# Impact Evaluation of DME Beacons on BeiDou B2a Signal Reception Performance
## 1 Introduction  
Global Navigation Satellite Systems (GNSS) signal processing is susceptible to electromagnetic interference, which can severely degrade signal quality, positioning accuracy, and overall system reliability [1, 2, 3]. In the context of civil aviation, identifying and quantifying the impact of radio frequency interference (RFI) on the performance of navigation signals reception is essential [4, 5]. The BeiDou-3 Navigation Satellite System (BDS-3), which achieved full operational capability in 2020, marked a major milestone in China's satellite navigation technology. Operating at 1176.45 MHz, the newly implemented B2a signal is spectrally compatible with Global Positioning System (GPS) L5 and Galileo E5a signals through identical frequency allocation and similar modulation characteristics. This enables improved positioning accuracy and precise timing for airborne navigation systems [6]. However, the frequency resources available for aeronautical radio navigation services arehighly constrained and exhibit significant overlap with the operating frequency bands of ground-based navigation systems. Distance Measuring Equipment (DME), serving as essential ground-based navigation infrastructure, operates in the 1151-1213 MHz frequency band. This range creates substantial spectral overlap with the B2a signal (Figure 1), potentially causing significant interference. Consequently, DME has emerged as a primary RFI source for B2a signal in aviation applications [7]. Previous studies have confirmed the existence of RFI between DME and GNSS signals (GPS L5/BeiDou B2 bands) [8; 9]. It has been shown that the level of interference caused by DME beacons to aeronautical users is closely related to the geographical distribution of DME ground beacons [10; 11]. In high-altitude flight scenarios, aeronautical users are subject to the greater interference that results from the simultaneous reception of multiple DME ground beacon signals [12]. To quantify the interference effects of DME under worst-case scenarios, the Radio Technical Commission for Aeronautics (RTCA) evaluated the degradation of the carrier-to-noise ratio (\(C/N_{0}\)) of GNSS L5/E5a signals in high-altitude flight conditions [13]. The International Civil Aviation Organization (ICAO) conducted further studies on DME beacons in Europe and the United States, which clarified the impact distribution of European and US DME ground beacons on L5/E5a signal performance at high altitudes [14; 15]. The global deployment of BDS-3 has driven intensified research into B2a signal interference characteristics, particularly in aeronautical navigation contexts [16; 17; 18]. [PERSON] et al. proposed a method to evaluate B2a signal \(C/N_{0}\) degradation. Their approach specifically analyzed the interference from DME beacons in potential interference hotspots of the Chinese mainland, with particular consideration given to complex terrain effects [16]. [PERSON] et al. conducted a systematic analysis of DME-induced \(C/N_{0}\) degradation mechanisms in airborne receivers across different parameter configurations, extending the investigation to its consequential effects on aviation protection level performance [17]. [PERSON] et al. conducted a quantitative analysis of B2a signal interference from DME beacons co-located with VORs in the Chinese mainland based on the RTCA assessment framework [18]. However, this model is limited to single-parameter (\(C/N_{0}\)) evaluation and exhibits conservative estimations under multi-DME interference scenarios [19]. While significant progress has been made in interference assessment approaches and impact quantification, several critical issues remain to be addressed. The current evaluation systems primarily focus on a single dimension of \(C/N_{0}\) degradation, lacking systematic consideration of the impacts that are present throughout the entire signal processing chain. The spatial scope was limited to  
Figure 1: The spectrum conflict between DME and GNSS signals.