舰载雷达在对海面目标进行探测时，由于蒸发波导环境的存在，对雷达评估性能产生极大影响，它不仅能增加雷达传播距离，而且还造成大面积的探测盲区，因此能够准确可靠地预报大气波导，对提高舰船等海面军事目标的生存能力、打击能力有极大帮助。本文提出了一种新的预测方法来反演蒸发波导参数，从理论仿真和试验两方面验证此方法的可行性。本文研究的主要内容可分为几个部分：

首先从大气折射指数的角度出发，研究了大气波导的形成原因、条件及分类。给出了不同波导环境（表面波导，蒸发波导和悬空波导）的物理特性、波导参数和环境特征。重点研究了蒸发波导的物理特性和参数模型以及其对电磁波传播所产生的影响，给出了蒸发波导环境会对雷达评估性能产生极大影响的结论。

其次研究了抛物方程的由来和模型以及抛物方程的解法，分步傅里叶变换法（SSFT）、离散混合傅里叶变换法（DMFT）、有限差分方法等，同时开展了初始场求解和边界条件设置等理论工作。随后通过抛物方程的有限差分法仿真模拟了三种不同大气环境中的电波传播损耗，结果表明波导环境下的电波传播损耗明显比标准大气环境下的损耗小的多，阐明了超视距传播的本质。最后结合射线追踪法呈现了蒸发波导环境下的电磁波传播轨迹，并与抛物方程法的计算结果进行了比较，两种方法的计算结果整体趋势一致。

最后依据雷达反射点的轨迹分布特点，结合电磁波在蒸发波导环境下传输轨迹的特点来判断研究区域有无波导环境存在。在此基础上将雷达回波数据进行“分组”“聚类”，然后选取某一方位角上的回波数据，将Snell定律和射线传播的几何关系结合起来，来反演这一方位角上大气折射指数，再利用波导强度计算公式得出蒸发波导参数。并且通过模拟仿真理论和工程试验证明了这种方法的可行性。

本文提出的大气波导反演方法与现有大气波导反演技术相比，该方法不需要海杂波的实际测量数据和雷达电波传播模式正向模拟结果之间相互比拟，而是利用雷达海面杂波反射点轨迹分布，对探测方向上大气波导环境内的大气折射指数布计算，从而进行大气波导诊断。基于大气折射效应进行分析，分析过程更能体现大气波导内电波的传播特性且计算过程更为简洁，具有更高的效率和良好的精度，适用性更好，同时，实施本方法所需要的装置简便、成本低、易于实现。

When naval radar detects sea surface targets, the presence of evaporative waveguide environment greatly affects the radar evaluation performance, which not only increases the radar propagation distance, but also causes a large detection blind area, thus being able to accurately and reliably predict the atmospheric waveguide is of great help to improve the survivability and strike capability of naval surface military targets such as ships. In this paper, a new prediction method is proposed to invert evaporative waveguide parameters, and the feasibility of this method is verified from both theoretical simulations and experiments. The main elements of the research in this paper can be divided into several parts:

 

Firstly, the causes, conditions and classification of atmospheric waveguides are studied from the perspective of the atmospheric refraction index. The physical properties, waveguide parameters and environmental characteristics of different waveguide environments (surface waveguide, evaporative waveguide and overhanging waveguide) are given. The focus is on the physical characteristics and parametric modelling of evaporative waveguides and their effect on electromagnetic wave propagation, giving the conclusion that the evaporative waveguide environment can have a significant impact on the performance of radar evaluation.

 

Next, the origin and model of the parabolic equation and the solution methods of the parabolic equation, split step Fourier transform (SSFT), discrete mixed Fourier transform (DMFT) and finite difference methods are investigated, while theoretical work such as initial field solution and boundary condition setting is carried out. The finite difference method of parabolic equations is then used to simulate the wave propagation losses in three different atmospheric environments. The results show that the wave propagation losses in the waveguide environment are significantly smaller than those in the standard atmospheric environment, elucidating the nature of over-the-horizon propagation. Finally, the electromagnetic wave propagation trajectory in the evaporating waveguide environment is presented in combination with the ray-tracing method and compared with the calculation results of the parabolic equation method. The overall trend of the calculation results of both methods is consistent.

 

Finally, based on the trajectory distribution characteristics of radar reflection points, combined with the characteristics of electromagnetic wave transmission trajectory in the evaporative waveguide environment to determine the existence of waveguide environment in the study area. On this basis, the radar echo data are "grouped" and "clustered", and then the echo data at a certain azimuth are selected, and Snell's law and the geometric relationship of ray propagation are combined to invert the atmospheric refraction index at this azimuth, and then use The waveguide intensity formula is then used to derive the evaporative waveguide parameters. The feasibility of this method has been demonstrated by simulation theory and engineering tests.

 

Compared with existing atmospheric waveguide inversion techniques, the method proposed in this paper does not require a comparison between actual measurements of sea clutter and the forward simulation results of radar wave propagation patterns, but rather uses the point trajectory distribution of radar sea clutter reflections to calculate the atmospheric refraction index within the atmospheric waveguide environment in the detection direction, thus performing atmospheric waveguide diagnosis. Based on the atmospheric refraction effect, the analysis process better reflects the wave propagation characteristics within the atmospheric waveguide and the calculation process is more concise, with higher efficiency and good accuracy, better applicability, and at the same time, the implementation of the method requires simple, low-cost and easy to implement devices.

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