Theoretical limits on SAR imposed by the ionosphere

The ultimate theoretical limitations on space-based synthetic aperture radar (SAR) image formation that are imposed by the ionosphere are examined. The effects on the SAR image are derived from first principles, and it is shown that the ionosphere will cause defocusing in both the range and along track directions. The performance of an autofocus procedure is then examined, and it is shown that the range defocusing can always be removed, but the range time delay can only be determined for high percentage bandwidths and high signal-to-noise plus clutter ratios. It is also shown that the performance limits of autofocus are not determined by the absolute total electron content, but are given by the amount of ionospheric turbulence, which limits the along track resolution. The relationship between the requirement for a focussed SAR image and the S₄ index and the integrated strength of turbulence CkL is derived.