Spaceborne imaging of the Earth's surface by synthetic aperture radars (SAR) may be adversely affected by the ionosphere, because 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 the matched filter, which is normally taken as if the propagation were unobstructed. In turn, the mismatch between the filter and the signal causes a deterioration of the image.
To reduce or remove the ionospheric distortions, 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 subsequently correct the matched filter, and thus improve the quality of the image. More-over, robustness of the proposed approach can be considerably 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.
Time permitting, we will also talk about including the Ohm conductivity of the ionosphere into the analysis of SAR resolution (in addition to its temporal dispersion), about considering the true Kolmogorov spectrum of the ionospheric turbulence (as opposed to its approximate representation), and about taking into account the variation of the ionospheric TEC along the flight trajectory (satellite orbit).
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