| Imaging of the Earth's surface by spaceborne synthetic aperture radars (SAR) may be adversely affected by the ionosphere, as the temporal dispersion of radio waves gives rise to distortions of signals emitted and received by the radar antenna. Those distortions lead to a mismatch between the actual received signal and its assumed form used in the signal processing algorithm (known as the matched filter). In turn, the discrepancy between the filter and the signal causes a deterioration of the image.;In this dissertation, we conduct a thorough mathematical analysis of transionospheric SAR imaging, and accurately quantify the distortions of images due to the ionosphere. In doing so, we model the ionosphere as a dilute cold plasma. Then, to mitigate the ionospheric distortions of SAR images, we propose to probe the terrain, and hence the ionosphere, on two distinct carrier frequencies. The resulting two images appear shifted with respect to one another, and the magnitude of the shift allows one to evaluate the total electron content (TEC) in the ionosphere. Knowing the TEC, one can correct the matched filter, and hence improve the quality of the image. Robustness of the proposed approach can subsequently be improved by applying an area-based image registration technique to the two images obtained on two frequencies. The latter enables a very accurate evaluation of the shift, which, in turn, translates into a very accurate estimate of the TEC.;We also analyze a host of additional factors that affect the spaceborne SAR performance. Those include the Ohm conductivity of the ionosphere, which is due to the collisions of electrons with other particles, the horizontal variation of the ionospheric parameters, and the random fluctuations of the electron number density, i.e., the ionospheric turbulence. The effect of the latter on the SAR resolution is evaluated in the statistical sense.;Finally, we devote special attention to anisotropic phenomena. The ionospheric plasma becomes anisotropic (gyrotropic) due to the magnetic field of the Earth. The propagation of radio waves in a gyrotropic medium is accompanied by the Faraday rotation. For spaceborne SAR, the Faraday rotation presents an additional source of mismatch between the received signal and the filter, and hence causes additional image distortions. We propose to use the image autocorrelation analysis to quantify the impact of the Faraday rotation and obtain its parameters, which then allows us to correct the filter accordingly.;Scattering of radar signals at the target may also be affected by anisotropy. We interpret the target as a weakly conductive birefringent dielectric, and derive a necessary and sufficient condition under which this model allows one to reconstruct all the degrees of freedom in the scattered signal that the previous studies in the literature have introduced phenomenologically. This development can help construct a full- edged radar ambiguity theory for polarimetric SAR imaging, i.e., the type of imaging that exploits individual polarizations of the transmitted and received signals.;This dissertation is based on four journal articles published or submitted for publication between 2011 and 2013. Although every effort has been made to streamline the overall presentation, unify the notations and cross-references, and remove the redundancies, still the structure and content of individual chapters may inherit the style of those journal publications whence they originate. |
