Radar is commonly used in air traffic control to guide aircraft in all weather conditions. Most sea-going ships have radar onboard. All these radar are used to measure distances and relative angles for positioning. They can detect, but not recognize, specific objects. To get a radar picture, imaging radar were built.
They are flown in aircraft and on satellites. The ERS satellites built and launched by the European Space Agency are amongst them. The radar onboard obtains detailed images of the Earth's surface.
It is an active system, which means that it illuminates the Earth surface and measures the reflected signal. Therefore, images can be acquired day and night, completely independent of solar illumination. This is particularly important in high latitudes, where the long polar night means that traditional satellites get no data at all for six months of the year. Moreover, the radar signal easily penetrates clouds, and images can be acquired regardless of current weather conditions.
Radar images look similar to photographs but their interpretation is very different! Let us see how different:
First: Radars are active systems: the scene to be "photographed" is illuminated - not with light but with electromagnetic signals of a specific wavelength, with microwave. Microwave images provide information about the geometric and dielectric properties of the surface or volume studied. All this mainly relates to the roughness of a surface (see below) and to the kind of material of this surface (iron, concrete, wood, and also to how wet or dry the surface is).
Apart from radar, satellites also carry optical sensors, which obtain a different kind of image. In contrast to radar, these do not work at night, and cannot see through cloud.
Second: Our Radar images are merely black and white since they are operated in a unique "colour" or electromagnetic wavelength. It is microwave of 5.3 cm wavelength.
Colour images can be produced by combining different single shots to a multi-shot, giving each a different colour Usually the three colours used to form a colour picture are red, green and blue. In this way a multi-temporal image can be produced. It is formed by 3 images taken at different times (e.g.35 days apart) from the same area.
Third: What a radar sees is different to what our eyes observe. Imagine that you are at the beach and you look out into the water. Sea or lake surfaces might be smooth because of little or no wind, or they might be rough, with waves - high waves, depending the speed of the wind. This is exactly what the radar can observe: surface roughness.
Once the radar has emitted a microwave signal, the power reflected by the objects hit by the signal is measured. This is called the backscatter. The rougher the sea, the higher the backscatter and the brighter the image (have a look at the green arrows in the figure above, their size indicates the brightness of the image). A calm sea would appear black in the black-and-white image, a windy or stormy sea would be bright, due to the height of the waves. But also ships appear as bright points since they are made of metal and have a lot of right angles. There are of course other factors involved, as we'll see in some of the Exercises.
The interpretation of images of land is in many ways similar. Cities are very rough surfaces and appear generally very bright. Single houses are visible as bright points (high reflection), if not too densely built all together (the second image above shows a color composition of the city of Bucharest as seen from the spaceborne radar onboard the ERS-1 satellite).
Forests are also bright features, with gray levels being rather high and even. On the other hand, grassland are smooth surfaces and appear dark in the radar image (The right figure shows part of the Amazon forest close to the Rio Branco in Brasil; man-made deforestation is also visible as dark rectangular areas).
Even smoother are runways and highways, which appear nearly black. The accompany figure shows a radar satellite image of a cultivated area in The Netherlands, where a network of roads is clearly visible as dark linear features. But you may also find a fine bright line in this image. Guess, what is it? Of course it is a railway - made of iron, reflecting microwave strongly.
But practically speaking, what is the use of Radar satellites?
Radar Satellite data help us in monitoring our environment, even in the night and in bad weather conditions, when other satellites become blind:
Along the coast and on the open sea we can use these satellites to detect oil spills, long before they reach the coast.
In shallow waters radar data shows sea currents. This is used to map sandbanks dangerously hidden and in general to make charts of the sea bottom topography.
Moreover such satellites allow us to measured sea waves, used in weather forecasts, and to route ships safely and more economically on their journey.
Similarly, oil rig engineers need this kind of information, firstly for the planning of their construction and later in operation, when many dozen of people work on those artificial island in the middle of a sometimes very stormy sea.
Radar data from satellites are also very much appreciated by icebreakers, on their way through the Arctic or Antarctic ice. They use the images to find an optimal track avoiding the danger of getting stuck in too thick ice - even in the night and in fog.
On the land, radar data are used to monitor floods, and is used in relief operations (of course, floods usually happen in very wet weather, when clouds may make other satellites and even aircraft useless). Areas at high risk from landslides, earthquake and even from volcanoes can also be monitored, since special techniques (called interferometry) allow us to measure small earth movements. Such measurements are used as indications to issue warnings. With a similar method digital maps can be generated from satellite data.
In the tropics the sky is very often clouded and therefore only Radar satellites can see the Earth surface. In agriculture, especially for rice yield predictions, images from Radar satellites are also applied. Fields are clearly visible on an image and their dimensions can be measured and subsequently the amount of rice to be harvested estimated.
To protect but also exploit the forest in a sustainable manner, radar surveillance is necessary. Satellites can spot already rather small clearcuts In this way every logging or deforestation activity can be controlled.
In all, Radar satellites like ERS and in the near future ENVISAT help not only to understand but also to protect our environment on a global and on a local scale.