## 2 Current Status of GNSS Integrity Service
### Development Trends in GNSS Integrity Services  
Recently, all GNSS service providers have been working on improving the navigation integrity performance of their own GNSS constellations. Improving the performance of integrity services is one of the key objectives. This part will summarize the development trends in integrity services in different GNSS constellations and provide references for the development of the integrity services within the next-generation BeiDou constellation. Among all GNSS constellations, the GPS has the longest service history and the best integrity performance. The promising performance of GPS integrity services comes from two facts. First, the GPS has established a network of ground monitoring stations globally, providing high-quality ground integrity monitoring capabilities. Second, over decades of development, the GPS has accumulated maintenance experience, which helps to significantly reduce the probability of satellite faults and enable rapid responses in the event of faults. To further enhance the integrity performance of the GPS, the GPS modernization plan explicitly aims to improve integrity monitoring capabilities with the SAIM technique. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**System** & **Correction Parameters** & **Accuracy Parameters** & **Degradation Parameters** \\ \hline \multirow{3}{*}{**WAAS**} & Fast Corrections, Long-Term & \multirow{3}{*}{UDRE, GIVE, and Clock Ephemeris} & \multirow{3}{*}{Fast Correction Degradation Factor} \\  & Corrections, Mixed Corrections, and Ionospheric Delay Corrections & & Covariance Matrix Message & \\ \hline \multirow{3}{*}{EGNOS} & Fast Corrections, Long-Term & \multirow{3}{*}{UDRE and GIVE} & \multirow{3}{*}{Fast Correction Degradation Factor} \\  & Corrections, Mixed Corrections, and Ionospheric Delay Corrections & & \\ \hline \multirow{3}{*}{SDCM} & Fast Corrections, Long-Term & \multirow{3}{*}{UDRE, GIVE, and Clock Ephemeris} & \multirow{3}{*}{Fast Correction Degradation Factor} \\  & Corrections, Mixed Corrections, and Ionospheric Delay Corrections & & Covariance Matrix Message & and Degradation Parameters \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of SBAS integrity parameters among WAAS, EGNOS, and SDCM. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**System** & **Satellite Type** & **PRN Code** & **Satellite Position** & **Certified Service Level** \\ \hline \multirow{3}{*}{**WAAS**} & Eutelsat 117 West B & 131 & 117°W & \\ \cline{2-5}  & SES-15 & 133 & 129°W & Single-Frequency LPV-200 \\ \cline{2-5}  & Intelsat Galaxy 30 & 135 & 125°W & \\ \hline \multirow{3}{*}{**EGNOS**} & ASTRA 5B & 123 & 31.5°E & \\ \cline{2-5}  & INMARSAT 4F2 & 126 & 63.9°E & Single-Frequency NPA \\ \cline{2-5}  & HOTBIRD 13G & 136 & 5°E & \\ \hline \multirow{3}{*}{**SDCM**} & Luch-5B & 125 & 16°W & \\ \cline{2-5}  & Luch-5V & 140 & 95°E & Not Certified \\ \cline{2-5}  & Luch-5A & 141 & 167°E & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Certification status of WAAS, ENGOS, and SDCM. Also, America is actively developing LEO constellations such as PULSAR and TruePoint, considering the use of LEO constellations to enhance GPS integrity performance. Meanwhile, the European Space Agency (ESA) has initiated the development of the second-generation Galileo constellation (G2). The aim of the second-generation Galileo constellation is to provide more robust and resilient positioning, navigation, and timing (PNT) services for global users. To enhance navigation integrity, the G2 satellites will be equipped with six sets of enhanced atomic clocks and inter-satellite links to improve the capabilities of SAIM. Additionally, the ESA has proposed the LEO-PNT initiative, which aims to establish a Kepler constellation consisting of six high-value LEO satellites. One of the goals of this initiative is to use LEO satellites to enhance the integrity of the Galileo satellites.