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Radar technology 
Radar are commonly used in air traffic control systems 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 recognise, specific objects. To produce a radar image, a special type of radar system is required. Radar are flown in aircraft and on satellites, such as the ERS satellites. The radar onboard obtains detailed images of the Earth's surface. The radar is an active system, which means that it illuminates the Earth's 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 prevents traditional satellites from getting any data for six months of the year. Moreover, the radar signal easily penetrates clouds, so that 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. 1. Radars are active systems: the scene to be 'photographed' is illuminated - not with light but with electromagnetic signals of a specific microwave wavelength. Microwave images provide information about the geometric and dielectric properties of the surface or volume studied, which mainly depends on the roughness of a surface (see below), the kind of material (eg iron, concrete, wood, organic) and its moisture content.
Satellites can carry radar or optical sensors, acquiring different kinds of images. Optical images contrast with radar in that they do not work at night and cannot see through clouds.
Colour radar images can be produced by combining three single images from different dates (e.g. 35 days apart) to produce a multi-temporal composite. Each individual date/image is displayed in one of the three colours used to form any colour picture, that is red, green and blue. The different amounts of colour from each date combine to produce other colours that can be interpreted by experienced image processors.
Once the radar has emitted a microwave signal, the power with which an object reflects 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. Ships also appear as bright points since they are made of metal and have a lot of right angles that reflect the microwave energy better than water. There are of course other factors involved, as we'll see in some of the other exercises.
But in terms of practical use, what can radar satellites do? Radar satellite data help us to monitor our environment, 24 hours a day and in bad weather conditions, when other satellites are unable to operate.
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 dangerously hidden sandbanks and to make charts of sea bottom topography.
Similarly, oil rig engineers need this kind of information, first to plan construction work, and later, to help safeguard the many dozens of people who work on these artificial islands in the middle of a sometimes very stormy sea.
Radar data from satellites are also very much appreciated by icebreakers, on their way through Arctic or Antarctic sea ice. They use the images to find an optimal track which will help them avoid dangerous thick ice that may trap their ship - even at night and in fog.
Areas at high risk of landslides, earthquakes and volcanoes can also be monitored, using special techniques, one of which is called interferometry, which allows us to measure small Earth movements. Such measurements are used as indications to issue warnings. Using a similar technique, digital maps can be generated from satellite data.
To protect but also exploit the forest in a sustainable manner, radar surveillance is necessary. Satellites can spot small clearcuts and in this way every logging or deforestation activity can be monitored.
Finally, radar satellites like ERS and Envisat not only help to understand, but also to protect our environment on a global and on a local scale.
Last update: 6 April 2011
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