## 3 Results
### Performance of Four-System Integrated SPP with a Priori ISB Constraints
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Strategy**} & \multirow{2}{*}{**Directions**} & \multicolumn{6}{c}{**Elevation Mask Angles**} \\ \cline{3-6}  & & **10\({}^{\circ}\)** & **20\({}^{\circ}\)** & **30\({}^{\circ}\)** & **40\({}^{\circ}\)** & **50\({}^{\circ}\)** \\ \hline \multirow{4}{*}{Without A} & East (m) & 0.537 & 0.543 & 0.642 & 1.092 & 2.103 \\  & North (m) & 0.933 & 0.890 & 1.001 & 1.699 & 3.162 \\ \cline{1-1}  & Up (m) & 2.804 & 2.688 & 2.964 & 4.911 & 9.238 \\  & 3D (m) & 3.003 & 2.883 & 3.193 & 5.310 & 9.988 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.522 & 0.517 & 0.600 & 0.856 & 1.574 \\  & North (m) & 0.906 & 0.846 & 0.910 & 1.239 & 2.346 \\ \cline{1-1}  & Up (m) & 2.696 & 2.549 & 2.684 & 3.728 & 7.128 \\ \cline{1-1}  & 3D (m) & 2.892 & 2.735 & 2.897 & 4.020 & 7.667 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.532 & 0.529 & 0.614 & 0.873 & 1.612 \\  & North (m) & 0.907 & 0.849 & 0.918 & 1.249 & 2.384 \\ \cline{1-1}  & Up (m) & 2.690 & 2.548 & 2.702 & 3.792 & 7.321 \\ \cline{1-1}  & 3D (m) & 2.888 & 2.737 & 2.919 & 4.086 & 7.866 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.537 & 0.536 & 0.627 & 0.895 & 1.648 \\ \cline{1-1}  & North (m) & 0.908 & 0.852 & 0.926 & 1.264 & 2.416 \\ \cline{1-1}  & Up (m) & 2.695 & 2.553 & 2.727 & 3.860 & 7.526 \\ \cline{1-1}  & 3D (m) & 2.894 & 2.745 & 2.947 & 4.159 & 8.075 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.545 & 0.546 & 0.638 & 0.915 & 1.676 \\ \cline{1-1}  & North (m) & 0.906 & 0.853 & 0.928 & 1.270 & 2.434 \\ \cline{1-1}  & Up (m) & 2.705 & 2.569 & 2.768 & 3.939 & 7.695 \\ \cline{1-1}  & 3D (m) & 2.904 & 2.762 & 2.988 & 4.239 & 8.243 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.556 & 0.563 & 0.658 & 0.941 & 1.739 \\ \cline{1-1}  & North (m) & 0.913 & 0.865 & 0.947 & 1.305 & 2.517 \\ \cline{1-1}  & Up (m) & 2.708 & 2.588 & 2.826 & 4.058 & 8.058 \\ \cline{1-1}  & 3D (m) & 2.911 & 2.787 & 3.052 & 4.365 & 8.619 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.569 & 0.582 & 0.684 & 0.983 & 1.812 \\ \cline{1-1}  & North (m) & 0.914 & 0.874 & 0.966 & 1.344 & 2.596 \\ \cline{1-1}  & Up (m) & 2.702 & 2.597 & 2.884 & 4.227 & 8.430 \\ \cline{1-1}  & 3D (m) & 2.909 & 2.801 & 3.117 & 4.544 & 9.005 \\ \hline \multirow{4}{*}{With A} & East (m) & 0.575 & 0.591 & 0.692 & 1.002 & 1.861 \\ \cline{1-1}  & North (m) & 0.911 & 0.873 & 0.966 & 1.353 & 2.635 \\ \cline{1-1}  & Up (m) & 2.698 & 2.604 & 2.907 & 4.316 & 8.700 \\ \cline{1-1}  & 3D (m) & 2.906 & 2.809 & 3.140 & 4.633 & 9.279 \\ \hline \end{tabular}
\end{table} 
Table 4: Positioning accuracy in (m) of four-system integrated SPP with and without a priori ISB constraints under different elevation masks. Table 6 provides the availability, satellite number, and position dilution of precision (PDOP) value of four-system integrated SPP with and without a priori ISB constraints under different elevation masks. The average satellite number and average PDOP value over all the available epochs (from all the employed stations and days in this section) are calculated. As to the availability, it is defined as the percentage of the epochs at which the position solutions can be acquired over the total epochs. For the four-system SPP ignoring a priori ISB constraints, the availability starts to decrease at an elevation mask of 40\({}^{\circ}\) and is reduced to only 64.0% at an elevation mask of 50\({}^{\circ}\). By contrast, the availability still remains at 100.0% for the four-system SPP considering a priori ISB constraints when the cutoff elevation is increased to 40\({}^{\circ}\) and can be up to approximately 90.0% at an elevation mask of 50\({}^{\circ}\).