Stepping into the 21st century, because of the successful launches of the BeiDou and the Galileo satellites, the GNSS is changing from the bipolar competition situation between GPS and GLONASS to a new situation of competition and cooperation among four systems. Obviously, the compatibility and interoperability of navigation signals have already been the main features during the period of GNSS development. Against this background, open signals, with the function of interoperability, are expected to bring GNSS services of higher quality and better performance to GNSS users, especially in such places as urban canyons and mountainous areas where visual airspace of satellite is limited. Open signals can bring a significant increase in the number of visible navigation satellites, improving the reliability and availability of navigation and positioning services (Yang, 2010). So far, the main existing and being designed signals of GNSS Open Services are as shown in Table 1.
At present, the four satellite navigation systems have issued or are planning to issue their own specifications on open service performance. However, because of the diversity in their respective conditions and knowledge, there may be large differences in those specifications, either in form or in performance. As a result, users will get confused when they are using and it will be inconvenient for them to use. In addition, there are no performance specifications on GNSS open service signals. To ensure the safety of the usage and achieve the ultimate goal of interoperability of GNSS Open Services signals, it is essential for us to research the monitoring and assessment of GNSS Open Services.
Monitoring and assessment of GNSS Open Services could provide third-party information on performance for a single system and reliable decision-support
TABLE 1 Signals of GNSS Open Services
|Global Navigation Satellite Systems||Frequency||Center Frequency (MHz)||Modulation Mode||Interoperable or Not|