###### Abstract
In recent years, two newly emerging satellite systems, namely BeiDou Navigation Satellite System (BDS) and Galileo, have also joined the GNSS community. Following the \"three-step\" strategy, an official declaration of positioning, navigation and timing (PNT) services over the Asia-Pacific areas by regional BDS (BDS-2) was made on 27 December 2012. Six years later, the global BDS (BDS-3) officially provided the PNT services for global users. The last networking satellite of BDS-3 started to work after passing the phase of flight test on 23 June 2020, and the number of available BDS satellites is increased to 50, including 16 BDS-2, four experimental BDS-3 and 30 BDS-3 satellites. As of May 2021, there are 26 available Galileo satellites, including 4 In-Orbit Validation (IOV) and 22 Full Operational Capability (FOC) satellites. Actually, the Quasi-Zenith Satellite System (QZSS), which is one of regional navigation satellite systems (RNSSs), also supports the precise services. The design of QZSS constellation carefully considers the widespread mountainous areas and urban canyons in Japan, so that more satellites can be observed under high elevation masks. Since 2018, QZSS with four operational satellites has been officially providing the initial PNT services for the users located in Asia-Pacific regions. In order to make QZSS-only constellation capable of providing continuous and reliable PNT services, seven more QZSS satellites will be launched by 2023. Currently, the number of operational BDS-3, BDS-2, GPS, GLONASS, Galileo and QZSS satellites can be up to about 136, the joint use of which will significantly enhance the PPP solutions in terms of accuracy, convergence speed, reliability and availability, especially in constrained visibility environments, such as urban canyons and mountainous areas. Multi-system combined PPP was originally carried out on the GPS/GLONASS integration. Both the stochastic component and the functional component of GPS/GLONASS combined PPP model were characterized in Cai and Gao [9]. In the kinematic test conducted with a land vehicle, the inclusion of GLONASS observations could significantly improve the positioning accuracy by 57%, 69% and 65% (test)-2 ificantlyrespectively [11]. Since the official declaration of the regional PNT services of BDS-2, many researchers have focused on the GPS/BDS-2 combined PPP. [PERSON] et al. [12] investigated the modeling and prediction of inter-system bias (ISB) between GPS and BDS-2, and the convergence time of GPS/BDS-2 PPP could be reduced by 16.1%, 19.6% and 2.4% in the east, north and up directions when taking the predicted ISBs as a priori constraints, respectively. Following [PERSON] et al. [13], 90.6% of the sessions had the TTFF within 1350 s for GPS-only PPP AR, while 91.9% of the sessions could achieve the TTFF within 870 s for GPS/BDS-2 PPP AR. Since 27 December 2018, the global PNT services of BDS-3 have attracted the increasing attention from the GNSS community. [PERSON] et al. [14] evaluated the contribution of BDS-3 observations to the PPP processing, and GPS/BDS-2/BDS-3 PPP could provide a kinematic accuracy of 3.5, 4.6 and 6.0 cm in the three directions, respectively. As for Galileo, its integrated PPP processing with GPS based on single-frequency observations could provide sub-decimeter level position accuracy [15]. According to [PERSON] et al. [16], after adding Galileo measurements to the GPS PPP processing, both the position accuracy and convergence time of single-frequency PPP as well as the convergence time of dual-frequency PPP could be enhanced. [PERSON] et al. [17] assessed the performance of GPS/Galileo combined PPP during 2015-2018. The kinematic positioning accuracy of GPS/Galileo PPP was improved by about 5 mm in 2017 and 2018 in comparison with GPS-only PPP. Regarding QZSS (only four in-orbit satellites), [PERSON] et al. [18] evaluated its contribution to GPS kinematic PPP, and the positioning accuracy was slightly improved from 2/1/6 cm to 2/1/5 cm in the east/north/up directions, respectively.