## 2 Strategy
### PPP Integer Ambiguity Resolution (IAR)  
PPP IAR has the capacity of shortening the convergence time and improving accuracy. To do so, the so-called fractional cycle biases (FCB), _i.e._, the satellite hardware biases, are needed. In this study, the approach proposed by [13] is used. In the reference side, the satellite FCB products of wide-lane and narrow-lane are estimated by utilizing the averaging of fractional parts of all pertinent wide-lane and narrow-lane ambiguity estimates derived from the Melbourne-Wubbena and ionosphere-free combination measurements, respectively. In the mobile side, the wide-lane and narrow lane FCB  
\begin{table}
\begin{tabular}{l l} \hline \hline Observable & Undifferenced Ionosphere-free combination of phases and \\  & code for GPS/GLONASS L1 and L2 as well as BDS B1 and B2 \\ \hline Elevation angle cutoff & \(10^{\circ}\) \\ \hline Sampling rate & 30 s \\ \hline Weight & Elevation-dependent weighting for the observations under \(30^{\circ}\) \\  & according to \(1/2\) sin(E) \\ \hline Code biases & C1 observations are corrected to P1 \\ \hline Estimator & Square Root Information Filter (SRIF) \\ \hline Troposphere & A prior model plus wet-component delay \\  & Mapping function: Vienna Mapping Function(VMF) \\ \hline Satellite phase center correction & GPS, GLONASS from igso8.txt \\  & BDS: WHU estimated values [10,11] \\ \hline Receiver phase center correction & GPS and GLONASS from igso8.txt \\ \cline{2-3}  & Corrections for GPS L1 and L2 are also used for BDS B1 and B2 \\ \hline Initial standard deviations & 2.0 m and 2.0 cm for raw code and phase of GPS, GLONASS as well as BDS IGSO and MEO \\  & 4.0 m and 4.0 cm for raw code and phase of BDS GEO \\ \hline Receiver clock & Estimated as epoch-wise white noise \\ \hline ISB/IFB & Estimated as arc-dependent constants for each receiver or \\  & receiver-satellite pair \\ \hline Coordinates & Constants in static mode \\  & Epoch-wise parameters in kinematic mode \\ \hline \hline \end{tabular}
\end{table}
Table 1: The strategy used for PPP.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**PPP Solutions** & **Abbreviations** \\ \hline GPS only & GPS \\ BDS only & B \\ BDS GEO and IGSO & GI \\ BDS GEO and MEO & GM \\ BDS IGSO and MEO & IM \\ GPS plus BDS & G + B \\ GPS plus GLONASS & G + R \\ GPS plus BDS GEO & GPS + G \\ GPS plus BDS IGSO & GPS + I \\ GPS plus BDS MEO & GPS + M \\ GPS plus BDS GEO and IGSO & GPS + GI \\ GPS plus BDS GEO and MEO & GPS + GM \\ GPS plus BDS IGSO and MEO & GPS + IM \\ GPS, plus GLONASS, BDS IGSO and MEO & GPS + RIM \\ \hline \hline \end{tabular}
\end{table}
Table 2: PPP solutions used in this study and their abbreviations.  
are applied to the corresponding ambiguities in order to remove the satellite hardware delay. As far as the receiver biases, single-differences between satellites are performed via selecting a reference satellite. The integer resolution strategy can be applied to the wide-lane firstly, and then to the narrow-lane ambiguities with being applied the fixed integers of wide-lane. Once the wide-lane and narrow-lane ambiguities have been fixed, the least-squares ambiguity decorrelation adjustment (LAMBDA) approach is used for ambiguity research, and the common used ratio test is used for ambiguity validation. The threshold of the ratio value is set as 3.0 [14]. In this contribution, we only made an attempt to fix the integer ambiguities for GPS phase observables.  
In this study, once the positioning error in three dimensions is less than one decimeter, and the accuracy criterion is met in the next 10 epochs; the positioning result is viewed as converged, and the convergence time is calculated from the beginning of positioning to the converged epoch. It should be mentioned that the positioning error is nothing but the difference between the solution and the ground-truth value.