## 6 Conclusions  
Using multi-GNSS can substantially improve the fixing percentage of PPP-IAR. GLONASS is the second worldwide positioning system, and the use of GPS + GLONASS will contribute greatly to the performance of PPP-IAR. However, IFCB prevents GLONASS ambiguity fixing with heterogeneous receivers. This paper proposes a method of calibrating GLONASS IFCB, such that PPP-IAR can be performed across heterogeneous rover receivers. This will make it possible to provide commercial real-time PPP service with rapid ambiguity resolution using multi-GNSS constellations.  
We first assessed the efficiency of the method with 21-day data collected by a network of 66 Trimble NetR8 receivers. As homogeneous receivers are used for both UPD estimation and PPP-IAR, PPP-IAR can be performed without considering the IFCB effect and can be used to assess the performance of the IFCB calibration method. The results show that we need only one day of data to estimate the IFCB, and the estimated daily IFCB for different days is consistent for better than 0.05 cycles (RMS). Upon adding GLONASS without IFCB correction, the fixing percentage within 5 min improves from  
\begin{table}
\begin{tabular}{c c c c} \hline
**Solution Type** & **North** & **East** & **Up** \\ \hline GPS Only & 0.80 & 0.68 & 2.79 \\ GPS + GLONASS & 0.66 & 0.60 & 2.45 \\ With IFCB & & & \\ \hline \end{tabular}
\end{table}
Table 6: Position RMS (cm) for kinematic ambiguity fixed PPP solutions.  
13.23 to 69.94%. We then further applied the IFCB calibration and found that the percentage can even be slightly improved by 0.28%.  
We then used a network of 29 heterogeneous rover receivers to validate the method and the data collection also extended over 21 days. Before applying the IFCB calibration, the fixing percentage for GPS + GLONASS is even smaller than the GPS-only solution. Upon applying the IFCB calibration, the fixing percentage improves for receivers from each manufacturer. The average fixing percentage within 15 min for the 12 Leica, 5 TPS, and 12 Trimble receivers is 93.4%, 92.3%, and 92.6%, respectively. These values are comparable to results for reference stations of homogeneous receivers. Upon adding GLONASS, the positioning accuracy of ambiguity-fixed PPP solutions improves by 17.5%, 11.8%, and 12.1% for the north, east, and upward directions. These results confirm that the proposed IFCB calibration method is efficient for GPS + GLONASS PPP-IAR with heterogeneous rover receivers.  
We thank the Crustal Movement Observation Network of China (CMONOC), IGN and ESA for providing the observation data and precise products. This work was partially supported by the National Key Research and Development Program of China (Nos. 2016 YFB0800405, 2016 YFB0502203, 2016 YFB0501802), National Natural Science Foundation of China (Nos. 41704033, 41074008, 41504028, 41374034, 41404010, 41301511, 41604028, 41604017, 41401444), National 973 Program of China (No. 2012 CB95701), and Shenzhen future industry development funding program (No. 201507211219247860).  
[PERSON] conceptualized the initial idea and experimental design. [PERSON] performed the experiments and wrote the main manuscript text. [PERSON] helped with data collection and results analysis.  
The authors declare no conflict of interest.