## 4 Analysis And Assessment  
### Experimental Strategy  
#### 4.1.1 Configuration Strategy  
For the sake of improving the accuracy and convergence time, some errors in PPP can be corrected by corresponding methods. All error corrections are shown in Table 3.  
Our self-developed MATLAB processing software based on Prototyping RTKLIB2.4.3 is used to perform multi-GNSS globally distributed reference network experiments. The initial values and corresponding variance-covariance of XYZ and \(dt\) are for the least squares solution in the first epoch. The initial value of \(\mathit{Tw}\) is calculated by the Hopfield model in first epoch, the corresponding variance-covariance is set to arbitrary value (0.25 in experiment). \(N_{i}\) and \(I\) are derived from undifferenced observation equations [35]. The noises of pseudorange and carrier phase are usually set to 0.3 m and 0.003 m [10], the weight ratio of pseudorange and carrier phase is 100. And the variance of dynamic noise in receiver clock error is 900 m\({}^{2}\)/s\({}^{2}\), the variance of dynamic noise in troposphere Zenith wet path delay is \(10^{-8}\) m\({}^{2}\)/s\({}^{2}\), the variance of dynamic noise in ionosphere is \(10^{-6}\)m\({}^{2}\)/s\({}^{2}\)[36]. The verification used daily (24 h) solutions in day of year (DOY) 100, 2018. General settings adopted for the PPP validation are provided in Table 4. The igs14 atx file provided by International GNSS Service (IGS) only have the Antenna Phase Center Variations (PCVs) and Antenna Phase Center Offsets (PCOs) on first two frequencies (L1/L2) in GPS. The PCV and PCO on the third frequency L5 in GPS can not be gotten directly. The L5 frequency (1176.45 Mhz) is approximate to the L2 frequency (1227.6 Mhz), we simple presume that the L5 frequency shares the same PCOs/PCVs as those on the L2 frequency [18]. Meanwhile, the PCV and PCO in BDS are set to constant zero and [0.6, 0, 1.1], respectively [37]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Model** & **System** & **Signal Combination** & \(\mathbf{k_{1}}\) & \(\mathbf{k_{2}}\) & \(\mathbf{k_{3}}\) & \(\boldsymbol{\varepsilon}\) (Cycle)** & **Noise (m)** \\ \hline \multirow{4}{*}{CIF2} & GPS & \(P_{1}\), \(P_{2}\) & \(2.5457\) & \(-1.5457\) & 0 & 297.8 & 0.8934 \\  & \(L_{1}\), \(L_{2}\) & \(2.5457\) & \(-1.5457\) & 0 & 2.978 & 0.0089 \\  & BDS & \(P_{1}\), \(P_{2}\) & \(2.4872\) & \(-1.4872\) & 0 & 289.8 & 0.8694 \\  & \(L_{1}\), \(L_{2}\) & \(2.4872\) & \(-1.4872\) & 0 & 2.898 & 0.0087 \\  & GPS & \(P_{1}\), \(P_{2}\), \(P_{5}\) & \(2.3269\) & \(-0.3596\) & \(-0.9673\) & 254.5 & 0.7635 \\  & \(L_{1}\), \(L_{2}\), \(L_{5}\) & \(2.3269\) & \(-0.3596\) & \(-0.9673\) & 2.545 & 0.0076 \\  & BDS & \(P_{1}\), \(P_{2}\), \(P_{3}\) & \(2.5664\) & \(-1.2289\) & \(-0.3375\) & 286.5 & 0.8596 \\  & L\({}_{1}\), \(L_{2}\), \(L_{3}\) & \(2.5664\) & \(-1.2289\) & \(-0.3375\) & 2.865 & 0.0086 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Combinations coefficients of different fundamental signals in CIF2 and CIF3 for GPS and BDS. As to measurement noise (m), it is defined as \(\sigma_{p}=0.3\) m, \(\sigma_{L}=0.003\) m in both the Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS). \begin{table}
\begin{tabular}{c c} \hline \hline
**Error Corrections** & **Setting** \\ \hline DCB & Multi-GNSS Experiment (MGEX) \\ Cycle slip & Melbourne-Wubbena detecting (M-W) and Ionosphere Residuals \\ Clock slip & Guo [33] \\ Observation weighting & Witchayangkoon [34] and Helmert \\ Earth rotation & Sagnac effect \\ Relativistic effects & Xu and Xu [23] \\ Troposphere & Random walk + Hopfield + Global Mapping Function (GMF) \\ Antenna phase center & Antenna Phase Center Offsets (PCO)+Antenna Phase Center Variations (PCV) \\ Phase window & Corrected \\ Earth tides correction & Solid/Pole tide \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Error corrections. Differential code bias (DCB). There are 146 globally distributed reference stations at MGEX, of which 39 stations have the coordinates in the file gbm19962.clk.