GNSS’s full name is the Global Navigation Satellite System, it refer to all satellite navigation systems, including global, regional,and enhanced navigation system like American GPS, Russian Glonass, European Galileo, Chinese Beidou,and related enhanced systems, such as American WAAS, European EGNOS,and Japanese MSAS ect, it also covers others satellite navigation systems that will be built and built later. International GNSS system is a complex system with multi-system, multi-level, and multi- mode .