Improved PPP Ambiguity Resolution by a cascaded method of Separating Orbit Errors from Satellite FCBs

摘要

Precise point positioning (PPP) integer ambiguity resolution is able to significantly shorten the time-to-first-fix (TTFF) and improve the positioning accuracy when correction for fractional cycle biases (FCBs) are available to recover the integer feature of the ambiguities. When real-time orbit and clock products are fixed to estimate the satellite FCB corrections, the narrow-lane (NL) FCB corrections will be biased by the Line-Of-Sight (LOS) errors of the orbits and clocks. Then the biased NL FCB estimates will affect ambiguity resolution (AR) performance as well as positioning accuracy. We propose a cascaded method to effectively separate the orbit errors from the satellite FCBs and to broadcast both as improved satellite FCBs for PPP users to further improve ambiguity resolution performance. The satellite NL improved FCB corrections are modeled by one direction-independent component and three directionaldependent components for each satellite, instead of only one direction-independent component. The directionindependent component assimilates actual FCBs whereas the directional-dependent components are used to assimilate the orbit errors. Besides, the cascaded method avoids estimated large amount of parameters in the same time which guarantees the model strength. GPS measurements from both regional and global experimental networks have been processed with the IGS Real-Time Service (RTS) and rapid products. The improvements by the proposed FCB estimation method are validated in terms of the mean and RMS values of the estimated FCBs, the performance of recovering the integer property of ambiguities as well as the obtainable positioning accuracy. The numerical results confirm that the obtained FCBs using the proposed method outperform the original FCBs which improve not only the ambiguity fix rate but also the positioning accuracy. Compared to those with the original FCBs, the percentage of the fractions of the ambiguities within 0.15 are improved by up to 12.2%. The horizontal position accuracy can be improved by up 28.8%.