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

对于毫米波地区，在频率较高或降雨率较高的情况下，雨中多次散射对雷达信号的影响可能不能忽略。传统的极化雷达方程只考虑了一阶散射的贡献，理论上对降雨量的估计是基于偏振雷达方程的。然而，机载雷达对降雨的测量表明，在34.5 GHz处有很高的去偏振率，这表明包括了多次散射。本文的目的是在时间域上阐明多次散射对雷达信号的影响，并将其应用于高精度雷达系统的研制。为了考察多次散射效应，推导了线偏振入射波的辐射传输方程的二阶解。计算了共极化强度和交叉极化强度。理论结果表明，共极化强度相对于雷达距离的衰减较慢，而在雨层后缘附近的线性退偏率(LDR)迅速增加。对16.5 GHz和30 GHz随机分布的球面散射体的模拟降雨进行了受控散射参数下的实验室测量。测得的LDR很大，与理论计算符合得很好。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。联合偏振强度和交叉偏振强度的同时观测将导致较高精度的降雨率估计，并需要分析雨的细微结构。我们将进一步研究辐射传输方程的另一种近似解，并考虑处理非球形雨滴的情况。对于不同尺寸参数的雨滴，也应进行类似的实验。

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.