LAUSR

Abstract The mixed function consisting of the harmonic and polynomial terms is used to model and study the characteristics of the GNSS satellite clock. The harmonic terms are obtained from the Fast Fourier Transform (FFT) results. The modeling results of the GNSS satellite clocks indicate that the RMS of GLONASS is the largest, and that of GPS Cs clocks is at the same level. The results also show that the RMS of BDS-2 is close to that of Rb clocks onboard GPS satellites and the RMS of Galileo is the smallest. The amplitudes and initial phases of the harmonic coefficients for the Rb clocks carried on GPS Block IIRM and Block IIF satellites have obvious periodic characteristics at 12 h and 8 h over the year. For BDS-2 IGSO satellites, the amplitudes for the periods of 12 h, 8 h, and 6 h have obvious periodic characteristics, and also the initial phases for the periods of the 12 h and 8 h. The amplitudes for the periods of 12 h and 8 h for the full Galileo constellation have periodic characteristics. The correlation of modeling coefficients is used to analyze the stability of four GNSS systems. The results show that the correlation of Galileo is better and more stable than other systems. The 2 h, 6 h and 12 h GNSS satellite clock values are predicted with 8 and 4 periods mixed functions and these results are compared with the IGS final clock, Broadcast Ephemeris (BRDM) and Ultra-Rapid (ISU) products. The results show that the RMS of 8-period mixed function is slightly smaller than that of 4-period for most satellites and the most of the predicted values are better than that of the clock products.