# Impact Evaluation of DME Beacons on BeiDou B2a Signal Reception Performance
## 4 Evaluation of B2a Signal Reception Performance
### Evaluation Results  
A simulation evaluation platform for the B2a signal reception performance under DME interference was built based on the empirical data from the DME beacon network. The B2a signal reception performance degradation and \(P_{k}\) budget were considered. The platform divides the areas from 10\({}^{\circ}\)-to-50\({}^{\circ}\) N latitude and 70\({}^{\circ}\)-to-140\({}^{\circ}\) E longitude into 1000 \(\times\) 1000 grids. For each grid cell, the process begins with budgeting the received power of the DME signal, followed with using the interference evaluation model to calculate the degradation in B2a signal reception performance, and, finally, the heatmap values are then visualized. Figure 9 shows the spatial distribution of effective \(C/N_{0}\) degradation for the B2a signal induced by DME beacons across the Chinese mainland at an FL400 altitude, approximately 12,192 meters above standard barometric altitude. The calculations of \(PDC\), \(Ri\) and \((C/N_{0})_{\text{deg}}\) only consider the interference from DME signals, with \(l_{0,\text{WB}}=0\). Figure 10 indicates that the most severe effective \(C/N_{0}\) degradation at FL400 altitude occurs in China's major aviation hubs, particularly the Yangtze River Delta (e.g., Shanghai, Nanjing, and Hangzhou), the Pearl River Delta (Guangzhou), and the area around Chongqing. These regions exhibit both the high air traffic density and stringent navigation precision requirements, and they are particularly susceptible to significant DME interference. The peak degradation value of 3.25 dB in \((C/N_{0})_{\text{deg}}\) occurs at 32.9\({}^{\circ}\) N, 118.4\({}^{\circ}\) E (a location proximate to Nanjing, Jiangsu Province). In the worst-case scenario, the airborne antenna can receive signals from up to 17 DME ground beacons. Parameters for several areas experiencing severe interference are presented in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Distance (km)** & \(L_{lens}\)**(dB)** \\ \hline
50 & 0.05 \\
100 & 0.1 \\
500 & 0.18 \\
300 & 0.3 \\
350 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The lens effect loss of the DME signal. Figure 8: Equivalent filter transfer function response characteristics. Figure 9: A heatmap of \(PDC\) (**a**), \(R_{i}\) (**b**), and \((C/N_{0})_{\text{deg}}\) (**c**) degradation at FL400 altitude. The theoretical \(C/N_{0}\) at the front end of the B2a receiver is 33.898 \(\rm{dB}\cdot\rm{Hz}\) without the interference of the DME [17]. Taking into account the \((C/N_{0})_{\rm{deg}}\) caused by DME, the remaining \(C/N_{0}\) is calculated and can be used for further evaluation of B2a signal reception performance degradation. The pre-detection integration times for the acquisition and tracking phases were set to \(T_{coh1}=2\) ms and \(T_{coh2}=10\) ms respectively, while the carrier tracking loop integration time for demodulation was set to \(T_{coh3}=1\) ms to specify the worst scenario. The results of B2a signal reception performance degradation are shown in Figure 11.  
The spatial distribution of the B2a signal reception performance degradation at FL400 altitude strongly correlated with regions exhibiting significant effective \(C/N_{0}\) degradation. These interference hotspots predominantly coincide with China's developed regions and busy air traffic zones, where DME ground beacon density reaches high levels. The most severely interfered region was located at 32.9\({}^{\circ}\) N, 117.4\({}^{\circ}\) E (near Nanjing, Jiangsu Province), and it experienced a 33% decrease in signal detection probability, a 2.57\({}^{\circ}\) increase in carrier tracking error, and a 5.1% increase in demodulation bit error rate. Parameters for other severely interfered areas are presented in Table 3.