## 3 Results  
### Precise Products for BDS, GPS, GLONASS, Galileo and QZSS  
As GNSS moves towards multi-system integration, the MGEX project [28], aiming at the continuous tracking, collection and analysis of all GNSS (GPS, GLONASS, Galileo, BDS) and RNSS (QZSS) satellites, has been continuously expanded and improved. Currently, there are seven analysis centers capable of providing multi-system precise satellite orbit and clock products, including CNES/Collectic Localisation Satellites (CLS)/Groupe de Recherche de Geodesie Spatiale (GRGS) [29], Center for Orbit Determination in Europe (CODE) [30], GFZ [31], Information and Analysis Center (IAC) [32], Japan Aerospace Exploration Agency (JAXA) [33], Shanghai Observatory (SHAO) [34] and WH-U [35]. It should be noted that the analysis center Technische Universitat Munchen (TUM) has no longer provided the precise products for MGEX project since 10 November 2019. The analysis center IAC started to synchronize the multi-system precise products to the Crustal Dynamics Data Information System (CDDIS) data server on 12 September 2020, and these products were uploaded to the internal File Transfer Protocol (FTP) (ftp.glonass-iac.ru) before this. The multi-system precise products provided by these analysis centers are all stored at the CDDIS MGEX product archive ([[https://cddis.nasa.gov/archive/gnss/products/mgex/](https://cddis.nasa.gov/archive/gnss/products/mgex/)]([https://cddis.nasa.gov/archive/gnss/products/mgex/](https://cddis.nasa.gov/archive/gnss/products/mgex/)), accessed on 25 September 2021) as well as the mirror sites hosted by Institute Geographique National (IGN) ([[ftp://igs.ign.fr/pub/igs/products/mgex/](ftp://igs.ign.fr/pub/igs/products/mgex/)]([ftp://igs.ign.fr/pub/igs/products/mgex/](ftp://igs.ign.fr/pub/igs/products/mgex/)), accessed on 25 September 2021) and Ecole Nationale des Sciences Geographiques (ENSG) ([[ftp://igs.ensg.eu/pub/igs/products/mgex/](ftp://igs.ensg.eu/pub/igs/products/mgex/)]([ftp://igs.ensg.eu/pub/igs/products/mgex/](ftp://igs.ensg.eu/pub/igs/products/mgex/)), accessed on 25 September 2021). Besides, the European Space Operations Centre of the ESA [36] can also offer the multi-system precise products ([[http://navigation-office.esa.int/products/gnss-products/](http://navigation-office.esa.int/products/gnss-products/)]([http://navigation-office.esa.int/products/gnss-products/](http://navigation-office.esa.int/products/gnss-products/)), accessed on 25 September 2021). Because of the rapid growth in demand for real-time precise applications, several analysis centers start to transmit the real-time precise corrections for satellite orbits and clocks based on the State Space Representation (SSR) messages defined by Radio Technical Commission for Maritime Services (RTCM) through Networked Transport of RTCM via Internet Protocol (NTRIP). The analysis center CNES is the first institution to provide real-time precise products, and the transmitted real-time stream CLK93 can support all the four GNSSs. Although the CNES real-time products do not incorporate QZSS, they are used to assess the performance of real-time PPP under the current GNSS constellation status in this study. For simplicity, we employ the retrieved precise satellite orbit and clock products based on the real-time stream CLK93 and the broadcast ephemeris, which can be downloaded from the PPP-Wizard (Precise Point Positioning With Integer and Zero-difference Ambiguity Resolution Demonstrator) website ([[http://www.ppp-wizard.net/products/REAL_TIME/](http://www.ppp-wizard.net/products/REAL_TIME/)]([http://www.ppp-wizard.net/products/REAL_TIME/](http://www.ppp-wizard.net/products/REAL_TIME/)), accessed on 25 September 2021). Table 1 conducts an overview of the precise satellite orbit and clock products from multi-system analysis centers as of January 2021.