A Tightly Combined GPS/Galileo Model for Long Baseline RTK Positioning with Partial Ambiguity Resolution

摘要

Knowledge of differential inter-system biases (DISBs) is critical to integrate observations from mixed GNSS. If the corresponding DISB could be calibrated in advance, only one pivot satellite is sufficient for ambiguity resolution on overlapping frequencies, which is the so-called tight combining (TC) strategy. Considering that GPS and Galileo transmit signals on two identical frequencies (e.g. L1/E1 and L5/E5a), a tightly combined GPS/Galileo RTK positioning model is proposed in this paper. Traditional DD model has been slightly adjusted to avoid the hand-over problem of reference satellites. The estimation of code and fractional part of phase DISB is archived through zero and ultra-short baselines. Three long baselines were selected to verify the proposed model with DISB calibrated in advance. Moreover, to get better AR performance, a simple but robust procedure of PAR, where the satellite elevation, number of consecutive tracking, success rate and ratio test are all combined to determine the subset of ambiguity, is adopted in the long baseline experiments. Results shows that the code and fractional part of phase DISB is rather stable. The TC strategy do not significantly improve the value of ratio, but shorten the convergence time to reach the 100% success rate. Compared with results of loose combining (LC) strategy, time to first fix (TTFF) is further reduced by 54.3, 72.9, 69.0% respectively under TC strategy corresponding to different long baselines. Besides, TC strategy could slightly improve the fixing rate of epochs. In terms of accuracy, the precision in up direction is worse than that in north and east direction. Once the ambiguity is fixed correctly, both LC and TC strategy can achieve centimeter-level positioning accuracy.