from which a high-precision three-dimensional elevation model of the entire earth is generated.
In 2014, TerraSAR-X and its twin brother TanDEM-X mapped the surface of the earth completely in 3D after several circumnavigations. The terrain model of the entire earth with a grid of twelve by twelve meters and a height accuracy of more than two meters has been ready since 2016.
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