# Development Status and Service Performance Preliminary Analysis for BDSBAS
## 2 BDSBAS Architecture Overview  
BDSBAS is operated by the BDS control system and consists of a regional monitoring network in China, two master control stations and three GEO satellites. The two master control stations are backup to each other to enhance the system robustness. To meet the needs of current SBAS users as well as the next generation of SBAS system construction, two parallel threads are dealt with in each master control station processing center for SF service messages and DFMC service messages, respectively. BDSBAS has chosen a regional network for the SBAS services. To estimate and model the regional ionosphere for SF service, the receiver network is evenly distributed in the service area of China [23]. Each monitoring station is equipped with three independent receivers identified as A/B/C which can receive BDS and GPS pseudorange and carrier phase observation data at present. The observation data from monitoring stations are sent to the master control stations through ground and satellite network links for information processing. Some of monitoring stations equipped with hydrogen atomic clocks are identified as class I stations to ensure the system time reference stability, and the others are identified as class II stations which are widely distributed in the service area. Both the measurements of class I and class II monitoring stations are used to compute differential corrections and integrity information. The workflow of the BDSBAS system is shown in Figure 1. \begin{table}
\begin{tabular}{l c c} \hline \hline  & **SF Service** & **DFMC Service** \\ \hline Broadcast Signal & BDSBAS- & BDSBAS-B2a \\ \hline Broadcast Satellites & 3 GEO & 3 GEO \\ \hline Augmentation constellation(s) & GPS & BDS/GPS/Galileo/GLONASS \\ \hline SBAS Network Time & GPS Time & BDS Time \\ \hline Clock and Ephemeris slow corrections & Broadcast & Broadcast \\ \hline Clock fast corrections & Broadcast & Not Broadcast \\ \hline Clock–Ephemeris correction integrity & Broadcast & Broadcast \\ \hline Clock–Ephemeris covariance matrix & Broadcast & Broadcast \\ \hline Ionospheric grid delay corrections and integrity & Broadcast & Not Broadcast \\ \hline Degradation information & Broadcast & Broadcast \\ \hline SBAS (GEO) satellites ephemeris and almanacs & Broadcast & Broadcast \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the BDSBAS SF service and the DFMC service. The master control station is the data processing center. The received GNSS measurements are first dealt with at the preprocessing facility to identify and eliminate outliers, detect the cycle slip of carrier phase data, correct the code noise and multipath errors in real time, and compute the common errors in the propagation path, such as tropospheric delay, ionospheric delay, receiver antenna phase center corrections. The code noise and multipath errors are corrected in real time with the code noise and multipath correction (CNMC) algorithm which is a kind of carrier-smoothing pseudorage method detail described in [23]. The tropospheric delay is corrected with Black model and the ionospheric delay is corrected with dual-frequency iono-free combination. The preprocessed observations are then sent to the differential correction processing (CP) facility and integrity processing (IP) facility to calculate the differential corrections and evaluate the system output safety. The CP facility selects preprocessing measurements from A receivers of the monitoring stations, and calculates the differential corrections for SF and DFMC services. For SF service, GPS ephemeris and clock-slow corrections, clock-fast corrections and ionospheric grid delay corrections are determined with respect to L1 C/A signal. For DFMC service, ephemeris and clock corrections are calculated for multiple constellations with dual-frequency. The calculation methods of ephemeris and clock corrections were introduced by [24; 25]. GEO Cartesian and Keplerian parameters as well as almanacs are also computed. The corrections are then passed along to the IP facility for evaluation.