The Global Navigation Satellite System (GNSS) includes the American Global Positioning System (GPS), the Chinese BeiDou Navigation Satellite System (BDS), the Russian GLONASS system, and the European Galileo Satellite Navigation System, as well as regional augmentation systems such as the Japanese Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite (IRNSS). GNSS features all-weather, all-time, real-time, and high-precision capabilities with continuously transmitting L-band signals, and has been widely used in positioning, navigation, and timing (PNT). With the improvement of satellite navigation systems and the increase in constellations and observation stations, its application prospects are becoming increasingly broad. Today, GNSS has been widely used in transportation, power telecommunications, public safety, precision agriculture, basic surveying and mapping, resource surveys, Earth science, space science, and some new fields that are yet to be developed.