A radar is an active system that illuminates the Earth's surface and measures the reflected signal
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.
2. Radar images are black and white since they are acquired using a unique electromagnetic wavelength of 5.3 cm.
Colour radar images can be produced by combining three single images from different dates
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.
3. What a radar 'sees' is different to what our eyes observe. Imagine that you are at the beach and you look out towards the water. The water may be smooth or there may be waves, depending on the wind's speed. However, for the sensor, the rougher the water surface, the greater the reflection of the microwave energy that will be recorded, and therefore the brighter (whiter) the water will appear in the image.
What a radar 'sees' is different to what our eyes observe
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.
The interpretation of land images 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 together.
Color composition of the city of Bucharest, Romania
Forests are also bright features, with grey levels being rather high and even. On the other hand, grassland are smooth surfaces and appear dark in the radar image.
Human-made deforestation is seen as coloured rectangular areas. The city of Porto Velho (Brazil) is depicted in white.
Even smoother are runways and highways, which appear nearly black. The accompanying 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 identify a fine bright line in this image. Can you guess what it is? Of course it is a railway - made of iron, which strongly reflects microwave energy.
Radar satellite image of a cultivated area in The Netherlands
Applications of radar images
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. Moreover, such satellites allow us to measure 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, 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. On land, radar data are used to monitor floods, and can be used in relief operations. Of course, floods usually happen in very wet weather, when clouds may leave other satellites and even aircraft obselete.
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.
In the tropics, the sky is very often clouded. Consequently, only radar satellites can image the Earth's surface. In agriculture and especially for rice yield predictions, images from radar satellites are often used. 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 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.