中文摘要：

利用GPS、GLONASS、Beidou和Galileo四系统的观测数据以及MGEX精密轨道和钟差产品，研究多系统融合精密单点定位的理论模型，并分析其收敛速度和定位精度。结果表明，静态定位时，Beidou系统收敛较慢，收敛后平面精度优于5cm，高程精度优于8cm，四系统融合收敛速度最快，定位精度和GPS接近；动态定位时，Beidou平均收敛时间在110min以上，平面定位精度优于8cm，高程精度优于16cm，四系统融合显著提升了收敛速度，但是定位精度和GPS相比没有明显提升。在截止高度角大于30。条件下，GPS系统定位偏差较大，而多系统依然能够保证足够数量的可见卫星，从而保证可靠的定位精度。

英文摘要：

The theory and algorithms of multi-GNSS precise point positioning（PPP） are studied and the convergence time and positioning precision are also analyzed in this article. We use observations from GPS, GLONASS, Beidou and Galileo and the MGEX precise ephemeris and clock from GFZ. The results show that, in static PPP, the convergence speed of Beidou is slow and the positioning ac- curacy of Beidou is about 5 cm in horizontal, 8 cm in vertical. The convergence speed of G/R/C/E combined PPP is faster than any single system and the positioning accuracy is close to GPS. In kine- matic PPP, the average convergence time of Beidou is 110 min or more and the positioning accuracy of Beidou is about 8 cm in horizontal, 16 cm in vertical. The convergence speed of G/R/C/E combined PPP is also the fastest, whereas the positioning accuracy is not significantly improved compared to GPS. Under the circumstances of higher cut-off elevation, the positioning errors of GPS are larger than multi-GNSS, since multi-GNSS can observe more satellites than GPS.