Electromagnetic waves traveling through the ionosphere undergo a Faraday rotation of the polarization vector, which modifies the polarization and phase characteristics of the electromagnetic signal. Using L-band (/spl lambda/=24 cm), polarimetric synthetic aperture radar (SAR) data from the shuttle imaging radar C (SIR-C) acquired in 1994, the author simulates the effect of a change in the Faraday rotation angle /spl psi/ on spaceborne interferometric and polarimetric data. In one experiment, it was found that phase coherence is reduced by up to 33% when /spl psi/ changes between successive data acquisitions. If /spl psi/ changes by more than 40/spl deg/, a differential phase signal, which varies from field to field, appears in the interferogram and impairs the mapping of surface topography and/or the detection of ground deformation. This signal is caused by phase differences between horizontal-polarized and vertical-polarized radar signals from intermediate levels of vegetation canopy, similar to the phase difference measured between H-polarized and V-polarized signals on a single date. In a second experiment, data from the Japanese Earth Resources Satellite (JERS-1) L-band radar acquired in an area of active deforestation in Rondonia, Brazil, are compared with SIR-C L-band polarimetric data acquired at the same incidence, two weeks later, but from a lower orbiting altitude. Large differences in scattering behavior are recorded between the two datasets in the areas of slash and burn forest, which are difficult to reconcile with surface changes. A simulation with SIR-C polarimetric data, however, suggests that those differences are consistent with a Faraday rotation angle of about 30/spl plusmn/10/spl deg/ in the JERS-1 data and 0/spl deg/ in the SIR-C data. Based on these two experiments and on Global Positioning System (GPS) records of ionospheric activity, it is concluded that Faraday rotation should not affect the analysis of L-band spaceborne data during periods of low ionospheric activity (solar minima).
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