Extra information can also be gathered on a site based on the fact that radar signals can have different polarisations; just as visible light does as demonstrated by the high amount of glare screened out with polarized sunglasses.
Envisat's Advanced Synthetic Aperture Radar (ASAR) is a dual-polarisation instrument, meaning that it can transmit and receive in either Horizontal (H) or Vertical (V) polarisations. In Alternating Polarisation (AP) mode it transmits in the same way, but can receive back radar pulses in both H and V polarisations. Combinations of HH/VV HH/HV or VV/VH polarisation pairs are possible.
Depending on the physical and chemical properties of the surface, the responses from these different combinations of polarisation pairs can vary considerably, for example enabling the differentiation of separate crop species in the same field, or telling sea ice from open water.
The new generation of airborne radar sensors go one better than ASAR because they include full quad-polarisation capability: this means their signal can be retrieved simultaneously in H and V polarisations, so that by sending alternating H and V pulses a full spread of combinations can be obtained, generating more information-rich imagery.
These two techniques of InSAR and polarimetry have both been around a long time. Pol-InSAR is the combination of these two processes the performance of interferometry from multi-polarised radar images.
"Using Pol-InSAR brings us an extra dimension in the imaging of forests," said Scott Hensley of California's Jet Propulsion Laboratory (JPL). "The interferometry part gives us the height data, while the polarisation part provides information on depth and also the orientation of objects relative to the instrument. This can
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Contact: Mariangela D'Acunto
mariangela.dacunto@esa.int
39-069-418-0856
European Space Agency
1-Feb-2005