## 1 Introduction  
In the past two decades, precise point positioning (PPP) has gradually been developed for GPS [1; 2]. Through the PPP algorithm, only one receiver can achieve the absolute positioning of the target point. Compared to other positioning methods, PPP has massive application scenarios, including seismogeodetic, engineering survey, offshore exploration, geometric orbit estimation for satellites, emerging applications, etc. [3; 4; 5; 6; 7; 8]. The main disadvantage of using PPP algorithms is the interminable convergence durations required to attain improved positioning accuracy. With the development of satellite constellations and tracking infrastructure, PPP using multi-system observations has become increasingly popular. To profit from multi-system and multi-frequency observations, the user must be provided with all calibration parameters that are system-dependent [9; 10]. As a result, the research community and enterprise are working on developing GNSS processing algorithms, methodologies, and associated models. The International GNSS Service (IGS) launched a MGEX to track, collect, and evaluate all GNSS signals available [11]. MGEX analysis centers (ACs) already provide precise satellite products, including satellite orbits and clock corrections, for all GNSS constellations [12]. The difference between multi-system PPP and single-system PPP is that the clock deviation between different systems and the corresponding inter-system bias (ISB) and hardware delay need to be estimated. In order to obtain high-accuracy positioning results, precision ephemeris and orbit selection are important. Previous research has explored and compared the performance of PPP with products from various analysis centers (ACs) [13; 14; 15; 16; 17; 18]. With comprehensive reference to these studies, the precision ephemeris of GFZ is chosen to conduct a series of multi-system PPP experiments in this paper. Initially, the system combination PPP was mainly based on GPS and GLONASS [19; 20; 21; 22]. Researchers have been studying the performance of the combined GPS and GLONASS PPP, which shows improved accuracy and shorter convergence time when compared to GPS-only PPP. With the rapid development of muti-frequency BDS and Galileo in recent years, GPS combined with BDS [23; 24; 25; 26; 27] and GPS combined with Galileo [28; 29; 30; 31] have received more and more attention. Especially since the opening of BDS-3 in 2020, the sufficient number of satellites on BDS-3 can further improve the geometric distribution of the satellite constellation and increase the number of observations. Redundant observations can be better realized to improve the positioning accuracy and shorten the convergence time of PPP positioning. Furthermore, the latest research has examined the performance of the combined four constellations (GPS, GLONASS, Galileo, and BeiDou) PPP. Single-system PPP will be gradually replaced by multi-system PPP [32; 33; 34; 35; 36; 37]. These studies compare the positioning accuracy and convergence time of different system combinations with those of single systems, such as GPS, but most of them select a small number of stations and do not present the corresponding distribution pattern on a global scale. Therefore, in this paper, GPS, BDS-3, and Galileo are selected to study the gain of the system combination with respect to the single system. In order to provide a comprehensive assessment for the different system PPP solutions on a global scale, 143 MGEX tracking stations, which can receive signals from these three systems, have been selected worldwide. The positioning accuracy and convergence time distribution of the three single systems are analyzed. Then, the gain of the three dual-system combinations, including the GPS with BDS-3 combination (GC), GPS with Galileo combination (GE), and BDS-3 with Galileo combination (CE), with respect to the corresponding single-system combinations is evaluated.