# Evaluation of Network RTK Positioning Performance Based on BDS-3 New Signal System
## 1 Introduction  
As a part of the GNSS, the BDS is a satellite positioning system built in China. Its purpose is to provide PNT services for global users [1,2]. According to the three-step development strategy, the first phase (BDS-1) is composed of 2 GEO satellites, with the purpose of experimental demonstration. It was completed in 2000 and has now ended its service. The second phase (BDS-2) is composed of 5 GEO + 5 IGSO + 4 MEO satellites. The purpose is to provide PNT services for users in the Asia-Pacific region. It was completed at the end of 2012. The satellites currently in orbit are composed of 5 GEO + 7 IGSO + 3 MEO, which can provide PNT service accuracies on the level of 10 m, 0.2 ms, and 50 ns, respectively. In addition, the full constellation of BDS-2 can broadcast triple-frequency signals, namely B1I (1561.098 MHz) + B2I (1207.140 MHz) + B3I (1268.520 MHz). The third phase (BDS-3) is composed of 3 GEO + 3 IGSO + 24 MEO satellites. The purpose is to provide PNTservices for global users; it will start full constellation operation in June 2020. The satellites currently in orbit and providing normal services are composed of 2 GEO + 3 IGSO + 24 MEO. BDS-3 inherits the B1I and B3I frequency of BDS-2, which can realize the transition between the two generation systems. In addition, BDS-3 also broadcasts another three frequency signals to achieve compatibility and interoperability with other GNSS systems, namely B1C (1575.42 MHz) + B2a (1176.45 MHz) + B2b (1207.14 MHz). The frequency signals of BDS-ICD publicly provided services [1; 3] are shown in Table 1. In addition, information about BDS satellite PRN, satellite SVN, satellite type, space signal accuracy, system operation status, and other information can be viewed on the website (TARC): [[http://www.csno-tarc.cn/system/constellation](http://www.csno-tarc.cn/system/constellation)]([http://www.csno-tarc.cn/system/constellation](http://www.csno-tarc.cn/system/constellation)) (accessed on 18 August 2021). The footprints of BDS satellites are shown in Figure 1. It can be seen from Figure 1 that the BDS has basically covered most of the world's middle and low latitudes after the accession of BDS-3 satellites. Furthermore, in the Asia-Pacific region, due to the presence of GEO+IGSO (BDS-2) satellites, the average number of visible satellites is better. Since the end of 2018, the BDS-3 system has provided global service. A large number of scholars have studied the PNT service performance of BDS-3, and literature [4; 5; 6] has evaluated the signal quality of BDS-3. In the literature [7; 8], the precise orbital determination of the BDS-3 satellite was studied. The precise point positioning (PPP) performance of BDS-3 was studied in the literature [9; 10; 11]. Reference [12] made a preliminary evaluation of the RTK service performance under the new signal system of BDS-3 and pointed out that the positioning performance of BDS-3 was better than that of BDS-2. Reference [13] made a similar study, and pointed out that the positioning accuracy of the B1C+B2a combination of BDS-3 was better than that of B1I+B3I. Literature [14] initially evaluated the positioning performance of single-frequency and dual-frequency BDS-3 RTK based on an ultra-short baseline, pointing out that the poor signal quality of B2a frequency of BDS-3 would lead to poor positioning results, and reasonable weighting processing should be carried out in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Signals** & **Frequency (MHz)** & **Wavelength (cm)** & **System** \\ \hline B1I & 1561.098 & 19.20 & BDS-2/3 \\ B1C & 1575.420 & 19.03 & BDS-3 \\ B2a & 1176.450 & 25.48 & BDS-3 \\ B2b & 1207.140 & 24.83 & BDS-3 \\ B3I & 1268.520 & 23.63 & BDS-2/3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Frequency signals of BDS-2 and BDS-3 satellites. Figure 1: Footprints of BDS-2 and BDS-3 satellites (18 August 2021). practical use. Literature [15] suggests using B1C+B2a or B1I+B3I for BDS-3 dual-frequency processing.