In past decades, general tracking and navigation systems such as GPS, GLONASS, Galileo, BeiDou, or QZSS have only used signals in the L1 frequency band. The positioning accuracy is about 1 to 10 meters. With the maturity of positioning technology, modern GNSS receivers have added another low-frequency band, such as L2, L5 and E6. Receiving GNSS signals in multiple frequency bands, including high and low frequencies, to eliminate ionospheric errors, improve multi-path effects, and reduce narrow frequency band interference, and achieve sub-meter positioning accuracy.