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 ( = 24 cm), polarimetric synthetic aperture radar (SAR) data from the shuttle imaging radar C (SIR-C) acquired in 1994, we simulate the effect of a change in the Faraday rotation angle on spaceborne interferometric and polarimetric data. In one experiment, we find that phase coherence is reduced by up to 33% when changes between successive data acquisitions. If 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, we conclude that Faraday rotation should not affect the analysis of L-band spaceborne data during periods of low ionospheric activity (solar minima). Yet during ionospheric storms or near solar maxima, the effects should become significant and impair data analysis. Corrections for Faraday rotation are impractical in the case of single-channel SAR data. SAR data must be acquired with the full polarimetry to recover from these effects.