# Evaluation of BDS/GPS Multi-Frequency RTK Positioning Performance under Different Baseline Lengths
## 5 Conclusions  
In this paper, based on the five sets of baselines with seven-day observation data, four aspects of multi-GNSS multi-frequency RTK were evaluated successively in terms of the number of satellites and PDOP values, positioning accuracy, ratio values, and the time to first fix of ambiguity. We draw the following conclusions:  
* The positioning accuracy of BDS-2+BDS-3 (B1I/B3I) dual-frequency RTK has reached a comparable performance to GPS (L1L2) in short baseline RTK and is slightly better than GPS (L1L2) in long baseline RTK.
* The BDS-3 (B1C/B2a/B3I) triple-frequency RTK has the highest positioning accuracy and is higher than BDS-3 (B1C/B2a) and BDS-3 (B1I/B3I) in terms of both positioning accuracy and ambiguity fixing rate. However, the improvement relative to BDS-3 (B1C/B2a) is not significant.
* The positioning accuracies of single-frequency bands B1C, BII, L1, and B3I were about the same, while the accuracy for band B2a was lower.
* As the baseline length increases, the positioning accuracy of the five frequencies decreases, the ratio value becomes smaller, and the TTFF becomes longer.
* The ambiguity fixing rates of GPS (L1L2) and BDS-2+BDS-3 (B1I/B3I) are comparable and are at the highest level among the five frequencies. GPS (L1L2) has the shortest time to first fix and BDS-2+BDS-3 (B1I/B3I) has the second shortest.
* BDS-3 (B1C/B2a) dual-frequency RTK has better than 2 cm positioning accuracy in the N and E directions and better than 3 cm positioning accuracy in the vertical direction within 100 km.  
In this paper, the selection of stations was limited by the length of the baseline. All the stations selected are located in Europe, not involving the Asian region and the American region, which has certain regional limitations. For the analysis of positioning performance, we chose only BDS and GPS, and the baseline length only involves long baselines, and no in-depth study has been conducted for very long baselines (>500 km). Only static data are used; no real dynamic data are involved in the evaluation. Future research in this area should be oriented to a wider area and introduce more satellite navigation systems.  
[PERSON] researched the performance evaluation method. [PERSON]. ([PERSON]) validated the results and undertook the writing. [PERSON] researched the model and revised the written work. [PERSON] investigated the related work. [PERSON] prepared the data. [PERSON] provided the funding. [PERSON]. ([PERSON]) helped validated the results. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the National Natural Science Foundation of China (62173237, 71731001), the Open Fund of Key Laboratory of Civil Aviation Flights Wide Area Surveillance and Safety Control technology for of Civil Aviation University of China (202105), the Applied Basic Research Programs of Liaoning Province (2022063), the Talent Project of Revitalization Liaoning (CLYCI907022), the Key R & D projects of Liaoning Province (2020 JH2/10100045), the Natural Science Foundation of Liaoning Province (2019-MS-251), the Scientific Research Project of Liaoning Provincial Department of Education (JYT2020142), and the High-Level Innovation Talent Project of Shenyang (RC190030).  
All data can be available from Multi-GNSS Experiment (MGEX) or by contacting the primary author.  
The authors declare no conflict of interest.