# Analyses of GLONASS and GPS+GLONASS Precise Positioning Performance in Different Latitude Regions
## 1 Introduction  
Currently, four satellite navigation systems with global coverage have been developed: GPS, GLONASS, BDS, and Galileo. A notable design difference among the different constellations is the satellite orbital inclination angle. The inclination angle is \(55^{\circ}\) for GPS, \(56^{\circ}\) for Galileo, \(55^{\circ}\) for BDS, and \(64.8^{\circ}\) for GLONASS. Among the four constellations, the GLONASS has the largest orbit inclination angle, which is about \(10^{\circ}\) larger than other systems, to provide the availability of the high-latitude of the Soviet Union. The first Final Operational Capability (FOC) of GLONASS was achieved in 1995. However, due to the short satellite service life and the budget decrease, the GLONASS constellation dropped to 7 satellites by 2002 [1]. During 2001-2011, the GLONASS program progressed steadily, and by late 2011, GLONASS declared FOC again. During the period of several satellites, many studies have been performed to investigate the advantages and disadvantages of combining GPS and GLONASS [2, 3]. [PERSON] [4] concluded that using the GLONASS constellation of 13 satellites does not significantly improve the precision of the double-difference (DD) network solutions, and similarresults have been obtained with IGS and CODE (Center of Orbit Determination in Europe) orbits. [PERSON] [5] obtained similar results with 16 GLONASS satellites. Additionally, [PERSON] and [PERSON] [6] indicated that adding GLONASS satellites in Precise Point Positioning (PPP) would reduce the convergence time and improve the positioning accuracy. As GLONASS was gradually restored, in terms of DD network processing, [PERSON] et al. [7] illustrated that the GLONASS stand-alone baseline solutions are inconsistent compared to that of GPS and that solutions using the combined GPS and GLONASS constellations do not provide any superiority over stand-alone GPS. [PERSON] et al. [8] presented that the additional GLONASS observations add little improvement to the estimates of the coordinates when compared to GPS-only processing. [PERSON] et al. [9] concluded that the repeatability of GLONASS coordinates is slightly worse than that of GPS. The research on GLONASS PPP increased as the usage of PPP increased. [PERSON] and [PERSON] [10] indicated that integrating the GLONASS with GPS could not significantly improve the PPP accuracy if the stand-alone GPS has adequate visible satellites with good observation geometry. [PERSON] et al. [11] also revealed that the static PPP performance among GPS, GLONASS, and GPS+GLONASS with long observation periods was similar. [PERSON] et al. [12] further demonstrated that the benefits of combining GLONASS with GPS in daily static PPP are negligible. [PERSON] et al. [13] assessed the static PPP performance of GPS, GLONASS, and GPS+GLONASS, and concluded that the GLONASS PPP could achieve similar coordinate estimate accuracy as GPS and GPS+GLONASS in daily solutions. However, [PERSON] [14] provided a different conclusion that the accuracy of undifferenced ionosphere-free dual-frequency PPP with GPS and GLONASS observations is better than that of GPS. [PERSON] et al. [15] obtained similar results with single-frequency PPP. The analysis of PPP convergence time indicates that the combination of GPS and GLONASS significantly shortened the convergence time of static PPP solutions [10; 16]. [PERSON] and [PERSON] [17] studied the combination of GPS and GLONASS and illustrated that the convergence time of ambiguity-float static PPP could be reduced by 45.9% compared to GPS. There are also many studies concerning the contribution of GLONASS to three or more GNSS systems' combined constellations [18; 19; 20]. However, the previous research rarely considers the constellation characteristics of GLONASS, especially the effect of the large orbit inclination angle of the GLONASS constellation, which benefits the positioning performance in high latitude regions.