Key Facts - ASAR
Radar (RAdio Detection And Ranging) is an active remote sensing system "active" refers to a sensor that supplies its own source of energy or illumination the opposite to this would be a "passive" system (Lillesand et al., 2004). Gibson et al., (2000) highlights the benefits of using radar imaging :
- Radar being an active sensor does not depends on solar illumination and hence can operate in the night aswell as in the day.
- Radar generally images the same area from different directions therefore increasing the likelihood that all features have been recorded within a scene.
- Cloud cover can be a difficulty when imaging areas with a passive sensor e.g. Landsat however with some radar systems the electromagnetic radiation can penetrate the clouds and image the ground below.
Imagery obtained during the event of Hurricane Katrina provides an example of how information can be obtained from radar data which would not be possible from a passive sensor. Figure 1 below shows how the ASAR radar image illustrates the effects of the hurricane on the sea surface beneath the spiraling clouds captured by the MERIS optical sensor.
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Figure 1 Illustrates two separate views of Hurricane Katrina acquired 28 August 2005 from instruments aboard Envisat. The ASAR Wide Swath mode radar image (active sensor) of the sea surface shows how Katrina's wind fields are rippling the ocean. Beside it is the MERIS Reduced Resolution mode optical images (passive sensor) showing characteristic swirling cloud patterns around the central eye, with the eye walls visible (ESA, 2005).
