Direct Estimation of Faraday Rotation and Other System Distortion Parameters from Polarimetric Sar Data

One of the major impacts of the Earth’s ionosphere on spaceborne radar signals is the rotation of the polarization vector, also called the Faraday rotation (FR) effect. The FR effect increases with decreasing frequency. It is reported [1] that estimated maximum FR values under peak ionospheric total electron content (TEC) conditions range from 2.5 degrees (C-band) to more than 320 degrees (P-band). The FR effect depends on the electron density along the propagation path, the Earth’s magnetic field, the radio frequency and the path direction of the signal, all of which vary in space and time. Low-frequency radars – radars operating at L-band and lower – must therefore be well calibrated not only for system distortion terms, but also for FR. Several recent techniques have been proposed for estimating FR and removing its effects from radar signals, though in most cases the range of applicability of the techniques has been small. In this paper we present a method for direct estimation of FR, along with several other system distortion parameters, which is applicable to a large range of Faraday rotation angles and can therefore be applied to low frequency radars. The solution is posed as a nonlinear optimization problem for the polarimetric radar system model. The applicability of the model is demonstrated through several examples and sensitivity studies.