You can see satellites during the day

 
Radar is an active system, i.e. it illuminates the earth and then measures the reflected signal
Radar technology
radar
 
Radar is widely used in air traffic control systems to control aircraft in all weather conditions. Most ocean-going vessels have radar systems on board. All of these radar systems measure distances and relative angles to determine the position. They can detect specific objects, but not recognize them. A special type of radar system is required to generate images.

Airplanes and satellites are equipped with such systems, such as the ERS satellites, which are built and launched by the European Space Agency. The on-board radar unit takes detailed images of the earth's surface.

Radar is an active system, i.e. it illuminates the earth and then measures the reflected signal. So you can take pictures during the day or at night completely independently of sunlight. This is particularly important in high latitudes, as the long polar night completely prevents the collection of all data by traditional satellites for six months a year. In addition, radar signals can easily penetrate clouds, so images can be captured regardless of temporary weather conditions.

Radar images look similar to photos, but they are evaluated completely differently! We want to see how much they differ:

First: Radar systems are active systems: the scene to be “photographed” is illuminated - not with light, but with electromagnetic signals of a certain microwave length. Microwave images provide information about the geometric and dielectric properties of the surfaces or volumes examined. These are mainly determined by the surface roughness (see below), the type of material (e.g. iron, concrete, wood, organic substances) and the moisture content.

Satellites can be equipped with radars or optical sensors to record various types of images. Optical images differ from radar images in that they do not work at night and cannot see through clouds.
 

Colored radar images can be generated by taking three individual images of different dates
Second, radar images are black and white because they only use a single electromagnetic wavelength of 5.3 cm.

Colored radar images can be generated by combining three individual images of different dates (e.g. at intervals of 35 days) to form a multitemporal composite image. Each date / picture is displayed in one of the three colors that make up all color pictures, i.e. red, green and blue. The different amounts of color of each date combine to form other colors that can be evaluated by experienced image processing experts.
 
 

Radars "see" differently than the human eye
Third, radars "see" differently than the human eye. Imagine standing on the beach and looking out at the water. The water can be smooth or swell (depending on the wind speed). For sensors, however, the more agitated water surface increases the reflection of the detected microwave energy, and the water therefore appears brighter (whiter) on the image.

Once the radar has emitted pulses of microwaves, it measures the strength with which the object reflects the signal. This is known as backscatter. The more agitated the sea, the stronger the backscatter and the brighter the picture. (Look at the green arrows in the picture above. Their size tells you how bright the picture is). Without a swell, the sea would appear black in the black and white image, while windy or stormy seas would be light because of the height of the waves. Because ships are made of metal and have numerous right angles that reflect microwave energy better than water, they also appear as bright spots. As we see in the other exercises, other factors naturally also play a role.
 
 

Landscapes are interpreted in a very similar way. Cities are surfaces with particularly great roughness and usually appear very bright. Individual houses are depicted as luminous dots (strong reflection), provided they are not too close to each other Radar sensor on board the ERS-1 satellite).
 
 
The areas cut down by humans can be seen as colored rectangles. The city of Porto Velho (Brazil) is shown in white.
Forests also look bright with fairly high, even shades of gray. In contrast, pastureland is a smooth surface that looks dark on radar images. (The illustration shows part of the Amazon jungle near the Rio Branco in Brazil. The man-made deforestation can also be seen as a dark, rectangular area).
 
 
Runways and highways are even smoother and appear almost black. The illustration opposite shows the radar satellite image of a cultivation area in the Netherlands, on which a road network can be clearly seen as a dark linear representation. You may also see a thin, light line in the picture. Can you guess what it is? Of course, a railway line made of iron that strongly reflects microwave energy.
 
 
Applications for radar images
 
What can radar satellites do in practice?

Radar satellite data helps monitor our environment around the clock and in bad weather when other satellites are not working.

Along the coasts and on the high seas, these satellites can be used to detect oil spills long before they reach the coast. In shallow water, radar data shows the topography of the sea floor. This is used to map dangerous, hidden sandbanks and to create maps of the sea depths.
 
 

These satellites can also be used to measure the swell of the sea (for use in weather forecasting) and to point out safer and more economical sea routes for ships.

The engineers on oil rigs also need this type of information, first to plan the construction work and later during operation, to ensure the safety of many people who work on these artificial islands in the middle of the sometimes very stormy sea.

Radar data provided by satellites is also valued by icebreakers making their way through the Arctic or Antarctic pack ice. They use the images to find an optimal track even at night or in fog and to avoid the ship getting stuck in thick ice.
 
 

On land, radar images are used to monitor floods and are also useful in rescue operations (of course, floods usually occur in very wet weather, when clouds prevent other satellites from seeing the ground or aerial photography by airplanes).

Using special technology (called radar interferometry) that can measure small earth movements, one can monitor areas where there is a high risk of landslides, earthquakes or even volcanic eruptions. Warnings can be triggered on the basis of such measurements. A similar technique can be used to create digital maps from satellite images.
 
 

In the tropics, the sky is often cloudy, so you can only take pictures of the earth's surface with radar satellites. In agriculture, especially when predicting the yield of the rice harvest, radar satellite images are often used. You can see the fields clearly on the picture, measure their size and thus estimate the yield of the rice harvest.

Radar monitoring is necessary for the protection and also for the sustainable use of the forest. Satellites can even detect small clearings. The technology thus provides the ability to monitor any logging or deforestation activities.

Radar satellites such as ERS and ENVISAT not only contribute to an understanding of our environment on a global and local level, but also to the protection of the environment.