## 5 Conclusions  
This paper proposes a low-cost two-dimensional GNSS signal positioning method for tunnels, featuring seamless positioning continuity between the interior and exterior of the tunnel. The system relays GNSS signals from open areas via frequency division relays, separately transmitting signals from different GNSS systems through distinct ports on two LCX cables laid parallel along both walls of the tunnel. By leveraging the radiation characteristics of the LCX cables, the system ensures stable and uniform GNSS signal coverage within the tunnel. At the receiving end, a differential correction method is applied, utilizing inter-satellite pseudorange differences to account for variations in signal propagation paths, thereby achieving accurate two-dimensional positioning. To correct errors induced by signal propagation through the LCX cable, a VEM is employed. Additionally, reference points are established within the tunnel to eliminate positioning errors stemming from frequency division relay processing and other factors.  
The complexity of the system algorithm primarily lies in the receiver positioning algorithm. Compared to RTK and PPP algorithms, the inter-satellite pseudorange difference and differential correction algorithm used in this study has lower computational complexity, eliminating the need for the receiver to perform complex iterative calculations  
Figure 33: Dynamic point positioning deviation distribution.  
Figure 32: Dynamic point positioning deviation distribution.  
or historical data corrections. This algorithm requires only five or more satellites from three GNSS systems to achieve two-dimensional positioning within a tunnel. Since the algorithm uses pseudorange differences and differential corrections for the same satellite, receiver clock bias does not impact the positioning results. Leveraging existing cloud computing infrastructure, this algorithm provides higher real-time performance than RTK and PPP algorithms.  
Multiple independent simulation experiments were conducted under various noise variance levels, and the results demonstrate that the system can achieve meter-level positioning accuracy in a normal noise environment when using a dual T-type LCX cable parallel layout. Additionally, the proposed positioning system does not employ filtering techniques to improve positioning accuracy. Future improvements could involve using Kalman filter, machine learning, or building an error fingerprint database to achieve higher accuracy.  
This paper presents a new solution for two-dimensional positioning in tunnel environments, while also offering new ideas for indoor positioning and extended applications of satellite navigation.  
Conceptualization, K.J. and M.S.; methodology, K.J.; validation, K.J. and M.S.; formal analysis, S.X.; investigation, X.T.; resources, S.D.; data curation, K.J.; writing--original draft preparation, K.J.; writing--review and editing, K.J.; visualization, K.J. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding and the APC was funded by Nanjing University of Aeronautics and Astronautics.  
Not applicable.  
Data is contained within the article.  
The authors declare no conflicts of interest.