# Evaluation of Network RTK Positioning Performance Based on BDS-3 New Signal System
## 5 Discussion  
Since the BDS-3 system began to initially provide global service at the end of 2018, a large number of scholars have studied the PNT service performance of BDS-3, mainly focusing on the aspects of signal quality, orbit, clock error, PPP positioning performance, and conventional RTK service performance. Network RTK technology is an important real-time regional high-precision GNSS positioning technology, but there are few related studies based on the new BDS-3 signal system. In this paper, the algorithm model of network RTK is firstly derived in detail, and then the positioning performance of BDS-3 is evaluated based on the measured data of a provincial CORS network in central China, and a better positioning effect is obtained. The experimental analysis also points out the direction for further research:  
* Firstly, in the long-distance baseline calculation between reference stations, tropospheric wet delay and ionospheric delay were estimated as parameters to assist the fixation of ambiguity. Although random walk was used to estimate parameters and a variance constraint was applied to accelerate the convergence of the baseline, the initial parameter values were set to zero. Theoretically, adding an external correction model to provide relatively accurate initial values would further shorten the baseline convergence time.
* Secondly, the experimental analysis results in Sections 4.1 and 4.2 show that the ionospheric and troposphere atmospheric modeling and interpolation accuracy are negatively correlated with the satellite elevation angle. In Section 2.2 of atmospheric modeling strategy, this paper gives a preliminary method of ionospheric weighting based on elevation angle, but a more refined modeling weighting method considering the influence of elevation angle needs to be further studied.
* Finally, Section 4.3 of this paper takes the GPS system as reference and designs five schemes to preliminarily evaluate the positioning performance of network RTK of BDS-3. The horizontal error of each scheme is basically within 3 cm, and the elevation error is within 5 cm, which can meet the requirements of providing network RTK high-precision service independently. In addition, the terminal positioning effect statistics show that the positioning accuracy of BDS-2 is relatively poor, the positioning accuracy of BDS-3 is only slightly better than that of GPS, and the combined processing effect of BDS-2/3 using BII+B3I transition signal is the best. The reason why the positioning performance of BDS-3 is slightly better than that of GPS may be attributed to the fact that the experimental data in this paper were collected in central China. Compared with GPS, the number of BDS-3 satellites and the distribution of DOP values in the Asia-Pacific region are obviously more advantageous. The number of satellites and the distribution of PDOP values of Station SC17, shown in Figure 9, can also be seen as related laws. Further comparative analysis requires further data collection of more regions for experimental demonstration.  
In addition, the theoretical basis of network RTK in this paper is based on the double-difference mode. In the aspect of algorithm practice, the double-difference model theory is relatively simple and does not need the support of precise products. However, when there are too many reference stations, the double-difference model has a large amount of computation. With the development of PPP-RTK technology [30], the non-difference model is used to fix the non-difference ambiguity of reference stations, and then the independent non-difference atmospheric delay of each station is extracted. It can be compatible with the traditional double-difference network RTK technology, and the product generation strategy under different modes needs to be further studied.