**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◦ for GPS, 56◦ for Galileo, 55◦ for BDS, and 64.8◦ for GLONASS. Among the four constellations, the GLONASS has the largest orbit inclination angle, which is about 10◦ 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]. Bruyninx [4] concluded that using the GLONASS constellation of 13 satellites does not significantly improve the precision of the double-difference (DD) network solutions, and similar

**Citation:** Zheng, Y.; Zheng, F.; Yang, C.; Nie, G.; Li, S. Analyses of GLONASS and GPS+GLONASS Precise Positioning Performance in Different Latitude Regions. *Remote Sens.* **2022**, *14*, 4640. https:// doi.org/10.3390/rs14184640

Academic Editor: Xiaoli Deng

Received: 3 August 2022 Accepted: 12 September 2022 Published: 16 September 2022

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results have been obtained with IGS and CODE (Center of Orbit Determination in Europe) orbits. Habrich [5] obtained similar results with 16 GLONASS satellites. Additionally, Cai and Gao [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, Alcay 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. Nardo et al. [8] presented that the additional GLONASS observations add little improvement to the estimates of the coordinates when compared to GPS-only processing. Zheng 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. Cai and Gao [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. Yigit et al. [11] also revealed that the static PPP performance among GPS, GLONASS, and GPS+GLONASS with long observation periods was similar. Choy et al. [12] further demonstrated that the benefits of combining GLONASS with GPS in daily static PPP are negligible. Mohammed 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, Malik [14] provided a different conclusion that the accuracy of undifferenced ionosphere-free dualfrequency PPP with GPS and GLONASS observations is better than that of GPS. Hamed 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]. Li and Zhang [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 [18–20]. 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. Therefore, this paper aims to evaluate the performance of GLONASS and its contribution to GPS+GLONASS processing in different latitude regions in terms of satellite visibility and positioning performance. Three networks located in high, middle, and low latitude regions are employed. The performance of both daily DD network solutions and daily static PPP solutions is used for the study.

The structure of this article is arranged as follows. Section 2 describes the methods of data quality evaluation, the positioning strategies, and the evaluation indicators. Section 3 describes the data and data selection factors. Section 4 presents the data quality results, the constellation visibility of different systems, as well as the analysis and discussion of the performance of the DD network and PPP solutions. Finally, the main conclusions and findings are shown in Section 5.
