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

Abstract

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 nonnegligible. 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. INDEX TERMS P-band spaceborne SAR, spatiotemporally varying, background ionospheric effects, geometric distortion, focusing quality, disparity of background ionosphere.

Theoretical Feasibility Analysis of Fast Back-Projection Algorithm for Moon-Based SAR in Time Domain

“…For a more realistic scenario, the ephemeris are adopted among calculations in this paper. While adopting JPL DE430, or other ephemerides at specific moments for researching, the dual-path method is used, i.e., uplink and downlink components, are calculated separately, with Earth's rotation been considered in every pulse [19,20]. Here, its antenna bore-sight lies in the corresponding zero-Doppler plane after using at least the two-dimensional attitude steering method, and the plane is assumed to remain unchanged during synthetic aperture time.…”

“…Here, its antenna bore-sight lies in the corresponding zero-Doppler plane after using at least the two-dimensional attitude steering method, and the plane is assumed to remain unchanged during synthetic aperture time. The dual-path method from MBS to achievable EGT at the 𝑛 pulse is shown as Figure 5, where While adopting JPL DE430, or other ephemerides at specific moments for researching, the dual-path method is used, i.e., uplink and downlink components, are calculated separately, with Earth's rotation been considered in every pulse [19,20]. Here, its antenna boresight lies in the corresponding zero-Doppler plane after using at least the two-dimensional attitude steering method, and the plane is assumed to remain unchanged during synthetic aperture time.…”

Theoretical Analysis of the Spatial Baseline for Moon-Based SAR Cross-Track Interferometry

“…Up to now, light devices, i.e., the far-ultraviolet camera and extreme ultraviolet camera, have been launched on Moon surface to process Earth observation. However, for radar devices, its size, antenna's attitude calibration, atmosphere and ionosphere influence, power configuration, and Doppler parameters estimation are critical factors in imaging quality [10], [11], [12], [13], [14]. Notwithstanding, inspired by successful Earth-ground radar observation experiment [15], Moon-based synthetic aperture radar (MB-SAR) is still an important feasible way to monitor Earth phenomenon.…”

“…A number of literature works have already indicated (permanent) that low thermal environment, which favors devices' operation, exists in Moon's near side [16]; the Moon's orbit can also be calculated accurately [17]. Almost 100% Qinghai-Tibet Plateau and more than 50% Amazon Plain can be covered by MB-SAR, and P-band is a candidate in Earth observation [14], [18]. For full use of the SAR images, few research works have theoretical analysis on Moon-based along track interference (MB-ATI) and Moon-based repeat-track interferometry (MB-RTI), and their huge potentials in the macroscale solid Earth movement phenomenon monitoring are proved [19]; the two methods have at least two SAR images acquired at different times through a single SAR platform.…”