Real-time precise orbit and clock estimation of multi-GNSS satellites with undifferenced ambiguity resolution

Real-time precise orbit and clock products are necessary for Global Navigation Satellite System (GNSS) real-time precise applications. In the classical strategy, the real-time orbit is predicted from post-processed least-squares solutions and then the clock is estimated in real-time filtering, which are quite different and separate processes. We proposed an integrated filter method in which the satellite orbit and clock states are estimated simultaneously based on the undifferenced observation model. With the estimation of satellite and receiver uncalibrated phase delays (UPDs), the undifferenced ambiguities are resolved in real time, resulting in the ambiguity-fixed satellite orbit and clock solutions. One-month observations of 150 globally distributed stations from multi-GNSS experiment tracking network are processed using the proposed method. In the experiment, the RMS of wide-lane (WL) and narrow-lane (NL) UPD residuals is all less than 0.07 cycles and 92% of WL and NL UPD residuals are within +/- 0.1 cycle, contributing to a high fixing success rate of more than 90% for both GPS and Galileo satellites. Comparison with the IGS and CODE final orbit products shows that ambiguity resolution (AR) brings about 45% and 44% improvements to 3D RMS of the filter-based orbit solutions, from 8.2 to 4.7 cm and 9.5 to 5.4 cm for GPS and Galileo satellites, respectively. For comparison, the prediction orbits of the IGU and GBU products in the same periods are also evaluated. The average 3D RMS of the ultra-rapid products in the same periods is 5.3 cm and 7.8 cm for GPS and Galileo satellites, respectively, which are larger than that of the filter orbits. Compared to the float solutions, the STDs of GPS and Galileo satellite clocks are improved by more than 40% after AR. In addition, both convergence time and accuracy of kinematic precise point positioning AR by using filter-based products are better than that of using ultra-rapid products.