Analysis And Elimination Of Faraday Rotation Effect From Spaceborne Polarimetric SAR Observation At Low Frequency Bands

Microwave observations of subcanopy and subsurface (0-5m), such as subsurface moisture profiles, are key parameters for the study of global change, land hydrological process, the global carbon balance, etc. Space-borne microwave observation of subcanopy and subsurface requires the SAR (polarimetric synthetic aperture radar) technology at lower microwave frequencies, such as P band. However, SAR observation at P band is remarkably influenced by Faraday rotation (FR) effect through ionosphere.Firstly, FR angle formula is deduced in this paper according to the mechanism of low frequencies microwave propagation in ionosphere. After that, FR effects on polarimetric scattering matrix are presented, and the relationship between polarimetric Mueller matrix with FR and the one without FR is given. Then an example is given to illustrate why the measured polarimetric data with FR at P band cannot be directly applied to terrain surface classification, based on the current deorientation terrain classification method with the SAR image of Tabernash in United States. We further present that the parameters u, v, H,α, A for terrain surface classification derived from the polarimetric data without FR, which are recovered from the data with FR, can be applied to the surface classification, even there is a±p/2 ambiguity error unresolved. Based on gradual change of FR degree along geographical location, a method to eliminate the±p/2 ambiguity error is designed. Thus, the polarimetric scattering vector and Mueller matrix without FR and±p/2 ambiguity can be fully inverted from the measured polarimetric data with FR.Finally, the observation of vertical soil moisture profiles for subcanopy and subsurface at UHF/VHF bands is discussed. We use one-layered plant model with non-spherical particles to analyze the significant impact from plants at L band, which can not be deprived easily. After that, according to the multi-layered soil model, scattering simulation of layered surface at UHF/VHF bands is presented. The contribution of soil in different depth and the difference of penetration depth between two bands are given. Further more, an iterative way is suggested to estimate the profile of three-layered soil moisture model at UHF/VHF bands.