At present, the four major satellite navigation systems of the United States GPS, Russian GLONASS, Chinese Beidou, and European Galileo have provided global services. India’s Navlc (Indian Navigation Constellation) and Japan’s QZSS (quasi-zenith system) also provide regional services to users.
The positioning accuracy of a satellite navigation system is mainly determined by two factors, one is the position accuracy attenuation factor (PDOP), and the other is the user equivalent ranging error (UERE). PDOP mainly depends on the coverage characteristics of the constellation. At present, the average value of PDOP of the four major global satellite navigation systems is equivalent, which is generally around 2. The UERE consists of three parts: the satellite orbit clock difference in space, the ionospheric and tropospheric errors in transmission, and the receiver noise and multipath errors in user segment. The orbital clock difference of the space segment can be measured by the User Range Error (URE), which is an important indicator for evaluating the accuracy of the space signal and the main standard for the performance of major international systems.
The US GPS system was developed in the 1970s and was fully completed in 1994. At present, the GPS system has a total of 35 satellites in orbit and 30 are operating normally. The average value of the GPS system’s space signal accuracy is 0.5 meters (the worst is 0.66 meters, and the best is 0.36 meters).
The Russian GLONASS system was launched in 1976, and a full constellation deployment was achieved in 1995. At present, GLONASS has incorporated its star-based augmentation system (SDCM, differential correction monitoring system) into the system. The system has a total of 3 geostationary orbits (GEO) and 27 medium circular earth orbit (MEO) satellites. The positioning accuracy level of the GLONASS system is better than 5 meters and the elevation is better than 9 meters, which is comparable to that of other systems. Considering that the satellite constellations of the major systems are basically complete, the average PDOP values are equivalent, and after deducting the error factors of the transmission section and the user section, it can be known that the spatial signal accuracy of the GLONASS system is basically equivalent to other global systems.
3. Beidou System
The Beidou system currently has a total of 46 satellites in orbit, including 28 Beidou-3 satellites and 18 Beidou-2 satellites. The satellites are in good orbit and the quality of the space signal is excellent. The accuracy of Beidou-3 signals is better than that of Beidou-2 signals. At present, the average space signal accuracy of Beidou-3 satellite is 0.41m. Under the current constellation conditions, the B1I and B3I signals have a positioning accuracy level of about 3.6m, an elevation of about 6.6m, a speed measurement accuracy of about 0.05m/s, a timing accuracy of 9.8ns (95% confidence), and an accuracy of about 30% in the Asia-Pacific region; The B1C and B2a signals have a positioning accuracy level of about 2.4m, an elevation of about 4.3m, a speed measurement accuracy of about 0.06m/s, and a timing accuracy of 19.9ns (95% confidence). Compared with the US GPS, Russian GLONASS and other systems, the Beidou system’s spatial signal accuracy is equivalent, and its positioning, speed measurement and timing capabilities are excellent. It is believed that with the gradual improvement of the Beidou satellite constellation and the further increase of satellites connected to the network, the service accuracy of the Beidou system will be further improved.
Galileo currently has 22 satellites in orbit, including 4 satellites with initial operational capability (IOC) and 18 satellites with full operational capability (FOC). In terms of service performance, the Galileo system currently has an average spatial signal accuracy of 0.27m and a timing accuracy of 16.8ns. It is worth noting that in 2017, up to 9 out of 72 satellite clocks of 18 satellites (including cymbals and hydrogen clocks) failed in the Galileo system. In 2019, up to 117 hours of service interruption occurred, affecting service. Reliability has increased users’ doubts about using their services.