Abstract
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).
Highlights
E LECTROMAGNETIC waves traveling through the ionosphere interact with the electrons and the magnetic field with the result that the polarization vector of the electric field is rotated by an angle called the Faraday rotation angle [10, ch. 8, p. 272]
The altitude of ambiguity of the phase data is 450 m, meaning that each interferometric fringe in the interferogram represents a 450-m change in surface elevation. Comparing this interferometric map with a topographic map of the area produced by the U.S.G.S., we estimated the root mean square (RMS) error of the interferometrically-derived height map to be about 15 m
Using synthetic aperture radar (SAR) data from the shuttle imaging radar C (SIR-C) and JERS-1 missions, we evaluated the effect of changes in ionospheric density on polarimetric and interferometric SAR data at the L-band frequency
Summary
E LECTROMAGNETIC waves traveling through the ionosphere interact with the electrons and the magnetic field with the result that the polarization vector of the electric field is rotated by an angle called the Faraday rotation angle [10, ch. 8, p. 272]. E LECTROMAGNETIC waves traveling through the ionosphere interact with the electrons and the magnetic field with the result that the polarization vector of the electric field is rotated by an angle called the Faraday rotation angle [10, ch. Other effects of the ionosphere include propagation delays of the radar echoes, ray bending, radiowave scintillation, and phase changes. While these effects are all of importance. For the processing of multichannel and interferometric synthetic aperture radar (SAR) data, we focus the present discussion on the effect of changes in the Faraday rotation angle on the polarimetric and interferometric charateristics of SAR data acquired from a spaceborne platform. 275], the Faraday rotation angle may be expressed as TEC (1)
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