Synthetic Aperture Radar

Slamet Volcano, Java, Indonesia (right)

Synthetic Aperture Radar (SAR) sensors send out a microwave signal and build an image from the radiation reflected back to the satellite. This signal, typically with a wavelength of several centimetres penetrates cloud cover and does not need any solar illumination. This RADARSAT image of Slamet Volcano (3428m) in central Java demonstrates the usefulness for imaging areas in tropical zones, where a cloud free optical satellite image might take months or years to acquire.

The radar signal is transmitted perpendicular to the orbital track, and the distance of the point of reflection is determined from the time for the signal to return to the satellite. But because higher points are nearer to the satellite, they appear on the resulting image shifted towards the satellite track, resulting in a 'leaning' appearance termed 'foreshortening'. Image intensity is significantly dependent upon the surface roughness on the scale of the signal wavelength.

Oil Slicks, Atlantic Ocean (left)

Because SAR image brightness is dependent upon surface geometry, SAR data is extremely useful for observing the surface features of the ocean. This SAR image of the Atlantic Ocean, close to Angola, shows many slicks resulting from oil naturally seeping from the ocean floor. The image is dark where the slicks occur not because of the colour of oil, but because the oil damps down small surface waves and the smoother surface reflects more of the transmitted signal away from the satellite. SAR imagery will also readily detect oil slicks coming from ships, however the changing alignment of the slicks here result from multiple static sources in combination with a change in tidal flow. Such natural seepage can be used to indicate the presence of offshore oil reserves. The large dark areas in the north of this image are from rain affecting surface waves, the source cloud being transparent to the radar signal.

back | contents | next