## 3 Results
### Four-GNSS
The satellite visibility of 24-GLONASS and 30-GLONASS as well as the GDOP are shown in Figure 4. It can be seen that 7-10 satellites of 24-GLONASS can be observed worldwide. Fewer satellites can be observed over mid-latitude regions (\(5^{\circ}\leq|B|\leq 45^{\circ}\)) if compared to visibility at other latitudes. Adding six GLONASS-I to 24-GLONASS increases the number of visible satellites by 1.7-6 (average: 3.46) in the Eastern Hemisphere. In Russia and its surrounding areas (20\({}^{\circ}\)E-170\({}^{\circ}\)W, 40\({}^{\circ}\)N-85\({}^{\circ}\)N), the improvement is from 1.8 to 4.33 (average: 3.08). The GDOP of 24-GLONASS is smaller at latitudes of 45-70\({}^{\circ}\) than it is at lower latitudes and at the north and south ends. For 30-GLONASS, the GDOP decreases by 15.7% on average in the Eastern Hemisphere and by 14.4% in (20\({}^{\circ}\)E-170\({}^{\circ}\)W, 40\({}^{\circ}\)N-85\({}^{\circ}\)N) regions. Figure 3: Satellite visibility and GDOP of Galileo: (**a**) visible satellites of Galileo 1 st generation; (**b**) visible satellites of Galileo 2 nd generation; (**c**) GDOP of Galileo 1 st generation; (**d**) GDOP of Galileo 2 nd generation. The GPS modernization program includes 10 GPS III satellites and 22 GPS IIIF satellites. The previous generation orbit parameters are still adopted for the 32 MEO satellites [7,38]. The satellite visibility and GDOP of the GPS are plotted in Figure 5. More GPS satellites can be observed over low latitudes (\(|B|\leq 15^{\circ}\)) and high latitudes (\(|B|\geq 60^{\circ}\)), which is similar to the distribution of Galileo. Compared to BDS-3 and 30-GLONASS, the number of visible GPS and Galileo satellites is more globally balanced. However, although there are fewer satellites in the BDS-3 and 30-GLONASS constellations, they show better GDOP in the Asia-Pacific region and in the high latitudes of the Eastern Hemisphere, which are benefits of GEO and IGSO satellite-augmented MEO constellations. Figure 4: Satellite visibility and GDOP of GLONASS: (**a**) visible satellites of 24-GLONASS; (**b**) visible satellites of 30-GLONASS; (**c**) GDOP of 24-GLONASS; (**d**) GDOP of 30-GLONASS. Figure 5: (**a**) visible satellites of GPS; (**b**) GDOP of GPS. The satellite visibility and GDOP of the four-GNSS and four-GNSS-NG are illustrated in Figure 6. The number of visible four-GNSS satellites is 33-45 worldwide (average: 38.25). When the four-GNSS are upgraded, the average number visible satellites of the four-GNSS-NG will reach 44.86, which is an increase of 6-7 satellites compared to four-GNSS. With the help of the BDS-3I, BDS-3G, and GLONASS-I satellites, the Asia-Pacific region will become the hottest GNSS area in the world, and more than 45 satellites will be able to be observed. In contrast, the number of observed satellites in the North, Central, and South America regions will be fewer. However, at least 37 satellites will still be able to be observed. Overall, the combination positioning of the four-GNSS-NG can greatly enhance the availability of satellite navigation and can provide more observations for global users. A benefit of the satellite visibility increasing is that the four-GNSS-NG satellites exhibit a better geometrical structure, which is helpful for the significant decrease in the GDOP. The average GDOP of the four-GNSS-NG is 1.19, which is an improvement rate of 7.8% on average compared to that of four-GNSS (average: 1.29). Additionally, at high-latitude regions, the there is a distinct GDOP increase of nearly 1.2, which is mainly caused by the lower elevation of the satellites observed and an increase in the station latitude. Based on Equation (15), we calculated the DOPs of combined positioning of the four-GNSS-NG. The results are presented in Figure 7. From Figure 7a,b, we found that the distributions of PDOP and TDOP are similar to the GDOP. The average PDOP and TDOP are 1.03 and 0.60, respectively. As shown in Figure 7c,d, the HDOP is from 0.42 to 0.56 (average: 0.49), and the VDOP is between 0.73 and 1.13 (average: 0.90).