Numerical Study of the Spatiotemporally-Varying Background Ionospheric Effects on P-Band Satellite SAR Imaging

At the relatively low frequency of the P-band spaceborne SAR, the background ionosphere is no longer treated as time-invariant, together with the spatial variation in the background ionosphere, and complicates the spaceborne SAR imaging. In this paper, we systematically quantify the effects of the spatiotemporally varying background ionosphere on the imaging performance of the P-band spaceborne SAR. The signal model for a P-band spaceborne SAR is established in accordance with the range history taking account of the Earth's rotation, based on which the 2-dimensional (2-D) signal frequency spectrum under the influence of spatiotemporally varying background ionosphere is derived. Then, the geometric distortion and defocusing phenomenon in the range and azimuth directions are quantitatively evaluated. It is found that the range shift is mainly determined by the constant component of the background ionosphere, and the contribution from the spatiotemporally varying components of the background ionosphere is non-negligible. In contrast, the azimuth shift has no dependency on the constant part of the background ionosphere, but both the spatially varying and temporally varying components of the background ionosphere could give rise to considerable azimuth shifts. Regarding the focusing quality, it is seldom impaired by the background ionosphere for the P-band spaceborne SAR with a coarse spatial resolution on a decameter scale. However, the high spatial resolution makes it difficult to maintain good focusing qualities, especially at the high solar activity. Finally, numerical simulations in terms of the single-point target responses confirm the theoretical analysis; simulations of responses of array-point targets, together with 2-D image shifts along the satellite's track, show the disparity of background ionosphere can further affect the imaging performance of the P-band spaceborne SAR. In this respect, effects of spatiotemporally varying background ionosphere deserve special care in the signal processing of a P-band spaceborne SAR.