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January 18, 2006
This combination of ASAR IMP images demonstrates the benefits of the flexible swath positioning of the ASAR instrument onboard the ENVISAT satellite, highlighting the angular dependence of the radar backscatter. The image on the left side has been acquired with low incidence angle (image swath 1, i.e. large swath). The extract presented is a subset of the original swath. The image on the right side has been acquired with high incidence angle (Image Swath 7, i.e. narrow swath). The acquisitions are centered over the city of Naples and surrounding (bright patches in the center of the right image). One significant advantage of higher incidence angles is that terrain distortion is reduced. This is well illustrated by a comparison of Mount Vesuvius on both images (center right hand side of the images). Looking first at the left image, obtained with incidence angles of 15 degrees - 22.9 degrees from near to far range (IS1), one sees extreme distortion in the form of severe foreshortening and layover of the slope facing the radar, combined with significant lengthening of the slope facing away from the sensor. As a consequence the crater of the volcano is hardly visible. In contrast, these distortions are seen to be much less in the right image where the incidence angles vary between 42.5 degree to 45.2 degree from near to far range (IS7). The sea surface appears darker on the right image (high incidence angles) as the signal is specularly reflected away from the sensor. In contrast, multiple scattering due to low incidence angles allows highlighting the sea surface imprints of atmospheric features (bright tones within the sea in the center of the left image). Contrast between urban areas and vegetated zone is larger for high incidence angles, as indicated by the bright patches in the center of the right image.

Technical Information:
  • Satellite: Envisat
  • Instrument: ASAR
  • Acquisition: 22-Oct-2002
  • Orbit nr: 03368
  • Center coordinates: lat. 40.73, lon. 14.21

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