# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 2 Data, Methods, and Strategy  
In recent years, more IGS/MGEX stations were updated to track Galileo and BDS, as well as QZSS and Indian Regional Navigation Satellite System (IRNSS) satellites. The data observed from stations were archived in the DCs from IGS/MGEX, which can be downloaded freely. However, the distribution of the stations, which can track the BDS and Galileo, are not as good as GPS and GLONASS. Thus, in this contribution, the observation data from stations of iGMAS, which are distributed globally, and the Crustal Movement Observation Network of China (CMONOC), mainly distributed in the Chinese mainland, are used. Approximately 120 stations are used to generate the hourly ultra-rapid orbit and clock products in this study, which include 60-70 stations that can receive BDS and Galileo observation data, as shown in Figure 1. The blue circles represent the stations, which can track GPS satellites; the green circles represent stations tracking GLONASS; the yellow circles show the stations tracking Galileo, and the red circles represent the stations tracking BDS satellites. In order to improve the efficiency for downloading the observation data and broadcast ephemeris, the optimized parallel scripts are coded with a remote scanning method in the Linux server, which runs by an automatic operation mode to retrieve data from IGS/MGEX DCs, such as the Crustal Dynamics Data Information System (CDDIS), Instituto Geografico Nacional (IGN), The Federal Agency for Cartography and Geodesy (BKG) and Chinese tracking station networks from iGMAS and CMONOC. A new method--combined serial and parallel threads (CSPT)--is studied in this contribution. It is different from previous studies [8; 11] when it comes to improving the stability and reliability of the products--avoiding some problems that may be caused by satellites (from a single satellite system), and reducing the time consumption and latency of the ultra-rapid orbit and clock products. The CSPT method is introduced in the Appendix A.  
Figure 1: Distribution of the global tracking stations used to generate hourly ultra-rapid orbit and clock global navigation satellite system (GNSS) products. The blue circles represent the stations, which can track American Global Positioning System (GPS) satellites; the green circles represent the stations tracking Ruassian Global’naya Navigatsionnaya Sputnikova Sistema (GLONASS); the yellow circles represent the stations tracking European Union’s Galileo, and the red circles represent the stations tracking Chinese BeiDou navigation satellite system (BDS) satellites. Table 1 shows the strategy of the CSPT method for GNSS product generation in this study. This method includes four serial threads, which were operated in parallel mode, and arranged to generate the hourly orbit and clock products for GPS, GLONASS, Galileo, and BDS. Moreover, the GLONASS, Galileo, and BDS were coupled with GPS in their threads to improve the accuracy of their products. The inter-system biases (ISBs) were estimated in the coupled satellite systems for Galileo and BDS, while the inter-frequency bias (IFB) parameters were estimated as constants, without constrains, for each GLONASS satellite and station pair, for the Frequency Division Multiple Access (FDMA), applied by GLONASS to recognize the different satellites. Full operational capability (FOC) satellites should be included in the products as much as possible, in order to provide better service to users. In this study, about 32 and 24 Medium Earth Orbit (MEO) satellites were included for GPS and GLONASS, which were established earlier and showed a notable performance. Moreover, about 24 MEO satellites for Galileo, and five Geostationary Orbits (GEO), five satellites in Inclined Geosynchronous Orbit (IGSO), and four MEO for BDS-2 satellites were also included in this contribution.