Temporal-Spatial Varying Background Ionospheric Effects on the Moon-Based Synthetic Aperture Radar Imaging: A Theoretical Analysis

The Moon-based synthetic aperture radar (SAR) potentially offers an unprecedented high temporal resolution and wide spatial coverage, simultaneously, for earth observation. However, the complex geometry in relating Moon-SAR-Earth motion, where both the earth’s rotation and lunar revolution exert, poses a great challenge in signal processing and image formation. Besides, the background ionospheric effects contributing to the phase dispersion become even more critical because the ionospheric freezing model usually assumed in the low earth orbit SAR imaging is no longer valid for the Moon-based SAR, and the curved trajectory induced by the ultra-long synthetic aperture time further complicates the background ionospheric effects. In this paper, we investigate such effects on the Moon-based SAR imaging by establishing an accurate curved trajectory signal model. The image shift and image defocus caused by the temporal–spatial varying background ionosphere are evaluated in detail. Numerical analysis shows that the range shift can be up to hundreds of meters, while the azimuth shift is much less, on the order of tens of meters. As for image quality, the range defocusing is persistently significant, and yet the azimuth focusing is only slightly disturbed. Under the presence of background ionosphere, the geometric distortion and image defocusing given rise to a Moon-based SAR deserve a special care in the context of image quality.