Recent work has highlighted the simulated performance of passive synthetic aperture radar (SAR) using Jupiter’s radio emissions to probe the icy moons of Jupiter. Terrestrially, passive radar sounding using the Sun as a source for echo detection, ranging, imaging, and measuring ice thickness has also been recently demonstrated for the first time. With increasing advancements in passive radar sounders that use extended, incoherent radio-astronomical sources for echo detection, we revisit a potential limitation of the technique in terms of the sources’ spatial coherence properties. While previous work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening, there has been little work to date that has examined the spatial coherence constraints for passive sounding imposed by source size, wavelength, incidence angle, and altitude—all of which govern the potential performance of passive SAR focusing. Starting from antenna theory, the Van Cittert-Zernike (VCZ) theorem, and the coherence function for passive sounding, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes when using radio-astronomical sources; in particular, we analyze the scenarios for a spacecraft using the Sun and Jovian bursts as sources for passive sounding of the Earth, Mars, and Europa. While the results of our analysis and simulations show that the coherence requirements (in terms of both pulse broadening and spatial radius of coherence) are met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence at these greater altitudes creates an upper bound for orbital passive radar sounding. Our results therefore provide a richer understanding of the passive sounding technique, its viability, and a critical design constraint when planning future planetary and terrestrial passive sounding experiments.

中文翻译：

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射电天文源被动雷达探测的空间相干性约束


最近的工作重点介绍了利用木星无线电发射探测木星冰冷卫星的无源合成孔径雷达（SAR）的模拟性能。在地面上，使用太阳作为回声探测、测距、成像和测量冰厚度的源的无源雷达探测最近也首次得到展示。随着使用扩展的、非相干射电天文源进行回声检测的无源雷达探测器的不断进步，我们重新审视该技术在源空间相干特性方面的潜在局限性。虽然以前的工作已经考虑了被动探测的扩展源在脉冲展宽方面的空间相干效应，但迄今为止几乎没有研究过源尺寸、波长、入射角和高度对无源探测的空间相干约束的研究。 ——所有这些都决定着被动 SAR 聚焦的潜在性能。从天线理论、Van Cittert-Zernike (VCZ) 定理和无源探测的相干函数出发，我们推导出由这些参数设置的附加界限和预期源范围，以估计使用射电天文源时的最大轨道高度；特别是，我们分析了使用太阳和木星爆发作为地球、火星和木卫二被动探测源的航天器的场景。虽然我们的分析和模拟结果表明，大入射角的地面实验满足相干性要求（在脉冲展宽和相干空间半径方面），但在这些更高的高度上有限的空间相干性产生了轨道无源雷达探测的上限。 因此，我们的结果提供了对被动探测技术、其可行性以及规划未来行星和陆地被动探测实验时的关键设计约束的更丰富的理解。