On SAR Imaging through the Earth's Ionosphere

We analyze the effect of dispersion of radio waves in the Earth’s ionosphere on the performance (image resolution) of spaceborne synthetic aperture radars (SARs). We describe the electromagnetic propagation in the framework of a scalar model for the transverse field subject to weak anomalous dispersion due to the cold plasma. Random contributions to the refraction index are accounted for by the Kolmogorov model of ionospheric turbulence. A key consideration used when analyzing the statistics of waves is normalization of the probability distributions for long propagation distances. The ionospheric phenomena, both deterministic and random, are shown to affect the azimuthal resolution of a SAR sensor stronger than the range resolution; also, the effect of randomness appears weaker than that of the baseline dispersion. Specific quantitative estimates are provided for some typical values of the key parameters. Probing on two carrier frequencies is identified as a possible venue for reducing the ionospheric distortions.