## 3 Results
### ISB Estimates Derived from Low-Cost Receivers  
In order to investigate the ISB characteristics of low-cost receivers, a SPP experiment with the u-blox M8T receiver was conducted within the new campus of the Central South University in Changsha, China, on 31 October to 1 November 2021 (DOY 304 to 305 of 2021). Figure 12 presents the observation equipment for the SPP experiment. The u-blox M8T receiver with an active antenna with 3 m cable was connected with a laptop through the Universal Serial Bus (USB) socket to collect the GNSS data. The built-in USB interface can provide both power supply and high-speed data transfer. The observation equipment was set up on the roof of the Geoscience Building of the Central South University. To obtain the references for the SPP solutions based on the double-differenced RTK results, a MS352 receiver (manufactured by Guangzhou Hi-Target Satellite Navigation Technology Co., Ltd., Guangzhou, China) with a BY500 antenna (manufactured by Hunan BYNAV Technology Co., Ltd., Changsha, China) was set up as a base station, which was only several meters away from the rover station (u-blox M8T receiver). It is noted that the employed u-blox M8T receiver only supports the GPS and GLONASS satellites but cannot track the Galileo and BDS (BDS-3/BDS-2) satellites. The SPP experiment lasted for approximately 12 h from 2:00 to 14:00 (GPS Time) for each day. The sampling interval was 1 s.  
Figure 13 provides the epochwise ISB estimates between GLONASS and GPS derived from the dual-system integrated SPP solutions with the low-cost u-blox M8T receiver at an elevation mask angle of 10\({}^{\circ}\) on DOY 304 and 305 of 2021. The varying range can be up to approximately 30 ns for the epochwise ISBs, with a STD statistic over the 12 h of 4.900 and 4.565 ns on the two days, respectively. The average value of epochwise ISBs over the 12 h is taken as the daily ISB. The prediction accuracy of daily ISB with a time span of prediction of a day is about 4 ns. Both the short-term stability of epochwise ISB and the single-day prediction accuracy of daily ISB are worse than those obtained with the geodetic receivers (see Figures 5 and 9). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Strategy**} & \multirow{2}{*}{**Items**} & \multicolumn{6}{c}{**Elevation Mask Angles**} \\ \cline{3-6}  & & **10\({}^{\circ}\)** & **20\({}^{\circ}\)** & **30\({}^{\circ}\)** & **40\({}^{\circ}\)** & **50\({}^{\circ}\)** \\ \hline \multirow{2}{*}{Without A} & Availability (\%) & 100.0 & 100.0 & 100.0 & 97.9 & 64.0 \\  & Num. of Sats. & 30.7 & 23.9 & 17.8 & 12.6 & 9.3 \\ \multirow{2}{*}{Prior ISB} & PDOP & 1.0 & 1.3 & 2.1 & 3.8 & 7.0 \\ \hline \multirow{2}{*}{With A} & Availability (\%) & 100.0 & 100.0 & 100.0 & 100.0 & 89.3–89.6 \\ \multirow{2}{*}{Prior ISB} & Num. of Sats. & 30.7 & 23.9 & 17.8 & 12.5 & 8.4 \\ \multirow{2}{*}{(1–7 days)} & PDOP & 1.0 & 1.3 & 2.1 & 3.8 & 8.2–8.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Availability, satellite number, and PDOP value of four-system integrated SPP with and without a priori ISB constraints under different elevation masks. Figure 12: Observation equipment for the SPP experiment. To investigate the performance enhancement of a priori ISB constraints for the SPP solutions with the low-cost u-blox MST receiver, the GPS/GLONASS integrated SPP processing with and without a priori ISB constraints based on the 12-h data sets on DOY 305 of 2021 under the cutoff elevations from 10\({}^{\circ}\) to 50\({}^{\circ}\) in steps of 10\({}^{\circ}\) is carried out. The a priori ISBs are acquired by prediction with the daily ISB estimates of the previous day (DOY 304 of 2021). For the SPP solutions ignoring a priori ISBs, the availability decreases to 89.7% at an elevation mask of 40\({}^{\circ}\) and is further reduced to only 18.6% at an elevation mask of 50\({}^{\circ}\). After introducing the a priori ISB constraints, the corresponding availability increases to 92.9% and 28.5%, respectively.