Some earth observation satellites are sensing the earth with radar. This is done with a so-called SAR, a synthetic aperture radar sensor. The SAR is an active system and illuminates the earth surface and measures the reflected signal. The advantages are that it can “see” through clouds and does not need sunlight. Therefore radar data can always be collected: day or night, regardless of weather conditions. This makes radar satellites very suitable for monitoring and surveillance applications.
A radar satellite “sees” the earth surface different from how the human eye would see it. In fact a radar “feels” the earth surface. It is sensitive for surface roughness. The rougher the surface the more (reflected) radar signals it will measure. Because of that oil pollution (which makes the sea water less rough) can be detected with radar satellites. Also radar data is used for flood mapping, illegal fisheries, ice monitoring and agriculture.