Effect of Faraday rotation on L-band interferometric and polarimetric synthetic-aperture radar data

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 (λ=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 ψ on spaceborne interferometric and polarimetric data. In one experiment, it was found that phase coherence is reduced by up to 33% when ψ changes between successive data acquisitions. If ψ changes by more than 40°, 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±10° in the JERS-1 data and 0° 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)