Multiple Scattering Effects of Electromagnetic Waves from Randomly Distributed Particles and Its Application to a High Precision Radar

对于毫米波区域，对于更高的频率或更高的降雨率，雨中的多重散射对雷达信号的影响不可忽视。降雨率的理论估计是基于传统的极化雷达方程，它只考虑了一阶散射的贡献。然而，机载雷达对雨的测量显示在34.5GHz处有非常高的去极化比，这表明多重散射包括在内。本文的目的是阐明多重散射对雷达信号在时域上的影响，并将其应用于高精度雷达系统的开发。为了研究多重散射效应，导出了线极化入射波的辐射传递方程的二阶解。计算了共极化强度和交叉极化强度。理论结果表明，在雨层后缘附近，共极化强度随雷达距离的减小速度较慢，线性去极化比（LDR）迅速增加。在可控散射参数下，对16.5GHz和30GHz随机分布的球形散射体模拟雨进行了室内测量。实测的LDR非常大，与理论计算结果非常吻合。因此，从目前的分析来看，传统的雷达方程可能高估了降雨率。同时观测共极化和互极化强度可以估算出更精确的降雨率，并需要分析降雨的精细结构。我们将进一步研究辐射传递方程的另一种近似解，并考虑处理非球形雨滴的情况。对雨滴的各种尺寸参数也应进行类似的试验。

For millimeter wave regions, the effects of multiple scattering in rain on radar signals may not be ignored for higher frequencies or higher rainfall rates. The theoretical estimation of rainfall rate has been based on the conventional polarimetric radar equation, which only takes account of the first-order scattering contribution. However, air-borne radar measurements of rain have shown very high depolarization ratios at 34.5GHz and suggested that the multiple scattering is included. The purpose of this paper is to clarify the effects of multiple scattering on radar signals in the time domain and to apply to the development of the radar systems with higher precision. To examine multiple scattering effects, the second-order solution of the radiative transfer equation was derived for the linearly polarized incident waves. Both of co-polarized and cross-polarized intensities were calculated. The theoretical results indicate a slower decrease in the co-polarized intensity against the range from the radar and a rapid increase in the linear depolarization ratio (LDR) near the rear edge of the rain layr. The laboratory measurements under controlled scattering parameters were made for simulated rain of randomly distributed spherical scatterers at 16.5GHz and 30GHz. The measured LDR is very large and is in remarkably good agreement with theoretical calculations. Therefore, the conventional radar equation may overestimate the rainfall rates in view of the present analysis. The simultaneous observation of co-and corss-polarized intensity would lead to estimate the higher precise rainfall rate and would need to analyze the fine structure of rain. We will further examine an alternative approximate solution of the radiative transfer equation and consider to treat the case of the nonspherical raindrops. The similar experiment should be done for various kind of size parameters of the raindrops.