Abstract: The Instrument Landing System (ILS) is a critical navigation device for ensuring flight safety, whose signal quality directly affects the accuracy and safety of aircraft landings. However, the increasingly complex electromagnetic environment around airports has exacerbated multipath effects, significantly impacting the reliability and precision of ILS signals. Therefore, Taking the LOCalizer (LOC) as the research object, a signal propagation path analysis method based on the backward ray-tracing algorithm was proposed. By establishing an electromagnetic propagation model of the airport environment and incorporating ray-tracing and path validity determination rules, the propagation characteristics of ILS signals in multipath environments are systematically investigated. The study particularly analyzes the influence of signal reflection and diffraction effects caused by obstacles on the Difference in Depth of Modulation (DDM) of the LOC signals received onboard. Computer simulation results demonstrate that when obstacles are placed near the runway centerline, DDM experiences significant fluctuations, with the maximum jitter approximately 283.4% of the value specified by the International Civil Aviation Organization (ICAO). However, after appropriately adjusting the obstacle positions, interference between obstacles and the increased distance from the runway centerline led to a reduction in multipath interference. As a result, the fluctuation of DDM decreases significantly and meets the ICAO's prescribed limit, with the maximum jitter approximately 99% of the specified value. This method can effectively assess the impact of multipath propagation on ILS performance in complex airport environments, assist in airport construction layout and site planning, thereby providing theoretical support for ILS system performance optimization and electromagnetic environment management.

Key words: Instrument Landing System (ILS), LOCalizer (LOC), backward ray-tracing method, Depth of Modulation (DDM), multipath interference

摘要： 仪表着陆系统（ILS）是保障飞行安全的关键导航设备，它的信号质量直接影响飞机的着陆精度与安全性。然而，机场周边日益复杂的电磁环境导致多径效应加剧，严重影响了ILS信号的可靠性与精度。因此，以航向信标（LOC）为研究对象，提出一种基于反向射线追踪算法的信号传播路径分析方法。在该方法中，通过建立机场环境电磁传播模型，结合射线追踪与路径有效性判定规则，系统研究了多径环境下ILS信号的传播特性；重点分析了障碍物引起的信号反射与绕射效应对机载LOC接收信号调制度差（DDM）的影响规律。仿真实验结果表明：障碍物靠近跑道中线布设时，DDM会出现显著波动，最大抖动约为国际民用航空组织（ICAO）规定值的283.4%；而适当调整障碍物位置后，障碍物间存在遮挡以及障碍物与跑道中线距离增大，导致多径干扰衰减，DDM的波动幅度明显减小且满足ICAO规定的限制要求，最大抖动约为规定值的99%。验证了该方法可有效评估复杂机场环境中多径传播对ILS性能的影响。

关键词: 仪表着陆系统；航向信标；反向射线追踪法；调制度差；多径干扰